Date Published: February 15, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Mei Yang, Zhen Zhou.
Featured with unique mechanical, electronic and chemical properties, nitrogen‐doped carbon materials have become the research hotspot of energy storage. As electrode materials in supercapacitors (SCs), N‐doped carbons have demonstrated intriguing flexibility and superb performances in a wide electrochemical window, equipped with versatile properties as both cathodes and anodes for constructing high voltage devices. Compared with limited doping level, N‐rich and porous carbon materials (NPCs) are of great desire to release the restricted properties of N species and obtain high specific capacitances (>600 F g−1), pushing the energy density towards the battery level without scarifying the capacitor‐level power ability. In this Research News we firstly discuss the key factors influencing the performance of NPC electrodes to disclose related charge storage mechanisms. In addition, the trade‐off among N‐content, porous structure and electrical conductivity is involved as well as electrochemical behaviors in different electrolytes. Also, various progressive developments are highlighted systematically ranging from asymmetric to symmetric and hybrid configurations, covering both aqueous and non‐aqueous systems. Finally, some stubborn and unsolved problems are summarized, with prospective research guidelines on NPC‐based SCs.
Carbonaceous materials are proposed as the best option for energy‐related applications because of natural abundance, facile accessibility and attractive properties, such as large surface area, high electrical conductivity, and excellent chemical stability.1, 2, 3 In this respect, they play critical roles as electrode materials in supercapacitors (SCs) and can be reversibly operated within a fairly large voltage window as well as versatility as both cathodes and anodes; however, their energy storage ability is largely limited because of the physical charge storage mechanism of electric double layer capacitors (EDLCs, <10 W h kg−1).4, 5 Therefore, it is desperately urgent to develop advanced functional carbon materials with enhanced electrochemical characteristics. Heteroatom‐ especially nitrogen‐containing carbon materials have attracted intensive attention as research hotspots in materials science over these years owing to their unique electronic, mechanical and catalytic properties, generating a crowd of exciting performances in related realms.6, 7, 8, 9 Similar to all the energy storage devices, supercapacitors are assembled by the anode and cathode sandwiching a separator in‐between, wherein overall devices work in aqueous/non‐aqueous electrolytes. Classically, two categories are classified for supercapacitors, symmetric SCs with the same or similar electrodes at both ends and asymmetric ones with the configuration of different electrodes. Generally, energy density (E, W h kg−1) and power density (P, W kg−1) are both critically significant parameters for the evaluation of SCs regarding energy storage performance, which represent the storage capacity and power delivery, respectively.42, 43 Apparently, as the energy storage mechanism of SCs relies on merely surface charge storage, low energy density is always the greatest problem. In this new era of energy storage, nitrogen‐rich porous carbon materials offer great opportunities to promote SCs towards battery‐level energy density, demonstrating many breakthroughs in broad applications ranging from asymmetric to symmetric and hybrid SCs. With favorably tuned physicochemical properties through nitrogen functionalities, NPC materials always manifest simultaneous enhancement in electrical conductivity, porous structures as well as electrochemically‐active sites compared with common carbon materials, which further results in huge advance on capacitive performance by introducing redox reactions at/near carbon surface. Further, NPCs are equipped with versatile properties as both cathodes and anodes, showing intriguing flexibility and superb performance in a wide electrochemical potential regions for constructing high voltage devices. Notably, according to the role of different N‐species playing in electrolytes, tailored dopants regarding doping types and contents are required to balance conductivity‐porosity‐active sites trade‐off and attain optimal performance with the matched operation environment. Specifically, NPCs exert different functions as a promoter in SC devices. As for asymmetric configurations, NPCs with high capacitance tactfully reduce the cathode‐anode mass imbalance and further improve energy‐power delivery. It also revives the symmetric SCs by exploiting the redox reactions, and scaling the energy to a new height. Building at both sides of hybrid SCs, NPCs establish a bridge to solve capacity and kinetics mismatches between battery‐type and the capacitor‐type electrodes, leading to a more consistent energy storage performance than commercial carbons. Source: http://doi.org/10.1002/advs.201600408