CoS2 nanoneedle array on Ti mesh: A stable and efficient bifunctional electrocatalyst for urea-assisted electrolytic hydrogen production

doi:10.1016/j.electacta.2017.06.068

Electrochimica Acta

Volume 246, 20 August 2017, Pages 776-782

https://doi.org/10.1016/j.electacta.2017.06.068 Get rights and content

Abstract

In this work, we report that uniformly CoS₂ nanoneedle array grown on Ti mesh (CoS₂ NA/Ti) behaves as a stable and efficient bifunctional electrocatalyst for urea-assisted electrolytic hydrogen production via overall urea splitting. As a non-noble metal electrocatalyst, CoS₂ NA/Ti exhibited excellent urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) catalytic activity in 1.0 M KOH with 0.3 M urea solution. Only a cell voltage of 1.59 V is required to achieve a current density of 10 mA cm⁻² for full urea splitting in 1.0 M KOH with 0.3 M urea.

Graphical abstract

The CoS₂ nanoneedle array on Ti mesh (CoS₂ NA/Ti) was successfully synthesized via hydrothermal and sulfuration processes and utilized as a highly active and robust bifunctional catalyst for urea electrolysis in alkaline solutions.

Introduction

Water electrolysis is a consolidated process to produce highly pure hydrogen, which is an ideal choice to replace fossil fuels [1], [2], [3]. However, the anodic oxygen evolution reaction (OER) is still the major bottleneck in the overall water splitting process [4], [5] and needs much higher activation energy for O-O bond formation [6], [7], [8]. Replacing water with other more readily oxidizable species is a promising way for more energy-efficient electrolytic hydrogen production. Urea is an electro-oxidative species with the advantages of cheap, non-toxic and renewable [9], [10], [11], [12]. All these advantages make anodic urea oxidation reaction (UOR) an ideal alternative for OER.

Although noble metal-based catalysts such as Pt/C, RuO₂ or IrO₂ can effectively reduce the energy barrier to promote the activity for UOR and hydrogen evolution reaction (HER) of urea electrolysis, however, their scarcity and high cost severely impede the applications [10], [13], [14], [15]. Thus, there is an urgent call for alternative urea electrolysis catalysts based on the earth-abundant elements. In recent years, transition metal based composites, such as CoP₂ [16], Co-Mo-S [17], NiCo₂S₄ [18], Co(OH)₂@PANI [19] have been largely explored because of their good catalytic activity and low preparation cost [20], [21]. Among these catalysts, cobalt disulfide (CoS₂) outperforms the most transition metal dichalcogenides due to its excellent metallic nature and high catalytic activity, making it a uniquely superior electrocatalyst [22], [23], [24]. In addition, long-term stability of CoS₂ was observed in both acidic and alkaline operating environments [23], [25]. However, the development of efficient CoS₂ based bifunctional catalysts towards both UOR and HER for an energy-saving electrolytic hydrogen production has not been reported yet.

Herein, we immobilized 1D CoS₂ nanoneedle array on Ti mesh (denoted as CoS₂ NA/Ti) by a facile hydrothermal treatment, followed by a sulfuration process under N₂. The 1D morphology of CoS₂ could expose abundant surface active sites to accelerate the electrolyte transportation [26], [27] and facilitate the release of N₂ and CO₂ at anode and H₂ at cathode. The stable interaction between the Ti mesh and CoS₂ nanoneedle array could facilitate the electron transport [28], [29] and prevent CoS₂ agglomeration during the urea electrolysis process [20], [24], [30]. As a result, the electrocatalytic performance of CoS₂ NA/Ti can be achieved at a cell voltage of only 1.59 V to generate a current density of 10 mA cm⁻² for full urea splitting in 1.0 M KOH with 0.3 M urea.

Section snippets

Materials

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, AR), sublimed sulfur (S, CP), ammonium fluoride (NH₄F, AR) and urea (H₂NCONH₂, AR) were received from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Ti mesh was bought from Hang Xu Company in Hebei province (Hebei, China). Deionized water was utilized for preparation of all solutions. All reagents were analytical grade and utilized without further purification. Pt/C catalyst was purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai,

Results and discussion

In this work, the precursor of Co(OH)₂ nanoneedle array on Ti mesh (denoted as Co(OH)₂ NA/Ti) was firstly obtained via hydrothermal method, and then the CoS₂ NA/Ti was obtained through sulfuration in the presence of sublimed sulfur. Fig. 1A displays the X-ray diffraction (XRD) patterns of Co(OH)₂ NA/Ti and CoS₂ NA/Ti, and all peaks of CoS₂ NA/Ti were well matched with the standard diffraction patterns (JCPDS No. 41-1471). Besides CoS₂, the diffraction peaks at 30.6°, 35.3° and 46.8° could be

Conclusion

In summary, we have successfully synthesized CoS₂ NA/Ti through a facile hydrothermal treatment and a subsequent sulfuration process. As a non-noble metal catalyst, CoS₂ NA/Ti showed excellent catalytic activity for UOR and HER in the presence of urea. For full urea splitting, this bifunctional catalyst enabled a cell voltage of 1.59 V to achieve 10 mA cm⁻² in 1.0 M KOH with 0.3 M urea. Our work would provide a new avenue for developing other transition metal sulfides towards urea electrolysis, and

Acknowledgements

This work was supported by Qingdao Innovation Leading Expert Program, Qingdao Basic & Applied Research project (15-9-1-100-jch), and the Qingdao Postdoctoral Application Research Project (40601060003).

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¹: These authors contributed equally to this work.

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Research PaperCoS2 nanoneedle array on Ti mesh: A stable and efficient bifunctional electrocatalyst for urea-assisted electrolytic hydrogen production

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusion

Acknowledgements

J. Power Sources

Electrochim. Acta

J. Power Sources

Curr. Opin. Solid St. M.

Electrochim. Acta

J. Hydrogen Energy.

J. Alloy. Compd.

Electrochim. Acta

Electrochem. Commun.

Carbon

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Research Paper
CoS₂ nanoneedle array on Ti mesh: A stable and efficient bifunctional electrocatalyst for urea-assisted electrolytic hydrogen production