Elsevier

Electrochimica Acta

Volume 299, 10 March 2019, Pages 152-162
Electrochimica Acta

Co9S8–Ni3S2 heterointerfaced nanotubes on Ni foam as highly efficient and flexible bifunctional electrodes for water splitting

https://doi.org/10.1016/j.electacta.2019.01.001Get rights and content

Abstract

Large-scale commercialization of water splitting is still challenging due to the lack of efficient, cost-effective and robust electrodes. In this work, Co9S8–Ni3S2 heterointerfaced nanotubes on Ni foam (Co9S8–Ni3S2 HNTs/Ni) have been successfully developed via a two-step hydrothermal method as a highly efficient, robust and flexible bifunctional electrodes for water splitting. Co9S8–Ni3S2 HNTs are composed of inner 1D Co9S8 nanotubes surrounded by outer 2D Ni3S2 nanosheets. Co9S8–Ni3S2 HNTs/Ni electrode achieves 10 mA cm−2 at overpotentials of 85 mV for H2 evolution reaction (HER) and 50 mA cm−2 at overpotential of 281 mV for O2 evolution reaction (OER) in 1 M KOH solution, substantially better than that on single-phased Co9S8 nanotubes and Ni3S2 nanosheets on Ni foam. The reason is most likely due to formation of defect-rich heterointerfaces between Co9S8 nanotubes and Ni3S2 nanosheets, and the significant change of binding energies of Co 2p and Ni 2p level between the Co9S8 and Ni3S2, leading to the synergistic effect on the enhanced catalytic activity for HER and OER. The robust and flexibility of Co9S8–Ni3S2 HNTs/Ni electrodes are demonstrated in an alkaline electrolyzer, delivering 10 mA cm−2 electrolysis current at a cell voltage of 1.59 V, one of the best bifunctional electrodes for water splitting. Owing to the fact that reaction occurs at the defects-rich heterointerfaces, the Co9S8-Ni3S2 HNTs/Ni electrodes are structurally very stable.

Introduction

Hydrogen is the most clean and environmentally benign energy carrier with high calorific value of 142 kJ/kg and has a potential to replace the diminishing fossil fuels in the future [1,2]. Electrochemical water splitting through cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is probably the most efficient way to produce high purity hydrogen [3]. Hydrogen production via water splitting driven by the renewable electric energy such as solar and wind power is also most effective to store the renewable energy in the form of hydrogen fuel. So far the most widely commercial device used for the electrochemical water splitting is the alkaline electrolyzer. Unfortunately, the electric energy conversion efficiency of current alkaline electrolyzer is low (about 70%–80%) and polarization losses are high, resulting in excessive consumption of energy [4]. The grand challenge is to develop highly active and stable electrocatalysts for both HER and OER in order to reduce the overpotential for overall water splitting and thereby increase the electric energy efficiency. Pt and RuO2/IrO2 based precious metal or metal oxide catalysts are highly active for HER and OER, but their widespread application and scale-up deployment have been suffered from high cost and scarcity [[5], [6], [7]].

In recent years, earth-abundant and low-cost transition metal sulfides, such as Ni3S2 [[8], [9], [10], [11], [12], [13], [14]] and Co9S8 [[15], [16], [17], [18], [19], [20]], have been widely investigated as the alternative and non-precious metal oxide electrocatalysts for water splitting. For example, Zou et al. [21] investigated high-index faceted Ni3S2 nanosheet arrays on Ni foam (Ni3S2/NF) and indicated that the exposed high-index Ni3S2 facets promote water splitting. Further work showed that carbon-supported Co9S8 nanoparticles (Co9S8@C) is stable in a wide pH range and shows reasonable activity for HER [22]. However, the catalytic activity of Ni3S2 or Co9S8 single-phased catalysts is generally low and their activity is also limited by the low electrical conductivity. Introducing heterointerfaces and/or nanostructures is an efficient way to promote potential synergistic effect and thus to enhance the catalytic activity and reaction rates [[23], [24], [25]]. For instance, Feng et al. [26] synthesized MoS2/Ni3S2 nanostructures and observed that the MoS2/Ni3S2 interfaces are favorable for the chemisorption of hydrogen and oxygen-containing intermediates and thereby improve the catalytic activity for water splitting. On the other hand, nanostructures such as one-dimensional (1D) nanoneedles and nanowires have high length/diameter aspect ratio, leading to the effective channels for electron and ion transport [27]. Two-dimensional (2D) nanostructures such as nanosheets can also increase the active sites [28,29]. Thus combining 1D and 2D nanostructures could increase the active sites as well as accelerate the diffusion and mass transfer of reaction species. For instance, Co3O4@Ni3S2 on Ni foam (Co3O4@Ni3S2/NF) [30], MoS2-Ni3S2 nanorods on Ni foam (MoS2–Ni3S2 NRs/NF) [31] and MoS2/Ni3S2 on Ni foam (MoS2/Ni3S2/NF) [32] have been synthesized and investigated as electrocatalysts for the overall water splitting. It has been reported that the nanotubes have a kinetically favorable open structure as compared to the nanoneedles and nanowires [[33], [34], [35], [36]]. Electrolyte could intimately be in contact within the inner and outer surfaces of the catalyst and thereby improve its utilization efficiency [34]. This implies that combining the 1D nanotubes and 2D nanosheets would be an efficient strategy to substantially increase the efficiency and stability of the nanostructured catalysts for water splitting.

In addition to the electrocatalytic activity, constructing active materials on flexible and robust substrates, such as Ni foam [37] and carbon clothes [38] are also very important for water splitting in practical and industrial applications. A high flexibility and robustness electrode is essential to stand against the internal as well as external stress and strain generated during operation [37]. Flexibility of the electrodes also allows for the easy assembly of the electrodes in the limited space of electrolyzers and meets the requirements of different transportation conditions and in wearable electronics and devices [37].

Herein, we developed hybrid Co9S8–Ni3S2 nanostructures on Ni foam as flexible, robust and bifunctional electrodes for water splitting. In this case, 1D Co9S8 nanotubes are covered by 2D Ni3S2 nanosheets, forming a new Co9S8–Ni3S2 heterointerfaced nanotube catalyst on Ni foam, Co9S8–Ni3S2 HNTs/Ni. The Co9S8–Ni3S2 HNTs/Ni possesses excellent performance and stability for water splitting in alkaline media. The results indicate that the defect-rich interfaces are most likely the active sites for the HER and OER on the Co9S8–Ni3S2 heterointerfaced nanotube structured electrodes.

Section snippets

Materials

Ni foam was obtained from Kunshan Electronic Limited Corporation. Co(NO3)2·6H2O, Ni(NO3)2·6H2O, CO(NH2)2 (urea), NH4F, and CH3CSNH2 (thioacetamide, TAA) were acquired from Aladdin. Pt/C (50 wt% Pt/C) and RuO2 were purchased from Guangzhou Chemical Reagent Factory. All chemical reagents were directly used as received without further treatment.

Electrocatalyst and electrode synthesis

Ni foam (3 cm × 3 cm) was cleaned in 3 M HCl solution and absolute ethanol by sonication for 15 min each before use. Co(NO3)2·6H2O (0.873 g), urea

Synthesis and microstructure of heterointerfaced nanotube structured catalysts

Fig. 1 shows the scheme of the synthesis of Co9S8–Ni3S2 HNTs/Ni electrocatalysts by a two-step hydrothermal method. During the first hydrothermal treatment at 120 °C, Co2+ reacted with CO32− and OH released by the hydrolysis of urea and formed nanoneedles on the smooth Ni foam (Fig. 1b and c). As shown below the composition of nanoneedles is Co(OH)(CO3)0.5·xH2O. The diameter of Co(OH)(CO3)0.5·xH2O nanoneedles (Co–CHH NDs) is about 120 nm. In the presence of thioacetamide (TAA) as sulfur

Conclusions

New hybrid electrocatalysts based on defects-rich Co9S8–Ni3S2 heterointerfaced nanotubes on Ni foam, Co9S8–Ni3S2 HNTs/Ni, have been successfully synthesized by a facile two-step hydrothermal method. The Co9S8–Ni3S2 HNTs are composed of 1D hollow Co9S8 nanotubes covered by the 2D Ni3S2 nanosheets. XRD and high resolution TEM analysis shows the formation of crystalline Co9S8 nanotubes and Ni3S2 nanosheets with defects-rich heterointerfaces. The presence of defects in the interface region has been

Acknowledgements

This work was supported by National Natural Science Foundation of China (21376105 and 21576113) and Australian Research Council (DP180100731 and DP180100569). JL thanks to the financial support of Jinan University for the exchange program.

References (61)

  • S. Egelund et al.

    Int. J. Hydrogen Energy

    (2016)
  • J. Li et al.

    Chin. J. Catal.

    (2018)
  • C. Tang et al.

    Int. J. Hydrogen Energy

    (2015)
  • C. Ouyang et al.

    Electrochim. Acta

    (2015)
  • Y. Guo et al.

    Nano Energy

    (2018)
  • X. Zhou et al.

    Nano Energy

    (2017)
  • L. Ma et al.

    Nano Energy

    (2016)
  • B. Ni et al.

    Chem. Sci.

    (2018)
  • V. Tozzini et al.

    Phys. Chem. Chem. Phys.

    (2013)
  • M.G. Walter et al.

    Chem. Rev.

    (2010)
  • T. Yoon et al.

    Adv. Funct. Mater.

    (2016)
  • G.-F. Chen et al.

    Adv. Funct. Mater.

    (2016)
  • T. Kwon et al.

    Adv. Funct. Mater.

    (2017)
  • J.–J. Lv et al.

    Small

    (2017)
  • L. Zeng et al.

    J. Mater. Chem. A

    (2018)
  • W. Zhou et al.

    Energy Environ. Sci.

    (2013)
  • T. Zhu et al.

    J. Mater. Chem. A

    (2016)
  • D. Zhang et al.

    Nanotechnology

    (2018)
  • M. Li et al.

    J. Mater. Chem. A

    (2017)
  • B.K. Barman et al.

    Dalton Trans.

    (2016)
  • C. Wu et al.

    Nanoscale

    (2017)
  • P. Ganesan et al.

    ACS Catal.

    (2015)
  • X. Ma et al.

    Nanoscale

    (2018)
  • S. Huang et al.

    Adv. Funct. Mater.

    (2017)
  • L.–L. Feng et al.

    J. Am. Chem. Soc.

    (2015)
  • L.L. Feng et al.

    ACS Appl. Mater. Interfaces

    (2015)
  • Y. Guo et al.

    Chem. Mater.

    (2017)
  • J. Zhang et al.

    Angew. Chem.

    (2016)
  • S. Huang et al.

    Inorg. Chem. Front.

    (2017)
  • C. Tang, H. Zhang, K. Xu, Q. Hu, F. Li, C. He, Q. Zhang, J. Liu, L. Fan, Catal. Today...
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