Effect of particle size on the electrochemical properties of MmNi_3.8Co_0.75Mn_0.4Al_0.2 hydrogen storage alloy

Charge and discharge testing, linear polarization, electrochemical impedance spectroscopy (EIS) and potential step chronoamperometry (PSCA) were used to investigate the electrochemical properties of the Ce-rich mischmetal MmNi_3.8Co_0.75Mn_0.4Al_0.2 hydrogen storage alloy with different particle sizes. At a discharge current density of 900 mA/g, the alloy with small original particle size maintained a high rate dischargeability (HRD) above 86% while the alloy with large particle size could not discharge in the same potential region. The alloy with small original particle size also showed lower contact resistances and polarization resistance after full activation. Both the exchange current density and the hydrogen diffusion coefficient increased when the hydrogen concentration decreased. The charge-transfer reaction on the surface of alloy particles with different sizes should be mainly responsible for the differences in electrochemical properties, especially the HRD.