Background

Just about every gas and electricity company in the deregulated UK power market is offering customers discounts if they buy both utilities from the one supplier. Ironically, they may be unwittingly pre-empting the technology of the future, which will allow residential and office buildings to generate all their heating and electricity needs from a single supply of gas. The key to this cleaner, more efficient and potentially cheaper source of energy is the solid oxide fuel cell.
New Materials Developments

Solid oxide fuel cells generate power not by burning fuel for heat, but by drawing oxygen ions through a solid membrane to react with hydrogen from a fuel. Electricity and heat are produced by this flow of ions and the chemical reactions involved, and the only waste products are water and, unless pure hydrogen is used as the fuel, carbon dioxide.

AZoM - Metals, Ceramics, Polymers and Composites: Solid Oxide Fuel Cell Developments and Applications, Schematic of how a ceramic oxygen ion conducting electrolyte works.

Figure 1. Solid oxide fuel cells are conventionally based on ceramic oxygen ion conducting electrolytes. Oxygen ions react with the hydrogen at the anode to produce water and liberate electrons in the external circuit. The oxygen ions are formed at the cathode by reaction of oxygen from air with electrons recycled from the external circuit.

New materials developments for fuel cell electrodes and electrolytes are driving forward the field. Ceres Power, a spin-off company from Imperial College, London, is pushing ahead with one such development, the intermediate temperature solid oxide fuel cell, which uses a special doped ceria electrolyte and operates at just 500°C. Previously, intermediate temperature meant between 650-800°C, and high temperature meant between 800-1000°C. Operating at a temperature of 500°C is a big advantage in the smaller scale applications of SOFCs, such as uninterruptible power supplies, secure power supplies, portable power units and auxiliary power units for automotive applications.'

Another advantage of Ceres's new solid oxide fuel cells is ‘fuel flexibility’, as they can run on propane, methanol, ethanol or pure hydrogen. This means that no new infrastructure is required for delivering new fuel, as fuel cell devices could run off the natural gas supply. “Companies are already looking at replacing the domestic boiler with a fuel cell boiler,” says Brandon, “to generate electricity and heat for hot water and central heating.” This is of particular interest in Japan, says John Miner, head of the Department of Materials at Imperial. “It is much cheaper to get gas into city centres than electricity,” he explains, “so the Japanese want to just pipe in gas and generate electricity and heating on site for homes and office blocks.”
Displacement Technology

Environmental benefits are often cited as the reason for choosing fuel cells to replace other sources of power. But as the above example highlights, fuel cells still require a source of gas to generate electricity and heat, be it natural gas or pure hydrogen. And at the other end of the process, carbon dioxide is still produced as a waste product. “Fuel cells are a displacement technology,” says Miner. “No carbon dioxide and pollutants are produced in the city centre, say, or at point of use, but they are produced where the hydrogen is made.”

However, fuel cells can reduce the level of emissions greatly by doing away with many pollutants and giving more power for the amount of fuel consumed. “Basically, fuel cells are more efficient in some cases, and are cost competitive,” says Steve O'Dea, Operations Manager for Ceres Power. It is this economic competitiveness that is most important for the new SOFCs. “Let’s be clear,” says Brandon, “their development is not driven by their environmental benefits. In the stationary power sector, there are true commercial drivers.”
Reforming Efficiency

The overall efficiency of the latest solid oxide fuel cell systems enables them to compete with other technologies such as standard gas-fired power stations and, in automotive applications, the internal combustion engine. “With SOFC’s you can put natural gas in, and reform hydrocarbons to obtain hydrogen before using it to generate electricity' says Brandon. `This gives an efficiency of 50% at a production level of a few kilowatts.” Attach the solid oxide fuel cell system to a gas turbine and you can get an impressive overall efficiency of 70%.

Such figures are encouraging the growth in the use of solid oxide fuel cell systems around the world. The overall market for fuel cell technologies (including polymer fuel cells and other types) in the US alone is expected to reach $30 billion by 2010, and commercial systems for localised and domestic power production are likely to be available in Europe in the next 2-3 years, the same time scale anticipated by Ceres Power for their own systems to become commercially available.

The products being developed by Ceres are aimed at grabbing a slice of this potentially highly lucrative market. As Brandon explains, the company is tackling the smaller scale applications of solid oxide fuel cells with its low operating temperature systems. “The high temperature systems are not appropriate in applications in which you want to produce tens of kilowatts,” he says. O'Dea reiterates the point, All these different fuel cell technologies have their niches, for which they are very suitable.'
Cheap and Robust

Ceres is carving out its own niche by introducing new materials developments to improve its fuel cells. “In traditional intermediate temperature SOFC’s, the electrolyte is zirconia, 10-20µm thick. This is not free-standing and needs an anode ceramic support,” explains Brian Steele, emeritus professor of materials at the Centre for Ion Conducting Membranes and technical director of Ceres. “Ours is very different. The material is supported on stainless steel, which is cheap and more robust than ceramic substrates and can be heated up very rapidly.”

“The rate of heating is important for the smaller fuel cells that Ceres is aiming to produce, because it determines the startup time for the device or system. The fuel cells will not start producing power until the electrolyte materials reach the temperature at which they become sufficiently ion conducting. For, say, a portable device or an auxiliary power system in an automobile, this start-up time is preferably of the order of a few minutes, which can be achieved with a stainless steel support. A ceramic supported anode can take at least one hour to heat up,” says Steele.

Ceres Power is concentrating on using gadolinia doped ceria for its fuel cells, which achieves the necessary ionic conductivity at temperatures above 500°C. This compares to yttria stabilised zirconia, which must be heated to above 700°C. The 200°C lower operating temperature makes all the difference. “At the lower temperature, gasket materials can be used to seal the cells, because they can withstand these temperatures,” says Steele. “This makes our system much cheaper than cells that operate at 700°C, which are usually sealed instead with a glass ceramic.” A glass ceramic seal also adversely affects the rate at which the cell heats up.
Going Commercial

Beyond this, Ceres is reluctant to reveal too much more about the details of its new materials and processes. “It will be the autumn before we provide more information about the technology involved,” says Steele. “Then we intend to license the technology, or form joint ventures. There are people out there who know more about the markets than us.” Just because a commercial product is on the market doesn’t mean that the developments will stop, though. “Part of Ceres's technology is that we have developed a way to deposit ceria on steel,” says Brandon. “Work is ongoing, funded by the EPSRC, with three projects within the Department of Materials and CICM. Our future work concentrates on electrode developments, with new, more active electrodes to reduce oxygen quicker.”
Mobile Ions

Among the projects at Imperial is work being carried out by Steve Skinner, a lecturer in the Department of Materials, on new cathode materials. Skinner is using perovskite related oxides, in which ionic transport of oxygen occurs via interstitial sites in the lattice. “The materials ‘accommodate’ excess oxygen ions in interstitial sites. These interstitial oxide ions have been found to be very mobile,” he says. “The diffusion rate is pretty competitive with current materials.” Skinner is optimising these materials through doping, and is also looking at materials with oxygen interstitials as electrolytes. Using materials with similar structures should improve the compatibility between materials, producing better fuel cells. For example, a reaction zone between the materials will tend to increase the resistance of the cell, which is why research is looking into the same materials for both the electrolyte and the electrode, explains Kilner.

The research is not only interesting from the fuel cell point of view. “These new materials Steve is working on are very different in the way oxygen moves,” says Kilner. “Oxygen interstitials are very unusual in oxides, and so this is a very interesting pure materials project, but because of the ongoing SOFC work it has interesting implications too.”

Working on a parallel track is Mortaza Sahibzada, Royal Academy of Engineering Research Fellow, who is looking into solid oxide fuel cell materials, again perovskites, that work on the basis of proton conduction rather than oxygen ion conduction. These materials again rely on oxygen vacancies in the lattice. Water molecules react with oxygen vacancies on the anode surface of the material, forming hydroxyl groups. But in this case it is protons that skip from site to site in the solid structure forming hydroxyls as they go. Finally the protons react with oxygen forming water at the cathode. This compares with the current conventional solid oxide fuel cells in which oxygen is drawn towards the anode, where it reacts with the hydrogen to form water.

AZoM - Metals, Ceramics, Polymers and Composites: Solid Oxide Fuel Cell Developments and Applications, Schematic of how a ceramic oxygen proton conducting electrolyte works.

Figure 2. Solid oxide fuel cells may also function with a ceramic proton conducting electrolyte. In this case the hydrogen is dissociated at the anode to form protons in the electrolyte and electrons in the external circuit. The protons are joined again with the electrons as well as oxygen at the cathode to produce water.

The set up of the system also lends it to carrying out interesting reactions. “We can do reactions in situ,” Sahibzada explains, “such as the dehydrogenation reaction, using the anode as a catalyst for the reaction.”
Chemical Conversion

Kilner thinks such a set up could form part of an integrated industrial chemicals process. “Overall, you could convert a low grade bulk chemical and generate a valuable chemical product, plus heat and electricity too,” he says. Examples could be the conversion of ethane to ethylene or propane to aromatics.

However, these materials will not be appearing in commercial applications for some time. “The materials are several years behind oxygen ion conductors in terms of technical development,” says Sahibzada, the reason being that they only work up to a temperature of about 800°C. Before the advent of 500°C intermediate temperature solid oxide fuel cells, the original temperature range for operation was 800-1000°C, so the protonic conductors weren't considered.

When the materials do arrive on the scene, they'll be running alongside the oxygen conductors in similar applications, for portable power supplies and localised heat and electricity generation. The automotive industry will also be a big market for new intermediate temperature solid oxide fuel cells, but they won't be used to provide power to drive the vehicles along. “Polymer-based fuel cells are far more advanced for electric powertrain applications in vehicles,” says Kilner, “whereas at present, solid oxide fuel cells are favoured for the auxiliary power supply unit, for powering the air conditioning, the heating, and so on.”

A major internal automotive application could be in truck cabins. Long distance lorries can spend as much as half their time with the engines on at idle, to provide electrical power for cab facilities that enable drivers to sleep in their vehicles. According to a paper Brandon presented at a fuel cells seminar last year, this is estimated to cost $2.7 billion per annum in fuel and additional maintenance costs. A gasoline fuelled intermediate temperature solid oxide fuel cell could provide big savings by allowing the main engine to be switched off.
Sewage Power

Sahibzada foresees applications for solid oxide fuel cells in a totally different field, as well. “We are looking at combining solid oxide fuel cells with a sewage/sludge gasifier,” he says. “This generates fuel gas at 800°C, which can be fed to a fuel cell to generate electricity from sewage instead of sending it to landfill.” This clear environmental benefit is backed up by the conversion of methane gas into carbon dioxide in the process, the former being a much more potent greenhouse gas. Indeed, anywhere there is a fuel gas produced as a byproduct of a process, solid oxide fuel cells could be linked up to produce electricity.

“All these ceramic membrane technologies have multiple applications,” says Kilner. In the next decade or so, nearly two hundred years after the concept of the fuel cell was first proposed, we should at last start seeing their efficiency and environmental benefits in our everyday lives.