Study of the Oxygen Evolution Reaction Catalytic Behavior of CoxNi1

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A Study of Oxygen Evolution Reaction Catalytic Behavior of CoxNi1-xFe2O4 in Alkaline Medium Viruthasalam Maruthapandian, Mahendran Mathankumar, Velu Saraswathy, Balasubramanian Subramanian, and Srinivasan Muralidharan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 25, 2017

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ACS Applied Materials & Interfaces

A Study of Oxygen Evolution Reaction Catalytic Behavior of CoxNi1-xFe2O4 in Alkaline Medium Viruthasalam Maruthapandian,*,$,§ Mahendran Mathankumar,¥, § Velu Saraswathy,*,$,§ Balasubramanian Subramanian,¥, § and Srinivasan Muralidharan $ $

Corrosion and Materials Protection Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 003, Tamil Nadu, India.

¥

Electrochemical Materials Science Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 003, Tamil Nadu, India.

§

Academy of Scientific and Innovative Research (AcSIR), Karaikudi-630 003, Tamil Nadu, India.

ACS Paragon Plus Environment


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ABSTRACT Catalysts for the oxygen evolution reaction (OER) play an important role in the conversion of solar energy to fuel of earth abundant water into H2 and O2 through splitting/electrolyzer. The heterogeneous electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) exhibit catalytic activity that depends on the electronic properties, oxidation states and local surface structure. Spinel ferrites (MFe2O4; M = Ni and Co) based materials have been attractive for the catalytic water oxidation due to their well-known stability in alkaline medium, easy synthesis, existence of metal cations with various oxidation states, low cost and tunable properties by the desired metal substitution. To understand the better catalytic activity of MFe2O4 in detail and the role of Ni and Co was studied through the MxNi1-xFe2O4 (M = Co; 0 < x < 1); which was prepared by sol-gel method. The results showed that bare NiFe2O4 has better catalytic activity (η = 381 mV at 10 mA cm-2 and Tafel slope of 46.5 mV dec-1) compared to Co containing MxNi1-xFe2O4 (η = 450-470 mV at 10 mA cm-2 and Tafel slope of 50-73 mV dec-1) in alkaline medium and the substitution of Co is found to suppress the catalytic activity of NiFe2O4. The degradation of catalytic activity with increase in Co content was accounted in further detailed investigations. KEYWORDS: water electrolysis, oxygen evolution reaction (OER), electrocatalysts, spinel based metal oxides, alkaline medium, effect of metal substitution


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1. INTRODUCTION The growth of energy demand ranging from fossil fuels to renewable sources has inspired to develop newer energy storage and conversion systems for futuristic applications. In the meantime, zero pollution emission (CO2 free), earth abundance, high conversion efficiency and recyclability also have to be taken into account.1-2 In these aspects, molecular hydrogen (H2) and oxygen (O2) are ideal vectors and the production of H2 and O2 electrocatalytic water splitting (electrolysis) is nowadays attracted as state of the art as it utilizes water and electricity at the room temperature,1,3-5 hence it is an attractive and alternative way to generate and store energy from water which is an earth abundant resource.6,7 Water electrolysis/splitting is kinetically hindered by the rate as well as its efficiency by the anodic part of four electron transfer oxygen evolution reaction (OER) (basic medium: 4OH- → O2 + 2H2O + 4e-) compared to the cathodic part of two electron transfer hydrogen evolution reaction (HER) (2H+ + 2e- → H2).8,9 This splitting of water into H2 and O2 is an important kind of reaction which plays a vital role in renewable energy conversion technologies of water electrolysis, photo-electrochemical water electrolysis, and metal air batteries.10 In these reactions, especially OER, catalytic activity is limited because of sluggish kinetics associated with the breaking of O-H bond and the formation of O=O bond is governed by the metal-oxygen (M-O) bonds (covalent or ionic) and the electronic/orbital structure of the active sites of the catalysts.11 At present, precious metals and metal oxides like Pt, IrO2 and RuO2 are used as active and efficient catalysts in acidic and alkaline mediums; however it is limited to commercialization in real devices due to its high cost, poor availability, preciousness, and instability at long term OER condition.6,12,13 Compared to acidic medium, alkaline medium offers economically viable low-cost, earth abundant nonprecious transition metal and metal oxides as OER catalysts for water splitting on account of the greater stability as many materials are more stable in alkali.5,7,9



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Towards the developments of low overpotential (η) electrocatalysts (minimum potential required to produce 10 mA cm-2 (geometric area)), the recent progressive studies on different approaches give detailed characteristics to design catalysts which involves the optimization of composition, morphology, synthetic route, choice of substrate, electrolyte concentration and additives which offers a better understanding of the structural and chemical composition relationship in the heterogeneous catalyst system.14 In another way, the catalytic activities can be tuned by substitution of transition metals into their crystalline structure or forming mixed metal oxide as the electronic structure varies with change in composition.11 In this regard, recent reviews show the sustainable development of electrocatalysts by their combinational activity of metal, non-metal, stochimetric defects, substitution and composites and all of these combined.9,14,15 Thereby, properties beside to catalytic activity were defined for the future development. The first row transition metals and their oxides were employed for the application of OER and ORR due to their high abundance, less toxicity, eco-friendliness, variable oxidation state and stability in neutral and alkaline medium.8,16-18 Among these, Mn, Fe, Co and Ni containing spinels compounds (AB2X4; A and B = divalent (M2+) and trivalent (M3+) metal, X = chalcogens (especially O) where A occupies the tetrahedral sites and B occupies the available octahedral sites) were considered as potentially interested for OER catalysts due their unique properties of easy preparation, variable valence state associated redox characteristics, corrosion stability in aqueous alkaline medium and offering tuneable catalytic activities by metal substitution.6,7,16,19-23 The alkaline medium also brings out a series of intermediates (e.g., M-O, M-OO, M-OH and M-OOH) through the transition metal oxides catalyst surface sites. They play a vital role in the proton and electron transferring process and rate determining factor of the catalytic activities in the OER and ORR than compared to neutral medium.24,25


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Metal oxides in the form of spinel offers a wide range of material development choice for their desired applications because the properties synergistically depends on the host and substituent.26,27 In the case of development of catalysts, numerous spinel oxides were widely studied by the different combination of A and B in AB2X4; which includes A and B as Mn, Co, Ni, Fe, Zn, Cr and Cu.18,22,28-34 [more comparative spinel compounds with substituents are

shown

in

Table

S1

in

Supporting

information

(SI)].

Among

these,

Cr

substituted/containing spinels have showed considerably increased catalytic activity than compared to their parent spinels, meanwhile Cr free catalyst leads to eco friendly production of H2 and O2.23,30,32,33 Hence, earth abundant and eco-friendly Ni, Co, Fe and Mn containing spinel compounds have gained more importance because as M-OH bond strength increase/decrease towards the formation of OER reactive intermediates of (oxy)hydroxide in the 3d metal series in the order of Ni > Co > Fe > Mn.17,25,32,34 Hence, MFe2O4 (M = Ni and Co) shows defined catalytic activity (overpotential (η) in the range of 350-500 mV) in alkaline medium towards OER.22 [recent reports on spinels containing Ni, Co, Fe and Mn relevant to this are summarized in Table S2 in SI]. Catalytic activity of MFe2O4 (M = Ni and Co) is not reported widely and whether the catalytic activity comes from Ni or Co is not investigated in detail. Further, whether the addition of Co into this Ni and Fe bimetallic system (NiFe2O4) resulting in a trimetallic compound will alter the catalytic activity or not is the question. Therefore, the study and clarification of role of Co substitution in NiFe2O4 in their catalytic activity is basic and opens a newer lack of facts about their fundamental properties. In these aspects, we studied the effect of Co substitution in NiFe2O4 as MxNi1-xFe2O4 (M = Co; 0 < x < 1) prepared by sol-gel combustion method and its synergistic OER electrocatalytic activity in alkaline medium. This sol-gel combustion technique is comparably better than hydrothermal, ball milling, co-precipitation and reverse micelles methods and it



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allows controlled composition, purity, homogeneity, particle size and metal cations distribution.16,35 Their catalytic activities with associated parameters were also analyzed through different physical and electrochemical techniques and the results were compared with benchmark catalyst and recent literatures. We found that the bare NiFe2O4 shows better catalytic activity among the studied series of Co substituted system. The Co substitution in NiFe2O4 is suppressing the OER catalytic activities. This work results support the synergetic catalytic activity with guiding principles towards the development of newer catalyst in the field of energy storage and conversion. 2. EXPERIMENTAL SECTION 2.1. Catalysts preparation NiFe2O4 and Co substituted MxNi1-xFe2O4 (M = Co and x = 0.25, 0.5, 0.75 and 1) were prepared by a conventional citric acid assisted sol gel combustion method as follows: 0.02 mole Ni(NO3)2.6H2O (≥98%, Acros) was added into 100 mL of H2O (Milli Q) followed by addition of 0.04 mole of Fe(NO3)3.9H2O (≥98%, Fisher scientific, Mumbai, India) (the molar ratio of Ni/Fe = 1/2) and 0.03 mole of citric acid (C6H8O7) (>99%, SRL, Mumbai, India) dissolved in 100 mL H2O was added to the above solution with constant stirring with help of a Teflon lined magnetic pellet. During this time the pH of the reaction mixture was adjusted to 7 by adding ammonia (30%, Fisher scientific, Mumbai, India) solution. After constant stirring of the whole mixture for 3 hours, it was heated around 70 °C to remove the excess water until the mixture produced metal ions rich sol followed by the gel. This removal of excess water produced viscous gel and the continuous heating initiated the self-combustion of reaction mixture and finally foamy nickel ferrite was formed. This preparation method is illustrated with optical microscopic image in Figure 1 and it is similar to our previous studies.36


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The prepared sample was milled uniformly in an agate mortar followed by annealing at 700 °C for 3 h at the heating rate of 10 °C min-1 in air atmosphere to obtain pure and crystalline NiFe2O4 powders. After cooling, the sample was again ground into a fine powder for electrochemical measurements. A similar procedure was adopted for the preparation of Ni and Co substituted ferrite MxNi1-xFe2O4 (M = Co; 0 < x < 1; 0.25, 0.5, 0.75 and 1).

Figure 1. Schematic and optical illustration of MxNi1-xFe2O4 (M = Co; 0 < x < 1) synthesis through the sol-gel combustion method.

2.2. Physicochemical characterizations X-ray diffraction (XRD) patterns of the as-prepared samples were recorded using PANalytical diffractometer Cu Kα radiation with wavelength of 1.5406 Ǻ and the data were recorded between 10 to 90º of 2θ angles. Fourier transform infrared (FTIR) spectra of the as prepared samples were recorded in the range of 400–4000 cm-1 in the transmittance mode using Bruker Tensor 27 with opus 6.5 version software. Raman spectroscopy measurements of powder sample were performed using Renishaw Invia Raman microscope (Renishaw U.K)



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He–Ne Laser 633 nm and 18mW laser source at room temperature, in spectral range from 100 to 1000 cm-1. Elemental constituents and mapping of the as-prepared samples were analyzed by an energy dispersive X-ray fluorescence method using a HORIBA Scientific XGT-5200 X-ray analytical microscope with intuitive software. The surface morphology of the samples was observed through VEGA3 SB TESCAN Scanning Electron Microscope (SEM). Surface area and pore size distribution were calculated by the Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda (BJH) method respectively using a Gemini™ V series surface area analyzer. 2.3. Electrochemical characterizations of catalytic activates Electrocatalytic activities of the as prepared materials were measured in a three electrode cell setup using active material coated carbon paper (CP) as a working electrode, Pt mesh as counter electrode and saturated calomel electrode (SCE; 0.241 V vs RHE) as reference electrode. The catalyst ink was prepared by mixing of 70 wt% active materials, 15 wt% of acetylene black carbon (conductive support) and 15 wt% of PVDF (binder) in NMethyl-2-pyrrolidone (NMP) in a mortar until it becomes homogenous. The slurry was coated by the doctor blade method on to the CP covering an area of 1 cm2 and it was kept at 60 °C in air oven for 12 hours for drying purpose before measurements. Mass of the active materials on the CP was controlled around 2 mg cm-2. For OER catalytic activities measurement, electrode was subjected to cyclic voltammetric (CV) method between the potential regions of 0 to 1 V vs saturated calomel electrode (SCE) at scan rate of 5 mV s-1 in 1 M KOH (97%, Fisher scientific, Mumbai, India) electrolyte. For HER catalytic activities measurements, electrodes were subjected to CV between the potential of 0 to -2 V vs SCE. Each measurement was carried out after the pre-treatment of 10 cycles at a scan rate of 10 mV s-1 in the respective potential windows. The solution was stirred during the CV measurements to eliminate the H2 and O2 bubbles from the working electrode surface. The


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linear portion of anodic and cathodic sweep were taken and presented in this study as LSV. All the measured potentials were converted into RHE using the equation (1). RHE = EMeasured + (0.059 * pH) + ERef

(1)

Overpotential (η) was calculated at the point of current density of 10 mA cm-2 using the equation (2). η = EMeasured – 1.23

(2)

Electrochemical surface area (ECSA) of prepared catalyst were studied through the double-layer capacitance (Cdl) measurements through the series of cyclic voltammograms at non-Faradic region (0 to 0.2 V vs SCE) at different scan rates of 1, 2, 5, 10, 20, 40, 60, 80 and 100 mV s-1. Cdl were obtained from the slope of the plot of scan rate vs current plot, which is similar to the study proposed by McCrory et al.2 Cyclic stability of the catalyst was studied using cyclic voltammetery between the potential range and the resulting anodic sweep (positive sweep) was taken into account of LSV. Chronoamprometry stability was studied under the applied potential of 1.7 V vs RHE in the test solution. Electrochemical impedance spectrum (EIS) was measured under constant applied potential for one minute to ensure their stability with 5 mV of amplitude potential and the frequency response was scanned between 100 kHz to 10 mHz. Turn over frequency (TOF) was calculated by using the equation (3). TOF = jS/4Fn

(3)

where j is the measured current density (A cm-2) at desired η, S is the area (cm2) of electrode with active materials which is used as working electrode, F is the Faraday constant (96485 C mol-1), 4 is the number of electrons involved in the reaction and n is the number of moles of the catalytic material on the working electrode.37 Mass activity (catalytic current, A g-1) of the



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catalysts was calculated from the measured current density (A) by the weight of materials (g) at the η of 300 mV.38 All the electrochemical experiments were conducted at room temperature using a CHI Electrochemical Anlayzer (CHI6084C). All the potentials mentioned here are not iR corrected and the current density was calculated from the geometric area of catalyst presented in the CP substrate exposed to the electrolyte solution of 1 M KOH. 3. RESULTS AND DISCUSSION 3.1. Characterization of catalyst Figure 2(a) shows the XRD pattern of as prepared samples of NiFe2O4 and Co substituted NiFe2O4 as described by general formula of (MxNi1-xFe2O4; M = Co; 0 < x < 1). The major observed reflection peaks of X-ray diffraction (XRD) studies reveals that spinel structure of as prepared samples coincide with standard diffraction patterns of NiFe2O4 (JCPDS No: 86-2267) and CoFe2O4 (JCPDS No: 79-1744) (see Figure S1 in SI). It indicates that the prepared sample of NiFe2O4 has the cubic spinel structure with Fd3m space group and the CoFe2O4 has rhombohedral structure with R3m space group. This result resembles the recent studies on NiFe2O4 by Shan et al. and Hessien et al.39,40 and on CoFe2O4 studied by Xiao et al.41 In addition to this, no NiO, CoO, CoO2 and Co3O4 peaks were observed, which clearly indicates that the high purity of as prepared materials. In addition to this phase purity analysis, crystalline size was also calculated from the data using Scherrer’s formula given by the equation (4). D = 0.9 λ/β cos θ

(4)

where D is the crystalline size in nm, 0.9 is the constant, λ is the wavelength of X-ray used in the XRD studies (here we used Cu Kα with wavelength of 1.5406 Ǻ), β is the full width at


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half maximum and θ is diffraction angle. It was found to be 57.3, 57.3, 67.3, 72.9 and 69.4 nm for the substituent of Co content x = 0, 0.25, 0.5, 0.75 and 1 respectively. The increment of crystalline size with the increase of Co content is due to the difference in ionic radii of Ni2+ (0.69 Ǻ) and Co2+ (0.78 Ǻ), increases of saturation magnetization (Ms), residual magnetization (Mr) and coercivity (Hc) by the substitution of the smaller magnetic moment of Ni2+ ions (2 µB) with Co2+ ions (3 µB).42

Figure 2. Physicochemical characterization of as synthesized (MxNi1-xFe2O4; M = Co; 0 < x < 1). (a) X-Ray diffraction pattern, (b) FTIR and (c) Raman spectrum.

The presence of mixed metal oxide phase was studied through the FTIR spectroscopic studies. Figure 2(b) shows overall the FTIR spectrum of as prepared spinel mixed metal oxides in which the main absorption peaks are found to be at 470 to 590 cm-1 and is ascribed to the intrinsic stretching vibrations of metal-oxygen atoms at the tetrahedral site (Mtetr-O) and the octahedral site (Mocta−O) respectively. These results are in good agreement with the recent studies of Wei et al., and Sun et al.43,44



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Cationic distribution and strain states of synthesized samples were analyzed by Raman spectroscopy. Figure 2(c) shows the observed Raman spectra of as prepared NiFe2O4 and Co substituted MxNi1-xFe2O4. Peaks around 210, 334, 488, 574 and 702 cm-1 are indicative that these are the Raman active. 5 phonon modes (A1g + Eg + 3T2g) for spinel compunds can be predicted by the group theory and these observed peaks coincide with recent studies of NiFe2O4 and CoFe2O4.44-48 The results obtained from Raman spectroscopy is in good agreement with the XRD studies.

Figure 3. SEM image of prepared MxNi1-xFe2O4 (M= Co; 0 < x < 1); (a) x = 0, (b) x = 0.25, (c) x = 0.5, (d) x = 0.75 and (e) x = 1.

Figure 3 shows the SEM images of as prepared MxNi1-xFe2O4 (M= Co; 0 < x < 1)4. It can be seen that the particles are aggregated together with large kinks and ledges like surface defects which are features of sol-gel synthesis. In correspondence to the structural morphology, the existences of metal cations were examined through the XRF and Energy


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dispersive X-Ray (EDX) spectroscopic technique which is shown in Figure S2-S6 in SI. XRF and EDX analysis showed that the metal cations were distributed uniformly in the appropriate stoichiometric ratio which closely resembles precursor taken and the notation given in the article. Surface area, porous nature and pore size distribution of as prepared catalysts were evaluated through the BET and BJH method. Figure 4(a) shows the N2 adsorption- desorption isotherm which indicates that the isotherm plots resemble Type IV and are in accordance with the recent studies of Wang et al.49

Figure 4. (a) N2 adsorption desorption isotherm and (b) BJH pore volume distribution of as prepared catalyst (MxNi1-xFe2O4; M = Co; 0 < x < 1).

The hysteresis loop at high relative pressure of NiFe2O4 shows aggregated mesoporous structure of materials compared to other Co substituted materials.49,50 Figure 4(b) depicts the corresponding pore size distribution from BJH plots of bare NiFe2O4 and Co substituted NiFe2O4. It confirms that NiFe2O4 has more pores between the range of 5 to 15 and average pore diameter of ~12 nm compared to other Co substituted materials. Table 1 summaries the calculated surface area, pores size and pore volume of the samples.



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Table 1. N2 adsorption desorption isotherm results of as prepared catalyst (MxNi1-xFe2O4; M = Co; 0 < x < 1). Sample x=0 x = 0.25 x = 0.5 x = 0.75 x=1

BET surface area (m2 g-1) 6.60 8.74 5.28 10.97 6.64

Pore size (nm) 6.25 5.64 7.62 4.51 4.68

Pore volume (cm3 g-1) 0.0103 0.0123 0.0100 0.0123 0.0077

3.2. Oxygen evolution catalytic activity In order to study and compare the OER catalytic activity of the present study material MxNi1-xFe2O4 (M = Co; 0 < x < 1) with the benchmarker IrO2 (99.9%, Sigma-Aldrich) (for comparsion purpose), cyclic voltammetry was performed and the linear portion of anodic sweep were used as LSV curve (Figure 5(a)) in 1 M KOH with sweep rate of 5 mV s-1. From the Figure 5(a), bare CP did not show any catalytic activity (increase of current density) towards OER compared to catalyst coated CP. In the case of MFe2O4 and IrO2 coated CP shows different onset potential (increase of current density) towards the OER and which was found to decrease towards lower regions to produce 10 mA cm-2; which were comparably higher than η of benchmarked IrO2. The high catalytic activity and lower η (260 mV at 10 mA cm-2) of IrO2 is due to the well-known catalytic activity of the CP substrate and the increased current density than compared to others from the starting regions due to the capacitive behavior. In the present case of NiFe2O4 and the Co substituted samples (CoFe2O4), onset potential was increased from 381 mV to 473 mV with increase in Co content (Figure 5(a)), whereas for Co content of 0.25, 0.5 and 0.75 in NiFe2O4 (MxNi1-xFe2O4) the over potential were found 450, 477 and 460 mV respectively. This observed catalytic activity (in term of η) of NiFe2O4 (381 mV at 10 mA cm-2) is two times superior to that of NiFe2O4 NP (550 mV at 5 mA cm-2) and NiFe2O4 NF (470 mV at 5 mA cm-2); CoFe2O4 (473 mV at 10 mA cm-2) is


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superior to CoFe2O4 NP (520 mV at 5 mA cm-2) and CoFe2O4 NF (410 mV at 5 mA cm-2) studied by Li et al.20 Similarly catalytic activity of NiFe2O4 is closer to NixCo3−xO4 (337 mV at 10 mA cm-2), dp-MnCo2O4/CNT (454 mV at 10 mA cm-2) and Ni0.9Fe0.1Ox (336 mV at 10 mA cm-2) of previous reports.6,21,27 Further more the comparison of present results with recent reports of spinel catalysts catalytic activity towards the OER in terms of η is summarized in Table S2 in SI. These results suggest that with the increase of Co content in NiFe2O4, decrease of catalytic activity was observed by increasing onset potential to produce 10 mA cm-2. It was synergistically influenced by the Co content. This decrease in catalytic activities may be ascribed to the lower electronic conductivity, poor oxidation kinetics, surface irreversible process by the reduction of number of active site and dependence of surface accessibility of OH- during the OER, while forming a mixed metal ferrites (MFe2O4) by the Co substitution.9,17,27,51,52



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Figure 5. Electrochemical characterization of as prepared MFe2O4 (MxNi1-xFe2O4; M = Co; 0 < x < 1). (a) Liner polarization curve at the scan rate of 5 mV s-1, (b) relationship of anodic and cathodic charging current density measured at non-Faradic regions (0 to 0.2 V vs SCE) as function of scan rate. Cdl were determined from the average of the absolute value of the slope of the linear fits to the data. (c) comparison of OER current density of the catalyst at the η of 300 and 400 mV and (d) Tafel plots derived from liner polarization curve in 1 M KOH.

In Ni and Fe containing ferrites (NiFe2O4; where M = 0) better catalytic activity is achieved due to the charge transfer of Fe and Ni in the form of Fe3+/Fe4+ and Ni3+/Ni4+ under OER condition.53,54 In the case of Co containing MxNi1-xFe2O4 (M = Co) the variation of catalytic activity is due to Co resist the +2 oxidation state of Ni2+ to further oxidation. It leads to NiOOH which is more conductive than compared to CoOOH intermediate in the catalyst during the OER.9


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In order to study the origin and relationship between the catalytic activities and the surface accessibility of MxFe1-xFe2O4 (M = Co); ECSA was measured in terms of Cdl associated with the non-Faradic region charging current in the CV as shown in Figure S7 and the slope of current density vs scan rates. Figure 5(b) relates the Cdl of catalysts MxFe1-xFe2O4 and results are summarized in Table 2. The decrease of Cdl with respect to the increase of Co substitution indicates that there is suppression of catalytic activity of NiFe2O4 while substitution. In connection with the catalytic activity, the specificity at given η among the catalyst were studied by the calculation of TOF (mole of O2 generated per second per mole of catalyst) using the current density derived from the LSV at 300 mV of η by assuming that the all metal oxide atoms participated in the OER and it is summarized in Table 2 (TOF at 400 mV of η shown in Table S3 in SI). The TOF is found to decrease with increase of Co content from 5.74 × 10-4 to 1.87 × 10-4 s-1. Table 2. Electrocatalytic parameters of catalysts (MxNi1-xFe2O4; M = Co; 0 < x < 1). Sample

η5

η10

Cdl

Tafel slope

TOF

Mass Rs Rct activity x=0 338 381 0.0039 46.5 5.74 ×10-4 946 4.85 12.68 -4 x = 0.25 380 450 0.0020 50.5 1.87 × 10 308 5.49 27.45 x = 0.5 418 477 0.0019 73.0 0.70 × 10-4 119 5.56 90.22 -4 x = 0.75 401 460 0.0018 52.2 1.87 × 10 308 4.98 32.70 x=1 401 477 0.0013 71.1 1.87 × 10-4 308 6.45 48.42 η5 at 5 mA cm-2 and η10 at 10 mA cm-2 in mV, Cdl in mF cm-2, Tafel slope in mV dec-1, TOF at 300 mV of η (s-1), Mass activity in A g-1, Rs and Rct in ohm cm2 and respectively.

In the case of Co and Ni equimolar ratio (1:1; Ni0.5Co0.5Fe2O4), TOF was found to be of the lowest magnitude. This may be ascribed to the synergistic effect of Ni and Co in the catalyst system and these results are consistent with the OER catalytic behavior from LSV results. The highest TOF of 5.74 × 10-4 of NiFe2O4 is attributed to the small crystalline size,



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pore volume distribution, electronic conductivity during the OER. In relation to this TOF, we calculated mass activity (A g-1) at the 300 mV η and specific activity (catalytic current at desired η of 300 and 400 mV) of catalysts, which is shown in Table 2 and Figure 5(c) respectively. This was found higher for the NiFe2O4 than compared to Co substituted NiFe2O4. In connection with the LSV, Tafel plots (liner dependency of η vs log (i)) of as present catalyst were used to compare the catalytic activity. Figure 5(d) depicts the Tafel plot of these catalysts. It was found to be 46.4, 71.1, 73.0 and 50.5 mV dec-1 respectively for the Co substitution content of x = 0, x = 0.25, x = 0.5, x = 0.75 and x = 1 in the NiFe2O4. Among these, the minimum slope (46.4 mV dec-1) of NiFe2O4 (x = 0) indicates better catalytic activity towards OER due to their higher electronic conductivity and charge transfer process. In addition to this, the Tafel slope of 46.4 mV dec-1 is near to 40 mV dec-1 which indicates that the rate determining step is the second electron transfer process in the four electron/proton process during the OER and it suggests that OER follows second order kinetics in OH- concentration.53,54 This is similar to the previous reports of Mn, Co and Ni substituted Fe3O4.28,31,34 To support these catalytic activities studies, EIS was carried out which is described below. In addition to the catalytic activity of catalyst, stability and durability are also important factor in the long-term application of water splitting in the energy conversion system.55 Among the best of NiFe2O4 from the studied catalyst, the stability and durability (stability under the fast scan rate) was further assessed by cyclic voltammetry at a scan rate of 30 mV s-1 between the potential window of 1 to 2 V vs RHE in 1 M KOH up to 250 cycles. Figure 6(a) displays the cyclic voltammetry and durability studies of the NiFe2O4 and it suggests that the catalytic stability and durability is retained during the cycling time. On the other hand, cycling and durability test of continuous process enhances the NiFe2O4 catalytic


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activity. Notably, from the first cycle to 250 cycles η was decreased to around 50 mV at the current density of 10 mA cm-2. In association with the cyclic stability test, chronoamperometric stability test was also carried out at applied potential of 1.7 V vs RHE in 1 M KOH for the period of 2 hours and it is shown in Figure 6(b).

Figure 6. (a) cyclic voltammetry stability curve (at the scan rate of 30 mV s-1) and (b) chronoamperometry stability curve of NiFe2O4 catalyst on the CP substrate at the applied potential of 1.7 V vs RHE in 1 M KOH.

It can be seen that the current density has increased in the initial period and attains stability. This is clearly in consistence with the cyclic stability test while continuous process enhances the catalytic activity of the catalyst while increasing the duration to 30 minutes the catalytic current was increased and attained stability.55 Mean time we observed the formation of gas bubbles in the catalyst coated area on the CP substrate. In most of the cases, the formation and accumulation of gas bubbles at the electrode surface block the active sites and hinders the catalytic activity. This may be attributed to position of electrode during testing period. In comparison with benchmarking IrO2 catalyst, the NiFe2O4 show better stability. IrO2 and RuO2 are well known for their catalytic activity and instability during the long cycling.12,13,56,57



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Figure 7. Electrochemical impedance spectroscopy studies of MxNi1-xFe2O4; M = Co; 0 < x < 1) catalysts. (a) Nyquist plots (inset graph representative equivalent circuit) and (b) Bode plots measured at the applied potential of 300 mV of η in 1 M KOH solution.

The EIS technique allows better understanding of kinetics related to OER and it is a mirror like tool to elucidate associated parameters with OER.3,58,59 To know the factors behind catalytic activities of NiFe2O4 while substitution of Co, we performed EIS analysis under the identical condition with η of 300 mV vs RHE. Figure 7(a) and (b) shows the representative EIS of Nyquist (Real (Z’) vs Imginary (Z’’) and Bode plot (Phase angle vs Log Frequency) as measured in frequency range of 100 kHz to 10 mHz with applied potential of 300 mV of η. The electrodes shows two semicircles at high and low frequency regions which attributed to the metal oxide and OER properties.32 In which, semicircle at high frequency is the response to the charge transfer resistance and that at low frequency regions is associated with the adsorption/desorption (H+/OH-) of reactive intermediates by the diffusion and relaxation of charge associated surface intermediates.59,60 We used a simple and most common Randles cell model equivalent circuit to analyse the impedance spectra, which is shown in the inset of Figure 7(a), it includes a solution resistance (Rs), double layer capacitance (Cdl) and charge transfer or polarization resistance (Rct). Rs is related to the solution induced resistance at the interface of electrode/electrolyte, Rct and Cdl is associated


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with OER through adsorption/desorption of H+/OH- by diffusion, charge transfer resistance (lower charge transfer resistance gives fast charge transfer kinetics) at the electrode or catalyst surface.29,33 Further, we calculated solution induced resistance (Rs) and charge transfer resistance (Rct) through circle fit method from the EIS Nyquist plot and which is shown in Table 2. Among the studied catalysts, the decrease of diameter of semi circle (response to Rct and Cdl) from low frequency region and the phase angle shift (in Bode plot) of NiFe2O4 indicates better conductivity among the studies catalysts system.33 The decrease of Rct with η is well known through Butler-volmer kinetic relations. This is similar to the recent reports.1,3,37,52,59 Among the studied catalysts bare NiFe2O4 shows smaller Rs and Rct, which clearly demonstrates its better catalytic activity and faster electron transfer rate compared to the Co substituted NiFe2O4 and it evident that Co substitution effect in the decline of the catalytic activity. This is because Rct is inversely proportional to the electron transfer rate during the OER and it subsequently decrease the conductivity.37,52 This EIS studies also support the previously measured results of catalytic activities of MxNi1-xFe2O4 (M = Co; 0 < x < 1) through LSV, Tafel and TOF. 3.3 Mechanism of OER catalytic activity of MxNi1-xFe2O4; M = Co; 0 < x < 1 The electrochemical water splitting of OER is believed to occur at the electrode surface by electrochemically producing metal cations (Mn+) active sites, reaction proceeds via electrosorption of OH- ions, a series of intermediates of M−OH, M−O, M−OOH and M−OO via the bounded M-O followed by the formation of O-O bond.12,18,29,31,33,61,62 M + OH- → M-OH + eM-OH + OH- → M-O + H2O + eM-O + OH- → M-OOH + e-



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M-OOH + OH- → M-OO + H2O +eM-OO → M + O2 The more catalytic activity of NiFe2O4 than compared to Co substituted materials is due to in the presence of Ni. During OER, Ni in the octahedral sites of NiFe2O4 undergoes oxidation from +2 to +3/+4 and the Fe stabilize the lower valance state of Ni and these oxidized form of Ni2+ to Ni3+ and Ni4+ becomes more electronic conductive than compared to other Co containing materials.61,63 Because of the CoOOH intermediates, conductivity of NiOOH is decreased in presence of FeOOH.9 3.4. Hydrogen evolution catalytic activity and Overall water splitting using NiFe2O4 as an anode and cathode As discussed in the introduction, HER is hindered by the four electrons transfer of OER. Increased OER kinetic results in improved HER. Addition to this OER catalytic behavior, if the catalyst plays a role in HER it is recognized to be bifunctional. Hence, we study the HER catalytic activity behavior of Co substituted NiFe2O4 in 1 M KOH at the cathodic region.


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Figure 8. (a) Companied OER and HER catalytic behavior of MxNi1-xFe2O4 (M = Co; 0 < x < 1) at the scan rate of 5 mV s-1 and (b) optical image of electrocatalytic water electrolysis (generation of H2 and O2) through the catalyst of NiFe2O4 catalyst coated on a carbon sheet as anode and cathode in 1 M KOH at the applied potential of 1.7 V vs RHE.

Figure 8(a) shows the HER catalytic activity of LSV combined with OER region for better understanding. It can be seen, that the catalyst materials of NiFe2O4 and Co substituted NiFe2O4 shows a better catalytic current by HER than bare CP. Meanwhile, catalytic current at the near to 0 V vs RHE is anonymously greater than 3 mA cm-2 than compared to the CP (see the Figure S8, HER LSV sweep as measured with respect to SCE in SI). However, the cathodic HER catalytic current of over the 10 mA cm-2 appeared at the η of > 200 mV. Notably, the bare NiFe2O4 shows lowest η of 264 mV at 10 mA cm-2 compared to other Co substituted NiFe2O4. This η of 264 mV is comparably lower than that of Ni containing compounds of Ni3S2 and NiCo2S4.64,65 Further Table S4 summarises the comparison of this results of HER catalytic activity with recent reports. Herein, we skipped the evaluation of HER stability and kinetic parameter (Tafel) under as it was beyond the scope of the present study. However, we made water electrolysis based on this study of NiFe2O4 catalyst on the CP substrate as an anode as well as cathode in 1 M KOH as describe in Figure 8(b) at the applied potential of 1.7 V vs RHE. During water electrolysis, appreciable gas bubbles of H2



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and O2 on the electrode surface of cathode and anode respectively is seen. Meanwhile, the evolution of H2 at cathode is higher than that of O2 at the anode. This was accounted by the multiple charge transfer reaction of O2 formation and the high solubility nature of O2 in water (≈27 times higher than that of H2 at 20 °C).59,66 4. CONCLUSION In this study, NiFe2O4 and Co substituted MxNi1-xFe2O4 (x = 0 to 1) were synthesized through sol-gel combustion method and the structural, physical and chemical properties were studied. Electrocatalytic behavior towards OER was studied in alkaline medium in order to investigate the role of Co substitution in the MxNi1-XFe2O4 spinel system. Physicochemical characterization results showed that Co substitution synergistically plays a vital role while substituted in NiFe2O4. It is found to alter the particle size, surface area, pore volume distribution of NiFe2O4 depending on the Co concentration. Electrochemical characterizations reveal that the physical properties are relative in accordance to the catalytic activity of the catalysts. Among the investigated catalysts, bare NiFe2O4 showed better catalytic activity for OER with 386 mV of η, Tafel slope of 46.4 mV dec-1 and highest TOF of 5.74 × 10-4 at 300 mV of η. The decrease of catalytic activity as a result of Co substitution can be ascribed to lower surface area, pores size distribution, ECSA, and electronic conductivity associated electrode electrolyte interaction and charge transfer process under the OER condition. Similarly, Co substitution has considerably influenced HER in alkaline medium. This systematic evaluation of OER catalytic activity of MxNi1-xFe2O4 (M = Co) suggest that Co substitution is responsible for the surface and electronic properties which in turn alter the catalytic activities and the controlled tuning is an important factors to develop newer catalysts.


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ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS website at DOI: Recent studies of spinel compounds, XRD pattern with the JCPDS standard, EDAX and elemental mapping, scan rate dependent capacitive current density, comparison Table of OER catalytic activity, Table of TOF at 400 mV of η for OER, HER catalytic activity LSV with respect to SCE, HER catalytic activity comparison Table (PDF). AUTHOR INFORMATION Corresponding authors *Email: [email protected] *Email: [email protected] ORCID Viruthasalam Maruthapandian: 0000-0001-6160-2663 ACKNOWLEDGEMENT Authors would like to acknowledge Council of Scientific Industrial Research (CSIR), India for financial support through the MULTIFUN (CSC-0101) project. Author M. Mathankumar thank to UGC, India for the financial support through the UGC-SRF. Faculties of the Central Instrumentation Facility (CIF), CSIR-CECRI, are greatly acknowledged for their support for materials characterizations.



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Graphical Abstract


Study of the Oxygen Evolution Reaction Catalytic Behavior of CoxNi1

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