Anodic Electrochromic Nickel Oxide Thin Films: Decay of Charge Density upon Extensive Electrochemical Cycling

Abstract

Electrochromic (EC) Ni oxide thin film is a critical component in the "smart windows". However, long-term decay of the EC performance in aprotic electrolytes is persistent and poorly understood, and it is difficult to assess life-times of EC devices. Here we report on charge density decline upon electrochemical cycling. The charge density decay was modeled with a power law or, alternatively, a stretched exponential; both models describe a rapid drop of charge density during the first hundreds of cycles and a subsequent slower decline. The decay is independent of film composition and applied potential range as long as the upper limit of the potential is ≤4.4 V vs. Li/Li + . Our decay models are interpreted in terms of dispersive chemical reaction kinetics and point at ion diffusion as the rate-limiting step.Power-law exponents are consistent with diffusion. The results provide a framework for evaluating EC durability of Ni-oxide-based thin films and may be important for assessing the durability of EC devices.

Degradation Dynamics for Electrochromic WO3 Films under Extended Charge Insertion and Extraction: Unveiling Physicochemical Mechanisms

“…8 In order to describe and understand the decay of the electrochromism, it is necessary to fit the CV-cycle-dependent decrease of the charge capacity to a physically-motivated model. Following earlier work on EC Ni oxide films, [15][16][17] we adopt a power-law model which can be fitted to the data in Figure 1B by using an asymptotic value of Q not significantly different from zero. A stretched-exponential model might also be used, 16,17 but this representation requires non-zero asymptotic charge capacity in the case of our films, which we consider to be unlikely since no physicochemical mechanism prevents the film from eventually losing all of its charge capacity and become electrochemically inactive.…”

Electrochemical Rejuvenation of Anodically Coloring Electrochromic Nickel Oxide Thin Films

“…Technical details of the set-ups can be found in the literature. 25,26 Species identification was done via mass detection and permitted unambiguous assignments up to atomic numbers of about 20; for heavier nuclei, there were issues associated with limited resolution and with overlap due to isotope mass distributions. Absolute concentrations, independent of standards and without risks due to interference of signals originating from different species, were derived from time-of-flight/energy coincidence spectra (Figure 3b) by employing the software CONTES.…”

Cation‐/Anion‐Based Electrochemical Degradation and Rejuvenation of Electrochromic Nickel Oxide Thin Films

“…Combinations of W‐oxide‐based and Ni‐oxide‐based thin films are employed in most of today's EC glazing ,. Films of Ni oxide are also subject to significant degradation upon electrochemical cycling, in particular at high applied potentials above 4.5 V, and many uncertainties pertain to the durability of Ni‐oxide‐based EC films: thus it is not fully clear what ionic species are involved in the electrochromism, especially in the case of Li + ‐based electrolytes, and the physical and chemical origins of the irreversibility as well as the coloration mechanism are uncertain, experimentally as well as theoretically. However, it is commonly accepted that cations (Li + ions in the case of lithium‐based electrolytes) participate in the electrochromism of Ni oxide and that its optical properties are modulated by reversible charge‐transfer processes between Ni 2+ and Ni 3+ .…”