1. Introduction
The wider uptake of intermittent sources of renewable energy, such of wind and solar, requires
efficient and scalable means of storing the energy. Electrochemical devices such as electrolysers,
photoelectrolysers and metal air-batteries are ideally suited towards this purpose.1–4 Unfortunately,
it turns out that the efficiencies of these technologies are severely limited by the sluggish kinetics of
the oxygen evolution electrode.5 Furthermore, the choice of electrolyte, acid or alkaline, has a
significant impact on the materials available as catalysts.6–8 This is exemplified by the case of Proton
Exchange Membrane (PEM) electrolyzers, which are well suited for localized hydrogen production,
due to their superior efficiency at high current densities, ability to manage fluctuating power inputs,
and fast start up times.9,10 A major drawback of the technology is that the acidic nature of the
electrolyte severely limits the number of available electrode materials.11,12 Copious amounts of
precious metal oxides are required to catalyze the oxygen evolution reaction (OER), in particular
Ir.9,13–15 We recently estimated that in order for PEM electrolyzers to be scaled up to the terawatt
level (i.e. to make a serious impact to the global energy challenge), ten years of the annual Ir
production would be needed, solely for the oxygen electrode.5,169 Similar scale up limitations hold
for devices for photo-catalytic water splitting.6 One alternative would be to switch to hydroxide
conducting polymer electrolytes; under alkaline conditions, catalysts based on Ni, Fe and Co catalyze
the OER on par, or even better, than the noble metal oxides.17–24 However, despite their promise,
hydroxide conducting polymeric electrolytes are still in their technological infancy: they are less stable
and slightly less conductive than their proton conducting counterparts, and impose greater
overpotentials at the hydrogen electrode.25–27 It turns out that Ni, Fe and Co are unstable towards
dissolution in acid.11,28 In fact, even the noble metals and their oxides corrode under OER
conditions.29–32 A number of non-precious metal oxides, such as TiO2, SnO2, Ta2O5 and Nb2O5, are
stable in acid, but inactive towards the OER.14 As a consequence, it has long been recognized that
there is a great need to finding stable and active materials, based on non-precious metals, for wateroxidation
in acid; even so, to date, no obvious solutions or even strategies have been put forward.33
MnO2 presents an interesting example, as it shows some compromise between being active
and stable in both alkaline and acidic media.34–43 In particular, the Pourbaix diagram of Mn reveals