Synergistic Regulation of Oxygen Reduction Activity on Antimonene via Transition Metal-Nonmetal Dual-Atom Doping.

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

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

Searching for high-activity, stable and low-cost catalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of significant importance to the development of renewable energy technologies. By using the computational screening method based on the density functional theory (DFT), we have systematically studied a wide range of transition metal (TM) atoms doped a defective BC3 monolayer (B atom vacancy VB and C atom vacancy VC), denoted as TM@VB and TM@VC (TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt), as efficient single atom catalysts for OER and ORR. The calculated results show that all the considered TM atoms can tightly bind with the defective BC3 monolayers to prevent the atomically dispersed atoms from clustering. The interaction strength between intermediates (HO*, O* and HOO*) and catalyst govern the catalytic activities of OER and ORR, which has a direct correlation with the d-band center (εd) of the TM active site that can be tuned by adjusting TM atoms with various d electron numbers. For TM@VB catalysts, it was found that the best catalyst for OER is Co@VB with an overpotential ηOER of 0.43 V, followed by Rh@VB (ηOER = 0.49 V), while for ORR, Rh@VB exhibits the lowest overpotential ηORR of 0.40 V, followed by Pd@VB (ηORR = 0.45 V). For TM@VC catalysts, the best catalyst for OER is Ni@VC (ηOER = 0.47 V), followed by Pt@VC (ηOER = 0.53 V), and for ORR, Pd@VC exhibits the highest activity with ηORR of 0.45 V. The results suggest that the high activity of the newly predicted well dispersed Rh@VB SAC is comparable to that of noble metal oxide benchmark catalysts for both OER and ORR. Importantly, Rh@VB may remain stable against dissolution at pH = 0 condition. The high energy barrier prevents the isolated Rh atom from clustering and ab initio molecule dynamic simulation (AIMD) result suggests that Rh@VB can remain stable under 300 K, indicating its kinetic stability. Our findings highlight a novel family of efficient and stable SAC based on carbon material, which offer a useful guideline to screen the metal active site for catalyst designation.

Advancing efficient catalysts for oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) is imperative for commercializing emerging energy devices. Using density functional theory (DFT) calculations, we propose doping different transition metal (TM) atoms to regulate the electronic structures of the two-dimensional 1T-HfTe2 monolayer to achieve bifunctional catalysis for the ORR/OER. Due to the small electronegativity of the Hf atom, we found the doped TM atoms can generally form anion centers by accepting abundant charges from the Hf interlayer. At the same time, the highly conductive 1T-HfTe2 contributes to the charge transfer between the active center and the reaction intermediates, rendering the designed SACs the tunable activity for the reactions. By comparing the theoretical overpotentials of ORR and OER on 15 single-atom catalysts (SACs), Pt-doped system exhibits excellent catalytic activity for both ORR and OER, outperforming the traditional Pt(111) and RuO2(110) catalysts. Based on the charge transfer mechanism, we clarified that the doped TM atoms act as a ‘bridge’ to transfer the electrons from the substrate to the reaction intermediates, thereby effectively contributing to the improvement of catalytic activity. In summary, our study shows that, by doping appropriate TM atoms, the intrinsic inert HfTe2 can be activated toward efficient ORR/OER. This could provide some guidance for the design of new two-dimensional ORR/OER bifunctional catalyst materials.

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines Yang Wang†, Zisheng Zhang†, Xiao Zhang, Yubo Yuan, Zhan Jiang, Hongzhi Zheng, Yang-Gang Wang, Hua Zhou and Yongye Liang Yang Wang† Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zisheng Zhang† Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Xiao Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027 , Yubo Yuan Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zhan Jiang Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Hongzhi Zheng Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Yang-Gang Wang Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Hua Zhou X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439 and Yongye Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Southern University of Science and Technology, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202000590 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The two-electron electrochemical reduction of oxygen is an appealing approach to produce hydrogen peroxide. Metal and heteroatom-doped carbon (M–X/C) materials have recently been recognized as compelling catalysts for this process, but their performance improvement is generally hindered by the ill-defined structures of active sites. Herein, we demonstrate a theory-driven design of catalysts for oxygen reduction reactions based on molecularly dispersed electrocatalysts (MDEs) with metal phthalocyanines on carbon nanotubes. Density functional theory calculations suggest that nickel phthalocyanine (NiPc) favors the formation of *H2O2 over *O, thus acting as a selective catalyst for peroxide production. NiPc MDE shows high peroxide yields of ∼83%, superior to the aggregated NiPc and pyrolyzed Ni–N/C catalysts. The performance is further enhanced by the introduction of the cyano group (CN). NiPc–CN MDE exhibits ∼92% peroxide yields and good stability. Our studies provide a new perspective for the development of heterogeneous electrocatalysts for hydrogen peroxide production from metal macrocyclic complexes. Download figure Download PowerPoint Introduction Driving economically important chemical reactions with renewable electricity offers an intriguing opportunity to replace current energy-intensive processes.1–3 For example, the electrochemical oxygen reduction reaction (ORR) through the two-electron (2e−) pathway is considered an environmentally benign alternative to the industrial anthraquinone method to produce hydrogen peroxide (H2O2), which is widely used as a green oxidizer in bleaching, waste water treatment, and the chemical industry.4–6 An ideal electrocatalyst should possess high activity toward the 2e− pathway to the peroxide product and suppress the competing 4e− process to water. Platinum (Pt) or palladium (Pd) alloys with mercury (Hg) have been demonstrated as selective electrocatalysts for the 2e− pathway in ORR.7,8 However, due to the toxicity of Hg and the limited reserve of noble metals, these elements are not preferred for practical applications. Carbon-based materials doped with earth-abundant elements, however, are compelling candidates as efficient and affordable electrocatalysts.9–14 Carbon nanotube (CNT), graphene, and activated carbon with oxygen-containing functional groups were reported to be selective in 2e− ORR.15–18 Embedding coupled boron–nitrogen (BN) domains into graphitic carbon showed enhanced selectivity and activity for reducing O2 to HO2− compared with the catalysts with individual B or N doping.19 In addition to metal-free catalysts, metal and heteroatom-doped carbon (M–X/C) catalysts with isolated heteroatom-coordinated metal moieties, one type of single-atom catalysts (SACs), have also been exploited for H2O2 production.20–23 A series of M–N/C (M = Mn, Fe, Co, Ni, and Cu) catalysts with proposed M–N4 active sites were synthesized to investigate their performance in ORR. The Co–N/C catalyst showed preference for the 2e− pathway in acidic condition.24 Moreover, SACs with transition-metal centers coordinated by different heteroatoms, such as O and S, were also reported to show high selectivities for the 2e− reduction pathway in ORR.25–28 However, the lack of well-defined structures and the copresence of various types of active sites prevent understanding the structure–performance relationships and catalyst design principles in these SAC catalysts. Metal macrocyclic complexes, such as metal phthalocyanines (MPcs) and porphyrins with well-defined M–N4 moieties, have been attractive electrocatalysts since the report of cobalt phthalocyanine as an active ORR catalyst.29–31 For instance, iron phthalocyanine (FePc) has been reported to be efficient in catalyzing ORR through the 4e− pathway to water.32,33 However, the performances of metal complexes in heterogeneous form are often limited by their low electric conductivity.34–36 Hybridizing metal macrocyclic complexes with nanocarbon materials were found to promote their catalytic performances.35,37,38 In the carbon dioxide reduction reaction, achieving molecular dispersion on conducting supports is beneficial to reveal the intrinsic performance of molecular catalysts and establish catalyst design principles.39,40 In addition, previous reports of heterogeneous molecular ORR catalysts mainly focused on optimizing the performance toward the 4e− pathway with little exploration of the 2e− pathway to peroxide production.30,41 In this work, we present a theory-driven design of electrocatalysts based on an molecularly dispersed electrocatalyst (MDE) consisting of dispersed MPcs on CNTs for electrochemical production of peroxide. From density functional theory (DFT) calculations, we identify nickel phthalocyanine (NiPc) as a selective catalyst for 2e− ORR with experimental peroxide yields of ∼83% in the form of MDE, in contrast to FePc MDE that is selective for 4e− ORR. Achieving molecular dispersion of NiPc with well-defined Ni–N4 sites is important to the high peroxide selectivity as proven by the lower peroxide yields of the physically mixed NiPc and CNT (containing aggregated NiPc) and a pyrolyzed Ni–N/C SAC. Moreover, molecular engineering of NiPc MDE with the introduction of cyano groups (CNs) to the Pc ligand (NiPc–CN MDE) further enlarges the free-energy preference to the 2e− pathway and enhances the selectivity for the electrochemical production of peroxide. NiPc–CN MDE exhibits a high peroxide yield of ∼92% in the potential range of 0.70–0.20 V versus a reversible hydrogen electrode (RHE). Experimental Methods Preparation of MPc MDEs The preparation of MPc MDEs was based on a reported procedure with the control of the ratio between MPcs and CNTs.40 NiPc and FePc were obtained from commercial sources, and NiPc–CN was synthesized according to a reported method.40 Briefly, 30 mg purified CNTs were dispersed in 25 mL of N,N-dimethylformamide (DMF) with the assistance of sonication, in which a calculated amount of MPcs in 5 mL of DMF was added to obtain a well-mixed suspension. The mixture was further sonicated for 30 min and then stirred at room temperature for 20 h. Subsequently, the precipitate was collected by centrifuge and washed with DMF (three times) and ethanol (twice). Finally, the collected precipitate was lyophilized to yield the final product. Electrochemical measurements About 4 mg of MPc MDEs and 10 μL of 5 wt % Nafion solution were dispersed in 990 μL ethanol under ultrasonication to form a homogeneous ink. About 13 μL catalyst ink was loaded onto the glassy carbon (GC) disk electrode (5.5 mm in diameter) of a rotating ring-disk electrode (RRDE) to achieve a catalyst loading of ∼0.2 mg cm−2. The ink of NiPc + CNT was prepared by dispersing 2.8 mg of NiPc, 1.2 mg of CNT, and 10 μL of 5 wt % Nafion solution in 990 μL ethanol under ultrasonication, then loaded onto the GC electrode. The RRDE experiments were conducted with a four-electrode system using a saturated calomel electrode (SCE) as the reference electrode (calibrated with a homemade RHE), a graphite rod as the counter electrode, and the catalyst-modified GC disk electrode as the working electrode. Meanwhile, the Pt ring electrode was kept at 1.5 V (vs RHE, the same for following potentials unless otherwise stated) for all experiments. The disk and ring electrodes were rotated at a speed of 1600 rpm (Pine research). Electrolytes (0.1 M KOH) were saturated with O2 by bubbling for 30 min prior to each experiment, and a flow of O2 was maintained over the electrolyte during the reaction. Linear sweep voltammetry (LSV) was conducted by scanning the disk electrode potential with a scan rate of 5 mV/s. For the stability test, the disk electrode potential was kept at 0.5 V. Experiments were also performed under an argon environment to record the background currents of the disk and ring electrodes, which were subtracted from the currents under O2. The peroxide yield and electron transfer number (n) were determined by the following equations: Peroxide yield ( HO 2 − ) = 200 × ( I r / N ) I d + ( I r / N ) % n = 4 × I d I d + ( I r / N ) where Ir is ring current, Id is disk current, and N is current collection efficiency of the Pt ring electrode (0.28, calibrated with K3[Fe(CN)6]). Computational Methods DFT calculations of gas-phase MPc molecules catalyzing ORR were conducted using the Gaussian 09 program.42 B3LYP functional43 with D3 correction (Becke–Johnson damping)44 was adopted for calculation.45 The all-electron 6-31G* basis set (for H, C, N, and O)46–48 and the Stuttgart–Dresden (SDD) basis set containing all double-ξ valence with effective core potentials (ECPs)49 (for Ni and Fe) were used. The geometric structures were all optimized at 298.15 K and under 1 atm. The harmonic vibrational frequencies were computed with no imaginary frequency found for all reaction intermediates. The Gibbs free energies of high- and low-spin forms of all intermediates were calculated with the harmonic potential approximation to determine the ground states. The electrocatalytic mechanisms were investigated with the computational hydrogen electrode (CHE) model.50 Additional details of computational methods are available in the Supporting Information. Results and Discussion Theoretical calculations of MPcs catalyzing ORR To understand how the central metals in MPc molecules affect the product selectivity in ORR, DFT calculations of the free-energy changes of ORR through the 2e− and 4e− pathways were conducted on FePc and NiPc at 1.23 V versus RHE. The calculated free-energy diagrams suggest distinctly different ORR behaviors of FePc and NiPc (Figure 1a). On FePc, O2 is first adsorbed on the Fe center, followed by a proton-coupled electron transfer (PCET) process to form *OOH with an uphill free-energy change. The divergence of the 2e− and 4e− pathways came from the preference of *OOH reduction with a protonation mechanism to *H2O2 or an O–O cleavage to *O. The downhill free-energy change to form *O and the large free-energy increase required for the generation of *H2O2 indicate a high preference for the 4e− reduction pathway on FePc, consistent with high selectivities of O2 reduction to water/hydroxide of Fe macrocyclic complexes and Fe-based SACs in previous reports.34,51 By contrast, the *OOH intermediate (generated from O2 through an *O2 intermediate with two uphill free-energy changes) on NiPc shows a slight downhill free-energy change to generate *H2O2, while the formation of *O in the 4e− reduction pathway is energetically uphill (Figure 1a). In contrast to FePc, the reversed trend in free-energy changes to form *H2O2 and *O on NiPc suggests the preference for the 2e− reduction pathway. Therefore, NiPc molecules are predicted to be selective electrocatalysts for 2e− ORR to peroxide product (Figure 1b). Figure 1 | Theoretical calculations of ORR catalyzed by MPcs. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and FePc at 1.23 V. (b) Schematic presentation of ORR selectivity on NiPc and FePc based on DFT calculations. Download figure Download PowerPoint ORR performance of MPc MDEs and aggregated MPcs Dispersed NiPc and FePc molecules were supported on the CNTs via π–π interactions to fabricate MPc MDEs according to our previous method40 to examine the calculated trends in ORR. The metal contents in MDEs were controlled to be ∼0.7 wt % ( Supporting Information Table S1), which were measured by inductively coupled plasma mass spectrometry (ICP-MS). The electrochemical behaviors of NiPc MDE and FePc MDE were first investigated in O2-saturated 0.1 M KOH electrolytes with the RRDE setup (0.2 mg cm−2 catalyst loading). The MDEs were drop-coated on the disk electrode as the working electrode to reduce O2, while the ring electrode (Pt) was maintained at 1.5 V to detect the produced peroxide. FePc MDE shows more positive onset potential (0.94 V at −0.025 mA, corresponding to a current density of ∼−0.1 mA cm−2) than that of NiPc MDE (0.79 V) (Figure 2a). The current of FePc MDE is saturated to −1.41 mA at ∼0.58 V. The saturation current for NiPc MDE is about −0.63 mA at the same potential. The peroxide yield and n of MPc MDEs calculated from the disk and ring currents are depicted in Figure 2b and Supporting Information Figure S1, respectively. NiPc MDE shows good peroxide yields of ∼83% in the potential range of 0.70–0.53 V, which decline at more negative potentials. Correspondingly, n of NiPc MDE is below 2.34 in the potential range of 0.70–0.53 V, which gradually increases to 2.83 from 0.53 to 0.20 V ( Supporting Information Figure S1). In contrast, low peroxide yields of ∼1% together with n above 3.97 in the potential range of 0.70–0.20 V are observed with FePc MDE (Figure 2b and Supporting Information Figure S1), confirming its strong preference toward the 4e− reduction pathway. These results indicate that the preferred ORR pathways of NiPc MDE and FePc MDE are the 2e− and 4e− reduction pathways, respectively, which are consistent with the DFT calculations (Figure 1a). Figure 2 | ORR performance of MPc-based electrocatalysts on RRDE. (a) Disk and ring currents of NiPc, NiPc + CNT, NiPc MDE, and FePc MDE in O2-saturated 0.1 M KOH electrolytes on RRDE test rotating at 1600 rpm. (b) Calculated peroxide yields of NiPc MDE, NiPc + CNT, and FePc MDE. Download figure Download PowerPoint The effects of aggregation state of NiPc were further investigated. The NiPc molecules directly deposited on substrates easily formed aggregates due to their strong intermolecular interactions ( Supporting Information Figure S2). Due to the poor electric conductivity and limited exposure of active sites of aggregated NiPc, the neat NiPc electrode shows minimal activity in ORR (Figure 2a). Therefore, we physically mixed NiPc with CNTs (denoted as NiPc + CNT) to enhance the conductivity. LSV shows that the physically mixed NiPc + CNT possesses higher activity than that of neat NiPc (Figure 2a). Although NiPc + CNT exhibits even more positive onset potential than NiPc MDE, the low peroxide yields of NiPc + CNT (under 60% in the potential range of 0.70–0.20 V) suggests much less preference toward the 2e− pathway compared with NiPc MDE (Figure 2b). Topological defects and N-dopants have been considered as active sites for ORR.11 However, Pc MDE (prepared by anchoring Pc molecules on CNTs) without metal centers shows inferior ORR activity and selectivity for the 2e− pathway compared with NiPc MDE ( Supporting Information Figure S3). These results suggest the critical role of dispersed Ni centers rather than the N-dopants or topological defects in the selective electrocatalysis of 2e− ORR. To investigate the origin of the different ORR behaviors of neat NiPc, NiPc + CNT, and NiPc MDE, their structures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM ( Supporting Information Figure S4a) and SEM ( Supporting Information Figure S4b) images of NiPc MDE show the bundles of multiwalled CNTs and excluded the formation of nano- or microsized NiPc aggregates. The isolated bright spots in the high-angle annular dark-field (HAADF) image of NiPc MDE obtained with a Cs-corrected scanning TEM (STEM) indicate the existence of single-Ni sites, suggesting the molecular dispersion of NiPc on CNTs ( Supporting Information Figure S5). No appreciable signature peaks of NiPc molecules are observed in the Raman spectrum of NiPc MDE ( Supporting Information Figure S6), which could be due to the low content of NiPc in NiPc MDE. On the contrary, physically mixed NiPc + CNT contains microsized NiPc aggregates, as revealed by SEM and confirmed by energy-dispersive spectrometry (EDS) mapping of the Ni signals ( Supporting Information Figures S4c and S7). Electrochemical impedance spectroscopy (EIS) was further conducted to gain insights into the ORR kinetics of the NiPc catalysts in aggregated and dispersed states. The Nyquist plots in Supporting Information Figure S4d show that the charge transfer of NiPc MDE for ORR is more favorable than that of NiPc and NiPc + CNT. The neat NiPc exhibited the largest charge-transfer resistance, in agreement with its low activity from LSV (Figure 2a). It should be noted that DFT calculations with individual NiPc molecule-catalyzing ORR suggest high selectivity toward the 2e− transfer pathway, which is only observed in dispersed NiPc as in NiPc MDE but not in aggregated NiPc as in NiPc + CNT. These results emphasize the importance of correlating free-energy diagrams calculated with individual catalyst molecule with the electrocatalytic performance of dispersed molecular catalysts as opposed to aggregated molecules. Comparison with the pyrolyzed Ni–N/C SAC SACs have gained extensive attention recently due to their superior electrocatalytic properties. In electrocatalytic applications, SACs are generally fabricated by pyrolyzing metal salts and N-containing organic precursors at high temperatures. However, these pyrolyzed SACs often contain parasitic active sites due to the insufficient structural control during the high-temperature synthesis.52 For comparison, a nickel SAC with Ni–Nx structures (denoted as Ni–N/C) was synthesized by pyrolyzing a Ni-containing zeolitic imidazolate framework (ZIF) precursor according to the reported method with minor modifications.53 The Ni content of the Ni–N/C catalyst was also controlled to be ∼0.7 wt % (measured by ICP-MS) to compare with NiPc MDE. TEM images of Ni–N/C suggest the absence of metallic Ni particles in the catalyst ( Supporting Information Figure S8). The X-ray diffraction (XRD) pattern of Ni–N/C only shows broad features attributable to graphitic carbon ( Supporting Information Figure S9),53 further indicating the absence of metallic Ni or crystalline Ni-containing compounds. The Fourier-transformed extended X-ray adsorption fine structure (FT-EXAFS) curve of Ni–N/C exhibits a peak at ∼1.4 Å (without phase correction) corresponding to Ni–N coordination, but little signal at ∼2.1 Å corresponding to Ni–Ni coordination ( Supporting Information Figure S10), confirming the presence of single Ni sites in Ni–N/C. Although no Ni particles are observed, parasitic active sites such as N-doped carbon sites could still be present in the Ni–N/C catalyst,54,55 which are known to be active for 4e− ORR (Figure 3a). The ORR performance of the Ni–N/C catalyst was further characterized in the RRDE setup with identical conditions as NiPc MDE. Given the LSV curves (Figure 3b), Ni–N/C possesses more positive onset potential (0.83 V) than that of NiPc MDE (0.79 V). The peroxide yields of Ni–N/C are under 43% with n larger than 3.1 in the potential range of 0.70–0.20 V (Figure 3c and Supporting Information Figure S11). The much worse selectivity of the Ni–N/C catalyst toward the 2e− reduction pathway than that of NiPc MDE is attributed to the structural heterogeneity in the pyrolyzed Ni–N/C catalyst. Additionally, the stability tests were carried out at the constant potentials of 0.50 V for the disk electrode to conduct ORR and at 1.50 V for the ring electrode to detect generated peroxide. As shown in Figure 3d, Ni–N/C shows obvious decay of both the disk and ring currents in the first suggesting the of the pyrolyzed By contrast, NiPc MDE exhibits much stability without appreciable decay of the disk and ring currents during the Therefore, the molecularly dispersed and well-defined Ni–N4 sites in NiPc MDE a ORR catalyst for the 2e− reduction pathway than the pyrolyzed Ni–N/C. NiPc MDE be a catalyst system to establish the between the active structure and electrocatalytic Figure | Comparison of electrocatalytic ORR performance between NiPc MDE and Ni–N/C. (a) Schematic presentation of ORR with NiPc MDE and Ni–N/C. (b) Disk and ring currents of NiPc MDE and Ni–N/C in O2-saturated 0.1 M KOH Calculated peroxide yields of NiPc MDE and Ni–N/C. tests of NiPc MDE and Ni–N/C under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint engineering of NiPc MDEs for ORR A of the MDE system is the of the of the active sites and the electrocatalytic performance through molecular we to further the performance of NiPc MDE for 2e− ORR for peroxide production with the introduction of groups to the Pc with Figure for molecular as an ORR catalyst by DFT calculations. The free-energy diagrams suggest to NiPc, NiPc–CN shows strong preference toward the 2e− reduction pathway, and the of *H2O2 over *O is from with NiPc to with NiPc–CN (Figure indicating that NiPc–CN be a more selective catalyst for 2e− ORR. Figure 4 | ORR electrocatalysis with NiPc–CN MDE. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and NiPc–CN at 1.23 V. shows the molecular structure of (b) Disk and ring currents of NiPc and NiPc–CN MDEs in O2-saturated 0.1 M KOH Calculated peroxide yields and n of NiPc and NiPc–CN test of NiPc–CN MDE under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint NiPc–CN MDE was synthesized ( Supporting Information Figure and characterized with the RRDE The LSV curve shows more positive onset potential of NiPc–CN MDE V) than that of NiPc MDE (0.79 V) (Figure Moreover, the saturation current for NiPc–CN MDE in the potential range of V together with constant ring The peroxide yields of NiPc–CN MDE were calculated to be ∼92% from to 0.20 V, superior to NiPc MDE with peroxide yields below V (Figure The enhanced peroxide yields of NiPc–CN MDE are consistent with the preference toward the 2e− reduction pathway from DFT calculations with the introduction of (Figure NiPc–CN MDE also shows good stability in ORR with little decay in the disk and ring currents and the peroxide yields during the (Figure The performance of NiPc–CN MDE for oxygen reduction to peroxide is the for the reported and metal catalysts, high peroxide selectivity of ∼92% at a potential in conditions ( Supporting Information Table S2). by catalyst design with DFT calculations, we identify NiPc MDE as a good ORR catalyst for the 2e− reduction pathway and further enhance its performance through molecular The enhanced NiPc–CN MDE with shows superior selectivity with peroxide yield of ∼92% in the potential range of 0.70–0.20 V. The molecularly dispersed and well-defined Ni–N4 sites on CNTs NiPc MDEs with higher 2e− selectivities than the aggregated NiPc and pyrolyzed Ni–N/C catalysts. These results also that the MPc MDE system as electrocatalysts for the of the between active structures and electrocatalytic performances of molecular catalysts and Supporting Information Supporting Information is available and Figures and and of is no of to Information was supported by Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and the from Zhejiang University, and Guangdong Provincial Key Laboratory of TEM and were measured with maintained by The were obtained with the of the Advanced Photon a Department of Energy of Science for the of Science by Argonne National Laboratory under The computational is supported from the for Computational Science and the in Energy Materials for It for to Zheng Wang Zhang Wang Pt on for O2 Reduction to H2O2 in Wang of Peroxide from and Jiang Wang Design for Electrochemical Oxygen Reduction toward H2O2 through in the Electrochemical of and by Zheng Zhou with

Dual-atom catalysts (DACs) have emerged as a compelling frontier in the realm of the electrochemical carbon dioxide reduction reaction (CO2RR). However, elucidating the intrinsic properties of dual-atom pairs and their direct correlation with catalytic activity poses significant challenges. Herein, we investigate CO adsorption on 248 kinds of C2N-supported DACs and analyze the underlying structure-activity relationships of dual transition metal (TM) atoms based on density functional theory (DFT) calculations and machine learning (ML) models. Compared to the direct input of atomic features in the decision tree model of ML, we confirm that extra feature engineering with the introduction of the arithmetic combination of atomic features can better reflect the correlation of dual TM atoms on C2N-based DACs. Further feature importance analysis reveals a strong relationship between the last one occupied orbital radius (rv), group number (G) for dual TM atoms and the CO binding strength, as well as a potential connection with the d band centre (εd). Our work provides deeper insights into the design of DACs and highlights the significance of twofold feature engineering for the synergistic effects between dual TM atoms.

Searching for efficient and stable bifunctional electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for realizing large-scale applications of electrolytic water splitting processes for clean energy production. Herein, using density functional theory (DFT) calculations, we systematically investigate the catalytic activity of 22 different transition metal (TM) atoms anchored on WSe2/WS2 heterostructures, which are screened as possible catalysts for both OER and ORR processes. Energetic analysis and ab initio molecular dynamics simulations showed that the proposed materials are stable under ambient conditions. The Pd@WSe2/WS2 structure is predicted to be an outstanding candidate for a bifunctional OER/ORR electrocatalyst, with an OER overpotential of 0.41 V and an ORR overpotential of 0.49 V, which are comparable to those of traditional monofunctional catalysts. Perhaps even more promisingly, we obtained a structure that does not require expensive noble metals (Ni@WSe2/WS2) that exhibit low overpotentials for both the OER and ORR (0.48 V/0.78 V) and could be an interesting alternative to bifunctional catalysts. The mechanisms behind the reactions involved in the catalysis are explained in detail using several calculated electronic properties. Our results not only endow TM@WSe2/WS2 bifunctional electrocatalysts with excellent catalytic activities, but also provide important insights for advancing clean energy technologies.

Power generation is one of the key challenges in the current century due to decreasing natural energy resources, population growth and the goal to reduction of carbon emissions. Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention for potential power generation technology especially for mobile application. However, PEMFC currently faces several challenges: high cost of Platinum group metals catalysts, the slow kinetics of oxygen reduction reaction (ORR) at the cathode and the durability of cathode catalysts in harsh acidic environment.Alloying Pt modifies the surface electronic structures and at the same time reduces the use of expensive Pt from the catalyst design. Recently, 3d-transition metals (TMs) alloyed Pt catalysts have been reported to show superior ORR activity as compared to pure Pt catalyst.1-5 For example, the ORR activity enhancement for bimetallic Pt3Ni(111) extended single crystal surface is reported to be 10 times (10´) higher than pure Pt(111) surface and 90´ higher than the commercial Pt/C nanoparticle catalysts.1 With these encouraging studies, catalysts design by controlling composition, sizes and shapes of Pt-based alloy nanoparticle are now under extensive investigations.Within this contribution, we employed an atomistic Monte Carlo (MC) simulation method to predict the equilibrium surface structures of Pt alloy surfaces. Based on the calculated surface composition profile, we built Pt surface-segregated surface slab models for Pt and Pt alloy catalysts. The possible ORR mechanism on the (111) and (100) surfaces of Pt and Pt alloys are explained by first-principles density functional theory (DFT) calculations. Our DFT computations predict that alloying Pt with transition metal alters the adsorption energetics of the ORR intermediates and changes the mechanism of ORR compared to pure Pt catalyst. The ORR on Pt and Pt alloy catalysts can proceed via one of the following ORR mechanisms: O2 dissociation, OOH dissociation or H2O2 dissociation ORR mechanism.Our calculations show that the ORR occurs via an OOH dissociation mechanism on the pure Pt(111) surface. In contrast, we predict that the ORR on Pt(100) surface proceeds via an O2 dissociation ORR mechanism. Our calculated activation energy for the rate determining step (RDS) on the Pt(111) surface (0.79 e V)3 is very similar to that (0.80 eV)4 on the Pt(100) surface. This suggests that the (111) and (100) facets of Pt nanocrystals show similar catalytic activity for ORR.We further studied the mechanism of ORR on Pt surface segregated modified Pt/TM(111) (TM =Fe, Co, Ni) and Pt3TM(111) (TM=Ti, V, Fe, Ni) surfaces. We found that the activation energy of the RDS on modified Pt/TM(111) and Pt3TM(111) surfaces is always smaller than that on the pure Pt(111) surface. Thus modified Pt/TM(111) and Pt3TM(111) surfaces have superior catalytic activity for ORR compared to the ORR activity of pure Pt(111) surface. The DFT calculations further predict that the most favorable ORR mechanism on the modified Pt(111) surfaces is H2O2 dissociation ORR mechanism. We found that Ni modified Pt/TM(111) and Pt3TM(111) surfaces show highest catalytic activity for ORR. Contrarily, our DFT calculations demonstrates that the catalytic activity of modified Pt/Ni(100) surface is very similar to that of pure Pt(100) surface. All these predictions are in excellent agreement with previous experimental measurements. Acknowledgements This work was funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant no. DE-FG02-09ER16093). We also acknowledge the research grant from the EERE program of the U.S. Department of Energy (Grant no. DE-AC02-06CH11357). References (1) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493-497.(2) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241-247.(3) Duan, Z. Y.; Wang, G. F. Phys. Chem. Chem. Phys. 2011, 13, 20178-20187.(4) Duan, Z. Y.; Wang, G. F. J. Phys. Chem. C 2013, 117, 6284−6292.(5) Kattel, S.; Duan, Z. Y.; Wang, G. F. J. Phys. Chem. C 2013, 117, 7107−7113.

Exploring highly active and cost-efficient single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is critical for the large-scale application of Zn-air battery. Herein, density functional theory (DFT) calculations predict that the intrinsic ORR activity of the active metal of SACs follows the trend of Co > Fe > Ni ≈ Cu, in which Co SACs possess the best ORR activity due to its optimized spin density. Guided by DFT calculations, four kinds of transition metal single atoms embedded in 3D porous nitrogen-doped carbon nanosheets (MSAs@PNCN, M = Co, Ni, Fe, Cu) are synthesized via a facile NaCl-template assisted strategy. The resulting MSAs@PNCN displays ORR activity trend in lines with the theoretical predictions, and the Co SAs@PNCN exhibits the best ORR activity (E1/2 = 0.851V), being comparable to that of Pt/C under alkaline conditions. X-ray absorption fine structure (XAFS) spectra verify the atomically dispersed Co-N4 sites are the catalytically active sites. The highly active CoN4 sites and the unique 3D porous structure contribute to the outstanding ORR performance of Co SAs@PNCN. Furthermore, the Co SAs@PNCN catalyst is employed as cathode in Zn-air battery, which can deliver a large power density of 220 mW cm-2 and maintain robust cycling stability over 530 cycles.

Coordination engineering on novel 2D pentagonal NiN2 for bifunctional oxygen electrocatalysts

Graphdiyne Electrocatalyst

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Designing efficient and cost-effective bifunctional electrocatalysts for the bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) is crucial for sustainable and renewable energy technologies. In this study, we systematically investigate the potential of single transition metal (TM)-doped T-C3N2 as bifunctional ORR/OER electrocatalysts using density functional theory and machine learning. The results reveal that TM atoms can be stably incorporated into the N vacancy (TMN) and the central hexagonal hole (TMi) of T-C3N2, creating various coordination environments for the TM atoms, which can influence the ORR/OER electrocatalytic performance. The TM atom embedded in the central hexagonal hole (Cui) is a robust bifunctional ORR/OER electrocatalyst due to its low overpotentials (0.53 V for ORR and 0.52 V for the OER) and superior thermodynamic stability. The ORR/OER catalytic performance of Cui maintains well under the biaxial strain (−1% to +6%), as the ORR and OER overpotentials of Cui change slightly with the biaxial strain. Nevertheless, the ORR and OER overpotentials increase sharply once the biaxial compressive strain exceeds −1%. Hence, substrates with lattice constants equal to or larger than T-C3N2 are required to obtain good bifunctional ORR/OER activity in experimental equipment. Lastly, we employ the machine learning method with a gradient-boosted regression model to determine the origin of ORR and OER activity. The results indicate that the charge transfer of TM atoms (Qe) is the dominant descriptor for ORR activity, while the d-electron counts (Ne) and the d-band center (εd) are critical descriptors for OER. Our research highlights the efficiency of TM atom-doped T-C3N2 as bifunctional electrocatalysts and offers valuable insights for developing electrocatalysts for future clean energy conversion and storage applications.

Electrocatalysts for the oxygen reduction reaction (ORR) are extremely crucial for advanced energy conversion technologies, such as fuel cell batteries. A promising ORR catalyst usually should have low overpotentials, rich catalytic sites and low cost. In the past decade, single-atom catalyst (SAC) TM-N4 (TM = Fe, Co, etc.) embedded graphene matrixes have been widely studied for their promising performance and low cost for ORR catalysis, but the effect of coordination on the ORR activity is not fully understood. In this work, we will employ density functional theory (DFT) calculations to systematically investigate the ORR activity of 40 different 3d transition metal single-atom catalysts (SACs) supported on nitrogen-doped graphene supports, ranging from vanadium to zinc. Five different nitrogen coordination configurations (TM-NxC4-x with x = 0, 1, 2, 3, and 4) were studied to reveal how C/N substitution affects the ORR activity. By looking at the stability, free energy diagram, overpotential, and scaling relationship, our calculation showed that partial C substitution can effectively improve the ORR performance of Mn, Co, Ni, and Zn-based SACs. The volcano plot obtained from the scaling relationship indicated that the substitution of N by C could distinctively affect the potential-limiting step in the ORR, which leads to the enhanced or weakened ORR performance. Density of states and d-band center analysis suggested that this coordination-tuned ORR activity can be explained by the shift of the d-band center due to the coordination effect. Finally, four candidates with optimal ORR activity and dynamic stability were proposed from the pool: NiC4, CoNC3, CrN4, and ZnN3C. Our work provides a feasible designing strategy to improve the ORR activity of graphene-based TM-N4 SACs by tuning the coordination environment, which may have potential implication in the high-performance fuel cell development.