Synthesis of Novel Melilite-Type Iron/Cobalt Oxides and Their Oxygen Evolution Reaction Electrocatalytic Activity

Here, we report the synthesis and oxygen evolution reaction (OER) electrocatalytic activity of A₂Fe_2–2xCo_xGe_1+xO₇ (A = Sr and Ba) with a melilite-type structure. The iron/cobalt-mixed melilites were successfully synthesized for the first time and found to exhibit high OER activity in highly alkaline media. In particular, the OER performance of Sr₂Fe_0.67Co_0.67Ge_1.66O₇ (x = 0.67) is comparable to, or even superior to, that of a precious-metal catalyst, RuO₂. The remarkable OER activity of the melilites seems to originate from a synergetic effect of Fe/Co coexistence as well as the structural feature of melilites, where “coordinatively unsaturated” Fe/Co sites effectively promote the formation of catalytically active surface layers through the adsorption of OH^– ions in highly alkaline media.