Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium

Mixed nickel/palladium hexacyanoferrates have been prepared both as thin fi lms and bulk precipitates (powders) attached to electrode surfaces. The mix ed material does not seem to be a simple mixture of hexacyanoferrates of ni ckel and palladium, and it shows unique voltammetric and electrochromic cha racteristics when compared with the respective single-metal hexacyanoferrat es. Electrodeposition of a mixed film is achieved by potential cycling in t he solution for modification containing nickel(II), palladium(II) and hexac yanoferrate(III). It comes from elemental analysis that, in general, the st oichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparabl e with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans follow ed by simultaneous insertion of palladium ions as countercations into the s ystem. In such cases, palladium ions tend to substitute potassium counterca tions at interstitial positions in the electrodeposited nickel hexacyanofer rate microstructures. We have determined the following stoichiometric formu la, (K1.74-2yPdyNi1.13II)-Ni-II[Fe-II(CN)(6)] (where y < 0.72) for such fil ms. At higher molar fractions of palladium in solutions for modification, t he formation of a mixed phase of nickel/palladium hexacyanoferrate (in whic h both nickel(II) and palladium(II) are nitrogen-coordinated within the cya nometallate lattice) is expected. This seems to be more probable than simpl e codeposition of separate palladium hexacyanoferrate and nickel hexacyanof errate microstructures during the film growth. Mixed (composite) nickel/pal ladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cy cling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes. (C) 2000 Elsevier S cience S.A. All rights reserved.