Charge Transfer Kinetics of Photo‐Electrochemical Hydrogen Evolution Improved by Nonstoichiometric Ni‐rich NiO_x‐Coated Si Photocathode in Alkaline Electrolyte

Abstract The integration of electrocatalyst (EC) with light‐absorbing semiconductor photoelectrode is regarded as a representative framework for photoelectrochemical (PEC) devices. When considering the charge transfer pathways, PEC performance is governed by the charge kinetics at the EC/electrolyte and the semiconductor/EC interfaces. Here, systematic investigations are reported, made on the overall kinetics of the PEC hydrogen evolution reaction (HER) in an alkaline electrolyte. A non‐stoichiometric (Ni‐rich) NiO_x (x < 1) EC is deposited on a Si photocathode passivated with a SiO_x layer. A few distinctive features of the Ni‐rich NiO_x film are identified in contrast with the conventional O‐rich NiO_y (y ≥ 1) which could lower the series resistance along the charge transfer pathways. The Ni‐rich NiO_x is found to possess both NiO and NiNi (by oxygen‐vacancy) bonds, which act as suitable catalytic sites for dissociating water molecules and recombining two hydrogen atoms, respectively. In addition, the Ni‐rich NiO_x reveals both n‐type and metallic conduction behavior, a feature that may contribute to lowering bulk resistance as well as tunneling resistance through SiOx layer. As a result, this noble metal‐free EC‐integrated Si photocathode achieves the highest potential of 0.41 V vs. reversible hydrogen electrode to produce a photocurrent density of 10 mA cm².