Effect of phosphate variation on morphology and electrocatalytic activity (OER) of hydrous nickel pyrophosphate thin films

Journal of Alloys and Compounds 779 (2019) 49e58

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Effect of phosphate variation on morphology and electrocatalytic activity (OER) of hydrous nickel pyrophosphate thin films S.J. Marje a, P.K. Katkar a, S.B. Kale a, A.C. Lokhande b, C.D. Lokhande a, U.M. Patil a, * a b

Centre for Interdisciplinary Research, D. Y. Patil Education Society (Deemed to be University), Kasaba Bawada, Kolhapur 416 006 India Department of Material Science and Engineering, Chonnam National University, Yongbong-dong, Puk-Gu, Gwangju 500 757 South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 August 2018 Received in revised form 9 November 2018 Accepted 16 November 2018 Available online 19 November 2018

Simple, cost effective chemical bath deposition (CBD) method is used for synthesis of hydrous nickel pyrophosphate (Ni2P2O7$8H2O) thin films as an electrocatalyst on stainless steel (SS) substrate for OER application. The effect of phosphate variation is studied via preparing thin films with different nickel and phosphate molar weight ratio, as decreasing phosphate concentration in the reaction bath willow leaf like morphology changes to compact microflower with rectangular petals. As-prepared Ni2P2O7$8H2O at a molar weight ratio of 1:2 (nickel: phosphate) gives better results as an electrocatalyst with low overpotential of 308 mV at 25 mA cm
2 current density, as compare to other molar weight ratios (1:1 and 1:0.5). This excellent electrocatalytic performance of hydrous nickel pyrophosphate thin film electrodes is credited to high ECSA and low impedance of electrocatalyst. Also, it exhibits good stability over 9 h in 1 M KOH at constant potential. © 2018 Published by Elsevier B.V.

Keywords: Chemical bath deposition Electrocatalyst Hydrous nickel pyrophosphate Oxygen evaluation reaction Phosphate variation Thin film

1. Introduction In today's world, important goal is to get environment friendly and sustainable renewable energy [1]. It is easily achievable using hydrogen fuel, through splitting water into pure H2 and O2, because it emits negligible carbon in environment [2]. The energy conversion process through water splitting consist of two half reactions HER and OER. But at the anode of OER reaction rate is too slow, which requires an overpotential in substantial excess of its thermodynamic potential (1.23 V verses reversible hydrogen electrode (RHE)) to deliver an acceptable current density [3]. So, it is a challenge to prepare an efficient anode electrocatalyst for OER, which demands low overpotential with good stability. To increase efficiency and decrease cost of electrocatalyst for OER, a number of metals [4], metal oxides [5], hydroxides [6,7], carbides [8], nitrides [9,10], sulfides [11e13], borides [14,15], selenides [16,17], and phosphides [18e21] have been studied. However, some materials such as metal oxides, hydroxides and sulfides suffer from less conductivity. While, some requires higher overpotential and have low stability such as borides, selenides, etc. To fulfill the requirement of OER electrocatalyst, there is a need to develop a material

* Corresponding author. E-mail address: [email protected] (U.M. Patil). https://doi.org/10.1016/j.jallcom.2018.11.213 0925-8388/© 2018 Published by Elsevier B.V.

which has low cost, earth abundant and low overpotential with good stability. Most recently, metal phosphates are getting much attention because of their special open framework structure with large channels and cavities which show good catalytic behavior [22]. Phosphate material has not only distorted local metal geometry that useful for oxidation and adsorption of water, but also act as proton accepter for oxidizing metal atoms. So, they can replace expensive noble metal for a catalytic application. Earliest member discovered in phosphate group as an electrocatalyst was cobalt phosphate for OER application [23e28]. To explore fundamental properties and search high performance electrocatalyst, some other metal phosphates have been investigated. In which nickel based compound get much attention because of its low price, high elemental abundance, high strength, high corrosion resistance, high electrical conductivity, superior catalytic activity and stability [29]. Therefore, different polymorphs of nickel phosphate based catalysts such as, Ni2P2O12 [23], Ni11(HPO3)8(OH)6 [30], NiFeOH$PO4 [31], Ni:Pi [32] etc. are widely investigated for OER. Also, several methods have been adopted and reported in literature for synthesis of nickel phosphate with different morphologies. Huang et al. [23] prepared nanoparticles of nickel metaphosphate (Ni2P2O12) via chemical bath deposition (CBD) method and

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Menezes et al. [30] synthesized nickel phosphite nanorods(Ni11(HPO3)8(OH)6) using hydrothermal method. As well as, Zhanwu et al. [31] electrodeposited porous nanobelt like nickeliron hydroxylphosphate (NiFe-OH.PO4) on nickel foam. Amorphous nickel phosphate synthesized on nickel foam substrate via hydrothermal method then iron was doped using electrodeposition by Li et al. [32]. Moreover, in all above reported method, CBD is much easy to handle and more convenient in terms of any kind of substrate (metal/non-metal) can be used to deposite material. Also, large area deposition can be achieved by CBD method with any complex size and shape of substrates. At the best of our knowledge, hydrous nickel pyrophosphate thin film synthesis on stainless steel (SS) substrate via CBD method for OER have not been investigated. In this work, a well known but till not used hydrous nickel pyrophosphate material as an electrocatalyst, prepared for OER application. A binder free hydrous nickel pyrophosphate thin film synthesized via CBD method at 363 K temperature on SS substrate to study electrocatalytic performance. The SS substrate and binder free method used for deposition, to minimize total cost of electrocatalyst electrode. Effect of phosphate variation on structural, morphological and consequently electrocatalytical performance of hydrous nickel pyrophosphate thin films are investigated. 2. Experimental section 2.1. Synthesis of nickel phosphate thin films by CBD method Nickel chloride (NiCl2$6H2O), potassium dihydrogen orthophosphate (KH2PO4) and urea (NH2CONH2) were purchased from Sigma Aldrich (AR grade), and used as received without any purification. For the synthesis of hydrous nickel pyrophosphate 0.066 M NiCl2$6H2O, 0.066 M KH2PO4 and 0.075 M NH2CONH2 were dissolved in 50 mL of double distilled water respectively, with magnetic stirring upto homogeneous solution formation. For the analysis different composition effect of nickel and phosphate, three solutions were prepared by varying molar weight ratio of NiCl2$6H2O:KH2PO4 as, 0.033:0.066, 0.05:0.05 and 0.066:0.033 and indicated as NP1, NP2 and NP3, respectively. The CBD method was used to prepare hydrous nickel pyrophosphate thin films on stainless steel (SS) substrate using above mentioned three different solutions. The SS substrates are used, because it has several advantages over other substrates such as low cost, high conductivity, importantly very stable in most acids and alkaline solutions and makes it an ideal substrate for OER application. A well cleaned SS substrate (1  5 cm2) was immersed vertically in prepared solution bath and then bath placed in water maintained at temperature 363 K, for 2 h. A heterogeneous reaction is occurred during 2 h and material deposited on SS substrate. Furthermore, obtained material in the form of thin films were rinsed 2e3 times with double distilled water for the removal of residues on the surface of prepared thin films and then dried naturally at room temperature. 2.2. Characterization The X-ray diffraction pattern (XRD) obtained from Rigaku miniflex-600 with Cu Ka (l ¼ 0.15406 nm) radiation, operated at 40 kV, with scanning rate of 2 min
1, for the crystal structure analysis of prepared material. Fourier transform-infrared spectroscopy (FT-IR) was recorded using an Alpha (II) Bruker unit and used for the detection of functional groups present in the prepared material. Morphology, size and elemental analysis of the prepared material was investigated by field emission scanning electron microscopy (FE-SEM, JSM-7001F, JEOL) and Energy dispersive spectroscopy (EDS) (Oxford, X-max). All electrochemical activity was measured by using ZIVE MP1 multichannel electrochemical

workstation. 2.3. Electrochemical measurements A three electrode system was used to carry out all electrochemical measurements at room temperature. In this system, platinum plate used as a counter electrode, saturated calomel electrode (SCE, with saturated KCl solution) as a reference electrode and prepared hydrous nickel pyrophosphate thin films with different Ni:PO4 molar weight ratios as a working electrode. To carry out electrochemical measurements 1 M KOH (pH ¼ 14) aqueous solution used as an electrolyte. All potentials mentioned in this work measured verses SCE and converted in to a reversible hydrogen electrode (RHE) scale according to Nernst equation without iR-correction. Equation for conversion of potential as follow,

ERHE ¼ ESCE þ 0:059  pH þ E0 SCE

(1)

Where, ERHE, ESCE, and E0SCE are potential verses RHE, potential verses SCE, and standard redox potential of SCE at 298 K (0.244 V), respectively [33]. Before taking any electrochemical measurements prepared electrodes were activated in 1 M KOH electrolyte by cycling for 50 cyclic voltammetry (CV) cycles at scan rate of 100 mV s
1 within potential window of 0e0.5 V verses SCE. Then, linear sweep voltammetry (LSV) recorded at sweeping rate of 10 mV s
1. The overpotential (h) at a current density of the prepared samples were calculated from LSV curves using following equation,

h ¼ ЕRHE
1:23V

(2)

and Tafel slopes (b) were calculated by fitting linear portion of Tafel plot with Tafel equation as,

h ¼ blogj þ a

(3)

Where, ‘h’, ‘j’ and ‘a’ are overpotential, current density and fitting parameter, respectively. Electrochemical impedance spectra (EIS) were measured in the range of 100 mHze1 MHz for all sample. Also, electrochemical active surface area (ECSA) of the all prepared electrodes calculated by taking CV at different scan rates. A sample has best electrochemical performance choosen for chronoamperometric stability which recorded at its overpotential for 9 h. 3. Result and discussion 3.1. Reaction mechanism A facile CBD method was used for the preparation of hydrous nickel pyrophosphate thin films on SS substrate. Process of film growth in CBD includes first adsorption of ionic species on substrate then it lead to heterogeneous nucleation and consequent growth of material on substrate. To analyze effect of phosphate variation, hydrous nickel pyrophosphate thin films NP1, NP2, and NP3 are prepared. Above prepared three different reaction bath consisting hydrolyzing agent (0.075 M urea) heated at 363 K constant temperature for 2 h. Urea plays an important role that is to control hydrolysis through slow decomposition. This includes decomposition of urea at 363 K which forms CO2 and NH3 as follows,

NH2 CONH2 þ H2 O/2NH3 þ CO2 [

(4)

Furthermore, released ammonia reacts with water as follow,

S.J. Marje et al. / Journal of Alloys and Compounds 779 (2019) 49e58

NH3 þ H2 O/NH4 OH

(5)

Decomposition of urea makes solution alkaline and alkaline conditions are more favorable to form complex with metal ions [34].

2ðNiCl2 $6H2 OÞ þ 2NH4 OH/2½NiðNH3 Þ2þ þ 14H2 O þ 4Cl
(6) On other hand, decomposition of phosphate carried out as follows,

KH2 PO4 /Kþ þ H2 PO
4

(7) 2þ

Furthermore, amine complex [Ni(NH3)] is unstable and it get react with H2PO
4 ions which resulted into formation of hydrous nickel pyrophosphate (Ni2P2O7$8H2O) in thin film form, as per following reaction,

4½NiðNH3 Þ2þ þ 4H2 PO
4 þ 16H2 O/2ðNi2 P2 O7 $8H2 OÞ þ 4NH3 þ 2H2 O þ 2H2 (8) By optimizing deposition time at low temperature uniform light green colored film was achieved on SS substrate. It is observed that, prepared film was not uniform before 2 h reaction time and after that powdery film was formed which attributes overgrowth of material and leads to pill off the outer layer of film. In above reaction, NH3 act as a complexing agent which control release of Ni2þ ions in the reaction bath. So, reaction rate also depend on amount of NH3 in the solution. By gravimetric weight difference method, mass of deposited material on SS substrate was measured and it is observed that, the deposited mass of material decreases from NP3 to NP1 sample as increasing concentration of phosphate in reaction bath (deposited mass ¼ 0.0013, 0.0012, 0.0011 g, respectively). It is may be due increase in phosphate concentration, it leads to more homogeneous reaction than heterogeneous reaction on SS substrate. 3.2. Structural and morphological analysis Crystalline nature of prepared NP1, NP2 and NP3 thin films were examined by X-ray diffraction pattern (XRD) and shown in Fig. 1 (a). The XRD patterns show, intense narrow peaks that confirm

51

polycrystalline nature of prepared thin films with high intensity indicates good crystallinity of material and that can improve electrochemical performance of material [35]. Peak marked with ‘*’ corresponds to SS peaks in XRD pattern. For NP1, NP2 and NP3 sample XRD pattern shows diffraction peaks at an angle 11.99, 13.96, 18.89, 20.83, 22.77, 23.97, 28.81, 30.97, 34.34, 35.59, 36.55, 39.90, 41.77, 44.02 and 56.13⁰ are ascribed to the (010), (100), (110), (111), (020), (200), (210), (211), (022), (220), (221), (311), (222), (320) and (420) crystal planes. These results are well matches with JCPDS card no.49e0672 and confirm formation of hydrous nickel pyrophosphate (Ni2P2O7$8H2O) on SS substrate. Intensity of peak (at an angle 13.96⁰) decreases from NP1 to NP3 sample as decreasing phosphate amount in the sample. In XRD pattern, peaks other than phosphate are not observed, so it confirms formation of Ni2P2O7$8H2O in thin film form. Fig. 1 (b) shows FT-IR spectra in the range 600e4000 cm
1 of prepared thin films NP1, NP2 and NP3. It gives the information about present functional group in the prepared samples. Figure reveals that, peak at 870 cm
1 (g1) corresponds to vibration of P-O-P groups, 940 cm
1 peak (g2) can be assigned symmetric stretching and 1010 cm
1 (g3) to asymmetric stretching modes of 4
PO
anion [36,37]. Peak observed in the range 4 /P2O7 1580e1590 cm
1 corresponds to bending vibrational mode of water molecule (g4) [36]. The peak obtained at 3000 (g5) and 3432 cm
1 (g6) exhibit O-H stretching vibration mode of water molecule that confirms presence of water molecule in the samples [36e38]. This result confirms, prepared material is hydrous in nature and same range peak of prepared three materials revels they have same functional group. Not significant change in functional group and hydrous content of sample NP1 to NP3 confirms prepared material is hydrous nickel pyrophosphate (Ni2P2O7$8H2O) in thin film form. Change in morphology of prepared thin films NP1, NP2 and NP3 were examined by FE-SEM at different magnifications and shown in Fig. 2(aef). For NP1 sample well dispersed willow leaf like petals observed in SEM images (shown in Fig. 2 (a) and (b)) and average length, width and thickness are 31.13, 15.4 and 1.13 mm, respectively. Decreasing phosphate ratio in sample NP2, it shows increase in thickness of willow leaf like petals upto 1.53 mm. Also, willow leafs converted into rectangular petals which come closer and inter connected sheets are grow on micropetals, it seems like a microflower like structure (shown in Fig. 2 (c) and (d)). These microflowers have increased length 48.59 mm and breadth 24.16 mm as

Fig. 1. (a) XRD patterns of sample NP1, NP2 and NP3 (JCPDS card no.- 49e0672), (b) FT-IR spectra of samples NP1, NP2 and NP3.

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Fig. 2. The change in morphology as decreasing concentration of phosphate in hydrous nickel pyrophosphate thin films, FE-SEM images of sample NP1 (a, b), NP2 (c, d) and NP3 (e, f) at magnifications of X800 and X3000, respectively.

compare to micropetals of NP1 sample. Similarly, thickness of micropetals of NP3 samples is increases upto 2.17 mm and it shows much compact microflower like structure as compare to micropetals of sample NP2 (shown in Fig. 2 (e) and (f)). Length and breadth of micropetals in NP3 sample are 42.3 and 22.14 mm, respectively which is less as compare to NP2 sample. From above discussion it clears that, the thickness of micropetals goes increasing as decreasing amount of phosphate in the sample. Similarly, length and breadth of micropetals from 1:2 (NP1 sample) to 1:1 (NP2 sample) ratio of nickel and phosphate increases and below that ratio (1:0.5) size decreases. Schematic representation of change in morphology as decreasing phosphate content in samples is shown in Fig. 3. Similar change in morphology from microsheetsmicroflower-sheet and microflower observed in the preparation of Ni3(PO4)2 with increasing reaction time 60, 90 and 120 min, respectively by Peng et al. [39]. Also, Wu et al. [40] observed change in morphology in the preparation of Ni3(PO4)2$8H2O via hydrothermal. From sample NP1 to NP3 microstructure changes as willow leaf like architecture to compact microflower with rectangular micropetals with decreasing amount of phosphate in sample. The EDS analysis used to confirm elements present in the sample and shown in Fig. 4(aec). The atomic ratio of nickel and phosphorous (Ni:P) is found to be 1:1.1, 1:0.9 and 1:0.8 for sample NP1, NP2 and NP3, respectively. From that ratio, it confirms that phosphate ratio in the sample decreasing in order and this is the main reason to change in morphology from willow leaf like structure to compact microflower with rectangular petal. EDS revels nickel, phosphorous and oxygen elements are present in the sample and no other elements observed in EDS spectra, that confirms

Fig. 3. Schematic representation of growth and change in morphology with varying nickel and phosphate molar weight ratio.

formation of Ni2P2O7$8H2O thin film and it supports XRD and FT-IR analysis. Structural and morphological analysis results are attributes to hydrous nickel pyrophosphate formation over SS substrate. 3.3. Electrocatalytical performance To examine OER catalytic properties, prepared hydrous nickel

S.J. Marje et al. / Journal of Alloys and Compounds 779 (2019) 49e58

53

Fig. 4. EDS spectra of prepared hydrous nickel pyrophosphate thin films, (a) NP1, (b) NP2 and (c) NP3 sample.

pyrophosphate thin films were used as a working electrode in a three electrode system. The LSV curves of NP1, NP2 and NP3 sample are measured at swiping rate 10 mV s
1 and shown in Fig. 5 (a), where potential is converted into RHE and the overpotential of asprepared thin films calculated using equations (1) and (2) at different current densities. Notably, the LSV curves of hydrous nickel pyrophosphate thin films, shows an obvious anodic peak

located at ~1.5 V (vs RHE), corresponding to oxidation of Niþ2 to Niþ3, indicating larger amounts of active phases [23,41]. Overpotential of as-prepared thin films calculated at current density 10 mA cm
2 and 25 mA cm
2. Enlarged image of LSV curves shown in inset of Fig. 5 (a). Observed overpotential of prepared samples NP1, NP2, NP3 at current density 10 mA cm
2 are 239, 246, 255 mV and at 25 mA cm
2 are 308, 312, 323 mV, respectively. The NP1

Fig. 5. (a) LSV graph at sweeping rate 10 mV s
1 in 1 M KOH for samples NP1, NP2 and NP3 and enlarged LSV image in inset, (b) Tafel slope of respective samples.

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S.J. Marje et al. / Journal of Alloys and Compounds 779 (2019) 49e58

sample shows lowest (308 mV at 25 mA cm
2) overpotential as compare to NP2 and NP3 samples, because of well dispersed willow leaf like architecture of NP1 sample, it can offer more surface area to carry OER reaction as compare to compact flower like structure of NP2 and NP3 sample. So, it concludes that, well dispersed microstructures of Ni2P2O7$8H2O material offer less overpotential towards OER. Tafel slope shown in Fig. 5 (b) of as-prepared material calculated using LSV curves and equation (3). Tafel slope values of samples NP1, NP2 and NP3 are found to be 51.5, 60.6 and 65.5 mV dec
1, respectively. As increasing overpotential of sample from NP1 to NP3 Tafel slope value also increases, it means overpotential related to Tafel slope that is reaction kinetics and fast kinetics responsible for low overpotential. The NP1 sample shows lower Tafel slope because of well dispersed petals may allow high surface area for the reaction. Low Tafel slope value is main requirement of electrocatalyst for OER application [42]. Upto now some reports are available on nickel phosphate material for OER application. Nickel metaphosphate (Ni2P2O12) synthesized by Huang et al. [23] tested for OER in 1M KOH electrolyte and obtained overpotential 280 mV at 10 mA cm
2 current density with Tafel slope 207 mV dec
1. Nickel phosphite (Ni11(HPO3)8(OH)6) synthesized on nickel foam by Menezes et al. [30] and it shows overpotential 232 mV at 10 mA cm
2 current density and Tafel slope 91 mV dec
1. Lei et al. [31] tested OER performance of nickel-iorn hydroxylphosphate (NiFe-OH.PO4) in 1 M KOH solution and reported overpotential 249 mV at 20 mA cm
2 current density with Tafel slope of 41.7 mV dec
1. Also, Fe doped nickel phosphate prepared by Li et al. [32] and tested OER activity in 1M KOH electrolyte, it shows overpotential of 220 mV at 10 mA cm
2 current density with Tafel slope 37 mV dec
1. Such a low overpotential and Tafel slope of nickel-iorn hydroxylphosphate reported because of Fe doping in nickel phosphate material. In present work, we achieved low overpotential of

239 mV at 10 mA cm
2 current density with Tafel slope 51.5 mV dec
1 for as-prepared hydrous nickel pyrophosphate without any doping. Comparative results of OER performance are tabulated in Table 1 [43e50] and it is found that present work exhibits low overpotential than other electrodes. However, obtained overpotential little high as compared with Fe doped nickel phosphate electrocatalyst. So, in future, study can be done by doping Fe/N, which may further decrease the overpotential of the prepared material. To determine electrochemical active surface area (ECSA) of prepared material NP1, NP2 and NP3 sample, cyclic voltammetry (CV) carried out in 1 M KOH electrolyte within 0e0.2 V potential window at 20e100 mV s
1 scan rates. The CV graphs of prepared samples NP1, NP2 and NP3 in non faradic region are shown in Fig. 5 (a), (b) and (c), respectively. Non faradic window is a region, where oxidation and reduction of material cannot obtain. In this region CV takes place to determine non-faradic capacitance of prepared material and ECSA of material [51]. From Fig. 6(aec), it is observed that, area under the curve of CV for prepared material NP1 to NP3 sample increases with increasing scan rate. Current under the curve decreases from NP1 to NP3 sample and consequently ECSA of material (12.11, 8.67 and 6.98 cm
2, respectively). The graph of current density vs scan rate is shown in Fig. 6 (d), which derived from CV graph. It observed that, sample NP1 shows high current density as compare to other two samples. This result confirms that, NP1 sample has more active sites because of well dispersed willow leaf like morphology as compare to compact microflower like morphology, and it contributes for excellent OER activity. Nyquist plot of prepared material NP1, NP2 and NP3 samples are shown in Fig. 7 (a). EIS of prepared material was tested at open circuit potential (OCP) in the frequency range of 100 mHze1 MHz. Nyquist plot revel that, all semicircle starts from a nearly same point for all samples which called as solution resistance (the

Table 1 Nickel phosphate study for oxygen evolution reaction. Sr. No

Materials

Method of deposition

Morphology

Electrolyte Overpotential

Tafel slope (mV dec
1)

Ref.

1.

CBD

Nanoparticles

1 M KOH

207

[23]

91

[30]

3.

N-doped Ni2P2O12/ CC Ni11(HPO3)8(OH)6/ NF (NiFe-OH.PO4)/NF

41.76

[31]

4.

Ni:Pi-Fe/NF

37

[32]

5. 6.

NiNPs-Graphene 3D-OM NiSA

81 39

[43] [44]

7.

Hydrothermal NiPK(Co)-NTs-C NiPNa(Co)-NTs-C NiPNa(Co3Fe1)-NTs-C NiPNa(Co3Fe1)-NTs-C

74.2 68.8 55 56.3

[45]

8.

Ni11P5/Ni3(PO4)2

Water-in-oil microemulsion

59.6

[46]

9.

Solvethermal

45.4

[47]

10.

(NiPO/Fe(OH)x) SWNT Ni-P

Elecrodeposition

49

[48]

11.

CoNiPP

Hydrothermal and pyrolyzed

60

[49]

12.

(Co0.5Ni0.5)3(PO4)2/ NF Ni2P2O7·8H2O/SS

Hydrothermal

59.3

[50]

51.5

Present work

2.

13.

Hydrothermal Elecrodeposition Hydrothermal and doping by Elecrodeposition Elecrodeposition Replication

CBD

280 mV at 30 mA cm
2 Microrods 1 M KOH 232 mV at 10 mA cm
2 Porous nanobelt 1 M KOH 249 mV at 20 mA cm
2 Nanoparticles 1 M KOH 220 mV at 10 mA cm
2 Nanoparticles 0.1 M KOH 334 mV Mesoporous nanosphere 0.1 M KOH 254 mV at 10 mA cm
2 Nanotubes 1 M KOH 420 mV 400 mV 340 mV 300 mV at 10 mA cm
2 Hollow sphere 1 M KOH 318 mV at 10 mA cm
2 Nanotubes 1 M KOH 248 mV at 10 mA cm
2 Amorphous 1 M KOH 344 mV at 10 mA cm
2 Flower-like microsphere 1 M KOH 246 mV at 10 mA cm
2 Flower-like 1 M KOH 273 mV at nanostructure 10 mA cm
2 Micropetals 1 M KOH 239 mV at 10 mA cm
2 308 mV at 25 mA cm
2

S.J. Marje et al. / Journal of Alloys and Compounds 779 (2019) 49e58

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Fig. 6. Cyclic voltammetry curve in non-faradic region of hydrous nickel pyrophosphate thin films in 1 M KOH within potential window 0e0.2 V for (a) sample NP1, (b) sample NP2 and (c) sample NP3. (d) Using (aec) drawn current density as a function of scan rate to determine electric double layer capacitance.

resistance of electrolyte) and denoted by Rs. Solution resistance of samples NP1, NP2 and NP3 are 0.97, 1.11 and 1.17 U, respectively. Semicircles appear in high frequency region corresponds to charge transfer resistance (Rct) and straight line in low frequency region stands for Warburg diffusion resistance (W). The charge transfer resistance of samples NP1, NP2 and NP3 are 18.18, 40.34 and 42.63 U, respectively. Semicircle of NP1 sample is smaller than NP2 and NP3 sample, which may due to the different morphologies of samples. Well dispersed willow leaf like microstructure of sample NP1 shows a low charge transfer resistance as compare to compact microflower like structure of NP2 and NP3 samples [52]. Also, phosphate can facilitate the proton transport as a mediator and expose more active sites. So, increase in phosphate concentration it accelerate the kinetics [42]. Hence, by considering above reasons EIS might get influence and NP1 sample shows very less impedance than other samples. Low Rct (18.18 U) value is important for better electrochemical properties in terms of low overpotential and quick electrochemical reaction (Tafel slope) [53]. Because of excellent OER activity, NP1 sample is used for stability test in 1 M KOH electrolyte for 9 h. The stability of NP1 sample investigated by chronoamperometry at constant potential of 400 mV vs SCE for 9 h, as shown in Fig. 7 (b). Figure shows that,

initially current density increases from 10 mA cm
2 to 14 mA cm
2 upto 6 h, because of activation process. Furthermore, little decrement in current density (12 mA cm
2) for 2 h is observe. Moreover, current density decreases little (10 mA cm
2) after 7 h and then stabilized for 2 h at 10 mA cm
2 current density (total 9 h). Current of sample NP1 is stable after 7 h, which indicates the NP1 sample is a promising candidate for future water electrolysis. LSV curves before and after stability of sample NP1 are shown in Fig. 7 (c), from which it clears that, required overpotential to deliver an acceptable current density decreases after stability. Before stability NP1 sample shows overpotential of 239 mV at 10 mA cm
2 current density and 308 mV at 25 mA cm
2 current density, it decreases upto 225 mV at 10 mA cm
2 current density and 256 mV at 25 mA cm
2 current density after stability, which indicating increase in active sites. From above results it is observed that, hydrous nickel pyrophosphate (NP1 sample) shows low overpotential (308 mV at 25 mA cm
2) as compare to other sample, because of its well dispersed willow leaf like microstructure, which offers more surface area than compact microflower like structure, that required low overpotential to OER. Large surface area gives more active sites resulted into high ECSA and low impedance of prepared hydrous

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Fig. 7. (a) Nyquist plots of NP1, NP2 and NP3 sample in the frequency range of 100 mHze1 MHz at OCP, (b) Chronoamperometric stability of prepared sample NP1 at 400 mV for 9 h, and (c) LSV graphs before and after stability of NP1 sample.

nickel pyrophosphate (NP1 sample) material. Also, it shows low Tafel slope which improve the stability of prepared material. So, hydrous nickel pyrophosphate (NP1 sample) material in form of thin film is a promising electrocatalyst for replacing other expensive and less stable material for OER application. 4. Conclusions In summary, hydrous nickel pyrophosphate (Ni2P2O7$8H2O) thin film electrocatalysts are successfully prepared by facile CBD method on SS substrate with nickel and phosphate ratio variation. Structural and morphological analysis confirms hydrous nature of prepared material and morphology changes (micropetals to microflower with rectangular micropetals) with decreasing phosphate amount. Thickness of micropetals increases (1.13e2.17 mm) as decreasing phosphate amount in the reaction bath. The sample NP1 (1:2 nickel/phosphate ratio) shows less overpotential 308 mV at 25 mA cm
2 current density with Tafel slope of 51.5 mV dec
1 as compare to other samples with excellent (9 hour) stability. The well dispersed willow leaf like morphology (NP1 sample) shows high ECSA and low impedance leads to low overpotential. The excellent performance of hydrous nickel pyrophosphate (Ni2P2O7$8H2O) material suggests its promising application in the field of OER and this performance can be further improve by Fe/N doping. Thus, the facile synthesis approach (CBD method) may provide a convenient

route for fabrication of efficient hydrous nickel pyrophosphate thin film electrocatalyst for OER at industry level application. Acknowledgment Authors are thankful to theDepartment of Science and Technology, India-Innovation in Science Pursuit for Inspired (DSTINSPIRE), for financial support through research project sanction no. DST/INSPIRE/04/2016/000260. References [1] Q. Zhang, D. Yan, Z. Nie, X. Qiu, S. Wang, J. Yuan, D. Su, G. Wang, Z. Wu, Fedoped NiCoP porous nanosheet arrays as a highly efficient electrocatalyst for oxygen evolution reaction, Appl. Energy Mater. 1 (2018) 571e579. https://doi. org/10.1021/acsaem.7b00143. [2] C. Cai, M. Kuang, X. Chen, H. Wu, H. Ge, G. Zheng, Spray-drying of milk for oxygen evolution electrocatalyst and solar water splitting, J. Colloid Interface Sci. 487 (2017) 118e122. https://doi.org/10.1016/j.jcis.2016.10.017. [3] L. An, P. Zhou, J. Yin, H. Liu, F. Chen, H. Liu, Y. Du, P. Xi, Phase transformation fabrication of a Cu2S nanoplate as an efficient catalyst for water oxidation with glycine, Inorg. Chem. 54 (2015) 3281e3289. https://doi.org/10.1021/ ic502920r. [4] T. Reier, M. Oezaslan, P. Strasser, Electrocatalytic oxygen evolution reaction (OER) on Ru, Ir, and Pt catalysts: a comparative study of nanoparticles and bulk materials, ACS Catal. 2 (2012) 1765e1772. https://doi.org/10.1021/ cs3003098. [5] Y. Lee, J. Suntivich, K. May, E. Perry, Y. Horn, Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline

S.J. Marje et al. / Journal of Alloys and Compounds 779 (2019) 49e58

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

solutions, J. Phys. Chem. Lett. 3 (2012) 399e404. https://doi.org/10.1021/ jz2016507. R. Subbaraman, D. Tripkovic, K. Chang, D. Strmcnik, A. Paulikas, P. Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N. Markovic, Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts, Nat. Mater. 11 (2012) 550e557. https://doi.org/10.1038/nmat3313. C. Tang, H. Wang, H. Wang, Q. Zhang, G. Tian, J. Nie, Wei Fei, Spatially confined hybridization of nanometer-sized NiFe hydroxides into nitrogen-doped graphene frameworks leading to superior oxygen evolution reactivity, Adv. Mater. 27 (2015) 4516e4522. https://doi.org/10.1002/adma.201501901. T. Ma, J. Cao, M. Jaroniec, S. Qiao, Interacting carbon nitride and titanium carbide nanosheets for high-performance oxygen evolution, Angew. Chem. Int. Ed. 55 (2016) 1138e1142. https://doi.org/10.1002/anie.201509758. K. Xu, P. Chen, X. Li, Y. Tong, H. Ding, X. Wu, W. Chu, Z. Peng, C. Wu, Y. Xie, Metallic nickel nitride nanosheets realizing enhanced electrochemical water oxidation, J. Am. Chem. Soc. 137 (2015) 4119e4125. https://doi.org/10.1021/ ja5119495. X. Jia, Y. Zhao, G. Chen, L. Shang, R. Shi, X. Kang, G. Waterhouse, L. Wu, C. Tung, T. Zhang, Ni3FeN nanoparticles derived from ultrathin NiFe-layered double hydroxide nanosheets: an efficient overall water splitting electrocatalyst, Adv. Energy Mater. 6 (2016) 1e6. https://doi.org/10.1002/aenm.201502585. H. Wang, Y. Liang, Y. Li, H. Dai, Co1-xSeGraphene Hybrid: a high-performance metal chalcogenide electrocatalyst for oxygen reduction, Angew. Chem. 123 (2011) 11161e11164. https://doi.org/10.1002/ange.201104004. L. Feng, G. Yu, Y. Wu, G. Li, H. Li, Y. Sun, T. Asefa, W. Chen, X. Zou, High-index faceted Ni3S2 nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting, J. Am. Chem. Soc. 137 (2015) 14023e14026. https://doi.org/10.1021/jacs.5b08186. W. Zhou, X. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, J. Wang, H. Zhang, Ni3S2 nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution, Energy Environ. Sci. 6 (2013) 2921e2924. https://doi.org/10.1039/c3ee41572d. H. Li, P. Wen, Q. Li, C. Dun, J. Xing, C. Lu, S. Adhikari, L. Jiang, D. Carroll, S. Geyer, Earth-abundant iron diboride (FeB2) nanoparticles as highly active bifunctional electrocatalysts for overall water splitting, Adv. Energy Mater. 7 (2017) 1e12. https://doi.org/10.1002/aenm.201700513. J. Masa, P. Weide, D. Peeters, I. Sinev, W. Xia, Z. Sun, C. Somsen, M. Muhler, W. Schuhmann, Amorphous cobalt boride (Co2B) as a highly efficient nonprecious catalyst for electrochemical water splitting: oxygen and hydrogen evolution, Adv. Energy Mater. 6 (2016) 1e10. https://doi.org/10.1002/aenm. 201502313. A. Swesi, J. Masud, M. Nath, Nickel selenide as high-efficiency catalyst for oxygen evolution reaction, Energy Environ. Sci. 9 (2016) 1771e1782. https:// doi.org/10.1039/C5EE02463C. S. Li, S. Peng, L. Huang, X. Cui, A. Enizi, G. Zheng, Carbon-coated Co3þ-rich cobalt selenide derived from ZIF-67 for efficient electrochemical water oxidation, Appl. Mater. Interfaces 8 (2016) 20534e20539. https://doi.org/10. 1021/acsami.6b07986. Q. Liu, S. Gu, C. Li, Electrodeposition of nickel-phosphorus nanoparticles film as a Janus electrocatalyst for electro-splitting of water, J. Power Sources 299 (2015) 342e346. https://doi.org/10.1016/j.jpowsour.2015.09.027. X. Yu, Y. Feng, B. Guan, X. Lou, U. Paik, Carbon coated porous nickel phosphides nanoplates for highly efficient oxygen evolution reaction, Energy Environ. Sci. 9 (2016) 1246e1250. https://doi.org/10.1039/c6ee00100a. J. Masud, S. Umapathi, N. Ashokaan, M. Nath, Iron phosphide nanoparticles as an efficient electrocatalyst for OER in alkaline solution, J. Mater. Chem. A 4 (2016) 9750e9754. https://doi.org/10.1039/C6TA04025J. A. Dutta, A. Samantara, S. Dutta, B. Jena, N. Pradhan, Surface oxidized dicobalt phosphide nanoneedles as a non-precious, durable and efficient OER catalyst, Energy Lett. 1 (2016) 169e174. https://doi.org/10.1021/acsenergylett. 6b00144. X. Li, A.M. Elshahawy, C. Guan, J. Wang, Metal phosphides and phosphatesbased electrodes for electrochemical supercapacitors, Small 13 (2017) 1701530e1701554. https://doi.org/10.1002/smll.201701530. J. Huang, Y. Sun, Y. Zhang, G. Zou, C. Yan, S. Cong, T. Lei, X. Dai, J. Guo, R. Lu, Y. Li, J. Xiong, A new member of electrocatalysts based on nickel metaphosphate nanocrystals for efficient water oxidation, Adv. Mater. 30 (2018) 1e9. https://doi.org/10.1002/adma.201705045. M. Kanan, D. Nocera, In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2þ, Science 321 (2008) 1072e1075. https://doi.org/10.1126/science.1162018. D. Lutterman, Y. Surendranath, D. Nocera, A self-healing oxygen-evolving catalyst, J. Am. Chem. Soc. 131 (2009) 3838e3839. https://doi.org/10.1021/ ja900023k. H. Ahn, T. Tilley, Electrocatalytic water oxidation at neutral pH by a nanostructured Co(PO3)2 anode, Adv. Funct. Mater. 23 (2013) 227e233. https://doi. org/10.1002/adfm.201200920. S. Cobo, J. Heidkamp, P. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave, V. Artero, A janus cobalt-based catalytic material for electro-splitting of water, Nat. Mater. 11 (2012) 802e807. https://doi.org/10.1038/NMAT3385. G. Carroll, D. Zhong, D. Gamelin, Mechanistic insights into solar water oxidation by cobalt-phosphate-modified a-Fe2O3 photoanodes, Energy Environ. Sci. 8 (2015) 577e584. https://doi.org/10.1039/c4ee02869d. V. Vij, S. Sultan, A. Harzandi, A. Meena, J. Tiwari, W. Lee, T. Yoon, K. Kim,

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

57

Nickelebased electrocatalysts for energy related applications: oxygen reduction, oxygen evolution, and hydrogen evolution reactions, ACS Catal. 7 (2017) 7196e7225. https://doi.org/10.1021/acscatal.7b01800. P. Menezes, C. Panda, S. Loos, F. Bruns, C. Walter, M. Schwarze, X. Deng, H. Dau, M. Driess, A structurally versatile nickel phosphite acting as a robust bifunctional electrocatalyst for overall water splitting, Energy Environ. Sci. 11 (2018) 1287e1298. https://doi.org/10.1039/C7EE03619A. Z. Lei, J. Bai, Y. Li, Z. Wang, C. Zhao, Fabrication of nano-porous nickel-iron hydroxylphosphate composite as bifunctional and reversible catalyst for highly efficient intermittent water splitting, ACS Appl. Mater. Interfaces 9 (2017) 35837e35846. https://doi.org/10.1021/acsami.7b10385. Y. Li, C. Zhao, Iron-doped nickel phosphate as synergistic electrocatalyst for water oxidation, Chem. Mater. 28 (2016) 5659e5666. https://doi.org/10.1021/ acs.chemmater.6b01522. J. Wang, H. Zeng, CoHPi nanoflakes for enhanced oxygen evolution reaction, ACS Appl. Mater. Interfaces 10 (2018) 6288e6298. https://doi.org/10.1021/ acsami.7b17257. U. Patil, J. Sohn, S. Kulkarni, S. Lee, H. Park, K. Gurav, J. Kim, S. Jun, Enhanced supercapacitive performance of chemically grown cobalt-nickel hydroxides on three-dimensional graphene foam electrodes, ACS Appl. Mater. Interfaces 6 (2014) 2450e2458. https://doi.org/10.1021/am404863z. B. Yan, D. Bin, F. Ren, Z. Xiong, K. Zhang, C. Wang, Y. Du, Facile synthesis of MnPO4.H2O nanowire/graphene oxide composite material and its application as electrode material for high performance supercapacitors, Catalysts 6 (2016) 198e212. https://doi.org/10.3390/catal6120198. M. Al-Omair, A. Touny, M. Saleh, Reflux-based synthesis and electrocatalytic characteristics of nickel phosphate nanoparticles, J. Power Sources 342 (2017) 1032e1039. https://doi.org/10.1016/j.jpowsour.2016.09.079. D. Yang, Q. Yu, L. Gao, L. Mao, J. Yang, Synthesis, characterization and nonisothermal decomposition kinetics of manganese hypophosphite monohydrate, Appl. Surf. Sci. 416 (2017) 503e510. https://doi.org/10.1016/j. solidstatesciences.2008.02.020. P. Noisong, C. Danvirutai, T. Srithanratana, B. Boonchom, The additive effect of graphene in nickel phosphate/graphene composite and enhanced activity for electrochemical oxidation of methanol, Solid State Sci. 10 (2008) 1598e1604. https://doi.org/10.1016/j.apsusc.2017.04.208. X. Peng, H. Chai, Y. Cao, Y. Wang, H. Dong, D. Jia, W. Zhou, Facile synthesis of cost-effective Ni3(PO4)2.8H2O microstructures as a supercapattery electrode material, Mater. Today Energy 7 (2018) 129e135. https://doi.org/10.1016/j. mtener.2017.12.004. H. Wu, Y. Gao, H. Li, Controlled synthesis of nickel phosphate hexahedronal and flower-like architectures via a simple template-free hydrothermal route, CrystEngComm 12 (2010) 3607e3611. https://doi.org/10.1039/c002120b. Q. Liang, L. Zhong, C. Du, Y. Luo, Y. Zheng, S. Li, Q. Yan, Achieving highly efficient electrocatalytic oxygen evolution with ultrathin 2D Fe-doped nickel thiophosphate nanosheets, Nano Energy 47 (2018) 257e265. https://doi.org/ 10.1016/j.nanoen.2018.02.048. Z. Wang, M. Liu, J. Du, Y. Lin, S. Wei, X. Lu, J. Zhang, A facile co-precipitation synthesis of robust FeCo phosphate electrocatalysts for efficient oxygen evolution, Electrochim. Acta 264 (2018) 244e250. https://doi.org/10.1016/j. electacta.2018.01.124. Z. Pu, Q. Liu, A. Asiri, X. Sun, Ni nanoparticles-graphene hybrid film: one-step electrodeposition preparation and application as highly efficient oxygen evolution reaction electrocatalyst, J. Appl. Electrochem. 44 (2014) 1165e1170. https://doi.org/10.1007/s10800-014-0743-6. T. Sun, L. Xu, Y. Yan, A. Zakhidov, R. Baughman, J. Chen, Ordered mesoporous nickel sphere arrays for highly efficient electrocatalytic water oxidation, ACS Catal. 6 (2016) 1446e1450. https://doi.org/10.1021/acscatal.5b02571. H. Liu, H. Li, X. Wang, Electrostatic interaction-directed growth of nickel phosphate single-walled nanotubes for high performance oxygen evolution reaction catalysts, Small 12 (2016) 2969e2974. https://doi.org/10.1002/smll. 201600345. J. Chang, Q. Lv, G. Li, J. Ge, C. Liu, W. Xing, Core-shell structured Ni12P5/ Ni3(PO4)2 hollow spheres as difunctional and efficient electrocatalysts for overall water electrolysis, Appl. Catal., B 204 (2017) 486e496. https://doi.org/ 10.1016/j.apcatb.2016.11.050. W. Bian, Y. Huang, X. Xu, M. Din, G. Xie, X. Wang, Iron hydroxide-modified nickel hydroxylphosphate single-wall nanotubes as efficient electrocatalysts for oxygen evolution reactions, ACS Appl. Mater. Interfaces 10 (2018) 9407e9414. https://doi.org/10.1021/acsami.7b18875. N. Jiang, B. You, M. Sheng, Y. Sun, Bifunctionality and mechanism of electrodeposited nickele phosphorous films for efficient overall water splitting, ChemCatChem 8 (2016) 106e112. https://doi.org/10.1002/cctc.201501150. P. Feng, X. Cheng, J. Li, X. Luo, Calcined nickel-cobalt mixed metal phosphonate with efficient electrocatalytic activity for oxygen evolution reaction, ChemistrySelect 3 (2018) 760e764. https://doi.org/10.1002/slct.201702637. J. Li, W. Xu, D. Zhou, J. Luo, D. Zhang, P. Xu, L. Wei, D. Yuan, Synthesis of 3D flower-like cobalt nickel phosphate grown on Ni foam as an excellent electrocatalyst for the oxygen evolution reaction, J. Mater. Sci. 53 (2018) 2077e2086. https://doi.org/10.1007/s10853-017-1631-3. J. Theerthagiri, K. Thiagarajan, B. Senthilkumar, Z. Khan, R. Senthil, P. Arunachalam, J. Madhavan, M. Ashokkumar, Synthesis of hierarchical cobalt phosphate nanoflakes and their enhanced electrochemical performances for supercapacitor applications, ChemistrySelect 2 (2017) 201e210. https://doi. org/10.1002/slct.201601628.

58

S.J. Marje et al. / Journal of Alloys and Compounds 779 (2019) 49e58

[52] S. Zhang, H. Gao, J. Zhou, Reduced graphene oxide-modified Ni-Co phosphate nanosheet self-assembled microplates as high-performance electrode materials for supercapacitors, J. Alloys Compd. 746 (2018) 549e556. https://doi. org/10.1016/j.jallcom.2018.02.008.

[53] Y. Cheng, R. Li, D. Mu, J. Ren, J. Liu, C. Dai, Enhanced electrochemical performance of Li3V2(PO4)3/C coated with Li fast ion conductive Li7La3Zr2O12, J. Electrochem. Soc. 164 (2017) 1545e1551. https://doi.org/10.1149/2. 1121707jes.