: :  What Are Fuel Cells

What Are Fuel Cells?

A fuel cell is a generator that produces electricity by way of a chemical reaction.  It is an electrochemical cell that reacts a fuel with an oxidant in the presence of an electrolyte to create an electrical current.  This electrical current can then be used to power an external electronic device.  A fuel cell is analogous to a battery but with a crucial distinction: a battery is a thermodynamically closed system i.e. it stores chemical energy contained within the battery itself; a fuel cell, however, consumes a reactant from an external source.  Hydrogen, methanol, biogas, liquid hydrocarbons, and natural gas can all be used as its fuel.  As long as the reactant and oxidant are maintained, the fuel cell can be run continuously.

Fuel cells can vary greatly in power; from 1 mW to over 1 MW.  They are silent, vibration free, and, if pure hydrogen is used as a fuel source, emit only water and heat; there are no particulates or aromatics (note that if a hydrocarbon fuel is used, carbon dioxide will also be emitted).  Since heat is also produced as a by-product, systems can be designed to operate in cogeneration mode and be used wherever electricity and/or heat is needed, whether stationary or mobile.

Fuel cells can come in many varying forms such as: alkali, molten carbonate, phosphoric acid, solid oxide, and proton exchange membrane (PEM) fuel cells.  Each type of cell works slightly differently but they all work on the same basic concept.  To illustrate this, below is a description of how a PEM fuel cell works.

How a Proton Exchange Membrane (PEM) Fuel Cell works

• Hydrogen in its natural form (H2) enters the fuel cell at the anode while oxygen (O2) is pumped to the cathode.

• Using a catalyst on the anode, the H2 molecule is split into its two constituent atoms. The electrons (e-) are then stripped off the H atoms exposing the proton nucleus (H+).

• Excess hydrogen leaves the fuel cell to be recycled.

• The PEM allows only positively charged ions to pass through it to the cathode. The electrons, unable to pass through the membrane, are forced through an external circuit to the cathode, creating an electrical current to power the desired device(s).

• At the cathode the electrons combine with two hydrogen protons and an oxygen atom to form water.

• Water and heat are then given out as the only by-products.

Fig 1. A PEM fuel cell [1].

Types of Fuel Cell

Alkali fuel cells consume hydrogen and oxygen whilst producing water, heat, and electricity.  The electrolyte commonly used is potassium hydroxide (KOH) in water.  Their efficiency is about 70 %, making them one of the most efficient fuel cells available, their power output ranges from 300 W to 5 kW, and their operating temperature is within the range of 150 to 200 °C.  Interestingly, alkali fuel cells were used in the Apollo-series spacecrafts and the Space Shuttle program as a means of providing electricity and drinking water.  These fuel cells operate using pure hydrogen fuel and platinum electrode catalysts which are expensive.  

Molten Carbonate fuel cells (MCFC) operate using an electrolyte composed of a molten carbonate salt compound.  Examples of these include sodium or magnesium carbonates.  Their efficiency typically ranges from 60 to 80 % . Cells with an output of up to 2 MW have been built with future designs offering systems up to 100 MW.  The fuel cells operate at about 650 °C; this high temperature limits any damage caused by carbon monoxide and excess heat can be recycled to make additional electricity.  The nickel electrode catalysts used are relatively cheap but due to such a high operating temperature, they may be unsafe for certain applications; use in the home would typically not be recommended.  An additional consideration is that carbon dioxide needs to be injected to compensate for the fact that carbonate ions are used up in the reactions.

Phosphoric Acid fuel cells (PAFC) use liquid phosphoric acid for their electrolyte. Their efficiency can range from 40 to 80 %, power output is up to 200 kW with R & D designs providing 11 MW , and operating temperatures range from 150 to 200 °C.  Their design allows for concentrations of approximately 1.5 % carbon monoxide, thus allowing a wider choice of fuels to be used.  These cells use platinum electrode catalysts and their internal components must be capable of withstanding the corrosive acid used.  Since the electrodes consist of carbon paper coated with a platinum catalyst, they are expensive to manufacture.  However, given their operating temperature, expelled water can be converted to steam and then used for heating purposes thus providing a high co-generation efficiency of approximately 80 %.  Due to these attributes PAFC dominate the on-site stationary fuel cell market.

Proton Exchange Membrane (PEM) fuel cells, as described above, work using a polymer electrolyte.  Their efficiency is typically around 40 to 50 % , their output power ranges from 50 to 250 kW, and they operate at temperatures of about 50 to 100 °C.  Since the electrolyte used is solid and flexible, it will not fracture or leak (a potential problem in other cells using liquid electrolytes).  Also, due to their relatively low operating temperature, they can be safely used in automobiles or residential homes.  Platinum catalysts are, however, used on both sides of the membrane and thus increase financial cost.

Solid Oxide fuel cells (SOFC) use a hard or ceramic metal oxide electrolyte.  This may typically consist of a calcium or zirconium metal.  An advantage of this type of fuel cell is its relatively high efficiency (about 60 %), it is also of low cost, low emissions, has long-term stability, and flexibility in its choice of fuel source.  A distinct disadvantage is its very high operating temperature (around 1000 °C) which limits applications and can result in a long start-up time when compared to other cells.  At these temperatures, however, a reformer is not required to extract hydrogen from the fuel and excess heat can utilised in a beneficial way.  Given the solid state of the electrolyte it will not leak but is susceptible to fractures.

Ref [1]: http://ww1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.htmlw