Donor-Acceptor Couples of Metal and Metal Oxides with Enriched Ni³⁺ Active Sites for Oxygen Evolution

Exploiting precious-metal-free and high-activity oxygen evolution reaction (OER) electrocatalysts has been in great demands toward many energy storage and conversion processes, for example, carbon dioxide reduction, metal-air batteries, and water splitting. In this study, the simple solid-state method is employed for coupling Ni (electron donors) with lower-Fermi-level MoO₂ or WO (electron acceptors) into donor-acceptor ensembles with well-designed interfaces as robust electrocatalysts for OER. The resulting Ni/MoO₂ and Ni/WO electrocatalysts exhibit smaller overpotentials of 287 and 333 mV at 10 mA cm^-2 as well as smaller Tafel slopes of 51 and 65 mV/dec, respectively, with respect to the single Ni, MoO₂, WO, and even the benchmark RuO₂ in 1 M KOH. Specially, on account of a higher Fermi level of Ni in comparison with MoO₂ and WO, their strong electronic interaction results in directional interfacial electron transfer and increases the hole density over Ni, dramatically enriching the population of high-valence Ni³⁺ active sites and decreasing the Fermi level of Ni. The existence of Ni³⁺ can strengthen the chemisorption of OH^-, and the downshift of the Ni Fermi level can significantly expedite migration of electrons toward the surface of catalysts during OER, thus synergistically boosting the OER catalytic performance. Furthermore, the inner Ni/MoO₂ and Ni/WO heterostructures and the electrochemically induced surface layers of oxides/hydroxides collectively boost the OER kinetics. This study highlights the importance of designing highly efficient OER electrocatalysts with high-valence active species (Ni³⁺) and better matched energy levels induced by the work function difference through interfacial engineering.