Nanostructured RheniumâCarbon Composites as Hydrogen-Evolving

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Nanostructured rhenium-carbon composites as hydrogenevolving catalysts effective over the entire pH range Minju Kim, Zhijie Yang, Jun Heuk Park, Seok Min Yoon, and Bartosz A. Grzybowski ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00236 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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ACS Applied Nano Materials

Nanostructured

rhenium-carbon

composites

as

hydrogen-evolving catalysts effective over the entire pH range Minju Kim,1,2 Zhijie Yang,1 Jun Heuk Park,1,2 Seok Min Yoon,*,1 and Bartosz A. Grzybowski*,1,2 1Center

for Soft and Living Matter, Institute for Basic Science (IBS), Ulsan 44919, Republic of

Korea 2Department

of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan

44919, Republic of Korea

KEYWORDS: rhenium, nanoparticle clusters, electrocatalysis, hydrogen evolution reaction

ABSTRACT: In addition to platinum (Pt), many metals (e.g., Ru, Co, Mo, Ni, W, and Fe) and their complexes have been explored as catalysts for hydrogen evolution reaction (HER). Although the position of relatively inexpensive rhenium (Re) on the so-called Sabatier plot suggests its usefulness in HER, the performance of Re catalysts have, so far, been quite disappointing. The present work describes synthesis and characterization of clusters comprised of rhenium nanoparticles held together by an amorphous-carbon phase. Unlike standard HER catalysts, these Re/C clusters are characterized by small overpotantials and Tafel slopes not only under acidic but

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also neutral and basic conditions. In addition, they exhibit excellent durability over the entire pH range.

INTRODUCTION Hydrogen evolution reaction (HER) enables electrochemical water splitting and has been the focus of much recent research on sustainable and clean energy sources.[1,2] Owing to minimal overpotential and fast kinetics,[3,4] Pt-based catalysts have been most popular in HER, but they are expensive. Accordingly, other metals (e.g., Ru, Co, Mo, Ni, W, and Fe) and their chalcogenides, phosphides, and carbides have been explored as alternatives.[5-15] Ominously, this list does not include Re, although the so-called Sabatier plot (a.k.a. volcano plot) suggests its usefulness as an efficient HER catalyst. Rhenium exhibits optimal binding energy for adsorption and desorption of protons as well as exchange current density for HER that is comparable to Pt[16-19]; it is also about an order of magnitude less expensive than platinum.[20] On the other hand, the bulk-state metallic Re HER catalysts used to date have required high overpotentials[21-26] (> 200–300 mV at 10 mA cm−2). In addition, few reports investigated HER performance in non-acidic media. Here, we describe efficient HER catalysis on nanoparticle clusters in which Re is combined with electrically conductive carbon materials such as amorphous carbon (a-C) or multi-walled carbon nanotubes (MWNTs). These nanocomposites are not only characterized by low overpotentials and fast kinetics but also remain efficient in both acidic, buffered neutral, and basic conditions. Our results suggest that Re-based nanomaterials can become less expensive and sturdy (in terms of admissible pH range) alternatives to the currently used HER catalysts.


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EXPERIMENTAL Materials: Rhenium (VII) oxide (99.995%, metal basis) and rhenium powder (99.999%) were purchased from Alfa Aesar. Tetrahydrofuran (99.5%) was purchased from Samchun Chemical Co., Ltd. Sulfuric acid solution, potassium hydroxide, potassium phosphate monobasic, potassium phosphate dibasic, potassium phosphate tribasic and 5 wt% Nafion solution were purchased from Sigma-Aldrich. All aqueous electrolyte solutions were prepared with deionized (DI) water purified by Milli-Q purification system. All chemicals were used as received without further purification.

Synthesis of nanoparticle clusters held by amorphous carbon (NPC/a-C): ReO2-NPC/a-C were synthesized in two steps; 1) gelation of Re2O7 with tetrahydrofuran (THF), and 2) solovothermal processing of the formed gel. 0.8 mmol of Re2O7 and 2 mL of THF was placed into a 20 mL glass vial. The mixture was gently shaken to dissolve all Re2O7 precursor in THF and was left sealed for 24 hrs to gelate. Then, 1 mg of the gel was dissolved in 4 mL of THF in a 40 mL teflon autoclave container. The sample was heated at 200 °C for 2.5 hrs in a furnace and then left to cool down to room temperature. The products were collected by centrifugation and washed with pure THF five times. The precipitates were dried in the vacuum. To synthesize Re-NPC/a-C, annealing process was performed in a tube furnace. 10 mg of ReO2-NPC/a-C on a 5 mL ceramic combustion boat was placed into a quartz tube. The tube was purged with Ar for 20 min, heated at 500 °C (with temperature ramped up at the rate of 5 °C min−1) for 4 hrs, and then let to cool down to room temperature. All the annealing procedures were conducted under Ar atmosphere at 200 sccm gas flow rate.


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Characterization: Molecular structure of Re2O7-poly(THF) composites and pure THF were analyzed by nuclear magnetic resonance spectroscopy (400M MHz FT-NMR, Bruker AVANCE III HD) and infrared spectroscopy (FT-IR, Varian 670/620) with ATR mode. The morphology and structure of the product samples were imaged by scanning electron microscopy (SEM, Hitachi SU8220) and transmission electron microscopy (TEM, JEOL JEM-2100). For further analysis, highresolution TEM (HRTEM) images, high-angle annular dark-field (HAADF) image and energy dispersive X-ray (EDX) mapping were taken on a JEOL JEM-2100F microscope. X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX2500V/PC High Power XRD instrument with Cu Kα radiation. Scan rate, accelerating voltage and applied potential were 2 deg min−1, 40 kV and 200 mA, respectively. Raman spectra were obtained on a WITec alpha300R confocal Raman spectrometer with 532 nm excitation wavelength. Oxidation states in the NPCs were investigated by X-ray photoelectron spectroscopy (XPS; ESCALAB 250XI, Thermo Fisher Scientific) and the deconvolution analysis with associated database was performed using XPSPEAK program and Thermo Scientific Avantage software. BET surface areas of ReO2-NPC/a-C and Re-NPC/a-C were measured by a volumetric adsorption system (ASAP2420, Micromeritics Instruments) with N2 adsorptive at 77 K. Prior to such analyses, all samples were heated at 90 °C for 10 hrs under vacuum to remove any adsorbed moisture.

Electrochemical measurements: Hydrogen evolution reaction (HER) - Electrochemical HER measurements were carried out on an electrochemical workstation (PARSTAT MC multichannel potentiostat) using standard three-electrode setup. Ag/AgCl (in 3M NaCl solution) electrode and glassy carbon (GC) rotating disk electrode (RDE) were used as a reference electrode and working electrode, respectively. Graphite rod was used as a counter electrode instead of Pt-based electrode


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to eliminate any possibility of dissolution of or contamination by Pt. Prior to use, the glassy carbon RDE was polished using a 1.0 μm alumina suspension, followed by a 0.3 μm suspension. To prepare working electrode, catalyst ink was prepared. 4 mg of the catalyst powder was dispersed in 1mL of 3:1 v/v water/ethanol mixed solvent with 40 μL Nafion solution (Nafion® 117 solution, Sigma-Aldrich) and was sonicated for 30 min to produce homogeneous slurry. Then, 5μL of the ink was applied onto the glassy carbon electrode (3 mm diameter, 0.0707 cm2 geometrical area) and dried in air at room temperature. The resulting catalyst loading was 0.283 mg cm−2geometric. In case of nanocomposite mixture of Re-NPC/a-C and MWNT (Re-NPC/a-C/MWNT), 4 mg of ReNPC/a-C and MWNT with 5:1 w/w proportion was mixed and sonicated together in the same ink solution (1mL of 3:1 v/v water/ethanol with 40 μL Nafion solution). Before electrochemical measurements, all electrolytes were bubbled with N2 gas (99.999 %) for 30 min, then cyclic voltammetry (CV) in the range from 0.1 V to -1.0 V (vs. RHE) was conducted for electrochemical cleaning (20 repetitions, scan rate of 100 mV s−1). HER polarization curves were recorded in N2saturated 0.5 M H2SO4, 1.0 M phosphate buffer solution at pH 7 (PBS), or 1 M KOH electrolytes at scan rates of 2 mV/s and on a 1600 rpm rotating disk electrode (RDE). For stability tests, CV was conducted at a scan rate of 100 mV/s within potential range from 0.1 V to − 0.3 V (vs. RHE) in acidic, neutral and basic conditions. Electrochemical impedance spectroscopy (EIS) analyses were performed at overpotential of 173 mV (vs. RHE) from 100 KHz to 0.1 Hz in the same configuration. The LSV polarization curves were plotted after compensating for Ohmic drops with the series resistance obtained from the EIS measurements. All potential values which are reported in this work were converted to the reversible hydrogen electrode (RHE) from Ag/AgCl reference electrode (3M NaCl) by the equation of ERHE = EAg/AgCl + 0.0592×pH + 0.209 V. Additionally, generation of hydrogen was confirmed and quantified by gas chromatography (YL6500 GC, YL


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Instruments Co., Ltd.). In these GC experiments, the catalyst’s sample was loaded on 1×2 cm2 carbon clothes (ELAT Hydrophilic, FuelCellStore) and chronopotentiometric electrocatalysis at 10 mA/cm2 was performed in 0.5 M H2SO4 for 60 min. The amounts of evolved hydrogen and calculated Faradaic efficiencies are plotted as a function of time in Figure S23.

Electrochemically active surface area (ECSA): Values of electrochemically active surface area (ECSA) were determined from electrochemical capacitance measurements. To first determine double layer capacitance (Cdl) values, CVs were collected at 10 different scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mV s−1) in the potential range from 0.1 to 0.2 V vs. RHE without any Faradic current features. All current was assumed to be due to double layer capacitive charging. The charging current density differences at 0.15 V vs. RHE (ΔJ0.15V = ja − jc) were plotted as a function of scan rate, and slopes were then calculated from linear fits. According to the equation of “ic = vCdl”,[27-30] the difference in the linear slopes is equivalent to twice of the double layer capacitance (Cdl) and so it was divided by 2 to obtain average Cdl value from cathodic and anodic charging currents. The values of double layer capacitance thus obtained were converted into an electrochemical active surface area (ECSA) with the equation of “ECSA = Cdl/Cs”,[31-36] using the specific capacitance value for a flat standard with 1 cm2 of real surface area. Here, we took widely used specific capacitance values of 0.035 mF cm−2 and 0.040 mF cm−2 in, respectively, acidic and basic conditions based on reported typical values from metal electrodes in aqueous H2SO4 (for various metal electrodes, reported values range between Cs = 0.015 - 0.110 mF cm-2) and NaOH solutions (Cs = 0.022 - 0.130 mF cm-2).[34-36] The value of 0.040 mF cm−2 was chosen for neutral condition.


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RESULTS AND DISCUSSION To synthesize Re/C nanocomposites (Scheme 1), Re2O7 powder (99.995%, Alfa Aesar) was first dissolved in tetrahydrofuran, THF, and the 0.4 M solution was kept at ambient conditions for 24 hrs. This transparent solution gradually changed into a dark, viscous “gel” (for macroscopic image, see Figure S1), whose formation can be attributed to ring opening polymerization (ROP) of the THF molecules initiated by concentrated Re2O7 acting as a Lewis-acid.[37,38] As illustrated by the TEM images in Figure S2, the initially dispersed Re2O7 nanocrystals gradually aggregated by polymerizing THF and ultimately formed networks of nanoparticle clusters few-hundred-nm in size. Crosslinking of Re2O7 by the possibly “opened” THF molecules is evidenced (i) by 1H–NMR proton signals from the polymerized THF at 3.44 ppm and 1.64 ppm[39] (vs. 3.77 ppm and 1.88 ppm in THF monomers; see Figure S3a) and (ii) in FT–IR, by the appearance of a strong C-O stretch[40] at 1108 cm−1 which likely comes from aliphatic ether (vs. 1068 cm−1 ring stretch of THF monomer; see Figure S3b).

Scheme 1. Scheme illustrating synthesis of Re-based nanomaterials starting from the Re2O7poly(THF) composite and yielding catalytic ReO2-NPC/a-C and Re-NPC/a-C berry-shaped clusters. This precursor material was diluted in excess THF (1 g of gel per 4 mL of THF) and was then subjected to a solvothermal treatment at 200 °C for 2.5 hrs followed by annealing at 500 °C


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under Ar for 4 hrs (Experimental section, Figure S4a). As illustrated in Figure 1a-c, the first step produced ~ 200–500 nm berry-shaped clusters made of ~ 5 nm nanoparticles. The powder X-ray diffraction, PXRD, pattern of these nanoparticle clusters (black line in Figure 2a; for control experiments at different heating temperatures, see Figure S4b) indicates that the particles have the ReO2 crystal structure with monoclinic unit cell and P21/c space group (a = 5.615 Å, b = 4.782 Å, c = 5.574 Å, β = 120.13o).[41] High resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements (e.g., Figure 1c) evidence that each nanoparticle is a monocrystal with the spacing between the (011) planes equal to 3.39 Å and very close to the d–value of the (011) peak observed in the PXRD pattern of ReO2 nanoparticle clusters. Furthermore, X-ray photoelectron spectroscopy, XPS, in Figure 2b reveals that the oxidation state within these clusters is mainly Re (IV) (67.12 at.% vs. 32.88 at.% Re (VI)). The XPS peaks at 42.38 eV and 44.83 eV (blue curve) correspond to ReO2 surface oxide, whereas peaks at 43.23 eV and 45.88 eV (purple curve) correspond to ReO2 bulk phase[42] (Re (VI) corresponds to peaks at 45.08 eV and 47.68 eV). Finally, Raman spectrum of the clusters in Figure 2d features D- (1384 cm−1) and G-bands (1585 cm−1) that can be assigned to amorphous carbon (a-C),[43] which is congruent with the absence of a π-π stacking peak around ~25o in PXRD or any noticeable 1-D or 2-D carbon structures in TEM. This a-C phase likely forms by carbonization of the THF crosslinkers during the solvothermal reaction. The presence of carbon within ReO2 nanoparticle clusters is also evidenced by electron dispersive spectroscopy (EDS) mapping image in Figure S5.


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Figure 1. (a) SEM, (b) low-magnification, and (c) high-magnification HR-TEM images of the ReO2 nanoparticle clusters, ReO2-NPC, formed after the first part of the synthetic procedure described in the main text and held together by the a-C phase. (d) SEM, (e) low-magnification, and (f) high-magnification HR-TEM images of the Re-NPC/a-C formed after the second part. (c) and (f) are magnified HR-TEM images corresponding to regions in red rectangles in, respectively, (b) and (e). The inset images in (c) and (f) are FFTs of selected area electron diffraction patterns (SAED).


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Upon further annealing under Ar, the morphology of the clusters remains intact (Figure 1d) but the crystal structure of the constituent nanoparticles changes from ReO2 to metallic Re with P63/mmc space group[44] (red line of PXRD spectra in Figure 2a, a = b = 2.76 Å, c = 4.46 Å, α = β = 90o, γ = 120o; for control experiments at different annealing temperatures see Figure S4c). The SAED pattern on (010) zone plane (Figure 1f) is congruent with the hcp lattice and the 2.13 Å spacing between the (101) planes matches the d-value of (101) in PXRD. In the XPS spectra in Figure 2c, the dominant peaks at 40.98 eV and 43.48 eV correspond to the fully-reduced Re(0) peaks (79.24 at%), whereas less pronounced peaks at 42.28 eV and 44.83 eV correspond to surface oxides (for survey scans and oxygen bands see Figure S6). In addition, Raman scattering in the ranges from 302.85 cm−1 to 400.84 cm−1, and from 870.45 cm−1 to 1005.57 cm−1 was observed which indicates the presence of Re contents on the surface (cf. Raman spectrum of a commercial, 99.999% Re powder in Figure S7; for SEM image, see Figure S8). The spectrum also features the D- and G-bands of amorphous carbon. The presence of both the clustered Re nanoparticles and a-C throughout the aggregates is directly visualized by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and EDS mapping (Figure 2e-g; for EDS mapping of ReO2 clusters, see Figure S5). Based on Brunauer–Emmett–Teller (BET) method, the thus prepared Re-NPC/a-C nanocomposite has the surface area of 33.1067 m2 g−1 (Figure S9).


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Figure 2. (a) PXRD spectra of nanocomposites comprised of ReO2 (black curve) and Re (red curve) nanoparticle clusters, NPC, held within a-C phase. (b) and (c) are Re 4f XPS spectra of the ReO2-NPC/a-C and the Re-NPC/a-C nanocomposites, respectively. (d) Raman spectra of the Re2O7-THF “gel” precursor (black), the ReO2-NPC/a-C (green), and the Re-NPC/a-C (blue). (e) HAADF-STEM image of a Re-NPC/a-C (scale bar = 20 nm) along with corresponding EDS mapping images for (f) Re and (g) C.

To study electrocatalytic HER activity of either ReO2-NPC/a-C or Re-NPC/a-C nanocomposites, polarization curves were obtained by linear scan voltammetry (LSV, scan rate of 2 mV s−1) using NPC ink loaded on a rotating disk electrode (RDE) serving as a working electrode (mass loading: 0.283 mg cm−2geometric). The ink was prepared by adding 4 mg of the


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nanocomposite’s powder and 40 μL of Nafion solution into 1 mL of a 3:1 v/v water-ethanol mixture (see Experimental section). The HER activities of the ReO2-NPC/a-C or Re-NPC/a-C nanocomposites were measured in acidic (pH = 0, 0.5 M H2SO4), alkaline (pH = 14, 1 M KOH), and neutral (pH = 7, 1 M PBS) solutions with iR correction. In the acidic electrolyte, ReO2-NPC/a-C exhibits moderate HER activity with onset overpotential η = 218 mV and overpotential at 10 mA cm−2 η10 = 243 mV vs. reversible hydrogen electrode (RHE) (green line in Figure 3a). The HER activity becomes worse in the alkaline solution (overpotential increased to η10 = 330 mV; green line in Figure 3c) or in the neutral solution (η10 = 273 mV, green line in Figure 3e). In contrast to these oxide-base materials, metallic Re-NPC/a-C under acidic conditions shows much lower values of η = 102.7 mV and η10 = 133 mV (purple line in Figure 3a), evidencing their enhanced activity in HER. Remarkably, the performance in an alkaline medium is even better (η = 91 mV and η10 = 122 mV; purple line in Figure 3c) and remains satisfactory in a neutral solution (η = 114 mV and η10 = 164 mV; purple line in Figure 3e). We note that high HER activity of metallic Re-NPC/a-C in acidic medium can be explained on the basis of optimal hydrogen adsorption energy at active sites, which has been confirmed by numerous theoretical studies on this topic.[16-19,45-47] On the other hand, the origin of high activity observed in alkaline or neutral conditions can be attributed to the ability of the nanoclusters to adsorb and dissociate water molecules, with moderate affinity towards hydrogen.[5,48-53] The excellent ability of rhenium to adsorb and dissociate water molecules, the key factors of HER electrocatalysis in basic or neutral electrolytes, is supported by electron spectroscopic studies which evidence facile dissociative adsorption of water on rhenium’s surface.[54,55] Regarding the performance in KOH or H2SO4 being (slightly) better than in neutral PBS, this difference can be expected as extreme pH conditions are known to maximize water


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electrolysis efficiency; this is why most studies in this field have been performed in highly acidic and alkaline environments.[56]

Figure 3. Polarization curves of bulk Re powder (99.999 %, Alfa Aesar), ReO2-NPC/a-C, ReNPC/a-C, and Pt/C in (a) 0.5 M H2SO4 aqueous solution and (b) corresponding Tafel plot. (c) Polarization curves in 1 M KOH aqueous solution and (d) corresponding Tafel plot. (e)


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Polarization curves in 1 M PBS aqueous solution and (f) corresponding Tafel plot. (g) Electrochemical impedance spectra of different electrodes at overpotential 0.17 V versus RHE in 0.5 M H2SO4 and with alternating current (AC) amplitude of 5 mV. Inset: Equivalent circuit used for data analysis. Rs: solution resistance, Rct: charge transfer resistance, CPE: constant-phase element, Zw: Warburg impedance. Figure S13 (Supporting Information) shows full impedance spectra for bulk Re powder and ReO2-NPC/a-C in 0.5 M H2SO4. (h) Capacitive current measured by CV at 0.15 V vs. RHE (ΔJ0.15V), plotted as a function of scan rate in 0.5 M H2SO4.

Next, we obtained the Tafel plots from the LSV polarization curves (Figure 3b,d,f). In control experiments, the Tafel slope recorded for Pt/C is 29.7 mV dec−1 in 0.5 M H2SO4, 46.3 mV dec−1 in 1 M KOH and 47.9 mV dec−1 in 1 M PBS – that is, in agreement with reported values and implying that H2 is evolved at the Pt/C electrode according to the Volmer-Tafel reaction mechanism (Volmer step: H+(aq) + e− → Had in acid, H2O(l) + e− → Had + OH−(aq) in base; Tafel step: 2Had → H2(g); Heyrovsky step: H+(aq) + Had + e− → H2(g) in acid, H2O(l) + Had + e− → H2(g) + OH−(aq) in base). The Tafel slopes for the Re-NPC/a-C are 56.3 mV dec−1 in 0.5 M H2SO4, 53.8 mV dec−1 in 1 M KOH and 69.6 mV dec−1 in 1 M PBS, indicating fast kinetics of electrochemical hydrogen adsorption and desorption processes in acidic, and water adsorption and dissociation processes in alkaline and neutral media. HER kinetic models suggest that Tafel slope of about 30, 40 or 120 mV dec−1 will be obtained when the Tafel, Heyrovsky or Volmer reactions are the rate-determining steps (rds), respectively.[33,57] The fact that the Tafel slopes of the catalyst are close to the theoretical value of 40 mV dec−1 and above 38 mV dec−1 implies that Heyrovsky-step-determined (electrochemical desorption of Had being the rate limiting step) Volmer-Heyrovsky reaction mechanism may be dominant at the Re-NPC/a-C electrode.[3,4] In the Volmer (discharge) reaction,


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the fact that the Tafel slopes are ~ 60 mV dec−1 indicates that the adsorption of hydrogen at an electrode follows the Temkin type adsorption isotherm.[58-61] Additionally, the values of 53.8 mV dec−1 and 69.6 mV dec−1 in basic and neutral media indicate facile dissociation of water in the Volmer step, the initial stage of HER.[62] Overall, the observed overpotential and Tafel-slope parameters are, expectedly, worse than for Pt (η = ~ 0 mV and η10 = 30 mV versus RHE) and recently reported Ru-based catalysts[5] but are significantly better than for other known Re-based HER catalysts[21-26] (see Table S1). In addition, although catalytic HER activities in non-acidic media are usually about 2–3 orders of magnitude lower than in acidic ones with significantly increased overpotential,[63-66] the unique feature of the Re-NPC/a-C nanocomposite is that it performs comparably well over the entire pH range, from acidic, through neural, to basic. Of note, Re-NPC/a-C performs significantly better than bulk Re powder – orange curves in Figure 3a,c,e, evidence this improved performance for mass loading of 0.283 mg cm−2geometric whereas comparisons against different mass loadings of Re powder are included in Figure S10 of the Supplementary Information. Such results indicate that combining nanosized Re particles with conductive a-C not only exposes abundant catalytic sites, but also improves electrical conductivity.[67] Following this logic and to further enhance electrical conductivity of the catalysts, the clustered Re nanoparticles were interconnected with multi-walled carbon nanotubes (MWNT; electrical resistivity ~ 3.00×10−5 Ω·cm[68,69]). The samples were prepared by mixing of Re-NPC/aC and MWNT (see Experimental section). The SEM images in Figure S11 show that in the materials thus made, the Re nanoclusters are enmeshed in a web of MWNTs. Importantly, the HER activity of these Re-NPC/a-C/MWNT constructs is further improved in both acidic (η = 85 mV, η10 = 107 mV, Tafel slope = 43.5 mV dec−1) and alkaline (η = 80 mV, η10 = 107 mV, Tafel slope


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= 50.7 mV dec−1) solutions, and roughly unchanged in a neutral medium (η = 115 mV, η10 = 163 mV, Tafel slope = 69.2 mV dec−1) (Figure S12a,b,c). The faster kinetics of electrocatalysis is expected to correspond to lower charge-transfer resistance (Rct). To study charge-transfer effects, we conducted a series of electrochemical impedance spectroscopy (EIS) studies at frequencies varying from 0.1 Hz to 100,000 Hz. The ReNPC/a-C catalyst shows Rct value of 19.8 Ω in acidic solution (Figure 3g) and exhibits much lower value of Rct than ReO2-NPC/a-C (530.6 Ω) or bulk Re powder (1042.3 Ω) (Figure S13). Similar trend is observed in basic and neutral electrolytes (Figure S14). Comparison between bulk Re powder and Re-NPC/a-C suggests that the electrically conductive a-C phase (resistivity 5.0– 8.0×10−4 Ω·m[70,71]) not only supports the Re NPs within the high-surface-area berry-shaped clusters but also facilitates charge transfer at the nanocomposite’s surface.[67,72] We observe that under all conditions, the value of Rct for Re-NPC/a-C/MWNT is lower Rct than for Re-NPC/a-C, which is in line with the expected role of MWNTs in enhancing charge transfer at the material’s surface (Figure S12d,e,f, Supporting Information). Although geometric current density is a well-known and accepted measure of total catalytic activity from the catalyst electrode, we also analyzed current density normalized by electrochemically active surface area (ECSA) to study the intrinsic activity of the samples. The ECSA values are determined as Equation (1)[31-36]: ECSA = Cdl/Cs

(1)

where Cdl and Cs are the double layer capacitance and the specific capacitance (for description of measurements, see Experimental section; for experimental data, Figures 3h, S15, S16 and Table S2). In our case, it must be remembered that amorphous carbon present in the nanocomposites also contributes to the double layer capacitance (Cdl). Therefore, rather than directly comparing ECSA


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or Cdl values of Re-NPC/a-C vs. bulk Re powder, we analyzed the intrinsic activity by dividing geometrical current densities by their respective ECSA. Figure S17 shows the HER polarization curves normalized with respect to ECSA (for Re-NPC/a-C/MWNT composite, see Figures S18, S19). In acidic media, the catalytic currents per ESCA for Re-NPC/a-C are much larger than for the bulk Re powder (Figure S17a). The enhanced catalytic activity for Re-NPC/a-C is also verified by lower Tafel slope exhibiting faster kinetics (Figure S17d). Enhanced intrinsic catalytic activity is also observed in basic and neutral media (Figure S17b,c,e,f), though it is not as pronounced as in the acidic media.

Figure 4. Polarization curves of Re-NPC/a-C after preparation (solid lines) and after 1000 CV cycles (dotted lines) in (a) 0.5 M H2SO4, (b) 1 M KOH, and (c) 1 M PBS.

Finally, we tested the durability of the catalysts by comparing initial polarization curves with the curves recorded after 1000 CV cycles in the potential range from 0.1 V to − 0.3 V vs. RHE. As shown in Figure 4, the Re-NPC/a-C catalyst exhibited only negligible current loss under all pH conditions (in fact, the HER performance is even slightly enhanced after stability tests under acidic conditions; see Figure 4a), thus confirming their robustness and usefulness for electrocatalytic HER (for Re-NPC/a-C/MWNT composite, see Figure S20). We also verified that


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neither the morphology (imaged by SEM; Figure S21, Supporting Information) nor surface compositions (quantified by XPS; Figure S22, Supporting Information) after HER change significantly; in XPS, there is only slight increase in the Re 4f peak probably due to the oxidation of rhenium metal under ambient conditions. Additionally, we confirmed the presence of the generated hydrogen by gas chromatography and quantified the amounts of evolved H2 gas as well as the Faradaic efficiency (Figure S23). In summary, we synthesized and characterized Re-NPC/a-C nanocomposite that is the bestknown HER catalyst based on the Re element. Within this nanocomposite, the berry-shaped ReNPC/a-C clusters expose abundant catalytic sites for HER whereas the a-C phase allows for fast charge transfer. Although the performance of this nanostructured catalysts is worse than either Pt or few other catalysts,[73-77] its distinctive feature is that it functions well and are durable over the entire pH range.


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ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. Description of experimental methods, optical images of samples, NMR, IR spectra, additional TEM, SEM, EDX, XRD, XPS, Raman, BET, and electrochemical data (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Institute for Basic Science (IBS-R020-D1). We thank W. J. Byun and J. M. Yu for help with gas chromatography measurements.


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