Fuel Cell Energy Generators 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 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 is now likely that fuel cells, which combine and ; 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 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 ), the other with oxygen

Platinum Metals Rev., 1978, 22, (2), 3846 38 (the ). 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 pair is an , 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 () 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 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 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, 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. Region BC is normally quite linear, and results from ohmic resistive losses in the electrode and electrolyte. A voltage gradient is necessary both to drive charged ions through the electrolyte and electrons through the electrode material. The thickness of the electrolyte layer is normally the minimum required to prevent gas crossing from one electrode to another. A thin porous barrier of inert electrically in- sulating material may also be interposed between the electrodes to assist in this function, and to prevent electrodes shorting together. The region CD represents a voltage drop caused by poor transport of reactants. At high current densities the gases fail to diffuse through the electrodes and pass to the reaction sites sufficiently quickly to maintain the reaction. Most activation overpotential problems occur at the oxygen electrode, the hydrogen reaction being considerably faster. Electrodes operating on air also suffer from lowered oxygen partial pressures, caused by the diluting effects of nitrogen.

Practical Fuel Cells To obtain useful voltages, pairs of cells are connected in series to form stacks. These are normally flat, packed back to back in a classical filter press configuration. Electrodes may be supported by channelled backing plates, which collect current and distribute gas uniformly over the electrode surfaces. Reactant supplies are normally maintained higher than stoichiometric, that is greater than is consumed in the reaction, and the excess gases carry away heat, product water and trace impurities. Alkaline and acid cells are usually operated at temperatures between 50 and 250°C to obtain a compromise between heat removal,

Platinum Metals Rev., 1978, 22, (2) 41 electrode activity and corrosion rate. To coverage is reduced, so it has proved feasible develop commercially viable power sources, to operate fuel cells on hydrogen obtained construction materials and catalyst costs must from a reformer system, even though this be minimised. Air is used as oxidant, and the contains a small percentage of carbon mon- modes must be tolerant of impure fuels, oxide, providing the operating temperature while still maintaining an adequate lifetime. is maintained at about 190°C (11). These criteria are being met in a system developed by the United Technologies High Temperattlre Fuel cells Gr~omtionwhich makes extensive use of Fuel cells which operate at high tempera- carbon as a cheap, corrosion resistant and tures can be divided into those which employ conducting material (5). molten carbonate and operate at use Operation on air demands the of acid about 600 to 7m"C, and solid electrolyte electrolyte to avoid carbonation problems types operated at above IOOO"~. from atmospheric carbon dioxide. Phos- Molten carbonate cells rely on high temper- phoric acid is used, despite its relatively low atures to enhance reaction rates and thereby conductivity and highly corrosive properties, decrease electrode polarisation. Carbonate in preference to other acids which are mostly mixtures, such as lithium and potassium, are unstable in a fuel cell environment. used because other species such as hydroxides, Alternative electrolytes under evaluation chlorides, nitrates and sulphates do not give include halogenated sulphonic acids, such as an acceptable combination of conductivity trifluorometlianesulphonic acid monohydrate and stability in the molten state. (TFMSA). Solid polymer electrolytes have Most systems envisaged involve produc- also been widely used, for example in the tion of hydrogen from hydrocarbons or coal General Electric Company's fuel cells (6). by an external reformer to yield hydrogen: Noble metal catalysts have now been carbon dioxide mixtures which are fed to the accepted as being the best choice, although anode : other systems, such as tungsten carbide, have also been evaluated (7-9). Effort has been *HzS aCOg+iH8-t-dCOY te' (anode reaction) concentrated on lowering costs by optimum utilisation of very low metal loadings. ~O,+~CO,+e'+~CO," (cathode reaction) Highly effective noble metal utilisation is The combusted anode gas is then enriched obtained by dispersing the metal evenly on with air and fed to the cathode where it a carbon support. By this means metal areas replaces the carbonate in the electrolyte. greater than 150 m2/g of platinum can be Electrodes are normally porous plaques of obtained, compared with only 30 mZ/g of sintered nickel or stainless steel, with silver, platinum for platinum blacks. The structure copper oxide or copper for oxygen reduction. also has improved mass transport properties For hydrogen oxidation, of nickel or and resistance to metal crystallite sintering silverised stainless steel may be used. during cell operation. Although loss of metal Problems encountered with the system area by sintering may be partially offset by include electrolyte losses due to evaporation increased activity of the resulting larger and leakage during operation, electrode sinter- crystallites, this problem is still the most ing, and gas crossover. While it is relatively crucial aspect governing the durability of the simple to fabricate and run a single cell, acid fuel cell (10). durable interconnections between multiples Carbon monoxide is preferentially adsorbed of cells are difficult to achieve. At present the on platbum, poisoning the catalytic sites for area of three-phase interface is greatly limited hydrogen oxidation. However, at higher by materials of construction, and this leads temperatures the degree of carbon monoxide to poor electrode performance.

Platinum Metals Rev., 1978, 22, (2) 42 At temperatures of about IOOO~Ccertain conductivity. The most widely studied solid oxide materials exhibit solid state electrical electrolytes are based on zirconia. This

The First Fuel Cell The fuel cell is generally considered to be a product of mid-twentieth century technology and to be yet some distance away from attaining its full potential as a source of energy. But its origin goes back some hundred and forty years to a remark- able man, both scientist and lawyer, who not only conceived the idea by careful experimentation but actually constructed a working fuel cell in 1842. William Robert Grove, born in in 1811, educated at Brasenose College, Oxford, and called to the Bar in 1835, spent several of the following years at home in Swansea on investigations in electro- , including the invention of the Grove cell. In a postscript dated January 1839 to a letter to The Philosophical Maga- zine he described an experiment on “an important illustration of the combination Sir William Robert Grove of gases by platinum” in which a gal- vanometer was permanently deflected the first practical fuel cell, employing when connected with two strips of plati- platinum foil coated with spongy platinum num covered by tubes containing oxygen produced by electrolysis of the chloride. and hydrogen. By 1840 he had been A series of tifty pairs, constructed as shown elected F.R.S., and had been appointed here in Grove’s original diagram, with Professor of Experimental Philosophy at dilute sulphuric acid as the electrolyte, the London Institution in Finsbury was found “to whirl round” the needle of Circus. It was from there that he addressed a galvanometer, to give a painful shock to a further letter, dated October 29, 1842, five persons joining hands, to give a to The Philosophical Magazine, “On a brilliant spark between charcoal points, Gaseous Voltaic Battery”. This paper and to decompose hydrochloric acid, (Phil. Mug., 1842, 21, 417-po) describes potassium iodide and acidulated water. Grove concluded his short paper with a modest but pregnant thought: “Many other notions crowd upon my mind, but I have occupied suficient space and must leave them for the present, hoping that other experimenters will think the subject worth pursuing.” In later years Grove had a distinguished legal career, becoming a Q.C. in 1853 and a judge in 1871. He was knighted a year later, was made a Privy Councillor in 1887, and died at the age of 85 at his house in Harley Street, London, on the first of August, 1896. L. B. H.

Platinum Metals Rev., 1978, 22, (2) 43 material is stable as the monoclinic form at together with various carbides and cermets. room temperature, and is transformed into Because the problems of charge transfer the tetragonal form at 115ooC, accompanied polarisation-that is -are not serious by a 9 per cent volume contraction. Both at high temperatures, molten carbonate and these structures are similar to a cubic lattice, solid oxide fuel cells offer the long-term possi- and by the addition of other oxides in solid bility of eliminating noble metal catalysts from solution, zirconia can be stabilised to give a the fuel cell. High operating temperatures cubic fluorite type of structure over certain also offer greater thermal efficiency (7500 ranges of concentration. The latter does not B.tu/kWh) than low temperature fuel cells exhibit the volume contraction and thus (9300 Btu/kWh) (5). To achieve this, how- minimises thermal cracking. Typical stabil- ever, a number of very difficult problems will isers are calcium and yttrium oxides. have to be overcome. Corrosion, electrolyte The fuels generally used are hydrogen and instability and electrode sintering have up to oxygen, thus : now prevented high temperature cells demon- strating reliable, long-term operation. Coun- Ha tO"+H,O+ae' (anode reaction) tering the extreme corrosion problem entails 0,+4e'-+zo" (cathode reaction) expensive construction materials, which are In this case, oxide ions (0")act as the also difficult to fabricate and assemble. current carrying species in the electrolyte. If the oxygen content becomes depleted during Methanol Cells cell operation, the material may become a Methanol is the most promising fuel for low semiconductor, resulting in a loss of cell power terrestrial applications, such as vehicle efficiency. traction and portable generators. It is easily Normally, the hydrogen is produced by an transported and stored, and is available in external steam reformer system. At 10oo"C bulk. It can be electrochemically oxidised to it would be possible to decompose hydro- carbon dioxide and water, the thermodynam- carbons thermally within the cell, but there ically reversible potential being 1.20 volts. is a danger of pyrolytic carbon deposition in Commercially attractive systems have not yet the anode compartment. been produced, but suitable platinum metal The cells consist of porous, sintered elec- catalysts are now being developed to yield trodes situated on either side of thin, flat discs high current densities with an acceptably low of the solid electrolyte, The electrolyte has polarisation. ReG,ently, active catalysts have to be chemically and physically stable at the been made based on the platinum-tin (12,13) operating temperature and impermeable to and platinum-ruthenium systems (14). gases, but as thin as possible to minimise Direct methanol cells, where the fuel is ohmic resistance. For the cathode catalyst dissolved in the electrolyte, have improved and current collector, the only metals suffi- mass transfer characteristics compared with ciently resistant to oxidation are the noble types. Unfortunately, metals. However, various oxides are suffi- methanol also diffuses to the cathode and is ciently catalytic and conducting to perform chemically oxidised, but this can be prevented this task. These include copper and man- if a is imposed ganese oxides, lithium doped nickel oxide, between the anode and cathode. and perovskite structures such as LaCoO, and A number of small indirect methanol cells PrCoO,. have also been produced. In these a water- For the anodes various powdered metals methanol mixture is reformed to hydrogen such as platinum, nickel, cobalt, iron, before passing into a normal hydrogen air manganese, and metal oxides such as fuel cell (15). ferric and titanium oxides are suitable, The first fuel cell applications for motor

Platinum Metals Rev., 1978, 22, (2) 44 vehicles are likely to be for methanol fuelled former, powerplant, and power conditioner units for public utility transport. However, sections. The latter converts direct current from power to weight, power to volume, and from the powerplant into alternating current cost considerations, the internal combustion suitable for distribution to the consumer. The engine is likely to predominate for many years three components will be available in modular to come, except of course for specialised form to build up the required generation application for the military and the aerospace capacity. Conventional steam or gas turbine industry. generators operate at severely reduced effi- ciency under part load conditions. Geaera- Current Technology tion is normally carried out with a mixture of The United States of America has a widely different types of generators of varying sizes, dispersed population. The country also faces each operated as close as possible to full load, a rapidly dwindling supply of fossil fuels, and in order to equate capacity with load demands. increasingly severe legislation against environ- Fuel cells, however, are efficient over a range mental pollution. It is hardly surprising of output levels from 25 per cent to 100 per therefore that development of efficient, cent of rated output. Lowered current flow pollution-free fuel cells is receiving support through the cell results in decreased ohmic both from the electricity utilities and from losses, and in fact efficiency actually increases the U.S. Government. Two major pro- at part load. Gas flows to the cell are auto- grammes are in progress. The first is the matically and rapidly regulated to balance construction and evaluation of a 4.8 MW unit consumption. Equally, the fuel cell can which is intended to lead to the construction respond to sudden increases in load demand. of larger 27 MW stations, each sufficient for Normally, a utility must maintain a propor- an urban locality of 20,ooo people. This size tion of spare generating capacity to allow for was chosen as being suitable for the gener- these sudden demand changes in the form of ating needs of 80 per cent of the municipal rotating alternators, or spinning reserve. This and rural producers in the United States. capacity, which inevitably runs at low effi- The programme is being carried out by ciency, would be eliminated by the presence United Technologies Corporation (U.T.C.) of fuel cell reserve power. and nine U.S. electric utility companies, The modular construction of fuel cells and backed by the Energy Research and Develop- associated plant increases its mobility, and ment Administration (DOE) and the Electric should enable adaptation of power stations to Power Research Institute (EPRI). meet changing demands caused by factors In addition, the TARGET programme aims such as population growth. Modular con- to develop on-site generators of 40 kW struction also increases system reliability, capacity for residential, commercial and allowing partial shutdown for maintenance or industrial applications. TARGET consists of subsystem failure while still operating at a number of gas supply utilities together with limited capacity. This high reliability factor U.T.C. would allow utilities to function with smaller This programme, which has now been standby reserves than are normally kept running for over 11 years has gained con- available at a few hours notice in case of plant siderable experience from the operation of breakdown. 65 unattended 12.5 kW units during 1972 and 1973 in the U.S.A., Canada and Japan. Prospects for Fuel Cell Applications Both the systems are based on low temper- It has been predicted (2) that in the future ature phosphoric acid cells, running on electric power utilities will use nuclear or impure hydrogen from steam reformed fossil fuel base load generators, running con- hydrocarbons. The system consists of re- tinuously at full load, and optimum efficiency.

Platinum Metals Rev., 1978, 22, (2) 45 These will be supplemented by fuel cell range from 2 to 7 per cent yearly growth rate. generators capable of providing the bulk of The degree of‘market penetration achieved intermediate and peaking power require- by fuel cell generators will depend not only ments, thus utilising their ability to operate on costs, which are estimated to be com- efficiently at part load. Because of their low parable with gas turbine units, but also on a pollutant emissions and silent operation, these combination of imponderables such as reli- fuel cells could be located close to centres of ability, fuel savings and power transmission population, eliminating a high proportion of cost savings. Forecasts of the number of 27 power transmission costs. Many city centre MW powerplants installed by 1985 range generating plants are over 30 years old and from 54 to 540, and by 1990, from 146 to have high atmospheric and thermal pollution 1460 units (16). characteristics. Possible options are to The first stage of the Demonstration Pro- attempt to clean up existing plant, incurring gramme is the construction of a 4.8 MW unit, further efficiency penalties, to eliminate the incorporating catalysts supplied by Matthey plant completely and bring in power via Bishop, the Johnson Matthey affiliate in the underground transmission lines, or build new United States of America. This unit will be plant on site. Fuel cells, available as relatively evaluated on-site by Consolidated Edison in small units, but with efficiencies equalling New Yorlr, as the prototype of over 50 of the large generating stations, make the third 27 MW units provisionally ordered by supply option most feasible. utilities across the U.S.A. Demands for replacement generating units Some eighteen years ago, F. T. Bacon in the period 1980 to 1985 are: baseload units described his fuel cell as being “at about the 10 per cent, intermediate units 20 per cent, same stage of development as Eleriot’s aero- and peaking units 40 per cent. This forecast plane’’ (17). By analogy, we may now be (16) is based on lifetimes of 50 years, 30 yezrs witnessing the beginning of the era of civil and 15 years for the respective units, taking aviation, and the evidence at present available into account the age histories of present plant suggests that platinum catalysts will have a in the United States. This is excluding new major part to play in this most essential generating capacity, for which the estimates development.

References I A. C. Ching, A. P. Gillis and F. M. Planche, Negas and A. R. Siedle, “Mixed Oxides for Seventh Intersociety Energy Conversion Fuel Cell Electrodes”, NTIS PB 248 744 Engineering Conference, San Diego, Cali- 10 Conference Proceedings : Fuel Cell Catalysis fornia, 1972 Workshop, Palo Alto, 1975, EPRI SR-13 z National Benefits Associated with Commercial Special Report, August 1975 Application of Fuel Cell PowerPlants, ERDA I I H. R. Kunz, Electrochemical Society Spring 76-54, February 1976 Meeting, Philadelphia, May 1977 3 W. R. Grove, Phil. Mag., 1839, 17, 127 12 M. R. Andrew, J. S. Drury, B. D. McNicol, 4 J. P. Hoare, “The of C. Pinnington and R. T. Short, 3. Awl. Oxygen”, Interscience Publ., New York, 1968 Electrochem., 1976, 6, (2), 99 5 Advanced Technology Fuel Cell Programme, 13 M. Watanabe and S. Motoo. 3. Electroanal. EPRI-EM-335. Project 114-1 Final Report, Chem. Znterfac. Electrochem., 1975~60,(3), 267 October 1976, (Electric Power Research Insti- 14 M. M. P. Janssen and J. Moolhuysen, Elec- tute, Palo Alto, California) trochim. Actn, 1976, 21, (11), 869 6 J. F. McElroy, National Fuel Cell Seminar, 15 S. Abens, P. Marchetti and M. Lambrech, Boston, U.S.A., 1977 National Fuel Cell Seminar, Boston, U.S.A., 7 P. N. Ross, K. Kinoshita, A. J. Scarpellino and 1977 p. Stonehm, 3- Electroanal. Chem. InWfac. 16 Assessment of Fuels for Power Generation by Electrochem., 1975, 63, (I), 97 Electric Utility Fuel Cells, EPRI 318-Final 8 H. Bohm,J. Power Sources, 1976-77, I, (z), Report, October 1975 I77 17 F. T. Bacon, New Scientist, 1959, August 27th, 9 U. Bertucci, M. Cohen, W. S. Horton, T. 6, (I45), 271

Platinum Metals Rev., 1978, 22, (2) 46