In this Thesis, I have investigated multicomponent alloy based thin films synthesized by magnetron sputtering. The studies in the thesis are centered around the phase diagram of the CrFeCoNi nitrogen containing system. Theoretical and experimental methods were employed to understand the phase formation in this system which is related to the archetypical Cantor alloy (CrMnFeCoNi). CrFeCoNi thin films of approximately equimolar composition crystallize with fcc structure when grown at room temperature. This structure, however, is not retained when nitrogen (x) is added into the lattice. Density functional theory calculations together with the experimental investigation on the (CrFeCoNi)N_x system revealed the stabilization of the metallic fcc when x ≤ 0.22 and the stabilization of the NaCl B1 structure when x > 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films. The importance of the phase diagram is realized when attempting to grow much more complex structures for application-oriented research such as irradiation resistance, corrosion resistance as well as epitaxial films for a fundamental understanding of the material system. The phase diagram of the CrFeCoNi system indicated that higher stability of the single-phase solid solution Cantor nitride lay in a limited temperature range of 200 to 300 °C. In order to compensate for the higher deposition temperature required to grow epitaxial films magnetic field assisted dc magnetron sputtering was used. This technique allows for the control of the flux of Ar ions bombarding the substrate during growth thereby providing the growing film with kinetic energy as opposed to thermal. The results from the study indicated that the quality of epitaxy can be improved by increasing low ion energy, high ion-flux bombardment. The thesis in whole, gives a fundamental understanding of the nitride cantor alloy material system in terms of crystal structure, mechanical and electrical properties.

Catalytic water recombination and electrolysis play an important role in the transition towards green, renewable, fossil-free energy production. The processes are kinetically limited by the oxygen reactions, i.e. Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER), thus these reactions have higher demand for electrocatalysts than the hydrogen reactions. There is a particular focus on finding abundant and cheap alternatives to noble metal oxygen reaction catalysts used today. Alternatives such as Co, Ni, Fe and Mn oxides, are promising candidates and in some cases found to be bifunctional oxygen catalysts, i.e. that they are active towards both reactions. In this thesis, these catalytically active elements have been alloyed together in multicomponent, high entropy thin films deposited by magnetron sputtering. 

Anodization profiles have been designed to activate the thin films by tuning the surface oxidation states. This considerably enhanced the catalytic activity towards both ORR and OER. For pure Co films, the electrochemical modification resulted in the formation of Co3O4 platelets with the two Co cations as active sites. In the CoCrFeNi films, an enrichment of the surface in Co and Ni cations was observed after anodization. In addition, the presence of Fe and its synergy with the active sites was identified as key to the catalytic activity.   

Another critical aspect was the film structure. Here the substrate bias was varied which affected the ion bombardment during the deposition. This changed the typical columnar structure of the films towards denser and more faceted films with larger grains. Both the columnar and denser structure were found catalytically active, in particular towards OER. The denser films had better long-term stability towards OER compared to the columnar films.   To understand in depth the catalytic mechanism for the CoCrFeNi and MnCrFeNi films, the ORR pathway was also investigated. It was first observed that replacing Co with Mn reduced the overpotentials for both ORR and OER. The as-deposited films follow a (2+1) electrons pathway whereas the anodized films shifted towards either 4 or 2 electrons pathway. This was also correlated to the active sites and film structure. 

Furthermore, the films synthesized in this thesis have also a high corrosion resistance in alkaline and neutral chloride containing environments. The films presented a passive behavior due to the formation of a protective oxide layer. Once again, film composition and film structure, in particular grain size affected the corrosion performance. The presence of defects due to lattice distortion of CoCrFeNi significantly improved the corrosion resistance in NaCl. The smaller grain size of the films led to a higher corrosion resistance in KOH. Additions of Mo to CoCrFeNi significantly improve the corrosion resistance in acidic environments at elevated temperature. This was attributed to the suppression of secondary phases and presence of Mo in the passive films.  

In summary, this thesis focuses on tuning thin film composition, structure, and post-deposition oxidation to improve both the catalytic activity towards oxygen reactions and the corrosion resistance in environments relevant for cata-lytic water recombination and electrolysis. The focus on abundant and cheap elements for material synthesis aims to contribute to the development of non-noble metal oxygen electrocatalysts for potential use in future green energy technologies.