Evaluation of the stabilizing effect of CeO2 on the electrochemical performance of Pt/CeO2/C as a catalyst for the oxygen reduction reaction in the cathodic compartment of proton exchange membrane fuel cells

Proton Exchange Membrane Fuel Cells are the best candidates for replacing the traditional combustion engines used in the automotive sector. The most widespread catalysts implemented in these systems rely on platinum, which is nanosized and supported on high surface area carbons. The main hindrances of PEM Fuel Cells commercialization lie firstly on the high costs of platinum, secondly on the sluggish oxygen reduction reaction (ORR) at platinum-based cathode and finally on the short-term durability associated with degradative mechanisms, which cause the loss of cell performance during its working. Carbon corrosion, due to an oxidative process caused by the operating conditions of PEMFCs, is maybe the worst since it accelerates further degradative phenomena such as sintering and detachment of platinum nanoparticles. Therefore, research has investigated on supports capable of guaranteeing a long-term durability throughout the operating conditions of PEM. Metal oxides have been demonstrated as promising supports as they manifest an elevated corrosion resistance and avoid the migration and detachment of platinum nanoparticles owing to the strong metal-support interaction created at the interface. It has been found that this effect can have an impact on the catalytic activity, as well since electron transfer phenomena can occur at the interface between the metal and metal oxide. Nevertheless, metal oxides taken alone does not afford a sufficient electron conductivity, which is the main requirement of an electrode. Therefore, metal oxide-carbon hybrid supports have been developed in the last few years to achieve an appreciable electrical conductivity. Among the most promising metal oxide ceria, CeO2, utilized in a wide variety of industrial applications, is rated as an interesting compound due to its intriguing feature of easily switching from Ce4+ to Ce3+ and its high oxygen storage capacity. These unique peculiarities have led scientists to study the effects of its presence on the cathodic compartment of PEM fuel cell. It has been proven that its capability of easily changing the oxidation states promotes the quenching of aggressive radicals that can destroy the proton conductive membrane (Nafion) over time. Furthemore, the addition of ceria to Pt/C catalysts has been found to be helpful for improving the CO tolerance in the oxidation of methanol in direct methanol fuel cells and for enhancing the ORR catalytic activity and durability of Pt/C catalysts thanks to the oxygen spillover and the strong metal support interaction. In this thesis ceria was deposited together with platinum by one pot or two-step approach on a commercial carbon, Vulcan XC72 and on home-made biomass-based carbon supports. Home-made carbons were prepared via a pyrolysis treatment of cheap carbon precursor, namely agarose and chitosan, mixed with a template, Pluronic F127 and a graphitizing agent, iron, which was selectively removed from the carbon matrix through an acid washing leaving intact the morphology of carbon. The best performing catalyst of this thesis Pt_3h/CeO2_750_15/CC was synthesized by firstly depositing ceria at 750 °C for 15 min and afterwards platinum at 300 °C for 3 h on a highly graphitized chitosan-based carbon. The final catalyst contains up to 5 wt% of CeO2 and 25 wt% of platinum loading. By performing a linear sweep voltammetry at 1600 rpm and 50 mV s-1 on a rotating disk electrode, its catalytic activity surpassed that of standard Pt/C (with 30 wt% of Pt) showing a mass activity 2.7 times higher and a half-wave potential shift of 30 mV towards more positive compared to Pt/C standard. In gas diffusion electrode the best performance in terms of activity and stability was obtained by using the same conditions but depositing on carbon VulcanXC72.