Tuning of Redox Conductivity and Electrocatalytic Activity in Metal

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Tuning of Redox Conductivity and Electrocatalytic Activity in MetalOrganic Framework Films Via Control of Defect Sites Density ran shimoni, Wenhui He, Itamar Liberman, and Idan Hod J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12392 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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The Journal of Physical Chemistry

Tuning of Redox Conductivity and Electrocatalytic Activity in Metal-Organic Framework Films Via Control of Defect Sites Density Ran Shimoni, Wenhui He, Itamar Liberman and Idan Hod* Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology, BenGurion University of the Negev, Beer-Sheva, 8410501, Israel. ABSTRACT Redox-active Metal-Organic Frameworks (MOFs) are considered as promising platforms for assembling high quantities of solution-accessible molecular catalysts on conductive surfaces, toward their utilization in electrochemical solar fuel related reactions. Nevertheless, slow redox hopping based conductivity often constitutes a kinetic bottle-neck hindering the overall electrocatalytic performance of these systems. In this work, we show that by a systematic control of MOF defect site density, one can modulate the spatial distribution of post synthetically-installed molecular catalyst and hence accelerate charge transport rates by an order of magnitude. Moreover, the improved MOF-conductivity also yields an enhancement in its intrinsic electrocatalytic activity. Consequently, these results offer new possibilities for designing efficient MOF-based electrocatalytic systems.

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INTRODUCTION Metal–organic frameworks (MOFs),1-4 also known as porous coordination polymers, are arousing a great deal of scientific interest over the last two decades, mainly due to their exceptional properties such as high surface area, ordered crystallinity, and versatile chemical and physical modularity.2,

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As a consequence, the use of MOFs in several types of applications as gas

storage12 and separation,13 chemical catalysis,14-17 sensing,18 and photocatalysis19-23 have been widely explored. Recently, increasing efforts have been put in order to design and develop MOFbased systems for electrocatalysis24-34 and artificial photosynthesis.35-48 Conceptually, one could envision a highly porous MOF scaffold, containing both a light-harvesting and an electrocatalytic moieties. Under light illumination, the system will be able to convert absorbed photons into multiple redox equivalents, while rapidly mediate them to drive a desired catalytic reaction to produce chemical fuels. Full realization of this concept however, will first require an efficient means for charge transportation within the MOF film, in order to deliver electrons to/from the catalytically-active sites. Up to date, several approaches have been suggested for inducing conductivity in MOFs, including: 1) π-stackings of MOF-based linkers and incorporated conductive guests,49-52 2) charge delocalization in two-dimensional MOFs53 3) charge transport through MOF-installed conductive polymers,54 and 4) conduction through a redox hopping mechanism, between spatially-isolated redox-active moieties (MOF nodes or linkers).24, 55-59 Yet, despite the significant progress achieved in this field, charge transport within MOF films remains a major bottleneck which impedes the overall activity of MOF-based electrocatalytic systems.26,

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Hence, there is a clear need for

introducing new approaches to tune and accelerate the rate of charge transport in MOF films.


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In this work, we have postulated that the rate of redox hopping based conductivity could be tuned by a careful, systematic modulation of the density of the MOF’s defects sites61-62 (in the form of missing linkers). In principle, each defect site could also be considered as an anchoring-spot for covalent modification of the MOF’s nodes with a molecular electrocatalyst. In that manner, one could control the concentration and the distribution of MOF-installed redox-active catalysts, and thus affect the rate of charge hopping between one catalyst to another. As a case study, we have chosen to use the well-known hexa-Zr-oxo based UIO-66 MOF,61 due to its high chemical stability, and the ability to manipulate the coordination-based connectivity of the MOF’s node. We show that the hopping diffusion coefficients of Fe-porphyrin based (Hemin)-modified UIO-66 (UIO-66@Hemin) correlates to the level of defects in the parent MOF. Noticeably, it was found that the intrinsic oxygen reduction electrocatalytic activity of the Hemin@UIO-66 could also be tuned as a function of hopping kinetics.

EXPERIMENTAL SECTION Materials. The chemicals: lithium perchlorate (LiClO4 ≥ 95%), zirconium chloride (ZrCl4 ≥ 99.5%), Terephthalic acid (C6H4(CO2H)) 98%), Hemin (C34H32ClFeN4O4 ≥ 99.8%) were purchased from TCI. Acetonitrile (CH3CN ≥ 99%) from J.T Baker, dimethylformamide (C3H7NO 98%) and ethanol (C2H6O 99.7%) were purchased from Bio-Lab. hydrochloric acid (HCl), nafion (99.8%) from Sigma Aldrich.

1. Electrodes preparation 1.1 Synthesis of different defect level UIO-66. ZrCl4 (125 mg, 0.54 mmol) were sonically dissolved in 5 ml of DMF with different amount of concentrated HCl (0.1, 0.25, 0.5, 0.75, 1 ml).


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Then, Terephthalic acid (123 mg, 0.75 mmol) and 10 ml of DMF were added and the vial was sonicated. The solution was placed in an oven at 80°c over night. The as-prepared UIO-66_1-5 were isolated from the solution by filtration and washed 3 times with DMF, and 3 times with ethanol. For the removal of solvents residues, the synthesized material was filtered and dried in vacuum oven overnight. 1.2 Post-synthetic modification of UIO-66 with Hemin. Hemin (34mg, 0.052 mmol) was dissolve in 10 ml of DMF containing 200 mg of UIO-66. The solution was placed in an oven at 70°c over night. The prepared UIO-66@Hemin 1-5 were isolated from the solution by filtration and washed 3 times with DMF, and 3 times with ethanol. For the removal of solvents residues, the synthesized material was filtered and dried in vacuum oven overnight. 1.3 Procedure for preparation of UIO-66@Hemin films on carbon-cloth electrodes. UIO66@Hemin/carbon cloth electrodes were prepared via a general drop-coating method. To prepare the UIO-66@Hemin ink, 30 mg UIO-66@Hemin powder were dispersed in a mixture solution of 0.5 ml isopropyl alcohol, 0.5 ml DI water and 60 µl nafion solution (5 wt. %) via ultrasonication for about 20 min. A 166 µl portion of the as-prepared ink was drop-casted on the surface of the carbon cloth for obtaining a loading of 5 mg/cm2 after air-drying. The UIO-66@Hemin/carbon cloth were directly used as the working electrode in the following electrochemical processes. 1.4 Procedure for preparation of UIO-66@Hemin glassy carbon electrodes Glassy carbon electrodes were prepared via a general drop-coating method. To prepare the UIO66@Hemin ink, 5 mg UIO-66@Hemin powder were dispersed in the mixture solution of 0.75 ml isopropyl alcohol, 0.25 ml DI water and 60-µl nafion solution (5 wt. %) via ultra-sonication for about 20 min. A 8 µl portion of the as-prepared ink was dropped on the surface of the glassy carbon


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for obtaining a loading of 40 µg after air-drying. The UIO-66@Hemin glassy carbon were directly used as the working electrode in the following ORR electrochemical measurements.

2. Physical characterization Powder X-ray diffraction (PXRD) and thin film XRD were measured on a Panalytical's Empyrean multi-purpose diffractometer using Cu-Ka (0.15405 nm) radiation. N2 physisorption isotherms were taken at 77 K after the UIO-66_1-5 and UIO-66@Hemin_1-5 sample were degassed at 120 °C in vacuum for 17 hours, using a Quantachrome autosorb IQ2. The surface area was calculated by applying the Brunauer-Emmett-Teller (BET) model to the isotherm data points of the adsorption branch. The pore size distribution was measured by applying the Discrete Fourier transform-DFT model to the isotherm. Scanning electron microscopy (SEM) images were recorded using a JSM-7499F ultrahigh resolution cold FEG-SEM scanning electron microscope operating at an acceleration voltage of 3 V. X-ray photoelectron spectroscopy (XPS) analysis was collected on a ESCALAB 250 Thermo Fisher Scientific instrument. an EX05 inert ion gas gun fitted in this instrument was used to etch away surface layers of samples for composition analysis as a function of depth (etching time). The XPSPEAK software (Version 4.1) was used for further analysis the XPS spectra of the samples.

3. Electrochemical characterization 3.1 electrochemical testing. Electrochemical measurements were performed in a threeelectrode system. Ag wire and a Pt foil (active area of 1 cm2) were used as the quasi-reference electrode and the counter electrode respectively. UIO-66@Hemin/carbon cloth (active area of 1 cm2) was used as the working electrode. The data were recorded using a SP-150 Potentiostat,


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BIOLOGIC (Britain). The electrolyte was bubbled with argon for at least 30 min prior to the electrochemical measurements, and the flow of argon was maintained over the electrolyte (0.1M lithium perchlorate in acetonitrile) during the recording of electrochemical data. Cyclic voltammograms (CV's) are reported in potentials vs. Fc/Fc+ (E𝑉 𝑣𝑠. 𝐹𝑐\𝐹𝑐 + = E𝐴𝑔\𝐴𝑔 + + E𝐹𝑐\𝐹𝑐 + (V vs Ag)) were conducted at the scan rate of 50 mV s‒1. 3.2 Electrochemical ORR measurements. ORR measurements were conducted using an PGSTAT AUTOLAB instrument, using a Rotating ring disk electrode (RRDE) setup (glassy carbon disk and Pt ring). UIO-66@Hemin was deposited on the disk electrode (0.5 mm dimeter) by drop casting (see procedure in section 1.4). All potentials are reported as V vs NHE ( ENHE Acetonitrile = 0.630 ― E𝐹𝑐\𝐹𝑐 + (V vs Ag)). Linear sweep voltammetry (LSV) of the glassy carbon disk between 0.6 V to -0.9 V vs NHE while applying a constant potential of 0.45 V vs. NHE on the ring, both in Ar and O2 environments, under rotation of 1600 rpm. The number of transferred electrons was calculated by eq.S1 Eq.S1 𝑖𝑅 (𝑖𝐷 ― ) 𝑖𝑅 𝑁 𝑁𝑢𝑚𝑏𝑒𝑟 𝑂𝑓 𝑇𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 = ∗ 𝑊𝑃 + ∗ 𝑃𝑃 𝑖𝐷 𝑁 ∗ 𝑖𝐷 Were 𝑖𝐷 is the current on the glassy carbon disk, 𝑖𝑅 is the current on the Pt ring, N is the collection efficiency of the RRDE electrode, WP and PP represent the number of electrons transferred for water (4) and peroxide (2) production respectively. The percentage of produced peroxide during ORR was calculated by eq.S2 Eq.S2

𝑖𝑅

%𝐻2𝑂2 = 𝑁 ∗ 𝑖𝐷 ∗ 100

Were 𝑖𝐷 is the current on the glassy carbon disk, 𝑖𝑅 is the current on the Pt ring and N is the theoretical collection efficiency of the RRDE electrode.


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RESULTS AND DISCUSSION A series of 5 UIO-66 MOFs with varying density of defect sites (termed UIO-66_1-5 from low to high defect density respectively) were synthesized by a previously reported procedure61 (see experimental section for more details). Briefly, different amounts of hydrochloric acid additive (0.25, 0.1, 0.5, 0.75 and 1 ml) were added to the reaction solution in order to accelerate the MOF’s crystallization kinetics and thus induce the formation of undercoordinated Zr6-oxo nodes (defect sites) in the crystalline structure of the MOF. Hereafter, we have chosen to modify our MOFs with Hemin, due its well-known redox activity and its function as a molecular oxygen reduction electrocatalyst. Hence, UIO-66_1-5 were subjected to post-modification with Hemin (termed UIO66@Hemin_1-5) using the Solvent Assisted Ligand Incorporation (SALI)6 method (Figure 1). Optical images of the Hemin-modified MOFs clearly correlate the increased catalyst loading with higher UIO-66 defect density (Figure 1b). As can be seen by the Powder X-Ray Diffraction (PXRD) patterns in Figure S1, the as-synthesized UIO-66_1-5 possess the crystalline structure of the standard UIO-66, while UIO-66@Hemin_1-5 samples retain MOF crystallinity upon catalyst installation. As can be seen in Figure S2a, Indication for the control of MOF defect level was obtained by N2 physisorption isotherm analysis, which showed gradual rise in BET surface area from UIO-66_1 (1040 m2/g, low defect density) up to UIO-66_5 (1432 m2/g, high defect density). In addition, pore-size distribution analysis shows a good correlation between the formation of an additional micropore (̴1.4 nm) at higher density of MOF defects (Figure S2b).63 Scanning Electron Microscopy (SEM) images show that all MOF samples exhibit similar nanoparticulate crystal morphology with an average size of 100-200 nm (Figure S3).


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Figure 1. a) Schematic illustration of the structure of UIO-66 MOF having a missing linker (defect site), which is utilized to install a molecular catalyst (Hemin) at the coordinatively-unsaturated Zr6-oxo node. b) Optical image of varying loadings of Hemin obtained by MOF defect level modulation.

Thin films of the different MOFs (UIO-66@Hemin_1-5) were deposited on conducting carbon cloth electrodes (1cm2) by a drop casting of an ink containing UIO-66@Hemin and a nafion binder (see experimental procedure in the supporting information). Each electrode was loaded with 5 mg of UIO-66@Hemin_1-5. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to determine the Hemin surface concentration for each of the electrodes. As can be seen in Table S1, Hemin’s surface loading increased by a factor of 4 (from 7.8 up to 33×10-8 mol/cm2) going from low defect density (UIO-66@Hemin_1) to high defect density (UIO-66@Hemin_5) MOFs respectively. Moreover, X-ray Photoelectron Spectroscopy (XPS) depth-profiling of UIO66@Hemin_1-5 revealed that the modification of UIO-66 with Hemin is non-homogeneous throughout the MOF’s particles, as determined by the Fe/N to Zr atomic ratios (Figure S4). Interestingly, the higher the UIO-66’s defect level is, the deeper the penetration of Hemin into the bulk of the MOF particle. In other words, the fraction of Hemins residing in close proximity to the


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surface of the MOF is higher when the MOF’s defect level is lower. As will be further discussed below, this phenomenon impacts the electrochemical activity of the different MOFs. We first set to examine the electroactivity of our UIO-66@Hemin samples. To do so, Cyclic Voltammetry (CV) of all thin films (UIO-66@Hemin_1-5) was measured, in a 0.1M LiClO4 acetonitrile (MeCN) solution under Ar environment, with a three-electrode setup which includes a Pt counter and an Ag-based quasi-reference electrode (referenced against Fc/Fc+ redox couple) respectively. The applied potential was scanned in the cathodic direction between 0.7 to -0.2 V vs. Fc0/+ at a scan rate of 100 mV/s (Figure 2). In all UIO-66@Hemin samples, a quasi-reversible, broad redox peak is clearly observed, attributed to the electrochemical reduction of the MOFinstalled Hemin-based ligand (porphyrin-based Fe+3/Fe+2 couple), thus confirming that all obtained MOFs are indeed electrochemically-active and electronic charge could be delivered to drive the redox activity of the Hemins. Additional information on the mechanism of charge transport in the UIO-66@Hemin_1-5 systems was obtained by measurements of CVs at different potential scan rates (10, 25, 50, 75, 100, 200 and 300 mV/s). As can be seen in Figure S5 and S6, for all films, plotting of Hemin’s redox peak’s currents as a function of the square root of CV’s scan rate reveals a linear dependency, thus providing an indication for the fact that charge transport in UIO66@Hemin_1-5 is governed by a diffusional process (either counter ion in electrolyte or charge hopping between neighboring redox active Hemin sites in the MOF).


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Figure 2. cyclic voltammetry (CV) of UIO-66@Hemin_1-5 in Ar environment at 100 mV/s. UIO66@Hemin_1 (black), UIO-66@Hemin_2 (red), UIO-66@Hemin_3 (blue), UIO-66@Hemin_4 (pink), and UIO-66@Hemin_5 (green).

As a consequence, we were interested in comparing between the charge transport kinetics of the different UIO-66@Hemin_1-5 samples. Accordingly, potential-step chronoamperometry measurements were recorded in order to determine the rates of charge hopping between neighboring Hemin-based ligands in the MOF films. We have stepped the potential from fully oxidizing (0.5 V vs Fc/Fc+, Fe3+ state in all Hemins) to fully reducing (-0.2V vs Fc/Fc+, Fe2+ state in all Hemins) and vice-versa (cathodic and anodic potential steps respectively), while recording the current decay over time. The resulting current transients obey the Cottrell relation:64-65 𝐼(𝑡) =

(𝑛𝐹𝐴𝐶𝐷ℎ𝑜𝑝𝑝𝑖𝑛𝑔1/2) (𝜋1/2 𝑡1/2)

Where n is the number of electrons transferred per hopping event (1 for Hemin-based Fe+3/Fe+2 couple), F is Faraday constant, A is the geometrical area of the MOF working electrode (1 cm2), C is the molar concentration of the redox-active Hemin molecules in the film (in mol/cm3), and


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Dhopping is the diffusion coefficient (in cm2/s). At short enough measurement time scales, where the diffusion obeys semi-infinite conditions, I(t) vs t-1/2 behaves linearly and one can extract the value of Dhopping from the curve’s slope (See Figure S7). Figure 3 and table S2 summarizes the extracted Dhopping values (for both cathodic and anodic potential steps) for the different UIO-66@Hemin samples, as a function of Hemin surface concentration (as determined from ICP-OES characterization). Noticeably, for both cathodic and anodic redox hopping events, an inverse relation between Dhopping and Hemin concentration exists at low Hemin loadings (UIO-66@Hemin_1-2), while for higher loadings (UIO-66@Hemin_3-5) Dhopping do not vary significantly. Overall, Dhopping was modulated by a factor of 11 Between the slowest (0.36x10-9 cm2/s, UIO-66@Hemin_5) to the fastest diffusion rate (4x10-9 cm2/s, UIO66@Hemin_1). A reasoning for this behavior could be found from the spatial distribution of installed Hemins within the UIO-66 particles, as presented in Figure S4. As discussed earlier, compared to UIO-66 having high defect density (UIO-66@Hemin_3-5), at low defect density (UIO-66@Hemin_1-2) larger fraction of Hemin is installed close to the MOF’s outer-surface. Being a counter-ion-diffusion coupled reaction, a redox hopping event occurring at the bulk of the MOF’s particle, is impeded by the slow ingress/egress of the charge balancing ions through the framework’s pores. Meaning, a redox-active moiety (Hemin) residing at the proximity of the MOF’s surface will be encountered with faster supply of electrolyte species, and hence will exhibit higher hopping diffusion rates compared to a bulk-residing redox-active moiety.55


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Figure 3. Dhopping of UIO-66@Hemin 1-5, extracted from both cathodic and anodic potential steps measurements.

It is well-known that a redox hopping charge transport mechanism often constitutes a kinetic bottleneck for MOF-based electrocatalytic systems. As such, having a series of UIO66@Hemin_1-5 with varying hopping diffusion rates, we were interested in gaining further understanding on the manner in which tuning of charge transport kinetics influences the overall electrocatalytic activity of the system. Metal porphyrin-based complexes have been widely explored as molecular catalysts capable of accelerating a variety of important, energy-related electrocatalytic reactions.66-67 Specifically, it was found that Fe-porphyrins are highly active for catalyzing the oxygen reduction reaction (ORR),68 initiated by the electro-generation of the catalytically-active Fe2+-porphyrin site (Fe3+/Fe2+ reduction). Knowing that, as a model catalytic reaction, we have chosen to study the electrocatalytic ORR of our UIO-66@Hemin_1-5 samples. The electrocatalytic ORR performance of UIO-66@Hemin_1-5 was studied using a rotating ring-disc electrode (RRDE) setup. For each MOF, thin films were fabricated by drop-casting 40 µg of MOF on the glassy-carbon disc electrode (surface loadings ranging from 3.1 to 13.2×10-9 mol/cm2). First, CVs of the UIO-66@Hemin_1-5 series were recorded both under Ar and O2


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atmospheres in 0.1M LiClO4 MeCN containing 5.6M water (H2O) as an added proton source for the 4e-/4H+ catalytic reaction (Figure 4). As can be clearly seen, for all MOFs, purging the electrolyte solution with O2 imposes a significant change in the measured CV. At potentials negative to 0 V vs NHE, a steep rise in current is observed, serving as a strong indication for electrocatalytic ORR activity. For all samples, a peak current (6.3 – 7.3 mA/cm2) is achieved at 0.4 - 0.5 V vs NHE, as a result of mass-transport limitation of O2 reactants toward the MOFinstalled Hemin catalysts. Interestingly though, despite the fact that the amount of installed Hemin in UIO-66@Hemin_1-5 varies significantly, the overall ORR activity (as obtained from the CVs) of all MOFs is fairly similar.

Figure 4. Cyclic voltammetry (CV) of UIO-66@Hemin_1-5 on glassy carbon electrode (diameter = 0.5cm, surface loading of 40 μg). For ORR evaluation, CVs of all samples were measured both under Ar and O2 environments (scan rate of 100 mV/s). UIO-66@Hemin_1 (black), UIO-66@Hemin_2 (red), UIO66@Hemin_3 (blue), UIO-66@Hemin_4 (pink), and UIO-66@Hemin_5 (green).


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Moreover, the catalytic ORR selectivity for the production of H2O (4e- process) and H2O2 (2eprocess) was evaluated using RRDE voltammetry. Measurements were performed by scanning the MOF-containing disc electrode in the cathodic direction to reduce O2, while applying a constant anodic potential on the Pt ring (0.63 V vs NHE) in order to selectively oxidize produced H2O2. In order to avoid mass-transport limitations toward the ring electrode and achieve maximum product collection efficiency, the RRDE was rotated at a rate of 1600 rpm. Figure 5 presents the RRDE linear sweep voltammetry (LSV) data of the UIO-66@Hemin_1-5 samples. For all samples, at potential negative to 0 V vs NHE, a rise in cathodic current at the disc electrode (blue curves) is immediately followed by an increase in ring anodic current (red curves), thus confirming the production of H2O2 during ORR (Figure 5). Using this RRDE data, one could obtain the fraction of produced H2O2 as well as the average number of electrons transferred during the electrocatalytic reaction as a function of applied potential (Figure 6). All samples exhibit values very close to 4 transferred electrons and very low percentage of produced H2O2 (less than 10%), implying for the high product selectivity toward H2O in the UIO-66@Hemin systems. The ORR activity of a thin film of a bare UIO-66 (without installed Hemin) was also characterized, showing only a trace of catalytic activity and low product selectivity, thus confirming that the Hemin catalysts are responsible for the electrocatalytic activity of the MOF-based systems (Figure S8).


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Figure 5. RRDE ORR characterization of UIO-66@Hemin_1-5 (glassy carbon disc electrode diameter = 0.5cm, surface loading of 40 μg). The disc electrode’s potential was scanned cathodically (0.5 to -0.9 vs. NHE) to reduce oxygen, while a constant potential (0.63 vs. NHE) was applied on the Pt ring for H2O2 detection.


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Figure 6. Plots of produced peroxide percentage (blue curve) and number of transferred electrons (black curve) for a) UIO-66@Hemin_1, b) UIO-66@Hemin_2, c) UIO-66@Hemin_3, d) UIO-66@Hemin_4, and e) UIO-66@Hemin_5.

Yet, the results presented so far can only provide a representation of the overall electrocatalytic activity of the entire UIO-66@Hemin_1-5 systems. However, one should perform a comparison between the intrinsic activity of the Hemin molecular catalysts at the different MOFs. To do so, for each MOF, the ORR turnover frequency (TOF), defined as the amount of electrocatalytically generated ORR products per active sites per second, was extracted and plotted vs overpotential (Figure S9). Herein, a clear difference in the ORR intrinsic electrocatalytic activity of the different MOFs appears. Noticeably, UIO-66@Hemin_1 (lowest defect density) exhibits the highest


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maximum TOF, 7.7 s-1 at -0.78 V vs NHE, while UIO-66@Hemin_5 (largest defect density) shows the lowest TOF, 2 s-1 at -0.78V vs NHE. We note that to the best of our knowledge, these TOF values are the highest reported to-date for MOF-based electrocatalytic ORR systems (albeit TOFs are reported at varying overpotentials in other reports, and hence it is difficult to compare).25, 29, 34 Moreover, plotting maximum TOF (at -0.78 V vs NHE) together with Dhopping for UIO66@Hemin_1-5 samples reveals a striking correlation between them (Figure 7). It clearly shows that by accelerating the rate of redox hopping one could also enhance the electrocatalytic activity of the MOF-based films, thus further emphasizing the important interplay between charge transport and electrocatalytic reaction kinetics in these systems.

Figure 7. A plot of ORR Turnover Frequency (TOF) (blue curve) and Dhopping (black curve) as a function of Hemin surface loading.


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CONCLUSIONS In this work we show that by varying the density of defects in the Zr-based UIO-66 MOF, we were able to tune the concentration of ligand anchoring sites, and thus prepare a series of 5 different MOFs with altered surface loadings of post-modified ligands (Hemin based molecular catalyst). Thin films of all Hemin-modified MOFs were electrochemically characterized. In all 5 MOFs, the installed Hemins retained their redox activity. XPS depth profiling analysis revealed that in MOFs having low defect site density, installed Hemins mainly reside close to the MOF crystal surface. However, for high defect density MOFs, larger fraction of Hemin is immobilized dipper into the bulk of the crystal. Consequently, variances in counter-ion diffusion rates toward bulk or surfaceinstalled electroactive Hemins, allowed us to manipulate the kinetics of the MOF’s redox hopping by an order of magnitude. In addition, we have studied the electrochemical oxygen reduction activity of the Hemin-modified MOFs. Interestingly, we were able to show a direct correlation between the tuned redox based conductivity and the resulting intrinsic electrocatalytic activity of the MOFs. Future efforts will be focused on accelerating the rate-limiting counter-ion diffusion process by designing electroactive MOF-based systems having larger pore/channel diameters. This study sheds light on the mechanisms that govern charge transport properties in electroactive MOFs, and in turn will open new possibilities for designing more efficient MOF-based electrocatalytic systems for a variety of solar fuel related applications.

ASSOCIATED CONTENT Supporting Information


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Experimental

procedures,

material

synthesis

and

characterization,

and

additional

electrochemical measurements are included in the supporting information. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Idan Hod: [email protected]

ACKNOWLEDGMENT We thank the Ilse Katz Institute for Nanoscale Science and Technology for the technical support in material characterization. This research was supported by the Israel Science Foundation (ISF) (grant No. 603/18). W. He thanks the Planning and Budgeting Committee’s (PBC) fellowship for the financial support.

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Biography Idan Hod have completed his Ph.D. studies in Photo-electrochemistry at Bar-Ilan University, Israel (2013). After a 3-year postdoctoral spell at Northwestern University (Department of Chemistry), IL, USA, he was appointed as a senior faculty member at the Chemistry Department in Ben-Gurion University, Israel. His main research interests focus on the development of new functional, porous materials and their implementation in (photo)-Electrochemical solar fuel generation.


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Tuning of Redox Conductivity and Electrocatalytic Activity in Metal

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