A Functionally
Stable Manganese Oxide Oxygen Evolution
Catalyst in Acid
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Abstract
First-row
metals have been a target for the development of oxygen
evolution reaction (OER) catalysts because they comprise noncritical
elements. We now report a comprehensive electrochemical characterization
of manganese oxide (MnOx) over a wide pH range, and establish MnOx
as a functionally stable OER catalyst owing to self-healing, is derived
from MnOx redeposition that offsets catalyst dissolution during turnover.
To study this process in detail, the oxygen evolution mechanism of
MnOx was investigated electrokinetically over a pH range spanning
acidic, neutral, and alkaline conditions. In the alkaline pH regime,
a ∼60 mV/decade Tafel slope and inverse first-order dependence
on proton concentration were observed, whereas the OER acidic pH regime
exhibited a quasi-infinite Tafel slope and zeroth-order dependence
on proton concentration. The results reflect two competing mechanisms:
a one-electron one-proton PCET pathway that is dominant under alkaline
conditions and a Mn<sup>3+</sup> disproportionation process, which
predominates under acidic conditions. Reconciling the rate laws of
these two OER pathways with that of MnOx electrodeposition elucidates
the self-healing characteristics of these catalyst films. The intersection
of the kinetic profile of deposition and that of water oxidation as
a function of pH defines the region of kinetic stability for MnOx
and importantly establishes that a non-noble metal oxide OER catalyst
may be operated in acid by exploiting a self-healing process