Water transport characteristics of polymer electrolyte membrane fuel cell

This paper describes the performance of a polymer electrolyte membrane fuel cell (PEMFC) system without humidification of the reactants which consumes a lot of parasitic power, increases the weight of the PEMFC system and thus adds complexity. Such PEMFC systems are preferable for portable applications. The results indicate that dry gas operation depends on various factors like reactant flow field design, area of the electrode and equilibration time for the product water. The performance of the fuel cell can be improved by giving some equilibration time for the product water, produced by the electrochemical reactions, to get transported across the membrane to the anode side, thus increasing the conductivity of the membrane. The water transported through the membrane across the cell was investigated by measuring the amount of product water at the anode side which allows humidification for the anode gas and less condensed water in the fluid flow channels of the cathode.     

Calculations of transport phenomena and reaction distribution in a polymer electrolyte membrane fuel cell

The performance of Polymer Electrolyte Membrane fuel cells depends on the design of the cell as well as the operating conditions. The design of the cell influences the complex interaction of activation effects, ohmic losses, and transport limitations, which in turn determines the local current density. Detailed models of the electrochemical reactions and transport phenomena in Polymer Electrolyte Membrane fuel cells can be used to determine the current density distribution for a given fuel cell design and operating conditions. In this work, three-dimensional, multicomponent and multiphase transport calculations are performed using a computational fluid dynamics code. The computational results for a full-scale fuel cell design show that ohmic effects due to drying of polymer electrolyte in the anode catalyst layer and membrane, and transport limitations of air and flooding in the cathode cause the current density to be a maximum near the gas channel inlets where ohmic losses and transport limitations are a minimum. Elsewhere in the cell, increased ohmic losses and transport limitations cause a decrease in current density, and the performance of the fuel cell is significantly lower than that which could be attained if the ohmic losses and transport limitations throughout the cell were the same as those near the gas channel inlets. Thus overall fuel cell design is critical in maximizing unit performance.     

A novel membrane transport model for polymer electrolyte fuel cell simulations

This work presents the development of a 1D model describing water and charge transport through the polymer electrolyte membrane (PEM) in the fuel cell. The considered driving forces are electrical potential, concentration and pressure gradients. The membrane properties such as water diffusion and electro-osmotic coefficients, water sorption and ionic conductivity are treated as temperature dependent functions. The dependencies of diffusion and electro-osmotic coefficients on the membrane water concentration are described by linear functions. The membrane conductivity is computed in the framework of the percolation theory under consideration that the conducting phase in the PEM is formed by a hydrated functional groups and absorbed water. This developed membrane model was implemented in the CFD code AVL FIRE using 1D/3D coupling. The simulated polarization curves at various humidification of the cathode are found in good agreement with the experiments thus confirming the validity of the model.     

Dynamic modeling and simulation of air-breathing proton exchange membrane fuel cell

Small fuel cells have shown excellent potential as alternative energy sources for portable applications. One of the most promising fuel cell technologies for portable applications is air-breathing fuel cells. In this paper, a dynamic model of an air-breathing PEM fuel cell (AB-PEMFC) system is presented. The analytical modeling and simulation of the air-breathing PEM fuel cell system are verified using Matlab, Simulink and SimPowerSystems Blockset. To show the effectiveness of the proposed AB-PEMFC model, two case studies are carried out using the Matlab software package. In the first case study, the dynamic behavior of the proposed AB-PEMFC system is compared with that of a planar air-breathing PEM fuel cell model. In the second case study, the validation of the air-breathing PEM fuel cell-based power source is carried out for the portable application. Test results show that the proposed AB-PEMFC system can be considered as a viable alternative energy sources for portable applications.     

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.     

Anion exchange membrane fuel cell modelling

A parametric model predicting the performance of a solid polymer electrolyte, anion exchange membrane fuel cell (AEMFC), has been developed, in Matlab environment, based on interrelated electrical and thermal models. The electrical model proposed is developed by modelling an AEMFC open-circuit output voltage, irreversible voltage losses along with a mass balance, while the thermal model is based on the energy balance. The proposed model of the AEMFC stack estimates its dynamic behaviour, in particular the operating temperature variation for different discharge current values. The results of the theoretical fuel cell (FC) stack are reported and analysed in order to highlight the FC performance and how it varies by changing the values of some parameters such as temperature and pressure. Both the electrical and thermal FC models were validated by comparing the model results with experimental data and the results of other models found in the literature.     

Dynamic modelling and controlling strategy of polymer electrolyte fuel cells

A comprehensive dynamic control oriented model of a polymer electrolyte fuel cell (PEFC) is developed, considering the mixed effects of temperature, gas flow and capacitance. The fractional relationship between the dynamic output voltage and the capacitance, which escaped the attention in most previous studies is also addressed. Moreover, based on the developed PEFC model, a proportional integral and derivative (PID) controller is designed to stabilize the output voltage at a determined value by regulating the input hydrogen mass flow rate under a series of disturbance in the input. The dynamic PEFC model and controller are simulated in Matlab/Simulink. The simulation results illustrate that the PEFC system model is capable of characterizing dynamic properties of PEFCs. Additionally, the developed PID controller is effective in stabilizing the output voltage with a rather small overshoot and rather faster response, which also proves that the developed model is suitable for PEFC control algorithms development.     

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.     

Modelling of coupled electron and mass transport in anisotropic proton-exchange membrane fuel cell electrodes

As one of the key components of proton-exchange membrane fuel cells, the gas-diffusion layer (GDL) that is made of carbon fibres usually exhibits strong structural anisotropy. Nevertheless, the GDL has traditionally been simplified as a homogeneous porous structure in modelling the transport of species through the GDL. In this work, a coupled electron and two-phase mass transport model for anisotropic GDLs is developed. The effects of anisotropic GDL transport properties due to the inherent anisotropic carbon fibres and caused by GDL deformations are studied. Results indicate that the inherent structural anisotropy of the GDL significantly influences the local distribution of both cathode potential and current density. Simulation results further indicate that a GDL with deformation results in an increase in the concentration polarization due to the increased mass-transfer resistance in the deformed GDL. On the other hand, the ohmic polarization is found to be smaller in the deformed GDL as the result of the decreased interfacial contact resistance and electronic resistance in the GDL. This result implies that an optimum deformation needs to be achieved so that both concentration and ohmic losses can be minimized.     

Dynamic modelling of PEM fuel cell system for simulation and sizing of marine power systems

This article presents a model of a proton exchange membrane fuel cell (PEMFC) system for marine power systems. PEMFC in marine hybrid power sources can have various power ranges and capacities in contrast with vehicle applications. Investigating PEMFCs behaviour and performance for various conditions and configurations is demanded for proper sizing and feasibility studies. Hence, modelling and simulation facilitate understanding the performance of the PEMFC behaviour with various sizes and configurations in power systems. The developed model in this work has a system level fidelity with real time capabilities, which can be utilized for simulator approaches besides quasi-static studies with a power-efficiency curve. Moreover, the model can be used for scaling the PEMFC power range by considering transient responses and corresponding efficiencies. The Bond graph approach as a multi-disciplinary energy based modelling strategy is employed for the PEMFC as a multi domains system. In the end, various PEMFC cell numbers and compressor sizes have been compared with power-efficiency curves and transient responses in a benchmark.     

X-ray computed tomography reconstruction and analysis of polymer electrolyte membrane fuel cell porous transport layers

A commercially available porous transport layer (SGL carbon group Sigracet^® GDL 30BA), is investigated using X-ray computed tomography reconstruction. A novel aspect of this study is an investigation of the effects of non-homogeneous compression of the GDL 30BA sample including effective transport properties. Non-homogeneous compression is typical in polymer electrolyte fuel cells as the flow field plates consist of a series of lands and channels which apply an uneven loading to the porous transport layers. The X-ray computed tomography technique provides input data for the computer reconstruction procedures integrating image post-processing and iso-surface reconstruction. The resulting tomographic and surface reconstruction is converted into the computational volume/grid for microstructural and computational fluid dynamics (CFD) analysis. The heterogeneous compression effects on effective geometric and transport properties are investigated for various compression levels and effective transport properties are compared to theoretical studies such as Bruggeman [1] and Tomadakis and Sotirchos [2]. The effects of non-homogeneous compression are significant, with the transport properties differing by a factor of about 2 between the land and the channel regions. It is found that the effective transport properties are significantly lower than predicted by commonly used relations, with the lowest values representing only 15% of the predictions from the Bruggeman relation.     

Free vibration analysis of a polymer electrolyte membrane fuel cell

A free vibration analysis of a polymer electrolyte membrane fuel cell (PEMFC) is performed by modelling the PEMFC as a 20 cm × 20 cm composite plate structure. The membrane, gas diffusion electrodes, and bi-polar plates are modelled as composite material plies. Energy equations are derived based on Mindlin's plate theory, and natural frequencies and mode shapes of the PEMFC are calculated using finite element modelling. A parametric study is conducted to investigate how the natural frequency varies as a function of thickness, Young's modulus, and density for each component layer. It is observed that increasing the thickness of the bi-polar plates has the most significant effect on the lowest natural frequency, with a 25% increase in thickness resulting in a 17% increase in the natural frequency. The mode shapes of the PEMFC provide insight into the maximum displacement exhibited as well as the stresses experienced by the single cell under vibration conditions that should be considered for transportation and stationary applications. This work provides insight into how the natural frequencies of the PEMFC should be tuned to avoid high amplitude oscillations by modifying the material and geometric properties of individual components.     

Dynamic numerical simulation of a molten carbonate fuel cell

The paper presents a set of two-dimensional molten carbonate fuel cell mathematical models, considering the electrical, mass and heat transfer characteristics of a molten carbonate fuel cell. A dynamic simulation model was built under the VC++ environment. The steady state and dynamic simulation results for a cross-flow molten carbonate fuel cell were obtained using the simulation model. The results are appropriate for the design and operation of molten carbonate fuel cells.     

Lattice Boltzmann simulations of water transport in gas diffusion layer of a polymer electrolyte membrane fuel cell

The effect of wettability on water transport dynamics in gas diffusion layer (GDL) is investigated by simulating water invasion in an initially gas-filled GDL using the multiphase free-energy lattice Boltzmann method (LBM). The results show that wettability plays a significant role on water saturation distribution in two-phase flow in the uniform wetting GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially. The resulting water saturation distributions show that the saturation in the GDL has little change after water breaks through the GDL, and further confirm that the selective introduction of hydrophilic passages in the GDL would facilitate the removal of liquid water more effectively, thus alleviating the flooding in catalyst layer (CL) and GDL. The LBM approach presented in this study provides an effective tool to investigate water transport phenomenon in the GDL at pore-scale level with wettability distribution taken into consideration.     

Development of polymer electrolyte membrane fuel cell stack

The proton exchange membrane fuel cell (PEMFC) is one of the strongest contenders as a power source for space, electric vehicle and domestic applications. Since 1988 intensive research is being carried out at our centre to develop PEMFCs. The main RandD activities are: (i) to develop a method for the electrode preparation (ii) to enhance platinum utilisation using low platinum loading and (iii) to design multicell stacks. The results of RandD development of the above activities are discussed in this paper.     

Microscale simulations of reaction and mass transport in cathode catalyst layer of polymer electrolyte fuel cell

The resistance of the cathode oxygen reduction reaction in polymer electrolyte fuel cells must be reduced for improving the performance. Therefore, it is important to thoroughly understand the relationship between the heterogeneous structures and the cell performance. However, it is difficult to obtain such an understanding using experimental approaches and typical uniform porous simulations. In this study, numerical analysis was used to simulate a three-dimensional catalyst layer (CL) with carbon black (CB) aggregate structures and ionomer coating models, and a cathode reaction and mass transport simulation model incorporating the heterogeneous structure was developed. Moreover, the relationship between the electrode structure and the cell performance, including the reaction distribution and output performance, was examined. The current density distribution depended on the CB structure and ionomer adhesion shape. From the viewpoint of enhancing both the Pt utilization and the mass transport performance, an adequate heterogeneous pore structure in the CL is necessary. These results were used to determine the optimal material properties for the high performance cell.     

Analysis and control of a hybrid fuel delivery system for a polymer electrolyte membrane fuel cell

A polymer electrolyte membrane fuel cell (PEM FC) system as a power source used in mobile applications should be able to produce electric power continuously and dynamically to meet the demand of the driver by consuming the fuel, hydrogen. The hydrogen stored in the tank is supplied to the anode of the stack by a fuel delivery system (FDS) that is comprised of supply and recirculation lines controlled by different actuators. Design of such a system and its operation should take into account several aspects, particularly efficient fuel usage and safe operation of the stack.     

Proton transport of porous triazole-grafted polysulfone membranes for high temperature polymer electrolyte membrane fuel cell

Introduction of porous structure to high temperature polymer electrolyte membranes is one of effective pathways to increase their proton conductivity under elevated temperature. However, the effect of the porous structure on the proton diffusion mechanism of these membranes is still unclear. In this work, the proton transport behaviour of a series of porous triazole-polysulfone (PSf) membranes under elevated temperature is comprehensively investigated. The functional triazole ring in the framework of porous triazole-PSf acts a proton acceptor to form acid-base pair with phosphoric acid (PA). In addition, the proton diffusion coefficient and proton conductivity of PA-doped porous triazole-PSf is an order of magnitude higher than that of the PA-doped dense triazole-PSf membrane. Percolation theory calculation convinces that the high proton conductivity of PA-doped porous triazole-PSf is due to the formation of continuous long-range proton diffusion channels under high pore connectivity and porosity. On the contrary, excessive pore connectivity also results in high gas permeability, leading to decrease of the open circuit voltage and cell performance of the single cell. Consequently, the optimum porosity for the PA-doped porous triazole-PSf membrane is 75% for fuel cell operating with the maximum peak power density of 550 mW·cm^?2 and great durability for 120 h under 140 °C.     

Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study

Chitosan has been considered attractive in polymer electrolyte membrane fuel cells (PEMFCs) due to its excellent film forming and fuel barrier properties. Reflecting the limitation of its low proton conductivity, various materials were used to improve the proton conductivity of chitosan, through combination with inorganic materials like graphene oxide. We present an ideal molecular model for bio-nanocomposites and their mechanism of proton conductivity in PEMFCs. In this study, the diffusion behavior of hydronium ions in chitosan/graphene complex systems at various temperatures, concentrations and pH values were studied systematically using 3 ns long molecular dynamics (MD) simulations with an aim to provide the mechanisms of proton conductivity of chitosan/graphene composite at an atomistic scale. Various amounts of water content (10%, 20%, 30% and 40%), pH values (achieved by adjusting the protonation degree of amino groups of chitosan by 20%, 40%, 60%, 80% and 100%) and numbers of graphene sheets (1, 2, and 3) were considered during MD simulations at 4 temperatures (298 K, 320 K, 340 K and 360 K). Our results indicated that the chitosan system containing 40% water was the most suitable polymer electrolyte membrane and temperature was a key factor affecting diffusion proton. Adding graphene to the chitosan system and adjusting the pH values of chitosan were demonstrated to have a significant effect on improving the proton conductivity of the membrane.     

Humidification studies on polymer electrolyte membrane fuel cell

Two methods of humidifying the anode gas, namely, external and membrane humidification, for a polymer electrolyte membrane fuel (PEMFC) cell are explained. It is found that the water of solvation of protons decreases with increase in the current density and the electrode area. This is due to insufficient external humidification. In a membrane-based humidification, an optimum set of parameters, such as gas flow rate, area and type of the membrane, must be chosen to achieve effective humidification. The present study examines the dependence of water pick-up by hydrogen on the temperature, area and thickness of the membrane in membrane humidification. Since the performance of the fuel cell is dependent more on hydrogen humidification than on oxygen humidification, the scope of the work is restricted to the humidification of hydrogen using Nafion^® membrane. An examination is made on the dependence of water pick-up by hydrogen in membrane humidification on the temperature, area and thickness of the membrane. The dependence of fuel cell performance on membrane humidification and external humidification in the anode gas is also considered.