Impact of silver incorporation on cobalt rich 3-D porous carbon arising from solid state thermolysis of ZIF-67 as a pseudocapacitor electrode: Improvement of diffusion-controlled charge storage

Highlights

• Silver incorporated cobalt rich porous carbon composite prepared by thermolysis.

• Only 3 wt% of Ag incorporating enhanced the specific capacitance to 135.5C/g.

• HT-Ag/ZIF(H)//AC delivered an energy density of 8.50 Wh/kg at power density of 7037.04 W/kg.

• 92% capacitance retention after 3000 cycles showed the cycling stability of the assembled ASC.

Abstract

Here, for improvement of electrochemical energy storage properties of ZIF-67, it has been incorporated with low and high concentrations of silver ions, followed by a solid state thermolysis in an inert atmosphere to synthesis of Ag-incorporated cobalt/porous carbon composites. These as-synthesized active materials have been deposited on a nickel foam to electrochemical measurements including, Cyclic Voltammetry, Galvanostatic Charge-Discharge and Electrochemical Impedance Spectroscopy. The results revealed that the sample with trace amounts of Ag incorporation enhanced the supercapacitive performance and improved the specific charge capacity fourfold in all conducted scan rates. An increase in Ag ion improved the contribution of diffusion-controlled charge storage mechanism. Also, to investigate practical capabilities of these materials, the asymmetric supercapacitor was set up using one of the electrodes as the positive electrode and an activated carbon electrode as the negative electrode which performed maximum specific energy of 8.50 Wh/kg and maximum specific power of 7037.04 W/kg at gravimetric currents of 1 and 10 A/g respectively. Furthermore, this hybrid supercapacitor showed a high-capacity retention of 92% after 3000 cycles.

Introduction

The over depletion of fossil fuels caused by the constantly increasing energy demands and the consequential environment crisis has motivated researchers to develop renewable and eco-friendly energy sources such as wind, solar and water powers. In order to fulfill the storage requirement of such systems, the development of efficient energy storage systems is essential [1]. In this concern, in the wide spectrum of electrochemical energy storage technologies, supercapacitors have become increasingly pervasive owing to their unique features such as high power uptake and delivery which makes supercapacitors a practical candidate among other energy storage systems for applications [2].

Supercapacitors on the basis of the charge storage principles are divided into two groups: Electrostatic double layer capacitors (EDLCs) and Pseudo-capacitors. The former type stores energy in the charge separation staking place in Helmholtz double layer formed in the electrolyte/electrode interface [3], while the latter involves in reversible redox reactions at the presence of electroactive species on the accessible surfaces of the electrode [4,5]. However there is a third category that usually consists of materials combining EDLC and pseudo-capacitive capacitance to achieving both high energy and power densities in the whole device [6], therefore enhancing both the double layer capacitance and faradaic redox reactions is vital for achieving large energy and power densities in supercapacitors [7].

Metal organic frameworks (MOFs) are a class of coordination polymers composed of metallic ions or clusters as nodes and organic ligands as bridging units, which are extended in one, two or three dimensions creating potential vacant spaces. MOFs are characterized by unique features like considerable specific surface area, well-regulated pore size and high thermal and chemical stabilities and so on [8]. These properties have made MOFs great candidate for applications like sensing [9], drug delivery [10], gas storage [11], optoelectronics [12] and heterogeneous catalysis [13]. Zeolitic imidazolate frameworks (ZIFs) are one of the most investigated subclass of MOFs, composed of metal ions with the imidazolate ligands as the coordinator which are very rich in carbon, nitrogen, and metal ion components and have presented promising performance in the electrochemical applications [14]. In supercapacitor applications, MOFs are ideal candidates to be directly used as the active electrode material, as they exhibit high surface areas and large values of reaction sites which provide higher redox activities and improve the capacitance [15]. Despite the aforementioned features, most MOFs suffer from lack of the electrical conductivity and do not efficiently transfer the charge to the current collector which limits the effective utilization of built-in redox centers, and therefore restricts their application in supercapacitors [16]. A general approach to addressing this challenge is to use MOFs as sacrificial templates for preparing porous carbon compounds [17], metal oxides [18], or metal nanoparticles [19] which can provide high conductivity and electrochemical reactivity. Hence, some researchers have overcome the low conductivity limitation of MOFs by forming their composites with other conductive materials such as conducting polymers [20], carbon nano-tubes [21], graphene [22] and highly conductive metals [23,24]. Dai et al. have constructed a nanostructure in which ultrathin carbon layers were coated on cobalt NPs by using a Co-based MOF as both precursor and self-sacrificing template, and used as symmetric supercapacitor electrode which provided a specific energy of 8.8 Wh/kg at a specific power of 375 W/kg [25]. Also Wang and coworkers have derived CuC nanocomposites from Cu-MOFs and obtained the specific capacitance of 318 F/g at a current density of 1 A/g [26].

Due to their intrinsically high conductivity, the composition of noble metals (e.g. Silver) with MOFs to overcome their low electrical conductivity, has been rising. For example Mukhiya et al. reported a ternary nanocomposite of carbon nanofibers, silver NPs and mesoporous cobalt oxide nanohairs, which provided a high specific energy of 53.8 Wh/kg at a high specific power of 797.9 W/kg [27]. In other work, a supercapacitor with ternary composite of Graphene/Silver nanoparticles/Polypyrrole which exhibited a high specific capacitance of 450 F/g at 0.9 mA/g gravimetric current [28].

In the present work, silver ions have been incorporated on Co-based Zeolitic Imidazolate Framework (ZIF-67) in two different concentrations. Then a thermolysis in an inert atmosphere was applied on this highly porous composite resulting in Ag loaded Co/C composite. Then the fabricated active materials were deposited on nickel foam. The comparison of the electrochemical data showed that incorporation of Ag particles, made an impressive enhancement in the capacitance by turning into electroactive sites and decreasing the charge transfer resistance by raising the conductivity of the system.