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

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Available online at www.sciencedirect.com

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Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer Yoo Sei Park a,b,1, Jeong Hun Lee a,b,1, Myeong Je Jang a,c, Jaehoon Jeong a, Sung Min Park a,b, Woo-Sung Choi a, Yangdo Kim b,**, Juchan Yang a,***, Sung Mook Choi a,* a

Materials Center for Energy Convergence, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon, 51508, Republic of Korea b Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea c Advanced Materials Engineering, Korea University of Science and Technology (UST), Daejeon, 34113, Republic of Korea

highlights

graphical abstract


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.

article info

abstract

Article history:

Co3S4 nanosheets on Ni foam (NS/NF) were prepared by sulfurization for various time after

Received 3 August 2019

calcination of electrodeposited Co(OH)2. In our FE-SEM images, we observed that Co3S4 NS

Received in revised form

was vertically, or obliquely, deposited on the Ni foam. As a result, the structure contained

2 October 2019

more active sites, and active sites were highly accessible to the electrolyte for the hydrogen

Accepted 20 October 2019

evolution reaction (HER). Furthermore, results of XPS and XRD analysis confirmed S-con-

Available online xxx

version from Co3O4 to Co3S4 during sulfurization. 3-Co3S4 NS/NF with sulfurization for 3 h

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Y. Kim), [email protected] (J. Yang), [email protected] (S.M. Choi). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.10.169 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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Keywords: Co3S4 nanosheets

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

Electrocatalyst

93 mV at 10 mA/cm2, with a Tafel slope of 55.1 mV/dec in N2-purged 1 M KOH. Also, the

Electrodeposition

single cell anion exchange membrane water electrolyzer (AEMWE) showed a high current

Hydrogen evolution reaction(HER) AEM water electrolyzer

density of 431 mA/cm2 with cell voltage 2.0 Vcell at 40e45  C. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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 [16e19], metal phosphide [20e23], and metal nitride [24e26] 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 [27e32]. 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 [35e37]. 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.

Scheme. 1 e Schematic illustration for the synthesis process of Co3S4 NS/NF. 1,2,3 and 4-Co3S4 NS/NF obtained by the different sulfurization time (1,2,3 and 4 h).

Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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Experimental 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 mm, 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 (Cobalt nitrate hexahydrate, Sigma Aldrich) electrolyte for 5 min at a potential of 1 V. Electrodeposition of Co(OH)2 NS was performed with an electrochemical workstation (VSP, Bio-Logic Science Instruments, France) in a three-electrode cell consisting of an etched Nifoam working electrode (0.5 cm  0.5 cm), a Pt-mesh counter electrode, and a saturated calomel reference electrode (SCE). Co3O4 NS/NF was synthesized by calcination of electrodeposited Co(OH)2 NS/NF at 200  C for 10 min at a heating rate of 10  C/min in an Ar atmosphere. Synthesis of 1, 2, 3, and 4Co3S4 NS/NF was performed by heating Co3O4 NS/NF to 250  C and sulfur powder (Sigma Aldrich) to 180  C at a rate of 3  C/min for 1e4 h in an Ar atmosphere. The Co3S4 NS/NF samples obtained by sulfurization for 1 h, 2 h, 3 h, and 4 h were designated 1-Co3S4 NS/NF, 2-Co3S4 NS/NF, 3-Co3S4 NS/NF, and 4-Co3S4 NS/NF, respectively. To fabricate a single AEMWE cell, a Cu0$81Co2$19O4 anode was synthesized onto Ni foam substrate in 0.01 M Co(NO3)2$6H2O and 0.002 M Cu(NO3)2$5H2O at 1 V during 5 min. The deposited sample was annealed at 250  C in air atmosphere for 3 h [38], and then used as anode in an AEMWE single cell.

Physicochemical characterization The morphologies of the synthesized Co(OH)2 NS/NF, Co3O4 NS/NF, and 1e4 Co3S4 NS/NF samples were analyzed with a JSM-7001F (JEOL, Tokyo, Japan) field emission scanning electron microscope (FE-SEM) and a JEM-2100F (JEOL, Tokyo, Japan) high-resolution transmission electron microscope (HRTEM). The crystallinity of each sample was investigated by Xray diffraction (XRD) analysis on a D/MAX 2500 diffractometer (Rigaku, Tokyo, Japan) equipped with a Cu target. Scans were performed from 20 to 70 (2W) at a rate of 0.02 /s at 40 kV and 250 mA. Depth profiling was performed by adjusting the divergence slit (D. S.) to 1/6 , 1/4 , 1/2 , and 1 to control the incident X-ray angle. Elemental composition was determined by X-ray photoelectron spectroscopy (XPS) with a K-Alpha XPS system (ThermoFisher Scientific, Waltham, MA) equipped with an Al Ka (1486.6 eV) X-ray source.

Electrochemical characterization To evaluate the catalytic performance of Co3O4 NS/NF and 1e4 Co3S4 NS/NF, linear sweep voltammetry (LSV) and durability tests were performed with electrochemical workstation (VMP-3, Bio-Logic, France). Co3O4 NS/NF and Co3S4 NS/NF (0.5 cm  0.5 cm) were used as working electrodes. A graphite

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rod and an Hg/HgO (1 M KOH) electrode were used as the counter and reference electrodes, respectively. LSV was conducted from 0 to 0.4 V (vs. RHE), and durability tests were performed by applying a constant current of 10 mA/cm2 for 220 h. The HER activity of Pt/C was evaluated using RDE (rotating disk electrode) three-electrode system. Threeelectrode systems were configured with graphite rod as a counter electrode and a commercial Hg/HgO for alkaline media as a reference electrode. Aqueous solution of 1 M KOH was used as electrolyte. The rotating disk electrodes (RDEs) were polished consecutively with two different alumina powders (0.3 and 0.05 mm) and cleaned by 20 min sonication in deionized water for removing alumina powders from electrodes. 13.5 mg of catalyst powders (40 wt % Pt/C, Johnson Matthey) was dispersed in a mixture of 950 mL of ethanol with 50 mL of water and 16.62 mL of 5 wt % Nafion suspension in alcohol (Sigma- Aldrich) by sonication for 30 min 5 mL droplet of the catalyst ink (Pt loading amount: 0.332 mg/cm2) placed on disk of polished RDEs was dried in an oven at 80  C for 5 min to evaporate solvent. Electrolytes were purged with nitrogen for measuring the HER currents. All potentials were reported versus reversible hydrogen electrode (RHE) (VRHE ¼ E (vs. Hg/HgO) þ 0.098 þ 0.059 pH).

Anion exchange membrane water electrolyzer (AEMWE) To evaluate the catalytic performance of a single cell, AEMWE consisting of cathode, anode and gas diffusion layer (GDL) was used. The cathode and anode were prepared by the catalystcoated diffusion layer (CCD) method by electrodeposition without binder. 3-Co3S4 NS/NF was used as a cathode. The previously reported copper cobalt oxide (Cu0$81Co2$19O4) was used as the anode on nickel foam [38]. The GDL was prepared by pressing nickel foams. Sustainion® X37-50 (Dioxide Materials, USA) was used as the anion exchange membrane (AEM). For fabrication of membrane electrode assembly (MEA), all components were constructed in the form of zero gap. The configuration of the MEA is as follows: GDL k 3-Co3S4 NS/NF cathode k AEM k Cu0$81Co2$19O4 NS/NF anode k GDL. A bipolar plate and endplate were made of stainless steel to avoid corrosion. The electrolyte of 1M KOH was supplied to the AEMWE with feed rate of 25 mL/min and operation temperature was 45e48  C. To investigate the electrocatalytic performance of single cell AEMWE, LSV and durability test was performed with a BP2C electrochemical workstation (ZIVE LAB, Korea). The LSV was conducted from 1.4 Vcell to 2.2 Vcell at 10 mV/s and a durability test were performed by applying a constant current of 500 mA/cm2 for 12 h.

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 3Co3S4 NS/NF was 1.094 mm (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).

Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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Fig. 1 e (a) The FE-SEM image of the 3-Co3S4 NS/NF. (b) The cross-sectional FE-SEM image of 3-Co3S4 NS/NF. (c) Bright field TEM image of Co3S4 NS in 3-Co3S4; upper inset is the magnified planar view in the marked area; lower inset is the corresponding fast fourier transform (FFT) patterns. (d) TEM image and EDS mapping of Co3S4 NS in 3-Co3S4 for Co, O and S.

Furthermore, the three-dimensional (3D) structure of Co3S4 NS grown obliquely or vertically on the Ni foam increased the accessibility of the Co3S4 NS to the electrolyte during the HER and thus provided more active sites [39e41]. This will be discussed in more detail later. The XRD patterns of Co3O4 NS/NF and 3-Co3S4 NS/NF are shown in Fig. 2. Peaks from the (222) and (440) planes of Co3O4 (JCPDS # 74-1657) appeared at 38.1 and 64.88 in the Co3O4 NS/ NF pattern. On the other hand, peaks from the (220), (311), (400), (511), and (440) planes of Co3S4 (JCPDS # 42-1448) were observed at 26.52 , 31.28 , 37.92 , 49.88 , and 54.78 in the 3Co3S4 NS/NF pattern. It was peculiar that no distinct Co3O4 pattern was observed in the 3-Co3S4 NS/NF data. Furthermore, characteristic Co3O4 peaks were not observed in the patterns of all 1, 2, 3 and 4-Co3S4 NS/NF samples after sulfurization (Fig. S2), and only the characteristic patterns of Co3S4 were observed. However, peaks from the (115), (044), and (244) planes of Co3O4 appeared in the depth profiling results from the 3-Co3S4 NS/NF as the D. S. was adjusted from 1/6 up to 1 (Fig. S3). In light of these results, we concluded the NS on the 3Co3S4 NS/NF surface consisted of Co3S4, while Co3O4 comprised the inner bulk of the Co3S4 NS. Therefore, it could be seen that S-conversion occurred as predicted on the surfaces of 1, 2, 3, and 4-Co3S4 NS during sulfurization of Co3S4 NS/NF. TEM analysis was performed to confirm the crystal structure of 3-Co3S4 in more detail. The 3-Co3S4 NS/NF TEM image

revealed the 3-Co3S4 NS consisted of multiple thin NS layers (Fig. 1c). As shown in the upper inset, the (311), (444) planes of Co3S4 and (400) plane of Co9S8 were observed with interlayer distances of 0.27 nm, 0.13 nm and 0.25 nm, respectively. The interlayer distance of the (SAED) pattern (lower inset) also

Fig. 2 e The XRD patterns of 3-Co3S4 NS/NF (upper) and Co3O4 NS/NF (lower) over the 2W range of 20e70 . The characteristic peaks of substratal Ni foam in XRD spectra of Co3O4 NS/NF and 3-Co3S4 NS/NF were hidden.

Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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showed the (311) plane of Co3S4 and the (400) plane of Co9S8 in a ring pattern, which confirmed that Co3S4 and Co9S8 were present in a polycrystalline phase. The TEM-EDS and mapping results shown in Fig. 1d and Fig. S4 indicated the 3-Co3S4 NS/ NF was composed of Co, O, and S, and the quantity of O was greater than that of S. This was likely because S reacted only on the Co3O4 NS surface during sulfurization of Co3O4 NS/NF, which was consistent with the depth-profiling results as the D. S. was adjusted. As can be seen in Fig. S5b, the Co3O4 NS of Co3O4 NS/NF prior to sulfurization was composed of multiple thin NS layers. The interlayer distances determined from the TEM and SAED patterns confirmed that polycrystalline Co3O4 was synthesized. Furthermore, EDS mapping (Fig. S4c and Fig. S5c) showed that it consisted only of Co and O and contained no S. XPS analysis was performed to examine the composition and chemical binding environments of each catalyst. Fig. S7ad shows a survey XPS spectrum of 1,2,3 and 4-Co3S4 NS/NF, indicating the presence of C, Co, Ni, O and S. The Co 2p XPS spectrum of 3-Co3S4 NS/NF is shown in Fig. 3a. The Co3þ peaks in the spectrum appeared at 780.77 eV and 795.97 eV, while Co2þ peaks were observed at 782.37 eV and 797.27 eV. In addition, satellite peaks appeared at 787.12 eV and 802.42 eV. These were consistent with previously reported peaks in the Co 2p spectrum of Co3S4 [42,43]. A peak corresponding to OH-

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oxygen appeared at 531.17 eV of the O 1s spectrum in Fig. 3b [44]. Furthermore, distinct peaks were observed at 162.62 eV and 163.77 eV in the S 2p spectrum, and a satellite peak at 167.77 eV can be seen in Fig. 3c. These results were consistent with previously reported characteristic peaks in the S 2p spectra of Co3S4 [45]. These findings verified that Co3S4 had formed. The Co 2p, O 1s, and S 2p XPS spectra from the Co3O4 NS/NF are shown in Fig. S6. The characteristic Co3O4 peak was observed in the Co 2p spectrum, and the characteristic peaks of oxygen in M-O, OH, and structural water were visible in the O 1s spectrum [46e48]. However, no peaks were observed in the S 2p spectrum. A comparison of the O 1s and S 2p spectra of 3-Co3S4 NS/NF and Co3O4 NS/NF therefore confirmed that peaks characteristic of oxygen in M-O and structural water disappeared after sulfurization, and that the characteristic Co3þ and Co2þ peaks of Co3S4 appeared in the S 2p spectrum. This suggested that Co3O4 was converted to Co3S4 on the surface of Co3O4 NS during the sulfurization process. Fig. 3d shows the correlations between sulfurization time and the S atomic ratios of 1e4 Co3S4 NS/NF that were identified by XPS determination of the degree of sulfurization. The highest S atomic ratio of approximately 15% was found in 3Co3S4 NS/NF. This was also indicated by the Co 2p3/2 peak shift in the XPS spectra of the Co3S4 NS/NF catalysts for 3 h sulfurization time. (Fig. S8). The Co3þ and Co2þ peak profile in

Fig. 3 e The XPS analysis results of 3-Co3S4 NS/NF after sulfurization for 3 h. The high-resolution spectra of (a) Co 2p, (n) O 1s, (c) S 2p. (d) the changes in atomic ratio of sulfur in Co3S4 NS/NF with different sulfurization times obtained from XPS results. Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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the Co 2p spectrum of 1-Co3S4 NS/NF was similar to the characteristic profile of Co3O4. After sulfurization for >2 h, the Co3þ and Co2þ peaks appeared at energies corresponding to those characteristic of Co3S4 and were observed in the Co 2p spectra collected with 2, 3, and 4-Co3S4 NS/NF. We therefore concluded that after >2 h of sulfurization, S-conversion was effective on the NS surface of Co3O4 NS/NF. From Co3S4 to Co9S8, the ratio of S to Co gradually reduced with an annealing time when the sulfurization was carried out for 4 h [33]. The relative ratio of S in the synthezied catalysts with different sulfurization time were confirmed using the integral area and relative sensitivity factor of the XPS survey spectrum. (Fig. 3d, Fig. S7). HER activity in N2-purged 1 M KOH was examined to evaluate the electrocatalytic performance of the Co3O4 NS/NF and the synthesized Co3S4 NS/NF over time (Fig. 4). The HER polarization curves of Co3O4 NS/NF and Co3S4 NS/NF are shown in Fig. 4a. Co3O4 NS/NF did not have catalytic activity, but the 3-Co3S4 NS/NF electrocatalyst exhibited better catalytic activity with a low overpotential of 93 mV at 10 mA/cm2 than the other synthesized catalysts. Fig. 4b shows the Tafel slopes of Co3O4 NS/NF and Co3S4 NS/NF. The Tafel slope of 3-Co3S4 NS/NF was the lowest (55.1 mV/dec) compared to those of Co3O4 NS/NF (81.3 mV/dec), 1-Co3S4 NS/NF (57.4 mV/dec), and Pt/C (66.4 mV/dec). This indicated that 3-Co3S4 NS/NF had more favorable kinetics and

superior catalytic activity. In addition, 3-Co3S4 NS/NF were compared with the reported cobalt sulfide electrocatalyst in Table S1 and it has lowest overpotential and Tafel slope among reported cobalt sulfide electrocatalysts. Durability is also important for electrocatalysts [49]. To investigate the durability of the 3-Co3S4 NS/NF catalyst under hydrogen production conditions, the experiment was carried out at a constant current for 220 h, as shown in Fig. 4c. The y-axis in Fig. 4c means the changes of overpotential compare with initial overpotential and after overpotential during durability test (hi: the initial overpotential, ha: the after overpotential). The value of zero in the y-axis is not theoretical value of 0 VRHE which generated by hydrogen. The electrocatalyst with superior durability has a small increase in overpotential. Therefore, when comparing the durability of Pt/C, Co3O4 NS/ NF and 3-Co3S4 NS/NF, 3-Co3S4 NS/NF has excellent durability because the smallest change in the y value of the 3-Co3S4 NS/ NF is observed. At a constant current of 10 mA/cm2, the 3Co3O4 NS/NF maintains a relatively constant potential without increasing the overpotential gap (ha- hi) during the durability test. On the other hand, for commercial Pt/C catalysts, a large overpotential change occurred during the durability test. The durability of Co3O4 NS/NF was found to drop during the constant current condition through continuous y-axid decrease.

Fig. 4 e HER half cell test in N2-saturated 1M KOH. (a) LSV polarization curves for hydrogen evolution reaction of Co3O4 NS/ NF, 1e4 Co3S4 NS/NF and 40 wt% Pt/C. The geometric areas of Co3O4 NS/NF, 1e4 Co3S4 NS/NF were always 0.25 cm2. Polarization curve of 40 wt% Pt/C obtained with rotating disk electrode of glassy carbon in 0.2 cm2. (b) Corresponding tafel plots of Co3O4 NS/NF, 1e4 Co3S4 NS/NF and 40 wt% Pt/C derived from the polarization curves of A. (c) Chronopotentiometry curves of Co3O4 NS/NF, 3-Co3S4 NS/NF and 40 wt% Pt/C under constant current of ¡10 mA/cm2. (d) Plot of the atomic concentrations for sulfur and the overpotentials with various sulfurization time.

Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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The correlations between the S atomic ratio and the electrocatalytic overpotential according to sulfurization time are shown in Fig. 4d. The overpotential was significantly lower after sulfurization than it was prior to sulfurization. Most notably, the lowest overpotential was observed following sulfurization for 3 h, and the S atomic ratio was highest in this sample. Therefore, S-conversion from Co3O4 NS/NF to Co3S4 NS/NF was most effective when sulfurization was performed for 3 h at 250  C. Sulfur in cobalt sulfide is strongly electronegative, and the compound is polarized by deflected electrons. The negatively charged sulfur can capture a positively charged proton in the electrochemical hydrogen evolution reaction. Therefore, the increase of the atomic ratio of sulfur in the metal sulfide can explain the more effective HER because more protons are adsorbed to the electrode than in the case of metallic cobalt or cobalt oxide. In addition, the sulfur is improving the electrical conductivity and reducing the energy barrier of H2 formation which lead to the enhancement of HER activity [50,51]. Consequently, 3-Co3S4, in which the largest quantity of S was incorporated in Co3S4, afforded the highest electrocatalytic activity and the lowest overpotential. To evaluate electrocatalytic performance of 3-Co3S4 NS/NF as cathode in single cell AEMWE, LSV and durability test were

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performed (Fig. 5). Fig. 5a shows the schematic illustration of AEMWE single cell system with 3-Co3S4 NS/SF electrocatalyst as cathode part. Addtionally, Fig. 5b shows photograph for components of single cell AEMWE. The AEMWE was operated at 45e48  C by electrolyte temperature. The operating temperature of water electrolyzers is an important factor in performance of AEMWE. When the operating temperature of AEMWE is high, kinetics, ion conductivity and mass transport are improved, so that it can exhibit excellent performance. However, at too high temperature, the degradation of the electrode and AEM is promoted. Therefore, the assembled AEMWE was operated below 50  C [52]. The single cell AEMWE exhibited a current density of 431 mA/cm2 at 2.0 Vcell (Fig. 5c). In addition, Fig. 6 indicates the performance of AEMWE for comparison with other system [53e60]. Our single cell AEMWE showed high performance among reported single cell AEMWEs using non-noble metal catalyst. The performance of single cell AEMWE is affected by various factors such as MEA fabrication method, electrolyte flow rate, type of AEM, and catalyst ink formula [9,38,61]. Therefore, the detailed conditions of the single cell AEMWE compared in Fig. 6 were summarized in Table S2. The durability test result of single cell AEMWE shown in Fig. 5d performed by chronopotentiometry at 500 mA/cm2.

Fig. 5 e Schematic illustration, photograph and HER activity of single cell AEMWE in 1M KOH. The Co3S4 and Cu0·81Co2·19O4 NS/NF were applied to HER and OER electrode, respectively. (a) Schematic illustration for the single cell AEMWE. (b) Photograph of single cell AEMWE. HER activity of single cell AEMWE in 1M KOH. The Co3S4 and Cu0·81Co2·19O4 NS/NF were applied to HER and OER electrode, respectively. (c) The LSV polarization curves. (d) Chronopotentiometry curves under constant current of 500 mA/cm2 for durability test. Please cite this article as: Park YS et al., Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.169

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Fig. 6 e Comparison of the reported single cell AEMWE performance using non-precious metal catalyst.

The high current density of 500 mA/cm2 was chosen to evaluate the durability under actual AEMWE operationg conditions. As shown in Fig. 5d, the initial cell voltage was 2.01 Vcell and increased by only 0.05 Vcell.

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.

Conclusion

Appendix A. Supplementary data

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. It was confirmed that Sconversion was effective with sulfurization times longer than 2 h. Additionally, desulfurization occurred with sulfurization times greater than 4 h, and Co3S4 was converted to Co9S8. The 3-Co3S4 NS/NF obtained following sulfurization for 3 h showed the highest HER activity in purged 1M KOH, with an overpotential of 93 mV at 10 mA/cm2 and a Tafel slope of 55.1 mV/dec. Also, this catalyst demonstrated excellent durability without significant changing the overpotential for 220 h. Furthermore, the single cell AEMWE with 3-Co3S4 NS/NF cathode achived a current density of 431 mA/cm2 with cell voltage 2.0 Vcell at 40e45  C and this single cell AEMWE demonstrated excellent durability without changing the cell voltage for 720 min.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.169.

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

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