Electrochemical Nanogravimetric Studies of Ruthenium(III) Trichloride Microcrystals

Ruthenium (III) trichlorid solid crystals have been mechanically attached to gold and paraffin-impregnated graphite surfaces and studied in the presence of aqueous solutions of M⁺Cl⁻ electrolytes, where M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and K₂SO₄ by cyclic voltammetry and electrochemical nanogravimetry at a quartz crystal microbalance (EQCM). The electrochemical reduction of the layered α-RuCl₃ microcrystals causes drastic changes in the composition and the structure of crystals. The comparison of the current—potential and surface mass change—potential functions belonging to the first reduction-reoxidation cycle with the subsequent ones reveals that the simple intercalation scheme described in the literature cannot be entirely valid. During the first reduction step at ca. 0.2 V vs. SCE the charge consumption is substantially higher than in the course of the further potential cycling, and the simultaneous rapid and intense mass decrease indicates that considerable chemical and structural transformations occurs. Although a loss of the surface mass cannot be entirely excluded, the frequency increase most likely is not related to the dissolution of the microcrystals, however, large amounts of water molecules and—to a much smaller extent—chloride ions leave the crystal phase, and in fact a new material, which remains strongly attached to the gold or graphite surface, is formed. The extremely high frequency change at the first reduction process during the first cycle is most likely related to the stress effect originating in the phase transition of the surface layer and/or the removal of the water rigidly coupled to the surface into voids of the immobilized microcrystals. Depending on the amount of microcrystals on the electrode surface and the experimental conditions (the nature and concentration of the contacting electrolyte, scan rate, and potential range) used, after the “break-in” cycle stable electrochemical and nanogravimetric responses develop. The several reduction and reoxidation pairs of waves in the cyclic voltammograms and the simultaneous mass changes are in connection with the wide variety of intercalation reactions and complex formation during the electrochemical transformations. The mass change was reversible, in general, during reduction mass increase, while during oxidation mass decrease occurred at medium electrolyte concentrations in three or more steps. The mass excursions are rather complicated, involving different mass increase/decrease regions as a function of potential and the composition of the contacting solution. Taking into account the layered structure of RuCl₃, the electrochemical reduction can be explained as an intercalation reaction in that mixed valence intercalation phases with a general formula K Ru [Ru Cl_3z-y (H₂O)_y] • dH₂O are formed from z (RuCl₃ · H₂O). The reduction/reoxidation waves are related to the redox transformations of Ru(III) to Ru(II) sites and the insertion/deinsertion of cations and water molecules, while the composition of the polynuclear complexes and the structure of microcrystals change.