Investigation of Concentration Overvoltage Increase in Low Pt-Loading PEFC

Abstract

Introduction Although fuel cell vehicles (FCVs) have already been commercialized, the cost reduction of PEFCs is required for full-scale distribution of FCVs, and so it is essential to reduce the amount of Pt, which is an expensive precious metal used as an electrode catalyst. So far, our research group has been working on preparation and performance evaluation of membrane electrode assembles (MEAs) with different cathode Pt-loading in order to determine the minimum requirement of Pt1. Then, we have found that concentration overvoltage increases in the very low Pt-loading region, lower than 0.20 mgPt/cm2. This is considered that resulting low carbon content (thin cathode layer) has led to increase in the water volume fraction in the catalyst layer under the same current density, even though the gas transfer distance is rather shorten. Therefore, in this study, we are aiming to investigate the mechanism of increase in concentration overvoltage in detail in order to reduce concentration overvoltage. Experimental The 1 cm2 MEAs were fabricated by a spray printing method2 using the Pt/C catalyst and Nafion® electrolyte. Here, the amount of anode Pt/C was fixed to 0.30 mgPt/cm2 (0.35 mgC/cm2), and the amount of cathode Pt/C was varied from 0.05 to 0.60 mgPt/cm2 (0.06-0.70 mgC/cm2). For the Pt/C catalyst, on the purpose of examining the influence of hydrophilic/ hydrophobic characters of carbon, TEC10E50E (46.5%Pt/KB) and TEC10E50EA (46.6%Pt/Graphitized KB (GKB)) were used. Then, current-voltage (I-V) characteristics and ECSA of MEAs were evaluated. Furthermore, impedance was also measured, and ohmic, activation, and concentration overvoltages were separately analyzed. Results and discussion For successful drain of water from the cathode layer in order to reduce concentration overvoltage, two approaches, such as changing evaluation conditions of MEAs and controlling the surface character of carbon supports in MEAs. First, the humidity condition of MEA evaluation was changed from RH100% to 57%, and I-V performance was evaluated. As a result, no change in concentration overvoltage was observed on changing relative humidity. This is probably because the generated water is rather effective on concentration overvoltage than relative humidity of MEA evaluating condition. Next, MEAs prepared using Pt/KB and Pt/GKB for the cathode and I-V performance was evaluated. As a result, as shown in Fig. 1, reduction of concentration overvoltage was confirmed by using Pt/GKB with 0.23 mgC/cm2, where increase in concentration overvoltage was already seen in case of Pt/KB. This is most likely because successful water drain derived by the hydrophobic character of GKB in comparison to KB. In other words, this result has suggested that controlling the hydrophobic character of carbon supports can reduce the concentration overvoltage even for low Pt-loading MEAs. Detailed discussion will be reported through further changing the amount of carbon. Figure 1