Non-carbonaceous supported platinum electrocatalyst as alternative to carbon base materials toward oxygen reduction reaction for low-temperature fuel cells

Abstract This dissertation is focused on investigation of comparative evaluation of structure, composition and electrochemical performance of new electrocatalysts approach Platinum (Pt) supported on metal oxides which are specifically Titanium suboxide and Titanium nanotube suboxide (TNTS). The Pt/MOx (M: Nb, Ti, Mn, Cu and Ni…) and Pt/TixO2x-1 with intrinsic electrocatalytic activity, conductivity and excellent stability are studied in acidic media toward the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). Specifically, a Pt thin layer (monolayer) is homogeneously deposited on the surface of TixO2x-1-Mo and TNTS supports by using different methods. For almost all applied methods the Pt atoms utilization, involved in the reactions, is very high. These efforts indicate that both the Pt morphology and the support structure can significantly enhance the catalytic activity and stability of electrocatalysts toward ORR and MOR. The interest in ORR and MOR is motivated by the application of these novel electrocatalysts at the cathodes and anode side, respectively, of proton exchange membrane fuel cells (PEMFCs). In fact, PEMFCs are extensively considered as potential highly efficient devices for clean energy production in many different applications, from portable to transportation and stationary. The mechanism of PEMFCs is to convert directly the chemical energy of hydrogen (or methanol) fuel and oxygen (air) oxidant into usable electrical energy. At the cathode (the positive electrode), a considerable amount of platinum is generally required to catalyse the sluggish ORR, because of its slow kinetics. Due to the excellent conductivity and high specific surface area, carbon materials are extensively employed in both the anode and cathode to support Pt electrocatalyst in PEMFCs. A major drawback of Pt/C electrocatalyst is at the cathodic site in which the electrode potential and pH are relatively high: carbon materials suffer corrosion (C + H2O → CO2 + 4H+ + 4e–, 0.207 V vs NHE at 25 °C) with consequent dissolution of Pt metal catalyst. Such a dissolution, which is an anodic reaction, causes problems of mixed potential formation at cathodic side with subsequent decreasing of the long-term stability of the Pt electrocatalyst performance and shifting the ORR reversible potential to lower values. This drawback is one of major technical barriers to the widespread commercialization of PEMFCs as energy source. Apart from carbon corrosion, in case of methanol as fuel, DMFCs suffer from methanol cross-over through the polymeric membrane, which causes a further reduction in electroactivity performance. In fact, the oxidation of methanol at the cathode involves the formation of intermediate products, mainly CO, which are strongly adsorbed to the surface of Pt-based catalysts reducing the number of active sites of Pt and consequently the ORR activity. These phenomena, together with the high cost of Pt, are considered the primary limiting factors to the developing the large-scale commercialization of DMFCs and PEMFCs. In order to overcome the problems associated to carbon corrosion and alleviate CO poisoning, this PhD thesis demonstrates the application of metal oxides, in particular the sub-oxide TixO2x-1-M, as a potential electrocatalyst support, promoting the electrocatalytic activity, stability and poison resistance of Pt electrocatalysts. TixO2x-1, which is a non-carbonaceous material with high conductivity, stability and durability in acidic media, has great potential to be an alternative promising material to carbon supports for Pt electrocatalyst in order to further boost PEMFCs and DMFCs application.