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

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) NiOx (x < 1) EC is deposited on a Si photocathode passivated with a SiOx layer. A few distinctive features of the Ni-rich NiOx film are identified in contrast with the conventional O-rich NiOy (y >= 1) which could lower the series resistance along the charge transfer pathways. The Ni-rich NiOx is found to possess both NiO 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 NiOx 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 cm(2).