The chemistry, recent advancements and activity descriptors for macrocycles based electrocatalysts in oxygen reduction reaction

Coordination Chemistry Reviews 402 (2020) 213047

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

The chemistry, recent advancements and activity descriptors for macrocycles based electrocatalysts in oxygen reduction reaction Anuj Kumar a, Ying Zhang b, Wen Liu a,⇑, Xiaoming Sun a,⇑ a State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China b School of Chemistry, Monash University, Wellington Road, Clayton 3800, VIC, Australia

a r t i c l e

i n f o

Article history: Received 25 April 2019 Accepted 28 August 2019

Keywords: Fuel cells Electrocatalysts Macrocycles Nanocomposites ORR Activity descriptors

a b s t r a c t The oxygen reduction response (ORR) shows sluggish kinetics on cathode surface, which welcomes a noteworthy commitment to the efficiency loss of the fuel cell devices. The advancements in the designing and developments of the competent electrocatalysts for ORR suggest that the macrocycles is the class of versatile materials for the rapid ORR. The hypothetical investigations and trial results specify that the ORR activity of MN4-macrocycles (M = Mn, Fe and Co) can be regulated by directing its MII/III formal potential through proper substitution on the macrocyclic framework and metal center. In this work, we tended to the principal dialog beginning from the fundamental chemistry of fuel cell, ORR, macrocycles, dioxygen, and metal–dioxygen (M–O2) bond communication, as the establishment of this survey. At that point, the emphasis on the comprehensive designs and developments on current macrocycles-based nanocomposites for ORR and the molecular understanding of macrocycles-based ORR descriptors, was considered as the core of this survey. The significance of this survey lies in objective planning the electrocatalysts beginning from the most essential normal for the electronic auxiliary designing to an increasingly reasonable dimension of nanofabrication. Ó 2019 Published by Elsevier B.V.

Contents 1.

2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Hydrogen fuel cell (HFC) and oxygen reduction reaction (ORR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Macrocyclic chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3. Dioxygen and metal–dioxygen bond chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Role of macrocycles in oxygen reduction reaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Natural occurring macrocycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Synthetic Mn, Fe and Co macrocycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Macrocycles-modified carbon materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4. Heat treated-macrocycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Relation between ORR activity and Macrocycles parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.1. The MIII/MII redox potential vs M–O2 binding energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2. Number of d electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.3. Intermolecular hardness (ɳDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.4. Theoretical Study’s contribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Prospects and research direction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

⇑ Corresponding author. E-mail addresses: [email protected] (W. Liu), [email protected] (X. Sun). https://doi.org/10.1016/j.ccr.2019.213047 0010-8545/Ó 2019 Published by Elsevier B.V.

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1. Introduction In modern society, the excessive consumption of fossil fuels results in rapid depletion of fossil conserve and degradation of the environment due to air pollution and greenhouse gas emission. To address these issues, development of low cost, clean and sustainable energy supply is now becoming an urgent and important need [1]. Since the past decades, several advanced technological including the metal–air batteries, fuel cell and water electrolytic devices have been developed to meet the energy demands as of a future need. Hydrogen fuel cell (HFC) devices particularly polymer electrolyte membrane (PEM) fuel cells are viewed as a standout amongst the most encouraging improvements in energy unit innovation used in versatile applications [2,3]. At the beginning of the twenty first century, Pt based catalysts were still considered as the most active catalysts in fuel cell technology [4]. But scarce reserves in earth crust and prohibitive cost limit the wide application of Pt based catalysts [5,6]. Therefore, searching for new potentially alternative electrocatalyst with high activity, long work life and low cost is becoming an obvious choice [7,8]. Recent advancements in the macrocyclic chemistry have encouraged scientists and researchers to design and develop the materials as superior alternatives for Pt based catalysts. In such manner, numerous macrocycles have as of late been explored which demonstrated fundamentally improved electrocatalytic activity towards ORR [9]. But there is a requirement for the quantitative appraisal on macrocycles electrocatalysts, especially those intended to address current vitality challenges. Inside this system, the electrocatalytic movement can be enormously influenced by the nearness of the central metal ion in macrocyclic framework [10]. Another key to streamline the macrocycles based electrocatalysts is changing macrocyclic system substituents to tune their electronic structures. Furthermore, these electrocatalysts can likewise be effectively immobilized on the conductive surfaces either by covalent or non-covalent communication to additionally advance their electrocatalytic movement [11,12]. To propel the field further not far off, identification and comprehension of the principal atomic interrelation between the material properties and the reactant execution are the requirements for the normal plan of superior macrocycles based electrocatalysts. Obviously, such a multidimensional issue can be tended to just with multidisciplinary approaches that ought to at any rate consolidate (Fig. 1): 1) macrocycles chemistry; for the controllable blend and portrayal of macrocycles based electrocatalysts; 2) carbon material chemistry; for understanding the genuine impact of carbon support on the structure of macrocycles based electrocatalysts and their exhibitions and 3) electrochemistry; for the reasonable trial and hypothetical depiction of the related electrochemical procedures. In this review, we address a short discussion on the chemistry of hydrogen fuel cell, ORR and macrocycles in Section 1 as the establishment for further areas. In Section 2, the imperative advancements on ORR utilizing macrocycles and their composites inside the most recent twenty years are secured. In Section 3, we discussed about all the announced descriptors on macrocyclesbased electrocatalysts for ORR in detail and the executions of DFT estimation on macrocycles-based ORR electrocatalysts have likewise talked about. In Section 4, the unsolved issues on macrocycles-based ORR electrocatalysts are given explicit difficulties and research bearings based on the above ends.

Fig. 1. Schematic illustration of a combined macrocyclic chemistry, carbon material chemistry and electrochemistry approach to address the issue of activity, stability and abundance of the macrocycles-based ORR electrocatalysts.

up of non-consumable terminals specifically cathode, anode and a sensible electrolyte. The cathode involves the catalyst layer and a porous gas dispersion layer which comprised of particularly electronic conductive materials, for instance, the penetrable carbon fiber paper [14]. For the most part, anode electrocatalysts are Pt or Ni based materials, working at the low or high temperatures individually, while the electrolyte should have especially high ionic conductivity. The electrolyte can be aqueous acidic/basic media or solid polymer layer. The charge carriers are found to be different, depending upon the characteristics of fuel cells [15]. As indicated by the idea of different ionic conducting materials, HFCs can be ordered into five most basic classes, for example, polymer electrolyte membrane (PEM) fuel cells or (PEMFCs) [16,17], alkaline (A) fuel cells or (AFCs), phosphoric acid (PA) fuel cells or (PAFCs), molten carbonate (MC) fuel cells or (MCFCs) and solid oxide (SO) fuel cells or (SOFCs) [14]. Among them, PEMFC is the most vital class of HFCs which comprises a polymer backbone holding some acidic/basic side-chains [18,19]. Fig. 2 demonstrates the schematic depiction of a hydrogen power module and the ORR pathways. The distinctive highlights of PEMFCs incorporate generally under 90 °C, showing sufficiently high power density, a reduced framework with a compatible system useful in taking care of fluid fuel [20]. A PEMFC contains an electrolyte between the anode and cathode in the design of sandwich engineering where two significant individual reactions happen on both electrode surfaces. The general redox reaction is displayed as follows (Eqs. (1)–(3)) [21,22]. Anodic Reaction: Hydrogen Oxidation Reaction (HOR)


 H2 ! 2Hþ þ 2e Ea ¼ 0:0 V v s SHE

ð1Þ

Cathodic Reaction: Oxygen Reduction Reaction (ORR)


 1=2O2 þ 2Hþ þ 2e  ! H2 O Ec ¼ 1:23 V v s SHE

ð2Þ

Therefore, the overall fuel cell reaction

1.1. Hydrogen fuel cell (HFC) and oxygen reduction reaction (ORR)


 H2 þ 1=2O2 ! H2 O E ¼ 1:23 V v s SHE

In HFC devices, the free energy of an electrochemical reaction gets converted into electrical energy and the produced energy can be used in running devices, etc. [13]. The HFC device is made

With the harmony standard electromotive power determined to be 1.23 V. Where, E°a, E°c and SHE represent the anodic standard electromotive force, cathodic standard electromotive force and standard hydrogen electrode respectively.

ð3Þ

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O2 þ 4Hþ þ 4e ! 4H2 O


 E ¼ 1:23 V; DG ¼ 113:5 kcal=mol ð4Þ

Fig. 2. Schematic illustration of a hydrogen fuel cell and the ORR pathways.

The research for the electrocatalysts which can accelerate the electrode reactions with their effect on the proficiency, sturdiness and cost of the fuel cells, is considered as a focal issue in this field. Normally Pt/C or platinum alloys were used as catalysts for hydrogen electrooxidation at anode as well as for oxygen electroreduction at cathode in PEMFCs [23]. The sluggish kinetics of ORR can be attributed to the poisoning effects of the adsorbed anions on ORR activity, which usually present in the electrolyte. This adsorption phenomenon is structural sensitive with the surface of the catalysts and may be proceed via reversible or partially reversible pathway. Such types of adsorption cause the site blocking. Thus, higher amount of catalyst is required on the cathode to achieve adequate action contrasted with the quick HOR on the anode. A large number of the reports have been distributed on the improvement of suitable cathode electrocatalyst however Pt is so far considered as the best option [24]. Consequently, the anode and cathodes of an ordinary PEMFC are made out of Pt based catalyst layer where little Pt particles are adsorbed onto the carbon black surface. On the other hand, the catalysts along with carbonsupport, in terms of permeable and high electronic conductivity, such as carbon cloths and carbon fiber paper (known as a gas diffusion layer (GDL)), were found to be effective. There are two extra species such as polytetrafluoroethylene (PTFE) and Nafion solution [25,26]. Those can be added during the catalyst layer preparation. PTFE is hydrophobic species which helps to expel the water molecules from the cathode surface, while due to the available protons on –SO3H group, Nafion act as a fastener for the polymer electrolyte membrane and electrocatalysts layer and additionally expands the ionic movements at the catalyst layer [27]. A good catalyst layer should have three highways at the same time, that of electrons, ions and gas [28]. In this way, to approach our interest in ORR, the essential destiny of dioxygen in the higher natural creatures is its conversion into water, occurs with an overall transfer of 4e (Eq. (4)), which is highly exothermic under both standard condition ([H+] = 1.0 M) and at organic convergences of [H+] (pH 7.4, E° = 0.79 V and Gibbs free energy (DG) = 72.9 kcal/mol) [58].

However, ORR is a sluggish reaction and shows slow reaction kinetics. Therefore, to enable this reaction to be happened, a substantial reduction overpotential is inevitable without suitable catalysts, affecting the energy production efficiency of the cutting edge PEMFCs. Thus, it creates enough challenges for the researchers who focus on PEMFCs research. The electrochemical ORR in acidic/basic solutions happens via two primary pathways: one including 4e pathway to give H2O/ OH (Eq. (5)) and another one is the exchange of 2e to give H2O2 (Eqs. (6) and (7)). The 4e pathway mechanism of ORR involves the breaking of the O–O bond in the dioxygen molecule, providing the most extreme released free energy which favors the real development of fuel cell innovations. Therefore, for a complete ORR, the catalyst should assist the well conversion of O2 to H2O through the 4e pathway. The two-electron ORR discharges practically 50% of the free energy contrasted with that of the four-electron ORR because of the relatively high breaking energy (118 kcal/mol) for the O–O bond. This procedure not just prompts a low energy conversion efficiency yet in addition delivers a receptive intermediate of the road that can be additionally converted to harmful free radical species and subsequently cause a decrease in the stability of an electrocatalyst specially the carbonaceous substances. Usually, the conversion of O2 into H2O, followed by 4e pathway involving the burst of the O–O bond in the O2, can include the collaboration of O2 with one or two active sites simultaneously on a cathode surface, know as single atom site or duel site actions respectively [8,29]. Path A – direct pathway, involves four-electron reduction

O2 þ 4Hþ þ 4e ! 2H2 OðEo ¼ 1:23 V v s SHEÞ

ð5Þ

Path B – indirect pathway, involves two two-electron reductions

O2 þ 2Hþ þ 2e  ! H2 O2 ðEo ¼ 0:69 V v s SHEÞ

ð6Þ

H2 O2 þ 2Hþ þ 2e ! 2H2 OðEo ¼ 1:77 V v s SHEÞ

ð7Þ

where, E° and SHE represents the corresponding standard electromotive force and standard hydrogen electrode respectively. 1.2. Macrocyclic chemistry In the last century, researchers have built up some conceivable materials through the synthetic chemistry, those could possibly be utilized as catalysts for fuel cells [10,30–32]. However, the macrocyclic chemistry that has been rehearsed in the previous couple of decades is of without giving noteworthy consideration towards the molecular-based electrocatalysts. Nowadays, specialists are progressively mindful of electrocatalytic ability and stability of the prepared electrocatalysts. It is vital to take note of that macrocyclic chemistry has changed the perspective in the way of molecular material design approach in the field of nanotechnology (supramolecular architectures) and electrocatalysis [33,34]. As to advance high-performance HFCs, there is a requirement for quick and specific strategies/advances to structure and create effective ORR electrocatalysts. Macrocycles are widely applied for such types of electrocatalytic developments [35,36], where the metal center with N4 moiety was supposed to be an active site for ORR, which can bind with O2 molecule and electron transfer proceeds via peripheral conjugated aromatic rings [37]. For what reason to pick macrocycles-based electrocatalysts [38–40]?

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 Nature holds the macrocycles systems e.g. heme and hemocyanines in several biological ORR systems which inspired to build the similar synthetic models.  In these macrocycles, the metal particle is held unflinchingly in the macrocycles pit with the ultimate objective that the biological job of these species can’t further debilitate for instance fighting demetallation responses.  The engineering of macrocyclic chemistry, associated with mimicking the unusual spectral, kinetic or thermodynamic and redox behavior of the macrocycles, founded upon the various developments of supramolecular chemistry for various catalysis. Macrocyclic frameworks are polydentate ligands which possess donor atoms either incorporated in or, less commonly, attached to a cyclic backbone. ‘‘Macrocycles” are defined as molecules having at least one large ring containing nine or more than nine atoms with at least three donor atoms such as N, O atoms [41–43]. Macrocyclic ligands have attracted widespread attention due to following unique properties such as (a) ability to discriminate among closely related metal ions based on the metal ion radius (ring size effect) and (b) the significant enhancement in complex stability which is generally exhibited by optimally-fitting macrocyclic ligands relative to their open-chain analogues (macrocyclic effect) [44–46]. A number of systems have been reported in which the metal ion is fully occupied in the macrocyclic cavity and possess maximum stability [47]. Macrocycles have several features that make them interesting in efforts to tackle ‘‘difficult” targets with extended binding sites [43,48,49]. Because of their size and complexity, they can be engaged as targets through numerous spatially distributed binding interactions, thereby increasing both binding affinity and selectivity [50–52]. Furthermore, cyclization provides a degree of structural pre-organization that may reduce the entropy cost of receptor binding compared to their linear analogues. Macrocyclic ligands based upon the porphyrin ring system and close structural relations such as chlorophylls and corrins are ubiquitous [53]. Iron complexes of these ligands are involved in electron transport, dioxygen storage and enzyme catalysis [49,54,55]. Fig. 3 shows an overview of the intensity of research in the use of metal porphyrins (MPs) [54] and metal phthalocyanines (MPcs) as molecular electrocatalysts for ORR during the last four decades [56]. Interestingly, the graph shows an exponential rise in the number of publications in the past

Fig. 3. Graphical representation of publication growth of metallophthalocyanine and metalloporphyrin based ORR electrocatalysts (% growth = number of publications per 3-year period divided by the total number of publications in the last 23 years, multiplied by 100) from 1995 until 2018. Raw data obtained from Web of Science.

few years indicating the research in ORR will continue to increase. The increased global awareness about the environmental impact of fossil fuels and fossil energy reserves has contributed partly to the increased intensity of research in ORR for fuel cells [57]. 1.3. Dioxygen and metal–dioxygen bond chemistry The comprehension of dioxygen and metal–dioxygen (M–O2) bond communication is exceptionally fundamental to structure the potential macrocycles based ORR electrocatalysts. Generally, the vacancy of a pair of electrons in the antibonding orbitals of dioxygen molecule make it paramagnetic in nature, having a triplet 3  Ʃg ground state [58]. The molecular orbital (MO) description of dioxygen molecule’s ground state 3Ʃ g level (Fig. 4a) is presented as O2KK(2srg)2(2sru*)2(2prg)2(2ppu)4(2ppg*)1(2ppg*)1, leaving two bond order between both oxygen atoms (where the used symbols Ʃg is the term symbols of the ground states, r and p represents the sigma and pi bonding orbitals, *, u and g represents the corresponding antibonding orbital, ungerade and gerade orbitals respectively and the KK expression demonstrates that the K shells of the both oxygen atoms are fully occupied) [59,60]. The MO electronic configuration of O2 demonstrates an opportunity to add a solitary e into both 2ppg* orbitals. The addition of a couple of electrons into O2 results the superoxide (O2) and peroxide (O2 2 ) species with a bond order of 1.5 and 1, respectively. Predictable with this task of bond orders, the bond lengths and bond strengths 2 of O–O link [58] fall in the order O2 > O 2 > O2 . When, O2 interacts with the MN4-macrocycles based electrocatalysts, the specific binding mode of O2 with metal (end-on or sideon) depends on the available coordination sites and frontier dorbitals energy of metal center and electronic properties of macrocyclic framework. Further, O2 can also interact via the ‘‘bridge-cis” and ‘‘bridge-trans” conformations, involving two metal active sites for example in case of Pt catalyst and face-to-face porphyrin systems. Since crystallographic studies suggested the optimal M-M separation for this type of interactions. Regarding the M–O2 binding stoichiometry, it is conceivable to imagine the exchange of none or a couple of electrons from the metal (active site) to the adsorbed O2. In this way, the association of O2 with the active center organize the electron exchange process between both metal and O2. This phenomenon includes the formal oxidation of metal center followed by the formation of superoxide (end-on interaction) or peroxide (side-on interaction) species [61]. Particularly, for the end-on M–O2 interaction, the r interaction (Sigma bond) and p back bonding between the metal atom and dioxygen molecule take place. The Sigma bond formation is the e density transfer from the 3dz2 orbital of metal donor site to the p*sg orbital of O2 acceptor molecule, while the p back bonding is the partially p bond formation through the transfer of electron density from pga orbital of O2 to 3dyz orbital of metal center (where the superscript ‘‘s” and ‘‘a” referred to symmetric and antisymmetric nature of orbital with standard reference to the M–O2 plane respectively) [62,63]. A systematic orbitalic representation for end-on and side-on interaction between N4-macrocycles and dioxygen is shown in Fig. 4b. Upon these conceivable associations, the O–O bond order diminishes, favoring O–O link break since electrons acknowledged by the O2 into the its p* orbitals [59,64]. Moreover, the superoxide formation via 1e transfer process to O2, occurs in the outer Helmholtz layer (OHL) without involving a direct M–O2 interaction between O2 and the active site of the cathode surface, this phenomenon known as an outer-sphere ORR [65]. Usually, nonaqueous or strongly alkaline media promotes such sort ORR and does not appear to be pertinent for the improvement of fuel cell innovations since minimal liberated energy is freed simultaneously. In contrast, the direct M–O2 interaction between dioxygen or intermediates and the active site like FePc macrocycles

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Fig. 4. (a) The molecular orbital energy-level description for O2. (b) Systematic Orbital diagram of electron donation and back donation in oxygen-complexed transition metal bonding via side-on and end-on [62].

involves the 2/4e followed by inner–sphere (I–S) reaction mechanism [66]. In this manner, it is critical to grow minimal effort electrocatalysts that decline the overpotential of ORR which can consequently advance the 4e ORR [67]. 2. Role of macrocycles in oxygen reduction reaction 2.1. Natural occurring macrocycles In hemoglobin (tetramers of myoglobin) O2 binds to the Fe2+ ion and formed Fe–O2 adduct, get reduce. This biological ORR is the responsible for the human being life by providing energy to human body tissues. Such kinds of M–O2 (M = metal) adducts are also important in other biological reactions in terms of O2 activation and reduction [68,69,59,70]. The biological models such as porphyrins, hemocyanines for oxygen transportation has inspired to the researchers to prepare similar synthetic macrocyclic models [69]. To address the trails ORR, noteworthy synthetic efforts have been made to design the macrocycles that would imitate the active center of ‘cytochrome c’. In the last step of respiration, ‘cytochrome c’ reduce the dioxygen into water followed by 4e to provide power for its H+ pump activity [71,72,73]. The 4e reduction pathway is important to keep away from the toxic ORR intermediates  like O 2 , H2O2, OH and so on. Inspired by these biological systems, extensive research progress has been made on similar synthetic macrocycles for the development of ORR electrocatalysis at the cathode. The electron transport mechanism can be easily understood with an enzyme in the respiratory e transport chain that is located in the mitochondrial membrane. The enzyme obtains an electron from each of four ‘cytochrome c’ molecules and transports them to one oxygen molecule converting O2 to H2O (Fig. 5). The iron is Penta-coordinated and binds with O2 (along with the Cu) prior to reduce it. Moreover, cyanide is a very strong ligand binding at the active site during cyanide poisoning stabilizing the Fe3+ state and preventing its redox (Fe2+/Fe3+) activity. The Eq. (8) showed the outline of the mechanism of ‘cytochrome c [73,74,62]. Summary reaction [72]:

Fe2þ cytochrome þ 8Hþin þ O2 ! Fe3þ cytochrome þ 2H2 O þ 4Hþout ð8Þ

2.2. Synthetic Mn, Fe and Co macrocycles In this subsection, we survey some critical reports on Mn, Fe and Co macrocycles without carbon support for ORR and found that the phthalocyanines, porphyrins and corroles [75,76] macrocycles of Mn, Fe and Co transition metals are mainly used for ORR [77,37,78]. Fig. 6 showed a typical structural comparison between Metallophthalocyanine, metalloporphyrin and metallocorrole. A standout amongst the most alluring highlights of these macrocycles is that the ligand can be altered to improve their auxiliary, electronic and catalytic properties [79,80]. Metal phthalocyanines (MPcs) is a typical class of macrocyclic complex containing nitrogen-carbon at the alternating position in the ring [33] structures [79]. Jasinski et al. first noticed the catalytical activity of metallophthalocyanines toward the ORR in alkaline media [35]. They found that metallophthalocyanines have excellent ORR activity and their electrocatalytic activities depend upon the characteristics of the central cations which act as the ORR active sites. Further, Zagal et al. have generally examined the MPcs macrocycles-based electrocatalysts for ORR. This research group has completed a notable commitment to comprehend the base to top parameters of macrocycles-based ORR electrocatalysts at the atomic dimension. They suggested that the interaction path of O2 with the metal ion ought to be a key factor that determines the efficiency of the 4e ORR process for metallophthalocyanines [38]. The side-on configuration of O2 on metal endorses back bonding of d-electrons from metal to O2 and facilitates O–O bond breaking. Due to this geometric/configuration effect, the orientation of O2 against the metal adsorption site can be controlled by designing the ligand structures. Further, the corroles macrocycles have similar skeleton structure with corrin ring of vitamin B12 with a direct pyrrole–pyrrole connection (one methine carbon atom is absent in porphyrin framework). Because of this distinction in the structure of corrole, it is a tri-anionic macrocyclic ligand which demonstrated the diverse coordination science when contrasted with the di-anionic porphyrin [52,81]. Corrole showed smaller macrocyclic cavity bearing smaller metal–nitrogen (M–N) bond length as a result it can effectively stabilize the high-valent metal ions. The MN4-square planar geometry is an ideal platform for the smaller substrate (like O2) binding and activation [82,61].

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Fig. 5. The possible mechanism of O2 Reduction by Cytc in the Presence of the FeCuPhOH CcO Model Complex. Reproduced with permission [71]. Copyright 2019, ACS Publishing Group.

Fig. 6. The typical structures Metallophthalocyanine [79], metalloporphyrin [52] and metallocorrole [78].

The macrocyclic structure of porphyrin and corrole can be efficiently changed with the electron-giving/pulling substituents [83,82]. This one of a kind features of porphyrins and corroles makes them a superb stage to research the structure–function relationship which favored for the ORR. Kadish, Ou, et al., systematically investigate the effect of e-giving/pulling substituents, at meso-site of Co corrole, on the ORR activity of Co corroles by adsorbing these molecules on an EPG electrode. The cyclic voltammograms of the molecules showed well-defined O2 reduction peak in between 0.34 and 0.39 V vs SHE which were approximately more positive as compared to the bare electrode (0.37 V vs SHE). In similar account for porphyrin-based ORR catalysts, Schuhmann et. al, carried out a methodical assessment of porphyrinbased ORR electrocatalysts by preparing a series of tetra(4-

aminophenyl)-porphyrin macrocycles of first row transition metals. This study examined the ORR activity difference with the different central metal ion substitution in the same macrocyclic framework [61]. The outcomes exhibited a volcano-shaped curve arranged by diminishing reactivity for example Co > Mn, Fe, Ni > Cr, Cu tetra(4-aminophenyl)-porphyrin macrocycles. The results suggested that the interaction of O2 molecule with metal ion is a critical viewpoint in the synergist procedure. In this way, for the best catalysis, the cooperation of the metal ion with the oxygen containing species should be neutral (neither too weak nor too strong). The e giving/pulling side chains on the porphyrin macrocyclic framework may either diminish or improve the electron density on metal active site and in like manner influence the synergist activity in light of the fact that the O2 activation followed the transfer of e density from metal to O2.

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Generally, Mn–O bond is more strengthen than the M–O bond (M = Fe and Co) for the MnPs as a result these are less effective ORR electrocatalysts. But it is interesting that the Mn–O bond could be debilitated by adjusting macrocyclic core using electron-giving groups. It is well established that electron-giving side chains at the meso-site of porphyrin framework enhance the e density on the metal ion and it is opposite for the electronpulling substituents. For the Mn complexes of tetra(4-pyridyl) porphyrin ligand and its analogues, the ORR activity was observed to be an element of the formal capability of the MnIII/MnII redox potential. Such type investigation clarified that the nature of metal ion and substitution on porphyrin framework decide the ORR electrocatalytic properties [45,84,85]. Further, along with monomeric cobalt macrocycles, there have been an exclusive amount literature published on covalently linked cofacial dimeric Co porphyrins as ORR electrocatalysts. In binuclear cobalt complex, face-to-face porphyrin forms a trans-peroxo intermediate which favors access to H+ and O–O bond rupture [86]. Early reports on dicobalt cofacial macrocycles were based on the use of these macrocylces as heterogeneous ORR electrocatalysts supported on the electrode however recent trends in literature have focused on homogeneous ORR catalysis with similar macrocyclic structures [20]. The inspirations for amalgamating these co-facial structures are integrating the idea that the CoIII/II redox couples in both structures would stabilize a l-peroxide intermediate. Fig. 7 showed (a) the inside binding (b) binding above with co-facial assembly respectively and (c) hypothetical and trialbased ORR mechanism at co-facial biscobalt bisporphyrin. For most of the co-facial systems, the Co–Co distance and bond angle are the important factors for the superior OOR electrocatalysis [87]. Ruili Liu et al. reported [89] a novel versatile class of triangular polymeric N4-macrocycles of Fe and Co metal ions ([Fe/CoN4]n) (Fig. 8a). The sole triangular tri-nuclear structure of the polymeric CoN4-macrocycle ([CoN4]3/C) showed the well-defined oxygen reduction peak (Fig. 8b) along with excellent oxygen reduction activity and higher stability in the alkaline media as compare to traditional Pt/C cathode catalyst (Fig. 8c). Such well-defined structured [MN4]n complexes provide the higher density of active site

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with suitable distance between to metals which welcomes the ORR in cis-bridge fashion followed by 4e ORR pathway. Further, the effect of different stoichiometry of Co(II) ions with metal-free triangular polymeric N4-macrocyclic ligand ([CoN4]1.45 and [CoN4]3) on the ORR activity was also explored in terms of ORR onset potential, (Fig. 8d). The analysis of onset potential for the [CoN4]3/C and [CoN4]1.45/C structures indicated that the ORR can be tuned by varying the stoichiometry of metal ion and triangular polymeric ligand. Such type conjugated polymeric MN4-based structure gives more insights about the well-defined identity of active site for ORR and can be considered as ideal molecular design for further advancement of cathode electrocatalysts. Xiang et al., reported another appealing approach in which they have synthesized a 2D covalent organic polymer (COP) systems with Mn, Fe, and Co metal ions by using Yamamoto polycondensation reaction [90]. This complex was utilized as ORR electrocatalyst and further to get a standard metal/nitrogen conveyance and phenomenal dependability in both acidic and basic media, an extra heat treatment was also likewise performed (Fig. 9a). A ORR comparison study between metal free 2D-COP (C–COP–P) and transition metal containing C–COP–P–M (M = Mn, Fe and Co) molecular framework showed that C–COP–P–M system is efficient ORR electrocatalyst showing an anodic shift in ORR peak with high current density as compared to metal free C–COP–P molecular framework. Further, to suggest the optimal carbonization temperature to achieve max ORR activity for these molecular systems, they also applied different carbonization temperatures on the similar sample. At below the 950 °C temperature, the sufficient graphitization (high conductivity) could not observed, while at higher than 950 °C temperature, the molecular framework C–COP–P decomposed and loss the conductivity, results poor ORR activity in contrast the ORR activity was significantly improved at optimal 950 °C temperature. The polarization curve of C–COP–P–Fe molecular framework showed a comparable onset potential (0.98 V vs. RHE) with Pt/C, while C–COP–P–Co molecular framework showed a higher limiting current density as compare to Pt/C (Fig. 9b). Further, the K–L plot calculations for C–COP–Fe and C–COP–Co molecular frameworks showed that 3.81 and 3.56 electrons were

Fig. 7. (a) Optimized structures at the DFT–PBE level of inside (‘‘dock-in” path) and outside (‘‘dock-on” path). (b) O2 bonded Co (DPX) adducts. (c) Dock-On/Dock-In Mechanism for Oxygen Reduction Catalyzed by Co-facial Biscobalt Bisporphyrin Based on Computational and Experimental Results. Reproduced with permission [88]. Copyright 2019, ACS Publishing Group.

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Fig. 8. (a) Structure of triangular trinuclear MN4 complexes [MN4]3 (M = Co(II) and Fe(II)), and (b) Cyclic voltammograms of [CoN4]3/C on a glassy-carbon RDE electrode at a scan rate of 100 mVs1 in Ar-saturated and O2-saturated aqueous solution of 0.1 M KOH. (c) ORR polarization curves for [MN4]3/C catalysts (M = Co and Fe) and Pt/C (20 wt%) in 0.1 M KOH saturated with O2, (d) ORR polarization curves for [CoN4]3/C, [CoN4]1.45/C, and 10/C, recorded with 1600 rpm rotation rate at the scan rate of 10 mV/s. A catalyst loading of 25.45 lg/cm2 was used for all RDE voltammograms., Reproduced with permission [89]. Copyright 2019, ACS Publishing Group.

Fig. 9. (a) The incorporation of non-precious metals (Fe, Co, or Mn) into C-COP, (b) LSV curves of metal-incorporated C-COP-P–M in O2-saturated 0.1 M KOH at 1600 rpm at a sweep rate of 5 mV s1, and (c) a comparison of electrochemical activity given as the kinetic-limiting current density (jk) at 0.75 V (vs RHE) for the metal-incorporated C– COP–P–M, included the calculated electron transfer numbers., Reproduced with permission [90]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

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involved in the trial ORR, respectively (Fig. 9c). Moreover, C–COP– P–M molecular frameworks were also found to be unaffected under methanol crossover [91]. This work opens the best approach to structure 1D and 2D nano-array of active sites through the substitution availability between the equivalent or distinctive macrocycles. 2.3. Macrocycles-modified carbon materials In this subsection, we talked about the cooperation between graphene, CNTs and macrocycles, several designs and developed macrocycles modified graphene and CNTs nanocomposites for ORR. The carbon materials like graphene and CNTs assume an essential job as conductive help for the macrocycles-based electrocatalysts for ORR. Graphene and CNTs contain sp2-hybridized carbon structure with high surface area [92] by showing excellent conductivity towards heat and [93]electricity [94,95]. These captivating properties of graphene and CNTs pulled in scientists for the balanced plan of macrocycles adjusted graphene and CNTs composites for the electrocatalysis to achieve the maximum activity,

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durability and stability of macrocycles-based electrocatalysts [20]. The interaction between graphene, CNTs and macrocycles depends on the functionalization of carbon support with macrocycles [96]. Fig. 10 showed a systematic scheme for the functionalization of graphene and CNTs with macrocycles including covalent and non-covalent functionalization methods [97–99]. The covalent functionalization of graphene and CNTs includes the longing functionalization treatment of carbon materials to make some leaving utilitarian gatherings (like –COOH, –NH2, –OH, –Cl etc.) on the surface. Superficially. At that point, the functionalized carbon materials are additionally permitted to respond with the leaving bunches containing macrocycles which thusly frames a covalent linkage with carbon material [100]. This functionalized graphene and CNTs can go about as e-donating/ withdrawing carbon support for the macrocycles by affecting the electron transfer processes of metal [101] center [102]. Fig. 11a and b show a schematic representation covalent functionalization of CNTs and graphene respectively [103,104]. The Noncovalent functionalization of graphene and CNTs with macrocycles elective way includes the p–p stacking among macro-

Fig. 10. Systematic scheme for the functionalization of carbon materials with macrocycles.

Fig. 11. Systematic illustration of covalent functionalization of (a) MWCNTs, Reproduced with permission [103]. Copyright 2019, Elsevier Publishing Group, and (b) Graphene. Reproduced with permission [104]. Copyright 2019, Elsevier Publishing Group.

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cycles and carbon materials and it is an alluring methodology since it offers the likelihood of connecting utilitarian gatherings to graphene without aggravating the entire electronic system of macrocycles and carbon materials [105]. Fig. 12 shows the schematic illustration of non-covalent functionalization of graphene and CNTs with macrocycles [106,107]. Mostly, this interaction decreases the band gap (HOMO–LUMO energy gap, HOMO = Highest Occupied Molecular Orbital and LUMO = Lowest Unoccupied Molecular Orbital) of the macrocycles, and the macrocycles modified carbon material (nanocomposite) more prone to donate the e density into the p* orbital of O2, thus more weaken the O–O bond [66,97,108]. This in turn means, carbon support increase ORR catalytic activity and stability by facilitating the electron transfer processes. Hijazi et al., reported a composite (MWCNTs–CoP) in which a covalent molecular network containing Co porphyrin (CoP) units fully covered the MWCNTs surfaces (Fig. 13). This composite was prepared by using the dimerization assembly of the triple bonds through the Hay-coupling reaction on the MWCNTs template [109,110]. The cooperative binding effect of very strong p–p stack-

Fig. 12. Systematic illustration of non-covalent functionalization of (a) CNTs, Reproduced with permission [106]. Copyright 2019, Elsevier Publishing Group, and (b) Graphene materials. Reproduced with permission [107]. Copyright 2019, Elsevier Publishing Group.

Fig. 13. The schematic representation of the nanotubes coated with the porphyrin polymer, Reproduced with permission [109]. Copyright 2019, ACS Publishing Group.

ing between CoPs and MWCNTs provided the distinct stability feature to the composite in favor of higher ORR activity and stability as compare to unsupported CoP. This work can be served to present a good strategy to design a polymeric macrocycle modified CNTs for ORR electrocatalysis. Recently, Tang et al. reported a composite for ORR which contains reduced graphene oxide (rGO) sheet with multilayers of the cobalt porphyrin CoTHPP macrocycle (where, THPP = 5,10,15,20-t etrakis(4-hydroxyphenyl) porphyrin) by utilizing a layer on layer congregation method. The systematic synthesis scheme of the composite (rGO/Co2+–THPP)n is shown in Fig. 14a [111]. The hypothetical and electrochemical results suggested that an optimized number of five layers of Co2+-THPP has shown to improve ORR activity while after the fifth layer, the diffusion of O2 and electrolyte with the inner layers of Co2+-THPP could be disturbed, causing the saturation in ORR performance. However, after the fifth layers of rGO/Co2+-THPP, the limiting current density was improved but equivalent to Pt/C catalyst and greater than other rGO composites (Fig. 14b). Further, the composite also showed higher stability and methanol crossover compared with Pt/C [112–114] (Fig. 14c). This work is an important one in the continued development of hydrogen fuel cells to prepare a layer by layer assembly of macrocycles modified graphene electrocatalyst for ORR (Fig. 15). In similar account, Wang et al. reported [115] an easy methodological approach to synthesize a 2D MtTMPyP/rGOn (Mt = MnIII, FeIII and CoII, and TMPyP = 5, 10, 15, 20-tetrakis (Nmethylpyridinium-4-yl) porphyrin ligand) layered hybrid composite for ORR electrocatalysis. In this approach, 2D MtTMPyP/rGOn composite was prepared by the self-assembly driven by p–p stacking and electrostatic forces between the MtTMPyP macrocycles and the rGO sheets (Fig. 15a). Particularly the CoTMPyP/rGO hybrid demonstrated outstanding ORR electrocatalytic activity, stability and higher methanol tolerance as compare to commercial Pt/C electrocatalyst (Fig. 15b). This study revealed that the selfassembled macrocycles architecture supported on conductive carbon materials can be considered as the non-noble metal superior ORR catalysts which is the primary demand of fuel cells developments. and LSV curves. 2.4. Heat treated-macrocycles M-NC (M = metal) based catalysts including MN4-macrocycles have been extensively studied as potentially ORR catalysts and considered as most promising non-noble alternative of traditional ORR catalyst, Pt/C [116,117]. However, the issue related to the stability and conductivity of macrocycles hindered their further practical applications [35]. In contrast, their carbon-rich backbones make them as precursors for the synthesis of highly active pyrolyzed M-N4/C materials. Yeager et al. proposed a strategy to improve the stability and electronic conductivity of MN4macrocycles by pyrolyzing them with carbon-support in an inert gaseous atmosphere [118]. After that, to develop efficient M–N–C ORR catalysts, many tremendous efforts have been made by using metal and nitrogen sources including MN4-macrocycles [119,3,120]. Pyrolyzed iron and cobalt porphyrins on carbon support revealed excellent ORR performance than non-Pyrolyzed ones and founded that temperature is a crucial factor to determine ORR activity in pyrolysis strategy. It was observed that the highest activity appeared around 600 °C while above 800 °C, activity declined and metallic clusters began to deposit on carbon support [121,122]. J. M. Ziegelbauer et al., reported a pyrolyzed CoTMPP (TMPP = 5,10,15,20-Tetrakis(4-methoxy-phenyl)21H, 23H porphyrine) material that was produced via self-templating process [123,124]. The results indicated that the material prepared at 700 °C displayed the highest ORR activity, whereas the material

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Fig. 14. (a) Preparation procedure of rGO/(Co2+-THPP)n. (b) LSV on RDE of rGO, rGO/Co2+, rGO/THPP, rGO/Co3O4, rGO/(Co2+-THPP)7, and Pt/C in O2-saturated 0.1 M KOH solution with a sweep rate of 10 mV s1 at a RDE rotation rate of 1600 rpm, and (c) normalized I-t chronoamperometric responses of rGO/(Co2+-THPP)7 and C/Pt at 0.25 V in O2 saturated 0.1 M KOH solution at a rotating rate of 1000 rpm., Reproduced with permission [111]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 15. (a) Schematic illustration of the preparation of MtMPyP/rGOn film through the self-assembly of MtTMPyP cations and GO sheets, filtration into a film and reduction with hydrazine vapor, and (b) LSV curves of oxygen reduction on MtTMPyP (Mt: MnIII (a), FeIII (b), CoII (c), rGO (d), MtTMPyP/rGOn (M t = MnIII (e), FeIII (f), CoII (g)), and Pt/C (h) in 0.1 M KOH O2 saturated solution with scan rate 10 mVs1 at rotation speed 1600 rpm (deducting the background currents in N2 saturated 0.1 M KOH solution)., Reproduced with permission [115]. Copyright 2019, Elsevier Publishing Group.

prepared at 800 °C was not found suitable ORR catalyst because the Co–N4 active site could be broke into Co–N2. Fig. 16a demonstrated the TEM image of pyrolyzed CoTMPP pyropolymer at 700 °C temperature, while Fig. 16 b and c demonstrated the CV and LSV curves of pyrolized CoTMPP at 700 °C recorded O2 saturated 1 M HClO4 respectively. The experimental results indicated that the chelate structure of MN4 was not totally destroyed during heat-treatment as long as the temperature was not too high, and some stable reaction sites remained in the material including metal center on carbon surfaces. Some authors suggested that during the high temperature pyrolysis, the macrocylic complexes dissociate into their small gaseous fragments. Hence the autogenic pressure of gaseous frag-

ments facilitates the deposition of N-contents on carbon support again in the template MNx fashion. Further, they suggested that metal ion helps to aggregate the N-contents via coordination [125,126]. However, a lot of discussions are available in the literature for the identification of basic structural feature of prepared M–NxCy material that fulfills the requirements for ORR active sites, but the real structure of the material is still remain unsolved [127]. Overall, in comparison of macrocycles with other electrocatalysts such as M–NC materials, metal oxides and Pt-based catalysts, macrocycles based materials showed excellent ORR activity, low cost except for the poor stability. Macrocycles based materials are well defined structure and easy to design and to understand the mechanism of the reaction of interest. Table 1 showed the

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Fig. 16. (a) TEM micrograph of the prepared CoTMPP pyropolymer following 700 °C thermal treatment. Top left inset: flattened schematic of Co(N4C24)4. Bottom right inset: space-filling model of the porous pyropolymer, (b) CoTMPP (700 °C) in room-temperature O2-saturated 1 M HClO4. Scan rate: 50 mVs1. Total pyropolymer loading: 55 mg per 0.246 cm2., and (c) Representative complete ORR curves for CoTMPP (700 °C) in room-temperature O2-saturated 1 M HClO4. Scan rate: 10 mVs1. Total pyropolymer loading: 55 mg per 0.246 cm2., Reproduced with permission [123]. Copyright 2019, ACS Publishing Group.

Table 1 Summary for some MN4-macrocycles based Materials for ORR Catalysis. Macrocycles-based Electrocatalysts

Media

Eonset

EHalfwave

Epeak

n

Ref.

Iron Porphyrin/graphenea Iron Porphyrin/MWCNTsb Cobalt Porphyrin/graphenea Cobalt Porphyrin/rGOa Cobalt Porphyrin/MWCNTsa Iron Phthalocyanine/graphenea Iron Phthalocyanine/MWCNTsc Iron Porphyrin/rGOa Cobalt(III) – amido-macrocyclic complex/graphenea [CoN4]3 Macrocyclic complex/Carbona Cobalt Porphyrin/polyanilinea CoHMTAA-16/Ca

0.1 M KOH 0.1 M HClO4 0.5 M H2SO4 0.5 M KOH 1.0 M H2SO4 0.1 M KOH 0.1 M NaOH 0.1 M KOH buffer with pH = 2 0.1 M KOH 1.0 M HCl 0.1 M KOH

0.0 V +1.0 V +0.46 V 0.12 V 0.44 V 0.14 V 0.05 V 0.05 V +0.11 V 0.14 V +0.32 V +0.93 V

0.12 V +0.88 V +0.32 V 0.18 V +0.31 V 0.30 V 0.09 V 0.14 V 0.0 V 0.21 +0.24 V +0.82 V

0.23 V +0.75 V +0.14 V 0.22 V +0.25 V 0.28 V 0.12 V 0.25 V 0.14 V 0.23 +0.17 V +0.83 V

3.82 4.0 3.9 3.85 3.17 3.73 3.81 3.71 4.04 3.70 3.70 –

[128] [129] [130] [111] [131] [132] [133] [134] [135] [89] [136] [137]

Where, a-Potentials are reported vs Ag/AgCl, b-Potentials are reported vs RHE, c-Potentials are reported vs SCE, HMTAA-16-hexamethyl-dibenzotetraaza-annulene [16].

Summary for some MN4-macrocycles based Materials for ORR Catalysis. 3. Relation between ORR activity and Macrocycles parameters The ORR reactivity of macrocycles-based electrocatalysts can be upgraded by varying the central metal ion, macrocyclic framework and axial ligation. These impacts can be clarified by connecting the MIII/MII redox potential (M = Mn, Fe and Co) with the oxygen bind-

ing energy Eb(O2), and reactivity and volcano relationships for MN4-macrocycles. Such sort parameters are straightforwardly determines the level of electronic communication between the active site and O2 with other reaction intermediates [138]. 3.1. The MIII/MII redox potential vs M–O2 binding energy A standout amongst the most essential components which control the ORR electrocatalytic activity (on the active site in terms of

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metal–oxygen binding energy, Eb(O2)), is the MIII/MnII redox potential of MN4 macrocycles based electrocatalysts. Fig. 17a showed a linear relationship of MIII/MnII redox potential with M–O2 binding energy for the MN4 macrocycles [139]. It was viewed as that increasingly positive the MIII/MII redox potential, higher the activity of MN4–macrocycles however the MIII/MII redox potential ought to be neither excessively positive (more difficult to oxidize the metal center) nor excessively negative (easy to oxidize the metal center) to support the ORR catalysis [38]. In this way, MIII/MII redox potential ought to be situated in a suitable window to accomplish unrivaled ORR activity. Much equivalent to by virtue of CoPcs, CoIII is likely not framed upon its relationship with the oxygen molecule since the CoIII/CoII redox potential is extensively more positive than the MIII/II redox potentials for Mn and Fe macrocycles [140,141]. However, numerous reports have proposed such sort relationship between the MIII/II formal potential and ORR activity of the macrocycles based electrocatalysts but it is yet misty that whether the formal potential of catalysts should be equal or close to ORR to achieve the maximum activity. However, in a large portion of the MN4–macrocycles (M = Mn and Fe), the ORR beginning potential in all respects firmly situated with the reduction of the metal ion. Because the metal ion in these macrocycles showed MII state which required the reduction of MIII in alkaline media according to Equation (9).

MIII  OH þ e ! MII þ OH

ð9Þ

where, MIII–OH is latent towards O2, as it is harmed by firmly adsorbed OH. So, O2 binds with MII form of metal center via an electron transfer step and formed an adduct according to Eq. (10).

MII þ O2 ! MIII  O2 or MðIIÞ  O2

ð10Þ

This M(II)–O2 adduct must be fleeting. Else it will obstruct further dioxygen molecules from connecting with the active site. The M(II)–O2 adduct will experience reduction as equation (11).

MIII  O2 þ e ! MII þ Intermediate

ð11Þ

The plan above is relevant to Mn and Fe macrocycles and in acidic media, these procedures will include a proton (H+). The last reaction step demonstrates the procedure in alkaline media and could include MII–O2 rather than MIII–O2 particularly when Co is

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the metal center. Next, as we discussed above, the Mphthalocyanines (Pc) (M = Fe, Mn, and Cr) promoted a 4e oxygen reduction [67] whereas CoPc follows 2e oxygen reduction process. In contrast, the dimeric CoN4-macrocycles arranged in the face-to-face geometric fashion showed the 4e ORR in acidic medium [20]. Biomolecules containing Co–N4 moiety such as Vitamin B12 has only one active site which proceeds through the 4e reduction over a wider pH range at high polarizations [62]. The predominating interactions between these macrocyclic moieties and O2 molecule strongly decrease the O–O bond order, lengthen the O– O link. Further, Fig. 17b showed another correlation of MIII/MII redox potential with the ORR activity for the MN4-macrocycles (M = Cr, Mn, Fe and Co), where the MIII/MII redox potential for some MN4macrocycles (M = Cr, Mn, Fe and Co) were found to be linear with corresponding ORR activity in the fashion of the rising portion of the volcano-shaped curve (labeled as 1). In contrast, on other side of the plot, some CoN4 macrocycles also pursued a similar linearity between ORR activity and CoIII/CoII redox potential (labeled as 2) but parallel to the labeled 1. These CoN4-macrocycles shown to have erroneous affinity with green dashed line which illustrates a promising volcano correlation in the fashion of a departure from the volcano correlation [142,143,62]. In this way, this projection of volcano-type plot clarified the left-hand sided macrocycles (labeled 1) promote the 4e except Cr–TSPc while the macrocycles lied right hand side (labeled 2) follow the 2e ORR [138]. Similarly, the volcano correlation evaluating ORR activity of several MN4-macrocycles in alkaline solution vs Eb(O2) is shown in Fig. 18a. Further, the DFT based calculations of metal–oxygen binding energy Eb(O2) for these macrocycles were reported [144,145]. These experimental and theoretical evidences also fully supported to the volcano plot followed by the separation of two sides with the 4e electrocatalysts from 2e electrocatalysts, where CrPc was considered as an exception, which is a 2e electrocatalyst. In summary of this section, for the particular MN4macrocycles based electrocatalyst confined to electrode surfaces, the descriptors for their ORR activity and reactivity have established [104]. These descriptors have connected the redox potential of MIII/MII redox couple with the M–O2 binding energy specially for the MnN4 and FeN4-macrocycles.

Fig. 17. Correlation between the MIII/MII formal potential of the MN4-macrocyclic complex and the binding energy of O2 to the MII metal center, Reproduced with permission [128]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim, and (b) Plots of catalytic activities for the ORR in alkaline media as current densities divided by the number of electrons involved in the reaction (n) for different catalysts and recorded at E = 0.24 V vs SCE. n = 4 for Cr, Mn, and Fe catalysts, and n = 2 for Co catalysts., Reproduced with permission [138]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

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Fig. 18. (a) Activity volcano correlation for the reduction of O2 in 0.1 M NaOH on different molecular MN4-catalysts adsorbed on ordinary pyrolytic graphite. Reproduced with permission [138]. Copyright, 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim., (b) Volcano plot for the electrocatalytic activity of different sulphonated MPcs adsorbed on graphite for O2 reduction in 0.1 M NaOH, as a function of the number of d-electrons in the metal., Reproduced with permission [146]. Copyright 2019, Nature Publishing Group., (c) Relative energies of frontier orbitals of O2 and Co Pcs. For simplicity, only one electron is shown on the SOMOs of the phthalocyanines, and (d) Plot of log k at constant potential for the ORR on different CoN4 catalysts confined on the surface of ordinary pyrolytic graphite, versus the donor-acceptor intermolecular hardness. Electrolyte: 0.1 M NaOH. Reproduced with permission[152]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

3.2. Number of d electrons Generally, number of d-electrons in transition metal of the MN4-macrocycles, is one of the factors contemplated in the literature. The plot (Fig. 18b) between experimentally measured potential at a particular current density and available d-electrons in transition metal of MN4-macrocycles showed a parabolic curve [146,38,138]. This kind of behavior was observed for both MPcs and MPs and indicated that the higher ORR activity for FeN4macrocycles can be attributed to the partially filled 3d orbitals of Fe metal ion [147]. Because of this reality, Mn and Co based N4macrocycles are near the zenith of the allegorical connection. This relationship connotes that FeN4-macrocycles are the most encouraging electrocatalysts for the ORR, due to their higher action as well as in light of the fact that they advance the 4e ORR with the burst of O–O bond [148]. 3.3. Intermolecular hardness (ɳDA) Another factor which controls the ORR activity of MN4macrocycles is the donor–acceptor intermolecular hardness (in terms of HOMO–LUMO energy gap) of M–O2 adduct. Pearson et al., suggested [149] that the molecule with low HOMO–LUMO energy gap (soft-molecule) can react rapidly as compare to hard molecule (high HOMO–LUMO energy gaps). The hardness (ɳDA) of a molecule can also be predicted in terms of ionization potential (IP) of donor (MN4-macrocycles) and electron affinity (EA) of acceptor (O2) [150] and can be calculated as ɳDA = 1/2 (ESOMO/D  ESOMO/A), (ESOMO/D = the energy of the singly occupied molecular orbital and ESOMO/A = the energy of a partially unoccupied molecu-

lar orbital) [151,147]. Fig. 18c showed the calculated energies of the SOMOs energy levels of the several CoPcs along with the degenerate SOMO energy levels of O2. According to this illustration, the electron-withdrawing substituents on the Pcs macrocyclic framework stabilize the SOMO energy levels by decreasing the e density on the cobalt center as a result decrease in the gap between SOMO of the CoPc and SOMO of O2, whereas it’s also true for opposite electron-donating substituents. Further, the plot between the rate constant and theoretical values of the hardness for these CoPcs is shown in Fig. 18d. On the other hand, the perturbation theory suggested that the similar energies frontier orbitals HOMO and LUMO would give the optimal interaction between to each other. In order, the ORR catalytic activity increases with decreases the donor–acceptor intermolecular hardness as estimated by using Pearson’s principle. Although the reactivity factor showed a good linear relationship between the ORR activity and hardness but these calculations cannot be acceptable in the presence of the solvent and applied potential [152,153,140]. 3.4. Theoretical Study’s contribution The comprehensive descriptions for the 4e ORR mechanism and parameters that control its competition with the 2e ORR, the breaking mode of O–O bond, stability of the electrocatalyst in  terms of intermediates such as H2O2, O 2 and OH are still not clearly understood [154,152]. The efforts to answer these questions should be considered as a central issue for ORR in fuel cells henceforth. In this regard, the understanding of DFT calculation can play an important role to solve the above problems and further design of desirable molecular based materials for ORR [155].

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DFT calculations revealed that the ORR electro-catalytic activity is particularly associated with: (i) the local environment (M–N coordination number) around the metal center, (ii) electro-negativity of the nearest neighboring atoms and (iii) the electronic effect of the substituents on the metal macrocyclic framework [144,145,151]. The DFT calculations not only suggest an approach that is aimed at synthesis of highly active non-precious ORR electrocatalysts, (e.g. Fe-pyridine/pyrrole-N4) but also offer the fundamental to design the macrocycles that can be utilized as a substitute for single-atom catalysts. Sun et al. effectively applied DFT calculations [156] on FePc, CoPc, FeP and CoP. They demonstrated the catalytic performance and proposed that FePc and FeP could reduce the O2 directly into H2O through a 4e transfer process whereas CoPc and CoP could reduce the O2 into H2O via 2e pathway [126]. Sun and coworkers further determined the ORR ability FePc, CoPc, FeP and CoP macrocycles. The optimized structures of the M–O2 adducts of the B3 state of Fe and Co Phthalocyanines are shown in Fig. 19a and b respectively with corresponding energy levels (a = spin-down orbital; b = spin-up orbital). The theoretical studies on these macrocycles revealed that the Pcs framework has 1u (occupied) and 2eg (unoccupied) MOs and a1g (d2z ), b1g (d2–y x ), 1eg (dzx, dyz), and b2g (dxy) as 3d-orbitals. The DFT calculations revealed that the eg-orbitals in the b state of FePc is more energetic than that of CoPc. Therefore, FePc can easily give away electrons from 3d-orbital (of metal) to p* orbital (of O2 bond) when compared to CoPc, which further results into weakening the O–O bond on FePc. Consequently, FePc showed higher ORR activity as compare to CoPc. Similarly, FeP has superior ORR activity than CoP. Conclusively, the study revealed that the higher energy level 3d-orbital (of the metal) resulted in the greater ability of oxygen reduction. Further, Zheng Shi et al., suggested [145] that the ORR activity of MN4-macrocycles is also interrelated to their ionization potential (IP) and O2-binding capacity. The IP and charge on macrocycles are the key factors for their O2-binding capacity. It was suggested that the greater the IP and O2-binding energy, better the ORR activity. Theoretical study revealed that the O2-binding abilities of the macrocycles depend upon the types of central metal ion,

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ligand and substituents such electron giving and electron pulling groups [81]. The electron-donating substituents increase the O2binding capacity and electron-withdrawing substitutes decrease the O2-binding capacity for CoPc systems. The ORR activity pattern watched for phthalocyanines and porphyrin frameworks can be justified with IP and O2-binding capacity. For porphyrin frameworks, cobalt subordinates have higher IP and, in this manner, have higher activity. For phthalocyanine frameworks, iron subsidiaries have great IP and extensive O2-binding energy and accordingly have better ORR activity [157]. Shi et al. applied the DFT estimation on different subordinates of phthalocyanines and porphyrines to propose the connection between the determined Mulliken charges on the transition metal ion and the principal IP of the framework (Fig. 20). For CoPc frameworks, a little difference in the charge on the cobalt molecules is related with the vast change in IP. Utilizing this relationship, one can estimated the IP and redox potential from determined Mulliken charge.

Fig. 20. Relationship between the calculated Mulliken charges on central metal and the first IPs of the systems., Reproduced with permission [145]. Copyright 2019, ACS Publishing Group.

Fig. 19. The optimized structures of the M–O2 adducts at the B3 state for (a) FePc, (b) CoPc, with corresponding Energy levels (a = spin-down orbital; b = spin-up orbital), Reproduced with permission [146]. Copyright 2019, ACS Publishing Group.

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Fig. 21. Optimized end-on configurations of (a, b) the O2 molecule and (c, d) the OH molecule adsorbed on MPc macrocyclic molecule. The central ball represents the transition-metal Fe or Co atom, blue balls represent N atoms, gray balls represent C atoms, and red balls represent O atoms and (e) Correlation plot showing the variation of the calculated adsorption energy of the OH (blue circles) and H2O2 (red squares) molecules as a function of the calculated adsorption energy of the O2 molecule on the Fe and Co macrocyclic complexes. The dashed lines are used to guide the eye, Reproduced with permission [158]. Copyright 2019, ACS Publishing Group.

Hui et. al, performed [158] a theoretical task to confirm the electronic effect of O2, OH and H2O2 species on the Fe/Co–N4–macrocycles by using DFT calculations. The theoretical study revealed that O2 and OH species an adopted the end-on adsorption configuration on the phthalocyanines and porphyrins macrocycles (shown in Fig. 21a, b, c and d). Further, the O–O and O–H bond projections in the plane of macrocycles were found to be aligned with respect to the bisect projection over the central metal and two closest Natoms in the macrocyclic framework. In contrast, H2O2 attains the Side-on fashion [145], the O–O link axis expands in the plan of macrocycles, both the oxygen atoms of O–O moiety are equidistant from the central metal or while, in the End-on fashion, one oxygen atom appeared right above and another one is far apart from the parent central metal and this configuration was suggested as the most promising optimized structure for the catalysts molecules. Fig. 21e shows a relationship between the hypothetical adsorption energy of the ORR intermediates (OH and H2O2) and O2 molecule adsorbed on the same macrocycles. The trend indicated that the adsorption energy of the ORR intermediates (OH and H2O2) directly proportional to the adsorption energy of dioxygen molecule on the macrocycles. This method of the theoretical studies on ORR of macrocycles drew the following conclusions: (1) The type of metal center is the key factor in determining the adsorption energies of dioxygen and ORR intermediates (OH and H2O2) molecules on the macrocycles. (2) The substitution on macrocyclic framework with electron donating and electron withdrawing groups can affects adsorption of dioxygen and ORR intermediates (OH and H2O2) on the macrocycles. (3) The N–M–N like structures, with a suitable N to N distance and N to M electronic collaboration, are required to break O–O link and thus endorse the efficient 4e–ORR pathway [159]. 4. Prospects and research direction In summary, the craving to locate an option of expensive Pt electrocatalyst for the ORR is the still in discussion for the advancement of energy unit innovations. The synthetic MN4-macrocycles are inconceivably versatile materials and could be used as incred-

ible models for ORR electrocatalytic activity. This survey deals with the fundamental chemistry of dioxygen, N4-macrocycles, M–O2 bond, ORR and macrocycles modified carbon material and makes an incredible photograph in the mind for the difficult round of material plan for ORR electrocatalysis. Further, the effectively reasonable and shortly explained macrocycles-based ORR descriptors in terms of redox potential, number of d-electrons and HOMO– LUMO ideas will be useful to comprehend that in what manner can the active site control the response of interest. Through investigating the ongoing advances in N4-macrocycles based frameworks for ORR catalysis, a few ends could be attracted and identified with the plan for the improvement of logically gainful electrocatalysts and activity descriptors. (a) MN4-macrocycles (M = Mn, Fe and Co metal ions) supported on conductive carbon material have been predominantly examined as cathode electrocatalysts for fuel cells. FeN4macrocycles catalyze the 4e ORR while CoN4-macrocycles catalyze the 2e ORR. This could be due to (i) low energy gap between the 3d-orbital (HOMO) of FePcs and p* orbital (LUMO) of O2, (ii) Due to the high stability of Fe–oxo intermediate, the O–O link breaking in the Fe–O2H2 unit is very simpler contrasted and Co–O2H2 unit. In contrast of monomeric CoN4-macrocycles, the dimeric CoN4-macrocycles arranged in the face-to-face geometric fashion could be catalyzed the ORR followed by 4e transfer mechanism due to a proper CoCo instances for O2 binding. (b) The metal centers (in terms of d-electrons), axial ligation (in terms of fifth ligand on the MN4-active site), distal pocket effect (in terms of push effect) and electron withdrawing/donating substituents on the core of MN4-macrocycles (in terms MII/III redox potential) could be considered as the key factors to alter the ORR activity. The e donor ligand at trans-axial position transfer the electron density at the central metal atom (well-known ‘‘push effect”), assists the binding of O2 by successive inner sphere mechanism and leads to easily O–O bond breaking. (c) The substitutions on macrocyclic framework play an important role to determine the reduction level of O2. Generally, electron-withdrawing groups on MN4-macrocycles such as porphyrins and corroles decreases the electron density on

A. Kumar et al. / Coordination Chemistry Reviews 402 (2020) 213047

the macrocyclic core, enhance its stability during catalysis, leads anodic shift in the reduction potential, favored for 4e ORR vice versa for electron donating groups. Further, the efforts in this survey resulted in some following challenges and future research direction related to the comprehension of description of the 4e ORR mechanism and parameters that control its competition with the 2e, (i) at what phase O–O bond cleavage takes place? (ii) What is the H+/e transport series during ORR? (iii) In addition, a main quest is related to stability, durability, and substantial upgrades are as yet unsolved about the ORR intervened by MN4-metallomacrocycles. The endeavors to address these inquiries identified with the nature and systems of these undesirable responses ought to be considered as a central issue for ORR in fuel cells henceforth. However, N4-macrocycles bolstered on the conductive carbon substrate (composites) is a great strategy to protect their ORR activity and can be a promising option in contrast to the Pt cathode fuel cells because of their high ORR execution yet they are still in shy towards the long-term stability. In any case, it may be conceivable to take care of this issue later on by thinking of some as vital components that can to be helpful in planning of non-Pt cathode electrocatalysts for ORR, for example, (i) MN4–macrocycles structures well stabilized on conductive carbon surfaces, (ii) nearby M–M locales that can adsorb O2 molecule with side-on fashion as in Pt, (iii) collaboration of MN4-macrocycles reaction sites with graphene surfaces that show high electrical conductivity and solid electronic cooperation with MN4-macrocycles. Acknowledgements This work was financially supported by the National Natural Science Foundation of China and Ministry of Foreign Affairs and International Cooperation, Italy (NSFC-MAECI 51861135202), Beijing University of Chemical Technology (start-up grant buctrc201901), the National Key Research and Development Project (Grant No. 2016YFF0204402), the Program for Changjiang Scholars and Innovative Research Team in the University (Grant No. IRT1205), the Fundamental Research Funds for the Central Universities, the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccr.2019.213047. References [1] C.W.B. Bezerra, Z. Lei, K. Lee, H. Liu, A.L.B. Marques, E.P. Marques, H. Wang, J. Zhang, Electrochim. Acta 53 (2008) 4937–4951. [2] E. Antolini, F. Cardellini, E. Giacometti, G. Squadrito, J. Mater. Sci. 37 (2002) 133–139. [3] C.W.B. Bezerra, Z. Lei, K. Lee, H. Liu, J. Zhang, S. Zheng, A.L.B. Marques, E.P. Marques, S. Wu, J. Zhang, Electrochim. Acta 53 (2008) 7703–7710. [4] A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catal. 2 (2012) 864–890. [5] Y. Garsany, O.A. Baturina, K.E. Swiderlyons, S.S. Kocha, Anal. Chem. 82 (2010) 6321–6328. [6] Y. Wang, Y. Zou, L. Tao, Y. Wang, G. Huang, S. Du, S. Wang, Nano Res. (2019) 1– 12. [7] J.H. Wee, K.Y. Lee, S.H. Kim, J. Power Sources 165 (2007) 667–677. [8] B. Šljukic´, C.E. Banks, R.G. Compton, J. Iran. Chem. Soc. 2 (2005) 1–25. [9] S. Walch, A. Dhanda, M. Aryanpour, H. Pitsch, J. Phys. Chem. 112 (2008) 831– 842. [10] C.E. Banks, A.H. Wylie, R.G. Compton, Ultrason. Sonochem. 11 (2004) 327– 331. [11] R.L. Doyle, I.J. Godwin, M.P. Brandon, M.E.G. Lyons, Phys. Chem. Chem. Phys. 15 (2013) 13737. [12] R. Baker, D.P. Wilkinson, J. Zhang, Electrochim. Acta 53 (2008) 6906–6919. [13] W. Schnurnberger, Chem. Ing. Tech. 69 (1997), 852-852.

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