Preparation of WO3, BiVO4 and reduced graphene oxide composite thin films and their photoelectrochemical performance

Synergistic doping effects of a ZnO:N/BiVO4:Mo bunched nanorod array photoanode for enhancing charge transfer and carrier density in photoelectrochemical systems

One-dimensional heterojunction nanorods are highly attractive as photoanodes for developing efficient photoelectrochemical (PEC) systems for the effective photogeneration of charge carriers and transport. ZnO/BiVO4 nanorod arrays (NRAs) are excellent candidates if their charge transferring and recombination issues can be improved. In the current work, we have studied the synergistic doping effects of N-doped ZnO/Mo-doped BiVO4 NRAs for enhancing the photoanode activity in PEC devices. The N-doping of ZnO NRs enhances the charge carrier density ∼3-fold over undoped ZnO NRs through increased oxygen vacancies induced by N dopants. The Mo dopants in BiVO4 improve the mobility of photogenerated charge carriers and contribute to reducing charge recombination. The synergistic doping effects of both ZnO and BiVO4 could increase the charge transfer rate constant (kct) of the ZnO:N/BiVO4:Mo heterojunction by ∼40% and decrease the charge transfer resistance ∼1.9-fold compared to those of undoped ZnO/BiVO4, which were confirmed by time resolved photoluminescence (PL) and electrochemical impedance (EIS) analyses. Our optimally fabricated ZnO:N/BiVO4:Mo NRA photoanode could achieve an excellent photocurrent of 3.62 mA cm-2 without the application of any co-catalysts. This work presents a useful strategy for designing efficient heterojunction photoanodes in PEC systems.

Enhancing Durability and Photoelectrochemical Performance of the Earth Abundant Ni-Mo/TiO2 /CdS/CIGS Photocathode under Various pH Conditions

Cu(In,Ga)(S,Se)₂ (CIGS) is a promising photocathode material owing to its high absorption coefficient, adjustable band gap, and suitable band edge for the hydrogen evolution reaction (HER). However, most CIGS photocathodes have suffered from instability in applications that require a wide range of pH conditions and have utilized noble metal HER catalysts to achieve a high performance. Thus, improving the durability of the CIGS photocathode under various pH conditions and developing a cost-effective non-noble metal catalyst are critical issues in the photoelectrochemical (PEC) application of this promising photocathode material. Here, we catalyze the CIGS photocathode with Ni-Mo as a non-noble metal to enhance the PEC efficiency, and we employ atomically grown TiO₂ to passivate the CdS/CIGS surface and improve the stability under a wide range of pH conditions. Our Ni-Mo alloy exhibits the best HER catalytic activity among reported earth-abundant HER catalysts in both acidic and alkaline solutions. The Ni-Mo/CdS/CIGS photocathode yields an onset potential of 0.5 V (vs. RHE) and a short-circuit photocurrent density as high as 15-25 mA cm^-2 under various pH conditions ranging from 0.4 to 14, which is highly comparable to that of Pt/CdS/CIGS. Furthermore, the passivation of CdS/CIGS with a thin TiO₂ layer, obtained by atomic layer deposition, effectively prevents the photocorrosion of CdS and the dissolution of the Mo back contact, which are the main causes of the degradation of the photocathode. The optimized Ni-Mo/TiO₂ /CdS/CIGS photocathode produces a stable photocurrent density at 0 V_RHE for 100 minutes except under strong alkaline conditions. The current work presents a very useful method to improve the efficiency and durability of the CIGS photocathodes with an earth-abundant metal catalyst, which completely replaces Pt.

Three-Dimensional Bicontinuous BiVO4/ZnO Photoanodes for High Solar Water-Splitting Performance at Low Bias Potential

A photoanode capable of high-efficiency water oxidation at low bias potential is essential for its practical application for photocathode-coupled tandem systems. To address this issue, a photoanode with low turn-on voltage for water oxidation and high charge separation efficiency at low bias potential is essential. In this study, we demonstrate the photoanode of the BiVO₄/ZnO three-dimensional (3D) bicontinuous (BC) structure. ZnO has a relatively cathodic flat-band potential, which leads to low turn-on potential; the BiVO₄/ZnO 3D BC photoanode shows an onset potential of 0.09 V versus the reversible hydrogen electrode ( V_RHE). Moreover, we achieve remarkably high charge separation efficiency at low bias potential (78% at 0.6 V_RHE); this is attributed to the application of thin-film BiVO₄ shells by high light-scattering properties of the 3D BC structure. As a result, the BiVO₄/ZnO 3D BC photoanode generates a high water oxidation photocurrent of up to 3.4 ± 0.2 mA cm^-2 (with CoPi catalyst coating). This photocurrent value is reproducible, and the photocurrent-to-O₂ conversion efficiency is over 90%. To the best of our knowledge, this is the highest value among the values of the photocurrent at 0.6 V_RHE in previous BiVO₄-based heterojunction photoanodes.