US 5,683,829 AGrant
Redox Promoters for Methonol Fuel Cells
Issue Date:1997-11-04
•12 Claims
•18 Drawing Sheets
Abstract
The invention refers to a fuel cell electrode for oxidation preferably methanol or methanol reformate in a fuel cell with acid electrolytes. The electrode material comprises of a) carbons modified by the chemisorption or covalent attachment of metallocomplexes, over which a noble metal catalyst such as platinum is deposited, b) carbon-noble metal systems mixed with metallocomplexes, and c) mixtures of noble metal blacks and metallocomplexes. An improvement of 100-200 mV is seen over controls using platinum alone in full tests using methanol and oxygen or air. The advantages of using the disclosed promoters is the use of low loaded (20% Pt on carbon) electrodes rather than pure noble metal or alloy blacks.
Metadata
Inventor
- Shantha Sarangapani
Application Information
Application Number:US 7169159
Filing Date:1996-09-11
Priority Date:1995-05-22
Art Unit:112
Classifications
IPC:
H01M 486
Field of Search:
204429291;290 R;292;29342;43;44
Patent Drawings (18 sheets)
Description
Background of the Invention
Fuel cells are electrochemical cells that oxidize certain fuel molecules at the anode and reduce an oxidant such as oxygen or air. From thermodynamics point of view the fuel cells constitute the most efficient form of conversion of fuel into electrical energy.
Direct Methanol fuel cell powered vehicles (DMFC) are foreseen as the choice of the future because of its environmentally benign combustion products, potential for high efficiency and above all for the unlimited supply of methanol in the United States. The major obstacle in the development of methanol fuel cells is the methanol electrode, particularly the platinum catalyst for methanol oxidation. Quick loss of activity due to the adsorption of poisoning intermediates related to carbon monoxide like species is well established.
It is well understood at this point that a promoter metal capable of oxygen transfer to the platinum or that could oxidize the adsorbed residues from methanol oxidation ideally should improve the performance of the methanol oxidation electrode. Adsorbed water appears to play an important role in this process. Recently, platinum ruthenium alloys have shown promise as such a promoter metal. The mechanism invoked is as follows:
Dissociative water adsorption on Pt alone occurs at about +0.8V vs. NHE at 50.degree. C. and this is why Pt alone is not a good catalyst for methanol oxidation. The reaction rate of methanol oxidation in the presence of promoting alloy atoms is assumed to be proportional to the number of Ru sites located at the nearest neighbor.
The oxidation of small organic molecules at a platinum electrode occurs through the formation of a chemisorbed residue, which poisons the Pt sites and makes further adsorption of reactants difficult. Consequently, the catalytic activity falls with time. The reformate gas, which is predominantly hydrogen and carbon dioxide, contains several impurities such as formaldehyde, formic acid, in ppm levels. However, in a fuel cell operating over several thousand hours, the adsorption of even small quantities can result in the loss of cell performance. Even with pure hydrogen/CO.sub.2, a problem exists in the form of"Reduced CO.sub.2," a concept introduced by J. Giner in J. Electrochimica Acta, 8, 857 (1963). At the hydrogen electrode potential, carbon-dioxide can get reduced forming an adsorbed intermediate that is similar to the adsorbed intermediate formed during the oxidation of small organic molecules. The nature of the intermediates formed from the reduction of CO.sub.2 is identical to that formed during the oxidation of CO.sub.2, methanol, formaldehyde, etc. Therefore, a promoter that works well with methanol oxidation, should do well in tolerating impurities as well as the 30% CO.sub.2 present in the reformate.
This invention relates to macrocyclic complexes of well-defined stereochemistry in stable ligand environments and redox potentials. The metal centers of the promoters used in this invention are capable of existing in oxidation states ranging from (II) to (VI) at well-defined potential ranges. They also have the ability to bind and oxidize the poisoning species thus releasing the active platinum sites. This contrasts the ill-defined and irreversible redox chemistry of oxide species of metals such as ruthenium, molybdenum, etc.
Summary of the Invention
This invention discloses a fuel cell in which higher current densities are achieved in a methanol anode that uses platinum electrocatalysts, by incorporating in the electrodes certain redox promoters.
The redox promoters include various metal complexes derived from a variety of organic ligands that a) are directly chemisorbed from their solution on high area carbons used for electrodes or directly chemisorbed on platinized high area carbons, or b) are covalently attached using a coupling agent to the carbon surface --OH groups.
The platinum can be included into these compositions in two ways. One method involves depositing Pt as a high area powder using acidic H.sub.2 0.sub.2 and Pt sulfite acid from prior art, in the presence of promoter chemisorbed carbon. Another method involves mixing a constant composition of platinum (20% platinum on carbon or Pt black) directly with a known weight of the promoter solution, thoroughly mixing by sonication and drying the mixture before electrode fabrication. The second method ensures a constant platinum/promoter ratio whereas by the first method, the platinized platinum amounts could vary.
The promoter/platinum compositions once obtained could be fabricated into polytetrafluoroethylene (PTFE) bonded gas diffusion electrodes, known from prior art, for their performance study.
The performance of the promoted electrodes are initially screened by measuring the polarization (the electrode potential vs. RHE (reversible hydrogen electrode) of the electrode at various applied current densities. The E (potential) vs. log i (i=current density) are compared to the behavior of unpromoted electrodes under the same conditions.
The best promoted electrodes based on their polarization behavior in half cells compared to controls are then assembled in these sandwich-type membrane electrode assemblies (MEAs), known from prior art, where the separating membrane is an ionically conducting polymer such as a Nation (Dupont.TM.) film. The full cell is tested with methanol as the anode feed and oxygen or air as the cathode feed in a conventional fuel cell.
Anode: CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.-
Cathode: O.sub.2 +4e+4H.sup.+ .fwdarw.2H.sub.2 O
Net: 2CH.sub.3 OH+3O.sub.2 .fwdarw.4H.sub.2 O+CO.sub.2
This invention uses as redox promoters, ligands, and their combinations with metals such as ruthenium, osmium, iridium, tin, molybdenum, manganese, rhodium, rhenium, and iron.
The ruthenium promoters typically show reversible electrochemistry in the potential region where methanol undergoes oxidation on platinum 0 V to 1.0 V vs. NHE.
The affinity of ruthenium species toward CO or CO.sub.2 may be responsible for the poison removal phenomena; it could occur by the removal of CO-like poisons from platinum-active sites and/or the metal centers providing water, hydroxo or oxo groups to facilitate their further oxidation.
The present invention discloses the use of:
1) Directly chemisorbed mono or multimetallic complexes of metals from group VIIIA and group VIIA of the periodic table (IUPAC version) on platinized high area carbon.
2) Covalently linked metallocomplexes that contain --NH.sub.2, NH or OH groups directly to a high area surface preferably carbon, followed by platinization to the desired extent.
3) Chemisorbed complexes as in 1, followed by platinization to the desired extent.
Gas diffusion electrodes fabricated using the above three types of materials, conforming to known designs for solid polymer electrolyte fuel cells show superior performance over controls with platinum alone.
The present invention provides an anode that is resistant to poisoning species when methanol or methanol reformate is oxidized using a platinum catalyst. It has been discovered that at low Pt loadings (20% w/w Pt on carbon) the presence of( 10% w/w) certain redox promoters enhance the performance of the electrode on par to 100% noble metal blacks, such as Pt black or Pt ruthenium black.
The ruthenium content in the promoted carbons from the present invention is less than 4%.
These and other features and objects of the present invention will be more fully understood from the following detailed description which should be read in connection with the drawings identified below.
Brief Description of the Drawings
FIGS. 1a-1l are representations of structures of ligands used to synthesize redox promoters incorporated in an electrode of the present invention.
FIGS. 2a-2f are graphs showing redox characteristics of selected promoters.
FIG. 3 is a representation of a covalent attachment scheme.
FIG. 4 is a sectional view of an experimental cell used for half cell polarization studies.
FIG. 5 is an exploded view of an electrode holder used in the cell shown in FIG. 4a.
FIGS. 6c-6h are graphs showing the results of half-cell polarization studies on electrodes constructed in accordance with the present invention.
FIGS. 7a-7c are graphs showing the behavior of full cells including anodes treated in accordance with the present invention.
Detailed Description of the Invention
According to the present invention a fuel cell electrode is provided which makes use of a class of `redox promoters` that enhance the activity of noble metal catalysts such as platinum during the electroxidation of methanol or methanol reformate. The redox promoters a) are characterized by well defined oxidation states at a given potential that have affinity for CO like species and/or oxygen; b) are coordinated to water, hydroxo or oxo groups that can catalytically oxidize poisoning impurities; and c) that are electrochemically reversible.
The redox promoters utilized in the present invention belong to ruthenium or tin complexes of macrocyclic ligands such as pyrazine (FIG. 1a), phthalocyanines (FIG. 1b), porphyrins (FIG. 1c), and cyclam types (FIG. 1d), as well as heterocyclic ligands derived from pyrazine (FIG. 1e), pyrimidine (FIG. 1f), pyridazine (FIG. 1g), and imidazoles (FIGS. 1h, 1i, & 1j) and polymeric imines such as polyethyleneimine (FIG. 1k (branched) and FIG. 1l (linear)).
Ligands such as bipyrimidine (FIG. 1f) are capable of binding two metals and can form bridges between two metals coordinated to other ligands. These complexes have been synthesized. The biimidazole and bibenzimidazole (FIGS. 1h & 1i) have been also synthesized as bridge systems.
One of the objects of the invention is to use some of the ligand systems to construct ML.sub.1, M(L.sub.1).sub.2, ML.sub.2 L.sub.1, or M.sub.2 (L.sub.3).sub.2 complexes (where M stands for Ru, Sn, Ir, Os, Mo, Mn, Rh, Pt, Co or Fe) that will have open coordination at the axial sites, where L.sub.1 refers to a ligand system that can form an --N.sub.4 coordination around the metal centers, L.sub.2 to simple coordinating species such as Cl.sup.-, H.sub.2 O, etc., and L.sub.3 to ligand systems such as DPPN (FIG. 1g) which are capable of forming binuclear complexes.
The syntheses of the metal complexes derived from the ligands from FIG. 1 are described in detail in Example 1. Ligands, such as tetrapyridyl pyrazine (FIG. 1a), porphyrins (FIG. 1b), phthalocyanines (FIG. 1e), cyclam types (FIG. 1d), dipyridyl pyrazine (FIG. 1e) and pyrimidines (FIG. 1f), and dimethyl biimidazoles (FIG. 1j) and polyethyleneimine (FIG. 1k), can be purchased from commercial sources indicated in Example 1. The other ligands can be synthesized using published procedures.
The principles of the covalent attachment scheme is shown in FIG. 2. This coupling of --NH.sub.2 to --OH groups on surfaces is achieved by the use of cyanuric chloride as the coupling agent. The --NH.sub.2 or --NH groups on the ligands could be thus anchored onto high area carbon surfaces. Tetraminophenylporphyrins containing Ru or Sn were anchored to carbon surfaces. The water soluble ligand cyclam could be successfully attached to carbon and complexed with ruthenium in a second step.
The Example 3 describes the redox behavior of some of the promoters. The Ru(III)/(II) couples are reversible and occur at different potentials for different ligand environments, in the range of +0.0V and +0.6V vs. SCE. At higher potentials around 1.0V, futher oxidation to Ru(IV) and Ru(VI) species occurs in some cases.
A method of making promoter modified carbons by chemisorption is described in detail in Example 4. The promoter complexes are dissolved in suitable solvents with quantities representing 10% w/w to the carbon. After this step the promoter modified carbons are platinized using prior art. This method results in different degrees of platinization. Such platinization can also be carried out on promoters bound to carbon by covalent attachment.
In order to maintain a constant platinum, another method where the redox promoters are directly chemisorbed onto platinized carbon (where Pt content is fixed), directly is described in Example 5.
The preparation of porous gas diffusion electrodes for testing in fuel cells is based on known designs. Such gas diffusion electrodes are described by, for example, Watanabe et al., J. Electroanal. Chem., 183, 391-394 (1985). These electrodes are produced by sintering carbon black together with carbon black particles covered with the catalyst, polytetrafluoroethylene on a suitable conductor, for instance carbon paper. The resulting electrode consists of a porous network in which the catalyst particles are in contact with one another, forming a percolation network. A percolation network has permanent connection between the catalyst particles.
Such electrodes prepared from Examples 4 and 5 were tested in a methanol half cell setup shown in FIG. 4, and FIG. 5 shows the electrode holder structure.
The ruthenium complexes in different ligand environments, when present as promoters along with platinum, show improvements to varying degrees when methanol is oxidized at a half cell and galvanostatic polarizations are carried out. Such differences are attributed to the electronic effects of the ligand. One striking discovery was that when tin and ruthenium are bound to certain ligands or when tin or ruthenium are bound to polyethyleneimine performances excelling that of the state of Ru--Pt black were observed. The details are described in Example 6 .
The full cell tests were carried out in a conventional fuel cell structure, conforming to known designs for solid polymer electrolyte (SPE) fuel cells having gas diffusion type anodes and cathodes capable of providing a three-way interface/gas/electrocatalyst/SPE for the oxidation of fuel at the anode and oxygen to water at the cathode. At the anode the alcohol is fed in a liquid such as water and a gas diffusion type structure can accommodate a liquid fell. In this invention it is generally preferred that alcohol be in the liquid mode for methanol and gaseous methanol reformate for methanol reformate oxidation. The results of full cell tests are discussed under Example 7.
The gas diffusion electrode is in contact with solid polymer acidic electrolytes such as sulfonated fluorinated polymer membrane-like materials under trademark Nation by E. I. DuPont de Nemours and Co. of Wilmington, Del., USA. Such membrane-like materials have a fluorinated olefin homopolymer or copolymer "backbone" and pendent partially or fully fluorinated hydrocarbon or ether chains or groups terminated with acidic groups, preferably --SO.sub.3 H, but phosphonic or boric acid type groups are known from the patent literature.
The anode for fuel cell from this invention can therefore be used in a conventional fuel cell.
The fully assembled fuel cell can have stack designs. Any conventional way of producing a steady stream of fuel or oxygen to the cathode can be used. Electrode to the anode leads and extended circuitry are conventional.
By fine-tuning such promoters in terms of the producing of an active redox state at potentials where methanol oxidation occurs, the poisoning effect is overcome.
The same redox promoters that clean platinum during methanol oxidation could be used for anodes that use H.sub.2 contaminated by small amounts of CO.sub.2. This is quite common in hydrogen-oxygen fuel cells that use methanol reformate at the anode (H.sub.2, CO, CO.sub.2 and trace organics). The platinum catalyst under these conditions become poisoned. By producing an electrode cell in accordance with the teachings of this invention the CO.sub.2 tolerance of platinum in the presence of redox promoters is considerably improved, especially at high current densities.
Example 1
The ruthenium complex of ligand la is synthesized as follows: The ligand 2,3,5,6 tetrakis (2'-pyridyl) pyrazine (TPPZ, GFS chemicals, USA) (3 mmoles) 1.165 g is dissolved in ethanol (95%) and RuCL.sub.3.3H.sub.2 O (Johnson Matthey) (3 mmoles) 0.783 g in 95% ethanol is slowly added under reflux with stirring. The mixture is refluxed under Argon for 24 h and the precipitate formed is filtered and washed with cold 95% ETOH five times and dried at 110.degree. C.
The ligands shown in FIGS. 1b and 1c phthalocyanines and porphyrins and their metallocomplexes can be obtained by custom order from Mid-Century Chemicals, Posen, Ill., or Porphyrin Products, Utah.
The ligand cyclam or tetramethyl cyclam can be obtained from Aldrich Co. The preparation of the ruthenium complexes are described in the literature (Che et al., Inorg. Chem., 24, 1797-1800 (1985) and references therein).
The synthesis involves the addition of stoichiometric quantities of tetramethyl cyclam (TMC) or cyclam dissolved in absolute ethanol to a refluxing solution of K.sub.2 RuCI.sub.5 H.sub.2 O (Strem Chemicals, R.I., USA). The metal ligand ratio is 1:1. The addition of the ligand to the metal solution is done dropwise over an extended period of over 6-8 h. The resulting solution is concentrated, and the solid filtered and recrystallized from 2MHCl.
The ligand shown in FIG. 1e is purchased from Aldrich Co. or Alfa.
Refluxing equimolar amounts of RuCl.sub.3.3 H.sub.2 O (Alfa, USA) and the ligand 1e in 95% ethanol for 72 h gave a maroonish precipitate which can be recrystallized from 1:1 ethanol, water. A bimetallic complex could be prepared by reacting 2 equivalents of the metal with one equivalent of the ligand. A similar preparation and purification of a related complex is described by Braunstein et al., Inorg. Chem., 23 (1984).
The ligand FIG. 1f can be purchased from Alfa, USA. Typically the ligand 3 mmoles is dissolved in 95% ethanol and 3 millimoles of RuCl.sub.3.3H.sub.2 O (6 mmoles for the bimetallic complex) is added and stirred under reflux for about 16-18 h under argon. The precipitated complex is filtered and washed with a small amount of 95% ETOH and dried at 110.degree. C.
The ligand FIG. 1g is prepared according to literature (Butte and Case, J. Org. Chem., 26, 4690 (1961). Ru.sub.2 (dppn).sub.2 Cl.sub.4 : The precursor of 1 g, 3,6-Bis(2'-pryidyl)-1,2,4,5-tetrazine was prepared as follows: A solution of 10.4 g. (0.1 mole (of 2-cyanopyridine and 13.4 g. (0.4 mole) of 95% hydrazine in 59 ml. of absolute ethanol was refluxed gently for 6 hr. The resulting orange precipitate was removed and recrystallized from ethanol providing 9.1 g. (76%) of the dihydro base, large yellow needles, m.p. 193.degree.-194.degree. C. (m.p. 194.2.degree. C.). The dihydro base obtained above was dissolved in 50 ml. of warm glacial acetic acid, and 8.0 ml. of concentrated nitric acid was added dropwise and the mixture was made distinctly alkaline by the addition of sodium bicarbonate. The crystalline precipitate was separated and washed with ethanol to give 5.5 g. (64%) of 3,6-Bis(2'-pryidyl)-1,2,4,5-tetrazine, a deep red solid, m.p. 222.degree. C. (m.p. 224.5.degree. C.).
The ligand 3,6-Bis(2'-pyridyl)pyridazine(dppn) was prepared from the precursor as follows. Acetylene was slowly bubbled through refluxing dimethylformamide containing 1.0 g. of finely divided precursor tetrazine. The disappearance of the red color indicated the completion of the reaction. The solvent was recovered by distillation and the solid residue was recrystallized from absolute ethanol producing 0.79 g. (84%) of colorless needles, m.p. 179.degree.-180.degree. C.
The ligand dppn, (2 mmoles) was dissolved in DMF (50 ml), warmed, and an acetonitrile solution containing 2 mmoles of RuCl.sub.3. 3H.sub.2 O (100 ml) was added slowly while vigorously stirring. The resulting brown solution was refluxed while vigorously stirring for 10-12 hours. The reflux flask was covered with aluminum foil to avoid light. At the end of this period, a maroon purple microcrystalline deposit was obtained, which was insoluble in several common organic solvents. Absolute ethanol was used to wash the precipitate which was dried in an oven.
Ru-Biimidazole Rillema et al., Inorg. Chem., 29(2), 167-173 (1990)!: Biimidazole was prepared as described by Fieselmann et al., full reference from recent Phase II (1978). Biimidazole (0.2 g; 1.5 mmol) was added to 30 ml of a 2:1 ethanol/water mixture in a 3 neck flask fitted with a gas inlet tube and a reflux condenser. Ruthenium chloride (0.38 g; 1.5 mmol) was dissolved in 15 ml 2:1 ethanol/water and added to the biimidazole suspension. The mixture was refluxed overnight to give a blue-purple solution, then cooled to room temperature and filtered. The solvent was evaporated on a hot plate until only a few ml remained and filtered to give 0.36 g of solid. Yield 80%.
Ru.sub.2 -Biimidazole: Biimidazole (2 mmol) was dissolved in dry ethanol containing Li isopropoxide (1 mmol), to deprotonate the protons on the ligand. RuCl.sub.3 (4 mmol) was added to the deaerated refluxing ligand and the mixture refluxed overnight under argon. On concentration, a solid separated out that was filtered, washed with water and dried.
Example 2
The redox behavior of some promoters obtained by cyclic voltammetry are shown in FIGS. 2a-2f.
The Ru.sub.2 bpmCl.sub.4 shows two redox couples approximately around 0.2V and 0.4V(FIG. 2a). The mononuclear complex shows a redox couple around 0.4V (FIG. 2b).
The ruthenium biimidazole complexes show redox couples around +0.2V (FIG. 2c). The binuclear species show a two step wave indicating two metal centers (FIG. 2d).
The redox behavior of Ru.sub.2 (dppn).sub.2 Cl.sub.4 is shown in FIG. 2e and its electrochemical oxidation states are indicated in the same figure.
The cyclic voltammogram of Ru.sub.2 TPPZ species is shown in FIG. 2f. The two redox couples around +0.3V and +0.9V are due to the two ruthenium.
Example 3
This example describes the attachment of --OH, NH.sub.2, --NH-bearing ligands to --OH groups of carbon surface on the carbon that is used for electrode fabrication using cyanuric chloride as the coupling agent.
Covalent binding of complexes
About 2-4% of cyanuric chloride (Sigma Co.) in dry acetone was equilibrated as a slurry with a known weight of dry Vulcan XC-72 distributed under the trade name (oxidized) (Cabot Co., Billerica, Mass.) or M-1300 (Cabot Co.) and allowed to react overnight at 4.degree. C. with stirring.
The resulting slurry was filtered the next day, washed with dry acetone to remove excess cyanuric chloride. The filtered carbon was then allowed to react with the following solutions. Cyclam in NaHCO.sub.3 /NaOH pH.about.8, tin or ruthenium tetramino phenyl porphyrin slurry in NaHCO.sub.3 at pH=8. The mixtures were heated up to 80.degree. C. to enable full coupling (FIG. 2) for about 6-8 hours with vigorous stirring. The solid is then filtered, washed several times with warm acetone, and dried.
The analysis of the surface modified carbons showed increase nitrogen contents and the infrared spectra showed the signals for the respective ligands.
Example 4
This example describes the preparation of promoter modified carbons by simple chemisorption, followed by platinization.
Fabrication of the promoter core
For compounds that were soluble in methylene chloride or methanol or dilute sulfuric acid, the following procedure was adopted. The quantities of promoters used are around at 10% w/w with respect to the carbon.
Dissolved 250mg each in separate solutions of ruthenium octaethyl porphyrin (OEP), tin octaethyl porphyrin, ruthenium tetraphenyl porphyrin (TPP), ruthenium or tin (VI) tetramino phenyl porphyrin (TAPP), ruthenium or tin tetramethoxy phenyl porphyrin (TMPP) in 30 mL of methylene chloride or methanol. After sonication, the solutions were mixed each with 2.5 g of Vulcan XC-72 carbon. The slurries were sonicated for a standardized period of time.
Complex Ru.sub.2 (dppn).sub.2 Cl.sub.4 was soluble only in 2M H.sub.2 SO.sub.4 and 2.5 g of the carbon was mixed and sonicated. Since most of the complex adsorbed very strongly on high area carbon, the acid could be washed off with a dilute solution of NaHCO.sub.3 (1M) followed by DI water.
The resulting carbons were allowed to equilibrate under stirring for about 2 hours and filtered. The carbons were washed with water and dried 110.degree. C. A loading of 10% w/w complex on carbon was aimed at.
This was prepared by adding 0.3 g of PEI polyethylene imine polysciences MW 1800 in water to 1.7 g of 20%Pt/C followed by the addition of 0.3 g RuCl.sub.3.3H.sub.2 O. Heated to boiling and evaporated the solvent to obtain semi-dry solid, which is further dried in an oven at 110.degree. C.
Platinization of promoter modified carbons: The Prototech method which is described in Petrow and Allen, U.S. Pat. No. 4,004,193 was adopted. This procedure refers to the deposition of a controlled amount of platinum black on carbon. Prepared 200 mL of platinum acid sulfite is mixed (Strem Chemical Company) in water to give a platinum metal concentration of 2.5 g/L. The solution was divided into 20 mL batches to which 0.45 g of each of the chemically modified carbons and the unmodified controls is added.
The slurry was stirred and sonicated. To the well dispersed slurry 1 mL of 30% H.sub.2 O.sub.2 (without stabilizer) was added dropwise slowly as described in U.S. Pat. No. 4,004,193. The resulting solution was heated to boiling while stirring for one hour.
The platinized carbon was filtered, washed with distilled water and dried in an oven at 110.degree. C. followed by vacuum drying. The quantities used represent an aim to load 10 wt. % platinum. The same procedure for platinization was used for surface modified carbons from Example 3. The amounts of Pt from this platinization procedure varied from 4-9% for various promoted carbons.
Example 5
This example describes the preparation of mixtures of 20% Pt on carbon or Pt black with 10% promoter w/w.
Solutions of macrocycles in methylene chloride or acetonitrile were mixed with 20% Pt on carbon. Typically 0.1 g of the promoter complex is dissolved in methylene chloride or acetonitrile and mixed with 20% Pt on carbon (Johnson-Matthey Co.) (0.9 g) and sonicated. The methylene chloride is slowly evaporated and the carbon vacuum dried (Table II). (Note: Complexes soluble in methanol or any oxidizable organic solvent cannot be exposed to 20% platinized carbon as platinum tends to react violently|) Pre-wetting the platinum/C or Pt black with water and using 50:50 alcohol-water mixture is recommended.
This table shows the amounts of platinum and promoted metal contents in the mixtures used for the electrodes. Since the amount of platinum that is used to make the mixture is well defined, the percentage of platinum in the mixture can be calculated easily. The promoter metal content is calculated from the formula weight of the complex and its weight percentage in the mixture.
Example 6
Fabrication of electrodes and half cell tests in liquid methanol
Preparation of Gas Diffusion Electrode
The catalyst (200 mg) is weighed into a 100 ml beaker and wetted with approx. 50 ml of distilled water. The mixture is stirred with a magnetic stirrer for 10 minutes and subsequently sonicated with 40 watts of power for 3 minutes to obtain a uniform wetted slurry. The pH of this suspension is then measured and adjusted to 3, with a dilute solution of sulfuric acid or NaOH. 67 mg (properly adjusted for the solids content) of Teflon T-30 suspension is added to the pH adjusted catalyst slurry while stirring with a magnetic stirrer. The addition is carried out slowly to allow proper and uniform mixing. The catalyst-Teflon suspension is then sonicated with 40 watts of power for 3 minutes to initiate the flocculation process. At the end of sonication, the suspension is set aside.
The flocculated slurry is now filtered through a Millipore filtration setup (available from Millipore Corporation, MA) with fluoropore 5 micron membranes. To make electrodes, the slurry is filtered through this set up onto a microporous Teflon sheet or to the carbon paper itself. The filtered slurry is washed several times to remove acid and surfactants. The filter cake is then transferred to a carbon paper support. The filtered cake is then pressed at 100 psi first at room temperature for 10 minutes, then at 110.degree. C. for another 10 minutes.
The baking and sintering of the electrode are carried out in a furnace equipped with inert atmosphere. The electrode is placed in a furnace pre-heated to 270.degree. C., and baked for 20 minutes; the temperature is then raised to 340.degree. C. and the electrode is maintained another 20 minutes at that temperature.
Polarization Studies with liquid phase methanol: The apparatus shown in FIGS. 4 and 5 was used for this study. The IR drop was measured using PAR potentiostat on several different occasions with a fixed cell configuration. Several measurements, over a time period gave values around 0.5 kohm, the average of which was 0.5 ohm. All data have been corrected for IR assuming this average value of 0.5 ohm.
This example describes the effect of various promoters on the platinum catalyst. Ligand effects and metal effects are discussed.
The half cell polarization data obtained in liquid phase methanol is shown in FIGS. 6a-6g.
The FIG. 6a uses electrodes from Example 4 where tetaminophenyl tin porphyrin was covalently bound to carbon and platinized. The Pt content was found to be 6.78 and the control carbon had a Pt content of 9.36. In spite of the lower Pt content in the former, the polarization behavior shows a remarkable improvement over the control. Similarly in FIG. 6b, compares the behavior of Rubpm chemisorbed carbon after platinization (Pt content) 5.86 vs. Pt control.
The behavior of promoted carbons with fixed amounts of Pt (from Example 5) are shown in FIG. 6c. The effect of various ligand environments are shown (FIG. 6d).
The effect of various metals in combination with the bipyrimidine ligand on the oxidation of methanol is shown in FIG. 6e. It is interesting to note that Snbpm and Rubpm show about the same improvement in polarization. The ligand biimidazole in combination with Ru showed the best performance also (FIG. 6f). It is thus found that 20% Pt/C combined with suitable promoters could match or exceed the performance of pure 100% Ru/Pt blacks. It is noteworthy that tin and ruthenium bridged by bpm ligand appears more effective (FIG. 6g). The polyethyleneimine-Ru promoted electrode also shows performance on par with ruthenium-Pt black (FIG. 6h).
Example 7
Electrodes were fabricated using standard procedures for Teflon bonded electrodes.
The catalyst carbon powder is teflonated with TFE-30 (25 w%) by mixing Teflon suspension dispersed in a mixture of isopropyl alcohol and water approximately 50/50. The mixture is ultrasonically dispersed and resulting slurry is filtered and dried. The dried powder is spread on Toray carbon paper and the powder pressed (500 psi) and sintered at 340.degree. C. approximately for 30 minutes in the absence of oxygen.
The results of the full cell testing are shown in FIGS. 7a-7c. The bpm-Ru combination was studied in detail both with 20% Pt/C as well as with Pt black (FIGS. 7a and 7b). FIG. 7c compares the performance of a supported 20 w% Pt, 10 w%Ru (Johnson Matthey) catalyst at 0.56 mg Pt/cm.sup.2 to Rubpm 10% on 20% Pt (0.24 mg/cm.sup.2 Pt). A substantially better performance by the Rubpm promoted system is seen compared to the high ruthenium catalyst.
Claims
I claim:
1. A fuel cell electrode for use in the oxidation of methanol or methanol reformate in a fuel cell, said electrode comprising an anode with a platinum electrocatalyst including and a redox promoter.
2. The fuel cell electrode of claim 1 wherein said redox promoter is a metal complex derived from one or more organic or polymeric ligands.
3. The fuel cell electrode of claim 2 wherein said anode further comprises high area carbons and wherein said metal complex is directly chemisorbed from a solution of said ligands onto said high area carbons.
4. The fuel cell electrode of claim 3 wherein said one or more ligands are covalently attached to said high area carbons using a coupling agent to carbon surface-OH groups.
5. The fuel cell electrode of claim 2 wherein said metal complex is a ruthenium or tin complex of macrocyclic ligands.
6. The fuel cell electrode of claim 1 wherein said anode further comprises high area carbons and wherein said platinum is deposited as a high area powder on said carbon.
7. An anode of a fuel cell electrode used for oxidizing methanol or methanol reformate in a fuel cell, said anode comprising a platinum electrocatalyst and a redox promoter.
8. The anode of claim 7 wherein said redox promoter is a metal complex derived from one or more organic or polymeric ligands.
9. The anode of claim 8 wherein said anode further comprises high area carbons and wherein said metal complex is directly chemisorbed from a solution of said ligands onto said high area carbons.
10. The anode of claim 8 wherein said one or more ligands are covalently attached to said high area carbons using a coupling agent to carbon surface-OH groups.
11. The anode of claim 8 wherein said metal complex is a ruthenium or tin complex of macrocyclic ligands.
12. The anode of claim 7 wherein said anode further comprises high area carbons and wherein said platinum is deposited as a high area power on said carbon.
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