Redox(II): Fuel Cells

Edexcel A level Chemistry (2017)

Topic 14: Redox(II): Fuel Cells

Here are the learning objectives relating to fuel cells:

14/16. To be able to understand that the energy released on the reaction of a fuel with oxygen is utilized in a fuel cell to generate a voltage.

Knowledge that methanol and other hydrogen-rich fuels are used in fuel cells is expected.

14/17. To know the electrode reactions that occur in a hydrogen-oxygen fuel cell.

Knowledge of hydrogen-oxygen fuel cells with both acidic and alkaline electrolytes is expected.

The Proton Exchange Fuel Cell

Proton exchange membrane fuel cells are a type of fuel cell being developed to replace conventional alkaline fuel cells of the type used in the Space Shuttle.

Their distinguishing features include lower temperature ranges (50 to 100 °C) and a special polymer electrolyte membrane.

How the proton exchange membrane fuel cell works

Fuel cells essentially transform the chemical energy released in the reaction between hydrogen and oxygen into electrical energy.

A stream of hydrogen is delivered to the cathode side of the cell. At the cathode side it is catalytically split into protons and electrons. An oxidation half—cell reaction takes place reaction (loss of electrons) at the cathode:

H₂ ⟶  2H⁺   +   2e^—

The newly formed protons permeate through the polymer electrolyte membrane to the anode side. The electrons travel along an external circuit to the anode side of the cell.

Meanwhile, a stream of oxygen is delivered to the anode side of the cell.

At the anode side, oxygen molecules react with the protons permeating through the polymer membrane and the electrons arriving by the external circuit and combine to form water molecules.

A reduction half-cell reaction or oxygen reduction takes place at the anode:

½O₂  + 2H⁺   +  2e^—  ⟶  H₂O

The overall effect is for hydrogen and oxygen to combine to form water:

H₂  +   ½O₂    ⟶  H₂O

Properties of the polymer membrane

To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "short circuit” the fuel cell.

The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover.

Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode.

The Direct Methanol Fuel Cell

Direct-methanol fuel cells or DMFCs are similar to proton exchange fuel cells but methanol is used as the fuel. Their main advantage is the ease of transport of methanol (over hydrogen gas) because methanol is an energy-dense yet reasonably stable liquid in most environmental conditions.

Methanol is a liquid from -97.0 °C to 64.7 °C at atmospheric pressure. The energy density of methanol is an order of magnitude greater than even highly compressed hydrogen, and 15 times higher than lithium ion batteries.

The chemistry of how the DMFC works

The DMFC relies upon the oxidation of methanol on a Platinum/Ruthenium catalyst layer to form carbon dioxide.  

Water is consumed at the anode and is produced at the cathode. Protons (H⁺) are transported across the proton exchange membrane—often made from polymer—to the cathode where they react with oxygen to produce water. 

Electrons flow through the external circuit from cathode (—) to anode (+), providing power to connected devices.

Cathode (—) reaction:  CH₃OH   +   H₂O  ⟶  6H⁺   + 6e^—  +  CO₂  Oxidation

Anode (+) reaction:  1½O₂   +   6H⁺   +   6e^—  ⟶   3H₂O

Reduction

Overall reaction:  CH₃OH    +   1½O₂   ⟶   2H₂O   +   CO₂

You can see that the overall reaction is just the combustion of methanol in pure oxygen.

Methanol and water are adsorbed on a catalyst usually made of platinum and ruthenium particles, and lose protons until carbon dioxide is formed. 

As water is consumed at the cathode in the reaction, pure methanol cannot be used without provision of water via either passive transport such as osmosis or pumping. The need for water limits the energy density of the fuel.

Cell efficiency is quite low, so they are targeted at portable applications, where energy and power density are more important than efficiency.