Hydrogen crossover in high-temperature PEM fuel cells

In this paper, hydrogen crossover was measured in an environment of high-temperature proton exchange membrane (PEM) fuel cells using a steady-state electrochemical method at various temperatures (T) (80–120 °C), backpressures (P) (1.0–3.0 atm), and relative humidities (RH) (25–100%). An H₂ crossover model based on an MEA consisting of five layers – anode gas diffusion layer, anode catalyst layer, proton exchange membrane (Nafion 112 or Nafion 117), cathode catalyst layer, and cathode gas diffusion layer – was constructed to obtain an expression for H₂ permeability coefficients as a function of measured H₂ crossover rates and controlled H₂ partial pressures. The model analysis suggests that the dominant factor in the overall H₂ crossover is the step of H₂ diffusing through the PEM. The H₂ permeability coefficients as a function of T, P, and RH obtained in this study show that the increases in both T and P could increase the H₂ permeability coefficient at any given RH. However, the effect of RH on the permeability coefficient seems to be more complicated. The T effect is much larger than that of P and RH. Through experimental data simulation an equation was obtained to describe the T dependencies of the H₂ permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H₂ crossover through both Nafion 112 and 117 membranes were also evaluated. Both Nafion 112 and Nafion 117 showed similar values of such parameters, suggesting that membrane thickness does not play a significant role in the H₂ crossover mechanism.     

Polymer electrolyte fuel cells based on phosphoric acid doped polybenzimidazole (PBI) membranes

In order to make fuel cells with high power density the structure and morphology for the three-dimensional gas diffusion electrodes (GDEs) are very important. A preparation technique for GDEs for phosphoric acid doped polybenzimidazole (PBI) is presented. Teflon treatment of the backing material was found to be beneficial for the performance of the electrodes, and explained by higher total porosity. In general the open circuit voltage (OCV) with PBI-based cells is 0.9 V. The observed low OCV was explained by slow kinetic for the oxygen reduction and cross over of the reactants. The performance of the fuel cells is found to increase with increasing temperature; this was explained by faster reaction kinetic and higher membrane conductivity. A typical power output was 0.3–0.4 W cm⁻² at 0.6 V and 175 °C.     

Investigation of parameter effects on the performance of high-temperature PEM fuel cell

The performance of high-temperature PEM fuel cell (HT-PEMFC) is substantially influenced by physical parameters. In this work, the effects of three parameters on the performance of HT-PEMFC are studied by using a 3D model in COMSOL. The parameters are the operating temperature, membrane's thickness and catalyst layer's thickness. The polarization curves are adopted to analyze the effects on the performance. The results show that the increase of temperature can enhance the performance of fuel cell. For the effect of catalyst layer thickness, the cell performance is promoted as the catalyst layer's thickness decreases. For the effect of the thickness of membrane, it is found that the thinner membrane of fuel cell can achieve better performance. These findings can be further extended to guide the operation and design of HT-PEMFC in practical applications.     

Dependence of high-temperature PEM fuel cell performance on Nafion® content

Operating a proton exchange membrane (PEM) fuel cell at elevated temperatures (above 100 °C) has significant advantages, such as reduced CO poisoning, increased reaction rates, faster heat rejection, easier and more efficient water management and more useful waste heat. Catalyst materials and membrane electrode assembly (MEA) structure must be considered to improve PEM fuel cell performance. As one of the most important electrode design parameters, Nafion^® content was optimized in the high-temperature electrodes in order to achieve high performance. The effect of Nafion^® content on the electrode performance in H₂/air or H₂/O₂ operation was studied under three different operation conditions (cell temperature (°C)/anode (%RH)/cathode (%RH)): 80/100/75, 100/70/70 and 120/35/35, all at atmospheric pressure. Different Nafion^® contents in the cathode catalyst layers, 15–40 wt%, were evaluated. For electrodes with 0.5 mg cm⁻² Pt loading, cell voltages of 0.70, 0.68 and 0.60 V at a current density of 400 mA cm⁻² were obtained at 35 wt% Nafion^® ionomer loading, when the cells were operated at the three test conditions, respectively. Cyclic voltammetry was conducted to evaluate the electrochemical surface area. The experimental polarization curves were analyzed by Tafel slope, catalyst activity and diffusion capability to determine the influence of the Nafion^® loading, mainly associated with the cathode.     

Mechanism of action of polytetrafluoroethylene binder on the performance and durability of high-temperature polymer electrolyte fuel cells

In this work, new insights into impacts of the polytetrafluoroethylene (PTFE) binder on high temperature polymer electrolyte fuel cells (HT-PEFCs) are provided by means of various characterizations and accelerated stress tests. Cathodes with PTFE contents from 0 wt% to 60 wt% were fabricated and compared using electrochemical measurements. The results indicate that the cell with 10 wt% PTFE in the cathode catalyst layer (CCL) shows the best performance due to having the lowest mass transport resistance and cathode protonic resistance. Moreover, cyclic voltammograms show that Pt (100) edge and corner sites are significantly covered by PTFE and phosphate anions when the PTFE content is higher than 25 wt%. Open-circuit and low load-cycling conditions are applied to accelerate degradation processes of the HT-PEFCs. The PTFE binder shows a network structure in the pores of the catalyst layer, which reduces phosphoric acid leaching during the aging tests. In addition, the high binder HT-PEFCs more easily suffer from a mass transport problem, leading to more severe performance degradation.     

The performance of miniature metallic PEM fuel cells

A prototype of metallic PEM fuel cell with thin stainless steel bipolar plates was tested for their potential applications in portable electronic products. The flow field pattern was grown from the stainless steel plates by the electroforming process. The main flow channel has the dimensions of 300 μm (width) × 300 μm (depth). The dimensions of the micro-features were 100 μm width × 50 μm depth and 50 μm width × 50 μm depth. The material of the electroformed flow field pattern is nickel. A prototype of a single cell with total thickness of 2.6 mm, overall reaction area of 4 cm² and bipolar plate area of 16 cm² was assembled for this study. In order to improve its corrosion resistance, the bipolar plates were coated with 5 μm thick of multi-layered corrosion resistant material.     

Effect of accelerated ageing tests on PBI HTPEM fuel cells performance degradation

The study presented in this paper aims to evaluate the performance degradation of Polybenzimidazole (PBI) based High Temperature PEM (HTPEM) fuel cells subjected to different ageing tests, according to a methodology already used by the authors. Three HTPEM Membrane Electrode Assemblies (MEAs) were characterized before and after different aging tests and performance compared. The three MEAs have been named MEA C, MEA D and MEA E. MEA C was subjected to 100,000 triangular sweep cycles between Open Circuit Voltage (OCV) and 0.5 A/cm² with 2 s of permanence at OCV at each cycle. MEA D and MEA E were subjected to 440 h of operation at constant load of 0.22 A/cm². In order to assess the cell performance, polarization curves, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were recorded during the ageing tests. Degradation rates have been obtained for MEA C (44 μV/h), for MEA D (30 μV/h) and for MEA E (29 μV/h). ECSA (Electrochemical Surface Area) has been calculated for the three MEAs showing a reduction of approximately 50% for MEA C and of approximately 30% for MEA D and MEA E. Polarization curves during aging tests confirm that load cycling is more detrimental. A comparison with data obtained by the authors in a previous research seems to confirm the repeatability of the test protocol used.     

Mechanical properties of a reinforced composite polymer electrolyte membrane and its simulated performance in PEM fuel cells

The hygro-thermo-mechanical properties and response of a class of reinforced perfluorosulfonic acid membranes (PFSA), that has potential application as an electrolyte in polymer fuel cells, are investigated through both experimental and numerical modeling means. A critical set of material properties, including Young's modulus, proportional limit stress, break stress and break strain, is determined for a range of temperature and humidity levels in a custom-built environmental test apparatus. The swelling strains are also determined as functions of temperature and humidity level. To elucidate the mechanical response and the potential effect these properties have on the mechanical durability, mechanics-based simulations are performed using the finite element method (ABAQUS). The results indicate that the relatively high strength of the experimental membrane, in combination with its relatively low in-plane swelling due to water absorption, should have a positive influence on membrane durability, potentially leading to longer life times for polymer electrolyte membrane fuel cells (PEMFC).     

Anhydrous proton-conducting electrolyte membranes based on hyperbranched polymer with phosphonic acid groups for high-temperature fuel cells

The two different molecular weight hyperbranched polymers (HBP(L)-PA-Ac and HBP(H)-PA-Ac) with both phosphonic acid group as a functional group and acryloyl group as a cross-linker at the chain ends were successfully synthesized as a new thermally stable proton-conducting electrolyte. The cross-linked electrolyte membranes (CL-HBP-PA) were prepared by their thermal polymerizations using benzoyl peroxide and their ionic conductivities under dry condition and thermal properties were investigated. The ionic conductivities of the low molecular weight CL-HBP(L)-PA membrane and the high molecular weight CL-HBP(H)-PA membrane were found to be 1.2 × 10⁻⁵ and 2.6 × 10⁻⁶ S cm⁻¹, respectively, at 150 °C under dry condition, and showed the Vogel–Tamman–Fulcher (VTF) type temperature dependence. Both membranes were thermally stable up to 300 °C, and they had suitable thermal stability as electrolyte membranes for the high-temperature fuel cells under dry condition. Fuel cell measurements using a single membrane electrode assembly cell with both cross-linked membranes were successfully performed.     

High performance and durability of polymer-coated Pt electrocatalyst supported on oxidized multi-walled in high-temperature polymer electrolyte fuel cells

The low durability and fuel cell performance of the platinum electrocatalyst block the widespread application of high-temperature polymer electrolyte fuel cells (HT-PEFCs). Here, we utilize a facile method to improve the durability as well as fuel cell performance of the platinum electrocatalyst supported on oxidized multi-walled carbon nanotubes (ox-CNT/Pt), in which the electrocatalyst is coated by polybenzimidazole assisted by the COOH groups on the surface of ox-CNT. The coated and non-coated electrocatalysts remain 50% of the initial electrochemical surface areas (ECSAs) after 350,000 and 50,000 potential cycles from 1.0 to 1.5 V vs. RHE, respectively, indicating that polymer coating enhances the durability of the electrocatalyst. Meanwhile, power density of PBI-coated electrocatalyst measured under anhydrous condition is 1.6 times higher compared to that of non-coated electrocatalyst due to the PBI layer acted as proton conductor in the catalyst layer. This study offers useful knowledge for enhancement of durability and performance of Pt electrocatalyst used in HT-PEFCs.     

Effects of the gas diffusion-layer parameters on cell performance of PEM fuel cells

The characteristic parameters of the gas diffusion-layer (GDL) on cell performance and mass transfer of a proton exchange membrane fuel cell have been investigated numerically. A two-dimensional, isothermal and multi-phase numerical model has been established to investigate the influence of the GDL parameters on the transport phenomenon and cell performance of PEM fuel cells. The porosity and thickness of the GDL are employed in the analysis as the parameters. In addition, the effects of liquid water and the flow direction of the fuel and air on the performance are also considered in this paper. The results show that both the porosity and thickness of the GDL affect the fuel cell performance significantly, especially the water mass transfer. It is shown that the cell performance with consideration of a liquid water effect is always less than that without consideration of the liquid water effect. In addition, the cell performance with a co-flow pattern of fuel and air is better than that with a counter flow pattern.     

Experimental study on the performance of PEM fuel cells with interdigitated flow channels

In this work, the effects of interdigitated flow channel design on the cell performance of proton exchange membrane fuel cells (PEMFCs) are investigated experimentally. To compare the effectiveness of the interdigitated flow field, the performance of the PEM fuel cells with traditional flow channel design is also tested. Besides, the effects of the flow area ratio and the baffle-blocked position of the interdigitated flow field are examined in details. The experimental results indicate that the cell performance can be enhanced with an increase in the inlet flow rate and cathode humidification temperature. Either with oxygen or air as the cathode fuel, the cells with interdigitated flow fields have better performance than conventional ones. With air as the cathode fuel, the measurements show that the interdigitated flow field results in a larger limiting current density, and the power output is about 1.4 times that with the conventional flow field. The results also show that the cell performance of the interdigitated flow field with flow area ratio of 40.23% or 50.75% is better than that with 66.75%.     

Prediction performance of PEM fuel cells by gene expression programming

In the present study, gene expression programming has been utilized to evaluate the output voltage of different PEM fuel cells as the performance symbol of these structures. A total number of 843 data were collected from the literature, randomly divided into 682 and 161 sets, and then trained and tested, respectively by different models. The used data as input parameters were consisted of current density, fuel cell temperature, anode humidification temperature, cathode humidification temperature, operating pressures, fuel cell type, O₂ flow rate, air flow rate and active surface area of the PEM fuel cells. According to these input parameters, in the gene expression programming models, the voltage of each PEM fuel cell in different conditions was predicted. The training and testing results in the gene expression programming model have shown an acceptable potential for predicting voltage values of the PEM fuel cells in the considered range.     

High performance electrocatalysts supported on graphene based hybrids for polymer electrolyte membrane fuel cells

In this study, new electrocatalysts for PEM fuel cells, based on Pt nanoparticles supported on hybrid carbon support networks comprising reduced graphene oxide (rGO) and carbon black (CB) at varying ratios, were designed and prepared by means of a rapid and efficient microwave-assisted synthesis method. Resultant catalysts were characterized ex-situ for their structure, morphology, electrocatalytic activity. In addition, membrane-electrode assemblies (MEAs) fabricated using resultant electrocatalysts and evaluated in-situ for their fuel cell performance and impedance characteristics. TEM studies showed that Pt nanoparticles were homogeneously decorated on rGO and rGO-CB hybrids while they had bigger size and partially agglomerated distribution on CB. The electrocatalyst, supported on GO-CB hybrid containing 75% GO (HE75), possessed very encouraging results in terms of Pt particle size and dispersion, catalytic activity towards HOR and ORR, and fuel cell performance. The maximum power density of 1090 mW cm^?2 was achieved with MEA (Pt loading of 0.4 mg cm^?2) based on electrocatalyst, HE75. Therefore, the resultant hybrid demonstrated higher Pt utilization with enhanced FC performance output. Our results, revealing excellent attributes of hybrid supported electrocatalysts, can be ascribed to the role of CB preventing rGO sheets from restacking, effectively modifying the array of graphene and providing more available active catalyst sites in the electrocatalyst material.     

Impact of high frequency current ripples on the degradation of high-temperature PEM fuel cells (HT-PEMFC)

In order to achieve the goal of reducing the environmental footprint of the transport sector, new low-carbon energy systems including fuel cells and power converters are proposed. The sizing and operation of such systems have to take into account the aging of the fuel cell. This paper focuses on the study of a potential impact of high frequency current ripples (HFCR) on the degradation of a high-temperature proton exchange membrane fuel cell (HT-PEMFC). A 2600 h long endurance test was carried out on 4 HT-PEMFC single cells with and without HFCR. The degradation of two cells operated with a triangular current waveform of frequency 20 kHz and amplitude 20 %_pp of the mean current density (0.2 A/cm²) is compared to the degradation of two cells operated at the same constant mean current density. In addition to endurance phase, several characterization phases (polarization curves, electrochemical impedance spectroscopy and cyclic voltammetry) are used to analyse the impact of the current harmonics. The obtained results show that the high frequency current ripples do not seem to accelerate the degradation of the HT-PEMFC single cells.     

Evaluation of electrochemical performance for surface-modified carbons as catalyst support in polymer electrolyte membrane (PEM) fuel cells

The electrochemical performance of platinum (Pt) catalyst deposited on various functionalized carbon supports was investigated and compared with that of a commercial catalyst, Pt on Vulcan XC-72 carbon. The supports employed were graphitic or amorphous with a wide range of surface areas. Cyclic voltammetry (CV) and rotating disk electrode (RDE) studies on the supported catalysts indicated equivalent platinum catalyst activities. Fuel cell performance was determined for membrane electrode assemblies (MEA) fabricated from the supported catalysts. The use of high surface area supports did not necessarily translate into a higher electrochemical utilization of platinum. Electrochemical impedance spectroscopy (EIS) measurements indicated lower ohmic losses for low surface area carbon MEAs. This is explained by the supported catalyst electrode microstructures and their intrinsic resistivities. Correlation of all data indicates that for low surface carbons, nature of the support does not significantly affect the Pt catalytic activity. The influence of the support is more critical when high surface area carbons are used because of the vastly different electrode morphology and resistivity.     

Low Pt loading high performance cathodes for PEM fuel cells

A simple direct mixing of carbon-supported catalysts with Nafion without adding any additional organic solvents was used to make electrodes for oxygen reduction in PEM fuel cells. For E-TEK 20% Pt/C, a Nafion content of 30% in the catalyst layer exhibited the best performance. Electrode dried from 90 to 150 °C showed little difference in performance. Highest power densities increased almost linearly with cell temperature, and values of 0.52, 0.60, 0.63, and 0.72 W/cm² were achieved at 35, 50, 60, and 75 °C, respectively, for a cathode with a Pt loading of 0.12 mg/cm² and operated using air at ambient pressure. A maximum performance was achieved with Pt loadings of 0.20±0.05 and 0.35±0.05 mg/cm² for 20 and 40% Pt/C, respectively, while the maximum performance using 40% Pt/C was only slightly better than that using 20% Pt/C. A Nafion/carbon sublayer with up to 30% Nafion content added between ELAT and the catalyst layer did not show any effect on performance.     

Enhancement of the performance and reliability of CO poisoned PEM fuel cells

CO poisoning is a major issue when reformate is used as a fuel in PEM fuel cells. Normally, it is necessary to reduce the CO to very low levels (∼5 ppm) and to use CO tolerant catalysts, such as Pt–Ru alloys. As an alternative approach, we have studied the use of pulsed oxidation for the regeneration of CO poisoned cells. Results are presented for the regeneration of Pt and Pt–Ru anodes in a PEM fuel cell fed with CO concentrations as high as 10,000 ppm. The results show that periodic removal of CO from the catalyst surface by pulsed oxidation can increase the average cell potential and overall efficiency.Although use of pulsed techniques has been studied before, the careful control of each cell's voltage that this approach requires has limited its use in large fuel cell stacks. When uniform pulsing is done on a stack of fuel cells in series, the variations in voltage across the cells can limit the usefulness of this approach. A novel method that allows each cell in a stack to be separately pulsed under controlled conditions has been developed to overcome this problem. Weak or defective cells in a fuel cell stack can also be supplemented to enhance the power output and reliability of fuel cells. We present the results of experiments and calculations that quantify these benefits, specifically as they relate to PEM fuel cells operating on impure hydrogen produced by reforming fuels.