Study On New Type Metal Hydride Electrode Of CeMnAl

The purpose of this work is to improve electrochemical characteristics of rare earth-based AB2-type Laves phase CeMnAl hydrogen storage alloy.CeMn_1-xAl_1-xNi_2x, CeMn_0.25Al_0.25Ni_1.5+x, CeMn_0.25Al_0.25Ni_2.4-xCo_x, Ce_1-xTi_xMn_0.35-Al_0.25Ni_1.5 and Ce_1-xZr_xMn_0.35Al_0.25Ni_1.5 alloys were prepared.The phase structure, surface morphology and compositions of the CeMn_1-xAl_1-xNi_2x alloys, CeMn_0.25Al_0.25Ni_1.5+x over-stoichiometric alloys and CeMn_0.25Al_0.25Ni_2.4-xCo_x alloys were investigated by means of XRD and SEM-EDS analyses. It has been shown that Ni substitution leads to a phase change of the CeMn_1-xAl_1-xNi_2x alloys and new phase to be formed. Single C15 cubic Laves phase for x=0 alloy has transformed into a multi-phase structure for x=0, 0.25, 0.50, 0.75 alloys. Both the CeMn_0.25Al_0.25Ni_1.5+x alloys and the CeMn_0.25Al_0.25Ni_2.4-xCo_x alloys consist mostly of hexagonal CeAl phase and cubic CeNi phase, and the peaks of XRD shift to smaller angle with increasing Ni content and Co content, respectively.The influences of different elements and their various contents on electrochemical performance of the alloy are also investigated by measuring P-C-T curve, discharge capacity, high-rate dischargeability, capacity decay and high-temperature discharge capacity. The discharge capacity of the CeMn_1-xAl_1-xNi_2x alloy electrode increases as Ni substitution content increases while the capacity decay is getting lower and the HRD is improved. For CeMn_0.25Al_0.25Ni_1.5+x alloy electrode, with increasing Ni over-stoichiometric content, the discharge capacity increases, the capacity decay becomes faster and the HRD is enhanced. For CeMn_0.25Al_0.25Ni_2.4-xCo_x alloy electrodes, when Co substitution content is increasing, the cycle stability and HRD are improved at a little loss of the decrease of discharge capacity. It has also been revealed that an appropriate substitutional amount of Ti, Zr for Ce in A-side of CeMn_0.35Al_0.25Ni_1.5 can improve the high temperature activation and cycle stability of the alloy electrode, and raise the discharge capacity at 338 K.The P-C-T curves show that the hydrogen equilibrium pressure of free-Ni CeMnAl hydride is much lower and no hydrogen equilibrium pressure plateau has been observed. The slope of hydrogen equilibrium pressure plateau of the alloy becomes flatter and longer and hydrogen equilibrium pressure gets raised with increasing Ni content. This is due to the Ni partial substitution and the formation of a secondary phase. With increasing Ni content, the△H^θas well△S^θshift to the positive direction and make alloy hydrides more unstable and electrochemical hydrogen desorption more easilier. The shifts of the△H^θand△S^θare the thermodynamic factors to improve the electrochemical characteristics of CeMnAl alloy.