Extremely fast electrochromic supercapacitors

During materials engineering, a network of tiny holes or pores can improve the energy storage capacity of materials for applications as smart windows. Smart windows are platforms whose light transmission properties can be altered when light, voltage or heat is applied. Scientists can control the fraction of light passing through the material using an electrical voltage to electrically switch from transparent to opaque materials during charge transfer. While this feature is associated with storage and release of energy, the same materials can be used for energy storage as well. In a new report, Jeon-Woo Kim and a team of scientists at the Pohang University of Science and Technology in South Korea developed and improved electrochromic supercapacitors made from tungsten trioxide (WO₃). They used an evaporation-induced self-assembly process to deposit a film of tungsten trioxide with pores, where the porous architecture increased the speed of switching and capacitance in the material compared to conventional tungsten trioxide thin films. The work is now published on Nature Asia Materials.

Photonics: smart windows and energy storage

During this work, Kim et al. demonstrated the ultrafast response of electrochromic supercapacitors by exploring the mesoporous structure of the constituent materials. Electrochromic devices (ECDs) can generate reversible color changes that correspond to electricity with promising applications across smart windows, displays and military camouflage. The devices can also control light transmittance to build materials for climate adaptive energy-efficient buildings. The functionality of ECDs can be extended to energy storage devices known as electrochromic supercapacitors (ECS). Such supercapacitors are increasingly under investigation as next-generation electrochemical components capable of changing their own optical properties and storing the energy supplied. Their inherent optical characteristics can therefore directly reveal the real-time energy levels stored within. Researchers had developed such high-performance devices using electrochromic chromophores based on transition metal oxides such as tungsten trioxide due to their superior electrochemical properties. The electrochromic displays developed here can change color based on their stored levels of energy and the product will have broad implications as next-generation smart window materials for buildings and portable energy storage.

Developing the new materials and building the device

The scientists detailed the fabrication process using a mixed solution of tetrahydrofuran and polystyrene-block-polyethylene oxide and ethanol-based tungsten hexachloride (WCl₃) as a precursor of tungsten trioxide. The resulting film contained inorganic-organic composites. They then calcinated the composite to partially remove the organic components and transform the remainder to amorphous carbon. The inorganic components underwent condensation to form tungsten trioxide and the resulting composite film contained a carbon/tungsten trioxide structure. The team subsequently exposed the film to oxygen plasma to eliminate the amorphous carbon, which they confirmed using Raman spectroscopy. Using scanning electron microscopy (SEM), the scientists supported the mesoporous structure of the resulting tungsten trioxide (WO₃) film with small pores (less than 30 nm) and a thickness approximating 250 nm.

Kim et al. expected ultrafast dynamics with mesoporous WO₃-based electrochromic supercapacitors (denoted meso-WO₃-ECs), and for comparison, they also developed a compact device denoted compact-WO₃-ECs using WO₃ nanoparticles. Thereafter they recorded the UV-vis transmittance spectra at various applied voltages to understand the electrochromic behavior of the two devices. When the applied voltage increased, the transmittance gradually decreased across the entire range of visible light wavelengths due to redox reactions in the setup. The team could then recover the transparent bleached state of the device by applying a 2.3 voltage.

Comparing device functionality

To compare the electrochromic dynamic response of the two devices, Kim et al. recorded the transmittance profiles at 700 nm, and at alternating potentials. The meso-WO₃-ECS device showed large optical modulation and ultrafast coloration in 0.8 seconds and a bleaching time of 0.4 seconds, notably faster than previous reports. The team did not obtain a similarly stable state of coloring and bleaching under the same conditions with compact-WO₃-ECS. The results depended on the surface area of the devices, where the meso-WO₃-ECS device consumed less energy compared to the compact-WO₃-ECS.

Typically, electrochromic supercapacitor devices must maintain cycling stability under fast response conditions. Additional tests under quick switching conditions between coloration and bleaching for 1000 cycles therefore showed how the mesoporous device retained 85.5 percent of their original optical modulation, while the optical modulation of compact devices dropped. The team credited the excellent stability of the mesoporous device to its characteristic architecture with a large surface area, well-suited for dynamic applications that require a fast response.

Charge transfer dynamics

Kim et al. next compared the charge transfer and ion kinetics of the devices and the results showed a smaller contact resistance, smaller charge transfer resistance and lower ion diffusion resistance for the mesoporous devices. The devices showed significantly different charge-storing capabilities as the functional current density increased. The work implied the mesoporous supercapacitors to be more promising compared to the compact devices to form quick charging and discharging devices with outstanding long-term stability. The team then directly viewed the stored energy levels of the supercapacitors. The mesoporous device showed no significant degradation of optical contrast, which they credited to its effective and fast ion transport properties. With compact devices, the optical modulation dramatically decreased while the current density increased, the compact devices were therefore not as efficient for high-rate functionality due to their inefficient ion transport and slow charge transfer.

Printing and evaporation-induced self-assembly

The team then combined printing and evaporation-induced self-assembly to develop the highly functional, energy-storing, electrochromic supercapacitor displays. This printing process produced a micellar structure through the nozzle after evaporation, which they then subjected to sequential calcination and oxygen plasma treatment to form a patterned mesoporous WO₃ device for energy-storing applications. When they charged the device, the patterns turned dark blue to indicate the charged state. To prove its mechanism of action, the team connected the device to a white-light emitting diode (LED) that initially emitted light, when the stored energy was consumed, the device returned to its original transparent state.

Outlook: next-generation smart electronics.

In this way, Jeon-Woo Kim and colleagues developed multifunctional electrochromic supercapacitors based on amorphous mesoporous WO₃ films. Compared to the compact version of electrochromic supercapacitors (compact-WO₃-ECS), the mesoporous electrochromic supercapacitors (meso-WO₃-ECS) showed superior performance. The scientists credited this to its large surface area and amorphous nature. The mesoporous devices functioned rapidly to serve as electrochemical reflective displays and to store electrical charge. This setup can also power other electronic devices, as the color intensity of the pattern on the device indicated the level of stored energy within. The outcomes will have tremendous potential to form next-generation smart electronics.

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