Mechanism of Oxygen Evolution Catalyzed by Cobalt Oxyhydroxide: Cobalt Superoxide Species as a Key Intermediate and Dioxygen Release as a Rate-Determining Step

The oxygen evolution reaction (OER) is the performance-limiting half reaction of water splitting, which can be used to produce hydrogen fuel using renewable energies. Whereas a number of transition metal oxides and oxyhydroxides have been developed as promising OER catalysts in alkaline medium, the mechanisms of OER onco these catalysts are not well understood. Here we combine electrochemical and in situ spectroscopic methods, particularly operando X-ray absorption and Raman spectroscopy, to study the mechanism of OER on cobalt oxyhydroxide (CoOOH), an archetypical unary OER catalyst. We find the dominating resting state of the catalyst as a Co(IV) species CoO2. Through oxygen isotope exchange experiments, we discover a cobalt superoxide species as an active intermediate in the OER. This intermediate is formed concurrently to the oxidation of CoOOH to CoO2. Combing spectroscopic and electrokinetic data, we identify the rate-determining step of the OER as the release of dioxygen from the superoxide intermediate. The work provides important experimental fingerprints and new mechanistic perspectives for OER catalysts.

Stability profiles of transition metal oxides in the oxygen evolution reaction in alkaline medium

Xile Hu, Aliki Moysiadou

The electrochemical water splitting reaction can convert renewable electricity into clean hydrogen fuel. The efficiency of water splitting is limited to a large extent by the sluggish oxygen evolution reaction (OER). Numerous transition metal oxides have been developed as electrocatalysts for the OER in alkaline medium. However, in-depth studies of the stability of these catalysts have rarely been performed. Here we report a systematic investigation of the stability profiles of five archetypical OER catalysts including CoOx, CoFeOx, CoFeNiOx, NiOx, and NiFeOx. We combine measurements of electrochemical activity, electrochemical quartz crystal microbalance (eQCM), inductively coupled plasma optical emission spectrometry (ICP-OES), and electrochemical impedance spectroscopy (EIS). We find that the eQCM analysis gives incorrect information about the mass change during the OER due to a non-ideal response, and confirms that activity is not a valid descriptor of stability. Of the five oxides, CoOx and CoFeOx lose some mass during the initial period of the OER while CoFeNiOx, NiOx, and NiFeOx maintain their mass. However, all five catalysts undergo noticeable compositional changes due to a dynamic exchange of metal ions with the electrolyte solutions. Partial dissolution of CoOx and incorporation of Fe ions are the main processes of this exchange. The dynamic exchange reaches equilibrium after 6 h, and the catalysts are stable afterwards.

ROYAL SOC CHEMISTRY2019

Oxygen evolution reaction catalyzed by first-row transition metal oxides: stability and mechanism

Aliki Moysiadou

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

Synthesis and Application of Transition Metal Oxides as Oxygen Evolution Catalysts

Laurent Liardet

Nowadays, mankind is facing an important energy challenge. Depletion in fossil fuels reserves and increasing energy demand, coupled with the problematic greenhouse effect induced by massive anthropogenic release of carbon dioxide into the atmosphere, are driving forces for the development of renewable and carbon-free energy technologies. Hydrogen gas is considered as the energy carrier of the future and is expected to replace fossil fuels in order to create a Hydrogen Economy. Hydrogen is viable for decarbonization of our society only if it is produced from sustainable and renewable energies. Water splitting, the redox reaction producing hydrogen and oxygen gas by reducing and oxidizing water, respectively, is a promising technique to produce clean hydrogen. However, the kinetics of the oxidative half-reaction, namely the oxygen evolution reaction (OER), are slow and catalysts are required in order to increase the global efficiency of water splitting. Moreover, in order to perform water splitting at large scale, catalysts based on Earth-abundant elements should be developed. This thesis focuses on the development of transition metal oxides catalysts, by different synthesis techniques, for electrochemical and photoelectrochemical oxygen evolution. Chapter 2 focuses on the atomic layer deposition (ALD) of different cobalt-based metal oxides and the development of procedures to fabricate bi- and trimetallic oxides catalysts. Cobalt vanadium oxide (CoVOx), cobalt iron oxide (CoFeOx) as well as cobalt vanadium iron oxide (CoVFeOx) are deposited by ALD and evaluated for OER. The ALD deposition method allowed us to apply these material on high-surface area nickel foam substrates. Among them, CoFeOx shows the highest catalytic activity for water oxidation and can compete with state-of-the art electrocatalysts for OER in alkaline solution. In Chapter 3, we focused on developing a photoelectrodeposition method to deposit CoFeOx on nanocauliflower hematite photoanodes. This photoelectrodeposition method allowed the deposition of an ultrathin, amorphous and optically transparent CoFeOx layer that induced a large cathodic shift of the photocurrent onset potential of hematite and a high enhancement of the photocurrent at 1.0 V vs RHE. Additionally, the reason of the improvement of the onset potential was explored in detail in order to differentiate between possible activity enhancement effects such as an increase of charge transfer kinetics, a reduction of surface recombination, a modification of the flat band potential or an increase of the photovoltage. We concluded that CoFeOx improved the OER efficiency of nanocauliflower hematite photoanodes by purely enhancing the kinetics of water oxidation. Chapter 4 details the development of a hydrothermal route to fabricate CoVOx. On the basis of a volcano plot that correlates the OER activity to the M-OH bond strength of metal oxides, CoVOx was predicted to be a highly active electrocatalyst that could compete with the most active OER electrocatalysts in alkaline solution. After characterization, this hydrothermally synthesized cobalt vanadium oxide was seen to be amorphous and provided high catalytic activity towards OER. This chapter, in addition to increasing the library of OER electrocatalysts, demonstrates the usefulness of M-OH bond strength as a simple descriptor for experimental development of OER catalysts.