The structural properties of CoFe composites fabricated from inexpensive Co(II) and Fe(III) precursors using a Prussian blue analogue (PBA) strategy without additional reductants were investigated. Microporous CoFe-200 and microporous/mesoporous CoFe-550 structures of the CoFe catalysts (CoFe-PBA) were produced by calcination for 1 h in N 2 at 200°C or 550°C, respectively. The electrocatalytic activities of the CoFe catalysts produced for the oxygen evolution and reduction reactions (OER/ORR) were studied in alkaline media. The OER measurements revealed the CoFe-200 catalyst to be superior to CoFe-PBA and CoFe-550, and even surpass the activity of commercial Ir/C in terms of the overpotential at 10 mA cm À 2 and onset potential (E OER onset). On the other hand, the ORR activity of CoFe-550 exhibited a more positive half-wave potential (0.837 V vs. RHE) and E ORR onset (0.942 V vs. RHE) than CoFe-200. The Tafel slope (À 55.9 mV dec À 1) of CoFe-550 was lower than that of Pt/C (À 77.8 mV dec À 1). A comparison of CoFe-200 and CoFe-550 suggested that the microporosity of CoFe-200 (average pore diameter, d � 2 nm) was beneficial in terms of the OER. In contrast, the mesoporous (d � 35.6 nm) structure of CoFe-550 promoted the mass-transport kinetics of oxygen through the electrode surface. CoFe nanocubes with tunable porosity are potential catalysts that can be utilized selectively for the OER and ORR.

The Cover Feature shows that microporous CoFe‐200 nanocubes (mean pore diameter, d≈1.2 nm) are beneficial for the oxygen evolution reaction (OER) because of the improved active site densities despite their larger diffusion path, whereas microporous/mesoporous CoFe‐550 (d≈35.6 nm) favors the oxygen reduction reaction (ORR) by enabling the mass transport of oxygen through the catalyst. More information can be found in the Article by J. H Shim and co‐workers.