Regulation of the discharge reservoir of negative electrodes in Ni–MH batteries by using Ni(OH) (x= 2.10) and γ-CoOOH
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Depth of discharge
Depth of discharge
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Chronoamperometry
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Graphite-based electrodes were prepared using synthetic graphite (MCMB 1028) or natural graphite (NG) powder using a dimensionally stable anode (DSA) as a substrate. Their electrochemical properties were investigated in vanadiumbased electrolytes to determine how to increase the durability and improve the energy efficiency of redox flow batteries. Cyclic voltammetry (CV) was performed in the voltage range of -0.7 V to 1.6 V vs. SCE at various scan rates to analyze the vanadium redox reaction. The graphite-based electrodes showed a fast redox reaction and good reversibility in a highly concentrated acidic electrolyte. The increased electrochemical activity of the NG-based electrode for the $V^{4+}/V^{5+}$ redox reaction can be attributed to the increased surface concentration of functional groups from the addition of conductive material that served as a catalyst. Therefore, it is expected that this electrode can be used to increase the power density and energy density of redox flow batteries.
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Scanning Electrochemical Microscopy
Reactivity
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The pasted foam-nickel electrodes were prepared with various amounts of the addition of cobalt metallic powder to nickel hydroxide powder. Constant current charge/discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrochemical performances of the pasted nickel electrodes. Results show that in comparison with that of the nickel electrode without cobalt addition, the nickel electrodes with cobalt powder addition exhibit better performances in the specific discharge capacity, active material utilization, resistance to swelling with cycling and cycle stability. Meanwhile, the cobalt addition also lowers the discharge potential of the pasted nickel electrodes, so the amount of cobalt addition to the nickel electrodes should be controlled, and an appropriate amount of addition is 3%~5%(mass percentage). The superior electrochemical performances of the nickel electrodes with the cobalt addition can be attributed to the higher reaction activity and lower electrochemical impedance.
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The effects of nickel powders on the electrochemical performance of Ni(OH)2 electrode were studied by constant charge/discharge,cyclic voltammetry(CV) . And the structure changes of the nickel powders during charge/discharge were characterized with X-ray diffraction(XRD). The results showed that the nickel powder was oxidized into β-Ni(OH)2 in alkaline solution which made contribution to the discharge capacity of the electrode. At the same time,the oxidization decreased the conductivity of the nickel powders and reduce the discharging effect of the electrode. It was suggested that the specific capacity of Ni(OH)2 could be corrected by excluding the contribution of the nickel powder at the 4th to 6th charging-discharging cycles to obtain a precise discharge capacity.
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Electrochemical impedance spectroscopy measurements under a smal alternating current were performed to investigate the electrochemical reactions on poly-crystalline nickel surfaces in nickel sulfamate electrolyte. The Nyquist diagrams exhibit the absence of inductive loops for the frequency range from 5MHz to 100 kHz. A condition for the existence of inductive loops derived in this study suggests that the electrocrystallization of Ni2+ions may take place in the slow reaction step Ni2++e-→ Ni+adsand fast reaction step Ni+ads+e-→Ni. In addition, the reaction impedances are found to have two exponents that slightly deviate from the values predicted by a homogeneous charge-transfer reaction model.
Nyquist plot
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