Research PapersRapid synthesis of nickel‑copper phosphate electrode by microwave-assisted hydrothermal reaction for supercapattery
Graphical abstract
Introduction
Supercapacitors and rechargeable batteries are the energy storage devices that have drawn global attention owing to their superior energy storage capabilities for a wide range of applications [1]. Supercapacitors are of two types: electrical double-layer capacitors (EDLCs) and pseudocapacitors [2]. The working principle of EDLCs is based on forming a double layer of charges at the electrode-electrolyte interface [3]. The charge storage is mainly due to the physical adsorption/desorption of electrolyte ions on the electrode surface, and the contribution of charge storage due to chemical reaction is negligible [4]. In pseudocapacitors, the charge storage is due to the surface-based redox reactions or intercalation of ions which renders electrochemical signature (CV and GCD) similar to EDLCs [2]. On contrary, the working principle of rechargeable batteries is based on the deep redox reactions in the bulk of electrode material, which are reversible in nature [5]. During the charging of rechargeable batteries, the electrons are released at the cathode via redox reaction, which move towards anode through the external circuit [6]. At the same time, the Li+/Na+ (from lithium-/sodium-ion battery) also move towards anode through the electrolyte [6]. There are three types of reaction mechanisms that occurred at the anode of the rechargeable batteries during charging: alloying, conversion, and intercalation, which depend on the type of electrode materials. Alloying reaction is the bonding between the Li+/Na+ and the host element A (A for Si, Ge, and Sn), forming the Li-A/Na-A alloys at the anode [7]. Conversion reaction occurs when Li+/Na+ reacts with the transition metal compounds (TMX, where TM is the transition metal, such as Mn, Fe, Co, and Ni, and X represents the oxide, selenide, sulphide and fluoride), resulting in the reduction of TMX to metallic state TM, and X reacts with the Li+/Na+ to form LiX/NaX [8]. Additionally, intercalation is the diffusion of Li+/Na+ into the lattice of the electrode materials (such as graphite) without destroying its crystal structure [9]. By taking graphite as an example, one Li+/Na+ intercalates into one carbon ring and forms LiC6 [9].
Although these two devices have been utilized in multiple applications, their individual efficiency is still insufficient to meet the requirement for balancing the energy supply and demand [10]. Supercapacitors store charges either by physical adsorption/desorption of electrolyte ions (in EDLCs) or intercalation/surface-confined faradaic reactions (in pseudocapacitors) [2]. This leads to the fast charge kinetics providing high power density but low energy density of supercapacitors. On the other hand, rechargeable batteries have higher energy density due to the deep faradaic reactions in the bulk of the electrode materials [10], [11]. Therefore, the reaction kinetics are comparatively slower compared to supercapacitors. This renders the low power density of the rechargeable batteries but the high energy density.
Numerous strategies have been adopted to fabricate the devices with the hybrid energy charge storage mechanism to enhance their energy and power densities. Consequently, this type of energy storage device was established, and the specific name of this device is “supercapattery” [12]. Supercapattery is fabricated by merging the battery-type electrode from a rechargeable battery and the capacitive-type electrode from EDLC [13]. Thus, the charge storage mechanism of the device comprises of faradaic reaction and physical adsorption/desorption of the electrolyte ions, which furnishes higher energy and power densities, respectively [14].
The faradaic charge storage kinetics of a battery-type electrode is one of the important factors in supercapattery, which governs the energy density of the device [15]. Therefore, selecting electrode material with excellent faradaic behavior is essential to fabricate a supercapattery with high energy density. Until today, transition metal hydroxides [16], oxides [17], [18], sulphides [19], selenides [20], [21], and phosphates [22], [23] have been studied as the battery-type electrode materials owing to their multivalent metal cations and reversible faradaic behavior [24]. Among these materials, transition metal phosphates (TMPs) are promising battery-type electrode materials due to their unique structure, composition and low-cost [25]. TMPs offer open framework structures with various morphologies that provide abundant active sites for faradaic reactions, endowing the electrodes to have outstanding electrical conductivity and charge storage capability [25], [26]. Besides, they have high chemical stability because of their strong PO covalent bond [27], [28]. These make TMPs the ideal candidates as battery-type electrode materials for supercapattery.
Numerous TMPs such as nickel, cobalt, and manganese phosphates are utilized as electrode materials for battery-type electrodes attributed to their high electrical conductivity, fast ion transport, and environmentally friendly [29]. Among all the TMPs, nickel phosphate has been mostly studied owing to the presence of Ni2+, offering a high theoretical capacity (QT) with the value of 791 C/g, which is calculated using Eq. (1) modified from Banerjee et al. [30], [31],where n represents the number of electrons involved in the electrochemical process, F is the Faraday constant (∼ 96,500 C/mol), and Mw implies the molecular weight of the electrode material (g/mol). The tetrahedral geometry of the nickel phosphate can deliver many interstitial spaces that can lower the volumetric expansion during the diffusion of electrolyte ions [32]. Although nickel phosphate has significant faradaic behavior, it also faces some challenges when used as the electrode material [33]. One of them is its low ion transfer rate which increases the charge transfer resistance and reduces the charging-discharging kinetics of the electrode [34]. Additionally, nickel phosphate has a lower rate capability that could hinder its application in high-performance energy storage applications [33], [35]. In order to overcome these issues, the fabrication of nickel-based binary TMP electrode material is an effective way to improve the performance of nickel phosphate. The synergistic effect of nickel and suitable transition metal can improve the electrochemical performance of the electrode [36]. Copper phosphate displays better electrical conductivity than other TMPs, as it can promote higher electron transfer [37]. Hence, the binary TMP of nickel and copper (nickel‑copper phosphate, NCP) electrode material is more likely to deliver better and more stable electrochemical performance compared to nickel phosphate, and it is utilized as the battery-type electrode material for supercapattery in this study.
Electrode fabrication is another factor that determines the electrochemical performance of the supercapattery. Generally, the conventional fabrication of electrode is mainly based on the slurry coating method, by mixing binder, conducting carbon, and electrode material, and then coating on the surface of the current collector [38]. Binder ensures the strong adhesion of electrode material on the current collector, which can reduce the chances of detachment upon continuous charge-discharge cycles [39]. Nevertheless, using binders will reduce electrode conductivity due to their insulating properties. Therefore, the preparation of binder-free electrodes by the direct growth of electrode material on the current collector can address this problem [40]. The absence of binders can facilitate the charge transfer between the current collector and the electrode material, thus, improving the electrode conductivity and charge transfer kinetics [41].
There are several techniques for the synthesis of binder-free electrodes, for instance, electrostatic spray deposition (ESD), sputtering, chemical vapor deposition (CVD), and hydrothermal methods [42]. Hydrothermal is the most promising fabrication method for binder-free electrode due to its facile steps, mild synthesis temperature, and less chemical waste [43]. However, the hydrothermal method requires a longer reaction time owing to its slow conventional heating, resulting in high energy consumption [44]. Thus, the recent research is focused on the microwave-assisted hydrothermal method for synthesizing binder-free electrodes. This method uses rapid microwave heating instead of slow conventional heating. The sample can easily be penetrated by the microwave energy, which is subsequently absorbed by the sample and transformed into heat energy [45]. Therefore, this technique can increase the reaction speed and shorten the synthesis time (from hours to minutes) compared to hydrothermal method [45], [46]. Indirectly, the microwave-assisted hydrothermal method with a short reaction time can also reduce energy consumption, making this technique a time- and energy-saving and environmentally friendly method [44], [46].
In this study, the binder-free nickel‑copper phosphate (NCP) electrode is fabricated via microwave-assisted hydrothermal method and applied as the battery-type electrode for supercapattery. The NCP electrodes with various precursor ratios of nickel and copper (Ni:Cu of 4:0, 3:1, 1:1, 1:3, 0:4) were fabricated at 90 °C for 12.5 min. X-ray diffractometer (XRD), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Field emission scanning electron microscope (FESEM) and Energy-dispersive X-ray (EDX) spectroscopy were utilized to identify the crystallinity, structure, functional group, chemical state composition, morphology, and purity of the electrodes, respectively. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were used to examine the electrochemical performance of the NCP binder-free electrodes. The NCP electrode that exhibited the best electrochemical performance was selected as the battery-type electrode for supercapattery.
Section snippets
Materials
Nickel (II) acetate tetrahydrate (Ni(CH3CO2)2·4H2O) and copper (II) acetate monohydrate (Cu(CH3CO2)2·H2O) were acquired from Acros organics (Belgium) and Alfa Aesar (United States), respectively. Di‑sodium hydrogen phosphate (Na2HPO4) and potassium hydroxide (KOH) were obtained from Merck (Germany). N-methyl-2-pyrrolidone (NMP), poly (vinylidene fluoride) (PVDF), carbon black and activated carbon (AC) were acquired from Sigma-Aldrich (Malaysia). Hydrochloric acid (HCl) and acetone (C3H6O) were
Formation of NCP binder-free electrodes
The binder-free electrodes are formed by the direct growth of NCP on the nickel foam surface. The growth mechanism of the NCP electrode materials involves the processes of nucleation and material growth [47]. At first, Ni(CH3CO2)2·4H2O, Cu(CH3CO2)2·H2O, and Na2HPO4 were dissolved under stirring and produced Ni2+, Cu2+, and PO43−, as shown in Eqs. (2), (3), (4), respectively.
The Ni2+, Cu2+, and PO43− were
Conclusion
Herein, the binder-free nickel‑copper phosphate (NCP) battery-type electrodes were prepared using the microwave-assisted hydrothermal technique. The Cu-P electrode was initially showed a uniform flower-like morphology with crystalline structure. With the addition of Ni contents into the Cu-P electrode material, the morphology was changed from flower-like to irregular microspheres. Besides, the crystallinity of Cu-P was changed to amorphous structure with the addition of Ni contents. The
CRedit authorship contribution statement
Ong Gerard: Methodology, Investigation, Writing – original draft.
Arshid Numan, S. Ramesh: Project administration, Resources, Supervision review & editing, Conceptualization.
K. Ramesh: Methodology, review & editing.
Muhammad Amirul Aizat Mohd Abdah, Mohammad Khalid: Resources, review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors appreciate the funding support of Sunway University collaborative research fund: MRU 2019 (STR-RMF-MRU-001-2019) and Sunway University Research Cluster Grant Scheme (STR-RCGS-E_CITIES[S]-004-2021). The authors also thank Ministry of Science, Technology and Innovation (MOSTI), Malaysia for providing the TED 1 grant (MOSTI002-2021TED1). We would also wish to thank Sunway University and Universiti Malaya for providing the facilities for this study.
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