Effect of cathode catalyst layer thickness on methanol cross-over in a DMFC

The effects of cathode catalyst layer (CCL) thickness on the detrimental effect of methanol cross-over in a direct methanol fuel cell (DMFC) under various operating conditions are studied. Three membrane electrode assemblies (MEAs) with different CCL thicknesses but identical catalyst loading and identical anode catalyst layer are used. The results show that, when a thicker CCL, approximately twice the thickness of the base case, is used, the fuel cell performance increases significantly. The increase in performance with a thicker CCL is attributed to the oxidation of the methanol crossed-over in part of the catalyst layer and leaving the rest of the catalyst layer free from methanol contamination, leading to mitigations of the effects of mixed potentials. The results of electrochemical impedance spectroscopy (EIS) show that the charge transfer resistance for the fuel cell with twice the thickness of CCL is 30% lower compared to that for the base case, indicating that the active catalyst area available for oxygen reduction reaction (ORR) is indeed greater. The results of the electrochemical active surface areas (ECA) show that without methanol contamination, the ECA of the thicker CCL is actually lower, indicating that the better performance and the lower charge transfer resistance are not caused by a higher original ECA, but a higher active area for ORR. A much thicker CCL, about 5 times of that for the base case, is also used and the cell performance is also higher than that for the base case. The experimental results show that there exists an optimum cathode catalyst layer thickness and the thickness of cathode catalyst layer has a significant effect on DMFC performance.     

Analysis of membraneless formic acid microfuel cell using a planar microchannel

A novel design of membraneless microfuel cell employing a planar microchannel has been investigated theoretically in this study. The fuel and oxidant are, respectively, formic acid and oxygen and both dissolved in dilute sulfuric acid solutions. Both liquid streams enter the planar microchannel and flow in parallel without the need of a membrane to separate them. A theoretical model is developed to simulate the species transport in both anode and cathode streams and the cell performance is analyzed accordingly by examining the effects of flow rate, concentration, and the geometric size of the system. The results show that the cell performance is mainly restricted by the high transport resistance in the cathode stream. It is found that the transport of oxygen to the cathode electrode can be improved significantly by using a higher flow rate or oxygen concentration, or a thicker cathode catalyst layer. However, the effectiveness of the flow rate and thickness of catalyst layer diminishes gradually which indicates that there exist optimal conditions of these parameters. The influences of thickness and length of the microchannel on cell performance are also examined in detail.     

Factors affecting the discharge lifetime of lithium-molten nitrate thermal battery cells using soluble cathode materials

The use of soluble cathode materials in molten nitrate electrolyte thermal battery cells presents several problems related to cathode material diffusion into the anolyte. A chemical reduction of the soluble cathode material by molten lithium anodes is observed to degrade cell performance. Factors affecting cell lifetime were found to include discharge rate and temperature, cell thickness, activation into load versus open circuit and the presence or absence of a Cl^– rich separator layer.     

Oxygen mass transport limitations at the cathode of polymer electrolyte membrane fuel cells

Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O₂/N₂ ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O₂/N₂ mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

直接甲醇液流燃料电池的二维两相模型

直接甲醇燃料电池(DMFC)的阴极水淹、甲醇渗透及贵金属催化剂的成本问题是DMFC商业化的主要障碍。直接甲醇液流燃料电池(DMRFC)使用Fe³⁺/Fe²⁺氧化还原电对取代DMFC阴极,克服了阴极的水淹、甲醇渗透和电池成本等问题。使用Comsol Multiphysics4.2a模拟软件,建立了一个DMRFC二维两相模型来预测电池性能,模拟结果显示增加阳极催化层厚度、减少阳极扩散层厚度和提高Fe³⁺浓度有利于提高电池的性能,但当阳极催化层厚度和甲醇浓度分别大于5×10^-5 m和1.41 mol/L时,电池性能并不能显著提高。     

Preparation of cost-effective Pt–Co electrodes by pulse electrodeposition for PEMFC electrocatalysts

Low loading platinum–cobalt (Pt–Co) cathode catalyst on a Nafion(Na⁺)-bonded carbon layer is fabricated by using galvanostatic pulse technique to show the advantage of electrodeposition for high utilization of catalyst in proton exchange membrane fuel cell (PEMFC). We observed that Pt–Co catalysts evenly exist on the surface of carbon electrode and its thickness is about 5.8 μm, which is four times thinner than conventional Pt/C. Improved single cell power performance of Pt–Co cathode catalysts with a ratio of 3.2:1 compared with Pt/C is clearly presented.     

催化层厚度对质子交换膜燃料电池性能影响的数值研究

采用一维稳态宏观均相模型,首次对不同阴极催化层（CCL）厚度的质子交换膜燃料电池（PEMFC）在全操作电流范围内的功率输出进行了计算和比较。在催化层中各相的体积比不变的前提下,催化层厚度以2种方式变化：一是变化单位面积催化层内的Pt载量;二是变化Pt在碳载铂（Pt/C）催化剂颗粒中的质量分数。2种方式的模拟结果均表明,催化层较厚的PEMFC具有较高的功率输出。但是从提高催化剂利用率角度考虑,第1种催化层厚度变化方式中催化层越薄催化剂利用率越高,第2种方式相反。在本研究设定的CCL参数范围内,最适宜厚度催化层在整个电流密度范围内都使PEMFC有最高的功率输出。     

The effect of magnetic field on the oxygen reduction reaction and its application in polymer electrolyte fuel cells

The effect of magnetic field gradients on the electrochemical oxygen reduction was studied with relevance to the cathode gas reactions in polymer electrolyte fuel cells. When a permanent magnet was set behind a cathode, i.e. platinum foil or Pt-dispersed carbon paper for both electrochemical and rotating electrode experiments and oxygen was supplied to the uphill direction of the magnetic field, electrochemical flux was enhanced and the current increased with increasing the absolute value of magnetic field. This magnetic effect can be explained by the magnetic attractive force toward O₂ gas. When magnet particles were included in the catalyst layer of the cathode and the cathode was magnetized, the current of oxygen reduction was higher than that of nonmagnetized cathode. A new design of the cathode catalyst layer incorporating the magnet particles was tested, demonstrating a new method to improve the fuel cell performance.     

Effects of hydrophobicity of the cathode catalyst layer on the performance of a PEM fuel cell

The effects of hydrophobicity of the cathode catalyst layer on the performance of a PEM fuel cell are studied. The surface contact angle is measured to understand the changes of the hydrophobicity of the cathode catalyst layer upon the addition of hydrophobic dimethyl silicone oil (DSO). The results show that the contact angle increases with the DSO loadings in the cathode catalyst layer ranging from 0 to 0.65 mg/cm². The subsequent electrochemical measurements of the fuel cells with various cathodes reveal that the addition of DSO in the cathode catalyst layer can effectively prevent the cathode flooding at high current density, thus leading to a much higher limiting current density and the maximum power density when compared to the fuel cell with a normal cathode. An optimal DSO loading in the cathode catalyst layer is found to be around 0.5 mg/cm² under the testing conditions in this work. The fuel cell with cathode loaded with 0.5 mg/cm² can reach the maximum power density of 356 mW/cm² in H₂/air (or 709 mW/cm² in H₂/O₂) at room temperature, which is around 2.5 times in H₂/air (or 1.8 times in H₂/O₂) of that with normal cathode. All of the results indicate that the hydrophobicity of the cathode catalyst layer plays a crucial role in the water management of the fuel cell. The possible function of the DSO on improved oxygen solubility for the oxygen starved cathode during flooding warrants some further investigation.     

Investigation on cathode degradation of direct methanol fuel cell

The cathode degradation of a direct methanol fuel cell (DMFC) was investigated after a 240 h discontinuous galvostatic operation at 80 °C. The catalyst coated membrane (CCM) and the cathode diffusion layer were not combined so as to isolate electrochemical and mass transport processes. It was indicated by the EDS and SEM tests that the loss of the cathode electrochemical surface area (ESA) was associated with the decays of the Pt/C catalyst and the interfacial contact. Furthermore, Ru crossover and higher methanol crossover resulting from the anode failure aggravated the degradation of the cathode. On the other hand, the change of the pore structure led to a higher wettability of the cathode microporous layer. Therefore, the oxygen transport was suppressed due to the decrease of hydrophobic passages.     

液相进样直接甲醇燃料电池性能研究

报道了用研制的Pt-Ru/C催化剂, 采用特殊工艺制备了膜电极, 并组装了直接甲醇质子交换膜单电池系统。考察了电极扩散层制备方法、催化剂层中催化剂、Teflon-C以及Nafion液的用量等电极制备工艺条件以及空气作为氧化剂对单电池性能的影响。结果表明：采用刷涂法制备电极扩散层比喷涂法好,催化剂层中催化剂的优化含量为0.6mg·cm-2,Teflon-C、Nafion液的最佳用量分别为0.3 mg·cm-2、0.5 mg·cm-2。当工作温度为80℃时,输出电压为0.3V,氧气作为阴极气体的输出电流密度为36mA·cm-2;而空气作为阴极气体的输出电流密度为22.5mA·cm-2。膜电极有效面积为9cm2的的液相进样直接甲醇/氧气燃料电池三电池电堆的最大功率为0.285W,此时输出电压为0.7V,输出电流为0.407A;而液相进样直接甲醇/空气三电池电堆的输出电压为0.635V,输出电流为0.252A时,最大功率为0.160W。     

采用常规条形流场的质子交换膜燃料电池阴极数值模拟(Ⅰ)模型建立

对采用常规条形流场的H₂-Air PEMFCs阴极建立了二维数学模型,模型的控制方程耦合了连续性方程、Darcy方程、电传导方程以及O₂和H₂O的对流-扩散方程,对氧的电化学还原反应过程采用Butler-Volmer方程描述.利用模型计算了阴极扩散层中电流密度、O₂和H₂O浓度、催化层界面上局部电流密度的分布,分析了采用常规条形流场时气体在阴极扩散层中的传递机制及各组分浓度分布的特点.     

结构参数对质子交换膜燃料电池阴极有序催化层的影响

建立了质子交换膜燃料电池阴极有序催化层的稳态数学模型,目的是研究催化层厚度、铂载量、电解质体积含量和碳载体直径等设计参数对催化层性能的影响。模型方程涉及质子、电子和氧气的传递以及电化学反应等过程。计算结果与实验数据吻合。模拟表明,一定范围内较薄的催化层有利于性能提高,但厚度太小反而不利;提高电解质体积含量和铂载量可以明显改进催化层工作特性,但存在优化值,高于此值,催化层性能迅速下降;较细的碳载体直径会适当改善催化层性能。     

质子交换膜燃料电池冷启动的数值模拟

冷启动是质子交换膜燃料电池（PEMFC）商业化所面临的挑战之一，在PEMFC冷启动实验中，通过中子成像技术已经观测到电池内部存在过冷水，因此本文模型重点考虑过冷水对电池冷启动性能的影响。通过引入结冰概率函数对过冷水结冰过程的随机性进行描述，从而建立了PEMFC冷启动的三维、瞬态和多相流动数学模型。基于该模型，研究电池阴极催化层中离子聚合物的体积分数和质子交换膜的厚度对电池冷启动性能的影响。研究结果表明，增加阴极催化层中离子聚合物的体积分数，可有效促进阴极催化层中的反应生成水向质子交换膜中进行扩散，从而充分利用膜内的储水空间；减少质子交换膜的厚度，能促进质子交换膜中的离聚物水向阳极催化层扩散，在大电流密度工况下可有效缓解阳极的“膜干”现象。     

微生物燃料电池阴极催化剂研究进展

针对微生物燃料电池中铂碳阴极催化剂的局限性,阐述了新材料催化剂的研究进展,列举了多种催化剂均表现出了良好的ORR性能,且成本低廉,解决了Pt/C阴极价格昂贵的问题,使微生物燃料电池的推广应用成为可能。     

Effect of cathode GDL characteristics on mass transport in PEM fuel cells

The effect of the content of the hydrophobic agent in the cathode gas diffusion layer (GDL) on the mass transport in the proton exchange membrane fuel cells (PEMFCs) was studied using mercury porosimetry, scanning electron microscopy, and electrochemical polarization techniques. The mercury intrusion data and SEM micrograph indicated that the hydrophobic agent alters the surface and bulk structure of the GDL, thereby controlling gas-phase void volume and liquid water transport. The electrochemical polarization curves were measured and quantitatively analyzed to determine the oxygen transport limitation both in the catalyst layer and the GDL. Evaluation of the parameter ζ, which represents the cathode GDL characteristics for liquid water transport, indicated that the optimized content of the hydrophobic agent and effective water management results from a trade-off between the hydrophobicity and the absolute permeability for faster water drainage.     

Modelling the PEM fuel cell cathode

Two models of the cathode of the proton exchange membrane fuel cell, a pseudohomogeneous film model and an agglomerate model, have been compared. The influence of different parameters on the shape of the polarization curves has been shown. Curves simulated by use of the two models and different values of oxygen permeability, effective conductivity and thickness of the active layer as well as thickness of the Nafion® film surrounding agglomerates have been presented and compared with the experimental results. On the basis of the simulations and the SEM study of the structure of the active catalyst layer it has been concluded that the agglomerate model is a better representation of the active catalyst layer than the pseudohomogeneous film model.     

Performance improvement of ultra-low Pt proton exchange membrane fuel cell by catalyst layer structure optimization

Reducing the loading of noble Pt-based catalyst is vital for the commercialization of proton exchange membrane fuel cell (PEMFC).However,severe mass transfer polarization loss resulting in fuel cell perfor-mance decline will be encountered in ultra-low Pt PEMFC.In this work,mild oxidized multiwalled carbon nanotubes (mMWCNT) were adopted to construct the catalyst layer,and by varying the loading of carbon nanotubes,the catalyst layer structure was optimized.A high peak power density of 1.23 W·cm 2 for the MEA with mMWCNT was obtained at an ultra-low loading of 120 μg·cm-2 Pt/PtRu (both cathode and anode),which was 44.7％ higher than that of MEA without mMWCNT.Better catalyst dispersion,low charge transfer resistance,more porous structure and high hydrophobicity of catalyst layer were ascribed for the reasons of the performance improvement.     

Effects of operational and structural parameters on cell voltage of industrial magnesium electrolysis cells

Electric field is the energy foundation of the electrolysis process and the source of the multiphysical fields in a magnesium electrolysis cell. In this study, a three-dimensional numerical model was developed and used to calculate electric field at the steady state through the finite element analysis. Based on the simulation of the electric field, the operational and structural parameters, such as the current intensity, anode thickness, cathode thickness, and anode-cathode distance (ACD), were investigated to obtain the minimum cell voltage. The optimization is to obtain the minimum resistance voltage which has a significant effect on the energy consumption in the magnesium electrolysis process. The results indicate that the effect of the current intensity on the voltage could be ignored and the effect of the ACD is obvious. Moreover, there is a linear decrease between the voltage and the thicknesses of the anode and cathode; and the anode-cathode working height also has a significant effect on the voltage.