Effects of operating parameters on transport phenomena and cell performance of PEM fuel cells with conventional and contracted flow field designs

A contracted parallel flow field design was developed to improve fuel cell performance compared with the conventional parallel flow field design. A three-dimensional model was used to compare the cell performance for both designs. The effects of the cathode reactant inlet velocity and cathode reactant inlet relative humidity on the cell performance for both designs were also investigated. For operating voltages greater than 0.7 V because the electrochemical reaction rates are lower with less oxygen consumption and less liquid water production, the cell performance is independent of the flow field designs and operating parameters. However, for lower operating voltages where the electrochemical reaction rates gradually increase, the oxygen transport and the liquid water removal efficiency differ for the various flow field designs and operating parameters; therefore, the cell performance is strongly dependent on both the design and operating parameters. For lower operating voltages, the cell performance for the contracted design is better than for the conventional design because the reactant flow velocities in the contracted region significantly increase, which enhances liquid water removal and reduces the oxygen transport resistance. For lower operating voltages, as the cathode reactant inlet velocity increases and the cathode reactant inlet relative humidity decreases, the cell performance for both designs improves.     

Effects of gas diffusion layer deformation on the transport phenomena and performance of PEM fuel cells with interdigitated flow fields

In this study, a three-dimensional, non-isothermal, two-phase flow mathematical model is developed and applied to investigate the effect of the GDL deformation on transport phenomena and performance of proton exchange membrane (PEM) fuel cells with interdigitated flow fields. The thickness and porosity of the GDL is decreased after compression, and the corresponding transport parameters (permeability, mass diffusivity, thermal conductivity and electrical conductivity) are affected significantly. The alterations in geometry and transport parameters of the GDL are considered in the mathematical model. The oxygen concentration, temperature, liquid water saturation and volumetric current density distributions of PEM fuel cells without compression are investigated and then compared to the PEM fuel cells with various assembly forces. The numerical results show that the cell performance is considerably improved with increasing assembly forces. However, the pressure drops in the gas flow channels are also substantially increased. It is concluded that the assembly force should be as small as possible to decrease the parasitic losses with consideration of gas sealing concern.     

Local transport phenomena and cell performance of PEM fuel cells with various serpentine flow field designs

The flow field design in bipolar plates is very important for improving reactant utilization and liquid water removal in proton exchange membrane fuel cells (PEMFCs). A three-dimensional model was used to analyze the effect of the design parameters in the bipolar plates, including the number of flow channel bends, number of serpentine flow channels and the flow channel width ratio, on the cell performance of miniature PEMFCs with the serpentine flow field. The effect of the liquid water formation on the porosities of the porous layers was also taken into account in the model while the complex two-phase flow was neglected. The predictions show that (1) for the single serpentine flow field, the cell performance improves as the number of flow channel bends increases; (2) the single serpentine flow field has better performance than the double and triple serpentine flow fields; (3) the cell performance only improves slowly as the flow channel width increases. The effects of these design parameters on the cell performance were evaluated based on the local oxygen mass flow rates and liquid water distributions in the cells. Analysis of the pressure drops showed that for these miniature PEMFCs, the energy losses due to the pressure drops can be neglected because they are far less than the cell output power.     

Effects of internal flow modification on the cell performance enhancement of a PEM fuel cell

This study presents a numerical investigation on the cell performance enhancement of a proton exchange membrane fuel cell (PEMFC) using the finite element method (FEM). The cell performance enhancement in this study has been accomplished by the transverse installation of a baffle plate and a rectangular block for the modification of flow pattern in the flow channel of the fuel cell. The baffle plates (various gap ratios, λ = 0.005–10) and the rectangular block (constant gap ratio, λ = 0.2) are installed along the same gas diffusion layer (GDL) in the channel at constant Reynolds number for the purpose of investigating the cell performance. The results show that the transverse installation of a baffle plate and a rectangular block in the fuel flow channel can effectively enhance the local cell performance of a PEMFC. Besides, the effect of a rectangular block on the overall cell performance is more obvious than a baffle plate.     

Unsteady three-dimensional numerical study of mass transfer in PEM fuel cell with spiral flow field

Bipolar plate design and its flow field shape have an important effect on the fuel cell performance. In this work, a FORTRAN program has been developed to investigate the effects of the channel width, the number of turns of the spiral channel and the flow direction on the reactants consumption in a proton exchange membrane fuel cell (PEMFC) with a spiral flow field design. The governing equations are discretized using the finite volume method in cylindrical coordinates. The results show that the channel-rib width ratio influences the cell performance; the higher ratio, the more important contact area between the channel and the GDL, the more reactants quantity seeped to the GDL and more uniform reactants distribution is. The increasing the spiral channel turns number improves the reactants distribution uniformity. The channel spiral shape engenders a centrifugal force which enhances the cell performances in the case when the reactants are injected from the external side of the spiral channel and ejected from its internal one.     

Numerical study of cell performance and local transport phenomena in PEM fuel cells with various flow channel area ratios

Three-dimensional models of proton exchange membrane fuel cells (PEMFCs) with parallel and interdigitated flow channel designs were developed including the effects of liquid water formation on the reactant gas transport. The models were used to investigate the effects of the flow channel area ratio and the cathode flow rate on the cell performance and local transport characteristics. The results reveal that at high operating voltages, the cell performance is independent of the flow channel designs and operating parameters, while at low operating voltages, both significantly affect cell performance. For the parallel flow channel design, as the flow channel area ratio increases the cell performance improves because fuel is transported into the diffusion layer and the catalyst layer mainly by diffusion. A larger flow channel area ratio increases the contact area between the fuel and the diffusion layer, which allows more fuel to directly diffuse into the porous layers to participate in the electrochemical reaction which enhances the reaction rates. For the interdigitated flow channel design, the baffle forces more fuel to enter the cell and participate in the electrochemical reaction, so the flow channel area ratio has less effect. Forced convection not only increases the fuel transport rates but also enhances the liquid water removal, thus interdigitated flow channel design has higher performance than the parallel flow channel design. The optimal performance for the interdigitated flow channel design occurs for a flow channel area ratio of 0.4. The cell performance also improves as the cathode flow rate increases. The effects of the flow channel area ratio and the cathode flow rate on cell performance are analyzed based on the local current densities, oxygen flow rates and liquid water concentrations inside the cell.     

Influence of geometric parameters of the flow fields on the performance of a PEM fuel cell. A review

The proton Exchange membrane fuel cell (PEMFC) performance depends not only on many factors including the operation conditions, transport phenomena inside the cell and kinetics of the electrochemical reactions, but also in its physical components; membrane electrode assembling (MEA) and bipolar plates (BPs). Among the PEM stack components, bipolar plates are considered one of the crucial ones, as they provide one of the most important issues regarding the performance of a stack, the homogeneous distribution of the reactive gases all over the catalyst surface and bipolar plate areas through, the so call, flow channels; physical flow patterns or paths fabricated on the BPs surfaces to guide the gases all along the BPs for its correct distribution. The failure in flow distribution among different unit cells may severely influence the fuel cell stack performance. Thus, to overcome such possible failures, the design of more efficient flow channels has received considerable attention in the research community for the last decade.     

Effect of height/width-tapered flow fields on the cell performance of polymer electrolyte membrane fuel cells

Water flooding at the cathode is a critical issue in polymer electrolyte membrane fuel cells, since it will block the oxygen transport, resulting in a large concentration loss. To address this issue, two tapered flow fields with varying height or width are proposed in this study, as the tapered channels can gradually increase flow velocity which is beneficial in water removal. To confirm the proposed function, both experiments and simulation are performed. The results prove that the tapered flow fields with high velocity at the downstream region can significantly enhance water removal in flow channels, which avoids the mass transport limitation and improves the cell performance at high current densities. Additionally, the tapered design can also enhance the under-rib convection between adjacent channels, which helps to remove accumulated water in the gas diffusion layer.     

质子交换膜燃料电池传输现象的数值分析

建立了一个具有蛇形通道,采用Nafion117膜单体质子交换膜燃料电池的三维数学模型,该模型同时考虑了流动、传热、传质、电化学动力学和多组分传输现象。通过求解传输方程组,并耦合电化学动力学方程,获得了电池的极化性能曲线和电池内部的反应物浓度、温度、速度分布。计算结果表明,增加电极孔隙率、提高电池运行温度和压力有助于改善电池性能。估算的极化性能与献中的实验数据基本符合。分析了运行条件对电池性能的影响。     

Numerical evaluation of a PEM fuel cell with conventional flow fields adapted to tubular plates

In this research a 3D numerical study on a PEM fuel cell model with tubular plates is presented. The study is focused on the performance evaluation of three flow fields with cylindrical geometry (serpentine, interdigitated and straight channels) in a fuel cell. These designs are proposed not only with the aim to reduce the pressure losses that conventional designs exhibit with rectangular flow fields but also to improve the mass transport processes that take place in the fuel cell cathode. A commercial computational fluid dynamics (CFD) code was used to solve the numerical model. From the numerical solution of the fluid mechanics equations and the electrochemical model of Butler-Volmer different analysis of pressure losses, species concentration, current density, temperature and ionic conductivity were carried out. The results were obtained at the flow channels and the catalyst layers as well as in the gas diffusion layers and the membrane interfaces. Numerical results showed that cylindrical channel configurations reduced the pressure losses in the cell due to the gradual reduction of the angle at the flow path and the twist of the channel, thus facilitating the expulsion of liquid water from the gas diffusion layers and in turn promoting a high oxygen concentration at the triple phase boundary of the catalyst layers. Moreover, numerical results were compared to polarization curves and the literature data reported for similar designs. These results demonstrated that conventional flow field designs applied to conventional tubular plates have some advantages over the rectangular designs, such as uniform pressure and current density distributions among others, therefore they could be considered for fuel cell designs in portable applications.     

Performance of micro-PEM fuel cells with different flow fields

Four designs of flow fields were applied to micro-proton exchange membrane fuel cells (μ-PEMFCs) using microelectromechanical system (MEMS) technology. The flow fields and membrane electrolyte assembly (MEA) of 2.25 cm² active area were assembled to μ-PEMFCs. Electrochemical behaviors of these μ-PEMFCs were investigated by polarization method at reactants flow rates of 15 ml min⁻¹, 30 ml min⁻¹ and 50 ml min⁻¹, respectively. This study emphasized the effects of different topologies of flow fields on performance of μ-PEMFCs. Results demonstrated that μ-PEMFCs with different flow fields have similar behavior at reactants flow rates of 50 ml min⁻¹. However, at reactants flow rates of 15 ml min⁻¹ and 30 ml min⁻¹, performance of the μ-PEMFC with long and narrow micro-channels rapidly deteriorated due to the flooding in micro-channels. The mixed serpentine design had a good ability to resist the flooding, but it displayed a low maximum power density because of its short effective length of micro-channels. The results in this study suggested that the μ-PEMFC with a mixed multichannel design flow field and long micro-channels yielded the best performance.     

Three-dimensional numerical analysis of PEM fuel cells with straight and wave-like gas flow fields channels

Using a three-dimensional computational model, numerical simulations are performed to investigate the performance characteristics of proton exchange membrane fuel cells (PEMFCs) incorporating either a conventional straight gas flow channel or a novel wave-like channel. The simulations focus particularly on the effect of the wave-like surface on the gas flow characteristics, the temperature distribution, the electrochemical reaction efficiency and the electrical performance of the PEMFCs at operating temperatures ranging from 323 K to 343 K. The numerical results reveal that the wave-like surface enhances the transport of the reactant gases through the porous layer, improves the convective heat transfer effect, increases the gas flow velocity, and yields a more uniform temperature distribution. As a result, the efficiency of the catalytic reaction is significantly improved. Consequently, compared to a conventional PEMFC, the PEMFC with a wave-like channel yields a notably higher output voltage and power density.     

Numerical study on cell performance and local transport phenomena of PEM fuel cells with novel flow field designs

In this work, a novel style of straight flow channel tapered in height or width is proposed to improve the efficiency of fuel utilization for PEM fuel cells. Fuel channels of various height and width taper ratios are numerically analyzed to understand their effects on fuel transport characteristics and cell performance. Influences of the liquid water formation on the transport phenomena and cell performance are included in the numerical model. The present results demonstrated that, with the tapered channel designs, the flow area contraction along the flow channel leads to increase in fuel velocity and thus enhances the fuel transport through porous layers, fuel utilization, and the capability of the liquid water removal. The results also reveal that the cell performance can be improved by either decreasing height taper ratio or increasing width taper ratio. If the power loss due to pressure drop is not considered, the performance of the fuel cell with the tapered flow channels is consistently improved with height taper ratios decreased and width taper ratio increased. With the pressure loss considered, however, the best performance can be obtained at the height taper ratio (Λ_x) of 0.5 and the width taper ratio (Λ_z) of 1.8 among the taper ratios studied in the present work.     

Performance study of solid oxide fuel cell with various flow field designs: numerical study

A comprehensive 3D mathematical model has been developed to study the performance of the planar anode-supported solid oxide fuel cell (SOFC) with different flow field designs such as helical, single-entry serpentine, traditional parallel, modified parallel design, double-entry serpentine and triple-entry serpentine. The model includes charge transport (electron and ion), conservation of mass, momentum, and energy. The developed model is numerically simulated and the predicted results are validated using the available experimental data from previous work. Results showed that single-entry cells suffer of back flow at the outlet of the cathode flow side in both helical and single-entry serpentine designs due to early full fuel consumption. To avoid back flow, increasing the number of entry ports at inlets and outlets in different designs is performed to increase the inlet mass flow rate. It is found that the triple-entry serpentine design attains good uniform distributions for both fuel and oxygen throughout the active surface area and achieves a high collected current of about 23.3 A with a percentage increase of 5.18% compared to the other designs at low voltage. Comparison with other designs indicates that the triple-entry serpentine gives better performance.     

Numerical investigation of the performance of symmetric flow distributors as flow channels for PEM fuel cells

This work reports on the performance of a single PEM fuel cell using symmetric flow patterns as gas delivery channels. Three flow patterns, two symmetric and one serpentine, are taken from the literature on cooling of electronics and they are implemented in a computational model as gas flow channels in the anode and cathode side of a PEMFC. A commercial CFD code was used to solve the physics involved in a fuel cell namely: the flow field, the mass conservation, the energy conservation, the species transport, and the electric/ionic fields under the assumptions of steady state and single phase. An important feature of the current modeling efforts is the analysis of the main irreversibilities at different current densities showing the main energy dissipation phenomena in each cell design. Also, the hydraulic performance of the flow patterns was studied by evaluating the pressure drop and pumping power. The first part of this work reveals the advantages of using a serpentine pattern over the base symmetric distributors. The second part is an optimization of the symmetric patterns using the entropy minimization criteria. Such an optimization led to the creation of a flow structure that promotes an improved performance from the point of view of power generation, uniformity of current density, and low pumping power.     

Reticulated porous and parallel channel cathode flow fields in real scale polymer electrolyte membrane fuel cells: A comparative experimental study

This paper focuses on understanding the effect of reticulated porous cathode flow fields in real scale close and open cathode polymer electrolyte membrane fuel cells (PEMFCs) in terms of their thermo-electrical performance. This research contributes to addressing challenges with PEMFCs linked to oxygen supply to the cathode and proper mixing of gasses as well as water removal issues. Parallel channel and porous cathode flow fields in both open and closed cathode PEMFCs of medium scale (active area of 15 × 15 cm²) have been investigated. The porous material consisted of 20 pores per inch with a porosity level of 80%. The cells’ polarisation and impedance characteristics have been analysed. The porous flow field has been found providing better electrical performance in closed cathode PEMFC compared to the open cathode. Improvements in gas diffusion and temperature uniformity were observed with porous flow field; however, water removal has been observed challenging, which need to be addressed before the benefits of using porous flow field are fully realised.     

Investigation of water droplet kinetics and optimization of channel geometry for PEM fuel cell cathodes

The kinetics and transport mechanisms of water droplets in model flow field channels of a low temperature polymer electrolyte fuel cell were investigated. The pressure drop at different air flows was measured for different channel geometries in a graphite plate as employed for fuel cell bipolar plates. The minimum air flow required for the movement of a water droplet in the flow channel was identified. From the experimental findings, recommendations for the development of flow fields with high condensate removal capabilities combined with low pressure differences were drawn to allow for an efficient operation of PEM fuel cells.     

Effect of humidity of reactants on the cell performance of PEM fuel cells with parallel and interdigitated flow field designs

This study investigates the effects of the relative humidity (RH) of the reactants on the cell performance and local transport phenomena in proton exchange membrane fuel cells with parallel and interdigitated flow fields. A three-dimensional model was developed taking into account the effect of the liquid water formation on the reactant transport. The results indicate that the reactant RH and the flow field design all significantly affect cell performance. For the same operating conditions and reactant RH, the interdigitated design has better cell performance than the parallel design. With a constant anode RH = 100%, for lower operating voltages, a lower cathode RH reduces cathode flooding and improves cell performance, while for higher operating voltages, a higher cathode RH maintains the membrane hydration to give better cell performance. With a constant cathode RH = 100%, for lower operating voltages, a lower anode RH not only provides more hydrogen to the catalyst layer to participate in the electrochemical reaction, but also increases the difference in the water concentrations between the anode and cathode, which enhances back-diffusion of water from the cathode to the anode, thus reducing cathode flooding to give better performance. However, for higher operating voltages, the cell performance is not dependent on the anode RH.     

Three-dimensional transport model of PEM fuel cell with straight flow channels

In this work, an isothermal, steady-state, three-dimensional (3D) multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with straight gas channels. The model computational domain, includes anode flow channel, membrane electrode assembly (MEA) and cathode flow channel. The catalyst layer within the domain has physical volume without simplification. A comprehensive set of 3D continuity equation, momentum equations and species conservation equations are formulated to describe the flow and species transport of the gas mixture in the coupled gas channels and the electrodes. The electrochemical reaction rate is modified by an agglomerate model to account for the effect of diffusion resistance through catalyst particle. The activation overpotential is predicted locally in the catalyst layer by separately solving electric potential equations of membrane phase and solid phase. The model is validated by comparison of the model prediction with experimental data of Ticianelli et al. The results indicate the detailed distribution characteristics of oxygen concentration, local current density and cathode activation overpotential at different current densities. The distribution patterns are relatively uniform at low average current density and are severely non-uniform at higher current density due to the mass transfer limitation. The local effectiveness factor in the catalyst layer can be obtained with this model, so the mass transport limitation is displayed from another point of view.