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Skip to content. Search for books, journals or webpages All Pages Books Journals. View on ScienceDirect. Hardcover ISBN: Imprint: Woodhead Publishing. Published Date: 5th December Page Count: Practical Electronics for Inventors Electronics.
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Electronics for Dummies 3rd Edition. Cradle to Cradle Patterns of Life. Petroleum Law in Australia. Cyclops Dirk Pitt Series : Book 8. Energy A Human History. Valve Radio and Audio Repair Handbook. Control Systems Engineering. View Wishlist. If the fuel cell is to be operated on pure hydrogen then this is normally stored either within a metal hydride or light weight compressed hydrogen cylinder. Other fuels under consideration include bio-fuels such as ethanol, synthetic hydrocarbon fuels such as methanol and non-hydrocarbon fuels such as ammonia Brown, ; Choudhary et al.
For fuel cells operated on the non-hydrogen fuels, with the exception of direct methanol fuel cells, a fuel processor is required to convert the fuel into either pure H 2 or a mixture of H 2 and CO with the later only being suitable for use in HT fuel cell systems. The use of a fuel processor can often greatly increase the complexity of the device but simplifies the storage of the fuel, particularly in the case of liquid fuels which can often have exceptionally high energy densities and low cost in comparison to either batteries or gaseous hydrogen storage solutions. However, due to the stringent requirements relating to the purity of hydrogen, the cost of the fuel processor can often significantly increase the overall cost of the device with the fuel processer potentially being greater than the cost of the fuel cells stack itself.
Similarly, any additional weight from the processor can be offset by the far higher energy density of the fuel storage solution. These small and portable fuel cell systems are being developed for a range of end-user applications including stationary backup generators, battery charging, remote area power, auxiliary power units, soldier packs, portable electronic appliances, and small transport applications. There are an increasing number of these devices now commercially available, however, lack of fuel infrastructure and high cost when compared to battery or battery generator combinations remain key challenges that need to be overcome for this market to expand further.
Future fuel cell designs should be able to operate directly on a greater variety of commonly available fuels without the requirement for significant amounts of fuel pre-processing. This should lead to far greater efficiencies and hence lower operating costs of fuel cell power systems when compared to conventional power generating technologies which are likely to remain lower cost in terms of capital investment in the medium to long term. The Alkali-Metal Thermo-electrochemical Converter AMTEC is an electrochemical device which utilizes heat from a solar or a nuclear source or from combustion of fossil fuels to generate electricity and is an excellent technology for conversion of heat to electricity Weber, ; Cole, ; Ryan, ; Lodhi and Daloglu, ; El-Genk and Tournier, ; Wu et al.
Some applications of AMTEC devices include dispersed small scale power generation, remote power supplies, aerospace power systems, and vehicle propulsion. The liquid metal is supplied to one side of the solid electrolyte. The sodium vapors are condensed and cycled back to the anode side for revaporization and the cycle is repeated.
There are no moving parts within the cell and therefore the device has low maintenance requirements. The AMTECs are modular in construction and in many respects have common features with batteries and fuel cells.
The technology has been under development since late s with initial effort going into liquid sodium anode based devices. However, due to low cell voltage and power density, more recent effort has been directed toward vapor phase anode or vapor fed liquid anode systems with significant advances made in the development and manufacturing with performance of multi tube modules demonstrated for several thousand hours of operation Wu et al.
Despite the simple operating principle of the AMTEC device and demonstration of the technology at multi kW level, the technology is quite complex with several severe issues still contributing to the cost, system efficiency, and lifetime. These include: stability of electrodes, electrolyte, and other materials of construction during operation leading to cell power degradation with time; sodium fluid flow management including heat removal during condensation on the cathode side to heat input on the anode side; power controls; system design; and low cost technology up-scaling.
The electrode materials play a critical role for charge exchange at the electrode—electrolyte interface and contribute significantly to cell performance efficiency and degradation.
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A number of different materials ranging from metals to ceramics or composites of metals and ceramics have been tried with varying degrees of success Wu et al. The electrolyte material is also prone to changes in electrical, chemical, and thermo-mechanical properties with extended operation leading to degradation with time.
Thus, although the technology offers many advantages for an extensive range of applications, further improvements to lifetime, reliability, power density, and efficiency are required. The implementation of energy storage for applications including transportation and grid storage has strong commercial prospects. A number of market and technical studies anticipate a growth in global energy storage Yang et al. The main forecasted growth of energy storage technologies is primarily due to the reduction in the cost of renewable energy generation and issues with grid stability, load leveling, and the high cost of supplying peak load.
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Additionally, the demand for energy storage technologies such as rechargeable batteries for transportation has also added to the forecasted growth. A number of battery technologies have been commercialized and additionally a large number are still under development. The development of nearly all electrically powered devices has closely followed that of the batteries that power them.
By way of example, the size and form of today's mobile phones is largely determined by the dimensions of the lithium-ion cells that have the required capacity. Electric vehicles for passenger transportation are an obvious exception. Here, the batteries and electric drive are replacing systems based on liquid-fuel fed combustion engines that provide levels of performance acceleration, distance between refueling, etc.
There is general reluctance by vehicle owners to embrace electric cars offering considerably less all-round performance. This is the main factor that drives researchers to look well-beyond current lithium-ion technology to a range of new metal-air batteries. By virtue of removing much of the mass of the positive electrode, metal-air batteries offer the best prospects for achieving specific energy that is comparable with petroleum fuels.
In its simplest form, the lithium-air cell brings together a reversible lithium metal electrode and an oxygen electrode at which a stable oxide species is formed. There are two variants of rechargeable Li-air technology—a non-aqueous and an aqueous form, both of which offer at least ten times the energy-storing capability of the present lithium-ion batteries Girishkumar et al. In both, the cathode is a porous conductive carbon which acts as the substrate for the reduction of oxygen, while the anode is metallic lithium. For the non-aqueous system, the reduction of oxygen ends with formation of peroxide, so that the overall reaction follows Equation 1.
A cell based on this reaction has an open circuit voltage of 2. During discharging, the cell draws in oxygen and thereby gains mass, while it loses mass during charging, so that specific energy reaches a maximum when fully charged. In the aqueous form of lithium-air battery, water is involved in the reduction of oxygen, while the lithium electrode must be protected from reaction with water, usually by means of a lithium-ion-conducting solid electrolyte such as LISICON. Typically the electrolyte solution is a saturated solution of LiCl and LiOH and the favored reduction product is a hydrated lithium hydroxide, according to Equation 2.
While this is still an impressive level of performance, the main problem with the aqueous form of lithium-air is the difficulty of maintaining separation of lithium metal from the aqueous medium. In addition they contribute significantly to cell impedance—reducing the thickness of this protective layer ameliorates this effect but is limited by the poor mechanical strength of very thin layers.
For these reasons, most research effort in lithium-air batteries is focusing on the non-aqueous form. Clearly a key aspect to the realization of the very high specific energy of lithium-air battery is that the lithium metal anode can be made to operate safely and at full utilization. Many early studies used the organic carbonate electrolytes from lithium-ion battery technology, until it was eventually discovered that these compounds ethylene carbonate, propylene carbonate, etc. Solvents with ether functionality have since taken precedence given that they are more stable during charging and also less likely to promote the growth of dendritic morphologies at the lithium electrode Abraham and Jiang, Nevertheless, both carbonates and ethers are flammable which ultimately makes these devices hazardous under conditions where they become hot.
It is not surprising therefore that interest has turned to the use of ionic liquids, which are essentially non-volatile and able to dissolve appreciable concentrations of most lithium salts. In addition, lithium electrodes operate with a high degree of reversibility in a range of low viscosity ionic liquid media, without the formation of dendrites, due to the formation of a durable solid electrolyte interphase SEI on lithium Howlett et al.
The positive electrode of a lithium-air cell represents a complex challenge in that it must provide for: i access to oxygen; ii wetting by the electrolyte; and iii displacement by reaction products. The properties of the main product of discharge, lithium peroxide, Li 2 O 2 , also pose a number of problems with regard to cell longevity. First, it is an insulating solid, which means that conditions must be adjusted to prevent the formation of massive deposits during discharging. Second, lithium peroxide is a strong oxidant that tends to react with electrolyte components, including any adventitious water, to form irreversibly a variety of materials that severely degrade the lifetime of a Li-air cell.
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In the last few years, researchers have been able to extract something close to the high levels of performance that the lithium-air system offers, but only for brief periods before rapid capacity loss occurs. The reversibility of oxygen reduction is still the key issue Mo et al. Accordingly, there is still considerable investigation required into the exact mechanism of oxygen reduction, and the oxidation of a range of oxide species, with the aim of greatly improving the energetics of these processes.
The reversible sodium electrode is well-known in the history of battery development as it is featured in some of the very earliest examples of high performance secondary batteries. Both the sodium-sulfur and the Zebra sodium-nickel chloride systems employ molten sodium electrodes which give reversible behavior at values of potential that are sufficiently negative for useful device voltages Ellis and Nazar, Recently, the sodium electrode has again become the focus of attention, now coupled with an oxygen electrode in the sodium-air cell.
These numbers are derived from the overall cell reaction shown in Equation 3. The identification of the superoxide as the main product of reduction has been verified experimentally Hartmann et al. Many of the limitations on performance of the air cathode in Li-O 2 cells also define the behavior of this electrode in Na-O 2 cells. The use of carbonate and ether electrolyte solutions has been hampered by problems of insufficient stability during charging Hartmann et al. While the preferential formation of sodium superoxide during discharging clearly lowers the overpotential associated with charging, it is not clear whether this compound will be stable on the longer timescale of a typical device service life, or whether the discharge product will gradually be converted to the more stable, and less easily recharged, sodium peroxide.
While the molten sodium electrode offers many advantages in terms of electrochemical characteristics, reality for rechargeable energy storage devices demands that maximum performance is delivered at ambient temperature. What is known of the behavior of solid sodium electrodes in conventional battery electrolytes suggests that it readily generates dendritic morphologies thereby posing a significant risk to further development of this battery technology. By analogy with lithium electrochemistry, it seems likely that more attention will be given to examining the behavior of sodium in ionic liquid electrolytes, in an attempt to replicate the benefits of generating a protective SEI in a medium that is inherently safer with respect to volatility and reactivity.
Although it is very early in the development cycle for sodium-air batteries, there are sound reasons for pursuing further progress.