Electrochemical and gas evolution characteristics of direct methanol fuel cells with stainless steel mesh flow beds

Carbon dioxide gas management and bipolar plate/(flow bed) design are important to the development of direct methanol fuel cell (DMFC) systems. If the gas produced at the anode is not removed rapidly and efficiently a gradual deterioration in electrical performance can occur. This paper examines the feasibility of using stainless steel mesh materials as flow beds for the DMFC. A flow visualization study, using a high-speed video camera and appropriate computer software, of the anode side, carbon dioxide gas evolution and flow behaviour with flow beds based on stainless steel mesh is reported. The electrochemical behaviour of the direct methanol fuel cell with stainless steel flow beds is also reported. A number of the flow bed designs, based on stainless steel mesh, showed promising behaviour in terms of gas removal characteristics and electrical performance.     

Experimental and Numerical Study of Serpentine Flow Fields for Improving Direct Methanol Fuel Cell Performance

Methanol crossover is an important issue as it affects direct methanol fuel cell (DMFC) performance. But it may be controlled by selecting a proper flow field design. Experiments were carried out to investigate the effect of single, double and triple serpentine flow field configurations on a DMFC with a 25 cm² membrane electrode assembly (MEA) with a constant open ratio. A three dimensional model was also developed for the anode of the DMFC to predict methanol concentration and cell current density distributions. Experimental and model results show that at lower methanol concentrations (0.25–0.5M), single serpentine flow field (SSFF) provides high peak power density, while a double serpentine flow field (DSFF) gives high peak power density at a high methanol concentration (1–2M). Single and double serpentine flow fields exhibit the same peak power density (33 mW cm⁻²) at 1M. But the cell efficiency of double serpentine flow field is 12.5% which is 3.5% point greater than single serpentine flow field. This is attributed to reduced mixed potential. triple serpentine flow field (TSFF) shows the lowest peak power density and cell efficiency, which is attributed to high mass transfer resistance.     

Performance Tests and Pressure Drop Measurements in the Anode Flowfield of a μDMFC

Cell performance tests and measurements of the pressure drops in the anode flow channels of a custom‐made microdirect methanol fuel cell (μDMFC) are conducted and studied for different methanol concentrations (0.5–2 M), flow rates (10–20 sccm) and operating temperatures (40–80 °C). The anode flowfields consist of three channel/four pass flow channels with widths of 500–2000 μm and a total length of 300–400 mm. Moreover, flow characteristics of the CO₂ gas bubbles and methanol solution in the anode flow channels are identified and analysed for CO₂ fraction through visualisation. Finally, an optimal channel size for the present μDMFC is obtained.     

Water flooding and pressure drop characteristics in flow channels of proton exchange membrane fuel cells

Water flooding of the flow channels is one of the critical issues to the design and operation of proton exchange membrane fuel cells (PEMFCs). The liquid water and total pressure drop characteristics both in the anode and cathode parallel flow channels of an operating PEMFC were experimentally studied. The gas/liquid two-phase flow both in the anode and cathode flow channels was observed, and the total pressure drop between the inlet and outlet of the flow field was measured. The effects of cell temperature, current density and operating time on the total pressure drop were investigated. The results indicated that the total pressure drop in the flow channels mainly depends on the resistance of the liquid water in the flow channels to the gas flow, and the different flow patterns distinguish the total pressure drops in the flow field. Clogging by water columns result in a higher pressure drop in the flow channels. The total pressure drop measurement can be considered as an in situ diagnoses method to characterize the degree of the flow channels flooding. The liquid water in the cathode flow channels was much more than that in the anode flow channels. The pressure drop in the cathode flow channels was higher than that in the anode flow channels. During the fuel cell operation, the cell performance decreased gradually and the pressure drop both in the anode and the cathode flow channels increased. The rate of flooding at the cathode side reached 49.56% under experimental conditions after 78 min of operation. However, it was zero at the anode side.     

The Design and Usage of a Visual Direct Methanol Fuel Cell

In order to better understand the influence of gas evolution on the performance of the direct methanol fuel cell (DMFC) anode, a visual DMFC, comprising of a transparent anode and a cathode endplate with an integrated heat exchanger, and a picture analysis methodology were developed. The result was an inexpensive, but very powerful, tool for analyzing the role of two-phase flow. An important finding is that gas bubbles do not appear uniformly throughout the fluid flow matrix, but rather only at a few active sites. Another important finding is that the gas saturation (volume fraction of gas/volume fraction of liquid) increases along the streamwise direction.     

The role of under-rib convection in mass transport of methanol through the serpentine flow field and its neighboring porous layer in a DMFC

Numerical simulation of a direct methanol fuel cell (DMFC) operating under discharging conditions is challenged by the difficulties in modeling of complicated liquid-gas two-phase flows and coupled electrochemical kinetics. Under open-circuit conditions, the net electrochemical reactions in the DMFC anode cease, but, owing to the methanol concentration difference between the anode and cathode, the mass transport of methanol remains, creating a mass transport process of methanol in a single-phase liquid flow with no electrochemical reactions in the DMFC anode. Consequently, an accurate simulation of mass transport of methanol under such open-circuit conditions becomes possible. In this work, we performed a 3D numerical simulation of mass transport of methanol in the DMFC anode under open-circuit conditions and obtained the mass flux of methanol through the porous layer for different values of permeability. We also measured the mass flux of methanol permeation from the anode flow field to the cathode under open-circuit conditions. The comparison between the numerical and measured mass flux of methanol made it possible to in situ determine the permeability of the typical commercial porous layer. Using this in situ determined permeability, we then investigated numerically the effect of methanol feed rates on mass transport and found that the in-plane under-rib convection plays an important role, even at low methanol feed rate, to make the reactant evenly distributed over the entire catalyst layer.     

微型直接甲醇燃料电池阳极传质分析

针对微型直接甲醇燃料电池,将阳极流场板简化为规则结构的多孔介质,运用多孔介质理论建立了包括流场板在内的阳极传输模型。模型考虑了阳极流道内液体饱和度沿流动方向的变化、催化层的厚度以及甲醇渗透,计算并讨论了阳极流道内液体饱和度的分布和流量对电池电流密度的影响,分析了阳极过电位对甲醇浓度分布和电池性能的影响以及质子交换膜内的传质特性。     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

Three-dimensional two-phase mass transport model for direct methanol fuel cells

A three-dimensional (3D) steady-state model for liquid feed direct methanol fuel cells (DMFC) is presented in this paper. This 3D mass transport model is formed by integrating five sub-models, including a modified drift-flux model for the anode flow field, a two-phase mass transport model for the porous anode, a single-phase model for the polymer electrolyte membrane, a two-phase mass transport model for the porous cathode, and a homogeneous mist-flow model for the cathode flow field. The two-phase mass transport models take account the effect of non-equilibrium evaporation/ condensation at the gas-liquid interface. A 3D computer code is then developed based on the integrated model. After being validated against the experimental data reported in the literature, the code was used to investigate numerically transport behaviors at the DMFC anode and their effects on cell performance.     

Portable Size DMFC‐Stack

A small, low temperature, direct methanol fuel cell stack for portable applications has been developed. Several flow field designs were investigated with respect to stable operation and high performance. Due to carbon dioxide and water production on the anode and cathode, respectively, methanol and oxygen access to the electrodes is hindered. During single cell operation the effect of both carbon dioxide evolution and water production on the current output was observed. The difference between parallel and serial feeding of both fuel and oxidant to the DMFC stack was also investigated. It was found that it is very important to remove reaction products from the active cell surface in order to ensure stable stack operation at low temperatures. The maximal power realised with the 12‐cell direct methanol fuel cell stack was 30 W.     

CFD Simulation of Liquid Film Flow on Inclined Plates

A two‐phase flow CFD model using the volume of fluid (VOF) method is presented for predicting the hydrodynamics of falling film flow on inclined plates, corresponding to the surface texture of structured packing. Using the proposed CFD model the influence of the solid surface microstructure, liquid properties and gas flow rate on the flow behavior was investigated. From the simulated results it was shown that under the condition of no gas flow the liquid flow patterns are dependent on the microstructure of the plates, and proper microstructuring of the solid surface will improve the formation of a continuous liquid film. It was also found that liquid properties, especially surface tension, play an important role in determining the thin‐film pattern. However, there are very different liquid film patterns under the action of gas flow. Thinner liquid films break easily, but thicker liquid films can remain continuous even at higher gas flow rates, which demonstrates that all factors affecting the liquid film thickness will affect the liquid film patterns under conditions of counter‐current two‐phase flow.     

铝电解槽内电解质运动的数值模拟

通过适当的简化,建立了铝电解槽内电解质运动的物理模型和数学模型,以商业CFD软件为平台,分别对电磁力作用、阳极气体作用以及电磁力和阳极气体共同作用3种情况下的电解质运动进行了数值模拟. 研究结果表明,电磁力作用时,电解质以水平运动为主,半槽的电解质流场基本上呈现一个大涡的结构;阳极气体作用时,电解质运动主要是以每个阳极周围的小循环为主;电磁力和阳极气体共同作用时,电解质流场与阳极气体作用时几乎完全相同,这说明阳极气体对电解质流场起主要作用.     

CO_2气泡脱离扩散层孔口过程的数值模拟

在直接甲醇燃料电池(DMFC)中,阳极催化层表面反应生成的CO2气体通过扩散层,及时排出阳极通道,对提高DMFC电流密度具有重要意义,因此研究气泡脱离孔口的过程很有益。今采用Fluent6.2.16对CO2气泡脱离扩散层孔口过程、两孔时气泡形成及聚并过程进行了数值模拟,考察了阳极通道内液体流速、扩散层孔道直径等因素对气泡脱离的影响。结果表明,阳极通道内液体流速越大,气泡脱离扩散层孔口所需的时间越短;扩散层孔道直径越大,气泡脱离扩散层孔口所需的时间越长,且生成的气泡越大;由于从相邻两扩散层孔道出来的气泡的阻挡和挤压作用,使得两气泡周围的压力分布与单气泡不同,气泡脱离过程与从单个扩散层孔口的脱离过程有所不同,脱离时间更早。     

Direct methanol fuel cell (DMFC): analysis of residence time behaviour of anodic flow bed

This paper studies the residence time behaviour and concentration distribution in a simplified rhomboidal DMFC anode flow bed by 3D numerical flow simulations and by experimental measurements. The rhomboidal DMFC anode flow bed and the applied volume flow are discussed with regard to data given in the literature. Simulations with CFX, based on Finite Volume Method, and MooNMD, based on Finite Element Method, show strongly similar results and the reliability of the computed residence time distributions (RTD) is proved by showing that they depend only slightly on parameters of the numerical schemes applied. The realisation of the RTD and concentration distribution measurements are described. Experimentally obtained RTD results are in good agreement with the numerical simulations. Also, the experimentally obtained concentration distribution inside the anode flow bed is very similar to the computed distribution. By analysing the RTDs and concentration distributions, the obtained results provided evidence of weaknesses of the flow bed design.     

Effect of anode and cathode flow field depths on the performance of liquid feed direct methanol fuel cells (DMFCs)

The effect of the anode and cathode flow field depths on the performance of a single cell Direct methanol fuel cell (DMFC) of 45 cm² active area were experimentally investigated. Double serpentine flow fields (DSFFs) with varying channel depth namely, 0.2, 0.4, 0.6, 0.8, and 1 mm but with fixed channel and rib width each of 1 mm on both anode and cathode were designed, fabricated, and tested. The experimental study involved measurement of pressure drops across anode and cathode flow field plates, polarization, and carbon dioxide concentration measurements at various current densities. The mass transport at both anode and cathode were found to increase with increase in pressure drop across the flow field on account of reduced channel depth from 1.0 to 0.4 mm at all current densities. However, further decrease to a channel depth of 0.2 mm was found to be counter-productive with different phenomena operating on either side viz., increased CO₂ slug length on the anode flow channel and increased methanol crossover on the cathode side. Hence, the maximum performance for DMFCs was observed for a channel depth of 0.4 mm on anode and cathode flow fields. A decrease in flow field channel depth at cathode was found to increase the methanol crossover due to convective mass transfer effect.     

Effect of Serpentine Multi‐pass Flow Field Channel Orientation in the Liquid Water Distributions and Cell Performance

A commercial 50 cm² polymer electrolyte membrane (PEM) fuel cell with serpentine flow fields was operated at 2.0 bar and 60 °C with two orientations of the flow field channels with respect to gravity, i.e. horizontal and vertical channels. A 3 × 3 test matrix of anode and cathode reactants relative humidity was used for the performance assessment of the cell in both orientations. The cell performance and operating data, including cell voltage and resistance, were measured, and neutron radiographs were recorded during the entire operation in order to gain knowledge of the liquid water distributions within the cell for both orientations. A quantitative analysis of the results is presented in this work, comparing the cell operation for both flow field orientations. It is observed that the configuration with horizontal cathode flow field channels presents a better cell performance, and less amount of liquid water blocking the flow field channels. Thus, the results show that the selection of the cell orientation has an influence on the final performance, and it is therefore, a design parameter to be considered for a real application. The differences in the cell water content are quantitatively analyzed and discussed.     

A two-dimensional, two-phase mass transport model for liquid-feed DMFCs

A two-dimensional, isothermal two-phase mass transport model for a liquid-feed direct methanol fuel cell (DMFC) is presented in this paper. The two-phase mass transport in the anode and cathode porous regions is formulated based on the classical multiphase flow in porous media without invoking the assumption of constant gas pressure in the unsaturated porous medium flow theory. The two-phase flow behavior in the anode flow channel is modeled by utilizing the drift-flux model, while in the cathode flow channel the homogeneous mist-flow model is used. In addition, a micro-agglomerate model is developed for the cathode catalyst layer. The model also accounts for the effects of both methanol and water crossover through the membrane. The comprehensive model formed by integrating those in the different regions is solved numerically using a home-written computer code and validated against the experimental data in the literature. The model is then used to investigate the effects of various operating and structural parameters, such as methanol concentration, anode flow rate, porosities of both anode and cathode electrodes, the rate of methanol crossover, and the agglomerate size, on cell performance.     

In Situ Investigation of Two-Phase Flow Patterns in Flow Fields of PEFC’s Using Neutron Radiography

A novel in situ method for the investigation of the two-phase flow patterns occurring in the flow fields of polymer electrolyte fuel cells (PEFC’s) has been developed. In these experiments, the gas / liquid two-phase flow in hydrogen fuel cells as well as in direct methanol fuel cells (DMFC’s) has been studied. Unique results and the promising use of neutron radiography for PEFC research are shown. Its current limitations but also future developments and opportunities are discussed.     

Gas evolution and power performance in direct methanol fuel cells

The use of acrylic cells and a CCTV camera for visually investigating the carbon dioxide gas evolution process inside an operating direct methanol fuel cell environment is demonstrated. Also, the effect of operating parameters on the system gas management, using a series of tests with different gas diffusion layer supporting materials, flow bed designs, cell sizes and exhaust manifold configurations, is studied. Carbon dioxide gas management is an important issue obstructing progress in viable direct methanol fuel cell systems development. Gas evolution mechanisms and gas management techniques are discussed and analysed with reference to several video picture and performance data. The data demonstrate that Toray carbon paper is not a suitable material for DMFCs due to its poor gas removal properties. A type carbon cloth shows relatively good gas removal behaviour. Increasing the liquid phase inlet flow rate is beneficial for gas removal. Increasing the current density results in higher gas production and in the formation of gas slugs, especially at low flow rates, which can lead to blocking of the channels and hence deterioration in the cell performance. A new flow bed design, based on a heat exchanger concept, is affective for gas management and gives a more uniform flow distribution in the flow bed channels. Using the results of this study, and the modelling techniques developed by our group, will are able to determine suitable operating conditions for our prototype 0.5kW cell DMFC stack.     

Systematic analysis of the direct methanol fuel cell

The dynamic operating behaviour of the direct methanol fuel cell (DMFC) is governed by several physico-chemical phenomena which occur simultaneously: double layer charging, electrode kinetics, mass transport inside the porous structures, reactant distributions in the anode and cathode flowbeds etc. Therefore it is essential to analyse the interactions of these phenomena in order to fully understand the DMFC. These phenomena were initially analysed independently by systematic experiments and model formulations. Electrode kinetics were determined by fitting models of varying complexity to electrochemical impedance spectroscopy (EIS) measurements. Reaction intermediates adsorbed on the catalyst seem to play a key role here. To describe mass transport across the DMFC a one-dimensional model was formulated applying the generalised Maxwell–Stefan equations for multi-component mass transport and a Flory–Huggins model for the activities of mobile species inside the membrane (PEM). Also swelling of the PEM as well as heat production and transport were considered. Finally, the anode flowbed was analysed by observing flow patterns in different flowbed designs and measuring residence time distributions (RTDs). Detailed CFD models as well as simpler CSTR network representations were used to compare to the experimental results. Even the simpler models showed good agreement with the experiments. After these investigations the results were combined: the electrode kinetics model was implemented in the mass transport model as well as in the CSTR network flowbed model. In both cases, good agreement, even to dynamic experiments, was obtained.