The production of solar fuels from CO 2 using sunlight and electricity provides one promising route for reducing atmospheric carbon emissions and storing intermittent solar energy. The rational design of an efficient and inexpensive electrocatalyst is the key. We developed a binary copper−iron catalyst for photoelectrochemical CO 2 reduction toward methane. The theoretical calculations suggest that Cu and Fe in the binary system can work in synergy to spontaneously favor CO 2 activation and conversion for methane synthesis. The earth-abundant CuFe catalyst exhibits a high current density with an impressive methane Faradaic efficiency using industry-ready planar silicon photoelectrodes under one-sun illumination. This work presents a unique, highly efficient, and inexpensive route for solar fuels synthesis.

A rational design of an electrocatalyst presents a promising avenue for solar fuels synthesis from carbon dioxide (CO 2 ) fixation but is extremely challenging. Herein, we use density functional theory calculations to study an inexpensive binary copper−iron catalyst for photoelectrochemical CO 2 reduction toward methane. The calculations of reaction energetics suggest that Cu and Fe in the binary system can work in synergy to significantly deform the linear configuration of CO 2 and reduce the high energy barrier by stabilizing the reaction intermediates, thus spontaneously favoring CO 2 activation and conversion for methane synthesis. Experimentally, the designed CuFe catalyst exhibits a high current density of −38.3 mA⋅cm −2 using industry-ready silicon photoelectrodes with an impressive methane Faradaic efficiency of up to 51%, leading to a distinct turnover frequency of 2,176 h −1 under air mass 1.5 global (AM 1.5G) one-sun illumination.

The production of clean solar fuels from carbon dioxide (CO 2 ) and water via photoelectrocatalysis (PEC) provides a promising route for alleviating our society’s reliance on fossil fuels and reducing atmospheric carbon emissions (1⇓⇓–4). A rational design of an electrocatalyst is the key for achieving high performance of CO 2 reduction reactions (CO 2 RR) (5⇓–7). It is worth noting that, among various products formed from PEC CO 2 RR, the most reduced methane is highly energy dense (∆H C o = 891 kJ/mol), and its storage, transportation, and combustion are compatible with the existing industrial infrastructure, thus being an ideal solar fuel (8). However, the production of methane requires complicated 8-electron/proton coupling transfer, which is both kinetically and thermodynamically unfavorable (9⇓–11). The development of an efficient electrocatalyst is thus highly desirable (12).

Over the past few decades, a large number of electrocatalysts, including molecular complexes (13, 14), enzymes (15, 16), metals (17, 18), and transition metal chalcogenides (19, 20), have been developed for CO 2 RR. Among these materials, copper (Cu) is well known to be a state-of-the-art electrocatalyst for producing methane from CO 2 RR (21⇓⇓⇓⇓⇓–27). To date, however, the use of Cu catalyst for PEC methane synthesis (28, 29) has suffered severely from low current density, inferior Faradaic efficiency, low turnover frequency, and high overpotential. This is because Cu with monofunctional site generally possesses a very weak interaction with CO 2 , which is not capable of concurrently activating CO 2 molecules and stabilizing the subsequent reaction intermediates (30⇓–32). Recently, binary catalyst of Cu with secondary metals and their derivatives has emerged as a possible approach to enhance the performance of PEC CO 2 RR. For example, Chu et al. (33) demonstrated that oxide-derived Cu−Zn electrocatalyst exhibited a remarkable enhancement on tunable syngas formation with a benchmark turnover number of 1,330 compared to Cu alone. Kong et al. (34) described directed assembly of CuAu nanoparticles on silicon nanowire (NW) photoelectrodes, exhibiting an evidently accelerated CO 2 -to-CO conversion with high selectivity of 80% at −0.2 V. Yin et al. (35) developed a Cu−Zn alloy for selectively reducing CO 2 toward HCOOH with a Faradaic efficiency of 79.11% through PEC, which is superior to either Zn or Cu. Nevertheless, these reported binary systems are still not efficient at improving the interaction with CO 2 for methane synthesis from PEC CO 2 RR. Therefore, a rational design of a novel binary catalyst of Cu for simultaneous CO 2 activation and stabilization of various intermediates to effectively synthesize methane is of fundamental and practical interest (30, 36), but has remained a grand challenge.

In this work, we present the discovery of a binary CuFe electrocatalyst for the selective reduction of CO 2 to CH 4 . Density functional theory (DFT) calculations reveal that Cu and Fe in the binary system work in synergy to induce a significantly distorted O−C−O angle of 126.05° from its original linear configuration at the interface to render a strong interaction with CO 2 , and a drastic reduction in the reaction energy barrier, thus greatly facilitating methane synthesis. Experimentally, the CuFe binary electrocatalyst is shown to exhibit high current density of −38.3 mA⋅cm−2 for silicon-based photoelectrodes with high Faradaic efficiency of up to 51% and high turnover frequency (TOF) of 2,176 h−1 for PEC CO 2 RR toward CH 4 under simulated solar light (air mass 1.5 global [AM 1.5G], 100 mW⋅cm−2) at −1.2 V versus reversible hydrogen electrode (RHE), which is superior to that of both Cu and Fe catalyst individually. In addition, the photocathode is made entirely of Earth-abundant materials by industrial semiconductor manufacturing process, presenting one promising route for producing clean fuels in aqueous solution using solar energy.

Results

CO 2 Adsorption/Activation over Cu(111) and Fe x O y /Cu(111). Since the initial activation of the inert CO 2 is crucial for the subsequent reactions, CO 2 adsorption characteristics were first investigated using DFT calculations. As iron appears to be in its oxidation state, Fe x O y was used in the analysis, and the preferred orientation of Cu surface with the lowest surface energy, that is, Cu(111), was adopted. Therefore, an inverse hydrogenated Fe 3 O 6 H 6 /Cu(111) was utilized as a representative model for CuFe electrocatalyst (please see Computational methods for more details), by taking the aqueous CO 2 RR environment (37) and the preferable H spillover from metal particles to oxide support into consideration (38). Illustrated in Fig. 1A and Fig. 1B are the optimized structures for CO 2 adsorption on Cu(111) and Fe 3 O 6 H 6 /Cu(111), respectively. For the case of CO 2 on Cu(111), we see that CO 2 remains the original linear configuration with the O−C−O angle of 179.67°, and the 2 C−O bond lengths are similar to the CO 2 isolated gas phase (Fig. 1A and SI Appendix, Fig. S1 and Table S1). On the other hand, it is found that, for the case of CO 2 at the Fe 3 O 6 H 6 /Cu(111) interface, the C atom strongly binds to the Cu atom underneath, with a bond length of 1.98 Å, and one O atom attaches to the Fe atom with a shorter bond length of 1.96 Å. This signifies a much stronger bonding, which results in a significant distortion of CO 2 away from its original linear form to a bent form with an O−C−O angle of 126.05° (Fig. 1B and SI Appendix, Table S1). A bidentate configuration is therefore formed, which facilitates the subsequent reactions (36, 39). In addition, it is observed that the interaction of CO 2 with the Fe 3 O 6 H 6 /Cu(111) interface weakens the 2 C−O bonds of CO 2 , leading to elongated C−O bonds (1.28 and 1.25 Å) from the original bond length of 1.18 Å in an isolated CO 2 . The weakened C−O bonds and the distorted CO 2 configuration together highlight an obvious activation of CO 2 upon chemisorption at the interface, which is in stark contrast to the negligible activation of CO 2 on Cu(111) that is highly beneficial for CO 2 RR. The CO 2 activation mechanism at Fe x O y /Cu(111) interface reveals a similarity to that of individual metal oxide, for example, TiO 2 (38), with surface oxygen vacancies, in which an undercoordinated Fe atom at the edge of the oxide cluster (i.e., essentially an O vacancy) acts as the active center to bind one of the O atom in CO 2 (40, 41). It is also worth noting that the coexistence of iron oxides and Cu nanoparticles facilitates the formation of the bifunctional Fe x O y /Cu(111) interface. On one hand, Fe x O y /Cu(111) interface allows multiple adsorption sites and directly participates in stabilizing the key reaction intermediates, such as *CO 2 , *C x H y O z , and *C x H y . On the other hand, the strong interaction between iron oxides and Cu nanoparticles results in a unique electronic structure that differs from those of isolated components, which is suitable for CO 2 activation and its subsequent transformation (39). These results are consistent with the observation in thermal CO 2 catalysis at metal/oxide interface (42⇓–44), and can be further verified by the CO 2 adsorption capacity measurement (SI Appendix, Fig. S2) showing much larger CO 2 adsorption capacity of CuFe@GaN NWs/Si than that of Cu/GaN NWs/Si. Fig. 1. CO 2 adsorption and activation over Fe x O y /Cu(111). Side and top views of optimized configurations of CO 2 activation on Cu(111) (A) and Fe 3 O 6 H 6 /Cu(111) (B). Cu, blue; Fe, orange; O, red; C, brown; and H, white.

Synthesis and Characterization of the Binary CuFe Electrocatalyst. Inspired by the theoretical results above, we developed a binary CuFe catalyst monolithically integrated with GaN NW arrays on planar n+-p silicon wafer, which was achieved by combing highly controlled molecular beam epitaxy with facile electrodeposition (SI Appendix, Fig. S3). As illustrated in Fig. 2A and SI Appendix, Fig. S4, one-dimensional (1D) GaN NWs are first grown on planar n+-p silicon junction with a length of ∼300 nm and diameters varying from 30 to 40 nm, using molecular beam epitaxy. Transmission electron microscope (TEM) images show that GaN NWs are nearly defect-free with lattice space of ∼0.26 nm, suggesting the c-axis growth direction (SI Appendix, Fig. S5) (33). Using these NWs as support, Cu and Fe were facilely codeposited via electrocatalysis. After the electrodeposition, the morphology of the GaN NW arrays remains largely unchanged (Fig. 2B). Scanning TEM high-angle annular dark-field (STEM-HAADF) image and elemental distribution mappings illustrate that both Cu and Fe are clearly dispersed on GaN NW with a unique alloyed geometry (Fig. 2 C–G). The binary CuFe catalyst loading could be optimized by the 1D GaN NWs. In particular, 1D nanostructure is favorable for exposing cocatalyst with high-density active sites. What is more, the ultrahigh surface-to-volume ratio of 1D nanostructure helps to reduce the loading amount of the catalyst (45). The inductively coupled plasma atomic emission spectrum (ICP-AES) indicates that the content of the binary CuFe catalyst is 0.041 μmol·cm−2 with Fe/Cu ratio of 6.3/1. X-ray photoelectron spectroscopy (XPS) measurement was conducted to further analyze the chemical states of Cu and Fe (SI Appendix, Fig. S6). It is clearly shown that the characteristic peaks of Cu 2p 3/2 and Cu 2p 1/2 appear at 933.2 and 953.1 eV (Fig. 2H), due to metallic copper and/or partially oxidized copper. Meanwhile, the peaks of ∼711 and 725 eV are associated with Fe 2p 3/2 and Fe 2p 1/2, respectively (Fig. 2I). As suggested by previous studies, these peaks originate from iron oxides and/or hydroxides (Fe x O y /Fe x (OH) y ) (46). X-ray diffraction spectrum measurement in SI Appendix, Fig. S7 illustrates that only a featured peak of GaN (002) at ∼34° was observed for both GaN/Si and CuFe@GaN NWs/Si (33). This may originate from both the low content of Cu and Fe and their amorphous phase, which agree well with TEM and ICP-AES characterizations. The amorphous copper−iron catalyst supported on one-dimensional GaN NW arrays could provide sufficient surface defects as well as a large number of low-coordinated atoms of the catalyst, and, consequently, abundant active sites can be produced for CO 2 RR (47, 48). Fig. 2. Structure and chemical characterization. Scanning electron microscopy (SEM) images of bare GaN NWs/Si (A) and CuFe@GaN NWs/Si (B) with magnified Insets. (Scale bars: A and B, 1 μm; Insets, 500 nm.) STEM-HAADF image of GaN nanowire NW modified with binary CuFe catalyst (C). The elemental distribution mappings of Ga (D), N (E), Fe, (F) and Cu (G) are described as well; the full horizontal width of D–G is 20 nm. XPS measurement of Cu 2p (H) and Fe 2p (I) in CuFe@GaN NWs/Si. a.u. denotes arbitrary unit; in H, the gray and pink lines represent original and fitting data of Cu 2p, respectively; in I, the gray and green lines represent original and fitting data of Fe 2p while orange, deep blue, light blue, pink, and purple lines represent various iron oxides and/or hydroxides.

Photoelectrochemical CO 2 Reduction Reaction. The PEC CO 2 RR performance of CuFe@GaN NWs/Si as well as other photocathodes was examined in CO 2 -saturated 0.5 mol/L KHCO 3 aqueous solution. As shown in Fig. 3A, it is obvious that, among all 5 of the tested photocathodes, CuFe@GaN NWs/Si exhibits the best current density–voltage (J–V) curve under standard one-sun illumination. Compared to bare n+-p silicon junction, GaN NWs/Si shows an evidently improved J−V curve with an onset potential of −0.33 V (corresponding to a current density of −0.1 mA⋅cm−2) but still suffers from rapid surface recombination and slow reaction kinetics because of the lack of catalysts. The introduction of catalysts could significantly improve the J−V behavior. It is noted that the binary CuFe catalyst shows an obvious enhancement compared to both Fe and Cu individually, confirming the synergetic effect of Cu and Fe for the reaction. The superior onset potential of +0.23 V of CuFe@GaN NWs/Si is 200 and 290 mV higher than that of Fe/GaN NWs/Si and Cu/GaN NWs/Si, respectively. Importantly, the current density of CuFe@GaN NWs/Si reaches −38.3 mA⋅cm−2 at −1.2 V, which is close to the light-limited current of the silicon-based photocathode (∼−45 mA⋅cm−2) under one-sun illumination (49). The origin of the improved performance comes primarily from the fact that the CuFe catalyst offers active centers to promote the kinetics (8). Moreover, photoluminescence (PL) spectra in SI Appendix, Fig. S8 illustrate that the featured peak intensity decreased in the order of GaN NWs/Si > Cu/GaN NWs/Si > CuFe@GaN NWs/Si. It indicates that a Schottky junction is formed between the loaded cocatalysts and GaN semiconductor, which is capable of greatly promoting the electron−hole separation (50). Furthermore, the dramatic reduction in PL intensity of CuFe@GaN NWs as compared to Cu/GaN NWs suggests that the binary CuFe catalyst is more favorable than Cu catalyst to promote electron−hole separation of GaN NWs. Additionally, it is found that the light intensity affected the J−V curve significantly (SI Appendix, Fig. S9). The current density increased with the increasing intensity because more electron−hole pairs could be formed under illumination with higher intensity. In contrast, there is nearly no current observed in the dark during the entire potential range examined. These results suggest that light-driven generation of electron−hole pairs is a critical step for CO 2 RR. Moreover, control experiments confirm that the linear sweep voltammetry (LSV) behavior under CO 2 atmosphere is superior to that under argon atmosphere (SI Appendix, Fig. S10), which further suggests the strong adsorption and activation of CO 2 over the binary CuFe catalyst (39). Based on Faradaic efficiency measurements, both GaN NWs/Si and Fe/GaN NWs/Si do not produce any methane (Fig. 3B). Hydrogen was the main byproduct, with a trace amount of CO (Faradaic efficiency <1%). Although Cu is catalytically active for methane synthesis, Cu/GaN NWs/Si only shows a low Faradaic efficiency of ∼20%, which is consistent with previous work (28). In stark contrast, the binary CuFe catalyst gives rise to more than 2-fold improvement in Faradaic efficiency, to 51% with a high current density of −38.3 mA⋅cm−2. As a consequence, the partial current density of CuFe@GaN NWs/Si for CH 4 formation is as high as −19.5 mA⋅cm−2 (Fig. 3C), which is remarkably higher than the previously reported silicon photocathode for PEC CO 2 RR toward CH 4 (28, 29). The optimal productivity of CuFe@GaN NWs/Si for CH 4 approaches 88.8 μmol·h−1·cm−2, which is 3.7 times larger than that of Cu/GaN NWs/Si, while Fe/GaN NWs/Si did not show any productivity under the same experimental conditions (Fig. 3D). These results undoubtedly suggest that the binary CuFe catalyst plays a crucial role in promoting methane production. Electronic properties evaluation of Cu using X-ray photoelectron spectrum demonstrates a considerable shift of about +0.3 eV. Cu 2p 3/2 was shifted from 932.9 to 933.2 eV by incorporating Fe species, suggesting that Cu in CuFe@GaN NWs/Si is electron-deficient compared to Cu/GaN NWs/Si (SI Appendix, Fig. S11) (51). Such a notable change of electronic properties may contribute to tuning the catalytic properties of Cu (52), and thus facilitates the CO 2 RR toward methane. It is noted that there is an optimized CuFe catalyst for maximum activity and methane selectivity. At a low loading amount of ∼0.033 μmol·cm−2 with Fe/Cu ratio of 4.5/1 (SI Appendix, Fig. S12), the active sites of CuFe@GaN NWs/Si are insufficient for suppressing charge carrier surface recombination and improving the kinetics, resulting in limited activity (7). However, at higher Fe/Cu ratio of 12.9/1 with CuFe overloading of 0.075 μmol⋅cm−2 (SI Appendix, Fig. S13), the light absorption of the silicon semiconductor would be suppressed (53), and the inherent catalytic activities would be lowered (54) (SI Appendix, Figs. S14 and S15). Therefore, there is an appropriate loading amount of 0.041 μmol⋅cm−2 with Fe/Cu ratio of 6.3/1, enabling optimal optical and catalytic activity for highly efficient PEC CO 2 RR toward CH 4 . Fig. 3. Photoelectrocatalytic performance measurements. J−V curves (A), Faradaic efficiencies (B), partial current density (C), and CH 4 productivity (D) of GaN NWs/Si, Cu/GaN NWs/Si, Fe/GaN NWs/Si, and CuFe@GaN NWs/Si. The gray curve in A corresponds to CuFe@GaN NWs/Si under dark. Variations of Faradaic efficiencies (E) and turnover frequency (F) for methane synthesis versus applied bias for CuFe@GaN NWs/Si. Experimental conditions: CO 2 -purged 0.5 M KHCO 3 aqueous solution (pH ≈ 8), one-sun illumination (AM 1.5G, 100 mW⋅cm−2). The dependence of Faradaic efficiency on the applied potentials is studied, and the results are illustrated in Fig. 3E. It is discovered that the applied potentials play a significant role in the Faradaic efficiency. The onset of CuFe@GaN NWs/Si for methane synthesis is −0.4 V with a methane Faradaic efficiency of 1.2%, which is more positive than that of −0.7 V for Cu alone. It reveals that a significantly lower driving force (by as much as 0.3 V) is required for the binary CuFe catalyst for CO 2 reduction reaction. The underlying cause is that the binary CuFe catalyst can initially activate the stable CO 2 molecule and reduce the high energy barrier, which is in excellent agreement with the theoretical calculation. At potentials more positive than −0.4 V, the driving force is sufficient for hydrogen production but not for overcoming the high energy barrier for methane synthesis. Methane was hence not formed. As the potential shifts negatively, Faradaic efficiency of CH 4 formation is continuously improved with the increasing driving force and approaches a maximum of 51% at −1.2 V. A more negative potential, however, leads to a mild reduction in Faradaic efficiency to 42% because of the severe competition of hydrogen evolution under high overpotential as well as the CO 2 mass transport limitation (55, 56). High turnover frequency is one distinct highlight of this work. As shown in Fig. 3F, an appreciable TOF of 9.5 h−1 is achieved under standard one-sun illumination at the onset potential of −0.4 V. The negative shift of potential results in increasing TOF. At −1.2 V, a maximum TOF, which is as high as 2,176 h−1, is achieved at a high current density of −38.3 mA⋅cm−2 and high Faradaic efficiency of up to 51% despite a slight reduction at more negative potential. Herein, the superior TOF mainly originates from the unique synergy of Cu and Fe in the binary catalytic system. Additionally, the pronounced sunlight absorption ability and efficient charge carrier extraction of the GaN/Si platform also play an important role, which will be discussed next.

CO 2 Conversion at the Interface over Cu(111) and Fe x O y /Cu(111). To gain fundamental insights into what underlines the superior performance of the binary CuFe catalyst, we have studied the reaction pathways, reaction intermediates, potential-determining steps (PDSs), and free-energy diagrams of the catalytic CO 2 RR to CH 4 on Fe 3 O 6 H 6 /Cu(111) in comparison with those on Cu(111). Fig. 4A shows the optimized structures of adsorption configuration for each reaction intermediate on Cu(111) and Fe 3 O 6 H 6 /Cu(111). On the Fe 3 O 6 H 6 /Cu(111), it was discovered that the interfacial sites directly participate in binding and stabilizing all of the reaction intermediates. Specifically, the O-bound species (*O and *OH) prefer to bind to reduced Fe2+ cation in the metal oxide with the η 1 − O F e 2 + configuration, while, for C- and O-bound species (species bound through both C and O, i.e., *COOH, *CO, *CHO, *CH 2 O, and *CH 3 O), the metal/oxide interfacial sites are favored with the η 2 − C C u O F e 2 + configuration. Consequently, the Fe 3 O 6 H 6 /Cu(111) interfacial sites are beneficial for methane synthesis via stabilizing all of the reaction intermediates during the complex 8-electron/proton coupling transfer process (30, 36). Fig. 4. Calculated free-energy diagrams for CO 2 RR on Cu(111) and Fe 3 O 6 H 6 /Cu(111) under zero (A) and applied electrode potentials (B). The values in B (i.e., 0.85 and 0.51 eV) show the potential-determining energy barriers that should be overcome for the CH 4 production on Cu(111) and Fe 3 O 6 H 6 /Cu(111). Cu, blue; Fe, orange; O, red; C, brown; and H, white. Fig. 4A demonstrates the free-energy diagram of the lowest-energy pathways of CO 2 reduction on the Cu(111) and Fe 3 O 6 H 6 /Cu(111) under zero electrode potential (U = 0 V). For the case of Cu(111), the protonation of CO species (i.e., *CO → *CHO) is the PDS, exhibiting a free-energy change of 0.85 eV. On the other hand, for CO 2 at the interface of Fe 3 O 6 H 6 /Cu(111), the PDS remains the same, but with an appreciably reduced free-energy change of 0.51 eV. By increasing the stability of the *CHO species relative to *CO, it is expected that the energy efficiency of PEC reduction of CO 2 on the Fe 3 O 6 H 6 /Cu(111) interface would surpass the pure metals, due to the various structures with complementary chemical properties in the metal/oxide interfacial sites that work in synergy to facilitate the CO 2 reduction into CH 4 (39, 42). Meanwhile, it is worth noting that Fe 3 O 6 H 6 /Cu(111) may hinder further reaction steps toward oxygen reduction due to an increased free-energy change associated with the proton/electron transfer step of *OH [i.e., *OH protonation to H 2 O(g)] in Fig. 4A. For this step, the Cu(111) surface requires 0.14 eV, while the Fe 3 O 6 H 6 /Cu(111) demands 0.33 eV. Nonetheless, it would not alter the PDSs of the CO 2 reduction on the Cu(111) surface and Fe 3 O 6 H 6 /Cu(111) interface with both of them laying in the *CO/*CHO step. Fig. 4B shows the corresponding free-energy diagrams of CO 2 reduction at applied electrode potentials of U = – 0.85 and –0.51 V for the Cu(111) and Fe 3 O 6 H 6 /Cu(111), respectively. These 2 electrode potentials are the required voltages for eliminating the free-energy change of the PDSs (*CO/*CHO). It illustrates that the CH 4 -forming reaction from CO 2 might occur at −0.85 and −0.51 V (vs. RHE) on the Cu(111) surface and Fe 3 O 6 H 6 /Cu(111) interface, respectively. It suggests that, for methane synthesis, the onset potential of Fe 3 O 6 H 6 /Cu(111) is 0.34 V more positive than that of Cu(111), which is in excellent agreement with the experimental results that the onset of the binary CuFe catalyst is 0.3 V lower than that of Cu. In addition to Fe 3 O 6 H 6 /Cu(111), we have also investigated CO 2 RR at other possible hydrogenated Fe x O y /Cu interfaces, that is, Fe 3 O 3 H 3 /Cu(111) and Fe 6 O 7 H 7 /Cu(111). The results show that the reaction energetics on Fe 3 O 3 H 3 /Cu(111) and Fe 6 O 7 H 7 /Cu(111) are similar to that of Fe 3 O 6 H 6 /Cu(111) (SI Appendix, Fig. S16). Additionally, to consider the effect of partially oxidization on Cu as characterized in the XPS data, we have also conducted a series of DFT calculations by constructing iron oxide clusters with varying atomic ratios of Fe, Cu, and O on the surface of partially oxidized Cu, that is, Fe x O y /Cu 2 O(111), similar to the cases of Cu(111). A similar conclusion has been found on the Fe x O y /Cu 2 O(111) interfaces; that is, in spite of quantitative variations among different systems, the similar qualitative trend confirms the critical role of Fe x O y /Cu or Fe x O y /Cu 2 O(111) interface in activating CO 2 and stabilizing the reaction intermediates to facilitate the CO 2 RR for methane synthesis (SI Appendix, Fig. S17). Specifically, the CO 2 RR on pristine Cu 2 O(111) is bottlenecked by the hydrogenation of both *CO to *CHO and *OH to H 2 O with a free-energy change for PDS being 1.02 and 1.12 eV, respectively. In contrast, free-energy change of the hydrogenation of *CO to *CHO has been lowered to 0.89, 0.76, and 0.63 eV on Fe 3 O 3 H 3 /Cu 2 O(111), Fe 3 O 6 H 6 /Cu 2 O(111), and Fe 6 O 7 H 7 /Cu 2 O(111), respectively (SI Appendix, Fig. S18). And the free-energy change for another PDS of hydrogenation of *OH to H 2 O has also been decreased due to a selective destabilization for the reaction intermediate of *OH. It is worth noting that the reaction mechanism of Fe x O y H z /Cu 2 O(111) is presumably the same as that of Fe x O y H z /Cu(111), since all of the reaction intermediates share similar adsorption configurations and react with the Cu atoms on Cu 2 O(111) surface.