On efficiency of both single fuel cells and stacks I

A method to estimate the efficiency of a stack of several identical cells is described on the basis of the electrochemical behavior of a single cell. Efficiency of fuel cell stacks is defined by means of a combination of semi-empirical models of polarization curves and dimensionless variables such as reaction extent and utilization. The connection of flows among the cells is basically divided in two extreme cases and one intermediate case. Higher efficiencies are obtained when the same main flow (both anodic and cathodic) passes consecutively through the stack cells (Chain Flow), because it is favored thermodynamically. It is less favored when the main flow is strictly divided among all the cells (Separate Flow). In the intermediate case, the main flow is divided among all the stack cells and all the outlets are collected in one flow. The latter can spontaneously evolve to the more thermodynamically stable behavior of Chain Flow.     

Polarization effects in electrolyte/electrode-supported solid oxide fuel cells

The effects of activation, ohmic and concentration polarization on the overall polarization in solid oxide fuel cells are presented. A complete analysis was conducted based on thermodynamic principles for the calculation of cell voltage. Treating the fuel cell as a control volume, the irreversibility term in a steady flow thermodynamic system was related to the overall polarization. The entropy production was calculated and related to the lost work of the fuel cell, while the heat loss from the cell was determined from the entropy balance. To generalize the cell voltage–current density expression, the Butler–Volmer model was used in the calculation of activation polarization and both ordinary and Knudsen diffusions were considered in the calculation of concentration polarization. The overall cell resistance was deduced from the generalized cell voltage–current density expression. The concentration resistance at the anode can be minimized by humidifying the hydrogen with an appropriate amount of water, depending on the thickness of the anode used. Comparison of polarization effects on the cell performance between the electrolyte-supported and anode-supported cells showed that the latter would give a better cell performance.     

质子交换膜燃料电池冷启动堆栈温度预测模型

为了快速准确地预测出质子交换膜燃料电池(proton exchange membrane fuel cell,PEMFC)在冷启动过程中的启动时长及启动方法的应用效果,提出了以堆栈温度和温度增量分别作为BP(back propagation)神经网络预测目标的堆栈温度实时预测模型,分别为模型T和模型K,并采用四个不同的预测精度评估标准来评估预测结果的准确性。基于文献中三种冷启动工况实验数据对预测模型进行验证,结果表明,模型K的平均相对误差在三种工况下均低于模型T,分别为0.4553、0.9537和1.0844。模型T在早期预测阶段缺乏训练样本,预测结果的堆栈温度变化趋势为零,因而模型K在早期预测阶段具有更大优势。堆栈温度变化趋势预测方法能够为用户当前的PEMFC冷启动实现效果提供参考。     

阳极支撑微管式固体氧化物燃料电池的研究进展

近些年来,微管式固体氧化物燃料电池(SOFC)由于其具有密封简单、体积能量密度高、抗热震性好、启动时间快等优点备受关注。本文主要介绍了微管式SOFC的优势,并重点概述了阳极支撑型微管式SOFC的制备方法、研究现状和未来的发展方向。分别对采用塑性挤出法和相转化法制备的阳极支撑微管式SOFC的技术进展进行了综述。介绍了阳极支撑微管式SOFC电池堆的设计理念,并对未来微管式SOFC的发展方向进行了展望。     

质子交换膜燃料电池研究进展

质子交换膜燃料电池（proton exchange membrane fuel cell, PEMFC）因具有效率高、功率密度大、排放产物仅为水、低温启动性好等多方面优点,被公认为下一代车用动力的发展方向之一。然而,目前PEMFC在耐久性和成本方面距离商业化的要求还存在一定差距。为攻克上述两大难题,需要燃料电池全产业链的共同努力和进步。本文回顾了近年来质子交换膜燃料电池从催化剂、膜电极组件、电堆到燃料电池发动机全产业链的研究进展和成果,梳理出单原子催化剂、非贵金属催化剂、特殊形貌催化剂、有序化催化层、高温质子交换膜、膜电极层间界面优化、一体化双极板-扩散层、氢气系统循环等研究热点。文章指出,催化层低铂/非铂化、质子交换膜超薄化、膜电极组件梯度化/有序化、燃料电池运行高温化、自增湿化是未来的发展趋势,迫切需要进一步的创新与突破。     

Exploring single electrode reactions in polymer electrolyte fuel cells

Utilising a pseudo-reference electrode in polymer electrolyte fuel cells allows for the separation of anodic and cathodic contributions to the entire cell impedance. Modelling the impedance responses by using equivalent circuits inhibits the investigation of kinetic parameters of the basic electrochemical reactions, which take place at single electrode-electrolyte interfaces. Therefore, we evaluate single electrode impedance measurements by a kinetic model, which is based on specific reaction pathways, either for the oxygen reduction reaction (ORR) or the hydrogen oxidation reaction (HOR). As a consequence, it is possible to obtain kinetic parameters for the specific reaction of interest. Furthermore, the information gained from the single electrode impedance measurements and the kinetic model can give insight into single reactions steps. In particular, the ORR has to include a chemical step in the reaction pathway.     

固体氧化物燃料电池阳极材料Ni-SDC的稳定性实验及热力学分析

以Ni-SDC作为固体氧化物燃料电池（SOFC）的阳极,研究了该阳极粉末在制备过程中以及5% H2S-N2硫化后的产物,并用热力学软件绘制相图对其在各种温度下的产物变化进行分析。结果表明：NiO-SDC在800 ℃煅烧和在850 ℃还原的产物与热力学分析结果是一致的。对比在5%的H2S-N2中硫化12 h前后的XRD表明Ni已经转化为NiS2,热力学分析验证了该结论。比较Ni-SDC和SDC硫化前后的Raman光谱和XRD结果得到：SDC硫化后主峰型没有发生明显变化,但强度变弱,说明粒径变大,可能因为有Ce-O-S键生成。     

Electrochemical impedance spectra of solid-oxide fuel cells and polymer membrane fuel cells

Electrochemical impedance spectroscopy (EIS) is a very useful method for the characterization of fuel cells. The anode and cathode transfer functions have been determined independently without a reference electrode using symmetric gas supply of hydrogen or oxygen on both electrodes of the fuel cell at open circuit potential (OCP). EIS are given for both polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) at current densities up to 0.76 A cm⁻² (PEFC) and 0.22 A cm⁻² (SOFC). With increasing current density the PEFC-impedance decreases significantly in the low frequency range reaching a minimum at 0.4 A cm⁻². At even higher current densities an increasing contribution of water diffusion is observed: the cell impedance increases again. From EIS of SOFC a finite diffusion behavior is observed even at OCP, depending on water partial pressure of the anodic gas supply. This additional element reflects the influence of water partial pressure on the cell potential. The simulation of the measured EIS with an equivalent circuit enables the calculation of the individual voltage losses in the fuel cell.     

Hydrodynamic modelling of direct methanol liquid feed fuel cell stacks

A model for the liquid feed, direct methanol fuel cell (DMFC), based on the homogeneous two-phase flow theory and mass conservation equation, which describes the hydraulic behaviour of internally manifolded cell stacks, is presented. The model predicts the pressure drop behaviour of the anode side of an individual DMFC cell and is used to determine the channel depth and width for fast and efficient carbon dioxide removal with minimum pressure drop. The model is used to calculate flow distribution through fuel cell stack internal manifolds. The effect of inlet and outlet manifold diameters on flow distribution is also determined. Two types of manifold design are compared, reverse flow and parallel flow. An iterative numerical scheme is used to solve the differential equations for longitudinal momentum and continuity.     

Two-dimensional analysis of PEM fuel cells

This study reports a two-dimensional numerical simulation of a steady, isothermal, fully humidified polymer electrolyte membrane (PEM) fuel cell, with particular attention to phenomena occurring in the catalyst layers. Conservation equations are developed for reactant species, electrons and protons, and the rate of electrochemical reactions is determined from the Butler–Volmer equation. Finite volume method is used along with the alternating direction implicit algorithm and tridiagonal solver. The results show that the cathode catalyst layer exhibits more pronounced changes in potential, reaction rate and current density generation than the anode catalyst layer counterparts, due to the large cathode activation overpotential and the relatively low diffusion coefficient of oxygen. It is shown that the catalyst layers are two-dimensional in nature, particularly in areas of low reactant concentrations. The two-dimensional distribution of the reactant concentration, current density distribution, and overpotential is determined, which suggests that multi-dimensional simulation is necessary to understand the transport and reaction processes occurring in a PEM fuel cell.     

Mathematical modeling of solid oxide fuel cells at high fuel utilization based on diffusion equivalent circuit model

Mass transfer and electrochemical phenomena in the membrane electrode assembly (MEA) are the core components for modeling of solid‐oxide fuel cell (SOFC). The general MEA model is simply governed with the Stefan‐Maxwell equation for multicomponent gas diffusion, Ohm's law for the charge transfer and the current‐overpotential equation for the polarization calculation. However, it has obvious discrepancy at high‐fuel utilization or high‐current density. An advanced MEA model is introduced based on the diffusion equivalent circuit model. The main purpose is to correct the real‐gas concentrations at the triple‐phase boundary by assuming that the resistance of surface diffusion is in series with that of the gaseous bulk diffusion. Thus, it can obtain good prediction of cell performance in a wide range by avoiding the decrement of effective gas diffusivity via unreasonable increment of the electrode tortuosity in the general MEA model. The mathematical model has been validated in the cases of H₂? H₂O, CO? CO₂ and H₂? CO fuel system. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

On the repeatability and reproducibility of experimental two-chambered microbial fuel cells

The capability of the experimental systems used in two-chambered microbial fuel cell experimentation was tested in terms of repeatability and reproducibility. The optimal number of replicates needed to discriminate between responses of technical interest, both in open-circuit and closed-circuit experiments was studied. For N = 4 replicates, these differences were set to 9.0% COD_R units, 261 mV and 63 mg/L in VFAs for open-circuit experiments and 3.6%, 30.2 mV and 45 mg/L in closed circuit experiments. Cycling operation with several reactor refills using fresh wastewater and keeping the same biofilm between cycles almost has no influence in COD_R and VFAs but voltage standard deviation reduces by one half between the first and fourth cycle. This study takes part by the option of increasing the number of replicates because although it may have lower repeatability, the amount of data generated per unit time is larger than running the experiments in cycles.     

Predicting chemical reaction equilibria in molten carbonate fuel cells via molecular simulations

It has been recently suggested that hydroxide ions can be formed in the electrolyte of molten carbonate fuel cells when water vapor is present. The hydroxide can replace carbonate in transporting electrons across the electrolyte, thereby reducing the CO₂ separation efficiency of the fuel cell although still producing electricity. In this work, we obtain the equilibrium concentration of hydroxide in five molten alkali carbonate salts from molecular simulations. The results reveal that there can be a substantial amount of hydroxide in the electrolyte at low partial pressures of CO₂ . In addition, we find that the equilibrium concentration of molecular water dissolved in the electrolyte is over two orders of magnitude higher than that of CO₂ . Increasing the size and polarizability (or in other words reducing the “hardness”) of the cations present in the electrolyte can reduce the hydroxide fraction, but at the cost of lowering ionic conductivity.     

Comparative study of different hydro-dynamic flow in microbial fuel cell stacks

This work has investigated the scale-up potential of microbial fuel cells (MFCs) under stacking mode. Stacking was done in batch mode and continuous mode. Batch feeding mode stacks were operated in electrical series (S) and parallel (P) mode. Continuous feeding mode stacks were kept in electrically par-allel mode with different hydro-dynamic patterns. The two continuous stacks were connected hydro-dynamically in series (i.e. Parallel Dependent;PD) and parallel (i.e. Parallel Independent;PID) configura-tions. The performance of the continuous stacks was evaluated on the basis of COD consumption rate, power generation and coulombic efficiency. PID obtained highest power (0.47 mW) which was approx-imately 3.6 times that of PD configuration (0.13 mW). The rate of COD consumption was also highest in PID stack (3091.75 mg·L-1·d-1). Coulombic efficiency of the PID stack was 14.26％ which was approxi-mately 292.8％ of the PD stack. The results confirmed that the parallel electrical connection hybridized with the independent hydro-dynamic flow gives the best possible results when working with stacking of MFCs.     

An H-shaped design for membraneless micro fuel cells

The geometry of micro fuel cell design influences the reactants’ mixing and the depletion at downstream of the channel and thus effects the cell performance. This paper proposes a design for membraneless micro fuel cells with an H-shaped cross-section and a small passage between the anode and cathode channels. The small passage restricts the mixing of the anode and cathode fluids in the main channel. Numerical simulations with electro-chemical reactions have been carried out to investigate the distribution and crossover of the reactants and also the mixing and depletion regions in the system. Results show that optimizing the size of the passage between the anode and cathode channels plays an important role in reducing the mixing of reactants and in increasing fuel utilization. The H-shaped design shows that the mixing region is reduced in size by about 20%, so the H-shaped design has 10 times less fuel crossover than the conventional rectangular design. Moreover, fuel utilization is increased by about 8% with respect to that of the conventional rectangular design. 90° angles between the passage and the anode and cathode channels provide the best layout for this H-shaped design. The aspect ratio 0.083 for the anode and cathode channels exhibits 23% higher fuel utilization than the conventional rectangular design. Moreover, the size of the passage has a significant influence on the boundary layer thickness, the depletion region and the current density. A micro fabrication of the H-shaped design was made and the open circuit voltages were measured. The results are compared with those in the available literature.     

Membrane fuel cells - concepts and system design

The low power range can be an interesting application market for the PEM fuel cell in the near future. With a possible function as battery in portable devices the banded structure PEM fuel cell will be an advantageous alternative to the conventional stack. The new system provides high output voltages, a flat cell geometry and can be optimized with respect to smaller cell volumes. The article describes the principle function of such a system and the necessary parameter for optimizing the cell geometry. A comparison with a conventional stack design is given and experimental results with an eight cell banded structure stack are presented.     

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.     

Improved gas diffusion electrodes for hybrid polymer electrolyte fuel cells

In this study, the performance of the anionic electrodes for hybrid polymer electrolyte fuel cells was improved. The anion exchange membrane (AEM) electrodes were initially characterized as the cathode on a proton exchange membrane (PEM) anode/membrane half-assembly (i.e. hybrid polymer electrolyte fuel cell). The electrode performance was improved by tailoring the ionomer distribution within the electrode structure so as to better balance the electronic, ionic, and reactant transport within the catalyst layer. An ionomer impregnation method was used to achieve a non-uniform ionomer distribution and higher performance. Traditional electrode fabrication methods (i.e. thin-film method) lead to a uniform ionomer distribution. The peak power density at 70 °C for a H₂/O₂ hybrid fuel cell was 44 mW cm⁻² using the thin-film electrode, and 120 mW cm⁻² using the ionomer impregnated electrode. A hydrophobic additive used in the catalyst layer further improved the electrode performance, giving a peak power density of 315 mW cm⁻² for H₂/O₂ at 70 °C. Electrochemical impedance spectroscopy was used as an in situ diagnostic tool to help understand the origin of the electrode improvements. The increase in performance was attributed to improved catalyst utilization due to the creation of facile gas transport domains in the AEM electrode structure. Similarly, the AEM anode prepared by ionomer impregnation with polytetrafluoroethylene resulted in a three-fold increase in the peak power density compared to ones made by the thin-film method, which has no polytetrafluoroethylene.     

Preliminary comparative studies of zinc and zinc oxide electrodes on corrosion reaction and reversible reaction for zinc/air fuel cells

Even though Zn/air energy system is considered to be a promising power energy source, it has been limited to be applied for an electrically rechargeable system basically due to the problem of the irreversible reaction and the corrosion reaction. In this paper a novel attempt has been made to compare the behavior of zinc electrode with a zinc oxide electrode and a modified zinc oxide electrode containing zinc oxide and lead oxide. The hydrogen overpotential is favorable in the zinc electrode, and the modified zinc oxide electrode shows the improved properties showing the more negative potential than the case of the zinc oxide electrode. Investigations of cyclic voltammogram reveal that the pure zinc electrode is irreversible, while both the zinc oxide and the modified zinc oxide electrodes are reversible. However, as far as dendrite formation is concerned there is no marked improvement in case of the zinc oxide and the modified zinc oxide electrodes.