Cobalt-Iron Cyanometallates as Active Components for Water Electrolysis through Oxygen Evolution in Acid Medium
Authors: Kulesza Pawel J. , Rutkowska Iwona A. , Krech Marzena 
Published: September 2023

Electrochemical water splitting (oxidation) leading to the oxygen evolution at anode and hydrogen generation at cathode is a topic of growing scientific and technological interest. The reaction provides not only means for producing oxygen but, more importantly, for generation of highly pure hydrogen, a carrier for storing renewable energy and further utilization in low-temperature fuel cells. Among important issues are choice of the catalytic material, its morphology and operating conditions including temperature, electrolyte, pH etc. In some cases of electrolysis cells, at cathode, the hydrogen evolution could be accompanied by CO2-reduction (to simple organic molecules), regardless of water oxidation, at anode.
The suitability of polynuclear mixed-valence inorganic materials (e.g. polyoxometallates, infinite metal oxides, cyanometallates) for the preparation of thin electrocatalytic films on electrode surfaces has been recognized in recent years. Among such systems, Prussian Blue (iron hexacyanoferrate) and the metal-substituted (e.g. with Co, Ni, or Ru) analogues known as transition metal cyanometallates have received considerable attention owing to their physicochemical stability, well-defined redox transitions, counter-ion-sorption selectivity and catalytic electroactivity. In the present work, we explore Prussian-Blue-like cobalt(II,III) hexacyanoferrate(II,III) cocatalytic overlayers together with ruthenium oxide (conventional electrocatalysis) and tungsten oxide n-type semiconductors (photoelectrochemistry). While cobalt hexacyanoferrate has already been demonstrated to exhibit some activity toward oxidation of water under photoelectrochemical conditions, its intrinsic electrocatalytic activity is unclear, and it will be addressed during presentation.
The mixed, or multi-layered, cyanometallates of cobalt, ruthenium and were fabricated through electrodeposition in the appropriate mixtures for modification. Special attention was paid to their morphological, structural properties and the presence of counter-cations (e.g., potassium or sodium, in addition to protons). Electrochemical measurements were performed using cyclic voltammetry, chronoamperometry and chronocoulometry. The systems’ structural modifications as well as their effect on the electroactivity of the catalytic systems towards the oxygen evolution reaction in acid media were addressed. It is noteworthy that some of the hybrid cobalt-ruthenium hexacyanoferrate compositions showed remarkable catalytic ability towards the oxygen evolution reaction in acid electrolyte. In other words, the cyanide-linked ruthenium-oxo species was used to promote oxygen evolution while cobalt hexacyanoferrates acted as stabilizing agents. 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.