that it is stable, in the form of MnO2, at pH 0 to 14 from 1.3-1.7 VRHE,28,44 a highly relevant range for
OER. There are few reports of MnOx catalysts used for OER in low pH. In Figure 1, a comparison of
these catalysts and a RuO2 thin film is shown, using a turnover frequency basis, ������������. Due to
differences in thickness and preparation techniques, a direct comparison is complicated. The upper
limit, ������������������, is found by assuming that only the surface participates in the reaction, whereas
������������������ is found by assuming all of the catalytic material can contribute. Although these assumptions
are crude, it is evident that the MnO2 film in this work has a significantly higher activity than the other
MnOx references, with the exception of the Morita et al. catalyst. However, their catalyst is prepared
by thermal composition and likely had a high roughness factor.45 See more details in the
supplementary information.
Figure 1. Comparison of the MnOx-based catalysts reported in the literature, based on both
������������������ and ������������������. Data from Morita et al. is shown in blue,46 from Takashima et al. in
green,47 Huynh et al. in yellow 35 and Najafpour et al. in magenta.48 A benchmark RuO2
catalyst from 32 is shown in teal. The catalyst reported in this work is shown in red (measured
with 5 mV/s, 1600 RPM and 0.05 M H2SO4).