Recent Progress in Carbon Dots‐Based Materials for Electrochemical Energy Storage Toward Environmental Sustainability

Carbon dots (CDs), an emerging category of carbon nanomaterials, have bright destiny in a vast diversity of engineering areas due to their great variety in design, arrangement, and characteristics. Their possible implementations have recently traversed from electrochemical energy storage (EES), fluorescent probing, and catalysis, particularly as materials into the critical elements of the electrochemical system. Herein, the current investigation based upon the preface of CDs in batteries, supercapacitors, hydrogen/oxygen evolution reaction, oxygen reduction reaction, electromagnetic interference shielding, and solar‐assisted energy generation used as electrode materials integrated with an active substance as an auxiliary mechanism is shown. Different aspects conferred upon selected illustrations outline the electrochemical activity, and eventually, current issues and future viewpoints are recollected toward the following optimization method of electrode substances. This review article is anticipated to demand broad attention within active CD materials and encourage the growth of high‐performance EES systems.

Carbon dots (CDs), an emerging category of carbon nanomaterials, have bright destiny in a vast diversity of engineering areas due to their great variety in design, arrangement, and characteristics. Their possible implementations have recently traversed from electrochemical energy storage (EES), fluorescent probing, and catalysis, particularly as materials into the critical elements of the electrochemical system. Herein, the current investigation based upon the preface of CDs in batteries, supercapacitors, hydrogen/oxygen evolution reaction, oxygen reduction reaction, electromagnetic interference shielding, and solar-assisted energy generation used as electrode materials integrated with an active substance as an auxiliary mechanism is shown. Different aspects conferred upon selected illustrations outline the electrochemical activity, and eventually, current issues and future viewpoints are recollected toward the following optimization method of electrode substances. This review article is anticipated to demand broad attention within active CD materials and encourage the growth of high-performance EES systems.
superior catalytical activity and advised that CDs would be employed as conductive agents. These investigations have revealed that CDs have significant assurance for implementation in electrochemistry. [8] CDs, a unique photoluminescent substance, have drawn significant concentrations because of their tunable photoluminescence (PL), remarkable electron transfer capabilities, economical, nontoxicity, and suitable hydrophilicity. [9] Lately, Liu et al. [10] suggested a known way to design low-ruthenium-content bimetallic materials for broad pH HER via carbon quantum dots. In addition, Lu and Wang [11] focus on the current consequences of PL CDs and systematically examine the preparation of CDs. In particular, they highlight different transformation behaviors of fluorescence and afterglow emission.
The CDs may be instantly added to the preparation methods or developed utilizing facile (hydro/solvothermal, microwaveassisted preparation, elevated heat carbonization), electrophoretic deposition (EPD) and electrochemical deposit. The relationship among CDs' amalgamation, role, and effects is shown in a graphic chart illustrated in Figure 1. [12] Current investigations have suggested different methods for the large-range and economical preparation of CDs. [12] For instance, Zhai et al. [13] prepared CDs as new structure blocks for EES and electrocatalysis. Cheng et al. [14] presented CDs' configuration and interface conversion for electrochemical energy utilization.
Here, this study will show the recent investigation based upon the preface of CDs within batteries, SCs, HER/OER, oxygen reduction reaction (ORR), electromagnetic interference (EMI) shielding, and solar-assisted energy generation used as electrode materials integrated with an active substance as an auxiliary mechanism. Different aspects conferred upon selected illustrations outline the electrochemical activity, and eventually, current hurdles and future outlooks are recollected toward the following development methods for electrode materials. This review article encourages the growth of high-performance EES systems and anticipated wide interest within active CD-based materials.

Carbon Dots in Different Dimensions
CDs, usually distinct as small carbon nanoparticles (CNPs) with numerous surface passivation arrangements, use and improve the inherent characteristics of the CNPs. Among different nanoscale carbon allotropes, fullerenes have intrinsically assumed the designation of CNMs on the 0D. Nevertheless, one can claim that fullerenes are, actually, stoichiometrically specified particles of not exclusive molecular arrangements but also different electronic networks and effects, basically distinct from CNMs in other sizes, such as carbon nanotubes (CNTs) and graphene nanosheets (GNSs) that evidently cannot be viewed as particles within any elasticity of the inventiveness. [15] In contrast, small CNPs convey a critical structural characteristic with CNTs and GNSs, that is, structural and boundary faults ( Figure 2a) is not feasible within fullerenes. Therefore, the NPs should deserve the label of nanorange carbon on the 0D. [16] All the produced carbon outcomes, primarily the support system and the construction tools of the 1D N, S-coincorporated carbon microrods, 2D N-, S-codoped carbon nanosheets (CNSs), and 3D N-, S-codoped carbon framework, are shown in Figure 2b. The construction of the 1D, 2D, and 3D CNMs may be separated into three phases. 1) The CDs were gathered and hidden upon the exterior of the supports. 2) The assembled CD sets obscured upon the template exteriors were restored to the related CNMs on high heat during pyrolysis. 3) The prototypes were released to get the matching CNMs with the congenital surfaces. [17] 2.1. Structure Owing to the vast assortment of publications associated with CDs, there is various and relatively confusing language within this field. CDs can be directed to carbon quantum dots (CQDs), carbon nanodots (CNDs), CNPs, etc. As there is no proof that quantum detention recreates a part within the PL tool toward these CDs, CQDs could not be correct, and CNP is a Figure 1. Graphic illustration of the association among the characteristic, function, and amalgamation approaches of CDs for EES and catalysis applications. Reprinted with permission under a Creative Commons CC BY 4.0 License. [12] Copyright 2021, Wiley Online Library. broader term that does not supply much data concerning the particles. Likewise, to the problems with terminology, there are numerous hypothesized configurations for CDs examined upon a rare occurrence. [18] Variable details for the structural arrangements that are barely unexpected as numerous reaction techniques and precursors are broadcast within the existing publications. One could predict divergent views, particularly for various top-down and bottom-up CDs. Further, a dispersal of dimensions is seen between all CDs, suggesting discrepancies, on the very minor, in the size of the arrangements seen in a sole CD trial. These concerns should be assessed while connecting data to CD design.
Mintz et al. [19] intensely studied the configuration of CDs. This must be mentioned that there are outlying additional oxygen atoms upon the particle's exterior, but only specimen groups have been established within for clearness Figure 3a. Also, this is assumed that there is an s-triazine segment of the network based upon Fourier transform infrared spectroscopy (FTIR) and 13 C nuclear magnetic resonance (NMR). The model was designed, as shown in Figure 3b. Regarding lattice classes, there should be two significant kinds: N 2 -doped sp 2 carbon lattices and s-triazine networks that can unsystematically exist within the model. Only a mild oxidant exists toward Y-CDs reaction, and the process is just ultrasonication; therefore, these necessities would direct to the more kinetically prevented branched configuration. The model in Figure 3c was designed for viewing these components. It is considered that the diverged polymer may times on itself, relying upon the inflexibility of distinct branches of the polymer. This comparison will start with the three CDs' ultraviolet-visible (UV-vis) absorption effects (B-CDs, CNDs, and Y-CDs). There are distinctions even among identical peak places; nevertheless, absorption peaks of all 3-CDs lie within the 200-300 nm area (Figure 3d). The PL spectrum of individually CDs exhibited within Figure 3e-g indicates relatively disparate characteristics. However, the maximum excitation toward every species is 50 nm individually; the extremer lease wavelength varies between 450 nm (CNDs), and 565 nm (Y-CDs), including B-CDs retaining, are lease utmost about 520 nm. This broad emission scope shows CDs' assortment and exhibits a remarkably diverse structural basis.
There are different characteristics and application areas of CNMs; within this study, we shortly concentrate upon the synthesis processes, properties, and energy storage applications of CDs. Hopefully, this study will offer practical understanding and facilitate a more profound quest for CDs.

Synthesis
Synthetic approaches for CDs may typically be categorized within two primary classes: top-down and bottom-up approaches. Topdown strategies direct the cutting of CNMs into CNPs; the techniques hold laser extirpation, arc liberation, electrochemical methods, and plasma processing. Bottom-up approaches contain the pyrolytic operation, template approach, reinforced synthetic process, microwave-supported process, chemical oxidation, inverse micelle process, etc. Some specific techniques are synthetic approaches, precursors, etc. [20] This process requires numerous steps in corrosion and exfoliation of cheap and readily obtainable carbon substances in extreme circumstances, essential oxidants, concentrated acids, and elevated temperatures. Instances of the top-down approach contain acidic exfoliation, [21] electrochemical oxidation, [22] metalgraphite embolism, [23] hydrothermal combination, [24] and robust physical ways, like arc release, [25] electron ray lithography, [26] laser excision, [27] and nanolithography through reactive ion etching (RIE). [28] However, it cannot precisely control the size diffusion and surface of the assembled particles with the top-down approach. It can utilize bottom-up approaches to prepare CDs from more small precursor molecules, such as the cyclodehydrogenation of polyphenylene [29] and carbonization of distinct organic particles. [30] Other bottom-up approaches comprise carbonization, pyrolysis, step-by-step chemical fusion, and solution chemistry. [31] However, a combination problematic during carbonization occurs.

Properties of Carbon Dots
CD-based materials have numerous engaging optical effects and superior biocompatibility. [32] Notably, CDs exhibit steady performance within aqueous media with PL at visible wavelengths; the light discharged from CDs relies upon the excitation wavelength. These remarkable edges create CDs a good prospect compared with established organic pigments and semiconductor QDs that deliver possible use for visualization, photocatalysis, drug transfer investigation, and spectroscopic pursuits. [33]

Structural and Compositional Properties
CDs, also comprehended as CNDs or CQDs, are quasispherical CNPs with particle dimensions under 10 nm and lattice spacing varying from 0.18 to 0.24 nm. [34] They contain unstructured or nanocrystalline cores, including sp 2 groups. [35] Few investigations have also documented diamond-similar networks constructed through carbon fractions having sp 3 hybridization. [36] Qu et al. [37] designed CDs via a single-step microwave preparation method containing sp 2 -and sp 3 -hybridized carbon bits. The dimensions and surface of CDs are illustrated utilizing transmission electron microscopy (TEM) and atomic force microscopy (AFM) (Figure 4a,c). The TEM pictures show that the CD particles are spheroidal with an average dimension of around 2.6 AE 0.2 nm (Figure 4b). In addition, none of the lattice edges is marked within the related high-resolution TEM (HRTEM) picture (inset of Figure 4a), suggesting the unstructured character of the CDs. The AFM picture also sustains spherical surface, with intermediate sizes of around 1.8 AE 0.2 nm ( Figure 4d). [38] FTIR or X-Ray photoelectron spectroscopy (XPS) may select functional surface clusters. Generally, moieties like C─O, C═O, and C-OH may typically present during the preparation procedure of prototypes and are present on the exterior of the prototype. In addition, distinct functional clusters may also be presented upon the exterior of CDs by choosing appropriate carbon predecessors or dopant precursors for the preparation operation. Dong et al. [39] designed CQDs through low-heat pyrolysis of citric acid (CA) and radiated polyethyleneimine (BPEI). This one-step process checked the consequent CQDs with aminoabundant BPEI upon the exterior. CDs may also be functionalized postpreparation, usually involving numerous stages.

Optical Properties
Under the outstanding optical characteristics shown through fluorescent CDs, they have been harshly employed in various  [19] Copyright 2021, Elsevier Ltd. healthcare implementations, particularly in biosensing, bioimaging, and treatment consequence. This is of significance to studying and comprehending CDs' optical effects to design a combination of CDs to help various bioapplications. Generally, CDs effectively absorb light in the 230-340 nm short-wavelength regions with a tail lengthening within the visible parts. Due to changes in π-π* of C═C interactions of the core of carbon, the first absorption band of about 230-280 nm is discovered, and due to n-π* changes on the surface of C═O, the second band was obtained at nearly 300-340 nm. [40] With the help of exterior passivation and doping of heteroatom, the absorption range may be modulated. For example, Das et al. [41] designed three distinct kinds of incorporated CQDs, that is, incorporated with boron (BCQDs), coincorporated with boron and nitrogen(BNCQDs), and codoped with boron, nitrogen, and sulfur (BNSCQDs). The former two materials showed two bands while the latter showed three centered bands at 260, 320, and 440 nm. π-π* and n-π* evolutions of nonbonding orbitals and aromatic sp 2 domains are equal for the two more high-energy BNSCQDs, while because of the trapping of excited-state energy, the shifted band was obtained at 440 nm through specific surface conditions.
CDs' room-temperature phosphorescence (RTP) results are significant owing to their long lifetime. Two factors should be ideally taken into concern toward receiving RTP. The first involves suppressing nonradiative transitions by limiting process and vibration, while the second one desires to reduce the intersystem junction capability by augmenting the spin-orbit coupling via transition metals. [42] On the other hand, RTP may also be accomplished by CDs with crosslinked networks possessing non-conjugated classes. [43] The construction of RTP within an aqueous solution is complex as phosphorescence quenching is generally followed in water because of the solvent-supported relaxation and liquefied oxygen. [44,45] Generally, the wavelength range of absorption peak of the CQDs is marked within 220-270 nm. As shown in Figure 5a, the absorption peak of electronic transitions from C-O or C═O bonds to π* orbital lies within 280 % 350 nm. The electron transitions of CQDs' exterior functional groups have an absorption peak wavelength in the range of 350-600 nm, representing that the surface chemical moieties can contribute to the absorption within the UV-vis region. Few investigations show that after the remediation with functional surface classes or by adjusting the sizes, the absorption peaks of CQDs redshift, as showcased in Figure 5b. Several notable CQDs display 600-800 nm longwavelength absorption spectra originating from aromatic ringcontaining structures, as shown in Figure 5c. Unlike earlier noted CDQs, the N-CQDs showed a robust excitonic absorption band owing to quantum-size consequences ( Figure 5d). [46] 4. Electrochemical Energy Storage Applications Toward Environmental Sustainability

Batteries
Carbon is one of the considerable significant elements within nature. CNMs, for example, CNTs and graphene (Gr), have www.advancedsciencenews.com www.advenergysustres.com constantly been constituents of great respect to investigators and are broadly utilized in various areas. CDs are new types of CNMs, which stand out between the carbon allotropes. They are illustrated by their quantum dimensions, large external practical groups, consistent distribution, flexible configuration and arrangement, better biocompatibility, and PL effects. In 2004, a set of fluorescent NPs with a dimension of less than 18 nm was initially discovered by Xu et al. [47] while filtering singlewalled CNTs (SWCNTs) from arc fume cataphoresis. This unique kind of fluorescent CNM soon drew more concentration from investigators, and a type of synthesis strategy for CDs has been dug out.
In 2006, comparable CNPs with a width of %5 nm and improved fluorescence emission consequence were composed. [27] They utilized argon (Ar) as a transport gas within laser excision of carbon targets with water vapor. Then the effects went via some processing, with exterior passivation and functionalization, and the CNPs with a unique character CD would be received eventually. Afterward, they performed a more extensive investigation upon nano-CDs. They discovered that surface passivation processing would remarkably enhance the quantum output of CDs, and the quantum output of CDs after cleavage and sanctification through column chromatography was elevated as 60%. [48] Although, these substances have their intrinsic imperfections. Inspired by the implementation of CDs in SCs, investigators initiated involving multifunctional CDs in battery designs in 2014, [49] as illustrated in Figure 6. It was approved that electrode substances incorporated, including CDs, were practical approaches to crack multiple issues, for example, improving electronic conductivity, enhancing ion communication, and regulating the dimensions and surface of the metal and metal oxides (MO x ). In addition, to electrode substances, the partitions and electrolytes also have a high effect on the catalytic activity of batteries. Astoundingly, CDs may also be utilized as transformers for partitions to accelerate ion communication and prevent dendrites' construction and electrolyte additives toward lithiumsulfur (Li-S) batteries to hinder the repeal and shuttle development of polysulfide-generated consistent Li deposit.

Supercapacitors
The most promising next-generation EES strategies, SCs, have drawn comprehensive investigation engagement owing to their high-energy density and storage capability, durable life, ecological cordiality, lightweight, and minor extent. [50] SCs may typically be categorized into two types with various energy-storage tools: 1) electric double-layer capacitors (EDLCs) and 2) pseudocapacitors. In EDLCs, CNMs, in individual Gr and GO, are the select options because of their outstanding conductivity, elevated explicit exterior area, and extended cycle resilience. [51] However, specific capacitances of EDLCs are determined owing to the non-Faradaic charge repository tool. In other words, pseudocapacitors exhibit 10-100 higher specific capacitances than the EDLCs due to the Faradaic charge storage tool's involvement with redox reaction taking a position within the bulk of active substances. [52] CDs are a unique family of CNPs below 10 nm that typically contain a nanocrystalline essence with central sp 2 -hybridized Reprinted with permission. [46] Copyright 2021, MDPI. carbon and an unshaped shell having large exterior operational groups. [53] They show substantial PL with high quantum results and tunable bandgaps, therefore generally named CQDs, where GQDs are also incorporated. Multiple carbon-based substances (graphene, activated carbon, reduced graphene oxide, etc.) are used for SC utilization. [54] Zhang et al. [55] prepared a ternary material of GO/CDs/PPy and utilized it as an active electrode composite toward SCs. Figure 7a shows that GO/CDs/PPy composite was incorporated through an improved in situ polymerization approach. Figure 7b illustrates the TEM photograph of CDs with a limited size dispersal histogram, confirming that as-prepared CDs are relatively consistent, having nearly 8.7 nm average sizes and having a variation of 1.4 nm. Figure 7c illustrates an expected TEM picture of the GO/CDs-2, where CDs particles may be noticed very obviously, adorning homogenously upon both flanks of GO layers. Also, an HRTEM picture (see the inset of Figure 7c) indicates the CDs with a lattice spacing of 0.215 nm into the crystalline body conforming the (100) characteristic of graphite. However, as for the GO/PPy representative, Figure 7d displays thick PPy layers that have been covered at equal edges of the GO sheet, and no CDs are located from its HRTEM picture (Figure 7e). On the other hand, black CDs may be noticed in the GO/CDs/PPy compound illustration from its HRTEM photograph, as illustrated in Figure 7f. Similarly, the ternary material GO/CDs/PPy hold the layered surface, likely owing to the asset of the GO sheet ( Figure 7g). As demonstrated in Figure 7h, galvanic charge-discharge (GCD) arcs were estimated utilizing 1 mol L À1 aqueous LiCl media as an electrolyte in a threeelectrode system. Similarly, GCD arcs of GO/CDs/PPy-2 at distinct current densities are offered within Figure 7i. Likewise, the catalyst's cyclic voltammetry (CV) curves into a three-electrode design at a sweep rate of 10 mV s À1 are exhibited in Figure 7j. A model EDLC displays a rectangular CV field, while the form transitions to a quadrilateral for EDLCs with resistive failures. In addition, CV arcs of GO/CDs/PPy-2 at different sweep rates of 5, 10, 20, 50, and 100 mV s À1 (Figure 7k) contain established virtuous and steady capacitive manners under the circumstances of 1.0 V working potential. The catalysts were cycled 5000 runs on a current density (cd) of 10.0 A g À1 to analyze their cycle strength ( Figure 7L). The electrochemical impedance spectroscopy (EIS) study has been identified as one of the main approaches to investigate the essential manners of electrode substances for SCs. The resultant Nyquist plots ( Figure 7m) contain a semicircle bend at the higher-frequency area and a linear bar at the lower-frequency site. Then the cadenced EIS graph was investigated by fitting the information by the equivalent circuit (the inset of Figure 7f ).
As a result of the adsorption of CDs on the Gr's surface, CD-Gr (CDG) hybrid media can be obtained. This adsorption primarily can take place via π-π relations ( Figure 8a). The carbonization of ammonium citrate synthesized the CDs via easy dry  Figure 8cc1) within water is relatively low, owing to its positively aquaphobic character. CD-based materials possess polar functioning assemblies upon their exterior, which cause them to be soluble in water (Figure 8cc2), where the delocalized π-electrons of a carbonenriched core having sp 2 character may reduce their connection with Gr via π-π relations. As displayed within Figure 8cc3, Gr comprises a satisfactorily spread and steady suspension within an aqueous CD media. Equal dispersion of CDs upon Gr is shown in the TEM pictures (inset of Figure 8bb2). Furthermore, the promising stretchy electronics technology and planar energy apparatuses require better than planar SC appliances. So, they simulated a solid-state SC established upon manganese oxide CD graphene nanocomposites (MnO x -CDGs) as the vigorous substance, N-methyl-2-pyrrolidone-polyvinylidene difluoride (NMP-PVDF) as dissoluble composite, and poly(vinylalcohol (H 3 PO 4 /PVA) as electrolyte solution (Figure 8d) to enhance elasticity and cyclic resilience. As may be noticed from CVs in Figure 8e, the solid-state SC maintained a 98.4% current value while the twisted slant was 90°, and it decreased to 81.2%, while the twisted slant was 135°. So, we may imagine that its catalytic activity was nearly not impacted till the twisting angle was capable of 90°, indicating that it may be utilized as a supple SC for relatively bending energy instrument implementations. Having a scope of sweep speed of 100-1600 mVs À1 , the solid-state SC (ss-SC) led to high current durability (Figure 8f ). After 10 000 runs, MnO x -CDGs achieved superior GCD cyclic resilience owing to the binder and gel media through a capacitive retaining value of 94.7% (Figure 8g), with cd around 1.2 A g À1 that was twofold more elevated compared with SC in the absence of NMP-PVDF binder and within 1 M Na 2 SO 4 media. So, they combined two appliances into succession that delivered good capacitive characteristics within the voltage window between 0 and 2.0 V, as illustrated in Figure 8h. Figure 8i indicates the employment of an lightemitting diode (LED) driven by two appliances combined into succession. [56] Here, Table 1 shows the applications and different parameters of CD-based materials in SCs. i) GCD arcs of GO/CDs/PPy-2 at distinct current densities; j) CV arcs at a scan rate of 10 mV s À1 ; k) CV arcs of GO/CDs/PPy-2 recognized on different sweep rates; l) maintained specific capacitances and coulombic efficacy of anodes within 5000 GCD runs on a current density of 10.0 A g À1 ; m) Nyquist graphs within the frequency coverage from 100 mHz to 100 kHz (inset: the electrical equivalent circuit). Reprinted with permission. [55] Copyright 2017, Elsevier Ltd. CDs have become the formidable challenger for other carbonbased materials and other materials such as Pt-and Ir/Ru-based electrocatalysts due to chemical inertness, high electron mobility, cost-effectiveness, larger surface area, abundant surface defects, and active sites. The electrocatalytic performance of pure CD electrocatalysts can be enhanced via doping of heteroatoms like B, S, N, and P, which are utilized for electron-coupling interactions and to modulate electrical structures. [44] Presently CDs are primarily categorized as GQDs, CQDs, and carbonized polymer dots (CPDs) as per their various construction tools, micro-/nanostructures, and characteristics (Figure 9a). At the same time, we can build connections between them by adjusting the graphene film and carbonization degree. In comparison, the converted N and O co-doped CPDs within available porous 3D turbostratic graphene grids to manufacture SCs (Figure 9b), ultimately acquiring quick charging paces with a relaxation period of 3.44 ms and energy density of 17.7 W h kg À1 . It was a considerably more elevated comparison to the retail aluminium-based capacitor. Through developing hierarchical porous carbon (HPC) substances from N-, P-, and O-codoped CPDs and polyacrylamide hydrogel (Figure 9c).
CDs can improve the storage capacity and electrocatalytic performance of nanomaterial when incorporated with other carbon materials, especially in SCs. For ion intercalation/deintercalation, improving the capacitance of SCs lattice strip with defects in CDs acts as layered structures. [14] The incorporation of CDs provides more active sites for capacitance, and doped CDs provide additional faradaic capacitance, which improves the capacitance immensely. They can be used as electrode materials by incorporating them on different substrates along with their electrochemical performances. For instance, the GO/CDs/PPy composite was fabricated by Bi et al. [55] under mild conditions via in situ polymerization of GO/CDs with pyrrole. The incorporated CDs surge the dielectric constant of the composite and facilitate the electron transfer through the active layer, which result in a high specific capacitance of 576 F g À1 at a current density of 0.5 A g À1 . It also exhibited outstanding cycling stability after 5000 cycles with 30.1 Wh kg À1 of energy density at a power density of 250 W kg À1 . Similarly, a micro-SC based on GDs/graphene fiber was constructed by Chen et al. [68] CD acts as a reinforcing nanofiller exhibiting superior mechanical strength of 109.93 MPa, which is three times larger as compared with bare graphene fiber. The as-synthesized material delivered In addition to this, GQDs//GQDs material were synthesized by Liu [24] and have outstanding power response, advanced rate capability up to 1000 V s À1 , and a high capacitance of 468.1 mF cm À2 in an aqueous solution.

Hydrogen/Oxygen Evolution Reactions
The complete water oxidation comprises two half-cell reactions: OER and HER. Thermodynamical and dynamic investigations of these responses indicated lethargic reactions and required a higher voltage of 1.23 V to flow-a broad assortment of composites, for example, MO x , [76] metal selenides, [77] metal tellurides, [78] metal sulfides, [79] metal phosphides, [80] metal-organic framework (MOF), etc. were used to overwhelm the energy borders of that response. This was found that Pt and oxides of Ir and Ru demonstrated more significant catalytic performance to overcome the energy barriers of HER and OER concurrently. [81] However, the issue persisted, as these stimuli related to noble alloys were not economical. Their origins were insufficient, and they were considerably expensive between the other metallic element. These characteristics restricted their usage on a retail scale. Below these possibilities, an excursion to manufacture an Earth-abundant, ecological, vigorous, and profitable compound that can substitute these noble metal compounds and has the disposition to reach their electrochemical performance is on the form.

Hydrogen Evolution Reaction
Developing practical and affordable HER electrocatalysts for prospective renewable power systems is positively expected. CNMs with exceptional edge locations, better chemical strength, elevated electrical performance, and synergetic consequences show tremendous potential. However, rare investigations have demonstrated the effectiveness of the CNMs upon catalytic performance. Toward renewable energy, H 2 is an illustrative energy porter, while for the accomplishment of good evolution of H 2 causes the creation of extremely functional electrocatalysts for HER acute. [82] The emission of carbon monoxide (CO) and carbon dioxide (CO 2 ) has evolved as the primary reason for environmental change, increasing global temperature. In addition, sulfur emission from fossil fuels directs to acid precipitation and sulfur mists. The surplus energy obtained by the periodic energy sources, for example, wind and sunlight, would be hydrogen kept by water splitting. Hydrogen gas created through steam reforming and coal gasification also leads to CO 2 release. Therefore, the electrocatalytic HER appears to be the weird approach toward collecting hydrogen. [83] The reaction appears on the catalyst's exterior, and the lower overpotential (η) and more elevated current density are achievable within the platinum (Pt) or its corresponding substances. However, Pt is very costly and not general smoothly. Typically, it lacks strength in a highly concentrated acidic environment  of polymer electrolyte fuel cells, which shows the reduction of H 2 in an enormous scale exhibition. Thus, studying an efficient and in situ way to simulate homogeneously spread metal NPs is definitely contained in a nanocarbon material through an economical path. CQDs are promising nanomaterials that exhibit significant assurance toward bioimaging, [84] optical sensing, [85] catalysis [86] and photovoltaic usages. [87] CQDs with high strength, straightforward surface functionalization, and better electrical performance are appealing substances toward the configuration and incorporation of photoelectrocatalyst. [88] The working assemblies (like, -COOH, -NH 2 , -OH, -CO, and many more) upon the characters of the CQDs may correspond, including metal ions or semiconductors, having vacant d-orbitals for creating the reasonably steady CDQ-metal ions or semiconductors synchronization compound. [89] The metal NPs are limited among the CQDs to create tremendous nanocrystals by steady networks that control the accumulation and development of the metal NPs throughout the reaction. Thus, CQDs can be an excellent transport for preparing metal HER materials. [90] Figure 10a illustrates the preparation of the compounds. Especially, CDs designed utilizing CA (CA; named CA-CDs), CA and ethylenediamine (EDA) (named CE-CDs), and CA and melamine (MA) (marked as CM-CDs) as feedstocks were combined by a Mo and PA and after pyrolyzed for acquiring the definitive composites. Hexagonal structures of MoP are confirmed by the consequent X-ray diffraction (XRD) patterns (Figure 10b). Figure 10c indicates exact D-and G-bands on 1338 and 1590 cm À1 , correspondingly. The estimated I D /I G intensity proportion of MoP/0.5CM-CDs1100 of 1.13 shows multiple defects and disruptions within the Gr films. Figure 10d-f shows TEM photographs at various ranges of MoP/0.5CM-CDs1100, exhibiting the related carbon coatings that are conductive and advantageous toward reducing the core resistance of the amalgam composite.
Within 1.0 M KOH, a typical three-electrode technique was defined for the HER catalytic performances of the MoP/CDs materials. All voltages here direct to reversible hydrogen voltage. Figure 10h indicates that the related overpotential of 614 mV at a cd is about 10 mA cm À2 while no CDs were counted. The Tafel slant estimated by polarization arcs and EIS (Figure 10i,j) has the identical frequency as Figure 10h, indicating that the MoP/0.5CM-CDs1100 composite enriched with N content had the most promising HER activity. The strength of the MoP/0.5CM-CDs1100 composite was after that explained for 1000 CV runs (Figure 10k), while its η varied slightly on a cd of nearly 10 mA cm À2 . The HER catalytic activity of MoP/0.5CM-CDs800 to MoP/0.5CM-CDs1100 was chosen  utilizing a typical three-electrode configuration within an aqueous media 1.0 M KOH ( Figure 10L). H 2 generation in water oxidation into a basic media was estimated (Figure 10m). [91] CDs are structurally mixed, inexpensive, readily incorporated (with N, B, S, and P), and nonhazardous. Their distinctive electron transfer effects and high explicit exterior area construct them as exceptional composites. [92] They may sustain various surface-active groups (like, ÀNH 2 , ÀOH, and ÀCOOH) and host numerous favorable locations towards catalysis. Heteroatomincorporated CDs may change the catalytically functional center's electronic configuration, facilitating adsorption of hydrogen intermediaries. The multicomponent nanostructures' relations could improve catalysis by reducing intermolecular electron transmission, essential for catalytic activity. Numerous earlier information on the hydrogen generation execution of RuP 2 amalgams have concentrated upon a one reaction procedure and have not assumed a sole composite toward H 2 generation through mutually the electrochemical HER of H 2 O and the hydrolysis of ammonia borane. [93]

Oxygen Evolution Reaction
Demand to follow the origins of energy fascinates humankind to see a renewable source with tremendous reservoirs during its carriages with no dangerous consequence to humanity. These days, the energy-lacking globe is desiring an electrocatalyst with a facile and straightforward preparation process with outstanding catalytic activity and outstandingly sustained strength toward the positively sluggish OER. [94] The OER is a favorable energy transformation strategy examined in the current years. Nevertheless, due to the elevated η and lethargic kinetics of the OER, an effective catalyst is essential toward lowering the η and accelerating the reaction. This description informs cobalt phosphide (CoP)/CD materials as a composite toward the OER. [95] CDs have produced tremendous concentration as fluorescent searches for detecting heavy metals because of their exceptional characteristics, like cost effectiveness, easy operating ways, useful photostability, simple functionalization, low toxicity, adequate water solubility, and superior conductivity. [96] These remarkable characteristics have evolved widespread into bioimaging, optoelectronic appliances, photocatalysis, sensors, and electrocatalytic water splitting (OER). [97] Nowadays, different precursors and techniques have been used to manufacture CDs. However, most outcomes show short emission beneath UV-vis radiation that is not conducive to functional usage. [98] OER composites based upon transition metal oxides (TMO x ), particularly NiO, have improved consideration owing to their extensive resources, low loss, and similar catalytic performance. [99] NiO, a standard semiconductor with the expansive bandgap energy of 3.6-4.0 eV, maintains superior redox reversibility, enormous exterior areas, and better performance and has been broadly investigated as a reason for an excellent electrochemical composite toward OER. [100] For instance, Figure 11a shows the incorporation of polarized-SO 3 -CDs/NiFe LDH (P-SO 3 -CDs/NiFe LDH) composite anode Figure 11. a) Graphic representation of working electrode incorporation procedure. Description of NiFe LDH materials. b) XRD patterns of the synthesized materials. c,d) TEM pictures and e) HRTEM picture of SO 3 -CDs/NiFe LDH. The catalytic activity of NiFe LDH-based OER composites: f ) IRamended polarization arcs of different composites within alkaline media. g) η of the synthesized OER composites. h) Tafel slopes of the synthesized OER composites. i) Chronopotentiometry arcs of synthesized materials on 10 mA cm À2 , correspondingly. Reprinted with permission. [101] Copyright 2021, Elsevier Ltd. electrodes. Figure 11b illustrates the XRD patterns for P-SO 3 -CDs/NiFe LDH, SO 3 -CDs/NiFe LDH, and NiFe LDH. The surface, dimensions, and configuration of the SO 3 -CDs/ NiFe LDH were also described via TEM, as depicted in Figure 11c-e, indicating sized nanofilms of 50-200 nm. This is nearly translucent beneath the e À -ray owing to its very thin character. Three reference specimens were also experimented under identical circumstances for comparison. Figure 11f shows the IR-corrected linear sweep voltammetry (LSV) graphs of OER of synthesized materials. As demonstrated in Figure 11f,g, in P-SO 3 -CDs/NiFe, better OER activity was shown by LDH, including η of 200 mV on a cd of 10 mA cm À2 , that is, 46 mV less than SO 3 -CDs/NiFe LDH (246 mV), 91 mV less as compared with NiFe LDH (291 mV) and 110 mV less as compared with the retail RuO 2 (310 mV). As demonstrated in Figure 11h, Tafel plot of P-SO 3 -CDs/NiFe LDH was obtained at 55.5 mV dec À1 and showed the most rapid kinetics, which is smaller as compared with SO 3 À CDs/NiFe LDH (58.5 mV dec À1 ), NiFe LDH (80.3 mV dec À1 ), and the retail RuO 2 (86.7 mV dec À1 ), correspondingly. Furthermore, P-SO 3 -CDs/NiFe LDH showed the uppermost performance of the composite estimated and achieved better electrochemical durability. Figure 11i indicates that the stability of the synthesized compound was estimated on the cd of 10 mA cm À1 . Likewise, after the count of sulfonated CDs around Fe atoms of NiFe LDH, there are also three middle forms within the OER method. The better catalytic performance was primarily ascribable toward NiFe LDH, while the accumulation of SO 3 -CDs improved OER performance. [101]

Oxygen Reduction Reaction
The need for competitive options for noble platinum as a positively active ORR material has improved because the retail implementations of Pt-based composites are restricted via their high price, CO inactivation, and insufficient stability. [102] Different methods have been investigated to substitute retail Pt-based composites with metal-free CNMs. Metal-free-based composites retain necessary characteristics, for example, better electrical performance, high exterior area, and moderately good strength. [103] Within the carbon material family, CQDs, or CDs, a new type of 0D nanocarbon with dimensions under 10 nm exhibited better solubility, low cytotoxicity, excellent quality compatibility, and facile preparation functionalized by exterior passivation. CDs have positively inferior oxygen-possessed moieties dispersed unsystematically on the basal plane and border areas. [104] Current research indicates that N-doped CNMs appear to be an exemplary process for promoting ORR. For example, Qu et al. [105] prepared N-doped GQDs (N-GQDs) through CV cycles. The as-synthesized N-GQDs with an N/C atomic proportion of %4.3% showed outstanding electrocatalytic capability toward ORR. Lee et al. [106] showed vigorous hydrothermal preparation of N-doped CDs (N-CDs) by the unripe peach toward fluorescent bioimaging and catalytic ORR. Likewise, N-doped Gr (NG) was prepared via distinct processes such as chemical vapor deposition, [107] plasma processing, [108] and thermal strengthening, including NH 3 , which was broadly used as ORR composite. However, strict constraints or special tools are usually needed for this method. Thus, creating metal-free catalysts with better performance, cost-effectiveness, strong stability, and gentle essentials stays a hurdle. [109] Niu et al. [110] proposed positively sturdy N-doped CNTs emanated by CDs and MOFs to superior, effective electrocatalyst toward ORR. Figure 12a illustrates a diagram of N-CNTs derived from N-CDs and MOFs for positively effective material of ORR. Within Figure 12b, ZIF-67 prototypes were developed within homogenous rod-like atoms with an exposed end. Precisely equivalent to the bottom-up organic process, this is clear from the bigger particles with a turbulent formation, wherever the ZIF-67 dodecahedron was finally transformed within thin N-CNTs with N-CDs through the pyrolysis method. An intermediate width of around 15 nm was measured for the as-synthesized N-CNTs, as illustrated in Figure 12c. Figure 12d shows HRTEM pictures of complicated organizational details and the atomic lattice edges of N-CNTs. Interlayer arrangement of graphite (002) along with the matrix space, which is around 0.311 nm, is illustrated in Figure 12e. During the matrix space (%0.205 nm) it is also constant through the Co (111) facade, as illustrated in Figure 12f.
As earlier said, it is expected that the designed N-CNTs can be employed as fuel cells and metal-air batteries as they can show notable ORR catalytic performance within rechargeable appliances. Thus, CV measures evaluate the ORR electrocatalytic performance. For the comparison, unadorned glassy carbon electrode (GCE), N-CDs, Co-N-C nanohybrids and viability known Pt/C catalysts were also strained below the identical circumstances. Figure 12g shows the CVs for N 2 -and O 2 -saturated N-CNT electrodes into 0.1 M KOH on a sweep rate of 10 mV s À1 . To additionally explain the part of N-CNTs within ORR, LSV measures were documented upon a rotating disk electrode (RDE) within an O 2pervaded alkaline solution at a spinning speed of 1600 rpm. In contrast, ORR polarization curves were also received for N-CDs, unmodified GCE, traditionally known as Pt/C and Co-N-C nanohybrids probes. As shown in Figure 12h, in comparison with GCE, N-CDs, and Co-N-C nanohybrids probes, the initial voltage (E 0 ¼ 0.88 V) and half-wave voltage (E 1/2 ¼ 0.82 V) of N-CNTs are significantly more favorable. Also, to thoroughly comprehend the electron transfer tool concerned with the activity of ORR, the ORR polarization plots on special rotating rates (ω) were examined in the segment. As demonstrated in Figure 12i, the limiting current density ( j L ) grows with higher spinning speeds toward all materials because of the condensed scattering extent at higher rates. In Figure 12j, significances of n within the current investigation change for N-CDs and Co-N-C nanohybrids between voltage ranges of 1.3 and 1.7 that lead to overwhelmed ORR two-electron approaches with HO 2 À upon the electrode. Consequently, the j K significance of N-CNTs may acquire 8.10 mA cm À2 on 0.3 V, which is 2.2-folds higher than N-CDs, 1.6 times more extensive than Co-N-C nanohybrids, and 1.3 times more significant compared with Pt/C, as shown in Figure 12k, correspondingly. In addition, the Tafel plot is an essential element to consider catalytic performance. Consequently, the slant of N-CNTs (%81 mV dec À1 ) can be seen as lower compared with the Pt/C composite (%132 mV dec À1 ), as illustrated in Figure 12L; this indicates the quick kinetics of N-CNTs. Consequently, the 4e À ORR method shows that N-CNTs deliver excellent catalytic performance.
Further, Table 2 shows the applications and different important parameters of CD-based materials in water splitting.

Electromagnetic Interference Absorption/Shielding
Carbonaceous byproducts within the previous years have yielded excellent concentration for the design of functioning catalysts owing to their QD surface characteristics, ultrathin organizational component, notably better strength-to-weight proportion, anticorrosion characteristics, exceptional transporter mobility (%200 000 cm 2 V À1 s), and significantly better thermal conductivity (around 5300 W m À1 K). [123] Hybrid materials possessing carbonaceous inclusions usually work as more suitable shielding composites toward their lightweight, high specific exterior area, and flexibility and earlier noted electronic features. [124] CNSs, CNTs, Gr, and Gr byproducts are broadly investigated [125] and also examine how they affect EM absorption/EMI shielding in core-shell system characteristics.
Singh et al. [126] reported CdSe@V2O5 core-shell QDs with reduced GO (rGO-V-CdSe) to design a light EMI shielding substance. A solvothermal method was utilized to prepare this core-shell design. This system's increased dielectric failure and protecting efficacy were attributed to different defects that exist in it, which showed dipolar and interfacial polarization. Complete protecting efficacy up to %38 dB has been documented within the X band.

Solar-Assisted Energy Generation
Global warming, mainly driven by CO 2 emission from the ignition of fossil fuels, is anticipated to have overwhelming effects on the Earth. The lessening of CO 2 emission and the quest toward hygienic and bearable options for fossil fuels are thus attractive and incredibly critical, particularly under the very enterprising international environment guidelines for the phase-down of coal usage. Hydrogen (H 2 ), a universal energy conveyance, is regarded as one of the most favorable origins of pure energy to attack multiple energy hurdles and has newly acquired exceptional awareness globally owing to its highly gravimetric energy density, about negligible greenhouse gas emissions properties, and ability to be utilized for living manufacturing set-up or power factories powered through fossil fuels. [127] The manufactured process of MXene-derived monodots (MXDs), CQDs, and the MXene@carbon(MX@C) mixture is schematically shown in Figure 13a. Shortly, CQDs were manufactured through a bottom-up process based upon hydrothermal removal at 200°C utilizing fumaronitrile as the carbon origin  Figure 13d illustrates the core-shell-designed MX@C amalgam nanodots at a shell consistency of %1.6 nm. The optic and electronic characteristics were examined through UV-vis and PL spectrometry, as illustrated in Figure 13e,f. The physically miscellaneous CQD and MXD (CQD-MXD) were used as control specimens. Figure 13g demonstrates the LSV arcs within 1 M KOH solution at a pH of 14 after IR recompense. Tafel slope, log (|j|) versus η was used to confirm the catalytical performance of the HER. A shorter Tafel plot shows that a lower η is needed for the composite to reach the desired current. As shown in Figure 13h, the Tafel plot of MX@C was 32 mV dec À1 and more inferior compared with pure MXD (49 mV dec À1 ), CQD (46 mV dec À1 ), and CQD-MXD (35 mV dec À1 ). Furthermore, we corresponded the η of composites at 10 and 100 mA cm À2 , as illustrated in Figure 13i. 205 and 328 mV were η for the MX@C mixture, lesser as compared with the CQDs, MXDs, and CQD-MXD at 10 and 100 mA cm À2 . As demonstrated in Figure 13j, Nyquist slops for MX@C hybrid at the open-circuit voltage exhibit an inferior charge transfer resistance (79 Ω) than those of additional nanodots, reducing the charge transportation performance toward the improved HER movement. In contrast, the MX@C hybrid composite operates virtually and is positively involved at all pH ranges, as illustrated in Figure 13k. Electrochemical stability is an influential factor in evaluating that an electrocatalyst shows HER activity. Therefore, the cycling stability of CQD-MXD and MX@C was conducted over 1000 runs, utilizing constant CV within an alkaline media, as demonstrated in Figure 13L. [128]

Conclusion, Challenges, and Outlook
This study suggests various kinds and synthesis techniques of CDs and their implementations as electrode substances in EES (such as SCs and batteries) and electroactive materials of HER, OER, and ORR into water oxidation cells, EMI shielding, and fuel cells. These study results indicate that CDs are among the multiple-competent NMs with conspicuous characteristics: considerable specific exterior area, flexible nanoscale dimensions, quick electron transfer capability, quantum dimension consequence, large exterior practical groups, and different defects that ensure their significant potential within EES applications. CDs may work as electron contributors and acceptors. This distinctive essence has also created perfect catalysts within other catalytic responses, such as photocatalysis, electrocatalytic O 2 or CO 2 removal, and overall water splitting. Most of the CDsdependent catalysts with high catalytic performance are present within the structure of CD-metal composites. Higher catalytic performance is anticipated via advanced synthesis processes and a profound acquaintance of the synergistic tool among metal and CDs into various catalytic responses. In contrast, economic CD-based nanomaterials catalysts can substitute the retail Pt/C catalyst within a few specific reactions (such as electrocatalytic O 2 deduction), and superior performance may be acquired by sensible reasoning structural composition and structural constraint. In addition, bare CDs should usually be incorporated (like heat processing) before being utilized as electrocatalysts within aqueous media to evade accumulating and liquifying.
Finally, CDs have demonstrated outstanding prospects toward EES and catalysis implementations, although far from being ultimately used. Continued study and growth of controllable synthesis techniques and attaining a more reasonable interpretation of the connection among their design and arrangements and their functional tools as the EES substances and electrocatalysts are required. In this respect, few in situ depictions of CD-based composites and their operating mechanisms during the EES and electrocatalysis methods should be utilized for essential acquaintance and growth of unique substances. Indeed, this may be considered that these distinctively outstanding components of CDs will prompt more exciting developments in the upcoming future.