Hydrogen Storage And Fuel Cell Technology
To start with the explantion of hydrogen storage and fuel cell technology, lets first of all discuss, hydrogen energy storage (emphasising the latest technological advances) after which an elaborate treatise on fuel cell technology is presented.Storing the gasIts importance cannot be overemphasised. It has higher energy density than petroleum and other common fuels and can be generated from water with little or no pollution affecting the environment. Hydrogen, along with oxygen, can be produced by the electrolysis of water, but this requires more energy than that released during the combustion of hydrogen. However, it is projected that the decreasing cost of solar cells will make it economical to use solar-generated electricity to produce hydrogen by electrolysis. Seawater could provide an almost inexhaustible source of water.Hydrogen can be stored in a number of ways, including compressed gas, chemical compounds, liquid hydrogen or metallic hydrogen. Due to excessive liquefaction costs, one of the most attractive methods for bulk storage of hydrogen produced from substantial non-oil based primary energy sources is compressed gas in underground caverns (a concept similar to the storage of natural gas). Current large-scale hydrogen storage is carried out in compressed hydrogen tanks, usually at pressures up to 70 bars. This is usually the case for chemical plants or the aerospace industry. Metal hydrides can be stored till about two per cent hydrogen by weight and, thus, can be bulky. Some carbon structures are under investigation for hydrogen storage but these have yet to be commercialised. There is a technology which shows promise for the future by involving the use of carbon nano fibers that have the ability to store 30-40 per cent of hydrogen by weight. High storage densities can also be obtained from chemical hydrides that typically consist of alkali or alkali earth metallic compounds.
Another approach,, involves storing hydrogen as a liquid or a solid. The process, called "Hydrogen on Demand", safely generates high-purity hydrogen from environment-friendly raw materials. The produced hydrogen can then be consumed in a fuel cell, or hydrogen-burning engine, to produce power.
Another approach to hydrogen storage as a solid involves the use of recycled plastics (known as power balls). Power balls are small solid balls or pellets of sodium hydride that is coated with a waterproof plastic coating or skin. They are stored directly in water and can remain in water for months with little or no change to the coatings. As soon as the plastic coating is fractured (through some external means), the sodium hydride inside reacts vigorously with water to produce hydrogen.
How it works
The upcoming application for hydrogen energy storage appears to be in the development of fuel cells. The electrochemical conversion of hydrogen and oxygen to produce electrical energy has little pollution associated with it and, thus, can be considered for applications where pollution levels (especially from burning hydrogen at high temperatures and producing NO) needs to be reduced. Fuel cells are lightweight, are well suited for producing electricity and have been used for years in spacecraft. They depend on an oxidation-reduction reaction which converts chemical energy directly into electrical energy. The oxidation of hydrogen by oxygen is the basis of the fuel cells used in space.
The fuel cell was first invented in 1839 by Sir William Grove, a professor of experimental philosophy at the Royal Institute in London. His experiments turned out to be the precursor to the phosphoric acid fuel cell by enclosing platinum in tubes of hydrogen and oxygen gas while submerging the tubes in sulphuric acid. Unfortunately, he was hampered by the inconsistency of cell performance (a common feature of cells today), but realised the importance of the three-phase contact (gas, electrolyte and platinum) to energy generation. He spent most of his time searching for an electrolyte that would produce a more constant current. He found several electrolytes which produced current, but struggled with consistent results. He also noted the potential of the energy production method commercially, that is, whether or not hydrogen could replace coal and wood as energy sources.
Since that time, researchers around the world have attempted to increase cell performance electrically, chemically as well as physically. Their experiments ranged from an improved three-phase contact to smart materials and the adoption of off gases from other power sources. After over 150 years of research, fuel cells can be divided into five major categories named after the electrolyte used in each: polymer electrolyte (including the direct methanol type), alkaline, phosphoric acid, molten carbonate and solid oxide. The five types resulted from the knowledge that heat accelerates chemical reaction rates and, thus, the electrical current. The types of fuel cells are described below:
1. Polymer Electrolyte Membrane (PEM) fuel cells are also called proton exchange membrane fuel cells which deliver high-power density and offer the advantages of low weight and volume as compared to other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen and oxygen from the air and water to operate and do not contain corrosive fluids as do some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks. PEM is manufactured by DuPont under the trade name of Nafion. The PEM fuel cell uses platinum as the electrode material. Platinum is a unique material because it is sufficiently reactive in binding H and O intermediates as required to facilitate the electrode processes and is also capable of effectively releasing the intermediate to form the final product. Despite this advantage, platinum costs three times the price of gold, which makes it very expensive. Most PEM development is focused on lowering the amount of platinum necessary for each cell.
PEM fuel cells operate at relatively low temperatures, approximately 80°C. The low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. The platinum catalyst is also extremely sensitive to carbon monoxide poisoning, making it necessary to employ an additional reactor to reduce carbon monoxide in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds to its cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to carbon monoxide.
PEM fuel cells are used primarily for transportation applications and some stationary applications. Due to their fast startup time, low sensitivity to orientation and favourable power-to-weight ratio, PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.
2. Phosphoric Acid Fuel Cells (PAFC) are used for liquid phosphoric acid as electrolytes in which the acid is contained in a teflon-bonded silicon carbide matrix and porous carbon electrodes containing a platinum catalyst. PAFC is considered to be the first generation of modern fuel cells. It is one of the most thoroughly developed cell types and is the first to be used commercially. This type of fuel cell is typically used for stationary power generation but some PAFCs have been used to power the large vehicles such as city buses.
They are more tolerant of impurities in reformed hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide. They are 85 per cent efficient when used for the co-generation of electricity and heat, but less efficient at generating electricity (37-42 per cent). This is only slightly more efficient than combustion-based power plants, which typically operates at 33-35 per cent efficiency. They are also less powerful than other fuel cells, given the same weight and volume. As a result, these are typically large and heavy. Like PEM, PAFC require an expensive platinum catalyst, which raises the cost of the fuel cell.
3. Direct Methanol Fuel Cells (DMFC) are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode. Despite numerous advantages, it has some technological problems. The amount of platinum necessary to achieve a high current is much greater than the amount used in the PEM fuel cell, making the DMFC very expensive. Methanol can permeate through the membrane material and cross from the anode to the cathode, decreasing the performance of the air cathode and wasting fuel.
Methanol is also easier to transport and supply to the public because it is a liquid, in comparison to gasoline or hydrogen. This advantage in terms of simplicity and cost means that DMFC presents an attractive alternative to hydrogen. It is relatively a new technology compared to that of fuel cells powered by pure hydrogen and research and development are roughly three to four years behind other fuel cell types.
4. The Alkaline Fuel Cell (AFC) was one of the first technologies ever developed and the first type widely used in the US space programme to produce electrical energy and water on board spacecrafts. AFC uses a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C; however, more recently AFC designs operate at lower temperatures of roughly 23°C to 70°C.
AFCs are high-performance fuel cells because of the rate at which chemical reactions take place. They are also very efficient in reaching efficiently 60 per cent in space applications. The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide. In fact, even the smallest amount of carbon dioxide in air can affect the cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. But the purification process is costly. Susceptibility to poisoning also affects the cell's life and it further adds to the cost.
However, cost is less of a factor for remote locations such as space or underwater. To compete effectively in most commercial markets, these fuel cells will have to become more cost-effective. AFC stacks have been shown to maintain sufficiently stable operations for more than 8,000 operating hours. To be economically viable in large-scale utility applications, they need to reach the operating times exceeding up to 40,000 hours. This is possibly the most significant obstacle in commercialising this technology at present.
5. Molten Carbonate Fuel Cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial and military applications. MCFCs are high-temperature and use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since, they operate at extremely high temperatures of 650°C and above; non-precious metals can be used as catalysts at the anode and cathode, reducing costs.Improved efficiency is another reason for which MCFC offers significant cost reduction over PAFC. The plants can reach efficiencies approaching 60 per cent, considerably higher than the 37 per cent to about 42 per cent efficiencies of a PAFC plant. When the waste heat is captured and used, the overall fuel efficiencies can be as high as 85 per cent. MCFCs are not prone to carbon monoxide or carbon dioxide "poisoning" but they can even use carbon oxides as fuel by making them more attractive for fueling them with coal-made gasses. Although, they are more resistant to impurities than other fuel cell types, scientists are looking for ways to make MCFC resistant to impurities from coal, such as sulphur and particulates. The primary disadvantage of the current MCFC technology is its durability.
6. Solid Oxide Fuel Cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration that is typical of other fuel cell types. SOFCs are expected to be approximately 50 per cent to 60 per cent efficient at converting fuel to electricity. In applications designed to capture and use the system's waste heat (co-generation), overall fuel use efficiencies can reach 80 per cent to 85 per cent.
They operate at very high temperatures, approximately 1000°C. High-temperature operation removes the need for precious metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. SOFCs are also the most sulphur-resistant type; they can tolerate several orders of magnitude more sulphur than other cell types. In addition, they are not poisoned by carbon monoxide which can even be used as fuel. This allows SOFCs to use coal-derived gases.
The SOFC high-temperature operation has disadvantages. It results in a slow start-up and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for the utility applications but not for the transportation and small portable applications. The high-operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology. Scientists are currently exploring the potential for developing lower temperature SOFCs operating at or below 800°C that have fewer durability problems and cost less. Lower temperature SOFCs produce less electrical power; however, stack materials which will function in lower temperature range have not been identified.
Plenty of researches and developments are going on in the field of hydrogen energy storage and its application in fuel cells throughout the world. Although many commercial tests have been conducted successfully with fuel cells in North America and other parts of the developed world, but technical and economical constraints still remain to be addressed in order for this technology to render the internal combustion engine a relic from the past.
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