Modifying the Electron-Trapping Process at the BiVO4 Surface States via the TiO2 Overlayer for Enhanced Water Oxidation

An MgO passivation layer and hydrotalcite derived spinel Co2AlO4 synergically promote photoelectrochemical water oxidation conducted using BiVO4-based photoanodes

MxCo3-xO4 co-catalysed photoanodes with high potential for improvement in PEC water-oxidizing properties are reported. However, it is difficult to control the recombination of photogenerated carriers at the interface between the catalyst and cocatalyst. Here, an ultra-thin MgO passivation layer was introduced into the MxCo3-xO4/BiVO4 coupling system to construct a ternary composite photoanode Co2AlO4/MgO/BiVO4. The photocurrent density of the electrode is 3.52 mA cm-2, which is 3.2 times that of BiVO4 (at 1.23 V vs. RHE). The photocurrent is practically increased by 0.86 mA cm-2 and 1.56 mA cm-2 in comparison with that of Co2AlO4/BiVO4 and MgO/BiVO4 electrodes, respectively. Meanwhile, the Co2AlO4/MgO/BiVO4 electrode has the highest charge separation efficiency, the lowest charge transfer resistance (Rct) and best stability. The excellent PEC performance could be attributed to the inhibitive effect provided by the MgO passivation layer that efficaciously suppresses the electron-hole recombination at the interface and drives the hole transfer outward, which is induced by Co2AlO4 to capture the electrode/electrolyte interface for efficient water oxidation reaction. In order to understand the origin of this improvement, first-principles calculations with density functional theory (DFT) were performed. The theoretical investigation converges to our experimental results. This work proposes a novel idea for restraining the recombination of photogenerated carriers between interfaces and the rational design of efficient photoanodes.

Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoOx-loaded bismuth vanadate

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoOx) loaded on bismuth vanadate (BiVO4) to investigate the impact of CoOx on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V5+ in BiVO4 promotes the formation of a surface state on CoOx that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoOx-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoOx(OH)y layer dominates over single sites. Therefore, CoOx is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.

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

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.

WO3 Photoanode with Predominant Exposure of {202} Facets for Enhanced Selective Oxidation of Glycerol to Glyceraldehyde

The photoelectrocatalytic (PEC) oxidation of glycerol into highly value-added products is attractive, but it is extremely challenging to limit the oxidation products to the valuable C3 chemicals. The hole concentration and surface atomic arrangement of a photoanode can be modulated by controlling facet exposure, thus tuning the activity and selectivity. Herein, we report for the first time the formation of a WO3 photoanode with predominant exposure of {202} facets by a secondary hydrothermal method. The photoanode exhibits superior PEC glycerol conversion efficiency, giving an 80% selectivity to glyceraldehyde with a production rate of 462 mmol h-1 m-2. Also, the faraday efficiency for the C3 product reaches 98.6%. We made comparison between the {202} facets and the commonly studied {200} facets using experimental and theoretical methods. It is disclosed that the former enhances not only the adsorption and activation of glycerol via the terminal hydroxyl groups but also the desorption of glyceraldehyde.