Investigation of Reduced Graphene Oxide with F-Doped SnO2 as Catalyst Support in Fuel Cells

Abstract

A main challenge in commercialisation of fuel cells is the performance loss with time of operation. The platinum catalyst is subjected to electrochemical instability, and carbon black as commonly used catalyst support suffers from corrosion.[1,2] To improve the fuel cell stability, alternative support materials can be used. Application of graphene-based carbon in low temperature PEMFCs has demonstrated higher durability than conventional carbon black.[3] The combination with corrosion resistant metal oxides provides additional binding sites for Pt nanoparticles[4,5] and can prevent graphene materials from restacking causing mass transport limitations in fuel cells.[6] In this study, reduced graphene oxide (rGO) is synthesised in two steps. Graphene oxide is received via Hummer’s method[7] and then thermally reduced to rGO. In the following synthesis, fluorine-doped tin (IV) oxide (FTO) nanoparticles are deposited on rGO sheets. Therefore, a sol-gel method using tin tetrachloride precursor followed by addition of NH4F and hydrothermal treatment is carried out on rGO.[8,9] Figure 1a shows the achieved crystalline FTO structure which is similar to cassiterite tin (IV) oxide. Using Scherrer equation an average FTO particle size of 4.8±0.4 nm is calculated. Transmission electron microscopy (TEM) in Figure 1b reveals successful deposition of the FTO nanoparticles on rGO. In the last synthesis step, Pt is precipitated on FTO-rGO via chemical reduction of chloroplatinic acid with ethylene glycol.[4] Then, Pt/FTO-rGO catalyst is intensively investigated for electrochemical stability under fuel cell operation conditions by the use of an accelerated stress test (AST). Within an electrochemical cell the catalyst is exposed to 1,000 potential cycles between -0.02 and 1.40 VRHE at a scan rate of 500 mV s-1under oxygen saturation. To ensure a holistic analysis of possible catalyst and support degradation, this AST is performed twice. During the first test, a rotating disk electrode allows the study of the oxygen reduction reaction (ORR) mechanism and activity at different rotation speeds (400, 900, 1200, 1600, 2000 and 2500 rpm). The electrochemical active surface area (EASA) is carefully determined by hydrogen underpotential deposition as well as carbon monoxide stripping voltammetry. In the second test run, the rotating disk electrode as working electrode is replaced by a catalyst coated grid for identical location TEM analysis. First, the untested Pt/FTO-rGO catalyst on the grid is analysed by TEM and then the catalyst on grid is stressed by exactly the same potential cycling. After performing AST, the same Pt/FTO-rGO particles are investigated again under the microscope. Thus, possible Pt degradation processes like agglomeration can be revealed which allows the supplementation of electrochemical investigations. Fig. 1: XRD pattern of FTO-rGO with comparison to crystalline structure of cassiterite SnO2 (ICDD 98-000-9463) (a) and TEM image of deposited FTO particles on rGO (b). [1] D. Schonvogel, M. Rastedt, P. Wagner, M. Wark, A. Dyck, Fuel Cells 2016, 16, 480-489. [2] Y. Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, E. F. Holby, D. Morgan, Top. Catal. 2007, 46, 285-305. [3] Y. Shao, S. Zhang, C. Wang, Z. Nie, J. Liu, Y. Wang, Y. Lin, J. Power Sources 2010, 195, 4600-4605. [4] R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V. V. Viswanathan, S. Park, I. A. Aksay, Y. Lin, Y. Wang, J. Liu, J. Am. Chem. Soc. 2011, 133, 2541-2547. [5] Y. Liu, W. E. Mustain, J. Am. Chem. Soc. 2013, 135, 530-533. [6] S. Park, Y. Shao, H. Wan, P. C. Rieke, V. V. Viswanathan, S. A. Towne, L. V. Saraf, J. Liu, Y. Lin, Y. Wang, Electrochem. Commun. 2011, 13, 258-261. [7] S. Ratso, I. Kruusenberg, M. Vikkisk, U. Joost, E. Shulga, I. Kink, T. Kallio, K. Tammeveski, Carbon 2014, 73, 361-370. [8] D.-J. Guo, Z.-H. Jing, J. Colloid Interface Sci. 2011, 359, 257-260. [9] H. Xu, L. Shi, Z. Wang, J. Liu, J. Zhu, Y. Zhao, M. Zhang, S. Yuan, ACS Appl. Mater. Interfaces 2015, 7, 27486-27493. Figure 1