GREEN AND FACILE SYNTHESIS OF CERIUM DOPED Ni3Fe ELECTROCATALYST FOR EFFICIENT OXYGEN EVOLUTION REACTION

Electrochemical water splitting is the most promising pathway to produce high-purity hydrogen to alleviate global energy crisis. This reaction demands inexpensive, efficient and robust electrocatalyst for its commercial use. Herein, we demonstrate an effective, facile and scalable method for the synthesis of cerium doped Ni3Fe nanostructures as an electrocatalyst for oxygen evolution reaction (OER) by following simple chemical bath deposition route. The different molar ratios (3, 6 and 12 mM) of cerium in the chemical bath were used to study its effect on the structural and the electrochemical properties of the Ni3Fe nanostructured films. Doping of cerium contents induced variations in the morphology of deposited Ni3Fe nanostructures. The optimized electrocatalyst Ni3Fe/Ce-6 yielded high surface area catalyst nanosheets uniformly deposited on threedimensional conductive scaffold to ensure increase in the exposure of doped Ni3Fe catalytic sites with high electrical conductivity. As a result, this earth-abundant electrocatalyst affords high OER performance with a small overpotential of 310 mV versus reversible hydrogen electrode (RHE) at 10 mA cm and retains good stability up to ~ 10 h in alkaline electrolyte. This scalable strategy has great potential in future advancement of efficient and low-cost electrocatalysts for their large-scale application in energy conversion systems.


INTRODUCTION
Splitting of water is the most promising pathway to get hydrogen as green energy carrier, to address the energy crisis across the globe due to diminishing fossil fuels. Water splitting is classified into two half reactions: the oxygen evolution reaction (OER) at the anode; and hydrogen evolution reaction (HER) at the cathode. OER is the bottleneck reaction in the water splitting and it involves many reaction intermediates and barriers (4OH -→ 2H 2 O + 4e -+ O 2 ) during the anodic polarization process; and hence need large overpotential above the thermodynamic requirement of 1.23 V. For commercial use, the highly efficient OER electrocatalyst as key component of the water electrolyser is crucial to accelerate the reaction kinetics and to effectively lower the overpotential for energy conversion [1][2][3][4]. Currently, the state-of-the-art electrocatalysts to fulfil the technological requirements are IrO 2 and RuO 2 for OER. But the scarcity and the high cost of these noble metals are major barriers for their implementation at large scale. Motivated by these challenges with non-renewable resources of energy and the noble metals based electrocatalysts, tremendous research has been conducted in this field in the development of advanced renewable technologies with inexpensive electrocatalyst systems that are highly active and robust for water oxidation [5][6][7][8][9][10][11][12].
Among all non-noble metals alternatives, NiFe-based electrocatalysts have appeared to be potential candidates with eco-friendly property and high electrocatalytic activity in alkaline electrolyte [1,7,[13][14][15][16][17][18][19][20]. The OER activity, in general, depends on the high oxidation states and the catalytic active sites in the transition metal elements [21][22][23][24][25][26]. The third element doping from 3d transition metals can modify the electronic structure of NiFe-based electrocatalysts by introduction of defects to facilitate the reaction intermediates with low reaction barriers for OER [27,28]. Recently several groups reported the introduction of cerium in the host catalyst with modified electronic structure for improved OER performance. For instance, CeO 2 doping in Ni(OH) 2 shows great contribution in the enhancement of OER owing to strong electronic interaction between these two phases [29]. The introduction of cerium induced crystal structure amorphourization of CoO x with increased cerium related oxygen storage capacity. These modifications lead to increase in electrochemical surface area of nanostructures and consequently high intrinsic OER activity of the resultant catalyst [30]. The promotion of NiFe-LDH for significantly enhanced OER activity by doping with cerium and interfaced with CNTs to form hierarchical nanocomposite has been demonstrated due to induced lattice defects, flexible coordination number of Ce 3+ and strongly coupled interface [31]. Although various strategies have been adopted to introduce cerium into host catalyst, however, a scalable, costeffective and simple synthetic protocol with precise control over the growth of trimetallic electrocatalyst without coalescence of nanostructures on conductive scaffolds is rare. The selfaggregation of catalyst nanostructures causes blockage of the active sites due to accumulation of generated gas bubbles that obstructed the diffusion of both reactants and products, which is the major source to increase the overpotential [23,25].
In this work, we demonstrate an effective, facile and inexpensive strategy for synthesis of cerium doped NiFe-based electrocatalyst nanosheets. Our strategy has following key features (1) amorphous phase of cerium in the host catalyst and dramatic variations in Ni 3 Fe morphology, (2) the nucleation of homogeneous deposition of electrocatalyst nanosheets with more accessible catalytic sites, and (3) the superior performance of cerium doped catalyst in catalyzing the OER in comparison to control sample of Ni 3 Fe. Thus, our low-cost and simple strategy developed an electrocatalyst that affords superior water oxidation activity with a low over potential of 310 mV RHE at 10 mA cm -2 and a good stability in alkaline electrolyte.

Materials
Hydrochloric acid (HCl), ethanol, iron sulfate heptahydrate (FeSO 4 .7H 2 O), cerium nitrate (Ce(NO 3 ) 3 .6H 2 O), trisodium citrate dehydrate (Na 3 C 6 H 5 O 7 ) and Ni foam were used. Deionized water, paraffin oil were the materials required for carrying out the synthesis. All chemicals were of lab grade and were utilized without any purification.

Experimental details
Prior to synthesis, pieces of Ni foam (1 cm × 3 cm) were washed with 3 M HCl solution, deionized water and ethanol in succession, each for 10 min in ultrasonic bath to remove any oxide layer. The chemical bath solutions with different atomic ratio of Fe to Ce were prepared by mixing 50 mmol of FeSO 4 .7H 2 O and 3 mM of Na 3 C 6 H 5 O 7 with varied contents of Ce(NO 3 ) 3 .6H 2 O (i.e., 3, 6 and 12 mM ) in 100 mL of deionized water and stirred them for 0.5 h. The cleaned pieces of Ni foam were inserted in the above prepared solutions followed by the oil bath heating at 90 o C for the development of different products and named as Ni 3 Fe/Ce-3, Ni 3 Fe/Ce-6 and Ni 3 Fe/Ce-12, respectively.
For the synthesis of control sample Ni 3 Fe, the method is similar to that of all above cerium doped Ni 3 Fe but without adding Ce(NO 3 ) 3 .6H 2 O into the chemical bath solution. After deposition, all prepared products were washed with deionized water and ethanol; and then dried at 60 o C overnight.

Structural characterization techniques
X-ray diffraction (XRD) patterns were obtained from X'pert powder diffractometer using CuKα radiations (λ = 0.15418 nm) with an operating voltage of 40 kV and an operating current of 40 mA at the scan rate of 0.05° 2θ s -1 . Field emission scanning electron microscope (FESEM, Hitachi, Japan) at an accelerating voltage of 10 kV was employed for morphology analysis of Ni 3 Fe, Ni 3 Fe/Ce-3, Ni 3 Fe/Ce-6 and Ni 3 Fe/Ce-12 films on the Ni foam surface.

Electrochemical measurements
To evaluate the electrocatalytic performance of synthesized electrode materials for oxygen evolution reaction (OER), a number of electrochemical measurement techniques were employed. Generally, the techniques involve the recording of electric response of the catalyst to an applied potential. From this current potential behavior, salient electrochemical properties such as overpotential and Tafel slope were extracted out.
All electrochemical measurements were performed in alkaline electrolyte (1.0 M KOH, pH 13.6) using 3-electrode cell was used to carry out these measurements. That system comprises of working, counter electrode and reference electrodes for the purpose of completion of the electrical circuit. All the electrochemical activities were conducted on a Zahner Zennium electrochemical workstation at a sweep rate of 50 mV.s -1 .
In our study, Pt sheet was employed as the counter electrode, saturated calomel electrode (SCE) as the reference electrode and synthesized samples were as working electrodes, in dark. For the conversion of potential against the reversible hydrogen electrode (RHE) from the SCE, following equation is used: Linear sweep voltammetry (LSV) technique was used to determine the current response as a result of applied voltage. This technique involves the unidirectional voltage sweep from an initial potential to final potential. LSV were corrected for OER with ohmic drop using relation: where V app is the applied potential and R s is the solution resistance. The electrocatalytic performance of the electrode either for OER is evaluated by Tafel slope. Tafel plot was calculated from the corresponding LSV polarization curves and it account for the changes in mechanism at different overpotentials. Tafel slope determines current response of the system to the applied voltage (V) and can be calculated by using following equation 3: where η represents the overpotential, b is the Tafel slope in mV.dec -1 , j being the current density. Low Tafel slope and large current density indicate the efficient performance. The overpotential was calculated by using the following relation: The electrochemical active surface area (ECSA) of electrodes CF, t-CF and NiFe@t-CF were calculated from the electrochemical double layer capacitance (C dl ). The scan-rate (ν) dependent capacitive current, I c is associated with the C dl according to relation: where I c = I anodic -I cathodic , calculated in the middle of potential domain as a function of scan rate ν, that yielded a linear slope equal to 2 × C dl .

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Electrochemical impedence spectroscopy (EIS) is a electrochemical response of a system to an applied alternating potential. EIS is basically employed to determine the resistance of the electrode reaction due to the transfer of charges R The EIS measurements were carried ou kHz to 0.1 Hz with the amplitude of the sinosoidal wave 10 mV.
Oxygen evolution reaction (OER) is energetically the most inefficient part of the water electrolysis 4OH -→ 2H 2 O + O scalable, facile and low-cost synthesis route. The synthesis was initially monitored by the color change of Ni foam from silvery a conductive substrate and assisted as synthesis process Fe +2 oxidize into Fe While cerium doping facilitates effectively to modulate the morphol nanostructures by increasing cerium related oxygen storage capacity [32,33].

X-Ray diffraction (XRD)
The as-synthesized films were characterized by XRD (Figure 1b). The presence of diffraction peaks at 44.2•, 51.4•, and 75.7 respectively, indicated the formation of Ni crystallization peaks were observed with the exception of the diffraction peaks associated with Ni 3 Fe. This analysis evident that the cerium phase in the as is amorphous; and this strongly suggests that they are likely small grain (oxy)hydroxide phases at OER potentials [34].

Field emission scanning electron analysis
The morphologies of the as scanning electron microscopy (SEM). In Figure 2a, the SEM image of Electrochemical impedence spectroscopy (EIS) is a versatile tool to evalute the electrochemical response of a system to an applied alternating potential. EIS is basically employed to determine the resistance of the electrode reaction due to the transfer of charges R The EIS measurements were carried out at 1.45 V RHE . The frequency scan range was from 100 kHz to 0.1 Hz with the amplitude of the sinosoidal wave 10 mV.

RESULTS AND DISCUSSION
Oxygen evolution reaction (OER) is energetically the most inefficient part of the water O 2 + 4e -. Chemical bath deposition route was adopted as a green, cost synthesis route. The synthesis was initially monitored by the color change of Ni foam from silvery white to brownish yellow (Figure 1a). Ni foam was employed as a conductive substrate and assisted as the source of nickel (Ni) in the reaction. During the oxidize into Fe +3 and Fe +4 , then oxidization of Ni foam to obtain While cerium doping facilitates effectively to modulate the morphology of the as-grown catalyst nanostructures by increasing cerium related oxygen storage capacity [32,33]. Fe. This analysis evident that the cerium phase in the as-synthesized electrocatalyst samples is amorphous; and this strongly suggests that they are likely small grain (oxy)hydroxide phases emission scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM- The morphologies of the as-grown catalyst structures were investigated by field-emission scanning electron microscopy (SEM). In Figure 2a, the SEM image of Ni 3 Fe shows that without versatile tool to evalute the electrochemical response of a system to an applied alternating potential. EIS is basically employed to determine the resistance of the electrode reaction due to the transfer of charges R CT .
. The frequency scan range was from 100 Oxygen evolution reaction (OER) is energetically the most inefficient part of the water . Chemical bath deposition route was adopted as a green, cost synthesis route. The synthesis was initially monitored by the color (Figure 1a). Ni foam was employed as nickel (Ni) in the reaction. During the , then oxidization of Ni foam to obtain Ni +2 .
grown catalyst synthesized films were characterized by XRD (Figure 1b). The presence of diffraction • corresponding to diffraction plans of (111), (200) and (311), 0419). No other obvious crystallization peaks were observed with the exception of the diffraction peaks associated with synthesized electrocatalyst samples is amorphous; and this strongly suggests that they are likely small grain (oxy)hydroxide phases characteristics and enhancement in the OER performance. Ce loading may create a synergistic effect by perturbing the electronic structure of the host cat exposed active sites on the surface of catalyst [29,30]. As no diffraction peak is observed in due to the amorphorization of the cerium related phase in the deposit we perform its EDX to confirm the presence of Ce contents in the electrocatalyst film. The EDX spectrum of Ce elements in Figure 2e. In addition, the inset elements. The ratios of Ni/Fe and Ni/Ce are found to be 3 and 1.5, respectively, for the incorporation of Ce during synthesis, the catalyst film exhibits no distinct morphology and is apparently less porous. The electrode sample Ni 3 Fe/Ce-3 shows better morphology in . Figure 2b illustrates the homogeneous dispersion of small globules mounted vertically on the surface of Ni foam. However, there is no full coverage of these globular structures on the 3D scaffold of Ni. SEM images of Ni 3 Fe/Ce-6 ( Figure 2c) clearly reveal that the entire surface of globules on Ni scaffold is homogenously covered with densely vertically aligned nanosheets. The nanoplatelet morphology with cavities in between the nano walls exhibits high surface area and porosity. Such open nanosheets architecture built by 6 afford high surface area and more exposed catalytic active sites, an easy of electrolyte penetration and fast adsorption and desorption of the generated gas bubbles to While Figure 2d presents the globular nanoparticles morphology of with rough surface that are stacked over each other on the conductive surface of Ni template. The catalyst particles are homogeneously dispersed on 3D network of Ni foam. SEM analysis of all samples indicated that Ce contents played vital role to modulate the morphology of the grown nanostructures. The different morphologies offer different geometrical characteristics and enhancement in the OER performance. Ce loading may create a synergistic effect by perturbing the electronic structure of the host catalyst Ni 3 Fe surface and the number of exposed active sites on the surface of catalyst [29,30].
As no diffraction peak is observed in Ni 3 Fe/Ce-6 to confirm the presence of cerium may be due to the amorphorization of the cerium related phase in the deposited electrocatalyst film. So we perform its EDX to confirm the presence of Ce contents in the Ni 3 Fe/Ce-6 deposited . The EDX spectrum of Ni 3 Fe/Ce-6 confirmed the presence of O, Ni, Fe and Ce elements in Figure 2e. In addition, the inset table in Figure 2e shows the percentages of these elements. The ratios of Ni/Fe and Ni/Ce are found to be 3 and 1.5, respectively, for Ni 3 Fe/Ce  the incorporation of Ce during synthesis, the catalyst film exhibits no distinct morphology and is 3 shows better morphology in on of small globules However, there is no full coverage of these 6 ( Figure 2c) clearly old is homogenously covered with densely vertically aligned nanosheets. The nanoplatelet morphology with cavities in between the nanowalls exhibits high surface area and porosity. Such open nanosheets architecture built by rea and more exposed catalytic active sites, an easy of electrolyte penetration and fast adsorption and desorption of the generated gas bubbles to presents the globular nanoparticles morphology of with rough surface that are stacked over each other on the conductive surface of Ni template. The catalyst particles are homogeneously dispersed on 3D network of Ni foam. The ate the The different morphologies offer different geometrical characteristics and enhancement in the OER performance. Ce loading may create a synergistic Fe surface and the number of 6 to confirm the presence of cerium may be ed electrocatalyst film. So 6 deposited 6 confirmed the presence of O, Ni, Fe and table in Figure 2e shows the percentages of these /Ce-6.

Electrochemical measurements
The electrochemical properties of developed cerium doped Ni 3 Fe nanostructures deposited directly on the porous 3D scaffold of the conductive Ni foam towards OER were unveiled with the help standard analytical methods such as linear sweep voltammetry (LSV), Tafel plots and electrochemical impedance spectroscopy (EIS). The potentials were measured versus saturated calomel electrode (SCE) and are reported here versus reversible hydrogen electrode (RHE).

Linear sweep voltammetry (LSV)
The catalytic activities of the electrocatalysts towards OER are compared based on three key parameters deduced from the LSV curves are the onset potential, minimum potential required to attain a current density of 10 mA.cm -2 and overpotential.

Tafel slope
To achieve the more insight into OER performance of the prepared catalysts samples, Tafel plots were calculated. Tafel plots for all catalyst samples were generated by plotting the log of current density (log I) against applied potentials (V SCE ). The Tafel slope (b) was extracted by fitting the linear portion of the plots to the Tafel equation as described in equation 3. Figure 4 displays the Tafel slopes of all electrodes derived from their respective LSV curves by fitting the linear portion of their Tafel Every step from Eq. 1-4 is assumed to be the rate determining to define the electric current. Based on the microkinetic analysis for the correlation of Tafel slope and the rate determining step, Eq. 4 should be the rate determining step for the Ni 3 Fe/Ce-3 since its Tafel slope is 55 mV dec -1 [35].

Electrochemical impedance spectroscopy (EIS)
Further, electrochemical impedance spectroscopy (EIS) was carried out to study the electrocatalytic behavior of Ni 3 Fe, Ni 3 Fe/Ce-3, Ni 3 Fe/Ce-6 and Ni 3 Fe/Ce-12. Nyquist plots were obtained from Ni 3 Fe, Ni 3 Fe/Ce-3, Ni 3 Fe/Ce-6 and Ni 3 Fe/Ce-12 electrocatalysts deposited on the porous 3D structure of Ni foam are shown in the Figure 5. These plots showing two semicircles corresponding to the charge transfer resistance at the anode/electrolyte interface and in the anode, respectively. The high-frequency semicircle for the Ni 3 Fe/Ce-6 is smallest indicating the enhanced charge transport with very less charge transfer resistance (R ct ) of ~ 2.63 Ω in comparison to Ni 3 Fe (~ 12 Ω), Ni 3 Fe/Ce-3 (~ 4.58 Ω) and Ni 3 Fe/Ce-12 (~ 3.76 Ω). It also demonstrates the higher conductivity and the good electrical contact between the conducting scaffold of Ni and the deposited Ni 3 Fe nanoplatelets in Ni 3 Fe/Ce-6.
Electrochemical active surface area (ECSA) measurements Where for these CV measurements, CV curves were run at multiple scan rates (ν) of 20, 40, 60, 80 and 100 mV s applied potential window versus ν gives C Equation 5. Figure 6c shows that in comparison to pristine Ni foam (0.98 mF cm surface area catalyst nanosheets of NiFe@t inferred in its SEM images.  KOH solution. Where for these CV measurements, CV curves were run at multiple scan rates (ν) of 20, 40, 60, 80 and 100 mV s -1 . The plot of capacitive current (I c ) at the middle of the applied potential window versus ν gives C dl that has direct relation with ECSA as given in Equation 5. Figure 6c shows that Ni 3 Fe/Ce-6 exhibits the 3.5 times higher ECSA (3.52 mF cm in comparison to pristine Ni foam (0.98 mF cm -2 ). The large ECSA is resulting from the high anosheets of NiFe@t-CF uniformly deposited on Ni foam surface as Fe (black curve), Ni 3 Fe/Ce-3 (red curve), Ni 3 Fe/Ce-6 (blue curve) (magenta curve).
Faradaic region in 1.0 M KOH for a) pristine Ni foam and b) 6. c) Calculated C dl plots of Ni (black curve) and Ni 3 Fe/Ce-6 (red line).
KOH solution. Where for these CV measurements, CV curves were run at multiple scan rates ) at the middle of the that has direct relation with ECSA as given in exhibits the 3.5 times higher ECSA (3.52 mF cm -2 ) ). The large ECSA is resulting from the high deposited on Ni foam surface as (blue curve) KOH for a) pristine Ni foam and b) 6 (red line).

Chronoamperometric measurements
As stability is an important parameter to evaluate the feasibility of the material for practical applications. The long-term stability of optimized electrocatalyst was examined by the chronoammperometry response at 1.54 V in 1.0 M KOH solution. As shown in Figure 6, Ni 3 Fe/Ce-6 retains its current density up to 10 h (36,000 s) of continuous operation with slight degradation in the value of its original current density (~9.8 mA cm -2 ), suggesting its good operational stability in the alkaline solution.

CONCLUSION
In conclusion, we have used a facile, simple and effective strategy for the synthesis of Ni 3 Fe as highly efficient electrocatalyst for OER in the alkaline media. Their morphology can be effectively tuned by the doping of cerium. Different doping percentages of cerium effectively tuned the morphology to form nanoparticles, globules and nanoplatelets. The high OER activity of the optimum sample attributed to the high surface area with more exposed active catalytic sites, low charge transfer resistance between the electrocatalyst and electrolyte solution owing to the non-aggregated morphology of deposited nanostructures. Considering the efficiency of our synthesized electrocatalyst, the present study will undoubtedly open opportunities for the development of non-precious and high performance electrocatalysts by adopting a simple synthetic route at moderate temperature.