Synthesis and Characterisation of Cobalt Ferrite Coatings for Oxygen Evolution Reaction

: In this paper, two novel procedures based on powder sedimentation, thermal treatment, and galvanostatic deposition were proposed for the preparation of porous cobalt ferrite (CoFe 2 O 4 ) coatings with a metallic and organic binder for use as catalysts in the oxygen evolution reaction (OER). The electrochemical properties of the obtained electrode materials were determined as well, using both dc and ac methods. It was found that cobalt ferrite coatings show excellent electrocatalytic properties towards the oxygen evolution reaction (OER) with overpotential measured at a current density of 10 mAcm − 2 from 287 to 295 mV and a Tafel slope of 35–45 mVdec − 1 . It was shown that the increase in the apparent activity of the CoFe 2 O 4 coatings with an organic binder results mainly from a large electrochemically active area. Incorporation of the nickel binder between the CoFe 2 O 4 particles causes an increase in both the conductivity and the electrochemically active area. The Tafel slopes indicate that the same rate-determining step controls the OER for all obtained coatings. Furthermore, it was shown that the CoFe 2 O 4 electrodes exhibit no signiﬁcant activity decrease after 28 h of oxygen evolution. The proposed coating preparation procedures open a new path to develop high-performance OER electrocatalysts.


Introduction
The oxygen evolution reaction (OER) plays an essential role in state-of-art energy storage and conversion devices. Nowadays, the major challenge is to obtain low-cost and high-performance catalysts for those reactions. Electrodes made of Pt, IrO 2 , and RuO 2 are known OER catalysts, however they are not appropriate for large-scale applications because of their cost. Transition metals, such as Fe, Ni, Co, and Mn, show high electrocatalytic activity and stability in alkaline solutions. Thus, they are promising candidates for replacing precious metals catalysts. Sulphides, selenides, nitrides, carbides, hydroxides, oxy-hydroxides, and oxides of transition metals are extensively used as efficient electrocatalysts [1][2][3][4][5][6]. Because of their high catalytic activity, relatively low cost, and environmental friendliness, numerous studies of transition metal oxides have been performed. For example, it has been found that NiO [7], Co 3 O 4 [8], MnO 2 [9], and Fe 2 O 3 [10] exhibit high catalytic activity toward OER compared to RuO 2 and IrO 2 . It was also found that nickeland cobalt-based composites usually produce overpotentials in the OER reaction around 350-450 mV at a current density of 10 mAcm −2 [4][5][6].
Generally, spinel mixed metal oxides with formula AB 2 O 4 (A, B = metal) exhibit higher electrochemical activity than the single-metal oxides. This is because of the electron jump among different valence states of ions in octahedral sites and additional metallic redoxactive centres [11][12][13][14][15][16]. Between various spinel oxides, Fe-based spinels, e.g., CuFe 2 O 4 , NiFe 2 O 4 , and CoFe 2 O 4 , have been reported as promising electrocatalysts for OER [17]. However, the CoFe 2 O 4 ferrite is one of the most interesting, especially for its excellent chemical stability, efficient electrocatalytic properties, high specific capacitance, low cost, and environmental friendliness. Therefore, CoFe 2 O 4 ferrite is a promising candidate as an electrode in water splitting, lithium-ion batteries, and supercapacitors [18][19][20]. Most of the mixed transition metal oxides catalysts are powders obtained by different methods, e.g., solvothermal, hydrothermal, electrodeposition, or spin coating [21,22]. Usually, to obtain an electrode, those powders are applied to a conductive substrate using a particle binder. However, polymeric binders are characterized by relatively poor electrical conductivity, which may influence the entire electrode's electrocatalytic activity. Additionally, the low durability of polymeric binders may also cause the detachment of coating that is especially noticeable at higher current densities and vigorous O 2 evolution [23]. Another frequently used method for electrode preparation is to anchor metal oxides on a carbon substrate. In this case, to assure sufficient functional groups, the carbon substrate is treated with harsh oxidative agents, which can deteriorate the electrical conductivity of the substrate and cause environmental pollution [24]. To improve the performance of these materials, various methods of their modifications have been reported, e.g., by creating nanoporous structures, synergistic metal-metal interactions, or embedding nanoparticles of catalysts in carbon nanomaterials [24][25][26]. However, there is still a need to design new MFe 2 O 4 electrocatalysts with a very porous surface, high electrical conductivity, and durability. This work has been carried out to study the electrocatalytic activity of the CoFe 2 O 4 coatings during oxygen evolution reaction. Two different methods of electrode preparation have been used. In the first method, a coating consisting of a combination of CoFe 2 O 4 powder and PEG 4000 (as a binder) was deposited by sedimentation on a metallic substrate. Subsequently, such prepared coating was subjected to heat treatment at 800 • C in an inert atmosphere. The second method used was the electrolytic co-deposition of nickel and CoFe 2 O 4 powder. Electrodeposition seems to be an interesting alternative in coatings production that contain mixed metal oxides powders and metallic binders. Additionally, a nickel substrate subjected to the analogous treatment like CoFe 2 O 4 coatings was also investigated.

CoFe 2 O 4 Powder and Coating Characterization
The phase composition of the as-obtained and annealed (800 • C, 1 h, air) CoFe 2 O 4 powder is shown in Figure 1. In both cases, X-ray diffraction patterns show the presence of the reflexes coming from the single-phase spinel cobalt ferrite with cubic crystal structure (space group: Fd − 3m, lattice constant: a = 8.4101 Å, reference code: 04-009-8438 ICDD PDF4+ 2015). Note that XRD lines obtained for as-obtained CoFe 2 O 4 particles are broadened due to some lattice strain [27]. The values of crystallite size calculated using the Williamson-Hall method [28] for as-obtained and annealed CoFe 2 O 4 powders were 10(1) nm and 15(1) nm, respectively. Thus, one can state that CoFe 2 O 4 powders are nanocrystalline phases and a 50% increase in the crystallite size is observed after annealing. Figure 2 shows the SEM images of the examined samples, i.e., CFO PEG , CFO Ni , and CFO a-r Ni , and Ni substrate. One can see that the images obtained for CFO PEG and CFO Ni electrodes represent a relatively homogenous distribution of cobalt ferrite particles, while the image obtained for CFO a-r Ni shows some agglomerates of CoFe 2 O 4 particles separated from each other. Thus, heat treatment of CoFe 2 O 4 powder (800 • C, 1 h, air) influences the shape of the electrode surface. Note that CoFe 2 O 4 particles, independent of fabrication procedure applied, distinctly enlarge the surface roughness of the electrode in comparison with nickel substrate. The increased surface area should increase the electrocatalytic activity of the CoFe 2 O 4 coatings towards oxygen evolution.      Figure 2 shows the SEM images of the examined samples, i.e., CFOPEG, CFONi, and CFO Ni a-r , and Ni substrate. One can see that the images obtained for CFOPEG and CFONi electrodes represent a relatively homogenous distribution of cobalt ferrite particles, while the image obtained for CFO Ni a-r shows some agglomerates of CoFe2O4 particles separated from each other. Thus, heat treatment of CoFe2O4 powder (800 °C, 1 h, air) influences the shape of the electrode surface. Note that CoFe2O4 particles, independent of fabrication procedure applied, distinctly enlarge the surface roughness of the electrode in comparison with nickel substrate. The increased surface area should increase the electrocatalytic activity of the CoFe2O4 coatings towards oxygen evolution.

Electrochemical Characterisation of CoFe 2 O 4 Coatings
Investigations of the electrolytic oxygen evolution on the Ni and CoFe 2 O 4 electrodes were carried out using dc and ac methods. In Figure 3a, quasi-stationary polarisation curves j vs. E are shown. It can be observed that the CoFe 2 O 4 electrodes are more efficient than the Ni electrode. Among the CoFe 2 O 4 electrodes, the lowest and the highest performance is observed for CFO a-r Ni and CFO Ni electrodes, respectively. For the linear part of polarisation curves, the Tafel equation η = a + blog|j| can be used to determine characteristic parameters of the electrode-electrolyte system. The parameters provide information about the mechanism (Tafel slope b) and the rate (apparent exchange current density j 0 ) of OER. It can be seen from Figure 3b that all investigated electrodes show a well-defined Tafel region with a slope changing from 35 to 56 mVdec −1 (see also Table 1). then the Tafel slope b = 2.3RT/βF. However, if the reaction (3) is rds, then b = 2.3RT/(1 + β)F [29]. Assuming transfer coefficient β = 0.5 and temperature 25 °C, the theoretical Tafel slope for the reaction (1) is equal to 120 mVdec −1 and for the reaction (3) 40 mVdec −1 . The Tafel slopes b obtained for all investigated CoFe2O4 coatings indicate that the OER proceeds via the same mechanism, with the third reaction step as the rate-determining step. The experimental b values higher than 40 mVdec −1 indicate that the transfer coefficient is lower than 0.5. According to literature reports, it can be caused by anion adsorption and the nonuniform distribution of the surface electric field on the rough electrodes [29].
Catalysts 2021, 11, x FOR PEER REVIEW The parameter j0 obtained for CFONi and CFO Ni a-r is ca. one order of ma in comparison with nickel electrode and ca. two orders of magnitude h CFOPEG. Such behaviour could be explained by the higher electrical condu (σ ~ 10 6 Ω −1 cm −1 ) in comparison with cobalt ferrite particles (σ ~ 10 −7 Ω −1 cm fore, for CFONi and CFO Ni a-r electrodes, the electrolytically deposited nicke ticle binder causes an increase in the apparent exchange current density of trode. The apparent activity of different electrodes could be compared usin η determined at selected current density (in this work, j = 10 mAcm −2 ). Fo trodes, the parameter η10 is from 80 mV to 150 mV lower in comparison substrate (see Table 1). Thus, one can state that CoFe2O4 electrodes charact parent activity towards OER with respect to Ni electrode. It was also fou parent activity of CFONi electrode modified by nickel binder is the highes vestigated electrodes. For comparison, different kinds of CoFe2O4 electro the literature for OER in alkaline media (1 M KOH or 1 M NaOH) are gath All these data confirm that the CFOPEG, CFONi, and CFO Ni a-r electrodes reve towards OER.    The OER mechanism consists of the following steps [29]: Catalysts 2022, 12, 21

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Either (1) or (3) electrochemical reaction may be the rate-determining step (rds). When the surface coverage by OH is much smaller than one and the reaction (1) is rds, then the Tafel slope b = 2.3RT/βF. However, if the reaction (3) is rds, then b = 2.3RT/(1 + β)F [29]. Assuming transfer coefficient β = 0.5 and temperature 25 • C, the theoretical Tafel slope for the reaction (1) is equal to 120 mVdec −1 and for the reaction (3) 40 mVdec −1 . The Tafel slopes b obtained for all investigated CoFe 2 O 4 coatings indicate that the OER proceeds via the same mechanism, with the third reaction step as the rate-determining step. The experimental b values higher than 40 mVdec −1 indicate that the transfer coefficient is lower than 0.5. According to literature reports, it can be caused by anion adsorption and the nonuniform distribution of the surface electric field on the rough electrodes [29].
The parameter j 0 obtained for CFO Ni and CFO a-r Ni is ca. one order of magnitude higher in comparison with nickel electrode and ca. two orders of magnitude higher than for CFO PEG . Such behaviour could be explained by the higher electrical conductivity of nickel (σ~10 6 Ω −1 cm −1 ) in comparison with cobalt ferrite particles (σ~10 −7 Ω −1 cm −1 ) [30]. Therefore, for CFO Ni and CFO a-r Ni electrodes, the electrolytically deposited nickel used as a particle binder causes an increase in the apparent exchange current density of the whole electrode.
The apparent activity of different electrodes could be compared using overpotential η determined at selected current density (in this work, j = 10 mAcm −2 ). For CoFe 2 O 4 electrodes, the parameter η 10 is from 80 mV to 150 mV lower in comparison with the nickel substrate (see Table 1). Thus, one can state that CoFe 2 O 4 electrodes characterise higher apparent activity towards OER with respect to Ni electrode. It was also found that the apparent activity of CFO Ni electrode modified by nickel binder is the highest among all investigated electrodes. For comparison, different kinds of CoFe 2 O 4 electrodes reported in the literature for OER in alkaline media (1 M KOH or 1 M NaOH) are gathered in Table 2. All these data confirm that the CFO PEG , CFO Ni , and CFO a-r Ni electrodes reveal high activity towards OER. In practical terms, an important criterion is the long-term stability of the catalysts. The stability of the cobalt ferrite electrodes in 1 M KOH solution was tested using the chronopotentiometry method at the constant current density of 50 mAcm −2 (see Figure 3c). Percentage change of the measured potential determined for time period from 1 to 28 h is 5% for Ni, 3% for CFO PEG , 1% for CFO Ni and 7% for CFO a-r Ni . Thus, it can be stated that all investigated electrodes are relatively stable in an alkaline environment. Figure 4 shows the SEM images and corresponding maps of Ni, Fe, Co elements distribution for the CFO PEG and CFO Ni electrodes after 28 h of oxygen evolution. Comparison with images obtained before OER (Figure 2) indicates that the surface of CFO PEG and CFO Ni electrodes remained almost unchanged. Thus, SEM images confirm the conclusion obtained from electrochemical measurements. The distribution maps of elements indicate that the electrode surface consists of the porous coating made of CoFe 2 O 4 particles (or their agglomerates) and nickel substrate in the case of CFO PEG or nickel binder in the case of CFO Ni .
the SEM images and corresponding maps of Ni, Fe, Co elements distribution for the CFOPEG and CFONi electrodes after 28 h of oxygen evolution. Comparison with images obtained before OER (Figure 2) indicates that the surface of CFOPEG and CFONi electrodes remained almost unchanged. Thus, SEM images confirm the conclusion obtained from electrochemical measurements. The distribution maps of elements indicate that the electrode surface consists of the porous coating made of CoFe2O4 particles (or their agglomerates) and nickel substrate in the case of CFOPEG or nickel binder in the case of CFONi. Impedance spectra measured for selected potentials are shown in Figure 5. For all investigated electrodes, two semicircles on Nyquist plots are observed. There is a small potential-independent semicircle in the high-frequency region, and a potential-dependent semicircle in the low-frequency region. Such a shape of spectra is typical for porous electrodes. The potential-independent semicircle can be related to the geometry of the electrode surface, whereas the potential-dependent semicircle corresponds to the Faradaic reaction [31][32][33]. Thus, a model with two-time constants can adequately describe the response of porous CoFeO4 electrodes during OER. Impedance spectra measured for selected potentials are shown in Figure 5. For all investigated electrodes, two semicircles on Nyquist plots are observed. There is a small potential-independent semicircle in the high-frequency region, and a potential-dependent semicircle in the low-frequency region. Such a shape of spectra is typical for porous electrodes. The potential-independent semicircle can be related to the geometry of the electrode surface, whereas the potential-dependent semicircle corresponds to the Faradaic reaction [31][32][33]. Thus, a model with two-time constants can adequately describe the response of porous CoFeO 4 electrodes during OER. An electrical equivalent circuit used to describe the investigated process is shown in Figure 6. It contains solution resistance (Rs), coating capacitance (CPEl.), coating resistance (Rl), charge transfer resistance (Rct), and double layer capacitance (CPEdl). Note that the deviation of solid electrodes from purely capacitive behaviour (caused by physical nonu- deviation of solid electrodes from purely capacitive behaviour (caused by physical nonuniformity or uneven distribution of active sites) has been taken into account using CPE elements instead of capacitors. The following equation describes the impedance of the CPE element: ZCPE = 1/T(jω) ϕ , where T is the capacity parameter and ϕ is the dispersion parameter related to the depression angle [31][32][33]. An electrical equivalent circuit used to describe the investigated process is shown in Figure 6. It contains solution resistance (R s ), coating capacitance (CPE l. ), coating resistance (R l ), charge transfer resistance (R ct ), and double layer capacitance (CPE dl ). Note that the deviation of solid electrodes from purely capacitive behaviour (caused by physical nonuniformity or uneven distribution of active sites) has been taken into account using CPE elements instead of capacitors. The following equation describes the impedance of the CPE element: Z CPE = 1/T(jω) φ , where T is the capacity parameter and φ is the dispersion parameter related to the depression angle [31][32][33].
For all obtained electrodes, the value of charge transfer resistance decreases with increasing overpotential, which corresponds to the decreasing diameter of the low-frequency semicircle (see Figure 5). Furthermore, comparing R ct with the R l , it was found that R ct >> R l . Thus, R ct governs the electrode kinetic. The relation η vs. log(R ct −1 ) for all investigated electrodes is shown in Figure 7. It was found that R ct values obtained for CoFe 2 O 4 at lower overpotentials can be over 100 times lower than that obtained for Ni electrode and in both cases gradually decreases with increasing overpotential. Additionally, η vs. log(R ct −1 ) curves show well-defined linear regions with slopes from 40 to 48 mVdec −1 . Note that the obtained slopes correlate with Tafel coefficients obtained from η vs. log(j) curves. For all obtained electrodes, the value of charge transfer resistance decreases with increasing overpotential, which corresponds to the decreasing diameter of the low-frequency semicircle (see Figure 5). Furthermore, comparing Rct with the Rl, it was found that Rct >> Rl. Thus, Rct governs the electrode kinetic. The relation η vs. log(Rct −1 ) for all investigated electrodes is shown in Figure 7. It was found that Rct values obtained for CoFe2O4 at lower overpotentials can be over 100 times lower than that obtained for Ni electrode and in both cases gradually decreases with increasing overpotential. Additionally, η vs. log(Rct −1 ) curves show well-defined linear regions with slopes from 40 to 48 mVdec −1 . Note that the obtained slopes correlate with Tafel coefficients obtained from η vs. log(j) curves. The double layer capacitance Cdl can be determined using the equation: Cdl = [T/((Rs + Rl) −1 + Rct −1 ) (1-ϕ) ] 1/ϕ [32]. The relation between Cdl and η for all obtained coatings is shown in Figure 8. It was found that the smallest value of Cdl was obtained for the nickel electrode, as should be expected. CFO and CFO coatings exhibit, respectively, ca. 200 times and ca. 150 times higher Cdl (average value) in comparison with nickel electrode. The double layer capacitance C dl can be determined using the equation: [32]. The relation between C dl and η for all obtained coatings is shown in Figure 8. It was found that the smallest value of C dl was obtained for the nickel electrode, as should be expected. CFO Ni and CFO PEG coatings exhibit, respectively, ca. 200 times and ca. 150 times higher C dl (average value) in comparison with nickel electrode. Note that double layer capacitance is directly proportional to the electrochemically accessible surface area. Assuming that the double layer capacitance of the smooth oxide surface is 60 µFcm −2 [29,33], the roughness factor Rf can be calculated as Rf = Cdl/60 µFcm −2 . The intrinsic exchange current density (calculated as j0/Rf) indicates that the superior catalytic properties of CFOPEG electrode are mainly the result of the large electrochemically accessible surface area. In turn, the excellent CFONi electrode properties result from both high intrinsic catalytic activity and large electrochemically accessible surface area. Note that the most useful case for the electrocatalysis is when the whole area of an electrode is accessible to reactants. As can be seen in Figure 8, the double layer capacitance obtained for all electrodes decreases with increasing overpotential. This may result from the fact that oxygen bubbles partially block the electrode surface. However, it should be emphasized that even at high overpotentials, CFOPEG and CFONi electrodes are characterized by much higher values of Cdl compared to the nickel substrate.

Cobalt Ferrite Powder Synthesis
The CoFe2O4 powder was synthesised using the coprecipitation method [27] and reagents of analytical purity (POCh, Gliwice, Poland). Hence, 0.6 moldm −3 of FeCl3·6H2O and 0.3 moldm −3 of CoCl2·6H2O solutions were mixed in ultrapure water with a resistivity of 18.2 MΩcm. Precipitating agent, i.e., NaOH (2 moldm −3 ), was slowly added to the stirred chlorides solution. Coprecipitation was conducted in a Teflon vessel at a temperature of 50 °C for two hours. Subsequently, the synthesised powder was rinsed in distilled water, centrifuged for five minutes at 300 rpm, and dried at 50 °C for 48 h. The powder was also ground in an agate mortar to break agglomerates into smaller particles. In further study, the as-received and post-thermal treatment (800 °C, 1 h, air) CoFe2O4 powder was used. The phase composition of CoFe2O4 powder was determined by X-ray diffraction technique (XRD) using a Philips X'Pert PW 3040/60 diffractometer (PANalytical, Almelo, Netherlands) equipped with CuKα radiation. The ICDD cards were used for phase identification.

Preparation of Cobalt Ferrite Coatings
Cobalt ferrite coatings were deposited on a nickel plate (Nickel 201, Ni ≥ 99.0%). Before the deposition process, the nickel plate with a working area of 0.5 cm 2 was mechanically polished using abrasive papers (P320, P600, P1000), and subsequently rinsed in acetone in an ultrasonic bath for 10 min. The studied coatings were produced by applying Note that double layer capacitance is directly proportional to the electrochemically accessible surface area. Assuming that the double layer capacitance of the smooth oxide surface is 60 µFcm −2 [29,33], the roughness factor R f can be calculated as R f = C dl /60 µFcm −2 . The intrinsic exchange current density (calculated as j 0 /R f ) indicates that the superior catalytic properties of CFO PEG electrode are mainly the result of the large electrochemically accessible surface area. In turn, the excellent CFO Ni electrode properties result from both high intrinsic catalytic activity and large electrochemically accessible surface area. Note that the most useful case for the electrocatalysis is when the whole area of an electrode is accessible to reactants. As can be seen in Figure 8, the double layer capacitance obtained for all electrodes decreases with increasing overpotential. This may result from the fact that oxygen bubbles partially block the electrode surface. However, it should be emphasized that even at high overpotentials, CFO PEG and CFO Ni electrodes are characterized by much higher values of C dl compared to the nickel substrate.

Cobalt Ferrite Powder Synthesis
The CoFe 2 O 4 powder was synthesised using the coprecipitation method [27] and reagents of analytical purity (POCh, Gliwice, Poland). Hence, 0.6 moldm −3 of FeCl 3 ·6H 2 O and 0.3 moldm −3 of CoCl 2 ·6H 2 O solutions were mixed in ultrapure water with a resistivity of 18.2 MΩcm. Precipitating agent, i.e., NaOH (2 moldm −3 ), was slowly added to the stirred chlorides solution. Coprecipitation was conducted in a Teflon vessel at a temperature of 50 • C for two hours. Subsequently, the synthesised powder was rinsed in distilled water, centrifuged for five minutes at 300 rpm, and dried at 50 • C for 48 h. The powder was also ground in an agate mortar to break agglomerates into smaller particles. In further study, the as-received and post-thermal treatment (800 • C, 1 h, air) CoFe 2 O 4 powder was used. The phase composition of CoFe 2 O 4 powder was determined by X-ray diffraction technique (XRD) using a Philips X'Pert PW 3040/60 diffractometer (PANalytical, Almelo, Netherlands) equipped with Cu Kα radiation. The ICDD cards were used for phase identification.

Preparation of Cobalt Ferrite Coatings
Cobalt ferrite coatings were deposited on a nickel plate (Nickel 201, Ni ≥ 99.0%). Before the deposition process, the nickel plate with a working area of 0.5 cm 2 was mechanically polished using abrasive papers (P320, P600, P1000), and subsequently rinsed in acetone in an ultrasonic bath for 10 min. The studied coatings were produced by applying two different procedures. In the first case, a mixture of the thermally treated CoFe 2 O 4 powder (20 mg per 10 mL of solution) and polyethylene glycol 4000 (PEG 4000) (20 mg per 10 mL of solution) in acetone was prepared. Additionally, 5 mg of sodium dodecyl sulfate (SDS) was added as a surfactant to the mixture. To obtain a homogeneous colloid, the prepared mixture was placed in an ultrasonic bath for 30 min. Subsequently, the nickel substrate was immersed in the mixture. The coating was deposited by sedimentation on a metallic substrate until all the solvent had evaporated. The obtained coating was subjected to heat treatment at 800 • C for 1 h in an argon atmosphere. In this paper, coating obtained in that way will be referred to as CFO PEG . In the second case, to obtain CoFe 2 O 4 coating with a metallic binder, electrochemical deposition was used. Nickel was deposited from the bath with the following composition (concentrations in gdm −3 ): NiSO 4 ·7H 2 O-84, NiCl 2 ·6H 2 O-10, H 3 BO 3 -8, to which 20 mg of cobalt ferrite powder and 5 mg of SDS was added. Note that nickel electrodeposition started when CoFe 2 O 4 powder settled by sedimentation on the surface of the metallic substrate. Nickel binder was deposited under galvanostatic conditions at current density j = 10 mAcm −2 and time 20 min. The deposition process was carried out at the temperature 20 • C. The Faradaic yield of nickel deposition was ca. 70%. In this paper, coatings obtained in that way will be referred to as CFO Ni (in the case of thermally treated CoFe 2 O 4 powder) and CFO a-r Ni (in the case of as-received CoFe 2 O 4 powder). Using a JEOL JSM-6480 (JEOL Ltd., Tokyo, Japan) scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) attachment, the surface morphology and chemical composition of the obtained CoFe 2 O 4 coatings were determined.

Electrochemical Measurements
Electrochemical tests were performed in 1 M KOH solution using a three-electrode cell with saturated calomel electrode (SCE) as a reference electrode and platinum mesh as a counter electrode. Working electrodes were CoFe 2 O 4 coatings with a geometric surface area of 0.5 cm 2 . The ohmic drop between the reference and working electrode was reduced using a Luggin capillary. All electrochemical experiments were carried out at a temperature of 20 • C. For the data registration, a PARSTAT 2273 system and the PowerSuite 2.58 software (Princeton Applied Research, Oak Ridge, TN, USA) were used. Values of measured potentials were converted from SCE to reversible hydrogen electrode (RHE) according to the following formula: E RHE = E SCE + 0.059pH + 0.241. The overpotential (η) for the oxygen evolution reaction was calculated using the equation: η = E RHE − 1.23.
Quasi-stationary polarisation curves j vs. E were recorded using the linear sweep voltammetry (LSV) technique within the potential range from 1.22 V to 1.85 V vs. RHE and sweep rate v = 10 mVmin −1 . Before measurements, CoFe 2 O 4 electrodes were conditioned at an anodic potential of 1.85 V for 5 h. The ohmic drop compensation was conducted during measurements using the current interrupt technique [4].
The spectra were registered using the electrochemical impedance spectroscopy (EIS) method. The impedance spectra were recorded potentiostatically at selected dc potentials from the interval where the OER takes place. Before the recording of each spectrum, electrodes were held at the appropriate potential for 5 min. Spectra were registered in the frequency range from 20 kHz to 10 mHz with a density of 10 points per decade. The amplitude of the ac signal was 10 mV rms . For the quantitative analysis of obtained data, the ZSimpWin 3.21 software was used.
To evaluate long-term stability of the CoFe 2 O 4 electrodes, the chronopotentiometry technique was used. The test was carried out at the current density j = 50 mAcm −2 for 28 h.

Concluding Remarks
This paper reports on two new procedures for the preparation of porous cobalt ferrite coatings with a metallic and organic binder for use as catalysts in the oxygen evolution reaction (OER). The parameter η 10 indicates that all investigated CoFe 2 O 4 coatings exhibit significantly higher apparent activity towards OER than nickel substrate in 1 M KOH. In particular, CoFe 2 O 4 coating with PEG applied as a binder shows overpotential η 10 = 295 mV at a current density of 10 mAcm −2 and Tafel slope b = 35 mVdec −1 . CoFe 2 O 4 coating with Ni binder (η 10 = 287 mV and b = 45 mVdec −1 ) shows comparable catalytic activity towards OER. It was stated that the main reason for the superior catalytic activity of CFO PEG coating is the large electrochemically active surface area. In contrast, the excellent catalytic activity of CoFe 2 O 4 coating containing nickel binder between the ferrite particles is caused by increasing both the conductivity and the electrochemically active surface area. It was also shown that the obtained CoFe 2 O 4 electrodes maintain their catalytic activity for at least 28 h at a current density of 50 mAcm −2 . It can be stated that this work offers a new path for the design of high-performance OER electrocatalysts using both sintering and electrodeposition techniques.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.