NF electrocatalysts for its

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Bifunctional electrodeposited 3D NiCoSe2/NF electrocatalysts for its applications in enhanced oxygen evolution reaction and for hydrazine oxidation Kamran Akbar, Jae Ho Jeon, Minsoo Kim, Junkyeong Jeong, Yeonjin Yi, and Seung-Hyun Chun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00644 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Bifunctional electrodeposited 3D NiCoSe2/NF electrocatalysts for its applications in enhanced oxygen evolution reaction and for hydrazine oxidation Kamran Akbar †,‡, Jae Ho Jeon† , Minsoo Kim§, Junkyeong Jeong§, Yeonjin Yi§ and Seung-Hyun Chun †* †

Department of Physics, Sejong University, Seoul 05006, Republic of Korea

‡

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea

§

Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Republic of Korea

Corresponding Author * E-mail: [email protected]

ABSTRACT: The development of stable and efficient oxygen evolutional electrocatalysts is fundamental to the production of hydrogen by water electrolysis. However, so far the majority of electrocatalyst require a substantial overpotential (η) (~ above 250 mV) to catalyze the bottleneck oxygen evolution reaction (OER). To overcome this large overpotential for OER, herein we report the growth of nickel-cobalt-selenide (NiCoSe2) nanosheets over 3D nickel foam (NF) via a facile and scalable electrodeposition method. The resulted 3D NiCoSe2/NF hybrid electrode requires an overpotential of merely 183 mV to reach the current density (J) of 10 mA cm-2. To the best of our knowledge, this is the lowest η value reported so far for any earth

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abundant material based OER electrocatalyst to attain the same current density. Moreover, a significant reduction in Tafel slope (88 mV dec-1) is being observed between bare NF and NiCoSe2/NF. Hence as a result, the 3D hybrid NiCoSe2/NF OER electrode outperforms the previously reported electrocatalysts including the expensive state of the art OER electrocatalysts like RuO2 and IrO2. Such enhancement in the OER catalytic efficiency of NiCoSe2 nanosheets over NF can be attributed to its enormous electrochemical active surface area (ECSA) (108 cm2), large roughness factor (270), highly conductive NF substrate and the presence of multiple catalytically active OER species (NiOOH and CoOOH) on its surface. In addition, 3D hybrid NiCoSe2/NF electrocatalyst was tested for hydrazine oxidation for its bifunctional utilization. Much lower onset potential values (-0.7 V vs. SCE) and high current densities (> 200 mA cm-2) are being observed for 3D hybrid NiCoSe2/NF when benchmarked against bare NF (-0.4 V and < 50 mA cm-2). Furthermore, 3D hybrid NiCoSe2/NF OER electrode shows excellent stability of 50 h for continuous OER in strongly alkaline solutions while maintaining its enormous ECSA, chemical composition and structural morphology. The excellent bifunctional electrocatalytic activity, long term stability and facile preparation method enable NiCoSe2/NF hybrid electrode to be a viable candidate for its widespread use in various water splitting technologies.

KEYWORDS:

Oxygen

evolution

reaction,

Nickel

cobalt

selenide,

water

splitting,

electrocatalysis

Introduction Electrochemical water splitting is widely recognized as a viable route to produce clean and renewable hydrogen fuel.1,2 However, the oxygen evolution reaction is the bottleneck halfreaction in the overall water splitting reaction due to its sluggish reaction kinetics owing to the


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complex four-electron oxidation process. To lower the activation barrier, and thus, lowering the overpotential for an OER, an electrocatalyst is normally needed.3,4 Even the state-of-the-art electrocatalysts (IrO2 and RuO2) require a significant η to triumph the viable working current densities (~ 300 mV @ 10 mAcm-2).5,6 Moreover, the increased price and the lack of earth abundancy of these precious-metals possess serious challenges to their application on large industrial scales.7 Thus, the significance of developing high performance, long-lasting, low cost and earth abundant OER electrocatalysts can hardly be overstated. In response, in the last decade, extensive research efforts have been focused on utilization of earth abundant 3d transition metal oxides/hydroxides due to their reduced cost, remarkable catalytic activity and stability for oxygen evolution reaction.8–11 However, these oxides/hydroxides of transition metals suffer from their relatively high onset potentials for OER reaction because of their lower electronic conductivities, which ultimately poses serious challenges for their utilization in widespread industrial usage due to their poor energy harvesting efficiency.12–14 The lower electronic conductivities of these transition metal oxides/hydroxides can be tailored by encapsulating these materials with conductive carbon based agents such as graphene or carbon nanotubes, but rather high temperatures required for synthesis of these materials hamper their large scale applications.15–17 Recently, the focus in the search for efficient and stable OER catalysts have been shifted toward the chalcogens (selenides/sulphides) of 3d transition metals because of their inherent corrosion stability in strongly alkaline electrolytes, high intrinsic electronic conductivities, enhanced catalytic performance, and the lower onset potential toward OER, especially when compared against their corresponding oxides/hydroxides.14,18,19 Moreover, the lower electronegativity of Se dictates that the diffusion of OH- species toward electrode surface would be faster compared to bimetallic oxides.20 For instance, Swesi et al. showed that Ni3Se2 on


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Au substrate requires an η of only 290 mV to attain a J of 10 mA cm-2.19 Tang et al. reported that NiSe nanowires supported on nickel foam require 270 mV to reach the current density of 20 mA cm-2.12 Xia et al. demonstrates that Ni doping in Co0.85Se can induce additional catalytically active sites for OER and resulted in an even lower η of 255 mV @10 mA cm-2 which is the lowest value achieved for an earth abundant metal selenide.13 In addition, bimetallic nickel cobalt diselenide has been successfully synthesised by various research groups and has been mainly demonstrated for hydrogen evolution reaction (HER).14,21 However, these studies utilize hydrothermal method which normally require high temperatures and prolonged synthesis time (> 12 h). In another study, Wang et al. synthesized porous NiCo diselenide nanosheets via similar hydrothermal method on conductive carbon cloth and utilized it for OER but fairly high overpotential of 258 mV is yet required to attain the current density of 10 mAcm-2. However, for the following reasons, we believe that still there is a substantial room for further development in the synthesis and catalytic performance of these selenides OER catalysts. At first, as mentioned earlier, normally hydrothermal route had been adopted for the synthesis of these metallic/ bimetallic selenides materials which thus require hours to synthesize the material. Thus, it is essential to come up with novel approaches to reduce synthesis time and temperatures to overcome high costs. Secondly, for many OER electrocatalysts, a conductive agent and a binder is normally needed. However, due to inactivity of binder/conductive agent for OER, the overall activity of electrocatalyst is lowered, owing to the presence of added “dead volume”.22 Furthermore, at higher current density operations when the oxygen evolution at electrode surface is violent, the glued electrocatalyst peels off from the surface and hence results in its poor stability. To circumvent this issue, one could utilize an inexpensive electrodeposition method and avoid the hassle of using binders or other additional treatments, like high temperature and longer


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synthesis times.13 Thirdly, for most of OER catalysts, the η@10 mA cm-2 is still greater than ~ 250 mV which is significantly high enough to hamper the overall energy harvesting efficiency for a water splitting reaction, and thus needed to be lowered for the further development of efficient and high performance OER electrocatalysts.23 In addition to OER, catalysts with dual functionalities are obvious choice instead the prepared catalyst serving only one purpose. Hydrazine as a fuel stands out among other liquid fuels owing to its high energy and power density 24,25. We had also tested our prepared 3D NiCoSe2/NF for hydrazine oxidation to achieve dual catalytic functionalities. Normally, hydrazine oxidation is performed under alkaline conditions and metals tend to passivate and become inactive due to oxide growth on the surface

24

. As metal selenides sustain harsh alkaline environments during

OER, they are conceptually capable for hydrazine oxidation which require lower onset potentials for its oxidation against RHE scale when compared to OER theoretical potentials.26,27 Based on these similar environments and constraints, we had further tested our hybrid electrocatalyst for hydrazine oxidation to attain bifunctional capabilities. Experimental Details Nickel foam (NF) (1.6mm thickness, Surface Density: 346 g/m2) was sonicated in 3M HCl solution for 10 minutes to remove the oxide layer on the surface, and rinsed successively with DI water and ethanol, then dried with N2 gas. The electrodeposition was carried out in a standard three-electrode electrochemical cell containing NF as the working electrode, a Ag/AgCl (Sat. KCl) as the reference and Pt wire as counter electrode. The electrodeposition bath contained 10mM of Ni(NO3)2.6H2O, Co(NO₃)₃·6H₃O and SeO2 each, with pH maintained at 2 by addition of HCl. Chronoamperometry was employed to electrodeposit NiCoSe2 on NF at constant potential of -1V (vs. Ag/AgCl) at 25°C for 600s. Shorter deposition time results in


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insufficient formation of NiCoSe on NF with certain places with no deposition at all, whereas prolonged deposition time leads to thick films which can inhibit the charge transfer between NiCoSe2 and NF. After deposition, the electrode was withdrawn from the deposition bath, rinsed with water and ethanol and dried with gentle N2 gas pressure. Materials characterizations The scanning electron microscopy (SEM) was performed with JEOL JSM-7001F at an acceleration voltage of 10 kV. Energy dispersive spectroscopy (EDS) was also obtained from the SEM microscope. Transmission electron microscope (TEM) samples were prepared using Cu grids. The atomic structure was characterized by JEOL- 2010F TEM with 200 keV accelerating voltage. X-ray Photoelectron Spectroscopy (XPS) were performed using a K-alpha (Thermo VG, U.K.) X-ray Photoelectron Spectrometer with monochromatic Al X-ray source with 12 kV power. XRD was performed with an X-ray diffractometer (Rigaku RINT 2000) with Cu Kα radiation. For ICP-AES measurements, NiCoSe2/NF was dissolved in 1M HCL to remove NF substrate and then black powered was ingested in con. HNO3 for further analysis. Electrochemical characterizations All electrochemical measurements were carried out with Biologic SP-300 workstation. The working electrode was either bare NF or NiCoSe2/NF and was directly used without additional treatment. Linear sweep voltammograms (LSV) were recorded in 1 M KOH solution saturated with O2 using Ag/AgCl (sat. KCL) and Pt wire as reference and counter electrode, respectively. Scan rate was fixed at 10 mVs-1. All potentials measured were converted to reversible hydrogen electrode (RHE) scale using the following equation: ERHE = E

Ag/AgCl

+ E°Ag/AgCl + 0.059 pH.

Tafel slopes were derived from LSV curves with 85% iR compensation (after taking impedance spectra at open circuit voltage) based on the following equation.


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ɳ=+

2.3 log() (1)

where ɳ is the overpotential, j is the current density and the other symbols have their usual meanings. Electrochemical impedance was performed at frequencies ranges from 1 MHz to 1 Hz at overpotential of 400 mV. Calculation of ECSA The electrochemically active surface area (ECSA) was calculated on the basis of measured double layer capacitance in non-Faradaic regime of cyclic voltammetry (CV)

5,10

. Non faradaic

regime was first determined by taking CV of NiCoSe2/NF in 1M KOH. Multiple CVs were then conducted at various scan rates for estimation of charging current (ic). The relation between ic, the scan rate (ν) and the double layer capacitance (Cdl) is given in equation 2. Hence, the slope of ic as a function of scan rate is a straight line and is equal to Cdl. c = Cdl (2) The measured Cdl for NiCoSe/NF is 4.43mF, which is converted to ECSA according to equation 3. A specific capacitance (Cs) value Cs = 0.040 mF cm-2 in 1 M NaOH is adopted from previous reports and hence ECSA of 108 cm2 is obtained 5. =

(3)

Finally, the roughness factor (RF) is calculated on the basis of equation 4, given the geometric surface area (GSA) of NiCoSe2/NF (0.4 cm2), RF of 270 is obtained. =

(4) !

Results and Discussions


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Herein, for the first time, we report nanostructured NiCoSe2 on Ni foam (NiCoSe2/NF) as a highly active OER catalyst via a facile, one-step, ultrafast electrodeposition method at room temperature. Ni foam acts as a highly conductive 3D scaffold for growth of NiCoSe2 nanoparticles. The resulting NiCoSe2/NF electrode possesses extremely small overpotential of only 183 mV to attain a J of 10 mA cm-2 for the OER. According to the best of our knowledge, this is the lowest overpotential value reported so far for any OER catalysts in alkaline solutions. In addition, the hybrid electrocatalyst shows remarkable stability for continuous OER reaction. In fact, NiCoSe2/NF performs significantly better than all of the reported OER electrocatalysts including the benchmark IrO2 and RuO2. Furthermore, the 3D NiCoSe2/NF hybrid electrode has further been tested for hydrazine oxidation reaction. Lower onset potential values of -0.7 V vs SCE has been observed for NiCoSe2/NF electrode compared to -0.4 V for bare NF for hydrazine oxidation.

The electrodeposition of NiCoSe2 onto NF was undertaken at constant deposition potential of 1.0 V versus Ag/AgCl (see experimental details in Supporting Information, SI). The direct growth of NiCoSe2 on macroporous NF substrates proves to be a better technique for preparation of electrode due to its room temperature working conditions along with its binder-free nature. Furthermore, high intrinsic conductivity of NF would result in a lower contact resistance with the deposited electrocatalyst which ultimately reduce the overall ohmic loses of the system. 28


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Figure 1. FE-SEM images of (a) bare NF. (b) NF covered with NiCoSe2. (c) Magnified SEM image of NiCoSe2 nanoparticles. (d) HR-TEM image of a NiCoSe2 nanoparticles.

Figure 1a-b shows the scanning electron microscopy (SEM) image for NF before and after electrodeposition. It can be clearly seen that surface of NF is fully covered with densely packed NiCoSe2 film after electrodeposition (Figure S1 in the SI). The high magnification SEM (Figure 1c) shows that the particle size of NiCoSe2 is in the nm range with average diameter of approximately 50-100 nm. It is also evident that the nanoparticles are interconnected with each other and offer a hierarchical structural morphology. Furthermore, Ni, Co and Se are


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homogeneously distributed within the discrete grains of NiCoSe2 as shown by energy-dispersive X-ray (EDX) mapping analysis (Figure S3, SI). A uniform coverage of NiCoSe2 on NF was attained for 10 min electrodeposition time as shown in Figure S4. The deposition of shorter time (5min) resulted in non-uniform coverage of NiCoSe2 onto NF, while the longer deposition time (15 min) resulted in thick films with the presence of wider cracks. HR-TEM image of NiCoSe2 deposited for 10 min confirms the high crystallinity of prepared nanoparticles with clearly visible lattice fringes as shown in Figure 1d.

Figure 2 (a) XRD patterns of NiCoSe2 on NF. (b) XPS spectra of the NiCoSe2: Ni 2p, (c) Co 2p, and (d) Se 3d.


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Figure 2a shows the X-ray diffraction (XRD) patterns of NiCoSe2/NF. The two peaks at 2θ = 44.58 and 51.88 can be indexed to NF (JCPDS No.65-2865). The diffraction peaks of NiCoSe2 could be assigned to hexagonal NiSe (JCPDS No. 70-2870) and hexagonal CoSe (JCPDS No. 89-2004), indicating that bimetallic NiCoSe2 possesses the same hexagonal crystal structure as the individual Ni and Co-selenide.29 It is worth noting that peaks are slightly shifted towards a larger diffraction angle (with respect to NiSe), suggesting a minor adjustment within the lattice parameter. This variation in the lattice parameter could be the result of the gradual replacement of Ni ions with Co ions having larger ionization energy in NiSe lattice and hence resulting in the formation of NiCoSe2. Moreover, further estimation of the composition of NiCoSe2 was performed by comparing the lattice spacing of (100) planes (d=0.316 nm from the HRTEM image in Fig. 1(d)) and applying the Vegard’s law on (100) plane of NiSe (d =0.317 nm) and on CoSe (d =0.313 nm) to obtain Ni0.77Co

0.23Se2.

Similarly, EDX elemental analysis of NiCoSe2

(Figure S5) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis indicate identical Ni/Co ratios within NiCoSe2. Hence, it is reasonable to say that our NiCoSe2 is rich in Ni-content when considering the accuracy. The NiCoSe2/NF was also characterized by X-ray photoelectron spectroscopy (XPS) and the corresponding survey spectrum (Figure S6, SI) demonstrates the presence of Ni, Co, and Se within the sample lattice. Oxygen peaks in the spectra are originated from the exposure of the sample to the ambient air. The high-resolution XPS spectra of Co 2p, Ni 2p can be fitted with two spin–orbit doublets as shown in Figure 2b–c. For Ni 2p, the peak fitting analysis reveals that the peaks at 874.0 eV in Ni 2p1/2 and 856.1 eV in Ni 2p3/2 arise due to the presence of Ni+2 chemical species.7,29 A similar fitting analysis for Co2p suggests that presence of peaks at 797.1 eV in Co 2p1/2 and 781.3 eV in Co 2p3/2 is due to the existence of Co+2 within sample lattice. 29,30


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Furthermore, two types of metal–selenium bonds (Se 3d 5/2 at 53.92 and 54.7 eV, respectively) were detected for the high resolution spectra of Se 3d (Figure 3d) which indicates the presence of a single crystal lattice with both Co and Ni attached to Se 13. This fact when corroborated with XRD data confirms the formation of NiCoSe2 on NF substrate. The OER catalytic activity and stability of the NiCoSe2/NF electrode is presented in Figure 3. A standard three-electrode electrochemical cell was used where NiCoSe2/NF serves as working electrode. All the potentials were adjusted against the reversible hydrogen electrode (RHE) reference while the currents were also corrected for ohmic losses. It can be seen from Figure 3a that NiCoSe2/NF clearly demonstrate high performance toward OER compared with bare NF. The appearance of a sharp oxidation peak before the start of oxygen evolution process (Figure 3a and Figure S7, SI) is ascribed to the transformation of NiII to NiIII in the case of NF.31 Peak broadening and the appearance of an additional oxidation peak in the case of NiCoSe2/NF could be assigned to the transformation of CoII to CoIII and CoIII to CoIV.32 In fact, this peak broadening is beneficial because it advocates the presences of additional active sites in NiCoSe2/NF compared with bare NF. Furthermore, rapid increment in the peak current density of NiCoSe2/NF is indicative of its much larger active surface area along with the presence of numerous catalytically active centers for OER. In general, the OER catalytic efficiency is evaluated on the basis of minimum overpotential required to attain a current density of 10 mAcm-2, which is equivalent to the upper limit of a realistic solar device having 12% solar to hydrogen efficiency.33,34 NiCoSe2/NF requires an overpotential (η) of merely 183 mV to reach J = 10 mA cm-2 (Figure 3a and Figure S8, SI). To the best of our knowledge, this is the lowest η value reported so far to attain J of 10 mA cm-2, indicating significantly diminished OER activation energies. The catalytic activity of


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NiCoSe2/NF is further assessed against the benchmark OER catalysts on the basic of overpotential required to attain a current density of 10 mA cm-2 (Table S1 in the SI). The catalytic performance of the NiCoSe2/NF electrode is significantly higher than all of the previously reported OER electrocatalyst, including the state-of-the-art IrO2 and RuO2 catalysts.5,6,35 Furthermore, the comparison of RuO2 deposited on NF has been performed for OER performance as shown in Figure S9. 3D NiCoSe2/NF outperformed the RuO2/NF in terms of OER performance at similar overpotential values. In addition, NiCoSe2/NF also outperforms the reported Ni and Co-based electrocatalysts including corresponding selenides.9,13,20,36–39 The electrodeposition time stronly affects the OER performance as displayed in Figure S10 with the best performance being observed for 10 min deposition time. It is due to the uniform growth of NiCoSe2 nanocrystals as shown in the FESEM image (Figure S4c). The deposition of 5 min results in poor coverage of NiCoSe2 and hence displays a degraded OER performance. The OER performance of thick films (Figure S4d) is also poor, because thick films loose nanocrystalline nature of NiCoSe2 and hence result in reduced active surface area.


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Figure 3 (a) Polarization curves for OER on NiCoSe2/NF and Ni foam at a scan rate of 10 mV s-1. (b) Corresponding Tafel plot (overpotential versus log current) derived from (a). (c) Cyclic voltammograms of NiCoSe2/NF at different scan rate (d) Chronoamperometry curves obtained with the NiCoSe2/NF electrode with the applied potential at 0.4 V (vs. Ag/AgCl). All experiments were carried out in O2 -saturated 1.0 M KOH.

To gain further insight into OER kinetics, Tafel slopes have been plotted as shown in Figure3b. The Tafel slope for NiCoSe2/NF is 97 mVdec-1, which in fact is much lower than the bare NF (185 mVdec-1), indicating the faster kinetics and more favourable OER at NiCoSe2/NF


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compared with bare NF. It should be kept in mind in that despite the wide discrepancies in the reported values of Tafel slopes of bare NF, our NiCoSe2/NF system exhibits a much larger reduction of 48 % (88 mV/dec) in its Tafel slope value (against bare NF) than NiSe/NF (30%) and Ni3S2/NF (39%).12,23 Beside the lower Tafel value, the NiCoSe2/NF can afford a rapid increment in OER performance at overpotential > 200 mV simply because its current density rises more swiftly with an increment in overpotential. In nanostructured catalysts, the enhanced catalytic activity is often the direct consequence of the electrochemically active surface area and the corresponding roughness factor (RF). Larger RF normally provides better OER performance because of the greater electrochemical surface area, providing more catalytically active sites for a particular reaction. The ECSA of NiCoSe2/NF was assessed by measuring the double-layer capacitance (Cdl) of the solid–liquid interface as reported previously (Figure 3c and Figure S12, SI).5 An ECSA of 108 cm2 and the corresponding RF of 270 are obtained for NiCoSe2/NF electrode. It is noteworthy that these values of RF and ECSA are larger than those of the previously reported nanostructured OER catalysts.7,9,10,19,34 The presence of such an enormous ECSA on nanostructured NiCoSe2/NF provides plentiful electrocatalytic active sites for conversion of anions into the molecular oxygen. Moreover, AC impedance spectra shows the smaller semicircle for NiCoSe2/NF compared to the larger semicircle for NF at overpotential of 400 mV (Figure S11). This implies an improved electrical conductance of NiCoSe/NF over NF, in accord with the better catalytic activity towards OER. Electrochemical stability of an OER catalyst is the prime factor to estimate its life time in strongly alkaline solutions. The durability of the NiCoSe2/NF electrode is evaluated at a constant current density of 10 mA cm-2 by chronoamperometry, as displayed in Figure 3d. NiCoSe2/NF


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demonstrates outstanding long term stability for water electrolysis, over the duration of the 50 h. Even though, vigorous oxygen evolution is observed under high current density operations, gas bubbles leave the electrode surface quite rapidly with virtually no accumulation on the electrode, hence avoiding any reduction in the ECSA (see the movie clip in the SI). On the other hand, bare NF exhibits lower performance for OER owing to the formation of the NiOx in alkaline solutions which ultimately passivate its surface and results in gradual decrease in its catalytic activity (Figure S13, SI).10,40 The remarkable physical stability of the NiCoSe2/NF is further verified by the macrostructural analysis and SEM of NiCoSe2/NF electrode after 50 h of constant OER (Figure S14, S15). The structural morphology of NiCoSe2 remains well-preserved after electrolysis, and no detachment or dissolution of nanoparticles was observed from the electrode. For bifunctional catalytic applications, NiCoSe2/NF hybrid electrode was further tested for its applications in hydrazine oxidation as displayed in Figure 4a, b. It is evident that NiCoSe2/NF is far superior in its performance towards hydrazine oxidation compared to bare NF. Sharp increase in the current density at equivalent given voltage shows the high catalytic performance of hybrid electrode. In addition, a low onset potential (-0.7 V) is also observed in case of NiCoSe2/NF compared to the high onset potential for bare NF (-0.4 V) as displayed in Figure S16. Stability of the prepared electrode is also an integral part for its widespread industrial utilization. To that end, the stability of NiCoSe2/NF for hydrazine oxidation was tested at fixed 0 V as shown in Figure 4c. Relative stable current densities indicate that the hybrid NiCoSe2/NF electrode is capable of catalyzing hydrazine for a fairly prolonged period of time. It is worth noting that these low onset potential values are superior to many metallic electrocatalysts previously used for hydrazine oxidation.41–43


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Figure. 4 (a) Hydrazine oxidation in the presence of 100 mM hydrazine in 0.5 M KOH (b) Long term stability of hydrazine oxidation at constant 0 V vs. SCE (c-d) XPS spectra of the NiCoSe2/NF after 50 hours of constant OER. (a) Ni2p. (b) Co2p

To better evaluate the mechanism of enhanced OER capabilities of NiCoSe2/NF, the XPS spectra after 50 h of continuous water electrolysis were measured and displayed in Figure 4c, d. It can be seen that the majority of Ni and Co in NiCoSe2/NF are present in their divalent electronic states, emphasizing the stability of NiCoSe2/NF in the harsh alkaline environment for a prolonged time period. The presence of Se in EDS mapping of NiCoSe2/NF after 50 h of continuous oxygen evolution (Figure S15) further confirms the stability of hybrid catalyst. Small


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contributions from Co+3 and Ni+3 in XPS spectra (Figure 4c, d) may be evidences of CoOOH and NiOOH on the surface of NiCoSe2/NF, which is beneficial for an OER catalyst. The presence of these CoOOH / NiOOH species on the surface of our electrocatalyst is consistent with previously reported bimetallic selenide electrocatalysts carrying similar species on their surfaces while displaying long term stability.13 Numerous prior studies pointed out that the presence of CoOOH and NiOOH is crucial in achieving high OER activity in Co and Ni based electrocatalysts since they serve as electrocatalytic active sites for an OER.6,8,12,13,22 These data further suggest that NiCoSe2/NF can be a stable electrocatalyst for both OER and hydrazine oxidation. Particularly, the stability of this bimetallic selenide suggests that its surface remains catalytically active and is not prone to surface passivation faced by metallic catalysts under similar oxidizing conditions.25,26 The enhanced activity of NiCoSe2/NF can be explained to the following aspects: (i) intrinsically high activity of the NiCoSe2 nanoparticles, (ii) high ECSA offered by unique nanoarchitectured surface, (iii) extremely rough surface with excellent gas bubble dissipation ability, (iv) possible presence of both electrocatalytically active (NiOOH and CoOOH) species on the surface of catalyst, (v) firmly bonded NiCoSe2 on highly conductive NF with low electrical resistance for overall water splitting and the absence of dead volume in electrocatalyst arisen from binder free electrodeposition approach of NiCoSe2 on NF. Furthermore, the macroporous 3D surface provided by NF permits rapid dissipation of the larger oxygen bubbles into the solution. Conclusions In summary, a NiCoSe2/Ni foam composite electrode is prepared via a facial and scalable electrodeposition route. The resulting NiCoSe2/NF electrode exhibits enhanced electrocatalytic


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performance for OER and hydrazine oxidation with excellent stability. Moreover, the overpotential to attain a viable current density of 10 mA cm-2 is merely 183 mV for OER. The presence of multiple catalytically active centers, high electrochemically active surface area, and synergistic coupling effects between NiCoSe2 nanoparticles and NF support might be the reason for improved catalytically activity of the NiCoSe2/NF composite electrode. Utilization of earthabundant raw materials, low manufacturing costs, excellent stability, low overpotential and large oxidation current make NiCoSe2/NF a viable candidate for its widespread use in various water oxidation technologies. Supporting Information Digital photographs of bare NF and NiCoSe2/NF (Figure S1); SEM and corresponding EDS data of the electrodes (Figure S2, S3); effect of electrodeposition time on the growth of prepared electrocatalyst on NF (Figure S4); XPS elemental mapping analysis and XPS survey spectra of hybrid electrode (Figure S5, S6); electrochemical measurements (Figure S7-S13); optical images of electrode before and after 50 h stability testing (Figure S14); EDS mapping of electrocatalyst after 50 h stability testing (Figure S15); close up view of corresponding CV (Figure 4a) curves for hydrazine oxidation (Figure S16); comparison of

the OER performance of numerous

electrocatalysts with our hybrid electrocatalyst (Table S1). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT


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This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Nos. 2010-0020207, 2011-0030786, 2016R1E1A1A01942649, and 2017R1A2B4002442). REFERENCES (1)

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Table of Contents

Synopsis Water splitting and fuel cells belongs to renewable technologies and thus emphasis the efficient utilization of earth resources.


26

NF electrocatalysts for its

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