Fuel Cells

By: Leah DeMares

Do you own a vehicle that only produces water and electricity as emissions? If you do not own one right now soon you will. New technology, such as fuel cells, derived from electrochemistry will make this process possible to obtain. Fuel cells are electrochemical devices that directly convert chemical energy into electrical energy. Fuel cells are small in nature therefore they can be used to provide electricity in many different applications like vehicles, power plants, or laptops. Electrochemistry, sources of fuels and oxidants, and the construction of the cells all play a role in the creation of power that fuel cells can provide in automobiles. Along with these fuel cells come many environmental impacts and advantages.

Fuel cells are generated through electrochemical processes. They have similar structures as a battery. They contain two porous electrodes, which are separated by an electrolyte. Electricity is produced by a chemical reaction. In most fuel cells that reaction is between hydrogen based fuel and an oxidant, oxygen, inside the cell. Most electrochemical fuel cells are related to the thermodynamics of chemical galvanic cells.[1] A galvanic cell consists of two electrodes in contact with an electrolyte and an electric current can flow over an external wire when they are connected. The difference of potentials at each interface is the electrode potential and the sum of the differences gives the cell potential. The electric potential produced by a cell is proportional to the Gibbs free energy change of the cell reaction.[2] This means that the cell potentials are important in thermodynamics of fuel cells.

The first law of thermodynamics is stated as DU=q + w. Where DU is the change in internal energy of the system, q is the heat absorbed by the system, and w is the work done by the system. In normal thermodynamics only mechanical work is dealt with. Examples of these are gases being compressed under pressure or the expansion of a surface area. Nevertheless, other kinds of work are possible instead of typical mechanical work. The work we are interested in is the electrical work, the work done when an electrical charge is moved through an electric potential difference.[3] Look at this series of equations undergoing a reversible process at a constant temperature and pressure in both mechanical and electrical work.[4]

1.Electrochemical cell thermodynamics states w= -PDV + w (electrical)

2. Since it is a reversible process at a constant temperature then q=TDS so internal energy is DU= TDS - PDV + w (electrical)

3.At constant pressure the system’s enthalpy change is DH= DU + PDV

4.At a constant temperature the Gibbs free energy is DG=DH - TDS

If you combine the last three equations the Gibbs free energy is equal to the electrical work, DG=w (electrical). Now, let us see how the electrical work is derived.

Moles of electrons and potential differences calculate electrical work. When a charge Q is moved through a potential difference E, the work done is w (electrical)=EQ. If the charge is assumed to be carried by electrons then Q=nF, n is the number of moles of electrons transferred and F is Faraday’s constant. Work is negative if the system transfers energy to the surroundings so then w (electrical)=-nFE, by substituting nF in for Q. Now we get the final equation DG=-nFE. Therefore, electrochemical fuel cells are essentially galvanic devices in which two half-cell reactions at the individual electrodes result in the direct conversion of the fee energy of the overall cell reaction into electrical energy as useful work.[5] Through the use of this equation, DG=-nFE, the ideal efficiency can be determined.

When hydrogen or hydrocarbon fuel burns in a heat engine, the heat is partially converted to work. The efficiency of the engine can be defined as the work done on the surroundings to the heat evolved, efficiency=w/q. In contrast to a heat engine the efficiency for an electrochemical cell is efficiency=DG/DH =(1- TDS)/DH. The Energy Educators of Ohio stated that the fuel cell process was more efficient than the traditional thermal power plants, converting up to 80% of the chemical energy into electricity compared to the power plants 40%.[6] The efficiency decreases with increasing temperature, but is still greater than a heat engine. Thus a hydrogen-oxygen fuel cell would have a higher potential and efficiency when it runs at a lower temperature.

The basic process that is happening in a hydrogen-oxygen fuel cell is an electrochemical reaction. Hydrogen (H₂) flows over the anode, negative electrode, and splits into hydrogen ions and electrons that carry a negative charge. Those electrons in turn flow through the anode to the external circuit. This produces useful work, the current generated, while the hydrogen ions pass through the anode and into the electrolyte. This moves them toward the cathode, also known as the positive electrode. The electrons return to the cathode that is supplied with oxygen (O₂). Electrons, hydrogen ions, and oxygen at the cathode react to form water (H₂O) and heat. This process can happen continuously as long as the fuel cell is supplied with hydrogen and oxygen.

The design is similar for each fuel cell except for the electrolyte that is used. There are five types of fuel cells determined by their electrolyte. They are alkaline, solid polymer, phosphoric acid, molten carbonate, and solid oxide fuel cells. Vehicle fuel cells typically contain the alkaline and the solid polymer fuel cells.[7] They are meant to operate at lower temperatures between 50°-260°C. Once the electrolyte is chosen you then need to choose a fuel.

In order to have a fuel cell operate at a maximum efficiency you would want to use pure hydrogen and pure oxygen. This would be an expensive process. Pure hydrogen and oxygen also create storage problems. In place of oxygen, air is used and in place of pure hydrogen, gaseous mixtures of hydrogen and carbon dioxide are used. The most economical hydrogen-carbon dioxide mixtures are gaseous hydrocarbons such as natural gas and propane. Also, liquid hydrocarbon liquids such as naphtha, methanol, and heavier hydrocarbons are used.

There are three methods to process hydrocarbon fuels to be used in fuel cells to create the hydrogen-carbon dioxide mixture. The first process is steam reforming. This involves the reaction of a light liquid hydrocarbon fuel with steam. The next process is partial oxidation. This is the incomplete burning of a fuel and is used to process heavier hydrocarbon liquids by reacting coal with oxygen and steam at high temperatures. Fuel processing can be performed at any stage before the fuel is added to the fuel cell. The last process is point of use. This is most common because it eliminates the storage factor of the hydrogen fuel. One thousand cubic feet of hydrogen can be obtained from five gallons of methanol and four gallons of fuel oil in a methyl alcohol steam convertor used for the military.[8] Liquid hydrocarbons, such as methanol, are preferred for fuel cells having to deal with vehicles due to the above fact. They can easily be transported and stored. In addition, they can be steam reformed at the point of use. When fuel cells are designed the types of fuels used are very important due to economic reasons and the need for simplicity in creating them.

The construction of the fuel cell plays a role in how much power the cell will produce. There are five basic requirements to take into consideration when building a fuel cell.[9] One requirement is the loss due to polarization and internal resistance of the cell should be small. Losses due to polarization are more in low temperature cells. Another requirement is the electrodes should be able to resist any type of corrosion. Corrosion often occurs in the liquid phase in fuel cells with aqueous electrolytes and carbonate electrolytes. The next requirement states that the electrolyte should be invariant. The specific conductivity of electrolytes in fuel cells should be large. This allows the ohmic drops in the solution between the anode and cathode to be relatively small. It is desired to have the most ionic current to come from the transport of the ion that participated in the electrochemical reaction. Electrolytes in low temperature fuel cells as we are dealing with here are mainly strong acids. Some examples are sulfuric acid, phosphoric acid, or strong alkali like sodium or potassium hydroxide solutions. In cases of fuel cells used for automobiles the formation of water from the cell tends to dilute the electrolyte. This causes special precautions to be taken in the design of the cell for removing those products if they change the composition of the electrode. The last two prerequisites declare the costs for electrocatalysts used and the prices of the fuels should be low and reasonable. As already stated above to cut down on the prices of fuels, air and hydrocarbons can be utilized. When forming a cell you must take into consideration all these requirements to provide a powerful fuel cell.

A hydrogen-oxygen fuel cell has a certain type of cell design. The hydrogen and oxygen electrodes consist of thin porous sintered-nickel plaques impregnated with appropriate mixtures of platinum and palladium.[10] All the nickel plaques are backed with a metal electrode holder. The holder holds the electrode in position, distributes the gas across the face of the electrode, serves as a bipolar electrical lead, is a heat transfer medium, and is the structural support of the cell.

The electrolyte, mostly a potassium hydroxide solution, is held in an asbestos electrolyte vehicle. A controlled amount of the electrolyte has been put into the asbestos before the cell has been assembled. The volume normally depends on the pore amount available in the electrolyte vehicle, the pore volume of the electrodes, and the temperature range of the cell’s operation.[11]

When the cell is assembled the holders are put in contact with the electrolyte vehicle. This way the vehicle can be in direct contact with one of the surfaces of each electrode. The other surface of the electrode comes in contact with the electrode holder. Pressure is applied to the electrode holder so good surface contact is maintained and so there is minimum ohmic losses. During the pressure stage a controlled amount of electrolyte penetrates the porous electrodes. Through the control of the volume and the placement of it in the interstices of the vehicle, the electrode will no longer have a flooding problem. Inside the high porous electrode the electrode-electrolyte reactant boundary occurs.

A continuous cell operation requires the removal of water produced by the chemical reaction as rapidly as it is formed. Due to the electrolyte being held stationary the best way to remove water is by evaporation through the porous electrode into a gas stream. The hydrogen gas is then cycled through a condensor and returned to the cell. This way the cell doesn’t need to be recharged at all as long as the hydrogen and oxygen are fed into the system. The cell operation makes sure that happens. Once the cell is operating the power of the cell can be determined by how many of them there are.

Single cells do not produce that much voltage. They produce only about one volt worth. This can decrease with and increasing value of current density. The total amount of current depends on the cross-sectional area of the electrode. Therefore, the fuel cell should be connected in series to build up the voltage, and connected in parallel to increase the current.[12] This power unit that is derived from connecting multiple fuel cells are called modules. In automobiles that contain fuel cells are modules. The combinations of individual cells provide about 75 kilowatts of power.[13] This module is packaged under the floor of the car, along with a reformer that can convert methanol fuel into hydrogen gas and carbon dioxide. The reason methanol is starting to replace pure hydrogen-oxygen cells is it is easier to transport and store than gaseous hydrogen. Plus, pure hydrogen needs to be stored in highly pressurized tanks. That might get damaged in accidents and explode. This is known as the Hindenberg problem. Methanol can also be obtained from biomass so carbon dioxide will now be added into the atmosphere. As you can see the combining of cells into a module creates more power than a cell by itself, and it can create many advantages over normal combustion engines.

Along with fuel cells come many environmental impacts and advantages. The environmental impacts depend on what type of hydrogen-rich fuel is used. If pure hydrogen is used then no other emissions are produced but water. If methanol is used from biomass the fuel cells have no net emissions of carbon dioxide because the carbon released is taken from the atmosphere by plants. This saves the greenhouse effect from worsening. Currently vehicles consume 6 million barrels of oil each day, which is 85% of oil imports.[14] Fuel cells are low emission and high efficiency machines so this would cut down on oil consumption. Fuel cells would reduce urban pollution. If 10% of automobiles were powered by fuel cells air pollution would be cut by one million tons per year and 60 million tons of carbon dioxide would be eliminated.[15] Fuel cells help the environment in numerous ways this is why many new cars are now being powered through this process. Just imagine the effects if all cars would be powered by fuel cells. How much air pollution would be cut down? There are also many advantages fuel cells provide over normal combustion engines.

The following are some advantages that low temperature fuel cell devices supply. One advantage is it provides efficiency. The chemical energy can be converted directly to electricity without having to have a conversion into heat first. This conversion is not subject to the Carnot cycle, which determines efficiency by the temperature limits between which the engine operates. The efficiency of the fuel cell is determined by the free energy divided by the enthalpy. Thermal efficiencies as high as 90% can be obtained through this procedure.[16] Also, the efficiency of a small cell operates equivalently to a larger one.

Another gain one would receive by using fuel cells is the low cost of manufacturing. There are no movable parts in a cell so no bearing problems can exist and the sealing problems are at a minimum. The fuel and oxidant manifolds and diffusers can be pressed, formed into metal or plastic, and punched. Which is ideal for mass production.

Low maintenance and noise are big edges in the use of fuel cells. Since there are no movable parts the cells need little or no maintenance. They can provide for long repair free services. Since there are no moving parts there is basically no noise produced when running. This allows for a quiet ride.

The use of fuel cells provides cleanliness. The only products that can be produced are water, carbon dioxide, or nitrogen. The unspent oxidants and fuels are recirculated and the electrolytes are kept separate from the exhaust systems.

The last benefit fuel cells have are their idling effect. The only time fuel cells consume fuel and oxidant is when power is drawn from the system.[17] This means that when the car is idle no fuel is being consumed. This way you could drive for longer periods without wasting fuel when the car is sitting in an idle position. This will help keep the environment clean from unwanted air pollution that normal combustion vehicles yield when they are just sitting around turned on. There are many advantages to using fuel cells in vehicles normal engines could not possibly supply.

Through the use of electrochemistry fuel cells can be constructed. The sources of fuel and the construction of the cell also play a big part in the power that the cells can supply. These cells are important technology for the future since they operate at high efficiencies and are able to run on a variety of fuels at low temperatures. As this technology improves fuel cells will be found in more and more devices as their primary source of power. Soon all vehicles will be converted to this technology making it easier for one to obtain an electrical powered vehicle.

I hope, that with all the benefits that fuel cells provide, we may all choose an electric powered vehicle over one that has a combustion engine that just adds to air pollution and needs high maintenance.