A High-Performance, Low Defected, and Binder-free Graphene-based Supercapacitor Obtained via Synergistic Electrochemical Exfoliation and Electrophoretic Deposition Process.

An integrated electrochemical exfoliation and electrophoretic deposition (EPD) method is developed to achieve a high performance graphene supercapacitor. The electrochemical delamination of graphite sheet has obtained a low defected few-layer graphene adorned with oxygen-containing functional groups. Then, the EPD process produced a binder-free electrode to alleviate the graphene restacking problem. The electrode prepared using a deposition voltage of 5V exhibits the highest specific capacitance of 145.95 F/g at 0.5 A/g from three-electrode measurement. Moreover, this EPD-prepared electrode also demonstrates superior electrochemical properties compared to electrodes fabricated using PVDF binder. In the real symmetrical cell, the EPD-prepared electrode also shows excellent performance with a high rate capability of 82.31% (from 0.5 A/g to 10 A/g), high cycling stability of 95.00% (at 5 A/g) after 10,000 cycles, and rapid frequency response with short relaxation time (τ0) of 9.73 ms. These results indicate that this integration method is beneficial to construct a high performance binder-free supercapacitor electrode consisting of low-defected graphene materials, low electrode resistance, and less agglomeration of graphene sheets by utilizing an environmentally friendly process.

Novel strategy for high-performance supercapacitors through the polyvinylpyrrolidone (PVP)-assisted in situ growth of FeS2.

Iron disulfide or pyrite (FeS2) has emerged as a promising transition metal sulfide-based supercapacitor owing to its abundance and superb electrochemical properties. However, FeS2 still faces major hurdles in realizing its full potential, such as a low energy density and poor conductivity. In this study, we report a high-performance FeS2 supercapacitor synthesized by a direct one-step process with the help of polyvinylpyrrolidone (PVP). The incorporation of PVP on the active materials prevented dendritic expansion and acted as a binding for solving the current FeS2 limitations, while facilitating a one-step synthesis process. Additionally, PVP could enhance the electrochemical performance by enabling faster ion movement. An FeS2/PVP nanocomposite was successfully synthesized, and used in an asymmetric supercapacitor, demonstrating a high specific capacity of 735 F g-1 (at 2 A g-1) and a high energy density of 69.74 W h kg-1 (at 911 W kg-1). The superior electrochemical properties of FeS2/PVP were enabled by the lower charge-carrier resistance and better surface passivation by PVP, as demonstrated by both electrochemical experiments and first-principles calculations. The high-performance supercapacitor of FeS2 presented in this study synthesized in situ by an efficient method provides a new insight into novel supercapacitor electrodes.

A Sustainable Approach for Preparing Porous Carbon Spheres Derived from Kraft Lignin and Sodium Hydroxide as Highly Packed Thin Film Electrode Materials.

A green synthetic strategy to design biomass-derived porous carbon electrode materials with precisely tailored structure and morphology has always been a challenging goal because these materials can fulfill the demands of next-generation supercapacitors and other electrochemical devices. Potassium hydroxide (KOH) is extensively utilized as an activator since it can produce porous carbon with high specific surface area and well-developed porous channels. The exploitation of sodium hydroxide (NaOH) as an activating agent is less referenced in the literature, although it offers some advantages over KOH in terms of low cost, less corrosiveness, and simple handling procedure, all of which are appealing particularly from an industrial viewpoint. The motivation for this present study is to fabricate porous carbon spheres in a sustainable manner via a spray drying approach followed by a carbonization process, using Kraft lignin as the carbon precursor and NaOH as an alternative activation agent instead of the high-cost and high-corrosive KOH for the first time. The structure of carbon particles can be accurately transitioned from a compact to hollow structure, and the surface textural properties can be easily tuned by altering the NaOH concentration. The obtained porous carbon spheres were applied as highly packed thin film electrode materials for supercapacitor devices. The specific capacitance value of porous carbon spheres with a highly compact structure (high packing density) is 66.5 F g-1, which is higher than that of commercial activated carbon and other biomass-derived carbon. This work provides a green processing for producing low-cost and environment-friendly porous carbon spheres from abundant Kraft lignin and important insight for selecting NaOH as an activator to tailor the morphology and structure, which represents an economical and sustainable approach for energy storage devices.

Effect of H2SO4/H2O2 pre-treatment on electrochemical properties of exfoliated graphite prepared by an electro-exfoliation method.

The effect of pre-treating graphite sheets in a H2SO4/H2O2 solution before electro-exfoliation is reported. It was revealed that the volume fraction of H2SO4 to H2O2 during pre-treatment could control the degree of exfoliation of the resulting exfoliated graphite (EG). X-ray diffraction (XRD), Raman, and Fourier transform infrared (FTIR) spectroscopy analyses have suggested that EG produced by first pre-treating the graphite sheet in H2SO4/H2O2 solution with the H2SO4 : H2O2 volume fraction of 95 : 5 demonstrates the highest exfoliation degree. This sample also demonstrated excellent electrochemical properties with good electrical conductivity (36.22 S cm-1) and relatively low charge transfer resistance (R ct) of 21.35 Ω. This sample also showed the highest specific capacitance of all samples, i.e., 71.95 F g-1 at 1 mV s-1 when measured at a voltage range of -0.9 to 0 V. Further measurement at an extended potential window down to -1.4 V revealed the superior specific capacitance value of 150.69 F g-1. The superior morphology characteristics and the excellent electrical properties of the obtained EG are several reasons behind its exceptional properties. The pre-treatment of graphite sheets in H2SO4/H2O2 solution allegedly leads to easier and faster exfoliation. The faster exfoliation is allegedly able to prevent massive oxidation, which frequently induces the formation of graphite/graphene oxide (GO) in a prolonged process. However, too large H2O2 volume fraction involved during pre-treatment seems to cause excessive expansion and frail structure of the graphite sheets, which leads to an early breakdown of the structure during electrochemical exfoliation and prohibits layer by layer exfoliation.