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Article Cite This: Langmuir 2018, 34, 14143−14150

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Nickel−Carbon−Zirconium Material Derived from Nickel-Oxide Clusters Installed in a Metal−Organic Framework Scaffold by Atomic Layer Deposition Rebecca H. Palmer,† Chung-Wei Kung,† Jian Liu,† Omar K. Farha,†,‡,§ and Joseph T. Hupp*,† Department of Chemistry and ‡Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Downloaded via UNIV OF GOTHENBURG on December 4, 2018 at 04:53:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Atomic layer deposition is employed to install nickel oxide into NU-1000. Upon heating to 900 °C under nitrogen, a carbon material containing ZrO2 and Ni is formed. In notable contrast to the parent metal−organic framework, the pyrolyzed material is: (a) stable in highly alkaline solutions (typical conditions for water electro-oxidation) and (b) electrically conductive and thus able to deliver oxidizing equivalents (holes) to catalytic sites located far from the underlying conductiveglass electrode. The pyrolysis-derived material was characterized and its electrocatalytic activity for oxygen evolution was investigated.

1. INTRODUCTION Metal−organic frameworks (MOFs), consisting of organic linkers and metal or oxy-metal ions or clusters as nodes, define an enormous class of crystalline materialsboth in silico1,2 and in real space. Appropriately chosen MOFs can provide a high density of identical, potentially catalytic, photocatalytic, or electrocatalytic sites, either by postsynthetic catalyst grafting or by deploying MOF linkers or nodes themselves as catalysts.3−5 Notably, the typically molecular-scale porosity of many MOFs can give molecular reactants access not only to catalytic sites on the external surfaces of MOF crystallites, but also to sites deep within crystallites. Recently, MOFs have been shown to be excellent platforms for high-density support of small, siteisolated clusters of metal oxides or metal sulfides, capable of catalyzing a variety of gas and condensed-phase chemical reactions.6−10 Although MOFs can support considerable quantities of candidate catalytic sites, the electronically insulating character of many, if not most, MOFs can limit their usefulness specifically as supports for catalysts for electrochemical reactions. Thus, a scientific challenge of increasing practical interest is to render MOFs redox-conductive or electronically conductiveideally in forms that permit them to be interfaced with conventional electrochemical or photoelectrochemical electrodes.6,11−18 We recently reported on the electrocatalytic activity of oxycobalt clusters installed via ALD-like chemistry (ALD = atomic © 2018 American Chemical Society

layer deposition) involving aqua and hydroxy ligands on the hexa-zirconium(IV) nodes of the mesoporous platform MOF, NU-1000. Termed AIM (for ALD in MOFs),19 the process yields well-defined clusters, as does an analogous solutionphase process, termed SIM. Tetra-cobalt(II)oxy clusters were successfully installed throughout conductive-glass-supported crystallites of NU-1000. The clusters proved competent for electrochemical oxidation of aqueous hydroxide to O2, but only marginally effective for oxidation of water. As such, electrocatalysis experiments were run under alkaline conditions. Unfortunately, above pH 11 the linker-node bonds [carboxylic oxygen/zirconium(IV) bonds] proved susceptible to attack by OH−, with concomitant dissolution of the MOF. Because of insufficient hydroxide at pH 11, the catalytic current for oxygen evolution was limited to between 1 and 2 mA/cm2well below a target current density of 10 mA/cm2 for possible application of the catalyst in photoelectrochemical water-splitting at an intensity of 1 sun. Further complicating matters, the insulating character of the MOF itself rendered all but the first four or five layers of catalytic clusters electrochemically inaccessible.6 The primary benefit of involving additional clusters as electrocatalysts would be to Received: June 27, 2018 Revised: September 20, 2018 Published: November 1, 2018 14143

DOI: 10.1021/acs.langmuir.8b02166 Langmuir 2018, 34, 14143−14150

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h and was held for 8 h. After allowing the temperature of the furnace to cool, the sample was removed. Prior to framework pyrolysis, oxy-nickel(II) clusters were installed, via AIM, throughout crystallites of NU-1000 by using a Savannah model 200 (Cambridge Nanotech) ALD tool. Briefly, 60 mg of solvent-evacuated NU-1000 was placed in an ALD powder sample holder and into the reactor chamber. Bis(N,N′-di-tbutylacetamidinato)nickel(II) and water were used as ALD precursors. To ensure chemical saturation of all interior reactive sites, an automated, exposure-type pulse sequence was used. The stop valve was first closed; then a 1 s pulse of a nickel precursor was introduced into the ALD reactor chamber followed by a 500 s wait time, allowing the precursor to diffuse through the sample. The stop valve was then opened for 500 s to evacuate any unreacted precursor; this sequence was repeated 100 times. Then, the same pulse cycle was used for 0.15 s water pulses and the sequence was repeated 15 times. After installation of nickel oxide clusters, the sample was placed in a quartz tube and the same temperature profile for bare NU-1000 was followed (heat to 900 °C for 10 hours, hold for 8 h). 2.3. Characterization of Materials. Apparent surface areas were evaluated via Brunauer−Emmett−Teller (BET) analysis of nitrogen isotherm measured on a Tristar Instrument (Micromeritics) at 77 K. Pore sizes were calculated from the isotherms by using Micromeritics software appropriate for an N2-density functional theory (DFT) model that assumes slit geometry. Prior to isotherm measurements, each sample was activated on a Smart VacPrep Instrument (Micromeritics) at 120 °C for at least 12 h. Powder X-ray diffraction (PXRD) data were collected on either an ATXG or a SmartLab (Rigaku) instrument using the Cu Kα line, 0.154 nm. ICP-optical emission spectrometry (ICP-OES) was analyzed on a Thermo iCAP 7600 ICP-OES instrument (Thermo Fisher Scientific) monitoring Zr (at 327.305 and 257.139 nm, axial) and Ni (221.647, 231.604, and 231.604 nm, axial). For these measurements, samples of ∼3 mg size were placed in 1 mL each of 3:1 H2SO4/H2O2 and digested in a microwave reactor (Biotage). An SU8030 scanning electron microscope (SEM, Hitachi) featuring energy dispersive X-ray spectroscopy (EDS) capability was used for imaging samples and conducting elemental line scans. Prior to imaging, samples were coated in Au/Pd by using a Desk III TSC sputter coater (Denton Vacuum). Thermogravimetric analysis (TGA) was accomplished with a Star instrument (Mettler Toledo) under air. Simple two-point measurements of electrical resistance were made by connecting an Ohm meter to a pair of metal plates (each 0.7 cm2 in area), sandwiching samples of ca. 0.05 cm thickness. Raman spectra were measured using a LabRAM Confocal Raman system (Horiba). Each sample was placed onto a microscope slide and irradiated at 633 nm for 10 s. 2.4. Conditions for Electrochemical Measurements. Fluorine-doped tin oxide (FTO) glass (7 Ω, 2.2 mm, Hartford Glass Company Inc.) was used as the working electrode for electrochemical studies. FTO was cleaned by sonicating for 20 min each in soapy water, ethanol, and acetone, followed by drying with nitrogen and then heating at 100 °C for at least 10 min. Kapton tape (Fisher Scientific) was used to mask the FTO, resulting in an exposed geometric area of 0.2 cm2. A suspension of 15 μL of Nafion 117 solution, 10 μL of deionized water, and 2 mg of sample was sonicated for 30 min, and then 2 μL of the obtained suspension was drop-casted onto the masked FTO. The tape remained on the FTO during measurements to prevent naked FTO from contributing to electrochemical signals. After drying of the cast solutions, the cyclic voltammetric (CV) responses of the resulting FTO-supported films were measured using a ModuLab instrument (Solartron Analytical). A conventional three-electrode setup was utilized, and 10 mL of 1 M NaOH was used as the electrolyte. A saturated calomel electrode (SCE) (CH Instruments, Inc.) was used as a reference with a platinum wire (BASi) as the counter electrode. The potential for voltammetry measurements was converted to reversible hydrogen electrode (RHE) by adding (0.242 + 0.059 × 13.8) V to the potential measured versus SCE.

diminish the overpotential. In the simplest circumstances, each factor-of-ten increase in the amount of participating catalyst should decrease the overpotential at a given current density by an amount numerically equal in millivolt to the Tafel slope for the electrocatalytic reaction (i.e., the slope of a plot of overpotential vs log10 of current density). Notably, behavior of this kind (i.e., substantial lowering of overpotentials based on the electrochemical availability of catalysts at unusually high density) has been observed previously for the MOF-based electrocatalytic reduction of CO2.20 One approach to overcoming of the combined problems of base instability and marginal electrical conductivity is to use the MOF as a template for pyrolytic formation of material rich in graphitic carbon, but potentially retaining catalytic inorganic components; thus, heating in inert atmosphere at several hundred degrees celsius drives linker (and node) dehydrogenation as well as consolidation of remnant carbon into high-area graphene or graphite-like material.21−31 Here, we report on application of the strategy to the aforementioned oxy-nickelcluster-functionalized MOF, NU-1000, and on evaluation of the resulting material (Scheme 1). We find that the desired Scheme 1. Simplified Representation of (a) NU-1000 Including the Pyrene Linker and Zr6O8 Node, (b) Ni-AIMNU-1000 after Nickel Oxide Has Been Deposited on the Nodes of NU-1000 via ALD, and (c) cNi-AIM-NU1000_900 °C after Heating to 900 °C under Nitrogen; the Drawing in Panel (c) Is Meant To Convey Dehydrogenation and Conversion of Organic Linkers to Graphitic Carbon and Is Not Meant To Imply Retention of an MOF-Like Topology

electrical conductivity and chemical stability in aqueous base are obtained, and that AIM-installed nickel ions show reasonable electrocatalytic activity for evolution of molecular oxygen. In terms of catalyst internal surface area and fraction of nickel sites available for electrocatalysis, however, there remains room for improvement.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Chemicals, [benzoic acid, zirconyl oxide, N,Ndimethylformamide, hydrochloric acid (HCl), acetone, bis(N,N′-di-tbutylacetamidinato)nickel (II) (Strem Chemicals), Nafion 117 solution (5% in lower aliphatic alcohols, Sigma-Aldrich), sodium hydroxide (NaOH) (98.9%, Fisher), sulfuric acid], were used as received. Deionized water was used for making solutions. Millipore water was used for inductively coupled plasma (ICP) measurements. The ligand for NU-1000 was synthesized (by Dr. Timothy Wang) as previously reported.32 2.2. Synthesis of Materials. After synthesizing NU-1000 according to the previous report,32 100 mg of NU-1000 was freshly evacuated in a vacuum oven overnight at 80 °C. For carbonization of NU-1000, the sample was placed in a quartz tube (cut to 30.5 cm, 10 mm outer diameter, 1 mm thick, AdValue Technology) with one end sealed. This quartz tube was then placed within a tube furnace with house nitrogen flowing. The temperature was stepped to 900 °C in 10 14144

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3. RESULTS AND DISCUSSION 3.1. Installation of Nickel Oxide and Pyrolysis. The nickel-functionalized MOF, termed Ni-AIM-NU-1000, was found to contain 3.7 ± 0.9 Ni ions per hexa-zirconium node, as measured by ICP-OES. Following nickel installation, the MOF surface area decreased to 1420 m2/g (from 2070 m2/g NU1000); however, the material remained crystalline, consistent with previous reports.7 Samples were heated to 900 °C; samples are denoted cNi-AIM-NU-1000_900 °C and cNU1000_900 °C. Several temperatures (700−1000 °C) for MOF pyrolysis were examined, but treatment at 900 °C was found to give the highest surface area (see Table S1 and Figure 1).

Figure 1. Isotherms of NU-1000 (yellow, circle), Ni-AIM-NU-1000 (blue, diamond), cNU-1000_900 °C (brown, circle), and cNi-AIMNU-1000_900 °C (grey, diamond). Filled shapes indicate the adsorption plot (ads), whereas open shapes indicate the desorption plot (des).

Figure 2. PXRD patterns, from top to bottom, of (a) cNi-AIM-NU1000_900 °C (grey), cNU-1000_900 °C (brown), Ni-AIM-NU-1000 (blue), NU-1000 (yellow), simulated NU-1000 (black), and (b) abscissa from 25 to 80° of cNi-AIM-NU-1000_900 °C (gray) and cNU-1000_900 °C (brown).

3.2. Characterization. Nitrogen isotherms, shown in Figure 1, reveal substantial decreases in BET surface area for both nickel-free and nickel-functionalized versions of NU-1000 treated at 900 °C (also see Table S2). Thus, the BET area of NU-1000 drops to ∼100 m2/ga twenty-fold decrease. NiAIM-NU-1000 decreases to ∼10 m2/ga disappointingly low value for a putatively porous form of carbon. Clearly, this is a point that will require further attention in any follow-up study. Absent from the isotherms for the pyrolyzed samples is the mesoporous step that signifies the presence of 31 Å channels in the pre-pyrolysis samples. Differential pore volume plots in Figure S1 show that mesoporous and microporous volumes are greatly diminished by pyrolysis. PXRD measurements (Figure 2a) indicate that after heating to 900 °C under nitrogen, all peaks characteristic of NU-1000 are lost. Peaks indicative of tetragonal ZrO2 (JCPDS: 00-0501089), arising after heating, are present in both powder patterns for cNi-AIM-NU-1000_900 °C and cNU-1000_900 °C, with additional peaks characteristic of Ni (JCPDS: 04-0106148) apparent for cNi-AIM-NU-1000_900 °C (Figure 2b). These signatures of ZrO2 and Ni indicate agglomeration of both the Zr6O8 nodes and installed nickel oxide, as well as conversion of nickel oxide to Ni. At high temperatures and in the presence of C, CO, or H2, nickel oxide can be reduced to nickel.33,34 The powder patterns of cNU-1000_900 °C show ZrO2 peaks which are much broader than for cNi-AIM-NU1000_900 °C, presumably indicating smaller grain sizes for the

former. One would expect higher mobility for neutral nickel atoms than for oxy-hexa-zirconium(IV) clusters; agglomeration/migration of Ni after pyrolysis is also apparent from SEM images and EDS line scans. Figure 3a,b shows the SEM images of NU-1000 and cNU-1000_900 °C, respectively. It can be seen that NU-1000 crystallites remain rodlike after pyrolysis, with a shrinking of diameter but not length. EDS line

Figure 3. SEM images of (a) NU-1000 (b) cNU-1000_900 °C, (c) Ni-AIM-NU-1000, and (d) cNi-AIM-NU-1000_900 °C. EDS line scans across the crystals are presented in (b−d), where the signal from Zr is yellow, Ni is blue, and Mn (as a baseline) is white. 14145

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Langmuir scans show that the initially uniform distribution of Zr survives pyrolysis. The behavior of nickel is somewhat different. Loading of Ni(II) extends through the crystallite, but with diminished loading in the central section of the rod (Figure 3c). NU-1000, as synthesized here, is known to contain a minority second phase19,35,36 that likely corresponds to a topological isomer of NU-1000 (csq topology)possibly NU901 (scu topology). The primary channels of the latter are narrower than those for NU-1000a difference that affects molecular transport, conceivably attenuating Ni(II) loading. In any case, the central region, upon pyrolysis, radially constricts more than does the rest of the rod. Consistent with greater constriction and presumably a preferential loss in micropore volume from the center portion of the rod, this region is largely depleted in Ni(0) (Figure 3c; see also Figure 4). SEM images

Figure 5. XPS depth profiling of cNi-AIM-NU-1000_900 °C. An Ar+ beam was used to etch the sample.

pyrolysis, Ni-AIM-NU-1000 is easily digested), the loading of Ni in cNi-AIM-NU-1000_900 °C is 11 wt %. Finally, thermally driven dehydrogenation of linkers and consolidation of remnant carbon into graphene- or graphitelike structures should render the pyrolyzed materials electrically conductive. (The conductivity of untreated NU-1000 was previously measured to be 9.1 × 10−12 S/cm.)37 Using Raman spectroscopy, we observed broad bands around 1300 and 1600 cm−1 in all our carbonized samples, which are consistent with the D and G bands of disordered graphene (Figure S5).38−41 We attribute any conductivity in our pyrolyzed materials to this disordered graphene. Single-run screening experiments at each of several pyrolysis temperatures (Figure 6; see also Table

Figure 4. SEM−EDS mapping of cNi-AIM-NU-1000_900 °C showing (a) SEM image, (b) C Kα1 map, (c) Zr Lα1 map, and (d) Ni Kα1.

of cNi-AIM-NU-1000_900 °C show discrete particles ranging in size from about 30 to 95 nm (Figures 3d and S2). For cNU1000_900 °C (Figure 3b), the particles are notably absent. We ascribe the particles, therefore, to Ni(0), and note again that agglomeration via Ostwald ripening should be considerably easier for metallic nickel than for zirconium oxide. Depth-profiling XPS measurements of the cNi-AIM-NU1000_900 °C show more nickel after etching with an Ar+ beam. Increasing nickel content with increasing etching depth also suggests that nickel is largely enshrouded by carbon. Figure 5 (see also Figure S3) summarizes data for a range of etching depths. The combined results suggest that the majority of the installed nickel is, unfortunately, rendered physically inaccessible by pyrolysis. ICP-OES proved impractical for gauging total nickel loading in cNi-AIM-NU-1000_900 °C because of resistance by the carbon-enshrouded metals to dissolution/digestion. Instead, TGA measurement of the cNi-AIM-NU-1000_900 °C was run under oxygen to remove all carbon material and oxidize all metals (Figure S4). The remaining mass was assumed to be NiO and ZrO2. On the basis of the ratio between nickel and zirconium measured by ICP prior to pyrolysis (prior to

Figure 6. Log of resistance, in ohms, for NU-1000 carbonized at various temperatures (gray, solid) and for cNi-AIM-NU-1000_900 °C (blue, diagonals).

S3) showed that pyrolysis at 700 °C is sufficient to impart substantial conductivity or, equivalently, a resistance of modest magnitude compared with the parent material, NU-1000. Measurements for samples subjected to progressively higher pyrolysis temperature indicate that resistance is lowest for samples pyrolyzed at 900 °C. As such, this temperature exclusively was used for preparation of candidate electrocatalysts from Ni-AIM-NU-1000. Data for cNi-AIM-NU1000_900 °C represent an average of three samples (Table S3). 14146

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the TOF is ∼0.01 s−1; thus, there clearly is room for improvement based on electrochemically accessing a larger fraction of the candidate catalytic centers. Under conditions where the catalytic material itself presents negligible electrical resistance, slopes of Tafel plots [potential (E) vs log of catalytic current density (J)] can provide mechanistic insight.47 For cNi-AIM-NU-1000_900 °C as the electrocatalyst and aq 1 M NaOH as the electrolyte, the Tafel slope for oxygen evolution at low to moderate current densities (J < 0.5 mA/cm2) is ∼120 mV/decade of current density, based on CV scans at 10 mV/s; see Figure S7. Representatives of previous studies of “NiO” are reported Tafel slopes of 30, 55, 60 mV/decade.45,48,49 Previous studies showed Tafel slopes for NiO on graphene of 8538 and 116 mV/decade for NiO on carbon cloth.50 The Tafel slope can be converted to a dimensionless quantity, β, that reports on the kinetic reaction order in electrons here for the overall four-electron (and four-proton) extraction from pairs of hydroxide ions to yield O2. Briefly, β = (2.3RT/ F)/(Tafel slope), where R is the gas constant and F is the Faraday. At room temperature, the quantity (2.3RT/F) equals 59 mV. Under conditions of stepwise transfer of individual electrons, β also equals np‑rds + α, where np‑rds is the number of electrons extracted prior to the rate-determining step and α, ideally, is either ∼0.5 or else zero. If α ≅ 0, the ratedetermining step is chemical (e.g., proton transfer) rather than electrochemical. If α ≅ 0.5, the rate-determining step is electron transfer. Thus, a Tafel slope of 120 mV/decade translates to β ≅ 0.5 and implies that extraction of the first of four electrons is rate-limiting. The reported literature value of 30 mV/decade48 equates to β ≅ 2 and implies that two electrons are reversibly transferred in advance of a ratedetermining chemical step. A Tafel slope of 40 mV/decade is equivalent to β ≅ 1.5 and implies that one electron is reversibly transferred, with extraction of a second electron constituting the rate-determining step. The most effective non-precious-metal-containing electrocatalysts for oxygen evolution from aqueous basefor example, layered double-hydroxides of nickel doped with iron51,52typically function via rate-determining transfer of the second or third electron and/or proton. The observation here of rate-determining transfer of the first electron suggests that there is room for improvement based on alteration of the nanoscopic structure of the oxy-nickel catalystperhaps to resemble more closely the hydrated, layered double-hydroxide form of oxy-nickel. For this preliminary study, however, we have viewed the assessment and modification of the electrocatalyst nanostructure falling outside the scope. Returning to Tafel results for cNi-AIM-NU-1000_900 °C, we find that the slope increases as the current density and overpotential increase. At J = 10 mA/cm2, for example, the Tafel slope is about 270 mV/decade, corresponding to a β value of ∼0.22. Simple Marcus theory considerations for an isolated initial electron-extraction step imply that β should decrease as the reaction drive, η, increases. From other studies of electrocatalytic oxygen evolution,46,53,54 however, the increase in Tafel slope (decrease in β) observed here is likely too substantial to be attributable fully to Marcus effects. We speculate that at high currents, i, an uncompensated resistance, Ru, may contribute to the observed large Tafel slope. Thus, a portion of the applied potential would be lost as iRu, with the portion increasing as the catalytic current increases.

3.3. Electrochemical Investigation of Oxygen Evolution. To investigate cNi-AIM-NU-1000_900 °C as an electrocatalytic material for oxygen evolution, cNi-AIM-NU1000_900 °C was drop-casted, as a Nafion suspension, onto FTO. The obtained thin film of was then continuously cycled in 1 M NaOH aqueous solution at 25 mV/s, between 1.06 and 1.63 V versus RHE. As shown in Figure S6, one set of reversible redox peaks is present in the CVs. On the basis of runs with three samples, an average peak potential (anodic & cathodic) of 1.38 V versus RHE was obtained. To first order, that is, within a few tens of millivolts or better, this potential can be equated with the formal potential of the nickel redox couple (see also Figure 7).42 Current for catalytic oxygen

Figure 7. Cyclic voltammetry of Nafion-enshrouded cNi-AIM-NU1000_900 °C (black) and cNU-1000_900 °C (red) drop-casted on FTO, measured in 1 M NaOH at a scan rate of 10 mV/s. The reported overpotentials are based on averaging forward and reverse voltammetric scans.

evolution is clearly evident at ∼1.55 V and beyond. The catalytic current increases for the first several cycles (ca. 20 cycles; Figure S6). Similar behavior has been attributed elsewhere: the electrochemical conversion of marginally catalytic nickel oxide to higher activity, hydrated NiOOH or similar species.43−45 Integration of the nickel III/II feature in the CV indicates that 4 ± 1 × 1015 nickel ions per square centimeter of electrode are electrochemically active. This corresponds to 2% of the total nickel presented in the dropcasted film and is consistent with the conclusion above that many nickel atoms are carbon-enshrouded and therefore electrochemically inaccessible. Scanning over a wider potential window (Figure 7) reveals an overpotential (η) of 550 mV to reach a benchmark46 current density of 10 mA/cm2 for oxygen evolution. At a current density of 0.5 mA/cm2, η is 344 mV. For comparison, cNU-1000_900 °C was drop-casted on the FTO substrate via the same procedure, and the CV of the obtained thin film was measured in the same electrolyte. As shown in Figure 7, the drop-casted cNU-1000_900 °C produces a much smaller catalytic current compared to the cNi-AIM-NU-1000_900 °C, which suggests that the nickelrich nanoparticles incorporated in carbonized NU-1000 exhibit promising electrocatalytic activity for oxygen evolution. For photoelectrochemical conversion of solar energy to storable chemical energy (e.g., via water-splitting), typical targets are 10 mA/cm2, at an overpotential on the oxidative side of 400 mV or less. At η = 400 mV, the turnover frequency (TOF) per electrochemically accessible nickel ion, assuming delivery of four holes per dioxygen molecule generated, is ca. 0.1 s−1. At η = 550 mV, the TOF is 0.5 s−1. If all nickel ions are considered, 14147

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Langmuir Use of the potentiostat’s iR compensation function should largely compensate for voltage losses attributable to electrolyte resistance (which, no doubt, are small for aq 1 M NaOH). A second, and more substantial, source of uncompensated resistance, however, might be reactant (hydroxide) concentration polarization. Although likely insignificant for an electrocatalyst immersed directly in 1 M NaOH, the story may be different for Nafion-enshrouded electrocatalysts. Solvent-swollen Nafion presents fixed sulfonate sites at high concentration, that is, on the order of 4 M.55 As such, Nafion is capable of functioning as a Donnan exclusion membrane for electrolyte anions (i.e., a cation-permselective and anionexcluding membrane). It is conceivable that electrolyte−anion (hydroxide) exclusion is significant enough at current densities above ca. 1 mA/cm2 to engender concentration polarization and exaggerated Tafel slopes. In principle, the putative concentration polarization effect could be eliminated by omitting Nafion from the catalytic electrode assembly. In practice, we found that depositing cNiAIM-NU-1000_900 °C without a Nafion shroudwhether by spin-casting or electrophoretic depositionleft the film-based electrocatalyst susceptible to delamination. If Donnanexclusion behavior and associated concentration polarization are the culprits with Nafion-embedded electrocatalysts, the problem could be overcome with a suitable anion-permselective (fixed-cation) polymeric host matrix for cNi-AIM-NU1000_900 °C.

has the unfortunate side-effect, at the pyrolysis stage, of substantially further lowering the surface area (to ∼20 m2/g). Such low areas suggest that the overwhelming majority of nickel ions may be carbon-enshrouded, and therefore isolated from the electrolyte solution. Clearly, a focus going forward will be to retain surface area during pyrolysis and, concomitantly, to expose a much larger percentage of potentially catalytic metal sites to the reaction solution. A further limitation on electrocatalyst performance, especially at high current density is an excessively large Tafel slope.48 We speculate that the large slope may be an artifact of catalyst immobilization in Nafion, a material known to be capable of displaying Donnan exclusion of electrolyte anions,56 for example, reactive hydroxide ions. Thus, the excessive Tafel slope might be a manifestation of Nafion-engendered concentration polarization. Elimination of Nafion, or replacement of Nafion with a similarly robust and water-swellable host polymer featuring fixed cationic sites in place of anionic ones, should eliminate overpotential contributions attributable to concentration polarization. Finally, identification and control of the nanostructure of the supported oxy-nickel catalyst merits attention. Particularly desirable may be a layered doublehydroxide structure, potentially doped with iron, a known cocatalyst.42,51,52

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02166. BET surface areas of carbonized NU-1000 at different temperatures; summary of BET surface areas from nitrogen isotherms shown in Figure 1; differential pore volumes; SEM image of cNi-AIM-NU-1000_900 °C showing particles; XPS depth profiling of cNi-AIM-NU1000_900°C; TGA of cNi-AIM-NU-1000_900 °C; resistance of powder samples; Raman spectra; multiple CVs of cNi-AIM-NU-1000_900 °C drop-casted onto FTO; and Tafel plot for cNi-AIM-NU-1000_900 °C (PDF)

4. CONCLUSIONS Zr-based MOFs are, in principle, attractive scaffolds for heterogenizing electrocatalysts, and especially catalysts consisting of AIM-synthesized oxy-metal or sulfur-metal clusters. For the particular case of electrochemical conversion of water (hydroxide) to dioxygen, however, the available Zr-MOFs are plagued by: (a) insufficient electrical conductivity to permit more than a tiny fraction of installed catalysts to be electrochemically addressed and (b) chemical degradation when solution pH values exceed ca. 11. We find that hightemperature pyrolysis of oxy-Ni-AIM-functionalized NU-1000 effectively overcomes both problems, yielding supports that are stable in aq 1 M NaOH (pH ≅ 13.8) as well as electrically conductivethe latter behavior finding precedent in the recent literature on electrocatalysts for O2 reduction (albeit, typically in highly acidic solutions). As electrocatalysts, the carbonsupported nickel species are capable of delivering 0.5 mA/cm2 at an overpotential of ∼344 mV and 10 mA/cm2 at η = 550 mV. At 10 mA/cm2, the TOF per electroactive nickel ion is ∼0.5 O2 molecules per second. The overpotential at 10 mA/cm2 substantially exceeds the target value of η = 400 mV for direct, solar-driven photoelectrochemical water-splitting. The available results suggest multiple causes for excessive overpotential or, equivalently, smaller-than-desired current density at the benchmark overpotential of 400 mV. Most notably, the percentage of installed nickel that is electrocatalytically active is only about 2%a limitation that may be intertwined with the magnitude of the electrode surface area obtained here by MOF pyrolysis. Thus, pyrolyzed samples of catalyst-free NU1000 exhibit modest gravimetric surface areas (ca. 100 m2/ g)values that are unfavorably influenced by retention of high-mass, electroinactive zirconium oxide. A subsequent design ideally would omit electroinactive metals from the MOF composition. Installation of oxy-nickel species by AIM

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rebecca H. Palmer: 0000-0001-5054-7467 Chung-Wei Kung: 0000-0002-5739-1503 Jian Liu: 0000-0002-5024-1879 Omar K. Farha: 0000-0002-9904-9845 Joseph T. Hupp: 0000-0003-3982-9812 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported as part of the ArgonneNorthwestern Solar Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, via grant DESC0001059. We thank Dr. Timothy Wang for synthesizing the ligand for NU-1000. R.H.P. acknowledges an NSF fellowship (grant number: DGE-1324585). C.-W.K. acknowledges 14148

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Langmuir

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support from the Postdoctoral Research Abroad Program (105-2917-I-564-046) sponsored by the Ministry of Science and Technology (Taiwan). Metal analysis was performed at the Northwestern University Quantitative Bio-Element Imaging Center generously supported by NASA Ames Research Center NNA06CB93G. This work made use of the J. B. Cohen X-Ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205). This work made use of the EPIC and Keck-II facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.

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ABBREVIATIONS MOF, metal−organic framework; TOF, turnover frequency; FTO, fluorine-doped tin oxide; ALD, atomic layer deposition; CV, cyclic voltammetry; RHE, reversible hydrogen electrode

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DOI: 10.1021/acs.langmuir.8b02166 Langmuir 2018, 34, 14143−14150

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DOI: 10.1021/acs.langmuir.8b02166 Langmuir 2018, 34, 14143−14150