Ampere‐Level Syngas Synthesis by Controllable Active Hydrogen Supply to Regulate CO₂ Reduction Depth on High‐Entropy (CuZnAlZrCe)O₂ Oxide Nanosheets

ABSTRACT

Syngas synthesis via CO₂ electroreduction offers a low‐temperature carbon‐neutral route, yet with poor H₂/CO ratio control and CH₄ byproduct. Herein, we decoupled ^*H generation and binding to modulate its supply and CO₂ reduction depth, steering efficient CO₂‐to‐syngas conversion. As a prototype, (CuZnAlZrCe)O₂ high‐entropy oxide (HEO) nanosheets (NSs) were synthesized via liquid‐phase templating and mild thermal decomposition. The multi‐cation disorder facilitates CO₂ activation and subsequent protonation into ^*COOH. Concurrently, HEO promotes water activation and accelerates ^*H generation, which in turn drives ^*COOH protonation into moderately‐protonated CO. Importantly, HEO weakens ^*H adsorption, suppressing H₂ overproduction and the formation of CH₄, a deeply‐hydrogenated byproduct. Consequently, (CuZnAlZrCe)O₂ HEO achieves 58.2% CO Faradaic efficiency and 88.6% syngas selectivity, retaining > 80% syngas yield at ampere‐level current density. This work presents a robust high‐entropy catalyst that provides tunable syngas at industrially current densities, demonstrating a novel ^*H‐supply‐modulation strategy to regulate CO₂ reduction depth for efficient CO₂‐to‐syngas electrolysis.

An entropy‐driven strategy for controllable ^*H supply, which decouples ^*H generation and adsorption behaviors to precisely regulate CO₂ reduction depth, was proposed over (CuZnAlZrCe)O₂ HEO nanosheets (NSs), enabling over 80% syngas selectivity even at ampere‐level current densities.

1. Introduction

Syngas, a stoichiometric mixture of H₂ and CO, functions as a pivotal platform intermediate that bridges fossil‐based and renewable carbon resources [1, 2]. Its molecular flexibility enables the selective synthesis of bulk chemicals such as NH₃ via the Haber–Bosch route [3] and methanol, as well as drop‐in fuels through Fischer–Tropsch polymerization [4]. State‐of‐the‐art production still relies on (i) coal/biomass gasification and (ii) natural‐gas steam reforming, both of which operate at 800–1300°C and 15–40 bar [5]. These thermochemical routes are energy‐intensive (ΔH >200 kJ mol⁻¹ CH₄ converted) and emit significant CO₂ and solid residues, thereby imposing a heavy environmental burden [6].

Electrocatalytic CO₂ reduction reaction (CO₂RR) offers a promising, low‐carbon route to close the anthropogenic carbon loop by converting emitted CO₂ into value‐added feedstocks such as syngas [7, 8, 9]. To yield syngas, the cathode must co‐generate CO and H₂ at a tunable H₂/CO ratio [10] (typically 0.5‐3 for coal‐derived syngas and natural gas‐derived syngas), while suppressing reaction intermediates. However, although CO formation proceeds via the 2e⁻/2H⁺ pathway (CO₂ + 2H⁺ + 2e⁻ → CO + H₂O) [11], the competing hydrogen evolution reaction (HER: 2H⁺ + 2e⁻ → H₂) is thermodynamically more favorable and often dominates under high proton availability [12]. Furthermore, an abundant supply of ^*H active species tends to drive CO₂ through the 8e⁻/8H⁺ pathway toward deep reduction, steering the formation of CH₄ (CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O) [13]. Therefore, a precisely controllable ^*H supply is critical to regulating the depth of CO₂ reduction, which in turn enables the highly selective production of syngas while suppressing H₂ overproduction and minimizing CH₄ formation.

High‐entropy oxides (HEOs) have recently attracted attention as a tunable platform for CO₂RR because the high‐entropy effect stabilizes single‐phase lattices that incorporate five or more principal cations in near‐equimolar proportions [14, 15, 16]. The resulting configurational entropy (ΔS ≥ 1.5R) counteracts the enthalpy penalty of multicomponent mixing, while the flexible anionic sub‐lattice (typically cubic rock‐salt or fluorite) tolerates large ionic‐radius mismatches without phase segregation [14]. Consequently, HEOs exhibit a continuum of electronic structures, which enables on‐demand modulation of ^*H generation and binding strength, thereby regulating the CO₂ reduction depth. To date, phase‐pure HEOs are almost exclusively obtained by energy‐intensive routes such as high‐temperature (≥ 1000°C) solid‐state reactions [17] or ultrafast Joule‐heating quenching [18], both of which offer limited control over particle size, morphology, and cation short‐range disorder. A low‐temperature, solution‐based synthesis that simultaneously enforces precise stoichiometry and uniform nanostructure is therefore an outstanding challenge for deploying HEOs as next‐generation CO₂RR electrocatalysts.

In this work, we proposed an entropy‐driven strategy for controllable ^*H supply in HEO, which decouples the generation and absorption behaviors ^*H species to enable precise CO₂‐to‐syngas conversion (Scheme 1). Surfactant‐directed liquid‐phase templating combined with mild thermal decomposition was employed to synthesize (CuZnAlZrCe)O₂ HEO nanosheets (NSs). In situ attenuated total reflection Fourier‐transform infrared (ATR‐FTIR) spectroscopy confirms that the ^*H species generated on (CuZnAlZrCe)O₂ HEO NS surface are immediately engaged in the protonation of ^*CO₂ intermediates. This rapid consumption prevents the surface accumulation of ^*H species, thus inhibiting the undesired ^*H dimerization and subsequent CH₄ formation via deep hydrogenation. Density‐functional theory (DFT) calculations further reveal that the multicomponent cation disorder in HEOs lowers the energy barrier for CO₂ activation. More importantly, the HEO surface promotes water dissociation to enhance ^*H generation, while simultaneously weakening the adsorption strength of ^*H species. As a result, the excessive H₂ production and CH₄ byproduct formation are effectively suppressed. Consequently, the (CuZnAlZrCe)O₂ HEO NSs achieve a CO Faradaic efficiency (FE) of 58.2% and an overall syngas selectivity of 88.6% at 0.1 A cm⁻². Remarkably, the catalyst preserves a syngas selectivity > 80% even at ampere‐level current density. This work establishes an entropy‐driven approach to regulating CO₂ reduction depth by modulating ^*H supply within a robust anionic framework, which enables highly selective syngas production via CO₂RR, providing a rational design paradigm for targeted CO₂RR pathways.

SCHEME 1.

Schematic illustration of boosted CO₂‐to‐syngas conversion via regulating ^*H supply and CO₂ reduction depth on (CuZnAlZrCe)O₂ HEO NSs.

2. Results and Discussion

2.1. Synthesis and characterizations of (CuZnAlZrCe)O₂ HEO NSs

To achieve efficient CO₂‐to‐syngas conversion, the rational element selection of HEO is critically important. Cu is widely recognized as the active center for CO₂ adsorption and its initial reduction towards the key ^*CO intermediate. Ce generally functions as a favorable site for water dissociation to generate ^*H, and simultaneously modulates the electron density of Cu sites to weaken ^*CO binding, favoring ^*CO desorption rather than deep hydrogenation to CH₄. Zn can optimize the ^*H binding energy on HEO surface, balancing ^*H supply for CO₂ reduction while suppressing the competing HER. As a lattice stabilizer, Al can maintain the intact single‐phase fluorite structure, inhibit phase segregation, and improve the long‐term durability. Zr can enhance structural robustness and regulate the band structure as well as charge transfer efficiency. Meanwhile, Zr can also cooperate with Al and Ce to stabilize the high‐entropy solid‐solution structure against segregation, and further tunes lattice strain and surface charge redistribution. (CuZnAlZrCe)O₂ HEO NSs were synthesized through a liquid‐phase template route, with polyvinylpyrrolidone (PVP) acting as a soft, two‐dimensional (2D) structure‐directing agent. During the aging process, PVP chains spontaneously self‐assemble into lamellar micelles through hydrogen bonding and π–π stacking, thereby creating 2D nanoreactors that sequester and homogenize the incoming metal ions (Figure S1). The resulting polymer‐ion precursor was immediately quenched by freeze‐drying to kinetically trap the 2D architecture and immobilize the metal ions on PVP substrates. Subsequent calcination at 450°C removes the organic scaffold and steers the complete decomposition of nitrate salts, yielding a single‐phase, entropy‐stabilized oxide. The formation process of (CuZnAlZrCe)O₂ HEO NSs is illustrated in Figure 1a.

FIGURE 1.

Preparation process and characterizations of (CuZnAlZrCe)O₂ HEO NSs. (a) Schematic illustration of the synthesis process. (b) TEM image. (c) HAADF‐STEM image and elemental mappings. The scale bar is 50 nm. (d) HRTEM image. (e), XRD patterns. Inset: atomic model of high‐entropy cubic fluorite phase. (f), Ce 3d orbital XPS spectra. (g), Raman spectra.

Transmission electron microscopy (TEM, Figures 1b and S2) images reveal that the as‐synthesized HEO NSs possess lateral dimensions reaching up to micrometers, similar to those of pristine CeO₂ NSs (Figure S3). Despite their multi‐element composition, (CuZnAlZrCe)O₂ HEO NSs retain the same ultrathin geometry and present a particulate, rough surface with abundant mesopores, features that are likewise observed for the CeO₂ counterpart (Figure S4). High‐angle annular dark‐field scanning TEM (HAADF‐STEM) image and energy‐dispersive x‐ray spectroscopy (EDS) elemental mappings (Figures 1c and S5) show the uniform distribution of all five constitutive metals, while inductively coupled plasma optical emission spectroscopy (ICP‐OES) gives an empirical formula of Cu_0.23Zn_0.15Al_0.22Ce_0.26Zr_0.14O_2‒x (Figure S6). High‐resolution TEM (HRTEM, Figure 1d) of (CuZnAlZrCe)O₂ HEO NSs displays continuous lattice fringes with an interplanar spacing of 0.309 nm, indexed to the CeO₂(111) plane. Notably, the fluorite lattice parameter of 0.316 nm in pure CeO₂ is contracted due to the incorporation of additional cations (Figure S7). X‐ray diffraction (XRD) patterns (Figure 1e) indicate that (CuZnAlZrCe)O₂ HEO NSs exhibit characteristic peaks corresponding to the cubic fluorite phase of CeO₂ (JCPDS No. 43‐1002) [19]. All peaks are shifted to higher angles without any detectable secondary phases when compared to CeO₂ NSs, corroborating the HRTEM observation and confirming that the multi‐cation disorder is entropy‐stabilized within a single‐phase solid solution.

To further elucidate the electronic structure of (CuZnAlZrCe)O₂ HEO NSs, x‐ray photoelectron spectra (XPS) analysis was performed (Figure S8). As displayed in Figure 1f, the Ce 3d orbital spectra can be deconvoluted into eight contributions using the conventional v/u notation [20]. The Cu 2p_3/2 spectra (Figure S9a) exhibit a dominant peak at 934.0 eV (Cu²⁺), a low‐binding‐energy shoulder at 931.4 eV (Cu^0/+1), and pronounced shake‐up satellites centered at 942.6 eV. Zn 2p_3/2 (1022.2 eV, Figure S9b) [21], Al 2p (74.4 eV, Figure S9c), and Zr 3d_5/2 (181.9 eV, Figure S9d) spectra [22] each exhibit a single spin‐orbit doublet consistent with Zn²⁺, Al³⁺, and Zr⁴⁺, respectively. O 1s region (Figure S10) shows an obvious increase in the ratio of vacancy‐associated oxygen (O_v, ∼ 531.1 eV) to lattice oxygen (O_L, ∼ 529.4 eV) [23] compared to CeO₂ NSs, confirming that the entropy‐driven lattice accommodates a substantial population of oxygen vacancies. Additionally, Raman spectra were also conducted on both (CuZnAlZrCe)O₂ HEO and CeO₂ NSs in Figure 1g. The F_2g vibrational mode of the cubic fluorite structure [24] shifts from 459.4 cm⁻¹ (CeO₂ NSs) to 455.1 cm⁻¹ (HEO NSs). The redshift originates from local bond‐length disorder and the mass‐defect effect introduced by the smaller substituents (Al³⁺ and Zn²⁺). A broad, weak feature at 600.0 cm⁻¹ (D), attributed to the Raman‐active defect mode of doubly ionized oxygen vacancies [25], becomes more noticeable in HEO, in agreement with the O 1s spectra results. Nonetheless, the absence of additional Raman‐active modes confirms that the long‐range M─O sub‐lattice remains intact, satisfying the structural criterion for an entropy‐stabilized, single‐phase HEO.

2.2. Electrocatalytic CO₂RR performances of (CuZnAlZrCe)O₂ HEO NSs

The electrocatalytic CO₂RR performances of (CuZnAlZrCe)O₂ HEO and CeO₂ NSs were evaluated in a flow cell under alkaline conditions. As shown in the cyclic voltammetry (CV) curve (Figure S11), (CuZnAlZrCe)O₂ HEO NSs exhibits a pair of obvious redox peaks centered at −0.06 and −0.28 V, which can be attributed to the reversible redox transition of Cu species (Cu²⁺/Cu⁺) [26]. As shown by the linear sweep voltammetry (LSV) curves in Figure 2a, (CuZnAlZrCe)O₂ HEO NSs achieve a marked increase in the current density from −6.0 to −54.0 mA cm⁻² at −2.0 V (vs. Ag/AgCl) in CO₂, as compared to that in Ar. This significant increase indicates the superior catalytic response toward CO₂ reduction [27]. An enhanced catalytic activity of (CuZnAlZrCe)O₂ HEO NSs is further highlighted by the increased current density compared to CeO₂ NSs (33.0 mA cm⁻²) in CO₂. The current density‐dependent FEs of various products generated on (CuZnAlZrCe)O₂ HEO and CeO₂ NSs are illustrated in Figures 2b and S12, respectively. The primary products of (CuZnAlZrCe)O₂ HEO NSs at various current densities are CO and H₂, with the syngas ratio exceeding 80%. At 0.1 A cm⁻², CO is produced with a FE of 58.2 % with syngas ratio climbing to 88.6%, yielding a H₂/CO ratio of 0.5. Notably, the H₂/CO ratio at ampere‐level current density approaches 2.0, ideal syngas stoichiometry for Fischer–Tropsch or methanol synthesis [28]. In contrast, CeO₂ NSs (Figure 2c) switch from CH₄‐selective behavior at low current (FE_CH4 = 55% at 0.1 A cm⁻²) to nearly pure H₂ evolution above 0.3 A cm⁻². At 0.5 A cm⁻², H₂ accounts for 80% of the charge, giving an H₂/CO ratio of near 30 (Figure 2d). The tight H₂/CO window (0.5–1.7) accessible with (CuZnAlZrCe)O₂ HEO catalyst, readily tunable via applied current density, highlights its promise as a scalable, alkaline‐stable syngas generator that circumvents the wide ratio fluctuations typically encountered with single‐metal oxide cathodes. In addition, a few liquid‐phase products were detected as well on both (CuZnAlZrCe)O₂ HEO and CeO₂ NSs (Figure S13). Kinetic insights into the CO₂RR pathway were obtained by electrochemical impedance spectroscopy (EIS). EIS spectra recorded at open circuit potential (Figure 2e) reveal a charge‐transfer resistance of 33.1 Ω cm² for HEO catalyst, less than half 66.6 Ω cm² determined for CeO₂ NSs, confirming superior interfacial charge transport [29]. The smaller semicircle diameter correlates with the higher vacancy density and enhanced electronic conductivity conferred by the mixed‐valent Ce³⁺/Ce⁴⁺ couples embedded in the high‐entropy lattice.

FIGURE 2.

Electrocatalytic CO₂RR performances of (CuZnAlZrCe)O₂ HEO and CeO₂ NSs. (a) LSV curves. (b) FEs of various products at different current densities from 0.1 to 1.0 A cm⁻² for (CuZnAlZrCe)O₂ HEO NSs. (c) FEs of various products at different current densities from 0.1 to 0.6 A cm⁻² for CeO₂ NSs. (d) The H₂/CO ratios over (CuZnAlZrCe)O₂ HEO and CeO₂ NSs. (e) EIS curves. (f) Stability measurement at 0.2 A cm⁻².

Long‐term electrolysis at 0.2 A cm⁻² in a flow cell shows no perceptible degradation over 4 h, as shown in Figure 2f. The CO and H₂ FEs remain constant at 55% and 31%, respectively, yielding a syngas ratio > 80%. Post‐electrolysis TEM images (Figure S14) and post‐electrolysis XRD pattern (Figure S15) evidence the retention of the sheet‐like morphology with no particle detachment or crystallographic phase separation. To better illustrate the catalyst stability under potential cycling, we also evaluated the activity retention of (CuZnAlZrCe)O₂ HEO NSs after multiple CV cycles. As shown in Figure S16, after 400 CV cycles, the current density decreased slightly from 55.4 to 49.8 mA cm⁻², retaining up to 90% of its initial activity, while the onset potential remained almost unchanged. Additionally, Figure S17 indicates that (CuZnAlZrCe)O₂ HEO NSs well preserve their intact 2D sheet‐like structure after CV cycling. Meanwhile, no phase segregation or new impurity phases are observed, further underscoring the robust structural and chemical stability of (CuZnAlZrCe)O₂ HEO NSs under aggressive alkaline CO₂‐reduction conditions [30].

2.3. Intermediates detection during CO₂RR

To probe the intermediates during CO₂RR on both (CuZnAlZrCe)O₂ HEO and CeO₂ NSs, in situ ATR‐FTIR measurement was conducted. Figure 3a shows five vibrational features for (CuZnAlZrCe)O₂ HEO NSs located at 1020, 1178, 1290, 1400, and 1630 cm⁻¹, respectively. The dominant bands at 1630, 1400, and 1290 cm⁻¹ are assigned to the asymmetric and symmetric O─C─O stretches of ^*COOH, the key precursor for CO formation [31]. Obviously, HEO NSs exhibit a stronger intensity at these positions, indicating an enhanced reduction of CO₂ into ^*COOH than CeO₂ NSs (Figure 3b). The peak at 1020 cm⁻¹, corresponding to C─H bending of ^*CHO, is markedly weaker than that on CeO₂ NSs, implying that ^*CO is rapidly expelled from the HEO surface rather than undergoing further hydrogenation. In agreement, no discernible ^*CO stretching band (2000 ∼ 2200 cm⁻¹) is detected on the CeO₂ sample, confirming that CO generation and aggregation are restrained [32]. Moreover, the time‐resolved in situ ATR‐FTIR spectra in Figure S18 reveal that the intensity of the ^*CO band increases continuously with reaction time, directly verifying the sustained generation and accumulation of ^*CO intermediates on the (CuZnAlZrCe)O₂ HEO surface. Conversely, CeO₂ NSs exhibit pronounced ^*CH₃O (1178 cm⁻¹) signature, intermediate that is transformed into CH₄ via a sequential proton‐coupled electron transfer (PCET) process [33]. The peak at 1520 cm⁻¹ on CeO₂ NSs can be attributed to ^*OCCO group [34], indicating minor C₂ products formation, which is consistent with the detection of C₂H₅OH in the corresponding liquid product (Figure S13). These spectral observations are consistent with experimental results, which corroborate that (CuZnAlZrCe)O₂ HEO NSs deliver CO as the primary product, whereas CeO₂ NSs preferentially reduce CO₂ to CH₄.

FIGURE 3.

Potential‐resolved in situ ATR‐FTIR and Raman spectra of (CuZnAlZrCe)O₂ HEO and CeO₂ NSs. (a,b) In situ ATR‐FTIR spectra of (CuZnAlZrCe)O₂ HEO (a) and CeO₂ NSs (b) within the wavenumber of 1000–2200 cm⁻¹. (c,d) In situ Raman spectra of (CuZnAlZrCe)O₂ HEO (c) and CeO₂ NSs (d).

To further investigate the generation of CO on (CuZnAlZrCe)O₂ HEO and CeO₂ NSs, in situ Raman spectra were collected. For (CuZnAlZrCe)O₂ HEO NSs (Figure 3c), a weak vibrational band of δ ^*CO₂ ⁻ is detected at 705 cm⁻¹ [35]. More importantly, a sharp and potential‐dependent feature at 370 cm⁻¹ emerges at −1.6 V and intensifies continuously up to −3.0 V. This peak is assigned to the frustrated rotational mode of Cu‐^*CO [36], confirming that CO generated during CO₂RR is chemisorbed on Cu sites within the high‐entropy lattice. As anticipated, CeO₂ NSs show no counterpart in this region, consistent with the absence of Cu (Figure 3d). Moreover, a distinct peak at 2080 cm⁻¹ is observed on (CuZnAlZrCe)O₂ HEO NSs, which is more prominent compared to that on CeO₂ NSs, further confirming the enhanced generation of CO (Figure S19). On the contrary, CeO₂ NSs exhibit a strong F_2g vibrational mode of fluorite lattice at 455 cm⁻¹, which is significantly attenuated on (CuZnAlZrCe)O₂ HEO surface. This suppression reflects the substantial lattice disorder and altered surface polarity introduced by the multi‐cation framework, further supporting that high‐entropy mixing modulates both adsorbate binding and interfacial speciation.

2.4. Exploration of ^*H supply during CO₂RR

To investigate the ^*H generation behavior on the catalyst surface during CO₂RR, the O─H stretching vibration of H₂O was monitored within 2800–3800 cm⁻¹ (Figures 4a and S20). Three typical components were resolved, including the strongly hydrogen‐bonded H₂O (∼3200 cm−¹), moderately hydrogen‐bonded H₂O (∼3400 cm⁻¹), and free H₂O (∼3550 cm⁻¹). As the potential shifts from −1.4 to −3.0 V, HEO NSs exhibit a higher ratio of free H₂O, whereas CeO₂ NSs contain a large amount of strongly hydrogen‐bonded H₂O (Figure S21). As is well‐known, a weaker hydrogen bond network is more favorable for water dissociation and protonation during CO₂RR [37, 38]. Therefore, HEO NSs weaken the hydrogen‐bond network at the interface, which in turn facilitates the PCET process, and thus the initial protonation of CO₂ into ^*COOH. Furthermore, time‐resolved in situ ATR‐FTIR spectra (Figure 4b,c) reveal that the peak intensity of HEO NSs increases initially (due to enhanced H₂O adsorption) and then decreases (attributed to rapid ^*H consumption for ^*COOH formation), whereas that of CeO₂ NSs remains relatively high throughout (suggesting significant ^*H accumulation and H₂ recombination). This difference further confirms that abundant ^*H from H₂O dissociation on the HEO surface rapidly participates in CO₂RR steps (CO₂ + ^*H → ^*COOH; ^*COOH + ^*H → ^*CO), avoiding excessive ^*H accumulation and consistent with the superior CO₂RR activity and CO selectivity of HEO NSs.

FIGURE 4.

In situ ATR‐FTIR and in situ EIS spectra of (CuZnAlZrCe)O₂ HEO and CeO₂ NSs. (a) Potential‐resolved in situ ATR‐FTIR spectra of (CuZnAlZrCe)O₂ HEO NSs within the wavenumber of 2800–3800 cm⁻¹. (b,c) Time‐resolved in situ ATR‐FTIR spectra of (CuZnAlZrCe)O₂ HEO (b) and CeO₂ NSs (c). (d) In situ EIS spectra. (e) The calculated C _φ. Inset: the equivalent circuit.

In addition, the ^*H transfer behavior was quantitatively evaluated by in situ EIS spectra, as illustrated in Figure 4d,e (fitting parameters summarized in Tables S1 and S2). Specifically, R _s represents the solution resistance, CPE denotes the double‐layer capacitance, R _ct corresponds to the charge‐transfer resistance, C _φ refers to the pseudocapacitance associated with surface ^*H coverage, and R₂ stands for the mass‐transport resistance encountered by ^*H migrating from bulk electrolyte to electrode surface [39]. During CO₂RR, ^*H species are derived from water dissociation, which undergo dynamic processes of robust binding, desorption, and spillover to C₁ intermediates. As expected, (CuZnAlZrCe)O₂ HEO NSs exhibit a smaller R₂ than CeO₂ NSs, which indicates that ^*H transport in the former is facilitated and more ^*H species are involved ^*CO₂ protonation. Furthermore, Figure 4e demonstrates that (CuZnAlZrCe)O₂ HEO NSs consistently yield a smaller C _φ compared to CeO₂ NSs, which corresponds to a diminished surface ^*H coverage. Collectively, these results suggest that the fast local ^*H generation and weak ^*H binding affinity on (CuZnAlZrCe)O₂ HEO NSs synergistically ensure the suitable H₂ production and moderate CO₂ reduction, suppressing the excessive HER and deep protonation into CH₄. In contrast, the abundant ^*H reservoir on CeO₂ NSs sustains a continuous ^*H flux that facilitates both H₂ generation and sequential hydrogenation steps (^*CO→^*CHO→^*CH_xO), thus steering the reaction pathway preferentially toward CH₄ formation.

2.5. Mechanism of desired syngas formation on (CuZnAlCeZr)O₂ HEO NSs

DFT calculations [40] were constructed to determine the thermodynamic preference of CO₂ conversion to CO and CH₄ on pure CeO₂ and (CuZnAlZrCe)O₂ HEO (computational details provided in Figures S22–S25). The energy barriers of water dissociation were comparatively evaluated on two surfaces. As shown in Figure 5a, (CuZnAlZrCe)O₂ HEO(111) presents a substantially lower free‐energy barrier (ΔG = −1.76 eV) for ^*H, implying the boosted initial protonation to ^*COOH and then ^*CO. In contrast, CeO₂(111) exhibits a positive ΔG (0.03 eV), evidencing a thermodynamically unfavorable protonation pathway into ^*CO. Next, the adsorption strength of key intermediates, ^*H, on both CeO₂(111) and (CuZnAlZrCe)O₂ HEO(111) was quantified. As depicted in Figure 5b, ^*H is stabilized by 0.41 eV on the HEO(111) surface compared to CeO₂(111), which promotes surface hydrogenation activity and impresses competitive H₂ evolution. These trends create a favorable environment for syngas (CO + H₂) production, where water dissociation and ^*H desorption are both thermodynamically favorable.

FIGURE 5.

DFT calculation results and mechanisms illustration. (a) Free energy diagrams for water dissociation on (CuZnAlZrCe)O₂ HEO(111) and CeO₂(111). (b) ^*H absorption energies on (CuZnAlZrCe)O₂ HEO(111) and CeO₂(111). (c) Free energy diagrams for CO₂RR on (CuZnAlZrCe)O₂ HEO(111) and CeO₂(111). (d) Mechanisms of CO₂RR on (CuZnAlZrCe)O₂ HEO and CeO₂ NSs.

To comprehensively map the full reaction landscape, the complete CO₂RR pathway on (CuZnAlZrCe)O₂ HEO(111) and CeO₂(111) was computed, as illustrated in Figure 5c. The rate‐determining step (RDS) on (CuZnAlZrCe)O₂ HEO(111) surface is ^*CO → ^* + CO, with an energy barrier of 1.07 eV, which is lower than that of the RDS on CeO₂(111) (^*COOH → ^*CO, with an energy barrier of 1.32 eV). Moreover, (CuZnAlZrCe)O₂ HEO(111) exhibits a lower free‐energy barrier for the formation of ^*CO₂ species, with the spontaneous generation of ^*COOH intermediates. When ^*COOH is reduced to ^*CO, the CeO₂(111) surface requires an energy input of 1.32 eV, which is significantly higher than 0.36 eV on the HEO surface, indicating that HEO is more favorable for the moderate protonation process (CO₂ → ^*CO₂ → ^*COOH → ^*CO). Importantly, the successive hydrogenation step (^*CO → ^*CHO → ^*CH₄) proceeds spontaneously, confirming that CeO₂ surface intrinsically favors deep reduction to CH₄ rather than partial reduction to CO. Consequently, the high‐entropy composition simultaneously promotes water dissociation and suppresses deep hydrogenation, thereby shifting the product selectivity from CH₄‐dominated products on CeO₂(111) to tunable H₂/CO ratios on (CuZnAlZrCe)O₂ HEO(111) surface.

Figure 5d illustrates the CO₂RR pathway on both catalysts. Benefiting from the unique high‐entropy effect, including abundant oxygen vacancies, strong interelement electronic interactions, lattice distortion, and high‐entropy stabilization against phase separation, (CuZnAlZrCe)O₂ HEO NSs effectively weaken the hydrogen‐bond network of interfacial water. This facilitates water dissociation and rapid ^*H generation, which promptly participates in the initial CO₂ hydrogenation to form the key ^*COOH intermediate and avoids excessive ^*H accumulation, thereby producing only a moderate amount of H₂. In contrast, CeO₂ NSs offer a sufficient ^*H reservoir to provide continuous ^*H flux, which promotes H₂ generation and sequential hydrogenation, ultimately favoring the CH₄ pathway.

3. Conclusion

In summary, we proposed the novel entropy‐driven synergistic strategy to simultaneously facilitate ^*H generation and modulate ^*H surface binding, which in turn regulates ^*H supply and CO₂ reduction depth, thereby enabling robust CO₂‐to‐syngas conversion at industrial current densities. As an example, high‐quality (CuZnAlZrCe)O₂ HEO NSs were fabricated through a simple solution‐phase route followed by mild annealing. The high‐entropy configuration steers a combination of multiple synergistic effects, such as rich oxygen vacancies, strong intermetallic electronic modulation, lattice distortion, and robust phase‐segregation resistance. DFT calculations confirm that (CuZnAlZrCe)O₂ HEO NSs exhibit a significantly lower free energy barrier for water dissociation and moderately weakened ^*H binding. In situ electrochemical spectra studies reveal that the rapidly generated ^*H immediately participates in the initial protonation of CO₂ to form the key ^*COOH intermediate, and this fast consumption avoids the excessive accumulation of ^*H and overproduction of H₂, as well as deep protonation into the CH₄ byproduct. As a result, (CuZnAlZrCe)O₂ HEO NSs deliver 58.2% CO FE with 88.6% overall syngas selectivity at 0.1 A cm⁻² and maintain > 80% syngas yield at ampere‐level current density. These findings establish HEOs as a versatile platform for decoupling the surface ^*H generation and binding, offering a practical strategy for selective and energy‐efficient CO₂ electroreduction to customizable syngas compositions.

Author Contributions

Peipei Li: writing – original draft, investigation, data curation. Wenya Fan: data curation, investigation. Haochen Zhang: data curation, investigation. Changjing Wang: data curation. Mengqian Li: data curation. Zequn Han: data curation. Liang Chen: writing – review and editing, software. Xingchen Jiao: supervision, conceptualization, funding acquisition, writing – review and editing. Qingxia Chen: supervision, conceptualization, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72527‐sup‐0001‐SuppMat.docx.

Data Availability Statement

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