WO2015170987A1 - Process for forming a leaf-shaped nanoparticulate material, an electrochemical cell and a process to convert water - Google Patents

Abstract

The invention is directed to a s process for forming a copper oxide laminar leaf- shaped nanoparticulate material on an amorphous copper surface by electrolytic anodization and /or self-deposition, the method comprising the steps of: a) immersing a copper substrate as anode, and a cathode in a reaction medium range of from pH=10 to pH=13, and b) applying a current over the anode and cathode suitable for electrolytically depositing the copper oxide onto the surface of the anode. The invention is also related to the materials thus obtainable, as well as to the various uses thereof.

Description

PROCESS FOR FORMING A LEAF-SHAPED NANOPARTICULATE MATERIAL, AN ELECTROCHEMICAL CELL AND A PROCESS TO CONVERT WATER

The present invention relates to a process for forming a metal oxide nano- particulate material on an amorphous copper surface by electrolytic anodization and /or self-deposition.

Processes are known to convert water by electrochemistry or photo- electrochemical (PEC) methods 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 and storage 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) which is multi-electron transfer process and requires a high overpotential to drive it. Hence catalytic electro-driven and photo-electrochemical water oxidation systems to make hydrogen and oxygen with high efficiency, rate and stability, and at a moderate overpotential are crucial processes in pursuit of solar to fuel conversion devices. Besides making hydrogen, the electrons and protons released from water oxidation reaction can also be combined with CO2 to reduce it directly into liquid fuels like methanol or formic acid.

Recently, many molecular complexes, transition metals oxides and inorganic materials have been evaluated for their activity in water oxidation under electrochemical conditions, providing a route to renewable and alternative energy carriers obtainable from abundant water and sunlight. However, owing to the difficulties in the abstraction of four electrons from two water molecules and subsequent formation of an 0-0 bond, this process poses a great challenge to develop state of the art water splitting systems.

So, there is a continuous effort to develop a stable and robust catalytic system that could be derived from cheap and earth abundant materials, while operating at a modest overpotential and at a high oxygen evolution current density at the same time. Known transition metal oxide based electrocatalysts that had been developed on a conducting substrate such as ITO (Indium tin oxide) or on a glassy carbon (GC) anode for the anodic water oxidation from phosphate, borate or other aqueous electrolytes in the presence of metal ions show very slow rate of metal oxide catalyst deposition and catalysis, and operate at a high overpotential for electrochemical oxygen evolution.

Moreover, during the electrodeposition phase, metal ions possibly also get deposited onto the cathode as a contamination blocking or poisoning the reduction sites. To avoid the interaction of metal ions with the cathode, membranes or separators are usually employed, which make the system more complex and introduce resistance and diffusion limitations in the electrochemical process.

Accordingly, there is a need for robust catalyst systems with low overpotential, high rate for anodic oxygen evolution and high stability.

Applicants have now surprisingly found a novel leaf-shaped nanoparticulate metal oxide material that may advantageously be employed as catalytic anode material.

Summary of the Invention

Accordingly, in a first aspect, the present invention relates to a process for forming a copper oxide laminar leaf-shaped nanoparticulate material on an amorphous copper surface by electrolytic anodization and /or self-deposition, the method comprising the steps of: immersing a copper substrate as an anode, and a cathode in an aqueous solution comprising a carbonate buffer at a pH in the range from 10 to 13, and applying a current over the anode and cathode suitable for electrolytically self- depositing the copper oxide onto the surface of the anode.

In a further aspect, the present invention relates to a copper material having copper oxide nano-leaf-shaped structures with an ellipsoidal shape and an average aspect ratio in the range of from 13: 1 to 3.0: 1.

In yet a further aspect, the present invention relates to a copper derived material comprising copper oxide nano-leaf-shaped structures associated to its surface, whereby the nanoparticles have an average thickness of from 5 to 50 nm. Both values may advantageously be determined by SEM microscopy. In yet a further aspect the present invention relates to a copper-based material having an overpotential (η) from 0.3 to 0.5 V when employed as catalytic electrode in water electrolysis.

In again a further aspect the present invention relates to the use of the copper-derived material as an electrolysis catalyst.

In yet a further aspect the present invention relates to an electrochemical cell comprising an anode comprising the copper oxide nanoparticulate material according to the invention.

In a further aspect the present invention relates to a process to convert water into oxygen, and releasing electrons and protons, comprising providing an electrochemical cell according to the invention, and applying a suitable voltage to the anode and cathode, using a power source.

Brief Description of the Figures

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. Unless indicated as representing the prior art, the figures represent aspects of the invention. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures:

Figure 1 discloses a schematic representation of the direct electro-generation of copper oxide nano-leaves electrocatalyst systems for anodic water oxidation.

Figure 2 shows cyclic voltammetry (CV) curves for the generation of the copper oxide nano-leaves derived electrocatalyst on a Cu foil in 0.1 M carbonate buffer (pH~11.5), while the inset shows the magnified view of the CV and catalytic wave for electro-formation of a Copper oxide nano-leaves based electrocatalyst, determined at a scan rate of 50 mV sec^-1.

Figure 3 shows an SEM (scanning electron microscopy) image of a starting copper foil. Figure 4 shows a scanning electron microscopy image of the electro- generated material comprising the copper oxide nano-leaves based electro-catalyst layer on a copper foil, at 40 x 10³ times magnification.

Figure 5 shows a scanning electron microscopy image of the electrodeposited copper oxide nano-leaf material, at 120 x 10³ magnification.

Figure 6 shows the extended period controlled-potential water electrolysis for the copper oxide nano-leaves based electrocatalyst in deoxygenated 0.1 M carbonate buffer (pH of 1 1.5) at 1.21 V (vs. NHE), for a copper oxide nano-leave based electrocatalytic system generated in-situ on a Cu foil during the CPE at 1.10 V (vs. NHE) for 1 1 minutes.

Figure 7 shows a cyclic voltammetry (CV) curve for the copper-oxide nano- leaves derived electrocatalyst on a Cu-foil in 0.1 M carbonate buffer (at a pH of 1 1.5), at a scan rate of 50 mV sec^-1.

Figure 8 shows a CV curve for the copper-oxide nano-leaves based electrocatalyst on a larger potential window under the same conditions as for Figure 7, at a scan rate of 50 mV sec^-1.

Figures 9 to 1 1 show the XPS (X-ray photoelectron spectroscopy) spectrum of an electrodeposited copper oxide nano-leaves film.

Figure 12 shows the EDX (energy dispersive X-ray) spectrum of the copper- oxide nanoleaves film.

Figure 13 shows a Raman spectrum for the electro-generated copper-oxide nanoleaves. The CuOx-nanoleaves were generated in situ on a Cu foil during the CCE@J>3 mA cm^"2 for 20 minutes.

Figure 14 shows STEM-EELS spectrum imaging analysis of CuOx-NLs. (a): HAADF-STEM image of an area from where spectrum image was acquired, (b): The corresponding Cu elemental map generate by analyzing the Cu-L23 edge at the energy-loss of 73 eV. (c): O elemental map generated by using the O-K edge at the energy-loss of 532 eV. (d): C elemental map generated with C-K edge at the energy- loss of 284 eV. The CuOx-nanoleaves were generated in situ on a Cu foil during the CCE@J>2 mA cm^-2 for 35 minutes.

Figure 15. (a) Cyclic voltammetry curve for the copper-oxide nanoleaves in 0.5 M carbonate buffer (pH~10-12). Inset shows the enlarged view of the CV and catalytic wave, (b) CV's for CuOx-NLs in carbonate and borate buffers. (CV's are taken at a scan rate of 20 mV sec-1 )

Figure 16 shows the 1^st and 50^th consecutive CV's for CuOx-NLs in 0.5 M carbonate buffer (pH~1 1.5) at a scan rate of 20 mV sec^-1.

Figure 17 shows a tafel plot or log i vs overpotential (η) curve obtained for electrocatalytic CuOx-NLs sample while CV in carbonate buffer. Inset shows extended Tafel plot for the current density ranging from 0.1 to 10 mA cm^-2. (The CuOx-nanoleaves are generated in situ on a Cu foil during the CCE@J«3 mA cm^-2 for 20 minutes). Other aspects, embodiments, and features of the invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings. The accompanying figures are schematic and are not intended to be drawn to scale. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. All patent applications and patents incorporated herein by reference are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will prevail.

Detailed Description of the Invention

Applicants have found a readily electro-assembled and easy accessible catalytic material for water oxidation that can be generated in situ on simple copper metal surfaces without requiring the presence of added metal ions, and under benign conditions. Surprisingly, the novel leaf-shaped nanoparticulates are self-assembling on a simple anode substrate, and form in absence of added metal ions in the electrolyte solution. The formation of the catalytic material according to the present invention may proceed until the potential applied to the current collector is turned off, or the catalytic material has reached a critical thickness beyond which additional film formation does not occur or is very slow.

M. Le et al. reported in "Electrochemical Reduction of C02 to CH30H at Copper Oxide Surfaces", Journal of The Electrochemical Society, 25 maart 201 1 (201 1 -03-25), page E45, the preparation of copper materials similar to those according to the present invention. However, the disclosed procedure uses 0.5 M KHCO₃ solution, with a pH <9, and a constant potential at 1 .25 V vs. SCE. Furthermore, the reported process shows a release of carbon, while the resent process withdraws carbon from the medium, and introduces it into the deposited electrocatalytic material, where it was found to be relevant for the catalytic activity. In my opinion is difficult to reconcile with procedures leading to the same materials, as the examiner suggests. The thus obtained materials are not nanoleaves according to the present invention.

Shaohuang Weng et al. disclose in "CuO nanoleaf electrode: facile

preparation and nonenzymatic sensor applications", MIKROCHIMICA ACTA, 180, 5- 6, 2013-04-01 , 371-378 the preparation of CuO nanoleaf electrodes from immersing cleaned copper foils in a solution containing 10 g-L-1 Na₂S₂0_s, 20 g^*L^"1 NaCI and 50 g^*L^"1 NaOH to react at 85 °C in water bath for 10 min, i.e. in absence of an electrical current. The copper foil covered with black thin film was then rinsed thoroughly with water and dried in a vacuum at 80 °C overnight. The morphologies and structures of the CuO nanoleaves disclosed in this publication do not comprise carbon, and are hence clearly different from those according to the present invention.

A voltage is advantageously be applied to the copper foil for a suitable period of time until a sufficiently thick layer of copper nanoleaves is formed.

Preferably, the voltage is applied for a time in the range from 0.1 to 100 minutes, preferably at least 1 , more preferably 5 minutes, more preferably 10 minutes, more preferably 20 minutes, more preferably 30 minutes, again more preferably 60 minutes, about 2 hours, about 4 hours, about 8 hours, about 12 hours, about 24 hours, and the like. In some cases, a potential may be applied to the current collector between 24 hours and about 30 seconds, between about 12 hours and about 1 minute, between about 8 hours and about 5 minutes, between about 4 hours and about 10 minutes, and the like.

The voltages provided herein are generally provided with reference to a normal hydrogen electrode (NHE), unless stated otherwise.

Once formed, the nanoleaves material was found to operate at remarkably low overpotential for the electrocatalytic water oxidation process and to generate oxygen at a high rate, and without loss of activity over a prolonged period of time.

The catalytic material according to the invention typically accumulates in the form of a leaf shaped solid on or at the anode surface upon exposure to carbonate ions in solution and application of a voltage under appropriate conditions as described herein below.

These conditions advantageously involve exposing the anode to the forming conditions for a period of time, and at a voltage, such that a suitable amount of catalytic material collates on the anode surface.

The copper starting material used in the present process comprising the anode preferably is an amorphous copper metal or a copper metal alloy.

The anode material preferably comprises at least 97% by weight of copper, more preferably at least 98%, more preferably at least 99%, yet more preferably at least 99.5% and even more preferably at least 99.7% by weight of copper metal.

The self-forming leaf-shaped nano-structured material according to the invention has the appearance of so called linear, or linear-lanceolate shaped botanical leaves, i.e. whereby the term "aspect ratio", herein denotes the ratio of the major axis to the minor axis of the leave shaped particles .

The average aspect ratio of the structures is preferably in the range of from

1.3:1 to 3.0:1 , or higher. It preferably is at least 1.4: 1 , more preferably at least preferably at least 1.5:1 , yet more preferably at least 2: 1. It also preferably is at most 3.0:1 , more preferably at most 2.9:1 , again preferably at most 2.8:1 , and yet more preferably at most 2.5:1.

The average thickness of the leaf shaped structures at thickest median point is typically in the range of from 10 nm to 50 nm. Preferably, the average thickness is of from 12 to 45, yet more preferably of from 15 to 45 nm.

The average aspect ratio of the structures is in the range of from 1.3: 1 to 3.0:1 , whereby the term "aspect ratio", herein denotes the ratio of the major axis to the minor axis of the leave shaped particles. It preferably is at least 1.4:1 , more preferably at least preferably at least 1 .5: 1 . It also preferably is at most 3.0:1 , more preferably at most 2.8:1 , and yet more preferably at most 2.5:1.

The leaf-shaped nano-material according to the invention may

advantageously be generated on a copper metal foil in a reaction medium

comprising carbonate ions, and at a pH in the range of from 10 to 13. This may preferably be done essentially in absence of added copper or other heavier metal ions in the solution. In the deposited copper oxide material, Cu and oxygen are preferably present in an amount of from 0.95 to 1.15, preferably 1 :1 composition in the film, as determined by an EDX spectrum.

In the copper oxide material according to the invention, carbon is present in amount in the range of from 10 to 12, preferably 10.5 to 1 1.5, more preferably 1 1 % by weight.

The nano-structured copper oxide material is typically deposited or generated in a film or layer on the anode material. The film or layer preferably has an average thickness in the range of from 100 nm to 500 Mm. The thickness is preferably at least 100 nm, preferably 200 nm, more preferably 300 nm, yet more preferably 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 10 pm, 20 pm, 100 pm, 200 pm, 500 pm . The layer thickness is advantageously determined from SEM micrographs.

Aspect ratio and thickness of the particles or structures largely depends on the time that the current is applied to allow for the deposition.

The layer or film of the material according to the invention film is preferably generated by applying a current to the copper foil under anodic conditions in a reaction medium comprising carbonates.

The reaction medium used in the method is preferably aqueous, i.e. water alone or a mixed aqueous solution containing 90% by weight or less of an organic compound other than water. The reaction medium further comprises a carbonate buffer system, at a pH in the range of from 10 to 13. The pH preferably is at least 10.0, yet more preferably 10.3; 10.5; 10.7; 10.9; 1 1.0; 1 1 .1 , 1 1.2, 1 1.3, 1 1.4, 1 1.5 1 1.6, 1 1.7 or 11 .8.

The pH is preferably at most 13, yet more preferably at most 12.8, 12.7, 12.6,

12.5, 12.4 12.3, 12.2, 12.1 , or 12.0.

The preferred range of the amount of water in the mixed aqueous solution is at least 80% by weight. The use of other solvents than water present in the reaction medium may however affect the average primary and secondary particle diameters of the resulting copper oxide nanoparticles.

The optimum reaction temperature in the process is preferably lower than 85°C, and preferably above 5°C. If the reaction temperature is lower than 5°C, the solubility of the copper ions may decrease too far, and the deposition becomes very slow. At or above 85°C, the particle diameter of the resulting copper oxide tends to increase. Preferably, a preferred temperature range is of from 15 to 35°C.

The reaction medium suitable for use in the practice of the present invention comprises as its essential constituents carbonate compounds, as well as mixtures thereof, at a pH of from 9 to 13. The reaction medium preferably is essentially free from added metal ions, specifically other than alkali or earth alkali metal ions.

The medium preferably comprises an aqueous electrolyte solution comprising a carbonate buffer. The ions are introduced during makeup of the electrolyte medium by employing any one or mixtures of compatible carbonate or bicarbonate salts, and/or by dissolving carbon dioxide in the medium, as well as other compatible salts such as oxides, hydroxides, and the like.

Preferably the medium comprises a 0.1 M to 1.0M carbonate buffer/system (Na₂C0₃ + NaHC0₃), with a pH of above 10.0, preferably pH 11 to 12, while preferably performing cyclic voltammetry (CV), more preferably at a scan rate of 50 mV sec^-1.

The medium may further comprise a soluble and compatible stabilizing and buffering agent including carbonate and bicarbonate compounds as well as mixtures thereof.

Preferably, ammonium, sodium carbonate and potassium carbonate are employed to stabilize the electrolyte against pH fluctuations and to further serve as a carrier for copper ions released from the copper foil into the medium during the electrodeposition operation.

The use of the aforementioned buffering agents has further been observed to inhibit the formation of amorphous copper oxide deposit on the anode.

Ammonium ions may be undesirable in some instances because of a potential loss of adhesion of the electrodeposited nano-leaves, while calcium ions are undesirable because of the tendency to form precipitates in the medium.

The concentration of the buffer can broadly range from 3 g/l up to 100 g/l, based on the sodium carbonate. The amount is preferably of from 10 to 25 g/l.

Concentrations of the buffering agent below the recommended minimum concentrations will result in pH fluctuations whereas concentrations above the maximum range specified do not appear to have any adverse effects on the operation of the electrolyte. Since the buffering agents are subject to depletion by both decomposition and eventual removal when the materials may be taken out, a replenishment of these components to maintain the medium within appropriate composition limits is necessary during commercial operation.

This can conveniently be performed on an intermittent or continuous basis in response to an analysis of bath composition by adding the constituents separately or by admixture in appropriate proportions.

The medium is preferably adjusted to provide a pH in the range of from 9 to 13, with a pH of from 10 to 12 being preferred. Preferably the process is operated at a pH of at least 9.5, more preferably at least 9.8, yet more preferably at least 10.5. The pH preferably is at most 13, more preferably at most 12.8, yet more preferably at most 12.5.

Typically an operating pH of 1 1.0 to 11.6 has been found particularly satisfactory. The appropriate pH of the electrolyte can be maintained by adding an alkali metal hydroxide to the electrolyte to raise the pH of which potassium hydroxide is preferred.

In order to reduce the pH within the desired range, carbon dioxide or an alkali metal bicarbonate can be employed of which potassium bicarbonate constitutes a preferred material. When the operating pH decreases below the recommended level, it has been observed that the medium tends to promote poor adhesion of the copper oxide deposition onto the substrate. On the other hand, at an operating pH above the recommended range, it has been observed in some instances, that the copper oxide deposit becomes grainy and has a much reduced surface area.

The elemental detection on the X-ray photoelectron spectroscopy (XPS) survey for the electro-generated copper oxide nanoleaf layer indicates the presence of copper, oxygen and carbon in the anodically formed film on the copper substrate, see Figures 9 to 1 1.In the XPS spectrum of the Cu2p core levels are present in the XPS region from 928 eV to 968 eV (Figure 10).

The two main peaks with binding energy peak positions centered at 933.8 eV and 953.8 eV correspond to the spin-orbit splitting between the Cu2p₃/₂ and Cu2pi_/2 levels, respectively. The high binding energy side of the copper core line 2p_3/2 observed at 933.8 eV is a typical indication of the presence of Cu²⁺ species. Also, the difference between the spin-orbit coupling energy for the Cu2p_3/2 and Cu2pi_/2 levels is about 20 eV, which is in good agreement with the standard value reported for CuO film. These observations are very characteristics XPS signatures for the presence of Cu(ll)-oxides on a pure copper metal surface.

Next to the Cu2p_{3 2} XPS signal, there are two satellite peaks of higher binding energy values that overlap between 940 eV and 943.9 eV (Figure 10). These satellite peaks are characteristic for Cu²⁺ materials with a d9 configuration in the ground state (absent in case of CU2O) and are in good agreement with the values reported for Cu2p levels in a CuO phase.

This copper oxide nanoleaf sample exhibits a lattice O 1 s peak at a binding energy of 531.8 eV that is associated with the surface bound hydroxide species (Figure 1 1 ). This is assigned to the presence of H₂0 molecules that are physically adsorbed on the surface of the CuO nanoleaves.

In addition to the foregoing constituents, the medium may optionally further comprise additives such as wetting agents or surfactants that are soluble and compatible with the other constituents.

As set out in Figure 2, the cyclic voltammetry (CV) plot for the copper oxide nanoleaves on metallic copper surface shows onset of the catalytic wave between 0.90-1.0 V vs. NHE in a carbonate buffer medium at a pH of 1 1.5.

The enlarged view of CV shows the copper metal oxidation features between

0.5-0.7 V vs. NHE followed by the onset current that increases sharply at about 0.95 V, see the inset Figure 2, reaching a value of 19 mA cm^-2 at 1.40 V vs. NHE

The reverse scan reproduces the voltammogram well excluding the kinetic barriers that may cause the overlap of the reduction current with the forward sweep as observed for an electrodeposited amorphous copper oxide film.

The thus prepared material proved to be highly suitable as a stable catalyst, as exemplified by repetitive scans reproduced with similar currents on CV's indicating no degradation of the catalytic system occurred over time, see Figure 6. Further potential cycling the electrocatalytic system reveals an 0₂ generation current density approaching 40 mA cm^-2 at 1.60 V vs. NHE, as depicted in Figure 7.

The oxygen generation onset potential for the copper oxide nanoleave material (ca. 1.63 vs. RHE) was found to be lower compared than other known metal oxide electrocatalysts, such as Co-Pi (1.67 V vs. RHE), Ni-Bi (1.71 V vs. RHE). The O2 evolution onset potential for copper oxide nanoleaves is also much lower than for known amorphous CuO films (1.74 V vs. RHE) deposited on glassy carbon or on boron-doped diamond substrate.

So, the copper oxide nanoleaves derived electrocatalytic film displays remarkably reduced oxygen onset overpotential, with a tremendous electrocatalytic water oxidation performance and very high current density for anodic 0₂ evolution.

The catalytic material may be described as a function of mass of catalytic material per unit area of electrode.

Preferably, the mass of catalytic material per area of the anode material may be 0.01 pg/cm², more preferably 0.05 pg/cm², more preferably 0.1 pg/cm², more preferably 0.5 pg/cm², yet more preferably 1.0 pg/cm², more preferably 1.5 pg/cm² , more preferably 2.5 pg/cm², more preferably 3.0 pg/cm², more preferably 4.0 pg/cm², again more preferably 5.0 pg/cm² and up to 10 pg/cm², and/or 100 pg/cm².

Where the amount of catalytic material is defined or investigated in terms of mass per unit area, and the material is present non-uniform ly relative to the copper surface, and thus the mass per unit area may be averaged across the entire surface area within which catalytic material is found. The invention provides not only new catalytic materials and compositions, but also related electrodes, devices, systems, kits, processes, etc. Non-limiting examples of electrochemical devices provided by the invention include electrolytic devices and fuel cells. Energy can be supplied to electrolytic devices by photovoltaic cells, wind power generators, or other energy sources. More preferably the nanoleaves are integrated in photoelectrochemical systems, even more preferably operating at high quantum efficiencies and

correspondingly high energy conversion. These and other devices are described herein.

The present invention provides a novel class of nano-structured materials that, when employed as catalyst, facilitate the production of oxygen and/or hydrogen gas from water at low energy input, i.e. at low "overpotential".

The invention not only permits to generate the novel materials in situ, and in absence of any sophisticated or difficult to prepare material sources. The novel catalytic materials facilitate the electrolysis of water with very low overpotential required. The invention thus also relates to a process for the facile, low-energy conversion of water to hydrogen gas and/or oxygen gas, whereby this process may be easily performed using renewable resources, e.g. standard photovoltaic cells, or any other power source that provides an electrical output, such as for instance wind powered generators or hydro-electricity. The photovoltaic cells, or any other power source can be used to directly provide energy to a user, and/or energy can be stored, via a reaction catalyzed by materials of the invention, in the form of oxygen gas and/or hydrogen gas. In some cases, the hydrogen and oxygen gases may be recombined at any time, for example, using a fuel cell, whereby they form water and release significant energy that can be captured in the form of mechanical energy, electricity, or the like.

The catalytic materials provided by the invention are made of readily available, low-cost material, and are easy to prepare, even in situ. Accordingly, it is believed that the invention will be useful in the field of energy capture, storage, and use, as well as oxygen and/or hydrogen production, and/or production of other oxygen and/or hydrogen-containing products or fuels, and operate with very high current density for oxygen evolution. Additionally, it offers a possible combination with a suitable reduction catalyst, to produce methanol or formic acid from water and C0₂.

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one". Herein above, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or,

equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); and so on.

The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to "A and/or B," when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); and so on.

As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of" or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of".

The following, non-limiting example illustrates the process and materials according to the invention.

Example 1 :

The following example describes the formation of a catalytic material on copper foil. Copper foils (thickness 0.25 mm or less or more, 99.99%) and

electrolytes for various pH solution / buffers were purchased from Sigma Aldrich. 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 and dried, as disclosed in K.S. Joya, N.K. Subbaiyan, F. D'Souza, H.J.M. de Groot, Angew. Chem. Int. Ed. 2012, 51, 9601- 9605. A three-electrode configuration Pyrex glass cell was employed for cyclic voltammetry (CV) studies. The catalytic water electrolysis experiments were carried out in a three electrode double junction H-type glass electrolysis cell as disclosed in K.S. Joya, Y.F. Joya, H.J.M. de Groot, Adv. Energy Mater. 2014, 4,

DOI:10.1002/aenm.201301929. Copper foils (1 cm x 2.5 cm, exposed surface area 1.0 cm²) were used as working electrodes (WE). A platinum wire (thickness: 1 mm), shaped into a spiral, was used as a counter electrode (CE). A silver-silver chloride electrode (SSCE: Ag/AgCI/KCIsat) was applied as reference electrode. However, the potentials are referred to the normal hydrogen electrode (NHE) or reversible hydrogen electrode (RHE) scale. Cyclic voltammetry experiments were performed with an Autolab PG-stat10 potentiostat controlled by GPES-4 software.

Before the preparation of Cu-oxide nano-leaves derived electrodes, the copper foils were cleaned in 0.1 -0.5 M HCI (2-5 min), followed by methanol and acetone (while sonication), and ultimately washed with Millipore water. Spiral platinum counter electrodes were flame annealed and washed with pure water before placing them into the cell. Prior to the water splitting investigations and before each measurement, the aqueous solutions were purged with high-purity argon (Linde Gas, 6.0) for at least 30 min to deoxygenate the solution.

The morphology of the catalytic film was analyzed by scanning electron microscopy (SEM).

The SEM images were taken 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.

The elemental detection and analyses of the (electro-generated) copper oxide laminar leaf-shaped nanoparticulate material on an amorphous copper surface was conducted with X-ray photoelectron spectroscopy (XPS).

The XPS 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 aluminum anode (Al Ka = 1486.6 eV) operating at 72W and a spot size of 400μιτι.

Survey scans were measured at a constant pass energy of 200 eV and region scans were collected at 50 eV. Background pressures of 2 x 10^"9 mbar or 3 x 10^"7 mbar Argon were used during the measurements when operating the charge compensation dual beam source. Early in the course of electrolysis a relatively uniform film was observed with a thickness of approximately 1 m upon passage of about 6 C/cm² Prolonged electrolysis (approximately 40 C/cm² passed) produced a film approximately 3 urn thick with the concomitant formation of spherical nodules of about 1 to about 5 urn in diameter on the surface of the film. The above results show conclusively that the present process allows to prepare a novel nano-structured material that is not only aesthetically pleasing, but in particular represents a larger improvement for the electrolysis of water by providing a novel class of catalytic materials that allow to facilitate the production of oxygen and/or hydrogen gas from water at "low overpotential", i.e. low energy input.

Moreover, the materials are robust, can be formed in situ, and do not require any particular difficult to obtain or rare starting materials, thus representing a sustainable and renewable resource.

In particular, the novel material that is accessible through the novel and simple process according to the invention, comprises copper oxide nanoleaves that are directly formed on, and from a simple copper foil, and are highly efficient as a stable water oxidation electrocatalyst, showing an unprecedented low overpotential among the members of the class of electrodeposited catalysts.

The electrocatalytic copper oxide nano-leaf shaped particles are preferably generated in carbonate buffer system, at a pH from 9 to 13 without the need for presence of copper ions in the solution, and exhibit remarkable activity for anodic oxygen evolution, as an oxygen onset of about 1 .63 V vs. RHE and oxygen evolution peak current density is approaching 40 mA cm^-2 at just 1.60 V vs. NHE, equivalent to 2.28V vs. RHE, measured at a pH of 1 1.5.

This shows that copper oxide nanoleaf based material, when used as an electrocatalyst, exhibits the lowest oxygen generation onset potential known so far as compared to recently reported electrocatalysts like cobalt-phosphate (Co-Pi, 1 .67 V vs. RHE) and nickel-borate (Ni-Bi, 1.71 V vs. RHE).

During long-term controlled-potential electrolysis (CPE) of water at 1.20 V (vs. NHE), the copper oxide nanoleaves showed an excellent oxygen evolution current density of ca. 7.1 mA cm^"2, which could be sustained for at least several hours, i.e. as long as the test was conducted. The bulk elemental composition of the CuOx-NLs (nanoleaves) film is analyzed by EDX (energy dispersive X-ray) measurements. The EDX spectrum shows Cu and oxygen with a 1 : 1 composition in the film (Fig. 12). Interestingly, there is about 1 1 % carbon content present in the catalytic CuOx-NLs deposits. This makes CuOx-NLs altogether a new copper based material for electrocatalytic water oxidation that have carbon assimilation. Carbon based materials are often used in electrochemical applications and catalysis due to their support in electron transfer. The carbon incorporation induces superior material properties such as high surface areas and electrical conductivity, structural flexibility and high mechanical strength. Our data provide converging evidence that the carbon incorporation in the electrocatalyst film is related to the high efficiency and stability of the CuOx-NLs for oxygen evolution and excellent stability even at high operating current density for long-term catalytic water electrolysis.

The bulk elemental composition of the CuOx-NLs film is analysed by EDX (energy dispersive X-ray) measurements. The EDX spectrum shows Cu and oxygen with a 1 : 1 composition in the film (Fig. 12). Interestingly, there is about 1 1 % carbon content present in the catalytic CuOx-NLs deposits. This makes CuOx-NLs altogether a new copper based material for electrocatalytic water oxidation that have carbon assimilation. Carbon based materials are often used in electrochemical applications and catalysis due to their support in electron transfer. The carbon incorporation induces superior material properties such as high surface areas and electrical conductivity, structural flexibility and high mechanical strength. Our data provide converging evidence that the carbon incorporation in the electrocatalyst film is related to the high efficiency and stability of the CuOx-NLs for oxygen evolution and excellent stability even at high operating current density for long-term catalytic water electrolysis.

Further, the catalytic nanoscale copper-oxide deposits are characterized by the Raman spectroscopy (Fig. 12). The broad Raman peak with a relatively high intensity at approximately 295 cm^-1 is assigned to the Ag band (in the CuOx). The small peak at 340-344 cm^"1 and a broad band centered at 628.5 cm^"1 are assigned to the 2Bg modes inthe metal oxide. The high Raman intensities of these peaks indicate a pure single phase of the CuOx film having high crystallinity.

Furthermore, the distribution of elemental constituents in the CuOx-NLs deposits is then analyzed by using spectrum imaging technique in the scanning transmission electron microscopy (STEM) mode combined with electron energy loss spectrosocpy (EELS). The high-angle annular dark-field (HAADF) STEM image of CuOx-NLs sample (Fig. 14a) and its corresponding elemental maps of copper (Cu), oxygen (O) and carbon (C) are presented in Fig. 14. The STEM-EELS spectrum imaging datasets were acquired in such a way that the acquired EELS spectra contained Cu-L23 edge, C-K edge, and O-K edge of Cu, C, and O elements. Theses edges were then analzed to generate the elemental maps of Cu, C, and O elements (Figs. 14b-14d). It is evident from the results that all three elements are

homogeneously distributed in the nanostructured copper-oxide electrocatalytic deposits.

The anodic water oxidation performance of the electrocatalytic CuOx-NLs is tested in clean carbonate solution (pH 10-12) and in near-neutral borate buffer (pH ~ 9). In carbonate electrolyte, cyclic voltammetry (CV) for the electro-generated CuOx- NLs shows appearance of the catalytic current at about 1.60 V (vs. RHE), followed by a sharp rise in the current density (Fig. 15a). The current density further grows rapidly reaching > 50 mA cm^-2 just under 1.9 V vs. RHE and at 2.0 V vs. RHE it approaches 120 mA crrf² (Fig. 15a). During this positive potential scan, an abundant stream of oxygen bubbles is leaving the dark catalytic surface. A magnified view of the CV shows a copper oxidation polarization prefeature between 0.7-0.9 V, followed by a small potential window of steady current and the pronounced catalytic wave at ca. 1 .6 V vs RHE (inset Fig. 15a). These oxidative current prefeature can be assigned to the surface oxidation of metallic copper Cu° into Cu', and leading to Cu" type species with potential increment. During the deposition process, there is a possible involvement of the metal hydroxide type species along with incorporation of carbon from carbonate source. The backward potential sweep, without major current hysteresis avoiding the kinetic barriers during reversible electrochemical reaction, produces a broader reduction wave below 1 .65 V indicating some reduction process of the catalytic oxides in the copper-oxide deposit.

In order to integrate an electrocatalytic system with a solar water-splitting device, a current density of 10 mA cm^"2 is an optimal requirement to achieve 10% efficiency for the solar-to-fuel conversion system. For electro-generated CuOx-NLs presented here, it is noticeable that a current density of 10 mA cm^"2 is obtained at just 1.7 V vs RHE (η » 470 mV). Furthermore, a higher oxygen evolution current approaching 25 mA cm^-2 is observed at 1.8 V vs RHE. These potentials for the O₂ onset voltage (E_on) and higher current regime (10 and 25 mA cm^-2) for CuOx-NLs sample are the lowest compare to other electrodeposited metal-oxide catalysts such as Co-Pi (Eon = 1 .67 V vs RHE), Ni-Bi (E_on = 1.71 V vs RHE). Hence, the CuOx-NLs derived electrocatalytic film displays remarkably reduced oxygen onset overpotential, with a tremendous electrocatalytic water oxidation performance and very high current density for anodic O2 evolution.

The CuOx-NLs sample for water oxidation is also tested in near-neutral borate (Bi) buffer. The anodic scan for CuOx-NLs electrocatalysts in Bi solution shows a slightly higher potential for the initiation of the anodic current for oxygen evolution than for the CV in carbonate buffer during water electrolysis (Fig. 15b). With catalytic onset at 1.66 V vs RHE for CuOx-NLs electrocatalyst in Bi buffer, the current grows rapidly reaching 90 mA cm^-2 at ~ 2.2 V vs RHE (Fig. 15b). The bench mark current density of 10 mA cm^-2 is obtained at ~ 1.8 V vs RHE. The anodic water oxidation response for the CuOx-NLs sample and peak current density is almost ten times higher than for the amorphous phase CuOx film electrodeposited from the

Cu²⁺/borate system. Thus a C0₃ ² HC0₃ ^~ system is essential for the generation and enhanced activity of the CuOx-nanoleaves electrocatlayst for water splitting reaction. The high catalytic activity of the CuOx-NLs electrocatalyst in carbonate and borate solutions is assigned to the special nanostructured surface morphology, presence of carbon contents in the film and to the single phase CuOx matrices exhibiting good crystallinity. The superior performance of the carbonate system relative to the borate system in terms of onset potential and wave current confirms the synergistic role of the carbonate in the catalytic cycle, and the strong catalytic response suggests a mediating role in the proton management.

In addition, the repetitive potential sweeps (forward and backward) reproduce the similar current trends on the 1^st and 50^th CV's indicating stability and robustness of the catalytic system (Fig. 16).

The oxygen evolution reaction in water splitting is kinetically more demanding than proton reduction, owing to its requirement of four redox equivalents per catalytic turnover. It is also energetically demanding due to difficulties in inducing a rapid oxygen-oxygen bonds formation and release the 0₂. Therefore most of the activation overpotential in water splitting originates from the oxygen evolution reaction. In order to accommodate the redox demand of the four electron water oxidation (and 0₂ evolution) steps, the WOEc is desired to operate at a high rate over a narrow potential range. In this context, a small current vs voltage window, the Tafel slope, is important for the integration of WOEc with photo-responsive materials. The current - overpotential (log i vs η) plot or Tafel slope for the CuOx-NLs sample, during OER in the current region 0.1 mA cm^-2 to 1.0 mA cm^-2, reveals an attractively small Tafel slope of 34 mV dec^-1, which is very unique for Cu-oxide based water oxidation catalyst (Fig. 17). Although a 60 mV dec^-1 Tafel slope represents a mechanism of one electron removal coupled with one proton transfer. But a Tafel slope close to of approximately 31 mV dec^-1 probably indicates an oxygen evolution mechanism involving two electron and a proton transfer and representing a very high rate of oxygen evolution reaction accompanying a rapid rate of electron transfer. For the current density above 1.1 mA cm^-2, the current - overpotential character is shifted and a new Tafel behaviour is observed, exhibiting a 85 mV dec^-1 Tafel slope (in set Fig. 17). This diferent electron transfer is attributed to the electrolyte resistance into the electronic communications across the catalytic site and the aqueous medium at higher driving energy.

As the catalytic electrochemical data of CuOx-NLs for anodic water oxidation indicate superior performance in borate and carbonate solutions, we did a long-term performance and stability test of the electro-generated CuOx-NLs via constant- potential electrolysis of water. For a performance comparison in other electrolyte regarding stability and oxygen evolution current density, water electrolysis in near- neutral borate buffer system were also conducted. Fig. 18 shows that during long- term CPE at 1 .83 V vs RHE (rp600 mV), the CuOx-NLs sample exhibits an excellent oxygen evolution current density approaching 9.0 mA cm^"2 right from the beginning of the CPE operation, and sustained for many hours of electrolysis. Meanwhile, a rich stream of oxygen bubbles was also coming out of the nanostructured copper- oxide anode surface.

During > 24 hours of the anodic catalytic operation, the current density remains highly stable and there is no sign of a decrease in catalytic performance for water oxidation. Hence the CuOx-NLs represent an attractive water electrolysis system with sustained performance at room temperature with very high and stable current density. Previously, we showed that cobalt and nickel derived

electrocatalysts developed in near-neutral HC0₃ C0₂ system or HC0₃ C0₃ ² buffers (Ni-Ci and Co-Ci) also exhibit excellent performance for water electrooxidation, and they are amazingly robust in other neutral or near-neutral buffers such as phosphate and borate solutions. Long-term CPE (1 .83 V vs RHE; η«600 mV) for CuOx-NLs electrocatalyst in borate solution shows an oxygen evolution current density above 3.0 mA cm^-2 in the beginning, that is reduced to > 2.5 mA cm^-2 after one hour of operation, and remains stable for about 17 hours of CPE testing (Fig. 18). The anodic performance of this copper-oxide based catalytic film grown directly from Cu foil reveals more than 100 % enhanced water oxidation activity as compared to electrodeposited Cu-oxide films from a Cu²⁺/borate solution. Thus, a unique feature of the CuOx-NLs electrocatalytic system is the remarkable activity and stability in a clean electrolyte solution without having metal ions in the solution.

Following long-term electrolysis tests, online oxygen generation

measurements were conducted for CuOx-NLs electrocatalysts in both electrolyte systems. Online gas chromatographic (GC) measurements show that the

electrocatalytic water oxidation experiment has yielded more than 1200 μηιοΙ of molecular oxygen in 20 hours CPE, with a Faradaic efficiency of > 98 %, considering the current is due to 4e^~ oxidation involving two water molecules to make one molecule of 0₂ (Fig. 19).

This can also be translated into an oxygen generation rate of 61 μηιοΙ of oxygen gas per hour coming out of 1 cm² area of the eletrocatalyst surface. In near- neutral borate system (pH~9), the water electrolysis with CuOx-NLs produces > 450 μηιοΙ of oxygen in 20 hours experiment. This oxygen generation rate, which is considerably lower than for the carbonate system, coroborates the CPE and CV data.

Claims

A process for forming a copper oxide laminar leaf-shaped nanoparticulate material on an amorphous copper surface by electrolytic anodization and /or self-deposition, the method comprising the steps of:

a) Immersing a copper substrate as anode, and a cathode in a reaction

medium range of from pH=9 to pH=13, and

b) applying a current over the anode and cathode suitable for electrolytically depositing the copper oxide onto the surface of the anode.

The process according to claim 1 , wherein the current is applied for a time period of from 0.1 minutes and 24 hours.

The process according to claim 1 or 2, wherein a voltage is applied at a potential of at least about 1.0 V vs. NHE.

The process according to any one of claims 1 to 3, wherein essentially no metal ions, preferably no transition metal ions, other than alkali or earth alkali metal ions are added to the medium.

The process according to any one of claims 1 to 4, wherein the pH is in the range of from 10 to 13.

The process according to any one of claims 1 to 4, wherein the composition reaction medium comprises an aqueous solution comprising carbonate and/or bicarbonate and/or hydroxide ions.

7. The process according to any one of claims 1 to 6, wherein the reaction

medium comprises an aqueous carbonate and bicarbonate buffer.

8. The process according to any one of claims 1 to 6, wherein the copper substrate comprises at least 99% by weight of copper metal.

9. The process according to any one of claims 1 to 8, wherein the copper oxide formed is a crystalline copper oxide.

10. The process according to any one of the proceeding claims, wherein the

electrolytic anodization and /or self-deposition is controlled-potential and controlled-current electrolytic anodization and /or self-deposition.

1 1 . An amorphous copper material having crystalline copper oxide nano-sized leaf-shaped structures associated to its surface exhibiting an ellipsoidal shape with an average aspect ratio in the range of from of 1.3:1 to 3:1.

12. A copper oxide material according to claim 1 1 , whereby Cu and oxygen are present in an amount of from 1 :1 composition in the film, as determined by EDX spectrum

13. A copper oxide material according to claim 1 1 or claim 12, wherein carbon is present in amount in the range of from 10 to 12, preferably 10.5 to 1 1.5, more preferably 1 1 % by weight.

14. A copper material according to any one of claims 1 1 to 13, whereby the

structures have an average thickness of from 10 nm to 50 nm.

15. A copper material according to any one of claims 1 1 to 14, wherein the film or layer thickness is in the range of from 100nm to 25pm, preferably 50nm to 25pm.

16. A copper material according to any one of claims 1 1 to 15, whereby the

structures have a laminar leaf-shaped nanoparticulate configuration.

17. A copper material according to any one of claims 1 1 to 16, having an overpotential (η) in the range of from 0.3 to 0.6 V when employed in a water electrolysis process, as determined by cyclic voltammetry.

18. A catalytic electrode, comprising a copper based material according to any one of claims 11 to 17, having copper oxide laminar leaf-shaped

nanoparticulate material associated to its surface, capable of catalytically producing oxygen gas from water at about 10-13 pH, with an overpotential (η) of from 0.3 to 0.6 volts at an electrode current density of at least 1 mA/cm²

19. Use of the material according to any one of claims 1 to 17 as an electrolysis catalyst, and/or and oxygen evolution reaction (OER) material.

20. An electrochemical cell comprising an anode comprising the copper oxide nanoparticulate material according to any one of claims 1 1 to 17.

21 .A process to convert water into oxygen, comprising providing an

electrochemical cell according to claim 20, and applying a suitable voltage to the anode and cathode, using a power source.

22. A process according to claim 21 , wherein an overpotential of less than 0.4 volts is applied at an electrode current density of at least 1 mA/cm².