Preparation and Electrochromic Properties of MnO₂/PPy Composite Films with Coral-like Structures

MnO₂/polypyrrole (PPy) composite films were deposited on fluorine-doped tin oxide (FTO) conductive glasses by a two-step wet-chemical method, including electrochemical deposition and chemical bath deposition (CBD). The porous MnO₂ films were first grown on FTO glasses by an electrodeposition method. Second, polypyrrole nanoparticles were polymerized by the oxidation-reduction reaction between MnO₂ and pyrrole, using the presynthesized MnO₂ as the skeleton. Then, MnO₂/PPy composite films with coral-like structures were obtained. The electrochemical and electrochromic (EC) properties of the prepared films were investigated. The results show that, compared to the single MnO₂ or PPy film, the MnO₂/PPy composite film has a larger optical modulation (67.3% at a wavelength of 900 nm), faster response times (4 s for coloration and 3 s for bleaching), and a higher coloration efficiency (218.16 cm²·C^-1). The high coloration efficiency attests to the exceptional performance of the composite film in converting electrical signals into vivid color changes. The electrochemical stability test results show that the composite film maintains a stable EC performance after 200 coloration/bleaching cycles. The coral-like structures of the composite film are responsible for the better EC properties.