Figure 2a shows the N2 adsorption-desorption BET isotherm from the TiO2 nanopowder. The BET surface area was calculated to be 269 m2 g− 1. The profile resembles a type IV isotherm according to the IUPAC classification. Fig. 2b shows a pore size distribution between 25 Å to 100 Å with a dominant peak at around 55 Å. This corresponds well with the isotherm in Fig. 2a, which demonstrates the mesoporous nature (2 nm – 50 nm) of the powder.
Cyclic voltammetry of two TiO2 electrodes was performed at various scan rates, ν, in 1 mol dm− 3 AlCl3 aqueous solution. One electrode was scanned between 0 V to − 1.3 V vs SCE and the other between the potential range of 0 V to − 1.0 V vs SCE. By limiting the potential window, charge storage may be limited to a capacitive or surface controlled mechanism. Figure 3a presents the profiles measured from TiO2 at the 5th, 18th and 25th cycles when swept between the extended potential range of 0 V to − 1.3 V vs SCE. Between the 5th and 18th cycle there is small reduction in the cathodic peak, from − 13.0 A g− 1 to − 11.8 A g− 1, while the anodic peak potential shifts from − 1.03 V, during the 10th scan, to − 0.97 V vs SCE during the 18th. Figure 3b shows the profile from TiO2 during the 5th, 25th and 80th scan at 10 mV s− 1 between 0 V to − 1.0 V vs SCE. The profiles can be seen to be nearly identical irrespective of scan number, suggesting improved stability from TiO2 when cycled at a more positive minimum potential.
Figure 4a shows the CV scans at 3, 9 and 16 mV s− 1 between 0 V to − 1.3 V vs SCE. The profile shapes at these three scan rates closely resemble each other. During the cathodic sweep, current curves down between − 0.55 V to − 0.95 V where there is a brief plateau till ca. -1.1 V. The current curves down to a prominent peak between − 1.15 V and − 1.20 V vs SCE. At 3 mV s− 1, the reverse sweep gives rise to a prominent peak at − 1.05 V. The position of this peak becomes more positive with increasing scan rate with peak position being approximately − 0.95 V at 16 mV s− 1. As with the cathodic sweep, the anodic sweep gives rise to a slight shoulder and plateau – between ca. -0.9 V and − 0.75 V, when current drops steadily to zero at approximately − 0.5 V. Fig. 4b gives the peak currents against the square root of the scan rates. A linear fit, with an x-y intercept set to zero, shows there is an approximately linear relationship between the measured current and square root of the scan rate for both cathodic and anodic sweeps. A linear relationship suggests a diffusion-limited process, as described by the power law given by equation… (1), where a and b are adjustable values, i is the measured current and ν the scan rate [27, 28].
A b-value of 0.5 is often measured from intercalation electrodes, with the measured current limited by the solid-state diffusion (intercalation) of the cation through the electrode. This may be true for the case of TiO2 and Al3+, given the use of a relatively high concentration electrolyte, which should negate the possibility of a reaction being limited by the diffusion of Al3+ through the electrolyte to the electrode surface.
However, the greater stability of TiO2 when scanned with the more positive minimum potential of − 1.0 V vs SCE, compared to − 1.3 V, suggests the possibility of a separate charge storage mechanism compared to when the electrode is scanned to − 1.3 V. That is, the redox reaction of Ti4+ to Ti3+ may only take place once more negative potentials are reached. As such, further CV scans were performed between 0 V to − 1.0 V vs SCE. Fig. 4c shows these CV profiles at scan rates between 2 mV s− 1 to 100 mV s− 1, normalised by scan rate. That the profiles do not fall onto a single profile means that charge storage in this potential range is not purely capacitive. Further analysis of the CV profiles can be performed by calculating the capacity of the electrodes at different scan rates. This technique has previously been used in the literature with materials, such as Nb2O5, NiCo2O4, LaB6, conductive polymers and for Li+ insertion into mesoporous titania [29,30,31,32]. The analysis can provide an indication of charge storage arising from bulk or surface mechanisms at given scan rates. Fig. 4d shows the cathodic and anodic voltammetric capacities against ν-1/2. For the cathodic charge input, the volumetric capacity is linearly proportional to ν-1/2 at scan rates up to 30 mV s− 1, (0.182 mV s− 1)-1/2. Extrapolation of the linear fit to 0 (mV s− 1)-1/2 suggests a surface charge storage contribution of approximately 12 mA h g− 1. Therefore, at a scan rate of 10 mV s− 1, for example, the surface contribution to capacity would be approximately 50%. The remaining charge could then be the result of a bulk process such as intercalation. Alternatively, it could suggest there are areas of the electrode, such as narrow pores, that are difficult to access. At scan rates above 30 mV s− 1, the charge vs ν-1/2 plot deviates from linearity, suggesting a change in the rate-limiting charge storage process or that charge storage is almost entirely dominated by a semi-infinite diffusion. At lower scan rates, between 2 to 30 mV s− 1, the extrapolation of the linear dependence of cathodic capacity vs ν-1/2, to approximately 12 mA h g− 1, suggests that charge storage is diffusion controlled. Given the low capacities, it is still unlikely that this diffusion limitation is a result of Al3+ intercalation through the crystal structure of anatase-TiO2 but may instead be due to the limited diffusion of electrolyte and Al3+, due to the short time constants at these high scan rates, through the electrodes pores. While there may be a capacitive contribution, as deduced from the extrapolation of the infinite scan rate capacity, the non-conformity of the normalised scan rates suggests there is also a diffusion controlled charge storage mechanism.
The existence of a surface controlled storage mechanism, along with the mesoporous structure of the 5 nm TiO2 powder (Fig. 2), suggests performance can be improved through greater electrolyte-electrode contact. In order to achieve this, a simple vacuum impregnation technique was employed to ensure proper electrode wetting. The experimental set-up and proposed schematic of forced electrode wetting were presented in Fig. 1. It is proposed that electrode pores previously inaccessible to electrolyte, due to surface tension and the hydrophobicity of the nanopowder electrode, are filled with electrolyte due to the removal of air and creation of low pressure voids within the electrode. Constant current cycling was then performed on a vacuum impregnated electrode in a 3-electrode cell between 0.4 V to − 1.0 V vs SCE. The coulombic efficiency and discharge capacity of the vacuum impregnated electrode when cycled at specific currents between 0.2 to 40.0 A g− 1 is shown in Fig. 5a. The figure shows the 10th cycle at a given specific current between cycles 70–120 for as-manufactured TiO2 and cycles 70–200 for impregnated TiO2. For comparison, the performance of an as-manufactured electrode, when cycled up to 6.0 A g− 1, is also shown in Fig. 5a. Additional file 1: Figure S1 shows the discharge capacity and coulombic efficiency of the two electrodes vs cycle number. Between 0.2 A g− 1 and 1.0 A g− 1, discharge capacity from the vacuum impregnated electrode decreases from 21.8 mA h g− 1 to 19.8 mA h g− 1, with coulombic efficiency increasing from 89.8 to 96.9%. At 2.0 A g− 1, coulombic efficiency was 99.4%, though discharge capacity was also measured at 19.8 mA h g− 1. Between 1.0 A g− 1 to 25 A g− 1, discharge capacity decreased by only 12.2% to 17.4 mA h g− 1. At 40.0 A g− 1, discharge capacity was measured at 15.3 mA h g− 1. Above 2.0 A g− 1, coulombic efficiency remained around 99.9%, though some error will be present due to the rapid charge discharge times, i.e. at 40.0 A g− 1 discharge occurs in 1.43 s, even at the used measurement rate of 80 data points per second. Coulombic efficiency of an as-manufactured electrode is lower throughout and while discharge capacity is comparable up to 2.0 A g− 1, once cycled at 6.0 A g− 1, discharge capacity was measured at 15.7 mA h g− 1 compared to 19.33 mA h g− 1 for the vacuum impregnated electrode.
The voltage profiles from the vacuum impregnated electrode between 1.0 A g− 1 to 40 A g− 1 are given by Fig. 5b. Voltage profiles can be seen to be similar, irrespective of the specific current used. The initial IR-drop at 1 A g− 1 is minimal, being less than 10 mV and only becoming noticeable at higher specific currents. At 10.0 A g− 1, the IR-drop is measured as 44 mV, increasing to 162 mV at 40.0 A g− 1, with the average charge and discharge potentials at 40.0 A g− 1 being − 0.826 V and − 0.627 V, respectively. For comparison, the IR drop from the as-manufactured electrode at 6 A g− 1 was already 124 mV. The results presented in Fig. 5 show a clear improvement in rate capability of electrodes subjected to the vacuum impregnation technique. This specific currents reached are considerably higher than have been previously reported for TiO2 in aqueous Al3+-containing electrolytes. It should also be noted that the experiment was performed on an electrode with a relatively high mass loading of 6.5 mg cm− 2, so that the corresponding current density at 40 A g− 1 is a very high value of 260 mA cm− 2. In comparison, capacities of 50 mA h g− 1 and ca. 62 mA h g− 1 were measured from MnHCF (positive) and graphene (negative) electrodes were achieved at the current density of 5 mA cm− 2 in LiNO3 [33, 34]. These capacities and current densities are toward the maximum reported for aqueous capacitive devices. Furthermore, the relative stability of the voltage profiles and capacity, where discharge capacity drops by < 25% over an order of magnitude increase in specific current, provides evidence that charge storage from these TiO2 electrodes in aqueous Al3+ electrolyte are predominantly capacitive or controlled by surface reactions at high currents, similar to psuedocapacitive materials. However, care should be taken in describing TiO2 as psuedocapacitive given the relatively clear voltage plateaus observed during constant current cycling, in aqueous Al3+-containing electrolytes, which is in contrast to the electrochemical characteristics of a capacitor.
The vacuum impregnation process was also repeated on a CuHCF electrode, envisaged as a potential positive electrode, with the effect on voltage profiles and capacities at various rates shown in Additional file 1: Figure S2. Capacity from the vacuum impregnated electrode, which had a mass loading of 8.8 mg cm− 2, was measured at 47.08 mA h g− 1 at 0.5 A g− 1 and maintained a capacity of 28.2 mA h g− 1 at 8 A g− 1. The capacity of the as-manufactured CuHCF electrode, with a mass loading of 8 mg cm− 2, was 44.42 mA h g− 1 at 0.5 A g− 1 and decreased to 14.1 mA h g− 1 at 6 A g− 1. The results demonstrate the applicability of the vacuum impregnation process for improving the performance of alternative electrodes.