Design and synthesis of MnN4 macrocyclic complex for efficient oxygen reduction reaction electrocatalysis

Journal Pre-proofs Design and Synthesis of MnN4 Macrocyclic Complex for Efficient Oxygen Reduction Reaction Electrocatalysis Vinod Kumar Vashistha, Anuj Kumar PII: DOI: Reference:

S1387-7003(19)31051-2 https://doi.org/10.1016/j.inoche.2019.107700 INOCHE 107700

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Inorganic Chemistry Communications

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15 October 2019 26 November 2019 27 November 2019

Please cite this article as: V. Kumar Vashistha, A. Kumar, Design and Synthesis of MnN4 Macrocyclic Complex for Efficient Oxygen Reduction Reaction Electrocatalysis, Inorganic Chemistry Communications (2019), doi: https://doi.org/10.1016/j.inoche.2019.107700

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Design and Synthesis of MnN4 Macrocyclic Complex for Efficient Oxygen Reduction Reaction Electrocatalysis Vinod Kumar Vashisthaa, Anuj Kumara,b,*,[email protected] aDepartment of Chemistry, GLA University, Mathura-281406, India bBeijing University of Chemical Technology, Beijing, 100029, PR China

Abstract The Pt-based materials were considered to be the most promising catalysts for the oxygen reduction reaction (ORR) in fuel cell technology but poor abundance and prohibitive cost limit their further use. Herein, we designed and synthesized a novel MnN4 macrocyclic complex and its nanocomposite, MnN4@rGO, with reduced graphene oxide (rGO). Both the MnN4 complex and MnN4@rGO composite were characterized by using multiple spectroscopies. On the basis of spectral analysis, octahedral geometry was assigned to MnN4 complex. Relevant to their performance for ORR electrocatalysis, the composite MnN4@rGO showed significant good ORR performance (+0.91 V and +0.86 V, onset and formal potential, respectively). Further, the Mn3+/Mn2+ redox potential and its role as activity indicator for ORR were correlated to the low energy gap between HOMO of Mn metal ion and LUMO of dioxygen, leading to effective overlapping between Mn3+/Mn2+ and ORR onset potentials for 4eORR. The SCN- poison effect on the ORR active sites showed the establishment of the Mn3+ state that could anticipate the redox (Mn3+/Mn2+) process, and decrease in available active site for oxygen adsorption, ORR declined. Keywords: Macrocycles, Electrocatalysts, Oxygen Reduction Reaction, Fuel Cells, Nanocomposite, Clean Energy. 1 Introduction Due to the depletion and over-exploitation of natural resources, the world is currently facing an energy crisis. Consequently, it is desired to develop some new alternative energy resources to meet future energy demand. The progress in the development of efficient, reasonable despicable, eco-friendly sustainable energy techniques has become an urgent need [1]. Over the last few decades, a number of advanced technologies include “fuel cells, metal-air batteries, and water splitting” were introduced to fulfill the target of future energy requirements [2]. Currently, fuel cells, in particular polymer electrolyte fuel cells (PEFCs) are most valued technique to exploit as a potential energy device for green vehicles; which may be due to their superior energy efficiency and negligible air pollution [3]. But PEFCs suffer efficiency loss in

cathodic over-potential due to the lethargic kinetics of oxygen reduction reaction (ORR). Ptbased electrocatalysts undergo serious limitations including poor abundance, high-priced cost, and low viable accessibility which restricted the application of Pt-based electrocatalysts for marketable uses [4-5]. Consequently, seeking for new potential alternate, Pt-based catalysts become an apparent preference [6]. Macrocyclic complexes have been largely considered as alternates for Pt-based ORR catalysts [7]. Tetraazamacrocyclic (N4-macrocyclic) complexes have gained much concentration due to their unique structure which is planar, highly conjugated as well as redox rich chemistry. The redox chemistry of N4-macrocycles complexes encourages the direct ORR on the active N4-M2+ (M=Fe and Mn) site by inducing “inner sphere electron transfer reactions” and other chemical reactions [8-9]. As a result, the ORR catalytic activity of the electrode sturdily depends on the N4-M2+ environment and redox potential of metals [10]. The alteration in the macrocyclic skeleton can tune their redox potential and catalytic activity. Such as for FePc (iron phthalocyanine), the electron-withdrawing substituents in the skeleton showed an anodic shift in the Fe3+/Fe2+ redox potentials while electron-donating substituents showed opposite shift [11]. In other words, the catalytic activity depends on the electronic coupling between HOMO of metal ion (M3+/M2+ redox potential or eg orbital) and LUMO of dioxygen (π*). The perturbation theory proposed that similar energy frontier orbitals (i.e. HOMO and LUMO) would offer the optimal interaction with each other. Prior reports have indicated a relationship between the formal potential and ORR activity of the catalysts although it is still uncertain that the formal potential of catalysts should be equal or close to ORR to realize the highest activity [12]. Further, the major challenges in the use of macrocycles onto the electrode surface are the retention of the ORR activity; principally because of the poor stability of these macrocycles in the electrolytic environment [13]. A literature survey of the last few years revealed that several nanocomposites have been designed to improve the stability as well as ORR activity of macrocyclic complexes by combining with carbon nanotubes and graphene [14-15]. A good amount of research work has been done on the synthesis of electrocatalysts using melamine as an N precursor, particularly for the preparation of MNC material for ORR [16-20]. Melamine is very stable, N-rich and widely used as potential ligand [21]. But the use of the melamine for the synthesis of macrocyclic complexes is very difficult due to its poor solubility in most of the solvents. Though, at some occasion’s melamine have been utilized for the synthesis of macrocyclic complexes [14, 22]. Considering the importance of N density and possibility of the synthesis of macrocyclic complexes through melamine, we designed and synthesized a novel melamine Schiff bases macrocyclic complex of Mn (MnN4) and its

nanocomposite (MnN4@rGO) using microwave heating. The synthesized complex and nanocomposite were characterized by using FT-IR, UV–visible spectra, MS spectra, 1H and 13C NMR, EPR and XRD, XPS and TEM techniques, respectively. As prepared MnN4 complex and MnN4@rGO composite were investigated as electrocatalysts towards ORR in alkaline solution. The coordination science, Sabatier's hypothesis, and intermolecular hardness were also taken into account in order to understand the mechanism of ORR. 2 Result and discussion 2.1 Synthesis and Characterization of MnN4 macrocyclic complex and MnN4@rGO composite The MnN4 macrocyclic complex was synthesized according to the literature method (template method in which metal ion directs the condensation reaction between amine and carbonyl groups followed by coordination cyclization) [23-24] and the synthesis scheme for MnN4 macrocyclic complex and MnN4@rGO composite is illustrated in Fig. 1 (please see the SI information for the detail of synthesis and analytical data of macrocyclic complex). The Fourier transform infrared spectroscopy (FT-IR) study was utilized to confirm the functionality of the MnN4 macrocyclic complex. The IR spectra of the MnN4 macrocyclic complex didn’t show any strong absorption band near 1700-1710 cm-1, indicating the absence of free >C=O of benzil. This evidence confirms the condensation reaction between >C=O groups of benzil and –NH2 groups of melamine. A strong absorption band in the region ~1595-1615 cm-1 can be attributed to the C=N group. The observed lower values of ν(C=N) can be credited to the drift of free lone pair density of C=N nitrogen towards Mn metal ion. However, the absorption band in the region ~1638-1645 cm-1 can be attributed to the C=N groups of triazine. The bands correspond to ν(C-H) vibrations of phenyl groups were observed at 2850-3050 cm-1. In addition, the bands present in the range ~1350-1000 cm-1 are assigned for the ν(C-N) vibration [23]. The IR spectra of the complex do not show any change in the intensity as well as vibrations of triazine ring and interestingly enough, it appears that in this complex triazine nitrogen does not involve in Mn-N coordination, otherwise it would have resulted in the formation of unstable four-membered rings. Further, the far IR spectra showed the (Mn-N) azomethine vibrational modes in the region ~430-450 cm-1, showing the coordination Mn-N. Moreover, ν(M-Cl) vibrations were observed [24] at 300-315 cm-1. The FT-IR spectra MnN4@rGO composites showed all characteristics of MnN4 complex with some new appeared peaks at 1000-1100, 1265 and 1540-1560 cm-1 due to the stretching vibration of rGO. The little shift in free -NH2 peaks can be attributed to the intramolecular hydrogen bonding within some

hydroxyl groups present at rGO surface. These results confirmed that the MnN4@rGO composite preserved the corresponding MnN4 macrocyclic complex on the rGO surface without any change in the structural features of the MnN4 complex [25] (Fig. S1). The mass spectra of the MnN4 macrocyclic complex showed a molecular ion peak corresponding to (M+1) at m/z 725. The mass spectral data is in good agreement with the formation of MnN4 macrocycle (Fig. S2). In addition to confirming the structure of MnN4 macrocycle, 1H- &

13C-NMR

spectral

analysis was performed. The 1HNMR spectrum of MnN4 complex (Fig. S3) showed two signals at δ7.1-8.0 and δ3.4 corresponding to the benzylidenimin and –NH2 (melamine) protons in the macrocyclic framework. Additionally, 13C-NMR spectrum of MnN4 complex also confirmed the structure of N4 macrocyclic framework as it showed four signals at δ175.5, δ158-161, δ147.38 and δ 127-138; corresponding to the N=C1=N, NH2-C-C2=N, C-C3=N and C-C4=C characteristics in the N4-macrocyclic ligand (the used carbon numbering is mentioned in

13C-

NMR spectra (Fig. S4) [21, 22, 24]. The electron paramagnetic resonance (EPR) spectra of polycrystalline MnN4 complex was recorded at 280 K and 100 K under liquid nitrogen temperature (Fig. S5) and found to be broad. The Mn2+ ion is an odd-electron system (d5) and shown to have Kramer’s doublet as the ground state. In this case, the degeneracy can be totally nullified by a magnetic field and resonance can be identified even for the larger zero-field splitting. Therefore, the broad signals in the polycrystalline MnN4-macrocycle may be due to the forbidden transitions for which Δm = ±M and Δm = 0 (M = electron spin quantum number; m = nuclear spin quantum number). As a result, the observed broadening in the spectra can also be ascribed to the immobilized freeradicals, e.g. MnN4-macrocycle of concanavalin where the control of M = ±M ion in the TM framework because the rotational motion of Mn2+ is highly restricted. In the DMF solution, the EPR spectra of MnN4-macrocycle gave well separated six lines which were obtained due to the hyperfine interaction between the unpaired electrons with the Mn2+ nucleus (I = 5/2) (Fig. 2a). These resolved six lines can be assigned as -5/2, -3/2, -1/2, +1/2, +3/2 and +5/2 transitions from low to a high field. The observed giso value of 2.03 is in agreement with Mn2+ N4-environment in the complex and its lower value indicated the covalent nature of the metal-ligand bond. In addition, the EPR spectra recorded at different temperatures showed that the values of giso and Aiso did not change with the temperature [25-26]. The magnetic moment for MnN4 complex was found in the range 4.89 B.M [27], which indicated S = 5/2 contributions at both experimental conditions [28]. Thus, these spectral studies are in good

harmony with the proposed structure of MnN4-macrocyclic complex, as also supported by elemental data. Ultraviolet-visible spectroscopic (UV-vis) study was performed in ethanolic solution to confirm the electronic transitions in MnN4 complex and the interaction between MnN4 macrocyclic complex and rGO in the MnN4@rGO composite. The electronic spectrum of the MnN4 complex displayed the weak absorption bands at 19,801, 21,739, 27,777 and 38,461 cm-1. These bands can be assigned to the 6A1g → 4T1g (4G), 6Ag → 4Eg; 4A1g (4G) (10B + 5C), 6A

g

→ 4Eg (4D) (17B + 5C) and 6Ag → 4T1g (4P) (B + 7C) electronic transitions, respectively

[26] as supported by EPR. On the basis of these bands, a distorted octahedral geometry can be assigned to this complex. The assignment of octahedral geometry for this complex also supported by the value of (v2/v1) ratio = 1.09, which is close to the expected value for octahedral geometry. Further, the value of molar conductance, measured in DMSO, was found to be 15 ohm-1 cm2 mol-1, showing it to be non-electrolyte nature and the axial positions being possessed by two chloride ions. Next, the UV–Vis spectra of MnN4@rGO composite showed the longer wavelength shift in main band from 395 nm to 457 nm, probably due to increasing π conjugation via π-π stacking effect between MnN4 complex and rGO [29] (Fig. S6). The structural characterization of the composite was investigated by using X-ray diffraction (XRD). Fig. 2b showed the XRD patterns of both MnN4 complex and MnN4@rGO composite. The unsupported MnN4 complex showed the sharp diffraction characteristic in the range ca. 05-38 (2θ), indicating the varying degrees of crystallinity. Whereas, the XRD spectrum of MnN4@rGO showed a similar characteristic of MnN4 complex with an additional diffraction peak [30, 31] at 2θ =25.2 which can be ascribed to rGO characteristic. These results again supported the adsorption of MnN4 macrocyclic complex on the rGO surface without any considerable structural change in MnN4 complex [32]. Further, the X-ray photoelectron spectroscopy (XPS) studies were taken into account to characterize the chemical composition of the composites. The XPS spectra for the MnN4@rGO composite displayed in Fig. 2(c-d) found to be inconsistent with the composition of C, N, and Mn for the MnN4@rGO composite. The XPS spectra of N1s state of MnN4@rGO showed three peaks at around 398.1 eV, 399.2 eV and 400.6 eV that can be consigned to the presence of N1s; neutral amine, neutral imine, and triazine imine, respectively. Further, a fine probe of Mn2p showed the presence of Mn2p3/2 and Mn2p1/2 states near 641.5 eV and 653.1 eV, respectively [33]. The observed group of a state in Mn2p3/2 and Mn2p1/2 regions may be due to possible photo-reduction of MnN4 complex by X-rays to different Mnn+ electronic states [34].

The morphologies of the synthesized rGO and MnN4@rGO were observed via a transmission electron microscope (TEM). The TEM image of rGO (Fig. S7) showed a welldefined rGO layer representing successful alteration of graphite into rGO by using ‘modified Hummer method’. While the TEM image of the MnN4@rGO is very similar to rGO, it can be attributed to the less appropriate to show the aggregation of the MnN4 complex on the rGO surface [35]. 2.2 Oxygen Reduction Activity of MnN4 macrocyclic complex and MnN4@rGO composite The ORR performance of MnN4 macrocyclic complex and MnN4@rGO composite catalysts was examined in the alkaline electrolyte (20 wt% of each catalyst loading at carbon electrode; see supporting information for the detail). The cyclic voltammetric (CV) curves (Fig. 3a) of the MnN4 macrocyclic complex, MnN4@rGO composite, and 20% Pt/C catalysts were recorded in an O2-saturated 0.1 M KOH electrolyte at a scan rate of 50 mV/s. The CV of MnN4@rGO demonstrated a distinct cathodic ORR signal near +0.87 V which are relatively more positive than MnN4/C (+0.77 V). The ORR kinetics of the MnN4@rGO catalyst was studied in O2-saturated 0.1 M KOH electrolyte by using linear sweep voltammetry (LSV). Fig. 3b shows the polarization curves of the MnN4/C, MnN4@rGO, and 20% Pt/C catalysts at a rotation speed 1600 rpm with the same amount of the catalysts by loading 0.25 µg/cm2 onto the glassy carbon electrode. The onset potential of MnN4/C and MnN4@rGO showed at +0.87 V and +0.91 V, which is close to +0.84 V and +0.92 V values obtained from cyclic voltammetry, respectively. The formal potential (E1/2) of MnN4@rGO is +0.86 V which equals 20% Pt/C (+0.86 V) and more positive than MnN4/C (+0.78 V), indicating the good ORR performance of MnN4@rGO. Fig. 3(c-d) showed the LSV curves of MnN4@rGO at a different scan rate and the linearity of the ‘KauteckyLevich’ plots at several electrode-potentials, respectively [35]. The slope of 1/B was obtained by plotting the graph between 1/J and ω−1/2 (see supporting information in detail), suggested that about 3.8 electrons were used in the experimental ORR process. Therefore, these studies specified the better ORR catalytic activity of MnN4@rGO than MnN4/C and comparable with 20% Pt/C (a traditional catalyst). Further, we also compared the ORR activity of MnN4@rGO with other reported MN4macrocycles@rGO (Table 1). The reason why the MnN4@rGO composite showed better ORR catalytic activity than the MnN4 complex can be ascribed to the presence of highly conductive rGO support. The ‘π-π

stacking’ interaction between rGO and MnN4 complex may be tailored to the electronic environment of the active site in MnN4 via intermolecular charge transfer. The rGO may be offered a highly conductive pathway for e- transfer, MnN4 complex showed highly efficient ORR catalysis [40]. Further, the CV in the N2-atmosphere showed the conversion of N4-Mn2+ to N4-Mn3+O(H), N4-Mn3+-O(H) to N4-Mn2+ (Fig. 4a) where the pivotal position is free for the O2 chemisorption (Similar to 20% Pt/C, Fig. S8a). Thus, the Mn3+/Mn2+ redox couple was found to be extremely sensitive with oxygen where N4-Mn2+ being the active site and ORR was catalyzed by the Mn2+ formal oxidation followed by 4e− inner-sphere electron transfer mechanism. In addition, this redox couple is invisible in the CVs of rGO modified electrode (Fig. S8b), suggesting that MnN4-macrocyclic complex was responsible for this redox couples and the π-π interaction between the rGO and MnN4-macrocyclic complex provides the structural stability to the MnN4@rGO composite. Based on these electrochemical results in N2/O2-environment and various approximations found in the literature, the following mechanism was proposed for ORR (Equation 1, 2 and 3) in alkaline media [41-42]. [N4-Mn3+-OH]ad + e- → [N4-Mn2+]ad + OH- (1) [N4-Mn2+]ad + O2 + e- → [N4-Mn3+-O2-]ad

(r.d.s.) (2)

[N4-Mn3+-O2-]ad + e- → prods. + [N4-Mn2+]ad (3) Thus, it is clear that the macrocyclic complex shifted the redox potential of Mn metal ion in the anodic direction, keeping Mn3+/Mn2+ state, as a result, it would facilitate the electronic coupling between the HOMO of Mn metal ion and LUMO (π*-orbital of O2), and insist for the 4e- ORR pathway. In addition, KSCN was used as an ORR inhibitor, in order to verify the ORR active center in the composite. The effect of SCN- ion on the ORR activity of the g-MnN4 and 20% Pt/C was probed by submerging modified electrodes for 30 min in KSCN (5 mM) containing 0.1 M KOH electrolyte. The CVs and polarization curves of MnN4@rGO (Fig. 4(a-b)) and 20% Pt/C, recorded before and after exposed in KSCN, suggested that MnN4@rGO lost the 100 mV informal potential (165 mV for 20% Pt/C) (Fig. S8(c-d)) and 1.0 mA/cm2 in current density. The results are indicating that (i) MnN4@rGO is less sensitive towards SCN- as compared to 20% Pt/C and (ii) during the SCN- poisoning, the establishment of the Mn3+ state could anticipate the redox (Mn2+/ Mn3+) redox process, decreases the availability of active sites for ORR, worse the ORR [43] (equation 4 and 5).

[N4-Mn3+-O2]ad + SCN → [N4-Mn3+-SCN]ad + O2- (4) [N4-Mn3+-SCN]ad + e → [N4-Mn2+-SCN]ad (5) Furthermore, the stability of MnN4@rGO was evaluated by CV, since it is also a major concern in practical application of fuel-cell technology. Figure 4(c-d) showed the CV and LSV of MnN4@rGO before 4000 CV cycles, respectively, indicating that MnN4@rGO composite losses 70 mV in formal potential and 0.25 mAcm-2 in current density which are reasonable as compared to reported 20% Pt/C. The nature of ORR active sites holds the key to the development of single-atom catalysts (SACs). In comparison with the recently reported high-performance SACs electrocatalysts and macrocycles based electrocatalysts, the macrocycles are well-defined structure and nature of ORR active sites, these facts can provide more information about the relationship between molecular structure and the catalytic activity [44]. 3 Conclusions In this work, we demonstrate a novel MnN4@rGO composite shown to have good ORR electrocatalytic activity in alkaline media. The π-π stacking between rGO and MnN4 macrocyclic complex provided the structural stability to the MnN4@rGO composite, by modifying the electronic structure of the active site (Mn metal center) and facilitated the electron transfer in ORR process. Further, we demonstrated the Mn3+/Mn2+ redox potential and its role as activity indicator for ORR and the Mn3+/Mn2+ redox potential of MnN4@rGO composite was found to be very close to ORR peak potential. It results the low gap between HOMO of Mn metal ion and LUMO of dioxygen, and higher overlapping between Mn3+/Mn2+ and ORR onset potentials, indicating 4e ORR pathway. We also studied the SCN poison effect on the ORR active sites and it can be concluded that during the SCN poisoning, the establishment of the Mn3+ state could anticipate the redox (Mn3+/Mn2+) process, decrease in available active site for oxygen adsorption, ORR declined. Such well-defined molecular structures at the carbon support offer a platform for the ORR molecular electrocatalysts which are efficient, non-precious and better alternatives of commercial Pt/C catalyst. Thus, this macrocyclic complex MnN4 can serve as an active cathode catalyst for the development of fuel cell technology. Declarations of interest None Acknowledgment

We are very thankful to the Beijing University of Chemical Technology, GLA University, Mathura, India for providing all kinds of support for this work. We are also thankful to IIT Roorkee, India and SAIF, Chandigarh for all types of characterization techniques. References 1. M. A. J Cropper., S. Geiger, D. M. Jollie, Fuel cells: a survey of current developments. Journal of Power Sources 131 (1-2) (2004) 57-61. 2. L. Carrette, K. A. Friedrich, U. Fuel cells–fundamentals and applications, Fuel cells 1.1 (2001) 5-39. 3. F. Zhao, F. Harnisch, U. Schröder, F. Scholz, P. Bogdanoff, I. Herrmann, Challenges and Constraints of Using Oxygen Cathodes in Microbial Fuel Cells, Environmental science & technology 40,17 (2006) 5193-5199. 4. K. C. Neyerlin, R. Srivastava, C. Yu, P. Strasser, Electrochemical activity and stability of dealloyed Pt-Cu and Pt-Cu-Co electrocatalysts for the oxygen reduction reaction (ORR), Journal of Power Sources 186.2 (2009): 261-267. 5. A. Rabis, P. Rodriguez, T. J. Schmidt, Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges, Acs Catalysis 25 (2012) 864-890. 6. S. Walch, A. Dhanda, M. Aryanpour, H. Pitsch, Mechanism of Molecular Oxygen Reduction at the Cathode of a PEM Fuel Cell: Non-Electrochemical Reactions on Catalytic Pt Particles, Journal of Physical Chemistry C 112 (2008) 8464-8475. 7. R. Baker, D.P. Wilkinson, J. Zhang, Electrocatalytic activity and stability of substituted iron phthalocyanines

towards

oxygen

reduction

evaluated

at

different

temperatures.

Electrochimica Acta 53(23) (2008) 6906-6919. 8. J. H. Wee, K. Y. Lee, S. H. Kim, Fabrication methods for low-Pt-loading electrocatalysts in proton exchange membrane fuel cell systems, Journal of Power Sources 165 (2007) 667677. 9. A. Kumar, Y. Zhang, W. Liu, X. Sun, The chemistry, recent advancements and activity descriptors for macrocycles based electrocatalysts in oxygen reduction reaction, Coordination Chemistry Reviews, https://doi.org/10.1016/j.ccr.2019.213047. 10. R. Jasinski, A New Fuel Cell Cathode Catalyst, Nature 201 (1965) 1212-1213. 11. R. Baker, D.P. Wilkinson, J. Zhang, Facile synthesis, spectroscopy and electrochemical activity of two substituted iron phthalocyanines as oxygen reduction catalysts in an acidic environment. Electrochimica Acta 54 (11) (2009) 3098-3102.

12. X. Yan, X. Xu, Z. Zhong, J. Liu, X. Tian, L. Kang, J. Yao, The effect of oxygen content of carbon nanotubes on the catalytic activity of carbon-based iron phthalocyanine for oxygen reduction reaction. Electrochimica Acta, 281 (2018) 562-570 13. J. H. Zagala, F. Javier Recio, C. A. Gutierrez, C. Zuñiga, M. A. Páez, C. A. Caro, Towards a unified way of comparing the electrocatalytic activity MN4 macrocyclic metal catalysts for O2 reduction on the basis of the reversible potential of the reaction, Electrochemistry Communications, 41 (2014) 24-26. 14. V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril, K. S. Kim, Functionalization of graphene: covalent and non-covalent approaches, derivatives and applications. Chemical Reviews, 112 (2012) 6156-214. 15. A. Kumar, V. K. Vashistha, Design and synthesis of CoIIHMTAA-14/16 macrocycles and their nanocomposites for oxygen reduction electrocatalysis, RSC Advances, 9(23) (2019) 13243-13248. 16. J. Xu, B. Li, S. Li, Jianhua Liu,From melamine sponge towards 3D sulfur-doping carbon nitride as metal-free electrocatalysts for oxygen reduction reaction, Material Research Express, 4 (2017) 0763-0775. 17. Z.-H. Sheng, L. Shao, J.-J. Chen, W.-J. Bao, F.-B. Wang, X.-H. Xia, Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis, ACS Nano, 5 (2011) 4350-4359. 18. T. C. Nagaiah, A. Bordoloi, M. D. Sánchez, M. Muhler, W. Schuhmann, Mesoporous nitrogen-rich carbon materials as catalysts for the oxygen reduction reaction in alkaline solution, ChemSusChem, 5 (2012) 637-641. 19. H. Zhao, K. S. Hui, K. N. Hui, Synthesis of nitrogen-doped multilayer graphene from milk powder with melamine and their application to fuel cells, Carbon, 76 (2014) 1-9. 20. L. Shang, W.J.-T. Chen, R.-X. Zhao, W. Qian, L. M. Pan, Catalytic Performance of HeatTreated Fe-Melamine/C and Fe-g-C3N4/C Electrocatalysts for Oxygen Reduction Reaction, Acta Physico-Chimica Sinica, 29 (2013) 792-798. 21. S. Uysal, Synthesis and Characterization of S-Triazine Cored Tripodal Keto-oxime and Its Trinuclear Complexes, Synthetic Communication, 42 (2012) 84-92. 22. İ. Kaya, M. Yıldırım, Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties, Synthetic Metals, 159 (2012) 1572-1582. 23. S. Chandra, Vandana, S. Kumar, Synthesis, spectroscopic, anticancer, antibacterial and antifungal studies of Ni(II) and Cu(II) complexes with hydrazine carboxamide, 2-[3-methyl-

2-thienyl methylene], Spectrochimica Acta A; Molecular and Biomolecular Spectroscopy, 135 (2015) 356-363. 24. U. Saban, U. H. Ismet, The synthesis and characterization of single substitute melamine cored Schiff bases and their [Fe(III) and Cr(III)] complexes, Journal of Inclusion Phenomenon Macrocyclic Chemistry, 68 (2010) 165–173. 25. A. Costa, A. Luís, P. Silva, R. B. Viana, A. A. Tanaka, J. J. G. Varela, Theoretical study of the interaction between molecular oxygen and tetraazamacrocyclic manganese complexes, Journal of Molecular Modeling, 22 (2016) 217-227. 26. W. R. Paryzek, V. Patroniak, J. Lisowski, .Metal complexes of polyaza and polyoxaaza Schiff base macrocycles, Coordination Chemistry Reviews, 249 (2005) 2156–2175. 27. B. Bouzerafa, A. Ourari, D. Aggoun, R. R. Rosas, Y. Ouennoughi, E. Morallon, Novel nickel(II) and manganese(III) complexes with bidentate Schiff-base ligand: synthesis, spectral, thermogravimetry, electrochemical and electrocatalytical properties, Research on Chemical Intermediates, 42 (2016) 4839-4858. 28. S. Gautam, S. Chandra, H. Rajor, S. Agrawal, P. K. Tomar, Structural designing, spectral and computational studies of bioactive Schiff's base ligand and its transition metal complexes, Applied Organometalic Chemistry, 32 (2017) 3915-3931. 29. R. Kumar, R. Johar, Structural elucidation and coordination abilities of Co(II) and Mn(II) coordination

entities

of

2,6,11,15-tetraoxa-9,17-diaza-1,7,10,16-(1,2)-

tetrabenzenacyclooctadecaphan-8,17-diene, Spectrochimica Acta A, 79 (2011) 1042-1049. 30. J. Ding, S. Ji, H. Wang, D. J. Brett, B. G. Pollet and R. Wang, MnO/N-Doped Mesoporous Carbon as Advanced Oxygen Reduction Reaction Electrocatalyst for Zinc–Air Batteries. Chemistry–A European Journal, 25 (2019) 2868-2876. 31. J. Ding, P. Kannan, P. Wang, S. Ji, H. Wang, Q. Liu, H. Gai, F. Liu, R. Wang, Synthesis of nitrogen-doped MnO/carbon network as an advanced catalyst for direct hydrazine fuel cells. Journal of Power Sources, 413 (2019) 209-215. 32. C.Y. Wang, C.P. Cho, T.P. Perng, Structural transformation and crystallization of amorphous copper phthalocyanine nanostructures, Thin Solid Films, 518 (2010) 6720-6728. 33. Y. Chen, H.Wang, F. Liu, H. Gai, S. Ji, V. Linkov, R. Wang, Hydrophobic 3D Fe/N/S doped graphene network as oxygen electrocatalyst to achieve unique performance of zincair battery. Chemical Engineering Journal, 353 (2018) 472-480. 34. Y. Chen, S. Huo, H. Wang, Engineering morphology and porosity of N, S-doped carbons by ionothermal carbonisation for increased catalytic activity towards oxygen reduction reaction. Micro & Nano Letters, 13 (2018), 530-535.

35. A.P. Dementjev, A. De Graaf, M.C.M. Sanden, K.I. Maslakov, A.V. Naumkin, A.A. Serov, X-Ray photoelectron spectroscopy reference data for identification of the C3N4 phase in carbon–nitrogen films. Diamond and related materials, 9 (2000) 1904-1907. 36. M. Jahan, Q. L. Bao, K. P. Loh, Electrocatalytically active graphene–porphyrin MOF composite for oxygen reduction reaction, J. Am. Chem. Soc., 134, (2012), 6707 – 6713. 37. Kim SK, Jeon S. Improved electrocatalytic effect of carbon nanomaterials by covalently anchoring with CoTAPP via diazonium salt reactions. Electrochemistry Communications. 2012 Aug 1;22:141-4. 38. H. Tang, H. J.; Yin, H. J.; Wang, J. Y.; Yang, N. L.; Wang, D.; Tang, Z. Y. Molecular Architecture of Cobalt Porphyrin Multilayers on Reduced Graphene Oxide Sheets for HighPerformance Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 5585 − 5589. 39. Kumar, A., Zhang, Y., Liu, W. and Sun, X., 2020. The chemistry, recent advancements and activity descriptors for macrocycles based electrocatalysts in oxygen reduction reaction. Coordination Chemistry Reviews, 402, p.213047. 40. D. Wei, X. Wang, J. Zhan, X. Sun, L. Kang, F. Jiang, X. Zhang, Biological cell template synthesis of nitrogen-doped porous hollow carbon spheres/MnO2 composites for highperformance asymmetric supercapacitors, Electrochimica Acta, 296 (2019) 907-915. 41. B. Merzougui, A. Hachimi, A. Akinpelu, S. Bukola, M.S. Merzougui, A Pt-free catalyst for oxygen reduction reaction based on Fe–N multiwalled carbon nanotubes composites, Electrochima Acta. 107 (2013) 126-132. 42. M.P. Anderson, P. Uvdal, New scale factors for harmonic vibrational frequencies using the B3LYP density functional method with the triple- ξ basis set 6 – 311+G(d, p). Journal of Physical Chemistry, 109 (2005) 2937-2941. 43. M. Tsuda, E. Dy, H.K. Kasai, Side-on O2 interaction with heme-based nanomaterials. The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics, 38 (2006) 139-141. 44. Z.D. Rutkowska, M. Witko, From activation of dioxygen to formation of high-valent oxo species: Ab initio DFT studies, Journal of Molecular Catalysis A; Chemical, 275 (2007) 113-120.

Fig. 1. Synthesis scheme for MnN4 macrocyclic complex

Fig. 2. (a) EPR spectra recorded in DMF at 100K and 298K of MnN4 macrocyclic complex, (b) XRD Pattern of MnN4 macrocyclic complex and MnN4@rGO composite, (c) XPS spectra of N1s (inset Fig. is the XPS spectra of Mn2p) of MnN4@rGO composite and (d) XPS survey of MnN4@rGO composite. Fig. 3. (a) CV of MnN4/C and MnN4@rGO recorded in O2-saturated 0.1 M KOH electrolyte at a scan rate of 50 mVs-1 (b) ORR polarization curves of MnN4/C, MnN4@rGO and 20% Pt/C recorded in O2-saturated 0.1 M KOH electrolyte at a scan rate of 1600 mVs-1, (c) Polarization curves of MnN4@rGO supported on a GC electrode in O2-saturated 0.1 M KOH electrolyte at different rotation rates, and (d) ‘Koutechy-Levich’ plot of J1 vs w1/2 at different electrode potentials. Fig. 4. (a) CVs of MnN4@rGO recorded in N2 and before and after submerging in 5 mM 0.1 M KSCN in O2-saturated 0.1 M KOH electrolyte at a scan rate of 50 mV/s, and (b) Polarization curves of MnN4@rGO recorded before and after submerging in 5 mM 0.1 M KSCN in O2saturated 0.1 M KOH electrolyte at a scan rate of 5 mV/s with 1600 rpm rotation speed, (c) CVs and (d) of MnN4@rGO recorded in O2-saturated 0.1 M KOH electrolyte, at 50 mV/s and 5 mV/s scan rates, respectively, before and after 4000 cycles.

Table 1: A comparison of MnN4-macrocycle with traditional molecular Materials for ORR Catalysis MN4-Complexes@rGO Electrocatalysts

Media

Eonset (V)

EHalfwave (V)

Epeak (V)

n

Ref.

MnN4-macrocycle@rGO

0.1 M KOH

+0.91

+0.86

+0.87

3.80

This work

Iron Porphyrin@G

0.1 M KOH

+0.98

+0.88 V +0.78

3.82

36

Cobalt Porphyrin/rGO

0.5 M KOH

+0.98

+0.88

+0.77

3.85

37

Iron Phthalocyanine/G

0.1 M KOH

+0.87

+0.86

+0.85

3.73

38

Iron Porphyrin/rGO

0.1 M KOH

+0.95

+0.86

+0.75

3.71

39