Superaerophobic Quaternary Ni-Co-S-P Nanoparticles for Efficient

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Superaerophobic Quaternary Ni-Co-S-P Nanoparticles for Efficient Overall Water Splitting Yumeng Tian, Zaiwen Lin, Jing Yu, Sijia Zhao, Qi Liu, Jingyuan Liu, Rongrong Chen, Yunfei Qi, Hongsen Zhang, Rumin Li, Junqing Li, and Jun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02556 • Publication Date (Web): 10 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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ACS Sustainable Chemistry & Engineering

Superaerophobic Quaternary Ni-Co-S-P Nanoparticles for Efficient Overall Water Splitting Yumeng Tian

a,b,

Zaiwen Lin

a,b,

Jing Yu

*a,b,

Sijia Zhao

b,e,

Qi Liu

a,b,

Jingyuan Liu

a,b,

Rongrong Chen a,b,c, Yunfei Qi d, Hongsen Zhang a,b, Rumin Li a,b, Junqing Li *c, Jun Wang a,b,c

a. Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, China b. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China c. Institute of Advanced Marine Materials, Harbin Engineering University, 150001, China. d. China Nuclear Power Engineering Co., Ltd, Beijing, 100840, China. e. Lingyun Science and Technology group Co., Ltd, Yichang, 443000, China. MAILING ADDRESS: No. 145, Nantong Rd., Harbin Engineering University, Harbin 150001, China.

EMAIL:

[email protected];

1

ACS Paragon Plus Environment

[email protected]

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The aggravation of environmental problems leads to an increasing demand for clean hydrogen fuel. Simplification of the synthesis process is the key to realize mass production. In this work, quaternary Ni-Co-S-P nanoparticles were directly grown on carbon fiber cloth via a summary one-step electrodeposition process. The interconnected nanoparticles are instrumental in increasing the specific surface area and accelerating mass transfer, thus enhancing the electrocatalytic overall water splitting performance. Moreover, the superhydrophilic and superaerophobic properties could facilitate the diffusion of medium and the rapid delivery of the generated bubble. As a result, bifunctional Ni-Co-S-P nanoparticles (NPs) achieve a current density of 10 mA cm-2 at low overpotentials of 280 mV and 78 mV for OER and HER, respectively, along with the Tafel slopes of 69 and 89 mV dec-1. Besides, the Ni-Co-S-P displays an excellent performance with a cell voltage of 1.61 V to reach 10 mA cm-2 for overall water splitting. Moreover, benefiting from the superhydrophilic and superaerophobic characteristics, Ni-Co-S-P also exhibits robust stability and durability for both OER and HER. This paper provides a new idea for simplifying the synthesis process of electrocatalysts.

KEYWORDS: Quaternary Ni-Co-S-P, Superhydrophilicity, Superaerophobic surface, Overall water splitting.

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INTRODUCTION Environmental problems are getting worse, leading to a growing demand for developing clean energy to supersede decreasing fossil energy.1-2 Hydrogen is considered as one of the most promising alternatives owing to the zero environment concern and high power density.3-4 Electrochemical water splitting is a simple, efficient and environmentally friendly technology of hydrogen production. It has attracted many attentions by transforming electrical energy into chemical energy, which is more convenient for transportation and storage.5-6 The theoretical minimum voltage of electrolytic water is 1.23 V.7-8 However, overpotential often exists in practical application due to the influence of uphill reaction. The use of electrocatalysts can reduce the overpotential and promote the energy conversion efficiency. Generally, noble metals such as Pt-based materials are the most efficient catalysts for HER (hydrogen evolution reaction), and Ir/Ru based materials perform best for OER (oxygen evolution reaction).9-14 However, on account of the excessive cost and low total content in the earth, it is difficult to achieve large-scale production.15-16 Consequently, the development of environmental and efficient low-cost electrocatalysts is crucial. Transition metal sulfides (TMSs) have been widely researched in the past several decades.17-18 The auto-oxidation of TMSs could facilitate the OER kinetics by in-situ generating active sites.19 For instance, Xu et al. prepared graphene decorated Ni3S2 pyramids (Ni3S2@G) by a simple solvothermal method. The as-synthesized catalyst exhibits an overpotential of  294  mV to achieve a current density of 20 mA cm-2 for OER.20 3



However, the HER performance of the TMSs still needs to be improved to the level comparable to Pt-based materials as a bifunctional electrocatalyst. The kinetics of phosphide-based materials are faster than that of the common sulfides.21-23 The negatively charged P atoms are propitious to the desorption of hydrogen by interacting with positive charge in hydrogen evolution reaction. Yamauchi et al. proposed the doping P atoms could activate the basal plane and S edge sites to optimize the ΔGH* of metal sulfides, thus accelerate the HER performance.24 In addition, doping with P moderately not only can improve the stability of electrocatalysts, but also can ameliorate the electrical conductivity in the electrocatalytic process.25-27 Accordingly, the HER activity and poor stability of TMSs affected by the dissolution of catalyst in the OER process are expected to enhance by the phosphorus doping. 28 Bimetallic doping is also an effective way to ameliorate the performance of catalysts by the optimized electronic structures from the incorporation of the second metal atom.24,2830

In recent years, the performance of nickel-cobalt bimetallic compounds is very

prominent in the research of electrocatalysts. Ye et al. reported Co-Ni-P synthesized by a one-step constant current density electro-deposition technique. And the as-synthesized bimetallic Co-Ni-P film exhibited excellent catalytic activities and kinetics for not only HER, but also OER in alkaline condition, which was better than Co-P and Ni-P.31 Xu et al. synthesized NixCoyP ternary phosphides films with various Ni/Co ratios by an electrodeposition method. Benefiting from the optimized electronic structure due to the bimetallic effect, the phosphide with Ni:Co=0.51:0.49 showed the best performances for 4


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HER, OER, and overall water splitting.28 The synergism of bimetal and double-anions may further improve the performance of electrocatalysts. Kim et al. prepared graphene coupled three-dimensional structures of quaternary Co-Ni-S-P compounds (Co1−xNix)(S1−yPy)2/G). They confirmed that the nickel and phosphorus substitution into cobalt and sulphur sites leads to higher catalytic activity and stability for bifunctional water splitting.32 However, the complex preparation process and the employ of binder might make it challenging as the potential candidate in practical application. Li et al. greatly simplified the process of multi-elements electrocatalysts. Moreover, they proposed that the controllable conditions for the structure and electronic properties of electrocatalysts could be achieved by regulating the ratio of S and P of FeCoNiPxSy samples. 33 Despite considerable progress, there are still few reports about the effect of interface modification on bubble detachment. The in-situ generated H2 and O2 gather on electrolyte-catalyst interface during the process of large-scale electrocatalytic water splitting at high current density. This critically impedes the exposure of instantaneous active sites, retarding the mass transfer efficiency and slowing the electron transfer rate, thereby seriously affecting the performance of electrocatalysts.34 Moreover, continuous generation-detachment of bubbles has a great impact on the surface morphology of materials, seriously going against the stability of electrocatalysts. Based on the above concerns, we prepared bimetal and double-anions Ni-Co-S-P by simple one-step electrodeposition. The as-prepared electrocatalysts own interconnected nanoparticles structure with superhydrophilic and superaerophobic surface, which can 5



enhance the active specific surface area, improve the stability and increase the active sites of catalysts. The Ni-Co-S-P shows the overpotentials of 78 mV and 79.9 mV at 10 mA cm-2 in acidic and alkaline conditions for HER, respectively. Ni-Co-S-P also exhibits excellent OER activity with the overpotential of 280 mV, as well as overall water splitting performance with the low cell voltage of 1.61 V at the current density of 10 mA cm-2. In addition, benefiting from the superhydrophilic and superaerophobic characteristics, the NiCo-S-P also shows excellent stability and durability. EXPERIMENTAL SECTION Preparation of Ni-Co-S-P nanoparticles: Bare carbon fiber clothes (CC) as the electrocatalytic substrate were carefully cleaned several times by acetone, deionized water and ethanol to dislodge the impurities attached to the surface. The electrolyte solution consists of 0.025 M nickel chloride hexahydrate and cobalt chloride hexahydrate, as well as 0.125 M thiourea and sodium hypophosphite. Ni-Co-S-P nanoparticles onto the aswashed 1  1 cm2 carbon fiber cloth were prepared by a simple one-step electrodeposition technique. The electrodeposition was carried out in a standard three-electrode system, which composed of bare CC (working electrode), platinum foil (counter electrode), and saturated calomel electrode (SCE, reference electrode). The cyclic voltammetry technique (CV) was implemented under the scan rate of 15 mV s-1 from 0.5 V to -1.1 V for ten cycles. Then the CC was rinsed by large amount of distilled water and dried at 60°C for 12h. The contrasted ternary materials were deposited at the same condition except the lack of one of the electrolyte components. 6


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RESULTS AND DISCUSSION The synthesis process of the Ni-Co-S-P nanoparticles is shown in Scheme 1. Interconnected nanoparticles were electrodeposited on cleaned CC, which are beneficial to the improvement of HER activity and superaerophobic property.35 In the XRD patterns, the characteristic peaks of various materials have not been clearly shown, which may be due to the low crystallinity and low loading of the synthesized materials (Figure S1). The morphologies of the as-prepared samples are characterized by the scanning electron microscopy (SEM). As showed in Figure 1a and b, the carbon cloth fiber is evenly covered with a layer of Ni-Co-S-P nanoparticles. The single nanoparticle size is about 100-150 nm. The protruding and interconnected particles are conducive to enlarge the specific surface area, accelerate mass transfer, and facilitate the diffusion of bubbles, thus enhancing the electrocatalytic performance.36 The construction was further characterized by transmission electron microscopy (TEM). Figure 1c exhibits the nanoparticles dropped from carbon fiber cloth by ultrasonic oscillation, echoing the previous SEM images. As Figure 1d shows, the Ni-Co-S-P nanoparticles consists of numerous small granules with the size of several nanometers. The high-resolution TEM (HRTEM) was further carried out to measure the interplanar spacing. The lattice fringe is 0.221 nm, pertain to the (111) plane of NiCoP. Moreover, the EDS elemental mappings (Figure 1e) of Ni-Co-S-P from TEM demonstrate the uniform distribution of nickel, cobalt, sulfur and phosphorus in Ni-Co-S-P. In order to determine the composition of Ni-Co-S-P, we retested the Selected Area Electron Diffraction of the sample (Figure S3). The (001) plane is exposed in NiCoP which spacing 7



is 0.34 nm. The (222) (0.271 nm) and (400) (0.235 nm) planes are exposed in CoNi2S4. And the (110) plane with the spacing 0.171 nm belongs to NiS. It can be seen that Ni-CoS-P is a homogeneous mixture of active components. X-ray photoelectron spectroscopy (XPS) was further carried out to discuss the surface chemical compositions of the catalysts. The XPS proves the existence of nickel, cobalt, sulfur and phosphorus elements, suggesting the successful synthesis of Ni-Co-S-P, and the atomic of Ni-Co-S-P is Ni1Co4.23S13.48P1.71. As shown in Figure 2a, there were two peaks located at 873.4 eV and 855.8 eV belong to Ni 2p1/2 and Ni 2p3/2, respectively.37 The two peaks located at 879.5 eV and 861.6 eV are satellite peaks. The peaks at binding energies of 794.1 eV and 779.0 eV are attributed to Co 2p1/2 and Co 2p3/2(Figure 2b).38 And the peak at 781.1 eV can be defined as surface oxidation.39-40 As Figure 2c exhibits, the binding energy of P is 130.0 eV, and the peak at 134.7 eV is related to oxidize P species.41 The binding energies corresponding to S 2p are located at 162.1 eV and 162.8 eV as displayed in Figure 2d. The peak at binding energy of 168.5 eV can be associated with S-O bond.42 Comparing to NiCo-S and Ni-S-P (Figure S4 and S5), the corresponding peaks in Ni-Co-S-P exhibit obvious shifts, suggesting the strong electron interactions between bimetal and double anions, which induces the change in electronic structure of the catalyst.43 The nitrogen adsorption-desorption isotherms were further measured as Figure S6 shows. Ni-Co-S-P exhibit a reversible type IV isotherm, which is one of the properties of mesoporous materials. Besides, In order to reveal the origin of performance enhancement, the turnover frequency (TOF) of Ni-Co-S and Ni-Co-S-P were calculated from the formula: 8


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TOF=JA/2nF. Where J is the current density at overpotential of 100 mV. A is the area of CC (1 1 cm-2), F is the Faraday constant (96485 C mol-1), n is the mole number of active sites on the catalyst. At overpotential of 150mV, the current density of N-Co-S, Ni-Co-P, Ni-S-P, CoS-P and Ni-Co-S-P are 0.019 A cm-2, 0.022 A cm-2, 0.044 A cm-2, 0.036 A cm-2 and 0.068 A cm-2, respectively. Accordingly, the TOF of Ni-Co-S-P was calculated as 0.031 s-1, better than that of Ni-Co-S (0.011 s-1), Ni-Co-P (0.013 s-1), Ni-S-P (0.022 s-1) and Co-S-P (0.019 s-1). Bubble-detachment property is also an essential factor for catalytic efficiency in largescale applications. Whether bubbles can adhere to the catalyst surface depends on the competition between bubbles and the intermediate liquid film over the catalyst surface. Therefore, the underwater aerophilic of catalysts is closely related to the wettability in air. According to the derivation of classical Young's equation,44 the contact angle of underwater bubbles can be followed:

(1) SV: solid–vapor; SL: solid–liquid; LV: liquid–vapor. According to Wenzel wetting state theory, the electrolyte will wet the microstructure when the superaerophobic catalyst immersed in the electrolyte. According to Cassie wetting state theory, bubbles on the tip of the morphology can only maintain the spherical structure with the lowest energy.45 In order to examine the bubble adhesion behavior on Ni-Co-S-P surface, the contact angle of 9



bubble and interface in 1M KOH medium was measured. As Figure 3a shows, the underwater contact angle is 151.6°, indicating the superaerophobic property of Ni-Co-S-P. We also measured the adhesive behavior of Ni-Co-S-P to the bubble (Figure 3b and c, Video S1). As showed in Figure 3b and c, there is no adhesion between the bubble and surface of electrocatalyst. Furthermore, gas bubbles slipped out along the inclined Ni-CoS-P surface with little resistance as Video S2 exhibited, proving that the adhesive force between bubbles and catalyst surface is extremely low.46 This is on account of the discontinuous state of the three-phase contact line (TPCL), reducing the contact area between bubbles and surface of electrocatalyst.47 In addition, the electrolyte droplets immersed the catalyst instantaneously when dropped on Ni-Co-S-P surface (Figure 3d and e), indicating the excellent superhydrophilicity of Ni-Co-S-P, which is advantageous to increase contact area of Ni-Co-S-P and electrolyte, thus reducing adhesive force.48 The electrocatalytic HER performance of Ni-Co-S-P, as well as homologous ternary mono-metallic and mono-anion electrocatalysts were first evaluated in the standard threeelectrode system under 0.5 M H2SO4 media at room temperature. The linear sweep voltammetry (LSV) curves in Figure 4a show that, the overpotential of Ni-Co-S-P is 163 mV versus RHE at the current density of 100 mA cm-2 and 78 mV at 10 mA cm-2, lower than Ni-Co-S, Ni-Co-P, Ni-S-P and Co-S-P with the overpotentials of 307, 284, 232 and 229 mV at 100 mA cm-2, respectively. Besides, the HER performances of Ni-Co-S-P are excellent compared with most of other reported multi-elements electrocatalysts, such as Co-MoS3 (171 mV@10 mA cm-2),49 Co2P/CoN-in-NCNTs (98 mV@10 mA cm-2),50 10


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CoPS@NPS-C(93 mV@10 mA cm-2) (detailed in Table S1).51 In order to further measure the kinetics for HER, the Tafel plots of Ni-Co-S-P were fitted (Figure 4b). The Tafel slope value of Ni-Co-S-P is 89 mV dec-1, much smaller than 248 mV dec-1 for Ni-Co-S, 179 mV dec-1 for Ni-Co-P, 99 mV dec-1 for Ni-S-P and 137 mV dec-1 for Co-S-P. This indicates that bimetal and double-anions have a significant effect on kinetics52. The Ni-Co-S-P conforms to the Volmer-Heyrovsky mechanism, illustrating the rate determining step is the reaction of hydrated proton and the Hads formed in Volmer step, and an electron is obtained from the surface of the catalyst to generate H2.53 The electrochemical impedance

spectroscopy (EIS) at -160 mV was then measured (Figure 4c). Compared with homologous ternary mono-metallic or mono-anion electrocatalysts, the Rct of Ni-Co-S-P (7.52 Ω) is much smaller, suggesting an expeditious charge transfer upon Ni-Co-S-P. Furthermore, the electrochemical surface area (ECSA) is an indispensable criterion to evaluate the performance of electrocatalysts. Ordinary, the ECSA is proportional to doublelayer capacitance (Cdl). Hence, the cyclic voltammetry techniques (CV) under various scan rates were carried out to calculate the double-layer capacitance. As Figure 4d displays, the Cdl of Ni-Co-S-P is 10.0 mF cm-2, higher than the 1.1 mF cm-2 of Ni-Co-S, 1.6 mF cm-2 of Ni-Co-P, 2.4 mF cm-2 of Ni-S-P and 2.6 mF cm-2 of Co-S-P, indicating Ni-Co-S-P owns higher ECSA, which could provide abundant active sites for electrocatalytic reaction. Figure 4e exhibits a steady overpotential at a current density of 100 mA cm-2 during 25 h of chronopotentiometry test. Moreover, the LSV after 1000 cycles of CV under the scan rate of 100 mV s-1 are basically coincide (Figure 4f), revealing excellent durability and 11



stability of Ni-Co-S-P. Moreover, In order to determine the optimal synthesis conditions, performances of Ni-Co-S-P with different electrodeposition parameters and element ratio were shown in Figure S7-9. Generally, the thermodynamic uphill reaction of HER in alkaline condition is stronger than that in acid. Accordingly, the electrocatalytic HER properties of the as-synthesized catalysts were further evaluated in alkaline condition. The LSV curves in Figure S12a illustrate that the overpotential of quaternary Ni-Co-S-P is 79.9 mV at 10 mA cm-2. In contrast, Ni-Co-S, Ni-Co-P, Ni-S-P and Co-S-P own the overpotentials of 147.9, 190.9, 193.7 and 87.6 mV at the current density of 10 mA cm-2, respectively. Moreover, the Tafel slope of Ni-Co-S-P is quite smaller than that of these counterparts, suggesting the more favorable reaction kinetics upon Ni-Co-S-P in basic solution. The electrocatalytic OER activities of Ni-Co-S-P, Ni-Co-S, Ni-Co-P, Ni-S-P and CoS-P were investigated in 1 M KOH at a scan rate of 5 mV s-1. The polarization curves (Figure 5a) show the overpotential of Ni-Co-S-P is 280 mV to obtain the current density of 10 mA cm-2, which is close to RuO2 (278 mV), and lower than Ni-Co-S (354 mV), NiCo-P (386 mV), Ni-S-P (401 mV) and Co-S-P (460 mV), respectively. Moreover, this performance is better than most of similar works, such as Ni/Mo2C-PC (368 mV@10 mA cm-2),54 NiFe-LDH/NiCo2O4 (290 mV@50 mA cm-2),55 Co-Nx/P-GC/FEG (320 mV@10 mA cm-2),56 NiCo2S4@graphene (470 mV@10 mA cm-2) (detailed in Table S2 ).57 The Tafel slopes calculated on the basis of potential against log J were shown in Figure 5b. The Tafel slope of Ni-Co-S-P is 69 mV dec-1, which is much lower than that of the homologous 12


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ternary mono-metallic and mono-anion electrocatalysts. As a supplement to Tafel slope, the EIS further indicated the OER kinetics of Ni-Co-S-P is the best among the assynthesized electrocatalysts (Figure 5c). Stability is also one of the significant indexes for evaluating an OER catalyst. The long-term stability is measured by chronopotentiometry a current density of 100 mA cm-2. And as shown in Figure 5d, the overpotential remained stable during 20 hours, suggesting an excellent stability of Ni-Co-S-P nanoparticles. The above analyses exhibit that, both HER and OER properties of Ni-Co-S-P are excellent. Therefore, the overall water splitting performance of as-synthesized electrocatalysts was then measured in alkaline condition (1 M KOH). As Figure 6a exhibits, Ni-Co-S-P displayed an excellent performance with a low cell voltage of 1.61 V to obtain 10 mA cm-2, lower than Ni-Co-P (1.93 V), Ni-Co-S (1.72 V), Ni-S-P (1.8 V) and Co-S-P (1.76 V). The long-term stability of Ni-Co-S-P was studied by Chronopotentiometry. As Figure 6b shows, the Ni-Co-S-P kept stable water splitting under a current density of 100 mA cm-2 for 25 hours. The performance of Ni-Co-S-P is more excellent than many of reported multi-elements electrocatalysts, whether in HER, OER or overall water-splitting. The detailed comparisons are given in Table S1, S2 and S3. The high electrocatalytic performance of quaternary Ni-Co-S-P can be attributed to the following aspects. (1) The interconnected nanoparticles of quaternary Ni-Co-S-P can increase the specific surface area and accelerate the mass transfer, thus enhance the electrocatalytic activity. (2) The bimetal and double-anions have a significant effect on electrocatalytic activity and kinetics of Ni-Co-S-P, substantially improving both HER and 13



OER performance, as long as the stability and electrical conductivity. (3) The superhydrophilic and superaerophobic properties are beneficial to the diffusion of electrolyte and the rapid release of generated bubble, effectively reducing the temporary impact of bubbles on the active specific surface area, thus ameliorating the electrocatalytic activity and stability of Ni-Co-S-P. CONCLUSION In summary, quaternary Ni-Co-S-P nanoparticles were successfully synthesized by a simple one-step electrodeposition method. The interconnected nanoparticles are conducive to enlarge the specific surface area, accelerate mass transfer, and facilitate the diffusion of bubbles. Profiting from the bimetallic and double-anions effect, the electronic structure of Ni-Co-S-P is well optimized. In addition, the unique structure features endows Ni-Co-S-P with superhydrophilic and superaerophobic properties, further promoting the stability and electrocatalytic activity. Combining the advantages above, superaerophobic Ni-Co-S-P exhibits a low overpotential of 78 mV for HER at a current density of 10 mA cm-2 along with the Tafel slope of 70 mV dec-1 under acidic medium, and OER overpotential of 280 mV at 10 mA cm-2. Moreover, when constructed in a two-electrode electrolyzer, Ni-CoS-P requires the cell voltage as low as 1.61 V to afford the current density of 10 mA cm-2, indicating outstanding overall water splitting ability. The quaternary Ni-Co-S-P nanoparticles provide a promising strategy to design non-noble metals electrocatalysts.

ASSOCIATED CONTENT 14


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Supporting Information SEM images of contrast samples, XRD patterns, XPS spectra, CV curves, LSV curves and Tafel plots. Experiment video, AVI.

AUTHOR INFORMATION Corresponding Author Jing Yu, E-mail: [email protected]; Junqing Li, E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No 51603053), Natural Science Foundation of Heilongjiang Province (LH2019E025), Fundamental Research Funds of the Central University (3072019CF1003), China Postdoctoral Science Foundation (2019M651260), Defense Industrial Technology Development Program (JCKY2016604C006, JCKY2018604C011) and The National Key Research and Development Program of China (2016YFE0202700). 15



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25



Scheme 1. Schematic diagram for synthesis process of Ni-Co-S-P nanoparticles on carbon fiber cloth.

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Figure 1. (a,b) SEM images, (c,d) TEM images and (e) EDS elemental mapping of Ni-Co-S-P.

27



Figure 2. XPS spectra of Ni-Co-S-P. (a) Ni 2p, (b) Co 2p, (c) P 2p and (d) S 2p spectra.

Figure 3. Superaerophobic and superhydrophilic measurements: (a) bubble contact angle, (b,c) bubble adhesive force and (d,e) wetting ability of Ni-Co-S-P.

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Figure 4. HER performance of Ni-Co-S-P and corresponding contrast ternary electrocatalysts in 0.5M H2SO4. (a) Polarization curves under a scan rate of 5 mV s-1, (b) corresponding Tafel plots, (c) EIS

curves of Ni-Co-S-P, (d) double-layer capacitance, (e) long-term stability at 100 mA cm-2 and 100mV for 25 h and (f) polarization curves before and after 1000 cycles of CV under the scan rate of 100 mV s-1.

29



Figure 5. OER performance of Ni-Co-S-P and corresponding contrast ternary electrocatalysts in 1M KOH. (a) OER polarization curves under a scan rate of 5 mV s-1, (b) corresponding Tafel plots, (c) Nyquist plots and (d) long-term stability at 100 mA cm-2 for 25 h.

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Figure 6. (a) Polarization curve of overall water splitting using the as-prepared samples as both anode and cathode electrocatalysts in a two-electrode system. (b) Long-term stability at 100 mA cm-2. (c) Photograph of generation H2 and O2 bubbles are detaching from Ni-Co-S-P surface.

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For Table of Contents Use Only

The bifunctional Ni-Co-S-P with the superhydrophilic and superaerophobic characteristics has high efficiency of H2 and O2 production.

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Superaerophobic Quaternary Ni-Co-S-P Nanoparticles for Efficient

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