High energy density supercapacitor electrode materials based on mixed metal MOF and its derived C@bimetal hydroxide embedded onto porous support

Highlights

• Ready-to-use and binder-free Ni,Co-MOF/NF was fabricated through one-pot deposition.

• C@Ni_1−xCox(OH)₂ was prepared though chemical treating Ni,Co-MOF/NF electrode.

• C@Ni,Co-hydroxide/NF electrode exhibited 1825 F/g at 1 A/g.

• High-rate capability of 66.5% was observed.

• C@Ni,Co-hydroxide/NF showed 87.6% of its capacitance at the end of 8000th cycle.

Abstract

In the recent years, metal-organic frameworks (MOFs) and their derived mixed metal hydroxides/oxides structures have emerged as an engrossing category of functional materials with some unique properties such as high porosity and high specific surface area for energy storage applications. Here, a cathodic electrodeposition strategy was utilized to prepare a bimetallic NiCo metal-organic framework (NiCo-MOF) with flower-like morphology consisting of nanopetals onto Ni foam (NF) as free-standing electrode. Structural characterization revealed that Ni²⁺ and Co²⁺ metal ions are uniformly distributed on the deposited films on the nickel foam. Post-chemical treatment of NiCo-MOF/Ni foam electrode under basic condition (i.e. 4 M KOH) was concluded carbon coated hierarchical mixed hydroxide (i.e. C@Ni_1-xCo_x(OH)₂) structures onto Ni-foam with similar morphology as its pristine binary MOF. Both ready-to-use fabricated electrodes were characterized with various techniques of X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Field-emission scanning electron microscope (FE-SEM), Energy dispersive X-ray analysis (EDAX), Energy dispersive spectroscopy (EDS) mapping and Thermogravimetric-differential scanning calorimetry (TG-DSC). Supercapacitive behaviors of pristine NiCo-MOF/NF and its derived C@NiCo-hydroxide/NF electrodes were also measured using cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) measurements in 2 M KOH as electrolyte. The C@NiCo-hydroxide/NF electrode exhibited better performance as compared to pristine sacrificial binary metallic MOF electrode, which was 1825 F g^–1 at a discharge current density of 1 A g^–1 which still preserved 66.5% of its initial capacitance even at a high-rate load of 15 A g^–1. Additionally, the C@NiCo-hydroxide/NF showed 87.6% of its capacitance at the end of 8000th cycle.

Introduction

Due to the rapid growth of the global economy, challenges associated with sustainable energy resource supply, environmental pollution crisis caused by rising greenhouse gases, along with the fast development of renewable energy sources, the need to develop energy storage systems is more highlighted than ever [1]. To address these essential needs, batteries and supercapacitors as two major electrochemical energy storage systems are developed in the recent years [2], [3]. The storing mechanisms of batteries are based on oxidation/reduction of electrode active materials and due to intrinsic nature of redox reactions, the energy and power densities and of course cycle life of batteries are limited [4], [5], [6], [7]. Supercapacitor as an emerging technology in the field of energy storage systems, can offer energy and power density 10–100 times larger than of traditional batteries and capacitor than batteries [5], [6]. For a lifespan comparison, traditional batteries and capacitor have a number of charge cycles average between 500 and 10,000 cycles while. Supercapacitors have a lifespan ranging from 100,000 to a million cycles with life cycles 25–30 years [8]. Also, supercapacitors have a much broader effective operating temperature (from roughly −40 to +85 °C) and high efficiency near to 90% [9], [10]. Other advantages of supercapacitors over the battery include less weight, fast charging and fast accessing to the stored energy [11]. The charge storage mechanisms of supercapacitors include electrical double layer capacitance (EDLC) and Faradaic redox reactions on the surface or electrode-electrolyte interface (pseudocapacitance) [12]. The charge-transfer reactions in the supercapacitors occurring at the electrode surfaces and so the suitable materials for supercapacitors electrode must necessarily have a high porosity and specific surface area.

Research on the metal-organic frameworks (MOFs) as organic/inorganic crystalline porous materials that consist of a regular array of positively charged metal ions surrounded by organic 'linker' molecules, has dramatically increased over the recent years [12]. The metal ions as a fulcrum bind the arms of organic ligand together to form a repeating, cage-like structure. Due to this permanent porous structure which is due to strong coordination band energy, MOFs have an extraordinarily large internal surface area and high stability. Their controllable porosity and high surface areas make MOFs potential materials for many applications such as gas storage and separation, liquid separation and purification, electrochemical energy storage, catalysis, optics, electronics and sensing [13], [14], [15], [16].

Transition-metal such as Ni, Co, Fe, Cd and Zr are widely used to design and synthesize various novel MOFs for reasons such as have many coordination numbers, large surface areas, low cost and high mass density. Up to now, various one metallic based MOFs have been synthesized via different methods and used as electro-active material for supercapacitors [17]. For example, Lee et al. have synthesized a Co-based MOF electrode with a specific capacitance of 206.76 F/g at a current density of 0.6 A/g using 1 M LiOH as an aqueous electrolyte [18]. Tan et al. prepared a Zr-based MOF (UiO-66) by solvothermal method at various reaction temperatures and reported the maximum specific capacitance (1144 F/g) at a scan rate of 5 mV/s [19]. In another report, Campagnol et al. exhibited a specific capacitance of 34 F/g at the scan rate 5 mV/s using 0.1 M Na₂SO₄ for MIL-100 pristine MOF [20]. Furthermore, bimetallic MOFs with synergistic effect of two different metal ions have been reported to possess high specific capacitances and good cycling performances [21]. Yang et al. [22] reported that doping the Ni-MOF with Zn improves its electrochemical performance, where the fabricated Ni,Zn-MOF delivered specific capacitances of 1620 and 854 F/g at current densities of 0.25 and 10 A/g, respectively.

In this paper, we develop a practical method to synthesize porous NiCo-BTC MOF flower-like structures using cathodic electrodeposition on the nickel foam (NF). After that, the NiCo-BTC MOF onto was etched in basic media containing 8 M KOH and carbon coated mixed NiCo-hydroxide onto Ni foam was prepared. Both fabricated electrodes were used as the binder-free ready use electrodes for supercapacitive evaluations. The C@NiCo-hydroxide/NF electrode showed highest specific capacitance of 1825 F g^–1 at a current density of 1 A g^–1 and still showed 1216 F g^–1 at a high current density of 15 A g^–1. The NiCo-hydroxide/NF preserved 75.6% of its initial capacitance after 8000 consecutive cycles at current density of 15 A g^–1.