Water electrolysis represents one possibility to store electric energy by transforming it in chemical energy. One of the major obstacles, which make the process uneconomical at this time, is the overvoltage loss for the oxygen evolution reaction (OER). In terms of overpotential, IrO2 and RuO2 are still the best dark water oxidation catalysts but they are rare and highly priced.
First-row transition metal oxides and hydroxides could be identified as a promising alternative since competitive catalytic performances were realized. It is known, that the redox switching behavior of transition metal oxides in contact with aqueous solutions is complex and is influenced not only by the nature of oxide but also on the manner of its preparation which define the composition, the structure of the oxide, available oxidation states and its morphology. As a consequence a catalytic system can show a different phase transformation in dependence of its preparation method. Combining surface science techniques like XPS with the electrochemical application of the catalysts by measuring XPS from the initial state and the electrochemical treated one, we can get information of a change in the oxidation states and the effect on the electrochemical activity. These results contribute to the development of a better mechanistic understanding. Our work focuses on the one hand on the preparation of thin cobalt films by PECVD and on the other hand on the change of the oxidation state of the active site during the electrochemical water-splitting. We could find that the PECVD deposition using air as a reactive gas leads to the deposition of a Co(II)OxOH2/Ti which is able to transform almost completely to a highly active Co(III)OOH surface (Co(III)max= 95%) under operating conditions.
The in-situ transformed CoOOH shows an overpotential of 0.37 V at 10 mA/cm2. In contrast to CoOx only showed minor changes on the surface after OER, the Co(III) content increased to max 77% and only slight hydroxide formation could be detected. The activity of the CoOx was lower compared to the deposited CoOxOH2, it shows an overpotential of 0.47 V at 10 mA/cm2.
These results identify Co(III) as the active state, which makes sense with regard to the in literature proposed active redox coupled of Co(III)/Co(IV). First results on Ni-catalysts show the same effect, the deposition using air as a reactive gas led to the formation of an highly active Ni(OOH) which transformed to a Ni(OH)2 showing an overpotential of 0.36 V at 5 mA/cm2 while the deposited NiO, using oxygen as a reactive gas, showed an overpotential of 0.45 V at 5 mA/cm2. Further optimization of the catalytic activity and stability will be investigated by depositing binary transition metal hydroxides.