What are Fuel Cells
A fuel cell is a device that converts chemical energy into electricity through a reduction-oxidation (redox) reaction. In a redox reaction, electrons from one atom or molecule are transferred to another atom or molecule. The element that is oxidized loses electrons (generally speaking) while the element that gains electrons is reduced. An easy way to remember the difference between oxidation and reduction is the mnemonic:-
OIL RIG - Oxidation Is Loss Reduction Is Gain
Rusting metal is an example of a redox reaction that occurs between iron and oxygen. In many cases, oxygen is the “oxidizing agent”, which means that it is the compound that will gain an electron and thereby oxidize the other element by removing electrons from it. Oxygen is a common oxidizing agent in fuel cells, though there are several others that can be used including hydrogen peroxide. More about the role that redox reactions play in fuel cells can be found on the how fuel cells work page.
Fuel cells are not the only devices that convert chemical energy into electrical energy. Batteries do the same thing. The difference between a fuel cell and a battery is that batteries only store energy, they do not create energy. The chemical reaction in a battery is reversible, which means it can proceed forward or backward. In a fuel cell the reaction generally only proceeds in one direction. Once a battery is fully discharged, the chemical reaction can be reversed by adding electricity. The battery can then be used again. In a fuel cell, the reaction continues to run so long as new chemicals are supplied. There is a class of fuel cells, called regenerative fuel cells, which can be “recharged.” The process is slightly different from recharging a battery.
It is helpful to think of a fuel cell as a kind of blend between a standard internal combustion engine and a battery. Rather than burn fuel like a normal engine does, fuel cells break fuel apart in an ordered and specific process to create electricity. This is more efficient than burning the fuel. Whereas batteries simply store electricity, fuel cells are able to extract and produce electricity, making them useful for power generation.
Types of Fuel Cells
Fuel cells actually come in different types. Just like batteries, which can be lead-acid, nickel metal hydride, lithium ion, etc., there are different technologies that can be used to create fuel cells and which are suited to different settings. In general, fuel cells can be divided into two groups based on operating temperature: normal temperature fuel cells and high temperature fuel cells.
Normal Temperature Fuel Cells
Fuel cells that operate at “normal” temperatures (less than about 220 C or 430 F) include proton exchange membrane fuel cells or PEMFCs and alkaline fuel cells (AFCs). PEMFCs can consist of many varied materials, but all work on the same general principle. The positively charged cathode is separated from the negatively charged anode by a solid membrane that conducts protons (positive charge), but nothing else. The membrane ensures a one-way flow of electricity. The basic operating principle for this type of fuel cells is as follows.
At the anode, hydrogen (usually) is broken down into protons and electrons. The protons can flow through the membrane, but the electrons cannot. Both wish to reach the cathode, but the electrons are forced to take an external route since they cannot cross through the polymer membrane. As the electrons flow through the external circuit, they create an electrical current that can be used to do work. This is all there is, in concept, to the operation of a fuel cell.
The second type of normal temperature fuel cell, referred to as an alkaline fuel cell, is similar in many ways to a battery. In fact, many of these fuel cells are batteries with the capability of replacing the fuel (usually a metal in this case) to make them act more like fuel cells than batteries. They operate differently from PEMFCs. In AFCs, oxygen is injected at the cathode and is converted to hydroxyl ions. These ions then travel through the alkaline electrolyte that acts as the membrane to the anode. At the anode, they interact with hydrogen to produce water and electrons. The electrons then flow through an external circuit back to the cathode to be reused. Look for in-depth discussions in the section on alkaline fuel cells.
High Temperature Fuel Cells
High temperature fuel cells are broken down into solid fuel cells (SFCs) and molten carbonate fuel cells (MCFCs). SFCs have no liquid components at all, which means they can be installed in unique positions and do not have to lie flat. They can even be designed as tubes, making them ideal for applications where space is limited. Their solid nature requires operating temperatures as high as 1000 degrees Celsius (1800 F). Oxygen is used to produce electricity in most SFCs, rather than the much more explosive hydrogen of normal temperature fuel cells.
MCFCs operate at temperatures around 650 C. The high temperature is needed to create molten salt (lithium potassium carbonate) that acts as the electrolyte in these fuel cells (similar to the membrane in PEMFCs). MCFCs use fossil fuels to produce hydrogen-rich gas that acts much like the hydrogen in a normal temperature fuel cell, producing protons and electrons for energy.
Efficiency and Environmental Impact
Despite the fact that many fuel cells rely on fossil fuels to produce energy, their efficiency when compared to simply burning fuel gives them an edge in terms of environmental impact. Whereas the standard internal combustion engine is 25% efficient at best, fuel cells commonly reach efficiencies of 40 to 60%. They are theoretically capable of 85 to 90% efficiency, but technology has yet to reach that threshold.
Stationary fuel cells are much more likely to approach the theoretical efficiency than are mobile fuel cells. Nevertheless, fuel cells achieve twice to three times the efficiency of simply burning fossil fuels, which means that the energy returned on energy invested is much greater. The result is less greenhouse gas emissions for fuel cells for the same amount of energy produced, making them much more environmentally friendly. In addition, because the reaction is controlled, it is possible to reduce emissions even further by collecting them for safer disposal.
Cost and Economics
Fuel cells are costly due to the fact that they require relatively rare and expensive components, like platinum. The platinum content and materials used in the anode and cathode make up 70% of the cost of a fuel cell. At current prices, fuels cells cost approximately $73 - $100 per kilowatt to run. To be competitive with standard internal combustion engines, that price needs to drop to around $35 per kilowatt.
The other thing that makes fuel cells less competitive in the current market is infrastructure. Because mobile fuel cells need to run at normal temperatures, hydrogen is the most efficient fuel and has been the target of most research efforts. While it is possible to run mobile fuel cells on hydrocarbons, this reduces many of the benefits that they offer. The U.S. government has estimated that installing a reasonable hydrogen infrastructure to supply fuel for 10 million fuel cell vehicles would cost $8 billion over the next 10 years.
Research and Future Directions
Research into fuel cell technology focuses primarily on two areas: reducing the cost of the components by developing alternatives to platinum and technology for transporting and storing hydrogen fuel safely. In both cases, progress is being made with backing from government agencies. The world’s first hydrogen refueling station opened in Reykjavik, Iceland in April 2003. Iceland is a leader in the “hydrogen economy” and may become the first country to switch to hydrogen as its primary fuel source. Iceland’s access to substantial geothermal resources for generating hydrogen fuel is an important asset that most countries cannot rely upon.
The United States, Japan, Canada, Sweden, Norway, and Korea have all invested substantially in hydrogen infrastructure as well as research into fuel cell catalysts to replace platinum. Developments in 2009 promise to help move fuel cells away from platinum toward less expensive materials, which will make them both more affordable and more reliable.
Most estimates are that fuel cells will be viable alternatives to internal combustion engines around 2025. They already find application in certain settings, such as back-up generators and power for remote installations. These real world uses are providing invaluable information about how to move fuel cells forward.