Effect of nanoparticle size and active site position on the thermodynamics and charge dynamics of NiOOH catalyst for oxygen evolution reaction

Nanoparticles of nickel oxyhydroxide ( β -NiOOH) have advantages for efficient water oxidation catalysis owing to their larger catalytic surface area to volume ratio, and small length scale for charge carrier transport. Though oxygen evolution reaction (OER) mechanism pathways on different facets of bulk β -NiOOH were proposed, the relation between thermodynamics and charge transport dynamics with nanoparticle size and the location of active site remains elusive. OER modeled on (0001) surface of staggered β -NiOOH using density functional theory (DFT) + U study among two sizes of nanoparticles with a fixed width and length of ~ 1 nm each and varying heights of 1.35 nm and 0.38 nm, and three locations of the active sites, the following relation for overpotential ( η ) is manifested: η1 layer (0.38 nm) < η 3 layers (1.35 nm) and for active site ηOH site < ηO site < η edge site. Hence both the size of the nanocluster as well as the active site location influences the overpotential. The lowest overpotential of 0.57 V was observed in one layered OH terminated site. On the edge site, the potential determining step is transition from *OH to *O, and *O to *OOH on OH and O terminated sites. When more number of Ni atoms contributing to the reaction and more Ni atoms changing oxidation state from 3 + to + 4, showed highest amount of hole flux on OH terminated site. Since Ni exhibited multiple oxidation states, magnetism can be used as a control handle over catalytic efficiency in the oxygen evolution reaction mechanism.