Supercapacitor Electrodes: Is Nickel Foam the Right ...

3.1. Electrooxidation of Nickel&#;Experimental Results and Literature Consideration

2O4 in 3 M KOH at different scan rates (2&#;100 mV/s) are shown in 2O4, while CVs of NixMg1&#;xFe2O4/NF with x being 0.1, 0.3, 0.5, 0.7, 0.9, and 1 recorded with a scanning rate of 20 mV/s are shown in 2O4. The most notable feature is that the peaks in all of the CVs increased with the number of cycles. This is illustrated in xMg1&#;xFe2O4. The highest obtained capacitance was 242 F/g for MgFe2O4/NF at a scanning rate of 2 mV/s. However, after learning that Ni foam may also participate in redox reactions in the same potential range [

CVs of MgFein 3 M KOH at different scan rates (2&#;100 mV/s) are shown in Figure 1 a. CVs show a clearly differentiated anodic peak at 0.35&#;0.45 V and a cathodic peak at 0.25&#;0.28 V. The anodic peak position shifted to the right, while the cathodic peak position shifted to the left with an increase in the scan rate from 2 to 100 mV/s, as expected for a diffusion-controlled process [ 60 ]. Figure 1 b shows the CV of MgFe, while CVs of NiMgFe/NF with x being 0.1, 0.3, 0.5, 0.7, 0.9, and 1 recorded with a scanning rate of 20 mV/s are shown in Figure S1a&#;f , respectively. All of the CVs show redox peaks similar in shape and position to the peaks of MgFe. The most notable feature is that the peaks in all of the CVs increased with the number of cycles. This is illustrated in Figure 1 b and Figure S1 as a comparison between CVs of the first cycles and CVs after tens of stabilization cycles and final cycles. It is important to note that this increase did not disappear, even after 100 cycles. In some of the CVs, the cathodic peak split into two components with cycling. At first, the high current response was attributed to the electrochemical activity of NiMgFe. The highest obtained capacitance was 242 F/g for MgFe/NF at a scanning rate of 2 mV/s. However, after learning that Ni foam may also participate in redox reactions in the same potential range [ 61 62 ], some of the samples were tested on platinum and GCE substrates.

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2O4/GCE and Ni0.5Mg0.5Fe2O4/GCE compared to the CVs of bare GCE substrate. The calculated capacitance of MgFe2O4/GCE substrate was 8 F/g, while the calculated capacitance of Ni0.5Mg0.5Fe2O4/GCE was 7 F/g, which is multiple times lower than the values obtained when Ni foam was used (150 and 75 F/g at 20 mV/s). Both CVs show no peaks that were originally noticed when Ni foam was used as a substrate. 0.5Mg0.5Fe2O4/Pt compared to a CV of bare Pt. CV of the material, again, shows no peaks resembling the ones obtained with Ni0.5Mg0.5Fe2O4/NF (

CVs on a GCE were recorded in a potential range between &#;1.2 V and 0.5, 0.6, or 0.7 V, depending on the onset of HERs and OERs, with a scanning rate of 20 mV/s. Figure 2 a,b show the CVs of MgFe/GCE and NiMgFe/GCE compared to the CVs of bare GCE substrate. The calculated capacitance of MgFe/GCE substrate was 8 F/g, while the calculated capacitance of NiMgFe/GCE was 7 F/g, which is multiple times lower than the values obtained when Ni foam was used (150 and 75 F/g at 20 mV/s). Both CVs show no peaks that were originally noticed when Ni foam was used as a substrate. Figure 2 c shows a CV of NiMgFe/Pt compared to a CV of bare Pt. CV of the material, again, shows no peaks resembling the ones obtained with NiMgFe/NF ( Figure S1c ). Even after subduing active materials to more cycles, no new peaks appeared. Therefore, it was concluded that the investigated Ni-Mg ferrites are not electrochemically active in 3 M KOH, contrary to the original conclusions made when Ni foam was used as a substrate.

2O4/NF in 6 M KOH after various numbers of cycles are shown in xMg1&#;xFe2O4/NF. An increase in the redox peaks and cathodic peak split with the number of cycles was observed again. The measurement was repeated four times. The calculated capacitance values for four probes were 32, 64, 158, and 165 F/g at 20 mV/s. After obtaining inconsistent results, NiMn2O4 was tested on a GCE substrate with the same experimental setup. There was no current response in the CV of NiMn2O4/GCE in the first cycle, but after 10 cycles in the potential window &#;1&#;0.4 V, redox peaks emerged. After stabilization, this material was subdued to cycling in the potential window of &#;0.4&#;0.42 V (2O4/GCE was calculated to be 33 F/g at 20 mV/s, which is much lower than NiMn2O4/NF (165 F/g at 20 mV/s). Since a modest capacitance value was obtained, it was concluded that NiMn2O4 synthesized via sol&#;gel combustion synthesis is electrochemically active. Its capacitance is due to electrochemical reactions of nickel, which will be discussed in more detail. Further research was directed to clarifying nickel foam behavior in alkaline media. Bare Ni foam, used &#;as received from the manufacturer&#;, was subjected to cycling in 3 M KOH in various potential intervals. 2O4, uncoated Ni foam underwent one voltammetric cycle in 6 M KOH. The resulting CV had no current response.

The obtained CVs for NiMn/NF in 6 M KOH after various numbers of cycles are shown in Figure 3 a. CVs contain anodic and cathodic redox peaks, with the shapes and positions of the peaks being similar to the ones obtained for NiMgFe/NF. An increase in the redox peaks and cathodic peak split with the number of cycles was observed again. The measurement was repeated four times. The calculated capacitance values for four probes were 32, 64, 158, and 165 F/g at 20 mV/s. After obtaining inconsistent results, NiMnwas tested on a GCE substrate with the same experimental setup. There was no current response in the CV of NiMn/GCE in the first cycle, but after 10 cycles in the potential window &#;1&#;0.4 V, redox peaks emerged. After stabilization, this material was subdued to cycling in the potential window of &#;0.4&#;0.42 V ( Figure 3 b) since the current below &#;0.4 V was negligible. It is important to note that in this case, no further increase in peak intensity was noticed. The capacitance of NiMn/GCE was calculated to be 33 F/g at 20 mV/s, which is much lower than NiMn/NF (165 F/g at 20 mV/s). Since a modest capacitance value was obtained, it was concluded that NiMnsynthesized via sol&#;gel combustion synthesis is electrochemically active. Its capacitance is due to electrochemical reactions of nickel, which will be discussed in more detail. Further research was directed to clarifying nickel foam behavior in alkaline media. Bare Ni foam, used &#;as received from the manufacturer&#;, was subjected to cycling in 3 M KOH in various potential intervals. Figure 4 shows various CVs (0.0&#;0.3 and 0.4 V). Redox peaks similar to the ones in Figure 1 Figure 2 , and Figure S1 appeared. These peaks also grew with each cycle ( Figure 4 ). This indicated that uncoated Ni foam was undergoing electrochemical redox processes in alkaline media. It is worth mentioning that before conducting the measurements on NiMn, uncoated Ni foam underwent one voltammetric cycle in 6 M KOH. The resulting CV had no current response.

2O4 and NixMg1&#;xFe2O4 were at first assigned to fast redox reactions of metal oxides [21,22,23,24,25,xMg1&#;xFe2O4/NF have shown capacitance up to 242 F/g at 2 mV/s, they have shown no electrochemical activity when loaded on a GCE or Pt substrate (2O4/GCE exhibited some capacitance (33 F/g at 20 mV/s), but much less than the value obtained with NiMn2O4/NF (165 F/g at 20 mV/s) (

The obtained redox peaks in the CVs of NiMnand NiMgFewere at first assigned to fast redox reactions of metal oxides [ 63 ]. The considerably high capacitance values are in line with the already published results [ 20 64 ]. While NiMgFe/NF have shown capacitance up to 242 F/g at 2 mV/s, they have shown no electrochemical activity when loaded on a GCE or Pt substrate ( Figure 1 Figure 2 , and Figure S1 ). It is worth mentioning that other researchers also obtained the capacitive performance of mixed Ni-Mg ferrites [ 41 64 ]. On the other hand, NiMn/GCE exhibited some capacitance (33 F/g at 20 mV/s), but much less than the value obtained with NiMn/NF (165 F/g at 20 mV/s) ( Figure 3 ). Yadav et al. also obtained better capacitance results when Ni foam was used [ 65 ]. The conclusion can be reached that most of the current response actually originated from the redox reactions of Ni foam.

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Ni + 2   OH &#; &#; Ni ( OH ) 2 at the potentials around &#;0.865&#;&#;0.565 V, represented as the peak at the potential of &#;0.7 V vs. Hg/HgO in 2 occurs, and &#;passive&#; (&#;0.65 V < E < 0.35 V vs. Ag/AgCl), where α-Ni(OH)2 irreversibly transforms to β-Ni(OH)2. This part of the CV is featureless and is evidence of a passivized surface [69, Ni ( OH ) 2 + OH &#; &#;   NiOOH + H 2 O + e &#; occurs. This reaction is represented as the anodic peak at 0.45 V vs. Hg/HgO and a corresponding cathodic peak at 0.42 V vs. Hg/HgO (1 M KOH) in +2 changed to Ni+3 oxidation state [ Ni ( OH ) 2 and NiOOH [45,46,66,71,72,73,74,75,76,77,2 and solid oxide NiO phases.

A literature review provides more insight into the electrochemistry of nickel in alkaline media. While nickel etches in acid solutions [ 42 ], it offers good corrosion resistance in alkaline media [ 16 ] because it quickly forms a passive oxide layer [ 44 ]. Nickel goes through oxidationat the potentials around &#;0.865&#;&#;0.565 V, represented as the peak at the potential of &#;0.7 V vs. Hg/HgO in Figure 5 (note: figure republished with permission. Seghiouer et al. [ 66 ] declared potentials towards a Hg/HgO reference electrode). The standard electrode potential for this reaction is &#;0.72 V vs. SHE (&#;0.925 vs. Ag/AgCl) [ 67 ]. Alsabet et al. classified nickel behavior at different potentials in alkaline media as &#;active&#; (E < &#;0.65 V vs. Ag/AgCl), where reversible electro-oxidation of Ni into α-Ni(OH)occurs, and &#;passive&#; (&#;0.65 V < E < 0.35 V vs. Ag/AgCl), where α-Ni(OH)irreversibly transforms to β-Ni(OH). This part of the CV is featureless and is evidence of a passivized surface [ 68 70 ]. The third region is &#;transpassive&#; (E > 0.35 V vs. Ag/AgCl), where oxidationoccurs. This reaction is represented as the anodic peak at 0.45 V vs. Hg/HgO and a corresponding cathodic peak at 0.42 V vs. Hg/HgO (1 M KOH) in Figure 5 with Nichanged to Nioxidation state [ 66 ]. After NiOOH was obtained, peaks in the negative potentials disappear, the surface is passivized (no metallic nickel is exposed), and the only electrochemical oxidoreduction that happens during cycling occurs betweenand 46 ]. Continuous cycling in alkaline solutions caused growth of the nickel hydroxide layer on the electrode surface [ 44 78 ]. Alsabet et al. [ 70 ] discovered that a surface oxide layer on a Ni electrode is comprised of solid hydroxide Ni(OH)and solid oxide NiO phases.

2 layer grows during cycling [2 formed on the Ni foam surface would probably grow and participate in the sum current with its redox reaction. If the active materials&#; peaks are expected in this exact region of potentials where nickel electrooxidation occurs, then Ni foam should be used with caution. The term &#;synergy&#; that is sometimes used in the literature, pertaining to the combined capacitance effect of the active material and Ni foam, is controversial since it is hard to distinguish capacitance originating from the active material and from the Ni foam. Active materials should be investigated on planar inert electrodes and then compared to the activity when loaded on Ni foam [61,2/NiOOH since the oxygen synthesized during OERs reacts with Ni(OH)2, enhancing the generation of NiOOH [2 to metallic nickel, which reacts with hydroxyl anions in the electrolyte when the potential reverses and forms more Ni(OH)2, which, again, generates more NiOOH in the positive potential region.

The possibility of Ni foam contributing to capacitance values by its own redox reactions is mostly disregarded in the papers reporting on supercapacitor or pseudocapacitive materials. Some papers do actually consider the influence of Ni foam by putting it through one voltammetric cycle, with an almost negligible current response. However, since the Ni(OH)layer grows during cycling [ 16 70 ], those researchers might have made the mistake of not putting uncoated Ni foam through more cycles. The authors of this article underline the problem of galvanostatic charge&#;discharge (GCD) cycling. If, for example, charge&#;discharge cycles are obtained with a nickel electrode freshly coated with the active material, there might not be a large portion of the current originating from Ni foam oxidation. But, if the material along with Ni foam is put through tens and hundreds of cycles, in the potential region where nickel electrooxidation occurs, Ni(OH)formed on the Ni foam surface would probably grow and participate in the sum current with its redox reaction. If the active materials&#; peaks are expected in this exact region of potentials where nickel electrooxidation occurs, then Ni foam should be used with caution. The term &#;synergy&#; that is sometimes used in the literature, pertaining to the combined capacitance effect of the active material and Ni foam, is controversial since it is hard to distinguish capacitance originating from the active material and from the Ni foam. Active materials should be investigated on planar inert electrodes and then compared to the activity when loaded on Ni foam [ 48 62 ]. If not, then the results may be overestimated or even completely untrue. Our results concur with the results obtained by Ali et al. [ 90 ], who claim that testing on Ni foam gives misleading results and interferes with the active materials&#; electrochemistry. The potential window width on Ni foam electrochemistry was investigated. Figure 6 a shows the CV of bare Ni foam, cycled in 3 M KOH, with the cathodic limit being fixed at 0.0 V and with the anodic limit values ranging from 0.36 V to 0.45 V. Widening the potential limit into positive potentials increases the peak of Ni(OH)/NiOOH since the oxygen synthesized during OERs reacts with Ni(OH), enhancing the generation of NiOOH [ 16 ]. Figure 6 b shows repetitive cycling with the same setup but with an anodic potential limit fixed at 0.4 V and a cathodic potential limit varying in the range &#;0.5&#;&#;1.3 V. With the cathodic potential limit increase in the negative potential region, the HER is more pronounced. Hydrogen reduces NiO and Ni(OH)to metallic nickel, which reacts with hydroxyl anions in the electrolyte when the potential reverses and forms more Ni(OH), which, again, generates more NiOOH in the positive potential region.

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