Rational Electrocatalyst Design for Coupled Hydrogen Evolution and Alcohol Oxidation with In Situ Perspective

ABSTRACT

The design of effective hydrogen generation systems with low energy consumption is pivotal for sustainable energy conversion. The slow oxygen evolution reaction hinders conventional water electrolysis, resulting in high energy consumption. Combining the processes of hydrogen evolution reaction (HER) and alcohol oxidation reaction (AOR) provides an effective approach that involves substituting the oxygen evolution reaction with more energetically favorable organic oxidation reaction, which can achieve lower cell voltages and simultaneous production of value‐added chemicals. Recent advances in electrocatalyst design including nanostructured, bimetallic, and atomically dispersed systems have significantly enhanced catalytic performance through optimized adsorption energetics and improved charge transfer kinetics. Rational control over composition, morphology, and electronic structure is vital for enhanced activity, selectivity, and long‐term durability. In addition, in situ characterization techniques provide vital insights into catalyst reconstruction and active phase evolution under working conditions. This review outlines the recent progress in the rational design of electrocatalysts for HER–AOR systems, highlighting the role of in situ studies in elucidating structure–activity relationships and providing insight into their potential applications in efficient hydrogen production and organic transformations with reduced energy consumption

This review summarizes recent advances in electrocatalysts for coupled hydrogen evolution and alcohol oxidation reactions as energy‐efficient alternatives to conventional water electrolysis. It highlights nanostructured and bimetallic catalyst design, in situ characterization insights, structure–activity relationships, and opportunities for low‐voltage hydrogen production with simultaneous value‐added chemical generation.

1. Introduction

The increasing rate of global energy consumption, fueled by population growth and industrialization, has accelerated the pursuit of sustainable and clean energy sources. Although fossil fuels, such as coal, oil, and natural gas, remain the dominant energy sources in the global energy mix, their finite reserves and detrimental effects on the environment, including emissions of greenhouse gases and global warming, make it imperative to shift toward renewable energy resources [1, 2]. Because of its high gravimetric energy density (285.8 kJ mol⁻¹), carbon neutrality, and ability to facilitate massive energy storage, hydrogen has been recognised as a promising energy carrier among the various alternatives [3, 4, 5]. Water electrolysis, consisting of the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode, is widely regarded as a very effective and scalable method for producing high‐purity hydrogen [6]. The thermodynamically challenging nature of the OER, which needs a theoretical potential of 1.23 V vs. reversible hydrogen electrode (RHE) but actually requires much higher overpotentials in practical electrolyzers, is the primary cause of water electrolysis's slow reaction rates and high energy costs [7, 8, 9].

To address these challenges, the use of alcohol oxidation reaction (AOR), in place of OER has proven to be an efficient approach. Generally, AOR occurs at lower potentials, hence lowering the total cell voltage and improving the hydrogen production performance [10, 11, 12]. Furthermore, it allows for the production of valuable chemicals such as aldehydes, carboxylic acids, and formates from biomass‐derived alcohols such as methanol [12, 13], ethanol [14], and glycerol [15, 16, 17]. The combination of HER and AOR has several advantages, including reduced energy consumption, avoidance of explosive H₂/O₂ mixtures, reduced membrane aging due to the absence of reactive oxygen species, and the ability to operate under energy‐efficient conditions without the need for high pressure and temperature [18]. In addition, it improves economic viability by allowing for the concurrent production of H₂ and value‐added chemicals via electrorefining or biomass valorization routes [19]. Recently, advanced catalyst engineering strategies, including nano structuring, heteroatom doping, and dual‐metal active site design, have been widely explored to improve the activity, selectivity, and stability of HER‐AOR electrocatalysts [20, 21].

This review concentrates on the latest trends in the rational design of electrocatalysts for HER integrated with AOR as an efficient alternative to conventional water electrolysis. HER‐AOR systems can dramatically reduce the cell voltage and, consequently, energy demand by replacing the sluggish oxygen evolution reaction with more thermodynamically favorable organic oxidation reactions. In addition, they provide an opportunity for the paired production of H₂ and valuable chemicals such as formate, acetate, etc. Special attention is placed on the rational design strategies of advanced electrocatalysts, such as nanostructured, bimetallic, and atomically dispersed electrocatalysts, which demonstrate superior electrocatalytic performance for integrated HER‐AOR systems. In addition, in situ characterization is emphasized in understanding catalyst reconstruction, active phase evolution, and reaction mechanisms. The review is intended to give a complete understanding of catalyst structure‐activity relationships and to propose a perspective for designing efficient HER‐AOR catalysts.

2. Electrocatalytic Reaction Mechanisms and Key Parameters

As shown in Figure 1a, the conventional water splitting process mediated by dual‐metal‐site catalysts consists of two half‐cell reactions in an electrochemical cell. Water is reduced to H₂ at the cathode by the HER and oxidized to oxygen gas at the anode by the OER [22]. The overall water splitting reaction is given by:

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+1/2{\mathrm{O}}_2\left({\mathrm{E}}^0=1.23\mathrm{V}\mathrm{vs}\mathrm{RHE}\right)$$ (1)

FIGURE 1.

(a) A schematic representation of conventional water splitting. (b) Schematic illustration of HER coupled with alcohol oxidation reaction for simultaneous hydrogen and value‐added chemical generation.

From thermodynamic perspective, the minimum driving potential required for water splitting at room temperature is 1.23 V [8]. In this reaction, one mole of hydrogen gas and half a mole of O₂ are produced from one mole of water [5].

2.1. HER Mechanism

The HER is a two‐electron transfer reaction that occurs at the cathode. In an acidic environment, the reaction involves the direct reduction of protons (H⁺), while in an alkaline medium, the reaction includes the water reduction, which serves as a proton source [22]. The HER reaction involves three elementary steps: Volmer (electrochemical adsorption), Heyrovsky (electrochemical desorption), and Tafel (chemical recombination). In an acidic environment, the reaction involves the following steps: H⁺ + e^─ → H* (Volmer), followed by either H⁺ + H* + e^─ → H₂ (Heyrovsky) or 2H* → H₂ (Tafel) (Figure 2a) [23]. In an alkaline condition, the Volmer step includes water dissociation (H₂O + e^─ → H* + OH^─) (Figure 2b), which is kinetically slower because of the additional energy barrier for H─OH bond cleavage, thus making HER less favorable in alkaline media compared to acidic media.

FIGURE 2.

Schematic representation of HER and OER processes in an acidic and alkaline environment: (a,b) The HER process involves Volmer, Heyrovsky, and Tafel reactions, which include proton adsorption, electrochemical desorption, and recombination to form H₂ molecules, and (c,d) The OER process includes a multi‐step oxidation of water via *OH, *O, and *OOH intermediates to form O₂ gas at the catalyst surface.

2.2. OER Mechanism

The OER is a complicated four‐electron transfer oxidation reaction that takes place at the anode [24]. The reaction involves several adsorbed intermediates (*OH, *O, *OOH) and involves proton‐coupled electron oxidation process. In an acidic environment, the oxidation of water involves the evolution of O₂ with the creation of intermediates *OH, *O, and *OOH (Figure 2c) [25]. In an alkaline environment, the reaction entails the oxidation of OH^─ ions, resulting in intermediate formation *OH, *O, and *OOH (Figure 2d). The OER reaction kinetics are slow because of multiple electron‐transfer steps and high activation energy barriers, making the half‐reaction particularly control the overall water‐splitting reaction.

2.3. Alcohol Oxidation Reactions

The coupling of HER with AOR (Figure 1b) is an energy‐efficient method of producing hydrogen by substituting the thermodynamically more favorable organic oxidation reactions for OER [26]. This method greatly lowers the cell voltage while producing value‐added chemicals like aldehydes, ketones, and carboxylates simultaneously. AOR occurs at lower potentials than OER (E⁰ = 1.23 V vs RHE), boosting the electrolysis's overall efficiency [3]. The reaction mechanism includes the adsorption of alcohol molecules, followed by stepwise dehydrogenation and oxidation by surface‐active species.

The overall anodic reaction for alcohol oxidation in alkaline conditions is expressed as:

$$\mathrm{RC}{\mathrm{H}}_2\mathrm{OH}+3\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{RCO}{\mathrm{O}}^{-}+2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$$ (2)

This reaction occurs through an adsorbate evolution process involving the following key intermediates:

$$\mathrm{RC}{\mathrm{H}}_2\mathrm{OH}+\mathrm{O}{\mathrm{H}}^{-}\rightleftharpoons \mathrm{RC}{\mathrm{H}}_2{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}$$ (3)

$$\mathrm{M}\ast +\mathrm{O}{\mathrm{H}}^{-}\rightleftharpoons \mathrm{MO}{\mathrm{H}}^{\ast }+{\mathrm{e}}^{-}$$ (4)

$$\mathrm{RC}{\mathrm{H}}_2{\mathrm{O}}^{-}+\mathrm{MOH}\ast \to \mathrm{RCHO}+{\mathrm{H}}_2\mathrm{O}+\mathrm{M}\ast +{\mathrm{e}}^{-}$$ (5)

$$\mathrm{RCHO}+\mathrm{MOH}\ast +\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{RCO}{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}+\mathrm{M}\ast$$ (6)

Here, M* denotes an active catalytic site on the electrode surface. The generation of alkoxide (RCH₂O^─) increases nucleophilicity, while MOH* represents the active oxidizing intermediate (Equations (3), (4), (5), (6)). Step (5) is generally considered the rate‐determining step involving C─H bond activation and oxidation of the alkoxide intermediate [27]. This adsorbate evolution mechanism highlights the efficient conversion of alcohols into value‐added chemicals with improved reaction kinetics, making AOR a thermodynamically favorable anodic alternative to the oxygen evolution reaction in hybrid electrolysis systems [18].

2.4. Key Parameters

2.4.1. Overpotential

Overpotential (η) can be defined as the voltage required beyond the thermodynamic requirement of 1.23 V to facilitate electrochemical reactions at a given current density [28]. It is a key factor in determining the feasibility and energy efficiency of hydrogen production. Although the operating voltage for water splitting is 1.23 V, the actual voltage required in practical systems is higher (≈1.6‐2.0 V) due to kinetic barriers and resistive losses (η _HER = E _RHE – 0 V and η _OER = E _RHE − 1.23 V). These equations are used to calculate the overpotential of HER and OER at specific current densities without iR compensation [29].

2.4.2. Tafel Slope

The Tafel slope is an important kinetic parameter in electrocatalysis that relates the overpotential to the current density (j), usually given in units of mV dec^─1 [30]. The Tafel slope is the additional potential needed to raise the current density by an order of magnitude. A smaller Tafel slope denotes stronger catalytic activity and faster reaction rates. The Tafel equation, which is derived from the Butler‐Volmer equation, is given by η = a + b log j, where b is the Tafel slope. The Tafel slope is also used to understand the rate determining step (RDS) of a reaction [29]. For instance, in the HER, the Tafel slopes of 120, 40, and 30 mV dec^─1 represent the Volmer, Heyrovsky, and Tafel steps, respectively [31, 32].

2.4.3. Stability

Stability is a key consideration in determining the applicability of catalysts for hydrogen and oxygen evolution in water electrolysis. Short‐term stability is usually measured by electrochemical cycling between a fixed potential window by cyclic voltammetry (CV), while long‐term stability is measured by chronopotentiometry (at a constant current density) or chronoamperometry (at a constant overpotential), usually performed for more than 12 h. The stability criterion is much more demanding for the OER than for the HER. HER takes place in a mild reducing environment at a relatively lower potential, where the catalysts can easily survive thousands of cycles. On the other hand, OER takes place at highly oxidative anodic potentials, which are prone to corrosion, surface reconstruction, and metal dissolution. Thus, stability beyond 250–1000 cycles is highly desirable for OER catalysts, and less than 1000 cycles is often used as a criterion for stability assessment [33].

2.4.4. Turnover Frequency

Turnover frequency (TOF) is another key kinetic parameter in electrocatalysis, which denotes the number of catalytic cycles, or the moles of H₂ or O₂ evolved per unit time per active site at a given overpotential. TOF quantifies the activity of active sites and is not affected by the amount of catalyst used. For the HER as well as the OER, higher TOF values are preferred, as they denote higher catalytic efficiency rates per unit time [34]. TOF can be calculated using the following equation: TOF = j × N _A/n × F × Γ, where j is the current density, n is the number of electrons transferred (2 for HER, 4 for OER), N_A  is Avogadro's number, F is Faraday's constant, and Γ represents the surface concentration of electrochemically active sites [10, 35].

2.4.5. Faradaic Efficiency

Faradaic efficiency (FE) is a significant factor in electrocatalysis that measures the selectivity of the catalyst by calculating the percentage of total charge utilized to generate the intended product. For water splitting, the HER is responsible for the production of H₂ gas, whereas the OER is accountable to produce O₂ gas. Gas chromatography or the water displacement method can be used to determine the volume of these gases. A good electrocatalyst should have an FE value close to 100%, which reveals that almost all the passed charge is used for the production of the desired gaseous products [36, 37].

3. Synthesis Strategies

3.1. Electronic Structure Modulation

Electronic structure modulation is a key factor in enhancing the electrocatalytic performance of materials by tuning adsorption energetics, charge distribution, and reaction kinetics. Among these, atomically dispersed dual metal site (DMS) engineering is a reliable technique to overcome the fundamental limitations of Single Metal Sites (SMSs), particularly in complex multi‐electron electrocatalytic reactions, by electronic interactions between two adjacent metal centres. DMSs are systematically classified into homonuclear and heteronuclear types based on the chemical identity of their metal centres [38, 39]. Homonuclear DMSs are a single type of metal‐forming pair of atom sites. Sa et al. [40] synthesised Fe₂‐N‐C DMS, which was fabricated through the confinement of a binuclear Fe₂(CO)₉ precursor within ZIF‐8, followed by controlled pyrolysis. This method produced well‐defined Fe─Fe pairs confined in N‐doped carbon, exhibiting remarkable oxygen reduction reaction that was attributed to a critical shift in O₂ adsorption mode transition from superoxo‐like to peroxo‐like species facilitated by the electronic coupling between the two Fe centres. Similarly, Wang et al. [41] synthesized atomically precise dual‐site catalysts to promote the electrochemical carbon dioxide reduction reaction via electronic control. Compared with traditional single‐atom catalysts, the Fe₂–N₆–C dual‐atom site showed superior performance, with CO Faradaic efficiency exceeding 80% throughout an extended potential window, as well as high turnover frequency of 26 637 h^─1 and remarkable durability (>20 h). Structural analysis revealed that two Fe atoms were strongly bound to a nitrogen‐doped carbon support, creating clear dual active sites. The orbital hybridization between two Fe atoms, which reduces the bonding–antibonding energy gap orbitals of *CO intermediates, is notably responsible for the activity boost. This dual‐site design allows for efficient *CO desorption, a major challenge for single‐atom catalysts. As shown in Figure 3a, the Fe₂–N₆ dual‐site design enables synergistic effects between neighboring metal atoms, which can be used to optimize the adsorption energetics and reduce the reaction barriers. How dual atom site engineering achieves precise control of the electronic structure opens an emerging strategy for the development of extremely effective electrocatalysts for carbon dioxide reduction and other reactions.

FIGURE 3.

Schematic representation of (a) Fe₂‐N₆ dual‐site catalyst structure for optimal CO₂ reduction, Reproduced with permission [41]. Copyright 2022, American Chemical Society, (b) Ni‐Mo aerogel synthesis through NaBH₄‐mediated gelation and gas templating, Reproduced with permission [50]. Copyright 2021, Elsevier, (c) FeCo‐NC dual‐atom catalyst synthesis via two‐step pyrolysis, Reproduced with permission [52]. Copyright 2025, American Chemical Society, and (d) Synthesis route of Ni/MoC/Ti₃C₂T_x@C involving MXene etching, precursor assembly, aerogel formation, and carbonization. Reproduced with permission [57]. Copyright 2023, American Chemical Society.

Heteronuclear DMSs : Pei et al. [42] used a sequential templating, atom migration, and trapping strategy to develop atomically isolated Ni/Co dual sites on nitrogen‐doped carbon (NiCo/NC) hollow prisms. First, NiCo precursor prisms were coated with polydopamine, followed by partial etching to create hollow structures and subsequent carbonization. NiCo nanoparticles in carbon were then converted to atomically dispersed dual sites at high temperatures. This controlled synthesis allowed for uniform dispersion of heteronuclear Ni‐Co pairs, facilitating strong electronic coupling and optimal catalytic performance. Wang et al. [43] introduced a general “metal ion targeting coordination” method for controlled fabrication of heteronuclear dual‐atom catalysts. The method involves the initial coordination of bipyridyl ligands to a central Fe ion to yield a (bpy)Fe(μ₂‐OH) complex, where oxygen atoms operate as binding sites for a secondary metal ion (Co, Ni, Mn, or Cu), establishing clear Fe─O─M arrangements. A Zn‐based MOF host framework built from a combination of organic linkers allows for controlled spatial distribution of catalytic sites and suppresses site aggregation. Pyrolysis of these precursors yields atomically dispersed Fe─M dual sites. This technique offers a solid basis for the advancement of heteronuclear dual‐atom catalysts by enabling exact control over catalytic site coordination and electronic structure.

3.2. Doping

Doping strategy has shown itself to be a very successful way to overcome the intrinsic limitations of transition metal catalysts, where inadequate binding strength with reaction intermediates, particularly adsorbed hydrogen (H^*), often results in sluggish kinetics and poor catalytic performance [44]. This method strategically incorporates foreign atoms into the host lattice via substitutional or interstitial mechanisms, resulting in composite materials that maintain the original phase structure while acquiring uniquely tailored physicochemical properties [45]. Heteroatom doping directly modifies the catalyst's electronic structure, by causing charge redistribution and accelerating electron transfer kinetics, hence improving the adsorption and desorption energetics of critical HER intermediates [46]. Doping strategies are divided into three categories according to the identity of the dopants: cation doping (e.g., Fe, Mn, or Ru substituting for transition metal sites), anion doping (e.g., N, P, S, or B replacing chalcogen or oxygen sites), and cation‐anion dual doping, which synergistically integrates both methods to facilitate enhanced electronic restructuring [47, 48]. Weng et al. [49] engineered atomically dispersed iridium on α‐MnO₂ through high‐valence manganese‐mediated anchoring, resulting in a stable Ir─O─Mn coordination structure that exhibited exceptional activity and stability in acidic OER. The strong catalyst support interaction induced by this coordination environment modulated the local electronic configuration of Ir sites, effectively weakening the adsorption strength of ‐OOH intermediates, a crucial measure for OER overpotential, thereby promoting the rate‐determining step of oxygen–oxygen bond formation.

Zhang et al. [50] synthesized a cost‐effective bifunctional Ni─Mo─P aerogel serving as an electrocatalyst for overall water splitting. The preparation method is shown in Figure 3b, where the Ni─Mo aerogel was initially prepared through a NaBH₄‐mediated gelation process and subsequent freeze‐drying. During the reaction, NaBH₄ functioned as both a reducing agent and a gelation facilitator. The H₂ gas produced in situ from the hydrolysis of NaBH₄ served as a dynamic gas template, which helped to form a three‐dimensional sponge‐like porous aerogel structure (Figure 3b). Finally, in situ P doping was carried out using NaH₂PO₄ to obtain the Ni─Mo─P aerogel. The catalyst exhibited an exceptionally low overpotential of 1.46 V at 10 mA cm^─2 for HER. Density functional theory (DFT) computation reveal that P doping induced favorable charge migration across the Ni‐Mo matrix, which fine‐tuned the hydrogen adsorption free energy (ΔG_H) toward thermoneutrality, thereby accelerating proton discharge and H₂ desorption kinetics.

3.3. Defect Engineering

Defect engineering core concept is that defects, especially edge‐rich, asymmetric geometric arrangements, act as high‐energy anchoring sites that stabilize two nearby metal atoms while concurrently influencing their electronic interaction and maximizing their exposure to reactants. The control of geometric construction, electronic structure modification, and active site exposure underlies the improved catalytic behavior observed in defect‐engineered DMSs [51]. Zhang et al. [52] described an Fe/Co dual‐atom catalyst (FeCo–NC) made using a two‐step pyrolysis approach, as shown in Figure 3c. First, ZnFe–ZIF was pyrolyzed to retain the carbon structure from the ZIF and introduce a high density of negatively charged defects by Zn evaporation. The defective carbon structure provided sites for the electrostatic adsorption of nitrogen‐rich [Co(en)₃]³⁺ guest molecules. In the second step, a pyrolysis process introduced isolated Fe and Co atoms into the N‐coordinated carbon lattice, creating Fe─Co dual atom sites. The confined space prevented metal atom aggregation. Characterization showed that the precursor retained a rough, spherical ZIF‐derived morphology, while FeCo‐NC had a high density of carbon nanotubes that improved electron and mass transport. Electrochemical tests showed high ORR performance, with half‐wave potentials of 0.92 V in alkaline solution and 0.76 V in acidic solution.

Kong et al. [53] synthesized 3D hierarchical NiMo₃S₄ nanoflowers arrayed on carbon textiles (NiMo₃S₄/CTs) via a hydrothermal process and subsequent annealing. Ni²⁺ doping in the Mo‐S lattice introduced defect‐rich nanoflakes with increased (002) d‐spacing, promoting conductivity and providing rich Ni³⁺ catalytic sites. In alkaline media, the self‐supported cell delivered overpotentials of only 149.5 mV for HER and 126.2 mV for OER at 10 mA cm^─2. For overall water splitting, only 1.55 V was needed at 10 mA cm^─2 with durability up to 72 h. Jiao et al. [54] demonstrated a Cu atom‐pair catalyst (Cu₁ ⁰–Cu₁ ^x+) supported on Pd₁₀Te₃ alloy nanowires. When an optimal Cu concentration (0.10%) was reached, neighboring Cu₁ ⁰–Cu₁ ^x+ sites were coupled by Te surface defects, facilitating the joint adsorption of CO₂ and H₂O molecules. The Cu atom‐pair structure greatly facilitated CO₂ activation, reaching a Faradaic efficiency for CO exceeding 92% with a remarkably suppressed HER reaction. DFT calculations and experimental studies both revealed lowered activation energy barriers and improved selectivity under low overpotential.

3.4. Synergistic Effects

Synergistic interactions between multiple catalytic components can significantly improve electrocatalytic activity by optimizing charge transfer and intermediate adsorption behavior [55, 56]. The cooperative electronic interaction between neighboring active sites can effectively tune the local electronic structure and accelerate reaction kinetics. In addition, synergistic effects can facilitate the adsorption and conversion of reaction intermediates, thereby enhancing catalytic activity and stability during multi‐step electrochemical reactions. Xu et al. [57] synthesized a multicomponent Ni/MoC/Ti₃C₂T_x@C electrocatalyst through electrostatic assembly combined with high‐temperature calcination, as shown in Figure 3d. Ti₃C₂T_x MXene nanosheets were first synthesized by etching Ti₃AlC₂, followed by the adsorption of Ni²⁺ and Mo precursors, freeze‐drying, and subsequent carbonization to obtain the hybrid material. In an alkaline solution, the electrocatalyst showed high HER and OER activity with overpotentials of 128 and 338 mV at 10 mA cm^─2. The boosted catalytic activity stems from the synergistic role of Ni and MoC, the conductive MXene scaffold, and the protective carbon shell. Moreover, the urea‐assisted electrolysis proceeded at a low voltage of 1.56 V to drive 10 mA cm^─2. The superior performance arises from a combined interaction between the Ni and MoC components, which collectively optimise the electronic structure at the interface, improve charge transfer kinetics, and leading to a higher density of catalytic sites. Liu et al. [58] developed Ni₃Cu₁@NG‐NC nano catalysts via controlled calcination, which include Ni‐Cu alloy nanoparticles encapsulated within nitrogen‐doped graphene layers formed from a porous N‐doped carbon matrix. This architecture provided exceptional HER activity in both alkaline and acidic media, achieving overpotentials of 122 mV and 95 mV at current densities of 10 mA cm^─2 respectively, with Tafel slopes of 84.2 and 77.1 mV dec^─1. The bimetallic Ni‐Cu core and the N‐doped graphene shell had synergistic electronic interactions that optimized the electronic structure and improved electron transport kinetics, which led to the high activity.

4. HER Coupled with AOR

The coupling of the hydrogen evolution reaction (HER) with alcohol oxidation reactions (AOR) offers a promising alternative to conventional water electrolysis by replacing the sluggish oxygen evolution reaction with more thermodynamically favorable anodic processes [6]. This strategy reduces the overall cell voltage and enables the simultaneous production of hydrogen and value‐added chemicals. Therefore, HER‐AOR systems provide an efficient platform for low‐energy hydrogen generation and biomass‐derived chemical upgrading [10, 11].

4.1. Methanol Oxidation Reaction

The Methanol Oxidation Reaction (MOR) is considered among the most efficient, widely studied anodic reactions that can be coupled with HER because of its low thermodynamic barrier, high kinetic rate, and high hydrogen content. In the anodic oxidation reaction, OER is replaced by the MOR, resulting in a dramatic decrease in the overall cell voltage while allowing for the concurrent generation of hydrogen. The MOR involves a multi‐step dehydrogenation mechanism with adsorbed intermediates CH₃OH, CHO, and CO, where the efficient removal or conversion of poisoning species is essential for the long‐term reaction sustainability [59, 60]. Designing these types of catalysts is a crucial factor in promoting reaction kinetics and stability in alkaline or neutral media. Atomically dispersed dual‐metal catalysts have been shown to exhibit outstanding performance in HER‐MOR coupling reactions because of their unique synergistic properties. In these catalysts, one metal site is generally responsible for methanol adsorption and C‐H bond activation, while the other metal site is responsible for the oxidation of intermediates and facilitating charge transfer, which can efficiently alleviate CO poisoning and promote long‐term reaction stability.

Linghu et al. [61] presented a rationally designed bifunctional electrocatalyst for HER and MOR, showing an efficient approach for low‐voltage hydrogen production. DFT studies were first used to screen and optimize Co‐Ni DMS, indicating the synergistic catalytic effects of Ni and Co for MOR and HER, respectively. Based on the theoretical understanding, leaf‐like Co₃₀Ni₆₀ nanosheets were successfully prepared on carbon cloth using a simple two‐step electrodeposition strategy (Figure 4a). Electrochemical evaluation showed that, at current densities of 10 and 100 mA cm^─2, the overpotentials required for MOR on Co₃₀Ni₆₀/CC (1.323 and 1.403 V, respectively) were significantly lower than those of Co₃₀/CC (1.392 and 1.560 V, respectively) and Ni₆₀/CC (1.335 and 1.457 V, respectively), confirming its superior MOR activity (Figure 4b). Notably, these values represent the average of three parallel measurements, ensuring reliability. For HER in 1.0 M KOH, Co₃₀Ni₆₀/CC exhibited lower overpotentials (95 mV/237 mV at 10/100 mA cm^─2) compared to Ni₆₀/CC (149 mV/269 mV at 10/100 mA cm^─2) and Co₃₀/CC (220 mV/344 mV at 10/100 mA cm^─2), though slightly higher than Pt/C/CC (49 mV/226 mV at 10/100 mA cm^─2), highlighting its superior HER activity (Figure 4c). Mechanistically, Ni acts as the primary active site for MOR, while Co enhances ECSA and promotes selective formic acid formation. Conversely, Co serves as the dominant HER active site, with Ni improving charge transfer kinetics through strong electronic coupling. A self‐assembled Co₃₀Ni₆₀/CC || Co₃₀Ni₆₀/CC electrolyzer was constructed using CH₃OH (1.0 M) + KOH (1.0 M) electrolyte (Figure 4d). The system required cell voltages of only 1.394 and 1.645 V at 10 and 100 mA cm^─2, respectively, which are substantially lower than those for conventional alkaline water electrolysis (1.592 and 1.868 V at 10/100 mA cm^─2), demonstrating significant energy savings (Figure 4e). The anodic product was confirmed as high‐value formic acid by ¹H NMR analysis (Figure 4f). Furthermore, the electrolyzer exhibited excellent durability, with only an 8.5% voltage increase after 150 h of continuous operation and a current retention of 91.4% (Figure 4g). The system maintained stable performance for over 150 h at 100 mA cm^─2, delivering high FE of 98.6% for MOR and 99.8% for HER. The outstanding performance was ascribed to the leaf‐like nanosheet structure with high active site density, monolithic electrode architecture for improved stability, and charge transfer from Co to Ni to tune the electronic structure, confirmed by XPS and density of states analysis. This work demonstrates the efficiency of DFT‐guided design and DMS synergy in designing high‐performance HER‐MOR dual‐functional electrocatalysts for efficient hydrogen production and selective formic acid synthesis.

FIGURE 4.

(a) Schematic of Co₃₀Ni₆₀/CC synthesis via two‐step electrodeposition, (b) MOR performance comparison showing lower onset potentials for Co₃₀Ni₆₀/CC at 10 and 100 mA cm^─2, (c) HER polarization curves indicating superior activity of Co₃₀Ni₆₀/CC in 1.0 M KOH, (d) Schematic of Co₃₀Ni₆₀/CC || Co₃₀Ni₆₀/CC electrolyzer using alkaline methanol, (e) Cell voltage comparison of alkaline water electrolysis, (f) ¹H NMR confirming formic acid formation, and (g) Long‐term stability showing 8.5% voltage increase and 91.4% current retention after 150 h. Reproduced with permission [61]. Copyright 2025, Elsevier.

Zhang et al. [60] developed ligand‐free PtBi nanoalloy catalysts that enabled efficient hydrogen production coupled with methanol oxidation during electrochemical seawater splitting. Metastable PtBi alloy nanoparticles with an average size of 2 nm and adjustable composition were readily prepared through femtosecond laser liquid ablation, providing defect‐rich and ligand‐free surfaces that provide maximum access to intrinsic active sites. Among them, Pt₄Bi/C showed high MOR reactivity in alkaline solution, with a mass activity of 17.7 A mg^─1 _pt, which is 11.5 times greater compared to that of commercial Pt/C, and a high specific activity of 54.9 mA cm^─2. In situ FTIR and ambient‐pressure XPS measurements confirmed a CO‐free MOR mechanism facilitated by Bi addition, while DFT calculations showed that Bi causes a negative shift of the Pt d‐band center by ∼0.37 eV, reducing the *CO adsorption energy, increasing the *OH adsorption energy, thereby decreasing the activation energy barrier for the rate‐determining *CHO to *HCOOH step. A hybrid MOR‐HER electrolyzer (Pt₄Bi/C || Pt/C) demonstrated an exceptionally low cell voltage of 545 mV at 10 mA cm^─2as a proof‐of‐concept demonstration. This is much lower than that of traditional alkaline electrolysis and other MOR‐assisted electrolyzers. Moreover, the electrolyzer operated stable in natural seawater for 54 h with cell voltages below 1.1 V. A Faradaic efficiency close to 100% for hydrogen evolution and efficient inhibition of chlorine evolution were also observed. This work demonstrates the efficiency of ligand‐free PtBi nanoalloys and hybrid MOR‐assisted electrolysis as a promising approach for efficient and chlorine‐free hydrogen generation from seawater.

Yang et al. [62] demonstrated the efficient methanol‐assisted hydrogen production system through the Pt nanoparticle modification on hierarchical MoSe₂@mesoporous hollow carbon spheres (Pt/MoSe₂@MHCS). Electrochemical characterization showed a negative shift of the MOR initiation potential at 0.58 V (vs. RHE), with a high forward‐scan current density of 81.1 mA cm^─2, outperforming Pt/MoSe₂, Pt/MHCS, and commercial Pt/C. CO‐stripping and cyclic voltammetry studies confirmed significantly improved anti‐poisoning property, as indicated by the lower CO oxidation onset and peak potentials and higher electrochemically active surface area (72.1 m² gPt^─1). Kinetic studies indicated lower Tafel slopes and R _ct, reflecting faster electron transfer at the interface and surface reaction kinetics. For hydrogen evolution in methanol‐containing acidic electrolyte, the catalyst needed only 28 mV to attain 10 mA cm^─2, with a low Tafel slope (∼25 mV dec^─1) and low impedance, characteristic of a Volmer‐Tafel mechanism. DFT calculations further demonstrated that the strong Pt‐MoSe₂ interaction influenced the electronic states of Pt sites, reducing CO* and H* adsorption but facilitating OH* formation due to enhanced oxophilicity, thus explaining the experimental results of improved activity, kinetics, and durability. When used in a two‐electrode methanol electrolysis cell, Pt/MoSe₂@MHCS needed only 0.62 V to attain 10 mA cm^─2, reflecting the synergistic electrochemical and theoretical rationale for low‐energy hydrogen production.

Dual‐monodispersed Pt‐Ni₃S₂ heterojunction nanocrystals were reported by Zhao et al. [63] as extremely effective bifunctional electrocatalysts for methanol oxidation‐assisted hydrogen generation. The interfacial electronic coupling between the ∼2 nm Pt and the ∼9.6 nm Ni₃S₂ led to an asymmetric charge distribution, which highly promotes adsorption and conversion of reaction intermediates. Electrochemically, the bifunctional electrocatalyst showed selective methanol oxidation to formate at low potentials, reaching 100 mA cm^─2 at 1.45 V with ∼98% electron efficiency and without CO₂ evolution, along with a low overpotential of 61 mV for HER at 10 mA cm^─2. It was highly stable over a wide range of potentials. XAFS and DFT studies showed that the oxidation process was accelerated by the positively charged Ni₃S₂, while the electron‐rich Pt was beneficial for H* adsorption and H₂ desorption. In the MOR//HER dual‐functional electrolyzer, the Pt‐Ni₃S₂ || Pt‐Ni₃S₂ cell functioned at a low applied potential of 1.60 V to reach 50 mA cm^─2, and it was highly stable, indicating that charge modulation at the heterointerface is an effective strategy for low‐voltage hydrogen production with value‐added formate co‐production.

Rao et al. [64] demonstrated Ni‐MoN nanorod arrays grown on nickel foam (Ni‐MoN/NF) as an efficient dual‐activity catalyst for methanol oxidation‐assisted hydrogen evolution. The Ni‐MoN/NF bifunctional electrocatalyst showed high electrochemical activity, needed only 49 mV overpotential to achieve 10 mA cm^─2 for HER in an acidic electrolyte and reaching 100 mA cm^─2 at 1.48 V for selective methanol oxidation to formate in an alkaline electrolyte. A hybrid acid/base MOR‐HER electrolyzer using Ni‐MoN/NF as an electrode achieved 10 mA cm^─2 at a low cell voltage of 0.56 V, which was significantly lower than that of the OER‐HER electrolyzer (0.70 V), with almost 100% FE for H₂ and ∼95% for formate. DFT calculations showed that the Ni incorporation modified the electronic structure of MoN, which was highly favorable for lowering hydrogen adsorption free energy and accelerating HER kinetics. The electrolyzer was also highly stable, maintaining its performance for 100 h, indicating that Ni‐MoN/NF was a promising dual‐activity catalyst for energy‐efficient H₂ with value‐added methanol upgrading.

4.2. Ethanol Oxidation Reaction

The ethanol oxidation reaction (EOR) is an attractive anodic process for coupling with HER owing to ethanol's low toxicity, high energy density, and widespread availability from biomass‐derived sources. Compared to MOR, EOR involves more intricate reaction pathways due to C─C bond, making complete oxidation kinetically challenging but highly rewarding in terms of electron transfer and energy efficiency. When integrated into HER–AOR systems, EOR effectively lowers the overall cell voltage while enabling the electrochemical upgrading of ethanol into value‐added products such as acetate, acetaldehyde, acetic acid, or carbonate species, depending on reaction conditions and catalyst design [14, 65]. Zhang et al. [14] demonstrated Ni‐Co bimetallic oxide nanoparticles as advanced electrocatalysts for the combination of ethanol oxidation and HER in alkaline conditions. The optimized Ni_0.67Co_0.33O_x anode showed a remarkably high current density of 126 mA cm^─2 at 1.6 V (vs. RHE) with an ethanol‐to‐acetate Faradaic efficiency of 85.74%, which was sustained at more than 80% selectivity after 15 h of testing. When combined in a complete ethanol electrolysis cell, coupling anodic EOR with cathodic HER, the Ni_0.67Co_0.33O_x cell reached a maximum current density of 145 mA cm^─2 at a low cell voltage of 1.6 V, surpassing the performance of traditional OER‐coupled cells. DFT calculations showed that proper Co doping could lower the energy barrier of ethanol dehydrogenation and facilitate the formation of key intermediates, which explained the observed activity enhancement.

Zulfiqar et al. [65] showed an ethanol‐assisted hybrid seawater electrolysis process that proceeded simultaneously with the EOR and HER, effectively short‐circuiting the slow OER and suppressing the competing CER in chloride‐containing medium. Employing a bifunctional NiCu hierarchical nanostructure supported on nickel foam (NiCu–HNS@NF, 1:1) as anode and cathode, the catalyst displayed high EOR activity in the simulated alkaline seawater (1.0 M KOH + 0.5 M NaCl + 1.0 M ethanol), requiring ∼220 mV lower than OER to achieve 20 mA cm^─2. The EOR proceeded with faster kinetics with smaller Tafel slope (81 mV dec^─1) and lower R _ct, selectively oxidizing ethanol via a C₂ pathway to acetate, as revealed by ¹H NMR analysis, with CO₂‐free product formation. On the cathodic side, the same NiCu–HNS@NF (1:1) electrode displayed high HER performance, requiring only 97 mV overpotential to attain 20 mA cm^─2 with high FE of ∼97%, exhibiting Volmer‐step‐limited kinetics and optimal charge‐transfer properties. In a two‐electrode arrangement (EOR || HER), the ethanol‐assisted seawater electrolyzer operated at a low cell voltage of 1.45 V to achieve 20 mA cm^─2, a ∼180 mV lower voltage than that needed for traditional seawater electrolysis. The electrolyzer operated stable for 24 h, emphasizing the complementary role of the Ni‐Cu bimetallic composition and hierarchical nanostructure in facilitating both EOR and HER.

Zhou et al. [66] demonstrated an interface‐engineered VO_x/Ni₃N heterostructure as an effective dual‐activity catalyst for ethanol‐assisted hydrogen production (Figure 5a). The electrocatalyst reached 50 mA cm^─2 at a low anodic potential of 1.38 V vs. RHE, greatly outperforming OER (≈1.65 V vs. RHE). Electrochemical impedance spectroscopy analysis showed that the VO_x/Ni₃N interface favored increased OH^─ adsorption and reactive O* generation. Density functional theory calculations showed a dual‐polarization mechanism, including interfacial and in‐plane charge transfer, which adjusted the Ni d‐band center to improve intermediate adsorption for EOR. The heterostructure also showed high alkaline HER activity, only requiring 60.3 mV to drive 10 mA cm^─2 with high stability. An EOR || HER electrolyzer constructed from VO_x/Ni₃N achieved 50 mA cm^─2 at ∼1.5 V, suggesting considerable energy benefits and efficient co‐production of H₂ and valuable chemicals. Jia et al. [67] demonstrated a non‐precious bifunctional electrocatalyst Sn‐Co‐P/NF for efficient alkaline electrolysis, where the HER is coupled with the EOR. The coral‐like Sn‐Co‐P catalyst, electrodeposited on nickel foam, had a porous and rough surface, which favored mass/electron transfer and offered abundant active sites. Using a three‐electrode setup, the HER electrocatalytic activity has been evaluated in 1 M KOH and contrasted with Sn–P/NF, Co–Sn/NF, Co–P/NF, Pt/NF, and bare Ni foam. (Figure 5b). In addition, LSV curves were also observed in 1 M C₂H₆O + 1 M KOH electrolyte to assess catalytic behavior under EOR conditions (Figure 5c). In an alkaline solution, the electrocatalyst showed high HER and OER activity, with small overpotentials of 59 mV (HER) and 304 mV (OER) at 10 mA cm^─2, and lower Tafel slopes, indicating fast reaction kinetics according to the Volmer‐Heyrovsky mechanism for HER. The OER replaced with EOR at the anode reduced the energy requirement, lowering the anodic overpotential by 240 mV. In a two‐electrode cell with Sn‐Co‐P/NF as the anode (EOR) and cathode (HER), the water‐ethanol electrolyzer achieved a current density of 10 mA cm^─2 at a low cell voltage of 1.49 V, which was 130 mV less than that for traditional water electrolysis (Figure 5d). At higher current densities, the voltage difference decreased due to ethanol consumption during the reaction. The electrocatalytic system showed excellent durability, with robust performance for up to 28 h at 10 mA cm^─2 (Figure 5e). Comparable stability was observed in pure 1 M KOH electrolyte, confirming the robustness of the electrocatalyst under both water and water–ethanol electrolysis conditions.

FIGURE 5.

(a) Schematic of VO_x‐Ni₃N synthesis. Reproduced with permission [66]. Copyright 2024, Wiley, (b) HER polarization curves of Sn–Co–P/NF and reference samples in 1 M KOH. (c) LSV curves of catalysts in 1 M C₂H₆O + 1 M KOH electrolyte, (d) Cell voltage comparison of Sn–Co–P/NF || Sn–Co–P/NF electrolyzer in water–ethanol system and alkaline water electrolysis, and (e) Stability test at 10 mA cm^─2 for 28 h in both water–ethanol and 1 M KOH electrolytes. Reproduced with permission [67]. Copyright 2024, Elsevier.

4.3. Benzyl Alcohol Oxidation

Benzyl alcohol oxidation reaction (BAOR) is a particularly attractive anodic reaction for coupling with HER because it enables the selective production of high‐value aromatic aldehydes under mild electrochemical conditions. Unlike small aliphatic alcohols, benzyl alcohol oxidation proceeds predominantly via a two‐electron pathway, converting benzyl alcohol into benzoic acid and benzaldehyde with high selectivity. When coupled with HER, this reaction significantly reduces the overall energy input while simultaneously generating hydrogen and valuable chemical feedstocks, making it highly appealing for integrated chemical energy conversion systems [68, 69, 70].

Yan et al. [71] demonstrated a high‐performance electrochemical hydrogen evolution coupled with organic oxidation (EHCO) system using a field‐induced enrichment strategy to address the issue of slow anodic kinetics. A synergistic nanostructured catalyst of Au nanoparticles supported on CuO nanowires (Au/CuO NWs) was developed to integrate benzyl alcohol oxidation and hydrogen evolution reaction. The nanowire morphology provided enhanced localized electric fields, which greatly promoted the surface enrichment of benzyl alkoxide and OH^─ ions, thus facilitating reaction kinetics. Under an applied potential of 1.5 V vs. RHE, the Au/CuO NWs catalyst achieved 734 mA cm^─2 and efficiently oxidized benzyl alcohol at a rate of 4.74 mmol cm^─2 h^─1, with 91% FE for benzoic acid. At higher applied voltages, current densities were close to 933 mA cm^─2. Multiphysics simulations and experimental findings both confirmed that the local electric field was essential to the activity enhancement. In addition, a membrane‐free flow electrolyzer cell showed stable industrial‐scale performance (>300 mA cm^─2), suggesting the potential and scalability of the EHCO.

Shanker et al. [72] introduced a sophisticated strategy for low‐energy hydrogen production through the combination of acidic benzyl alcohol electrooxidation and hydrogen evolution. The bismuth electrocatalyst was modified with a thin UiO‐66 metal‐organic framework layer, which improved the microenvironment of the catalyst and enabled highly selective benzyl alcohol (BnOH) electrooxidation to benzaldehyde (BnCHO) in 0.1 M H₂SO₄. The MOF layer improved the surface hydroxide ion reactivity and reduced the adsorption of BnCHO, preventing overoxidation to benzoic acid. This strategy improved the current density by ∼73% and reached a FE of 98% at 0.45 V vs. RHE. The combination of anodic BnOH electrooxidation and cathodic hydrogen evolution enabled the production of H₂ at a remarkably low potential of 0.65 V vs. RHE with close to 100% efficiency. Chronoamperometric measurements demonstrated the stability of the catalyst, where the current density returned to its original value upon the addition of fresh BnOH, suggesting that the catalyst was not deactivated. This work demonstrates the effectiveness of microenvironment engineering for selective organic electrooxidation‐assisted hydrogen production in acidic media.

Zhai et al. [73] demonstrated a highly efficient bifunctional electrocatalyst, Mo‐doped biphasic Ni₂P–Ni₁₂P₅ on nickel foam (Mo‐Ni₂P/Ni₁₂P₅@NF), for the coupling of BAOR and HER. Figure 6a shows the development of Mo‐doped biphasic nickel phosphide heterostructure using a simple two‐step procedure that combines hydrothermal and phosphating techniques. Mo doping had a profound effect on the modulation of the electronic configuration of Ni sites, which optimized the d‐band center and improved the adsorption of benzyl alkoxide (Ph‐CH₂O^─) and H* intermediates, thus boosting BAOR and HER activities. This was evidenced by the superior BAOR performance of Mo‐Ni₂P/Ni₁₂P₅@NF compared to Ni₂P/Ni₁₂P₅@NF (Figure 6b). The LSV results reveal that Mo‐Ni₂P/Ni₁₂P₅@NF achieved a current density of 10 mA cm² at an overpotential of 130 mV for HER, outperforming Ni₂P/Ni₁₂P₅@NF (204 mV) and bare NF (281 mV) (Figure 6c). Given its excellent bifunctional activity, a membrane‐free two‐electrode electrolyzer using Mo‐Ni₂P/Ni₁₂P₅@NF was constructed (Figure 6d). In 1 M KOH, the system required cell voltages of 1.526, 1.774, and 1.856 V at 10, 50, and 100 mA cm^─2, respectively. Surprisingly, when BA was added, these values dropped dramatically to 1.38, 1.565, and 1.64 V, respectively (Figure 6e), reaching 10 mA cm^─2 at an extremely small voltage of 1.38 V. Furthermore, the system exhibited excellent catalytic efficiency with BAOR conversion of 96.4%, benzoic acid (yield = 94.1%), and Faradaic efficiency of 96.5%, along with outstanding durability. The improved reaction kinetics and reduced R _ct, highlighting Mo‐Ni₂P/Ni₁₂P₅@NF as a promising candidate for energy‐efficient H₂ production coupled with valuable chemical synthesis. A highly efficient electrocatalyst, NiMoO‐Ar/NF nanorod array, was designed by Hu et al. [68] for the electrochemical BAOR to produce benzoic acid with the HER as a coupled reaction. The electrocatalyst was synthesized by the annealing of a hydrated Ni‐Mo oxide precursor, which removed crystal water and introduced highly accessible active sites, as well as a porous structure to enhance surface area and mass transport. Electrochemical tests in 1 M KOH with 15 mM BA showed that NiMoO‐Ar/NF had a low overpotential of 1.39 V at 10 mA cm^─2 vs. RHE, outperforming NiMoO/NF and calcined NF. The Tafel plot had a small slope of 42 mV dec^─1, and EIS data indicated a small R _ct of 1.45 Ω, indicating fast kinetics. Chronoamperometric measurements with gas chromatographic analysis indicated a near‐complete conversion of BA to benzoic acid in 2 h with 94.27% BA conversion, 100% selectivity, and 91.25% FE. Large‐cycle and electrolyte‐refresh tests proved the high stability of NiMoO‐Ar/NF. NiMoO‐Ar/NF is a stable and effective electrocatalyst for sustainable hydrogen production and green chemical synthesis.

FIGURE 6.

(a) Schematic of Mo‐Ni₂P/Ni₁₂P₅@NF synthesis via hydrothermal and phosphating steps, (b) BAOR polarization curve, (c) HER polarization curves showing overpotentials of Mo‐Ni₂P/Ni₁₂P₅@NF, Ni₂P/Ni₁₂P₅@NF, and NF, (d) Membrane‐free two‐electrode configuration using Mo‐Ni₂P/Ni₁₂P₅@NF as bifunctional electrodes, and (e) Cell voltages in 1 M KOH with and without BA at different current densities. Reproduced with permission [73]. Copyright 2024, Elsevier.

4.4. Ethylene Glycol Oxidation Reaction

Ethylene glycol oxidation reaction (EGOR) is an appealing anodic process for HER coupling due to its high hydrogen content and relatively simple molecular structure compared to polyols such as glycerol. EGOR proceeds through multi‐electron transfer pathways, yielding value‐added products such as glyoxal, glycolic acid, and oxalic acid, depending on catalyst composition and reaction conditions. When used as the anodic reaction, EGOR effectively replaces the OER, enabling substantial reductions in overall cell voltage while simultaneously producing hydrogen and high‐value oxygenated chemicals [74, 75, 76].

Guo et al. [77] designed a super‐efficient bifunctional electrocatalyst, N–PdRh@Ni(OH)₂ nanosheets (NSs), for both EGOR and HER. The N‐coordinated PdRh bimetallic sites allow for accurate electronic control, which optimizes the adsorption energies of important intermediates and improves glycolic acid (GA) selectivity. The negatively charged Pd sites favored ethylene glycol adsorption and facilitate *OC–CH₂OH desorption, while the positively charged Rh sites favored OH⁻ adsorption and facilitate *OH radical formation, thus accelerating EGOR. Moreover, the Pd‐Rh synergistic effect improved the hydrolytic dissociation of *H atoms, thus improving HER activity. In a membrane‐free flow cell, the system reached 100 mA cm^─2 at a low voltage of 0.85 V, with an energy consumption of 2.52 kWh m^─3 and GA selectivity above 97.6%. Electrochemical analysis showed low Tafel slope of 85.92 mV dec^─1, high ECSA of 66.22 mF cm^─2, and low R _ct of 5.91 Ω, suggesting fast reaction rates and excellent hydrogen adsorption ability. Long‐term chronoamperometry tests showed high stability with 96.62% current retention after 12 h, suggesting that N‐PdRh is a potential catalyst for low‐energy hydrogen formation and biomass transformation. A self‐supported Fe₃S₄/Ni₃S₂ (NiFeS) heterojunction electrocatalyst for energy‐saving hydrogen evolution and formate production was demonstrated by Ren et al. [78] involving EGOR. The one‐step solvothermal route was used to synthesize NiFeS nanosheets with the high accessible electrocatalytic active surface area. The electrocatalyst could reach 100 mA cm^─2 at a low overpotential of 1.37 V vs. RHE in 1 M KOH + 1 M EG solution. Raman spectroscopy results revealed that metal sulfides were the active sites for EGOR, while NiOOH and FeOOH, which were generated from surface reconstruction, acted as catalysts for OER. The electrocatalyst showed a high FE of 92.4% and formate production rate of 0.652 mmol cm^─2 h^─1 at 1.50 V, with stability of 120 h. Successful energy‐saving H2 evolution was demonstrated when EGOR and HER were combined in a two‐electrode cell, resulting in a cell voltage of 1.55 V at 100 mA cm^─2, which was 140 mV lower than that of conventional water splitting. NMR spectroscopy results revealed that formate was the main product, with traces of methanol and glycolate, indicating that NiFeS is a highly efficient heterojunction electrocatalyst for sustainable H₂ and value‐added chemical synthesis.

Wang et al. [79] studied the role of Mn doping in Ni(OH)₂ catalysts to improve the electrocatalytic EGOR activity and formate selectivity. Mn doping caused distortion in the NiO₆ octahedral structure, resulting in broadening of the e_g* band and an increase in the Ni 3d band center approaching the Fermi level, which favored the adsorption of glyoxal intermediates and inhibited the formation of glycolate. XANES and FT‐EXAFS results revealed that the Ni valence was unaffected, but Mn‐doped catalysts showed higher structural distortion and oxygen vacancy concentration, ascertained by EPR and O 1s XPS spectra. Raman spectroscopy results showed that NiOOH with Mn doping had higher disorder, further ascertaining lattice distortion. The above results showed that the modulation of electronic states and lattice distortion in catalysts was a crucial approach to optimize polyol electro‐oxidation efficiency and selectively produce formate. Among the various compositions, Ni_0.9Mn_0.1 LDH showed the highest structural distortion, oxygen vacancy concentration, and e_g* band broadening, which were consistent with improved EGOR activity. Yuan et al. [80] offered a sustainable approach to upcycling polyethylene terephthalate (PET) waste into an ultrathin bimetallic 2D metal‐organic framework (2D‐CoNi‐PET) electrocatalyst for the EGOR to simultaneously remediate plastic waste and produce valuable chemicals. The terephthalic acid from PET waste was used as the organic ligand, and the incorporation of Ni into Co‐based MOFs was crucial for modulating the electronic structure and catalytic properties. Ni helped to create high‐valence Co³⁺ ions, improved deprotonation ability, and boosted the interaction with oxygenated intermediates, which together lowered the initial potential of EGOR and improved reaction kinetics. Electrochemical tests, such as CV, pulse voltammetry, operando EIS, and in situ Raman spectroscopy, demonstrated that Co³⁺ ions were the dominant active sites, while Ni facilitated rapid EG adsorption and spontaneous chemical oxidation reactions. In comparison to 3D‐CoNi‐BDC, the 2D nanosheet structure provided a significantly larger surface area, higher density of unsaturated metal nodes, faster charge transfer rates, and better EG and intermediate adsorption capabilities. Consequently, 2D‐CoNi‐PET exhibited a glyoxal‐mediated EGOR pathway that preferred efficient C–C bond cleavage to formic acid, suppressing glycolate and carbonate by‐products. The electrocatalyst exhibited a low operating potential of 1.41 V (vs. RHE) at 100 mA cm^─2, high Faradaic efficiency of ∼91%, high FA selectivity, and long‐term stability for 72 h. This study showcases the effectiveness of combined geometry and electronic modulation strategies for highly selective EGOR and provides a promising pathway for PET waste conversion.

4.5. Glycerol Oxidation Reaction

Glycerol Oxidation Reaction (GOR) is a highly effective anodic process for coupling with HER due to glycerol's abundance, low cost, and high functionality as a biodiesel‐derived byproduct. Unlike simple alcohols, glycerol oxidation involves multiple hydroxyl groups and complex multi‐electron transfer pathways, enabling the formation of diverse valuable products such as formate, glyceraldehyde, glyoxylic acid, glyceric acid, and tartronic acid [15, 81]. When integrated with HER, GOR significantly reduces the overall cell voltage while enabling simultaneous hydrogen generation and electrochemical upgrading of biomass‐derived feedstocks [16, 82].

Jiang et al. [83] designed a bifunctional self‐standing NC@NiFeN_x/CF electrocatalyst, which consisted of nitrogen‐doped carbon‐coated NiFeN_x nanosheets supported on carbon fibers. In the GOR as the anodic reaction, NC@NiFeN_x/CF had a low onset potential of 1.37 V at 10 mA cm^─2 vs. RHE, a lower Tafel slope (62 mV dec^─1), and excellent charge‐transfer properties with a low R _ct of 3.22 Ω, suggesting fast interfacial electron transfer. The high electrochemical active surface area further facilitated glycerol‐to‐formate oxidation, achieving a Faradaic efficiency of ∼95%. For the HER, the catalyst showed 10 mA cm^─2 at an overpotential of 195 mV with favorable kinetics and stability. As a complete NC@NiFeN_x/CF || NC@NiFeN_x/CF electrolyzer, the cell voltage was only 1.38 V at 10 mA cm^─2, efficiently producing H₂ and value‐added formate. Lu et al. [84] employed Co‐ZIF/CF as a precursor to synthesize CoRuO/CF nanosheet arrays as the anode material for GOR, while CoRuCN/CF nanosheets were used as the efficient cathode for the HER. Figure 7a shows the synthesis procedure for the CoRu nanosheet array. CoRuO/CF showed excellent GOR performance, with 10 mA cm^─2 at 1.14 V and a FE of ∼100% for formate production, with faster kinetics than OER. Controlled experiments with and without glycerol showed significant differences in activity (Figure 7b), while ¹H NMR analysis confirmed glycerol consumption and the production of formic acid as the dominant product, in addition to trace methanol (Figure 7c). The reaction mechanism over CoRuO/CF in alkaline media is shown in Figure 7d. Notably, LSV curves showed that glycerol had very little effect on HER activity (Figure 7e). Electrochemical impedance spectroscopy analysis revealed an ultralow R _ct of 2.09 Ω, indicating fast electron transfer. For HER, CoRuCN/CF had a low overpotential of 64 mV at 10 mA cm^─2 with favorable kinetics and stability. When combined into a two‐electrode CoRuO/CF || CoRuCN/CF system (Figure 7f), a low cell voltage of 1.19 V at 10 mA cm^─2 was achieved in 1 M KOH with 0.5 M glycerol, outperforming conventional OER‐based electrolysis. The system also demonstrated great longevity, operating steadily for 100 h at 100 mA cm^─2 (Figure 7g), indicating its potential for efficient glycerol‐assisted hydrogen production.

FIGURE 7.

(a) Schematic of CoRu nanosheet array synthesis, (b) Polarization curves with and without glycerol, (c) ¹H NMR analysis showing glycerol consumption and formate formation. (d) Proposed GOR mechanism on CoRuO/CF, (e) HER Polarization curves with/without glycerol, (f) Two‐electrode CoRuO/CF || CoRuCN/CF performance, and (g) Long‐term stability test over 100 h. Reproduced with permission [84]. Copyright 2024, Elsevier.

Deng et al. [85] demonstrated a glycerol‐assisted alkaline water electrolysis system using CoMoP ternary nanocomposite decorated on 304 stainless steel mesh, targeting reduced energy consumption for concurrent hydrogen and value‐added formate production. The CoMo_0.10P catalyst showed outstanding GOR performance, driving the current to 10 mA cm^─2 at a mere 1.325 V, which is much lower than that of the traditional OER. The enhanced GOR kinetics were demonstrated by the decreased Tafel slope and much smaller R _ct, suggesting accelerated interfacial electron transfer. For the HER, CoMo_0.10P required an overpotential of 192 mV at 10 mA cm^─2 with desirable kinetics and stability in alkaline solution. When integrated into a two‐electrode CoMo_0.10P || CoMo_0.10P electrolyzer cell, the device enabled hydrogen evolution at a low cell potential of 1.60 V at 10 mA cm^─2, with a formate Faradaic efficiency of ∼60%. DFT calculations demonstrated that the Co, Mo, and P components cooperated to promote HER and GOR activities.

4.6. Other Alcohol Oxidation Reactions

The oxidation of 1,3‐propanediol (1,3‐PD) represents an emerging anodic reaction for coupling with HER, offering opportunities for the selective synthesis of valuable chemicals such as 3‐hydroxypropionaldehyde and corresponding acids. As a biomass‐derived diol, 1,3‐propanediol exhibits favorable oxidation kinetics and high hydrogen content, making it a promising alternative to conventional anodic reactions. Coupling 1,3‐propanediol oxidation with HER enables low‐voltage hydrogen production while facilitating the electrochemical upgrading of renewable feedstocks.

Zhang et al. [86] reported an efficient way to substitute the slow oxygen evolution reaction with (glycerol, isopropanol, propanol, 1,3‐propanediol) propanol oxidation reaction (PrOR) to decrease energy consumption and facilitate the green production of value‐added chemicals. A self‐supported N‐CoFeP/NF electrocatalyst was prepared on nickel foam through hydrothermal treatment, accompanied by nitriding and phosphating, to produce highly crystalline nanorod arrays (Figure 8a). Electrochemical measurements revealed that N‐CoFeP/NF electrocatalysts possessed superior glycerol, isopropanol, propanol, and 1,3‐propanediol performance, with a low operating voltage of 1.38–1.42 V (vs. RHE) at 100 mA cm^─2 (Figure 8b,c), which is 80–120 mV lower than that for OER. The electrocatalyst also showed good HER activity with a small overpotential and desirable Tafel slope, allowing for dual‐functional PrOR/OER performance. Electrochemical impedance spectroscopy and C _dl measurements validated enhanced charge transfer rates, reduced R _ct values, and increased electrochemically active surface areas by nitrogen‐induced electronic modulation. Product analysis showed that glycerol oxidation achieved a high FE of ∼74% toward formic acid (Figure 8d). Additionally, other alcohols produced valuable products such as acetic acid, propionic acid, and 3‐hydroxypropionic acid, as confirmed by ¹H NMR, with corresponding Faradaic efficiencies summarized in Figure 8e. Secondary hydroxy alcohols groups favored C─C bond cleavage, while primary alcohols underwent selective oxidation. The N‐CoFeP/NF electrocatalyst supported stable PrOR/OER performance at high current densities of up to 400 mA cm^─2 for over 100 h, which indicated great potential for energy‐efficient AOR‐coupled hydrogen production.

FIGURE 8.

(a) Schematic of N‐CoFeP/NF synthesis and alcohol‐assisted electrolysis, (b) Polarization curves of different alcohol oxidation reactions, (c) Comparison of AOR and OER potentials at 100 mA cm^─2, (d) ¹ H NMR measurement of glycerol oxidation to formic acid, and (e) Faradaic efficiencies of various alcohols. Reproduced with permission [86]. Copyright 2024, Elsevier.

Mahmoudian et al. [87] described the electrocatalytic joint production of acrylate and hydrogen through the reforming of 1,3‐propanediol with a Pd/C‐CeO₂ electrocatalyst. By optimizing cell potential, temperature, and flow conditions, high selectivity for acrylate production was obtained. At 80°C and 400 mV with single‐pass flow conditions, maximum acrylate selectivity of 77% was reached, concurrently producing hydrogen. Zhao et al. [88] showed atomically thin Rh nanosheets to be a dual‐function electrocatalyst with high HER activity and high selectivity for isopropanol oxidation in alkaline solution. At a very low cell voltage of 0.4 V, hydrogen evolution was accomplished by substituting the thermodynamically more advantageous isopropanol oxidation process for OER, which preferentially produced valuable acetone at the anode. This study is significant in demonstrating organic oxidation‐assisted water electrolysis as an effective approach for energy‐saving H₂.

Kitano et al. [89] developed a bifunctional NiO@Rh‐M electrocatalyst consisting of ultrathin Rh metallene nanosheets embedded with low‐crystalline NiO nanodomains, forming abundant Rh–NiO heterointerfaces. The synergistic interaction between Rh sites and NiO domains enhanced both alkaline HER and isopropanol oxidation reaction (IOR) activities by promoting hydrogen adsorption, water dissociation, and OH^─ enrichment. The catalyst exhibited a low HER overpotential of 39 mV at 10 mA cm^─2 and initiated IOR at 0.08 V vs. RHE. In the coupled IOR–HER system, NiO@Rh‐M achieved stable electrolysis at a remarkably low cell voltage of 0.6 V for 10 h. These results demonstrate the effectiveness of metallene‐based heterointerface engineering for energy‐efficient hydrogen production and electrochemical alcohol oxidation reactions.

Abramczyk et al. [90] investigated the influence of isopropanol (IPA) on HER kinetics over Ni foam and Pd‐modified Ni foam electrodes in an alkaline medium. The addition of low concentrations of IPA significantly enhanced HER performance, which was attributed to the partial electrooxidation of IPA into acetone that facilitated hydrogen bubble detachment from the electrode surface. Importantly, IPA showed minimal electrosorption on nickel‐based catalysts, thereby avoiding active‐site blocking during HER. These findings demonstrate the potential role of simple alcohol molecules in improving the efficiency of alkaline hydrogen evolution systems. Yang et al. [91] developed an electrocatalytic membrane structured reactor (EMSR) using MnO₂ immobilized within Ti membrane pores (MnO₂@Ti EMSR) for n‐propanol oxidation coupled with hydrogen evolution. The MnO₂@Ti EMSR exhibited enhanced electron transfer and larger electrochemically active surface area, leading to a 44% increase in n‐propanol conversion and 33% higher propionic acid selectivity under flow‐through operation. In addition, the Faradaic efficiency increased by 59% due to improved contact and mass transfer within the membrane structure. Compared with conventional water electrolysis, the coupled system significantly reduced the cell voltage and energy consumption, demonstrating its potential for efficient co‐production of hydrogen and value‐added chemicals.

Overall, coupling HER with alcohol oxidation reactions offers a compelling pathway to reduce energy consumption in water electrolysis while simultaneously producing value‐added chemicals. A wide range of alcohol oxidation reactions involving methanol, ethanol, benzyl alcohol, glycerol, and ethylene glycol are summarized in Table 1. The energy advantage position HER–AOR coupled systems as a promising platform for next‐generation, low‐voltage hydrogen production integrated with sustainable chemical synthesis.

TABLE 1.

Summary of electrocatalysts for HER–AOR systems, including electrolyte, products, Faradaic efficiency, and operating potential.

Catalyst	Electrolyte	Cathodic Product	Anodic Product	FE (%) (Anode / H2)	Stability (h)	Potential (V) (η1 0, otherwise mentioned in the brackets)	Refs.
Mn‐doped NiFe‐LDH	1 M KOH + 0.5 M CH3OH	H2	Formate	99.5	120	1.5	[106]
NiCo2O4	1 M KOH + 1 M CH3OH	H2	Formate	∼100	100	1.274	[107]
Co30Ni60/CC	1 M KOH + 1 M CH3OH	H2	Formate	98.6	100	1.323	[61]
Pt‐Ni3S2	1 M KOH + 1 M CH3OH	H2	Formate	98	72	1.45	[63]
Ni‐MoN/NF	1 M KOH + 0.5 M CH3OH	H2	Formate	95	100	1.48	[64]
Pd3Pb@Pt	1 M KOH + 1 M C2H5OH	H2	Acetic acid	98	20	0.75	[108]
NiOOH‐CuO	1 M KOH + 1 M C2H5OH	H2	Acetate	79.1	70	1.34 (η 200)	[109]
AuCu	1 M KOH + 1 M C2H5OH	H2	Acetate	92	300 min	1.5 (η 1000)	[110]
Ni0.67Co0.33Ox	1 M KOH + 1 M C2H5OH	H2	Acetate	85.74	15	1.6 (η 125)	[14]
Sn‐Co‐P/NF	1 M C2H5OH + 1 M KOH	H2	Ethyl acetate	98	28	1.44	[67]
NiCo2O4/NF	1 M NaOH + 0.1 M Glycerol	H2	Formate	97	120	1.23	[111]
PtSA‐NiCo LDH/NF	1 M KOH + 0.1 M Glycerol	H2	Formate	88.7	24	1.37 (η 100)	[112]
Au/Ni (OH)2	3 M KOH + 1 M Glycerol	H2	Lactic acid	47	10	0.95 (η 317)	[113]
Ag‐Co3O4/ NF	1 M KOH + 0.1 M Glycerol	H2	Formic acid	96.79	80	1.35 (η 100)	[114]
Pd‐CoFe	1 M KOH + 0.5 M Glycerol	H2	Glyceric acid	60	200	0.64 (η 100)	[115]
NC@NiFeNx/CF	1 M KOH + 0.1 M Glycerol	H2	Formate	95	16	1.37	[83]
CoRuO/CF	1 M KOH + 0.5 M Glycerol	H2	Formate	100	83	1.14	[84]
CoMo0.10P	1 M KOH + 0.1 M Glycerol	H2	Formate	60	24	1.325	[85]
Au/CuO NWs	1 M KOH + 0.1 M Benzyl alcohol	H2	Benzoic acid	91	3600 s	1.5 (η 734)	[71]
Au/CoOOH	1 M KOH + 0.1 M Benzyl alcohol	H2	Benzoic acid	98	24	1.3	[116]
Mo‐Ni2P/Ni12P5@NF	1 M KOH + 0.1 M Benzyl alcohol	H2	Benzoic acid	96.5	24	1.38	[73]
NiMoO‐Ar/NF	1 M KOH + 15 mM Benzyl alcohol	H2	Benzoic acid	91.25	8	1.39	[68]
NiCo2O4/NF	1 M KOH + 1 M EG	H2	Formate	94.89	120	1.35	[117]
NiCu/NF	1 M KOH + 0.3 M EG	H2	Formate	95.8	24	1.52	[118]
PdFe/N‐CNTs	1 M KOH + 0.3 M EG	H2	Formate	87	40	1.35	[119]
Pt1/Ni (OH)2	1 M KOH + 0.1 M EG	H2	Formate	90	20	1.44 (η 1000)	[120]
Pt−Ni (OH)2/NF	1 M KOH + 0.1 M EG	H2	Formate	93	150	0.69 (η 100)	[121]
N–PdRh@Ni(OH)2	1 M KOH + 0.1 M EG	H2	Glycolic acid	95.14	12	0.9 (η 1093)	[77]
Fe3S4/Ni3S2	1.0 M KOH + 1.0 M EG	H2	Formate	92.4	120	1.37 (η 100)	[78]
MnNi(OH)2	1.0 M KOH + 1.0 M EG	H2	Formate	85.8	600 s	1.37	[79]
2D‐CoNi‐PET	1.0 M KOH + 0.3 M EG	H2	Formic acid	91	72	1.28	[80]

5. In Situ Systems for HER Coupled with AOR

Understanding the dynamic evolution of active sites and reaction intermediates during HER coupled with AOR is crucial for the rational design of high efficiency electrocatalysts. Conventional ex situ characterization techniques often fail to capture transient intermediates, catalyst reconstruction, and interfacial changes occurring under realistic operating conditions. Therefore, in situ and operando spectroscopic techniques have emerged as indispensable tools for elucidating reaction mechanisms, identifying active catalytic sites, and monitoring structural and electronic evolution during electrocatalytic reactions. Theethagiri et al. [92] highlighted that in situ spectroscopic methods provide real‐time insight into electrode–electrolyte interactions, intermediate adsorption, catalyst evolution, and reaction kinetics in electrochemical energy conversion systems. Among these techniques, in situ FTIR spectroscopy is widely used to monitor reaction intermediates and product evolution during alcohol oxidation reactions, while in situ Raman spectroscopy is particularly effective for tracking catalyst reconstruction, bond evolution, and phase transformations under different applied potentials. In addition, in situ infrared spectroscopies such as in situ infrared absorption spectroscopy (IRAS) and attenuated total reflection surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) provide enhanced surface sensitivity for identifying adsorbed intermediates and interfacial reaction pathways during HER–AOR processes. Therefore, combining multiple operando spectroscopic techniques is essential for establishing structure–activity relationships and understanding catalytic mechanisms in HER‐coupled alcohol oxidation systems.

5.1. In Situ Infrared Spectroscopies

5.1.1. In Situ Fourier Transform Infrared (FTIR) Spectroscopy

In situ FTIR spectroscopy is extensively used to detect reaction intermediates and product evolution during alcohol electrooxidation. This technique enables real‐time identification of key functional groups, such as C─O, C═O, and O─H species, providing direct mechanistic insights into alcohol oxidation pathways. In HER–AOR coupled systems, FTIR analysis helps distinguish selective oxidation routes leading to aldehydes, acids, or carbonate species, while simultaneously confirming the suppression of poisoning intermediates. The ability to correlate intermediate evolution with applied potential makes in situ FTIR particularly valuable for catalyst screening and optimization.

Wang et al. [93] used in situ FTIR spectroscopy to investigate the GOR mechanism on Ni‐MnO₂ catalysts. In situ FTIR experiments were carried out to clarify the reaction mechanism and monitor intermediate evolution (Figure 9a). When the applied potential was ≥1.30 V vs. RHE, bands at ∼1351 and 1383 cm^─1, attributed to O─C─O symmetric stretching of carboxylates, and a band at ∼1586 cm^─1, attributed to C═O stretching of aldehyde groups, appeared. The increase in the intensity of these bands with increasing potential indicated a continuous production of formate and formaldehyde. From these FTIR observations, a novel *OH‐assisted reaction mechanism including the successive cleavage of C─C bonds via glycolaldehyde and formaldehyde intermediates was deduced, producing formate as the major product. The in situ FTIR observations underscored the importance of the dual‐site Ni─O─Mn structure in coordinating *OH and glycerol adsorption, facilitating efficient intermediate production and high formate yield.

FIGURE 9.

(a) In situ FTIR spectra of Ni–MnO₂ during GOR, showing potential‐dependent formation of carboxylate and aldehyde intermediates. Reproduced with permission [93]. Copyright 2026, Elsevier, (b) In situ FTIR spectra of PdNi/NF showing glycolate formation and intermediates during EG oxidation. Reproduced with permission [94]. Copyright 2025, Elsevier. (c) In situ electrochemical IRAS spectra of Ni(OH)₂–Ni₃S₂/NF during EG oxidation. Reproduced with permission [96]. Copyright 2025, Springer, (d) In situ electrochemical IRAS spectra of NiFe‐based electrode during EOR at 0.9–1.7 V vs. RHE. Reproduced with permission [97]. Copyright 2024, Springer, (e) Proposed EGOR pathway via CoOOH involving GA intermediate formation and C─C bond cleavage, and (f) In situ ATR‐SEIRAS spectra showing evolution of aldehyde and formate intermediates. Reproduced with permission [98]. Copyright 2026, Elsevier.

Similarly, the EGOR on PdNi/NF electrodes was probed by in situ FTIR spectroscopy (Figure 9b) by Chang et al. [94] As the potential was raised above 0.4 V, peaks at ∼1340 and 1580 cm^─1, attributed to the symmetric and asymmetric –COO^─ stretching of glycolate, confirmed glycolate as the major product. Additional peaks at ∼1258 cm^─1 (C─O stretching of glycolate), ∼1700 cm^─1 (C═O stretching of 2‐hydroxyacetyl), and ∼1077 cm^─1 (–CHO stretching) indicated aldehyde‐type intermediates. The appearance of these peaks indicated an oxidative dehydrogenation mechanism, where ethylene glycol was first oxidized to a 2‐hydroxyacetyl intermediate, followed by •OH‐assisted oxidation to glycolate, consistent with the high C₂ selectivity of PdNi/NF. In situ electrochemical FTIR spectroscopy was further employed by Zhou et al. [95] to study the EOR on amorphous CrO_x‐protected Pd nanowires. As the potential was raised from 0 to 1.2 V vs. RHE, peaks formation at ∼1030 and 1140–1160 cm^─1, corresponding to CH₃CH₂OH vibrations, indicated continuous ethanol oxidation. Simultaneously, peaks at ∼1700 cm^─1 (C═O stretching of CH₃CHO/CH₃COOH) and acetate‐related peaks at ∼1560, 1420, and 1280 cm^─1 confirmed selective C₂ oxidation to acetate. The lack of C₁ product‐related peaks (CO₂, HCO₃ ^─, CO₃ ^2─) and linearly bonded CO, despite a weak bridge‐bonded CO peak at ∼1866 cm^─1, indicated efficient CO oxidation and high CO tolerance. These in situ FTIR analyses offer clear mechanistic information on selective C₂ oxidation reactions facilitated by rational catalyst design.

5.1.2. In Situ Infrared Absorption Spectroscopy

In situ infrared absorption spectroscopy (IRAS) serves as a powerful technique to probe the dynamic evolution of reaction intermediates and elucidate the mechanisms of electrocatalytic alcohol and polyol oxidation reactions. By monitoring potential‐dependent changes in vibrational bands, IRAS enables the direct identification of surface‐adsorbed species, reaction intermediates, and product formation, providing insight into pathway selectivity, rate‐limiting steps, and the influence of electronic or structural modifications in the catalyst. For example, IRAS can distinguish symmetric and asymmetric O─C─O stretching vibrations in carboxylate species, aldehyde C═O stretches, or other functional groups, offering a molecular‐level understanding of the oxidation process under operating electrochemical potentials.

Sun et al. [96] employed IRAS to analyze PET‐derived EGOR on a heterojunction Ni(OH)₂‐Ni₃S₂/NF catalyst (Figure 9c). The IRAS results showed that there were symmetric O─C─O stretches at 1350 and 1381 cm^─1 for GA and formate, and an asymmetric stretch at 1578 cm^─1, while a weak peak at 1063 cm^─1 suggested aldehyde groups in glycolaldehyde and glyoxal. A weak peak at 1305 cm^─1 suggested a small amount of oxalate. Mechanistic studies using GA and glyoxal confirmed that EG first dehydrogenated to glycolaldehyde, which selectively converted to GA and then to formate through C─C bond cleavage. The increasing intensity of formate peaks with the applied potential indicated that the built‐in electric field at the Ni(OH)₂‐Ni₃S₂ interface facilitated EG and *OH adsorption, thus accelerating the reaction kinetics and promoting selective formate production. At 616 mA cm^─2 and 1.50 V vs. RHE, a FE of 89% for formate was obtained, which highlighted the importance of electronic modulation and optimized d‐band alignment in high‐performance EGOR. Li et al. [97] employed IRAS to analyze ethanol oxidation on Ni‐Fe bimetallic alloy nanoparticles (Figure 9d). Asymmetric O─C─O stretches at 1552 cm^─1 and symmetric stretches at 1350–1405 cm^─1 indicated CH₃COO^─ production, with no evidence for C₁ products or adsorbed CO. Acetate production started at 1.35 V, which corresponded to the formation of NiOOH, and the current increased sharply at ∼1.40 V. DFT calculations showed that Fe addition improved the Ni electronic structure, facilitated electron transfer, and reduced the energy barrier, which enabled selective and stable ethanol‐to‐acetate conversion. These studies clearly showed that in situ IRAS can directly detect intermediate formation, selectivity, and the role of electronic modulation in optimizing alcohol and polyol electrooxidation.

5.1.3. In Situ Attenuated Total Reflection

In situ attenuated total reflection surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) has emerged as a powerful operando technique to directly identify surface‐bound intermediates and soluble products during electrooxidation reactions, enabling unambiguous discrimination of reaction pathways that are often inaccessible by ex situ methods. By tracking potential‐dependent vibrational fingerprints of adsorbed species at the electrified interface, ATR‐SEIRAS especially when combined with complementary Raman spectroscopy and NMR provides molecular‐level insight into dehydrogenation steps, C─C bond cleavage events, and product selectivity, which is essential for rational catalyst design in alcohol oxidation reactions.

Bai et al. [98] discussed the mechanism of EGOR on Fe(0.3 wt.%)‐CoPO/NF catalysts. In situ ATR‐SEIRAS (Figure 9e,f) analysis showed a weak peak at 1076 cm^─1, ascribed to aldehyde C═O stretching of glycolaldehyde or glyoxal, which confirmed the first step of EG dehydrogenation. Strong peaks at 1527 cm^─1 (ν _s(COO^─)) and 1271 cm^─1 (C─O stretching) indicated the presence of formate and GA, respectively, identifying GA as an important intermediate. By integrating the ATR‐SEIRAS data with Raman and ¹H NMR spectra, the authors proposed a GA‐mediated mechanism, where EG adsorbed on electrochemically reconstructed CoOOH sites, proceeded with stepwise dehydrogenation to CH₂OH–CHO, oxidized to GA, and finally split C─C bond to produce formate as the major product. Notably, Fe addition prevented Co³⁺ to inactive Co⁴⁺ through overoxidation, and maintained active CoOOH species, thus improving the efficiency of C─C bond splitting and achieving 100% Faradaic efficiency with lower energy input in EGOR || HER couple. Li et al. [99] employed in situ ATR‐SEIRAS, IRAS, and NMR to decipher the MOR mechanism on branched Ni₃C electrocatalysts. ATR‐SEIRAS spectra taken at potentials above 0.2 V vs. Ag/AgCl showed peaks at ∼2924, 2906, and 2853 cm^─1 and bands at 1530, 1450, and 1407 cm^─1, which were attributed to surface‐adsorbed CH₃O* and CH₃OH molecules, respectively, signifying active methanol adsorption and dehydrogenation. The absence of features in the 1000–1200 cm^─1 range indicated continuous methanol oxidation. Importantly, no features associated with adsorbed CO (1700‐2000 cm^─1) or CO₂ (2341 cm^─1) were detected. IRAS analysis of the electrolyte solution revealed characteristic bands at 1558, 1377, and 1342 cm^─1, which were attributed solely to formate, and whose intensity changes were directly correlated to the onset of the MOR current. Follow‐up ¹³C NMR spectroscopy analysis unequivocally identified formate as the sole liquid product. Collectively, these operando spectroscopic findings provided irrefutable evidence for the highly selective, CO‐free methanol‐to‐formate reaction on Ni₃C, underscoring the pivotal importance of in situ ATR‐SEIRAS.

5.2. In Situ Raman Spectroscopy

In situ Raman spectroscopy offers a powerful approach to monitor surface‐bound intermediates and vibrational fingerprints during alcohol oxidation reactions. This technique is particularly effective for identifying adsorbed species such as *CO, *CHO, *COOH, and alkoxide intermediates formed during methanol, ethanol, and glycerol oxidations. When applied to HER–AOR systems, in situ Raman spectroscopy reveals potential‐dependent adsorption behavior and provides direct evidence of synergistic interactions at dual‐metal sites. Additionally, changes in Raman band intensity and position under applied bias offer valuable information on catalyst reconstruction and surface oxidation states.

Zhang et al. [100] discussed the mechanism of EGOR on CoFe‐LDH/NF through in situ electrochemical Raman spectroscopy (Figure 10a–d). At anodic potentials above 1.30 V vs. RHE, Raman peaks at ∼458 and 569 cm^─1 attributed to the E_g and A_1g modes of CoOOH appeared, clearly indicating the formation of high‐valent Co active sites. Simultaneously, the decrease of ethylene glycol peaks (867–1090 cm^─1) showed continuous EG consumption. At higher potentials (>1.70 V vs. RHE), prominent peaks at 1550 cm^─1 assigned to the symmetric O─C─O stretching of formate, along with weaker glycolate peaks, indicated a potential‐dependent pathway with dominant C─C bond cleavage. These results clearly confirmed that CoOOH sites controled selective EG oxidation to formate. Similarly, Yuan et al. [80] carried out in situ Raman spectroscopy to monitor the evolution of active species during EGOR on PET‐based 2D‐CoNi‐PET MOFs. Under OER conditions, M³⁺OOH peaks appeared, whereas under EGOR conditions, these species were quickly consumed, leaving only M²⁺ lattice peaks. Ni doping facilitated EG adsorption and deprotonation, favoring fast interfacial EGOR kinetics and selective formic acid production. In a complementary study, Xu et al. [101] showed the bifunctional electrocatalytic performance of NiCo₂O₄/NF for benzyl alcohol oxidation and HER, illustrating the strategic role of catalyst design in achieving simultaneous organic oxidation and hydrogen evolution with high selectivity and efficiency.

FIGURE 10.

In situ Raman spectra and 2D contour plots of EGOR on NF‐10: (c and e) 300–800 cm^─1, and (d and f) 300–1600 cm^─1 at different potentials. Reproduced with permission [100]. Copyright 2025, Elsevier.

Several recent studies and reviews have emphasized the growing importance of in situ and operando characterization techniques for understanding electrocatalytic reactions under realistic working conditions. Xu et al. [102] summarized advances in in situ/operando Raman and FT‐IR spectroscopy for understanding catalytic mechanisms and structure–activity relationships in electrocatalytic systems. Reuss et al. [103] further highlighted the application of in situ resonant inelastic X‐ray techniques for tracking electrocatalyst coordination environments and adsorbate interactions during electrochemical reactions. Ismail et al. [104] reviewed several in situ analytical techniques, including XAS, XPS, Raman scattering, and SR‐FTIR, for identifying active sites and intermediates in acidic OER systems. Haryanto et al. [105] discussed the use of in situ FT‐IR, Raman, and XAS techniques to monitor catalyst phases, reaction intermediates, metal valence states, and coordination environments during HER. In addition, Theethagiri et al. [92] comprehensively discussed the role of in situ spectroscopy techniques in elucidating the mechanistic pathways of electrochemical energy conversion reactions, particularly in understanding catalyst evolution, intermediate adsorption, and reaction kinetics under various operating conditions.

In situ and operando analyses have greatly improved the understanding of HER‐AOR coupled systems by revealing the dynamic behavior of electrocatalysts under working conditions, which cannot be fully captured through ex situ characterization. Future research integrating advanced in situ techniques with theoretical calculations and data‐driven approaches will provide deeper insight into the relationship between electronic structure, surface chemistry, and reaction kinetics, thereby enabling the rational design of next‐generation electrocatalysts for sustainable hydrogen production and value‐added chemical synthesis.

6. Summary and Future Prospects

This review has emphasized recent developments in electrocatalysts for HER in combination with AOR. By overcoming the OER with a more thermodynamically advantageous AOR has been acknowledged as a successful and efficient alternative to traditional water electrolysis. This approach not only lowers the cell voltage and energy requirements but also allows for the concurrent production of valuable chemicals such as formate, acetate, and other oxygenates. Significant progress has been achieved through rational electrocatalyst design, including nanostructured, bimetallic, and atomically dispersed systems. These catalysts exhibit enhanced activity, selectivity, and stability due to synergistic electronic interactions, optimized adsorption energetics, and improved charge transfer kinetics. These catalysts display low overpotentials, reduced charge transfer resistance, and enhanced Faradaic efficiencies for a variety of substrates such as methanol, ethanol, glycerol, and ethylene glycol, reflecting their versatility in integrated electrolysis systems.

However, several key challenges persist despite these achievements. The long‐term electrochemical durability still remains a challenge, as metal atoms have a tendency to form clusters or undergo structural reconstruction. Furthermore, the high selectivity of target products in multi‐carbon alcohol oxidation reactions is a challenge that requires careful control of reaction intermediates and surface properties. Comparing the effectiveness of various catalysts is additionally challenging due to the absence of standardized testing procedures. Future investigations should also focus on developing non‐noble metal catalysts with long‐term operational stability under industrial current densities. More attention should be given to selective C─C bond cleavage and controlled oxidation pathways for targeted value‐added products. Furthermore, membrane‐free and flow‐electrolyzer configurations should be explored to improve practical applicability and reduce system cost. Moving forward, future studies must focus on scalable methods for preparing robust, highly active catalysts with defined coordination environments. The combination of cutting‐edge operando methods with DFT calculations and data‐driven analysis will play a vital role in uncovering the active sites and mechanisms of these systems. Moreover, future catalyst development can be accelerated through machine learning‐assisted material screening and data‐driven catalyst design approaches [122]. Optimization of membrane–electrode assembly (MEA) configurations, electrolyte management, and mass transport properties will also be essential for improving device efficiency under industrial operating conditions [123]. In addition, detailed techno‐economic analysis and life‐cycle assessment studies [124] are necessary to evaluate the commercial feasibility and sustainability of HER‐AOR systems for practical hydrogen production. Additionally, the convergence of catalyst and electrolyzer designs will help to expedite the implementation of HER‐AOR for large‐scale systems.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.