On the Design of a Comb-Shaped, Poly(phenylene oxide)-Based Anodic Binder for Anion-Exchange Membrane Direct Methanol Fuel Cell (AEM-DMFC)

Abstract

Anion-exchange ionomers are used in wide range of applications such as water treatment (desalination, purification, decontamination), chromatography and fuel cells. They consist of polymer chains functionalized with cationic groups to carry anionic molecules and can therefore be used as membrane and catalyst binder in anion-exchange membrane fuel cells (AEMFCs). In the last years there were many improvements related to alkaline stability and ion conductivity of anion-exchange membranes making these fuel cells more feasible. Anion-exchange membrane direct methanol fuel cells (AEM-DMFCs) are a subcategory of AEMFCs and convert the chemical energy of methanol into electric current by oxidizing methanol and reducing oxygen on the anode and cathode, respectively. Advantages of the alkaline over the acidic environment are enhanced methanol oxidation kinetics, lower overpotentials for oxygen reduction reaction, usage of cheaper catalysts than platinum, no need of highly alloyed steels due to less corrosive medium and possible usage of alcohol tolerant oxygen reduction reaction catalysts. It was recently demonstrated that AEM-DMFCs free from platinum reach power densities of up to 0.1 W cm-2 [1,2] making them an interesting alternative as power source for mobile and back-up power applications. In AEM-DMFCs, an ionomeric binder is needed to fix the carbon supported catalyst onto the liquid/gas diffusion layer. The catalyst layer consisting of the ionomer and the catalyst is supposed to build a three-phase boundary (TPB). The TPB facilitates the simultaneous transport of hydroxide ions, electrons and reactants/products. Research groups working on AEM-DMFCs currently rely on highly stable polytetra-fluoroethylene (PTFE) as binder as it is forming a microporously structured catalyst layer. On the downside, PTFE is non-anion conductive polymer. Hence, AEM-DMFCs with PTFE as binder need KOH added to the anodic fuel as electrolytic supplement to reach high current densities. As most anion-exchange membranes are not stable in highly alkaline media at elevated temperatures the addition of KOH to the fuel is considered to be problematic leading to the failure of the cell. Besides this, the addition of KOH is not consumer friendly and could therefore lead to problems in commercialization of the AEM-DMFCs. Thus, it is favorable to eliminate KOH as a fuel supplement. To reach this, ionomeric binders have to be designed for the needs of the AEM-DMFC especially on the anode compartment as the liquid methanolic fuel and the methanol oxidation reaction demand special properties from the ionomeric binder: Anionic conductivity, low swelling in water/methanol, high stability in alkaline environment and formation of a microporous catalyst layer structure. We herein present the synthesis, characterization and implementation in single cells of a comb-shaped anion-conductive ionomer with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) as backbone polymer for the use as anodic binder in AEM-DMFCs. To synthesize the ionomer, PPO was brominated to further functionalize it. The comb-shaped ionomer resulted from a subsequent reaction of linking (2-methylimidazole, 2MIm), side chain (octafluoro-1,4-diiodobutane, DIFB) and quaternization (1,2,4,5-tetramethylimidazole, TMIm) agents. Successful synthesis was confirmed by 1H-NMR-spectroscopy. In Figure 1 the alkaline stability of comb-like PPO-2MIm-DIFB-TMIm is compared with a non-comb-shaped ionomer (PPO-TMIm). It is observed that the alkaline stability is greatly enhanced especially at high temperatures when the cationic group is not directly linked to the PPO backbone. Besides this, the comb-shaped ionomer shows several other advantageous properties over PPO-TMIm like lower swelling ratios while having similar water/methanol uptakes, higher thermal stability and higher ionic conductivity. Further investigations were done by implementing the ionomer as anodic binder in membrane electrode assemblies and conducting CO stripping and performance experiments on single cell level. Resulting UI and power curves (Fig. 2) showed a significant performance gain for the single cells with the comb-shaped ionomer used as binder. Fig. 1 (A) Loss of ionic conductivity of PPO-TMIm (black) and PPO-2MIm-DIFB-TMIm membranes (red) immersed in 3M KOH solution for 10 days at room temperature (straight lines), 60 °C (dashed lines) and 80 °C (dotted lines). (B) UI and power density curves of single cells fed with 4M CH3OH and O2 as anodic and cathodic fuels in dependency of the ionomer content in the anodic catalyst layers. Anodes consisted of various amounts of comb-like ionomer or PTFE and 2 mgPt cm-2 Pt/C. Cathodes consisted of 2 mgPt cm-2 Pt/C and 12.5wt% PTFE binder. Membrane: Tokuyama A201.