Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation

Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of high interest, inter alia for OER—CO2-reduction coupling in non-fossil production. To address pertinent stability problems and insufficiently understood structure-activity relations, we investigate the influence of moderate annealing (100 – 300 C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of the birnesite type became as OER active as the non-heated Oxide 2 which has a highly disorderd atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure-reactivity relations. Oxide structures featuring both di-µ-oxo bridged Mn ions and (close to) linear mono-µ-oxo Mn3+-O-Mn4+ connectivity seem prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potential and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in strongly accelerated rates of Mn oxidation and O2 formation. Lacking charge mobility, which may result from a low-level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures relating to the perovskite-like ζ-Mn2O3 was formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower that 4+. This rare phase was previously found only at high pressure (20 GPa) and temperatures (1200 C) and this is the first report where it was stable at ambient conditions.