nanomaterials Review Carbon-Based Quantum Dots for Supercapacitors: Recent Advances and Future Challenges Fitri Aulia Permatasari 1 , Muhammad Alief Irham 1,2 , Satria Zulkarnaen Bisri 2,* and Ferry Iskandar 1,3,* 1 Department of Physics, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Jalan Ganesha 10, Bandung 40132, Indonesia; [email protected] (F.A.P.); aliefi[email protected] (M.A.I.) 2 RIKEN Center of Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3 Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Jalan Ganesha 10, Bandung 40132, Indonesia * Correspondence: [email protected] (S.Z.B.); ferry@fi.itb.ac.id (F.I.); Tel.: +81-48-462-1111 (S.Z.B.); +62-22-251-5032 (F.I.) Abstract: Carbon-based Quantum dots (C-QDs) are carbon-based materials that experience the quantum confinement effect, which results in superior optoelectronic properties. In recent years, C-QDs have attracted attention significantly and have shown great application potential as a high- performance supercapacitor device. C-QDs (either as a bare electrode or composite) give a new way to boost supercapacitor performances in higher specific capacitance, high energy density, and good durability. This review comprehensively summarizes the up-to-date progress in C-QD applications either in a bare condition or as a composite with other materials for supercapacitors. The current state of the three distinct C-QD families used for supercapacitors including carbon quantum dots, carbon dots, and graphene quantum dots is highlighted. Two main properties of C-QDs (structural and electrical properties) are presented and analyzed, with a focus on the contribution to supercapacitor performances. Finally, we discuss and outline the remaining major challenges and future perspectives for this growing field with the hope of stimulating further research progress. Citation: Permatasari, F.A.; Irham, M.A.; Bisri, S.Z.; Iskandar, F. Keywords: carbon; quantum dots; quantum capacitance; supercapacitor Carbon-Based Quantum Dots for Supercapacitors: Recent Advances and Future Challenges. Nanomaterials 2021, 11, 91. https://doi.org/ 1. Introduction 10.3390/nano11010091 Fossil fuel shortages and environmental concerns regarding its uses for energy genera- tion are among the most severe challenges in achieving sustainable development. Therefore, Received: 4 November 2020 pursuits for alternative energy sources and energy storage devices are essential. Several Accepted: 28 December 2020 critical parameters need to be achieved by those alternative energy storage devices, includ- Published: 3 January 2021 ing high energy density, high power density, long lifecycle, environmental safety, and low Publisher’s Note: MDPI stays neu- cost [1–3]. To meet these critical parameters, the development of electrochemical-based tral with regard to jurisdictional clai- energy storage devices (i.e., batteries and supercapacitors) still possesses fundamental ms in published maps and institutio- research challenges, even though the earliest electrochemical supercapacitor patent was nal affiliations. filed in 1957 by General Electric [4]. The Ragone plot (Figure1), utilized as a figure-of-merit of energy storage devices, shows that the supercapacitor performance lies between those of conventional capacitors and batteries. A Supercapacitor (SC) provides a higher en- ergy density and higher power density than batteries (>10 kW kg−1) and longer lifecycles Copyright: © 2021 by the authors. Li- (>105 cycles) than a traditional capacitor [5]. Therefore, supercapacitors hold great prospect censee MDPI, Basel, Switzerland. as ubiquitous energy storage devices for future electronic systems which requires high This article is an open access article power and energy density yet durability and fast charge/discharge (in seconds). distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Nanomaterials 2021, 11, 91. https://doi.org/10.3390/nano11010091 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021,, 1111,, 91x 2 of 3534 Figure 1. RagoneRagone plot plot for for several several types types of of electrochemical electrochemical energy energy storage storage devices: devices: adapted adapted from from Reference [[5].5]. Copyright Amer Americanican Chemical Society, 2004.2004. Hybrid electric electric vehicles vehicles (EVs), (EVs), memory memory backup backup systems, systems, and and portable portable electronic electronic de- vicesdevices are are demanding demanding electronic electronic systems systems [6–8]. [6– In8]. those In those systems, systems, SCs can SCs be can integrated be integrated with withprimary primary high-energy high-energy batteries batteries or fuel or cells fuel to cells provide to provide temporary temporary energy storage energy storagedevices withdevices a high with power a high capability. power capability. Also, SCs Also, could SCs overcome could overcomethe battery the problem battery in problem high cycle in ratehigh operations. cycle rate operations. Nowadays, Nowadays, batteries are batteries the primary are the energy primary storage energy for storageEVs. Neverthe- for EVs. less,Nevertheless, in high cycle in highrate operations, cycle rate operations, batteries significantly batteries significantly heat up, which heat raises up, which many safety raises concerns.many safety On concerns.the other hand, On the high other cycle hand, rate highoperations cycle ratealso operationssignificantly also decrease significantly the ca- pacitydecrease of batteries. the capacity These of batteries.situations Theseare in cont situationsrast with are SCs incontrast that usually with have SCs significantly that usually betterhave significantly cycling performance better cycling and performancedurability for and high durability rates of operations. for high rates Although of operations. it has Althoughmany advantages it has many and advantages a larger power and adensity, larger power an SC density,still has ana lower SC still energy has a lowerdensity energy than adensity battery. than It is a battery.the real Itmajor is the challenge real major for challenge the development for the development and applications and applications of SCs. We of shouldSCs. We overcome should overcome this problem this problem by employin by employingg composite, composite, doping, doping, or other or other surface surface en- hancementsenhancements into into materials. materials. In general, anan SCSC comprisescomprises two two electrodes electrodes that that are are immersed immersed in in an an electrolyte. electrolyte. It may It may be beelectrically electrically isolated isolated by aby separator, a separator, which which can alsocan playalso anplay essential an essential role in role SC in performance. SC perfor- mance.Based on Based the storageon the storage mechanism, mechanism, SCs are SCs classified are classified into three into types, three i.e., types, the i.e., electric-double- the electric- double-layerlayer capacitors capacitors (EDLCs), (EDLCs), pseudocapacitors, pseudocapa andcitors, hybrid and supercapacitors hybrid supercapacitors [9,10]. [9,10]. EDLC is is the the conventional conventional supercapacitor supercapacitor in which in which capacitance capacitance arises arises solely solely from fromelec- trostaticelectrostatic charge charge accumulation accumulation between between ions ionsat the at electrode/electrolyte the electrode/electrolyte interface interface [11]. This [11]. mechanismThis mechanism allows allows infinite infinite time time charge/discharge charge/discharge and andis thus is thus stable stable in the in theviewpoint viewpoint of lifecycleof lifecycle stability. stability. However, However, EDLCs EDLCs suffer suffer lower lower energy energy density density than pseudocapacitors owing to limited specific surface area and the compatible electrode/electrolyte [12,13]. The owing to limited specific surface area and the compatible electrode/electrolyte [12,13]. The most common electrode materials used in EDLC are porous carbon and its derivatives most common electrode materials used in EDLC are porous carbon and its derivatives with large specific surface areas [14]. On the other hand, a pseudocapacitor relies on the re- with large specific surface areas [14]. On the other hand, a pseudocapacitor relies on the versible faradaic redox originating from the electroactive phases at the electrode/electrolyte reversible faradaic redox originating from the electroactive phases at the electrode/elec- interface. Even though it possesses high specific capacitance, a pseudocapacitor suffers trolyte interface. Even though it possesses high specific capacitance, a pseudocapacitor challenging cycling stability because of its reversible faradaic redox process in the potential suffers challenging cycling stability because of its reversible faradaic redox process in the window [8]. Metal oxides, notably transition metal oxides, and conductive polymers are potential window [8]. Metal oxides, notably transition metal oxides, and conductive pol- among the most common materials used in pseudocapacitors [15,16]. Another type is the ymers are among the most common materials used in pseudocapacitors [15,16]. Another combination of both the EDLC electrode and pseudocapacitive electrode in a single device type is the combination of both the EDLC electrode and pseudocapacitive electrode in a with an intermediate SC performance.