Solid Oxide Fuel Cells

Introduction

Since the Industrial Revolution, human beings have been releasing vast amounts of carbon dioxide (CO2) and other "Greenhouse" gases into Earth's atmosphere through the burning of fossil fuels. These gases have contributed to the increase in average global temperature known as Global Warming. If left unchecked these temperature increases could have devastating effects on our planet. Few people now dispute that ways of generating energy which do not rely on fossil fuels (so-called renewable energy, or at least reduce the amount of pollutants produced, need to be put in place. Fuel cells are one such alternative technology being developed by various companies and governments around the world (see Links).

A solid oxide fuel cell (SOFC) is a type of fuel cell constructed entirely from solid-state materials, in particular, a group of advanced ceramics known as electroceramics. Within this group of materials, fast ion conductors (also known as "rapid ion conductors" or "superionic conductors") are able to conduct charged atoms (ions) through their crystal structures. Solid oxide fuel cells contain a fast ion conductor that rapidly conducts oxide ions (O2−). Incidentally, these materials are also used in a range of other applications such as oxygen gas sensors, oxygen pumps, and automotive and industrial exhaust catalysts. Scientists, engineers and technologists are working hard to make these efficient, environmentally-friendly devices a reliable means of electricity production for tomorrow's industries and consumers.

Further information about advanced ceramics in Australia and the UK.

What are Fuel Cells?

A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel and an oxidizing gas via an ion-conducting electrolyte. The chief characteristic of a fuel cell is its ability to convert chemical energy directly into electrical energy without the need for combustion, giving much higher conversion efficiencies than conventional thermomechanical methods (for example, steam turbines and internal combustion engines). Consequently fuel cells have much lower carbon dioxide emissions than fossil—fuel–based technologies for the same power output. They also produce negligible amounts of SOx and NOx, the main causes of acid rain and photochemical smog.

The first fuel cell was demonstrated by Sir William Grove in 1839, but it has only been in the last few decades that research into fuel cells has (literally) taken off. Several types of fuel cell are now being developed around the world, the chief difference between them being the material used for the electrolyte. Different electrolytes operate best in different temperature regimes. Small, alkaline fuel cells (utilizing a liquid electrolyte) were developed by NASA to power the Apollo and other space missions in the 1960s, and improved versions are still used on the Space Shuttle today. SOFCs, in contrast, are constructed entirely from solid-state materials; they utilize a fast oxide-ion−conducting ceramic as the electrolyte, and operate in the temperature range 800–1000oC. SOFCs provide the following advantages compared with other fuel cell types:

few problems with electrolyte management (cf liquid electrolytes, which are typically corrosive and difficult to handle)
highest efficiencies of all fuel cells (50–60%)
high-grade waste heat is produced that can be used for combined heat and power (CHP) applications
internal reforming of hydrocarbon fuels (to produce hydrogen and methane) is possible.

SOFC Design and Operation

A single fuel cell unit consists of two electrodes (an anode and cathode) separated by the electrolyte (Figure 1). Fuel (usually hydrogen, H2, or methane, CH4, or carbon monoxide, CO) arrives at the anode, where it reacts with oxide ions from the electrolyte, thereby releasing electrons (e−) to the external circuit. On the other side of the fuel cell, oxidant (usually O2 or air) is fed to the cathode, where it supplies the oxide ions (O2−) for the electrolyte by accepting electrons from the external circuit. The electrolyte conducts these ions between the electrodes, maintaining overall electrical charge balance. The flow of electrons in the external circuit provides useful power.

Figure 1: Concept diagram for a solid oxide fuel cell

Today's technology employs several ceramic materials as the active SOFC components. The anode is typically formed from an electronically conducting nickel/yttria-stabilised zirconia (Ni/YSZ) cermet (ie, a ceramic/metal composite). The cathode is based on a mixed (ie, both electronic and ionic) conducting perovskite, lanthanum manganate (LaMnO3). Yttria-stabilised zirconia (YSZ) is used for the ion-conducting electrolyte.

To generate a reasonable voltage, SOFCs are not operated as single units but as an array of units or "stack", with a doped lanthanum chromite (eg, La0.8Ca0.2CrO3) interconnect joining the anodes and cathodes of adjacent units. Although several stack designs are being considered around the world, the most common configuration is the planar (or "flat-plate") SOFC illustrated in Figure 2.

Figure 2: Planar design for a solid oxide fuel cell (exploded view)

Future Development of SOFCs

For the YSZ electrolyte to provide sufficient ionic conductivity, it must be heated to high temperature (900–1000oC). This means that expensive superalloys must be used to house the fuel cell, increasing its cost substantially. These costs could be reduced if the operating temperature were lowered to between 600 and 800oC, allowing the use of cheaper structural components such as stainless steel. A lower operating temperature would also ensure a greater overall system efficiency and a reduction in the thermal stresses in the active ceramic structures, leading to a longer service life for the system.

To lower the operating temperature, either the conductivity of YSZ must be improved, or alternative electrolytic materials must be developed that can replace YSZ. One way of improving the conductivity is to make the electrolyte thinner; the shorter the distance the oxide ions have to travel, the less overall resistance they encounter. Nowadays state-of-the-art thin film technology is used to produce electrolytes between 10 and 100 microns (0.01 to 0.1 mm) thick. The major challenge is to ensure that the electrolyte remains fully dense so that the system isn't short-circuited by gases passing from one side of the cell to the other.

A concerted effort is also being made to develop new electrolyte materials. One method is to use zirconia doped with compounds other than yttria, notably scandia, Sc2O3. Other ceramics that are currently being investigated include gadolinium-doped ceria (Gd-CeO2), barium indate (Ba2In2O5), and strontium- and magnesium-doped lanthanum gallate (Sr,Mg-doped LaGaO3 or "LSGM"). However, these novel materials all have serious drawbacks, and it is most likely that the first commercial SOFC units will use zirconia-based ceramics as the electrolyte.