Low pH Electrolytic Water Splitting Using Earth-Abundant Metastable Catalysts That Self-Assemble in Situ

Typical catalysts for the electrolysis
of water at low pH are based
on precious metals (Pt for the cathode and IrO2 or RuO2 for the anode). However, these metals are rare and expensive,
and hence lower cost and more abundant catalysts are needed if electrolytically
produced hydrogen is to become more widely available. Herein, we show
that electrode-film formation from aqueous solutions of first row
transition metal ions at pH 1.6 can be induced under the action of
an appropriate cell bias and that in the case of cobalt voltages across
the cell in excess of 2 V lead to the formation of a pair of catalysts
that show functional stability for oxygen evolution and proton reduction
for over 24 h. We show that these films are metastable and that if
the circuit is opened, they redissolve into the electrolyte bath with
concomitant O2 and H2 evolution, such that the
overall Faradaic efficiency for charge into the system versus amounts
of gases obtained approaches unity for both O2 and H2. This work highlights the ability of first row transition
metals to mediate heterogeneous electrolytic water splitting in acidic
media by exploiting, rather than trying to avoid, the natural propensity
of the catalysts to dissolve at the low pHs used. This in turn we
hope will encourage others to examine the promise of metastable electrocatalysts
based on abundant elements for a range of reactions for which they
have traditionally been overlooked on account of their perceived instability
under the prevailing conditions.