Platinum and Platinum alloy electrocatalysts for oxygen electrodes in Proton Exchange Membrane Fuel Cells : electrochemical and x-ray absorbtion spectroscopic investigation

Abstract

Research on proton exchange membrane fuel cells (PEMFCs) has gained momentum due to the prospects of attaining high energy and power densities, which are essential for transportation and space applications. One of the major problems of PEMFCs is the high overpotential of the cathodic oxygen reduction reaction (ORR) on the Platinum electrocatalyst. Platinum has been the most favored electrocatalyst for all low and medium temperature fuel cells. Two possible approaches for further improving the ORR activities in PEMFCs are as follows: (i) optimization of the structure of the electrode/electrolyte interface by sputter-deposition of a thin layer of Pt onto the front surface of the electrode; and (ii) use of Pt alloy electrocatalysts. The primary objective of this dissertation is to investigate the electrocatalysis of ORR using the mentioned approaches. The first investigation consisted of electrochemical characterization to determine the electrode kinetic parameters (Tafel slopes, exchange current densities, activation energies and reaction orders) on the fuel cell electrodes with a thin layer of Pt on the front surface. The second investigation was to determine the role of alloying element on the ORR electrocatalysis using electrochemical and in-situ X-ray absorption spectroscopic (XAS) techniques. Of special significance is that the two complementary parts of XAS, the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) were used to provide information on the electronic and geometric parameters of the electrocatalysts in the in-situ electrochemical environment. The results demonstrated that sputter-deposition of Pt on the front surface of the electrode improved the ORR electrocatalysis, and is in all probability due to the non teflonized morphology of the electrode/electrolyte interface. Binary alloys of Pt with the first row transition elements (Cr, Mn, Fe, Co and Ni) show up to three fold improvement in performance for the electrocatalysis of ORR as compared to that on Pt/C electrocatalyst. Correlation of the electrocatalytic activities with electronic and geometric properties as obtained from in-situ XAS studies show volcano type relationship for both Pt and Pt alloy electrocatalysts. All the Pt alloys show higher Pt 5 d-orbital vacancies per atom as compared to the Pt/C electrocatalyst. The alloys also exhibited lattice contractions, in terms of the Pt-Pt bond distances, relative to the Pt/C electrocatalyst. There was no evidence of any redox type process involving the alloying element. The higher electrocatalysis by the Pt alloys as compared to Pt/C electrocatalyst could be accounted on the basis of the interplay of the electronic & geometric parameters and their combined effect on the chemisorption characteristics of oxygenated species.