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Summary As the polymer electrolyte membrane fuel cell (PEMFC) matures performance keeps increasing—mainly in terms of conversion rate rather than efficiency. Increased conversion results in increased heat release. Fuel cell performance and degradation is a delicate balance of heat and work manipulated significantly by the transport and the state of water in the system. With high heat release rates and low thermal conductivities the maximum temperature can be up to 20 °C warmer than the coolant. While much work has been done in understanding the thermal conductivity of fuel cell materials, even more is needed. Particular needs for continued research include: the thermal implications of material ageing, the nature and importance of the composite interface between the microporous layer and the porous transport layer, the impact of liquid water on thermal conductivity, a better understanding of phase change in electrodes and the heat capacities of fuel cell materials. Finally, the heat release in a fuel cell presents a significant opportunity to better understand the system and changes in the system. Fuel cell calorimetry has only begun to be used to explore heat and work in PEMFC and should be explored in greater detail.

Code stability is a matter of concern for three-dimensional (3D) fuel cell models operating both at high current density and at high cell voltage. An idealized mathematical model of a fuel cell should converge for all potentiostatic or galvanostatic boundary conditions ranging from open circuit to closed circuit. Many fail to do so, due to (i) fuel or oxygen starvation causing divergence as local partial pressures and mass fractions of fuel or oxidant fall to near zero and (ii) nonlinearities in the Nernst and Butler–Volmer equations near open-circuit conditions. This paper describes in detail, specific numerical methods used to improve the stability of a previously existing fuel cell performance calculation procedure, at both low and high current densities. Four specific techniques are identified. A straight channel operating as a (i) solid oxide and (ii) polymer electrolyte membrane fuel cell is used to illustrate the efficacy of the modifications.

The thermal conductivity and the thickness change with pressure of several different micro porous layers (MPL) used for the polymer electrolyte membrane fuel cell (PEMFC) were measured. The MPL were made with different compositions of carbon and polytetrafluoroethylene (PTFE). A one-dimensional thermal PEMFC model was used to estimate the impact that the MPL has on the temperature profiles though the PEMFC. The thermal conductivity was found to vary from as low as 0.05 up to as high as 0.12 W K−1 m−1 while the compaction pressure was varied from 4 bar and up to around 16 bar resulting in a decrease in thickness of approximately 40%. The PTFE content, which varied between 10 and 25%, did not result in any significant change in the compression or thermal conductivity. Both the thickness and the thermal conductivity changed irreversibly with compaction pressure. Considering a MPL thermal conductivity of 0.1 W K−1 m−1, a MPL thickness of 45 μm, a current density of 10 kA m−2 (1.0 A cm−2), liquid water (production and sorption), and a 30 μm membrane it was found that the MPL is responsible for a temperature increase of up to 2 °C. This contribution can be lowered by integrating the MPL into the porous transport layer.

The through-plane thermal conductivity of artificially aged SGL Porous Transport Layers, PTLs, are measured and reported. NMR measurements of PTFE content and water/PTL contact angles are also reported. The PTLs were artificially aged in air rich water at 80 °C for 0, 200, 400, 600, 800, and 1000 h. For the dry samples, it was found that the through-plane thermal conductivity did not change significantly in response to the ageing treatment. For the samples containing water, the through-plane thermal conductivity increased by a factor of two to three and the more aged samples had the highest thermal conductivities. The through-plane thermal conductivity of dry PTLs is known to decrease with increasing PTFE content, which was also seen in this study. The chosen ageing procedure is known to wash PTFE from the PTL, and this verified using NMR. Water-PTL contact angle measurements also demonstrate that the PTL becomes less hydrophobic with ageing. This coincides with the increase in the through-plane thermal conductivity. Because the dry PTL retains its thermal conductivity while the PTFE content decreases we suggest that the PTFE remains in the fibre to fibre contact region and is removed predominantly elsewhere.

Impedance measurements of H2/N2 cell for the determination of PEMFC catalyst layer (CL) resistance can exhibit significant deviation from those predicted assuming homogeneous CL structure with uniform resistance and capacitance. Predictions from model with homogeneous structure but distributed resistance and/or capacitance shows deviation from the ideal mid-frequency response (45o line) but expected low-frequency response (vertical line). A distributed model considering agglomerate-type structure shows deviation from ideal mid-frequency response ((45o line) as well as from ideal vertical-line like response at experimentally employed frequencies although a vertical-line response is predicted at frequencies significantly lower than typically accessible in experiments.

A calorimeter was used to measure the heat production in polymer electrolyte membrane (PEM) fuel cells operated on hydrogen and oxygen at 50 °C and 1 bar. Two cells were examined, one using a 35 μm thick Nafion membrane and a catalyst loading of 0.6/0.4 mg Pt cm−2, for the cathode and anode layer, respectively, the other using a 180 μm thick Nafion membrane and loading of 0.4/0.4 mg Pt cm−2. The cells investigated thus had different membranes and catalyst layers, but identical porous transport layers and micro-porous layers. The calorimeter is unique in that it provides the heat fluxes out of the cell, separately for the anode and the cathode sides. The corresponding cell potential differences, ohmic cell resistance and current densities are also reported. The heat fluxes through the current collector plates were decomposed by a thermal model to give the contributions from the ohmic and the Tafel heats to the total heat fluxes. Thus, the contributions from the reversible heat (the Peltier heats) to the current collectors were determined. The analysis suggests that the Peltier heat of the anode of these fuel cell materials is small, and that it is the cathode reaction which generates the main fraction of the total heat in a PEM fuel cell. The entropy change of the anode reaction appears to be close to zero, while the corresponding value for the cathode is near −80 J K−1 mol−1.

A design procedure is presented that improves the energy efficiency and saves catalyst material of a polymer electrolyte membrane fuel cell (PEMFC). The method is demonstrated for a single cell in a stack and uses the theorem of equipartition of entropy production to maximize energy efficiency. The gas supply and water outlet systems, designed to produce entropy uniformly, have a fractal structure inspired by the human lung. The tree-like gas distributor engraved in the bipolar plates may eliminate the need for porous transport layers. Mathematical solutions are given for the optimal height, macroporosity, and nanoporous column width of the electro-catalytic layer beneath the gas supply system. It is shown that the optimal macroporosity of the catalytic layer is equal to 1/2 for the model chosen and that the optimal height of the catalytic layer depends upon the coefficient for first-order reaction kinetics at the cathode, the diffusion constant for oxygen in the gas phase, and the oxygen concentration of...

This paper presents 2D thermal model of a fuel cell to elucidate some of the issues and important parameters with respect to temperature distributions in PEM fuel cells. A short review on various properties affecting the temperature profile and the heat production in the polymer electrolyte fuel cell is included. At an average current density of 1 A cm−2, it is found that the maximum temperature of the MEA is elevated by between 4.5 and 15 K compared to the polarisation plate temperature. The smallest deviation corresponds to one dimensional transport, while the largest corresponds to the two dimensional transport considering anisotropic thermal conductivity. The two dimensional thermal model further predicts increased lost work. While most of the heat generation is allocated in the cathode, it is shown that the heat effect may be balanced by the water phase change in the anode. The most significant factor in determining the temperature distribution is the gas channel geometry (width and channel type), followed by the thermal conductivity of the porous transport layer and state of the water in the cell.

In this paper thermal properties for materials typically used in the proton exchange membrane fuel cell (PEMFC) are reported. Thermal conductivities of Nafion membranes were measured ex situ at 20 ◦ Ct o be 0.177 ± 0.008 and 0.254 ± 0.016 W K−1 m−1 for dry and maximally wetted membranes respectively. This paper also presents a methodology to determine the thermal conductivity of compressible materials as a function of applied load. This technique was used to measure the thermal conductivity of an uncoated SolviCore porous transport layer (PTL) at various compaction pressures. For the dry PTL at 4.6, 9.3 and 13.9 bar compaction pressures, the thermal conductivity was found to be 0.27, 0.36 and 0.40 W K −1 m −1 respectively and the thermal contact resistivity to the apparatus was determined to be

A calorimeter has been constructed and used to measure the total heat production of a single polymer electrolyte fuel cell that is operated on hydrogen and oxygen at 50 °C and 1 bar. The cell had a SolviCore Catalyst Coated Backing and Nafion membranes 112, 115 and 117. We report that the total heat production plus the power production corresponds to the enthalpy of formation of water for cell potentials above 0.55 V. For cell potentials less than 0.55 V, we measured a linear decrease in the reaction enthalpy with decreasing cell potential. This effect was obtained independently of membrane thickness and current density. We propose therefore that the main power loss at low cell potentials and the inflection point in the polarisation curve is due to hydrogen peroxide formation at the cathode. The total heat production was decomposed into reversible and irreversible effects (non-ohmic and ohmic). The non-ohmic part was evaluated using Tafel plots. We show that it is possible to determine the overpotential of an electrode also from its thermal signature.

This paper compares three different approaches for describing the cathode catalyst layer of a PEMFC, using a three-dimensional CFD model. The three catalyst treatments include: a thin-film model, a discrete-catalyst volume model and an agglomerate model. It is shown that, within a single-phase approach using physically meaningful parameters, the commonly employed thin-film or discrete-volume descriptions of the catalyst layer do not show the significant mass transport limitations which occur at higher current densities; while this region is observed using a catalyst agglomerate approach. Further, an in-depth analysis of the current density distributions indicates that for a given electrode overpotential the thin-film model significantly over-predicts the current density, compared to the discrete and agglomerate approaches. The thin-film model also greatly exaggerates the variation in current density both along and across the channel. Finally, the agglomerate model predicts noticeable mass transport losses even at very low current densities despite the use of high stoichiometric ratios and thin-electrolyte films.

A highly reliable experimental system that consistently closed the overall water balance to within 5% was developed to study the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the water transport and performance of a standard 100 cm2 active area PEM fuel cell. Various combinations of cells were built and tested with PTLs at the electrodes using either carbon fibre paper with a MPL (SGL 10BB) or carbon fibre paper without a MPL (SGL 10BA). The net water drag coefficient at three current densities (0.3, 0.5 and 0.7 A cm−2) for two combinations of anode/cathode relative humidity (60/100% and 100/60%) and stoichiometric ratios of H2/air (1.4/3 and 1.4/2) was determined from water balance measurements. The addition of a MPL to the carbon fibre paper PTL at the cathode did not cause a statistically significant change to the overall drag coefficient although there was a significant improvement to the fuel cell performance and durability when a MPL was used at the cathode. The presence of a MPL on either electrode or on both electrodes also exhibited similar performance compared to when the MPL was placed at the cathode. These results indicate that the presence of MPL indeed improves the cell performance although it does not affect the net water drag coefficient. The correlation between cell performance and global water transport cannot be ascertained and warrants further experimental investigation.

A test station was modified to collect water from the anode and cathode exhaust streams of a PEM fuel cell. Various combinations of cells were built and tested with (SGL10BA) and without microporous layers (SGL10BB) on the electrodes. Cells with a MPL at the cathode performed much better compared to cells with no MPL and the degradation rates were decreased. Water balance experiments were conducted with two combinations of inlet relative humidities (RH): 100% anode-60% cathode and 60% anode-100% cathode at current densities of 300, 500 and 700 mA/cm2. For the range of current densities studied, the net drag coefficient at 60{degree sign}C was -0.08-+0.08 mol H2O/mol H+ for RH 60% anode and 0.01-0.2 mol H2O/mol H+ for RH 100% anode. There was no significant change in the drag coefficient attributable to the presence of the MPL on the cathode. A MPL on the anode did affect the net water drag.

This paper reviews the approach taken in the literature to model the effective transport coefficients – mass diffusivity, electrical conductivity, thermal conductivity and hydraulic permeability – of carbon-fibre based porous electrode of polymer electrolyte membrane fuel cells (PEMFCs). It is concluded that current PEMFC model do not account for the inherent anisotropic microstructure of the fibrous electrodes. Simulations using a 2-D PEMFC cathode model show that neglecting the anisotropic nature and associated transport coefficients of the porous electrodes significantly influences both the nature and the magnitude of the model predictions. This emphasizes the need to appropriately characterize the relevant anisotropic properties of the fibrous electrode.

A recently constructed 2D calorimeter was used to measure the work and the total heat production of a single PEM fuel cell that is operated on hydrogen and oxygen at 50 °C and 1 bar. The cells had different membranes and catalyst layers, but the same porous transport layer and micro-porous layer. In this paper the distribution of the reaction entropy between the anode and the cathode, also known as the Peltier heats, was determined. This was done by the use of the measured heat through the anode and the cathode polarisation plates, the measured cell resistance, the measured overpotential and a fuel cell thermal model. The results show that the reaction entropy is almost solely related to the cathode and that the Peltier heat of the anode is near zero.

Efficient water management is crucial for the good performances of proton-exchange membrane fuel cells (PEMFCs). The geometric and physical characteristics of the components of a PEMFC as well as operating conditions have an impact on the transport of water through the porous transport layer (PTL) and the two-phase flow regimes in the microchannels. One parameter of importance is the local temperature, which affects properties such as surface tension and is coupled with phase change. Indeed, a temperature difference of about 5K is expected across the PTL, with spatial variations due to the geometry of the flow field plate. We present preliminary results obtained with a first experimental setup for the ex-situ characterization of two-phase flow regimes in the flow channels. Water is pushed through the PTL, which is sandwiched between a porous metal foam and the flow field plate. The air flow rate, temperature and humidity can be controlled. The cell can be heated up by applying an electrical current through the metal foam. A transparent window is located on top of the flow channel. The two-phase flow within the micro-channels is visualized using a high-speed camera and laser-induced fluorescence. Preliminary results obtained under isothermal conditions at room temperature show that different two-phase flow regimes occur in the channels depending on the operating conditions, in good qualitative agreement with data from the literature. Eventually, a new visualization cell is presented that is expected to correct the flaws of the previous design and will allow a better thermal control. It will be possible to adjust the temperature gradient and the mean temperature in order to observe their impact on two-phase flow regimes for different types of PTL and flow rates. The results will provide a better understanding of water transport in PEMFC and benchmark data for the validation of numerical models.Copyright © 2010 by ASME

The flow phenomena in a serpentine microchannel segment attached to a porous transport layer in a micro proton-exchange membrane fuel cell is investigated. Due to the presence of a porous transport layer, the fluid flow for this configuration exhibits different characteristics compared with that through a simple serpentine channel. The pressure drop and friction factor variation in the channel is examined for various values of Reynolds number and radii of curvature. Also, the effect of variation of permeability is investigated. There are two modes of fluid transport in this geometry—one through the serpentine channel and other via the porous media. With increasing permeability, more fluid is convected through the porous transport layer.

A 1-D MEA Performance model was developed that considered transport of liquid water, agglomerate catalyst structure, and the statistical variation of the MEA characteristic parameters. The model was validated against a low surface area carbon supported catalyst across various platinum loadings and operational conditions. The statistical variation was found to play a significant role in creating noise in the validation data and that there was a coupling effect between movement in material properties with liquid water transport. Further, in studying the low platinum loaded catalyst layers it was found that liquid water played a significant role in the increasing the overall transport losses. The model was then further applied to study platinum dissolution via potential cycling accelerated stress tests, in which the platinum was found to dissolve nearest the membrane effectively resulting in reaction distribution shifts within the layer.

An improved catalyst deposition methodology based on a piezo-electric printing technique has been developed and used to fabricate catalyst coated membranes (CCM) with thin catalyst layers (1-5 μm) and ultra-low Pt loadings (0.02-0.12 mg Pt /cm 2 ). The performance of these CCMs was examined in proton exchange membrane fuel cells (PEMFCs). The catalyst utilization was observed to increase with decreasing catalyst layer thickness (decreasing Pt loading). The printed CCM with two layers containing an ultra-low Pt loading (0.02 mg Pt /cm 2 ) exhibited Pt utilizations of 100%. Neglecting the anode contributions, the mass activity at 850 mV for the printed CCM is nearly 76.5 mA/mg Pt which is 3.5 times higher than that for the CCM fabricated by conventional spraying method (22.5 mA/mg Pt ).

The influence of humidity of supplied gases on on electrochemically active surface area and charge transfer resistance in cathode process of PEM fuel cell was studied. Impedance spectra for cells operated with various gas stream combinations - H2/Air, H2/O2, H2/H2 and H2/N2 - was analyzed to determine the physical/chemical origin of the spectra features. Cathode charge transfer resistance increased with a decrease in the humidity of supplied gases. As well, a reduction in the electrochemically active surface area with a decrease in relative humidity was observed.

This experimental study was undertaken to resolve the contrasting viewpoints on the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the net water transport in a polymer electrolyte membrane fuel cell (PEMFC). Experimental results on single cells with and without cathode MPL show no statistically significant change in the net drag coefficient that could be attributed to the presence of the MPL. In contrast to the two prevailing but contrasting viewpoints, our results indicate that the MPL on the cathode neither enhances back-diffusion nor increases water removal from the cathode catalyst layer to the PTL.