Study Of Electrochemical Properties On Co-free La_1-xCe_x(NiMnFeAl)_5.6 Rapid Quenching Hydrogen Storage Alloys

For the commercial AB₅-type electrode alloys, cobalt is a key element in improving the cyclic stability of electrodes, however the cobalt is very expensive and shares the 40-50% of the total cost of alloys, so the research and development of low-Co or Co-free alloy has been the focus of the study on AB₅-type alloys and the focus of industrialization. It is regretful that the electrochemical properties of low-Co or Co-free AB₅-type alloys, especial for the cyclic stability, haven't been improved satisfactorily unit now.In this paper, the LaNi_4.17Mn_0.93Al_0.1Fe_0.4 (the quenching rate 5m/s, 10m/s, 15m/s, 20m/s, 30m/s),La_1-xCe_xNi_4.17Mn_0.93Al_0.1Fe_0.4(x=0-0.5) and LaNi_4.27Mn_0.93Al_xFe_0.4-x(x=0.1 -0.3) rapid quenching Co-free over-stoichiometric rare earth hydrogen storage alloys were studied. By means of XRD analysis,P-C-T and electrochemical measurements, the effects of rapid quenching rate and element Substitution on the phase structures and electrochemical properties were studied systemically , enhance the hydrogen storage capacity and electrochemical capacity of alloys.The phase structures of the LaNi_4.17Mn_0.93Al_0.1Fe_0.4 (the quenching rate 5m/s, 10m/s, 15m/s, 20m/s, 30m/s) rapid quenching over-stoichiometric rare earth hydrogen storage alloys are only composed of CaCu₅ type phase, the lattice parameters a decreased,c increased and the unit cellvolume decreased after a trend of increase, The crystal latice constants a and c exhibit a anisotropic change. With the increase of quenching speed while the discharge capacities of the alloys decreased, but the cycle stability of the alloy increased dramatically. For alloys with quenching rate of 5 m/s and 10 m/s, the maximum discharge capacities of the quenched alloys reach 325 mAh/g and 337 mAh/g respectively, the rates of capacity retention (S₁₀₀) of the alloys raised from 73% with the quenching rate of 5m/s to 92% with the quenching rate of 30m/s after 100 charge/discharge cycles. High-rate dischargeability (HRD) of the alloy electrodes showed a trend of declining with the increasing of quenching speed.The phase structures of the La_1-xCe_xNi_4.17Mn_0.93Al_0.1Fe_0.4(x=0-0.5) rapid quenching over-stoichiometric rare earth hydrogen storage alloys are only composed of CaCus type phase, and with the increase of Ce, the unit cell volume decrease linearly. With the increase of Ce, the hydrogen absorption capacity of the alloys reduced and platform pressure increased. Electrochemical testing and analysis results show that with the increase of Ce, the activation performance and discharge capacity of the alloy electrodes decreased, however, the electrode cycle life improved significantly. For the alloys with x=0 and x=0.5, the largest discharge capacity reach 328.9 and 305.4mAh/g respectively; After 100 charge/discharge cycles, the rates of discharge capacity retention(S₁₀₀) raised from 80%( x=0) to 92-96%; High-rate dischargeability (HRD) of the alloy electrodes showed a trend of declining first when x<0.3, and rising when x≥0.3.The phase structures of the LaNi_4.27Mn_0.93Al_xFe_0.4-x(x=0.1 - 0.3) rapid quenching over-stoichiometric rare earth hydrogen storage alloys are only composed of CaCu₅ type phase, and with the increase of Al, the lattice parameters a, c value and the unit cell volumeâ…¤of melt-spun alloy phase CaCu₅ are increasing. With the increase of Al, the hydrogen absorption capacity of the alloys reduced and platform pressure decreased.Electrochemical testing and analysis results show that with the increase of Al, the discharge capacity of the alloy electrodes decreased, But with x=0 and x=0.2, the discharge capacity of the alloy electrodes than more the discharge capacity of LaNi_4.17Mn_0.93Al_0.1Fe_0.4(15m/s). For the alloys with x=0 and x=0.3, the largest discharge capacity reach 340 and 326mAh/g respectively , the electrode cycle life improved significantly. After 100 charge/discharge cycles, the rates of discharge capacity retention(S₁₀₀) raised from 65%( x=0.1) to 78%; High-rate dischargeability (HRD) of the alloy electrodes showed a trend of declining with the increasing of quenching speeds.Through the systematical investigation, the cycling stability of Co-free AB₅-type electrode alloys has been resolved preferably. The over-stoichiometric quenched alloys electrode we developed can exhibit higher discharge capacity, good cyclic stability and better overall electrochemical properties.