Activity and Stability of Dispersed Multi Metallic Pt-based Catalysts for CO Tolerance in Proton Exchange Membrane Fuel Cell Anodes

Anais da Academia Brasileira de Ciências (2018) 90(1 Suppl. 1): 697-718 (Annals of the Brazilian Academy of Sciences) Printed version ISSN 0001-3765 / Online version ISSN 1678-2690 http://dx.doi.org/10.1590/0001-3765201820170559 www.scielo.br/aabc | www.fb.com/aabcjournal Activity and Stability of Dispersed Multi Metallic Pt-based Catalysts for CO Tolerance in Proton Exchange Membrane Fuel Cell Anodes AYAZ HASSAN and EDSON A. TICIANELLI Instituto de Química de São Carlos-USP, Avenida Trabalhador São Carlense, 400, Parque Arnold Schimidt, Caixa Postal 780, 13560-970 São Carlos, SP, Brazil Manuscript received on July 21, 2017; accepted for publication on September 6, 2017 ABSTRACT Studies aiming at improving the activity and stability of dispersed W and Mo containing Pt catalysts for the CO tolerance in proton exchange membrane fuel cell (PEMFC) anodes are revised for the following catalyst systems: (1) a carbon supported PtMo electrocatalyst submitted to heat treatments; (2) Pt and PtMo nanoparticles deposited on carbon-supported molybdenum carbides (Mo2C/C); (3) ternary and quaternary materials formed by PtMoFe/C, PtMoRu/C and PtMoRuFe/C and; (4) Pt nanoparticles supported on tungsten carbide/carbon catalysts and its parallel evaluation with carbon supported PtW catalyst. The heattreated (600 oC) Pt-Mo/C catalyst showed higher hydrogen oxidation activity in the absence and in the presence of CO and better stability, compared to all other Mo-containing catalysts. PtMoRuFe, PtMoFe, PtMoRu supported on carbon and Pt supported on Mo2C/C exhibited similar CO tolerances but better stability, as compared to as-prepared PtMo supported on carbon. Among the tungsten-based catalysts, tungsten carbide supported Pt catalyst showed reasonable performance and reliable stability in comparison to simple carbon supported PtW catalyst, though an uneven level of catalytic activity towards H2 oxidation in presence of CO is observed for the former as compared to Mo containing catalyst. However, a small dissolution of Mo, Ru, Fe and W from the anodes and their migration toward cathodes during the cell operation is observed. These results indicate that the fuel cell performance and stability has been improved but not yet totally resolved. Key words: Heat treatment, CO tolerance, stability, carbides, quaternary catalysts. INTRODUCTION Nowadays different varieties of fuel cells are in use for different applications, but the proton exchange membrane fuel cells (PEMFCs) have gained considerable attention among them due to Correspondence to: Edson Antonio Ticianelli E-mail: * Contribution to the centenary of the Brazilian Academy of Sciences. characteristics properties, such as low operating temperature, high power density, high efficiency in energy conversion etc. Among these PEMFCs, the fuel cell that operates with hydrogen in the anode and oxygen/air in the cathode is the most efficient one (Peighambardoust et al. 2010). Pt is the most active metal for these reactions (Ralph and Hogarth 2002), but an expensive metal, therefore various attempts have been made to use its reduced amount An Acad Bras Cienc (2018) 90 (1 Suppl. 1) 698 AYAZ HASSAN and EDSON A. TICIANELLI in the electrodes by alloying with other transition metals and supporting the resultant composites on substrates with high surface area, such as carbon black (Antolini 2009). Due to the very low anode overpotential of the hydrogen oxidation reaction (HOR) on Pt, particularly in acid media, H2 is supposed to be the best fuel, which can be used in the anode of PEMFC for obtaining sufficient high performance in these cells, but unfortunately the use of pure H2 in these system is highly unfavorable d u e t o the lack of required infrastructure for i t s p r o d u c t i o n . In order to overcome this problem, some alternative methods have been proposed such as the reforming of fuels such as methanol, ethanol or natural gas, allowing H2 to be obtained in situ or on board, resulting in t h e formation of a gas mixture of composition with 75% H2, 25% CO2 and 1-2% CO (Baschuk and Li 2003). The CO concentration in the resultant gas mixture can be further reduced through succeeding clean up steps to sufficient low levels (10-100 ppm), even such a l o w concentration of CO in the anode feed stream can poison the Pt catalyst deployed in the anode. This blocks the active sites of the electrocatalyst available for H 2 chemisorption and subsequent electro-oxidation, considerably decreasing the catalyst activity, especially i n l o w temperatures operation systems (70-150 °C) (Manasilp and Gulari 2002, Urian et al. 2003, Wee and Lee 2006, Ticianelli et al. 2005). Therefore, CO poisoning is of fundamental concern in the PEMFCs anode and hence a major fence in the large scale commercialization of PEMFC systems. Several methods have been reported in the literature for overcoming the CO poisoning effect, the most common being the physicochemical changes in Pt electrocatalyst by adding a second or third transition metal or with the formation of oxide or carbide phase employing as the catalyst support (Chung et al. 2007, Leng et al. 2002). An Acad Bras Cienc (2018) 90 (1 Suppl. 1) In general, the catalysts e m p l o y e d in H2/ O 2 fuel cell anodes are largely composed of nanoparticles of PtM bimetals (where M=transition metal), such as PtMo, PtRu, PtW, PtRh, etc. (Hassan et al. 2014a, Lee et al. 2005, Pereira et al. 2006, 2009, Santiago et al. 2003), with the PtMo giving the best catalytic performance, when the PEMFC single cell anode is supplied with H2 containing CO. Other more recently investigated materials are Pt3Co, Pt/FeOx and Pt/BeO (Liu et al. 2016, Kwon et al. 2016, Zhang et al. 2017). Earliest works involving the use of PtMo alloys were made in the end of the 1990 by Grgur et al. (1997, 1998). They used well-characterized PtxMo (100-x) (x = 70 and 75) bulk alloys and rotating disk electrode for investigating the hydrogen oxidation reaction in the presence of CO. In these works, it was observed that the existence of Mo atoms alongside Pt on the surface of the catalyst results in an enhancement of H2 oxidation caused by the oxidative decrease of the adsorbed CO coverage on Pt. Researches were also conducted involving testing of carbon-supported PtMo nanocrystals in liquid electrolyte (Grgur et al. 1999) and in single PEMFC under operational conditions (Mukerjee et al. 1999). Here, a two-to-three fold enhancement of CO tolerance in the PEMFC is reported for a PtMo/C catalyst (Pt:Mo atomic ratio of 87:13), compared to PtRu/C. On the other hand, theoretical calculations had shown that in cubo-octahedrical PtMo nanoparticles (Wang et al. 2005), the Pt atoms weakly segregate to the particle surface forming core-shell nanostructures with the shell enriched by 5-14% Pt, compared to the bulk composition. Nowadays, it is generally accepted that the association between Pt and a second metal improves the electrocatalytic activity, because it all (...truncated)