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Materials for electrochemical power systemsA briefing document prepared for the Royal Society and Association of British Science WritersFabian Acker
May 1996 Materials for electrochemical power systemsSummary The development of new materials and ways of processing and structuring them for improving electrochemical energy storage systems were discussed at a recent Royal Society discussion meeting; these in turn are necessary components in optimising energy use. However, delegates differed about the relative values of traditional lead-acid systems, configured and engineered to yield maximum storage efficiency, and systems based on newer materials such as lithium, and fuel cells. While everyone accepted that electrochemical energy storage could make for better use of existing and new renewable and non-renewable energy sources, some expressed concern that the advantages of more efficient energy usage might be negated by the pollution caused in the manufacture and disposal of some chemicals used in the storage processes. Another related aspect covered was materials used in high speed energy recovery for military applications, and supercapacitors for both civilian and military purposes. Neither of these systems is involved in bulk energy storage, such as envisaged for the better use of existing energy sources, but have quite different functions, and consequently have developed in different ways. Nevertheless, many aspects and materials are common to all chemical energy storage systems whatever the proposed end use.
Materials for electrochemical power systems
The dominance of the lead-acid battery was underlined by Dr R M Dell of AEA Technology who pointed out that in a wider context, liquid electrolyte batteries, irrespective of the materials, also dominate the market. In either context, he noted, the primary battery (ie non-rechargeable) is easier to design than the secondary battery (rechargeable) which is often required to have a life of more than 1000 charge/discharge cycles with little loss of capacity or power output. The secondary battery, having such onerous demands made upon it, is often designed for a specific use to optimise its performance, and within the different electrode/electrolyte combinations there are further subdivisions according to the proposed end use. Extensive development of the lead-acid battery has taken place mainly in non-active materials (such as the case and the separators) and design (such as automatic watering) to improve efficiency and reduce the power/weight ratio. But few changes have been made to the basic chemicals involved; only to the way the electrodes are manufactured, formed and assembled. On the primary side, the traditional Leclanché cell (zinc/manganese dioxide), first proposed in 1886, is still widely used despite some drawbacks in its storage life and low temperature performance. The zinc chloride primary operates better at low temperatures and also has a higher discharge rate, but these characteristics are offset by the higher manufacturing costs. The alkaline manganese cell, although marketed as a primary battery, can be recharged 20 or 30 times under certain critical conditions. It performs better than zinc/carbon at low temperatures and offers a higher current drain. Compared with nickel-cadmium (NiCad) it is cheaper and has twice the initial capacity, although NiCad batteries have a recharge cycle of more than 200. It does not contain toxic materials, and therefore does not create a disposal problem. Magnesium or aluminium can be substituted for zinc in primary cells, giving higher specific capacities, but there are problems resulting from the formation of oxide films on open circuits. The use of mercury oxide or silver oxide as cathodes in primary cells leads to high volumetric capacities, but disposal presents problems for the mercury, while the use of silver puts the cost up. Dr D A J Rand of the Institute of Materials in Victoria, Australia, discussed the value of electrochemical storage for helping electricity generating systems cope with high demand, by supplying some of the peak loads. At times of low demand they can remain quiescent or recharge themselves from the main system. He argued that the lead-acid battery was still the best form for fulfilling these strategies and therefore to reach some of these goals, he said, the lead-acid battery had to be developed still further, despite the many improvements already achieved. Compared with its competitors, lead-acid had a low specific energy, partly because of the weight of lead, and partly because of low utilization of the active material; in electric vehicles less than 40% of the active material could be accessed. The target set by the Advanced Lead-Acid Battery Consortium of 50 Wh/kg had not yet been met. Reducing the inert components, could reduce the battery weight, as these accounted for 30% of the total. Reconfiguration of current pathways, some of which are now being implemented would also help. Better utilization of the active material involved both chemical and manufacturing strategies. But there was a conflict between maximising a battery's energy/power and its cycle life, and for electric vehicles, the dual battery concept was now being evaluated, in which the main unit would be optimised for range (that is specific energy) and the subsidiary one for acceleration and hill climbing (specific power). Continuing research was needed, he concluded, to improve not only its efficiency, weight : power ratio and capacity, but also to develop faster charging techniques, particularly for electric vehicle applications. All of these areas, he said will "engage the electrochemist and materials scientist alike". Dr C Delmas from the Institut de la chemie de la matiére condensée de Bordeaux, France, discussed the nickel hydroxide electrode from the solid chemistry point of view. Dual storage systems were also considered by Professor R A Huggins as a way of optimising a system so as to handle high transient power demands in addition to storing as much energy as possible. The voltage of electro-chemical systems that can store large amounts of energy generally drops rapidly as demand increases. On the other hand, systems that can provide high power typically have relatively low energy storage capability. Since demands for high power are generally of short duration, the two systems can be combined to cater for short periods of high demand while still storing appreciable amounts of energy, the voltage of the high energy system rising when power demand falls, thus recharging the high power component. He put forward the concept of "energy quality", drawing an analogy with "heat quality" in which high-temperature heat is more useful than low-temperature heat. In the same way, low-voltage energy is not generally useful. Professor C A Vincent of the University of St Andrews surveyed the battery chemistry, materials, and design of lithium based primary and reserve cells, and of lithium or sodium based rechargeable batteries. The burgeoning microelectronics market, he said, had driven up the demand for small portable primary and secondary cells, while the perceived need for more efficient electric traction, as well as stationary requirements and military applications had generated corresponding demand for larger high energy and high power density batteries. Alkali systems, particularly those based on lithium or sodium were the most likely materials for these diverse applications. Lithium, for instance, has a higher capacity for storing energy than zinc or lead. But the chemical reactivity of these newer materials preclude the use of aqueous electrolytes. Lithium primary cell applications range from primary cell pacemakers, which deliver low power pulses for 10 years and typically weigh 22g, to reserve batteries which may have to operate after 25 years inactivity. More than 350 000 pacemakers based on lithium are emplaced every year, and, in aerospace, thermal reserve batteries generate a market of £50 million a year. Batteries in this latter category are designed either to give a high power discharge, typically lasting less than 30 seconds, or a long discharge of about one hour's duration. Professor Vincent went on to outline some of the problems of secondary batteries, categorising them as "ambient", that is within the range of 60-80 ºC and "high temperature", that is operating in the range of 250-500 ºC. He pointed out that while the active components of high temperature cells were relatively inexpensive, the separators, seals and current collectors are made from highly engineered specialised materials, and are expensive because of the highly corrosive properties and high temperatures of the active materials. Sodium-sulphur batteries and sodium-nickel chloride and related batteries, operating at the lower end of the high temperature range, are being tested for electric vehicles and the Zebra battery, using nickel chloride is now at the pilot production stage. He concluded by noting that while non-liquid primary cells were well developed with substantial markets, the position of secondary cells was less successful. Their chemical reactions were well understood, but much further work was needed if they were to have a wider application. This problem was explored in greater detail by Professor P G Bruce of the University of St Andrews who noted that the commercial success of the world's first rechargeable lithium battery, recently introduced by Sony and which has an energy density three times higher than conventional batteries, was just the first step in a new electrochemical technology. Rechargeable lithium cells are of two basic kinds. One is designed to strip off and replate lithium metal for the charging/discharge cycle. The second, more recent version, is based on what is sometimes known as the "rocking chair" process, where charging/discharging only involves an exchange of lithium ions between two inert electrodes. This second version is a safer design than the first because the lithium is inserted not plated, avoiding problems such as "whisker" formation associated with plating processes. In addition, the rocking chair cells are manufactured in the discharged state, and can be transported to the point of use and stored with little difficulty. These newer cells are successfully meeting the requirements of the consumer electronics market, with applications in lap-top PCs, portable telephones, camcorders and so on. But there are drawbacks when the technology is scaled up for applications such as electric traction for a number of reasons such as its lower cell voltage. A strong argument for the merits of high temperature batteries was put forward by Mr J L Sudworth of Beta Research and Development Limited, Derby. If the system was one which needed a stored energy of at least 10 kWh, then such batteries have substantial advantages. They have high specific energies and their performance is independent of ambient temperatures. But for use in vehicles, hot batteries need to be kept at their operating temperature to avoid bringing them to the correct temperature every time before starting the vehicle, and for this reason they have to be well insulated, and/or subjected to continuous use. Sudworth went on to describe some of the research and development that had been carried out on certain high temperature cells, which although not resulting in commercially viable products, led to the development of more potentially useful cells. The solid electrolyte sodium-sulphur cell was amongst these, and its successful operation depends essentially on two variants of aluminium. Of the five known fuel cell technologies, only one is beginning to compete in the market place, according to Professor H Wendt of the Institut fü}r Chemische Technologie in Darmstadt, Germany, and that is the phosphoric acid fuel cell (PAFC). This operates at about 200 ºC, and with the alkaline fuel cell (AFC) and the proton exchange membrane fuel cell (PEMFC), is considered to fall into the low temperature categories. In the high temperature category, two cells make up the total; one (MCFC) is based on molten carbonates, and the other, a solid oxide fuel cell (SOFC) is based on an oxygen anion conducting yttria stabilized zirconia as the electrolyte. In all acid fuel cells, the splitting of the hydrogen molecule into positively charged hydrogen, liberates two electrons: this reaction is catalysed by platinum, and the transition metals, iron, cobalt and nickel, although iron reacts in H2/H2O mixtures, and is therefore not suitable. The ageing and deterioration of electrocatalysts is no different to that of chemical catalysts, although it is important to distinguish between deterioration by poisoning and that caused by slowly changing morphology, the latter being a true ageing effect.
Dr A G Ritchie of the Defence Research Agency considered specialized forms of batteries, quite different in application from that of the previous speakers, in that the devices had to remain inactive for long periods, possibly measured in years, and then become active for very short periods, measured in seconds or minutes. The essential characteristic of both, is that some action, such as adding, or exposing a chemical component, to the cell elements, activates the discharge process. A common form is the zinc-air cell for hearing aids. The hole which admits the air to the battery is sealed until the point of use. Lead-acid batteries can also be stored inactively until the electrolyte is added. In explosive shells, the electrolyte is forced out of a glass ampoule and into the cell by the rifling motion as the cell moves out of the gun barrel. In anti-tank or anti-personnel mines, the electrolyte may be contained in bellows, which on activation, is compressed either by gas pressure or a spring. Another type is the high temperature thermal battery, which is inactive when cold, but can be brought to its operating temperature by activating a pyrotechnic. This in turn requires triggering energy, which can be provided by mechanical shock or electrical impulse. The chemistry is similar to the kinds of batteries discussed earlier, and lithium too is beginning to replace the traditional materials in many applications. The system in which the electrolyte is added to activate the system gives a higher performance than the usual materials, but lithium/thionyl or sulphuryl chloride systems are not as successful, because of the high temperature of the reaction. Where many such cells are used in a confined space, the emitted heat may cause problems. High temperature cells suffer from a weight disadvantage because of their stainless steel cases. This could be replaced by titanium which, of course, is more costly.
Professor A Hamnett of the University of Newcastle upon Tyne expressed doubts about how far fuel cells would penetrate the energy generation market. The inherent difficulty was the nearly universal use of hydrogen as the feed fuel. This has to be generated by reformation of the primary cell, which is costly and complicated. However, a major fillip to the practicability of developing fuel cells, he noted, had come with the discovery of substantial natural gas fields, as methane can be reformed with high efficiency. Also, its innate purity reduces the problems of poisoning. But methanol as a fuel was even more attractive, as it is more reactive than methane and can be reformed at about 300 ºC and is sufficiently electroactive for direct oxidation at the anode at temperatures above 100 ºC. Dr G J K Acres and Dr G A Hards of Johnson Matthey Technology Centre, considered the use and development of catalysts for fuel cells of which there were six types: alkaline (operating temperature 50-90 ºC); proton exchange mechanism (50-125 ºC); phosphoric acid (190-210 ºC); molten carbonate (630-650 ºC); solid oxide (900-1000 ºC); biological (ambient); and direct methanol (50-120 ºC). They noted that there was a strong potential for catalyst loading reduction and improved performance in small 5-50 cm2 cells, and emphasised the importance of the electrode backing structure in the efficient mass transport performance in the cathode. Platinum alloys, the paper concluded, showed an intrinsic kinetic benefit of at least 20 mV in solid polymer cells. The role of platinum in polymer cells was discussed in detail by Professor A J Appleby of Texas A&M University System, USA, who said that the high cost of its component materials originally inhibited mass production of these forms of cells. However, platinum loadings had been reduced by a factor of 100 since 1986 and by a further factor of 10 since 1991, with no deterioration in cell performance. But he warned that the costs of the other materials would also have to be reduced to bring this cell into wide commercial application. He detailed legislative measures now started in the USA that are driving the development of electrically powered vehicles. Hybrid systems could meet some of the legislation (which will become progressively stringent), and a fuel cell could provide an ideal component of such a system. But of the five practical classes of fuel cells, the only one suited to a small vehicle was that based on the proton exchange membrane, also known as the polymer electrolyte membrane (PEM). Humidification and rapid water removal are essential activities in the operation of fuel cells, he noted, and development at Ballard had demonstrated practical techniques for both. Self-humidified cells can operate close to the practical upper temperature limit of 90 ºC using a recycle loop with a condenser. Development work on solid oxide (SO) fuel cells could make them promising candidates for combined heat and power applications and for electric cars, according to Professor B C H Steele of Imperial College, London. The high temperatures (1000 ºC) at which they operate make them more efficient than their low temperature equivalents, which need to be supplied with hydrogen. Most research and development in this field is directed to state-of-the-art material such as nickel-alloy anodes, nickel oxide cathodes, and alkali-carbonate (with alkaline-earth additions) as electrolytes, and common alloys as structural materials. Demonstration plants indicate that the operating life of these cells seems to be no more than 10 000 h, with a degradation of 3 mV/1000 h. While the high operating temperature may be advantageous for combined heat and power systems and certain manufacturing processes, he said, the high cost of balance-of-plant components has limited the market. There was an urgent need, he concluded, to develop lower temperature reactions and investigate more economical processing routes for supported electrolyte designs.
Contacts
AEA Technology 429 Harwell Didcot tel: 01235 434931 Oxfordshire OX11 0RA fax: 01235 432278
Institute of Materials, Energy and Construction CSIRO 339 Williamstown Road PO Box 124 Port Melbourne Victoria 3207 tel: 00 61 3647 0211 AUSTRALIA fax: 00 61 3646 3223
Institute de chimie de la matiéere condensée Chateau Brivazac Avenue du Docteur A Schweitzer 33600 Pessac tel: 00 33 5684 6296 FRANCE fax: 00 33 5684 6634
Technical Faculty Christian-Albrechts-Universitat zu Kiel Kaiserstrasse 2 24143 Kiel tel: 00 49 775 72304 GERMANY fax: 00 49 775 72379
School of Chemistry University of St Andrews Purdie Building North Haugh tel: 01334 463804 St Andrews KY16 9ST fax: 01334 463808
School of Chemistry University of St Andrews Purdie Building North Haugh tel: 01334 463825 St Andrews KY16 9ST fax: 01334 463808
Beta Research 50 Goodsmoor Road Sinfin tel: 01332 770500 Derby DE24 9GN fax: 01332 771591
Institute fü}r Chemische Technologie Technische Hochschule Petersenstrasse 20 64287 Darmstadt tel: 00 49 6151 162265 GERMANY fax: 00 49 6151 164298
Defence Research Agency Haslar Gosport tel: 01252 392787 Hampshire PO12 2AG fax: 01252 393947
Department of Chemistry University of Newcastle upon Tyne tel: 0191 222 7701 Newcastle upon Tyne NE1 7RU fax: 0191 222 6929
Johnson Matthey Technology Centre Blount's Court Sonning Common tel: 01734 242134 Reading RG4 9NH fax: 01734 242306
Centre for Electrochemical Systems and Hydrogen Research Texas A&M University System 238 Wisenbaker Engineering Research Centre College Station Texas 77843 3402 tel: 00 1 409 845 8281 UNITED STATES OF AMERICA fax: 00 1 409 845 9287
Department of Materials Imperial College of Science, Technology and Medicine South Kensington tel: 0171 589 5111 London SW7 2AZ fax: 0171 584 3194
Enquiries to: Ref: PR 24 (96)
Miss Anna Link Science Promotion Section The Royal Society 6 Carlton House Terrace London SW1Y 5AG
Direct line: 0171 451 2581 14 May 1996
INFORMATION NOTE Materials for electrochemical power systems On 4 and 5 October 1995, the Royal Society held a scientific meeting on Materials for electrochemical power systems. The enclosed document was prepared afterwards to summarize key issues raised by the speakers and to provide a list of helpful contacts for future reference. The document does not necessarily constitute the views of the Royal Society and views expressed in it should not be attributed to the Society. The document is free of copyright and may be used without reference to source. |
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