Faradaic and/or capacitive: Which contribution for electrochromism in NiO thin films cycled in various electrolytes?

This study compares the electrochromic performance of NiO thin films deposited by RF magnetron sputtering cycled in lithium, sodium and small cations free based electrolytes, namely 1:9 LiTFSI in EMITFSI, 1:9 NaTFSI in EMITFSI and EMITFSI respectively. Regardless of the electrolyte nature, NiO thin films show similar electrochromic properties associated with an optical switch from colorless to brownish upon oxidation correlated to a modulation of transmittance close to 40 % associated with a good electrochemical stability. Interestingly, depending on the electrolyte nature, the EC behavior is correlated with various CV shapes raising the question of the mechanism involved. In particular, the reversible coloration mechanism cannot be only described by a single insertion/extraction of lithium ions, as observed for tungsten oxide in lithium media. The rectangular shape of the CV curves of NiO thin films in neat electrolyte and the behavior of NiO with various scan rates suggest that the EC properties result from a combination of both faradaic and capacitive contributions, with redox reactions largely occurring at the surface. Integration of NiO thin films in a full device is illustrated in double sided PANI/white electrolyte-EMITFSI/NiO EC devices showing simultaneous progressive color changes from blue to green and white to brown, on PANI and NiO sides, respectively.