Characteristics and performance of membrane electrode assemblies with operating conditions in polymer electrolyte membrane fuel cell

The degradation behavior of a membrane-electrode assembly (MEA) was investigated in accelerated degradation tests under constant voltage (0.8 V and 0.7 V) and load cycling (from open circuit voltage to 0.35 V) conditions. Changes in the structural and electrochemical characteristics of MEA after the durability tests give information as to the degradation mechanism of MEAs. The results of cyclic voltammogram and postmortem analysis by X-ray diffraction and high resolution-transmission electron microscopy indicate that the cathode catalyst layers of the MEAs showed no extreme degradation under constant voltage mode, whereas MEAs under repetition of load cycling mode showed very severe degradation after 280 h. However, the single cell performance of the MEA under repetition of load cycling mode was higher than under constant voltage mode. In addition, although the Pt band in the membrane of the MEA under repetition of load cycling mode was observed by field emission scanning electron microscopy, it did not affect the ohmic resistance.     

Characterization of catalysts and membrane in DMFC lifetime testing

The lifetime and performance of a direct methanol fuel cell (DMFC) were investigated to understand the correlation between the structure of catalysts/membrane and cell performance versus time. The cell polarization and performance curves were obtained during the DMFC operation with the time. The catalysts and Nafion^® membrane of the membrane electrode assembly (MEA) from the lifetime test were comprehensively examined by XRD, HRTEM, FTIR and Raman spectroscopy techniques. The results revealed that there was significant performance degradation during the first 200 h operation; while the degradation was slowing down between 200 and 704 h operation. The degradation became worse after 1002 h operation. The increases of the catalyst particle size from both anode and cathode catalysts were observed after the DMFC lifetime test. The changes of microstructure, surface composition, the interfacial structure of the MEA, and the aging of Nafion^® under the DMFC lifetime tests were also observed.     

Microscopic characterizations of membrane electrode assemblies prepared under different hot-pressing conditions

The durability of the membrane electrode assembly (MEA) for direct methanol fuel cells (DMFCs) is one of the most critical issues to be addressed before widespread commercialization of the DMFC technology. In this work, we investigated the effect of the hot-pressing duration on the performance and durability of the MEA prepared by hot-pressing technique. It was found that the 60-min hot pressing at 135 °C under the pressure of 4.0 MPa yielded a significantly improved MEA durability than did the 3-min hot pressing (a typical duration in practice) under the same condition, but no substantial difference was found in the cell performance of the MEAs prepared with the two different hot-pressing durations. The reason why the hot-pressing duration had no significant effect on cell performance is explained based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) characterizations of the changes in the physiochemical properties of MEAs and their constituent components, including the anode, cathode and Nafion membrane, before and after hot pressing with different durations.     

Preparation of low-platinum-loading electrocatalysts using electroless deposition method for proton exchange membrane fuel cell systems

One promising preparative method that offers the potential for improved platinum (Pt) dispersion of electrocatalysts is electroless deposition (ED). In this study, the effects of multiwalled carbon nanotubes (MWCNTs) pretreatment and synthesis procedure on properties of the four catalysts, synthesized by ED method, have been considered. The results of energy-dispersive X-ray spectroscopy (EDS), X-ray dot-mapping, X-ray fluorescence (XRF) and cyclic voltammetry (CV) analyses showed that using palladium (Pd) precursor during two-step sensitization-activation coating procedure gives uniform Pt particles distribution on MWCNTs with low aggregation and high specific surface area (∼80 m² g⁻¹). In addition, to investigate the performance of the synthesized catalysts in experimental fuel cell system, thin-film method was used to fabricate the membrane electrode assemblies (MEAs). Obtaining the polarization curves for the fabricated MEAs (Pt loading ∼0.4 mg cm⁻²) and a commercial MEA (ElectroChem, Pt loading ∼1 mg cm⁻²) demonstrated that the catalyst prepared by two-step sensitization-activation coating procedure possesses a good performance despite of its lower Pt content.     

直接涂膜技术用于质子交换膜燃料电池膜电极制备

引　言质子交换膜燃料电池 (PEMFC)是极具吸引力的电化学能量转换装置 ,是未来电动汽车的主要动力源 ,也是洁净高效的新型化学电源 .对于电动汽车的应用 ,要求PEMFC提供高能量密度、低催化剂负载量 ,以降低系统体积和成本[1] .膜电极(membraneandelectrodeassembly ,简称MEA)是由聚合物电解质膜、电极催化剂和扩散层材料组合而成的三明治式结构组件 ,类似于计算机的芯片 ,是燃料电池的核心部件 ,长期以来大量的研究集中于MEA新材料设计与制备 ,以提高电池的性能 .近年来 ,对MEA的微观结构分析、MEA制备工艺与电池性能的关系研究工作明显增多[2～ 7] .从PEM FC研究实践中发现 ,如何减少电极中Pt催化剂负载量并能继续保持或者提高电池性能的MEA制备技术开发至关重要 .其中超薄Pt层沉积法[8～ 10 ] 是MEA的制备新技术之一 .与传统的基于墨水涂布(based inkprinting)的方法相比 ,喷溅沉积法(sputterdeposit) [9] 制备的MEA提高了电池的性能和催化剂的利用率 ,它是用直接喷溅沉积法 (directlydeposit) [10...     

Radiation-grafted membrane/electrode assemblies with improved interface

Recent progress is reported in preparing membrane/electrode assemblies for polymer electrolyte fuel cells based on radiation-grafted FEP-g-poly(styrenesulfonic acid) membranes. MEAs with an improved interface between the membrane and commercially available gas diffusion electrodes were obtained by Nafion®-coating of the membrane and hot-pressing. These improved MEAs showed both, performance data comparable to those of MEAs based on Nafion® 112 and an operation lifetime in H₂/O₂ fuel cells of more than 2000 h at 60 °C and 500 mA cm⁻² current density.     

Silica-sol-templated mesoporous carbon as catalyst support for polymer electrolyte membrane fuel cell applications

Mesoporous carbon (MC) samples having especially high specific surface area, pore size, and pore volume (e.g. pore volume in excess of 4 cm³ g⁻¹) were prepared and their suitability as Pt catalyst supports in polymer electrolyte membrane fuel cells was examined. Pt particles on the MC support were slightly larger than those on commercial samples of Pt on carbon black, and they showed a greater tendency to agglomerate on the MC support than on carbon black. Ex situ cyclic voltammetry gave values for electrochemically active surface area that were about half that for a commercial Pt-on-carbon black sample. Preliminary attempts to prepare thin-film electrodes from Pt/MC samples with a Nafion binder using conventional ink formulations failed, probably because much of the Nafion electrolyte was taken up inside support pores and was not available to bind the support particles together. An alternate approach involving painting of catalyst inks directly onto gas diffusion layers was used to prepare membrane electrode assemblies (MEAs) from Pt/MC samples, which were tested using single-cell test hardware. Performance of these Pt/MC sample MEAs was compared with that prepared by decal transfer method with commercially obtained Vulcan XC-72R supported Pt catalyst. The reasons for the lower performance of Pt/MC were discussed.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part II: preparation and characterization of MEAs

Cathode catalyst layers were prepared and characterized as part of membrane electrode assemblies (MEA) and catalyst coated membranes (CCM) on the basis of carbon supported methanol tolerant RuSe _x catalysts. Preparation parameters varied were: catalyst loading (0.5–2 mg RuSe _x cm⁻²), PTFE content (0, 6, 18 wt.%), carbon support (Vulcan XC 72 or BP2000), and fraction of RuSe _x in the carbon supported catalysts (20, 44, 47 wt.%). The MEAs and cathode catalyst layers were electrochemically characterized under Direct Methanol Fuel Cell (DMFC) operating conditions by recording polarization curves, galvanostatic measurements, and impedance spectra. The morphology of the catalyst layers was investigated by means of confocal laser scan microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. MEAs with Ru(44.0 wt.%)Se(2.8 wt.%)/VulcanXC72 cathode catalyst achieved the highest performance of all RuSe _x catalysts investigated, i.e. ∼40 mW cm⁻² at 80 °C under ambient pressure and λ_MeOH = λ_air = 4. This is 40% of the value obtained with commercial platinum cathode catalyst under the same operating conditions. The RuSe _x catalysts investigated are stable over a period of more than 1,000 h. This was confirmed by TEM and XRD measurements, where no increase in mean RuSe _x particle size (∼5 nm) after fuel cell operation was found. Enhancement of specific catalyst activity, mass transport, and active surface offer potential for a further improvement of RuSe _x catalyst layers.     

Degradation of high temperature MEA with PBI-H3PO4 membrane in a life test

The article reports on the results of a 780 h life test of high temperature MEA with PBI-H₃PO₄ membrane. The MEA was loaded by current density 0.2 A cm⁻² for 763 h at 160 °C in hydrogen-air feed. The load was discontinued 14 times during the life test including three complete shut downs. In the course of the life test MEA characteristics were studied by electrochemical methods. Pt particle size growth was evaluated by ex situ measurements of electrochemical hydrogen adsorption/desorption with the cathode catalyst sampled after the life test and with pristine catalyst. Possible changes of electrochemically active surface area (ESA) of carbon support were monitored by electrochemical impedance studies (EIS) performed in the course of the MEA life test. Average Pt particle diameter was found 3.8 and 7.8 nm for pristine catalyst and for catalyst sampled after the life test, respectively. ESA of carbon support remained unchanged, membrane resistance decreased by ∼20%, hydrogen crossover increased by a factor of 14, although remained insignificant. Voltage loss rate in the life test was ∼25 μV h⁻¹. The major cause of the MEA degradation was identified as a loss of Pt ESA by particle size growth.     

Do not forget the electrochemical characteristics of the membrane electrode assembly when designing a Proton Exchange Membrane Fuel Cell stack

The membrane electrode assembly (MEA) is the key component of a PEMFC stack. Conventional MEAs are composed of catalyzed electrodes loaded with 0.1–0.4 mg_Pt cm⁻² pressed against a Nafion^® membrane, leading to cell performance close to 0.8 W cm⁻² at 0.6 V. Due to their limited stability at high temperatures, the cost of platinum catalysts and that of proton exchange membranes, the recycling problems and material availability, the MEA components do not match the requirements for large scale development of PEMCFs at a low cost, particularly for automotive applications.Novel approaches to medium and high temperature membranes are described in this work, and a composite polybenzimidazole–poly(vinylphosphonic) acid membrane, stable up to 190 °C, led to a power density of 0.5 W cm⁻² at 160 °C under 3 bar abs with hydrogen and air. Concerning the preparation of efficient electrocatalysts supported on a Vulcan XC72 carbon powder, the Bönnemann colloidal method and above all plasma sputtering allowed preparing bimetallic platinum-based electrocatalysts with a low Pt loading. In the case of plasma deposition of Pt nanoclusters, Pt loadings as low as 10 μg cm⁻² were achieved, leading to a very high mass power density of ca. . Finally characterization of the MEA electrical properties by Electrochemical Impedance Spectroscopy (EIS) based on a theoretical model of mass and charge transport inside the active and gas diffusion layers, together with the optimization of the operating parameters (cell temperature, humidity, flow rate and pressure) allowed obtaining electrical performance greater than 1.2 W cm⁻² using an homemade MEA with a rather low Pt loading.     

Comparison of catalyst-coated membranes and catalyst-coated substrate for PEMFC membrane electrode assembly:A review

Catalyst-coated membranes (CCMs) have gained popularity among membrane electrode assembly (MEA) fabri-cators for their abilities and advantages compared with those of other methods, such as catalyst-coated sub-strates (CCSs). CCMs show a profound new analysis for reducing platinum (Pt) catalyst loading. In addition, they increase the total number of reactions that occur on the MEA because of their active area amplification, which leads to an improved catalyst-utilization efficiency rate. Moreover, several characteristics are involved in the MEA fabrication methods. Material-manufacturing effects with regard to catalyst inks and analysis of the overall performance of MEAs prepared by the CCM and CCS methods are deliberated. This deliberation empha-sizes the practical approaches in minimizing performance deterioration during the fabrication of MEAs using the CCM method and converses the commercialization of the CCM fabrication method toward developing an end product. Novel research is required for MEA fabrication using the CCM methods to ensure that the fuel cell performance is improved. Therefore, this review is focusing on the pros and cons of both distinguished methods, that is, CCM and CCS fabrication, for better comparison.     

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.     

Novel carbon-supported Fe-N electrocatalysts synthesized through heat treatment of iron tripyridyl triazine complexes for the PEM fuel cell oxygen reduction reaction

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPTZ) was used as a ligand to prepare iron-TPTZ (Fe-TPTZ) complexes for the development of a new oxygen reduction reaction (ORR) catalyst. The prepared Fe-TPTZ complexes were then heat-treated at temperatures ranging from 400 °C to 1100 °C to obtain carbon-supported Fe-N catalysts (Fe-N/C). These catalysts were characterized in terms of catalyst composition, structure, and morphology by several instrumental methods such as energy dispersive X-ray, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. With respect to the ORR activity, the Fe-N/C catalysts were also evaluated by cyclic voltammetry, as well as rotating disk and ring-disk electrodes. The results showed that among the heat-treated catalysts, that obtained at a heat-treatment temperature of 800 °C is the most active ORR catalyst. The overall electron transfer number for the catalyzed ORR was determined to be between 3.5 and 3.8, with 10-30% H₂O₂ production. The ORR catalytic activity of this catalyst was also tested in a hydrogen-air proton exchange membrane (PEM) fuel cell. At a cell voltage of 0.30 V, this fuel cell can give a current density of 0.23 A cm⁻² with a maximum MEA power density of 0.070 W cm⁻² indicating that this catalyst has potential to be used as a non-noble catalyst in PEM fuel cells.     

Electrosynthesis and efficient electrocatalytic performance of poly(neutral red)/ordered mesoporous carbon composite

A novel composite film which contains ordered mesoporous carbon (OMC) along with the incorporation of poly(neutral red) (PNR) has been synthesized on glassy carbon electrode by potentiostatic method. This composite film was characterized by scanning electron microscope (SEM) and cyclic voltammetry (CV). Two pairs of the redox peaks appear at formal potential E^0′ = +0.045 V (peak I) and E^0′ = −0.49 V (peak II) at the PNR/OMC/GC electrode. And it is found that only the redox waves (peak I) exhibits good electrocatalytic activity towards nicotinamide adenine dinucleotide (NADH) and 2-mercaptoethanol (2-ME). Under a lower operation potential of +0.07 V, amperometry method was used to determine the concentration of NADH and 2-ME, respectively. In pH 7.0, sensors for two molecules under their corresponding optimized conditions were developed with acceptable sensitivity and low detection limits in large determination ranges. In addition, these sensors have good reproducibility and stability.     

Investigation of electrode composition of polymer fuel cells by electrochemical impedance spectroscopy

In order to optimize the electrode composition and performance of Polymer Fuel Cells and to reduce the production cost of membrane electrode assemblies (MEAs), different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt). The electrochemical characterization was performed by recording current-voltage curves and electrochemical impedance spectra (EIS) in the galvanostatic mode of operation at 500 mA cm⁻². The evaluation of the measured impedance spectra with an adequate equivalent circuit shows that the cathode of the fuel cell is very sensitive to the electrode composition whereas the contribution of the anode is very small and invariant to the electrode composition. Furthermore, it could be shown for the first time using electrolyte powder in the electrodes that the charge transfer of the cathode decreasing monotonically with increasing electrolyte content in the cathode. These findings suggest that with increasing electrolyte content in the electrodes, in particular in the cathode, the utilization degree of the catalyst increasing linearly with increasing electrolyte content in the electrode.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

Effect of cathode binder IEC on kinetic and transport losses in all-SPEEK MEAs

The effect of ion exchange capacity (IEC) and loading of sulfonated polyether ether ketone (SPEEK) binder on PEFC cathode performance was studied. MEAs were prepared by decal transfer onto a SPEEK membrane (IEC-1.75 mequiv./g). The IEC of SPEEK binder in the MEA cathode was varied between 1.3 and 2.1 mequiv./g. Cathodes prepared with 30 wt.% SPEEK loading had an electrochemically active surface area (ECA) that was 25% lower than a Nafion^® bonded electrode with similar loading. Polarization curves were obtained at 80 °C and 75% RH with hydrogen as fuel and air and oxygen (O₂) as oxidants. Polarization data was analyzed to determine the relative contributions of different sources of polarization, namely membrane ohmic losses, electrode ohmic losses, and mass transport losses in the gas diffusion layer, binder film and electrodes. The electrode ohmic and mass transport losses decreased with increase in SPEEK IEC. However, even for the highest SPEEK IEC, these losses were higher than those obtained in a Nafion^® bonded electrode. This was attributed to the lower proton activity and O₂ permeability in SPEEK. The loading of SPEEK in the electrode was found to influence performance in the activation controlled region, with a loading of 7.5 wt.% giving the highest performance. However, gains in this region were negated at higher current densities due to enhanced ohmic and transport losses and MEAs with all binder loadings between 7.5 and 30 wt.% had similar limiting currents.     

Simulations and study of electrochemical hydrogen energy conversion in EasyTest Cell

An EasyTest Cell concept is applied to study the performance characteristics of the electrochemical processor for polymer electrolyte membrane electrochemical hydrogen energy converters (PEM EHEC), broadly known as a membrane electrode assembly (MEA). A series of MEAs consisting of Nafion 117 polymer electrolyte and magnetron sputtered Pt, IrO_x, and composite IrO_x/Pt/IrO_x catalysts with varying catalytic loadings were investigated. The partial electrode reactions proceeding in the real PEM EHEC, namely hydrogen oxidation (HOR), hydrogen evolution (HER), oxygen reduction (ORR), and oxygen evolution (OER), are simulated and studied in a recently developed test cell with a unitized gas compartment. The EasyTest Cell design gives possibilities for strict control of the experimental conditions by avoiding the usage of any auxilliary gas conditioning equipment. By varying the thickness of the sputtered Pt film, the catalyst loading is remarkably reduced (from 0.5 to 0.06 mg cm⁻² or about 8 times) for both HOR and HER without any sacrifice of the electrode performance. The electrode with 0.2 mg cm⁻² sputtered IrO_x shows the best OER performance. The composite IrO_x/Pt/IrO_x electrode demonstrated a bi-functional catalytic activity toward both OER and ORR, as well as improved gas diffusion properties toward ORR compared to the single Pt layer with the same catalytic loading.A phenomenological criterion for evaluating the gas diffusion properties of the electrodes is proposed. The applied testing approach is validated via comparison of the results obtained in the EasyTestCell and the common laboratory PEM electrolytic cell.     

Electrochemical study of a hydrogen diffusion anode-membrane assembly for membrane electrolysis

The membrane electrode assembly (MEA) studied was constituted with a gas diffusion electrode (E-TEK) impregnated with Nafion^® solution which was assembled with a Nafion^® 117 cation exchange membrane under heat and pressure. The MEA was used as anode in a membrane electrolysis (ME) cell with the objective to regenerate HCl and NaOH from NaCl. Current efficiency for hydrogen oxidation was determined and its value is 100%, which indicates that the only reaction occurring at the anode is the oxidation of hydrogen. Current-potential curves, recorded in different conditions, showed a linear variation in the range 0-3000 A m⁻² when hydrochloric acid concentration is below 2 mol dm⁻³. In this case, the overvoltage was shown to be mainly due to the ohmic drop in the membrane and in the layer where Nafion^® impregnation was performed. MEA overvoltage necessary to reach 3000 A m⁻² current density was about 0.12 V. For high HCl concentration (6-8 mol dm⁻³), the MEA overvoltage increased sharply with current density due to the adsorption of chloride anions on platinum catalyst.     

Enhanced performance and improved interfacial properties of polymer electrolyte membrane fuel cells fabricated using sputter-deposited Pt thin layers

For this study, catalyst layers for polymer electrolyte membrane fuel cells (PEMFC) were prepared by spraying and sputtering to deposit Pt amount of 0.1 and 0.01 mg cm⁻², respectively. These Pt layers were then assembled to fabricate membrane electrode assemblies (MEA) having either single- or double-layered catalysts. The PEM fuel cell with double layers showed a current density of 777 mA cm⁻² at a cell voltage of 0.6 V, which is a higher current density than state-of-the-art fuel cells at 643 mA cm⁻². These results indicate that Pt loading in state-of-the-art PEMFCs could be reduced by approximately 50% with no performance loss by using both spraying and sputtering method in the MEA fabrication process.