Morphology effects on electrocatalysis of anodic water splitting on nickel (II) oxide

Oxygen evolution reaction (OER) is critical for producing high purity hydrogen and oxygen via electrocatalytic water splitting. In this work, single crystalline, nanoporous nickel oxide (NiO) was prepared using a hydrothermal, soft-templated synthesis route followed by calcination at different temperatures. It is shown that the NiO crystals have a cubic lattice, and the pore size can be tuned from ∼1 to ∼70 nm by varying the calcination temperature, i.e. variation from micro to macroporosity. The NiO's catalytic performance as electrocatalysts was evaluated in OER, both thermodynamically and kinetically. Mesoporous NiO with calcination temperature of 400 °C had the lowest overpotential (335 mV) required @ 10 mA/cm2 accompanied with the highest turnover frequency value and mass activity among of the obtained NiO electrocatalysts. The study shows that the electrocatalytic activity of nanoporous NiO outperforms that of commercial catalyst Ir/C (∼360 mV @ 10 mA/cm2). Microporous NiO possess the highest specific surface area and electrical double layer capacitance, while the nonporous NiO particles have the highest specific activity and BET activity of the catalysts. It is concluded that the minimization of voltage losses by the nanoscale enlargement of the electrocatalyst surface area shows the coherence between gas adsorption and electrocapacitive measurements. Conversely, the OER kinetics showed deterioration with surface area maximization due to the impediment of ionic transport inside the micropores. This work demonstrates the importance of morphology optimization to obtain an efficient OER electrocatalyst with low required overpotential and kinetic loss.