Proton Exchange Membrane Fuel Cells (PEMFC)

Proton Exchange Membrane Fuel Cells or Polymer Electrolyte Membrane Fuel Cells are the quintessential fuel cell and that which is most often implied when the term “fuel cell” is used. They operate on a relatively simple principle of extracting protons and electrons from hydrogen atoms.

General Operation of PEMFCs

At the anode, hydrogen is broken down to yield a single proton and single electron. The source of the hydrogen for this step is what determines the subtype of PEMFC. For instance, pure liquid hydrogen is used in hydrogen fuel cells while hydrogen derived from methanol is used in methanol fuel cells (whether direct or indirect).

After the proton and electron are separated, the proton is free to travel through the proton exchange membrane (also called a polymer eletrolyte membrane and both abbreviated PEM). The PEM is most commonly made of perfluorosulfonic acid, which acts as the electrolyte in these fuel cells. The proton moves to the cathode side of the fuel cell, leaving the electron behind.

The electron is unable to cross the PEM and as a result cannot reach the cathode, which is positively charged thanks to all of the protons migrating through the PEM. This difference in charges sets up an electrochemical gradient, which is commonly referred to as a voltage. A voltage is nothing more than an imbalance in positive and negative charge that can be used to move electrons around.

Once an external circuit is created, electrons will flow through it to the cathode, doing useful work on their way. PEMFCs all rely on platinum or other precious metal catalysts to boost the reaction that breaks apart hydrogen. This allows these fuel cells to operate at relatively low temperatures (less than 40 C to 250 C).

Benefits of PEMFCs

Proton exchange membrane fuel cells can operate at temperatures of 80 to 100 C, which is a tremendous benefit when compared to high temperature fuel cells. The ability to operate at low temperatures means short warm-up periods, which makes PEMFCs suitable for transportation solutions.

Depending on the fuel type, PEMFCs are also environmentally friendly. Those that burn pure hydrogen produce only water as an end product.

PEMFCs have high power densities. They have power ranges from 5 watts to well over 500 kilowatts. What is more, this power is generated in a relatively small volume fuel cell when compared to other types like solid oxide or molten carbonate. This high power density makes PEMFCs ideal for use in transportation.

Drawbacks of PEMFCs

There are several challenges to overcome with PEMFCs, the most substantial of which is the expense and rarity of the catalysts used. Platinum or a similar catalyst is necessary to facilitate the breakdown or hydrogen. Without a catalyst, the reaction proceeds to slow to be useful in generating electricity.

Another problem with platinum is that it is sensitive to carbon monoxide, which is a problem in any PEMFC that does not utilize pure hydrogen. Carbon monoxide at a level of even 1 part per million can “poison” the platinum catalyst and dramatically lower its efficiency. There is ongoing research to find alternatives to platinum, the most promising of which is an iron-based catalyst.

In Quebec, Canada, researchers at the Institut National de la Recherche Scientificque have been able to improve the efficiency of an iron-based catalyst by 35 fold. This has allowed this far less expensive catalyst to perform as well as platinum. Its widespread use, however, is held back by the fact that it is extremely fragile. After only 100 hours of operation, its efficiency drops by half.

Water management is a critical component of PEMFCs and a factor that makes these fuel cells somewhat more complex and less durable. The polymer electrolyte membrane (PEM) looks much like a sheet of plastic that has been specially treated to allow only protons to pass through it. To work properly, the PEM requires a certain saturation level of water. Too much or too little water will affect the efficiency of a PEMFC and can drastically affect the lifespan of the fuel cell.

The final challenge to overcome with PEMFCs (and fuel cells in general) is cost. The U.S Department of Energy has estimated that fuel cells will not be economically viable unless they reduce their use of platinum four fold. Currently, PEMFCs can produce electricity a rate of $100 per kilowatt, even with efficiencies of economy factored in. To be competitive in price with internal combustion engines, the cost would have to drop to $35 per kilowatt. There are currently experiments aimed at altering the size and shape of platinum catalysts to see if the quantity of platinum in a PEMFC can be reduced.

PEMFC Subtypes

Name

Fuel

Operating Characteristics

Direct Formic Acid Fuel Cell (DFAFC)

Formic Acid

Less than 40 C with production capacity of 50 W max

Direct Methanol Fuel Cell

Methanol

90 to 120 C with production capacity of 100 mW to 1 kW

Direct Ethanol Fuel Cell

Ethanol

90 to 120 C with production capacity of 140 mW per square centimeter

Reformed Fuel Cell

Hydrogen extracted from Methanol or Ethanol

250 to 300 C with production capacity of 5 W to 100 kW

Hydrogen Fuel Cell

Hydrogen

50 to 220 C with production capacity of 100 W to 500 kW

Microbial Fuel Cell

Organic matter

Less than  40 C

Regenerative Fuel Cell

A fuel cell run in reverse, which basically makes it a battery

Less than 50 C