Title of Invention

A PROCESS FOR DEPOSITING PLATINUM ON CARBON BLACK FOR FUEL CELLS

Abstract

A process for depositing platinum on carbon black for fuel cells, comprising of activating carbon black in presence of carbon dioxide at 700°C to 1000°C for 1 to 3 hours so that the specific surface area of carbon black becomes 320 m2/g (meter square/gram)to 400 m2/g; loading water by talking 8 to 12% w/v of said activated carbon black with water at 40°C for half an hour; impregnating 12 to 100% w/v of water loaded carbon black by adding drop by drop 1.0 to 3.0% w/v of neutralized chloroplatinic acid to make a slurry and agitating it for half an hour in an ultrasonic bath at 50 °C to 60°C followed by stirring on a magnetic stirrer at 50° C to 60° C for 3 to 4 hours till dryness, finally powdering the said dried impregnated power; reducing salt of platinum by adding finely ground platinum salt impregnated carbon powder to 50% - 75% v/w of 5% sodium borohydride solution, whose pH is maintained at 12 to 12.5 at 0°C + 1°C, and ensuring complete reduction by stirring it vigorously on magnetic stirrer for 1 to 2 hours; washing the said platinum loaded carbon black 4 to 5 times with distilled water at a temperature of 40°C to 50 °C to remove chloride ions, drying at 70°C for 4 hours, then soxheleting the said platinum loaded washed and dried carbon black for 12 to 18 hours to remove residual chloride, borate and other trace impurities, drying the said soxheleted platinum loaded carbon black for 20 to 28 hours in an air oven at 50 ° C to 70 ° C.

Full Text

FIELD OF INVENTION This invention relates to a process for depositing platinum on carbon black for use in fabrication of electrodes for fuel cells, particularly Phosphoric Acid fuel Cell (PAFC). PRIOR ART Fuel cell is a device in which chemical energy of the reaction is converted directly into electrical energy. A typical fuel cell comprises of two electrodes, a cathode and an anode, and an electrolyte interposed between the two electrodes. To avoid leaks and spillage of electrolyte, the electrolyte is usually held on a porous support, e.g. in case of phosphoric acid fuel cell (PAFC), the phosphoric acid acts as the electrolyte and is held in a porous silicon carbide matrix. When a fuel, typically hydrogen, and an oxidant, such as oxygen are supplied to the anode and cathode of the cell respectively, the cell produces electrical power, water and heat. At the anode, ionisation of hydrogen takes place, forming hydrogen ions, which are transported to the cathode through the electrolyte. The electrons released at the anode in this process flow to the cathode via the external circuit. The hydrogen ion reacts with oxygen in the presence of electrons at the cathode, to form water. Performance of a fuel cell depends on the catalytic activity of catalyst. Increasing the cathode activity results in higher power.density of any fuel cell. So high performance cathode catalyst would be of great significance in increasing the power density of any fuel cell system It also helps in reducing the overall capital cost per unit power. Noble metal catalysts, particularly platinum (Pt) or Pt based alloys are generally used for both the electrodes, cathode and anode, in low temperature range fuel cells like Polymer Electrolyte Membrane Fuel Cell (PEMFC) or PAFC, which operate between 50 to 200 C. Generally the platinum used is in the form of small crystallites, dispersed on the conducting support, which is stable in the electrolyte media. The catalytic activity of platinum catalyst generally increases with increase in surface area and to achieve this it is necessary to decrease the platinum crystallite size and optimize the same. There are several known techniques available to deposit platinum particles of size as small as 20 Å on carbon black. The method commonly known as colloidal deposition technique, (US patent Nos 4136059, 4137373, -3992551and 4044193) . uses chloroplatinic acid as a precursor to produce an intermediate platinum sulphite complex, which is subsequently adsorbed on carbon black and further reduced to give finely divided platinum crystallites below 25 Å. The disadvantage: of this known colloidal deposition technique is that during operation of fuel cell, the force between the substrate carbon black and the platinum particles of size less than 50 Å is weakened, and the catalyst particles migrate on the surface of the support to fonn larger agglomerates of size 120 - 140 Å, whichidecreases the performance of fuel cell. Another disadvantage of the above known technique is that it involves elaborate and complex steps. In another known method the platinum surface area has been retained constant by preventing the platinum migration through heat treatment of the catalyst in inert atmosphere The main disadvantage of the above known art is that it adds one more step to already elaborateand complex process. According to a method proposed by Blurton (J. Electrochem. Soc. 119 i. (1972) 559) and Bregoli (Electrochim. Acta 23 (1978) 489) a decrease in specific activity for oxygen reduction was found by diminishing the particle size of platinum crystallites from 120 Å to 3.0 Å and 2.0 Å respectively. The disadvantage of the above technique as reported by Peuckert (J. Electrochem. Soc. 13(5) (1986) 944-947) is that by reducing platinum crystallite size to less than 50 Å, the performance of a fuel cell, particularly for long term operation is not enhanced. OBJECTS OF INVENTION The main object of the present invention is to provide a process for depositing platinum on carbon black for fabrication of electrodes of fuel cells. Another object of the present invention is to provide a process for depositing platinum on carbon black wherein platinum crystallites having an average diameter of'50 Å to 80 Å/are dispersed uniformly on carbon support. Yet another object of the present invention is to provide a process for depositing platinum on carbon black which utilizes a catalyst that does not require heat, treatment for sustained performance. Still another object of the present invention is to provide a process for depositing platinum on carbon black which facilitates/loading of catalyst to as high as 20% on the substrate Further, object of the present invention is to provide a process for depositing platinum on carbon black which is simple and inexpensive. To achieve the said objective this invention provides a process for depositing platinum on carbon black for fuel cells, comprising for a) activating carbon black in presence of carbon dioxide at 700°C to 1000°C for 1 to 3 hours so that the specific surface area of carbon black becomes 320 m2/g (meter square/gram)to 400 m7g; define b) loading water on carbon black by talking 8 to 12% w/v of said activated carbon black obtained in step (a); with water at 40°C for half an hour; c) impregnating 12 to 100% w/v of water loaded carbon black from step (b) by adding drop by drop 1.0 to 3.0% w/v of neutralized chloroplatinic acid to make a slurry, agitating the said slurry for half an hour in an ultrasonic bath at 50°C to 60°C followed by stirring on a magnetic stirrer at 50°C to 60°C for 3 to 4 hours till dryness, then finely powdering the said dried impregnated power; d) reducing salt of platinum by adding the said finely ground platinum salt impregnated carbon powder prepared in step (C) to 50% - 75% v/w of 5% sodium borohydride solution, whose pH is maintained at 12 to 12.5 at 0°C ± 1°C, and ensuring complete reduction by stirring it vigorously on magnetic stirrer for 1 to 2 hours; e) washing the said platinum loaded carbon black fronl step (d) 4 to 5 times with distilled water at a temperature of 40°C to 50°C to renl0ve chloride ions, drying at 70°C for 4 hours, then soxheleting the said platinum loaded washed and dried carbon black for 12 to 18 hours to remove residual chloride, borate and other trace impurities, drying the said soxheleted platinum loaded carbon black for 20 to 28 hours in ah air oven at 50°C to 70°C. Broadly, this invention provides a process for depositing platinum on carbon black for fuel cell electrodes by impregnation and electro-less deposition technique. Sodium salt of platinum is adsorbed on a high surface area carbon black, this salt is then reduced with sodium borohydride at controlled pH and temperature to give platinum crystallites having diameter between 50A to 80A, uniformly distributed on the support. The platinum loaded carbon catalyst of this invention is uniformly dispersed on carbon support to be used in fuel ceil electrodes and the cell output is 200mA/cm2 at 0:670V and 300mA/cm2 at 0.6V at an operating temperature of 150°C of the cell. Description of the Invention The process of deposition of platinum on carbon black comprises of following steps: (a) Activating carbon black Carbon black having specific surface area in the range of 50m2/g to l000m2/g preferably 200m2/g to 300m /g is activated in presence of carbon di-oxide at 700°C to 1000°C preferably at 900°C for 1 to 3 hours so that the specific surface area of carbon becomes 320m2/g to 400m2/g preferably 324m2/g. (b) Loading of water on carbon black 8 to 12% w/v preferably 10% w/v of activated carbon from step a) is treated with water at 40°C for about half an hour so that the inner most pores of carbon black are filled with water. Impregnating carbon black with chloroplatinic acid 1.0 to 3.0 w/v preferably 2.0% w/v of chloroplatinic acid is neutralized with 5 to 15% of sodium carbonate, pH 6.8 to 7, Iso-propanol 40 to 60% v/v preferably 50% of that of chloroplatinic acid, is then added to increase the wetability. This neutralized chloroplatinic acid is added drop by drop to 12 to 100% w/v preferably 27% of carbon black obtained from step b) and agitated for about half an hour in an ultrasonic bath at 50 to 60°C. This slurry is then stirred on a magnetic stirrer at 50 to 60°C for 3 to 4 hours till it is completely dry. The rate of drying is controlled to give a water loss of l0ml/hr/gm of the powder. This dried impregnated powder is then finely powdered. To 50-75v/w/of; 5% sodium borohydride solution, whose pH is maintained at 12 to 12.5 by adding ammonia solution and the temperature is thermostatically controlled at 50 ± 1°C, the dried and finely ground platinum salt impregnated carbon powder prepared in step c) is added and stirred vigourously on a magnetic stirrer for 1 to 2 hours to ensure complete reduction. Washing and soxhelting the catalyst The platinum loaded carbon from step d) is separated by filtration and washed 4 to 5 times with distilled water at a temperature of 40 to 50°C to remove chloride ions. It is then dried at about 70°C for about 4 hours. This platinum loaded washed and dried carbon black is soxheleted for 12 - 18 hours preferably 15 hours for through cleaning of residual chloride, borate and other trace impurities. This:is then finally dried for 20 to 28 hours preferably 24 hours in an air oven at 50 to 70°C preferably 60°C to gel platinum loaded carbon catalyst. This invention will now be illustrated with a working example, which is intended to be typical example to explain the technique of the present invention and is not intended to be taken restrictively to imply any limitation to the scope of the present invention. WORKING EXAMPLE 100 gm of Carbon black (Vulcan XC-72R) having specific surface area in the range of 200 m2g to 300 m /g was taken in a quartz crucible and heated in a furnace, where a carbon di oxide atmosphere was maintained, at 900 JC for about 1 hour. The specific surface area of carbon black after this activation step became 324 m7g. 4.5 gm of this activated carbon black (Vulcan XC-72R) was taken in a flask and 50 ml of distilled water was added to it. This slurry of carbon black was heated at 40°C for half an hour with continuous stirring. Dissolved 1.25 gm of chloroplatinic acid in 60 ml of distilled water and neutralized it with 3.5 ml of 1M sodium bicarbonate. Added 25 ml of iso-propanol to this neutralized chloroplatinic acid and then added this solution drop wise to the water loaded carbon black. Agitated the slurry ultrasonically for half an hour at 50°C and then stirred the slum' for 3 hours at 50 to 60°C for complete absorption of the sodium salt of chloroplatinic acid on carbon material Dissolved 2 5 gm of sodium borohydride in 50ml of water and adjusted the pH to 12 by ammonia while maintaining the temperature of the solution at 50°C. Added the pretreated carbon powder slowly to this reducing solution, stirred it for half an hour and then filtered the carbon powder. Washed the carbon powder with 400 ml of warm distilled water at a temperature of 40°C, 3 to 4 times, dried it at 70°C in an air oven for 4 hours. Soxheleted the catalyst loaded carbon powder for 15 hours and then dried the resultant powder for 24 hours in an air oven at a temperature of 60°C. Evaluation The weight percentage of platinum loaded on the carbon black is analyzed by colorimetric and gravimetric methods. Transmission electron micrograph (TEM) showed that platinum crystallite on carbon support had a size distribution between 50 A to 80 A. The electrochemical surface area of the catalyst was measured by cyclic voltammetry and was found to be between 70-80 m2 /gm of platinum. The electrochemical characterization is earned out in a unit cell, by interposing a phosphoric acid filled silicon carbide matrix between two porous gas diffusion electrodes - anode and cathode. Test conditions Operating temperature of the cell -150°C Area of the electrode - 20 cm2 Fuel - Hydrogen Oxidant - Oxygen Cell out put - 200mA/cm2 at 0.670 V And 300mA/cm2 at 0.6 V It is to be understood that the process of the present invention is susceptible to modifications, changes, adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention which is further set forth under the following claims: We Claim: 1. A process for depositing platinum on carbon black for fuel cells, characterized in that: a. activating carbon black in presence of carbon dioxide at 700ºC to 1000ºC for 1 to 3 hours so that the specific surface area of carbon black becomes 320 m2/g (meter square/gram)to 400 m2/g; b. loading water on carbon black by taking 8 to 12% w/v of said activated carbon black obtained in step (a); with water at 40ºC for half an hour; c. impregnating 12 to 100% w/v of water loaded carbon black from step (b) by adding drop by drop 1.0 to 3.0% w/v of neutralized chloroplatinic acid to make a slurry, agitating the said slurry for half an hour in an ultrasonic bath at 50ºC to 60ºC followed by stirring on a magnetic stirrer at 50ºC to 60ºC for 3 to 4 hours till dryness, then finely powdering the said dried impregnated power; d. reducing salt of platinum by adding the said finely ground platinum salt impregnated carbon powder prepared in step (C) to 50% - 75% v/w of 5% sodium borohydride solution, whose pH is maintained at 12 to 12.5 at 0ºC ± 1ºC, and ensuring complete reduction by stirring it vigorously on magnetic stirrer for 1 to 2 hours; e. washing the said platinum loaded carbon black from 1 step (d) 4 to 5 times with distilled water at a temperature of 40°C to 50ºC to renl0ve chloride ions, drying at 70°C for 4 hours, then soxheleting the said platinum loaded washed and dried carbon black for 12 to 18 hours to remove residual chloride, borate and other trace impurities, drying the said soxheleted platinum loaded carbon black for 20 to 28 hours in an air oven at 50ºC to 70ºC. 2. A process as claimed in claim 1, wherein carbon black is activated at 900ºC. 3. A process as claimed in claim 1, wherein specific surface area of activated carbon is 324 m2/g. 4. A process as claimed in claim 1, wherein loading water on carbon black by taking 10% w/v of the activated carbon with water at 40°C for half an hour. 5. A process as claimed in claim 1, wherein neutralized chloroplatinic acid is 2% w/v. 6. A process as claimed in claim 1, wherein 27% w/v of water loaded carbon black is impregnated by adding drop by drop neutralized chloroplatinic acid. 7. A process as claimed in claim 1, wherein soxheleted platinum loaded carbon black is washed and dried for 15 hours. 8. A process as claimed in claim 1, wherein soxheleted platinum loaded carbon black is dried for 24 hours in an air oven. 9. A process as clainled in claim 1, wherein carbon black is dried at 60 C. 10. A process as claimed in claim 1, wherein specific surface area of carbon black taken for activation is 50m2/g to 1000m2/g. 11. A process as claimed in claim 1, wherein specific surface area of carbon black taken for activation is 200m2/g to 300m2/g. 12.A process as claimed in claim 1, wherein chloroplatinic acid is neutralized with 5% to 15% w/v sodium carbonate, pH 6.8.to 7. 13.A process as claimed in claim 1, wherein to said neutralized chloroplatinic acid 40% to 60% v/v of Iso-propanol is added to increase the wetability. 14. A process as claimed in claim 1, wherein to said neutralized chloroplatinic acid 50% v/v of Iso-propanol is added to increase the wetability. 15. A process as claimed in claim 1, wherein the rate of drying of the said impregnated carbon black is controlled to give a water loss of 10ml/hr/gm of the powder. 16. A process for depositing platinum on carbon black for fuel cells as substantially herein described with reference to examples.

Documents:

1491-del-2004-abstract.pdf

1491-del-2004-claims.pdf

1491-del-2004-complete specification (granted).pdf

1491-DEL-2004-Correspondence-Others-(29-12-2010).pdf

1491-del-2004-correspondence-others.pdf

1491-del-2004-correspondence-po.pdf

1491-del-2004-description (complete).pdf

1491-del-2004-form-1.pdf

1491-DEL-2004-Form-15-(29-12-2010).pdf

1491-del-2004-form-19.pdf

1491-del-2004-form-2.pdf

1491-del-2004-form-3.pdf

1491-DEL-2004-GPA-(29-12-2010).pdf

1491-del-2004-gpa.pdf

1491-del-2004-pa.pdf