Spray-Coated Thin-Film Ni-Oxide Nanoflakes as Single Electrocatalysts for Oxygen Evolution and Hydrogen Generation from Water Splitting

Abstract

Electrochemical water splitting is a key process in many electrochemical energy conversion and storage phenomena. Simple synthesis methods to make highly porous and active nanostructured catalytic materials with large electroactive surface areas are very important to implement water-to-fuel conversion schemes. Herein, ultrafine, transparent thin-film nickel-oxide (NiOx) nanoflakes are facilely synthesized following a simple spray-coating method from a solution-phase precursor. The NiOx nanoscale structures are grown on the FTO surface in the form of highly uniform smooth thin films. They are employed as promising bifunctional electrocatalysts for the overall water splitting process under alkaline conditions. During water oxidation catalysis, NiOx-SC/FTO initiates the oxygen evolution reaction (OER) at an overpotential η of just 250 mV while generating current decade at just 300 mV and demonstrates well-balanced kinetics toward OER. 10 mA cm-2 current density remains persistent for many hours of continuous electrolysis at just 1.53 VRHE illustrating high robustness of the system. The catalyst also showed substantial activity and durability toward the hydrogen evolution reaction (HER) under the same electrochemical conditions. Tafel slopes of just 57 and 89 mV dec-1 for OER and HER in 0.5 M aqueous KOH solution, respectively, showing high intrinsic kinetics for electrocatalysis. Having high electrochemical surface area and an optimum number of electrochemically active sites, these transparent NiOx thin films can be advantageously combined with photoelectrochemical devices for light-driven water-to-fuel conversion systems.