Closely Arranged 3D–0D Graphene–Nickel Sulfide Superstructures

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Closely Arranged 3D−0D Graphene−Nickel Sulfide Superstructures for Bifunctional Hydrogen Electrocatalysis Jingjing Duan, Sheng Chen,* Yibing Li, and Chuan Zhao* School of Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia

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ABSTRACT: Electrocatalysts with both dense active sites and abundant transport channels are crucial for the realization of high activity and fast kinetics in electrochemical applications but are very challenging to produce. Here, we illustrate a gas-emission-assisted synthetic strategy to produce such a hybrid architecture constructed by closely arranged 0D nickel sulfide nanodots (∼9 nm) in a 3D porous graphene framework. Owning to these remarkable features, the material demonstrates superior electrocatalytic performance toward both hydrogen evolution (HER) and oxidation reactions (HOR). For HER, it requires a small overpotential of 105 mV to achieve a current density of 10 mA cm−2, features favorable kinetics with a Tafel slope of 42 mV dec−1, and survives for at least 50 h. For HOR, it needs a low overpotential of 46 mV for achieving 5 mA cm−2 and has strong durability for operation of 12 h. KEYWORDS: electrochemistry, reversible hydrogen reactions, bifunctional electrocatalyst, 0D−3D superstructure, graphene, nickel sulfide

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The efficiency of a reversible hydrogen electrocatalyst is usually dependent on, among many other factors, accessible active centers, porosity, and electrode geometry. Improvement in catalytic efficiency requires each parameter to be optimized, which simultaneously involves dense active sites, abundant porosity, and robust electrode architecture;8 however, optimizing some of them without compromising the others has been proven to be difficult. For example, bulk transition metal silicide has been examined for promoting HER/HOR due to its good stability,5 but its efficiency is rather low because of limited accessible active sites for electrocatalysis. Lowdimensional architectures like carbon nanotube−graphitic carbon hybrid7 have been developed, benefiting from enormous low-coordination atoms that can function as accessible active sites. Nevertheless, these nanosized materials usually suffer from severe aggregation to minimize specific surface energy. Recently, one useful strategy was reported to disperse nanoparticles onto highly porous carbon supports such as nitrogen-doped carbon nanotubes or Vulcan-XC72,8,14 followed by being decorated onto a two-dimensional (2D) planar scaffold such as glassy carbon. However, the catalyst loadings of these electrodes are significantly limited (0.1−1 mg cm−2). Herein, we synthesized a nickel sulfide−graphene electrocatalyst (denoted as NiS−G) with abundant active sites, hierarchically porosity, and extremely large catalyst loading of

he grid-scale application of renewable energy resources (like solar and wind) has stimulated research into a number of energy techniques such as regenerative fuel cells1 and metal−hydrogen batteries,2 both of which are associated with the core process of reversible hydrogen reactions. In regenerative fuel cells, hydrogen evolution reaction (HER) transforms renewable electricity into hydrogen gas through electrolysis function; while at the same electrode, hydrogen oxidation reaction (HOR) takes place to convert hydrogen back into electricity via the fuel cell function.1,3 Similarly, in metal−hydrogen batteries, HER and HOR also proceed at the same electrode, which plays a crucial role during charging and discharging processes.2 As a consequence, it is of vital importance to develop bifunctional hydrogen electrocatalysts for promoting the efficiencies of both HER and HOR.4−7 Thus far, the benchmark bifunctional electrocatalysts for hydrogen reactions are precious metals like platinum and iridium owing to their high activities.8,9 However, their high price and scarcity restrict large scale commercialization. As a consequence, extensive studies have been focused on their lowcost alternatives with comparable performance but much cheaper prices.5,7,10 Particular interests have been directed to transition metal materials (like nickel5) because of their outerlayer electronic structure similar to that of precious metals.11−13 The electrocatalytic performances of these transition metal materials have been enhanced through optimization of their chemical, structural, and morphological properties.4−7 Although moderate performances have been achieved with these materials, their HER/HOR activities are still inferior to those of precious metals. © XXXX American Chemical Society

Received: August 13, 2018 Accepted: October 11, 2018

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DOI: 10.1021/acsaem.8b01348 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Structure characterization of NiS−G. (a) Schematic microstructure of NiS nandot/graphene superstructures (NiS−G). (b−d) TEM images and (e−g) XRD patterns of the intermediate products obtained at different durations in hydrothermal process. (h) TGA analysis of graphene (G), NiS−G, and NiS at a heating rate of 10 °C min−1 in N2 atmosphere; the inset of (h) shows the histogram of the mass loss of corresponding materials. (i) Raman spectrum of NiS−G. (j) XPS spectrum of Ni 2p. (k) Nitrogen adsorption−desorption isotherms (expressed in cm3 STP g−1) of NiS−G.

46.5 mg cm−2. The material shows characteristic feature of “0D−3D” structures,15−19 which are composed of small-size 0D nickel sulfide nanodots closely arranged into 3D macroscopic architecture, followed by coating with 3D porous graphene framework. The unique topological feature of 0D NiS nanodots is useful for providing abundant active sites, while 3D macro/mesoporosity facilitates ion and gas transport. Moreover, the 3D electrode can provide large surface area for accommodating active species. Owing to these remarkable properties, NiS−G has demonstrated excellent electrocatalytic activity toward reversible hydrogen reactions and is among the most active bifunctional electrocatalysts reported in the literature. In a typical synthetic procedure, the sulfur precursor of thioacetamide (TAA, CH3CS-NH2) was mixed with nickel foam in aqueous solutions, which then decomposed to produce hydrogen sulfide (H2S) gas during hydrothermal treatment and transformed nickel foam into NiS nanodots via the following equations:20

crystal particle size of NiS.21,22 On the other hand, the vigorous gaseous emissions could raise the overall atmosphere pressure of the reactor, similar to an early report of carbon dioxide emission assisted process,22 and thus produce densely arranged NiS nanodot framework. To enhance the structure durability, NiS nanodot framework was then infiltrated with Na4EDTA (C10H12N2Na4O8) followed by thermal annealing in nitrogen atmosphere to generate few-layer graphene coated on its outer surface (Figures 1a and S1).23 The formation and structure of NiS−G was characterized by a number of techniques. Initially, pristine nickel foam shows bulk macroporous architecture with all XRD diffraction peaks corresponding to metallic nickel (JPCDS card no. 65-2865; Figures 1b and e). At the reaction duration of 6 h, nickel foam has been converted to mixed phases of rhombohedral NiS (JCPDS card no. 12-0041) and metallic Ni (Figures 1c and f). While at 12 h, only small 0D NiS nanodots were observed in the form of rhombohedral crystal phase (JCPDS card no. 120041, Figures 1d and g), suggesting nickel foam was entirely converted to nickel sulfide nanodots. Particularly, nickel element has the oxidation valence of +2, as revealed by its characteristic X-ray photoelectron spectroscopy (XPS) Ni 2p3/2 peak at 853.4 eV, while sulfur has the oxidation valence of −2, derived from its S 2p2/3 peak at 161.7 eV (Figures 1j and S2).24 In the next step, NiS was coated by few-layered graphene generated from NaEDTA, which accounts for 17 wt % inside NiS−G hybrid according to thermogravimetric analysis (TGA; Figures 1h and S1).23 The presence of graphene was further verified by Raman spectroscopy of

CH3CS − NH 2 + H 2O → CH3CO − NH 2 + H 2S(g) (1)

Ni + H 2S(g) → NiS + H 2(g)

(2)

During the above reaction process, the gaseous participants of H2S, H2, and water vapors play dual roles in the formation of NiS−G superstructures. On one hand, these gases can disrupt the aggregation and growth of NiS nuclei at the interfaces of thioacetamide and nickel foam and substantially reduce the B

DOI: 10.1021/acsaem.8b01348 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 2. Morphology of the 0D−3D superstructure. (a and b) SEM images of nickel foam. (c−g) SEM images and corresponding element mapping of NiS−G. The inset of (c) is the optical image of NiS−G. (h−n) TEM images and corresponding element mapping of NiS−G. The inset of (h) is the histogram showing size distribution of NiS nanodots. (o) Contact angle measurements of NiS and NiS−G. (p) SEM EDS spectrum of NiS−G.

NiS−G (Figure 1i), i.e., the diagnostic peaks centered at 1369 and 1596 cm−1 for D and G bands of graphene23 and the Raman modes of Eg and Ag below 1000 cm−1 for NiS.25 The XPS C 1s spectrum of graphene is shown in Figure S2a, which can be deconvoluted into three peaks: CC (284.6 eV), C− O(N) (286.1 eV), and CO (287.9 eV). Further, NiS−G has a Brunauer−Emmett−Teller (BET) surface area of 25 m2 g−1, as determined by nitrogen adsorption−desorption isotherms (expressed in cm3 STP g−1, Figure 1k), which exceeds that of individual NiS (18 m2 g−1).26,27 The 3D−0D morphology of NiS−G was clearly observed on scanning electron microscopy (SEM; Figures 2a−g). In contrast to silver-gray pristine nickel foam, NiS−G is black in color with the 3D freestanding architecture typically of 1 cm in width and several centimeters in length (Figures 2c and d). The morphology of NiS−G resembles the topotactic macroporosity of nickel foam (Figures 2a and b), despite of the roughened surface originated from small-size NiS nanodots. Therefore, the 0D nanodots inside NiS−G were examined using TEM (Figures 2h−n, S3), which show densely packed patterns across the whole surface of carbon supported TEM grids. These nanoparticles have an average diameter of 9 nm through the measurements of 100 locations. High resolution TEM (HRTEM) images show these nanoparticles with a dspacing of 0.27 nm corresponding to the (300) plane of rhombohedra-phase NiS crystals, which is consistent with XRD

analysis. Moreover, almost all NiS nanoparticles were encapsulated by few-layered graphene shells that lead to a hydrophobic surface characteristic with a contact angle at 127.5° (Figures 2o). Both SEM and TEM element maps show homogeneous distribution of carbon, nickel, and sulfur atoms across NiS−G at both micro- and nanoscopic level (Figures 2e−g, p, k−n, and S4). The remarkable 3D−0D structure of NiS−G motivated us to explore their potential applications for electrocatalytic reversible hydrogen reactions (Figure 3a). The NiS−G directly served as working electrode and coupled with carbon rod counter electrode and Ag/AgCl reference electrode in a threeelectrode system. The electrolyte was selected as sulfuric acid with abundant protons for favorable HER and HOR processes, which was purged with nitrogen or hydrogen gas for 30 min before testing. The internal resistance of overall system was below 5 ohms; therefore, all data were presented without iR correction. To perform electrocatalytic HER (2H+ + 2e → H2), NiS−G electrode was polarized to negative potential, where a favorable reaction process was observed with vigorous evolution of hydrogen bubbles at the electrode/electrolyte interfaces (Figures 3b, 3d, and S6 and inset of Figure 3e). In a typical linear scanning voltammetry (LSV) plot, the cathodic current of NiS−G simultaneously increases as the potential became more negative, which was commonly observed for HER C

DOI: 10.1021/acsaem.8b01348 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 3. Applications of NiS−G superstructures for bifunctional hydrogen electrocatalysis. (a) Schematic HER and HOR processes. (b) LSV plots of NiS−G for HER in comparison with NiS, 2D NiS−G, and Pt/C at 10 mV−1 in 0.1 M H2SO4. (c) Tafel plots of NiS−G in comparison to Pt/C. (d) GC spectra of the hydrogen gas collected in a separated chamber at the operation durations of 10 and 1800 s. (e) Chronoamperometric response of HER at −0.2 V (vs RHE) for 50 h; the inset shows the optical image of NiS−G electrode during HER with the evolution of hydrogen gas bubbles. (f) LSVs for HOR in H2 and N2 atmospheres. (g) Roughness factor study of NiS−G. The CVs measured at different scan rates from 2 to 10 mV s−1 in the potential region of 0.4−0.45 (vs RHE); the inset shows the current density at 0. 45 V (vs RHE) plotted against the scan rate. (h) Chronoamperometric response of HOR at 0.05 V (vs RHE) in H2 and N2 atmospheres for 12 h. (i) EIS spectra of NiS−G for HOR before and after 12 h.

catalysis in acidic media.4,7 It is noteworthy that NiS−G shows excellent electrocatalytic activity with a small overpotential of 105 mV to obtain a current density of 10 mA cm−2, which outperforms other counterparts such as NiS alone (347 mV) and NiS−G powder decorated onto a glassy carbon macrodisc electrode (2D NiS−G, >400 mV, Figure 3b). The Tafel slope of NiS−G derived from corresponding LSV is even comparable to that of commercial Pt/C benchmark (Figure 3c), indicating favored hydrogen evolution activity, which renders it among the most active metal sulfide electrocatalysts for HER reported in the literature (Table S1). The intrinsic HER catalytic activity of NiS−G was determined by the turnover frequency (TOF), which is 5 × 10−4 s−1 at the overpotential of 300 mV by assuming that all the Ni sites inside NiS−G are involved in HER. Further, NiS−G showed excellent electrode stability in chronopotentiometry testing for 50 h with little morphology or crystal structure alternation (Figures 3e and S4), which is different from NiS alone with sharp activity decay after operation for 1 h (Figure S5). The electrocatalytic HOR at NiS−G was tested by purging the electrolytes with hydrogen gas for 30 min to form H2saturated electrolyte. Similar to HER, HOR is also a conceptually simple process as comparison to many other

electrochemical reactions;28−30 therefore, it can be used as a model system to investigate the performance of our synthesized 0D−3D NiS−G superstructure. First, NiS−G electrode was polarized positively from 0 to 0.35 V (vs RHE) at the scan rate of 5 mV s−1, which shows obvious current increase in the whole potential range, indicating the occurrence of HOR in the electrochemical system.8 This result was further confirmed by the LSV plots of NiS−G in nitrogen gassaturated electrolyte with little current change in a similar testing condition (Figure 3f), indicating hydrogen gas plays an important role in the HOR process. From the LSV plots, NiS− G electrode can catalyze the HOR (H2 → 2H+ + 2e) with a small onset potential of 40 mV (vs RHE) and achieve the current densities of 5 mA cm−2 at the overpotential of 46 mV, which is comparable to Pt/C benchmark (9 mV) and outperforms other counterparts like individual NiS (87 mV) and 2D NiS−G (>350 mV, Figure S8). This performance has set NiS−G at the top of nonprecious metal electrocatalysts for HOR in the literature (53−98 mV; Table S2). The TOF of NiS−G toward HOR is 4.6 × 10−4 s−1 by assuming that all the Ni sites involved in the electrochemical reactions. Further, the electrode was polarized at 0.05 V (vs RHE) in H2-saturated electrolyte, which demonstrates robust durability for HOR in D

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an acidic solution for operating 12 h with insignificant component and resistance change, as revealed by XPS and electron impedance spectroscopy (EIS) spectra (Figures 3h, 3i, and S7). The significantly enhanced HER/HOR performances for NiS−G originated from its hierarchically porous yet densely arranged 3D−0D structure. According to the literature, NiS is recognized as a traditional non-noble metal catalyst for hydrogen reactions where the Ni−S bonds can reversibly adsorb hydrogen-related species (proton, hydrogen, or water molecules) with charge transfer between them as the ratedetermining step (RDS) for the overall reaction.31 On one hand, the 0D nanodots offer highly exposed surfaces with a BET surface area of 25 m2 g−1 (vs less than 1 m2 g−1 for nickel foam) that feature with numerous unsaturated atoms to serve as active centers. These 0D nanodots are densely packed together so that substantially increase the catalyst loading of NiS material on the electrode. The geometrical catalyst loading of NiS−G was determined as high as 46.5 mg cm−2, which is in great contrast to common catalyst electrodes for HER/OER (0.1−1 mg cm−2).7−9 Consequently, the HER current density of NiS−G is an order of magnitude higher than that of its 2D counterpart (Figure 3b). On the other hand, the 0D−3D superstructure has hierarchical porosity for facilitating the transport of reagents and products; large macropores of several hundred micrometers derived from the morphology-reserved transformation of nickel foam (Figures 2a−d) are beneficial to vigorous hydrogen gas evolution/transportation,32 while these mesopores inside NiS−G can provide sufficient ionic transport channels for easy infiltration by electrolyte ions during the reaction.33 These interconnected macro-mesopores can substantially increase the utilization of active species, as revealed by the tripled roughness factor of NiS−G in comparison to that of its 2D counterpart (595 vs 174, Figures 3g and S8). Interestingly, graphene showed three functions during electrochemical process. First, it has excellent electrical conductivity to facilitate the charge transport of electrode; consequently, the system showed a small internal resistance of 0.8 ohm (Figure 3i). Second, graphene has good mechanical property that can increase electrode stability during electrochemical cycling; therefore, NiS−G demonstrated excellent stability for 50-h chronopotentiometry test (Figures 3e and h). Third, graphene has worked as spacers between NiS nanodots and inhibits the aggregation of NiS during synthetic operation; thus, NiS nanoparticles can have a small diameter of ∼9 nm inside the hybrid materials (Figure 2h), which can promote HER/HOR through highly favorable Volmer−Heyrovsky pathway with a Tafel slope of 42 mV dec−1 (Figure 3c).34,35 All these attractive features make this class of 3D−0D hybrid materials promising for large-scale hydrogen batteries and reversible fuel cell applications. In conclusion, we demonstrated a facile and effective approach for the fabrication of 3D−0D superstructures by chemical conversion from bulk nickel foam, which were successfully synthesized by a gas-emission-assisted morphology-transformation method. The material exhibits attractive topotactic properties that demonstrate excellent electrocatalytic performances toward reversible hydrogen reactions. This work opens up more pathways for the efficient production of mixed-dimensional materials with promising applications in electrochemistry, catalysis, energy storage, and conversion.

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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/acsaem.8b01348. Material synthesis, physical and electrochemical characterizations; supplementary data (SEM, TEM, EDS, XRD, and XPS data of related materials), tables for comparison of HER and HOR performance with reported materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sheng Chen: 0000-0002-0402-6107 Yibing Li: 0000-0002-1729-5963 Chuan Zhao: 0000-0001-7007-5946 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the Australian Research Council (DP160103107, C.Z.) and a UNSW ViceChancellor’s Research Fellowship (S.C.). This research used equipment located at the UNSW MWAC analytic center.

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DOI: 10.1021/acsaem.8b01348 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX