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Letter www.acsami.org
Study of Electrocatalytic Properties of Metal−Organic Framework PCN-223 for the Oxygen Reduction Reaction Pavel M. Usov, Brittany Huffman, Charity C. Epley, Matthew C. Kessinger, Jie Zhu, William A. Maza, and Amanda J. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *
ABSTRACT: A highly robust metal−organic framework (MOF) constructed from Zr6 oxo clusters and Fe(III) porphyrin linkers, PCN223-Fe was investigated as a heterogeneous catalyst for oxygen reduction reaction (ORR). Films of the framework were grown on a conductive FTO substrate and showed a high catalytic current upon application of cathodic potentials and achieved high H2O/H2O2 selectivity. In addition, the effect of the proton source on the catalytic performance was also investigated.
KEYWORDS: metal−organic framework, electrochemistry, Fe porphyrin, oxygen reduction reaction, catalysis
H
O2 + 4H+ + 4e− → H 2O
(1)
attractive candidates. MOFs comprise metal ions or clusters bridged by organic linkers to form highly ordered multidimensional arrays.5 Compared to other heterogeneous catalysts, MOFs typically exhibit permanent porosity. This feature allows for rapid diffusion of substrate throughout the material and could give rise to unparalleled active site densities per unit area. Furthermore, despite the fact that the majority of frameworks reported to date are electrical insulators,6,7 it was demonstrated that electrons could still be supplied to the catalytic centers through a redox-hopping mechanism.8 One of the strategies for designing electrocatalytic MOFs is incorporation of active sites into the existing structures, which can be achieved through the modification of the linkers, the metal nodes, or introduction of active guests into the pores. Several framework families have been investigated for their catalytic activity toward ORR, both as standalone catalysts9,10 and as part of composites with other materials.11 Transition metal porphyrin complexes offer a versatile platform for the design of electrocatalytic materials, owing to their rich chemistry and ability to catalyze a wide array of reactions.12−14 In particular, Fe(III) porphyrins were found to be highly efficient molecular catalysts for ORR.15,16 These complexes are structurally related to hemoproteins, a motif ubiquitous in nature, and exhibit strong interaction with the O2 molecules.17 Several MOFs incorporating Fe(III) porphyrin as
O2 + 2H+ + 2e− → H 2O2
(2)
Special Issue: Hupp 60th Birthday Forum
ydrogen-based power sources have long been proposed to replace the current hydrocarbon-based fuels because of their higher gravimetric energy densities and less harmful emissions. Fuel cells have emerged as one of the most attractive technologies to convert hydrogen feedstock into useful energy because they display greater thermodynamic efficiency over the alternatives.1 Despite considerable advances in this field, several challenges to the wider implementation of fuel cells remain. One of the major limitations of the current technology is the oxygen reduction reaction (ORR) taking place at the cathode, which requires the use of electrocatalysts in order to achieve high performance. At present, the Pt-based systems still represent a benchmark in ORR catalysis efficiency,2 however, sluggish reaction kinetics, as well as scarcity and high cost of Pt make these materials poorly suitable for commercial fuel cells. Recent research efforts in catalyst development have focused on low-cost alternatives, such as first-row transition metals3 and entirely metal-free carbon-based materials.4 The key design parameters of the desired catalytic system include low overpotential for ORR, high turnover frequency (TOF), longterm stability in electrolyte media and selectivity for the fourelectron pathway (1), generating H2O, versus the two-electron pathway (2), which produces H2O2.
Among the multitude of solid-state materials investigated for heterogeneous ORR catalysis, metal−organic frameworks (MOFs) offer several unique characteristics that make them © 2017 American Chemical Society
Received: January 31, 2017 Accepted: March 29, 2017 Published: March 29, 2017 33539
DOI: 10.1021/acsami.7b01547 ACS Appl. Mater. Interfaces 2017, 9, 33539−33543
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ACS Applied Materials & Interfaces
framework structure (Figure 1c). The SEM imaging revealed that the PCN-223-Fe film consists of spindle-shaped MOF particles 0.5−1.0 μm in length deposited onto the FTO slide (Figure 1d). This particle morphology is consistent with previously reported PCN-223 frameworks.21,23 The electrochemical properties of PCN-223-Fe films were investigated using cyclic voltammetry (CV) in 0.1 M LiClO4/ DMF supporting electrolyte. This combination was selected over more commonly utilized aqueous electrolytes because similar DMF-based electrolytes were used to study Fe(III) porphyrins in solution.15 In addition, an Fe-free analogue, PCN-223-fb, was also examined under the same conditions, providing a point of comparison and helping differentiate Fe centered redox processes from those localized on the porphyrin macrocycle. The cathodic scan of the PCN-223-Fe films under Ar atmosphere revealed a broad quasi-reversible redox wave with an onset at ca. −0.5 V vs NHE (Figure 2a) which was
linkers have been reported. Most notably, Zr-based frameworks MOF-52518 and PCN-22219 were investigated for their electrocatalytic activity toward CO2 reduction and ORR, respectively. The advantages of these materials for electrocatalytic applications include their chemical and structural stabilities, high density of available catalytic sites, and stabilization of the active catalyst through immobilization within the MOF structure, which limits its deactivation pathways.20 In the current work, we investigated the catalytic activity of a porphyrinic MOF, PCN-223-Fe (Zr 6 O 4 (OH) 4 (Fe(III)(TCPP)3)21 toward ORR. This framework contains large triangular channels lined up with Fe(III) porphyrin moieties with the Fe−Fe distances of 10.7 Å (Figure 1a, b). In addition,
Figure 1. Crystal structure of PCN-223-Fe MOF viewing along (a) (001) and (b) (100) directions (C, gray; O, red; N, blue; Fe, orange; and Zr, light blue polyhedra). (c) PXRD patterns of PCN-223-Fe and PCN-223-fb compared to the predicted pattern. (d) SEM image of PCN-223-Fe film deposited on FTO slide.
Figure 2. Cyclic voltammograms of PCN-223-Fe (blue) and PCN223-fb (red) films measured at 100 mV s−1 scan rate using 0.1 M LiClO4/DMF as a supporting electrolyte, without proton source ((a) Ar, (b) O2), 0.3 M AA ((c) Ar, (d) O2), and 0.3 M TCA ((e) Ar, (f) O2).
this MOF exhibits a permanent porosity and a Brunauer− Emmett−Teller (BET) surface area of 1600 m2 g−1 allowing rapid diffusion of the substrate and the product throughout the framework.21 The porphyrin linkers are connected to Zr6 oxo clusters giving rise to a highly stable structure. This property makes it particularly suitable for the acidic environment of ORR. Furthermore, the redox inertness of Zr4+ ions means that the secondary building units (SBU) of PCN-223-Fe should remain unaffected during the redox cycling. This consideration is particularly important for selecting MOFs for electrocatalytic applications, since SBUs play a significant role in supporting the framework structural integrity and any changes in the oxidation states of metal ions would often lead to structural collapse.22 For electrocatalytic studies, PCN-223-Fe and PCN-223-fb (fb = free base) MOFs were solvothermally grown on a conductive fluorine-doped tin oxide (FTO) substrate using a modified literature procedure (Experimental Section in the Supporting Information).21 Briefly, FTO slides were heated in a mixture of ZrCl4 and either Fe(III)TCPP (for PCN-223-Fe) or H2TCPP (for PCN-223-fb) dissolved in DMF with formic acid added as a modulator. The resultant films were activated and subsequently used as working electrodes in electrochemical experiments. Powder X-ray diffraction (PXRD) confirmed that the structure of the synthesized materials matches the expected
assigned to the Fe(III)TCPP/Fe(II)TCPP process. This process becomes more resolved at faster scan rates (>400 mV s−1) (Figure 3a) with E1/2 = −0.56 V vs NHE. At these scan rates, the electrochemical activity of PCN-223-Fe would be dominated by the outermost porphyrin linkers on the outer surface of the MOF particles, since counterions would have less time to diffuse into the pores. As a result, the voltammetric response would be less affected by the internal MOF
Figure 3. (a) Variable scan rate cyclic voltammograms of PCN-223-Fe films measured using 0.1 M LiClO4/DMF as a supporting electrolyte. (b) Plot of log(current density) vs log(scan rate) for the PCN-223-Fe films. The line of best fit (blue) has a slope of 0.53. 33540
DOI: 10.1021/acsami.7b01547 ACS Appl. Mater. Interfaces 2017, 9, 33539−33543
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atmosphere and without a proton source (Figure S3). The position of this peak is at lower potential compared to the MOF. Addition of O2 and the proton source (AA or TCA) caused a pronounced current increase, however, the magnitude of measured current does not exceed that of the bare FTO slide (Figure S1). These results demonstrate that the PCN-223-Fe exhibit a higher density of accessible sites than a monolayer of Fe(III)TCPP. Thus, MOFs provide a superior platform for immobilization of molecular catalysts on conductive substrates. Rotating ring-disk electrode (RRDE) voltammetry was utilized to determine the selectivity of ORR for the production of H2O vs H2O2. The measurements were performed on films of PCN-223 powder drop cast onto the glassy carbon working electrode (disk) using 0.1 M LiClO4/DMF + 0.3 M proton source (AA or TCA) under O2 atmosphere. A rotation rate of 500 rpm was used, which ensures that all electrogenerated H2O2 is swept to the ring electrode (Pt), which was held at 1.0 V vs NHE where it would be oxidized. The cathodic scan of PCN-223-Fe shows a rapid current drop below −0.3 V vs NHE (Figure 4a, b) characteristic of electrocatalytic ORR. The ring
environment and resemble that of the Fe(III) porphyrin in solution. The position of this process appears at more reductive potentials compared to other Fe porphyrins reported in the literature,24,25 including those inside other frameworks.18 It was noted, however, that the reduction potential of FeIII/II couple is highly sensitive to the chemical environment around the metal center. As a general rule, the half-wave potential shifts anodically with increasing binding strength of the solvent and counterions present in the electrochemical experiment.26 Moreover, the confinement of Fe porphyrin inside the PCN223 framework could also modify the interactions with surrounding electrolyte, thus causing further shifts in its reduction potential. Another possibility could be chemical transformation of electrogenerated Fe(II)TCPP species causing broadening and shift of the redox peak. This explanation is unlikely, however, because PCN-223-Fe was found to be stable toward reduction (vide infra). A linear dependency in the log(scan rate) vs log(current density) plot with the slope of 0.53 was obtained for PCN-223-Fe (Figure 3b). This trend suggests that the framework reduction is limited by diffusion which would involve counterion diffusion, into the MOF pores coupled with charge hopping between redox centers.27,28 A redox peak was observed at −0.9 V vs NHE in the voltammetry data of the free base analogue, and is attributed to the reduction of the porphyrin core to its radical anion state. It is likely that this process also occurs during the reduction of PCN-223-Fe, due to its close proximity to the Fe-based peak which would explain its broadness. Addition of O2 to the CV experiments caused marked change to the electrochemical response of PCN-223-Fe (Figure 2b). A pronounced current increase is observed below −0.4 V vs NHE, which is due to the reduction of MOF-captured O2. The FeIII/II reduction cannot be clearly distinguished which means that electrogenerated Fe(II)TCPP is rapidly reacting with O2. Electrochemistry of PCN-223-fb exhibits similar behavior, however, the current drop occurred at more cathodic potentials (−0.55 V vs NHE) and its magnitude was considerably lower. This result suggests that the presence of Fe is essential to achieving high catalytic activity toward ORR. Because protons are necessary to promote ORR, the effect of proton source on the catalytic performance PCN-223-Fe was investigated. For this purpose, acetic (AA) and trichloroacetic (TCA) acids were added to the supporting electrolyte, because of their compatibility with Zr-based MOFs. Under oxygen-free conditions (Figure 2c, e) PCN-223-Fe exhibit a distinct reversible redox process at E1/2 = −0.32 for AA and −0.49 V vs NHE for TCA assigned to the Fe(III)TCPP/Fe(II)TCPP redox couple. The anodic shift of this process compared to the acid-free voltammogram is ascribed to the stronger interaction of acetate or trichloroacetate with the Fe center in the porphyrin linkers.26 When both O2 and a proton source are present, the current undergoes a 2 orders of magnitude increase during the cathodic scan (Figure 2d, f). The onset of the catalytic current occurs immediately after the reduction of Fe(III)TCPP. TCA resulted in a higher catalytic current and earlier onset relative to AA which is attributed to the fact that it is a stronger acid. PCN-223-fb on the other hand, generated significantly less current compared to the bare FTO slides (Figure S1), which suggest that the framework hinders ORR by acting as an insulating layer on the electrode surface. PCN-223Fe was also compared with a self-assembled monolayer (SAM) of Fe(III)TCPP on FTO. The latter shows a quasi-reversible reduction process at E1/2 = −0.22 V vs NHE under Ar
Figure 4. Rotating ring-disk electrode (RRDE) voltammograms of PCN-223-Fe (blue) and PCN-223-fb (red) measured in 0.1 M LiClO4/DMF at 500 rpm and 10 mV s−1 scan rate (the ring current was magnified 5 times), (a) 0.3 M AA and (b) 0.3 M TCA. Changes in the number of transferred electrons (solid line) and H2O2 (%) (dashed line) with applied potential, (c) 0.3 M AA and (d) 0.3 M TCA.
current experienced an increase in the same potential range, mirroring the response of the disk electrode, which is evidence of H2O2 formation. PCN-223-fb, in comparison, generated less current, which is consistent with its CV data. The 4 e− ORR TOF vs Potential plot (Figure S4) shows that the electrocatalytic activity of PCN-223-Fe is comparable to other reported MOFs.9 It is important to note, however, that the calculated TOFs are dependent on the applied potential and therefore are not directly comparable. The percentage of generated peroxide (H2O2(%)) and the number of electrons transferred during the catalytic reaction can be quantified from the disk and ring currents.29 The results show that H2O2(%) steadily decreases before plateauing out when the potential is below −0.6 V vs NHE (Figure 4c, d). At the same time, the number of transferred electrons increases, approaching 4, the number required to produce H2O. This behavior resembles the literature reports of related Fe 33541
DOI: 10.1021/acsami.7b01547 ACS Appl. Mater. Interfaces 2017, 9, 33539−33543
Letter
ACS Applied Materials & Interfaces porphyrin-based systems.15,16,29 In the presence of AA, the H2O2(%) fell below 6%, which is comparable to the Fe(III)TCPP in related DMF-based electrolyte.16 The usage of TCA as a proton source on the other hand, resulted in significantly poorer selectivity, with H2O2(%) reaching 34%, despite the fact that a higher current was detected. In fact, below −0.66 V vs NHE PCN-223-fb exhibits higher selectivity for H2O production. One possible explanation could be partial instability of PCN-223-Fe in the presence of TCA (vide infra), leading to deterioration of its catalytic performance. Also, this finding suggests that other factors, besides acid strength, must be considered in the selection of suitable proton source for MOF-catalyzed ORR. The stability of PCN-223-Fe toward ORR conditions was investigated using a variety of techniques. It is an important consideration in assessing electrochemical behavior of MOFs in order to ensure that the electrocatalytic activity is an intrinsic property of a framework and is not arising from possible MOFderived decomposition products. For this purpose, PCN-223Fe films were subjected to 6 h controlled potential electrolysis (CPE) experiment at −1.0 V vs NHE using 0.1 M LiClO4/ DMF + 0.3 M proton source (AA or TCA) as a supporting electrolyte in the presence of O2. The CPE data collected in the presence of AA revealed that the current gradually decreases over time, whereas the use of TCA resulted in much higher initial current which rapidly dropped off before stabilizing above 1 h (Figure S2). This behavior indicates that TCA could be reacting with PCN-223-Fe (vide infra) but the framework still remains redox-active, as evidenced by the amount of generated current in the CPE data. After the electrolysis, films where characterized to confirm that the framework retained its structural integrity. X-ray photoelectron spectroscopy (XPS) showed that the chemical environments and oxidation states of Fe and N atoms in the linker remained unchanged as evidenced by the position of Fe 2p3/2 and N 1s peaks, matching the pristine PCN-223-Fe (Figure S5). In particular, the N 1s excitation of the porphyrin can be diagnostic to the presence of a metal inside the macrocycle.30 The N 1s signal of the PCN223-fb is split into two distinct peaks attributed to protonated (−NH−) and deprotonated (−N) pyrrolic N atoms. When Fe occupies the porphyrin macrocycle, the N atoms have identical chemical environments and the two peaks merge into one. Similar behavior was observed for the free Fe(III)TCPP and H2TCPP ligands (Figure S6). The fact that the N 1s signal of PCN-223-Fe did not split after CPE experiment means Fe(III) remained inside the TCPP ligand and did not leach into the solution. Therefore, catalytic activity due to released Fe ions in the electrolyte can be ruled out. This conclusion is further supported by the ICP-MS of the digested PCN-223-Fe films (Figure S8). The SEM imaging of the films showed that the morphology of the MOF particles did not change when AA was used as a proton source (Figure S9), which suggests that PCN223-Fe confirming its stability. TCA on the other hand, caused the appearance of amorphous coating on the surface of MOF particles (Figure S10). This reaction appears to be limited to the framework surface, since XPS spectra indicated that the bulk of PCN-223-Fe remained unchanged. The powder patterns of the scraped films (Figure S12) show that the peaks corresponding to the PCN-223-Fe structure can still be detected after the CPE experiment. This observation suggest that PCN-223-Fe does not lose its structural integrity. The low intensity of diffraction peaks is attributed to the lack of sufficient quantity of microcrystalline material on the films.
Furthermore, the absence of any peaks above 15° indicate that no dense crystalline phases have formed during the CPE experiment. Finally, UV−vis absorption spectroscopy of the supporting electrolytes after electrolysis confirmed that Fe(III)TCPP does not get released into the solution as evidenced by the absence of characteristic porphyrin absorption bands (Figure S13). Overall, the combination of characterization techniques demonstrated that PCN-223-Fe did not significantly degrade under the electrocatalytic ORR condition and therefore, can be considered the active catalytic species. In summary, porphyrinic PCN-223-Fe framework was solvothermally grown on a conductive FTO substrate and subsequently used as a working electrode to catalyze ORR. The electrochemical characterization determined that the redox activity and catalytic properties of Fe(III)TCPP were retained inside the framework. PCN-223-Fe films exhibited high H2O/ H2O2 selectivity, producing less than 6% of peroxide. Interestingly, TCA resulted in poorer selectivity compared to AA, even though it is a stronger acid and generates higher current. This finding opens up further avenues for optimization of MOF-based ORR catalysts through selection of a proton source more suitable for the internal pore environment. For example, installation of proton relays in close proximity to the catalytic centers could be used to improve the proton management during the catalytic cycle. The stability of PCN223-Fe after 6 h of electrolysis under ORR conditions was confirmed using several characterization techniques. In this study, the viability of utilizing highly robust MOF scaffolds to support a catalytically active moiety has been demonstrated. With further development, these materials can make a significant impact in the heterogeneous ORR catalysis field.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01547. Experimental details describing the sources of all chemicals and the synthesis of PCN-223-Fe films; description of the experimental methods and techniques; CVs of FTO slides, CPE of PCN-223-Fe with different proton sources; CVs of Fe(III)TCPP SAMs on FTO; XPS spectra of PCN-223-Fe before and after electrolysis, PCN-223-fb, Fe(III)TCPP and H2TCPP ligands, and Fe(III)TCPP SAM on FTO; ICP-MS data of digested PCN-223-Fe before and after electrolysis; SEM images of PCN-223-Fe before and after electrolysis, and PCN-223fb; PXRD of the scraped PCN-223-Fe films before and after electrolysis; UV−vis absorption spectra of the supporting electrolytes before and after electrolysis, and Fe(III)TCPP in DMSO solution (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Amanda J. Morris: 0000-0002-3512-0366 Notes
The authors declare no competing financial interest. 33542
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Efficient and Stable Electrocatalyst for the Oxygen Reduction Reaction in Acidic Media. ChemCatChem 2016, 8, 2356−2366. (20) Gao, W.-Y.; Chrzanowski, M.; Ma, S. Metal-Metalloporphyrin Frameworks: A Resurging Class of Functional Materials. Chem. Soc. Rev. 2014, 43, 5841−5866. (21) Feng, D.; Gu, Z.-Y.; Chen, Y.-P.; Park, J.; Wei, Z.; Sun, Y.; Bosch, M.; Yuan, S.; Zhou, H.-C. A Highly Stable Porphyrinic Zirconium Metal−Organic Framework with Shp-a Topology. J. Am. Chem. Soc. 2014, 136, 17714−17717. (22) Usov, P. M.; McDonnell-Worth, C.; Zhou, F.; MacFarlane, D. R.; D’Alessandro, D. M. The Electrochemical Transformation of the Zeolitic Imidazolate Framework ZIF-67 in Aqueous Electrolytes. Electrochim. Acta 2015, 153, 433−438. (23) Kelty, M. L.; Morris, W.; Gallagher, A. T.; Anderson, J. S.; Brown, K. A.; Mirkin, C. A.; Harris, T. D. High-Throughput Synthesis and Characterization of Nanocrystalline Porphyrinic Zirconium MetalOrganic Frameworks. Chem. Commun. 2016, 52, 7854−7857. (24) Munoz, R. A. A.; Banks, C. E.; Davies, T. J.; Angnes, L.; Compton, R. G. The Electrochemistry of Tetraphenyl Porphyrin Iron(III) within Immobilized Droplets Supported on Platinum Electrodes. Electroanalysis 2006, 18, 649−654. (25) Kadish, K.; Morrison, M.; Constant, L.; Dickens, L.; Davis, D. G. A Study of Solvent and Substituent Effects on the Redox Potentials and Electron-Transfer Rate Constants of Substituted Iron MesoTetraphenylporphyrins. J. Am. Chem. Soc. 1976, 98, 8387−8390. (26) Bottomley, L.; Kadish, K. Counterion and Solvent Effects on the Electrode Reactions of Iron Porphyrins. Inorg. Chem. 1981, 20, 1348− 1357. (27) Usov, P. M.; Ahrenholtz, S. R.; Maza, W. A.; Stratakes, B.; Epley, C. C.; Kessinger, M. C.; Zhu, J.; Morris, A. J. Cooperative Electrochemical Water Oxidation by Zr-Nodes and Ni-Porphyrin Linkers of a PCN-224 MOF Thin Film. J. Mater. Chem. A 2016, 4, 16818−16823. (28) Lin, S.; Pineda-Galvan, Y.; Maza, W. A.; Epley, C. C.; Zhu, J.; Kessinger, M. C.; Pushkar, Y.; Morris, A. J. Electrochemical Water Oxidation by a Catalyst-Modified Metal−Organic Framework Thin Film. ChemSusChem 2017, 10, 514−522. (29) Lefèvre, M.; Dodelet, J.-P. Fe-Based Catalysts for the Reduction of Oxygen in Polymer Electrolyte Membrane Fuel Cell Conditions: Determination of the Amount of Peroxide Released During Electroreduction and Its Influence on the Stability of the Catalysts. Electrochim. Acta 2003, 48, 2749−2760. (30) Li, Y.; Xiao, J.; Shubina, T. E.; Chen, M.; Shi, Z.; Schmid, M.; Steinrück, H.-P.; Gottfried, J. M.; Lin, N. Coordination and Metalation Bifunctionality of Cu with 5,10,15,20-Tetra(4-Pyridyl)Porphyrin: Toward a Mixed-Valence Two-Dimensional Coordination Network. J. Am. Chem. Soc. 2012, 134, 6401−6408.
ACKNOWLEDGMENTS This material is based upon work supported by U.S. Department of Energy, Office of Basic Energy Sciences, under Award DE-SC0012446.
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