Materials Science and Engineering/Diagrams/Power Generating Devices

Fuel Cell

A fuel cell is an electrochemical energy conversion device. It produces electricity from various external quantities of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.

Fuel cells are different from batteries in that they consume reactant, which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable.

Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.

Fuel Cell Design

In essence, a fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, hence converting them to electrical power. The catalyst is typically comprised of a platinum group metal or alloy. Another catalytic process takes the electrons back in, combining them with the protons and the oxidant to form waste products (typically simple compounds like water and carbon dioxide).

Types of Fuel Cell

Metal Hydride Fuel Cell

Metal hydride fuel cells are a subclass of alkaline fuel cells that are currently in the research and development phase. A notable feature is their ability to chemically bond and store hydrogen within the cell. This feature is shared with direct borohydride fuel cells, although the two differ in that MHFCs are refueled with pure hydrogen. Though the absorption characteristics of metal hydrides (around 2%) is far lower than sodium-borohydrides and other "light" metal hydrides (around 10,8%), prototypes have been claimed to demonstrate a number of interesting characteristics:

Ability to be recharged with electrical energy (similar to NiMH batteries);
Low operating temperatures (down to -20°C);
Fast kinetics;
Extended shelf life;
Fast "cold start" properties;
Ability to operate for limited periods of time with no external hydrogen source, enabling "hot swapping" of fuel canisters.

Electro-Galvanic Fuel Cell

A chemical reaction occurs in the fuel cell when the potassium hydroxide in the cell comes into contact with oxygen. This creates an electric current between the lead anode and the gold-plated cathode through a load resistance. The voltage produced is proportional to the concentration of oxygen present.

Formic Acid Fuel Cell

Direct-formic acid fuel cells or DFAFCs are a subcategory of proton-exchange fuel cells where, the fuel, formic acid, is not reformed, but fed directly to the fuel cell. Their applications include small, portable electronics such as phones and laptop computers.

Advantages

Similar to methanol, formic acid is a small organic molecule fed directly into the fuel cell, removing the need for complicated catalytic reforming. Storage of formic acid is much easier and safer than that of hydrogen because it does not need to be done at high pressures and (or) low temperatures, as formic acid is a liquid at ambient temperature.

There are two important advantages that formic acid possesses over methanol for use in the fuel cell. First, formic acid does not cross over the polymer membrane, so its efficiency can be higher than that of methanol. Second, formic acid does not cause blindness as does methanol, making it a somewhat safer fuel in case of leakage.

Zinc-Air Battery

Zinc-air batteries ( non-rechargeable), and zinc-air fuel cells, ( mechanically-rechargeable) are electro-chemical batteries powered by the oxidation of zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. They are used in hearing aids and in experimental electric vehicles. They may be an important part of a future zinc economy.

Particles of zinc are mixed with an electrolyte (usually potassium hydroxide solution); water and oxygen from the air react at the cathode and form hydroxyls which migrate into the zinc paste and form zincate (Zn(OH)42-), at which point electrons are released and travel to the cathode. The zincate decays into zinc oxide and water is released back into the system. The water and hydroxyls from the anode are recycled at the cathode, so the water serves only as a catalyst. The reactions produce a maximum voltage level of 1.65 volts, but this is reduced to 1.4–1.35 V by reducing air flow into the cell; this is usually done for hearing aid batteries to reduce the rate of water drying out.

Microbial Fuel Cell

A microbial fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature.

Generating Electricity

When micro-organisms consume a substrate such as sugar in aerobic conditions they produce carbon dioxide and water. However when oxygen is not present they produce carbon dioxide, protons and electrons as described below (Bennetto, 1990):

C₁₂H₂₂O₁₁ + 13H₂O ---> 12CO₂ + 48H⁺ + 48e^- Eqt. 1

Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and steal the electrons that are produced. The mediator crosses the outer cell lipid membranes and plasma wall; it then begins to liberate electrons from the electron transport chain that would normally be taken up by oxygen or other intermediates. The now-reduced mediator exits the cell laden with electrons that it shuttles to an electrode where it deposits them; this electrode becomes the electro-generic anode (negatively charged electrode). The release of the electrons means that the mediator returns to its original oxidised state ready to repeat the process. It is important to note that this can only happen under anaerobic conditions, if oxygen is present then it will collect all the electrons as it has a greater electronegativity than the mediator.

Reversible Fuel Cell

A reversible fuel cell (RFC) is a fuel cell that is designed to consume chemical A to produce electricity and chemical B and be reversed to consume electricity and chemical B to produce chemical A. A hydrogen fuel cell, for example, uses hydrogen (H2) and oxygen (O2) to produce electricity and water (H2O); a reversible hydrogen fuel cell could also use electricity and water to produce hydrogen and oxygen.

By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Fuel cells operated backwards generally do not make very efficient systems. Because of this, fuel cells operated in forward-reverse mode are not suited for energy storage systems in small and medium scale. Most fuel cells operated in the reverse mode are sold as learning kits or curiosities.

Direct Borohydride Fuel Cell

Direct borohydride fuel cells (DBFCs) are a subcategory of alkaline fuel cells that use a solution of sodium borohydride for fuel. The advantage of sodium borohydride over conventional hydrogen in an alkaline fuel cell is that the highly alkaline fuel and waste borax prevents poisoning of the fuel cell from carbon dioxide (CO2) in the air.

Sodium borohydride could potentially be used in more conventional hydrogen fuel cell systems as a means of storing hydrogen. The hydrogen can be regenerated for a fuel cell by catalytic decomposition of the borohydride:

$NaBH_{4}+2H_{2}O\rightarrow NaBO_{2}+4H_{2}$

Direct borohydride fuel cells decompose and oxidize the borohydride directly, side-stepping hydrogen production and even producing slightly higher energy yields:

Cathode: $2O_{2}+4H_{2}O+8e^{-}\rightarrow 8OH^{-}$ $2O_{2}+4H_{2}O+8e^{-}\rightarrow 8OH^{-}$
- $E_{0}=.4V\;$
Anode: $NaBH_{4}+8OH^{-}\rightarrow NaBO_{2}+6H_{2}O+8e^{-}$ $NaBH_{4}+8OH^{-}\rightarrow NaBO_{2}+6H_{2}O+8e^{-}$
- $E_{0}=1.24V\;$
- Total $E_{0}=1.64V\;$

DBFCs could be produced more cheaply than a traditional fuel cell because they do not need expensive platinum catalysts. In addition, they have a higher power density. Unfortunately, DBFCs do produce some hydrogen from a side step reaction of NaBH4 with water heated by the fuel cell. This hydrogen can either be piped out to the exhaust or piped to a conventional hydrogen fuel cell. Either fuel cell will produce water, and the water can be recycled to allow for higher concentrations of NaBH4.

After releasing its hydrogen and being oxidized, NaBO2 or borax is produced. Borax is a common detergent and soap additive and is relatively non-toxic. Borax can be hydrogenated back into borohydride fuel by several different techniques, some of which require nothing more than water and electricity or heat. These techniques are still in active development.

Sodium borohydride costs US$50 per kg, but with borax recycling and mass production projected prices for the fuel are as low as US$1/kg.

Alkaline Fuel Cell

The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, is one of the most developed fuel cell technologies and is the cell that flew Man to the Moon. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. AFCs consume hydrogen and pure oxygen producing potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.

Direct Methanol Fuel Cell

Direct-methanol fuel cells or DMFCs are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is not reformed, but fed directly to the fuel cell. Because methanol is fed directly into the fuel cell, complicated catalytic reforming is unneeded. Storage of methanol is much easier than that of hydrogen because it does not need to be done at high pressures or low temperatures, as methanol is a liquid from -97.0 °C to 64.7 °C (-142.6 °F to 148.5 °F). The energy density of methanol, the amount of energy contained in a given volume of methanol, is an order of magnitude greater than even highly compressed hydrogen.

However, the efficiency of direct-methanol fuel cells is low due to the high permeation of methanol through the membrane, which is known as methanol crossover, and the dynamic behaviour is sluggish. Other problems include the management of carbon dioxide created at the anode. Current DMFCs are limited in the power they can produce, but can still store a high energy content in a small space. This means they can produce a small amount of power over a long period of time. This makes them ill-suited for powering vehicles, but ideal for consumer goods such as mobile phones, digital cameras or laptops.

Reactions

The DMFC relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Positive ions (H+) are transported across the proton exchange membrane (often Nafion) to the cathode where they react with oxygen to produce water. Electrons are transported via an external circuit from anode to cathode providing power to external devices.

The half-reactions are:

Anode: CH₃OH + H₂O → CO₂ + 6H⁺ + 6e^-

The methanol is adsorbed on a catalyst, usually made of platinum particles, and deprotonized until carbon dioxide is formed. Usually, the catalyst consists of another metallic component, usually ruthenium, which is used to catalyze methanol oxidation (see last paragraph for more details.

Cathode: (3/2)O₂ + 6H⁺ + 6e^- → 3H₂O

Net reaction: CH₃OH + (3/2)O₂ → CO₂ + 2H₂O

Because water is consumed at the anode in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back diffusion (osmosis), or active transport such as pumping. The need for water limits the energy density of the fuel.

Currently, platinum is used as a catalyst for both half-reactions. This contributes to the loss of cell voltage potential, as any methanol that is present in the cathode chamber will oxidize. If another catalyst could be found for the reduction of oxygen, the problem of methanol crossover would likely be significantly lessened. Furthermore, platinum is very expensive and contributes to the high cost per kilowatt of the fuel cell.

In one of the steps of the methanol oxidation reaction, a CO species is produced, which adsorbs strongly on the platinum catalyst, reducing the surface area for the catalyst reaction. The addition of another components, such as ruthenium or gold, to the catalyst, tends to ameliorate this problem because, according to the most well-established theory in the field, these catalysts oxidize water to yield OH radicals: H2O → OH• + H+ + e-. The OH species from the oxidized water molecule oxidizes CO to produce CO2 which can then be released as a gas: CO + OH• → CO2 + H+ + e-.

Reformed Methanol Fuel Cell

Reformed methanol fuel cell systems or RMFCs are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is reformed, before being fed into the fuel cell. RMFC systems offer advantages over DMFC systems including higher efficiency, smaller cell stacks, no water management, better operation at low temperatures, and storage at sub-zero[vague] temperatures. The tradeoff is that RMFC systems operate at hotter temperatures and therefore need more advanced heat management and insulation.

Direct-Ethanol Fuel Cell

Direct-ethanol fuel cells or DEFCs are a subcategory of Proton-exchange fuel cells where, the fuel, ethanol, is not reformed, but fed directly to the fuel cell.

Advantages

DEFC uses Ethanol in the fuel cell instead of the more toxic methanol. Ethanol is an attractive alternative to methanol because it comes with a supply chain that's already in place. Ethanol also remains the easier fuel to work with for widespread use by consumers.

Proton Exchange Membrane

Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells (PEMFC), are a type of fuel cell being developed for transport applications as well as for stationary and portable applications. Their distinguishing features include lower temperature/pressure ranges and a special polymer electrolyte membrane.

Molten Carbonate Fuel Cell

Molten-carbonate fuel cells (MCFCs) are high-temperature fuel cells, that operate at temperatures of 600°C and above.

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix of beta-alumina solid electrolyte (BASE). Since they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Protonic Ceramic Fuel Cell

The protonic ceramic fuel cell or PCFC is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures.