There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

It is of great interest to develop new carbon-based materials as electrodes for supercapacitors because the conventional electrodes of activated carbons in supercapacitors cannot meet the ever-increasing demands for high energy and power densities for electronic devices. Due to their high electronic conductivity and improved hydrophilic properties, together with their easy syntheses and functionalization, N-doped carbons have shown a great potential in energy storage and conversion applications. In this review, after a brief introduction of electrochemical capacitors, we summarize the advances, in the recent six years, in the preparation methods of N-doped carbons for applications in supercapacitors. We also discuss and predict futuristic research trends towards the design and syntheses of N-doped carbons with unique properties for electrochemical energy storage.

Review on recent advances in nitrogen-doped carbons: preparations and applications in supercapacitors

The exploration of renewable, cost-effective, and environmentally friendly electrode materials with high adsorption, fast ion/electron transport, and tunable surface chemistry is urgently needed for the development of next-generation biocompatible energy-storage devices. In recent years, biomass-derived carbon electrode materials for energy storage have attracted significant attention because of their widespread availability, renewable nature, and low cost. More importantly, their inherent uniform and precise biological structures can be utilized as templates for fabricating electrode materials with controlled and well-defined geometries. Meanwhile, the basic elements of biomass are carbon, sulfur, nitrogen, and phosphorus. The special naturally ordered hierarchical structures as well as abundant surface properties of biomass-derived carbon materials are compatible with electrochemical reaction processes such as ion transfer and diffusion. To date, a series of novel porous carbon materials with different dimensions have been prepared by various methods using biomass as the raw material, which is an important field in the fabrication of supercapacitor electrode materials. Herein, we summarized recently reported biomass-derived carbon materials with one-dimensional, two-dimensional, and three-dimensional structures and their applications as carbon-based electrode materials for supercapacitors. Finally, the current challenges and future perspectives of the carbon-based electrode materials with respect to the supercapacitor's performance were closely highlighted.

Biomass-derived porous carbon materials with different dimensions for supercapacitor electrodes: a review

Advanced electrical energy storage technology is a game changer for a clean, sustainable, and secure energy future because efficient utilization of newable energy hinges on cost-effect and efficient energy storage. Further, the viability of many emerging technologies depends on breakthroughs in energy storage technologies, including electric vehicles (EVs) or hybrid electric vehicles (HEVs) and smart grids. Lithium-ion batteries (LIBs), a great success in the portable electronics sector, are believed also the most promising power sources for emerging technologies such as EVs and smart grids. To date, however, the existing LIBs (with LiCoOx cathode and graphite anode) are still unable to meet the strict requirements for safety, cycling stability, and rate capability. The development of advanced anode materials, which can overcome the shortcomings of graphite anode (such as formation of dendritic lithium during charge and undesirable solid electrolyte interface), is of critical importance to enhancing the cycling stability and operational safety of LIBs. Lithium titanate (Li4Ti5O12) has recently attracted considerable attentions as a potential anode material of LIBs for high power applications due to several outstanding features, including a flat charge/discharge plateaus (around 1.55 V vs. Li/Li+) because of the two-phase lithium insertion/extraction mechanism and minimum chance for the formation of SEI and dendritic lithium, dramatically enhance the potential for high rate capability and safety. In addition, there is almost no volume change during the lithium insertion and extraction processes, ensuring a high cycling stability and long operational life. However, the electronic conductivity of Li4Ti5O12 is relatively low, resulting in large polarization lose, more so at higher cycling rates, and poor rate performance. Currently, considerable research efforts have been devoted to improving the performance of Li4Ti5O12 at fast charge/discharge rates, and some important progresses have been made. In this review, we first present a general overview of the structural features, thermodynamic properties, transport properties, and the electrochemical behavior of Li4Ti5O12 under typical battery operating conditions. We then provide a comprehensive review of the recent advancements made in characterization, modification, and applications of Li4Ti5O12 electrodes to LIBs, including nanostructuring, surface coating, morphological optimization, doping, and rational design of composite electrodes. Finally, we highlight the critical challenges facing us today and future perspectives for further development of Li4Ti5O12-based electrodes. It is hoped that this review may provide some useful guidelines for rational design of better electrodes for advanced LIBs.

A comprehensive review of Li4Ti5O12-based electrodes for lithium-ion batteries: The latest advancements and future perspectives

Carbon materials, owing to their excellent electrical conductivity, tailorability, inexpensiveness and versatility, have been extensively studied as electrode materials for supercapacitors. The capacitance of carbon-based supercapacitor electrodes has remained at a mediocre level between 100 and 200 F g−1 for decades. Until recently, a new family of carbon materials termed hierarchical porous carbons has pushed the capacitance to new benchmark values beyond 300 F g−1, and has revitalized the exploration of carbon materials for supercapacitors. Hierarchical porous carbons contain different scales of pores (from micropores to macropores) inter-connected together and assembled in hierarchical patterns. Experimental studies coupled with theoretical investigations have elucidated that the presence of micropores is responsible for offering a large surface area to enhance charge storage capability, whilst mesopores, macropores and the hierarchical structure improve electrolyte infiltration and facilitate ion diffusion. This review will start by introducing different pore types and the definition of hierarchical porous structures, followed by discussion and exemplification of major synthesis strategies. In addition, recent molecular-level understanding of the relationship between pore size, functionalities inside pores, pore spatial distribution and capacitive performance is presented. Finally, challenges and future opportunities associated with hierarchical porous carbons for supercapacitors are discussed.

Revitalizing carbon supercapacitor electrodes with hierarchical porous structures

A cost-effective approach to obtain electrode materials with excellent electrochemical performance is critical to the development of supercapacitors (SCs). Here we report the preparation of a three-dimensional (3D) honeycomb-like porous carbon (HLPC) by the simple carbonization of pomelo peel followed by KOH activation. Structural characterization indicates that the as-prepared HLPC with a high specific surface area (SSA) up to 2725 m2 g−1 is made up of interconnected microporous carbon walls. Chemical analysis shows that the HLPC is doped with nitrogen and also has oxygen-containing groups. Electrochemical measurements show that the HLPC not only exhibits a high specific capacitance of 342 F g−1 and 171 F cm−3 at 0.2 A g−1 but also shows considerable rate capability with a retention of 62% at 20 A g−1 as well as good cycling performance with 98% retention over 1000 cycles at 10 A g−1 in 6 M KOH. Furthermore, an as-fabricated HLPC-based symmetric SC device delivers a maximum energy density of ∼9.4 Wh kg−1 in the KOH electrolyte. Moreover, the outstanding cycling stability (only 2% capacitance decay over 1000 cycles at 5 A g−1) of the SC device makes it promising for use in a high-performance electrochemical energy system.

A honeycomb-like porous carbon derived from pomelo peel for use in high-performance supercapacitors

Nitrogen-enriched porous carbon nanofiber networks (NPCNFs) were successfully prepared by using low-cost melamine and polyacrylonitrile as precursors via electrospinning followed by carbonization and NH3 treatments The NPCNFs exhibited inter-connected nanofibrous morphology with a large specific surface area, well-developed microporous structure, relatively high-level nitrogen doping and great amount of pyridinic nitrogen As free-standing new anode materials in lithium-ion batteries (LIBs), the NPCNFs showed ultrahigh capacity, good cycle performance and superior rate capability with a reversible capacity of as high as 1323 mA h g−1 at a current density of 50 mA g−1 These attractive characteristics make the NPCNFs materials very promising anode candidates for high-performance LIBs and, as free-standing electrode materials to be used in other energy conversion and storage devices

Nitrogen-enriched electrospun porous carbon nanofiber networks as high-performance free-standing electrode materials

A high performance Li-ion capacitor constructed with Li4Ti5O12/C hybrid and porous graphene macroform

A supercapacitor that possesses excellent performance in a wide temperature range is constructed with a partially graphitized porous carbon (PGPC) as the electrode material and tetraethylammonium tetrafluoroborate/propylene carbonate (Et4NBF4/PC) as the electrolyte. The PGPC-based electrode shows a very consistent performance in a wide temperature range of −20 to 50 °C. At working temperatures as high as 50 °C and as low as −20 °C, it delivers specific capacitances of ∼148 and ∼135 F g−1, respectively, and the supercapacitors exhibit high energy densities of ∼46 and ∼43 W h kg−1. In particular, the low-temperature capacitive performance and rate capability of PGPC are much better than those of the normally used activated carbon and among the best ones ever-reported for carbon-based supercapacitors. More significantly, the PGPC-based supercapacitor shows excellent cycling performance at all tested working temperatures, with a capacitance retention of ∼98% after 1000 cycles at 5 A g−1 even when the working temperature decreases to −20 °C.

A supercapacitor constructed with a partially graphitized porous carbon and its performance over a wide working temperature range

Electrochemical performance of a porous graphitic carbon-based supercapacitor in three different electrolytes

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