Stylianos G. Neophytides

Abstract:

Pt supported on carbon electrocatalysts are the most efficient and stable materials for both the oxygen reduction reaction (ORR) at the cathode and the hydrogen oxidation reaction (HOR) at the anode of polymer electrolyte membrane fuel cells (PEMFCs) (1). In this respect, there is increasing demand to reduce cost and therefore, the amounts of Pt used. This can be achieved by increased catalyst activity and/or utilization (2). To reach this goal, there are two approaches: (a) enhancing the specific activity or (b) increasing the specific surface area of the catalyst by forming a fine dispersion. The performance and stability of the (electro) catalysts strongly depends on the physicochemical characteristics, such as the surface area, the crystalline structure, the size and shape of the particles, and the interactions with the support. Both approaches for Pt reduction can be followed separately or combined by exploiting the differentiations induced to the metal by the surface chemistry of the support to result in customized properties and control its performance. When the dispersion of the metal is high, its metal atom is accessible to reactants and available for catalysis, maximizing the efficiency of the metal and minimizing the cost. Reducing the size of the metal in atoms or small groups of atoms can significantly increase both the active surface and the activity of the catalyst through diversification or strengthening of the metal-support interactions3.
In this work, we have developed Pt/f-MWCNTs (f-MWCNTs=covalently functionalized MWCNTs) based electrocatalysts with different surface functionalities and Pt loadings. The deposition of the metal was achieved by using the polyol synthetic procedure: reduction of metal precursor salts in an ethylene glycol solution. Through a structural and chemical characterization study of the materials, the introduction of certain groups on the sidewalls of the carbon support resulted in differentiation of the properties, not only in terms of quantitative deposition and dispersion, but also with respect to metal-support interactions, platinum crystal properties and/or oxidative states. The present work addresses scientific issues regarding the most challenging core component of a PEM fuel cell: the Pt based electrocatalyst. This work proposes a comprehensive effort to explore a new approach and exploit the differentiations induced on the metal by the surface chemistry of the support. The introduction of pyridines on the sidewalls of the carbon support can differentiate the metal deposition, not only in terms of dispersion and the obtained morphology, but also with respect to metal-support interactions on platinum properties and its oxidative state. The aim is the interpretation of the catalyst’s electrochemical behavior through a structural and physicochemical characterization study. It is shown that the substrate can play a decisive role on the size and functionality of the electrochemical interface. This approach constitutes a promising route for developing materials with innovative features aiming to a serious reduction in the Pt loads, thus resulting into increased catalyst activity and metal utilization.

1. Yang C., Costamagna P., Srinivasan S., Benziger J., Bocarsly A.B. (2001). J Power Sources, 103:1-9.
2. Notter A D., Kouravelou K., Karachalios T., Daletou M.K., Haberland N.T. (2015). Energy Environ Sci, 8:1969-1985.
3. Flytzani-Stephanopoulos M., Gates B.C. (2012). Annu. Rev. Chem. Biomol. Eng., 3:545-574.