Molybdenum single-atom-bridged ReS2–graphene heterostructures for boosting supercapacitor performance

Journal Pre-proof Molybdenum single-atom-bridged ReS2–graphene heterostructures for boosting supercapacitor performance Mahima Khandelwal, Ievgen Obraztsov, Aby Cheruvathoor Poulose, Shashank Sundriyal, Gabor Ersek, Oleg Usoltsev, Laura Simonelli, Giuseppe Portale, Radek Zbořil, Aristides Bakandritsos PII: S1385-8947(25)06636-7 DOI: https://doi.org/10.1016/j.cej.2025.165798 Reference: CEJ 165798 To appear in: Received date: 23 April 2025 Revised date: 26 June 2025 Accepted date: 7 July 2025 Please cite this article as: M. Khandelwal, I. Obraztsov, A.C. Poulose, et al., Molybdenum single-atom-bridged ReS2–graphene heterostructures for boosting supercapacitor performance, (2024), https://doi.org/10.1016/j.cej.2025.165798 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. 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Journal Pre-proof Journal Pre-proof 1 Molybdenum Single-Atom-Bridged ReS2–Graphene Heterostructures for Boosting Supercapacitor Performance Mahima Khandelwal1*, Ievgen Obraztsov1, Aby Cheruvathoor Poulose1, Shashank Sundriyal1, Gabor Ersek2, Oleg Usoltsev3, Laura Simonelli3, Giuseppe Portale2, Radek Zbořil,1,4* Aristides Bakandritsos1,4* 1Regional Center of Advanced Technologies and Materials, The Czech Advanced Technology and Research Institute (CATRIN), Palacký University Olomouc, Slechtitelu 27, 779 00 Olomouc, Czech Republic 2Zernike Institute for Advanced Materials, University of Groningen, Nijemborg 3, 9747, Groningen, the Netherlands 3ALBA Synchrotron Light Facility, Carrer de la Llum 2-26, 08290, Cerdanyola del Vallès, Spain 4Nanotechnology Centre, Centre for Energy and Environmental Technologies, VSB−Technical University of Ostrava, 17. listopadu 2172/15, 708 00, Ostrava Poruba, Czech Republic *Correspondence: [email protected] (Dr. Mahima Khandelwal); [email protected] (Prof. Radek Zboril); [email protected] (Dr. Aristides Bakandritsos) Abstract Rhenium disulfide (ReS2), a two-dimensional transition metal dichalcogenide (TMD), possesses weak interlayer interactions and tunable band gap, however its limited charge storage and cycling stability hinder its application in supercapacitors (SCs). This work demonstrates a paradigm shift in the performance of ReS2 by introducing molybdenum (Mo) single-atom reactive centers, leading to strong interfacial coupling with a nitrogen-doped graphene (GN3). Notably, Mo-nitrogen bonds are formed enhancing charge storage in the Mo-ReS2@GN3 (MRG2) heterostructure. The incorporation of Mo also induces p-type doping (beneficial for coupling with the n-type GN3), lowers the band gap, and enhances the activity of the in-plane sulfur Journal Pre-proof Journal Pre-proof 2 atoms. The MRG-2 electrode delivered 87% and 31% increase in capacitance compared to pristine ReS2 and Mo-ReS2, respectively, along with a four-fold enhancement in electrical conductivity. An asymmetric SC cell using MRG-2 as the negative and NiCo2S4/graphene as the positive electrode achieved exceptional gravimetric (54.3 Wh kg−1 at 1.0 kW kg−1), volumetric (110.5 mWh cm−3 at 0.940 W cm−3), and areal (54.3 µWh cm−2 at 1.0 mW cm−2) energy densities, and remained stable for at least 10,000 cycles. These findings establish interfacial single-atom engineering as a transformative approach to optimizing TMD-based heterostructures for next-generation energy storage technologies. Keywords: 2D transition metal dichalcogenides; single-atom engineering; interfacial coupling; N-doped graphene; supercapacitors Introduction Electrochemical energy storage has gained significant attention due to the increasing demand for sustainable and efficient energy solutions. Among various energy storage technologies, supercapacitors (SCs) stand out due to their rapid charge/discharge cycles, high power density, and exceptional longevity. However, their low gravimetric and volumetric energy density remains a significant challenge, limiting their applicability in energy-demanding applications.[1,2] To address this issue, extensive research efforts have been directed toward the effective tuning of the physicochemical properties of structurally tailored materials, and on the efficient integration thereof, such as in electrodes based on graphene, transition metal dichalcogenides (TMDs), oxides or sulfides, MXenes, layered double hydroxides, covalent and Journal Pre-proof Journal Pre-proof 3 metal-organic frameworks (COFs and MOFs).[3–12] The development of such materials is crucial not only for supercapacitors, but also in energy conversion processes, such as in water splitting, fuel cells, alcohol oxidation, and zinc-air batteries.[13–16] Two-dimensional (2D) TMDs have gained substantial attention in SC research, owing to their rich redox chemistry, unique layered structures, and 2D permeation channels that facilitate rapid ion diffusion.[17–19] Among TMDs, MoS2 can be stabilized in the conductive 1T phase and intercalate ions with extraordinary efficiency, delivering a volumetric energy density of 16 mWh cm−3 at a power density of 620 mW cm−3 in aqueous electrolytes.[5] Quantum dots of MoS2 rich in defects demonstrated 0.55 mWh cm−3 at 3 W cm−3.[20] Oxygen-incorporation in the mixed 1T/2H MoS2 phase and growth on graphite foil increased the interlayer spacing and conductivity, delivering a gravimetric energy density of 39.7 Wh kg−1 at 450 W kg−1.[21] The usually decoupled interfaces between the inorganic electroactive species and their conductive counterparts (e.g., carbon black, graphene) is one of the bottlenecks in electrochemical processes.[7,22] Therefore, heterostructures are developed with the aim to improve the charge transfer at the phase boundaries.[22–25] MoS2 grown on reduced graphene oxide (rGO), delivered 0.78 mWh cm−3,[22] while the 1T/2H MoS2 grown on pristine graphene reached 46.3 mWh cm−3.[23] Moreover, the 1T-MoS2 was grown on Ti3C2 to leverage the excellent conductivity of MXenes, which exhibited an areal energy density of 17.4 µWh cm−2 at 0.6 mW cm−2.[24] Rhenium disulfide (ReS2) is a relatively newer member in the family of 2D TMD materials[26] with unique properties, such as thermodynamically stable distorted 1T (1T’) phase with inherent anisotropic structure leading to an extremely weak interlayer coupling (18 meV for ReS2 vs. 460 meV for MoS2) and large interlayer spacing.[27] Moreover, the rich density of states of ReS2 around the Fermi level shows that the band structure can be drastically modulated.[28] Journal Pre-proof Journal Pre-proof 4 These characteristics enhance ion diffusion between layers, and facilitate access to electrochemically active sites.[27,29] However, ReS2 has a band gap of ca. 1.43 eV, leading to limited electronic conductivity, similar to other TMDs [26,27] The electrochemical activity of the in-plane sulfur atoms is low due to their limited contribution to the density of states near the Fermi level, which is mainly populated by the orbitals of the edge sulfur atoms.[30] Importantly, effective coupling between ReS2 and conductive carbon counterparts has not yet been achieved, which is critical for fully exploiting the active material’s electrochemical properties. As a result, so far, ReS2 as a SC electrode material has demonstrated 25.3 Wh kg−1 at 1.0 kW kg−1, even in a high voltage organic electrolyte with 95.6% capacitance retention after 5,000 cycles.[31] In aqueous electrolyte, 2D ReS2 nanosheets embedded in a porous carbon framework delivered 4.2 Wh kg−1 at 0.5 kW kg−1, retaining 91.5% capacitance after 6,000 cycles.[32] A symmetric SC based on a ReS2/MoS2 2D heterostructure achieved an areal energy density of 1.94 µWh cm−2 at 250 µW cm−2, maintaining 66.5% capacitance after 10,000 cycles.[33] Additionally, ReS2 combined with conducting black phosphorous demonstrated 4.26 µWh cm−2 but at lower power density of 41 µW cm−2.[34] Despite the intrinsic structural advantages of 2D 1T’ ReS2, its performance still lags behind leading systems in the field, as shown by recent literature (Tables S1 and S2).[7,8,35] Fully realizing ReS2’s electrochemical potential requires enhanced charge transfer, higher intrinsic activity, and strong interfacial coupling with a conductive counterpart. On this basis, we sought to modulate the structure of 2D 1T’ ReS2 by incorporating molybdenum (Mo) single-atoms, which significantly enhance conductivity through pronounced band-gap reduction.[28] Mo incorporation also boosts the contribution of the in-plane sulfur (S) atoms to the density of states around the Fermi level, rendering them as reactive as the edge S atoms in ReS2.[30] Moreover, Mo-ReS2 is a p-type semiconductor, which is particularly beneficial Journal Pre-proof Journal Pre-proof 5 for strong interlayer coupling with n-doped systems,[34,36] such as nitrogen-doped graphene. To exploit this synergy, a structurally and chemically tailored nitrogen-superdoped (16 at.%) graphene derivative (GN3)[37] was strongly coupled with Mo single-atom engineered ReS2 (MoReS2), affording for the first time a highly electrochemically active Mo-ReS2@GN3 heterostructure. Extended X-ray absorption fine-structure spectroscopy (EXAFS) revealed the single-atom state of Mo and its remarkable coordination with the nitrogen (N) functionalities of GN3. The Mo-ReS2@GN3 electrode demonstrated an 87% improvement in capacitance compared to pristine ReS2 and a four-fold enhancement in conductivity. When integrated into an asymmetric SC (ASC), the device delivered an outstanding gravimetric (54.3 Wh kg−1 at 1.0 kW kg−1), and volumetric (110.5 mWh cm−3 at 0.940 W cm−3) energy density. The energy density was retained as high as 47.9 mWh cm−3 even at 37.7 W cm−3, with excellent long-term cycling stability (96.7% after 10,000 cycles). The areal density was also particularly high, reaching 54.3 µWh cm−2 at 1.0 mW cm−2. This Mo-ReS2@GN3 based ASC outperformed not only previously reported ReS2-based SCs, but also other 2D TMDs, as well as top-tier materials from the broader family of transition metal sulfides,[7,8,38] and advanced SC electrode materials (Tables S1 and S2).[6,9,10,39] This work introduces a previously unexplored approach, of Mo single-atom engineering in 1T’ ReS2 as a transformative strategy to mitigate the intrinsic limitations of low conductivity and electrochemical activity. Strong coupling with conductive, nitrogen-doped graphene fundamentally reshapes interfacial interactions and unlocks a previously unattainable level of charge-storage performance. Journal Pre-proof Journal Pre-proof 6 Results and Discussions Mo single-atoms were successfully incorporated into the 2D ReS2 network, and seamless coupling with GN3 was achieved through a facile hydrothermal route for 24 h at 240 °C (Figure 1a). The metal precursors used were ammonium perrhenate (NH4ReO4) and ammonium heptamolybdate ((NH4)6Mo7O24), while thiourea served as the sulfur source, enabling the formation of 2D Mo-ReS2 nanosheets. Initially, three Mo-ReS2 samples were synthesized (MR1, MR-2, and MR-3) by varying the mass ratio(s) of NH4ReO4 to (NH4)6Mo7O24 (Table S3). Of the three, the optimally doped sample (Mo0.22Re0.78S2; designated as MR-2), exhibited the highest capacitance and the lowest impedance (Figure S1; details discussed therein). This composition was obtained by using a precursor mass ratio of NH4ReO4 to (NH4)6Mo7O24 equal to 5.5 in the reaction (Table S3). This optimum ratio was employed for the in-situ growth of MR-2 in the presence of selectively and densely functionalized graphene derived via fluorographene chemistry i.e., highly conductive nitrogen-doped graphene (GN3). After testing three different GN3 loadings, the optimal Mo0.22-Re0.78S2/GN3 ratio was identified according to capacitance and impedance measurements (Figure S2, sample name MRG-2). The MRG-2 heterostructure contained 9.2 wt.% GN3. The X-ray diffraction (XRD) patterns of the synthesized pure 1T’ ReS2 (designated as PR), the Mo-doped system (MR-2), and the heterostructure with GN3 (MRG-2) exhibit main diffraction peaks at 14.3°, 32.5°, 43.7°, and 57.5°, corresponding to the (002), (-220), (210), and (-422) crystal planes, respectively, confirming the formation of 1T’ ReS2 (JCPDS 89-0341, Figure 1b).[40,41] The XRD reflection at 14.3° corresponds to a d-spacing of 0.61 nm, attributed to the (002) crystal plane of PR. The same pattern was observed in MR-2 and MRG-2, confirming the crystal structure of ReS2 after Mo doping and GN3 coupling, with no formation Journal Pre-proof Journal Pre-proof 7 of secondary phases or impurities. Wide-angle X-ray scattering (WAXS) measurements revealed a slight reduction of 0.10 Å in the interatomic distance of the (002) plane in MRG-2 compared with PR, as expected since Mo induces stronger interlayer interactions and makes exfoliation more difficult (Figure S3).[27] The Raman spectrum of PR featured two characteristic peaks at 147.2 cm−1 and 207.4 cm−1 assigned to the in-plane (Eg) and out-of-plane (Ag) vibration modes of 1T’ ReS2 (Figure 1c).[41–44] However, MR-2 showed slightly blue-shifted Ag and Eg vibrational modes by 2–3 cm−1 due to p-type Mo doping in 1T’ ReS2, attributed to electronphonon coupling in TMDs.[44,45] The p-type Mo doping in ReS2 enhances hole carrier density, enabling interaction with incoming photons and causing a Raman shift toward higher frequencies, as similarly observed in chemical vapor deposited Mo-doped ReS2 films[44] (Figure 1c). Similar Raman shifts were also observed in MRG-2, along with the presence of D and G bands from GN3 (Figure S4). The G band in MRG-2 was redshifted by 7 cm−1 compared to GN3, indicating strong electronic interactions and charge transfer from N atoms of GN3 to Modoped ReS2 (Figure S4), similar to observations in graphene quantum dots acting as charge transfer bridge in MoSe2 heterostructures.[46] The Fourier transform infrared (FTIR) spectroscopy (Figure S5) exhibited bands at 1110 and 1417 cm−1 corresponding to Re-S bonds[47] in MR-2, which shifted to lower energy vibrations (1082 and 1402 cm−1 in MRG-2), indicating a weakening of the Re-S bonds due to the changes in the coordination environment. Additionally, a new band appeared in MRG-2 at around 1560 cm−1 (C=C, sp2),[37] corresponding to the aromatic carbons in GN3. The small-angle X-ray scattering (SAXS) measurements demonstrated increased scattering in the low-q region of MRG-2 compared to PR, suggesting the development of a more hierarchical and interconnected porous network in the MRG-2 heterostructure (Figure 1d). Journal Pre-proof Journal Pre-proof 8 Figure 1. (a) Schematic illustration of the synthesis of MRG-2 showing the Mo single metal atoms incorporated into the ReS2 sheets, and their covalent interactions with N atoms in GN3. The Mo-N coordination illustrated here is confirmed by EXAFS analysis (vide infra). The energy density values of the supercapacitor are based on the electrochemical results discussed later (see Figure 6). (b) XRD and (c) Raman measurements of PR, MR-2, and MRG-2. (d) SAXS measurements of PR and MRG-2. The scanning electron microscopy (SEM) image of PR revealed the formation of 2D nanosheets assembled into flower-like arrangement (Figure 2a). The sheet-like morphology of PR remained intact after the incorporation of Mo single metal atoms and seamless coupling with GN3 (Figure 2b, c). Transmission electron microscopy (TEM) of PR also showed nanosheets 125 150 175 200 225 250 Intensity (a.u.) Raman shift (cm-1) PR MRG-2 MR-2 Ag Eg 10 20 30 40 50 60 70 Intensity (a.u.) 2 Theta (degree) PR MR-2 (-422) (210) (-220) MRG-2 (002) 0.1 1 10-1 100 101 102 103 104 105 106 Intensity (a.u.) q (nm-1) MRG-2 PR a bcd Journal Pre-proof Journal Pre-proof 15 The Fourier-transformed (FT)-EXAFS spectrum of MRG-2 shows a dominant peak at around 2.0 Å, similar to the distances observed in MoS2 (Figure 3g), corresponding to the primary Mo-S bonding (Figure 3g and S9a).[56] Its asymmetry toward lower R is compatible with the coexistence of Mo-N coordination bonds (Figure 3h). It should be noted, however, that it is difficult, if not impossible, to distinguish between Mo-N, Mo-C, and Mo-O coordination bonds (Figure 3h and S9b, c). The k3-weighted FT-EXAFS spectrum of MRG-2 shows no second-shell contribution at ≈2.5 Å, indicating the absence of Mo–Mo or Mo–Re shells (Figure 3i) and confirming that Mo exists as single-atom sites coordinated to S and likely to a lighter element—probably N—within the 2D MRG-2 heterostructure. To further quantify the results, the EXAFS signals were fitted using the standard equation in the single scattering approximation,[57] (Table S4 and Figure 3j and S10). Fits for Mo foil and MoS2 references converged quickly with the experimental data (Figure S10).[57] However, the EXAFS fit of the MRG-2 sample with the Mo-S shell did not yield sufficient agreement with experimental data. The inclusion of Mo-N bonds dramatically improved the fit agreement (Figure 3j), confirming the previously speculated presence of Mo-N coordination bonds. The MRG-2 hybrid system can be represented considering a 6-coordinated Mo atom bonded to S (91%) and N (9%) atoms. Electrochemical Measurements The electrochemical performance of the MRG-2 hybrid and its individual counterparts was evaluated in a three-electrode cell using 3 M KOH aqueous electrolyte within a potential range of -1.0 to -0.3 V vs. Ag/AgCl. The comparative cyclic voltammetry (CV) curves for PR, MR-2, GN3, and MRG-2 at a scan rate of 50 mV s−1, showed the largest area under the CV curve Journal Pre-proof Journal Pre-proof 16 for MRG-2 (Figure 4a). The galvanostatic charge discharge (GCD) profiles recorded at a current density of 1 A g−1 also displayed the longest discharge time for the MRG-2 sample (Figure 4b). Evidently, MRG-2 exhibited the highest specific capacitance (Csp) calculated from the GCD curve (228.6 F g−1), followed by MR-2 (175 F g−1), GN3 (161.5 F g−1), and PR (122 F g−1), all at 1 A g−1 (Figure 4b). The Csp values in all cases were calculated from the GCD curves with the non-linear equation 1, provided in the SI. The Csp decreased with increasing current density, typically attributed to insufficient time for electrolyte ions to penetrate into the pores of the electrode material (Figure 4c).[58] In view of the coordination bond observed between Mo in ReS2 and N in GN3 from the EXAFS (Figure 3), we hypothesized that using a different type of graphene (such as rGO) without N atoms, would result in inferior properties. Thus, a control sample of 2D Mo-ReS2 grown in-situ on rGO was synthesized under the same experimental conditions as MRG-2. This material (MR-rGO) exhibited a reduced area under the CV curve (Figure S11a) and a 20% decrease in Csp compared to MRG-2 at 1 A g−1 (Figure S11b). The equivalent series resistance (Rs) values obtained from the Nyquist plots derived from electrochemical impedance spectroscopy (EIS, Figure S11d) indicated a lower Rs for MRG-2 (0.81 Ω) compared to MRrGO (0.99 Ω). Furthermore, the steeper slope of the curve in the low-frequency region for MRG2, compared to that of the MR-rGO hybrid, demonstrated a very effective charge-transport profile within the bulk of the electrode. These control experiments unveiled the significance of the Mo-N coordination bonds between the Mo in ReS2 and N in GN3, leading to the significantly enhanced charge storage properties of MRG-2. In addition to the strong coupling between MoReS2 and GN3 through the Mo-N bond, other factors such as weak interlayer interactions in ReS2 compared to MoS2 and the improved electronic conductivity due to Mo p-doping are also crucial Journal Pre-proof Journal Pre-proof 17 for enhancing the charge storage properties. For example, the control MoS2-GN3 hybrid sample achieved merely a 25.8% increase in Csp over pristine MoS2 (Figure S12), in contrast to the 87% increase in Csp between the PR and MRG-2, clearly highlighting the importance of precisely controlling the presence of Mo only as single-atoms and at a certain content (Figure S1). Furthermore, the experimentally measured electronic conductivity of MR-2 was two-fold higher than that of PR (Figure S13). These observations are also supported by theoretical calculations[44] of the band-structure changes upon Mo-doping of ReS2 because: i) the Fermi level shifts downward into the valence band,[44] with Mo filled d-orbitals populating the nearFermi density of states[42] and thus improving the interlayer coupling of the p-doped Mo-ReS2 with the n-doped GN3 systems,[34,36] ii) the band gap of ReS2 decreases substantially upon Modoping, leading to an increase in conductivity.[28] This highlights the importance of establishing covalent coordination bonds (Mo-N), which significantly enhance the charge storage performance of the MRG-2 hybrid, resulting in an 87% improved Csp compared to PR. The Nyquist plot of MRG-2 showed the lowest Rs of 0.81 Ω compared with PR (1.32 Ω), MR-2 (1.23 Ω), and GN3 (1.02 Ω). The low Rs value for the MRG-2 electrode material indicates faster electronic and ionic transport and reduced interfacial resistance in the electrode material, manifesting the strong coupling between GN3 and Mo0.22Re0.78S2. Additionally, the absence of a semicircle in the high-frequency region demonstrates very low charge transfer resistance at the electrode/electrolyte interface. Meanwhile, the steeper slope for the MRG-2 hybrid compared to PR and MR-2 in the low frequency region suggests a more efficient ion diffusion process within the bulk of the electrode material. The EIS data, presented as a Bode impedance plot (Figure S13d), further indicate strong coupling between GN3 and Mo-ReS2, thereby enhancing the conductivity of the MRG-2 heterostructure, as evidenced by (i) a downward shift of the high- Journal Pre-proof Journal Pre-proof 18 frequency Re(Z) plateau in the Bode magnitude plot (Figure S13d, bottom panel); (ii) the higher phase angle for the MRG-2 across a broader intermediate frequency range, indicating minimal resistive losses; (iii) a shift of the peaks in the phase angle curve towards higher frequencies in the MRG-2 > MR-2 > PR order (Figure S13d, upper panel). The reduction in electronic resistance within the bulk of the electrode was further corroborated by the enhanced electronic conductivity of the MRG-2 hybrid, measured by the four-probe method (Figure S13). The MRG-2 (6.1 S m−1) exhibited more than four-fold and two-fold higher electrical conductivity than PR (1.5 S m−1) and MR-2 (3.01 S m−1), respectively (Figure S13). Figure 4. Electrochemical measurements of PR, MR-2, GN3, and MRG-2 in an open cell, three electrode configuration using 3 M KOH (a) Comparative CV curves of PR, MR-2, GN3, and MRG-2 at a scan rate of 50 mV s−1. (b) GCD curves of PR, MR-2, GN3, and MRG-2 at a current density of 1 A g−1. (c) Specific capacitance as a function of current density for PR, MR-2, GN3, and MRG-2. (d) Nyquist plots of PR, MR-2, GN3, and MRG-2 samples. The inset is a zoom in the high frequency region from where the ESR is determined. -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -15 -10 -5 0 5 10 Current density (A g-1) Potential (V vs. Ag/AgCl) MRG-2 MR-2 GN3 PR 0 5 10 15 20 25 30 0 5 10 15 20 25 30 -Z'' (Ω) Z' (Ω) PR MR-2 GN3 MRG-2 MRG-2 MR-2 GN3 PR 050 100 150 200 250 300 350 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 Potential (V vs. Ag/AgCl) Time (s) 0 2 4 6 8 10 0 50 100 150 200 250 MRG-2 MR-2 GN3 PR Specific capacitance (F g-1) Current density (A g-1) ab c 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0 0 -Z'' (Ω) Z' (Ω) d Journal Pre-proof Journal Pre-proof 19 The CV curves of the MRG-2 hybrid at different scan rates displayed a nearly rectangular shape (Figure 5a), which was well-maintained even at higher scan rates, indicating good rate capability. Furthermore, the nearly symmetrical triangular GCD profiles of MRG-2 at different current densities suggest capacitive behavior and good rate capability (Figure 5b). To gain insight into the operational mechanism of the MRG-2 electrode material, electrochemical kinetics were evaluated from the CV curves recorded at different scan rates (Figure 5a), using a power law analysis. The b-value was calculated from the slope of the log-log plot of current (i) versus scan rate (ν) at different potentials, according to the equation ip = aνb.[59] The MRG-2 sample demonstrated the cathodic potential sweep b-values of 0.71 and 0.67 at -0.8 V and -0.4 V potentials, respectively (Figure S14a, b). This suggests that the current response involves both surface-controlled (capacitive) and diffusion-controlled processes. For the anodic potential sweep, the b-values were 1.05 and 0.95 at -0.8 V and -0.4 V, respectively (Figure S14a, c), suggesting a dominant capacitive contribution.[24,60] The relative contributions of the diffusionand surface-controlled processes were quantified using the Dunn equation (i = k1v + k2v1/2).[24,59] The k1v and k2v1/2 represent the surface-controlled and diffusion-limited current contributions, respectively, to the total current; v is the potential sweep rate, and k1 and k2 are constants independent from the potential sweep-rate. The shaded portion of the CV curve of MRG-2 at 20 mV s−1 represents the contribution from diffusion-controlled processes (Figure 5c). The capacitive contribution increased from 33% at 5 mV s−1 to 67% at 50 mV s−1, and levelled off at 72% at 100 mV s−1 (Figure 5d). Furthermore, the dQ/dV plots for MRG-2, derived from the GCD data in a three-electrode cell at different current densities, exhibit a rectangular shape with only a slight decrease in area at higher current Journal Pre-proof Journal Pre-proof 20 densities (Figure S15). This behavior further confirms that the charge storage is predominantly governed by a surface-controlled (capacitive) process. Figure 5. (a) CV curves of MRG-2 at different scan rates. (b) GCD profiles of MRG-2 at different current densities. (c) CV curve of MRG-2 showing diffusion-controlled contribution to total current (shaded area) at a scan rate of 20 mV s−1. (d) diffusionand surface-controlled charge storage contribution of MRG-2 at different scan rates. The practical applicability of the MRG-2 heterostructure in terms of energy and power density was evaluated by assembling an asymmetric supercapacitor full cell (ASC) device using 3 M KOH aqueous electrolyte. The ASC device was assembled by employing MRG-2 as a negative (-1.0 V to -0.3 V) and NiCo2S4/graphene (NCSG) as a positive (-0.3 V to 0.5 V) electrode, resulting in a broader potential window up to 1.5 V, with a mass loading of 1 mg cm−2. NCSG was selected for its well-documented performance as a positive electrode.[7] The total mass density considering the thickness of both electrodes was estimated to be 1.88 g cm−3, which is used to calculate the volumetric characteristics of the electrodes for the full cell. The synthesis and characterization of NCSG are provided in the SI (Figures S16 and S17). The charges Journal Pre-proof Journal Pre-proof 21 between the negative (MRG-2) and positive (NCSG) electrodes were balanced by recording their CV responses in their respective potential windows at 10 mV s−1 (Figure S18). Accordingly, the mass ratio was evaluated to be m+ = 0.27 m- (equation 2, SI). The CV of the MRG-2//NCSG device, recorded at scan rates ranging from 5 to 100 mV s−1, showed that the overall capacitance is a combination of pseudoand electric double-layer capacitance (Figure 6a). The GCD profiles at different current densities showed almost symmetrical curves, indicating good electrochemical reversibility of the ASC device. Additionally, the small IR drop of 0.037 V at 5 A g−1 (Figure 6b), demonstrates low resistivity of the electrodes and efficient ion transport at the electrode/electrolyte interface. The ASC device achieved a high gravimetric capacitance (Cg) of 187.5 F g−1 at 0.5 A g−1, retaining 116 F g−1 at 10 A g−1 (Cg was calculated according to nonlinear equation 3, SI), demonstrating an excellent rate performance of 62.0%. Even at a very high current density of 20 A g−1, the device maintained a Cg of 99.9 F g−1 (Figure 6c). The excellent rate capability of the MRG-2//NCSG ASC device is evident when compared with previously reported full-cell ASC devices based on similar systems.[8,61] For example, NiMo3S4/BP//NiCo2S4/Ti3C2S4 retained 59.5% of its initial capacitance at its maximum reported current density of 1 A g−1,[8] and P-mediated MoS2//MnO2 retained 34% at 16 A g−1,[61] which is lower compared to the MRG-2//NCSG ASC device at a higher current density of 20 A g−1 (53%) (Figure 6c). The Ragone plot (energy density vs. power density) is crucial for evaluating the performance of SC devices, with the herein MRG-2//NCSG ASC delivering a high gravimetric energy density (Eg) of 58.6 Wh kg−1 at a power density (Pg) of 0.5 kW kg−1, retaining 25.4 Wh kg−1 even at 20.0 kW kg−1 (Figure 6d). The volumetric characteristics of SC are critical for advancing portable energy storage solutions.[1,2] The MRG-2//NCSG ASC device delivered an extremely high volumetric energy density (Ev) of 110.5 mWh cm−3 at a power density (Pv) of Journal Pre-proof Journal Pre-proof 22 0.94 W cm−3, retaining as high as 47.9 mWh cm−3 at 37.7 Wcm−3. Interestingly, the areal energy density (Ea) is also particularly high, reaching 54.3 µWh cm−2 at a power density (Pa) of 1.0 mW cm−2. Notably, the gravimetric, volumetric, and areal performance of the MRG-2//NCSG ASC device not only outperformed the previously reported ReS2-based systems, but also other toprated advanced electrode materials, including 2D TMDs and transition metal sulfide systems (Figure 6e-g, Tables S1 and S2). The detailed comparisons also revealed that the gravimetric and volumetric performances of MRG-2//NCSG ASC device operating in a low-cost aqueous electrolyte outperform devices operating even under a wide potential window using organic and water-in-salt electrolytes (Tables S1 and S2). Journal Pre-proof Journal Pre-proof 23 Figure 6. Electrochemical studies of MRG-2//NCSG ASC device: (a) CV curves at variable scan rates. (b) GCD profiles at different current densities. (c) Rate performance. (d) Ragone plot. (e-g) Comparison of the gravimetric, volumetric, and areal energy and power density output obtained with the MRG-2//NCSG ASC device with respect to advanced electrode materials selected from the literature. More details are given in Tables S1 and S2. (h) Cycling stability of MRG2//NCSG ASC device at a constant current density of 8 A g−1. (i) Nyquist plots of MRG2//NCSG ASC device before and after cycling stability test and magnified high-frequency region (inset-i). (panels e-g, explanation of abbreviations of materials h-MoO3//CuCoHCF: asymmetric pseudocapacitor device based on hexagonal molybdenum oxide/Prussian blue analogue,[6] Ti3C2@CNT//CNT: ASC based on MXene-knotted carbon nanotube//carbon nanotube,[9] CoNiMOF//AC: asymmetric SC based on oriented cobalt nickel-MOF on carbon fiber paper//activated carbon (Adv. Energy Mater.[11]), ReS2: rhenium disulfide,[31] NMS/BP//NCS/TCX: asymmetric SC based on nickel molybdenum sulfide/black phosphorous//nickel cobalt sulfide/MXene,[8] Ni/Co-N-350//PC: asymmetric SC based on Ni-doped cobalt-cobalt nitride//porous carbon,[62] GCNAS//GCFS: asymmetric SC based on graphene/cobalt nickel aluminum sulfide//graphene/cobalt iron sulfide,[38] 1T MoS2/Ti3C2: symmetric SC based on molybdenum disulfide/MXene,[24] COF/rGO: symmetric SC device based on covalent organic framework/reduced graphene oxide,[10] GNRib: MOF derived Graphene nanoribbons,[63] MoS2/rGO//Fe2O3/MnO2: Molybdenum disulfide/reduced graphene oxide//iron oxide/ manganese oxide core shell nanorods based asymmetric SC,[22] 1T MoS2: 1T molybdenum disulfide,[5] Ni-HAB: Nickel-hexaaminobenzene MOF,[64] CNT-GP: carbon nanotubes/graphene petals,[4] Ni3(HITP)2: Nickel-Hexaimino triphenylene,[39] Ni(OH)2/G//MoS2/G: asymmetric SC based on nickel hydroxide/graphene//molybdenum disulfide/graphene,[23] Graphene/MoS2: compact graphene/molybdenum disulfide,[65] AC//HTMC-SCS: activated carbon//hierarchical transition metal (Cu-Ni) chalcogenide shell-core-shell,[66] GNS1: graphene nanospheres,[67] MXrHGO3: MXene/holey graphene,[68] ReS2/MoS2: rhenium disulfide/molybdenum disulfide,[33] ReS2/BP: rhenium disulfide coupled with black phosphorus,[69] PPy/GF: polypyrrole coated graphene foam.[70] Journal Pre-proof Journal Pre-proof 24 Another important parameter for evaluating the performance of an ASC device is its cycling stability. For the MRG-2//NCSG ASC device, the capacitance was initially increased by up to 150 % due to the electrochemical activation of the electrode material.[71] The activation of the electrode material during the initial cycles was confirmed by the increased area under the peaks in the differential capacity plot (Figure S19), which is likely attributed to the more effective participation of active material in the Faradaic redox processes.[72] This activation is usually connected to the progressively deeper and more effective penetration of the electrolyte ions into the pores of the material during charging/discharging, enhancing charge storage.[71] The device retained 96.7% of its capacitance even after 10,000 charge-discharge cycles at 8 A g−1 (Figure 6h). The cycling stability exceeds that of current state-of-the-art systems (Tables S1 and S2). For instance, NiMo3S4/BP//NiCo2S4/Ti3C2Tx-based ASC device retained 86% of its initial capacitance after 5,000 cycles at 10 A g−1,[8] h-MoO3//Cu0.82Co0.18HCF maintained 83% of initial capacitance after 10,000 cycles at 10 A g−1,[6] and a (Ni,Co)3S4/graphene ASC retained 85% after 10000 cycles,[7] and many other systems mentioned in Tables S1 and S2. The EIS measurements of the full cell at open circuit potential were conducted before and after the cycling stability test (Figure 6i and inset) and fitted using a modified Randles circuit (Figure S20 and Table S5). Before cycling, the Nyquist plot revealed a charge transfer resistance (Rct) of 0.42 Ω (Table S5), indicating fast electrochemical kinetics (Figure 6i and inset). The Warburg region displayed a slope greater than 45°, suggesting the presence of diffusion restrictions (Figure 6i). After the stability test, the Rct substantially decreased to 0.25 Ω (Table S5), demonstrating excellent electrochemical stability of the electrode. Additionally, the Warburg slope shifted to ~45° (Figure 6i), indicating improved ion diffusion due to material activation during cycling, as also suggested by the cycling stability test (Figure 6h). [71] Journal Pre-proof Journal Pre-proof 31 Journal Pre-proof Journal Pre-proof 32 Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Journal Pre-proof Journal Pre-proof 33 Table of Contents This work demonstrates the modulation of 2D 1T’ ReS2 structure through Mo single-atom engineering, followed by strong interfacial coupling with GN3, improving charge transport kinetics, porosity and charge storage. The assembled asymmetric SC device achieved exceptional gravimetric (54.3 Wh kg−1), volumetric (110.5 mWh cm−3), and areal (54.3 µWh cm−2) energy densities unlocking the potential of ReS2 in energy storage. Journal Pre-proof Journal Pre-proof 34 Table of Contents Highlights  The electronic structure of 2D 1T’ ReS2 was modulated through Mo single-atom engineering, followed by strong interfacial coupling with N-doped graphene (GN3).  The 2D Mo-ReS2@GN3 heterostructure exhibits improved charge transport kinetics, porosity, and charge storage.  The assembled asymmetric SC device achieved exceptional gravimetric (54.3 Wh kg−1), volumetric (110.5 mWh cm−3), and areal (54.3 µWh cm−2) energy densities unlocking the potential of ReS2 in energy storage.