High Power Density Fuel Cell Systems for Portable Electric Generators

Design and development of modular fuel cell stacks for various applications

Polymer electrolyte membrane fuel cell (PEMFC) systems for heavy-duty vehicles (HDV) are attracting attention for several reasons including tighter emission regulations and, compared with light duty vehicles (LDVs), possibility of a gradual build-up of hydrogen infrastructure during the market introduction phase, and higher allowable cost (60 $/kWe vs. 30 $/kWe) and Pt loadings.1 However, competing with the incumbent diesel technology requires longer stack lifetimes than for LDVs (30,000 h vs. 8,000 h) and higher efficiencies to compensate for the likely price differential between diesel and hydrogen.2 Heat rejection in fuel cells for HDVs can also be an issue. We conducted simulations to compare radiator heat loads in two propulsion systems for a Class-8 HDV: a 450-hp turbocharged diesel engine, and a hybrid 275-kWe FCS with a 35-kWh battery. The simulations showed that the radiator heat load at rated power is higher for the hybrid propulsion system even though it is 20% more efficient than the diesel engine. Higher heat load combined with lower operating temperature makes heat rejection in an HDV radiator more challenging for the hybrid powertrain. We conducted system analysis to determine the stack operating conditions (cell voltage and temperature) and the radiator fan power required to reject the waste heat in the HDV radiator at rated power. Figure 1 summarizes the important results of the analysis of an FCS with a compressor-expander module (CEM) for air management, 2.5-atm stack inlet pressure, no cathode humidifier, de-alloyed PtCo cathode catalyst supported on a high surface-area carbon, and 0.25 mg/cm2 total Pt loading.3, 4 Figures 1a and 1b show that the heat load and radiator fan power are strong functions of the cell voltage and stack coolant temperature. These indicate a heat transfer limit that sets the minimum coolant temperature which only depends on the cell voltage. For example, at 0.7 V cell voltage, the minimum coolant temperature is 88oC corresponding to 265 kW radiator heat load and 30 kWe fan power. The cell voltage needs to be higher than 0.75 V for stacks that operate below 80oC. Figures 1c and 1d present the net stack power (stack power minus the parasitics including radiator fan power) density and system efficiency (net power produced divided by the lower heating value of H2 consumed) as functions of cell voltage and coolant temperature. Increasing the cell voltage at fixed coolant temperature improves the system efficiency at the expense of lower power density. For a fixed cell voltage, there is an optimum temperature at which the net power density is highest. The net power density is smaller below the optimum temperature because of higher fan power, and above it because of the adverse effects of low relative humidity on Ohmic losses and ORR kinetics. As a reference point, the stack can reach 1200 mW/cm2 gross and 1015 mW/cm2 net stack power density for 0.7 V cell voltage at rated power and 88oC optimum coolant temperature. The projected power density and operating conditions are based on a model formulated using the experimental data for d-PtCo/C cathode catalysts with 0.05, 0.1 and 0.15 mg/cm2 Pt loading. Further work is required to confirm these projections by building and optimizing electrodes with 0.2 mg/cm2 Pt loading. References Marcinkoski, R. Vijayagopal, J Adams, B. James, J. Kopsz, and R. Ahluwalia, DOE Advance Truck Technologies https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf L. Borup, A. Kusoglu, K. C. Neyerlin, R. Mukundan1, R. K. Ahluwalia, D. A. Cullen, K. L. More, A. Z. Weber, and D. J. Myers, “Recent Developments in Catalyst-related PEM Fuel Cell Durability,” Current Opinion in Electrochemistry, 21, 192–200 (2020)Ahluwalia, X. Wang and J-K Peng. "Fuel Cell System Modeling and Analysis," 2019 DOE Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation Meeting, Arlington, VA, April 29 - May 1, 2019.K. Ahluwalia, X. Wang, and A.J. Steinbach, “Performance of Advanced Automotive Fuel Cell Systems with Heat Rejection Constraint,” Journal of Power Sources, 309 (2016), 178-191. Figure 1

This study presents a comparative evaluation of Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel cell (SOFC) stacks coupled with a micro gas turbine (MGT). For the analysis, it is assumed that the fuel supply to the stacks is constant in all the analyzed conditions. The components of the system have been sized using the first law of thermodynamics to meet the thermal conditions required to maintain fuel cell stack operation, while an exergetic analysis has been implemented in order to assess the components in terms of irreversibilities. Furthermore, an economic analysis to estimate the Levelized Cost of Electricity (LCOE) has been carried out to indicate the most feasible option between the two analyzed systems. The capacity of the fuel cell stacks is 500kWdc operating at 350 kPa and 600 °C/650 °C. The results indicate that MCFC stacks are more efficient than SOFC stacks, but are considerably more expensive. Nevertheless, the SOFC/MGT system has a global efficiency higher than that of the MCFC/MGT; also its Total Capital Investment (TCI) is 2.5–3.5 times lower, thus making the SOFC/MGT coupling more attractive. The only product considered coming out from the systems to determine the LCOE is electricity. The LCOE for the SOFC/MGT system is 0.339–0.402$/kWh and for the MCFC/MGT is 0.875–0.897$/kWh, which for both configurations is still higher than the current electricity prices. One aspect that increases the investment and consequently the LCOE is the high cost of the stacks and their replacements which is around 19 % and 10 % of the total Purchased Equipment Cost (PEC) for MCFC and SOFC stacks, respectively.

Proton exchange membrane fuel cell systems are increasingly recognized as a promising alternative to conventional energy technologies, including electric batteries and other renewable energy sources, in the global transition away from fossil fuel dependence. However, their practical deployment remains constrained by challenges such as water accumulation and contaminant buildup within the fuel cell stack, which obstruct gas diffusion pathways and degrade performance. The dead-end anode (DEA) configuration, while widely employed for its structural simplicity, introduces risks that include hydrogen depletion, localized fuel starvation, and carbon corrosion of the cathode catalyst, issues frequently exacerbated by insufficient purge control. In this study, we present an experimental investigation aimed at optimizing the operational parameters of proton exchange membrane fuel cells (PEMFCs) operating under DEA mode. Specifically, the effects of hydrogen supply pressure, purge duration, and purge interval are systematically evaluated across a range of load currents. The objectives are to mitigate water flooding, maintain voltage stability during purge intervals, facilitate rapid voltage recovery after purge, and enhance overall energy efficiency.

Our dependency on road transportation of people and goods is huge. Unfortunately, this transportation is mainly fed by fossil fuels, with as accompanying disadvantages undesired local and global emissions and politically less desired dependencies. Electric propulsion systems can help to cover the disadvantages of fossil fuel based transportation. Still, battery based, grid charged electric vehicles will not provide an overall solution. Charging takes time and puts a signicant demand on the local electric infrastructure. In addition, such vehicles have a modest range. Therefore, our future mobility is expected to be driven by a more diverse palette of energy carriers, including hydrogen. Hydrogen can be provided by renewable sources and fueling a vehicle with hydrogen takes little time. As the energy density of hydrogen is large compared to batteries, also fuel cell hybrid vehicles provide a signicant longer driving range compared to battery only electric vehicles. In addition, local production of hydrogen by electrolysis with electricity from the local grid, provides new possibilities for demand side management. A fuel cell hybrid propulsion system consists of a fuel cell system to convert hydrogen into electricity, a battery to cover variations in the demand for electric power, a DC/DC converter to match the battery and the fuel cell stack voltages, and an electric motor with inverter to propel the vehicle. Apart from the fuel cell stack, the fuel cell system itself comprises a hydrogen tank to store hydrogen under pressure, a control valve to reduce the hydrogen pressure, a recirculation pump to reuse the surplus of hydrogen leaving the fuel cell stack, a humidier to prevent the fuel cells from dehydration, an air compressor to supply oxygen to the stack and a cooling system to keep the stack at the right operating temperature. The fuel consumption of such a fuel cell hybrid propulsion system is determined by the operation of the vehicle, the management of powers from fuel cell stack and battery, and the sizes of the fuel cell stack and battery. The operation of the vehicle depends on its purpose, its dimensions, the trac conditions and how the driver drives the vehicle. Although how to drive the vehicle is up to the driver, the resulting demand for electric power can still be characterized. This study suggests to characterize the power for traction as a normal distribution. Both simulations and measurements support this assumption, but conclusive evidence is not part of this study. The energy management system denes the management of powers from fuel cell stack and battery when driving the vehicle. This study concludes a close to optimal operation of fuel cell stack and battery is achieved, when the point of operation of the fuel cell stack is chosen equal to the average power demand, and when all variations from this average power demand are distributed over fuel cell stack and battery according to the ratio in their internal losses. The real-time implementation of this energy management strategy determines the value of the average power demand on the measured changes in the amount of energy stored in the battery. The resulting feedback loop provides a robust solution against disturbances, model errors and slowly changing parameters due to issues related to aging. The proposed solution to the energy management problem is validated on a full-scale test facility and demonstrated on a fuel cell hybrid delivery van. The found analytic solution to the energy management problem enables the derivation of an expression for the third factor in fuel consumption; the sizes of fuel cell stack and battery. This derivation makes use of the previously mentioned characterization of the power demand as a normal distribution. The resulting relation expresses the fuel consumption in terms of: properties of the driving cycle such as average speed, parameters of the vehicle such as weight and air resistance, and the number of fuel cells and battery cells. The optimum sizes of fuel cell stack and battery are derived from this relation. The resulting expression shows that, to obtain a minimum fuel consumption, the optimum size of the fuel cell stack does not match the average power demand, nor the maximum power demand, two choices commonly observed for prototypes currently available. Under ordinary condition, its optimum size is located between these two extremes. The sensitivity of the fuel consumption on the fuel cell stack size around this optimum value is limited, which makes the choice for the fuel cell stack less critical. The corresponding optimum size of the battery is the smallest size for which fuel cell stack and battery still can provide the maximum in the power demand and absorb the regenerated power when decelerating the vehicle. The optimum sizes found for the fuel cell stack and the battery are veried through brute force computer simulations. Based on an existing fuel cell hybrid vehicle, for dierent driving cycles, the fuel consumption for all feasible combinations of fuel cell stack size and battery size are derived by simulation. The results match the results of the proposed method. This not only substantiates the proposed sizing method, but also supports a normal distributed power demand as suitable characterization when optimizing the sizes of fuel cell stack and battery in terms of a minimum fuel consumption. An important contribution of the study is the analytical approach of both the energy management problem and the sizing problem. This analytical approach provides a solution to the sizing problem. Moreover, it provides an understand ing how parameter values aect the fuel consumption. For example, a reduction of the specic weights of battery cells and fuel cells oers the best opportunity for additional fuel savings, followed by a reduction of the auxiliary power and a reduction of the internal resistance of battery cells and fuel cells. The observation the fuel consumption is not very sensitive against variations in the fuel cell stack size around its optimum value, can help manufacturers of fuel cell stacks and systems to reduce the number of products over a certain power range. Consequently, the turnover per product will increase and production costs per product will reduce. This will help fuel cell hybrid propulsion systems to faster contribute to the desired reduction of our dependency on fossil fuels and to a further reduction of unwanted (local) emissions.

A total cost of ownership model is described for low temperature proton exchange membrane stationary fuel cell systems for combined heat and power (CHP) applications from 1-250kW and backup power applications from 1-50kW. System designs and functional specifications for these two applications were developed across the range of system power levels. Bottom-up cost estimates were made for balance of plant costs, and detailed direct cost estimates for key fuel cell stack components were derived using design-for-manufacturing-and-assembly techniques. The development of high throughput, automated processes achieving high yield are projected to reduce the cost for fuel cell stacks to the $300/kW level at an annual production volume of 100 MW. Several promising combinations of building types and geographical location in the U.S. were identified for installation of fuel cell CHP systems based on the LBNL modelling tool DER CAM. Life-cycle modelling and externality assessment were done for hotels and hospitals. Reduced electricity demand charges, heating credits and carbon credits can reduce the effective cost of electricity ($/kWhe) by 26-44percent in locations such as Minneapolis, where high carbon intensity electricity from the grid is displaces by a fuel cell system operating on reformate fuel. This project extends the scope of existing cost studies to include externalities and ancillary financial benefits and thus provides a more comprehensive picture of fuel cell system benefits, consistent with a policy and incentive environment that increasingly values these ancillary benefits. The project provides a critical, new modelling capacity and should aid a broad range of policy makers in assessing the integrated costs and benefits of fuel cell systems versus other distributed generation technologies.

This study presents both a hybrid microgrid system design with renewable energy and their control methods, analysis result. This renewable energy resources (RES) consist of 33kW PVs, 100kW fuel cell stack and a 50kW wind turbine with permanent magnet synchronous generator (PMSG). PV plant includes the PV arrays and DC-DC boost converter. Fuel cell plant includes the fuel cell stacks and DC-DC boost converter. The wind energy plant contains the wind turbine, PMSG, uncontrolled rectifier and DC-DC boost converter. The boost converter connected to PV plant has been controlled by using incremental conductance maximum power point tracking algorithm (IC-MPPT A). Both the boost converters of the wind energy system and fuel cell system have been operated with PI controllers. The switching element of all boost converters is M OSFE T. The switching frequency for boost converters of the wind energy system and fuel cell system is 30 kHz and 50 kHz for boost converter connected PV array. The hybrid microgrid has been coupled on 1000V DC-bus bar. 400V/120 kV transformer and 120 kV, 50 Hz AC supply have been used to create the grid model. To convert from DC to AC, as the topology, full bridge inverter circuit has been used and IGBT has selected as the switching element. Phase locked loop (PLL) algorithm has been used as a control for the AC voltage generated at the inverter output to be the same phase, frequency and amplitude with the grid. The system has been operated under the various operating conditions such as wind speed and solar irradiation. And despite these variables, the desired results have been obtained from the system.

Space power applications historically include fuel cells due to the high energy storage density of hydrogen and oxygen compared to batteries. Fuel cells are continuously under development to incorporate latest technology and focus on specific details of fuel cells systems relevant to harsh space transportation environments. The National Aeronautics and Space Administration is developing proton exchange membrane fuel cells systems for space power applications because of the potential for longer life, reduction in cost, and increase in safety compared to current alkaline fuel cell technology. Space fuel cell applications utilize oxygen instead of air, which introduces better performance but greater hazards. Circulation of reactants is beneficial for these systems to aid in removal of product water from the fuel cell stack and to humidify reactant fluid streams. Current space fuel cell prototype systems use a simple but effective pump for reactant recirculation known as a gas ejector. A gas ejector uses a high-pressure primary gas supply to produce suction to a secondary fluid at a lower pressure. A gas and water separator is then necessary to remove the fuel cell product water from the unutilized recirculated oxygen. The National Aeronautics and Space Administration is analyzing and testing several different means to separate the oxygen gas and water in both microgravity and increased gravity conditions. This paper addresses specific components and design concerns for proton exchange membrane fuel cell systems for space power applications.

Review A Review of Fuel Cell System Technology: From Fuel Cell Stack to System Integration Weiqun Ren 1,*, Jun Shen 2, Xuebing Li 1, and Changqing Du 2 1 Dongfeng Commercial Vehicle Co. Ltd., 10 Dongfeng Avenue, Wuhan, China 2 Wuhan University of Technology, Wuhan, China * Correspondence: renweiqun@tsinghua.org.cn; Tel.: +86-139-1820-4209 Received: 14 October 2022 Accepted: 8 November 2022 Published: 18 December 2022 Abstract: The technology of hydrogen fuel cell vehicles (FCV) is the ultimate direction of clean energy vehicle development, and commercial vehicles are the most important application area for fuel cell commercialization. This paper summarizes the key components, technologies and development trends of the fuel cell stack, fuel cell system and vehicle integration at home and abroad, and points out that key materials (such as bipolar plates and membrane electrodes), key system components (such as air compressors and ejectors), high-power modular integration technologies, and fuel cell control technologies are the main factors influencing the commercialization of FCVs. Particularly, we put the main fouce on variable ejector or multi-stage ejector technology, integrated and optimized control technology, and multi-energy cooperative control technology due to their crucial roles in meeting the industrial development of FCVs in China. Furthermore, some guidance opinions are also provided for reference about the development and industrialization of FCVs.

Toshiba’s Stationary Fuel Cell SystemToshiba Energy Systems & Solutions Corporation is developing hydrogen solution, which is a key technology to achieve carbon neutral. Now we are manufacturing the pure hydrogen fuel cell systems for stationary application. This fuel cell systems have some models up to 100kW, and in order to provide the larger electricity than 100kW, the 100kW system can be operated connecting several units up to 1MW.Toshiba's fuel cell systems have been achieved stable, long-term operation, high durability, and high energy efficiency. These features are realized by internal water management cell stack conflagration using porous carbon bi-polar plates. The porous bi-polar plates can humidify reactant gases supplied as vapor from the surface of the porous bi-polar plates and removes produced water through the porous bi-polar plates by the pressure difference between the reactant gases and the coolant water. This technology realizes ideal conditions in terms of water activity entire active area, suppressing overpotentials due to flooding and temperature and water activity distribution. This technology enables stable continuous operation up to 1 week and design life of 80,000 hours confirmed by demonstration tests, which is one of the highest in the world.Recent trends in fuel cell development include the growing expectation and demand for higher capacity and long durability fuel cell systems for stationary applications such as carbon neutral complexes and data centers, etc. In addition, demand for high power density fuel cell systems for large commercial mobility vehicles, such as buses and trucks, is also increasing. In order to meet this demand, Toshiba is developing elemental technologies for higher performance fuel cells.Development of advanced high performance fuel cellsIn order to improve cell performance, it is necessary to reduce ohmic, activation, and diffusion polarizations. Among these three overpotentials, a reduction of diffusion polarization is particularly important to achieve operation up to the high current density. To reduce diffusion polarization, the flow field of bi-polar plates must be designed to improve the limiting current density by efficient gas supply to the catalyst layer and to reduce mass transport resistance in the gas diffusion layer and catalyst layer.Toshiba has focused on reducing this diffusion polarization and is working on the flow field design using numerical simulation. As an example, Toshiba has developed the advanced numerical simulation by introducing mass transfer models of water evaporation and water absorption on the surface of a porous bi-polar plates into a commercially available electrochemical reaction and thermal fluidics 3D simulator.This porous bi-polar plates model was then used to perform parametric study of the flow field design, and determine parameters in terms of gas diffusivity, electrical resistance, and temperature and humidity distribution. And then, the advanced cell that incorporating designed bi-polar plates, thinner components, and a new catalyst was fabricated and evaluated. Figure 1 shows comparison of cell performance between conventional and advanced cells. The limiting current density of advanced cell is 3.6 times larger than that of conventional cell due to simulation-based flow field design and optimization of operating pressure. The application of these development items and optimization of operating conditions reduced each polarization and achieved a current density twice that of conventional cell. In the future, we aim to achieve even higher performance by improving catalytic activity and reducing cell resistance through thinner components. Figure 1

Fuel cell systems appear as a good alternative for future power generation. This is mainly due to their high energy conversion efficiency and simple operation. Since the output of the fuel cell is low voltage DC, fuel cell systems supplying power to AC loads are usually constructed using a two stage approach cascading a DC-DC converter and an inverter. It can be shown that in such a system the current drawn from the fuel cell under normal single phase (or unbalanced three phase) operation contains a large second harmonic component. Due to the internal impedance of the fuel cell stack the presence of low frequency current ripple has as side effect additional heating, and more importantly, a reduction in its output power thus diminishing the power availability and power conversion efficiency of the system. To mitigate this problem an auxiliary power converter is proposed in this paper. The proposed converter operates as an active filter supplying the low frequency ripple current demanded by the power converter allowing the fuel cell stack to supply the DC component, which in turn optimizes system operation. In addition the proposed converter, if suited with an energy storage element such as a supercapacitor, can be controlled to supply transient load changes further improving stack characteristics.

The commercialization of fuel cell products for a variety of applications including forklifts, buses, cars and heavy duty trucks is well underway. Particularly in the heavy duty fuel cell market, recent studies suggest that in Europe, fuel cell electric buses (FCEB) total cost of ownership (TCO) will breakeven with battery electric buses (BEB) by 2023, and with internal combustion engine buses (ICEB) by 20241. For applications such as long-distance and heavy-duty transportation, the hydrogen route appears the decarbonisation option of choice, and can become cost competitive with internal combustion engines by 20302.Over the past 30 years, Ballard has developed a wide array of technology “building blocks” for Fuel Cell stacks components. The application requirements (duty cycle, speed, vehicle weight, lifetime and environmental exposure) will determine the key stack features and result in customization of components where economically viable. In contrast with fuel cell stacks developed for passenger cars, Fuel Cells for heavy duty vehicles are particularly challenged by high durability requirements3, with MEA attributes such as catalyst loading and peak efficiency point significantly affecting stack and fuel costs.Building on proven and demonstrated stack durability in the field of more than 35,000 hours in heavy duty vehicles4 Ballard’s MEA development activities draw from extensive field experience to support step changes in durability with each subsequent product iteration. The use of an accelerated testing and modeling approach developed and validated over numerous product iterations enables continuous cost decreases and gains in power density, operational robustness and reliability, with a calculated risk tolerance for new technology introduction.Those MEA durability prediction models along with a good understanding of failure mechanisms are a cornerstone to developing the next generation of Fuel Cells for heavy duty vehicles. Of particular importance is the use of the voltage degradation model to evaluate the time to reach the degradation target for heavy duty applications (USDOE Ultimate target of 10% degradation at 30,000 hours5). Simulation results as a function of the electrode composition show the relationship between cathode catalyst loading and lifetime, and assist in making electrode design and composition choices for a successfully durable product. A sharp increase in voltage loss is observed when local oxygen transport resistance becomes limiting once the cathode reaches low surface area. Maximizing beginning-of-life catalyst electrochemical surface area (ECSA) utilization in the electrode structure, as well as optimizing surface area retention, can have significant benefits for lifetime. A combination of design choices or processing parameters during catalyst layer fabrication can be effective ways to optimize the electrode6.The same voltage degradation model is used to demonstrate the relationship between lifetime and the peak operating current density of the product, which limits the stack design power density within the allocated voltage degradation constraints of heavy-duty vehicles.The simulation results demonstrate that the ideal Fuel Cell stack for heavy duty applications is the result of complex trade-offs between cost, stack design attributes (e.g. catalyst loading), active area sizing (peak efficiency point) and durability, with the ultimate goal to achieve an attractive TCO for wide acceptance of the technology. References Deloitte and Ballard. (2020). Fueling the Future of Mobility: Hydrogen and fuel cell solutions for transportation. Volume 1 [White paper]. Deloitte China. https://www2.deloitte.com/content/dam/Deloitte/cn/Documents/finance/deloitte-cn-fueling-the-future-of-mobility-en-200101.pdf McKinsey & Company and Hydrogen Council. (2020). Path to hydrogen competitiveness: A cost perspective. https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdfMacauley, N., Lauritzen, M., Knights, S. and Kjeang, E., J. Electrochem. Soc., 165(10), F780 (2018).https://www.ballard.com/about-ballard/newsroom/news-releases/2020/01/27/ballard-powers-fuel-cell-electric-vehicles-for-unprecedented-30-million-kilometers-enough-to-circle-the-globe-750-timesUSDOE Hydrogen Class 8 Long Haul Truck Targets https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf M. Dutta, A. Young, V. Colbow, J. Bellerive, S. Knights, Examining Catalyst Layer Design Strategies for Improved Fuel Cell Performance and Durability, ECS PRiME 2020

12 - Stack and bipolar plates design for direct methanol fuel cells

The NASA Glenn Research Center is developing advanced passive thermal management technology to reduce the mass and improve the reliability of space fuel cell systems for the NASA exploration program. The passive thermal management system relies on heat conduction within the cooling plate to move the heat from the central portion of the cell stack out to the edges of the fuel cell stack rather than using a pumped loop cooling system to convectively remove the heat. Using the passive approach eliminates the need for a coolant pump and other cooling loop components which reduces fuel cell system mass and improves overall system reliability. Previous analysis had identified that low density, ultra-high thermal conductivity materials would be needed for the cooling plates in order to achieve the desired reductions in mass and the highly uniform thermal heat sink for each cell within a fuel cell stack. A pyrolytic graphite material was identified and fabricated into a thin plate using different methods. Also a development project with Thermacore, Inc. resulted in a planar heat pipe. Thermal conductivity tests were done using these materials. The results indicated that lightweight passive fuel cell cooling is feasible.

Challenges and opportunities in modelling of proton exchange membrane fuel cells (PEMFC)