Investigation of the water splitting reactions on multi-heteroatom doped cobalt-based carbon catalysts

In the context of energy transition, hydrogen is addressed as a clean future energy carrier that enables the emission-free production of energy to become independent of fossil fuels. The development of hydrogen production technologies like the electrolysis is strongly driven by the two main criteria: sustainability and economy. Over the past decades, various technological achievements resulted in a reduction of costs which has dramatically improved the economic potential of hydrogen produced by electrolysis. Especially in the field of material development, great effort was devoted to replace the precious state of the art catalyst materials with abundant cost-effective catalysts accelerating sluggish water splitting reactions. This dissertation focuses on the investigation of carbon-based cobalt catalysts with multi-heteroatom doping for the oxygen evolution reaction (OER) and hydrogen evolution reactions (HER). Within this study, two major synthesis approaches, one with metal organic framework (MOF) and another with polyaniline (PANI), were investigated in terms of structural and electrochemical characterization. Moreover, the catalysts were analyzed in detail by active site identification and mechanistic understanding of the reactions within the scope of each project. Within the MOF approach, the role of the metal species on HER activity was investigated using X-ray photoelectron spectroscopy (XPS). The discussion was further supported by density functional theory (DFT) calculations resulting in structure-activity correlations with emphasis on the importance of the nature of the metal. Besides, bimetallic catalysts with optimal hydrogen binding energies were suggested as a promising active catalyst toward HER . The PANI approach was proposed to investigate multi heteroatom doping influence on the catalytic activity and material properties. Within this approach, cobalt catalysts with variation of cobalt loading and sulfur loading in the precursors were prepared. The catalysts were highly active toward both HER and OER, though the origin of activity might be different. Several physico-chemical characterization techniques combined with post mortem analysis were carried out in order to get insight into the origin of the activity. It was found that the high HER activity of PANI-based samples is attributed to MeN4 sites, and the OER activity is originating from a hybrid cobalt complex depending on the synthesis route