Bioinspired charge reservoir enables efficient CO_2 photoreduction with H_2O via tungsten valence oscillation

Domain: physics
Published: 2026-01-31
Editorial outcome: minor_revision

Abstract
Solar-driven conversion of CO2 and H2O into chemicals is a promising strategy, while achieving simultaneous and efficient CO2 reduction and H2O oxidation remains challenging. Here, inspired by the role of plastoquinone in temporarily storing electrons during natural photosynthesis, we design a silver-modified tungsten trioxide (Ag/WO3) that functions as a charge reservoir through reversible W6+/W5+ transitions under irradiation. When coupled with various active components, Ag/WO3 significantly enhances their CO2 conversion performance, indicating the universality of this strategy. Specifically, coupling Ag/WO3 with cobalt phthalocyanine (CoPc), the CoPc/Ag/WO3 catalyst achieves a CO production rate of ~1.5 mmol g−1 h−1, representing a 100-fold enhancement over pure CoPc. Mechanistic studies reveal that electrons stored in Ag/WO3 efficiently scavenge photogenerated holes from CoPc, thereby maintaining a high electron density at CO2 reduction sites of CoPc. This work establishes a bioinspired charge reservoir strategy for efficient CO2 photoreduction, providing a universal approach to solar fuel production.

Introduction
Solar-driven photocatalytic reduction of CO2 into value-added chemicals has attracted considerable interest, as it offers a promising route to mitigate greenhouse gas emissions and alleviate resource constraints1–6. In recent decades, the use of organic sacrificial agents (e.g., triethanolamine or triethylamine) has significantly enhanced photocatalytic CO2 conversion performance, achieving rates ranging from tens to thousands of micromoles per gram per hour7,8. However, their irreversible consumption and potential interference with accurate performance evaluation hinder practical application9.

Water (H2O), being earth-abundant, represents an ideal hole scavenger. Nevertheless, its four-electron oxidation process, with an oxidation potential of +0.83 V (vs. NHE, pH = 7, hereafter the same), presents substantial thermodynamic and kinetic limitations, impeding efficient hole extraction and sustainable CO2 reduction10–12. To address these challenges, photocatalytic systems must overcome two inherent limitations in CO2 reduction with H2O: both half-reactions involve multiple electron transfers and require simultaneous accumulation of electrons and holes at distinct surface sites. Furthermore, the two half-reactions are tightly coupled and interdependent, necessitating not only matched migration rates of photogenerated electrons and holes to the surface but also kinetically balanced surface redox reactions7. Constructing a Z-scheme heterojunction, inspired by natural photosynthesis and integrating two narrow-bandgap semiconductors, has been regarded as an effective strategy to overcome these obstacles13,14. Exemplary systems include 3D-SiC@2D-MoS2 7, AgVO3/TiO2 15, MoS2/Fe2O3 16, among others. However, a key challenge still remains: the significant difference in electron and hole transport dynamics requires spatiotemporal coupled charge migration17, which results in the absence of a universal strategy for constructing composite structures applicable to the vast majority of active components.

Natural photosynthesis achieves remarkable efficiency in coupling CO2 reduction and H2O oxidation through sophisticated charge management: Photosystem II (PSII) and Photosystem I (PSI) are spatially and temporally separated and combine with reversible electron carriers for directional energy transfer18,19. In this system, PSII exclusively drives H2O oxidation (2H2O → O2 + 4H+ + 4e⁻), while the liberated electrons are temporarily stored and buffered by quinone carriers like plastoquinone (PQ) through reversible redox reactions before being delivered to PSI for the synthesis of reduced nicotinamide adenine dinucleotide phosphate (NADPH)20,21, which is essential for subsequent CO2 reduction (Fig. 1a). In this process, PQ enables the temporary storage of short-lived photogenerated electrons, ensuring efficient electron transfer between PSI and PSII. Inspired by this natural strategy, we considered introducing a similar charge storage strategy into an artificial photosynthetic system through a redox medium, enabling the reaction of the composite material to decouple the functions of H2O oxidation and CO2 reduction in time, and developed a general interfacial charge transfer strategy.

Our previous studies demonstrated that the conversion of W6+ to W5+ in tungsten oxide (WO3) allows for the transient trapping and storage of photogenerated electrons22,23. If these stored photogenerated electrons can be extracted to recombine with photogenerated holes in the CO2 reduction active components, they would function analogously to PQ. To validate this, we synthesized a silver single-atom-anchored WO3 material (Ag/WO3) and demonstrated that Ag/WO3 can effectively store photoexcited electrons through irradiation-induced W6+/W5+ valence oscillation (Fig. 1b). As verification experiments, subsequent separate coupling of Ag/WO3 with extensively studied active components, including cobalt phthalocyanine (CoPc), graphitic carbon nitride (C3N4), and cuprous oxide (Cu2O), yields composite catalysts exhibiting substantially enhanced performance relative to the pristine catalysts. Detailed characterization of the most effective model catalyst, CoPc/Ag/WO3, reveals that stored photoexcited electrons in Ag/WO3 migrate to recombine with photogenerated holes in CoPc. This process maintains a high density of reducing electrons at the CO2 reduction sites of CoPc. The CoPc/Ag/WO3 catalyst achieves a 100-fold enhancement in photocatalytic CO2 reduction activity compared to pure CoPc, matching the performance of optimized CoPc systems employing organic sacrificial agents. This bioinspired charge reservoir strategy demonstrates universal applicability across different catalytic systems, providing a sustainable and universal strategy for photocatalyst development in CO2 conversion.

Results
Synthesis and characterization of Ag/WO3
Unmodified WO3 was synthesized via a solvothermal method, utilizing WCl6 as the precursor. Subsequently, Ag/WO3 was synthesized via in situ reduction of Ag+ by low-valence W5+24. The detailed synthetic procedures can be found in the Methods section. As shown in Fig. 2a, the X-ray diffraction (XRD) patterns of obtained samples exclusively exhibit peaks corresponding to monoclinic WO3 (JCPDS#83-0950). After the introduction of Ag, neither the peak positions nor the intensities in the XRD patterns show significant changes. To further analyze the W–O configuration, Raman spectroscopy was performed (Fig. 2b). The peak at 944 cm−1 corresponds to the W=O stretching vibration, while bands at 718 cm−1 and 794 cm−1 are characteristic of O–W–O stretching modes. Additionally, the signals at 261 cm−1 and 321 cm−1 represent the W–O–W stretching vibration25,26. The synthesized samples were further characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption analysis. SEM images indicated that Ag/WO3 exhibits a spherical morphology with diameters ranging from 500 to 1000 nm (Supplementary Fig. 1). TEM-energy dispersive X-ray spectroscopy (EDS) mapping confirmed the uniform distribution of Ag on the surface (Supplementary Fig. 2). Moreover, the presence of W and O was verified by XPS analysis (Supplementary Fig. 3). In comparison with pure WO3, the Ag/WO3 showed a slight decrease in specific surface area, which can be attributed to the repeated calcination process (Supplementary Fig. 4).

We further employed XPS to detect Ag elements in the sample. As illustrated in Fig. 2c, the high-resolution Ag 3d XPS spectrum of Ag/WO3 confirms the presence of the Ag element on the surface of Ag/WO3. The amount of Ag element is 1.68 wt%, as determined by inductively coupled plasma mass spectrometry (Supplementary Table 1). To look into the local coordination structure of Ag element in Ag/WO3, the X-ray absorption fine structure (XAFS) spectra were collected. In the Ag K-edge X-ray absorption near edge structure (XANES) spectrum (Fig. 2d), the white-line of the Ag K-edge for the Ag/WO3 catalyst is higher than that of the Ag foil and is close to that of Ag2O. This observation suggests that Ag element carries a fraction of positive charge, suggesting that there is charge transfer between Ag sites and carrier material27,28. Through density functional theory (DFT) calculations, we can clearly see that there is a charge transfer phenomenon between Ag and the support WO3 (Supplementary Fig. 5). The Fourier-transformed k3-weighted extended X-ray absorption fine structure (EXAFS) spectra show the main peak at 1.51 Å (Fig. 2e), corresponding to the first coordination shell of isolated Ag atoms27. The Ag−Ag peak at 2.65 Å is significantly weaker than the first-shell peak, confirming that Ag in the catalyst exist predominantly as dispersed single atoms. The wavelet transform (WT) plot of Ag/WO3 exhibited an intensity maximum at ~1.5 Å, corresponding to the Ag−O coordination (Supplementary Fig. 6), and pattern distinctly contrasts with the maxima observed in Ag foil (metallic Ag−Ag bonds), further confirmed the atomic dispersion state of Ag species in the Ag/WO3 system. Quantitative least-squares EXAFS analysis was further employed to investigate the coordination environment of single Ag atoms in Ag/WO3 (Fig. 2f). The fitting results indicate a first coordination shell with a coordination number of 3.86 for Ag−O and a second shell with 4.06 for Ag−W (Supplementary Table 2; Supplementary Fig. 7).

The light absorption properties of the synthesized catalysts were investigated using UV-Visible diffuse reflectance spectroscopy (UV-Vis DRS). Both WO3 and Ag/WO3 catalysts exhibit primary excitation by light with wavelengths below 500 nm (Supplementary Fig. 8). Interestingly, upon simulated sunlight irradiation, Ag/WO3 displays an increased absorption band within the 450–800 nm range (Fig. 2g). This phenomenon is attributed to the ability of visible or near-infrared light to induce polaron transitions, specifically the hopping of polarons from W5+ to adjacent W6+ positions23,26. During the initial 3 min of light irradiation, the peak intensity in the 450–800 nm range significantly increases, indicating an increased W5+ content in Ag/WO3. To further confirm that W5+ is involved in this process, in situ XPS analysis was performed on Ag/WO3 catalysts. The deconvolution of the W 4f XPS spectra revealed two valence states under both conditions, consisting of W5+ (with characteristic doublet at 35.9 eV for 4f 7/2 and 38.0 eV for 4f 5/2) and W6+ (with characteristic doublet at 34.5 eV for 4f 7/2 and 36.6 eV for 4f 5/2)29,30. Three-minute irradiation increased the W5+ proportion from 14.45% to 16.98% (Fig. 2h; Supplementary Fig. 9 and Supplementary Table 3). The switching of tungsten valence states between W6+ and W5+ indicates that the Ag/WO3 sample stores electrons in the catalyst during irradiation.

To investigate whether the stored electrons in Ag/WO3 can spontaneously release for reduction reaction, we employed 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) as a trapping agent to capture the stored electrons in the catalyst. TEMPO contains unpaired electrons, and typical three peaks can be observed via electron paramagnetic resonance (EPR) spectroscopy31. Initially, we stirred TEMPO in the dark for 40 min, and no significant change in the EPR peak intensity was observed (Supplementary Fig. 10a), indicating that subsequent EPR signal variations are related to the added species. Subsequently, we introduced the newly prepared Ag/WO3 into the TEMPO solution. Upon initial addition, a slight decrease in the unpaired electron signal in the EPR was observed (Supplementary Fig. 10b). However, after continuing to stir, there was no further decrease in the EPR signal, suggesting that no electrons were released during the stirring process. Interestingly, when Ag/WO3 was irradiated with simulated sunlight for 10 min and then added to the TEMPO solution, the unpaired electron signal intensity in the EPR spectra continued to decrease during stirring, reaching a stable state after 30 min (Fig. 2i). This sustained decrease indicates that photo-generated electrons stored in Ag/WO3 can be released for reduction process.

Sustainable CO2 conversion performance of catalysts containing Ag/WO3 composites
To determine whether the electrons stored in the Ag/WO3 nanoparticles can effectively recombine with the photogenerated holes produced by the active components, we constructed a model catalyst, CoPc/Ag/WO3, by combining Ag/WO3 with the widely studied CoPc. The fundamental physicochemical properties of the CoPc/Ag/WO3 composite catalyst were characterized using XRD, XPS, SEM, Fourier-transform infrared (FTIR) spectroscopy, and UV-Vis DRS (Supplementary Figs. 11–15). All these characterization results confirm that the synthesized composite material CoPc/Ag/WO3 is composed of Ag/WO3 and CoPc, which is consistent with the expected structure.

The CO2 reduction coupling with H2O oxidation performance of synthesized samples was evaluated under simulated solar irradiation by using a Xenon lamp source with an AM 1.5G filter. As shown in Fig. 3a, CoPc and Ag/WO3 exhibit minimal CO production during photocatalytic CO2 reduction, consistent with literature reports32,33. Under simulated solar irradiation, the CO production efficiency of the CoPc/Ag/WO3 catalyst with the optimal ratio (10 wt%) reaches 0.148 mmol g−1 h−1. This observation implies that the combination of Ag/WO3 and CoPc can fulfill the requirements for CO2 reduction with H2O. When the content of CoPc in the CoPc/Ag/WO3 catalyst is less than 10 wt%, there is an approximate linear relationship between the content of CoPc in the CoPc/Ag/WO3 catalyst and the CO yield, showing a rate of ~1.5 mmol CO produced per gram of CoPc per hour (Supplementary Fig. 16). The efficiency of this catalyst is considered high compared to previous reports using CoPc in pure H2O systems, and it approaches the efficiency catalytic system using organic sacrificial agents (Fig. 3d; Supplementary Table 4). Additionally, we explored the catalytic performance of the composite catalyst (CoPc/WO3) composed of CoPc and WO3, which demonstrates a CO production rate of 0.017 mmol g−1 h−1. The CO generation rate of the CoPc/Ag/WO3 catalyst is eight times higher than that of the CoPc/WO3 catalyst, indicating that Ag at the interface also plays an important role. By systematically varying the Ag content in CoPc/Ag/WO3, we observed that Ag has a significant impact on the catalytic efficiency of CO2 conversion (Supplementary Fig. 17), further confirming this point and highlighting the crucial role of the Ag in enhancing the CO2 reduction performance.

In addition to detecting CO, we also observed trace quantities of methane (CH4) for CoPc/Ag/WO3 catalyst (Supplementary Fig. 18). CO remains the predominant product, exhibiting a carbon selectivity exceeding 90%. Through DFT calculations, we discovered that CoPc selectively produces CO rather than CH4, because converting CO to CH4 requires overcoming an energy barrier that is 0.65 eV higher than that for producing CO (Supplementary Fig. 19). This energy difference explains why CO becomes the dominant product. Additionally, we did not detect H2 in the products, which may be attributed to limited H2O-catalyst surface mass transfer process during the gas-solid phase process28. Moreover, we examined O2 evolution during H2O oxidation by CoPc/Ag/WO3, with the O2:CO molar ratio approaching 1:2, indicative of the synergistic coupling between CO2 reduction and H2O oxidation (Fig. 3b, Supplementary Fig. 20)7,34. To ascertain whether the production of CO stems from CO2 reduction, we conducted performance tests on the CoPc/Ag/WO3 catalyst under high-purity Ar conditions. The results revealed only trace amount of CO, which could potentially originate from carbon impurities present within the catalyst35. To further explore this process, we replaced 12CO2 with 13CO2 and attempted to detect 13CO in the products. Following gas chromatography (GC) separation, mass spectrometry (MS) confirmed the existence of ion fragments (m/z = 13, 16, and 29) corresponding to 13CO (Fig. 3c). Based on the aforementioned evidence, we conclusively attribute the production of CO to CO2 reduction.

In practical applications, stability is of paramount importance of catalyst. Consequently, we subjected the CoPc/Ag/WO3 catalyst to cyclic testing under simulated sunlight, over six cycles spanning 72 h of CO2 reduction reaction, the catalyst exhibited no significant deactivation (Supplementary Fig. 21). In addition, characterizations of the used CoPc/Ag/WO3 catalyst using XRD, FTIR, XAFS and XPS revealed no observable changes compared to the fresh one (Supplementary Figs. 22–25), affirming the excellent stability of the catalyst.

Effects of charge reservoir in Ag/WO3 for CoPc catalysts
Base on the characterization of Ag/WO3 and the evaluation of CO2 reduction performance in its composite catalysts, we have demonstrated that Ag/WO3 enables the storage and release of photogenerated electrons, and significantly enhances the CO2 reduction activity of the active component. This prompts us to investigate whether a potential link exists between the electron storage/release properties in Ag/WO3 and its improved CO2 reduction performance. Therefore, we selected the CoPc/Ag/WO3 catalyst for further study to elucidate the mechanism of performance improvement.

We first determined the valence band (VB) position of Ag/WO3 using XPS VB spectra36. The obtained VB value of Ag/WO3 is ~2.93 V (Supplementary Fig. 26). Similarly, we determined the highest occupied molecular orbital (HOMO) of CoPc to be 0.90 V. Further analysis using the Kubelka–Munk function revealed that the band gaps of CoPc and Ag/WO3 are 1.89 eV and 2.75 eV, respectively (Supplementary Fig. 27). Based on the above information, we have obtained the band structure of CoPc and Ag/WO3, as shown in Fig. 4a. From the band structure, it can be seen that the charge transfer along the Z-scheme structure between CoPc and Ag/WO3 is the most conducive to achieving efficient CO2 reduction and H2O oxidation7,11,13. The formation of Z-scheme heterojunctions can be verified through the redox capabilities of generated electrons and holes. Photogenerated electrons can reduce O2 to ·O2− radicals (−0.33 V), while holes can oxidize H2O or OH− to ·OH radicals (2.39 V)12. Therefore, EPR spectroscopy was used to detect ·O2− and ·OH radicals using DMPO (5,5-dimethyl-1-pyrroline N-oxide) as the spin-trapping agent. Under light irradiation, the characteristic peaks of DMPO-·O2− and DMPO-·OH in the CoPc/Ag/WO3 system appeared (Fig. 4b, c). For the CoPc catalyst, only the DMPO-·O2− peak appears; while in the Ag/WO3 system, only the DMPO-·OH peak is observed. In the CoPc/Ag/WO3 system do both DMPO-·O2− and DMPO-·OH peaks occur simultaneously, and their intensities are significantly higher than those of individual components. This suggests that in the CoPc/Ag/WO3 photocatalytic system, the electron-hole transfer pathway follows the Z-scheme, with the lowest unoccupied molecular orbital (LUMO) of CoPc accumulates strongly reducing electrons, while VB of Ag/WO3 accumulates strongly oxidizing holes, resulting in increased production of ·O2− and ·OH radicals, consistent with our expectations.

To gain sight into the photoinduced electron transfer between Ag/WO3 and CoPc components, we employed femtosecond time-resolved transient absorption (fs-TA) spectroscopy. As shown in Fig. 4d, g, the positive signals observed in the 425–525 nm range are attributed to the excited-state absorption (ESA) of the CoPc component37, which originates from π electrons transitioning in the phthalocyanine (Pc) ring from the S0 state to the S1 state (S0 → S1). Meanwhile, the positive peak appearing in the 525–545 nm range corresponds to the ESA associated with the WO3 component (Supplementary Fig. 28). The negative ground-state bleaching (GSB) peaks in the 550–650 nm range arise from electron transfer from the Pc ligand to the Co sites (Pc → Co)37. In the fs-TA spectra of simulated sunlight irradiated CoPc/Ag/WO3 (Fig. 4e, h), the ESA peak at 489 nm significantly enhanced compared to those of unirradiated CoPc/Ag/WO3 (Fig. 4d, g). This increase in the number of electrons in the excited state of CoPc indicates that the recombination of electrons and holes within the phthalocyanine ring is effectively suppressed. The ESA signal in the 525–545 nm range, attributed to WO3 component, is also enhanced, implying an increased number of electrons in the excited state of WO3 and supporting the hypothesis that photogenerated electrons are stored in WO3. Additionally, the GSB peak of irradiated CoPc/Ag/WO3 in the range of 550–650 nm range is also significantly enhanced (Fig. 4d, g). The stronger GSB peaks in irradiated CoPc/Ag/WO3 catalyst compared to fresh CoPc/Ag/WO3 catalyst suggests a greater amount of photoinduced electron transfer from Pc to Co atom. These results demonstrate that the Ag/WO3 charge reservoir effectively scavenges photogenerated holes from CoPc, thereby maintaining a high electron density at the Co centers. This conclusion is further confirmed by time-resolved photoluminescence (TRPL) measurements, which show a reduced photoluminescence lifetime (Supplementary Fig. 29 and Supplementary Table 5).

To elucidate the role of the interfacial Ag, we also characterized the irradiated CoPc/WO3 catalyst using fs-TA spectroscopy, as shown in Fig. 4f, i. The ESA signal of the CoPc component was reduced, even though the ESA signal of the WO3 component remained largely unchanged. This result suggests that the electrons stored in WO3 cannot effectively extract or scavenge the photogenerated holes in the CoPc component, thereby leading to a decrease in the excited-state electron density of CoPc. Correspondingly, the GSB signal of the CoPc component in the range of 550–650 nm was significantly weaker in the pre-irradiated CoPc/WO3 sample than in the pre-irradiated CoPc/Ag/WO3 sample, indicating suppressed electron transfer from the Pc ring to the Co atom. It is possible that the charge excited on Pc is primarily recombined rather than being transferred to the active sites, suggesting that single Ag atoms can serve as electron bridges, facilitating electron transfer across the interface. This interpretation is further corroborated by the smaller reduction in the average fluorescence lifetime in the Ag-free sample compared to the Ag-containing counterpart (Supplementary Fig. 30 and Supplementary Table 5). In addition, photocurrent and electrochemical impedance spectroscopy measurements are consistent with this conclusion (Supplementary Figs. 31–32). To further explore the role of Ag, we conducted in situ FTIR spectroscopy study on the CO2 conversion process over CoPc/Ag/WO3 and CoPc/WO3. The results showed that the introduction of Ag did not create new active sites for CO2 (Supplementary Fig. 33). This indicates that the main role of the interfacial Ag single atoms is to kinetically enhance the electron transfer at the interface.

The light-assisted Kelvin Probe Force Microscopy (KPFM) was employed to study the separation and transport behavior of photo-induced charge carriers of the CoPc/Ag/WO3 catalyst before and after irradiation. The surface photovoltage (SPV) distribution of CoPc/Ag/WO3 is consistent with the height profile displayed by atomic force microscopy (AFM) (Supplementary Fig. 34), indicating that the changes in photovoltage before and after irradiation are related to the catalysts. After light irradiation, the SPV signal of CoPc/Ag/WO3 decreased by approximately −50 mV, while the catalyst was irradiated by simulated sunlight for 10 min decreased significantly by −100 mV. According to previous reports, a more negative SPV signal change represents a higher concentration of photogenerated electrons after light irradiation17,38. The negative SPV response of preirradiated CoPc/Ag/WO3 is ~50 mV greater than that of fresh CoPc/Ag/WO3, indicating that photo-charged catalyst accelerates the concentrated accumulation of photogenerated electrons on the catalyst surface.

The universality of the charge reservoir strategy
Following the discovery that stored photogenerated electrons enhance the catalytic activity, assessing the universality of this charge storage mechanism becomes crucial. Therefore, we employed XPS VB spectra to investigate the distribution of accumulated electrons39. Through the XPS VB spectrum test, we found that after the sites of W5+ were introduced into the catalyst, a new peak appeared in the range of 0.3–1.5 V (centered at 0.9 V) (Fig. 5a, b). This means that as long as the oxidation potential of the photogenerated holes produced by the active components is greater than 0.3 V, accumulated electrons can be consumed, which greatly broadens the selection of active components.

To explore the potential application of Ag/WO3 in diverse catalytic systems, we prepared composite catalysts by combining it with two representative catalyst types: the polymer catalyst C3N4 (yielding C3N4/Ag/WO3) and the metal oxide Cu2O (yielding Cu2O/Ag/WO3). The CO2 reduction performance of these composites was evaluated under simulated sunlight. The C3N4/Ag/WO3 composite exhibited substantially enhanced CO2 reduction activity compared to unmodified C3N4. At the optimal C3N4 loading of 50 wt%, the CO and CH4 production rates reached 0.123 and 0.064 mmol g−1 h−1, respectively (Fig. 5c). This represents a 5.5-fold increase in CO yield and a 23.8-fold increase in CH4 yield over the pure C3N4, which produced CO and CH4 at only 0.022 and 0.003 mmol g−1 h−1, respectively. The pronounced enhancement in CH4 production, which involves an 8-electron transfer process compared to the 2-electron pathway for CO, suggests a significant improvement in the electron supply capability of the active sites. Similarly, the optimal composite with 40 wt% Cu2O loading achieved a CO production rate of 0.070 mmol g−1 h−1 (Fig. 5d), which is 11.7 times higher than that of the pure Cu2O catalyst (0.006 mmol g−1 h−1). These results collectively confirm that the Ag/WO3 can be used in diverse catalyst systems, highlighting the potential for further applications in high-value product synthesis.

To investigate whether Ag/WO3 remains effective under natural sunlight, we conducted an outdoor CO2 reduction test on CoPc/Ag/WO3 using natural sunlight. A demonstration experimental device was constructed, incorporating a Fresnel lens to concentrate sunlight onto the catalyst surface (Supplementary Fig. 35). Solar irradiance was recorded at one-minute intervals throughout the test, as illustrated in Fig. 5e. Gas samples (2 mL each) were collected on the hour from 6:00 to 20:00 for analysis (Fig. 5g, h). Before solar irradiation reached the catalyst (6:00–7:00), no products were detected. Although sunlight began to illuminate the catalyst around 7:25, the CO signal remained negligible at 8:00 due to low light intensity. Measurable CO production was not observed until 9:00. The hourly CO yield reached a maximum of 9.3 μmol between 13:00 and 14:00. After 17:50, solar irradiation was no longer sufficient to maintain the reaction. Accordingly, no further increase in CO concentration was observed after the 18:00 sampling point. The variation in CO generation rate closely followed the trend of solar light intensity (Fig. 5e, g), confirming that natural sunlight can effectively trigger the CO2-to-CO reduction reaction. Although there is still plenty of room to improve overall efficiency, likely due to limitations in light collection by the simple reaction setup, our results suggest the synthesized catalyst can be trigger by nature light.

Methods
Synthesis of the WO3 catalyst
1 g of tungsten hexachloride (WCl6) was dissolved in 60 mL of ethanol. The solution was continuously stirred at room temperature for 30 min. Subsequently, the solution was transferred to a Teflon-lined autoclave, where it underwent treatment at 180 °C for 12 h. Following this process, the suspension was subjected to centrifugation, followed by repeated washes with ethanol and deionized H2O. Finally, the obtained material was dried at 70 °C and subsequently calcined in an air atmosphere at 300 °C for 1 h, resulting in the formation of the desired WO3 catalyst.

Synthesis of Ag/WO3 catalyst
100 mg of WO3 was added to 50 mL of deionized H2O, followed by ultrasonic dispersion for 1 h to achieve a homogeneous dispersion solution; the mixture was purged with Ar for 1 h and irradiated using a 300 W xenon lamp for 1 h, during which the color changed to light blue, indicating the formation of reduced W5+. Subsequently, 3 mL of silver nitrate solution (AgNO3, 1 mg/mL) was introduced into the dispersed solution and stirred at room temperature for 1 min; after stirring, the solid catalyst was separated via centrifugation and alternately washed three times with absolute ethanol and deionized H2O. Finally, the washed catalyst was dried at 70 °C in air and calcined in an air atmosphere at 300 °C for 1 h, yielding a light-yellow product corresponding to Ag/WO3; the procedure was replicated to prepare samples with varying Ag contents by adjusting the AgNO3 addition amount accordingly.

Synthesis of composite catalyst
10 mg of cobalt phthalocyanine (CoPc, analytical grade) was dissolved in 50 mL of absolute ethanol, followed by 3 h of ultrasonic treatment at room temperature. Subsequently, 90 mg of Ag/WO3 was added to the mixture, and an additional hour of ultrasonic treatment was applied. The dispersion was then ball-milled at 500 rpm for 5 h. Next, the ultrasonically treated catalyst was stirred at 85 °C until all the ethanol had evaporated. The resulting dried catalyst was washed three times with absolute ethanol and air-dried at 70 °C. The resulting catalyst is denoted as CoPc/Ag/WO3. The synthesis steps for other proportional CoPc catalysts remain the same as described above, with the only variation being the ratio of CoPc added. By substituting Ag/WO3 with WO3, while keeping the other steps unchanged, the resulting catalyst is CoPc/WO3. The detailed synthesis procedures for Cu2O and highly crystalline C3N4 are described in the Supporting Information. Upon obtaining the individual Cu2O and C3N4 monomers, the Cu2O/Ag/WO3 catalyst and the C3N4/Ag/WO3 composite catalyst were respectively prepared following a synthesis procedure analogous to that used for CoPc/Ag/WO3.

General characterization
The XRD patterns were tested using a Panalytical X’pert Pro X-ray diffractometer with Cu Kα radiation (λ=1.5406 Å, 40 kV, 40 mA). Raman spectra were recorded using a Raman microscope imaging spectrometer (Thermo Scientific DXR2xi, USA). XPS data were recorded using a Thermo ESCALAB 250 X-ray photoelectron spectrometer (USA). All the binding energies were referenced to the C 1s peak at 284.8 eV. To present the data intuitively, the data presented in the VB spectrum have been calibrated using the contact potential difference and the work function of the instrument36,40. EPR spectra were collected by a ELEXSYS E500 spectrometer (Bruker, Germany) at room temperature. Ag K-edge XAFS spectrum was collected at the 14W1 beamline in Shanghai Synchrotron Radiation Facility. UV-Vis DRS spectra were carried on a Varian Cary 100 Scan UV-Vis system with BaSO4 as the reflectance standard. The transient absorption setup utilized an Yb:KGW laser in conjunction with the pump-probe transient absorption spectrometer (TA100, Time-Tech Spectra, LLC). The fundamental beam was split into two branches. One branch was directed into an optical parametric amplifier to generate the pump pulse. Meanwhile, the broadband probe pulse (ranging from 300 to 1000 nm) was produced by focusing the fundamental pulse onto a sapphire crystal. At the sample position, the focal size of pump was adjusted to be significantly larger than that of the probe, ensuring nearly homogeneous excitation density in the probed area. The samples were pumped at 360 nm and coated onto quartz glass for testing.

Evaluation of CO2 conversion performance
The photocatalytic activity of the as-prepared samples for CO2 reduction was investigated using a custom-made cylindrical stainless-steel reactor (volume = 200 mL) equipped with a quartz window. The photocatalysts (5 mg) were uniformly dispersed in 3 mL of deionized H2O and then transferred to a dish (50 mm in diameter). The dish was subsequently dried under vacuum at 70 °C for 24 h to coat it with the sample. Next, the dish with the sample was placed inside the reactor, and 1 mL of deionized H2O was added to the reactor. The reactor was purged with high-purity CO2 three times. Following this, a specific amount of high-purity CO2 was introduced into the reactor to maintain a pressure of 1 bar. Simulated sunlight was generated using a 300 W Xe arc lamp with an AM 1.5G filter, which irradiated the sample through the quartz window. The resulting products were analyzed using gas chromatography (7890B, Agilent). The instrument was equipped with a flame ionization detector (FID) to detect CH4 and CO, as well as a thermal conductivity detector (TCD) to detect O2 and H2. In the isotope labeling experiment, we substituted 13CO2 for 12CO2 as the reactant, and the resulting product was detected using gas chromatography-mass spectrometry (7890A and 5975C, Agilent).

Data availability
Source data are provided in this paper. Data are available from the corresponding authors upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors reported a bioinspired charge reservoir system that enables efficient CO2 photoreduction with H2O via tungsten valence oscillation. The study demonstrates a well-structured experimental design with comprehensive characterizations and relevant analyses that support the proposed mechanism. The work contributes meaningfully to the field of photocatalytic CO2 reduction by introducing a novel approach that enhances charge separation and conversion efficiency. However, several issues related to data interpretation, figure consistency, and the use of abbreviations require attention and clarification. Some figure labels, spectral peak positions, and statements in the results and discussion sections need to be revised for accuracy and consistency. These concerns are not fundamentally detrimental to the quality or novelty of the work but should be addressed to ensure scientific clarity and reliability.\n1. line 73: Please specify what the full name of NADPH before start using the abbreviation.\n2. line 79: Please consistently use the previously introduced abbreviation for H₂O and CO₂.\n3. line 114: It was mentioned that in Fig. 2b, the bands at 645 cm⁻¹ and 820 cm⁻¹ represent the characteristics of the O–W–O stretching mode. However, in Fig. 2b, the presented peaks are not exactly at 645 cm⁻¹ and 820 cm⁻¹. They seem to be around 710 cm⁻¹ and 790 cm⁻¹ instead. Please correct accordingly.\n4. line 115: The peak at 230 cm⁻¹ is said to represent W–O–W stretching, but it appears closer to 260 cm⁻¹. Please correct accordingly.\n5. lines 117–121: Three different characterization methods were mentioned. However, the statement “Ag element was uniformly distributed on the material’s surface” should clarify which specific characterization method supports this observation.\n6. line 154: Is the hyphen necessary in “transitions-specifically”?\n7. line 158: Is the hyphen necessary in “states-W⁵⁺”?\n8. line 218: The term “silver” has already been abbreviated as Ag; please use Ag consistently throughout the manuscript.\n9. line 300: In Fig. 4e and 4h, there is no peak at 511 nm as mentioned. Please correct the peak location.\n10. line 301: The text says “To those of unirradiated samples (Fig. 4f and 4i)”, but those graphs are identified as irradiated, not unirradiated.\n11. line 304: The peak at 565 nm was referred to which figure? There doesn’t appear to be a peak at 565 nm in any presented Fig. 4.\n12. lines 304 and 309: The stated “pre-irradiated catalyst” refers to which figure?\n13. line 351: The peak at 0.3–1.5 V was mentioned, but the reader must read to the last paragraph to know which figure is being referred to. It would be better to introduce the figure earlier.\n14. line 360: It was mentioned that “compared to unmodified C₃N₄, C₃N₄/Ag/WO₃ exhibited substantial improvements… (Fig. 5c)”. However, in Fig. 5c there is no graph trend for unmodified C₃N₄, only for C₃N₄/Ag/WO₃. Please justify.\n15. line 364: It was mentioned that the Cu₂O/Ag/WO₃ sample produced an 11.7-fold increase in CO product with 0.070 mmol g⁻¹ h⁻¹ compared to C₃N₄/Ag/WO₃ with 0.123 mmol g⁻¹ h⁻¹. Isn’t 0.123 higher than 0.070 mmol g⁻¹ h⁻¹? Please recheck.\n16. line 373: It was mentioned that between (6.00–7.00), no products were generated, but it seems that up to 8.00 no product was formed. Please recheck.\n17. line 375: It was mentioned that after 18.00, no further CO was produced, but Fig. 5f shows otherwise. Please justify.", "reviewer_2": "The bio-inspired concept of a \"charge reservoir\" to decouple the kinetic bottlenecks between water oxidation and CO2 reduction is not only innovative but also demonstrated with compelling experimental evidence across multiple catalytic systems. The observed universal enhancement in performance is a significant finding that could have broad implications for the field of artificial photosynthesis. The manuscript is generally well-written, and the data are presented clearly. I am supportive of publication, pending satisfactory responses to the following points,s which aim to further strengthen the mechanistic claims and contextualize the impact of the work.\n1. The authors have only considered the charge transition between W5+ and W6+. Please comment on the possibility of further reducing the state to a W4+ state.\n2. The demonstration of catalyst performance under real-world outdoor conditions is a notable strength of this work. To fully appreciate these results, a detailed description of the testing parameters (e.g., light intensity, duration, weather conditions, and reactor setup) is essential.\n3. We recommend a thorough review of the supporting information to correct minor typographical errors, particularly concerning superscripts/subscripts and capitalization, as exemplified in the caption of Supplementary Fig. 25.\n4. To substantiate the “decoupled” mechanism, time-resolved charge recombination kinetics (fs-TA decay fitting or photoluminescence lifetime) before and after Ag/WO₃ coupling could be discussed quantitatively.\n5. Six-cycle tests (72 h) are promising but relatively short for a solar system. Extending to ~200 h continuous operation or including post-reaction EXAFS would prove structural robustness of W⁵⁺/W⁶⁺ cycling.\n6. Clarify whether O₂ was quantified via GC-TCD or mass spectrometry, and if leakage corrections were applied, to support the claim of stoichiometric coupling (O₂:CO ≈ 1:2).", "reviewer_3": "This study presents an original bio-inspired charge reservoir strategy in the field of solar energy conversion. The conceptual design is elegant and logically rigorous, offering a novel approach to overcome the reliance on sacrificial agents in photocatalytic CO2 reduction. The experimental work is particularly impressive, demonstrating the effectiveness of the charge storage and controlled release mechanism through multi-system universality validation, a hundred-fold performance enhancement. The experimental designs are scientifically sound, with mutually corroborating data forming a complete evidence chain. It meets the standards for publication in Nature communications and I am pleased to recommend acceptance after addressing the following revisions:\n1. The introduction adequately discusses the limitations of organic sacrificial agents but offers a somewhat generalized overview of existing charge separation strategies. A more in-depth comparison with recent representative works is recommended.\n2. The relatively high silver loading (1.68 wt%) in Ag/WO3 may raise concerns about aggregation. Further justification of its stability is needed.\n3. XRD and Raman results do not effectively distinguish WO3 crystal phase changes. Supplementing with high-resolution TEM or selected-area electron diffraction data would help verify crystal structure integrity.\n4. Inconsistent reporting of CO production rates may cause confusion. When comparing with literature data (Table S4), note that most studies report μmol-level yields, whereas this work uses mmol. Units should be clearly defined.\n5. The O2:CO molar ratio close to 1:2 supports the overall reaction, but background oxygen interference should be ruled out via dark control experiments to exclude residual air effects.\n6. Isotope labeling experiments (13CO2 → 13CO) provide solid evidence, but the MS peaks at m/z = 13, 16, and 29 lack specific assignments. More detailed labeling in the figures is suggested.\n7. Stability tests over 6 cycles (72 hours) show good recyclability, but post-reaction surface element analysis (e.g., XPS to track valence states) is missing.\n8. In situ FTIR shows stronger *COOH intermediate signals, supporting accelerated reaction rates, but other product assignments should be described in detail and compared with DFT-calculated pathways.\n9. The outdoor natural light experiment is well-conceived, but insufficient description of key parameters affects reproducibility. Additional details are recommended."}, "author_response": "We sincerely appreciate your positive evaluation and insightful suggestions on our work. We have thoroughly addressed all the points you raised, with particular attention to data interpretation, figure consistency, and abbreviation usage. Through necessary corrections to the figures, labels, and text, we have strived to ensure accuracy and consistency across the manuscript. We hope these revisions have strengthened the paper, and we are truly grateful for your valuable input, which has been instrumental in enhancing its quality.\n1. line 73: Please specify what the full name of NADPH before start using the abbreviation.\nAuthor response: We thank you for pointing this out. The abbreviation “reduced nicotinamide adenine dinucleotide phosphate (NADPH)” has now been defined at its first occurrence in the revised manuscript (see Line 64–65 of the revised manuscript).\nLines 64–65 of the revised manuscript:\n…, while the liberated electrons are temporarily stored and buffered by quinone carriers like plastoquinone (PQ) through reversible redox reactions before being delivered to PSI for the synthesis of reduced nicotinamide adenine dinucleotide phosphate (NADPH).\n2. line 79: Please consistently use the previously introduced abbreviation for H₂O and CO₂.\nAuthor response: We thank you for this helpful comment. We have now consistently used the abbreviations H₂O and CO₂ throughout the manuscript following their initial full definition, as you suggested.\nLines 64–65 of the revised manuscript:\n…, enabling the reaction of the composite material to decouple the functions of H₂O oxidation and CO₂ reduction in time, and developed a general interfacial charge transfer strategy.\n3. line 114: It was mentioned that in Fig. 2b, the bands at 645 cm⁻¹ and 820 cm⁻¹ represent the characteristics of the O–W–O stretching mode. However, in Fig. 2b, the presented peaks are not exactly at 645 cm⁻¹ and 820 cm⁻¹. They seem to be around 710 cm⁻¹ and 790 cm⁻¹ instead. Please correct accordingly.\nAuthor response: We thank you for pointing out this discrepancy. The peak labels in the text have been corrected to 718 cm⁻¹ and 794 cm⁻¹, as determined by automated peak detection software, to accurately match Fig. 2b. We have also applied this objective method across the manuscript to ensure the accuracy of all spectral assignments.\nLines 104–106 of the revised manuscript:\nThe peak at 944 cm-1 corresponds to the W=O stretching vibration, while bands at 718 cm-1 and 794 cm-1 are characteristic of O−W−O stretching modes.\n4. line 115: The peak at 230 cm⁻¹ is said to represent W–O–W stretching, but it appears closer to 260 cm⁻¹. Please correct accordingly.\nAuthor response: We thank you for highlighting this additional inaccuracy. For consistency with other objectively determined peaks mentioned above, this value has also been corrected to 261 cm-1.\nLines 106–107 of the revised manuscript:\nAdditionally, the signals at 261 cm-1 and 321 cm-1 represent the W−O−W stretching vibration.\n5. lines 117–121: Three different characterization methods were mentioned. However, the statement “Ag element was uniformly distributed on the material’s surface” should clarify which specific characterization method supports this observation.\nAuthor response: We appreciate your comment regarding the need for clarity in attributing the characterization results. As suggested, we have revised the paragraph to distinctly separate the findings from each technique. It is now explicitly stated that the uniform distribution of Ag is based on the TEM-EDS mapping analysis (Supplementary Fig. 2), as shown in the revised text below.\nLines 113–114 of the revised manuscript:\nTEM-energy dispersive X-ray spectroscopy (EDS) mapping confirmed the uniform distribution of Ag on the surface (Supplementary Fig. 2).\n6. line 154: Is the hyphen necessary in “transitions-specifically”?\nAuthor response: Thank you for pointing out this detail. We have revised the expression “transitions–specifically” and have further engaged a native English-speaking editor to refine similar expressions throughout the manuscript to improve overall clarity and natural phrasing.\nLines 148–150 of the revised manuscript:\nThis phenomenon is attributed to the ability of visible or near-infrared light to induce polaron transitions, specifically the hopping of polarons from W5+ to adjacent W6+ positions\n7. line 158: Is the hyphen necessary in “states-W⁵⁺”?\nAuthor response: We thank you for this correction. We have revised it to “W5+” to ensure it is both grammatically correct and scientifically clear. This change has been applied consistently across the text.\nLines 154–157 of the revised manuscript:\nThe deconvolution of the W 4f XPS spectra revealed two valence states under both conditions, consisting of W5+ (with characteristic doublet at 35.9 eV for 4f 7/2 and 38.0 eV for 4f 5/2) and W6+ (with characteristic doublet at 34.5 eV for 4f 7/2 and 36.6 eV for 4f 5/2).\n8. line 218: The term “silver” has already been abbreviated as Ag; please use Ag consistently throughout the manuscript.\nAuthor response: We thank you for this note. We have replaced “silver” with “Ag” throughout the manuscript to ensure consistency. The text has been updated accordingly.\nLines 107–108 of the revised manuscript:\nAfter the introduction of Ag, neither the peak positions nor the intensities in the XRD patterns and Raman spectra show significant changes.\nLines 213–216 of the revised manuscript:\n…, we observed that Ag has a significant impact on the catalytic efficiency of CO2 conversion (Supplementary Fig. 17), further confirming this point and highlighting the crucial role of the Ag in enhancing the CO2 reduction performance.\n9. line 300: In Fig. 4e and 4h, there is no peak at 511 nm as mentioned. Please correct the peak location.\nAuthor response: We thank you for this insightful observation. You are correct that a distinct peak at 511 nm is not present in Figs. 4e and 4h, as the signal in that region is overlapped by adjacent, more intense peaks. We have re-analyzed the data and updated both the figures and the text accordingly.\nLines 296–298 of the revised manuscript:\nIn the fs-TA spectra of simulated sunlight irradiated CoPc/Ag/WO3 (Figs. 4e and 4h), the ESA peak at 489 nm significantly enhanced compared to those of unirradiated CoPc/Ag/WO3 (Fig. 4d and Fig. 4g).\n10. line 301: The text says “To those of unirradiated samples (Fig. 4f and 4i)”, but those graphs are identified as irradiated, not unirradiated.\nAuthor response: We thank you for catching this error in figure referencing. The text has been corrected to compare against the unirradiated samples in Figs. 4d and 4g, and we have verified all figure citations in this section for accuracy.\nLines 296–298 of the revised manuscript:\nIn the fs-TA spectra of simulated sunlight irradiated CoPc/Ag/WO3 (Figs. 4e and 4h), the ESA peak at 489 nm significantly enhanced compared to those of unirradiated CoPc/Ag/WO3 (Fig. 4d and Fig. 4g).\n11. line 304: The peak at 565 nm was referred to which figure? There doesn’t appear to be a peak at 565 nm in any presented Fig. 4.\nAuthor response: Thank you for pointing out this lack of clarity. In our revision, we have added a clarification in the text to specify that the peak refers to the broad feature shown in Figs. 4d and 4g. Furthermore, we have revised the phrase “the peak at 565 nm” to “the peak in the range of 550–650 nm” to more accurately describe its position.\nLines 303–304 of the revised manuscript:\nAdditionally, the GSB peak in the range of 550–650 nm range is also significantly enhanced (Figs. 4d and 4g).\n12. lines 304 and 309: The stated “pre-irradiated catalyst” refers to which figure?\nAuthor response: Thank you for this feedback. We have clarified the text by replacing the ambiguous “pre-irradiated catalyst” with the specific terms “irradiated CoPc/Ag/WO3” (line 304) and “irradiated CoPc/WO3” (line 313).\nLines 306–309 of the revised manuscript:\nAdditionally, the GSB peak of irradiated CoPc/Ag/WO3 in the range of 550–650 nm range is also significantly enhanced (Figs. 4d and 4g). The stronger GSB peaks in irradiated CoPc/Ag/WO3 catalyst compared to fresh CoPc/Ag/WO3 catalyst suggests a greater amount of photoinduced electron transfer from Pc to Co atom (Pc→Co).\nLines 313–314 of the revised manuscript:\nTo elucidate the role of the interfacial Ag, we also characterized the irradiated CoPc/WO3 catalyst using fs-TA spectroscopy, as shown in Figs. 4f and 4i.\n13. line 351: The peak at 0.3–1.5 V was mentioned, but the reader must read to the last paragraph to know which figure is being referred to. It would be better to introduce the figure earlier.\nAuthor response: Thank you for this helpful suggestion. We have now moved the citation for Fig. 5a to the first mention of the peak at 0.3–1.5 V.\nLines 359–363 of the revised manuscript:\nThrough the XPS VB spectrum test, we found that after the sites of W5+ were introduced into the catalyst, a new peak appeared in the range of 0.3–1.5 V from the Fermi level (centered at 0.9 V) (Figs. 5a and 5b).\n14. line 360: It was mentioned that “compared to unmodified C₃N₄, C₃N₄/Ag/WO₃ exhibited substantial improvements… (Fig. 5c)”. However, in Fig. 5c there is no graph trend for unmodified C₃N₄, only for C₃N₄/Ag/WO₃. Please justify.\nAuthor response: We appreciate this constructive feedback. The comparison was initially based on the performance of unmodified C₃N₄ (represented by the \"0%\" column) versus Ag/WO3 (the “100%” column). To make this comparison immediately clear to the reader, we have revised the figure to directly label the columns as “C₃N₄” and “Ag/WO3”, and have updated the figure caption accordingly to ensure clear interpretation.\nFigure 5 of the revised manuscript:\nFig. 5. Research results on the universality of the charge reservoir strategy. (a) XPS VB spectra of irradiated and fresh Ag/WO3. (b) Energy-level diagram of electron migration between Ag/WO3 and active component. The photocatalytic performance for CO2 reduction to CO was assessed for composite catalysts comprising varying mass ratios of the active components: (c) C₃N₄, and (d) Cu₂O in combination with Ag/WO3. Values on the x-axis correspond to the mass percentage of each active component within the composite catalysts. Error bars represent the standard deviation of three measurements. (e) Solar light intensity profile recorded during the outdoor experiment. (f) Cumulative CO yield over time during the outdoor experiment under natural sunlight. (g) Hourly CO evolution rate, calculated as the difference in cumulative yield between consecutive hourly measurements.\n15. line 364: It was mentioned that the Cu₂O/Ag/WO₃ sample produced an 11.7-fold increase in CO product with 0.070 mmol g⁻¹ h⁻¹ compared to C₃N₄/Ag/WO₃ with 0.123 mmol g⁻¹ h⁻¹. Isn’t 0.123 higher than 0.070 mmol g⁻¹ h⁻¹? Please recheck.\nAuthor response: We thank you for this careful observation and for pointing out the lack of clarity in our original wording. You are correct to question this, as the value of 0.123 mmol g-1 h-1 is indeed higher than 0.070 mmol g-1 h-1. The confusion arose because the “11.7-fold increase” for the Cu₂O/Ag/WO3 composite was in comparison to the pure Cu₂O catalyst, not to the C₃N₄/Ag/WO3 composite. To prevent any misunderstanding, we have thoroughly revised the relevant paragraph to explicitly state the comparison baseline for both composites in the revised manuscript.\nLines 370–381 of the revised manuscript:\nThe CO2 reduction performance of these composites was evaluated under simulated sunlight. The C₃N₄/Ag/WO3 composite exhibited substantially enhanced CO2 reduction activity compared to unmodified C₃N₄. At the optimal C₃N₄ loading of 50 wt%, the CO and CH4 production rates reached 0.123 and 0.064 mmol g-1 h-1, respectively (Fig. 5c). This represents a 5.5-fold increase in CO yield and a 23.8-fold increase in CH4 yield over the pure C₃N₄, which produced CO and CH4 at only 0.022 and 0.003 mmol g-1 h-1, respectively. The pronounced enhancement in CH4 production, which involves an 8-electron transfer process compared to the 2-electron pathway for CO, suggests a significant improvement in the electron supply capability of the active sites. Similarly, the optimal composite with 40 wt% Cu₂O loading achieved a CO production rate of 0.070 mmol g-1 h-1 (Fig. 5d), which is 11.7 times higher than that of the pure Cu₂O catalyst (0.006 mmol g-1 h-1).\n16. line 373: It was mentioned that between (6.00–7.00), no products were generated, but it seems that up to 8.00 no product was formed. Please recheck.\nAuthor response: Thank you for pointing out this issue. You are correct that no CO product was detected up to 8:00. The reason for this is that the solar irradiance intensity was below 180 W/m2 before 8:00, which is exceptionally weak. This low irradiation level resulted in a cumulative CO production rate that was too low to be detected by our gas chromatograph. Since product analysis was performed at hourly intervals, a measurable CO signal was not observed until the 9:00 time point. We have revised the manuscript to clarify this point and to include the explanation for the results.\nLines 387–398 of the revised manuscript:\nSolar irradiance was recorded at one-minute intervals throughout the test, as illustrated in Fig. 5e. Gas samples (2 mL each) were collected on the hour from 6:00 to 20:00 for analysis (Figs. 5g and 5h). Before solar irradiation reached the catalyst (6:00–7:00), no products were detected. Although sunlight began to illuminate the catalyst around 7:25, the CO signal remained negligible at 8:00 due to low light intensity. Measurable CO production was not observed until 9:00. The hourly CO yield reached a maximum of 9.3 μmol between 13:00 and 14:00. After 17:50, solar irradiation was no longer sufficient to maintain the reaction. Accordingly, no further increase in CO concentration was observed after the 18:00 sampling point. The variation in CO generation rate closely followed the trend of solar light intensity (Fig. 5e and Fig. 5g), confirming that natural sunlight can effectively trigger the CO2-to-CO reduction reaction.\n17. line 375: It was mentioned that after 18.00, no further CO was produced, but Fig. 5f shows otherwise. Please justify.\nAuthor response: We thank you for pointing out the apparent discrepancy between our statement and the visual presentation in Fig. 5f. Upon carefully re-examining the original data, we confirmed that the cumulative CO yield remained constant at 37.45 μmol at 18:00, 19:00, and 20:00, which aligns with our conclusion that no further CO was produced after 18:00. The impression of a continuous increase in the original figure was caused by the positioning of the data labels directly above the time points, which may have introduced a visual misinterpretation. To resolve this issue, we have revised Fig. 5g by relocating the hourly yield labels to the center of each corresponding time interval. This adjustment provides a more accurate representation of the data and clearly shows that CO generation ceased after 18:00. We appreciate your valuable feedback, which has helped improve the clarity of our figures.\nFig. 5 of the revised manuscript:\nFig. 5. Research results on the universality of the charge reservoir strategy. (a) XPS VB spectra of irradiated and fresh Ag/WO3. (b) Energy-level diagram of electron migration between Ag/WO3 and active component. The photocatalytic performance for CO2 reduction to CO was assessed for composite catalysts comprising varying mass ratios of the active components: (c) C₃N₄, and (d) Cu₂O in combination with Ag/WO3. Values on the x-axis correspond to the mass percentage of each active component within the composite catalysts. Error bars represent the standard deviation of three measurements. (e) Solar light intensity profile recorded during the outdoor experiment. (f) Cumulative CO yield over time during the outdoor experiment under natural sunlight. (g) Hourly CO evolution rate, calculated as the difference in cumulative yield between consecutive hourly measurements.\nReviewer #2\nThe bio-inspired concept of a \"charge reservoir\" to decouple the kinetic bottlenecks between water oxidation and CO2 reduction is not only innovative but also demonstrated with compelling experimental evidence across multiple catalytic systems. The observed universal enhancement in performance is a significant finding that could have broad implications for the field of artificial photosynthesis. The manuscript is generally well-written, and the data are presented clearly. I am supportive of publication, pending satisfactory responses to the following points, which aim to further strengthen the mechanistic claims and contextualize the impact of the work.\nAuthor response: We sincerely appreciate your positive feedback on our work and your valuable suggestions for improving the manuscript. We have carefully addressed all of your comments and revised the text accordingly. We hope these revisions have strengthened the paper, and we are truly grateful for your valuable input, which has been instrumental in enhancing its quality.\n1. The authors have only considered the charge transition between W5+ and W6+. Please comment on the possibility of further reducing the state to a W4+ state.\nAuthor response: We thank you for this insightful question. Our careful XPS spectral deconvolution (Supplementary Fig. 9 and Table S2 of the revised manuscript) specifically considered the potential presence of W4+. According to the reference (Proc. Natl. Acad. Sci. U. S. A., 2024, 121, e2319751121), the characteristic peaks for W4+ are expected at binding energies of 33.6 eV and 36.0 eV, which are distinct from those of W5+ (34.52 eV and 36.62 eV) and W6+ (35.94 eV and 38.04 eV). Our analysis revealed no spectral signatures at the positions attributable to W4+.\nSupplementary Fig. 9 and Table S3 of the revised manuscript:\nSupplementary Fig. 9 Deconvoluted W 4f XPS spectra for the corresponding conditions in panel a with (a) as-synthesized state, (b) after 1 min of irradiation, (c) after 2 min of irradiation, (d) after 3 min of irradiation.\nTable S3. Quantitative deconvolution results of W 4f XPS spectra for Ag/WO3 under varied processing conditions.\nConditions W6+ 4f 7/2 B.E. Area W6+ 4f 5/2 B.E. Area W5+ 4f 7/2 B.E. Area W5+ 4f 5/2 B.E. Area Ratio W6+ Ratio W5+\nFresh sample 35.94 73527.6 38.04 55145.7 34.52 12414.8 36.62 9311.1 85.55% 14.45%\nLight on 1 min 35.94 72785.8 38.04 54589.3 34.54 12710.9 36.64 9533.2 85.13% 14.87%\nLight on 2 min 35.94 71707.1 38.04 53780.3 34.56 13194.2 36.66 9895.6 84.46% 15.54%\nLight on 3 min 35.95 69847.9 38.05 52385.9 34.60 14288.9 36.70 10716.7 83.02% 16.98%\n*B.E. denotes binding energy (eV).\n2. The demonstration of catalyst performance under real-world outdoor conditions is a notable strength of this work. To fully appreciate these results, a detailed description of the testing parameters (e.g., light intensity, duration, weather conditions, and reactor setup) is essential.\nAuthor response: We thank you for this valuable suggestion regarding the description of outdoor testing conditions. Following your comment, we have now included a new section (Supplementary note 7) in the Supporting Information, which provides a detailed account of the experimental setup and parameters used in the outdoor test.\nSupplementary note 7 of the revised manuscript:\nThe outdoor experiment was carried out on June 5, 2024, in Xi'an, China (latitude 34.22°N, longitude 109.00°E) under clear sky conditions. Solar irradiance was continuously recorded using a solar radiometer (QTS-4, Jinzhou Sunshine Technology Co. Ltd). A custom-made cylindrical stainless-steel reactor with a volume of 200 mL and a quartz window was used. To increase the energy density, a Fresnel lens made of acrylic material with a diameter of 30 cm was employed for solar concentration. It should be noted that no solar tracking or active temperature control was applied during the test. For catalyst preparation, 5 mg of the photocatalyst was uniformly dispersed in 3 mL of deionized water and coated onto a dish (50 mm in diameter) by vacuum drying at 70°C for 24 hours. The coated dish was placed inside the reactor along with 1 mL of deionized water. The reactor was purged three times with high-purity CO2 and then pressurized to 1 bar with CO2. Gas sampling was performed hourly from 6:00 to 20:00, with 2 mL of gas collected each time for product analysis. All other testing conditions were consistent with those used in the laboratory experiments.\n3. We recommend a thorough review of the supporting information to correct minor typographical errors, particularly concerning superscripts/subscripts and capitalization, as exemplified in the caption of Supplementary Fig. 25.\nAuthor response: We thank you for your careful reading and for pointing out the need for formatting improvements in the Supporting Information. We have thoroughly reviewed the entire document and corrected the identified typographical errors, including issues related to superscripts/subscripts and capitalization as highlighted in the caption of Supplementary Fig. 25 (Supplementary Fig. 34 of the revised manuscript). We have also carefully checked the complete text to ensure consistency and to prevent similar issues throughout the manuscript.\nSupplementary Fig. 34 of the revised manuscript:\nSupplementary Fig. 34 SPV images of (a) fresh CoPc/WO3 and (b) pre-irradiated CoPc/Ag/WO3. (c) AFM image of CoPc/Ag/WO3. (d) SPV values extracted across dashed lines in (a) and (b).\n4. To substantiate the “decoupled” mechanism, time-resolved charge recombination kinetics (fs-TA decay fitting or photoluminescence lifetime) before and after Ag/WO3 coupling could be discussed quantitatively.\nAuthor response: Thank you for this suggestion. To substantiate the proposed \"decoupled\" mechanism, we have performed time-resolved photoluminescence (TRPL) measurements, and the corresponding fitting results are provided in Supplementary Fig. 29 and Supplementary Table 5. As you pointed out, the TRPL lifetime decreased significantly after irradiation. Specifically, the short-lived (τ1) and long-lived (τ2) components decreased from 1.2 ns to 1.1 ns and from 6.4 ns to 3.0 ns, respectively, leading to a reduction in the average lifetime from 2.7 ns to 1.4 ns for the CoPc/Ag/WO3 sample. This pronounced shortening of the TRPL lifetime confirms the formation of efficient electron transfer pathways from the phthalocyanine rings to the Ag/WO3 substrate following irradiation, supporting the decoupled charge transfer mechanism.\nSupplementary Fig. 29 and Table S5 of the revised manuscript:\nSupplementary Fig. 29 (a) Time-resolved PL decay curves for the fresh and irradiated CoPc/Ag/WO3 samples. (b) Fitting result of the fresh CoPc/Ag/WO3. (c) Fitting result of the irradiated CoPc/Ag/WO3.\nTable S5. The fluorescence lifetimes and relative percentage of fresh and irradiated CoPc/Ag/WO3.\nSample τ1 (ns) A1 τ2 (ns) A2 τave (ns)\nfresh CoPc/Ag/WO3 1.2 92.8% 6.4 7.2% 2.7\nirradiated CoPc/Ag/WO3 1.1 94.1% 3.0 5.9% 1.4\nfresh CoPc/WO3 1.3 96.3% 8.7 3.7% 3.5\nirradiated CoPc/WO3 1.2 96.7% 10.7 3.3% 2.7\n5. Six-cycle tests (72 h) are promising but relatively short for a solar system. Extending to ~200 h continuous operation or including post-reaction EXAFS would prove structural robustness of W5+/W6+ cycling.\nAuthor response: Thank you for this valuable suggestion. Following your advice, we have performed post-reaction EXAFS analysis to probe the local structure of the catalyst. As shown in Supplementary Fig. 24 of the revised manuscript, the Fourier-transformed k3-weighted EXAFS spectrum of the post-reaction CoPc/Ag/WO3 exhibits a main peak at 1.7 Å, which corresponds to the first coordination shell of isolated Ag atoms. The Ag–Ag peak at 2.7 Å is significantly weaker than the first-shell peak, confirming that Ag predominantly exists as highly dispersed single atoms in the catalyst. These results clearly demonstrate that the local atomic structure remains intact during the reaction, providing direct evidence for the structural stability of this key charge-transfer mediator. To further substantiate the overall robustness of the catalyst, we have also included complementary post-reaction characterizations. The XRD patterns confirm that the crystalline phase of the catalyst is well-preserved before and after the reaction. The FTIR spectra demonstrate that the molecular structure of the CoPc co-catalyst remains stable. Additional XPS analysis was conducted to investigate the valence state changes of the major elements in the catalyst, further supporting its stability under reaction conditions. Collectively, the data from EXAFS, XRD, FTIR, and XPS provide strong and multi-faceted evidence for the excellent structural stability of our catalyst, successfully addressing your concern regarding the robustness of the W5+/W6+ cycle.\nSupplementary Figs. 22–25 of the revised manuscript:\nSupplementary Fig. 22 XRD patterns of CoPc/Ag/WO3 before and after the CO2 reduction reaction.\nSupplementary Fig. 23 FTIR spectra of CoPc/Ag/WO3 before and after the CO2 reduction reaction.\nSupplementary Fig. 24 EXAFS spectra of Ag foil and CoPc/Ag/WO3 after the reaction.\nSupplementary Fig. 25 (a) The survey XPS spectrum, (b) W 4f XPS spectrum, (c) O 1s XPS spectrum, and (d) Ag 3d XPS spectrum of CoPc/Ag/WO3 before and after the CO2 reduction reaction.\n6. Clarify whether O2 was quantified via GC-TCD or mass spectrometry, and if leakage corrections were applied, to support the claim of stoichiometric coupling (O2:CO ≈ 1:2).\nAuthor response: We thank you for pointing out the need to clarify the O2 quantification method and leakage correction procedures. In our study, O2 was quantitatively analyzed using gas chromatography with a thermal conductivity detector (GC-TCD). To ensure measurement accuracy and address potential air leakage concerns, we implemented the following approaches throughout the photocatalytic reaction. First, all gas sampling was performed using an automated online system to prevent leakage during the sampling process. Second, we continuously monitored N2 levels as an indicator of potential air intrusion. Prior to the reaction, the reactor was rigorously purged with high-purity CO2 (>99.999%) through multiple evacuation-refill cycles. Under our GC-TCD configuration, O2 and N2 exhibit well-resolved peaks at approximately 4.4 min and 5.9 min, respectively (Supplementary Fig. 20a). Throughout the photocatalytic tests, the N2 peak area remained consistently stable (Supplementary Fig. 20b), confirming the absence of significant air leakage into the reactor. To further ensure accuracy, the detected O2 concentration was corrected by subtracting the contribution from any residual air based on the measured N2 signal, using the following equation:\nn_reaction_O2 = n_O2 - n_N2 * (20.9% / 78.1%)\nwhere n_reaction_O2 is the final amount of O2 produced from the reaction, n_O2 is the amount of O2 measured by GC-TCD, and n_N2 is the amount of N2 measured by GC-TCD. The factors 20.9% and 78.1% represent the volume fractions of O2 and N2 in air, respectively.\nThis correction, along with the consistent N2 level and clear O2 evolution trend, strongly supports the stoichiometric coupling observed between O2 and CO production (O2:CO ≈ 1:2). We have included a detailed description of the methodology and validation data in the revised Supporting Information.\nSupplementary Fig. 20 of the revised manuscript:\nSupplementary Fig. 20 Quantification of O2 production during photocatalytic CO2 reduction. (a) Gas chromatographic peaks of O2, N2, and air. (b) Time-dependent growth of O2 and N2 peaks monitored by automated online sampling during the photocatalytic reaction.\nReviewer #3\nThis study presents an original bio-inspired charge reservoir strategy in the field of solar energy conversion. The conceptual design is elegant and logically rigorous, offering a novel approach to overcome the reliance on sacrificial agents in photocatalytic CO2 reduction. The experimental work is particularly impressive, demonstrating the effectiveness of the charge storage and controlled release mechanism through multi-system universality validation, a hundred-fold performance enhancement. The experimental designs are scientifically sound, with mutually corroborating data forming a complete evidence chain. It meets the standards for publication in Nature communications and I am pleased to recommend acceptance after addressing the following revisions:\nAuthor response: We sincerely appreciate your positive evaluation of our work and are grateful for your insightful suggestions, which have helped us substantially improve the quality of our manuscript. We hope that our careful revisions have satisfactorily addressed all of your concerns.\n1. The introduction adequately discusses the limitations of organic sacrificial agents but offers a somewhat generalized overview of existing charge separation strategies. A more in-depth comparison with recent representative works is recommended.\nAuthor response: Thank you for this suggestion. We have now expanded the introduction to include a more in-depth discussion and comparison with several recent representative works on charge separation strategies, as referenced in the revised manuscript. This addition provides a better context for our work and highlights its position within the current research landscape.\nLines 50–53 of the revised manuscript:\nConstructing a Z-scheme heterojunction, inspired by natural photosynthesis and integrating two narrow-bandgap semiconductors, has been regarded as an effective strategy to overcome these obstacles 12, 13. Exemplary systems include 3D-SiC@2D-MoS2 7, AgVO3/TiO2 14, MoS2/Fe2O3 15, among others. However, a key challenge still remains: the significant difference in electron and hole transport dynamics requires spatiotemporal - coupled charge migration 16, which results in the absence of a universal strategy for constructing composite structures applicable to the vast majority of active components.\n2. The relatively high silver loading (1.68 wt%) in Ag/WO3 may raise concerns about aggregation. Further justification of its stability is needed.\nAuthor response: Thank you for raising this valid concern regarding the potential aggregation of silver due to the relatively high loading (1.68 wt%) in our Ag/WO3 catalyst. To address this, we have performed additional XAFS spectroscopy on the catalyst after the cyclic stability tests for photocatalytic CO2 reduction. Analysis of the EXAFS spectra revealed no detectable Ag–Ag coordination, ruling out the formation of metallic Ag clusters. This result provides strong evidence for their structural stability and effectively addresses the concern of aggregation.\nSupplementary Fig. 24 of revised manuscript:\nSupplementary Fig.24 EXAFS spectra of Ag foil and CoPc/Ag/WO3 after the reaction.\n3. XRD and Raman results do not effectively distinguish WO3 crystal phase changes. Supplementing with high-resolution TEM or selected-area electron diffraction data would help verify crystal structure integrity.\nAuthor response: Thank you for this insightful suggestion. You rightly pointed out that XRD and Raman spectroscopy alone are not sufficient to conclusively distinguish the crystal phase of WO3. To directly address this and unequivocally verify the structural integrity of the WO3 support, we have performed high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) characterizations on the Ag/WO3 catalyst. As shown in the HRTEM image, the clearly resolved lattice fringes with a measured interplanar spacing of approximately 0.38 nm are indicative of the (020) crystallographic plane of monoclinic WO3. This observation is further corroborated by the SAED pattern, which exhibits sharp diffraction rings that can be assigned to the (020) planes of monoclinic WO3. Together, these results provide direct visual and diffraction evidence that the WO3 crystal phase remains stable without any observable phase transformation, thereby fully addressing your concern regarding the crystal structure integrity. This new evidence has now been included as Supplementary Figure 2, conclusively confirming the existence of the monoclinic WO3 structure.\nSupplementary Fig. 2 of revised manuscript:\nSupplementary Fig. 2 (a-c) TEM images of Ag/WO3. (d) Selective area electron diffraction images. (e) TEM-EDS elemental mappings.\n4. Inconsistent reporting of CO production rates may cause confusion. When comparing with literature data (Table S4), note that most studies report μmol-level yields, whereas this work uses mmol. Units should be clearly defined.\nAuthor response: Thank you for pointing out this important issue. We agree that the inconsistent reporting of units between our original manuscript and the literature data in Table S4 could cause confusion for readers. We have now standardized the units of CO production rate across the entire manuscript and the supporting information. All relevant values, including those in the main text, figures, and Table S4, are now consistently reported in mmol g-1 h-1 to ensure a clear and direct comparison.\nTable S4. Comparison of the state-of-the-art works on photocatalytic CO2 reduction.\nCatalyst Reaction condition Products evolution Reference\nCoPc/Ag/WO3 Under a 300 W Xe arc lamp with an AM 1.5G filter; Water as sacrificial agent CO: 0.148 mmol g-1 h-1 This work\nCoPc/Bi2WO6 Under a 300 W Xenon lamp with cutoff filter (λ≥420 nm); water as sacrificial agent CO: 0.008 mmol g-1 h-1; CH4: 0.003 mmol g-1 h-1 20\nCoPc/ZnO/RGO Under 300 W Xe lamp; water as sacrificial agent CO: 0.026 mmol g-1 h-1 21\nCoPc/BiOBr Under UV–vis light (approximately 200 mW cm−2); Water as sacrificial agent CO: 0.059 mmol g-1 h-1; CH4: 0.006 mol g-1 h-1 19\nCoPcPDA/CMP Under a 300 W xenon lamp with a cut filter (keep range of 400 to 800 nm); Water as sacrificial agent CO: 0.014 mmol g−1 h−1; H2: 0.001 mmol g-1 h-1 22\nCoPc/g-C3N4 Under a 300W xenon lamp equipped with a cut-off filter of 420 nm; water as sacrificial agent CO: 0.012 mmol g−1 h−1; H2: 0.001 mmol g-1 h-1 18\nCoPc/0.6PO4−/WS2 Under a 300 W xenon arc lamp with 420 filter; water as sacrificial agent CO: 0.028 mmol g-1 h-1; CH4: 0.007 mmol g-1 h-1 23\nCoPc/Ni/NiO Under a visible light source (85 W/m2) triethylamine (TEA) as sacrificial agent CO: 0.151 mmol g−1 h−1 24\nCoPc/ZnIn2S4 Under LED light (55.3 mW/cm2); BIH as sacrificial agent CO: 0.026 mmol g-1 h-1; H2: 0.0056 mmol g-1 h-1 25\nCoPc-COP0.02@mpg-C3N4 Under xenon lamp (350 nm<λ<780 nm); TEOA sacrificial agent CO: 0.134 mmol g-1 h-1 26\n5. The O2:CO molar ratio close to 1:2 supports the overall reaction, but background oxygen interference should be ruled out via dark control experiments to exclude residual air effects.\nAuthor response: We thank you for pointing out the need to clarify the O2 quantification method and leakage correction procedures. In our study, O2 was quantitatively analyzed using gas chromatography with a thermal conductivity detector (GC-TCD). To ensure measurement accuracy and address potential air leakage concerns, we implemented the following approaches throughout the photocatalytic reaction.\nFirst, all gas sampling was performed using an automated online system to prevent leakage during the sampling process. Second, we continuously monitored N2 levels as an indicator of potential air intrusion. Prior to the reaction, the reactor was rigorously purged with high-purity CO2 (>99.999%) through multiple evacuation-refill cycles. Under our GC-TCD configuration, O2 and N2 exhibit well-resolved peaks at approximately 4.4 min and 5.9 min, respectively (Supplementary Fig. 20a). Throughout the photocatalytic tests, the N2 peak area remained consistently stable (Supplementary Fig. 20b), confirming the absence of significant air leakage into the reactor. To further ensure accuracy, the detected O2 concentration was corrected by subtracting the contribution from any residual air based on the measured N2 signal, using the following equation:\nn_reaction_O2 = n_O2 - n_N2 * (20.9% / 78.1%)\nwhere n_reaction_O2 is the final amount of O2 produced from the reaction, n_O2 is the amount of O2 measured by GC-TCD, and n_N2 is the amount of N2 measured by GC-TCD. The factors 20.9% and 78.1% represent the volume fractions of O2 and N2 in air, respectively.\nThis correction, along with the consistent N2 level and clear O2 evolution trend, strongly supports the stoichiometric coupling observed between O2 and CO production (O2:CO ≈ 1:2). We have included a detailed description of the methodology and validation data in the revised Supporting Information (Supplementary note 8).\nSupplementary Fig. 20 Quantification of O2 production during photocatalytic CO2 reduction. (a) Gas chromatographic peaks of O2, N2, and air. (b) Time-dependent growth of O2 and N2 peaks monitored by automated online sampling during the photocatalytic reaction.\n6. Isotope labeling experiments (13CO2 → 13CO) provide solid evidence, but the MS peaks at m/z = 13, 16, and 29 lack specific assignments. More detailed labeling in the figures is suggested.\nAuthor response: Thank you for this helpful suggestion. We have now clearly labeled the MS peaks at m/z = 13 (13C), 16 (16O), and 29 (13C16O) in the relevant figures (e.g., Fig 3c) as recommended. These specific assignments should now provide a clearer guide for readers when interpreting the isotopic tracing data.\nFig. 3 of the revised manuscript:\nFig. 3. Evaluation of photocatalytic CO2 reduction performance. (a) The photocatalytic performance for CO2 reduction to CO was assessed for composite catalysts comprising varying mass ratios of the CoPc in CoPc/Ag/WO3 catalyst. Values on the x-axis correspond to the mass percentage of each active component within the composite catalysts. (b) The yields of CO and O2 generation on CoPc/Ag/WO3. Error bars represent the standard deviation of three measurements. (c) MS spectra of CO from the photocatalytic reduction of 13CO2 and 12CO2 over CoPc/Ag/WO3. (d) Comparison of photocatalytic CO2-to-CO reduction activities among different photocatalyst formulations. The last eight test results are from previous reports, and more relevant information is in Supplementary Table 4.\n7. Stability tests over 6 cycles (72 hours) show good recyclability, but post-reaction surface element analysis (e.g., XPS to track valence states) is missing.\nAuthor response: Thank you for this insightful comment. Following your suggestion, we have conducted a detailed post-reaction XPS analysis. The results indicate that the main elemental composition and concentrations in the catalyst remained largely unchanged after the reaction (Supplementary Fig. 25). We did observe a slight shift of the W 4f, O 1s, and Ag 3d XPS peaks toward lower binding energies in the post-reaction CoPc/Ag/WO3 catalyst. We attribute this phenomenon primarily to the incomplete release of stored electrons within the catalyst system, which is a known effect that can cause a slight reduction in measured binding energies. This observation is consistent with the effective function of the W5+/W6+ redox cycle as an electron mediator and does not indicate structural degradation. To further substantiate the overall robustness of the catalyst, we have included complementary post-reaction characterizations. The FTIR spectra demonstrate that the molecular structure of the CoPc co-catalyst remains stable (Supplementary Fig. 22). The XRD patterns confirm that the crystalline phase of the catalyst is well-preserved (Supplementary Fig. 23). Furthermore, the Fourier-transformed k3-weighted EXAFS spectrum of the post-reaction sample exhibits a main peak at 1.7 Å, corresponding to the first coordination shell of isolated Ag atoms (Supplementary Fig. 24). The Ag–Ag peak at 2.7 Å is significantly weaker than the first-shell peak, confirming that Ag remains predominantly as highly dispersed single atoms. These results clearly demonstrate that the local atomic structure remains intact during the reaction, providing direct evidence for the structural stability of this key charge-transfer mediator. Collectively, the data from XPS, EXAFS, XRD, and FTIR provide strong and multi-faceted evidence for the excellent structural and chemical stability of our catalyst under the reported reaction conditions.\nSupplementary Fig. 22 XRD patterns of CoPc/Ag/WO3 before and after the CO2 reduction reaction.\nSupplementary Fig. 23 FTIR spectra of CoPc/Ag/WO3 before and after the CO2 reduction reaction.\nSupplementary Fig. 24 EXAFS spectra of Ag foil and CoPc/Ag/WO3 after the reaction.\nSupplementary Fig. 25 (a) The survey XPS spectrum, (b) W 4f XPS spectrum, (c) O 1s XPS spectrum, and (d) Ag 3d XPS spectrum of CoPc/Ag/WO3 before and after the CO2 reduction reaction.\n8. In situ FTIR shows stronger *COOH intermediate signals, supporting accelerated reaction rates, but other product assignments should be described in detail and compared with DFT-calculated pathways.\nAuthor response: We thank you for this insightful comment. We have now provided a thorough analysis and discussion of the other key intermediates in Supplementary note 3. This section details the identification of species such as *CO, *HCOO, based on their characteristic vibrational frequencies. Furthermore, as you aptly pointed out, these experimental observations are directly compared with our DFT-calculated reaction pathways in Supplementary note 4. The theoretical calculations excellently support the experimental FTIR data, not only confirming the stability of the *COOH intermediate but also validating the proposed formation and conversion pathways for the other observed species. This combined experimental-theoretical analysis provides a cohesive and robust mechanistic picture, strongly supporting our conclusion of an accelerated reaction rate.\nSupplementary note 3. FTIR test and result analysis\nTo investigate the reasons behind the increased CO production rate, we conducted in-situ FTIR spectroscopy studies on the catalyst surface conversion process of CO2.The setup consisted of a Harrick in situ diffuse reflectance cell equipped with a liquid nitrogen-cooled Mercury Cadmium Telluride (MCT) detector. Prior to measurement, all samples were pretreated at 120 °C under Ar atmosphere for 120 min to obtain a clean surface. The background spectrum was collected after the sample cooled to room temperature under Ar atmosphere. The reaction was initiated by introducing a flow of CO2 saturated with water vapor, which was achieved by bubbling the CO2 gas through a container of deionized water, thereby delivering both CO2 and H2O to the catalyst surface. The catalyst was then irradiated using a light source whose spectral output matched the wavelength distribution used in the CO2 reduction performance tests. The light intensity was kept consistent for all samples. Spectral data were collected at 2-minute intervals throughout the irradiation period.\nAs shown in Supplementary Fig. 33, on the CoPc/Ag/WO3 surface, absorption peaks corresponding to monodentate carbonate (m-CO3 2-) at 1311 cm-1 and bidentate nitrate (b-CO3 2-) at 1505 cm-1 were observed 3, 4. Additionally, absorption peaks at 1117, 1153, 1272, and 1452 cm-1 were attributed to bicarbonate species (HCO3 -) 3, 5, 6. In addition, the figure also shows an absorption peak attributed to water at 1630 cm-1 5. These peaks indicate that the reactants CO2 and water have successfully adsorbed to the surface of the catalyst. Of greater significance, under illumination, absorption peaks associated with *COOH were detected at 1202, 1362, and 1561 cm-1 7-11. The position of the CoPc/WO3 peak closely resembled that of CoPc/Ag/WO3, indicating similar CO2 conversion processes on both samples. However, under identical test conditions, the intensity of the *COOH species on the CoPc/Ag/WO3 surface was significantly stronger than that on CoPc/WO3, implying a faster reaction rate for this catalyst 11, 12. The close similarity in CO2 conversion intermediates between the CoPc/Ag/WO3 and CoPc/WO3 surfaces confirms thermodynamic consistency. The clear enhancement in reaction rates upon Ag incorporation, however, demonstrates that the interfacial Ag single atoms are key to improving kinetics, definitively serving as a kinetic promoter for the CO2 reduction reaction.\nSupplementary note 4. DFT calculation details\nAll density functional theory (DFT) computations were executed using the Vienna Ab Initio Simulation Package (VASP) 13. The Perdew-Burke-Ernzerhof exchange-correlation functional (PBE-GGA) was adopted 14. Structural models were constructed based on experimental results from XRD, Raman, and X-ray absorption fine structure (XAFS) spectroscopy, while possible reaction pathways were initially proposed according to insights from in situ FTIR spectroscopy. A substantial 20 Å vacuum region was implemented in surface models to prevent artificial interactions between periodic replicas. Specific computational parameters included an energy cutoff of 520 eV and a Monkhorst-Pack k-point mesh of 3×3×1. For geometry optimization, convergence criteria required energy differences per atom below 10-5 eV and forces less than 0.03 eV/Å. To determine the Gibbs free energy relevant to reaction conditions, we applied the formula:\nG = E_DFT + E_ZPE + E_enthalpy - T*S (1)\nHere, E_DFT corresponds to the static electronic energy at absolute zero, E_ZPE to the zero-point energy correction, E_enthalpy to the enthalpy contribution, and T*S to the entropy contribution. The VESTA software package was used for visualizing all structural models 15.\n9. The outdoor natural light experiment is well-conceived, but insufficient description of key parameters affects reproducibility. Additional details are recommended.\nAuthor response: Thank you for pointing out the need for more detailed parameters in the outdoor natural light experiment. We have added a new section (Supplementary note 7) in the Supporting Information, where you will find a comprehensive description of the experimental setup and key ambient conditions recorded during the test to ensure full reproducibility.\nSupplementary note 7 of the revised manuscript:\nThe outdoor experiment was carried out on June 5, 2024, in Xi'an, China (latitude 34.22°N, longitude 109.00°E) under clear sky conditions. Solar irradiance was continuously recorded using a solar radiometer (QTS-4, Jinzhou Sunshine Technology Co. Ltd). A custom-made cylindrical stainless-steel reactor with a volume of 200 mL and a quartz window was used. To increase the energy density, a Fresnel lens made of acrylic material with a diameter of 30 cm was employed for solar concentration. It should be noted that no solar tracking or active temperature control was applied during the test. For catalyst preparation, 5 mg of the photocatalyst was uniformly dispersed in 3 mL of deionized water and coated onto a dish (50 mm in diameter) by vacuum drying at 70°C for 24 hours. The coated dish was placed inside the reactor along with 1 mL of deionized water. The reactor was purged three times with high-purity CO2 and then pressurized to 1 bar with CO2. Gas sampling was performed hourly from 6:00 to 20:00, with 2 mL of gas collected each time for product analysis. All other testing conditions were consistent with those used in the laboratory experiments."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This is the revised manuscript previously submitted to Nature Communications. I am satisfied with the current revision, as the authors have addressed the majority of the reviewers’ concerns. Therefore, I am pleased to recommend acceptance of the manuscript.", "reviewer_2": "The authors have addressed the necessary concerns and comments raised during the manuscript review, and the manuscript now meets the requirements, making it suitable for publication in Nature Communications.", "reviewer_3": "The revision can be accepted as it is."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nThis is the revised manuscript previously submitted to Nature Communications. I am satisfied with the current revision, as the authors have addressed the majority of the reviewers’ concerns. Therefore, I am pleased to recommend acceptance of the manuscript.\nAuthor response: We sincerely appreciate the reviewer’s insightful comments and the time dedicated to evaluating our manuscript. We are pleased that our revisions have met the referee’s satisfaction and are grateful for the recommendation for publication.\nReviewer #2 (Remarks to the Author):\nThe authors have addressed the necessary concerns and comments raised during the manuscript review, and the manuscript now meets the requirements, making it suitable for publication in Nature Communications.\nAuthor response: We really appreciate the referee for his/her participation in the review of this manuscript. We are pleased that our revisions have met the referee’s satisfaction and are grateful for the recommendation for publication.\nReviewer #3 (Remarks to the Author):\nThe revision can be accepted as it is.\nAuthor response: We sincerely appreciate the reviewer for his/her insightful comments and the time dedicated to evaluating our manuscript. We are grateful for the recommendation for publication."}]

Decision letter