Core-shell CoFe2O4@Co-Fe-Bi nanoarray: a surface-amorphization water oxidation catalyst operating at near-neutral pHElectronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c7nr02929b

The exploration of high-performance and earth-abundant water oxidation catalysts operating under mild conditions is highly attractive and challenging. In this communication, core-shell CoFe 2 O 4 @Co-Fe-Bi nanoarray on carbon cloth (CoFe 2 O 4 @Co-Fe-Bi/CC) was successfully fabricated by in situ surface amorphization of CoFe 2 O 4 nanoarray on CC (CoFe 2 O 4 /CC). As a 3D water oxidation electrode, CoFe 2 O 4 @Co-Fe-Bi/CC shows outstanding activity with an overpotential of 460 mV to drive a geometrical catalytic current density of 10 mA cm −2 in 0.1 M potassium borate (pH 9.2). Notably, it also demonstrates superior long-term durability for at least 20 h with 96% Faradic efficiency. Density functional theory calculations indicate that the conversion from OOH* to O 2 is the rate-limiting step and the high water oxidation activity of CoFe 2 O 4 @Co-Fe-Bi/CC is associated with the lower free energy of 1.84 eV on a Co-Fe-Bi shell. Core-shell CoFe 2 O 4 @Co-Fe-Bi nanoarray on carbon cloth (CoFe 2 O 4 @Co-Fe-Bi/CC) acts as a durable water oxidation electrocatalyst with an overpotential of 460 mV to drive 10 mA cm −2 in 0.1 M potassium borate (pH 9.2).