Dynamic semiconductor-electrolyte interface for sustainable solar water splitting over 600 hours under neutral conditions

Abstract

Photoelectrochemical (PEC) water splitting that functions in pH-neutral electrolyte attracts increasing attention to energy demand sustainability. Here, we propose a strategy to in situ form a NiB layer by tuning the composition of the neutral electrolyte with the additions of nickel and borate species, which improves the PEC performance of the BiVO₄ photoanode. The NiB/BiVO₄ exhibits a photocurrent density of 6.0 mA cm^-2 at 1.23 V_RHE with an onset potential of 0.2 V_RHE under 1 sun illumination. The photoanode displays a photostability of over 600 hours in a neutral electrolyte. The additive of Ni²⁺ in the electrolyte, which efficiently inhibits the dissolution of NiB, can accelerate the photogenerated charge transfer and enhance the water oxidation kinetics. The borate species with B─O bonds act as a promoter of catalyst activity by accelerating proton-coupled electron transfer. The synergy effect of both species suppresses the surface charge recombination and inhibits the photocorrosion of BiVO₄.

Fig. 1.. Improved neutral PEC performances of B/BVO.

(A) J-t curve of BVO photo-polarized potentiostatically at 0.8 V_RHE for 5 hours in NaB (pH 7). (B) J-V curves of B/BVO in NaB and Na₂SO₄ (pH 7). BVO was performed in Na₂SO₄ for comparison. (C) Stability measurements for the corresponding samples polarized at 0.8 V_RHE. Inset shows the contents of B/BVO in Na₂SO₄ before and after stability by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis. (D) J-t curves of BVO photo-polarized potentiostatically at 0.8 V_RHE for 5 hours in NaB + V and NaB + Bi (pH 7). (E) J-V curves of the corresponding samples in NaB (pH 7). (F) J-V curves and (G) J-t curves of B/BVO after photocharging, storing in vacuum for 2 hours, and rephotocharging of J-t for 5 hours. (H and I) J-V curves of B/BVO in NaB scanning from 0 to 1.23 V_RHE and from 0.3 to 1.23 V_RHE. All measurements were performed under AM 1.5G (100 mA cm⁻²) illumination with back side.

Fig. 2.. Borate-Fe/Co/Ni synergized improvement of neutral PEC performances.

(A) J-t curves of BVO photo-polarized potentiostatically at 0.8 V_RHE in NaB containing 5 μM Fe²⁺, 1 μM Co²⁺, and 10 μM Ni²⁺ (pH 7). (B) J-V curves of FeB/BVO, CoB/BVO, and NiB/BVO in NaB (pH 7). (C) ABPE of NiB/BVO. (D) Cross-sectional SEM and (E) high-resolution TEM images of NiB/BVO. (F) Stability measurements for FeB/BVO, CoB/BVO, and NiB/BVO polarized at 0.8 V_RHE in NaB (pH 7). (G) J-t curves potentiostatically polarized at 0.8 V_RHE in stage I (Na₂SO₄ + B and Na₂SO₄ + Ni²⁺) and stage II (Na₂SO₄ + Ni²⁺ and Na₂SO₄ + B). (H) J-V curves of corresponding samples in NiB after stages I and II. All measurements were performed under AM 1.5G (100 mA cm⁻²) illumination with back side.

Fig. 3.. Long-term neutral PEC stability under borate-nickel synergy.

(A) Stability measurements of NiB/BVO applied at 0.8 V_RHE in NaB, NaB + V, and NaB + Ni (pH  7). Inset shows the related J-t curves at the initial 30 min. (B and C) J-V curves of NiB/BVO before and after stability in (B) Na₂SO₄, (B) NaB, (C) NaB + V, and (C) NaB + Ni. (D) Six hundred–hour operation of NiB/BVO applied at 0.8 V_RHE in NaB + Ni. The electrolyte was replaced with a fresh electrolyte during the test of 335 hours. Inset of (D) shows the schematic illustration of B/BVO and NiB/BVO. (E) Current ratio at 0.8 V_RHE as function of time. All measurements were performed under AM 1.5G (100 mA cm⁻²) illumination with back side.

Fig. 4.. IMPS analysis of BVO, B/BVO, and NiB/BVO under neutral water oxidation.

(A to C) IMPS responses of (A) BVO, (B) B/BVO, and (C) NiB/BVO at various applied potentials. (D) Charge transfer rate constants (k_trans), (E) charge recombination rate constants (k_rec), and (F) charge transfer efficiencies of corresponding photoanodes derived from IMPS data.

Fig. 5.. Neutral photoelectrochemical properties and TAS measurements of BVO, B/BVO, and NiB/BVO.

(A) Surface charge separation. (B) PEC impedance spectroscopy (PEIS) in NaB (pH 7). (C) Photoluminescence spectra. PL, photoluminescence; a.u., arbitrary units. (D) Time-resolved transient absorption spectra with 350-nm light. (E) 510-nm and (F) 650-nm decays. The fits for the decays were calculated with a global four-exponential decay model, shown as the solid lines, whereas the circles represent the experimental data. (G to I) Time-resolved transient absorption spectra of (G) BVO, (H) B/BVO, and (I) NiB/BVO when excited with 350-nm light. The time scale was logarithmic on longer delay times.

Fig. 6.. Proposed charge carrier kinetics mechanismACDEG) H

(A to C) Optimized structures of (A) BVO, (B) B/BVO, and (C) NiB/BVO. (D) PDOS of corresponding samples. (E to G) Evolution of populations of VBM and CBM for charge recombination in (E) BVO, (F) B/BVO, and (G) NiB/BVO. (H) Free energies of OER reaction steps for BVO, B/BVO, and NiB/BVO. TDTSv: virtual TOF dependent transition state; TDI: TOF dependent intermediate.