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

CO₂ electroreduction (CO₂RR) into value-added chemical feedstocks and fuels, driven by local-generated renewable energy, is a highly promising strategy for realizing the carbon-neutral cycle together with earning potential economic returns^1,2,3,4. Among all CO₂RR products, CH₄ is of considerable interest based on its well-established infrastructure toward storage, distribution, and utilization^5,6. Up to date, mainly Cu-based catalysts are able to generate appreciable CH₄ via stabilizing and subsequently hydrogenating the *CO species during CO₂RR⁷. However, owing to the involvement of complicated 8-electron transfer steps and structural degradations (e.g., fragmentation, dissolution, agglomeration), most Cu-based catalysts (e.g., oxide-derived Cu) still suffer from unsatisfactory Faradaic efficiency for CH₄ and poor stability^8,9.

Perovskite oxides (typically ABO₃), featuring distinct merits (e.g., diverse chemical compositions, flexible crystal and electronic structures, and governable physicochemical properties), have provided an attractive platform for accessing high-performance catalysts toward numerous electrochemical reactions^10,11,12,13. Upon most occasions, the nature of B-site cations or B–O bonding determines the electrocatalytic properties of perovskite oxides in essence^14,15,16. Based on the above characteristics of Cu-based catalysts and perovskite oxides, if the B sites could be occupied entirely or partly by Cu element, the corresponding Cu-based perovskite oxides would be active toward CO₂ electromethanation^{17,18,19,20,21,22}. Typical examples involve Cu-based Ruddlesden–Popper perovskite oxides (e.g., La₂CuO_4−δ)^{17,18,19,20,21,22}. Nonetheless, these catalysts with B sites wholly occupied by Cu exhibit low activity and selectivity for CH₄, owing to the distance of their adjacent Cu sites not far enough to inhibit the competitive C–C coupling^{17,18,19,20,21,22}. Furthermore, like traditional Cu-based oxides (e.g., CuO and Cu₂O)^23,24,25, since the electrode-supplied electrons attack or break the Cu–O bond to reduce the Cu sites, these catalysts also undergo uncontrollable reconstructions (e.g., metallic Cu exsolution) during CO₂RR^20,21. Such reconstructions could make the active sites unmaintainable, causing lowered catalytic performance or even deactivation^8,26,27.

Substitution of another cation (B’) for B to form doped perovskite oxides (e.g., AB_1−xB’_xO₃) has been intensively proved as a tried-and-true strategy to optimize the catalytic performance of perovskite oxides^{10,11,14,15,16}. Accordingly, for Cu-based perovskite oxides, partly occupying their initial Cu sites by the doping cations (B’) could also availably modulate or enhance their catalytic properties toward CO₂-to-CH₄ conversion. In general, if the B’ and Cu cations are almost equal in molar content, while they are sufficiently different in size and/or charge, Cu-based double perovskite oxides (A₂CuB’O₆) with B-site rock-salt ordering will be produced^28,29,30. The formation of a double perovskite structure is very likely to introduce important benefits to the physicochemical properties, affecting activity, selectivity, and stability in CH₄ production^{28,29,30,31,32,33,34}. Specifically, in the rock-salt-type arrangement, the B-site cations alternate in all three crystallographic dimensions, markedly widening the distance between adjacent Cu cations, theoretically almost doubling relative to the undoped ones^28,29,30. This increased distance could suppress *CO dimerization and promote activity and/or selectivity for CH₄ production^31,32. Moreover, the B-site rock-salt ordering could bring superexchange interaction between Cu and B’ cations (mediated by intermediate O anions) and give rise to the redistribution of charge densities of the B-site cations via electron transfer^33,34. During CO₂RR, this superexchange interaction may transfer the electrode-supplied electrons accumulated around the Cu sites to B’ sites and stabilize the Cu sites, thereby boosting the catalytic stability. However, to our knowledge, such Cu-based double perovskite oxides have not been reported in CO₂RR, so the vital roles of their unique physicochemical properties in catalytic performance are yet to be fully uncovered.

Here we present Cu-based double perovskite oxides (A₂CuB’O₆) with B-site rock-salt ordering and superexchange interaction to facilitate efficient and stable CO₂-to-CH₄ conversion. As the proof of concept, we employed W⁶⁺ cations as the B’ sites, mainly because of their low-lying unoccupied 5d states that strongly hybridized with O 2p states, and synthesized a double perovskite oxide of Sr₂CuWO₆ as the model catalyst for CO₂RR. As expected, for the Sr₂CuWO₆, its corner-linked octahedra of CuO₆ and WO₆ were rock-salt ordered. This unique structure made the nearest Cu cations very far apart from each other with a minimum distance of 5.4 Å and introduced superexchange interaction that was mainly manifested by O-anion-mediated electron transfer from Cu to W cations. When evaluated as a catalyst toward CO₂RR, relative to its physical-mixture counterpart and the reported Cu-based perovskite oxides, the Sr₂CuWO₆ delivered remarkable enhancements in activity and selectivity for CH₄, together with boosted stability. Our experiments and theoretical calculations suggested that such performance improvements were mainly attributed to the following aspects: the sufficiently long Cu–Cu distances promoting *CO hydrogenation but inhibiting C–C coupling; the superexchange interaction transferring the electrons (around Cu sites) to W sites during CO₂RR and thus stabilizing the Cu sites (e.g., Cu⁺).

Crystal structure and long Cu–Cu distances

The Sr₂CuWO₆ catalyst was synthesized through a facile and scalable solid-state reaction (combined high-energy ball milling) process. Note that, according to the tolerance factor rule¹⁰, another alkaline-earth metal cation, i.e., Ba²⁺, can also be selected as the A-site cation to form a double perovskite of Ba₂CuWO₆. Since our work mainly focused on uncovering the key roles of superexchange-stabilized long-distance Cu sites in enhancing CO₂RR property, either Sr₂CuWO₆ or Ba₂CuWO₆ 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., Sr₂CuWO₆. The as-prepared sample had uniform particle size with an average value of around 300 nm together with a specific surface area of about 3 m² g⁻¹ (Supplementary Fig. 1). According to the inductively coupled plasma mass spectroscopy analysis, the chemical constituent of the Sr₂CuWO₆ 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 Sr₂CuWO₆ sample. The Sr₂CuWO₆ 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 ų⁵. Here we also showed the crystal structure of Sr₂CuWO₆ in Fig. 1a. The structure consisted of alternating corner-sharing WO₆ and Jahn–Teller distorted CuO₆ 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 CuO₆ octahedra³⁵ (Fig. 1b and Supplementary Fig. 2). These distances between adjacent Cu species were far enough to inhibit the C–C coupling and facilitate CO₂-to-CH₄ conversion, as to be discussed below.

a Rietveld refinement plot of XRD data and schematic illustrations of crystal structure for Sr₂CuWO₆. Sr, Cu, W, and O are represented by green, blue, gray, and red dots, respectively. The blue and gray octahedra represent CuO₆ and WO₆ motifs, respectively. b Schematic illustrations of distances between the probably nearest Cu cations. c HRTEM image of Sr₂CuWO₆ (scale bar: 10 nm). d Enlarged HRTEM image of Sr₂CuWO₆ taken from the region marked in (c) (scale bar: 2 nm). e SAED pattern of Sr₂CuWO₆ (scale bar: 5 1/nm). f EDX mappings of Sr₂CuWO₆ (scale bar: 100 nm).

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We validated the crystal structure of Sr₂CuWO₆ using high-resolution transmission electron microscopy (HRTEM) and a selected-area electron-diffraction (SAED) pattern along the [1\(\bar{1}\)0] 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 Sr₂CuWO₆ 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 Sr₂CuWO₆ catalyst. A physical mixture of CuO/WO₃ was prepared as a control sample (Supplementary Fig. 5), carrying the same molar ratio of Cu and W elements as the Sr₂CuWO₆. Figure 2a, b shows Cu 2p and W 4f spectra of the Sr₂CuWO₆. The peaks at 934.1 and 35.1 eV could be assigned to Cu²⁺ 2p_3/2 and W⁶⁺ 4f_7/2, respectively, illustrating the approximate valence states of Cu (+2) and W (+6) in Sr₂CuWO₆. Relative to the CuO/WO₃, the Cu²⁺ 2p_3/2 peak of Sr₂CuWO₆ shifted 0.33 eV to higher binding energy, whereas their W⁶⁺ 4f_7/2 peak underwent a negative shift of 0.26 eV. Such XPS peak shifts preliminarily suggest that there is electron redistribution (from Cu²⁺ to W⁶⁺) in the Sr₂CuWO₆.

a Cu 2p XPS spectra of Sr₂CuWO₆ and CuO/WO₃. b W 4f XPS spectra of Sr₂CuWO₆ and CuO/WO₃. c Cu K-edge XANES spectra of Sr₂CuWO₆. d W L₃-edge XANES spectra of Sr₂CuWO₆. e Enlargement of Cu K-edge XANES spectra. f Enlargement of W L₃-edge XANES spectra. g Top view of charge distribution between CuO₆ and WO₆ octahedra in Sr₂CuWO₆. 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 Sr₂CuWO₆. The blue and gray octahedra represent CuO₆ and WO₆, respectively. i Schematic illustration of electronic DOS contributions (from O 2p and Cu 3d states) and Cu–O bond covalency for Sr₂CuWO₆ and CuO.

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Figure 2c, d shows the normalized Cu K-edge and W L₃-edge X-ray absorption near-edge structure spectra (XANES) for Sr₂CuWO₆. The absorption edges (i.e., Cu K-edge and W L₃-edge) of Sr₂CuWO₆ were nearly identical to those of CuO and WO₃ references, respectively, confirming the valence states of Cu and W species in Sr₂CuWO₆ 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 Sr₂CuWO₆, as compared to the CuO reference, indicative of the existence of a higher valence state of Cu species in Sr₂CuWO₆. On the contrary, the spectrum of the W L₃-edge for Sr₂CuWO₆ exhibited a slight shift towards lower energy and a weaker white-line peak intensity relative to the WO₃ reference (Fig. 2f), indicating a minor reduction of W valence state in Sr₂CuWO₆. These XAS results demonstrate electron interaction between CuO₆ and WO₆ octahedra or electron transfer in the direction from Cu to W species in the Sr₂CuWO₆. Combined with the above crystal structure characterization, one can believe that this electron transfer between rock-salt-ordered CuO₆ and WO₆ 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 Sr₂CuWO₆ 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 WO₃ (Supplementary Table 3). These phenomena also indicate the charge redistribution from Cu to W sites (mediated by O sites) in Sr₂CuWO₆, consistent with the XPS and XAS results. Thus, the B-site rock-salt-ordered double perovskite lattice was endowed with significant superexchange interaction (Cu–O–W) between alternate CuO₆ and WO₆ octahedra, mainly characterized by the O-anion-mediated electron transfer from Cu to W cations (Cu²⁺ + W⁶⁺ → 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 CO₂RR. 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 CO₂RR. 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 CH₄

We carried out density functional theory (DFT) calculations to predict CO₂RR properties of Sr₂CuWO₆ catalyst (Fig. 3a, Supplementary Figs. 7 and 8). The DFT calculations were implemented on CuO₂/WO₂-terminated Sr₂CuWO₆(001) surface (Supplementary Fig. 7) since such a surface was usually observed and stable^37,38. We took the full reaction pathways for CH₄ and C₂H₄ formation starting from *CO as analysis objects and calculated their corresponding energy profiles at the Cu sites^39,40. On the Sr₂CuWO₆(001) surface, the energy difference between *CO and *CHO was about 0.64 eV, much lower than the energy barrier (1.08 eV) for C₂H₄ production (i.e., 2*CO to the TS) (Fig. 3a and Supplementary Fig. 8). As a result, CH₄ formation was more favorable on the Sr₂CuWO₆(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 Sr₂CuWO₆(001) surface were able to intensify the single-atomic feature of Cu, thereby inhibiting the C–C coupling but facilitating the CH₄ production. To this end, associated with its actual physicochemical properties, we can predict that the Sr₂CuWO₆ catalyst with B-site rock-salt-ordered structure will offer remarkable activity and selectivity toward CO₂-to-CH₄ conversion.

a DFT-calculated energy diagrams for CH₄ and C₂H₄ formation on Sr₂CuWO₆(001) surface starting with *CO (TS: transition state). b FEs for various gas products over Sr₂CuWO₆ at different applied current densities. c \({{{\mbox{FE}}}}_{{{\mbox{C}}}{{{\mbox{H}}}}_{4}}\) of Sr₂CuWO₆ and CuO/WO₃ at different applied current densities. d \({{{\mbox{FE}}}}_{{{\mbox{C}}}{{{\mbox{H}}}}_{4}}/{{{\mbox{FE}}}}_{{{{\mbox{C}}}}_{2}{{{\mbox{H}}}}_{4}}\) or \({j}_{{{\mbox{C}}}{{{\mbox{H}}}}_{4}}/{j}_{{{{\mbox{C}}}}_{2}{{{\mbox{H}}}}_{4}}\) of Sr₂CuWO₆ and CuO/WO₃ at different applied current densities. e \({{{\mbox{FE}}}}_{{{\mbox{C}}}{{{\mbox{H}}}}_{4}}\) and \({j}_{{{\mbox{C}}}{{{\mbox{H}}}}_{4}}\) of Sr₂CuWO₆, in comparison with those of Cu-based perovskite oxides reported in the literature (Supplementary Table 4). The error bars represent the mean ± standard deviation (SD, n = 3 replicates).

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We preliminarily checked the probability of CO₂RR occurring over the Sr₂CuWO₆ catalyst by linear sweep voltammogram (LSV) curves recorded in a CO₂- 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 CO₂-flowed electrolyte, suggesting that the Sr₂CuWO₆ catalyst is indeed active toward CO₂RR. We then systematically evaluated CO₂RR properties of the Sr₂CuWO₆ catalyst at various applied current densities in CO₂-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 CO₂RR activity and selectivity, relative to the bicarbonate/carbonate electrolytes¹³. In the applied current density range (from 100 to 600 mA cm⁻²), the main product was CH₄, with high Faradaic efficiencies (FEs) more than 42.3% (Fig. 3b). At a current density of 400 mA cm⁻², the CH₄ 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 \({{{{{\rm{FE}}}}}}_{{{{{{\rm{C}}}}}}_{2}{{{{{\rm{H}}}}}}_{4}}\) 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 CO₂RR, the Sr₂CuWO₆ is prone to generate CH₄ rather than C₂H₄, in line with the above DFT calculations (Fig. 3a).

We also benchmarked the CO₂RR properties of the Sr₂CuWO₆ against the CuO/WO₃. The detailed CO₂RR properties of the CuO/WO₃ were shown in Supplementary Fig. 14. The Sr₂CuWO₆ significantly promoted CO₂-to-CH₄ 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/WO₃, the Sr₂CuWO₆ exhibited 3.7- to 14.1-fold higher \({{{{{\rm{FE}}}}}}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) or \({j}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) (Fig. 3c and Supplementary Fig. 13), together with much lower \({{{{{\rm{FE}}}}}}_{{{{{{\rm{C}}}}}}_{2}{{{{{\rm{H}}}}}}_{4}}\) or \({j}_{{{{{{\rm{C}}}}}}_{2}{{{{{\rm{H}}}}}}_{4}}\) (Supplementary Fig. 15). And the values (10.8–26.2) of \({{{{{\rm{FE}}}}}}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}/{{{{{\rm{FE}}}}}}_{{{{{{\rm{C}}}}}}_{2}{{{{{\rm{H}}}}}}_{4}}\) or \({j}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}/{j}_{{{{{{\rm{C}}}}}}_{2}{{{{{\rm{H}}}}}}_{4}}\) for the Sr₂CuWO₆ were almost 13.2–59.8 times higher than those (0.44–1.98) for the CuO/WO₃ (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 Sr₂CuWO₆ that regulated the adsorption/activation of key intermediates, thus inhibiting C–C dimerization and promoting *CO hydrogenation. We compared the \({{{{{\rm{FE}}}}}}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) and \({j}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) of Sr₂CuWO₆ catalyst with those of the reported Cu-based perovskite oxides (Fig. 3e and Supplementary Table 4). The Sr₂CuWO₆ performed much better than all these perovskites reported in the literature. For instance, the \({j}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) of Sr₂CuWO₆ was about 2.5–1562.5 times higher than that of the reported perovskite-based catalysts. To our knowledge, the Sr₂CuWO₆ was the most effective Cu-based-perovskite catalyst for CO₂-to-CH₄ conversion. Moreover, Supplementary Fig. 16 highlights that the activity and selectivity for CH₄ of Sr₂CuWO₆ 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 Sr₂CuWO₆ and stabilization of Cu sites during CO₂RR (Fig. 4). The reduction tolerance of Sr₂CuWO₆ was probed under a high-temperature reducing atmosphere. At 300 °C (in H₂/Ar for 1 h), the Cu²⁺ in Sr₂CuWO₆ 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/WO₃ was gradually reduced to Cu₂O/WO₃ (at 250 °C) and Cu/WO₃ (at 300 °C) (Supplementary Fig. 19). According to the Rietveld refinement analysis (Supplementary Fig. 20 and Supplementary Table 7), the Sr₂CuWO₆ 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 CuO₆ octahedra induced by reduction of smaller-size Cu²⁺ (0.87 Å) to larger-size 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 or even be strengthened due to the easier electron transfer from Cu⁺ to W⁶⁺ sites relative to that from Cu²⁺ to W⁶⁺ 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 Sr₂CuWO₆. These may partly imply the high structural stability of Sr₂CuWO₆ during CO₂RR.

a XRD pattern of Sr₂CuWO₆ after thermochemical reduction. b Schematic illustration of phase transition of Sr₂CuWO₆ 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, CuO₆, and WO₆, respectively. c Schematic illustration of the distances between the nearest Cu cations in Fm-3m phase of Sr₂CuWO₆. d XRD patterns of Sr₂CuWO₆ after CO₂RR at different current densities (GDE: gas diffusion layer). e Cu LMM XPS spectra of Sr₂CuWO₆ after CO₂RR at different current densities. f W XPS spectra of Sr₂CuWO₆ after different current densities. g CO₂RR stability test of Sr₂CuWO₆ and CuO/WO₃ in a flow cell at 400 mA cm⁻². In-situ Raman spectra of h Sr₂CuWO₆ and i CuO/WO₃ as a function of CO₂RR time at −1.2 V vs. RHE (RHE: reversible hydrogen electrode).

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The possible structural changes of Sr₂CuWO₆ after CO₂RR were analyzed using ex-situ XRD and XPS (Fig. 4d–f). Similar to the thermochemical reduction, after CO₂RR (e.g., at 200 mA cm^-2), part of the I4/m phase of Sr₂CuWO₆ was converted into Fm-3m phase (Fig. 4d), without detectable impurity, and the Cu²⁺ and part W⁶⁺ species on the surface were reduced to Cu⁺ and W⁵⁺, 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 CO₂ methanation. By contrast, as fully evidenced by previous studies, the CuO (in CuO/WO₃) can completely be reduced to metallic Cu under similar CO₂RR conditions^26,27. As a result, the Cu sites of oxidation states in double perovskite structure are well stabilized by the superexchange interaction during CO₂RR. On this basis, we evaluated CO₂RR stability of the Sr₂CuWO₆ in comparison with the CuO/WO₃ through chronopotentiometric polarization in the CO₂-flowed liquid-electrolyte flow cell (Fig. 4g). During 20,000 s of electrolysis (at 400 mA cm⁻²), for the Sr₂CuWO₆, the applied potential was stable at 1.23 ± 0.15 V vs. RHE, and the \({{{{{\rm{FE}}}}}}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) was maintained at 64% ± 6%. Whereas the CuO/WO₃ showed severe deterioration in potentials and obvious fluctuations in \({{{{{\rm{FE}}}}}}_{{{{{\rm{C}}}}}{{{{{\rm{H}}}}}}_{4}}\) during 8000 s of electrolysis. These results demonstrate excellent CO₂RR stability of the Sr₂CuWO₆. As a note, our gas diffusion layer suffered flooding issues when the CO₂RR stability test of Sr₂CuWO₆ catalyst ran for more than 20,000 s. Since the flooding issues can cause an essential failure of the CO₂RR system, we terminated the electrolysis on the Sr₂CuWO₆ catalyst at about 20,000 s; but the steady CO₂RR testing time for the Sr₂CuWO₆ 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 CuO₆ octahedra of Sr₂CuWO₆ during CO₂RR 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 Sr₂CuWO₆ 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 W⁶⁺ to W⁵⁺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 above³⁶. However, for the CuO/WO₃ 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 Cu₂O appeared and then disappeared, and the intensity of Cu–CO peaks at 294.2 and 385.6 cm⁻¹ was gradually improved⁴⁵. These indicate that the CuO in CuO/WO₃ is initially reduced to Cu₂O 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 Sr₂CuWO₆ during CO₂RR (Fig. 5). As shown in Fig. 5a, once the CO₂RR was initiated, the Cu²⁺ on Sr₂CuWO₆ surface began to be reduced to Cu⁺ by the electrode-supplied electrons. At the same time, the Sr₂CuWO₆ surface was transformed from the I4/m to Fm-3m phase, still being the B-site rock-salt ordered structure. Further reduction was not able to convert the Cu⁺ to Cu⁰ but rather led to the transformation of W⁶⁺ to W⁵⁺ on the surface while maintaining the Fm-3m phase. The most plausible reason was that, during CO₂RR, the superexchange interaction effectively transferred the electrode-supplied excessive electrons accumulated around the Cu⁺ sites to W⁶⁺ sites (to form W⁵⁺) 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/WO₃ without superexchange interaction was successively reduced to Cu₂O/WO₃ and Cu/WO₃ (Fig. 5b). Taken together, during CO₂RR, although these changes occurred on the Sr₂CuWO₆ surface, the superexchange interaction prevented structural collapse, stabilized the Cu⁺ sites, and maintained the long Cu–Cu distances, thereby promoting the efficient and stable CO₂-to-CH₄ conversion.

a Sr₂CuWO₆. b CuO/WO₃. Sr, Cu, W, and O are represented by green, blue, gray, and red dots, respectively. The red dotted circle represents oxygen vacancy.

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Employing Sr₂CuWO₆ as the proof-of-concept catalyst, we have developed Cu-based rock-salt-ordered double perovskite oxides for efficient and stable CO₂-to-CH₄ conversion and uncovered the key roles of their unique physicochemical properties in boosting activity, selectivity, and stability toward CH₄ production. In the rock-salt-ordered structure, the corner-linked CuO₆ and WO₆ 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 CO₂RR, relative to its physical-mixture counterpart, the Sr₂CuWO₆ featured not only enhancements in terms of activity and selectivity for CH₄ but also significantly boosted stability. Moreover, the Sr₂CuWO₆ was the most effective Cu-based-perovskite catalyst for CO₂ 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 CO₂RR, providing a new avenue for the rational design of more advanced Cu-based catalysts.

Chemicals and materials

All chemicals were used directly without any further purification. SrCO₃ (AR, ≥99%) and isopropanol (AR, ≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethyl sulfoxide (DMSO, ≥99.9%) was purchased from Shanghai Macklin Biochemical Co., Ltd. CuO (AR, 99%) and WO₃ (AR, 99.8%) were purchased from Shanghai Aladdin Biochemical Technology. Nafion 117 solution (5 wt%) and D₂O (99.9 atom% D) were purchased from Sigma-Aldrich Biochemical Technology. HNO₃ (AR, 65–68%), H₂O₂ (AR, 30%), HF (AR, ≥40%), and HCl (AR, 36–38%) were purchased from Xilong Scientific. High-purity CO₂ gas (99.999%), Ar gas (99.999%), and 10 vol% H₂–Ar gas (99.999%) were purchased from Qingdao Dehaiweiye Technology Co., Ltd.

Synthesis

Sr₂CuWO₆ was synthesized by a high-temperature solid-state reaction/high-energy ball-milling process. In a typical procedure, stoichiometric SrCO₃, CuO, and WO₃ 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 Sr₂CuWO₆ were processed in a sealed tube furnace in 10 vol% H₂–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 Sr₂CuWO₆, CuO, and WO₃. 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 equation⁴⁸:

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, high-resolution 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 = 1486.6 eV) 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 L₃-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, WO₃, 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 program⁴⁹. 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 Sr₂CuWO₆. The samples for ICP–MS analysis were obtained by dissolving 10 mg sample powder with the mixture of 5 mL HNO₃, 1 mL H₂O₂, 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% HNO₃ 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 cm²) 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 CO₂ 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 CO₂ 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 CO₂ 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 H₂, CO, CH₄, C₂H₄, C₂H₆, and 95 vol% CO₂) was used to calibrate the gas products. The Faradaic efficiency (FE) of each gas product under different current densities was gained based on 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 D₂O 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 CO₂RR 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 three-electrode 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 CO₂-saturated 0.1 M KHCO₃ electrolyte. The 0.1 M KHCO₃ 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.