(Invited) Structure and Reactivity of Cobalt-Containing Nanocomposites for Electrocatalytic Oxygen Evolution in Acid Medium
Authors: Rutkowska Iwona A. , Krech Marzena , Siamuk Olena , Kulesza Pawel 
Published: December 2024

Electrochemical water splitting involves two heterogeneous multi-step half-reactions, which are referred to as the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). It should be noted that the OER under acidic conditions has the following two advantages compared to the process in neutral and alkaline solutions. The kinetics of the OER in acidic media could be much faster due to the higher proton transfer rate between anode and cathode. The proton exchange membrane (PEM) is an acidic solid polymer electrolyte membrane characterized by good proton conductivity, excellent electrochemical durability, and high mechanical strength.
Polynuclear iron(III) hexacyanoferrate(II), or Prussian blue, and its analogues possess unique open framework structures with the general chemical formula of AxMy[Fe-(CN)6]z·nH2O (where A = alkali metal cation, M = transition metal cation). The possibility to accommodate different transition metal cations within the coordination framework renders them with appealing electrochemical, ion-exchange, sensing, or photomagnetic properties, which have been the subject of intense research for decades. Despite their rigid structure, these mixed-metal coordination polymers are usually nonstoichiometric. This chemical variety makes them a versatile type of molecule-based materials. Prussian Blue type cobalt hexacyanoferrates seem to activate water molecules during photoelectrochemical oxidations. Here we report the water oxidation catalytic activity found in Prussian blue-type cobalt hexacyanoferrate modified electrodes both under conventional electrochermcal and photoelectrochemical conditions (using WO3 n-type semiconductor) in acid medium. It is noteworthy that some of the hybrid cobalt-ruthenium hexacyanoferrate compositions showed remarkable catalytic ability towards the oxygen evolution reaction in acid electrolyte. The enhanced catalytic activities of the as-synthesized electrodes should also be attributed to such features as high population of hydroxyl groups and high Broensted acidity (due to presence of Ru or W oxo sites) and related fast electron transfers coupled to unimpeded proton displacements. The possibility of metal-metal interactions between nanosized metals (Co and Ru or Co and W) cannot be excluded
For comparison, we also show that mixed-metal oxide nanocomposites combining the favorable catalytic properties of Co3O4 and CeO2, nanocomposites (with different phase distribution and Co3O4 loading) can modify the cobalt-oxo-species and enhance its intrinsic oxygen evolution reaction activity. Synthesis of Co3O4-modified CeO2, which was addressed through three different sol-gel based routes, each with 10.4 wt% Co3O4 loading, yielded three different nanocomposite morphologies: CeO2-supported Co3O4 layers, intermixed oxides, and homogeneously dispersed Co. It is reasonable to expect that the local bonding environment of Co3O4 can be modified after the introduction of nanocrystalline CeO2, which allows the CoIII species to be easily oxidized into catalytically active CoIV species, thus avoiding the undesirable surface reconstruction process. Here, ceria is likely to regulate the surface oxygen concentration, due to its oxygen buffer (storage/release) capacity, associated with the fast CeIV/CeIII redox transition. Our research addresses the problems related to the fabrication of efficient earth-abundant catalysts for oxygen evolution reaction, and it provides strategies for designing more active and stable catalytic systems.