ON THE SEARCH FOR EFFICIENT CATALYSTS FOR OXYGEN REDUCTION REACTION

Abstract

Cathodic oxygen reduction reaction (ORR) has technological importance in the development of electrochemical devices such as fuel cells, zinc-air batteries and many industrial electrolytic processes. The sluggish kinetics of oxygen reduction reaction under experimental conditions causes the deterioration of performance of the devices. In order to improve the performance with enhanced kinetics, a better electrocatalyst is necessary. Though various compositions and configurations of the electrocatalysts have been proposed, the search for an efficient electrocatalyst has always been continuing. Even though, one tends to believe that the basic principle of oxygen reduction reaction is known, the formulation of a compositional electrode, satisfying the basic requirements of the components of the overall reaction has not yet been successful. This could be due to various factors inherent in extrapolation as well as the gap existing between theoretical formulation and experimental realization. Conventional carbon supported platinum is the best catalyst for oxygen reduction reaction. However, it is associated with several drawbacks besides the high cost. Even though it is commercially exploited, the overpotential is found to be around 300-400 mV and the kinetics is not fast enough. In addition, Pt is expensive and less abundant. The main objective of the present work is to develop both Pt and non-Pt based oxygen reduction electrocatalysts for exploitation in electrochemical devices. Conventional metals and metal alloys, chalcogenide and macrocycle based systems have been prepared in nano-size range and examined for the oxygen reduction reaction in this study. The prepared materials are characterized by using variety of techniques such as X-ray diffraction (XRD), Transmission electron spectroscopy (TEM), Scanning electron spectroscopy (SEM), Energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), Ultraviolet-Visible (UV-Vis) spectroscopy. The electrochemical measurements are carried out in aqueous media at room temperature using cyclic voltammetry. The ORR activity of the electrocatalyst was evaluated from the steady-state cyclic voltammograms and the evaluation of the performance in kinetically controlled region was measured by chronoamperometry. In the present work, carbon supported Pt nanoparticles of different sizes were synthesized by polyol reduction method. The optimum size of Pt for maximum ORR activity is determined. To improve the ORR activity and reduce Pt loading, carbon supported Pt-M (M = Fe, Co and Cr) alloys were synthesized by polyol reduction method and the role of alloying element towards enhanced ORR activity was studied. As an alternative to Pt, carbon supported Pd, Pd-Co-M (M = Mo and Au) alloys, Ru and RuxSey (x = 1 and y = 0-1) were synthesized by reverse microemulsion (RME) method and the ORR activity was investigated. Density functional calculations were performed to study the facile reduction of oxygen on MN4 (M = Fe and Co) clusters. MNxCx (M = Fe and Co) clusters were prepared by pyrolyzing the corresponding metal macrocycles supported on carbon and the ORR activity was investigated.