• Energy Research

We have used environmental scanning electron microscope to observe vapor condensation and liquid water morphology and breakthrough in porous layers of polymer electrolyte membrane fuel cell. These suggest presence of large droplets and high liquid saturation at interface of the catalyst layer (CL) and gas diffusion layer (GDL), due to jump in pore size. We develop a model for morphology of liquid phase across multiple porous layers by use of both continuum and breakthrough (percolation) treatments. Using the results of this model we show the liquid morphologies deteriorate the efficiency of electrochemical reactions in CL and increase the water saturation in GDL. Then we show that inserting a microporous layer between CL and GDL reduces both the droplet size and liquid saturation and improves the cell performance.

Abstract The effect of relative humidity of the cathode (RHC) on proton exchange membrane (PEM) fuel cells has been studied focusing on automotive operation. Computational fluid dynamics (CFD) simulations were performed on a 300-cm 2 serpentine flow-field configuration at various RHC levels. The dependency of current density, membrane water contents, net water flux on the performance and the uniformity was investigated. The uniformity of current density and temperature was evaluated by employing standard deviation. The water balance inside a fuel cell was examined by describing electro-osmotic drag and back diffusion. It was concluded that the RHC has strong effect on the cell performance and uniformity. The dry RHC showed low cell voltage and non-uniform distributions of current density and temperature, whereas high RHC presented increased cell performance and uniform distributions of current density and temperature with well-hydrated membrane electrode assembly (MEA). Also the local current density distribution was strongly dependent on the local membrane water contents distribution that has complex phenomena of water transport. The elimination of external humidifier is desirable for the automotive operation, but it could degrade cell performance and durability due to dehydration of the MEA. Therefore a proper humidification of the reactant is necessary.

Abstract A uniform temperature distribution is important to obtain better control and higher performance of polymer electrolyte membrane fuel cells (PEMFCs). In PEMFCs, more than half of the chemical energy of hydrogen is converted into heat during the electrochemical generation of electricity. If not being properly exhausted, this reaction heat overheats the PEMFCs and thus impairs their performance and durability. In general, large-scale PEMFCs are cooled by liquid water that circulates through coolant flow channels in bipolar plates or in dedicated cooling plates. In this study, detailed fluid flow and heat transfer in large-scale cooling plates with 18 cm × 18 cm square area was simulated using a commercial computational fluid dynamics (CFD) code. Based on the CFD simulations, the performances of six different coolant flow field designs were assessed in terms of the maximum temperature, temperature uniformity, and pressure drop characteristics. The results demonstrated that multi-pass serpentine flow field (MPSFF) designs could significantly improve the uniformity of temperature distribution in a cooling plate compared with the conventional serpentine flow field designs, while maintaining the coolant pressure drop similar.

Abstract In this study, a two-dimensional micro/macroscale model is developed to simulate the operation of anode-supported, planar, intermediate-temperature solid oxide fuel cells (IT-SOFCs) fed with partially reformed methane fuel. The previous micro/macroscale model for hydrogen-fueled IT-SOFCs is extended to take into account the direct internal reforming (DIR) of methane inside the porous cermet anode and the multi-component mass transport and reforming reaction heat consumption. The intrinsic reaction kinetics for steam methane reforming (SMR) at the nickel catalyst surface is fully considered based on the micro/macroscale calculation framework under the assumption of fully-developed laminar channel flow. Using the developed micro/macroscale model, a detailed investigation of the methane-fueled IT-SOFC operation is conducted, followed by parametric studies on the effects of the inlet temperature, the co- or counter-flow configuration, the air flow rate, and the cell length on performance.

A comprehensive model for detailed description of micro-scale transport and electro-chemical reaction in intermediate temperature SOFCs (solid oxide fuel cells) was developed by combining many relevant theoretical and experimental researches. Dependence of electro-chemical performance of PEN (positive electrode/electrolyte/negative electrode) on micro-structural parameters of electrodes was investigated through numerical simulation. Spatial distribution of transfer current density confirmed that TPBs (three phase boundaries) at electrode/electrolyte interface were most active for electro-chemical reaction and its contribution to overall reaction increased at higher current densities. Spatial gradient of total pressure in cathode was found to facilitate oxygen transport while that in anode hinder hydrogen transport. Among various micro-structural parameters for electrodes, particle diameter was found to be the most important one that governs the PEN performance; smaller particle diameter decreased activation overpotential with larger TPB length, while increasing mass transport resistance and concentration overpotential with smaller pore diameter. The proposed micro-model was found successful in micro-structural characterization of PEN performance, and thus believed to serve as a bridge connecting micro-scale models and macro-scale calculations.

Abstract The formation–distribution of condensed water in diffusion medium of proton exchange membrane fuel cells, and its tendency to reduce the local effective mass diffusivity and to influence cell performance, are studied. First the local effective mass diffusivity of a fibrous diffusion medium is determined as a function of the local porosity and local water saturation, using the network model for species diffusion. Then using this along with the hydrodynamics of capillary, two-phase flow in hydrophobic porous media, the water formation rate (hydrogen–oxygen reaction), and condensation kinetics, the one-dimensional distribution of water saturation is determined and roles of fiber diameter, porosity, and capillary pressure on cell performance are explored. The results point to a two-layer medium (similar to the added conventional microlayer) which is then analyzed for optimum performance.

The water management role of a microporous layer (MPL) in a polymer electrolyte membrane fuel cell (PEMFC) is demonstrated experimentally by visualizing the drainage behaviors of a non-wetting fluid through multiple porous layers. An intermediate layer inserted between a fine layer and a coarse layer is observed to reduce the number of (the non-wetting fluid) breakthrough sites towards the coarse layer by merging many transport paths. Then, the reduced number of the breakthrough sites decreases the non-wetting fluid saturation in the coarse layer by minimizing capillary fingering process. These results clearly demonstrate the water management role of an MPL: An MPL reduces the liquid water breakthrough into a gas diffusion layer (GDL) by merging many paths from a catalyst layer (CL), and thereby reduces the liquid water saturation in the GDL.

Abstract A pore-network model was developed to study the water transport in hydrophobic gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The pore structure of GDL materials was modeled as a regular cubic network of pores connected by throats. The governing equations for the two-phase flow in the pore-network were obtained by considering the capillary pressure in the pores, and the entry pressure and viscous pressure drop through the throats. Numerical results showed that the saturation distribution in GDLs maintained a concave shape, indicating the water transport in GDLs was strongly influenced by capillary processes. Parametric studies were also conducted to examine the effects of several geometrical and capillary properties of GDLs on the water transport behavior and the saturation distribution. The proper inlet boundary condition for the liquid water entering GDLs was discussed along with its effects on the saturation distribution.

To achieve a high energy density for Li-ion batteries, it is important to optimize the electrode thickness and electrode density. It is common to design the electrodes to be thick and dense to achieve a high energy density. However, highly tortuous transport paths in thick and dense electrodes can lead to severe transport losses, which negatively affect the cell performance. In this work, we investigated the effects of varying the electrode thickness and density on lithium ion transport in the electrolytes by means of both experiments and simulations. Both results indicated that an additional capacity loss occurs from the electrode with low porosity because the effective diffusivity decreased in the electrolyte phase. Optimal ranges of the electrode thickness and density (porosity) that can be used to help design high-power LiFePO4/graphite batteries were also suggested in this work.