The CND/CQD particles with sp²/sp³ hybridized carbon core act as efficient catalysts. Most CND/CQD electrocatalysts are hetero atom doped and their activities are often better than commercial metallic catalysts. The electronic behavior of N-doped CNDs (termed as SP2) was utilized by Gao et al. for oxygen reduction catalysis (ORR) through a four electron path in 0.1 M KOH medium [32]. The SP2 particles, the conglomeration of nanodots, showed a conductivity of 0.466 S cm⁻¹. A characteristic ORR peak appeared at ca. ~0.22 V in the O₂ saturated solution, while a featureless character was observed in the O₂-absent medium indicating initialization of ORR by SP2 (Fig. 3).

Lie et al. [28] synthesized uniformly N-doped CQDs where they used solvent (N-methyl-2-pyrrolidone and dimethyl-imidazolidinone, with cyclic structures and rich amino groups) as carbon source. They examined its catalytic activity towards ORR in O₂-saturated 0.1 M KOH with 3 M CH₃OH. The peak potential was −160 mV which remained almost unaltered after 1000 cycles. The presence of pyridine N-atoms caused efficient activity for ORR. Kakaei et al. used CQD to fabricate a gas diffusion electrode for reduction of oxygen in 0.5 M KOH [33]. CQD was pasted on carbon paper as an electrode for ORR in fuel cell. At high current region, the Tafel slope was calculated as 111 mV/decade, indicating the significant activity of the catalyst. The results were attributed to the ability of CQD to enhance the solubility and permeability of oxygen in the reaction layer due to its high porosity. Bhattacharyya et al. [34] synthesized a N-doped CQD from a Mg^II based anionic MOF, {[Mg₃(ndc)_2.5(HCO₂)₂(H₂O)]·[NH₂Me₂]·2H₂O·DMF, ncd=1,4-naphthalenedicarboxylate}, to ensure the high porosity and ease of doping. Traces of metallic Co nanoparticles were deliberately incorporated as a part of post synthetic treatment that helped the activity of as-synthesized CQDs for ORR reaction. It was reflected in the Tafel slope values; at higher and lower current density, the slopes were 141 mV dec⁻¹ and 63 mV dec⁻¹, respectively. Atchudan et al. synthesized N-doped CQDs from unripe peach and applied them for electrocatalytic ORR reaction in alkaline medium [35]. They found a prominent O₂ reduction peak at 0.45 V vs. RHE for the O₂ saturated KOH solution with an onset potential of ORR at 0.94 V. They obtained a two electron pathway of O₂ reduction to hydrogen peroxide. Martínez-Periñán et al. [36] reported the effect of surface functional groups on ORR in acidic and alkaline medium. They fabricated amide (CD-CONH₂) and carboxylic acid functionalized (CD-COOH) CND particles and examined their electrocatalytic properties towards ORR in acidic and alkaline medium. A suitable onset potential of −0.28 V (vs. SCE) was obtained for CD-COOH modified the screen-printing electrode in alkaline medium. Datta et al. [37] showed a metal free water oxidation by CQD with entrapped 5-hydroxymethyl-2-furaldehyde (5-HMF) at carbon skeleton. They proposed that the 5-HMF molecules in close vicinity to the catalytically active sites containing C=O groups could extract the ^·OH/^·OOH radicals and increase the in situ H₂O concentration to facilitate the O₂ evolution. The electrocatalytic behavior of the CQDs was dependent on 5-HMF concentration. The highest concentration of 5-HMF decently exhibited low 0.21(±0.03) V overpotential and generated current density up to 33.6 (±2.3) mA cm⁻². Shen et al. [38] prepared a CND from melamine and glutaraldehyde and used them for electrocatalytic ORR in alkaline medium with an onset potential and current density of −0.169 V and 2.69 mA cm⁻², respectively. The CND particles were then coupled with silver nanoparticles to achieve superior catalytic activity with onset potential of −0.081 V and a current density of 4.09 mA cm⁻² at −0.5 V.

In their study, Shen et al. [38] reported that the composite of the as-synthesized N-doped CND with silver nanoparticle (AgNP) provided better activity towards ORR compared to N-CND. The CND possessed both p-and n-type conductivities from the Mott-Schottky relationship. The catalytic property of metal was synergistically improved by the high surface area and facile electron transport property of CND particles. There are certain other reports of such synergistic interaction within CND-metal/metal oxide/metal phosphide etc. towards electrocatalysis. Along with that, nonmetal carbonaceous counter parts have also been studied as the alternative for metal. Karim Kakaei reported the enhanced activity of platinum (Pt) nanoparticles combined with CQDs towards ORR [39]. The activity of Pt/CQD composite was compared with Pt/C catalyst. The durability was checked by chronoamperometric measurements at 0.5 V vs Ag/AgCl in an O₂-saturated 0.1 M HClO₄ electrolyte (89.4% relative current persisted in acidic medium after 21000s compared to 81.6% for Pt/C), which showed that the composite can further be used for fuel cells. Bao et al. [40] prepared CQD/nickelcobalt phosphide (CQDs@NiCoP) catalysts through a facile annealing method for electrocatalytic hydrogen evolution reaction (HER), which yielded a lower initial overpotential than pure for NiCoP (108 vs.182 mV) and a lower Tafel slope (80 mVdecade⁻¹). CQD aided high catalyst conductivity and significantly accelerated charge transfer. Li et al. found CQD loaded ruthenium nanoparticles (Ru@CQDs480) to be efficient electrocatalyst for reduction of hydrogen at alkaline medium [41]. They varied the medium pH and observed that even at extreme alkaline condition the as-formed Ru@CQDs480 exhibited significant catalytic behavior with onset overpotential of 0 mV, a Tafel slope of 47 mV decade⁻¹, and good durability (Fig. 4).

Li et al., reported CQD supported enhanced activity of ruthenium (Ru) nanocatalyst for HER [31]. The catalytic activity largely depended on the crystallinity and confinement effects. The experimental results showed that Ru nanoparticles were successfully anchored by strong coordination interactions between the d orbitals of Ru and the surface functional groups of CQDs, which is beneficial to the confinement of Ru nanoparticles around the CNDs and to prevent their aggregation. The Tafel slope with the catalyst at its optimized condition was ~ 63 mV decade⁻¹. From the experimental data, they proposed Volmer-Heyrovsky mechanism for HER. A rhodium (Rh)/silicon quantum dot/CQD nanocomposite was reported by Dang et al. for HER [42]. The silicon quantum dots limited the size of Rh and prevented the aggregation and the CQDs greatly increased the electron transportation during the catalytic process. The catalyst showed a low overpotential of 36 mV at 10 mAcm⁻², low Tafel slope of 26 mV.dec⁻¹ and long-term stability. Nanocomposites based on transition metal oxide and carbon nanostructure are of low cost and provide high activity and stability. Liu et al., prepared a general route for synthesis of Ru-M (M=Ni, Cu, Mn) bimetallic nanoparticle and CND hybrid, and out of three kind of composite, RuNi/CQD exhibited highest performance for HER. [43]. The overpotentials of 13, 58, and 18 mV were shown by RuNi/CQDs at a current density of 10 mAcm⁻² in 1M KOH, 0.5M H₂SO₄, and 1M PBS, respectively, for Ru loading at 5.93 μgRucm⁻². The role of CQD particles was tested by replacing CQD with other carbon materials, but it was observed that CQD based particles showed high activity and durability. This might be due to strong interaction within d-orbital of Ru and functional groups of CQD which further prevented the aggregation of the particles. Theoretical studies revealed that Ni doping caused a moderate weakening of the hydrogen bonding energy of nearby surface Ru atoms, that played a critical role in improving the HER activity. Li et al. [44] prepared a CQDs decorated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ-perovskite nanofibers (CQDs@BSCF-NFs) catalyst for OER. The catalyst achieved a low overpotential (0.35 V at 10 mA cm⁻²), high current density (140.8 mAcm⁻² at a potential of 1.65 V vs. RHE), and high stability. The high catalytic activity was attributed to high surface area of hollow nanofibers further increased by CQD and the numerous surface oxygen vacancies as well as the synergy between CQD and BSCF. Tian et al. [45] incorporated CQDs in MnO₂ nanoflower, increasing the surface area, electron transfer speed, and electrical conductivity of the catalyst and used for OER in alkaline medium: with an overpotential of 343 mV and a low Tafel slope of 43.6 mV dec⁻¹, it was a good catalysis. Zhang et al. [46] showed enhanced specific surface area and electric conductivity in a N,S-codoped CQD supported Co₉S₈ nanoparticles (Co₉S₈/CD@NSC) where CQD helped in smooth distribution of Co₉S₈ nanoparticles in the matrix. The obtained Co₉S₈/CD@NSC exhibited an excellent ORR and OER bifunctional catalytic activity and a great long-term durability, with a half-wave potential of 0.84 V vs. reversible hydrogen electrode (RHE) for the ORR and a low potential of 1.62 V versus RHE at 10 mA cm⁻². These results were better than those exhibited by commercial Pt/C and RuO₂ catalysts. Later, they used the Co₉S₈/CD@NSC catalyst for zinc/air battery with low charge/discharge voltage gap of 0.62 V and great cycling stability of over 125 h at 10 mA cm⁻². Zhao et al prepared CQD/SnO₂-Co₃O₄ composite for highly efficient electrochemical water oxidation [47]. Their experimental results showed that the Co atoms were considered as the active center, the nano sized SnO₂ enhanced electronic conductivity and the insoluble CQDs layer on the surface effectively protected the catalyst. CQD also enhanced the electrocatalytic property with surface modification and doped hetero atoms. Tang et al. [48] reported a CQD-NiFe layered double-hydroxide (NiFe-LDH) composite for electrocatalytic water splitting with a high electrocatalytic activity (overpotential of ~235 mV in 1 M KOH at a current density of 10 mA cm⁻²) and stability for oxygen evolution. CQD increased the electroactivity and durability of NiFe-LDH through the bonding and interaction between the CQDs and the pure NiFe-LDH nanoplate. Canton-Vitoria et al. synthesized a CQD/MoS₂ ensemble and used it for HER [49]. The quenching in CD fluorescence by MoS₂ indicated electronic interaction between the two components. Yang et al. [16] prepared a nickel nanoparticle/CQD hybrid for hydrogen evolution in alkaline medium. The obtained hybrid showed good catalytic ability for HER with an onset potential comparable to that of platinum wire and a low Tafel slope of 98 mV dec⁻¹. The good catalytic activity might be attributed to the synergistic interaction between nickel nanoparticles and CQDs at the Ni-O-C interface. Shih et al. [50] designed Cu_2−xS/CQD nanoparticles that was used as cathode electrode material for ORR in direct methanol fuel cells. The ORR was carried out in acidic medium. The synthesized nanocomposite contained Cu₉S₅ and Cu₂S. The catalyst provided a limiting density of −1.77 mA cm⁻² at a scan rate of 5 mV s⁻¹ and rotation rate of 3600 rpm. Wang et al. [51] developed a heteroatom doped CQD-metal sulfide hybrid for HER in acidic medium. They prepared a N,P-doped CQD/CoS₂ hybrid through hydrothermal method, which showed long term stability and low overpotential during the HER. Zhang et al. [52] prepared a CQD/molybdenum phosphide (MoP) nanohybrid for HER in alkaline medium. The CQDs helped to alleviate the agglomeration and surface oxidation of MoP nanoparticles, ensuring that each MoP nanoparticle could be electronically addressed to elevate the electrocatalytic activity of the nanohybrid. The optimized CQDs/MoP hybrid synergistic catalysis toward HER in 1 M KOH electrolyte with a low onset potential of −0.08 V and a small Tafel slope of 56 mV dec⁻¹. The nanohybrid showed high durability with negligible current loss for at least 24 h. Zhao et al. [53] prepared a CQD supported cobalt phosphate (CoPi) nanocomposite for electrocatalytic water splitting. The nanocomposite showed efficient catalysis for OER in both alkaline and neutral medium. The activity was a result of the synergistic interaction within CQD and CoPi. The CQD layer on the CoPi surface averted the CoPi from dissolution of enhancing the stability of the composite electrocatalyst. In neutral condition, the Tafel slope was 59 mV per decade. Cao et al. [54] prepared a N-doped CQD/graphene (N-CQD/G) composite and increased its activity for ORR by combining it with Pt nanoparticles (Pt/N-CQD/G). The composite was advanced with highly distributed Pt nanoparticles with small size, N-doping, and strong interaction between metal and support, as well as the unique structure of N-CQD/G hybrids with abundant edges and doping sites, high surface area, large pore size and high electrical conductivity. The structural advantages reflected in their high activity and durability (the mass activity decreased a ~32% after 5000 cycles). A three component electrocatalyst composed of Co₃O₄-CQD-C₃N₄ was designed by Guo et al. [55] for reduction of carbon dioxide to syngas. The composite was able to control the balance between the HER channel and CO₂ reduction channel and initiated the reaction of CO₂ reduction to CO in aqueous solutions at a low overpotential (0.17V) while the total current density reached up to 15 mA/cm² at a potential of −1.0 V. A sandwich-like electrode with tungsten nitride nanosheets decorated with CQD (N-CDs@WN hybrid) was designed by Zhang et al. [56] for oxygen reduction. The hierarchical structure with large specific area, numerous active sites, and synergistic effect between CQD and WN might facilitate the conductivity and mass transfer during the ORR process. The low Tafel slope of 79 mV dec⁻¹ indicated high catalytic efficiency. Even after 20000s, the catalyst retained ~82.3% of its original ORR performance.

Nevertheless, metal free CQD/carbon nanoparticle composites also showed efficient electrocatalytic activity. Hu et al. reported a N-doped CQD decorated graphene composite for ORR [57]. The graphene was support and CQD provided the active sites. The catalyst showed comparable catalytic activity, durability, stability and selectivity with commercial platinum electrocatalyst during alkaline ORR. Zhang et al. [27] examined the superior behavior of CQD/reduced graphene oxide (rGO) composite compared to CQD doped rGO. They prepared a N,S-dual doped CQD/rGO nanocomposite and analyzed its catalytic activity towards ORR in N₂-saturated and O₂-saturated 0.1 M KOH. The doping of hetero atom played an important role in catalysis. The low Tafel slope (71 mVdecade⁻¹) indicated high catalytic activity. Wang et al. [58] reported that the effect of defect in the CQD/graphene composite helped to achieve high catalytic activity towards ORR. The composite was used as an air cathode material for zinc/air battery. Zhou et al. [59] synthesized a CQD decorated reduced graphene oxide using microorganisms. Both CQD and graphene were doped with hetero atoms; N, S, and P. This hybrid had long term stability and significant catalytic activity towards reduction of oxygen through a four electron pathway in alkaline medium. The catalytic property was as a result of the combined effect of heteroatom doping and numerous surface/edge defects that induced the high-density distribution of highly active and stable catalytic sites. Again, the strong coupling effects between CQDs and graphene facilitated the charge transport and ionic motion during the reduction of oxygen. The electrochemical studies and fabrication process of the catalyst are shown in Fig. 5.

Niu et al. [60] prepared a nitrogen rich CQD supported graphene oxide for reduction of oxygen in alkaline medium. The as-prepared hybrid showed positive onset potential (0.13 V vs. Ag/AgCl) and high kinetic current density (up to 18.4 mA cm⁻² at −0.70 V) comparable to commercial Pt/C catalyst during ORR. A N-doped CQDs supported carbon nanosheet (N-CNA) particle was used as electrocatalyst for ORR in an O₂-saturated 0.1 M KOH solution by Zhang et al. [61]. The N-CNA showed a small over potential of −0.08 V that could be compared to intrinsic low over potential for ORR. The catalytic activity was comparable to commercial Pt/C. The wide current plateau within a wide potential range indicated high performance of the N-CNA electrocatalyst. Another synergistic interaction within N-doped CQD and graphene was reported by Liu et al. [62], showing that the synergistic interaction within CQD and graphene oxide boosted the catalytic activity and the doping could modify the charge density on the electrocatalyst and weaken the adsorption of reaction intermediates (O and OOH). The composite was used for ORR process, through a four-electron way mechanism, with an onset potential of −95 mV and kinetic current density of 12.7 mA cm⁻² at −0.7 V (vs. Ag/AgCl). The results indicated good catalytic behavior. The catalyst also possessed significant stability (~88% retention of current density even after 40000 s), durability and methanol tolerance.

Colloidal GQDs are new addition to 0-D carbonaceous nanoparticles. GQDs with high electrical conductivity and active sites have been used for electrocatalysis. Li et al. [63] applied solution chemistry approach for the synthesis of N-doped GQDs and explored their size with respect to their catalytic property for ORR in 0.1M KOH. The doping of hetero atom and size of the catalyst played a major role. The linear sweep voltammetry showed that the N-doped GQDs have high, size-dependent electrocatalytic activity for the ORR They prepared three kinds of GQDS (1, 2 N-doped and 3-undoped) and demonstrated via both experimental and theoretical data, that the size and abundance of nitrogen atom with specific bonding configuration have an important role in the catalysis. Figure 6 shows the results of electrochemical studies and the structure of GQDs.

Yan et al. prepared GQDs from graphene flakes for ORR [64]. They found that the GQD catalyst showed a high electrocatalytic activity for the ORR, with a 70 times enhancement in voltammetric current in oxygen saturated KOH compared to that in N-GQD. The ORR onset potential was −0.25 V with the reduction peak at −0.45 V. The ORR followed a two-electron mechanism. Guin et al. [65] used green luminescent GQDs as impending electrocatalyst for reduction of K₃[Fe(CN)₆]. The standard rate constant of this heterogeneous reaction was 2.69 × 10⁻³ cm s⁻¹ within a scan rate of 25–300 mV s⁻¹. They proposed that the edge functionalities and the basal graphitic π-units facilitated the outer-sphere heterogeneous electron transfer kinetic of the bench-mark redox mediator, which could be further used for various electrochemical applications. Favaro et al. [66] synthesized singly and doubly doped graphene oxide quantum dots with N and B hetero atoms for ORR. Through DFT calculation, they determined that the selectivity of the reaction is controlled by the oxidation states of the dopants. The as-prepared GQDs follow a two-electron reduction path that leads to the formation of hydrogen peroxide, whereas after the reduction with NaBH₄, the same materials favor a four-electron reduction of oxygen to water.

The electronic interaction between GQD and metallic nanoparticles results in the modification of the catalytic properties of the metals and makes GQD an active support. Deming et al. [67] prepared a Pd nanoparticle supported N-doped GQD for the electroreduction of oxygen in alkaline medium. The catalyst worked better than Pd supported on undoped GQDs and Pt/C. The Pd atoms had a strong interaction with the pyrrole ring. The p-type pyridinic and pyrrolic N-centers in N-doped GQDs increased the catalytic activity in terms of manipulating the Pd-oxygen intermediate. The highest specific current density of 25.4 A/m² and best Tafel slope of 120 mV/dec were obtained for the nitrogen centers felicitated charge transfer and provided optimal binding condition within metal surface and oxygenated intermediates. Deming et al. also reported about Pd supported GQD nanocatalyst [68]. Voltammetric studies showed apparent electrocatalytic activity toward oxygen reduction in alkaline medium. The peak activity of the sample was attributed to the optimal interactions between Pd and GQD, where the GQD defects promoted charge transfer from metal to GQDs and weakened interactions with oxygenated intermediates, leading to enhanced ORR activity. The corresponding defect concentration was higher than that identified with the platinum counterparts due to the stronger affinity of oxygen to palladium. The best Tafel slope was 71.6 mV/dec and the highest kinetic current density of 1.08 A/m² at + 0.90V for optimal interactions between the GQD structural defects and the palladium nanoparticles was recorded. He et al. [69] prepared nanosized GQDs through acid etching of carbon fibers and used these as effective substrate-supports for platinum nanoparticles (Pt/G), synthesized by thermolytic reduction of platinum (II) chloride in ethylene glycol. The as-synthesized Pt/G particles showed enhanced catalytic activity during reduction of oxygen, with an onset potential of +1.05 V. This result was 70 mV more positive than that of Pt/C, and a specific activity almost nine times higher, showing better activity of Pt/G over commercial Pt/C catalysts. These results were ascribed to the abundant structural defects of the nanosized GQDs that manipulated the dissociative adsorption of oxygen and the binding of reaction intermediates O* and HO* on platinum surfaces. Another GQD supported platinum nanocatalyst (Pt/G) was prepared by Song et al. [30] for ORR. The electrocatalytic performance was accounted by the deliberate manipulation of the adsorption of oxygen and reaction intermediates on platinum by the GQD structural defects through partial charge transfer. Pt-GQD catalysts were prepared at different conditions, however, materials at two reaction conditions (160°C for 12 h and 180°C for 6 h) exhibited maximum efficiencies (kinetic current densities of 29.2 and 24.6 A/m² at + 0.90 V, respectively). Apart from platinum noble metal, gold has also been used for catalysis due to its lower cost and effective catalytic activity. The activity of gold nanoparticles with GQD support has been explored by Luo et al. [70]. They prepared GQD/gold nanoparticle hybrid (Au@GQDs core-shell nanostructure with an average diameter of 15.6 nm) and used it for catalytic HER in 0.5 M H₂SO₄ medium. The Tafel slopes were 78 and 75 mV/dec, respectively for Au@GQD and Au nanoparticles, but the former had a lower over potential compared to latter. The higher activity of the composite was attributed to the strong chemical coupling/interactions between components GQD and Au nanoparticles. The strong GQDs coating layer at Au surface prevented the aggregation of Au nanoparticles and boosted the activity. Liu et al. synthesized a Cu₂O/CuO-GQD composite for alkaline electroreduction of oxygen [71]. The Cu/C ratio played an important role during catalysis. The comparison of the activity of Cu/GQD nanocomposite with Pt/GQD and Pt/C showed that the composite had the lowest onset potential towards ORR. Non-noble metal based GQD catalysts also showed enhanced catalytic activity. Koh et al. conjugated iron (II) phthalocyanine (FePC) with GQDs and used the catalyst for ORR in alkaline medium [72]. The catalyst also showed potential tolerance against methanol and carbon dioxide. The FePC-GQD catalyst followed a four-electron path and reduced oxygen directly to OH⁻ in alkaline electrolyte, with the Tafel slope of the reaction being 23 mV dec⁻¹. When GQD followed a two-electron transfer process yielding HO₂⁻, the Tafel slope was 28.9 mVdec⁻¹, indicating the four-electron mechanism to be the preferable path. A GQD loaded and molybdenum doped porous Ni₃S₂ hybrid (G-Mo-Ni₃S₂) was in situ grown on Ni foam by GQDs and was used for overall water splitting by Li et al. [73]. The in situ coupling of GQD with metal hybrid modified the electronic behavior of the catalyst and exposed more active sites for reaction. This boosted the overpotential of HER and OER by 68 mV at 10 mA cm⁻² and 326 mV at 20 mA cm⁻² in 1.0 M KOH, respectively. The two-electrode cell, using G-Mo-Ni₃S₂, was constructed to deliver low cell voltage of 1.58 V at water-splitting current of 10 mA cm⁻² with a significant stability for more than 50 h. Chen et al. used GQDs to enhance the activity of Bi₂O₃ nanosheet for reduction of CO₂ to formate [74]. The as prepared Bi₂O₃-NGQDs demonstrated a nearly 100% formate Faraday Efficiency (FE) at a moderate overpotential of 0.7 V with a good stability and a high activity. The DFT calculation suggested that the increase in activity could be attributed to better adsorption of reaction intermediate at the GQD surface. In Figure 7, the experimental results and schematics of synthesis are documented.

Vinoth et al. [75] decorated N-doped GQDs (N-GQDs) on a 3D MoS₂-reduced graphene oxide (rGO) framework via a facile hydrothermal method for ORR. They concluded that the chemical interaction between the electroactive N-GQDs and MoS₂-rGO increased surface area and pore size of the N-GQDs/MoS₂-rGO nanohybrid synergistically, which in turn improved the ORR onset potential to +0.81 V vs reversible hydrogen electrode (RHE). Moreover, the N-GQDs/MoS₂-rGO nanohybrid showed better ORR stability for up to 3000 cycles with negligible deviation in the half-wave potential. Lv et al. [76] fabricated a noble metal-free electrocatalyst via in situ formation of nanocomposite of N-doped GQDs (NGQDs) and Ni₃S₂ nanosheets on the Ni foam (Ni₃S₂-NGQDs/NF). The resultant Ni₃S₂-NGQDs/NF served as an active, binder-free, and self-supported catalytic electrode for direct water splitting. At a current density of 10 mA cm⁻², the overpotential was 216 mV for OER and 218 mV for HER in alkaline media. This bifunctional electrocatalyst enabled the construction of an alkali electrolyzer with a low cell voltage of 1.58 V vs. reversible hydrogen electrode (RHE) at 10 mA cm⁻².

Apart from GQD metal/metal oxide composites, metal free GQD/carbon nanoparticle composites were also investigated for different electrocatalytic reactions. Luo et al. synthesized a GQD supported N,S-co doped reduced graphene oxide (N,S-RGO/GQDs) by microwave treatment for ORR in alkaline medium [77]. The doping enhanced the catalytic activity. The onset potential, reduction peak, and the limiting current of N,S-RGO/GQDs hybrids potential of were −0.10 V and −0.25 V, and 4.53 mA cm⁻² (at −0.8 V a rotary speed of 1600 rpm), respectively. These results showed that N,S-RGO/GQDs have good catalytic activity and are potential metal-free catalyst for the ORR in an alkaline solution through a four electron mechanism. Yao et al. prepared S-doped GQD decorated carbon nanoparticles (S-GQDs/CNPs) from dehydration of fructose and H₂SO₄ for ORR [78]. The low reduction peak at −0.396 V and high peak current density of −2.02 mA/cm² showed the efficiency of the composite. From the experimental data, the layer by layer modified method could inhibit the occurrence of a two-electron pathway and cause a four-electron pathway for ORR.

The S-GQDs/CNP nanohybrid showed better methanol tolerance and long-term durability than commercial 20% Pt/C. Wang et al. [79] reported synergistic within GQD and graphene hydrogen composite during electroreduction of oxygen. The nanocomposite exhibited fine ORR activity and durability in alkaline solution, which showed a comparable discharge performance of zinc/air batteries with Pt/C under a current density of 20 mA cm⁻². The high performance was attributed to the rich edge defects present at the composite surface. Graphene nano ribbons (GNR) were also reported to get support from GQD and perform as excellent electrocatalyst for oxygen reduction. Jin et al. [80] prepared a GQD/GNR composite for ORR in 0.1 M KOH medium, with the numerous surface/edge defects on the GQD/GNR surfaces and at their interface acting as the active sites. This, coupled with efficient charge transfer between the intimately contacted GQDs and GNRs, rationalized the observed electrocatalytic performance. The Tafel slope was measured to be ~87.7 mV per decade, indicating high efficiency. The catalyst showed significant stability over 26 hrs. Hasanzadeh et al. prepared a GQD functionalized by chitosan (CS) and beta-cyclodextrin (β-CD) as a new support nanocomposite material for efficient methanol electrooxidation [81]. The anodic peak currents showed a linear relationship with the square root of the scan rate for characteristic diffusion-controlled process. The overall electron transfer coefficients were 0.44 and 0.5 while the Tafel slopes were 0.7 mV and 0.63 V for CS and β-CD support, respectively towards methanol oxidation. The observed peak current values indicated the essential roles of CS and β-CD in the observed electrocatalytic behavior. Zhou et al. prepared another metal free electrocatalyst for reduction of oxygen [82]. The synergistic effect of GQD and multi walled carbon nanotube (MWCNT) was used for the catalysis in O₂-saturated 0.1 M KOH solution. The GQD intake was optimized by considering the ratio of lattice and adsorbed oxygen in the composite. At optimized ratio of GQD-MWCNT, the ORR peak was observed at −0.25 V. The highest current density was obtained as 3.47 mA cm⁻². Fei et al. synthesized a B,N-doped GQDS/Graphene hybrid nanoplatelets for electrocatalysis of oxygen [83]. The electrochemical studies and the synthetic protocol are shown in Fig. 8.

The GQD particles self-assembled on graphene and the hybrid material combined the advantages of both components, such as abundant edges and doping sites, high electrical conductivity, and high surface area, making the resulting materials excellent oxygen reduction electrocatalyst with activity even higher than that of commercial Pt/C in alkaline media. The optimum activity of the catalyst was obtained by tuning the concentration of doped atoms. The onset potential of composite at optimized condition, measured from the rotating disk electrode was ~15 mV more positive than of Pt/C. In addition, the diffusion-limited current density and current density of the composite was larger than that of Pt/C within the range of 0.20 to −0.15 V at the same mass loading, suggesting the higher ORR activity of the composite than Pt/C. For the four-electron transfer mechanism, the kinetic current density was obtained as 11.1 mA cm⁻² for the composite.