• Pasupathi, Sivakumar
• Hacker, Viktor
• Kapun, Gregor
• Grandi, Maximilian
• Gatalo, Matija

Abstract

Introduction<br/>For a large scale commercialization of fuel cells as clean energy conversion system, considerable reductions in production and system costs are necessary. In a recent cost analysis of the U.S. Department of Energy (DOE) it was confirmed that the cost of a 80 kW PEFMC stack at production volumes between 100,000 and 500,000 units per year can be reduced by changing the catalyst, increasing power output by 47%, simultaneously lowering the Pt content by 7% (using PtCo-alloy) and changing the membrane manufacturing process [1]. In total this leads to a projected price of 47$/kW or 45$/kW for 2020/2025 respectively, approaching the 2020 cost targets of the DOE. <br/>Ultrasonic spray-coating has attracted great attention as a scalable and flexible method to produce very homogeneous catalyst layers with good porosity control [2]–[4]. Through vibration of a metal tip at 120 kHz a solid/liquid suspension is atomized with lower droplet diameters and narrower size distribution than with pneumatic atomization. It can be used to either, directly coat the gas diffusion layer (GDL), or the membrane with very thin (2-6 µm) active layers. This is important for lowering mass transport related voltage losses and increase power output. As an example of its potential, a combined process using ultrasonic spray-coating and electrospinning (for membrane fabrication) resulted in a membrane electrode assembly (MEA) with the highest achieved platinum utilization so far with 88 kW gPt-1 [4]. <br/><br/>Experimental<br/>Automated ultrasonic spray-coating (see Figure 1) is used in the course of this work, to study the influence of catalyst structure on the layer thickness, proton conductivity and platinum utilization. The catalysts used in this study were a commercially available Pt/C (50 wt% platinum on carbon) and a bimetallic PtCu3/C (6 wt% platinum, 8 wt% Pt on carbon) prepared at the National Institute of Chemistry (NIC) in Ljubljana. The latter showed very high oxygen reduction activity and cycling stability in previous ex-situ studies [5].<br/>Both catalysts were dispersed in a mixture of 2-propanol and Nafion ionomer in the right amount, to obtain electrodes with 0.2 mgmetal cm-2 and 30 wt% of Nafion, after the coating process. <br/>Physical characterization consisted of cryo-cut SEM cross sections. Electrochemical characterization was performed in 5 cm² test cells and included recording of polarization curves and electrochemical impedance spectroscopy (EIS). <br/><br/>Results<br/>The MEA using Pt/C (50 wt%) performed better than the PtCu3 (8 wt%) catalyzed MEA. While 667.5 W gPt-1 were achieved with the first one, the PtCu MEA reached 256.7 W gPt-1. This is in contrast to the results obtained in ex-situ tests at the NIC, were PtCu3/C clearly outperformed Pt/C. The kinetic region of the polarization curves revealed, that kinetic Voltage losses were the same for the same metal content, meaning PtCu3 outperformed Pt. However, the polarization curve and impedance spectra of the PtCu3 MEA indicates strong diffusion limitations. SEM cross sections revealed that catalyst layers fabricated with PtCu3/C are 18 times thicker than Pt/C electrodes, explaining high mass transport losses seen in the polarization curves. This is a direct consequence of the lower metal content in the catalyst material (8wt% vs. 50 wt%).<br/><br/>Conclusions<br/>Automated ultrasonic spray-coating is a promising, scalable technique for industrial manufacturing of polymer electrolyte fuel cells, producing highly uniform- and thin layers, with great reproducibility.<br/>To achieve high platinum utilizations, catalysts need a metal content higher than 10 wt% regardless of the activities measured in ex-situ studies, to achieve thin layers. For this reason, further studies will be performed using PtCu3 and Pt catalysts with varying metal content.<br/><br/>References:<br/>[1] A. Wilson, G. Kleen, and D. Papageorgopoulos, “DOE Hydrogen and Fuel Cells Program Record,” 2017.<br/>[2] L T C Joseph M. Nolan, “Maximizing the Use of Simulations,” Infantry, no. December, pp. 39–42, 2011.<br/>[3] B. Britton and S. Holdcroft, “The Control and Effect of Pore Size Distribution in AEMFC Catalyst Layers,” J. Electrochem. Soc., vol. 163, no. 5, pp. F353–F358, 2016.<br/>[4] M. Breitwieser, M. Klingele, B. Britton, S. Holdcroft, R. Zengerle, and S. Thiele, “Improved Pt-utilization efficiency of low Pt-loading PEM fuel cell electrodes using direct membrane deposition,” Electrochem. commun., vol. 60, pp. 168–171, 2015.<br/>[5] M. Gatalo et al., “Positive Effect of Surface Doping with Au on the Stability of Pt-Based Electrocatalysts,” ACS Catal., vol. 6, no. 3, pp. 1630–1634, 2016.<br/><br/>

Topics

• Deposition
• pore
• surface
• polymer
• Carbon
• scanning electron microscopy
• simulation
• Oxygen
• Platinum
• Hydrogen
• ultrasonic
• electrochemical-induced impedance spectroscopy
• porosity
• electrospinning
• atomization