Use of a Segmented Cell for the Development of PGM-Free Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells

Abstract

To reduce the Pt content in polymer electrolyte fuel cells (PEFC), the development of highly active and stable platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) is crucial. In the last decade a significant amount of work has been published focusing on PGM-free catalyst synthesis and characterization, and excellent RDE performances were recently achieved.1,2 A smaller, but still considerable number of work showing H2/air and/or H2/O2 PEFC polarization curves data is available.3–5 However, only few papers have been published that report on systematic durability studies,6,7 and electrode layer fabrication optimization to maximize the PEFC performance.8 Durability assessment and electrode fabrication optimization are demanding tasks in terms of time and effort for execution of the experiments because of the high number of variables that can influence the performance. Examples for such variables are the ionomer-to-catalyst ratio, catalyst loading, catalyst deposition method, ink concentration and composition in terms of solvents. Furthermore, each catalyst may have a different optimum electrode fabrication condition. This means that a separate optimization is necessary for each newly developed PGM-free catalyst showing good potential to perform well in PEFCs. Due to this vast parameter space, no systematic studies about the catalyst layer optimization related to the final PEFC performance, have been published so far. Recently, NREL’s fuel cell group has upgraded its segmented cell with improved electrical signal measuring features, and an innovative segmented parallel straight channel flow field. The segmented cell hardware consists of a 50 cm2 square electrode active area, divided into a total of 121 segments (11 x 11, each segment = 0.413 cm2).9 Each single segment has a current collector and a voltage sensor, which enables the separate measurement of current density and potential. The upgraded segmented cell can operate in both active (each segment connected to a single load duty) and passive (applying a single electric load and collecting the signals through each single segment) modes. With these features, the segmented cell becomes an attractive tool to implement a fast and systematic screening for PGM-free catalyst layers that were fabricated differently or with different material sets. Screening and optimization entails simultaneously testing pieces of electrodes synchronously in the same cell. BOL performance and the durability behavior of different catalyst layers can also be assessed in parallel. In this work, we report on the utilization of NREL’s segmented cell for screening PGM-free cathode catalyst. The results demonstrate the effectiveness of the segmented cell in accelerating preliminary optimization studies. First, we demonstrate segmented cell operation under differential cell conditions, eliminating the spatial effects due to the position of the segments along the flow field. In particular, we demonstrate homogeneous current distribution in two sets of seven segments located along a straight channel section of the cell. The current densities match those of a 5 cm2 differential single cell with an identical PGM-free electrode. We further demonstrate the parallel screening of (i) two different PGM-free catalysts and (ii) catalyst layers with different ionomer contents (15 – 30 – 45 %). The results for (i) reveal significantly different behavior of the two catalysts in terms of activation time and short-term durability. The results for (ii) indicate a clearly increasing performance trend with increasing ionomer content, i.e. performance increases in the order 45 > 30 > 15 % ionomer content. The data establish the segmented cell as a useful tool for PGM-free electrode optimization and durability screening. More details, and further applications of the segmented cell to PGM-free electrode optimization and durability will be given in the Meeting presentation. Figure 1. a) Polarization curves measured on 7 segments in one row of the segmented cell, compared to the polarization curve measured on a single 5 cm2 differential cell. Electrodes are fabricated in the same way using the same PGM-free catalyst. b) H2/air polarization curves measured in the segmented cell for electrodes fabricated using the same PGM-free catalyst with different ionomer wt. %.