Optimised Ni3+/Ni2+ and Mn3+/Mn2+ Ratios in Nickel Manganese Layered Double Hydroxide for Boosting Oxygen and Hydrogen Evolution Reactions

Abstract NiMn layered double hydroxide (LDH) grown hydrothermally on nickel foam substrate for water splitting application is reported. The Ni 3+ /Ni 2+ and Mn 3+ /Mn 2+ ratios are tuned to boost oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) active sites. The best‐optimized half‐cells with NiMn of 0.05 M (OER) and 0.025 M (HER) have the lowest overpotential, but a full‐cell fabricated from them has a higher cell voltage (1.68 V) than a symmetric cell made from 0.1 M NiMn LDH (1.56 V) at 10 mA cm −2 . It is found that the best half cells made from NiMn of 0.05 M (OER) and 0.025 M (HER) have higher series and charge transfer resistance on the HER and OER side, respectively. The calculated specific activity and turnover frequency normalized by electrochemical active surface area also support the individual half‐cell electrode performance, such as 0.05 M (20.19 A g −1 and 0.69 s −1 ) and 0.025 M (−41.91 A g −1 and −1.53 s −1 ) at 550 and −150 mV for OER and HER, respectively. An H‐type cell constructed with an anion exchange membrane delivers robust stability even at a substantially higher current density of 200 mA cm −2 for over 56 hours with the highest Faradaic efficiency of 98.64 % at 1.6 V vs. reversible hydrogen electrode in 1 M KOH.