Trifunctional catalytic properties of nickel cobalt oxide/cobalt oxide electrocatalysts for rechargeable zinc-air battery and water splitting applications

Aqueous zinc-air batteries (ZABs) are used in energy storage applications because of their high energy density, reliability, and eco-friendly feature. Developing highly active and effective electrocatalysts for slow kinetics toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is critical to improving the efficiency of ZABs. Herein, we report effective NCO/CO-500 spheres encircled nanorod flower-like structured electrocatalyst materials synthesized using a simple hydrothermal technique, followed by calcination. First, the NCO-120 nanorod flower-like structures were synthesized and then calcined at 500 °C. Calcination temperature transforms the nanorod flower-like structures into the spheres encircled by them. The NCO/CO-500 electrocatalyst exhibited excellent trifunctional activities for OER (η₁₀ = 274 mV) in 1.0 M KOH, ORR (E_onset = 0.93 V and E_1/2 = 0.76 V vs. reversible hydrogen electrode) in 0.1 M KOH, and hydrogen evolution reaction (HER (η₁₀ = 163 mV)) in 1 M KOH due to its increased active sites and rapid ion/electron transport capability. The Tafel slopes of the NCO/CO-500 electrocatalyst (OER = 131 mV dec⁻¹, ORR = 120 mV dec⁻¹, and HER = 125 mV dec⁻¹) were smaller than those of the counter electrocatalysts. Additionally, the NCO/CO-500 electrocatalyst was employed as an air cathode for ZABs to achieve a good durability of 120 h. Owing to their electrical conductivity and electron configurations, transition metal particles comprising Co and Ni offer many active sites, leading to long-term stability as well as higher OER, ORR, and HER activities in ZABs. This provides a deep insight into the design of multifunctional electrode catalysts with excellent performance, low cost, and well-designed structure.