(397ar) Zeolite-Confined Sulfonated Graphene-Nafion Composite Membrane for Self-Humidifying PEMFC

Sim, V., The Hong Kong University of Science and Technology
Han, W., The Hong Kong University of Science and Technology
Poon, H. Y., The Hong Kong University of Science and Technology
Yeung, K. L., The Hong Kong University of Science and Technology

Perfluorosulfonic acid (PFSA) membrane is the most common electrolyte membrane used for PEMFC. Nafion (PFSA) membrane displays high proton conductivity (about 0.1 S·cm-1) and excellent long-term stability (about 6000 hours) under fully hydrated conditions. However, the proton conductivity of Nafion membrane decreases sharply under low humidity, whereas excessive water can result in electrode flooding. External humidification equipment and special-designed water drainage unit are often required for hydrogen PEMFC resulting in complicated design and operation of system, and a lower overall energy efficiency. Self-humidifying proton exchange membrane that can regulate better hydration in PEMFC is attractive.

Many self-humidifying composite membranes using Pt-containing inorganic fillers such as Pt/Carbon-Nafion, Pt/Oxide-Nafion and Pt/Zeolite to Nafion matrix generate and retain water by catalysis and adsorption. However, increased transport resistance, particle aggregation and phase separation are major concerns for these traditional composite membranes. There are also increasing evidences that platinum deposited in proton exchange membrane promotes the formation H2O2 and HO∙ free radicals that are responsible for membrane degradation. Moreover, electron-conducting carbon material can cause internal short-circuit. Recently, we explored the effects of narrow confinement of PFSA in subnanoliter volume of zeolite-coated pores. These zeolite-confined Nafion composite membranes displayed improved thermal and mechanical stabilities as well as much higher proton conductivity under the conditions of high temperature and low humidity. The hierarchical design could be used to uniformly distribute inorganic fillers within PFSA and isolate electron-conducting fillers in a discontinuous phase. Heteroatom (N, B, or P)-doped carbon nanotube, graphene and graphite are shown to be interesting oxygen reduction electrocatalysts. Graphene derivatives in PFSA could therefore in principle catalyze water generation. This work examines a series of zeolite-confined sulfonated graphene-Nafion composite membranes and their performances for self-humidifying PEMFCs.

A silicalite-1 (Sil-1) coating was hydrothermally grown on porous stainless steel mesh (SSM) as an electron-insulating and anticorrosive layer. Graphene was prepared via modified Hummers method from commercial graphite flakes, followed by sulfonation with sulfanilic acid. The sulfonated graphene was dispersed in Nafion PFSA and casted into Sil-1-coated SSM to prepare Sil-1-confined sulfonated graphene-Nafion composite membranes. For comparison, Sil-1-confined sulfonated graphite (Sgrap)-Nafion and sulfonated carbon nanotube (SCNT)-Nafion were prepared. The membrane-electrode assembly was fabricated by hot-press method using 0.5 mg/cm2 Pt/Vulcan XC-72 catalyst.

The results of micro-Raman spectroscopy and transmission electron microscopy show that sulfonated graphene oxide (SGO) and sulfonated graphene (SG) have multi-layered structures of less than 10 layers. All sulfonated carbon materials were well dispersed in the Nafion PFSA, and the composite membranes are uniform black in color. Compared to Sil-1-confined Nafion composite membrane, Sil-1-confined SGO-Nafion and SG-Nafion composite membranes have weaker fluorescence due to the quenching effect of graphene materials, which further indicates membrane homogeneities. The fuel cell with Sil-1-confined SGO-Nafion composite membrane gives a power density of 728 mW/cm2 at 70oC without humidification, which is 16.5 times higher than those of standard fuel cell with Nafion 117 membrane. It can also tolerate much higher temperature even compared to Sil-1-confined Sgrap-Nafion and SCNT-Nafion composite membranes. Improved glass transition temperatures (verified by differential scanning calorimetry curves) and oriented proton transport channels (suggested by EDX elemental mapping images) are contributing reasons for the excellent performance.