An alkaline anion exchange membrane fuel cell (AAEMFC), also known as anion-exchange membrane fuel cells (AEMFCs), alkaline membrane fuel cells (AMFCs), hydroxide exchange membrane fuel cells (HEMFCs), or solid alkaline fuel cells (SAFCs) is a type of alkaline fuel cell that uses an anion exchange membrane to separate the anode and cathode compartments.
Alkaline fuel cells (AFCs) are based on the transport of alkaline anions, usually hydroxide , between the electrodes. Original AFCs used aqueous potassium hydroxide (KOH) as an electrolyte. The AAEMFCs use instead a polymer membrane that transports hydroxide anions.
Mechanism
In an AAEMFC, the fuel, hydrogen or methanol, is supplied at the anode and oxygen through air, and water are supplied at cathode. Fuel is oxidized at anode and oxygen is reduced at cathode. At cathode, oxygen reduction produces hydroxides ions (OH−) that migrate through the electrolyte towards the anode. At anode, hydroxide ions react with the fuel to produce water and electrons. Electrons go through the circuit producing current.
Electrochemical reactions when hydrogen is the fuel
At Anode: H2 + 2OH− → 2H2O + 2e−
At cathode: O2 + 2H2O + 4e− → 4OH−
Electrochemical reactions when methanol is the fuel
At anode: CH3OH + 6OH− → CO2 + 5H2O + 6e-
At cathode: 3/2O2 + 3H2O + 6e− → 6OH−
Mechanical Properties
Measuring mechanical properties
The mechanical properties of anion exchange membranes are relevant for use in electrochemical energy technologies such as polymer electrolyte membranes in fuel cells. Mechanical properties of polymers include the elastic modulus, tensile strength, and ductility. Traditional tensile stress-strain test used to measure these properties are very sensitive to the experimental procedure because the mechanical properties of polymers are heavily dependent on the nature of the environment such as the presence of water, organic solvents, oxygen, and temperature. Increasing the temperature generally results in a decrease of elastic modulus, a reduction of tensile strength, and an increase of ductility, assuming there is no modification of the microstructure. Near the glass transition temperature, very significant changes in mechanical properties is observed. Dynamic Mechanical Analysis (DMA) is a widely used complimentary, characterization technique for measuring the mechanical properties of polymers including the storage modulus and loss modulus as functions of temperature.
Methods of Increasing Mechanical Properties
One method of increasing the mechanical properties of polymers used for anion exchange membranes (AEM) is substituting conventional ternary amine and anion exchange groups with grafted quaternary groups. These ionomers results in large storage and Young's moduli, a high tensile strength, and high ductility. Exchanging the counterion from hydroxide to hydrogen carbonate, carbonate, and chloride ions further enhances the strength and elastic modulus of the membranes. Narducci and colleagues concluded that the water uptake, related to the type of anion, plays a very important role for the mechanical properties. Zhang and colleagues prepared a series of robust and crosslinked poly(2,6-dimethyl-1,4-phenylene oxide)s (PPO) AEMs with chemically stable imidazolium cations through quaternization of C1, C3, C4-substituted imidazole and crosslinking them via "thiol-ene" chemistry. These crosslinked AEMs showed excellent film forming properties and exhibited a higher tensile strength owing to the increased entanglement interactions in the polymer chains which in turn increased the water up take. This strong relation between water uptake and mechanical properties mirrors the findings of Narducci and colleagues. The findings of Zhang et al. suggest that the crosslinking of anion conductive materials with stable sterically-protected organic cations is an effective strategy to produce robust AEMs for use in alkaline fuel cells.