自呼吸式直接甲醇燃料电池性能及其传质特性

针对有效面积为1 cm2的自呼吸式直接甲醇燃料电池（direct methanol fuel cell,DMFC）单电池,阳极采用燃料罐供液,将阴极侧集流体和夹具设计为一体式结构,并用自制的七合一膜电极组件对其进行测试,讨论了催化剂类型、扩散层材料、集流体结构等因素对其性能的影响,分析了电池内部的传质特性,优化了电池特别是其在中高电流密度条件下的性能。实验结果表明：采用Pt黑、Pt-Ru黑催化剂制作的自呼吸式DMFC能强化反应物的传质;采用碳布制作的膜电极更倾向于获得更高的极限电流密度;低电流密度时,因甲醇渗透电池电压随着甲醇浓度的增加而降低,但在中高电流密度下,电池性能随甲醇浓度的增大先升高后降低;平行集流体有利于阴阳极生成物的排出和反应物的传质,因此易获得较高的电池性能。     

High-loading Pt-alloy catalysts for boosted oxygen reduction reaction performance

To improve performance of membrane electrode assembly (MEA) at large current density region, efficient mass transfer at the cathode is desired, for which a feasible strategy is to lower catalyst layer thickness by constructing high loading Pt-alloy catalysts on carbon. But the high loading may induce unwanted particle aggregation. In this work, H-PtNi/C with 33% (mass) Pt loading on carbon and monodisperse distribution of 3.55?nm PtNi nanoparticles, was prepared by a bimodal-pore route. In electrocatalytic oxygen reduction reaction (ORR), H-PtNi/C displays an activity inferior to the low Pt loading catalyst L-PtNi/C (13.3% (mass)) in the half-cell. While in H₂-O₂ MEA, H-PtNi/C delivers the peak power density of 1.51?W·cm^?2 and the mass transfer limiting current density of 4.4?A·cm^?2, being 21% and 16% higher than those of L-PtNi/C (1.25?W·cm^?2, 3.8?A·cm^?2) respectively, which can be ascribed to enhanced mass transfer brought by the thinner catalyst layer in the former. In addition, the same method can be used to prepare PtFe alloy catalyst with a high-Pt loading of 36% (mass). This work may lead to a range of catalyst materials for the large current density applications, such as fuel cell vehicles.     

PEM fuel cell reaction kinetics in the temperature range of 23-120 °C

The performance of a Nafion 112 based proton exchange membrane (PEM) fuel cell was tested at a temperature range from 23 °C to 120 °C. The fuel cell polarization curves were divided into two different ranges based on current density, namely, <0.4 A/cm² and >0.4 A/cm², respectively. These two ranges were treated separately with respect to electrode kinetics and mass transfer. In the high current density range, a linear increase in membrane electrode assembly (MEA) power density with increasing temperature was observed, indicating the advantages of high temperature operation.Simulation based on electrode reaction kinetic theory, experimental polarization curves, and measured cathodic apparent exchange current densities all gave temperature dependent apparent exchange current densities. Both the calculated partial pressures of O₂ and H₂ gas in the feed streams and the measured electrochemical Pt surface areas (EPSAs) decrease with increasing temperature. They were also used to obtain the intrinsic exchange current densities. A monotonic increase of the intrinsic exchange current densities with increasing temperature in the range of 23-120 °C was observed, suggesting that increasing the temperature does promote intrinsic kinetics of fuel cell reactions.There are two sets of cathode apparent exchange current densities obtained, one set is for the low current density range, and the other is for the high current density range. The different values of cathode current densities in the two current density ranges can be attributed to the different states of the cathode Pt catalyst surface. In the low current density range, the cathode catalyst surface is a Pt/PtO, and in the high current density range, the catalyst surface becomes pure Pt.     

An approximate model for mass transfer with reaction inporous gas diffusion electrodes

An approximate model for reactant mass transfer in fuel cell electrodes has been formulated which considers gaseous diffusion in the hydrophobic Teflon phase, diffusion across a thin electrolyte film at the surface of the catalyst phase, and internal diffusion with first order reaction in the electrolyte-filled catalyst phase. An analytical solution is presented which indicates the fraction of the “activation only” current density that is obtained from the electrode. Application of this model for low reactant concentrations should allow a more quantitative evaluation of important mass transfer processes in fuel cell electrodes.     

Modeling of the anode side of a direct methanol fuel cell with analytical solutions

In this work, analytical solutions were derived (for any methanol oxidation reaction order) for the profiles of methanol concentration and proton current density, by assuming diffusion mass transport mechanism, Tafel kinetics, and fast proton transport in the anodic catalyst layer of a direct methanol fuel cell. An expression for the Thiele modulus that allows to express the anodic overpotential as a function of the cell current and kinetic and mass transfer parameters was obtained. For high cell current densities, it was found that the Thiele modulus (?²) varies quadratically with cell current density; yielding a simple correlation between anodic overpotential and cell current density. Analytical solutions were derived for the profiles of both local methanol concentration in the catalyst layer and local anodic current density in the catalyst layer. Under the assumptions of the model presented here, in general, the local methanol concentration in the catalyst layer cannot be expressed as an explicit function of the position in the layer. In spite of this, the equations presented here for the anodic overpotential allow the derivation of new semi-empirical equations.     

生物燃料电池酶电极的研究进展

综述了生物燃料电池酶电极的研究进展,尤其是近年来在氧化还原酶的种类、电子介体电极、直接电子传递电极以及固定化酶等方面的研究成果。从提高生物燃料电池的转换效率出发,分析各因素对酶电极性能的影响,包括针对不同底物燃料使用相应的氧化还原酶实现电极之间的电子传递、小分子或聚合物中介体存在下提高电流密度、导电聚合物等修饰电极对直接电子传递效率的贡献,以及物理或化学的酶固定化方法增加酶的稳定性等。因此采用新材料及新工艺构筑酶电极,最大程度上保持酶的活性,提高载酶量及电子传递效率,将成为该领域未来的发展方向。     

Carbon aerogel supported Pt-Ru catalysts for using as the anode of direct methanol fuel cells

A carbon aerogel (CA) loaded with platinum nanoparticles can achieve good catalytic performance in proton exchange membrane fuel cells. Pt-Ru bimetallic nanoparticles were loaded onto a carbon aerogel through a simple process. The PtRu/CA achieved good cell performance when used as a direct methanol fuel cell anode catalyst. The advantages of carbon aerogel may be attributed to the mesopore structure that can facilitate the mass transportation in the electrode. The Ru content in the catalyst has a great influence on its performance. The PtRu/CA with 25 at.% Ru achieves the best cell performance at 30 °C.     

Balance Equations for Molecular Distillation

Abstract

We have derived balanced equations for continuous molecular (short-path) distillation of a binary mixture in a falling film evaporator. The relation of heat and mass transfer in liquid films on both the evaporating and condensing surfaces were taken into account, as well as the mass transfer in the vapor phase in the distillation gap. Individual balance equations were coupled by boundary conditions into a closed set of partial differential equations. By introducing the streams, we have transformed partial differential equations into the simple form of ordinary differential equations. These were solved numerically or approximately for some selected problems in isothermal (e.g., for turbulent film on evaporation surface) and nonisothermal regimes and for the influence of reevaporation from the condenser. These developed mathematical formulations can be used to model various operational situations in a molecular evaporator or to evaluate the influence of some parameters on the molecular distillation process.     

Evaluation of methods to increase the oxygen partial pressure in PEM fuel cells

This paper compares the performance of a hydrogen–air fuel cell system with the oxygen electrode operating under different conditions of pressure, stoichiometry and oxygen enrichment. This paper shows that the net power density can be improved using a pressurized or oxygen enrichment system when the oxygen electrode is limited by oxygen mass transfer. If the current density is determined by kinetics, then the ambient pressure system has a higher net power density at the same fuel efficiency.     

Uniform Thin Film Electrodeposition onto Large Circular Wafer Substrates

A wafer-scale electrochemical flow cell specially suited for uniform thin-film electrodeposition is presented. The cell consists of a porous disk injector set parallel and coaxial to a working disk electrode with a narrow separation gap between them. The porous injector was designed to supply a uniform axial flow of electrolyte onto the surface of the working disk electrode. The mass transfer characteristics of the cell and the current distribution on the working disk electrode were studied for various operating conditions, geometric parameters and bath chemistries. The experimental results confirmed the simple scaling properties that were theoretically predicted. It has also been shown that this cell can provide nearly uniform current distribution on the working electrode under conditions of both migration and convective-diffusive mass transfer control. The results indicate that this cell is suitable for uniform thin-film electrodeposition onto large circular wafer substrates.     

Solution of nonlinear boundary value problems—VII A novel method: differentiation with respect to boundary condition

A simple numerical method for construction of the dependence of solutions to nonlinear boundary value problem on a paremeter will be developed. The set of differential equations is differentiated with respect to the boundary condition chosen and the resulting partial differential equations are solved by a finite-difference method. The method is illustrated by an example of heat and mass transfer in a porous catalyst.     

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.     

固体氧化物燃料电池中的阳极电化学过程数学模拟研究

在质量守恒、动量守恒和能量守恒定律及Butler-Voulmer方程组的基础上,加上边界条件和初始条件,通过数学模型对SOFC中的阳极催化层内部的燃料反应气体的气相扩散及产物的气相扩散的基本动态规律进行了描述。其偏微分方程只能通过数值计算求解,而无法得到解析解。增大阳极孔隙率ε可提高多孔电极中的有效气体扩散系数。当阳极较薄时,阳极的总极化电阻与单位体积内的电化学活性区的面积A成反比,增大阳极的电化学活性区的面积有利于降低其总极化电阻。该数学推演结论对阳极的优化制备具有重要的参考价值。     

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.     

直接利用生物质的化学燃料电池研究进展

近些年燃料电池技术有了长足的发展,利用燃料电池处理废弃生物质并产电是一种新型途径,可以达到废物处理、能源回收的目的。然而,受限于燃料种类、电池性能、产物分离等因素,传统的燃料电池难以直接用于处理废弃生物质。本文首先针对中低温燃料电池如碱性燃料电池、质子交换膜燃料电池的研究现状进行了综述,结果表明,碱性燃料电池在以小分子有机物作为燃料时性能良好,但是容易受到产物CO₂酸化影响;液相催化燃料电池在催化剂耐受性、生物质处理、电池功率密度等方面表现出优异的性能。然后介绍了电催化剂如过渡金属氧化物、多酸等研究现状,此类催化剂具有较强的氧化性、布朗斯特酸性和路易斯酸性等,具有很强的催化分解生物质的能力,针对液相催化剂不易分离的局限,介绍了催化剂固载化、纳米复合材料等研究进展。之后介绍了电极材料和膜材料的研究进展,碳极板因其综合性能和成本成为当前的主流选择,全氟磺酸膜性能优异,成为实验探究应用的理想材料,同时对一些复合材料的研究现状进行了简要介绍。最后,对化学燃料电池应用于生物质处理的方向进行了展望,液相催化燃料电池综合性能突出,在可处理生物质种类、催化剂循环等问题进一步优化之后,有望成为一种废弃生物质处理的新途径。     

Patterned catalyst layer boosts the performance of proton exchange membrane fuel cells by optimizing water management

Mass transport is crucial to the performance of proton exchange membrane fuel cells, especially at high current densities. Generally, the oxygen and the generated water share same transmission medium but move towards opposite direction, which leads to serious mass transfer problems. Herein, a series of patterned catalyst layer were prepared with a simple one-step impressing method using nylon sieves as templates. With grooves 100 μm in width and 8 μm in depth on the surface of cathode catalyst layer,the maximum power density of fuel cell increases by 10% without any additional durability loss while maintaining a similar electrochemical surface area. The concentration contours calculated by finite element analysis reveal that the grooves built on the surface of catalyst layer serve to accumulate the water nearby while oxygen tends to transfer through relatively convex region, which results from capillary pressure difference caused by the pore structure difference between the two regions. The separation of oxidant gas and generated water avoids mass confliction thus boosts mass transport efficiency.     

Numerical optimization of proton exchange membrane fuel cell cathodes

A fuel cell gradient-based optimization framework based on adaptive mesh refinement and analytical sensitivities is presented. The proposed approach allows for efficient and reliable multivariable optimization of fuel cell designs. A two-dimensional single-phase cathode electrode model that accounts for voltage losses across the electrolyte and solid phases and water and oxygen concentrations is implemented using an adaptive finite element formulation. Using this model, a multivariable optimization problem is formulated in order to maximize the current density at a given electrode voltage with respect to electrode composition parameters, and the optimization problem is solved using a gradient-based optimization algorithm. In order to solve the optimization problem effectively using gradient-based optimization algorithms, the analytical sensitivity equations of the model with respect to the design variables are obtained. This approach reduces the necessary computational time to obtain the gradients and improves significantly their accuracy when compared to gradients obtained using numerical sensitivities. Optimization results show a substantial increase in the fuel cell performance achieved by increasing platinum loading and reaching a Nafion mass fraction around 20-30 wt.% in the catalyst layer.     

Concentration and potential distributions in the active layer of proton exchange membrane fuel cell electrodes

This work is focussed on modelling mass and charge transfer limitations within an active layer considering uniform distribution of catalyst phase (classical model) or a more realistic discrete distribution (modified model). A model is proposed here based on soft-coupled equations describing diffusion, ionic ohmic drop and interfacial charge. It is applied to the practical case of oxygen reduction or hydrogen oxidation for PEM fuel cells. Simulation shows that the modified model has to be used for fast kinetics, that is, when the local limitations become predominant. In contrast, the classical flooded homogeneous model remains suitable when mass and charge transport resistances are negligible at the particle level.     

质子交换膜燃料电池高稳定性低铂载量膜电极的研究进展

质子交换膜燃料电池由于高能量转化率、零污染、低温启动等优点在新能源领域备受关注,但其成本和耐久性仍是本领域的挑战性课题。本文首先回顾了近年来国内外研究者在降低燃料电池成本和提高其耐久性方面取得的成就,从催化剂制备技术、膜电极结构优化、耐久性提升三个方面介绍了近年来国内外在降低膜电极铂载量、提高膜电极功率密度和耐久性方面的发展趋势,通过构筑铂基合金、核壳结构和纳米结构等催化剂能有效地降低铂载量,从而降低燃料电池成本;通过构筑多孔结构催化层或气体扩散层可以改善膜电极的微结构,从而提高电池的功率密度;通过开发新型质子交换膜、更换催化剂载体等方法可以提高膜电极的耐久性。最后,本文针对目前研究进展阐述提高膜电极稳定性仍然是目前的研究难题,并对未来的研究方向进行了展望。     

Applying Shape‐Controlled Pt Nano‐dendrites Supported on Carbon for Membrane‐Electrode Assembly in a Proton Exchange Membrane Fuel Cell

Platinum nano‐dendrites of various sizes were applied as cathode catalysts in a proton exchange membrane fuel cell (PEMFC). The membrane electrode assembly (MEA) fabricated with Pt nano‐dendrites showed very high ORR activity at high potential ranges due to lower activation overpotential. But it showed the cell performance only comparable to commercial Pt/C due to Ohmic and mass transfer resistance at high current density ranges. These results demonstrate that both high intrinsic activity of the catalysts and the formation of three‐phase interface with efficient proton, electron, and mass transfer should be considered together when shape‐controlled metal nanoparticles are used as cathode catalysts in a PEMFC.