Cobalt-tuned engineering of nanorod arrays in Ni MOF films for high-contrast electrochromics

Metal-organic frameworks (MOFs) are promising electrochromic materials owing to their rapid response and multicolor switching capabilities. However, their performance in thin-film configurations is often limited by slow switching speeds and insufficient optical contrast. In this study, a conjugated biphenyl-based ligand, 4,4′-dihydroxy-3,3′-biphenyldicarboxylic acid (DHBDC), was employed to synthesize Co/Ni MOF films directly on fluorine-doped tin oxide (FTO) substrates via a one-pot solvothermal method. The introduction of Co 2+ and the Ni/Co molar ratio are identified as decisive factors for steering the mesoscopic architecture, enabling precise tuning from agglomerated particles to a well-aligned nanorod arrays. The film fabricated with an equimolar (1:1) Ni/Co ratio at 150 °C delivers optimal performance, exhibiting a high optical contrast of 84% at 900 nm with rapid coloration/bleaching times of 2.9 s and 7.0 s, respectively, alongside good stability over 1000 cycles. Electrochemical analysis confirms that this optimal morphology yields the highest charge transport kinetics and Li + diffusion coefficient. This work provides a viable strategy and fundamental insights for developing high-performance MOF-based electrochromic materials.