Films of nickel-based and iridium-based oxides were deposited using standard DC and RF reactive magnetron co-sputtering from 99.95 % pure targets of Mg, Al, Si, Ni, Zr, Nb, Ag, Ta, Ir, Ni(61.5 %)-Al(38.5 %), and Ni(93 %)-V(7 %). The depositions were carried out from 5-cm-diameter targets in a mixture of Ar, O2, and H2, all being 99.998 % pure. The total sputter pressure was approximately 4.7 Pa (30 mTorr). Further details on the preparation and optimization of the nickel-oxide-based films are given elsewhere [11-13]. Unheated glass substrates, pre-coated with ITO having a resistance of 60 Omega/square, were used for optical and electrochemical work, whereas polished carbon substrates were used for compositional analysis. The film thickness was maintained at approximately 200 nm. Elemental compositions were determined by Rutherford Backscattering Spectrometry, and it was verified that atomic ratios according to Mg/Ni = 0.80 and Al/Ni = 0.59 ± 0.03 led to optimized electrochromic properties [13]. It should be noted that these compositions correspond to non-magnetic sputter targets, which is advantageous with regard to manufacturing.

Cyclic voltammograms were recorded by use of a three-electrode electrochemical cell with a working electrode being the oxide-based film, a counter electrode of Pt, and a reference electrode of Ag/AgCl. The voltammograms were obtained in a 1 M KOH solution. After some initial voltammetric cycles to stabilize the electrochromic performance, spectral transmittance T(A) and reflectance R(A) were measured in the 300 < A < 800 nm range with the samples in their fully bleached and colored states. The data were taken at normal incidence using an integrating sphere. The reference for the reflectance measurements was a plate of barium sulfate.