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Enhanced Catalysis of Electrochemical Overall Water Splitting in Alkaline Media by Fe Doping in Ni3S2 Nanosheet Arrays Geng Zhang, Yu-Shuo Feng, Wang-Ting Lu, Dan He, CaoYu Wang, Yong-Ke Li, Xun-Ying Wang, and Fei-Fei Cao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00413 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Catalysis

Enhanced Catalysis of Electrochemical Overall Water Splitting in Alkaline Media by Fe Doping in Ni3S2 Nanosheet Arrays

Geng Zhang,a Yu-Shuo Feng,a Wang-Ting Lu,b,c Dan He,c Cao-Yu Wang,a Yong-Ke Li,a Xun-Ying Wangd and Fei-Fei Cao*a

a

Department of Chemistry, College of Science, Huazhong Agricultural University, 430070,

Wuhan, P. R. China. b

Institute for Interdisciplinary Research, Jianghan University, 430056, Wuhan, P. R. China.

c

Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education,

School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, China. d

Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key

Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, 430062, Wuhan, P. R. China.

*To whom correspondence should be addressed. E-mail: [email protected] (Fei-Fei Cao)

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Abstract The development of bifunctional electrocatalyst with high performances for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with earth-abundant elements is still a challenge for the electrochemical water splitting technology. Herein, we fabricated a free-standing electrocatalyst in the form of vertically oriented Fe-doped Ni3S2 nanosheet array grown on 3D Ni foam (Fe-Ni3S2/NF), which presented a high activity and durability for both HER and OER in alkaline media. On the basis of systematic experiments and calculation, the Fe-doping was evidenced to increase the electrochemical surface area, improve the water adsorption ability and optimize the hydrogen adsorption energy of Ni3S2, which resulted in the enhancement of HER activity on Fe-Ni3S2/NF. Moreover, metal sites of Fe-Ni3S2/NF were proved to play a significant role in the HER process. During the catalysis of OER, the formation of Ni-Fe (oxy)hydroxide was observed on the near-surface section of Fe-Ni3S2/NF, and the introduction of Fe element dramatically enhanced the OER activity of Ni3S2. The overall-water-splitting electrolyzer assembled by Fe-Ni3S2/NF exhibited a low cell voltage (1.54 V @ 10 mA cm-2) and a high durability in 1 M KOH. This work demonstrated a promising bifunctional electrocatalyst for water electrolysis in alkaline media with potential application in the future.

Keywords: Sulfide; Doping; Bifunctional electrocatalyst; HER; OER

1. Introduction Hydrogen is one of the most promising alternatives to traditional fossil fuels due to its high energy density (~282 kJ mol-1) as well as clean and efficient utilization process through fuel cells.1-3 The development of efficient, clean and low-cost hydrogen production technology is of crucial importance to the application of hydrogen energy.4 Electrolysis of water by renewable energy, such as solar and wind energy, is believed to be an ideal approach to produce hydrogen of high quality.5-6 Currently, two main types of water electrolysis are available, which is based on alkaline liquid electrolyte and proton exchange membrane (PEM), respectively.7-8 In comparison with PEM water electrolysis, the usage of expensive noble metal catalysts and perfluorinated Nafion-based PEM can be avoided in alkaline water electrolysis.7,

9

Moreover, the future

employment of alkaline solid electrolyte (i.e., anion exchange membrane) will further eliminate 2


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the shortcomings of water electrolysis caused by liquid electrolyte (e.g., gas crossover and alkaline liquid corrosion).9 Even so, the key factor determining whether water electrolysis technology can be widely used is the energy consumption, i.e., cell voltage. The cell voltage of water electrolysis largely depends on the overpotential of hydrogen evolution reaction (HER) at the anode and oxygen evolution reaction (OER) at the cathode of electrolyzer. Pt and Ir/Ru oxide is the most efficient electrocatalyst for HER and OER, respectively. However, the usage of scarce noble metals is undoubtedly an obstacle to the large-scale application of water electrolysis technologies. Traditionally, alkaline water electrolyzers use stainless steel, Raney Ni and Ni alloys as catalysts, which are cheap but not active enough.8, 10-12 Therefore, it is of highly importance to develop efficient electrocatalyst for both HER and OER with earth-abundant elements under alkaline conditions. Owing to the historic application of metallic Ni electrode in alkaline water electrolysis as mentioned above, tremendous research efforts have been invested to develop high-performance Ni-based materials as alternatives to noble metals,12-15 in which Ni sulfides (e.g., NiS, NiS2 and Ni3S2) were paid much attention due to their low cost and facile fabrication, and several promising candidates have been reported.16-28 Among various Ni sulfides, Ni3S2 stood out because of its intrinsic metallic behaviors with high conductivity, an important property for electrocatalyst.16, 18-19, 26, 29

To further improve the activity of electrocatalysts, element doping has been well

established as an effective strategy by means of increasing active surface area, enhancing electronic conductivity, optimizing adsorption/desorption energetics of intermediates, etc.30-35 Without exception, Fe,16, 36 Sn,37 Mo,38 Zn39 or V40 doped Ni3S2 showed superior activity to pure Ni3S2. However, we found that most of these doped Ni3S2 only presented an improved performance in either HER or OER process, but highly active Ni3S2-based electrocatalyst for both HER and OER was rarely reported.38 It is believed that the use of bifunctional electrocatalyst in water splitting can help to simplify the system and reduce the cost of manufacturing, but it is still a tough challenge to achieve such materials that are stable and catalytic efficient for both HER and OER in the same electrolyte simultaneously.6,

41

Hence, the development of Ni3S2-based

bifunctional electrocatalyst with high performances for overall water splitting is an attractive but challenging work. Revealing the actual active sites or species is always an important task in the development of 3


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new electrocatalyst. Some researchers have found that metallic oxide and/or (oxy)hydroxide is in-situ formed on the surface of metal sulfides, selenides, nitrides and phosphides under OER conditions, which is believed to be responsible for OER catalysis actually.42-43 In contrast, the attention focused on the catalytic sites of HER was inadequate, because it was assumed that electrocatalyst was stable under HER conditions in comparison with that in harsh OER conditions.21 However, Zheng’s group recently found that NiS2 was converted to metallic Ni during the catalysis of HER, and Ni(0) was believed to be the inherent active species.21 In addition, the role of metal and sulfur atom on sulfide material in the catalysis of HER was not well defined yet. Many reports speculated the active site based on theoretical calculations.19, 23, 26, 44-45 As an improvement, Zhang et al. adopted operando poison experiments besides calculation, and observed that HER-inert sulfur atoms of CoS2 were activated after Cu doping.46 Therefore, it is highly necessary to carry out a comprehensive study on the real active species of electrocatalyst during HER and OER process. Herein, we provided a free-standing electrocatalyst in the form of vertically oriented Fe-doped Ni3S2 (Fe-Ni3S2) nanosheet array grown on 3D Ni foam (NF) with robust activity for both HER and OER in alkaline media. Although Fe-Ni3S2 has been reported as active material for OER,16, 47 the application of Fe-Ni3S2 in the catalysis of HER and overall water splitting has rarely been reported. In order to determine the actual active species during catalytic process, the structure change of Fe-doped Ni3S2 after long-term operation for HER and OER was analyzed. On this basis, we studied the origin of high activity of Fe-Ni3S2 in HER via systematic experiments and theoretical calculation. The water electrolyzer using Fe-Ni3S2/NF as the anode and cathode presented a comparable initial performance to benchmark Pt/C||IrOx couple but better durability.

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Figure 1 (a) Schematic illustration of the fabrication of Fe-Ni3S2/NF. (b) A two-electrode configuration of Fe-Ni3S2/NF for overall water splitting in alkaline media.

Table 1 The Fe/Ni feed ratio and atomic ratio for Fe-doped Ni3S2/NF with various Fe content. Fe-doped Ni3S2

a

Fe(NO3)3/Ni(NO3)2 feed

Fe/Ni atomic ratio in a

Corresponding

ratio (mol/mol)

Fe-Ni3S2 from EDS

NiFe LDH/NF

Fe4.9%- Ni3S2/NF

0.1/2.125 = 1/21.3

1/19.5

NiFe LDH/NF-1

Fe7.9%- Ni3S2/NF

0.225/2 = 1/8.9

1/11.7

NiFe LDH/NF-2

Fe17.5%- Ni3S2/NF

0.45/1.775 = 1/3.9

1/4.73

NiFe LDH/NF-3

Fe25.9%- Ni3S2/NF

0.75/1.475 = 1/1.97

1/2.86

NiFe LDH/NF-4

Fe36.6%- Ni3S2/NF

1.1/1.125 = 1/1.02

1/1.73

NiFe LDH/NF-5

Fe24%-Ni3S2/NF(w/o Ni(NO3)2)

2/0

1/3.16

NiFe LDH/NF-6

The EDS test was performed on Fe-Ni3S2 nanosheets peeled off from Ni foam by sonication.

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Figure 2 (a) SEM images of NF. (b-e) SEM images of Fe17.5%-Ni3S2/NF. (f) EDS spectrum of Fe17.5%-Ni3S2/NF. (g) XRD patterns of Ni3S2/NF and Fe-Ni3S2/NF with different Fe content. (h) The XPS signal taken at binding energy between 700 and 740 eV from Fe17.5%-Ni3S2/NF and Ni3S2/NF; the inset shows the Fe 3p signal of Fe17.5%-Ni3S2/NF. (i) High-resolution S 2p spectra of Fe17.5%-Ni3S2/NF and Ni3S2/NF.

2. Results and Discussion 2.1 Catalyst Synthesis and Structure Analysis The fabrication of Fe-Ni3S2/NF was realized by a facile two-step strategy (Figure 1a). NF with macropores and 3D conductive network was adopted as the substrate for the growth of electrocatalyst (Figure 2a). First, NiFe LDH nanosheet array was deposited on NF (NiFe LDH/NF) by a hydrothermal treatment of NF in a solution containing Ni(NO3)2, Fe(NO3)3, urea and NH4F with various Fe/Ni feed ratio.48 After reaction, the color of NF was changed from silver white to yellow green at low Fe/Ni feed ratio or yellow at high Fe/Ni feed ratio (Figure S1). The SEM, EDS and XRD techniques proved the successful synthesis of NiFe LDH on NF (Figure S2). In contrast, a Fe-free sample named as Ni(OH)2·0.75H2O/NF was also fabricated by similar 6


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procedures (Figure S3). In order to obtain sulfide, NiFe LDH/NF was then treated in a solution of Na2S, during which hydroxide will be transformed to sulfide via the ion exchange of OH- by S2-. After sulfuration, the color of NiFe LDH/NF was changed to black (Figure S1). The SEM detected that the sulfide anchored on the NF also presented in the form of vertically aligned nanosheet array (Figure 2b-2e), similar with the morphology of NiFe LDH (Figure S2) and also the Ni3S2 nanosheet array (Figure S4) derived from Ni(OH)2·0.75H2O. The EDS confirmed the presence of Fe and S element in sulfurated product (Figure 2f), and the Fe/Ni atomic ratio measured by EDS was close to that of Fe/Ni feed ratio (Table 1). Interestingly, the NiFe LDH after sulfuration showed XRD patterns the same with those of Ni3S2 (PDF #44-1418) and no peaks ascribed to Fe sulfides can be detected with Fe content up to at least 36.6% (Table 1), suggesting that Fe atoms are probably stabilized in a doping state in the lattice of Ni3S2 (Figure 2g and Figure S5), thus the sulfide product was denoted as Fe-Ni3S2/NF. Furthermore, we used XPS to analyze the near surface composition of Fe-Ni3S2/NF. Unfortunately, the commonly used Fe 2p signal (the most intense XPS peak of Fe element) can’t be used to analyze Fe element in the product, because the binding energy range where Fe 2p signal exists is overlapped by the Ni LMM signal (Figure 2h).49 Fortunately, a weak Fe 3p signal can be detected, which proved the presence of Fe element in Fe-Ni3S2/NF. The S 2p XPS spectra are shown in Figure 2i. The components locating at 161.8 and 163.4 eV come from Sn2- and metal-S bonds in Fe-Ni3S2/NF.50-51 It should be pointed out that these two peaks shifted negatively compared with that of Ni3S2/NF (162.4 and 164.0 eV), suggesting an extra electron transfer to S atoms in Fe-Ni3S2/NF, which is likely due to the existence of doped Fe atoms in Fe-Ni3S2/NF, because the electronegativity of Fe (1.83) is lower than that of Ni (1.92), thus the electron transfer from Fe to neighboring S atoms is more remarkable than that between Ni and S atoms, which also support the doping of Fe in the Ni3S2 matrix. All of the above results definitely proved the successful fabrication of Fe doped Ni3S2 nanosheet array on NF. The facile doping of Fe in Ni3S2 may arise from the uniform distribution of Fe atoms in NiFe LDH, where no other Fe compounds except NiFe LDH have been detected with the increase of Fe/Ni feed ratio, even when Ni salt is excluded in the synthesis (Table 1 and Figure S2d). As a result, in the following sulfuration process, the NiFe LDH with uniform element distribution is easily transformed into Fe-doped Ni3S2.

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Figure 3 (a) TEM image, (b) SAED pattern and (c) HRTEM image of Fe17.5%-Ni3S2 nanosheet. The inset of panel (c) is the fast-fourier-transform (FFT) pattern of selected area. (d) HAADF-STEM image and elemental mapping of Ni, Fe and S of Fe17.5%-Ni3S2 nanosheet.

The structure of Fe-Ni3S2 was further characterized by TEM (Figure 3). As shown in Figure 3a, a sheet-like morphology of Fe-Ni3S2 was confirmed by TEM. The selected area electron diffraction (SAED) pattern of Fe-Ni3S2 nanosheet in Figure 3b presents a series of diffraction rings assignable to the crystal planes of Ni3S2, which agrees well with the XRD result and confirms the polycrystalline nature of Fe-Ni3S2 nanosheet. The clear lattice fringes shown in Figure 3c indicate high crystallinity of Fe-Ni3S2 nanosheet. The interplanar spacing of 0.29, 0.24 and 0.23 nm can be ascribed to the (110), (003) and (021) crystalline plane of Ni3S2 (PDF #44-1418), respectively. It is well known that the crystalline plane exposed is of great importance for catalyst. The observed angle between two (110) planes is 60o, indicating the plane exposed in this area is {001} plane, while the angle between (003) and (021) crystalline plane is ~70.5o, indicating a high-index {-210} plane is exposed.26 We also detected the catalyst at other positions and found {-210} plane exposing frequently (Figure S6). As for the Ni3S2 nanosheet, many {-210} planes were identified as well (Figure S7). As reported previously, the high-index {-210} of Ni3S2 was more active for HER and OER compared with thermodynamically stable {001} plane,26 8


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which is beneficial to the activity of Fe-Ni3S2/NF. Moreover, the HAADF-STEM technique provided the mapping of every element in the Fe-Ni3S2 nanosheet. It is displayed in Figure 3d that Fe, Ni and S element distributes homogeneously throughout the whole nanosheet, which is in favor of the uniform distribution of active sites on Fe-Ni3S2 surface.

Figure 4 (a) iR-corrected LSV curves of Fe-Ni3S2/NF with various Fe doping level, Ni3S2/NF and Pt/C/NF for HER in 1 M KOH at 5 mV s-1. (b) Tafel plots for Fe17.5%-Ni3S2/NF, Ni3S2/NF and Pt/C/NF. (c) The durability of Fe17.5%-Ni3S2/NF in dynamic and constant potential testing. The potential cycling is performed between 0.067 and -0.347 V (w/o iR correction) for 1000 times, and the constant potential is controlled at η = 173 mV (w/o iR correction). (d) SEM image and (e) XRD pattern of Fe17.5%-Ni3S2/NF after hydrogen evolution for 20 h. High-resolution (f) Ni 2p and (g) S 2p XPS spectra of Fe17.5%-Ni3S2/NF before and after hydrogen evolution for 20 h; Ar+ sputtering is used to remove surface materials.

2.2 Catalytic Performance for HER The electrocatalytic performance of various catalysts for HER was evaluated in 1 M KOH (Figure 4a). In comparison with Ni3S2/NF, all of the LSV curves of Fe-Ni3S2/NF electrodes are shifted obviously to the positive direction, approaching that of Pt/C (the benchmark catalyst for HER). It is also shown that the HER activity of Fe-Ni3S2/NF is increased with Fe content up to 17.5%, and further increasing the Fe content fails to enhance its HER activity any more. Specifically, the overpotential (η) at 10, 20 and 100 mA cm-2 for Fe17.5%-Ni3S2/NF is 47, 142 and 9


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232 mV, respectively, which outperforms many Ni3S2-based materials, such as N-Ni3S2/NF (η@10 mA cm-2 =

110 mV),22 Sn-Ni3S2/NF (η@10 mA cm-2 =137 mV),37 V-Ni3S2-NW (η@20 mA cm-2 = 203 mV),40

200-SMN/NF (Mo-doped Ni3S2, η@100 mA cm-2 = 278 mV), mV)27 and NixCo3−x S4/Ni3S2/NF (η@10

mA cm-2

38

MoOx/Ni3S2/NF (η@10 mA cm-2 = 106

= 136 mV and η@100

mA cm-2

= 258 mV).19 It is

important to highlight that Fe-Ni3S2/NF is also comparable or favorable to many other recent reported transition metal sulfides (Table S1). The reaction kinetics of HER can be analyzed by using Tafel plots: η = a + b log j, where j and b stands for the measured current density (mA cm-2) and Tafel slope, respectively. The Tafel slope of Fe17.5%-Ni3S2/NF is 95 mV dec-1, lower than that of Ni3S2/NF (147 mV dec-1), indicating a more facile reaction on Fe-Ni3S2/NF (Figure 4b). The exchange current density (j0) of Fe-Ni3S2/NF, calculated by extrapolating the Tafel plot to η = 0 V, is 0.77 mA cm-2, which is 5.1 times that of Ni3S2/NF (0.15 mA cm-2) and outperforms many reported catalysts (Table S2). Moreover, Fe17.5%-Ni3S2/NF presents an electron transfer resistance (Rct) of 0.69 ohm at η = 333 mV (Figure S8), much smaller than that of Ni3S2/NF (1.09 ohm). These results demonstrated that a more rapid HER kinetics was achieved on Fe-Ni3S2/NF in contrast to Ni3S2/NF, proving a significant effect of Fe doping on increasing the HER activity of Ni3S2. In addition, we evaluated the HER activity of Fe17.5%-Ni3S2/NF in 1 M phosphate-buffered saline (PBS, pH=7). The overpotential (η) required to achieve a HER current at 10 and 100 mA cm-2 is 145 and 337 mV, respectively (Figure S9a). The performance of Fe17.5%-Ni3S2/NF is superior to those of reported materials under neutral conditions (Table S3), including FeMoS4 NRA/CC (η@10 mA cm-2 = 204 mV),52 a-Ni3S2@NPC (η@2 mA cm-2 = 193 mV)

18

and CoP NW/Hb

(η@100 mA cm-2 = ~400 mV).53 The durability of Fe17.5%-Ni3S2/NF was first evaluated by a potentiostatic testing, and no obvious decline of current density was observed in a period of 20 h (inset of Figure 4c). After another dynamic potential testing for cycling 1000 times, only < 10 mV variation in overpotential was observed on the LSV curve (Figure 4c). The SEM technique confirms that the nanosheet array morphology of Fe17.5%-Ni3S2/NF is well maintained after hydrogen evolution for 20 h (Figure 4d). In addition, the Fe, Ni and S element can still be detected (Figure S10), and the XRD peaks and Raman peaks are consistent with those of pristine Fe17.5%-Ni3S2/NF (Figure 4e and Figure S11). Furthermore, we adopted XPS coupled with Ar+ sputtering technique to analyze the composition change of Fe-Ni3S2 after long-term hydrogen evolution process. No obvious change occurred on 10


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the binding energy and component type for both Ni and S element in the used Fe17.5%-Ni3S2/NF after Ar+ sputtering (Figure 4f and 4g), indicating the same composition between the surface and inner section of the used Fe-Ni3S2 nanosheet. More importantly, the Ni and S signal of used Fe17.5%-Ni3S2/NF was nearly identical to that of pristine Fe17.5%-Ni3S2/NF, demonstrating that Fe-Ni3S2 was stable during the catalysis of HER. All the above results indicated that Fe-Ni3S2/NF possessed a superior stability during HER process in both structure and composition.

Figure 5 (a) Double layer capacitance (Cdl) of Fe17.5%-Ni3S2/NF and Ni3S2/NF. (b) Comparison of j/Cdl between Fe17.5%-Ni3S2/NF and Ni3S2/NF where j stands for the hydrogen evolving current density at a given overpotential. (c) Top and side view of {-210} plane model that simulates the surface of Fe-Ni3S2. (d) Calculated free-energy diagram of HER over {-210} plane of Ni3S2 and Fe-Ni3S2. (e) Calculated water adsorption energy of Ni3S2 and Fe-Ni3S2. (f) The effect of KSCN on the HER current of Fe17.5%-Ni3S2/NF and Ni3S2/NF under potentiostatic conditions.

It is important to reveal the inherent reason why Fe doping could increase the activity of HER. The effect of electrochemical surface area (ECSA) on the activity was first analyzed. It is well known that the ECSA is positive correlated with the double layer capacitance (Cdl) of catalyst.3 We found that the Cdl of every Fe-Ni3S2/NF electrode with different Fe doping level was larger than that of Ni3S2/NF (Figure 5a and Figure S12), i.e., Fe-Ni3S2/NF has larger ECSA than Ni3S2/NF, indicating that the Fe doping efficiently improves the roughness of Ni3S2 nanosheet, which is in 11


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favor of making more catalytic sites accessible. To exclude the contribution from ECSA to activity, we normalized the hydrogen evolving current (j) of a catalyst by its Cdl, and the j/Cdl obtained should be positively correlated with the area-specific activity of this catalyst. It is found that the j/Cdl of Fe-Ni3S2/NF is larger than that of Ni3S2/NF (Figure 5b and Figure S13), indicating that the improvement in the HER activity of Fe-Ni3S2/NF results not only from increased ECSA, but also from enhanced intrinsic activity of Ni3S2 caused by Fe doping. On this basis, DFT calculation was performed to research intrinsic reasons for the improvement of HER activity after Fe doping. As mentioned above, Fe-Ni3S2 is stable during HER process, thus it is safe to consider that Fe-Ni3S2 is the actual active species for HER. The HRTEM demonstrated the exposure of {-210} and {001} planes on the surface of Fe-Ni3S2 (Figure 3), and it has been reported that the high-index {-210} plane was more active in comparison to low-index {001} plane, thus we built the model based on {-210} surface to perform the DFT calculation. Considering the crystalline structure of Ni3S2, there are two types of {-210} surface: Ni atom-terminated and S atom-terminated. For both of them, there are three possible doping sites for Fe atoms, and only the most stable configurations are adopted and exhibited in Figure 5c based on the substituted energy of various doping conditions (Figure S14). The HER usually proceeds via the Volmer-Tafel or Volmer-Heyrovsky mechanism in alkaline media on the basis of three elementary steps: discharge step (H2O + e- → H* + OH-, Volmer reaction), electrochemical desorption step (H2O + H* + e- → H2 + OH-, Heyrovsky reaction) and recombination step (2H* → H2, Tafel reaction).2, 10, 14, 54 Generally, the HER rate of catalyst was affected by the adsorption/desorption energetics of H* on its surface. It is widely accepted that the value of adsorption free energy of H*(∆G(H*)) on the catalyst can be used as a good descriptor for its activity towards HER, i.e., the optimal HER catalyst should adsorb H* neither too strongly nor too weakly with a thermoneutral ∆G(H*) just like Pt.26, 55 In this work, we found that the ∆G(H*) was significantly decreased from 0.89 and 0.60 eV (Ni3S2) to -0.07 and 0.31 eV (Fe-Ni3S2), all close to thermoneutral ∆G(H*), after Fe doping for Ni atom-terminated and S atom-terminated surface, respectively (Figure 5d), indicating the adsorption of H on Ni3S2 is optimized by Fe doping, which is beneficial to increase the activity of Fe-Ni3S2/NF. A more significant factor of the improvement in HER activity is probably an accelerated Volmer step on catalyst after Fe doping. It has been found that the catalytic activity for HER in 12


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alkaline media is usually much lower than that of the same catalyst in acid media, and it is believed to be resulted from the more complicated forming process of adsorbed hydrogen atom (i.e., Volmer step) in alkaline media (H2O + e- → H* + OH-) in comparison to that in acid media (H+ + e- → H*), because a sluggish water activation process must be involved in alkaline media, which may be the rate determining step for large number of non-Pt HER catalysts.10, 14 The Tafel slope of Ni3S2/NF was 147 mV dec-1 (Figure 4b), close to the theoretical value of Tafel slope (120 mV dec-1) when Volmer step is the rate determining step of HER,3, 56 indicating that the Volmer step is the main process that drags HER on Ni3S2/NF. However, the Tafel slope of Fe17.5%-Ni3S2/NF is decreased to 95 mV dec-1, implying that the rate of Volmer step is accelerated, i.e., water is activated more easily on Fe-Ni3S2 surface. The water adsorption energy can be used to evaluate the energy barrier of water activation on the catalyst.22, 57 As shown in Figure 5e, the water adsorption energy on Fe-Ni3S2 is -0.95 and -0.73 eV, which is much lower than that of Ni3S2 (-0.46 and -0.29 eV) for Ni atom-terminated and S atom-terminated surface, respectively, indicating an easier water activation process on the surface of Fe-Ni3S2 to start the HER. In alkaline media, it is speculated that there is a synergetic mechanism for Volmer step on transition-metal sulfides, phosphides and selenides.45, 57-58 Briefly, the H-OH bond is broken under the interaction between positive-charged metal site and O atom of water as well as the interaction between negative-charged non-metal site and H atom of water (Figure S15). To give a direct evidence on the role of metal site in the catalysis of HER, we evaluated the effect of SCN- on the HER activity, because SCN- can selectively poison metal sites of catalyst during HER process.46, 59-60

As shown in Figure 5f, once KSCN is introduced to the electrolyte, the HER current of both

Fe-Ni3S2/NF and Ni3S2/NF is remarkably decreased from ~38 to ~8 mA cm-2, confirming that metal sites of Fe-Ni3S2/NF and Ni3S2/NF play a significant role in the catalysis of HER. In summary, the high catalytic efficiency towards HER of Fe-Ni3S2/NF can be ascribed to the following four points (Figure 1b): (1) Fe-Ni3S2 possessed a metallic behavior with high electronic conductivity (Figure S16), and the in-situ growth of Fe-Ni3S2 on the NF surface was also very important, because the supported Fe-Ni3S2 powder on NF presented a much lower activity in comparison with Fe-Ni3S2/NF (Figure S17); (2) the NF supported nanosheet array with open 3D porosity benefited mass transfer;35, 61 (3) Fe doping increased the ECSA of Ni3S2; (4) Fe doping optimized the adsorption of water and H* on the catalyst surface, which increased the intrinsic 13


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activity of catalytic active sites.

Figure 6 (a) iR-corrected CV curves of Fe17.5%-Ni3S2/NF, Ni3S2/NF, NiFe LDH/NF-3, IrOx/NF and NF for OER in 1 M KOH at 5 mV s-1. (b) Tafel plots of electrocatalysts shown in panel (a). (c) The durability of Fe17.5%-Ni3S2/NF in dynamic and constant potential testing. The potential cycling is performed between 1.367 and 1.717 V (w/o iR correction) for 1000 times, and the constant potential is controlled at η = 270 mV (w/o iR correction). (d) SEM image and (e) XRD pattern of Fe17.5%-Ni3S2/NF after oxygen evolution for 20 h. (f) Raman spectra of Fe17.5%-Ni3S2/NF before and after oxygen evolution for 20 h. (g) High-resolution XPS S 2p spectrum of Fe17.5%-Ni3S2/NF before and after oxygen evolution for 20 h; Ar+ sputtering is used to remove surface materials.

2.3 Catalytic Performance for OER The electrocatalytic performances of Fe-Ni3S2/NF for OER were also evaluated (Figure 6a). Due to the interference of Ni(II)/Ni(III) redox peak at ~1.4 V, the OER overpotentials for electrodes are determined based on the polarization curves scanned in the negative direction.24 It is found that the OER activity of Fe-Ni3S2/NF is not sensitive to the Fe content (Figure S18). Only a low Fe doping level (4.9%) is able to enhance the OER activity of Ni3S2 significantly, and further increasing the Fe content up to 36.6% has little effect on decreasing the overpotential of OER remarkably. This discovery in the effect of Fe content on the OER activity of Ni3S2 is consistent with the results reported by other work concerned with Fe doped Ni-based materials.13 Specifically, 14


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the η required to achieve an OER current density at 10, 20 and 100 mA cm-2 for Fe17.5%-Ni3S2/NF is 214, 222 and 249 mV, respectively, which is not only much smaller than that of Ni3S2/NF (287, 306 and 363 mV), but also outperforms the benchmark OER catalyst IrOx (347, 377 and 465 mV). The Tafel slope for Fe17.5%-Ni3S2/NF, Ni3S2/NF and IrOx is 42, 82 and 107 mV dec-1, respectively (Figure 6b). Surprisingly, Fe17.5%-Ni3S2/NF presented a superior or comparable activity to many recently reported OER catalyst (Table S4), such as Fe-Ni3S2/FeNi (η@10 mA cm-2 = 282 mV, Tafel slope = 54 mV dec-1),16 Fe11.8%-Ni3S2/NF (η@100 mA cm-2 = 253 mV, Tafel slope = 65.5 mV dec-1),47 Zn-Ni3S2/NF (η@100 mA cm-2 = 300 mV, Tafel slope = 87 mV dec-1),39 N-Ni3S2/NF (η@100 mA cm-2 = ~340 mV, Tafel slope = 70 mV dec-1),22 200-SMN/NF(Mo-doped Ni3S2, η@100 mA cm-2 = ~400 mV, Tafel slope = 45.5 mV dec-1)38 and MoS2/Ni3S2 heterostructures (η@10 mA cm-2 = 218 mV, Tafel slope = 88 mV dec-1),24 MnO2-CoP3/Ti (η@10

mA cm-2

= 288 mV, Tafel slope = 65 mV dec-1)62 and

Cu(OH)2@CCHH NW/CF (η@50 mA cm-2 = 270 mV, Tafel slope = 78 mV dec-1).63 In addition, the much lower Rct on Fe17.5%-Ni3S2/NF (0.12 ohm) than that of Ni3S2/NF (0.46 ohm) also indicates a faster electron transfer process between Fe17.5%-Ni3S2/NF and electrolyte during oxygen evolution (Figure S19). The above results demonstrated that a robust OER activity can be achieved after Fe doping in Ni3S2 nanosheet arrays. In addition, the OER activity of Fe17.5%-Ni3S2/NF under mild conditions was evaluated in 1 M KHCO3 (pH=8.3). The η@10 mA cm-2 is 490 mV (Figure S9b), comparable or superior to those of OER catalysts in carbonate electrolyte, including Ni3N NA/CC (η@20 mA cm-2 = 540 mV),64 CoCHH/NF (η@10 mA cm-2 = 414 mV),65 Fe-Ci (η@10 mA cm-2 = 560 mV)66 and Co-Ci (η@9.1 mA cm-2 = ~770 mV).67 The durability of Fe17.5%-Ni3S2/NF during OER process was evaluated by dynamic and constant potential testing (Figure 6c). The XRD and SEM analysis show that the crystalline structure and nanosheet array of Fe-Ni3S2 are well maintained after durability testing (Figure 6d and 6e). The EDS demonstrates the presence and uniform distribution of Fe, Ni and S on Fe17.5%-Ni3S2/NF after long-term OER process (Figure S20). Although S element was observed by EDS, the surface-sensitive techniques, Raman spectroscopy and XPS, provided some different results. The Raman spectroscopy exhibits that the peaks at 290 and 374 cm-1 from Fe-Ni3S2 disappears after long-term oxygen evolution and a new peak at 561 cm-1 ascribed to Ni-Fe (oxy)hydroxide emerges (Figure 6f).68 The XPS peak centered at 855.9 and 873.5 eV in the Ni 2p XPS spectrum of Fe17.5%-Ni3S2/NF after long-term OER process (Figure S21) can be attributed to 15


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the Ni 2p3/2 and 2p1/2 of Ni (oxy)hydroxide, respectively.20, 35, 68 The S 2p XPS spectra show that the signal intensity of S element on the surface of Fe-Ni3S2/NF is significantly reduced after oxygen evolution for 20 h (Figure 6g). Moreover, the S signal of used Fe17.5%-Ni3S2/NF after Ar+ sputtering is still much lower than that of pristine Fe17.5%-Ni3S2/NF, suggesting the near-surface sulfide has transformed to Ni-Fe (oxy)hydroxide during OER process. Note that this transformation has already occurred even after the LSV is recorded. The reason why no diffraction peaks assignable to Ni-Fe (oxy)hydroxide was observed in XRD may be explained by the low content and low crystallinity of the in-situ formed Ni-Fe (oxy)hydroxide.20 Similar phenomena have been reported that the transition metal sulfides will be transformed to oxides or (oxy)hydroxides after OER catalysis due to the high oxidation environment, which are believed to the actual catalytic species for OER.42-43 This was also supported by a similar variation trend of oxygen evolving current in the KSCN poison experiment of Fe17.5%-Ni3S2/NF with that of NiFe LDH/NF-3 (Figure S22), and the fact that the OER activity of Fe17.5%-Ni3S2/NF was close to that of NiFe LDH/NF-3 (Figure 6a). As mentioned above, Fe-Ni3S2/NF had larger ECSA than that of Ni3S2/NF, and the introduction of Fe can give rise to bimetallic Ni-Fe (oxy)hydroxide during OER catalysis, a more active material than single component Ni (oxy)hydroxide,13 both of which are beneficial for enhancing the OER activity of Fe-Ni3S2/NF in comparison with Ni3S2/NF (Figure 1b). 2.4 Overall Water Splitting The overall water splitting performance of Fe-Ni3S2/NF with different Fe content was evaluated in a two-electrode configuration by using Fe-Ni3S2/NF as both the anode and cathode. It is found that all Fe-Ni3S2/NF couples possess better performance than Ni3S2/NF couple (Figure S23), confirming the beneficial effect of Fe doping on the water splitting ability of Ni3S2. Specifically, the cell voltage of Fe17.5%-Ni3S2/NF couple at 10, 20 and 100 mA cm-2 is 1.54, 1.60 and 1.70 V, respectively (Figure 7a and Video S1), which agrees well with the sum of the overpotential of HER and OER measured in the aforementioned three-electrode system (Figure 7b). This performance enabled Fe17.5%-Ni3S2/NF to be one of the most active sulfide-based bifunctional electrocatalysts for overall water splitting in alkaline media. Notably, Fe-Ni3S2/NF also shows better or comparable performances in comparison with NiSe/NF (1.63 V at 10 mA cm-2),69 Co-B@CoO/Ti (1.56 V at 20 mA cm-2)70 and other state-of-the-art water-splitting 16


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electrocatalysts shown in Figure 7c. During electrolysis, Fe17.5%-Ni3S2/NF presents a faradaic efficiency quite close to 100% for both HER and OER process (Figure 7d), indicating side reactions are negligible. The stable i-t curve (Figure 7e) and nearly overlapped polarization curves before and after durability testing (Figure 7a) demonstrate a high durability of Fe17.5%-Ni3S2/NF under the overall water splitting conditions. In comparison with the benchmark Pt/C||IrOx counterpart, Fe17.5%-Ni3S2/NF electrolyzer presented a comparable performance but better durability (Figure 7a). Black powder was found at the bottom of electrolytic cell after durability testing for Pt/C||IrOx couple (Figure S24), which may be the catalyst peeled off from the NF substrate caused by the impact of generated gas bubbles, demonstrating the advantage of the binder-free Fe-Ni3S2/NF electrode. Finally, a commercial D-size battery (~1.5 V) was used to power the Fe17.5%-Ni3S2/NF||Fe17.5%-Ni3S2/NF electrolyzer, and continuous evolution of H2 and O2 bubbles can be observed on both of electrodes (Figure S25 and Video S2).

Figure 7 (a) iR-corrected polarization curves of Fe17.5%-Ni3S2/NF||Fe17.5%-Ni3S2/NF and Pt/C/NF||IrOx/NF water electrolyzer in 1 M KOH at 5 mV s-1. The inset shows H2 and O2 bubbles evolving from Fe17.5%-Ni3S2/NF electrodes. (b) The cell voltage in a two-electrode configuration and the sum of the electrode potential of HER and OER measured in a three-electrode system for Fe17.5%-Ni3S2/NF. (c) Comparison of the water splitting performance of Fe17.5%-Ni3S2/NF with other recent reported bifunctional electrocatalysts. (d) The amount of gas collected and calculated from electric quantity for Fe17.5%-Ni3S2/NF during water splitting. (e) i-t curves of 17


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Fe17.5%-Ni3S2/NF||Fe17.5%-Ni3S2/NF and Pt/C/NF||IrOx/NF electrolyzer at 1.65 V. References cited in panel (c): porous MoO2,71 2.5H-PHNCMs,72 Cu@NiFe LDH,73 2-cycle NiFeOx/CFP,74 np-(Co0.52Fe0.48)2P,75 FeB2,76 NSWANs (Nickel Diselenide),77 Ni3FeN-NPs,78 Ni@NC-80079 and Fe-Ni3C-2%80.

3. Conclusions In this work, we provided a Fe-doped Ni3S2 bifunctional electrocatalyst in the form of nanosheet array vertically anchored on 3D NF, which presented high performances for both HER and OER in alkaline media. On the basis of systematic experiments and DFT calculation, we believed that the Fe doping increased the ECSA, improved the water adsorption ability and optimized the hydrogen adsorption energy of Ni3S2, which gave rise to an enhanced HER activity on Fe-Ni3S2/NF. Moreover, metal sites of Fe-Ni3S2/NF and Ni3S2/NF were proved to play a significant role in the catalysis of HER. In addition, it was found that the near-surface Fe-Ni3S2 were converted to Ni-Fe oxyhydroxide during oxygen evolution, which was responsible for the catalysis of OER actually. The introduction of Fe element also resulted in a remarkable enhancement on the OER activity of Ni3S2. Fe-Ni3S2/NF also presented a high performance in overall water splitting testing.

4. Experimental Section 4.1 Material synthesis All reagents used in the material synthesis are of analytical grade and used as-received from Sinopharm Chemical Reagent Co. Ltd. without further treatment. Ultrapure water (18.25 MΩ cm) was used in all experiments. The Ni foam (NF, 99.8%, 1.0 mm in thickness, 320 g m-2, Li Zhi Yuan, Taiyuan, China) was first cut into pieces at a size of 1 cm × 3 cm. Then the NF was successively sonicated in acetone, 10% HCl and water, followed by drying under vacuum at 60oC. The synthesis of NiFe LDH/NF was performed according to the method reported previously with modification.48 In brief, a given amount of Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, 4 mmol NH4F and 10 mmol urea was added into 40 mL water. After completely dissolution, the solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and two pieces of treated NF were 18


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put into the solution. The autoclave was then sealed and heated at 120oC for 6 h. After natural cooling, the NiFe LDH loaded NF was taken out, washed by water and dried at 60oC under vacuum. The feed ratio of Fe and Ni salts as well as the name of NiFe LDH/NF obtained correspondingly is listed in Table 1. If the Fe salt was removed in the synthesis, Ni(OH)2·0.75H2O/NF will be obtained at last. For the sulfuration of NiFe LDH/NF and Ni(OH)2·0.75H2O/NF, a 0.2 mol L-1 Na2S aqueous solution was first prepared. Subsequently, the 40 mL Na2S solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and the NiFe LDH/NF was put into this solution. Then the sealed autoclave was heated at 100oC for 8 h. After natural cooling, the sulfide loaded NF was taken out, thoroughly washed by water and dried at 60oC under vacuum. The loading of sulfide catalyst on NF was in the range of 4~6 mg cm-2 calculated from the mass change of NF before and after the growth of catalyst. The Fe/Ni atomic ratio in the final sulfide was detected by EDS, and the sulfurated electrode was denoted according to the Fe content in the catalyst (see Table 1 for the detail). 4.2 Physical characterizations XRD patterns were obtained on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. SEM images and EDS spectra were recorded by a Hitachi SU8010 microscope. A JEOL JEM-2100F microscope was used to perform the high-resolution TEM (HRTEM) and scanning TEM (STEM) analysis. A Renishaw inVia spectrometer with exciting laser at 532 nm was adopted to get the Raman spectra. XPS spectra were obtained on a Thermo Scientific ESCALAB 250Xi spectrometer. 4.3 Electrochemical measurements The electrochemical measurement was performed by a three-electrode system controlled by a CHI-760E electrochemical station in N2-purged 1 M KOH. The catalyst-loaded NF acted as the working electrode. The counter and reference electrode was a graphite rod and a saturated calomel electrode, respectively. All electrode potentials were given versus the reversible hydrogen electrode (RHE) unless otherwise mentioned.81 Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) was adopted to determine the activity of electrocatalyst towards HER and OER, respectively. Electrochemical impedance spectra (EIS) were obtained with a frequency between 105 Hz and 1 Hz at a given potential. The double layer capacitance of electrocatalyst was 19


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calculated based on CV curves recorded in a non-Faradaic region with scan rates from 10 to 40 mV s-1. For comparison, the activity of 20%Pt/C and IrOx (Johnson Matthey) was evaluated. The working electrode of the commercial catalyst was prepared by ultrasonicating Pt/C or IrOx powder in ethanol with Nafion ionomer (Sigma-Aldrich) for 1 h followed by dipping the catalyst slurry on a piece of NF (1 cm × 3 cm). The catalyst loading was controlled at ~0.5 mg cm-2. 4.4 Theoretical calculations For investigating the effect of Fe-doping on the electronic structure and HER catalytic process of Ni3S2, first principles quantum mechanics calculations were performed on the basis of density functional theory (DFT) plane-wave pseudopotential method. The calculation details are available in the Supporting Information. Acknowledgements This work was financially supported by the National Natural Science Foundations of China (Program No. 21603080, 21773078), the Fundamental Research Funds for the Central Universities of China (Program No. 2662017JC025, 2662015PY163) and Beijing National Laboratory for Molecular Sciences (BNLMS20160102). Supporting Information Theoretical calculation details; Table S1-S4; Figure S1-S25. (PDF) Video S1: Fe17.5%-Ni3S2/NF||Fe17.5%-Ni3S2/NF water electrolyzer at high current density. (AVI) Video S2: Fe17.5%-Ni3S2/NF||Fe17.5%-Ni3S2/NF water electrolyzer powered by a commercial D-type battery. (AVI)

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148-153. (22) Chen, P.; Zhou, T.; Zhang, M.; Tong, Y.; Zhong, C.; Zhang, N.; Zhang, L.; Wu, C.; Xie, Y. 3D Nitrogen-Anion-Decorated Nickel Sulfides for Highly Efficient Overall Water Splitting. Adv. Mater. 2017, 29, 1701584. (23) Yan, J. Q.; Wu, H.; Li, P.; Chen, H.; Jiang, R. B.; Liu, S. Z. Fe(III) doped NiS2 nanosheet: a highly efficient and low-cost hydrogen evolution catalyst. J. Mater. Chem. A 2017, 5, 10173-10181. (24) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X.; Feng, X. Interface Engineering

of

MoS2/Ni3S2

Heterostructures

for

Highly

Enhanced

Electrochemical

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