Mechanical endurance of polymer electrolyte membrane and PEM fuel cell durability

The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long‐term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain‐to‐failure of the chemically aged and RH‐cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge‐constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2346–2357, 2006     

聚合物电解质膜燃料电池薄电极制备技术的研究

为降低聚合物电解质膜燃料电池 (PEMFC)电极中铂的载量 ,本文建立一种新的薄电极制备技术 (TEFT) ,制备了表面平滑、颗粒分布均匀的低铂载量电极 .结果表明当电极的铂载量为 1mg/cm2 ,用Nafion 117膜作电解质时 ,电池的最大功率密度达 0 30W·cm-2 .系统地考察了阴极中不同PTFE和Nafion含量对PEMFC性能的影响 .     

Characterization of fibril reinforced membranes for fuel cells

Characteristics of fibril reinforced membranes developed by Asahi Glass Company are reviewed. PTFE-fibrils <1 μm in diameter are dispersed in ion-exchange membranes uniformly. Mechanical properties, such as tensile strength, tear strength, creep property and compressive property were examined and compared with non-reinforced membranes. Fibril reinforced membranes, even by the addition of a small amount of PTFE-fibrils (2.7 wt.%), show excellent mechanical strength, especially in creep and tear strength. Cell performance is nearly equal to the one using a non-reinforced membrane and cell voltage stays about the same during the cell operation at 80 °C for 3000 h.     

采用交流阻抗法对质子交换膜燃料电池(PEMFC)电化学行为的研究

通过对PEMFC运行于不同状态下的交流阻抗分析,提出其等效电路可用LR₁[QR₂]表示.对各参数模拟表明,随放电电位升高感抗L下降,而系统电阻R₁和电荷转移阻抗R₂均呈上升趋势,改进催化层结构设计及在大电流运行时加大排水量有利于减小电池内部感抗;加湿温度上升有利于降低R₁和R₂,但低放电电位下R₂的偏离程度较大,电池内阻是LR₁[QR₂]的综合结果,在高放电电位时,随加湿温度上升内阻下降,低放电电位时内阻上升的原因在于水抑制了反应气体到达催化剂表面及电荷内部转移的过程,使电池内部趋向于电容状态.LR₁[QR₂]微观分析和宏观现象得到较好的统一,高放电电位时表现尤为明显.     

Anode Catalytic Dependency Behavior on Ionomer Content in Direct CO Polymer Electrolyte Membrane Fuel Cell

In this work, the effect of Nafion ionomer content on the structure and catalytic performance of direct CO polymer electrolyte membrane fuel cell(CO-PEMFC) by using Rh-N-C single-atom catalyst as the anode catalyst layers was studied. The ionic plaque and roughness of the anode catalyst layers increase with the increase of Nafion ionomer content. Furthermore, the contact angle measurement results show that the hydrophilicity of the anode catalyst layers also increases with the increase of Nafion ionomer content. However, when the Nafion ionomer content is too low, the binding between microporous layers, catalyst layers and membrane cannot meet the requirement for either electric conductivity or mass transfer. While Nafion ionomer content increased above 30%, the content of water in anode is difficult to control. Therefore, it was found that AN 30(30% Nafion ionomer content of anode) is the best level to effectively extend the three-phase boundary and improve CO-PEMFCs performance.     

有机溶剂对PEM燃料电池CCM结构和性能的影响

采用直接喷涂法将催化剂涂覆在质子交换膜上形成CCM(catalyst coated membrane),CCM与碳纸扩散层组成膜电极用于质子交换膜燃料电池.制备CCM的混合液由质量分数20%的Pt/C催化剂、质量分数5%的Nafion溶液、有机溶剂和水组成.不同的有机溶剂(乙醇、异丙醇和叔丁醇)、有机溶剂的含量、溶剂的...     

Preparation of 4(5)-vinylimidazole-co-acrylic acid copolymer and thermal performances related to applicability as PEM fuel cells

A series of acrylic acid and 4(5)-vinylimidazole copolymers for a non-hydrous proton transferring membrane used in polymer electrolyte membrane for fuel cell (PEMFC) are reported. The feed ratio of each monomer results in the variation of copolymer as evaluated by Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (¹H-NMR). Differential scanning calorimeter and thermal gravimetric analyzer confirm the thermal properties of copolymer films with T_g at 105-177 °C and T_d above 230 °C. The simultaneous analysis of wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) suggests the thermal performance about the decrease in domain size as a consequence of the loss of moisture content in the membrane and the increase in domain size as a consequence of chain mobility after T_g. The proton conductivities under anhydrous condition of the copolymer membranes are in the range of 10⁻² S/cm even up to 120 °C.     

电催化剂纳米碳基载体的研究进展

质子交换膜燃料电池(PEMFCs)因具有工作温度低、结构紧凑、无腐蚀、启动快和灵活性好等优点,受到人们广泛关注.但其工作时动力学迟缓且易受CO毒化影响,往往需要负载Pt等贵金属催化剂,导致PEMFCs的成本高昂,阻碍了其商业化应用.为提高Pt贵金属的利用率,通常将Pt负载在载体材料上来提高Pt的分散性以减少Pt颗粒集聚...     

质子交换膜燃料电池膜电极组催化层结构

膜电极组件(MEA)是质子交换膜燃料电池(PEMFC)的核心元件,而催化层是MEA的核心部分。催化层既是电化学反应的场所,同时也为质子、电子、反应气体和水提供运输通道,其结构对PEMFC的成本及性能有很大的影响。本文综述了近年来国内外催化层结构方面的研究进展,介绍了催化层中聚合物电解质(Nafion)含量、溶剂的性质和其他添加剂对MEA结构和性能的影响,MEA热压参数的研究进展以及目前常见的催化层涂布方法。     

Sulfonated poly(ether sulfone)/sulfonated polybenzimidazole blend membrane for fuel cell applications

Polymer blending is used to modify or improve the dimensional and thermal stability of any two different polymers or copolymers. In this study, both sulfonated polybenzimidazole homopolymer (MS-p-PBI 100) and sulfonated poly(aryl ether benzimidazole) copolymers (MS-p-PBI 50, 60, 70, 80, 90) were successfully synthesized from commercially available monomers. The chemical structure and thermal stability of these polymers was characterized by ¹H NMR, FT-IR and TGA techniques. Blend membranes (BMs) were prepared from the salt forms of sulfonated poly(ether sulfone) (PES 70) and MS-p-PBI 100 using dimethylacetamide (DMAc). These blend membranes exhibited good stability in boiling water. The blending of 1 wt.% of MS-p-PBI 100 and 99 wt.% of PES 70 to produce the blend membrane BM 1 reduced membrane swelling, thus leading to good dimensional stability and comparable proton conductivity. Hence, BM 1 was chosen for the fabrication of a membrane electrode assembly (MEA) for proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) applications. This paper reports on PEMFC and DMFC performance under specific conditions.     

低维纳米材料在质子交换膜燃料电池电催化反应中的应用

在聚合物膜燃料电池中,电催化剂决定了电池的运行寿命及性能。本文按照材料维数的不同,分别介绍了近年来零维、一维和二维材料在聚合物膜燃料电池电极反应中应用的最新研究进展;并对各类聚合物膜燃料电池电催化反应的机理和研究侧重点进行了详细的介绍。     

Improving the efficiency of a direct ethanol fuel cell by a periodic load change

We present a simple method to increase the efficiency of a direct ethanol fuel cell by a periodic modulation of the load (pulsed mode). The fuel cell was periodically short circuited with a resistor (1...     

周期性负载变化提高乙醇燃料电池效率(英文)

We present a simple method to increase the efficiency of a direct ethanol fuel cell by a periodic modulation of the load(pulsed mode). The fuel cell was periodically short circuited with a resistor(1 Ω) for a few seconds(high load period) followed by a low load period of up to 100 s when the resistor was disconnected. The open circuit voltage(OCV) values before and after the short circuit of the cell showed an increase of up to 70 mV. The higher OCV was due to the oxidation and removal of strongly adsorbed CO during the electric short circuit when the electric potential of the anode was increased to be close to the cathode potential. The depoisoned anode surface was much more active directly after the short circuit. The slow decrease of the OCV observed after the short circuit was caused by the subsequent poisoning of the anode surface, which can be neutralized by another short circuit. In general, a stable increase in cell performance was obtained by repetition of the electric short circuit. The data showed that the pulse mode gave an increase in the power generated by the direct ethanol fuel cell by up to 51% and was 6% on average. It is anticipated that this mode of operation can be used also in different types of polymer electrolyte membrane fuel cells where CO poisoning is a problem, and after optimization of the parameters, a much higher gain in efficien-cy can be obtained.     

Gas diffusion electrodes for polymer electrolyte fuel cell using sulfonated polyimide

Gas diffusion electrodes for high temperature polymer electrolyte fuel cells (PEFCs) have been prepared by using a novel proton conductive sulfonated polyimide (SPI) electrolyte. The catalyst layer was composed of Pt-loaded carbon black (Pt-CB) and SPI ionomer. The polarization properties and the microstructure of the catalyst layer were investigated as a function of the SPI/CB weight ratio. The anodic polarization was found to be negligibly small for all the compositions examined. The highest cathode performance was obtained at SPI/CB = 0.5 (by weight), where the best balance of high catalyst utilization and oxygen gas diffusion rate through the ionomer was obtained.     

The challenges in reliable determination of degradation rates and lifetime in polymer electrolyte membrane fuel cells

The durability of polymer electrolyte membrane fuel cells (PEMFCs) needs to be further improved to cope with application requirements and economic competitiveness. This article highlights the challenges in the reliable determination of degradation rates and lifetime. The reliable evaluation of performance degradation rates is fundamental to quantify and benchmark durability and to allow comparisons between PEMFC durability tests performed using different materials or in different laboratories. The use of efficient recovery procedures enables the discrimination of reversible and irreversible voltage losses and facilitates the understanding of recovery mechanisms. In the end, recent contributions about lifetime diagnoses and prediction are presented, which are promising to be implemented in PEMFC applications.     

PEFC catalyst layers: Effect of support microstructure on both distributions of Pt and ionomer and cell performance and durability

In order to improve the performance and durability of polymer electrolyte fuel cells (PEFCs), various improvements in the microstructures of cathode catalyst layers (CLs) were initiated in the early 1990s. More recent advances in CL materials are highlighted, including carbon supports for improved accessibility of Pt nanoparticles (NPs), adsorption of ionomer on the Pt surface, high-oxygen-permeability ionomers, corrosion resistance of mesoporous and microporous carbons, and conductive ceramic supports with a fused-aggregate network structure. These approaches are summarized as stepwise improvements. The influences of the support structure on the distribution of Pt NPs and ionomer are reviewed, as well as their effects on performance and durability. These approaches for carbon supports are extended to conductive ceramic supports and the unique advantages are discussed.     

Effect of pretreatments on the anode structure of solid oxide fuel cells

An important objective in the development of solid oxide fuel cell (SOFC) is to produce thin stabilized zirconia electrolytes that are supported upon the nickel–zirconia composite anode. Although this will reduce some of the problems associated with SOFCs by permitting lower temperature operation, this design may encounter problems during start- up. The first step in a start-up involves the reduction of nickel oxide in the anode to metallic nickel and increase of three-phase boundary will be beneficial for further reaction. In this study, two pretreatment methods are investigated for their effects on the performances of SOFC. Performances of the SOFCs are influenced by the pretreatment conditions, which included exposure of the cells to dilute H₂/O₂ either under open-circuit or closed-circuit conditions before their performance studies. By carrying out the methods, the pretreatment using the closed circuit is found to attain much higher performances effectively and efficiently. Accompanying with SEM and element analysis, increase of three-phase boundary is considered to give rise to changes in the anode microstructure, leading to activation of the anode. Mechanisms of NiO in anode reducing to Ni and porous structure via different pretreatments and their effects on the anode microstructure are proposed.     

The Role of PANI/Nafion on the Performance of ORR in Gas Diffusion Electrodes of PEM Fuel Cell

In this work, gas diffusion electrodes were fabricated from Nafion and polyaniline (PANI) nanofibers. The goal of this work was to find the optimal combination of Nafion and polyaniline in gas diffusion electrodes. The electrodes were evaluated for oxygen reduction reaction effectiveness, and their electrochemical properties were investigated by using electrochemical techniques, and PEM single cell. The results revealed that electrodes containing both Nafion and polyaniline worked more efficiently than electrodes containing either Nafion or polyaniline only. The optimum combination is noted as 0.6 mg/cm² PANI and 0.4 mg/cm² Nafion.     

An NMR study of methanol diffusion in polymer electrolyte fuel cell membranes

Methanol diffusion in two polymer electrolyte membranes, Nafion 117 and BPSH 40 (a 40% disulfonated wholly aromatic polyarylene ether sulfone), was measured using a modified pulsed field gradient NMR method. This method allowed for the diffusion coefficient of methanol within the membrane to be determined while immersed in a methanol solution of known concentration. A second set of gradient pulses suppressed the signal from the solvent in solution, thus allowing the methanol within the membrane to be monitored unambiguously. Over a methanol concentration range of 0.5–8 M, methanol diffusion coefficients in Nafion 117 were found to increase from 2.9 × 10⁻⁶ to 4.0 × 10⁻⁶ cm² s⁻¹. For BPSH 40, the diffusion coefficient dropped significantly over the same concentration range, from 7.7 × 10⁻⁶ to 2.5 × 10⁻⁶cm² s⁻¹. The difference in diffusion behavior is largely related to the amount of solvent sorbed by the membranes. Increasing the methanol concentration results in an increase in solvent uptake for Nafion 117, while BPSH 40 actually excludes the solvent at higher concentrations. In contrast, diffusion of methanol measured via permeability measurements (assuming a partition coefficient of 1) was lower (1.3 × 10⁻⁶ and 6.4 × 10⁻⁷ cm² s⁻¹ for Nafion 117 and BPSH 40 respectively) and showed no concentration dependence. The differences observed between the two techniques are related to the length scale over which diffusion is monitored and the partition coefficient, or solubility, of methanol in the membranes as a function of concentration. For the permeability measurements, this length is equal to the thickness of the membrane (178 and 132 μm for Nafion 117 and BPSH 40 respectively) whereas the NMR method observes diffusion over a length of approximately 4–8 μm. Regardless of the measurement technique, BPSH 40 is a greater barrier to methanol permeability at high methanol concentrations.     

Low-temperature direct ammonia fuel cells: Recent developments and remaining challenges

Low-temperature direct ammonia fuel cells (DAFCs) are fueled directly by ammonia, a carbon-neutral fuel stored in the liquid state under low pressure. Liquid ammonia has advantages over compressed hydrogen gas, including higher energy density and facilitated distribution and refill. The beginning-of-life performance reported until recently for low-temperature DAFCs has been substantially lower than that of polymer electrolyte fuel cells fueled by hydrogen. We discuss here promising recent advances in electrocatalyst development, cell performance, and cell performance stability for low-temperature DAFCs, including beginning-of-life peak power density of 420 mW/cm², and operation over several days at constant current. In addition, we describe technology gaps that must be closed for low-temperature DAFCs to achieve the performance required for practical applications.