Surface fluorination of BiVO4 for the photoelectrochemical oxidation of glycerol to formic acid

Abstract

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.

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.

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).

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: A_N = 15.1 and A_H = 14.8 G for the hydroxyl radical (•OH); A_N = 16.1 and A_Hβ = 23.2 G for the hydroxymethyl radical (•CH₂OH)). Kinetics of the •CH₂OH formation. C₀ = relative amount of •CH₂OH at 30 s. C_t = •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.

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.

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.