Review: Development of Non-Perfluorosulfonic Acid Membranes for High Temperature Proton Exchange Membrane Fuel Cells

综述：高温质子交换膜燃料电池用非全氟质子膜的制备

柯茜1,2,冯威2

（1.东岳上海先进氟硅材料研发中心 上海 201108;2.含氟功能膜材料国家重点实验室,山东 淄博255000）

中文说明：

温度是影响燃料电池的关键因素之一,根据不同的工作温度聚合物电解质薄膜可被划分为高温和低温质子交换膜。低温质子交换膜主要依赖水进行离子传导,因此使用温度通常低于100℃。相比而言,可于120℃以上工作的高温质子交换膜由于其出众的特性,比如简化的水管理设备、加速的电极反应动力以及铂催化剂对CO耐性和系统中热能回收的提高而备受青睐。众所周知,目前高温质子交换膜领域的大量研究均以磷酸掺杂的聚合物薄膜为基础。本篇综述就适用于高温质子交换膜燃料电池的各类膜材料,特别是它们的合成、性能、技术关键点和相应的解决手段进行了讨论。涉及的膜结构包括聚苯并咪唑薄膜、其他碳氢结构薄膜以及预辐射接枝技术制备的含氟聚合物薄膜。

关键词：燃料电池,磷酸,聚苯并咪唑,碳氢化合物,预辐射引发接枝

     

An easy parameter estimation procedure for modeling a HT-PEMFC

Fuel cells are a very complex system in which many phenomena of different nature occur simultaneously and within a small space, so a truthful measurement of some variables is not feasible using state-of-the-art technology. If a deep knowledge of the unit is desired, modeling can be of great help when it is properly used as it is possible to calculate the value of the variables of interest by adjusting experimental data. However, when models are complicated, it is not trivial to identify in which way a certain parameter alters the model results and then it is necessary to resort to sensitivity analysis before approaching an adequate parameter estimation procedure. In this work, a parameter estimation procedure has been proposed with the results obtained from the sensitivity analysis applied to a high temperature proton exchange membrane fuel cell (HT-PEMFC) model. The procedure has been demonstrated to be straightforwardly applicable as well as effective, which makes it suitable to be used as engineering tool to obtain a first consistent value of the parameters that define the main characteristics of a HT-PEMFC.     

On the activation of polybenzimidazole-based membrane electrode assemblies doped with phosphoric acid

Three different activation procedures of phosphoric acid-doped polymer fuel cells have been analyzed: a discontinuous procedure with daily startup and shutdown, a continuous procedure without interruption and a continuous procedure after 1 h of direct exposure of the MEA to the atmosphere. The investigation was carried out analyzing the voltage profiles over time, the high frequency impedance and complete impedance spectra. The results show that the voltage increase during activation is due to the reduction of the kinetic resistance. The continuous activation leads to the best performance (0.678 V in 100 h) while the discontinuous one leads to a lower performance in a shorter time (0.665 V in 50 h). The initial exposure of the MEA to the atmosphere alters the initial acid distribution and leads to a final performance of (0.664 V in 200 h) in spite of the continuous activation.     

Polybenzimidazole-modified carbon nanotubes as a support material for platinum-based high-temperature proton exchange membrane fuel cell electrocatalysts

We fabricate polybenzimidazole (PBI) wrapped carbon nanotubes (MWCNTs) as support material for platinum-based fuel cell electrocatalyst. With the aid of microwave-assisted polyol reduction, we obtain very fine platinum (Pt) nanoparticles on PBI/MWCNT support while reducing the amount of Pt waste during synthesis. Cyclic voltammetry (CV) concludes that Pt-PBI/MWCNT has 43.0 m² g⁻¹ of electrochemically active surface area (ECSA) to catalyze hydrogen oxidation. Furthermore, after the 1000th cycle, Pt-PBI/MWCNT preserves almost 80% of its maximum ECSA, meaning that Pt-PBI/MWCNT is much more durable than the Pt/MWCNT and commercial Pt/C. High-temperature proton exchange membrane fuel cell (HT-PEMFC) performance tests are conducted under H₂/Air conditions at the temperatures ranging from 150 °C to 180 °C. Nevertheless, tests conclude that the maximum power density values of the Pt-PBI/MWCNT are found inferior to the Pt/C at all temperatures (e.g., 47 vs. 62 mW cm⁻² at 180 °C), suggesting that some balance between durability and performance has to be taken into consideration.     

Study of flow channel geometry using current distribution measurement in a high temperature polymer electrolyte membrane fuel cell

To improve fuel cell design and performance, research studies supported by a wide variety of physical and electrochemical methods have to be carried out. Among the different techniques, current distribution measurement owns the desired feature that can be performed during operation, revealing information about internal phenomena when the fuel cell is working. Moreover, short durability is one of the main problems that is hindering fuel cell wide implementation and it is known to be related to current density heterogeneities over the electrode surface. A good flow channel geometry design can favor a uniform current density profile, hence hypothetically extending fuel cell life. With this, it was thought that a study on the influence of flow channel geometry on the performance of a high temperature polymer electrolyte membrane (PEM) fuel cell using current distribution measurement should be a very solid work to optimize flow field design. Results demonstrate that the 4 step serpentine and pin-type geometries distribute the reactants more effectively, obtaining a relatively flat current density map at higher current densities than parallel or interdigitated ones and yielding maximum powers up to 25% higher when using oxygen as comburent. If air is the oxidant chosen, interdigitated flow channels perform almost as well as serpentine or pin-type due to that the flow conditions are very important for this geometry.     

Cell resistances of poly(2,5-benzimidazole)-based high temperature polymer membrane fuel cell membrane electrode assemblies: Time dependence and influence of operating parameters

Time-dependent measurements of cell impedance of a HT-PEFC based on ABPBI were performed at constant frequencies close to the high-frequency (h.f.) intercept of the corresponding Nyquist plots with the real axis. The h.f. impedances approximate the ohmic resistance of the cell and they decrease, when current (140 mA cm⁻²) is switched on. Steady-state values are attained after 10 min. Vice versa, when current is switched off (OCV), the h.f. impedances instantaneously increase but reach steady-state values only after about 1 h. These values rise with increasing gas flow rates. The results are discussed in terms of hydration/dehydration processes, changing the equilibrium between orthophosphoric and pyrophosphoric acid and thus the conductivity of the electrolyte as well as the mobility of molecules and charge carriers. Impedance spectra were recorded after each time-dependent measurement under OCV conditions. The fit of these impedance data based on an equivalent circuit revealed ohmic resistances corrected by h.f. inductances and low frequency impedances associated with the cathode oxygen exchange reaction. The charge transfer resistances deduced from the low frequency impedances strongly depend on both air and hydrogen flow rates.     

Study of the influence of the amount of PBI–H3PO4 in the catalytic layer of a high temperature PEMFC

The influence of the amount of polybenzimidazole (PBI)-H₃PO₄ (normalized with respect to the PBI loading, which expressed as C/PBI weight ratio) content in both the anode and cathode has been studied for a PBI-based high temperature proton exchange membrane (PEM) fuel cell. The electrodes prepared with different amounts of PBI have been characterized physically, by measuring the pore size distribution, and visualizing the surface microstructure. Afterwards, the electrochemical behaviour of the electrodes has been evaluated. The catalytic electrochemical activity has been measured by voltamperometry for each electrode prepared with a different PBI content, and the cell performance results have been studied, supported by the impedance spectra, in order to determine the influence of the PBI loading in each electrode. The best results have been achieved with a C/PBI weight ratio of 20, for both the anode and the cathode. A lower C/PBI weight ratio (larger amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and impaired the mass transport processes, due to the large amount of polymer covering the catalyst particle, lowering the cell performance. A higher C/PBI weight ratio (lower amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and slightly increased the ohmic resistance. The low amount of the polymeric ionic carrier PBI–H₃PO₄ limited the proton mobility, despite of the presence of large amounts of “free” H₃PO₄ in the catalytic layer.     

Modeling and analysis of a 5 kWe HT-PEMFC system for residential heat and power generation

We present a high-temperature proton exchange membrane fuel cell (HT-PEMFC) system model that accounts for fuel reforming, HT-PEMFC stack, and heat-recovery modules along with heat exchangers and balance of plant (BOP) components. In the model developed for analysis, the reaction kinetics for the fuel reforming processes are considered to accurately capture exhaust gas compositions and reactor temperatures under various operating conditions. The HT-PEMFC stack model is simplified from the three-dimensional HT-PEMFC CFD models developed in our previous studies. In addition, the parasitic power consumption and waste heat release from the various BOP components are calculated based on their heat-capacity curves. An experimental fuel reforming reactor for a 5.0 kW_e HT-PEMFC system was tested to experimentally validate the fuel reforming sub model. The model predictions were found to be in good agreement with the experimental data in terms of exhaust gas compositions and bed temperatures. Additionally, the simulation revealed the impacts of the burner air-fuel ratio (AFR) and the steam reforming reactor steam-carbon ratio on the system performance and efficiency. In particular, the combined heat and power efficiency of the system increased up to 78% when the burner AFR was properly adjusted. This study clearly illustrates that an HT-PEMFC system requires a high degree of thermal integration and optimization of the system configuration and operating conditions.     

PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation,Characterization and Fuel Cell Demonstration

Proton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid‐doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown‐restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.     

操作条件对PBI/H3PO4体系高温PEMFC的性能影响

全面研究了PBI/H3PO4体系高温质子交换膜燃料电池(PEMFC)的操作性能,研究了温度、压力、阴极气体成分(空气/氧气)对电池稳态操作性能的影响,同时,测试了电池的变压变温和变负载操作曲线,并且运用电化学交流阻抗谱的方法解释了操作条件对电池性能影响的原因,同时应用薄膜旋转圆盘电极技术初步研究了氧还原反应在两种不同的反应界面(Pt/C-PBI/H3PO4和Pt/C-Nafion/H2SO4)的差别。