Graphene Oxide Modified Pt/C and Pt2Ru3/C for Enhanced Carbon Monoxide Tolerable Fuel Cell Anode Catalyst

Abstract

The commercial viability of residential fuel cell technology operating on reformed H2 strongly demands to find efficient, CO tolerable and long lasting anode catalyst.1 Currently, Pt-based electrocatalysts are widely used for this purpose. However, one of the major drawbacks of Pt-based anode is the surface poisoning by carbon monoxide (CO) leading to severe decrease in efficiency and long-term performance of catalyst.2 This problem has been mitigated by either (i) synthesis of catalysts with higher CO oxidation capability or (ii) suppressing the adsorption of CO on the catalyst surface. Within the first context, additives often partially cover the alloy surface and lead to a decrease in the electrochemically active Pt surface area and thus the HOR activity. Therefore, in the current work, the second strategy has been pursued. Here we use graphene oxide (GO) to improve the CO tolerance of Pt/C and Pt2Ru3/C catalysts by suppressing the adsorption of CO on the catalyst surface. Graphite oxide was prepared from graphite (Z-5F, Average particle size 4μm, ≥98.8 %, Ito Graphite Industry Co., Ltd.) by a modified Hummers' oxidation method.3 Graphite oxide was then dispersed in 2-propanol to exfoliate graphite oxide into GO. Commercial Pt/C (TKK TEC10E50E, 46.6 mass% Pt) and Pt2Ru3/C (TKK TEC61E54, 30 mass% Pt and 23.3 mass% Ru) were used as-received. GO–Pt/C and GO–Pt2Ru3/C were prepared by stirring, sonication and drying of a mixture of GO colloid with Pt/C and Pt2Ru3/C powder respectively. COad stripping measurement was performed by cyclic voltammetry (CV) at 10 mV s-1 in N2 saturated 0.1 M HClO4. Chronoamperometry (CA) was conducted in 0.1 M HClO4 saturated with either pure H2 or 250 ppm CO/H2 at 20 mV vs. RHE with 400 rpm rotation. Accelerated durability test (ADT) was conducted by square-wave potential stepping between 5 and 400 mV vs. RHE for 3000 cycles with a holding time of 3 seconds at each potential. TEM images confirm that GO wraps around Pt/C or Pt2Ru3/C and are distributed uniformly. XRD and XPS data support the co-existence of GO and Pt/C or Pt2Ru3/C. COad stripping of Pt/C and GO–Pt/C suggest that in contrast to Pt/C, in case of GO–Pt/C a pre-oxidation peak was observed at 0.5-0.7 V before the main CO oxidation peak (Fig. 1a). This pre-oxidation peak may be attributed to oxidation of weakly adsorbed CO. The apparent HOR activities in pure H2 for Pt/C (190 A (g-Pt)−1) and Pt2Ru3/C (148 A (g-PtRu)−1) do not change after the addition of GO, thus GO does not interfere with the HOR. Fig. 1b compares CA (HOR current) of Pt/C, GO–Pt/C, Pt2Ru3/C and GO–Pt2Ru3/C in 250 ppm CO/H2 saturated HClO4 after 3000 cycles of ADT. The HOR current for GO–Pt/C was 109 A (g-Pt)−1, which is 1.3 times higher than that of Pt/C (84 A (g-Pt)−1. The HOR current for GO–Pt2Ru3/C (89 A (g-PtRu)−1) is 1.3 times higher than that of commercial Pt2Ru3/C (68.4 A (g-PtRu)−1). The enhanced CO tolerance and durability of GO–Pt/C and GO–Pt2Ru3/C may be attributed to i) GO protects the coagulation of Pt particles and ensure the sufficient stability of Pt/C catalyst, ii) steric hindrances arise from oxygen-containing functional groups of GO surface decrease the adsorption of CO on Pt surface and iii) the electron transfer between the oxygen atoms and Pt nanoparticles could lead to the enhanced electron density of Pt via a ligand effect which in turns decrease the attraction towards CO. The current findings suggest that the prepared GO–Pt/C and GO–Pt2Ru3/C catalysts have great potential for use in fuel cell anode. Acknowledgments This work was supported in part by the ‘Polymer Electrolyte Fuel Cell Program’ from the New Energy and Industrial Technology Development Organization (NEDO), Japan. References E. Antolini, Appl. Catal. B Environ., 74, 324 (2007), E. Antolini, Appl. Catal. B Environ., 74, 337 (2007).M. Watanabe, T. Sato, K. Kunimatsu, and H. Uchida, Electrochem. Acta, 53, 6928 (2008).S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 80, 1339(1958). Figure 1