High-performance electrode materials of heteroatom-doped lignin-based carbon materials for supercapacitor applications

Supercapacitors are the preferred option for supporting renewable energy sources owing to many benefits, including fast charging, long life, high energy and power density, and saving energy. While electrode materials with environmentally friendly preparation, high performance, and low cost are important research directions of supercapacitors. At present, the growing global population and the increasingly pressing issue of environmental pollution have drawn the focus of numerous researchers worldwide to the development and utilization of renewable biomass resources. Lignin, a renewable aromatic polymer, has reserves second only to cellulose in nature. Ten million tonnes of industrial lignin are produced in pulp and paper mills annually, most of which are disposed of as waste or burned for fuel, seriously depleting natural resources and polluting the environment. One practical strategy to accomplish sustainable development is to employ lignin resources to create high-value materials. Based on the high carbon content and rich functional groups of lignin, the lignin-based carbon materials generated after carbonization treatment display specific electrochemical properties as electrode materials. Nevertheless, low electrochemical activity of untreated lignin precludes it from achieving its full potential for application in energy storage. Heteroatom doping is a common modification method that aims to improve the electrochemical performance of the electrode materials by optimizing the structure of the lignin, improving its pore structure and increasing the number of active sites on its surface. This paper aims to establish theoretical foundations for design, preparation, and optimizing the performance of heteroatom-doped lignin-based carbon materials, as well as for developing high-value-added lignin materials. The most reported the mechanism of supercapacitors, the doping process involving various types of heteroatoms, and the analysis of how heteroatoms affect the performance of lignin-based carbon materials are also detailed in this review.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Facile fabrication of hierarchical porous carbon for a high-performance electrochemical capacitor

A facile and rapid pyrolysis method is developed for the synthesis of 3D hierarchical porous carbon, which exhibits a high specific capacitance, good rate capability and good cycling performance.

Controlling the BET Surface Area of Porous Carbon by Using the Cd/C Ratio of a Cd-MOF Precursor and Enhancing the Capacitance by Activation with KOH

Herein, four new cadmium metal-organic frameworks (Cd-MOFs), [Cd(bib)(bdc)]_∞ (1), [Cd(bbib)(bdc)(H₂ O)]_∞ (2), [Cd(bibp)(bdc)]_∞ (3), and [Cd₂ (bbibp)₂ (bdc)₂ (H₂ O)]_∞ (4), have been constructed from the reaction of Cd(NO₃ )₂ ⋅4 H₂ O with 1,4-benzenedicarboxylate (H₂ bdc) and structure-related bis(imidazole) ligands (1,4-bis(imidazol-1-yl)benzene (bib), 1,4-bis(benzoimidazol-1-yl)benzene (bbib), 4,4'-bis(imidazol-1-yl)biphenyl (bibp), and 4,4'-bis(benzoimidazol-1-yl)biphenyl (bbibp)) under solvothermal conditions. Cd-MOF 1 shows a 2D (4,4) lattice with parallel interpenetration, whereas 2 displays an interesting 3D interpenetrating dia network, 3 exhibits an unusual 3D interpenetrating dmp network, and 4 presents a 3D self-catenated pillar-layered framework with a Schäfli symbol of [4³ ⋅6³ ]₂ ⋅[4⁶ ⋅6¹⁶ ⋅8⁶ ]. The structural diversity indicates that the backbone of the bis(imidazole) ligand (including the terminal group and spacer) plays a crucial role in the assembly of mixed-ligand frameworks. By using the pore-forming effect of cadmium vapor, for the first time we have utilized these Cd-MOFs as precursors to further prepare porous carbon materials (PCs) in a calcination-thermolysis procedure. These PCs show different porous features that correspond to the topological structures of Cd-MOFs. Significantly, it was found that the specific surface area and capacitance of PCs are tuned by the Cd/C ratio of the MOF. Furthermore, the as-synthesized PCs were processed with KOH to obtain activated porous carbon materials (APCs) with higher specific surface area and porosity, which greatly promoted the energy-storage capacity. After full characterization, we found that APC-bib displays the largest specific surface area (1290 m²  g^-1 ) and total pore volume (1.37 cm³  g^-1 ) of this series of carbon materials. Consequently, APC-bib demonstrates the highest specific capacitance of 164 F g^-1 at a current density of 0.5 A g^-1 , and also excellent retention of capacitance (≈89.4 % after 5000 cycles at 1 A g^-1 ). Therefore, APC-bib has great potential as the electrode material in a supercapacitor.

One-step nanocasting synthesis of nitrogen and phosphorus dual heteroatom doped ordered mesoporous carbons for supercapacitor application

N and P dual heteroatom doped ordered mesoporous carbon was synthesized and exhibited enhanced specific capacitance (220 F g⁻¹ at 1 A g⁻¹), good rate capability (178 F g⁻¹ at 16 A g⁻¹ with 81% capacitance retention) and excellent cycling stability (91% after 3000 cycles).