Mono- and Bimetallic Nickel-Cobalt Metal-Organic Framework Electrocatalysts for Overall Water Splitting

The use of carbon-neutral energy carriers could reduce the ecological footprint of some of the most polluting industries (e.g., ammonia, cement, and steel production). A promising energy carrier is "green" hydrogen, i.e., H₂ produced by electrochemical water splitting driven by electricity from renewable sources. Metal-organic frameworks (MOFs) are constructed by the assorted coordination of metal clusters and polytopic organic linkers. MOFs containing 3d transition metals, like Fe, Co, and Ni, can be active catalysts in the hydrogen evolution reaction (HER) and/or the oxygen evolution reaction (OER). In this study, we fabricated nickel-, cobalt-, and bimetallic nickel- and cobalt-metal-organic frameworks (Ni/Co-MOFs) by a simple wet-chemical method and deposited them onto porous nickel foam (NF) and carbon paper substrates via drop-casting and electrochemical deposition. The alkaline water splitting activity of the electrodes was studied in a standard 3-electrode electrochemical cell by cyclic voltammogram, linear sweep voltammetry, electrochemical impedance spectroscopy, etc. The bimetallic Ni/Co-MOF drop-cast on NF showed the highest activity and the lowest overpotential among the investigated samples. Onset potentials of 1.52 and 0.24 V (vs reversible hydrogen electrode) at current densities of 20 and 10 mA cm^-2 were obtained in OER and HER, respectively, maintaining stable oxygen and hydrogen evolution for 50 h without any deterioration in the catalytic performance. In two-electrode overall water splitting, the NF-supported Ni/Co-MOF exhibits high HER and OER activity and enables efficient overall water splitting, requiring only 1.41 V to reach 10 mA cm^-2, demonstrating its practical electrocatalytic applicability.