Alkaline anion-exchange membrane fuel cell

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 are 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. 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. == Comparison with traditional alkaline fuel cell ==

The alkaline fuel cell used by NASA in 1960s for Apollo and Space Shuttle program generated electricity at nearly 70% efficiency using aqueous solution of KOH as an electrolyte. In that situation, CO2 coming in through oxidant air stream and generated as by product from oxidation of methanol, if methanol is the fuel, reacts with electrolyte KOH forming CO32−/HCO3−. Unfortunately as a consequence, K2CO3 or KHCO3 precipitate on the electrodes. However, this effect has found to be mitigated by the removal of cationic counterions from the electrode, and carbonate formation has been found to be entirely reversible by several industrial and academic groups, most notably Varcoe. Low-cost CO2 systems have been developed using air as the oxidant source. In alkaline anion-exchange membrane fuel cell, aqueous KOH is replaced with a solid polymer electrolyte membrane, that can conduct hydroxide ions. This could overcome the problems of electrolyte leakage and carbonate precipitation, though still taking advantage of benefits of operating a fuel cell in an alkaline environment. In AAEMFCs, CO2 reacts with water forming H2CO3, which further dissociate to HCO3− and CO32−. The equilibrium concentration of CO32−/HCO3− is less than 0.07% and there is no precipitation on the electrodes in the absence of cations (K+, Na+). The absence of cations is, however, difficult to achieve, as most membranes are conditioned to functional hydroxide or bicarbonate forms out of their initial, chemically stable halogen form, and may significantly impact fuel cell performance by both competitively adsorbing to active sites and exerting Helmholtz-layer effects. In comparison, against alkaline fuel cell, alkali anion-exchange membrane fuel cells also protect the electrode from solid carbonate precipitation, which can cause fuel (oxygen/hydrogen) transport problem during start-up. The large majority of membranes/ionomer that have been developed are fully hydrocarbon, allowing for much easier catalyst recycling and lower fuel crossover. Methanol has an advantage of easier storage and transportation and has higher volumetric energy density compared to hydrogen. Also, methanol crossover from anode to cathode is reduced in AAEMFCs compared to PEMFCs, due to the opposite direction of ion transport in the membrane, from cathode to anode. In addition, use of higher alcohols such as ethanol and propanol is possible in AAEMFCs, since anode potential in AAEMFCs is sufficient to oxidize C-C bonds present in alcohols. However, high ion-exchange capacity leads to excessive swelling of polymer on hydration and concomitant loss of mechanical properties. Management of water in AEMFCs has also been shown to be a challenge. Recent research has shown that careful balancing of the humidity of the feed gases significantly improves fuel cell performance. ==See also==