Pt Immobilization within a Tailored Porous-Organic Polymer

Oct 24, 2017 - Chemistry Department, Faculty of Science, Ain-Shams University, Abbasia, Cairo 11566, Egypt. § Laboratoire de Chimie des Processus Biol...
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Research Article Cite This: ACS Catal. 2017, 7, 7847-7854

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Pt Immobilization within a Tailored Porous-Organic Polymer− Graphene Composite: Opportunities in the Hydrogen Evolving Reaction Ahmed B. Soliman,†,‡,¶ Mohamed H. Hassan,† Tran Ngoc Huan,§ Arwa A. Abugable,†,∥ Worood A. Elmehalmey,† Stavros G. Karakalos,⊥ Manuel Tsotsalas,# Marita Heinle,# Mady Elbahri,¶ Marc Fontecave,§ and Mohamed H. Alkordi*,† †

Center for Materials Science, Zewail City of Science and Technology, Sheikh Zayed District, Giza 12588, Egypt Chemistry Department, Faculty of Science, Ain-Shams University, Abbasia, Cairo 11566, Egypt § Laboratoire de Chimie des Processus Biologiques, Collège de France, Université Pierre et Marie Curie, CNRS UMR 8229, 11 Place Marcelin Berthelot, 75005 Paris, France ∥ Center of Genomics, Helmy Institute, Zewail City of Science and Technology, Sheikh Zayed District, Giza 12588, Egypt ⊥ College of Engineering and Computing, Swearingen Engineering Center, University of South Carolina, Columbia, South Carolina 29208, United States # Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany ¶ Nanochemistry and Nanoengineering, School of Chemical Engineering, Department of Chemistry and Materials Science, Aalto University, Kemistintie 1, 00076 Aalto, Finland ‡

S Supporting Information *

ABSTRACT: A facile, postsynthetic treatment of a designed composite of pyrimidine-based porous-organic polymer and graphene (PyPOP@G) with ionic Pt, and the subsequent uniform electrodeposition of Pt metallic within the pores, led to the formation of a composite material (PyPOP-Pt@G). The pyrimidine porous-organic polymer (PyPOP) was selected because of the abundant Lewis-base binding sites within its backbone, to be combined with graphene to produce the PyPOP@G composite that was shown to uptake Pt ions simply upon brief incubation in H2PtCl6 solution in acetonitrile. The XPS analysis of PyPOP@G sample impregnated with Pt ions confirmed the presence of Pt(II/ IV) species and did not show any signs of metallic nanoparticles, as further confirmed by transmission electron microscopy. Immediately upon electrochemical reduction of the Pt(II/IV), metallic Pt (most likely atomistic Pt) was observed. This approach stands out, as compared to Pt monolayer deposition techniques atop metal foams, or a recently reported atomic layer deposition (ALD), as a way of depositing submonolayer coverage of precious catalysts within the 1−10 nm pores found in microporous solids. The prepared catalyst platform demonstrated large current density (100 mA/cm2) at 122 mV applied overpotential for the hydrogen evolution reaction (HER), with measured Faradaic efficiency of 97(±1)%. Its mass activity (1.13 A/mgPt) surpasses that of commercial Pt/C (∼0.38 A/mgPt) at the overpotential of 100 mV. High durability has been assessed by cyclic and linear sweep voltammetry, as well as controlled potential electrolysis techniques. The Tafel plot for the catalyst demonstrated a slope of ∼37 mV/decade, indicating a Heyrovsky-type rate-limiting step in the observed HER. KEYWORDS: hydrogen evolution reaction, porous-organic polymers, graphene, Pt immobilization, electrocatalysis

1. INTRODUCTION

(HER). Large efforts have been devoted to abundant metal catalysts, targeting commercializing the process.3−6 As Pt is by far the most efficient catalyst for the HER, and the most

In the quest for abundant and clean energy sources, electrocatalytic water splitting can potentially offer a great opportunity to store solar energy through conversion into hydrogen.1−3 An essential component to such actively investigated technology is the construction of novel materials acting as catalysts to facilitate the hydrogen evolution reaction © XXXX American Chemical Society

Received: July 8, 2017 Revised: September 26, 2017

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Scheme 1. Synthesis of the PyPOP-Pt@G. C (Orange, Teal in Graphene Sheet), N (blue), Br (Break Red), Cl (Yellow), Pt (Green), H (White)

Figure 1. (a)TEM image for the PyPOP-Pt@G, (b) SEM image, and EDX maps for Pt (c) and C (d) indicating homogeneous loading of Pt into the composite.

nanoparticles. The ionic Pt species impregnated within the composite can easily be reduced to their metallic state, affording the composite denoted here as (PyPOP-Pt@G), Scheme 1. This approach represents a step forward from the current state-of-the-art designs where Pt is commonly loaded as a metallic monolayer deposited atop 3D metal foam support. In our system, Pt loading within the micropores of the material was directed by ligand−metal ion coordination interactions, designed a priori and further successfully attained in a simple one-pot setup. Key to facilitating this approach was the utilization of chemical complementarities between the aromatic nitrogen atoms within the pores of the PyPOP and the ionic Pt species, all deposited on the conductive G sheets. A Pt-loaded microporous N-containing polymer was previously described as an efficient catalyst toward methane C−H activation under thermal conditions,31 highlighting the significance of our approach to expand the realms of tailored microporous solids into electrochemical processes.

expensive, wide scientific interest is directed toward novel approaches to construct Pt electrodes of high mass activity, to keep the cost to minimum through limiting the mass of Pt utilized. One of the most promising approaches is the deposition of a Pt monolayer on top of 3D metal foams,7 where the metal foam acted primarily as a support with moderate surface area. Key to attain high surface area in solids is to introduce the porosity at the molecular level,8,9 with microporous solids constructed through bottom up assembly from molecular precursors representing a rich arena for such investigations.9 In addition to providing solids with high surface area and tailorable pore size and chemistry,10 microporous solids provided platform materials for efficient heterogeneous catalysis applications.2,11,12However, as microporous solids commonly show poor electrical conductivity, several distinct approaches were made to introduce some degree of electrical conductivity to such solids3,13−16 to access applications in electrochemical processes,17−20 materials for supercapacitors,21,22 or an electrode material in batteries.15 An example of microporous solids with covalent backbone structure,23−25 high surface area, chemical stability, and tailorable pore chemistry is the family of porous-organic polymers (POPs).11 We previously reported on the combination of a designed pyrimidine-based POP (PyPOP) and graphene (G), as an efficient electron conductor,26−28 to construct the composite [email protected] This highlighted the potential for such composites to merge the realms of both solids with different characteristics targeting novel materials for specific applications.30 In this report, we outline a facile approach to construct an efficient heterogeneous, Pt-containing catalyst for HER. The PyPOP@G was straightforwardly impregnated with Pt through wetness incipient impregnation in an acetonitrile solution of H2PtCl6. The sample prepared through soaking in this solution was analyzed by X-ray photoelectron spectroscopy (XPS) and was shown to contain Pt(II/IV) species, with no signs of metallic Pt as bulk or

2. RESULTS AND DISCUSSION The synthesis of the PyPOP@G composite reported here was closely similar to the one we described earlier,29 with the exception of the relative amounts of G and the molecular building units for the polymer in order to attain a thinner coverage of the G sheets by the PyPOP. This design was employed to ensure closer interactions between the to-be-built catalytic Pt sites within the pores of the PyPOP and the G sheets. To confirm the construction of the targeted composite, Fourier-transform infrared spectroscopy (FT-IR) was conducted on the PyPOP@G, Supporting Information (SI, Figure S1), and showed similarity between the PyPOP and the composite. However, due to the relatively low coverage of the G by the PyPOP, strong interference from the G background hindered full detailed analysis of the IR spectrum of the composite. Despite this interference, several characteristic peaks 7848

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detected that could be assigned to Pt nanoparticles or clusters. This strongly supports that Pt is exclusively present as ionic/ atomic species homogeneously dispersed within the porous organic matrix. Utilization of cryogenic high-resolution TEM led to sample damage that was unavoidable within our setup due to the organic nature of the microporous PyPOP matrix, and thus, it was not useful in providing further atomistic resolution information about the PyPOP-Pt@G before or after electrolysis. Moreover, the X-ray powder diffraction pattern (XRD) recorded for the sample after electrolysis, Figure S2, did not show the characteristic peaks for Pt nanoparticles, further supporting the presence of atomistic Pt in the PyPOP-Pt@G. The elemental analyses conducted on the three solids (PyPOP, PyPOP@G, and PyPOP-Pt@G) indicate detectable variation to the CHN content as shown in Figure S3−S5. The CHN analysis conducted on graphene indicated a carbon content of 94.3 wt %. The observed elemental wt % for the PyPOP/PyPOP@G/PyPOP-Pt@G was the following: C (71.55/80.91/59.23%); H (2.66/1.82/1.74); N (21.14/3.3/ 2.57). The increase in C content and decrease in N and H content moving from PyPOP to the PyPOP@G is expected due to incorporation of the graphene in the composite. The noticeable decrease in the CHN content sum (from 95.35 wt % in the PyPOP to 63.55 wt % in the PyPOP-Pt@G) can be ascribed to formation of inorganic, nonvolatile products during the combustion of the Pt-loaded sample. The thermogravimetric analysis (TGA) conducted under N2 atmosphere (Figure S6) indicated an initial loss of 7 wt % below the temperature of 120 °C, potentially attributed to adsorbed moisture within the porous solid. The composite demonstrated a gradual weight loss of ∼10 wt % up to 200 °C, indicating thermal stability of the composite up to 200 °C. The residual of 40 wt % observed at 800 °C was further analyzed by SEM and EDX techniques, revealing the structures of graphene sheets and microstructured Pt, Figure S7. In order to determine the Pt loading into the PyPOP-Pt@G, inductively coupled plasma optical emission spectroscopy (ICP-OES) was attempted on a digested sample of the composite and indicated a Pt loading of 5.4 (±0.22) wt %. To gain more information about the elemental composition and chemical environment of the constituting elements of the composite, X-ray photoelectron spectroscopy (XPS, Figure 3) was recorded (see Figure S8 for survey spectrum) and indicated a near surface Pt loading of 7.8 wt % (0.6 atom %), and a corresponding Cl loading of 9.6 wt % (3.8 atom %). The measured Cl: Pt atom ratio with XPS was found to be 6 and can be ascribed to the presence of the potential Pt−Cl complexes in the Pt4+or Pt2+ oxidation states, with concomitant pyrimidinium chloride complexes, preserving the initial Cl:Pt atom ratio in the hexachloroplatinic acid used for Pt loading. In the XPS spectrum, the observed pattern for the Pt 4f peak consists of two components at high binding energy (BE) compared to metallic platinum expected at 71.2 eV.35 These two Pt 4f peak components indicated the presence of Pt in ionic state and excluded the presence of subnanometer particles, as previously reported.32,36−38 The spin orbit splitting for Pt 4f leads to the detection of the 4f5/2 (higher BE) and 4f7/2 (lower BE) contributions. Two types of Pt, potentially Pt4+ (74.3 eV BE for 4f7/2) and Pt2+ (72.4 eV BE for 4f7/2)39 can be detected. Similarly, the Cl 2p peaks could be deconvoluted into two major components with BE at 197.9 and 200.6 eV, respectively. The Cl species detected at low BE (197.9 eV) can be assigned to Cl counterions and/or those coordinated to

were commonly observed in both solids, including several peaks at the fingerprint region, the νC=N stretching at ∼1670 cm−1, internal ν CC stretching at 2190−2250 cm−1, and no terminal ν C−H stretching could be found at ∼3200 cm−1 for the starting triethynylbenzene. Loading of the composite with Pt was accomplished simply through incubation of a known dry weight of the composite in acetonitrile solution of H2PtCl6 for 80 min and at room temperature, Scheme 1. Subsequent thorough washings with acetonitrile were made to ensure that weakly adsorbed Pt species are removed from the sample prior to further characterization. The same treatment was applied to G as a control. The transmission electron microscopy (TEM) images for the PyPOP-Pt@G, Figure 1a, did not show any signs of metallic nanoparticles, indicating that the Pt loading within the composite was achieved on the atomic/ionic level.32 Indeed, this result is to be anticipated considering the design of the Pt loading into the material which avoided unfavorable formation of metallic clusters or nanoparticles. In a recently reported system where atomistic Pt was anchored over g-C3N4, the atomistic nature of Pt (loaded through wet chemistry approach similar to the one reported here) was evident through highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM).33 Presence of Pt and Cl was confirmed through EDX analysis within the TEM chamber. Detailed EDX analysis of the composite could not be extracted from this particular analysis due to the presence of Cu and C in the TEM sample grid. To probe the sample morphology and homogeneity and loading extent of Pt into the composite, scanning electron microscopy (SEM, Figure 1b) and energydispersive X-ray spectroscopy (EDX, Figure 1c,d) were utilized. The SEM image of the composite indicated homogeneous coating of the PyPOP on G, Figure 1b. Additionally, the EDX maps for Pt and C showed homogeneous distribution of Pt within the sample. In order to evaluate whether Pt nanoparticles were present or not within the polymer matrix, dark-field STEM imaging was acquired on two PyPOP-Pt@G samples, before and after electroreduction of Pt(II/IV). For that purpose the material was deposited on a FTO electrode and ten consecutive cyclic voltammetry cycles were carried out followed by controlled potential electrolysis for 300 s at 10 mA/cm2. In principle, unlike the bright-field imaging, contrast in dark-field imaging is based on a combination between crystalline and mass-deflected electrons. Such technique facilitates the detection of dense inorganic Pt nanoparticles, if present, dispersed within the organic matrix.34 The dark-field STEM micrographs are shown in Figure 2a and Figure 2b for the PyPOP-Pt@G before and after electrolysis, respectively. As shown, no bright spots were

Figure 2. Dark-field STEM images for (a) the PyPOP-Pt@G before and (b) after electrolysis. 7849

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Figure 3. XPS detailed spectra for Pt 4f, Cl 2p, C 1s, N 1s, and O 1s peaks in the PyPOP-Pt@G.

Figure 4. (a) N2 Gas sorption isotherms for the PyPOP, PyPOP@G, and PyPOP-Pt@G (closed symbols for adsorption and open symbols for desorption) and (b) PSD histograms.

Pt(IV) ions,39 while those with the high BE (200.6 eV) can be assigned Pt−Cl species attached to Pt(II) ions.39 The N 1s

peak can be deconvoluted into two components (398.9 and 400.5 eV) with the earlier ascribed to N atoms within the free 7850

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Figure 5. (a) First few CV scans for a fresh electrode of the PyPOP-Pt@G on GCE showing initial performance enhancement upon reduction of Pt(IV) in cycle progression, (b) the CVs for GCE casted with the PyPOP-Pt@G with scan rate 100 mV/s, (c) LSV for GCE casted with PyPOPPt@G with scan rate 10 mV/s with inset showing the Tafel plot and the linear fit. All measurements were conducted under N2 atmosphere in predegassed aqueous solutions of 1 M H2SO4, scan rate of 100 mV/s for the CVs and 10 mV/s for the LSVs.

corresponding pore size distribution (PSD), Figure 4b. The PSD histograms demonstrated closely similar distribution of pores within the PyPOP and PyPOP@G, supporting the above argument for major contribution of the organic polymer to the microporosity of the composite. However, for the PyPOP-Pt@ G, a noticeable change in PSD is observed for pores below 1.2 nm, suggesting that incorporation of Pt has occurred within such pores. The BET apparent surface area measured for PyPOP-Pt@G (122 m2/g) is exceptionally high as compared to some Ni foams with a reported surface area of 0.02 m2/g.45 Using the BET surface area for the Pt-loaded composite and assuming the density of graphene (1.91 g/cm3), the calculated surface area per cm3 was found to be 638 × 103 cm2/cm3. This calculated specific surface area is rather high even when compared to one of the highest observed for Ni metal foams (9 × 103 cm2/cm3).46 This observation indicates a dramatic enhancement of accessible surface area within our system as compared to other solid supports, specifically the Ni foams, targeting Pt immobilization. To characterize the electrochemical performances of the PyPOP-Pt@G with regard to HER, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques were utilized in N2-saturated aqueous solution of 1.0 M H2SO4. The working electrode was prepared by drop casting an ink of finely dispersed PyPOP-Pt@G on top of a glassy carbon electrode (GCE). For the controlled potential bulk electrolysis experiment, the electrode was prepared by depositing the ink of the PyPOP-Pt@G on a gas diffusion layer (GDL) electrode. Two control experiments were conducted: one using PyPOP@G and the other one using only graphene treated with H2PtCl6 in essentially the same way used to prepare the PyPOP-Pt@G. Both control samples did not show any appreciable catalytic activity toward HER, verifying that the observed HER activity was truly due to immobilized Pt within the PyPOP-Pt@G. With a freshly prepared PyPOP-Pt@G on a GCE, starting the CV measurement at 1.2 V and sweeping the potential into the cathodic direction, we noticed a nonreversible reduction wave that was ascribed to both Pt(II/IV)/ Pt(0) reduction and H2 evolution, Figure 5a. Upon progression of the cycles, the subsequent CVs demonstrated a catalytic wave (presumably HER activity, see below) with current activity larger than that observed in the first two cycles. This is consistent with metallic

pyrimidine rings, and the latter to those present in protonated or H-bonded rings, or coordinated to Pt ions.32,33,40,41 The deconvolution of the XPS C 1s peak indicated presence of peaks corresponding to the aromatic C−C (284.7 eV), defects in the G sheets42 or CN in pyrimidine (285.5 eV),43 C− O(286.5 eV), as well as a small component (288.5 eV) that can be assigned to C2 of the pyrimidine ring (N−CN)44 or to CO bonds, potentially arising from carbonyl and other C−O functionalities from the graphene sheets support.42 Deconvolution of the XPS O 1s peak confirmed the presence of some carboxylic and hydroxyl functions onto the surface CO (533.4 eV)42 and C−O (531.8 eV),42 respectively, and was found to be essentially similar to those observed for the nonmetalated PyPOP@G. The total oxygen amount on the sample was 2.8 atom %. The N2 sorption isotherms were measured for the PyPOP, PyPOP@G, and the PyPOP-Pt@G to assess their microporosity. The isotherms for the PyPOP and PyPOP@G demonstrated type-I like isotherms, with notable H2-type hysteresis and with calculated surface area (SA) using the Brunauer−Emmet−Teller (BET) model of 664 m2/g and 463 m2/g, respectively, Figure 4a. The BET SA for G was 445 m2/g, as measured previously. As the composite contained about 42 wt % of the PyPOP and 58 wt % G, calculated from the isolated dry weight of the composite (87 mg) and the initial weight of G used in the synthesis (50 mg), the observed SA of the composite reflected the mass contribution from each of its two components. In accordance, the shape of the isotherm for PyPOP@G resembled that of PyPOP in the early to midcoverage, followed by a noticeable increase in uptake near the saturation pressure, which can be explained assuming a major contribution from the PyPOP to the overall microporosity of the composite; however, the late part of the isotherm indicated interparticle condensation due to the platelike nature of the G support. The N2 isotherm measured for the composite after loading with Pt (PyPOP-Pt@G) is included in Figure 4a, with a calculated BET SA of 122 m2/g. This observation indicated the maintained microporosity even after loading with Pt, although with decreased SA as expected upon loading with the heavy metal. The nonlocal density function theory (NLDFT)-carbon finite pores model was applied to the early adsorption points in the three samples to calculate the 7851

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Figure 6. (Left) Controlled potential bulk electrolysis experiment performed in 0.5 M H2SO4 solution under N2 atmosphere at −0.3 V vs RHE. (Right) Plot demonstrating time-dependent accumulation of H2 produced in relation to charge passed through the electrode and the corresponding Faradaic efficiency.

plot was fitted to the Tafel equation using the electrochemical fitting functions of the EC-lab software and the extracted Tafel slope was ∼37 mV/decade. This observed value pointed toward a Heyrovsky-type rate-determining step for the HER.50,51 The above-mentioned proposed mechanism is unlike the most commonly observed Tafel-type rate-limiting step for HER on metallic Pt surfaces.52 The Heyrovsky-type rate-limiting step suggested here can be attributed to retardation of the hydrated proton diffusion within the microporous matrix to access the electroactive catalytic site(s). Central to the potentially operating rate-limiting step in our composite is the assumption of a single-metal-atom active site, see eqs 1−3 below.

Pt immobilized within the PyPOP-Pt@G acting as a proton reduction catalyst. The CVs for the PyPOP-Pt@G, Figure 5b, demonstrated appreciable low onset potential approaching the theoretical value for HER on Pt, and a high current density reaching up to 0.6A/cm2 at −0.45 V vs RHE. Using the current density values from the LSV scan, the calculated mass activity of the PyPOP-Pt@G in terms of A/mg Pt was found to be 0.373 A/mgPt at an applied overpotential of 50 mV and 1.13 A/mg Pt at an applied overpotential of 100 mV, considerably higher than those of commercial 20% Pt/C as reported earlier to be within the range of 0.27 A/mg Pt and 0.38 mA/mg Pt, respectively (see SI for more details). The enhanced mass activity, 1.4−3.6 times that of 20% Pt/C, is impressive and comparable to what was recently reported for a material obtained by atomic layer deposition (ALD) of atomistic Pt on graphene, where a sample prepared through 100 cycle ALD demonstrated an enhanced mass activity of ∼2.12 A/mgPt at an applied overpotential of 50 mV.47 Comparing the facile wet chemistry approach utilized here for the metalation of the PyPOP@G to the more expensive ALD technique reveals the advantage of our approach, not only in the cost of developing such highly efficient catalyst but also in terms of the scalability of our approach to potentially meet industrial scale production. The composite demonstrated high durability/stability under the conditions utilized, as was evident from overlaying the first CV scan (after electrode conditioning) and that after 1250 scans (Figure 5b), where no deterioration of the electrocatalytic performance was observed. A control experiment was also conducted to asses if the catalytic activity of the composite could be due to cross contamination by Pt(II) ions leached from the Pt counterelectrode, as was recently demonstrated to be a potential source of Pt(II) ions contamination in a similar setup.48 In the control experiment, the PyPOP@G was used as the electrode material and showed no catalytic response under the experiment conditions utilized, Figure S9. The composite demonstrated rather interesting catalytic proton reduction activity as indicated by its low onset potential approaching the theoretical limit of H+ reduction on Pt, as compared with recent reports for Pt-based systems either deposited on Ni foam (−40 mV vs RHE)7 or on carbon films.49 In order to gain more insight into the mechanism of proton electroreduction with our catalyst, the Tafel plot for the PyPOP-Pt@G was constructed, insert in Figure 5c. The Tafel

H3O+ + e− + M ↔ M−H + H 2O

Volmer step

M−H + H3O+ + e− ↔ H 2 + H 2O + M

(1)

Heyrovsky step (2)

2 M−H ↔ H 2 + 2 M

Tafel step

(3)

The above-mentioned proposed rate-limiting step is in agreement with our hypothesis of highly active and accessible atomistic Pt sites within the pores of the composite, facilitated and stabilized by the abundant N atom binding sites of the PyPOP. The presence of such Lewis-base binding sites as integral part of the composite backbone is thus a key factor in establishing the noticeable durability of the catalyst. Such functionality of the POP provided anchoring sites for Pt, preventing leaching or migration from the support and hence hindered their sintering and aggregation and leading to maintained activity. The presence of G in the composite was essential to access its electrochemical catalytic property, in contrast to other systems demonstrating efficient photochemical activity of a similar single-atom catalyst.33 To evaluate the Faradaic efficiency of the composite toward HER, a GDL electrode was utilized in controlled potential bulk electrolysis at a fixed potential of −0.3 V vs RHE, Figure 6. We observed an initial drop in current (from −40 mA·cm−2 to −25 mA·cm−2 within a few seconds), then followed by stable activity during a 1 h experiment. In order to calculate the Faradaic efficiency for the HER, the quantity of evolved hydrogen in the electrolysis solution was monitored through gas chromatography, and the calculated Faradaic efficiency was 97(±1)%, Figure 6. 7852

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3. CONCLUSIONS A facile approach to construct an efficient heterogeneous catalyst based on a ternary system of PyPOP/Pt/G is described. The system was designed such that the N-rich PyPOP will act as the microporous host for immobilizing ionic/atomic Pt species, while G providing desirable electrical properties to the composite. It is thus through synergy between the three distinct but complementary components of the composite that the targeted enhancement of the Pt mass activity, and hence projected favorable impact on catalyst price, was attained. This approach opens more doors for accessing heterogeneous singlesite catalysts in electrochemical processes, with the ability to fine-tune, and further design, the catalyst−host−support interactions.

Aldrich). After sonication, the suspension was drop casted on a 1 cm2 carbon paper electrode (a carbon-based gas diffusion layer (GDL)) and dried in air for 1 h. Long-Term Electrolysis. A long-term electrolysis (1 h) was performed at −0.3 V vs RHE in 0.5 M H2SO4 saturated-N2 solution to investigate the stability of the catalyst. The production of H2 was determined throughout the electrolysis by GC chromatography, and the result was used to evaluate the Faradaic yield. Sample Electrolysis for STEM and XRD Measurements. An ink of the PyPOP-Pt@G was deposited on FTO slide, with an electrode surface area of 3 cm2. It was allowed to dry and then utilized as the working electrode in a solution of 1 M H2SO4. Next, 10 CV cycles were recorded from 0 to −0.7 V vs SSCE at a scan rate of 100 mV/s. CP electrolysis was conducted for 300 s at −10 mA/cm2. XPS Measurements. X-ray photoelectron spectroscopy measurements were performed using a Kratos AXIS Ultra DLD XPS system with a monochromatic Al Kα source operated at 15 keV and 150W and a hemispherical energy analyzer. The X-rays were incident at an angle of 45° with respect to the surface normal. Samples were placed in small powder pockets on the holder, and analysis was performed at a pressure below 1 × 10−9 mbar. High-resolution core-level spectra were measured with a pass energy of 40 eV. The XPS experiments were performed by using an electron beam, directed on the sample, for charge neutralization.

4. EXPERIMENTAL SECTION Synthesis of PyPOP@G. A slightly modified procedure to that we published earlier was utilized to introduce thinner coating of the PyPOP on G sheets.29 Briefly, to a suspension of G (50 mg) in DMF/Et3N mixture (15 mL/5 mL) was added 4,6-dibromopyrimidine (34 mg, 0.14 mmol), 1,3,5-triethynylbenzene (26 mg, 0.17 mmol), and the tube content was sonicated for 10 min. The solution was then degassed and maintained under N2, followed by addition of CuI (5 mg, 0.026 mmol), PPh3 (5 mg, 0.019 mmol), and PdCl2(PPh3)2 (5 mg, 0.014 mmol), and the tube was sealed and heated at 80 °C for 24 h. The solid precipitate was then filtered, washed, then guest exchanged in heated acetonitrile before drying (dry weight 72 mg). Pt Loading. A solution of H2PtCl6 (16 mg) in 10 mL of acetonitrile was prepared, to which was added 20 mg of the dry PyPOP@G and the solid left for Pt impregnation at room temperature for 80 min, followed by filtration and thorough washing with acetonitrile and drying at 80 °C oven for 1 h. Graphene Control Sample. A graphene control sample was fabricated by incubating G into acetonitrile solution of H2PtCl6 in a similar way as described earlier for the metalation of PyPOP@G, followed by the same washing procedure. Electrochemical Measurements. Electrochemical measurements were conducted in a three-electrode setup using Pt wire as the counter electrode, glassy carbon as working electrodes, and SSCE as the reference electrode on a Biologic SP-50 potentiostat/galvanostat. Degassing of the solution was achieved through bubbling high purity nitrogen (AlphaGaz 2, 99.999%) for 30 min prior to analysis, with maintained nitrogen flow above the solution during the measurements. All measurements reported were not compensated for iR drop. All measurements were recorded against SSCE and converted to RHE according to the relation (E(RHE) = 0.059*pH + 0.236 for SSCE + EHg|Hg2Cl2)53 Preparation of the GCE Electrode. PyPOP-Pt@G (2.25 mg) was dispersed in a solution of 1 mL of isopropanol and 40 μL of a Nafion perfluorinated resin solution (5 wt % in mixture of lower aliphatic alcohols and water, containing 5% water, Sigma-Aldrich). After sonication, a 40 μL portion of the suspension was drop casted on a 0.07 cm2GCE electrode (ALS co., Ltd.) and let to dry in air. Preparation of the Carbon-Based Gas Diffusion Layer (GDL) Electrode. Initially, 1.5 mg of the material was dispersed in a solution of 200 μL of ethanol and 10 μL of a Nafion perfluorinated resin solution (5 wt % in mixture of lower aliphatic alcohols and water, containing 5% water, Sigma-

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02246. Detailed experimental procedure and further characterizations including FTIR, SEM, and TGA (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mails for M.H.A.: [email protected], Malkordi@ mail.usf.edu. ORCID

Arwa A. Abugable: 0000-0003-1275-3328 Stavros G. Karakalos: 0000-0002-3428-5433 Manuel Tsotsalas: 0000-0002-9557-2903 Mohamed H. Alkordi: 0000-0003-1807-748X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the funds from Zewail City of Science and Technology, Center for Materials Science, and Egypt Science and Technology Development Fund (STDF, USC17-43).

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REFERENCES

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DOI: 10.1021/acscatal.7b02246 ACS Catal. 2017, 7, 7847−7854

Research Article

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DOI: 10.1021/acscatal.7b02246 ACS Catal. 2017, 7, 7847−7854