The making of a high performance supercapacitor active at negative potential using sulphonic acid activated starch-gelatin-TiO₂ nano-hybrids

2.1 Design of activated starch and gelatin nanoparticles

The hydrolysis method, which involves sulfuric acid, was adapted and modified from literature (Ding et al., 2010; Tay et al., 2012). A mixture of 30 g starch, 1.5 g of para-toluene sulphonic acid (PTSA) and 500 mL of 3.1 M H₂SO₄, was stirred for 2 h at 100 rpm (stirring speed) at 40 °C. The mixture was then placed in a microwave oven, and subjected to a microwave treatment protocol that consisted of a 30-second exposure to 100 W of microwave energy, and then a rest cycle for 1 min. This was repeated several times over a period of an hour, until the mixture changed from white to a brown colour. The suspension produced was mixed with 15% methanol without stirring and successively centrifuged and stored under a fume hood until the gelatine was ready for the next step (Ding et al., 2010; Tay et al., 2012). The polyol emulsion method was adopted and modified (Houshyari et al., 2018). A mixture of 10 g of gelatine and 200 mL of mixed polyols (50% tergitols and 50% of styrol solution) was heated to a temperature of 60 °C for 1 h using a hot plate. The suspension was treated with 20 mL of 0.1 M HCl and 1.0 g of para-toluene sulphonic acid and left standing, without any agitation, under a fume hood for a period of 2 hours. The mixture was then centrifuged and the gelatine material stored under ambient conditions.

2.2 Fabrication of nanoarchitectures of starch, gelatin and TiO₂

Distilled water, 200 mL, was mixed with 1 g of urea in a double-necked round bottom flask and the resultant mixture was heated at 75 °C and allowed to reflux for fifteen minutes. Then, 50 mL of Titanium isopropoxide was added drop-wise, over a period of 5 min, and the suspension produced was then stirred for one hour at 90 °C, in a water bath. The obtained powder was then separated (centrifuge, 2500 gcf) and dried at 80 °C in an oven. The synthesis of TiO₂ was adapted from the literature (Dodoo-Arhin et al., 2018; REDDY et al., 2001). Activated starch nanoparticles (AS-NPs), activated gelatin nanoparticles (AG-NPs), activated starch-gelatin nanohybrids (ASG-NH), activated starch-TiO₂ (AS-TiO₂ NH), activated gelatin-TiO₂ (AG-TiO₂ NH), and activated starch-gelatin-TiO₂ (ASG-TiO₂ NH) were designed by stabilization of respective nano-biopolymers in 20 mL of 50:50 tween-80 and isobutanol solution under stirring for 2 h. To obtain the nanocomposite powders, the mixtures were dried at 50 °C in an oven (∼8 hours), and then kept in a desiccator until characterization.

2.3 Material characterizations

Attenuated Total Reflection-Fourier Transform Infra-Red (ATR-FTIR) was performed on a Spectrum-100 Perkin Elmer (USA) instrument, from 4000 to 600 cm⁻¹ wavenumbers. The X-ray diffraction (XRD) measurements were done using a Rigaku Ultima IV, X-ray diffractometer (Japan) using Cu-Kα radiation of 1.5406 Å generated at 45 kV and 40 mA. The diffraction patterns were obtained over a 2-theta values ranging from 5° to 90°, using a step size of 0.01° and 1°/min scan rate. A TESCAN VEGA (Czech Republic) scanning electron microscope (SEM) was employed to study the surface morphology of the samples, using a 20 kV accelerating voltage. The particle sizes of the nano-hybrids were investigated using a Dynamic Light Scattering (DLS) instrument, Zetasizer, model: ZEN 3600.

2.4 Electrochemical studies

The principle of push loading was employed to coat 20 mg of homogenous pastes, of the prepared biopolymeric nanoarchitectures, on dried nickel foams to form working electrodes. Pastes were prepared by mixing the samples (160 mg) with polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) (20 mg PVDF: 1.0 g NMP). The electrochemical properties of the nanocomposites were initially investigated using a three-electrode configuration, with a 1 M Na₂SO₄ aqueous electrolyte (at room temperature), a Ag/AgCl reference electrode, and a platinum wire coil as the counter electrode. Cyclic voltammetry measurements, electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge (GCD) experiments were performed using a Gamry 1010 E potentiostat. In the two-electrode configuration, the electrodes were prepared as per the method used for the working electrodes in the three-electrode configuration. A glass fibre separator was soaked in 1 M Na₂SO₄ electrolyte and sandwiched in between two symmetrical working electrodes. The symmetrical supercapacitor was aligned and pressed before setting it up for electrochemical measurements.

3.1 Effect of activation of starch and gelatin biopolymers

The starch and gelatin nanoparticles were first activated in the presence of para-toluene sulphonic acid and ammonium chloride, then tested for energy storage applications. As reported in previous works (Romero et al., 2013), the activation step leads to p-type doping which creates structural distortions and re-alignment of the polymer chains, and this results in the generation of radical cations (polarons) and double radicals (bipolarons). The activation was confirmed through conductivity studies carried out using the four-probe method, where a four-fold conductivity increase was obtained after activation. At a temperature of 100 °C, the conductivity of starch moved from 6.02 to 30.05 $\mu Sc{m}^{-1}$ after activation, while that of gelatin moved from 6.31 to 68.17 $\mu Sc{m}^{-1}$ . Hence, the activation led to an overall increase in conductivity of the biopolymers (see Table 1).

At 30 °C, the effect of activation resulted in a conductivity of 16. 31 $\mu Sc{m}^{-1}$ in starch, and 18.09 $\mu Sc{m}^{-1}$ in gelatin, indicating that they will not lose their conductance at room temperature, for energy applications. The activation blocks the swelling points of starch and gelatin through ring attachments and creates faster electronic transport channels. This will enable the starch and gelatin biopolymers to undergo numerous, and possibly higher, charge and discharge cycles. This can be observed in the conductivity values of the un-activated starch and gelatin at temperatures of 90 and 100 °C, where a decrease from 6.10 and 6.37 to 6.02 and 6.31 were observed. This is because the un-activated (ordinary) starch and gelatin biopolymers are now being distorted at high temperatures, hence, they start losing their conductance. After activation, both starch and gelatin retained their high conductivity behaviours, because all swelling points have been blocked.

As depicted in the FTIR spectra presented in Fig. 1a of the starch and gelatin before and after activation, the non-activated starch displayed the expected bands for C—H stretching (between 2750 and 3000 cm⁻¹), C—O stretching, C—H symmetric bending, CH₂ twisting (1250–1500 cm⁻¹), and C—C stretching (500–750 cm⁻¹). After activation, a strong peak was observed at 1708 cm⁻¹ to confirm the oxidative products of the starch carbons. Sulphone peaks were also noted at 1311 cm⁻¹ and there was a general shift in the C—H vibrational energies of the starch biopolymer into the lower wavelength regions. Similar results were reported in the literature (Fan et al., 2012; Warren et al., 2016). Similar observations were seen with the activated gelatin at the fingerprint region, there are new peaks due to the para-aromatic dopant.

Close

Fig. 1 (a) FTIR of starch, before and after activation (b) FTIR of gelatin, before and after activation.

Export to PPT

The XRD results are presented in Fig. 2a and b. Two prominent peaks were observed with the starch, before activation, at 19.4° and 22.8° 2θ, and the gelatin before activation had wide characteristic peaks at 23.1°, 38.3°, 48.4° and 53.7° 2θ. After activation, the type-A starch was converted into type-B starch. Type-B starches are more crystalline and ordered, as seen with the additional peaks (Fig. 2a), and the characteristic peaks of the type-B starch were noted at 22.8°, 33.3°, and 35.4° 2θ. In addition, after activation, the gelatin also switched from type-B gelatin (or alkaline derived gelatin) into type-A gelatin, which is an acid derived gelatin. The type-A gelatin possess higher degree of crystallinity, as seen with the increased number of peaks. The XRD confirms a phase transition in both the starch and gelatin after activation.

Close

Fig. 2 (a) XRD of starch, before and after activation (b) XRD of gelatin, before and after activation.

Export to PPT

3.2 The design of activated starch and gelatin nano-hybrids

Fig. 3 shows the FTIR spectra of all six fabricated nano-hybrids, and biopolymer-based, electrode materials. Activated starch and activated gelatin were first converted to nano-forms, to make activated starch nanoparticles and activated gelatin nanoparticles. These hybrids were fabricated to form activated starch-gelatin nanohybrids, and three of them were incorporated with TiO₂. For the activated starch nanoparticles, the transmittance peaks at 3408 and 2914 cm⁻¹ represents the O—H and C—H stretch vibrations, while the bending vibrations of the methylene groups are shown at 1431 and 1460 cm⁻¹ respectively for in and out-of-plane bends (see Fig. 3). The C—O—C linkages of the glycosidic rings are confirmed from the 1068 cm⁻¹ peak (Hoyos-Leyva et al., 2017; Sondari et al., 2018).

Close

Fig. 3 (I) FTIR and (II) XRD of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin naohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids.

Export to PPT

The successful restructuring of the gelatin, to form nanoparticles, was confirmed using FTIR. The vibrations of amide A, amide I and amide II were seen at 2783 cm⁻¹, 1635 cm⁻¹ and 1522 cm⁻¹ respectively, and these peak positions are similar to other literature reports (Subara et al., 2018). The activated starch-gelatin nanohybrids showed no change in the amide I peak environment indicating no major change in the secondary structure of activated gelatin nanoparticles after blending with the activated starch nanoparticles. This is in agreement with the report of Chen et al., and Sharma et al., but Kumar et al., further reported the formation of aldimine as a result of the interactions between the carbonyl and the amine groups of the gelatin biopolymers. The C—H overtones between 1400 and 800 cm⁻¹ confirmed the mixed helices of both biopolymers without a change in their transmittances as shown from FTIR results (Chen et al., 2017; Kumar et al., 2019; Sharma et al., 2018).

After the incorporation of TiO₂ nanoparticles, the O—H and methylene groups displayed reduced intensities and transmittances, which confirms intermolecular interactions between the Ti—O, C—O and O—H bonds of the starch and gelatin nano-hybrids. Some of the amide bands of the gelatin disappeared, but the glycosidic linkages remained intact and showed a shift towards the left at 907 cm⁻¹. The shift is an indication of tight bonding interactions within and between the prepared nano-architectures. The C—O—Ti bond was seen at 1054 cm⁻¹ confirming stable Ti—O functionalities after incorporation of TiO₂. Oleyaei et al., (Oleyaei et al., 2016) reported that electrostatic interactions between Ti²⁺ and —OH groups of starch caused wider and flattened bands at 450–800 cm⁻¹ while O—H and C—H stretching vibrations moved to longer wavenumbers, similar observations were noted with the nano-architectures prepared in this work. The electromeric effect and hydrogen bonding between the amide groups of gelatin and Ti²⁺ further caused the reduced transmittance and intensity observed in the FTIR spectra of the hybrid nano-architectures, similar results were reported by W. Li et al. (2019).

Histograms displaying the size distributions for the prepared nano-architectural samples are presented in Fig. 4. The activated starch nanoparticles showed an average size of 13 nm, the activated gelatin nanoparticles showed an average size of 79 nm, and the hybrids formed (activated starch-gelatin nanohybrid) had an average particle size of 66 nm. After incorporating TiO₂, the starch-TiO₂ nanohybrid showed an average particle size of 74 nm, the gelatin-TiO₂ nanohybrids had an average size of 93 nm, and the activated starch-gelatin-TiO₂ nanohybrid showed average sizes of 87 nm. The size distributions revealed the nano-hybrids have good dispersibility and low aggregation, favouring a fast electron transfer and kinetics behaviour during impedance studies.

Close

Fig. 4 Particle size histogram of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin nanohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids.

Export to PPT

The statistics of the particle sizes indicated a normal distribution, this was similar to the study by Liu et al., 2009. From Gu et al., normal size distributions confirms good capacitive behaviour of the activated polymer matrixes (Gu et al., 2019). In addition, previous works have shown that uniform coating of electrodes can be achieved when the polymers exist at nano-size dimensions; retention capacity decreases quickly when particle sizes are too large. In addition, interfacial impedance of nano-sized electrodes, which have incorporated transition metal oxides and hydroxides are very low during electrode dependant applications (Capone et al., 2019; Hu and Lei, 2007; Wu et al., 2018).

The XRD of the activated starch nanoparticles (Fig. 3II) showed characteristic diffraction peaks at 19.4° and 22.7° as a singlet and doublet respectively to represent the [1 1 1] and [2 2 0] crystal planes (JCPDS 64-1453). The gelatin nanoparticles showed predominant peaks at 26.1°, 38.3° and 49.5° for [1 1 1], [2 0 0] and [2 2 0] respective crystal planes (Marvizadeh et al., 2017; Xia et al., 2015). The diffraction peaks at 26.3°, 35.7° and 46.1° confirms the incorporation of titanium dioxide nanoparticles for [1 0 1], [0 0 4] and [2 0 0] crystal planes (JCPDS card no. 78-2486) (Soliman and Furuta, 2014).

The percentage crystallinity of the starch nanoparticle helix remained constant after the blending of both starch and gelatin biopolymers together. A crystallinity of 74 and 71.8% for starch was obtained from the XRD peaks before and after blending. Although, the intensity of the characteristic crystalline peak of the starch helices remained fairly constant, the intensity of the triple helical structure in the crystalline gelatin at 26.1° decreased progressively with blending (Mendes et al., 2016). The XRD patterns for the TiO₂ incorporated into the various nano-architectures (Fig. 3(II), patterns d, e, and f) appeared to be very similar, and this is due to the strong effect of the TiO₂ crystallinity over the biopolymers.

The TEM analysis was used to establish the successful design of the nano-structured starch and gelatin based architectures. Fig. 5a shows the activated starch nanoparticles with well arranged lamellar and no traces of internal aggregations. The activated gelatin nanoparticles were seen to be oval nanocrystals, and similar to other observation reported in the literature (Jeong and Park, 2014). The absence of aggregation in both activated starch and gelatin nanoparticles corresponds with the particle size distribution results obtained in Fig. 4. In the activated starch-gelatin nanohybrids, the gelatin were seen to be encapsulated within the starch lamellar (Kašlík et al., 2018; Yang et al., 2013). After TiO₂ incorporation, The TiO₂ nanoparticles stayed in well-spaced localized regions within the lattices of the activated starch and gelatin nanoparticles. This good dispersion of the TiO₂ nanoparticles is required for better transfer and storage of charges within supercapacitors (Liu et al., 2009, 2017).

Close

Fig. 5 TEM of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin nanohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids.

Export to PPT

3.3 Three electrode configuration electrochemical study

The electrochemical properties of the nano-hybrids prepared were evaluated using a three electrodes configuration. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) experiments, and electrochemical impedance spectroscopy (EIS) were done in a 1 M Na₂SO₄ electrolyte solution. As seen in Fig. 6, the CV curve shows quasi rectangular shapes from 5 to 200 mV/s, indicative of supercapacitive behaviour of the nano-hybrids (Xu et al., 2019). This also shows that the fabricated nano-starch-gel-TiO₂ is free of structural and conformational defects (Xu et al., 2019).

Close

Fig. 6 (Three electrode configuration) CV of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin nanohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids, at scan rates of 5, 10, 20, 50, 100 and 200 mVs⁻¹.

Export to PPT

In general, any redox peaks that may be observed with polymer based nanohybrids are due to the presence of polarons and free electrons created after activation (Wang et al., 2015). The CV curves of the electrodes were recorded in the potential range of −0.8 to −0.4 V at scan rates of 5, 10, 20, 50, 100 and 200 mV/s. Surface faradaic reactions, which are responsible for the good performance of the electrodes, include interactions between TiO₂, H⁺, and polarons to form TiOOH complex, interaction between TiO₂, N—H and O—H from starch and gelatin to form charge storage domains in excited states (Nagamuthu et al., 2013).

Specific capacitance values were obtained using the area of the CV curves, scan rates and potential windows. Activated starch nanoparticles had specific capacitances (C_sp) values of 41, 89, 128, 174, 206 and 237 Fg⁻¹ at scan rates of 200, 100, 50, 20, 10 and 5 mVs⁻¹ respectively. Activated gelatin nanoparticles had C_sp of 77, 96, 125, 169, 212 and 246 Fg⁻¹. Activated starch-gelatin nanohybrids had higher specific capacitance values than those of activated starch and gelatin nanoparticles respectively, showing C_sp values of 129, 146, 228, 273, 301 and 349 Fg⁻¹. Values are summarised in Table 2.

After TiO₂ incorporation, activated starch-TiO₂ nanohybrid showed C_sp values (Table 2) of 380, 487, 517, 549, 609 and 686 Fg⁻¹. The effect of TiO₂ incorporation led to a greater than five-times increase in the specific capacitance values, with the activated starch. Activated gelatin-TiO₂ nanohybrid showed C_sp values of 397, 461, 522, 593, 641 and 691 Fg⁻¹, while activated starch-gelatin-TiO₂ nanohybrids showed C_sp values of 286, 502, 581, 644, 726 and 808 Fg⁻¹. Whenever the activated starch and gelatin nano-hybrids were combined, there was an increase in the amount of surface oxygen available, hence, higher pseudo-capacitive faradaic charges which contributes to their higher specific capacitance values. This is the first time such a high specific capacitance has been reported for electrochemical tests utilising negative voltages for supercapacitor applications. This confirms that our conductive biopolymer-based nano-architecture can be applied in areas which depend on negative voltages like op-amp circuits, signal control systems, smart switches, and corrosion inhibition.

At scan rates of 200, 100, 50, 20, 10 and 5 mVs⁻¹, the activated starch nanoparticles showed energy density (E_d) values of 12.3, 19.7, 22.8, 36.1, 57.3 and 80.6 WhKg⁻¹. The activated gelatin nanoparticles showed E_d values of 12.9, 24.8, 26.1, 43.5, 70.41 and 108.3 WhKg⁻¹. The nanohybrids of both the activated starch and gelatin nanoparticles showed E_d values of 19.1, 26.2, 26.8, 59.2, 93.6 and 126.5 WhKg⁻¹. After TiO₂ incorporation, activated starch-TiO₂ nanohybrid had E_d values of 30.8, 38.7, 43.2, 71.0, 103.8 and 137.2 WhKg⁻¹, activated gelatin-TiO₂ hybrid had E_d values of 36.3, 42.4, 49.0, 78.6, 109.3 and 159.4 WhKg⁻¹, while activated starch-gelatin-TiO₂ nanohybrid had E_d values of 52.1, 61.9, 73.6, 92.1, 145.7 and 208.3 WhKg⁻¹ (Table 2).

High energy densities in supercapacitors, which are based on conductive polymers, are a result of the presence of permissive polarons and bipolarons which gives room for fast electronic kinetics and diffusion (Ramadoss and Kim, 2014) (Xiong et al., 2016). The power density was 2704, 2791, 2912, 3461, 3813, and 4213 WKg⁻¹ for the respective activated forms of starch nanoparticles, gelatin nanoparticles, starch-gelatin nanohybrids, starch-TiO₂ nanohybrid, gelatin-TiO₂ nanohybrid, starch-gelatin-TiO₂ nanohybrid. In Table 2, W is the Warburg impedance, which estimates the diffusional behaviour of the electrodes, CPE is constant phase element, which estimates the capacitance of an unsmooth electrode surface, and n is a CPE index which shows the capacitive capability of the electrode system.

Nyquist plots can be used to investigate the electron transfer kinetics within supercapacitor electrodes (Kim et al., 2016), Fig. 7 presents the Nyquist plots for the biopolymer-based nano-hybrids in 1 M-Na₂SO₄. The EIS showed ohmic resistance values of 0.110, 0.107, 0.104, 0.101, 0.101, 0.087 ohms corresponding to the equivalent series resistances for the nano-hybrids a to f. The diameter of the semicircles at the moderate frequency region indicates the type of ionic diffusion and level of charge transfer resistance (Xu et al., 2015). Generally, high charge transfer resistances also known as electrochemical system resistance show low capacitances. The charge transfer resistances of starch-TiO₂ nanohybrid, gelatin-TiO₂ nanohybrid, starch-gelatin-TiO₂ nanohybrid were 0.34, 0.31, and 0.29 micro-ohms, which is relatively lower when compared with 0.43, 0.40, 0.39 micro-ohms of starch nanoparticles, gelatin nanoparticles, starch-gelatin nanohybrids without TiO₂ (see Fig. 7).

Close

Fig. 7 (a) EIS (b) GCD (c) Retention capacity of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin nanohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids, at scan rates of 5, 10, 20, 50, 100 and 200 mVs⁻¹.

Export to PPT

The electrochemical performances of the nanohybrids were investigated using galvanostatic charge–discharge (GCD). The GCD curves are shown in Fig. 7 (b) at 1 Ag⁻¹ current density. Generally, all the GCD curves showed nearly symmetrical patterns indicating a good super-capacitive behaviour (Kumar et al., 2019; Nagamuthu et al., 2013). At 1 Ag⁻¹, the activated starch nanoparticles, gelatin nanoparticles, and starch-gelatin nanohybrid showed iR drop of 0.2, 0.16 and 0.13 ohms. After TiO₂ incorporation, Starch-TiO₂, gelatin-TiO₂ and starch-gelatin-TiO₂ nanohybrids showed a lower iR of 0.13, 0.08, 0.06 ohms respectively. Also, the potential drop decreases as the ESR decreases for all the nanohybrid electrodes due to rapid diffusion pathways (Sun et al., 2018), which was confirmed from the previously EIS experiments (see Fig. 7(a)). The relatively high polarization resistance might be due to uneven distribution of active materials at higher charging levels, higher viscosity of the electrolyte as the biopolymers are leached into the electrolyte rates (Kim et al., 2016; Zhang et al., 2019). This can be reduced through use of solid electrolytes.

The cycling stabilities of the nanoarchitectures were studied by using 5000 cycles. After 5000 cycles, a capacitance retention of 95.5, 94, 92, 87, 83, 81% was observed for the six respective nanoarchitectures fabricated. The capacitance loss in nano-hybrids without TiO₂ incorporation has been demonstrated to arise from the volume swelling and the structure relaxation of the materials caused by the intercalation and dissociation of dopant anions during the long-time charging and discharging processes (Xu et al., 2011, 2015). Although, TiO₂ frameworks helped to stabilize starch-gelatin templates thereby reducing the swelling and shrinking rate of the starch and gelatin, during the charging/discharging period.

3.4 Two electrode configuration electrochemical study

The supercapacitive performance of the novel materials was also studied using two electrode systems to establish the specific capacitance, energy density, power density and retention capacity of the starch-gelatin-TiO₂ nano-hybrids. A symmetric supercapacitor assembly was designed to achieve this study. Overall, the specific capacitances followed similar trends as was observed with the three electrode configurations, and the cyclic voltammetry curves that were quasi rectangular maintained their shape even at higher scan rates. Activated starch nanoparticles had C_sp of 47, 83, 106, 148, 171 and 194 Fg⁻¹. The activated gelatin nanoparticles had C_sp of 70, 107, 122, 156, 173, and 204 Fg⁻¹. The activated starch-gelatin had C_sp of 133, 170, 204, 238, 260, and 287 Fg⁻¹. After TiO₂ incorporation, activated starch-TiO₂ had a C_sp of 203, 335, 427, 515, 531 and 541 Fg⁻¹, while the activated gelatin-TiO₂ showed C_sp of 231, 356, 431, 522, 540 and 570 Fg⁻¹, and the starch-gelatin-TiO₂ nanohybrids showed C_sp of 286, 371, 442, 537, 548 and 617 Fg⁻¹. The data is summarised in Table 3.

Elmouwahidi et al., prepared a series of carbon xerogels doped with different percentages of TiO₂. A high specific capacitance of 137F/g at 0.250 A/g was obtained for 20% TiO₂ incorporation. He further reported a retention capacity of 66–80%. Naeem et al., reported an energy density of 81 Wh/Kg and a specific capacitanc eof 2332F/g for a TiO₂ nano-memberane with controlled thickness fabricated by atomic layer deposition. Our work showed a higher specific capacitance and energy density than most of the similar literatures available on carbons, starch, gelatin and TiO₂ nanomaterials. This is as a result of the activation process carried out to reduce the overall resistance of the biopolymers used. This succesful electrochemical testing on two electrode configurations at negative potential windows has helped to confirm that supercapcitors can now be applied in areas of voltage amplifier circuits which depends on negative voltages for gain stability and decreased output impedance. The effect of positive charges at the oxygen vacancy sites within the starch-gelatin-TiO₂ bio-nanoarchitectures also contributes to this high electrochemical performance at two electrodes configurational level (Elmouwahidi et al., 2018; Fukuhara et al., 2016; Naeem et al., 2019) (see Fig. 8).

Close

Fig. 8 (Two electrode configuration) CV of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin nanohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids, at scan rates of 5, 10, 20, 50, 100 and 200 mVs⁻¹.

Export to PPT

In the two-electrode configuration, at scan rates of 200, 100, 50, 20, 10 and 5 mVs⁻¹, the activated starch nanoparticles showed energy density (E_d) values of 9.6, 12.4, 16.0, 26.5, 34.2 and 57.9 WhKg⁻¹. The activated gelatin nanoparticles showed E_d values of 10.8, 19.2, 25.5, 38.4, 51.8 and 89.5 WhKg⁻¹, while activated starch-gelatin-nanohybrid showed E_d values of 13.8, 25.1, 31.0, 49.7, 76.9, and 105.7 WhKg⁻¹. After TiO₂ incorporation, activated starch-TiO₂ had E_d values of 17.1, 33.4, 42.7, 63.6, 94.8, and 122.6 WhKg⁻¹. The activated gelatin-TiO₂ nanoparticles had E_d values of 20.1, 40.8, 44.2, 69.1, 105.9 and 138.7 WhKg⁻¹. The activated starch-gelatin-TiO₂ nanohybrid had E_d values of 22.3, 53.1, 65.0, 79.6, 116.4, and 147.4 WhKg⁻¹, as summarised in Table 3. The power density values were 1983, 2554, 2804, 3005, 3527 and 3742 WKg⁻¹ for the six respective nanohybrids,.

The EIS (Fig. 9), was carried out to analyse the electrostatic contribution and the resistance elements of the nano-hybrids prepared in the symmetric supercapacitor forms from 0.1 Hz to 1 MHz. The data fitted well into a vertically linear Nyquist plot as produced by the Constant Phase Element model used. Also, from Fig. 9, a rapid increase in the imaginary impedance was seen as compared with the real impedance at the lower frequency regions (Srivastava and Karna, 2015). This capacitive behaviour near the −90° phase angle is a clear confirmation that the starch-gelatin-TiO₂ based supercapacitive nano-hybrids are good materials for supercapacitor fabrication. The charge transfer resistance is 0.91, 0.90, 0.88, 0.68, 0.65 and 0.51 ohms; for the six respective nanohybrids.

Close

Fig. 9 (a) EIS (b) GCD (c) Retention capacity of the activated forms of (a) starch nanoparticles (b) gelatin nanoparticles (c) starch-gelatin nanohybrids (d) starch-TiO₂ nanohybrids (e) gelatin-TiO₂ nanohybrids (f) starch-gelatin-TiO₂ nanohybrids, at scan rates of 5, 10, 20, 50, 100 and 200 mVs⁻¹.

Export to PPT

The GCD plot in Fig. 9 showed similar shapes to those observed with the three electrode configurations, and the charge–discharge times decreased after the incorporation of nano-TiO₂. The increased charge–discharge time after the nano-TiO₂ incorporation is due to the abundance of nano-level tunnels, which are being filled up by the diffusion of polaron and solitons, formed from the diffusion of oxygen radicals within the starch-gelatin-TiO₂ nano-hybrids. The iR drops reduced after the nano-TiO₂ was incorporated, and minor deviations were observed from the GCD linearity, which is expected for supercapacitors because of the rapid anodic oxidation peaks of the oxygen groups in the biopolymers used. The discharge times for all the prepared nanohybrids were shorter than the charge time, typical for energy devices with fast power delivery.

The effect of the rapid anodic oxidation before titanium oxide nanoparticles incorporation was clearly seen in the retention capacity plot. The starch and gelatin nano-architectures had lower retention capacities of 81, 83, and 85% compared with 88, 92, 95% after nano-TiO₂ incorporation. This is because of charge leakages, cracks, surface tears and most importantly rapid anodic oxidation of the available oxygen species, which are slowed down apparently after the oxidation. The retention capacities obtained generally were very good when compared with past works on the use of starch biopolymers for energy storage applications. This high capacity might be due to the use of all our biopolymers in the nano-scale since the quantum-size effect helps to further strengthen the internal interactions thereby improving the overall strength of capacitance retention in the nano-hybrids (Liu et al., 2017).