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

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.