Supercritical CO2 Deposited Pt-Ir-Co into Carbon Aerogel As a Potential Catalyst for Methanol Oxidation and Oxygen Reduction Reactions

Proton Exchange Membrane (PEM) fuel cells and Direct Methanol Fuel Cells (DMFCs) are emerging sustainable energy technologies that have potential to replace rechargeable batteries in portable electronics, and in automobiles. In PEMFCs, high noble metal (Pt) loading is required at cathode due to major voltage losses associated with slow Oxygen Reduction Reaction (ORR) kinetics. On other hand, Pt-Ru/C has been successfully used as state of art anode catalysts in Direct Methanol Fuel Cell (DMFC), to oxidized chemisorbed CO (byproduct of methanol oxidation) on Pt surface via bi-functional mechanism. Nevertheless, high PtRu loading is necessary for improved DMFC performance. To make these fuel cell technologies commercially viable, one strategy is to reduce the noble metal loading by replacing (or alloying) with non-precious transition metals. Consequently, the study of Pt- based bimetallic and trimetalic catalyst such as PtFe, PtCo, PtAu, PtNi, PtVFe, PtNiFe, etc. have gained much importance to develop active and chemically stable catalysts via scalable synthesis methods.

There are several considerations to choose trimetalic PtIrCo alloy as the material of interest. Among them: (i) Pt and Ir in the bulk phase form a stable alloy; (ii) Alloy formation  prevents acidic dissolution of transition metal and (iii) Electronic interaction among Pt, Ir and Co in PtIrCo improves electrochemical behavior. In addition, synergistic electrochemical performance of PtIrCo and decrease in binding strength of OH species especially for Pt₂IrCo (111) have been observed from recent studies  ADDIN EN.CITE  ADDIN EN.CITE.DATA [1, 2] . In this context, our work is focused on PtIrCo impregnated into carbon aerogel via supercritical CO₂method. The study of structure dependent electrochemical oxidation of methanol and oxygen reduction kinetics is emphasized.

Organic precursors of platinum, iridium and cobalt were used in 2:2:1 ratio in order to impregnate into mesoporous carbon aerogel from Aspen aerogels^® in supercritical CO₂ vessel at 3000 psi at 80^oC. The resultant composite was thermally treated at 600^oC and 900^oC in inert atmosphere (N₂-gas flowing at 100 ml/min) for 1 hour at 10^oC/min ramp rate.

Energy dispersive spectrum of as synthesized materials (Fig. 1) shows the relative composition in terms of weight and atomic percentage of Pt, Ir and Co along with scanning electron micrograph image. X-ray diffraction spectrum of as-synthesized catalyst (indicated as PtIrCo/C-SCF in Fig. 2) is compared with that of heat treated materials at 600^oC and 900^oC. As synthesized materials are mostly amorphous and show partial crystalline phase of PtRuCo along (111). PtIrCo/C catalyst heat treated at 600^oC (PtIrCo/C-600^oC) improved crystal structure along (111), (220) and (331) planes while PtIrCo/C-900^oC forms complete fcc-structure having peaks along (111), (200), (220), (331) and (222) directions. Absence of separate iridium and cobalt peaks confirms the presence of PtIrCo alloy in carbon aerogel.

Morphology of the catalysts from BET surface area analysis and pore size distribution, catalyst structure from TEM study, and X-ray photo-electron spectroscopy (XPS) of the materials will be presented. Electrochemical behavior by means of cyclic voltammetry, linear sweep voltammetry, rotating ring disc electrode (RRDE), CO-stripping, and chronoamperometry in acidic and alkaline media will be discussed. The discussion will specially emphasized on structure dependent electrochemical surface area (ESA), onset potentials for methanol and CO-oxidation, CO-tolerance, ORR over-potential, ORR kinetics regarding 4 electron path, mass activity and specific activity of methanol oxidation and oxygen reduction.

Acknowledgements:

This work is financially supported by NSF EPSCoR grant no: 0903804. Authors would also like to acknowledge partial financial support from Dr. Fong’s group at SDSM&T.

References:

[1] A. L. Smirnova, Y.-L. Hu, L. Zhang, M. Aindow, P. Menard, P. Singh, et al., "Synthesis of Novel Electrode Materials Using Supercritical Fluids," ECS Transactions, vol. 19, pp. 9-21,  2009.

[2] R. Loukrakpam, B. N. Wanjala, J. Yin, B. Fang, J. Luo, M. Shao, et al., "Structural and Electrocatalytic Properties of PtIrCo/C Catalysts for Oxygen Reduction Reaction," ACS Catalysis, vol. 1, pp. 562-572, 2011.