Carbonate and Bicarbonate Ion Transport in Alkaline Anion Exchange Membranes

Abstract

Anion exchange membranes (AEMs) are being developed for potential use in fuel cell systems which include portable power applications. In a fuel cell, these membranes transport hydroxide ions from the cathode to the anode. If carbon dioxide is present, carbonate and bicarbonate ions can form, displacing the hydroxide ions. Among the challenges this presents, the carbonate and bicarbonate are less mobile than the hydroxide and therefore the ionic conductivity of the membrane suffers. A procedure is outlined to take data from a permeation based water flux experiment and determine diffusion coefficients and the ionic conductivity of the membrane. The water-membrane diffusion coefficients can be measured from a water flux experiment. Using principles from kinetic theory, the water-membrane diffusion coefficient can be converted to an appropriate ion-membrane diffusion coefficient. Finally, an equation derived from the dusty fluid model can be used to calculate the ionic conductivity of the membrane in different counter ion forms. The calculated ionic conductivities have been shown to agree well with reported values for proton and anion exchange membranes.Anion exchange membrane fuel cells (AEMFCs) have received increased attention in recent years. The AEMFC typically operates at low temperatures, below 80 • C, and can utilize alcohol fuels; making it of possible appeal for portable power applications. Operating in a high pH environment allows for favorable alcohol oxidation kinetics and the ability to use non-platinum oxygen reduction catalysts. 1,2 Despite recent improvements, there are still several challenges confronting the technology. The low hydroxide ionic conductivity of the AEM and the formation of carbonate and bicarbonate species which further reduce the membrane's ionic conductivity are two such challenges that are examined in this study. 3,4 Current AEMs often use a polymer hydrocarbon backbone with benzyl-trimethylammonium fixed side chain groups. This cation is a strong base (pK b ≈ 1) which allows for reasonable dissolution of the hydroxide ions (OH − ) from the membrane and easy transport through the membrane. 5 The polymer backbone can range from several polymers including poly(ethylene-co-tetrafluoroethylene) (ETFE), poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), polypropylene, and polysulphone. 6-10 In one study, a fully hydrated AEM with an ETFE backbone was reported to have an ionic conductivity of roughly 30 mS/cm at 30 • C. When comparing this to Nafion 115 proton exchange membrane (PEM), which has a similar IEC, the PEM has a much higher ionic conductivity around 90 mS/cm. 7 If carbon dioxide (CO 2 ) is present, then the formation of carbonate (CO 3 −2 ) and bicarbonate (HCO 3 − ) ions can affect the membrane in several ways. One effect is the decrease in pH, which might actually work to increase the stability of the membrane. 6 However, the same ion exchange process also reduces the ionic conductivity of the membrane. This happens because the carbonate species displace the hydroxide ...

Importance of balancing membrane and electrode water in anion exchange membrane fuel cells

“…This performance characteristic, avoiding cathode dry out even at high current density and reduced RH feeds, may be different than observed with AEMFCs containing other materials because of the novelty of the electrode fabrication method used in this work as well as the use of the powder AEI and high ionic conductivity of the ETFE-based AEM used in this study; the water mobility and resulting water back diffusion are, therefore, significantly enhanced [15,17], leading to balanced anode and cathode water with avoidance of anode flooding and cathode dry out. Fig.…”

“…To avoid cathode dry-out and/or anode flooding in AEMFCs, it would be preferable for the AEM to have high water back diffusion. However, many AEMs in the literature do not have the same efficient phase separation as Nafion ® and limited OH À conductivity (Table 1), which translates directly to low water backdiffusion rates [15,17]. Therefore, engineering solutions have been explored in a number of studies, including running commercial systems at very low current density [16], pressurizing the gas streams, or even feeding condensed water through the cathode [10] e none of which are tenable long-term solutions to high performing AEMFCs.…”

“…Therefore, engineering solutions have been explored in a number of studies, including running commercial systems at very low current density [16], pressurizing the gas streams, or even feeding condensed water through the cathode [10] e none of which are tenable long-term solutions to high performing AEMFCs. Compared to many modern AEMs (Table 1), radiation-grafted ETFE-based AEMs have been reported to have high conductivity [13,18] and high water back diffusion rates [15,17], which may be utilized to alleviate the water gradient that is intrinsic to operating AEMFCs. However, high water back diffusion risks the introduction of new variables to be considered, including the possibility for cathode flooding.…”

Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

“…The effective cathode resistance (R c ) against water transport from the cathode inlet to the cathode catalyst layer can be described as [15] Note that the resistance of vapor absorption into AEM ionomers is not included in calculating the effective resistance here. The expression for the water transport resistance in the ionomer and gas phases can be referenced to the previous work.…”

“…16 However, AEMFCs still suffer a decline in performance due to bicarbonate/carbonate formation in the membrane and catalyst layers (CLs) leading to additional ohmic and kinetic losses. [13][14][15] In an AEMFC, the following electrochemical reactions occur in the CLs as follows:Net :Hydrogen combines with hydroxide ions to produce water in the anode. Oxygen reacts with water to generate hydroxide ions in the cathode.…”