Nickel Iron Phosphide/Phosphate as an Oxygen Bifunctional Electrocatalyst for High-Power-Density Rechargeable Zn–Air Batteries

The evolution of an effective oxygen electrocatalyst is of great importance for the widespread application of Zn–air batteries but remains an immense challenge. Thus, an efficient catalyst toward the oxygen evolution reaction and oxygen reduction reaction (OER and ORR) is highly essential for high-performance Zn–air batteries. Here, we have reported bifunctional nickel iron phosphide/phosphate (NiFeP/Pi) catalysts with various Ni/Fe ratios toward oxygen electrocatalysis in alkaline media. These catalysts are highly active toward OER and ORR, wherein NiFe(1:2)­P/Pi exhibits a low OER overpotential of 0.21 V at 10 mA cm^–2 and a high ORR onset potential (0.98 V vs RHE) with the lowest potential difference (ΔE = E₁₀ – E_1/2) of 0.62 V, which surpasses that of the benchmark Pt/C and RuO₂ catalyst as well as those of most previously reported bifunctional catalysts. Furthermore, the NiFe(1:2)­P/Pi-based Zn–air battery demonstrates a very high power density of 395 mW cm^–2 and outstanding discharge capacity of 900 mAh g^–1@10 mA cm^–2 along with steady cyclability, maintaining 98% of the round trip efficiency over 300 cycles. These results are helpful for a good understanding of the composition–activity relation with a certain band gap toward high-performance Zn–air batteries.