Electrocatalytic oxidation of aliphatic alcohols: Application to the direct alcohol fuel cell (DAFC)

The electrooxidation of some low molecular weight alcohols, such as ethanol, ethylene glycol and n-propanol, is discussed in terms of reaction mechanisms and catalytic activity of the anode material. Some examples of a single cell, using a proton exchange membrane (PEM) as electrolyte, are given to illustrate interesting results, particularly for the direct electrooxidation of ethanol. This alcohol may replace methanol in a direct alcohol fuel cell.     

Spreading and sorption of a droplet on a porous substrate

In this paper, a model for spreading and sorption of a droplet on a thick porous substrate is derived in the frame of lubrication theory. Flow in the saturated part of the porous material is governed by Darcy's law, assuming a discontinuous wetting front separating the saturated from the unsaturated regions. Numerical results are presented for spreading and sorption of droplets in their dependence on the material and process parameters for axisymmetric and two-dimensional configurations.     

液滴撞击微结构疏水表面的动态特性

对去离子水滴撞击不同几何尺寸显微结构方柱和方孔状疏水表面的动态特性进行了研究。结果表明：当液滴以不同速度撞击微方柱疏水表面时,液滴展现铺展和回缩过程,且随着韦伯数（We数）增大,最大铺展直径增大,并伴随卫星液滴出现,但到达最大铺展直径的时间一致;而当液滴以相同的速度（We数相同）撞击间距不同的微方柱疏水表面时,液滴的最大铺展直径随着间距的增大而减小,且铺展过程会液滴浸润状态变得不稳定,发生由Cassie向Wenzel状态的浸润转变。当微方柱间距较小时,液滴受到的黏附功越小,越易发生向Cassie状态的转变;液滴撞击微方孔疏水表面时,液滴以规则的圆环状向外铺展和回缩,最后呈现近似规则的椭球状,不会发生向Wenzel状态的浸润转变,利用建立的物理模型对前述现象进行了分析。     

波纹状基底上含不溶性活性剂液滴的铺展过程

针对波纹状基底上含不溶性活性剂液滴的铺展过程,采用润滑理论建立了液膜厚度和浓度演化模型,通过PDECOL程序数值求解了演化方程组,得到了液滴的铺展特性及基底结构参数的影响规律。研究表明:当液滴铺展进入中后期,Marangoni效应减弱,此时基底的作用范围相应增大,基底对液滴铺展过程的影响逐渐显著。与平整基底相比,波纹状基底上的最小液膜厚度明显降低,而铺展前沿处的子波数显著增多,子波波峰高度呈单驼峰形的模态变化;而且,增加波纹状基底的高度或减小波数具有加剧液滴铺展不稳定性的作用。     

The effect of methanol and ethanol cross-over on the performance of PtRu/C-based anode DAFCs

In the present work, the cross-over rates of methanol and ethanol, respectively, through Nafion^®-115 membranes at different temperatures and different concentrations have been measured and compared. The changes of Nafion^®-115 membrane porosity in the presence of methanol or ethanol aqueous solutions were also determined by weighing vacuum-dried and alcohol solution-equilibrated membranes. The techniques of anode polarization and adsorption stripping voltammetry were applied to compare the electrochemical activity and adsorption ability, respectively. To investigate the consequences of methanol and ethanol permeation from the anode to the cathode on the performance of direct alcohol fuel cells (DAFCs), single DAFC tests, with methanol or ethanol as the fuel, have been carried out and the corresponding anode and cathode polarizations versus dynamic hydrogen electrode (DHE) were also performed. The effect of alcohol concentration on the performance of PtRu/C anode-based DAFCs was investigated.It was found that ethanol shows lower cross-over rates than methanol through the Nafion^® membrane in spite of the higher membrane porosity resulted in presence of ethanol aqueous solutions. Furthermore, it was found that ethanol presents less negative effect on the cathode performance due to both its smaller permeability through Nafion^® membrane and its slower electrochemical oxidation kinetics over Pt/C cathode.     

Numerical study of drop spreading over saturated pores

Numerical study of the dual effect of pores in liquid spreading over porous surface (flat and spherical) whereby liquid movement is facilitated as well as restricted (visualised by Khanna and Nigam [Khanna and Nigam, Chem. Eng. Sci. 57, 3401–3405 (2002)]) is presented to improve the present understanding of wetting. Using the volume of fluid (VOF) method in a two‐dimensional solution domain, the influence of various parameters viz. drop volume, pore density, surface wettability and liquid properties on the liquid spreading over 2D pellets with saturated pores was numerically investigated. Simulation results were found to capture the key features of the liquid spreading over non‐porous and porous surfaces qualitatively. The variation of spread factor and the apex height of liquid film with time were in a good qualitative agreement with the concept of dual action of pores. The liquid spreading was observed to have a direct proportionality with the pore density, that is, higher the number of pores, better was the liquid spreading. It was also observed that the influence of the pores on the liquid spreading was reduced with decrease in the surface wettability. It is expected that this numerical analysis of the liquid droplet spreading over saturated porous surface will be useful for better understanding of the physics of drop and pore (saturated) interactions.     

Positioning the reference electrode in proton exchange membrane fuel cells: calculations of primary and secondary current distribution

The primary and secondary current distribution study indicates the geometry of a thin electrolyte in a proton exchange membrane (PEM) fuel cell has a direct relation to the measured electrode polarization, thus making the positioning of the reference electrode and ohmic compensation critical. The different kinetic overpotentials on the electrodes can also affect the potential distribution and therefore affect the measurement accuracy. The measurement error can be significant for the fuel cell system with different kinetic overpotentials and with electrode misalignment. The measurement error for both hydrogen and direct methanol fuel cells (DMFC) has been analyzed over the current density region with no mass transfer effects. By using two reference electrodes, the measurement error can be substantially decreased for both anode and cathode measurement in a direct methanol fuel cell, and for the cathode measurement in a hydrogen/air fuel cell.     

Non-isothermal effects of single or double serpentine proton exchange membrane fuel cells

Mathematical models on transport processes and reactions in proton exchange membrane (PEM) fuel cell generally assume an isothermal cell behavior for sake of simplicity. This work aims at exploring how a non-isothermal cell body affects the performance of PEM fuel cells with single and double serpentine cathode flow fields, considering the effects of flow channel cross-sectional areas. Low thermal conductivities of porous layers in the cell and low heat transfer coefficients at the surface of current collectors, as commonly adopted in cell design, increase the cell temperature. High cell temperature evaporates fast the liquid water, hence reducing the cathode flooding; however, the yielded low membrane water content reduces proton transport rate, thereby increasing ohmic resistance of membrane. An optimal cell temperature is presented to maximize the cell performance.     

直接甲醇燃料电池质子交换膜研究进展

直接甲醇燃料电池(DMFC)以其燃料来源丰富、储存方便、结构简单、安全等优点而日益受到广泛的关注。预计将在很多领域中能得到广泛的应用。过去,人们对DMFC做了很多研究,针对直接甲醇燃料电池中的质子交换膜(PEM)的阻甲醇性能方面的研究进展作如下评述。     

On the modeling of water transport in polymer electrolyte membrane fuel cells

Water management is a critical issue in polymer electrolyte membrane (PEM) fuel cells, and water transport through the membrane, catalyst layer and gas diffusion layer has significant impact on the cell performance and durability. In this study, the mechanism of water transport processes in PEM fuel cells has been analyzed through 3-D unsteady non-isothermal simulations, along with a comprehensive examination of various modeling approaches in literature. It is shown that the finite rates of sorption/desorption of water in membrane affect the cell current output and the cell response time. Water dissolved in the membrane should be taken as the proper mechanism of water formation in the cathode of practical PEM fuel cells. Capillary pressure and relative permeability have significant impact on the distribution of liquid water saturation and transport, implying a need for their determination under specific PEM fuel cell conditions.     

Effect of process parameters on droplet spreading behaviour over porous surface

Droplet spreading behaviour over a porous surface is a complex phenomenon, and is a basic component of many industrial processes, for example the spray coating process. The coating process has wide applications and this includes coating of urea fertilizer to produce slow release urea. The quality of coating film in such applications is affected by many factors, one of them being droplet spreading on the substrate. Droplet spreading behaviour is affected by process parameters such as viscosity, density, surface tension, impact velocity, porosity, etc. Droplet spreading on a porous surface involves penetration into the porous surface and spreading on the surface. Previously, the effect of individual process parameters has been studied. The current work aims at finding the interactive effect of process parameters on droplet spreading behaviour by using response surface methodology. The combined effect of liquid viscosity, impact velocity, and surface porosity has been studied on contact angle, spreading factor, and residual drop volume. The results show that minimum contact angle can be achieved with maximum impact velocity, minimum porosity, and minimal liquid viscosity. Similar behaviour was observed with droplet residual volume. Maximum spreading factor was attained at minimum viscosity and porosity while impact velocity was at maximum level.     

Performance of a direct methanol fuel cell

The performance of a direct methanol fuel cell based on a Nafion® solid polymer electrolyte membrane (SPE) is reported. The fuel cell utilizes a vaporized aqueous methanol fuel at a porous Pt–Ru–carbon catalyst anode. The effect of oxygen pressure, methanol/water vapour temperature and methanol concentration on the cell voltage and power output is described. A problem with the operation of the fuel cell with Nafion® proton conducting membranes is that of methanol crossover from the anode to the cathode through the polymer membrane. This causes a mixed potential at the cathode, can result in cathode flooding and represents a loss in fuel efficiency. To evaluate cell performance mathematical models are developed to predict the cell voltage, current density response of the fuel cell.     

Ethanol crossover and direct ethanol PEM fuel cell performance modeling and experimental validation

In the present work, a one-dimension, steady-state and single phase model is developed with the purpose of describing the mass transport within a PtRu/Nafion^®-115/Pt membrane-electrode assembly and the performance of a direct ethanol proton exchange membrane fuel cell (DE-PEMFC). The effect of the most important cell operating parameters on the ethanol crossover rate and the fuel cell performance is investigated. According to the results, in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover could pose serious problems to the DEFC operation. Moreover, it was pointed out that the ethanol crossover rate dependence on the ethanol feed concentration is an almost linear function presenting a maximum at about . A further increase of the ethanol feed concentration leads to a steep decrease of ethanol crossover rate. This behavior could be attributed to the membrane swelling which is responsible for the membrane volume fraction decrement. It was also found that by the aid of the same model the performance of a direct ethanol PEM fuel cell over three different anode catalysts can be predicted. A relatively good agreement between theory and experimental results related to both ethanol crossover rates and direct ethanol fuel cell performance was found.     

PEM燃料电池阴极中的液态水及其影响因素

阴极多孔介质中液态水的含量对PEM燃料电池阴极中的传质及其性能具有极其重要的影响。提出了一个二维、两相、稳态数学模型,研究PEM燃料电池阴极中两相水的传递及其对电池性能的影响。模型耦合了连续方程、动量方程和组分守恒方程,并将质子膜中的净水迁移通量作为边界条件之一来处理。通过实验的方法和数值模拟的方法,研究了电池操作压力和温度对电池性能的影响,同时验证了模型的有效性。模拟发现:提高操作压力和升高阴极加湿温度使电池阴极催化剂层(CTL)和扩散层(GDL)界面上的液态水含量大幅提高;升高阳极加湿温度,电池阴极CTL和GDL界面上的液态水含量变化不明显;而升高燃料电池的操作温度,阴极CTL和GDL界面上液态水的含量降低。     

Optimization of platinum dispersion in Pt–PEM electrodes: application to the electrooxidation of ethanol

Nafion® can be used as a solid polymer electrolyte in a PEM fuel cell. Direct platinization of the membrane was realized by chemical reduction of a platinum compound. The platinization procedure was modified to enhance the roughness factor and thus to improve the electrocatalytic activity towards ethanol electrooxidation. The Pt–PEM electrodes were characterized by TEM, atomic absorption analysis, cyclic voltammetry and their polarization curves for ethanol electrooxidation.     

Absorption of picoliter droplets by thin porous substrates

Absorption of picoliter (pL) droplets into porous substrates is studied experimentally and numerically. In the case of pL droplets, major phenomena involved in the interaction between droplet and porous media develop at different time scales: spreading and wetting at microseconds, absorption and wicking at milliseconds, and evaporation at seconds. Therefore, one can decouple these processes to minimize the complexity of the study. A high‐speed imaging system capable of 1 million frames per second is used to visualize individual droplets impacting, spreading, and imbibing on substrates. To simulate droplet dynamics, the governing equations for flow outside and inside porous media are proposed and solved using an in‐house developed computational fluid dynamics solver. The simulation results are in good agreement with the experimental data. The effect of drop impact velocity and fluid properties on final dot shape in the porous substrates is investigated through a series of parametric numerical studies. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1690–1703, 2017     

The liquid spreading on porous solids: Dual action of pores

Experimental observation of the dual effect of pores in liquid spreading over porous substrates, whereby liquid movement is facilitated as well as restricted is presented based on spreading of micro-liter-sized liquid drops on substrates that have saturated (filled) millimeter-sized pores. The drops were put on porous and nonporous parts of solid substrate. The substrate was then rotated in vertical direction and the resulting motion of drops was recorded by a video camera. The analysis of the recorded images revealed that depending on whether the drop edge is moving toward the pore or away from the pore, the pore acts as accelerator or brake for the drop edge. This dual nature of the saturated pores can be ascribed to the attraction between the liquid in the drop and the liquid inside the pore. Qualitative changes in the morphology of the drop as it slides over saturated pores are also presented to highlight the process. This dual effect of pores is expected to play a major role in processes such as flow through a trickle bed of porous catalyst where it manifests itself in increased wetting efficiencies as well as pronounced hysteresis [Khanna, R., Nigam, K.D.P., 2002. Partial wetting in porous catalyst: wettability and wetting efficiency. Chemical Engineering Science 57, 3401-3405; Maiti et al., 2004. Enhanced liquid spreading due to porosity. Chemical Engineering Science 59, 2817-2820; 2005. Trickle-bed reactors: Porosity induced hysteresis. Industrial and Engineering Chemistry Research, in press.].     

多孔有机笼在聚丙烯腈纳米纤维表面固载及其复合质子交换膜

质子交换膜（PEM）是质子交换膜燃料电池的核心部件,需具备选择性地快速传递质子的特性。多孔有机笼具有高比表面积、良好的化学稳定性和高吸水特性以及三维连通的质子传递路径,可提升PEM的质子传导性能。本文将多孔有机笼（CC3）原位固载到聚丙烯腈（PAN）纳米纤维表面,与Nafion复合制备了CC3/PAN-Nafion复合质子交换膜,对其结构和性能进行了研究,结果表明：CC3的固载改变了纤维的微观形貌,增加了纤维直径,使纳米纤维比表面积从9.57m²/g增加到113.6m²/g;将CC3/PAN引入复合膜显著提升了CC3/PAN-Nafion的热稳定性、吸水性、阻醇性以及质子传导性能,其中CC3/PAN-Nafion12在100%RH,80℃时质子传导率可达0.165S/cm,较Nafion膜提升了一倍。     

Optimization of platinum dispersion in Pt–PEM electrodes: application to the electrooxidation of ethanol

Nafion® can be used as a solid polymer electrolyte in a PEM fuel cell. Direct platinization of the membrane was realized by chemical reduction of a platinum compound. The platinization procedure was modified to enhance the roughness factor and thus to improve the electrocatalytic activity towards ethanol electrooxidation. The Pt–PEM electrodes were characterized by TEM, atomic absorption analysis, cyclic voltammetry and their polarization curves for ethanol electrooxidation.     

High-surface-area CoTMPP/C synthesized by ultrasonic spray pyrolysis for PEM fuel cell electrocatalysts

Ultrasonic spray pyrolysis (USP) was used to synthesize a high-surface-area CoTMPP/C catalyst for oxygen reduction reaction (ORR). SEM micrographs showed that the USP-derived CoTMPP/C consists of spherical, porous and uniform particles with a diameter of 2-5 μm, which is superior to that with a random morphology and large particle sizes (up to 100 μm) synthesized by the conventional heat-treatment method. BET results revealed that the USP-derived catalyst had a higher specific surface area (834 m² g⁻¹) than the conventional one. Cyclic voltammetric, rotating ring-disk electrode (RRDE) and H₂-air PEM fuel cell testing were employed to evaluate the USP-derived CoTMPP/C. The kinetic current density of the USP-derived catalyst at 0.7 V versus NHE was two times higher than that of the conventional catalyst. Compared to Pt/C catalyst, the USP-derived CoTMPP/C catalyst showed a strong methanol tolerance and a higher ORR activity in the presence of methanol. In a H₂-air PEM fuel cell with USP-derived CoTMPP/C as the cathode catalyst, the cell performance was much higher than that with conventional heat-treated CoTMMP/C as the catalyst.