Proton Conductivity of Graphene-Based Polymer Electrolyte Membrane

A polymer electrolyte membrane fuel cell (PEMFC) is one of the promising alternative energy systems, but the one of the bottlenecks of the PEMFC performance is poor electrochemical reaction and key transport properties, i.e., proton conductivity in catalyst layers. Recently, it has been reported that an addition of the rapidly emerging material, i.e., graphene, significantly improves the performance¹, but the understandings of the role of the graphene on the performance improvements are poor. Here, we examine the role of the graphene in PEM for enhanced performance. The graphene-based, thin PEM, i.e., Nafion^®, is coated on the bulk Nafion^®membrane, and the proton conductivity of the graphene-based, thin PEM is measured using an impedance spectroscopy with respect to the graphene content from 0 to 3 wt% at the ambient temperature and liquid-water equilibrated state. It shows the proton conductivity improvement with the increasing graphene content. The enhancement is also compared using carbon black, showing that graphene-based PEM provides higher proton conductivity than the carbon-black-coated PEM. The obtained results provides an insight into an optimal material for improving the proton transport of PEM.

Experimental

Graphene flakes were purchaced from Angstrong materials, were added to Nafion^® solution which was purchased from Sigma-Aldrich by weight percentages of 0, 0.1, 0.5, 1, 2, and 3. This solution was then sonicated for 90 minutes in order to ensure desired mixture of graphene with Nafion^® solution. The graphene-Nafion^® mixture was then sprayed on either side over a dry 2, 7, and 10 mil thick Nafion^®membrane which was purchased from Ion Power Inc. was soaked in DI water for one hour to ensure for liquid equilibrium state, and it is placed between the cell. The proton conductivity is measured using an impedance spectroscopy with a frequency range between 10 and 300 kHz with a voltage peak-to-peak variation of 10mV at 0 V. The same wt% of the carbon black is added to the PEM for 2 mil, 7 mil, and 10 mil thick membranes. The real part resistance at the high frequency is used for the proton conductivity, given as σ = 1/ρ where, σ is the proton conductivity, and ρ is the resistivity.

Results

With the addition of graphene nano-flake particles to the Nafion^®, the proton conductivity increases in all the Nafion^® thicknesses as shown in Fig 1(a). The proton conductivity increases significantly up to the 2 wt% of graphene, and when it is greater than 2 wt% it increases nearly linearly as it reaches a percolation threshold.² It is also observed that the proton conductivity improvement of the thicker Nafion^® membrane with respect to the graphene content is smaller than those of the thinner membrane. Perhaps, this is caused by the different ratio of the proton conductivity between the bulk membrane and graphene-based Nafion^® coating.³Thus, the 2 mil membrane offers more proton conductivity than the 10 mil.

The measured proton conductivity of the carbon-black-based Nafion^® as a function of the carbon-black content is shown Fig 1(b), comparing with the graphene-based-Nafion^® on 7mil thick membrane. The carbon black in Nafion^® almost linearly increases the proton conductivity up to 3 wt%, and the degree of the proton conductivity enhancement at the same weight percentage is smaller than that of the graphene. It is also noted that the proton conductivity enhancement by carbon black is much less than the ones of graphene at the low carbon black and graphene content. Perhaps, this is related to the fact that the graphene provides an improved interaction with water compared to the carbon black, which in turn results in the effective proton conductivity enhancement. Also, in the graphene surface, the presence of –O-, -OH, and –COOH functional groups having hydrophilic sites may facilitates the proton transport through hydrogen-bonding networks of the water clusters in the membrane⁴.