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Journal of Science: Advanced Materials and Devices 5 (2020) 361e367 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Short Communication Hydrogen evolution reaction at extreme pH conditions of copper sulfide micro-hexagons Karthik S. Bhat*, H.S. Nagaraja** Department of Physics, National Institute of Technology Karnataka, P.O. Srinivasnagar, Surathkal, Mangaluru, 575 025, India a r t i c l e i n f o a b s t r a c t Article history: Received 19 January 2020 Received in revised form 5 June 2020 Accepted 16 June 2020 Available online 23 June 2020 Electrochemical hydrogen evolution reaction (HER) using non-precious compounds has gained substantial interest in the development of water electrolyzers. Herein, we report the synthesis of Copper sulfide (Cu2S) micro-hexagons via a hydrothermal method, followed by some of the important physiochemical characterizations and electrochemical measurements towards the HER. Cu2S micro-hexagons could catalyze the HER in both basic (1 M KOH) and acidic solutions (0.5 M H2SO4), corresponding to the extreme pH values of 14 and 0, respectively. As manifested from the polarization curve, Cu2S microhexagons required an overpotential of
330 mV and
312 mV to deliver a benchmark catalytic current density of 10 mA cm
2 in basic and acidic solutions, respectively. Furthermore, lower overpotentials are complemented with the prominent long-term stability of 24 h, as evident from chronopotentiometric analysis. The superior electrochemical performance of these Cu2S micro-hexagons demonstrates their promising suitability for water-splitting applications. © 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Keywords: Copper chalcogenides Sulfides Hydrogen evolution reaction 1. Introduction Hydrogen evolution reaction (HER) via an electrochemical approach has received considerable and persistent attention in the last decade owing to their non-toxicity, efficiency, and high energy density [1,2]. Noble metal platinum (Pt) remains the first choice for the HER owing to their superior activity [3,4], while their nonabundancy, high-cost, and stability limit their commercial applications. Subsequently, these factors encourage researchers the search on other possible alternatives. Recently, various transition metal-based compounds such as oxides [5], layered double-hydroxides [6], carbides [7], chalcogenides [8] and phosphides [9] of different shapes and sizes are in the limelight as an active electrocatalyst for the HER. Of which, transition metal chalcogenides deliver competent electrocatalytic activity ascribing to their various synergistic features such as electronic conductivity, tunable morphology, layered structure, superior surface area, and porous structures [10e12]. Furthermore, among different available transition metal chalcogenides, * Corresponding author. ** Corresponding author. E-mail addresses: kasubhat@gmail.com (K.S. Bhat), nagaraja@nitk.edu.in (H.S. Nagaraja). Peer review under responsibility of Vietnam National University, Hanoi. chalcogenide compounds of cobalt [13], nickel [14], and molybdenum [15] are enormously explored with numerous reports. While, to the best of our knowledge, copper chalcogenides are the least explored with seldom reports. Copper (Cu) possesses important and attractive properties such as high earth-abundance [16], multiple oxidation states [17], good redox kinetics [18], and rich coordination chemistry [19]. However, metallic Cu demonstrates poor HER activity owing to their weak hydrogen adsorptionedesorption at their catalytic sites [20,21]. In this regard, the optimization of hydrogen adsorption property of catalytic active sites for efficient HER is crucial. Among several successful attempts, tuning the coordination environment of catalytic sites by different electronegative elements such as phosphorus (P), oxygen (O) and nitrogen (N) was effective in improving the hydrogen adsorption of Cu, which in turn enhanced the HER [22e24]. Herein, electron density drift from the Cu metal atoms to anions lead to the positively-charged Cu atoms and negativelycharged anions; which functions as hydride and proton acceptors, respectively [25,26]. Based on this hypothesis, several Cu based HER electrocatalysts such as Cu3P [27], Cu3N [28], Cu2O [29], etc. were reported. With this framework, it is also anticipated to observe the enhanced HER performance of Cu atoms coordinated with different chalcogen atoms (S, Se, and Te), since it is believed that the increase in covalency and declination of electronegativity favors the superior HER [15]. https://doi.org/10.1016/j.jsamd.2020.06.004 2468-2179/© 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 362 K.S. Bhat, H.S. Nagaraja / Journal of Science: Advanced Materials and Devices 5 (2020) 361e367 Copper sulfides as an electrocatalyst were prepared using different techniques such as hydrothermal [30], electrodeposition [31], chemical vapor deposition [31] and ioneexchange reaction [32], among which some of the processes are relatively expensive and complex. Considering the electrochemical properties of synthesized copper sulfides, the HER activities are measured in different electrolytes, such as acidic, alkaline, and neutral solutions. However, no effort has been put up to compare the intrinsic catalytic activity of the synthesized copper sulfides at extreme pH conditions. In the present manuscript, we propose the synthesis of copper sulfide (Cu2S) with a unique morphology of micro-hexagons using copper sulfate, sulfur powder as precursors and hydrazine hydrate as a reducing agent via hydrothermal method. Used as an electrocatalyst, Cu2S micro-hexagons could deliver the catalytic current density of 10 mA cm
2 at an overpotential of
330 mV in basic solution, with a Tafel slope of 106.93 mV dec
1. On the other hand, Cu2S micro-hexagons required an overpotential of
312 mV to deliver catalytic current density of 10 mA cm
2 in acidic solution, with a Tafel slope of 49.79 mV dec
1. In addition, Cu2S microhexagons are complimented with their long-term stability of 24 h in both basic and acidic solutions, as evident from chronopotentiometric measurements. 2. Experimental 2.1. Synthesis of copper sulfide (Cu2S) micro-hexagons Copper sulfide (Cu2S) micro-hexagons were synthesized employing a one-pot hydrothermal method. About 1 mmol (250 mg) of copper sulfate pentahydrate (CuSO4.5H2O) was added to a beaker containing 30 mL of DI water at room temperature under continuous stirring. On the other hand, 0.5 mmol (16 mg) of sulfur powder (S) was added to a vial containing 5 mL of hydrazine hydrate (NH2NH2.H2O), followed by ultra-sonication for a short time interval of ~2e3 min. After this, the as-formed solution was added dropwise to a beaker containing pre-prepared 30 mL of CuSO4.5H2O solution, resulting in an immediate color change from green to dark brown and then black. The final black colored solution was then transferred into a Teflon-lined stainless steel autoclave of 50 mL capacity, followed by a hydrothermal process at a temperature of 180  C for 12 h. After the reaction, the autoclave was allowed to cool down naturally, and the collected precipitates were washed in excess of DI water and dried in vacuum at 60  C. The successful formation of Cu2S-micro-hexagons may be as given below: S þ N2 H4 :H2 O / H2 S ðaqÞ þ 4H þ ðaqÞ þ N2 ðgÞ (1) 2CuSO4 :5H2 O þ 2H2 S/2Cu2 S þ þ2H2 SO4 þ 5H2 O (2) 2.2. Electrodes preparation for HER measurements The synthesized Cu2S micro-hexagons are assembled on cylindrical graphite rod substrates for the successful electrochemical measurements. Graphite rod substrates of ~1 cm diameter were finely polished to obtain a mirror polish, followed by cleaning with acetone and DI water through ultra-sonication. About ~5 mg of the synthesized catalyst was added into 1 mL of 3:1 (v/v) watereethanol solvent containing ~40 mL of perforated Nafion resin as a binder. The mixture was then ultra-sonicated for ~30 min, ensuring homogeneous dispersion of the synthesized Cu2S catalyst. About ~100 mL of catalyst ink was then drop-cast onto the surface of graphite rod substrates, followed by drying at room temperature overnight. With the above electrodes preparation conditions, the mass loading on graphite rod substrates was estimated to be about ~0.5 mg cm
2. 3. Results and discussion 3.1. Physiochemical analysis Fig. 1 represents various physiochemical characterizations employed for the synthesized Cu2S micro-hexagons. Structural characterization using Powder X-ray diffraction (PXRD) measurements as seen in Fig. 1a indicates the indexed multiple diffraction peaks, which are commensurate with standard JCPDS reference pattern #033e0490. The recorded PXRD pattern of Cu2S is indexed into their monoclinic phase, with lattice constants a ¼ 15.23 Å, b ¼ 11.88 Å and c ¼ 13.49 Å of P21/c space group. Further, energy dispersive X-ray spectroscopy (EDS) measurements are employed to determine elemental composition and phase purity of the synthesized Cu2S micro-hexagons. As displayed in Fig. 1b, EDS spectra indicate elemental peaks corresponding to Cu and S along with low intense peaks of C and O. The representative SEM image wherein EDS measurements are performed are provided in (See Fig. S1, supplementary content). The existence of C and O peaks could be ascribed to the use of carbon tape during FESEM-EDS measurements and surface oxygen moieties of Cu or S, respectively. Furthermore, the calculated atomic ratio of Cu to S is 2.06:1, which is very close to the stoichiometric ratio of Cu2S. Besides, the weight and atomic percentages of all elements are provided as a bar diagram in the inset of Fig. 1b and tabulated in (see Table T1, supplementary content). Next, field emission scanning electron microscopy (FESEM) images confirm the morphology of synthesized Cu2S micro-hexagons. FESEM images (Fig. 1c and d) at two different magnifications demonstrate the uniformly structured and well-ordered Cu2S structures of hexagonal shape with obvious thickness at a micron level. The average width and thickness of hexagons were about ~1.32 mm and ~0.65 mm, respectively. Furthermore, BrunauereEmmetteTeller (BET) and BJH (BarrettJoyner-Halenda) measurements (see Fig. S2, Supplementary content) indicate the superior surface area of ~33.97 m2 g
1 and average pore size of ~2.02 nm, respectively. 3.2. Hydrogen evolution reaction (HER) The HER performance of synthesized Cu2S micro-hexagons was evaluated in two different solutions namely 1 M KOH (basic) and 0.5 M H2SO4 (acidic), which correspond to the extreme pH values of 14 and 0, respectively. As a primary evaluation of the electrocatalyst, LSV polarization curves were recorded at a slower scan rate of 5 mV s
1. Further, manual 100% iR compensation was applied to remove resistive and background losses. As such, Fig. 2a and b demonstrates the polarization curves of Cu2S microhexagons in 1 M KOH (basic) and.5 M H2SO4 (acidic) solutions, respectively. As evident from the polarization curve in a basic solution (Fig. 2a), Cu2S micro-hexagons require the different overpotentials of
330 mV,
399 mV,
492 mV and
596 mV to deliver benchmark catalytic current densities of 10, 20, 50 and 100 mA cm
2, respectively. Whereas, in acidic solution (Fig. 2b), the overpotentials were
312 mV,
376 mV,
416 mV, and
443 mV to deliver the same benchmark current densities of 10, 20, 50 and 100 mA cm
2, respectively. It is also worth noting that, Cu2S microhexagons deliver a relatively high current density of ~440 mA cm
2 at a moderate overpotential of
505 mV in 0.5 M H2SO4 solution. Furthermore, Tafel plots (Fig. 2c and d) obtained by redrawing the polarization curves represent Tafel slope values of 106.93 mV dec
1 K.S. Bhat, H.S. Nagaraja / Journal of Science: Advanced Materials and Devices 5 (2020) 361e367 363 Fig. 1. Physiochemical characterization of Copper sulfide micro-hexagons: (a) PXRD pattern, (b) EDS spectra, (ced) FESEM images at two different magnifications. Fig. 2. (aeb) LSV polarization curves of Cu2S micro-hexagons in 1 M KOH and 0.5 M H2SO4 corresponding to pH values of 14 and 0, respectively, and (ced) Tafel plots derived from polarization curves. 364 K.S. Bhat, H.S. Nagaraja / Journal of Science: Advanced Materials and Devices 5 (2020) 361e367 Fig. 3. (aeb) Nyquist plot and corresponding Bode phase angle plot in a basic solution (1 M KOH) and (ced) Nyquist plot and corresponding Bode phase angle plot in acidic solution (0.5 M H2SO4). Fig. 4. (a) Simulated electrochemical equivalent circuit (EEC) for the Nyquist plots and (b) Plot of difference in double-layer charging current densities vs. scan rate for the estimation of ECSA. Table 1 EEC parameters of Cu2S micro-hexagons in a basic solution (1 M KOH, pH ¼ 14). Table 2 EEC parameters of Cu2S micro-hexagons in acidic solution (0.5 M H2SO4, pH ¼ 0). Overpotential (mV) R S ( U) QCPE1 (mF) RCT1 (U) Overpotential (mV) RS (U) QCPE1 (mF) RCT1 (U)
430
480
530
580
630 3.507 3.643 3.723 3.774 3.850 3.194 3.550 3.774 3.681 3.644 15.79 7.70 5.11 3.73 3.05
355
380
405
430
455 1.606 1.617 1.609 1.603 1.604 39.7 31.9 44.2 38.4 50.5 16.5 5.91 5.84 3.74 3.37 K.S. Bhat, H.S. Nagaraja / Journal of Science: Advanced Materials and Devices 5 (2020) 361e367 and 49.79 mV dec
1 in basic and acidic solutions, respectively. These results suggest Cu2S micro-hexagons has better kinetic HER interface in acidic solution than in alkaline solution. 3.3. HER kinetics To study the HER interface, potentiostatic electrochemical impedance spectroscopy (PEIS) measurements are employed. Nyquist plots were recorded in frequency range of 0.2 Hze100 kHz at different applied bias Vs. RHE/V. Fig. 3a and b and Fig. 3c and d represents the Nyquist plots and Bode phase angle plots of Cu2S micro-hexagons in basic and acidic solutions, respectively. The Nyquist plots of Cu2S micro-hexagons in both solutions are semicircles, representing charge-transfer resistance [33]. The declination of resistive semicircle diameter with applied 365 bias determines these semicircles are related to HER kinetics [34]. Also, the downfall of phase angle and phase angle shift towards higher frequencies at an higher applied bias demonstrates faster HER kinetics [35]. Furthermore, the recorded Nyquist plots of Cu2S micro-hexagons was fit into the Randle's electrochemical equivalent circuit (EEC) represented by RS(QCPERCT) (Fig. 4a), wherein RS, RCT and QCPE portray the solution resistance, charge-transfer resistance and constant phase element/pseudocapacitive element, respectively. At an applied bias of
430 mV, Cu2S micro-hexagons reveal the resistance values RS and RCT of ~3.9 U and ~16.5 U in basic solution, which is reasonably higher than the resistance values in acidic solution (RS ¼ ~1.6 U and RCT ¼ ~3.74 U). Furthermore, other EEC fitting parameters extracted by fitting the recorded Nyquist plots at different applied bias voltages in basic and acidic solutions are Fig. 5. (a) Chronopotentiometric stability tests of Cu2S micro-hexagons in 1 M KOH, (b) LSV curve, and (c) Nyquist plot and recorded before and after watereethanol stability tests in 1 M KOH. (d) Chronopotentiometric stability tests of Cu2S micro-hexagons in 0.5 M H2SO4, (e) LSV curve, and (f) Nyquist plot and recorded before and after chronopotentiometric stability tests in 0.5 M H2SO4. 366 K.S. Bhat, H.S. Nagaraja / Journal of Science: Advanced Materials and Devices 5 (2020) 361e367 tabulated in Table 1 and Table 2, respectively. All of these values complement the superior HER performance of Cu2S microhexagons in acidic solution. On the other hand, these results also indicate the high RS and RCT values of Cu2S micro-hexagons in basic solution could be disparaging for the HER. These results are further supported by the electrochemically active surface area (ECSA) measurements, which indicate effective surface area available for the HER. The ECSA is determined in the double-layer chagrining region of the voltammetry curve, wherein no Faradaic reaction takes place [36]. The CV curves of Cu2S micro-hexagons in the non-Faradaic region are provided in (See Fig. S3, supplementary content). The ECSA values are estimated by plotting the difference in double-layer charging current densities Dj ¼ (ja-jc) Vs. scan rates as displayed in Fig. 4b. The difference in double-layer charging current densities were estimated at a potential of
0.55 V Vs. SCE and at
0.3 V Vs. SCE in basic and acidic solutions, respectively. As awaited, Cu2S micro-hexagons demonstrate the high ECSA of 5.83 mF cm
2 in acidic solution than in basic solution (3.89 mF cm
2), which further supports the superior HER as confirmed from Tafel plots and HER kinetic analysis. realized from PXRD and FESEM measurements. The HER performances of the Cu2S micro-hexagons were evaluated in basic and acidic solutions corresponding to the extreme pH values of 14 and 0, respectively. In addition, the Cu2S micro-hexagons are complimented with their long-term stability of 24 h as evident from chronopotentiometric measurements. Therefore, the Cu2S microhexagons could be a promising and effective electrocatalyst for water-splitting applications. Declaration of Competing Interest The authors declare that they have no known Conflict of Interest that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2020.06.004. References 3.4. Stability of the electrocatalysts After evaluating the comparative HER activities of Cu2S microhexagons in basic and acidic solutions, it is necessary to evaluate their long-term stability, since it is one of the figure-of-the-merit to evaluate performance of the electrocatalysts. For this chronopotentiometry is a vital tool, in which a constant current density is applied for certain hours. As a constant current density is applied, the overpotential attained by electrocatalyst depends upon characteristics of the catalysts, redox couple and may vary with time [37,38]. The achieved overpotential may change due to a decrease in the concentration of Hþ ions or due to bleach of the catalyst under long-term operations [39,40]. In the present case, chronopotentiometry measurements of Cu2S micro-hexagons are recorded at an applied current density of
35 mA cm
2 for the duration of 24 h as pictured in Fig. 5a and Fig. 5d, respectively. Results demonstrate vigorous evolution of hydrogen with no obvious drift in the overpotential. The polarization curve (Fig. 5b) and Nyquist plot (at
580 mV bias) (Fig. 5c) recorded after chronopotentiometry measurements in basic solution could retrace the initial curve, confirming their promising stability. However, as evident from the Nyquist plot, a small increment in RS of ~3.7 Ue~4.27 U may be noted. While, the RCT remained the same before the after chronopotentiometry measurements, which further endorse the promising stability of Cu2S micro-hexagons in basic solution. On the other hand, a small downward shift is observed for the chronopotentiometry in acidic solution, which can be necessarily attributed to the enhanced wettability and/or activation of electroactive sites under long-term operations. Also, the polarization curve recorded after chronopotentiometry measurements (Fig. 5e) represents an enhanced overall current density by about ~13%. This enhancement was further supported by a decrease in chargetransfer resistance RCT by ~40% (Fig. 5f) as validated from the Nyquist plot at
455 mV bias. Therefore these results successfully promote the promising stability of Cu2S micro-hexagons in both basic and acidic solutions. 4. Conclusion In summary, the Cu2S micro-hexagons were successfully synthesized via a hydrothermal reaction of 12 h. The monoclinic phase and hexagonal shape of the synthesized Cu2S micro-hexagons were [1] F. Safizadeh, E. Ghali, G. Houlachi, Electrocatalysis developments for hydrogen evolution reaction in alkaline solutionsea review, Int. J. Hydrogen Energy 40 (2015) 256e274. [2] G. Zhao, K. Rui, S.X. Dou, W. Sun, Heterostructures for electrochemical hydrogen evolution reaction: a review, Adv. Funct. Mater. 28 (2018) 1803291. [3] W. Sheng, H.A. Gasteiger, Y. Shao-Horn, Hydrogen oxidation and evolution reaction kinetics on platinum: acid vs alkaline electrolytes, J. Electrochem. Soc. 157 (2010) B1529eB1536. [4] K.S. Bhat, H.S. Nagaraja, Recent trends and insights in nickel chalcogenide nanostructures for water-splitting reactions, Mater. Res. Innovat. (2019) 1e24. [5] H. Jin, J. Wang, D. Su, Z. Wei, Z. Pang, Y. Wang, In situ cobaltecobalt oxide/Ndoped carbon hybrids as superior bifunctional electrocatalysts for hydrogen and oxygen evolution, J. Am. Chem. Soc. 137 (2015) 2688e2694. [6] R. Subbaraman, D. Tripkovic, D. Strmcnik, K.-C. Chang, M. Uchimura, A.P. Paulikas, V. Stamenkovic, N.M. Markovic, Enhancing hydrogen evolution activity in water splitting by tailoring Liþ-Ni (OH) 2-Pt interfaces, Science 334 (2011) 1256e1260. [7] C. Wan, Y.N. Regmi, B.M. Leonard, Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction, Angew. Chem. Int. Ed. 53 (2014) 6407e6410. [8] Q. Lu, Y. Yu, Q. Ma, B. Chen, H. Zhang, 2D transition-metal-dichalcogenidenanosheet-based composites for photocatalytic and electrocatalytic hydrogen evolution reactions, Adv. Mater. 28 (2016) 1917e1933. [9] P. Xiao, W. Chen, X. Wang, A review of phosphide-based materials for electrocatalytic hydrogen evolution, Adv. Energy Mater. 5 (2015) 1500985. [10] H. Wang, H. Yuan, S.S. Hong, Y. Li, Y. Cui, Physical and chemical tuning of twodimensional transition metal dichalcogenides, Chem. Soc. Rev. 44 (2015) 2664e2680. [11] N.A. Vante, W. Jaegermann, H. Tributsch, W. Hoenle, K. Yvon, Electrocatalysis of oxygen reduction by chalcogenides containing mixed transition metal clusters, J. Am. Chem. Soc. 109 (1987) 3251e3257. [12] S. Bag, I.U. Arachchige, M.G. Kanatzidis, Aerogels from metal chalcogenides and their emerging unique properties, J. Mater. Chem. 18 (2008) 3628e3632.
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