A fuel cell is a device that converts a chemical fuel (generally pure hydrogen) directly into electricity. A fuel cell is like a battery that never runs down. The chemicals that are consumed (hydrogen & oxygen) are continually fed into the cell, rather than being a component that is used up.
Fuel cells may also be thought of as "reverse electrolyses". When two electrodes are put into a salty water solution and a current is passed, water is broken down into hydrogen and oxygen. This process is called electrolysis. Fuel cells perform the reverse action they combine hydrogen & oxygen to form electricity and water.
Fuel Cell Vehicles
Battery electric vehicles can solve some of our transportation problems, but they have three major flaws, all related to energy storage: batteries are expensive, heavy, and even the best offer only limited vehicle range.
In the short term, hybrid battery electric vehicles with small internal combustion engine "range extenders" will be used to provide the vehicle range and performance that we are used to. By the year 2000, developments in fuel cell technology promise a cleaner, more efficient alternative to the internal combustion engine and thus a new age of pollution-free driving.
The Key: Efficiency
The laws of thermodynamics limit internal combustion engines to a maximum efficiency (the mechanical work output divided by the chemical energy in) of about 30%. Practical engines are closer to 20% efficient, and when stop-start driving is considered, efficiency drops to about 15%. Fuel cells are not limited by the thermodynamic Carnot cycle, and can convert fuel to electricity at up to 80% efficiency. Efficiencies of more than 50% have been demonstrated to date. This means that you can go three times as far in a fuel cell car as in a gasoline car, on the same amount of fuel.
There are two ways of storing the hydrogen needed to run a fuel cell car. Either pure hydrogen can be stored in gas, liquid, or "metal hydride" form, or hydrogen can be generated onboard from hydrocarbon fuels such as compressed natural gas or methanol.
The "reforming" of methanol or other hydrocarbons to produce hydrogen and carbon dioxide has the advantage of easy fuel storage but the disadvantages of needing a small, onboard chemical processing plant, and still polluting the atmosphere with carbon dioxide.
Storage of pure hydrogen in cryogenic liquid or high-pressure gaseous forms poses safety hazards that are unacceptable for general transportation. Storage in metal hydrides, where hydrogen atoms lodge in the atomic lattice of metals such as magnesium and titanium, offers safety and ease of use, but carries the penalty of high costs and much added weight (only 2-5% of the weight of the storage system is actually hydrogen). When the system is looked at as a whole, however, this extra weight is compensated by the reduced weight of the drive system (the fuel cell, electric motor and motor controller) when compared to a gasoline engine and transmission. An overall lower weight results in reduced fuel requirements.
Typically, fuel cells are categorized according to the kind of electrolyte that is utilized within these devices. The electrolyte may consist of a liquid solution or a solid membrane material. In any case the electrolyte serves the vital function of ionic transfer of electrical charge.
Some of the technologies are relatively advanced while others are still in their infancy. There are basically five fuel cell versions:
PAFCs: The Most Mature Approach. Phosphoric acid fuel cells (PAFCs) probably represent the most mature fuel cell technology. Westinghouse, International Fuel Cells, and at least a trio of Japanese manufacturers have been refining the design of mid-sized PAFC cogeneration plants. They are intended to fill the niche for stand-alone power generation for utility substations, factories, restaurants, hotels, and hospitals.
The fuel choice for PAFCs is not restricted to pure hydrogen. Typically, these near-term plants will use natural gas, methanol, or light distillates derived from fossil fuel sources. These cells operate at moderate temperatures (less than 200°C) with auxiliary reformers. Reformers convert the hydrocarbons to a mixture of hydrogen and carbon dioxide gases for the cells. The requirement for the initial reformation step sacrifices some efficiency, but the advantage of PAFCs is that they are tolerant of CO2 and other reformate impurities. The overall efficiency improves above the 4050% range if the installations are used as cogeneration plants, and the waste heat is used to make hot water and/or steam.
AFCs: Extraterrestrial & Terrestrial Applications. Another fuel cell technology which has been with us since the 1960s is the alkaline fuel cell (AFC) system. AFCs were first developed for spaceflight applications as part of the Gemini program to produce reliable on-board power and fresh water for the astronauts.
International Fuel Cells and Siemens are currently major players in this field. AFCs operate at relatively low temperatures, and dont require noble metal catalysts, strong advantages in their favor. Highly purified hydrogen, such as electrolytic hydrogen, is required as the fuel source. Unfortunately, AFCs also require pure oxygen as the oxidant, not air. AFCs are intolerant of even meager amounts of CO2 which effectively poisons them. If air is to be used as the oxidant, expensive CO2 scrubbers would have to be used to prevent a degradation of AFC performance.
The use of AFCs in transportation applications is doubtful; it is generally assumed that oxygen will not be stored on-board light vehicles. In home systems with solar hydrogen production, oxygen will also be produced in most cases, so this may not be a problem.
MCFCs: The New Hot Shots on the Block. Little will be said here about molten-carbonate fuel cells (MCFCs) and solid-oxide fuel cells (SOFCs). These second generation fuel cell strategies require very high temperatures for operation, (6001200°C). This allows for the internal reformation of fuels such as natural gas, methanol, petroleum, and coal. These devices tolerate CO2 without requiring any further treatment and are possible substitutes for large to mid-sized thermal power plants, substations, or as cogenerators for factories. MCFCs and SOFCs are less likely to be utilized for remote home power generation by you or me, even in the distant future.
PEMFCs: Promise for Home Power Generation. One remaining fuel cell design approach has been saved for last. It is the solid polymer fuel cell, perhaps more commonly referred to as the proton exchange membrane fuel cell (PEMFC). This technology deserves the most careful scrutiny by advocates of decentralized renewable energy and alternative transportation.
Proton exchange membrane fuel cells (PEMFCs) appear to be the "new kids on the block". In reality they represent a technology that was virtually "forgotten" for about a decade. This was an area of fuel cell research that languished in relative obscurity, and which received minimal R&D funding until only recently.
General Electric pioneered the early work. The interest really revived in the last few years when Ballard Power Systems of Vancouver B.C., Canada went public with their results. Other private organizations which have gotten into the act in recent years include: H-Power, Ergenics, Energy Partners, Lynntech, Siemens, and Billings (International Academy of Science). United States educational and public institutions which have on-going laboratory research in this field include the Schatz Fuel Cell Project at California State University at Humboldt, the Center for Electrochemical and Hydrogen Research at Texas A&M, and Los Alamos National Laboratory. New players are entering and exiting this field so frequently that this lineup may already be out of date.
One can hardly examine PEMFCs without being impressed with their elegantly simple design concept. Yet, closer study reveals their complexities and potential pitfalls in operation. Although PEMFCs are currently available commercially from a few vendors on special order, dont rush for your checkbooks unless you have deep pockets and a strong heart. PEMFCs are currently in the prototype development stage, although laboratory research continues as well.
The Pregnant Promise of Fuel Cells
We can only hope that fuel cell research coupled with engineering refinements continues at an accelerated pace. The inefficiency of the internal combustion engine cannot be tolerated much longer. Atmospheric pollution, global warming resulting from greenhouse gas emissions, and the steadily declining reserves of petroleum are all part of the legacy left us by dependence on fossil fueled IC engines. Many scientists and energy analysts believe that a solar based hydrogen energy system is the answer to these problems. The timely maturity of hydrogen fuel cell technologies will be of critical significance, if the world is going to successfully wean itself from fossil fuels. An appropriate analogy might be made between the development of integrated circuits and fuel cells. The first integrated circuits were a landmark advance that ushered in the electronic and information age. As fuel cells replace IC engines, I believe a Solar Hydrogen Age will blossom from the dust of the passing fossil fuel era.