NiS Heterostructures: An Efficient and Stable Electrocatalyst for

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NiO/NiS Heterostructures: An Efficient and Stable Electrocatalyst for Oxygen Evolution Reaction Nadeem Asghar Khan, Naghmana Rashid, Muhammad Junaid, Muhammad Nadeem Zafar, Muhammad Faheem, and Iqbal Ahmad ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Energy Materials

NiO/NiS Heterostructures: An Efficient and Stable Electrocatalyst for Oxygen Evolution Reaction Nadeem Asghar Khan,† Naghmana Rashid,† Muhammad Junaid,‡ Muhammad Nadeem Zafar,§ Muhammad Faheem,∥ and Iqbal Ahmad*,† †Department of Chemistry, Allama Iqbal Open University, Islamabad 44000, Pakistan ‡ College of Physics, Changchun University of Sciences and Technology, Changchun 130000, China §Department of Chemistry, University of Gujrat, Gujrat, 50700 Pakistan School of Chemistry, Northeast Normal University, Changchun 130000, China

∥

*Corresponding author: Email: [email protected][email protected] Phone:0092-51-9057874 Postal Address: Department of Chemistry, Allama Iqbal Open University, Islamabad, 44000, Pakistan

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The intervening barrier to produce hydrogen from water is the frustratingly slow kinetics of the water splitting reaction. Further, insufficient understanding of the key obstacle of the oxygen evolution reaction (OER), is an obstruction to perceptive design of efficient OER electrocatalysts. In this research, we present synthesis, characterization and electrochemical evaluation of nickel oxide/nickel sulphide (NiO/NiS) heterostructures and its counterparts nickel oxide (NiO) and nickel sulphide (NiS) as low cost electrocatalysts for electrochemical water splitting. These electrocatalysts have been characterized using powder x-ray diffraction (XRD), fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The NiO/NiS is found to be highly efficient and stable electrocatalyst, which initiates the OER at an amazingly low potential of 1.42 V (vs. RHE). The NiO/NiS electrocatalyst provides a current density of 40 mA cm-2 at 209 mV overpotential for OER in 1.0 M KOH with Tafel slope of 60 mV dec-1, outperforming its counterparts (NiO and NiS) under same electrochemical conditions. These results are better than those of benchmark Ni based and even noble metals based electrocatalysts. The continued oxygen generation for several hours with an applied potential of 1.65 V (vs. RHE) reveals the long-term stability and activity of NiO/NiS electrocatalyst towards OER. This development provides an attractive non noble metal, highly efficient and stable electrocatalyst towards OER. Keywords: Nickel sulphide, nickel oxide, oxygen evolution, heterostructures, water splitting


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INTRODUCTION Depletion of fossil fuels and their adverse effects on the environment are the biggest problems of the day. To address these problems, an extensive research on renewable energy sources has been carried out by different research groups

1-3.

Among renewable energy

sources, hydrogen is the most suitable and clean energy source due to its high energy density and zero environmental pollution. One of its biggest sources is reforming process which is neither efficient nor pollution free and almost 90% hydrogen is still produced by this method. There is need of clean and environmentally friendly source of hydrogen production. Electrochemical water splitting is the best choice for this purpose and in recent years, a lot of research, has been carried out in this field 4, 5. However, the oxygen evolution reaction (OER) at electrode surface is not much efficient and requires high overpotential due to its sluggish kinetics 6. To derive OER with faster rate and at low overpotential, much effort has been devoted to finding out efficient electrocatalysts by tuning both the morphological and electronic features

7-9.

Among the various electrocatalyst reported to

date, the noble metals like Ru, Ir and their oxides demonstrated the best OER performance, but to achieve a current density ≥ 10 mA cm-2, they still need overpotential of ≥ 250 mV 10, 11.

The scarcity and high cost are the major barriers in wide spread applications of Ru and

Ir. So, there is a need of low cost and earth abundant alternative electrocatalysts like Mn, Fe, Co and Ni

12-15.

Particularly, nickel-based electrocatalysts have recently been exhibited

similar or even better OER activity 16-18. In basic medium, the NiOOH has been proved an active OER electrocatalyst

19-21.

It has

22,

NiFe

been reported that most of the Ni-based electrocatalysts like atomic Ni oxyhydroxides

23,

oxides

24,

hydroxides

25,

selenides

26, 27,


sulphides

28,

nitrides

29,


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phosphides 30, phosphates 31, MOFs 32, borides 33, 34 and borates 35 can undergo conversion from Ni(II) to Ni(III) generating disordered NiOOH centeres

36.

The Ni oxyhydroxides

serve as active sites for oxygen evolution as the metal ions in higher oxidation state can assist the generation of oxyhydroxide intermediates, the key players in the electrochemical water oxidation process

37.

Attempts have already been made to activate the Ni-based

electrocatalysts via in situ formation of active NiOOH specie under alkaline conditions 7, 38, 39.

In addition to the NiOOH active centers, the rational design of electrode structure is another aspect which can determine the performance of electrocatalyst. Xiao et al. revealed that low charge transfer resistance, large number of active sites and strong electronic interactions between Ni2P and NiS in NiS/Ni2P heterostructure, play an important role to the improved OER performance

40.

Sirisomboonchai et al. reported the heterostructures of NiO@NiFe-

LDH with excellent OER performance and stability due to low charge transfer resistance 41. Konkena et al. revealed that accessible active metallic edge and structural defects in NiPS3@NiOOH core-shell heterostructures are key performance parameters in OER process 42. On the basis of above stated reasons, it is quite appropriate to fabricate a novel heterostructures in search of improved electrocatalytic performance. Additionally, Ni-based oxides and sulphides have been shown to be cost effective and earth-abundant electrocatalysts for electrochemical water oxidation under alkaline conditions. Herein, we report a fabrication of NiO/NiS heterostructures, an efficient electrocatalyst, for OER. The fabricated NiO/NiS electrocatalyst requires overpotential of just 209 mV to attain current density of 40 mA/cm-2 for OER in 1.0 M KOH solution, outperforming its


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counterparts (NiO and NiS) under same electrochemical conditions. It is demonstrated to be an efficient and stable electrocatalyst and is better than the reported benchmark Ni-based and even noble metals based electrocatalysts 27, 34, 43-45. Our findings demonstrate the origin of the electrocatalytic performance of the OER electrocatalyst and insights in the rational design of Ni and other earth abundant elements based electrocatalysts. EXPERIMENTAL SECTION Materials Nickel nitrate hexahydrate (Ni(NO)3.6H2O, >99%) was purchased from BDH Chemicals Ltd. Potassium hydroxide (KOH, 99%) was obtained from Scharlau. Sodium hydroxide (NaOH, 97%) and sodium sulphide pentahydrate (Na2S.5H2O, 98%) were purchased from Daejung Chemicals. Nafion solution (5 wt%) was purchased from Sigma Aldrich. Nickel foam (0.5 mm in thickness, 98% in porosity) was purchased from Shenzhen Lifeixin Environment Material Co. All chemicals were used as received without any further purification. Synthesis of Electrocatalysts All the electrocatalysts were synthesized by the simple hydrothermal method. For synthesis of NiO nanoparticles, 25 mL solution of 0.5 M Ni(NO)3.6H2O was added in glass container and continuously stirred for 30 min. Subsequently, 2.0 M NaOH solution was added dropwise to maintain pH of the solution at 12. Then, the resultant reaction mixture was transferred to 40 mL Teflon lined stainless steel autoclave and was kept in the oven at 180 °C for 16 h. After that the resultant product was collected via filtration and washed with doubly distilled water to remove soluble impurities. Afterwards, obtained precipitates were



dried in oven at 80 °C overnight. Finally, precipitates were calcinated at 500 °C for 1 h to obtain NiO nanoparticles. For synthesis of NiS electrocatalyst, 12 mL solution of 0.5 M Ni(NO)3.6H2O was added in 0.5 M Na2S solution and the resultant reaction mixture was kept in glass container with continuously stirring for 1 h. After that, the reaction mixture was transferred to autoclave and was kept at 180 °C for 16 h. Finally, as prepared NiS was collected and dried in oven at 60 °C for 12 h. To obtain NiO/NiS heterostructures, previously prepared 0.373 g of NiO was added into a 10 mL solution of 0.5 M Ni(NO)3.6H2O and resultant dispersion was sonicated for 30 min. After that, 10 mL solution of 0.5 M Na2S was added into reaction mixture which was kept on stirring for 1 h. Then, the reaction mixture was transferred to autoclave for further reaction at 180 °C for 16 h to form desired product. Two additional experiments have also been performed to prepare two other electrocatalysts by the above stated hydrothermal method. One electrocatalyst (1) has been prepared by reaction of NiO nanoparticles with 0.55 M of Na2S and another one (2) has been prepared by mixing 10 mL of 0. 5 M Ni(NO3) with 10 mL of N2S. Materials Characterization The x-ray diffraction (XRD) measurements were carried out on a RigakuSmartlab diffractometer with Cu-Kα radiation operating at a voltage of 40 kV and a current of 30 mA. The scanning electron microscopic (SEM) studies were performed on a HITACHI SU8010 microscope. FTIR spectroscopic studies were carried out by a Nicolet iS50 110V/InGaAs Fourier transform infrared spectrometer. The BET surface area of prepared


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electrocatalysts has been calculated by N2 adsorption–desorption technique by Quantachrome NOVA2200e. Electrochemical Measurements All electrochemical experiments were performed by electrochemical workstation, Autolab PGSTAT 302 (Utrecht, The Netherlands). A conventional three-electrode electrochemical cell was used for all electrochemical experiments. The samples (prepared electrocatalysts) modified Ni foam with dimensions 2 x 2 cm2 was used as working electrode. The Ag/AgCl and Pt wire electrodes were used as reference electrode and the counter electrode respectively. The 1.0 moldm-3 KOH solution was used as electrolyte in all electrochemical experiments. All the experiments of cyclic voltammetry (CV) and linear sweep voltammetry were performed at scan rate of 5 mV/s. In present study, calibration of all the potentials was performed by the Nernst equation: ERHE = EAg/AgCl + (0.1976 + 0.059pH) V

(1)

The value of overpotential was calculated by following equation: η = ERHE – 1.23 V

(2)

where η is overpotential. For Tafel slope calculation, following well known Tafel equation was used. η = blogj + a

(3)

where, j is current density, b is the Tafel slope and a is constant. Chronoammperometry was also used for controlled potential electrolysis under static conditions. Electrochemical



impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 10 mHz at open circuit potentials. In EIS experiments, amplitude was set at 5 mV. To fabricate modified Ni foam electrode, 5 mg of the electrocatalyst was added in 1.0 mL of ethanol and subsequently sonicated for 1 h. After that, 10 μL of Nafion solution (5 wt%) was added in the resultant dispersion of electrocatalyst. It was then dropped on both sides of a piece of Ni foam with dimensions of 2 x2 cm2. Afterwards, it was kept overnight at room temperature before measurements. RESULTS AND DISCUSSION Structural Analysis Powder x-ray diffraction (XRD) studies have been performed to confirm the crystal structures of electrocatalysts. XRD spectra of NiO, NiS and NiO/NiS nanocrystals are displayed in Figure 1. X-ray diffractogram of NiO nanoparticles (Figure 1a) shows that it exhibits diffraction peaks having 2θ values of 37.29, 43.24, 62.88, 75.27 and 79.68 which correspond to the planes (111), (200), (220), (311) and (222) respectively. Such type of diffraction pattern is characteristic of pure cubic phase of NiO nanocrystals 46, 47 and is well matched with JCPD card number 01-1239. The absence of any extra peak ruled out the presence of Ni(OH)2 as it is converted into NiO by annealing at 500 oC and it revels the formation of pure cubic phase of NiO. The major diffraction peaks observed for NiS (Figure 1b) are centered at 2θ values of 31.05, 34.98, 46.13 and 53.77 and are indexed as (100), (101), (102) and (110) planes respectively which is attributed to the hexagonal phase of NIS 48, 49 (JCPD card number 02-1280). Apart from these major diffraction peaks of NiS, other three peaks centered at 2θ values of 20.12, 22.38 and 25.28 have also been observed


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and may be attributed to Ni(OH)2. While, XRD pattern of NiO/NiS (Figure 1c) contains diffraction peaks of both NiO and NiS along with Ni(OH)2. Fourier transformed infrared (FTIR) spectroscopy is very useful and informative technique to identify the surface functional groups

50.

Therefore, to gain insight into bonding of

electrocatalysts, FTIR spectroscopy has been used. FTIR spectra of NiO, NiS and NiO/NiS electrocatalysts are displayed in Figure 2. In FTIR spectra of all three samples, a broad band observed at about 3436 cm-1 is assigned to stretching vibration of O-H group of water molecules which is due to adsorbed moisture

51.

Another peak located at 1620 cm-1 is

attributed to bending vibration of O-H of adsorbed water molecules

52.

For NiO and NiS,

the peak found at 2342 cm-1 is correlated to CO2 which is thought to be adsorbed on surface of samples from atmosphere during sample preparation

53, 54

and its intensity is variable

depending upon concentration of CO2 present in the sample. However, this peak is absent in the FTIR spectrum of NiO/NiS (Figure 2c) which might be due to low concentration of CO2 in this electrocatalyst. In FTIR spectra of all the samples, a peak located at 1085 cm-1 which is much more intense for NiS (Figure 2b) is associated with stretching vibration of C=O of adsorbed CO2 55. The characteristic absorption peak of Ni-O is located at 633 cm-1 56.

The presence of this peak in NiS FTIR spectrum is due to Ni(OH)2 and its presence in

this electrocatalyst has already been confirmed by XRD results. The peak at about 1130 cm1

in FTIR spectrum of NiS (Figure 2b) which is due to bending vibration of sulphide group

50

is merged with peak of C=O of adsorbed CO2 and is not fully resolved.

The surface morphological features of an electrocatalyst play an important role in its activity. To study the surface morphology of fabricated electrocatalysts, scanning electron microscopy (SEM) has been employed. SEM micrographs of NiO, NiS and NiO/NiS



electrocatalysts are shown in Figure 3. It is clear from the SEM images that particles of NiO (Figure 3a) are spherical in shape and are randomly distributed. The average particle size of NiO is found to be 55 nm. Particles of NiS (Figure 3b) are highly agglomerated. Due to their overlapping and aggregation quite large particles are formed with a large density of interconnected porous cavities. These cavities will act as a reservoir for electrolyte which may result in enchased electocatalytic activity. In SEM micrograph of NiO/NiS (Figure 3c) spherical NiO nanoparticles are embedded in the cavities of NiS. The surface area of electrocatalyst is a key factor to determine its electrochemical performance for OER. In this regard, adsorption-desorption studies on electrocatalysts have been performed employing N2 gas as adsorbate and resultant Brunauer-Emmett-Teller (BET) isotherms are presented in Fig. S1. The BET curve < P/Po = 0.4 is almost straight and this region has been selected for BET surface area calculation. The values of BET surface area for NiO, NiS and NiO/NiS are calculated to be 109.27, 145.58 and 172.06 m2g1

respectively. Large surface area of NiO/NiS electrocatalyst than its counterparts (NiO and

NiS) suggests its potential candidature for electrochemical water splitting. All the electrocatalysts exhibited type IV isotherms with H3-type hysteresis loop with both mesoporous and macroporous characteristics. Electrochemical Studies Electrochemical studies have been performed to evaluate the water electocatalytic oxidation performance of NiO, NiS and NiO/NiS nanoparticles fabricated on Ni foam by employing cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in 1.0 M KOH solution at a scan rate of 5 mV/s. The iR uncompensated cyclic voltammograms (CVs) and linear sweep


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voltammograms (LSVs) of Ni foam modified NiO, NiS and NiO/NiS electrocatalysts are presented in Figure 4 (A, B). In case of NiO, during anodic potential sweep of CV, preoxidation feature of water, i.e. an oxidation peak centered at Epa = 1.43 V (vs. RHE) is observed and it is attributed to oxidation of Ni (II) to Ni (III) which results in an active electrocatalyst structure

44.

Exactly similar oxidation signature has also been observed in

LSV of NiO (Figure 4 B). During reverse scan of CV, a reduction peak of NiO centered at Epc = 1.32 V (vs. RHE) is observed as shown in Figure 4 A. This redox couple observed in CV for NiO electrocatalyst is due to inter conversion of Ni(OH)2/NiOOH 57-59. The similar redox pair has also been observed in the CVs of NiS and NiO/NiS (Figure 4 A). Anodic peak potentials are observed at 1.43 and 1.41 V, while those of cathodic at 1.22 and 1.25 V for NiS and NiO/NiS respectively. Values of ipa/ipc for NiO, NiS and NiO/NiS are found to be 0.55, 0.40 and 0.60 respectively, which indicate that reaction might be quasi-reversible rather than reversible. Additionally, value of Epa- Epc is quite larger than 0.059 V which further support that reaction is not exactly reversible. Value of peak width at half of the maximum height (W1/2) for oxidation peak observed in both CV and LSV for all electrocatalysts is almost equal to 90 mV. It indicates that one electron is involved in preoxidation electron transfer process of water. From iR uncompensated CVs and LSVs of NiO, NiS and NiO/NiS (Figure 4), the onset potential for oxygen evolution reaction (OER) is found to be 1.53, 1.52 and 1.49 V (vs. RHE) respectively. Just like to other reported Ni based electrocatalysts, there is a very small potential difference between oxidation peak potentials and onset potentials of prepared electrocatalysts 44, 60. After this potential, a sharp increase in current density is observed and it happens due to contribution to catalytic current generated from OER of water. However, an increase in current density for NiO is



not very sharp as compared to NiS and NiO/NiS. In case of NiO, the overpotential (η) at current densities of 10 and 40 mAcm-2 for OER is 350 and 470 mV respectively. For NiS and NiO/NiS electrocatalyst, the presence of oxidation peak in the region of 10 mAcm-2 current density, hinders the determination of value of overpotential at this current density. Due to this reason, we are unable to report overpotential for these electrocatalysts at a current density of 10 mAcm-2. Therefore, overpotential for these electrocatalyst has been reported at a current density of 40 mAcm-2. The values of overpotential observed at this current density for NiS and NiO/NiS are 294 and 279 mV respectively. In case of NiO/NiS, the increase in current density is very sharp and at overpotential of 490 mV only, > 100 mAcm-2 current density is achieved which is quite larger than that of NiO and NiS. This improved performance is attributed to its activation before OER. It might be due to the formation of active NiOOH which is generated in stitu from the oxidation of Ni2+. It is thought that formation of NiOOH phase speed up the electrochemical oxidation reaction of water. As soon as it is generated, shift in overpotential of OER takes place towards lower value 22. Additionally, there is no need of pre-conditioning to obtain higher oxidation state of Ni unlike to many other Ni based already reported electrocatalysts 44. It is pertinent to mention here that in iR uncomposated CV or LSV results there has been always contribution of solution resistance and most of the researchers do iR correction while reporting their results. However, according to our point of view, it is better to report both iR corrected and uncorrected overpotential values for comparison purpose. For this purpose, electrochemical impedance spectroscopy (EIS) has been used to determine value of solution resistance. EIS results in the form of Nyquist plots recorded in 1.0 mol/dm3 solution of KOH at open circuit potential are shown in Figure 5. Solution resistances for


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NiO, NiS and NiO/NiS have been found to be 5.1, 4.8 and 5.2 ohm respectively. For iR correction, these values of solution resistance are multiplied by the current. The 100 % iR corrected CVs and LSVs of Ni foam modified NiO, NiS and NiO/NiS electrocatalysts are presented in Figure 6 (A, B). Interestingly, the onset potential values of all the electrocatalysts are quite less than those of observed for without iR compensation. The iR componsated onset potential values are found to be 1.50, 1.43 and 1.42 V (vs. RHE) for NiO, NiS and NiO/NiS respectively. After onset potential, very sharp increase in current density is observed for all elerocatalysts. This increase is much sharper than that of observed for iR uncompensated CV and LSV. The current density of >100 mAcm-2 is achieved just at 1.55 V (vs. RHE) for NiO/NiS electrocatalyst. The values of overpotential are observed to be 400, 220 and 209 mV at 40 mAcm-2 current density for NiO, NiS and NiO/NiS respectively. These values are significantly lower than that of observed for iR uncomposated results. The enhanced electrochemical performance of NiO/NiS might be due to the several reasons. One of the reasons might be cavities present within its structure which may serve as a reservoir for electrolyte. Other possible reasons of enhanced electrocatalytic activity for OER might be due to a large surface area which provides a large number of electrocatalytic sites and strong interactions between NiO and NiS in NiO/NiS heterostructures. Additionally, charge transfer resistance of NiO/NiS (observed from EIS results), is lower than its counterparts and results in its better electrocatalytic performance. Electrochemical performance of electrocatalyst 1 and 2 towards OER is similar to NiO and NiS respectively (Fig. S2). To gain insight into the kinetics of OER, the Tafel slope analysis has been performed. Tafel plots (η vs logj) for NiO, NiS and NiO/NiS are presented in Figure 7. Tafel slope is 60



mV/dec for NiO/NiS and is smaller than those for NiO (70 mV/dec) and NiS (65 mV/dec). The smallest value of the Tafel slope of NiO/NiS suggests that OER is more facile for this electrocatalyst under same electrochemical conditions. Controlled potential electrolysis (Chronoamperomety) has been used to perform a stability test of the best electrocatalyst. As NiO/NiS electrocatalyst demonstrated excellent performance, therefore, its stability test has been conducted. A constant potential of 1.65 V (vs. RHE) was selected and maintained throughout the water electrolysis, while observing the current density response of the system at the same time. Extended period controlled potential electrolysis of water for NiO/NiS electrocatalyst is shown in Figure 8. Chronoamperometric experiment was performed for 14 h. A stable current density of 50 mA cm-2 has been observed for an extended period of 10 h. However, a slight decrease in current density has been found after 10 h. Overall, NiO/NiS demonstrated excellent stability for long time water oxidation operation. Comparison of electrochemical performance of prepared electrocatalysts with other reported benchmark Ni based electrocatalysts, recently appeared, is shown in Table 1. As shown in the Table, the electrocatalysts reported in the present study demonstrate the lowest onset potential (E) and overpotential (η) [NiO/NiS; E = 1.42 V vs RHE, η = 209 mV and NiS; E = 1.43 V vs RHE, η = 220 mV] for the OER comparative to other benchmark Ni based water oxidation electrocatalysts. Although, Tafel slopes of NiCoON/NF and Ni4(PET)8/GCE are just 35 and 38 mV/dec respectively, but their onset potential for oxygen evolution is higher than our (NiO/NiS and NiS) electrocatalysts 7, 22.


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CONCLUSION We have developed the NiO/NiS electrocatalyst for OER at low overpotential. NiO/NiS heterostructure has found to be highly efficient, stable, cost effective and low overpotential oxygen evolution electrocatalyst. The NiO/NiS electrocatalyst showed OER activity at low overpotential of 209 mV@40 mAcm-2 in 1.0 M KOH. The oxygen evolution onset happened at a low potential of 1.42 V (vs. RHE) and this oxygen evolution potential is the lowest for Ni based electrocatalysts reported so far. Long term controlled potential water oxidation shows stable oxygen evolution current density of 50 mAcm-2 at 1.65 V vs. RHE. The synthesis of NiO/NiS heterostructure is very simple making it very appealing and competitive nonprecious catalyst for OER. These results are very inspiring for non noble metal electrocatalyst for water oxidation and it is foreseen that this exploration will further promote the methods and knowledge to develop highly efficient, stable and robust electrocatalyst for water oxidation. ASSOCIATED CONTENT Supporting Information Available: **[ N2 adsorption-desorption isotherms of NiO, NiS and NiO/NiS. Linear sweep voltammograms of (1) and (2) ]** ACKNOWLEDGEMENT We are highly thankful to the Department of chemistry, Allama Iqbal Open University, Islamabad, Pakistan for providing us laboratory and space facilities. REFERENCES 1.

Grätzel, M., Photoelectrochemical cells. nature 2001, 414, 338.



2.

Zhao, D.; Chen, C.; Wang, C.; Junda, M. M.; Song, Z.; Grice, C. R.; Yu, Y.; Li, C.;

Subedi, B.; Podraza, N. J.; Zhao, X.; Fang, G.; Xiong, R.-G.; Zhu, K.; Yan, Y., Efficient two-terminal all-perovskite tandem solar cells enabled by high-quality low-bandgap absorber layers. Nat. Energy 2018, 3, 1093-1100. 3.

Segev, G.; Beeman, J. W.; Greenblatt, J. B.; Sharp, I. D., Hybrid

photoelectrochemical and photovoltaic cells for simultaneous production of chemical fuels and electrical power. Nat. Mater. 2018, 17, 1115-1121. 4.

Huang, H.; Yu, C.; Zhao, C.; Han, X.; Yang, J.; Liu, Z.; Li, S.; Zhang, M.; Qiu, J.,

Iron-tuned super nickel phosphide microstructures with high activity for electrochemical overall water splitting. Nano Energy 2017, 34, 472-480. 5.

Swierk, J. R.; Mallouk, T. E., Design and development of photoanodes for water-

splitting dye-sensitized photoelectrochemical cells. Chem. Soc. Rev. 2013, 42, 2357-2387. 6.

Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y., A

perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 2011, 334, 1383-1385. 7.

Li, Y.; Hu, L.; Zheng, W.; Peng, X.; Liu, M.; Chu, P. K.; Lee, L. Y. S., Ni/Co-based

nanosheet arrays for efficient oxygen evolution reaction. Nano energy 2018, 52, 360-368. 8.

Xiong, X.; You, C.; Liu, Z.; Asiri, A. M.; Sun, X., Co-Doped CuO Nanoarray: An

Efficient Oxygen Evolution Reaction Electrocatalyst with Enhanced Activity. ACS Sustainable Chem. Eng. 2018, 6, 2883-2887. 9.

Cui, B.; Lin, H.; Li, J. B.; Li, X.; Yang, J.; Tao, J., Core–ring structured NiCo2O4

nanoplatelets: synthesis, characterization, and electrocatalytic applications. Adv. Funct. Mater. 2008, 18, 1440-1447.


Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.

Stoerzinger, K. A.; Rao, R. R.; Wang, X. R.; Hong, W. T.; Rouleau, C. M.; Shao-

Horn, Y., The role of Ru redox in pH-dependent oxygen evolution on rutile ruthenium dioxide surfaces. Chem 2017, 2, 668-675. 11.

Park, H.-S.; Yang, J.; Cho, M. K.; Lee, Y.; Cho, S.; Yim, S.-D.; Kim, B.-S.; Jang, J.

H.; Song, H.-K., RuO2 nanocluster as a 4-in-1 electrocatalyst for hydrogen and oxygen electrochemistry. Nano Energy 2019, 55, 49-58. 12.

Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.;

Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M., Trends in activity for the water electrolyser reactions on 3d M (Ni, Co, Fe, Mn) hydr (oxy) oxide catalysts. Nat. Mater. 2012, 11, 550. 13.

Liu, K.; Zhang, C.; Sun, Y.; Zhang, G.; Shen, X.; Zou, F.; Zhang, H.; Wu, Z.;

Wegener, E. C.; Taubert, C. J.; Miller, J. T.; Peng, Z.; Zhu, Y., High-Performance Transition Metal Phosphide Alloy Catalyst for Oxygen Evolution Reaction. ACS Nano 2018, 12, 158-167. 14.

Morales-Guio, C. G.; Liardet, L.; Hu, X., Oxidatively electrodeposited thin-film

transition metal (oxy) hydroxides as oxygen evolution catalysts. J. Am. Chem. Soc. 2016, 138, 8946-8957. 15.

Joya, K. S.; Takanabe, K.; De Groot, H. J., Surface Generation of a Cobalt‐Derived

Water Oxidation Electrocatalyst Developed in a Neutral HCO3−/CO2 System. Adv. Energy Mater. 2014, 4, 1400252. 16.

Qiu, Y.; Xin, L.; Li, W., Electrocatalytic Oxygen Evolution over Supported Small

Amorphous Ni–Fe Nanoparticles in Alkaline Electrolyte. Langmuir 2014, 30, 7893-7901.



17.

Maruthapandian, V.; Mathankumar, M.; Saraswathy, V.; Subramanian, B.;

Muralidharan, S., Study of the Oxygen Evolution Reaction Catalytic Behavior of CoxNi1– xFe2O4

in Alkaline Medium. ACS Appl. Mater. Interfaces 2017, 9, 13132-13141.

18.

Chen, G. F.; Ma, T. Y.; Liu, Z. Q.; Li, N.; Su, Y. Z.; Davey, K.; Qiao, S. Z.,

Efficient and stable bifunctional electrocatalysts Ni/NixMy (M= P, S) for overall water splitting. Adv. Funct. Mater. 2016, 26, 3314-3323. 19.

Görlin, M.; Ferreira de Araújo, J.; Schmies, H.; Bernsmeier, D.; Dresp, S. R.;

Gliech, M.; Jusys, Z.; Chernev, P.; Kraehnert, R.; Dau, H., Tracking catalyst redox states and reaction dynamics in Ni–Fe oxyhydroxide oxygen evolution reaction electrocatalysts: The role of catalyst support and electrolyte pH. J. Am. Chem. Soc. 2017, 139, 2070-2082. 20.

Smith, R. D.; Berlinguette, C. P., Accounting for the dynamic oxidative behavior of

nickel anodes. J. Am. Chem. Soc. 2016, 138, 1561-1567. 21.

Zhang, Q.; Zhang, C.; Liang, J.; Yin, P.; Tian, Y., Orthorhombic α-NiOOH

Nanosheet Arrays: Phase Conversion and Efficient Bifunctional Electrocatalysts for Full Water Splitting. ACS Sustainable Chem. Eng. 2017, 5, 3808-3818. 22.

Joya, K. S.; Sinatra, L.; AbdulHalim, L. G.; Joshi, C. P.; Hedhili, M. N.; Bakr, O.

M.; Hussain, I., Atomically monodisperse nickel nanoclusters as highly active electrocatalysts for water oxidation. Nanoscale 2016, 8, 9695-9703. 23.

Swierk, J. R.; Klaus, S.; Trotochaud, L.; Bell, A. T.; Tilley, T. D., Electrochemical

Study of the Energetics of the Oxygen Evolution Reaction at Nickel Iron (Oxy)Hydroxide Catalysts. J. Phys. Chem. C 2015, 119, 19022-19029.


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24.

Diaz-Morales, O.; Ledezma-Yanez, I.; Koper, M. T.; Calle-Vallejo, F., Guidelines

for the rational design of Ni-based double hydroxide electrocatalysts for the oxygen evolution reaction. ACS Catal. 2015, 5, 5380-5387. 25.

Xu, Y.; Xie, L.; Li, D.; Yang, R.; Jiang, D.; Chen, M., Engineering Ni(OH)2

Nanosheet on CoMoO4 Nanoplate Array as Efficient Electrocatalyst for Oxygen Evolution Reaction. ACS Sustainable Chem. Eng. 2018, 6, 16086-16095. 26.

Xu, R.; Wu, R.; Shi, Y.; Zhang, J.; Zhang, B., Ni3Se2 nanoforest/Ni foam as a

hydrophilic, metallic, and self-supported bifunctional electrocatalyst for both H2 and O2 generations. Nano Energy 2016, 24, 103-110. 27.

Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X., NiSe nanowire film supported on

nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting. Angew. Chem. 2015, 127, 9483-9487. 28.

Chen, J. S.; Ren, J.; Shalom, M.; Fellinger, T.; Antonietti, M., Stainless steel mesh-

supported NiS nanosheet array as highly efficient catalyst for oxygen evolution reaction. ACS Appl. Mater. Interfaces 2016, 8, 5509-5516. 29.

Xu, K.; Chen, P.; Li, X.; Tong, Y.; Ding, H.; Wu, X.; Chu, W.; Peng, Z.; Wu, C.;

Xie, Y., Metallic nickel nitride nanosheets realizing enhanced electrochemical water oxidation. J. Am. Chem. Soc. 2015, 137, 4119-4125. 30.

Sun, H.; Xu, X.; Yan, Z.; Chen, X.; Cheng, F.; Weiss, P. S.; Chen, J., Porous

multishelled Ni2P hollow microspheres as an active electrocatalyst for hydrogen and oxygen evolution. Chem. Mater. 2017, 29, 8539-8547. 31.

Li, Y.; Zhao, C., Iron-Doped Nickel Phosphate as Synergistic Electrocatalyst for

Water Oxidation. Chem. Mater. 2016, 28, 5659-5666.



32.

Wang, L.; Wu, Y.; Cao, R.; Ren, L.; Chen, M.; Feng, X.; Zhou, J.; Wang, B., Fe/Ni

Metal–Organic Frameworks and Their Binder-Free Thin Films for Efficient Oxygen Evolution with Low Overpotential. ACS Appl. Mater. Iinterfaces 2016, 8, 16736-16743. 33.

Nsanzimana, J. M. V.; Peng, Y.; Xu, Y. Y.; Thia, L.; Wang, C.; Xia, B. Y.; Wang,

X., An Efficient and Earth‐Abundant Oxygen‐Evolving Electrocatalyst Based on Amorphous Metal Borides. Adv. Energy Mater. 2018, 8, 1701475. 34.

Masa, J.; Sinev, I.; Mistry, H.; Ventosa, E.; de la Mata, M.; Arbiol, J.; Muhler, M.;

Roldan Cuenya, B.; Schuhmann, W., Ultrathin high surface area nickel boride (NixB) nanosheets as highly efficient electrocatalyst for oxygen evolution. Adv. Energy Mater. 2017, 7, 1700381. 35.

Bediako, D. K.; Surendranath, Y.; Nocera, D. G., Mechanistic studies of the oxygen

evolution reaction mediated by a nickel–borate thin film electrocatalyst. J. Am. Chem. Soc. 2013, 135, 3662-3674. 36.

Sun, K.; Saadi, F. H.; Lichterman, M. F.; Hale, W. G.; Wang, H.-P.; Zhou, X.;

Plymale, N. T.; Omelchenko, S. T.; He, J.-H.; Papadantonakis, K. M., Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films. Proc. Natl. Acad. Sci. 2015, 201423034. 37.

Yeo, B. S.; Bell, A. T., In situ Raman study of nickel oxide and gold-supported

nickel oxide catalysts for the electrochemical evolution of oxygen. J. Phys. Chem. C 2012, 116, 8394-8400.


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38.

Wang, H. Y.; Hsu, Y. Y.; Chen, R.; Chan, T. S.; Chen, H. M.; Liu, B., Ni3+‐Induced

Formation of Active NiOOH on the Spinel Ni–Co Oxide Surface for Efficient Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5, 1500091. 39.

Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W., Nickel–iron

oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. 40.

Xiao, X.; Huang, D.; Fu, Y.; Wen, M.; Jiang, X.; Lv, X.; Li, M.; Gao, L.; Liu, S.;

Wang, M., Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting. ACS Appl. Mater. Iinterfaces 2018, 10, 4689-4696. 41.

Sirisomboonchai, S.; Li, S.; Yoshida, A.; Li, X.; Samart, C.; Abudula, A.; Guan, G.,

Fabrication of NiO Microflake@NiFe-LDH Nanosheet Heterostructure Electrocatalysts for Oxygen Evolution Reaction. ACS Sustainable Chem. Eng. 2018. 42.

Konkena, B.; Masa, J.; Botz, A. J. R.; Sinev, I.; Xia, W.; Koßmann, J.; Drautz, R.;

Muhler, M.; Schuhmann, W., Metallic NiPS3@NiOOH Core–Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction. ACS Catal. 2017, 7, 229-237. 43.

Pu, Z.; Luo, Y.; Asiri, A. M.; Sun, X., Efficient electrochemical water splitting

catalyzed by electrodeposited nickel diselenide nanoparticles based film. ACS Appl. Mater. Interfaces 2016, 8, 4718-4723. 44.

Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W., Solution-cast

metal oxide thin film electrocatalysts for oxygen evolution. J. Am. Chem. Soc. 2012, 134, 17253-17261.



45.

Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.;

Tilley, S. D.; Fan, H. J.; Grätzel, M., Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014, 345, 1593-1596. 46.

Roy, A.; Ray, A.; Saha, S.; Ghosh, M.; Das, T.; Satpati, B.; Nandi, M.; Das, S.,

NiO-CNT composite for high performance supercapacitor electrode and oxygen evolution reaction. Electrochim. Acta 2018, 283, 327-337. 47.

Meng, T.; Ma, P.-P.; Chang, J.-L.; Wang, Z.-H.; Ren, T.-Z., The Electrochemical

Capacitive Behaviors of NiO Nanoparticles. Electrochim. Acta 2014, 125, 586-592. 48.

Huang, T.; Fang, H.; Xu, L.; Wang, Z.; Xin, Y.; Yu, J.; Yao, S.; Zhang, Z.,

Electrocatalytic performance of cubic NiS2 and hexagonal NiS for oxygen reduction reaction. J. Catal. 2018, 359, 223-232. 49.

Harish, S.; Naveen, A. N.; Abinaya, R.; Archana, J.; Ramesh, R.; Navaneethan, M.;

Shimomura, M.; Hayakawa, Y., Enhanced performance on capacity retention of hierarchical NiS hexagonal nanoplate for highly stable asymmetric supercapacitor. Electrochim. Acta 2018, 283, 1053-1062. 50.

Tang, Y.; Chen, T.; Yu, S.; Qiao, Y.; Mu, S.; Zhang, S.; Zhao, Y.; Hou, L.; Huang,

W.; Gao, F., A highly electronic conductive cobalt nickel sulphide dendrite/quasi-spherical nanocomposite for a supercapacitor electrode with ultrahigh areal specific capacitance. J. Power Sources 2015, 295, 314-322. 51.

Khan, S. B.; Faisal, M.; Rahman, M. M.; Jamal, A., Low-temperature growth of

ZnO nanoparticles: photocatalyst and acetone sensor. Talanta 2011, 85, 943-949. 52.

Chen, X.; Shen, Y.; Zhang, W.; Zhang, J.; Wei, D.; Lu, R.; Zhu, L.; Li, H.; Shen,

Y., In-situ growth of ZnO nanowire arrays on the sensing electrode via a facile


Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hydrothermal route for high-performance NO2 sensor. Appl. Surf. Sci. 2018, 435, 10961104. 53.

Wan, L.; Li, X.; Qu, Z.; Shi, Y.; Li, H.; Zhao, Q.; Chen, G., Facile synthesis of

ZnO/Zn2TiO4 core/shell nanowires for photocatalytic oxidation of acetone. J. Hazard. Mater. 2010, 184, 864-868. 54.

Mishra, A. K.; Ramaprabhu, S., Palladium nanoparticles decorated graphite

nanoplatelets for room temperature carbon dioxide adsorption. Chem. Eng. J. 2012, 187, 10-15. 55.

Linganiso, E. C.; Mwakikunga, B. W.; Coville, N. J.; Mhlanga, S. D., Observation

of the structural, optical and magnetic properties during the transformation from hexagonal NiS nano-compounds to cubic NiO nanostructures due to thermal oxidation. J. Alloys Compd. 2015, 629, 131-139. 56.

Sabouri, Z.; Akbari, A.; Hosseini, H. A.; Darroudi, M., Facile green synthesis of

NiO nanoparticles and investigation of dye degradation and cytotoxicity effects. J. Mol. Struct. 2018, 1173, 931-936. 57.

Corrigan, D. A., The catalysis of the oxygen evolution reaction by iron impurities in

thin film nickel oxide electrodes. J. Electrochem. Soc. 1987, 134, 377-384. 58.

Corrigan, D. A.; Bendert, R. M., Effect of coprecipitated metal ions on the

electrochemistry of nickel hydroxide thin films: cyclic voltammetry in 1 M KOH. J. Electrochem. Soc. 1989, 136, 723-728. 59.

Wehrens-Dijksma,

M.;

Notten,

P.,

Electrochemical

Quartz

Microbalance

characterization of Ni(OH)2-based thin film electrodes. Electrochim. Acta 2006, 51, 36093621.



60.

McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F., Benchmarking

heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. Captions of Figures Figure 1. XRD patterns of (a) NiO, (b) NiS and (c) NiO/NiS nanoelectrocatalysts Figure 2. FTIR spectra of (a) NiO, (b) NiS and (c) NiO/NiS nanoelectrocatalysts Figure 3. SEM micrographs of (a) NiO, (b) NiS and (c) NiO/NiS nanoelectrocatalysts Figure 4. The iR uncompensated (A) CVs and (B) LSVs obtained in 1.0 mol/dm3 solution of KOH for NiO, NiS and NiO/NiS electrocatalysts loaded on Ni foam Figure 5. EIS Nyquist plots obtained in 1.0 mol/dm3 solution of KOH for NiO, NiS and NiO/NiS electrocatalysts loaded on Ni foam Figure 6. The 100 % iR drop compensated (A) CVs and (B) LSVs obtained in 1.0 mol/dm3 solution of KOH for NiO, NiS and NiO/NiS electrocatalysts loaded on Ni foam Figure 7. Tafel plots obtained for NiO, NiS and NiO/NiS electrocatalysts loaded on Ni foam Figure 8. Extended period oxygen evolution during controlled potential electrolysis (1.65 V (vs. RHE) at surface of NiO/NiS electrocatalyst loaded on Ni foam in 1.0 mol/dm3 KOH solution


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Figure 1



Figure 2

Figure 3


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Figure 4

Figure 5



Figure 6

Figure 7


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Figure 8 Table 1. Comparison of electrochemical performance of prepared electrocatalysts with other reported Ni based electrocatalysts Electrocatalysts

Electrolyte

Onset potential

Overpotential

Tafel slope

E/V (vs. RHE)

η (mV)

b (mV/dec)

Reference

NiO/NiS/NF

1.0 M KOH

1.42

209@40 mAcm-2

60

This work

NiS/NF

1.0 M KOH

1.43

220@40 mAcm-2

65

This work

NiSe2/Ti

1.0 M KOH

1.50

295@20 mAcm-2

82

43

NiFe LDH/NF

1.0 M KOH

~1.43

240@10 mAcm-2

-

45

NiSe/NF

1.0 M KOH

~1.46

270@20 mAcm-2

64

27

Ni4(PET)8/GCE

0.1 M KOH

1.51

280@10 mAcm-2

38

22

NiCoON/NF

1.0 M KOH

~1.46

247@10 mAcm-2

35

~1.44

mAcm-2

NixB/NF

1.0 M KOH

280@20

NF= Nickel foam, GCE = Glassy carbon electrode


89

7 34


Graphical abstract 201x146mm (150 x 150 DPI)


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NiS Heterostructures: An Efficient and Stable Electrocatalyst for

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