Fuel Cells, a 93-page paperback book authored by Paul Breeze, could have been entitled “An Introduction to Fuel Cells.” It contains basic essential information for an engineer who wants to learn about fuel cells without being burdened by heavy theoretical descriptions. It is a concise guide to fuel cells with a minimum of chemical equations. Author Paul Breeze has written seven other books covering electric power generation.
This book provides some of the historical background on the development of fuel cells for the U.S. space program. Going forward, the subject of fuel cells is gaining more attention because this technology is beginning to enter the mainstream of automobile designs.
Toyota has introduced a fuel-cell car called the Mirai with an output power density of 3.0 kW/L, delivering more than 100 kW output. It uses Toyota’s proprietary, small, lightweight fuel-cell stack and two 70 MPa high-pressure hydrogen tanks placed beneath the specially designed body. It can accommodate up to four occupants.
In addition, General Motors Co. and Honda announced establishment of the auto industry’s first manufacturing joint venture to mass-produce an advanced hydrogen fuel-cell system that will be used in future products from each company.
In the future, technical information about the use of fuel-cell power will be of increased interest to power electronics engineers. A basic understanding of this technology is covered in the first two chapters of this book. These chapters describe the principles of the fuel cell, which is an electrochemical device that employs a chemical reaction to generate electricity. The fuel-cell mechanism merely provides a chamber in which the fuel-cell reaction can take place. The actual chemicals that cause the generation of electricity are provided externally. The fuel cell has to control carefully the way in which the chemical reactants introduced into the cell interact with one another so that electrical power can be extracted. As long as there is a source of the chemical fuel, the fuel cell will generate power. When the fuel is exhausted it must be “recharged” by the addition of more fuel.
The next chapters describe various types of fuel cells.
Cell reaction in the Alkaline Fuel Cell (AFC) (Fig. 1) is different than the standard fuel-cell reaction. Molecular hydrogen is supplied to the anode where it splits into hydrogen atoms where each releases an electron to the cell’s external circuit. These positively charged hydrogen ions then react with hydroxyl ions from the electrolyte to form water molecules at the anode. Meanwhile at the cathode, oxygen molecules dissociate to form oxygen atoms that take electrons from the external circuit and then react with water molecules in the electrolyte to create hydroxyl ions.
Proton Exchange Membrane Fuel Cells (PEMs) exploit the reaction between hydrogen and oxygen typical of most fuel cells. The electrolyte of the cell is an acidic polymer membrane that is permeable to protons and it is the transfer of protons from one electrode of the cell to the other across the membrane that permits the reaction to proceed. The cell is shown in Fig. 2.
Phosphoric Acid Fuel Cells (PAFCs) use an acid electrolyte to produce the reaction between hydrogen and oxygen (Fig. 3). The electrolyte is pure phosphoric acid, which is a solid at room temperature but melts at 42°C and is stable in liquid form to just above 200°C. It is a proton conductor, with a relatively low overall conductivity. To overcome this, the electrolyte is normally loaded into an inert matrix so that it forms a thin wafer between the electrodes. Catalysts are applied to the electrodes to accelerate the cell reactions.
Molten Carbonate Fuel Cells (MCFCs) have the most complex fuel-cell reaction of all the cells available commercially (Fig. 4). The electrolyte is a mixture of alkali metal carbonates (typically 62% lithium carbonate and 38% potassium carbonate by molecular proportions, a eutectic1 that melts at 550°C) that is heated to between 600°C and 1000°C and in its molten state is capable of conducting carbonate ions (CO32-). The molten carbonate mixture is held by capillary action within a solid ceramic matrix that is commonly made from lithium aluminum oxide (LiAlO2, also known as lithium aluminate).
The primary component of the Solid Oxide Fuel Cell (SOFC) is a solid ceramic electrolyte. This solid metal oxide is an insulator that will not conduct electricity. However, it is capable of conducting oxygen ions and this makes it suitable for a fuel cell electrolyte (Fig. 5).
Because it is a solid, the ceramic has significant advantages over all other types of fuel cells. The electrolyte itself is expected to remain stable for very long periods, making the lifetime of a solid oxide cell the longest of any current design. There are no liquids to seal, eliminating a major engineering problem as well as removing a source of electrode erosion common in other cell types. In addition, a fully solid-state device makes fabrication simpler, in principle at least, as the electrodes can be applied directly to the ceramic electrolyte surface using simple solid-state deposition techniques.
Direct methanol fuel cells (DMFCs) (Fig. 6) are a polymer membrane fuel cell, similar in concept to the proton exchange membrane fuel. The major difference is that in the DMFC the fuel supplied to the anode of the cell is not gaseous hydrogen but methanol in the liquid form. The methanol can react directly at the cell electrode without the need for external reforming. This simplifies the cell, reducing costs. The use of a liquid rather than a gaseous fuel is extremely attractive as it makes fuel handling much easier. The main application for the DMFC today is as a portable power supply with fuel provided in cartridges but it is of interest to the automotive industry too where the use of a high energy density, liquid fuel in a fuel cell-powered engine has many attractions as a replacement for gasoline.
Fuel Cells and the Environment
Fuel cells appear to be the ideal clean energy source for providing electricity in situations where electricity generated from renewable sources such as wind, solar, or hydro power is either not available or not appropriate. Most fuel cells require only two inputs, hydrogen and oxygen, the latter from air. When operating, their only reaction products are water and some heat. They are simple, compact, and easily scalable so that the efficiency of a small fuel cell is nominally the same at that of a large cell. Moreover, the overall efficiency of a fuel cell when fed with pure hydrogen is comparable or better than that of most fossil fuel power plants, while advanced hybrid fuel cell power plants may be the most efficient type of power plant yet devised.
Cost of Electricity from Fuel Cells
The cost of electricity from a power plant of any type depends on a range of factors. First there is the cost of building the power station and buying all the components needed for its construction. In addition, most large power projects today are financed using loans so there will also be a cost associated with paying back the loan, with interest. Then there is the cost of operating and maintaining the plant over its lifetime, including fuel costs. Finally, the overall cost equation should include the cost of decommissioning the power station once it is removed from service.
The reality today is somewhat more complex. Hydrogen is not widely available for fuel-cell power plants and so most have to generate hydrogen by reforming natural gas or another hydrocarbon fuel. This reduces overall efficiency because energy is required to drive the reforming process. The latter will also generate similar volumes of carbon dioxide as would result from burning the fuel directly in a fossil-fuel power plant, although the catalytic reforming process leads to lower emissions of other pollutants such nitrogen oxides. Over the long term, perhaps looking at a 50-year horizon, hydrogen may become a readily available fuel. If that happens, then the full benefits of fuel cells will be available. In the meantime, they still have many advantages.