PEMFC catalyst layers: the role of micropores and mesopores on water sorption and fuel cell activity

The effects of carbon microstructure and ionomer loading on water vapor sorption and retention in catalyst layers (CLs) of PEM fuel cells are investigated using dynamic vapor sorption. Catalyst layers based on Ketjen Black and Vulcan XC-72 carbon blacks, which possess distinctly different surface areas, pore volumes, and microporosities, are studied. It is found that pores <20 nm diameter facilitate water uptake by capillary condensation in the intermediate range of relative humidities. A broad pore size distribution (PSD) is found to enhance water retention in Ketjen Black-based CLs whereas the narrower mesoporous PSD of Vulcan CLs is shown to have an enhanced water repelling action. Water vapor sorption and retention properties of CLs are correlated to electrochemical properties and fuel cell performance. Water sorption enhances electrochemical properties such as the electrochemically active surface area (ESA), double layer capacitance and proton conductivity, particularly when the ionomer content is very low. The hydrophilic properties of a CL on the anode and the cathode are adjusted by choosing the PSD of carbon and the ionomer content. It is shown that a reduction of ionomer content on either cathode or anode of an MEA does not necessarily have a significant detrimental effect on the MEA performance compared to the standard 30 wt % ionomer MEA. Under operation in air and high relative humidity, a cathode with a narrow pore size distribution and low ionomer content is shown to be beneficial due to its low water retention properties. In dry operating conditions, adequate ionomer content on the cathode is crucial, whereas it can be reduced on the anode without a significant impact on fuel cell performance.     

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells

Increasing catalytic activity and durability of atomically dispersed metal–nitrogen–carbon (M–N–C) catalysts for the oxygen reduction reaction (ORR) cathode in proton-exchange-membrane fuel cells remains a grand challenge. Here, a high-power and durable Co–N–C nanofiber catalyst synthesized through electrospinning cobalt-doped zeolitic imidazolate frameworks into selected polyacrylonitrile and poly(vinylpyrrolidone) polymers is reported. The distinct porous fibrous morphology and hierarchical structures play a vital role in boosting electrode performance by exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transport of reactant. The enhanced intrinsic activity is attributed to the extra graphitic N dopants surrounding the CoN₄ moieties. The highly graphitized carbon matrix in the catalyst is beneficial for enhancing the carbon corrosion resistance, thereby promoting catalyst stability. The unique nanoscale X-ray computed tomography verifies the well-distributed ionomer coverage throughout the fibrous carbon network in the catalyst. The membrane electrode assembly achieves a power density of 0.40 W cm⁻² in a practical H₂/air cell (1.0 bar) and demonstrates significantly enhanced durability under accelerated stability tests. The combination of the intrinsic activity and stability of single Co sites, along with unique catalyst architecture, provide new insight into designing efficient PGM-free electrodes with improved performance and durability.     

质子交换膜燃料电池关键材料的研究进展EI北大核心CSCD

燃料电池是一种非常有前景的新能源体系。燃料电池不使用热力发动机,利用电极和电解质界面发生的化学反应直接将燃料的化学能转换成电能,反应不受卡诺循环限制,因此,具有高的能量转换效率。在燃料电池中,质子交换膜燃料电池(PEMFC)在便携式设备、交通运输以及固定装置领域具有重要的应用前景。然而,目前的PEMFC还存在一些问题,主要包括高成本、功率不足、稳定性差等问题,限制了其大规模商业化应用。这些问题的根本原因在于PEMFC中阴极催化剂、气体扩散层、质子交换膜和双极板等关键材料的成本和性能还不能满足PEMFC商业化的要求。要实现PEMFC的大规模应用,需要开发先进的阴极催化剂、气体扩散层、质子交换膜和双极板等关键材料。针对PEMFC对低成本、高性能先进材料的需求,本文综述了阴极催化剂、气体扩散层、质子交换膜和双极板等关键材料的研究进展以及应用面临的问题,并指出了未来的发展方向:加强铂合金催化剂以及金属-氮-碳(M-N-C)化合物催化剂的规模化制备工艺的探索;制备兼具高质子传导率和优异力学性能的质子交换膜;详细研究改性气体扩散层在不同的工况条件下对PEMFC性能的影响;开发具有优良耐蚀性和导电性的涂层或新型金属材料用于双极板。     

Highly Water‐Stable Lanthanide–Oxalate MOFs with Remarkable Proton Conductivity and Tunable Luminescence

Although proton conductors derived from metal–organic frameworks (MOFs) are highly anticipated for various applications including solid‐state electrolytes, H₂ sensors, and ammonia synthesis, they are facing serious challenges such as poor water stability, fastidious working conditions, and low proton conductivity. Herein, we report two lanthanide–oxalate MOFs that are highly water stable, with so far the highest room‐temperature proton conductivity (3.42 × 10^?3 S cm^?1) under 100% relative humidity (RH) among lanthanide‐based MOFs and, most importantly, luminescent. Moreover, the simultaneous response of both the proton conductivity and luminescence intensity to RH allows the linkage of proton conductivity with luminescence intensity. This way, the electric signal of proton conductivity variation versus RH will be readily translated to optical signal of luminescence intensity, which can be directly visualized by the naked eye. If proper lanthanide ions or even transition‐metal ions are used, the working wavelengths of luminescence emissions can be further extended from visible to near infrared light for even wider‐range applications.     

Synthesis and characterization of composites of DBSA-doped polyaniline and polystyrene-based ionomers

The synthesis of composites of n-dodecylbenzene sulfonate-doped polyaniline (PANI-DBSA) and poly(styrene–metal acrylate) ionomers is presented. The ionomers of lithium, sodium and potassium were prepared by emulsion polymerization at different styrene-to-metal acrylate weight ratios. The composites made with the potassium ionomer exhibit the largest conductivity due to the higher content of acid groups that allows stronger interactions with the PANI chains compared to the Na and Li ionomers. IR spectroscopy suggests that hydrogen bonding interactions take place between PANI-DBSA chains and that amine salt groups form by chemical reactions between the amine groups of PANI and the acid groups of the ionomer. X-ray diffraction reveals that the ionomer affects the structural ordering of PANI-DBSA. All the PANI-DBSA–ionomer composites show higher thermal stability than the PANI-DBSA material. SEM shows a characteristic agglomerate morphology in all the composites. The composite showing the highest electrical conductivity was mixed with poly(n-butyl methacrylate) (PBMA) by extrusion and the films obtained have higher electrical conductivity than that of films of the same system without ionomer.     

Nitrogen‐Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells

Due to the Fenton reaction, the presence of Fe and peroxide in electrodes generates free radicals causing serious degradation of the organic ionomer and the membrane. Pt‐free and Fe‐free cathode catalysts therefore are urgently needed for durable and inexpensive proton exchange membrane fuel cells (PEMFCs). Herein, a high‐performance nitrogen‐coordinated single Co atom catalyst is derived from Co‐doped metal‐organic frameworks (MOFs) through a one‐step thermal activation. Aberration‐corrected electron microscopy combined with X‐ray absorption spectroscopy virtually verifies the CoN₄ coordination at an atomic level in the catalysts. Through investigating effects of Co doping contents and thermal activation temperature, an atomically Co site dispersed catalyst with optimal chemical and structural properties has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in challenging acidic media (e.g., half‐wave potential of 0.80 V vs reversible hydrogen electrode (RHE). The performance is comparable to Fe‐based catalysts and 60 mV lower than Pt/C ‐60 μg Pt cm^?2). Fuel cell tests confirm that catalyst activity and stability can translate to high‐performance cathodes in PEMFCs. The remarkably enhanced ORR performance is attributed to the presence of well‐dispersed CoN₄ active sites embedded in 3D porous MOF‐derived carbon particles, omitting any inactive Co aggregates.     

Application of low-voltage electrophoretic deposition to fabrication of direct methanol fuel cell electrode composite catalyst layer

In this study, the application of a low-voltage electrophoretic deposition (EPD) approach to the fabrication of DMFC electrode composite (i.e., catalyst/ionomer) catalyst layers using a CNT-supported PtRu (PtRu/CNT) anode nanocatalyst was investigated. In the operation of EPD, the PtRu/CNT electrocatalyst was first well mixed with a suitable amount of Nafion^® solution (ionomer dispersion) with or without the addition of HClO₄ as a supporting electrolyte and then electrophoretically deposited onto a non-catalyzed electrode base at a low applied DC voltage range of 0–5 V for 0–60 min. The resultant composite catalyst layer appeared to be thin and quite smooth exhibiting a lustrous texture particularly when the supporting electrolyte was employed in the suspension. Electrochemical impedance spectroscopy (EIS), however, showed that the coated composite catalyst layer exhibited a fairly high resistance indicating an excessive amount of ionomer was preferably deposited. Application of the fabricated electrode to a DMFC resulted in a cell performance with low but reasonable power density. These test results suggested that the low-voltage EPD could be a feasible approach to effective fabrication of DMFC electrode composite catalyst layers incorporated with CNT-supported electrocatalysts, although significant improvements are deemed to be necessary.     

一种新型的磺化聚芳醚酮质子交换膜材料

利用先聚合后磺化的方法合成了一种新型磺化聚芳醚酮,FTIR和HNMR结构表征表明,其磺酸基只连在悬挂侧链上.利用浇铸法将该材料制备成膜,对膜的离子交换容量(IEC)、平均当量重量(EW)、磺化度(SD)、吸水性、线性膨胀率及其电导率进行了表征,结果表明这种膜材料具有良好的吸水性和较低的线性溶胀率,所制得的膜在100℃、100%相对湿度时的质子电导率与Nafion-117~(R)膜相近,有望作为质子交换膜使用.     

环境对自呼吸式MEMS微型H2/空气质子交换膜燃料电池的影响

利用MEMS技术制备了一种自呼吸式微质子交换膜燃料电池(PEMFC),阳极采用点蛇混合结构,阴极采用双层镂空微流场结构,阴极靠近膜电极侧微孔尺寸从5～50m不等。鉴于自呼吸式电池的性能受环境的影响很大,本文着重研究了环境湿度和温度对电池性能的影响。结果表明阴极微孔尺寸为11m和15m的电池孔径适度,在环境20℃、30%-70%RH时两电池的极限电流密度(Jmax)和峰值功率密度(Pmax)均表现出较高值,性能良好;阴极微孔尺寸为11m的电池在空气维持50%RH下,温度由10℃升到40℃时Pmax逐渐增大,增幅达14.9%;若不维持空气湿度而改变温度,则温度由10℃升高到40℃时Pmax先增大后减小,20℃时达最大。     

Thin plasma-polymerized layers of hexamethyldisiloxane for humidity sensor development

The response of resistive-type sensors based on thin hexamethyldisiloxane layers to relative humidity (RH) was evaluated. Humidity sensitive layers were plasma polymerized at low frequency glow discharge using a capacitively coupled parallel plate reactor. The sensor design comprises the absorbing layer deposited on clean glass substrate with comb-shape aluminum electrodes (interdigitated structure). The change in electrical impedance of the sensing film was monitored as the device was exposed to humidity. The variation of the plasma-polymerization parameters resulted in different humidity sensing properties which could be correlated to the results of Fourier transform infrared spectroscopy (FTIR). The deposited films exhibited a detectable response to RH ranging from 30 to 95% with low hysteresis, good reproducibility and stability in long-term use. Films with a greater thickness showed a significant decrease in the humidity sensing capability. FTIR analysis revealed the presence of SiH bonding groups, which are frequently linked to the film density. The increase in the plasma discharge power induced also a significant decrease in the diffusion process of water vapor inside the sensitive layer bulk.     

First Principles Computations of the Oxygen Reduction Reaction on Solid Metal Clusters

An improvement in the catalytic process of oxygen reduction reactions is of prime importance for further progress in low temperature fuel cell performance. This paper intends to investigate this problem from a fundamental quantum mechanics viewpoint. For this purpose, a hybrid density functional theory is employed to analyze the catalytic mechanism of the oxygen reduction at the fuel cell cathode. Major steps in the oxygen reduction that include the oxygen adsorption on solid metal clusters (e.g. Cu and Pt) and complete four proton transfer steps are simulated. Proton transfer processes from hydroniums to the adsorbed oxygen molecules to produce water are observed to be mainly influenced by the electronic cloud at the catalyst. The first principles computation results reveal a difficulty in choosing the catalyst material and how the catalytic mechanism limits the performance of current low temperature fuel cells.     

氧化石墨烯/笼型聚倍半硅氧烷星型嵌段共聚物复合质子交换膜的制备与性能

通过共混的方法制备了含笼型聚倍半硅氧烷(POSS)星型拓扑结构嵌段共聚物的氧化石墨烯(GO)/笼型聚倍半硅氧烷-(聚甲基丙烯酸甲酯-共聚-磺化聚苯乙烯)(POSS-(PMMA₂₆-b-SPS₁₅₆)₈)复合质子交换膜。通过研究复合质子交换膜的离子交换容量(IEC)、质子传导率、吸水率与溶胀率,考察了GO含量对复合质子交换膜性能的影响。研究发现:复合质子交换膜的离子交换容量随GO含量的增加而升高,吸水率和溶胀率随着GO加入而降低,在测定温度范围内复合质子交换膜均表现出较高的尺寸稳定性,GO的添加改善了纯聚合物膜在80℃失水导致传导率下降的问题,提高了质子交换膜的质子传导率,发现在相对湿度为100%、80℃时,GO含量为0.3wt%的复合质子交换膜的质子传导率约为纯聚合物膜的3.2倍。      

SPPO/SiO2/PWA复合质子交换膜的制备及性能

利用溶胶-凝胶法制备出了SPPO/SiO2/PWA复合质子交换膜,对膜的离子交换容量(IEC)、平均当量重量(EW)、磺化度(SD)、吸水性、溶胀率、质子电导率、Tg进行了表征,此外,还对膜的结构进行了FT-IR、SEM表征,结果表明,所制得的掺杂2%～5%SiO2和3%PWA的SPPO复合膜在100℃、100%相对湿度时的质子电导率与Nafion-117?膜相近,有望作为质子交换膜使用。     

Sulfonated poly(arylene ether sulfone ketone) multiblock copolymers with highly sulfonated blocks. Long-term fuel cell operation and post-test analyses

The stability of poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes having highly sulfonated hydrophilic blocks was tested in an operating fuel cell. The electrochemical properties and drain water were monitored during the test, followed by post-test analyses of the membrane. During a 2000-h fuel cell operation test at 80 °C and 53% RH (relative humidity) and with a constant current density (0.2 A cm(-2)), the cell voltage showed minor losses, with slight increases in the resistance. In the drain water, anions such as formate, acetate, and sulfate were observed. Post-test analyses of the chemical structure by NMR and IR spectra revealed that the sulfonated fluorenyl group with ether linkage was the most likely to have degraded during the long-term operation, producing these small molecules. The minor oxidative degradation only slightly affected the proton conductivity, water uptake, and phase-separated morphology.     

Carbon-nanofiber composite electrodes for thin and flexible lithium-ion batteries

Addition of vapor-grown carbon nanofiber (VGCF) into a LiCoO₂ composite electrode increases electrode’s conductivity and adhesion strength significantly. These increases are attributed to the uniform distribution of network-like VGCF of high conductivity; VGCF not only connects the surface of the active materials, its network penetrates into and connects each active material particle. VGCF composite electrode also improves the electrochemical performance of thin and flexible lithium-ion batteries such as discharge capacity at high current densities, cycle-life stability, and low-temperature (at −20 °C) discharge capacity. These improved electrochemical properties are attributed to the well-distributed network-like carbon nanofibers, VGCF, within the cathode. The addition of VGCF reduces the electron conducting resistance and decreases the diffusion path for lithium ions, hence increases the utilization of active materials during high-current discharge and low-temperature discharge. In addition, network-like VGCF forms a more uniform cathode structure so as to have a lower deterioration rate and correspondingly better life cycle stability.     

Nafion and modified-Nafion membranes for polymer electrolyte fuel cells: An overview

Polymer electrolyte fuel cells (PEFCs) employ membrane electrolytes for proton transport during the cell reaction. The membrane forms a key component of the PEFC and its performance is controlled by several physical parameters, viz. water up-take, ion-exchange capacity, proton conductivity and humidity. The article presents an overview on Nafion membranes highlighting their merits and demerits with efforts on modified-Nafion membranes. Energy security refers to various security measures that a given nation, or the global community as a whole, must carryout to maintain an adequate energy supply     

化学镀制备高温质子导体镍电极及其电化学性能

采用化学镀在高温质子导体CaZr_0.9In_0.1O_3-δ (CZI)的电解质陶瓷表面沉积金属镍电极, 通过SEM显微结构分析比较了酸刻蚀和还原工艺对电极形貌以及电极-电解质界面的影响。结果表明, 使用HNO₃-HCl混合刻蚀液, 并以水合肼为还原剂的二次化学镀可获得颗粒均匀细小且界面结合良好的镍电极。通过电化学阻抗谱并结合浓差电池等方法研究比较了以化学镀镍电极和涂覆焙烧铂电极为电极, CZI为电解质的对称电池的电导率和质子迁移率。工作温度为800℃时, 镍电极高温质子导体的总电导率为4.131×10^-4 S/cm, 并且工作温度在400℃以上时, 镍电极对称电池的质子迁移率均接近100%。这些结果表明, 二次化学镀制备的镍电极具有与铂电极相近的电化学性能, 而成本则更低, 可以取代Pt电极用于高温质子导体的电化学器件中。     

Supported and unsupported platinum catalysts prepared by a one-step dry deposition method and their oxygen reduction reactivity in acidic media

The present study examines the physical and electrochemical properties of platinum particles generated by a combustion method for use in oxygen reduction on the cathode side of a proton exchange fuel cell (PEMFC). This method employs a one-step, open-atmosphere, and dry deposition technique called reactive spray deposition technology (RSDT). The objective of this study is to characterize the intrinsic activity of the platinum produced for incorporation into low-loading cathode electrodes in high performing membrane electrode assemblies (MEA). The process allows for independent real-time control of the carbon, platinum, and ionomer ratios in the final electrode. In this research work we examine the oxygen reduction reaction via a rotating disk three electrode set-up to understand the intrinsic activity of the as-sprayed platinum as well as platinum condensed onto a carbon support. The mass and specific activities were measured in a 0.1 M perchloric acid electrolyte under different deposition conditions and loading was verified by atomic emission spectroscopy inductively coupled plasma (AES-ICP). Microscopy results indicate that the platinum particle sizes are 5 nm (σ = 2.8 nm) in diameter while TEM and XRD show that the platinum generated by the process is pure and crystalline without bulk oxides or precursor material present. The initial rotating disk electrode result shows that the RSDT technique is capable of producing catalysts with an oxygen reduction mass activity at 0.9 V of 200 mA/mg_Pt rotating at 1600 rpm and 30 °C. The electrochemically active surface area approaches 120 m²/g for the platinum, carbon, and ionomer samples and the unsupported sample with only platinum has an active area of 92 m²/g. The rather larger surface area of the unsupported sample exists when the platinum is deposited as a highly porous nanostructured layer that allows for high penetration of reactant.     

Cerium sulfophenyl phosphate, a novel inorgano-organic solid proton-conducting material

Cerium sulfophenyl phosphate (CeSPP), a novel inorgano-organic solid proton conductor, was synthesized from the reaction of m-sulfophenyl phosphonic acid and hydrated cerous nitrate. The synthesized CeSPP was characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis (TGA), scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The TGA results reveal that CeSPP has good thermal stability. The synthesized CeSPP exhibited considerable high-proton conductivity. The proton conductivity was about 0.13 S/cm at 150 °C under 100% relative humidity. The proton conductivity at 105 °C at different RH was also measured.     

UIO‐66‐NH2‐Derived Mesoporous Carbon Catalyst Co‐Doped with Fe/N/S as Highly Efficient Cathode Catalyst for PEMFCs

Efficient, low‐cost catalysts are desirable for the sluggish oxygen reduction reaction (ORR). Herein, UIO‐66‐NH₂‐derived multi‐element (Fe, S, N) co‐doped porous carbon catalyst is reported, Fe/N/S‐PC, with an octahedral morphology, a well‐defined mesoporous structure, and highly dispersed doping elements, synthesized by a double‐solvent diffusion‐pyrolysis method (DSDPM). The morphology of the UIO‐66‐NH₂ precursor is perfectly inherited by the derived carbon material, resulting in a high surface area, a well‐defined mesoporous structure, and atomic‐level dispersion of the doping elements. Fe/N/S‐PC demonstrates outstanding catalytic activity and durability for the ORR in both alkaline and acidic solutions. In 0.1 m KOH, its half‐potential reaches 0.87 V (vs reversible hydrogen electrode (RHE)), 30 mV more positive than that of a 20 wt% Pt/C catalyst. In 0.1 m HClO₄, it reaches 0.785 V (vs RHE), only 65 mV less than that of Pt/C. The catalyst also exhibits excellent performance in acidic hydrogen/oxygen proton exchange membrane fuel cells. A membrane electrode assembly (MEA) with the catalyst as the cathode reaches 700 mA·cm^‐2 at 0.6 V and a maximum power density of 553 mW·cm^‐2, ranking it among the best MEAs with a nonprecious metal catalyst as the cathode.