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Fuel Cell Energy Generators PLATINUM CATALYSTS USED in ALTERNATIVE ENERGY SOURCE

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Fuel Cell Energy Generators PLATINUM CATALYSTS USED in ALTERNATIVE ENERGY SOURCE Fuel Cell Energy Generators PLATINUM CATALYSTS USED IN ALTERNATIVE ENERGY SOURCE By D. S. Cameron Group Research Centre, Johnson Matthey and Co Limited The steam, in turn, is produced by burning Improvements in the preparation and fossil fuels (coal or hydrocarbons), or by utilisation of platinum metal catalysts nuclear fission. Unfortunately, the processes have now made megawatt scale fuel cell of converting fuel into heat, heat into steam, systems economically viable. The high and steam into electricity involve efficiency thermal efficiency and low pollution losses at every stage. The Carnot cycle characteristics of this novel power source shows that the thermal efficiency of any heat are likely to result in it forming an engine is limited to E where: increasing proportion of the generating capacity utilised for both peaking and intermediate applications. This paper T, and TI being the temperatures, in degrees briejly reviews the development of fuel kelvin, at which heat is supplied to, and cells; it also describes how they work, rejected by, the system. In practice, upper including the function of the catalyst, temperatures are limited to about 500'C and indicates the more important of the (773 K), lower temperatures are at least 30°C many factors which have to be considered (303 K), and efficiencies are limited to a when selecting a commercial system. maximum of 60 per cent. Indeed most modern large power stations are able to achieve only 38 to 40 per cent efficiency. Fuel cells have already achieved acceptance Fuel cells convert fuel directly into elec- as reliable, lightweight power sources for trical energy by an electrochemical route space applications, for example in the Apollo which is not limited by the Carnot cycle. and Gemini projects (I). Because their efficiency is not related to size, Considerable experience has also been cells being developed range from I to 2 kilo- obtained in operating terrestrial plants to watts to several megawatts. In contrast to power residential, industrial and office build- batteries, the cells generate power rather than ings, under the TARGET programme in the store it, and continue to do so as long as a United States of America. Currently in the fuel supply is maintained; there is no re- U. S.A., commercially viable, multi-megawatt charging cycle. generator systems are under construction. It The simplest fuel cells run on hydrogen is now likely that fuel cells, which combine and oxygen; their operation is the converse high thermal efficiency with low environ- of the electrolysis of water. This effect was mental pollution, will take a substantial share discovered in 1839 by Sir William Grove (3), of future generating capacity. By this means who found that electrolysis between platinum the ability of the power generating utilities to electrodes could be reversed, with the con- respond to changing power demands will be sequent generation of electric current and the increased, and this will help to conserve production of water. supplies of fossil fuels (2). The simple unit cell, Figure I, consists of Most electrical energy used today is gener- two electrodes, one of which is supplied with ated by alternators driven by steam turbines. hydrogen (the anode), the other with oxygen Platinum Metals Rev., 1978, 22, (2), 3846 38 (the cathode). These electrodes are made and F is the Faraday constant of 96,500 electrically conducting, and porous so that coulombs per equivalent. The standard free gases can diffuse through them. Between the energy change for the chemical reaction electrode pair is an electrolyte, usually a forming water is -113.38 kcal/mol of oxygen strong acid or alkali. The purpose of this is consumed, n=4, and I calorie is 4.18 joules. to prevent mixing of the molecular gases fed The standard thermodynamic reversible to the electrodes, while permitting charged potential for the hydrogen-oxygen fuel cell at chemical species (ions) to pass from one 25°C is therefore 1.229 volts. electrode to the other carrying an electrical In practice, this reversible potential is current. Connecting an external load across rarely attained, due to the presence of trace the electrodes completes a circuit, the passage impurities and side reactions such as peroxide of electrons through the load resistance con- formation, mainly at the oxygen electrode (4). stituting the work done by the cell. The overall chemical reaction for this pro- cess is: The Role of the Catalyst 2H,+02-+2H,0 If a piece of inert metal (M) is immersed In an acid electrolyte cell, the hydrogen and in acid and covered by hydrogen gas bubbles, oxygen electrode reactions are respectively: some of the hydrogen molecules dissolve in the acid under equilibrium conditions. At the 2H2+- 4H++4e’ (at the anode) metal surface, some of these dissolved 0, +4H+ k4e’ --f 2H,O (at the cathode) hydrogen molecules attach themselves loosely Current is carried through the electrolyte to the metal by donating electrons from their between the electrodes by the positively valency, or bonding, orbitals. A number of charged protons (H b) and round the external these atoms then become detached, leaving load by the electrons (e’). an electron in the metal, and exist in the Considering the thermodynamics of the solution as positively charged ions (H+): system for a cell with no current flowing, that BHz(gas) %*Ha(dissolved) is, at reversible equilibrium; it may be shown that the standard free energy change (AGO) &Hz(dissolved)+(M)%(M)-H%M+H++e’ is related to the standard electromotive force The process is reversible, the exchange of (E”) by the relationship : electrons (e’) constituting a current flowing AGO= -nFE” into and out of the metal. Under equilibrium where n is the number of electrons involved, conditions, the rate at which electrons are Platinum Metals Rev., 1978, 22, (2) 39 discharged at the metal surface equals the clearly it is desirable to limit overpotential rate at which they are released back into the and other voltage losses as far as possible. TO electrolyte. This rate is termed the exchange achieve this, exchange current densities can current density, When a current flows to or be increased by increasing reactant concen- from the electrode to an external source, the trations, pressures and temperatures. In rates of reaction become unbalanced. The addition, the most catalytically active metal change in electrode potential required to available should be used for the reaction, and achieve this inequality is termed rhe over- the active surface area of the catalyst should potential. be as large as possible. The highest exchange current densities, Although the relatively simple hydrogen about ~o-~A/cm~,are obtained with the ionisation reaction has been used as an platinum group metals, where hydrogen is example of the function of the catalyst, many weakly adsorbed on the metal surface, reactions proceed via a series of intermediate enabling a rapid formation and breaking of steps involving a variety of chemical species. metal-hydrogen bonds. For metals having a Any one of these steps may be slower than high positive heat of adsorption, such as the others, and the rate of the reaction will mercury, lead, tellurium, zinc and cadmium, be governed by this slowest step. In such a the metal-hydrogen bond is very weak com- situation the function of the catalyst may be pared with the hydrogen-hydrogen bond of to increase the rate of this limiting step so the molecules. Only a small proportion of the that the overall reaction will proceed much molecules have sufficient energy to dissociate faster; although again it may well be limited and bond to the metal and the exchange by another step in the process. current is correspondingly low ( Io-lZA/cm2). With metals to which hydrogen is strongly Electrode Operation bonded, such as molybdenum, tantalum and Fuel cell electrodes perform a number of tungsten, hydrogen is so firmly adsorbed functions simultaneously. In addition to that further reaction is restricted, and the supporting the catalyst and carrying current, exchange current is again low (Io-eA/cmz). It they must act as a barrier to prevent electro- should be noted that the exchange currents lyte escape into the gas compartment, and quoted are for real area of metal surface. Due provide maximum area of interface between to roughness factors, these may be several reactant gases, electrolyte, and catalyst orders of magnitude higher than geometric surface. areas. The structure is therefore made porous to Noble metal catalysts are particularly allow gas to diffuse to the reaction sites, with effective in fuel cells due to their high ex- only a thin film of electrolyte covering the change current densities, and resistance to catalyst. Gases can then dissolve and diffuse oxidation and dissolution under operating rapidly to the sites during cell operation. This conditions. electrolyte/gas /solid three-phase interface may It may be shown that as an electrode is be established by incorporating a wetproofing moved away from equilibrium conditions by agent, such as polytetrafluoroethylene, into the the application of an external current, metals structure, Figure 2. Alternatively, the pore with high exchange current densities exhibit size distribution may be varied through the lower overpotentials than metals with low thickness of the electrode; in which case the exchange current densities. position of the electrolyte interface is rnain- Since the magnitude of 'the overpotential tained by balancing capillary forces with a represents a voltage-and therefore efficiency pressure difference across the two electrode -loss, and operating cells have to conduct faces. high currents in order to do useful work, The effect of drawing current from a fuel Platinum Metals Rev., 1978, 22, (2) 40 cell is illustrated in Figure 3. At the oxygen electrode, region AB represents voltage losses at low currents due to activation overpotential caused by the slowness of one or more inter- mediate steps in the electrode reaction.
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