Performance and Durability of Pt-Ti Catalyst for Oxygen Electroreduction

Abstract

Fuel cells are expected to be a key next-generation energy source used for vehicles and homes, offering high energy conversion efficiency and minimal pollutant emissions. The high efficiency arises from the fact that fuel cells convert chemical energy directly into electrical energy without the Carnot limitation of thermal engines. However, due to the extreme operating environment, Pt was almost exclusively the only practical catalyst for molecular oxygen electroreduction in PEM fuel cells. Recently, considerable efforts have been made to investigate synergy effects of platinum alloyed with base metals to improve the sluggish kinetics of oxygen reduction reaction of Pt catalyst. The more active Pt-alloy catalysts, in return, may arise the durability issue under the extreme environment of PEM fuel cell operation. In this presentation, results of a more active yet durable PtTi alloy catalyst will be discussed for oxygen reduction reaction in acidic media, including (i) combinatorial high-throughput discovery of PtTi alloy composition; (ii) synthesis and characterization of nano-scale PtTi particles with controllable size, phase, and individual particle composition; and (iii) electrochemical STM and ICP characterization of coarsening and corrosion of PtTi alloy catalyst in comparison with Pt and PtCo catalysts. A novel combinatorial high-throughput electrocatalyst discovery workflow and associated tools were developed [1-4], where thin films of alloys can be made through a Multi-sources Physical Vapor Deposition system [5], and the resulted materials can be electrochemically screened by a Multi-channel Rotating Disk Electrode system [6]. PtTi binary system has been screened using this methodology for activity-stability-composition relationship and the best alloy composition was identified. In addition to the discovery of alloy compositions, engineering the PtTi alloy particles in nanoscale has been a challenge. Several synthesis technologies were studied and developed to achieve capabilities of controlling particle size and particle microcomposition [7]. The results show that by careful engineering the particle size and microcomposition in nanoscale, it is able to achieve the superior electrocatalytic activity predicted by the combinatorial high-throughput discovery. Electrochemical STM technique, in combination with electrochemical potentiostating, potential cycling, and ICP analysis, was used to monitor the coarsening and base metal corrosion under dynamic fuel cell operation conditions and compared with phenomena observed from Pt and PtCo catalysts [8-10]. The results show that PtTi not only is more active than Pt and much stable than PtCo, but also can mitigate the catalyst coarsening. Reference [1] T. He and E. Kreidler, Phys. Chem. Chem. Phys. 10, 3731 (2008). “Combinatorial screening of PtTiMe ternary alloys for oxygen electroreduction”. [2] E. Kreidler and T. He, in Catalysts for Oxygen Electroreduction – Recent Developments and New Directions (Ed. T. He, Transworld Research Network) Chapter 4, p.63 (2009). “Oxygen reduction on Pt alloys: high throughput combinatorial screening of Pt binary alloys”. [3] T. He, E. Kreidler, L. Xiong and E. Ding, J. Power Sources 165, 87 (2007). “Combinatorial screening and nano-synthesis of platinum binary alloys for oxygen electroreduction”. [4] T. He, E. Kreidler, L. Xiong, J. Luo and C.J. Zhong, J. Electrochem. Soc. 153, A1637 (2006). “Alloy electrocatalysts: combinatorial discovery and nano-synthesis”. [5] T. He, E.R. Kreidler and T. Nomura, US 8,944,002 (2015). “High throughput physical vapor deposition system for material combinatorial studies”. [6] T. He, US 7,077,946 (2006). “High throughput multi-channel rotating disk or ring-disk electrode assembly and method”. [7] E. Ding, K.L. More and T. He, J. Power Sources 175, 794 (2008). "Preparation and characterization of carbon-supported PtTi alloy electrocatalysts". [8] L. Tang, B. Han, K. Persson, C. Friesen, T. He, K. Sieradzki and G. Ceder, J. Am. Chem. Soc. 132, 596 (2010). “Electrochemical stability of nanometer-scale Pt particles in acidic environments”. [9] Q. Xu, E. Kreidler, D.O. Wipf and T. He, J. Electrochem. Soc. 155, B228 (2008). "An in-situ electrochemical STM study of potential-induced coarsening and corrosion of platinum nano crystals". [10] Q. Xu, T. He and D. Wipf, Langmuir 32, 9098 (2007). "An in-situ electrochemical STM study of the coarsening of platinum islands at double-layer potentials".