Effects of gas-diffusion layer properties on the performance of the cathode for high-temperature polymer electrolyte membrane fuel cell

The role of the gas-diffusion layer (GDL) in high-temperature polymer electrolyte fuel cell (HT-PEMFC) differs from that in low-temperature PEMFC GDL due to operating conditions and environment. Determining the GDL's structural parameters that affect its transport properties, and how these properties impact HT-PEMFC performance was urgently required. Four commercial GDLs were employed in HT-PEMFC cathode's GDE and was examined using X-μCT, mercury intrusion porosimetry, and an optical microscope to analyze structural parameters and characteristics. Fractal theory was applied to comprehend the gas transmission property of GDL, and the validity of the theory was confirmed through ex-situ through-plane gas permeability measurement. The analysis indicated that the porosity of GDL influenced by the crack region of the MPL has more impact on the GDL's gas transmission than its thickness. After that, we established a correlation between HT-PEMFC cathode performance and GDL porosity and theoretical gas transmission properties using R² coefficient of determination.     

Investigation of the effects of the anisotropy of gas-diffusion layers on heat and water transport in polymer electrolyte fuel cells

The aim of this work is to study the effects of gas-diffusion layer (GDL) anisotropy and the spatial variation of contact resistance between GDLs and catalyst layers (CLs) on water and heat transfer in polymer electrolyte fuel cells (PEFCs). A three-dimensional, two-phase, numerical PEFC model is employed to capture the transport phenomena inside the cell. The model is applied to a two-dimensional cross-sectional PEFC geometry with regard to the in-plane and through-plane directions. A parametric study is carried out to explore the effects of key parameters, such as through-plane and in-plane GDL thermal conductivities, operating current densities, and electronic and thermal contact resistances. The simulation results clearly demonstrate that GDL anisotropy and the spatial variation of GDL/CL contact resistance have a strong impact on thermal and two-phase transport characteristics in a PEFC by significantly altering the temperature, water and membrane current density distributions, as well as overall cell performance. This study contributes to the identification of optimum water and thermal management strategies of a PEFC based on realistic anisotropic GDL and contact-resistance variation inside a cell.     

Steady saturation distribution in hydrophobic gas-diffusion layers of polymer electrolyte membrane fuel cells: A pore-network study

A pore-network model is developed to simulate liquid water transport in a hydrophobic gas-diffusion layer (GDL) during the operation of polymer electrolyte membrane fuel cells (PEMFCs). The steady saturation distribution in GDLs is determined through a numerical procedure using a pore-network model combined with invasion-percolation path-finding and subsequent viscous two-phase flow calculation. The simulation results indicate that liquid water transport in hydrophobic GDLs is a strongly capillary-driven process that almost reaches the pure invasion-percolation limit with zero capillary number. A uniform flux condition is found to better reflect the actual phenomenon occurring at the inlet boundary for liquid water entering a GDL than a uniform pressure condition. The simulation further clarifies the effect of the invaded pore fraction at a uniform-flux inlet boundary in modifying water transport in GDLs. Finally, the effect of the GDL thickness on the steady saturation distribution is investigated.     

Platinum-sputtered electrode based on blend of carbon nanotubes and carbon black for polymer electrolyte fuel cell

A novel Pt-sputtered electrode based on a blend layer of carbon black (CB) and carbon nanotubes (CNTs) is developed for polymer electrolyte fuel cells. The Pt is sputtered on the surface of the blend to form a catalyst layer. The CNTs generate a pore in the blend layer, and the CB provides a high surface roughness for the blend layer. At a CNT content of 50 wt.%, the maximum value (20.6 m² g⁻¹) for the electrochemical area of the Pt is obtained, which indicates that the surface area of the blend layer exposed for Pt deposition is the largest. The power density of a membrane-electrode assembly (MEA) employing the Pt-sputtered electrodes shows a linear increase with electrochemical area. The mass activity of the optimized Pt-sputtered electrode with a Pt loading of 0.05 mg cm⁻² is 8.1 times that of an electrode with a Pt loading of 0.5 mg cm⁻² prepared using a conventional screen-printing technique. Excellent mass transfer is obtained with the Pt-sputtered electrode.     

One-dimensional thermal model of cold-start in a polymer electrolyte fuel cell stack

A transient, one-dimensional thermal model for a generic polymer electrolyte fuel cell (PEFC) stack is developed to investigate the cold-start ability and the corresponding energy requirement over different operating and ambient conditions. The model is constructed by applying the conservation of energy on each stack component and connecting the component's relevant boundaries to form a continuous thermal model. The phase change of ice and re-circulation of coolant flow are included in the analytical framework and their contribution to the stack thermal mass and temperature distribution of the components is also explored. A parametric study was conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, and anticipated temperature distribution in the stack at start-up for various operating conditions. Results indicate that 20 cells were sufficient to accurately experimentally and computationally simulate the full size stack behavior. It was observed that an optimum range of operating current density exists for a chosen stack design, in which rapid start-up of the stack from sub-zero condition can be achieved. Thermal isolation of the stack at the end plates is recommended to reduce the start-up time. Additionally, an end plate thickness exceeding a threshold value has no added effect on the stack cold-start ability. Effect of various internal and external heating mechanisms on the stack start-up were also investigated, and flow of heated coolant above 0 °C was found to be the most effective way to achieve the rapid start-up.     

Measurement and modelling of carbon monoxide poisoning distribution within a polymer electrolyte fuel cell

The distribution of carbon monoxide (CO) across the anode of a polymer electrolyte fuel cell with a single channel flow-field is modelled and compared with experimental results obtained using localised stripping voltammetry. Good agreement is observed between experiment and model over a wide range of CO carrier gas flow rates. The model is used to predict the effect of CO transient poisoning, expected to occur during system start-up, when high CO slippage is likely to occur from the fuel processor and the fuel cell is not electrically loaded.     

Modification of hydrocarbon structure for polymer electrolyte membrane fuel cell binder application

The development of novel hydrocarbon polymer membranes needs to be accompanied by catalyst layer hydrocarbon binder research so that the resultant membrane electrode assembly (MEA) can be durable. Hydrocarbon polymers which show high performance levels as membranes, however, are inadequate as catalyst layer binders as they are designed for low fuel penetration. Modification to the hydrocarbon polymer structure of high performing hydrocarbon polymers such as Sulfonated poly(arylene ether sulfone) (SPAES) can take advantage of its high conductivity while increasing gas permeability and maintaining interfacial compatibility with the membrane. The incorporation of a biphenyl fluorene group into the polymer backbone of SPAES successfully increased d-spacing which led to an increase in gas crossover. In the catalyst layer, the modified polymer ionomer showed higher penetration into primary pore volume thus increasing ESA. Higher catalyst utilization due to easier fuel access and ionomer coverage led to higher fuel cell performance. Durability tests revealed that structural modification did not hinder interfacial compatibility as well as performance.     

Controlling gas diffusion layer oxidation by homogeneous hydrophobic coating for polymer electrolyte fuel cells

Reduced production costs and enhanced durability are necessary for practical application of polymer electrolyte fuel cells. There has been a great deal of concern about degradation of the gas diffusion layer located outside the membrane electrode assembly. However, very few studies have been carried out on the degradation process, and no suitable methods for improving the durability of the cell have been found.In this work, the influence on the cell performance and factors involved in the degradation of the gas diffusion layer has been clarified through power generation tests.Long-term power generation tests on single cells for 6000 h were carried out under high humidity conditions with homogeneous and inhomogeneous hydrophobic coating gas diffusion layers. The results showed that the increase in the diffusion overvoltage from the gas diffusion layer could be controlled by the use of a homogeneous coating. Post-analyses indicated that this occurred by controlling oxidation of the carbon fiber.     

Expert diagnosis of polymer electrolyte fuel cells

Diagnosing faulty conditions of engineering systems is a highly desirable process within control structures, such that control systems may operate effectively and degrading operational states may be mitigated. The goal herein is to enhance lifetime performance and extend system availability. Difficulty arises in developing a mathematical model which can describe all working and failure modes of complex systems. However the expert's knowledge of correct and faulty operation is powerful for detecting degradation, and such knowledge can be represented through fuzzy logic. This paper presents a diagnostic system based on fuzzy logic and expert knowledge, attained from experts and experimental findings. The diagnosis is applied specifically to degradation modes in a polymer electrolyte fuel cell. The defined rules produced for the fuzzy logic model connect observed operational modes and symptoms to component degradation. The diagnosis is then tested against common automotive stress conditions to assess functionality.     

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.     

Reduced mass transport resistance in polymer electrolyte membrane fuel cell by polyethylene glycol addition to catalyst ink

Effects of Polyethylene glycol (PEG) addition to cathode catalyst ink were investigated by changing the addition amount of PEG. Performance of the polymer electrolyte membrane fuel cells (PEMFCs) increased and then decreased at the higher current density than 1.5 A/cm² as the amount of PEG addition increased. However, durability was not changed by the addition of PEG to the catalyst ink. Three different molecular weights of PEG were compared for PEG additives to cathode catalyst ink. Performance at high current density region increased and then decreased as PEG molecular weight increases from 200 to 10000. Increased performance by addition of PEG was attributed from reduced mass transport resistance. However, addition of large molecular weight PEG to catalyst ink reduced the performance because it lowered ionomer conductivity in the catalyst layer and then reduced proton transport resistance. Increased pore size in the catalyst layer and increased hydrophilicity on the electrode were also analyzed by addition of PEG to catalyst ink.     

Non intrusive diagnosis of polymer electrolyte fuel cells by wavelet packet transform

Fuel cell is a promising technology for both automotive and stationary applications. However, its reliability and its lifetime remain major hurdles to its wide access to these markets.It is therefore necessary to develop reliable diagnosis tools adapted to these two applications’ requirements. More particularly, online and real time tools for diagnosis will permit an early faults diagnosis and therefore an increase of the system reliability and performance.Most of the existing fault diagnosis methodologies in fuel cells require the knowledge of numerous parameters that may lead to a special inner parameter monitoring setup, which is difficult, even impossible to obtain, considering constraints like fuel cell stacks’ geometry. Moreover, considering the final fuel cell stack end-uses, for instance in transportation applications in which the “on-board” instrumentation has to be minimized, a model using a minimal number of parameter is highly desirable.In this paper, a simple and low-cost flooding diagnosis method applied to a PEFC (Polymer Electrolyte Fuel Cell) is described. This method only uses the stack voltage and can be adapted to a large set of fuel cell configurations and applications.Coming from the signal-processing domain, the diagnosis consists in a signal feature extraction by multiscale decomposition using discrete wavelet transform, followed by fault identification and classification. Results obtained in this work showed that the wavelet analysis method allows the identification of the flooding based on the patterns obtained from the wavelet packet coefficients.The application of wavelet theory to fuel cell diagnosis is innovative and very promising and the experimental results obtained in this study proved its feasibility and reliability to classify correctly PEFC experimental states into flooded and non-flooded state of health.     

Characterization of interfacial morphology in polymer electrolyte fuel cells: Micro-porous layer and catalyst layer surfaces

The interface between the micro-porous layer (MPL) and the catalyst layer (CL) can have an impact on thermal, electrical and two-phase mass transport in a polymer electrolyte fuel cell (PEFC). However, there is scant information available regarding the true morphology of the MPL and CL surfaces. In this work, optical profilometry is used to characterize the MPL and CL surfaces at the sub-micron level scale to gain a better understanding of the surface morphology. Selected MPL and CL surfaces were sputtered with a thin layer of gold to enhance the surface reflectivity for improved data acquisition. The results show that, for the materials tested, the MPL surface has a relatively higher roughness than the CL surface, indicating the potential dominance of the MPL surface morphology on the local transport and interfacial contact across the MPL|CL interface. The level of roughness can be on the order of 10 μm peak height, which is significant in comparison to other length scales involved in transport, and can result in significant interfacial water storage capacity (approximately 6-18% of the total water content in a PEFC [37]) along this interface. Another surface characteristic that can have a profound influence on multi-phase transport is the existence of deep cracks along the MPL and CL surfaces. The cracks on MPL and CL surfaces are observed to differ significantly in terms of their orientation, size, shape, depth and density. The areal crack density of the CL tested is calculated to be 3.4 ± 0.2%, while the areal crack density of the MPL is found to vary from 2.8% to 8.9%. The results of this study can be useful to understand the true nature of the interfacial transport in PEFCs.     

Direct three-dimensional reconstruction of a nanoporous catalyst layer for a polymer electrolyte fuel cell

The direct three-dimensional reconstruction of a polymer electrolyte fuel cell cathode catalyst layer from focused ion beam/scanning electron microscope (FIB/SEM) images is presented. The carbon and pore distribution is shown and quantitatively analysed. A new catalyst layer sample (Fumapem-F950/HiSpec13100) is sliced with FIB and a series of SEM images is taken. The images are registered, segmented and a three-dimensional stack is reconstructed. The three-dimensional carbon and pore distribution is shown. Based on the reconstruction the pore size distribution is evaluated. The total porosity and the unconnected pores space is analysed. The fully segmented 2D images are provided as supplemental material to this paper for future analysis and modeling work.     

Through-plane thermal conductivity of the microporous layer in a polymer electrolyte membrane fuel cell

Understanding the thermal properties of the microporous layer (MPL) is critical for accurate thermal analysis and improving the performance of proton exchange membrane (PEM) fuel cells operating at high current densities. In this study, the effective through-plane thermal conductivity and contact resistance of the MPL have been investigated. Gas diffusion layer (GDL) samples, coated with 5%-wt. PTFE, with and without an MPL are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Effective thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15 bar at 0.30 W/m°K and 55 μm, respectively. The effective thermal conductivity of the GDL substrate containing 5%-wt. PTFE varied from 0.30 to 0.56 W/m°K as compression was increased from 4 to 15 bar. As a result, GDL containing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL.     

Effect of anode iridium oxide content on the electrochemical performance and resistance to cell reversal potential of polymer electrolyte membrane fuel cells

An ideal polymer electrolyte membrane fuel cell (PEMFC) is one that continuously generates electricity as long as hydrogen and oxygen (or air) are supplied to its anode and cathode, respectively. However, internal and/or external conditions could bring about the degradation of its electrodes, which are composed of nanoparticle catalysts. Particularly, when the hydrogen supply to the anode is disrupted, a reverse voltage is generated. This phenomenon, which seriously degrades the anode catalyst, is referred to as cell reversal. To prevent its occurrence, iridium oxide (IrO₂) particles were added to the anode in the membrane-electrode assembly of the PEMFC single-cells. After 100 cell reversal cycles, the single-cell voltage profiles of the anode with Pt/C only and the anodes with Pt/C and various IrO₂ contents were obtained. Additionally, the cell reversal-induced degradation phenomenon was also confirmed electrochemically and physically, and the use of anodes with various IrO₂ contents was also discussed.     

Effect of material properties on evaporative water removal from polymer electrolyte fuel cell diffusion media

This work is devoted to delineating the fundamentals of evaporative water removal from diffusion media (DM) to achieve highly efficient and durable gas purge. Multiphase water transport from DM during gas purge is characterized by a balance of internal capillary liquid water flow and water vapor diffusion. In this study, DM with polytetrafluoroethylene (PTFE) content ranging from 0 to 20 wt%, and DM with three different geometric pore structures are utilized to understand this material property effect. It is found that overall evaporative water removal rate increases as PTFE content decreases and as the geometric pore structure changes from a two- to a more three-dimensional structure. This is due to the increase of wettability and porous space favorable for the water transport. The effect of phase-change-induced (PCI) flow and capillary flow on water removal is compared, and it is found that PCI flow is dominant at lower saturation of DM, whereas capillary flow is dominant at higher saturation. The results of this study build upon a previous study by the authors (Cho and Mench [17]), and are useful to understand the competing phenomena of water removal in PEFC DM. The ultimate goal of this work is to guide material design to achieve purge that preserves membrane durability with reduced shutdown power requirements.     

A lumped parameter model of the polymer electrolyte fuel cell

A model of a polymer electrolyte fuel cell (PEFC) is developed that captures dynamic behaviour for control purposes. The model is mathematically simple, but accounts for the essential phenomena that define PEFC performance. In particular, performance depends principally on humidity, temperature and gas pressure in the fuel cell system. To simulate accurately PEFC operation, the effects of water transport, hydration in the membrane, temperature, and mass transport in the fuel cells system are simultaneously coupled in the model. The PEFC model address three physically distinctive fuel cell components, namely, the anode channel, the cathode channel, and the membrane electrode assembly (MEA). The laws of mass and energy conservation are applied to describe each physical component as a control volume. In addition, the MEA model includes a steady-state electrochemical model, which consists of membrane hydration and the stack voltage models.     

Effect of temperature uncertainty on polymer electrolyte fuel cell performance

The temperature of operation is a key parameter in determining the performance and durability of a polymer electrolyte fuel cell (PEFC). Controlling temperature and understanding its distribution and dynamic response is vital for effective operation and design of better systems. The sensitivity to temperature means that uncertainty in this parameter leads to variable response and can mask other factors affecting performance. It is important to be able to determine the impact of temperature uncertainly and quantify how much PEFC operation is influenced under different operating conditions. Here, a simple lumped mathematical model is used to describe PEFC performance under temperature uncertainty. An analytical approach gives a measure of the sensitivity of performance to temperature at different nominal operating temperatures and electrical loadings. Whereas a statistical approach, using Monte Carlo stochastic sampling, provides a ‘probability map’ of PEFC polarisation behaviour. As such, a polarisation ‘area’ or ‘band’ is considered as opposed to a polarisation ‘curve’. Results show that temperature variation has the greatest effect at higher currents and lower nominal operating temperatures. Thermal imaging of a commercial air-cooled stack is included to illustrate the temporal and spatial temperature variation experienced in real systems.     

Numerical analysis of gas crossover effects in polymer electrolyte fuel cells (PEFCs)

The gas crossover phenomenon in polymer electrolyte fuel cells (PEFCs) is an indicator of membrane degradation. The objective of this paper is to numerically investigate the effects of hydrogen and oxygen crossover through the membrane in PEFCs. A gas crossover model is newly developed and implemented in a comprehensive multi-dimensional, multi-phase PEFC model developed earlier. A parametric study is carried out to investigate the effects of the crossover diffusion coefficients for hydrogen and oxygen as well as the membrane thickness. The simulation results demonstrate that the hydrogen crossover induces an additional oxygen reduction reaction (ORR) and consequently causes an additional voltage drop, while the influence of oxygen crossover on PEFC performance is relatively insignificant because it leads to the hydrogen/oxygen chemical reaction at the anode side. Finally, using the time-dependent gas crossover data that are available in the literature (measured in days), we conduct gas crossover simulations to examine the effects of increased gas crossover due to membrane degradation on PEFC performance and successfully demonstrate decaying polarization curves with respect to time. This study clearly elucidates the detailed mechanisms of the hydrogen and oxygen crossover phenomena and their effect on PEFC performance and durability.