Water splitting offers the opportunity for storing solar energy and, thus, producing carbon-neutral and renewable solar fuels. The process, known as artificial photosynthesis, is limited by the electrocatalytic conversion of water into molecular oxygen. Electrocatalysts are essential for reducing energy losses and improving the efficiency of the oxygen evolution reaction. This thesis describes the catalytic properties of first-row transition metal oxides for the OER and provides new insights into the mechanism of the reaction.
Decades of research have yielded various transition metal oxides as efficient electrocatalysts for the OER, yet their stability is not well established. Here, a combination of techniques was employed to investigate the stability of unary, and mixed metal oxides; electrochemical activity measurements, electrochemical quartz crystal microbalance, inductively coupled plasma optical emission spectrometry and electrochemical impedance spectroscopy. The change of mass, composition, and activity metrics over time resolved the stability profiles of CoOx, CoFeOx, CoFeNiOx, NiOx, and NiFeOx. According to ICP-OES, the composition of all the five catalysts changed over time, while the CoOx and CoFeOx exhibited the most significant mass loss during an initial period of electrolysis. During a dynamic exchange of metal ions between the catalyst and the electrolyte, cobalt ions dissolve into the electrolyte, and iron from the electrolytic solution incorporates into the catalytic film. Once this dynamic exchange reaches equilibrium, the catalysts are stable.
The mechanism of the OER mediated by amorphous cobalt oxyhydroxide was investigated by in situ spectroscopic and electrokinetic experiments. In situ surface-enhanced Raman and X-ray absorption spectroscopy supported a Co(IV)-O species at potentials before the onset of OER. The gradual transformation of the catalyst into a CoO2 concurred with the appearance of a superoxidic species. In situ SERS coupled with 18O and H/D isotope experiments provided evidence for the participation of the superoxidic species in the oxygen evolution reaction. The catalyst exhibits a Tafel slope of 60 mV/dec, a first-order dependence of the catalytic activity, and a proton-coupled electron-transfer pre-equilibrium. These findings propose a new OER mechanism, in which the rate-determining step is the release of oxygen from a Co-superoxo intermediate. Iron incorporation into the cobalt lattice enhances the catalytic activity greatly. Even though there are several studies on the OER mechanism in Co-Fe oxides, many vital questions on the role of iron, the true nature of active sites, and the oxidation states of Co and Fe remain. In situ SERS and XAS indicated the presence of Co(IV)-O species and a Fe valency higher than three at potentials prior to the oxygen evolution. An O-O- band, attributed to the superoxidic species, emerged at potentials lower than the onset of OER. In situ SERS coupled with 18O isotope experiments confirmed that both the lattice oxygen and superoxidic species participate in the oxygen evolution. The Tafel slope is about 37 mV/dec indicating that both the pre-equilibrium and rate-determining step involve a single-electron transfer. These findings support that neither the Co(III)/Co(IV) oxidation nor the O-O bond formation are controlling the rate of the reaction.
EPFL2020
