Design and development of modular fuel cell stacks for various applications

Polymer electrolyte membrane fuel cell (PEMFC) stacks are conventionally made by assembling a large numbers of cells together connected electrically in series. The number of cells and the area of the electrodes determine the capacity of such a stack. One of the reasons for non-proliferation of the fuel cell systems in many applications is due to the high cost involved in making these units. Cost reduction of the fuel cell components, fuel cell stacks and fuel cell system are being extensively pursued. One of the options for cost reduction is thro’ fabricating the fuel cells in smaller scale industrial units as their overheads are low. To attract small scale entrepreneurs (SSE) the capital investment has to be minimum. If compact fuel cell stacks of size ranging from 200 to 300 W can be produced comfortably, the SSE's can be encouraged to set up manufacturing units and they can become original equipment manufacturer (OEM) supplier to system integrators who could develop fuel cell systems of various capacities by simply connecting these compact stacks electrically in series or parallel depending on the end use. Such modular architecture would reduce the cost and improve the reliability of manufacture and increase the range of applications. This modular construction not only allows the materials and manufacturing technologies for components and stacks for uniformity but also suitable for homogenous large volume production. The geometry of the stacks allows easy installation even in crowded or compact areas. Centre for Fuel Cell Technology (CFCT) has demonstrated PEMFC stacks of 1–3 kW capacities. These stacks are single units with fixed voltage and current capabilities. CFCT is now embarked on a program to explore the possibility of introducing modular architecture for various applications of fuel cell systems. In this context, CFCT has recently developed a 250 W module which can be cooled either by air or water. The present paper discusses the concept and design of a modular architecture.     

Development of a galvanostatic analysis technique as an in-situ diagnostic tool for PEMFC single cells and stacks

A new galvanostatic analysis technique was developed for PEMFC single cells and stacks, while conventional potentiodynamic techniques, such as cyclic voltammetry for an electrochemical active surface area (EAS) and linear sweep voltammetry for a crossover current (iH₂)$\left(i_{{\text{H}}_2}\right)$, cannot be directly utilized for stacks. Using a developed relationship for double-layer charging region, the iH₂$i_{{\text{H}}_2}$ and C_dl (double-layer capacitance) of a PEMFC single cell could be determined from the galvanostatic data under an atmosphere of nitrogen (cathodes) and hydrogen (anodes). Then, simply from the elapsed time in hydrogen adsorption/desorption region, EAS or roughness factors could be analyzed for a PEMFC single cell. For a 5-cell PEMFC stack, it was experimentally confirmed that the same analysis technique can be applied to analyze performance distribution in PEMFC stacks. As the characteristics of catalyst layers (EAS and C_dl) and polymer electrolyte membranes (iH₂)$\left(i_{{\text{H}}_2}\right)$ of individual cells can be analyzed without stack disassembly, the developed galvanostatic technique is expected to be utilized for the degradation study and performance monitoring of practical PEMFC stacks.     

High performance PEMFC stack with open-cathode at ambient pressure and temperature conditions

An open-air cathode proton exchange membrane fuel cell (PEMFC) was developed. This paper presents a study of the effect of several critical operating conditions on the performance of an 8-cell stack. The studied operating conditions such as cell temperature, air flow rate and hydrogen pressure and flow rate were varied in order to identify situations that could arise when the PEMFC stack is used in low-power portable PEMFC applications. The stack uses an air fan in the edge of the cathode manifolds, combining high stoichiometric oxidant supply and stack cooling purposes. In comparison with natural convection air-breathing stacks, the air dual-function approach brings higher stack performances, at the expense of having a lower use of the total stack power output. Although improving the electrochemical reactions kinetics and decreasing the polarization effects, the increase of the stack temperature lead to membrane excessive dehydration (loss of sorbed water), increasing the ohmic resistance of the stack (lower performance).     

Sublayers for Pt catalyst electrodeposition electrodes in PEMFC

The effect of sublayers on the deposited platinum (Pt) catalyst layer fabricated by electrodeposition, and on the resulting fuel cell performance, was investigated. The substrate was prepared by applying a hydrophobic sublayer, composed of polytetrafluoroethylene (PTFE) and carbon black, and a hydrophilic sublayer, composed of Nafion and glycerol, onto an uncatalyzed gas diffusion layer prior to the electrodeposition of the Pt catalyst. The hydrophilic sublayer was found to play a substantial role in the Pt electrodeposition, since the structure of the resultant Pt catalyst significantly depended on the presence of the hydrophilic sublayer, the total loading amount and the Nafion to glycerol weight ratio, which in turn affected the fuel cell performance. The hydrophobic sublayer, which did not directly contact with the plating solution, did not show as marked an effect on the Pt deposit structure compared to that of the hydrophilic sublayer, except at high PTFE to carbon black weight ratios (≥70:30). However, the suitable PTFE to carbon black weight ratio in the hydrophobic sublayer was still important for the water management, mass transport of reactant gases and ohmic resistance of the membrane electrode assembly (MEA) during fuel cell operation. In this study, a total hydrophilic loading of 0.8 mg cm⁻² with a Nafion to glycerol weight ratio of 50:50, and a PTFE to carbon black weight ratio in the hydrophobic sublayer of 30:70 was found to yield the best Pt catalyst layer for PEMFC.     

A preliminary study of a miniature planar 6-cell PEMFC stack combined with a small hydrogen storage canister

The fabrication and performance evaluation of a miniature 6-cell PEMFC stack based on Micro-Electronic-Mechanical-System (MEMS) technology is presented in this paper. The stack with a planar configuration consists of 6-cells in serial interconnection by spot welding one cell anode with another cell cathode. Each cell was made by sandwiching a membrane-electrode-assembly (MEA) between two flow field plates fabricated by a classical MEMS wet etching method using silicon wafer as the original material. The plates were made electrically conductive by sputtering a Ti/Pt/Au composite metal layer on their surfaces. The 6-cells lie in the same plane with a fuel buffer/distributor as their support, which was fabricated by the MEMS silicon–glass bonding technology. A small hydrogen storage canister was used as fuel source. Operating on dry H₂ at a 40 ml min⁻¹ flow rate and air-breathing conditions at room temperature and atmospheric pressure, the linear polarization experiment gave a measured peak power of 0.9 W at 250 mA cm⁻² for the stack and average power density of 104 mW cm⁻² for each cell. The results suggested that the stack has reasonable performance benefiting from an even fuel supply. But its performance tended to deteriorate with power increase, which became obvious at 600 mW. This suggests that the stack may need some power assistance, from say supercapacitors to maintain its stability when operated at higher power.     

蛇形流场结构质子交换膜燃料电池的性能研究

建立包括催化层、扩散层、质子膜在内的三维质子交换膜燃料电池模型,通过Fluent软件模拟4种不同结构的蛇形流场,通过对速度、膜中水含量以及功率密度等分析得出蛇形流场的最优结构,并对最优结构进行参数优化。研究表明,4种不同蛇形流场结构中,Multi-serpentine II为最优,随着温度、压强的增加,这种流场结构的燃料电池呈现出良好的性能,从而为质子交换膜燃料电池双极板的设计提供依据。     

Optimal topology and distribution of catalyst in PEMFC

The equations that govern the various transport phenomena occurring in a polymer electrolyte membrane fuel cell (PEMFC) were formulated and implemented in a commercial finite element software, in order to predict the fuel cell current density with respect to the operating conditions. The numerical model showed polarization curves in accordance with literature. The catalyst utilization was then improved by optimizing the platinum distribution (design variable) in the fuel cell, so as to maximize current density (objective function) for a fixed total amount of platinum (constraint). The first analysis showed that, for equal anode and cathode catalyst layer thicknesses, maximal current density was achieved by placing more catalyst in the cathode than in the anode. The second analysis showed that, for equal anode and cathode catalyst layer density, maximal current density was achieved by using a catalyst layer that is thicker on the cathode side than that on the anode side. Finally, a topological optimization of the platinum density within the cathode catalyst layer was performed with a gradient based algorithm, and the results showed that at a high stoichiometric ratio, the best design has most of its platinum placed where the reaction rate is the highest, i.e., close to the membrane layer.     

Characterisation and modelling of a 5 kW PEMFC for transportation applications

In the framework of the French inter lab SPACT project (fuel cell systems for transportation applications), a 10 kW PEM fuel cell testing bench has been installed in 2002 in the national fuel cell test platform located in Belfort, France. The behaviour of a 5 kW fuel cell, fed with humidified pure hydrogen gas and compressed air, has been investigated by the Laboratory of Electrical Engineering and Systems (L2ES) in association with the French National Institute for Transport and Safety Research (INRETS).     

Experimental analysis of cathode flow stoichiometry on the electrical performance of a PEMFC stack

The paper describes an experimental analysis on the effect of cathode flow stoichiometry on the electrical performance of a PEMFC stack. The electrical power output of a PEMFC stack is influenced by several independent variables (factors). In order to analyse their reciprocal influence, an experimental design methodology was adopted in a previous experimental session, to determine which factors deserve particular attention. In this work, a further experimental analysis has been carried out on a very significant factor: cathode stoichiometry. Its effects on the electrical power of the PEMFC stack have been investigated. The tests were performed on a 3.5 kW_el ZSW stack using the GreenLight GEN VI FC Test Station. The stack characteristics have been obtained running a predefined loading pattern. Some parameters were kept constant during the tests: anode and cathode inlet temperature, anode and cathode inlet relative humidity, anode stoichiometry and inlet temperature of the cooling water. The experimental analysis has shown that an increase in air stoichiometry causes a significant positive effect (increment) on electric power, especially at high-current density, and up to the value of 2 stoichs. These results have been connected to the cathode water flooding, and a discussion was performed concerning the influence of air stoichiometry on electrode flooding at different levels of current density operation.     

CFD investigating the effects of different operating conditions on the performance and the characteristics of a high-temperature PEMFC

The effects of different operating conditions on the performance and the characteristics of a high-temperature proton exchange membrane fuel cell (PEMFC) are investigated using a three-dimensional (3-D) computational fluid dynamics (CFD) fuel-cell model. This model consists of the thermal-hydraulic equations and the electrochemical equations. Different operating conditions studied in this paper include the inlet gas temperature, system pressure, and inlet gas flow rate, respectively. Corresponding experiments are also carried out to assess the accuracy of this CFD model. Under the different operating conditions, the PEMFC performance curves predicted by the model correspond well with the experimentally measured ones. The performance of PEMFC is improved as the increase in the inlet temperature, system pressure or flow rate, which is precisely captured by the CFD fuel cell model. In addition, the concentration polarization caused by the insufficient supply of fuel gas can be also simulated as the high-temperature PEMFC is operated at the higher current density. Based on the calculation results, the localized thermal-hydraulic characteristics within a PEMFC can be reasonably captured. These characteristics include the fuel gas distribution, temperature variation, liquid water saturation distribution, and membrane conductivity, etc.     

Performance and transient behavior of the kW-grade PEMFC stack with the PtRu catalyst under CO-contained diluted hydrogen

In the study, a self-made kW-class 40-cell proton exchange membrane fuel cell (PEMFC) stack, with an active area of 112.85 cm² for each membrane electrode assembly and with the anodic PtRu catalyst, was tested under different simulated reformate gases of different CO concentrations and different hydrogen concentrations. The performances and the transient voltages of the stack and the individual cells under different CO/N₂/H₂ mixtures were studied. The results show that increasing the CO concentration or decreasing the H₂ concentration of the CO-contained reformate gas negatively affects the performance of the PEMFC stack. Moreover, the PEMFC stack with the PtRu anodic catalyst can tolerate a CO concentration of up to 50 ppm under non-diluted H₂. However, it can only tolerate 10 ppm CO under diluted H₂. The CO tolerance decreases dramatically with an increase in the H₂ dilution level. In addition, increasing the CO concentration in diluted H₂ or decreasing the H₂ concentration in CO-contained H₂ accelerates the occurrence of potential oscillation. The potential oscillation is owing to the interactions of CO electro-oxidation and adsorption reactions on the catalyst. This work is also the first to report that the potential oscillation phenomenon initially occurs at the upstream cells of the stack.     

C16MI.OTf ionic liquid on Pt/C and PtMo/C anodes improves the PEMFC performance

The effect of 1-hexadecyl-3-methylimidazolium trifluoromethanesulfonate (C₁₆MI.OTf) ionic liquid (IL) on the catalytic activity of Pt/C or PtMo/C anodes is studied in a proton exchange membrane fuel cell (PEMFC). PtMo nanoparticles (NPs) are synthesized with two different Pt:Mo proportions (13 or 31 at.% Mo) by a borohydride method on the carbon support. The composition, crystalline structure, morphology of the PtMo/C are evaluated by energy-dispersive X-ray spectroscopy, X-ray diffraction and transmission electron microscopy, respectively. The stability tests of the electrocatalysts are carried out in acid medium using cyclic voltammetry measurements. Pt/C or PtMo/C electrocatalysts containing C₁₆MI.OTf are assessed in the anode in a H₂/air PEMFC by polarization curve and ac electrochemical impedance spectroscopy. The synthesized PtMo nanoparticles show spherical shape and average particle size of 3.5 nm. The PEMFC performance of PtMo (13 at.% Mo) at anode is very similar than of Pt/C anode. The presence of 15 wt% C₁₆MI.OTf in the Pt/C or PtMo/C (13 at.% Mo) anodes let to an increase of the maximum power values, 71 and 107 W g_Pt^?1 cm^?2, respectively. The catalytic surfaces of nanoparticles are modified due to C₁₆MI.OTf presence which improved the PEMFC performance. This result agrees with the EIS analysis, where the resistances of charge transfer and mass transfer decrease in the C₁₆MI.OTf presence. However, this effect is more pronounced for PtMo/C (13 at.% Mo) catalyst, demonstrate that PtMo/C anodes with a small amount of Mo and C₁₆MI.OTf ionic liquid improve significantly the PEMFC performance.     

PEFC stacks as power sources for hybrid propulsion systems

In this paper the performance of two polymeric electrolyte fuel cell systems (FCS) for hybrid power trains are presented and discussed. In particular, an experimental analysis was effected on 2.4 and 20 kW stacks with the aim to investigate the energy management issues of the two FCSs for utilization as power sources in electric power trains for scooter and minibus, respectively. The stack characterizations permitted the effect of the main operative variables (temperature, pressure and stoichiometric ratio) on mean power density of cells to be evaluated. The FCS efficiency was evaluated and compared for the two traction systems, individuating the optimal operative conditions for automotive application and specifying the energy losses of the auxiliary components. The efficiency of both fuel cell systems resulted higher than 40% in a wide range of loads (100–600 mA/cm²), with maximum values close to 50%. The experimental characterization of the two power trains was carried out on dynamic test benches, able to simulate the behaviour of the two vehicles on the European R40 driving cycle. The characterization of the two propulsion systems on R40 driving cycle evidenced that the overall efficiency was not affected significantly by the hybrid configuration adopted, as the efficiency values ranged from 27 to 29% in the different procedures analyzed.     

Study of the impact of reactants utilization on the performance of PEMFC commercial stacks by impedance spectroscopy

Reactants utilization is a key stake for a PEMFC system: a too low utilization leads to a waste of reactant but a too high utilization may result in a detrimental starvation. To study these impacts, two commercial stacks were characterised by impedance spectroscopy under different hydrogen and oxygen utilizations (from nominal conditions to quasi-starvation). One was fresh while the other was operated on-field during 10,000 h. This study shows that the two capacitive loops in the lowest frequency range (1 Hz and below) correspond respectively to oxygen and hydrogen mass transfer limitations: the limiting reactant can be clearly identified from the impacted frequencies. The size of these loops was increased by up to 30% when the cell operated at high reactant utilizations. These results could therefore pave the way to the development of algorithms able to estimate the degree of starvation of some cells.     

Mitigation studies of sulfur contaminated electrodes for PEMFC

The performance of Polymer Electrolyte Membrane Fuel Cell (PEMFC) is highly influenced by the contaminants present in the air, like sulfur compounds, nitrogen compounds, carbon oxides, chlorides etc. In particular, sulfur dioxide (SO₂) on the cathode side has a severe effect on the performance of PEMFC. In the present paper we have attempted to mitigate the effect of SO₂ poisoned catalyst layer of the fuel cell for both half cell and for single cell using a novel catalyst support. The mitigation strategies for the contaminated MEAs were studied by three different methods viz., electrochemical cycling, increased air stoichiometry and by successive polarization. The platinum on graphene as a catalyst support was found to have a better tolerance towards SO₂ contamination when compared with the conventional platinum supported on Vulcan XC carbon catalyst. Electrochemical and single cell contamination tests were done to study the extent of SO₂ poisoning on both the catalysts. The sulfur coverage was found to be less for Pt/G based catalyst, around 54% compared with Pt/C. The recovery by successive polarization was found to be better compared to air flow regulation. This may be attributed to the indirect removal of sulfur ions from Pt through scavenged OH⁻ ions.     

Multiphysics DC and AC models of a PEMFC for the detection of degraded cell parameters

The present paper proposes a new 2D modelling of ac impedance spectra of polymer electrolyte fuel cells (PEMFC). The computational domain includes the Membrane Electrode Assembly, the Gas Diffusion Layers and the channels on both the anode and cathode sides. The model takes into account the main fuel cell phenomena, i.e. reactants, charges transport and transfer and electrochemical reactions. First, the partial differential equations are solved in the steady state regime, then in the frequency domain in order to obtain the cell dynamic behaviour at different potentials. Experimental PEMFC impedance spectra are satisfactory reproduced over a relative large potentials range using only one set of model parameters. Numerical analysis of the key model parameters linked to the cell flooding state has been done. It is concluded that at least two impedance spectra at low and high potential are needed in order to discriminate the nature and the location of the cell degradations (anode or cathode, electrode or GDL). Based on a least square criterion, the model inversion is presented and several cell flooding scenarios have been precisely identified.     

Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation

The damage caused by cell reversal during proton exchange membrane fuel cell (PEMFC) operation with fuel starvation was investigated by a single cell experiment. The samples from degraded membrane–electrode assemblies (MEAs) were characterized. Chemical analysis of the anode catalyst layer of MEA samples by energy dispersive X-ray analysis (EDX) clearly showed ruthenium dissolution from the anode catalyst particles. Severe ruthenium loss was observed especially in the fuel outlet region. A reduced carbon monoxide (CO) tolerance was found by CO stripping voltammetry and measurement of deteriorated the fuel cell performance. Surface area loss of the cathode platinum by sintering was also detected by transmission electron microscopy (TEM) analysis and cyclic voltammetry.     

Sulfonated poly(ether sulfone) electrolytes structured with mesonaphthobifluorene graphene moiety for PEMFC

Sulfonated poly(ether sulfone)s containing a mixture of cis and trans mesonaphthobifluorene moiety were synthesized, and their properties were characterized. The mesonaphthobifluorene graphene moiety contained 6 phenyl rings and was conjugated together to form planar sheets of sp²-bonded carbon. Poly(arylene ether sulfone)s containing a mixture of cis and trans tetraphenyl ethylene units were synthesized by polycondensation, and converted into graphene by intramolecular Friedel–Craft cyclization with Lewis acid (FeCl₃). The sulfonation was taken selectively on mesonaphthobifluorene units with concentrated sulfuric acid. The structural properties of the sulfonated polymers were investigated by ¹H NMR spectroscopy. The membranes were studied with regard to ion exchange capacity (IEC), water uptake, and proton conductivity.     

Performance analysis of a 5 kW PEMFC with a natural gas reformer

Power systems based on fuel cells have been considered for residential and commercial applications in energy Distributed Generation (DG) markets. In this work we present an experimental analysis of a power generation system formed by a 5 kW proton exchange membrane fuel cell (PEMFC) unit and a natural gas reformer (fuel processor) for hydrogen production. The performance analysis developed simultaneously the energy and economic viewpoints and enabled the determination of the best technical and economic conditions of this energy generation power plant, and the best operating strategies, enabling the optimization of the overall performance of the stationary cogeneration fuel cell unit. It was determined the electrical performance of the cogeneration system in function of the design and operational power plant parameters. Additionally, it was verified the influence of the activation conditions of the fuel cell electrocatalytic system on the system performance. It also appeared that the use of hydrogen produced from the natural gas catalytic reforming provided the system operation in excellent electrothermal stability conditions resulting in increase of the energy conversion efficiency and of the economicity of the cogeneration power plant.     

Analysis of degradation in PEMFC caused by cell reversal during air starvation

The damage caused by cell reversal during proton exchange membrane fuel cells (PEMFCs) operation with air starvation was investigated by a single-cell experiment. Samples from degraded membrane–electrode assemblies (MEAs) were characterized. The loss of electrochemical surface area of the cathode platinum was detected by in situ cyclic voltammetry, and platinum sintering was detected by transmission electron microscopy (TEM) analysis. Degradation at the anode was not detected in the chemical analysis of the anode catalyst layer of MEA samples by energy dispersive X-ray analysis (EDX) and TEM. An obvious decrease in the performance of PEMFC was observed in a sample degraded by cell reversal for 120 min.