Minimalist and nanoparticle-free selenium-based photocathodes for record performance solar-driven hydrogen evolution

Abstract

This work reports the highest photoelectrochemical (PEC) performance for selenium (Se)-based photocathodes, achieved through a simple, sustainable, and nanoparticle-free design. A half-cell solar-to-hydrogen (HC-STH) efficiency of 2.78 ± 0.01% and a photocurrent density of 11.35 ± 0.01 mA cm⁻² at 0 V_RHE were obtained with bare Mo/Se devices tested in H₂SO₄, surpassing the previous Se-based (FTO/Se/TiO₂/Pt) HC-STH benchmark by over a factor of seven. To improve sustainability and device safety, the deposition of a thin TiO₂ passivation layer enabled comparable performance (2.76 ± 0.01%), even in neutral phosphate buffer, allowing to obtain the highest photoelectrocatalytic onset potential reported so far (0.74 V_RHE). Unlike most PEC devices that rely on complex multilayer stacks and costly noble metals, which limit scalability and environmental compatibility, this work demonstrates that high performance can be achieved with a fully earth-abundant and low-toxicity materials set. A systematic screening of back contacts, Se phases, absorber thickness, protective overlayers, and electrolyte formulations revealed the crucial role of Mo in enhancing Se orientation, charge extraction, and photovoltage generation. These results establish multiple benchmarks for Se-based PEC water splitting and highlight the potential of streamlined and scalable architectures for efficient and sustainable green hydrogen production.