Water oxidation is the key step in natural and artificial photosynthesis for solar-energy conversion. As this process is thermodynamically unfavorable and is challenging from a kinetic point of view, the development of highly efficient catalysts with low energy cost is a subject of fundamental significance. Herein, we report on iron-based films as highly efficient water-oxidation catalysts. The films can be quickly deposited onto electrodes from Fe II ions in acetate buffer at pH 7.0 by simple cyclic voltammetry. The extremely low iron loading on the electrodes is critical for improved atom efficiency for catalysis. Our results showed that this film could catalyze water oxidation in neutral phosphate solution with a turnover frequency (TOF) of 756 h À1 at an applied overpotential of 530 mV. The significance of this approach includes the use of earth-abundant iron, the fast and simple method for catalyst preparation, the low catalyst loading, and the large TOF for O 2 evolution in neutral aqueous media.Catalytic water splitting is an appealing way of using solar energy to produce hydrogen fuels as a clean, carbon-free, and renewable energy source and thus to eliminate the energy and environmental problems caused by burning fossil fuels. [1][2][3] Electrochemical water splitting has received extensive attention because this process can convert electric energy from renewable but intermittent sources into chemical energy for easier storage and delivery. [4][5][6] As one of the half reactions, water oxidation becomes the bottleneck for large-scale water splitting, as it is challenging from both thermodynamic and kinetic points of view. [1,7] Extensive efforts have been made to develop efficient and robust water-oxidation catalysts (WOCs) that can catalyze oxygen evolution at considerable rates at low overpotentials (h). Examples include homogeneous and heterogeneous systems based on noble metals, such as Ru [8][9][10] and Ir, [10][11][12] and first-row transition metals, such as V, [13] Mn, [14][15][16][17][18][19][20] Fe, [21][22][23][24][25] Co, [2,[26][27][28][29][30] Ni, [4,6,31,32] and Cu, [5,33,34] and mixed-metal materials. [35][36][37][38][39] However, substantial improvement of catalyst activity and stability is still needed, in particular for WOCs made of cheap earth-abundant transition-metal elements.Iron is the most abundant transition metal in the earths crust, and it is less toxic than Co and Ni. Meanwhile, iron is attractive for water oxidation because of its rich redox properties and the extensive biological/biomimetic activity of iron enzymes/complexes for oxygen activation. [40,41] Ironbased films have been much less explored as electrocatalysts for water oxidation than those of Co and Ni, although iron oxides (Fe 2 O 3 ) have been shown to be photocatalysts for this process [22,23] and are known to be important cocatalysts for Co and Ni oxides. [35,36,38,39] One possible reason is that the direct preparation of iron-based films by electrodeposition, a method generally employed to deposit other metal...
