Influence of single-nanoparticle electrochromic dynamics on the durability and speed of smart windows

Abstract

Nanomaterials have tremendous potential to increase electrochromic smart window efficiency, speed, and durability. However, nanoparticles vary in size, shape, and surface defects, and it is unknown how nanoparticle heterogeneity contributes to particle-dependent electrochromic properties. Here, we use single-nanoparticle-level electro-optical imaging to measure structure–function relationships in electrochromic tungsten oxide nanorods. Single nanorods exhibit a particle-dependent waiting time for tinting (from 100 ms to 10 s) due to Li-ion insertion at optically inactive surface sites. Longer nanorods tint darker than shorter nanorods and exhibit a Li-ion gradient that increases from the nanorod ends to the middle. The particle-dependent ion-insertion kinetics contribute to variable tinting rates and magnitudes across large-area smart windows. Next, we quantified how particle–particle interactions impact tinting dynamics and reversibility as the nanorod building blocks are assembled into a thin film. Interestingly, single particles tint 4 times faster and cycle 20 times more reversibly than thin films made of the same particles. These findings allow us to propose a nanostructured electrode architecture that optimizes optical modulation rates and reversibility across large-area smart windows.

Quantifying Capacitive‐Like and Battery‐Like Charge Storage Contributions Using Single‐Nanoparticle Electro‐optical Imaging

“…For E more positive than −0.4 V, ΔOD d ( t ) does not increase with time, which indicates that those potentials are not sufficiently cathodic to drive the ion insertion reaction in Equation (1). However, ΔOD d ( t ) increases at E =−0.4 V. We quantitatively define the onset potential ( E onset ) as the value of E that causes ΔOD( t ) to exceed a value of three times the standard deviation of background noise plus the mean background noise, as described in our previous work . Upon applying a potential more negative (cathodic) than −0.4 V, ΔOD d ( t ) quickly increases, and then rises gradually with increasing cathodic polarization time.…”

“…Following the methodology and model explained in our previous work, we obtain Δ Q diff and Δ Q pseudo (in units of charge, C) from the integrated Li‐ion fluxes from the pseudocapacitive and diffusion‐limited processes [ J pseudo and J diff according to Eq. ]. …”

“…H ( t ‐ t wait ) is the Heaviside function that switches on the fast pseudocapacitive coloration term to fit the OD burst feature that onsets at t wait . Following the fitting procedure that was developed in our previous study, we fit the potential‐dependent ΔOD d ( t ) data using Equation (5) to obtain the charge inserted into WO 3 at a rate that is consistent with pseudocapacitive and diffusion‐limited processes. The fit result to the experimental data is shown in Figure a.…”

“…In a perfect hexagonal prism morphology, the hexagonal window sites are confined to the nanorod end caps. A distribution of hexagonal window sites along the nanorod length could be more beneficial if an interfacial electric field at the step edge promotes the electrochemical reaction . Surface adsorbed or structural water present in larger quantities on longer nanorods could also improve kinetics .…”

“…We used a widefield electro-optical imaging technique to study the electrochromic dynamics of single WO 3 nanoparticles due to the faradaic ion insertion reaction in Equation (1). [26] The detailed experimental and image analysis procedures are described in our prior work. [27] In a typical experiment, nanorods are spin-cast onto an indium tin oxide (ITO) working electrode that is assembled into a three-electrode electrochemical cell and mounted on an inverted microscope.…”