Life-cycle assessment of fuel cell stacks

Life-cycle assessment is a useful instrument to evaluate the ecological performance of innovative energy systems. This paper investigates the production process of polymer electrolyte fuel cell (PEFC) stacks, identifies the ecological contributions of various components and materials and compares the results with impacts due to utilization of the stacks in a vehicle (i.e. hydrogen or methanol production and direct emissions). The production of fuel cell stacks leads to environmental impacts which cannot be neglected compared to the utilization of the stacks in a vehicle (the actual driving process). These impacts are mainly caused by the platinum group metals for the catalyst and, to a lesser degree, the materials and energy for the flow field plates. The paper identifies several options how to further enhance the environmental advantages of fuel cells.     

Modelling compression pressure distribution in fuel cell stacks

A general purpose 3D finite element method model has been developed for the estimation of the compression pressure distribution in fuel cell stacks. The model can be used for the optimisation of any type of fuel cell structure at any temperature. The model was validated with pressure sensitive film measurements using PEFC stack components that had low rigidity and were highly deformable.     

Development of small polymer electrolyte fuel cell stacks

The results on the research and development of small polymer electrolyte fuel cell stacks, including the assembly of single cell. 6-cell and 21-cell modules, are described. The important characteristics of the systems are: (i) membrane and electrode assemblies were made with Nafion^® 115 and 117 membranes and particularly low catalyst loading electrodes presenting a geometric area of 20 cm² and a catalyst loading of 0.4 mg Pt/cm²: (ii) bipolar plates were fabricated using a nonporous graphite material in which a series/parallel flow field was machined out: (iii) external distribution of gases to the cells was done using parallel manifolding; (iv) cooling systems were tested employing water/air cooling plates distributed every three cells throughout the stack; (v) the reactant gases were externally humidified using temperature controlled humidification bottles. Testing of the stacks was conducted in a specially designed test station employing nonpressurized H₂/O₂ reactants and measuring the individual and the overall cell voltage vs. current under several conditions for the overall system operation.     

PEM fuel cell stacks operated under dry-reactant conditions

A double-path-type flow-field design that has two gas inlets and two gas outlets is presented for PEM fuel cells. The two paths are arranged in such a way that the inlet of one flow-field is adjacent to the outlet of the other flow-field and, within any section of the electrode active area, there are always adjacent channels with reactant flowing in opposite directions. Such a design enables the dry entering gas to become hydrated by acquiring some moisture from the exiting moist gas; and, within any section of the active area, the drier gas in one flow-field can share the moisture in the wetter gas flowing in the other flow-field. Such a design effectively uses the water produced by the stack to hydrate the membrane and the catalyst layers. The effectiveness of this design was demonstrated by running multiple-cell stacks where the stack could run stably at a current density up to 0.33 A/cm² using dry hydrogen and dry air.     

Upscaling of microfluidic fuel cell using planar single stacks

The commercialization of microfluidic fuel cells remains difficult because of their low‐power density. In this study, microfluidic fuel cells with a planar single‐stack structure are proposed to improve the power density. The proposed stacks connect multiple cells in series, parallel, series–parallel, and parallel–series configurations. The electrolyte flow patterns of the stacks were numerically analyzed, and cell performances were experimentally measured with a platinum electrode using formic acid as the fuel. With a minimum size, these planar cell single stacks provide better power density than a single cell. The cell stack connected in parallel and then in series, where the velocity and pressure distributions of the electrolytes were simulated as almost uniform and few inner electrical connections existed, produced the best scaling‐up efficiency of 1.93. Additionally, a common feed inlet configuration was developed to further reduce the size of the cell stack further. The results show that well‐balanced fluid flow between inlets is necessary to obtain high scalability.     

Experimental investigation of coupling phenomena in polymer electrolyte fuel cell stacks

Propagation of performance changes to adjacent cells in polymer electrolyte fuel cell stacks is studied by means of voltage monitoring and local current density measurements in peripheral cells of the stack. A technical fuel cell stack has been modified by implementing two independent reactant and coolant supplies in order to deliberately change the performance of one cell (anomalous cell) and study the coupling phenomena to adjacent cells (coupling cells), while keeping the working conditions of the later cell-group unaltered.Two anomalies are studied: (i) air starvation and (ii) thermal anomaly, in a single anomalous cell in the stack and their coupling to adjacent cells. The results have shown that anomalies inducing considerable changes in the local current density of the anomalous cell (such as air starvation) propagate to adjacent cells affecting their performance. The propagation of local current density changes takes place via the common bipolar plate due to its finite thickness and in-plane conductivity. Consequently, anomalies which do not strongly influence the local current density distribution (such as a thermal anomaly under the studied working conditions) do not propagate to adjacent cells.     

Reliability computing of polymer-electrolyte-membrane fuel cell stacks through Petri nets

In this paper a model is introduced which computes reliability data of PEMFC (polymer-electrolyte-membrane fuel cell) stacks, especially the average lifetime of a single stack or the reliability of stacks of a whole fuel cell vehicle fleet within a given timing. The stack and its behaviour over time is modelled by a Petri net. The behaviour is divided into degradation, spontaneous and reversible events. Through the worsening over time the characteristics voltage, internal and external leakages, which are assigned to the components MEA (membrane electrolyte assembly) and BIP (bipolar plate), are changed. Thresholds for every characteristic monitor the operating ability of the whole stack.     

Development of planar air breathing direct methanol fuel cell stacks

In this paper, design criteria and development techniques for planar air breathing direct methanol fuel cell stacks are described in detail. The fuel cell design in this study incorporates a window-frame structure that provides a large open area for more efficient mass transfer and is modular, making it possible to fabricate components separately. The membrane electrode assembly and gas diffusion layers are laminated together to reduce contact resistance, which eliminates the need for heavy hardware. The composite current collector is low cost, has high electrical conductivity and corrosion resistance. In the interest of quick and cost-efficient prototyping, the fabrication techniques were first developed on a single cell with an active area of 1.0 cm². Larger single cells with active areas of 4.5 and 9.0 cm² were fabricated using techniques based on those developed for the smaller single cell. Two four-cell stacks, one with a total active area of 18.0 cm² and the other with 36.0 cm², were fabricated by inter-connecting four identical cells in series. These four-cell stacks are suitable for portable passive power source applications. The performance analysis of single cells as well as stacks is presented. Peak power outputs of 519.0 and 870.0 mW were achieved in the stacks with active areas of 18.0 and 36.0 cm², respectively. The effects of methanol concentration and fuel cell self-heating on the fuel cell performance are emphasized.     

Computational analysis of the gas-flow distribution in solid oxide fuel cell stacks

A computer model is developed, capable of predicting the flow distribution along the height of a fuel-cell stack. The model distinguishes a number of hydraulic resistances that are linked in series and parallel, thus forming a network to simulate the gas flow and pressure distribution in a stack. The hydraulic resistances are found from analytical solutions or from tabulated data inferred from measurements. The electrochemistry of the stack is simulated by adding or subtracting gas in the active cell area. The model is tested on an assumed separator-plate geometry and stacking height.     

Modeling electrochemical performance in large scale proton exchange membrane fuel cell stacks

The processes, losses, and electrical characteristics of a Membrane-Electrode Assembly (MEA) of a Proton Exchange Membrane Fuel Cell (PEMFC) are described. In addition, a technique for numerically modeling the electrochemical performance of a MEA, developed specifically to be implemented as part of a numerical model of a complete fuel cell stack, is presented. The technique of calculating electrochemical performance was demonstrated by modeling the MEA of a 350 cm², 125 cell PEMFC and combining it with a dynamic fuel cell stack model developed by the authors. Results from the demonstration that pertain to the MEA sub-model are given and described. These include plots of the temperature, pressure, humidity, and oxygen partial pressure distributions for the middle MEA of the modeled stack as well as the corresponding; current produced by that MEA. The demonstration showed that models developed using this technique produce results that are reasonable when compared to established performance expectations and experimental results.     

Computational study of forced air-convection in open-cathode polymer electrolyte fuel cell stacks

A mathematical model for a polymer electrolyte fuel cell (PEFC) stack with an open-cathode manifold, where a fan provides the oxidant as well as cooling, is derived and studied. In short, the model considers two-phase flow and conservation of mass, momentum, species and energy in the ambient and PEFC stack, as well as conservation of charge and a phenomenological membrane and agglomerate model for the PEFC stack. The fan is resolved as an interfacial condition with a polynomial expression for the static pressure increase over the fan as a function of the fan velocity. The results suggest that there is strong correlation between fan power rating, the height of cathode flow-field and stack performance. Further, the placement of the fan - either in blowing or suction mode - does not give rise to a discernable difference in stack performance for the flow-field considered (metal mesh). Finally, it is noted that the model can be extended to incorporate other types of flow-fields and, most importantly, be employed for design and optimization of forced air-convection open-cathode PEFC stacks and adjacent fans.     

Rapid self-start of polymer electrolyte fuel cell stacks from subfreezing temperatures

Polymer electrolyte fuel cell (PEFC) systems for light-duty vehicles must be able to start unassisted and rapidly from temperatures below −20 °C. Managing buildup of ice within the porous cathode catalyst and electrode structure is the key to self-starting a PEFC stack from subfreezing temperatures. The stack temperature must be raised above the melting point of ice before the ice completely covers the cathode catalyst and shuts down the electrochemical reaction. For rapid and robust self-start it is desirable to operate the stack near the short-circuit conditions. This mode of operation maximizes hydrogen utilization, favors production of waste heat that is absorbed by the stack, and delays complete loss of effective electrochemical surface area by causing a large fraction of the ice to form in the gas diffusion layer rather than in the cathode catalyst layer. Preheating the feed gases, using the power generated to electrically heat the stack, and operating pressures have only small effect on the ability to self-start or the startup time. In subfreezing weather, the stack shut-down protocol should include flowing ambient air through the hot cathode passages to vaporize liquid water remaining in the cathode catalyst. Self-start is faster and more robust if the bipolar plates are made from metal rather than graphite.     

Multiscale modeling of degradation of full solid oxide fuel cell stacks

Limiting the degradation of solid oxide fuel cells is an important challenge for their widespread use and commercialization. The computational expense of long-term simulation of a full stack with conventional models is immense. In this study, we present a multiscale three-dimensional model of a degrading full stack of solid oxide cells, where we integrate degradation phenomena of nickel particle coarsening in the anode electrode, chromium poisoning of the cathode electrode, and oxidation of the interconnect into a multiscale model of the stack. This approach makes this type of simulation computationally feasible, and 38 thousand hours of the stack operation can be simulated in 1 h and 15 min on a high-end workstation. Hereby one can start to explore the optimum operating conditions for a range of parameters. The model is validated with experimental data from an 18-cell Jülich Mark-F stack experiment and predicts common trends reported in the literature for evolutions of the stack performance, degradation phenomena, and the related model variables. Moreover, it captures how different regimes in the full stack degrades at different rates and how the various degradation phenomena interact over time. The model is used to investigate the effects of galvanostatic and potentiostatic operation modes, operating conditions, and flow configurations on the long-term performance of the stack. Results demonstrate, as expected, that potentiostatic operation mode, moderate temperature, lower load current, and counter-flow configuration improve the long-term performance of the stack.     

Longevity test results for reformate polymer electrolyte membrane fuel cell stacks

Polymer electrolyte membrane fuel cell (PEMFC) stacks offer a great potential for combined heat and power (CHP) applications because of their good performance and technical maturity of the key components. Nonetheless, some developmental issues have remained open. Among those are the long-term stability with respect to performance degradation and sudden death phenomena like membrane rupture.In a development program for domestic CHP systems, PEMFC stacks intended for long-term operation on reformate were developed. Development targets were high performance, high media utilization, good longevity and low degradation rates. In this paper, results on long-term performance tests of these stacks are reported. Operating times of more than 15,000 h with degradation rates of approx. 10 μV h⁻¹ have been achieved.     

Two strategies for multi-objective optimisation of solid oxide fuel cell stacks

This paper focuses on multi-objective optimisation (MOO) to optimise the planar solid oxide fuel cell (SOFC) stacks performance using a genetic algorithm. MOO problem does not have a single solution, but a complete Pareto curve, which involves the optional representation of possible compromise solutions. Here, two pairs of different objectives are considered as distinguished strategies. Optimisation of the first strategy predicts a maximum power output of 108.33 kW at a breakeven per-unit energy cost of 0.51 $/kWh and minimum breakeven per-unit energy cost of 0.30 $/kWh at a power of 42.18 kW. In the second strategy, maximum efficiency of 63.93%at a breakeven per-unit energy cost of 0.42 $/kWh is predicted, while minimum breakeven per-unit energy cost of 0.25 $/kWh at efficiency of 48.3% is obtained. The present study creates the basis for selecting optimal operating conditions of SOFC under the face of multiple conflicting objectives.     

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.     

A critical review of cooling techniques in proton exchange membrane fuel cell stacks

Effective cooling is critical for safe and efficient operation of proton exchange membrane fuel cell (PEMFC) stacks with high power. The narrow range of operating temperature and the small temperature differences between the stack and the ambient introduce significant challenges in the design of a cooling system. To promote the development of effective cooling strategies, cooling techniques reported in technical research publications and patents are reviewed in this paper. Firstly, the characteristics of the heat generation and cooling requirements in a PEMFC stack are introduced. Then the advantages, challenges and progress of various cooling techniques, including (i) cooling with heat spreaders (using high thermal conductivity materials or heat pipes), (ii) cooling with separate air flow, (iii) cooling with liquid (water or antifreeze coolant), and (iv) cooling with phase change (evaporative cooling and cooling through boiling), are systematically reviewed. Finally, further research needs in this area are identified.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

Factors of cathode current-collecting layer affecting cell performance inside solid oxide fuel cell stacks

Factors of cathode current-collecting layer (CCCL) affecting cell performance are studied by investigation of solid oxide fuel cell (SOFC) stacks with various (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) as CCCL in-suit. A larger real contact area between cathode and interconnect appears when the LSM is coated on cathode side as CCCL through characterization of a 2-cell stack. The result reveals that the real contact area depends on the surface roughness match (SRM) between CCCL and its neighboring components (active cathode and interconnect). A 6-cell stack using CCCLs with various levels of surface roughness is assembled and characterized further. The results show a higher electrical output performance of the stack repeating unit can be obtained when the surface roughness of the CCCL matches that of its neighboring components better, i.e. the surface roughness match (SRM) is the factor of cathode current collector affecting cell performance inside stack. Accordingly, the cell performance inside SOFC stack can be regulated by designing the SRM to its neighboring components.