nanomaterials Review Nanoarchitectonics of Nanoporous Carbon Materials in Supercapacitors Applications Rekha Goswami Shrestha 1,*, Subrata Maji 1, Lok Kumar Shrestha 1 and Katsuhiko Ariga 1,2,* 1 International Center for Materials Nanoarchitectonics (WPI MANA), National Institute for Materials − Science (NIMS), 1 1 Namiki, Tsukuba 305 0044, Japan; [email protected] (S.M.); − − [email protected] (L.K.S.) 2 Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277 8561, Japan − * Correspondence: [email protected] (R.G.S.); [email protected] (K.A.); Tel.: +81-029-860-4597 (K.A.) Received: 5 March 2020; Accepted: 26 March 2020; Published: 29 March 2020 Abstract: High surface area and large pore volume carbon materials having hierarchical nanoporous structure are required in high performance supercapacitors. Such nanoporous carbon materials can be fabricated from organic precursors with high carbon content, such as synthetic biomass or agricultural wastes containing cellulose, hemicellulose, and lignin. Using recently developed unique concept of materials nanoarchitectonics, high performance porous carbons with controllable surface area, pore size distribution, and hierarchy in nanoporous structure can be fabricated. In this review, we will overview the recent trends and advancements on the synthetic methods for the production of hierarchical porous carbons with one- to three-dimensional network structure with superior performance in supercapacitors applications. We highlight the promising scope of accessing nanoporous graphitic carbon materials from: (i) direct conversion of single crystalline self-assembled fullerene nanomaterials and metal organic frameworks, (ii) hard- and soft-templating routes, and (iii) the direct carbonization and/or activation of biomass or agricultural wastes as non-templating routes. We discuss the appealing points of the different synthetic carbon sources and natural precursor raw materials derived nanoporous carbon materials in supercapacitors applications. − Keywords: carbon; energy storage; fullerene; nanoarchitectonics; nanoporous 1. Introduction Various social demands including environmental problems [1–5], energy managements [6–9], device development [10–14], and biomedical applications [15–19] have been handled with various scientific and technological developments. This includes organic synthesis [20–24], polymer synthesis [25–27], energy production [28–31], energy storage [32–34], sensing [35–39], photocatalysis [40–43], organic synthetic catalyst [44–47], self assembly [48–51], and biological − investigation [52–54] in addition to analytical approach [55–57] and theoretical consideration [58–61]. In order to develop highly efficient systems, synthesis and organization of nanomaterials becomes much more important [62–65]. Especially, an emerging concept, nanoarchitectonics, has crucial roles for these challenges to fabricate functional materials from nano-units [66,67]. This concept was initially developed by Masakazu Aono [68,69] to combine nanotechnology with the other research fields such as supramolecular chemistry and nanofabrications [70,71]. This concept has been applied to many research fields such as materials synthesis [72–74], structure organization [75,76], catalysts [77], sensing [78,79], device [80,81], energy [82,83], environment [84], and biological applications [85,86]. In this review, recent developments on important subject, nanoarchitectonics of nanoporous carbon materials for Nanomaterials 2020, 10, 639; doi:10.3390/nano10040639 www.mdpi.com/journal/nanomaterials Nanomaterials 20202020,, 1010,, 639x FOR PEER REVIEW 2 of 2726 materials for supercapacitors applications, are overviewed. Different synthetic methods using supercapacitorsdifferent starting applications, materials and are general overviewed. design and Diff basicerent principles synthetic for methods the production using diff oferent nanoporous starting materialscarbon materials and general for supercapacitors design and basic applications principles forthrough the production nanoarchitectonics of nanoporous concept carbon (Scheme materials 1) are forillustrated. supercapacitors applications through nanoarchitectonics concept (Scheme1) are illustrated. Scheme 1.1. Schematic illustration of the nanoarchitectonicsnanoarchitectonics concept in whichwhich nanoporousnanoporous carboncarbon materials are fabricated by didifferentfferent synthetic approachesapproaches fromfrom variousvarious syntheticsynthetic and natural biomass as carbon sourcessources forfor supercapacitorssupercapacitors applications.applications. Supercapacitors (SCs) are thethe currentcurrent state-of-the-artstate-of-the-art electrochemicalelectrochemical energyenergy storagestorage systemssystems thatthat havehave beenbeen extensivelyextensively usedused inin hybridhybrid electricelectric vehicles,vehicles, portableportable electronicelectronic devices,devices, memorymemory backup systems, and alsoalso inin industrialindustrial energyenergy andand powerpower managements.managements. ComparedCompared toto lithium-ionlithium-ion batteries, SCs are more convenient devicesdevices for the energy storage because of their high power density 1 (410(410 kW kg kg−1),), extraordinary extraordinary cycling cycling stability stability (>10 (> 10000 000 cycles), cycles), low-cost, low-cost, low lowinternal internal resistance, resistance, fast fast charging discharging dynamics, and high rate capability [87]. Based on the energy storage charging−discharging− dynamics, and high rate capability [87]. Based on the energy storage mechanism, there are twotwo distinctdistinct typestypes ofof supercapacitorssupercapacitors calledcalled electricalelectrical double-layerdouble-layer capacitorscapacitors (EDLCs)(EDLCs) andand pseudocapacitors pseudocapacitors (PCs). (PCs). In EDLCs,In EDLC thes, energy the energy storage storage mechanism mechanism involves theinvolves formation the offormation electrical of double electrical layer double charges layer at thecharges electrode at the/electrolyte electrode/electrolyte interface due interface to electrostatic due to electrostatic adsorption ofadsorption electrolyte of ions.electrolyte As a result, ions. As the a EDLCs result, allowthe EDLCs fast charging–discharging allow fast charging–discharging rate. On the rate. other On hand, the inother PCs hand, reversible in PCs Faradic reversible reactions Faradic occur reactions at the electrodeoccur at the surface electrode followed surface by thefollowed charge by transfer the charge and, thus,transfer they and, off erthus, higher they specificoffer higher capacitance specific and capacitance energy density and energy compared density to compared those of EDLCs to those [88 of]. ManyEDLCs pseudocapacitive [88]. Many pseudocapacitive materials including materials transition including metals, transition their oxides,metals, rate-earththeir oxides, metal rate-earth oxides, andmetal conducting oxides, and polymers conducting have polymers been explored have been as ex electrodeplored as materials electrode of materials the PCs of [89 the]. ForPCs example, [89]. For manganeseexample, manganese oxides (MnO oxides2 and (MnO Mn2 2andO3), Mn cobalt2O3), oxidescobalt oxides (CoO and(CoO Co and3O4 Co), nickel3O4), nickel oxides oxides (NiO (NiO and Niand2O Ni3),2O cerium3), cerium oxides oxides (CeO (CeO2), vanadium2), vanadium pentoxide pentoxide (V (V2O25O),5), and and iron iron oxide oxide (Fe (Fe2O2O33)) areare commonlycommonly used electrodeelectrode materialsmaterials for PCsPCs [[90].90]. Recently, hybridhybrid supercapacitorssupercapacitors thatthat essentiallyessentially comprisecomprise ofof EDLC andand PCPC materialsmaterials areare inin commoncommon practicepractice forfor betterbetter energyenergy storage.storage. HybridHybrid supercapacitivesupercapacitive materials generally include binary or ternary composites of metal oxides and also carbon composites withwith metalmetal oxidesoxides [[91].91]. Despite thethe aforementioned aforementioned several several advantages, advantages, SCs SCs suff ersuffer from from the delivery the delivery of low of energy low densityenergy 1 1 (1 10 W h kg ) compared−1 to the lithium-ion batteries (160 W h kg )[92−1 ]. Therefore, it is yet a density− (1−10 W− h kg ) compared to the lithium-ion batteries (160 W −h kg ) [92]. Therefore, it is yet keya key challenge challenge to to fabricate fabricate supercapacitive supercapacitive materials materials that that can can deliver deliver highhigh energyenergy densitydensity sustainingsustaining other intrinsic properties of high power density, long cycling stability and fast charging discharging other intrinsic properties of high power density, long cycling stability and fast charging−−discharging dynamics. Special and extensive eeffortffortss have been givengiven to solve this issue by the fabrication of novel supercapacitor electrode materials, which can achieve high specific capacitances ‘Cs’, and also can be Nanomaterials 2020, 10, 639 3 of 27 supercapacitor electrode materials, which can achieve high specific capacitances ‘Cs’, and also can be operational over a wide potential window ‘V’. This is because the energy storage is proportional to 2 CsV [93]. Therefore, the design of novel functional nanoporous carbon materials with high specific capacitances is expected to facilitate the assembly of effective energy storage systems that can deliver high energy densities. Another strategy to achieve high energy density is the assembly