Optimization of the efficiency and degradation rate of an automotive fuel cell system

In this contribution an approach for the analysis of the operation parameters of a fuel cell system is presented, which can be used for a lifetime and efficiency optimization. For this purpose, a physically-based polarization curve model of an automotive fuel cell stack is derived, which enables a realistic simulation study. Furthermore, the influence of degradation based on semi-empirical correlations for the loss of catalytic activity is included. This stack model is combined with a simplified fuel cell system model and used for a subsequent simulation study with focus on the system efficiency on the one hand and the lifetime on the other hand. The results show that an adaption of the operation parameters of the system can partly counteract the deterioration of the efficiency due to degradation. Furthermore, the lifetime of the stack could be enhanced at the cost of lower efficiency.     

Fuel cell/battery passive hybrid power source for electric powertrains

The concept of passive hybrid, i.e. the direct electrical coupling between a fuel cell system and a battery without using a power converter, is presented as a feasible solution for powertrain applications. As there are no DC/DC converters, the passive hybrid is a cheap and simple solution and the power losses in the electronic hardware are eliminated. In such a powertrain topology where the two devices always have the same voltage, the active power sharing between the two energy sources can not be done in the conventional way. As an alternative, control of the fuel cell power by adjusting its operating pressure is elaborated. Only pure H₂/O₂ fuel cell systems are considered in this approach. Simulation and hardware in the loop (HIL) results for the powertrain show that this hybrid power source is able to satisfy the power demand of an electric vehicle while sustaining the battery state of charge.     

A quick evaluating method for automotive fuel cell lifetime

Fuel cell vehicle commercialization and mass production are challenged by the durability of fuel cells and could be promoted by accelerated lifetime evaluating methods. In this paper, an arithmetic equation of fuel cell lifetime is presented, which is relating with load changing cycles, start–stop cycles, idling time, high power load condition and the air pollution factor. Basing on the practical data gathered from a fuel cell bus and the test results of a fuel cell stack in laboratory, the calculated lifetime fits the bus real running lifetime very well. It is shown that the automotive fuel cell lifetime mightily depends on driving cycles, and the potential lifetime in different operating mode can be effectively predicted by using this method with about 300 h test time. The test results also show that the effect of start–stop cycling on fuel cell lifetime can be almost ignored if the stack open circuit voltage is dispelled quickly after fuel cell stops operating. It is worthwhile that from this quick lifetime-evaluating method we can find many possible directions to improve fuel cell durability.     

Comparative study of energy management systems for a hybrid fuel cell electric vehicle - A novel mutative fuzzy logic controller to prolong fuel cell lifetime

Hybrid fuel cell battery electric vehicles require complex energy management systems (EMS) in order to operate effectively. Poor EMS can result in a hybrid system that has low efficiency and a high rate of degradation of the fuel cell and battery pack. Many different types of EMS have been reported in the literature, such as equivalent consumption minimisation strategy and fuzzy logic controllers, which typically focus on a single objective optimisations, such as minimisation of H₂ usage. Different vehicle and system specifications make the comparison of EMSs difficult and can often lead to misleading claims about system performance. This paper aims to compare different EMSs, against a range of performance metrics such as charge sustaining ability and fuel cell degradation, using a common modelling framework developed in MATLAB/Simulink - the Electric Vehicle Simulation tool-Kit (EV-SimKit). A novel fuzzy logic controller is also presented which mutates the output membership function depending on fuel cell degradation to prolong fuel cell lifetime – the Mutative Fuzzy Logic Controller (MFLC). It was found that while certain EMSs may perform well at reducing H₂ consumption, this may have a significant impact on fuel cell degradation, dramatically reducing the fuel cell lifetime. How the behaviour of common EMS results in fuel cell degradation is also explored. Finally, by mutating the fuzzy logic membership functions, the MFLC was predicted to extend fuel cell lifetime by up to 32.8%.     

A parameter optimized model of a Proton Exchange Membrane fuel cell including temperature effects

An electrical equivalent circuit model of the proton exchange membrane (PEM) fuel cell system with parameters extracted through optimization is presented in this paper. The analytical formulation of the fuel cell behavior is based on a set of equations which enables to estimate his overall performance in terms of operation conditions without extensive calculations. The approach uses a set of parametrical equations and related parameters in order to characterize and predict the voltage–current characteristics of the fuel cell operation without examining in depth all physical/chemical phenomena, but including within the model different components and forms of energy actuating in the generation process. Although many models have been reported in the literature, the parameter extraction issue has been neglected. However, model parameters must be precisely identified in order to obtain accurate simulation results. The main contribution of this work is the application of Simulated Annealing (SA) as optimization method focused on the extraction of the PEM model parameters. Model validation is carried out comparing experimental and simulated results. The good agreement between the simulation and experimental results shows that the proposed model provides an accurate representation of the static and dynamic behavior for the PEM fuel cell. Therefore, the approach allows at getting the set of parameters within analytical formulation of any fuel cell. In consequence, fuel cell performance characteristics are well described as they are carried out through a methodology that simultaneously calibrates the model.     

Coking-free direct-methanol-flame fuel cell with traditional nickel–cermet anode

This paper presents a systematic study of a direct-flame solid oxide fuel cell (DF-SOFC) operating on methanol and ethanol flames by SEM, EIS, I-V polarization and mass spectrometer (MS) characterizations and numerical simulation. The experimental study demonstrated that, by adopting a conventional Ni + Sm_0.2Ce_0.8O_1.9 (SDC) anode, irreversible carbon deposition and a drop of cell performance was observed when running the cell on an ethanol flame, while no carbon was deposited by operating on a methanol flame. Fuel cell stability tests indicated significant degradation in performance after 3 h of operation on an ethanol flame, while no degradation was observed after 30 h of operation on a methanol flame. A simple qualitative explanation of the difference observed in the electrochemical performance for the fuel cell operating on a methanol flame and an ethanol flame is presented based on numerical simulation.     

Evaluation of the power generation efficiency and the amount of CO2 discharge from a microgrid using a fuel cell cascade system

The purpose of this study was to combine fuel cells with different operating temperatures into fuel cell cascade systems in order to analyze their power generation efficiency and environmental impact (CO₂ emissions). Nine fuel cell cascade systems were investigated by numerical analysis. We also proposed the use of these systems in microgrids. The power generation efficiency of a compound system containing a solid‐oxide fuel cell, a micro‐gas turbine, a reformer, and a proton‐exchange membrane fuel cell showed great improvement compared with simplex operation of each component of the system. Moreover, a fuel cell cascade system can use alcohol fuels with low CO₂ emission factors. The fuel cell cascade systems tested showed that CO₂ emission reductions are possible. Copyright © 2011 John Wiley & Sons, Ltd.     

Study on LiI and KI with low melting temperature for electrolyte replenishment in molten carbonate fuel cells

Molten carbonate fuel cells (MCFCs) are regarded as the closest fuel cell to commercialization due to their high capacity and energy efficiency. However, they are operated at a high temperature (620 °C or higher), where liquid electrolyte loss occurs during operation; hence, their lifetime is limited. For the long-term operation of MCFCs, it is essential to develop a novel method to replenish the electrolyte during operation. However, it is very difficult to directly inject the electrolyte, (Li_0.62K_0.38)₂CO₃, into each unit cell of the stack unless it is supplemented through liquid or gas phase at low temperature. It was verified whether LiI and KI, which have low melting points and high vapor pressures, could replenish the lost electrolyte in MCFCs. In this study, the LiI and KI injected into the unit cell in liquid phase showed a similar tendency to the Li/K carbonate electrolyte. This is because LiI and KI react with the CO₂/O₂ gases supplied to the cathode during MCFC operation to form Li/K carbonate electrolytes.     

A practical model for evaluating the performance of proton exchange membrane fuel cells

Several models have been proposed in the literature to predict the performance of proton exchange membrane fuel cells (PEMFC). These models have different levels of complexity and can be divided basically into two groups: (i) mechanistic (theoretical); and (ii) semi-empirical models. The mechanistic models are obtained from electrochemical, thermodynamic, and fluid dynamic equations, and describes, with a high level of details, the processes in the operation of the fuel cell. The main drawback of the mechanistic approach is that, in general, the models are very complex, requiring the knowledge of parameters that are difficult to be obtained. Semi-empirical models, on the other hand, are easier to be obtained and can also be used to accurately predict the fuel cell system performance for engineering applications. In this paper, a new semi-empirical model, that is simpler than others presented in the literature, is proposed. It is derived by using semi-empirical equations and the resulting empirical coefficients are calculated through linear least squares. The model can be used in the evaluation of performance of small-distributed electrical generation systems, and also for the design of fuel cell systems for vehicles and portable electronics.     

Modeling and optimization of a typical fuel cell–heat engine hybrid system and its parametric design criteria

A theoretical modeling approach is presented, which describes the behavior of a typical fuel cell–heat engine hybrid system in steady-state operating condition based on an existing solid oxide fuel cell model, to provide useful fundamental design characteristics as well as potential critical problems. The different sources of irreversible losses, such as the electrochemical reaction, electric resistances, finite-rate heat transfer between the fuel cell and the heat engine, and heat-leak from the fuel cell to the environment are specified and investigated. Energy and entropy analyses are used to indicate the multi-irreversible losses and to assess the work potentials of the hybrid system. Expressions for the power output and efficiency of the hybrid system are derived and the performance characteristics of the system are presented and discussed in detail. The effects of the design parameters and operating conditions on the system performance are studied numerically. It is found that there exist certain optimum criteria for some important parameters. The results obtained here may provide a theoretical basis for both the optimal design and operation of real fuel cell–heat engine hybrid systems. This new approach can be easily extended to other fuel cell hybrid systems to develop irreversible models suitable for the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Data driven models for a PEM fuel cell stack performance prediction

The operating principles of polymer electrolyte membrane (PEM) fuel cells system involve electrochemistry, thermodynamics and hydrodynamics theory for which it is not always easy to establish a mathematical model. In this paper two different methods to model a commercial PEM fuel cell stack are discussed and compared. The models presented are nonlinear, derived from a black-box approach based on a set of measurable exogenous inputs and are able to predict the output voltage and cathode temperature of a 5 kW module working at the CNR-ITAE. A PEM fuel cell stack fed with H₂ rich gas is employed to experimentally investigate the dynamic behaviour and to reveal the most influential factors. The performance obtained using a classical Neural Networks (NNs) model are compared with a number of stacking strategies. The results show that both strategies are capable of simulating the effects of different stoichiometric ratio in the output variables under different working conditions.     

Direct methanol fuel cell bubble transport simulations via thermal lattice Boltzmann and volume of fluid methods

Carbon dioxide bubble removal at the anode of a direct methanol fuel cell (DMFC) is an important technique especially for applications in the portable power sources. This paper presents numerical investigations of the two-phase flow, CO₂ bubbles in a liquid methanol solution, in the anode microchannels from the aspect of microfluidics using a thermal lattice Boltzmann model (TLBM). The main purpose is to derive an efficient and effective computational scheme to deal with this technical problem. It is then examined by a commercially available software using Navier-Stokes plus volume of fluid (VOF) method. The latter approach is normally employed by most researchers. A simplified microchannel simulation domain with the dimension of 1.5 μm in height (or width) and 16.0 μm in length has been setup for both cases to mimic the actual flow path of a CO₂ bubble inside an anodic diffusion layer in the DMFC. This paper compares both numerical schemes and results under the same operation conditions from the viewpoint of fuel cell engineering.     

Proton exchange membrane fuel cell performance investigation considering internal heterogeneity of current density – A novel method study

The remaining useful lifetime (RUL) of proton exchange membrane fuel cell (PEMFC) has been influenced by the heterogeneous distribution. In this presented paper, a novel method considering the internal heterogeneity is proposed and investigated to manage the proton exchange membrane fuel cell (PEMFC) operation thus prolonging the remaining useful lifetime. This method including the mathematic steps of quantification, normalization, and coordinate transformation, converts the conventional power-current density curve into a novel power-heterogeneity curve. The electro-thermal mapping device is applied to measure the physical-field heterogeneity of a single cell during the polarization curve tests under different temperature conditions. This method is validated by the results in the current region of 750–860 mA cm^?2 of the polarization curves under temperature conditions of 50 °C and 60 °C. The novel method shows the effectiveness to make the fuel cell operate at a lower heterogeneity extent meanwhile a similar performance.     

Degradation analysis of commercial interconnect materials for solid oxide fuel cells in stacks operated up to 18000 hours

Despite being a mature technology, solid oxide fuel cell (SOFC) devices are still limited by lifetime issues. In SOFC stacks, cell/interconnect interaction is the main responsible for voltage degradation at the oxygen electrode side. Corrosion and chromium evaporation might in fact increase ohmic and charge transfer losses. This study presents the evolution of the degradation phenomena inside four SOFC short-stacks tested respectively for 45, 2700, 4800 and 10000 hours. An additional stack which underwent 124 thermal cycles is also analyzed to assess the mechanical reliability of the interconnect/ceramic coupling. Metal interconnect was made of K41/AISI441 ferritic stainless steel coated with MnCo₂O₄ porous barrier layer. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) characterization is applied to examine the degradation process. Observations indicate that despite a harsh initial red-ox interaction between the cathode materials and the interconnect, after 5000 h of operation the kinetic of the degradation process in the electrical contact areas slows down dramatically. An empirical model based on the scale thickness at different interconnect location gives estimation for the oxide thermal growth for a stack lifetime period. From the mechanical properties point of view, no spallation was observed and local delamination was mainly due to the sample preparation process.     

A preliminary life cycle assessment of PEM fuel cell powered automobiles

This paper provides a preliminary life cycle assessment (LCA) of polymer electrolyte membrane (PEM) fuel cell powered automobile. Life cycle of PEM fuel cell automobile not only includes operation of the vehicle on the road but also include production and distribution of both the vehicle and the fuel (e.g. hydrogen) during the vehicle’s entire lifetime. Assessment is based on the published data available in the literature. The two characteristics of the life cycle, which were assessed, are energy consumption and greenhouse gases (GHGs) emissions. Greenhouse gases (GHGs) emissions considered in the present assessment are CO₂ and CH₄. In addition, conventional internal combustion engine (ICE) automobile is also assessed based on similar characteristics for comparison with PEM fuel cell automobile. It is found that the energy utilized to generate the hydrogen during fuel cycle for the PEM automobile is about 3.5 times higher than the energy utilized to generate the gasoline during its fuel cycle. However, the overall life cycle energy consumption of PEM fuel cell automobile is about 2.3 times less than that of ICE automobile. Similarly, the GHGs emissions of PEMFC automobile are about 8.5 times higher than ICE automobile during the fuel cycle, but the overall life cycle GHGs emissions are about 2.6 times lower than ICE automobile.     

An innovative membrane-electrode assembly for efficient and durable polymer electrolyte membrane fuel cell operations

An innovative membrane-electrode assembly, based on a polyoxometalate (POM)-modified low-Pt loading cathode and a sulphated titania (S-TiO₂)-doped Nafion membrane, is evaluated in a polymer electrolyte membrane fuel cell. The modification of fuel cell cathode with Cs₃HPMo₁₁VO₄₀ polyoxometalate is performed to enhance particles dispersion and increase active area, allowing low Pt loading while maintaining performance. The POM's high surface acidity favors kinetics of oxygen reduction reaction. The mesoporous features of POM allow the embedding of Pt inside the micro-mesopores, avoiding the Pt aggregation during fuel cell operation and delaying the aging process, with consequent increase of lifetime. On the other hands, commercial Nafion is modified with superacidic sulphated titanium oxide nanoparticles, allowing operation at low relative humidity and controlled polarization of the MEA. Further MEAs, formed by unmodified Nafion membrane and the POM-based cathode, as well as sulphated titanium-added Nafion and commercial Pt-based electrodes, are used as terms of comparison. The cell performances are studied by polarization curves, electrochemical impedance spectroscopy, Tafel plot analysis and high frequency resistance measurements. The dependence of cell performances on relative humidity is also studied. The catalytic and transport properties are improved using the coupled system, despite the reduced Pt loading, thanks to rich proton environment provided by cathode and membrane.     

Current density and polarization curves for radial flow field patterns applied to PEMFCs (Proton Exchange Membrane Fuel Cells)

A numerical solution of the current density and velocity fields of a 3-D PEM radial configuration fuel cell is presented. The energy, momentum and electrochemical equations are solved using a computational fluid dynamics (CFD) code based on a finite volume scheme. There are three cases of principal interest for this radial model: four channels, eight channels and twelve channels placed in a symmetrical path over the flow field plate. The figures for the current–voltage curves for the three models proposed are presented, and the main factors that affect the behavior of each of the curves are discussed. Velocity contours are presented for the three different models, showing how the fuel cell behavior is affected by the velocity variations in the radial configuration. All these results are presented for the case of high relative humidity. The favorable results obtained for this unconventional geometry seems to indicate that this geometry could replace the conventional commercial geometries currently in use.     

Demonstrating feasibility of a high temperature polymer electrolyte membrane fuel cell operation with natural gas reformate composition

Experimental data on the performance of a single cell PBI-based HT-PEMFC operated with a fuel composition similar to natural gas reformate and oxygen enriched cathode air are presented. A test studying the effect of CO₂, H₂O and CO in the fuel on fuel cell performance revealed that the presence of CO₂ mainly worsens mass transport, H₂O improves proton conduction and CO influences reaction kinetics as well as causing mass transport limitations. A small increase of the O₂ concentration in the oxidant provided a boost on performance. Electrical efficiency of the fuel cell was improved from 36.6% with H₂/air operation up to 38.2% with synthetic reformate gas/30% O₂ enriched air. Three 1000 h long-term tests at constant load conditions were performed. The first test showed a degradation rate of ?21.4 μV/h and was operated with H₂/30% O₂. The second test was performed with the same kind of MEA but different fuel composition (54% H₂, 15% CO₂ and 31% H₂O) and exhibited a reduction of the degradation rate to ?5.5 μV/h. The main reason for this lifetime improvement is H₂O because its transport from anode to cathode may sweep along PA that soaks catalyst active sites and limits HOR. Moreover, water in rich H₂ reformate streams also relieves formation of CO from CO₂ via RWGS. The third test was performed with a different kind of MEA (extra PTFE content in GDE) but the same fuel composition than the second one. A higher degradation rate of ?22.2 μV/h was observed but it was mainly caused by unprotected shut-downs during operation. Two preliminary long-term tests were also performed with a fuel composition similar to natural gas reformate (54% H₂, 14% CO₂, 1% CO and 31% H₂O). These latest tests revealed that the fuel cell should be operated at higher temperatures to diminish CO catalyst coverage, and that anode purge with dry gases avoids water condensation in gas pipes. In addition, CO poisoning on anode catalyst is time dependent and operation at high current densities enhances CO catalyst coverage.     

An overview of hydrogen as a vehicle fuel

As hydrogen fuel cell vehicles move from manifestation to commercialization, the users expect safe, convenient and customer-friendly fuelling. Hydrogen quality affects fuel cell stack performance and lifetime, as well as other factors such as valve operation. In this paper, previous researcher's development on hydrogen as a possible major fuel of the future has been studied thoroughly. Hydrogen is one of the energy carriers which can replace fossil fuel and can be used as fuel in an internal combustion engines and as a fuel cell in vehicles. To use hydrogen as a fuel of internal combustion engine, engine design should be considered for avoiding abnormal combustion. As a result it can improve engine efficiency, power output and reduce NO_x emissions. The emission of fuel cell is low as compared to conventional vehicles but as penalty, fuel cell vehicles need additional space and weight to install the battery and storage tank, thus increases it production cost. The production of hydrogen can be ‘carbon-free’ only if it is generated by employing genuinely carbon-free renewable energy sources. The acceptability of hydrogen technology depends on the knowledge and awareness of the hydrogen benefits towards environment and human life. Recent study shows that people still do not have the sufficient information of hydrogen.