MOF Derived Co₃O₄@Co/NCNT Nanocomposite for Electrochemical Hydrogen Evolution, Flexible Zinc-Air Batteries, and Overall Water Splitting

Toward the goal of clean and sustainable energy source, the development of a trifunctional electrocatalyst is a boon for energy storage and conversion devices such as regenerative fuel cells and metal-air batteries. MOF-derived semiconducting-metallic core–shell electrocatalyst <b>Co</b>_<b>3</b><b>O</b>_<b>4</b><b>@Co/NCNT</b> (NCNT = nitrogen-doped carbon nanotube), which was shown to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is also found to be an active electrocatalyst for hydrogen evolution reaction (HER) with a low overpotential of 171 mV. Here, the HER activity of <b>Co</b>_<b>3</b><b>O</b>_<b>4</b><b>@Co/NCNT</b> is presented and is shown as highly efficient and robust trifunctional electrocatalyst. The detailed theoretical calculation has found N-center of Co–N₄ moiety to be the H⁺ binding active site and thus proves <b>Co</b>_<b>3</b><b>O</b>_<b>4</b><b>@Co/NCNT</b> to be active for HER. Further, the ORR and OER bifunctionality of <b>Co</b>_<b>3</b><b>O</b>_<b>4</b><b>@Co/NCNT</b> helped in fabricating secondary Zn-air battery with high power density of 135 mW/cm². Also, an all-solid-state flexible and wearable battery with <b>Co</b>_<b>3</b><b>O</b>_<b>4</b><b>@Co/NCNT</b> as cathode and electrodeposited Zn on carbon fiber cloth as anode was shown to withstand its performance even under stressed conditions. Finally, the material being trifunctional in nature was used both as an anode and cathode material for the electrolysis of water, which was powered by the Zn-air batteries with <b>Co</b>_<b>3</b><b>O</b>_<b>4</b><b>@Co/NCNT</b> as the cathode material. It is believed that the development of a trifunctional catalyst would help in wide commercialization of regenerative fuel cells.