Fuel Cells

Prof. Park

UTI-111

Essex County College

What Is A Fuel Cell?

• In principle, a fuel cell operates like a battery. Unlike a battery, a fuel cell does not run down or require recharging. It will produce energy in the form of electricity and heat as long as fuel is supplied.

• A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat.

What Is A Fuel Cell?

What Is A Fuel Cell?

• Hydrogen fuel is fed into the "anode" of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode.

• Encouraged by a catalyst, the hydrogen atom splits into a proton and an electron, which take different paths to the cathode.

• The proton passes through the electrolyte. The electrons create a separate current that can be utilized before they return to the cathode, to be reunited with the hydrogen and oxygen in a molecule of water.

What Is A Fuel Cell?

• A fuel cell system which includes a "fuel reformer" can utilize the hydrogen from any hydrocarbon fuel - from natural gas to methanol, and even gasoline.

• Since the fuel cell relies on chemistry and not combustion, emissions from this type of a system would still be much smaller than emissions from the cleanest fuel combustion processes.

Types of Fuel Cells

1. Phosphoric Acid 2. Proton Exchange Membrane 3. Molten Carbonate 4. Solid Oxide 5. Alkaline 6. Direct Methanol 7. Regenerative8. Zinc Air 9. Protonic Ceramic 10.Microbial Fuel Cell

Phosphoric Acid fuel cell (PAFC)

• Phosphoric acid fuel cells are commercially available today.• Hundreds of fuel cell systems have been installed in 19 nations - in

hospitals, nursing homes, hotels, office buildings, schools, utility power plants, landfills and waste water treatment plants.

• PAFCs generate electricity at more than 40% efficiency - and nearly 85% of the steam this fuel cell produces is used for cogeneration - this compares to about 35% for the utility power grid in the United States.

• Phosphoric acid fuel cells use liquid phosphoric acid as the electrolyte and operate at about 450°F.

• One of the main advantages to this type of fuel cell, besides the nearly 85% cogeneration efficiency, is that it can use impure hydrogen as fuel.

• PAFCs can tolerate a CO concentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must be removed.

Proton Exchange Membrane fuel cell (PEM)

• These fuel cells operate at relatively low temperatures (about 175°F), have high power density, can vary their output quickly to meet shifts in power demand, and are suited for applications, such as in automobiles, where quick startup is required.

• According to the U.S. Department of Energy (DOE), "they are the primary candidates for light-duty vehicles, for buildings, and potentially for much smaller applications such as replacements for rechargeable batteries."

• This type of fuel cell is sensitive to fuel impurities. Cell outputs generally range from 50 watts to 75 kW.

Molten Carbonate fuel cell (MCFC)

• Molten carbonate fuel cells use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert matrix, and operate at high temperatures - approximatelly 1,200ºF.

• They require carbon dioxide and oxygen to be delivered to the cathode.

• To date, MCFCs have been operated on hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine diesel, and simulated coal gasification products.

• 10 kW to 2 MW MCFCs have been tested on a variety of fuels and are primarily targeted to electric utility applications.

Solid Oxide fuel cell (SOFC)

• Solid oxide fuel cells use a hard, non-porous ceramic compound as the electrolyte, and operate at very high temperatures - around 1800°F.

• One type of SOFC uses an array of meter-long tubes, and other variations include a compressed disc that resembles the top of a soup can.

• Tubular SOFC designs are closer to commercialization and are being produced by several companies around the world.

• SOFCs are suitable for stationary applications as well as for auxiliary power units (APUs) used in vehicles to power electronics.

Alkaline fuel cell (AFC)

• Long used by NASA on space missions, alkaline fuel cells can achieve power generating efficiencies of up to 70 percent.

• They were used on the Apollo spacecraft to provide both electricity and drinking water.

• Alkaline fuel cells use potassium hydroxide as the electrolyte and operate at 160°F.

• However, they are very susceptible to carbon contamination, so require pure hydrogen and oxygen.

Direct Methanol fuel cell (DMFC)

• These cells are similar to the PEM cells in that they both use a polymer membrane as the electrolyte.

• However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer.

• Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120-190°F.

• This is a relatively low range, making this fuel cell attractive for tiny to mid-sized applications, to power cellular phones and laptops.

• Higher efficiencies are achieved at higher temperatures. • Companies are also working on DMFC prototypes to be

used by the military for powering electronic equipment in the field.

Regenerative fuel cell

• Regenerative fuel cells are attractive as a closed-loop form of power generation.

• Water is separated into hydrogen and oxygen by a solar-powered electrolyzer.

• The hydrogen and oxygen are fed into the fuel cell which generates electricity, heat and water.

• The water is then recirculated back to the solar-powered electrolyzer and the process begins again.

• These types of fuel cells are currently being researched by NASA and others worldwide.

Zinc Air fuel cell (ZAFC)

• In a typical zinc/air fuel cell, there is a gas diffusion electrode (GDE), a zinc anode separated by electrolyte, and some form of mechanical separators.

• The GDE is a permeable membrane that allows atmospheric oxygen to pass through.

• After the oxygen has converted into hydroxyl ions and water, the hydroxyl ions will travel through an electrolyte, and reaches the zinc anode.

• Here, it reacts with the zinc, and forms zinc oxide. • This process creates an electrical potential; when a set

of ZAFC cells are connected, the combined electrical potential of these cells can be used as a source of electric power.

• This electrochemical process is very similar to that of a PEM fuel cell, but the refueling is very different and shares characteristics with batteries.

Zinc Air fuel cell (ZAFC)

• ZAFCs contain a zinc "fuel tank" and a zinc refrigerator that automatically and silently regenerates the fuel.

• In this closed-loop system, electricity is created as zinc and oxygen are mixed in the presence of an electrolyte (like a PEMFC), creating zinc oxide.

• Once fuel is used up, the system is connected to the grid and the process is reversed, leaving once again pure zinc fuel pellets.

• The key is that this reversing process takes only about 5 minutes to complete, so the battery recharging time hang up is not an issue.

• The chief advantage zinc-air technology has over other battery technologies is its high specific energy, which is a key factor that determines the running duration of a battery relative to its weight.

Protonic Ceramic fuel cell (PCFC)

• This new type of fuel cell is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures.

• PCFCs share the thermal and kinetic advantages of high temperature operation at 700 degrees Celsius with molten carbonate and solid oxide fuel cells, while exhibiting all of the intrinsic benefits of proton conduction in PEM and phosphoric acid fuel cells.

• The high operating temperature is necessary to achieve very high electrical fuel efficiency with hydrocarbon fuels.

• PCFCs can operate at high temperatures and electrochemically oxidize fossil fuels directly to the anode.

• This eliminates the intermediate step of producing hydrogen through the costly reforming process.

Protonic Ceramic fuel cell (PCFC)

• Gaseous molecules of the hydrocarbon fuel are absorbed on the surface of the anode in the presence of water vapor, and hydrogen atoms are efficiently stripped off to be absorbed into the electrolyte, with carbon dioxide as the primary reaction product.

• Additionally, PCFCs have a solid electrolyte so the membrane cannot dry out as with PEM fuel cells, or liquid can't leak out as with PAFCs.

Microbial fuel cell (MFC) • Microbial fuel cells use the catalytic reaction of microorganisms

such as bacteria to convert virtually any organic material into fuel.  • Some common compounds include glucose, acetate, and

wastewater.  • Enclosed in oxygen-free anodes, the organic compounds are

consumed (oxidized) by the bacteria or other microbes.  • As part of the digestive process, electrons are pulled from the

compound and conducted into a circuit with the help of an inorganic mediator. 

• MFCs operate well in mild conditions relative to other types of fuel cells, such as 20-40 degrees Celsius, and could be capable of producing over 50% efficiency. 

• These cells are suitable for small scale applications such as potential medical devices fueled by glucose in the blood, or larger such as water treatment plants or breweries producing organic waste that could then be used to fuel the MFCs. 

Applications of Fuel Cells

• Stationary: Hotel, College, House– Telecommunications – Landfills/Wastewater Treatment Plants/Breweries

• Transportation – Cars, Buses, and Scooters– Forklifts/Materials Handling– Auxiliary Power Units (APUs)– Trains, Planes, and Boats

• Portable Power: campsite, military • Micro Power: Consumer Electronics

Applications of Fuel Cells• There are many uses for fuel cells — right now, all of the

major automakers are working to commercialize a fuel cell car.

• Fuel cells are powering buses, boats, trains, planes, scooters, forklifts, even bicycles.

• There are fuel cell-powered vending machines, vacuum cleaners and highway road signs.

• Miniature fuel cells for cellular phones, laptop computers and portable electronics are on their way to market.

• Hospitals, credit card centers, police stations, and banks are all using fuel cells to provide power to their facilities.

• Wastewater treatment plants and landfills are using fuel cells to convert the methane gas they produce into electricity.

• Telecommunications companies are installing fuel cells at cell phone, radio and 911 towers. The possibilities are endless.

Benefits of Fuel Cells

• Low to Zero Emissions• High Efficiency • High Reliability/High Quality Power • Fuel Flexibility• Security • Modularity/Scalability/Flexible Siting • Lightweight/Long-lasting Battery

Alternative

Fuel Cell Name Electrolyte Qualified Power (W)Working Temperature (°C)

Electrical efficiency

Metal hydride fuel cell Aqueous alkaline solution (e.g.potassium hydroxide)above -20

(50% Ppeak @

0°C)

Electro-galvanic fuel cell Aqueous alkaline solution (e.g., potassium hydroxide) under 40

Direct formic acid fuel cell (DFAFC) Polymer membrane (ionomer) to 50 W under 40

Zinc-air battery Aqueous alkaline solution (e.g., potassium hydroxide) under 40

Microbial fuel cell Polymer membrane or humic acid under 40

Upflow microbial fuel cell (UMFC) under 40

Regenerative fuel cell Polymer membrane (ionomer) under 50

Direct borohydride fuel cell Aqueous alkaline solution (e.g., sodium hydroxide) 70

Alkaline fuel cell Aqueous alkaline solution (e.g., potassium hydroxide) 10 kW to 100 kW under 80Cell: 60–70%System: 62%

Direct methanol fuel cell Polymer membrane (ionomer) 100 mW to 1 kW 90–120Cell: 20–30%

System: 10–20%

Reformed methanol fuel cell Polymer membrane (ionomer) 5 W to 100 kW(Reformer)250

–300(PBI)125–200

Cell: 50–60%System: 25–40%

Direct-ethanol fuel cell Polymer membrane (ionomer) up to 140 mW/cm²above 25? 90–120

Proton exchange membrane fuel cellPolymer membrane (ionomer) (e.g., Nafion or Polybenzimidazole fiber)

100 W to 500 kW(Nafion)50–

120(PBI)125–220

Cell: 50–70%System: 30–50%

Fuel Cell Name Electrolyte Qualified Power (W)Working Temperature (°C)

Electrical efficiency

RFC - RedoxLiquid electrolytes with redox shuttle & polymer membrane (Ionomer)

1 kW to 10 MW

Phosphoric acid fuel cell Molten phosphoric acid (H3PO4) up to 10 MW 150-200Cell: 55%

System: 40%Co-Gen: 90%

Molten carbonate fuel cellMolten alkaline carbonate (e.g., sodium bicarbonate NaHCO3)

100 MW 600-650Cell: 55%

System: 47%

Tubular solid oxide fuel cell (TSOFC)O2--conducting ceramic oxide (e.g., zirconium dioxide, ZrO2)

up to 100 MW 850-1100Cell: 60–65%

System: 55–60%

Protonic ceramic fuel cell H+-conducting ceramic oxide 700

Direct carbon fuel cell Several different 700-850Cell: 80%

System: 70%

Planar Solid oxide fuel cell

O2--conducting ceramic oxide (e.g., zirconium dioxide, ZrO2 Lanthanum Nickel Oxide

La2XO4,X= Ni,Co, Cu.)up to 100 MW 850-1100

Cell: 60–65%System: 55–60%

Enzymatic Biofuel CellsAny that will not denature the enzyme (usually aqueous buffer).

under 40