CoOx Encapsulated in Nitrogen-, Oxygen

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Large-scale synthesis of Co/CoOx Encapsulated in N-, O-, and S-Tridoped 3D Porous Carbon as E#cient Electrocatalysts for Hydrogen Evolution Reaction Tao Zhang, Yiqiang Sun, Lifeng Hang, Yu Bai, Xinyang Li, Lulu Wen, Xiaomin Zhang, Xianjun Lyu, Weiping Cai, and Yue Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01272 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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ACS Applied Energy Materials

Large-scale synthesis of Co/CoOx Encapsulated in N-, O-, and S-Tridoped 3D Porous Carbon as Efficient Electrocatalysts for Hydrogen Evolution Reaction Tao Zhang,ab Yiqiang Sun,ab Lifeng Hang,a Yu Bai,a Xinyang Li,a Lulu Wen,ab Xiaomin Zhang,c Xianjun Lyu,d Weiping Cai,a and Yue Li*a a Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, P. R. China b University of Science and Technology of China, Hefei, 230026, P. R. China c College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, Xi’ an, 710055, P.R. China d College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, P.R. China

KEYWORDS: filter paper, pyrolysis, porous, tridoped carbon, hydrogen evolution reaction ABSTRACT: A facile and scalable one-step thermal treatment method is developed to produce Co/CoOx encapsulated in N-, O-, and S-tridoped three-dimensional porous carbon material to catalytic hydrogen evolution reaction. Notably, the process is simply performed via pyrolysis of

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solid carbon source of filter paper with adsorbed moderate cobalt chloride and thiourea. Benefiting from the high conductivity and porosity of N-, O-, and S-tridoped three-dimensional porous carbon, the high stability of carbon-protected nanomaterials and the strong interactions between Co and CoOx, the resulting Co/CoOx encapsulated in N-, O-, and S-tridoped threedimensional porous carbon material (synthesized at 900 °C) can yield the current densities of 10 mA cm-2 at a small overpotential of 61 mV and show a low Tafel slope of 78 mV dec-1, as well as a considerable stability in alkaline medium (1.0 M KOH solution) for hydrogen evolution reaction. Compared with other referential catalysts, the electrochemical impedance spectroscopy of Co/CoOx encapsulated in N-, O-, and S-tridoped three-dimensional porous carbon material (synthesized at 900 °C) further reveals a favorable kinetics during electrolysis. Moreover, these findings may inspire the exploration of low-cost, efficient and high yield three-dimensional porous electrodes for practical hydrogen production. 1. INTRODUCTION Ever-increasing energy security and environmental issues have motivated extensive attention on the development of alternative clean and sustainable energy sources.1-8 Hydrogen (H2) can serve as a desired alternative for fossil fuel, owing to its high gravimetric energy density, environment friendly and renewability.9-12 Among various hydrogen production strategies, electrochemical water splitting technique is extraordinarily promising for the future hydrogen economy because it can be easily connected with other renewable sources such as solar and wind.13,14 Although platinum (Pt)-based nanomaterials are most effective hydrogen evolution reaction (HER) electrocatalysts in acidic condition, their high cost and poor durability restrict their large-scale applications.15-17 Furthermore, hydrogen generated from water using alkaline electrolysis possess enormous potential in industries by consuming a moderate amount of


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energy.18,19 As a consequence, novel low-cost and earth-abundant HER electrocatalysts under alkaline environment, which possess comparable catalytic performance to Pt, are extremely desired to accelerate the development of green energy conversion systems.20-24 Recently, plenty of efforts have been concentrated on exploring the noble-metal-free electrocatalysts for alkaline water electrolysis, and a great achievement has been made through developing highly active novel low-cost materials for catalysis. Among various earth-abundant electrocatalytic materials including transition metal sulfides,25,26 selenides,27-29 oxides,30-32 and phosphides,33-34 have been generally considered as attractive candidates for Pt-based HER electrocatalysts because of their high activity. In terms of the stability of electrocatalysts, recent researches showed that carbon encapsulated nanoparticles (NPs) can effectively preserve catalytic performances in rigorous electrochemical reaction environment.35-40 Additionally, heteroatoms-doped (N, O, and S) carbon nanomaterials can possess unique electronic structures, which can promote the adsorption of intermediates on the surfaces of material during electrocatalytic reaction and improve the electronic conductivity between catalyst surfaces and reaction intermediates, resulting in enhanced electrocatalytic activity and durability.41-45 In previously reports, metallic cobalt (Co) has been proven to possess a proper energy barrier for H adsorption, which is closed to that of Pt,18,46 while the CoO and Co3O4 (CoOx) can preferentially adsorb OH− generated by H2O decomposition owing to strong electrostatic affinity between Co2+/Co3+ and OH−,31,47 leading to valid strong interactions of metal Co and CoOx for HER. These findings indicated that the combination of the protective heteroatom-doped carbon layers and the strong interactions of metallic Co and CoOx could further enhance HER process. However, due to the limitation of present synthesis procedures, the N-, O-, and S-tridoped


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carbon-encapsulated Co/CoOx nanomaterials with electrocatalytic activities toward HER have not been realized. Herein, we develop a facile straightforward approach to the synthesis of Co/CoOx NPs encapsulated in N-, O-, and S-tridoped three-dimensional (3D) porous carbon (Co/CoOx-NOSC) materials through filter paper adsorbed moderate cobalt chloride and thiourea for the first time, followed by one-step thermal treatment. Notably, the O atoms in carbon were derived from filter paper consisted of cellulose with many hydroxyls, while the N and S atoms came from thiourea. On account of the presence of cellulose, the valence state of cobalt can be easily tuned via pyrolysis treatment. The resulting Co/CoOx-NOSC-900 (synthesized at 900 °C) electrode can achieve current densities of 10 mA cm-2 at a small overpotential of 61 mV and show a low Tafel slope of 78 mV dec-1, as well as long-term cycling performance in alkaline medium for hydrogen evolution reaction. The intrinsic HER performance of Co/CoOx-NOSC-900 electrocatalyst could be attributed to the high conductivity and porosity of N-, O-, and S-tridoped 3D porous carbon structure, the strong interactions of metallic Co and CoOx, and the enhanced stability of carbonencapsulated NPs. The present straightforward method developed in this study is suitable for synthesizing more cost-efficient, large-scale non-precious electrocatalysts for industrial hydrogen production. 2. EXPERIMENTAL SECTION 2.1. Materials. Cobalt chloride hexahydrate (CoCl2·6H2O), thiourea (CH4N2S), potassium hydroxide (KOH), ethanol (C2H5OH) were all purchased from Sinopharm Chemical Reagent Corporation. Filter papers (quantitative # 203) were obtained from Hangzhou Xinhua Paper Industry Co., Ltd., China. Nafion (5.0 wt%) and commercial Pt/C (20 wt%) were purchased from


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Sigma-Aldrich. Deionized water (H2O) was generated via a Milli-Q water purification system. All chemicals were used without further purification. 2.2 Synthesis of Co/CoOx encapsulated in N-, O-, and S-tridoped 3D porous carbon. Co/CoOx encapsulated in N-, O-, and S-tridoped 3D porous carbon materials were prepared via one-step pyrolysis of solid carbon source from filter paper with adsorbed moderate cobalt chloride and thiourea. Typically, the filter paper (size: 1 × 2 cm) was immersed in ethanol and underwent ultrasonic treatment for 30 min and then dried in an oven at 60 oC for 1 h. 0.47 g CoCl2·6H2O and 1.22 g CH4N2S were dissolved in 10 mL deionized water. The cleaned filter paper was soaked in above mixture solution for 3 h (Scheme 1a-1c). After that, the tissue paper was used to remove the excess mixture solution and then the filter paper with mixtures was dried at 50 °C for 2 h in an oven and finally annealed at of 700, 800, 900, and 1000 °C in a tubular furnace under N2 flow for 2 h with a heating rate 2 °C min-1 to obtain the final products, which was designated as Co/CoOx-NOSC-T (Co/CoOx-NOSC-700, Co/CoOx-NOSC-800, Co/CoOxNOSC-900, and Co/CoOx-NOSC-1000, respectively), as indicated in Scheme 1d and 1e. Additionally, filter paper with only adsorbed CoCl2·6H2O or CH4N2S were annealed at 900 °C with the same condition to obtain Co/CoOx-OC-900 and NOSC-900, respectively. In order to prove the enhanced stability of catalysts induced by carbon shells, we synthesized Co/CoOx NPs depositing on the outer surface of the NOSC material. Firstly, the Co NPs were synthesized. 0.1g sodium dodecyl sulfate (SDS), 0.848 g NaH2PO2·6H2O, and 0.475g CoCl2·6H2O were dissolved in 10 mL deionized water (solution A). 0.05 g NaOH was dissolved in 5 mL DMF (solution B). The mixed solution of A and B was then transferred into a Teflonlined stainless steel autoclave and kept in an oven at 160 ºC for 10 h. After cooling down to room


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temperature, the Co NPs were obtained. Then the as-prepared Co NPs were exposed to air at 300 °C for 30 min to get Co/CoOx NPs. 2.3 Characterization. The products were characterized by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM, Sirion 200). Energy dispersive X-ray spectroscopy (EDS) elemental mapping images of final products were performed by transmission electron microscopy (FEI, Tecnai G2 F20). X-ray diffraction (XRD) data were performed on a Philips X’pert Pro X-ray diffractometer with Cu Kα radiation (λ=0.15419 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALABMK II X-ray photoelectron spectrometer, in which the Mg served as the exciting source. The nitrogen adsorption and desorption isotherms of the various materials were obtained with a N2 porosimeter (TriStar II 3020, Micromeritics Instrument Corporation). From the adsorption/desorption data, the Brunauer-Emmett-Teller (BET) surface areas of the materials were then determined. All the electrochemical measurements were conducted with a threeelectrode configuration on CHI 760e electrochemical workstation. 2.4 Electrochemical measurements. The electrocatalyst ink was made by adding 10 mg catalytic material into 500 µL solution consist of deionized water, absolute ethanol, and Nafion (5%) (v/ v / v = 8 : 1 : 1) under sonication for 30 min. 100 µL electrocatalyst ink was uniformly pipetted onto a Ni foam substrate (1 × 2 cm) for catalysis. The electrocatalytic activity toward HER was evaluated with a traditional three-electrode cell configuration by using a CHI 760e electrochemical workstation in 1 M KOH at room temperature. During this electrochemical test, catalyst coated on Ni foam was used as the working electrode (WE), graphite rod was employed as the counter electrode (CE), and a saturated Ag/AgCl electrode (RE) was employed as the reference electrode. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS),


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chronopotentiometric measurements were performed, and linear sweep voltammetry (LSV) was conducted at a scan rate of 2 mV s-1. All the potentials were converted to values with the reference to a reversible hydrogen electrode (RHE). All potentials and voltages were IR corrected.

Scheme 1. Schematic illustration of the synthesis of Co/CoOx-NOSC: (a) filter paper, (b) tailored filter paper, (c) filter paper with adsorbed moderate cobalt chloride and thiourea, (d) waterless filter paper with moderate cobalt chloride and thiourea, and (e) Co/CoOx-NOSC obtained after pyrolysis in N2. 3. RESULTS AND DISCUSSION As demonstrated in Scheme 1, the fabrication of Co/CoOx-NOSC-900 material was realized by facile pyrolysis treatment of the low-cost initial materials (filter paper, CoCl2·6H2O and CH4N2S) at 900°C in N2 atmosphere. Figure 1a and 1b show the representative field emission scanning electron microscopy (FESEM) images of the as-prepared Co/CoOx-NOSC-900. They reveal that the Co/CoOx nanoparticles with average diameter of 35.3 nm were embedded in the three-dimensional (3D) porous carbon. The further structural information about Co/CoOxNOSC-900 was interpreted by the XRD patterns (Figure 1c). Particularly, the obvious broader


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diffraction peak located at 25.8° corresponds to the plane of C (002), suggesting the presence of graphitic carbon after the pyrolysis. Moreover, the peaks located at 44.3°, 51.5° and 75.9° can be ascribed to Co (111), (200), and (220) planes, the distinct peaks at 36.5° and 42.4° are well coincident with CoO (111) and (200) planes, and the peaks appeared at 36.9° and 55.7° belong to Co3O4 (311) and (422) planes, indicating the co-existence of Co, CoO, and Co3O4. To further reveal the morphology and structure of Co/CoOx-NOSC-900, TEM and HRTEM are shown in Figure 1d-1f. Low-resolution TEM image displays that the NPs were clearly encapsulated in 3D porous carbon (Figure 1d), which was consistent with the previous SEM images. The HRTEM image in Figure 1e clearly reveals that lattice fringe spaces of 0.205 nm consistent with the (111) plane of the Co crystal, and also discovers the resolved lattice fringes of CoO (111) and (200) planes with a spacing of 0.246 and 0.213 nm, respectively. What’s more, a distinctive lattice fringe with an interplanar distance of 0.244 nm are also observed, corresponding to (311) facets of typical Co3O4 crystal structure. It is also evident that Co/CoOx NPs are embedded in 3D porous carbon, which is beneficial for the stability of catalysts. The corresponding energydispersive X-ray spectroscopy (EDS) elemental mapping analyses performed by HAADF-STEM demonstrate that C (red), N (blue), O (yellow), and S (orange) were homogeneously distributed in the hybrids (Figure 1g), proving the N-, O-, and S-tridoped 3D porous carbon. It was clearly seen that the Co (green) and O (yellow) intensively distribute only in few regions, indicating formation of CoOx NPs. Besides, the Brunauer_Emmett-Teller (BET) surface areas of NOSC900 (Figure S1), Co/CoOx-OC-900 (Figure S2), and Co/CoOx-NOSC-900 products obtained from the N2 adsorption and desorption isotherms are 287.8, 277.8, and 492.9 m2/g, respectively, indicating Co/CoOx-NOSC-900 with the highest BET surface area (Figure S3).


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Figure 1. (a) Low- and (b) high-magnification FESEM images of Co/CoOx-NOSC-900 achieved by annealing the filter paper adsorbed moderate cobalt chloride and thiourea at 900 °C for 2 h. (c) Typical XRD pattern of Co/CoOx-NOSC-900. (d)-(f) TEM and HRTEM images of Co/CoOxNOSC-900. (g) HAADF-STEM image and EDS elemental mapping of Co/CoOx-NOSC-900. Furthermore, X-ray photoelectron spectroscopy (XPS) was performed to identify the surface compositions and the chemical states of Co/CoOx-NOSC-900 materials. As illustrated in Figure S4, the XPS survey spectrum showed main peaks centered at 164.8, 284.6, 400.2, 531.8 and 780.9 eV, which could be assigned to S2p, C1s, N1s, O1s, and Co2p, respectively. S and N atoms were generated from CH4N2S and O atoms in the materials were due to cellulose with many hydroxyls, which were commonly observed in carbon materials.48 The high-resolution XPS spectra of C1s for Co/CoOx-NOSC-900 was displayed in Figure 2a. All the C1s peaks are further deconvoluted into four peaks, corresponding to C-C (284.6 eV), C-S (285.7 eV), C-O/C-


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N (286.4 eV) and O-C=O (289.3 eV) species, respectively.42,44 The observed peaks associated with C-N ， C-O and C-S moieties suggested that N, O, and S atoms were successfully incorporated into the carbon materials. Moreover, the high-resolution N 1s spectra (Figure 2b) can be well-matched into three special peaks at 398.3, 400.1 and 401.3 eV, corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, respectively.49,50 In case of deconvoluted S2p spectra, two peaks located at 164.0 and 165.2 eV show an intensity ratio of 2:1, well coincident with S2p3/2 and S2p1/2, revealing the formation of thiophene-like structures between S atoms and their neighboring C atoms arising from spin-orbit coupling.44,51 In addition, the presence of metallic Co and CoOx is also confirmed by XPS technique. As indicated in Figure 2d, the two peaks located at 778.7 eV and 793.7 eV were attributed to Co2p3/2 and Co2p1/2, which was signed to Co phase. Moreover, the peaks around 781.1 and 796.7 eV are ascribed to Co3O4 phase. On the basis of integral area, the proportion of Co3+/Co2+ is 1.18, further implying the presence of Co3O4 and CoO.18 All of these observations manifest that Co/CoOx-NOSC materials have been successfully fabricated via one-step thermal treatment strategy.


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Figure 2. XPS spectra of the Co/CoOx-NOSC-900: (a) C1s, (b) N1s, (c) S2p, and (d) Co2p energy regions. The electrocatalytic activities toward hydrogen evolution reaction (HER) of the NCOS-900, Co/CoOx-OC-900, and Co/CoOx-NOSC-900 catalysts uniformly pipetted onto Ni foam (NF) substrates were systematically investigated in a 1.0 M KOH solution using a typical threeelectrode system. For comparison, bare NF and Pt/C electrodes were also measured at the same condition. As shown in Figure 3a and 3b, the linear sweep voltammetry (LSV) curves and histogram of the bare NF and NOSC electrodes exhibited inappreciable catalytic performance, whereas their current density clearly increases for Co/CoOx-OC-900 and Co/CoOx-NOSC-900 materials. One can find that a very small overpotential of 61 mV can drive the current density of 10 mA cm−2 for the Co/CoOx-NOSC-900 electrode, but higher overpotentials of 162, 270, and 304 mV are required for Co/CoOx-OC-900, NOSC, and NF electrodes, respectively. The HER behavior of Co/CoOx-OC-900 was superior to that of recently reported HER catalysts, requiring a smaller overpotential to drive the current density of 10 mA cm−2 under alkaline media (Table S1). The Tafel plot is an effective tool to estimate the HER reaction kinetics of electrocatalysts. In this study, the Tafel slope of Co/CoOx-NOSC-900 is 78 mV dec-1, smaller than that of Co/CoOx-OC-900 (96 mV dec-1), NOSC (124 mV dec-1), and NF (150 mV dec-1), respectively, which indicates the comparatively favorable catalytic kinetics, as shown in Figure 3c. The Tafel slope value of the Co/CoOx-NOSC-900 also indicated that the Volmer-Heyrovesky pathway was the prominent rate limiting step for the HER.18,52 Moreover, the turnover frequency (TOF) value Co/CoOx-NOSC-900 was calculated to be 0.088 s-1 (at an overpotential of 100 mV), which was higher than some recent reported work (Table S2). The small overpotential, lower Tafel slope,


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and higher TOF indicated that Co/CoOx-NOSC-900 showed good electrocatalytic activity toward HER. Electrochemical impedance spectroscopy (EIS) measurements were investigated to assess the HER kinetics of the catalysts at the electrode/electrolyte interface, as shown in Figure 3d. The Nyqusit plots reveal that the diameter of the semicircle of the Co/CoOx-NOSC-900 was obviously smaller than that of Co/CoOx-OC-900, NOSC, and NF, suggesting that the Co/CoOxNOSC-900 catalyst possesses a faster charge-transfer capacity from strong interactions between NOSC layers and Co/CoOx NPs during the HER process. Electrochemical double-layer capacitance (Cdl) measurements were conducted to confirm Co/CoOx-NOSC-900 with high exposure active sites. The results show that Co/CoOx-NOSC-900 (7.8 mF cm-2) exhibit a 3.3, 8.7 and 9.8 times Cdl than Co/CoOx-OC-900 (2.4 mF cm-2), NOSC (0.9 mF cm-2), and NF (0.8 mF cm-2), respectively, suggesting that Co/CoOx-NOSC-900 had more exposed catalytically active sites for electrolysis during HER (Figure 3e and Figure S5). It is worth noting that the stability of an electrochemical catalyst is another significant indicator for practical applications. Here, for the sake of assessing the electrochemical stabilities of the Co/CoOx-NOSC-900 electrodes, longterm cyclic voltammogram (CV) cycling and chronoamperometric response measurements were further performed, respectively. The current density of Co/CoOx-NOSC-900 electrode exhibited negligible degradation after 2000 CV cycles, as illustrated in Figure 3f. Moreover, their stability was also evaluated by a chronopotentiometric method. The long-term stability was evaluated by sustaining a current density of 10 mA cm-2 for 20 h. Figure S6 shows that the initial current was retained without visible change after 20 h, indicating that the Co/CoOx-NOSC-900 possesses excellent stability. Moreover, the as-prepared Co/CoOx NPs were deposited onto the outer surface of the NOSC material, proved by the SEM image and XRD pattern (Figure S7). Their


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stability test exhibited that the current density (10 mA cm-2) dropped ca. 50 % during the 20 h continuous test, showing more decarase than Co/CoOx-NOSC-900 (the current density dropping 10% at 10 mA cm-2). This indicated that the carbon shells-coated metal NPs structure was beneficial for the stability. The SEM, STEM and XRD analyses further reveal that the morphology and structure of the Co/CoOx-NOSC-900 material remained unchanged after the long-term HER catalysis (Figure S8). Based on above results, the as-obtained Co/CoOx-NOSC900 electrode has been demonstrated to be an effective electrocatalyst for enhancing HER performance under alkaline condition.

Figure 3. (a) IR-corrected LSV curves for HER and (b) Overpotential at current density of 10 mA cm−2 of NF, NCOS-900, Co/CoOx-OC-900, Co/CoOx-NOSC-900, and Pt/C. (c) Tafel plots,


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(d) EIS, and (e) Capacitive currents as a function of scan rate of NF, NCOS-900, Co/CoOx-OC900 and Co/CoOx-NOSC-900. (f) HER polarization curves of Co/CoOx-NOSC-900 before and after 2000 CV tests. All the measurements were tested in 1.0 M KOH solution at a scan rate of 2 mV s-1. In order to investigate the influence of the annealing temperature on the HER performances, the Co/CoOx-NOSC electrocatalysis were obtained by only changing the pyrolysis temperature. As shown in Figure 4, with increasing pyrolysis temperature, the average particle size of these materials gradually increased from 14.1, 26.4 35.3 to 73.7 nm, which may reduce the catalytic activity of materials. Additionally, the porous carbon structure exhibited perfectly at 900 °C. This is due to that carbon thermal reduction at relatively low temperature could not cause obvious consumption of carbon. However, the higher temperature extremely raised the consumption of carbon, leading to the destruction of 3D porous carbon. The corresponding XRD patterns of the Co/CoOx-NOSC-T materials illustrated that all the materials exhibited similar XRD patterns, with peaks that can be assigned to Co, CoO, and Co3O4 phase (Figure S9). The XPS results were used to analyze the surface compositions of Co/CoOx-NOSC-T materials. The N:C and S:C atomic ratios were found to decrease significantly (from 5.6% to 2.6% and from 1.7% to 1.1%, respectively) when the annealing temperature was increased from 700 to 1000 °C, suggesting that more N and S atoms were lost during higher temperature pyrolysis (Figure S10). However, the contents of pyridinic-N follow the opposite trend with the temperature increasing from 700 to 900 °C, when the temperature was increased to 1000 °C, the percentage of pyridinicN was slightly decreased, the percentage of pyridinic-N at 900 °C reached the highest value (Figure S11). It is reported that the pyridinic-N can enhance the electron transfer, boosting the HER activity.53,54 Raman spectra of all the Co/CoOx-NOSC obtained at different annealing


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temperatures show two remarkable peaks around 1555 and 1310 cm-1, which can be ascribed to the graphitic G band and the disordered structures/structural defects (or D band), respectively (Figure S12). Moreover, the intensity ratio (ID/IG) of the D and G bands of Co/CoOx-NOSC- 700, Co/CoOx-NOSC-800, Co/CoOx-NOSC-900, and Co/CoOx-NOSC-1000 are 0.81, 0.92, 0.96, and 1.11, respectively. The increase of ID/IG is due to the formation of defects in graphitic carbon, further confirming the successful doping of N, O, and S atoms.42,54 Besides, the BET surface areas of Co/CoOx-NOSC-700, Co/CoOx-NOSC-800, Co/CoOx-NOSC-900, and Co/CoOxNOSC-1000 products obtained from the N2 adsorption and desorption isotherms were 418.3, 422.9, 492.9, and 275.2 m2/g, respectively, indicating Co/CoOx-NOSC-900 with the highest BET surface area (Figure S13).

Figure 4. SEM images of the Co/CoOx-NOSC synthesized at different annealing temperature: (a) 700, (b) 800, (c) 900, and (d) 1000 °C. The insets were the corresponding particle size distribution histograms.


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As indicated in Figure 5a, with increasing annealing temperature from 700 to 900 °C, the HER onset potential of these materials shifted to more negative values and the current density at a given potential increased gradually while that slightly decreased at 1000 °C. In particular, the overpotential needed for Co/CoOx-NOSC-900 for HER at a current density of 10 mA cm-2 was 61 mV, which was much smaller than those needed for other catalysts (92 mV for Co/CoOxNOSC-700, 74 mV for Co/CoOx-NOSC-800, and 79 mV for Co/CoOx-NOSC-1000) under the same conditions, as shown in Figure 5b. The Tafel plots of these Co/CoOx-NOSC-T electrodes were performed to estimate the HER reaction kinetics, as illustrated in Figure 5c. Compared with the Tafel slope of Co/CoOx-NOSC-700 (116 mV dec-1), Co/CoOx-NOSC-800 (93 mV dec-1), and Co/CoOx-NOSC-1000 (99 mV dec-1), the Co/CoOx-NOSC-900 exhibited the smallest Tafel slope (78 mV dec-1), showing the comparatively better reaction kinetics. Besides, EIS measurements were also carried out on the Co/CoOx-NOSC-T electrodes. The as-obtained Nyqusit plots indicated that Co/CoOx-NOSC-900 showed a relatively smaller diameter of the semicircle than Co/CoOx-OC-700, Co/CoOx-OC-800, and Co/CoOx-OC-1000, suggesting that the Co/CoOx-NOSC-900 catalyst possesses a faster charge-transfer capacity (Figure 5d). The best catalytic performance of Co/CoOx-NOSC-900 was caused by the synergy of more defects in graphitic carbon, smaller size of Co/CoOx NPs, highest BET surface area, and highest percentage of pyridinic-N.


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Figure 5. (a) IR-corrected LSV curves of Co/CoOx-NOSC-T materials annealed at 700, 800, 900, and 1000 °C. (b) Potentials required over Co/CoOx-NOSC-T materials to produce a 10 mA cm-2 current densities. (c) Tafel plots of Co/CoOx-NOSC-T materials. (d) The corresponding EIS tested in 1.0 M KOH solution at a scan rate of 2 mV s-1. Based on the above results, the excellent catalytic activity toward HER exhibited by Co/CoOx-NOSC-900 can be attributed to the following factors. Firstly, it has been proved the metallic Co possess a proper hydrogen adsorption free energy,18,46,55 and CoO and Co3O4 species can be beneficial for dissociating H2O.31,47 As a result, the CoO and Co3O4 can preferentially adsorb OH− species derived from H2O splitting owing to strong electrostatic affinity between the Co2+/Co3+ and OH−. Meanwhile, the metallic Co around the CoOx could availably promote H adsorption, which can facilitating the HER process through the valid strong interactions of Co and CoOx.18 Secondly, Co/CoOx NPs embedded in the N-, O-, and S-tridoped 3D porous carbon were prevented from aggregation, resulting in the high BET surface area and improvement of electron transmission efficiency.42-44 The 3D porous carbon is beneficial for the ions transport owing to its interconnected nanochannels.56 Meanwhile, Co/CoOx-NOSC-900 exhibited a


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relatively larger effective electrochemical surface area (ECSA), exposing more catalytically active sites for electrolysis during HER. Thirdly, it has been proven heteroatom doping in carbon structure could tune the electronic structures of carbon.45,57-59 The reported work showed that the existing N-containing species and S-doping in the porous carbon could serve as electrochemically active sites to boost the HER activity.53,60-63 Thus, proper amount of N, O, and S dopant in carbon materials can not only results in unique electronic structures and lower local work function but also cause amounts of defects in the carbon, which finally leads to the enhanced electrochemical activity. 4 Conclusions In summary, a kind of 3D porous Co/CoOx-NOSC electrocatalysts composed of only earthabundant elements have been successfully synthesized with a simple and scalable one-step thermal treatment method. On account of the presence of cellulose, the valence state of cobalt can be tactfully tuned via pyrolysis. Particularly, Co/CoOx-NOSC-900, which was achieved at annealing temperature of 900 °C, has been found to serve as an efficient electrocatalyst for HER in alkaline medium with a near-zero onset potential, small overpotential of 61 mV at a current density of 10 mA cm-2, low Tafel slope of 78 mV dec-1, faster charge-transfer capacity, as well as larger ECSA. It also displays excellent stability after long-term electrochemical tests. The excellent electrocatalytic activity toward HER exhibited by Co/CoOx-NOSC-900 has been attributed to the synergistic effects between the N-, O-, and S-tridoped carbon layers and the Co/CoOx NPs in maintaining both outstanding conductivity and amounts of surface active sites. With the good electrochemical performance of materials and environment friendly synthesis method, this work will open up new, facile opportunities for us to design earth-abundant, costefficient and large-scale non-precious electrocatalysts for realizing hydrogen economy.


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ASSOCIATED CONTENT Supporting Information.

Additional TEM, HAADF-STEM, FESEM, EDS mapping data, SERS result are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Yue Li); Author Contributions All the authors have made contributions to the manuscript and given their approval of the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge financial support from the National Key Research and Development Program of China (Grant No. 2017YFA0207101), the Natural Science Foundation of China (Grant Nos. 51771188, 51571189), the Major Program of Development Foundation of Hefei


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Table of Contents Entry

A facile and scalable one-step thermal treatment method is developed to produce Co/CoOx encapsulated in N-, O-, and S-tridoped 3D porous carbon material (Co/CoOx-NOSC) applied to efficient hydrogen evolution reaction (HER).


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CoOx Encapsulated in Nitrogen-, Oxygen

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