Types of Fuel Cells

How does it work?

History

What is a fuel cell?

A fuel cell is a device that converts the chemical energy of a fuel (hydrogen, natural gas, methanol, gasoline, etc.) and an oxidant (air or oxygen) into electricity. In principle, a fuel cell operates like a battery. Unlike a battery however, a fuel cell does not run down or require recharging. It will produce electricity and heat as long as fuel and an oxidizer are supplied.

Both batteries and fuel cells are electrochemical devices. As such, both have a positively charged anode, a negatively charged cathode and an ion-conducting material called an electrolyte. Fuel cells are classified by their electrolyte material. Electrochemical devices generate electricity without combustion of the fuel and oxidizer, as opposed to what occurs with traditional methods of electricity generation.

Fuel cell construction generally consists of a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion-conducting membrane. Oxygen passes over one electrode, and hydrogen over the other, generating electricity, water and heat. Fuel cells chemically combine the molecules of a fuel and oxidizer without burning or having to dispense with the inefficiencies and pollution of traditional combustion.

Basic Characteristics

Some of the general characteristics of fuel cells have been introduced above; however, to understand the difference between types of fuel cells, several other characteristics must be explained.

Charge Carrier

The charge carrier is the ion that passes through the electrolyte, and for several types of fuel cells, the charge carrier is a hydrogen ion, H+, which is simply a single proton. The charge carrier differs between different types of fuel cells.

Poisoning by Contamination

Fuel cells can be "poisoned" (experience severe degradation in performance) by different types of molecules. Because of the difference in electrolyte, operating temperature, catalyst and other factors, different molecules can behave differently in different fuel cells. The major poison for all types of fuel cells is sulfur-containing compounds such as hydrogen sulfide (H₂S) and carbonyl sulfide (COS). Sulfur compounds are naturally present in all fossil fuels, and small quantities remain after normal processing and must be almost completely removed prior to entering the fuel cell.

Fuels

Hydrogen is the current fuel of choice for all fuel cells. Some gases, such as nitrogen from the air, have only a dilution effect on the performance of the fuel cell. Other gases, such as CO and CH₄, have different effects on fuel cells, depending on the type of fuel cell. For example, CO is a poison to fuel cells operating at relatively low temperatures, such as the Proton Exchange Membrane Fuel Cell (PEMFC). However, CO can be used directly as a fuel for the high-temperature fuel cells such as the Solid Oxide Fuel Cell (SOFC). Each fuel cell with its specific electrolyte and catalysts will accept different gases as fuels and experience poisoning or dilution. Therefore, the gas supply systems must be tailored to a specific type of fuel cell.

Performance Factors

The performance of a fuel cell depends on numerous factors. The electrolyte composition, the geometry of the fuel cell (particularly the surface area of the anode and cathode), the operating temperature, gas pressure and many other factors. For reference material that covers introductory to highly technical information on different types of fuel cells, refer to the Fuel Cell Handbook, Fifth Edition, published by the U.S. Department of Energy in October 2000.

Fuel Reformers

Low-temperature fuel cells (<h;200ºC, 390 ºF) operate on hydrogen as the fuel. At the present time, there are no readily available sources of hydrogen with widespread delivery infrastructure. There are two major approaches to solving this issue. In the shorter term, use of fossil fuels to generate the hydrogen is required.

The transformation of fossil fuels to hydrogen is generally called fuel reforming. Steam reforming is one example in which steam is mixed with the fossil fuel at temperatures around 760ºC. The chemical formula of this reforming reaction for natural gas composed primarily of methane (CH₄) is:

CH₄ + 2 H₂O => CO₂ + 4 H₂

In the high-temperature fuel cells (MCFC and SOFC), CO in the fuel stream acts as a fuel. However, it is likely that the water-gas shift reaction is occurring and the fuel for the actual fuel cell is actually hydrogen.

CO+ H₂O => CO₂ + H₂

Fuel reforming can be done in facilities of different scales. The reforming can be done at a large scale in a central facility like a chemical plant. This can result in pure hydrogen, either as a high-pressure gas or as a liquid. This would then be delivered to fuel cell users.

The fuel reforming can also be performed on an intermediate scale in a location such as a gasoline station. In this example, gasoline or diesel fuels would be refined and delivered to the station with the current infrastructure. Onsite equipment would reform the fossil fuel into a mixture composed primarily of hydrogen, but could include other molecular components such as CO₂ and N₂. The purity of this hydrogen will depend on ongoing developments in techniques to cost-effectively separate H₂ from other gases. This hydrogen would likely then be delivered to customers as a high-pressure gas.

Finally, the fuel reforming process can be performed on a small scale on an as-needed basis immediately before its introduction into the fuel cell. One example would be for a fuel cell-powered vehicle to have a gasoline tank on board that would use the existing infrastructure of gasoline delivery. An on-board fuel processor would reform the gasoline into a hydrogen-rich stream that would be fed directly to the fuel cell. At the present time, it is not practical to perform separation of other products of the reforming process from the hydrogen at this small scale.

In the longer term, most, if not all, of the hydrogen used to power fuel cells could be generated from renewable resources such as wind or solar energy. The electricity generated at a wind farm could be used to split water into hydrogen and oxygen. This electrolysis process would produce pure hydrogen and pure oxygen. The hydrogen could then be delivered by pipeline to all end-users. Such a shift in source of energy has been described as a hydrogen economy. Much has been written about the future potential of this energy use.

Fuel Cell Functionality

Fuel cells generate electricity from a simple electrochemical reaction in which an oxidizer, typically oxygen from air, and a fuel, typically hydrogen, combine to form a product, which is water for the typical fuel cell. Oxygen (air) continuously passes over the cathode and hydrogen passes over the anode to generate electricity, by-product heat and water. The fuel cell itself has no moving parts – making it a quiet and reliable source of power.

The electrolyte that separates the anode and cathode is an ion-conducting material. At the anode, hydrogen and its electrons are separated so that the hydrogen ions (protons) pass through the electrolyte while the electrons pass through an external electrical circuit as a Direct Current (DC) that can power useful devices. The hydrogen ions combine with the oxygen at the cathode and are recombined with the electrons to form water. The reactions are shown below.

Anode Reaction: 2H₂ => 4H⁺ + 4e^-
Cathode Reaction: O₂ + 4H⁺ + 4e^- => 2H₂O
Overall Cell Reaction: 2H₂ + O₂ => 2H₂O

Individual fuel cells can then be combined into a fuel cell "stack." The number of fuel cells in the stack determines the total voltage, and the surface area of each cell determines the total current. Multiplying the voltage by the current will yield the total electrical power generated.

Power (Watts) = Voltage (Volts) X Current (Amps)

Fuel Cells vs. Traditional Electricity Methods

In traditional methods of generating electricity, the fuel and air are burned, generating a high-temperature gas. In the case of a coal-burning power plant, heat is transferred from this hot gas to high pressure liquid water that is boiled. In the case of a gasoline, diesel or gas turbine engine, the hot gas itself is at high pressure. The high-pressure steam, or hot gas, is expanded in a mechanical device (e.g., cylinder, turbine) and ultimately turns an electrical generator.

In a fuel cell, the same basic chemical reactions occur, but generate electricity directly as an electrochemical device and therefore, never goes through the step of being a high-temperature gas through normal burning. This direct conversion of chemical energy to electrical energy is more efficient and generates much less pollutants than do traditional methods that rely on combustion.

Which is Better?

As mentioned above, the direct conversion of fuel and air to electricity is much more efficient than internal combustion engines and other methods of generating electricity. Therefore, fuel cells can generate more electricity from the same amount of fuel.

Furthermore, by skipping the combustion process that occurs in traditional power-generating methods, the generation of pollutants during the combustion process is avoided. Some of the pollutants that are significantly lower for fuel cells are oxides of nitrogen and unburned hydrocarbons, (which together cause ground-level ozone), and carbon monoxide (a poisonous gas).