Effects of different electrolytes containing Na2WO4 on the electrochemical performance of nickel hydroxide electrodes for nickel–metal hydride batteries

• Na2WO4 can significantly improve the high-temperature performance of Ni–MH batteries.
• The mechanism of Na2WO4 on nickel electrode in KOH and NaOH electrolytes is proposed.
• Na2WO4 in electrolyte forms WO3·2H2O solid film on the surface of nickel electrode.
• Electrolyte added Na2WO4 results in lower impedance and slower oxygen evolution.

To improve the high-temperature performance of the nickel hydroxide electrodes in nickel–metal hydride batteries, sodium tungstate (Na2WO4) used as an electrolyte additive has been added into two types of binary electrolytes (KOH–LiOH and NaOH–LiOH) in this study. The effects of electrolyte composition on the electrochemical performance of nickel electrodes have been systematically investigated via a combination of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and charge/discharge tests. It is found that by adding (1.0 wt.%) Na2WO4, the performance of nickel electrodes is significantly improved in both NaOH and KOH electrolytes at 70 °C. The improved performance can be attributed to the deposition of WO3·2H2O solid film on the surface of nickel electrode, which is beneficial to the increase in oxygen evolution overpotential, the slow-down of oxygen evolution rate and the decrease in charge-transfer resistance.