HYDROGEN - A clean, green fuel for the future?

Transcription

1 HYDROGEN - A clean, green fuel for the future? Unless you live in your own little isolated bubble, you are probably all too aware from reports in the media that our continued and increased reliance on fossils fuels could be accelerating global warming. Though there is still a great deal of debate about exactly how much of an influence our use of these fuels, in particular the amount of greenhouse gases like carbon dioxide that we are pumping into the atmosphere, is having on climate change, one this is certain: fossil fuels are finite fuels and one day they will run out, or become uneconomical to use! For many years now we have been looking at developing sustainable and renewable energy sources, using natural resources like the wind, sun, water and the heat of the earth to generate power. This article appeared in Issue 28 of the SAS newsletter in Spring One area that has been of particular interest is the possible use of hydrogen as a fuel. Hydrogen, the first element to be formed in the early Universe, is the simplest element consisting of just one proton and one electron. So how exactly could this substance change the way we produce and use energy? What is the Hydrogen Economy? The Hydrogen Economy is a term that has been around now for many years and basically describes an economy in which hydrogen replaces traditional fossil fuels as the main source of energy. Energy from hydrogen could be used to generate electricity for use around the home or office, or for more mobile applications ranging from transport to portable electronic devices. Obviously making such a drastic change in the type of primary energy source that we use would not be easy. A whole new infrastructure for production, storage and distribution would be required and existing systems would need to be replaced. However, research teams around the world are working right now on ways of overcoming these obstacles one at a time. The chief advantage of using hydrogen as a fuel is that carbon dioxide is not produced at the site of end use. However, this greenhouse gas may be generated during the

2 processes that are used to produce the hydrogen in the first place. But, it is possible to produce hydrogen cleanly and eliminate the generation of green house gases altogether. How can we produce hydrogen? Hydrogen is the most abundant element in the Universe; however, its low density means that it is only found in the Earth s atmosphere, in its gaseous form, at a concentration of about 1ppm. There are many abundant and easily obtainable hydrogen-containing compounds, the most important if which is water. Hydrocarbons such as methane and indeed oil are also rich in hydrogen. Hydrogen has been produced commercially for many years by mixing methane with steam at a temperature of around 1000 C, where the following chemical reaction occurs: CH 4 + H 2 0 3H 2 +CO Although this does produce lots of hydrogen gas, the byproduct, carbon monoxide is an important greenhouse gas and this is exactly what the hydrogen economy aims to eliminate. This is the key issue with producing hydrogen as a fuel - how can hydrogen be produced without generating greenhouse gases? There are really three main options for producing hydrogen. Firstly, it can be extracted from methane in the reaction described above and this technique has been used commercially for many years for producing hydrogen for use in the Haber Process to make ammonia. As mentioned already though, this slightly defeats the object of using hydrogen as a fuel, as greenhouse gases are produced as a by-product of hydrogen generation. Secondly, hydrogen gas can be filtered from natural gas where it occurs in its diatomic form. Doing this requires the natural gas to be passed through a filter with sufficiently small pores to only allow the hydrogen through. Research is being carried out to develop a material which is capable of doing this on a commercial level. The downside to both of these options is that they still rely on fossil fuels as the raw material from which hydrogen is extracted. The third option could completely remove our reliance on fossil fuels and uses water as the hydrogen source. Electrolysis of water provides a sustainable source of clean hydrogen, but it does require the input of electrical energy to drive the process. The ultimate solution to this would be to use energy from a sustainable source (e.g.

3 geothermal energy, solar energy etc.). Producing hydrogen in this way could be done centrally on a large scale for distribution to end users or it could be produced locally and used directly by the end user. In either case the hydrogen produced would need to be stored, perhaps only for a short amount of time and this brings with it a whole host of other problems which need to be solved. How can we store hydrogen? One of the biggest hurdles to overcome if we are to use hydrogen on a large scale is how to store it prior to use. Hydrogen gas has a low density and storing it in its ambient form would require tanks which would simply be impractical in size. It is possible to store compressed hydrogen gas or even liquefied hydrogen in tanks, but these methods also have problems. The tanks needed to store the hydrogen under pressure would need to be pretty substantial and very heavy and the tanks needed to store liquid hydrogen would need to be very well insulated to prevent it from evaporating. All three of these techniques have one serious safety consideration. Hydrogen is a very combustible element and storage in either its liquid or gaseous form would be dangerous (liquid hydrogen and liquid oxygen are mixed together in rocket fuel!). There are two other methods for storing hydrogen which are less risky. In the first method hydrogen is stored chemically as a hydride or other hydrogen-containing compound. In the form of a stable metal hydride, for example, the hydrogen can be stored and transported safely to the point of use, where it can be liberated. Again this technique has problems associated with it. At the moment high temperatures and pressures are required to get hydrides to form in the first place and to then decompose and liberate hydrogen for use. The final method uses a solid material to contain hydrogen in its molecular form. Storage materials include carbon nanotubes and metal-organic frameworks. These materials contain pores in which hydrogen gas can be stored. Since the hydrogen is stored in its molecular form this method does not have the same problems associated with it as using a hydride and similar storage densities to liquefied hydrogen can be achieved. Once a safe and effective method of storing and transporting hydrogen is found it may then be used as a fuel.

4 How can we use hydrogen? The chief way of using hydrogen as a fuel is in a fuel cell. Fuel cells have been around for many years and are basically a way of converting chemical energy into electrical energy. Unlike a battery, which is also an electrochemical conversion device, fuel cells do not store energy, they generate a DC voltage and must be connected to the equipment they will power or to batteries which will store the energy. The hydrogen-oxygen fuel cell was invented in 1839 by Sir William Grove. He knew that he could separate water into its constituent elements using electrolysis and suggested that electricity and water could be made by recombining hydrogen and oxygen. Using his very primitive gas voltaic battery he proved his theory to be true. A number of different types of fuel cells exist. Some of these are better for large stationary power generation plants and others are better for more portable devices. Alkaline fuel cells (AFC) have been used by the United States Space program since the 1960s and are the oldest design of fuel cell. However they are very susceptible to contamination and require the hydrogen and oxygen to be very pure. Solid oxide fuel cells (SOFC) again use the combination of hydrogen and oxygen to generate electricity but they do this at very high temperatures ( C). When in continuous use this type of system is very stable and could be used to generate electricity on a large scale. Electricity can be generated from a SOFC in two ways increasing their overall efficiency. Firstly, electricity is generated by the recombination of hydrogen and oxygen in the fuel cell itself, and secondly the high temperature steam produced by the fuel cell is used to drive a more conventional generator. Proton exchange membrane fuel cells (PEMFC) offer the most promise as they operate at room temperature and can be made in a variety of sizes. It is also one of the simplest types of fuel cell consisting of just four parts. The anode has channels etched in to it, to ensure that the hydrogen gas is dispersed equally over its surface, where the following reaction occurs: H 2 2H + + 2e - The cathode is also etched to ensure that the oxygen is distributed evenly. At the cathode the electrons from the

5 external circuit recombine with hydrogen ions that have diffused through the membrane and oxygen to produce water. The catalyst facilitates the reaction between hydrogen and oxygen and is usually made from platinum in the form of nanoparticles coated on to carbon paper or cloth. The proton exchange membrane is the electrolyte between the anode and cathode. It only allows the passage of positively charged particles and forces electrons to flow around the external circuit. A schematic diagram of a PEMFC is shown below. Where can hydrogen be used? The main reason for using fuel cells is to reduce pollution, whether this is from a power station or exhaust fumes from a car. However, it is important the make sure that the fuel cells used are as energy efficient as possible. In theory, using pure hydrogen a fuel cell can achieve an efficiency of 80%. In reality, when connected to systems which will convert the electrical energy into mechanical work an overall efficiency of about 64% is more realistic. Fuel cell powered vehicles are not very common at the moment and indeed it could be another 5 or 10 years before they are a practical solution. In these vehicles the electricity generated by the fuel cell would be used to power electric motors, making the vehicles very quiet.

6 As well as being used to power cars one of the main suggested uses of fuel cells is to power buses. Many cities across the world are now looking at investing in this technology for clean, pollution-free public transport. It has also been suggested that one day we will all have a bank of fuel cells in our home to generate electricity when we need it to power our appliances. Can we learn from the stars? Using hydrogen in a fuel cell is just one way of producing clean energy, but we could look to the starts to find an alternative way of generating energy on a large scale. At the heart of our Sun hydrogen is combining to produce helium which generates huge amounts of energy. Nuclear fusion has been the subject of scientific research for many years and we may be getting closer to finding a way of using hydrogen fusion on a practical scale. The JET (Joint European Torus) project, located at the UK AEA Culham Science Centre in Oxfordshire is home to the largest and most powerful magnetic confinement fusion device in the world and is carrying out fusion experiments on a scale close to that of a possible commercial fusion reactor. Where can I find out more? Both Wikipedia and How Stuff Works have good articles looking at all aspects of using hydrogen as a fuel, however, the data that is quoted by both of these sites is very focussed on the USA. The Hydrogen Materials Group in the Materials Department at the University of Birmingham is carrying out research on all aspects of hydrogen production, storage and use. They even have a working fuel cell-powered narrow boat! You can find out more at For more information about fusion you can visit The AEAs laboratories at Culham in Oxfordshire offer a range of resources, activities and visits for schools.