일렉트로크로믹 WO₃전극과 Ce/V oxide 대전극의 전기화학적 광학적 특성

Abstract

가시광선 영역에서 투과율을 조절할 수 있는 일렉트로크로믹을 이용한 에너지 효율성 스마트 윈도우는 화석연료로 인한 환경오염의 감소와 에너지 절약에 기여할 것으로 생각되며, 주거문화 및 사무환경 개선을 통한 삶의 질적 향상을 가능하게 할 것이다. 본 연구에서는 일렉트로크로믹 소자의 활동전극 (Working Electrode)으로 활용한 WO₃와 대전극 (Counter Electrode, Ion storage layer)으로 활용한 CeO₂-V₂O_(5)를 가격이 저렴하고 대형화에 용이한 Sol-Gel법을 이용하여 전극을 만들었고 전하용량, charge balance, 가역도를 고려하여 full cell을 구성하여 소자가 1000사이클 이상 구동 할 수 있는 것을 목표로 하였다. 활동전극으로 사용한 WO₃는 H₂O₂가 0.16mole이상이 포함되어 있는 오렌지색의 투명한 PTA sol을 활용하였으며 딥코터의 인상속도를 100mm/min로 박막을 형성하고 200℃에서 1시간동안 열처리하여 전극을 만들었는데, 두께는 250nm, 밀도는 5.3g/cm³, 기공률은 25%, 밴드갭 에너지는 3.53eV 인 비정질의 박막을 사용하였다. 비정질의 WO₃ 박막은 자외선/가시광선/근적외선 영역 중에서 적외선 영역에서 차폐율과 착색 효율이 가장 우수하게 나타났고, XRD를 이용하여 결정성을 관찰하였으며 Raman Scattering Spectroscopy를 이용하여 탈색 상태 (bleaching state)의 W^(6+)상태와 W^(5+)상태로 존재하다가 착색 시에는 W^(5+)상태와 W^(4+)상태로 환원 되는 것을 확인하였다. 또한 WO₃ 격자 속으로 리튬이온을 1/3만큼 인터컬레이트 시킬 경우, 장시간의 내구성 테스트에서 적합 하였다. 리튬이온이 인터컬레이트되었을 때 착색은 하지 않고 이온을 저장하는 수동형 (passive type)의 CeO₂-V₂O_(5)를 대전극으로 활용하여 full cell을 구성하였는데, WO₃의 전하밀도가 17mC/cm²이고, 가역도가 97%인 것을 고려하여 전하용량이 16.9mC/cm²로 WO₃와 전하용량이 균형을 이루고 가역도가 90%이상인, V:Ce의 몰비가 1:1인 전극을 이용하여 박막을 형성하였으며, 400℃에서 2시간동안 열처리를 해 결정성 박막이 형성되도록 하였다. 이렇게 형성한 양극과 음극에 1mole의 LiClO₄와 PC를 혼합한 전해질을 주입하여 full cell을 구성하여 내구성을 평가해본 결과, 100사이클까지는 초기 열화현상으로 전하량이 약간은 감소하였지만, 100사이클 후부터 리튬이온의 활성화로 인해 1000사이클까지 전하 용량이 꾸준히 늘어서 1000번째 사이클에서는 5.8mC/cm²에 착색 시 투과율은 27%에서 탈색 시 투과율은 66%로 39%의 투과율 변화를 나타내었고 응답속도도 상승하였다.; Electrochromic device is a excellent smart window which controls light transmission artificially by applying electric potential. It means that we could save energy for maintaining buildings by controlling infrared energy in and out through window. Normally, electrochromic device consists of five layers, transparent conducting oxide layer (TCO), active electrochromic layer (EC), electrolyte, ion storage (IS, or counter electrode), and transparent conducting oxide layer (TCO). In this study, WO₃ and CeO₂-V₂O_(5)thin film were prepared as a active electrochromic layer and a ionic storage layer, respectively, by sol-gel coating method which is cost-effective and has advantage of its scalability. We studied of the effects of film thickness and heat-treatment conditions of WO₃ and CeO₂-V₂O_(5)to its electrochromic properties. Durability of electrochromic full cell device were also evaluated. WO₃ thin film was amorphous phase below 400℃ and crystallized as monoclinic at 400℃ and as orthorhomic at 500℃. In amorphous phase, the increase in heat-treatment temperature causes the densification of thin film and the decrease in porosity. Although lower temperature heat-treatment generates porous films, it results in the remaining of organics within the thin film and inhibits the electrochromic properties. Crystallization above 400℃ makes the thin film denser and decreased its electrochromic properties. Based on these facts, we think that tungsten oxide thin film prepared at 200℃ has the optimal electrochromic properties. WO₃ thin film thickness could be controlled to be in the range of 120 and 350nm by varying withdrawal speeds. Densities of tungsten oxide thin film showed almost the same value of 5.3g/cm³ in 170 and 300nm thickness region. The quantity of inserted lithium ion into the thin film tended to increase linearly up to 250nm and then decrease with the thickness. It means that the increase in thickness offers more active sites for lithium ion insertion up to critical thickness of sol-gel prepared WO₃ thin film, but organics and impurities can not be removed enough above the critical thickness, which degrades electrochromic properties. Thickness of CeO₂-V₂O_(5)thin film increased from 30nm to 120nm by changing withdrawal speeds, but did not show further increase in thickness more than 200mm/min speed, having almost saturating thickness of 120nm. Charge density increased with thickness from 2.3 to 5.2mC/cm². The heat-treatment conditions of CeO₂-V₂O_(5)thin film is considered to be very important factor to charge density. For 1:1 CeO₂-V₂O_(5), 1 hour heat treatment at 400℃ is not enough to remove impurities and crystallize the thin film. The remaining impurities and the partial crystallization are considered to cause small charge density and irreversibility. 2 hours heat-treatment at 400℃ promotes the crystallization of tetragonal CeVO₄ and removes the remaining impurities, which result in obvious improvement of charge density and reversibility. However, in the case of 2:1 CeO₂-V₂O_(5), the longer heat-treatment time causes the formation of CeO₂, which inhibit the insertion of lithium ion and showed irreversibility. Full cell device is comprised of WO₃ for working electrode, CeO₂-V₂O_(5)for counter electrode and 1M LiClO₄ and propylene carbonate as electrolyte. In the results of durability test up to 100 cycles, full cell device with 1:1 CeO₂-V₂O_(5)ion storage layer showed good stability of charge density, about 6mC/cm² with no degradation, but full cell device with 2:1 CeO₂-V₂O_(5)showed the degradation of charge density from 4 to 3.5mC/cm². The most optimal full cell device with 1:1 CeO₂-V₂O_(5)maintained its charge density of 5.8mC/cm² and had 39% of transmittance modulation from 27% at colored state to 66% at bleached state. In conclusion, remaining impurities, amorphous or crystalline phase, densification of thin film are the main factors to electrochromic properties of WO₃ and CeO₂-V₂O_(5)prepared by sol-gel coating method. For WO₃, about 250nm thick amorphous phase with no organics has the most optimal electrochromic properties.