Investigation Of Counter Electrode Reactions And Their Improvement Of Electrochromic Devices’ Performance

Under the dual pressure of energy crisis and environmental crisis,the application potential of smart windows with excellent energy efficiency has attracted the attention of researches,considering that buildings account for more than half of the annual carbon emissions.Among them,electrochromic(EC)technology is seen as the best smart window technology proposal owing to its artificially controllable optical modulation capability.Electrochromism can be described as a stable,reversible and significant variation in the optical properties(transmittance,absorbance and reflection)of a material under an applied electric field,usually manifested externally as a change in color or transparency.Nowadays,depending on the state of the EC material and charge-balancing material in the electrochromic device(ECD),it can be divided into solid and solution types.Among them,WO3 and viologen are the commonly used electrochromic materials(ECM)in these two types of devices for their excellent cycling stability,remarkable optical modulation ability and high coloring efficiency.Newtheless,WO3based ECDs suffer from the lack of matching counter electrodes,and viologen-based solution type ECDs exist with difficulty in rapid bleaching under a reverse electric field.Therefore,through the design and optimization of ECDs structure,this paper solves the problem of poor cycling performance of ECDs due to the mismatch of electrochemical processes between electrodes,and tries to expand the types of guest ions used in ECDs,realize the electrochromic electrochemical energy storage bi-functional devices,and explore the "directionality" of electrochemical reactions in solution-tyoe ECDs,and is divided into four parts as follows:In Chapter 3,we performed a failure analysis of the classical WO3//NiO ECD.WO3 and NiO films were found to undergo electrochemical activation and ion trapping,respectively,under electrochemical cycling,and the contradictory electrochemical activity trends make them difficult to match.Thus,based on the hybrid-type ECDs,we introduced the concept of catatlytic counter electrode(CCE)to accelerate the reactivity of redox couple to match the electrochemical process of WO3 electrode,and designed the ECDs with ECM/Redox/CCE structure.The ECDs with this structure posseses robust cycles,low driving voltage,and abundant structural scalability.In Chapter 4,in order to solve the problem of slow intercalation rate of multivalent ions in WO3 films and the increase of charge capacity in the corresponding ECDs,we achieved 20,000 cycles of recession-free stabilization by using the synergistic effects of CCE with redox couple to accelerate the diffusion rate of Al3+ in WO3 film and maintain the self-adaption to the reuired charge of WO3 film.The multi-electron reaction of multivalent ions helps the WO3 film to generate W4+ with higher absorbance,resulting in ECD with-0%transmittance in the 500～1500 nm waveband.The ECD also possess the advantages of extremely low driving voltage(-0.1 V),independent dual-band modulation capability,and stable memory effect.To verify the compatibility of the ECM/Redox/CCE structure with the guest ions species,we also tried to introduce Na+with large ionic radius into WO3-based ECD,and the ECD exhibited electrochromic properties and stability similar to that of Li+-based ECD.In Chapter 5,we focused on the realization of all-black state ECDs and try to design a class of ECDs’ structure that can recycle electrical energy by combining EC technology and electrochemical energy storage.We regard the Mn2+/MnO2 reversible deposition reaction as a redox couple with anodic EC properties and introduce it into WO3-based ECD.The charge-adaptive Mn2+in the coloring process enables the ECD to ensure stable cycling performance,and the complementary colors of MnO2 and WO3 films in the coloring state result in～0%transmittance in the 300～1500 nm waveband,while 138.3 mC cm-2 of electrical energy is stored.Similar,this structure shows good scalability,and we attempted to use Pb2+/PbO2 as the counter electrode in ECD and obtained excellent EC and cycling performance.In Chapter 6,we investigated and designed a solution-type ECD with ’directional’electro-chemical reactions by using the opposite electrochemical activities of viologen and TMTU/TMFDS2+redox couple on the surfaces of ITO and MoS2 elctrode,respectively,to deal with the phenomenon that solution-type ECDs are difficult to achieve rapid bleaching by applying reverse electric fields.Since the viologen and TMTU/TMFDS2+can only gain/loss electrons on the corresponding electrode,this is equivalent to binding the two materials to the corresponding electrode surface,inhibiting the bleached viologen from recoloring on the other electrode.This strategy reduces the bleaching duration of the ECD to as quarter of the self-bleaching mode.This research can be seen as another successful example of ECM/Redox/CCE structure in ECD applications.