Figure 4. SEM images showing the surface morphology of the samples. a) A pure MnO2 film as
prepared. b) A pure MnO2 after electrochemical testing at both 1.8 and 1.9 VRHE. c) Ti-MnO2
film as prepared. d) Ti-MnO2 film after electrochemical testing at both 1.8 and 1.9 VRHE. Each
graph has a scale bar indicated in the bottom. All four images were acquired with an
acceleration voltage of 3 kV.
We previously reported the catalytic activity of the pure MnO2 films in 1 M KOH;32 which we found to
exhibit comparable activity to the best performing manganese-based OER catalysts reported in
literature.34,42,46,75,76 Several manganese-based catalysts were recently compared by Gorlin et al. on
the basis of turnover frequencies (TOF), which were in the order of 0.002 to 0.006 s-1 at an
overpotential of 0.4 V.77 In comparison, the MnO2 reported here exhibit an initial ������������������ of 0.007 s-1
when using the same normalization procedure (see Supporting Information for details). Figure 5b
shows the initial activity of the unmodified MnO2 thin film, tested in 1M KOH and 0.05 M H2SO4.