Charge transfer and mass transfer reactions in the metal hydride electrode

Charge transfer reaction across the electrode/electrolyte interface and hydrogen diffusion in the negative MH alloy electrode dominate the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. The mass transfer process in the MH electrode mainly involves hydrogen diffusion in the bulk MH alloy. The charge transfer reaction in the negative electrode reflects the capability of hydrogen reduction and oxidation reactions at the surface of the MH alloy powder. In this study, an AB₅-type hydrogen-absorbing alloy was used as the negative electrode material. The rate-determining mass transfer process in the bulk MH alloy electrode was studied and analyzed using anodic polarization measurements. The exchange current density, which is related to the charge transfer reaction, was analyzed by using the hydrogen equilibrium pressure. The estimation of hydrogen diffusion coefficient in the MH alloy is strongly dependent on the value of the effective reaction area of charge transfer reaction at the surface of the alloy powder.     

Electrochemical characteristics of the interface between the metal hydride electrode and electrolyte for an advanced nickel/metal hydride battery

The characteristics of the negative electrode of a Ni/MH (metal hydride) battery are related to the charge transfer and mass transfer processes at the interface between the MH electrode and the electrolyte. With increasing number of charge/discharge cycles, the MH alloy powders micro-crack into particles that are several microns in diameter and this then influences the exchange current density. A polarization experiment was used to analyze the charge transfer and mass transfer processes. The exchange current densities of uncoated and Pd-coated Mm_0.95Ti_0.05Ni_3.85Co_0.45Mn_0.35Al_0.35 alloy electrodes increase with increasing number of charge/discharge cycles before reaching a constant value after 20–30 cycles.     

Electrochemical and kinetic properties of as-cast and quenched CeMg11Ni hydrogen storage alloys

采用感应熔炼法制备CeMg₁₁Ni铸态合金，对部分铸态合金二次重熔并经过快速凝固处理获得快淬态合金，对快淬态合金进行X射线衍射表征（XRD），运用高分辨透射电镜（HRTEM）技术分析合金的微观形貌特征及晶态结构，并通过充放电容量及其循环稳定性分析、交流阻抗谱测试和动电位极化曲线对合金CeMg₁₁Ni的性能进行详细研究。研究结果表明：铸态合金由多相结构组成，快淬工艺可促使合金内部形成大量非晶纳米晶结构、改善合金活化性能与电化学循环稳定性、减少合金氢化物释放焓值、降低合金表面活化能、增强电荷传递反应同时增强合金内部氢原子扩散能力，进一步解释了合金拥有较强电化学反应动力学性能原因。     

Investigations on kinetics properties of hydrogen adsorbing/desorbing reactions for metal hydride electrodes

As we know, the kinetics properties of hydrogen adsorbing/desorbing reactions for metal hydride electrodes are determined by the rates of the charge transfer, hydrogen transfer and hydrogen diffusion reactions. In previous studies, not only the hydrogen transfer process was always ignored, but also the values of the kinetics parameters for the charge transfer and hydrogen diffusion processes were quite different. Therefore, the purpose of this work is to investigate the key issues of the kinetics properties for hydrogen adsorbing/desorbing reactions. Firstly, the hydrogen transfer process was thoroughly studied by electrochemical impedance spectrum (EIS) method, in which it emphasized the corresponding relationship between the electrode process of hydrogen adsorbing/desorbing reactions and the time constants presented in different frequency regions of impedance spectrum. The values of the hydrogen transfer resistance were calculated as a function of depth of discharge (DOD). Meanwhile, almost all the electrochemical techniques including linear polarization curves (LP), constant potential step method (CPSM), galvanostatic intermittent titration technique (GITT), cyclic voltammetry method (CV) and EIS were used to measure the kinetics parameters for the charge transfer and hydrogen diffusion processes. Moreover, the factors causing the discrepancy of the kinetics parameters were analyzed in detail.     

有机溶剂浸泡法回收再利用锂离子电池钛酸锂负极材料

利用有机溶剂法回收了废旧锂离子电池中的钛酸锂负极材料,并对回收的钛酸锂材料的结构、形貌和电化学性能进行了测试。XRD结果表明,材料除炭后添加适量锂源进一步合成得到的产物具有尖晶石结构,且不含其他的杂质。SEM图像显示,其颗粒分布均匀、无团聚现象。EIS结果表明,最终回收的钛酸锂电极材料比未添加锂源进行煅烧处理的材料具有较小的电荷转移阻抗和较高的锂离子扩散系数。在0.1 C倍率下,经过100次循环后其容量保持率为92.4%,具有优异的循环稳定性和可逆性,可以实现循环利用。     

The electrochemical properties of /polyaniline composites

The La_0.7Mg_0.25Ti_0.05Ni_2.975Co_0.525 (AB₃) alloy was modified with different contents of polyaniline (PANI) through ball milling. XRD, SEM and FTIR were used to characterize the properties of the AB₃/x PANI composites (x=1, 2, 3 and 4 wt%). The effects of PANI on the electrochemical properties of AB₃ alloy electrode were studied by charge–discharge, electrochemical impedance spectroscopy (EIS), linear polarization (LP) and potentiostatic discharge experiments. The XRD, SEM and FTIR results showed that ball milling did not change the characterizations of PANI and AB₃ alloy but decreased the average particle size of AB₃ alloy. The charge–discharge results indicated that the maximum discharge capacity of AB₃ alloy electrode decreased with the addition of PANI. However, the discharge cycle stability of AB₃/PANI composite electrodes increased firstly and then decreased with the increase in PANI content. The EIS and LP curves showed the same trends with the discharge cycle stability. The hydrogen diffusion coefficients of AB₃/PANI composites were estimated from the potentiostatic discharge curves, indicated the opposite trend with the discharge cycle stability. When the PANI content was equal to 3 wt%, the AB₃/PANI composite electrode exhibited an optimal electrochemical kinetic property.     

Preparation and electrochemical characteristics of LiNi1/3Mn1/3Co1/3O2 coated with metal oxides coating

LiNi_1/3Mn_1/3Co_1/3O₂ prepared by a spray drying method exhibited poor cyclic performance when it was operated at rates of 0.5C and 2C in 3–4.6 V. A metal oxide (ZrO₂, TiO₂, and Al₂O₃) coating (3 wt%) could effectively improve its cyclic performance at both 0.5C and 2C. Electrochemical impedance spectroscopy (EIS) studies suggested that both the surface resistance and the charge transfer resistance of the bare LiNi_1/3Mn_1/3Co_1/3O₂ significantly increase after 100 cycles, whose origin is mainly related to the change in both the particle surface and electrode morphologies. The presence of a thin metal oxide layer could remarkably suppress the increase in the total resistance (sum of the surface resistance and the charge transfer resistance), which was attributed to the improvement in good cyclic performances.     

The effect of annealing on the electrochemical properties of Zr0.5Ti0.5Mn0.5V0.3Co0.2Ni1.1 alloy electrodes

The annealing treatment was found to result in the improvement in the cyclic stability but the degradation of discharge capacity, activation and high-rate dischargeability for Zr_0.5Ti_0.5Mn_0.5V_0.3Co_0.2Ni_1.1 alloy electrode. A lower discharge potential in the annealed alloy electrode was found owing to a more homogeneous alloy, which is consistent with the pressure–composition isotherms (P–C–T) measurement. We found that the annealed alloy also had lower and flatter pressure plateaus, and larger pressure hysteresis. At high discharge rates, the hydrogen diffusion in the bulk of the alloy was the rate-determining step. The diffusion coefficients for hydrogen in the annealed and as-cast alloys were calculated to be 1.4×10⁻¹² cm² s⁻¹ and 4.3×10⁻¹² cm² s⁻¹, respectively. The lowering of high-rate discharge capacity can be ascribed to the reason that the hydrogen diffusion coefficient is lower due to homogeneous microstructure in the annealed alloy.     

Electrocatalytic characteristics of the metal hydride electrode for advanced Ni/MH batteries

The electrocatalytic characteristics of a metal hydride (MH) electrode for advanced Ni/MH batteries include the hydrogen adsorption/desorption capability at the electrode/electrolyte interface. The hydrogen reactions at the MH electrode/electrolyte interface are also related to factors such as the surface area of the MH alloy powder and the nature of additives and binder materials. The high-rate discharge capability of the negative electrode in a Ni/MH battery is mainly determined by the mass transfer process in the bulk MH alloy powder and the charge transfer process at the interface between the MH alloy powder and the electrolyte. In this study, an AB₅-type hydrogen-absorbing alloy, Mm (Ni, Co, Al, Mn)_5.02 (where Mm denotes Mischmetal, comprising 43.1 wt.% La, 3.5 wt.% Ce, 13.3 wt.% Pr and 38.9 wt.% Nd), was used as the negative MH electrode material. The MH electrode was charged and discharged for up to 200 cycles. The specific discharge capacity of the alloy electrode decreases from a maximum value of 290–250 mAh g⁻¹ after 200 charge/discharge cycles. A cyclic voltammetry technique is used to analyze the charge transfer reactions at the electrode/electrolyte interface and the hydrogen surface coverage capacity.     

Characterization and performance of (La,Ba)(Co,Fe)O3 cathode for solid oxide fuel cells with iron–chromium metallic interconnect

(La_0.6Ba_0.4)(Co_0.2Fe_0.8)O₃ (LBCF) is synthesized by a sol–gel method as a Cr-tolerant cathode for intermediate-temperature solid oxide fuel cells (ITSOFCs). The electrochemical performance and Cr deposition process for the O₂ reduction reaction on LBCF cathodes in the presence and absence of a Fe–Cr alloy interconnect are investigated in detail, in comparison with a (La,Sr)(Co,Fe)O₃ (LSCF) electrode. Cr deposition occurs for the O₂ reduction reaction on LBCF electrodes in the presence of Fe–Cr alloy. Very different from that observed for the reaction on the LSCF cathode, Cr deposition on the LBCF electrode/gadolinia-doped ceria (GDC) electrolyte system is very small and shows little poisoning effect for O₂ reduction on LBCF electrode. The results demonstrate that the LBCF electrode has a high resistance towards Cr deposition and high tolerance towards Cr poisoning.     

Effects of surface treatments of MlNi4.0Co0.6Al0.4 hydrogen storage alloy on the activation,charge/discharge cycle and degradation of Ni/MH batteries

The effects of the surface treatment of the hydrogen storage alloy on the activation property and cycle life of nickel/metal-hydride (Ni/MH) batteries were investigated by means of the electrochemical impedance spectra. It was found that the oxide layer on the alloy surface affected its electrochemical properties and catalysis for the oxygen combination. Therefore, Ni/MH battery employed the untreated alloy as negative electrode material exhibited bad activation property, short cycle life and high internal pressure. Because of the improvement in the metal hydride electrode electrochemical characteristics and catalysis for oxygen recombination by the surface treatment of the alloy in 0.02 M KBH₄+6 M KOH or 6 M KOH solution, the battery used the treated alloy as negative exhibited good activation, long cycle life and low internal pressure. The composition and dissolution of the alloy surface were analyzed by an electron probe microanalysis (EPMA) and induced coupled plasma spectroscopy (ICP). It was found that the Ni-rich surface layer was an important factor to improve the activation and cycle life of battery.     

Effects of pretreatment of LM-Ni3.9Co0.6Mn0.3Al0.2 alloy powders in a KOH/NaBH4 solution on the electrode characteristics and inner pressure of nickel-metal-hydride secondary batteries

The discharge capacities of lanthanum-rich mischmetal (LM)-Ni_3.9Co_0.6Mn_0.3Al_0.2 alloy electrodes are significantly degraded by an increase in the C rate. Nevertheless, the discharge capacity of alloy electrodes pretreated with KOH/NaBH₄ is maintained higher than that of raw alloy electrodes, with the difference in discharge capacities between the raw and pretreated alloy electrodes more prominent at higher C rates. The charge retention of the electrodes decreases with increasing rest time. In particular, the charge retention of the pretreated alloy electrode is lower than that of the raw alloy electrode due to the higher self-discharge rate. The overvoltage for hydrogen evolution of the pretreated alloy electrode is superior to that of the raw alloy electrode, particularly at higher temperatures. This phenomenon indicates that the charge efficiency of the electrode was significantly improved by the surface pretreatment, resulting from its high surface catalytic activity. Repeated charge-discharge increases the inner pressure of the battery. Nevertheless, due to its higher charge efficiency and faster recombination rate, the inner pressure of the battery made using the pretreated alloy electrode is much smaller than that of the battery made using a raw alloy electrode.     

Effect of particle size on the electrochemical properties of MmNi3.8Co0.75Mn0.4Al0.2 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.     

A paste type negative electrode using a MmNi5 based hydrogen storage alloy for a nickel–metal hydride (Ni–MH) battery

Different conducting materials (nickel, copper, cobalt, graphite) were mixed with a MmNi₅ type hydrogen storage alloy, and negative electrodes for a nickel–metal hydride(Ni–MH) rechargeable battery were prepared and examined with respect to the discharge capacity of the electrodes. The change in the discharge capacity of the electrodes with different conducting materials was measured as a function of the number of electrochemical charge and discharge cycles. From the measurements, the electrodes with cobalt and graphite were found to yield much higher discharge capacities than those with nickel or cobalt. From a comparative discharge measurements for an electrode composed of only cobalt powder without the alloy and an electrode with a mixture of cobalt and the alloy, an appreciable contribution of the cobalt surface to the enhancement of charge and discharge capacities was found.     

Principle and application of electrochemical impedance spectroscopy method

电化学阻抗测定技术是一种研究电极反应动力学和电化学体系中物质传递与电荷转移的有效方法,通过电化学阻抗数据所提供的信息,能够分析电极过程的特征,包括动力学极化,欧姆极化和浓差极化,为电化学过程设计,电极材料开发和电极结构研究提供基本依据.本文在介绍电化学阻抗谱技术原理的基础上,分别以液流电池,空气扩散电极过程为对象,阐述电化学阻抗谱中电荷转移,物质传递等过程以及多孔电极本身的电荷传递电阻等,并综述阻抗技术在设计电池结构,优化电极材料等方面的应用实例.     

Impedance measurements of air-oxidized and H2-annealed Raney-Ni catalysts for alkaline fuel cells

The catalytic properties and the long-term performance of Raney-NiTi2 catalysts being used in H₂ electrodes of alkaline H₂---O₂ fuel cells can be significantly improved by slow air oxidation and subsequent annealing in an H₂ atmosphere at 300°C. Individual reaction steps are investigated by means of impedance measurements. Theoretical estimations, on the basis of a simple equivalent circuit of a supported electrode, result in a frequency-response relationship which is in very good agreement with the experimental data referring to the relevant frequency range of 10⁻³−10⁻¹ Hz. A method to evaluate the impedance spectra is described in some detail. Calculated and measured impedance data are in good agreement, thus indicating the validity of the charge transfer resistance, the diffusion resistance, as well as the chemisorption capacity and the double-layer capacity. Experiments on the influence of catalyst annealing in an H₂ atmosphere at 350°C show a strong increase in the charge transfer resistance and an obvious decrease in the diffusion resistance, depending on the annealing time. A similar influence on the chemisorption capacity and the double-layer capacity is not observed.     

Improvement of the catalytic properties of Ti-doted Raney-Nickel for H2-Anodes of alkalione fuel cells by Ni(OH)2-surface coating

This paper belongs to a series of three dealing with the latest improvements in the alkaline H₂---O₂ fuel cells operating under mild conditions thanks to their Raney-Ni-catalysts. The first of these papers describes the benefication of a Ni(OH)₂ surface coating on the catalytic activity of Ti-doted Raney-Ni in supported electrodes. This Ni(OH)₂ surface coating is produced by carefully optimized oxidation. A Ni(OH)₂-content of 5 up to 6wt% increases the attainable current density by the factor 3–4. In addition, the exchange current density is markedly enlarged up to a Ni(OH)₂-fraction by 5%, but remains unchanged when further increasing the Ni(OH)₂ percentage. Thus, one may conclude that the Ni(OH)₂ surface coating improves markedly the charge exchange reaction. On the other side, the surface diffusion of H-atoms on the pore walls to the location of charge exchange reaction is hindered by too much Ni(OH)₂.     

Carbon nanotube buckypaper/MmNi5 composite film as anode for Ni/MH batteries

An efficient and cost-effective method has been developed to produce high quality buckypapers from multi-walled carbon nanotubes. The buckypapers were then used as a substrate for AB₅ hydrogen storage alloy electrodes, and electrochemical performances of these composite films in Ni/MH batteries have been investigated. AB₅ alloy was coated on the buckypaper using magnetron sputtering. The buckypapers prepared by our approach were thin, highly flexible and provided sufficient strength as substrates for the hydrogen storage alloy film. A good contact between the buckypaper and the MmNi₅ alloy was established. The electrochemical results show that the buckypapers can be a versatile replacement for conventional metal substrates for the anode in Ni/MH batteries. They provide exceptional electrical conductivity and significant reduction in weight and cost. The obtained maximum discharge capacity of 276 mAh/g for BM-1 electrode is higher than what was previously obtained on electrode with metal substrate of 220 mAh/g. Amongst the two different thickness of AB₅ film studied, it was found that the reduction of MmNi₅ layer thickness enhanced the discharge capacity of the electrode, but the high rate discharge capability was irrelevant to the film thickness. However, the thicker film exhibits better chargeability and cycle stability. Thus all these are beneficial for the miniaturisation of the Ni/MH batteries.     

First-principles analysis of electrochemical hydrogen storage behavior for hydrogenated amorphous silicon thin film in high-capacity proton battery

Silicon material electrodes as proton carriers for high-capacity proton battery have only been proposed for such a short period of time that their physicochemical properties and electrochemical hydrogen storage behavior during charge and discharge processes remain nearly uncharted territory. Herein, the hydrogenated amorphous silicon (a-Si:H) thin film electrodes are prepared by radio frequency sputtering followed by ex-situ hydrogenation. The electrochemical properties of a-Si:H electrodes are tested experimentally, and the electrochemical hydrogen storage behaviors of a-Si:H electrodes are analyzed by first-principles calculations. The results show that the hydrogenation process significantly increases the electrochemical capacity of the electrodes and reduces the band gap of the electrode structure. The electrode exhibits weak conductivity during the initial charging, but the instability of the electrode electronic structure during the later charging results in a slight fluctuation of the electrochemical charging process. The a-Si:H electrode have better electrochemical hydrogen storage/release reversibility than non-hydrogenated electrodes, but this reversibility is weakened by oxygen atoms covered on the electrode surface. The electrochemical hydrogen storage process is easier to accomplish than the electrochemical desorption process of hydrogen evolution reaction for the a-Si:H electrodes. The a-Si:H thin film electrode is more stable on the Ni(111) substrate surface and the good conductivity of the electrode/substrate interface provides convenient conditions for the free transport of electrons in the electrochemical charge/discharge processes. We believe that these results perfectly explain the microscopic mechanisms responsible for the electrode reaction and electrochemical behavior of a-Si:H electrodes in this type of proton battery, and have a certain reference value in understanding the physicochemical properties and electrochemical hydrogen storage behavior of silicon material electrodes applied to other types of batteries during charge/discharge processes.     

Electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 hydrogen storage alloy

The electrochemical properties of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 hydrogen storage alloy have been studied through chronopotentiometric, chronoamperometric and cyclic voltammogram measurements. The maximum capacity value obtained was 265 mAh g⁻¹ at rate C/6 and the capacity decrease was recorded by 1.5% after 30 cycles. The values of the hydrogen diffusion coefficient D_H obtained through cyclic volammogram and chronoamperometric techniques were, respectively, 7.01 × 10⁻⁸ cm² s⁻¹ and 4.23 × 10⁻¹¹ cm² s⁻¹.