Electrodeposited Nanostructured CoFe2O4 for Overall Water Splitting and Supercapacitor Applications

Catalysts 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts Electrodeposited Nanostructured CoFe2O4 for Overall Water Splitting and Supercapacitor Applications Chunyang Zhang1, Sanket Bhoyate1, Chen Zhao 1, Pawan K. Kahol2, Nikolaos Kostoglou3*, Christian Mitterer3, Steven J. Hinder4, Mark A. Baker 4, Georgios Constantinides5, Kyriaki Polychronopoulou6,7, Claus Rebholz8, Ram K. Gupta1* Electronic Supplementary Information


Introduction
According to the report presented by the International Renewable Energy Agency (IRENA), the Renewable Energy Policy Network for the 21st Century (REN21), and the International Energy Agency, transport is the second largest energy end-use sector, accounting for 29% of the total final energy consumption in the year 2015 [1].About 96% of this energy comes from petroleum products, making it heavily reliant on fossil fuels.Several companies such as Tesla, Toyota, Nissan, Mercedes-Benz, Hyundai, Honda, and others have targeted their automotive market in the way that global dependence on petroleum resources is reduced [2,3].Thus, major research focuses on multifunctional materials for green energy applications to be used for electric motor and fuel cell vehicles.
Spinels have recently attracted considerable research attention for energy applications [4][5][6].They represent a group of composite metals with such multifunctional capabilities, having a formula of AB 2 O 4 (A and B are metal ions) [7].Mixed phase composition of transition metals into spinel structure provides advantageous electrical and catalytic properties required characteristics for energy conversion and storage [4][5][6].Previously, electroactive materials such as structured carbons [8][9][10][11][12][13][14], transition metal oxides, and sulfides [15][16][17][18] have been studied for applications in batteries, supercapacitors, and fuel cells, as well as for water splitting.However, the lower activity of carbon-based materials, the inferior electronic conductivity of transition metal oxides, and the poor stability of transition metal sulfides limit their applicability for future energy devices [14,[19][20][21].Transition metal-based systems including oxides, hydroxides, or sulfides of cobalt and iron have been studied in the past few years for energy conversion or storage applications [22][23][24][25][26][27].However, these materials show limited electrochemical performance.Various studies suggest that spinels of cobalt and iron provide better electronic conductivity, a higher population of active sites, and thus better electrochemical performance.For example, Kumbhar et al. synthesized spinel CoFe 2 O 4 nano-flakes on a stainless steel substrate by a simple chemical route using an alkaline bath containing Co 2+ and Fe 2+ ions for supercapacitor application [28].An impedance test of spinel electrodes showed the low equivalent series resistance of about 1.1 Ω. Yu et al. reported a novel hierarchical Cu on a CoFe layered double hydroxide (LDH) core-shell nanostructured catalyst for oxygen and hydrogen evolution reaction [29].The unique structure of LDH provides more active sites, leading to better catalytic performance.Another report suggests the use of iron-cobalt oxide nanosheets (Fe x Co y -ONSs) as a stable water oxidation catalyst [30].Excellent oxygen evolution reaction (OER) activity was observed as a result of mass diffusion/transport of OH -ions facilitated by ultrathin nanosheets, improved electronic conductivity, and adsorption of H 2 O onto close Co 3+ sites.
Several methods such as solvothermal, hydrothermal, chemical co-precipitation, and solution reduction reaction are reported for the synthesis of iron-cobalt oxide [30][31][32][33][34][35].Most of the synthesized iron-cobalt oxide samples were powders, which are coated onto conductive substrates by dip-coating.The use of electrically insulating binders reduces the conductivity and thus affects the electronic activity of the material.Furthermore, the stability and flexibility of the electrodes are also compromised.However, direct electrodeposition of materials on conductive substrates could overcome this problem.For example, Lu et al. fabricated a hierarchically structured three-dimensional nickel-iron composite on Ni-foam for efficient oxygen evolution catalyst electrode using a one-step electrodeposition [36].Higher conductivity and active surface area of the material over the electrode resulted in better electrochemical performance.
In this work, CoFe 2 O 4 was deposited on Ni-foam and carbonized cotton cloth using a facile electrodeposition method.The synthesized CoFe 2 O 4 on Ni-foam exhibited high electrocatalytic activity for OER and hydrogen evolution reaction (HER) with small overpotential.In addition, the CoFe 2 O 4 electrode showed high specific capacitance of 768 F/g at the current density of 0.5 A/g, maintaining stable performance with 80% retention up to 10,000 charge-discharge cycles.Our results suggest that electrodeposited CoFe 2 O 4 on Ni-foam could be used as a multifunctional material for both water splitting and supercapacitor applications.

Results and Discussion
X-ray diffraction (XRD) was used to study phase purity and crystallinity of the synthesized samples.Figure S1  An X-ray photoelectron spectroscopy (XPS) wide scan of the CoFe 2 O 4 surface is presented in Figure S2.The Co 2p and O 1s peaks are given in Figure 1.Considering the Co 2p peak shape, this is very consistent with spectra of Co 3 O 4 standards given by Gu et al. [37] and Yang et al. [38], showing a 2p 3/2 peak binding energy of 779.9 eV and a broad plateau-like satellite structure to higher binding energies between the 2p 3/2 and 2p 1/2 peaks.Thus, this peak shape and satellite structure is representative of mixed Co 2+ /Co 3+ chemical states.Compared with the Co 2p spectrum for CoFe 2 O 4 given by Gu et al. [37] and Sagu et al. [39], the peak shape of the electrodeposited CoFe 2 O 4 shows a peak shape that is more consistent with that of Co 3 O 4 , and hence a 1:1 stoichiometric Co 2+ /Co 3+ fraction rather than the Co 2+ rich compound observed in these other studies.However, it should be noted that quantification of the XPS spectra shows an enrichment of Co at the surface, indicating that the XPS spectra and outermost surface composition are not fully representative of the bulk material.The O 1s spectrum (Figure 1b) shows a peak at 530.1 eV corresponding to the metal oxide, with peaks at 531.6 and 532.7 eV corresponding to hydroxyl and adsorbed water species, respectively.CoFe2O4 layer or the lack of sufficient diffracted X-ray signal from the coating on the porous Ni-foam structure.An X-ray photoelectron spectroscopy (XPS) wide scan of the CoFe2O4 surface is presented in Figure S2.The Co 2p and O 1s peaks are given in Figure .1. Considering the Co 2p peak shape, this is very consistent with spectra of Co3O4 standards given by Gu et al. [37] and Yang et al. [38], showing a 2p3/2 peak binding energy of 779.9 eV and a broad plateau-like satellite structure to higher binding energies between the 2p3/2 and 2p1/2 peaks.Thus, this peak shape and satellite structure is representative of mixed Co 2+ /Co 3+ chemical states.Compared with the Co 2p spectrum for CoFe2O4 given by Gu et al. [37] and Sagu et al. [39], the peak shape of the electrodeposited CoFe2O4 shows a peak shape that is more consistent with that of Co3O4, and hence a 1:1 stoichiometric Co 2+ /Co 3+ fraction rather than the Co 2+ rich compound observed in these other studies.However, it should be noted that quantification of the XPS spectra shows an enrichment of Co at the surface, indicating that the XPS spectra and outermost surface composition are not fully representative of the bulk material.The O 1s spectrum (Figure .1b) shows a peak at 530.1 eV corresponding to the metal oxide, with peaks at 531.6 and 532.7 eV corresponding to hydroxyl and adsorbed water species, respectively.Figure 2(a-c) present scanning electron microscopy (SEM) images of electrodeposited CoFe2O4 over Ni-foam at different magnifications.The coating over the Ni-foam is relatively uniform and dense, as observed from the images.The cracks in the coating are most likely the result of dehydration, associated with removal of the electrode from solution.The energy dispersive X-ray spectroscopy (EDS) O elemental image in Figure 3 confirms the general uniform distribution and composition of CoFe2O4 over the Ni-foam substrate.However, the Fe and Co images in Figure 3 show that there are differences in the Fe/Co ratio between the 'smoother' surface regions and underlying 'mottled' areas of the coating.Considering the XPS results, it seems most likely that the smooth outer layer is CoFe2O4 and this predominantly covers the Ni-foam.However, in some areas, this layer has become detached (possibly because of stresses associated with dehydration or transfer to the analytical laboratory), exposing a more Co-rich underlying layer.SEM images for CoFe2O4 electrodeposited on CCC are given in Figure S3(a-c), evidencing evenly coated fibers.The reduced impedance over increasing potential can be observed from the Nyquist plot for the CoFe 2 O 4 ED@NF electrode given in Figure S4.The kinetics of the OER reaction were analyzed using Tafel slopes (Figure 4b).The Tafel slope for CoFe 2 O 4 ED@NF, CoFe 2 O 4 @NF, and CoFe 2 O 4 ED@CCC was calculated to be 31, 47, and 119 mV/dec, respectively.The low Tafel slope value for the CoFe 2 O 4 ED@NF electrode indicates faster OER kinetics and can be correlated to the high electrocatalytic surface area of the electrodeposited Ni-foam electrode [15].The electrochemical effective surface area was calculated using cyclic voltammetry (CV) curves in the non-faradic region (Figure S5) and the plotting relation of current density with respective scan rate is given in Figure 4d.A linear relationship was observed, showing a higher slope for CoFe 2 O 4 ED@NF (65 mC/cm 2 ) than CoFe 2 O 4 @NF (40 mC/cm 2 ) and CoFe 2 O 4 ED@CCC (3 mC/cm 2 ), thus indicating a larger effective electrochemical surface area.The higher surface area of CoFe 2 O 4 ED@NF is consistent with the excellent OER performance.The CoFe 2 O 4 ED@NF electrode of this study required a lower overpotential for achieving a current density of 10 mA/cm 2 compared with other FeCo-based reports.For example, Yan et al. synthesized a CoFe 2 O 4 hybrid strongly coupled with carbon nanotubes and observed a higher overpotential (>0.6 V) to achieve 5 mA/cm 2 in 0.1 M KOH [31].Another hybrid of CoFe 2 O 4 nanoparticles supported on nitrogen/sulfur dual-doped three-dimensional (3D) reduced graphene oxide networks (CFO/NS-rGO) showed an overpotential of about 0.55 V to achieve the current density of 10 mA/cm 2 [32].The iron-cobalt oxide nanosheets (FeCo-ONS) prepared by Zhuang et al. required an overpotential of 308 mV to reach 10 mA/cm 2 with a Tafel slope of 36.8 mV/dec [30].Table S1 shows a detailed comparison of catalytic electrodes synthesized within this work with previously reported CoFe 2 O 4 samples.It can be observed that the CoFe 2 O 4 ED@NF electrode requires a low overpotential and shows higher kinetics of reaction compared with all the other reported values.
rather than the Co rich compound observed in these other studies.However, it should be noted that quantification of the XPS spectra shows an enrichment of Co at the surface, indicating that the XPS spectra and outermost surface composition are not fully representative of the bulk material.The O 1s spectrum (Figure .1b) shows a peak at 530.1 eV corresponding to the metal oxide, with peaks at 531.6 and 532.7 eV corresponding to hydroxyl and adsorbed water species, respectively.3 show that there are differences in the Fe/Co ratio between the 'smoother' surface regions and underlying 'mottled' areas of the coating.Considering the XPS results, it seems most likely that the smooth outer layer is CoFe2O4 and this predominantly covers the Ni-foam.However, in some areas, this layer has become detached (possibly because of stresses associated with dehydration or transfer to the analytical laboratory), exposing a more Co-rich underlying layer.SEM images for CoFe2O4 electrodeposited on CCC are given in Figure S3(a-c), evidencing evenly coated fibers.The OER activity of the CoFe2O4 samples was studied by linear sweep voltammetry (LSV).The polarization curves for CoFe2O4 ED@NF, CoFe2O4@NF, and CoFe2O4 ED@CCC in 1M NaOH are shown in Figure 4a.The CoFe2O4 ED@NF electrode required a low overpotential of 270 mV to achieve a current density of 10 mA/cm 2 .This value is significantly lower than for CoFe2O4@NF (308 mV) and  The effect of bending on the OER properties of the CoFe2O4 ED@NF electrode was studied to investigate its potential to be used as a flexible electrode.The polarization curves in Figure 5a show almost identical behavior for curves before and after bending the electrode by a 60 o angle.Moreover, the durability of the electrode was assessed for 17 h using chronoamperometry (Figure 5b).The chronical current measurement showed almost constant values without any detriment over the catalytic performance of the electrodes, suggesting the durable performance of the electrode for longterm use.As several reports suggest the use of spinels for multifunctional catalytic application [7,31,40], herein we evaluated the performance of the CoFe2O4 ED@NF electrode for HER catalysis.Figure S6a   The effect of bending on the OER properties of the CoFe 2 O 4 ED@NF electrode was studied to investigate its potential to be used as a flexible electrode.The polarization curves in Figure 5a show almost identical behavior for curves before and after bending the electrode by a 60 o angle.Moreover, the durability of the electrode was assessed for 17 h using chronoamperometry (Figure 5b).The chronical current measurement showed almost constant values without any detriment over the catalytic performance of the electrodes, suggesting the durable performance of the electrode for long-term use.The effect of bending on the OER properties of the CoFe2O4 ED@NF electrode was studied to investigate its potential to be used as a flexible electrode.The polarization curves in Figure 5a show almost identical behavior for curves before and after bending the electrode by a 60 o angle.Moreover, the durability of the electrode was assessed for 17 h using chronoamperometry (Figure 5b).The chronical current measurement showed almost constant values without any detriment over the catalytic performance of the electrodes, suggesting the durable performance of the electrode for longterm use.As several reports suggest the use of spinels for multifunctional catalytic application [7,31,40], herein we evaluated the performance of the CoFe2O4 ED@NF electrode for HER catalysis.As several reports suggest the use of spinels for multifunctional catalytic application [7,31,40], herein we evaluated the performance of the CoFe 2 O 4 ED@NF electrode for HER catalysis.Figure S6a shows the LSV polarization curves.A small overpotential of −270 mV was required to achieve the current density of −10 mA/cm 2 .Moreover, the reaction kinetics using the Tafel slope yielded a low value of 94 mV/dec (Figure S6b).The value observed for the CoFe 2 O 4 electrode as HER catalyst was lower compared with other similar catalytic systems.For example, a hydrothermally synthesized CoFe 2 O 4 NPs-on-CFP electrode by Kargar and co-workers required an overpotential of −356 mV to achieve a current of −10 mA/cm 2 [41].The comparison table of other reports based on Fe, Co, and other non-precious transition metals in alkaline media can be observed from Table S2.Moreover, the chronoamperometry test indicated a stable HER performance of the catalyst electrode for 17 h, suggesting long-term durability of the electrode (Figure S6c).This indicates that the CoFe 2 O 4 ED@NF electrode could be used as a bifunctional water-splitting catalyst.
An electrolyzer cell was fabricated using a two-electrode system to study water-splitting of catalytic electrodes.In this study, the CoFe 2 O 4 ED@NF electrode was used as an anode and a standard platinum carbon (Pt-C) electrode was used as a cathode.Figure 6a shows polarization curves using the LSV test for different electrolyzer cells.The CoFe 2 O 4 //Pt-C electrolyzer system required a low potential of 1.56 V to obtain a current density of 10 mA/cm 2 compared with benchmark electrolyzer cells such as IrO 2 //Pt-C (1.57V), IrO 2 //IrO 2 (1.77V), and Pt-C//Pt-C (1.64 V), which all required higher potentials.Moreover, the chronoamperometry test yielded stable performance of the CoFe 2 O 4 //Pt-C electrolyzer system for more than 17 h, as evidenced by Figure 6b.shows the LSV polarization curves.A small overpotential of −270 mV was required to achieve the current density of −10 mA/cm 2 .Moreover, the reaction kinetics using the Tafel slope yielded a low value of 94 mV/dec (Figure S6b).The value observed for the CoFe2O4 electrode as HER catalyst was lower compared with other similar catalytic systems.For example, a hydrothermally synthesized CoFe2O4 NPs-on-CFP electrode by Kargar and co-workers required an overpotential of −356 mV to achieve a current of −10 mA/cm 2 [41].The comparison table of other reports based on Fe, Co, and other non-precious transition metals in alkaline media can be observed from Table S2.Moreover, the chronoamperometry test indicated a stable HER performance of the catalyst electrode for 17 h, suggesting long-term durability of the electrode (Figure . S6c).This indicates that the CoFe2O4 ED@NF electrode could be used as a bifunctional water-splitting catalyst.
An electrolyzer cell was fabricated using a two-electrode system to study water-splitting of catalytic electrodes.In this study, the CoFe2O4 ED@NF electrode was used as an anode and a standard platinum carbon (Pt-C) electrode was used as a cathode.Figure 6a shows polarization curves using the LSV test for different electrolyzer cells.The CoFe2O4//Pt-C electrolyzer system required a low potential of 1.56 V to obtain a current density of 10 mA/cm 2 compared with benchmark electrolyzer cells such as IrO2//Pt-C (1.57V), IrO2//IrO2 (1.77 V), and Pt-C//Pt-C (1.64 V), which all required higher potentials.Moreover, the chronoamperometry test yielded stable performance of the CoFe2O4//Pt-C electrolyzer system for more than 17 h, as evidenced by Figure 6b.The energy storage capacity of the CoFe2O4 ED@NF electrode was measured using CV.The specific capacitance values can be predicted based on the area under the CV curves.Figure 7a depicts the specific capacitance values as a function of varying scan rates.Higher specific capacitance values were observed with decreasing scan rates.A similar behavior was observed in other reports of energy storage systems [28,[42][43][44].The CV curves of CoFe2O4 ED@NF in 3M KOH electrolyte at various scan rates are given in Figure .S7a.Pairs of redox peaks in the CV curves indicate the pseudocapacitive behavior of the material.This resulted in a high specific capacitance of 2246 F/g at a scan rate of 1 mV/s.The charge storage behavior was further analyzed using the galvanostatic charge-discharge test.Figure 7b shows the relationship of specific capacitance at different current densities.At lower charging current, high capacitance values were obtained.A similar relation of current density with discharge time can be observed from Figure S7b.This could be the result of sufficient time provided for redox reactions at low current densities.Redox reactions promoted at low current densities lead to better charge storage compared with other non-faradic systems [43][44][45][46].At a current density of 0.5 A/g, a high specific capacitance of 768 F/g was observed.This result is comparable to other reported values for electrodes within an Co-Fe based system, as summarized in Table S3.Moreover, the stability test of the electrode showed high capacitance retention of about 80% after 10,000 cycles (Figure S8), which is better compared with other reports based on Co-Fe systems.For example, The energy storage capacity of the CoFe 2 O 4 ED@NF electrode was measured using CV.The specific capacitance values can be predicted based on the area under the CV curves.Figure 7a depicts the specific capacitance values as a function of varying scan rates.Higher specific capacitance values were observed with decreasing scan rates.A similar behavior was observed in other reports of energy storage systems [28,[42][43][44].The CV curves of CoFe 2 O 4 ED@NF in 3M KOH electrolyte at various scan rates are given in Figure S7a.Pairs of redox peaks in the CV curves indicate the pseudocapacitive behavior of the material.This resulted in a high specific capacitance of 2246 F/g at a scan rate of 1 mV/s.The charge storage behavior was further analyzed using the galvanostatic charge-discharge test.Figure 7b shows the relationship of specific capacitance at different current densities.At lower charging current, high capacitance values were obtained.A similar relation of current density with discharge time can be observed from Figure S7b.This could be the result of sufficient time provided for redox reactions at low current densities.Redox reactions promoted at low current densities lead to better charge storage compared with other non-faradic systems [43][44][45][46].At a current density of 0.5 A/g, a high specific capacitance of 768 F/g was observed.This result is comparable to other reported values for electrodes within an Co-Fe based system, as summarized in Table S3.Moreover, the stability test of the electrode showed high capacitance retention of about 80% after 10,000 cycles (Figure S8), which is better compared with other reports based on Co-Fe systems.For example, flower-like CoFe 2 O 4 /FeOOH nanocomposites maintain 91.3% retention after 1000 cycles [47].For reduced graphene oxide-CoFe 2 O 4 composites, the capacitance retention is about 78.1% after 1000 cycles [35].This suggests that the synthesized CoFe 2 O 4 ED@NF electrodes showed higher charge storage capacities and stable performance to be used as energy storage electrode materials.
Catalysts 2018, 8, x FOR PEER REVIEW 7 of 11 flower-like CoFe2O4/FeOOH nanocomposites maintain 91.3% retention after 1000 cycles [47].For reduced graphene oxide-CoFe2O4 composites, the capacitance retention is about 78.1% after 1000 cycles [35].This suggests that the synthesized CoFe2O4 ED@NF electrodes showed higher charge storage capacities and stable performance to be used as energy storage electrode materials.

Materials
All chemicals were of analytical grade purity and were used without any further purification.Cobalt (II) nitrate and iron (III) nitrate (99.9%) were purchased from Sigma Aldrich, USA.All other chemicals were purchased from Fisher Scientific, USA.

Preparation of Electrodeposited CoFe2O4 Electrodes
Nanostructured CoFe2O4 was electrodeposited on Ni-foam and carbonized cotton cloth (CCC) using a standard three-electrode system.Before electrodeposition, Ni-foam was thoroughly cleaned [15].In the three-electrode system, Ni-foam as a working electrode, a platinum plate as a counter electrode, and an Ag/AgCl electrode as a reference electrode were used.The electrolyte consists of an aqueous solution of 0.582 g of cobalt nitrate and 1.616 g of iron nitrate in 20 ml deionized water.The electrodeposition was carried out at −1.0 V (vs.Ag/AgCl) for a period of 5 min to obtain a thin brown film covering the Ni-foam [48].The average weight of the deposited material was 1.2 mg/cm 2 .
Electrodeposition of CoFe2O4 on the CCC was carried out using the above setup, where CCC was used instead of Ni-foam as a substrate.The CCC was prepared by calcination of carbon cloth in a tube furnace at 800 °C for two hours under N2 atmosphere (at 5 °C/min) [49].Electrodeposition of CoFe2O4 on CCC was carried out under similar conditions, but for 50 min to achieve 1.2 mg/cm 2 of the deposited material.

Preparation of Dip-Coated CoFe2O4 Electrode
To compare the effect of the direct growth of CoFe2O4 on a conducting surface, we have dipcoated CoFe2O4 powders on the Ni-foam after synthesizing CoFe2O4.For this reason, CoFe2O4 was synthesized by a simple hydrothermal method, where 0.582 g of cobalt nitrate, 1.616 g of iron nitrate, and 7.2 g of urea were dissolved in a mixture of 30 ml of isopropyl alcohol and 6 ml of water.The mixture was stirred for 30 min and transferred into a 45 mL Teflon-lined stainless-steel autoclave.The reaction was carried out at 120 °C for 12 h.The resultant precipitates were washed several times using water and ethanol.The separation was carried out using centrifugation at 10,000 rpm, followed by drying under vacuum at 80 °C for 6 h and calcination at 350 °C in air for 2 h (5 °C/min).The CoFe2O4 electrode was prepared by dipping the Ni-foam in a slurry of CoFe2O4 powder (80 wt.%),

Materials
All chemicals were of analytical grade purity and were used without any further purification.Cobalt (II) nitrate and iron (III) nitrate (99.9%) were purchased from Sigma Aldrich, USA.All other chemicals were purchased from Fisher Scientific, USA.

Preparation of Electrodeposited CoFe 2 O 4 Electrodes
Nanostructured CoFe 2 O 4 was electrodeposited on Ni-foam and carbonized cotton cloth (CCC) using a standard three-electrode system.Before electrodeposition, Ni-foam was thoroughly cleaned [15].In the three-electrode system, Ni-foam as a working electrode, a platinum plate as a counter electrode, and an Ag/AgCl electrode as a reference electrode were used.The electrolyte consists of an aqueous solution of 0.582 g of cobalt nitrate and 1.616 g of iron nitrate in 20 ml deionized water.The electrodeposition was carried out at −1.0 V (vs.Ag/AgCl) for a period of 5 min to obtain a thin brown film covering the Ni-foam [48].The average weight of the deposited material was 1.2 mg/cm 2 .
Electrodeposition of CoFe 2 O 4 on the CCC was carried out using the above setup, where CCC was used instead of Ni-foam as a substrate.The CCC was prepared by calcination of carbon cloth in a tube furnace at 800 • C for two hours under N 2 atmosphere (at 5 • C/min) [49].Electrodeposition of CoFe 2 O 4 on CCC was carried out under similar conditions, but for 50 min to achieve 1.2 mg/cm 2 of the deposited material.

Preparation of Dip-Coated CoFe 2 O 4 Electrode
To compare the effect of the direct growth of CoFe 2 O 4 on a conducting surface, we have dip-coated CoFe 2 O 4 powders on the Ni-foam after synthesizing CoFe 2 O 4 .For this reason, CoFe 2 O 4 was synthesized by a simple hydrothermal method, where 0.582 g of cobalt nitrate, 1.616 g of iron nitrate, and 7.2 g of urea were dissolved in a mixture of 30 ml of isopropyl alcohol and 6 ml of water.The mixture was stirred for 30 min and transferred into a 45 mL Teflon-lined stainless-steel autoclave.The reaction was carried out at 120 • C for 12 h.The resultant precipitates were washed several times using water and ethanol.The separation was carried out using centrifugation at 10,000 rpm, followed by drying under vacuum at 80 • C for 6 h and calcination at 350 • C in air for 2 h (5 • C/min).The CoFe 2 O 4 electrode was prepared by dipping the Ni-foam in a slurry of CoFe 2 O 4 powder (80 wt.%), acetylene black (10 wt.%), and polyvinylidene difluoride (10 wt.%) in N-methyl pyrrolidinone.The material loading on the Ni-foam was about 1.2 mg/cm 2 .Sample electrodes fabricated by deposition of CoFe 2 O 4 over Ni-foam using electrodeposition and dip-coating are termed in the following as CoFe 2 O 4 ED@NF and CoFe 2 O 4 @NF, respectively, while the sample electrodeposited over carbonized carbon cloth is referred to as CoFe 2 O 4 ED@CCC.

Characterization
The synthesized samples were characterized using using various methods.X-ray diffraction (XRD) was used to study the crystallinity and phase purity of the samples.X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was used to investigate structural, chemical, and morphological properties.XRD patterns were recorded using a Shimadzu X-ray diffractometer (Lenexa, Kansas, USA).CuKα 1 radiation (λ = 1.5406Å) in the 2θ-θ mode was used to study the crystallinity and phase purity of the synthesized samples.Microstructural properties of the samples were investigated using a Quanta 200 SEM (FEI, Hillsboro, Oregon, USA).The elemental compositions and elemental mapping of the synteized samples were recorded for the sysnthesized samples using EDS.The chemical composition of the synthesized samples was determined using XPS analysis using a ThermoFisher Scientific Instruments (East Grinstead, UK).A monochromated Al Kα radiation source (hν = 1486.6eV) and an X-ray spot of ~400 µm in radius were used for data recording.The elemental survey spectra were recorded using a pass energy of 200 eV.A passing energy of 50 eV was used for recording high-resolution core level spectra for all the elements.The Advantage software was used for the peak fittings.
Electrochemical performance was studied using a standard three-electrode system and a Versastat4-500 electrochemical workstation (Princeton Applied Research, TN, USA).CoFe 2 O 4 electrodes were used as working electrodes.Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were performed to examine the energy storage performance of electrodes in 3M KOH electrolyte.The electrocatalytic behavior of the CoFe 2 O 4 electrodes for OER and HER was measured in 1M NaOH electrolyte.Electrocatalytic testing includes linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS).LSV was performed at a scan rate of 1 mV/s in both the OER and HER region.The potential was converted to a reversible hydrogen electrode (RHE) using the Nernst equation.All the EIS measurements were recorded in a frequency range of 0.05 Hz to 10 kHz with an applied 10 mV of AC amplitude.

Conclusions
In summary, we synthesized an efficient catalyst and energy storage material using a facile, ultra-fast, and one-step electrodeposition method to grow CoFe 2 O 4 over Ni-foam and carbon cloth.A uniform distribution of CoFe 2 O 4 electrochemically deposited over the surface of the substrate was observed through SEM and EDX.A low overpotential of 270 mV was calculated for OER when CoFe 2 O 4 was grown over Ni-foam using electrodeposition.The obtained low Tafel slope of 31 mV/dec compared with a dip-coated sample represents excellent reaction kinetics.This could be attributed to a high conductive pathway stemming from the CoFe 2 O 4 deposited over Ni-foam.Flexibility and chronoamperometry evidenced the stable catalytic performance of the electrodes.The energy storage capacity of the material using galvanostatic charge-discharge test show high specific capacitance of 768 F/g at a current density of 0.5 A/g.Stability of electrodes after 10,000 cycles showed capacitance retention of 80%.The obtained results reveal the promising potential of efficient, multi-functional, and cost-effective CoFe 2 O 4 catalysts for energy generation and storage systems.
shows the XRD patterns of CoFe 2 O 4 , CoFe 2 O 4 ED@NF, and CoFe 2 O 4 ED@carbonized cotton cloth (CCC).The XRD pattern of CoFe 2 O 4 shows characteristic peaks of (111), (220), (311), (400), (422), (511), (440), and (533) planes, corresponding to the cubic structure of FeCo 2 O 4 .No peaks other than those stemming from CoFe 2 O 4 were observed, confirming phase purity of the synthesized CoFe 2 O 4 .The XRD patterns of CoFe 2 O 4 ED@NF and CoFe 2 O 4 ED@CCC do not show any peaks attributable to CoFe 2 O 4 , indicating either the amorphous nature of the electrodeposited CoFe 2 O 4 layer or the lack of sufficient diffracted X-ray signal from the coating on the porous Ni-foam structure.

Figure
Figure 2a-c present scanning electron microscopy (SEM) images of electrodeposited CoFe 2 O 4 over Ni-foam at different magnifications.The coating over the Ni-foam is relatively uniform and dense, as observed from the images.The cracks in the coating are most likely the result of dehydration, associated with removal of the electrode from solution.The energy dispersive X-ray spectroscopy (EDS) O elemental image in Figure 3 confirms the general uniform distribution and composition of CoFe 2 O 4 over the Ni-foam substrate.However, the Fe and Co images in Figure 3 show that there are differences in the Fe/Co ratio between the 'smoother' surface regions and underlying 'mottled' areas of the coating.Considering the XPS results, it seems most likely that the smooth outer layer is CoFe 2 O 4 and this predominantly covers the Ni-foam.However, in some areas, this layer has become detached (possibly because of stresses associated with dehydration or transfer to the analytical laboratory), exposing a more Co-rich underlying layer.SEM images for CoFe 2 O 4 electrodeposited on CCC are given in Figure S3a-c, evidencing evenly coated fibers.The OER activity of the CoFe 2 O 4 samples was studied by linear sweep voltammetry (LSV).The polarization curves for CoFe 2 O 4 ED@NF, CoFe 2 O 4 @NF, and CoFe 2 O 4 ED@CCC in 1M NaOH are shown in Figure 4a.The CoFe 2 O 4 ED@NF electrode required a low overpotential of 270 mV to achieve a current density of 10 mA/cm 2 .This value is significantly lower than for CoFe 2 O 4 @NF (308 mV) and CoFe 2 O 4 ED@CCC (410 mV) electrodes.The observed difference could be the result of the high conductive pathway between the CoFe 2 O 4 catalyst and the Ni-foam, compared with the electrode synthesized by dip-coating.The Nyquist plot in Figure 4c at 0.5 V (versus SCE) shows lower charge transfer resistance for the CoFe 2 O 4 ED@NF electrode, which could be responsible for better catalytic

Figure 2 (
Figure2(a-c) present scanning electron microscopy (SEM) images of electrodeposited CoFe2O4 over Ni-foam at different magnifications.The coating over the Ni-foam is relatively uniform and dense, as observed from the images.The cracks in the coating are most likely the result of dehydration, associated with removal of the electrode from solution.The energy dispersive X-ray spectroscopy (EDS) O elemental image in Figure3confirms the general uniform distribution and composition of CoFe2O4 over the Ni-foam substrate.However, the Fe and Co images in Figure3show that there are differences in the Fe/Co ratio between the 'smoother' surface regions and underlying 'mottled' areas of the coating.Considering the XPS results, it seems most likely that the smooth outer layer is CoFe2O4 and this predominantly covers the Ni-foam.However, in some areas, this layer has become detached (possibly because of stresses associated with dehydration or transfer to the analytical laboratory), exposing a more Co-rich underlying layer.SEM images for CoFe2O4 electrodeposited on CCC are given in FigureS3(a-c), evidencing evenly coated fibers.

Figure 3 .
Figure 3. EDX elemental maps of electrodeposited CoFe2O4 on Ni-foam revealing the distribution of Co, Fe, and O.

Figure 3 .
Figure 3. EDX elemental maps of electrodeposited CoFe 2 O 4 on Ni-foam revealing the distribution of Co, Fe, and O.

Figure 6 .
Figure 6.(a) Polarization and (b) chronoamperometry plots for overall water splitting of a two-electrode electrolyzer.

Figure. 7 .
Figure. 7. Variation of specific capacitance as a function of (a) scan rate and (b) current density for FeCo2O4 ED @ Ni-foam.

Figure 7 .
Figure 7. Variation of specific capacitance as a function of (a) scan rate and (b) current density for FeCo 2 O 4 ED @ Ni-foam.