Nanostructured Fuel Cell Catalysts

Abstract: A long term challenge for society is to develop sustainable energy systems to reduce the dependence on fossil fuels and to decrease air pollution. This requires new alternatives regarding the supply of energy and increased efficiency of the future energy systems. Fuel cell technology is one such possible alternative. Fuel cells convert chemical energy directly to electrical energy with small environmental effects. Today, the greatest disadvantage with the fuel cell technology is the cost. Low temperature fuel cells, such as polymer electrolyte fuel cells (PEMFC), require expensive noble metal electrocatalysts. Improved catalyst activity with lower Pt content could contribute to a decrease in fuel cell cost. Real fuel cell catalysts are not suited for fundamental studies they consist of nm sized particles deposited on porous supports with a three dimensional structure. To develop improved and cost-effective fuel cells detailed studies on catalyst reactions are important. Model systems of well defined nanostructured (electro) catalysts are well suited for such studies. The main objective of this work was to explore and establish a platform to fabricate stable model electrocatalysts, using modern nanotechnology methods, and to test them in electrochemical / fuel cell systems. Nanofabrication techniques, such as colloidal lithography and thin film deposition, were used to prepare well-defined nanostructures. Surface science and microscopy methods were used for characterization. The electrocatalytic properties were evaluated with respect reduction of oxygen, and oxidation of methanol, formaldehyde, and CO. A general result from this study is the establishment of a nanotechnology platform for evaluation of various aspects of fuel cell performance, particularly in PEMFCs. Experiments with nanostructured catalysts on glassy carbon supports demonstrate their suitability for studying electrocatalytic reactions and transport processes. In addition, results on catalysts prepared by thin film deposition on Nafion membranes show that this approach is well suited for material screening and for studies regarding material stability and distribution in the catalyst layer. In particular, I found that the active surface area and performance were increased by deposition of TiO2 in between the Nafion membrane and the Pt layer. The TiO2 layer was also shown to have good proton conduction properties. Material screening for oxygen reduction showed that the activity for the single material films ranks as Pt > Ir > Au > TiO2. When Pt is mixed with Ir or TiO2, the activity is similar to pure Pt samples with twice the amount of Pt. Pt mixed with Au results in poor activity. Interesting extensions of this work include smaller particles, new material combinations and theoretical modelling. Keywords: PEMFC, model catalyst, electrocatalyst, nanofabrication, screening, Pt, TiOx, polymer electrolyte, colloidal lithography, thin film evaporation, Nafion, oxygen reduction, methanol oxidation, formaldehyde oxidation, acetic acid oxidation, CO oxidation