Electrochromic NiO z and WO y thin films were prepared by sputtering and were used in a feasibility study aimed at investigating mixtures of these two oxides. The object was to identify a suitable electrolyte, compatible with both NiO z and WO y . To that end we carried out cyclic voltammetry in potassium hydroxide (KOH), propionic acid, and lithium perchlorate in propylene carbonate (Li-PC). WO y could be coloured in propionic acid and Li-PC, while NiO z could be coloured only in KOH. Both films showed best stability in Li-PC, which hence is well suited for further studies of mixed NiO z and WO y .

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Potentiostatic intermittent titration technique ͑PITT͒ and electrochemical impedance spectroscopy ͑EIS͒ were investigated as methods to determine solid phase chemical diffusion coefficient ͑D͒ and electronic density of states ͑DOS͒. These techniques were then applied to iridium oxide ͑IrOx͒ and iridium-tantalum oxide ͑IrTaOx͒ thin films prepared by sputter deposition. The experiments, performed in 1M propionic acid between −0.2 and 0.8 V vs Ag/ AgCl, showed effects of interfacial side reactions, whose contribution to the electrochemical response could be identified and corrected for in the case of PITT as well as EIS. It was found that D is strongly underestimated when using PITT with the common Cottrell formalism, which follows from non-negligible interfacial charge transfer and Ohmic resistances. EIS indicated an anomalous diffusion mechanism, and D was determined to be in the 10 −11 -10 −10 cm 2 / s range for IrOx and IrTaOx. Both PITT and EIS showed that the intercalated charge as a function of potential exhibits a shape that resembles the theoretical DOS of crystalline iridium oxide, especially for IrTaOx.

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