Molybdate hexacyanoferrate (MoOHCF) thin film: A brownish red Prussian blue analog for electrochromic window application

• First molybdate hexacyanoferrate (MoOHCF) film for electrochromic window.
• Uniform electrodeposited coating characterized by XPS, SEM, and in situ method.
• Highly reversible red-to-transparent electrochromism in NaClO4/PC.
• Assembled into a complementary device with Prussian blue (PB)/PEDOT:PSS.
• The device is capable of 40% ΔT modulation over most visible wavelengths.

Molybdate hexacyanoferrate (MoOHCF), a brownish red Prussian blue (PB) analogue based on molybdate (MoO), has been prepared as an electrodeposited film for electrochromic window application. The electrochromic mechanism of MoOHCF is similar to that of PB, involving alkali metal ions which intercalate into the lattice structure at different applied potentials to bring a vivid color change. MoOHCF presents a brownish red color at its reduced state (0.51 V) and becomes yellowish at the oxidized state (1.17 V). Our electrodeposited MoOHCF thin film exhibits wide transmittance change (ΔT=ca. 48%) at 480 nm, a fast response time (2.1 s) and high cycling stability that exceeds 300 cycles. In addition, it can be assembled into an electrochromic window capable of a broad-band (300–900 nm) transmittance modulation with PB. In this study, a PB/MoOHCF electrochromic device (PMECD) with the NaClO4/propylene carbonate (PC) electrolyte is fabricated. The device changes its color between deep green and yellowish (almost transparent). By controlling the potential between ˗0.5 V and +1.0 V (PB vs. MoOHCF), the PMECD shows the capacity of modulating average 40% optical transmittance for whole visible wavelengths. This suggests that MoOHCF is an ideal counter electrode for PB on assembling a promising energy-saving smart window.

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