Low-temperature and instantaneous high-rate output performance of AB5-type hydrogen storage alloy with duplex surface hot-alkali treatment

Abstract

The excellent low-temperature and instantaneous high-rate output performance of MmNi3.7Co0.7Mn0.3Al0.3 alloy are obtained via hot-alkali treatment (K alloy) and duplex hot-alkali treatments with reducing agents, including N2H4 (KN alloy), NaH2PO2 (KP alloy) and NaBH4 (KB alloy). The charge transfer process on the alloy surface is the primary control step that influences the electrochemical properties at −40 °C. Scanning electron microscopy and energy dispersive X-ray analyses demonstrate that a high electrocatalytic Ni-and Co-rich layer is formed via the preferential dissolution of Al and Mn and that the alloy surface oxide is effectively removed via the duplex surface hot-alkali treatments. Thus, the charge-transfer resistance decreases and the exchange current density increases obviously in the treated alloys, making the K, KN, KP and KB alloys exhibit better activation property, high-rate dischargeability and instantaneous high-rate output performance at 20 °C, as well as higher discharge capacity, low-temperature dischargeability and high-power delivery at −40 °C. Based on the overall electrochemical properties of the alloys at various temperatures, a duplex surface modification of the hot-alkali treatment with NaH2PO2 is determined to be the best.