An investigation of the composite polymer electrolytes and electrocatalysts for the proton exchange membrane fuel cell

Abstract

Durability is one of the major issues for the successful commercialisation of polymer electrolyte membrane fuel cells (PEMFCs) and it mainly depends on the stability of the individual cell components. In order to minimise the durability issues, the development of new materials or modification to replace the existing fuel cell components is required. The typically used proton exchange membrane (PEM) is the perfluorosulfonated polymer such as Nafion® and electrocatalysts for PEMFC is high surface area carbon supported platinum electrocatalyst (Pt/C). A higher temperature of operation (>80 oC) of PEMFC would boost their performance by enhancing the electrochemical kinetics and also improve the carbon monoxide tolerance of platinum catalysts. A Nafion® type membrane is not suitable for higher temperature operation as its proton conductivity mainly depends on the hydration level. An approach to improve the proton conductivity of Nafion® based membranes is the incorporation of hydrophilic inorganic oxide materials into the Nafion® polymer matrix. A composite membrane based on graphite oxide (GO) has been developed and demonstrated as an alternative PEM for high temperature operation up to 120 oC. GO is an insulator and hydrophilic in nature. GO exhibits proton conductivity due to the presence of acidic functional groups like, carboxylic acid, hydroxyl groups and epoxy groups. Further functionalisation of GO with sulfonic acid (called SGO) improves the proton transport properties of GO which in turn improves the composite membrane proton conductivity. Free standing GO and SGO papers were fabricated and evaluated to understand their proton transport mechanism. The in-plane and through-plane proton conductivities of GO paper were 0.008 and 0.004 S.cm-1 at 30 oC and 25% RH respectively. The in-plane and through-plane proton conductivities of SGO paper were 0.04 and 0.012 S.cm-1 at 30 oC and 25% RH respectively. The fuel cell performance of a membrane electrode assembly made with SGO paper gave a maximum power density of 113 mW cm-2. GO/Nafion composite membranes were fabricated with different GO content. The composite membranes with an optimum of 4 wt% GO showed better mechanical strength (tensile strength of 8.17 MPa) and water uptake (37.2%) compared to recast Nafion. A GO (4 wt%) /Nafion composite membrane gave a high ion exchange capacity (IEC) value of 1.38 meq g-1. The proton conductivity of GO (4 wt%) /Nafion was 0.026 S.cm-1 at 120 oC. SGO/Nafion composite membrane showed improved proton ii conductivity (0.029 S.cm-1). The SGO/Nafion composite membrane gave peak power density of 240 mW cm-2, whereas GO/Nafion composite membrane gave a power density of 200 mW cm-2 at 120 oC and 25% RH. The stability and durability of GO and SGO/Nafion composite membranes was investigated under fuel cell operating conditions and compared with recast Nafion. A non fluorinated proton exchange membrane based sulfonated poly ether-ether ketone (SPEEK) was used to develop a composite membrane with SGO. SGO (4 wt%) /SPEEK composite membrane showed high IEC of 2.3 meq g-1 and proton conductivity of 0.055 S.cm-1 at 80 oC and 30% RH. SGO (4 wt%) /SPEEK composite membrane gave a power density of 378 mW cm-2 at 80 oC and 30% RH, which was higher than that of recast SPEEK (254 mW cm-2). Transition metal nitride based electrocatalyst support such as titanium nitride (TiN), has been used to replace carbon to support Pt and Pt-Co alloy for PEMFC cathode. Nafion® stabilised Pt nanoparticles supported on TiN (Pt/TiN) were prepared and evaluated as cathode electrocatalyst for PEMFC. Pt/TiN showed better electrocatalytic activity, stability and durability under fuel cell operating conditions compared to commercial Pt/C. Pt/TiN retained 66% of electrochemical active surface area (ECSA) after 1000 potential cycles (cycled between the potential range of +0.6 to +1.20 V vs. RHE) under fuel cell operating conditions. The ECSA of the Pt/C catalyst fell by 75%. Pt/TiN was also evaluated for its suitability in phosphoric acid based PEMFCs. Pt/TiN showed better durability than Pt/C under fuel cell operating conditions. Pt/TiN showed a two-fold increase in mass and specific activities than Pt/C as calculated from oxygen reduction reaction data at 0.9 V. An improved durability of Pt/TiN resulted from a Nafion® layer surrounding the Pt protecting from phosphate ion adsorption. Alloying of Pt with 3d transition metals changes the electronic structure of Pt (Pt becomes e- deficient) and enhances the electrocatalytic activity of PtM alloy compared to Pt. 3d transition metals such as Fe, Co and Ni are reported to be more active than other metals. Pt-Co alloy supported on TiN was prepared and evaluated. Pt-Co/TiN showed about +21 and +32 mV positive shifts in half-wave potential compare to Pt/TiN and conventional Pt/C respectively. After 5000 potential cycles, the ECSA of Pt-Co/TiN had decayed by about 55%, whereas Pt/TiN and Pt/C showed a greater loss in ECSA of 70%.