Membranes and Membrane Electrode Assemblies Based on Sulfonated Poly (ether ketone ketone) and Heteropolyacids for Polymer Electrolyte Fuel Cells
Organic sulfonated polyether ketone ketone SPEKK membranes with different ion-exchange capacities IECs, and composite membranes prepared by the addition of 20 wt % phosphotungstic acid PTA to SPEKK were used to prepare membrane electrode assemblies MEAs. The proton conductivity of the membranes increased with increasing IEC of the SPEKK, and with the addition of PTA. The proton conductivity attained at 80°C and 75% relative humidity was 20 2 mS/cm. The feasibility of using SPEKK in the cathode layer of the MEAs was investigated. The electrochemically active surface areas ECAs of the SPEKK-based cathodes were lower than that of the Nafion-based cathode and decreased further as the operating relative humidity was lowered. These observations were reflected in the single-cell polarization data, which indicated that the MEAs with the SPEKK-based electrodes were outperformed by their Nafion-based counterparts. Furthermore, a mismatch in SPEKK IEC be-tween the membrane and cathode resulted in immiscibility at the interface. While the additive stability in the composite membrane was very good, the long-term stability of the membranes was poor when compared to perfluorosulfonic acid membranes such as Nafion, with failure occurring by scission along the gasket edges of the MEA after limited operation. Sulfonated hydrocarbons offer an alternative to perfluorosulfonic acid membranes such as Nafion for polymer electrolyte fuel cell PEFC and direct methanol fuel cell DMFC applications. Ad-vances in the area of hydrocarbon-based membranes for fuel cell applications have been summarized in several recent reviews. 1-8 Composite membranes based on Nafion and heteropolyacids HPAs have been previously studied 9-11 in an attempt to reduce the depen-dence of membrane conductivity on water content. Stabilization of the HPA additive within the ionomeric matrix, designed to prevent HPA leaching in aqueous environments, has recently been demon-strated in Nafion-based systems. 12 Studies have also been performed on composite membranes prepared using hydrocarbon-based matri-ces that contain HPAs. 13-15 These studies attest to the utility of HPA additives in membranes designed for medium to high relative hu-midity RH PEFC operation. Sulfonated polyether ketone ketone SPEKK 16 is a proton-conducting material with good film-forming properties. However, the conductivity of SPEKK is strongly dependent on the water up-take of the membrane and, concomitantly, on the ion exchange ca-pacity which in turn is a function of the degree of sulfonation. The operating range and composition of SPEKK membranes are there-fore limited to fully saturated conditions and high ion exchange capacities IECs, respectively. Perfluorosulfonated polymer electro-lyte membranes PEMs such as Nafion IEC = 0.9 mequiv/g in-trinsically have a higher acid ionization constant lower pKa when compared to SPEKK, which implies that a higher IEC is needed for SPEKK to obtain the same conductivity under a given set of oper-ating conditions. However, the mechanical properties of the SPEKK films deteriorate with increasing IEC. This results in low operational lifetimes for membrane electrode assemblies MEAs prepared us-ing high IEC SPEKK. Hence, a trade-off exists between proton con-ductivity and operational lifetime in the case of SPEKK membranes. One approach to get around this trade-off is to prepare composite membranes with a low IEC organic matrix supplemented with inor-ganic additives to enhance proton conductivity. An added benefit of this approach is that the inorganic particles will also serve to impede the crossover of methanol from anode to cathode. To this end, SPEKK/HPA composite membranes using low IEC SPEKK were prepared and studied, with the results obtained discussed in this paper. While significant attention has been devoted to membrane devel-opment, many of the hydrocarbon-based membranes detailed in the references above have been evaluated ex situ to determine proton conductivity. MEAs have been prepared using these alternate membranes. 17-20 However, until recently the electrode layers in these MEAs have been either devoid of proton-conducting material or employed perfluorosulfonic acid ionomers such as Nafion as the proton-conducting component and binder. While Nafion is an admi-rable proton-conducting binder, concerns exist about the integrity of the membrane electrode interface due to the mismatch that exists between the hydrocarbon-based membranes and the fluorocarbon-based Nafion. Commercialization of hydrocarbon-based membranes could well be contingent on the concomitant development of com-patible proton-conducting materials to be used in the electrode lay-ers. Recognizing this, Lakshmanan et al. 21 and Easton et al. 22 inves-tigated the influence of the addition of sulfonated polyether ether ketone SPEEK as a binder in PEM gas diffusion electrodes. Jung et al. 23 have investigated the effect of SPEEK as an electrode binder in DMFCs. In this study, MEAs were prepared for the first time using SPEKK binders with varying IECs in the cathode. Unlike prior stud-ies involving SPEEK, the MEAs were prepared by applying the catalyst layer directly to the membrane as opposed to applying the catalyst layer to the gas diffusion layer, followed by hot pressing onto the membrane. This technique improves the utilization of elec-trocatalyst in the MEA. The MEAs were characterized by linear sweep voltammetry LSV, cyclic voltammetry C-V, and polariza-tion tests to determine the feasibility of using SPEKK as the proton-conducting material in the cathode layer to promote interfacial sta-bility.