Bias-free Si-based photocathode for efficient photoelectrochemical ammonia synthesis and HMF oxidation†

Abstract

The photoelectrochemical nitrate reduction reaction (NO_xRR) has emerged as a promising route for achieving an eco-friendly solar-to-ammonia conversion process. Despite its potential, the approach has been hindered by poor stability and the requirement of an external bias voltage for the coupled NO_xRR and oxygen evolution reaction (OER). In this study, an oxygen vacancy (V_O)-rich TiO₂ catalyst was synthesized and conformally deposited onto a silicon photocathode using the simple yet effective atomic layer deposition (ALD) technique. The resulting Si–TiO₂ photocathode exhibited promising NO_xRR performance, achieving a faradaic efficiency exceeding 89% at 0 V vs. RHE, along with a solar-to-ammonia productivity (SAP) of 15 μmol h⁻¹ cm⁻². Additionally, the Si-based photoanode demonstrated commendable stability, maintaining performance for over 44 hours. To further advance the system, a bias-free photoelectrochemical cell was developed, coupling 5-hydroxymethyl furfural oxidation with NO_xRR. This configuration yielded a working current density of 1.5 mA cm⁻² and an SAP of 3.6 μmol h⁻¹ cm⁻², demonstrating the viability of a sustainable, self-powered approach for solar-to-ammonia conversion.