Oxygen reduction reaction (ORR) has considerable potential for the pro-duction of electricity, issues with water splitting and many other applications in the energy sector. But in order to increase the efficiency of the reaction an electrocatalyst needs to be introduced.

In today’s industrial devices precious and costly metals such as platinum (Pt) are used as catalysts. Other more abundant and cheaper alternatives, for example cobalt oxides, are therefore being investigated. In this thesis, pure cobalt (Co) thin films were synthesised to investigate if thin films can be used for the catalysis of ORR. This was successfully carried out by electrochemically modifying the thin films and grow catalytically ac-tive hexagonal cobalt oxide nanoparticles.  

Multicomponent system CoCrFeNi is an emerging alloy system with high research interest for its high corrosion resistance suitable for harsh environments in which the applications for ORR are found. In this thesis, CoCrFe_xNi were synthesised as thin films. The corrosion resistance of the films was investigated in addition to their catalytic activity. The effect of Fe content on these properties was also studied. The presence of Fe was crucial for the electrochemical activation of films and catalytic activity towards ORR. 

In summary, this thesis shows that cobalt based thin films can be used as catalysts combined with corrosion resistance for ORR applications.

Corrosion resistance and catalytic activity toward the oxygen reduction reaction (ORR) in an alkaline environment are two key properties for water recombination applications. In this work, CoCrFexNi (0 <= x <= 0.7) thin films were deposited by magnetron sputtering on polished steel substrates. The native passive layer was 2-4 nm thick and coherent to the columnar grains determined by transmission electron microscopy. The effect of Fe on the corrosion properties in 0.1 M NaCl and 1 M KOH and the catalytic activity of the films toward ORR were investigated. Electrochemical impedance spectroscopy and potentiodynamic polarization measurements indicate that CoCrFe0.7Ni and CoCrFe0.3Ni have the highest corrosion resistance of the studied films in NaCl and KOH, respectively. The high corrosion resistance of the CoCrFe0.7Ni film in NaCl was attributed to the smaller overall grain size, which leads to a more homogeneous film with a stronger passive layer. For CoCrFe0.3Ni in KOH, it was attributed to a lower Fe dissolution into the electrolyte and the build-up of a thick and protective hydroxide layer. Scanning Kelvin probe force microscopy showed no potential differences globally in any of the films, but locally, a potential gradient between the top of the columns and grain boundaries was observed. Corrosion of the films was likely initiated at the top of the columns where the potential was lowest. It was concluded that Fe is essential for the electrochemical activation of the surfaces and the catalytic activity toward ORR in an alkaline medium. The highest catalytic activity was recorded for high Fe-content films (x >= 0.5) and was attributed to the formation of platelet-like oxide particles on the film surface upon anodization. The study showed that the combination of corrosion resistance and catalytic activity toward ORR is possible for CoCrFexNi, making this material system a suitable candidate for water recombination in an alkaline environment.

Catalysts and electmcatalysts are crucial for energy production and storage. To develop cost-efficient systems taking advantage of functionalized surfaces, the catalysts can be synthesized as nanomaterials or thin films. In this work, cobalt thin films were deposited on low-alloyed steel using magnetron sputtering. The films are uniform with a smooth surface and a thickness of similar to 400 nm. The films were electrochemically oxidized via anodization to a mix of cobalt oxides, one of them being Co3O4, at room temperature in an alkaline solution. The electrocatalytic performances of the anodized films were evaluated in 1 M KOH electrolyte saturated with oxygen. Cathodic currents in -0.5 mA/cm(2) range, corresponding to oxygen reduction reaction (ORR) activity, were measured with cyclic voltammetry. The catalytic activity of the films was evaluated as a function of time. The anodized Co coating exhibited three times higher activity than the steel substrate. The kinetics for the ORR were evaluated through Tafel plots and a slope of 226 mV/decade was found. Post-ORR characterization of the films revealed hexagonal plate-like oxide particles on the surface. After 50 cyclic voltammograms, the film was further oxidized, indicating that the ORR activity also affects the overall surface state of the film. This study demonstrates that thin films, after electrochemical modification, can be electrocatalysts for the oxygen reduction reaction and potentially used for applications in energy production and storage.