Fuel Cells are electrochemical devices that produce electricity directly from the chemical energy of a fuel, that is without combustion or the use of rotating machinery. Similar in operation to a battery, they do not go flat or need recharging, as long as they are supplied with fuel and oxidant (usually air).
There are three essential components of an individual fuel cell: two electrodes - an anode and a cathode, and an electrolyte. The electrolyte is sandwiched between the two electrodes. This is shown schematically in Fig. 1. At the anode of an acid electrolyte fuel cell, the hydrogen fuel gas ionises, releasing electrons and creating H+ ions (or protons).
2H2 = 4H+ + 4e- 
This reaction releases energy. At the cathode, oxygen reacts with electrons taken from the electrode, and H+ ions from the electrolyte, to form water.
O2 + 4e- + 4H+ = 2H2O 
Clearly, for both these reactions to proceed continuously, electrons produced at the anode must pass through an electrical circuit to the cathode. Also, H+ ions must pass through the electrolyte. An acid is a fluid with free H+ ions, and so serves this purpose very well. Certain polymers can also be made to contain mobile H+ ions. These materials are called proton exchange membranes, as an H+ ion is also a proton.
Comparing equations 1and 2 we can see that two hydrogen molecules will be needed for each oxygen molecule if the system is to be kept in balance. This is shown in Figure 2. It should be noted that the electrolyte must only allow H+ ions to pass through it, and not electrons. Otherwise, the electrons would go through the electrolyte, not a round the external circuit, and all would be lost.
Fig. 1 Schematic of a single hydrogen fuel cell
Fig. 2. Electrode reactions and charge flow for an acid electrolyte fuel cell.
The current produced by a single fuel cell is proportional to the cross sectional area of the electrodes. The voltage is limited electrochemically to about 1.23 volts per cell. To produce a higher voltage, cells can be “stacked” or electrically connected in series.
Leaving aside practical issues such as manufacturing and materials costs, the two fundamental technical problems with fuel cells are
• slow electrochemical reaction rates, leading to low currents and power
• hydrogen is not a readily available fuel.
To solve these problems, many different fuel cell types have been developed. The different fuel cell types are usually distinguished by the electrolyte that is used, though there are always other important differences as well. The situation now is that six classes of fuel cell have emerged as viable systems for the present and near future. Basic information about these systems is given in Table 1.
Table 1. Data for the main types of fuel cell
The AFC became prominent in the 1960s when it was used to power the Apollo spacecraft. Running on pure hydrogen and oxygen, it produced an important by-product - water, which was used in the life support system of the astronauts. Pratt and Whitney, who had produced the Apollo fuel cells, identified phosphoric acid as a better electrolyte for terrestrial power generation in the 1970s. Using electrodes that incorporate platinum catalysts, development and demonstration of PAFC technology took place in the USA and Japan. To enable the fuel cells to run on natural gas, a fuel processor was developed to convert the natural gas to hydrogen.
Compared to the PAFC, the MCFC operates at higher temperatures, can operate at or slightly above ambient pressure, and does not require the use of platinum catalysts. The MCFC can run directly on natural gas and other hydrocarbon fuels without the need of a external fuel processor. The use of this “internal reforming” leads to high system efficiencies of 50 or 60 percent (LHV). This far surpasses conventional generation technologies such as gas turbines, internal combustion engines, and steam turbines, which generate power at a maximum efficiency of 33 or 35 percent, limited by the Carnot cycle.
The SOFC operating at a higher temperature still, uses a solid electrolyte of ceramic zirconia. Ceramic electrodes are used and as with the MCFC, internal reforming of fuel enables them to run at high efficiencies with a high grade recoverable exhaust heat product.
The PEM fuel cell operates at near ambient temperature and pressure and has proven to be the best available technology for vehicles. When hydrogen is stored on-board, the only emission from the tailpipe of a fuel cell vehicle is water. Fuel cell forklift trucks are now commercially available, and nearly all of the major automotive manufacturers are developing fuel cell electric vehicles, with many demonstrations having taken place over the past 15 years. The first commercial offerings are expected in 2015.
Advantages of fuel cell technology
Environmental Acceptability - Because fuel cells are so efficient, CO2 emissions are reduced for a given power output. The fuel cell is quiet, current systems emit typically 60 decibels at 30 metres. Emissions of SOx and NOx are 0.003 and 0.0004 pounds/megawatt-hour respectively. Fuel cells can be designed as water self-sufficient.
Efficiency - Dependent on type and design, the fuel cells direct electric energy efficiency ranges from 40 to 60 percent. Fuel cell operates at near constant efficiency. For hybrid fuel cell/gas turbine systems, electrical conversion efficiencies are expected to achieve over 70 percent. The PEM fuel cell is very responsive - cold starting to full load within minutes, making it ideal for fuel cell vehicles.
Distributed Capacity - Distributed generation reduces the capital investment and improves the overall conversion efficiency of fuel to end use electricity by reducing transmission losses. In high growth or remotely located areas, distributed generation could reduce or eliminate transmission and distribution problems by reducing the need for new capacity or laying new power lines. Presently 8-10 percent of the generated electrical power is lost between the generating station and the end user. Also many smaller units are statistically more reliable than one larger generating unit since the probability of all distributed units failing at one time is negligible.
Modularity - The fuel cell is inherently modular. A fuel cell power plant can be configured in a wide range of electrical outputs, ranging from less than 1 watt to several MW.
Fuel Flexibility - The primary fuel source for the fuel cell is hydrogen, which can be obtained from natural gas, coal gas, methanol, landfill gas, and other fuels containing hydrocarbons. This fuel flexibility means that power generation can be assured even when a primary fuel source is unavailable.
Cogeneration Capability - High-quality heat is available for cogeneration, heating, and cooling. Fuel cell exhaust heat is suitable for use in residential, commercial, and industrial cogeneration applications.
Adapted from: Larminie and Dicks “Fuel Cell Systems Explained” 2nd Edition, 2003. John Wiley. London.
Fuel Cells in Australia
In 1992 the CSIRO established a spin-out company, Ceramic Fuel Cells Ltd (CFCL), to develop solid oxide fuel cell (SOFC) technology. Based in Victoria, over the past 22 years this company has developed an internationally leading position in SOFCs, which has led to the production of a domestic scale cogeneration system that will run on natural gas. While the technology has been developed in Australia, in October 2009 the Company officially opened a plant in Heinsberg, Germany for the volume assembly of its fuel cell stacks. This is principally aimed at the European market. CFCL has now sold its BlueGen systems in more than seven countries around the world including Europe and Japan. And it has units in operation in Victoria, New South Wales and Queensland. More information on CFCL and the BlueGen systems can be obtained from the company website at www.cfcl.com.au.
PEM fuel cells have not been developed in Australia to the same extent as SOFCs, although research has been undertaken for many years by CSIRO and several university groups have been active in developing the membranes and catalyst materials and in testing PEM fuel cell stacks obtained from elsewhere. Similarly AFC and PAFC systems have been field tested, including a 200kW PAFC system at Sydney Technology Park. Early commercial PEM fuel cell systems have also been installed in demonstration projects for the likes of Telstra, Ergon Energy and Queensland Rail. It is intended that the AAHE will provide on-going source of information not only for research associated with fuel cell technologies but also their demonstration and commercialization in Australia.