Integrable utilization of intermittent sunlight and residual heat for on-demand CO_2 conversion with water

Domain: earth_science
Published: 2024-11-22
Editorial outcome: major_revision

Abstract
Abundant residual heat from industrial emissions may provide energy resource for CO2 conversion, which relies on H2 gas and cannot be accomplished at low temperatures. Here, we report an approach to store electrons and hydrogen atoms in catalysts using sunlight and water, which can be released for CO2 reduction in dark at relatively low temperatures (150−300°C), enabling on-demand CO2 conversion. As a proof of concept, a model catalyst is developed by loading single Cu sites on hexagonal tungsten trioxide (Cu/WO3). Under light illumination, hydrogen atoms are generated through photocatalytic water splitting and stored together with electrons in Cu/WO3, forming a metastable intermediate (Cu/HxWO3). Subsequent activation of Cu/HxWO3 through low-temperature heating releases the stored electrons and hydrogen atoms, reducing CO2 into valuable products. Furthermore, we demonstrate the practical feasibility of utilizing natural sunlight to drive the process, opening an avenue for harnessing intermittent solar energy for CO2 utilization.

Introduction
Upcycling CO2 emissions into valuable chemicals is a highly important process toward carbon neutrality1–6. Despite the development of various techniques, energy input and hydrogen resources are two factors limiting their ultimate applications. Considering conversion rates, thermal catalysis using H2 gas is the most effective approach to CO2 conversion7–9, which is, however, limited by these two factors. To this end, abundant residual heat from industrial emissions is a potential energy resource for thermally driving CO2 conversion. Such an approach encounters the limitation that CO2 hydrogenation can hardly be accomplished when the industrial residual heat is within the low-temperature range10. Photocatalysis and electrocatalysis provide alternative approaches to utilize renewable energy for CO2 conversion using water as a hydrogen source11–14. However, the intermittent feature of solar energy and other renewable energy cannot sustain continuous chemical transformations, forcing the process to be coupled with energy storage devices, which inevitably results in a tremendous increase in costs15. For this reason, it is imperative to develop an approach to store intermittent solar energy as well as use readily available and cost-effective water as a hydrogen source, which can be compatible with the thermal catalysis system driven by low-temperature heat to complete the overall CO2 conversion process.

Natural photosynthesis, which provides a paradigm for separating light absorption and CO2 reduction, gives us important inspiration. Within the intricate mechanism of photosynthesis, particularly during the light-dependent reactions occurring in chloroplasts, the synthesis of reduced equivalents takes place16–18. These reduced equivalents play a pivotal role in the generation of reduced nicotinamide adenine dinucleotide phosphate (NADPH), a crucial molecule that serves as a potent reducing agent essential for subsequent CO2 assimilation pathways. Through this process, solar energy is efficiently captured and converted into chemical energy, facilitating the transformation of CO2 into organic molecules essential for sustaining life processes. Thus, the fundamental principles underlying natural photosynthesis provide valuable insights for the development of efficient solar-driven CO2 utilization technologies. To simulate natural photosynthesis, state-of-the-art industrial photosynthesis has proposed a multi-step CO2 reduction pathway mediated by H2, which involves the generation of H2 through photocatalysis followed by CO2 hydrogenation9,17. However, the H2 generated from photocatalysis is mixed with O2, necessitating further separation and purification before being utilized in conventional CO2 hydrogenation reaction systems19–21, which significantly augments the purification costs associated with the end product. Moreover, the dissociation of H2 during the CO2 hydrogenation process requires extra energy input, the magnitude of which relies on the type of catalyst employed8,22. This scenario underscores compelling opportunities for the refinement of catalytic methodologies, aiming at optimizing CO2 conversion for practical implementation.

Here, we demonstrate an approach for tandem solar energy harvesting and CO2 conversion utilizing a catalyst with the capability of storing electrons and hydrogen atoms, as shown in Fig. 1. In the presence of solar irradiation, hydrogen atoms are generated through photocatalytic water splitting and subsequently stored alongside electrons within the catalyst. In scenarios where CO2 reduction is needed, the stored electrons and hydrogen atoms are utilized to reduce CO2 into value-added products with low-temperature heat. This approach is experimentally validated using Cu-loaded tungsten trioxide (Cu/WO3) as a model catalyst. The Cu/WO3 material functions akin to a “reservoir”, effectively storing hydrogen atoms and electrons generated during photocatalytic water splitting under solar irradiation. This storage process results in the formation of a Cu/HxWO3 intermediate. When CO2 conversion is required, heating the Cu/HxWO3 intermediate allows the hydrogen atoms and electrons to be released to the catalyst surface for CO2 reduction. This work opens an avenue for the controllable acquisition of solar energy for CO2 utilization.

Results
Synthesis and characterization of model catalyst
According to the literature report, the hexagonal WO3 was synthesized via a one-step hydrothermal process23. Then, Cu atoms were introduced to the catalyst surface under dark conditions by photo-reduced W species with a low-valence state, forming a Cu/WO3 catalyst. The crystal structure of the obtained catalyst was examined by X-ray diffraction (XRD), as shown in Fig. 2a. The Cu/WO3 displays clear diffraction peaks of hexagonal WO3, which aligns with previous reports23–25. No diffraction peaks related to Cu crystals were observed despite the Cu content of 0.14 wt% determined by inductively coupled plasma mass spectrometry (ICP-MS). We further employed X-ray photoelectron spectroscopy (XPS) to detect elements in the sample. As illustrated in Fig. 2b, the high-resolution Cu 2p XPS spectrum of Cu/WO3 confirms the presence of the Cu element on the surface of Cu/WO3. In addition, the signals of W and O elements can be seen from the XPS spectra (Supplementary Fig. 1). To further characterize the W–O structure, Raman spectra were collected (Fig. 2c). The band at 953 cm−1 can be assigned to the stretching vibration of the terminal W=O groups, while the peaks at 645 cm−1 and 820 cm−1 are ascribed to the O−W−O stretching vibration, and the peak at 230 cm−1 is attributed to W–O–W stretching mode24,26. There is no obvious change in the intensity and position of the peaks after Cu introduction, which indicates that Cu does not affect the crystal structure of the sample.

We further characterized the morphology of Cu/WO3 using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show that Cu/WO3 is composed of irregular nanosheets (Supplementary Figs. 2, 3). To look into the atomic structure, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to characterize the catalyst. As shown in Fig. 2d, e, the arrangement of WO6 octahedra sharing corners forms a structure with stacked tunnels along the c axis, which is favorable for the insertion and extraction of hydrogen atoms23. In addition, the steep increase in adsorbed volume at low relative pressure also indicates the presence of microporosity (Supplementary Fig. 4). Due to the light weight of Cu element relative to W element, no Cu-related species can be seen in the HAADF-STEM image, but the corresponding energy-dispersive spectroscopy (EDS) mapping of Cu/WO3 clearly proves the uniform dispersion of Cu elements on the catalyst (Fig. 2f).

To analyze the local structure of Cu in Cu/WO3, X-ray absorption fine structure (XAFS) spectroscopy was performed. The Cu K-edge absorption edge position of Cu/WO3 in the X-ray absorption near edge structure (XANES) spectra is closer to that of CuO than Cu foil (Fig. 2g), indicating that the Cu element carries a positive charge and has a valence state approaching +227. Fourier-transformed (FT) extended X-ray absorption fine structure (EXAFS) spectra were used to determine the local structures of the Cu element (Fig. 2h). The Cu/WO3 does not show the peak of Cu–Cu bond at 2.2 Å in reference to standard Cu foil, revealing the absence of Cu clusters or particles in the catalyst28–30. According to the EXAFS fitting results (Supplementary Fig. 5 and Supplementary Table 1), the Cu atoms most likely are located in the hexagonal holes of WO3 through coordination with 4 O atoms. Such an introduction of Cu can slightly affect the photoelectric properties of WO3 (Supplementary Figs. 6, 7). We also characterized the interaction of samples with hydrogen atoms using H2 temperature programmed reduction (H2-TPR) measurement. The H2-TPR peaks shift to low temperatures after Cu is deposited onto WO3 (Fig. 2i), indicating the spillover of hydrogen atoms from Cu sites to the WO3 support31. This feature will promote the storage of H atoms rather than their release as H2. Taken together, the characterizations suggest that the well-designed Cu/WO3 catalyst has the potential to serve as a reservoir for storing hydrogen atoms and electrons.

Storage of electrons and hydrogen atoms in the catalyst
Upon gaining the structural information of the Cu/WO3 catalyst, we further assessed its capacity for storing hydrogen atoms and electrons. To this end, we dispersed the catalyst in water and subjected it to illumination, with the resulting sample named Cu/HxWO3 (Fig. 3a). Following the illumination process, the catalyst undergoes a discernible color transformation, transitioning from gray to deep blue (Supplementary Fig. 8). Compared to Cu/WO3, Cu/HxWO3 exhibits significantly enhanced absorption across the visible light spectrum, as evidenced by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) measurements (Supplementary Fig. 9). According to the literature, WO3 can form polarons due to electron localization at the d-states of tungsten, resulting in the conversion of W(6+) to W(5+)32,33. Visible or near-infrared light can excite polaronic transitions, which prompts the migration of polarons from W(5+) to neighboring W(6+) sites, consequently engendering light absorption. To further validate this phenomenon, we employed electron paramagnetic resonance (EPR) spectroscopy to examine the W(5+) species in the catalyst during light irradiation (Fig. 3b). The EPR signal at approximately 3720 G (g value = 1.895) corresponding to W(5+) exhibits variation with the duration of illumination, reaching maximal intensity after 10 min23,34. This observation suggests an increase in the W(5+) content within the catalyst during the irradiation process. In addition, the deconvoluted W 4f XPS spectra for Cu/WO3 and Cu/HxWO3 can be fitted into two oxidation states, W(6+) (4f7/2, 35.82 eV) and W(5+) (4f7/2, 34.81 eV) (Supplementary Fig. 10)35. The W(5+) content in Cu/WO3 experiences a notable increase, rising from 0.7% to 4.5% in Cu/HxWO3. This shift implies that a portion of W(6+) undergoes reduction to W(5+) during the illumination process. The transformation from W(6+) to W(5+) under illumination signifies the successful storage of photoinduced electrons in the catalyst.

To maintain charge balance, the storage of electrons may be accompanied by the participation of cations. In fact, the H2 and O2 produced during the photocatalytic process do not match stoichiometrically (Supplementary Fig. 11)36. To investigate whether hydrogen atoms are stored in the catalyst, we collected 1H solid-state nuclear magnetic resonance (NMR) spectra from Cu/WO3 and Cu/HxWO3. The intensification of the peaks for Cu/HxWO3 clearly indicates the enhancement of surface terminal hydroxyl groups (1.6 ppm) and bridging hydroxyl groups (4.4 ppm) (Fig. 3c)37, signifying the successful incorporation of hydrogen atoms into the catalyst following exposure to light. This conclusion gains further support from the enhanced peaks corresponding to O–H species in the Fourier-transform infrared (FTIR) spectra, specifically in the range of 3500–3700 cm−1 (Fig. 3d). We further carried out the measurements by replacing water (H2O) with deuterium oxide (D2O) to gain a deeper understanding of the origin of hydrogen atoms. The resulting product, denoted as Cu/DxWO3, exhibits an FTIR spectrum where the peak associated with O–D species appears in the range of 2500–2700 cm−1 after light irradiation38. Remarkably, the peaks related to O−H species remains consistent with those of Cu/WO3, indicating that the hydrogen atoms inserted into the catalyst indeed originate from water splitting. Furthermore, we assessed the energy storage efficiency by analyzing the wavelength-dependent apparent quantum efficiency (AQE) of H2O to electron−hydrogen pairs (Fig. 3e). Notably, the maximum AQE was observed at 320 nm, reaching 0.66%, underscoring the excitation of catalyst by ultraviolet light. In addition, we assessed the storage of hydrogen atoms under intermittent illumination conditions (Supplementary Fig. 12). This observation underscore the capability of our designed system to mitigate the challenges posed by intermittent supply of solar energy.

To explore the impact of storing electronic and hydrogen atoms in catalysts, we conducted tests on the WL3-edge XAFS spectra of Cu/WO3 and Cu/HxWO3. From the XANES spectra (Fig. 3f), it can be observed that the absorption edge position of Cu/HxWO3 is slightly lower compared to Cu/WO3, but both are higher than that of W foil. This suggests a slight reduction in average valence state of W element, primarily attributed to an increase in W(5+) content, which is consistent with the findings from EPR and XPS results. In addition, the EXAFS spectra show that the intensity of W−O coordination at 1.35 Å slightly decreases, primarily due to insertion of hydrogen atom (Fig. 3g). No significant changes can be found in XRD patterns and Raman spectra, indicating that the overall crystal structure of the catalyst does not change obviously after the storage of electrons and hydrogen atoms (Supplementary Fig. 13). Taken together, the above results indicate that the storage of hydrogen atoms and electrons does not lead to the collapse of crystalline structure, which is crucial for the recyclability of catalyst.

Catalytic CO2 reduction performance
After confirming the ability to store hydrogen atoms and electrons in the catalyst using light and water, we further explored the utilization of low-temperature residual heat to thermally activate Cu/HxWO3 for CO2 conversion. To this end, the catalytic CO2 reduction performance by the stored hydrogen atoms and electrons was evaluated under heated conditions in a homemade reactor (Supplementary Fig. 14, see the experiment section for details). Specifically, a gas-solid phase reaction was implemented, wherein only gaseous products—such as CH4, CO, H2, and C2H4—are detected to assess the efficacy of catalyst39. It turns out that the CO2 conversion can be achieved at a temperature as low as 90°C, but the production rate is very limited. For this reason, we chose a temperature above 150°C for the demonstration (Supplementary Fig. 15). When CO2 reacts with Cu/HxWO3 at 150°C, a small amount of CO (0.13 mL/g) and CH4 (0.11 mL/g) are generated (Fig. 4a). As the reaction temperature is elevated to 180°C, C2H4 becomes detectable (0.07 mL/g). Remarkably, by increasing reaction temperature, product selectivity for CO2 conversion gradually rises—from 11.4% at 150°C to 91.8% at 300°C (Fig. 4b). At 270°C, a variety of products are yielded at relatively high levels (Supplementary Table 2), with the stored electrons and hydrogen atoms contributing to the formation of C2H4. As the formation of C2H4 requires the transfer of 12 electrons and 12 hydrogen atoms, this observation underscore that the elevated temperatures can facilitate the provide additional electrons and protons for the sites of CO2 activation. To better understand this aspect, we determined the conversion rates of hydrogen atoms and electrons by analyzing the yield of final products, which show a positive correlation with the temperatures (Supplementary Fig. 16). This suggests that thermal energy input enhances the migration and release of electrons and hydrogen atoms. Our further exploration was fixed at 270°C to uphold a relative balance between efficiency and electron/hydrogen utilization.

Stability is a key parameter determining the practical feasibility of a catalyst. The performance of the catalyst remains stable after six testing cycles—an encouraging sign of its endurance and potential for practical applications (Supplementary Fig. 17). Furthermore, we underscored the catalyst stability by employing characterization techniques, including XRD, Raman spectroscopy, XPS, and FTIR spectroscopy (Supplementary Fig. 18). Notably, no significant structural changes can be detected in the catalyst after undergoing cyclic reactions, indicating excellent stability of the catalyst.

To gain a comprehensive understanding of CO2 reduction by Cu/HxWO3, a series of comparative experiments were conducted. We first conducted a thorough assessment using WO3 and Cu/WO3 for CO2 conversion. Apart from a small amount of CO, no other carbonous products were detected (Fig. 4c). This product may be attributed to residual carbon impurities on the surface that have not been entirely eliminated40. Subsequently, we examined WO3 after storing electrons and hydrogen atoms (noted as HxWO3). Notably, a small number of carbonous products emerged with primarily H2 due to the scarcity of suitable active sites. When high-purity CO2 was replaced by Ar, only a negligible amount of CO was detected (0.012 mL/g) at 270°C. To strengthen this conclusion, we conducted in-situ FTIR measurements for the formation of intermediate products. Only CO intermediate was generated in a CO2 atmosphere, indicating that the carbonous product indeed originates from CO2 reduction (Supplementary Figs. 19, 20). To trace the carbon source in the products, we performed tests using 13CO2 instead of 12CO2 and analyzed the ion fragment distribution of CO, CH4, and C2H4. The detection of 13C-associated ion fragments indicates that the product is indeed derived from CO2 reduction (Fig. 4d–f)11,41. Furthermore, during the stage of collecting electrons and hydrogen atoms, H2O was substituted with D2O, resulting in the formation of a Cu/DxWO3 catalyst. We conducted CO2 reduction reactions using Cu/DxWO3 and observed that the primary hydrocarbon products were CD4 and C2D4 (Supplementary Fig. 21). As such, we can safely attribute the carbonous products to CO2 reduction by the hydrogen atoms and electrons in the catalyst.

Catalytic mechanisms
Following the validation of the stored hydrogen atoms and electrons for CO2 reduction, the investigation of the reaction mechanism is warranted. We employed the kinetic isotope effect (KIE, H/D) to elucidate the rate-limiting step38,42. By using Cu/HxWO3 and Cu/DxWO3 for CO2 conversion reactions, based on the consumption of hydrogen atoms and electrons in product generation, the KIE (H/D) was calculated to be 5.24 (Fig. 5a). This understanding of the reaction process underscores the pivotal role of hydrogen transfer within the catalytic framework37,43.

To look into the hydrogen transfer process from the carrier material (WO3) to the active site (Cu sites), we conducted density functional theory (DFT) calculations to discern the energy barrier associated with hydrogen transfer. We scrutinized the sites within the WO3 carrier material where hydrogen atoms reside and discovered a substantial number of catalyst sites capable of adsorbing hydrogen atoms (Supplementary Fig. 22). This raises a key question: how do hydrogen atoms reach the catalyst surface to participate in the reaction? The Gibbs free energy variation at each stage of hydrogen transfer—from the interior of WO3 to the surface sites—was meticulously computed (Fig. 5b). The hydrogen adsorption shows a remarkable weakness in the vicinity of the six-octahedron tunnels of WO3 (sites 2 and 4), while it shows a considerate strength along the four-octahedron channels (sites 1 and 5), indicating that the latter is more favorable for hydrogen adsorption. The migration of a hydrogen atom from the inner WO3 layer to the outer WO3 layer requires crossing the intermediate O layer (site 3). The computed Gibbs free energy difference associated with adsorbed hydrogen between site 2 and site 3 is 0.93 eV, highlighting the inherent difficulty of hydrogen transfer from the WO3 interior to the surface sites. Interestingly, the Gibbs free energy difference of adsorbed hydrogen between site 2 and site 3 on Cu/WO3 is lower than that on WO3, with a barrier of 0.60 eV. This outcome suggests that the hydrogen transfer process is significantly facilitated on Cu/WO3. Furthermore, we observed stronger hydrogen adsorption on the Cu site (site 5) of the Cu/WO3 surface compared to the six octahedron channels (site 4), thereby favorably influencing the CO2 reduction reaction. To unravel the intricacies of this facilitated hydrogen transfer process, we examined the charge density difference and revealed distinct electron accumulation regions on the surface layer of Cu/WO3 upon the introduction of a Cu atom (Fig. 5c). The change in the electron distribution can affect the trapped hydrogen atom by interacting with unsaturated electrons in the H 1s orbital of the hydrogen atom44, which can account for the change in the free energy of the hydrogen transfer process.

Demonstration of solar-driven application
To validate the practicality of our concept, we designed a process to integrate utilization of intermittent sunlight and residual heat for on-demand CO2 conversion (Fig. 6a). In addition, we investigated the solar energy harvesting process through a demonstration experiment under cloudy weather (Supplementary Fig. 23). As depicted in Fig. 6a, the initial sample exhibits a subdued gray hue prior to solar exposure (at 6:00). When exposed to sunlight, the color of catalyst gradually changes (at 10:00). Despite fluctuations in sunlight intensity due to passing clouds wherein irradiance nears 0 W/m², the catalyst consistently accumulates solar energy, reaching saturation around 16:00 (Supplementary Table 3). Subsequently, we used the catalyst to perform CO2 reduction tests. Remarkably, the amount of hydrogen stored in the catalyst using both natural sunlight and simulated sunlight is comparable (Fig. 6b), confirming the successful use of solar energy. These results demonstrate the feasibility of collecting hydrogen atoms directly from atmospheric water vapor.

To verify that the catalyst can perform CO2 reduction on demand, we tested the catalyst after 15 days and 90 days of storage. Although the catalytic performance slightly decreases, it still maintains over 80% of the stored hydrogen atoms and electrons (Fig. 6c). Moreover, the consumption of the stored electrons and hydrogen atoms is mainly in the initial stage (0–15 days), with very little change over the subsequent extended period, indicating that long-term storage is feasible. The reduced charge loss during storage suggests that this method holds promise for mitigating the uneven seasonal distribution of solar energy. In addition, the catalyst used for CO2 conversion was scaled up by two orders of magnitude, with a corresponding increase in product yield (Fig. 6d and Supplementary Fig. 24). The results have confirmed the theoretical feasibility of large-scale application. It should be acknowledged that the purpose of the aforementioned experiments is mainly to verify the feasibility of integrable utilization of intermittent sunlight and residual heat for on-demand CO2 conversion with water, without targeted optimization of material costs and reaction devices. Achieving large-scale applications necessitates substantial efforts, particularly in catalyst preparation and reactor design, which are the focal points of our future research. Overall, our results have confirmed the practicality of directly collecting, storing, and deploying catalysts when CO2 conversion is required, significantly increasing the flexibility of catalytic CO2 reduction.

Methods
Materials preparation
1.5 g of Na2WO4·2H2O was added to 30 mL of deionized (DI) water and stirred for 30 min. Subsequently, 265 µL of hydrazine hydrate solution (50%) was added and stirred continuously for 2 h. Next, a certain amount of HCl was added to adjust the pH to 0.5, and the solution was stirred for another 12 h. Then, the solution underwent a hydrothermal reaction for 12 h at 120°C. The final precipitation was collected centrifugally, then washed alternately three times with deionized water and absolute ethanol, and finally dried under vacuum at 70°C. The dried sample was then calcinated in air at a temperature of 320°C for 12 h, followed by an additional calcination in high-purity N2 for another 12 h, to obtain WO3.

To prepare the Cu/WO3, 1 g of WO3 catalyst was dispersed in 30 mL solution (5 mL of absolute ethanol and 25 mL of DI water), followed by ultrasonic treatment for 6 h at 20°C. Then, the solution was irradiated by a 300 W xenon lamp for 1 min. Subsequently, a certain amount of Cu(NO3)2·3H2O solution (the Cu content is 0.5 wt% of the WO3 content) was added dropwise, and the mixture was stirred for 12 h under dark. The resulting sample was collected centrifugally, then washed alternately three times with deionized water and absolute ethanol, and finally dried under vacuum at 70°C. After drying, the samples were calcinated at 320°C for 12 h, first in air and then in high-purity N2 to obtain Cu/WO3 catalyst. All samples were stored in the dark for further experiments.

Collection of electrons and hydrogen atoms from simulated sunlight
To verify the feasibility of sunlight harvesting, 0.5 g of as-synthesized catalyst was dispersed in 30 mL of DI water and subjected to 10 min of ultrasonic treatment to ensure uniform dispersion. The sample was then irradiated by a 300 W xenon lamp with an AM 1.5 filter for 10 min under continuous stirring. The optical power of the light source was recorded by the optical power meter (PM100D, Thorlabs, USA), and the corresponding optical power density was 0.45 W/cm2. The wavelength distribution information of the light source is shown in Supplementary Fig. 25. Subsequently, the sample was centrifuged and vacuum-dried for 24 h at 55°C. According to the different catalysts added, the products are noted as HxWO3 (added WO3) and Cu/HxWO3 (added Cu/WO3). Wavelength-dependent AQE of H2O to electron−hydrogen pairs was calculated by: AQE (%) = (Number of electron−hydrogen pairs) / (Number of incident photons) × 100%. Number of electron−hydrogen pairs was calculated by the number of H atoms obtained by ion exchange using an excess of K+ in water23.

Evaluation of CO2 conversion performance
The synthesized samples were used for CO2 reduction reactions in a homemade reactor consisting of a temperature-controlled heater and a double-sealed quartz tube. The sample was placed in the middle of the quartz tube, and gas was injected or extracted through sampling ports located at the end of the quartz tube. The reactor was purged with high-purity CO2 (99.999%) ten times before reaction. Then, 90 mL of high-purity CO2 was injected into the reactor to keep the pressure at 0.1 MPa. The temperature of the samples was regulated by the heater and monitored using a thermocouple. After a certain period of reaction, 2 ml of gas was extracted from the reactor to measure the product yield. The products were detected by gas chromatography (7890B, Agilent). Gaseous products such as CH4, C2H4, CO, and CO2 were detected using a flame ionization detector (FID), while O2 and H2 were detected using a thermal conductivity detector (TCD). The amount of the CO2 reduction product was determined by comparing the peak area with the standard curve of the gas with known concentration (Supplementary Fig. 26). For the isotope-labeling experiment, high-purity 13CO2 was used as the reactant instead of 12CO2 for the reaction, and the products were analyzed using gas chromatography-mass spectrometry (GC-MS, 7890A and 5975C, Agilent). The volume of the products has been converted to the volume under standard temperature and pressure (STP), which corresponds to a temperature of 0°C and a pressure of 1 atmosphere.

Cyclic stability test
The cyclic stability test of the catalyst comprises two stages: the collection of electrons and hydrogen atoms under simulated sunlight illumination, and the thermal reaction at 270°C in a high-purity CO2 atmosphere. The specific procedures for these stages are detailed in the above sections. A cycle was considered complete when the product yield ceased to increase under heating conditions. Subsequently, the catalyst was irradiated by simulated sunlight illumination to initiate the next cycle.

Calculation details
All calculations were carried out using the Vienna Ab Initio Simulation Package (VASP)45. The exchange-correlation interaction was described by Perdew−Burke−Ernzerhof exchange-correlation functional (GGA-PBE)46. To prevent interactions between periodic structures, a vacuum of 20 Å was introduced during the construction of surface models. The energy cutoff and Monkhorst-Pack k-point mesh were set to 450 eV and 3×3×1, respectively. The convergence criteria for geometry optimization was set as 1×10−5 eV/atom for energy and 0.03 eV/Å for force. The Gibbs free energy was determined under reaction conditions using the following equation: G = EDET + EZPE + Eenthalpy − TS. Here, EDET represents the static electron energy of the system at 0 K, EZPE is zero-point energy, Eenthalpy is the enthalpy contribution, and TS is the entropy contribution. All the structural model visualization is achieved using VESTA47.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Inspired by natural photosynthesis, the authors reported an interesting strategy of storing electrons and hydrogen atoms using sunlight and water, which can be then released for CO2 reduction by residual heat. The concept is impressive and the paper is well written. I would like to recommend its publication once the following issues are addressed. Please see below the detailed comments.\n1. Obviously, the author deliberately divided the reaction process into the photocatalytic process and the thermal catalytic process by using Cu/HxWO3. How the catalyst will perform if the reaction is carried out via a heat-assisted photocatalytic process?\n2. Energy conversion efficiency needs to be considered by taking the output of chemical energy and the input of thermal energy and light energy into consideration.\n3. The author claimed that the stability of Cu/HxWO3 during the four cycles is almost unchanged, but what is the specific time of each cycle?\n4. In the absence of an additional hydrogen source, the H atom in Cu/HxWO3 should be continually consumed for CO2 conversion in the stability test. In this case, how can the stability be guaranteed? Please elaborate on the stability test.\n5. As shown in Figure S18, there is no significant change in the structure of the catalyst after the reaction. However, this cannot prove whether the content and valence state of Cu were changed, according to Fig.2a, 2c, S1b, and S1c. Please include this key information.\n6. Why is in-situ FTIR measurement carried out at 120 °C. Increasing the temperature seems likely to see characteristic peaks belonging to CH4, C2H4 or their intermediates.\n7. In Fig. 4d-f, the value of C13/C12 rather than only C13 may make more sense.\n8. It is recommended to use Cu/DxWO3 for CO2 conversion to detect D-containing products such as CH3D, C2H3D or others possible.\n9. A writing error in Fig. 2i. The Y-axis label should be TPR rather than TPD.\n10. Some recent important discoveries (Nat. Catal. 2023, 6, 987-995, Proc. Natl. Acad. Sci. U. S. A. 2022, 119, e2121174119.) are highly related to this topic. Appropriate citations of these discoveries are recommended.", "reviewer_2": "This work reports an approach to store electrons and hydrogen atoms in catalysts using sunlight and water, which can be released for CO2 reduction in dark in a controlled manner at relatively low temperatures 150ºC – 300ºC for on-demand CO2 conversion. Overall, the manuscript is well written and well organized presenting an in-depth analysis of the catalytic system consisting of loaded single Cu sites on hexagonal tungsten trioxide. It is noteworthy that unlike in conventional photocatalysis, separation of oxygen is not necessary and that the stored electrons and hydrogen atoms can be used on demand by simple heating. The stability of the catalysts is also very promising.\nFigure 2 depicts the concept of this approach. This concept is not aligned with the goal of reaching carbon neutrality, as the world needs to stop using fossil fuels for energy production. Thus, in Figure 2 “fossil fuel” should be replaced by renewable fuels/biofuels or renewable energy. Furthermore, the direction of one of the arrows should be inverted Cu/WO3 to light reaction and not the opposite.\nAlthough other systems were reported with similar characteristics e.g. Bi-Hx ref. 35 of the paper of Li et al. and Ir-Hx-WO3 ref. 36 of the paper of Zheng et al. the very detailed investigation of the Cu-Hx-WO3 catalyst including structural characterization, determination of storage capacity of hydrogen and electrons and of the catalytic mechanism adds to the knowledge base of the field.\nThe work supports the conclusions and most of the claims, however it seems to be limited in terms of scalability. The catalyst was scaled-up by two orders of magnitude, but product yield did not increased proportionally. At this stage, the ratio product yield/quantity of catalysts is not suitable for a commercial application as described in Figure 6a. The possibility of overcoming this problem with the present system Cu-Hx-WO3 is doubtful. The authors should comment on this issue.\nIn Figure 6a from 10:00 onwards no further colour change can be perceived from the figure.The quality of the figure should be improved. The authors should indicate how they reached the conclusion that at 16:00 the catalyst reached saturation, if by colour change or by quantitative measurement.\nIn the methods section or in the supplementary information a detailed description of the determination and calculation of hydrogen storage capacity should be provided for the work to be reproduced. Quantitative values of storage capacity should be provided to allow a comparison with other materials that are being developed for hydrogen storage.\nThe methodology is sound and in accordance with the standards in the field.\nI recommend the work for publication after revision.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The authors reported a Cu/WO3 photocatalyst which could store electron and proton equivalents under irradiation. Using stored electrons and protons, CO2 reduction delivered value-added fuels. This work demonstrated a facile pathway for effective CO2 reduction by stepwise storage and utilization of electron and proton equivalents. In-depth study was carried out to unravel how electrons and protons were stored and utilized. The following questions are clarified:\n1. Is the storage of electrons and protons stoichiometric? How many equivalents of electrons and protons could be stored with a given amount of photocatalyst?\n2. A very low loading of Cu (0.14 wt% based on ICP, line 104) was applied. Does the loading of Cu affect the capacity of electron and proton storage?\n3. A probable estimate of the H atom content in the metastable intermediate Cu/HxWO3 should be made to evaluate the efficiency of the subsequent CO2 reduction process.\n4. Why is the absence of additional peaks in the XRD pattern of Cu/WO3 compared to WO3 indicative of uniform dispersion of Cu in Cu/WO3?\n5. How is the recyclability of Cu/WO3 photocatalyst for electron and proton storage and release? The Figure S17 only presents four sets of data from cyclic experiments, whereas the text indicates the existence of six sets of cyclic experiments.\n6. The authors need complete the error bar.\n7. Is Cu/WO3 photocatalyst able to perform CO2 reduction with low-concentration CO2?\n8. In the discussion section, the author emphasized the formation process of C2H4, but the mechanism of CO2 reduction to C2H4 was not sufficiently discussed in the manuscript.\n9. Why does the N2 adsorption capacity of Cu/WO3 decreased compared to WO3?\n10. How to prove that Cu is reduced by W at reduced state?\n11. Why did the amount of catalyst and the products increase in different proportions in Figure 6d?\n12. During the synthesis of Cu/ WO3, why is it important to expose WO3 with xenon lamp irradiation for 1 minute before adding Cu(NO3)2? Why is it necessary to calcine first in air and then in N2?\n13. The reaction between water and Cu/WO3 yields oxygen and Cu/HxWO3. In Fig.1, the arrow between light reaction and Cu/WO in Figure 1 may be drawn upside down.\n14. In Fig. 3d, three kinds of the FTIR spectra of materials should be separated in order to better observe the difference.\n15. Please add the standard curve for the detection of CO2 reduction products by gas chromatography to the Supporting Information.\n16. The authors attribute the peak at 2500-2700 cm-1 in the FTIR data to the signal of O-D. References should be provided.\n17. The information of the wavelength and optical power of the light source used in the light reaction is requested.\n18. What is the meaning of the percentage of H atoms to products and H/e- conversion rate in Figure S16? Please provide the formulae for these two parameters. When the reaction temperature is raised from 240℃ to 270℃, why do both parameters show a similar trend?\n19. The authors should give the calibration and binding energies calibrated for the carbon 1 s peak for XPS.\n20. The W(6+) of W 4f XPS spectra for Cu/WO3 and Cu/HxWO3 should be clearly marked.\n21. In Supplementary Fig. 18c, the W 4f XPS spectra has a significant shift after undergoing cyclic reactions. Why?"}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nInspired by natural photosynthesis, the authors reported an interesting strategy of\nstoring electrons and hydrogen atoms using sunlight and water, which can be then\nreleased for CO reduction by residual heat. The concept is impressive and the paper\n2\nis well written. I would like to recommend its publication once the following issues are\naddressed. Please see below the detailed comments.\nAuthor response: We really appreciate the referee’s highly positive evaluation of our\nwork, and are grateful to the referee for his/her insightful suggestions to help us further\nimprove the quality of our manuscript. We have made all the revisions as suggested by\nthe referee. We sincerely hope that our careful revisions have satisfactorily addressed\nthe referee’s concerns.\n1. Obviously, the author deliberately divided the reaction process into the\nphotocatalytic process and the thermal catalytic process by using Cu/H WO . How the\nx 3\ncatalyst will perform if the reaction is carried out via a heat-assisted photocatalytic\nprocess?\nAuthor response: We thank the referee for his/her insightful question. Following the\nreferee’s suggestion, we have performed a heat-assisted photocatalytic CO reduction\n2\nwith H O reaction using Cu/WO catalyst. As shown in Figure R1-1, the CO reduction\n2 3 2\nreaction can be carried out under the reaction conditions of heat-assisted photocatalysis.\nAdditionally, the types of products do not change, and the main products include H ,\n4 2 4\nyield of Cu/WO catalysts is still lower than that of state-of-the-art heat-assisted\n3\nphotocatalytic CO reaction catalysts (Refs: Angew. Chem. Int. Ed. 62, e202313784\n2\n(2023); Nat. Commun. 15, 4675 (2024)). As described by the referee, the main purpose\nof this study is to demonstrate the separation of light reaction and thermal reaction\nprocesses. Therefore, the Cu/WO catalyst requires further optimization before it can\n3\nbe applied in heat-assisted photocatalytic systems.\nFigure R1-1. The products yields of heat-assisted photocatalytic CO reduction with\ntest tube and sonicated for 2 h. The mixture was then transferred to a quartz Petri dish\n(inner diameter: 40 mm) and dried at 80°C to form a thin film of Cu/WO catalyst. The\n3\nresulting film was then placed in a 160 mL reaction chamber (CEL-HPR100T miniature\nreactor, Aulight, Ltd.), which was evacuated 5 min prior to the introduction of CO .\n2\n0.1 Mpa, and 2 mL of deionized water was added as a proton source. The temperature\nof the catalyst was raised to the desired level, and the catalyst was then irradiated by a\nextracted from the reactor to measure the products yield.\n2. Energy conversion efficiency needs to be considered by taking the output of chemical\nenergy and the input of thermal energy and light energy into consideration.\nAuthor response: We thank the referee for his/her valuable suggestion. The energy\nconversion efficiency was calculated as follows:\n𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑐𝑐𝑐𝑐𝐸𝐸𝑐𝑐𝐸𝐸𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸 𝐸𝐸𝑒𝑒𝑒𝑒𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸𝐸𝐸𝑐𝑐𝐸𝐸 (%) = (𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑒𝑒 𝑐𝑐ℎ𝐸𝐸𝑒𝑒𝑐𝑐𝑐𝑐𝑒𝑒𝑒𝑒 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 / (𝐼𝐼𝐸𝐸𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑒𝑒 𝑂𝑂ℎ𝐸𝐸𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸+𝐼𝐼𝐸𝐸𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑒𝑒 𝑒𝑒𝑐𝑐𝐸𝐸ℎ𝑂𝑂 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸)) ×100%\nThe output chemical energy, stored in the gas molecules, including H , CO, CH and\n2 4\nC H , was calculated by the following formula:\n2 4\n𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑒𝑒 𝑐𝑐ℎ𝐸𝐸𝑒𝑒𝑐𝑐𝑐𝑐𝑒𝑒𝑒𝑒 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 = 𝐸𝐸(𝐻𝐻2)+𝐸𝐸(𝐶𝐶𝑂𝑂)+𝐸𝐸(𝐶𝐶𝐻𝐻4)+𝐸𝐸(𝐶𝐶2𝐻𝐻4)\nwhere , , and represent the chemical energy stored in\nH , CO, CH and C H , respectively. These energies can be calculated by the change in\n2 4 2 4\n𝐸𝐸(𝐻𝐻2) 𝐸𝐸(𝐶𝐶𝑂𝑂) 𝐸𝐸(𝐶𝐶𝐻𝐻4) 𝐸𝐸(𝐶𝐶2𝐻𝐻4)\nGibbs free energy ( G) for the reaction generating the molecule (Ref: Proc. Natl. Acad.\nSci. U.S.A. 121, e2408183121 (2024)). The Gibbs free energies of the various reactions\n△\nare summarized in Table R1-1.\nTable R1-1. Change of Gibbs free energy when CO and H O are reduced to different\n2 2\nproducts (Ref: Chem. Rev. 119, 3962-4179 (2019)).\nThe chemical energy stored in the H molecules was calculated as follows:\n2\n𝐸𝐸(𝐻𝐻2) = Product of 𝐻𝐻2 (mol)×ΔG𝐻𝐻2(𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 19.85×10^-6 (𝑒𝑒𝑐𝑐𝑒𝑒/𝐸𝐸)×0.5(𝐸𝐸)×237×10^3 (𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 2.352 𝐽𝐽\nThe chemical energy stored in the CO molecules was calculated as follows:\n𝐸𝐸(𝐶𝐶 𝑂𝑂) = Product of 𝐶𝐶 𝑂𝑂 (mol)×ΔG𝐶𝐶𝐶𝐶 (𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 36.41×10^-6 (𝑒𝑒𝑐𝑐𝑒𝑒/𝐸𝐸)×0.5(𝐸𝐸)×257×10^3 (𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 4.679 𝐽𝐽\nThe chemical energy stored in the CH molecules was calculated as follows:\n4\n𝐸𝐸(𝐶𝐶𝐻𝐻4) = Product of 𝐶𝐶𝐻𝐻4 (mol)×ΔG𝐶𝐶𝐻𝐻4(𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 6.41×10^-6 (𝑒𝑒𝑐𝑐𝑒𝑒/𝐸𝐸)×0.5(𝐸𝐸)×818×10^3 (𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 2.622 𝐽𝐽\nThe chemical energy stored in the C H molecules was calculated as follows:\n2 4\n𝐸𝐸(𝐶𝐶2𝐻𝐻4) = Product of 𝐶𝐶2𝐻𝐻4 (mol)×ΔG𝐶𝐶2𝐻𝐻4 (𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 7.91×10^-6 (𝑒𝑒𝑐𝑐𝑒𝑒/𝐸𝐸)×0.5(𝐸𝐸)×1323×10^3 (𝐽𝐽/𝑒𝑒𝑐𝑐𝑒𝑒) = 5.232 𝐽𝐽\nBased on the above results, we can determine the total output energy.\n𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑒𝑒 𝑐𝑐ℎ𝐸𝐸𝑒𝑒𝑐𝑐𝑐𝑐𝑒𝑒𝑒𝑒 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 = 𝐸𝐸(𝐻𝐻2)+𝐸𝐸(𝐶𝐶𝑂𝑂)+𝐸𝐸(𝐶𝐶𝐻𝐻4)+𝐸𝐸(𝐶𝐶2𝐻𝐻4) = 14.885 𝐽𝐽\nThe input of light energy was calculated as follows:\n𝐼𝐼𝐸𝐸𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑒𝑒 𝑒𝑒𝑐𝑐𝐸𝐸 ℎ𝑂𝑂 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 = 𝑂𝑂𝑂𝑂𝑂𝑂𝑐𝑐𝑐𝑐𝑒𝑒𝑒𝑒 𝑂𝑂𝑐𝑐𝑝𝑝𝐸𝐸𝐸𝐸 𝑑𝑑𝐸𝐸𝐸𝐸𝑐𝑐𝑐𝑐𝑂𝑂𝐸𝐸(𝑊𝑊/𝑐𝑐𝑒𝑒^2) × 𝐼𝐼𝐸𝐸𝐸𝐸𝑒𝑒𝑑𝑑𝑐𝑐𝑒𝑒𝑂𝑂𝐸𝐸𝑑𝑑 𝑒𝑒𝐸𝐸^2 (𝑐𝑐^2)×𝑂𝑂𝑐𝑐𝑒𝑒𝐸𝐸 (𝑐𝑐) = 0.45 (𝑊𝑊/𝑐𝑐𝑒𝑒^2)×3.14×3.5^2 (𝑐𝑐^2)×600 (𝑐𝑐) = 10,385.55 𝐽𝐽\nWe used differential scanning calorimetry to test the endothermic behavior of the\nCu/H WO catalyst in a CO atmosphere. The reaction was conducted by using an SDT\nx 3 2\nQ600 instrument (TA Instruments Corp., USA) with a heating rate of 10 °C/min,\nheating the catalyst to 270°C and maintaining this temperature for a certain time until\nthe complete consumption of stored species, simulating the CO reduction process. By\n2\nintegrating the heat absorption at different times, the total heat absorption during the\nreaction process was determined to be 676.7 J/g (Figure R1-2). Given the catalyst mass\nof 0.5 g, a heat consumption of 338.35 J was calculated for the photoreaction test. It\nshould be noted that we only considered the heat energy required to heat the catalyst\nand initiate the reaction, excluding the heat loss from the reaction equipment, resulting\nin a lower heat input than the actual application scenario. The thermal energy\nconsumption of the reaction equipment requires further optimization.\nFigure R1-2. (a) The mass and temperature of Cu/H WO catalyst change with heating\nx 3\ntime. (b) Heat absorption and integral heat consumption of Cu/H WO catalyst with\nx 3\ndifferent heating time.\nBased on these results, we can calculate the energy conversion efficiency as follows:\n𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑐𝑐𝑐𝑐𝐸𝐸𝑐𝑐𝐸𝐸𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸 𝐸𝐸𝑒𝑒𝑒𝑒𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸𝐸𝐸𝑐𝑐𝐸𝐸 (%) = (14.885 𝐽𝐽 / (338.35 J+10,385.55 𝐽𝐽)) ×100% = 0.139%\nThis efficiency can be further optimized, particularly by focusing on the morphology\nand structure of the catalyst, as well as the method of illumination. We will continue to\nimprove the energy conversion efficiency in our future efforts.\n3. The author claimed that the stability of Cu/H WO during the four cycles is almost\nx 3\nunchanged, but what is the specific time of each cycle?\nAuthor response: We thank the referee for his/her thoughtful question. During the\ncyclic tests, two main processes are involved: the collection of electrons and hydrogen\natoms from simulated sunlight, and the thermo-catalytic conversion of CO . Each cycle\n2\nbegins with a photocatalytic reaction, where the catalyst is irradiated by a 300 W xenon\nlamp with an AM1.5 filter for 10 min under continuous stirring. The subsequent thermal\nconversion process is conducted at 270°C until no further products are formed, typically\ntaking approximately 40 min.\n4. In the absence of an additional hydrogen source, the H atom in Cu/H WO should be\nx 3\ncontinually consumed for CO conversion in the stability test. In this case, how can the\n2\nstability be guaranteed? Please elaborate on the stability test.\nAuthor response: We thank the referee for his/her helpful suggestion. The stability test\ninvolves two key processes: the collection of electrons and hydrogen atoms from\nsimulated sunlight, and the thermo-catalytic conversion of CO . Consequently, the\n2\nhydrogen source for the stability test comes from hydrogen atoms collected via\nphotocatalytic reactions under illumination during each cycle. We have included a\ndetailed description of cyclic stability test in the revised manuscript.\nIn the Methods section of the revised manuscript:\n“Cyclic stability test. The cyclic stability test of the catalyst comprises two stages: the\ncollection of electrons and hydrogen atoms under simulated sunlight illumination, and\nthe thermal reaction at 270°C in high purity CO atmosphere. The specific procedures\n2\nfor these stages are detailed in the above sections. A cycle was considered complete\nwhen the product yield ceased to increase under heating conditions. Subsequently, the\ncatalyst was irradiated by simulated sunlight illumination to initiate the next cycle.”\n5. As shown in Figure S18, there is no significant change in the structure of the catalyst\nafter the reaction. However, this cannot prove whether the content and valence state of\nCu were changed, according to Fig.2a, 2c, S1b, and S1c. Please include this key\ninformation.\nAuthor response: We thank the referee for his/her valuable suggestion. The valence\nstate of Cu was studied by Cu 2p XPS, and the content of Cu was determined by ICP-\nMS. It was found that the valence state and content (0.14 wt%) of Cu in the catalyst did\nnot change significantly before and after the cycle tests, indicating that the Cu/WO\n3\ncatalyst has good stability. These key results have been incorporated into\nSupplementary Figure 18 of the revised manuscript.\nSupplementary Figure 18. (a) XRD patterns, (b) Raman spectra, and high-resolution\n(c) O 1s, (d) Cu 2p and (e) W 4f XPS spectra of Cu/WO before and after cycle tests.\n3\n(f) FTIR spectra of Cu/WO before and after cycle tests.\n3\n6. Why is in-situ FTIR measurement carried out at 120°C. Increasing the temperature\nseems likely to see characteristic peaks belonging to CH , C H or their intermediates.\n4 2 4\nAuthor response: We thank the referee for his/her thoughtful suggestion. During the\nin-situ FTIR experiments, data were collected at 30°C intervals between 120°C and\n300°C to gather more information on CO conversion. As illustrated in Figure R1-3,\n2\neven when the reaction temperature reached 300°C, no intermediates related to CH\n4\nand C H were observed. Attempts to increase the time of FTIR data collection also\n2 4\nfailed to detect intermediates associated with CH and C H . This phenomenon may be\n4 2 4\ndue to the rapid desorption of intermediates related to CH and C H at high\n4 2 4\ntemperatures, resulting in low concentrations of surface intermediates beyond the\nresolution capability of in-situ FTIR. Therefore, we present the results obtained at\n120°C, where the CO intermediate signal is the most obvious.\nFigure R1-3. In situ FTIR spectra collected for Cu/H WO at different temperatures in\nx 3\na CO atmosphere.\n2\n7. In Fig. 4d-f, the value of C13/C12 rather than only C13 may make more sense.\nAuthor response We thank the referee for his/her insightful suggestion. We conducted\nreduction reactions using unlabeled 12CO as the reactant, and then analyzed the ion\n2\nfragment distribution of CO, CH and C H by GC-MS. As suggested by the referee,\n4 2 4\ncomparing 12CO isotopic labeling results with those obtained from 13CO -labeled\n2 2\nexperiments more effectively highlights the differences due to labeling. Therefore, we\nhave included these findings in Figs. 4d-f of the revised manuscript.\nFig. 4 Assessment of catalytic performance. (a) Product yields for Cu/H WO at\nx 3\ndifferent temperatures. (b) Selectivity of products at different reaction temperatures. (c)\nProduct yields for WO , Cu/WO , H WO and Cu/H WO at 270°C. Mass spectra of\n3 3 x 3 x 3\n(d) CO, (e) CH and (f) C H over Cu/H WO with 13CO or 12CO as the reactant at\n4 2 4 x 3 2 2\n270°C. The error bar represents the standard deviation of the measurements.\n8. It is recommended to use Cu/D WO for CO conversion to detect D-containing\nx 3 2\nproducts such as CH D, C H D or others possible.\n3 2 3\nAuthor response: We thank the referee for his/her thoughtful question. We have\nemployed Cu/D WO for CO conversion reaction and characterized the ion fragment\nx 3 2\ndistribution of methane and ethylene using GC-MS. The results indicate that the\nproducts contain CD and C D , further confirming that the hydrogen atoms in the\n4 2 4\nproducts predominantly originate from the hydrogen stored within the catalyst. We have\nincluded this result in the Supplementary Figure 21 of the revised manuscript.\nSupplementary Figure 21. Mass spectra of (a) CH and (b) C H over Cu/D WO with\n4 2 4 x 3\n12CO as the reactant at 270°C.\n2\n9. A writing error in Fig. 2i. The Y-axis label should be TPR rather than TPD.\nAuthor response: We thank the referee for pointing out this issue. We have changed\nthe Y-axis label in Fig. 2i from TPD to TPR.\nFig. 2 Characterization of catalysts. (a) XRD patterns, (b) high-resolution Cu 2p XPS\nspectra and (c) Raman spectra of WO and Cu/WO . (d) HAADF-STEM image of\n3 3\nCu/WO . (e) The structure of hexagonal WO . (f) EDS mapping for Cu, W and O\n3 3\nelements of Cu/WO . (g) Normalized Cu K-edge XANES spectra of Cu/WO , Cu foil\n3 3\nand CuO. (h) Cu K-edge EXAFS spectra of Cu/WO , Cu foil and CuO. (i) H -TPR\n3 2\nspectra of WO and Cu/WO .\n3 3\n10. Some recent important discoveries (Nat. Catal. 2023, 6, 987-995, Proc. Natl. Acad.\nSci. U. S. A. 2022, 119, e2121174119.) are highly related to this topic. Appropriate\ncitations of these discoveries are recommended.\nAuthor response: We thank the referee for his/her valuable suggestion. We have cited\nthese important research advances in the revised manuscript.\nIn the References section of the revised manuscript:\n6. Zhou B. et al. Light-driven synthesis of C H from CO and H O on a bimetallic AuIr\n2 6 2 2\ncomposite supported on InGaN nanowires. Nat. Catal. 6, 987-995 (2023).\n13. Rashid R.T. et al. Tunable green syngas generation from CO and H O with sunlight\n2 2\nas the only energy input. Proc. Natl. Acad. Sci. U.S.A. 119, e2121174119 (2022).\nReviewer #2 (Remarks to the Author):\nThis work reports an approach to store electrons and hydrogen atoms in catalysts using\nsunlight and water, which can be released for CO reduction in dark in a controlled\n2\nmanner at relatively low temperatures 150ºC–300ºC for on-demand CO conversion.\nanalysis of the catalytic system consisting of loaded single Cu sites on hexagonal\ntungsten trioxide. It is noteworthy that unlike in conventional photocatalysis,\nseparation of oxygen is not necessary and that the stored electrons and hydrogen atoms\ncan be used on demand by simple heating. The stability of the catalysts is also very\npromising.\nAuthor response: We really appreciate the referee’s highly positive evaluation of our\nwork, and are grateful to the referee for his/her insightful suggestions to help us further\nimprove the quality of our manuscript. We have made all the revisions as suggested by\nthe referee.\nFigure 2 depicts the concept of this approach. This concept is not aligned with the goal\nof reaching carbon neutrality, as the world needs to stop using fossil fuels for energy\nproduction. Thus, in Figure 2 “fossil fuel” should be replaced by renewable\nfuels/biofuels or renewable energy. Furthermore, the direction of one of the arrows\nshould be inverted Cu/WO to light reaction and not the opposite.\nreplaced “fossil fuel” with “renewable fuels/biofuels” in the revised manuscript to align\nwith long-term carbon neutrality goals. Additionally, we have inverted the mentioned\narrow to accurate direction.\nFig. 1 Schematic illustration of on-demand CO conversion, based on\n2\nelectron−hydrogen storage in catalysts with sunlight and water and their release with\nresidual heat.\nAlthough other systems were reported with similar characteristics e.g. Bi-H ref. 35 of\nx\nthe paper of Li et al. and Ir-H -WO ref. 36 of the paper of Zheng et al. the very detailed\nx 3\ninvestigation of the Cu-H -WO catalyst including structural characterization,\nx 3\ndetermination of storage capacity of hydrogen and electrons and of the catalytic\nmechanism adds to the knowledge base of the field.\nAuthor response: We thank the referee for his/her positive comment.\nThe work supports the conclusions and most of the claims, however it seems to be\nlimited in terms of scalability. The catalyst was scaled-up by two orders of magnitude,\nbut product yield did not increased proportionally. At this stage, the ratio product\nyield/quantity of catalysts is not suitable for a commercial application as described in\nFigure 6a. The possibility of overcoming this problem with the present system Cu-H -\nx\nWO is doubtful. The authors should comment on this issue.\nsignificant differences in yield with different catalyst amounts, the Y-axis in the data\nvisualization was chosen to be logarithmic in order to display as much data as possible\n(Figure R2-1, Fig. 6d of the previous version). This makes the increase of product yield\nappear less obvious as the catalyst dosage increases. In fact, when the catalyst amount\nincreases by two orders of magnitude, the product yield also increases by almost two\norders of magnitude although the yield per unit mass of catalyst decreases slightly\n(Supplementary Figure 24). This decrease is likely related to the mass transfer process\nof the catalyst, which could be addressed by optimizing the reactor design. To make the\nrelationship between catalyst amount and product yield more intuitive, we changed the\nY-axis from logarithmic to linear in the revised manuscript (Figure R2-2, Fig. 6d of the\nrevised manuscript) and added a graph showing the relationship between catalyst\namount and product yield in the Supplementary Information (Supplementary Figure 24).\nAdmittedly, realizing the application scenario described in Figure 6a remains\nchallenging at this stage. Achieving large-scale application will require significant\nefforts, particularly in catalyst preparation and reactor design, which will be the focus\nof our future research. In the revised manuscript, we have commented on this issue.\nFigure R2-1. Product yields for Cu/H WO with different catalyst dosage (Fig. 6d of\nx 3\nthe previous version).\nFigure R2-2. Product yields for Cu/H WO with different catalyst dosage (Fig. 6d of\nx 3\nthe revised manuscript).\nSupplementary Figure 24. Product yields for per gram of Cu/H WO with different\nx 3\ncatalyst dosage.\nIn Figure 6a from 10:00 onwards no further colour change can be perceived from the\nfigure. The of the figure should be improved. The authors should indicate how they\nreached the conclusion that at 16:00 the catalyst reached saturation, if by colour\nchange or by quantitative measurement.\nAuthor response: We thank the referee for bringing this to our attention. In the revised\nmanuscript, we have added a new set of high-resolution photographs (Fig. 6a). We also\nmeasured the amount of hydrogen atoms stored in the catalyst every two hours to\nfurther confirm that the catalyst had reached saturation. The hydrogen atom content\ndata have been added to Supplementary Table 3.\nFig. 6 Demonstration of utilizing natural light for the process. a Proposed block\nflow diagram to achieve on-demand CO conversion based on electron−hydrogen\n2\nstorage in catalysts with sunlight and water and their release with residual heat. The\nimages on the left show the catalyst under natural light demonstrating the process of\ncollecting hydrogen atoms and electrons. b Product yields for Cu/H WO by exposure\nx 3\nto natural sunlight and simulated sunlight. c Product yields for Cu/H WO after being\nx 3\nstored in the dark for 15 days and 90 days. d Product yields for Cu/H WO with\nx 3\ndifferent catalyst dosage. The error bar represents the standard deviation of the\nmeasurements.\nSupplementary Table 3. Change of the amount of stored H in catalyst during outdoor\ntest.\nTime The amount of stored H (mmol/g)*\n6: 00 0.007\n8: 00 0.009\n10: 00 0.956\n12: 00 2.821\n14: 00 3.593\n16: 00 4.431\n18: 00 4.390\n20: 00 4.400\n*The amount of stored H was measured by means of ion exchange.\nIn the methods section or in the supplementary information a detailed description of\nthe determination and calculation of hydrogen storage capacity should be provided for\nthe work to be reproduced. Quantitative values of storage capacity should be provided\nto allow a comparison with other materials that are being developed for hydrogen\nstorage.\nAuthor response: We thank the referee for his/her thoughtful suggestion. We have\nprovided a detailed description of the determination and calculation of hydrogen storage\ncapacity in the Supplementary Information of the revised manuscript.\nIn the Supplementary Note 7 of the revised manuscript:\nThe amount of stored H was measured by means of ion exchange (Ref: Nat. Commun.\n10, 327 (2019)). 10 mg of photocatalyst powder was dispersed in 30 mL of deionized\nwater and then ultrasonically dispersed for 30 min. Subsequently, the air in the reactor\nwas removed by vacuum pump for 10 min while the pH of the solution was close to 7.\nThen, excessive KNO solution (0.5 M, 4 mL) was dropped into the obtained solution\n3\nin the dark with constant stirring. After stirring for 3 h until the pH no longer changed,\nthe sample was separated by centrifugation, and the pH of the separated solution was\nmeasured. This value was used to evaluate the amount of released H through the\nfollowing equation:\n𝑀𝑀(𝐻𝐻) = 10 ×0.034×10^-6\nwhere M(H) is the amount of hydrogen− atoms (μmol). Three tests were conducted on\nthe Cu/H WO catalyst, yielding pH values of 3.839, 3.876, and 3.846, respectively.\nx 3\nThe amount of stored hydrogen atoms in Cu/H WO catalyst was calculated to be 0.477\nx 3\nmmol/g.\nThe methodology is sound and in accordance with the standards in the field.\nI recommend the work for publication after revision.\nAuthor response: We really appreciate the referee’s highly positive evaluation of our\nwork, and are grateful to the referee for his/her insightful suggestions to help us\nsubstantially improve the quality of our manuscript. We sincerely hope that our careful\nrevisions have satisfactorily addressed the referee’s concerns.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nAuthor response: We really appreciate the referee for his/her participation in the\nreview of this manuscript. In addition, the choice of training in peer review for Early\nCareer Researchers is commendable. We sincerely hope that our careful revisions have\nsatisfactorily addressed the referee’s concerns.\nReviewer #4 (Remarks to the Author):\nThe authors reported a Cu/WO photocatalyst which could store electron and proton\n3\nequivalents under irradiation. Using stored electrons and protons, CO reduction\n2\ndelivered value-added fuels. This work demonstrated a facile pathway for effective CO\n2\nreduction by stepwise storage and utilization of electron and proton equivalents. In-\ndepth study was carried out to unravel how electrons and protons were stored and\nutilized. The following questions are clarified:\nAuthor response: We really appreciate the referee’s positive evaluation of our work,\nand are grateful to the referee for his/her insightful suggestions to help us substantially\nimprove the quality of our manuscript. We sincerely hope that our careful revisions\nhave satisfactorily addressed the referee’s concerns (see below).\n1. Is the storage of electrons and protons stoichiometric? How many equivalents of\nelectrons and protons could be stored with a given amount of photocatalyst?\nAuthor response: We thank the referee for his/her insightful question. Given the\nchallenges in accurately quantifying electron storage, we have indirectly demonstrated\nthe stoichiometry of electrons and hydrogen atoms. Two primary mechanisms of\nelectron storage in catalysts have been reported in the literature (Ref: Adv. Energy Mater.\n11, 2003049 (2021)). The first mechanism involves co-storage with cation to maintain\noverall charge balance, resulting in a stoichiometric relationship between electrons and\nhydrogen atoms (Mechanism 1). The second mechanism involves electrons being\ncaptured by defect sites in the catalyst without the participation of protons (Mechanism\n2). In our work, we observed that the synchronization of changes of electrons and\nhydrogen atoms during the reaction, which supports the existence of Mechanism 1.\nAdditionally, we tested the EPR spectra of Cu/WO and Cu/H WO and found no\n3 x 3\nformation of oxygen vacancies (g-factor = 2.002), thereby ruling out Mechanism 2\n(Figure R4-1). Based on these results, we infer that the electrons and protons stored in\nthe catalyst are stoichiometric.\nWe quantified the storage amount of electrons and protons by measuring the amount of\nhydrogen atoms in the catalyst. Following the method reported in the literature (Ref:\nNat. Commun. 10, 327 (2019)), the test results show that the hydrogen and electron\nstorage in the Cu/H WO catalyst is 0.477 mmol/g. We have provided a detailed\nx 3\ndescription of the determination and calculation of hydrogen storage capacity in the\nSupplementary Information of the revised manuscript.\nIn the Supplementary Note 7 of the revised manuscript:\nThe amount of stored H was measured by means of ion exchange. 10 mg of\nphotocatalyst powder was dispersed in 30 mL of deionized water and then ultrasonically\ndispersed for 30 min. Subsequently, the air in the reactor was removed by vacuum pump\nfor 10 min while the pH of the solution was close to 7. Then, excessive KNO solution\n3\n(0.5 M, 4 mL) was dropped into the obtained solution in the dark with constant stirring.\nAfter stirring for 3 h until the pH no longer changed, the sample was separated by\ncentrifugation, and the pH of the separated solution was measured. This value was used\nto evaluate the amount of released H through the following equation:\n𝑀𝑀(𝐻𝐻) = 10 ×0.034×10^-6\nwhere M(H) is the amount of hydrogen− atoms (μmol). Three tests were conducted on\nthe Cu/H WO catalyst, yielding pH values of 3.839, 3.876, and 3.846, respectively.\nx 3\nThe amount of stored hydrogen atoms in Cu/H WO catalyst was calculated to be 0.477\nx 3\nmmol/g.\nFigure R4-1. EPR spectra of Cu/WO and Cu/H WO\n3 x 3\n2. A very low loading of Cu (0.14 wt% based on ICP, line 104) was applied. Does the\nloading of Cu affect the capacity of electron and proton storage?\nAuthor response: We thank the referee for his/her thoughtful question. We synthesized\ncatalysts with different Cu content, and tested the storage capacity of H atoms in these\ncatalysts under simulated sunlight. The results are summarized in Table R4-1. The test\nresults indicate a significant increase in the storage of electrons and hydrogen atoms\nupon the introduction of Cu, due to the hydrogen spillover effect induced by Cu.\nHowever, varying the Cu content form 0.14 wt% to 1.01 wt% does not significantly\naffect the amount of stored hydrogen atoms and electrons in the catalyst. This is mainly\nbecause the amounts of stored electrons and hydrogen atoms are related to the\nproperties of the storage sites of the carrier material rather than the Cu sites. In the\nsynthesis method reported in this work, the loaded metal sites tend to agglomerate as\nthe loading of metal elements increases (Ref: J. Am. Chem. Soc. 134, 6508−6511\n(2012)). In order to maximize the utilization of Cu element and maintain the uniformity\nof the active sites, a catalyst with a low Cu loading (0.14 wt%) was selected as the\nmodel material for the experiment.\nTable R4-1. Hydrogen storage capacity for catalysts with varying Cu content.\nCu content (wt%)* 0 0.14 0.29 0.71 1.01\nThe amount of hydrogen 0.306 0.477 0.475 0.468 0.482\nstored （mmol/g）**\n*The value was obtained by ICP-MS\n**The value is obtained by ion exchange experiment\n3. A probable estimate of the H atom content in the metastable intermediate Cu/H WO\nx 3\nshould be made to evaluate the efficiency of the subsequent CO reduction process.\nmeasured the H atom content in the metastable intermediate Cu/H WO , which was\nx 3\ndetermined to be 0.477 mmol/g. Based on this result and the hydrogen consumption\nrequired for the formation of products, we evaluated the hydrogen release rate and the\neffective utilization of hydrogen atoms during the subsequent CO reaction. These\n2\nresults have been incorporated into Supplementary Figure 16 of the revised manuscript.\nIn Supplementary Figure 16 of the revised manuscript:\nSupplementary Figure 16. (a) The release rate of stored H atoms and electrons, and\n(b) the percentage of stored H atoms to products at different temperature for Cu/H WO .\nx 3\nThe error bar represents the standard deviation of the measurements.\nRelease rate of stored H atoms refers to the rate of transfer of stored hydrogen atoms to\nthe products. The formula for calculating this parameter is shown as follows:\n𝑅𝑅𝐸𝐸𝑒𝑒𝐸𝐸𝑒𝑒𝑐𝑐𝐸𝐸 𝐸𝐸𝐸𝐸𝑂𝑂𝐸𝐸 𝑐𝑐𝑒𝑒 𝑐𝑐𝑂𝑂𝑐𝑐𝐸𝐸𝐸𝐸𝑑𝑑 𝐻𝐻 𝑒𝑒𝑂𝑂𝑐𝑐𝑒𝑒𝑐𝑐 = (𝑁𝑁(𝐻𝐻2)×2+𝑁𝑁(𝐶𝐶𝑂𝑂)×2+𝑁𝑁(𝐶𝐶𝐻𝐻4)×8+𝑁𝑁(𝐶𝐶2𝐻𝐻4)×12) / 𝑅𝑅𝐸𝐸𝑒𝑒𝑐𝑐𝑂𝑂𝑐𝑐𝑐𝑐𝐸𝐸 𝑂𝑂𝑐𝑐𝑒𝑒𝐸𝐸\nThe percentage of H atoms to products refers to the ratio of stored hydrogen atoms\nconverted into final products such as CO, CH , H and C H . The calculation formula\n4 2 2 4\nof the percentage of H atoms to products is shown as follows:\n𝑇𝑇ℎ𝐸𝐸 𝑂𝑂𝐸𝐸𝐸𝐸𝑐𝑐𝐸𝐸𝐸𝐸𝑂𝑂𝑒𝑒𝐸𝐸𝐸𝐸 𝑐𝑐𝑒𝑒 𝐻𝐻 𝑒𝑒𝑂𝑂𝑐𝑐𝑒𝑒𝑐𝑐 𝑂𝑂𝑐𝑐 𝑂𝑂𝐸𝐸𝑐𝑐𝑑𝑑𝑂𝑂𝑐𝑐𝑂𝑂𝑐𝑐(%) = ((𝑁𝑁(𝐻𝐻2)×2+𝑁𝑁(𝐶𝐶𝑂𝑂)×2+𝑁𝑁(𝐶𝐶𝐻𝐻4)×8+𝑁𝑁(𝐶𝐶2𝐻𝐻4)×12) / 𝑁𝑁(𝐻𝐻 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎)) ×100%\nThe , , and 𝑁𝑁 (r𝐻𝐻ep 𝑎𝑎r𝑎𝑎e𝑎𝑎s𝑎𝑎e𝑎𝑎n)t the number of molecules of H 2 ,\nCO, CH and C H in the products, respectively. refers to the number of H\n4 2 4\n𝑁𝑁(𝐻𝐻2) 𝑁𝑁(𝐶𝐶𝑂𝑂) 𝑁𝑁(𝐶𝐶𝐻𝐻4) 𝑁𝑁(𝐶𝐶2𝐻𝐻4)\natoms stored in the catalyst.\n4. Why is the absence of additional peaks in the XRD pattern of Cu/WO compared to\n3 3\nAuthor response: We thank the referee for bringing this to our attention. In the original\nmanuscript, the XRD results were described with reference to previous studies showing\nthat agglutination of loaded metal sites results in metal-correlated peaks in the XRD\npattern (Refs: Nat. Commun. 9, 1252 (2018); Nature 589, 396-401 (2021)). In this study,\nthe Cu loading is low (0.14 wt%), making it difficult for Cu-related diffraction peaks to\nappear in the XRD pattern. Therefore, this inaccurate description has been revised in\nthe revised manuscript.\n5. How is the recyclability of Cu/WO photocatalyst for electron and proton storage\n3\nand release? The Figure S17 only presents four sets of data from cyclic experiments,\nwhereas the text indicates the existence of six sets of cyclic experiments.\nAuthor response: We thank the referee for pointing out this issue. The cyclic stability\ntest of the catalyst comprises two stages: the collection of electrons and hydrogen atoms\nunder simulated sunlight illumination, and the thermal reaction at 270°C in high purity\nCO atmosphere. A cycle was considered complete when the product yield ceased to\n2\nincrease under heating conditions. Subsequently, the catalyst was irradiated by\nsimulated sunlight illumination to initiate the next cycle. We have included a detailed\ndescription of cyclic stability test in the revised manuscript.\nAdditionally, the data from the last two cycle tests were omitted by mistake in the\noriginal Supplementary Figure 17. We have replaced the figure in the revised version\nand conducted a thorough review of the entire manuscript to prevent similar issues.\nIn the Methods section of the revised manuscript:\n“Cyclic stability test. The cyclic stability test of the catalyst comprises two stages: the\ncollection of electrons and hydrogen atoms under simulated sunlight illumination, and\nthe thermal reaction at 270°C in high purity CO atmosphere. The specific procedures\n2\nfor these stages are detailed in the above sections. A cycle was considered complete\nwhen the product yield ceased to increase under heating conditions. Subsequently, the\ncatalyst was irradiated by simulated sunlight illumination to initiate the next cycle.”\nIn Supplementary Figure 17:\nSupplementary Figure 17. Stability of Cu/WO during six reaction cycles. The cyclic\n3\nstability test comprises two processes: Collection of electrons and hydrogen atoms\nunder simulated sunlight illumination, and the thermal reaction at 270°C in high purity\nCO atmosphere. The error bar represents the standard deviation of the measurements.\n2\n6. The authors need complete the error bar.\nAuthor response: We thank the referee for his/her valuable suggestion. We have\nincluded error bars in the CO reduction performance evaluation results, including Figs.\n2\n4a-c, Figs. 6b-d, Supplementary Figure 15, Supplementary Figure 16, Supplementary\nFigure 17, and Supplementary Figure 24.\nFig. 4 Assessment of catalytic performance. a Product yields for Cu/H WO at different\nx 3\ntemperatures. b Products selectivity for Cu/H WO at different reaction temperatures. c Product\nx 3\nyields for WO , Cu/WO , H WO and Cu/H WO at 270°C. Mass spectra of d CO, e CH and f\n3 3 x 3 x 3 4\nC H over Cu/H WO with 13CO or 12CO as the reactant at 270°C. The error bar represents the\n2 4 x 3 2 2\nstandard deviation of the measurements.\nFig. 6 Demonstration of utilizing natural light for the process. a Proposed block\nflow diagram to achieve on-demand CO conversion based on electron−hydrogen\n2\nstorage in catalysts with sunlight and water and their release with residual heat. The\nimages on the left show the catalyst under natural light demonstrating the process of\ncollecting hydrogen atoms and electrons. b Product yields for Cu/H WO prepared\nx 3\nunder natural sunlight and simulated sunlight. c Product yields for Cu/H WO after\nx 3\nbeing stored in the dark for 15 days and 90 days. d Product yields for Cu/H WO with\nx 3\ndifferent catalyst dosage. The error bar represents the standard deviation of the\nmeasurements.\nSupplementary Figure 15. Product yield for Cu/H WO at different temperatures. The error bar\nx 3\nrepresents the standard deviation of the measurements.\nSupplementary Figure 16. (a) The release rate of stored H atoms, and (b) the percentage of stored\nH atoms to products at different temperature for Cu/H WO . The error bar represents the standard\nx 3\ndeviation of the measurements.\nSupplementary Figure 17. Stability of Cu/WO during six reaction cycles. The error bar represents\n3\nthe standard deviation of the measurements.\nSupplementary Figure 24. Product yield for per gram of Cu/H WO with different catalyst dosage.\nx 3\n7. Is Cu/WO photocatalyst able to perform CO reduction with low-concentration CO ?\n3 2 2\nAuthor response: We thank the referee for his/her thoughtful question. We conducted\nCO reduction reactions using low concentrations of CO (10 vol%, 1 vol%, and 0.1\n2 2\nvol%, balanced with Ar). As shown in Figure R4-2, the results indicate that as the CO\n2\nconcentration decreases, the yield of carbon-containing products decreases\nsignificantly while the yield of H increases. This suggests that CO selectivity\n2 2\ndecreases at low CO concentrations and H becomes the primary product under these\n2 2\nconditions although CO conversion still occurs.\n2 2\nyield and (b) product selectivity.\n8. In the discussion section, the author emphasized the formation process of C H , but\n2 4\nthe mechanism of CO reduction to C H was not sufficiently discussed in the\n2 2 4\nmanuscript.\nAuthor response: We thank the referee for his/her valuable comments. The formation\nof C H involves two key processes (Refs: Chem. Soc. Rev. 52, 1773-1946 (2023); Nat.\n2 4\nCommun. 15, 2422 (2024)), including proton-coupled electron transfer processes and\nC−C coupling processes. First, C H formation requires the consumption of 12\n2 4\nelectrons and 12 protons. We observed that with increasing temperature, the supply of\nhydrogen atoms and electrons increases (Supplementary Figure 16), resulting in a\nhigher C H content in the product, consistent with experimental observations. For the\n2 4\nC−C coupling process, we investigated the reaction intermediates by using in situ FTIR\nspectroscopy (Figure R4-3). Unfortunately, we did not detect the key intermediates of\nthe C−C coupling process. In this work, we primarily present a conceptual approach for\non-demand CO conversion using electron-hydrogen reservoirs. We will therefore\n2\ncontinue to accomplish this task in our future efforts. We have discussed the mechanism\nof CO reduction to C H as much as possible in the revised manuscript.\n2 2 4\nFigure R4-3. In situ FTIR spectra collected for Cu/H WO at different temperatures in\nx 3\na CO atmosphere.\n2 4\nthis observation underscores that the elevated temperatures can facilitate to provide\nadditional electrons and protons for the sites of CO activation. To better understand\n2\nthis aspect, we determined the conversion rates of electrons and hydrogen atoms by\nanalyzing the yield of final products, which show a positive correlation with the\ntemperatures (Supplementary Fig. 16). This suggests that thermal energy input\nenhances the migration and release of electrons and hydrogen atoms.\n9. Why does the N adsorption capacity of Cu/WO decreased compared to WO ?\n2 3 3\nAuthor response: We thank the referee for his/her insightful question. First, we\nsuspected that the measurement error caused the lower N adsorption capacity of\n3 3 2\nisotherms of WO and Cu/WO , we found that the adsorption capacity of WO remains\n3 3 3\nslightly higher than that of Cu/WO (Supplementary Figure 4). Further analysis\n3\nrevealed that this variation in adsorption capacity is due to changes in the specific\nsurface area of the catalysts. Specifically, the BET method determined the specific\nsurface areas of WO and Cu/WO to be 84.3 m2/g and 79.2 m2/g, respectively. This\n3 3\nfeature may be attributed to slight alterations in the surface sites of the catalyst (Refs:\nNat. Catal. 1, 781–786 (2018); Environ. Sci. Technol. 53, 11391−11400 (2019)). The\nrelevant analysis results have been included in the Supplementary Information of the\nrevised manuscript.\nSupplementary Figure 4. N sorption isotherms of WO and Cu/WO .\n2 3 3\n10. How to prove that Cu is reduced by W at reduced state?\nAuthor response: We thank the referee for his/her thoughtful suggestion. In the EPR\nspectra, both the reduced state of W(5+) and the high-valence state of Cu(2+) exhibit\nparamagnetic signals (Refs: Nat. Mater. 18, 620-626 (2019); Nat. Commun. 10, 327\n(2019)). Therefore, we stirred Cu(NO ) ·3H O in the liquid phase with irradiated WO\n3 2 2 3\nfor 12 h, followed by EPR spectroscopy analysis of the obtained sample. Additionally,\nwe separately stirred the irradiated WO and Cu(NO ) ·3H O solution (containing the\n3 3 2 2\nsame amounts of Cu or W elements as the control sample) under the same conditions.\nAs shown in Figure R4-4, the irradiated WO displays a distinct W(5+) signal, while\n3 2 2 3\nwith Cu(NO ) ·3H O sample, the signals of W(5+) and Cu(2+) both significantly\n3 2 2\ndecreased, indicating a reaction between Cu(2+) and W in the reduced state.\nFurthermore, when Cu(NO ) ·3H O solution was added to the irradiated WO solution,\n3 2 2 3\nthe color of the WO solution changed from deep blue to light blue (Figure R4-5),\n3\nfurther supporting the conclusion that Cu was reduced by W in the reduced state.\nMoreover, a previous study has also reported the role of the reduced state of W in Cu\nreduction (Ref: Chem. Eng. J. 451, 138801 (2023)).\nFigure R4-4. EPR spectra of Cu(NO ) ·3H O, WO solution after irradiation and the\n3 2 2 3\nproduct of the reaction of Cu(NO ) ·3H O with irradiated WO .\n3 2 2 3\nFigure R4-5. (a) Photograph of the WO solution after irradiation by a 300 W xenon\n3\nlamp for 1 min. (b) Photograph of the irradiated WO solution after the addition of\n3 2 2\n11. Why did the amount of catalyst and the products increase in different proportions in\nFigure 6d?\nAuthor response: We thank the referee for bringing this to our attention. Due to the\nsignificant differences in yield with different amounts of catalyst, the Y-axis of Figure\n6d was chosen to be logarithmic in the data visualization to display as much data as\npossible (Figure R4-6, Fig. 6d of the previous version). This makes the increase of\nproduct yield appear less obvious as the catalyst dosage increases. In fact, when the\ncatalyst amount increases by two orders of magnitude, the product yield also increases\nby almost two orders of magnitude although the yield per unit mass of catalyst\ndecreases slightly (Supplementary Figure 24). This decrease is likely related to the mass\ntransfer process of the catalyst, which could be addressed by optimizing the reactor\ndesign. To make the relationship between catalyst amount and product yield more\nintuitive, we changed the Y-axis from logarithmic to linear in the revised manuscript\n(Figure R4-7, Fig. 6d of the revised manuscript) and added a graph showing the\nrelationship between catalyst amount and product yield in Supplementary Figure 24 of\nthe revised manuscript.\nFigure R4-6. Product yields for Cu/H WO with different catalyst dosage (Fig. 6d of\nx 3\nthe previous version).\nFigure R4-7. Product yields for Cu/H WO with different catalyst dosage (Fig. 6d of\nx 3\nthe revised manuscript).\nSupplementary Figure 24. Product yields for per gram of Cu/H WO with different\nx 3\ncatalyst dosage.\n12. During the synthesis of Cu/WO , why is it important to expose WO with xenon lamp\n3 3\nirradiation for 1 minute before adding Cu(NO ) ? Why is it necessary to calcine first in\n3 2\nair and then in N ?\nwas irradiated under a xenon lamp for 1 min to controllably generate a specific amount\nof W(5+), which can induce copper species to load onto the catalyst surface. Due to the\npresence of H atoms and W(5+) in the catalyst after hydrothermal treatment or xenon\nlamp irradiation, the product yield in subsequent reactions becomes uncontrollable.\nTherefore, calcination in air was employed to remove these residual species.\nCalcination under N was conducted to simulate the anaerobic conditions of CO\n2 2\nreduction, thereby enhancing the stability of the catalyst under anoxic conditions and\npreventing significant structural changes during the CO reduction process.\n2\n13. The reaction between water and Cu/WO yields oxygen and Cu/H WO . In Fig.1,\n3 x 3\nthe arrow between light reaction and Cu/WO in Figure 1 may be drawn upside down.\nthe direction of the arrows in Figure 1 to ensure that the correct reaction process is\nindicated.\nFig. 1 Schematic illustration of on-demand CO conversion, based on\n2\nelectron−hydrogen storage in catalysts with sunlight and water and their release with\nresidual heat.\n14. In Fig. 3d, three kinds of the FTIR spectra of materials should be separated in order\nto better observe the difference.\nAuthor response: We thank the referee for his/her helpful suggestion. As shown in\nFigure R4-8, we have separated the FTIR spectra of the three materials to better observe\nthe differences. In the revised manuscript, we have made corresponding modifications\nto Fig. 3d.\nFigure R4-8. FTIR spectra of Cu/WO , Cu/HxWO and Cu/D WO (Fig. 3d of the\n3 3 x 3\nrevised manuscript)\n15. Please add the standard curve for the detection of CO reduction products by gas\n2\nchromatography to the Supporting Information.\nAuthor response: We thank the referee for his/her valuable suggestion. The standard\ncurves of CO reduction products such as CO, CH , C H and H detected by gas\n2 4 2 4 2\nchromatography have been added to Supplementary Figure 26 of the revised manuscript.\nSupplementary Figure 26. Standard curves for the detection of (a) CO, (b) CH , (c)\n2 4 2\n16. The authors attribute the peak at 2500-2700 cm-1 in the FTIR data to the signal of\nO-D. References should be provided.\nAuthor response: We thank the referee for bringing this to our attention. We have\nsupplemented the necessary references in the revised manuscript.\nIn the “Catalytic mechanisms” section of the revised manuscript:\nThe resulting product, denoted as Cu/D WO , exhibits an FTIR spectrum where the\nx 3\npeak associated with O–D species appears in the range of 2500-2700 cm-1 after light\nirradiation37.\nIn the “References” section of the revised manuscript:\n37. Yin S. et al. Elucidating protonation pathways in CO photoreduction using the\n2\nkinetic isotope effect. Nat. Commun. 15, 437 (2024).\n17. The information of the wavelength and optical power of the light source used in the\nlight reaction is requested.\nAuthor response: We thank the referee for his/her valuable suggestion. We have\nincluded the details of the wavelength and optical power of the light source used in the\nlight reaction in the revised manuscript.\nIn the Methods section of the revised manuscript:\nThe optical power of the light source was recorded by the optical power meter\n(PM100D, Thorlabs, USA) and the corresponding optical power density was 0.45\nW/cm2. The wavelength distribution information of the light source is shown in\nSupplementary Figure 25.\nIn Supplementary Figure 25 of the revised manuscript:\nSupplementary Figure 25. Wavelength information of the 300 W xenon lamp with\nAM1.5 filter used in this work.\n18. What is the meaning of the percentage of H atoms to products and H/e- conversion\nrate in Figure S16? Please provide the formulae for these two parameters. When the\nreaction temperature is raised from 240℃ to 270℃, why do both parameters show a\nsimilar trend?\nAuthor response: We thank the referee for his/her thoughtful suggestion. The\npercentage of H atoms to products refers to the ratio of stored hydrogen atoms\nconverted into final products such as CO, CH , H and C H . The calculation formula\n4 2 2 4\nof the percentage of H atoms to products is shown as follows:\n𝑇𝑇ℎ𝐸𝐸 𝑂𝑂𝐸𝐸𝐸𝐸𝑐𝑐𝐸𝐸𝐸𝐸𝑂𝑂𝑒𝑒𝐸𝐸𝐸𝐸 𝑐𝑐𝑒𝑒 𝐻𝐻 𝑒𝑒𝑂𝑂𝑐𝑐𝑒𝑒𝑐𝑐 𝑂𝑂𝑐𝑐 𝑂𝑂𝐸𝐸𝑐𝑐𝑑𝑑𝑂𝑂𝑐𝑐𝑂𝑂𝑐𝑐(%) = ((𝑁𝑁(𝐻𝐻2)×2+𝑁𝑁(𝐶𝐶𝑂𝑂)×2+𝑁𝑁(𝐶𝐶𝐻𝐻4)×8+𝑁𝑁(𝐶𝐶2𝐻𝐻4)×12) / 𝑁𝑁(𝐻𝐻 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎)) ×100%\nThe H/e- conversion rate refers to the rate at which the stored hydrogen atoms and\nelectrons are transferred to the products. The calculation formula for this parameter is\nshown as follows:\nH/e conversion rate = (𝑁𝑁(𝐻𝐻2)×2+𝑁𝑁(𝐶𝐶𝑂𝑂)×2+𝑁𝑁(𝐶𝐶𝐻𝐻4)×8+𝑁𝑁(𝐶𝐶2𝐻𝐻4)×12) / 𝑅𝑅𝐸𝐸𝑒𝑒𝑐𝑐𝑂𝑂𝑐𝑐𝑐𝑐𝐸𝐸 𝑂𝑂𝑐𝑐𝑒𝑒𝐸𝐸\nwhere , , and represent the number of molecules of H ,\n4 2 4\n𝑁𝑁(𝐻𝐻2) 𝑁𝑁(𝐶𝐶𝑂𝑂) 𝑁𝑁(𝐶𝐶𝐻𝐻4) 𝑁𝑁(𝐶𝐶2𝐻𝐻4)\natoms stored in the catalyst.\nAs the reaction temperature rises from 240°C to 270°C, the percentage of H atoms to\nproducts becomes stable. This phenomenon is primarily attributed to the complete\nconsumption of competing species that utilizes H atoms on the catalyst surface above\n240°C. However, the release rate of stored H atoms continues to rise with increasing\ntemperature. Therefore, over the temperature range within the margin of error, the two\nparameters show what appears to be a consistent trend.\nIn Supplementary Figure 15:\nSupplementary Figure 15. (a) The release rate of stored H atoms and electrons, and\n(b) the percentage of stored H atoms to products at different temperature for Cu/H WO .\nx 3\n19. The authors should give the calibration and binding energies calibrated for the\ncarbon 1 s peak for XPS.\nAuthor response: We thank the referee for his/her valuable suggestion. All the binding\nenergy values of XPS spectra were calibrated referring to the C 1s peak at 284.8 eV.\n(Refs: Nat. Commun. 14, 4209 (2023); Nature Energy 8, 504–514 (2023)). We have\nadded this important information to the Supporting Information of revised manuscript.\nIn the Supplementary Note 3. Characterizations:\nXPS spectra were collected on an X-ray photoelectron spectrometer (Thermo\nESCALAB 250, USA). All the binding energy values of XPS spectra were calibrated\nreferring to the C 1s peak at 284.8 eV.\n20. The W(6+) of W 4f XPS spectra for Cu/WO and Cu/H WO should be clearly\n3 x 3\nmarked.\nAuthor response: We thank the referee for bringing this to our attention. We have\nclearly marked W(6+) in the revised manuscript.\nIn Supplementary Figure 10 of the revised manuscript:\nSupplementary Figure 10. High-resolution W 4f XPS spectra of Cu/WO and\nx 3\n21. In Supplementary Fig. 18c, the W 4f XPS spectra has a significant shift after\nundergoing cyclic reactions. Why?\nAuthor response: We thank the referee for his/her thoughtful question. In the W 4f\nXPS spectra of the catalyst, the binding energies of W 4f were slightly shifted to lower\nenergies after the reaction compared to the results of Cu/WO before the cyclic stability\n3\ntest, indicating the incomplete consumption of stored electrons. In order to prove this\nconclusion, we examined the FTIR spectra of the Cu/WO before and after the reaction,\n3\nand found that the corresponding O−H vibration peak increased slightly after the cyclic\nreaction, proving that the hydrogen atom content in the catalyst increased. We have\nadded these results to the Supplementary Information for the revised manuscript.\nIn Supplementary Figure 18 of the revised manuscript:\nSupplementary Figure 18. (a) XRD patterns, (b) Raman spectra, and high-resolution\n(c) O 1s and (d) Cu 2p XPS spectra of Cu/WO before and after cycle tests. High-\n3\nresolution (e) W 4f XPS spectra of Cu/WO and (f) FTIR spectra of Cu/WO before\n3 3\nand after cycle tests.\nXRD patterns and Raman spectra of the Cu/WO show no significant changes\n3\nbefore and after the reaction, indicating that the composition and structure of the\ncatalsyt remain unchanged. Additionally, the O 1s and Cu 2p XPS spectra reveal no\nsignificant changes in the valence state of these elements. However, in the W 4f XPS\nspectra of the catalyst, the binding energies of W 4f were slightly shifted to lower\nenergies after the reaction compared to the results of Cu/WO before the cyclic stability\n3\ntest, indicating the incomplete consumption of stored electrons. In order to prove this\nconclusion, we examined the FTIR spectra of the Cu/WO before and after the reaction,\n3\nand found that the corresponding O−H vibration peak increased slightly after the cyclic\nreaction, proving that the hydrogen atom content in the catalyst increased."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am satified with the revision and recommend the publication.", "reviewer_2": "I’ve co-reviewed this paper with an early career researcher and we have the following comments:\nOur concerns have been very well addressed with only one exception. Despite what is said in the rebuttal letter, we could not find in the manuscript a discussion relative to the realisation of the large scale application illustrated in Figure 6a. This is important to provide an overall perspective and impact of the results of the paper. After addressing this issue we recommend publication of this very interesting manuscript.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The authors provided a detailed response to the comments raised. I recommend its acceptance as it is."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nI am satified with the revision and recommend the publication.\nAuthor response: We sincerely appreciate the reviewer’s insightful comments and the\ntime dedicated to evaluating our manuscript. We are pleased that our revisions have\nmet the referee’s satisfaction and are grateful for the recommendation for publication.\nReviewer #2 (Remarks to the Author):\nI’ve co-reviewed this paper with an early career researcher and we have the following\ncomments:\nOur concerns have been very well addressed with only one exception. Despite what is\nsaid in the rebuttal letter, we could not find in the manuscript a discussion relative to\nthe realisation of the large scale application illustrated in Figure 6a. This is\nimportant to provide an overall perspective and impact of the results of the paper.\nAfter addressing this issue we recommend publication of this very interesting\nmanuscript.\nAuthor response: We greatly appreciate the reviewers’ positive evaluation of our\nwork and their insightful suggestions for improving our manuscript. In response to the\nreviewers’ comments, we have discussed the challenges to realizing the large-scale\napplication illustrated in Figure 6a and outlined future research directions, providing a\nperspective on the overall impact and future implications of our results. We sincerely\nhope that our revisions have satisfactorily addressed the reviewers’ concerns.\nIn the last paragraph of the Results section:\nTo verify that the catalyst can perform CO reduction on demand, we tested the\n2\ncatalyst after 15 days and 90 days of storage. Although the catalytic performance\nslightly decreases, it still maintains over 80% of the stored hydrogen atoms and\nelectrons (Fig. 6c). Moreover, the consumption of the stored electrons and hydrogen\natoms is mainly in the initial stage (0-15 days), with very little change over the\nsubsequent extended period, indicating that long-term storage is feasible. The reduced\ncharge loss during storage suggests that this method holds promise for mitigating the\nuneven seasonal distribution of solar energy. In addition, the catalyst used for CO\n2\nconversion was scaled up by two orders of magnitude, with a corresponding increase\nin product yield (Fig. 6d, Supplementary Fig. 24). The results have confirmed the\ntheoretical feasibility of large-scale application. It should be acknowledged that the\npurpose of the aforementioned experiments is mainly to verify the feasibility of\nintegrable utilization of intermittent sunlight and residual heat for on-demand CO\n2\nconversion with water, without targeted optimization of material costs and reaction\ndevices. Achieving large-scale application necessitates substantial efforts, particularly\nin catalyst preparation and reactor design, which are the focal points of our future\nresearch. Overall, our results have confirmed the practicality of directly collecting,\nstoring and deploying catalysts when CO conversion is required, significantly\n2\nincreasing the flexibility of catalytic CO reduction.\nI co-reviewed this manuscript with one of the reviewers who provided the listed\nreports. This is part of the Nature Communications initiative to facilitate training in\npeer review and to provide appropriate recognition for Early Career Researchers who\nco-review manuscripts.\nAuthor response: We really appreciate the referee for his/her participation in the\nreview of this manuscript.\nReviewer #4 (Remarks to the Author):\nThe authors provided a detailed response to the comments raised. I recommend its\nacceptance as it is.\nAuthor response: We sincerely appreciate the reviewer for his/her insightful\ncomments and the time dedicated to evaluating our manuscript. We are grateful for\nthe recommendation for publication."}]

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