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.     

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.     

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.     

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.     

Filter paper membrane based microfluidic fuel cells: Toward next-generation miniaturized and low cost power supply

Micro Fuel cells or microfluidic fuel cells (μMFCs) are one of the most promising power supplies for portable electronics. However, the necessary electrode spacing is required to prevent fuel-crossover and maintain the stable operation, introducing the unavoidable ohmic resistance and retarding the miniaturization. Herein, we propose a novel μMFC device combining the cellulose paper as separator, with selective catalysts at the cathode side to eliminate the unwanted side reactions and increase the system compactness. One single reactant solution containing fuel and electrolyte is applied to keep the device stable operation. The power-generation properties are evaluated in typical alkaline conditions. A great construction simplification makes the device a substantial high-power density of 2.14 W cm^?3 and maximum current density of 15.82 A cm^?3. The μMFC stacks are arranged in series and parallel manners, which delivers a maximum power output of 23.6 mW and current of 194.6 mA. It is expected that innovative and customizable performance from commercial paper and low-cost carbonaceous catalysts can provide a forum for future advancement in chip-based electrochemical energy generation and storage devices.     

A membraneless microfluidic fuel cell stack

A membraneless microfluidic fuel cell stack architecture is presented that reuses reactants from one cell to a subsequent one, analogous to PEMFC stacks. On-chip reactant reuse improves fuel utilization and power densities relative to single cells. The reactants flow separately through porous electrodes and interface with a non-reacting and conductive electrolyte which maintains their separation. The reactants remain separated downstream of the interface and are used in subsequent downstream cells. This fuel cell uses porous carbon for electrocatalysts and vanadium redox species as reactants with a sulfuric acid supporting electrolyte. The overall power density of the fuel cell increases with reactant flow rate and decreasing the separating electrolyte flow rate. The peak power, maximum fuel utilization, and efficiency nearly double when electrically connecting the cells in parallel.     

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.     

Microfluidic fuel cells: A review

A microfluidic fuel cell is defined as a fuel cell with fluid delivery and removal, reaction sites and electrode structures all confined to a microfluidic channel. Microfluidic fuel cells typically operate in a co-laminar flow configuration without a physical barrier, such as a membrane, to separate the anode and the cathode. This review article summarizes the development of microfluidic fuel cell technology, from the invention in 2002 until present, with emphasis on theory, fabrication, unit cell development, performance achievements, design considerations, and scale-up options. The main challenges associated with the current status of the technology are provided along with suggested directions for further research and development. Moreover, microfluidic fuel cell architectures show great potential for integration with biofuel cell technology. This review therefore includes microfluidic biofuel cell developments to date and presents opportunities for future work in this multi-disciplinary field.     

Copper hexacyanoferrate as cathode material for hydrogen peroxide fuel cell

The current need for handheld electronic devices with high energy autonomy has amplified research into clean and mobile energy source developments. Among suitable and promising technologies for this application, fuel cells, FCs are highlighted because of their minimal emission of pollutants and high efficiency. One type of FCs that has yet to be studied is the hydrogen peroxide/direct hydrogen peroxide fuel cell (DPPFCs). The present work is dedicated to the development of DPPFCs of one compartment using copper hexacyanoferrates (CuHCFs) as cathodic material and a Ni grid as anodic material. CuHCFs containing Fe^II and/or Fe^III were synthesized, characterized and their electrocatalyst performances were compared in 0.1 mol L⁻¹ HCl and 0.5 mol L⁻¹ H₂O₂. The maximum power densities reached for the CuFe^II was 8.3 mW cm⁻² and for the CuFe^IIFe^III was 2.9 mW cm⁻². The CuHCFs cathode materials show promising results, standing out as innovative materials for such an application.     

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 review on membraneless laminar flow-based fuel cells

The review article provides a methodical approach for understanding membraneless laminar flow-based fuel cells (LFFCs), also known as microfluidic fuel cells. Membraneless LFFCs benefit from the lamination of multiple streams in a microchannel. The lack of convective mixing leads to a well-defined liquid-liquid interface. Usually, anode and cathode are positioned at both sides of the interface. The liquid-liquid interface is considered as a virtual membrane and ions can travel across the channel to reach the other side and complete the ionic conduction. The advantage of membraneless LFFC is the lack of a physical membrane and the related issues of membrane conditioning can be eliminated or becomes less important. Based on the electrode architectures, membraneless LFFCs in the literature can be categorized into three main types: flow-over design with planar electrodes, flow-through design with three-dimensional porous electrodes, and membraneless LFFCs with air-breathing cathode. Since this paper focuses on reviewing the design considerations of membraneless LFFCs, a concept map is provided for understanding the cross-related problems. The impacts of flow and electrode architecture on cell performance and fuel utilization are discussed. In addition, the main challenges and key issues for further development of membraneless LFFCs are discussed.     

A development of direct hydrazine/hydrogen peroxide fuel cell

A direct hydrazine fuel cell using H₂O₂ as the oxidizer has been developed. The N₂H₄/H₂O₂ fuel cell is assembled by using Ni-Pt/C composite catalyst as the anode catalyst, Au/C as the cathode catalyst, and Nafion membrane as the electrolyte. Both anolyte and catholyte show significant influences on cell voltage and cell performance. The open-circuit voltage of the N₂H₄/H₂O₂ fuel cell reaches up to 1.75 V when using alkaline N₂H₄ solution as the anolyte and acidic H₂O₂ solution as the catholyte. A maximum power density of 1.02 W cm⁻² has been achieved at operation temperature of 80 °C. The number of electrons exchanged in the H₂O₂ reduction reaction on Au/C catalyst is 2.     

Performance improvement in direct borohydride/peroxide fuel cells

Direct borohydride/peroxide fuel cells (DBPFCs) show progressively deteriorating performance during operation for various reasons such as decreasing reactant concentrations, gas evolution and uneven distribution of liquids. The present study aims to emphasize the importance of certain design parameters, such as bipolar plate materials, flow fields and manifold design, in determining the DBPFC performance. Bipolar materials and flow channel design have been investigated. A power density of 67 mW cm^?2 has been obtained with composite graphite and parallel flow channel bipolar plates. It has increased to 87 mW cm^?2 using sintered graphite and then to 93.3 mW cm^?2 using sintered graphite with serpentine flow fields. The stacking of DBPFCs results in a loss of performance and unstable output. The performance has remained nearly unchanged as the cell number was increased by applying an independent cell liquid distribution network (ICLDN). Using an ICLDN, power densities of 98.3, 83.3 and 82 mW cm^?2 have been obtained for single-cell, 3-cell and 6-cell stacks, respectively. Finally, a controlled oxidant feeding system (COFS) has been developed to provide stable output power, and it has demonstrated a stable output power of 6 W for 2.5 h.     

An enhanced microfluidic control system for improving power density of a hydride-based micro fuel cell

Microfuel cells (MFCs) can potentially power emerging technologies that require power sources in the microliter size range. The recent development of a microfluidic mechanism for self-regulated generation of hydrogen has enabled fabrication of MFCs orders of magnitude smaller than previously possible. In this study, we report an order of magnitude enhancement in the power density of a microliter-scale fuel cell incorporating a new microfluidic design. The microfluidic mechanism is part of an on-board hydrogen generator that uses a reaction between a metal hydride, LiAlH₄, and water vapor to generate hydrogen. The hydrogen generated exits the hydride reactor through a porous silicon wall to reach a Nafion-based membrane electrode assembly (MEA). The microfluidic design increased the water vapor release rate to the hydride reactor by one order of magnitude over a previous design. A 9 μL device incorporating the enhanced microfluidic design delivered a power density of 92 W L⁻¹. Details of a parametric study conducted to improve the water vapor release rate of the microfluidic mechanism and performance analysis of the integrated device are presented in this paper.     

Counter flow membraneless microfluidic fuel cell

A counter flow membraneless microfluidic fuel cell is presented, where a non-reacting electrolyte separates the reacting streams. In this fuel cell design, vanadium reactants flow through porous carbon electrocatalysts. A sulfuric acid stream is introduced in the gap between the electrodes and diverts the reactants to opposite and independent outlets. This fuel cell differs from other membraneless designs in its ability to maintain a constant separation between the reactants without diffusive mixing.     

In situ visualization of biofilm formation in a microchannel for a microfluidic microbial fuel cell anode

A novel in situ approach is proposed to visualize biofilm formation in the microchannel for the microfluidic microbial fuel cell (MMFC) anode, which could reflect a more precise biofilm formation during start-up process in real-time. A microchannel reactor was designed and fabricated based on a transparent indium-tin-oxide (ITO) conductive membrane. In situ visualization of biofilm formation under various anolyte flow rates was captured by a phase contrast microscope combined with a custom long working distance objective. The results show that no steady biofilm is formed on the surface of anode under low flow rate of 50 μL min^?1 because of the insufficient nutrient supply. With increasing the anolyte flow rate, more attached bacteria on the anode surface and denser biofilm are observed in the microchannel. Less bacteria are attached on the surface of anode along flow direction due to the entrance effect. However, denser biofilm leads to larger mass transfer resistance of the anolyte and product in biofilm. Therefore, a superior bioelectrochemical performance is yielded for the biofilm formed under a moderate flow rate during start-up process.     

Electrochemical analysis of hydrogen membrane fuel cells

An electrochemical analysis was conducted with respect to a hydrogen membrane fuel cell with SrZr_0.8In_0.2O_3−δ electrolyte, which is a new type of fuel cell featuring an ultra-thin proton conductor supported on a dense metal anode. Most of the voltage loss derives from the cathode and the electrolyte, and a small amount of anode polarization was observed only in regions with high current density. The cathode polarization was approximately an order of magnitude lower than that of SOFCs. Furthermore, the conductivity of the film electrolyte was almost identical to that of the sinter at 600 °C; however, it was several times as large at 400 °C. A TEM micrograph revealed that the film electrolyte consists mainly of long columnar crystals, and this crystal structure can be related to the conductivity enhancement below 600 °C.     

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.     

Investigation of bubble effect in microfluidic fuel cells by a simplified microfluidic reactor

This study experimentally examines the influence of two-phase flow on the fluid flow in membraneless microfluidic fuel cells. The gas production rate from such fuel cell is firstly estimated via corresponding electrochemical equations and stoichiometry from the published measured current–voltage curves in the literature to identify the existence of gas bubble. It is observed that O₂ bubble is likely to be generated in Hasegawa’s experiment when the current density exceeds 30 mA cm^?2 and 3 mA cm^?2 for volumetric flow rates of 100 μL min^?1 and 10 μL min^?1, respectively. Besides, CO₂ bubble is also likely to be presented in the Jayashree’s experiment at a current density above 110 mA cm^?2 at their operating volumetric liquid flow rate, 0.3 mL min^?1. Secondly, a 1000-μm-width and 50-μm-depth platinum-deposited microfluidic reactor is fabricated and tested to estimate the gas bubble effect on the mixing in the similar microchannel at different volumetric flow rates. Analysis of the mixing along with the flow visualization confirm that the membraneless fuel cell should be free from any bubble, since the mixing index of the two inlet streams with bubble generation is almost five times higher than that without any bubble at the downstream.     

A fuel cell with selective electrocatalysts using hydrogen peroxide as both an electron acceptor and a fuel

We have realized a novel hydrogen peroxide fuel cell that uses hydrogen peroxide (H₂O₂) as both an electron acceptor (oxidant) and a fuel. H₂O₂ is oxidized at the anode and reduced at the cathode. Power generation is based on the difference in catalysis toward H₂O₂ between the anode and cathode. The anode catalyst oxidizes H₂O₂ at a more negative potential than that at which the cathode catalyst reduces H₂O₂. We found that Ag is suitable for use as a cathode catalyst, and that Au, Pt, Pd, and Ni are desirable for use as anode catalysts. Alkaline electrolyte is necessary for power generation. The performance of this cell is clearly explained by cyclic voltammograms of H₂O₂ at these electrodes. This cell does not require a membrane to separate the anode and cathode compartments. Furthermore, separate paths are not needed for the fuel and electron acceptor (oxidant). These properties make it possible to construct fuel cells with a one-compartment structure.