Abstract
Recently, attention has been diverted to clean, alternative and renewable technologies due to the depletion of petroleum resources and increase of the green house gases from the combustion of fuel resources. Fuel cell is the fore-runner of these technologies and has received immense impetus due to its relatively clean source of energy. Fuel-cells are devices that produce electrical energy from chemical transformation reactions. Electrodes are placed within an electrolyte medium that facilitates oxidation of fuel. In these electrochemical reactions, ions are generated that generate electricity. This electrical energy can be effectively harnessed to carry out a myriad of functions. Since there is no mixing of the fuel and oxidant, there is no combustion and therefore yield 100 % efficiency. Fuel cells are classified on the basis of the electrolyte material utilised in the process and the electrolyte material used is dependent on the use and the operating temperatures of the fuel cell. The different types and the materials used are discussed in the following article. It is commonly said that 19th Century was the century of the steam engine, while the 20th Century is the century of the internal combustion engine. However, the 21st Century shall most likely be called as the century of Fuel cells.
Introduction – What is Fuel cell?
A fuel cell is an electrical cell that works continuously, since the fuel is supplied in this cell, unlike a storage cell. The primary electrical energy generating molecule is hydrogen (H2). These fuel cells convert electrical potential of hydrogen through electrochemical reaction to generate electricity. The principal reaction in a fuel cell is the combination of hydrogen (H2) and oxygen (O2) to generate electrical energy, heat and water. In general, the reaction can be shown as:
2 H2 + O2 → 2 H2O + energy
In this reaction, hydrogen gas (H2) reacts with oxygen gas (O2) to produce water (H2O) and energy. This energy is in the form of electrical and heat energy that is tapped as a more efficient source as compared to heat engines. Since there is water produced at the end to this electrochemical process, it does not cause any emissions that can act as a pollutant. Fuel cells have other advantages like higher efficiencies and virtually silent operations. If the hydrogen used is obtained from renewable resources, then fuel cells would become the cleanest and most sustainable technology designed till date (Cook 2001).
Background, history and advantages of Fuel cells:
The concept of fuel cell develops from the thought that as the content of hydrogen in fuel goes on increasing, the efficiency of the fuel increases. It was observed that on the combustion of hydrocarbons from petroleum produced water and carbon dioxide. With the development of the fuel used engines, the percentage of hydrogen in the fuel also increased. Therefore, as the content of hydrogen increased in the fuel, the emissions consisted of increased content of water. Hence, a cell was proposed that can utilize 100 % hydrogen to produce electricity and thus be used as a clean source of energy.
The first fuel cell was designed by Sir William Grove, a British scientist, in 1939. It was called as the “Gas battery” and was assembled by disconnecting the battery of an electrolysis machine and connecting the two electrodes together. This ‘gas battery’ generated 1 volt and was made of platinum electrodes immersed in dilute sulphuric acid and hydrogen and oxygen gases trapped in test-tubes. In the further research these gas batteries were connected in series to generate enough voltage to power an electrolysis machine. This formed the basis of the fuel cell technology.
In 1959, a better version of fuel cells was developed by Francis Bacon, which consisted of alkaline electrolyte in place of dilute sulphuric acid and sintered nickel electrodes. It was called as the “Bacon fuel cell” that used molten potassium hydroxide (KOH) as the electrolyte, while sintered nickel electrode allowed higher diffusion of gases through the electrode. Since nickel was economical as compared to platinum and KOH being comparatively less corrosive, the Bacon fuel cell was considered to a best model for fuel cell (Larminie 2000).
The next big leap came from the utilization of fuel cells by NASA, as fuel cells would provide several times more energy as compared to conventional batteries per equivalent unit of weight. In 1960s, NASA used fuel cells in the Gemini space program, where purified hydrogen and oxygen gases were used. The International Fuel Cells were developed as a power plant in Apollo spacecraft, which was used as a source of electricity and water for the astronauts. It could supply 1.5kilowatts of continuous electricity and could operate for 10,000 hours. By 1970s, the International Fuel Cells developed powerful fuel cells with stronger alkaline fuel for NASA’s Space Shuttle Orbiter. There were 3 fuel cell power plants installed and together these fuel cells were capable of generating 12 kilowatts of electricity continuously. They were able to generate 16 kilowatts of electricity for short periods and thus fuel cells were only used in this mission without backup batteries, proving the reliability of fuel cells (Cook 2001).
In the energy crisis of 1970s, fuel cell provided the alternative of conventional combustion engines and therefore, since then the fuel cell research has been used successfully in wide variety of applications.
Fuel cells are compared to batteries and combustion engines in terms of their performance and efficiencies. The advantages of fuel cells are as follows:
- Fuels cells operate on pure hydrogen and therefore do not produce any pollutant. The only emissions are water and heat. However combustions engines burn fossil fuels that emits carbon dioxide and other green house gases, which are harmful to the environment.
- The thermodynamic efficiency of fuel cells is higher than heat engines. The heat generated in the engine by burning fuel is used in mechanical movement. However, fuel cells generate electrical energy that is used to do work.
- Fuels cells also exhibit higher part load efficiency, where fuel cells do not display a drastic drop in efficiency as the size of power plant decreases. This property is like that of batteries; however the replenishing of gases in fuel cells provides an advantage over batteries.
- Fuel cells have higher load following characteristics that makes fuel cell ideal for the generation of electrical energy and not mechanical energy.
- Fuel cells operate even at low temperature, as the warm-up time and temperature hazards are reduced in fuel cells.
- Fuel generation is used in co-generations processes, like generation of heat or water along with the electrical energy.
- There is very less tuning required for fuel cells.
- The fuel cells do not require charging, only needs to be re-fueled thus making the process faster as compared to charging batteries.
Applications of fuel cells
Stationary Power plants: Stationary power plants are one of the obvious applications of fuel cells. Currently coal and natural gas is used for generation of electricity that is non-renewable energy resource. The depletion of these non-renewable resources has led to the new variety of fuel cell technologies that can be utilized as power plants. Ballard Generations Systems has the largest 250 kilowatts electricity generation facility in a number of cities worldwide. These power plants are typically used for emergency back-up power for facilities that are critical like hospitals, schools, etc. These power plants shall act as the “decentralized” power plants, which further reduces the losses during distribution. Stationary fuel cell power plants produce large amounts of hot water as waste, which could be used directly in the surrounding community and thus increasing the effectiveness of these plants.
Submarines and space shuttles: The concept of these power plants can be extended further for its application in space shuttles and submarines. Application of fuel cell in military submarines is more as fuel cells have very low noise and infrared signatures. The water generated from the fuel cells can be used for domestic purposes on-board in submarines and space stations.
Transportation: Fuels cells have very low emissions and can meet requirements of any alterative technology. The prototype fuel cell automobile is developed by Daimler Chrysler with liquid methanol being converted into hydrogen and used. It has two times higher efficiency as compared to internal combustion engine. Automobiles with fuel cells as considered as zero emission vehicles (ZEVs), these ZEVs have virtually no pollutant emissions. Automobiles with fuel cells are known to generate more than 100 horsepower power outputs and speeds higher than 90 mph. Automobile manufacturing companies like Honda, General Motors, Toyota, Volkswagen, Nissan, etc have presented their models of fuel cell vehicles.
The best application of fuel cell in transportation is utilized by buses developed by XCELLSiS Fuel Cell Engines Inc., in Vancouver (Canada) and Chicago (USA). These buses uses pure hydrogen gas stored in cylinders and have on-board reformer systems. Similar designs have also been used in parts of Europe thus evidently exhibiting the on ground application and acceptance of fuel cells. Recently the demand for fuel cells in transportation has increased by 2.6 % in Canada, while by 2.4 % in USA (IEA 1997). The annual demand for fuel cells increases by 6 % in developing countries of Asia and Europe (Khatib 1998).
Non-grid applications (Portable Power Systems): There are many non-grid applications like stand alone, back-up power generators, portable power systems etc. These portable fuel cells can supply 1.2 kilowatts of power thus providing electrical energy whenever and wherever required. These fuel cell plants can be started in seconds and can supply clean and silent electrical supply for as long as the hydrogen can be replenished. Recently power systems are being developed with as high as 50 kilowatts of energy. These portable power systems have less moving parts, high durability and very low emissions. Therefore these systems are smaller, lighter and with methanol power fuel they are safe to use.
Residential Power: Manufacturers are now developing fuel cells that can be compacted and used at single-family homes. These modern-day homes shall deliver base-load power to the electricity in the house. With the co-generation advantage of fuel cells, the efficiency increases to as high as 85 %. These fuel cell generators are popular in the Japanese markets. Plug Power (New York, USA) have developed fuel cell power plant that can supply 7 kilowatts of electrical power to the home, which is enough to supply the electrical needs of a modern-day energy efficient home. These technologies are being developed further and in the future these shall be no electrical requirements form the grid and an energy efficient home shall be independent of the grid.
The fuels are used almost anywhere, where the electrical energy has to be generated to do work. The fuel cells are used as stationary power plants and in space stations, submarines, automobile engines (buses and cars), portable power systems, etc. there are many applications of fuel cells and is dependent of the engineer to utilize it successfully in any process that requires continuous electrical energy. Thus, with all these applications and advantages, the fuel cell research and technology has been growing unabatedly since the latter half of the 21st century.
Fuel cell technology – working and chemistry
Presently the most successful fuel cell technology is that of polymer electrolyte membrane (PEM) fuel cell. In PEM, the two half cell reactions take place concurrently in the cell. The oxidation reaction occurs at the anode where electrons are lost and the reduction reaction occurs at the cathodes where electrons are gained. This redox reaction (oxidation-reduction) completes the circuit to generate electrical energy with the formation of water using hydrogen and oxygen.
The cathode and anode separated by the electrolyte solution, as seen in an electrolyzer. The ions generated at the electrodes are transferred from onside to another in this electrolyte solution. However in the PEM, the electrolyte fuel is an acid supported within a solid membrane. The solid acid membrane is saturated with water for an uninhibited and easy passage of ions across the membrane. From figure 1, it can be observed that, as the electrical supply is attached across the electrode of the fuel cell, a redox reaction occurs across the electrodes. Around 0.5 – 0.8 volts of power is generated from the PEM fuel cells. This load is dependent on the load of fuel in PEM and the power can be increased by stacking these cells together to generate practical working voltages.
Figure 1: A single PEM fuel cell (Source: Cook 2001)
The redox reactions taking place in a PEM fuel cell is as follows:
Anode reaction: H2 (gas) → 2 H+ + 2e-
Cathode reaction: ½ O2 (gas) + 2 e- + 2 H+ → H2O (liq)
Overall reaction: H2 + ½ O2 → H2O (liq)
The hydrogen gas molecules interact with the platinum electrode (anode) surface to form hydrogen-platinum (H-Pt) bonds. These bonds are weak bonds and the hydrogen atom undergoes oxidation to release electron. This electron travels the external circuit to reach the cathode. If there is a free flow of these electrons across this external circuit, then it is called as an electrical current. Meanwhile, the hydrogen ion (proton) in the fuel cell combines with the water molecule to form a hydronium ion (H3O+). The hydronium ion travels across the membrane material to the cathode, thus leaving the platinum electrode site free to the next hydrogen molecule.
At the other electrode (cathode), platinum interacts with oxygen molecule to catalyze the formation of oxygen-platinum (O-Pt) bonds. These bonds break-down by taking up two electrons that have travelled from the external circuit. The oxygen atom undergoes reduction at the cathode. The reduced oxygen ions and hydrogen ion crossed across the membrane complete this redox reaction by combing these two atoms together to from water. The platinum catalysts are also free for the next oxygen molecule and carry out the next redox reaction.
This electrochemical reaction is an exothermic reaction which produces water and 286 kilojoules of energy per mole of water formed. This energy is in the form of electrical energy that is utilized to do work. The energy generated is a function of temperature and at 25 ºC and 1atmosphere pressure 237 kilojoules of energy is generated.
In PEM, the membrane used as a solid material is a polymer that is obtained as large sheets. The electrodes are applied as sheets on either side of the membrane, thus forming a sandwich of polymer between the electrodes. These PEM sheets are normally 50-175 microns in thickness. Nafion® is a commonly used material in PEM sheets, which is made up of poly-tetrafluoroethylene (PTFE). PTFE was developed by DuPont in 1970s and is called as Teflon®. Teflon forms the backbone of the PEM fuel cells. To this backbone, sulphonic acid (HSO3) side chains are attached. On observing through an electron microscope, the Teflon molecules are seen as long chains on which there is sulphonic acid side chains attached on the ends. The PTFE long chain is hydrophobic (repel water), while sulphonic acid is hydrophilic (attract water) in nature. This amphoteric nature allows the membrane to conduct ions with higher efficiencies and also absorb higher amounts of water. These water rich regions, allows an easy movement of hydrogen ion across the membrane.
Figure 2: Structure of PFTE in PEM fuel cell (Source: Cook 2001)
Efficiency and Voltage
Under the ideal conditions and a perfectly energy transferring system the PEM fuel cell can generate 1.23 volts at 25 ºC and 1 atmosphere pressure. The thermodynamic reversible cell potential of the PEM fuel cell drops as the temperature increases. However, it still manages to generate 1.18 volts of voltage at around 80 ºC. The voltage generated is dependent on many other factors and the electrical efficiency of PEM is a good measure of the voltage output. As the efficiency drops more and more chemical energy is utilized in generation of water and heat. The factors that reduce electrical efficiency are:
Activation losses: The energy required for the initiation of the electrochemical reaction dependent on the catalyst. If the energy required for the initiation is higher there shall be activation losses thus reducing the efficiency of the fuel cell. Therefore, catalysts that have very low activation energy are selected. Platinum is one of the best materials that can be used as the catalyst. However, platinum is expensive, which had led to researching of many new and better materials as the source of catalyst for PEM fuel cells. The reaction at the cathode is 100 times slower as compared to that of anode and thus a more and more research is conducted for identification of cathode materials with higher densities. Reduction of oxygen at the cathode is the rate limiting step in the study activation losses in fuel cells.
Fuel crossover and internal currents: The fuel (ions) must cross through the polymer membrane and if these ions are lost in the external circuit then the efficiency of the fuel cell decreases. There should be minimum loss of electrons and ions in the fuel cell. Therefore a lot of material sciences research is being conducted on identifying abundant and effective polymer membranes for the reduction of losses during fuel crossover in the internal circuits.
Ohmic losses: Ohmic losses are those losses where the components of the fuel cell themselves provide resistance to electrical flow and therefore decrease the efficiencies. The electrode material and interconnections causes resistance, which can be reduced by the using of high conducting materials.
Mass transfer: The concentration of the hydrogen and oxygen gas at the electrodes is very important for the functioning of fuel cells. If there is reduction in the concentration of gases at the cathode or anode, then the fresh molecules are not readily available at the reaction site thus reducing the efficiency of fuel cells. The accumulation of water at the cathode can reduce the transport of oxygen to the cathode reducing the current. This problem is solved by application of porous membranes for easy movement of water across the fuel cells.
Methanol fuel cells
A new development in PEM fuel cell is the use of methanol as a fuel. The electrical energy is generated using hydrogen. However the hydrogen is generated from breakdown of methanol. In these fuel cells, an anode other than platinum is used to generate free electrons and hydrogen ions. The electrons travel across the external circuit and the hydrogen travels across the membrane and react at the cathode. Hydrogen ions combine with the oxygen atoms at the cathode to generate water. Carbon dioxide is also generated in this reaction. The reactions occurring in a methanol fuel cell are as follow:
Anode reaction: CH3OH + H2O → CO2 + 6H+ + 6e-
Cathode reaction: 3/2 O2 + 6H+ + 6e- → 3H2O
Overall reaction: CH2OH + 3/2 O2 → CO2 + 2 H2O
Jet Propulsion Laboratory (USA) invented and developed direct methanol fuel cells which are developed to supply electricity to the Army. The primary advantage of methanol fuel cell over hydrogen fuel cell is that of the fuel source. Methanol is a liquid fuel source which stable and easy to store as compared to pressurized hydrogen gas cylinder. This increases the safety and usability of the fuel cells. Presently the methanol fuel cells have lower efficiencies compared to hydrogen fuel cells, but with new developments in material sciences the efficiencies of methanol fuel cells can be significantly increased. Companies like Ballard Power, Motorola and Manhattan Scientific are involved in the development of portable power systems made of methanol fuel cells. They also claim that the methanol fuel cells shall be commercially available in the next 5 years.
Classification of Fuel cells
The fuel cells are classified on the basis materials utilized in the cell and the material used for the fuel cell is dependent on the application and the operating temperature of the system. The different types of fuel cells are given below:
Hydrogen sources for the fuel cell
The foremost important question in the fuel cell technology is the source of hydrogen gas required as the fuel. Reports indicate that the sources of hydrogen play a crucial role of the effect of hydrogen fuel cell on the environment (Ramani 2006). There are methods of hydrogen gas generation plays a critical role in the environmental impacts on the methods of hydrogen generation. The commonly used process of hydrogen generation is electrolysis of water. However, the electrolyzer uses the energy from the grid and therefore the overall process shall not reduce the carbon dioxide emissions, since the electricity of the grid is obtained from burning of coal.
This leads to renewable processes of hydrogen generation that would reduce the carbon emissions and thus prove to be a significant in safeguarding the environment. Renewable energy sources like solar panel, wind turbine, hydroelectric turbines, etc. can be used to power the electrolyzer used for hydrogen generation. Therefore, the carbon emissions would be drastically reduced, flourishing the fuel cell industry.
Renewable Energy Systems
Renewable hydrogen fuel cell is gaining immense importance recently due to its sustainability and application. For example if a solar panel is utilized to generate electricity that is used to operate the electrolyzer. The electrolyzer breaks down water to produces hydrogen and oxygen. This hydrogen is stored and then used in the fuel cell, which in turn, generates electricity and water is produced as emissions. There shall absolutely no pollutants produced in the overall process. Thus there shall be generation of oxygen and water as the products of this process that actually would increase the sustainability of the environment.
The power generated from the hydrogen fuel cell is clean and the water formed in the process can be recycled in the electrolyzer to breakdown into hydrogen. Therefore essentially energy is created from the water to produce water as the end-product and electricity as the by-product. Hence, utilization of renewable energy for fuel cell would be the best source of energy that has ever been invented by humans. These systems would be truly sustainable and as long as there shall be these renewable sources, there shall the energy required to do work.
Biohydrogen
There are biological methods also used for hydrogen gas generation. It is called as biohydrogen and clean, renewable and alternative energy fuel which does not produces CO2 emissions. Hydrogen is also produced by many bacteria and algae and includes direct biophotolysis, indirect biophotolysis and dark-fermentation (Kapdan & Kargi 2006).
Direct biophotolysis: In this process, there is hydrogen production from water by the photosynthetic process where sunlight is used to fix atmospheric CO2 to form sugars. Green algae under anaerobic conditions produce H2 during CO2-fixing. The hydrogen synthesis is induced in green algae by incubation in dark and anaerobic conditions and may take few minutes to few hours. The H2 metabolism is dependent of the hydrogenase enzyme which combines protons (H+) with electrons to form H2 gas, subsequently producing ATP. The hydrogenase enzyme is very sensitive to O2, hence photosynthetic production of H2 and O2 must be temporally and spatially separated (Levin 2004).
Indirect biophotolysis: Cyanobacteria (blue-green algae, cyanophyceae, or cyanophytes) are unicellular, filamentous algal species which produce H2 through photosynthesis. These cyanobacteria fix atmospheric N2 and O2 and hydrogen is produced as a by-product during reduction of N2 to ammonia. H2 metabolism is dependent on nitrogenases and the hydrogenases and is influenced by many factors and the variety of species and strains used for cultivation (Tamagnini 2002).
Fermentation: Fermentation could in the presence of light (photo-fermentation) or dark fermentation. Purple non-sulfur bacteria are used for H2 production in photo-fermentation, where H2 is evolved in presence of light by nitrogenase enzyme under nitrogen limitation. This process uses waste organic compounds as substrate for fermentations with batch, continuous or immobilized cultures (Kapdan & Kargi 2006). The dark-fermentation process involves production of H2 by anaerobic bacteria using carbohydrate rich substrates. It is produced by many bacterial species (Enterobacter, Bacillus and Clostridium) and can be operated from 25-80 °C. This process produces mixed biogas and hydrogen from sugars (hexoses, starch or cellulose) with H2 yields depending on fermentation pathway and end-products (Beer 2009).
Conclusion: Benefits and obstacles of hydrogen fuel cell based economy
Fuels cells are highly efficient machines (50 – 60 %) that convert hydrogen and oxygen gases to electricity and water. They have double the efficiency as compared to combustion engines. It is a clean source of energy with virtually no pollutants and can be sustainably developed. Since there are no moving parts in this machine, the fuel cells do not make any noise and are extremely silent machines. Their size can be compacted and can be made modular thus making then extremely good for modern day homes. Therefore fuel cells can act as the best environmentally friendly and sustainable technology.
Currently, there are few uncertainties with regards to the fuel cell and development of hydrogen economy. The present market acceptance for fuel cell is limited due to lack of knowledge and expensive technology. There are queries about price, reliability, longevity of fuel cells and their accessibility. However, market acceptance for hydrogen economy is surely on a rise and shall provide immense revenues in the future. There developments of new age materials shall provide durable and less expensive fuel cells.
At present, fuel cell technology is accepted by the automobile manufacturing companies, but very soon power plants systems will also gain status and grow further. The reduction in the cost of catalysts and developments of government policies shall provide for the growth for hydrogen fuel cells in the future. The demand for electrical supply is rising worldwide. One of the key points is the production of this energy is to meet these demands responsibly and safely. Hydrogen fuel cells shall provide one of the best tools to accomplish our energy from clean sources efficiently and sustainably.
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