Superexchange-stabilized long-distance Cu sites in rock-salt-ordered double perovskite oxides for CO2 electromethanation

Cu-oxide-based catalysts are promising for CO2 electroreduction (CO2RR) to CH4, but suffer from inevitable reduction (to metallic Cu) and uncontrollable structural collapse. Here we report Cu-based rock-salt-ordered double perovskite oxides with superexchange-stabilized long-distance Cu sites for efficient and stable CO2-to-CH4 conversion. For the proof-of-concept catalyst of Sr2CuWO6, its corner-linked CuO6 and WO6 octahedral motifs alternate in all three crystallographic dimensions, creating sufficiently long Cu-Cu distances (at least 5.4 Å) and introducing marked superexchange interaction mainly manifested by O-anion-mediated electron transfer (from Cu to W sites). In CO2RR, the Sr2CuWO6 exhibits significant improvements (up to 14.1 folds) in activity and selectivity for CH4, together with well boosted stability, relative to a physical-mixture counterpart of CuO/WO3. Moreover, the Sr2CuWO6 is the most effective Cu-based-perovskite catalyst for CO2 methanation, achieving a remarkable selectivity of 73.1% at 400 mA cm−2 for CH4. Our experiments and theoretical calculations highlight the long Cu-Cu distances promoting *CO hydrogenation and the superexchange interaction stabilizing Cu sites as responsible for the superb performance.


Results
Crystal structure and long Cu-Cu distances The Sr 2 CuWO 6 catalyst was synthesized through a facile and scalable solid-state reaction (combined high-energy ball milling) process.Note that, according to the tolerance factor rule 10 , another alkaline-earth metal cation, i.e., Ba 2+ , can also be selected as the A-site cation to form a double perovskite of Ba 2 CuWO 6 .Since our work mainly focused on uncovering the key roles of superexchange-stabilized long-distance Cu sites in enhancing CO 2 RR property, either Sr 2 CuWO 6 or Ba 2 CuWO 6 can serve as the model catalyst in our work.For the proof of concept, here we designed and synthesized one of these two, i.e., Sr 2 CuWO 6 .The asprepared sample had uniform particle size with an average value of around 300 nm together with a specific surface area of about 3 m 2 g −1 (Supplementary Fig. 1).According to the inductively coupled plasma mass spectroscopy analysis, the chemical constituent of the Sr 2 CuWO 6 sample was compatible with its nominal compositions (Supplementary Table 1).Figure 1a shows the X-ray diffraction (XRD) pattern and corresponding Rietveld refinement analysis (Supplementary Table 2) of the Sr 2 CuWO 6 sample.The Sr 2 CuWO 6 was characterized by a pure tetragonal B-site rock-salt-ordered double perovskite phase that was indexed to a space group of I4/m with lattice parameters of a = 5.436 Å and c = 8.400 Å 35 .Here we also showed the crystal structure of Sr 2 CuWO 6 in Fig. 1a.The structure consisted of alternating cornersharing WO 6 and Jahn-Teller distorted CuO 6 octahedra (with short Cu-O ab bonds in the ab-plane and long Cu-O c bonds along the c-axis), with Sr cations situated at the void positions between these octahedra.As a result, the probably nearest Cu cations were far apart from each other, with two different distances of 5.4 and 5.7 Å that were induced by the Jahn-Teller distortion of CuO 6 octahedra 35 (Fig. 1b and Supplementary Fig. 2).These distances between adjacent Cu species were far enough to inhibit the C-C coupling and facilitate CO 2 -to-CH 4 conversion, as to be discussed below.
We validated the crystal structure of Sr 2 CuWO 6 using highresolution transmission electron microscopy (HRTEM) and a selectedarea electron-diffraction (SAED) pattern along the [1 10] zone axis.In Fig. 1c-e, the tetragonal phase was observed, presenting clear crystal fringes with interplanar spacings of about 0.284 and 0.420 nm, corresponding to its (112) and (002) diffraction planes, respectively.Raman spectra further suggested the phase structure of Sr 2 CuWO 6 crystallized with tetragonal I4/m symmetry (Supplementary Fig. 3) 36 .The energy dispersive X-ray (EDX) mappings in Fig. 1f suggested the existence and homogeneous distribution of Sr, Cu, W, and O elements in the sample.Wide-scan X-ray photoelectron spectra (XPS) (Supplementary Fig. 4) also indicated that the sample was composed of the Sr, Cu, W, and O elements, without any detected signal of other elements except the reference C element.

Superexchange interaction
We conducted XPS and synchrotron-based X-ray absorption spectra (XAS) to explore electronic structure information and superexchange interaction of Sr 2 CuWO 6 catalyst.A physical mixture of CuO/WO 3 was prepared as a control sample (Supplementary Fig. 5), carrying the same molar ratio of Cu and W elements as the Sr 2 CuWO 6 .Figure 2a, b shows Cu 2p and W 4f spectra of the Sr 2 CuWO 6 .The peaks at 934.1 and 35.1 eV could be assigned to Cu 2+ 2p 3/2 and W 6+ 4f 7/2 , respectively, illustrating the approximate valence states of Cu (+2) and W (+6) in Sr 2 CuWO 6 .Relative to the CuO/WO 3 , the Cu 2+ 2p 3/2 peak of Sr 2 CuWO 6 shifted 0.33 eV to higher binding energy, whereas their W 6+ 4f 7/2 peak underwent a negative shift of 0.26 eV.Such XPS peak shifts preliminarily suggest that there is electron redistribution (from Cu 2+ to W 6+ ) in the Sr 2 CuWO 6 .
Figure 2c, d shows the normalized Cu K-edge and W L 3 -edge X-ray absorption near-edge structure spectra (XANES) for Sr 2 CuWO 6 .The absorption edges (i.e., Cu K-edge and W L 3 -edge) of Sr 2 CuWO 6 were nearly identical to those of CuO and WO 3 references, respectively, confirming the valence states of Cu and W species in Sr 2 CuWO 6 close to +2 and +6.In the enlarged spectrum of the Cu K-edge (Fig. 2e), a positive-energy shift and higher white-line peak intensity were observed for the Sr 2 CuWO 6 , as compared to the CuO reference, indicative of the existence of a higher valence state of Cu species in Sr 2 CuWO 6 .On the contrary, the spectrum of the W L 3 -edge for Sr 2 CuWO 6 exhibited a slight shift towards lower energy and a weaker white-line peak intensity relative to the WO 3 reference (Fig. 2f), indicating a minor reduction of W valence state in Sr 2 CuWO 6 .These XAS results demonstrate electron interaction between CuO 6 and WO 6 octahedra or electron transfer in the direction from Cu to W species in the Sr 2 CuWO 6 .Combined with the above crystal structure characterization, one can believe that this electron transfer between rock-saltordered CuO 6 and WO 6 octahedra must be mediated by the intermediate oxygen anions, thus being defined as a superexchange interaction.
We further performed Bader charge analysis to investigate charge density redistribution.In Fig. 2g, the light-blue regions, surrounding the Cu, O, and W sites, clearly depicted the Cu-O-W charge transfer channels.The Bader charges of Cu and W sites in Sr 2 CuWO 6 were calculated to be 1.27 and 2.95 |e|, respectively, which were different from 1.08 |e| for Cu sites in CuO and 3.08 |e| for W sites in WO 3 (Supplementary Table 3).These phenomena also indicate the charge redistribution from Cu to W sites (mediated by O sites) in Sr 2 CuWO 6 , consistent with the XPS and XAS results.Thus, the B-site rock-saltordered double perovskite lattice was endowed with significant superexchange interaction (Cu-O-W) between alternate CuO 6 and WO 6 octahedra, mainly characterized by the O-anion-mediated electron transfer from Cu to W cations (Cu 2+ + W 6+ → Cu >2+ + W <6+ ), as schematically illustrated in Fig. 2h.As a result, we infer that the superexchange interaction could suppress the accumulation of electrode-supplied electrons around Cu sites via fast electron transport channels (light-blue regions in Fig. 2g), thereby protecting the Cu sites during CO 2 RR.Besides, in light of the increased valence state (or electronegativity) of Cu sites reducing the electronegativity difference between Cu and O sites, the superexchange interaction could strengthen Cu-O bond covalency and thus maintain the Cu-O lattice integrity during CO 2 RR.We proved the strengthened Cu-O bond covalency by the computed density of states (DOS) and band centers of Cu 3d and O 2p (Fig. 2i and Supplementary Fig. 6), using CuO as a reference.

Activity and selectivity for CH 4
We carried out density functional theory (DFT) calculations to predict CO 2 RR properties of Sr 2 CuWO 6 catalyst (Fig. 3a, Supplementary Figs. 7  and 8).The DFT calculations were implemented on CuO 2 /WO 2 -terminated Sr 2 CuWO 6 (001) surface (Supplementary Fig. 7) since such a surface was usually observed and stable 37,38 .We took the full reaction pathways for CH 4 and C 2 H 4 formation starting from *CO as analysis objects and calculated their corresponding energy profiles at the Cu sites 39,40 .On the Sr 2 CuWO 6 (001) surface, the energy difference between *CO and *CHO was about 0.64 eV, much lower than the energy barrier (1.08 eV) for C 2 H 4 production (i.e., 2*CO to the TS) (Fig. 3a and Supplementary Fig. 8).As a result, CH 4 formation was more favorable on the Sr 2 CuWO 6 (001) surface based on the presumption that the energy of TS for the *CO hydrogenation was not significantly different from the energy of *CO step.This could be ascribed to the fact that the long Cu-Cu distances (at least 5.4 Å) on Sr 2 CuWO 6 (001) surface were able to intensify the single-atomic feature of Cu, thereby inhibiting the C-C coupling but facilitating the CH 4 production.To this end, associated with its actual physicochemical properties, we can predict that the Sr 2 CuWO 6 catalyst with B-site rock-salt-ordered structure will offer remarkable activity and selectivity toward CO 2 -to-CH 4 conversion.
We preliminarily checked the probability of CO 2 RR occurring over the Sr 2 CuWO 6 catalyst by linear sweep voltammogram (LSV) curves recorded in a CO 2 -and Ar-flowed liquid-electrolyte (1 M KOH) flow cell, respectively (Supplementary Figs. 9 and 10).Relative to Ar-flowed electrolyte, there were higher current densities as well as a less negative onset potential in CO 2 -flowed electrolyte, suggesting that the Sr 2 CuWO 6 catalyst is indeed active toward CO 2 RR.We then systematically evaluated CO 2 RR properties of the Sr 2 CuWO 6 catalyst at various applied current densities in CO 2 -flowed liquid-electrolyte (1 M KOH) flow cell (Fig. 3b, Supplementary Figs.11 and 12).As a note, the 1 M KOH was adopted as the electrolyte in the flow cell because it was able to improve charge transfer, inhibit HER, and thus give rise to marked improvements in CO 2 RR activity and selectivity, relative to the bicarbonate/carbonate electrolytes 13 .In the applied current density range (from 100 to 600 mA cm −2 ), the main product was CH 4 , with high Faradaic efficiencies (FEs) more than 42.3% (Fig. 3b).At a current density of 400 mA cm −2 , the CH 4 product displayed a maximum FE of 73.1%, corresponding to a high partial current density of 292.4 mA cm −2 exceeding the industrial-level requirements (>200 mA cm −2 ) (Fig. 3b and Supplementary Fig. 13).Meanwhile, the FE C 2 H 4 and FE liquid C2+ ranged from 2.2% to 7.1% (Fig. 3b and Supplementary Fig. 12), indicating an efficient suppression of C-C coupling.These results reveal that upon serving as a catalyst toward CO 2 RR, the Sr 2 CuWO 6 is prone to generate CH 4 rather than C 2 H 4 , in line with the above DFT calculations (Fig. 3a).We also benchmarked the CO 2 RR properties of the Sr 2 CuWO 6 against the CuO/WO 3 .The detailed CO 2 RR properties of the CuO/WO 3 were shown in Supplementary Fig. 14.The Sr 2 CuWO 6 significantly promoted CO 2 -to-CH 4 conversion, whereas its physical-mixture counterpart enhanced C-C coupling (similar to oxide-derived Cu catalysts 23,24 ).To be specific, in the applied current density range, relative to the CuO/WO 3 , the Sr 2 CuWO 6 exhibited 3.7-to 14.1-fold higher FE CH 4 or j CH 4 (Fig. 3c and Supplementary Fig. 13), together with much lower FE C 2 H 4 or j C 2 H 4 (Supplementary Fig. 15).And the values (10.8-26.2) of FE CH 4 =FE C 2 H 4 or j CH 4 =j C 2 H 4 for the Sr 2 CuWO 6 were almost 13.2-59.8times higher than those (0.44-1.98) for the CuO/WO 3 (Fig. 3d).Combined with the above physicochemical property characterizations (Figs. 1 and 2) and DFT calculations (Fig. 3a), we could attribute these results to the sufficient-long Cu-Cu distances of Sr 2 CuWO 6 that regulated the adsorption/activation of key intermediates, thus inhibiting C-C dimerization and promoting *CO hydrogenation.We compared the FE CH 4 and j CH 4 of Sr 2 CuWO 6 catalyst with those of the reported Cu-based perovskite oxides (Fig. 3e and Supplementary Table 4).The Sr 2 CuWO 6 performed much better than all these perovskites reported in the literature.For instance, the j CH 4 of Sr 2 CuWO 6 was about 2.5-1562.5 times higher than that of the reported perovskite-based catalysts.To our knowledge, the Sr 2 CuWO 6 was the most effective Cu-based-perovskite catalyst for CO 2 -to-CH 4 conversion.Moreover, Supplementary Fig. 16 highlights that the activity and selectivity for CH 4 of Sr 2 CuWO 6 are comparable to or higher than those of most reported representative Cu-based catalysts in flow cells (Supplementary Table 5).

Cu sites stabilized by superexchange interaction
We performed a series of ex-situ and in-situ characterizations to investigate the structural evolution of Sr 2 CuWO 6 and stabilization of Cu sites during CO 2 RR (Fig. 4).The reduction tolerance of Sr 2 CuWO 6 was probed under a high-temperature reducing atmosphere.At 300 °C (in H 2 /Ar for 1 h), the Cu 2+ in Sr 2 CuWO 6 was reduced to Cu + , instead of metallic Cu (Fig. 4a and Supplementary Fig. 17), with the generation of oxygen vacancies (Supplementary Fig. 18 and Supplementary Table 6).Whereas the CuO/WO 3 was gradually reduced to Cu 2 O/WO 3 (at 250 °C) and Cu/WO 3 (at 300 °C) (Supplementary Fig. 19).According to the Rietveld refinement analysis (Supplementary Fig. 20 and Supplementary Table 7), the Sr 2 CuWO 6 underwent a phase transition from I4/m to Fm-3m during thermochemical reduction but still belonged to the category of B-site rock-salt-ordered double perovskites (Fig. 4a, b) 41 .This phase transition could be ascribed to lattice expansion of CuO 6 octahedra induced by reduction of smaller-size Cu 2+ (0.87 Å) to largersize Cu + (0.91 Å).Notably, in the newly generated structure, the Cu-Cu distance (about 5.8 Å) was still very long (Fig. 4c and Supplementary Fig. 21), and the superexchange interaction (Cu-O-W) could still exist   4).The error bars represent the mean ± standard deviation (SD, n = 3 replicates).
or even be strengthened due to the easier electron transfer from Cu + to W 6+ sites relative to that from Cu 2+ to W 6+ sites (Supplementary Fig. 22).These results indicate that the superexchange interaction can inhibit deep reduction of the Cu sites and thus avoid structural collapse of the Sr 2 CuWO 6 .These may partly imply the high structural stability of Sr 2 CuWO 6 during CO 2 RR.The possible structural changes of Sr 2 CuWO 6 after CO 2 RR were analyzed using ex-situ XRD and XPS (Fig. 4d-f).Similar to the thermochemical reduction, after CO 2 RR (e.g., at 200 mA cm -2 ), part of the I4/m phase of Sr 2 CuWO 6 was converted into Fm-3m phase (Fig. 4d), without detectable impurity, and the Cu 2+ and part W 6+ species on the surface were reduced to Cu + and W 5+ , respectively (Fig. 4e, f, Supplementary Fig. 23, and Supplementary Table 8).This suggests that the Cu + species (in the Fm-3m phase) might be active sites for CO 2 methanation.By contrast, as fully evidenced by previous studies, the CuO (in CuO/WO 3 ) can completely be reduced to metallic Cu under similar CO 2 RR conditions 26,27 .As a result, the Cu sites of oxidation states in double perovskite structure are well stabilized by the superexchange interaction during CO 2 RR.On this basis, we evaluated CO 2 RR stability of the Sr 2 CuWO 6 in comparison with the CuO/WO 3 through chronopotentiometric polarization in the CO 2 -flowed liquidelectrolyte flow cell (Fig. 4g).During 20,000 s of electrolysis (at 400 mA cm −2 ), for the Sr 2 CuWO 6 , the applied potential was stable at 1.23 ± 0.15 V vs. RHE, and the FE CH 4 was maintained at 64% ± 6%.Whereas the CuO/WO 3 showed severe deterioration in potentials and obvious fluctuations in FE CH 4 during 8000 s of electrolysis.These results demonstrate excellent CO 2 RR stability of the Sr 2 CuWO 6 .As a note, our gas diffusion layer suffered flooding issues when the CO 2 RR  stability test of Sr 2 CuWO 6 catalyst ran for more than 20,000 s.Since the flooding issues can cause an essential failure of the CO 2 RR system, we terminated the electrolysis on the Sr 2 CuWO 6 catalyst at about 20,000 s; but the steady CO 2 RR testing time for the Sr 2 CuWO 6 catalyst itself was supposed to be for more than 20,000 s by considering the well-stabilized Cu sites.Usually, the flooding issues can be mitigated by washing away the carbonate precipitation.This has been widely used to reactivate the electrodes 42,43 .
Moreover, the stability of Cu sites in CuO 6 octahedra of Sr 2 CuWO 6 during CO 2 RR was further verified by in-situ Raman spectroscopy in an operando electrolyzer (Supplementary Fig. 24).As expected, the in-situ Raman spectroscopic analyses were consistent with the above ex-situ characterizations.To be specific, as the applied potential negatively shifted (from −0.8 to −1.2 V vs. RHE), the Raman spectra of Sr 2 CuWO 6 displayed no change in characteristic peaks and no formation of any new peak (Supplementary Fig. 25).In addition, these characteristic peaks were also retained during 1200 s of electrolysis at −1.2 V vs. RHE (Fig. 4h).Note that, within the electrolysis time, only a new peak at about 308.2 cm −1 appeared (Fig. 4h), possibly originating from the electrochemical reduction of W 6+ to W 5+44 .It was also observed that a main characteristic peak at 864.1 cm −1 gradually moved to lower Raman shifts (Fig. 4h), probably corresponding to the reduction-induced lattice expansion and phase transition (from I4/m to Fm-3m) as mentioned above 36 .However, for the CuO/WO 3 during 1200 s of electrolysis (Fig. 4i), the characteristic peaks at 278.1 and 331.2 cm −1 of CuO gradually disappeared, while a characteristic peak at 613.6 cm −1 of Cu 2 O appeared and then disappeared, and the intensity of Cu-CO peaks at 294.2 and 385.6 cm −1 was gradually improved 45 .These indicate that the CuO in CuO/WO 3 is initially reduced to Cu 2 O and then to metallic Cu.
In light of the above analyses, we plotted out the structural evolution diagrams to graphically describe the key role of superexchange interaction in stabilizing the Cu sites in Sr 2 CuWO 6 during CO 2 RR (Fig. 5).As shown in Fig. 5a, once the CO 2 RR was initiated, the Cu 2+ on Sr 2 CuWO 6 surface began to be reduced to Cu + by the electrodesupplied electrons.At the same time, the Sr 2 CuWO 6 surface was transformed from the I4/m to Fm-3m phase, still being the B-site rocksalt ordered structure.Further reduction was not able to convert the Cu + to Cu 0 but rather led to the transformation of W 6+ to W 5+ on the surface while maintaining the Fm-3m phase.The most plausible reason was that, during CO 2 RR, the superexchange interaction effectively transferred the electrode-supplied excessive electrons accumulated around the Cu + sites to W 6+ sites (to form W 5+ ) through the fast electron transport channels (Fig. 2g), thereby protecting the Cu sites from electron attack and preserving the double perovskite phase.By contrast, the CuO/WO 3 without superexchange interaction was successively reduced to Cu 2 O/WO 3 and Cu/WO 3 (Fig. 5b).Taken together, during CO 2 RR, although these changes occurred on the Sr 2 CuWO 6 surface, the superexchange interaction prevented structural collapse, stabilized the Cu + sites, and maintained the long Cu-Cu distances, thereby promoting the efficient and stable CO 2 -to-CH 4 conversion.

Discussion
Employing Sr 2 CuWO 6 as the proof-of-concept catalyst, we have developed Cu-based rock-salt-ordered double perovskite oxides for efficient and stable CO 2 -to-CH 4 conversion and uncovered the key roles of their unique physicochemical properties in boosting activity, selectivity, and stability toward CH 4 production.In the rock-saltordered structure, the corner-linked CuO 6 and WO 6 octahedra alternated in all three crystallographic dimensions, leading to sufficiently long Cu-Cu distances (at least 5.4 Å) and marked Cu-O-W superexchange interaction.When explored as a catalyst toward CO 2 RR, relative to its physical-mixture counterpart, the Sr 2 CuWO 6 featured not only enhancements in terms of activity and selectivity for CH 4 but also significantly boosted stability.Moreover, the Sr 2 CuWO 6 was the most effective Cu-based-perovskite catalyst for CO 2 methanation and performed comparably to or better than most reported representative Cu-based catalysts.According to the experiments and theoretical calculations, the superb performance could be attributed to the following factors: (i) the long-distance Cu sites facilitating *CO hydrogenation while inhibiting C-C coupling; (ii) the superexchange interaction stabilizing the Cu sites and preventing structural collapse.This work discovered efficient and stable Cu-based double perovskite oxides for CO 2 RR, providing a new avenue for the rational design of more advanced Cu-based catalysts.

Synthesis
Sr 2 CuWO 6 was synthesized by a high-temperature solid-state reaction/ high-energy ball-milling process.In a typical procedure, stoichiometric SrCO 3 , CuO, and WO 3 were well mixed by ball-milling process and then calcined at 900 °C in Air for 12 h.The admixture was ground again and then pressed into pellets under the pressure of 10 MPa for subsequent re-calcination at 1050 °C in Air for 24 h.Finally, the as-prepared powder was ground with the high-energy ball-milling process (900 rpm) to obtain uniform nanoparticles.The high-temperature reducing-atmosphere treatments of Sr 2 CuWO 6 were processed in a sealed tube furnace in 10 vol% H 2 -Ar mixture with a flow rate of 20 mL min −1 for 1 h.

Theoretical calculations
First-principles calculations were carried out on the basis of periodic density functional theory (DFT) using a generalized gradient approximation within the Perdew-Burke-Ernzerh of exchange correction functional with Vienna ab initio simulation package (VASP) 46,47 .Geometry optimization was conducted in Sr 2 CuWO 6 , CuO, and WO 3 .The wave functions were constructed from the expansion of plane waves with an energy cutoff of 450 eV.Gamma-centered k-point of 3 × 3 × 1 has been used.The consistence tolerances for the geometry optimization were set as 1.0 × 10 −6 eV/atom for total energy and 0.02 eV/Å for force, respectively.In order to avoid the interaction between the two surfaces, a large vacuum gap of 15 Å has been selected in the periodically repeated slabs.Static calculations were conducted with a convergence condition of 1.0 × 10 −6 eV for density of state (DOS), Bader charge, and electron localization function analysis.The band center of Cu 3d or O 2p was calculated using the following equation 48 : where T(E) is the density of states (DOS) of orbitals.E corresponds to the occupied state ranges below the fermi energy level (E F ) in DOS.
Climbing image nudged elastic band (CI-NEB) was used for transition state searching.In free energy calculations, the entropic corrections and zero-point energy (ZPE) have been included.The free energy of species was calculated according to the standard formula: where ZPE is the zero-point energy, ΔH is the integrated heat capacity, T is the temperature of the product, and S is the entropy.

Characterization
X-ray diffraction (XRD) patterns were recorded by Rigaku Miniflex 600 (Hitachi) diffractometer with Cu Kα radiation (1.5418 Å).The Rietveld refinements of obtained data were conducted using FullProf software.Scanning electron microscopy (SEM) images were taken by a Hitachi S4800 microscope.Transmission electron microscopy (TEM) images were taken by a JEOL 2010F microscope (operated at 200 kV).To further confirm the structure and elements distribution, highresolution TEM (HRTEM) and energy dispersive X-ray (EDX) spectra/ mappings were performed on a JEOL ARM 300 F microscope equipped with dual EDX detectors.X-ray photoelectron spectroscopy (XPS) analyses were carried out by the Thermo ESCALAB 250Xi spectrometer with monochromated Al Kα radiation (hv = 1486.6eV) operating at 150 W. The energies of each element were calibrated by the adventitious C1s (284.8 eV).Raman spectra were performed on a Renishaw Qontor spectrometer equipped with a 532 nm laser beam and a ×63 water-immersion objective lens.X-ray absorption spectroscopy (XAS) of Cu K-edge and W L 3 -edge were obtained in a Singapore synchrotron light source (SSLS), using an XAFCA Beamline (operated at 700 MeV) with a maximum current of 200 mA.The reference samples, such as CuO, Cu foil, WO 3 , and W foil were also measured for comparison and energy calibration.All XANES data were measured in transmission mode using an ion chamber detector with a Si 111 monochromator and analyzed by the Athena program 49 .The nitrogen adsorption and desorption processes were recorded on an Autosorb-iO (Quantachrome) device at the boiling point of liquid nitrogen to calculate the specific surface areas by the Brunauer-Emmett-Teller (BET) method.The inductively coupled plasma mass spectrometer (ICP-MS) (Agilent 730) was applied to test the metal contents of Sr 2 CuWO 6 .The samples for ICP-MS analysis were obtained by dissolving 10 mg sample powder with the mixture of 5 mL HNO 3 , 1 mL H 2 O 2 , 1 mL HCl, and 0.5 mL HF in the oven at 180 °C for 8 h.The cooled-down solution was further diluted to a level of 100 ppb by using a 1% HNO 3 solution.

Preparation of working electrodes
The working electrodes were prepared by coating the catalyst ink onto the hydrophobic carbon paper (i.e. the gas diffusion layer, GDL).To be specific, for the preparation of the catalyst ink, 10 mg sample powder was homogenously dispersed into a mixed solution of isopropanol (1 mL) and Nafion (50 μL) by ultrasonic processing for 1 h.The catalyst ink was then coated on the hydrophobic carbon paper (Toray, YLS-30T, 1.5 × 1.5 cm 2 ) with a loading amount of 0.5 mg cm −2 and dried under the infrared lamp.This method was used to prepare the working electrodes for both electrochemical measurements and the in-situ Raman spectroscopic measurements.

Electrochemical measurement
The CO 2 electrochemical reduction measurements were processed in a homemade flow cell with a three-electrode system controlled by a CS310M electrochemical workstation (Wuhan, Corrtest).The Ag/AgCl electrode (filled with saturated KCl solution) and Pt mesh were used as the reference and counter electrodes, respectively.1 M KOH was used as the electrolyte, filling, and cycling in the flow cell with a pumped rate of 20 mL min −1 controlled by a double channel peristaltic pump (Hebei, Leadfluid, BQ80s).An anion-exchange membrane (Hefei, ChemJoy Polymer Materials Co., Ltd., SYMA-2) was used for separating the anodic and cathodic compartments to avoid crossover pollution.High-purity CO 2 gas was continuously supplied into the gas chamber with a flow rate of 35 mL min −1 controlled by a mass flow controller (D07-19B, Sevenstar Electronics Co., Ltd, Beijing) and the flow rate was further verified by a soap bubble flowmeter.The LSV curves were also recorded in the flow-cell configuration flowed with Ar or CO 2 gas at a scan rate of 10 mV s −1 .All applied potentials were converted into the standard reversible hydrogen electrode (RHE) potentials by the equation of E RHE = E Ag/AgCl + 0.197 V + 0.0591 V × pH, with 70% iR compensation.The cell resistance was measured using the function of Rs measurement in the measurement soft of Corrtest CS310M electrochemical workstation (the value at 10,000 HZ from the electrochemical impedance spectroscopy) under open circuit potentials before every independence test.

Quantification of products
The gas products were detected by online gas chromatography (GC2060, Ramiin, Shanghai) equipped with flame ionization (FID) and thermal conductivity (TCD) detectors.A standard gas mixture (containing 1 vol% each of H more than three parallel experiments.After the reaction, the catholyte was collected for liquid product analyses by a Nuclear magnetic resonance spectrometer (NMR, Bruker, AVANCE-III 600 Hz).Typically, 2 mL catholyte was mixed with 100 μL 5 mM DMSO (as an internal standard substance).And then 250 μL mixture was mixed with 350 μL D 2 O for the NMR measurement.The FEs of the products were calculated by the following equation: where Q product and Q total present the charge consumption of the target product and CO 2 RR process, respectively, n presents the electron transfer number of the target product, N presents the amount of substance for the product and can be calculated from the product concentration, and F presents the faradaic content (96,485 C mol −1 ).

In-situ Raman test
The in-situ Raman test was processed in an electrochemical operando cell (C031-2, Tianjin Gaoss Union Technology Co. Ltd.) with a threeelectrode system.The in-situ Raman spectra were recorded by the Renishaw Qontor spectrometer using a 532 nm laser beam and a ×63 water-immersion objective lens.The Ag/AgCl (filled with saturated KCl solution) and graphite electrodes were used as reference and counter electrodes, respectively.The carbon paper coated with catalyst ink was used as the cathode, immersed in the CO 2 -saturated 0.1 M KHCO 3 electrolyte.The 0.1 M KHCO 3 was filled in the cathodic compartment while flowing in the anodic compartment with a flow rate of 20 mL min −1 to remove bubbles.The water-immersion objective lens was immersed in the cathodic compartment to directly observe the surface of the catalyst.The cathodic and anodic compartments were separated by a Nafion 117 proton exchange membrane.The power of the laser was kept at 1 mW to avoid irradiation damage on the catalyst.The surface exposure time was 20 s, and each signal line was collected twice.All Raman raw data were recorded and processed by Wire soft.

Fig. 1 |
Fig. 1 | Crystal structure and composition of Sr 2 CuWO 6 .a Rietveld refinement plot of XRD data and schematic illustrations of crystal structure for Sr 2 CuWO 6 .Sr, Cu, W, and O are represented by green, blue, gray, and red dots, respectively.The blue and gray octahedra represent CuO 6 and WO 6 motifs, respectively.b Schematic illustrations of distances between the probably nearest Cu cations.c HRTEM image of Sr 2 CuWO 6 (scale bar: 10 nm).d Enlarged HRTEM image of Sr 2 CuWO 6 taken from the region marked in (c) (scale bar: 2 nm).e SAED pattern of Sr 2 CuWO 6 (scale bar: 5 1/nm).f EDX mappings of Sr 2 CuWO 6 (scale bar: 100 nm).

Fig. 2 |
Fig. 2 | Superexchange interaction in Sr 2 CuWO 6 .a Cu 2p XPS spectra of Sr 2 CuWO 6 and CuO/WO 3 .b W 4f XPS spectra of Sr 2 CuWO 6 and CuO/WO 3 .c Cu K-edge XANES spectra of Sr 2 CuWO 6 .d W L 3 -edge XANES spectra of Sr 2 CuWO 6 .e Enlargement of Cu K-edge XANES spectra.f Enlargement of W L 3 -edge XANES spectra.g Top view of charge distribution between CuO 6 and WO 6 octahedra in Sr 2 CuWO 6 .Cu, W, and O are represented by blue, gray, and red dots, respectively.The light-blue regions (surrounding the Cu, O, and W sites) depict the electron transfer channels.h Schematic illustration of Cu-O-W superexchange interaction (electron transfer from Cu to W cations mediated by O anions) in Sr 2 CuWO 6 .The blue and gray octahedra represent CuO 6 and WO 6 , respectively.i Schematic illustration of electronic DOS contributions (from O 2p and Cu 3d states) and Cu-O bond covalency for Sr 2 CuWO 6 and CuO.
Fig. 3 | Activity and selectivity for CH4 over Sr 2 CuWO 6 .a DFT-calculated energy diagrams for CH 4 and C 2 H 4 formation on Sr 2 CuWO 6 (001) surface starting with *CO (TS: transition state).b FEs for various gas products over Sr 2 CuWO 6 at different applied current densities.c FE CH 4 of Sr 2 CuWO 6 and CuO/WO 3 at different applied current densities.d FE CH 4 =FE C 2 H 4 or j CH 4 =j C 2 H 4 of Sr 2 CuWO 6 and CuO/WO 3 at

Fig. 4 |
Fig. 4 | Stability of the Cu sites.a XRD pattern of Sr 2 CuWO 6 after thermochemical reduction.b Schematic illustration of phase transition of Sr 2 CuWO 6 after reduction.Sr, Cu, W, and O are represented by green, blue, gray, and red dots, respectively.The red-dotted circle, blue, and gray octahedra represent oxygen vacancy, CuO 6 , and WO 6 , respectively.c Schematic illustration of the distances between the nearest Cu cations in Fm-3m phase of Sr 2 CuWO 6 .d XRD patterns of Sr 2 CuWO 6 after
Article https://doi.org/10.1038/s41467-024-45747-5Nature Communications | (2024) 15:1565 3 | Activity and selectivity for CH 4 over Sr 2 CuWO 6 .a DFT-calculated energy diagrams for CH 4 and C 2 H 4 formation on Sr 2 CuWO 6 (001) surface starting with *CO (TS: transition state).b FEs for various gas products over Sr 2 CuWO 6 at different applied current densities.c FE CH 4 of Sr 2 CuWO 6 and CuO/WO 3 at different applied current densities.d FE CH 4 =FE C 2 H 4 or j CH 4 =j C 2 H 4 of Sr 2 CuWO 6 and CuO/WO 3 at different applied current densities.e FE CH 4 and j CH 4 of Sr 2 CuWO 6 , in comparison with those of Cu-based perovskite oxides reported in the literature (Supplementary Table 2 CuWO 6 after reduction.Sr, Cu, W, and O are represented by green, blue, gray, and red dots, respectively.The red-dotted circle, blue, and gray octahedra represent oxygen vacancy, CuO 6 , and WO 6 , respectively.c Schematic illustration of the distances between the nearest Cu cations in Fm-3m phase of Sr 2 CuWO 6 .d XRD patterns of Sr 2 CuWO 6 after CO 2 RR at different current densities (GDE: gas diffusion layer).e Cu LMM XPS spectra of Sr 2 CuWO 6 after CO 2 RR at different current densities.f W XPS spectra of Sr 2 CuWO 6 after different current densities.g CO 2 RR stability test of Sr 2 CuWO 6 and CuO/WO 3 in a flow cell at 400 mA cm −2 .In-situ Raman spectra of h Sr 2 CuWO 6 and i CuO/WO 3 as a function of CO 2 RR time at −1.2 V vs. RHE (RHE: reversible hydrogen electrode).