Engineering the active sites by tuning the Ni and Mn composition in hierarchical heterostructured composites for electrocatalytic water splitting

Hydrogen production via electrochemical water splitting is a potentially clean and sustainable energy technology. Composition tuned binary NiMn oxides of Ni0.75Mn0.25O and Ni0.8Mn0.2O as an anode and cathode, respectively, demonstrate an excellent bifunctional electrocatalytic performance with a long term stability of 77 hours at a higher current density of 200 mA cm−2 toward overall water splitting (OWS) in an alkaline 1M KOH electrolyte solution. An anion exchange membrane H-type electrolytic cell requires a cell voltage of 1.74 V in order to generate a current density of 10 mA cm−2. The manganese oxide-modified nickel oxide nanosheets expedite the O–O bond formation via synergistic interaction between Ni3+ (t2g6eg1) and Jahn–Teller active Mn3+ (t2g3eg1). Moreover, due to their single electron occupancy in the eg orbital, the higher Mn3+ and NiO active sites could readily adsorb OH− ions from the electrolyte and promote a higher oxidation state, thereby accelerating the OER reaction. During OER, the energy and occupancy of these antibonding orbitals result in strong σ-bonds between these orbitals and oxygen-related adsorbate. Furthermore, the highest concentration of Ni(OH)2 attracts H+ ions, promoting the formation of H2.