Can NiFe-Layered-Double-Hydroxide Catalysts Suppress Carbon Corrosion in Electrochemical Oxygen Evolution?

Abstract

Sustainable energy societies demand rechargeable batteries using ubiquitous-material electrodes of geopolitical-risk-free elements. We aim to develop low-overpotential oxygen-evolution-reaction (OER) catalysts that suppress carbon corrosion of gas-diffusion electrodes (GDEs) to realize two-electrode rechargeable Zn-air batteries (r-ZABs). Herein, single-walled-carbon-nanotube (SWNT) thin films are used as a scaffold for a benchmark OER catalyst, doping-free NiFe-layered double hydroxide (NiFeLDHs), operating in r-ZABs using alkali aqueous electrolytes. Metal compositions of NiFeLDHs are controlled with an atomic-level quality using Prussian-blue-analog nanoparticles of Ni_xFe_1-x[Fe(CN)₆]_0.67 (x = 0-1). The nanoparticles with dimensions of ∼8 nm adhere to SWNTs on carbon paper as a GDE model by a drop-casting method using their aqueous dispersion solutions. Ni_0.6Fe_0.4[Fe(CN)₆]_0.67 shows OER activity by hydrolysis for generating NiFeLDH nanodots of metal compositions between Ni_0.5Fe_0.5 and Ni_0.6Fe_0.4 with a size distribution of 1.75 ± 0.26 nm and exposing OER-active (018) and (015) planes on SWNTs. The activity is investigated by regulating the loading amounts of the NPs to avoid aggregating the nanodots. An optimal low-loading amount of 270 nmol cm^-2 minimizes iR-corrected overpotential to 156 mV at 10 mA cm^-2. The iR-uncorrected overpotential is 260 mV and suppresses carbon corrosion of SWNTs and carbon black. Using an r-ZAB half-cell with a Zn foil, OER-driven charging stably proceeds at 10 mA cm^-2 over 3 h with an average voltage of 1.99 V vs Zn/Zn²⁺. Limited metal electrodes have further improved OER overpotentials by third-element doping, while carbon electrodes still offer room for discovering intrinsically high OER activities of NiFeLDHs without doping.