Photo-electrochemical hydrogen production from neutral phosphate buffer and seawater using micro-structured p-Si photo-electrodes functionalized by solution-based methods

Abstract

Solar fuels such as H₂ generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix. We herein report a systematic analysis of the photo-electrochemical performance of TiO₂ coated, microstructured p-Si photo-electrodes (p-Si/TiO₂) that were functionalized with CoO_x and NiO_x for H₂ generation. These photocathodes were synthesized from commercial p-Si wafers employing wet chemical methods. In neutral phosphate buffer and standard 1 sun illumination, the p-Si/TiO₂/NiO_x photoelectrode showed a photocurrent density of −1.48 mA cm⁻² at zero bias (0 V_RHE), which was three times and 15 times better than the photocurrent densities of p-Si/TiO₂/CoO_x and p-Si/TiO₂, respectively. No decline in activity was observed over a five hour test period, yielding a Faradaic efficiency of 96% for H₂ production. Based on the electrochemical characterizations and the high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and emission spectroscopy measurements performed at the Ti Kα₁ fluorescence line, the superior performance of the p-Si/TiO₂/NiO_x photoelectrode was attributed to improved charge transfer properties induced by the NiO_x coating on the protective TiO₂ layer, in combination with a higher catalytic activity of NiO_x for H₂-evolution. Moreover, we report here an excellent photo-electrochemical performance of p-Si/TiO₂/NiO_x photoelectrode in corrosive artificial seawater (pH 8.4) with an unprecedented photocurrent density of 10 mA cm⁻² at an applied potential of −0.7 V_RHE, and of 20 mA cm⁻² at −0.9 V_RHE. The applied bias photon-to-current conversion efficiency (ABPE) at −0.7 V_RHE and 10 mA cm⁻² was found to be 5.1%.