Recent Advances on Graphene Quantum Dots for Electrochemical Energy Storage Devices

Noura Zahir,Pierre Magri ,Wen Luo,Jean Jacques Gaumet*,and Philippe Pierrat

Graphene quantum dots(GQDs)which are nanofragments of graphene with an average size between 2 and 50 nm have attracted much attention due to their outstanding properties such as high conductivity,high surface area,and good solubility in various solvents.GQDs combine the quantum confinement and edges effects and the properties of graphene.Therefore,GQDs offers a broad range of applications in various fields(medicine,energy conversion,and energy storage devices).This review will present the recent research based on the introduction of GQDs in batteries,supercapacitors,and microsupercapacitors as electrodes materials or mixed with an active material as an auxiliary agent.Tables,discussed on selected examples,summarize the electrochemical performances and finally,challenges and perspectives are recalled for the subsequent optimization strategy of electrode materials.This review is expected to appeal a broad interest on functional GQDs materials and promote the further development of high-performance energy storage device.

Keywords

energy storage,graphene quantum dots,lithium ion batteries,sodium ion batteries,supercapacitor

1.Introduction

Graphene as a material is not only the thinnest ever but also the strongest.It corresponds to an infinite 2D monolayer of hexagonal sp2bonded carbon network,which shows interesting properties of zero band gap due to the delocalized electrons.This gives it a semi-metallic behavior and,consequently,electrons move through graphene with virtually no resistance leading to electrical conductivity that is higher than copper.While displaying other outstanding features such as high strength,lightness,and high thermal conductivity,graphene fabrication nevertheless still constitutes the key technical hurdle to overcome in order to permit graphene to reach industry.Indeed,it is still a major challenge to set up stable and homogeneous dispersions of graphene without agglomeration.The recent discovery of graphene quantum dots(GQDs),[1]a new member of the allotropic carbon family(diamond,graphite,fullerene,nanotube,graphene etc.),and the rapid advances in their synthetic preparation do offer a unique opportunity for investigating their applications.These carbonaceous quantum dots combine several favorable attributes of traditional semiconductor-based quantum dots(namely nanoscale size,size-and wavelengthdependent luminescence emission,resistance to photobleaching,ease of bioconjugation)without incurring the burden of intrinsic toxicity or elemental scarcity,and without the need for stringent,costly,or inefficient preparation steps.Moreover,GQDs are highly soluble graphene substitutes,which is a crucial property for an easy transfer to industrial development.

GQDs are functionalized nanofragments of graphene with lateral size generally below 10 nm.Their anisotropic morphology originates from lateral dimension larger than their height.GQDs systematically possess graphitic lattices within their structures,as evidenced by high-resolution transmission electron microscopy HRTEM(Figure 1).Their height is usually ranging from 0.4 to 4 nm as evidenced by statistical AFM analysis,which corresponds to few GQDs(from 1 to 10)stacked on the top of each other.Chemists have developed GQDs synthetic approaches(either bottom-up or top-down method),in most cases in one step,which end in the preparation of a mixture of GQDs having statistical size and chemistry distribution.Consequently,macroscopic properties arise from the whole GQDs population.

Figure 1.HRTEM images(and related FFT image in the inset)of in-house fabricated GQDs(unpublished results).

GQDs preparation methods could be classified in two distinct categories and thus rely either on top-down or bottom-up synthetic approaches.We invite readers interested in further details to consult selected reviews which give an overview of the synthetic possibilities toward the access of GQDs.[2-7]Brie fly,the top-down synthesis deals with chemical breakdown of large carbon-based materials(carbon fibers,[8]graphene oxide GO,[9]coal,[10]fullerenes,[11]graphite,[12]．．．)into small fragments with concentrated acids.GO is typically the ideal starting material according to the presence of many oxygen-containing functional groups,which facilitate the chemical cleavage toward nanosized GQDs.Nevertheless,GO does not naturally occur and has thus to be prepared from various materials such as coal or anthracite by the complex Hummer’s chemical approach.In that context,the use of graphite as natural source has been evaluated as GO substitute with lesser success to date according to lower synthetic yields.Top-down syntheses are reported to be possible through hydrothermal or solvothermal cutting,[13]microwave-assisted exfoliation,[14]electrochemical methods,[15]and oxidation.[16]On the other hand,one-step bottom-up synthesis deals with the carbonization of organic precursors(citric acid,[17]glucose,[18]glutamic acid,[19]hexa-peri-hexabenzocoronene,[20]．．．)by microwave-assisted pyrolysis,solvothermal heating or under pulsed laser irradiation.However,bottom-up approaches generally suffer from lower yields associated with purification hurdles to remove unreacted small organic materials.

In the particular context of global warming and increasing energy demand,it is very appealing to develop efficient and stable energy storage technologies in order to respond to the intermittency of alternative renewable energy sources(wind,sunlight,tides．．．)under development.In general,electrochemical energy storage(EES)systems,beyond their intrinsic performances,could display some limitations such as capacity fading and increased charge transfer resistance during cycles.Rapidly,next to the first paper reporting their first synthesis,[1]GQDs have been studied as an advanced material for electrodes in EES systems such as batteries and supercapacitors.Herein,we collected the literature results in different tables,highlighting the main appropriate metrics that give a real picture of the performance of related EES systems.For each system,both reaction type(top-down/bottom-up)and precursors are systematically given in all tables.

2.Batteries

Batteries represent one of the energy storage devices that stored the energy in form of chemical energy and converted it to electricity via redox reactions or intercalation processes as observed generally in lithium ion batteries(LIBs)and in sodium ion batteries(SIBs)(Figure 2a,b).They consist of two electrodes separated by an electrolyte.[21]There is a large range of different battery types such as leadacid,Nickel-Cadmium,and Nickel-Metal-hydride(NiMH),lithium ion,and lithium metal batteries.They display different properties in terms of volumetric and gravimetric energy densities(Figure 2c).

Figure 2.Schematic diagrams of batteries working principle:a)LIBs.[22]Copyright 2018,The Royal Society of Chemistry.b)SIBs.[23]open access 2019,MDPI.c)Parameters value range for common batteries(energy density vs gravimetric energy density).[24]Copyright 2012,Elsevier.

Among all of them,LIBs are already in our daily life as they are powering our portable devices(phones,tablets,laptops．．．)and electric vehicles.They display many advantages such as high cycling stability,high capacity,and high operating voltage.[25]Recently,the utilization of GQDs as anode material or mixed with an active material as an auxiliary agent in secondary rechargeable batteries has attracted much attention.[26]Table 1 summarized the LIBs performances using GQDs.For example,Yin et al.[27]designed a multilayered bimetal oxides NiO@Co3O4modified with GQDs and they used it as an anode material for LIBs.The NiO@Co3O4@GQDs exhibits excellent cycling property and a large reversible capacity of 1158 mA h g-1that remain stable even after 250 cycles at 0.1 A g-1as shown in Figure 3a.This result should be compared with the device made of pure NiO@Co3O4as an anode material,for which,the charge capacity decreased from 1093 to 521 mA h g-1after 250 cycles.

Figure 3.a)Preparation route and structural illustration of the multilayer NiO@Co3O4@GQDs microspheres and cycling capabilities of NiO@Co3O4and NiO@Co3O4@GQDs.[27]Copyright 2019,The Royal Society of Chemistry.b)Schematic illustration for the preparation of Co3O4@CuO@GQDs and cyclic performance of Co3O4@CuO and Co3O4@CuO@GQDs.[28]Copyright 2019,Elsevier.

Table 1.Comparison of LIBs performances using GQDs.

Wu et al.[28]prepared a GQDs modified yolk shell Co3O4@CuO microspheres(Co3O4@CuO@GQDs).The Co3O4@CuO@GQDs as an anode material displays an initial discharge/charge capacities of 1352/816 mA h g-1at 0.1 A g-1(Figure 3b).Furthermore,it exhibits good cycling performance,no capacity fading during charging/discharging process and a reversible charging specific capacity of 1054 mA h g-1was remained after 200 cycles.By comparison,for Co3O4@CuO without GQDs as an anode material,a capacity fading was observed at the 15th cycle and only 414 mA h g-1of capacity was measured after 200 cycles at 0.1 A g-1.

GQDs doped with nitrogen or boron atoms were also evaluated in lithium battery storage.In that context,Vijaya et al.prepared undoped GQDs,boron GQDs(B-GQDs),and nitrogen GQDs(N-GQDs)and used them as pure anode materials for LIBs.[29]Figure 4 illustrates the preparation methods(Figure 4a),TEM images of B-GQDs,N-GQDs,and GQDs(Figure 4b)and the electrochemical performances as well.B-GQDs and N-GQDs exhibit higher specific capacity compared to undoped GQDs, 1896 mA h g-1, 1500 mA h g-1, and 697 mA h g-1at 50 mA g-1,respectively(Figure 4c).Moreover,as depicted in Figure 4d,B-GQDs and N-GQDs retain 95.7% and 90% from the initial capacity,respectively,while undoped GQDs maintain only 86% of their initial capacity.It is postulated that the addition of heteroatoms enhances the Li ions storage capacity.A consistent explanation is the doping of graphene backbone with boron and nitrogen creates electron-deficient and electron-rich sites in carbon lattices which permits to improve Li ions adsorption and storage capacity.

Figure 4.a)Preparation route of B-GQDs,N-GQDs,and GQDs.b)TEM images of i)GQDs,ii)B-GQDs,and iii)N-GQDs.c)Galvanostatic charge-discharge profiles of i)B-GQD,ii)N-GQDs,and iii)GQDs for the first cycles at 50 mA g-1for LIB anodes.d)Cyclic stability for B-GQD,N-GQD,and GQD at 200 mA g-1.[29]Copyright 2020,Elsevier.

To conclude,the addition/coupling of GQDs has systematically a positive impact on lithium storage capacity as it offers more catalytically active site and increases the specific surface area.It enhances the contact area between the electrolyte and the active layer and increases the electrochemical conductivity and thus improves the cycling stability of the electrode as well as the specific capacity.[28,35,36]

Although LIBs devices are widely used nowadays,many reports suggest a supply scarcity of lithium as the demand increases exponentially.Some estimates expected that the demand will reach to 900 thousand tons per year by 2025 which will be three times higher than 2018 and considering that Li is not a naturally abundant element its price will skyrocket.[42]In that context,sodium ion batteries(SIBs)has attracted more and more attention as an alternative to LIBs due to sodium higher natural abundance and low cost.In terms of cathode materials during the discharge or anode materials during the charge,SIBs have the same working principle than LIBs,that is based on intercalation of sodium ions[43,44](Figure 2a,b).Recently,various materials for SIBs have been studied.Concerning the cathode,a wide range of potential efficient materials are available.[45]In that context,Chao et al.used graphene foam supported VO2@GQDs as cathode material.The electrode displays a high discharge capacity of 306 mA h g-1at 100 mA g-1(1/3 C).[31]The most challenging task in SIBs batteries development relates on the finding of appropriate anode materials.In this review,we present the recent developments for anodes incorporating GQDs as dopant material,with the main properties depicted in Table 2.For example,Kong et al.fabricated a binder free anode via N-doped GQDs decorated Na2Ti3O7nanofibers arrays directly grown on flexible carbon textiles(Na2Ti3O7@N-GQDs/CTs).The results showed that the anode material delivers a high initial discharge capacity of～488 mA h g-1at 1 C,while the anode without GQDs(Na2Ti3O7/CTs)displays～300 mA h g-1at 1 C(Figure 5).Regarding the cycling stability performances,the Na2Ti3O7@N-GQDs/CT retained 92.5% of its initial reversible capacity after 1000 cycles,which is much higher than the capacity retention of Na2Ti3O7/CTs(68% of its initial capacity after 1000 cycles).[44]Thus,the introduction of GQDs enhances the electrochemical performance of SIBs and,as outlined in the cases of LIBs,is particularly beneficial on the cycling stability of SIBs devices.

Table 2.Electrochemical performances of SIBs using GQDs.

Figure 5.a)TEM image of N-GQDs with the size distribution in the inset.b)HRTEM images of NTO@N-GQD NFAs with the FFT pattern in inset.c)schematic illustration of the prepared material Na2Ti3O7@N-GQD NFAs on the flexible carbon textile.d)SEM image of Na2Ti3O7@N-GQD NFAs.e)longterm cycling performances of Na2Ti3O7@N-GQDs/CTs-20 and Na2Ti3O7/CT electrodes at different current densities of 0.5 C and 4 C,respectively.f)galvanostatic charge/discharge profiles during the first five cycles of the Na2Ti3O7@N-GQDs/CTs-20 at a current density of 1 C.g)Schematic illustration of the as-fabricated full cell.[44]Copyright 2019,The Royal Society of Chemistry.

Kong et al.fabricated also a flexible full battery using the prepared material as an anode(Figure 5g).The Na2Ti3O7@N-GQDs//Na3V2(-PO4)3@N-doped carbon full cell provides high discharge capacity of 104.8 mA h g-1and a remarkable cycling performance with approximately 95.7% of the initial capacity which was retained after 50 cycles.Furthermore,the full battery displays a higher energy density 273.5 W h kg-1and at power density 5097.6 W kg-1.In terms of comparison,Chao et al.studied the electrochemical performances of the graphene foam supported VO2@GQDs electrode for both LIBs and SIBs.For LIBs,the specific capacity of the electrode was 421 mA h g-1at 100 A g-1,and the capacity retention is 94% after 1500 cycles at 18 A g-1.And for sodium storage performance,the specific capacity was 306 mA h g-1at 100 A g-1and 88% of the initial capacity was retained after 1500 cycles at 18 A g-1.[31]In summary,the SIBs storage technology seems to be a promising candidate for the replacement of LIBs but still need to be developed to have the same performances than LIBs.

3.Supercapacitors

Supercapacitors(SCs)also called as electrochemical capacitors or ultracapacitors have been attracting much attention due to their outstanding properties such as high-power density,fast charge and discharge and long cycle life.SCs consist of two electrodes separated by an ion permeable separator and an electrolyte(Figure 6a).There are two main types of SCs,electrical double layer capacitance(EDLC)and pseudocapacitors.In EDLC,the charge is stored in Helmholtz double layer at the electrode-electrolyte interface while,in pseudocapacitors,the charge is stored through redox reactions.[21,47]The major challenge to overcome for SCs is their low-energy density compared to batteries(Figure 6b)which limits their use in some applications.[48]As a result,tremendous research efforts have been devoted to develop and enhance SCs performances.This part of the review will concern more specifically designed supercapacitors using GQDs-based materials.

Figure 6.a)Schematic representation of a supercapacitor and b)Ragone plot showing the specific power vs specific energy of various energy storage devices.[21]Copyright 2019,The Royal Society of Chemistry.

GQDs are promising and attractive materials for their uses in supercapacitors due to their excellent electrical properties,high surface area,abundant active sites,high conductivity,and their high solubility in various solvents.[49,50]Therefore,much research has been developed on novel capacitors including all-solid-state supercapacitors and microsupercapacitors(MSCs)using GQDs materials.The GQDs-based supercapacitors can deliver an energy density close to that of batteries.[51]Table 3 summarizes electrochemical performances parameters of the assembled SCs such as specific capacitance,stability,energy,and power density found in recent literature.GQDs have been used as an electrode material for SCs,MSCs and even as an electrolyte.Zhang et al.[52]developed SCs employing GQD film as solid-state electrolyte with a specific capacitance of 6 F g-1at a current density of 1 A g-1.Very interestingly,the specific capacitance for GQDs film neutralized with KOH was 45 F g-1at 1 A g-1.The full ionization of the weak acidic oxygenbearing functional groups may explain this improvement allowing a high enhancement of the ionic conductivity and ion-donating ability of GQDs.In general,SCs and MSCs having GQDs as electrode material correspond to electrochemical double layer type.Xu et al.developed electrodes materials-based N-doped reduced graphene oxide(NrGO)combined with GQD.The obtained NrGO/GQD exhibits a high specific capacitance of 344 F g-1at current density 0.25 A g-1with a good 82% cycling stability of its capacitance which was retained after 3000 cycles.The elaborated electrodes display remarkably improved electrochemical performance compared to NrGO without GQD which exhibit a specific capacitance of 254 F g-1at 0.25 A g-1.[53]Moreover,Liu et al.[54]designed symmetric micro-supercapacitor using GQDs//GQDs as electrodes with 534.7 μF cm-2at current density 15 μA cm-2,and 98% of the initial capacitance retained after 5000 cycles.The electrochemical test of the assembled SC reveals that the introduction of GQDs enhances the electrochemical performance of SCs and leads to high rate capability and excellent cycling stability.Qing et al.[55]further developed a new strategy to enhance the electrochemical performances of activated carbon by embedding crystallized GQDs.The assembled symmetric SC shows high capacitance of 388 F g-1at a current density of 1 A g-1and achieves excellent cycle stability with no obvious capacitance fading after 10 000 cycles.Moreover,the symmetric SCs show high energy density of 13.47 Wh kg-1at a power density of 125 W kg-1.[55]

Table 3.Comparison of electrochemical performances of the assembled SCs.

Table 3.Continued

Table 3.Continued

On the other hand,SCs and MSCs designed using GQDs doped with nitrogen or sulfur and asymmetric SCs belong to pseudocapacitors.Yin et al.[27]prepared multilayer NiO@Co3O4hollow spheres modified with GQDs(NiO@Co3O4@GQDs)and investigated their electrochemical performance.As a working electrode in a conventional three-electrode cell,NiO@Co3O4@GQDs shows an impressive capacitance of 1361 F g-1at 1 A g-1,and a capacity retention rate of～76.4% after 3000 cycles at 10 A g-1.This is much higher than NiO@Co3O4,which maintains only 54.7% of its initial capacitance(Table 4).Moreover,they fabricated all-solid-state asymmetric SC,NiO@Co3O4@GQDs//Activated Carbon(AC)using polyvinyl alcohol(PVA/KOH)as an electrolyte.The asymmetric supercapacitor(ASC)device exhibits a specific capacitance of 123 F g-1at 1 A g-1and an energy density of 38.44 Wh kg-1at a power density of 750 W kg-1and a high cycle stability(84% retention of its initial capacitance after 10 000 cycles).The superior performances can be ascribed to the bimetallic oxides(NiO and Co3O4)together with the introduction of carboxyl functionalized GQDs which provide more electrochemical active sites,facilitate ion accessibility and enhance the electronic conductivity.Qiu et al.[56]fabricated an ASC,based on a flower like ball histidine functionalized GQDs/Ni-Co layered double hydroxides(LDH)(His-GQDs/LDH)//AC(Figure 7).The specific capacitance of the device is 138 F g-1at 1 A g-1.The assembled ASC delivers a high energy density of48.89 Wh kg-1at a power density of 0.80 kW kg-1and shows good cycling performances(93% of the initial capacitance was retained after 6000 cycles).The ASC exhibits good electrochemical properties,probably due to the special structure of His-GQDs/LDH,which can facilitate electrolyte ions transfer according to its high surface area.This is also due to the introduction of His-GQDs,which improves the electrical conductivity of the electrode,and prevents the volume expansion of nickel and cobalt ions during charge and discharge processes.

Table 4.Electrochemical performance of typical GQD-based electrodes for supercapacitors.

Figure 7.a)HRTEM.b)TEM images of His-GQD/LDH.c)performances supercapacitors:GCD.d)cycling property.[56]Copyright 2020,Elsevier.

Interestingly,some authors[88-91,99,100]use the term carbon dots(CDs)or carbon nanodots(CNDs)to the prepared materials and according to high-resolution TEM results,these materials have a similar crystallinity to GQDs.For example,Wei et al.[90]have synthesized CDs and the high-resolution TEM images(Figure 8)reveals the CDs with a lattice spacing of 0.34 nm which corresponds to the(002)plane of graphite.

Figure 8.a)TEM and b)HRTEM images of CDs.[90]Copyright 2016,Wiley.

4.Conclusions and Perspectives

We have reviewed the interesting potentiality of combining GQDs in electrochemical energy storage devices focusing on batteries and supercapacitors.From all papers described herein,the addition of GQDs brings nearly systematically enhancement of lifetime and electrochemical performances.For instance,the specific capacity of lithium ion batteries has an increase of a 1.4 factor.Furthermore,the cycling stability after addition of GQDs has an increase≈15% in lithium ion batteries.Among the large number of articles dedicated to the use of GQDs-based electrodes for EES systems,it appears that GQDs improve the electrical conductivity and thus facilitate the charge transfer within the composite electrodes evaluated.It is also postulated that incorporating GQDs within the electrode matrix is acting as a binder,which sustain the overall electrode microstructure leading to enhanced cycling stability.Moreover,from the published results,it is clear that the interaction between the electrolyte,with the electrode material,and the electron charge transfer are enhanced by the addition of GQDs within the active matrix.This could be explained by a higher specific area or by the enhanced porosity of the electrode.

GQDs should be considered,as a material with inner and edge functionalized groups with heteroatoms,especially oxygen.The edges are suitable for improving solubility in various solvents and are useful for reducing aggregation phenomena.The inner functional groups,which are mainly epoxy groups,are statistically distributed on the GQDs surface.Controlled reduction of those oxygen-bridges could be a road to further improve the positive impact of GQDs material in EES systems,owing to the formation of larger conductive sp2domains.

Finally,it is now time to develop maturely this approach and to extend it industrially in the near future.Low-cost industrial production is highly demanded for these applications to be largely developed.Moreover,one has to be able to produce GQDs by controlling as much as possible their size for managing their macroscopic features,as well as their inner and edge chemistry.

Acknowledgements

This work was supported by the L2CM,UMR 7053,a partner of the Jean Barriol Institute at the Université de Lorraine(France).

Conflict of Interest

The authors declare no conflict of interest.