Characterization of bubble formation in microfluidic fuel cells employing hydrogen peroxide

In order to examine bubble evolution and discuss the effects of bubbles effect on the performance of microfluidic fuel cells, two 1.2-mm-depth microfluidic fuel cells employing 0.1-M H₂O₂ dissolved in 0.1-M NaOH solution and 0.05-M H₂SO₄ solution as fuel and oxidant, respectively, with transparent lids having width of 1.0 mm and 0.5 mm, are fabricated in the present study for both cell performance measurement and flow visualization. The results show that the present cells operating at either a higher volumetric flow or a smaller microchannel width yield both better performance and more violent bubble growth. The bubble growth rate, Q_g, in a given microfluidic fuel cell is almost the same at different regions of that cell at a given volumetric flow rate, i.e. 10⁻⁵ cm³ s⁻¹ and 5 × 10⁻⁵ cm³ s⁻¹, respectively, for cells having widths of 0.5 mm and 1.0 mm at Q_l = 0.05 mL min⁻¹, and slightly increases at higher volumetric flow rates. Furthermore, the present study reports approximately constant values of Q_g/C_dA at various volumetric flow rates, which are 2 × 10⁻² and 5 × 10⁻² cm³ s⁻¹ A⁻¹, respectively, for cells having channel widths of 0.5 mm and 1.0 mm. In addition, the 0.5-mm-wide cell has higher cell output and performs more tortuous polarization curve.     

The effect of wetting properties on bubble dynamics and fuel distribution in the flow field of direct methanol fuel cells

We investigate CO₂ bubble dynamics on the anode side of a direct methanol fuel cell (DMFC). In contrast to previous studies, we analyse the effect of both channel wall and diffusion layer wettability by observing two-phase flow from the side at different mean velocities of the fuel supply. Hydrophobic and hydrophilic flow channel surfaces are compared experimentally. The hydrophilic flow channel leads to a minimum pressure drop along the channel. Bubbles show virtually no pinning and consequently travel at approximately the mean fuel velocity inside the channel. In contrast to this, we observe bubble pinning in the hydrophobic flow channels. The critical fuel velocities necessary for detachment of the bubbles mainly depends on bubble length. We identify and describe a new bubble bypass configuration where fuel bypass channels are solely generated in a favourable position underneath a blocking bubble along the diffusion layer. This enforces fuel to bypass the CO₂ bubble at a large relative velocity close to the diffusion layer, thus enhancing mass transfer. Our experimental findings are in excellent agreement with a CFD/analytical model. This model allows for quantitative prediction of average bypass flow velocity.     

Analyses of fuel utilization in microfluidic fuel cell

A microfluidic fuel cell is a miniature power source, which potentially could be used in micro electronic equipments, laptop computers, mobile phones and video cameras. In recent reports, the idea of a microfluidic fuel cell without using a polymer electrolyte membrane is proposed, whereby the laminar nature of the flow in the micro-channels is used to keep the anode and cathode streams separated such that adverse electrochemical reactions do not take place at the two electrode polarities. Since such cells are restricted by their size, improvement in fuel utilization would increase the cell efficiency by several degrees. In the present study, an improvement in fuel utilization is proposed by altering the design of the microfluidic fuel cell. In particular, a sulfuric acid stream is introduced between the fuel (HCOOH) and oxidizer (O₂ in H₂SO₄) streams to improve fuel utilization. Further improvement in fuel utilization is possible by changing the aspect ratio of the cell from 0.1 to 1. The fuel utilization of a cell with an aspect ratio of 0.1 is 14.1%, which increases to 16% when a sulfuric acid stream is introduced to prevent mixing of the fuel and oxidizer streams. The fuel utilization increases to 19% with the change in aspect ratio from 0.1 to 10, which further increases to 32% with the introduction of a sulfuric acid stream.     

Chip-embedded thin film current collector for microfluidic fuel cells

Microfluidic fuel cells are an attractive candidate for low-power applications and provide a unique advantage over traditional fuel cells by elimination of the membrane. More importantly, microfluidic fuel cells enable a simple single-layer structure similar to common lab-on-chip devices, which makes conventional microfabrication or micromachining techniques readily applicable. Microfabrication is a preferable fabrication tool for microscale devices due to the benefits of high precision and repeatability at relatively low cost. However, the performance of most microfluidic fuel cells reported to date was negatively influenced by intrinsic contact resistances arising due to the highly porous nature of the electrodes. In the present work, a chip-embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed, fabricated, and evaluated as a potential mitigation strategy. The micromachining based thin film process is compatible with the overall cell fabrication, comprising photolithography and soft lithography, and does not require a substantial modification of the original cell design. Cells with and without current collectors are directly compared experimentally: the cell with current collectors demonstrates a 79% increase in peak power density, indicating that the contact resistance is significantly reduced by this approach. A volume specific peak power density of 6.2 W cm⁻³ is achieved, which is significantly higher than for previously reported microfluidic fuel cells. Electrochemical impedance spectroscopy (EIS) analysis is carried out to measure the combined ohmic cell resistance and confirmed a 32% reduction using the current collectors, which shows a good agreement with slope decrements in the polarization curves.     

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.     

Polymer separator and low fuel concentration to minimize crossover in microfluidic direct methanol fuel cells

Microfluidic direct methanol fuel cell (DMFC) has some key issues, such as fuel crossover and water management, which typically hamper the development of conventional polymer electrolyte‐based fuel cells. Here a method of minimizing fuel crossover in microfluidic DMFC is reported. A polymer separator at the interface of the fuel and electrolyte streams in a single‐channel microfluidic DMFC is used to reduce the cross‐sectional area across where methanol can diffuse. Based on the optimized fuel rate, fuel concentration and pore radius, a maximum power density of 7.4 mW cm^?2 was obtained with the separator using 2 M methanol. This simple design improvement reduces the voltage loss at the cathode and leads to better performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Nanochannel arrays as supports for proton exchange membranes in microfluidic fuel cells

In this article, we report the use of nanochannel arrays as supports for proton exchange membranes in microfluidic fuel cells. The proposed design has been demonstrated by fabricating a sodium silicate based sol-gel structure within such an array bridging two microchannels containing the fuel (HCOOH) and the oxidant (KMnO₄) streams. A voltage was generated in this system by bringing two platinum electrodes in contact with these solutions and then connecting them through an external circuitry. With this current design, we have been able to generate an open circuit potential of 1.31 V and a maximum current of 31.2 μA at 25 °C.     

Types of simplified flow channels without flow obstacles in microbial fuel cells

Microbial fuel cells (MFCs) use microorganisms to convert organic matter into electricity. In order to enhance the mass transfer of MFCs, four types of simplified flow channels, without flow obstacles (square, circular, divergent and convergent), were designed and applied to the anode/cathode channels of MFCs. The simulation analysis showed that the four types of simplified flow channels without flow obstacles obtained a better flow mixing efficiency with an Aspect Ratio (AR) of 1 at a Reynolds number (Re) of 60. A maximum power density of 617.8 mW/m² and a COD (chemical oxygen demand) degradation ratio = 9.9% were obtained by the MFCs with the convergent types of flow channels without flow obstacles. This is because the flow mechanism (convection and vortex flow) generated by convergent types of flow channels decrease the mass transfer and ohmic losses. Therefore, this concept of the simplified flow channel without flow obstacles will be useful to the application of MFCs in the future.     

Numerical study of the effect of the channel and electrode geometry on the performance of microfluidic fuel cells

Using COMSOL Multiphysics 3.5, 3D numerical models of different microfluidic fuel cells have been developed in this paper to determine the effect of different modifications which have been implemented in the microfluidic fuel cell since its advent. These modifications include the channel geometry aspect ratio and electrode configuration, the third flow between the anolyte and catholyte in the channel (i.e., multi-stream laminar flow), and multiple periodically placed inlets. To be consistent with the convention, the output power of the device is normalized by the electrode surface area; however, the power density calculations are also performed through normalization by the device volume. It is shown that the latter method is more realistic and providing more information from the design point of view since the ultimate goal in designing the microfluidic fuel cell is to fabricate a compact, yet powerful device. Finally, a novel design of the microfluidic fuel cell with a tapered channel is suggested and compared to the non-tapered geometry through the polarization curves. The steps which have been taken in COMSOL to obtain these polarization curves are clearly and thoroughly explained. The Butler-Volmer equation was implemented to incorporate for the electrochemical reactions at the electrodes. The “Conductive Media DC” module, in COMSOL, is used to model the electric fields within the fuel cell. The concentration distributions of the reactant species are obtained using the “Incompressible Navier-Stokes” and “Convection and Diffusion” modules. Solving these equations together predicts the current density for given cell voltage values. The results demonstrate the cell voltage losses due to activation, ohmic and concentration overpotentials. It is shown that for a fixed value of the cell voltage (say 0.45 V), the fuel cell with multiple periodically placed inlets has the highest fuel utilization (i.e., 62.3%); while the “Simple square” geometry depicts 13.8% fuel utilization at this potential. Thus, the multiple-inlets design is particularly suitable for low-voltage applications which require high current. Also, the results of the tapered geometry proposed in this paper show that tapering the channel enhances the polarization curve comparing to the square cross-section geometry with extended electrodes. In essence, the fuel utilization of the “Extended square” geometry is increased from 15.4% to 57.6% by tapering the channel. This is due to the fact that the mixing region growth rate is restricted in the tapered geometry, and hence the electrodes on the top and bottom walls of the channel can be more extended toward the centre of the channel before the crossover occurs.     

A computational study of bifunctional oxygen electrode in air-breathing reversible microfluidic fuel cells

Air-breathing reversible microfluidic fuel cell (RMFC) provides flexibility to choose either acid or alkaline medium for the bifunctional oxygen electrode. A numerical model has been developed and validated to predict the performance of an air-breathing RMFC. Half-cell J-V characteristics of the RMFC using different pH media for the oxygen electrode are compared. The model results suggest that when the RMFC is operated in fuel cell (FC) mode, alkaline medium is preferred for the oxygen electrode, and when operated in electrolysis-cell (EC) mode, acid medium is preferred. By further analyzing the round-trip energy efficiency and major potential loss of the half-cell, it is found that adopting acid medium for oxygen electrode can maximize the overall charging/discharging cycle efficiency and performance of RMFC, due to much lower activation overpotential in the EC mode. Heat and mass transport characteristics of the half-cell are also investigated. It is found that the flowing electrolyte can efficiently remove the heat generated by various sources in the RMFC, leading to the mass convection in the oxygen electrode and surrounding environment solely driven by concentration gradient. Due to the presence of water vapor as the reaction product, FC mode operation in acid medium yields the most intensive breathing process of the oxygen electrode. The results provide implications to further optimizations of RMFC.     

Hydrodynamic focusing in microfluidic membraneless fuel cells: Breaking the trade-off between fuel utilization and current density

Microfluidic membraneless fuel cell (MFC) is a promising fuel cell type due to its simple structure without the need of proton conducting membrane. However, the common disadvantage is the low fuel utilization. Previous works have shown that adopting a conventional method to increase the fuel utilization would cause a low power density. This study shows that the use of hydrodynamic focusing technology can overcome the trade-off problem between the fuel utilization and the current density. A numerical model has been developed to simulate the MFC operation with the fuel stream being hydrodynamically focused by a buffer stream. The results indicate that both fuel utilization and current density can be increased by properly adjusting the buffer flow rate to enhance the flow focusing. The optimal performance is achieved when the buffer-to-fuel flow rate ratio is around 25. Moreover, high fuel flow rate and shallow channel shape have proven beneficial to the cell performance with the use of hydrodynamic focusing technology. It is predicted that a MFC with a current density above 100 mA cm⁻² is capable of achieving fuel utilization up to 50%, which is considerably higher than the previously reported value of 5-8%.     

Investigation of degradation mechanisms in PEM fuel cells caused by low-temperature cycles

Environmental influences, especially temperatures below the freezing point, can affect the performance and long-term stability of PEMFCs. Within the scope of this research, a completely new test procedure was developed to characterize PEMFC single cells with respect to their long-term stability at temperature cycles between 80 °C and ?10 °C. Using this procedure, the behavior of PEMFC single cells (active surface area of 43.6 cm²) with different cathode-ionomer-to-carbon (I/C) weight ratios (0.5/1.0/1.5) was evaluated. The generated in-situ measurement data clearly demonstrate that the performance of each PEMFC single cell changes individually as a function of the cathode I/C-ratio during the 120 stress cycles. While the MEA with an I/C ratio of 0.5 showed a power loss of ~1.49%, the MEAs with an I/C ratio of 1.0 and 1.5 showed a power loss of about ~7.75% and ~24.7%, respectively. The subsequent post-mortem ex-situ analyses clearly showed how the test procedure and the different I/C-ratios affected the changes in the catalyst layers (CL). The destructive mechanisms responsible for the changes can be divided into two categories: One part was driven by rapid enthalpy change leading to mechanical failure, and the other part, which led to the reduction of cathode CL thickness, was driven by rapid potential changes and potential shifts (overpotentials). This reduction in cathode CL thickness ultimately leads to an accumulation and excessive load of ionomer in the direction of GDL, resulting in a reduction in pore size, a shift in the core reaction area, and high O₂ transport resistance.     

A simplified method for solving anisotropic transport phenomena in PEM fuel cells

A simplified isotropic numerical treatment for solving the anisotropic electron transport phenomenon in PEM fuel cells has been proposed. In order to maintain appropriate lateral current distribution, the in-plane electronic conductivity in the catalyst and gas diffusion layers is utilized, while an extra contact resistance is added between the gas diffusion layer (GDL) and the current-collecting land to compensate the reduced through-plane electronic resistance. This simplified method is also applicable for solving the anisotropic heat transfer phenomenon in PEM fuel cells, and it improves numerical convergence and stability in three-dimensional large-scale simulations.     

Boosting cell performance and fuel utilization efficiency in a solar assisted methanol microfluidic fuel cell

Solar generated hydrogen from an optimized P25 thin film of 3.2 mg/cm² with 0.25% of platinum as co-catalyst improves the peak power output of a methanol microfluidic fuel cell operated with a methanol to water ratio of 1:1 almost ninefold, from 22 mW/cm² to 213 mW/cm². Different methanol to water ratios in the fuel tank generate similar amounts of hydrogen, but the cell performance has large variations due to the different oxidation kinetics of hydrogen and methanol in the fuel breathing anode, resulting in a mixed-potential anodic performance. The trade-off between power output and fuel utilization diminishes in this system. The methanol utilization efficiency at peak power operation increases from 50% (for 0.2 V) to 78% (for 0.5 V) for methanol to water ratio of 1:1. The result indicates that in-situ generation of hydrogen by solar light can be applied to both portable and large-scale stationary fuel cells.     

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.     

Current density distribution in air-breathing microfluidic fuel cells with an array of graphite rod anodes

Scale-up is required in the practical application of microfluidic fuel cells. Using an array of electrodes is demonstrated as a promising way. However, the non-uniform current density distribution in array anodes will significantly limit the power output. In this study, current density distribution in air-breathing microfluidic fuel cells with an array of graphite rod anodes is tested under acidic and alkaline conditions. The array anode is divided into four layers according to their distance to cathode. Current density of each layer is recorded individually. The cell performance under alkaline media is better than that under acidic media and various current density distributions are found under different media. When the air-breathing microfluidic fuel cell is operated under acidic media, at current densities lower than 50 mA cm^?3, current densities of two-layer anodes far from the cathode are higher than that of the other layers, while the reverse happens at current densities higher than 50 mA cm^?3. This is mainly due to the enhanced fuel transport caused by CO₂ bubbles and the lower ohmic resistance. Moreover, the generated CO₂ bubbles lead to fluctuation of discharging densities especially at low voltages. However, for the air-breathing microfluidic fuel cell operated under alkaline media, two-layer anodes far from the cathode are main contributor to the total current density at all operation voltages.     

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.