Single Ru atom supported on defective graphene for water splitting: DFT and microkinetic investigation

DFT study on Al-doped defective graphene towards adsorption of elemental mercury

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

Self healing mechanisms of vacancy defects in graphene and silicene are studied using first principles calculations. We investigated host adatom adsorption, diffusion, vacancy formation and revealed atomistic mechanisms in the healing of single, double and triple vacancies of single layer graphene and silicene. Silicon adatom, which is adsorbed to silicene at the top site forms a dumbbell like structure by pushing one Si atom underneath. The asymmetric reconstruction of the single vacancy in graphene is induced by the magnetization through the rebonding of two dangling bonds and acquiring a significant magnetic moment through remaining unsaturated dangling bond. In silicene, three two-fold coordinated atoms surrounding the single vacancy become four-fold coordinated and nonmagnetic through rebonding. The energy gained through new bond formation becomes the driving force for the reconstruction. Under the external supply of host atoms, while the vacancy defects of graphene heal perfectly, Stone-Wales defect can form in the course of healing of silicene vacancy. The electronic and magnetic properties of suspended, single layer graphene and silicene are modified by reconstructed vacancy defects.

A DFT study of CO adsorption on the pristine, defective, In-doped and Sb-doped graphene and the effect of applied electric field

Stabilizing single‐atom metal catalysts with carbon materials and utilizing their synergistic effect remains challenging due to weak interactions between carbon‐based supports and metals. Density functional theory (DFT) calculations indicate that a single Ru atom was supported on a wide range of natural nanoporous carbon materials, including C2N, triazine‐C3N4 (T‐C3N4), and γ‐graphene. These carbon materials belong to a new generation of highly efficient electrocatalysts for the N2 reduction reaction (NRR) and are named Ru1@C2N, Ru1@T‐C3N4, and Ru1@γ‐graphyne, respectively. Ab initio molecular dynamic (AIMD) simulations show that a single Ru atom can be stably anchored in the nanopores of these carbon materials with strong cohesive energy. Compared with parallel adsorption configuration, the vertical adsorption configuration of N2 exhibits higher adsorption energy. The calculated Gibbs free energy reveals N2 reduction on the three catalysts via associative mechanisms. Despite the similar limiting potentials (−0.96, −0.94, and −0.98 V on Ru1@C2N, Ru1@T‐C3N4, and Ru1@γ‐graphynes, respectively), the limiting step differs, indicating the significant effects of carbon material substrates on electrochemical NRR. However, the competitive and efficient hydrogen evolution reaction (HER) changes the potential determining step and increases the overpotential for the electrochemical nitrogen reduction (NRR). This study provides insights for experimental synthesis of electrocatalysts for N2 reduction.

Designing and constructing high-density single-atom catalysts (SACs) is vital for electrochemical hydrogen evolution to meet the demand for fundamental research and practical applications of electrocatalysis. However, it is challenging to synthesize atomically dispersed electrocatalysts with high density and high performance. Herein, an integrated g-C3N4-C-TiO2 heterostructural nanosphere with oxygen-rich vacancies is constructed by a multicomponent assembly-calcination strategy. Abundant single Ru atoms (12.4 wt %) are then anchored via the occupation of partial oxygen vacancies, forming a unique Ru/g-C3N4-C-TiO2 heterostructure. A reasonable configuration is developed including single Ru atoms bonded with two oxygens and two nitrogens and coupled with jacent oxygen vacancies on the g-C3N4-C-TiO2 nanosphere. Density functional theory calculations reveal that the remaining oxygen vacancies are beneficial for water dissociation, while single Ru atoms facilitate hydrogen adsorption. As expected, the result exhibits high electrocatalytic activity, delivering overpotentials of 112 and 107 mV at 10 mA cm-2, Tafel slopes of 83 and 65 mV dec-1 in H2SO4 and KOH, and a turnover frequency of 0.28 H2 s-1 at -100 mV toward the hydrogen evolution reaction (HER). Benefiting from the outstanding electrocatalytic performance, such a unique heterostructure with dense single Ru sites and oxygen vacancies could serve as a prominent alternative HER catalyst for renewable energy applications.

Understanding reaction mechanism of oxygen evolution reaction using Ru single atoms as catalyst for Li-O2 battery

Watching the defectives: DFT studies on the growth of nickel clusters on different graphene substrates shows that graphene defects essentially determine the morphology of the clusters. Defects greatly improve the catalytic reactivity of the larger clusters Nin (5≤n≤10). Smaller clusters Nin (n<4) efficiently open the band gap of defective graphene, allowing a way to modulate the electronic properties of the graphene. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Downsizing a catalyst nanoparticle (NP) to a single atom (SA) has proven to be highly effective in increasing catalytic activity and decreasing the amount of catalyst required for various electrochemical reactions. However, insufficient stability of the single‐atom site catalysts (SACs) is still a significant challenge for their practical application. Here, SACs firmly bound to stable metal oxide NPs are proposed to dramatically increase the electrochemical activity and stability of SA‐based catalysts for hydrogen evolution reaction (HER). Starting from a Ru‐infiltrated, Zr‐based metal‐organic framework (MOF), the tetragonal zirconium oxide (ZrO2‐x) NPs‐embedded carbon matrix is fabricated as support through facile pyrolysis. Simultaneously, Ru SAs as active sites are well dispersed on the surface of ZrO2‐x NPs due to the generation of oxygen vacancies in the tetragonal ZrO2‐x. The Ru‐ZrO2‐x SAC exhibits a 4–5 times higher mass activity than commercial Pt and Ru catalysts and superior durability due to strong metal‐support interaction (SMSI) between Ru atoms and ZrO2‐x substrate.

Modulating interfacial charge distribution of single atoms confined in molybdenum phosphosulfide heterostructures for high efficiency hydrogen evolution

Maximizing the atom efficiency of noble metal species is crucial for their application, but directly reducing the size down to single atoms (SAs) is limited when facing complex situations requiring the simultaneous adsorption and activation of multiple reactants. Herein, Schiff base condensation of 1,3,5‐tris(4‐aminophenyl)benzene and 2,5‐dibromoterephthalaldehyde offered a Br‐tethered covalent organic framework (COF) TAPB‐DBDH for the co‐loading of ruthenium (Ru) SAs and nanoparticles (NPs). We demonstrated a triadic synergism of Ru SAs (Ru–C3N1), Ru NPs, and the TAPB‐DBDH framework in reductive aminations of carbonyl compounds with ammonia and hydrogen to primary amines, showing the high yield, large turnover number, and turnover frequency, stable recycling, and broad substrate compatibility. A systematic theoretic and experimental study revealed that the cooperation of Ru SAs and the TAPB‐DBDH framework provided a strong affinity toward the carbonyl compound and imine intermediate to promote their conversion, while the co‐existed Ru NPs lowered the energy barrier of H2 cleavage.

The correlation between metal loading and overall catalytic performance remains elusive for single-atom electrocatalysts (SACs), which hinders the oriented optimization of active site densities and scalable synthesis of them. To effectively address this issue, Ru single atoms with different loading are anchored on the graphene-like framework and tungsten carbide substrate (WC1-x) to investigate the synergistic effect among different local configurations. X-ray absorption spectroscopy demonstrated that the loading of Ru atoms critically governs the interatomic distance between adjacent metal active sites at second shell coordination. In situ Raman spectroscopy shows that WC1-x nanoparticles can break the hydrogen bond network by reorienting H2O molecule adsorption and promoting the availability of active H2O among electrode-electrolyte interface. Density functional theory (DFT) calculations demonstrated that the moderate distance between active sites could further lower the reaction barrier and enhance the catalytic activity. Consequently, the optimal sample Ru-WC1-x with 0.76wt% Ru loading exhibits a low overpotential of 7mV at 10mA⋅cm-2 and the anion exchange membrane electrolyzer to stably operate for 100h at 1A⋅cm-2. Such correlation of spatial effects between different active sites were universally demonstrated in similar systems anchored with either Pt, Ir, or Co elements.

Developing an inexpensive and efficient catalyst for a hydrogen evolution reaction (HER) is an effective measure to alleviate the energy crisis. Single-atom catalysts (SACs) based on Janus materials demonstrated promising prospects for the HER. Herein, density functional theory calculations were conducted to systematically investigate the performance of SACs based on the BiTeBr monolayer. Among the one hundred and forty models that were constructed, fourteen SACs with potential HER activity were selected. Significantly, the SAC, in which a single Ru atom is anchored on a BiTeBr monolayer with a Bi vacancy (RuS2/VBi-BiTeBr), exhibits excellent HER activity with an ultra-low |ΔGH*| value. A further investigation revealed that RuS2/VBi-BiTeBr tends to react through the Volmer-Heyrovsky mechanism. An electronic structure analysis provided deeper insights into this phenomenon. This is because the Tafel pathway requires overcoming steric hindrance and disrupting stable electron filling states, making it challenging to proceed. This study finally employed constant potential calculations, which approximate experimental situations. The results indicated that the ΔGH* value at pH = 0 is 0.056 eV for RuS2/VBi-BiTeBr, validating the rationality of the traditional Computational Hydrogen Electrode (CHE) method for performance evaluation in this system. This work provides a reference for the research of new HER catalysts.

The electrochemical synthesis of ethylene oxide (EO) using ethylene and water under ambient conditions presents a low‐carbon alternative to existing industrial production process. Yet, the electrocatalytic ethylene epoxidation route is currently hindered by largely insufficient activity, EO selectivity, and long‐term stability. Here we report a single atom Ru‐doped hollandite structure KIr4O8 (KIrRuO) nanowire catalyst for efficient EO production via a chloride‐mediated ethylene epoxidation process. The KIrRuO catalyst exhibits an EO partial current density up to 0.7 A cm−2 and an EO yield as high as 92.0 %. The impressive electrocatalytic performance towards ethylene epoxidation is ascribed to the modulation of electronic structures of adjacent Ir sites by single Ru atoms, which stabilizes the *CH2CH2OH intermediate and facilitates the formation of active Cl2 species during the generation of 2‐chloroethanol, the precursor of EO. This work provides a single atom modulation strategy for improving the reactivity of adjacent metal sites in heterogeneous electrocatalysts.

Open AccessCCS ChemistryRESEARCH ARTICLES22 Oct 2022Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N5 Site for Enhanced Electrocatalytic Reaction Bingyu Huang†, Senhe Huang†, Chenbao Lu, Longbin Li, Judan Chen, Ting Hu, Dirk Lützenkirchen-Hecht, Kai Yuan, Xiaodong Zhuang and Yiwang Chen Bingyu Huang† Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Senhe Huang† Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Chenbao Lu Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Longbin Li Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Judan Chen Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Ting Hu School of Materials Science and Engineering, Nanchang University, Nanchang 330031 , Dirk Lützenkirchen-Hecht Faculty of Mathematics and Natural Sciences-Physics Department, Bergische Universität Wuppertal, Wuppertal D-42119 , Kai Yuan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Xiaodong Zhuang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 and Yiwang Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 Institute of Advanced Scientific Research (iASR), Key Lab of Fluorine and Silicon for Energy Materials and Chemistry of Ministry of Education, Jiangxi Normal University, Nanchang 330022 https://doi.org/10.31635/ccschem.022.202202241 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metal porphyrins are star molecules that possess well-defined coordination metal centers for versatile catalytic reactions. However, most previous work has focused on the correlations between in-plane symmetric configuration of metal-N4 sites and their catalytic performance. Addressing the catalytic contribution of additional axial coordination to such symmetric configuration remains a challenge. Theoretical calculations revealed that axially anchoring an extra pyridine on the tetra-coordinated cobalt porphyrin (Co-N4) to construct penta-coordinated cobalt porphyrin (Co-N5) renders cobalt a higher electron density, thereby favoring the rate-determining O2 adsorption/activation and reducing the oxygen electroreduction barrier. Therefore, a well-defined Co-N5 site is rationally introduced into the azo-linked polymer framework for a fundamental structure–catalytic performance correlation study. As-prepared Co-N5 catalyst exhibits a 26 mV positive shift in half-wave potential compared with the pyridine-free Co-N4 counterpart, discloses a markedly higher power density (141.4 mW cm−2), and possesses better long-term durability (over 160 h cycles) in a Zn-air battery. Moreover, such a Co-N5 catalyst also showcases potential applications for CO2 reduction with high CO2-to-CO conversion faradic efficiency and better selectivity than the Co-N4 counterpart because coordination of the fifth pyridine evokes electronic localization that suppresses a competitive side reaction. This work proves the positive electrocatalytic contribution of axial penta-coordination on well-defined metal-porphyrin-based catalysts and offers atomic understanding of the structure–performance correlation on single atom catalysts for future catalyst design. Download figure Download PowerPoint Introduction Currently, many advanced electrocatalysts have been developed to facilitate the sluggish kinetics of the multiple-electron transfer process in energy conversion reactions, such as the oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2RR).1–4 Due to maximized atom utilization, mainstream research has focused on the isolated transition metal/nitrogen coordinated (M-N-C) single-atom catalysts (SACs) with high catalytic activity and selectivity.5–9 Despite extensive investigations, the rational design and controllable and precise synthesis of M-N-C catalysts continues to be the main obstacle to multiple-electron transfer processes.10–12 The preparation of M-N-C catalysts normally needs high-temperature pyrolysis to increase the graphitization degree for better conductivity.13–17 Unfortunately, the pyrolysis process inevitably evokes metal aggregation due to the thermal decomposition of metal precursors and high surface energy of single metal atoms, posing challenges for maintaining atomic-metal isolation.13,18,19 The inhomogeneity and indistinction of the catalytic environment of carbonaceous materials give rise to tremendous difficulties in simultaneously enhancing the catalysts' activity and selectivity. In addition, the inherently less-defined active sites formed after pyrolysis leaves an ambiguous structure–performance relationship and seriously precludes us from exploring the in-depth mechanism for different electrocatalytic reactions.20–24 Therefore, the above-mentioned challenges stimulate the vigorous search for developing cost-effective and high-performance pyrolysis-free electrocatalysts with well-defined active sites. Transition-metal macrocycles, such as cobalt porphyrins, which possess the distinct tetra-coordinated cobalt porphyrin (Co-N4) site, have been heavily studied.25–28 Benefitting from the production of fewer radical oxygen species during the electrocatalytic process, the Co-N4 structure is more robust and advantageous for electrocatalysis of ORR.16,20,29–31 However, the representative plane-symmetric electron configuration of Co-N4 is not the optimal structure for the chemisorption and activation of reactants.32–34 Breaking the structure symmetry with penta-coordination to regulate the charge redistribution of Co-N4 sites could promote the electrocatalytic process, but the accurate synthesis of this structural motif is still a noteworthy challenge. Besides, the role of penta-coordination and the exact local microenvironment of such active sites have not yet been ascertained.35,36 In view of this, offering a model catalyst as an ideal platform is urgently required to elucidate the contribution of axial penta-coordination on catalytic activity, selectivity, and durability of Co-N4 sites. In this work, we probed the positive effect of axial pyridinic penta-coordinated cobalt porphyrin (Co-N5) to achieve optimized electronic localization on the Co-N5 site for boosting ORR through density functional theory (DFT) calculations as well as electrochemical analysis. DFT calculations indicate Co-N5 with axial pyridinic coordination possesses obviously higher electronic density on the Co center in comparison with Co-N4. Taking advantage of the electronic-push effect of penta-coordination, the ORR rate-determining step of O2 adsorption/activation can be significantly promoted, ensuring better ORR performance of Co-N5 than the Co-N4 model. Hence, we innovatively designed and synthesized an azo-linked polymer framework with atomically dispersed electron-rich Co-N5 sites for ORR through a pyrolysis-free axial-pyridinic coordination strategy. The Co-N5 catalyst displays impressive ORR performance with a more positive half-wave potential of 0.811 V and lower Tafel slope of 39 mV dec−1 than the Co-N4 counterpart (0.785 V and 50 mV dec−1), which support the theoretical predictions. Encouragingly, such a penta-coordination-induced electronic localization strategy also promises the potential for improving the CO2-to-CO conversion faradic efficiency and selectivity of electrocatalytic CO2 reduction. This work provides new design strategies toward well-defined single-atom electrocatalysts with axial coordination and offers new models for fundamentally understanding the catalytic mechanism of asymmetric coordination systems. Experimental Methods Synthesis of [email protected] catalysts First, 20 mg (27.3 μmol) of CoTAPP and 80 mg of G-py were redispersed in 20 mL of dimethyl sulfoxide (DMSO). Then, the mixture was stirred for 15 h to ensure CoTAPP was fully coordinated to G-py. Immediately after the coordination, 35.2 mg (0.11 mmol, 4 equiv to CoTAPP) of PhI(OAc)2 was added to the mixed solution, and the mixture was stirred for 24 h. The resultant precipitate was filtered and washed with DMSO, methanol, deionized water, and ethanol for three times, and dried at 60 °C overnight. [email protected] was harvested as a black powder (86.8 mg, 87% yield). [email protected] and [email protected] were prepared with a procedure similar to [email protected] Experimental details, materials characterization methods, and synthesis of other samples are available in the Supporting Information. Results and Discussion Theoretical calculations and catalytic mechanism Based on cobalt porphyrin, two models with different coordination environments, axial pyridinic Co-N5 model and Co-N4 model were designed (Figure 1a and Supporting Information Figure S1). The elementary steps and corresponding ORR adsorption configurations on the two models are presented in Supporting Information Figure S2. The Gibbs free energies at different potentials for all elementary steps were evaluated to illustrate how the penta-coordination affects intrinsic ORR activity. As given in Figure 1b, all elementary steps are distinctly downhill at U = 0 V, thus the reaction is exothermic and able to proceed spontaneously. When the potential rises to 1.23 V, the rate-determining step of Co-N4 is the first step (* + O2 + H2O + e− → *OOH + OH−) with a high energy barrier of 0.34 eV. In contrast, the final release step (*OH + e− → OH− + *) of the Co-N5 model is the rate-determining step, and its free energy can be distinctly reduced to only 0.26 eV. The thermodynamic limiting potentials, which represent the maximum potential to ensure all steps downhill are 0.97 and 0.89 V for Co-N5 and Co-N4 models, respectively (Figure 1c and Supporting Information Figure S3), revealed the Co-N5 model requires the minimum overpotential to drive the oxygen reduction. This result implies that the axial pyridine coordination plays a crucial role in regulating the ORR intermediates adsorption strength and decreasing the reaction barrier of the rate-determining step. Similarly, axial pyridine coordination can also alter the Fe electronic microenvironment, thereby enhancing the catalytic activity of the Fe-N5 model ( Supporting Information Figures S4–S7), further validating the universality of additional axial coordination. Figure 1 | (a) DFT calculation models of Co-N5 and Co-N4 for the electrochemically catalyzed ORR. (b) Free energy diagrams at U = 0 V and U = 1.23 V, and (c) free energy diagrams for the thermodynamic limiting potentials of Co-N5 model and Co-N4 model. (d) PDOS of Co atom for Co-N5 model (top) and Co-N4 model (bottom); the d-band center is denoted by the dashed gray line. Differential charge density distribution after O2 absorption on (e) Co-N5 model and (f) Co-N4 model. Download figure Download PowerPoint To further study axial coordination induced changes in the Co center's electronic configuration and interaction with oxygen-containing intermediates, projected density of states (PDOS) was conducted. The axial coordination obviously tunes the Co 3d orbital according to the PDOS in Figure 1d. The d-band center of the Co-N5 model at −1.59 eV is closer to the Fermi level than that of Co-N4 at −1.76 eV, thereby leading to an increase in O-containing intermediates' adsorption.37 The enhanced adsorption ensures subsequent ORR steps proceed through a more efficient four-electron pathway. Hence, the thermodynamic onset potentials improvement can be ascribed to fine-tuned adsorption strength of the ORR intermediates, which directly determines the activity and selectivity of catalysts. In addition, the higher PDOS near the Fermi level represents more abundant charge carriers and better electronic conductivity for the Co-N5 model. During the ORR process, the activated Co d orbitals and their hybridization with O p orbitals co-determine the adsorption strength for oxygen-containing adsorbates.38–42 After O2 adsorption and *OH formation, the increased overlapping degree of the strong σ-bond that originates from the Co dz2 orbital and O p orbital, along with the decreased O2 antibonding orbital filling degree that appears above the Fermi level for Co-N5 model, can be observed. It theoretically suggests that the extra fifth pyridine coordination assures a tighter connection between Co centers and O2 ( Supporting Information Figures S8 and S9), thus enabling higher ORR selectivity towards the four-electron pathway.43 As verified by charge density differences ( Supporting Information Figure S10), obvious asymmetrical charge distribution caused by axial coordination can be found for the Co-N5 model in comparison with the symmetric Co-N4 model. An apparent charge accumulation on the Co center is found to form the electron-rich Co-N5 site due to the electronic-push effect of axial pyridine. As expected, the O2 only absorbs on the individual Co center (Figure 1e,f), which can provide superior ORR electrocatalytic sites. Furthermore, the axial pyridine can construct an electronic pathway that renders adequate charge transfer to oxygen molecules from the conductive graphene layer. In general, the electron-rich Co-N5 site promises stable chemisorption and activation of O2, which facilitates O–O bond cleavage, thus offering better selectivity for the four-electron reduction pathway. Therefore, the axial-pyridine coordination-induced electronic localization strategy is viable to efficaciously enhance the ORR kinetics. Synthesis and structural characterization To experimentally confirm the calculation results and demonstrate the significance of penta-coordination architecture in ORR, an azo-linked penta-coordinated cobalt porphyrin-based polymer (CoTAPP-Azo) anchored on pyridine functionalized graphene (G-py) ([email protected]) was synthesized. The synthesis route and structure of [email protected] and the counterpart [email protected] (directly grown CoTAPP-Azo on pristine graphene) are schematically revealed in Figure 2a. Tetrakis(4-aminophenyl) porphyrin (TAPP) was first chelated with cobalt cation to obtain CoTAPP ( Supporting Information Figure S11). TAPP coordinating with Fe (FeTAPP) and Ni (NiTAPP) were also successfully obtained using the same procedure. The formation of CoTAPP, FeTAPP, and NiTAPP were confirmed by mass spectrometry ( Supporting Information Figure S12) and Fourier-transform infrared (FTIR) spectroscopy ( Supporting Information Figure S13). The successful pyridine functionalization in G-py was confirmed by thermogravimetric analysis ( Supporting Information Figure S14), Raman spectroscopy ( Supporting Information Figure S15), and X-ray photoelectron spectroscopy (XPS) ( Supporting Information Figure S16). Subsequently, CoTAPP was anchored on G-py through axial pyridine coordination; following an azo-coupling reaction, [email protected] was finally obtained (see details in Supporting Information Scheme S1). For comparison, FeTAPP-based polymer hybridized with G-py ([email protected]) and graphene ([email protected]) and NiTAPP-based polymer hybridized with G-py ([email protected]), were synthesized by similar routes. Figure 2 | (a) Synthetic route of CoTA[email protected]y and [email protected] (b) HAADF-STEM image of [email protected] (c) High-resolution Co 2p XPS spectra of [email protected] and [email protected] Download figure Download PowerPoint The morphology of [email protected] was first identified by scanning electron microscopy (SEM). Compared with the thin-layered sheet structure of G-py, a thicker layer was observed, indicating G-py was covered by CoTAPP-Azo ( Supporting Information Figures S17 and 18). The transmission electron microscopy (TEM) image of [email protected] also presented representative lamellar plate morphology ( Supporting Information Figure S18c). To further discern the structural features at the atomic level, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out. HAADF-STEM images and the corresponding energy dispersive spectroscopy (EDS) of [email protected] indicated homogeneous spatial distribution of C, N, Co elements ( Supporting Information Figure S19). The abundant bright points (highlighted by red circles) offer direct evidence for the uniform distribution of single Co atoms (Figure 2b). Similarly, [email protected], [email protected], and [email protected] all exhibited similar lamella morphology with homogeneously distributed elements ( Supporting Information Figures S20–S22). The chemical structures of [email protected] were preliminarily examined by FTIR spectroscopy ( Supporting Information Figure S23). Compared with CoTAPP, the corresponding FTIR spectrum of [email protected] displayed a significant intensity increase of N=N stretching signals (1570 cm−1, 1215 cm−1) along with an obviously weakened N–H peak at 3400 cm−1, proving the successful polymerization of CoTAPP. The increased ID/IG ratio of 0.94 for [email protected] to 1.07 for [email protected] in the Raman spectra ( Supporting Information Figure S24) verifies surface lattice interference caused by penta-coordination. X-ray diffraction (XRD) confirmed that G-py was coated by CoTAPP-Azo, and the absence of crystalline cobalt species diffraction peaks demonstrated the highly dispersed state of Co atoms ( Supporting Information Figure S25), agreeing well with the HAADF-STEM observations. [email protected] and [email protected] both show similar XRD patterns ( Supporting Information Figures S26 and S27). Furthermore, XPS was extracted to unravel the nature of the chemical bonding of [email protected] and [email protected] ( Supporting Information Figures S28–S31). Compared with [email protected], the N 1s XPS spectrum for [email protected] displayed an obvious pyridinic N peak (398.3 eV, Supporting Information Figure S29). The C 1s XPS spectrum of [email protected] showed that the C–N peak shifts to higher binding energy (BE) than that of [email protected] (ΔBE = 0.4 eV), revealing the decreased electron density of C atom. In the Co 2p spectra (Figure 2c), [email protected] underwent a shift to lower BE for Co 2p3/2 (780.3 eV) and Co 2p1/2 (795.6 eV) peaks, relative to that of [email protected] (780.8 and 796.0 eV for Co 2p3/2 and Co 2p1/2, respectively), providing evidence for the electronic localization on the Co with penta-coordination.44,45 This result confirms that axial pyridine ligands act as channels between G-py and CoTAPP-Azo and boost the charge transfer from graphene to Co centers, which is consistent with the electron-rich Co-N5 model in theoretical calculations. Importantly, similar phenomena were found in [email protected] and [email protected], suggesting the universality of such an electron localization approach. The UV–vis spectrum of [email protected] revealed a clear porphyrin Soret band at 448 nm ( Supporting Information Figure S32). Interestingly, an appreciable peak change to 438 nm in [email protected] was observed due to the penta-coordination-induced electron transfer. Meanwhile, photoluminescence spectroscopy was conducted to explore the charge separation behaviors ( Supporting Information Figure S33). Significantly increased quenching occurred in [email protected] compared with [email protected], further demonstrating that axial pyridine can enhance the charge transfer between G-py and CoTAPP-Azo. The work function, which represents the minimum energy needed to draw one inner electron from the nucleus, was obtained by ultraviolet photoelectron spectroscopy. [email protected] showed a 0.51 eV shift to higher BE than [email protected] in the second electron cut-off edge ( Supporting Information Figure S34). Hence, with respect to 4.49 eV for [email protected], the smaller work function for [email protected] (3.98 eV) indicated that the electrons are more likely to be activated and transferred outward.46,47 We assessed the porosity of the samples by nitrogen physisorption isotherms ( Supporting Information Figure S35). [email protected] and [email protected] both displayed the typical type-IV isotherms with an evident hysteresis loop, indicative of the coexistence of micropores and mesopores. The Brunauer–Emmett–Teller specific surface areas for [email protected] and [email protected] were calculated to be 506.6 and 440.3 m²/g, respectively. Pore size distribution demonstrated by nonlocalized DFT showed the pore widths centered at 1.9 and 4.8 nm for [email protected] ( Supporting Information Figure S36). These results demonstrate the improved surface area and hierarchical porous structure of [email protected], which are conducive to increase the quantity of accessible active sites and enhance the mass transport. To investigate the local coordination environment of the Co center, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were examined. The stronger white-line peak intensity of [email protected] (7727 eV) compared with [email protected] was assigned to the larger coordination number of the Co center for [email protected] than [email protected] (Figure 3a).48 The quantitative fits ( Supporting Information Table S1) showed a Co atom in [email protected] was straightforwardly connected by N atoms with a coordination number of 5.1 and the average bond length of 1.98 Å. From the Co K-edge Fourier-transformed EXAFS spectra (Figure 3b), the first shell peak for [email protected] around 1.43 Å corresponded to the Co–N scattering path, accompanying the absent Co–Co peak at 2.18 Å, jointly signifying the isolated Co atoms' configuration. [email protected] displayed an obviously positive shift for the Co–N first shell in than [email protected], suggesting an Co–N bond length caused by the to the EXAFS results (Figure and Supporting Information Figures and in to an of for [email protected], the first shell intensity for [email protected] was enhanced with an to revealing the larger number of N coordination The was also for the coordination In Figure the intensity maximum at for [email protected] was assigned to Co–N Co–Co and corresponding coordination was demonstrating the of the Co The shift of [email protected] maximum compared with that of CoTAPP is caused by the in [email protected] Hence, on and EXAFS as well as the XPS we that [email protected] by Co-N5 sites was successfully synthesized. Figure | (a) Co K-edge spectra and (b) the of Co K-edge EXAFS spectra of [email protected] with Co and CoTAPP as (c) The corresponding EXAFS of [email protected] model with Co N and C (d) of [email protected] in comparison with Co and CoTAPP. Download figure Download PowerPoint Electrocatalytic ORR performance To how axial penta-coordination affects ORR [email protected] and [email protected] were first evaluated with a In ( Supporting Information Figure significant peaks in for [email protected], but a peak was observed in The (Figure and Supporting Information Figure that [email protected] the ORR performance with the most positive half-wave potential of 0.811 V which is 26 mV more positive than that of [email protected] (0.785 V These results indicate that [email protected] possesses superior catalytic performance that from the electron localization caused by pyridine penta-coordination. in comparison with [email protected], the density for [email protected] was enhanced because the axial pyridine as between G-py and CoTAPP-Azo, thus boosting the charge transfer from graphene to CoTAPP layer. Figure 4 | (a) ORR of [email protected] and [email protected] catalysts at (b) Tafel (c) density by electrochemically active surface and (d) comparison of Tafel and for [email protected] and [email protected] catalysts. (e) The of [email protected] and [email protected] and after (f) and power density specific for Zn-air at and long-term performance of Zn-air at a density of using [email protected], [email protected], and as catalysts. Download figure Download PowerPoint To further into the reaction kinetics of [email protected], the at from to were ( Supporting Information Figure The ( Supporting Information Figure which were from at different potentials, revealed the toward the of Based on the the number for [email protected] was which with the This proves that the Co-N5 site a four-electron pathway towards ORR, [email protected] has a smaller of ( Supporting Information Figure We the enhanced ORR selectivity to the Co-N5 configuration that can be with because of the effect between Co and pyridinic the resultant O–O bond with stronger stretching is thus providing higher selectivity for four-electron ORR. [email protected] has a more positive than [email protected] ( Supporting Information Figures that ORR activity can be improved by microenvironment through extra axial penta-coordination on different metal Moreover, the Tafel slope of 39 mV dec−1 the process of [email protected] for oxygen reduction (Figure and Supporting Information Figure by the of electrochemical spectroscopy ( Supporting Information Figure the decreased in the for [email protected], indicating the optimized and electron toward ORR kinetics. In addition, [email protected] exhibited the electrochemically active surface area ( Supporting Information Figure which is conducive to more active sites. density with an in the ORR can be found (Figure the intrinsic activity for [email protected] For further comparison, the performance density Tafel density and for different electrocatalysts are (Figure and