Tubular assemblies of N-doped carbon nanotubes loaded with NiFe alloy nanoparticles as efficient bifunctional catalysts for rechargeable zinc-air batteries.

Enormous research effort is presently being directed towards the discovery of low cost bifunctional electrocatalysts capable of efficiently driving the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), with such bifunctional electrocatalysts being particularly sought after for rechargeable metal-air batteries. Herein, we report the successful synthesis of a highly efficient bifuctional ORR/OER electrocatalyst, comprising tubular assemblies of 20-40 nm N-doped carbon nanotubes containing NiFe alloy nanoparticles (denoted herein as TA-NiFe@NCNT). To synthesize TA-NiFe@NCNT, we first prepared g-C3N4 nanotubes with a diameter ∼200 nm as a sacrificial template and nitrogen source, then decorated the nanotubes with NiFe-layered double hydroxide nanoparticles (NiFe-LDH). The NiFe-LDH/g-C3N4 composite obtained was then coated with a thin layer of glucose (an additional carbon source), then the resulting NiFe-LDH/g-C3N4@Glu composite was pyrolyzed at 900 °C in N2. The obtained TA-NiFe@NCNT product exhibited a low overpotential of only 310 mV at a current density of 10 mA cm-2 during OER in 0.1 M KOH (cf. 401 mV for IrO2) and an ORR activity in 0.1 M KOH (onset potential of 0.93 V and half-wave potential of 0.81 V vs. RHE) comparable to a commercial Pt/C catalyst (onset potential of 0.99 V and half-wave potential of 0.82 V vs. RHE). The remarkable bifunctional performance of TA-NiFe@NCNT can be attributed to the excellent OER and ORR activities of NiFe alloy nanoparticles and NCNTs, respectively, as well as the high porosity and excellent conductivity of the electrocatalyst that benefitted mass and electron transfer processes, respectively. A custom-built rechargeable zinc-air battery constructed using TA-NiFe@NCNT at the air electrode delivered a lower charge-discharge voltage gap (0.92 V) and longer cycling lifetime (170 h at 25 mA cm-2) than a battery fabricated using a mixture of IrO2 and Pt/C as air electrode catalysts.