Carbon Supported IrRu4 Catalyst for the Cell Reversal Tolerant Anode in Automotive Pemfcs

Abstract

Design of highly durable, electro-active and cost-effective catalyst to replace prevalent Pt has been a major issue to researchers in a polymer electrolyte membrane fuel cell (PEMFC) community. For past decades, the catalyst degradation, during the transient conditions, such as start-up/shut­down and cell reversal, in automotive fuel cells has gained large attention due to its irreversible consequences in the membrane electrode assembly (MEA). It was reported earlier that binary aIloy of Ir and Ru supported on carbon can be used for hydrogen oxidation reaction (HOR) catalysts for PEMFCs. Among various Ir:Ru atomic compositions, Ir:Ru=1:4 showed the best HOR activity, confirmed by rotating disk electrode (RDE) half-cell test. Furthermore, Ir and Ru are known to possess water oxidation properties, in other words, each can catalyze the oxygen evolution reaction (OER) from the water. Due to this OER-enabling feature, the Ir and Ru were often used as additives to promote the water oxidation over carbon oxidation reaction in order to protect the catalyst layer from the collapse when the catalyst layer was abruptly exposed to a high potential for various reasons. The Ir and Ru exhibit oxygen reduction reaction (ORR) activity to certain extent, however they typically show inferior ORR characteristics compared to the platinum so they are scarcely applied to the cathode catalysts in MEAs.In this presentation we propose to utilize multi­functional IrRu4 nanoparticles supported on carbon, IrRu4/C, as an anode catalyst for MEAs, particularly to be used in the fuel cell electric vehicles (FCEVs). IrRu­4’s material costs only as high as 40% of widely used platinum. Therefore during the normal fuel cell operation, IrRu4/C can be used as anode electro-catalyst at a reduced cost while showing the Pt-similar performance. And during the transient conditions of FCEV operations, the MEA durability can be retained with IrRu4’s other interesting properties.IrRu4/C was synthesized by a simple impregnation method using metal salts and carbon support and a successive reduction in a hydrogen atmosphere at higher temperature. The synthesized catalysts were characterized with XRD, ICP, TGA, and TEM for physicochemical properties. And it underwent the RDE half-cell tests for various electrochemical activity measurements, such as HOR, OER, and ORR. IrRu4/C and Pt/C were each applied to the anode in the MEA, and the single-cell IV performance and anode polarization tests were carried out at various operating conditions (cell temperature, relative humidity, and back pressure). The cell reversal tolerance of IrRu4/C and commercial Pt/C anode MEAs was also measured by subjecting each MEA to the anode’s fuel starvation condition. The IrRu4/C anode catalyst showed Pt-anode comparable MEA performance and Pt-similar hydrogen oxidation catalytic activity. Moreover, IrRu4/C anode exhibited superior durability (~120 times better) over Pt/C anode under cell reversal condition. This is because during the cell reversal condition, IrRu4 promoted water oxidation reaction so that carbon oxidation reaction was avoided. For Pt anode MEA, since Pt has low OER activity, the catalyst layer oxidation took place to severely impact the MEA during the fuel starvation. The prospective benefits earned from this work, that is replacing Pt/C anode to IrRu4/C anode for a PEMFC in a FCEV are the following: firstly, the cost down of the MEA while preserving the performance, secondly the anode catalyst layer protection during the cell reversal, thirdly, although not studied in detail in this work, the cathode catalyst layer protection during the start-up/shut-down.