Fuel cells have gained importance in the research field due to the arrival of portable electronic devices in the market, which require power supplies with high energy density and longer life span for the varied functionalities in these devices. The miniaturization of the electronic devices has also lead to research being directed towards miniaturization of fuels to provide energy for these devices. Over the past decade research in fuel cells has been focused on making micro fuel cells, which can be used in portable electronic devices. This research has gained importance as fuel cells are small, provide high energy density and use lighter materials such as alcohol, formic acid, gases such as hydrogen/oxygen and hydrides for fuel [1].
Although, currently batteries are used to supply energy to portable electronic devices, but fuel cells have certain advantages like the use of lighter materials to produce energy as compared to the batteries, which store their energy in heavy, voluminous materials such as metal oxides and graphitic materials. Moreover, the batteries are heavy, bulky, of limited charge and cause pollution. The two main hurdles that have to be overcome for commercialization of micro fuel cells are energy density and efficient utilization of fuel [1].
Fuel cells contain two types of chemical species- fuel and the oxidant. Some fuel cells may use an electrolyte to enhance the reaction kinetics by increasing the ionic conductivity. The research in micro fuel cells can be divided into two: (1) micro fuel cells- which use a membrane to separate the fuel and the oxidant; (2) membraneless micro fuel cells- which are membrane-less, and instead use co-laminar flow of the oxidant and the fuel to keep them from mixing with each other. The research in membraneless micro fuels is more recent in comparison to the micro fuel cells which use a membrane. In this paper, we will briefly describe the micro fuel cells and the advances in them, and then we devote the rest of the paper to membraneless micro fuel cells.
2. MICRO FUEL CELLS
The energy densities of micro fabricated fuel cells range from µW/ cm2 to hundreds of mW/ cm2. The basic components in all these cells are the same: electrodes with flow fields, current collectors, diffusion layers, catalyst layers, and proton exchange media (membrane). The design approaches for these cells are of two types: (1) the bipolar design, where all the components of the cell are stacked together and the fuel and oxidant are separated by the membrane electrode assembly; (2) the planar design where the fuel and oxidant channels are interdigitated and both the electrodes are on the same side. The two designs are shown in figure 1and 2 [1].
In the bipolar design, the fuel and the oxidant are separated by the proton exchange membrane, but all the components are fabricated separately and then put together. The planar design requires large surface area to give similar performance but is more suited for large scale integration [1].
Figure 1. Bipolar design [1]
Figure 2. Planar design [1]
2.1 DIRECT METHANOL FUEL CELLS
The fuels used in these cells are usually hydrogen, methanol, formic acid or ethanol. Methanol has high energy density, but reaction kinetics are poor, and to improve the output noble metals are used as catalysts, which increase the cost. Concentrated methanol cannot be used, as it leaks through the membranes, and the permeation of fuel through the electrolyte is hindered. Dilution leads to reduction in energy density, and the electro-osmotic drag causes water molecules to be transported across the membrane from anode to cathode. This decreases the energy density and causes water management problems [1].
Methanol otherwise can directly be used as fuel in fuel cells, and energy density as high as 100 mW/ cm2 at 60°C and 34 mW/ cm2 at room temperature have been reported with 4 M methanol. Most micro fuel cells use silicon as substrate on which to fabricate the electrodes and flow fields taking advantage of many microfabrication processes available. Apart from silicon, carbon paper, polydimethylsiloxane, SU-8 epoxy resin, copper and stainless steel metal foils have been used as substrates. In the micro fuel cell quoted above, silicon was replaced with thin stainless steel plates and the flow fields were fabricated by photochemical etching (Earlier when silicon was used the power density was found to be 16 mW/ cm2). A gold layer was deposited on the stainless steel plates to prevent corrosion. Nafion was used as the proton exchange membrane. The flow fields for these fuel cells are fabricated using silicon or metal or polymers using various microfabrication processes such as reactive ion etching, photolithography, wet or dry etching. Catalysts for improving reaction kinetics are deposited using physical vapor deposition or sputtering. The proton exchange membrane may be Nafion or its composite or a polymer material such as polyimides [1].
Researchers use various combination of microfabrication schemes to fabricate micro fuel cells. In another approach Liu et al. sandwiched a Nafion membrane between two silicon substrates. Photolithography followed by wet etching was used to modify the two substrates. Gold electrodes were deposited using physical vapor deposition. Laser beams were used to fabricate the flow channels on the anode side followed by silica-glass anodic bonding. After assembling the membrane and electrodes epoxy resin was used as sealant, and the Nafion membrane was modified by X-ray radiation and palladium deposition to reduce methanol crossover [1].
2.2 HYDROGEN FUEL CELLS
Hydrogen has storage problems, as it cannot be stored either as compressed gas or as liquid. Storage in the form of hydrides or other chemicals is possible, but this adds to the complexity and cost of the device. Hydrogen fuel cells have been manufactured using silicon, metals and polymers as substrates for the microchannels. The flow fields were fabricated using the microfabrication processes mentioned above. Power densities as high as 193 mW/ cm2, 250 mW/ cm2 and 315 mW/ cm2 have been reported with silicon, stainless steel and PMMA as substrates, respectively. With silicon substrate the membrane electrode assembly was sandwiched between micromachined Si substrates and depositing Cu between layers of gold to decrease the internal resistance. Such metal layers are needed for current collecting. Various researchers use metals such as Ni or Cu or Au layers with different microfabrication schemes to form current collectors.
The basic microfabrication steps in most micro fuels involve sandwiching the membrane electrode assembly between silicon substrates and then sealing them with epoxy. Researchers use different schemes to pattern the substrates initially and to develop the flow channels. The flow fields are fabricated by wet etching with KOH or reactive ion etching. The use of metals and polymers as substrates involve use of similar patterning techniques as mentioned above. The metals substrates can be used as current collectors. Micro fuel cells with metal substrates were found to have good mechanical strength. Reformed hydrogen fuel cells are those in which hydrogen is supplied from a previous process such as reforming of a hydrocarbon or from a chemical hydride such as NaBH4 [1].
2.3 FORMIC ACID FUEL CELL
With formic acid as fuel, power densities as high as 177 mW/ cm2 have been reported at 0.53 V with 10 M formic acid. This was achieved when Pd black was used as catalyst at the anode side. Nafion was used as membrane and the current collectors were fabricated using gold coated titanium. Earlier, when Pt-Ru black was used as anode catalyst, the maximum power density was found to be 33 mW/ cm2 with 8.8 M formic acid. Other researchers have reported power densities of 34 mW/ cm2 with 5 M formic acid (this was improved up to 94 mW/ cm2 by depositing gold Pd alloy film on both anode and cathode catalysts) and 28 mW/ cm2 [1]. The microfabrication procedures for these cells involve techniques previously discussed above.
A fuel cell operated with ethanol as fuel with platinum deposited porous silica electrodes produced a maximum power density of 8.1 mW/ cm2 with 8.5 M ethanol solution at room temperature [1].
Although, the micro fuel cells discussed above have high power density, they have several disadvantages like water management problems, the use of proton exchange membrane and catalysts such as platinum makes these cells expensive. Furthermore, one has to be selective with the type of electrolyte, fuel and membrane being used, as there is chance that the some chemical may be corrosive towards the membrane. The use of membrane makes the fabrication process more complicated. Moreover, fuel crossover across the membrane leads to losses and poor performance of the cell. Therefore, of late researchers have turned their focus towards fabricating membrane-less micro fuel cells. Such micro fuel cells make use of micro fluidics, where laminar flow of fuel and oxidant is used to prevent them from mixing with each other. These membraneless micro fuel cells are described in detail in the forthcoming section [1], [2], [3].
3. MEMBRANELSS MICRO FUEL CELLS
Membraneless micro fuel cells are also known as microfluidic fuel cells or laminar flow fuel cells. Membraneless micro fuel cells utilize the laminar flow of the cathode and anode streams, which prevents them from mixing. The laminar flow regime is determined by the dimensionless Reynolds number which is Re= DUρ/µ, where ρ is the fluid density, U is the fluid velocity, µ is the fluid viscosity and D is the hydraulic diameter. Laminar flow is said to occur when Re < 2300. The fluid flow in micro fluidic fuel cells is maintained at values of Re below 2300. At low Reynolds number, both fluids flow parallel to each other without much intermixing of the two fluids. There is some intermixing due to diffusion, and a diffusion boundary layer is formed near the interface of the two liquids. If this diffusion boundary layer gets close to the electrodes, then the cell performance is poor. Thus it should be ensured that the boundary layer does not penetrate too deep into either the anolyte or the catholyte stream [2], [3].
The membraneless micro fuel cell concept was invented in 2002. The first such fuel cell used vanadium species in both cathodic (V (V)/ V (IV)) and anodic (V (III)/ V (II)) compartments. Both the anodic and cathodic streams were made to flow in co-laminar fashion in a T or Y- shaped channel with graphite electrodes. The micro channels were fabricated by soft lithography in polydimethylsiloxane or in SU-8 photoresist using conventional lithography. The solution concentrations were 1 M for V (V) and V (II) solutions and 10-3 M for V (III) and V (IV) solutions. With three cells in series a power density of 192 mW/ cm2 was obtained, but the fuel utilization was found to be very low at ~0.1%. Fuel utilization is the amount of fuel consumed at the electrodes and converted to current to the total amount of fuel delivered to the cell. These results were obtained with a channel thickness of 200 µm. When the channel thickness was reduced to 50 µm, the fuel utilization improved and was found to be ~10% [4].
Since the advent of the membraneless micro fuel cell in 2002 various researcher have made efforts in this direction with different channel shapes and electrode configuration. Some of these are depicted in figure 3 and 4 [2].
Figure 3 (a) Electrodes on bottom; (b) Electrodes on sides; (c) Porous electrodes at the bottom; (d) F-channel configuration; (e) F-channel with porous air-breathing electrodes [2]
Figure 4 (f) An electrode array micro fluidic fuel cell; (g) A flow-through porous electrode membraneless micro fuel cell; (h) A radial porous electrode architecture [2].
Ever since its invention in 2002, several developments have occurred in the field of membraneless micro fuel cells. Various researchers have tried to make these fuel cells using different fuels, apart from the commonly used methanol, formic acid and hydrogen. Kjeang et al. fabricated a fuel cell on a polyurethane substrate with carbon electrodes. A Y-shaped micro channel was fabricated using microfabrication techniques on the polyurethane substrate. Polyurethane was coated on a microscope glass slide and cured on a hot plate at 60°C to solidify. The carbon electrodes were patterned on the substrate from toray carbon paper by fitting carbon strips on aluminum holders by heating the substrate to 160°C on another aluminum holder on a hot plate. The softened polyurethane containing aluminum holder was pressed together with the aluminum holder containing carbon strip to deposit the carbon electrodes on the polyurethane substrate. The top part of the fuel cell containing the 120 µm high and 2 mm wide micro channel was fabricated by replica molding using SU-8 negative photoresist in a polydimethylsiloxane mold using soft lithography tools. Aluminium clamps were used as reversible seals to join the two parts. This type of clamping facilitates cleaning and substitution of channel structure. Vanadium redox couple V2+/ V+3 and VO2+/ VO2+ were used as both fuel and oxidant. During cell operation at very low flow rates of 1000 µl/ min the power density was found to be 70 mW/ cm2 and at flow rates of 1 µl/ min a single pass fuel efficiency of 55% was reported. This value is very high compared to the one obtained with the first vanadium redox couple fuel cell [5].
The major advantages with such a fuel cell are that the aqueous fuel oxidant combination is soluble at relatively high concentrations of 5.4 M; expensive metal catalysts such as Pt and Pd are not required; a high open circuit voltage (~1.5 V) can be obtained at uniform pH; water management problems are eliminated; fuel loss due to crossover is reduced and since the cell is membrane-less the cost is reduced [5].
Kjeang et al. used the vanadium redox couple fuel cell to compare the working of a planar and three-dimensional cell architecture consisting of a hexagonal array of graphite electrodes. The micro channels were fabricated on polyurethane substrates using microfabrication techniques employed with the fuel cell mentioned above. The electrodes in the planar configuration were found to have an active area of 0.165 cm2.
In the three-dimensional cell architecture 29 graphite rods were mounted inside a CNC machined cavity drilled in a block of Delrin. Arrays of holes were drilled at the end plates to hold and separate the rods. The 0.5 mm diameter graphite rods were fitted in the cavity through these holes and were sealed using SPR 220-7.0 positive photoresist. The top wall of the cell was formed by a slab of polydimethylsiloxane and an aluminum clamp was used as seal during operation of the cell. Total active area exposed to the fluid was found to be 7.3 cm2 per electrode. In the planar configuration, the fuel cell was operated at flow rates varying from 2.5-300 µl/min. The highest power density was obtained at 0.6-0.9 V at a flow rate of 300 µl/min for 2 M vanadium concentrations and was found to be 35 mW/cm2. The highest fuel utilization obtained was 63% for 1 M solution at 2.5 µl/min. Thus the optimum operating conditions here are cell operating at 2.5 µl/min with 2 M vanadium solution with power density of 11 mW/cm2 at 0.6 V with single pass fuel utilization of 38%. For the array cell operation the flow rates were varied between 10-3000 µl/min, and it was found that the for all flow rates above 10 µl/min, the array configuration delivered higher power output than the planar configuration. The highest power output was found to be 28 mW at 0.6 V and 2000 µl/min. The highest fuel utilization was reported as 78% and at any flow rate above 30 µl/min, the fuel utilization for array cell was approximately an order of magnitude higher than the planar cell. Practical operating conditions for the array cell were identified to be at 30 µl/min, with power output as 6.6 mW at 0.5 V with 43% single pass fuel utilization. Thus the three dimensional array cell configuration was found to produce an order of magnitude higher power output than the planar cell with higher fuel utilization. Further, the array cell provides greater flexibility in power conditioning in series/parallel configurations and there is scope for vertical expansion without additional fluid manifold requirements or performance loss [6].
In further efforts on micro fluidic fuel cells by the Kjeang group, they fabricated a fuel cell with formic acid as fuel and hydrogen peroxide as oxidant. The advantage of hydrogen peroxide is that it is available in high concentrations, highly soluble and enables fuel cell operation under anaerobic conditions. The micro channel and electrodes were microfabricated on a gold slide, which was subject to etching and photolithography to attain the desired structure. The electrodes of platinum and palladium were patterned using SU-8 negative photoresist by electrodepositing them from plating solutions. The uncovered gold was removed using gold etching solution and chromium etching solutions. The remaining photoresist was removed by sonication in acetone. The T-shaped micro channel was molded in polydimethylsiloxane using soft-lithographic techniques. Ports for fluid handling were punched into the polydimethylsiloxane substrate prior to assembly. The substrate was plasma treated for 30s to yield silanol groups that covalently bind glass and polydimethylsiloxane to give hydrophilic channel walls.
The fuel cell performances for platinum and palladium electrodes were compared based on micro channel height (70 and 170 µm), flow rates (60 and 300 µl/ min) and channel cross section geometry. Six cells were fabricated and tested. One of the cells had a grooved cross section geometry restricted oxygen gas bubble growth, produced during the reaction, and transport to the vicinity of electrodes preventing cell performance from going down. This fuel cell could be operated on flow rates as low as 3 µl/ min. The maximum power density was reported to be 30 mW/ cm2 at 300 µl/ min for a 2 M hydrogen peroxide solution at room temperature [7].
Kjeang et al. improved on the fuel utilization efficiency obtained previously by increasing active electrode area by fabricating a fuel cell with flow-through porous electrodes [8] and comparing its performance with a previously fabricated flow-over porous electrode [5]. The porous carbon electrodes were fabricated from toray carbon paper with 78% porosity by replica molding in polydimethylsiloxane molds. The toray carbon is coated with polytetrafluoroethylene, to form hydrophobic surface, which is removed by depolymerization and annealing in propane flame. The channel structures were obtained using photolithography with negative SU-8 50 photoresist, which had two layers each 150 µm thick. The first layer contained all flow channels and electrode grooves and the second layer contained electrode grooves and channel patterns from inlets to the electrodes. The polydimethylsiloxane part was placed face up on microscopic glass slide with he heat treated carbon electrodes fitted in its grooves. This completed the bottom part of the cell. The top of the cell was sealed with a polydimethylsiloxane with punched holes for inlets, outlets and electrical contacts. The top and bottom parts were fused by plasma treatment to create a hydrophobic seal. Vanadium redox couple was used as catholyte and anolyte solutions. The flow-through electrodes were found to enhance the active electrode area and increase the species transport from bulk to the active sites. The highest power density was reported to be 121 mW/ cm2 at 300 µl/ min at 0.8 V. This value was found to be twice that of the power density obtained with flow over porous electrodes. At 1 µl/ min the power density was found to increase by four times. The single pass fuel utilization efficiency was found to be 94% at 0.8 V as compared to the 60% obtained for flow-over type fuel cell. Thus most of the fuel was found to be utilized during the fuel cell operation and co-laminar streams at the end contained mainly the waste products. The cell could be operated in an electrolytic format to carry out in-situ regeneration of the reactants [8].
Kjeang et al. fabricated a fuel cell based on formic acid and hypochlorite bleach under alkaline conditions to improve the reaction kinetics. Alkaline formic acid and hypochlorite bleach were found to be soluble at high concentrations. Flow through porous electrodes of gold and palladium were formed by electrodepositing these on a porous carbon electrode. The flow fields were fabricated by microfabrication techniques used earlier [7], [8]. The cell was operated over flow rate range of 2-300 µl/ min. Power density as high as 52 mW/ cm2 with single pass fuel utilization efficiency up to 30% were obtained with 1.2 M formate fuel and 0.67 M hypochlorite at room temperature. The advantage of using alkaline conditions is that gaseous CO2 formation is prevented by absorption of the carbonates while improving the reaction kinetics [9]. In all their experiments Kjeang et al. have used syringe pump with Teflon tubing to deliver the fuel and oxidant streams to the fuel cell.
Lee et al. did theoretical and experimental work to optimize the operating conditions for a micro fluidic fuel cell by studying the role of flow rate, micro channel geometry, and location of electrodes on cell performance. Their results indicate that with multiple electrodes the performance of second and subsequent electrodes increases as the gap between the electrodes increases and fuel cells with shorter electrodes offer the highest efficiency at the lowest cost of electrocatalyst. In fuel cell designs with electrodes on top and bottom surfaces of the micro channel the fuel consumption increases as compared to cells with electrodes only at the bottom surface. Their results show that cell designs with short electrodes on top and bottom are 3 times more efficient than cell designs with long electrodes only on the bottom of the micro channel. This leads to an increase in current density while reducing the amount of unconsumed fuel [10].
Choban et al. have performed experiments to compare a micro fluidic fuel cell with 2.1 M formic acid as fuel and 0.144 M potassium permanganate and oxygen dissolved in 0.5 M sulphuric acid as oxidants. The Y-shaped micro channel with the electrodes on the inner side wall was fabricated using soft lithographic techniques using SU-8 photoresist on polydimethylsiloxane substrate. First, a negative of the channel is obtained on the thick photoresist. This is then replicated into a polydimethylsiloxane mold to obtain a positive relief structure of the microfluidic channel. A liquid UV curable adhesive layer is applied over the elastomeric mold followed by an elastomeric material clamped on top. This assembly is UV treated and the elastomeric top layer and positive relief elastomeric mold are peeled away to give the central support structure of the Y-shaped microfluidic channel. Electrode adhesive layer was deposited by chromium sputtering, followed by gold deposition as seed layer. Platinum black was deposited as catalyst by contact electrodeposition. After applying the catalyst, the central support structure is clamped between polydimethylsiloxane support to form the top and bottom of the micro fuel cell. Polyethylene tubing is fitted into one of the two slabs of polydimethylsiloxane to guide the fluids into the channels [11].
The results achieved show that when permanganate is used as anode fuel with 10% formic acid as cathode fuel, a higher fuel efficiency of 32% is obtained, and the current density is 10 times higher when compared to fuel cell with formic acid as fuel and oxygen dissolved sulphuric acid as oxidant at flow rate of 0.5 ml/ min. these results could be improved if the transport of reactants from the boundary layer to the electrode is improved and low concentration of oxidant is used in the cathode stream [11].
Choban et al. demonstrated the use of nanoparticles as catalyst for micro fluidic fuel cell with 1 M methanol as fuel and oxygen dissolved in 0.5 M sulphuric acid as oxidant. Three different anode catalysts were used: (1) unsupported Pt: Ru nanoparticles (50: 50 atomic wt %); (2) 50: 50 mixture of unsupported Pt black and Ru nanoparticles; (3) unsupported platinum nanoparticles. Unsupported platinum nanoparticles were also used as cathode catalysts. The use of an external reference electrode enabled the measurement of performance of the overall fuel cell as well as the individual electrodes (cathode and anode). The microfluidic channels were fabricated as described in the cell mentioned above [11], except that graphite plates were used as catalyst support, current collector and edificial element. They reported current densities of 8-10 mA/ cm2 as compared to the earlier fuel cells operated with formic acid as fuel and single metal catalysts, which gave current densities of only 0.8 mA/ cm2. The use of bimetallic catalysts also addressed the problem of CO poisoning of the catalysts. The maximum power densities for these fuel cells were found to be 2.8 mW/ cm2 [12].
The use of dissolve oxygen as oxidant in membraneless micro fuel cells has a major disadvantage that oxygen has low solubility in liquids. Jayashree et al. tried to overcome this problem by fabricating an air-breathing laminar flow based fuel cell using formic acid as fuel. They addressed the problem of low oxygen solubility in liquids by using porous gas diffusion electrodes as cathode. Since diffusivity of oxygen in air is four orders of magnitude higher than in aqueous media, this cell is expected to have better performance than those which use oxygen dissolved aqueous solutions as oxidant. The best performance of the cell was observed at formic acid concentration of 1 M where the current density was found to be 130 mA/ cm2 and the power density was found to be 26 mW/ cm2. This was compared with a fuel cell operated without gas diffusion electrodes using oxygen dissolve in 0.5 M sulphuric acid as oxidant. The single pass fuel utilization with the gas diffusion electrodes was found to be 8%, and without the gas diffusion electrodes it was found to be <1%.With the gas diffusion electrodes a single pass fuel utilization of 33% can be achieved at low flow rates while compromising the power density to 10 mW/ cm2 [13].
Jayashree et al. also compared the operation of a laminar flow micro fluidic fuel cell under alkaline and acidic conditions with methanol as fuel. They reported that the fuel cell operation was better under alkaline conditions due to improved reaction kinetics. The current density under alkaline conditions was found to be 40% higher than that under acidic condition. The power densities under acidic and alkaline conditions were found to be 11.8 and 17.2 mW/ cm2, respectively. The improvement in the performance of the cell can be attributed to improved reaction kinetics at the anode under alkaline conditions. Another advantage of membrane-less operation under alkaline conditions is that any carbonates that are formed are washed away. Membrane clogging due to deposition of carbonates in the pores of the membrane was one of the major problems that hampered the development of micro fuel cells with membranes operating under alkaline media. The substrate used for micro channel fabrication was polymethylmethacrylate(PMMA). Graphite plates loaded with Pt-Ru (50: 50 wt %) was used as anode and platinum loaded carbon was used as cathode. The fluids were delivered using polyethylene tubes attached using glue. The entire assembly was held together with paper binder clips which prevented the bulging of gas diffusion electrode. The surface area of both electrodes was found to be 0.66 cm2 [14].
Jayashree et al. also fabricated a membraneless micro hydrogen fuel cell with oxygen dissolved sulphuric acid as the oxidant. Porous gas diffusion electrodes were used to overcome the problem of low solubility of oxygen in the aqueous media. Platinum black was used as the catalyst for both cathode and anode. The micro channels were fabricated in PMMA substrate with 1 mm thick graphite windows acting as current collectors. The entire assembly is held together with binder clips. They studied the effect of electrolyte concentration and flow rates of hydrogen, oxygen and electrolyte on the room temperature performance of the cell. The peak power density was found to increase from 69 to 187 mW/ cm2 when electrolyte concentration increased from 0.25 to 2 M. The maximum power was also found to increase with increasing flow rates up to 5 sccm of hydrogen and oxygen. An increase in electrolyte flow rate from 0 to 1 ml/ min was found to increase the power and current density of the cell. The maximum power density was reported to be 191 mW/ cm2. Other factors such as lowering electrode to electrode distance, lowering the catalytic loading to 0.1 mg/ cm2 and operation at higher temperatures can further enhance the performance of the cell [15].
Another effort at improving cell performance was done by Salloum et al. who fabricated as sequential radial flow through concentric porous electrodes. This is shown in figure 4(h). In this design the anolyte enters through the center of a disc shaped anode and is partially consumed at the anode before it mixes with catholyte stream prior to entering the porous electrode. This design allows independent control of fuel and oxidant flow rate, but the losses due to fuel crossover may be high [2].
One of the best examples for applications of membraneless micro fuels was demonstrated by Cardenas-Valencia et al. Their group fabricated a membraneless micro galvanic fuel cell for use as an on-chip power source. The galvanic cell uses an aluminium anode, alkaline electrolyte and air as cathode. The cell works on thermal actuation mechanism. The cell has certain advantages like environmentally safe chemistry, abundance and low cost of aluminium and oxygen and high energy density available from aluminium (25 kJ/g in alkaline media). The cell suffers from drawbacks such as spontaneous corrosion of aluminium in alkaline media and formation of oxide passivation layer during the progress of reaction. The microchannels were fabricated on a polydimethylsiloxane substrate using a series of photolithography and etching steps. The open circuit voltage of the cell was measured to be around 1.9 V. Three different supporting electrolytes namely, 0.1 M H2SO4, 0.1 M NaOH and 1 M NaOH were tested and it was found that the open circuit potential and current densities increased with the use of supporting electrolytes. Better performance was obtained under alkaline conditions with roughened Pt electrodes with roughness factor of around 90. The cell used in the experiments here had an electrode area of 0.75 cm2 and was found capable of providing 5 J of energy after 6 min when subjected to a load of 20 Ω [16].
4. MICRO FLUIDIC FUEL CELLS WITH BIOCATALYSTS
The biological molecules such as enzymes and microbes that catalyze chemical reactions can be used in membraneless micro fuel cells as an alternative to traditional catalysts. A properly designed fuel cell with biocatalysts should give performance equivalent to a fuel cell operated with traditional catalysts at high temperatures. As the biocatalysts are selective in their catalytic activity, laminar flow of fuel and oxidant may not be required. A mixed stream of fuel and oxidant would undergo oxidation at the electrodes where the catalyst for that species is present. Relatively, few papers have been published in this area. Most of the research so far has involved bioanode development. A bioanode based on NAD dependent alcohol dehydrogenase enzyme immobilized in a tetrabutylammonium bromide-treated nafion membrane. This was integrated on a carbon electrode integrated on a glass substrate. Polydimethylsiloxane micro channel was used to deliver the fuel solution. The open circuit voltage was measured to be 0.34 V and the maximum current density to be 53 µA/ cm2, when operated against a platinum cathode. Another effort on bioanode development involved vitamin K3 mediated glucose oxidation by the enzyme glucose dehydrogenase. The bioanode was integrated on a chip with polydimethylsiloxane coated platinum cathode with Ag/AgCl reference electrode. 32 µW/ cm2 of power was produced at 0.29 V with 1 ml/ min of air-saturated pH 7 buffered fuel solution containing 5 mM glucose and 1 mM NAD+ [2], [3].
Kjeang et al. have made an effort in biofuel cell research by utilizing a series of consecutive biocatalyzed reactions for current extraction. For this purpose they used strategic patterning of multiple enzyme electrodes. They performed rigorous computational studies on membraneless micro biofuel cell technology for design and fabrication of these cells. Analysis was done on separated and mixed enzyme patterns in different proportions for various Peclet numbers, Pe. At moderate values of Pe maximum current densities were obtained. Mixed enzyme patterning, that is patterning enzymes with same cofactors together such as methanol dehydrogenase, formaldehyde dehydrogenase and formate dehydrogenase enzymes; was found to give high current densities along with near complete fuel utilization [3].
5. CONCLUSION
In summary, though micro fuel cells with membranes are capable of providing higher energy densities, they suffer from severe disadvantages such as high cost due to the use of membrane and its wear and tear during operation, use of expensive metals such as platinum for electrode catalysts, complex and expensive fabrication techniques, losses due to fuel crossover across the membrane, water management problems, and clogging of pores due to carbonate production in alkaline conditions.
All these problems led to the invention of membraneless micro fuel cell technology in 2002. The major advantage of micro fluidic fuel cells is that they do not require a membrane for their operation; this factor alone reduces the cost of the cell and makes this design lucrative for commercialization. Other factors such as independent control of fuel and oxidant concentrations, reduced losses due to fuel crossover, cheaper fabrication techniques, and use of cheaper materials such as carbon and gold for electrodes make the micro fluidic fuel cell technology an interesting field of research. Apart from that due to flowing streams of fuel and oxidant any carbonates formed during operation in alkaline conditions are washed away. The research conducted in this field thus far indicates that there scope for commercialization of this technology. Though power densities produced so far are not as high compared to those possible with micro fuel cells which use a membrane, but the results are encouraging in the sense that some researchers have succeeded in raising the single pass fuel utilization from less than 1% to near complete fuel utilization per single pass. The maximum power density produce thus far has been 191 mW/ cm2. This value can be improved considerably if certain issues such as transport of species from the diffusion boundary layer to the electrode surface can be increased and efforts are on to reduce inter-diffusion of fuel and oxidant. One strategy that can be used is two phase flow of oxidant and fuel; that is if the fuel is in liquid phase, then an oxidant in gaseous phase such as atmospheric oxygen may be used as oxidant.
Researchers have already tried to improve the reaction kinetics using various cell designs, such as flow through porous electrodes which provide high active electrode surface area instead of the conventional flow over electrodes and using porous gas diffusion electrodes to supply oxygen in the oxidant stream. Other strategies could involve removal of reacted species from the stream so that the resistance to fuel and oxidant transport to the electrodes is reduced. It has been shown that the membraneless micro fuel cells perform better under alkaline conditions due to improved reaction kinetics and absorption of carbonates produced during the reaction. Efforts could be made to further improve design such that cells operating under alkaline condition give better performance. Another option may be to have high operation temperature, but this is not practically feasible as the devices in which these cells would be used would operate at room temperature, an operation at high temperature could lead to heating of the device. Another option may be to recirculate the product stream to reutilize the unreacted reactants, but this could add to the size and the complexity of the cell. Moreover, there are chances of contamination, unwanted mixing and leakage if the cell is not designed properly. The use of biocatalysts such as enzymes and microbes is recent area of interest and research is going on in this field to develop fuel cells which can produce high energy density. The fuel cell performance could be improved if new combinations of fuel and oxidant or new electrolytes with high ionic conductivity are used. Therefore, membraneless micro fuel cell technology has quite some distance to go before commercial fuel cells are made available in the market. But with the current interest being shown in this field and with available technology that day should not be far away.
