C bifunctional electrocatalysts for alkaline

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N-doped 3D porous Ni/C bifunctional electrocatalysts for alkaline water electrolysis Jieting Ding, Shan Ji, Hui Wang, Vladimir Linkov, Hengjun Gai, Fusheng Liu, Quanbing Liu, and Rongfang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05264 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Sustainable Chemistry & Engineering

N-doped 3D porous Ni/C bifunctional electrocatalysts for alkaline water electrolysis Jieting Ding,1 Shan Ji,1,2,* Hui Wang,1 Vladimir Linkov,3 Hengjun Gai,1 Fusheng Liu,1 Quanbing liu,4,* Rongfang Wang1*

1

State Key Laboratory Base for Eco-Chemical Engineering, College of Chemical

Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China 2

College of Biological, Chemical Science and Chemical Engineering, Jiaxing University,

Jiaxing, 314001, China 3 South

African Institute for Advanced Materials Chemistry, University of the Western

Cape, Cape Town, 7535, South Africa. 4

School of Chemical Engineering and Light Industry, Guangdong University of

Technology, Guangzhou 510006, China *E-mail: jishan@mail zjxu.edu.cn; [email protected]; [email protected]

Abstract: 1

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Water electrolysis offers a promising approach towards future sustainable energy mix utilizing hydrogen as energy carrier. Bifunctional catalyst which is active in both oxygen and hydrogen evolution reactions can significantly enhance total electrochemical water splitting efficiency and simplify electrolysis systems. In this study, nitrogen doped nickel containing porous carbon networks were prepared to fulfil this role. Peptone was used as carbon and nitrogen source during the synthesis and NaCl presence as a template resulted in the formation of open mesopores interconnected by thin carbon sheets with nano-sized Ni particles encapsulated in the carbon matrix. A low onset potential of 53 mV and a charge transfer resistance of 3.8 Ω with excellent cycling stability were demonstrated when the optimized catalyst Ni-2 was applied in HER. It was also shown that Ni-2 onset potential, charge transfer resistance and durability in OER were comparable to those of a commercial RuO2 catalyst. During electrolysis study conducted at 10 mA cm-2, Ni-2 maintained a cell voltage of 1.63 V with remarkable stability during 25 h operation. Keywords: Electrolysis; N containing carbon; Transition metal nanoparticles; Hydrogen generation; Oxygen evolution reaction. Introduction Energy security, universal access to energy and fossil fuels related emissions reduction challenges are being addressed through a worldwide roll-out of renewable and clean energy sources.1-3 The use of hydrogen as energy carrier which can be produced via direct splitting of water using excessive renewable power in an 2


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electrolyser is a promising option to reduce humankind’s dependence on diminishing fossil fuels and mitigate the global warming.4, 5 During water electrolysis, hydrogen evolution reaction (HER) occurs on the cathode accompanied by oxygen evolution reaction (OER) on the anode.6 Due to high overpotentials occurring on both cathodes and anodes, effective but costly PGM catalysts are required to improve HER and OER reaction rates, which complicates widespread applications of this green energy technology.7,

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Thus, the need to develop and design new PGM-free electrolysis

catalysts have attracted much research attention over the last decade.9, 10 Transition metal compounds, such as nitrides,11 phosphides,12 sulphides13 and selenides14 are becoming promising substitutes for PGM- based HER and OER catalysts in a wide pH range. However, in terms of their catalytic durability and activity, these new materials still do not match noble metal catalysts.15 Particles of transition metal compounds easily aggregate and lose their surface area and active sites. One effective way to address this limitation is to encapsulate transition metal nano-particles inside much more stable carbon materials. It was found recently that carbon compounds containing both metal and non-metal elements are promising electrolysis catalysts with the possibility of replacing PGM in these applications. These materials include N, P, S and Se doped carbon nanostructures,16-18 metal/non-metal compounds19-22 and carbon-encapsulated metal/metal oxides.23, 24 It was also discovered that strong interaction between transition metals and carbon networks resulted in significant electrochemical activity and stability of new catalysts.17 3



Additionally, the presence of highly porous carbon improves mass transfer and desorption of hydrogen and oxygen from electrocatalyst surfaces.25, 26 Therefore, the combination of transition metals with porous carbon materials improves the performance of electrolysis catalysts by complementing active sites on their surfaces.4, 24, 27 It was reported that in addition to avoiding particle aggregation, this combination improved electrocatalytic activity due to synergistic effect between carbon and deposited matter.28 Electrochemical performance of metal/carbon compounds was effectively tailored by fine tuning carbonaceous structures.29 Furthermore, a single catalyst with dual functionality to generate both hydrogen and oxygen simultaneously would minimize reaction overpotential and significantly simplify the overall water splitting system. Despite these advantages, transition metal catalysts have not been widely applied in electrochemical water splitting processes due to their complicated preparation procedures involving loss of active sites as well as high cost of their synthesis precursors. Therefore, simple and low cost synthesis method for carbon supported bifunctional catalysts preparation with numerous transition metal containing active sites could be of significant importance for achieving wider application of electrochemical water splitting. Here, Ni nanoparticles encapsulated by a 3D nitrogen-containing porous carbon network were prepared via a simple single step method utilizing peptone as a source of carbon, NaCl as a template and Ni(NO3)2·6H2O to introduce Ni in the porous structure. Ni nanoparticles were evenly distributed in newly obtained materials and encapsulated in 3D nitrogen enriched porous carbon. Because of the synergetic 4


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effect and increased surface area, as-prepared catalyst samples demonstrated elevated electrochemical activity in both HER and OER. Newly developed catalyst designated Ni-2 exhibited performance comparable to that of the commercial Pt/C in HER and RuO2 in OER for both stability in alkaline media and electrocatalytic activity. Experimental Synthesis To prepare three different catalyst samples, 0.2, 0.4 and 0.8 g of Ni(NO3)2·6H2O were added to 15 mL water and 1 g peptone powder (CAS 73049-73-7, grade R) was added to each solution. The homogeneous solutions were freeze-dried in liquid nitrogen. 10 g of NaCl was added to each dried mixture followed by separate ball-milling at 3000 rpm for 6 h. Obtained powders were heated up to 900 °C during 60 min in a furnace under the nitrogen atmosphere, rinsed thoroughly with water and dried in a vacuum oven at 80 °C overnight. The products were labelled as Ni-1 (0.2 g Ni(NO3)2·6H2O), Ni-2 (0.4 g Ni(NO3)2·6H2O) and Ni-3 (0.8 g Ni(NO3)2·6H2O). For comparison, pure carbon materials (designated as C) was synthesized in the same way without adding the Ni containing precursor. Characterization Shimadzu XD-3A X-ray diffractometer operating at λ = 0.15418 nm, 40 kV and 30 mA was used to study the crystal structure of obtained catalysts. Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and selected area electron diffraction (SAED) were performed using a JEM-2000 Electron Microscope. Brunauere-Emmette-Teller method (BET) and density functional theory 5



(DFT) were used to study specific surface areas and pore sized distributions on the Quantachrome Autosorb-1. The elemental composition of the catalysts was determined by X-ray photoelectron spectrometry (XPS) on a VG Escalab210 instrument. Elemental composition of the samples was determined from the data obtained on a Thermo Flash 2000 Organic Elemental Analyzer. Electrochemical analysis HER/OER electrocatalytic activity was evaluated in a three-electrode cell using the electrochemical workstation CHI660E, CH Instruments. The procedure for preparing electrode was as follows: catalyst (2mg) was added to 0.4 mL Nafion solution (0.25 % in ethanol) and ultrasonicated for 30 min. 8 μl of the ink was deposited on a rotating disc electrode to achieve catalyst loading of 0.204 mg cm-2 and dried at a room temperature. Pt/C and RuO2 containing electrodes were made in the same way. The cell was equipped by Hg/HgO (1 M KOH) reference electrode and a graphite counter electrode. N2 purged 1M KOH was used as electrolyte for all measurements. All obtained potentials were adjusted against the reversible hydrogen electrode (RHE) using the equation: ERHE = EHg/HgO + 0.059 pH + 0.14 V. All electrodes containing newly synthesised catalysts were prepared by first dispersing catalyst (2 mg), PTFE solution (3 μL) and acetylene black (1 mg) in isopropyl alcohol (300 μL) to form a paste. The films with consistent thicknesses were obtained by hand rolling the pastes on glass pans using a Swagelock pipe 2.5 cm in diameter. The films, dried at 80 oC in a vacuum oven, were pressed onto pieces 6


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of Ni foam at a pressure of 20.0 MPa. Pt/C (20 wt%, 2 mg, Johnson Matthey) and RuO2 (99.9 wt%, 2 mg, Aladdin) catalysts were also prepared using the same method.

Results and discussion

Figure 1. Catalysts synthesis routes. The preparation process for N-doped 3D porous Ni/C network is schematically illustrated in Figures 1 and S1 (Supporting Information). Initially, peptone powder was mixed with Ni(NO3)2·6H2O and freeze-dried with liquid nitrogen. Subsequently, NaCl, a template forming material, was added to the mixtures and ball-milled for 6 h to ascertain that the precursors and NaCl were completely mixed with each other. In the process of carbonization, NaCl acted as a template by forming nanodroplets and penetrating into the carbonized precursor at an elevated temperature, which enhanced the formation of pores in resulting carbon materials.30 Compared to using conventional organic compounds as templates, NaCl brings about low-cost and 7



simple procedure, since it is easy to remove from the final material without using high temperatures necessary to decompose organic molecules of more conventional templates.

Figure 2. SEM, TEM and HRTEM pictures of catalysts Ni-1 (a-c), Ni-2 (d-f) and Ni-3 (g-i); Inserts (b, e, h) are their particle size distributions. SEM and TEM micrographs revealing morphology and microstructure of as-prepared catalyst samples are presented in the supporting information section’s Figures S2a, 2a(S2b) and Figure 2d. Newly obtained materials possessed open-structured and porous networks constituted by interconnected thin carbon sheets. When Ni(NO3)2·6H2O content in the precursor was increased to 0.8 g, very dense, solid and irregular particles were formed in resulting Ni-3, as shown in Figure 2g. This indicates that high precursor concentrations of Ni(NO3)2·6H2O could impede the formation of porous structures in the catalysts. 8


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The SEM picture of the carbon prepared without NaCl presented in supporting information Figure S2c (Supporting Information), clearly demonstrates that the salt can act as efficient template to form an open porous structure where nanodroplets of NaCl forming at a high temperature assume a pore-forming role.31,

32

Without

using NaCl in the synthesis process only solid and dense carbon lumps were obtained. TEM micrographs of Ni-1, Ni-2 and Ni-3 shown in Figure 2 b, e and h prove that Ni nanoparticles were encapsulated in the carbon network. In Ni-1 and Ni-2 the particles appear quite uniform with their size distributions centered at ca. 16 nm. (The particle size calculation method is present in the (Supporting Information). However, as per Figure 2h, particle aggregation occurred when the Ni(NO3)2·6H2O precursor content was too high. It is visible from HR TEM images presented in Figure 2 c, f and i that Ni nanoparticles were wrapped by thin carbon shells. D-spacing of 0.2 nm, corresponding to Ni (111) lattice,33 is clearly visible in all three images. It is also clear that Ni nanoparticles are surrounded by ~10 layers of carbon shells with a spacing of 0.33 nm attributable to C (002) lattice.

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Figure 3. XRD of material C, and catalysts Ni-1, Ni-2 and Ni-3. XRD patterns of material C and catalysts Ni-1, Ni-2 and Ni-3 are shown in Figure 3 where all 3 Ni containing samples possess similar diffraction features, namely three characteristic peaks at 44.3 o, 52.0 o and 76.0 o. No diffraction peaks of nickel oxide were found in these XRD patterns. Adding to Ni peaks, the weak and broad peak at ca. 26

o

corresponding to C (002) plane was observed in XRD patterns of all four

samples, demonstrating that small quantities of graphite existed in them.

Figure 4. (a) N2 adsorption curves and (b) pore sizes of C, Ni-1, Ni-2 and Ni-3; (c,d) corresponding surface areas and pore volume change trends. N2 adsorption results representing materials’ porosity are given in Figure 4a. The curves are of a hybrid I/IV IUPAC type due to the presence of micropores and mesopores.34,

35

Obvious hysteresis loops appear at relatively high pressures in C,

and catalysts’ Ni-1 and Ni-2 isotherms due to high mesopore content of these 10


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materials which is further illustrated by pore size distributions presented in Figure 4b. Compared to Ni-3 sample, in which most of pores appear in the mesopore range only, both micropores and mesopores are present in samples C, Ni-1 and Ni-2. Surface areas of C, Ni-1, Ni-2 and Ni-3 were 936.3, 887.1, 884.2 and 478.4 m2 g-1, showing that the porosities of newly obtained materials decreased with an increase in the Ni(NO3)2·6H2O content in their precursors. Since reactants can’t get into some small micropores, the open pore area obtained from V-t graphs was another parameter to represent the real accessible surface areas.36,

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External surface areas of all four

samples presented in Figure 4c were 746 m2 g-1 for C, 677 m2 g-1 for Ni-1, 673 m2 g-1 for Ni-2 and 358 m2 g-1 for Ni-3. As per Figure 4d, pore volumes of C and catalysts Ni-1 and Ni-2 were larger than that of Ni-3. Since the electrocatalytic reaction occurs on the catalyst surface only, Ni-1 and Ni-2 with much larger surface areas can be expected to exhibit better electrocatalytic activity than Ni-3.

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Figure 5. XPS spectra of C and Ni-2 (a); high resolution XPS of Ni (b), C (c), N (d). The actual compositions of material C and Ni-2 catalyst were evaluated by elemental analysis. The results indicate that C consists of N (6.57 wt.%), C (86.82 wt.%) and H (1.27 wt.%), and Ni-2 consists of N (5.10 wt.%), C (66.83 wt.%), H (1.70 wt.%) and Ni (22.37 wt.%). Chemical states and compositions of as-prepared C and Ni-2 samples were studied by XPS. In Ni-2 spectrum (Figure 5a), signals of Ni, O, C and N were present, and no Ni signal was found in the carbon sample C. Surface chemical composition of Ni-2 was calculated from relative areas of its elements’ integrated intensities, and the fractions of C, O, N and Ni on the surface were 83.96%, 13.1%, 1.93% and 1.01% respectively. High-resolution Ni 2p XPS spectrum reveals two main peaks with satellites, which can be attributed to Ni 2p3/2 (854.9 eV) and Ni 2p1/2 (873.1 eV) representing typical Ni2+ and Ni3+ bound to oxygen.38 However, no 12


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diffraction peak of nickel oxide was found in the XRD pattern. It was previously reported that XPS method could provide surface composition and chemical state data within a depth range of 0.1-10 nm only.39 According to the XRD and XPS results, it could be assumed that metal phase Ni nanoparticles with surfaces covered by nickel oxides were formed within the carbon porous structure. As illustrated in Figure 5c, carbon XPS spectrum is represented by four peaks, such as C-O/C-N, sp2 C=C, -COO and C=O.40 The existence of C=O and C-O/C-N functional groups verified the presence of O and N dopant species in the nanocomposites. Figure S3a demonstrates that the proportion of C-O to C-N is higher for Ni-2 than in the sample C, suggesting that Ni nanoparticles promoted the formation of nitrogen and oxygen functional groups. Figure 5d exhibits N 1s spectra for Ni-2 and C, showing six individual nitrogen species existing on the surface, i.e. pyridinic-N, pyrrolic-N, pyridinic N-oxide and graphitic-N, entrapped NOx and π-π* satellite.41 The percentages of nitrogen groups in C and Ni-2 samples are shown in Figure S3b (Supporting Information). The ratios of pyrrolic and pyridinic groups in Ni-2 were greater than in C, suggesting that there are more pyrrolic and pyridinic species present in Ni-2 composite, which are commonly recognized as active sites for electrochemical reactions.28 Thus, it can be assumed that high density pyridinic and pyrrolic groups are the main reasons resulting in the improved activity of Ni-2 catalyst.

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Figure 6. (a) LSV curves of C, catalysts Ni-1, Ni-2 and Ni-3 and Pt/C tested in KOH; (b) Onset and overpotential histogram of; (c) Tafel plots of C and catalysts Ni-1, Ni-2 and Ni-3 and commercial Pt/C; (d) Nyquist plots of electrochemical impedance of C, catalysts Ni-1, Ni-2 and Ni-3 and Pt/C, -0.4 V vs. RHE; (e) Voltammogram of Ni-2 at different scanning rates; (f) Δj fitting for five samples (Δj = ja − jc) vs. scan rates at set potential (+0.566 V vs. RHE). HER electrocatalytic activity of C and Ni-containing catalyst samples 1, 2 and 3 was evaluated by linear sweep voltammetry (LSV) and compared with data obtained for commercial Pt/C as shown in Figure 6a. Figure 6a shows that Pt/C had the highest 14


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onset potential among all samples studied, pointing to its superior HER activity. Onset potentials and overpotentials of the catalysts are given in Figure 6b where Pt/C exhibits lowest values for both, 10 mV at 2 mA cm-2, and 25 mV at 10 mA cm-2, respectively. HER onset potentials of C and catalysts Ni-1, Ni-2 and Ni-3 were 327 mV, 76 mV, 53 mV and 132 mV, indicating that introduction of Ni into the carbon network improved HER activity of the catalysts. In addition, HER currents for all samples increased rapidly when potentials became more negative. At 10 mA cm-2 Ni-2 overpotential was relatively low (94 mV) in comparison with 407, 142 and 216 mV exhibited by C, Ni-1 and Ni-3 respectively, which could result from highly developed porous structure of these materials. The overpotential of Ni-2 (10 mA cm-2) was also higher than those of many transition metal catalysts listed in Table S1 (Supporting Information). To further understand HER activation mechanism occurring in Ni/C materials, Tafel plots of C, catalysts Ni-1, Ni-2 and Ni-3, and commercial Pt/C were obtained and are presented in Figure 6c. Tafel slopes of linear regions were calculated by the Tafel equation, such as η=b log j + a, whereη was overpotential, b Tafel slope, j current density. While Tafel slope for Pt/C was 32 mV dec-1, Ni-2 exhibited slightly higher 52 mV dec-1 which was still less than C (149 mV dec-1), Ni-1 (96 mV dec-1) and Ni-3 (122 mV dec-1), pointing to its fastest HER kinetic among all four newly produced catalysts. It was reported that the HER process comprises three steps represented by Volmer, Heyrovsky, and Tafel equations.42 In the Volmer step, proton absorption is a rate-limiting process at 120 mV dec-1, while Heyrovsky and Tafel slopes were 40 and 30 mV dec-1. For as-prepared Ni/C sample 15



HER follows Volmer and Heyrovsky mechanisms, in which desorption determines the rate of electrochemical process, and for Pt/C sample the reaction rate is determined by Heyrovsky-Tafel mechanism. Compared to other Ni/C samples, Ni-2 gave relatively low Tafel slope which could be attributed to strong electronic coupling of Ni particles and carbon. Lower Tafel slope indicates promising electrocatalytic properties of Ni-2 that create an opportunity for increasing HER rate in practical applications.43 Figure 6d represents electrochemical impedance spectroscopy (EIS), where Pt/C is characterized by the smallest Nyquist plot radius followed by Ni-2 pointing to lower charge transfer resistance for these two electrocatalysts. In numerical terms, charge transfer resistances of Pt/C, C, catalysts Ni-1, Ni-2 and Ni-3 were 1.72, 40.02, 4.58, 3.8 and 12.49 Ω respectively, with Ni-2 value being the lowest among as-prepared Ni/C samples. Compared to pure C material, the coupling of the conductive carbon support and Ni nanoparticles improves electron transfer capability of Ni/C type catalysts in the HER process.44 As-prepared samples’ effective surface areas were further investigated by a simple CV method of electrochemical double-layer capacitance measurement. The potential range applied for CV measurements was selected to avoid Faradic current and slow voltage sweep rate was chosen to increase the accuracy of experiments presented in Figures 6e and S4 (Supporting Information). At 0.566 V (RHE), Ja-Jc versus voltage scan rates were plotted and are shown in Figure 6f; corresponding electrochemical double-layer capacitances rose concurrently with an increase in active surface sites concentrations: C (4.91 mF cm-2) 16


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< Ni-3 (8.84 mF cm-2) < Ni-1 (19.25mF cm-2) < Ni-2 (20.5 mF cm-2) < Pt/C (23.25 mF cm-2).

Figure 7. (a) iR compensated RDE polarization curves of OER for C, catalysts Ni-1, Ni-2 and Ni-3, RuO2 and Pt/C in 1 M KOH; (b) mass-transport corrected OER Tafel plots; (c) Corresponding OER onset potential and overpotential values of different catalysts; (d) Nyquist plots tested at 1.65 V. OER activities of C and Pt/C presented in Figure 7a were relatively low in comparison with well performing RuO2 and Ni-2, both at similar onset potentials. The OER kinetics in the presence of new materials was further investigated using Tafel method. The slopes of Pt/C, C, Ni-1, Ni-2, Ni-3 and RuO2 were 413, 245, 74, 56, 76 and 58 mV dec-1 respectively as shown in Figure 7b. Again, Tafel slope of Ni-2 was the lowest among all samples due to its high OER activity. Since overpotential recorded at 10 mA cm-2 can be regarded as important parameter to evaluate OER 17



activity, Ni-2 stands out here as well, due to its lowest onset potential and overpotential among all samples studied (Figure 7c). The comparison of onset potential and overpotential of Ni-2 with those of many previously reported N-doped and Ni-based carbon materials is given in Table S2 (Supporting Information). That data further demonstrates superior OER activity of Ni-2 in KOH. EIS analysis results shown in Figure 7d were obtained at 1.65 V vs. RHE. The semicircles of a high frequency Nyquist plot

representing charge transfer resistance, demonstrate that

Ni-2 had the lowest impedance which probably resulted from large pore diameters and high external surface area of this material.

Figure 8. (a) HER LSVs and (b) OER LSVs of different cycle points on Ni-2 based electrode during stability testing; Structure of Ni-2 after 3000 cycles (c) SEM and (d) TEM. In order to evaluate its long-term stability during water splitting process, Ni-2 was subjected to repeated CV measurements in 1 M KOH and the LSV plots for 1000th, 18


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2000th and 3000th cycles are presented in Figure 8 a and b. After 3000 cycles, overpotential loss on Ni-2 was 16 mV for HER and 17 mV for OER at 10 mA cm-2. Commercial Pt/C and RuO2 were utilized as references for HER and OER respectively, and their LSV cycling measurements were done under similar alkaline conditions. According to Figure S5 presented in the supporting information section, after 3000 cycles at a 10 mA cm-2 the overpotentials deteriorated by 34 mV for Pt/C in HER and by 33 mV for RuO2 in OER. Both Pt/C and RuO2 exhibited higher overpotential corrosion after 3000 cycles than Ni-2, making the latter promising bi-functional HER/OER catalyst in alkaline media. HER/OER stability of C, Ni-1 and Ni-3 was investigated under the same conditions and results are shown in Figures S6, S7 and S8 (Supporting Information), their comparison to Figure S5 demonstrates superior stability of as-prepared N-doped 3D porous Ni/C networks in KOH electrolyte. The comparison of TEM and SEM images of Ni-2 after 3000 cycles treatment and images of fresh Ni-2 presented in Figure 2 revealed no obvious changes in morphology and Ni particle sizes. This is supported by the particle size distribution diagram of cycled Ni-2 shown in the insert of Figure 8d where no evidence of Ni nanoparticles agglomeration is visible. Similarly, no particle aggregation after stability experiments was observed in Figures S6-8 where SEM and TEM micrographs of C, Ni-1 and Ni-3 are shown, which can be attributed to effective encapsulation of Ni nanoparticles in the carbon matrices. Therefore, it can be concluded that introducing Ni nanoparticles into porous N-doped carbon is an effective way to develop stable bi-functional HER/OER electrocatalysts. 19



Chemical composition and crystalline structure of Ni-2 samples subjected to cycling stability testing were analyzed by XRD and XPS, as shown in Figure S9 (Supporting Information), and no obvious differences with the XRD pattern (Figure 3) and Ni 2p XPS spectrum (Figure 5) of the fresh Ni-2 could be seen, further testifying to its remarkable HER and OER stability.

Figure 9. (a) Water electrolysis polarization study using Ni-2||Ni-2 (Ni foam), Pt/C||RuO2 (Ni foam) and Ni foam||Ni foam in 1.0M KOH (5 mV s-1); (b) Chronopotentiometric study of Ni-2||Ni-2 (Ni foam) system (10 mA cm-2). Ni-2 catalyst was utilized for preparation of both the cathode and the anode in a two-electrode overall water electrolysis cell. According to Figure 9a, Ni-2||Ni-2 (Ni foam) cell reached 1.63 V at 10 mA cm-2. The cell performed almost as good as Pt/C||RuO2 cell which potential at the same current density was 1.55V and compared favorably to the one made of a bare Ni foam which reached 1.85 V. The stability of Ni-2 based electrodes is demonstrated in Figure 9b where a chronopotentiometric curve of the Ni-2||Ni-2 cell obtained at 10 mA cm-2 is presented. After 25 hours of operation, the cell voltage increased by 47 mV only, indicating its good stability in water electrolysis. During the whole water electrolysis 20


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process, vigorous bubble formation was observed on both electrodes as visible from the insert of Figure 9. To summarize, newly prepared N-doped 3D porous Ni/C network materials exhibited high electrocatalytic activity in HER and OER due to several reasons listed below. (1) High external surface area and well developed mesoporous structure provide plenty active sites for electrochemical HER and OER.4, 23 (2) Such structure can effectively enhance proton/electron transfer between the electrode electrolyte and inside the electrode due to the presence of 3D porous networks with short transport pathways. At the same time, the mesoporous structure enhances penetration of the electrolyte throughout the electrode resulting in low diffusion resistance and fast electrocatalytic kinetics.24,

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(3) Nanosized Ni particles are

encapsulated by N-doped carbon, which structure can significantly impede particles aggregation, and provide tight carbon wrapping for them, which manifests itself in superior long-term stability as demonstrated in the durability testing.28 (4) It was shown in electroanalytical experiments that high electrocatalytic performance of Ni-2 also results from the synergy of Ni particles with N-doped carbon.27 It can be concluded that careful design of mesoporous N-doped carbon nanostructure and its impregnation with transition metal nanoparticles resulted in a new promising type of bifunctional catalysts for water electrolysis in an alkaline media. Conclusions N-doped 3D porous Ni/C networks with mesoporous structure and high HER/OER catalytic activity have been successfully synthesized by thermal annealing of peptone 21



and NaCl, in which peptone provides nitrogen and carbon and NaCl serves the role of a template to form the porous structure. As-prepared Ni/C materials have well developed surfaces and high porosity which provide shorter pathways for mass transfer and electron transport during HER and OER regulated electrochemical processes. The study demonstrated outstanding electrocatalytic activity towards HER and OER in KOH, owning to intrinsic electrochemical properties, synergetic effect between Ni and N-carbon, and mesoporous structure which can facilitate mass transfer of reactants as well as release of hydrogen and oxygen. The optimized Ni-2 material was highly active and stable during HER and OER experiments conducted in KOH electrolyte. Due to its uncomplicated synthesis procedure and low cost, this newly developed Ni/C material can be regarded as a promising water electrolysis catalyst highly active in alkaline media.

Corresponding authors: Shan Ji (*): [email protected], Tel./fax: +86 (0)15024355548 Quanbing Liu (**): [email protected], Tel./fax: +86 (0)13610343516 Rongfang Wang (***): [email protected], Tel./fax: +86(0)17686458002 Associated Content The Supporting Information contains additional experimental information and data. Acknowledgements This is financially supported by the National Natural Science Foundation of China 22


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C bifunctional electrocatalysts for alkaline

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