Water management strategies for PGM-free catalyst layers for polymer electrolyte fuel cells

Platinum group metal–free (PGM-free) catalysts are promising candidates to catalyze the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs). Because of their low activity, larger loadings are used resulting in thicker catalyst layers. Transport, particularly water management, thereby becomes a more prominent performance factor. Currently, very few works attempted to understand water management in PGM-free catalyst layers, mainly because of other challenges that had to be overcome first, such as enhancing their activity and durability. The field has also been active in a hypothesis discussion of micropores flooding that led to the belief that poor stability of the PEFC performance is linked to active sites flooding within the micropores. We present here an overview of recent advances in understanding water management in the PGM-free catalyst layer for oxygen reduction reaction in PEFCs and provide an opinion on design guidance in optimizing catalyst layers to avoid flooding.     

Nb-TiO2 supported Pt cathode catalyst for polymer electrolyte membrane fuel cells

The Nb-doped TiO₂ nanostructure (Nb-TiO₂) was prepared as a support of metal catalyst in polymer electrolyte membrane fuel cells. Using the Nb-TiO₂ nanostructure support, we prepared the Nb-TiO₂ supported catalyst. The Nb-TiO₂ supported Pt catalyst (Pt/Nb-TiO₂) showed the well dispersion of Pt catalysts (∼3 nm) on the Nb-TiO₂ nanostructure supports (∼10 nm). The Pt/Nb-TiO₂ showed an excellent catalytic activity for oxygen reduction compared with carbon supported Pt cathode catalyst. The enhanced catalytic activity of Pt/Nb-TiO₂ in electrochemical half cell measurement may be mainly due to well dispersion of Pt nanoparticles on Nb-TiO₂ nanosized supports. In addition, from XANES spectra of Pt L edge obtained with the supported catalysts, the improved catalytic activity of Pt/Nb-TiO₂ for oxygen reduction may be caused by an interaction between oxide support and metal catalyst.     

Advances in the high performance polymer electrolyte membranes for fuel cells

This critical review tersely and concisely reviews the recent development of the polymer electrolyte membranes and the relationship between their properties and affecting factors like operation temperature. In the first section, the advantages and shortcomings of the corresponding polymer electrolyte membrane fuel cells are analyzed. Then, the limitations of Nafion membranes and their alternatives to large-scale commercial applications are discussed. Secondly, the concepts and approaches of the alternative proton exchange membranes for low temperature and high temperature fuel cells are described. The highlights of the current scientific achievements are given for various aspects of approaches. Thirdly, the progress of anion exchange membranes is presented. Finally, the perspectives of future trends on polymer electrolyte membranes for different applications are commented on (400 references).     

Assessment of transport-limited catalyst utilization for engineering of ultra-low Pt loading polymer electrolyte fuel cell anode

In this study, transport-limited catalyst utilization in polymer electrolyte fuel cell (PEFC) anodes is assessed via an agglomerate model with a broader view of designing ultra-low Pt loading, high performance anode. The model accounts for electrical and chemical potential-driven transport of electrons/protons and dissolved hydrogen, respectively and multi-component gas-phase transport in the catalyst layer. The model employs the kinetics of hydrogen oxidation reaction based on dual-pathway reversible reaction mechanism reported recently [J.X. Wang, T.E. springer, R.R. Adzic, J. Electrochem. Soc. 153 (2006) A1732]. The model predictions show that for conventional, randomly-structured catalyst transport limitations exist at two levels. At single-agglomerate level, the catalyst utilization is restricted by dissolved hydrogen diffusivity limiting the reaction to occur primarily in the outer shell of the agglomerate. At the catalyst layer level, the catalyst utilization is limited primarily by poor protonic conductivity. However, significant electronic potential gradients can exist in the catalyst layer thereby effectively reducing the available overpotential. Simulation results also show that by engineering the catalyst layer to overcome the transport limitations and, thereby, improving the effective catalyst utilization, high performance can be achieved in a PEFC anode at ultra-low Pt loading of 0.0225 mg/cm².     

Performance of catalyst layers of polymer electrolyte fuel cells: exact solutions

Performance of a catalyst layer of polymer electrolyte fuel cell under the assumptions of ideal transport of reactants and Tafel kinetics of electrochemical reaction is considered. Explicit expressions for the profiles of basic parameters (proton current density, overpotential and reaction rate) across the catalyst layer are obtained and a new conservation law is found. Exact expression for voltage–current curve of the catalyst layer is derived and simplified in the limiting cases of small and large current densities. The physics of transition from small to large currents is discussed.     

Development of polyoxadiazole nanocomposites for high temperature polymer electrolyte membrane fuel cells

Novel nanocomposite membranes were prepared with sulfonated polyoxadiazole and different amounts of sulfonated dense and mesoporous (MCM-41) silica particles. It has been shown that particle size and functionality of sulfonated silica particles play an important role when they are used as fillers for the development of polymer electrolyte nanocomposite membrane for fuel cells. No significant particle agglomerates were observed in all nanocomposite membranes prepared with sulfonated dense silica particles, as analyzed by SEM, AFM, TGA, DMTA and tensile tests. The T_g values of the composite membranes increased with addition of sulfonated silica, indicating an interaction between the sulfonic acid groups of the silica and the polyoxadiazole. Constrained polymer chains in the vicinity of the inorganic particles were confirmed by the reduction of the relative peak height of tan δ. A proton conductivity of 0.034 S cm⁻¹ at 120 °C and 25% RH, which is around two-fold higher than the value of the pristine polymer membrane was 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.     

燃料电池聚合物电解质膜的研究进展

本文根据聚合物电解质膜燃料电池操作温度、使用的电解质和燃料的不同,将其分为高温质子交换膜燃料电池、低温质子换膜燃料电池、直接甲醇燃料电池和阴离子交换膜燃料电池,综述了它们所用电解质膜的最新进展.第一部分简要介绍了这4种燃料电池的优点和不足.第二部分首先介绍了Nafion膜的结构模型,并对平行柱状纳米水通道模型在介观尺度上进行了修正;接着分别对应用于不同燃料电池的改性膜的改性思路作了分析;最后讨论了用于不同燃料电池的新型质子交换膜的研究,同时列举了性能突出的改性膜和新型质子交换膜.第三部分介绍了阴离子交换膜的研究现状.第四部分对未来聚合物电解质膜的研究作了展望.     

Nanostructured ultrathin catalyst layer with ordered platinum nanotube arrays for polymer electrolyte membrane fuel cells

Fabrication of novel electrode architectures with nanostructured ultrathin catalyst layers is an effective strategy to improve catalyst utilization and enhance mass transport for polymer electrolyte membrane fuel cells (PEMFCs).Herein,we report the design and construction of a nanostructured ultrathin catalyst layer with ordered Pt nanotube arrays,which were obtained by a hard-template strategy based on ZnO,via hydrothermal synthesis and magnetron sputtering for PEMFC application.Because of the crystallographically preferential growth of Pt (111) facets,which was attributed to the structural effects of ZnO nanoarrays on the Pt nanotubes,the catalyst layers exhibit obviously higher electrochemical activity with remarkable enhancement of specific activity and mass transport compared with the state-of-the-art randomly distributed Pt/C catalyst layer.The PEMFC fabricated with the as-prepared catalyst layer composed of optimized Pt nanotubes with an average diameter of 90(±10) nm shows excellent performance with a peak power density of 6.0W/mg~(Pt) at 1 A/cm~2,which is 11.6%greater than that of the conventional Pt/C electrode.     

介孔结构Fe/N/C作为质子交换膜燃料电池氧还原催化剂

燃料电池具有高效、低排放等优势,非常有希望作为未来电动汽车的能源转化装置.目前,燃料电池的商业化受制于昂贵的铂基催化剂,特别是动力学迟缓的阴极氧还原反应(ORR)铂催化剂. Fe/N/C被认为是最有潜力的ORR非贵金属催化剂,但其活性仍远低于Pt催化剂,必须依靠增加载量来弥补其与Pt催化剂的活性差距.然而,较厚的催化层(~100mm)会降低阴极传质速率.因此,改善Fe/N/C阴极的传质是提高电池性能的重要途径.
　　本文选择高N含量的2-氨基苯并咪唑(ABI)为氮源,通过水热聚合包覆在碳黑表面,然后掺入FeCl3,经高温热解/酸洗制备了Fe/N/C-ABI催化剂,并与基于间苯二胺的微孔型Fe/N/C催化剂(Fe/N/C-PmPDA)进行比较. Ar等温吸附-脱附结果表明, Fe/N/C-ABI催化剂具有较高的比表面积(662 m2/g)和丰富的双级孔结构(微孔和介孔);透射电镜表征显示Fe/N/C-ABI催化剂具有中空结构,介孔孔径大约为10–25 nm.而Fe/N/C-PmPDA催化剂具有相当的比表面积(656 m2/g),但以微孔为主,基本不含介孔.旋转环圆盘电极(RRDE)测试表明,在0.1 mol/L H2SO4溶液中, Fe/N/C-ABI催化剂的起始还原电位为0.92 V,在0.8 V电位下质量电流密度可达9.21 A/g;而Fe/N/C-PmPDA催化剂具有相近的起始电位,但具有更高的催化活性,质量电流密度为13.4 A/g.氢氧燃料电池(PEMFC)系统测试结果表明, Fe/N/C-ABI催化剂在1个背压和80oC测试条件下的最大功率密度达710 mW/cm2,高于Fe/N/C-PmPDA催化剂(616 mW/cm2).燃料电池与RRDE测试活性顺序的差异归结于Fe/N/C-ABI的中空球状结构. PEMFC工作时阴极会产生大量的水,很容易堵塞氧气传输通道. Fe/N/C-ABI的介孔结构可以作为水的产生和排除的缓存空间,也有利于提高O2传质,从而提高燃料电池性能.本文为具有高传质速率的Fe/N/C催化剂研制提供了一种新思路.     

New polymer electrolyte membranes for low temperature fuel cells

Partially fluorinated proton exchange materials were synthesised by pre-irradiation grafting of styrene into poly(vinylidene fluoride) films with subsequent sulfonation. The grafted and sulfonated membranes, PVDF-g-PSSA membranes, have been studied with respect to water uptake, ion and water clustering, ion conductivity and water diffusion coefficients. Water associates with the membranes in three different ways: bound non-freezable water, freezable bound water and freezable free water. The proton conductivity of the membrane is strongly dependent on the hydration, it decreases more rapidly than the water self diffusion with decreasing water content. Ion clusters with a Bragg distance of 25 Å form the conducting channels in the membranes.     

Fluorinated polyoxadiazole for high-temperature polymer electrolyte membrane fuel cells

For the first time a fluorinated polyoxadiazole doped with phosphoric acid as a proton-conducting membrane for operation at temperatures above 100 °C and low humidities for fuel cells has been reported. Fluorinated polyoxadiazole with remarkable chemical stability was synthesized. No changes in the molecular weight (about 200,000 g mol⁻¹) can be observed when the polymer is exposed for 19 days to mixtures of sulfuric acid and oleum. Protonated membranes with low doping level (0.34 mol of phosphoric acid per polyoxadiazole unit, 11.6 wt.% H₃PO₄) had proton conductivity at 120 °C and RH = 100% in the order of magnitude of 10⁻² S cm⁻¹. When experiments are conducted at lower external humidity, proton conductivity values drop an order of magnitude. However still a high value of proton conductivity (6 × 10⁻³ S cm⁻¹) was obtained at 150 °C and with relative humidity of 1%. In an effort to increase polymer doping, nanocomposite with sulfonated silica containing oligomeric fluorinated-based oxadiazole segments has also been prepared. With the addition of functionalized silica not only doping level but also water uptake increased. For the nanocomposite membranes prepared with the functionalized silica higher proton conductivity in all range of temperature up to 120 °C and RH = 100% (in the order of magnitude of 10⁻³ S cm⁻¹) was observed when compared to the plain membrane (in the order of magnitude of 10⁻⁵ S cm⁻¹).     

Freestanding sulfonated graphene oxide paper: a new polymer electrolyte for polymer electrolyte fuel cells

Sulfonated graphene oxide paper was fabricated by vacuum filtration of a colloidal solution of sulfonated graphite oxide. Layer by layer assembly of graphene oxide nano sheets interconnects the conduction paths and therefore sulfonated graphene oxide exhibits good proton conductivity and fuel cell performance.     

Effects of ionomer coverage on agglomerate effectiveness in catalyst layers of polymer electrolyte fuel cells

A large proportion of voltage losses in polymer electrolyte fuel cells (PEFCs) originates in cathode catalyst layers. Catalyst utilization and performance of conventional catalyst layers depend largely on their ionomer content and distribution. The present study explores effects of agglomerate size and ionomer distribution on reaction rate distributions and effectiveness factor of Pt utilization. To study the oxygen reduction reaction, we have developed an agglomerate model, which consists of coupled relations for proton and oxygen transport, metal charging behavior, and interfacial charge transfer kinetics. The model is considered under steady state conditions. Results show that higher effectiveness factor is attained for agglomerates with smaller size and larger oxygen partial pressure on the surface. In addition, low to medium coverage of the ionomer skin layer is beneficial in view of high effectiveness factors due to the optimized interplay of oxygen and proton supply.     

Use of a carbon nanocage as a catalyst support in polymer electrolyte membrane fuel cells

The corrosion-resistance of a carbon nanocage used as a catalyst support in a polymer electrolyte membrane fuel cell was investigated by measuring CO₂ generation using on-line mass spectrometry at a constant potential of 1.4 V for 30 min. Polarization curves of membrane electrode assemblies containing Pt/carbon nanocage were obtained and used to evaluate performance degradation. The carbon nanocage was found to possess significant resistance to electrochemical corrosion, exhibiting low performance degradation of only about 2.3% after the corrosion test. This high corrosion resistance is attributed both to the strong hydrophobic nature of the surface and the graphitic structure of the carbon nanocage.     

Asymmetric polymer electrolyte membranes for water management of fuel cells

Water management is one of the critical issues of polymer electrolyte membrane fuel cells because dehydration of a membrane increases membrane-resistance whereas excessive water flooding at the cathode impedes the gaseous diffusion of oxygen to reaction sites at the wetted catalyst surface. In this study, we have developed an asymmetric polymer electrolyte membrane that facilitates water management. The structural modification of the membrane strongly affected water management, due primarily to the fact that water must move through the membrane during fuel cell operation. The asymmetric membrane improved transport of water from the cathode to the anode when the hydrophilic side of the membrane located to the cathode, thereby enhancing overall fuel cell performance under both fully humidified and non-humidified conditions.     

Improving protonic conduction of membranes for polymer electrolyte fuel cells

Method for the modification of proton-conducting Nafion membranes by using a zirconium citrate one-substituted salt, aimed at the improving of characteristics of membranes for polymer-electrolyte-based fuel cells, is suggested. In the method, the membrane is impregnated first with zirconyl chloride and then with citric acid; an insoluble sol is thus formed in the membrane pores. The impregnation is carried out in ultrasound bath, using an isopropyl alcohol-water solvent, to make it more rapid and uniform. It is shown that the impregnation lowers the real component of the membrane impedance. The discharge characteristics of the impregnated and nonimpregnated membranes are compared.     

High performance nitrile copolymers for polymer electrolyte membrane fuel cells

This paper reports the fuel cells (DMFC and PEMFC) performance using sulfonated poly(arylene ether ether nitrile) (SPAEEN) copolymers containing sulfonic acid group arranged in structurally different ways. The membrane electrode assembly (MEA) fabricated from SPAEEN containing 60 mol% of angled naphthalenesulfonic acid group (m-SPAEEN-60) had superior performance over those derived from pendent naphthalenesulfonic acid group (p-SPAEEN) or sulfonated hydroquinone (HQ-SPAEEN) in H₂/air and/or DMFC conditions. For example, the current density of the MEA using m-SPAEEN-60 at 0.5 V and 2.0 M methanol was 250 mA/cm², whereas the current densities of the MEAs using p-SPAEEN-50 and HQ-SPAEEN-56 were 185 and 190 mA/cm², respectively. In addition, compared with the sulfonated polysulfone (BPSH-35) and Nafion membranes, the copolymer containing nitrile group showed the improved cell performance. For example, the power density of the MEA using m-SPAEEN-60 at 250 mA/cm² and 2.0 M methanol was 125 mW/cm², whereas the power densities of the MEAs using sulfonated polysulfone (BPSH-35) and Nafion were 115 and 113 mW/cm², respectively. m-SPAEEN-60 showed stable cell performance during extended operation (>100 h).     

直接甲醇燃料电池用PEMs的研究进展

本文介绍了用于直接甲醇燃料电池(DMFCs)的质子交换膜(PEMs)的工作原理与性能要求。讨论了影响DMFCs国PEMs的甲醇渗透性能的因素。综述了Nation、改性Nafion膜以及其它新品种膜的研究进展。