A review of polymer electrolyte membrane fuel cell stack testing

This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.     

Parametric study on geometric and material properties of polymer electrolyte membrane fuel cell with free vibration analysis

The mechanical aspect of understanding the dynamic behaviour is very important in stack assembly of fuel cell configuration. Designing such configuration would be very useful to tune the system accordingly to avoid catastrophic failure due to resonance. The dynamic analysis of a polymer electrolyte membrane fuel cell (PEMFC) is performed to understand its response during complex loading nature. The stack assembly of bipolar plate, gas diffusion electrolyte (GDE) and membrane are modelled and analysed using Mindilin plate element with finite element method (FEM). The parametric study on thickness, density and young's modulus and its influence on dominant modes of natural frequencies are analysed with a 5%–20% range of variations. It is observed that the bipolar plate's thickness plays a vital role in the vibration behaviour of the PEMFC.     

Estimation of crack propagation in polymer electrolyte membrane fuel cell under vibration conditions

In transportation applications, the main reasons of mechanical damage in polymer electrolyte membrane fuel cell (PEMFC) are road-induced vibrations and impact loads. The most vulnerable place of these cells is the interface between membrane and catalyst layer in the membrane electrode assembly (MEA). Hence, studies on mechanical strength of PEMFC should focus on that interface. The objective of present study lies in the fact that employing a prediction method to investigate the damage propagation behavior of vibration applied PEMFC using artificial neural network (ANN). The data available in the literature are used to constitute an ANN model. Three-layer model; input, hidden and output, are used for construction of ANN structure. Initial delamination length (a), amplitude (A), frequency (ω) and time (t) are used as input neurons whereas delamination length is output. Levenberg–Marquardt algorithm is selected as learning algorithm. On the other hand, number of hidden layer neuron is decided with the use of different neuron numbers by trial and error method. It is concluded that prediction capability of ANN model is in allowable limits and model can be suggested as efficient way of delamination length estimation.     

Analysis of the spatially distributed performance degradation of a polymer electrolyte membrane fuel cell stack

Herein we report the spatially uneven degradation of a polymer electrolyte membrane fuel cell (PEMFC) stack operated under load variation. Fifteen sub-membrane electrode assemblies (sub-MEAs) at various cell positions and various points within each cell were obtained from the original MEAs employed in the fuel cell stack. Polarization curves and the voltammetric charge of these MEAs were measured in order to correlate localized performances with the redistributed electrochemically active surface on Pt using the polarization technique and cyclic voltammetry. Several ex situ characterizations including electron probe microanalysis, environmental scanning electron microscopy, and X-ray diffraction were also performed to find evidence, supporting the inhomogeneous degradation of the fuel cell stack. Possible routes and processes for the non-uniform stack degradation during the PEMFC stack operation will also be discussed.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.     

Thermal conductivity and temperature profiles of the micro porous layers used for the polymer electrolyte membrane fuel cell

The thermal conductivity and the thickness change with pressure of several different micro porous layers (MPL) used for the polymer electrolyte membrane fuel cell (PEMFC) were measured. The MPL were made with different compositions of carbon and polytetrafluoroethylene (PTFE). A one-dimensional thermal PEMFC model was used to estimate the impact that the MPL has on the temperature profiles though the PEMFC.     

In-fibre Bragg grating sensors for distributed temperature measurement in a polymer electrolyte membrane fuel cell

A new application of in-fibre Bragg grating (FBG) sensors for the distributed measurement of temperature inside a polymer electrolyte membrane fuel cell is demonstrated. Four FBGs were installed on the lands between the flow channels in the cathode collector plate of a single test cell, evenly spaced from inlet to outlet. In situ calibration of the FBG sensors against a co-located micro-thermocouple shows a linear, non-hysteretic response, with sensitivities in good agreement with the expected value. A relative error of less than 0.2 ° C over the operating range of the test cell (∼20-80 °C) was achieved, offering sufficient resolution to measure small gradients between sensors. While operating the fuel cell at higher current densities under co-flow conditions, gradients of more than 1 ° C were measured between the inlet and outlet sensors. Due to their small thermal mass, the sensors also exhibit good temporal response to dynamic loading when compared with the thermocouple. Design and instrumentation of the graphite collector plate features minimal intrusion by the sensors and easy adaptation of the techniques to bipolar plates for stack implementation.     

Specific approaches to dramatic reduction in stack activation time and perfect long-term storage for high-performance air-breathing polymer electrolyte membrane fuel cell

Air-breathing polymer electrolyte membrane fuel cell (PEMFC) systems without the humidifier and air blower have been developed to overcome the cost and complexity of balance of plants (BOPs). Until now, there has been no specific way to improve the stack's initial performance through the specific activation protocol and maintain the initial performance for a very long time. Herein, we studied a technique for finishing the total activation within 1 h by using a pre-activation process (i.e., soaking the stack in a DI-water reservoir) and applying current at 0.65 V. The pre-activation procedure significantly increased the swelling of the polymer membrane and the Nafion binder in the catalyst layer, reducing the total activation time. Also, we showed that long-term storage using humidified N₂ gas in a closed box did not hinder the electrocatalytic activity of the Pt and the drying of the polymer membrane for 60 days.     

A graphite-coated carbon fiber epoxy composite bipolar plate for polymer electrolyte membrane fuel cell

A PEMFC (polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell) stack is composed of GDLs (gas diffusion layers), MEAs (membrane electrode assemblies), and bipolar plates. One of the important functions of bipolar plates is to collect and conduct the current from cell to cell, which requires low electrical bulk and interfacial resistances. For a carbon fiber epoxy composite bipolar plate, the interfacial resistance is usually much larger than the bulk resistance due to the resin-rich layer on the composite surface.In this study, a thin graphite layer is coated on the carbon/epoxy composite bipolar plate to decrease the interfacial contact resistance between the bipolar plate and the GDL. The total electrical resistance in the through-thickness direction of the bipolar plate is measured with respect to the thickness of the graphite coating layer, and the ratio of the bulk resistance to the interfacial contact resistance is estimated using the measured data. From the experiment, it is found that the graphite coating on the carbon/epoxy composite bipolar plate has 10% and 4% of the total electrical and interfacial contact resistances of the conventional carbon/epoxy composite bipolar plate, respectively, when the graphite coating thickness is 50 μm.     

Enhancement of polymer electrolyte membrane fuel cell performance by boiling a membrane electrode assembly in sulfuric acid solution

A catalyst-coated membrane (CCM) as used in the membrane electrode assembly (MEA) of a polymer electrolyte membrane fuel cell is treated by dilute sulfuric acid solution (0.5 M) at boiling temperature for 1 h. This treatment improves the single-cell performance of the CCM without further addition of Pt catalyst. The changed microstructure and electrochemical properties of the catalyst layer are investigated by field emission scanning electron microscopy with energy dispersive X-ray, mercury intrusion porosimetry, waterdrop contact angle measurement, Fourier transform-infrared spectrometry in attenuated total reflection mode, electrochemical impedance spectroscopy, and cyclic voltammetry. The results indicate that this pretreatment enhances MEA performance by changing the microstructure of the catalyst layer and thus changing the degree of hydration, and by modifying the Pt surface, thus enhancing the oxygen reduction reaction.     

Degradation behaviors of polymer electrolyte membrane fuel cell under freeze/thaw cycles

Degradation behaviors of polymer electrolyte membrane fuel cell (PEMFC) in high current density region were investigated under Freeze/Thaw cycles. Different dehumidification scenarios namely hot purge, cold purge and no purge were adopted for comparison. Micrographs from scanning electron microscopy proved little change in catalyst-coated membrane (CCM) integrity, no delamination or segregation occurred after many freeze/thaw cycles. Cyclic Voltammetry (CV) measurement revealed reduction in electrochemical active surface area of CCM. The observed performance decay in the high current density region was mainly attributed to the increased interface contact resistance and degraded electric and gas coupling characteristics at interfaces between CCM and GDL in this paper. Meanwhile, the performance degradation under low current densities (for example 400 mA cm⁻² or even lower) was mainly ascribed to the degraded characteristics of catalyst layers referring to CCM as cyclic voltammetry indicated. Proper dehumidification through gas purging is effective to maintain stable preference under subzero temperature.     

Humidification of polymer electrolyte membrane fuel cell using short circuit control for unmanned aerial vehicle applications

In the present study, a short circuit controller for use in the humidification of polymer electrolyte membrane fuel cells was developed for unmanned aerial vehicles (UAVs). Fuel cells (FCs) require an external humidifier to avoid drying up. Particularly in UAV applications, humidity control is more important because the boiling point of water decreases with increase in flight altitude. In this study, overcurrent was generated by short-circuiting an FC to humidify the electrolyte membrane and improve the electric power output. An FC controller incorporating a short circuit unit was developed, and a battery was hybridized with the FC to compensate the power when the latter was short-circuited. The performance of the FC was evaluated for the interval (period) and duration of short circuit. Using this method, the power output was improved by 16% when short circuit control was operated at the optimal condition.     

Electrochemical analysis of polymer electrolyte membrane fuel cell operated with dry-air feed

Electrochemical analysis of a commercial polymer electrolyte membrane fuel cell (PEMFC), operated at varying cathode relative humidity (RH) and current density, has been conducted to understand the factors that affect power performance when the PEMFC is operated with a dry-air feed. With a change in the cathode RH from 80 to 4%, the electrochemical area and double-layer capacitance of the cathode are reduced by 9 and 8%, respectively. This indicates that exclusion of the catalyst layer (CL) of the cathode from proton access occurs to some extent at low RH. It does not, however, explain the observed increase in activation loss. For the dry-air feed, the ionic resistances of the membrane and cathode CL are comparable in magnitude. Impedance analyses show that drying of the cathode at low RH and low current density leads not only to an increase in the ionic resistance of the CL, but also to increases in both charge-transfer and mass-transfer resistances. The simultaneous decrease in all the resistance components with decrease in the air permeability of the cathode diffusion layer highlights the importance of cathode design for operation with dry-air feed.     

Nonlinear frequency response analysis of dehydration phenomena in polymer electrolyte membrane fuel cells

Dehydration phenomena in a PEM fuel cell were investigated by nonlinear frequency response analysis (NFRA) in a differential H₂/H₂ cell. The linear H_1,0 spectra, which are equal to classic EIS spectra, showed not only an increase of the membrane resistance but also an increase of the anode reaction resistance, caused by dehydration leading to the decrease of the protonic conductivity of the polymer network in the catalyst layer. With this, active sites with long protonic pathes to the membrane become inactive. In order to further clarify this effect, modelling work was used. Therefore, proton transport was incorporated into an existing model of a differential H₂/H₂ cell. Finally, the key features of NFRA spectra under dehydration and CO poisoning are compared in order to discuss the suitability of NFRA for unambiguous diagnosis of PEMFC. It can be seen that while the linear spectrum is not sufficient to distinguish between both cases, the second order frequency response functions can be used for discrimination.     

Influence of local porosity and local permeability on the performances of a polymer electrolyte membrane fuel cell

In the literature, many models and studies focused on the steady-state aspect of fuel cell systems while their dynamic transient behavior is still a wide area of research. In the present paper, we study the effects of mechanical solicitations on the performance of a proton exchange membrane fuel cell as well as the coupling between the physico-chemical phenomena and the mechanical behavior. We first develop a finite element method to analyze the local porosity distribution and the local permeability distribution inside the gas diffusion layer induced by different pressures applied on deformable graphite or steel bipolar plates. Then, a multi-physical approach is carried out, taking into account the chemical phenomena and the effects of the mechanical compression of the fuel cell, more precisely the deformation of the gas diffusion layer, the changes in the physical properties and the mass transfer in the gas diffusion layer. The effects of this varying porosity and permeability fields on the polarization and on the power density curves are reported, and the local current density is also investigated. Unlike other studies, our model accounts for a porosity field that varies locally in order to correctly simulate the effect of an inhomogeneous compression in the cell.     

Optimizing polymer electrolyte membrane thickness to maximize fuel cell vehicle range

Automotive hydrogen polymer electrolyte membrane (PEM) fuel cell systems require periodic purges to remove nitrogen and water from the anode. Purging increases system performance by limiting anode hydrogen dilution, but reduces hydrogen utilization. State of the art fuel cell membrane electrode assemblies utilize thin ionomer membranes in an effort to increase performance and reduce cost. Thinner membranes also increase the required anode purge rates due to the increased transport of inert gases. A model was developed to examine the relationship between membrane thickness and vehicle range which takes into account anode purge rate. The model includes changes in efficiency and hydrogen utilization as a function of PEM thickness for a variety of operating conditions. The model predicts that an optimal membrane thickness which maximizes vehicle range exists, but this thickness is highly dependent on other system conditions. The results of this study can be extended to help optimize stack development and balance of plant design.     

Implementation of discrete wavelet transform-based discrimination and state-of-health diagnosis for a polymer electrolyte membrane fuel cell

This research investigates a new approach based on the discrete wavelet transform (DWT) that suitable for analyzing and evaluating output terminal voltage signal (OTVS) for discrimination analysis of a polymer electrolyte membrane fuel cell (PEMFC). Due to its ability for extracting information from the non-stationary and transient phenomena simultaneously in both time and frequency domain, the OTVS can be applied as source data in the DWT-based approach. By using the wavelet decomposition including the multi-resolution analysis (MRA) using the Daubechies wavelet (dB) as mother wavelet, the information on the electrochemical characteristics of a PEMFC can be extracted from the OTVS over a wide frequency range. Thus, the cells that have similar electrochemical characteristics can be eventually discriminated. In particular, this present research develops these investigations one step further by showing low-frequency components (approximation A_n) and high-frequency components (detail D_n) extracted from variable single cells with different electrochemical characteristics. Experimental results show that DWT-based approach is clearly appropriate for the reliable SOH diagnosis for a PEMFC.     

Performance enhancement of membrane electrode assemblies with plasma etched polymer electrolyte membrane in PEM fuel cell

In this work, a surface modified Nafion 212 membrane was fabricated by plasma etching in order to enhance the performance of a membrane electrode assembly (MEA) in a polymer electrolyte membrane fuel cell. Single-cell performance of MEA at 0.7 V was increased by about 19% with membrane that was etched for 10 min compared to that with untreated Nafion 212 membrane. The MEA with membrane etched for 20 min exhibited a current density of 1700 mA cm⁻² at 0.35 V, which was 8% higher than that of MEA with untreated membrane (1580 mA cm⁻²). The performances of MEAs containing etched membranes were affected by complex factors such as the thickness and surface morphology of the membrane related to etching time. The structural changes and electrochemical properties of the MEAs with etched membranes were characterized by field emission scanning electron microscopy, Fourier transform-infrared spectrometry, electrochemical impedance spectroscopy, and cyclic voltammetry.     

Performance equations for a polymer electrolyte membrane fuel cell with unsaturated cathode feed

A mathematical formulation for the cathode of a membrane electrode assembly of a polymer electrolyte membrane fuel cell is proposed, in which the effect of unsaturated vapor feed in the cathode is considered. This mechanistic model formulates the water saturation front within the gas diffusion layer with an explicit analytical expression as a function of operating conditions. The multi-phase flows of gaseous species and liquid water are correlated with the established capillary pressure equilibrium in the medium. In addition, less than fully hydrated water contents in the polymer electrolyte and catalyst layers are considered, and are integrated with the relevant liquid and vapor transfers in the gas diffusion layer. The developed performance equations take into account the influences of all pertinent material properties on cell performance using first principles. The mathematical approach is logical and concise in terms of revealing the underlying physical significance in comparison with many other empirical data fitting models.