Enhanced electrochromic performance of carbon-coated V₂O₅ derived from a metal–organic framework

Highlights

• Carbon-coated V₂O₅ nanoparticles are prepared using MOF as template and precursor.

• The carbon coating layer can enhance the electronic conductivity and dimensional stability of V₂O₅.

• Compared with the bare V₂O₅, carbon-coated V₂O₅ exhibits excellent electrochromic performance.

Abstract

In this study we synthesized a vanadium (V)-containing metal–organic framework (MOF) and used it as a template to prepare V₂O₅ NPs. We used MIL-47, a V-containing MOF having uniform C and V distributions, as a precursor for the preparation of carbon-coated V₂O₅ (C@V₂O₅) samples through annealing. The C@V₂O₅ structures were readily dispersed in poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) to form stable solutions, allowing the deposition of C@V₂O₅ on various substrates through simple low-temperature solution-based processes. The uniform coating of carbon on V₂O₅ was useful for two reasons: (i) the outer layer enhanced the electronic conductivity of a V₂O₅ electrode and its corresponding electrochemical properties and (ii) based on the electrochemical quartz crystal microbalance (EQCM) analysis, the carbon coating served as a buffer layer that allowed Li⁺ ion transport, but blocked migration of solvent into the V₂O₅ electrode, thereby improving the dimensional and electrochemical stability. Compared with the bare V₂O₅, C@V₂O₅ exhibited excellent electrochromic (EC) performance, with an EC contrast of 45.8%, a mean response time of 3.4 s, and a coloration efficiency of 89.3 cm²/C. C@V₂O₅ also displayed higher cycling stability (80.6% retention after 5000 cycles) and highly reversible ionic transport during redox reactions, compared with those of the bare V₂O₅.

Introduction

With the continued net emission of greenhouse gases, more energy is being consumed for cooling-dominated air conditioning. Heat gain and loss through windows is responsible for 25–30% of residential heating and cooling energy use [1]. Therefore, modern buildings should be built with energy-efficient windows. Unlike conventional windows, smart windows have the ability to modulate their optical properties dynamically in response to the climate or the user’s requirements. Among the various developed optically adaptive technologies, including electrochromic (EC) windows [2], [3], [4], liquid crystals [5], [6] and suspended-particle devices [7], [8], ECs have attracted the most research interest because of their high energy efficiency and their ability to change color in a persistent but reversible manner.

EC materials undergo a reversible change in their optical properties when a voltage is applied. A typical EC material consists of two active electrodes separated by an electrolytic layer. In general, EC materials can be divided into three categories: organic dyes [9], [10], [11], conjugated polymers [12], [13], [14], [15], and transition metal oxides (TMOs) [16], [17], [18], [19]. TMOs are particularly attractive because of their low cost, color variation, excellent stability, and reliability. Among the various TMOs, V₂O₅ is an excellent EC material because its layered structure allows the intercalation of Li⁺ and H⁺ ions [20]. Furthermore, V₂O₅ displays both anodic and cathodic electrochromism, making it quite useful as a material for EC devices. Unfortunately, the low electric and ionic conductivities of V₂O₅ restrict its response kinetics and cycling life, thereby limiting its practical applications.

Metal–organic frameworks (MOFs), which feature high surface areas and tunable microporosity, are promising materials for a wide range of applications, including energy storage [21], [22], [23], [24], [25], [26], sensors [27], catalysis [28], [29], CO₂ capture [30], and electrochromism [31], [32], [33], [34], [35], [36]. Recently, TMOs [37], [38], metal sulfides [39], [40], [41] and carbon materials [42], [43] have been synthesized by using MOFs as starting materials and sacrificial templates. The structure of a MOF precursor, with its periodically distributed metal centers, can endow porous structures with open channels and prevent the negative effects of aggregation [44], [45] upon thermal treatment. In addition, thermal pyrolysis of a MOF can provide a carbon matrix that coats the resultant TMOs, leading to higher electric conductivity [46], [47]. The unique properties of MOF-derived TMOs make them exhibit excellent electrochemical performance for application in lithium-ion batteries and supercapacitors. Similarly, MOF-derived V₂O₅ should have a favorable morphology—high surface area and percolating channels—to facilitate ionic transport for EC applications.

In this study, we synthesized the V-MOF MIL-47 from VCl₃ and terephthalic acid as the node and linker units, respectively. Because of the open framework and uniform carbon distribution, carbon-coated V₂O₅ (C@V₂O₅) NPs were readily obtained through thermal treatment of MIL-47 under a mixed atmosphere (N₂/O₂, 7:3). The as-prepared C@V₂O₅ powders could be dispersed in poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) through a simple sonication process, forming stable suspensions. The stable C@V₂O₅/PEDOT:PSS solutions could then be spray-coated onto indium tin oxide (ITO) substrates to fabricate EC electrode. The thin carbon layer on the V₂O₅ surface decreased the grain boundary resistance, leading to higher electric conductivity. As a result, the MIL-47–derived C@V₂O₅ EC electrodes exhibited a higher electroactivity, faster response times, and enhanced cycling stability, when compared with their unmodified counterparts.