Computational modeling of microfluidic fuel cells with flow-through porous electrodes

In the current work, a computational model of a microfluidic fuel cell with flow-through porous electrodes is developed and validated with experimental data based on vanadium redox electrolyte as fuel and oxidant. The model is the first of its kind for this innovative fuel cell design. The coupled problem of fluid flow, mass transport and electrochemical kinetics is solved from first principles using a commercial multiphysics code. The performance characteristics of the fuel cell based on polarization curves, single pass efficiency, fuel utilization and power density are predicted and theoretical maxima are established. Fuel and oxidant flow rate and its effect on cell performance is considered and an optimal operating point with respect to both efficiency and power output is identified for a given flow rate. The results help elucidate the interplay of kinetics and mass transport effects in influencing porous electrode polarization characteristics. The performance and electrode polarization at the mass transfer limit are also detailed. The results form a basis for determining parameter variations and design modifications to improve performance and fuel utilization. The validated model is expected to become a useful design tool for development and optimization of fuel cells and electrochemical sensors incorporating microfluidic flow-through porous electrodes.     

Numerical investigation of the chemical and electrochemical characteristics of planar solid oxide fuel cell with direct internal reforming

A fully three-dimensional mathematical model of a planar solid oxide fuel cell (SOFC) with complete direct internal steam reforming was constructed to investigate the chemical and electrochemical characteristics of the porous-electrode-supported (PES)-SOFC developed by the Central Research Institute of Electric Power Industry of Japan. The effective kinetic models developed over the Ni/YSZ anode takes into account the heat transfer and species diffusion limitations in this porous anode. The models were used to simulate the methane steam reforming processes at the co- and counter-flow patterns. The results show that the flow patterns of gas and air have certain effects on cell performance. The cell at the counter-flow has a higher output voltage and output power density at the same operating conditions. At the counter-flow, however, a high hotspot temperature is observed in the anode with a non-fixed position, even when the air inlet flow rate is increased. This is disadvantageous to the cell. Both cell voltage and power density decrease with increased air flow rate.     

Modeling of an air cathode for microfluidic fuel cells: Transport and polarization behaviors

Air-cathode microfluidic fuel cells are promising micro-scale power sources that unfortunately undergo substantial performance loss at the cathode. This study therefore develops a mathematical model to gain a better understanding of the fundamental processes and polarization characteristics associated with the MFC air cathode operation so as to find strategies to minimize the cathode polarization. The model is solved for the four regions of an MFC cathode compartment (i.e. gas channel, gas diffusion layer, catalyst layer and electrolyte microchannel), and considers microfluidic flow, species transport, charge transport and multi-step oxygen reduction reactions. Relying on the model, transport and chemical patterns inside the MFC cathode compartment are examined. Corresponding electrode polarization behaviors are analyzed over a wide operating potential range including different forms of resistance. Through a series of model-based parametric studies, it is found that the internal transfer resistance slightly decreases with increasing catalyst layer porosity but can be effectively reduced through a proper control of electrolyte hydrodynamic conditions, indicating microfluidic technology is a powerful tool for enhancing electrochemical cells.     

Energy and exergy analysis of microfluidic fuel cell

Microfluidic fuel cell (MFC) is a promising fuel cell type because its membraneless feature implies great potential for low-cost commercialization. In this study, an energy and exergy analysis of MFC is performed by numerical simulation coupling computational fluid dynamics (CFD) with electrochemical kinetics. MFC system designs with and without fuel recirculation are investigated. The effects of micropump efficiency, fuel flow rate and fuel concentration on the MFC system performance are evaluated. The results indicate that fuel recirculation is preferred for MFC to gain higher exergy efficiency only if the efficiency of the micropump is sufficiently high. Optimal cell operating voltage for achieving the highest exergy efficiency can be obtained. Parasitic effect will cause a significant reduction in the exergy efficiency. An increase in the fuel concentration will also lead to a reduction in the exergy efficiency. Increasing the fuel flow rate in a MFC with fuel recirculation will cause a fluctuating variation in the exergy efficiency. On the other hand, in a one-off MFC system, the exergy efficiency decreases with increasing fuel flow rate. The present work enables better understanding of the energy conversion in MFC and facilitates design optimization of MFC.     

Design of a radial vanadium redox microfluidic fuel cell: A new way to break the size limitation

Microfluidic fuel cell (MFC) suffers from small single cell output power due to the inherent cell size limitation as microscale geometries are prerequisite to prevent reactant crossover between the anode and cathode. To meet the power demand of practical applications, previous works mainly focus on the creating of MFC stacks with multiple cells connected in series, parallel, or mixture of both series and parallel to increase the output power. Yet, low energy efficiency is observed because of the flow distribution nonuniformity and shunt current losses. In this work, a high performance radial vanadium redox MFC is presented to address the size limitation issue by adding a separate layer between the porous electrodes of the conventional plate‐frame MFC. Specific cell characteristics are detailed by mathematical modeling, and parametric studies are performed to evaluate the influences of the geometrical and operational parameters on the cell performance. The results show that this new radial MFC can provide a higher fuel utilization and meanwhile an improved cell performance under a fixed electrode size compared with the conventional plate‐frame MFC. Moreover, the electrode size limitation due to the reactant crossover between the anode and cathode is broken as the influences of the electrode size on the mixing region are greatly reduced. In the case with the electrode size equal to 18 mm × 18 mm , single cell output power of 0.35 mW with a fuel utilization of 53.33% is obtained under the reactant concentration of 2 mol L^?1 and flow rate of 300 μL min^?1 .     

A membraneless microfluidic fuel cell with continuous multistream flow through cotton threads

In typical membraneless microfluidic fuel cells, the anolyte and catholyte are driven by syringe pumps, increasing the overall size of the system and limiting its miniaturization. In this study, a membraneless microfluidic fuel cell with continuous multistream flow through cotton threads was proposed. Cotton threads are simply laid in parallel to form flow channels. Multistream flow through cotton threads is formed without any external pumps. Cell performances under various operation conditions are evaluated. The results show that the middle stream could separate other two streams effectively to prevent the diffusive mixing of anolyte and catholyte. A peak power density of 19.9 mW cm⁻² and a limiting current density of 111.2 mA cm⁻² are delivered. Moreover, the performance improves with the sodium formate concentration rising up to 2M, while it declines at 4M fuel concentration due to the weakened convection transport and product removal caused by the low flow rate. With increasing the flow rate, the performance is enhanced because of the improved fuel transport at the anode. The good performance as well as the constant-voltage discharging curve indicates that the microfluidic fuel cell with cotton threads as flow channels provides a new direction for miniature power sources.     

Modelling and simulation of two-chamber microbial fuel cell

Microbial fuel cells (MFCs) offer great promise for simultaneous treatment of wastewater and energy recovery. While past research has been based extensively on experimental studies, modelling and simulation remains scarce. A typical MFC shares many similarities with chemical fuel cells such as direct ascorbic acid fuel cells and direct methanol fuel cells. Therefore, an attempt is made to develop a MFC model similar to that for chemical fuel cells. By integrating biochemical reactions, Butler–Volmer expressions and mass/charge balances, a MFC model based on a two-chamber configuration is developed that simulates both steady and dynamic behaviour of a MFC, including voltage, power density, fuel concentration, and the influence of various parameters on power generation. Results show that the cathodic reaction is the most significant limiting factor of MFC performance. Periodic changes in the flow rate of fuel result in a boost of power output; this offers further insight into MFC behaviour. In addition to a MFC fuelled by acetate, the present method is also successfully extended to using artificial wastewater (solution of glucose and glutamic acid) as fuel. Since the proposed modelling method is easy to implement, it can serve as a framework for modelling other types of MFC and thereby will facilitate the development and scale-up of more efficient MFCs.     

Efficient biohydrogen and bioelectricity production from xylose by microbial fuel cell with newly isolated yeast of Cystobasidium slooffiae

Although xylose is the secondary dominant sugar derived from biomass, the conversion of xylose to energy products is quite challenging. In this work, a new exoelectrogenic yeast strain (Cystobasidium slooffiae strain JSUX1) that can generate electricity in microbial fuel cell (MFC) by using xylose as the substrate was isolated and identified. After adaptation, it produced significant current output with rapid xylose metabolism. More surprisingly, this strain produced hydrogen gas either in anerobic flask incubation or in MFC, which delivered a 67 mW/m² power output and 23 L/m³ hydrogen gas in MFC with xylose as fuel. Further electrochemical analysis indicated that riboflavin was secreted by this strain as the electron mediator for efficient electron transfer between cells and electrode in MFC. This is the first microorganism identified that can simultaneously produce bio-hydrogen and bio-electricity from xylose, which would diversify the toolbox of biomass energy.     

Effects of force field and design parameters on the exergy efficiency and fuel utilization of microfluidic fuel cells

Microfluidic fuel cells (MFCs) are novel systems that satisfy the critical requirements of having small dimensions and substantial power output for use in portable devices. In this study, three-dimensional mathematical models of two types of MFCs (flow-over and flow-through) are developed, by coupling multiphysics consisting of microfluidic hydrodynamics, electrochemical reaction kinetics, and species transport of fluid. Moreover, gravity, exergy, and parametric sensitivity are studied, which have tremendous impact on fuel cell performance and have been frequently overlooked in previous literature. The reliability of the numerical model is demonstrated by the excellent consistency between simulation results and experimental data. First, a parametric analysis is conducted, which includes the design parameters and gravity effect. Following this, the fuel utilization and exergy efficiency are calculated for various design parameters. Finally, a sensitivity analysis is performed to evaluate the influence of the indicators on the cell performance. It is shown that a relatively stable performance is achieved with the flow-through MFC under interference from the external environment. The reactive sites of the flow-through MFC can be utilised effectively, whereas further promotion of the flow-over MFC is limited by its inherent drawback. In addition, the sensitivity analysis reveals that cell performance depends strongly on the flow rate and fuel concentration. The results can be beneficial for the investigation of cell performance optimization.     

Microbial fuel cells based on carbon veil electrodes: Stack configuration and scalability

The aim of this study was to compare the performance of three different sizes of microbial fuel cell (MFC) when operated under continuous flow conditions using acetate as the fuel substrate and show how small‐scale multiple units may be best configured to optimize power output. Polarization curve experiments were carried out for individual MFCs of each size, and also for stacks of multiple small‐scale MFCs, in series, parallel and series–parallel configurations. Of the three combinations, the series–parallel proved to be the more efficient one, stepping up both the voltage and current of the system, collectively. Optimum resistor loads determined for each MFC size during the polarization experiments were then used to determine the long‐term mean power output. In terms of power density expressed as per unit of electrode surface area and as per unit of anode volume, the small‐sized MFC was superior to both the medium‐ and large‐scale MFCs by a factor of 1.5 and 3.5, respectively. Based on measured power output from 10 small units, a theoretical projection for 80 small units (giving the same equivalent anodic volume as one large 500 mL unit) gave a projected output of 10 W m^?3, which is approximately 50 times higher than the recorded output produced by the large MFC. The results from this study suggest that MFC scale‐up may be better achieved by connecting multiple small‐sized units together rather than increasing the size of an individual unit. Copyright © 2008 John Wiley & Sons, Ltd.     

A new analytical approach to model and evaluate the performance of a class of irreversible fuel cells

An irreversible model of a class of hydrogen–oxygen fuel cells working at steady-state is established, in which the irreversibilities resulting from electrochemical reaction, electrical resistance, and heat transfer to the environment are taken into account. The entropy production analysis is introduced and applied to investigate the physical and chemical performances of the fuel cell by using the theory of electrochemistry and non-equilibrium thermodynamics. Expressions for the power output and efficiency of the fuel cell are derived by introducing the equivalent internal and leakage resistances. With the help of the model being applied to high temperature solid oxide fuel cells, the performance characteristic curves of the fuel cell are presented and the influence of some design and operating parameters on the performance of the fuel cell are discussed in detail. Moreover, the optimum criteria of some important parameters such as the power output, efficiency, and current density are given. The results obtained may provide a theoretical basis for both the optimal design and operation of real fuel cells. This new method can also be used in the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Performance assessment of a four-pass serpentine proton exchange membrane fuel cell with non-humidified cathode and cell state estimation without special measurement

Proton exchange membrane fuel cells are promising electrochemical energy conversion devices especially important for mobile technologies, including the automotive industry thanks to their quick start-up, low operation temperature, and relatively higher energy density characteristics. However, cell performance depends on many parameters like reactant temperature and humidification ratio, cell operating temperature, reactant feeding pressure, and flow field. In this study, the performance of a 50 cm² active area four-pass serpentine flow field hydrogen-air proton exchange membrane (PEM) fuel cell experimentally investigated for various cell operating temperatures and reactant back pressures without humidification on the cathode side. Dehydration or flooding condition of the cell is showed to be determined with tafel slope, limiting current density and types of voltage losses without using a special measurement. The results show that flooding, which is called mild flooding, is possible to be seen even at high cell temperature in a non-humidified cathode fuel cell, in case of exceeding operating pressures. Behavior of cell parameters under mild flooding and ongoing severe flooding are different from each other. Pressure increase at above 45 °C operating temperature is seen to served higher power output. However, at low back pressure with escalated operating temperature doesn't result with a substantial increase on performance since less amount of water is produced as a product of reaction causing membrane dehydration at relatively low current density levels thus increasing ohmic loss.     

Modeling of electrochemistry and steam-methane reforming performance for simulating pressurized solid oxide fuel cell stacks

This paper examines the electrochemical and direct internal steam-methane reforming performance of the solid oxide fuel cell when subjected to pressurization. Pressurized operation boosts the Nernst potential and decreases the activation polarization, both of which serve to increase cell voltage and power while lowering the heat load and operating temperature. A model considering the activation polarization in both the fuel and the air electrodes was adopted to address this effect on the electrochemical performance. The pressurized methane conversion kinetics and the increase in equilibrium methane concentration are considered in a new rate expression. The models were then applied in simulations to predict how the distributions of direct internal reforming rate, temperature, and current density are effected within stacks operating at elevated pressure. A generic 10 cm counter-flow stack model was created and used for the simulations of pressurized operation. The predictions showed improved thermal and electrical performance with increased operating pressure. The average and maximum cell temperatures decreased by 3% (20 °C) while the cell voltage increased by 9% as the operating pressure was increased from 1 to 10 atm.     

微生物燃料电池运行影响因素的研究

研究自行设计的微生物燃料电池在常温常压下,以厌氧污泥为接种源,以葡萄糖为底物原料,以不同溶液作为电子受体的条件下测试其稳定运行的影响因素与工艺条件。实验结果表明:该微生物燃料电池可稳定运行约30d,并在注入新的底物后,电压又快速回升至稳定电压。以铁氰化钾溶液作为电子受体,输出电压可达0.75V,输出功率为2100mW/m~2;以高锰酸钾溶液作为电子受体,输出电压为1.023V,输出功率为2638mW/m~2。     

平板式阳极支撑固体氧化物燃料电池传质传热仿真研究

利用流体力学计算软件F luen t建立平板式阳极支撑固体氧化物燃料电池(SOFC)的三维数学模型。在阳极与阴极多孔电极中使用尘气模型模拟气体质量传输并采用B rinkm an-Forschhe im er-D acy模型来模拟多孔电极中黏性与惯性效应对气体流动的影响。研究给出了燃料气与空气在同向流与反向流情况下组分浓度、电压与温度分布。结果显示在同向流情况下,电池的最大功率密度较大与温度分布较均匀合理。研究给出了多孔电极结构参数(孔隙率、曲折因子与孔径尺寸)对电池性能的影响。结果表明比较计算的极化性能与文献的实验数据两者较好的吻合。     

Microfluidic microbial fuel cells: Recent advancements and future prospects

Over the years, there has been a substantial increase in the demands of a portable, green source of energy for powering microelectronics to be used as sensors, medical implants and other lab-on-chip devices. Microfluidic microbial fuel cells have been identified as a genuine option to address these requirements. These cells operating at microscale level are characterised by laminar flow of fuel and oxidant which eradicates the requirement of a membrane ensuring higher performance and improved reaction rates than conventional fuel cells. Owing to these advantages, microsized microbial fuel cells have been extensively used to design micro power sources for environmental biosensors, point-of-care diagnostics, medical implants. However, the microfluidic microbial fuel cell technology suffers from some noteworthy disadvantages which need to be addressed before the commercialization of technology. The review comprehensively discusses the development, and advancements in microfluidic microbial fuel cell technology followed by their current applications, challenges, the possible solutions and future prospects.     

微生物燃料电池阴极电子受体与结构的研究进展

从工程应用的角度分析了微生物燃料电池的结构变化趋势;从电化学角度介绍了几种两室微生物燃料电池中阴极室采用不同电子受体对提高电池输出功率的影响和单室空气阴极微生物燃料电池的研究现状及应用前景;分析了电池组在电池放大过程中可能存在的串挠和电压反转等问题,为微生物燃料电池的工程应用提供了理论参考。     

Mid-baffle interdigitated flow fields for proton exchange membrane fuel cells

A new design of an interdigitated flow field, called as a mid-baffle interdigitated flow field, was built and tested for its effect on the performance of proton exchange membrane (PEM) fuel cells. The results were compared to the conventional interdigitated flow field. Their performances at different oxidant gas flow rates and operating pressures were also examined and compared by using both O₂ and air as the cathode fuel reactants. The experimental results showed that when air was used as the cathode reactant, the cell with the mid-baffle interdigitated flow field outperformed the conventional one, giving a power output approximately 1.2-1.3 times higher depending on the air flow rates. The polarization curves of the mid-baffle interdigitated flow field showed larger limiting current densities at every air flow rate tested in this work. However, the performances of both flow fields were almost the same when the cathode reactant gas was O₂. The test also demonstrated that the flow field performance could be enhanced by increasing the oxidant gas flow rate and cell operating pressure.     

Performance of a microfluidic microbial fuel cell based on graphite electrodes

A microfluidic microbial fuel cell utilizing the laminar flow to separate the anolyte and catholyte streams based on graphite electrode is proposed. The co-laminar flow of the two streams inside the microchannel is visualized under different flow rates. The effects of the acetate concentration and flow rate on the cell performance are investigated. The results show that the cell performance initially increases and then decreases with increasing influent COD concentration and the anolyte flow rate. The microfluidic microbial fuel cell produces a peak power density of 618 ± 4 mW/m² under the conditions of 1500 mg/L influent COD and an anolyte flow rate of 10 mL/h. The low internal resistance of fuel cell results from elimination of the proton exchange membrane and high surface-to-volume ratio of the microfluidic structure. Moreover, the thickness of biofilm decreases gradually along the flow direction of the microchannel due to the diffusive mixing of the catholyte.     

Improved performance of air-cathode microbial fuel cell through additional Tween 80

The ability of electron transfer from microbe cell to anode electrode plays a key role in microbial fuel cell (MFC). This study explores a new approach to improve the MFC performance and electron transfer rate through addition of Tween 80. Results demonstrate that, for an air-cathode MFC operating on 1 g L⁻¹ glucose, when the addition of Tween 80 increases from 0 to 80 mg L⁻¹, the maximum power density increases from 21.5 to 187 W m⁻³ (0.6-5.2 W m⁻²), the corresponding current density increases from 1.8 to 17 A m⁻², and the resistance of MFC decreases from 27.0 to 5.7 Ω. Electrochemical impedance spectroscopy (EIS) analysis suggests that the improvement of overall performance of the MFC can be attributed to the addition of Tween 80. The high power density achieved here may be due to the increase of permeability of cell membranes by addition of Tween 80, which reduces the electron transfer resistance through the cell membrane and increases the electron transfer rate and number, consequently enhances the current and power output. A promising way of utilizing surfactant to improve energy generation of MFC is demonstrated.