NL2011354C2

NL2011354C2 - Process for preparing an anode material, an electrochemical cell and a process to convert water.

Info

Publication number: NL2011354C2
Application number: NL2011354A
Authority: NL
Inventors: Khurram Saleem Joya; Hubertus Johannes Maria Groot
Original assignee: Univ Leiden
Priority date: 2013-08-29
Filing date: 2013-08-29
Publication date: 2015-03-03
Also published as: WO2015030591A1

Abstract

The invention is directed to a process for preparing an anode material suitable for the catalytic production of oxygen from water, comprising providing an aqueous solution comprising (i) ionic Group 8-11 metal, preferably nickel, silver and/or cobalt, (ii) anionic bicarbonate and (iii) dissolved carbon dioxide, providing an anode submerged in the aqueous solution, and causing a Group 8-11 metal comprising layer of catalytically active anode material to form on the surface of the anode by application of a voltage to the anode.

Description

PROCESS FOR PREPARING AN ANODE MATERIAL, AN ELECTROCHEMICAL CELL AND A PROCESS TO CONVERT WATER

The invention is directed to a Process for preparing an anode material, an electrochemical cell and a process to convert water by electrochemistry into oxygen and protons.

Processes are known to convert water by electrochemistry or photo-electrochemical (PEC) processes into oxygen and protons. Such processes, also referred to as catalytic water oxidation or water splitting, are for example promising and attractive for the direct conversion of light energy into renewable fuels and cheap energy carriers. In water electrolysis systems, the H2 is produced under harsh chemical conditions, i.e. highly acidic or strong alkali media at elevated temperature with expensive electrode assemblies and an electrochemical set-up. But, the overall efficiency of water oxidation process is largely limited by the oxygen evolution reaction (OER) that requires a high overpotential to drive it. WO-A-2012/050436 describes a catalytic oxidation using precious metal oxides like ruthenium oxide and iridium oxides as part of the catalyst. It is possible to perform the catalytic water oxidation at a low overpotential using such a catalyst. However, their use in large scale applications is impeded by their high prices and limited availability.

Systems based on non-noble metals are described by, for example, Kanan, M. W., et al., Science 321, 1072 (2008). This paper describes a method for preparing an anode suitable for the electro-catalytic production of oxygen from water, by providing a phosphate-buffered aqueous solution comprising cobalt (II) ions. Next a voltage is applied to an anode submerged in this solution causing a layer of catalytic material to form in situ on the anode by electrodeposition. It is reported that this catalyst enables one to perform the water oxidation at neutral (pH=7.1) conditions. A disadvantage of this process is that Co(ll) has a limited solubility in the phosphate-buffered aqueous solution. A further system based on a non-noble metal catalyst is described in Dinca, M., et al., PNAS 2010, 10337-10341. This paper describes a method for preparing an anode suitable for the electro-catalytic production of oxygen from water, by electrodeposition of a dilute N|2+ solution in borate electrolyte at pH 9.2. The actual water splitting process is performed at relatively high pH. A disadvantage of this process is that Ni(ll) has a limited solubility in the borate-buffered aqueous solution.

Chen, Z. et al., Angew. Chem. Int. Ed. 2013, 52, 700-703 describes the preparation of a catalytically active anode by electrodeposition from an aqueous solution comprising CUSO4 and Na2C03 at a pH of about 10.8. US2012/0031770 describes a process for reducing carbon dioxide to methane at a boron carbide based cathode of an electrochemical cell. The anode in this process is platinum. The use of the platinum anode results in an acceptable performance. However, large scale application is impeded by the high price and limited availability of platinum.

The object of the present invention is to provide a self-assembly catalyst for an anode suitable for the electro-catalytic production of oxygen and protons from water with good activity.

This object is achieved by the following process. Process for preparing an anode material suitable for the catalytic production of oxygen and protons from water, comprising providing an aqueous solution comprising (i) ionic Group 8-11 metal, (ii) anionic bicarbonate and (iii) dissolved carbon dioxide, providing an anode submerged in the aqueous solution, and causing a Group 8-11 metal comprising layer of catalytically active anode material to form on the surface of the anode by application of a voltage to the anode.

The invention is also directed to an electrochemical cell comprising an anode as obtainable by the above, a cathode, a power source connected to both anode and cathode and an aqueous solution in which both anode and cathode are submerged.

The invention is also directed to a process to convert water into oxygen using an electrochemical cell as described above by applying a voltage to the anode using a power source.

Applicants found that the catalytic water oxidation can be performed at a lower pH and with high activity when an anode is used as prepared by the process according to the present invention. Furthermore the anode as prepared by this process enables one to perform the catalytic water oxidation in the presence of dissolved carbon dioxide. This is especially advantageous when carbon dioxide is reduced to valuable hydrocarbons at the cathode of the same electrochemical cell enabling a simpler cell configuration. A further advantage is that the process requires easy-accessible materials, is very efficient for water splitting in a CO2 rich environment and capable for making liquid fuels in combination with a CO2 reduction system. Preparing hydrocarbon fuels by means of CO2 reduction at the cathode is advantageous if one does not desire to make hydrogen at the cathode. The thus prepared hydrocarbon fuels have less safety issues as hydrogen and are more easily stored and transported. Working in CO2 enriched condition is not compatible with the phosphate-buffered conditions or with the borate-buffered condition of the earlier described prior art processes.

Applicants further found that the anode as prepared enables one to perform the catalytic water oxidation at a high current density for oxygen evolution in a CO2 enriched condition. Applicants further found that the catalytic water oxidation using the anode as formed by the process may operate at moderate over-potential with high performance. The above is especially advantageous when such an anode is used in a system to prepare liquid fuels, like for example methanol or formic acid from water and CO2. In addition, the anode as prepared is also very efficient to make O2 and H2 by the water splitting reaction, offering an easy route to a gaseous fuel production system that delivers pure water as its waste product.

Other aspects, embodiments and features of the invention shall be described in more detail below.

The Group 8-11 metal is defined as the metals of Group 8-11 of the periodic system of elements in accordance with the 1984 IUPAC recommendations and includes for example Fe, Co, Ni, Cu, Ru, Ir, Os, Rh, Pd and Ag. Preferably the ionic Group 8-11 metal is ionic nickel, ionic cobalt, ionic iron and/or ionic silver, more preferably ionic cobalt and/or ionic nickel and most preferably ionic cobalt.

In the process a catalytic material is formed on the surface of the anode by submerging part of an anode in an aqueous solution comprising a Group 8-11 metal ionic species M with an oxidation state of n (Mn+), for example Ni(ll), Co(ll) and Ag(l), anionic bicarbonate (HCO3·) and dissolved carbon dioxide (CO2). As a voltage is applied to the anode, the metal ions may be oxidized to an oxidation state n+m and m electrons may be transferred from the anode to the cathode. It is believed that the metal ionic species interact with the bicarbonate and possibly also with the dissolved carbon dioxide to form a substantially soluble complex that facilitate in the formation of a catalytic active layer on the surface of the anode for water oxidation.

The aqueous solutions may thus comprise ionic nickel or ionic cobalt or ionic silver and more preferably ionic cobalt. The nickel ions are preferably present as Ni2+. The cobalt ions are preferably present as Co2+. The metal may be dissolved in the aqueous solution as part of a salt. The solubility of these salts will determine the suitability for use in the present invention. Preferably the KSp of such a salt is between 10-3 and 10"50, preferably below 1CH 0. Applicants found that the presence of CO2 in a HCO3· system shifts the pH from about 8.2 to a lower region, especially between 6.7 and 6.8. The lower pH enhances the solubility of the ionic metals, thereby enabling the use of higher concentrations of metal ions in the bulk of the aqueous solution. Applicants believe that this in turn is advantageous for obtaining a thicker and a more stable catalyst film on the electrode using the process according to the invention. Examples of suitable anions of such salts are NO3", SO42-, Cl", CIO4-, CH3COO-, CO32-, PO43-, BF4-, PF6- and OTf (triflate).

Examples of suitable nickel and cobalt salts are Co(N03)2-6H20, CoCl2.nH20 (n= 0 - 6), CoS04.nH20, Ni(NC>3)2.nH20, wherein n is from 0 to 6, N1SO4.6H2O and NiCl·? nH20 wherein n is from 0 to 6.

The bicarbonate ions may be added to the aqueous solution as a salt, preferably an alkali metal salt, for example NaHC03, KHCO3 and L1HCO3.

The concentration of the bicarbonate is preferably between 0.1 mol/l and 1 mol/l. The concentration of the ionic Group 8-11 metal in the aqueous solution is preferably between 0.1 and 5 mmol/l and suitably between 0.1 and 1 mmol/l. The pH of the aqueous solution is preferably between 6 and 8 and more preferably between 6.7 and 6.8. The concentration of the dissolved CO2 may range from partially to fully saturated in the aqueous solution. Preferably the concentration of CO2 is fully saturated or as high as possible. To further increase the CO2 concentration the temperature of the aqueous solution is kept low, suitably below 21 °C. The pressure at which the process is performed may be ambient or higher. Higher pressures may be advantageous to increase the concentration of CO2 in the solution according to

Henry’s Law.

Applicants found that it is possible to use high concentrations of cobalt and/or nickel ions without the risk of precipitation in the present process. It is believed that the dissolved carbon dioxide enhances the solubility of the ionic cobalt and/or nickel metal. When the process is performed using ionic cobalt or nickel, the aqueous solution is suitably prepared by first adding the ionic metal, for example as a corresponding salt, to the solution and subsequently contacting the solution with carbon dioxide, for example by continuously bubbling CO2 gas in the solutions, to facilitate the ionic metal solubility. Preferably the aqueous solution is prepared by first dissolving carbon dioxide and subsequently adding the ionic metal.

The anode is preferably made of a conductive or semi-conductive material. Examples of possible materials are metals like copper, titanium, tin, nickel, iron, steel, stainless steel, mild steel, Boron-doped Diamond (BDD) electrode, gold, platinum, silver, or on metal alloy (MA), carbon, glassy or vitreous carbon, simple or pyrolytic graphite and conductive polymers. More preferably the anode has a conductive oxide surface or semi-conductive oxide surface, for example Fe2C>3, TiC>2, simple and doped B1VO4, simple and doped TaON, simple and doped Ta3Ns, indium doped tin oxide (ITO), fluorine doped tin oxide (FTO) and/or aluminum-doped zinc oxide (AZO) or optically transparent films of tin oxide nanoparticles.

The voltage applied to the anode is preferably below 1.6 vs NHE and more preferably below 1.35 V vs NHE at a pH of between 6.7 and 6.8. The layer thickness of the catalyst formed on the surface of the anode will depend on the voltage and duration of the voltage. Layers having a thickness starting from 100 nm to even up to 10 pm may be formed by the process. Stable layers are found to be formed, which remain stable in, for example, cyclic voltammetry conditions and long-term electrolysis like chronoamperometry or chronopotentiometry.

The anode as obtained by the above process is believed to be novel. The invention is thus also directed to the anode obtainable by the above process. The anode may find application as an electrolysis catalyst. The term "electrolysis," as used herein, refers to the use of an electric current to drive an otherwise non spontaneous chemical reaction. For example, in some cases, electrolysis may involve a change in redox state of at least one species and/or formation and/or breaking of at least one chemical bond, by the application of an electric current. For example, the electrolysis of water can involve splitting water into oxygen gas and hydrogen gas, or oxygen gas and another hydrogen-containing species, or hydrogen gas and another oxygen-containing species, or a combination of such reactions.

The electric current as provided to the anode to perform the electrolysis reaction may be generated by any power supply, for example a battery, a generator, a photovoltaic cell and by a power grid. The anode may also be directly combined with a photoactive material, for example a semiconductor material. The electric current is generated by electromagnetic radiation, for example sun light, received by such a material. Such a cell is also referred to as a photo-electrochemical cell as for example described in US2010/0133111.

The above electrochemical cell or photo-electrochemical cell suitably comprises an anode as obtainable by the process according to the invention, a cathode, an electrolyte, a power source connected to both anode and cathode and an aqueous solution in which both anode and cathode are submerged. The pH of the aqueous solution is suitably equal or greater than the pH at which the anode is prepared. The pH may range from 6 to 14. Preferably the aqueous solution comprises dissolved carbon dioxide in a bicarbonate electrolyte. The concentration of CO2 may be the same as when preparing the anode. The temperature and pressure conditions may also be the same as when preparing the anode. The cell may be provided with means to supply carbon dioxide to the aqueous solution. Furthermore means to add ionic nickel and/or ionic cobalt and/or anionic bicarbonate either as a salt or in solution to the aqueous solution may be present. This will enable one to rejuvenate the catalyst on the anode in situ. Further means to measure the activity of the catalyst on the anode may be present. This can, for example, involve measurement of the concentration of the ionic nickel and/or ionic cobalt and/or anionic bicarbonate in the aqueous solution or by measurement of the current running through the anode.

The above electrochemical cell is suitably used to convert water into oxygen by applying a voltage to the anode using the power source. Preferably the aqueous solution comprises dissolved carbon dioxide. Carbon dioxide may be present at both the anode and the cathode. This makes the process advantageous because this enables one to perform an electrochemical reduction of carbon dioxide at the cathode while no special measures are required to separate the carbon dioxide from the anode.

In the electrochemical reduction protons, as generated at the anode, react with carbon dioxide and water to a hydrocarbon or mixtures of hydrocarbons. Exemplary hydrocarbons are formic acid, methane, ethane, ethanol and methanol. Such an electrochemical reduction of carbon dioxide is well known and for example described in Cyrille et al., Chem. Soc. Rev., 2013, 42, 2423-2436.

The cathode in such an electrochemical reduction may comprise copper (Cu), tin (Sn), silver (Ag) and their alloys. Other examples of catalysts comprise cobalt (Co) complexes, nickel (Ni) complexes or iron (Fe) complexes. Another example is US2012/0031770 that describes a cathode based on boron carbide, B4C in a process to prepare methane.

Figure 26 shows an example of the electrochemical cell 1. The cell comprises a cathode 2, an anode 3 as obtained by the process according to the invention, and a vessel 4. This vessel 4 stores an aqueous solution 5. The cathode 2 and the anode 3 are electrically connected to each other via a power supply 9 and in contact with the aqueous, electrolyte, solution 5. The aqueous solution 5 contains bicarbonate/C02 mixture. The vessel 4 may comprise a semi-permeable partition 6, for example a solid electrolyte membrane. The partition may also be deleted or be permeable for carbon dioxide and hydrogen protons. This partition 6 is disposed between the cathode 2 and anode 3. The electrochemical cell 1 comprises further a gas introduction tube 7 that functions as a gas inlet for carbon dioxide. One end of the gas introduction tube 7 is disposed in the aqueous solution 5. The electrochemical cell 1 shown in Figure 16 is a three-electrode cell provided further with an optional reference electrode 8 useful when performing experiments. As shown in Figure 16 water is electro-catalytically oxidised to oxygen at anode 3 and carbon dioxide is electro-catalytically reduced to hydrocarbons at cathode 2.

The invention shall be illustrated using the following non-limiting examples.

General conditions

Materials:

Co(N03)2 6H20 (99.999%), Ni(N03)2.6H20 (99.999%) and sodium bicarbonate (NaHC03, 99.5-100.5 %) were purchased from Sigma Aldrich. C02 gas (99.999%) was obtained from Linde, B.V. Netherlands. Indium-tin-oxide (ITO) coated glass slides (8-12 Ω/sq surface resistivity) were purchased from Aldrich. A glassy carbon disk (diameter d=0.5 mm) was obtained from Pine research instrumentation. All solutions were prepared in ultra-pure water (Millipore MilliQ® A10 gradient, 18.2 ΜΩ cm, 2-4 ppb total organic content) and all electrochemical measurements were performed in deoxygenated aqueous solutions at room temperature. The glassware and the electrochemical cell were cleaned as described in Joya, K S., et al. Angew. Chem. Int. Ed. 2012, 51, 9601-9605.

Experimental set-up A three electrode configuration pyrex glass cell was employed for cyclic voltammetry (CV). ITO coated glass slides (1 cm x 2.5 cm, exposed surface area 1.0 cm^) were used as working electrodes (WE). The catalytic water electrolysis experiments were carried out in a three electrode double junction H-type glass electrolysis cell. A platinum wire (thickness: 0.5 mm), shaped into a spiral, was used as a counter electrode (CE). A silver-silver chloride electrode (SSCE: Ag/AgCI/KCI) was applied as reference electrode. However, all potentials are referred to a normal hydrogen electrode (NHE). Cyclic voltammetry experiments were performed with an Autolab PG-stat10 potentiostat controlled by GPES-4 software. The oxygen was detected with a calibrated oxygen electrode connected with a digital 02 meter (YSI,

Inc., Model 550A).

The spiral platinum counter electrode was flame annealed and washed with pure water before placing it into the cell. The ITO slides were cleaned in succession with methanol and acetone, and ultimately washed with Millipore water. The glassy carbon disk was polished mechanically with an aqueous slurry of 0.3, 0.1 and 0.05 pm alumina (Buehler Limited) successively, on a Microcloth polishing fabric, until a mirror finish was achieved. After polishing, the electrode was ultrasonically cleaned in Milli-Q (Millipore) water for 15-20 minutes after each polishing step and rinsed thoroughly with pure water. Prior to the water splitting investigations, the aqueous solutions were purged with high-purity argon (Linde Gas, 6.0) for at least 30 min before each measurement to deoxygenate the solution. For the electrochemical measurements, CO2 gas was purged (1 atm) through the NaHC03 electrolyte solution throughout the course of entire experiment, and at least 20 minutes before each test (pH=6.7-6.8).

Example 1-6 A C0/HCO3/CO2 type of electrocatalyst was generated in the above experimental set-up in situ from a CO2 saturated (1 atm) bicarbonate solution at a varying concentration of between 0.1 M and 1.0 M, and containing Co++ at a varying concentration of between 0.4 mM and 2 mM. The anode was an ITO coated glass slide or a glassy carbon (GC) disk. The applied voltage during the cyclic voltammetry (CV’s) was between 0.0 V - 1.60 V (vs. NHE). The results are presented in Table 1. Example 7

The catalyst film of type C0/HCO3/CO2 (Table 1) was also generated on an ITO electrode, while maintaining the potential of the working electrode above 1.3 V (vs. NHE), from a CO2 saturated (1 atm) bicarbonate solution (0.1 M) containing

Co++ (1 mM). This type of experiments is known as controlled-potential electrolysis (CPE).

Table 1

*The catalyst film was generated in Example 7 (Table 1) while maintaining the potential of the working electrode above 1.3 V (vs. NHE).

In Table 1 the numbers listed for Experiment A, B and C relate to the best or representative results as derived from the prior art wherein: A Kanan, M. W. et al., Science 2008, 321, 1072-1075 B Dinca, M., et al., PNAS 2010, 10337-10341 C Chen, Z. et al., Angew. Chem. Int. Ed. 2013, 52, 700-703 Figure 1 shows the cyclic voltammetry curves for Example 1 on an Indium tin oxide anode in a CO2 saturated 0.2 M bicarbonate electrolyte (pH 6.7-6.8) with 1.5 mM Co++ (Cat-ITO) and without Co++ (ITO). ITO area A=1 cm2. Scan rate: 50 mV sec-1.

Figure 2 shows the cyclic voltammetry curves for Example 4 on a freshly polished glassy carbon (GC) electrode in a CO2 saturated 0.2 M bicarbonate electrolyte (pH 6.7-6.8) with 0.75 mM Co++ (Cat-GC) and without Co++(GC). Scan rate: 50 mV sec-1;GC disk diameter d=0.5 mm.

Figure 3 shows the Recurring cyclic voltammetry curves for Example 2 on an Indium tin oxide anode in a CO2 saturated 0.2 M bicarbonate electrolyte (pH 6.7- 6.8) with 1.0 mM Co++ (Cat-ITO) and without Co++ (ITO). Scan rate: 50 mV sec-1; ITO area A=1 cm2;

Example 8

In order to understand more about the surface morphology of the layer formed on the surface of the anode SEM (SEM=scanning electron microscopy) images were taken. The measurements were carried out with a FEI NanoSEM 200 microscope. After the electrodeposition of a catalytic film on the electrode surface, the catalyst samples were rinsed with deionized water and allowed to dry in air before loading into the instrument. The SEM images were taken with an acceleration voltage of 5-15 kV. The resolution at 15 kV is 1 nm and 1.8 nm at 1 kV.

Figure 4 shows SEM images of the electrodeposited catalytic film of C0/HCO3/CO2 on the ITO during first 5 CV cycles (0 V - 1.3 V; vs. NHE) according to Example 1.

Figure 5 is an enlarged view of Figure 4.

Figure 6 shows a SEM image of the electrodeposited catalytic film of C0/HCO3/CO2 on the ITO after 15 min CPE at 1.35 V (vs. NHE) according to

Example 7.

Figure 7 shows the surface of the anode of Example 7 after a 10 hours water electrolysis experiment.

Example 9

In order to understand more about the composition of the layer formed on the surface of the anode XPS (X-ray photoelectron spectroscopy) analyses were undertaken. The measurements were carried out with a Thermo Scientific K-Alpha, equipped with a monochromatic small-spot X-ray source and a 180° double focusing hemispherical analyzer with a 128-channel detector. Spectra were obtained using an aluminium anode (Al Ka = 1486.6 eV) operating at 72Wand a spot size of 400pm. Survey scans were measured at a constant pass energy of 200 eV and region scans collected at 50 eV. The background pressure was 2 x 10-9 mbar and during measurement 3 x 10~7 mbar Argon because of the charge compensation dual beam source.

Figure 8 shows the full survey of XPS spectra of the (a) electrodeposited C0/HCO3/CO2 film on an ITO anode of Example 1.

Figures 9-11 are the enlarged views of the XPS of Figure 8 for Co 2p, O 1 s and C 1s respectively, present in the electrodeposited C0/FICO3/CO2 film.

Figures 8-11 indicate the following. The surface nature of the electrodeposited C0/HCO3/CO2 film was analyzed by XPS. The elemental detection on the XPS survey for background (bare ITO with no deposition) and electro-deposited C0/HCO3/CO2 on ITO indicate the presence of cobalt and oxygen in the deposited film (Figure 8). The cobalt 2p (Co 2p) signals at 780 eV and 795.7 eV indicate the presence of Co2+ and/or Co3+ bound to oxygen (Figure 9). XPS signals at 63 eV and 101 eV are also consistent with the presence of Co 3s and Co 3p in the electrodeposited C0/HCO3/CO2 film. Likewise, oxygen binding energy at 529.3 eV and 531 eV are in the typical range for the presence of -OH type species (531-532) and metal bound oxide species (528-531), respectively (Figure 10). Carbon incorporation in the C0/HCO3/CO2 electro-deposits, as indicated by small trends for C(1s) at 286-290, shows a contribution from carbon as C(0)0- and/or -C-OH (Figure 11).

From the above it follows that the actual composition of the cobalt comprising film as deposited on the anode using the process according to the invention is not yet fully understood. Nevertheless applicants believe that the following composition is novel and therefore the invention is also directed to a water oxidation catalyst comprising Co2+ and/or Co2+ bound to oxygen and C(0)0" and/or -C-OH. The catalyst is preferably present as a film on an anode as described above. Preferably the presence of these species is detected by high resolution XPS. The presence of the Co2+ and/or Co^+ species is detected by the cobalt 2p (Co 2p) signals at 780 eV and 795.7 eV. The C(0)0- and/or -C-OH are suitably detected in the XPS region of between 286 - 290 eV.

Example 10

To test the long-term stability and catalytic efficiency and performance of the C0/HCO3/CO2 film type electrocatalyst, controlled-potential electrolysis (CPE) of waterwas conducted. CPE was carried out at 1.33 V (vs. NHE) in a bicarbonate solution continuously purging with the C02gas at near-neutral pH (pH=6.7-6.8) electrolyte solution. The C0/HCO3/CO2 electrocatalyst was deposited on ITO during 5 consecutive CV’s (0 V - 1.3 V vs. NHE). ITO area A=1 cm2

Figure 12 shows the controlled-potential water electrolysis with freshly electrodeposited C0/HCO3/CO2 catalyst film on the ITO electrode of example 10.

Figures 12 indicate the following. A stable oxygen evolution current density of > 0.95 mA cm-2 was obtained right from the beginning of the controlled-potential electrolysis and the catalyst film is stable and stays active for extended periods of CPE over 17 hours with high current density (Figure 12). No apparent O2 evolution current was observed on the ITO immersed in bicarbonate solution without cobalt ions, and the current density remains at < 35 μΑ cm-2 for the prolonged CPE operation. For the C0/HCO3/CO2 electrocatalyst, the catalytic film formation and oxygen generation (as detected with a calibrated oxygen probe, YSI 550A) commence within seconds of the CPE initiation in a bicarbonate solution (pH=6.7-6.8).

Example 11-13 A Ni/HC03/CC>2 based and a N1/HCO3 type electrocatalyst were generated in the above experimental set-up in-situ with and without a CO2 saturated (1 atm) bicarbonate solution having a varying concentration of between 0.1 M and 1.0 M, and containing Ni++ at a varying concentration of between 0.5 mM and 5 mM. The anode was a FTO or a glassy carbon (GC) disk. The applied voltage during the cyclic voltammetry (CV’s) was between 0.0 V - 1.6 V (vs. NHE). The results are presented in Table 2.

Table 2

Figure 13 shows the cyclic voltammetry curves for Example 11 on a freshly polished glassy carbon (GC) anode in a CO2 saturated 0.2 M bicarbonate electrolyte (pH 6.7-6.8) with 1.0 mM Ni++ (NiOx-Cat/GC) and without Ni++ (GC). GC disk diameter d=0.5 mm; Scan rate: 50 mV sec-1.

Figure 14 shows the cyclic voltammetry curves for Example 12 on a freshly polished glassy carbon (GC) electrode in a 0.2 M bicarbonate electrolyte (pH 8.2) with 1.0 mM Ni++(NiOx-Cat/GC) and without Ni++(GC). Scan rate: 50 mV sec-1; GC disk diameter d=0.5 mm.

Figure 15 shows the cyclic voltammetry curves for Example 13 on a FTO anode in a 0.2 M bicarbonate electrolyte (pH 8.2) with 1.0 mM Ni++ (NiOx-Cat/FTO) and without Ni++ (FTO). Scan rate: 50 mV sec-1; GC disk diameter d=0.5 mm.

Figure 16 shows the recurring cyclic voltammetry curves for Example

11 on a freshly polished glassy carbon (GC) anode in a CO2 saturated 0.2 M bicarbonate electrolyte (pH 6.7-6.8) with 1.0 mM Ni++ (NiOx-Cat/GC) and without Ni++ (GC). GC disk diameter d=0.5 mm; Scan rate: 50 mV sec-"·.

Figure 17 shows the recurring cyclic voltammetry curves for Example 12 on a freshly polished glassy carbon (GC) electrode in a 0.2 M bicarbonate electrolyte (pH 8.2) with 1.0 mM Ni++(NiOx-Cat/GC) and without Ni++(GC). Scan rate: 50 mV sec-1; GC disk diameter d=0.5 mm. The numbers of the curves in Figures 16 and 17 refer to the sequence of the cyclic voltammetry runs wherein 1 is the first and 5 is the final run in these examples.

Example 14

In order to understand more about the surface morphology of the layer formed on the surface of the anode SEM (SEM=scanning electron microscopy) images were taken. The measurements were carried out with a FEI NanoSEM 200 microscope. After the electrodeposition of catalytic film on the electrode surface, the catalyst samples were rinsed with deionized water and allowed to dry in air before loading into the instrument. The SEM images were taken with an acceleration voltage of 5-15 kV. The resolution at 15 kV is 1 nm and 1.8 nm at 1 kV.

Figure 18 shows SEM images of the electrodeposited catalytic film of a N1/HCO3/CO2 type of electrocatalyst on the ITO during the first 5 CV cycles (0 V - 1.3 V; vs. NHE) according to Example 11.

Figure 19 is an enlarged view of Figure 18.

Example 15

In order to understand more about the composition of the layer formed on the surface of the anode, XPS analyses were undertaken. The measurements were carried out with a Thermo Scientific K-Alpha, equipped with a monochromatic small-spot X-ray source and a 180° double focusing hemispherical analyzer with a 128-channel detector. Spectra were obtained using an aluminium anode (Al Ka = 1486.6 eV) operating at 72W and with a spot size of 400pm. Survey scans were measured at a constant pass energy of 200 eV and region scans were collected at 50 eV. The background pressure was 2 x 10"9 mbar and during measurement 3 x 10-7 mbar Argon because of the charge compensation dual beam source.

Figure 20 shows the full survey of XPS spectra of the (a) electrodeposited N1/HCO3/CO2 film on an ITO anode of Example 11.

Figures 21 -23 are the enlarged views of the XPS of Figure 20 for Ni 2p, O 1s and C 1s, respectively, present in the electrodeposited N1/HCO3/CO2 film.

Figures 20-23 indicate the following. The high resolution XPS for the Ni 2p signal at 855.2 eV indicates the presence of a mixed structure that contains Ni(OFI)2 (Ni2+ species) and NiOOH. Flowever, the XPS feature in the region 878 - 881 eV indicates the presence of a γ-NiOOFI (Ni9+ species) type catalytic film. In the O 1 s high-resolution spectrum of electro-deposited nickel hydroxide (Figure 22), the

oxygen binding energy at 530.7 eV indicates a hydroxide bound to Ni(OH)-. And the small XPS feature at 528.8 - 529.7 eV is attributed to the presence of some oxygen in the form of NiO and carbon bound oxygen that might come from the bicarbonate environment. So the film may be a mixture of Ni(OH)2 and NiOOH with some NiO that might transform into hydroxide during the course of catalysis. Carbon C(1s) incorporation in the C0/HCO3/CO2 electro-deposits is also vivid in the XPS region 286-290 eV (Figure 23). This indicates the presence of C(0)0" and/or -C-OH type carbonaceous species.

From the above it follows that the actual composition of the nickel comprising film as deposited on the anode using the process according to the invention is not yet fully understood. Nevertheless applicants believe that the following composition is novel and therefore the invention is also directed to a water

oxidation catalyst comprising Ni(OH)2, γ-NiOOH and C(0)0' and/or -C-OH. NiO may also be present. The catalyst is preferably present as a film on an anode as described above. Preferably the presence of these species is detected by high resolution XPS. The presence of Ni(OH)2 is detected in the 0 1s high-resolution spectrum of electro-deposited nickel hydroxide at the oxygen binding energy at 530.7 eV. The γ-NiOOH is suitably detected in the XPS region of between 878 - 881 eV. The C(0)0" and/or -C-OH are suitably detected in the XPS region of between 286 - 290 eV.

Example 16

To test the long-term stability and catalytic efficiency and performance of the electrodeposited N1/HCO3/CO2 film, controlled-potential electrolysis (CPE) of waterwas conducted. CPE was carried out at 1.41 V (vs. NHE) in a bicarbonate solution continuously purging with the C02gas at near-neutral pH (pH=6.7 - 6.8) electrolyte solution. ITO area A=1 cm2.

Figure 24 shows the controlled-potential water electrolysis with a freshly electrodeposited Ni/HC03/CC>2 catalyst film on the GC electrode of example 16.

Figures 24 indicate the following. An oxygen evolution current density of > 1.85 mA crrf2 was obtained at just 1.41 V (vs. NHE, pH=6.7 - 6.8) in the beginning few minutes of the CPE (Figure 24). Unlike the Ni-borate water oxidation catalyst, the NÏ/HC03/C02 type electrocatalyst film developed faster on the electrode surface and no turbidity was observed in the Ni++ solution. The slight decrease in the current density was due to the surface coverage with immense oxygen bubbles which was relieved by gently tapping the anode. The catalyst film is stable and stays active for extended periods of CPE over 16 hours with high current density for oxygen evolution. Without the Ni++ in the CO2 saturated bicarbonate solution, no apparent oxygen evolution current was observed, and the current density remains under 45 μΑ cm-2 for the entire CPE procedure.

Example 17

To test the long-term stability and catalytic efficiency and performance of the electrodeposited N1/HCO3 based electrocatalyst, controlled-potential electrolysis (CPE) of water was conducted. CPE was carried out at 1.35 V (vs. NHE) in a bicarbonate solution without purging with the CO2 gas (pH=8.2) electrolyte solution. ITO area A=1 cm2.

Figure 25 shows the controlled-potential water electrolysis with freshly electrodeposited Ni/HCC>3 type catalyst film on the GC electrode of example 17.

Figures 25 indicate the following. Long-term CPE for the Ni/HCC>3 based electrocatalyst on glassy carbon anode was also conducted in the bicarbonate solution without CO2 presence. Controlled-potential electrolysis was conducted at just 1.35 V (vs. NHE, pH=8.2) and an oxygen evolution current density of > 1.15 mA cm-2 was obtained.

Claims

A method for producing an anode material suitable for the catalytic production of oxygen and protons from water, comprising: providing an aqueous solution comprising: (i) an ionic group 8-11 metal, (ii) anionic bicarbonate and (iii) dissolved carbon dioxide, providing an anode immersed in the aqueous solution, and causing a group of 8-11 metal-containing layer of catalytically active anode material to be formed on the surface of the anode by a voltage to be applied to the anode.

The method of claim 1, wherein the ionic group 8-11 metal is ionic nickel, ionic cobalt, ionic iron, and / or ionic silver.

The method of claim 2, wherein the ionic group 8-11 metal is ionic cobalt and / or ionic nickel.

The method of claim 3, wherein the ionic group 8-11 metal is ionic cobalt.

The method according to any of claims 1-4, wherein the pH value of the solution is between 6 and 7.

The method of any one of claims 1-5, wherein the anode is provided with a conductive oxide surface or with a semiconductive oxide surface.

The method of any one of claims 1-6, wherein the concentration of the ionic cobalt or ionic nickel in the aqueous solution is between 0.1 and 1.0 mmol / l.

The method of any one of claims 1-7, wherein the voltage applied to the anode is less than 1.6 volts vs NHE.

A water oxidation catalyst comprising Νΐ (ΟΗ) 2, γ-ΝΐΟΟΗ and C (0) 0 'and / or -C-OH.

A water oxidation catalyst, comprising Co 2+ and / or Co 3+, bound for oxygen and C (O) 0 and / or C-OH.

Use of the anode material as obtained by a method according to any of claims 1-8, as an electrolysis catalyst.

An electrochemical cell comprising an anode such as can be obtained using the method of any one of claims 1-8, or an anode comprising the water oxidation catalyst of any one of claims 9 and 10, a cathode, a power source which is connected to the anode and the cathode, and an aqueous solution in which both the anode and the cathode are immersed.

The electrochemical cell of claim 12, wherein the aqueous solution comprises dissolved carbon dioxide.

A method of converting water into oxygen by using an electrochemical cell according to claim 12 or claim 13, by applying a voltage to the anode and to the cathode by using the power source.

The method of claim 14, wherein the aqueous solution comprises dissolved carbon dioxide, and wherein at the cathode the carbon dioxide reacts with water and the protons as generated at the anode to form a hydrocarbon.

The method of claim 15, wherein the hydrocarbon is any of methane, methanol, or formic acid.