MetalâOrganic Frameworks as Highly Active Electrocatalysts for High

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Metal-organic frameworks as highly active electrocatalysts for high-energy density, aqueous zinc-polyiodide redox flow batteries Bin Li, Jian Liu, Zimin Nie, Wei Wang, David Reed, Jun Liu, Pete McGrail, and Vincent L. Sprenkle Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01426 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016

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Metal-organic frameworks as highly active electrocatalysts for highenergy density, aqueous zinc-polyiodide redox flow batteries Bin Li*, Jian Liu*, Zimin Nie, Wei Wang, David Reed, Jun Liu, Pete McGrail, Vincent Sprenkle Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA. *

Corresponding author: Email: [email protected]; [email protected]

Phone: 1-509-375-2782 Abstract The new aqueous zinc-polyiodide redox flow battery (RFB) system with highly soluble active materials as well as ambipolar and bifunctional designs demonstrated significantly enhanced energy density, which shows great potential to reduce RFB cost. However, the poor kinetic reversibility and electrochemical activity of the redox reaction of I3-/I- couples on graphite felts (GFs) electrode can result in low energy efficiency. Two nanoporous metal-organic frameworks (MOFs)—MIL-125-NH2 and UiO-66-CH3—that have high surface areas when introduced to GF surfaces accelerated the I3-/I- redox reaction. The flow cell with MOF-modified GFs serving as a positive electrode showed higher energy efficiency than the pristine GFs; increases of about 6.4% and 2.7% occurred at the current density of 30 mA/cm2 for MIL-125-NH2 and UiO-66-CH3, respectively. Moreover, UiO-66-CH3 is more promising due to its excellent chemical stability in the weakly acidic electrolyte. This paper highlights a way for MOFs to be used in the field of RFBs. Keywords: Energy storage, redox flow battery, catalysts, metal-organic frameworks, polyiodide

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Intermittent renewable wind power and solar power are predicted to supply 12% of electricity in USA and 20% in Europe by 2020.1 The increased supply of renewable energy causes great concern about the performance and reliability of the electrical grid infrastructure. Large-scale electrochemical energy storage (EES) as a key technology can improve the flexibility of the grid for renewable energy, improve grid reliability and power quality, increase utilization of renewable energy, and extend the service life of the infrastructure. Among large-scale EES technologies, redox flow batteries (RFBs) are considered to be one of the most promising EES technologies because of their decoupled energy storage and/or power generation capabilities. The tunable energy and power ratio over a wide range can effectively reduce the cost of the whole battery system, and make RFBs well-suited for large-scale application and long duration storage.2-4 However, further commercial uptake of RFBs remains hampered by their high cost, which is partially due to their low energy density.

The traditional aqueous all-vanadium flow battery (VRB, pure sulfuric acid based) proposed by Skyllas-Kazacos et al. in the 1980s is the most investigated flow battery so far, owing to the minor cross-contamination of active species.5 In 2011, Li et al. introduced chlorine acid into the traditional VRB, thereby improving the solubility of the active element of vanadium. The improved energy density reached 25 Wh/L; but this is still only around 1/10 of that of Li-ion batteries.6, 7 The high cost of vanadium and low energy density of VRBs prevent its further market penetration.


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The energy density of RFBs is determined by the concentration of active materials in the electrolytes and the cell voltage. In an aqueous system, cell voltage is constrained by water electrolysis. For this reason, in the past few years some efforts have been put into the development of non-aqueous RFBs, because they potentially have high energy density due to their wider electrochemical window.8 But the low solubility of active species in non-aqueous solvents does not greatly improve energy density.8 Conversely, the poor safety, high cost, and low power density of non-aqueous RFBs are causes for concern.9 Accordingly, aqueous RFBs with appropriate redox potential, highly soluble and low-cost active species are attracting more attention. Low-cost water-soluble organic molecules, such as quinone derivative10, 11 and viologen derivative,12, 13 are reported to be very promising to replace expensive vanadium. Nevertheless, their poor chemical stability and low solubility in water are still issues to be addressed to achieve practical applications in the future.

In 2015, Li et al. reported a new environmentally friendly flow battery system.14 They developed a novel zinc-polyiodide redox flow battery (ZIB) system based on the following redox reactions: ℎ: + 2 ↔ 3 = 0.536 . : ! ↔ !

"#

(1)

+ 2 = −0.7626 . (2)

&'((: + ! ↔ 3 + !

"#

= 1.2986 . (3)

As shown in Fig. 1, the aqueous electrolyte in each battery half-cell is composed of zinc iodide salt dissolved in water. During the charging process, zinc is electroplated on the negative electrodes, while triiodide ions are formed in the positive half-cell. Upon discharge, the reverse


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process occurs. The zinc ions act as charge carriers transporting charge through the cation exchange membrane. The system exhibits ambipolar and bifunctional features, using zinc ions as both the redox active species and charge carriers, thereby eliminating the need for non-active counter ions. A high energy density of 167 Wh/L approaching that of Li-ion batteries is achievable based on the high solubility of zinc iodide electrolytes (> 7M). However, the energy efficiency is still very low, exhibiting with the values of 63% at a charge/discharge rate of 30 mA/cm2 at 2.5 M ZnI2. Thus, the low energy efficiency means that more active species are required for identical energy output, which eventually results in increased cost. Similar to other RFB systems, high electrode polarization resistance is one cause of low energy efficiency. Such resistance is primarily due to the poor kinetic reversibility and electrochemical activity of redox reaction between couples of I3-/I- on the electrode surfaces of graphite felts (GFs). An effective way to overcome this problem is to introduce an electrocatalyst on GF surfaces to facilitate I3-/Iredox reaction. Similar research on iodide/triiodide dye-sensitized solar cells (DSSCs) has been extensively reported.15 Catalysts, such as CoS16, WO217, TiN18, WC19, and MoC19 , as counter electrodes instead of expensive Pt, were proven to improve the energy conversion efficiency of DSSCs by catalyzing the redox reaction of I3-/I-.

Metal-organic frameworks (MOFs) also known as porous coordination polymers are novel porous materials with three-dimensional structures. They have been considered promising candidates for advanced adsorbents and catalysts because of their extraordinary surface area, tunable pore geometries, and unlimited chemical composition.20-23 The nodes of most MOFs are reasonably good Lewis acid sites. These acid sites tend to be exposed at MOF surfaces and may well function as catalysts. The uniform pore space exhibits shape and size selectivity toward 4 ACS Paragon Plus Environment

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catalytic reactions, and the unlimited structural topologies and diversified compositions may endow MOFs with many more possibilities to become designable catalysts.24 For instance, some MOFs have been studied as photocatalysts, working under visible light when photon absorption produces a state of charge separation with a positive hole in the valence band and an electron in the conduction band, like semiconductors. Silva et al., studied the Zr-based MOFs UiO-66 and its derivative UiO-66-NH2 as photocatalysts for hydrogen generation, and they found that the influence of the amino group produces a bathochromic shift in the optical spectrum.25 Similarly, the amino-functionalized Ti-based MOF MIL-125-NH2 was found to efficiently photocatalyze the hydrogen production reaction in an aqueous solution containing a sacrificial electron donor under visible light.26 These interesting catalytic properties are very likely to be used in electrochemical redox reactions as well.

In this paper, we use two high-surface area and hydrothermally stable MOFs, UiO-66-CH3 (calculated formula Zr6O4(OH)4[O2C-C6H2(CH3)2-CO2]6, space group Fm3,m) and MIL-125-NH2 (calculated formulaTi8O8(OH)4[(O2C-C6H5-CO2)]6, space group I4mmm) as catalysts in a zincpolyiodide redox flow battery to facilitate the redox reactions of I-/I3-, which has been proven to significantly increase the energy efficiency of flow cells. The chemical stabilities of both MOFs and their catalytic effects on the performance of flow cells are also investigated in detail.

MIL-125-NH2 and UiO-66-CH3 are successfully synthesized through solvothermal reaction using procedures reported in previous literature.27, 28 Fig. 2 (a) and (c) show the X-ray diffraction (XRD) patterns for the as-synthesized UiO-66-CH3 and MIL-125-NH2 powders, respectively. 5 ACS Paragon Plus Environment

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The XRD patterns match the theoretical patterns predicted from the crystal structures. No impurities were found in both MOFs. The crystal structures of both MIL-125-NH2 and UiO-66CH3 are illustrated in Fig. 2 (e) and (f), respectively. Their pore sizes are 0.6 and 1.2 nm, respectively, which are comparable to the radii of iodide (~0.21 nm) and triiodide ion (~0.58 nm). Accordingly, it is feasible for these ions to penetrate into MOFs to participate in electrochemical redox reaction.

Fig. 3 shows the cyclic voltammetry (CV) behaviors of the catalyst/graphite mixed powders and graphite powders deposited on glassy carbon at a scan rate of 20 mV/s by employing 0.25 M ZnI2 and 0.25 M ZnI6 mixed solutions. In Fig. 3, two peaks are exhibited in each curve, representing the redox reactions of the redox couples of I-/I3-. The positions and current intensities of anodic and cathodic peaks for different catalysts are listed in Table 1. The electrochemical activity and reversibility of catalysts toward the redox couple of I-/I3- can be reflected by the peak potential separation (∆E) and the ratio of the redox peak currents density (|ipa/ipc|). In comparison with pure graphite powders, the addition of UiO-66-CH3 and MIL-125NH2 enable the peak potential separation to be reduced by 138 mV and 121 mV, respectively. And the corresponding ratios of the redox peak currents for UiO-66-CH3 and MIL-125-NH2 are increased by 9.9% and 16.6%, respectively. These results suggest that both UiO-66-CH3 and MIL-125-NH2 exhibit improved electrochemical activity and reversibility toward the redox couple of I-/I3- compared to graphite powders. It seems that MIL-125-NH2 can better facilitate the redox reaction of I-/I3- than UiO-66-CH3, because of the similar peak potential separation and much-improved ratios of the redox peak current densities. That can be further confirmed by calculating reaction rate constant (k) of triiodide/iodide redox reaction (See Supporting 6 ACS Paragon Plus Environment

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Information and Fig. S1). The k values for different electrodes are listed in Table S1. It can be seen from Table S1 that k values for both oxidation and reduction reaction of triiodide/iodide couples are in the order of MIL-125-NH2 > UiO-66-CH3 >graphite, which agrees well with the above descriptions and our ensuing flow-cell performances.

Due to the low conductivity of UiO-66-CH3 and MIL-125-NH2, their distribution on the GF surfaces is of importance in improving electrode performances. Both MOFs were uniformly dispersed on the GF surfaces using in situ growth methods, as described in the Experimental Section in this paper. Basically, the graphite electrode was placed into precursor solutions. The typical MOF synthesis times were reduced from 24 h to 4 h in order to obtain nanoscale MOF seed crystals to achieve uniform distribution on the electrode. The loading amount of UiO-66CH3 and MIL-125-NH2 on GFs was calculated to be almost identical (~6%), based on the weight difference of GFs before and after depositing MOFs. Fig. 4 (a) and (b) show field emission scanning electron microscope (FE-SEM) images of UiO-66-CH3-modified and MIL-125-NH2modified GFs. Both catalyst particles are distributed uniformly on the GF surfaces. And the insets in Fig. 4 show that most particle sizes lie in the range of 100 nm to ~500 nm. Both MOFmodified GFs were used as positive electrodes in ZIBs and the corresponding cell performances were tested.

Because this work focuses on the study of electrodes used in ZIB, the charge capacity was fixed to be 60% of the state of charge (SOC) in order to avoid damaging the Nafion membranes that results from the growth of zinc dendrites. In agreement with the above CV results, as shown in 7 ACS Paragon Plus Environment

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Fig. 5 (a), the charge/discharge curves corresponding to MIL-125-NH2 demonstrated the smallest overpotentials (highest discharge voltage and lowest charge voltage) among all of the samples because of the superior catalytic effect of MIL-125-NH2. And the modification of GFs using UiO-66-CH3 also resulted in the reduction of cell overpotentials compared to using pristine GFs. The Coulomb efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) were also measured under current densities varying from 10 to 30 mA/cm2. The CE of a rechargeable battery is the ratio of the charge numbers that enter the battery upon charging compared to the number that can be extracted from the battery upon discharging, while VE is determined by the voltage difference between the charging and discharging processes. Fig. S2 shows CE and VE values for GFs with and without modification as a function of current density. The CE values for all of the samples are above 98%, and they slightly increase with increasing charge/discharge current densities, suggesting minor crossover of active species (iodide or triiodide ions), which we proved previously using X-ray photon spectroscopy.14 However, a fast charge/discharge rate can cause a substantial increase in charge and discharge overpotential, leading to a significant drop in VE. The introduction of catalysts onto the GF surfaces has little effect on CE values, but it can significantly influence the VE values. As shown in Fig. S2 and Fig.5 (a), the MIL-125NH2-modified GFs and UiO-66-CH3-modified GFs displayed higher VE values than the pristine GFs at the same charge/discharge current density resulting from the reduced charge/discharge overpotentials. The EE value is a derivative of the CE and VE (EE = VE × CE). As shown in Fig. 5(b), the trend of EE values with current density and GF surface modification is very similar to that of VE values, which is attributed to minor variations in CE values. The flow cells with MIL125-NH2-modified GFs and UiO-66-CH3-modified GFs serving as the positive electrodes both demonstrate higher EE values than those with pristine GFs; the EE values are improved by


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around 6.4% and 2.7%, respectively, at the current density of 30 mA/cm2. The enhancement may be due to the Lewis acid sites in the MOFs that can act as catalytic sites to accelerate the electron transfer.29 The Lewis acid sites can accept electrons and facilitate the transportation of electrons which we believe is the key to increase the transfer of electrons for the redox reactions. Our peak potential separation results and k values in Table 1 and S1 clearly showed that the MIL-125-NH2 has a better performance and this is could due to that the Lewis acid sites in the MIL-125-NH2 are less stronger than those in the UiO-66-CH3 so the it is easier for the electrons to dissociate from the Ti atoms.30 As we know, poor chemical stability of MOFs is one major obstacle to commercialization. Some MOFs may suffer from degradation during long-term soaking in water. Therefore, the chemical stability of MIL-125-NH2 and UiO-66-CH3 as catalysts should be tested in weakly acidic ZIB electrolytes (pH value ≈ 4). The cycling performances of ZIBs with pristine GFs and modified GFs as positive electrodes under a current density of 30 mA/cm2 are shown in Fig. 5(c). In Fig. 5(c), it can be seen that EE values are enhanced by the introduction of both electrocatalysts (MIL-125-NH2 and UiO-66-CH3) in the beginning. However, EE fading is observed for MIL125-NH2 from 25th cycle, and the EE value drops to ~64% over 45 cycles. Conversely, EE values keep nearly constant for UiO-66-CH3 over 50 cycles. Fig. S3(a) and (b) show the morphologies of MIL-125-NH2- and UiO-66-CH3-modified GFs after cycling. These MOFmodified GFs were washed with deionized (DI) water three times and dried at 423K for 24 h. We can still see that both MIL-125-NH2 and UiO-66-CH3 particles distributed well on the surfaces of the GFs. This suggests that the decay of EE values for MIL-125-NH2 is mainly related to the poor chemical stability of the catalysts rather than detachment of catalysts from GFs. UiO-66CH3 seems to have much better chemical stability than MIL-125-NH2 in ZIB electrolytes. 9 ACS Paragon Plus Environment

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To further understand the long-term chemical stability of the MIL-125-NH2 and UiO-66-CH3 catalysts used in our study, surface area, crystallinity, and inductively coupled plasma spectroscopy (ICP) tests of both of them were performed before and after they were soaked in the zinc iodide or triiodide (50% SOC) electrolytes for 7 days. The results for the surface area comparison of both MOFs are shown in Figure S4(a) and (b). The Brunauer–Emmett–Teller (BET) surface area of the UiO-66-CH3 remained almost the same after soaking in the ZIB electrolyte solution for 7 days, while the surface area for the MIL-125-NH2 dropped significantly by ~70%. Fig. 2(b) and (d) show the XRD patterns of MIL-125-NH2 and UiO-66-CH3 after soaking Similar to the results above, the UiO-66-CH3 maintained its crystal structure, while MIL-125-NH2 lost some and became more amorphous. Consistent with BET and XRD results, the ICP test results shown in Figure 5(d) verified that no Zr was detected in the electrolyte over time from 1 day to 7 days for the UiO-66-CH3, and Ti was detected leaching out of the MIL-125NH2 over time. The better chemical stability of UiO-66-CH3 can be ascribed to the 8-coordinated Zr-O bonds; in the case of MIL-125-NH2 the coordination number for Ti is 6. It has been reported that the higher the coordination number of metal atoms in the MOF structures the better stability the MOFs will have.31 Considering both the electrochemical performance and stability results, it might be worth focusing future research to enhance the electrochemical properties of the more stable UiO-66-CH3.

In conclusion, two kinds of high-surface area nanoporous MOFs, MIL-125-NH2 and UiO-66CH3, which were uniformly deposited onto the surfaces of GFs, can both act as electrocatalysts


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in aqueous ZIBs. Additionally, MIL-125-NH2 was proven to better accelerate the redox reaction of I-/I3- than UiO-66-CH3. Therefore, flow cells with MIL-125-NH2-modified GFs and UiO-66CH3-modified GFs serving as the positive electrode demonstrated enhanced EE values by ~6.4% and ~2.7%, respectively, at the current density of 30 mA/cm2 compared to those achieved with pristine GFs. However, UiO-66-CH3 was more chemically stable than MIL-125-NH2 in ZIB electrolytes, leading to more stable cycling performances. The current study represents an important potential that will boost the development and application of more MOFs in the field of RFBs. Experimental Section Materials preparation All chemicals were obtained from Sigma-Aldrich and Fisher Scientific and used without further purification. The UiO-66-CH3 powder was synthesized used a modified procedure from the previous literature.27 Basically, ZrCl4 (0.35 g, 1.5 mmol) and acetic acid (2.57 ml, 45 mmol) were first dissolved in 50 mL of dimethylformamide 2,5-dimethylterephthalic acid (0.29 g, 1.5 mmol) was then added to the solution. The reaction solution was heated at 393 K for 24 h. The resulting solid was isolated by centrifugation and dried at ambient temperature. In the GF electrode modification experiment, the GFs were placed inside the precursor solution and the reaction temperature was kept at 393 K for only 4 h. The MIL-125-NH2 was synthesized using the modified procedure reported by S.N. Kim et al.28 A solution of 3 mmol of titanium isobutoxide and 6 mmol of 2-amino benzene dicarboxylic acid in a 100 mL mixture of DMF and methanol (1:1, v/v) was prepared. The reaction solution was heated at 423 K for 24 h to obtain MIL-125-NH2 powder. Similarly, in the GF electrode modification experiment, GFs were placed


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inside the precursor solutions in a 200 mL Teflon-lined autoclave and heated at 423 K for 4 h. After cooling to room temperature, the GF electrodes modified with both MOFs were rinsed with DI water several times until no obvious MOF particles were observed in the wastewater stream. Then the modified GF electrodes were rinsed with methanol and dried in air.

Preparation of lab-scale zinc-polyiodide flow cells The makeup of the single flow cell has been described in detail in previously published papers.14 In each half-cell, porous GFs (SGL Carbon Group, Germany) with apparent areas of 40 cm2 served as the electrodes, and gold-coated copper serve as current collectors. The modified GFs acted as the cathode electrode. The commercially available cation exchange membranes (Nafion 115, Dupont, DE, USA) with a thickness of 125 µm were used as the membranes. Approximately 2 mm of distance between the anode surface and the membrane was reserved for the electrodeposition of zinc metal. The original electrolytes with 2.5 M ZnI2 were prepared by dissolving appropriate ZnI2 (Fisher Scientific, 98%) in DI water at room temperature. The single cell was connected to two glass reservoirs at both sides to store electrolytes.

Flow-cell test The electrochemical performance of the flow cell was carried out using a potentiostat/galvanostat (Arbin Instrument, USA) within a fixed voltage window between 0.3 V and 1.5 V under a constant current mode operated under current densities ranging from 10 to 30 mA cm-2. The uplimit of the charging process was determined by both voltage (1.5 V) and 60% of theoretical


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capacity. The definition of theoretical capacity can be seen in the previously published paper.3 Electrolytes were pumped at a flow rate of 100 mL/min through a peristaltic pump.

Characterizations A CV test was conducted in a three-electrode cell using a CHI660C workstation (CH Instruments, USA). A Pt wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The working electrodes were prepared by applying highly dispersed graphite or MOFs/graphite ink onto the pre-polished glassy carbon disk electrode. A Nafion thin film was coated on the surface by dropping 10 µL of 0.05 wt% Nafion on it after the ink was dried. The ink was prepared by dispersing graphite powder or the mixtures of MOFs and graphite powders in ethanol under strong ultrasonication. The active material loading on the disk electrode was 20 µg. CV testing was performed from 0 V to 1.1 V (vs. Ag/AgCl reference electrode) in water solutions with 0.25 M ZnI2 and 0.25 M ZnI6 at the different scan rates from 5 mV/s to 100 mV/s. The surface areas of MOF powders were determined by the BET method using nitrogen adsorption/desorption collected with a Quantachrome Autosorb-6B gas sorption system on degassed samples. The samples were activated at 423 K for 6 h with dynamic vacuum before the measurement. The XRD patterns of the MOF powders were collected with a Philips Xpert X-ray diffractometer using Cu-Kα radiation at ~1.54A. The morphologies of MOF-modified GFs before and after cycling were characterized by a FESEM (JEOLJSM-7600F).


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To study the chemical stability of MOFs in the ZnI2 electrolyte (50% SOC), the dissolved metal concentration was determined using ICP/atomic emission spectrometry (Optima 7300DV, Perkin Elmer) techniques after appropriate dilution. Three emission lines were chosen for each element as a crosscheck for spectral interference. The calibration standards were matrix-matched in water.

ASSOCIATED CONTENT Supporting Information. Additional information is available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE) Office of Electricity Delivery and Energy Reliability (OE) (under Contract No. 57558). We also are grateful for insightful discussions with Dr. Imre Gyuk of the DOE-OE Grid Storage Program. Pacific Northwest National Laboratory is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

Notes The authors declare no competing financial interest.

References


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1. IEA, World Energy Outlook. 2014. 2. Wang, W.; Luo, Q.; Li, B.; Wei, X.; Li, L.; Yang, Z. Advanced Functional Materials 2013, 23, (8), 970-986. 3. Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Chemical Reviews 2011, 111, (5), 3577-3613. 4. Dunn, B.; Kamath, H.; Tarascon, J.-M. Science 2011, 334, (6058), 928-935. 5. Skyllaskazacos, M.; Rychcik, M.; Robins, R. G.; Fane, A. G.; Green, M. A. Journal of the Electrochemical Society 1986, 133, (5), 1057-1058. 6. Li, L.; Kim, S.; Wang, W.; Vijayakumar, M.; Nie, Z.; Chen, B.; Zhang, J.; Xia, G.; Hu, J.; Graff, G.; Liu, J.; Yang, Z. Advanced Energy Materials 2011, 1, (3), 394-400. 7. Cheng, F.; Liang, J.; Tao, Z.; Chen, J. Advanced Materials 2011, 23, (15), 1695-1715. 8. Cappillino, P. J.; Pratt, H. D.; Hudak, N. S.; Tomson, N. C.; Anderson, T. M.; Anstey, M. R. Advanced Energy Materials 2014, 4, (1). 9. Soloveichik, G. L. Chemical reviews 2015, 115, (20), 11533-11558. 10. Huskinson, B.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X.; AspuruGuzik, A.; Gordon, R. G.; Aziz, M. J. Nature 2014, 505, (7482), 195-198. 11. Yang, B.; Hoober-Burkhardt, L.; Wang, F.; Prakash, G. S.; Narayanan, S. Journal of The Electrochemical Society 2014, 161, (9), A1371-A1380. 12. Liu, T.; Wei, X.; Nie, Z.; Sprenkle, V.; Wang, W. Advanced Energy Materials 2015. 13. Janoschka, T.; Martin, N.; Martin, U.; Friebe, C.; Morgenstern, S.; Hiller, H.; Hager, M. D.; Schubert, U. S. Nature 2015, 527, (7576), 78-81. 14. Li, B.; Nie, Z.; Vijayakumar, M.; Li, G.; Liu, J.; Sprenkle, V.; Wang, W. Nature communications 2015, 6. 15. Gong, J.; Liang, J.; Sumathy, K. Renewable and Sustainable Energy Reviews 2012, 16, (8), 58485860. 16. Wang, M.; Anghel, A. M.; Marsan, B. t.; Cevey Ha, N.-L.; Pootrakulchote, N.; Zakeeruddin, S. M.; Grätzel, M. J Am Chem Soc 2009, 131, (44), 15976-15977. 17. Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T. Chemical Communications 2011, 47, (15), 4535-4537. 18. Li, G. r.; Wang, F.; Jiang, Q. w.; Gao, X. p.; Shen, P. w. Angewandte Chemie International Edition 2010, 49, (21), 3653-3656. 19. Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T. Angewandte Chemie International Edition 2011, 50, (15), 3520-3524. 20. Li, J.-R.; Sculley, J.; Zhou, H.-C. Chemical reviews 2011, 112, (2), 869-932. 21. Kitagawa, S.; Kitaura, R.; Noro, S. i. Angewandte Chemie International Edition 2004, 43, (18), 2334-2375. 22. Liu, J.; Thallapally, P. K.; McGrail, B. P.; Brown, D. R.; Liu, J. Chemical Society Reviews 2012, 41, (6), 2308-2322. 23. Rowsell, J. L.; Spencer, E. C.; Eckert, J.; Howard, J. A.; Yaghi, O. M. Science 2005, 309, (5739), 1350-1354. 24. Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y. Chemical Society Reviews 2014, 43, (16), 6011-6061. 25. Gomes Silva, C.; Luz, I.; Llabrés i Xamena, F. X.; Corma, A.; García, H. Chemistry–A European Journal 2010, 16, (36), 11133-11138. 26. Fu, Y.; Sun, D.; Chen, Y.; Huang, R.; Ding, Z.; Fu, X.; Li, Z. Angewandte Chemie 2012, 124, (14), 3420-3423. 27. Huang, Y.; Qin, W.; Li, Z.; Li, Y. Dalton Transactions 2012, 41, (31), 9283-9285. 28. Kim, S.-N.; Kim, J.; Kim, H.-Y.; Cho, H.-Y.; Ahn, W.-S. Catalysis today 2013, 204, 85-93. 29. Tiwari, A.; Titinchi, S., Advanced Catalytic Materials. John Wiley & Sons: 2015. 30. Yang, G.; Pidko, E.A.; Hensen, E.J.M. J. Phys. Chem. C 2013, 117, 3976-3986. 31. Low, J. J.; Benin, A. I.; Jakubczak, P.; Abrahamian, J. F.; Faheem, S. A.; Willis, R. R. J Am Chem Soc 2009, 131, (43), 15834-15842. 15 ACS Paragon Plus Environment

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Table 1 The parameters obtained from CV curves for I-/I3- on graphite, MIL-125-NH2 and UiO66-CH3 electrodes. Epc (V)

Epa (V)

|ipc| (A/mg)

|ipa| (A/mg)

∆E (V)

|ipa/ipc| (%)

graphite

0.944

0.260

0.621

0.325

0.684

52.4

MIL-125-NH2

0.880

0.317

0.604

0.417

0.563

69.0

UiO-66-CH3

0.872

0.326

0.545

0.340

0.546

62.3


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Figure. 1 Schematic of the zinc-polyiodide redox flow battery (ZIB).


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Figure 2. XRD patterns for MIL-125-NH2 (a: before; b: after) and UiO-66-CH3 (c: before; d: after) before and after soaking in ZIB electrolyte for 7 days. Crystal structures for the MIL-125NH2 and UiO-66-CH3: (e) MIL-125-NH2, calculated formula Ti8O8(OH)4[(O2C-C6H5-CO2)]6, space group I4mmm. C: gray; O: red; Ti: silver; N: blue; (f) UiO-66-CH3, calculated formula Zr6O4(OH)4[O2C-C6H2(CH3)2-CO2]6, space group Fm3,m. C: gray; O: red; Zr: cyan. H atoms are omitted for clarity.


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0.6

Current Density /A/mg

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0.4 0.2 0.0 -0.2

Graphite MIL-125-NH2

-0.4

UiO-66-CH3

-0.6 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential /V (vs. Ag/AgCl) Figure 3. Cyclic voltammetry curves for the catalyst/graphite mixed powders and graphite powders deposited on glassy carbon at a scan rate of 20 mV/s in mixed solutions of 0.25 M ZnI2 and 0.25 M ZnI6.


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Figure 4. SEM images for MOF-modified GF electrodes. (a) MIL-125-NH2-modified GFs; (b) UiO-66-CH3-modified GFs.


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Figure 5. Electrochemical performances for the pristine GF and the MOF-modified GF electrodes. (a) Charge/discharge curves; (b) energy efficiency with different current densities; (c) energy efficiency with cycling numbers; and (d) metal ion leaching for MOFs soaked in electrolyte for different periods of time.


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MetalâOrganic Frameworks as Highly Active Electrocatalysts for High

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