MetalâOrganic Frameworks as Porous Templates for Enhanced

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Metal-Organic Frameworks as Porous Templates for Enhanced Cobalt Oxide Electrocatalyst Performance Luke Huelsenbeck, Shelby L. Hooe, Arian Ghorbanpour, Ashley M. Conley, Helge Heinrich, Charles W. Machan, and Gaurav Giri ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00127 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Energy Materials

Metal-Organic Frameworks as Porous Templates for Enhanced Cobalt Oxide Electrocatalyst Performance Luke D. Huelsenbecka‡, Shelby L. Hooeb‡, Arian Ghorbanpoura†, Ashley M. Conleya, Helge Heinrichc, Charles W. Machanb*, Gaurav Giria* a

Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904-

4319, United States b

Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United

States c

Department of Materials Science and Engineering, University of Virginia, Charlottesville,

Virginia 22904-4319 ‡ Equal

Contribution

*Corresponding Authors: [email protected] (ORCID 0000-0002-5182-1138); [email protected] (ORCID 0000-0002-8504-1033) KEYWORDS: electrocatalysis, oxygen evolution reaction, metal-organic framework, templating, electrodeposition

ACS Paragon Plus Environment

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ABSTRACT:

Water oxidation to dioxygen is an essential energy capture process in biological systems mediated by the oxygen-evolving complex. It continues to attract interest as a half-reaction in the electrochemical water-splitting to hydrogen and dioxygen in abiotic systems. There is a corresponding interest in improving the activity of non-precious metal electrocatalysts for this reaction. One of the most well-known water oxidation catalysts is amorphous cobalt oxide. This catalyst can operate in water at neutral pH with low overpotentials and is known to continually reform under catalytic conditions, a type of self-healing. The active sites for the electrocatalytic reaction are thought to be adjacent octahedral cobalt sites in the bulk material, meaning that only the surface of the material is active. Thus, catalytic activity could be improved by increasing the surface area to volume ratio of the active material. To this effect, co-deposited microporous materials such as metal organic frameworks (MOFs) can be used to control the morphology of cobalt oxide thin films. This study employs a highly stable MOF, UiO-66, to act as a templating agent for an electrodeposited cobalt oxide film. By depositing this porous crystalline MOF on the electrode surface prior to the electrochemical growth of the catalyst, we show the catalytic current density of the cobalt oxide material can be increased relative to the unmodified material. Through cyclic voltammetry (CV), controlled potential electrolysis (CPE), and electron microscopy studies, we show that the increase in current density is likely due to an increase in the effective surface area of the catalyst resulting from catalyst deposition around the MOF particles.


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INTRODUCTION: A rapidly changing energy landscape in the coming decades will require the diversification of renewable fuel sources, energy production, and energy storage. Splitting water into hydrogen and oxygen for use in fuel cells offers a cheap, long-term energy storage solution, and has been extensively studied for viability as a deployable renewable energy strategy.1–8 However, the lack of a robust, efficient and cost-effective water-splitting system has prevented ubiquitous implementation. Electrocatalysts have been developed to lower the activation energy of the water splitting reaction and drastically improve overall efficiency, but the highest performing electrocatalysts are composed of scarce elements and are not suitable for large-scale applications.9– 11

Earth-abundant first-row transition metals, specifically cobalt, have been extensively examined for viability in large-scale water splitting applications.12–18 One of the most promising cobalt-based compounds for the oxygen evolution reaction (OER) is a cobalt oxide cubane material deposited from phosphate buffer.19 The resultant amorphous cobalt oxide has demonstrated exceptional stability and activity.19 This includes a ‘self-healing’ property, where a significant driving force to reform the material occurs during the catalytic reaction.19 Catalytic activity is also strongly correlated with the cobalt oxide morphology, where higher surface area to volume ratio yields higher activity.20 Mechanistic and computational studies suggest the active species is a relatively smaller domain of the whole material: edge-sharing CoO6 motifs.15,21–23 To retain high catalytic performance, several supports and templates have been used to prevent aggregation during the ‘self-healing’ process and preserve a large number of surfaceactive sites.20,24,25 Vela et al. demonstrated a significant performance increase for Co3O4 when it


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was deposited as a part of a porous Co3O4/SiO2 core-shell nanoparticle construct.24 In this case, the improved performance was attributed to the higher surface area of the catalyst and prevention of catalyst aggregation.24 In a different approach, Zhao et al. created a hierarchically porous cobalt oxide film by electrodeposition on a polystyrene nanosphere template, which exhibited enhanced electrochemical performance as a Li-ion anode.25 Hence, controlling Co3O4 morphology and enhancing the catalyst surface area during the electrodeposition and catalytic process are promising ways to increase electrocatalytic activity for the OER. Metal-organic frameworks (MOFs) have been used in numerous fields to enhance the surface area of heterogeneous materials.

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They are highly porous, crystalline materials with

readily controlled pore geometry and tunable chemical properties.32 These frameworks are composed of metal ions or oxo-metallic secondary building units (SBUs) coordinated with organic linkers to form a porous, open framework. MOFs have been increasingly applied in the field of heterogeneous catalysis as catalyst supports and sacrificial templating agents to create hierarchical structures that enhance transport of reactants and products to active sites.33–37 Typical approaches for creating MOF supports involve immobilizing nanoparticles or catalysts within the MOF framework.20,38–40 Dolbecq et al. immobilized a sandwich-type polyoxometalate (POM) in a porphyrinic MOF, MOF-545, to show a high photocatalytic activity and stability for water oxidation.38 Other approaches have used MOFs as sacrificial templates to create hierarchical catalyst structures. Hu et al. created a mesoporous nanostructure Co3O4-MOF hybrid with a porous structure consisting of cobalt oxide nanoparticles embedded in a porous matrix.41 The porous framework and versatile chemistry of MOFs has proven to be advantageous in enhancing heterogeneous catalysts in a variety of systems.


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In this study, we aim to create a MOF-based microporous templating film to control the morphology of electrodeposited cobalt oxide films for OER (Figure 1). We chose UiO-66 as our templating MOF due to its high thermal, chemical, and mechanical stability.42–45 UiO-66 is composed of Zr6O4(OH)4 secondary building units (SBUs) in an octahedral geometry connected via benzene 1,4-dicarboxylate (bdc) linkers. This topology creates triangular pores around 6 Å in size with two pore cages exhibiting diameters of ~11 Å and ~8 Å.42 By controlling the deposition of the UiO-66 template and deposition of cobalt oxide films, we find a two-fold enhancement in current density for select cases.

Figure 1. (A) Schematic showing spray coating of a solution of UiO-66 particles onto a fluorine doped tinoxide (FTO) electrode (B) Closed packed films of UiO-66 particles on the electrode (C) Controlledpotential electrolysis (CPE) deposition of Co3O4 onto the UiO-66 films. (D) Conceptual diagram of Co3O4 electrocatalyst deposition onto the UiO-66/FTO layer. The catalyst grown on and around the MOF template increases the active site area present for catalysis.

RESULTS & DISCUSSION: MOF Structural Control: Spray coating allows for uniform and controlled deposition of a UiO-66 suspension onto the FTO substrate. Film thickness and coverage was controlled by varying the number of times the


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substrate is sprayed with a 1 second burst of solution, termed ‘passes’. Figure 2 shows how the film morphology changes as a function of pass number. It was found that increasing the number of passes increased the surface coverage and average film thickness, with a 20-pass spray-coated UiO-66 film resulting in complete coverage of the FTO electrode as seen by SEM (Figure 2D). Features resembling cracks were observed for UiO-66 films with complete coverage, likely due to capillary stresses developed in the film during the coating and drying processes.46 Cross sections of 20-pass samples as studied by SEM and profilimetry data typically yielded a UiO-66 layer thickness of several micrometers with high surface roughness and variability between sample batches (Figure S1, Table S1).

Figure 2. Topographic SEM images comparing (A) blank FTO substrate, (B) an FTO substrate with 1 pass of spray coated UiO-66, (C) an FTO substrate with 5 passes of spray coated UiO-66, and (D) an FTO substrate with 20 passes of spray coated UiO-66. Scale bar is 5 μm.

Cobalt Oxide Electrodeposition: The effects of UiO-66 film coverage on the behavior of a co-deposited cobalt oxide catalyst were analyzed by cyclic voltammetry (CV). Surface coverage of the UiO-66 on FTO was found to be proportional to the thickness of the UiO-66 on the FTO (Figure 2 and Table S1). With increasing thickness of the UiO-66 film from 300-1500 nm, an internal comparison of changes in Co oxide behavior shows a consistent shift in potential from 0.52 to 0.72 V vs Ag/AgCl for the Co(IV)/(III) reduction. There is also a 2.7x increase in catalytic current from 5.59 x 10-4 A/cm2


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for the cobalt oxide control to 1.51 x 10-4 A/cm2 at 1.2 V vs Ag/AgCl (Figure 3). This suggests the number of active sites increases with greater coverage of UiO-66. Furthermore, the observation that the reduction peak potential of the Co(IV)/(III) feature in the return sweep from catalytic potentials moves to increasingly positive potentials with increased UiO-66 film thickness is suggestive of different Co oxide growth mechanisms (Figure 3). We propose that the thickness of the UiO-66 changes the distribution of available chemical environments for the electroactive Co sites. It was also observed that when the film thickness exceeded 1500 nm, a suppression of current which is attributed to the insulating effect of the UiO-66 film (Figure S2).47–50

Figure 3. Topographic SEM images after 10-minute deposition of cobalt oxide (1 mM) on FTO substrates spray coated with (A) 1 pass (290 nm average thickness) and (B) 5 pass (1200 nm average thickness). The scale bar is 5 μm. (C) CV scan showing the effects of UiO-66 film thickness. Conditions: pH 7 phosphate buffer; glassy carbon rod counter electrode; Ag/AgCl reference electrode; referenced to Ag/AgCl; 100 mV/s scan rate.

A series of cobalt oxide electrodeposition experiments was performed on FTO substrates coated with UiO-66 from 1 to 10 minute time periods (Figure 4). It was found that more cracks developed in the films with increasing the cobalt oxide deposition times (Figure S3). This suggests


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the controlled-potential electrolysis (CPE) deposition either develops stresses in the UiO-66 films or exfoliates UiO-66 particles from the film during extended electrochemical deposition process. It should be noted that no significant change in diffraction pattern was observable using PXRD, indicating the UiO-66 structure is preserved during catalyst deposition (Figure S4). For all cobalt deposition times, cobalt oxide and UiO-66 particles can be observed in the film cracks, as confirmed by energy dispersive spectroscopy (EDS) (Table 1). We also studied the impact of cobalt oxide deposition times on CV performance. CV analysis showed that at increased deposition times of cobalt oxide, an increased catalytic current was observed at more positive potentials (Figure 4C). Consistent with previous observations, the amount of cobalt oxide deposited on the UiO-66/FTO film increases as more current is passed, resulting in higher catalytic activity.51,52 Indeed, on the return sweep, the Co(IV)/(III) reduction is observed to have a higher current for a 10-minute electrodeposition than a 1-minute electrodeposition time, a direct indication of an increased number of electroactive Co atoms in the material (Figure 4C).53


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Figure 4. Topographic SEM images after (A) 1-minute and (B) 10-minute deposition of cobalt oxide (1 mM) on FTO substrates spray coated with 20 passes of UiO-66. Scale bar is 5 μm. (C) Scan showing the effects of different deposition time (1 and 10 minutes) on the CV response. Conditions: pH 7 phosphate buffer; glassy carbon rod counter electrode; Ag/AgCl reference electrode; referenced to Ag/AgCl; 100 mV/s scan rate.

Characterization of the Heterogeneous Material: For the catalyst characterization study, films with a 10-minute deposition of cobalt oxide on FTO with a prepared 20-pass UiO-66 film were used unless otherwise noted. Films were probed using energy dispersive X-ray spectroscopy (EDS) during scanning electron microscopy (SEM) for areal sampling and transmission electron microscopy (TEM) for cross-sectional sampling. EDS mapping was used to show the relative elemental composition of the heterogeneous film (Figure 5). EDS maps for zirconium and cobalt suggests the spatial distribution of cobalt oxide is not significantly enhanced by the quantity of UiO-66 in a given region, as evidenced by the uniform distribution of cobalt coupled with a non-uniform distribution of zirconium (Figure 5A). Point measurements in cracked versus UiO-66 regions show the zirconium signature to cobalt signature ratio was lower in regions with cracks compared to those without (Table 1). The relative amount of cobalt, compared to the zirconium signature, was higher in the cracked regions.


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Figure 5. (A) Topographic SEM image of the cobalt oxide/UiO-66/FTO substrate with 10-minute deposition of cobalt oxide (1 mM) on FTO substrates spray coated with 20 passes of UiO-66 with EDS elemental mapping of zirconium and cobalt and (B) Topographic SEM image with EDS elemental mapping for the same conditions and sonicated for 10 minutes to remove excess UiO-66. Scale bar is 5 μm. Table 1. Zirconium and cobalt signature ratios with varied deposition times of cobalt oxide on FTO substrates spray coated with 20 passes of UiO-66, FTO substrate with a 10-minute cobalt oxide deposition and a 10-minutes cobalt deposition that was sonicated for 10 minutes to remove loose UiO-66 particles. ‡Data

averaged over two points

Deposition Time (min) 1 2 5 10 10, sonicated UiO-66

Zirconium (wt%)/Cobalt (wt%) Ratio UiO-66 Region Cracked Region 28.0 ± 5.8 20‡ 12 ± 4 5±2 10 ± 1 5±1 5.0 ± 0.7 1.5 ± 0.4 0.16 ± 0.05

To further study the cobalt distribution, the FTO electrode surface was characterized after sonication of the cobalt oxide/UiO-66 film in DI water. The sonication process mechanically removes both cobalt oxide and UiO-66 crystals that are weakly attached to the substrate. EDS point scans and mapping showed an increase in relative cobalt signature after sonication (Figure 5B, Table 1). The larger percentage of cobalt signature after sonication indicates that there is more cobalt oxide present near the FTO substrate than UiO-66 film. Additionally, the presence of a zirconium signature on the substrate after sonication indicates the presence of UiO-66 on the FTO


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substrate even after sonication, interspersed with the cobalt oxide layer. With this data, we hypothesize that the deposition of cobalt oxide starts near the electrode and grows into packing defects around the crystallites of the UiO-66 film. The UiO-66 particles act to guide cobalt oxide growth, directing it to form a layer with a higher surface area compared to that formed on a control FTO substrate without UiO-66 (Figure 1D). To confirm the preferential localization of cobalt oxide on the surface of the FTO substrate, a cross-section sample was characterized by TEM (Figure 6A). The TEM image cross-section was obtained with a focused ion beam (FIB) cutting procedure. A control TEM image of only cobalt oxide on an FTO substrate (Figure S5A) revealed a ~50 nm cobalt oxide layer is deposited on the FTO surface. Similarly, a ~50 nm layer can be seen in the TEM image taken of the cobalt oxide/UiO-66 film. EDS line scans indicate a strong cobalt signature centered around this layer for the cobalt oxide/UiO-66 film, confirming cobalt oxide is deposited on the FTO electrode regardless of the presence of UiO-66. It should be noted that these length scales reach the limit of the EDS spatial resolution due to beam scattering and nearest atomic neighbor interactions, leading to the blending of cobalt, tin, and zirconium signatures.54 The control sample in Figure S5 show the same blending for the tin and cobalt signature without the presence of UiO-66. Figure S5B also shows a blending of the tin and zirconium signature for UiO-66 that is spray coated onto the FTO. However, the decay of both tin and cobalt signatures as the zirconium signature increases indicated cobalt oxide is not uniformly distributed through the height of the UiO-66 film, and is instead preferentially located near the FTO substrate.


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Figure 6. (A) HRTEM of a cross sectional cut through an oxide/UiO-66/FTO film of cobalt prepared in a Focused Ion Beam (FIB) system, HRTEM micrograph of cobalt oxide/UiO-66/FTO film. Scale bar is 200 nm. B) EDX line scan showing the relative composition of cobalt, zirconium, and tin representing cobalt oxide, UiO-66, and FTO, respectively.

Catalytic Characterization: When the cobalt oxide is deposited on the UiO-66/FTO substrate, an increase in the catalytic response is observed during CV measurements relative to control experiments without the UiO-66 present (Figure 7). The CV data also show current increases with an increasing number of spray-coating passes of UiO-66, with an improvement observed for the 20 pass-coated samples (Figure 3C). The same result of improved current is observed with increasing the cobalt oxide deposition time (Figure 4C).


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Figure 7. CV comparison of a UiO-66 MOF sample on FTO (1100 nm average thickness) (red), cobalt oxide (10 min deposition) on FTO (black), and a UiO-66 MOF sample with cobalt oxide (10 min deposition) on FTO (1520 nm average thickness) (blue). Conditions: pH 7 phosphate buffer; glassy carbon rod counter electrode; Ag/AgCl reference electrode; referenced to Ag/AgCl; 100 mV/s scan rate.

The coverage of electroactive Co atoms was obtained from background current-corrected CV data from the Co(IV)/(III) reduction feature (Figure 8). Electrode surface coverage of electroactive cobalt atoms is a function of Faraday’s constant, electrode area and the integrated current profile reductive portion of the CV sweep upon the return from catalytic potentials. Calculations for a 20-pass UiO-66 sample with varied cobalt deposition times yielded electroactive cobalt surface coverage of 2.34 x 10-9 and 3.98 x 10-9 moles/cm2 for 1-minute deposition and 10minute deposition, respectively. Comparing these results to controls with no UiO-66 and the same deposition times shows a 22-fold increase in electroactive cobalt atoms (Figure S16). Furthermore, it can be seen two distinct peaks appear in the reductive portion of the CV curve with a distinct wave at lower potentials centered around ~0.5-0.6 V that increases with increased deposition time for UiO-66 samples. This trend suggests there are two different active species of


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cobalt atoms in the deposited cobalt oxide film. We hypothesize two chemically distinct environments exist on the UiO-66 covered electrode, one corresponding to a thin film of cobalt oxide on the FTO electrode surface similar to that of UiO-66 free FTO and the second is found growing around UiO-66 particles. However, more work is required to characterize the two sites and contribution of each to the total catalytic activity.

Figure 8. Reductive portions of CV used to calculate the number of electroactive Co atoms with the corresponding CV portion of a 20 pass UiO-66 subtracted out in order to remove capacitive current for a 1 minute (A) and 10 minute (B) deposition time of cobalt oxide on 20 pass UiO-66.

To compare the catalytic activities of these materials, CPE experiments were also performed with an O2 probe in a sealed cell configuration under Ar saturation conditions (Figure S7). The data show that O2 production rate, defined as the slope of O2 production over time, using the cobalt oxide/UiO-66/FTO substrate is higher than the cobalt oxide/FTO substrate. Control experiments with blank FTO and FTO with the spray coated UiO-66 film only, show a response that is comparable to the observed background diffusion of atmospheric O2 into the ‘sealed’ cell demonstrating no catalytic activity (Figure 9 and Figure S6). The solubility of O2 in aqeuous buffered conditions (1.3 mM in pure H2O) results in an induction period for O2 observation by the headspace probe under all catalytic and control conditions (Figure 9).55


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Figure 9. (A) Percent O2 time profiles for the reaction headspace in controlled potential electrolysis experiments in Figure S20 (B) Sealed-cell O2 background permeation data subtracted from the Co-oxide on FTO and Co oxide on UiO66 in (A), linear fit of the non-zero regions in the headspace for comparison. Conditions: pH 7 phosphate buffer; glassy carbon rod counter electrode; Ag/AgCl reference electrode; referenced to Ag/AgCl.

We also studied the catalytic stability of the cobalt oxide/UiO-66 films; CVs from before and after CPE are in qualitative agreement (Figure 10C). Because no significant morphological change beyond additional exfoliation was observed by scanning electron microscopy (Figure 10), we believe the structure of the cobalt oxide/UiO-66 film is unaffected by the CPE experiments (Figures S7-S14).


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Figure 10. Topographic SEM images before (A) and after (B) a CPE experiment at an applied potential of 1.4 V (vs. Ag/AgCl) on a cobalt oxide/UiO-66/FTO substrate. The scale bar is 5 μm. (C) CVs of cobalt oxide/UiO-66/FTO substrate, before (black) and after (red) a 10-minute controlled potential electrolysis experiment at an applied potential of 1.4 V (vs. Ag/Ag/Cl). Conditions: pH 7 phosphate buffer; glassy carbon rod counter electrode; Ag/AgCl reference electrode; referenced to Ag/AgCl; 100 mV/s scan rate.

Conclusion In conclusion, we have demonstrated that UiO-66 can act as a template for the enhanced catalytic activity of cobalt oxide films through an increased surface area, using a facile UiO-66 spray coating and subsequent cobalt oxide electrodeposition method. Greater surface coverage and film thickness of UiO-66 on the working electrode was found to enhance the catalytic activity over control films of cobalt oxide; current enhancement is observed at 1.2 V vs Ag/AgCl for the MOF film with cobalt oxide deposited in comparison to cobalt oxide deposited on a blank FTO slide (Figure 7). The presence of UiO-66 was not found to enhance or impede the deposition of cobalt oxide near the electrode, but rather guide the growth of the bulk material to yield cobalt oxide grown around the microporous UiO-66. This templating effect results in a larger active surface area of the electrodeposited cobalt oxide which has been shown to improve catalytic performance


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and a shift in reduction peak potential. While sacrificial templating techniques have been used to grow high surface area cobalt oxide previously, the templating agents must be removed for this surface area to be accessible to reactants.56 By employing a highly stable, microporous MOF template, this work is able to constrain the active morphology of cobalt oxide to maintain high surface area while allowing transport of products and reactants through micropores in the template to the active surface of the cobalt catalyst. The presence of the MOF is also advantageous as the catalytic reforming of the cobalt oxide will be slowed due to the heterogeneous substrate. This study suggests MOF structures can successfully be integrated with electrodeposited catalyst films as heterofunctional materials. Future work will focus on leveraging the size- and chemoselectivity of different MOF structures and depositing cobalt oxide within the framework to enhance the molecular transport properties and catalytic activities. Furthermore, we will investigate the active species to understand the change in electroactivity for this class of hybrid catalyst constructs.


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EXPERIMENTAL: General All chemicals and solvents (ACS or HPLC grade) for electrochemistry were commercially available and used as received unless otherwise indicated. The following chemicals were purchased from Sigma-Aldrich: dimethyl formamide (DMF, ≥ 99.8%), methanol (≥ 99.9%), zirconium (IV) chloride (ZrCl4, ≥ 99.9%), benzene-1,4- dicarboxylic acid (H2bdc, 98%). Ethanol was purchased from Koptec (190 proof). Chemicals were used as received without further purification. Gas cylinders were obtained from Praxair (Ar as 5.0). A FOSPOR-R oxygen probe from OceanOptics was used with NeoFox Viewer software. Electrochemistry All electroanalytical experiments were performed using a Metrohm Autolab PGSTAT302N potentiostat. Fluoride-doped Tin Oxide (FTO) glass slides were obtained from Sigma Aldrich( ~7 Ω/sq). An aqueous silver/silver chloride (Ag/AgCl) reference electrode (3 M KCl) was obtained from Metrohm. The counter electrode was a glassy carbon rod (⌀ = 3 mm) inside a glass diffusion tube. All CV experiments were performed in a 50 mL cell from Metrohm in a single-chamber configuration with ports for all electrodes and a sparging needle. All data were referenced to Ag/AgCl. The buffer solution used for electrochemical measurements was prepared on a 1.0 L scale by combining 6.81 g of NaH2PO4 (0.057 M) and 1.19 g of NaOH (0.030 M) in deionized H2O.


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Synthesis of UiO-66 Crystals UiO-66 was synthesized using the technique outlined by Cavka et al.42 In a typical synthesis, 24.9 g DMF was added 0.23 mmol ZrCl4 in a beaker and sonicated for 2 minutes before adding 0.23 mmol H2bdc. The mixture was stirred until fully dissolved. The solution was placed in a Teflon-lined acid digestion vessel (Parr Instruments) and heated at 120 °C for 24 hours. The vessel was cooled in air and the contents were centrifuged to isolate the solid product. The solid was centrifuged and washed once in DMF and twice in methanol before redispersion in methanol overnight to complete solvent exchange. A final centrifugation and resuspension in 12.6 g of methanol yielded a ~1 wt. % solution of UiO-66 in methanol. Spray Coating UiO-66 Films Fluorine doped tin oxide (FTO) slides were cut to 1 cm x 2 cm pieces, rinsed with ethanol and dried with a stream of air. Slides were masked to expose a 1 cm x 1 cm section and heated to 70 °C. An airbrush (Paasche H-CARD Single Action) was used to coat slides, where a ~1 second spray was equivalent to “1 pass”. Between 1 to 20 passes of the spray coater was used to control UiO-66 thickness and coverage. Catalyst Incorporation Co3O4 modified electrodes were prepared based on prior literature.57 Short (< 20 min) deposition times were utilized to ensure catalyst and UiO-66 film thicknesses were similar. In a representative experiment, an FTO electrode was prepared (with and without UiO-66 present) by attaching a nichrome wire with Parafilm and electrical tape. The counter electrode was a glassy carbon rod in a gas diffusion tube separated by a glass frit and the reference electrode was an


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aqueous Ag/AgCl electrode (3 M KCl). Deposition was carried out in a phosphate buffer solution (pH = 7) with 1 mM cobalt(II) nitrate hexahydrate (Co(NO3)2•6H2O). Controlled-potential electrolysis (CPE) was carried out at +1.4 V (vs. Ag/AgCl) for 10 minutes to deposit cobalt oxide onto the working electrode. Following deposition, electrodes were washed with deionized water and cyclic voltammetry (CV) and CPE were conducted in a fresh phosphate buffer solution. Characterization A Quanta 650 Scanning electron microscopy (SEM) (15kV, spot size 4, Everhart-Thornley detector (ETD)) with Energy Dispersive Spectroscopy (EDS) (Oxford Instruments, X-MaxN 80) was used to collect morphological and elemental analysis of Cobalt Oxide and UiO-66 films. Samples were prepared for SEM by coating with a protective layer (~20 nm thick) of goldpalladium using a Precision Etching and Coating system (PECS). Selected films were submersed in DI water and sonicated using a Bransonic Ultrasonic Bath (40kHz) to remove loose particles. All powder X-ray diffraction (PXRD) patterns were recorded on a PANalytical Empyrean X-ray diffractometer with a Cu K-α beam (λ = 1.54 Å). Scans were completed from 2θ of 5° to 50°. Cross sections of ~100 nm thickness were prepared from electrodes using a Helios Dual Beam FIB G4 UC. FIB milling and cleaning was performed with a Ga ion beam current ranging from 24 pA to 9.3 nA at 30 kV. To protect the surface, additional layers of protective material, consisting of carbon and platinum, were deposited. High resolution transmission electron micrographs (HRTEM), scanning transmission electron micrographs (STEM) and energy dispersive x-ray spectroscopy (EDS) (Ametek EDAX Titan 300ST) were taken of these 100 nm thick cross section samples using a FEI Titan (300kV).


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A Bruker Dektak XT Stylus Profiler was used to measure the film thicknesses. Using a blade, a line was cut along the center of the thin film to expose the substrate underneath. Using Vision64 software, an average height was calculated and compared against the bare substrate to determine the thin film thickness. Multiple thicknesses were determined along the line cut for each sample.

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Additional cross section and coverage micrographs, before and after cobalt deposition and CPE micrographs, CV graphs of varied cobalt and UiO-66 deposition combinations, CPE control and varied cobalt and UiO-66 experiments, Tafel plots for varied conditions, CVs before and after CPE of varied conditions, Oxygen evolution time profiles, and cobalt monolayer calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors


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*E-mail: [email protected] (G.G.)(ORCID 0000-0002-8504-1033), [email protected] (C.W.M.)(ORCID 0000-0002-5182-1138) Present Addresses †Institute for Sustainability, Energy, and Environment, University of Illinois UrbanaChampaign, Urbana, Illinois 61801 Author Contributions L.D.H. and S.L.H. authors contributed equally. A.C. obtained and analyzed thickness data. C.W.M. and G.G. directed the project, and analyzed the data. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Funding Sources G.G. and C.W.M. would like to acknowledge funding from the Research Innovations Award from the University of Virginia. G.G. and L.H. would like to acknowledge funding from the Virginia Space Grant Consortium (NNX15A120H).


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