Iridium oxide catalyst for hybrid electrochromic device based on tetramethylthiourea (TMTU) redox electrolyte

This work reports a study on sputter coated iridium oxide film as a counter electrode (CE) for the electrochemical catalysis of tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS²⁺) redox reaction in electrochromic (EC) devices. The iridium oxide (IrO_x) with the thickness of about 5 nm coated onto transparent conductive oxide, e.g., fluorine-doped tin oxide (F:SnO₂), has shown excellent catalytic properties and excellent electrochromic cycling stability in EC device. The electrochemical impedance spectroscopy (EIS) results revealed that the charge transfer resistance (R_ct) is mainly influenced by the oxygen flow rate during the sputtering process, and by the layer thickness. The IrO_X films coated with low oxygen flow rate has shown a lower charge transfer resistance compared to the fully oxidized iridium oxide film. However, sub-stoichiometric layers are less transparent, and therefore less appropriate for the application. In this regard, fully oxidized and highly transparent (T_visible = 86%) layer formed by the sputtering process at or above 50 sccm (standard cubic centimeters per minute) flow of oxygen having the thickness of 5 nm is used in EC devices. These layers have the R_ct of 25 Ω cm² at 1 V bias voltage for the redox electrolytes. The cyclic voltammetry technique has shown a typical quasi-reversible nature of redox electrolyte at the same IrO_x coated electrode. An electrochromic test for 550 cycles demonstrates that 5 nm of IrO_x is sufficient for the stabile EC window with TMTU/TMFDS²⁺ electrolyte.