Proton Conducting Nanocomposites Using Phosphonic Acid Grafted Polyhedral Oligomeric Silsesquioxane (POSS)
Authors: Youn Taeyung , Rhee Hee-Woo 
Published: February 2020

In PEMFC, nanocomposites are drawing attention as the alternative membrane because of selected various acid moiety functionalized additives, increased water retention and improved mechanical and thermal properties. But in modification of nanocomposites, when more than 100 nm-sized particles are introduced in ion channel, ion channel development is affected and hydrogen ion movement is reduced.

So this study explores the concept of improving fuel cell membrane performance without compromising proton conductivity by using a 1 ~ 3 nm sized polyhedral oligomeric silsesquioxane (POSS). Polyhedral oligomeric silsesquioxane (POSS) is a molecule that has a spatially defined, rigid and hydrophobic cube-octameric siloxane skeleton (about 1 ~ 3 nm in size) with eight organic vertex groups, one or more of which are reactive or polymerizable. [1 ~ 3]. These particular structural features render POSS be a versatile additive for acquiring enhanced thermomechanical properties, better thermal stability, atom oxygen resistance, abrasion resistance and low water uptake [4 ~ 6].

Only one POSS-based nanocomposite for PEMFC h as been reported [7]. An open-cage POSS carrying three glycidyl epoxy groups was reacted with 4-hydroxybenzenesulfonic acid, the resulting sulfonated POSS was blended with polyvinylalcohol and the blend was crosslinked using ethylenediaminetetracetic dianhydride (EDTAD). This system differs considerably from the system described in this study (open-cage versus closed-cage POSS, crosslinked versus noncross-linked structure and aliphatic versus aromatic composition) and also has several disadvantages as follows: it requires a multistep fabrication process and it contains chemically unstable methylene groups. Additionally, the mechanical and chemical stability was not reported.

In this study, POSS nanoadditives carrying phosphonic acid moieties were synthesized, characterized and formulated into Nafion®. As mention in the ref. [8 ~ 12], phosphonic acid groups are good candidate for high temperature proton carriers with the relatively high proton conductivity without water due to its self-dissociation natures. Also we developed Nafion®/3-aminopropyl triethoxysilane (APTES) hybrid membranes with phosphonic acid groups in the previous work in order to improved high temperature performances [11]. These hybrid membranes show high proton conductivities (0.028 S/cm) at 120 oC and 40% RH, so we can confirm the strong influence of phosphonic acid groups in low humid conditions.

Also, in contrast to common additives, it can be possible to progressive improvement of proton conductivity because of eight vertex groups which substitute by phosphonic acid groups. We expect to innovatively improve proton conductivities by molecular-closed particle size and many vertex groups of POSS.

The phosphonic acid grafted POSS prepared were characterized for particle size and chemical structure. Also Nafion®/POSS-PA nanocomposites were characterized for morphologies, mechanical strength, proton conductivities at different temperatures and humid conditions, and the cell performances at different temperatures.