可溶铅酸液流电池模拟与仿真

可溶铅酸液流电池是一种使用单个容器存储电解液并且不需要微孔隔膜的氧化还原液流电池,这使得电池设计简单并降低了成本。建立二维暂态可溶铅酸液流电池模型,模型基于对质量、电荷以及能量的转移与守恒以及包含铅离子反应的宏观动力学模型为基础,研究了电极间隔、电极形状、电流密度、实验温度、入口电解液流速和电解质初始浓度对电池性能的影响。研究表明：与平板电极相比,弧形电极明显提高了充电时的电池电压。在影响铅酸液流电池性能的诸多条件中,电池温度和电流密度可能是优化电池性能的重要因素。     

Experimental study on the effect of reactant flow arrangements on the current distribution in proton exchange membrane fuel cells

Current distribution in a proton exchange membrane fuel cell (PEMFC) is significantly influenced by reactant flow configurations. In this study, the current distribution has been measured experimentally using a segmented flow-field plate and printed circuit board (PCB). Local current distributions for a PEMFC with serpentine flow field and three different flow arrangements including co-flow, cross-flow, and counter-flow arrangements for the anode and cathode streams are investigated along with the effect of flow channel orientation. It is shown that the counter-flow arrangement yields most uniform distribution for the current density, whereas the co-flow arrangement results in a considerable variation in the current density from the reactant gas stream inlet to exit. Flow channel orientation can also impact the cell performance and the current distribution appreciably. The limiting hydrogen concentration at the anode side due to the low stoichiometry condition can have a predominant effect on the current distribution and cell performance.     

Numerical simulation of the current, potential and concentration distributions along the cathode of a rotating cylinder Hull cell

Numerical simulations of the non-uniform current, potential and concentration distributions along the cathode of a rotating cylinder Hull (RCH) cell (RotaHull^® cell) are performed using finite element methods. Copper electrodeposition from an acid sulfate electrolyte is used as a test system. Primary, secondary and tertiary current distributions are examined. The importance of controllable and uniformly accessible hydrodynamics along the length of the RCH cathode is demonstrated. Charge transfer kinetics are described by a Tafel approximation while mass transport is considered using a Nernstian diffusion layer expression. The effects of applied current density and electrode rotation speeds on the distribution of potential and current along the RCH cathode are investigated. An expression of the primary current distribution and a dimensionless mass transport correlation facilitate comparisons with the simulations.     

OXYGEN LIMITATION IN MICROFLUIDIC BIOFUEL CELLS

Biofuel cells are devices that use biocatalysts (enzymes or microbes) to convert biochemical energy directly into electrical energy. Microfluidic biofuel cells exploit the lack of active mixing at microscale dimensions to eliminate the use of proton exchange membranes that separate anolyte and catholyte streams. Simulation of this system, by solving the governing 3-D conservation equations (flow, species transport), reveals that oxygen availability limits the performance of the cathode. An exponential decay in the availability of oxygen at the cathode is observed along the length of the microchannel, indicating that increasing the number of electrode pairs reduces the overall current density. This conclusion is consistent with experimental observations. Increasing electrolyte flow rates can reduce the mass transport limitations by decreasing the diffusion boundary-layer thickness, but disparity between the flow rates of the anolyte and catholyte can induce wastage of dissolved oxygen.     

OXYGEN LIMITATION IN MICROFLUIDIC BIOFUEL CELLS

Biofuel cells are devices that use biocatalysts (enzymes or microbes) to convert biochemical energy directly into electrical energy. Microfluidic biofuel cells exploit the lack of active mixing at microscale dimensions to eliminate the use of proton exchange membranes that separate anolyte and catholyte streams. Simulation of this system, by solving the governing 3-D conservation equations (flow, species transport), reveals that oxygen availability limits the performance of the cathode. An exponential decay in the availability of oxygen at the cathode is observed along the length of the microchannel, indicating that increasing the number of electrode pairs reduces the overall current density. This conclusion is consistent with experimental observations. Increasing electrolyte flow rates can reduce the mass transport limitations by decreasing the diffusion boundary-layer thickness, but disparity between the flow rates of the anolyte and catholyte can induce wastage of dissolved oxygen.     

Effect of mass transfer on the current distribution in monopolar and bipolar electrochemical reactors with a gas-evolving electrode

The current distribution in an electrochemical reactor with vertical parallel-plate electrodes was experimentally determined. The research was performed with monopolar and bipolar electrodes. The reactor has a gas-evolving electrode and at the counter electrode an electrochemical reaction with combined diffusion and charge-transfer kinetic control, takes place. Therefore the kinetics at the counter electrode are influenced by the bubble-induced convection and by the forced convection of the electrolyte. These reactors are found in many electrochemical processes, for example, electrowinning of metals and electrosynthesis. The test reactions were hydrogen evolution at the cathode and the anodic oxidation of sulphite to sulphate from basic solutions. The current distribution shows a minimum at a distance of approximately six times the equivalent diameter of the reactor from the inlet region. This minimum is a consequence of the interaction between forced convection and the bubble-induced convection, which shifts the mass transfer coefficient of the anodic reaction along the reactor. The effects of the total current, the volumetric electrolyte flow rate and the metal phase resistance on the current distribution are also analysed. The experimental current distribution data are compared with theoretical expectations and good agreement is found.     

A Three‐Dimensional Two‐Phase Model for a Membraneless Fuel Cell Using Decomposition of Hydrogen Peroxide with Y‐Shaped Microchannel

A three‐dimensional, two‐phase, multi‐component mixture model in conjunction with a finite‐volume‐based computational fluid dynamics (CFD) technique is applied to simulate the operation of membraneless fuel cell with Y‐shape channel. Hydrogen peroxide is employed both as fuel and oxidant, which are dissolved in diluted sodium hydroxide and sulfuric acid solutions, respectively. Almost all transport phenomena occurring in the fuel cell such as fluid flows, mass transport, electrochemical kinetics, and charge transport are accounted in this model. The oxygen O₂ gas, which is a product on the anode electrode, is assumed to be insoluble. The presence of gas phase acts to prevent the processes of reactant supply and product removal. Thus, the cell performance is hindered, while it is operated at the normal current density situation. On the other hand, the capillary action is found to enhance the electrolyte transport in the anode porous electrode, which may slightly improve the cell performance at the high‐current density situation. Besides, a secondary vortex flow is induced due to the transportation of the gas phase, which drifts from the bottom to the top of the channel. The mixing zone is then inclined, which may result in serious fuel crossing phenomenon.     

Simulation of velocity profiles in a laboratory electrolyser using computational fluid dynamics

A commercial CFD code, Fluent, has been used to analyse the design of a filter-press reactor operating with characteristic linear flow velocities between 0.024 and 0.192 m s⁻¹. Electrolyte flow through the reactor channel was numerically calculated using a finite volume approach to solve the Navier-Stokes equations. The length of the channel was divided into 7 sections corresponding to distances of 0, 0.01, 0.04, 0.08, 0.12, 0.14 and 0.15 m from the electrode edge nearest to the inlet. The depth of the channel was divided into three planes parallel to the channel bottom. For each channel section, a velocity profile was obtained at each depth together with the average velocity in each plane. The flow predictions show that the flow development, as the electrolyte passes through the cell, is strongly affected by the manifold causing strong vortex structures at the entrance and exit of the channel. Although the flow disturbances are a function of the flow rate, they gradually disappear downstream along the channel length. Simulated velocity profiles are considered for the typical current density range used in the FM01-LC reactor.     

A computer simulation of continuous ion exchange membrane electrodialysis for desalination of saline water

A computer simulation program including the principle of ① mass transport, ② current density distribution, ③ energy consumption and ④ limiting current density is developed for predicting desalinating performance of a continuous (one-pass flow) electrodialysis process. In this simulation the following parameters are inputted; ① membrane characteristics such as overall transport number, overall solute permeability, overall electro-osmotic permeability, overall hydraulic permeability, direct current electric resistance etc. ② electrodialyzer specifications such as flow-pass thickness, flow-pass width and flow-pass length of a desalting cell etc. and ③ electrodialytic conditions such as current density, electrolyte concentration in a feeding solution, linear velocity in desalting cells, standard deviation of normal distribution of solution velocity ratio etc.In a practical-scale electrodialyzer, electrolyte concentration in a desalting cell is decreased along a flow-pass and it gives rise to electrolyte concentration distribution. It causes electric resistance distribution and current density distribution. Solution velocities in desalting cells vary between the cells, and give rise to solution velocity distribution. In this simulation, these distributions are taken into account assuming that the frequency distribution of solution velocity ratio is equated by the normal distribution. Further, the influences of electrodialyzer specifications and elctrodialysis conditions described above on the performances of an electrodialyzer (desalting ratio, current efficiency, electrolyte concentration at the outlets of desalting cells, cell voltage, energy consumption, electrolyte concentration distribution, current density distribution, and limiting current density) are predicted. The simulation model is developed on the basis of the experiments and its reasonability is supported by the performance of electrodialyzers operating in salt-manufacturing plants.     

Potential and current density distributions along a bipolar electrode

Potential and current density distributions were modelled and measured for an electrochemical cell with a bipolar electrode. The dimension of the bipolar electrode in the direction of current flow was extended, to enable experimental determination of the electrode potential and the local current densities at various positions inside the electrolyte and in the electrode body. The experimental results showed that the most active regions of the bipolar electrode are located at the ends of the bipolar electrode facing the terminal electrodes. The equations corresponding to the mathematical model of the experimental cell were solved using the finite volume method and gave very good qualitative agreement with the experimental data. However, some discrepancies between model predictions and experimental data were evident in the active parts of the bipolar electrode and in the variation of the terminal voltage with the total current. This was explained in terms of the active electrolyte cross-section and the electrode surface area being diminished due to the presence of gas bubbles in the system.     

Local Flooding Phenomena in Channel and Land Areas Occurring during Dynamic Operation of a PEFC

In this work, we report on flooding phenomena occurring during dynamic operation of a polymer electrolyte fuel cell (PEFC). The combination of high spatially and temporally resolved neutron radiography and submillimeter resolved current density distribution measurements enables the simultaneous observation of local liquid water content and current density transients in the channel and land areas of a differentially operated PEFC air cathode. The local transients of a triangular voltage sweep and a voltage step are presented here. Both results demonstrate that in the land area the current density is only marginally affected by the local liquid water content. In the voltage sweep experiment, at higher cell polarization a limiting current density is observed in the land area as a result of mass transport limitations due to the high lateral diffusion path length. In the channel area the corresponding transients of the liquid water content and the current density both exhibit a hysteresis. The transients of the voltage step indicate liquid water rearrangement in channel and land areas as a slow process occurring on a time scale of several minutes. Thereby, the local cell performance is primarily affected by the local liquid water content in front of the oxygen electrode.     

A printed circuit board approach to measuring current distribution in a fuel cell

A new method of measuring current distribution in a polymer electrolyte fuel cell of active area 100cm2 has been demonstrated, using a printed circuit board (PCB) technology to segment the current collector and flow field. The PCB technique was demonstrated to be an effective approach to fabricating a segmented electrode and provide a useful tool for analysing cell performance at different reactant gas flow rates and humidification strategies. In this initial chapter of work with the segmented cell, we describe measured effects on current distribution of cathode and anode gas stream humidification levels in a hydrogen/air cell, utilizing a NafionTM 117 membrane and single serpentine channel flow fields, and operating at relatively high gas flow rates. Effects of the stoichiometric flow of air are also shown. A clear trend is seen, apparently typical for a thick ionomeric membrane, of lowering in membrane resistance down the flow channel, bringing about the highest local current density near the air outlet. This trend is reversed at low stoichiometric flows of air. At an air flow rate less than three times stoichiometry, the local performance starts to drop significantly from inlet to outlet, as local oxygen concentration drop overshadows the lowering in resistance along the direction of flow.     

Enhanced mass transport to a reticulated vitreous carbon rotating cylinder electrode using jet flow

The mass transport characteristics of a porous, rotating cylinder electrode (RCE, 1.0 cm diameter; 0.5, 0.9 or 1.2 cm long; 1.25, 2.25, 3.00 cm³ overall volume; 250-2000 rpm speed) fabricated from reticulated vitreous carbon (RVC, 60 ppi or 100 ppi) were investigated. The deposition of copper from an acid sulfate electrolyte (typically, deoxygenated 1 mM CuSO₄ in pH 2, 0.5 M Na₂SO₄ at 298 K) was used as a test reaction. The effect of a jet flow of electrolyte towards the electrode and the introduction of polypropylene baffles in the electrochemical cell were studied at controlled rotation rates of the RCE. The product of mass transport coefficient and volumetric electrode area (k_mA_e) is related to the rotation speed of the electrode. For the 60 ppi RVC RCE, the jet electrolyte flow (3.5 cm³ s⁻¹) enhanced the mass transport rates by a factor of 1.46 at low rotation speeds; this factor was reduced to 1.08 at high rotation speeds. For a 100 ppi electrode, the enhanced mass transport decreased from 1.26 to 1.03 at low and high rotation rates, respectively. Under the experimental conditions, baffles showed little effect on the mass transport rates to the RVC RCE. Mass transport to jet flow at an RVC RCE is compared to other RCEs using dimensionless group correlation.     

Polarization characteristics and fuel utilization in anode-supported solid oxide fuel cell using three-dimensional simulation

A three-dimensional numerical simulation for anode-supported tubular solid oxide fuel cell (SOFC), which is characterized by good electrical conductivity, has been carried out. Performance results by simulation are in good agreement with those by experiments, reported in [7]. Effect of various process conditions such as operating temperature, inlet velocity of fuel, and flow direction of inlet gases on the cell performance and fuel utilization has been further scrutinized. Polarization curve rises with increasing temperature of preheated gases and chamber, resulting from the incremented activity of catalysts within electrode. An effective way to reduce the temperature variation in the single cell with increasing current density has been sought, considering the temperature-dependent thermal expansion of materials. It has also been found that the fuel utilization is enhanced by increasing the cell length and operating temperature and lowering the inlet velocity of fuel.     

Three‐Dimensional Computational Fluid Dynamics Modeling of a Planar Solid Oxide Fuel Cell

A three‐dimensional computational fluid dynamics model was developed to study the performance of a planar solid oxide fuel cell (SOFC). The governing equations were solved with the finite volume method. The model was validated by comparing the simulation results with data from literature. Parametric simulations were performed to investigate the coupled heat/mass transfer and electrochemical reactions in a planar SOFC. Different from previous two‐dimensional studies the present three‐dimensional analyses revealed that the current density was higher at the center along the flow channel while lower under the interconnect ribs, due to slower diffusion of gas species under the ribs. The effects of inlet gas flow rate and electrode porosity on SOFC performance were examined as well. The analyses provide a better understanding of the working mechanisms of SOFCs. The model can serve as a useful tool for SOFC design optimization.     

球形团聚物模型在质子交换膜燃料电池过程模拟中的应用

在建立直通道质子交换膜燃料电池(PEMFC)的二维全电池数学模型中,将球形团聚物模型应用至两极的催化剂层。通过调节团聚物中质子传导介质的比例和催化层孔隙率,预测了基准供气状态下单电池的极化曲线,与文献报道的实验数据吻合良好。研究了电池运行过程中,膜电极内各化学组分和电流密度的分布情况及流向,比较了不同供气压力、催化剂铂颗粒尺寸等参数对电池性能的影响。计算结果表明,在阴极及时排出反应产生的水,并在阳极对燃料气进行加湿是保证单电池正常运行的前提,提高阴极的氧化剂气体压力,可显著改善PEMFC单电池性能,特别是在受浓差极化影响较大的大电流密度区;在催化剂铂载量相同的情况下,减小铂颗粒的尺寸可以提高电池的性能。     

Large-scale simulation of polymer electrolyte fuel cells by parallel computing

A three-dimensional, electrochemical-transport fully coupled numerical model of polymer electrolyte fuel cells (PEFC) is introduced. A complete set of conservation equations of mass, momentum, species, and charge are numerically solved with proper account of electrochemical kinetics and water management. Such a multi-physics model combined with the need for a large numerical mesh results in very intense computations that require parallel computing in order to reduce simulation time. In this study, we explore a massively parallel computational methodology for PEFC modeling, for the first time. The physical model is validated against experimental data under both fully and low-humidified feed conditions. Detailed results of hydrogen, oxygen, water, and current distributions in a PEFC of 5-channel serpentine flow-field are discussed. Under the fully humidified condition, current distribution is determined by the oxygen concentration distribution. Cell performance decreases in low-humidity inlet conditions, but good cell performance can still be achieved with proper water management. Under low-humidity conditions, current distribution is dominated by the water distribution at high cell voltages. When the cell voltage is low, the local current density initially increases along the flow path as the water concentration rises, but then starts to decrease due to oxygen consumption. Under both fully and low-humidified conditions, numerical results reveal that the ohmic losses due to proton transport in anode and cathode catalyst layers are comparable to that in the membrane, indicating that the catalyst layers cannot be neglected in PEFC modeling.