Synthesis of transition metal compounds for efficient electrocatalytic water splitting

Abstract

Electrochemical catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) play a key role in highly-efficient water splitting and many other important energy conversion applications. Transition metal oxides are promising OER catalysts. In this work, Fe,W co-doped Co₃O₄ was grown on carbon fiber cloth (FeWCo₃O₄/CFC) and polypyrrole (PPy) coated carbon fiber cloth (FeWCo₃O₄/PPy/CFC) through a simple anodic electrodeposition method. The FeWCo₃O₄/CFC free-standing electrode reached an electrocatalytic current density of 30.7 mA cm⁻² at 400 mV overpotential with a Tafel slope of 177 mV dec⁻¹. The PPy can serve as conductive binder to improve the contact between FeWCo₃O₄ and substrate. The resulting FeWCo₃O₄/PPy/CFC free-standing electrode reached an electrocatalytic current density of 36.2 mA cm⁻² at 400 mV overpotential with a Tafel slope of 163 mV dec-1. The FeWCo₃O₄/PPy/CFC free-standing electrode shows low electric resistance and is able to catalyze OER at 10 mA cm⁻² for 12 hours without obvious decay under the optimized electrodeposition conditions. This study provides new insight for design and synthesis of highly-efficient OER catalyst. Nickel sulfides synthesized via a simple one step hydrothermal method in this work exhibit superior HER catalytic performance. The nickel sulfide flakes grown on Ni plate substrate as free-standing electrode shows 112 mV overpotential to reach a current density of 10 mV cm⁻² for HER. The nickel sulfides are mostly in a Ni₃S₂ form, which is found to be more catalytically active for HER than other nickel sulfides. EIS measurement indicates the small resistance of nickel sulfides. The nickel sulfides are highly stable under a current density of 10 mA cm-2 for 12 hours. OER and HER electrocatalysts are two crucial parts in electrochemical water splitting. This work is dedicated to improve the electrochemical water splitting efficiency by developing both OER and HER electrocatalysts.