Dynamic behaviour of Li batteries in hydrogen fuel cell power trains

A Li ion polymer battery pack for road vehicles (48 V, 20 Ah) was tested by charging/discharging tests at different current values, in order to evaluate its performance in comparison with a conventional Pb acid battery pack. The comparative analysis was also performed integrating the two storage systems in a hydrogen fuel cell power train for moped applications. The propulsion system comprised a fuel cell generator based on a 2.5 kW polymeric electrolyte membrane (PEM) stack, fuelled with compressed hydrogen, an electric drive of 1.8 kW as nominal power, of the same typology of that installed on commercial electric scooters (brushless electric machine and controlled bidirectional inverter). The power train was characterized making use of a test bench able to simulate the vehicle behaviour and road characteristics on driving cycles with different acceleration/deceleration rates and lengths. The power flows between fuel cell system, electric energy storage system and electric drive during the different cycles were analyzed, evidencing the effect of high battery currents on the vehicle driving range. The use of Li batteries in the fuel cell power train, adopting a range extender configuration, determined a hydrogen consumption lower than the correspondent Pb battery/fuel cell hybrid vehicle, with a major flexibility in the power management.     

Diagnosis of hydrogen crossover and emission in proton exchange membrane fuel cells

When hydrogen leaks through holes in membrane-electrode assemblies (MEAs) in proton exchange membrane (PEM) fuel cells, it recombines directly with air. This recombination results in a reduction in oxygen concentration on the cathode side of the MEA. In this paper, the signatures of electrochemical impedance spectroscopy (EIS) are analyzed in different multi-cell stack configurations to show the relation between hydrogen leak rate and reduced oxygen concentrations. The reduction in concentration was made by mixing oxygen with nitrogen at different rates, and the increase in hydrogen leak rate was made by controlling the differential pressure (dP) between anode and cathode. To analyze the impedance signatures, we fit the data of oxygen concentration and dP with the parameters of a Randles circuit. The correlation between the parameters of the two data sets allows us to understand the change in impedance signatures with respect to reduction of oxygen in the cathode side. To have a better insight on the effect of insufficient oxygen at the cathode, a model that establishes a relationship between impedance and voltage was considered. Using this model along with the impedance signatures we were able to detect the reduction of oxygen concentrations at the cathode with the help of fuzzy rule-base. However, resolution of detection was reduced with the reduction of leak rate and/or increases in the stack cell count.     

Diagnosis of hydrogen crossover and emission in proton exchange membrane fuel cells

When hydrogen leaks through holes or cracks in membrane-electrode assemblies (MEAs) in Proton Exchange Membrane (PEM) fuel cells, it recombines directly with air. This recombination results in a reduction in oxygen concentration on the cathode side of the MEA. In this paper, the signatures of electrochemical impedance spectroscopy (EIS) are analyzed in different multi-cell stack configurations to show the relation between hydrogen leak rate and reduced oxygen concentrations. The reduction in concentration was made by mixing oxygen with nitrogen at different rates, and the increase in hydrogen leak rate was made by controlling the differential pressure (dP) between anode and cathode. To analyze the impedance signatures, we fit the data of oxygen concentration and dP with the parameters of a Randles circuit. The correlation between the parameters of the two data sets allows us to understand the change in impedance signatures with respect to reduction of oxygen in the cathode side. To have a better insight on the effect of insufficient oxygen at the cathode, a model that establishes a relationship between impedance and voltage was considered. Using this model along with the impedance signatures we were able to detect the reduction of oxygen concentrations at the cathode with the help of fuzzy rule-base. However, resolution of detection was reduced with the reduction of leak rate and/or increases in the stack cell count.     

A model for hydrogen sulfide poisoning in proton exchange membrane fuel cells

A polymer-electrolyte fuel cell model that incorporates the effects of hydrogen sulfide contaminant on performance is developed. The model is transient, fully two-phase and non-isothermal and includes a complex kinetic mechanism to describe the electrode reactions. Comparisons between the simulation results and data in the literature demonstrate that known trends are well captured. The effects of temperature and relative humidity variations in the anode stream are investigated, with further comparisons to experimental data and a proposed explanation for the nonlinear behaviour observed in the experiments of Mohtadi et al. [R. Mohatadi, W.-K. Lee, J. van Zee, Appl. Catal. B 56 (2005) 37–42)]. Extensions to the model and future work are discussed.     

Heat transport characteristics of flow fields in proton exchange membrane fuel cells

Since the electrochemical reaction at electrodes of PEMFC is a form of exothermic reaction, three-dimensional non-isothermal numerical simulation was developed including the energy equation with all heat source terms such as reversible heat generation and irreversible heat release attributed to ohmic and activation polarization. The results show that the maximum temperature is observed at the cathode because of reaction heat from water formation reaction and temperature at the electrolyte along channel increases due to accumulative heat addition and heat is mainly generated by irreversible loss at a low voltage and by water formation loss at a low current density.     

Constructal flow distributor as a bipolar plate for proton exchange membrane fuel cells

A plate-type constructal flow distributor is implemented as a gas distributor for a proton exchange membrane fuel cell. A 3D complete model is simulated using CFD techniques. The fuel cell model includes the gas flow channels, the gas diffusion layers and the membrane-electrode assembly (MEA). The governing equations for the mass and momentum transfer are solved including the pertinent source terms due to the electrochemical reactions in the different zones of the fuel cell. Three constructal flow configurations were studied; each pattern is a fractal expansion of the original design, therefore, the only difference between them is the number of branches in the geometry. It was found that the number of branches is the key parameter in the performance of a fuel cell when using the constructal distributors as flow channels. The performance of the fuel cell is reported in I-V curves, power curves, and overpotential curves in order to determine which irreversibility is the main cause of energy losses. In terms of flow analysis, it was found that the constructal flow distributor presents a low pressure drop for a wide range of Reynolds number conditions at the inlet, as well as an excellent uniformity of flow distribution. Regardless of the outstanding hydrodynamic performance of the constructal distributors and the large current density values obtained, the implementation of these designs as flow patterns for PEMFCs need further optimization; first, the manufacturing of the plates have to be addressed in an efficient way; and secondly, the application in stacks will require an elaborate design to accomplish this task.     

Thermal characteristics of an air-cooled open-cathode proton exchange membrane fuel cell stack via numerical investigation

In light of stricter emissions regulations and depleting fossil fuel reserves, fuel cell vehicles (FCVs) are one of the leading alternatives for powering future vehicles. An open-cathode, air-cooled proton exchange membrane fuel cell (PEMFC) stack provides a relatively simple electric generation system for a vehicle in terms of system complexity and number of components. The temperature within a PEMFC stack is critical to its level of performance and the electrochemical efficiency. Previously created computational models to study and predict the stack temperature have been limited in their scale and the inaccurate assumption that temperature is uniform throughout. The present work details the creation of a numerical model to study the temperature distribution of an 80-cell Ballard 1020ACS stack by simulating the cooling airflow across the stack. Using computational fluid dynamics, a steady-state airflow simulation was performed using experimental data to form boundary conditions where possible. Additionally, a parametric study was performed to investigate the effect of the distance between the stack and cooling fan on stack performance. Model validation was performed against published results. The temperature distribution across the stack was identical for the central 70% of the cells, with eccentric temperatures observed at the stack extremities, while the difference between coolant and bipolar plate temperatures was approximately 10°C at the cooling channel outlets. The results of the parametric study showed that the fan-stack distance has a negligible effect on stack performance. The assumptions regarding stack temperature uniformity and measurement were challenged. Lastly, the hypothesis regarding the negligible effect of fan-stack distance on stack performance was confirmed.     

Transient model of oxygen-starved proton exchange membrane fuel cell for predicting voltages and hydrogen emissions

Transfer (crossover) leaks initiated by the chemical deterioration of the PEM and the resulting performance degradation has been previously identified as one the primary life-limiting factors in fuel cells. The leaks result in reduced oxygen levels in affected cells, where a secondary factor intimately related to this is high hydrogen emissions in the cathode exhaust when some cells operate in fully-oxygen-starved conditions. This paper builds on previous work that developed a unified fuel cell model that predicts cell voltage behavior under driving (normal) and driven (oxygen-starved) conditions, where this latest analysis now explicitly includes hydrogen pumping and emissions release when operating under oxygen-depleted conditions. In addition to considering diffusion effects and electrochemical effects, the model tracks the evolution of hydrogen in the cell cathode when no oxygen remains to generate water. The voltage response of the model under normal (non-starved) conditions is first validated for steady-state and transient (current step-change) conditions against previously published experiments, and then the model is used to simulate the cell voltage and stack hydrogen emissions behavior measured from three different commercially available fuel cell stacks. In the first fuel cell stack, a 9-cell commercial short stack, only one cell was fully oxygen-starved. Excellent agreement is seen between the measured and simulated hydrogen release concentrations (where air injection was used downstream of the stack to ensure adequate oxygen levels for measurement with a catalytic hydrogen sensor and to condense water vapor in the exhaust), where the role of hydrogen pumping is seen to contribute significantly to the release behavior. The first fuel cell stack is then used transiently in comparison with testing performed where the hydrogen injection level in the cell is changed quickly, where the model gives good agreement with the measured emission response and cell voltage behavior. Further comparisons with test data from a second and third 10-cell commercial short stack models operated with stack inlet hydrogen injection show good agreement with measured emissions onset versus current, where the observed threshold of starvation and emissions occurs a few percent sooner in the third model than the simulation, but the overall behavior is well predicted.     

A review of platinum-based catalyst layer degradation in proton exchange membrane fuel cells

Catalyst layer degradation has become an important issue in the development of proton exchange membrane (PEM) fuel cells. This paper reviews the most recent research on degradation and durability issues in the catalyst layers including: (1) platinum catalysts, (2) carbon supports, and (3) Nafion ionomer and interfacial degradation. The review aims to provide a clear understanding of the link between microstructural/macrostructural changes of the catalyst layer and performance degradation of the PEM fuel cell fueled with hydrogen under normal operating or accelerated stress conditions. In each section, different degradation mechanisms and their corresponding representative mitigation strategies are presented. Also, general experimental methods are classified and various investigation techniques for evaluating catalyst degradation are discussed.     

Conducting carbon/polymer composites as a catalyst support for proton exchange membrane fuel cells

Carbon/poly(3,4‐ethylene dioxythiophene) (C/PEDOT) composites are synthesized by in situ chemical oxidative polymerization of EDOT monomer on carbon black in order to decrease carbon corrosion that occurred in carbon‐supported catalysts used in proton exchange membrane fuel cell. The effects of different dopants including polystyrene sulfonic acid, p‐toluenesulfonic acid and camphorsulfonic acid with the addition of ethylene glycol or dimethyl sulfoxide on the properties of the composites are investigated. The synthesized composites are characterized by X‐ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, surface area analysis and scanning electron microscope. Electrical conductivity is determined by using the four‐point probe technique. Electrochemical oxidation characteristics of the synthesized C/PEDOT composites are investigated by cyclic voltammetry by applying 1.2 V for 24 h. The composite prepared at 25 °C with p‐toluenesulfonic acid and ethylene glycol shows the best carbon corrosion resistance. Platinum‐supported catalyst by using this composite was prepared using microwave irradiation technique, and it was seen that the prepared catalyst did not significantly lose its hydrogen oxidation and oxygen reduction reaction activities after electrochemical oxidation. Copyright © 2013 John Wiley & Sons, Ltd.     

Performances prediction study for proton exchange membrane fuel cells

In this work, we propose to study the influence of the membrane physical properties on the performance of a single PEM cell through the polarization curve. A thermal approach describing the main heat transfer aspects was also discussed. For this study, we have developed and used a simulation tool called Performances Prediction Fuel Cell tool (2PFC tool) based on simplified charge, mass and even heat transfer equations. This tool aims to visualize the main evolutions in the heart of a single cell, and the results should help users to understand the variation of some operating conditions and component properties on the output cell voltage by offering a variety of sensitivity parameter studies. For this sensitivity analysis, three separated simulations are launched. The first simulation regards the effect of the resistive losses and charge transfer coefficient on the cell voltage. The second simulation concerns the influence of the water content of the membrane and the cell operating temperature on it proton conductivity. The last simulation takes in consideration the effect of water activity variation on the proton membrane conductivity, and the results proved the direct and strong relation of the charge transfer coefficient and of the water content of the membrane on the output cell voltage. In the thermal approach part, we have proposed to study the temperature distribution between two cathodes with the presence of an implemented cooling channel.     

A mini-type hydrogen generator from aluminum for proton exchange membrane fuel cells

A safe and simple hydrogen generator, which produced hydrogen by chemical reaction of aluminum and sodium hydroxide solution, was proposed for proton exchange membrane fuel cells. The effects of concentration, dropping rate and initial temperature of sodium hydroxide solution on hydrogen generation rate were investigated. The results showed that about 38 ml min⁻¹ of hydrogen generation rate was obtained with 25 wt.% concentration and 0.01 ml s⁻¹ dropping rate of sodium hydroxide solution. The cell fueled by hydrogen from the generator exhibited performance improvement at low current densities, which was mainly due to the humidified hydrogen reduced the protonic resistivity of the proton exchange membrane. The hydrogen generator could stably operate a single cell under 500 mA for nearly 5 h with about 77% hydrogen utilization ratio.     

Stresses and their impacts on proton exchange membrane fuel cells: A review

The Proton Exchange Membrane (PEM) fuel cell is a promising substitute for combustion engines owing to its low level of output pollutants and high efficiency. Great efforts have been done toward the commercialization of PEM fuel cell. A number of review papers that investigate the electrochemical aspects of the fuel cell have already been published. However, the literature on the mechanical aspects is relatively limited. The durability of a PEM fuel cell is one of the significant factors that influence the industrialization of this technology. The PEM fuel cell is subjected to several mechanical stresses due to the different assembly procedures, operational and environmental aggressive conditions. Avoiding the high stress points is necessary for long term PEM fuel cell durability. The behavior of these generated stresses and how they affect each other is not well understood, including the compressive clamping stress, hygrothermal stress, freeze-thaw stress, and the stress due to vibration conditions. This paper reviews the developed stresses within the PEM fuel cell under different conditions. In addition, the various failure and damage mechanisms in the MEA, GDL, gas flow channel and bipolar plate due to these stresses are reviewed. The aforementioned stresses are discussed separately in the literature. The review shows that the combination of these stresses could be a key reason for the performance degradation and structural damage. This review suggests an effective tool to explore the correlation between the addressed stresses and to find out how they contribute to mechanical damage of PEM fuel cell systems and recommendations that can be implemented for improving the cell durability.     

Prospects for introducing hydrogen fuel cell vehicles in Malaysia

Sustainable economic development through adoption of hydrogen pathway is expected to simultaneously solve the double challenge of environment and energy security. The momentum of current research in this area is focused on optimizing the potential benefits in transportation sector through introduction of hydrogen fuel cell vehicles (FCVs). However the rapid adoption of critical cleaner energy system is still problematic in developing countries especially in transportation. This article sheds light on the Malaysian case by highlighting related issues and obstacles, and discusses the future prospects of hydrogen FCV. We argue that innovation and technological advancement of hydrogen FCV is important and ready for introduction in Malaysia; however it is not sufficient in explaining the conditions that will allow for rapid transition in Malaysia. The main message of this article is that hydrogen FCV in Malaysia can be expected and is possible when there is a thorough understanding of energy security issues, where energy policy integration for economic development and environmental objectives are achieved at the same time. The understanding of specific problems from the standpoint of developing country is crucial in encouraging government policies to induce development of hydrogen FCV.     

All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications

A portable electric power plant is developed using a NaBH₄ (sodium borohydride)-based proton exchange membrane fuel cell stack. The power plant consists of a NaBH₄-based hydrogen generator, a fuel cell stack, a DC-DC converter, a micro-processed controller and a data monitoring device. The hydrogen generator can produce 5.9 L/min pure hydrogen gas using catalytic hydrolysis of 20 wt% NaBH₄ to feed a 500-W scale fuel cell stack. Thus, the Co/γ-Al₂O₃ and Co-P/Ni foam catalysts in the hydrogen generator play significant roles in promoting hydrogen production rates that are as fast as necessary by enhancing the slow response that is intrinsic to using only Co-P/Ni foam catalysts. Moreover, different hydrogen production rates can easily be achieved during the operation by controlling NaBH₄ solution rates using a fuel pump so that the hydrogen storage efficiency can be improved by supplying required hydrogen gas in accordance with load demands. The specific energy density of the electric power plant was measured 211 Wh/kg. Therefore, the power plant described here can be a power source for mobile applications, such as cars and UAVs, as well as a stationary power supplier when electric energy is required.     

Investigation of bio-inspired flow channel designs for bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. The objective of this paper is to present two new flow channel patterns: a leaf design and a lung design. These bio-inspired designs combine the advantages of the existing serpentine and interdigitated patterns with inspiration from patterns found in nature. Both numerical simulation and experimental testing have been conducted to investigate the effects of two new flow channel patterns on fuel cell performance. From the numerical simulation, it was found that there is a lower pressure drop from the inlet to outlet in the leaf or lung design than the existing serpentine or interdigitated flow patterns. The flow diffusion to the gas diffusion layer was found be to more uniform for the new flow channel patterns. A 25 cm² fuel cell was assembled and tested for four different flow channels: leaf, lung, serpentine and interdigitated. The polarization curve has been obtained under different operating conditions. It was found that the fuel cell with either leaf or lung design performs better than the convectional flow channel design under the same operating conditions. Both the leaf and lung design show improvements over previous designs by up to 30% in peak power density.     

Modelling of proton exchange membrane fuel cell performance based on semi-empirical equations

Using semi-empirical equations for modeling a proton exchange membrane fuel cell is proposed for providing a tool for the design and analysis of fuel cell total systems. The focus of this study is to derive an empirical model including process variations to estimate the performance of fuel cell without extensive calculations. The model take into account not only the current density but also the process variations, such as the gas pressure, temperature, humidity, and utilization to cover operating processes, which are important factors in determining the real performance of fuel cell. The modelling results are compared well with known experimental results. The comparison shows good agreements between the modeling results and the experimental data. The model can be used to investigate the influence of process variables for design optimization of fuel cells, stacks, and complete fuel cell power system.     

Potential oscillations in a proton exchange membrane fuel cell with a Pd-Pt/C anode

We report in this paper the occurrence of potential oscillations in a proton exchange membrane fuel cell (PEMFC) with a Pd-Pt/C anode, fed with H₂/100 ppm CO, and operated at 30 °C. We demonstrate that the use of Pd-Pt/C anode enables the emergence of dynamic instabilities in a PEMFC. Oscillations are characterized by the presence of very high oscillation amplitude, ca. 0.8 V, which is almost twice that observed in a PEMFC with a Pt-Ru/C anode under similar conditions. The effects of the H₂/CO flow rate and cell current density on the oscillatory dynamics were investigated and the mechanism rationalized in terms of the CO oxidation and adsorption processes. We also discuss the fundamental aspects concerning the operation of a PEMFC under oscillatory regime in terms of the benefit resulting from the higher average power output.     

Willingness to pay for hydrogen fuel cell electric vehicles in China: A choice experiment analysis

The hydrogen energy is considered to be main power source of transport sector in the future, and a huge amount of funds have been invested into developing hydrogen fuel cell electric vehicles (FCEVs). Since FCEVs are in initial development stage and there're few FCEVs on the road, before their expansion this paper intends to conduct an economic analysis for FCEVs by using the choice experiment method. In the choice experiment, 1072 participants were required to select among two FCEVs and one conventional fuel vehicle. Logit models were estimated and then the results were used to calculate the willingness to pay for FCEVs. Results showed that purchase price, driving range, refueling time, fuel cost, emissions reduction, refueling accessibility are significant influences, and the marginal values for every 200 km improvement in driving range, 5 min reduction in refueling time, RMB 0.5/kilometer reduction in fuel cost, 20% reduction in emissions, and 20% improvement of refueling accessibility were estimated to be RMB 49,091, 12,727, 3818, 47,818, and 12,909, respectively. A range of FCEV configurations were calculated, and compared to a gasoline-powered counterpart the extra value that customers were likely to pay for a FCEV ranged from RMB 20,810 to 95,310. These results have significant implications for promoting FCEVs and contribute to better sustainability in transport sector.