Next‐Generation Aerogel Electrocatalysts for Oxygen Reduction Reaction

ABSTRACT

Aerogel‐based electrocatalysts have become a groundbreaking class of materials for the oxygen reduction reaction (ORR), distinguished by their ultralight nature, pore networks, and outstanding electrical conductivity. Their interconnected three‐dimensional frameworks offer abundant active sites, efficient diffusion channels, and continuous electron transport pathways—key attributes that overcome the kinetic and stability limitations traditionally faced in fuel cells and metal–air batteries. This study presents a comprehensive review of aerogel‐based electrocatalyst materials for ORR. The overview of how structural configuration, composition, and reaction mechanisms influence ORR performance was discussed. Various fabrication approaches, such as sol–gel processing, freeze‐drying, supercritical drying, and template‐guided methods, are examined. Recent developments in metallic, heteroatom‐doped carbon, and conductive hybrid aerogels are analyzed in detail, with emphasis on their performances for ORR, structure–property relationships, advanced characterization for identifying active centers, and mechanistic insights. Furthermore, the integration of these materials into functional devices, along with their operational stability, is critically discussed. Future progress will depend on the synergistic integration of scalable synthesis, mechanistic understanding, advanced characterization, and computational design. The convergence of materials informatics, green manufacturing, and bioinspired chemistry is expected to accelerate the transition of aerogel‐based electrocatalysts from laboratory prototypes to practical energy technologies powering sustainable societies.

1. Introduction

The oxygen reduction reaction (ORR) represents a critical electrochemical process that governs the performance of various renewable energy conversion and storage technologies, including proton exchange membrane fuel cells (PEMFCs), alkaline fuel cells, and metal–air batteries [1, 2]. Despite decades of intensive research, the ORR remains the primary kinetic bottleneck at the cathode of these devices due to its intrinsically sluggish multi‐electron transfer pathways, which necessitate high overpotentials and substantially limit the overall energy conversion efficiency [3, 4, 5, 6, 7, 8, 9]. This kinetic challenge has historically driven the field toward platinum‐group metal catalysts, particularly platinum and its alloys, which exhibit the highest known intrinsic activity for ORR. However, the widespread commercialization of fuel cell technologies continues to be severely constrained by the scarcity, high cost, and limited durability of platinum‐based catalysts, which can account for up to 40% of the total fuel cell system cost [10, 11, 12, 13].

The pursuit of alternative electrocatalyst architectures that can either reduce platinum loading while maintaining performance or completely replace precious metals has emerged as one of the most critical research frontiers in electrochemistry and materials science [14, 15]. Among various innovative approaches, aerogel‐based electrocatalysts have garnered significant attention in recent years due to their unique three‐dimensional (3D) hierarchical porous structures, ultrahigh surface areas, and exceptional mass transport properties (Figure 1) [16, 17, 18, 19, 20, 21, 22, 23, 24]. Aerogels are  characterized by their extremely low density (<0.1 g cm^{− 3}) and interconnected nanoscale networks [20, 25, 26, 27]. They offer a compelling platform for addressing the fundamental challenges associated with ORR electrocatalysis by simultaneously optimizing active site density, electronic conductivity, and reactant/product diffusion pathways [7, 28, 29].

FIGURE 1.

Unique characteristics of aerogel materials for electrocatalytic ORR.

The rational design of aerogel electrocatalysts leverages their distinctive structural features to overcome multiple limitations encountered in conventional catalyst systems. First, the specific surface area (often exceeding 500 m² g^{− 1}) enables the creation of unprecedented active site densities, including atomically dispersed metal centers that maximize metal utilization efficiency [7, 30, 31, 32]. Second, the pore architecture—spanning micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) facilitates rapid mass transport of oxygen molecules and protons/hydroxide ions to active sites while efficiently removing reaction products, thereby minimizing concentration polarization losses that plague dense catalyst layers [33, 34]. Third, the self‐supported 3D conductive framework provides continuous electron pathways throughout the catalyst structure, eliminating the need for conductive additives and reducing interfacial resistances that compromise performance in traditional carbon‐supported catalysts [35, 36].

Recent advances in aerogel synthesis and functionalization have enabled the development of three major classes of ORR electrocatalysts: (i) carbon‐based aerogels incorporating heteroatom dopants (N, S, P) or defect engineering to create metal‐free active sites; (ii) metallic aerogels composed of noble metal alloys (Pt─Ni, Pd─Cu─Fe) or noble metal‐free compositions that exploit strain and ligand effects to enhance intrinsic activity; and (iii) hybrid aerogels that integrate atomically dispersed metal—nitrogen–carbon (M–N–C) sites or molecular catalysts (porphyrins, phthalocyanines) within porous carbon scaffolds [37, 38, 39, 40]. Each class demonstrates distinct advantages and trade‐offs in terms of activity, selectivity, durability, and cost, with state‐of‐the‐art examples approaching or even surpassing the performance of commercial Pt/C catalysts in specific metrics [41, 42].

This review synthesizes recent progress in the application of aerogel electrocatalysts for ORR. It discussed the structural advantages of aerogel architectures with the corresponding synthesis strategies and structure control of aerogel materials for ORR. In addition, we provided extensive insights that emphasize structure–property relationships, mechanistic insights, and practical device integration. The evaluation of the strategies employed to enhance ORR performance in this study is anticipated to evoke considerable interest among researchers in the pursuit of designing and advancing functional aerogel‐based materials for electrocatalytic ORR. Furthermore, we discuss the current challenges limiting widespread adoption of aerogel electrocatalysts, such as scalability, durability under operating conditions, and standardized performance evaluation, and outline promising future directions for this rapidly evolving field.

2. Structural Advantages of Aerogel Architectures for ORR

The exceptional performance of aerogel electrocatalysts stems from their unique structural characteristics that directly address the fundamental requirements for efficient ORR. The 3D interconnected network structure provides several critical advantages over conventional catalyst designs [43, 44].

2.1. High Active Site Density and Accessibility

Aerogel morphologies enable the incorporation of ultrahigh concentrations of catalytically active sites while maintaining their accessibility to reactants. Unlike bulk materials or densely packed nanoparticle assemblies, the open porous structure of aerogels ensures that the majority of active sites remain exposed to the electrolyte and accessible to oxygen molecules [45]. Recent studies have demonstrated that aerogels can achieve atomically dispersed active site densities exceeding 10^{2 0} sites g^{− 1}, representing near‐theoretical metal utilization that is unattainable in traditional supported catalysts [46, 47].

2.2. Hierarchical Porosity for Enhanced Mass Transport

The pore characteristic of aerogels addresses one of the most significant challenges in practical fuel cell electrodes—simultaneous optimization of reaction kinetics and mass transport. Micropores (<2 nm) maximize surface area and host active sites; mesopores (2–50 nm) serve as highways for rapid oxygen diffusion and proton/hydroxide ion transport; and macropores (>50 nm) act as reservoirs that prevent flooding and facilitate gas‐phase oxygen delivery to the catalyst layer [48, 49]. This hierarchical organization substantially reduces concentration polarization losses that typically limit performance at high current densities, as demonstrated by studies reporting limiting current densities exceeding 7 mA/cm² in rotating disk electrode (RDE) measurements [2, 17].

2.3. Three‐Dimensional Conductive Framework

The continuous, self‐supported nature of aerogel networks provides percolating electron pathways throughout the catalyst structure, eliminating the contact resistance and charge transfer limitations associated with carbon‐supported nanoparticle catalysts [50, 51]. Metallic aerogels composed of noble metal alloys or conductive carbon aerogels offer electrical conductivities ranging from 10² to 10⁴ S m^{− 1}, ensuring rapid electron transport to active sites even in thick catalyst layers required for practical devices [52, 53]. This intrinsic conductivity also enhances mechanical robustness and stability under electrochemical cycling, as the monolithic structure resists the particle detachment and agglomeration mechanisms that degrade conventional catalysts [33].

3. Synthesis Strategies and Structure Control

The realization of high‐performance aerogel electrocatalysts requires sophisticated synthesis approaches that enable precise control over composition, morphology, pore structure, and active site distribution [54, 55]. Recent advances in synthesis methodologies have significantly expanded the design space for aerogel catalysts. Aerogels are an exceptional class of ultra‐light and highly porous materials that are produced when the liquid component of a wet gel is carefully replaced by a gas without collapsing the solid network structure. As illustrated in Figure 2, the key to aerogel formation lies in preventing capillary stress during solvent removal, which would otherwise destroy the fragile gel skeleton [44, 56, 57].

FIGURE 2.

Aerogel fabrication generally proceeds via a sol–gel process followed by supercritical drying to prevent the collapse of the particle network [58].

Typically, this transformation is achieved through controlled drying techniques, most notably supercritical drying or freeze‐drying, which allow the solvent to be removed under conditions that minimize surface tension forces. By maintaining the integrity of the nanoscale framework, these methods enable the production of aerogels that combine the structural features of solids with the lightness and permeability of gases. The resulting materials possess unique thermal, mechanical, and transport properties that make them particularly attractive for applications in catalysis, adsorption, energy storage, and environmental remediation.

Overall, the ability to convert wet gels into aerogels without structural collapse represents a critical advancement in materials science. It enables the design of highly tunable porous networks, where parameters such as pore size, surface chemistry, and connectivity can be engineered through careful control of precursor composition and drying conditions. Thus, aerogels exemplify a remarkable intersection of chemistry, physics, and engineering principles, offering vast potential for developing multifunctional materials with superior performance in diverse technological domains.

The study reported by Hussain et al. demonstrated a systematic and well‐integrated approach to the synthesis and characterization of carbon aerogel (CA)‐supported mono‐, bi‐, and trimetallic Pd–Ni–Al catalysts for oxygen reaction [59]. The CA support was first synthesized via a classical resorcinol–formaldehyde sol–gel polymerization, followed by controlled aging, solvent exchange, supercritical CO₂ drying, and high‐temperature pyrolysis (1000°C, N₂), yielding a high‐surface‐area (664 m²/g), mesoporous, graphitic network ideal for metal dispersion. Metal incorporation (Pd, Ni, Al) was achieved through a one‐step microwave‐assisted polyol method using ethylene glycol under alkaline conditions (pH 12), enabling rapid nucleation, uniform adsorption on CA, and reduced agglomeration, particularly in bimetallic and trimetallic systems. The ICP‐MS analytical technique confirmed effective metal loading, while TEM images (Figure 3a) show that the CA support (100 and 200 nm scales) consisted of interconnected carbon spheres forming a 3D porous network and revealed the nanoscale distribution and size of Pd, Ni, and Al particles on its surface. Al nanoparticles are uniformly dispersed without noticeable agglomeration (Figure 3b). Pd particles in Pd/CA are larger and partially aggregated, with agglomeration significantly reduced in the bimetallic and trimetallic catalysts due to improved metal–metal interactions. The Debye–Scherrer XRD analysis confirmed FCC Pd, γ‐Al₂O₃ phases, and peak shifts in multimetallic samples, indicating alloy formation and lattice distortion. The XPS revealed mixed metallic/oxidized species and strong metal–support interactions, while Raman verified the graphitic CA structure and increased defect density. These application methods collectively linked structural and electronic features of the catalysts to their enhanced catalytic performance.

FIGURE 3.

A display of TEM images (100–200 nm) of support material and catalysts (a) CA, (b) Al/CA, (c) Pd/CA, (d) Pd–Al/CA, (e) Pd–Ni/CA, and (f) Pd–Ni–Al/CA [59].

3.1. Sol–Gel Chemistry and Gelation Control

The fundamental synthesis route for most aerogels involves sol–gel transitions, where molecular or colloidal precursors undergo controlled polymerization or assembly to form interconnected networks [18]. For metallic aerogels, strategies such as destabilization‐induced aggregation, template‐assisted assembly, and ligand‐mediated gelation have been developed to direct the formation of desired morphologies [60]. The aqueous synthesis of Pt‐Ni bunched‐nanocage aerogels, for example, exploits pH‐controlled destabilization of metal salt precursors in the presence of structure‐directing agents to generate hierarchical nanocage assemblies [1]. Critical parameters, including metal precursor concentration, reducing agent choice, gelation temperature, and aging time, must be carefully optimized to achieve reproducible aerogel structures with controlled properties [61].

As shown in Figure 4a, the sol–gel route typically starts with preparing a sol by dissolving metal alkoxides or metal salts in an appropriate solvent under controlled temperature conditions. As hydrolysis and condensation reactions proceed, the initially dispersed species progressively assemble into a 3D network, transforming the liquid sol into a semi‐solid gel composed of interconnected particles [62]. After gel formation, the entrapped solvent must be removed with great care, commonly through supercritical drying or freeze‐drying to avoid structural collapse. These drying strategies help maintain the gel's porous architecture, a factor that plays a decisive role in determining the final porosity and overall structural characteristics of the resulting material.

FIGURE 4.

(a) Schematic illustration showing the synthesis of aerogels via the sol–gel route [66]. (b) Approaches for synthesizing noble metal gels through sol–gel techniques [18]: Single‐level structured gels are produced using routes 1 and 2, corresponding to the one‐step and two‐step synthesis methods. In contrast, multi‐level structured gels are formed via routes 3–6, which involve either bottom‐up assembly or post‐synthetic engineering. Here, NPs refer to nanoparticles, while SE‐NPs indicate specially engineered nanoparticles.

Compared with other fabrication strategies, the sol–gel approach has become one of the most versatile and widely adopted methods for constructing noble metal frameworks. Its growing prominence stems from its numerous advantages, such as simple experimental procedures, mild synthesis conditions, and the capacity for precise control over material composition and structural morphology. The sol–gel process provides an effective means of generating nanostructured materials with tunable physicochemical properties, allowing for fine manipulation of porosity, particle size, and surface functionality. These capabilities render the sol–gel route an attractive platform for producing next‐generation functional materials for catalysis, sensing, and energy conversion applications, where performance is often dictated by subtle changes in nanostructural features.

Despite the broad application of sol–gel chemistry in fabricating oxides, composites, and hybrid porous systems, the gelation mechanism of noble metals displays unique and somewhat anomalous behavior that is not yet completely understood [63, 64, 65]. Conventionally, the formation of noble metal gels occurs in aqueous systems through a two‐step process: the initial reduction of metal precursors into dispersed nanoparticles, followed by the self‐assembly of these nanoparticles into interconnected, 3D gel networks [55, 57]. In certain formulations, these steps overlap, leading to a one‐pot, spontaneous gelation pathway driven by interparticle interactions, van der Waals forces, and solvent‐mediated aggregation. The complexity of these dynamic processes has made it difficult to establish universal mechanistic models, thereby highlighting the need for continued studies on the physicochemical principles governing noble metal gelation.

The resulting network architectures typically consist of branched nanowires that serve as the fundamental building blocks of noble metal aerogels. However, under different reaction environments such as variations in pH, reductant concentration, or temperature, secondary structural motifs may emerge, including interconnected clusters, nanosheets, and hierarchical porous frameworks. These multilevel architectures impart distinct mechanical stability, electrical conductivity, and surface accessibility, which are crucial for optimizing performance in catalytic and electrochemical systems. Accordingly, the previous study has categorized the gel structures into single‐level and multi‐level hierarchical types, as schematically depicted in Figure 4b. It also reported the current mechanistic insights into the gelation pathways, providing a conceptual basis for the rational design and controlled assembly of noble metal foams (NMFs) with enhanced structural and functional attributes [18, 66].

Chen et al. presented a facile one‐step wet‐chemical synthesis of PtCu aerogels via room‐temperature NaBH₄ reduction of Na₂PtCl₄ and CuCl₂ in aqueous solution, where NaBH₄ acts simultaneously as reductant, initiator, and stabilizer, enabling rapid nucleation and gelation within minutes (Figure 5a) [67]. The formation process is visually tracked, showing the emergence of a self‐supported black foam, which evolves into a monolithic aerogel network after aging, followed by centrifugation, solvent washing (water and ethanol/methanol), freezing, and vacuum freeze‐drying sequentially, to preserve the porous structure. A control experiment using citrate as a stabilizer revealed slower gelation kinetics, highlighting the mechanistic role of rapid nucleation in aerogel network formation. The materials were structurally fully characterized (Figure 5b–e): XRD confirmed alloy formation and crystallinity; SEM and TEM (including HRTEM and HAADF‐STEM with EDX mapping) elucidated the interconnected porous morphology and nanoscale elemental distribution. The nitrogen adsorption–desorption measurements were applied to quantify the hierarchical porosity and specific surface area, and ICP‐AES was used to determine precise bulk composition following aqua regia digestion. The combination of rapid chemical reduction, controlled gelation, and freeze‐drying yields a high‐surface‐area, alloyed PtCu aerogel with preserved 3D architecture; while the multi‐technique characterization strategy correlates structural topology, alloying effects, with compositional uniformity and enhanced electrochemical performance.

FIGURE 5.

Synthetic route for PtCu aerogel formation in (a). Structural characterization of PtCu(CS) aerogels (b,c) SEM, (d) TEM, and (e) HRTEM images. The inset in (d) is the SAED pattern recorded from TEM. The inset in (e) is the corresponding FFT pattern of panel [67].

3.2. Freeze‐Drying and Supercritical Drying

Preservation of the delicate gel network during solvent removal is essential to maintain the high porosity and surface area characteristic of aerogels [68]. Freeze‐drying (lyophilization) offers a cost‐effective approach suitable for carbon‐based aerogels, where the gel is frozen, and the solvent sublimated under vacuum [69]. However, ice crystal formation during freezing can disrupt the network structure and reduce surface area. Supercritical drying, where the solvent is removed above its critical point to avoid capillary forces, provides superior preservation of the gel structure but requires specialized high‐pressure equipment and is more expensive [63]. Recent innovations in ambient pressure drying methods, which involve solvent exchange and surface modification to reduce capillary stress, offer promising alternatives that balance structural preservation with practical scalability [58].

Bigall et al. reported the first unsupported mono‐ and bimetallic noble‐metal aerogels formed directly from colloidal Au, Ag, and Pt nanoparticles [53]. The work is significant because it eliminates oxide supports and chemical cross‐linkers, enabling self‐supported, porous metallic networks composed of fused (3–10 nm) nanoparticles arranged in wire‐like, percolating architectures. The gels are formed via controlled destabilization using ethanol or H₂O₂ addition, or through spontaneous aging in bimetallic systems, followed by supercritical drying to yield ultralight aerogels with densities as low as ∼0.016 g cm^{− 3} and BET surface areas ∼46–48 m² g^{− 1}. Structural analyses (SEM, TEM, HRTEM, EDX) confirm alloy formation in Au/Ag systems and intimate nanoparticle fusion rather than simple aggregation. The study convincingly demonstrated that the entire metallic framework served as catalytically active surface, offering high conductivity and potential applications in catalysis and plasmonic.

In addition, Bigall et al. and other colleagues [53, 55, 70, 71] employed supercritical CO₂ drying, a critical step in preserving the delicate nanoparticle network. Conventional evaporative drying would introduce capillary forces during solvent removal that would lead to pore collapse, densification, and loss of the nanoscale wire morphology [55]. By replacing the solvent and drying under supercritical conditions, the liquid–gas interface is eliminated, thereby avoiding surface tension effects and maintaining the monolithic porous structure. The retention of macroscopic dimensions and ultralow density following drying confirms the effectiveness of this approach. Nonetheless, supercritical drying is energy‐intensive, requires specialized high‐pressure equipment, and may limit large‐scale production. This highlights an ongoing challenge to translate noble‐metal aerogels from laboratory synthesis to industrial application.

In another report, Fu et al. introduced a facile sol–gel polymerization followed by freeze‐drying and hydrogen‐assisted pyrolysis to synthesize ordered Pd/Pb nanoparticles embedded in a 3D carbon aerogel (Figure 6a) [72]. In this method, PdCl₂ and Pb(NO₃)₂ are used as precursors and incorporated into a polyvinyl alcohol (PVA)/graphene oxide or (GO)/carbon nanotube (CNT) double‐network hydrogel via coordination and hydrogen bonding interactions. The hydrogel is freeze‐dried to form a porous aerogel followed by pyrolysis at 600°C under 10% H₂/Ar, to yield ordered intermetallic Pd/Pb nanoparticles uniformly anchored within reduced graphene oxide (rGO)‐CNT aerogels. The hydrogel‐derived freeze‐drying–pyrolysis method effectively prevents nanoparticle agglomeration and promotes strong metal–support interaction. A comprehensive characterization (Figures 6b‐i) was conducted using XRD and confirmed ordered Pd/Pb intermetallic phase with superlattice reflections. Other complimentary experimental techniques were employed to determine the structures: TGA (metal loading ∼28 wt%), XPS (surface composition and oxidation states), SEM and TEM (3D interconnected porous architecture and ∼7.2 nm particle size), HRTEM with FFT (verification of ordered lattice structure), STEM‐EDX mapping (uniform Pd/Pb distribution), and BET analysis (surface area ∼134 m² g^{− 1} with mesoporous distribution). It appeared that, the synthesis strategy successfully integrated structural confinement, compositional control, and phase ordering, leading to a robust, highly dispersed intermetallic catalyst that is supported on a conductive 3D carbon framework.

FIGURE 6.

(a) Synthetic scheme for preparing Pd,Pb/rGO‐CNTs aerogels. (b‐c) SEM micrographs of Pd₃Pb/rGO–CNTs. (d–f) TEM images of Pd₃Pb/rGO–CNTs shown at various magnifications. (g) Particle size distribution histogram based on image (f). (h) HRTEM image with the corresponding fast Fourier transform pattern. (i) STEM image together with line‐scan analysis and EDX elemental mapping profiles [72].

Although supercritical drying has long been regarded as the gold standard for aerogel synthesis due to its superior pore structure preservation, the high capital and operational costs remain a significant barrier to large‐scale industrial adaptation. As mentioned earlier, supercritical drying eliminates capillary stress during solvent removal, thereby preserving the gel network without collapse. The methodology was first established in silica aerogels by Kistler and has since been widely applied to carbon and metal–organic framework derived aerogels [63, 68]. Some comprehensive experimental analysis confirmed that supercritical drying preserves hierarchical pore networks with minimal shrinkage and maintains >90%–95% of gel volume, leading to high surface areas and structural reproducibility [44].

Supercritical CO₂ drying, typically operates above the critical point of CO₂ (31.1°C, 7.38 MPa), industrial processes commonly operate between 8 and 20 MPa depending on the solvent exchange conditions [73, 74, 75]. Although industrial cost estimates vary depending on plant size and throughput, techno‐economic analyses consistently report that high‐pressure reactor systems and CO₂ recycling units constitute the dominant capital expenditures in supercritical drying processing [76]. However, continuous and semi‐continuous supercritical drying technologies have recently been developed to improve throughput and reduce operational costs, particularly through solvent recovery and CO₂ recycling efficiencies exceeding 90% [73]. These advances are important for repurposing aerogel electrocatalysts toward practical fuel cell and metal–air battery applications.

Freeze‐drying (lyophilization) represents a lower‐pressure alternative to supercritical drying and removes solvent via sublimation of frozen liquid under vacuum. The method has been widely applied in carbon aerogel and biomass‐derived aerogel synthesis due to its operational simplicity and reduced equipment cost compared with supercritical drying [77]. However, ice crystal formation during freezing can disrupt microporous frameworks and induce meso‐ to macropore formation; often reducing specific surface areas relative to supercritical drying samples [78]. Other strategies such as directional freezing, the use of tert‐butanol instead of water, and cryoprotectant additives have significantly improved structural control. In electrocatalytic systems, freeze‐dried Fe–N–C and heteroatom‐doped carbon aerogels have demonstrated ORR half‐wave potentials approaching those of their supercritical dried counterparts, thus indicating acceptable performance trade‐offs for cost‐sensitive applications [79, 80, 81].

Ambient pressure drying (APD) offers the most economically attractive approach by eliminating high‐pressure systems. In APD, surface modification—typically via sialylation using trimethylchlorosilane (TMCS) or hexamethyldisilazane (HMDS)—reduces capillary stress during solvent evaporation by rendering the gel network hydrophobic [82]. Although APD often results in greater shrinkage and partial micropore collapse compared to supercritical drying, optimized surface modification protocols have achieved high porosity retention in both silica and carbon aerogels [83, 84]. For carbon aerogels, high‐temperature carbonization can restore or even enhance porosity via devolatilization and structural rearrangement [85].

From an industrial readiness perspective, silica aerogel production via APD and freeze‐drying methods has already reached commercial deployment (Aspen Aerogels, Cabot Corporation), corresponding to Technology Readiness Levels (TRL 8–9) for insulation materials [86]. In contrast, supercritical processing for functional carbon and electrocatalytic aerogels remains at pilot‐to‐demonstration scale (TRL 5–7), primarily due to cost and throughput considerations [73, 74]. Therefore, while supercritical drying provides unmatched structural fidelity for high‐performance electrocatalysts, freeze‐drying and APD offer more economically viable pathways for large‐scale deployment, especially when moderate porosity loss does not critically compromise electrochemical activity.

3.3. Carbonization and Activation

For carbon‐based aerogels, high‐temperature carbonization (typically 600°C–1000°C) converts organic precursors into conductive carbon frameworks while incorporating heteroatom dopants [87]. The carbonization atmosphere (inert, reducing, or reactive gases), temperature ramp rate, and final temperature critically influence the resulting carbon structure, degree of graphitization, heteroatom content, and pore characteristics [88]. Subsequent activation using CO₂, steam, or chemical agents (KOH, ZnCl₂) can further develop porosity and increase surface area, though excessive activation may compromise mechanical integrity [89]. For M─N─C aerogels, careful control of carbonization conditions is essential to form and preserve the desired metal‐nitrogen coordination sites while avoiding formation of inactive metallic particles or carbides [90].

3.4. Template‐Assisted Synthesis

Hard and soft templating approaches provide additional control over aerogel pore structure and morphology [91]. Hard templates such as silica nanoparticles or metal–organic frameworks can be infiltrated with catalyst precursors and subsequently removed to create inverse replica structures with controlled pore sizes [92]. The integration of MXenes (Ti₃C₂T_x) or reduced graphene oxide sheets into N‐doped carbon aerogels exemplifies template‐assisted approaches that create hybrid structures combining two‐dimensional conducting sheets with 3D porous networks [31, 93].

Li et al. employed a template‐assisted strategy to fabricate ultrathin carbon aerogels with skeletons composed of few‐layer graphene, pyrolytically derived from a hydrogel precursor using an array of NaCl crystals as the template (Figure 7) [94]. The resulting carbon aerogels exhibited high electrical conductivity (869 S m^{− 1}) and an ultralow mass density (11.1 mg cm^{− 3}). These aerogels were prepared through controlled pyrolysis of a gelatin‐based hydrogel precursor, with NaCl crystal arrays serving as the sacrificial template [95, 96].

FIGURE 7.

Schematic illustration of the template‐assisted synthesis of the ultrathin carbon aerogel (NiFe‐LDH)_n/GA_x [94].

A further emerging strategy is the liquid‐templating technique, which proceeds through three sequential stages (Figure 8) [97] including (i) electrostatic assembly between nanoparticles (e.g., GO decorated with Fe₃O₄) and functional ligands such as PSS‐[3‐(2‐aminoethyl)amino]propyl‐heptaisobutyl‐substituted POSS‐NH₂ to generate a liquid template (Phase I); solidification of this template through freezing (Phase II); and freeze‐drying to obtain an aerogel featuring a dense outer skin and a porous internal framework (Phase III). The process begins by injecting an aqueous GO dispersion into a hexane phase containing POSS‐NH₂, which rapidly forms a positively charged POSS‐NH₃ ⁺ layer at the interface. pH‐dependent electrostatic interactions between protonated amine groups on GO and carboxylate groups on POSS drive the formation of a filament‐like complex (Figure 8b,c). The strong interfacial activity of POSS‐NH₂ promotes the rapid development of a solid shell that stabilizes the filament morphology. Subsequent freezing and freeze‐drying yield filamentous aerogels with a porous core enclosed by a robust skin. This approach offers tunable design parameters that enhance aerogel stability, structural integrity, and functional versatility.

FIGURE 8.

(a) Overview of the coined process for fabricating hierarchical 3D aerogels with tailored composition and functionality. The procedure begins by injecting an aqueous GO‐containing phase into an immiscible organic medium‐hexane enriched with POSS‐NH₂, forming a liquid template (Phase I). This is followed by freezing the template (Phase II) and subsequently subjecting it to freeze‐drying to obtain filamentous aerogels (Phase III). The resulting aerogels consist of core–shell filaments, characterized by a thin solid outer layer surrounding a highly porous interior. In Phase I, * marks the introduction of GO‐based aqueous inks into the POSS‐NH₂‐containing hexane phase, while ** highlights the interfacial complexation driven by electrostatic interactions between nanoparticles and ligands. During Phase II, cooling the aqueous template to −85°C solidifies it, whereas the hexane phase remains liquid due to its lower freezing point (−95.3°C), allowing removal prior to lyophilization. (b) Illustration of the electrostatic binding between POSS‐NH₂ and GO, enabled through deprotonation of GO's carboxyl groups and protonation of amine functionalities on POSS. (c) (I) Fragmentation of the liquid jet into droplets to reduce hydrodynamic instabilities. (II) Stabilization and preservation of the filament structure once nanoparticle‐ligand complexes accumulate at the interface, effectively locking in the non‐equilibrium shape. (d) Pendant‐drop behavior demonstrating interfacial structuring: I) an equilibrium droplet shape in the absence of POSS in the oil phase, and II) a non‐equilibrium, shape‐preserved droplet due to the formation of a jammed solid skin composed of interfacial nanoparticle assemblies. Black dots represent GO sheets dispersed within the aqueous phase [97].

Additionally, Xie et al. reported a hybrid strategy that couples template‐assisted fabrication with chemical vapor deposition (CVD) to produce ultralow‐density boron nitride aerogels [98]. Using borazine as the B/N precursor, the resulting materials displayed outstanding hydrophobic behavior, high oil uptake, and strong resistance to oxidation at elevated temperatures. Given the widespread use and scalability of CVD in the electronics sector, as well as its compatibility with diverse precursors [99], this method presents a promising and economical route for producing bulk quantities of high‐quality aerogels with exceptional surface area and porosity. Nonetheless, it should be noted that template‐assisted processes often require supplementary template‐removal steps, which can introduce unwanted residues or structural imperfections in the final product.

4. Building Blocks for Aerogels

The field of nanoscience has witnessed substantial improvements in the study of noble metal‐based nanocrystals, which are commonly used as nanobuilding blocks (NBBs) for aerogels. These nanocrystals were studied widely using a surfactant‐assisted precision synthesis method, which enables precise control over their size (monodispersity), surface property (such as alloying or specific facet), and hierarchical structures (such as dendritic, coreshell, hollow, etc.) [100, 101, 102, 103, 104, 105]. The as‐synthesized nanocrystals often demonstrate enhanced performance in their respective applications. For example, the utilization of Pt‐based hollow nanocrystals with extremely thin walls and adjustable facets has significantly decreased the reliance on precious metals while also demonstrating unique catalytic capabilities in the context of oxygen reduction [104]. Ultrathin core–shell nanowires composed of Pd‐Pt with a high aspect ratio showed significantly enhanced specific and mass activity for ORR, surpassing those of Pt/C by factors of 8.5 and 9.0, respectively [105].

Nevertheless, the use of these intricate nanocrystals in aerogel synthesis continues to pose a significant challenge. This is mostly due to the requirement of surfactants during the nanocrystal's synthesis, which hinders the process of destabilizing gelation for aerogels [106]. Consequently, the metal aerogels that have been documented so far predominantly originate from nanoparticles with straightforward shapes, often solid spherical particles, forming a nanowired backbone. The utilization of shape‐tuned nanocrystals as NBBs in producing an aerogel with the required characteristics has facilitated further research on nanocrystals‐based aerogels, known as hierarchical aerogels.

In contrast to traditional aerogels featuring nanowire‐like backbones, hierarchical aerogels derived from carefully engineered nanocrystals exhibit a combination of two distinct architectural levels. These aerogels possess a 3D interconnected porous structure on a macroscopic scale, along with a finely tuned configuration at the nanoscale, specifically in the local backbones. The integration of the aforementioned elements effectively preserves the intrinsic characteristics of the NBBs, hence ensuring the retention of advantageous genes acquired by nano‐engineering in the resultant hierarchical aerogels. By employing this approach, the favorable characteristics shown by nanocrystals originating from distinct morphologies may be transferred to aerogel monoliths, hence opening up new possibilities for applications that may have significant implications.

The integration of aerogel research with nanotechnology has resulted in the development of hierarchical aerogels (Figure 9a–g), including two distinct nanoconfiguration levels. These nanoconfigurations are achieved through the utilization of NBBs shape‐regulation or by the gel structural post‐engineering. The synthesis of bi‐metallic hierarchical aerogels, consisting solely of hollow nanoshell Pd/Ni alloy, was successfully achieved through a straightforward bottom‐up approach (see Figure 9d) [52]. The potential of Pd/Ni nanoshell aerogels as electrocatalysts was attributed to the combined effects of hollow building block, doping with transition metal, and 3D network structure. Notably, the Pd₈₃Ni₁₇ aerogel exhibits a significantly enhanced mass activity, approximately 5.6 times superior to the commercial Pd/C. The NBBs morphology may be modified in situ from HNSs to dendritic nanocrystals by compositional tuning.

FIGURE 9.

Summary of the conventional synthesis methodology and subsequent characterizations employed for aerogel electrocatalysts. (a) The schematic diagram illustrates the sol–gel synthesis pathway employed for the production of nanocrystal‐derived aerogel electrocatalysts. (b) A representative SEM image and (c) a TEM image depicting the overall structure of nanocrystal‐derived metallic aerogels. The insets provide optical pictures of the aerogel and hydrogel. The high‐angle annular dark‐field TEM images depict the hierarchical aerogels constructed on (d) Pd/Ni nanoshells. (e) The nanodendrites of Pt/Ni, (f) the structured core–shell aerogels of PdAu‐Pt, and (g) the perovskite oxide aerogels of LaMnO₃. The insets present the EDX elemental maps and an HRTEM image [107]. Reproduced with permission Elsevier.

Additionally, the galvanic replacement process was investigated to understand the structural growth mechanism, which involves nano‐welding the particulate reaction intermediates (Figure 9e) [106]. By leveraging the synergistic benefits derived from integrating a combined hollow–dendritic morphology, multi‐metallic alloy, and aerogel structure, Ni─Pd₆₀Pt₄₀ aerogel demonstrates a noteworthy level of electrocatalytic activity. Specifically, its performance surpasses the state‐of‐the‐art Pt/C and Pd/C electrocatalysts by factors of 7.6 and 10.6, respectively.

Furthermore, a hierarchical Pd_xAu─Pt structural core–shell aerogel constructed with an ultra‐thin Pt shell and a tunable composition alloyed core has been achieved by a post‐synthetic procedure. It is worth noting that this approach may be used for different Pd‐alloyed core compositions, as shown in Figure 4f [108]. The observed activities of as‐obtained electrocatalysts toward ORR demonstrated a volcano‐like correlation with the lattice parameters of the core substrates. The maximal mass‐specific activity and specific activity values are 5.25 A/mg_Pt and 2.53 mA/cm², respectively, which outperformed those observed in Pt/C [109]. The construction of electrocatalysts based on transition metals on conductive substrates have several advantages in terms of enhancing electron conduction, facilitating efficient mass and charge transfer, and offering a stable framework for active materials.

Gengtao et al. put forward the use of NiCo NPs supported by a porous fiber carbon aerogel, referred to as NiCo/PFC aerogel, as oxygen catalysts with dual functionality in Zn‐air batteries [110]. The synthesis of NiCo/PFC aerogel involved the utilization of a K₂Ni(CN)₄/K₃Co(CN)₆‐chitosan hydrogel‐derivative technique. The NiCo/PFC aerogel that has been developed demonstrates several notable advantages. These include the ample pore channels and open spaces, the presence of highly dispersed and ultrafine NiCo particles, efficient diffusivity of active species, higher electronic conductivity, and effective prevention of particle agglomeration and dissolution (Figure 10a). The remarkable recharging performance of the developed Zn‐air battery with NiCo/PFC aerogel electrocatalyst may be attributed to the synergistic impact resulting from its unique structural and compositional properties. The voltaic efficiency during the initial cycle, which is the ratio between the end voltage during charging and discharging, reaches a notable value of 65.4%. Furthermore, this efficiency was sustained at a commendable level of 56.7% even after undergoing 300 cycles. As depicted in Figure 10b,c, it demonstrated that the NiCo/PFC aerogel catalyst employed exhibits both good efficiency and durability. In addition, it is anticipated that future research will focus on the exploration of noncarbon supports that exhibit higher bulk electronic conductivity, excellent anti‐corrosion properties, and robust supporting electrocatalyst‐chemical interactions.

FIGURE 10.

Diagrammatic representation of the benefits of NiCo/PFC aerogel electrocatalyst. (b) The discharge and charge voltage profiles of Zn–air battery utilizing NiCo/PFC aerogel electrocatalyst [111]. Reprinted with permission from the American Chemical Society.

5. Different Aerogel‐Based Electrocatalyst Materials for Oxygen Reduction Reactions

The electrocatalytic ORR poses significant challenges in fuel cells and metal–air batteries due to its slow reaction kinetics and substantial overpotentials. The cathodic ORR exhibits a significantly lower reaction rate compared to the anode reaction, namely, hydrogen oxidation. This substantial disparity in reaction rates serves as a major constraint on the overall performance of fuel cells. Hence, significant research has been conducted in the field for developing efficient electrocatalysts for ORR [112, 113, 114]. The ORR can occur through different mechanisms depending on the specific catalytic material employed. In an acidic medium, it involves a four‐proton‐electron transfer process that reduces oxygen to water. In contrast, in an alkaline medium, the ORR proceeds through a four‐proton‐electron transfer process leading to the reduction of oxygen to OH‐ ions. Alternatively, in an acidic medium, the ORR can follow a two‐proton‐electron transfer pathway, leading to the formation of H₂O₂, while in an alkaline medium, the ORR can generate HO₂‐ ions as the products. Undoubtedly, a greater preference for the four‐electron pathway is advantageous in the context of energy conversion methodologies. Figure 11 presents the possible ORR reaction mechanisms in both non‐acidic and acidic environments [115].

FIGURE 11.

Possible ORR pathways in both non‐acidic and acidic environments.

Currently, fine platinum nanoparticles supported on carbon black (Pt/C) are generally recognized as the leading electrocatalyst for ORR. Nevertheless, commercializing this technology is impeded by the exorbitant price of platinum and its inadequate stability. In order to address these challenges, researchers have explored different types of electrocatalyst materials that possess desirable characteristics such as lower cost, higher activity, and stability. These materials can be classified into three categories: (i) carbon materials with heteroatom substitution, which exhibit significant activity primarily in alkaline environments; (ii) transition metal oxides, nitrides and carbides,, which demonstrate rather low activity; and (iii) platinum and its alloys, that continue to be the utmost effective electrocatalysts for ORR, but are constrained by their limited availability [39, 116, 117]. In the following subsections, we report advances in the performance of metallic aerogel electrocatalysts, heteroatom‐substituted carbon aerogel electrocatalysts, and conductive aerogel as electrocatalyst supports for ORR.

5.1. Metallic Aerogel Electrocatalysts

A potential approach to address the problem of supporting corrosion would be the application of catalysts without any support structure [118]. The use of unsupported electrocatalysts has garnered significant attention in the scientific and technical communities due to the remarkable properties exhibited by pure metallic aerogels. These properties include extremely low density, high surface area, and interconnected pores that are spacious and accessible. The metallic aerogels possess the characteristics of noble metals, such as electrocatalytic activity and electric conductivity, while also exhibiting a wide surface area that allows for more active sites. Additionally, their higher porosity facilitates mass transport, and their self‐supportability eliminates the requirement for carbon supports [55, 119].

The investigation by Liu et al. focused on Pd_xPt_y aerogels synthesized using a spontaneous gelation pathway, aiming to explore their potential as ORR electrocatalysts [120]. The specific surface area (SSA) of these aerogels ranged from 73 to 168 m²/g [120]. By optimizing the composition, the mass activity = 1.2 A/mg_metal at 0.9 VRHE was realized. This improvement in activity can be ascribed to a downward‐shifted d‐band center of the platinum. Using a comparable approach, Pt_xNi aerogel was synthesized, possessing a backbone diameter of approximately 5.6 nm with an SSA of 55 m² g⁻¹ [121].

Chattot et al. [122] proposed ‘surface distortion’ as an innovative structural descriptor that reconciles and unifies seemingly conflicting concepts and inconsistent experimental findings related to the electrocatalytic ORR reactivity (Figures 12a,b). In addition to its unifying nature, it was demonstrated that surface distortion is crucial for elucidating the electrocatalytic characteristics of an advanced PtNi/C nanocatalyst, which possesses varying atomic shape, composition, size, and level of surface defects in a simulated PEMFC cathodic environment. Through the adjustment of the Ni contents, Pt₃Ni aerogel exhibits a three‐fold improvement in specific reactivity compared to Pt/C catalysts available commercially, along with the mass activity = 65 A/g_Pt at 0.95 V (vs. RHE). The observed higher inherent activity of the Pt₃Ni aerogel could be ascribed to the increased micro‐strain effects resulting from the presence of structurally disordered surfaces. The strong correlation between experimental data and DFT calculation illustrated in Figure 12c indicates that chemical disordering is the sole origin of micro‐strain in structurally ordered electrocatalysts, thereby validating the defect‐free characteristics noted for PtNi electrocatalysts, though as a singular contribution exclusively for octahedron, sphere, and cube PtNi/C in Figure 12c. Recognizing that the density of bulk defective (chemical disorder or inhomogeneous alloy) was influenced by the nickel contents indicated by the DFT calculation (Figure 12d), and the mitigation of surface defective (surface dealloying, grain boundary) by the bulk.

FIGURE 12.

(a) Typical patterns of synchrotron wide‐angle x‐ray scattering (WAXS) experiment obtained structurally disordered and ordered hollow PtNi/C and cube Pt/C electrocatalysts. The patterns are represented as a function of the momentum transfer Q. (b) Possible origins of micro‐strain in bimetallic electrocatalysts include grain boundary inhomogeneous alloy or electrochemical surface degradation. (c,d) Analyzing the impact of the chemical disorder on micro‐strain value through a DFT calculation, (c) The micro‐strain values determined by the Rietveld refinement of the WAXS pattern are presented as a function of the averaged nickel content for all the electrocatalysts assessed in this investigation. (d) The technique employed in DFT calculation assesses the chemical disorder's impact on micro‐strain values [122]. Reproduced with permission from Springer Nature.

In contrast, when subjected to high potential conditions, such as those experienced during fuel starvation and start‐up/shutdown, the benchmark Pt/C material exhibited a substantial degradation of its active surface, while the Pt₃Ni aerogel showed insignificant degradation [123]. Furthermore, when utilized in cathodes of polymer electrolyte fuel cells, the Pt₃Ni aerogel retains 90% of its initial reactivity after undergoing an accelerated stress test, whereas the Pt/C material only retains 40% [124]. This finding underscores the superior stability of the Pt₃Ni‐aerogel, particularly in comparison to a commercial catalyst utilizing graphitized carbon support, which exhibits short‐term durability exceeding that of Pt/C.

5.2. Heteroatom‐Substituted Carbon Aerogel Electrocatalysts

Heteroatom‐doped graphitic carbon materials, such as graphene and carbon nanotubes, have emerged as a very promising category of electrocatalysts for ORR. In fact, certain heteroatom‐doped graphitic carbon materials have demonstrated superior performance to the current leading catalyst, platinum/carbon (Pt/C), particularly in alkaline environments. In the context of fuel anti‐poisoning, carbon‐based catalysts are often modified to enhance their effectiveness against fuels such as CO and CH₃OH [125, 126]. This enhancement is achieved by substituting carbon atoms with atoms of different electronegativity, which can alter the electronic structure of carbon materials [127]. Consequently, these modified catalysts exhibit improved performance in the ORR when exposed to a favorable chemical environment [128]. An illustration of this phenomenon involves the enhancement of positive charge density in neighboring carbon atoms due to the strong electron affinity of N₂ atoms. As a result, the adsorption of oxygen is favored, and the bonding between O─O is weakened [129], thereby facilitating the four‐electron reduction of O₂. Another mechanism involves the absorption of π* electrons by positively charged boron dopants, which are then transferred to chemisorbed O₂, thereby facilitating the ORR [125].

The incorporation of heteroatoms, particularly nitrogen doping, can improve the electrocatalytic stability and activity of multifunctional electrocatalysts. The synthesis of porous interface NiO/CoN nanowires (NiO/CoN PINWs) was achieved by an in situ nitrogenization approach [130]. The resulting NiO/CoN PINWs demonstrate remarkable performance and longer‐term stability toward ORR. Consequently, the utilization of NiO/CoN PINWs in the construction of Zn‐air batteries leads to the manifestation of remarkably elevated open‐circuit voltage, power density, and specific capacity, alongside exceptional stability. The enhancements observed in performances can be attributed to an improved oxygen vacancy concentration and the robustly interconnected interfaces between CoN and NiO [110, 130].

The large SSA of doping heteroatom carbon aerogels plays a vital role in designing higher‐performance ORR catalysts since the aerogel's skeleton serves as the electroactive center. The use of a doped‐heteroatom carbon material frequently leads to a reduction in the actual surface area due to π‐π restacking. This issue may be mitigated by the development of 3D aerogel frameworks. An example of a synthesized aerogel (Figure 13a) composed of carbon nanowires doped with nitrogen and fluorine was successfully synthesized, featuring a skeleton diameter approximated to 50 nm with an impressive SSA of 768.4 m²/g [131].

FIGURE 13.

(a) Functionalized aerogels synthesized by a hydrothermal carbonization method exhibit significant electrocatalytic activity [131]. Reproduced with permission from Wiley. (b) RRDE test of the ORR on various samples in O₂‐saturated 0.1 M KOH at the rotation speed of 1600 rpm with the scan rate of 10 mV/s [132]. Reproduced with permission from the RSC. (c) Schematic illustration of the synthesis procedure of Fe–N‐CNTAs. (d) Kinetic current density (j _k) and the electron‐transfer number (n) of the Fe–N‐CNTAs‐5‐900 and Pt/C at different potentials [133]. Reproduced with permission from Wiley.

Another study reported the doping of N and B with graphene aerogel, which was produced using a two‐step technique comprising a hydrothermal reduction‐gelatinating process of graphene oxides with chitosan and urea, followed by annealing with boric acid [132]. The as‐obtained aerogel demonstrated an onset potential of 0.86 V (vs. RHE) at 0.1 mA/cm², electrocatalyst loading of 212 ug/cm² for ORR, which was comparable to that of the Pt/C electrocatalyst. The RRDE technique enables precise determination of H₂O₂ yield from the disk and ring current during ORR. As presented in Figure 13b, the observed H₂O₂ yield for N, B‐GAs‐1000 is lower than 10% across −0.3 to −0.7 V (vs. SCE), corresponding to a higher transfer electron number around 3.87, signifying a four‐electron reduction route of O₂ on N, B‐GAs‐1000.

Through careful optimization of the dopant composition, the aerogels demonstrated a remarkable onset potential of 0.88 V_RHE at a geometric current density of 0.1 mA/cm² and electrocatalyst loading as low as 100 µg/cm². The decoration of carbon nanotubes (CNTs) with Fe─Nx active centers has demonstrated the potential to serve as structural foundations for the development of an aerogel catalyst using the hydrothermal method, Figure 13c [133]. A uniform distribution of active sites that possess single‐atom characteristics contributes to the enhanced performance of ORR on a Fe‐N‐CNT aerogel (Figure 13d). The catalyst even surpasses the performance of a Pt/C electrocatalyst and exhibits a low overpotential and improved durability.

Conversely, the utilization of heteroatom‐containing monomers or polymers in constructing aerogels ensures enhanced uniformity of the dopants. The fabrication of N, P‐doped carbon aerogels was achieved through the pyrolysis of polyaniline aerogel, which was fabricated in the presence of phytic acid, as depicted in Figure 14 [134]. In addition to its exceptional performance in the ORR, as observed in the RDE test, the utilization of this aerogel as the air electrode in a Zn–air battery resulted in 1.48 V open circuit potential and a specific capacity of 735 mAh/g. It is important to note that heteroatom‐doped carbon aerogels typically serve as catalysts for ORR in an alkaline environment. However, the applications in fuel cells are constrained due to the potential risk of electrolyte poisoning caused by CO₂. The investigation involving doping heteroatom with carbon aerogel catalyst in the context of acidic fuel cells presents a persistent difficulty [135]. Furthermore, there is still ample opportunity for further advancements in this area. Although aerogel electrocatalysts exhibit a substantial SSA, which implies a greater number of active sites, it appears that substantial loadings are necessary for electrode preparation to achieve satisfactory ORR efficiency. Nevertheless, such a scenario would result in an augmentation of the electrocatalyst layer's thickness in a fuel cell, which frequently triggers a decline in potential and a subsequent reduction in cell efficiency.

FIGURE 14.

(a) Schematic illustration of the preparation process for the NPMC foams. SEM images of (b) PANi aerogel and (c) NPMC‐1000. Inset in (c): Digital photo images of PANi aerogel before (left) and after (right) pyrolysis at 1000°C. (d) HRTEM image [134]. Reproduced with permission from Springer Nature.

5.3. Conductive Aerogel as Electrocatalyst Supports

Hybrid aerogel electrocatalysts employ aerogels as substrates, with the active sites being dependent on the presence of anchoring nanomaterials. The fundamental aspect of hybrid electrocatalysts is their ability to improve the dispersion and stabilization of active nanomaterials [136, 137, 138]. Nanocarbon‐based aerogels and other conductive aerogels possess notable characteristics such as extensive surface area, rapid electron‐transport kinetics, significant porosity, and strong affinity toward inorganic nano‐catalysts like nitrides and oxides. Consequently, they are very effective as electrocatalyst supports. The phenomenon of synergistic effects resulting from the interaction among the supporting materials and electrocatalysts, facilitated by coupling the electrical structure, is frequently observed in electrocatalysis. These effects contribute to augmenting both the electrocatalytic activity and stability of the system.

The synthesis of a conductive electrocatalyst by dispersing small particles of Ni‐MnO in a graphene hydrogel crosslinked with poly(vinyl alcohol) has been reported (Figure 15) [136]. This electrocatalyst, supported by graphene aerogel, demonstrated excellent performance in ORR in an alkaline electrolyte. The superior performance can be attributed to the presence of MnO, which enhances ORR (Figure 15). This electrocatalyst has potential applications in metal–air batteries. In order to enhance the level of support, the introduction of N‐doping into the graphene aerogel can be achieved by the process of nitridation in the presence of ammonia [137]. Incorporating nitrogen into the graphene structure results in robust contact with Fe_xN NPs, hence causing a reduction in charge transport resistance and fostering synergetic effects that enhance the ORR. The as‐obtained Fe_xN/N‐graphene aerogel exhibited an onset potential = 0.89 V versus RHE at a geometric current density of 0.1 mA/cm² with an electrocatalyst loading of just 50 ug/cm². As indicated in Table 1, several materials exhibiting electrocatalytic activity for ORR have been combined with non‐doped graphene, heteroatom dopants, or various nanocarbon aerogels. These materials include metal sulfides, spinel oxides, and transition metal carbides [139, 140, 141].

FIGURE 15.

(a) Preparation scheme of Ni–MnO/rGO aerogel. (b) FTIR spectra of GO, PVA, and GO–PVA aerogels. (c) Interaction scheme among GO and PVA. (d) XRD patterns of GO, PVA, and GO–PVA aerogels. (e) SEM and (f) TEM images of Ni–MnO/rGO aerogels. (g) Tafel plots as‐synthesis electrocatalysts under O₂‐saturated 0.1 M KOH. (h) Schematic illustration of Ni–MnO/rGO as a conductive ORR electrocatalyst [136]. Reproduced with permission from Wiley.

TABLE 1.

Performance of aerogel electrocatalysts toward ORR.

Aerogel electrocatalysts	Electrolytes	Electrocatalyst loading (ug/cm2)	BET (m2/g)	Current density (mA/cm2 geo) at 0.9 V versus RHE (acidic media)	Current density (mA/cm2 geo) at 0.8 V versus RHE (alkaline media)	Potential (V vs. RHE) at 1 mA/cm2	Refs.
Ca0.9La0.1Al0.1Mn0.9O3	0.1 M KOH	21	6.1	—	0.4	0.73	[149]
S‐carbon	0.1 M KOH	531	734	—	0.78	0.76	[150]
Co‐N‐graphene	0.5 M H2SO4	600	485	—	0.45	0.77	[151]
Crumpled N‐graphene	0.1 M KOH	242	—	—	1.25 a	0.81 a	[152]
N,S,Fe─carbon	0.1 M KOH	200	714.4	—	1.7	0.84	[153]
Fe─N─carbon	0.1 M HClO4	230	455.1		0.3	0.7	[154]
Fe─N─carbon	0.1 M KOH	230	455.1	—	1.8	0.85	[154]
Fe/Co─N─graphene	0.1 M KOH	714	170	0.2	2.3	0.85	[155]
N,B─graphene	0.1 M KOH	212	545.7	0.5 a	2.9 a	0.86 a	[132]
Co,N─graphene	0.1 M KOH	300	405	—	4	0.86	[156]
NH3‐act‐N‐graphene	0.1 M KOH	393	1049	0.45	3.3	0.87	[157]
N–carbon fiber	0.1 M KOH	400	918	0.49	2.6	0.87	[158]
N‐CNT/graphene	0.1 M KOH	79	497	—	2.2 a	0.88 a	[159]
N,F─carbon nanowire	0.1 M KOH	100	768.4	0.6	3.5	0.88	[131]
N─carbon fiber	0.1 M KOH	120	526	—	1.9 a	0.88 a	[160]
N,P─carbon (PANi)	0.1 M KOH	150	53.5	0.8	3.5	0.89	[131]
Fe,N‐CNT	0.1 M KOH	100	638.5	2	4.5	0.93	[133]
FeNx/CNTs‐Carbon	0.1 M HClO4	56	315	—	0.18 a	0.62 a	[161]
Fe3O4/graphene	0.1 M KOH	143	110	—	0.2 a	0.68 a	[162]
Ag NW/graphene	0.1 M KOH	200	—	—	0.45 a	0.73 a	[163]
CoNi2S4/S,N,Co‐graphene	0.1 M KOH	286	—	—	0.5	0.76	[164]
V3S4/graphene	0.1 M KOH	200	526.4	—	1.0 a	0.80 a	[165]
Co‐N‐graphene	0.1 M KOH	275	466.6	—	2.1	0.83	[166]
Co3O4/CNT	0.1 M KOH	1000	53	—	2.1	0.83	[167]
Co9S8/N,S,P‐graphene	0.1 M KOH	283	478	—	3.63	0.84	[168]
CoS/N,S‐graphene	0.1 M KOH	62.5	76	—	2.5	0.85	[140]
CoMn2O4/N,P‐graphene	0.1 M KOH	280	135.8	—	3.1 a	0.86 a	[141]
Fe3C/N‐graphene	0.1 M KOH	150	75.4	0.4	3	0.86	[169]
Fe3O4/N‐graphene	0.1 M KOH	100	494	0.5	2.4	0.87	[170]
CoOx/N‐graphene	0.1 M KOH	100	814	0.6	4.4	0.88	[139]
FexN/N‐graphene	0.1 M KOH	50	116.9	0.9 a	3.8 a	0.89 a	[137]
Ni/MnO/graphene	0.1 M KOH	250	109	0.8	2.5	0.89	[136]
Pt/graphene	0.1 M HClO4	85 (28 Pt)	708	0.8 a	—	0.87 a	[171]
Pt/graphene	0.1 M HClO4	219 (127 Pt)	—	0.1	—	0.9	[172]
PtIrCo/carbon	0.1 M HClO4	243 (30 Pt)	514	1.4 a	—	0.91 a	[143]
Pt/Sb─SnO2	0.1 M H2SO4	316 (60 Pt)	85	1.3	—	0.91	[29]
Au─Pt3Pd	0.1 M HClO4	n/a (25 Pt)	29.2	2.8	—	0.93	[173]
Pt/Sb─SnO2	0.1 M H2SO4	300 (60Pt)	85	2.6	—	0.95	[145]
Pt/Sb─SnO2	0.1 M H2SO4	300 (60Pt)	85	3.4	—	0.96	[174]
Pt3Ni	0.1 M HClO4	33 (30Pt)	55	4.5	—	0.96	[121]

^a V (vs. RHE) was converted from the original SCE, NHE, and Ag/AgCl.

Despite the limited availability of Pt metal, it continues to be indispensable for manufacturing commercially viable low‐temperature fuel cells up to the present day. The Pt/C electrocatalyst is now considered the most advanced in its field and employs carbon black as a support material. However, it is often observed that carbon black undergoes degradation during the operation of fuel cells. Therefore, CNTs and graphene exhibit enhanced conductivity, chemical durability, and a favorable affinity toward metal NPs, making them highly promising as supports for electrocatalysts [142]. For example, N‐doped graphene aerogel has been utilized to support monodispersed Pt NPs having a 2.8 nm average size. This configuration results in a remarkably high SSA of 1750 m/g, surpassing the surface area of Vulcan XC‐72 carbon black, which is approximately 300 m²/g [142].

Furthermore, it has been observed that the structure of the aerogel serves the dual purpose of facilitating the exposure of Pt active surface to O₂, thereby reducing the dispersion routes of O₂ in the pore channels. Additionally, it enhances the O₂ diffusion and adsorption onto the Pt/N–graphene aerogel electrocatalysts. Similarly, carbon aerogels have been employed as catalyst support for Pt‐based alloys, including PtIrCo [143] and PtCu [144], to produce the ORR electrocatalysts with enhanced performance.

Metal oxides that are devoid of carbon are seen as a viable option for supporting electrocatalysts used in the ORR. A particularly promising support material for fuel cell cathodes is Sb‐doped SnO₂ due to its thermal‐dynamic stability and higher electronic conductivity of 0.12 S/cm [145]. In this context, the use of Sb–SnO₂ aerogel as a supporting electrocatalyst for Pt NPs has been explored. It possesses a relatively higher SSA of 85 m²/g since the SnO₂ skeleton density and the pore size range from 20 to 45 nm. Under optimal circumstances, a mass activity of 27 A/gPt was obtained at 0.9 V versus RHE [29].

The one‐pot hydrothermal synthesis of S‐ and N‐doped carbon aerogels (Figure 16a–c), employed as potential ORR catalysts, has been documented [146, 147]. This approach employed two co‐monomers, specifically S‐(2‐thienyl)‐l‐cysteine (TC) and 2‐thienyl carboxaldehyde (TCA), for sulfur inclusion by the conventional solvothermal technique. The samples were subsequently positioned in a furnace under a nitrogen environment, purged for 30 min, and then heated to 900°C at 10 K/min. The secondary pyrolysis process was then utilized to adjust the conductivity of carbon aerogel and the binding state of the heteroatom. A comparative electrocatalytic test against a simple N‐doped aerogel for ORR in both alkaline and acidic environments was also conducted. Co‐doped materials were identified as superior candidates in catalytic studies, perhaps attributable to the synergistic impact of N and S [147].

FIGURE 16.

SEM images of the solid product after hydrothermal carbonizing technique (a) pure glucose, (b) glucose with cysteine, and (c) glucose with thienyl‐cysteine [146]. Reproduced with permission from the Royal Society of Chemistry. (d) Synthesis scheme of P‐CD/G nanocomposites using hydrothermal‐annealing process. (e) LSV curves of various samples obtained at different temperatures. (f) Theoretical mechanistic studies for ORR. The Gibbs free energy variation, as calculated using DFT, at various reaction stages for planar graphene, phosphorus‐doped graphene, carbon dots, and phosphorus‐doped carbon dots. The inset atomic structure depicts the adsorption of diverse radicals on the materials throughout the ORR process [148]. Reproduced with permission from Wiley.

The optimal ORR performance in a metal–air battery is frequently constrained by moderate catalytic activity and the absence of effective dispersion methods. To address this, Wang et al. [148] used a biomass‐derived method for an in situ P‐doped carbon dot and the concurrent integration onto a graphene matrix (Figure 16d). The resulting P‐doped carbon dot/graphene nanocomposite products, denoted as P‐CD/G, achieved an exceptionally higher level of P‐doping for carbon nanomaterial. The P‐CD/G composites demonstrate superior ORR activity equivalent to that of commercial Pt/C electrocatalysts (Figure 16e). The device demonstrates a notable power density of 157.3 mW/cm² when utilized as a cathode electrocatalyst in a primary liquid Al–air battery, in comparison to 151.5 mW/cm² for a similar Pt/C battery. The device demonstrates a consistent discharge voltage of approximately 1.2 V across various bending states. DFT was used to compute the free energy variations for the fundamental ORR steps on the four types of electrocatalysts. As shown in Figure 16f, it was posited that the adsorbed species (*) are invariably bound to the P site within the P‐doped materials. This research presents biomass‐derived materials that could substitute for noble‐metal electrocatalysts in future portable and wearable electrical devices.

Table 1 presents the relevant data obtained for the ORR performance of several aerogel catalysts. The ORR performance was evaluated using an RDE in an O₂‐saturated solution, with 1600 rpm and a scan rate of 10 mV/s. To assess the effectiveness of various aerogel catalyst materials, onset potential at 1 mA/cm² _geo and current density at 0.8 V versus RHE (for alkaline media) and 0.9 V versus RHE (for acidic media) were adjusted, since the electrochemical surface area for non‐platinum electrocatalysts poses challenges. All potentials are referenced using the RHE to facilitate comparing and integrating data obtained under both acidic and alkaline environments. The aerogel electrocatalyst materials may be categorized into three distinct groups, namely Pt‐containing aerogel, hybrid aerogel heteroatom‐doped carbon, and aerogel electrocatalysts. This is expected to facilitate a deeper comprehension of the enhancement capabilities exhibited by aerogel electrocatalysts.

6. Structure–Property Relationships and Mechanistic Insights

Understanding the fundamental relationships between aerogel structure and ORR performance is essential for rational catalyst design and optimization [175, 176]. The characterization of electrocatalysts using advanced experimental techniques combined with computational modeling provides increasingly detailed insights into the active sites and reaction mechanisms in aerogel electrocatalysis.

6.1. Active Site Identification and Characterization

Determining the nature and distribution of catalytically active sites in complex aerogel structures, requires combined‐experimental techniques [177]. The x‐ray absorption spectroscopy (XAS), including x‐ray absorption near‐edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS), provides element‐specific information about oxidation states, coordination environments, and nearest‐neighbor distances around metal centers [178, 179]. For Fe─N─C aerogels, EXAFS analysis confirms the presence of isolated Fe─N₄ sites with Fe─N bond lengths of approximately 2.0 Å, consistent with square‐planar or square‐pyramidal coordination geometries [180]. Mössbauer spectroscopy offers complementary insights into Fe oxidation states and spin states, enabling a clearer distinction among catalytically active Fe^{2 +}/Fe^{3 +} species from inactive Fe⁰ or Fe₃C phases [181].

Advanced electron microscopy techniques, including aberration‐corrected scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS), enable direct visualization of atomically dispersed metal sites and elemental mapping at the sub‐nanometer scale [182]. These methods confirm the atomic dispersion of metal centers in high‐performance M─N─C aerogels and reveal the distribution of heteroatom dopants within carbon frameworks [183, 184]. The x‐ray photoelectron spectroscopy (XPS) provides surface‐sensitive information about elemental composition and chemical states, which is crucial for identifying nitrogen speciation (pyridinic, pyrrolic, graphitic, oxidized) and to correlate specific nitrogen configurations with ORR activity [185].

A pivotal frontier in ORR electrocatalysis involves engineering aerogel‐based single‐atom catalysts (SACs) with precisely regulated active sites to maximize both activity and stability. Aerogels with their 3D structures, high porosity, and low‐density matrices, provide an ideal scaffold for immobilizing isolated metal atoms (e.g., Fe─N_x sites) with abundant mass‐transport pathways and high surface areas [186, 187, 188, 189]. As an example, single‐site Fe─N─C catalysts derived from polypyrrole carbon nanotube aerogel precursors exhibit impressive ORR activity and durability, thus leveraging well‐dispersed Fe─N_x centers and hierarchical porosity to facilitate efficient oxygen adsorption and to promote four‐electron transfer pathways, which outperform commercial Pt/C benchmarks in alkaline media [190]. Furthermore, Pt₁/CuS_x atomic aerogel in powder form demonstrates excellent ORR performance in acidic electrolytes, achieving over 92% selectivity for the two‐electron reduction process of O₂ to H₂O₂ [191, 192].

The strong covalent Pt–S interaction is critical to stabilize a high density of atomically dispersed Pt sites and to enhance the efficiency of electrochemical performance. Such intricate site regulation is achieved through careful precursor design, controlled pyrolysis, and engineering support to address concomitant challenges of SACs, namely, atom aggregation and electronic environment optimization. This ensures the existence of isolated metal active centers with tunable coordination environments, which enhance intrinsic catalytic kinetics and long‐term operational stability [187, 189, 193, 194, 195, 196]. The ongoing refinements in synthesis techniques and atomic‐scale characterization (e.g., in situ spectroscopy and microscopy) continue to redefine our understanding of how aerogel supports influence single‐atom site electronic structures and ORR pathways, therefore, laying the foundation for rational design strategies, which optimize activity, durability, and scalability of the aerogels.

A recent report describes the development of bioinspired catalysts engineered through a directed synthetic strategy to construct spatially adjacent Cu and Fe active sites within a covalent 3D framework assembled in aerogel form [3]. As a consequence of their intrinsically high surface area and hierarchically porous architecture, aerogels enable exceptionally high active‐site densities and facilitate efficient mass transport of reactants and products between catalytic centers. In a 0.1 M KOH, aerogel‐based catalyst demonstrated outstanding half‐cell performance, delivering an onset potential of 0.94 V versus RHE and a half‐wave potential (E₁/₂) of 0.80 V versus RHE, along with high selectivity toward the four‐electron oxygen reduction pathway to hydroxide, including remarkable durability. These advantages effectively translated into practical operation in an anion exchange membrane fuel cell (AEMFC), to achieve an open‐circuit voltage of 0.97 V and an iR‐corrected peak power density of 0.51 W cm^{− 2}. DFT calculations attribute the enhanced activity, relative to Fe‐porphyrin and Cu‐corrole analogues, to the synergistic influence of the extended conductive carbon framework and the close proximity of the dual‐metal sites.

6.2. Computational Modeling and Mechanism Elucidation

DFT calculations prove to be indispensable for understanding ORR mechanisms and predicting catalyst performance [197, 198]. Computational studies on M─N─C sites have established that the ORR activity is governed by the binding energies of key intermediates (O*, OH*, OOH*) in accordance with the Sabatier principle, the optimal catalysts exhibit intermediate binding strengths, which balance activation and desorption steps [199]. Volcano plots, which correlate experimental activity with computed binding energies, guide the selection of metal centers and coordination environments using near‐optimal thermodynamics [200].

For metallic aerogels, DFT calculations have elucidated the roles of strain and ligand effects in modulating activity [201]. Compressive strain in Pt─Ni alloys shifts the Pt d‐band center to lower energies, weakening the binding of OH species and simultaneously enhancing ORR kinetics [202]. Conversely, tensile strain in Pd‐based alloys can be exploited to reduce CO binding for anti‐poisoning reaction strategies (Figure 17) [28]. Computational screening approaches, which systematically evaluate thousands of potential formulations and structures, are positioned to accelerate the discovery of novel high‐performance aerogel catalysts [203].

FIGURE 17.

Schematic illustration for the mechanism of ORR and the anti‐poisoning properties on Pd MAs and Pd@PdBiCo MAs [28].

Currently, composition‐tunable high‐entropy alloy aerogels (HEAAs) are emerging as a transformative platform for ORR electrocatalysis, these exploit the configurational complexity intrinsic to high‐entropy systems. Traditional HEA research emphasizes that multicomponent alloys, which comprise of five or more principal elements in near‐equimolar ratios, promote unique synergistic effects, including lattice distortion, sluggish diffusion, and diverse local environments that collectively produce a broad spectrum of active sites with tunable adsorption energetics [204]. By extending HEA concepts into aerogel architectures promises to combine compositional optimization strategies with the inherent advantages of aerogels, to yield catalysts with enhanced active‐site diversity, robust structural stability, and rich electron‐transport networks. Although studies of HEAA electrocatalysts for ORR remain nascent, the broader HEA electrocatalysis literature highlights how multi‐element design can enhance ORR performance through optimized adsorption energies and durability under electrochemical cycling [205]. Future research will likely focus on systematic high‐throughput and machine‐learning‐guided approaches to navigate the vast compositional space, optimizing intrinsic properties such as d‐band structure, surface segregation, and electronic synergy to tailor reaction energetics for the ORR. The Integration of such HEA composition strategies with aerogel morphology control could unlock the next‐generation ORR catalysts, which could significantly surpass current materials in both performance and resource efficiency.

6.3. Pore Structure Effects on Performance

The hierarchical pore architecture of aerogels directly impacts both intrinsic activity and mass transport limitations [206]. As discussed earlier, micropores (<2 nm) maximize surface area and active site density but can restrict oxygen diffusion and create transport bottlenecks in thick catalyst layers [207]. Mesopores (2–50 nm) facilitate rapid mass transport while maintaining high surface area, making them particularly beneficial for high‐current‐density operation [208, 209]. Macropores (>50 nm) serve as oxygen reservoirs and prevent electrolyte flooding, though excessive macroporosity reduces volumetric activity [210].

Systematic studies correlating pore size distribution with RDE and fuel cells performance revealed that optimal aerogel catalysts exhibit bimodal or trimodal pore distributions, with micropores hosting active sites and meso/macropores providing transport highways [4]. The total pore volume, pore connectivity, and tortuosity also influence effective diffusivity and overall catalyst utilization [211]. Advanced characterization techniques, including gas sorption analysis (BET, BJH), mercury intrusion porosimetry, and small‐angle x‐ray scattering (SAXS), provide quantitative pore structure information essential for structure–property correlation studies [212].

In summary, the recent progress in the field demands that a truly critical discussion of aerogel electrocatalysts for the ORR must go beyond summarizing reported activities in order to confront two persistent debates in this field. First, the nature of active sites in Fe─N─C aerogels—often invoked as the primary ORR centers due to isolated Fe–N_x motifs—is still contested; while many studies assume Fe–N₄ configurations dominate activity. Operando spectroscopic work and ion‐poisoning experiments suggest that the coordination environment and dynamic evolution of Fe sites under operating conditions significantly influence reactivity. Some Fe(II) configurations are proposed as the true active species, and even transformations of coordination occurring during ORR (e.g., breaking of Fe─N bonds to facilitate O₂ activation) is an idea that is not universally agreed upon [213]. This uncertainty is compounded in aerogel systems, where high defect densities and heterogeneous porosity can lead to multiple co‐existing active species (Fe─N_x, Fe oxide clusters, carbon edge sites), and conventional ex situ characterization often lacks the resolution to distinguish between their individual contributions.

Second, performance comparability across test systems remains a significant methodological controversy; traditional RDE measurements—while useful for initial screening, suffers from mass transport limitations and can yield vastly different activity metrics than those obtained in gas diffusion electrode (GDE) or membrane electrode assembly (MEA) contexts, this leads to discrepancies in reported half‐wave potentials and kinetic currents that are difficult to reconcile without strict benchmarking protocols and standardized measurement conditions [214, 215]. To address these challenges, future research could adopt interdisciplinary, multiscale structural design and evaluation frameworks, which integrate in situ/operando spectroscopy to resolve dynamic active‐site behavior. Machine‐learning‐assisted synthesis could map out site distributions within aerogel matrices and estimate the cross‐platform performance benchmarking (RDE vs. GDE vs. full cells) to ensure that reported enhancements reflect intrinsic catalytic improvements rather than measuring artifacts. Such a framework could elevate aerogel ORR research from fragmented reports toward predictive design principles and reliable electrocatalyst comparisons.

7. Device Integration and Practical Performance

The ultimate measure of aerogel electrocatalyst performance is their behavior in practical energy conversion devices under realistic operating conditions [216, 217]. While rotating disk electrode measurements provide valuable insights into intrinsic activity and selectivity, translation to fuel cells and metal–air batteries introduces additional challenges related to catalyst layer engineering, membrane integration, and long‐term durability (Figure 18).

FIGURE 18.

Flowchart illustrating the integration pathway and operational performance of aerogel electrocatalysts in energy conversion systems.

The schematic outlines the transition from fundamental RDE evaluations, covering essential electrochemical parameters such as onset potential, mass and specific activities, and Tafel slope, to practical implementation in full‐scale devices. These include fuel cells, encompassing membrane electrode assemblies (MEAs) fabrication, single‐cell assessment, and stack‐level testing, as well as metal–air batteries, involving electrolyte optimization, cell construction, and long‐term cycling studies. The diagram further highlights critical stability considerations, including durability, cycling reliability, and degradation pathways that influence the overall efficiency and longevity of aerogel‐based electrocatalytic systems.

7.1. Fuel Cell Testing and Performance Metrics

Several studies have successfully integrated aerogel catalysts into MEAs for single‐cell fuel cell testing [3, 11, 218]. The iron phthalocyanine aerogel catalyst achieved a peak power density of 292 mW cm^{− 2} in a PEM fuel cell operating on H₂/O₂, demonstrating that molecular aerogels can deliver practical device‐level performance [11]. The biomimetic Fe─Cu porphyrrole aerogel reached 510 mW cm^{− 2} in an anion exchange membrane fuel cell, approaching the performance of Pt‐based cathodes in alkaline environments [3]. These results demonstrate that properly designed aerogel catalysts can bridge the notorious performance gap between RDE and MEA testing that has plagued many non‐precious metal catalysts [218].

Critical factors for successful device integration include catalyst layer thickness, ionomer content, and interfacial engineering [219]. The high porosity of aerogels must be balanced with sufficient mechanical integrity to withstand MEA fabrication and compression [220]. Ionomer distribution within the catalyst layer affects both proton/hydroxide conductivity and oxygen transport, requiring optimization specific to each aerogel structure [221]. The self‐supported nature of many aerogels offers advantages by eliminating the need for conductive carbon additives, simplifying catalyst layer fabrication, and reducing interfacial resistances [222].

7.2. Metal–Air Battery Applications

Aerogel catalysts have been extensively explored as cathode materials for zinc–air, lithium–air, and aluminum–air batteries, where ORR performance directly determines discharge voltage and power density [223, 224]. The 3D Fe₃O₄‐decorated nitrogen‐doped graphene aerogel demonstrated excellent performance in zinc–air batteries, delivering a peak power density of approximately 150 mW cm^{− 2} and stable discharge over multiple cycles [10]. The hierarchical structure facilitates oxygen ingress from the air electrode while accommodating the solid discharge products (e.g., ZnO in zinc–air batteries) that form during operation [225].

Rechargeable metal–air batteries impose even more stringent requirements, as the catalyst must exhibit bifunctional activity for both ORR (discharge) and oxygen evolution reaction (OER) charge [226]. Several aerogel catalysts have demonstrated promising bifunctional performance, with potential gaps (ΔE = E _OER, 10 mA/cm²—E _ORR, half‐wave) below 0.8 V indicating good reversibility [227]. The long‐term cycling stability of aerogel cathodes in metal–air batteries has shown substantial improvements over conventional catalysts, with some systems maintaining stable voltage profiles over hundreds of charge–discharge cycles [228].

7.3. Durability and Stability Assessment

Long‐term stability under operating conditions remains a critical challenge for all ORR catalysts, and aerogel structures offer both advantages and potential vulnerabilities [229]. The monolithic, self‐supported nature of aerogels resists the particle detachment and agglomeration mechanisms that degrade carbon‐supported nanoparticle catalysts [230]. Accelerated stress tests involving thousands of potential cycles have demonstrated that well‐designed metallic aerogels can retain 80%–90% of their initial activity, substantially outperforming commercial Pt/C [1, 231].

Despite their promising ORR activity, aerogel catalysts still suffer from significant durability limitations. Under electrochemical operating conditions, particularly in acidic media, non‐noble metal species are prone to oxidation and dissolution, while M–N–C active centers may undergo protonation‐induced demetallation and metal leaching. At elevated potentials, carbon frameworks are vulnerable to electrochemical corrosion, and prolonged cycling can induce structural degradation through mechanical stress, pore collapse, and electrolyte flooding [232, 233]. Additional deterioration pathways in carbon‐based aerogels include surface oxidation, above ∼0.8 V versus RHE, leading to the formation of quinone and carboxyl functionalities, which diminish electrical conductivity; increase oxidative etching of micropores, resulting in substantial surface area loss. Also, cleavage of C─N bonds, which reduces heteroatom dopant concentration and active site density, including fragmentation caused by gas evolution and wettability shifts from hydrophobic to hydrophilic states, which promote electrolyte permeation [124, 136, 190, 232, 233]. Table 2 presents the stability comparison of representative aerogel electrocatalysts for oxygen reactions.

TABLE 2.

Stability comparison of representative aerogel ORR electrocatalysts.

Aerogel electrocatalyst	Electrolyte/test	Stability metric/observation	Main degradation mechanism	Refs.
Pd3CuFe0.5 aerogel	ORR in alkaline/acidic media	∼4.9% decay after 16 000 s ORR test; stable cycling for ≈120 h	Retention of 3D aerogel structure	[17]
Pd3Cu@NC (N‐doped carbon aerogel)	1.0 M KOH	Outstanding long‐term ORR stability reported	Alloy aerogel + carbon matrix stabilizes active sites	[234]
Fe‐N‐C single‐site carbon nanotube aerogel	0.1 M KOH, alkaline	Superior stability than commercial Pt/C in alkaline ORR	Single‐atom Fe site dispersion within aerogel network	[190]
Fe–N‐C/TiN composite aerogel	PEMFC durability test	Only ∼15 mV loss after 30 000 cycles (ADT)	Radical scavenging + network protection	[235]
Fe─N─C from modified carbon aerogel	Phosphoric acid (acidic)	Mass activity & ORR selectivity optimized	Aerogel support with tailored N functionality	[236]
MoS2‐anchored S‐doped rGO aerogel (Pt/MoS2@S‐rGO)	N2‐saturated 0.1 m HClO4 electrolyte	∼90% mass activity retention after 10 000 cycles	Stabilization of Pt nanoparticles on aerogel	[237]

Accelerated durability tests (ADTs) underscore the severity of these degradation processes. For example, potential cycling between 0.6 and 1.0 V in O₂‐saturated alkaline electrolyte over several thousand cycles can lead to a 30%–50% decrease in half‐wave potential for unprotected carbon aerogels [17]. Similarly, chronoamperometric measurements at moderate operating potentials frequently show substantial current decay within relatively short timeframes. Such performance losses are incompatible with the long operational lifetimes (>5000 h) required for practical fuel cell applications [235].

In metal‐doped aerogels (e.g., Fe–N–C and Co–N–C systems), further instability mechanisms arise [190, 236, 238]. Transition metal centers may undergo oxidation state changes (e.g., M^{2 +} to higher valence states), altering ORR pathways and intrinsic activity. Metal nanoparticles can agglomerate through Ostwald ripening, reducing dispersion and active surface area. In acidic environments, soluble metal ions such as Fe^{3 +} or Co^{2 +} may leach into the electrolyte, while M–N_x moieties are susceptible to protonation‐driven demetallation. Moreover, peroxide intermediates formed during ORR can trigger Fenton‐type reactions, generating highly reactive hydroxyl radicals that accelerate carbon matrix degradation [235].

In order to mitigate these challenges, several stabilization strategies have been proposed. These include applying protective ultrathin carbon or oxide coatings to suppress corrosion and leaching, incorporating more corrosion‐resistant elements, and engineering robust carbon frameworks with enhanced graphitization to improve oxidative resistance [239, 240]. Importantly, advanced operando and in situ characterization techniques are now enabling real‐time monitoring of structural and compositional evolution of the electrochemical catalyst, providing critical insights into degradation mechanisms and informing the rational design of next‐generation durable aerogel catalysts [241].

8. Challenges and Future Perspectives

Aerogel electrocatalysts represent a rapidly advancing frontier in the development of next‐generation ORR materials, combining tunable architecture with controllable chemistry. Although these materials possess tunable porosity, high surface area, and hierarchical mass transport pathways, translating these intrinsic structural advantages from laboratory synthesis into practical, scalable electrocatalytic systems remains a significant challenge. Despite remarkable progress, several obstacles must be addressed before aerogel electrocatalysts can achieve widespread commercial adoption in fuel cells and metal–air batteries [242, 243].

While laboratory‐scale synthesis of aerogel ORR catalysts is well established, the difficulty of transforming these materials to industrial production still remains. Conventional fabrication involves multistep sol–gel polymerization, aging, solvent exchange, drying, and post‐treatments such as carbonization and activation, all of which must be redesigned for continuous or large‐batch processing to achieve commercial viability [244]. In particular, supercritical drying, commonly performed using supercritical CO₂, is highly effective for preserving hierarchical porosity but requires costly high‐pressure infrastructure and suffers from limited throughput, thus constraining scalability [245]. Consequently, increased attention has shifted toward ambient‐pressure drying (APD) and freeze‐drying strategies capable of maintaining structural integrity while reducing capital and operational costs [246]. Recent studies demonstrate that APD combined with solvent exchange and surface sialylation (e.g., trimethylchlorosilane modification) can suppress capillary stress during evaporation, enabling crack‐free carbon and M–N–C aerogels with high surface area and preserved mesoporosity, thereby offering a more industrially relevant pathway. Comprehensive techno‐economic analyses, accounting for precursor cost, energy input during pyrolysis, solvent recovery, and equipment amortization, are essential to evaluate the true economic feasibility of competing synthesis routes [247].

Beyond scalability, durability in acidic media and testing inconsistency must be addressed through concrete, literature‐supported strategies rather than general optimization statements. For Fe–N–C aerogels, maximizing Fe–N₄ active‐site density through controlled pyrolysis (typically 850°C–950°C), incorporation of secondary nitrogen precursors such as melamine for nitrogen enrichment, and dual‐heteroatom doping (e.g., S or P co‐doping) has been shown to modulate local electron density, strengthen Fe–N coordination, and suppress demetallation and carbon corrosion under acidic ORR conditions [248, 249, 250, 251]. Furthermore, structural stabilization approaches, including the formation of corrosion‐resistant graphitized carbon shells or the deposition of protective TiO₂ nanolayers, have demonstrated improved durability in 0.1 M HClO₄ by mitigating against peroxide‐induced carbon oxidation and metal leaching. By integrating scalable drying technologies, site‐density engineering, heteroatom modulation, and protective nanolayer design within standardized evaluation frameworks, aerogel electrocatalysts can transition from promising laboratory constructs to economically and operationally viable fuel cell materials.

Although numerous aerogel electrocatalysts exhibit outstanding ORR activity in alkaline electrolytes, their translation to the most commercially relevant platform, proton exchange membrane fuel cells (PEMFCs) for automotive applications, requires sustained performance under strongly acidic conditions (pH ≈ 0–1) [252]. In such environments, non‐precious metal aerogels, particularly Fe–N–C systems, undergo progressive degradation driven by Fe demetallation, protonation of nitrogen coordination sites, peroxide‐induced carbon oxidation, and structural corrosion of the carbon matrix [253]. Consequently, and for example, meeting the U.S. Department of Energy (DOE) targets for PEMFC cathode catalysts—namely mass activities exceeding 44 A g^{− 1} at 0.9 V and operational durability beyond 5000 h with less than 40% performance loss remains a formidable benchmark for non‐precious aerogel catalysts [180]. Addressing these challenges requires rational stabilization strategies, including encapsulation of Fe–N_x active centers within corrosion‐resistant graphitic shells, reinforcement of metal–nitrogen coordination environments, and the development of oxidation‐resistant conductive supports [180, 254].

On a more optimistic development, recent advances in aerogel‐derived Fe–N–C and related M–N–C catalysts demonstrate that tailored active‐site engineering combined with hierarchical porosity can yield competitive ORR performance even in acidic media, and relevant to low‐temperature PEMFCs. By creating ultrahigh surface area and open pore architectures, these aerogels maximize active‐site accessibility and facilitate mass transport under experimentally viable operating conditions. Several recent reports show that optimized aerogel‐modified catalysts achieve onset and half‐wave potentials approaching ∼0.9 V versus RHE in acidic electrolytes, including diluted phosphoric acid, while simultaneously exhibiting durability trends comparable to Pt/C benchmarks under PEM‐like stress protocols [236]. Enhanced acid stability has been attributed to controlled graphitization, protective carbon coatings, and precise modulation of Fe–N_x coordination to suppress demetallation and carbon corrosion pathways [255].

Beyond single‐metal systems, bimetallic aerogel architectures (e.g., Mn–Fe porphyrinic aerogels) introduce synergistic electronic interactions that help mitigate linear scaling constraints and improve both onset potentials and intrinsic mass activities [256]. More recently, Fe–Sn–N–C aerogel‐type catalysts designed for high‐temperature PEMFC operation have demonstrated that secondary metal incorporation and optimized nitrogen coordination can significantly enhance acidic ORR stability relative to conventional Fe–N–C materials [257].

Taken together, these developments underscore the importance of systematic structural and compositional tuning to achieve acid‐stable aerogel electrocatalysts. A balanced comparative assessment of key performance metrics, such as onset potential, half‐wave potential, kinetic current density at 0.9 V versus RHE, and long‐term durability, between acidic and alkaline systems is therefore essential to provide up‐to‐date and application‐relevant metrics on the evolution of non‐precious metals aerogel catalysts for next‐generation PEM fuel cells.

A comparison of ORR catalysts reported in different studies remains a challenge due to inconsistencies in testing protocols, electrolyte composition, reference electrode calibration, catalyst ink formulation, and performance metrics [258]. Notably, ORR activity derived from RDE measurements often fails to translate quantitatively into practical fuel cell performance because of disparities in catalyst loading, film thickness, ionomer distribution, mass transport regimes, and operational potential windows [259]. To address these limitations, the establishment of unified and transparent benchmarking standards for aerogel electrocatalysts is essential. Such standards should encompass harmonized procedures for sample preparation, electrode fabrication, RDE testing parameters, accelerated stress testing (AST), and membrane electrode assembly (MEA) integration [260]. Ongoing initiatives by institutions such as the Fuel Cell Industrialization Research Center (FC‐Cubic) and the U.S. Department of Energy's Electrocatalysis Consortium (ElectroCat) represent important progress toward globally aligned evaluation frameworks [261, 262].

In line with international fuel cell evaluation practices, this review recommends the adoption of explicit benchmark protocols to enhance reproducibility and cross‐study comparability. A standardized testing scheme should include: (i) RDE measurements conducted in O₂‐saturated 0.1 M HClO₄ (acidic) and 0.1 M KOH (alkaline) electrolytes at 1600 rpm; (ii) catalyst thin‐film loading of 0.6 mg cm^{− 2}; (iii) rigorous iR compensation and background subtraction procedures; (iv) durability assessment via 30 000 potential cycles between 0.6 and1.0 V versus RHE at 50 mV s^{− 1}; and (v) mandatory reporting of kinetic current densities at 0.9 V versus RHE normalized to both geometric area and electrochemically active surface area.

Furthermore, validation under practical MEA conditions, such as H₂–O₂ fuel cell operation at 80°C, fully humidified gases, and 150 kPa backpressure, should be required for high‐performing aerogel catalysts to bridge the gap between half‐cell evaluation and device‐level performance. By integrating clearly defined, parameter‐based testing benchmarks, future studies can move beyond descriptive reporting toward actionable, reproducible, and translationally relevant advancements in fuel cell electrocatalysis.

8.1. Mechanistic Understanding and Rational Design

Although significant progress has been made in identifying active sites and elucidating reaction mechanisms, many aspects of ORR electrocatalysis in aerogel systems remain poorly understood [263]. The heterogeneous nature of most aerogels, containing multiple types of sites (metal centers, heteroatom‐doped carbon, defects, interfaces), complicates mechanistic studies [264]. Operando characterization techniques that probe catalyst structure, composition, and electronic properties under realistic electrochemical conditions are essential for understanding dynamic changes during operation and degradation [265]. Integration of advanced characterization with machine learning approaches offers promising pathways for identifying structure–property relationships in complex aerogel systems and accelerating the discovery of optimized compositions [266, 267].

8.2. Emerging Directions and Opportunities

Several promising research directions are likely to drive future advances in aerogel electrocatalysts:

8.2.1. Single‐Atom Catalysts in Aerogel Scaffolds

The integration of atomically dispersed metal sites (Fe, Co, Ni, Cu) within hierarchical carbon aerogel supports represents a convergent approach that combines maximum metal utilization with optimal mass transport [268, 269]. Recent advances in atomic layer deposition and molecular precursor strategies enable precise control over single‐atom loading and coordination environment [270].

8.2.2. High‐Entropy Alloy Aerogels

High‐entropy alloys containing five or more elements in near‐equimolar ratios offer vast compositional space for optimizing catalytic properties [271]. The aerogel synthesis of high‐entropy alloys is in its infancy but could provide pathways to catalysts with unprecedented combinations of activity, selectivity, and stability [272].

8.2.3. Bioinspired and Biomimetic Designs

Further exploration of enzyme‐inspired active site structures, such as the binuclear Cu─Fe centers in cytochrome c oxidase, may yield aerogel catalysts with enhanced activity through cooperative multi‐site mechanisms [3, 273]. Incorporation of secondary coordination sphere effects, proton relay groups, and substrate channeling features could further improve performance.

8.2.4. In Situ and Operando Characterization

Expanding the application of operando spectroscopy (XAS, Raman, infrared), microscopy (electrochemical TEM), and scanning probe techniques to aerogel catalysts under realistic operating conditions will provide unprecedented insights into active site evolution, reaction intermediates, and degradation mechanisms. These insights will guide the design of more stable and active catalysts.

8.2.5. Computational Catalyst Screening and Design

High‐throughput computational screening using DFT and machine learning models can rapidly evaluate thousands of potential aerogel compositions and structures, identifying promising candidates for experimental validation. Integration of multi‐scale modeling from electronic structure calculations to continuum transport simulations will enable holistic optimization of aerogel catalyst properties.

9. Concluding Remarks

Aerogel electrocatalysts represent a transformative approach to addressing the critical challenges of ORR in fuel cells and metal–air batteries. The unique three‐dimensional hierarchical porous structures of aerogels provide simultaneous optimization of active site density, electronic conductivity, and mass transport requirements that are difficult to achieve in conventional catalyst architectures. Recent advances across carbon‐based, metallic, and hybrid aerogel systems have demonstrated that properly designed aerogels can match or exceed the performance of commercial Pt/C catalysts in specific metrics while offering advantages in cost, resource availability, and durability.

The field has progressed from proof‐of‐concept demonstrations to sophisticated structure–property understanding and practical device integration. State‐of‐the‐art aerogel catalysts now exhibit mass activities exceeding 1.5 A mgPt^{− 1} for noble metal systems, half‐wave potentials approaching 0.90 V versus RHE for non‐precious metal catalysts, and fuel cell power densities of 300–500 mW/cm² for both PEM and anion exchange membrane systems. These achievements reflect the successful implementation of advanced synthesis strategies, including atomic‐level site engineering, strain and ligand effect tuning, molecular catalyst integration, and hierarchical pore architecture control.

However, significant challenges remain before aerogel electrocatalysts can achieve widespread commercial deployment. Scalable manufacturing processes that maintain the structural and catalytic properties of laboratory‐scale aerogels must be developed and economically validated. The durability of non‐precious metal aerogels in acidic PEM fuel cell environments requires substantial improvement to meet automotive application targets. Standardized testing protocols and performance metrics are needed to facilitate meaningful comparison across studies and accelerate progress. Deeper mechanistic understanding through operando characterization and computational modeling will guide rational design of next‐generation catalysts.

The convergence of several promising research directions: single‐atom catalysts in aerogel scaffolds, high‐entropy alloy aerogels, bioinspired designs, multifunctional catalysts, advanced characterization, and computational screening suggests that the field is poised for continued rapid advancement. As synthesis methods mature, mechanistic insights deepen, and device integration strategies improve, aerogel electrocatalysts are likely to play an increasingly important role in enabling the widespread adoption of fuel cell and metal–air battery technologies for clean energy conversion and storage.

Author Contributions

Kayode Adesina Adegoke: Conceptualization, methodology, interpretation, validation, writing – original draft, writing – review, and editing. Potlaki Foster Tseki: review, final editing, validation, and supervision.

Funding

There is no funding available for this work.

Ethics Approval and Consent to Participate

This article does not contain any studies involving human or animal subjects.

Ethics Statement

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

All data and materials used in this study are available within this article.