Solvent-free MOF-CVD prepared ZIF-67 film with hollow and opened morphology for supercapacitor application

Highlights

• ZIF-67 films on foam Ni, Cu, Fe and Zn were successfully prepared using solvent-free MOF-CVD method.

• The morphology evolution of the ZIF-67 films has been investigated and the ZIF-67 film on Ni substrate presents a unique hollow and opened polyhedron morphology.

• The ZIF-67 film on foam Ni has superior electrochemical performances as active material for supercapacitors.

Abstract

Metal organic frameworks (MOFs) have demonstrated excellent properties for application in energy storage field. In this work, we use a facile and solvent-free method to fabricate ZIF-67 films on foam Ni, Cu, Fe and Zn substrates. The pre-electrodeposited films are exposed to 2-methylimidazole vapor to generate ZIF-67 films. The morphology of the ZIF-67 films depends on the substrates and reaction duration. Specially, a hollow and opened polyhedron appearance is generated on the foam Ni substrate, which presents a uniform and smooth distribution. All ZIF-67 films on different substrates demonstrate good electrochemical performances when they are utilized as the active materials in the supercapacitors. The ZIF-67 film on the foam Ni substrate has 105 mF/cm² areal specific capacitance at 10 mA/cm² current density, showing superior electrochemical properties than that on other substrates. The specific energy can reach 24.5 μWh/cm² at a high specific power of 14 mW/cm² in an asymmetric supercapacitor, with Ni@Z67 as the cathode material and active carbon as anode electrode material. The process combining a simple electrodeposition method with solvent-free CVD transformation will be useful for the fabrication of MOFs films and their potential applications in various fields.

Introduction

In recent years, many efforts have been attempted to apply metal organic frameworks (MOFs) in energy storage due to their large surface areas, controllable pore volumes, lots of active sites and chemical stability [1], [2], [3], [4], [5], [6], [7]. Some MOFs, including Ni-based [8], [9], [10], [11], [12], Mn-based [13], Co-based [14], [15], [16], [17], Ag-based [18] and Zn-based [19], [20], Zr-based [21], [22], bimetallic [23], [24], [25], [26] MOFs etc., have demonstrated prospective performances for application in supercapacitors. Ni-MOF microflowers were synthesized by a solvent-assisted approach [10], which presented high specific capacitance and good rate capability in supercapacitors. The hierarchical flower-like structure and the nickel hydroxide species were responsible for high electrochemical performance of the Ni-MOF electrode. Guo et al. reported a self-assembled Ni-doped Co-MOF spherical shell electrode via facile solvothermal method [16]. The optimized CoNi_0.5-MOF electrode showed a high energy density of 23.44 Wh/kg at a power density of 350 W/kg in the asymmetric solid-state supercapacitor, implying that CoNi_0.5-MOF nanosheets was effective for offering abundant active sites and beneficial to the transport of electrolyte ions. In Shim’s work, a three-dimensional hollow microspheres of nickel-zinc MOFs were fabricated in situ on carbon fabric and used as binder-free electrode in supercapacitors [24]. The asymmetric supercapacitor assembled with nickel-zinc MOFs had desirable electrochemical performances, providing excellent MOF-based binder-free energy storage materials.

MOFs were combined with other active materials to form composite, which presented far superior performances than that of individual component due to the synergistic effects [12], [27], [28], [29], [30], [31]. MnO₂ @Ni-HHTP nanoarrays with different lengths were designed and investigated. The symmetric supercapacitor with MnO₂ @Ni-HHTP nanoarrays as positive electrode had high energy density, excellent rate performance and cycle stability, due to high redox activity of MnO₂ and superior electron and ion conductivity in Ni-HHTP [12]. Fisher et al. reported a covalent graphene-MOF hybrids for high-performance supercapacitors [28]. It was revealed the amide linkage could facilitate charge transfer and offer good electrochemical properties. In an asymmetric supercapacitor, the hybrids electrode has a power density of up to 16 kW/kg and an energy density of up to 73 Wh/ kg, which is comparable to that of commercial Pb-acid and Ni/MH batteries. By the intercalation method, Mondal et al. synthesized vanadium intercalated cobalt trimesic MOF composite for supercapacitor [30]. The results indicated organic system precursors and the integration of annealing were important to the electrochemical storage response. In general, MOF-based electrode materials were promising and versatile, which is worth exploring widely and intensively.

ZIF-67 is one hot issue of MOFs, which commonly prepared in wet chemistry method with zinc salt and 2-methylimidazolate (2-MIM) ligands. A solvent-free process called ‘MOF-CVD’ was developed by Ameloot et al. to generate MOFs films on certain substrates [32]. This method is useful and promising because the generation of the MOFs films is solvent free, avoiding the problems in terms of adhesive and ions impurities. ZIF-67 films can be easily generated via MOF-CVD method. Furthermore, we noticed that the morphology and properties of the films are greatly influenced by different substrates and other experimental conditions [33], [34], [35]. Trukhanov et al. deposited nanostructured NiFe film on silicon via pulsed electrodeposition and annealing treatment. They found the opposite effect of annealing heat treatment on the mechanical properties of the films [35]. Therefore, it is meaningful and interesting to investigate the effects of the substrates on the films. In this work, we used the MOF-CVD method to generate ZIF-67 films on different substrates. The morphology, structure and electrochemical properties of as-prepared ZIF-67 films were systematically studied.