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.     

In-situ measurement of ethanol tolerance in an operating fuel cell

Ethanol is seen as an attractive option as a fuel for direct ethanol fuel cells and as a source for on-demand production of hydrogen in portable applications. While the effect of ethanol on in-situ electrode behavior has been studied previously, these efforts have mostly been limited to qualitative analysis. In alkaline fuel cells, several cathode catalysts, including Pt, Cu triazole, and Ag can be used. Here, we apply a methodology using a microfluidic fuel cell to analyze in-situ the performance of these cathodes as well as Pt anodes in the presence of ethanol and acetic acid, a common side product from ethanol oxidation. For a given concentration of ethanol (or acetic acid), the best cathode catalyst can be determined and the kinetic losses due to the presence of ethanol (or acetic acid) can be quantified. These experiments also yield information about power density losses from the presence of contaminants such as ethanol or acetic acid in an alkaline fuel cell. The methodology demonstrated in these experiments will enable in-situ screening of new cathodes with respect to contaminant tolerance and determining optimal operational conditions for alkaline ethanol fuel cells.     

PEM fuel cell performance with solar air preheating

Proton Exchange Membrane Fuel Cells (PEMFC) have proven to be a promising energy conversion technology in various power applications and since it was developed, it has been a potential alternative over fossil fuel-based engines and power plants, all of which produce harmful by-products. The inlet air coolant and reactants have an important effect on the performance degradation of the PEMFC and certain power outputs. In this work, a theoretical model of a PEM fuel cell with solar air heating system for the preheating hydrogen of PEM fuel cell to mitigate the performance degradation when the fuel cell operates in cold environment, is proposed and evaluated by using energy analysis. Considering these heating and energy losses of heat generation by hydrogen fuel cells, the idea of using transpired solar collectors (TSC) for air preheating to increase the inlet air temperature of the low-temperature fuel cell could be a potential development. The aim of the current article is applying solar air preheating for the hydrogen fuel cells system by applying TSC and analyzing system performance. Results aim to attention fellow scholars as well as industrial engineers in the deployment of solar air heating together with hydrogen fuel cell systems that could be useful for coping with fossil fuel-based power supply systems.     

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.     

Immobilization of Saccharomyces cerevisiae for application in paper-based microfluidic fuel cell

Microfluidic fuel cells that use microorganisms to oxidize different organic substances to generate electricity are gaining importance due to their versatility to use different fuels. Saccharomyces cerevisiae has used for various purposes due to its capacity to ferment broad spectrum of carbohydrates. In this research, the development of bioanodes based on the immobilization of this yeast was carried out to apply them in the evaluation of a paper lateral-flow microfluidic fuel cell. Immobilization was performed using two different supports, Vulcan carbon and graphene oxide, and four carbohydrates as fuel (saccharose, glucose, fructose, and maltose). The results indicated that the yeast is better distributed and reaches a higher capacity to oxidize carbohydrates when is immobilized on graphene oxide, this bioanode shows better performance in the microfluidic device, reaching a potential above 0.9V when saccharose are used as fuel, representing a promising approach to use microbial bioanodes in small energy conversion devices.     

A review of polymer electrolyte membrane fuel cells: Technology,applications, and needs on fundamental research

Polymer electrolyte membrane (PEM) fuel cells, which convert the chemical energy stored in hydrogen fuel directly and efficiently to electrical energy with water as the only byproduct, have the potential to reduce our energy use, pollutant emissions, and dependence on fossil fuels. Great deal of efforts has been made in the past, particularly during the last couple of decades or so, to advance the PEM fuel cell technology and fundamental research. Factors such as durability and cost still remain as the major barriers to fuel cell commercialization. In the past two years, more than 35% cost reduction has been achieved in fuel cell fabrication, the current status of $61/kW (2009) for transportation fuel cell is still over 50% higher than the target of the US Department of Energy (DOE), i.e. $30/kW by 2015, in order to compete with the conventional technology of internal-combustion engines. In addition, a lifetime of ∼2500 h (for transportation PEM fuel cells) was achieved in 2009, yet still needs to be doubled to meet the DOE’s target, i.e. 5000 h. Breakthroughs are urgently needed to overcome these barriers. In this regard, fundamental studies play an important and indeed critical role. Issues such as water and heat management, and new material development remain the focus of fuel-cell performance improvement and cost reduction. Previous reviews mostly focus on one aspect, either a specific fuel cell application or a particular area of fuel cell research. The objective of this review is three folds: (1) to present the latest status of PEM fuel cell technology development and applications in the transportation, stationary, and portable/micro power generation sectors through an overview of the state-of-the-art and most recent technical progress; (2) to describe the need for fundamental research in this field and fill the gap of addressing the role of fundamental research in fuel cell technology; and (3) to outline major challenges in fuel cell technology development and the needs for fundamental research for the near future and prior to fuel cell commercialization.     

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.     

Harvesting energy from real human urine in a photo-microfluidic fuel cell using TiO2–Ni anode electrode

The use of a liquid sample employed for analysis in microfluidic fuel cells has been increased because it can be used in order to obtain medical diagnostic and at the same time as fuel. This document presents the construction and evaluation of a photo-assisted microfluidic fuel cell (photo-μFC) that uses human urine as a fuel. For the construction of this photo-μFC, TiO₂ nanoparticles modified with Ni(OH)₂ were synthesized for use as a photoanode in the oxidation of the urea content in urine, finding an increase in the absorption of light in the visible spectrum with respect to TiO₂. Nanoparticles of TiO₂–Ni in a mixture of anatase (60%) and brookite (40%) phases were found with crystallite sizes of 9 and 15 nm, respectively. The photo-μFC proved with urine, showed an open-circuit potential of 0.70 V, a maximum current density of 1.7 mA cm^?2 and a maximum power density of 0.09 mW cm^?2. The photo-μFC developed was evaluated for 15 consecutive hours at room temperature to observe the lifetime and stability of the photoanode with respect to the generated current. In addition, the oxidation of the urea by the photogenerated holes (h⁺) in the TiO₂ was verified. This research shows the novelty of a promising advance in the use of a microfluidic fuel cell operated with a single-stream from human urine and using photoanodes (TiO₂–Ni) to obtain electrical power with a feasible application in low power portable medical devices.     

A personal retrospect on three decades of high temperature fuel cell research; ideas and lessons learned

In1986 the Dutch national fuel cell program started. Fuel cells were developed under the paradigm of replacing conventional technology. Coal-fired power plants were to be replaced by large-scale MCFC power plants fuelled by hydrogen in a full-scale future hydrogen economy. With today's knowledge we will reflect on these and other ideas with respect to high temperature fuel cell development including the choice for the type of high temperature fuel cell. It is explained that based on thermodynamics proton conducting fuel cells would have been a better choice and the direct carbon fuel cell even more so, with electrochemical gasification of carbon as the ultimate step. The specific characteristics of fuel cells and multisource multiproduct systems were not considered, whereas we understand now that these can provide huge driving forces for the implementation of fuel cells compared to just replacing conventional combined heat and power production technology.     

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.     

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.     

Manufacturing process assumptions used in fuel cell system cost analyses

This paper is a summary of the manufacturing processes used in recent automotive fuel cell system cost analyses funded by the U.S. Department of Energy (DOE). Through these analyses, DOE examines the projected cost of an 80-kW polymer-electrolyte fuel cell system manufactured at a rate of 500,000 systems per year. Directed Technologies Inc. (DTI) and TIAX LLC (TIAX) have been contracted independently to perform such analysis since 2006, and both have prior experience. This paper addresses the most recent fuel cell configurations envisioned by DTI and TIAX. DTI has recently presented their 2010 analysis results and TIAX has recently presented their 2009 results with preliminary 2010 results. Since these presentations do not document in full, the underlying details and assumptions, DTI and TIAX's most recent comprehensive written reports are used for the present discussion. DTI's most recent report detailed 2009 technology, and TIAX's most recent report detailed 2008 technology. The summary of manufacturing process assumptions is meant to impart a sense of the rigor of the cost analyses funded by the DOE, and to provide the reader with an overview of the manufacturing processes used for fuel cells.     

A plate-frame flow-through microfluidic fuel cell stack

We present a plate-frame microfluidic fuel cell architecture with porous flow-through electrodes. The architecture combines the advantages of recent microfluidic fuel cells with those of traditional plate-frame PEM fuel cells and enables vertical stacking with little dead volume. Peak current and power densities of 15.7 mA cm⁻² and 5.8 mW cm⁻² were observed. In addition to the new plate-frame architecture, microfabrication techniques have been used to create a new form of high performance electrode. Laser ablation of a polymer precursor followed by a pyrolysis process was used to create a thin, low-cost micro-porous electrode that provides for more rapid reactant transport. Here we show a 140% increase in power density compared to commercial carbon fiber paper.     

Market perspectives of stationary fuel cells in a sustainable energy supply system—long-term scenarios for Germany

Because of high efficiency, low environmental impacts and a potential role in transforming our energy system into a hydrogen economy, fuel cells are often considered as a key technology for a sustainable energy supply. However, the future framing conditions under which stationary fuel cells have to prove their technical and economic competitiveness are most likely characterised by a reduced demand for space heating, and a growing contribution of renewable energy sources to heat and electricity supply, which both directly limit the potential for combined heat and power generation, and thus also for fuel cells. Taking Germany as a case study, this paper explores the market potential of stationary fuel cells under the structural changes of the energy demand and supply system required to achieve a sustainable energy supply. Results indicate that among the scenarios analysed it is in particular a strategy oriented towards ambitious CO₂-reduction targets, which due to its changes in the supply structure is in a position to mobilise a market potential that might be large enough for a successful fuel cell commercialisation. However, under the conditions of a business-as-usual trajectory the sales targets of fuel cell manufacturers cannot be met.     

AN EVALUATION OF THE ECONOMICS OF FUEL CELLS IN URBAN BUSES

One of the contenders for a clean source of on-board electric power in a vehicle is the fuel cell: an electrochemical device that transforms the chemical energy stored in a fuel directly into electricity. While less widely known and less far advanced than batteries, fuel cells hold a considerable potential to provide the power for a novel generation of non-polluting vehicles. At present, fuel cell technology is entering the stage of commercialization, which is an appropriate moment to try and assess its economic potential in the field of transport. Based on a review of the present state of the technology, concentrating on the solid polymer fuel cell, a model is set up of a fleet of urban buses, widely regarded as one of the earliest applications of fuel cells in transport. Under the central assumption that the fuel cell stack cost is $300 per kilowatt, the fuel cell bus is found to be around 30% more expensive than its diesel counterpart. However, there are considerable cost reductions possible through economies of scale in the production of hydrogen, the fuel required for the solid polymer fuel cell. Remarkably, these economies of scale allow the cost of the fuel cell bus to drop below that of the diesel. What is more, the fleet size required for this—more than 25 vehicles—is by no means prohibitive. In a trade-off analysis, the possibility is investigated of reducing the cost by allowing one parameter to deteriorate, if this permits another one to improve. In particular, it is found that cheaper (mass-) production techniques for the fuel cell at the expense of reduced efficiency make economic sense if a relative drop in the efficiency of 1% is accompanied by a cost reduction of at least 1⋅06%—something that is likely to be attainable. A scenario for a direct methanol fuel cell, assuming a cost per kilowatt three times as high as for the solid polymer fuel cell, results in a slightly higher overall cost, without being able to offer the same economies of scale in fuel provision. The social costs are calculated also, taking into account the environmental externalities associated with the whole system including the fuel supply chain. The inclusion of externalities fails to shift the balance decisively in favour of the fuel cell, partly because of a cancellation effect between the diesel tax, which has to be excluded in a social cost calculation, and the externalities, which are added in. The main outcome is therefore that in order to succeed in the marketplace, the principal task for the developers of fuel cells is a further reduction of the cost. © 1997 by John Wiley & Sons, Ltd.     

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.     

Planar and three-dimensional microfluidic fuel cell architectures based on graphite rod electrodes

We propose new membraneless microfluidic fuel cell architectures employing graphite rod electrodes. Commonly employed as mechanical pencil refills, graphite rods are inexpensive and serve effectively as both electrode and current collector for combined all-vanadium fuel/oxidant systems. In contrast to film-deposited electrodes, the geometry and mechanical properties of graphite rods enable unique three-dimensional microfluidic fuel cell architectures. Planar microfluidic fuel cells employing graphite rod electrodes are presented here first. The planar geometry is typical of microfluidic fuel cells presented to date, and permits fuel cell performance comparisons and the evaluation of graphite rods as electrodes. The planar cells produce a peak power density of 35 mW cm⁻² at 0.8 V using 2 M vanadium solutions, and provide steady operation at flow rates spanning four orders of magnitude. Numerical simulations and empirical scaling laws are developed to provide insight into the measured performance and graphite rods as fuel cell electrodes.     

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.     

High performance formic acid fuel cell benefits from Pd–PdO catalyst supported by ordered mesoporous carbon

Formic acid as a renewable fuel can be converted to clean electricity in fuel cells by high-efficient electrochemical oxidation. The conversion rate is fundamentally dictated by the synergy of interactive aspects: catalytic activity, accessibility to active sites, electron transfer, and anti-poisoning stability. For the first time, ordered mesoporous carbon (OMC) is used as the substrate for Pd–PdO catalyst for fuel cells. The unique ordered pore-channel network of OMC can enhance the spatial dispersion of Pd nanoparticles on the pore-channel wall, while the hollow pore-channel can facilitate reactant transport. Microwave and annealing treatments are found to enhance the chemical reduction and to strengthen the anchoring of Pd–PdO catalyst on OMC substrate, respectively. The OMC supported Pd–PdO catalyst (Pd–PdO/OMC) shows 1.7-fold and an order of magnitude higher mass activity and stability as compared to commercial Pd/C catalyst. For fuel cell testing, the Pd–PdO/OMC catalyst is applied to an air-breathing microfluidic fuel cell and achieves a maximum power density of 63.0 mW cm⁻², at least one-fold higher than similar previous reports.     

Energy harvesting by implantable abiotically catalyzed glucose fuel cells

Implantable glucose fuel cells are a promising approach to realize an autonomous energy supply for medical implants that solely relies on the electrochemical reaction of oxygen and glucose. Key advantage over conventional batteries is the abundant availability of both reactants in body fluids, rendering the need for regular replacement or external recharging mechanisms obsolete. Implantable glucose fuel cells, based on abiotic catalysts such as noble metals and activated carbon, have already been developed as power supply for cardiac pacemakers in the late-1960s. Whereas, in vitro and preliminary in vivo studies demonstrated their long-term stability, the performance of these fuel cells is limited to the μW-range. Consequently, no further developments have been reported since high-capacity lithium iodine batteries for cardiac pacemakers became available in the mid-1970s. In recent years research has been focused on enzymatically catalyzed glucose fuel cells. They offer higher power densities than their abiotically catalyzed counterparts, but the limited enzyme stability impedes long-term application. In this context, the trend towards increasingly energy-efficient low power MEMS (micro-electro-mechanical systems) implants has revived the interest in abiotic catalysts as a long-term stable alternative. This review covers the state-of-the-art in implantable abiotically catalyzed glucose fuel cells and their development since the 1960s. Different embodiment concepts are presented and the historical achievements of academic and industrial research groups are critically reviewed. Special regard is given to the applicability of the concept as sustainable micro-power generator for implantable devices.