Coordination-Dependent Catalytic Activity and Design Principles of

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Coordination-Dependent Catalytic Activity and Design Principles of Metal−Organic Frameworks as Efficient Electrocatalysts for Clean Energy Conversion Chun-Yu Lin,† Jing Zhang,‡ and Zhenhai Xia*,†,‡ †

Department of Materials Science and Engineering, Department of Chemistry, University of North Texas, Denton, Texas 76203, United States ‡ School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China J. Phys. Chem. C 2019.123:214-221. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/12/19. For personal use only.

S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are promising as high performance electrocatalysts in clean energy conversion and storage owing to their unique porous structures, large surface area, and atomically distributed nonprecious transition metal ions (single atom catalysts). However, the ligands or coordinations of metal ions strongly influence their catalytic activities, making it complicated for selecting better catalysts. To rationally design the MOF catalysts, we have calculated the catalytic activities of zeolitic imidazolate frameworks (ZIFs), a subset of the MOF hybrids, with the density-functional theory (DFT) methods and identified an intrinsic descriptor that governs the catalytic activities of the ZIFs. Our DFT calculations show that the unsaturated metal sites exhibit higher catalytic activities than those of the best noble metal electrocatalysts. The efficiency and selectivity of the ZIF catalysts could be improved by controlling the number of ligands/coordinations and type of metal ions. The best catalysts are identified for fuel cells, water splitting, and direct prodiction of hydrogen peroxide. The theoretical predictions are supported by experimental data. This work provides a theoretical base and guiding principles for rational design of high-performance MOF-based electrocatalysts for clean energy conversion and storage.

1. INTRODUCTION The rising global energy consumption of traditional fossil fuels has posed serious challenges to energy security and environmental protection. One promising solution is the clean energy sources, such as hydrogen fuel generated by water splitting utilizing sunlight and fuel cells using hydrogen to generate electricity, without pollution or greenhouse gas emission. In these clean energy technologies, noble metal (e.g., Pt and RuO2) catalysts are usually needed to promote the key chemical reactions: oxygen reduction reaction (ORR) in fuel cells and oxygen evolution reaction (OER) in water splitting.1,2 However, the high cost and poor durability of the noble metal catalysts hinder the development of the clean energy technologies. It is therefore necessary to search for alternative materials that are abundant and have comparable or better catalytic activities than the noble metals. Metal−organic frameworks (MOFs) are appealing as electrocatalysts owing to their unique porous structures with nonprecious transition metals. MOFs are compounds composed of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. Among the MOFs, zeolitic imidazolate frameworks (ZIFs) are topologically isomorphic with zeolites, consisting of tetrahedrally coordinated transition metal ions (e.g., Fe, Co, Cu, and © 2018 American Chemical Society

Zn) connected by imidazolate linkers. The molecular structure of the ZIFs is similar to the natural zeolites with the transition metals as the central ions and four coordinate ligands which are imidazole compounds. Their high surface area, tunable porosity, and diversity in metal and functional groups make them especially attractive as heterogeneous catalysts,1,3,4 as well as for gas storage and separation and biosensors.1,5 ZIFs have diverse coordination geometries, and their organic linking groups can be modified and even exchanged with a new linker by ligand exchange or partial ligand exchange.6,7 This modification or exchange allows for the pores, and in some cases the overall frameworks of ZIFs, to be tailored for specific purposes. For example, ZIF-67, composed of coordinately saturated cobalt ions with four ligands, shows almost inert in catalytic activities, but after plasma etching to break some ligands of ZIF-67 to create the unsaturated metal sites, it exhibits good stability and improved catalytic performance comparable to the best OER catalyst, RuO2.8−13 This approach provides a strategy to design inexpensive and durable electrocatalysts for water splitting, metal−air batteries, and Received: October 24, 2018 Revised: December 5, 2018 Published: December 7, 2018 214

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The Journal of Physical Chemistry C fuel cells. Nevertheless, to rationally design high performance catalysts by modifying ligands or exchanging metal ions, it is necessary to understand how the metal ions and ligands determine the catalytic activities. Identification of descriptors that correlate the intrinsic properties of ligands/metal ions with the catalytic activities would establish the guiding principles for the rational design of the MOFs as highperformance electrocatalysts. A few of the effective activity descriptors have been proposed in different material systems, including the d-band center for metal surfaces,5 eg-filling for transition-metal-oxide perovskites,1 the crystal field stabilization energy (CFSE) for COFs,14 and the electronegativity and electron affinity for doped carbon materials.15 However, the principle has not been established to guide the catalytic design for the family of ZIFs. In this study, we have, for the first time, identified the effective ligand number that serves as the activity descriptor for predicting ORR/OER activities of the ZIFs and established a volcano relationship between the descriptor and the ORR/ OER catalytic activity of the ZIF catalysts, which has predictive power to guide the experiments. The predictions are consistent with the experimental reports from the literature.

Figure 1. (A) Illustratrion of a Co-containing ZIF, 4N4-Co1 ZIF with four ligand bonds on each metal ion and O2 adsorption on the active site, Co1 (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn). The red, pink, deep gold, silver, and blue colors represent O, H, C, N, and Co, respectively. Boxes are periodic in x, y, and z directions. (B) A simplified TM-containing ZIF structure highlighting the metal ions and their ligand bond connections.

reactions take place in both associate and dissociate pathways, depending on the type of metal ions and the number of ligands. Parts A and B of Figure 2 show the free energy diagrams for the 4e and 2e transfer reactions over the 4N4 ZIFs, respectively. For the 4e transfer, the free energy diagrams (Figure 2A) show that the rate limiting step is the third

2. METHOD A series of models were developed to simulate OER/ORR on ZIF67 with various numbers of nitrogen ligands (ZIF-xNy, x = 1−4, y = 2−4). Density functional theory (DFT) with Hubbard (DFT+U)27 calculations were carried out using soft projector-augmented-wave (PAW) pseudopotentials28,29 and the Perdew−Burke−Ernzenhof29 (PBE) exchange correlation functional, as implemented in the VASP code.30,31 The k-point setting of the Brillioun zone was obtained by a 3 × 3 × 1 grid generating meshes with their origin point at the gamma point. The plane wave kinetic energy had a high cutoff energy of 550 eV throughout the computations and a vacuum spacing of at least 20 Å in the z direction. Moreover, all of the spin-polarized calculations were converged to 0.01 eV Å−1 for all surfaces and geometries. 3. RESULTS 3.1. Reaction Pathways and Scaling Relationship. To predict the catalytic activities of the MOFs with ZIF units, we created a series of ZIF structures containing 3d transition metals (TMs) (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) with different numbers of ligands, as shown in Figure 1 and Figure S1. These ZIFs are denoted as xNy-TMz, where N represents the ligand number, integer x (x = 1−4) refers to the number of ligands on the metal ion in the box center of the models, y (y = 1, 4) is the number of ligands of the metal ions adjacent to the central metal ion, and z represents the location of the active site with z = 1 for the central metal ion and z = 2 or 3 for the neighboring metal ions (Figure S2). For example, 4N4-Co1 refers to a ZIF structure with four ligands (x = 4) on the central metal ion, Co, four ligands (y = 4) on each of the adjacent metal ions, and an active site on the central metal ion (z = 1) (Figure 1). Using the DFT methods, the elementary steps of ORR and OER at transition metal sites of the ZIFs with different ligands were simulated (Figures S3 and S4). Both four-electron (4e) transfer (2H2 + O2 → 2H2O) (Figure S4) and two-electron (2e) transfer (H2 + O2 → H2O2) (Figure S5) were simulated and the computational methods are described in detail in the Supporting Information. The

Figure 2. Free energy diagram of ZIF-67 with various TMs for (A) four-electron transfer and (B) two-electron transfer ORR in 4N4-TM ZIFs at U = 0 V. 215

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overpotential is UOER limit = 1.64 − 1.23 = 0.41 V, which sets the lower limit of OER and ORR overpotentials for 4N4-TM ZIFs. 3.2. Ligand Effect on the Catalytic Activities of the ZIFs. It is well-known that knocking off ligands from the metal ions in ZIFs will lead to coordinately unsaturated metal sites, which could affect OER/ORR catalytic activities of the ZIFs. We have built all of the possible types of unsaturated ZIF structures with different numbers of ligands (e.g., 4N2, 3N4, 3N3, and 2N2) to study the role of the ligands in the ORR/ OER catalytic activities, as shown in Figure 4. The overpotentials of these coordinately unsaturated ZIFs were calculated, and the results are listed in Tables S3−S6. Compared with those 4N4-TM ZIFs with coordinately saturated metal sites, removing one ligand of an active metal site results in the reduction of OER overpotential except for Cr-ZIFs, but knocking off more than one ligand would lead to an increase in the overpotential, as shown in Figure 5A. Therefore, the coordinately unsaturated metal sites with one missing ligand would enhance the OER catalytic activities of the ZIFs. This result is supported by the experiment that showed significantly enhanced OER catalytic activities comparable to Ru2O after the Co-ZIFs were treated to knock off some of the ligands by plasma treatment.18 Conversely, eliminating the ligands would lead to the increase in ORR overpotential, which would suppress ORR catalytic activities except for Ni-ZIF (Figure 5B). In addition to the number of ligands attaching to the active sites of the metal ions, the number of ligands on the metal ions adjacent to the active site also influences the overpotential and thus catalytic activities. Figure 5D shows OER and 4e ORR overpotentials as a function of the number of the ligands of metal ions adjacent to the active sites for 3Ny-Co1 ZIFs (y = 1−4). With a reduction in the number of ligands of the adjacent metal ions, the ORR and OER overpotentials increase gradually. Thus, reducing the number of ligands of the active sites enhances the OER catalytic activities, but reducing the number of ligands of the neighboring metal sites would suppress the activities. A similar trend has been found for other TM ions. Therefore, the catalytic activities of an active site are influenced by its environments (e.g., neighboring metal ions). It is possible to control the catalytic activities by introducing different types of metal ions (e.g., Co and Zn codoping) and controlling the number of ligands. 3.3. Ligand Effects on 2e Transfer ORR for H2O2 Production. Hydrogen peroxide (H2O2) can be used as an environmentally benign chemical oxidant in chemical synthesis, pulp and paper industry, and water treatment as well as a potential energy carrier for vehicles and space shuttles. H2O2 could be synthesized directly from its elements through a catalytic process.19,20 Currently, Pd nanoparticles are the best catalysts, showing up to 90% selectivity.4 Since 2e and 4e processes are competitive with each other in energy conversion and oxidizer production, any materials that promote one process, while suppressing the other reaction, could serve as an effective catalyst with good selectivity in either energy conversion or oxidizer production. ZIFs could be an excellent candidate catalyst for the direct production of H2O2. As can be seen from Figure 5B and C, while eliminating the ligands increases the 4e ORR overpotential, it actually lowers the 2e ORR overpotential for some metal ions. In particular, the 2e ORR overpotential for 3N3-Ni ZIFs (∼0.056 V) is much lower than that of Pd (0.08 V),17 while its 4e ORR overpotential is 0.586 V, larger than that of Pt (∼0.45 V).16 This ligand effect is

reaction step of ORR except for Mn and Co that are the fourth reaction step. In essence, the overpotential can be considered as an effective indicator of the catalytic activities.16 In general, an ideal catalyst will make water oxidation or oxygen reduction above the equilibrium potential (U0), with zero overpotential (η = 0), but the ideal catalyst rarely exists in OER/ORR due to the correlation of the binding energies of intermediates. Therefore, the lower value of η would indicate the better performance. We have calculated the overpotentials of OER/ ORR on the metal ions of coordination-saturated 4N4-TM ZIFs (Figure S3), as listed in Table S2. Overall, the 4N4-TM ZIFs have a much larger OER overpotential (∼0.79 V for 4N4Co, the lowest one among the 3d TMs) than that of benchmark Ru2O (∼0.42 V), indicating that their catalytic activities are much lower in catalyzing OER compared with Ru2O. This prediction is consistent with the experimental results for the ZIFs.8 In the case of ORR, 4N4-Cr ZIFs (∼0.42 V) have the lowest overpotential, comparable to that of Pt (∼0.45 V) for 4e ORR,16 but its overpotential for 2e transfer is also low (∼0.10 V), very close to that of the best 2e ORR catalyst, Pd (∼0.08 V).17 Thus, the reaction pathway of 4N4Cr ZIFs probably follows the mixture of 2e and 4e transfer. Interestingly, 4N4-Fe ZIFs exhibit the lowest 2e overpotential (∼0.05 V) but a relatively high 4e one, and they could be excellent non-precious-metal-containing catalysts for direct H2O2 production from H2 and O2 with good selectivity. The binding energies of the intermediates can be used to predict the lower limit of the overpotentials that the ZIF materials could possibly reach in OER/ORR. Figure 3 shows

Figure 3. Absorption energy of OOH* versus that of OH* for 4N4TM ZIFs.

the correlation of the adsorption energies of OOH* and OH* for the 4N4-TM ZIFs. Through the linear regression, the adsorption energy of OH* is correlated with that of OOH* by ΔG0OOH* = ΔG0OH* + 3.28, from which the relationship between the second and third elementary reactions (OH* → O* + H+ + e− and O* + H2O(l) → OOH* + H+ + e−) in OER are determined as ΔG2 + ΔG3 = 3.28 eV, where ΔG2 and ΔG3 are the free energies of the second and third reactions of OER, respectively. According to the above equation, the lowest free reaction energy, or overpotential, is achieved when ΔG2 = ΔG3 = 1.64 eV. Since the ideal free reaction energy in each step of charge transfer is 1.23 eV in acidic media, the lowest 216

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Figure 4. Adsorption of O2 on Co in ZIFs with various ligand numbers. (A) Saturated 4N4-Co1 ZIF with four ligands on each metal ion, (B) unsaturated 4N2-Co1 ZIF with four ligands on the active site (Co1) and two ligands connecting with each of the metal ions (Co2 and Co3) adjacent to the active site (Co1), (C) unsaturated 3N4-Co1 ZIF with three ligands on the active site (Co1) and four ligands connecting with each of the metal ions (Co2, Co3) adjacent to the active site (Co1), (D) unsaturated 3N3-Co1 ZIF with three ligands on the active site (Co1) and three ligands connecting with each of the metal ions (Co2, Co3) adjacent to the active site (Co1), (E) unsaturated 2N2-Co1 ZIF with two ligands on the active site (Co1) and two ligands connecting with each of the metal ions (Co2) adjacent to the active site (Co1). The red, pink, deep gold, silver, and gold colors represent O, H, C, N, and Co, respectively.

Figure 5. Overpotentials of (A) OER, (B) 4e ORR, and (C) 2e ORR as a function of the number of the ligands on the active sites. (D) OER and 4e ORR overpotential as a function of the number of ligands of metal ions neighboring to the active sites for 3Ny-Co1 ZIF (y = 1−4). 217

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Figure 6. Volcano plots of ORR/OER catalytic activities versus descriptors. (A) Volcano plot of OER catalytic activities versus absorption energy of OH* for Co-ZIFs. (B) Absorption energy of OH* versus ORR overpotentials as a function of the ligand number for 3N4-TM ZIFs. (C) Absorption energy of OH* versus the descriptor, the ligand number (ELN). (D) Volcano plot of OER catalytic activities versus the descriptor, the ligand number (ELN).

maps can also be found in other TM-ZIF systems for ORR. In particular, the catalytic activities of all of the TM-ZIF systems can be well described by the same volcano diagram, as shown in Figure 6B. This descriptor follows the Sabatier principle,22 which states that the interactions between the catalysts and the adsorbents should be neither too strong nor too weak. However, the descriptor ΔGOH* is not an intrinsic property of ZIFs, and inconvenient for predicting the catalytic activities of ZIFs directly from their ligand structures. Since the catalytic activities are affected by both the ligands of the active metal sites and their neighboring metal sites, we defined a parameter, the so-called effective ligand number (ELN), which is the number of ligands connecting to the active metal site, x, plus the number of ligands bonded to the neighboring metal site, y,

particularly useful to create highly effective catalysts with high selectivity for the direct production of H2O2. This offers a new route to create highly selective and highly effective catalysts by properly modifying the ligand structures of the ZIFs for the production of H2O2. 3.4. Intrinsic Descriptors for ZIFs. It is critical to find the intrinsic material properties that govern the catalytic behaviors of the ZIF system so that the best catalyst could be identified. As a starting point, we employ the descriptor ΔG0OH* that was previously selected to describe the catalytic behavior of doped carbon and transition metals.21 Figure 6A shows the OER overpotentials, ηOER, as a function of the descriptor ΔG0OH* for Co-ZIFs. A “volcano”-shaped curve is formed for the descriptor−activity relationship with 3N4-Co1 located on the top of the volcano, at which the adsorption energy of OH* is almost zero. Therefore, 3N4-Co1 ZIF is identified as the best catalyst in the Co-ZIFs. As shown in Figure 6A, eliminating one ligand from the active metal site leads to an increase in the adsorption energy of the OH intermediate, thereby increasing the catalytic activities, but further eliminating ligands would result in too high adsorption energy which in turn reduces the catalytic activities. Similar volcano-shaped descriptor−activity

ELN = a(x + by)

(1)

where a is the factor associated with properties (e.g., electronegativity) of metal ions, b is the factor related to the effect of the number of ligands on the metal ions adjacent to the active site, and x and y are the number of ligands on the active site and that of the metal ions adjancent to the active 218

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The Journal of Physical Chemistry C site. The second term of eq 1 on the right-hand side represents the contribution from the metal ions adjacent to the active site. For Co-ZIF, when b = 0.2, the ELN is nearly proportional to the adsorption energy, ΔGOH* (Figure 6C). We also found that the electronegativity of a metal ion is approximately linearly related to ΔGOH* for 4N4-TM ZIFs (Figure S7), and thus, the factor a can be replaced by the electronegativity in eq 1. Therefore, the ELN can be considered as an intrinsic descriptor to describe the catalytic behavior of TM-ZIFs. Figure 6D shows a volcano plot of OER catalytic activities versus the ELN for Co-ZIFs. Similar plots can be made for the ZIFs with different metal ions (TMs) by adjusting the parameters a and b in eq 1. These plots would be useful in predicting the performance of a catalyst with certain ligand structures.

4. DISCUSSION Our DFT simulations show that the catalytic activities of ZIFs strongly depend on the types of metal ions and the number of ligands. This dependence may be attributed to the charge coordination of the metal ions that determines the chemical interactions and bonding strength of the metal ions with the intermediates (e.g., O2, OOH*, O*, and OH*) during catalytic reactions. We have calculated the charge distribution over the ZIFs, and the results show that there are highly positive charges (0.37−2.35e) on metal sites and highly negative charges (−0.44 to −2.71e) on nitrogen, as shown in Figure 7 and Table S8. For the coordinately saturated metal sites (4N4 ZIFs), the charges increase with increasing number of valence electrons. The highly positively charged metal sites strongly interact with oxygen, facilitating O2 adsorption as an active center of ORR/OER, but too high of a charge would suppress the catalytic activities. The number of ligands on the metal sites also influences the coordination of the metal ions. When ligands are knocked off from a metal site, it becomes coordinately unsaturated. The coordinately unsaturated metal ions would interact with the intermediates more strongly, which affects the overpotentials of the ORR.5,23 Our calculations show that the charge on the active metal sites reduces with a decrease in the number of ligands (Figure S6). Therefore, the ELN is directly associated with the coordination state and the bonding energy of the transition metals. The factor b in the ELN represents the effect of the metal ions adjacent to the active site on the adsorption of reaction intermediates. The catalytic activities of the ZIFs can be well described by the ELN, which show a “volcano”shaped dependence on the descriptor (Figure 6D). This provides quantitative information that can be used to predict new effective catalysts for energy conversion. The above simulation results are supported by the experimental results. It is well-known that the onset potential Vonset is a critical indicator of the ORR and OER catalytic activities of the electrocatalysts in fuel cells, metal-air batteries, and water splitting system,24 and it can be measured by the linear scan voltammogram (LSV) method. We firstly compare the predictions with the experimental results available for coordinately saturated ZIF (4N4). To make a reliable comparison, the relative onset potential Vonset is used, which is defined as the onset potential of the benchmarked Pt/C electrode subtracted by that of the ZIFs, measured under the same conditions as in the same experiment. All of the experimental data are shown in Table S7.3,6,7,18,25,26 Similarly, the relative ORR overpotential is calculated by subtracting the

Figure 7. Charge distributions of (A) 4N4-Co, (B) 3N3-Co, and (C) 2N2-Co, where the colors of green, yellow, blue, and red refer to negative, positive, highly negative, and highly positive charges, respectively.

overpotential of Pt. Figure 8 shows the relative ORR overpotential predicted by the DFT calculations and the 219

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Computational Methods section, eight supplementary tables, seven supplementary figures, and nine references (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chun-Yu Lin: 0000-0002-6818-4681 Zhenhai Xia: 0000-0002-0881-2906 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported financially by the National Science Foundation (1662288, 1363123, 1561886). Computational resources were provided by UNT high-performance computing initiative.

Figure 8. Predicted relative 4e ORR overpotential and experimental relative onset potential for TM-ZIFs.7,18,26

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experimental onset potential. The DFT calculations predict that the catalytic performance of Co-ZIF is comparable to that of Fe-ZIF but better than that of Zn-ZIF, which is consistent with the experimental results (Figure 8). Secondly, we compared the ligand effects determined from the predictions and the experiments for OER. It has been reported that ZIF-67 (Co-containing MOFs) has a low OER catalytic activity, but after dielectric barrier discharge (DBD) plasma etching, its catalytic activities are significantly improved and it is comparable to the commercial precious RuO2.8 Interestingly, the OER activity of the ZIF-67 is reversible by adding the missing ligands. These results are also consistent with the predictions that removing a ligand lowers the overpotential from 0.8 to 0.38 V, which leads to the enhanced catalytic activity comparable to that of RuO2 (∼0.42 V), as shown in Figure 6A. It should be noted that these ZIF-67 catalysts have relatively good electrical conductivity and chemical stability.3,6,7,18,25,26

(1) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Design Principles for OxygenReduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal-Air Batteries. Nat. Chem. 2011, 3, 546−550. (2) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Chen, Y.; Liu, Z.; Bruce, P. G. A Stable Cathode for the Aprotic Li-O2 Battery. Nat. Mater. 2013, 12, 1050−1056. (3) Zhang, L.; Su, Z.; Jiang, F.; Yang, L.; Qian, J.; Zhou, Y.; Li, W.; Hong, M. Highly Graphitized Nitrogen-Doped Porous Carbon Nanopolyhedra Derived from ZIF-8 Nanocrystals as Efficient Electrocatalysts for Oxygen Reduction Reactions. Nanoscale 2014, 6, 6590−6602. (4) Jirkovský, J. S.; Panas, I.; Ahlberg, E.; Halasa, M.; Romani, S.; Schiffrin, D. J. Single Atom Hot-Spots at Au-Pd Nanoalloys for Electrocatalytic H2O2production. J. Am. Chem. Soc. 2011, 133, 19432−19441. (5) Hammer, B.; Norskov, J. K. Why Gold Is the Noblest of All the Metals. Nature 1995, 376, 238−240. (6) Li, X.; Niu, Z.; Jiang, J.; Ai, L. Cobalt Nanoparticles Embedded in Porous N-Rich Carbon as an Efficient Bifunctional Electrocatalyst for Water Splitting. J. Mater. Chem. A 2016, 4, 3204−3209. (7) Wang, X.; Zhou, J.; Fu, H.; Li, W.; Fan, X.; Xin, G.; Zheng, J.; Li, X. MOF Derived Catalysts for Electrochemical Oxygen Reduction. J. Mater. Chem. A 2014, 2, 14064−14070. (8) Tao, L.; Lin, C. Y.; Dou, S.; Feng, S.; Chen, D.; Liu, D.; Huo, J.; Xia, Z.; Wang, S. Creating Coordinatively Unsaturated Metal Sites in Metal-Organic-Frameworks as Efficient Electrocatalysts for the Oxygen Evolution Reaction: Insights into the Active Centers. Nano Energy 2017, 41, 417−425. (9) Yuan, S.; Feng, L.; Wang, K.; Pang, J.; Bosch, M.; Lollar, C.; Sun, Y.; Qin, J.; Yang, X.; Zhang, P.; et al. Stable Metal-Organic Frameworks: Design, Synthesis, and Applications. Adv. Mater. 2018, 1704303, 1−35. (10) Huang, X. C.; Lin, Y. Y.; Zhang, J. P.; Chen, X. M. LigandDirected Strategy for Zeolite-Type Metal-Organic Frameworks: Zinc(II) Imidazolates with Unusual Zeolitic Topologies. Angew. Chem., Int. Ed. 2006, 45, 1557−1559. (11) Pan, Y.; Liu, Y.; Zeng, G.; Zhao, L.; Lai, Z. Rapid Synthesis of Zeolitic Imidazolate Framework-8 (ZIF-8) Nanocrystals in an Aqueous System. Chem. Commun. 2011, 47, 2071−2073. (12) Wu, H.; Qian, X.; Zhu, H.; Ma, S.; Zhu, G.; Long, Y. Controlled Synthesis of Highly Stable Zeolitic Imidazolate Framework-67 Dodecahedra and Their Use towards the Templated Formation of a Hollow Co3O4catalyst for CO Oxidation. RSC Adv. 2016, 6, 6915−6920.

5. CONCLUSIONS We have calculated the OER and ORR activities of TM-ZIFs (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and found an intrinsic descriptor, ELN, which well describes the catalytic activities of the Co-ZIFs. The best OER and ORR catalysts are identified to be those with 0 and ∼0.5 eV in adsorption energy of OH*, respectively. The ligand number or coordination strongly affects the ORR/OER catalytic activities of TM-ZIFs. While eliminating one ligand from the active metal site results in an increase in the OER catalytic activity, removing two or more ligands would suppress the OER catalytic activities. Decreasing the ligand number/coordination would lower the 2e and 4e ORR catalytic activity of most of the ZIFs except for Ni-, Zn-, and Cu-ZIFs. Thus, the ORR catalytic activities of Ni-, Zn-, and Cu-ZIFs could be improved by modifying their ligands or coordination. The coordinately satuated Fe-, Cr-, and Mn-ZIFs are identified to be promising electrocatalysts for the direct production of H2O2.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10375. 220

DOI: 10.1021/acs.jpcc.8b10375 J. Phys. Chem. C 2019, 123, 214−221

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DOI: 10.1021/acs.jpcc.8b10375 J. Phys. Chem. C 2019, 123, 214−221