Quantitative Analysis of H+ Transportation of Nafion film on Different Substrate using Planar Inter Digitated Electrodes

Abstract

Introduction: Ionomer thin-film gained attraction because of their thickness- and morphology-dependent changes in ionic conductivity. In a fuel cell catalyst layer, ionomer thin films coating the carbon-supported Pt-based catalysts (Pt/C) is distributed in spatially heterogeneous manner with variation in thickness and connectivity. Thus, it is difficult to clarify the quantitative attribution of the true source of effective proton resistance among Pt and carbon-support. To evaluate the quantitative attribution of proton conductivity of a thin-film ionomer on relevant substrates (e.g., carbon), we have fabricated specially designed interdigitated electrodes (IDEs) to accommodate different substrates upon which ionomer film is coated. In this work, we present the Nafion (~55 nm) proton (H+) transportation over carbon and quartz substrate in N2 atmosphere. Also, we will discuss the true attribution of substrate-dependence to estimate the Nafion’s thin film conductivity. Experimental: Interdigitated electrodes (IDEs) with and without carbon pad were fabricated in clean room facilities. To prepare IDEs, the patterned carbon structure was developed and pyrolyzed at 1000 ºC in 2% H2/N2 gas before the Pt deposition. 1.2% Nafion dispersion was spin coated and dried at 40ºC under vacuum before electrochemical impedance spectroscopy (EIS) measurement in N2 atmosphere. Relative humidity (RH) dependence of in-plane H+ resistance was obtained and discussed through EIS measurements. Results and Discussion: Figure 1 (a) and (b) shows the side view of IDEs with (Pt_C_IDE) and without carbon pad (Pt_Q_IDE), and also Nafion’s (~55 nm) H+ resistance obtained from EIS measurement at various RH in N2 atmosphere. Nyquist plot (not shown) exhibited significant difference and ascribed film resistance (Mean value) for ionomer at carbon substrate was significantly lower (one order of magnitude) compared to that at the quartz substrate, as shown in Fig. 1 (c).In case of H+ resistance using Pt IDE with carbon (Pt_C_IDE), approximately 90% of the IDE area between the electrodes is carbon and the rest is quartz. Therefore, the method of estimation of Nafion’s thin film conductivity from impedance response must be reviewed carefully and will be discussed later in detail.