Improvement of the Pseudocapacitive Performance of Cobalt Oxide-Based Electrodes for Electrochemical Capacitors

: Cobalt oxide nanopowders are synthesized by the pyrolysis of aerosol particles of water solution of cobalt acetate. Cobalt nanopowder is obtained by subsequent reduction of obtained cobalt oxide by annealing under a hydrogen atmosphere. The average crystallite size of the synthesized porous particles ranged from 7 to 30 nm, depending on the synthesis temperature. The electrochemical characteristics of electrodes based on synthesized cobalt oxide and reduced cobalt oxide are investigated in an electrochemical cell using a 3.5 M KOH solution as the electrolyte. The results of electrochemical measurements show that the electrode based on reduced cobalt oxide (Re-Co 3 O 4 ) exhibits signiﬁcantly higher capacity, and lower Faradaic charge–transfer and ion di ﬀ usion resistances when compared to the electrodes based on the initial cobalt oxide Co 3 O 4 . This observed e ﬀ ect is mainly due to a wide range of reversible redox transitions such as Co(II) ↔ Co(III) and Co(III) ↔ Co(IV) associated with di ﬀ erent cobalt oxide / hydroxide species formed on the surface of metal particles during the cell operation; the small thickness of the oxide / hydroxide layer providing a high reaction rate, and also the presence of a metal skeleton leading to a low series resistance of the electrode.


Introduction
Energy in the form of electricity generated from renewable resources such as wind, tidal and solar energy will contribute prominently and play an important role in our future energy demands by: (i) offering enormous potential for powering our future systems to overcome the concerns associated with the future energy shortage due to the anticipated doubling of world energy consumption within the next 50 years [1] and (ii) the use of low-or even zero-emission energy sources to tackle the fundamental problems associated with the use of fossil fuels as the world strives to reduce greenhouse gas emissions. However the success and practicality of the employment of renewable energies as clean and efficient energy supplies in powering our future energy demands depend on the development and value due to its slow electron kinetics, fast capacity decay and impaired morphologies during its redox electrochemical reactions [51].
Thus, numerous efforts are being made to increase the electrochemical activity of electrode materials based on metal oxides [52,53], in particular, on the basis of cobalt oxide as a viable electrode material for supercapacitors [44][45][46][47][48][49][53][54][55]. In this article we demonstrate a new effect of enhancing the electrochemical activity of the Co 3 O 4 electrode as a result of hydrogen reduction of the oxide to metal and the subsequent formation of core-shell structure by forming a thin oxide/hydroxide layer obtained by the in situ electrochemical oxidation of the cobalt nanoparticles surface. This new and simple method of enhancing electrochemical activity is mainly related to the redox activity of electroactive material.

Experiment
Cobalt oxide nanopowders were prepared by the pyrolysis of aerosol of aqueous solutions of 0.06 M and 0.12 M cobalt acetate tetrahydrate Co(CH 3 COO) 2 ·4H 2 O (AcCo). The aerosol was obtained using a piezo ultrasonic atomizer. The mist of aerosol droplets was transferred by air stream into a tubular furnace heated to a temperature between 300 and 550 • C at a flow rate of about 1 L min −1 . The cobalt oxide particles formed in the hot zone of the furnace were carried by the gas stream into an electrostatic filter. The temperature of the filter was maintained at~200 • C to prevent water vapor condensing on the walls of the electrostatic filter. The synthesized powder trapped by the filter was collected for further investigation.
Samples of metal particles of cobalt were obtained by thermal hydrogen reduction of the cobalt oxide synthesized using 0.06 M AcCo solution. The reduction was carried out in a quartz furnace at a temperature in the range of 200-300 • C for between 1 and 2 h, and the samples then were cooled down to the room temperature. The entire process was performed under hydrogen flowing at 60 mL min −1 .
X-ray diffraction (XRD) analysis was carried out by a MiniFlex Rigaku diffractometer with current and voltage kept at 15 mA and 40 kV, respectively, using Cu Kα radiation to study the crystalline structure of the samples and identify their crystalline phases and their chemical composition. Raman spectra were taken by a Ntegra Spectra (NT-MDT) spectrometer with a 473-nm solid-state exciting laser in the range of 100-2600 cm −1 to investigate the chemical composition and crystallinity of the cobalt oxide samples synthesized at different temperatures. The surface morphology and microstructure of the samples were studied using a Quanta 3D 200i FEI scanning electron microscope (SEM). All electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed using an Elins P-40X potentiostat in conjunction with an FRA-24M electrochemical impedance measurement module in a conventional three-electrode electrochemical cell. A platinum electrode was used as the counter electrode along with Ag/AgCl electrode as the reference electrode. An electrode using a sample of cobalt oxide or reduced cobalt oxide as the electroactive material was used as the working electrode where a 3.5 M KOH solution was used as the electrolyte. CV measurements were used as the most appropriate method for characterizing redox processes during the cell operation. CV measurements were performed in the potential range of ±0.5 V at a scan rate of 50 mV s −1 . EIS measurements were performed at AC amplitude of 100 mV in the frequency range of 1-100 KHz. All cell measurements were performed at room temperature.
In order to prepare the working electrodes, 0.05 g of a powdered sample was added to 3 mL of ethanol and processed in an ultrasonic bath for almost 30 min until a homogeneous suspension was obtained, which was then deposited on a 5 mm × 12 mm nickel foam substrate. It should be noted that the initial cobalt oxide powder had very good wettability; as a result, a stable suspension was formed while the reduced cobalt oxide powders clumped together significantly and the suspension of the reduced powders in ethanol was unstable and quickly precipitated.
The powdered sample on the nickel foam substrate was dried and pressed under a pressure of~10 8 Pa. Subsequent treatment of the electrode in ethanol in an ultrasonic bath for 10 min was Energies 2020, 13, 5228 4 of 16 performed in order to remove any weakly attached particles from the electrode. The sample mass was determined as a difference between the mass of the manufactured electrode and the initial mass of the substrate. Figure 1 shows the XRD patterns of the samples obtained by the pyrolysis of cobalt acetate aerosol at the synthesis temperatures of T syn = 300 • C and 550 • C. Only XRD peaks of Co 3 O 4 (JCPDS card No-00-043-1003) were detected as shown in Figure 1. The average crystallite size estimated from the XRD patterns using a MiniFlex Rigaku diffractometer processing program based on the Williamson-Hall method [56] was 25 Å for the sample synthesized at 300 • C and 97 Å for the sample synthesized at 550 • C.

Results and Discussion
Energies 2020, 13, x FOR PEER REVIEW 4 of 16 Figure 1 shows the XRD patterns of the samples obtained by the pyrolysis of cobalt acetate aerosol at the synthesis temperatures of Tsyn = 300 °C and 550 °C. Only XRD peaks of Co3O4 (JCPDS card No-00-043-1003) were detected as shown in Figure 1. The average crystallite size estimated from the XRD patterns using a MiniFlex Rigaku diffractometer processing program based on the Williamson-Hall method [56] was 25 Å for the sample synthesized at 300 °C and 97 Å for the sample synthesized at 550 °C. The obtained cobalt oxide powder was then reduced by annealing in hydrogen atmosphere. XRD studies showed ( Figure 1) that annealing at temperatures as low as 250-300 °C results in the complete disappearance of the Co3O4 phase and the appearance of metallic cobalt phase, consisting of a mixture of cubic and hexagonal cobalt (PDF cart #00-015-0806 and #01-089-7094, respectively).

Results and Discussion
The SEM images of the cobalt oxide nanoparticles synthesized by the pyrolysis of AcCo solutions with different concentrations given in Figure 2a,b show that the samples form almost round islands with little coalescence between them with clean surfaces. The size of Co3O4 nanoparticles decreases with a decrease in the concentration of AcCo solution leading to a more compacted structure with strong competition for space at the grain boundaries due to the accommodation of smaller particles in the vicinity of each other. The SEM micrographs of Co3O4 nanoparticles synthesized at different synthesis temperatures are given in Figure 2c,d and as seen the size of nanoparticles is weakly dependent on the synthesis temperature Tsyn. The obtained cobalt oxide powder was then reduced by annealing in hydrogen atmosphere. XRD studies showed ( Figure 1) that annealing at temperatures as low as 250-300 • C results in the complete disappearance of the Co 3 O 4 phase and the appearance of metallic cobalt phase, consisting of a mixture of cubic and hexagonal cobalt (PDF cart #00-015-0806 and #01-089-7094, respectively).
The SEM images of the cobalt oxide nanoparticles synthesized by the pyrolysis of AcCo solutions with different concentrations given in Figure 2a,b show that the samples form almost round islands with little coalescence between them with clean surfaces. The size of Co 3 O 4 nanoparticles decreases with a decrease in the concentration of AcCo solution leading to a more compacted structure with strong competition for space at the grain boundaries due to the accommodation of smaller particles in the  Figure 2e,f show the morphology of the Re-Co 3 O 4 samples synthesized at 350 • C and then reduced by annealing in hydrogen atmosphere for 1 h at 250 • C and 300 • C, respectively. The SEM images of the annealed samples show amorphous-like clusters where the size of clusters increases with increasing the annealing temperature. According to XRD data, these samples consist of metallic cobalt. An enlargement of particle size and their sticking take place, and as seen the sintering process accelerates with increasing the reduction temperature. This clearly leads to a decrease in the surface area of the annealed sample. On the study of the cobalt oxide and its activity in the reduced form used as catalyst for the preparation of synthetic liquid fuels from hydrogenation of carbon monoxide, Shults et al. have shown that surface area of the cobalt oxide decreases 20-fold (from 67 to 3.2 m 2 g −1 ) upon hydrogen reduction mainly due to the sintering, shrinkage and reorientation of the reduced cobalt [57].
The Raman spectra of the Co 3 O 4 samples synthesized at different temperatures are shown in Figure 3. The detected Raman bands at 193.3, 474.6, 518.9, 611.7, and 679.8 cm −1 belong to the characteristics Raman-active 3F 2g , E g , 2F 2g , 1F 2g , and A 1g , respectively, which are related to the vibration modes of cobalt oxide [58]. The normal (unshifted) Raman bands of Co 3 O 4 at room temperature appear at 197, 485, 523, 624, and 693 cm −1 [59]. The main Raman bands of Co 3 O 4 samples obtained at 300-550 • C exhibit a red shift up to 30 cm -1 and more relative to the normal Raman bands of Co 3 O 4 prepared at a temperature range of 250-500 • C given in Table 1. The noticeable red shift of the Raman bands observed in all samples is typical for both single and agglomerated cobalt oxide nanoparticles and is caused by high laser power [60]. Thus, lowering the synthesis temperature increases the Raman band width and red shift, which indicates a decrease in crystallite size. This is consistent with the XRD results.
Energies 2020, 13, x FOR PEER REVIEW 6 of 16 area of the annealed sample. On the study of the cobalt oxide and its activity in the reduced form used as catalyst for the preparation of synthetic liquid fuels from hydrogenation of carbon monoxide, Shults et al. have shown that surface area of the cobalt oxide decreases 20-fold (from 67 to 3.2 m 2 g −1 ) upon hydrogen reduction mainly due to the sintering, shrinkage and reorientation of the reduced cobalt [57]. The Raman spectra of the Co3O4 samples synthesized at different temperatures are shown in Figure 3. The detected Raman bands at 193.3, 474.6, 518.9, 611.7, and 679.8 cm −1 belong to the characteristics Raman-active 3F2g, Eg, 2F2g, 1F2g, and A1g, respectively, which are related to the vibration modes of cobalt oxide [58]. The normal (unshifted) Raman bands of Co3O4 at room temperature appear at 197, 485, 523, 624, and 693 cm −1 [59]. The main Raman bands of Co3O4 samples obtained at 300-550 °C exhibit a red shift up to 30 cm -1 and more relative to the normal Raman bands of Co3O4 prepared at a temperature range of 250-500 °C given in Table 1. The noticeable red shift of the Raman bands observed in all samples is typical for both single and agglomerated cobalt oxide nanoparticles and is caused by high laser power [60]. Thus, lowering the synthesis temperature increases the Raman band width and red shift, which indicates a decrease in crystallite size. This is consistent with the XRD results.  The electrochemical performance of the electrodes prepared from the synthesized Co3O4 and reduced Co3O4 (Re-Co3O4) samples was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) measurements using the electrodes as the working electrode in a conventional three-electrode electrochemical cell with a 3.5 M KOH solution used as the electrolyte. Typical CV curves at a scan rate of 50 mV s −1 for the C3O4 electrode (curve 1) and the Re-C3O4 electrode (curve 2) are presented in Figure 4. The Co3O4 sample demonstrates two anodic peaks (A1 and A2) in the potential range of 0.2-0.4 V and the corresponding cathodic peaks denoted as C1 and C2. Anodic peaks A1 and A2 and corresponding cathodic peaks  The electrochemical performance of the electrodes prepared from the synthesized Co 3 O 4 and reduced Co 3 O 4 (Re-Co 3 O 4 ) samples was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) measurements using the electrodes as the working electrode in a conventional three-electrode electrochemical cell with a 3.5 M KOH solution used as the electrolyte. Typical CV curves at a scan rate of 50 mV s −1 for the C 3 O 4 electrode (curve 1) and the Re-C 3 O 4 electrode (curve 2) are presented in Figure 4. The Co 3 O 4 sample demonstrates two anodic peaks (A1 and A2) in the potential range of 0.2-0.4 V and the corresponding cathodic peaks denoted as C1 and C2. Anodic peaks A1 and A2 and corresponding cathodic peaks C1 and C2 can be attributed to the Co 2+ → Co 3+ and Co 3+ → Co 4+ reversible redox transitions, respectively. The two redox couples observed in the CV curves can be attributed to the following redox reactions according to the CV curves of the C 3 O 4 electrode in an alkaline solution reported in Samal et al. and Premlatha et al. works [61,62]: The first reaction is Co 2+ ↔ Co 3+ transformation (redox pair A1 and C1) that involves a reversible intercalation of OH − into the reduced form of Co 3 O 4 , forming CoOOH and leading to an improved charge storage capability of the electrode through pseudocapacitance. This is followed by a second reaction that is Co 3+ ↔ Co 4+ transformation (redox pair A2 and C2) as a result of the adsorption of OH − ions on the near-surface and the formation of CoO 2 during the reduction process. It corresponds to an irreversible non-faradaic process that still contributes to the capacitance. Samal et al. and Premlatha et al. have also observed similar redox peaks for the electrochemical activity of cobalt oxide in alkaline electrolytes [61,62]. Such redox couples are typical for the cyclic voltammetry of cobalt oxide and are in good agreement with other published works [63][64][65][66].
The first reaction is Co 2+ ↔ Co 3+ transformation (redox pair A1 and C1) that involves a reversible intercalation of OHinto the reduced form of Co3O4, forming CoOOH and leading to an improved charge storage capability of the electrode through pseudocapacitance. This is followed by a second reaction that is Co 3+ ↔ Co 4+ transformation (redox pair A2 and C2) as a result of the adsorption of OHions on the near-surface and the formation of CoO2 during the reduction process. It corresponds to an irreversible non-faradaic process that still contributes to the capacitance. Samal et al. and Premlatha et al. have also observed similar redox peaks for the electrochemical activity of cobalt oxide in alkaline electrolytes [61,62]. Such redox couples are typical for the cyclic voltammetry of cobalt oxide and are in good agreement with other published works [63][64][65][66]. Annealing of Co3O4 samples in hydrogen results in a significant change in their electrochemical properties. As can be seen in Figure 4, the Re-Co3O4 sample displays a CV curve with four anodic peaks A1-A4 and the corresponding cathodic peaks C1-C4 in the potential range of −0.25 to +0.4 V.
A number of oxidation states of cobalt in alkaline medium are stable, so different oxide/hydroxide species of Co(II) such as Co(OH)2 and CoO; Co(II,III)-Co3O4; Co(III)-Co2O3 and Co(OH)3; Co(IV)-CoO2 can be formed on metallic cobalt surface and involved in the electrochemical processes and different studies have confirmed this [67][68][69]. The formation of the oxide layer on the surface of the metal electrode is a well-known process and this process is repeatedly described for different metals in the literature [70][71][72][73]. Although all the reports on the electrochemical formation of cobalt oxide films are based on the formation of the oxide directly on a bare electrode, studies of the anodic formation of cobalt oxide in alkaline solutions suggest that the anodic oxide film is CoO and/or studies have confirmed this [67][68][69]. The formation of the oxide layer on the surface of the metal electrode is a well-known process and this process is repeatedly described for different metals in the literature [70][71][72][73]. Although all the reports on the electrochemical formation of cobalt oxide films are based on the formation of the oxide directly on a bare electrode, studies of the anodic formation of cobalt oxide in alkaline solutions suggest that the anodic oxide film is CoO and/or Co(OH) 2 at relatively low potentials and changes to Co 3 O 4 and Co 2 O 3 (or CoOOH) at relatively high potentials. The presence of CoO 2 has also been suggested in the oxygen evolution potential region [74]. Consequently, the formation of both oxide and hydroxide compounds on the metallic cobalt surface provides a wide range of redox reactions and increases the electrochemical activity of the Re-Co 3 O 4 electrode. Due to a wider extent of the redox activity of the Re-Co 3 O 4 sample leading to the large number of redox peaks as observed in CV curves, the capacity of the reduced C 3 O 4 sample is much higher than that of the initial C 3 O 4 sample. Peaks at high potential (~0.2-0.4 V) correspond to oxide spices, and peaks at low potential (from −0.2 V) correspond to hydroxide spices. Peaks at low potential have very low intensity in the initial sample since the sample consists of a thick layer of cobalt oxide, and there are few hydroxide spices. After H 2 reduction, all oxides transform into the metal, and a highly dispersed conductive electrode is formed. Then during in situ oxidation in the electrolyte, a thin layer consisting of a mixture of oxide and hydroxide grows on a metal surface, since there is a lack of oxygen and the presence of hydrogen. Therefore, additional processes of hydroxide recharging make a significant contribution and expand the range of possible redox reactions. A significant advantage is that this layer is very thin, and it is all involved in redox processes. In the study of pseudocapacitive behavior of different metal oxides for high-rate electrochemical energy storage, Augustyn et al. have also shown that thin layers exhibit more pseudo-capacitive behavior and can achieve very high specific capacitances [75].
Similar redox couples with good reversibility have been detected in the voltammetric profile of cobalt hydroxide nanosheet electrodes by Ji et al. [76], on the cyclic voltammograms of cobalt-nickel double hydroxide-based electrodes obtained through cathodic electrodeposition by Xiao et al. [77], and also on the CV curves of cobalt/nickel layered double hydroxides prepared through a facile one-step solvothermal method by Chu and his co-workers [78]. They have explained these redox peaks by the following Faradaic reactions: Co(OH) 2 CoOOH CoOOH Therefore, the observed redox couples at low potentials on the CV curves for reduced Co 3 O 4 electrode in this work shown in Figure 4 can be attributed to the Faradaic activities originated from the in situ oxidation and reduction of cobalt hydroxide and reversible transformation between different hydroxide spices, and these spices are located very close to the surface.
As seen in Figure 4 the redox couples on the CV curves of Re-Co 3 O 4 electrode shift towards lower voltages compared with those of Co 3 O 4 electrode presumably as a result of the formation of Co(OH) 2 . In their study on electrocatalytic performance evaluation of cobalt hydroxide and cobalt oxide thin films for oxygen evolution reaction, Babar et al., also found that Co(OH) 2 has much higher current density and lower over-potential than Co 3 O 4 [79]. The specific capacitance for the electrodes can be calculated using CV curves on the basis of following equation: where, C s is the specific capacitance (F g −1 ), m is the mass of the electroactive material of the electrode (g), ν is the scan rate (V s −1 ), V max − V min is the potential window (V), I is the current (A) and integration is performed over one CV cycle. The plot of C s versus scan rate for both Co 3 O 4 and Re-Co 3 O 4 electrodes is shown as the inset in Figure 4. It is known (Augustyn et al. [75]) that the peak current i in CV curves can be represented as a function of the sweep rate v as i = av b , where b = 0.5 corresponds to battery material with diffusion-controlled redox reactions, and b = 1 corresponds to surface controlled capacitive current. The data on the dependence of the A1-C1 peak current in Co 3 O 4 and A2-C2 in Re-Co 3 O 4 on the scanning speed ( Figure 5) allow us to estimate the parameter b. In both cases, the b value is in the range of 0.9-1.0, which indicates the predominance of capacitive currents at a sweep rate of at least more than 0.025 V s −1 .
Energies 2020, 13, x FOR PEER REVIEW 9 of 16 charging and discharging steps for peaks A4-C4 (Figure 5b) also increases much more slowly with increasing scanning speed than for the corresponding peaks A2-C2 in Figure 5a. This indicates that the new states in Re-Co3O4 are of a more surface-like nature. The improved capacity of the Re-Co3O4 sample compared to Co3O4 is further confirmed by galvanostatic charge-discharge measurements (GCD), as shown in Figure 6. The capacity of the electrodes is calculated by the following equation: where Q is the charge stored (expressed in mA h g −1 ), I the discharge current (A), t the discharge time (s), C the specific capacity (F g −1 ) and ΔU voltage drop. In this case the Re-Co3O4 sample exhibits a capacity of 60.2 mA h g −1 or ~509.6 F g −1 at a current density of 0.38 A g −1 while the Co3O4 sample shows a much lower capacity of 11.4 mA h g −1 or ~91.3 F g −1 at a current density of 0.24 A g −1 ( Figure  6). Although the capacity is not as high, as will be shown below, the stability of the capacity during cycling is very high.  It is also known that the surface pseudocapacitance is characterized by a small potential difference between the CV peaks of the charging and discharging steps (Augustyn et al. [75]). Figure 5 shows that the difference of the potentials between the oxidation and reduction peaks A1-C1 in Co 3 O 4 is 0.078, 0.103 and 0.125 V at scan rate of 0.025, 0.050 and 0.100 V s −1 , correspondingly. At the same time, for new oxidation-reduction peaks A2-C2 (Figure 5b) that emerge on the CV curves of Re-Co 3 O 4 , which are absent in the initial sample, the difference of the potential is 0.035, 0.061, and 0.082 V, respectively. It can also be noted that the potential difference between the CV peaks of the charging and discharging steps for peaks A4-C4 (Figure 5b) also increases much more slowly with increasing scanning speed than for the corresponding peaks A2-C2 in Figure 5a. This indicates that the new states in Re-Co 3 O 4 are of a more surface-like nature.
The improved capacity of the Re-Co 3 O 4 sample compared to Co 3 O 4 is further confirmed by galvanostatic charge-discharge measurements (GCD), as shown in Figure 6. The capacity of the electrodes is calculated by the following equation: where Q is the charge stored (expressed in mA h g −1 ), I the discharge current (A), t the discharge time (s), C the specific capacity (F g −1 ) and ∆U voltage drop. In this case the Re-Co 3 O 4 sample exhibits a capacity of 60.2 mA h g −1 or~509.6 F g −1 at a current density of 0.38 A g −1 while the Co 3 O 4 sample shows a much lower capacity of 11.4 mA h g −1 or~91.3 F g −1 at a current density of 0.24 A g −1 (Figure 6). Although the capacity is not as high, as will be shown below, the stability of the capacity during cycling is very high.
where Q is the charge stored (expressed in mA h g −1 ), I the discharge current (A), t the discharge time (s), C the specific capacity (F g −1 ) and ΔU voltage drop. In this case the Re-Co3O4 sample exhibits a capacity of 60.2 mA h g −1 or ~509.6 F g −1 at a current density of 0.38 A g −1 while the Co3O4 sample shows a much lower capacity of 11.4 mA h g −1 or ~91.3 F g −1 at a current density of 0.24 A g −1 ( Figure  6). Although the capacity is not as high, as will be shown below, the stability of the capacity during cycling is very high.  The electrochemical impedance of the samples was also measured in the frequency range of 1-10 5 Hz for further comparison of the Co3O4 and Re-Co3O4 electrodes. The Nyquist plots for both Co3O4 and Re-Co3O4 samples shown in Figure 7 consist of a high-frequency semicircle and an almost straight line in the low-frequency section. The semicircle is associated with charge-transfer resistance (Rct), and the straight line is attributed to the process of diffusion of ions from the electrolyte into the electrode. The Re-Co3O4 electrode has a smaller semicircle than the Co3O4 sample, meaning that the Re-Co3O4 electrode has a lower charge transfer resistance Rct. The intersection of the impedance spectra on the Re Z axis in the high-frequency region gives the value of equivalent series resistance (Rs) for the electrodes, which is a combination of the ionic resistance of the electrolyte, the internal resistance of the electrode material and the electrode/electrolyte interfacial contact resistance. The Rs value for the Re-Co3O4 electrode (0.90 Ohm) is almost the same as the Rs value for the Co3O4 electrode (0.87 Ohm) despite the fact that the wettability of the Co3O4 electrode is significantly higher than that of Re-Co3O4 [80,81].
In the low frequency range, the slope of the straight line corresponding to the Re-Co3O4 electrode is significantly higher than that of the Co3O4 electrode, indicating the resistance of ion diffusion in the Re-Co3O4 electrode is lower than that of Co3O4. This demonstrates that the charge storage process in The intersection of the impedance spectra on the Re Z axis in the high-frequency region gives the value of equivalent series resistance (R s ) for the electrodes, which is a combination of the ionic resistance of the electrolyte, the internal resistance of the electrode material and the electrode/electrolyte interfacial contact resistance. The R s value for the Re-Co 3 O 4 electrode (0.90 Ohm) is almost the same as the R s value for the Co 3 O 4 electrode (0.87 Ohm) despite the fact that the wettability of the Co 3 O 4 electrode is significantly higher than that of Re-Co 3 O 4 [80,81].
In the low frequency range, the slope of the straight line corresponding to the Re-Co 3 O 4 electrode is significantly higher than that of the Co 3 O 4 electrode, indicating the resistance of ion diffusion in the Re-Co 3 O 4 electrode is lower than that of Co 3 O 4 . This demonstrates that the charge storage process in the Re-Co 3 O 4 sample is more capacitive in nature than in Co 3 O 4 .
Cycling stability of the capacitor is an important parameter for its practical application. Figure 8 shows that a capacity retention of 95% was obtained after 1000 charge/discharge cycles for the Re-Co 3 O 4 electrode at a current density of 12 A g −1 . It can be seen that during the first~300 cycles, the capacity dropped by about 5%, and then remained constant indicating a good stability for the electrode with cycling.
Energies 2020, 13, x FOR PEER REVIEW 11 of 16 and the ion diffusion resistance of the Re-Co3O4 electrode compared with those in the Co3O4 electrode before reduction.   The inset shows the GCD curves at a current of 12 A g −1 during the first and last cycles.
In the experiments described above, the powder Co 3 O 4 samples were first reduced by annealing in hydrogen and then electrodes were fabricated. Additional experiments were also carried out to study the electrochemical property of a manufactured Co 3 O 4 electrode before and after hydrogen reduction. Figure 9 shows the GCD and EIS curves of the Co 3 O 4 electrode before and after annealing in hydrogen. As can be seen from Figure 9a, the capacity of the Co 3 O 4 electrode is increased about four times after hydrogen reduction. Figure 9b shows that the charge-transfer resistance R ct is decreased significantly, and the slope of straight line at low frequency region of the EIS spectrum is increased as a result of hydrogen reduction. This indicates improvement in the capacitive behavior and the ion diffusion resistance of the Re-Co 3 O 4 electrode compared with those in the Co 3 O 4 electrode before reduction.
The results of CV, EIS and GCD measurements show that reduced Co 3 O 4 provides significantly better electrochemical charge storage characteristics than the initial Co 3 O 4 sample. The greater capacity of the Re-Co 3 O 4 sample compared with the capacity of Co 3 O 4 sample can be attributed to the capacitive redox reactions associated with a thin submicron layer composed of various oxide and hydroxide spices covering the surface of metallic-Co particles obtained as a result of hydrogen reduction process. This layer is formed due to both natural oxidation and anodic oxidation during cyclic voltammetry measurements. The presence of oxide and hydroxide spices provides a wide range of redox reactions. This is evidenced by a significantly larger number of peaks on the CV curves corresponding to redox couples. The small thickness of the oxide/hydroxide layer provides a high diffusion rate of ions involved in redox reactions resulting in rapid kinetics of these reactions and low series resistance of the electrode. Therefore, applying metal particles instead of oxide particles in some cases can provide significant advantages in the manufacture of electrodes for high energy supercapacitors. Figure 8. Cycling stability of the Re-Co3O4 electrode tested at a current density of 12 A g −1 . The inset shows the GCD curves at a current of 12 A g −1 during the first and last cycles. Figure 9. The GCD curves at a current density of 2 A g −1 (a) and Nyquist plots (b) of Co3O4 electrode and the same electrode after reduction in H2 (Re-Co3O4), the inset indicates the Nyquist plots at high frequency region.
The results of CV, EIS and GCD measurements show that reduced Co3O4 provides significantly better electrochemical charge storage characteristics than the initial Co3O4 sample. The greater capacity of the Re-Co3O4 sample compared with the capacity of Co3O4 sample can be attributed to the capacitive redox reactions associated with a thin submicron layer composed of various oxide and hydroxide spices covering the surface of metallic-Co particles obtained as a result of hydrogen reduction process. This layer is formed due to both natural oxidation and anodic oxidation during cyclic voltammetry measurements. The presence of oxide and hydroxide spices provides a wide Urso et al. have also reported [82] a novel electrode material consisting of Ni(OH) 2 @Ni core-shell nanochains with promising high-rate energy storage performance. These discovered features of the Ni(OH) 2 @Ni core-shell nanochains obtained by reduction in forming gas are explained by the quicker electron transport, greater electric field and better use of the active material when compared with those of similar NiO and Ni(OH) 2 -based nanostructures. Our results on enhancing the electrochemical activity of reduced cobalt oxide electrodes presented in this work largely coincide with these results.

Conclusions
In summary, a new effect of a significant increase in the electrochemical activity of reduced Co 3 O 4 electrodes is demonstrated in the present work. It is shown that the reduced Co 3 O 4 can ensure a higher storage capacity compared to the initial Co 3 O 4 due to the presence of different forms of oxide and hydroxide spices formed on the surface of metallic Co particles. These spices provide a wide range of reversible redox reactions, all contributing to the overall storage capacity of the Re-Co 3 O 4 electrode. In addition to these, the small thickness of the oxide/hydroxide layer provides a high reaction rate, and the presence of a metal skeleton leads to a low series resistance of the electrode. The proposed simple method can be applied to other metal oxides (e.g., nickel [82] or tungsten [83]) which are able to recover under a hydrogen atmosphere, as well as to Co 3 O 4 electrodes obtained by other methods.