Effect of cathode channel dimensions on the performance of an air-breathing PEM fuel cell

A three dimensional, steady state, non-isothermal, single phase model was developed and simulations were carried out in order to find the effect of cathode channel dimensions (width, depth and height) on the performance of an air-breathing fuel cell. The model was solved using commercial CFD package Fluent (version 6.3). Separate user defined functions were written to solve the electrochemical equations and the water transport through the membrane along with the other governing equations. Analyses were carried out for three different channel widths (2, 4 and 6 mm), for three different channel depths (2, 6 and 10 mm) and for three different cell heights (15, 30 and 45 mm). Cell characteristics like current distribution, species distribution, oxygen mass transfer coefficient, cell temperature, cathode channel velocities and net water transport coefficients are reported. The results show that the cell performance improves with increase in cathode channel width, channel depth and with decrease in cell height. Maximum power density obtained was 240 mW/cm² for a channel width of 4 mm and channel depth of 6 mm. When the channel depth was 2 mm the performance was limited mainly due to the resistance offered by the channel for the buoyancy induced flow. For channel depths higher than 2 mm, the diffusion resistance of the porous GDL also contributed significantly to limit the performance to low current densities. At low current densities the fuel cell is prone to flooding whereas at high current densities ohmic overpotential due to dehydration of the membrane significantly contributes to the overall voltage loss.     

Three-dimensional modeling and experimental investigation for an air-breathing polymer electrolyte membrane fuel cell (PEMFC)

An air-breathing polymer electrolyte membrane fuel cell bears many advantages, which are important for portable-power applications. However, several barriers must be overcome before an air-breathing PEMFC achieve commercially wide-scale adoption. In this paper, with emphasis on improving the performance of air-breathing PEMFC, the simulation and experiment has been done simultaneously. Considering the natural convection in the cathode side, electrochemical reaction in the catalyst layer, water transport in the membrane, a coupled three-dimensional complex model has been developed in this work. The parameters which greatly affect the performance of an air-breathing PEMFC have been calculated for the base case such as the distribution of water and reactant, temperature and electrochemical performance. To validate the numerical result, the experiment test system have been designed to investigate the temperature distribution and cell performance. The results from this work show that the performance of air-breathing PEMFCs is strongly affected by natural convection feature. The concentration losses play a major role on the cell performance. The ambient relative humidity also has significant effect on the cell performance. The fields of water, temperature, velocity and electrochemical reaction have strong interaction on each others.     

Two-dimensional two-phase thermal model for passive direct methanol fuel cells

A two-dimensional two-phase thermal model is presented for direct methanol fuel cells (DMFC), in which the fuel and oxidant are fed in a passive manner. The inherently coupled heat and mass transport, along with the electrochemical reactions occurring in the passive DMFC is modeled based on the unsaturated flow theory in porous media. The model is solved numerically using a home-written computer code to investigate the effects of various operating and geometric design parameters, including methanol concentration as well as the open ratio and channel and rib width of the current collectors, on cell performance. The numerical results show that the cell performance increases with increasing methanol concentration from 1.0 to 4.0 M, due primarily to the increased operating temperature resulting from the exothermic reaction between the permeated methanol and oxygen on the cathode and the increased mass transfer rate of methanol. It is also shown that the cell performance upgrades with increasing the open ratio and with decreasing the rib width as the result of the increased mass transfer rate on both the anode and cathode.     

Effects of cathode channel configurations on the performance of an air-breathing PEMFC

A mathematical model is developed for evaluating the effects of various channel dimensions on the performance of an air-breathing polymer electrolytes membrane fuel cell (PEMFC). The model, which is based on Nguyen's model, has been extended to include the natural convection to consider buoyancy effect in the channels, electro-chemical reaction in the catalyst layer, and concentration overpotential due to mass transportation limitation. Results from the model indicate that the concentration loss is more serious in natural convection than in forced convection, especially at small channel width, and the performance of air-breathing PEMFC could be improved by increasing the channel width to some extend. Results also show that the temperature, channel size, and air flow rate influence each other, and the performance cannot be improved infinitely by increasing the channel size, and thus the cathode flow field should be optimized. This model provides insights into many design issues of air-breathing fuel cell, and can be easily used as an optimal design tool for air-breathing PEMFC.     

A lumped parameter model of the polymer electrolyte fuel cell

A model of a polymer electrolyte fuel cell (PEFC) is developed that captures dynamic behaviour for control purposes. The model is mathematically simple, but accounts for the essential phenomena that define PEFC performance. In particular, performance depends principally on humidity, temperature and gas pressure in the fuel cell system. To simulate accurately PEFC operation, the effects of water transport, hydration in the membrane, temperature, and mass transport in the fuel cells system are simultaneously coupled in the model. The PEFC model address three physically distinctive fuel cell components, namely, the anode channel, the cathode channel, and the membrane electrode assembly (MEA). The laws of mass and energy conservation are applied to describe each physical component as a control volume. In addition, the MEA model includes a steady-state electrochemical model, which consists of membrane hydration and the stack voltage models.     

质子交换膜燃料电池内部传热传质三维模拟

针对常规流场质子交换膜燃料电池提出了三维非等温数学模型。模型考虑了电化学反应动力学以及反应气体在流道和多孔介质内的流动和传递过程,详细研究了水在质子膜内的电渗和扩散作用。计算结果表明,反应气体传质的限制和质子膜内的水含量直接决定了电极局部电流密度的分布和电池输出性能;在电流密度大于0.3~0.4A/cm2时开始出现水从阳极到阴极侧的净迁移;高电流密度时膜厚度方向存在很大的温度梯度,这对膜内传递过程有较大影响。     

Simulation of species transport and water management in PEM fuel cells

A single phase computational fuel cells model is presented to elucidate three-dimensional interactions between mass transport and electrochemical kinetics in proton exchange membrane (PEM) fuel cells with straight gas channels. The governing differential equations are solved over a single computational domain, which consists of a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. Emphasis is placed on obtaining a basic understanding of how three-dimensional flow and transport phenomena in the air cathode impact the electrochemical process in the flow field. The complete cell model has been validated against experimentally measured polarization curve, showing good accuracy in reproducing cell performance over moderate current density interval. Fully three-dimensional results of the flow structure and species profiles are presented for cathode flow field. The effects of pressure on oxygen transport and water removal are illustrated through main axis of the flow structure. The model results indicate that oxygen concentration in reaction sites is significantly affected by pressure increase which leads to rising fuel cells power.     

Simulation and experiment of heat and mass transfer in a proton exchange membrane electrolysis cell

A full-scale, two-phase, single-channel model of proton exchange membrane electrolysis cell is established. The electrochemical model and the thermal model are coupled to explore the mass transfer of the channel, catalytic layer and diffusion layer, and the heat transfer of the entire electrolysis cell. Two different calculation models are compared, and it is found that the calculation results of the model with bipolar plates are closer to the actual values. Simultaneously, effective water and thermal management strategies are proposed: The temperature of the electrolysis cell can be reduced effectively by supplying water to the cathode side. The Counter-flow mode has a lower temperature than the Co-flow mode, but the temperature gradient in the Counter-flow mode is greater. Reducing the channel depth and increasing the channel width can improve the water transmission in the electrolysis cell and reduce the temperature of the electrolysis cell, but a larger channel width will increase the electrical loss. Therefore, the selection of appropriate channel size is of great significance to the long-term stable operation of the electrolysis cell.     

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.     

Experimental and numerical investigation on effects of cathode flow field configurations in an air-breathing high-temperature PEMFC

Air-breathing high-temperature proton exchange membrane fuel cell (HT-PEMFC) gets rid of the cumbersome air supplying systems and avoids the water flooding problem by directly exposing the cathode to air and operating the fuel cell at elevated temperature. Performance of the air-breathing HT-PEMFC is dependent on many factors particularly the cathode flow field configurations. However, studies about air-breathing HT-PEMFCs are quite limited in the literature. In the present study, an experimental testing system was setup for the performance measurement of the air-breathing HT-PEMFC. A 3D numerical model was established and validated by the experimental data. Effects of the cathode flow field configurations including the opening shape, end plate thickness, open ratio and opening direction on performance of the air-breathing HT-PEMFC were experimentally and numerically investigated. It was found that the cathode end plate thickness and upward or sideways orientation have the least effect on the performance. The maximum power density of 160 mW/cm² at the current density of 394 mA/cm² can be achieved for the cathode flow field with slot holes and an open ratio of 75%.     

Performance investigation on a novel 3D wave flow channel design for PEMFC

Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel.     

Computational fluid dynamics modeling of a solid oxide electrolyzer cell for hydrogen production

A 2D computational fluid dynamics (CFD) model was developed to study the performance of a planar solid oxide electrolyzer cell (SOEC) for hydrogen production. The governing equations for mass continuity, momentum conservation, energy conservation and species conservation were discretized with the finite volume method (FVM). The coupling of velocity and pressure was treated with the SIMPLEC (Semi-Implicit Method for Pressure Linked Equations – Consistent) algorithm. Simulations were performed to investigate the effects of operating/structural parameters on heat/mass transfer and the electric characteristics of a planar SOEC. It is found that the gas velocity at the cathode increases significantly along the main flow channel, as the increase in H₂ molar fraction decreases the density and viscosity of the gas mixture at the cathode. It is also found that increasing the inlet gas velocity can enhance the SOEC performance. Another important finding is that the electrode porosity has small effect on SOEC performance. The results of this paper provide better understanding on the coupled heat/mass transfer and electrochemical reaction phenomena in an SOEC. The model developed can serve as a useful tool for SOEC design optimization.     

Performance characteristics of air-breathing anion-exchange membrane direct ethanol fuel cells

An air-breathing direct ethanol fuel cell (DEFC) with an anion-exchange membrane (AEM) and Pt-free electrodes is designed and investigated. Particular attention is paid to studying the performance characteristics of the air-breathing AEM DEFC. Experimental results reveal that this air-breathing AEM DEFC yields a peak power density as high as 38 mW cm⁻² at room temperature, which is comparable to the conventional Pt-based proton exchange membrane direct methanol fuel cells (PEM DMFCs). The overshoot/undershoot behaviors of both the cell voltage and cell temperature are avoided in the air-breathing AEM DEFC due to the use of ethanol-tolerant cathode catalyst. It is also found that the cathode water flooding behavior occurs in this air-breathing AEM DEFC, thus lowering the cell performance.     

Numerical studies on an air-breathing proton exchange membrane (PEM) fuel cell

The objective of this article is to investigate the performance of an air-breathing proton exchange membrane (PEM) fuel cell operating with hydrogen fed at the anode and air supplied by natural convection at the cathode. Considering a dual-cell cartridge configuration with a common anode flow chamber, a comprehensive two-dimensional, non-isothermal, multi-component numerical model is developed to simulate the mass transport and electrochemical phenomena governing the cell operation. Systematic parametric studies are presented to investigate the effects of operating conditions, cell orientation and cell geometry on the performance. Temperature and species distributions are also studied to assist the understanding of the single cell performance for different conditions. It is shown that the cell orientation affects the local current density distribution along the cell and the average current density, particularly at lower cell voltages. The cell performance is shown to improve with increase of temperature, anode flow rate, anode pressure and anode relative humidity.     

Effect of breathing-hole size on the electrochemical species in a free-breathing cathode of a DMFC

A three-dimensional numerical model is developed to study the electrochemical species characteristics in a free-breathing cathode of a direct methanol fuel cell (DMFC). A perforated current collector is attached to the porous cathode that breathes the fresh air through an array of orifices. The radius of the orifice is varied to examine its effect on the electrochemical performance. Gas flow in the porous cathode is governed by the Darcy equation with constant porosity and permeability. The multi-species diffusive transports in the porous cathode are described using the Stefan–Maxwell equation. Electrochemical reaction on the surfaces of the porous matrices is depicted via the Butler–Volmer equation. The charge transports in the porous matrices are dealt with by Ohm's law. The coupled equations are solved by a finite-element-based CFD technique. Detailed distributions of electrochemical species characteristics such as flow velocities, species mass fractions, species fluxes, and current densities are presented. The optimal breathing-hole radius is derived from the current drawn out of the porous cathode under a fixed overpotential.     

Water flooding and two-phase flow in cathode channels of proton exchange membrane fuel cells

The water flooding and two-phase flow of reactants and products in cathode flow channels (0.8 mm in width, 1.0 mm in depth) were studied by means of transparent proton exchange membrane fuel cells. Three transparent proton exchange membrane fuel cells with different flow fields including parallel flow field, interdigitated flow field and cascade flow field were used. The effects of flow field, cell temperature, cathode gas flow rate and operation time on water build-up and cell performance were studied, respectively. Experimental results indicate that the liquid water columns accumulating in the cathode flow channels can reduce the effective electrochemical reaction area; it makes mass transfer limitation resulting in the cell performance loss. The water in flow channels at high temperature is much less than that at low temperature. When the water flooding appears, increasing cathode flow rate can remove excess water and lead to good cell performance. The water and gas transfer can be enhanced and the water removal is easier in the interdigitated channels and cascade channels than in the parallel channels. The cell performances of the fuel cells that installed interdigitated flow field or cascade flow field are better than that installed with parallel flow field. The images of liquid water in the cathode channels at different operating time were recorded. The evolution of liquid water removing out of channels was also recorded by high-speed video.     

A novel ribbed cathode polar plate design in piezoelectric proton exchange membrane fuel cells

Previous studies have indicated that a novel design for proton exchange membrane fuel cells with a piezoelectric (PZT) device, which is regarded as an actuator for pumping air onto the cathode channel, can offer better performance with higher current generation. These results indicate that piezoelectric proton exchange membrane fuel cells (PZT-PEMFCs) may compress more air into the catalyst layer and thus may enhance electrochemical reactions, resulting in higher current output. At the same time, produced water vapor is pumped out from the cathode channel during the compression process. Previous studies on PZT-PEMFCs without ribs showed the strong effect of ohmic and concentration losses. In this study, a shallow rib is chosen to reduce the aforementioned losses and pressure drop in the cathode channel. The rib design is an important parameter that can be used as the support for the membrane electrode assembly (MEA). A transient three-dimensional model is built to simulate and compare the performance of PZT-PEMFCs both with and without ribs. Water vapor, oxygen, and current density profiles in the PZT-PEMFC are studied in detail. The major operating parameters include the rib width and the PZT vibration frequency. Our results show that the ribbed cathode channel can reduce ohmic losses and double current generation. Moreover, at higher PZT vibration frequency (f = 64 Hz), an air-breathing PZT-PEMFC compresses more oxygen into the catalyst layer and thus enhances the electrochemical reaction, resulting in a higher current output (0.208 A cm⁻²).     

Numerical investigation of innovative 3D cathode flow channel in proton exchange membrane fuel cell

Two kinds of innovative 3‐dimensional (3D) proton exchange membrane fuel cell (PEMFC) cathode flow channel designs were proposed to improve the water removal on the surface of gas diffusion layer and enhance mass transfer between flow channel and gas diffusion layer. A validated 2‐phase volume of fluid model was used to investigate different water removal behaviors in flow channel. The optimal length of water baffle and other parameters of the proposed designs were determined. A validated 3D PEMFC performance model was adopted to assess the new designs. The results suggest that these 2 designs can improve PEMFC performance as to 9% when operating at the high current density because of the significant enhancement of mass transfer induced by air baffles.     

Analysis of membraneless fuel cell using laminar flow in a Y-shaped microchannel

In this study, we implemented a theoretical analysis for a novel microfluidic fuel cell that utilizes the occurrence of laminar flows in a Y-shaped microchannel to keep the separation of fuel and oxidant streams without turbulent mixing. The liquid fuel and oxidant streams enter the system at different inlets, and then merge and flow in parallel to one another through the channel between two electrodes without the need of a membrane to separate both streams. A theoretical model containing the flow kinetics, species transport, and electrochemical reactions within the channel and the electrodes is developed with appropriate boundary conditions and solved by a commercial CFD package. The performance of this novel fuel cell is analyzed by a systematic study with respect to some important physical factors and the geometric effect of channel size. Results indicate that the performance is primarily dominated by the mass transport to the electrodes especially at the cathode and could be raised significantly by using a high aspect ratio of cross-sectional geometry.     

Numerical study of optimized three-dimension novel Key-shaped flow field design for proton exchange membrane fuel cell

Key-shaped three-dimension (3D) flow field channel is designed to improve the performance and mass transfer of proton exchange membrane fuel cell (PEMFC). This study comprehensively analyses the impacts on the performance and mass transfer of the flow channel from multiple dimensions such as the size, shape, and placement of the blocks. In comparison with the conventional straight single flow field channel, the new channel with rectangular blocks can effectively improve performance by 30%. Semi-elliptical and quarter-elliptical blocks are designed to make forced convection and increase the diffusion area of oxygen. The results indicate that the flow velocity in the Z-axis direction can be increased to 0.08–0.2 m/s due to the narrow space formed by variable cross-sections. In conclusion, the Key-shaped design has a potential to improve the performance of mass transfer in the cathode channel, providing a new strategy for the development of flow field design in PEMFC field.