Patent Application
20080057381
Furthermore, a fuel cell system is typically composed of a multiplicity of fuel cell units that are stacked together with the negative terminal (e.g., a current collector similar to plate 12) of a unit cell in physical and electronic contact with the positive terminal (e.g., a current collector similar to plate 14) of another unit cell so that effectively the unit cell units are electrically connected in series to form a fuel cell stack. Most typically, Plates 12 and 14 are integrated into a single bipolar plate. There are really too many layers in a conventional DMFC fuel cell stack.
In one preferred embodiment, as illustrated in
The fuel anode 22 is preferably a non-consumable electrode that is electronically conducting so that it may serve two primary functions: (1) as a backing layer or carrier for electro-catalyst 36 that promotes the anode reaction (e.g., Eq.(1) or the like) to produce electrons and (2) as an electron collector through which the produced electrons are transported to an external load (e.g., a bulb 32 in
The oxidant electrode (cathode) 24 is made of a material selected so that oxygen can be easily engaged in an electro-reduction reaction on its surface. For example, such an electrode may include a carbon paper or cloth, nickel-dispersed carbon paper or cloth, nickel electrode, and the like, which preferably has a double-layer structure consisting of a hydrophilic side interfaced with the electrolyte 26 and a hydrophobic side interfaced with atmosphere (in the case of acid electrolyte). A hydrophobic material such as polytetrafluoroethylene (PTFE, e.g., Teflon®) may be added to one surface of the cathode layer. The oxidant cathode 24 should be permeable to oxygen gas molecules so that oxygen could migrate to the cathode catalyst layer 34 that promotes Eq.(2) to produce byproduct water. The oxygen cathode should comprise no or little catalyst that can significantly promote oxidation of the dissolved alcohol fuel at the cathode. Preferably, the cathode electro-catalyst 34 should comprise an alcohol-tolerant catalyst. We have found that an alcohol-tolerant oxidant reduction catalyst may be selected from a platinum-chromium alloy, a transition metal mixture or alloy (preferably with a particle size smaller than 2 nm and/or preferably that obtained from transition metal hexacyanometallate) dispersed on carbon black, a carbon-supported iron and/or cobalt, a MoxRuySez—(CO)n carbonyl cluster, a Ru—Se mixed metal, or a combination thereof. Some of these catalysts were originally developed to reduce the mixed potential effect in a conventional DMFC working on a solid acid electrolyte. Our diligent research work has demonstrated that these catalysts are also relatively insensitive to DMFC in liquid acid electrolyte environments and alkaline solution electrolyte environments. They are also found to be relatively insensitive to other alcohols in both liquid acid and alkaline electrolytes. The herein invented dissolved-fuel DAFCs containing an alcohol fuel all are capable of delivering a very impressive power output.
Schematically shown in
Another preferred embodiment of the present invention, schematically shown in
Another attractive design for a unit cell with a high-current output is schematically shown in
The acid electrolyte may comprise water, an acid, and an alcohol fuel. The acid may be selected from phosphoric acid, sulfuric acid, nitric acid, sulfonic acid, acetic acid, formic acid, hydrogen chloride, and the like. There is no theoretical limitation as to what type of acid that can be used in the present invention as long as it can readily provide hydrogen ions (protons) H+. However, some acid (e.g. sulfuric) can be more corrosive than others.
Another preferred embodiment of the present invention is a fuel cell as schematically shown in
The conventional direct methanol fuel cell typically operates on an acid-based solid electrolyte with the electrochemical reactions being summarized in Equations (1)-(3). Contrary to this conventional thinking, we felt that it was not necessary for a dissolved-fuel DMFC to be based on an acidic electrolyte. With a basic or alkaline solution-based electrolyte, the electrochemical reactions occurring in a DMFC (including a DF-DMFC) may be given as follows:
Anode: CH3OH+6OH−→CO2+5H2O+6e− (4)
Cathode: 3/2O2+3H2O+6e−→6OH− (5)
Overall: CH3OH+3/2O2→CO2+2 H2O (6)
The electrolyte that can be employed in the present invention may comprise an alkaline solution with pH>7; for example, KOH, NaOH, LiOH, or the like. A chemical species (alcohol fuel) that can react with OH− to produce electrons at the anode is added to the alkaline solution of electrolyte. This fuel may be selected from the group consisting of methanol, ethanol, propanol, 2-propanol (iso-propanol), etc. All members of this group have standard reduction potentials in water that are more negative than the standard reduction potential of a hydrogen electrode in water. They are all good hydrogen-releasing agents that promote anode reactions.
The selection of a fuel to be dissolved in the alkaline electrolyte solution may dictate the types of anode catalyst and cathode catalyst used. The anode catalyst should be selected to promote the anode reaction so that it may proceed at a sufficiently high rate even at ambient temperature or at a temperature not too much higher than the ambient temperature. The cathode catalyst should not promote the fuel oxidation reaction that produces electrons at the cathode, which electrons otherwise would be wasted or produce the mixed potential effect. For instance, platinum is an effective anode catalyst, but platinum alone should not be used at the cathode side. Other oxygen-reducing catalysts (e.g., nanometer-scaled Pt—V, Pt—Cr, and, preferably, Pt-free Ni—Co, Co—Fe, and Ru—Se particles) may be used instead.
Perhaps the most important features of the presently invented alkaline electrolyte-based DF-DAFCs are: (1) obviation of the need to utilize expensive catalysts such as platinum and platinum/ruthenium at the cathode and/or anode; (2) enhanced oxygen reduction kinetics at the cathode in an alkaline electrolyte environment as compared to in an acid environment (with or without Pt); and (3) permission of operating the fuel cell at a much higher temperature for improved electro-chemical reactions at both the anode and cathode. The first feature is significant since the electro-catalyst cost represents a high proportion of a fuel cell system cost. The utilization of platinum-free catalysts could significantly reduce the system cost. The second feature is also significant since cathode reactions are typically much slower than anode reactions in a low-temperature fuel cell (such as the hydrogen/oxygen PEM fuel cell and DMFC) and are the primary sources of over-potential or voltage losses. By using an alkaline electrolyte, the cathode reaction proceeds faster at a given operating temperature and pressure. The third feature allows both anode and cathode reactions to proceed faster, make better utilization of the fuel, and reduce the over-potential, resulting in much improved fuel cell power output. Conventional DMFCs could not be operated at a temperature higher than 80° C. since Nafion-type PEM materials undergo irreversible degradation reactions at elevated temperatures. The presently invented fuel cells eliminate the use of a PEM layer all together and are not subject to such a constraint.
In the case of an acid electrolyte DF-DAFC, the anode preferably includes a backing layer (e.g., carbon paper) and a platinum/ruthenium electro-catalytic film positioned at the interface between the anode backing layer and the electrolyte for promoting oxidation of the methanol fuel. Similarly, the cathode preferably includes a backing layer (e.g., carbon paper or carbon paper surface-treated with a hydrophobic material like Teflon) and a preferably non-platinum-based, alcohol-tolerant electro-catalyst coated on the backing layer. The oxidation or reduction electro-catalyst may be applied directly to the backing layer of its respective electrode or may be dispersed on a suitable catalyst support, such as a carbon, graphite or other electrically conductive support (e.g., nano-scaled carbon particles), which is in turn applied directly to the backing layer of its respective electrode. Most preferred alcohol-tolerant oxidant reduction catalysts are selected from a platinum-chromium alloy, a transition metal mixture or alloy (nano-scaled particles, preferably obtained from transition metal hexacyanometallate) dispersed on carbon black, a carbon-supported iron and/or cobalt, a MoxRuySez—(CO)n carbonyl cluster, a Ru—Se mixed metal, or a combination thereof. Other reduction electro-catalysts known to those skilled in the art, such as sodium platinate, tungsten bronzes, lead ruthenium oxides, lead iridium oxides, lanthanum oxide, and macrocyclic or porphyrin structures containing one or more metals, could also be used.
When a stack of multiple fuel cell units are desired, as is usually the case in real practice, anode catalyst-backing layer 22 may be in contact or integrated with plate 28 of the cathode (e.g., in
To achieve a desired output voltage level, a number of these unit fuel cells can be stacked together to form a fuel cell assembly. As shown in
In another design (
In another embodiment of the present invention, as schematically shown in
In a conventional fuel cell stack design, stacking and porting unit fuel cells may require complex flat stack arrangements and involve numerous parts (e.g., membranes, gaskets, bipolar plates with flow channels, and solid electrode layers) that may be difficult and expensive to fabricate and assemble. Traditional fuel cell stacks are highly prone to catastrophic failure of the entire system if a leak develops. The cost of fabricating and assembling fuel cells is significant, due to the materials and labor involved. In addition, it is difficult to transport the oxygen and fuel through the traditional stack, and increased gas or liquid transport requires pressurization, with attendant difficulties.
An alternative style of fuel cell has been recently proposed (e.g., Binder, et al., U.S. Pat. No. 5,783,324, Jul. 21, 1998 and Pratt, et al., U.S. Pat. No. 6,127,058, Oct. 3, 2000), which is a side-by-side configuration in which a number of individual cells are placed next to each other in a planar arrangement. This is an elegant solution to the problem of gas and fuel transport and mechanical hardware. However, a planar fuel cell configuration based on the conventional direct methanol fuel cell (DMFC) approach is still subject to the same drawbacks associated with all DMFCs (e.g., complex configuration or too many layers in a unit, expensive PEM, expensive platinum catalysts, etc.). An improved planar fuel cell that is less complex and can be made at a lower cost would be a significant addition to the field.
Hence, another preferred embodiment of the present invention is a planar or co-planar fuel cell configuration that features the alcohol-tolerant cathode catalysts described above. A co-planar fuel cell, also referred to as a strip or segmented fuel cell, comprises several series-connected cells that are fabricated on the same continuous, planar layer or pool of liquid electrolyte-fuel mixture. Alternatively, separate layers or pools of a liquid electrolyte-fuel mixture, together with an anode on one side thereof and a cathode on the opposite side thereof, may be used for each cell.
In a planar fuel cell (e.g.,
Edge current collection wires or cell interconnects are used to connect the individual cells in electrical series. Alternatively, cells or groups of cells may be connected in parallel. The anode and cathode catalyst backing layers (sheets of carbon paper or graphite fiber cloth) may perform additional functions such as serving as current collectors (or electron transporters). The anode assembly 64 consists of an electrically insulating plastic frame 65 that contains a plurality of current collectors (i.e., backing layers for anodes A′, B′, C′, and D′) embedded within the plastic frame. Each of the current collectors has an interconnect means (e.g., 68A, 68B, 68C, 68B) appended thereto. As shown in
In one preferred embodiment, the current collectors can be insert-molded into the plastic frame 65 with the interconnect means extending through the frame such that when the planar fuel cell is assembled, the current collector is within the perimeter of the assembly and the interconnect means is outside the perimeter of the assembly. One main advantage of this format is that the plastic frame 65 forms a gas tight integral seal around the interconnect means, thus eliminating the need to add other seals and/or gaskets. This novel approach provides for electrical connections between and within the fuel cell without traversing the thickness of the solid electrolyte sheet. No penetrations are made in the assembly, thus the liquid electrolyte chambers can be made in a single, continuous frame. There are no holes or apertures to seal, as in the prior art. This novel scheme allows the individual anodes and cathodes in each of the arrays to be placed very close together, thus utilizing a greater amount of the active area, as high as 95% of the total area of the current collector assembly. The individual anodes or cathodes can be spaced as close as 1 mm to each other. The feature that the electrolyte and fuel are mixed together further reduces the bulkiness of the fuel cell assembly system.
A conventional planar fuel cell is typically composed of a membrane electrode assembly (MEA) sandwiched between two current collector assemblies. By contrast, in one embodiment of the present invention, no current collector assembly or only one assembly is needed, significantly reducing the bulkiness and complexity of the fuel cell system. The current collectors may be supported by a plastic frame, and they have an interconnect tab that provides an electrical pathway beyond the perimeter of the assembly. The interconnect tab can be connected to or integral with the corresponding anode or cathode. The interconnect tab is situated to provide electron transfer between the anodes and the cathodes such that preferably the interconnect tab does not traverse the thickness of the liquid electrolyte chambers. When the planar fuel cell is assembled, the interconnect tab is properly sealed to prevent leaking of fuel/electrolyte. The presently invented planar fuel cell system has fewer parts, fewer layers, and lesser degree of complexity in design compared to those proposed by Binder, et al. (U.S. Pat. No. 5,783,324, Jul. 21, 1998) and Pratt, et al. (U.S. Pat. No. 6,127,058, Oct. 3, 2000).
One preferred class of the alcohol-tolerant electro-catalysts used in the present study was platinum-chromium or platinum-vanadium catalysts supported on carbon black.
Nano-scaled platinum-chromium alloy catalyst particles supported on carbon were prepared in the following manner: 20 grams of platinum-on-carbon-black (containing 10% platinum by weight) was dispersed in 1,000 ml of water followed by ultrasonic blending for 30 minutes. The pH of the solution was then raised to 8 with dilute ammonium hydroxide solution to counter the natural acidity of the supported catalyst. Stirring continued during and after pH adjustment. A solution of 12 g of ammonium chromate in 100 ml of water was then added to the pH adjusted solution. Following addition of the complete 100 ml of solution, dilute hydrochloric acid was added to the solution until a pH of 5.5 was attained to cause the adsorption of the chromium species on the supported catalyst. Stirring continued for one hour. After filtering, the solids were dried at 90° C. and sifted through a 100 mesh screen. The sifted solid was then heat-treated at 900° C. in flowing nitrogen for one hour to form the platinum-chromium alloy catalyst (particle sizes typically smaller than 1.2 nm). Graphitized Vulcan XC-72 (Cabot Corporation) was used in this Example, but other carbons in the graphitized or ungraphitized form or acetylene black could also be used as support material. X-ray diffraction data on the formed catalysts indicated that the alcohol-tolerant cathode catalysts developed were Pt—Cr alloys with up to about 30 atomic percent of chromium in the alloy. The alloy preferably has about 25 atomic percent of chromium.
As another example, one gram of V2O5 in 250 ml distilled water was dissolved by the addition of 1 N NaOH to bring the pH to 9. The dissolution rate was accelerated by heating the solution. The solution was chilled to 5-10° C. and 2 ml of 30 volume percent H2O2 and 15 ml of 4 weight percent Na2S2O4 were added into this solution. After a few minutes of mixing, the pH of this solution was decreased to about 1.5 by the addition of cold 1 N HCl. This solution had a light yellow, clear appearance that changed to a clear but very dark black-green color (believed to be V+3 sulfite complex) upon extended (longer than 30 minutes) stirring.
Meanwhile, in a separate beaker, twenty grams of catalyst consisting of 10% Pt by weight supported on carbon black was ultrasonically dispersed in 800 ml distilled water and cooled to 5-10° C. The surface area of this catalyst was 110 m2/g Pt or greater. The two suspensions were mixed together and stirred for a sufficient length of time (about one hour) for the V+3 sulfite complex to adsorb on the carbon black support in an appreciable quantity (about 50 atom percent vanadium based on platinum). The vanadium complex impregnated catalyst was then filtered and dried to obtain an intimate mixture of a highly dispersed vanadium complex and highly dispersed platinum on carbon. The mixture was then heated to 930° C. in flowing N2 (or H2) and held at this temperature for one hour. The product was cooled to room temperature before exposing it to atmospheric air. The Pt—V alloys were in the form of ultra-fine particles with particle size typically smaller than 1.5 nm. Similar procedures can be used for the preparation of other nano-scaled, noble metal-transition metal catalyst particles of Pt—Ti/C, Pt—Si/C, Pt—Al/C, Pt—Cr—Al/C, Pt—Ce/C (C=carbon support).
Zhu, et al. (U.S. Pat. No. 7,014,931, Mar. 21, 2006) disclosed that selected Pt—Cr alloys were methanol-tolerant in a solid acid electrolyte environment (e.g., using Nafion as the PEM). However, they did not recognize that Pt—Cr was also methanol-tolerant in an alkaline electrolyte environment. Nor did they recognize that Pt—Cr was relatively insensitive to other alcohols than methanol. Furthermore, they disclosed a conventional DMFC using Pt—Cr as a cathode catalyst, as opposed to the presently invented dissolved-fuel DMFC.
As an example, the carbon-supported platinum-chromium catalysts used in the present study contained about 30-44 wt % platinum-chromium. The corresponding anode catalyst was a PtRu/C (45 wt %). The anode and cathode catalysts were dispersed in appropriate amounts in water, with an added perfluorinated ion exchange polymer for ionic conduction adjacent the catalysts (e.g., 5% Naflon® solution) and for binding catalyst particles to carbon paper. Exemplary cathode ink compositions were 65 wt % Pt/C and 35 wt % Nafion® (for comparison) and 66 wt % Pt3Cr/C and 34 wt % Nafion®. The anode ink compositions were 85 wt % PtRu and 15 wt % Nation® or 70 wt % PtRu/C and 30 wt % Nafion®. The electrodes were prepared by painting the catalytic inks on sheets of carbon paper. The cathode catalyst inks were applied to obtain an experimental loading of about 0.6 mg/cm2. In all cases the geometric active area of the catalyzed electrode was 5 cm2. The anode catalyst used in the present study as primarily Pt/Ru supported on carbon black. However, in the cases of formic acid-based DD-DAFC, carbon supported palladium nano particles (alone or in combination with Pt) were also used.
Another class of alcohol-tolerant oxygen reduction catalysts can be prepared by dispersing a series of transition metal hexacyanometallates on carbon black (CB) and heat-treating the mixture at several temperatures under a nitrogen atmosphere. The precursors were prepared by adding 0.03 M M′SO4 (M′=Mn, Fe, Co, Ni, and Cu) aqueous solution into stirring dispersion of CB in 0.02 M K3M″(CN)6 (M″=Fe, Co) solution. The procedures were similar to those suggested by Sawai, et al., (J. of the Electrochemical Soc., 151 (5) (2004) A682-A688). In the present example, the catalysts used were obtained primarily from the groups with M′=Fe, Co, and Ni, and M″=Fe and Co. The loaded amount of the Prussian blue analogs (PB) was about 5×10−4 mol on 0.1 g of CB. The resulting mixture was filtrated with a filter paper, washed with distilled water several times, and dried in an oven at 80° C. The dried sample was wrapped with a copper foil, heated to the preset temperature at a heating rate of about 200°/h in a horizontal quartz tube under a nitrogen atmosphere, followed by keeping the sample at the preset temperature for approximately 10 min. It may be noted that a gas containing toxic HCN was evolved when heating the hexacyanometallate salt. Hence, HCN in the gas was collected in a gas washing bottle and the cyanide ion was decomposed by a sodium hypochlorite (NaClO) solution. The heat-treated sample was kept in a desiccator over blue silica gel until use. The particle sizes were typically between 0.5 nm and 2 nm.
Sawai, et al. observed that these catalysts were insensitive to methanol. We have found that they were also insensitive to other alcohols such as ethanol, propanol, and iso-propanol in both acid and alkaline electrolyte environments. In an acid electrolyte, they were also formic acid-tolerant when used as a cathode catalyst in the presently invented dissolved-fuel direct alcohol fuel cell.
A pre-treated carbon black sample was impregnated with RuCl3-xH2O solution (with 30% by weight metal content). Upon removal of the solvent, the dried powder was treated under hydrogen at 200° C. The resulting material was then dispersed or immersed in a H2SeO3 solution. After filtration and drying steps, the powder was annealed in a hydrogen environment at 300° C. for 60 minutes to obtain RuSex (with Se varied preferably between 5 to 20%). The particle size was found to be below 2 nm.
A comparison was made between dissolved-fuel direct methanol fuel cells (Fuel Cell-A: DF-DMFC containing methanol-tolerant Pt/Cr cathode catalyst), conventional DMFC (Fuel Cell-B: containing no methanol-tolerant cathode catalyst), and DF-DMFC (Fuel Cell-C: containing conventional Pt cathode catalyst). The anode and cathode were made by the procedures illustrated in Example 2. The electrolyte-fuel mixture used was methanol-water-nitric acid at a molar ratio of 0.5:1.0:0.1 for both Fuel Cell-A and Fuel Cell-C. For Fuel Cell-B, the fuel was methanol-water at a molecular ratio of 0.5:1.0 and the electrolyte was solid Nafion.
The corresponding power density curves for these three fuel cells are shown in
A comparison was made between an acid-electrolyte dissolved-fuel direct methanol fuel cell (Fuel Cell-D: DF-DMFC containing methanol-tolerant Pt/V cathode catalyst) and an alkaline solution electrolyte DF-DMFC (Fuel Cell-E: containing Pt/V cathode catalyst). The electrolyte-fuel mixture used was methanol-water-nitric acid at a molar ratio of 0.5:1.0:0.1 for Fuel Cell-D and that for Fuel Cell-E was methanol-water-KOH at a molar ratio of 0.5:1.0:0.2.
A comparison was made between an acid-electrolyte dissolved-fuel direct ethanol fuel cell (Fuel Cell-F: DF-DEFC containing Fe/Co cathode catalyst obtained in Example 3) and an alkaline solution electrolyte DF-DEFC (Fuel Cell-G: containing Fe/Co cathode catalyst). The electrolyte-fuel mixture used was ethanol-water-nitric acid at a molar ratio of 0.5:1.0:0.1 for Fuel Cell-F and that for Fuel Cell-G was ethanol-water-KOH at a molar ratio of 0.5:1.0:0.2.
A comparison was made between an acid-electrolyte dissolved-fuel direct 2-propanol fuel cell (Fuel Cell-H: DF-D2PFC containing Fe/Co cathode catalyst obtained in Example 3) and a corresponding DF-DMFC (Fuel Cell-I: containing Fe/Co cathode catalyst). The electrolyte-fuel mixture used was 2-propanol-water-formic acid at a molar ratio of 0.5:1.0:0.1 for Fuel Cell-H and that for Fuel Cell-I was methanol-water-formic acid at a molar ratio of 0.5:1.0:0.1.
The anode fuel oxidation reaction for 2-praponol may be given in Equation (7):
CH3CHOHCH3+5H2O→3CO2+18H++18e− (7)
For each 2-propanol molecule oxidized at the anode, 18 electrons are produced. In contrast, Equation (1) shows that for each methanol molecule oxidized at the anode, 6 electrons are produced. The molecular mass of 2-propanol (60.10 g/mol) is less than double that of methanol (32.04 g/mol). Further, the two fuel species have very similar physical densities (0.785 g/cm3 for 2-propanol vs. 0.791 g/cm3 for methanol). The electrochemical energy density of 2-propanol is more than 1.5 times that of methanol per unit mass or per unit volume, provided that the fuel is completely oxidized. In the near future when more effective electro-catalysts become available, 2-propanol will become a highly viable fuel for DF-DAFC cells.
Instead of Pt or Pt/Ru, palladium nano-particles were found to be more effective anode catalyst for direct formic acid fuel cell using formic acid (FA) as the fuel fluid at the anode side and a solid polymer (Nafion) as the electrolyte interposed between a cathode and an anode (Masel, et al., US Pat. No. 2005/0136309 (Pub. Date: Jun. 23, 2005)). Masel's fuel cell still employs a solid electrolyte. In contrast, in our DF-DAFC, liquid formic acid was interposed between the anode and cathode as both the electrolyte and the fuel.
A chemical reduction method was employed to make the palladium nano-particle catalyst. The multi-step procedure included (1) dispersing carbon black particles in deionized water in an ultrasonic bath, (2) adding a reducing agent solution (sodium formate) and a metal precursor solution (palladium nitrate) into the carbon black suspension while under sonication at 45-75° C., and (3) forming palladium nanoparticles on the nano-scaled carbon black surface. The anode and cathode catalyst inks were prepared by mixing appropriate amounts of catalyst powders with 5% recast Nafion solution. Both the anode and cathode inks were applied onto one side of a carbon paper. Nafion was used to bind the catalyst particles to the carbon paper surface. The palladium particles at the anode had particle sizes smaller than 3 nm. The cathode catalyst was RuSex (with approximately 15% Se) as prepared in Example 4.
A comparison was made between a dissolved-fuel formic acid fuel cell (Fuel Cell-J: DF-FAFC containing RuSex cathode catalyst obtained in Example 4 and carbon-supported palladium anode catalyst), a conventional FAFC with a Nafion PEM (Fuel Cell-K: containing RuSex cathode catalyst obtained in Example 4 and carbon-supported palladium anode catalyst), and a DF-DMFC (Fuel Cell-L: containing RuSex cathode catalyst and Pt/Ru anode catalyst). The electrolyte-fuel mixture used was formic acid-water at a FA concentration of 2M for Fuel Cell-J and that for Fuel Cell-L was methanol-water-formic acid at a molar ratio of 0.5:1.0:0.1 (formic acid serving primarily as electrolyte). For the conventional FAFC, the formic acid (2 M) was fed into the anode side as a liquid fuel while the anode and cathode are separated by a Nafion membrane.
A comparison between the presently invented DF-FAFC and the conventional FAFC indicates that the conventional FAFC has a higher voltage (lower activation overpotential) when the external circuit demands a current density less than 400 mA/cm2. When the required current density exceeds 400 mA/cm2, the DF-FAFC is capable of maintaining a higher voltage. It is of further interest to note that the presently invented DF-FAFC provides a relatively high, stable working voltage (>0.6 volts) over a very broad current density range (200-600 mA/cm2). This was achieved without having to use an expensive solid electrolyte membrane like Nafion, without expensive Pt-based catalyst, and with a simpler fuel cell design as compared to the conventional formic acid fuel cell (e.g., that developed by Masel, et al.) or DMFC.
The corresponding power density curves for both the DF-FAFC and the conventional FAFC are shown in
HCOOH→CO2+2H++2e− (7)
This is in contrast to 1.2 volts for the corresponding methanol-water reaction. The notion that DF-FAFC is capable of maintaining a relatively high, stable working voltage (>0.6 volts) over a very broad current density range also contributes to this exceptionally high power density.
Polyurethane (PU) was prepared by the reaction of toluene-2,4-diisocyanate with hydroxy-terminated oligomers. Oligomers were either liquid polybutadiene (MW 3000) or propylene oxide-based polyethers (MW 420 and 4800). Polyurethanes with linking segments formed predominantly by high-molecular-weight oligomers (MW 3000 or 4800) were rubbery materials with a glass transition temperature (Tg) lower than room temperature.
Cross-linked poly (vinyl alcohol) (PVA) material was prepared with poly (acrylic acid-co-maleic anhydride) (PAAM) serving as a polymeric cross-linking agent. Cross-linked materials were characterized by good water and methanol-retaining capabilities. Swelling ratio decreased with increasing cross-linking agent content since the swelling of water molecule is restricted by chemical cross-linking between PVA chains and polymeric cross-linking agent chains and physical cross-linking by entanglement between the chains. Both PU and cross-linked PVA were found to be particularly suitable for use as a electrolyte/fuel-retaining material. They did not show any significant sign of degradation after being impregnated with the liquid electrolyte/fuel for five months. They were used to prevent electrolyte from over-flooding the cathode pores. Both protons and hydroxyl ions are able to freely move through the retained electrolyte with an ionic conductivity being at least one to two orders of magnitude greater than in solid Nafion.
