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. This device is a so-called hybrid supercapacitor. single device with an intermediate SC performance. This device is a so-called hybrid su- Based on the abovementioned energy storage mechanisms, numerous approaches have percapacitor.been extensively Based investigated on the abovementioned to explore many energy different storage materials mechanisms, suitable for numerous SC elements. ap- proachesThose approaches have been include extensively nanostructuring investigated and to functionalizingexplore many different the known materials electrode suitable mate- forrials, SC designing elements. materials Those byapproaches considering includ the energetice nanostructuring mismatch ofand electrolytes/electrodes, functionalizing the and exploring newly emergent materials [17].
Nanomaterials 2021, 11, 91 3 of 35
Carbon-based Quantum Dots (C-QDs) emerged as materials that gained enormous scientific interest because of their unique properties [18,19]. This class of materials was firstly reported by Sun et al. in 2006 as carbon dots, owing to its nanometer-size diame- ter [20]. The carbon dots that were prepared from carbon nanotubes exhibited bright and colorful photoluminescence. These intriguing optical properties emerge from the quantum confinement effect, which typically occur in C-QDs with a diameter of around 10 nm [21]. However, some reports claimed that the quantum confinement effect was observed from carbon-based QDs with a larger diameter but less than 20 nm [22]. It should be noted that the occurrence of quantum confinement in this system is related to the significant increase in surface-to-volume ratio by reducing the C-QD diameter. The complex formation and structure of carbon-based QDs, which still largely remain unclear, make explanations of these material properties constrained and still debatable. To date, several excellent review articles on the application of carbon-based QDs in energy conversion and storage applications, including solar cells, battery, thermoelectric devices, and supercapacitor, are reported [23–25]. However, they focus on the synthe- sis and the recent progress of C-QDs in applications broadly. To best our knowledge, there is a distinct lack of reviews mainly focusing specifically on Carbon-based QDs in supercapacitor devices. The outstanding properties of C-QDs, i.e., their tunable electrical and optical prop- erties, high stability, and excellent biocompatibility, not only correspond to the quantum confinement occurrence but also are due to variations in their structure and surface pas- sivation [26,27]. There are various methods of producing C-QDs with different unique properties that lead this class of materials to become prospective for many applications such as optical devices, photocatalysts, sensors, and biomedicine [21,27,28]. Among the application avenues where the use of C-QDs might be distinguishable is its potential for components in electric and energy storage devices such as batteries and SCs. In this review, we provide comprehensive discussions regarding the progress and the prospect of C-QD supercapacitors. The first part of the review explains the basic principles of a supercapacitor, including the charge storing mechanism, and deliberations of the essential parameters that need to be satisfied for any material before application in SCs. Subsequently, the general classification of C-QDs, their properties, and their preparation are summarized systematically. Based on the structural properties, the terminology of C-QDs can be categorized into three different types, i.e., Carbon Dots (CDs), Carbon Quantum Dots (C-QDs), and Graphene Quantum Dots (GQDs). The electrical properties of C-QDs are elaborated thoroughly for use in SC applications. Then, we summarize the recent progress of C-QD supercapacitor developments, either using bare C-QDs or its derivative to construct excellent EDLC materials. We also sum up the use of C-QDs as performance- enhancing agents for pseudocapacitance materials like transition metal sulfide, transition metal oxide, and conductive polymer. Considering the excellent properties of C-QDs, we provide critical insight into the challenges and the future development and application of C-QDs in energy storage devices.
2. Fundamental of Supercapacitor 2.1. Electric Double Layer Capacitor Based on the electrical charge storage mechanism, supercapacitor devices are classified into three types: electric-double-layer capacitors (EDLCs), pseudocapacitors, and hybrid supercapacitors that combines both EDLCs and pseudocapacitors. Schematically, the mech- anism behind each supercapacitor is shown in Figure2a. EDLCs work via a phenomenon in which a charged object is placed inside a liquid. In the SC case, the liquid is an electrolyte, and the item is the electrode, which can also be a carbon-based material. The charges are stored electrostatically in the form of space charge accumulated at the electrolyte/electrode interface due to electric-double-layer formation. EDLCs can also be formed when a semi- conductor is interfaced with a liquid electrolyte [29]. There are several models to simulate interface phenomena between charged electrodes and electrolytes. Helmholtz model is Nanomaterials 2021, 11, x 4 of 34
Nanomaterials 2021, 11, 91 4 of 35 electrolyte/electrode interface due to electric-double-layer formation. EDLCs can also be formed when a semiconductor is interfaced with a liquid electrolyte [29]. There are several models to simulate interface phenomena between charged electrodes and electrolytes. theHelmholtz simplest model approximation is the simplest to model approximatio charge distributionn to model atcharge a metal–electrolyte distribution at interface.a metal– Theelectrolyte Helmholtz interface. layer modelThe Helmholtz is a well-known layer mo approximationdel is a well-known and is approximation the simplest model and is to explainthe simplest charge model distribution to explain on the charge electrode/electrolyte distribution on interfacethe electrode/electrolyte [30]. This theory interface explains that[30]. the This surface theory charge explains is neutralizedthat the surface by thechar oppositege is neutralized counterion by th situatede opposite at thecounterion surface. However,situated at some the surface. phenomena However, are hardlysome phen explainedomena withare hardly this theory explained alone with since this no theory rigid layersalone aresince formed no rigid on layers the surface, are formed as described on the surface, by this theory.as described by this theory.
FigureFigure 2. 2.(a ()a Illustration) Illustration of of the the Stern Stern model model for for electrolyte electrolyte ions ions on on a chargeda charged electrode electrode in (ini) ( ani) an electric-double-layer electric-double-layer capacitor capac- (EDLC)itor (EDLC) and (ii and) a pseudo(ii) a pseudo capacitance capacitance supercapacitor, supercapacitor, as well as aswell (b )as a ( schematicb) a schematic model model of ion of distribution ion distribution and mechanism.and mecha- nism. In 1913, Gouy-Chapman came up with a better model. This theoretical model assumes that theIn opposing1913, Gouy-Chapman counterions arecame not up rigidly with a attached better model. to the surfaceThis theoretical but tend model to diffuse as- intosumes the that liquid the phase. opposing The thicknesscounterions of theare resultingnot rigidly double attached layer to isthe affected surface by but the tend kinetic to energydiffuse of into the the counterions. liquid phase. This The model thickness is named of the the resulting diffuse double double layer layer model. is affected Boltzmann by the distributionkinetic energy is used of the to modelcounterions. the counterion This mode distributionl is named near the thediffuse charged double electrode layer model. [31,32]. Furthermore,Boltzmann distribution Stern proposed is used a better to model model the bycounterion modification distribution of the Gouy-Chapman near the charged diffuse elec- doubletrode [31,32]. layer model Furthermore, using finite Stern ion proposed sizes. In thisa better modified model diffuse by modification double layer of model,the Gouy- the firstChapman ion layer diffuse is not double precisely layer at mo thedel surface, using finite but it ion has sizes. a finite In this distance modified from diffuse the surface. dou- Theble surfacelayer model, may the also first adsorb ion laye somer is ions not precisely in the plane at the to formsurface, another but it has layer, a finite which distance is now commonlyfrom the surface. known The asthe surface Stern may layer also [33 adsorb]. The some ions are ions absorbed in the plane by the to electrodeform another and formlayer, layers which which is now can commonly be distinguished known intoas the Inner Stern Helmholtz layer [33]. Planes The ions (IHPs, are for absorbed specifically by adsorbedthe electrode ions) and and form Outer layers Helmholtz which can Planes be distinguished (OHPs, for nonspecifically into Inner Helmholtz adsorbed Planes ions) (Figure(IHPs, 2forb). specifically Overall, the adsorbed Stern model ions) combined and Outer the Helmholtz concepts Planes of the (OHPs, previous for two nonspecifi- models. Thiscally current adsorbed model ions) of the(Figure EDLC 2b). extends Overall, to manythe Stern fields model and provides combined better the concepts approximations of the toprevious the experimental two models. results This [ 10cu,rrent34,35 ].model of the EDLC extends to many fields and pro- videsTo better date, approximations many theories andto the simulations experimental have results been [10,34,35]. developed to understand better and toTo estimate date, many the capacitancetheories and valuessimulations of the ha electric-double-layerve been developed to capacitors understand (EDLCs). better Inand general, to estimate EDLCs the can capacitance be assumed values as a of parallel-plate the electric-double-layer capacitor so thatcapacitors its capacitance (EDLCs). can In begeneral, approximated EDLCs can using be theassumed following as a parallel-plate equation: capacitor so that its capacitance can be approximated using the following equation: ε0εr A C = (1) = d (1) wherewhereA Ais is the the surface surface areaarea ofof thethe electrode,electrode, andand dd definesdefines the effective thickness of of the the electricelectric doubledouble layerlayer (the(the DebyeDebye length).length). However, electrodes based on nanomaterials (e.g.,(e.g., nanowires, nanowires, nanoparticles,nanoparticles, nanotubes,nanotubes, quantum dots, etc.) etc.) have have a a high high volume-to- volume-to- surface-areasurface-area ratio ratio and and differentdifferent shapesshapes ofof surface area. Consequently, Consequently, another another formulation formulation isis needed needed toto rationalizerationalize thethe surfacesurface areaarea ofof shapesshapes andand thethe formedformed pores. For For example, example, Wang et al. modeled a Stern layer inside spherical pores, and the capacitance of the EDLC can be approximated as ε0εr Router + d CSSA,me = (2) d Router Nanomaterials 2021, 11, x 5 of 34
Nanomaterials 2021, 11, 91 5 of 35 Wang et al. modeled a Stern layer inside spherical pores, and the capacitance of the EDLC can be approximated as