Water balance in a free-breathing polymer electrolyte membrane fuel cell

Water balance in a free-breathing polymer electrolyte membrane fuel cell was studied, focusing on the effect of anode conditions. The methods used were current distribution measurement, water collection from the anode outlet, and the measurement of cell polarization and resistance. Current density levels were 100 and 200 mA cm^–2, temperature levels were 40 and 60 °C, and hydrogen stoichiometry range was from 1.5 to 2.5. The direction of hydrogen flow was varied. The fraction of product water exiting through the anode outlet varied from 0 to 58%, and it was found to increase with increasing temperature and hydrogen flow rate. When the general direction of hydrogen flow was against the direction of air flow, the percentage of water removal through the anode was smaller and the current distributions were more even than in the cases where the direction was the same as that of the air flow. This probably resulted from a more favorable distribution of water over the active area. The results also indicate that the net water transport coefficient varies across the active area. In further measurements, operation with the anode side in dead-end mode was investigated. It was also found that water distribution was more favorable when the general direction of hydrogen flow was against the air flow.     

Effect of ambient conditions on performance and current distribution of a polymer electrolyte membrane fuel cell

The performance and current distribution of a free-breathing polymer electrolyte membrane fuel cell (PEMFC) was studied experimentally in a climate chamber, in which temperature and relative humidity were controlled. The performance was studied by simulating ambient conditions in the temperature range 10 to 40 °C. The current distribution was measured with a segmented current collector. The results indicated that the operating conditions have a significant effect on the performance of the fuel cell. It was observed that a temperature gradient between the fuel cell and air is needed to achieve efficient oxygen transport to the electrode. Furthermore, varying the air humidity resulted in major changes in the mass diffusion overpotential at higher temperatures.     

PEM fuel cell voltage transient response to a thermal perturbation

This paper describes a transient model predicting PEMFC voltage response to a step change in the cooling water temperature. Its objectives are to put forward the main transport parameters and their corresponding time scales. The fuel cell is assumed isothermal with a time constant τ_t. The temperature variations result from the production of heat by the exothermic chemical reaction and by internal heat dissipation, and from heat transfer with the cooling circuit. The effects of temperature on fuel cell performances are taken into account through the variations in its thermodynamic voltage, in the kinetics of the half-reactions, and in the membrane ionic resistance. A dynamic and one-dimensional simulation of water transport in the membrane by electroosmotic drag and by diffusion is carried out: the relative humidity of gases varies with the cell temperature under the assumption that their specific humidity (i.e., the vapor content in the gas diffusion layers) remains unchanged.Two time constants characterize mass transfer in the membrane by water diffusion (τ_d) and by electroosmosis (τ_e). The Péclet number Pe which is equal to the ratio between τ_d and τ_e allows the comparison of the magnitude of these two transport mechanisms, both depending on current density and on the other operating conditions.The results of the model are compared to a set of experimental results obtained with a cell composed of a Nafion 115 membrane, and fed by hydrogen and pure oxygen. The average current density is 4000 A m⁻². In these conditions, the smallest time constant is the one characterizing the fuel cell thermal response τ_t (16 s). Therefore, the fuel cell voltage response to a temperature step occurs in two stages, the first one corresponding to the thermal regime. The second stage concerns water transport in the membrane; the best fit between numerical and experimental results yields to a Péclet number of about 16, which makes electroosmosis the most significant phenomenon.     

Realising a reference electrode in a polymer electrolyte fuel cell by laser ablation

This work presents a new concept for realising a reference electrode configuration in a PEM fuel cell by means of laser ablation. The laser beam is used to evaporate a small part of the electrode of a catalyst-coated membrane (CCM) to isolate the reference electrode from the active catalyst layer. This method enables the simultaneous ablation of the electrodes on both sides of the CCM because the membrane is transparent for the laser beam. Therefore, a smooth electrode edge without electrode misalignment can be realised. A test fuel cell was constructed which together with the ablated CCM enables the separation of the total cell losses during operation into the cathode, anode and membrane overpotentials in PEFC as well as in DMFC mode. The methanol tolerance of a selenium-modified ruthenium-based catalyst (RuSe _x ) was investigated under real fuel cell conditions by measuring polarisation curves, electrochemical impedance spectroscopy (EIS) and current interrupt measurements (CI).     

Influence of toluene contamination at the hydrogen Pt/C anode in a proton exchange membrane fuel cell

For fuel cells run on hydrogen reformate, traces of hydrocarbon contaminants in the hydrogen gas may be a concern for the performance and lifetime of the fuel cell. This study focuses on the influence of low concentrations of toluene on the adsorption and deactivation chemistry in a proton exchange membrane (PEM) fuel cell. For this purpose cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques were employed. Results from adsorption and desorption (by oxidation or reduction) experiments performed in a humidified nitrogen or hydrogen flow in a fuel cell test cell with a mass spectrometer system connected to the outlet are presented. The influence of adsorption potential, temperature, and humidity are discussed. The results show that toluene adsorbs on the catalyst surface in a broad potential window, up to at least 0.85 V versus RHE at 80 °C. Adsorbed toluene oxidizes to CO₂ with peak potentials above 1.0 V for temperatures below 95 °C. Some desorption of toluene (or reduced products) may take place at potentials below 0 V. In a hydrogen flow, toluene contamination in per mille concentrations leads to a continuous growth of the charge transfer resistance, while a 10-fold dilution of the toluene concentration resulted in a low and constant charge transfer resistance even for longer exposures. This indicates that a competition between toluene and hydrogen may take place on the active platinum surface at the anode.     

A printed circuit board approach to measuring current distribution in a fuel cell

A new method of measuring current distribution in a polymer electrolyte fuel cell of active area 100cm2 has been demonstrated, using a printed circuit board (PCB) technology to segment the current collector and flow field. The PCB technique was demonstrated to be an effective approach to fabricating a segmented electrode and provide a useful tool for analysing cell performance at different reactant gas flow rates and humidification strategies. In this initial chapter of work with the segmented cell, we describe measured effects on current distribution of cathode and anode gas stream humidification levels in a hydrogen/air cell, utilizing a NafionTM 117 membrane and single serpentine channel flow fields, and operating at relatively high gas flow rates. Effects of the stoichiometric flow of air are also shown. A clear trend is seen, apparently typical for a thick ionomeric membrane, of lowering in membrane resistance down the flow channel, bringing about the highest local current density near the air outlet. This trend is reversed at low stoichiometric flows of air. At an air flow rate less than three times stoichiometry, the local performance starts to drop significantly from inlet to outlet, as local oxygen concentration drop overshadows the lowering in resistance along the direction of flow.     

Reliability and accuracy of measured overpotential in a three-electrode fuel cell system

Numerical simulation was conducted to study the potential and current density distributions at the active electrode surface of a solid oxide fuel cell. The effects of electrode deviation, electrolyte thickness and electrode polarization resistance on the measurement error were investigated. For a coaxial anode/electrolyte/cathode system where the radius of the anode is greater than that of cathode, the cathode overpotential is overestimated while the anode overpotential is underestimated. Although the current interruption method or impedance spectroscopy can be employed to compensate/correct the error for a symmetric electrode configuration, it is not useful when dealing with the asymmetric electrode system. For the purpose of characterizing the respective overpotentials in a fuel cell, the cell configuration has to be carefully designed to minimize the measurement error, in particular the selection of the electrolyte thickness, which may cause significant error. For the anode-support single fuel cell, it is difficult to distinguish the polarization between the anode and cathode with reference to a reference electrode. However, numerical results can offer an approximate idea about the source/cause of the measurement error and provide design criteria for the fuel cell to improve the reliability and accuracy of the measurement technique.     

The regimes of catalyst layer operation in a fuel cell

A generalized Perry-Newman-Cairns model for performance of a generic catalyst layer (CL) with the Butler-Volmer conversion function is considered. The CL polarization curve, the rate of electrochemical conversion S(x) and the thickness of the conversion domain l_∗ are derived for the cases of ideal transport of ions or feed molecules. In both cases, the CL may work in the low- or high-current regime. In the low-current regime with poor ionic transport, l_∗ is given by the Newman’s current-independent reaction penetration depth. In the high-current regime, l_∗ is inversely proportional to the cell current, regardless of the origin of transport loss. The position and width of the transition region between the low- and high-current branches of the polarization curve are calculated. Based on these results, the features of catalyst layer performance in PEMFC, HT-PEMFC, DMFC and SOFC are discussed.     

Structural influence of hydrophobic diamine in sulfonated poly(sulfide sulfone imide) copolymers on medium temperature PEM fuel cell

Sulfonated poly(sulfide sulfone imide) copolymers containing flexible sulfide bond and six-membered imide ring were synthesized by random polycondensation. Two types of membranes were prepared by using sulfide (S-PSI) and sulfide sulfone (S-PSFI) based non-sulfonated diamines to investigate the effects of the hydrophobic component. IEC_w values were controlled to 1.51−1.94 meq g⁻¹ depending on the degree of sulfonation (DS) which was in the range of 50–80%. The membrane series showed good thermal stability up to 310 °C and mechanical properties (tensile strength >30 MPa). Dimensional stabilities were excellent with 23−35% increases, even at 100 °C. Proton conductivities of membranes composed of different hydrophobic diamines display a relatively good correlation with water content and morphology. In fuel cell tests, the S-PSI60 membrane shows relatively high current density of 250 mA cm⁻² at 0.6 V and maximum power density of 175 mW cm⁻² at 120 °C, 35% RH, 1.5 atm.     

Optimal shape of catalyst loading along the oxygen channel of a PEM fuel cell

A simple equation for the optimal shape of catalyst loading along the oxygen channel of a PEM fuel cell is derived. This shape g(z;λ) is a one-parametric, independent of cell current function of the distance z along the channel; λ is the stoichiometry of the oxygen flow. Optimal g homogenizes local cell current not affecting the cell polarization curve.     

质子交换膜燃料电池用催化剂及其电极的研究进展

膜电极(MEA)是质子交换膜燃料电池(PEMFC)的核心技术。膜电极包含的催化剂层、材料和结构等对PEMFC的性能影响很大。催化剂面层上供三相(质子、电子、气体)用的通道对于电池使用时的催化作用是必不可少的。介绍了近几年催化剂的研究进展,看重对三相通道进行了详细叙述。也回顾了一些用于改善催化剂活性的其他方法,如阴极催化、合金催化剂,根据这些进展,对今后的研究方向提出了建议。     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell during cold start processes

Cold start is critical to the commercialization of proton exchange membrane fuel cell (PEMFC) in automotive applications. Dynamic distributions of current and temperature in PEMFC during various cold start processes determine the cold start characteristics, and are required for the optimization of design and operational strategy. This study focuses on an investigation of the cold start characteristics of a PEMFC through the simultaneous measurements of current and temperature distributions. An analytical model for quick estimate of purging duration is also developed. During the failed cold start process, the highest current density is initially near the inlet region of the flow channels, then it moves downstream, reaching the outlet region eventually. Almost half of the cell current is produced in the inlet region before the cell current peaks, and the region around the middle of the cell has the best survivability. These two regions are therefore more important than other regions for successful cold start through design and operational strategy, such as reducing the ice formation and enhancing the heat generation in these two regions. The evolution of the overall current density distribution over time remains similar during the successful cold start process; the current density is the highest near the flow channel inlets and generally decreases along the flow direction. For both the failed and the successful cold start processes, the highest temperature is initially in the flow channel inlet region, and is then around the middle of the cell after the overall peak current density is reached. The ice melting and liquid formation during the successful cold start process have negligible influence on the general current and temperature distributions.     

Performance improvement of electrode for polymer electrolyte membrane fuel cell

The very high power density output available from polymer electrolyte membrane fuel cells combined with low cost has high potential for commercialization. Such high power densities are attained via better utilization of Pt crystallites in the reaction layer. This enhanced performance can be achieved by making a thin catalyst layer on the membrane surface. The robustness in the front surface catalysts is essential to minimize the coagulation of Pt particles when the fuel cells are subjected to long-term operation. This robustness of the catalyst structure depends on the manufacturing processes and also the organic solvents used to make the slurry. In this work, five different electrodes were fabricated by using different fabrication procedures, and the poison effect of CO was investigated at the anode interface.     

Multivariable optimization studies of cathode catalyst layer of a polymer electrolyte membrane fuel cell

The amount of current generated in a polymer electrolyte membrane fuel cell (PEMFC) depends strongly on the local conditions in a cathode such as available oxygen, surface area available for the reactions, amount of ionomer, and amount of electro-catalyst. In the present work, design parameters of a cathode catalyst layer are optimized to achieve the maximum current density at a given operating voltage. The decision variables are chosen such that they can be realized experimentally. To understand the effect of the model fidelity on the decision variables, optimization is performed with a single phase model and a two-phase model with and without membrane. Other objective functions such as maximization of current generation per catalyst loading, minimization of catalyst layer cost per power and minimization of cell cost per power are also considered to study the effects of the objective functions on the decision variables.     

Numerical methodology for proton exchange membrane fuel cell simulation using computational fluid dynamics technique

To analyze the physical phenomena occurring in the Proton Exchange Membrane Fuel Cell (PEMFC) using Computational Fluid Dynamics (CFD) technique under an isothermal operating condition, four major governing equations such as continuity equation, momentum conservation equation, species transport equation and charge conservation equation should be solved. Among these governing equations, using the interfacial boundary condition is necessary for solving the water transport equation properly since the concept of water concentration in membrane/electrode assembly (MEA) and other regions is totally different. It was first attempted to solve the water transport equation directly in the MEA region by using interfacial boundary condition; and physically-meaningful data such as water content, proton conductivity, etc. were successfully obtained. A detailed problem-solving methodology for PEMFC is presented and result comparison with experimental data is also implemented in this paper.     

Determination of mass diffusion overpotential distribution with flow pulse method from current distribution measurements in a PEMFC

The mass diffusion overpotential distribution in a free-breathing proton exchange membrane fuel cell (PEMFC) was determined from current distribution measurements using a flow pulse approach. The current distribution measurements were conducted with a segmented flow-field plate. Flow pulses were fed to the cathode channels to form a uniform oxygen concentration distribution along the channels. Simultaneously, the cell resistance was monitored using the current interruption method. From the experimental data, the mass diffusion overpotential distribution was calculated using the Tafel equation. The results show that the mass diffusion overpotential in different parts of the cell may vary considerably, for example, at 180 mA cm^–2 the mass diffusion overpotential difference between the bottom and top part of the cell was 0.1 V.     

Mass transport in the cathode of a free-breathing polymer electrolyte membrane fuel cell

In small fuel cell applications, it is desirable to take care of the management of reactants, water and heat by passive means in order to minimize parasitic losses. A polymer electrolyte membrane fuel cell, in which air flow on the cathode was driven by free convection, was studied by experimental and modelling methods. The cathode side of the cell had straight vertical channels with their ends open to the ambient air. A two-dimensional, isothermal and steady state model was developed for the cathode side to identify the limiting processes of mass transport. The modelled domain consists of the cathode gas channel and the gas diffusion layer. Experimental data from current distribution measurements were used to provide boundary conditions for oxygen consumption and water production. The model results indicate that at the cell temperature of 40 °C the performance of the cell was limited by water removal. At the cell temperature of 60 °C, the current distribution was determined by the partial pressure of oxygen.     

The internal resistance of a microbial fuel cell and its dependence on cell design and operating conditions

The internal resistance R_int of a mediator-less microbial fuel cell (MFC) has been determined as a function of cell voltage using electrochemical impedance spectroscopy (EIS) for a MFC with and without Shewanella oneidensis MR-1. The same tests were performed for a MFC containing small stainless steel (SS) balls in the anode compartment with a graphite feeder electrode as in a packed bed cell. It has been found that R_int decreased with decreasing cell voltage as the increasing current flow decreases the polarization resistance of the anode and the cathode. The ohmic components of R_int played a very minor role. In the presence of MR-1 R_int was lower by a factor of about 100 than R_int of the MFC with buffer and lactate as anolyte. R_int was also significantly lower for the anode containing SS balls with buffer and lactate as anolyte. For the MFC containing SS balls in the anode compartment no significant further decrease of R_int could be obtained when MR-1 was added to the anolyte since in this case the polarization resistance of the anode was lower than that of the cathode. Similar trends were observed in the cell voltage (V)-current (I) curves that were obtained using potentiodynamic sweeps and the power (P)-V curves that were calculated from the V-I curves.     

Mesoporous carbons as low temperature fuel cell platinum catalyst supports

Platinum catalysts supported on ordered mesoporous carbons (OMC) are described. The mesoporous carbon support, CMK3 type, was synthesised as an inverse replica of a SBA-15 silica template. The platinum catalysts (i.e. Pt 20 wt% and Pt 10 wt%, respectively), obtained through a conventional wet impregnation method, have been investigated to determine their structural characteristics and electrochemical behaviour. The electro-catalytic performance towards the oxygen reduction reaction (ORR) was compared to those of commercial Pt/C-Vulcan XTC72R (E-Tek) catalysts with the same Pt wt%, under the same experimental conditions. The two catalyst samples have allowed the effect of the variation of both the Pt to Nafion and Pt to the supporting carbon ratios to be studied. Electrochemical tests have been carried out in three different systems: a catalyst ink deposited on a glassy carbon rotating disk electrode (RDE), a gas diffusion electrode (GDE) in a three-electrode cell with H₂SO₄ as the electrolyte and a complete PEM single fuel cell. The first results indicate that the OMC performs slightly less well than commercial carbon supports, mainly in the complete fuel cell system. The data from the cell tests indicate a less effective distribution of Nafion on the OMC surface which, probably, decreases the platinum utilisation and the proton conductivity.     

Modelling and simulations of carbon corrosion during operation of a Polymer Electrolyte Membrane fuel cell

A two-dimensional model is developed to simulate the carbon corrosion reaction during operation of a Polymer Electrolyte Membrane fuel cell. Specifically, carbon corrosion caused by local fuel starvation and during a startup/shutdown (SUSD) procedure is investigated. The present model considers coupled transport of charged and non-charged species, and multiple electrochemical and chemical reactions. In the simulations, the same set of governing equations is solved for the local fuel starvation case and the SUSD case with appropriate boundary conditions and local properties applied to each case respectively. In the local fuel starvation case, a portion of the gas diffusion layer (GDL) is artificially set to have extremely low gas diffusivity in order to mimic the condition when locally the GDL is flooded by liquid water. For the SUSD case, a portion of the anode channel is filled with air, which simulates the purging/refilling in an SUSD procedure. Several mitigation techniques to reduce carbon corrosion are evaluated and it is found that for the local fuel starvation case, using OER (Oxygen Evolution Reaction)-favorable catalysts and using membranes with low O₂ diffusivity are two effective techniques for carbon corrosion mitigation. For the SUSD case, using OER-favorable catalysts appears to be the only effective mitigation technique.