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Parallelized Reaction Pathway and Stronger Internal Band Bending by Partial Oxidation of Metal Sulfide-Graphene Composite: Important Factors of Synergistic Oxygen Evolution Reaction Enhancement HyukSu Han, Kang Min Kim, Heechae Choi, Ghulam Ali, Kyung Yoon Chung, YURIM HONG, Junghyun Choi, Jiseok Kwon, Seung Woo Lee, Jae Woong Lee, Jeong Ho Ryu, Taeseup Song, and Sungwook Mhin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00017 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Catalysis

Parallelized Reaction Pathway and Stronger Internal Band Bending by Partial Oxidation of Metal Sulfide-Graphene Composite: Important Factors of Synergistic Oxygen Evolution Reaction Enhancement

HyukSu Han1,†, Kang Min Kim1,†, Heechae Choi2,*, Ghulam Ali3, Kyung Yoon Chung3, Yu-Rim Hong,1,4 Junghyun Choi,5 Jiseok Kwon,5 Seung Woo Lee,5 Jae Woong Lee6, Jeong Ho Ryu7, Taeseup Song5,*, and Sungwook Mhin6,*

1

Korea Institute of Industrial Technology, 137-41 Gwahakdanji-ro, Gangneung-si, Gangwon

25440, Republic of Korea 2

Korea Institute of Science & Technology, Center for Computational Science, 5 Hwarang-ro

14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea 3

Center for Energy Convergence Research, Korea Institute of Science and Technology,

Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea 4

Department of Chemistry, Seoul Women's University, Seoul 01797, Republic of Korea

5

Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea

6

Korea Institute of Industrial Technology, 156 Gaetbeol-ro, Yeonsu-gu, Incheon 21999, Republic of Korea 7

Department of Materials science and Engineering, Chungju National University, 50

Daehak-ro, Chungji-si, Chungbuk 28644, Republic of Korea

†These authors contributed equally to this work. Correspondence and request for materials should be addressed to H.C (email: [email protected]) [email protected]) or S.M (email: [email protected])

1

ACS Paragon Plus Environment

or

T.S

(email:

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ABSTRACT Electrocatalytic performance of transition metal sulfide (TMS)-graphene composites has been simply regarded as the results of high conductivity and the large surface/volume ratio. However, unavoidable factor such as degree of oxidation of TMSs has been hardly considered for the origin of this catalytic activity of TMS-graphene composite. To accomplish reliable application of TMS based electrocatalytic materials, clear understanding in thermodynamic stability of TMS and impacts of oxidation on catalytic activity are necessary. In addition, the mechanism of charge transfer at TMS-graphene interface must be studied in depth to properly design composite materials. Herein, we report a comprehensive study of the physical chemistry at the junction of a Co1-xNixS2-graphene composite, which is a prototype designed to unravel the mechanisms of charge-transfer between TMS and graphene. Specifically, the thermodynamic stability and the effects of oxidation of TMSs during oxygen evolution reaction (OER) on reaction mechanism is systematically investigated using density functional theory (DFT) calculations and experimental observations. Cobalt atoms anchored on pyridinic N sites in the graphene support form metal–semiconductor (SC) junctions, and the internal band bending at these junctions facilitates electron transfer from TMSs to graphene. The junction enables fast sinking of the excess electron from OH– adsorbate. Partially oxidized amorphous TMSs layer formed during OER can facilitate adsorption and desorption of OH and H atoms, boosting the OER performance of TMSs–graphene nanocomposite. From the DFT calculations, the enhanced electrocatalytic activity of TMSs-graphene nanocomposite originates from two important factors: i) increased internal band bending and ii) parallelized OER pathways at the interface of pristine and oxidized TMSs. 2


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ACS Catalysis

KEYWORDS: Aerogel, cobalt–nickel-sulfide, graphene, water splitting

1. INTRODUCTION Hydrogen production and its efficient storage for sustainable energy generation have the potential to solve current environmental issues such as rapid fossil fuel depletion and environmental

pollution.1-2

Among

various

approaches

for

hydrogen

production,

electrochemical water splitting is a promising path because it requires only earth-abundant natural resources and produces eco-friendly by-products.3 Simply, water splitting can be defined as a chemical reaction that separates oxygen and hydrogen from water, which occurs with the aid of external energy input. Potential electrical resources, including solar and wind energy, can be converted via water splitting into chemical energy stored as hydrogen fuel.4 The water splitting reaction is a thermodynamically uphill process requiring an energy input of 286 kJ mol−1 at ambient pressure and room temperature. The oxygen evolution reaction (OER) is the bottleneck of the overall water splitting process due to the sluggish kinetics of the multi-electron reaction.5-7 Thus, noble metal-based oxides such as RuO2 and IrO2 have been widely used as electrocatalysts to lower the overpotential (η) of the OER reaction.8 Unfortunately, the high cost, scarcity, and poor durability of noble metal-based electrocatalysts

are

roadblocks

hindering

widespread

large-scale

applications

of

electrochemical water splitting electrolyzers. Thus, it is important to explore inexpensive, robust, and efficient OER electrocatalysts based on earth-abundant nonprecious transition metals or carbon-based nanomaterials.9-15 Nitrogen-doped reduced graphene oxide aerogel (NGA) is a porous carbon-based material having excellent conductivity. Additionally, its high surface area-to-volume ratio, chemical 3


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stability, and multidimensional electron transport pathway make NGA an attractive template to incorporate inorganic nanomaterials for electrochemical applications.16-17 For example, the introduction of earth-abundant transition metal (especially Co and Ni)-based sulfides, oxides, and phosphates to NGA can boost OER electrocatalytic performance.18-21 Among these, transition metal sulfides (TMSs) have attracted particular interest due to their noble metallike conductivity and excellent overall water splitting activity.22-24 The improved OER activity of the TMSs is attributed to unsaturated transition metal sites that favor chemisorption of OH– and oxygen-containing intermediates on the surface.25 Extensive research has been carried out on nanocomposites of TMSs and NGA as high-performance electrocatalysts. The origin of high catalytic activity of TMSs-NGA composites have been simply regarded as high conductivity and geometrical feature (i.e. large surface/volume ratio). However, the corresponding physical model of the electronic band structure for OER has not been proposed for TMSs-NGA composites. Thus, the origin of the high catalytic activity of TMSs-NGA is still unknown. Moreover, TMSs is well known to be oxidized under alkaline OER condition, however the effect of oxidation of TMSs on OER mechanism has not been studied in details so far. Herein, we report maiden findings unravelling the origin of the high catalytic activity of nanocomposites of Ni-doped CoS2 (CNS, space group Pa3) nanoparticles, as a prototype for TMSs, integrated with a three-dimensional (3D) porous NGA template (CNS–NGA). Metallic CNS and p-type semiconducting NGAs can modulate the band structure of CNS– NGA nanocomposites by facilitating charge transfer from CNS clusters to the porous NGA support: the work function difference between two components leads to strong internal band bending. In such composites, Co atoms are anchored on pyridinic N sites of the NGA 4


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ACS Catalysis

supports, which results in the formation of metal–SC junctions that act as electron sinks when OH– ions become attached to the CNS–NGA surface. Also, a partially oxidized CNS cluster during OER can facilitate adsorption and desorption of OH and H atoms resulting in parallelized OER pathway. The CNS–NGA catalyst was prepared via a facile one-step hydrothermal process. It exhibited one of the highest OER activities reported among Co- and Ni sulfides and oxides as well as carbon-based catalysts. Experimental results and computational calculations demonstrated that the synergetic effects between the CNS and 3Dporous NGA supports, mainly contributed to the state-of-the-art catalytic performance of the CNS–NGA nanocomposites for electrochemical water splitting. . From our DFT calculations oxidation of CNS is found to be thermodynamically stable upto ~17 % of mole fraction of sulfur atoms. The workfunction of CNS increases as oxidation occurs which enhances internal band bending at the CNS and NGA interface.

2. EXPERIMENTAL SECTION Catalyst Synthesis: The CNS–NGA nanocomposite was synthesized via a facile one-step hydrothermal process. In a typical procedure, graphene oxide (GO) was prepared via chemical exfoliation of graphite powder following a modified Hummer’s method.[13] A solution of GO (20 mL, concentration of 0.2 g L−1) in 50 mL of deionized (DI) water was mixed with cobalt chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), and thiourea (CH4N2S) under vigorous stirring. The amount of CoCl2, NiCl2, and thiourea was controlled based on the targeted weight ratio of Co and Ni. The solution was further ultrasonicated for 20 min to homogeneously disperse the Co, Ni, and S precursors in the GO solution. The resulting mixture was then transferred to a 200-mL Teflon®-lined stainless-steel autoclave and hydrothermally reacted at 200°C for 24 h. The resultant product 5


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was washed several times with DI water and freeze-dried at −50°C for 3 days to obtain the porous aerogel. CNS was prepared by dissolving CoCl2·6H2O, NiCl2·6H2O, and thiourea in DI water (70 mL) using magnetic stirring and ultrasonication. The resultant suspension was transferred to a Teflon®-lined autoclave, which was heated at 200°C for 24 h. The resultant powder was centrifuged and washed several times with DI water, and then dried overnight at 80°C. Characterizations: Scanning electron microscopy (SEM; model S4800; Hitachi) was used to obtain microstructural images of each sample. Transmission electron microscopy (TEM) and corresponding energy dispersive X-ray (EDX) mapping images were acquired using a Talos F200X (Thermo Fisher Scientific) microscope equipped with an EDX analyzer at 200 kV. X-ray diffraction (XRD) patterns were measured using a D/MAX-2500/PC (Rigaku) diffractometer with Cu-Kα radiation (λ = 0.15418 nm) at 40 kV and 100 mA. Raman spectroscopy was performed via a dispersive laser spectrophotometer (model NRS-3100; JASCO) at room temperature with an excitation wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) spectra were recorded for the samples using a VG ESCALAB 200i instrument (Thermo Fisher Scientific). XPS survey and high-resolution scans were conducted with pass energies of 100 and 20 eV, respectively. The specific surface area was measured by nitrogen adsorption–desorption tests (model TriStar II 3020; Micromeritics) following the Brunauer–Emmett–Teller

(BET)

theory.

Experimental

details

for

electrochemical

measurements, computational calculations, ultraviolet photoelectron spectroscopy (UPS), and X-ray absorption spectroscopy (XAS) are summarized in Supporting Information.

3. RESULTS AND DISCUSSION 6


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ACS Catalysis

The synthesis of a CNS–NGA is illustrated in Scheme 1 (see the Experimental section for details).

Cobalt

chloride

hexahydrate

(CoCl2·6H2O),

nickel chloride

hexahydrate

(NiCl2·6H2O), and thiourea (CH4N2S) were used as the cobalt, nickel, and sulfur precursors, respectively. The CoCl2·6H2O and NiCl2·6H2O were first mixed with an aqueous solution of graphene oxide (GO), which was prepared following a modified Hummer’s method using an ultrasonication (step (i)).26 The resultant mixture was then hydrothermally reacted to form the CNS–NGA hydrogel (step (ii)). Finally, the CNS–NGA hydrogel was freeze-dried to form the highly porous aerogel (step (iii)). In this way, the CNS nanoparticles were encapsulated and anchored on reduced-GO (rGO) porous templates (step (iv)). Simultaneously, N atoms from the thiourea were doped into the rGO framework. Co atoms in the CNS were also incorporated at pyridinic N sites of the graphene layer, resulting in the formation of metal–SC junctions between the TMS (i.e., CNS) and the N-doped rGO (N-rGO) layer (step (v)). The proposed band structure at the junction is illustrated in Scheme 1(vi). According to this model, electrons transfer from the CNS clusters to the N-rGO (p-type SC) layer, where they can further transfer to the electrode (glassy carbon) via hopping conduction. The band structure of CNS–NGA is suitable for the efficient consumption of electrons from OH– ions adsorbed onto the surface, which thereby significantly enhances the OER activity of the CNS–NGA nanocomposite. Figure 1(a) shows a typical digital image of the CNS–NGA hybrid. Field-emission scanning electron microscopy (FE-SEM) images revealed the presence of an interconnected 3D-porous NGA network in the hybrid, with continuous macropores several-hundred nanometers in size (Figure 1(b)). Diamond-like CNS nanoparticles (300–500 nm) grew within the 3D-porous NGA frames. For comparison, the microstructure of the NGA before 7


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the integration of the CNS is also shown in Figure S1(a). Figure 1(c) shows that the CNS nanoparticles were mainly populated on the walls of the pores. The absence of significant aggregation of the CNS nanoparticles was attributed to the 3D-porous network of the NGA. Transmission electron microscopy (TEM) was used to further investigate the morphology of the hybrid. It appeared as crumpled NGA nanosheets decorated by diamond-shaped CNS nanoparticles (Figure 1(d, e)). Low-magnification TEM images revealed little agglomeration of the CNS nanoparticles (Figure S1(b)). Figure 1(f) shows high-magnification TEM images of the CNS nanoparticles; the inset represents the corresponding fast Fourier transform (FFT) pattern that was well-indexed to the crystalline planes of cubic-phase CNS (d(001) and d(111) spacings of 5.76 and 3.34 Å, space group Pa3, respectively). Energy dispersive spectroscopy (EDS) mapping images (Figure 1(g)) clearly revealed uniform distributions of Co, Ni, and S elements in the CNS. Furthermore, nitrogen was homogeneously distributed into the graphene nanosheets, confirming nitrogen doping of the GA. The 3D-porous network of the NGA facilitated mass transfer of the electrolyte and degassing of oxygen at the anode. The nitrogen adsorption–desorption isotherm of the NGA indicated a porous structure with a Brunauer–Emmett–Teller (BET) surface area of 499.3 m2g−1 (Figure S2(a)). After loading of the CNS nanoparticles, the BET surface area of the CNS–NGA decreased to 99.1 m2g−1 (Figure S2(b)). This reduced BET surface area was attributed to anchoring of the CNS–NGA nanoparticles to pores inside the NGA. Although the BET surface area of the CNS–NGA was not as high as that of NGA, this value is comparable to or better than that of other GA-based nanocomposites reported elsewhere.27-29 The electrolyte could easily diffuse into the pores of the 3D-porous CNS–NGA and thereby facilitate electrochemical reactions between the electrolyte and CNS nanoparticles anchored 8


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ACS Catalysis

in the pores. The X-ray diffraction (XRD) pattern of the CNS–NGA shows peaks at 27.5, 31.9, 35.8, 39.3, and 45.7° (Figure S3), which were assigned to the (111), (002), (021), (112), and (022) planes of the cubic CNS phase (CoNiS4, ICDD 98-602-4479). The peaks for CNS–NGA were observed at slightly higher 2θ angles than those of nickel sulfide–NGA (NS–NGA), and cobalt sulfide–NGA (CS–NGA). This result implies that Ni atoms were successfully incorporated into the CoS2 phase without forming a secondary phase. The broad XRD peak at 25.8°, corresponding to the (002) plane of rGO, indicated the reduction of GO during the hydrothermal process.30-31 The strong XRD peak observed at ~42˚ for hybrid samples may be due to the facilitated reduction of GO via nitrogen and sulfur doping from thiourea, which is used as sulfur precursor. This result indicates that the improved crystallinity of graphene structure of rGO support can be expected for hybrid samples compared to NGA support.32-33 Raman spectroscopy revealed two distinct bands at 1350 and 1589 cm−1 that corresponded to the characteristic disorder-induced D and graphitic G bands of graphene, respectively (Figure S4). The D-band is related to disordered graphene edges, while the G band is associated with first-order scattering of the E2g mode of sp2-carbon domains.34 The peak intensity ratios between the D and the G bands (ID/IG) were 1.40, 1.28, 1.26, and 1.21 for NGA, CNS–NGA, NS–NGA, and CS–NGA, respectively. Additionally, the D and G peaks of the CNS–NGA were downshifted from those of NGA. The decrease in the ID/IG ratio and the downshifts of the D and G bands for the CNS–NGA were attributed to the incorporation of Co or Ni atoms into defective sites in the N-doped graphene layer.29 Of the hybrids, the CNS– NGA had the highest ID/IG ratio, which may have contributed to its superior catalytic properties. 9


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The chemical composition and oxidation states of the CNS–NGA were studied by X-ray photoelectron spectroscopy (XPS). Incorporation of N into the rGO aerogel was confirmed by high-resolution C1s and N 1s XPS spectra of the CNS–NGA (Figure S5), which supported the EDS mapping results in Figure 1(g). Also, high-resolution N 1s XPS spectra showed that N in the CNS–NGA was primarily pyridinic nitrogen (399.0 eV, 72.3%) with a small amount of quaternary nitrogen (401.8 eV, 27.7%). Quantitative elemental analysis of the XPS data indicated N atomic content in the CNS–NGA of ca. 5.9%. The C1s XPS spectra was deconvoluted into four peaks with binding energies at 284.6, 285.2, 285.8, and 286.7 eV, which were assigned to sp2-hybridized carbon, sp2-C bonded to N, C–OH, and C–O–C bonds in the CNS–NGA, respectively.35-37 The S 2p XPS spectrum shows four peaks at binding energies of 162.5 (S–Co), 163.6 (C–S–C), 165.0 (C=S), and 168.5 eV (C–SOn–C), respectively, indicating the formation of chemical bonds between Co, C, and S atoms.38-40 Also, the high-resolution Co2p XPS spectra shows the presence of a shoulder near at 782.5 eV which can be attributed the N bonding to the cobalt (Co-N) (Figure S5(c)).41 To gain more accurate structural information about local bonding nature and electron transition mechanism in the CNS–NGA hybrid, synchrotron-based hard X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were employed. The CNS without NGA were synthesized using the same experimental conditions as CNS–NGA, except that only DI water was used as the solvent in the hydrothermal process. Figure S6 shows SEM images and the experimentally measured XRD pattern of the CNS nanoparticles. Those without NGA supports exhibited severe agglomeration, implying the important role of NGA supports for obtaining homogeneously dispersed CNS clusters. Low-resolution TEM image with selected-area diffraction (SAED) 10


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ACS Catalysis

patterns of CNS are shown in Figure S7. SAED patterns of CNS confirm polycrystalline nature of Co1-xNixS2. Also, elemental mapping images present that Co, Ni, and S are homogeneously distributed over CNS particles (Figure S7(c)). The Co K-edge XANES spectra of CNS and CNS–NGA along with reference samples (of Co3+, Co2+, and Co0) are shown in Figure 2(a). The XANES spectrum of CNS shows prominent pre-edge features (marked by vertical dash line A) which are caused by the dipole-forbidden 1s to 3d electronic transition, indicating disordered octahedral coordination of Co-S bonds. The pre-edge peak of CNS–NGA hybrid shows relatively weak intensity than CNS, suggesting well-ordered octahedral coordination of Co-S bonds and correspondingly less amount of 3d hole vacancy. The oxidation state of a specific metal can be determined by the absorption edge which is caused by the dipole-allowed 1s to 4p electronic transition (marked by C). The absorption edges marked by B and C occur due to the electronic transition to Co 4sp state and to Co 4sp mixed with S 3p states, respectively.42 The absorption edge of CNS was much broader than that of CNS–NGA and an additional absorption edge B was clearly observed. In contrast, the sharp absorption line of CNS–NGA well overlaps with the Co2+ standard spectrum and no additional absorption edge was observed. This indicates that Co atoms have the oxidation state of 2+ and the electronic transition in the CNS-NGA hybrid occur through a single transition related with the Co 4sp and S 3p states, however, multiple electronic transitions (A, B, and C) exist in the CNS. In the CNS-NGA hybrid, a large amount of hole vacancy in 3d states can be reduced, as indicated by the weak intensity of pre-edge peak A, enabling the electrons to be directly excited to the above Fermi level. This can significantly promote charge transfer in the CNS–NGA hybrid, which can contribute to the faster reaction kinetics. Similar XANES spectrum was previously observed in the case of Co-doped FeS2 where bare material exhibits broad spectrum, while Co-doped FeS2/CNT hybrid shows sharp absorption 11


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features.43 Furthermore, sharp XANES features of CNS–NGA suggests excessive charge can be induced due to the conductive NGA supports. Ni K-edge spectra (Figure 2(b)) shows similar XANES features to Co K-edge, suggesting Ni atoms with the oxidation state of 2+ are well incorporated at Co site in the CoS2 crystal structure without forming any secondary phases. Local structural information around Co and Ni atoms in CNS and CNS–NGA are investigated by fitting the EXAFS spectra. Quantitate structural information on coordination number (N), bond length (R) and Debye-Waller factor (σ2) are measured and listed in Table S1 and S2 (Supporting Information) for Co and Ni EXAFS, respectively. As shown in Figure S8, the first peak (around 1.8 Å and shown in dotted rectangle) in all the EXAFS spectra corresponds to M-S (M = Co/Ni) bonding in the octahedra. The first fitted peak of CNS and CNS–NGA implies that Co in CNS has a coordination number of 6 with an average distance of 2.3447 Å, while Co in CNS-NGA shows a relatively variable coordination number of 6 ± 0.5 with an average distance of 2.3429 Å (Table S1, Supporting Information). The next prominent peak (around 3.4 Å and shown in dotted rectangle) corresponds to M-S (M = Co/Ni) and M-M (M = Co/Ni) bondings. The Co-M bonding distance was calculated to be 4.1082 Å and 4.1115 Å for CNS and CNS–NGA hybrid (Figure S9), respectively. The EXAFS data of Ni edge shows almost similar trends to Co, hence suggesting crystal structure remains same after incorporation of Ni into CoS2. The electrocatalytic activity of the CNS–NGA in basic media (1 M KOH aqueous solution) was investigated using a typical three-electrode system with a rotating disk electrode (RDE). Linear sweep voltammetry (LSV) was performed at a scan rate of 0.5 mV s−1 on the CNS– NGA, NS–NGA, CS–NGA, NGA, and RuO2 samples. During the measurements, the RDE was continuously rotated at 2,000 rpm to remove the generated bubbles. All potential values 12


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ACS Catalysis

were iR-compensated and referenced to the hydrogen electrode (RHE). Figure 3(a) shows that the CNS–NGA exhibited remarkable catalytic activity for the OER. The overpotential (η) required to deliver a current density of 10 mA cm−2 (η10) is often used as a reference to evaluate electrocatalytic performance in terms of solar-to-fuel conversion.44 The CNS–NGA required an η10 of 330 mV, while the benchmark RuO2 electrocatalyst afforded the same current density at an η10 of 350 mV, indicating that the former had much better electrocatalytic activity than the latter. Also, the CNS–NGA demonstrated a much smaller η10 than the counterparts NS–NGA (370 mV), CS–NGA (400 mV), CNS (440 mV), and NGA (>570 mV). Importantly, the NGA showed negligible OER activity compared with CNS and the CNS–NGA hybrid, which demonstrated the significant enhancement of OER activity from the incorporation of the CNS compound with 3D-porous NGA supports. Additionally, the OER activity of CNS–NGA catalysts made with different ratios of Ni/Co was measured (Figure S10). The sample with an Ni/Co mass ratio of 2.33 exhibited the lowest OER activity, while the sample with a ratio of 0.42 displayed the highest OER activity. This implied that Co-rich CNS compounds were beneficial for the OER activity of the CNS– NGA composite. Furthermore, the OER activity of the CNS–NGA was better than that of the other OER catalysts derived from carbon nanomaterials such as NiCo2S4/N–S rGO (η10 = 355 mV),45 CoS2/N–S GO (η10 = 370 mV),46 NiCo2S4/carbon cloth (CC) (η10 = 340 mV),47 CoO/N-crumpled graphene (η10 = 340 mV),48 and N-graphene nanoribbons (η10 = 360 mV).49 Tafel slopes were estimated to investigate the kinetics of the OER (Figure 3(b)). This slope is a useful indicator for reaction kinetics; a smaller Tafel slope corresponds to faster kinetics for those electrochemical reactions involving charge transfer. The Tafel plots of the catalysts are derived from the LSV curves via the Tafel equation (η = b × logj + a, where η is the 13


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overpotential, j is the current density and b is the Tafel slope). The estimated Tafel slope for the CNS–NGA hybrid (47 mV dec−1) was much smaller than those for NS–NGA (98 mV dec−1), CS–NGA (72 mV dec−1), CNS (117 mV dec−1), and RuO2 (128 mV dec−1), which implied faster reaction kinetics in the CNS–NGA case. The Tafel slope of the CNS–NGA is the lowest among recently developed OER electrocatalysts other than a few materials such as Fe0.1NiS2/Ti and atomic layer-deposited (ALD) NiSx.50-51 Additionally, only CNS clusters without NGA displayed a much larger Tafel slope than the CNS–NGA hybrid. The Tafel slope as a function of η10 value for the catalysts is shown in Figure 3(c). This plot clearly indicates the synergistic effect between the CNS clusters and the conductive NGA supports and that the highly porous structure boosted the catalyst’s OER activity. Moreover, Faradaic efficiency (FE) of CNS-NGA was measured using a rotating ring disk electrode (RRDE) in order to demonstrate high electrocatalytic activity of the catalyst. For RRDE measurement, a ring potential of 0.4 VRHE was applied to detect O2 molecules generated by oxygen reduction reaction (ORR). As shown in Figure S11, a ring current of 0.114 mA was detected when a constant current of 0.57 mA was applied to the disk electrode for O2 generation. FE of CNSNGA was calculated using the following equation.

(

FE = iring / (idisk × N) 1)

where, idisk, iring, and N denote the disk current, the ring current, and the current collection efficiency of the RRDE which is generally about 0.2, respectively. The calculated result demonstrates that FE of CNS-NGA is above 99.9 % verifying the observed current is mostly originated from the OER of the catalyst. This result demonstrates that the CNS-NGA is a highly efficient OER catalysts. Such excellent OER activity of the CNS–NGA hybrid rivals 14


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or outperforms other reported state-of-the-art OER catalysts in alkaline media (Table S3, Supporting Information). In addition to good activity, long-term stability is also essential for practical applications of electrocatalysts. The electrocatalytic stability of the CNS–NGA catalyst was evaluated by chronoamperometric measurements. The current density dropped more slowly compared with RuO2 at a working potential of 1.6 V (Figures S12 and S13). After 7 h of OER, the CNS– NGA hybrid displayed about 100% of the current density output without any degradation, which demonstrated excellent stability in alkaline aqueous solution. Current fluctuation was observed during stability test possibly due to partial oxidation of CNS-NGA under OER condition. In sharp contrast, the benchmark RuO2 electrolyzer rapidly degraded with an OER activity loss of about 80% within only 1 h (Figure S13), indicating much better durability of the CNS–NGA hybrid. Additionally, the stability of the CNS–NGA sample was tested by continuous cycling in the OER potential window. Figure 3(d) shows that the polarization curve of the CNS–NGA catalyst exhibited negligible current loss compared after 1,000 cycles at a scan rate of 5 mV s−1. Overpotential at 10 mAcm2 was increased 0.11 % after 1,000 cycling potential sweeps in a range of 1.23 ~ 1.60 VRHE. Furthermore, from the thermodynamic point of view, it is impossible for metal sulfides to retain a pristine metalsulfur terminating surface under the strong oxidizing environment of OER. Therefore, we characterized structural and compositional changes of CNS-NGA after OER stability test. Figure S14 presents TEM images for CNS-NGA after long-term OER. CNS nanoparticles exhibiting round shape are deposited onto rGO sheets. HR-TEM images clearly show the totally different crystallinity at the two sides. At the crystalline area, lattice fringe of 0.32 and 0.31 nm which is associated with the (111) and (023) crystallographic planes of CNS phase 15


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are observed (Figure S14(d)). However, thin amorphous layer (i.e., Co-Ni-OH) was formed at the surface of CNS nanoparticles. The obtained EDS mapping images (Figure S14(e)) reveal the presence of Co, Ni, S, C, N, and O in CNS-NGA after long-term OER test. Based on the EDS result, O atoms present in the interior as well as the surface of CNS nanoparticle implying that partial oxidation may also occur and the crystalline phase of CNS is partially preserved as indicated by the crystal lattice of CNS in the TEM image. As shown in Figure S15(a), O atom was not observed in CNS nanoparticles, while Co and Ni exist in the particle. According to EDS mapping for O atom, O may exist in the rGO sheet (i.e, possibly unreduced functional groups or rGO layer) for pristine CNS-NGA. This result indicates that CNS nanoparticle is not oxidized before long-term OER tests. Also, quantitative analysis of EDS mapping results for CNS-NGA before and after long-term OER test is shown in Figure S15(a-b). The oxygen content was increased after long-term OER test from 5.61 to 22.95 at% indicating partial oxidation was occurred during OER in alkaline solution. XRD analysis was performed on CNS-NGA sample after long-term OER test and the result is shown in Figure S16. After OER, diffraction peaks for (Co1-xNix)S2 still distinct, while peak intensity was slightly reduced. This may be attributed to the fact that amorphous oxy- or hydroxide phases are mainly formed on the surface (outer) or grain boundary (interior) in CNS nanoparticles during OER due to high energy states. Thus, large volume of CNS nanoparticles can still retain highly crystalline (Co1-xNix)S2 phase. The surface amorphous hydroxide layers may protect the interior CNS phase for the further oxidation during long-term OER. The in-situ generated surface hydroxides or oxyhydroxides on the catalyst can be beneficial for OER reaction since it can function as the actual catalytic active sites. Figure S17 shows polarization curve and the derived Tafel slope for CNS-NGA after 10 hrs OER stability test. It can be clearly seen in Figure S17(a) that η10 for CNS-NGA (340 mV) shows negligible 16


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change after OER stability test compared to pristine CNS-NGA (η10 = 330 mV). The anodic peak centered at about 1.35 V is associated with the reversible redox process,CoሺOHሻଶ / NiሺOHሻଶ + OHି ↔ CoOOH/NiOOH + Hଶ O + e, which may be related with the formation of surface Ni-Co (hyro)oxide layers. Additionally, Tafel slope is calculated as 52 mV dec-1 (Figure S17(b)) from the polarization curve, indicating OER kinetics in CNS-NGA still remains unchanged after long-term OER. Generally, low conductivity of Ni-Co (hydro)oxide is one of main reason for their low intrinsic catalytic activity. Thus, although the formation of metal hydroxides in CNS-NGA during OER may not be avoided, the higher conductivity of the chalcogenide (i.e., the preserved crystalline CNS clusters) may be responsible for its excellent OER catalytic activity by facilitating the accessibility of surface active sites. Electrochemical impedance spectroscopy (EIS) was used to estimate the charge transfer resistance (Rct) of the catalysts. Generally, Rct can be determined from the semicircle diameter in the high-frequency region of a Nyquist plot (Zʹ vs. –Zʹʹ). The experimental data were fitted using Randles equivalent circuit models to extract Rct of the samples. Figure 4(a) shows the EIS data of samples, which clearly show that the CNS–NGA hybrid had the lowest Rct value of ca. 30 Ω among the tested samples. This indicated that better electrical transport and faster reaction kinetics were present in the CNS–NGA compared with the other materials. The NGA had a significantly higher Rct (higher than 500 Ω) than the CNS–NGA hybrid and the other materials studied in this work (Figure S18). Rct for the CNS-NGA after long-term OER test (ca. 32 Ω, Figure S19) remains almost constant, indicating the preserved CNS cluster can still possess high conductivity. Furthermore, we examined the electrochemical effective surface area (EESA) of the CNS–NGA and RuO2 catalysts through Cdl measurements using cyclic voltammetry (CV) at different scan rates. Scan-rate dependence of 17


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CVs is performed in the potential range of 1.0 – 1.1 V where redox processes do not occur in order to obtain the capacitive current related with double-layer charging (Figure S20). The ∆j = ja – jc at 1.05 V is plotted against the scan rate, where the linear slope is twice of the Cdl. Figure 4(b) shows that the EESA of the CNS–NGA hybrid (Cdl = 10.25 mF cm-2) was about 20-fold larger than that of RuO2, indicating greater numbers of catalytically active sites on the CNS–NGA hybrid. In addition, EESA was measured for CNS-NGA after long-term OER test. As can be seen in Figure S21, EESA (Cdl = 21.28 mF cm-2) was increased after long-term OER, which may possibly be attributed to the newly formed surface amorphous Ni-Co (hydro)oxide layers. The low Rct and high EESA values of the CNS–NGA can be attributed to the highly conductive CNS nanoclusters anchored on the 3D-porous NGA supports. Highly conductive electron pathways were possible through the porous structure of the CNS–NGA hybrid. Thus, the faster transfer of electrons and reactants as well as larger amount of catalytically active sties in the CNS–NGA structure was responsible for its excellent electrochemical OER performance. Also, since it is recently reported that Pt counter electrode can affect electrochemical properties of Pt-free catalysts due to dissolution of Pt under electrochemical cycling, we tested OER properties of CNS-NGA under the same conditions using a graphite rod as counter electrode. The results verify that OER properties (i.e., LSV, Tafel plot, and EIS) of CNS-NGA is almost comparable (Figure S22), indicating Pt dissolution from the counter electrode is not critical for the OER properties of CNS-NGA. The adsorption energies of OH, O, OOH, and OO were calculated to obtain the energy OER landscapes for the NGA, CNS, and CNS–NGA systems. The Gibbs free energies of the reactions (∆G) were calculated using the following: ∆G = ∆E + ∆ZPE – T∆S + ∆GU, 18


(1)

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where ∆E is the calculated total energy difference and ∆ZPE, T∆S, and ∆GU are the zeropoint energy correction, entropy, and free energy terms, respectively. According to experimental observations of our synthesized NGA, we considered only a pyridinic structure of graphene in the density functional theory (DFT) analyses for this study. A CNS cluster was modelled with a composition of Co9Ni5S28 following our synthesis conditions. We used a model of the CNS–NGA hybrid having cobalt atoms anchored onto the NGA, since calculated binding energy of the CNS and NGA was much larger for Co-anchorage (3.34 eV) than Ni-anchorage (1.03 eV) on the NGA. The calculated free energy diagrams and the atomic structures of OH–, O–, OOH–, and OO– adsorbed on the NGA, CNS, and CNS–NGA systems with U = 0 and 1.23 V are presented in Figure 5(a). To obtain the free energy diagrams for the energetically favored adsorption sites, we evaluated numerous adsorption energy calculations for different sites with varying angles of adsorbates on substrates. The reaction steps (I to IV) corresponding to OH, O, OOH, and OO adsorptions on NGA, CNS, CNS–NGA substrates are described as follows: (I)

H 2 O (l) → OH ads + H + + e -

(2)

(II)

OH ads → O ads + H + + e -

(3)

(III)

O + H 2 O (l) → HOO ads + H + + e -

(4)

(IV)

HOO → O 2 (g ) + H + + e -

(5)

The OER on the NGA substrate was expected to occur with adsorptions of OH, O, OOH, and OO on C atoms near doped N atoms in the pyridinic structure. Step III was predicted to be the rate-determining step (RDS) on the NDG at U = 0 V due to the large energy barrier of 2.66 eV (Figure 5(a)). Also, the reaction steps I and III for the NGA were endothermic at U = 19


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1.23 V, which would significantly suppress the water splitting reaction under the applied voltage and may be responsible for the high η10 of the NGA. On the CNS surface, the OER reaction can occur with the following geometries: (I) Co on top, (II) Co at the center of the surface, (III) S on top, and (IV) Ni on top (Figure 5(b)). Interestingly, with U = 0 V, reaction step II is spontaneous, as indicated by the negative energy change, and reaction step III has the highest energy barrier of 4.42 eV. With U = 1.23 V, reaction step III still has a high energy barrier (3.19 eV) and was expected to be the RDS. For the CNS–NGA system, the OER reaction pathway is different than that of the CNS system (Figure 5(b)), i.e., (I) S on top, (II) S on top, (III) Co on top, and (IV) Ni on top. The energy barrier for reaction step III is significantly reduced compared with the CNS system, i.e., 3.25 and 2.02 eV for U = 0 and 1.23 V, respectively (Figure 5(a)). In the CNS–NGA system, only step III has an energy barrier and the other steps are expected to be spontaneous; this may be responsible for the extremely fast reaction kinetics resulting in the lowest Tafel slope observed for the CNS– NGA sample. Charge transport is another key factor, in addition to the reaction energy barrier, that determines electrocatalytic activity.52 The spatial distributions of an extra electron from an OH– group adsorbed on NGA, CNS, and CNS–NGA surfaces are shown in Figure 5(c-e). For the NGA and CNS, the electron is localized at C or Co and Ni atoms, respectively. In contrast, the extra electronic charge in the CNS–NGA hybrid is highly delocalized on the CNS clusters as well as on the NGA supports. This indicates that localized excess electronic charge on the CNS can spread over the CNS–NGA composite through the conductive NGA supports, which remarkably improves charge transport. According to Allain et al., the formation of covalent bonds between metal atoms and a twodimensional semiconductor (2D-SC) metallizes the surface of the 2D-SC and thus eliminates 20


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the van der Waals gap between the metal and 2D-SC, resulting in a reduction of the contact resistance.53 The charge distribution near an anchored Co on a pyridinic N site after adsorption of OH– ions, and the corresponding density-of-state (DOS) plots, are shown in

Figure 6(a) and (b), respectively. Excess electrons accumulate at the anchored Co atoms, while electron depletion occurs around the C atoms near the pyridinic N sites. Furthermore, the partial DOS plots near the Fermi energy level (EF) show that strong hybridization is present between the Co 3d and N 2p orbitals of the CNS–NGA hybrid. The formation of covalent bonds between anchored Co and pyridinic N atoms significantly facilitates charge transport in the CNS–NGA composite by reducing contact resistance. Figure 6(c) shows the band diagram of the CNS–NGA before and after anchoring of Co atoms at the pyridinic N sites. Work functions for CNS and N-rGO were determined by using ultraviolet photoelectron spectroscopy (UPS) (Figure S23). The work functions were estimated using the following equation: Φ=hν-|Eonset -EF|

(6)

, where hν=21.2eV (He I source), Eonset is secondary electron energy onset. The calculated values of work function (N-rGO = 4.5 eV, CNS = 5.2 eV) are well matched with the theoretical value, which indicates appropriate band position between CNS and N-rGO. Also, the N-rGO band gap was estimated as 0.7 eV from density functional theory (DFT) calculations (Figure S24). When Co atoms anchored on the N-rGO layer, covalent bonds formed between the Co atoms and the pyridinic N atoms, and band bending occurred correspondingly at the conduction band maximum (CBM) and valence band maximum (VBM) of N-rGO. In this band structure, free electrons in CNS transferred to the VBM of NrGO, which has a high concentration of holes (Figure 6(c)). The transferred electrons hop 21


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with holes in the VBM of N-rGO, and further conduct through the GC electrode. Therefore, bonding between Co atoms and the pyridinic N sites in the CNS–NGA system acts as an electron sink for OH– ions during the OER process; OH– electrons, adsorbed onto the surface of CNS clusters, effectively spread over the conductive N-rGO support and sink toward the GC electrode (Figures 5(e) and 6(c)). In order to investigate the effect of oxidation of CNS on the OER performance, we also calculated the adsorption energies of OH, O, OOH, and OO to obtain the energy OER landscapes for CNSO-NGA (partially oxidized CNS after long-term OER reaction) systems. The calculated free energy diagrams and the atomic structures of OH–, O–, OOH–, and OO– adsorbed on the CNS–NGA and CNSO-NGA systems with U = 0 and 0.7 V are presented in

Figure 7(a). Sequential adsorptions of OH, O, OOH, and OO on CNS-NGA and CNSO-NGA are expected to occur on the equivalent sites: OH and O on S-site, OOH on Co-Ni bridge site, and OO on Ni-site. Overall, the adsorptions of OH, O, OOH, and OO are stronger on CNSO, compared to CNS. From the calculated energy landscape, steps I and II reactions are found to be more favorable on CNSO surface due to lower energy barrier. However, steps III and IV reactions are expected to be more favorable on CNS at U=0V, due to higher energy barriers from step II to III on CNSO: 3.25 and 4.05 eV on CNS and CNSO, respectively (Figure 7(a)). With U = 0.7 V, reaction steps I and II become spontaneous on CNSO, as indicated by the negative energy change, while CNS has still energy barriers, 0.03 and 0.45 eV. In addition, the energy barriers from reaction step II to III are reduced to 2.55 and 3.35 eV on CNS and CNSO. It is important to note here that the metallic property of CNS is not significantly altered after partial oxidation as indicated by the calculated DOS of CNS and CNSO clusters (Figure S25). In addition, we examined whether changed stoichiometry of CNS by oxidation 22


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can affect the work function value and energetically stable, which is an important factor to determine electrocatalytic activity of this hybrid system. The work function of CNS is gradually increased with oxidation mole fraction, the substitution amount of O for S (Figure

7(b)). The increased work function of CNS after oxidation might facilitate electron transfer from CNS to NGA support as band bending at the interface between CNS and NGA can be more significant (Figure 6(c)). Furthermore, the oxidation of CNS is predicted to be stable only upto 17% of substitution of O for S, from the calculated oxidation energy, indicating partial oxidation of CNS may occur at the surface (Figure 7(b)). Then, the good OER performance of CNS-NGA after long-term test (Figure S17(a)) can be interpreted as the parallelized reaction pathways: reaction steps I and II dominantly take place at the oxidized region, which may facilitate the adsorption and desorption of OH and H atoms due to the amorphized structure of thin oxide layers, while reaction steps III and IV occur at CNS exhibiting high conductivity (Figure 7(c)). Thus, oxidation of CNS, which is unavoidable for OER reaction in alkaline solution, will not significantly degrade OER performance of CNSNGA hybrid system. According to DFT results, Co, Ni, and S in CNS were found to have different roles in OER. Firstly, Co atom in CNS was anchored on NGA with high binding energy (3.34eV) by forming covalent Co-N bond. Hence, Co in CNS is found to make strong linkage between CNS and NGA. The role of sulfur in OER steps becomes more important when CNS is junctioned to NGA. The steps I and II of OER occur mainly on sulfur site of CNS-NGA whether it is oxidized or not. This can be attributed to the fact that sulfur becomes more catalytically active by losing electron when CNS is junctioned with NGA because electron sinks toward NGA support through band bending. Thus, electron deficient sulfur atoms in CNS-NGA should play important role in facilitating OER, especially for steps I and II. Also, Co and Ni sites are both in charge of OER before partial oxidation occurs, since step 23


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III and IV are favored on Co and Ni sites, respectively (Figure 5(b)). Ni becomes more important catalytically active site for OER after partial oxidation occurs because step III and IV take place on Co-Ni bridge and Ni sites (Figure 7(a)). However, based on our experimental results, Co-rich CNS-NGA shows higher OER performance than Ni-rich CNSNGA hybrid. Thus, one need to pay more attention on the role of Co incorporated onto rGO layer for OER, since internal band bending is caused by forming covalent bond between Co and pyridinic N sites on rGO layer. As internal band bending occurs more strongly, electrons of sulfur atoms can be more efficiently sink toward NGA support resulting in higher activity of sulfur for OER (especially for step I). Hence, electron deficient S atoms in CNS nanoparticles and Co atoms anchored on rGO layer may have significant impact on boosting the OER activity for CNS-NGA hybrid. These results can give insight into the role of internal band banding at the interface between TMSs-carbon supports and the partially oxidized amorphous layer of TMSs for the OER, and therefore can open a route into interface engineering for the OER electrocatalysts.

4. CONCLUSION In conclusion, a Co–Ni–S ternary system anchored on porous N-doped reduced graphene aerogel was prepared by a facile one-step hydrothermal synthesis. The CNS–NGA hybrid catalyst enabled a highly efficient OER in alkaline solution. The CNS–NGA hybrid exhibits an overpotential of 330 mV yielded a current density of 10 mA cm−2 with a Tafel slope of 47 mV dec−1. The high performance of the CNS–NGA catalyst was attributed to the following factors: a 3D-porous structure with a high surface area, higher EESA, lower charge transfer resistance, and delocalized charge distribution at anchored Co atoms on the NGA supports. Our DFT calculations suggested that coupling of the CNS with NGA supports facilitated the 24


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formation metal-SC junction at the anchored Co atoms on a pyridinic N site. This junction acted as electron sinks for excess electrons of OH- ions adsorbed on the surface, which significantly enhance the OER performance of CNS-NGA composite. Moreover, the formation of OOHads, OHads and Oads is thermodynamically favored for the CNS–NGA, which led to faster OER kinetics compared with CNS, NGA, CS–NGA, and NS–NGA. Effect of partial oxidation of CNS on the OER performance was also investigated by calculating OER energy land scape and work function after partial oxidation. Results indicated that thin amorphous oxide layer on the surface of CNS, which is formed during long-term OER, may facilitate the adsorption and desorption of OH and H atoms favoring the OER reaction steps i) and ii), while step iii) and iv) may occur on the pristine CNS clusters. This parallelized OER reaction pathway at the interface between partially oxidized amorphous and pristine CNS phases can be responsible for a high OER performance of CNS-NGA hybrid even after longterm OER reaction. We believe that this work can open up new opportunities for designing highly efficient, low-cost catalysts for OER.

Supporting Information Experimental details for electrochemical measurements, computational calculations, ultraviolet photoelectron spectroscopy (UPS), and X-ray absorption spectroscopy (XAS) are summarized in Supporting Information.

Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1C1B2007299). Dr. H. Choi was supported by the Korean Institute of 25


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Science and Technology Institutional project (Grant No. 2E26130). Authors want to acknowledge Hosang Chun for helpful discussion on this manuscript.

Author Contributions H.H., H.C., K.M.K., T.S., and S.M. conceived the project. H.H. designed experiments and prepared the samples. H.C. designed and performed the computational calculations. Y.R.H., J.W.L., S.W.L, and J.H.R. participated in interpreting and analyzing the data. G.A and K.Y.C designed and analyzed X-ray absorption spectroscopy experiments. J.C and J.J designed and analyzed ultraviolet photoelectron spectroscopy experiments. All authors commented on the manuscript. H.H., H.C., K.M.K., T.S., and S.M. wrote the manuscript.

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FIGURES

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Scheme 1. Schematic illustration of the preparation of a Co–Ni–S ternary system anchored on a porous reduced graphene oxide aerogel (CNS–NGA) hybrid.

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Figure 1. a) Digital and b) scanning electron microscopy (SEM) images of the threedimensional (3D)-porous CNS–NGA hybrid. c) SEM image of the CNS nanoparticles anchored on the NGA supports. d, e) Low-magnification transmission electron microscopy (TEM) images of the CNS–NGA hybrid. f) High-magnification TEM image of the CNS nanoparticles and the corresponding selected area electron diffraction (SAED) pattern (inset). g) Elemental mapping images for Co, Ni, S, and N.

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Figure 2. XANES of (a) CNS and CNS-NGA at Co K-edge and (b) CNS and CNS-NGA at Ni K-edge plotted with reference spectra.

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Figure 3. . a, b) Oxygen evolution reaction (OER) polarization curves (scan rate 0.5 mV s−1) and the corresponding Tafel plots of the NGA, CNS, NS–NGA, CS–NGA, CNS–NGA, and commercial RuO2 catalysts loaded on the rotating disc electrode (2,000 rpm) in an O2saturated 1.0 M KOH solution. c) Comparison of the η10 values and Tafel slopes for the tested samplesd) Initial OER polarization curve of the CNS–NGA and after 1,000 cycles in 1.0 M KOH.

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Figure 4. a) Nyquist plots of the CNS–NGA (orange), NS–NGA (red), CNS (green), and RuO2 (black) systems. The inset shows the Randles equivalent circuit model. b) The current difference between the anodic and cathodic sweeps as a function of scan rate. The dashed line is a linear fitting of the data; the slope of the fitted line was used to calculate the electrochemical effective surface area.

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Figure 5. a) Calculated energy landscapes for the NGA, CNS, and CNS–NGA systems with U = 0 and 1.23 V. b) Optimized structures of OH– adsorption on the NGA, CNS, and CNS– NGA systems at the four OER reaction steps. Isosurfaces of an excess electron on adsorbed OH–. c) NDA, d) CNS, and e) CNS–NDG systems. The isovalue was set at 0.003 e/Å3.

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Figure 6. a) Charge distribution near a Co atom anchored on the NGA supports. The purple color indicates the charge accumulation region, and the pink color represents the charge depletion region. b) Computed density-of-state (DOS) plots of the Co 3d, C 2p, and N 2p orbitals for a Co atom anchored to NGA supports. The red box shows the overlapping DOS plots between Co and N atoms near the Fermi level. c) Band diagram for CNS-NGA composite before and after the junction formation.

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Figure 7. a) Calculated energy landscapes for the NGA, CNS, and CNS–NGA systems with U = 0 and 0.7 V. b) Work function and oxidation energy of CNS as a function of oxidation mole faction at S site. c) OER reaction pathways at CNS and CNSO boundary.

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Parallelized Reaction Pathway and Stronger ... - ACS Publications

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