质子交换膜燃料电池温度和电流分布同步测定

发展了一种新的质子交换膜燃料电池（PEMFC）局部特性测试方法,该方法实现了在不改变PEMFC膜电极和电池结构的情况下对PEMFC电流密度和局部温度分布的同步测量。对一种PEMFC在自然冷却状态下,阳极流场板背面温度分布以及阴极电流密度分布进行了实验测定。实验结果表明,阳极流场板背面最大温差小于1℃;电流密度分布主要受电极内液态水分布影响;同时,水蒸气冷凝放热导致电池局部温度升高是造成电池温度分布不均的主要因素。     

基于直流内阻和交流阻抗特性的铝空气电池输出特性分析

基于二阶RC等效电路模型与工作机理,建立铝空气电池直流内阻特性模型、交流阻抗特性模型和U-I输出特性模型,研究操作条件变化对电池工作性能的影响。通过研究电池在不同操作条件、不同电流密度工作下的直流内阻特性曲线、电化学阻抗谱图以及U-I输出特性曲线的变化规律,分析操作条件变化对电池总内阻与输出性能的影响规律。仿真和实验结果表明,模型具有较高的准确性;操作条件对电池交流阻抗特性与U-I输出特性有明显的影响作用;不同操作条件下电池的交流阻抗谱图和U-I输出特性曲线具有对应的变化规律和量化关系,通过研究操作条件对这两者的影响作用,可为后续操作条件优化、电池输出性能提升奠定基础。     

基于直流内阻和交流阻抗特性的PEMFC水管理状态分析

基于Randles等效电路,研究质子交换膜燃料电池（PEMFC）操作温度和湿度耦合关系,建立电堆直流内阻和交流阻抗特性模型。通过两种方法相结合,研究不同操作条件下的电化学阻抗谱图和U-I输出特性曲线的变化规律,以及不同水管理状态在直流内阻和交流阻抗变化规律中体现出的对应关系,进而分析水管理状态对电堆输出性能的影响作用。仿真和实验结果表明,温湿度耦合关系下的不同水管理状态,在电化学阻抗谱图和U-I特性曲线中具有一致的变化规律和对应的量化关系;电堆输出性能中的膜干、水淹等现象,在直流内阻值和交流阻抗图的变化中具有明显的表现特征;通过研究水管理状态对两者的影响,能够实现操作条件的优化和电堆输出性能的优化控制。     

吹扫条件对PEMFC阻抗弛豫现象和低温启动的影响

停机吹扫作为质子交换膜燃料电池(PEMFC)零下温度启动前的关键流程，能够有效降低冷启动前膜电极(MEA)的含水量，以提高电池启动的成功率。为了研究PEMFC在停机吹扫后出现的阻抗弛豫现象，以及吹扫条件如何直接作用于PEMFC低温启动性能，以25 cm²的单片电池作为对象，进行实验研究。实验条件下的结果表明，随着吹扫气体流量的增加以及吹扫温度的提高，阻抗弛豫表现更加明显；吹扫流量和吹扫时间对PEMFC冷启动性能存在显著影响。在实验条件下，以1500 ml/min的干燥N₂吹扫15 min，测试电池在低温中的表现最佳。吹扫流量过大、过小以及吹扫时间过长、过短均不利于PEMFC的低温启动，存在最佳吹扫条件。     

阴、阳极加湿程度对PEMFC内部传质的影响

为了研究常规流场下阴、阳极增湿程度对电池内部水分布、传递、膜性能及水拖曳系数等的影响,对PEMFC进行二维建模,应用控制容积法对控制方程进行离散,然后求解,得到了电池内部水和反应气浓度、速度分布、膜中电流密度、电势分布及膜中水分布,考察了气体不同增湿程度对质子交换膜电导率及电池内部传质的影响.结果表明,PEMFC中水综合拖曳系数随着阳极加湿程度的增加而增大,随阴极增湿程度的增加而减小,但阳极增湿对水综合拖曳系数的影响比同增湿程度下阴极增湿对水综合拖曳系数的影响大得多.同时,随着阳极加湿程度的升高,质子交换膜（PEM）电导率急剧升高,而阴极加湿程度对PEM电导率的影响只是停留在较小的电流范围之内.故PEMFC在小电流密度工作时,应该使阳极气体充分增湿;而在大电流密度工作时,应该适当降低阳极的增湿程度以降低阴极两相流的机会,从而改善阴极的传质状况.     

不同温度下微生物燃料电池的运行特性

实验采用双室型微生物燃料电池（MFC）,以生活废水中厌氧菌作为生物催化剂,葡萄糖为燃料,通过5个不同温度条件下的间歇运行,应用循环伏安、交流阻抗、极化测试等电化学方法考察温度对电池产电性能的影响。结果表明,一定温度范围内,提高温度有助于增强微生物的电化学活性,降低传荷阻抗,提高电池输出功率密度和交换电流密度。32 ℃时,电池产电效能最佳,电池功率密度和交换电流密度分别达到156.2 mW/m2和8.02×10?5 mA/m2,温度太低或太高均不利于细菌的电化学活性。体系温度为18 ℃、25 ℃、32 ℃、39 ℃、46 ℃时,传荷阻抗Rct在阳极内阻中占的比例分别为97.99%、84.02%、47.36%、91.30%、99.61%,说明传荷阻抗在阳极内阻中占绝对份额,MFC是传荷过程控制下的电化学反应体系。     

质子交换膜燃料电池稳态自增湿性能分析

增湿及水管理系统使得燃料电池系统结构复杂,质子交换膜燃料电池（proton exchange membrane fuel cell,PEMFC）自增湿操作在实用化方面逐渐引起研究者的兴趣。提高PEMFC自增湿性能的关键在于对生成水的有效管理,保证质子交换膜的良好水合。实践证实采用自增湿膜电极组件是一个有效途径。本文建立催化层中增加保水层的水传递平衡模型预测膜中水的分布,考察自增湿操作的可行性和稳定性。数值分析表明：只有低于50 mm（如Nafion112）的薄膜能满足电池自增湿膜水合的要求。保证膜水合性能和电池操作稳定性的电池温度为60℃,操作压力为0.15 MPa,阴极气体过量系数可以增大到1.8。在上述操作条件下,电池自增湿性能与饱和增湿有可比性,与饱和增湿最佳条件有差距。因此PEMFC自增湿性能在综合考虑降低成本和费用,简化结构和操作时具有可行性,但不能替代增湿操作。     

氢气进气压力对空冷PEMFC性能的影响

空冷型质子交换膜燃料电池(PEMFC)具有自增湿、质量轻、系统操作简单等特点,适合应用于无人机等领域。氢气进气压力是影响空冷型PEMFC电堆性能的一个重要因素。以1 kW阴极开放式空冷型PEMFC电堆为研究对象,对比了不同氢气进气压力对单片电池电压及其一致性、电堆输出电压、输出功率以及氢气利用率的影响。研究结果表明,氢气进气压力越高,单片电池电压、电堆输出电压和功率越高,大电流下单片电池电压的一致性越好;此外,本实验利用排水法收集阳极尾气并计算氢气利用率,氢气进气压力越高,系统氢气利用率越低。     

阳极功能层厚度对中温固体氧化物燃料电池电性能的影响

采用水系流延成型工艺,研究了阳极支撑型中温SOFC阳极功能层厚度对中温SOFC电性能的影响,运用电化学工作站对单电池的电性能进行了表征。结果表明,在相同的运行温度下,单电池的功率密度随着功能层厚度的增加而减小,而极化阻抗则相应增加;单电池的功率密度随着运行温度的提高而增大,对应的极化阻抗则减小。以H2＋3%水蒸气为燃料气,空气为氧化气,在750℃运行条件下,功能层厚度为25μm、30μm和35μm的单电池的功率密度分别为0.31 W/cm^2、0.10 W/cm^2和0.07 W/cm^2,相应的极化阻抗则分别为1.05Ωcm^2、2.41Ωcm^2和3.08Ωcm^2;阳极功能层厚度为25μm的单电池的测试温度在700℃、750℃和800℃,其功率密度分别为0.22 W/cm^2、0.31 W/cm^2和0.45 W/cm^2,对应极化阻抗分别为1.90Ωcm^2、1.05Ωcm^2和0.67Ω/cm^2。     

1.5 kW质子交换膜燃料电池堆动态工况响应特性

考察了自制1.5 kW质子交换膜燃料电池(PEMFC)电堆在动态工况下的性能。研究了PEMFC电堆电压、功率、反应参数随车载工况运行出现的响应情况。发现在大电流密度下,由于各单电池的差异,电堆电压和功率出现比较明显的波动现象。在选定两个工况周期中,电堆各单电池电压的差异系数C_V最大达到4.23%,最高单电池电压和最低单电池电压相差0.106 V。数据分析表明在该动态工况下,PEMFC电堆的动态响应特性受到反应物和电堆温度变化、空气局部流量过大或不足以及电堆内部阳极和阴极出现明显压降等因素的影响。该研究不仅为后续耐久性测试提供分析依据,还对PEMFC电堆实际车载运行参数与控制策略的优化具有指导意义。     

Effects of Proton-Exchange Membrane Fuel-Cell Operating Conditions On Charge Transfer Resistances Measured by Electrochemical Impedance Spectroscopy

Abstract

Proton-exchange-membrane fuel cells (PEMFC) are highly dependent on operating conditions, such as humidity and temperature. This study employs electrochemical impedance spectroscopy (EIS) to measure the effects of operating parameters on internal proton and electron transport resistance mechanisms in the PEMFC. Current-density experiments have been performed to measure the power production in a 25 cm² Nafion 117 PEMFC at varying operating conditions. These experiments have shown that low humidity and low temperature contribute to decreased power production. EIS is currently employed to provide a better understanding of the mechanisms involved in power production by calculating the specific resistances at various regions in the PEMFC. Experiments are performed at temperatures ranging from 30 to 50°C, feed humidities from 20 to 98%, and air stoichiometric ratios from 1.33 to 2.67. In all experiments, the hydrogen feed stoichiometric ratio was approximately 4.0. EIS is used to identify which transport steps limit the power production of the PEMFC over these ranges of conditions. The experimental data are analyzed via comparison to equivalent circuit models (ECMs), a technique that uses an electrical circuit to represent the electrochemical and transport properties of the PEMFC. These studies will aid in designing fuel cells that are more tolerant to wide-ranging operating conditions. In addition, optimal operating conditions for PEMFC operation can be identified.     

Systematic study of back pressure and anode stoichiometry effects on spatial PEMFC performance distribution

A segmented cell system was applied to investigate the effects of the anode and cathode back pressure and hydrogen stoichiometry on fuel cell performance in terms of overpotential distributions along the flow field. The segmented cell system was designed with closed loop Hall sensors and a data acquisition system allowing simultaneous spatial electrochemical impedance spectra (EIS) measurements. It was determined that an increase in back pressure for the tested serpentine flow field design results in an improvement of the cell performance and uneven improvement of individual segments’ performance. In general, the performance and the overpotentials become more uniform downstream with an increase in the back pressure due to a decrease in activation and mass transfer losses. Spatial EIS data for the PEMFC operated at different back pressures support the overpotential analysis. Hydrogen stoichiometry variations do not affect the performance of the cell or the individual segments at low current density because there is no significant hydrogen concentration gradient in the flow field. However, at high current densities a reduction in hydrogen stoichiometry produces a slight decrease in performance for inlet segments while outlet segments showed a noticeable performance loss. The decrease in performance is attributed to an increase in mass transfer losses due to nitrogen diffusion from the cathode to the anode. This effect becomes more pronounced for the outlet segments due to a downstream nitrogen accumulation. Under high current density conditions, the cell is locally fuel starved even with a high fuel stoichiometry creating conditions leading to cell degradation by carbon corrosion. More importantly, this local degradation is masked by the overall cell performance which remains largely unaffected.     

Electrochemical and gas phase parameters of cathodes for intermediate temperature solid oxide fuel cells

A series of cyclic voltammetry, chronoamperometry and electrochemical impedance experiments have been carried out in order to investigate the effect of cathode composition and porosity on the electrochemical characteristics of strontium-doped lanthanum, praseodymium and gadolinium cobaltite cathodes. The impedance responses at different electrode potentials of the half cell and symmetric single cell setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution of impedance spectra for single cell cathode and anode reactions contributions based on the results of simultaneous analysis of half cells and symmetric single cells has been made by differential impedance real part vs. ac frequency plot analysis method. Noticeable influence of cathode chemical composition, meso-porosity and macro-porosity on the electrochemical activity of the oxygen electroreduction has been demonstrated. Seeming activation energy values have been calculated and discussed.     

Membrane electrode assembly with Pt/SiO2/C anode catalyst for proton exchange membrane fuel cell operation under low humidity conditions

In this work, a novel self-humidifying membrane electrode assembly (MEA) with Pt/SiO₂/C as anode catalyst was developed to improve the performance of proton exchange membrane fuel cell (PEMFC) operating at low humidity conditions. The characteristics of the composite catalysts were investigated by XRD, TEM and water uptake measurement. The optimal performance of the MEA was obtained with the 10 wt.% of silica in the composite catalyst by single cell tests under both high and low humidity conditions. The low humidity performance of the novel self-humidifying MEA was evaluated in a H₂/air PEMFC at ambient pressure under different relative humidity (RH) and cell temperature conditions. The results show that the MEA performance was hardly changed even if the RHs of both the anode and cathode decreased from 100% to 28%. However, the low humidity performance of the MEA was quite susceptible to the cell temperature, which decreased steeply as the cell temperature increased. At a cell temperature of 50 °C, the MEA shows good stability for low humidity operating: the current density remained at 0.65 A cm⁻² at a usual work voltage of 0.6 V without any degradation after 120 h operation under 28% RH for both the anode and cathode.     

Electrochemical impedance spectroscopy evidence of dimethyl-silicon-oil enhancing O2 transport in a porous electrode

Faster oxygen transport is critical to guarantee reliable power output of polymer electrolyte membrane fuel cells (PEMFCs). In order to enhance oxygen transfer in a porous electrode especially in the case of water flooding, water-proof oil (dimethyl-silicon-oil (DMS)) was introduced into the conventional Pt/C electrode. Owing to the capability of electrochemical impedance spectroscopy (EIS) in discriminating individual contribution of ohmic, kinetic, and mass transport from all PEMFC processes, EIS was carried out to evaluate the effect of the DMS on the oxygen reduction reaction (ORR). The equivalent circuits corresponding to the EIS spectra were employed. The parameters in the equivalent circuits were obtained by curve fitting to the EIS spectra with the aid of the frequency response analysis software (FRA) attached in the electrochemical station Autolab PGSYAT302. The EIS analysis has shown that the introduction of DMS reduces the oxygen diffusion resistance as well as the charge transfer resistance in the flooded state. The single cell tests show that even in the case of normal operating condition the accumulated water with PEMFC operation also worsens the oxygen transfer in the conventional Pt/C gas diffusion electrode (GDE) with more and more water produced at the cathode. GDE containing DMS, which is defined as a flooding tolerant electrode (FTE), is fortunately quite good at alleviating water flooding. Success of the FTE in alleviating water flooding is ascribed to (1) its high oxygen transfer flux due to the higher solubility of oxygen in DMS than in water as long as parts of pores are occupied beforehand by DMS rather than by water, and (2) enhanced hydrophobic property of the FTE with DMS adsorption on the walls of the pores, which makes more hydrophobic pores be open to oxygen transport.     

A “proton pump” concept for the investigation of proton transport and anode kinetics in proton exchange membrane fuel cells

A novel concept for the measurement of proton transport properties and electrode kinetics in proton exchange membrane fuel cells (PEMFC) is presented. The “proton pump” is essentially a fuel cell operated with pure nitrogen or very low hydrogen partial pressure instead of oxygen-containing gas on the cathode side, avoiding the complicated electrode kinetics of oxygen reduction. In this first study using this concept, we investigated the proton transport in high temperature PEMFC based on polybenzimidazole (PBI)/phosphoric acid membranes. The impedance spectra of the proton pump allow the clear distinction between anode and cathode kinetics and proton transport in the membrane. Identifying and analyzing the contribution of the anodic processes in the impedance spectra enabled the quantitative investigation of anode kinetics based on the Butler-Volmer equation. The proton transport was investigated in more detail in the current saturation region, where proton transport turned out to be the limiting process in case of sufficient H₂ supply at the anode. The maximum proton transport capacity of the PBI/phosphoric acid membrane was found to be comparable to those of Nafion^® membranes.     

基于频率正割角计算的燃料电池堆水热管理状态诊断方法

水热状态管理对提高燃料电池堆输出性能具有重要意义。本文基于电化学阻抗谱法原理,结合等效电路模型,通过获取两个阻抗谱特征频率点响应,计算出频率响应点连线与实轴夹角（频率正割角）,提出了一种频率正割角计算方法。理论证明该角度与电堆内阻变化具有唯一性对应关系。仿真和实验结果表明,频率正割角计算结果能够很好地描述堆内水热管理状态变化,以及操作条件变化对电堆输出性能的影响。该方法避免了交流阻抗试验测试步骤繁复、时间周期长、参数拟合困难等不足,给出一种特征明显、直接简便的诊断方法,为电堆操作条件优化和输出性能故障预警奠定了基础,具有良好的应用前景。     

Performance degradation studies on PBI/H3PO4 high temperature PEMFC and one-dimensional numerical analysis

Five hundred hours continuous aging test at constant discharge current (640 mA cm⁻²) was performed on PBI/H₃PO₄ high temperature PEMFC unit cell, electrochemical techniques-linear sweep voltammetry (LSV) and AC impedance measurement were used to investigate the changes of electrochemical surface area (ESA) and high frequency resistance (internal resistance) with time. Initial experimental results showed that during 500 h continuous aging test the main reason for cell performance degradation is the decrease of ESA caused by sintering. In addition, a one-dimensional mathematical model was constructed, the concentration distributions of cathode reactant gases (O₂ and gaseous H₂O) were calculated and polarization curves recorded during aging test were simulated based on the model, the simulated polarization curves compare well with the experimental results.     

Water flooding and pressure drop characteristics in flow channels of proton exchange membrane fuel cells

Water flooding of the flow channels is one of the critical issues to the design and operation of proton exchange membrane fuel cells (PEMFCs). The liquid water and total pressure drop characteristics both in the anode and cathode parallel flow channels of an operating PEMFC were experimentally studied. The gas/liquid two-phase flow both in the anode and cathode flow channels was observed, and the total pressure drop between the inlet and outlet of the flow field was measured. The effects of cell temperature, current density and operating time on the total pressure drop were investigated. The results indicated that the total pressure drop in the flow channels mainly depends on the resistance of the liquid water in the flow channels to the gas flow, and the different flow patterns distinguish the total pressure drops in the flow field. Clogging by water columns result in a higher pressure drop in the flow channels. The total pressure drop measurement can be considered as an in situ diagnoses method to characterize the degree of the flow channels flooding. The liquid water in the cathode flow channels was much more than that in the anode flow channels. The pressure drop in the cathode flow channels was higher than that in the anode flow channels. During the fuel cell operation, the cell performance decreased gradually and the pressure drop both in the anode and the cathode flow channels increased. The rate of flooding at the cathode side reached 49.56% under experimental conditions after 78 min of operation. However, it was zero at the anode side.