
==== Front
Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

39289360
52161
10.1038/s41467-024-52161-4
Article
Surface fluorination of BiVO4 for the photoelectrochemical oxidation of glycerol to formic acid
Liu Yang 1
Shang Huishan 2
Zhang Bing 2
http://orcid.org/0000-0001-8261-154X
Yan Dongpeng yandp@bnu.edu.cn

3
http://orcid.org/0000-0003-1089-6210
Xiang Xu xiangxu@mail.buct.edu.cn

14
1 grid.48166.3d 0000 0000 9931 8406 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, P. R. China
2 https://ror.org/04ypx8c21 grid.207374.5 0000 0001 2189 3846 School of Chemical Engineering, Zhengzhou Key Laboratory of Advanced Separation Technology, Zhengzhou University, 450001 Zhengzhou, P. R. China
3 https://ror.org/022k4wk35 grid.20513.35 0000 0004 1789 9964 Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, 100875 Beijing, P. R. China
4 Quzhou Institute for Innovation in Resource Chemical Engineering, 324000 Quzhou, P. R. China
17 9 2024
17 9 2024
2024
15 815510 2 2024
28 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
The C–C bond cleavage of biomass-derived glycerol to generate value-added C1 products remains challenging owing to its slow kinetics. We propose a surface fluorination strategy to construct dynamic dual hydrogen bonds on a semiconducting BiVO4 photoelectrode to overcome the kinetic limit of the oxidation of glycerol to produce formic acid (FA) in acidic media. Intensive spectroscopic characterizations confirm that double hydrogen bonds are formed by the interaction of the F–Bi–F sites of modified BiVO4 with water molecules, and the unique structure promotes the generation of hydroxyl radicals under light irradiation, which accelerates the kinetics of C–C bond cleavage. Theoretical investigations and infrared adsorption spectroscopy reveal that the double hydrogen bond enhances the C=O adsorption of the key intermediate product 1,3-dihydroxyacetone on the Bi–O sites to initiate the FA pathway. We fabricated a self-powered tandem device with an FA selectivity of 79% at the anode and a solar-to-H2 conversion efficiency of 5.8% at the cathode, and these results are superior to most reported results in acidic electrolytes.

This work reports a surface fluorination strategy to construct dynamic dual hydrogen bonds on semiconducting BiVO4 photoelectrode for glycerol oxidation to value-added C1 production formic acid in acidic solution.

Subject terms

Renewable energy
Sustainability
https://doi.org/10.13039/501100001809 National Natural Science Foundation of China (National Science Foundation of China) 22288102, 21978021 Xiang Xu issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The oxidative transformation of glycerol to higher-value chemicals is intriguing1–3. Among various products, formic acid (FA) is a key C1 product that has been widely used in the textile, agriculture, and pharmaceutical industries4–6. The reaction pathway from glycerol to FA is complex and involves C–H and O–H bond dehydrogenation, C=O bond adsorption/desorption, and C–C bond cleavage7. Among these steps, C–C bond cleavage is the most challenging.

The electrochemical oxidation of glycerol to generate FA has been extensively studied8–12. In general, the oxidation reaction occurs in the presence of strongly alkaline electrolytes (pH > 13) and at high potentials beyond 1.3 V vs. a reversible hydrogen electrode (RHE), which promotes the kinetics of C–C bond cleavage to yield 80% selectivity for FA10. However, this product exists in the form of formate. Other products, e.g., glyceraldehyde (GLYAD) and formaldehyde, are inevitably degraded to lactate and methanol via base-catalyzed dehydration and Cannizzaro rearrangement8,13. These issues complicate our understanding of glycerol-to-FA conversion in alkaline electrolytes. To directly obtain the desired FA product and avoid interference from unexpected side reactions, the reaction network should be simplified by using acidic electrolytes. However, catalyst deactivation and slow kinetics hinder glycerol-to-FA conversion under acidic conditions.

Compared to energy-intensive electrochemical methods, the photoelectrochemical (PEC) method uses sunlight to reduce the electrical energy that is required to drive alcohol oxidation by approximately 60%14. Recently, BiVO4 and WO3 were applied to the oxidation of PEC glycerol to generate dihydroxyacetone (DHA) and GLYAD via C–H activation in acidic electrolytes15–18. The slow oxidation kinetics led to a low FA selectivity of 10–40%. An alkaline electrolyte with pH >9.3 enables C–C bond cleavage and promotes FA selectivity to 70%16. Nevertheless, most semiconductor materials experience severe erosion under alkaline conditions19. Therefore, the limitations of glycerol-to-FA conversion in acidic media must be overcome.

Hydroxyl radicals are formed in situ on the surface of semiconductor materials during PEC water splitting and glycerol oxidation16,20. Additionally, hydroxyl radicals have been indicated to be powerful species for the oxidative C–C bond cleavage of organic molecules21. Nevertheless, the insufficient amount of hydroxyl radicals in acidic electrolyte solutions limits C–C bond cleavage kinetics. In addition, the C=O bond adsorption strength of the oxidative intermediate products, i.e., DHA and GLYAD, is usually weak, which inhibits further oxidation to generate FA. It is challenging to simultaneously address the insufficient number of hydroxyl radicals and the weak adsorption of C=O on the surface of semiconductors. A recent study showed that F-modified TiO2 enhanced water adsorption and dissociation via hydrogen bond interactions during water oxidation22. The incorporation of hydrogen bond acceptors is a promising approach for modulating the adsorption properties of semiconductors and initiating the desired oxidation pathways.

In this study, we designed a dual hydrogen-bond structure using a surface-fluorinated BiVO4 photoanode, i.e., BVO–F, to overcome the limitation of glycerol-to-FA conversion kinetics in acidic media. Comprehensive microscopic and spectroscopic characterizations revealed the atomic location and electronic regulation of fluorine in BVO. In situ experimental measurements and density functional theory (DFT) calculations revealed that the F–Bi–F sites formed dynamic double hydrogen bonds with water, which accelerated the production of hydroxyl radicals under light irradiation and subsequently enhanced C–C bond cleavage. Moreover, the double hydrogen bond structure improved the adsorption of the key intermediate product, namely, DHA, and its oxidation to generate FA. The self-powered PEC-photovoltaic (PV) tandem device with an integrated BVO–F photoanode achieved 79% selectivity for FA and a solar-to-H2 conversion efficiency of 5.8%. Our findings demonstrate that the PEC C–C bond cleavage of polyols can be initiated by constructing dual hydrogen bonds on the prevailing photoelectrodes.

Results

Synthesis and structural characterization

BVO photoanodes supported on fluorine-doped SnO2 (FTO) substrates were fabricated by combining electrodeposition and calcination23. BVO–F was obtained by immersing the prepared BVO in a sodium fluoride aqueous solution (details in the “Methods” section). No change in morphology was observed by scanning electron microscopy (SEM) (Supplementary Figs. 1 and 2) or transmission electron microscopy (TEM) (Supplementary Fig. 3) analyses of pristine BVO and BVO–F. The X-ray diffraction (XRD) patterns show that the typical diffraction peaks were indexed to monoclinic BVO (JCPDS No. 14-0688) (Fig. 1a and Supplementary Fig. 4). Interestingly, the intensity ratios of the (112) and (004) planes from the XRD patterns decreased to 3.0 relative to that of the pristine BVO sample (Fig. 1a), which clearly demonstrates the highly selective etching of the BVO (112) plane. According to the atomic arrangements (Supplementary Fig. 5), the fluoride ions could selectively replace the di-coordination/tri-coordination oxygen sites and bind to the unsaturated coordination terminal Bi owing to its high electronegativity and similar ionic radius to that of oxygen (Supplementary Note 1). High-resolution TEM (HRTEM) images show that the main exposed (112) facet had a lattice spacing of 0.308 nm (Fig. 1b, c). BVO–F exhibited an amorphous layer with a thickness of approximately 2 nm owing to the surface atom structure rearrangement24,25 and homogeneous dispersion of F, as shown by energy dispersive X-ray spectroscopy (EDS) elemental mapping (Fig. 1d).Fig. 1 Structural characterization of the BVO–F photoanode.

a XRD patterns of pristine BiVO4 and surface-fluorinated BiVO4 samples (BVO–F). The intensity ratios of (112) and (004) from the XRD patterns. HRTEM lattice images showing the d-spacings of the (112) facets of b BVO and c BVO–F. d EDS mappings of Bi, F, O, and V in BVO–F. e FT-IR spectra and f XPS spectra of F 1s for BVO and BVO–F samples. g Fourier transforms (FTs) of Bi L3-edge extended X-ray absorption fine structure (EXAFS) spectra for BVO, BVO–F, Bi foil, and Bi2O3. h Wavelet transform (WT) of BVO and BVO–F at the Bi L3 edge.

There was a downward trend in the intensity of the V–O stretching mode at approximately 828 cm−1 for BVO–F compared with BVO (Supplementary Fig. 6) in the Raman spectra, which suggests a weak F–V–O bond formation on BVO–F owing to the deformation of the VO4 tetrahedron26,27. The same obvious characteristic band changed at 839 cm−1 for the V–O symmetric vibrations in the FT-IR spectra (Fig. 1e, Supplementary Fig. 7a–c, and Supplementary Table 1)28. The changes in the characteristic bands of these spectra provide evidence that F replaces O in BVO. BVO–F showed strong absorption band intensities at 3550–2500 cm−1 and 1641 cm−1, which correspond to the linear and bridged H-bonded νOH oscillators29 and δH2O, respectively. The absorption edge, band gap, and observed color did not change significantly after the surface fluorination (Supplementary Fig. 8). X-ray photoelectron spectroscopy (XPS) revealed the effects of surface fluorination on the chemical states (Supplementary Fig. 9 and Supplementary Note 2). Compared with the pristine BVO photoanode (Supplementary Fig. 10), the Bi 4f and V 2p peaks shifted to a higher energy of 0.3 eV for BVO–F (Supplementary Fig. 11a, b), which indicates a change in the local coordination of Bi and V30,31. Additionally, we analyzed the XPS valence band spectra of the prepared photoanodes (Supplementary Fig. 12) and reported that the valence band maximum (VBM) increased from 1.3 eV for BVO to 1.6 eV for BVO–F. These findings suggest the upward band bending of BVO–F32,33. The F 1s peaks at 680.1 eV can be attributed to F− that adsorbed onto the terminal Bi atom (Bi–Fterminal)22. The peak at 682.5 eV was assigned to fluorine, which replaced the surface-bridging oxygen (Bi–Fbridge) (Fig. 1f and Supplementary Fig. 11c)34,35. The O 1s spectra (Supplementary Fig. 11d, e, and Supplementary Table 2) could be divided into lattice oxygen (M–O), hydroxyl groups bonded to metal cations in the oxygen-deficient region (O–H), and chemisorbed oxygen species from water molecules (H2O) in BVO36,37. The ratio of chemisorbed oxygen species from H2O increased significantly, which was consistent with the FT-IR results. The reason is the formation of hydrogen bonds between F and H2O, which enhanced the adsorption of H2O38,39.

The X-ray absorption near-edge structure spectra (XANES) and extended X-ray absorption fine structure (EXAFS) of the Bi L3 edge revealed the local structural characteristics of Bi. Compared with the height of the white line of the reference sample Bi2O3 (Supplementary Fig. 13a), the Bi valence states of BVO and BVO–F were approximately +3. The Bi L3-edge k2χ(k) oscillation curves of BVO and BVO–F showed that the atomic configurations of Bi were similar (Supplementary Fig. 13b)40. The Fourier transform (FT) curves of the EXAFS at the Bi L3 edge showed that the dominant peak was located at 1.62 Å for BVO, which corresponded to the Bi–O bonds in the coordination shell (Fig. 1g). The main peak of BVO–F was wider and located at 1.60 Å, which indicates that Bi had direct coordination with F and a shortened Bi–O bond41,42. The maximum value of the WT-EXAFS contour plot was approximately 4.3 Å−1 in the k space (Fig. 1h) for BVO–F, which corresponded to the contributions of the Bi–O and Bi–F bonds. A decrease in the coordination number of the Bi–O coordination shells and a slight increase in the coordination number of the Bi–F coordination shells were observed (Supplementary Table 3), which confirms that the F atom replaced the O atom. To determine where F replaces O in BVO, the formation energy (Eform) was calculated via DFT calculations (Supplementary Fig. 14a–e). The results show that the configuration of di-coordination O replaced by F is the energy-favorable and stable state.

PEC performance of glycerol oxidation

The performance of PEC glycerol oxidation was evaluated in a 0.5 M Na2SO4 electrolyte (pH = 2, adjusted with 0.5 M H2SO4) that contained 0.1 M glycerol under illumination (AM 1.5 G, 100 mW cm−2). The BVO and BVO–F photoanodes displayed stable photocurrents, with fast and reproducible responses upon each cycle of illumination at 1.2 V vs. RHE (Fig. 2a and Supplementary Fig. 15a). The applied bias photon‒to-current efficiency (ABPE) reached a maximum value of 0.77% at 0.78 V vs. RHE for the BVO–F photoanode, which is twice as high as that of BVO (0.36%) (Supplementary Fig. 16). The photoelectrode-electrolyte interface charge-transfer resistance (Rct) was 91.1 Ω for the BVO–F photoanode, which was much lower than that for BVO (208 Ω) (Fig. 2b and Supplementary Fig. 15b, c). This result reveals that the surface fluorine modification mainly reduces the transfer resistance of photogenerated holes on the surface of the BVO–F photoanode to the electrolyte (Supplementary Note 3).Fig. 2 PEC performance of the glycerol oxidation.

a Transient current-time curves of the BVO and BVO–F photoanodes at 1.2 V vs. RHE under light irradiation. b Impedance spectra of the photoanodes under illumination. Photoelectrochemical oxidation of glycerol under illumination on the c BVO and d BVO–F photoanodes at 1.2 V vs. RHE. e Comparison of the anodic O2 product and cathodic H2 evolution on BVO–F photoanodes with and without glycerol. f Selectivity of the PEC glycerol oxidation on BVO–F within 1 h in the presence of radical scavengers, 0.02 M Na2SO3 as a hole scavenger, 0.02 M K2S2O8 as an electron scavenger, and 0.2 M t-butyl alcohol (TBA) as an •OH scavenger. The error bars represent the standard deviations of triplicate experiments, whose values are within 5%. Reaction conditions: Xe lamp, AM 1.5 G, 100 mW cm−2, solution: 0.1 M glycerol in an aqueous solution of 0.5 M Na2SO4 (pH = 2, adjusted by 0.5 M H2SO4).

Prior to studying glycerol conversion in PEC, we excluded the spontaneous transformation of the reaction via experiments43 with no external field energy supply conditions (Supplementary Fig. 17 and Supplementary Note 4). Figure 2c, d shows the product selectivities of the BVO and BVO–F photoanodes after 4 h of PEC glycerol oxidation as a function of time at 1.2 V vs. RHE in an H-type cell under illumination. The electrolyte system was pretreated before the reaction, and N2 was injected into the solution for approximately 30 min to remove O2. The selectivities of GLYAD and glycolic acid (GCA) for the BVO and BVO–F photoanodes stabilized over time. For the BVO photoanode, the DHA selectivity remained at 52.7%, whereas the FA selectivity was 39.5%. The change in the trend was not significant. For BVO–F, the DHA selectivity decreased to 10.7%, and the FA selectivity increased to 74.7% over time. In addition, the hydrogen production rate of the cathode with the glycerol system (186.3 µmol h−1) was 1.5 times greater than that without the glycerol system (123.6 µmol h−1) (Fig. 2e). This behavior is related to the increased charge transfer rate at the electrode/electrolyte interface (Fig. 2b) and the accelerated transfer of electrons to the cathode for BVO–F. Furthermore, the prepared photoanodes were stable against photocorrosion at pH = 2 with stable photocurrent densities and structural durability (Supplementary Figs. 18 and 19). The BVO–F photoanode morphology and chemical composition during the reaction process were unchanged according to a series of in situ/operando characterizations (Supplementary Movies 1 and 2 and Supplementary Figs. 20 and 21). The fluorine anion content of BVO–F was 2.01 mg L−1 after 4 h of PEC glycerol oxidation, which was slightly lower than that of the newly prepared BVO–F (2.23 mg L−1) (see the “Characterizations” section for test methods). Compared with other types of catalysts and catalytic technologies that are used for glycerol-to-FA conversion in the presence of acidic electrolytes (Supplementary Tables 4 and 5), the prepared BVO–F had the highest FA production rate (65.2 µmol h−1 cm−2), which was higher than those of electrocatalysts and photoanodes in the presence of alkaline electrolytes.

When this oxidation reaction occurred only at a given potential of 1.2 V vs. RHE without illumination of the BVO and BVO–F photoanodes, the dark current values were 0.02 and 0.03 mA cm−2, respectively (Fig. 2a). The oxidation products and glycerol conversion could not be detected (Supplementary Fig. 22). This result suggests that dark oxidation at 1.2 V vs. RHE hardly occurs. Moreover, the comparison experiments indicate that, compared with the energy-intensive electrochemical method, the PEC method can utilize sunlight to reduce more than 1 V (a nearly 50% reduction) of electrical energy to drive the oxidation of glycerol to generate FA (Supplementary Fig. 23 and Supplementary Note 5). Under photocatalytic conditions, glycerol was oxidized to DHA with less than 5% FA on the BVO photoanode. Although the selectivity for the C–C bond cleavage product FA increased on BVO–F, the selectivity was only 25%, and the reaction kinetics were insufficient. In contrast, a high FA production rate was obtained under PEC conditions, and the selectivity of the oxidation product was not affected by the applied potential (Supplementary Fig. 22) or current density (Supplementary Fig. 24). The reason is that the high potential that was applied prevented the electron/hole recombination, and the holes that accumulated at the photoanode surface transferred to the electrolyte44. To determine whether the photogenerated holes play a key role in FA production, a series of radical quenching experiments was performed (Fig. 2f). When Na2SO3 was used as a hole scavenger, the FA production rate significantly decreased, which confirms the important role of photogenerated holes in glycerol oxidation. When t-butyl alcohol (TBA) was used as a quenching agent for •OH radicals, the selectivity of DHA and FA was markedly reduced, which indicates that •OH played an important role in the DHA and FA selective production in the BVO–F PEC glycerol oxidation reaction.

Role of dual hydrogen bonds

To investigate the effect of •OH on the selectivity of glycerol oxidation, room-temperature electron spin resonance (ESR) experiments were conducted to trace the photocatalytic reaction process, with 5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trapping agent. The formation of •OH on BVO and BVO–F without glycerol was the primary focus. The intensity ratio of the ESR spectrum was a 1:2:2:1 quadruple signal upon light irradiation45, and the signal peak of BVO–F was 3.4 times greater than that of BVO (Fig. 3a). This result occurred because of the special dual hydrogen bond adsorption between H2O and F–Bi–F sites on the BVO–F surface, which reduced the adsorption energy of water (Fig. 3b and Supplementary Fig. 25). Additionally, the dynamic formation of dual hydrogen bonds provide a direct hole-transfer channel from the BVO–F (112) surface to water molecules46, which accelerated the surface reaction kinetics of the photogenerated carriers and promoted the oxidation of water to •OH. A 0.2 M H2O2 solution was added to the BVO water oxidation system to quantitatively analyze the •OH generated during the photocatalytic water oxidation, and the peak intensity of •OH was close to that of BVO–F (Fig. 3a). This result confirms that the dynamic formation of a dual hydrogen bond structure on BVO–F has a similar effect on H2O2 decomposition, generates •OH under illumination and increases the production rate of •OH.Fig. 3 Identification of radicals during the PEC glycerol oxidation.

Room-temperature electron spin resonance (ESR) spectra of 0.5 M Na2SO4 (pH = 2, adjusted with 0.5 M H2SO4) with and without glycerol. a ESR detection of •OH radicals without glycerol with 5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trapping agent on BVO, BVO–F, and with 0.2 M H2O2 on BVO after 2 min of illumination. b Adsorption energies and simulated adsorption models when H2O was adsorbed on the BVO and BVO–F surfaces. ESR detection of radicals after glycerol had been added on c BVO and d BVO–F over time. The peaks labeled with “◆” are attributed to DMPO-OH radical adducts and those labeled with “●” are attributed to DMPO-CH2OH radical adducts. (Hyperfine parameters: AN = 15.1 and AH = 14.8 G for the hydroxyl radical (•OH); AN = 16.1 and AHβ = 23.2 G for the hydroxymethyl radical (•CH2OH)). Kinetics of the •CH2OH formation. C0 = relative amount of •CH2OH at 30 s. Ct = •CH2OH is the relative amount (fraction of C0) at t min. e GC‒MS signals of FA during the PEC glycerol oxidation with H2O and with H218O.

After glycerin had been added, in addition to the DMPO-OH adducts, a new signal with hyperfine splitting parameters of AN = 16.1 G and AHβ = 23.2 G was assigned to the DMPO-hydroxymethyl radical adduct (DMPO-CH2OH) (Fig. 3c, d)47–49. Almost no •CH2OH was initially observed; however, as the illumination time increased, the signal peak of •CH2OH increased. After the double integration of the two radicals, the relative amounts of •OH and •CH2OH exhibited a consumption-production relationship (Supplementary Fig. 26). The kinetic measurements indicate that the •CH2OH formation induced by •OH followed first-order kinetics on the BVO and BVO–F photoanodes (Supplementary Fig. 27). The •CH2OH formation versus time plots were fitted, and the •CH2OH rate constant for BVO–F was 0.051, which was 3.6 times greater than that for BVO. This result suggests that the dynamic formation of the double hydrogen bond structure of BVO–F accelerates the C–C bond cleavage kinetics and promotes the •CH2OH production rate. To track the migration of O atoms during PEC glycerol oxidation, 18O-labeled water was used as the tracer. When H216O was replaced with 10 wt.% H218O, both HCO18OH and HC18O18OH were detected using gas chromatography-mass spectrometry (GC-MS) (Fig. 3e). Additionally, the liquid chromatography-mass spectrometry (LC-MS) results revealed the presence of 18O in the DHA product (C3H6O218ONa) (Supplementary Fig. 28). These results demonstrate that the O atoms of FA partly originate from water via the PEC glycerol oxidation process.

Identification of active sites and the glycerol-to-FA pathway

To gain a mechanistic understanding of the glycerol-to-FA process and investigate the adsorption and desorption behaviors of the reactant, oxidation intermediate product, and FA product over the prepared photoanodes, a series of time-resolved FT-IR spectroscopic tests was performed. The activation of glycerol was first investigated by the adsorption of isopropanol for 30 min (Supplementary Fig. 29 and Supplementary Table 6), followed by Ar purge desorption. When the desorption time increased, one prominent absorption band at approximately 3776 cm−1 was observed, which was assigned to the terminal OH groups ν(OH)T of H2O and adsorbed onto the coordinately unsaturated surface Bi sites (Fig. 4a) on BVO. For BVO–F, three major absorption components were observed at 3745, 3618, and 3595 cm−1. The vibrations at 3745 cm−1 were assigned to ν(OH)T, whereas those at 3618 and 3595 cm−1 were attributed to the bridging OH groups ν(OH)B of H2O due to the double hydrogen bond structure (Fig. 4b)22. Moreover, the bands gradually increased in the 1345 cm−1 δ(CH) mode for the BVO and BVO–F photoanodes (Fig. 4a, b)50, which confirms that isopropanol was activated by the C–H bond adsorption at the Bi site. The C–H bond adsorbed on the surface of BVO–F more quickly reached a stable state than BVO (Supplementary Fig. 30a), which indicates that the double hydrogen bond structure on BVO–F accelerated the adsorption of the C–H bond of alcohols.Fig. 4 Proposed glycerol-to-FA pathway.

Time-resolved FT-IR spectra after the adsorption of isopropanol for 30 min followed by desorption on a BVO and b BVO–F in the dark and without an applied potential. Time-resolved FT-IR spectra of isopropanol on c BVO and d BVO–F for 60 min under AM 1.5 G, 100 mW cm−2 illumination without applied potential. e FT-IR spectra after the adsorption of acetone for 30 min on BVO and BVO–F photoanodes. f PDOS of the 2p states of surface O and F, 6p state of surface Bi, and 3d state of surface V in BVO and BVO–F. The dashed line represents the Fermi level. g Proposed mechanism of the selective oxidation of glycerol to generate FA on BVO–F.

A time-resolved FT-IR analysis of isopropanol was subsequently performed on the photoanodes under illumination (AM 1.5 G, 100 mW cm−2). The signal at 1742 cm−1, which is the characteristic band of the carbonyl group (C=O), on BVO gradually increased with increasing light exposure time (Fig. 4c)20,51 due to the photocatalytic oxidation of isopropanol to acetone. Notably, the ν(OH)T band of H2O shifted to a lower wavenumber (3754 cm−1), which indicates that light irradiation promoted ν(OH)T activation and induced oxidation on BVO. For BVO–F (Fig. 4d), in addition to the C=O band (1742 cm−1), new bands at 1555, 1537, and 1436 cm−1 corresponded to ν(OCO) of acetate surface species52,53, bident νa(OCO), and νs(OCO) of typical formate ions that adsorbed on the metal oxides, respectively54, and these bands significantly increased with increasing illumination duration (Supplementary Fig. 30b). Thus, under light irradiation, the BVO–F photoanode surface was oxidized to generate acetone, and C–C bond dissociation occurred at the Bi–O sites. The ν(OH)T and ν(OH)B bands of H2O shifted to a lower wavenumber, which indicates that light irradiation promoted the ν(OH)T and ν(OH)B activation on the dynamic double hydrogen bond structure of BVO–F, and ν(OH)B induced the C–C bond cleavage and further oxidation. The glycerol FT-IR test results were identical to those of the isopropanol chemical bond change (Supplementary Fig. 31).

To verify the presence of adsorption sites for the key intermediate product DHA, the adsorption behavior (acetone as a probe) of BVO and BVO–F was detected by FT-IR. The bands at 1735, 1378, and 1213 cm−1 were assigned to ν(C=O), δs(CH3), and ν(C–C) of the adsorbed acetone species on the BVO surface, respectively (Fig. 4e). For BVO–F, the band of the C=O bond shifted to a lower wavenumber. Generally, band shifts to lower frequencies (1732 cm−1) occur due to interactions with heavier atoms55. This result provides direct evidence that the Bi–O sites on BVO–F are conducive for the adsorption of C=O bonds. Theoretical calculations also revealed that DHA was preferentially adsorbed at the Bi–O site on BVO–F (Supplementary Fig. 32) with a high adsorption energy of −2.69 eV. From electronic-scale insight into the adsorption and activation ability of DHA on BVO–F, the projected density of state (PDOS) calculation revealed that F atoms enabled the O 2p state to significantly shift upward from the Fermi level (Fig. 4f). The more positive O 2p states provided O sites with higher adsorption and activation activities. Finally, the adsorption behavior of FA on the surface of BVO–F was investigated (Supplementary Fig. 33). In addition to νas(OCO) (1537 cm−1) and νs(OCO) (1436 cm−1) of typical formate ions adsorbed on the Bi–O site, the bands at 1744, 1731, 1362, 1213, and 1088 cm−1 were assigned to νas(C=O) of the most stable FA cyclic dimer, formic acid trimer, δ(CH), δ(OH), and ν(C–O) of the adsorbed FA species56, respectively. The characteristic bands of FA rapidly disappeared after 5 min of desorption from BVO–F, which accounted for its high FA selectivity.

Thermodynamically, considering the high bond dissociation energy of the C–C bond cleavage step of the process of DHA oxidation to generate FA (Supplementary Fig. 34 and Supplementary Table 6), this step was considered the rate-determining step, which required an uphill 1.23 eV reaction energy. In theory, the oxidation potential of •OH is approximately 2.8–2.9 V, which can overcome the reaction energy barriers and oxidize DHA to FA21. BVO hardly breaks C–C bonds under photocatalytic conditions (Supplementary Fig. 22). On one hand, there was a lack of adsorption sites on the surface of BVO for DHA. On the other hand, the low •OH concentration decreased the C–C bond cleavage kinetics. Notably, no oxidant (O2 or H2O2) was added to increase the •OH concentration in this study, even in the acidic reaction system, only by constructing dynamic dual hydrogen bonds, which is different from the results of other studies48,49,57. By combining time-resolved FT-IR spectroscopy with computational results, we proposed a PEC glycerol oxidation reaction pathway on the surface of BVO–F (Fig. 4g). First, the reaction started with the adsorption of the secondary carbon C–H of glycerin on the Bi sites in BVO–F. Then, •OH radicals were quickly produced via photogenerated holes on dual hydrogen bonds, followed by dehydrogenation and dehydration to produce the key intermediate product DHA. The reaction pathway of the conversion of glycerol to DHA was consistent with that of BVO16. Subsequently, the C=O bond of DHA was adsorbed on Bi–O sites, and the C–C bond broke into •CH2OH and GCA with the help of bridging OH and •OH. These intermediate products could be further oxidized by •OH to produce FA, which can be easily desorbed from the BVO–F surface.

Extended applications to polyol oxidation

To demonstrate the general applicability of C–C bond cleavage over the dynamic dual-hydrogen-bond structure on the BVO–F photoanode, several small-molecule polyols, including DHA, GLYAD, ethylene glycol, GCA, 1,2-propanediol and methylglyoxal, were used to replace glycerol under identical PEC conditions. When DHA or GCA was used as the substrate, the corresponding photocurrent density (Fig. 5a) and FA production rate (Fig. 5b) were greater than those obtained with glycerol (Supplementary Fig. 35). The selectivity for the FA was >98%. This result reveals that the glycerol oxidation intermediate products containing C–C=O bonds can be easily converted to FA in synergy with PEC technology and the dual hydrogen bond structure. Although the selectivity of GLYAD for FA was >95%, the production rate was not ideal, which is related to the higher C–C bond energy of GLYAD (362.4 kJ/mol)58 than that of DHA (334.4 kJ/mol)48. Compared with glycerol, ethylene glycol, a shorter-chain polyol, and it has a higher FA production rate and selectivity. The selectivity for FA was not high for alcohols, ketones, or aldehydes that contain electron-donating groups (Supplementary Fig. 36). The selectivities of FA for 1,2-propanediol and methylglyoxal were 64% and 37%, respectively. The selectivities for acetic acid (AA) were 29% and 60%, respectively.Fig. 5 Extended applications of polyol oxidation.

a Linear sweep voltammetry (LSV) curves of the oxidation of small organic substrates on a BVO–F photoanode. b Product selectivity and FA production rates of each small-molecule PEC oxidation process at 1.2 V vs. RHE. The error bars represent the standard deviations of triplicate experiments, whose values are within 5%. c Diagram of the self-powered PEC-photovoltaic (PV) tandem device for oxidation of glycerol to generate FA and production of H2. d Productivity of anodic products and H2 in the self-powered PEC-PV system with and without glycerol. Reaction conditions: BVO–F photoanode in a 0.5 M Na2SO4 electrolyte (pH = 2, adjusted by 0.5 M H2SO4) with 0.1 M glycerol under AM 1.5 G and 100 mW cm−2 illumination. e Comprehensive comparison of the self-powered PEC-PV system in this study with those in previous reports. The anodic products in the radar map are the main products in the literature.

As a proof-of-concept model, glycerol-to-FA at the BVO–F photoanode with a high H2 production rate at the Pt cathode without an external energy input required only solar energy. A self-powered PEC-PV tandem device was set up (Fig. 5c) and achieved an FA productivity of 41.9 µmol h−1 cm−2 with >79% selectivity and an H2 productivity of 44.2 µmol h−1 cm−2 after 4 h of testing (Fig. 5d). The measured H2 productivity was 25.4 µmol cm−2 h−1 without the glycerol PEC-PV system. This nearly two-fold difference in the H2 production rate occurred due to the large charge-transfer resistance in the glycerin-free system. When 1 kg of H2 (US$2.5/kg) is produced at the cathode59, the net value of the FA (24.3 kg, US$0.97/kg) product from the PEC-PV glycerol system is approximately US$26.1 (separation and purification are not considered), which is much greater than that of the O2 (US$0.11/kg) that is generated from the PEC-PV water splitting system (US$6.0), so this system has considerable economic potential. The comprehensive assessment (from the anodic productivity, H2 productivity, net value of the anodic product (vs. per kg H2), solar-to-H2 conversion efficiency (STH), and open-circuit voltage) of a PEC-PV system in recent years (Fig. 5e and Supplementary Table 7)60–64 suggests that the BVO–F photoanode is a promising candidate semiconductor material for PEC small organic molecule oxidation to generate FA with efficient hydrogen generation. Although some studies have shown higher H2 productivity, they incur higher costs by using more complex device configurations, increasing the light intensity, and adding optical filters.

Discussion

We developed a facile surface fluorination method to modify BVO photoanodes and found that it overcame the limitations of glycerol-to-FA kinetics in acidic electrolyte solutions (pH = 2) and achieved a high selectivity of 74.7%. After a series of characterizations of BVO–F, the locations and roles of F were identified. The coordination environment around Bi changed to form F–Bi–F sites, which adjusted the electronic structure of the Bi–O sites and water adsorption mode to form dual hydrogen bonds. The dynamic formation of the dual hydrogen bond structure accelerated the production rate of •OH and C–C bond cleavage kinetics under illumination. The FT-IR results and DFT calculations verified that C=O bonds adsorbed on the Bi–O sites and that FA strongly desorbed in BVO–F. We also extended the study to include the oxidation of other polyols to generate FA on BVO–F via PEC technology, which exhibited strong universality. The self-powered PEC-PV tandem device for glycerol oxidation at the BVO–F photoanode and the high H2 productivity rate at the Pt cathode showed significant economic potential. Preliminary analysis estimated that the net value of FA (24.3 kg) was approximately $26.1 when 1 kg of H2 was produced. This economic benefit is much greater than that of conventional electrolytic water-hydrogen production because power is not consumed during the reaction process. This study accelerates C–C bond cleavage kinetics by constructing dynamic dual hydrogen bonds in PEC polyol oxidation to value-added chemicals even under acidic conditions and enhances the reaction pathway of biomass-derived platform molecules.

Methods

Reagents and solvents

Acetone (C3H6O, ≥99.5%), isopropanol (C3H8O, ≥99.5%), bismuth nitrate pentahydrate (Bi(NO3)3•5H2O, ≥99.0%), potassium iodide (KI, ≥99.0%), absolute ethanol (C2H6O, ≥99.5%), p-benzoquinone (C6H4O2, ≥99.0%), dimethyl sulfoxide (DMSO, C2H6SO, ≥99.0%), vanadyl acetylacetonate (C10H14O5V, 98.0%), sodium hydroxide (NaOH, 97.0%), sodium fluoride (NaF, 99.99% metals basis), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, C6H11NO, 97.0%), sodium sulfate (Na2SO4, 99.0%), sodium sulfite (Na2SO3, 98.0%), potassium persulfate (K2S2O8, ≥99.0%), t-butyl alcohol (TBA, C4H10O, ≥97.0%), H2O2 (30 wt%), H18O2 (97 atom%), glycerol (C3H8O3, 99.0%), 1,3-dihydroxyacetone (DHA, 99.0%), formic acid (CH2O2, 99%), dl-glyceraldehyde (GLYAD, ≥90.0% (HPLC) aqueous solution), glycolic acid (C2H4O3, 98%) were purchased from Aladdin Chemical Inc. (Shanghai, China). Nitric acid (HNO3, 60 wt%) and sulfuric acid (H2SO4, 98.3%) were purchased from Beijing Chemical Works (Beijing, China). Deionized water (18.2 MΩ cm) was produced with a Millipore Q water purification system (Sigma-Aldrich). All the chemicals were used without further purification.

Catalyst preparation

BiVO4 photoanodes were fabricated by a two-step process via a modified method that was originally developed by Kim and Choi23. Initially, fluorine-doped conductive SnO2 (FTO; 14 Ω; light transmittance 90%; thickness 2 mm) was sequentially washed with a solution containing acetone, isopropanol, and deionized water (1:1:1) to remove surficial contaminants. Then, 40 mL of a 0.04 M Bi(NO3)3•5H2O solution was added to 10 mL of a 0.4 M KI solution after its pH had been adjusted to 1.7 by adding HNO3. This solution was mixed with 20 mL of absolute ethanol containing 0.23 M p-benzoquinone and vigorously stirred for a few minutes. A typical three-electrode cell was used for electrodeposition at −0.1 V vs. Ag/AgCl for 3 min to obtain BiOI. Subsequently, 100 μL of DMSO solution containing 0.2 M vanadyl acetylacetonate was impregnated into the prepared BiOI sample and calcined in air at 450 °C for 2 h at a ramping rate of 2 °C min−1. Finally, the obtained samples were soaked in a 1 M NaOH solution for 30 min with gentle stirring to remove impurities.

Surface-fluorinated BiVO4 (active area of 10 × 20 mm; mass loading of 1 mg cm−2) was obtained by simply dipping pristine BiVO4 into different molar concentrations (0.01 M, 0.05 M, 0.1 M, 0.3 M, or 0.5 M) of sodium fluoride aqueous solution. After 12 h, the treated BiVO4 electrode was removed from the NaF solution and rinsed with purified water until the pH reached 6.3 to yield BiVO4–F. The samples that were produced were labeled BVO-0.01, BVO-0.05, BVO-0.1, BVO-0.3, and BVO-0.5. The fluorine anion content on BiVO4 was determined to be 0.24, 1.25, 2.23, 6.29, and 10.95 mg L−1 according to HPIC measurements, respectively.

Characterizations

X-ray diffraction (XRD; Shinmadzu XRD-6000 diffractometer; Japan) measurements were performed by using Cu Kα as the radiation source (λ = 0.1541 nm) at 40 kV and 30 mA. XRD patterns were collected with a scanning angle (2θ) range of 10–70° at a scan speed of 10°·min−1. The surface structure and morphology of the catalyst were observed via field emission scanning electron microscopy (SEM; ZEISS SUPRA 55; Germany). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM; Tecnai G2 F30 S–Twin; Netherlands) images were obtained at 300 kV. Raman spectra (Renishaw inVia Reflexm; England) were recorded equipped with a laser (532 nm). The fluorine anion content was evaluated using high-pressure ion chromatography (HPIC; Thermo Fisher ICS-5000; China) with an analytical column (IonPacAS11 HC, 4*250 mm) and a guard column (IonPacAG11-HC, 4*250 mm), an automatic sample injector, an automatic KOH eluent generator, and a conductivity detector with a suppressor device (ASRS300). The flow rate was 1.2 mL min−1. XPS characterization was performed on a Shimadzu Kratos Axis Supra at a pressure of 2 × 10−9 Torr. The samples were charged in reference to the main line of the C 1s spectrum for aliphatic carbon from adventitious carbon, which was set to 284.8 eV, and all other core-level and valence bands were shifted accordingly. UV–vis spectroscopy was performed using a Shimadzu UV–3000 with a detection range of 800–200 nm. The L3-edge EXAFS spectra of Bi(III) were collected in the transmission mode on beam line 1W1B at the Beijing Synchrotron Radiation Facility (BSRF; China). Room-temperature electron spin resonance (ESR) measurements were performed on a JES–FA200 X ESR spectrometer (298 K, 9063.386 MHz). To detect •OH and •CH2OH, a 0.5 M Na2SO4 or glycerol solution containing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and samples scratched off of the FTO substrate was placed into a paramagnetic tube and irradiated with AM 1.5 G and 100 mWcm−2 light. FT-IR spectra (Bruker; Germany) to investigate the adsorption of isopropanol and acetone, as well as the desorption of acetone and FA on the surface of the photoanode, were recorded on a Nicolet iS50 spectrometer equipped with a cell fitted with BaF2 windows and an MCT-A detector cooled with liquid nitrogen. The spectrum was collected at a resolution of 4 cm−1 with 64 scans in the range of 4000–750 cm−1. The photoanode samples (0.2 × 0.2 cm−2) were pressed into a self-supported wafer for each measurement. Then, the sample was pretreated under an Ar flow for 40 min at room temperature to remove impurities on the surface. The background was collected at room temperature under an Ar flow. Isopropanol, acetone, and formic acid were bubbled into the sample by an Ar flow at 20 °C for 30 min to obtain a stable spectrum. Then, the system was purged with an Ar flow to remove physical adsorption, and the spectrum was collected. To investigate the reaction of isopropanol on the photoanode surface, time-resolved FT-IR spectra under light were recorded on a Nicolet iS50 spectrometer with a cell fitted with BaF2 windows and an MCT-A detector cooled with liquid nitrogen. The spectrum was collected at a resolution of 4 cm−1 with 64 scans in 4000–750 cm−1. The sample was pretreated under an Ar flow for 40 min at room temperature to remove the impurities absorbed on the surface. The background was collected at 20 °C under a flow of Ar. Isopropanol or acetone was bubbled into the sample by an Ar flow at 20 °C for 30 min to obtain a stable spectrum. After the spectrum had stabilized, the sample was irradiated with a 300 W xenon lamp (AM 1.5 G; 100 mW cm−2) for 1 h, and the spectrum change was recorded.

Photoelectrochemical (PEC) measurements

All PEC measurements were performed at room temperature in an H cell (volume: 12 mL) using an electrochemical workstation (CHI 660E, CH Instrument Co.). The electrolyte contained 0.1 M glycerol and 0.5 M Na2SO4 aqueous solution, and the pH was adjusted to 2 using a 0.5 M H2SO4 solution. The scan rate for all current-voltage measurements was 10 mV·s−1. All the potentials mentioned in this work were converted to potentials versus RHE (in volts) according to Eq. (1):1 ERHE=EAg/AgCl+EAg/AgClvs.NHE+0.059pH

where EAg/AgClvs.NHE in Eq. (1) is 0.197 V at 25 °C.2 ABPE(%)=[(1.23−Vapp)×Jlight/Plight]×100

where Vapp is the applied potential (vs. RHE), Jlight is the photocurrent density under AM 1.5 G light, and Plight is 100 mW cm−2 for the simulated sunlight65,66. The impedance spectroscopy measurements were conducted by applying a 10.0 mV amplitude perturbation to ensure a linear relationship between applied signal and system response. The frequency ranged from 100 kHz to 0.1 Hz since the applied bias was 1.2 V vs. RHE. A 300 W Xe lamp with an AM 1.5 G filter was used as the illumination source. The average power density at the photoanode was 100 mW cm−2. All the measurements were recorded without iR- compensation.

A Neo-FOX oxygen sensor system (Ocean Optics Inc.) with a FOX Y probe was used to measure the degree of oxygen production in the system. Before the measurement, a nitrogen purge was introduced, and the purge time was 30 min. Then, under 1.2 V vs. RHE bias voltage and light, the oxygen content was monitored in real time for 4 h.

Glycerol oxidation reactions

In a typical reaction, the prepared photoanodes were operated as the working electrode in contact with 0.1 M glycerol in 12 mL of 0.5 M Na2SO4. The electrolyte was continuously stirred using a magnetic stir bar. To eliminate interference by O2, nitrogen was bubbled through the solution for 20 min before the reaction. Glycerol oxidation was performed at room temperature by holding the illuminated photoanode at 1.2 V vs. RHE for 4 h. During the reaction process, 200 µL of the reaction mixture was collected from the cell at certain intervals, diluted with 0.01 M H2SO4 5 times, and filtered for the quantitative analysis. Each substance in the reaction mixture was quantitatively analyzed via high-performance liquid chromatography (HPLC; Shimadzu LC-20AT). The detector used a series-connected ultraviolet detector UV (210 nm) and a differential refractive index detector and was equipped with an Aminex HPX-87H column (300 mm × 7.8 mm × 9 µm, Bio-Rad) at 50 °C with 0.01 M aqueous H2SO4. The retention time of each compound via HPLC was determined by comparison with the corresponding pure sample. For quantification, an external calibration method was used for analysis.

The selectivity and conversion rate of the product were calculated as follows16:3 Productselectivity(%)=molesofCiproduct∑i=13molesofCiproduct×100

4 Glycerolconversion(%)=1−molesofCiproductmolesofglycerolloadedinitially×100

5 Faradaicefficiencyoftheliquidproducts(%)=Numberholestooxidizeglyceroltoliquidproductsnumberofallcollectedphotogeneratedholes×100=2×cDHA+2×cGLYAD+cGLYCA×2/3×5+8×cFA×1/3Q/e×N×V×100

where GLYAD is glyceraldehyde, GLYCA is glycolic acid, and FA is formic acid. c is the concentration obtained by HPLC. N is Avogadro’s constant (6.02 × 1023 mol−1), and V is the volume of the system. Q is the quantity of electric charge, and e is the elementary charge (1.60 × 10−19 C), referring to relevant literature67 The measurement potentials for the Faradaic efficiency and selectivity are 1.2 V vs. RHE.6 Carbonbalance(%)=nGLYAD+nDHA+nGLACA×2/3+nFA×1/3nglycerol×100

where n is the mole16.

Photoelectrochemical oxidation of glycerol with scavengers and isotope-labeled H218O

To explore the major active species involved in the oxidation of glycerol, small amounts of free radical scavengers were added to the PEC oxidation system. Na2SO3 was used as a hole scavenger, K2S2O8 was used as an electron scavenger, and tert-butanol alcohol was used as a hydroxyl radical scavenger. The concentration of each scavenger was 0.02 M, 0.02 M, and 0.2 M, respectively. H218O isotope-labeled experiment: isotope-labeled H2O was used to determine whether water participated in the oxidation of glycerol. Except for the replacement of the aqueous 0.5 M Na2SO4 electrolyte with H218O (VH218O/Vtotal=0.1), the H218O-labeling experiments were conducted under identical conditions to those used for the photoelectrochemical oxidation of glycerol. The oxidation products were identified via LC-MS. The relative abundances of FA and 18O-labeled FA were detected via gas chromatography‒mass spectrometry (GC-MS, Shimadzu GCMS-QP2010Plus, Japan) with an Rxi-5ms column.

Density functional theory (DFT) calculations

All the DFT calculations were conducted with the Vienna Ab-initio Simulation Package (VASP)68,69. The electron-ion interactions were described by the Projected Augmented-Wave (PAW) potentials, whereas the exchange-correlation interactions were calculated with the Perdew-Burke-Enzerhof (PBE) pseudopotentials of the Generalized Gradient Approximation (GGA)70,71. The plane-wave energy cutoff was set to 480 eV. The convergence threshold was set to 1.0 × 10−4 eV in energy and 0.02 eV per Angstrom in force. The Brillouin zone was sampled with 3 × 3 × 1 k-points. Supplementary Fig. 14b shows the DFT-optimized structure of BVO–F (112). It was built with the space group of C2/c, which indicates that α = γ = 90.00°, β = 134.87°, a = 7.34, b = 11.74, and c = 5.17. A vacuum space of 20 Å was inserted in the z direction to avoid interactions between periodic images. The reaction pathways of DHA to FA on BVO–F (112) were revealed by DFT calculations. The reaction begins with the adsorption of DHA on Bi20V20O78F2. The reaction pathway is as follows:*+HOCH2COCH2OH→*CO(CH2OH)2,

*CO(CH2OH)2+CH2O→*CO(CH2OH)2+2*H2O,

*CO(CH2OH)2→*COCH2OH+*CH2OH,

*COCH2OH+*CH2OH+*H2O→COOHCH2OH+H++*CH2OH+e−,

*CH2OH+H2O→*CH2(OH)2+H++e−,

*CH2(OH)2→*CH2O+H2O,

*CH2O→*CHO+H++e−,

*CHO+H2O,→HCOOH+H++e−.

The binding energy can be calculated as follows67:7 Eads=E(A+B)−E(A)−E(B)

where Eads is the adsorption energy, E(A+B) is the calculated energy of adsorption configuration, and E(A) and E(B) are the calculated energy of the substrate and adsorbent, respectively.

The Gibbs free energy change (ΔG) of the reaction was calculated as follows16,67:8 ΔG=ΔE+ΔZPE−TΔS+ΔGU+ΔGpH

where ΔE is the electronic energy difference that was directly obtained from the DFT calculations, ΔZPE is the zero-point energy difference, T is room temperature (298.15 K), and ΔS is the entropy change. ΔGU = −eU, where U is the applied electrode potential and was set to 0. ΔGpH = kBT × ln 10 × pH, where kB is the Boltzmann constant, and the pH value was set to 2. The standard entropy of the gas species was obtained from the NIST Chemistry Webbook. Supplementary Table 8 shows the calculated ΔE and TΔS.

Self-powered PEC-photovoltaic (PV) tandem device

For the constructed self-powered PEC-PV tandem devices, in the PV part, a 25 × 20-mm polycrystalline silicon solar cell with a 2.0 V open-circuit voltage was exposed to light irradiation (AM 1.5 G, 100 mW cm−2) to provide the voltage that drove the entire system64. Supplementary Fig. 37 shows its current-voltage characteristics. In PEC cells, a two-electrode test was performed in an H cell with a proton-exchange membrane (Nafion 117) that separated the anode and cathode, where the Pt wire was placed in a cathode cell filled with 0.5 M Na2SO4 (pH = 2, adjusted by 0.5 M H2SO4), while the BVO–F photoanode was placed in 0.5 M Na2SO4 (pH = 2, adjusted by 0.5 M H2SO4) without or with 0.1 M glycerol. Lengthening and focusing lens tubes can increase the illumination area while ensuring the same light intensity. Considering that the BVO–F photoanode can only use light <507 nm, a polycrystalline solar panel (absorption wavelength range 400–1100 nm) was used to efficiently absorb light in the solar spectrum that cannot be used by the photoanode.

The total reaction equation:C3H8O3+3H2O→3HCOOH+4H2

Photoanode:C3H8O3+8⋅OH→3HCOOH+5H2O+8h+

Cathode:8e−+8H2O→4H2+8OH−

The total solar-to-H2 (STH) conversion efficiency was calculated as follows:9 STH(%)=H2productionrate×ΔGr,H2Plight×100

where the unit of the H2 production rate is µmol h−1 cm−2, and ∆Gr,H2 is 474.2 kJ mol−1. The Gibbs free energy of formation (∆G0f) values are from a previous report72.

Supplementary information

Supplementary information

Peer Review File

Source data

Source Data

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-52161-4.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 22478025, 22288102, 21978021) and the Fundamental Research Funds for the Central Universities (XK1803).

Author contributions

Y.L. performed the catalyst preparation, characterizations, and catalytic evaluations. X.X. and H.S. analyzed the catalyst structure investigations. D.Y. and Y.L. performed the DFT theoretical calculations. B.Z. helped design the experiments. X.X. supervised the project, conceived the idea, analyzed the data, and wrote the manuscript. All authors commented on the manuscript and have given approval to the final version of the manuscript.

Peer review

Peer review information

Nature Communications thanks Rubén Rizo and the other anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information. The source data generated in this study are available in the figshare repository 10.6084/m9.figshare.25200872. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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