Dynamic Characteristic of an Electrochemical Cell with Gauze Electrodes in Convective Diffusion Conditions

A frequency dependence of the transfer function of an electrochemical cell is studied experimentally in the frequency span 0.02 to 40 Hz under conditions of controlled convective diffusion. Above the diffusion frequency, experimental data nicely conform to theoretical calculations, but below it the function's decay cannot be explained within existing theoretical notions.     

Convective Current in a Four-Electrode Electrochemical Cell at Various Boundary Conditions at Anodes

The frequency dependence of the transfer function of an electrochemical cell is studied experimentally and theoretically in the frequency region 0.005 to 1 Hz under conditions of controlled convective diffusion, at various boundary conditions on the anodes. It is shown that the results are independent of the conditions on the anodes at frequencies exceeding a diffusion frequency. On the other hand, the effect of the boundary conditions al low frequencies is substantial. In particular, it is shown that it is feasible to design a cell with a conversion efficiency indefinitely increasing in the direction of lower frequencies.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 987–992.Original Russian Text Copyright © 2005 by Agafonov, Nesterov.     

钠离子电池磷酸盐正极材料研究进展

近年来,钠离子电池因其原材料丰富、资源成本低廉及安全环保等突出优点,在电化学规模储能领域和低速电动车中具有广阔的应用前景。聚阴离子型磷酸盐具有稳定的框架结构、合适的工作电压和快速的离子扩散路径等特征,是一类极具研究价值和应用前景的钠离子电池正极材料。但是,磷酸盐正极材料电子导电性差和比能量偏低等缺陷限制了其走向实际应用。研究工作者通过体相结构调控和微纳结构设计等手段进行改性研究,旨在提升磷酸盐正极材料的性能表现、推动钠离子储能体系的研究开发。本文综述了钠离子电池磷酸盐正极材料的最新进展,包括正磷酸盐、焦磷酸盐、氟磷酸盐和混合磷酸盐化合物,通过对磷酸盐材料的晶体结构、储钠机理和改性策略等方面的综述,揭示材料成分、结构与电化学性能之间的本征关系,为聚阴离子磷酸盐正极材料的持续改性和新型磷酸盐高压正极材料的探索开发提供指导。     

PEMFC阴极中氧扩散和电化学反应模型研究—I．TFFA模型及其数值计算

将薄膜浸渍聚集体（TFFA）模型用于描述质子交换膜燃料电池（PEMFC）阴极中氧的扩散和反应过程，其中包括氧气在气体扩散层和反应层气体通道中的扩散，氧气在反应层薄膜中的溶解和扩散，氧在反应层浸渍聚休体中的扩散和反应以及电子和离子的传导，并根据PEMFC阴极的结构特点给出TFFA模型的数值解法。     

瞬时离子释放扩散模型及熔盐电解固态WS_2制备纳米W的液相扩散研究

固态化合物熔盐电解冶金在21世纪初被提出后受到学术界和工业界的广泛关注.根据固态化合物电解的动态三相电化学界线模型,固态金属氧(硫)化物阴极在电解还原过程中,涉及O2?(S2?)在电解生成的多孔金属层中的液相扩散,但由于一直以来缺乏方便可靠的理论和实验方法,相关传质过程动力学的研究鲜有文献报道.本文引入多孔电极瞬时离子释放扩散模型,以粉末微腔电极为微型多孔电极,设计双电势阶跃实验研究了WS2在等摩尔比NaCl+KCl熔盐中电解时固态阴极中的液相扩散.实验结果与理论模型符合良好,973 K时,硫离子在孔隙率为69%的多孔金属钨层中的扩散系数为0.92×10?5 cm2/s,扩散活化能为53.4 kJ/mol.研究表明,二硫化钨在NaCl+KCl混盐体系中能够快速电解还原生成纳米金属钨,其中,S2?的扩散传质是整个电解过程的速度控制步骤.     

Electrochemical noise of a hydrogen-air polymer electrolyte fuel cell operating at different loads

The electrochemical noise of a polymer membrane hydrogen-air fuel cell operating at different load currents was measured in serial experiments. Spectral power densities of the noise are shown to be divided into three regions. At frequencies greater than 3–10 Hz, the spectrum dependence has a constant slope of ??2 in the bilogarithmic coordinates. At frequencies 0.3–5 Hz, there is a horizontal plateau in which length is determined by the value of a load. At frequencies less than 0.3 Hz, the dependence of spectral power density has a slope of ??2. Medium-frequency plateau and high-frequency slope of spectral power densities of the noise were approximated by model RC circuits. The values of Faradic resistance and double-layer capacitance connected in parallel were obtained from the electrochemical impedance data. At load voltages higher 0.5 V, the height of the plateau was shown to be proportional to the 2.68 power of the load current value.     

锂离子电池正极材料中的极化子现象理论计算研究进展

In addition to their extensive commercial application in electronic devices such as cell phones and laptops, lithium-ion batteries (LIBs) are most suitable to fulfill the energy storage requirements of electric vehicles because of their recognized safety, portability, and high energy density. Cathodes are the most important part of LIBs, and various cathode materials have been widely investigated over the past decades. Polaron formation has been attracting increasing attention in the research of cathode materials, as it limits electron conduction. In particular, polarons are responsible for low electronic conductivity in cathode materials like olivine phosphate. Polaron is a typical crystal defect caused by the integrated motion of lattice distortion and its trapping electrons. Research on the mechanism of polaron formation will provide theoretical guidance for the design of high-electronic-conductivity cathode materials and improvement of the electrochemical performance of LIBs. Theoretical calculation is a direct and important method to study polaron formation in a specific crystal material, because the presence of polarons and their formation mechanisms can be effectively verified through this method. In this article, we first introduce the basic physical concept of polarons and their dynamical model according to the Marcus and Emin-Holstein-Austin-Mott theories. A comparison of the general properties of large and small polarons, summarized in this chapter, reveals that small polaron formation more likely occurs in cathode materials. Moreover, the theoretical characterization, electrical impact, control and challenges of polarons are reviewed. Although a universal necessary and suitable condition for the theoretical characterization of polarons has not yet been found, we still propose three criteria that are proven to be feasible and practical for the theoretical identification of polarons when applied in combination. Experimental characterizations are also introduced briefly for reference, because the comparison with the experiment is suggested to be necessary and mandatory. The electrical impact caused by polarons results in low electronic conductivity, which has been broadly reported in layered, olivine, and spinel cathode materials. Doping can weaken the influence of polarons and, thus, significantly enhance the electronic conductivity, thereby becoming the most prevalent strategy for tuning polarons. Although theoretical calculations have been widely and effectively conducted in the study of polarons, some challenges may still be faced because of the intrinsic shortcomings of the traditional density functional theory, which need to be addressed. Finally, further research on polarons from the perspective of basic theory and practical applications is prospected.     

Pre-potassiated hydrated vanadium oxide as cathode for quasi-solid-state zinc-ion battery

Zinc-ion batteries（ZIBs）, in particular quasi-solid-state ZIBs, occupy a crucial position in the field of energy storage devices owing to the superiorities of abundant zinc reserve, low cost, high safety and high theoretical capacity of zinc anode. However, as divalent Zn²⁺ions experience strong electrostatic interactions when intercalating into the cathode materials, which poses challenges to the structural stability and higher demand in Zn²⁺ions diffusion kinetics of the ...     

测氢扩散系数的电化学交流法研究

应用电化学交流法测定氢在纯铁中的扩散系数 .证明对电解氢在试样中的非稳态扩散 ,按菲克第二定律所设定的边界条件符合实际扩散行为 ,并论证了该方法的可行性     

Electrochemical Systems Based on Sol-Gel Silica Matrix Impregnated with Organic Solvent

Two electrochemical systems based on sol-gel silica matrix impregnated with organic solvent were prepared and studied. The first one is composed of tetramethylorthosilicate based material filled with ferrocene solution in polar solvent: propylene carbonate. Electrodes are immersed in this solid electrolyte during all stages of sol-gel process. Despite of the lack of the extra added salt, by using ultramicroelectrode, undistorted electrochemical signal corresponding to the electrooxidation of the ferrocene was obtained. Its diffusion coefficient within the sol-gel matrix depends on the time elapsed after gelation and it is not much below that in salt solution in the same solvent. The second system is based on graphite dispersion in hydrophobic sol-gel silicate matrix. This material was filled with mixture of liquid butylferrocene and hexadecane. After immersion in aqueous salt solution it serves as working electrode. The electrochemical signal corresponding to the electrooxidation of the butylferrocene within organic phase was obtained. Probably the electrode process occurs at three phase (carbon/organic phase/aqueous phase) junction and it is accompanied by anion transfer through the liquid-liquid interface.     

固体氧化物燃料电池电化学阻抗谱差异化研究方法和分解

电化学阻抗谱技术(EIS)在固体氧化物燃料电池(SOFC)中已获得广泛应用。在EIS分析过程中,研究者能够清楚地获得燃料电池内部因纯离子(电子)导电引起的欧姆电阻和因电化学过程、扩散作用引起的极化阻抗的大小,但是对于极化阻抗的构成缺乏进一步解析。本文选用传统的Ni-YSZ阳极支撑电池,通过改变测试温度、阳极运行气氛和阴极运行气氛,设计了一套完整的阻抗差异分析(ADIS)实验。并基于弛豫时间分布法(DRT)和阻抗差异分析法,系统地分析并解释了阻抗谱中各频率段对应阻抗的物理或(电)化学含义,将该类型电池阻抗谱以6个RQ并联电路予以拟合,为之后燃料电池性能稳定性的研究奠定基础。     

Model for the multicomponent gas diffusion in a fuel cell porous electrode

A simple analytically solvable model for the diffusion of a multicomponent vapor-air mixture is proposed. The model, which accurately accounts for the oxygen and water balance on the gas diffusion cathode of a fuel cell, is used for computing polarization characteristics of a fuel cell with a polymer electrolyte. An analytical solution for the cathodic overvoltage in the extreme cases of high and low current densities is derived. The results are compared with the available theoretical and experimental data. It is shown that the solutions of the proposed model coincide with the solutions provided by the Bernardi-Verbrugge model, which is far more involved.     

LiF-BaF2-LiCl熔盐体系中Li+的电化学行为

运用三电极在电化学工作站AUTOLAB以循环伏安法、计时电流法和计时电位法研究了LiF-BaF₂-LiCl熔盐体系中1203 K温度下锂在钨（W）电极上的电化学还原过程及其控制步骤. 结果表明,Li⁺在W电极上的还原过程是一步得电子的准可逆反应,析出电位在-1.0 V附近. 阴极过程受离子的扩散步骤控制,计算得出扩散系数为4.5 × 10^-6 cm²•s^-1.     

Na掺杂对LiFePO4/C复合正极材料的结构和倍率性能的影响

以Fe(NO3)3,LiNO3,NH4H2PO4和NaNO3为原料,采用简单的液相-碳热还原法合成Li0.97Na0.03FePO4/C复合正极材料.使用X射线衍射(XRD)、扫描电子显微镜(SEM)和充放电等测试技术研究了材料的结构及倍率充放电性能.通过循环伏安(CV)曲线和电化学阻抗谱(EIS)研究电极反应过程中的动力学特点.结果表明,Na掺杂形成了具有橄榄石结构的Li0.97Na0.03FePO4固溶体,并增大了晶格中Li+一维扩散通道,使LiFePO4/C的电荷转移电阻减小了约2/3,Li+扩散系数提高了3～4倍.因此,Li0.97Na0.03FePO4/C首次放电比容量在0.1 C和2 C倍率下分别达到152 mAh g-1和109 mAh g-1,比未掺杂的LiFePO4/C的放电比容量分别提高了4.83%和62.69%.     

Solid-state electrochemical behavior of Keggin-type borotungstic acid single crystal

We demonstrate the electrochemical characteristics of the, poorly described in the literature, mixed-valence proton conducting solid borotungstic acid single crystal, H₅BW₁₂O₄₀ xH₂O, in the absence of liquid electrolyte phase. We performed electrochemical measurements in an all-solid cell with a gold fiber ultramicrodisk (diameter 10, 25, and 40 μm) working electrode, a silver semi-reference disk electrode, and a glassy carbon ring counter electrode. Diagnostic experiments at different scan rates aimed at probing the model of mass transport and potential kinetic limitations. Such bulk parameters as the effective diffusion coefficient of charge propagation and the concentration of mixed-valence redox centers were determined by two methods. The first method is based on the analysis of both Cottrellian and steady-state currents (the mixed-regimes method), and the second method provides the true diffusion coefficient (transport coefficient free of the migration influence) for both the substrate and the product of the electrode reaction. Together, these methods constitute a double potential step chronoamperometry experiment. The data obtained with these electrochemical experiments (effective diffusion coefficients, concentration of mixed-valence redox centers, etc.) can support the results obtained with other techniques (XRD, FTIR, and TGA).     

导电聚合物/贵金属复合材料应用于C1小分子电催化氧化

低温燃料电池作为一种新型的能源装置,具有能量转换效率高、工作温度低、无污染、液体燃料处理简单、启动迅速等诸多优点,已成为世界各国竞相研究的热点。有机小分子的高效电催化氧化直接关系到低温燃料电池的发展和应用。低温燃料电池的电极材料主要是碳/贵金属复合材料,碳载体易导致贵金属粒子团聚、且易发生电氧化腐蚀等缺点降低了贵金属的利用率和电池的使用寿命。导电聚合物具有高的抗腐蚀性、高的表面积、低电阻和高稳定性得到很大关注。本文综述了近年来国内外导电聚合物/金属复合电极材料在燃料电池中的研究进展。     

Lithium deintercalation/intercalation processes in cathode materials based on lithium iron phosphate with the olivine structure

This review describes the current state of the studies of lithium deintercalation/intercalation processes in cathode materials based on lithium iron phosphate with olivine structure. The limiting factors of LiFePO₄ charge/discharge processes, as well as the main methods for their acceleration are considered. A partial replacement of iron cations in the structure improves the electrochemical characteristics of the cathode materials, including the discharge capacity, charge/discharge rate, and, in some cases, changes the charge/discharge mechanism. The use of nanoscale phosphate LiFePO₄ with the olivine structure considerably increases the charge/discharge rate of cathode materials based on it by reducing the diffusion path length. Methods for LiFePO₄ surface modification are considered. Particular attention is paid to the development of composite materials with electron-conducting additives. Combining of various approaches to the modification of the material in question makes it possible to obtain materials with a discharge capacity close to the theoretical value (170 mA h g^–1) at a low charge/discharge rate and to considerably increase its capacity at high charge and discharge currents.     

聚苯胺纳米管在阳极氧化铝模板中电聚合的生长机理

利用阶跃电位、循环伏安等电化学方法，应用金属电沉积理论,研究聚苯胺纳米管在阳极氧化铝模板（AAO）内电聚合的生长机理.结果表明：电聚合初始，苯胺二维非扩散控制的瞬时成核生长形成聚苯胺晶胞链段单层,单层形成以后，聚苯胺的生长过程变为线性扩散控制. XRD实验进一步证实电聚合的聚苯胺同时包含结晶相和无定型相结构.     

Diffusion Kinetics Study of Lithium Ion in the Graphdiyne Based Electrode

Herein,we report a comparative investigation of the electrochemical lithium diffusion within graphidyne(GDY)based electrodes.The transfer kinetic behaviors of lithium ions during the insertion/extraction process are analyzed through different methods including the galvanostatic intermittent titration technique(GITT)and the electrochemical impedance spectroscopy(EIS).GDY with the morphology of nanosheets(GDY NS)shows lithium diffusion coefficients in the orders range of 10-12-10-13 cm2/s through the GITT method.Meanwhile,EIS indicates quite a lower value of lithium diffusion coefficients between 10-13 and 10-15 cm2/s,which indicates that the analysis technique has an influence on the evaluation of GDY-based electrodes.In addition,under the same measurement condition of GITT,GDY nanoparticles(GDY NP)exhibit a lower value of Li+diffusion coefficient(10-14-10-16 cm2/s)during the charge-discharge process compared to those of GDY NS,which can be ascribed to the wide distributing range of particle size in GDY NP based electrodes.The analysis results in this work reveal that the aggregating forms of GDY electrode material have an important effect on the diffusion process of lithium ions,which provides a pathway to optimize the performance of GDY-based energy storage devices.     

Picoliter-volume aqueous droplets in oil: electrochemical detection and yeast cell electroporation

An electrochemical detection method was introduced for aqueous droplet analysis in oil phase of microfluidic devices. This method is based on the electrochemical signal difference between aqueous and oil. Applying a low alternating current (AC) voltage to a couple of Au microelectrodes, this method can offer size information and ion concentration range from 0.02 mmol/L to 1 mol/L of tens of picoliter to nanoliter aqueous droplets. Alternatively, applying a relative high AC voltage (18 Vpp) at a frequency of 1 kHz leads to electroporation of yeast cells encapsulated into picoliter droplets. We believe that this simple technique is useful for a number of aqueous droplet-based chemical and biological analyses as well as cell electroporation.