Can\nNiFe-Layered-Double-Hydroxide Catalysts Suppress\nCarbon Corrosion in Electrochemical Oxygen Evolution?

Sustainable\nenergy societies demand rechargeable batteries using\nubiquitous-material electrodes of geopolitical-risk-free elements.\nWe aim to develop low-overpotential oxygen-evolution-reaction (OER)\ncatalysts that suppress carbon corrosion of gas-diffusion electrodes\n(GDEs) to realize two-electrode rechargeable Zn-air batteries (r-ZABs).\nHerein, single-walled-carbon-nanotube (SWNT) thin films are used as\na scaffold for a benchmark OER catalyst, doping-free NiFe-layered\ndouble hydroxide (NiFeLDHs), operating in r-ZABs using alkali aqueous\nelectrolytes. Metal compositions of NiFeLDHs are controlled with an\natomic-level quality using Prussian-blue-analog nanoparticles of Ni<sub><i>x</i></sub>Fe<sub>1–<i>x</i></sub>[Fe(CN)<sub>6</sub>]<sub>0.67</sub> (<i>x</i> = 0–1). The nanoparticles\nwith dimensions of ∼8 nm adhere to SWNTs on carbon paper as\na GDE model by a drop-casting method using their aqueous dispersion\nsolutions. Ni<sub>0.6</sub>Fe<sub>0.4</sub>[Fe(CN)<sub>6</sub>]<sub>0.67</sub> shows OER activity by hydrolysis for generating NiFeLDH\nnanodots of metal compositions between Ni<sub>0.5</sub>Fe<sub>0.5</sub> and Ni<sub>0.6</sub>Fe<sub>0.4</sub> with a size distribution of\n1.75 ± 0.26 nm and exposing OER-active (018) and (015) planes\non SWNTs. The activity is investigated by regulating the loading amounts\nof the NPs to avoid aggregating the nanodots. An optimal low-loading\namount of 270 nmol cm<sup>–2</sup> minimizes <i>iR</i>-corrected overpotential to 156 mV at 10 mA cm<sup>–2</sup>. The <i>iR</i>-uncorrected overpotential is 260 mV and\nsuppresses carbon corrosion of SWNTs and carbon black. Using an r-ZAB\nhalf-cell with a Zn foil, OER-driven charging stably proceeds at 10\nmA cm<sup>–2</sup> over 3 h with an average voltage of 1.99\nV vs Zn/Zn<sup>2+</sup>. Limited metal electrodes have further improved\nOER overpotentials by third-element doping, while carbon electrodes\nstill offer room for discovering intrinsically high OER activities\nof NiFeLDHs without doping.