Effect of surface treatment on electrochemical properties of MmNi_3.8 Co_0.75 Mn_0.4 Al_0.2 hydrogen storage alloy

Surface treatment of the MmNi_3.8Co_0.75Mn_0.4Al_0.2 (Mm = Ce - rich mischmetal) hydrogen storage alloy is carried out by using the solution of HF and KF with a little addition of KBH₄. The electrochemical properties of the original and the treated hydrogen storage alloys are investigated. The discharge properties such as discharge capacity, potential plateau, and high rate dischargeability (HRD) are improved after the treatment. At a current rate of 900 mA/g, the HRD of the treated alloy still remains a high value of 80.2% while the original one cannot discharge. The result of the linear polarization measurement shows that the exchange current density increases from 80.99 to 124.98 mA/g and the polarization resistance decreases from 371.1 to 205.5 m Ω g after the treatment. The result of the electrochemical impedance spectroscopy (EIS) also demonstrates the same consequence. The hydrogen diffusion coefficient studied by potential-step measurement increases from 1.79 × 10^{- 9} to 3.29 × 10^{- 9} cm² / s after the treatment.