Systematic Investigation of Carbon Supported Ru Based Chalcogenides As ORR Catalysts and Their Application in Safc

Abstract

The primary consideration in enabling solid acid fuel cells operating at 250oC such as those using CsH2PO4 (CDP) from reaching commercial reality is based on the cost of the MEA, where noble metal loading is a critical factor. In addition there is the perennial issue of carbon corrosion. The former has been extensively considered in the seminal article by Gasteiger et. al.1 Typical operating conditions for CDP, 250oC, Reformate/Air are 200 mA/cm2, 0.7 V vs RHE. This translates to 120-140 mW/cm2. Considering the typical Pt loading of >3 mg/cm2 for CDP we have a requirement of 11-25 g(Pt)/KW. Contrast this with the current loading of 0.5-1.1 g(Pt)/KW at ≥ 0.65 V vs. RHE for a lower temperature (typically 800C) polymer electrolyte fuel cell, which represents a ten to twenty fold loading difference between the two types of fuel cell technologies. The goal therefore is to significantly lower Pt loading. This presentation critically examines the potential use of chalcogenides as oxygen reduction reaction (ORR) catalyst to either replace or embellish noble metal catalysts. Hence, the overall objectives of this effort and the associated portfolio of catalysts are specifically aimed at lowering the Pt loading requirement per KW as well as ensure durable performance over the lifetime of the fuel cell. While Pt based system is the focal point of ORR catalysis in acidic conditions, a wide range of non-noble group metals (non PGM) are viable options in alkaline condition. This disparity arises due to dissimilar double layer structures in acid and alkaline conditions, which provides a kinetic facility to the underlying electrode surface in alkaline media2. This presentation will demonstrate a detailed investigation of ruthenium based chalcogenide catalyst, which is a promising non PGM catalyst alternative. Mechanistic details showcasing ORR pathways in acid and alkaline electrolyte will be discussed. In addition to electrochemical data, in situ X- ray absorption spectroscopy (XAS) results will be discussed. Taking advantage of the XAS analysis, correlation with the electrochemical data will be drawn. The effect of chalcogen modification on Ru surface and a detailed description of the active site will be provided. Advantages such as controlled carbon content, limited carbon corrosion and resistance to anion poisoning suggest Ru based chalcogenide catalysts as potential cathode catalyst in fuel cells, such as solid acid fuel cell (SAFC). SAFC such as that using cesium dihydrogen phosphate as electrolyte exhibit anhydrous proton transfer (2) thereby enabling simpler humidification and balance of plant required for reformation of most fuels. Actual fuel cell polarization curves will be used to compare with comparable Pt/C cathodes. In conclusion, a comprehensive discussion on ruthenium based chalcogenide catalyst will be made, emphasizing on structural details, ORR mechanistic pathways and SAFC performance. Acknowledgement: The authors gratefully acknowledge the financial support from Arpa- E, US DOE under a grant led by SAF Cell Inc., Pasadena, CA. Use of Stanford Synchrotron Radiation Light Source (SSRL), Stanford Linear Accelerator Center is supported by U.S Dept. of Energy under contract number DE- AC02-76SF00515. Use of beamline 2-2 at SSRL was partially supported by National Synchrotron Light Source II, Brookhaven National Laboratory, under U.S Dept. of Energy under contract number DE-SC0012704.