FIELD OF APPLICATION OF THE INVENTION

The present invention relates to photocatalytic, electrocatalytic and photo- electrocatalytic materials and in particular, metal tellurium oxides and their use as (i) catalysts for water oxidation/splitting, and (ii) active materials for the conversion and storage of solar energy as electrochemical energy.

BACKGROUND TO THE INVENTION

An ever-increasing focus on preserving our planet and reducing our reliance on a fossil infrastructure has resulted in a global drive towards clean and renewal energy. Where the majority of electricity generation comes from coal, nuclear and other non renewable power plants, renewal energy sources offer an ideal alternative with fewer environmental impacts.

Solar energy has been found to be an effective source of energy, which is capable of sufficient scale to meet future global energy demand. Considered to be the pioneers of solar energy, the work of Fujishima and Honda (Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 5358, 37-38) showed how the processing of splitting water into molecular hydrogen and oxygen could provide an alternative energy source. During the splitting of water, energy is consumed to split water into its hydrogen and oxygen gas components, with hydrogen acting as an energy store, after which the gasses are available to recombine to again form water and release the stored energy. To facilitate the process of water splitting, catalysts are required to lower the energy required in the aforementioned process.

Of interest to the present invention is the technique of photo-electrocatalytic (PEC) water splitting, where hydrogen is produced from water using ultraviolet/visible light irradiation and the application of an applied potential in combination with photo- electrocatalytic materials. Ultimately, the effectiveness of converting solar energy into chemical energy will rely on the photo-electrocatalyst. The important role of the photo-electrocatalyst results from its involvement in the enhancement of the oxygen evolution reaction (OER) step, which in turn improves the conversion of solar to chemical energy and storage of the same.

To this end, many studies have focussed on creating novel photo-electrochemical materials that include transition metal oxides/hydroxides, metal phosphides and/or phosphates, perovskite oxides, metal chalcogenides, carbon nanomaterials, amongst many others. In addition to the aforementioned synthesized materials, studies have shown that metal doping and the introduction of electrocatalysts as cocatalysts could improve the activity of PEC materials. A significant drawback of the majority of these materials is that they immediately lose their activity once illumination is terminated. As such, there is a need for PEC materials that can be photocharged and continue to facilitate PEC splitting once illumination has ceased (in the dark). Lou et al (Lou, S. N.; Ng, Y. H.; Ng, C.; Scott, J.; Amal, R. Harvesting, storing and utilising solar energy using Mo03: Modulating structural distortion through pH adjustment. Chem. Sus. Chem. 2014, 7 (7), 1934-1941) reported that the a-Moq3 catalyst could be utilized as a thin film that is capable of storing and discharging its charge under dark conditions. In addition, it was also shown that a-Moq3 could be recharged with successive irradiation, which indicates that this material has battery like properties.

Two studies by Kriek et al (R. J. Kriek, M. Z. Iqbal, B. P. Doyle, and E. Carleschi ACS Applied Energy Materials 2019 2 (6), 4205-4214 & M. Z. Iqbal, E. Carleschi, B. P. Doyle, and R. J. Kriek ACS Applied Energy Materials 2019 2 (11), 8125-8137) reported that two binary-metal oxides, namely europium(lll) tellurium oxide and nickel(ll) tellurium oxide, are capable of maintaining their current density after illumination has ceased, which indicates that photocharging of the materials, and the subsequent utilization of the stored charge, could be used to drive the OER (in the dark) step in the process of water splitting. Prior to the publication on the properties of europium(lll) tellurium oxide, WO 2019/239235 was also filed which disclosed europium(lll) tellurium oxide and its use as a photo-electrocatalyst.

While the nickel(ll) tellurium oxide and europium(lll) tellurium oxide provided some interesting results, there is a need for a compound that is capable of utilizing a greater stored charge over a longer period of time. Furthermore, it would be advantageous if the OER step in the process of water splitting could be driven in the dark over a longer period of time. Given the above, it is clear that there exists a present need for novel photo electrocatalysts that are capable of converting solar to chemical energy, store it as such, and release the stored energy once the source of irradiation has been removed. In addition, such a photo-electrocatalyst should be capable of being recharged with successive irradiation to afford it battery-like properties.

OBJECT OF THE INVENTION

It is accordingly an object of the present invention to provide a photo-electrocatalyst and/or semiconductor that overcomes, at least partially, the abovementioned problems and/or which will be a useful alternative to existing photo-electrocatalysts and/or semiconductors.

SUMMARY OF THE INVENTION

According to a first aspect thereof, there is provided a cobalt tellurium oxide (Co3TeC>6) compound which retains an increased electro-active state induced by a preceding illumination step.

The cobalt tellurium oxide compound may retain an increased electro active stated induced during by the preceding illumination step for a period of at least 150 minutes after the illumination step has ceased.

According to a second aspect thereof, there is provided for the use of a cobalt tellurium oxide compound, according to the first aspect of the present invention, as a photo-electrocatalyst. The cobalt tellurium oxide compound may be used as a photo-electrocatalyst in a water splitting process.

The cobalt tellurium oxide compound may be used as a photo-electrocatalyst in an electrochemical reaction.

The electrochemical reaction may be an oxygen evolution reaction in an electrolysis reaction for the production of hydrogen.

The electrochemical reaction may also be an oxygen reduction reaction in fuel cells which is used for the supply of electricity.

According to a third aspect of the present invention, there is provided for the use of a cobalt tellurium oxide compound, according to the first aspect of the present invention, as a semiconductor.

It is envisaged that the cobalt tellurium oxide compound, when used as a semiconductor, may be used as an active material in a photovoltaic (PV) cell, whereby the material lends the PV cell battery-like characteristics in that it can be photocharged.

According to a fourth aspect of the present invention, there is provided for the use of the cobalt tellurium oxide compound in a coating solution for a working electrode of an electrochemical reaction. There is provided for the coating solution to be prepared by a process including the steps of:

(i) dispersing 30 mg of the cobalt tellurium oxide in 1 cm ³ of ethylene glycol under ultrasonication for 60 minutes;

(ii) adding 0.4 cm ³ of Nafion (sulfonated tetrafluorethylene) and 0.2 cm ³ of 0.1 mol. dm ³ NaOH were added to the suspension and sonicated for 60 minutes;

(iii) Polishing glassy carbon electrode inserts (GCEs) with a 0.05 pm alumina suspension; and

(iv) placing an 0.1 cm ³ (or 10 pL) aliquot of the suspension onto the GCEs and drying thereof overnight at 70 °C.

According to a fifth aspect thereof, there is provided a metal tellurium oxide compound wherein the metal compound may be platinum and bismuth, and wherein the metal tellurium oxide retains an increased electro-active state induced by a preceding illumination step.

The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described further, by way of example only, with reference to the accompanying drawings wherein:

Figure 1 shows the (a) UV-visible absorption spectra, and (b) Tauc plots of CTO-400 and CTO-900.;

Figure 2 shows a schematic diagram of the photo-electrochemical cell employed;

Figure 3 shows the current density-potential curves for (a) CTO-400, (b)

CTO-700, (c) CTO-900 and (d) CTO-1100 (conditions: O2- purged 0.1 mol.dnr ³ KOH electrolyte under UV-light illumination, scan rate of 10 rnV.s ¹ , at 0 rpm, 25 °C);

Figure 4 Current density-potential curves of (a) CTO-400, (b) CTO-700,

(c) CTO-900 and (d) CTO-1100 (conditions: 02-purged 0.1 mol.dnr ³ KOH electrolyte under UV-light illumination, scan rate of 10 mV. s ¹ , at 1600 rpm, 25 °C);

Figure 5 shows current density vs. illumination time plots for CTO-400,

CTO-700, CTO-900 and CTO-1100, at 1.8 V vs. RHE, for (a) 0 rpm, and (b) 1600 rpm, in oxygen-purged 0.1 mol.dnr ³ KOH electrolyte at 25 °C; Figure 6 shows graphs pertaining to light on-off LSVs of (a) CTO-400, (b) CTO-700, (c) CTO-900 and (d) CTO-1100 (conditions: 02- purged 0.1 mol.dnr ³ KOFI electrolyte under UV-light illumination, scan rate of 10 rmV.s ¹ , at 0 rpm, 25 °C);

Figure 7 shows graphs pertaining to light on-off LSVs of (a) CTO-400, (b) CTO-700, (c) CTO-900 and (d) CTO-1100 (conditions: 02- purged 0.1 mol.dm-3 KOFI electrolyte under UV-light illumination, scan rate of 10 mV.s-1 , at 1600 rpm, 25 °C);

Figure 8 shows current density vs. light on-off time plots for all the photo electrocatalysts at (a) 0 rpm, and (b) 1600 rpm, at 1.8 V vs. RFIE, in oxygen-purged 0.1 mol.dnr ³ KOFI electrolyte at 25 °C; and

Figure 9 shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample CTO-900) in an O2 purged 0.1 M KOFI solution after termination of UV illumination and at a working electrode rotation of 1600 rpm.

The presently disclosed subject matter will now be described more fully hereinafter with reference to the accompanying Examples, in which representative embodiments are shown. The presently disclosed subject matter can, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art.

DETAILED DESCRIPTION OF THE DRAWINGS

Synthesis of cobalt tellurium oxide compounds

Cobalt tellurium oxide compounds were synthesized at different calcination temperatures by employing the sol-gel process as described in the publication by Iqbal et al (Iqbal, M. Z.; Carleschi, E.; Doyle, B. P.; Kriek, R. J. Photocharged Water Splitting Employing a Nickel (II) Tellurium Oxide (Photo) anode in Alkaline Medium. ACS Appl. Energy Mater. 2019, 2 (11 ), 8125-8137).

This was done by pouring 0.1 mol.dnr ³ HQTQOQ to 50 cm ³ of a 2:1 (v/v) ethanol-water mixture and stirring the resulting solution at 30 °C for 30 minutes. An amount of 0.2 mol.dnr ³ Co(N03)2.6H20 was added to this solution, followed by the addition of the chelating agent (0.6 mol. dm ^-3 3 citric acid) to facilitate effective solution mixing. The solution was allowed to stir for another 30 minutes after which a cross-linking agent (2.5 cm ³ ethylene glycol) was added to the mixture. The pH of the solution was adjusted to 11 by the addition of an ammonia solution. The solution was then heated at 70 °C under constant stirring to obtain a gel, after which it was oven-dried overnight at 120 °C. The obtained powder was crushed by mortar and pestle, and heated in a furnace at 300 °C for 120 minutes. The powder obtained was crushed again and separated into four samples. These samples were calcined at increasing temperatures of 400 °C (CTO-400), 700 °C (CTO-700), 900 °C (CTO-900) and 1100 °C (CTO-1100) for 300 minutes (5 hours). After calcination, the samples were collected, ground, and stored in a desiccator for further use and analysis (see below) , where the latter identified the compounds as Co3TeC>6.

Analysis

UV-VIS

Figure 1a shows the UV-Vis absorption spectra of the CTO-400 and CTO-900 compounds. A prominent peak is observed at « 375 nm, with a red shift observed for CTO-900, calcined at a higher temperature. The occurrence of a small peak at -460 nm was also noticeable for CTO-900, which could be attributed to the presence of a trace amount of C03O4 on the surface of the compound.

Tauc plots

Figure 1b shows the Tauc plots obtained for CTO-400 and CTO-900. The optical band gap (Eg) of the compounds were estimated from the Tauc plots (Figure 1b), according to Equation 1 :

(ahu) ^A 2 = A(hu-Eg) (1) where Eg is the optical band gap, hu is the photonic energy, a is the absorption coefficient, and A is the proportionality constant for the material. From these data, the optical band gap of CTO-900 was estimated as 3.10 eV, which is indicative of the compound being UV-light active. This result was used in order to select the choice of lamp for further PEC studies. As such, a lamp that emits at 253.7 nm was selected for further studies.

Electrocatalvtic and photo-electrocatalvtic measurements

A catalyst 'ink' for (photo)electrocatalytic measurements was prepared by dissolving 30 mg of the (photo)electrocatalyst in 1 cm ³ of ethylene glycol; followed by ultrasonication for 60 min. Afterwards, 0.4 cm ³ of Nafion (sulfonated tetrafluorethylene) and 0.2 cm ³ of 0.1 mol. dm ^-3 NaOH solution were added to the suspension and sonicated for 60 minutes. Glassy carbon electrode inserts (GCEs) with a geometric surface area of 0.196 cm ² were prepared by polishing them with a 0.05 pm alumina suspension. A 0.1 cm ³ (or 10 pL) aliquot of the ink suspension was dropped onto GCEs with the aid of a micropipette, and dried overnight in a vacuum oven at 70 °C. Thereafter, the GCEs were cleaned sequentially in Milli-Q water, ethanol and isopropanol under ultrasonication for 20 minutes each, subsequent to which the GCEs were dried by exposing them to a stream of nitrogen gas.

Figure 2 shows an in-house built three-electrode jacketed electrochemical cell (manufactured from polypropylene) with a quartz tube slot to fit the UV lamp. The device was utilized in order to conduct electrochemical (EC) and photo electrochemical (PEC) measurements. The EC and PEC activity measurements of the prepared (photo)electrocatalysts were performed by using separate inks to limit any contamination. For example, the presence of any carbon may inhibit light absorption of the compounds. The temperature was kept constant at 25 °C.

A platinum wire and a Hg/HgO electrode were employed as the counter and reference electrodes, respectively. Electrocatalytic and photo-electrocatalytic measurements were performed on a rotating-disk working electrode (RDE) setup. The GCE was inserted into the RDE and linear polarization curves of the samples were recorded by employing a VSP double-channel potentiostat. A Philips UVC germicidal lamp (TUV PL-S 9W/2P) emitting short-wave UV radiation, with a sharp and dominant peak at 253.7 nm, was used as the light source (see discussion regarding lamp choice above).

EC and PEC measurements were conducted in 02-saturated 0.1 mol. dm ^-3 KOH at 25 °C at an RDE rotation speed of either 0 or 1600 rpm. The calibration of the Hg/HgO electrode vs. RHE (reversible hydrogen electrode) was conducted in a H2- saturated 0.1 mol dnr ³ KOH solution and was measured as -0.935 V vs. RHE. All potentials were IR-corrected by employing Equation 2.

E(IR corrected)= Eapp - IR (2)

Here I is the current, R is the ohmic resistance of the electrochemical cell, and Eapp is the applied potential. A single point high frequency impedance measurement was used to measure the ohmic resistance of the electrochemical cell (R), which generated 43.12 W (CTO- 400), 42.34 W (CTO-700), 40.62 W (CTO-900) and 44.22 W (CTO-1100) respectively, in a 0.1 mol dnr ³ KOH solution. Employing the OER as a model electrochemical reaction (equation 3), the EC and PEC activity of the material was probed by means of a series of linear sweep voltammetry (LSV) measurements, recorded in an alkaline medium (0.1 mol dnr ³ KOH) at a scan rate of 10 mV.s ¹ . Experiments were conducted from 0 to 1 V vs. Hg/HgO by rotating the RDE at either 0 or 1600 rpm.

Figures 3a to 3d shows LSV curves obtained at a scan rate of 10 mV.s ^-1 ; rotation of 0 rpm; and temperature of 25 °C. From these figures, it can be seen that CTO-900 provided the highest current density value of 1.2 mA.cnr ² after 150 minutes of UV- light illumination.

Figures 4a to 4d shows LSV curves obtained at a scan rate of 10 mV.s ^-1 ; rotation of 1600 rpm; and temperature of 25 °C. From these figures, it can be seen that CTO- 900 provided the highest current density value of 4.3 mA.cnr ² after 150 minutes of UV-light illumination.

Figure 3 and 4 clearly depict the greatest activity increase being observed for the CTO-900 sample. This is clearly evident when plotting the current densities, at 1.8 V (vs. RHE), for each (photo)electrocatalyst at different illumination times as seen in Figures 5a and 5b. From Figure 5a, and 0 rpm (absence of forced mass transfer), the PEC activity declined in the order of:

CTO-900 > CTO-700 > CTO-1100 > CTO-400.

From Figure 5b, and rotation of the working electrode at 1600 rpm (forced mass transfer), the PEC activity declined in the order of:

CTO-900 > CTO-1100 > CTO-700 ~ CTO-400.

Investigations into the photo-electrochemical properties of cobalt tellurium oxide compounds were performed by recording LSV curves under three different conditions, namely:

(i) in the absence of any initial illumination, i.e. under an EC condition;

(ii) under a PEC condition after 150 min of illumination; and

(iii) under an EC condition, in the dark (from 10 - 40 min, with 10 min intervals), subsequent to a 150 min illumination time, at 0 rpm and 1600 rpm.

Figures 6 and 7 show the current density of the cobalt tellurium oxide compounds that have been placed under conditions as set out in (iii) above for 0 rpm and 1600 rpm, respectively. Figure 8 provides a summary of Figures 6 and 7, from which only a small increase is observed at 0 rpm. Flowever, at 1600 rpm, a notable increase is observed for the current density of the CTO-900 compound. More specifically, there is more than a five-fold increase in the current density when comparing the CTO-900 compound that has been illuminated for 150 min to the non-illuminated EC current densities. It is to be understood that the enhanced PEC current after 150 min of illumination results from photo-induced charge separation that further results in an increased concentration of positive holes that contribute to the OER.

It should further be noted that at 1600 rpm, the CTO-900 compound exhibits an active OER dark current that is 73% of the 150 min PEC-value after 10 minutes in the dark. After 40 minutes in the dark, this decreased to 67% of the 150 min PEC- value. However, the decrease still maintains a current density that is almost a four fold increase compared to the non-illuminated EC current.

Chronoamperometric experiment

A chronoamperometric (CA) experiment was utilized to investigate the photo-induced charge storage by employing a sequential switching experiment in the presence of a sodium metal ion for the CTO-900 compound. The experiment was performed at 1 V (vs. Pt) in I\l2-purged 0.1 mol. dm ^-3 Na2SC>4 at 25 °C, with the termination of UV-light after every 10 minutes.

The results are summarized in Figure 9, which shows that the current density sharply increases after an induction period of 10 minutes, after which the same stabilizes after a period of approximately 150 minutes. The charge storage capacity of the CTO-900 material was confirmed by the process of on- and off-switching of the UV- light not resulting in an immediate drop in current back to the baseline. On permanent termination of the UV-light, an initial decrease can be observed in Figure 9, after which a slow discharge of the material can be observed over a prolonged period of time. As such, Figure 9 would serve to indicate that the cobalt tellurium oxide, and more specifically the CTO-900 compound, is capable of facilitating the conversion of solar energy to chemical energy. In addition, the same compound is capable of storing such energy and releasing it in the dark over time. From the above, it has been shown that in the presence of light, cobalt tellurium oxides are capable of facilitating higher efficiency water splitting compared to pure EC water splitting, and also continues in the dark.

The properties of the cobalt tellurium oxides materials could also find application dye sensitized solar cells as photovoltaic cells, wherein solar energy is converted and stored electrochemically during daylight hours and returned to the grid during night time hours.