A fuel cell is like a battery in that it generates electricity from an electrochemical reaction.
A fuel cell, uses an external supply of chemical energy and can run indefinitely, as long as it is supplied with a source of hydrogen and a source of oxygen (usually air). The source of hydrogen is generally referred to as the fuel and this gives the fuel cell its name, although there is no combustion involved. Oxidation of the hydrogen instead takes place electrochemically in a very efficient way. During oxidation, hydrogen atoms react with oxygen atoms to form water; in the process electrons are released and flow through an external circuit as an electric current.
Fuel cells can vary from tiny devices producing only a few watts of electricity, right up to large power plants producing megawatts. All fuel cells are based around a central design using two electrodes separated by a solid or liquid electrolyte that carries electrically charged particles between them. A catalyst is often used to speed up the reactions at the electrodes. Fuel cell types are generally classified according to the nature of the electrolyte they use. Each type requires particular materials and fuels and is suitable for different applications.
In the energy field, most hydrogen is used through Fuel Cells (FCs). A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, with water and heat as by-products. In its simplest form, a single fuel cell consists of two electrodes - an anode and a cathode - with an electrolyte between them. At the anode, hydrogen reacts with a catalyst, creating a positively charged ion and a negatively charged electron. The proton then passes through the electrolyte, while the electron travels through a circuit, creating a current. At the cathode, oxygen reacts with the ion and electron, forming water and useful heat.
Solid Oxide Fuel Cells (SOFC)
SOFCs use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is solid, the cells do not have to be constructed in the platelike configuration typical of other fuel cell types. SOFCs are expected to be around 50%–60% efficient at converting fuel to electricity 15 .
They operate at very high temperatures, typically between 500 and 1 000 °C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower-temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning (deactivation by impurities). However, vulnerability to sulfur has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.
SOFCs have a wide variety of applications ranging from auxiliary power units in vehicles to stationary power generation, with outputs from 100 W to 2 MW. The higher operating temperature make SOFCs suitable candidates for uses with heat engine energy recovery devices or combined heat and power, which further increases the overall fuel efficiency.
Proton Exchange Membrane Fuel Cells (PEMFC)
Proton Exchange Membrane Fuel Cells, also known as Polymer Electrolyte Fuel Cells or PEMFC, provide high-power density and have several advantages related to its low weight and volume, compared to other FCs 15 . PEMFCs use a polymeric membrane as an electrolyte, and porous carbon electrodes containing a platinum catalyst. These type of FCs only need hydrogen, oxygen from the air, and water to operate, and their operation do not involve corrosive fluids like some other FCs. They are typically fuelled with pure hydrogen supplied from storage tanks.
They operate at low temperatures, about 80°C, and they are suitable for mobility applications and other uses that require an initial high demand of power, which is of high density.
As of today, PEMFCs do not operate at high temperatures due to the deterioration of the current membranes, being a limitation for some FC applications. Their operation at low temperatures have an important advantage but also has some inconvenients. The main advantage is that the FC can quickly reach the operation temperature starting from ambient temperature. The main problem is the fact that they need the presence of a platinum catalyzer to be able to operate, adding costs. Moreover, the platinum catalyst is also very sensitive to CO poisoning, making it compulsory to use an additional reactor to reduce CO in the fuel gas if the hydrogen comes from an alcohol or hydrocarbon fuel. This step makes this type of FCs more expensive. Research efforts to reduce or even suppress the use of platinum are ongoing and the quantity of platinum used in PEM FC has already decreased very substantially. In addition, platinum catalyst can be recycled.
Today PEM fuel cell is the consensus choice for road transport application (car, bus, trucks, etc. ) PEM are also used in some stationary application.
Alkaline Fuel Cells
Alkaline Fuel Cells (AFCs) were one of the first developed FC technologies, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecraft 13 .
These FCs use a solution of potassium hydroxide in water as the electrolyte, and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C. However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C. The efficiency of an alkaline FC operating on pure hydrogen is 60%.
One of their advantages is that the water produced is drinkable and currently are the cheapest fuel cells to manufacture 14 . The reason lies in the relatively inexpensive materials used as catalyst on their electrodes, compared to the catalysts such as platinum required for other types of FCs.
One of the limitations of AFCs, is that they are sensitive to carbon dioxide (CO2) which may be present in the fuel or air. The CO 2 reacts with the electrolyte to form a carbonate which can decrease the conductivity.
Currently, this type of FC is being tested for stationary power applications.
Direct Methanol Fuel Cells (DMFC)
DMFCs are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode. Their novelty is the fuel used. The anode can be fed with liquid methanol or methanol vapours, whereas the cathode receives air. DMFCs belong to the family of low temperature FCs. They can be considered an evolution of the PEMFC, as they use a polymer membrane as an electrolyte. However, the platinum-ruthenium catalyst on the DMFC anode is able to draw the hydrogen from liquid methanol, eliminating the need for a fuel reformer. Therefore, pure methanol can be used as a fuel.
DMFCs have operating temperature ranges between 60°C and 130°C and tend to be used in applications with modest power requirements, such as mobile electronic devices or chargers and portable power packs 19 .
DMFCs could also be an alternative to PEMFCs and the H 2 onboard storage tanks in vehicles. In this line, recent projects aim to demonstrate the use of methanol driven fuel cells as possible range extenders for small battery electric city cars.
Phosphoric Acid Fuel Cells (PAFC)
Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte (the acid is contained in a teflon-bonded silicon carbide matrix) and porous carbon electrodes that contain a platinum catalyst.
PAFCs were developed in the mid 60s and tested since the 70s. Since then, features such as instability, performance and cost have been enhanced. These characteristics have made PAFCs good candidates for stationary applications.
They operate at a range between 150°C and 200 °C, the water resulting from the operation can be converted to steam for air and water heating (Combined Heat and Power, CHP). This feature allows efficiency increases of up to 70%. At lower temperatures phosphoric acid is a poor ionic conductor, and CO poisoning of the platinum catalyst in the anode becomes significant. However, they have the advantage that they are much less sensitive to CO than PEMFCs and AFCs. PAFCs admit fuels that contain CO and even can tolerate a CO concentration of about 1.5%, which increases the range of fuels that can be used (note: if gasoline is used, the sulfur must be removed first).
Molten Carbonate Fuel Cells (MCFC)
Molten Carbonate Fuel Cells (MCFCs) are being developed for Natural Gas (NG) and coal-based power plants for electrical utility, industrial, and military applications. MCFCs operate at high-temperature and use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO 2 ) matrix. Because MCFCs operate at extremely high temperatures (650°C and above), catalysts do not need to be precious metals such as platinum, making MCFC more affordable 20 .
They have an efficiency of 60% when producing electricity and 85% if they are used in cogeneration. The advantages of high temperature operation is that there is an increment of efficiencies and allows the use of catalyzers that are less expensive; however, such high temperatures shorten the useful life of the FC and promote corrosion. MCFCs can operate on fuels such as natural gas, biogas, syngas, methane and propane.
Disadvantages include a low power density and the aggressiveness of the electrolyte.
Source and picture: FuelCellToday