Effect of the CeO2 nanoparticles in microporous layers on the durability of proton exchange membrane fuel cells

In this work, the effect of the CeO₂ in the microporous layer (MPL) on the durability of proton exchange membrane (PEM) fuel cells is investigated. The 400 h dry-wet accelerated stress test (AST) and the open-circuit voltage (OCV) holding testing were used to identify the function of CeO₂ in the microporous layer (MPL) on the durability and performance of MEA. The results show that the performance decay of the sample with CeO₂ is much smaller than the sample without CeO₂ (e.g., 24 mV vs. 140 mV@ 1200 mA cm⁻²). More importantly, the OCV decrease rate for sample without CeO₂ is as high as 7.250μV/cycle, which is 9.6 times as the value of 0.752μV/cycle for sample with CeO₂. And it is interesting that the addition of CeO₂ in MPL does not increase the inner resistance in the cell. Therefore, the addition of CeO₂ to the MPL not only can significantly improve the cell durability but also can effectively alleviate the negative impact of Ce ions on the proton conductivity in proton exchange membrane.     

Detaching behaviors of catalyst layers applied in PEM fuel cells by off-line accelerated test

The detaching behavior of catalyst layers in membrane electrode assembly (MEA) for PEM fuel cells could affect the lifetime of both catalyst layers and membranes. However, this issue is always neglected. Therefore, the study of detaching behavior of catalyst layer is very conducive to investigate the failure mechanism of fuel cells. The detaching of catalyst layers was simulated by dipping membrane electrode assemblies (MEAs) into H₂O₂ solution with or without Fe²⁺. We observed the presence of detaching of catalyst layers and found the varied detaching behaviors with different accelerated testing solutions: a layered-type detaching behavior is shown for the catalyst layer treated with 30% H₂O₂ solution, whereas a crack-like detaching behavior in the case of 30% H₂O₂ solution with Fe²⁺ species (or Fenton's test). At the same time, the layered-type detaching of catalyst layers has a higher detaching rate than the crack-like detaching. These detaching behaviors should have an inherent link to degradation of recast-ionomer (Nafion) films in catalyst layers. In addition, the effect of detaching behaviors of catalyst layers on the lifetime of fuel cells has been studied by hydrogen crossover measurement, and shows that, for the crack-like detaching, the membrane has a shorter lifetime than that for the layered detaching.     

High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers

Metal-supported solid oxide fuel cells are believed to have commercial advantages compared to conventional anode (Ni-YSZ) supported cells, with the metal-supported cells having lower material costs, increased tolerance to mechanical and thermal stresses, and lower operational temperatures. The implementation of a metallic support has been challenged by the need to revise the cell fabrication route, as well as electrode microstructures and material choices, to compete with the energy output and stability of full ceramic cells.The metal-supported SOFC design developed at Risø DTU has been improved, and an electrochemical performance beyond the state-of-the-art anode-supported SOFC is demonstrated possible, by introducing a CGO barrier layer in combination with Sr-doped lanthanum cobalt oxide (LSC) cathode. Area specific resistances (ASR) down to 0.27 Ω cm², corresponding to a maximum power density of 1.14 W cm⁻² at 650 °C and 0.6 V, were obtained on cells with barrier layers fabricated by magnetron sputtering. The performance is dependent on the density of the barrier layer, indicating Sr²⁺ diffusion is occurring at the intermediate SOFC temperatures. The optimized design further demonstrate improved durability with steady degradation rates of 0.9% kh⁻¹ in cell voltage for up to 3000 h galvanostatic testing at 650 °C and 0.25 A cm⁻².     

A review of accelerated stress tests of MEA durability in PEM fuel cells

This paper is a review of recent work done on accelerated stress tests in the study of PEM fuel cell durability, with a primary focus on the main components of the membrane electrode assembly (MEA). The accelerated stressors for each component under different conditions are outlined, in an attempt to gain a detailed understanding of cell degradation with respect to microstructural change and performance attenuation in the perfluorosulfonic acid membrane, catalyst, and gas diffusion layers. Various techniques for evaluating the components' performance are presented, along with representative mitigation strategies. In addition, different degradation mechanisms proposed in recent publications are briefly reviewed.     

Accelerated durability tests of catalyst layers with various pore volume for catalyst coated membranes applied in PEM fuel cells

The effect of pore volume on the catalyst layer durability of PEM fuel cell was simulated by soaking the catalyst coated membrane (CCM) into H₂O₂/Fe²⁺ solution. Before this simulation, the CCM with various pore volumes in catalyst layer was fabricated. The structure of catalyst layers was optimized with an increase in pore volume, leading to an improvement of fuel cell performance. However, this treatment causes a negative effect on the lifetime of CCM especially when H₂O₂/Fe²⁺ introduced. As a result, the catalyst layer with high pore volume has a higher detaching rate than that with low pore volume. The detaching of catalyst layers could be attributed to degradation of both the recast Nafion in catalyst layers and the Nafion membrane. The catalyst layer with high pore volume accelerates the recast Nafion degradation. Thus, the durability of membrane electrode assembly should be considered when the catalyst layer is optimized.     

Comparison between two PEM fuel cell durability tests performed at constant current and under solicitations linked to transport mission profile

The first phase of a research program on PEMFC durability was devoted to the test of a 100 W stack operated in stationary regime during 1000 h. The second phase was dedicated to the test of another 100 W stack under dynamical current constraint. In this case, the load current cycle applied was computed from a standardised transportation mission profile and then adapted to the power of the investigated fuel cell. For both ageing experiments, the stack characterisations were based on polarisation curves, recorded for various stoichiometry rates, as well as on EIS measurements performed at regular time-spaced intervals throughout the ageing. Some analysis tools derived from the response surface methodology are employed to analyse and compare the results of the two durability experiments. Some numerical models are proposed for the degradation of the stack performances. They are finally used to optimise the fuel cell operating conditions versus ageing time.     

A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies

This paper reviews publications in the literature on performance degradation of and mitigation strategies for polymer electrolyte membrane (PEM) fuel cells. Durability is one of the characteristics most necessary for PEM fuel cells to be accepted as a viable product. In this paper, a literature-based analysis has been carried out in an attempt to achieve a unified definition of PEM fuel cell lifetime for cells operated either at a steady state or at various accelerated conditions. Additionally, the dependence of PEM fuel cell durability on different operating conditions is analyzed. Durability studies of the individual components of a PEM fuel cell are introduced, and various degradation mechanisms are examined. Following this analysis, the emphasis of this review shifts to applicable strategies for alleviating the degradation rate of each component. The lifetime of a PEM fuel cell as a function of operating conditions, component materials, and degradation mechanisms is then established. Lastly, this paper summarizes accelerated stress testing methods and protocols for various components, in an attempt to prevent the prolonged test periods and high costs associated with real lifetime tests.     

Experimental investigation of the role of a microporous layer on the water transport and performance of a PEM fuel cell

A highly reliable experimental system that consistently closed the overall water balance to within 5% was developed to study the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the water transport and performance of a standard 100 cm² active area PEM fuel cell. Various combinations of cells were built and tested with PTLs at the electrodes using either carbon fibre paper with a MPL (SGL 10BB) or carbon fibre paper without a MPL (SGL 10BA). The net water drag coefficient at three current densities (0.3, 0.5 and 0.7 A cm⁻²) for two combinations of anode/cathode relative humidity (60/100% and 100/60%) and stoichiometric ratios of H₂/air (1.4/3 and 1.4/2) was determined from water balance measurements. The addition of a MPL to the carbon fibre paper PTL at the cathode did not cause a statistically significant change to the overall drag coefficient although there was a significant improvement to the fuel cell performance and durability when a MPL was used at the cathode. The presence of a MPL on either electrode or on both electrodes also exhibited similar performance compared to when the MPL was placed at the cathode. These results indicate that the presence of MPL indeed improves the cell performance although it does not affect the net water drag coefficient. The correlation between cell performance and global water transport cannot be ascertained and warrants further experimental investigation.     

Improved PEM fuel cell performance with hydrophobic catalyst layers

Flooding of catalyst layers is one of the major issues, which effects performance of low temperature proton exchange membrane fuel cells (PEMFC). Rendering catalyst layers hydrophobic one may improve the performance of PEMFC depending on Pt percentage in the catalyst and Polytetrafluoroethylene (PTFE) loading on the electrode. In this study, effect of hydrophobicity in catalyst layers on performance has been investigated by comparing performances of membrane electrode assemblies prepared with 48% Pt/C. Ultrasonic coating technique was used to manufacture highly efficient electrodes. Power density at 0.45 V increased by the addition of PTFE, from 0.95 to 1.01 W/cm² with H₂/O₂ feed; while it slightly increased from 0.52 W/cm² to 0.53 W/cm² with H₂/Air feed. Addition of PTFE to catalyst layers while keeping Pt loading constant, enhanced performance providing improved water management. Kinetic activity increased by decreasing Nafion loading from 0.37 mg/cm² to 0.25 mg/cm² while introducing PTFE (0.12 mg/cm²) to the electrode. Electrochemical impedance spectroscopy (EIS) results proved that charge transfer resistance decreased with hydrophobic catalyst layers for H₂/O₂ feed. This is attributed to enhanced water management due to PTFE presence.     

Microstructure changes induced by capillary condensation in catalyst layers of PEM fuel cells

This paper proposes a hypothesis for explaining Pt/C particles’ coarsening inside the catalyst layers of a PEM fuel cell. The hypothesis includes the two parts: (1) due to capillary condensation a water-bridge could be formed between two neighboring nano-scale Pt/C particles at relative humidity under 100% when the surfaces of the Pt/C particles are hydrophilic; (2) the capillary force of the water-bridge tends to pull together the Pt/C particles. The relation is derived in this paper between the capillary force and the factors including the diameter of Pt/C particles, relative humidity, temperature, distance between the two neighboring Pt/C particles and water contact angle. A parametric study is performed showing some details about water-bridge formation. Finally, the stress level induced by the capillary force inside the Nafion thin-film connecting with the Pt/C particles is calculated. The result shows that the capillary force could be large enough to break apart the Nafion thin-film, facilitating the movement of Pt/C particles towards each other.     

Numerical investigation of delamination onset and propagation in catalyst layers of PEM fuel cells under hygrothermal cycles

Durability is one of the main obstacles that inhibits the commercialization of polymer electrolyte membrane (PEM) fuel cells for transport applications, in which the microstructure of the catalyst layers (CLs) deteriorates under dynamic loading operation. In this study, CLs’ naturally random porous structure is simplified to be a random three-phase microstructure consisting of ionomers, catalyst agglomerates and pores, and the onset and growth of delamination process between the ionomer and catalyst agglomerate is investigated numerically by considering the catalyst agglomerate as elastic while the ionomer is elasto-viscoplastic, influenced by the cell assembly force arising from the cell clamping and variations in temperature and relative humidity. It is found that increasing clamping stress delays the delamination onset but has marginal effect on delamination propagation. The amplitude of hygrothermal cycles is the dominating factor in delamination and more frequent startup/shutdown of PEM fuel cells alleviates the delamination. Correlation between the rate of plastic strain accumulation in the ionomer and the interface delamination has been observed.     

Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells

In this study on humidifiers for polymer electrolyte membrane (PEM) fuel cell application, the experimental outcome of two air-to-air planar membrane humidifiers with three different internal flow patterns including cross, parallel and counter flows are investigated under isothermal and insulated boundary conditions. At all temperatures and flow rates, the conditions of higher performance, corresponding to highest water recovery ratio (WRR) and lowest dew point approach temperatures (DPAT), are encountered in the counter flow case, in contrary to the cross flow configuration. The insulation condition with dry inlet temperature at 30 °C and wet inlet temperature at 60 °C has a higher WRR index compared to isothermal condition at 60 °C but is lower than isothermal condition at 30 °C. The DPAT in humidifier with insulation condition is approximately equal to that obtained in isothermal condition at 60 °C but is much higher than what results in isothermal condition at 30 °C. It can be deduced that the temperature of the wet side inlet plays a key role in the humidifier performance.     

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.     

X-ray computed tomography of gas diffusion layers of PEM fuel cells: Calculation of thermal conductivity

Three commercially available gas diffusion layers were investigated by 3D X-ray computed tomography (CT). The carbon fibers and the 3D structure of the gas diffusion layers were clearly resolved by this lab-based technique.     

PEM fuel cell performance with solar air preheating

Proton Exchange Membrane Fuel Cells (PEMFC) have proven to be a promising energy conversion technology in various power applications and since it was developed, it has been a potential alternative over fossil fuel-based engines and power plants, all of which produce harmful by-products. The inlet air coolant and reactants have an important effect on the performance degradation of the PEMFC and certain power outputs. In this work, a theoretical model of a PEM fuel cell with solar air heating system for the preheating hydrogen of PEM fuel cell to mitigate the performance degradation when the fuel cell operates in cold environment, is proposed and evaluated by using energy analysis. Considering these heating and energy losses of heat generation by hydrogen fuel cells, the idea of using transpired solar collectors (TSC) for air preheating to increase the inlet air temperature of the low-temperature fuel cell could be a potential development. The aim of the current article is applying solar air preheating for the hydrogen fuel cells system by applying TSC and analyzing system performance. Results aim to attention fellow scholars as well as industrial engineers in the deployment of solar air heating together with hydrogen fuel cell systems that could be useful for coping with fossil fuel-based power supply systems.     

Improving proton exchange membrane fuel cell performance with carbon nanotubes as the material of cathode microporous layer

The cathode microporous layer (MPL) is fabricated by various multiwall carbon nanotubes (CNTs), and its influence on the performance of a proton exchange membrane fuel cell (PEMFC) is evaluated. Three types of CNT with different dimensions are employed in the experiments, and the conventional MPL made by acetylene black (AB) is also considered for the purpose of comparison. The results show that the employment of CNT as MPL composition indeed may improve fuel cell performance significantly in comparison with the case of AB. The type of CNT with the largest tube diameter and straight cylinder in shape exhibits the highest cell performance. The corresponding optimal CNT loading and polytetrafluoroethylene (PTFE) content in the MPL are also evaluated. Results show that the case of cathode MPL composed of 1.5 mg cm^?2 CNT and 20 wt% PTFE exhibits the best performance in all the experimental cases. The present data reveal that the application of CNT for MPL fabrication is beneficial to promote PEMFC performance. Copyright © 2015 John Wiley & Sons, Ltd.     

Measurement of effective gas diffusion coefficients of catalyst layers of PEM fuel cells with a Loschmidt diffusion cell

In this work, using an in-house made Loschmidt diffusion cell, we measure the effective coefficient of dry gas (O₂-N₂) diffusion in cathode catalyst layers of PEM fuel cells at 25 °C and 1 atmosphere. The thicknesses of the catalyst layers under investigation are from 6 to 29 μm. Each catalyst layer is deposited on an Al₂O₃ membrane substrate by an automated spray coater. Diffusion signal processing procedure is developed to deduce the effective diffusion coefficient, which is found to be (1.47 ± 0.05) × 10⁻⁷ m² s⁻¹ for the catalyst layers. Porosity and pore size distribution of the catalyst layers are also measured using Hg porosimetry. The diffusion resistance of the interface between the catalyst layer and the substrate is found to be negligible. The experimental results show that the O₂-N₂ diffusion in the catalyst layers is dominated by the Knudsen effect.     

Impact of air contamination by NOx on the performance of high temperature PEM fuel cells

High temperature PEM fuel cells show enhanced tolerances regarding fuel impurities like CO for use in various applications. However, the impact of air impurities like NO_x on the cell behavior is not completely understood yet. This study provides systematic investigation during 500 h of operation in presence of cathode air containing 10 ppm NO or NO₂. Nitrogen oxides provoke a strongly and linearly decreasing voltage of 245.3 ± 18.5 μV h⁻¹ and highly comparable damage that verifies similar HT-PEMFC degradation via both oxides. Cyclic voltammetry and electron microscopy reveal the loss of electrochemical catalyst surface by selectively poisoned surface and enforced catalyst particle growth. Impedance spectroscopy reveals besides increased electrode charge transfer resistances an affected proton conductivity. In contrast, SO₂/NO₂ impurity mixture in real occurring ratio causes less voltage decay due to a positive SO₂ impact through H₂SO₄ formation causing further shown and discussed effects like nitrate formation and discharge.     

Analytic determination of the effective thermal conductivity of PEM fuel cell gas diffusion layers

Accurate information on the temperature field and associated heat transfer rates are particularly important in devising appropriate heat and water management strategies in proton exchange membrane (PEM) fuel cells. An important parameter in fuel cell performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL). Estimation of the effective thermal conductivity is complicated because of the random nature of the GDL micro structure. In the present study, a compact analytical model for evaluating the effective thermal conductivity of fibrous GDLs is developed. The model accounts for conduction in both the solid fibrous matrix and in the gas phase; the spreading resistance associated with the contact area between overlapping fibers; gas rarefaction effects in microgaps; and salient geometric and mechanical features including fiber orientation and compressive forces due to cell/stack clamping. The model predictions are in good agreement with existing experimental data over a wide range of porosities. Parametric studies are performed using the proposed model to investigate the effect of bipolar plate pressure, aspect ratio, fiber diameter, fiber angle, and operating temperature.     

Modelling water intrusion and oxygen diffusion in a reconstructed microporous layer of PEM fuel cells

The hydrophobic microporous layer (MPL) in PEM fuel cell improves water management but reduces oxygen transport. We investigate these conflict impacts using nanotomography and pore-scale modelling. The binary image of a MPL is acquired using FIB/SEM tomography. The water produced at the cathode is assumed to condense in the catalyst layer (CL), and then builds up a pressure before moving into the MPL. Water distribution in the MPL is calculated from its pore geometry, and oxygen transport through it is simulated using pore-scale models considering both bulk and Knudsen diffusions. The simulated oxygen concentration and flux at all voxels are volumetrically averaged to calculate the effective diffusion coefficients. For water flow, we found that when the MPL is too hydrophobic, water is unable to move through it and must find alternative exits. For oxygen diffusion, we found that the interaction of the bulk and Knudsen diffusions at pore scale creates an extra resistance after the volumetric average, and that the conventional dusty model substantially overestimates the effective diffusion coefficient.