Prussian-Blue Type Cyanometallate Systems with Cobalt and Ruthenium As Oxygen Evolution Electrocatalytic Components for Water Electrolysis in Acid Medium

Abstract

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) together with ruthenium oxide/cyanoruthenate, a mixed-valence polynuclear ruthenium oxide structure cross-lined with cyanides. While cobalt hexacyanoferrate has already been demonstrated to exhibit activity toward electrooxidation of water, the oxocyanoruthenium system is known as highly specific electrocatalyst during oxidations. The mixed, or multi-layered, cyanometallates of cobalt, ruthenium and iron (with and without nickel square-planar cross-linking ions) were fabricated through electrodeposition in the appropriate mixtures for modification. Special attention was paid to their morphological and structural properties. Electrochemical measurements were performed using cyclic voltammetry, chronoamperometry and chronocoulometry. The systems’ structural modifications as well as their effect on the electroactivity of the catalyst towards the oxygen evolution reaction in acid and neutral 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, co-electrodeposition of the cyanide-linked ruthenium-oxo species was used to promote the oxygen evolution on the cobalt hexacyanoferrates. 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-oxo sites) and related fast electron transfers coupled to unimpeded proton displacements. The possibility of metal-metal interactions between nanosized metals (Co and Ru) cannot be excluded. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627).