3.1 XRD study
Fig. 2 (A) presents the XRD spectra of different Nickel cobalt oxide nanostructures. The observed d-spacing values in XRD spectra are associated with standard JCPDS card 01-080-1543 for Co3O4 and 01-073-1702 after adding different Ni concentrations. The peaks are observed at an angle, 64.98, 59.19, 44.68, 36.79, 31.16,18.94° which applied to the cubic structure and assigned to the (440), (511), (400), (311), (220), and (111) crystal planes respectively. The crystallite size is calculated by using Scherer's formula 7,37. The calculated values for average crystallite size are 34.18, 36.61, 43.10, 30.96, and 34.18 nm for Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4, respectively.
3.2 FTIR study
The different Nickel cobalt oxide powders were further characterized by the FT-IR study. Fig. 2 (B) presents the FTIR spectra of all synthesized nickel cobalt oxide powders. The FTIR spectra display two sharp peaks at about 661 cm-1 and 565 cm-1 which are originated from Co-O bonds (stretching vibrations), confirms the formation of Co3O4 38,39. As the percentage of Ni increases, these two peaks shift towards a higher wavenumber. The corresponding change in the peak is mentioned in supplementary information Table S1. The relatively weak peaks at about 1632 and 3432 cm-1 are associated with O-H bending and stretching mode of vibration of water 40–42.
3.3 FE-SEM study
Fig. 3 shows FE-SEM pictures and EDS analysis of different nickel cobalt oxide powders. The Co3O4 in Fig. 3 (A) shows nanosheets like nature. As the different percentages of Ni are added into Co3O4, surface modifications are observed. In Fig. 3 (B), the nanosheet-like morphologies are observed, which are merged. As the percentage of Ni increases, the nanosheets are incorporated a lot in Fig. 3 (C). The flakes are started covered with nanoparticles in Fig. 3 (D), and finally, the number of nanoparticles increased as shown in Fig. 3 € corresponding to the Ni1.2Co1.8O4. Fig. 3 (F) shows the EDS spectra of Ni1.2Co1.8O4, which confirms the presence of Ni, Co, and O. Fig. 3 (G-I) presets the elemental distribution of Ni, Co, and O, respectively.
3.4 EIS study
The electrochemical behavior at the electrode-electrolyte interface was studied by using the EIS study. Fig. 4 presents the EIS study of synthesized Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes. EIS spectra were recorded within 1 MHz to 0.1 Hz frequency range in 1 M KOH electrolyte at applied potential 20 mV. Fig. 4 (A) presents the Nyquist plots of Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes. The Inset of Fig. 4 (A) shows the corresponding fitted circuit diagram. The solution resistance (Rs) values estimated from Nyquist plot for Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes are 0.08, 0.37, 0.36, 0.02, and 0.15 W cm-2. The charge transfer resistance values for Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes are 88.81, 91.51, 109.10, 79.51, and 90.47 W cm-2 respectively. The lower Rct and Rs values reveal the better electrical conductivity and good adhesion of coated material with current collector 43. The different EIS parameters are mentioned in supplementary information Table S2. Fig. 4 (B) presents the Bode plot of Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes, respectively.
3.5 Electrochemical supercapacitor study
The three-electrode system with SCE as a reference electrode, platinum wire counter electrode, and prepared CC electrodes as a working electrode is used to study electrochemical supercapacitor performance. The electrochemical performance is studied in 1 M KOH electrolyte.
Fig. 5 A presents the cyclic voltammetry (CV) curves of all nickel cobalt oxide electrodes studied at a 10 mV s-1 scan rate. The maximum capacitance values estimated from CV curves are 189.31, 204.22, 220.63, 516.51, and 312.32 F g-1 for Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes, respectively. All CVs, along with different scan rates, are mentioned in Fig. S1 (A-E). From the graphs in Fig. S1, it is clear that with an increase in scan rate, the area under the curve increases as the current drawn increases with the voltage in the CV graph. But the increase in scan rate limits the interaction time between electrolyte and electrode. This limited-time interaction between electrode and electrolyte also limits the utilization of the active surface area of the electrode. Hence specific capacitance decreases with the increase of scan rate 44.
The galvanostatic charge-discharge curves are displayed in fig. 5 B of different nickel cobalt oxide electrodes studied at current density 3 mA cm-2. The maximum specific capacitance values estimated from GCD curves are 89.10, 101.27, 165.38, 455.27, and 337.01 F g-1 for Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes, respectively. All GCD study with various current densities is mentioned in Fig. S2 (A-E). The cyclic stability is measured for the Ni0.9Co2.1O4 electrode as it exhibited a higher capacitance value than other nickel cobalt oxide nanostructured electrodes. The observed stability for the Ni0.9Co2.1O4 electrode is 87.7% over the 2000 charging-discharging cycles as depicted in Fig. 5 C. Inset image of Fig. 5C shows the first and last charging and discharging cycles. The higher specific capacitance and stability of Ni0.9Co2.1O4 electrode may be due to micro sheets covered nanospheres like morphology. The micro sheets and nanosphere morphology provide maximum surface area so that the interaction between electrode and electrolyte becomes easier, enhancing the specific capacitance value. The calculated specific capacitance from CV vs. scan rate is mentioned in fig. 5 D. In contrast, the variation of specific capacitance vs. current density is mentioned in fig. 5 E. Fig. 5 F presents the Regone plot. The maximum energy density for the Ni0.9Co2.1O4 electrode is 15.81 Wh kg-1 (at 0.75 kW kg-1 power density). The comparison of supercapacitor performance of present work and previously reported work is mentioned in Table. 1.
3.6 Nonenzymatic glucose biosensing study
Here the same three-electrode system is used for the glucose sensing study except for 0.1 M NaOH electrolyte. Fig. 6 (A-E) presents the CVs of Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes in the absence of glucose (red) and in the presence of 1.0 M glucose (blue). Fig. 7 (A-E) presents the CV curves of Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes at different concentration of glucose (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 M). Here we can observe that for all samples, peak currents are increased after the addition of glucose. The increase in the peak currents is explained as follows 45,
During the oxidation, Ni2+ and Co2+ ions present in the NiOOH and CoOOH compounds oxidize to Ni3+ and Co3+ with the release of two electrons (eq. 1-3). After adding glucose, the glucose molecules dissociate and are converted into gluconolactone with the release of two electrons (eq. 4). During reduction, the oxidized species Ni3+ and Co3+ reduce back to the Ni2+ and Co2+ by accepting electrons and returning to their original states. Fig. 8 (A-E) presents the CV of different nickel cobalt oxide electrodes in the presence of 1.0 M glucose at various scan rates (10-100 mV s-1). It can be seen that with scan rate, the anodic and cathodic peaks also increase. Fig. 9 (A-E) presents the chronoamperometry study (I vs. t) of each nickel cobalt oxide electrode at 0.4 V applied potential with the subsequent addition of 0.05 mM glucose into 0.1 M NaOH electrolyte. The inset of Fig. 9 (A-E) shows their calibration curves of Amperometric responses. The observed sensitivities for Co3O4, Ni0.3Co2.7O4, Ni0.6Co2.4O4, Ni0.9Co2.1O4, and Ni1.2Co1.8O4 electrodes are 370.7, 759.5, 747.4, 557.7, 688.1 µA mM-1 cm-2, respectively. The Nyquist plots in the absence of glucose and in the presence of 1.0 mM glucose are shown in supplementary information Fig. S3. The estimated glucose-sensing parameters are given in Table 2. As shown in Fig. 10 (A-E), the selectivity of samples was examined by addition 0.2 mM glucose and other interfering species [0.0125 mM of uric acid (UA), Ascorbic acid (AA), Fructose (FR), Sucrose (SA), and Lactic acid (LA)]. All the electrodes showed a negligible effect of interfering species as compared to glucose. The response times for all prepared electrodes are mentioned in Fig. S4 (Supplementary data). The comparative glucose-sensing study of current work and previously reported work is mentioned in Table. 3.