考虑空间电荷层效应的氧离子导体电解质内载流子传输特性

晶界或异质界面诱发的空间电荷层(space charge layer,SCL)效应,被认为是氧离子导体电解质内界面附近区域载流子传输特性显著区别于体相区域的关键原因之一.现有研究多采用Poisson-Boltzmann(PB)方程预测SCL效应的影响规律,但其基于载流子电化学平衡假设,无法用于载流子存在宏观运动的工况,极大限制了相关传输机理研究.本文耦合Poisson方程和载流子质量守恒方程,建立了适用于载流子具有宏观运动时氧离子导体内载流子传输过程的模型,推导了控制SCL效应的关键无量纲参数.聚焦固体氧化物燃料电池中常用的AO2-M2O3氧离子导体电解质,对比研究了传统PB方程和本文建立的Poisson-载流子质量守恒耦合方程的预测结果可靠性.进一步采用耦合模型深入分析了考虑SCL效应时氧离子导体内部氧空位传输机理,发现导体界面电流密度增大导致SCL电阻先减小后增大.增大无量纲Debye长度(表征空间电荷层厚度与导体厚度的比值)可显著增大SCL电阻.当驱动氧空位移动的过电势与热势数量级相当时,增大无量纲电势(表征过电势与热势的比值)导致SCL电阻增大;当过电势远小于热势时,改变无量纲电势对氧空位传输过程几乎无影响.本文研究结论可为通过合理设计晶界或异质界面以改善氧离子导体内载流子传输能力及最终提高相关电化学器件性能提供理论依据.     

固体氧化物燃料电池产功机理研究

本文基于电化学反应原理及热力学分析方法,研究固体氧化物燃料电池非热力循环产功机理.基于电化学反应原理,建立了SOFC性能分析模型,研究了SOFC电池性能与电化学参数之间的变化关系,从而揭示了SOFC化学能直接转变为电能的机理;分析了热力学参数及电化学参数对SOFC系统性能的影响规律,提出改善SOFC电池性能的途径,并揭示了通过SOFC与先进热力循环系统集成进一步提高动力系统性能的潜力.本文研究成果为开拓研究高效SOFC复合动力系统提供有益的参考.     

固体氧化物燃料电池阳极微结构三维数值模拟

固体氧化物燃料电池(SOFC)是一种清洁高效的发电设备,其电极微结构直接影响电池的电化学性能。本文通过X-ray技术获取了SOFC阳极微结构,将电荷和物质传导定义在体相材料,将电化学反应定义在三相边界线上,建立了SOFC阳极电化学–传质耦合的三维微观模型,对比了两个微结构在80?C条件下的极化特性。研究表明微结构对电极内部物理场分布有极大影响,越靠近电极电解质界面,活化极化和离子电势波动越强烈。电极孔隙相细小的喉附近存在较大传质阻力,形成明显浓度极化跳跃。活化极化和欧姆极化大小相当,各占据总损失的45%以上。本文模型可用于研究微结构改变引起的电池退化和电极的优化设计。     

多孔介质孔隙率对SOFC阳极管道化学/电化学反应与传递过程影响

固体氧化物燃料电池复合阳极管道由多孔层、燃料气体通道以及固体平板组成。在多孔层内部,存在着传热、多组分对流/扩散等多种传递过程,它们与CH_4的重整反应,CO、H_2的电化学反应耦合在一起,既受到设计参数,如孔隙率等的影响,同时对电池性能以及稳定性产生重要影响。编制了三维模拟程序,对阳极复合管道的化学反应以及气体流动、传热过程等进行了研究。研究结果显示,随着孔隙率的增大,在多孔层的较深区域内,重整与变换反应速度明显增加,受它的影响,H_2浓度与多孔层温度也相应提高。因此,在一定范围内提高多孔介质的孔隙率对阳极的各种传递过程与化学/电化学反应具有积极影响。     

连续梯度的功能层对燃料电池在初始还原过程中曲率及残余应力的影响

连续梯度的电极由于其相对于多层梯度电极能更加有效地缓解电极和电解质的热失配及改善界面黏接而受到特别的关注. 本文通过建立含连续梯度的阳极功能层的阳极支撑固体氧化物燃料电池的力学模型, 研究了连续梯度的阳极功能层对阳极支撑固体氧化物燃料电池半电池在初始还原过程中曲率及残余应力的影响. 结果表明电池的曲率在初始还原过程中随还原程度的增大而逐渐增大. 连续梯度的阳极功能层的引入不能同时改善电池的曲率和残余应力, 即连续梯度的阳极功能层在缓解应力的同时会导致曲率的增大, 反之亦然. 含有连续梯度的阳极功能层的电池在部分还原状态下, 梯度层/阳极支撑界面处具有最大的拉应力容易导致电池受损, 实际中应保证电池被完全还原.     

Efficient Thickness of Solid Oxide Fuel Cell Composite Electrode

利用一个物理模型计算了固体氧化物燃料电池复合电极的有效厚度. 此模型考虑了复合电极内部的电化学反应,电子和离子的传递,以及电极的微结构. 电极的有效厚度对应于电极性能最优时电极的理论厚度,经过模型计算表明此厚度同时为电荷转移电阻率、有效离子(电子)电阻率以及单位体积内三相线长度的函数. 通过与实验数据比较验证了模型的可用性. 通过模拟表明电极成分、粒子大小、电极材料的本征电阻率、不同的操作温度以及电极反应的机理都影响着复合电极的有效厚度.     

阳极传质对三合一微生物燃料电池性能的影响

物质传输是影响MFC性能的一个重要因素。本文构建三合一膜电极式微生物燃料电池(MFC),研究了阳极传质形式对MFC启动特性、阳极生物膜电化学活性和性能的影响。结果表明,与阳极采用大腔室结构的MFC-1相比,阳极采用蛇形流道的MFC-2由于在启动过程中阳极电解液传质较佳,不但启动较快而且输出电压更高。启动完成后,MFC-2阳极生物膜电化学活性较高,采取不同扫描速度的循环伏安扫描测试证明了这主要是由于蛇形流道较佳的传质所致。启动过程和产电过程中较佳的传质导致MFC-2最大功率密度(2676.2 mW·m~(-2))比MFC-1最大功率密度(2149.0 mW·m~(-2))约高24.5%。     

电化学阻抗谱在本科实验教学中的应用

电化学阻抗谱是常用的一种电化学测试技术,该方法具有频率范围广、对体系扰动小的特点,是研究电极过程动力学、电极表面现象以及测定固体电解质导电率的重要工具。本文以固体氧化物燃料电池的电化学阻抗谱的测试为例设计了一套在线分析电化学阻抗的应用课程。介绍了电化学阻抗谱的原理,数据采集方法和数据处理方法,阐述了电化学阻抗谱技术在本科实验教学中的可行性和重要性。     

梯度孔隙阳极固体氧化物燃料电池传质与电性能研究

本文通过建立梯度孔隙阳极固体氧化物燃料电池(Solid Oxide Fuel Cell,SOFC)的三维数学模型,模拟了SOFC内部气体传输现象及电性能,通过对比梯度孔隙阳极SOFC与均匀孔隙阳极SOFC的能质传递特性,揭示了梯度孔隙阳极的优越性。研究发现,梯度孔隙阳极设计在保证传质特性不降低的同时,有效调整了电极与其他部件特别是电解质热膨胀系数的匹配。在本文的参数设置下,梯度孔隙阳极设计的SOFC最大输出功率可达到1.005W·cm~(-2),相比于均匀孔隙阳极SOFC,至少可提高8.78%。因此,梯度孔隙阳极设计的SOFC可以显著提高电池的电性能。     

结合碳捕集的SOFC-sCO2布雷顿循环集成系统性能分析

固体氧化物燃料电池是将化学能转化成电能的全固态能量转换装置,被认为是极具前景的绿色发电系统。本研究提出了结合碳捕集的固体氧化物燃料电池-超临界二氧化碳布雷顿循环集成系统,通过阳极尾气富氧燃烧实现低能耗碳捕集,并利用s CO₂再压缩布雷顿循环回收燃烧室余热提高系统效率。模拟结果显示,该集成系统在设计工况下的净发电效率为59.74%,二氧化碳捕集量为134.50 kg/h。此外,关键工作参数对系统性能的影响分析结果表明,合理的阳极尾气再循环比、燃料利用率和燃料流量是确保系统安全高效运行的必要前提。     

XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells

In electrodes of low temperature fuel cells like polymer electrolyte membrane fuel cells (PEFC) or alkaline fuel cells (AFC) the reactants and the water must be transported. For this purpose the pore system in the electrodes needs a hydrophilic character for the transport of the water and a hydrophobic character for the transport of the gases. The degree of the hydrophobicity determines whether the pore system will be flooded by the reaction water. In the case of PEFC, this is also determined by the degree of the required humidification of the reaction gases. In AFC hydrophobicity determines the extension of the three-phase reaction zone. Caused by the strong influence of hydrophobicity on the transport processes, the electrochemical performance and the optimized operation conditions are also affected by hydrophobicity.Typically polytetrafluoro-ethylene (PTFE) is used to make the electrodes hydrophobic, because PTFE has a high chemical stability. Hydrophobicity depends on the concentration of PTFE on the electrode surface. The PTFE concentration, which is related to the hydrophobic character, can be determined by XPS. The changes in the PTFE content and structure of the electrode of a PEFC was investigated by cyclic voltammetry and XPS and correlated with the performance of the cell in long-term operation. With both methods an initial significant increase in free and electrochemically active surface platinum area is observed. This activation is associated with a degradation of the PTFE in the electrode which is responsible for the hydrophobic properties of the electrode. With further operation the performance of the cell decreases because the water management becomes more critical. Generally, it is shown that XPS can be used for the investigation of the hydrophobicity of electrodes prepared by various manufacturing techniques as well as of changes in their hydrophobicity induced by the electrochemical operation.     

水中脉冲放电金属电极烧蚀机理

采用板-板电极, 在放电间隙距离为2 mm、放电电流峰值为22 kA条件下, 对黄铜、钨铜电极的烧蚀特性进行了对比研究。利用高精度天平测量放电过程中的电极质量损失, 分别获取了阴极、阳极及总的平均烧蚀速率。通过放电后电极表面微观形貌、微观元素组成的分析及液体中金属离子的含量分析, 对水中脉冲放电金属电极的烧蚀机理进行了探讨。结果表明, 水中脉冲放电时, 钨铜电极的抗烧蚀性能明显高于黄铜电极。黄铜电极的主要烧蚀是以中心的大量孔洞及其边缘的波纹结构为表现形式的液体金属的溅射;钨铜电极的突出物及较平整的表面暗示了气相侵蚀的作用。以电弧的焦耳热效应为催化剂, 钨铜与水的电化学反应更为强烈, 因此电化学腐蚀是水中放电电极烧蚀的形式之一。     

Electrochemical cell with two layers cathode for NO decomposition

An electrochemical cell composed of an yttria-stabilized zirconia disk and two layers cathode was used for nitrogen monoxide decomposition. It was found that covering the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) leads to enhancement of the performance of the electrochemical cell for NO_x decomposition in the presence of excess oxygen. The decomposition activity was measured for the one-compartment cell oxide|(cathode)|YSZ|(anode) by applying a DC voltage lower than 3.7 V in the temperature range 550–700 °C. The microstructure of the YSZ-NiO mixed oxide electrodes was investigated in dependence of the cell operating condition and the working electrode sintering temperature. The correlation between the microstructure of the mixed oxide electrode and conversion rate of NO was studied. The phenomenon of self-optimization of the microstructure of the NiO-YSZ working electrode during the cell operation was observed and investigated. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Trigger devices for pseudospark switches

Effective triggering of pseudospark switches with long lifetime and low jitter remains an important problem. This paper presents results of investigations of trigger methods for pseudospark switches. based upon pulsed glow discharges in planar and hollow electrode geometry for charge injection. The influence of different wiring and geometries of the electrodes for preionization is investigated. The effect of additional blocking potentials in the hollow cathode to improve different trigger systems was measured. Calculations of the static potential in the hollow cathode with or without blocking potential are compared with parameters of the discharge     

Vernier阳极设计与成像模拟

光子计数成像探测器作为探测微弱光的重要手段,由微通道板,解码阳极以及后续的读出电路组成,其中解码阳极的性能直接影响着探测器的成像质量。作为一种电荷分割型阳极,Vernier阳极利用周期性的正弦电极区域替代了楔条形阳极(WSA)的线性电极,可获得高的成像分辨率和大的电极活动区域。根据Vernier阳极的设计原理对Vernier阳极进行了仿真和设计,首先,介绍了矢量形式的阳极解码,确定了阳极设计参量为阳极周期长度,电极振幅及电极波长;其次,分析了各阳极设计参量对探测器成像的影响,利用Labview软件分别模拟了电子云,Vernier阳极板以及其相互作用成像情况,确定了Vernier阳极周期长度与粗调波长之间的关系以及设计参数一定时,成像达到最佳的电子云大小,依照模拟结果和实际的加工条件,设计和制备了周期为891 μm,绝缘沟道为25 μm,振幅为50 μm,粗调数为5的九路Vernier阳极。     

Dependence of an electrochemical cell performance upon the thickness of the ionic conductor

The thickness of the ionic conductor in an electrochemical cell is shown to be among the governing parameters of the cell's performance. Internal field, I-V characteristics and p-n ‘junctions’ formed upon voltage application are investigated through a computer model based on fundamental charge transport equations including also Poisson's equation.     

Triode fuel cells

The triode fuel concept is discussed together with some of its first applications in the areas of SOFCs and PEMFCs. It is shown that, when the electrodes are polarizable, both the power output and the thermodynamic efficiency can be improved quite significantly via application of electrolytic currents between the anode or cathode and an auxiliary electrode.     

Mathematical modeling of the operation of SOFC Nickel-cermet anodes

A surface diffusion–reaction model is developed and solved to describe the steady-state operation of Nickel-cermet anodes in solid oxide fuel cells. The model accounts for the migration (backspillover) and diffusion of oxygen ions from the solid electrolyte onto the nickel surface and the concomitant reaction with the fuel over a finite reaction zone extending from the three-phase boundaries onto the Ni–gas interface. The model is developed for various nickel particle geometries and is compared with existing anode model predictions for flat geometries. The performance of the anode, expressed by an anodic effectiveness factor, is found to depend on two dimensionless numbers, which contain all the operational and structural information of the anode. The model is validated with literature experimental data regarding the dependence of the anode performance on the fuel partial pressure and predicts correctly the observed deviation from linearity of the dependence of cell current on fuel partial pressure.     

Oxide anode materials for solid oxide fuel cells

A major advantage of solid oxide fuel cells (SOFCs) over polymer electrolyte membrane (PEM) fuel cells is their tolerance for the type and purity of fuel. This fuel flexibility is due in large part to the high operating temperature of SOFCs, but also relies on the selection and development of appropriate materials — particularly for the anode where the fuel reaction occurs. This paper reviews the oxide materials being investigated as alternatives to the most commonly used nickel–YSZ cermet anodes for SOFCs. The majority of these oxides form the perovskite structure, which provides good flexibility in doping for control of the transport properties. However, oxides that form other crystal structures, such as the cubic fluorite structure, have also shown promise for use as SOFC anodes. In this paper, oxides are compared primarily in terms of their transport properties, but other properties relative to SOFC anode performance are also discussed.