Nucleation and Growth Mechanisms of an Electrodeposited Manganese Oxide Oxygen Evolution Catalyst

We investigate the mechanisms of nucleation and steady-state growth of a manganese oxide catalyst (MnOx) electrodeposited from Mn2+ solutions in a weakly basic electrolyte. Early catalyst growth was probed through chronoamperometry transients, which were fit to reveal a progressive nucleation mechanism for initial catalyst formation. Time-dependent atomic force microscopy snapshots of the electrode surface reveal a rapid increase in nucleus size together with a sluggish rise in coverage, which is also characteristic of progressive nucleation. Electrochemical kinetic studies of the catalyst growth yield a Tafel slope of approximately 2.3 x RT/2F and a rate law consisting of a second-order and inverse fourth-order dependence on [Mn2+] and proton activity, respectively. These results are consistent with a deposition mechanism involving rate-limiting disproportionation of aqueous Mn3+, resolving a longstanding ambiguity surrounding the deposition of manganese oxides under nonacidic conditions.