Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer

https://doi.org/10.1016/j.ijhydene.2019.10.169Get rights and content

Highlights

  • The Co3S4 nanosheets on Ni foam was prepared by electrodeposition and sulfurization for various time.

  • The Co3S4 nanosheets on Ni foam with sulfurization for 3 h indicated the highest sulfur content.

  • The Co3S4 nanosheets on Ni foam with sulfurization for 3 h showed a lowest overpotential of 93 mV at −10 mA/cm2 in 1 M KOH.

  • The single cell anion exchange membrane water electrolyzer (AEMWE) showed a high current density of 431 mA/cm2 at 2.0 Vcell.

Abstract

Co3S4 nanosheets on Ni foam (NS/NF) were prepared by sulfurization for various time after calcination of electrodeposited Co(OH)2. In our FE-SEM images, we observed that Co3S4 NS was vertically, or obliquely, deposited on the Ni foam. As a result, the structure contained more active sites, and active sites were highly accessible to the electrolyte for the hydrogen evolution reaction (HER). Furthermore, results of XPS and XRD analysis confirmed S-conversion from Co3O4 to Co3S4 during sulfurization. 3-Co3S4 NS/NF with sulfurization for 3 h exhibited the highest sulfur content, while Co3S4 began to desulfurize to Co9S8 after sulfurization for 4 h. The 3-Co3S4 NS/NF electrocatalyst showed a lowest overpotential of 93 mV at −10 mA/cm2, with a Tafel slope of −55.1 mV/dec in N2-purged 1 M KOH. Also, the single cell anion exchange membrane water electrolyzer (AEMWE) showed a high current density of 431 mA/cm2 with cell voltage 2.0 Vcell at 40–45 °C.

Introduction

Existing energy systems based on fossil fuels have caused problematic environmental pollution, resource depletion, and global warming. The hydrogen economy has been proposed as an alternative, as it employs hydrogen fuels as energy carriers instead of fossil fuels. One of the most fundamental problems that must be resolved to actualize the hydrogen economy is how to produce hydrogen fuels economically without emitting greenhouse gases or other pollutants [1]. Among the existing approaches to this problem, water electrolysis is the best means by which to produce large quantities of hydrogen, which is an infinite and clean energy source, from water [2,3]. Hydrogen production through water electrolysis is primarily accomplished with alkaline water electrolyzer (AWE) [4,5], proton exchange membrane water electrolyzer (PEMWE) [6,7], and anion exchange membrane water electrolyzer (AEMWE) [8,9]. The AWE has long been commercialized because of its non-noble metal catalyst, easy handing. However, the liquid based system causes drawback such as electrolyte leakage, low hydrogen purity and low stability. In case of PEMWE system, it can generate the high purity hydrogen and obtain high efficiency [10]. But, PEMWE system operates in a low pH and requires the use of noble metal catalyst such as platinum for high efficiency. So, the cost is a disadvantage for commercialization [11]. In view of the pros and cons of the two systems mentioned above, AEMWE has recently been developed as an alternative electrolysis system [12]. The AEMWE is a hybrid of the conventional AWE and the PEMWE. It can produce higher purity hydrogen than conventional AWE without undergoing a additonal reforming process. And also, this system advantage of producing high-pressure hydrogen with non-noble metal catalysts under alkaline conditions. However, the maturity of AEMWE technology is lower than AWE, it is still in the R&D stage. Therefore, much research is required until commercialization [12,13]. Currently, most studies are aimed at reducing the overpotential required for water electrolysis in catalytic units. Among the overpotentials generated from water electrolysis, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) generate the largest. Therefore, the greatest challenge to improve the efficiency of hydrogen production through water electrolysis is to minimize them. Noble metals, such as Pt, exhibit high HER activity due to low overpotential and excellent durability. However, the scarcity and high costs of noble metals limit their universal application [14,15]. Active research of metal sulfide [[16], [17], [18], [19]], metal phosphide [[20], [21], [22], [23]], and metal nitride [[24], [25], [26]] electrocatalysts is thus underway. These materials are produced by doping non-noble metals, such as Co, Ni, and Fe, with sulfur, phosphate, or nitrogen. Cobalt sulfides in particular have been reported to show excellent electrochemical activity and durability for the HER during alkaline water electrolysis [[27], [28], [29], [30], [31], [32]]. Furthermore, various cobalt sulfides, such as CoS2, Co3S4, and Co9S8, can be easily synthesized by adjusting the temperature and duration of sulfurization [33,34]. Among these, Co3S4 is known to be highly stable and demonstrates excellent electrocatalytic activity in the HER during alkaline water electrolysis [[35], [36], [37]].

In this study, we developed an effective strategy for synthesis of Co3S4 nanosheets on Ni foam (NS/NF) as HER electrocatalyst through electrodeposition and sulfurization as shown in Scheme 1. First, Co(OH)2 nanosheets were prepared on Ni foam by electrochemical deposition. Second, Co3O4 was obtained from Co(OH)2 by calcination. Finally, Co3S4 was synthesized from Co3O4 by sulfurization. By controlling the sulfurization conditions, Co3S4 NS/NF having excellent HER activity was achieved. We also confirmed the feasibility of using synthesized Co3S4 NS/NF electrocatalyst in a single cell AEMWE system as well as confirming the electrocatalytic performance in a half cell test.

Section snippets

Material synthesis

Co3S4 NS/NF was synthesized as shown in Scheme 1. The catalyst layer was electrodeposited onto a Ni foam substrate (Pore size: 580 μm, Alantum, South Korea). Prior to electrodeposition of Co(OH)2 NS onto the Ni foam, the foam was treated with 6 M HCl in an ultrasonic bath for 20 min to remove the NiO surface layer. The foam was then washed with acetone, ethanol and DI water in an ultrasonic bath for 20 min. Electrodeposition of Co(OH)2 NS onto the Ni foam was performed in 0.05 M Co(NO3)2·6H2O

Result and discussion

The surface morphology of 3-Co3S4 NS/NF imaged by FE-SEM is shown in Fig. 1a. A thin, curled Co3S4 NS was fabricated at a fixed size and high density on the Ni foam in 3-Co3S4 NS/NF. In addition, the average thickness of the Co3S4 NS layer on 3-Co3S4 NS/NF was 1.094 μm (Fig. 1b). The morphology of the catalyst was governed by the electrodeposited Co(OH)2 layer, and the Co3O4 NS/NF exhibited the same morphology before and after sulfurization for various times (Fig. S1, Fig. S5a). Furthermore,

Conclusion

In conclusion, we have demonstrated that Co3S4 NS/NF is a good electrode for the HER in alkaline water electrolysis and single cell AEMWE. Co3S4 NS/NF was synthesized through electrodeposition and sulfurization for various times. The synthesized Co3S4 NS/NF had a vertically or obliquely-grown 3D-structure with a large surface area, where curled Co3S4 NS was densely and evenly packed on Ni foam. Furthermore, S-conversion occurred on the NS surface of Co3O4 NS/NF during the sulfurization process.

Acknowledgement

This study was supported by the Fundamental Research Program of the Korean Institute of Materials Science (Grant PNK6130) and the New & Renewable Energy Core Technology Program of the KETEP (KETEP-20173010032080) in Republic of Korea. This research was supported by the Basic Science Research Program of the NRF (NRF-2017R1D1A1B03029419) in the Republic of Korea.

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    These authors contributed equally to this work.

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