Charge balancing and optical contrast optimization in Fe-MEPE/Ni1-xO electrochromic devices containing a Li reference electrode

A three-electrode cell setup enables to evaluate the redox behavior of both electrodes, i.e., iron-based metallopolymers (Fe-MEPE) and a complementary switching non-stoichiometric nickel oxide (Ni1-xO, multilayer structure), in detail during full cell operation. In this work, electrochromic devices (ECDs) with a Li reference electrode and different charge density ratios between the electrode materials have been studied (spectro-)electrochemically. The charge density ratio between the electrodes affects the cell voltage, optical contrast, response time, and cycling stability. Here, a neutral-colored bright state was obtained for the unbalanced configuration (ECD-1, charge density ratio Fe-MEPE:Ni1-xO_2 = 2:1, underbalanced) and the balanced configuration (ECD-2, charge density ratio Fe-MEPE:Ni1-xO_3 = 1:1, balanced). The operating voltage range of the ECDs can be narrowed by using a balanced or overbalanced Ni1-xO electrode, e.g., 1.5 V ↔ -1 V for ECD-3 (charge density ratio Fe-MEPE:Ni1-xO_5 = 1:3, overbalanced). Moreover, the cycling stability is enhanced from just a few cycles (ECD-1) to over 1000 potentiostatic switching cycles (ECD-2 and ECD-3).