A facile electrochemical method to synthesize nickel composites for high-performance energy storage

Abstract

Nickel-based compounds have drawn much attention as promising electrode materials for high performance supercapacitor. Currently, many fabrication methods including chemical precipitation, hydrothermal synthesis, sol-gel, thermal oxidation and electrochemical deposition have been used to produce various kinds of nickel-based nanostructures. However, these ways more or less suffers from some problems, such as the mixture of the nickel-based materials with binder and conducting agent, the relatively weak adhesion between the nickel-based materials and the current collector. Nevertheless, synthesizing nickel-based composites with high electrochemical performance utilizing a simple method still faces significant challenges. In this thesis, an anodization method in an organic electrolyte to synthesize porous layered nickel-based composite electrode for supercapacitor is demonstrated. These organic electrolytes provide slow diffusion and transfer rates of ions which contribute to the formation of the homogeneous oxide layer. Simultaneously, the absence of water in the electrolyte improves the rate performance of the nickel composite electrode. Additionally, due to the direct growth of nickel-based composite on the substrate, the contact resistance between the active materials and the substrate is reduced. As a result, the hierarchical porous Ni(OH)₂/Ni₂O₃ electrode shows a high specific capacitance of 3280 F g⁻¹ at 1 A g⁻¹. After 5,000 charge/discharge cycles, the as-prepared material remains 95.6% of the initial capacitance. By this fine controlled one-step and low cost electrochemical process without the need for hard/soft template, the hierarchical porous nickel composite shows good capacity. The strategy used here provides an easy and effective method to achieve excellent performance for supercapacitor materials. In order to further enhance the intrinsic conductivity of nickel-based composite based on a simple preparation method, we exploit the cathodic deposition. Cathodic deposition is a simple, cost-effective and easily scaled-up method to deposit active materials on many complex substrates. Normally the deposited materials come from electrolytes. Herein, we develop a novel electrodeposition method in a source-free electrolyte. Dendritic Ni@NiO core@shell electrode (DNE) is successfully fabricated by electrodeposition in a Ni-free electrolyte, with the Ni anode providing Ni ions through dissolution and diffusion. The as-prepared DNE demonstrates a high specific capacitance of 1930 F g⁻¹ and a high areal capacitance of 1.35 F cm⁻², with super-long cycle stability. Owing to the newly formed electrochemically active NiO and Ni(OH)₂ during the cycling test, the gravimetric capacitance of DNE hardly shows any decay after 70,000 cycles at a scan rate of 100 mV s⁻¹. It is also demonstrated that our method is universal to deposit dendritic Ni-compound on many other types of substrates, versatile for different applications. It is also noted that the fabrication process for supercapacitor electrode is always one-side, that is, either anodization or cathodic deposition. One-step process has been developed to produce two kinds of nickel composites by the simultaneous occurrence of anodization and cathodic deposition. The porous anodic film electrode and dendritic structure cathodic film electrode yield high areal capacitance of 3.25 and 2.02 Fcm⁻² at a scan rate of 5 mV s⁻¹, respectively. Moreover, the cathodic film electrode presents long cycling stability. Due to the continuous formation of electrochemically active no during the cycling process, the areal capacitance of the cathodic film electrode keeps very steady and almost exhibits no decay after 10,000 cycles. Furthermore, the anodic and cathodic films are directly contacted with current collector, therefore, it is avoided to use conducting agents or binders, which increases the effective active materials and facilitates the electrode preparation process. Finally, on the base of studies of single metal nickel-based composite, the introduction of extra cobalt to improve electrochemical performance is investigated. Hierarchical Ni-Co@Ni-Co layered double hydroxide (Ni-Co@Ni-Co LDH) nanotube arrays (NTAs) are fabricated on carbon fiber cloth (CFC) by template-assisted electrodeposition for high-performance supercapacitors. The synthesized Ni-Co@Ni-Co LDH NTAs/CFC (Ni:Co=1:1) shows high capacitance of 2385 F g-1 at a current density of 5 A g⁻¹, while 98.8% of its initial capacitance is retained after 5,000 cycles. When the current density increases from 2 to 20 A g⁻¹, the capacitance loss is less than 20%, demonstrating an excellent rate capability. A highly flexible all-solid-state asymmetric supercapacitor is successfully fabricated with Ni-Co@Ni-Co LDH NTAs/CFC as the positive electrode and electrospun carbon fibers/CFC as the negative electrode, showing a maximum specific capacitance of 319 F g⁻¹, a high energy density of 100 W h kg⁻¹ at 1.5 kW kg⁻¹, and good cycling stability (98.6% after 3,000 cycles). These fascinating electrochemical properties result from the novel structure of electrode materials and synergistic contributions from the two electrodes, showing great potential for energy storage applications. The material structure design can be hopeful for fabricating supercapacitors with excellent performance.