Nickel selenide-based composite nanomaterial, method of production and use thereof

Abstract

The present invention relates to a non-stoichiometric Ni0 .85Se / NaCI composite nanomaterial obtained from a nickel selenide and sodium chloride, useful as a multifunctional electrocatalyst and / or photocatalyst for the evolution of hydrogen and oxygen from water, and / or for urea oxidation, and which can also act as a mediator in the oxygen evolution reaction, and the simultaneous purification of aqueous solutions contaminated by organic pollutants. The invention also relates to the method for obtaining said composite nanomaterial, together with the use thereof for obtaining a multifunctional electrode for the evolution of hydrogen and oxygen from water, and / or for urea oxidation. The invention also relates to a multifunctional electrode comprising the non- stoichiometric Ni0 . 85Se / NaCI composite nanomaterial of the invention.

Description

[0001] DESCRIPTION

[0002] TITLE

[0003] “Nickel selenide-based composite nanomaterial, method of production and use thereof”

[0004] TECHNICAL FIELD

[0005] The present invention relates to a composite nanomaterial of a non- stoichiometric nickel selenide and sodium chloride, useful as a multifunctional electrocatalyst and / or photocatalyst for hydrogen and oxygen evolution from water, and / or for urea oxidation, and which can also act as a mediator in the oxygen evolution reaction, and the simultaneous purification of aqueous solutions contaminated by organic pollutants.

[0006] STATE OF THE ART

[0007] In order to achieve sustainable development, it is imperative to develop innovative green energy strategies. Such technologies must improve efficiency in the use of energy and address current environmental problems. In particular, hydrogen (H2) has been identified as a promising alternative to traditional fossil fuels by virtue of its high energy density and environmental benefits.

[0008] Production thereof can be achieved through various methods, including electrochemical water splitting, which is an efficient method for producing high-purity hydrogen without generating pollutants.

[0009] Water electrolysis, which involves the decomposition of pure water into hydrogen and oxygen, requires a considerable input of electrical energy due to the high electrical resistance of pure water (18 MQ-cm). This intrinsic resistance represents a significant obstacle, making direct electrolysis of pure water inefficient. In order to remedy this problem, water electrolysis is usually carried out in strongly acidic (pH 0) or strongly basic (pH 14) electrolytes. Under these conditions, use is made of salts containing inert anions such as sulphate or perchlorate in an acidic environment, and strong bases such as potassium hydroxides (KOH) or sodium hydroxide (NaOH), which create an alkaline environment, in order to facilitate the process.

[0010] The electrolysis process can be divided into two half-reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.

[0011] Under acidic conditions, the HER reaction follows the following equation: 2H++ 2e~ - H2 Eq. 1 with a standard electrode potential of 0.0 V, whereas the OER reaction proceeds as follows: 2H2O - O2+ 4H++ 4e“ Eq. 2 with a standard electrode potential of +1 .23 V.

[0012] Under basic conditions, the corresponding half-reactions are: 4H2O + 4e" - 2H2+ 4OH~Eq. 3 with a standard electrode potential of -0.828 V, and: 4OH — * O2+ 2H2O + 4e Eq. 4 with a standard electrode potential of +0.401 V.

[0013] This method, which makes use of chemical elements and / or compounds known as catalysts (for example platinum, Pt) to facilitate the splitting of water molecules into the components thereof, hydrogen and oxygen, is subject, however, to some problems which limit or preclude its application on a large scale. The main limitation is represented precisely by the lack of catalysts that are able to provide excellent performances at low costs.

[0014] Understanding the mechanisms of HER and OER is thus crucial for technological advances in these electrocatalytic processes.

[0015] As indicated above, HER begins with the adsorption of a proton or a water molecule onto the catalytic site, followed by the release of the adsorbed species.

[0016] In contrast, various mechanisms have been proposed for OER under both acidic and alkaline conditions, with widely accepted oxide / oxide electrochemical pathways (Y. Matsumoto, E. Sato, Mater. Chem. Phys. 1986, 14, 397). These pathways align with experimental results for structurally simple catalysts, though discrepancies may arise with more complex structures (H. Y. Qu, X. He, Y. Wang, S. Hou, Appl. Sci. 2021 , 11, 4320).

[0017] Unlike HER, OER requires that the catalytic site undergoes continuous oxidation and reduction cycles, as a complex structural reorganisation is necessary at the surface to maintain the catalytic structure active. Amorphous and / or composite electrocatalysts offer the necessary flexibility for that reorganisation, whereas crystalline catalysts have less of an inclination towards these processes (Y. Matsumoto, E. Sato, Mater. Chem. Phys. 1986, 14, 397).

[0018] However, the oxygen evolution reaction represents the limiting step in water splitting due to its slow multi-electron transfer process. To remedy this problem, it has been proposed to integrate a urea oxidation reaction (UOR) into the system, as urea is a chemical substance used as a redox mediator for electron transfer (P. Wang et al., Adv. Mater. 2024, 2404806). UOR is not only more favourable from a thermodynamic and kinetic viewpoint compared to OER, but also offers the dual advantage of clean energy generation together with treatment of urea-rich industrial and sanitary wastewater, when associated with water purification. From a thermodynamic viewpoint, the urea oxidation reaction requires a significantly lower potential than the oxygen evolution reaction. From a kinetic viewpoint as well, UOR is more efficient, as it has a lower activation energy than OER, which favours a higher reaction speed, thus making the process faster. The dual advantage of UOR resides in the fact that, on the one hand, it enables the production of clean energy in the form of hydrogen, useful as a fuel, whilst on the other it enables the treatment of wastewater. Urea, often present in industrial and agricultural wastewater, can be oxidised to less harmful compounds such as nitrogen and carbon dioxide, thus contributing to water purification. In this manner, UOR not only improves the energy efficiency of the electrolysis process, but also addresses an environmental problem by combining energy production and water purification in a single sustainable operation.

[0019] The urea oxidation reaction is crucial for the functioning of urea-based technologies. Optimisation of UOR catalysts implies lowering energy barriers (W. Xu, Z. Wu, S. Tao, Energy. Technol. 2016, 4, 1329).

[0020] Precious metal-based catalysts are commonly used for UOR catalysis in studies on hydrogen; however, their high cost and scarcity limit their applicability and commercialisation.

[0021] Therefore, there are substantial efforts underway to design and explore catalysts based on non-precious transition metals that can match the performances of precious metal-based catalysts.

[0022] In this regard, nickel is the most widely studied transition metal thanks to its promising performances in UOR catalysis (R. P. Forslund etal., ACS Catal. 2019, 9, 2664).

[0023] Numerous publications focus on the design and synthesis of efficient catalysts for hydrogen evolution, oxygen evolution and urea oxidation reactions (Yu, Z. et al., J. Colloid. Interf. Sci. 2024, 661, 629-661 ).

[0024] Therefore, the development of efficient catalysts capable of sustaining HER, OER and UOR is of fundamental importance for technological progress related to the production of sustainable energies and environmental reclamation.

[0025] For the catalysis of the hydrogen evolution reaction, as an alternative to traditional catalysts based, for example, on platinum, molybdenum disulphide (M0S2), iridium oxide (lrO2), ruthenium oxide (RUO2), and cobalt oxides (C03O4), some promising candidates consisting of transition metals, such as nickel (Ni), have been studied and introduced, in particular under alkaline conditions.

[0026] For example, the compound Nio.ssSe is a non-stoichiometric nickel selenide that offers excellent properties for catalytic applications (B. Yu et al., Electrochim. Acta. 2017, 242, 25-30). It has a high electrical conductivity, a large specific surface and considerable stability under different reaction conditions. The non-stoichiometric nature of Nio.ssSe allows for flexible electron structures, thus improving its catalytic activity (L. Zhao et al., J. Alloy. Compel. 2021 , 852, 156751 ).

[0027] However, the compound Nio.ssSe is difficult to obtain: it is usually obtained by synthesis using harmful reagents (e.g. hydrazine), and / or prohibitive experimental conditions (e.g. temperatures >1000 °C, synthesis times even of 2 weeks), and / or poor reaction yields, and / or using further supporting materials (e.g. carbon fibres, graphene, graphene oxide, rGO) onto which the Nio.ssSe must necessarily be anchored in order to be stable.

[0028] Notwithstanding the studies carried out, there is no evidence relating to the possibility of using a same, single catalyst in all the abovementioned reactions, namely HER, OER and UOR.

[0029] There is thus a felt need for a non-precious metal-based catalyst capable of catalysing reactions of hydrogen and oxygen evolution from water, possibly also catalysing the urea oxidation reaction and purifying aqueous solutions contaminated by organic pollutants.

[0030] The present invention solves the technical problem specified above thanks to a non-stoichiometric Nio.ssSe / NaCI composite nanomaterial obtained from nickel selenide and sodium chloride, useful as a multifunctional electrocatalyst and / or photocatalyst for the evolution of hydrogen and oxygen from water, and / or for urea oxidation, and which can also act as a mediator in the oxygen evolution reaction, and the simultaneous purification of aqueous solutions contaminated by organic pollutants.

[0031] DEFINITIONS

[0032] Unless otherwise defined, all the terms of the art, notations and other scientific terms used herein are intended to have the meanings commonly understood by those who are skilled in the art to which this description pertains. In some cases, terms with commonly understood meanings are defined herein for the sake of clarity and / or for ease of reference; the inclusion of such definitions in the present description should thus not be interpreted as representing a substantial difference from what is generally understood in the art.

[0033] The terms “comprising”, “having”, “including” and “containing” are to be understood as open-ended terms (i.e. the meaning of “comprising, but not limited to”) and are to be considered as a support also for terms like “consist essentially of”, “consisting essentially of”, “consist of” or “consisting of”.

[0034] For all the ranges specified in the text, figures and claims of the present patent application, it is understood that the endpoints of these ranges are included, as it is likewise understood that all the values within said ranges are described.

[0035] The terms “obtainable”, “obtained”, “obtainable directly from”, “obtained directly from” are considered equivalent.

[0036] The acronym “HER” stands for hydrogen evolution reaction, which takes place at the cathode of an electrochemical, electrolytic, photoelectrochemical, or photoelectrolytic cell.

[0037] The acronym “OER” stands for oxygen evolution reaction, which takes place at the anode of an electrochemical, electrolytic, photoelectrochemical, or photoelectrolytic cell.

[0038] The acronym “UOR” stands for urea oxidation reaction, which takes place at the anode of an electrochemical, electrolytic, photoelectrochemical, or photoelectrolytic cell.

[0039] The acronym “XRD” stands for X-ray diffraction.

[0040] The acronym “SEM” stands for scanning electron microscopy.

[0041] The acronym “FESEM” stands for field emission scanning electron microscopy.

[0042] The terms “SEM” and “FESEM” are used interchangeably in the present invention.

[0043] The acronym “EDX” stands for energy dispersive X-Ray spectroscopy.

[0044] “Electric overpotential” means the energy that must be supplied in order for HER, OER and / or UOR to take place.

[0045] The acronym “LSV” stands for linear sweep voltammetry. The acronym “LSV-RDE” stands for linear sweep voltammetry coupled to a rotating disk electrode.

[0046] The acronym “EIS” stands for electrochemical impedance spectroscopy.

[0047] The acronym “PEC” stands for the adjective photoelectrochemical.

[0048] In the present invention, the term “catalyst” means an “electrocatalyst”, a “photoelectrocatalyst”, and / or a “photocatalyst”. Therefore, the terms “catalyst” and “electrocatalyst” are used interchangeably with each other, just as the terms “photoelectrocatalyst” and “photocatalyst” are used interchangeably with each other.

[0049] In the present invention, the term “nanostructure” or “nanostructured material” means the Nio.ssSe / NaCI composite nanomaterial.

[0050] In the present invention, the term “multifunctional” is understood to mean the ability of the Nio.ssSe / NaCI composite nanomaterial to catalyse hydrogen evolution reactions, oxygen evolution reactions and / or urea oxidation reactions. This takes place both in the electrochemical field and in the photoelectrochemical field. Therefore, the term “multifunctional” field is also meant to indicate the possibility that the Nio.ssSe / NaCI composite nanomaterial of the invention may be comprised in the cathode, in the anode or in both in an electrolytic cell, and / or that it may be comprised in the photocathode, the photoanode or in both in an electrochemical cell.

[0051] In the present invention, the expression “basic characterisations” means a set of fundamental analyses conducted to understand the main properties of a nanomaterial, and which represent the starting point for more advanced or applied studies. In the context of energy production as in the present invention, these analyses typically comprise:

[0052] 1 ) structural characterisations, such as electron microscopy (SEM, TEM), to analyse particle morphology and size and X-ray diffraction (XRD) to determine crystallinity and the phases present.

[0053] 2) chemical characterisations, such as spectroscopy (EDX), to identify the elemental composition and / or state of oxidation. 3) electrical and electrochemical characterisations, such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements to analyse electrochemical activity and charge transfer mechanisms.

[0054] SUMMARY OF THE INVENTION

[0055] The present invention relates to a non-stoichiometric Nio.ssSe / NaCI composite nanomaterial, useful as a multifunctional electrocatalyst and / or photocatalyst for hydrogen and oxygen evolution from water, and / or for urea oxidation, which can also act as a mediator in the oxygen evolution reaction. The invention also relates to a method for the synthesis of Nio.ssSe / NaCI, comprising a step of co-precipitating a nickel precursor and a selenium precursor in a supersaturated aqueous NaCI solution, wherein the nickel precursor is selected from nickel acetate, nickel chloride and nickel nitrate, optionally hydrated and the selenium precursor is selected from H2Se, H2SeO3, H2SeO4 and NaSeH (sodium hydroselenide), and wherein the coprecipitation is carried out in the presence of a temperature comprised between 80° C and 150° C and / or after a time comprised between 1 and 4 hours.

[0056] Moreover, the invention relates to the use of the Nio.ssSe / NaCI composite nanomaterial as a multifunctional electrocatalyst and / or multifunctional photoelectrocatalyst for hydrogen evolution reactions, oxygen evolution reactions and / or urea oxidation reactions.

[0057] The invention also relates to the use of the Nio.ssSe / NaCI composite nanomaterial for obtaining a multifunctional electrode and / or a multifunctional photoelectrode for hydrogen evolution reactions, oxygen evolution reactions and / or urea oxidation reactions.

[0058] Finally, the invention also relates to a multifunctional electrode and / or a multifunctional photoelectrode comprising the Nio.ssSe / NaCI composite nanomaterial of the invention. BRIEF DESCRIPTION OF THE FIGURES

[0059] Figure 1A shows the XRD pattern of Nio.ssSe / NaCI and the XRD pattern of reference for Nio.ssSe and NaCL

[0060] Figure 1 B shows the Raman spectrum of the non-stoichiometric nickel- based Nio.ssSe / NaCI composite nanomaterial.

[0061] Figure 1C shows an SEM micrograph of the non-stoichiometric nickel- based Nio.ssSe / NaCI composite nanomaterial.

[0062] Figure 1 D shows an elementary mapping that highlights the presence of Se and Ni in the non-stoichiometric nickel-based Nio.ssSe / NaCI composite nanomaterial.

[0063] Figure 2 shows the reflectance curve of the non-stoichiometric nickel-based Nio.ssSe / NaCI composite nanomaterial. Square: Kubelka-Munk transform.

[0064] Figure 3 shows the Tafel diagram obtained from the LSV curves at a scan rate of 25 mV s1in a 0.5 M aqueous solution of H2SO4, for HER carried out using Nio.ssSe / NaCI as the electrocatalyst.

[0065] Figure 3A shows the polarisation curves obtained by linear sweep voltammetry (LSV), with a scan rate of 25 mV s-1in a 0.5 M solution of H2SO4, for HER carried out using Nio.ssSe / NaCI as the electrocatalyst.

[0066] Figure 3B shows the Tafel slope extrapolated from the linear portion of the Tafel graph corresponding to the start of HER, carried out using Nio.ssSe / NaCI as the electrocatalyst.

[0067] Figure 3C shows the Raman spectra of Nio.ssSe / NaCI obtained before and after HER, carried out using Nio.ssSe / NaCI as the electrocatalyst.

[0068] Figure 4A shows the LSV-RDE polarisation curves in 0.5 M H2SO4 at different rotation speeds (in rpm) for HER carried out using Nio.ssSe / NaCI as the electrocatalyst.

[0069] Figure 4B shows the Koutecky-Levich graph extrapolated for the Nio.ssSe / NaCI composite nanomaterials.

[0070] Figure 5A shows the Nyquist plot obtained from EIS measurements and which shows the charge transfer resistance for HER carried out using Nio.ssSe / NaCI as the electrocatalyst. Figures 5B and 5C show plots representing the Bode modulus (figure 5B) and the Bode phase plot (figure 5C) recorded in 0.5 M H2SO4 in the frequency range of 100 kHz to 100 mHz with an applied potential of -0.20 V and an alternating current amplitude of 10 mV, for HER carried out using Nio.ssSe / NaCI as the electrocatalyst.

[0071] Figure 6A shows the oxidation current density values associated with different concentrations of urea in the presence of Nio.ssSe / NaCI as the electrocatalyst.

[0072] Figure 6B shows the linear sweep voltammetry curve for Nio.ssSe / NaCI at a scan rate of 25 mV s-1in a 1 M solution of KOH 1 M (broken line) and in 1 M KOH in the presence of 20 mM of urea (black line).

[0073] Figure 6C shows the Tafel slope extrapolated from the linear portion of the Tafel graph corresponding to the start of OER and UOR carried out in the presence of Nio.ssSe / NaCI as the electrocatalyst.

[0074] Figure 7A shows the Nyquist plots obtained from EIS measurements, and which show the charge transfer resistance for OER and UOR carried out in the presence of Nio.ssSe / NaCI as the electrocatalyst.

[0075] Figures 7B and 7C show plots representing the Bode modulus (figure 7B) and the Bode phase plot (figure 7C) recorded in 1 M KOH and 1 M KOH + 20 mM urea in the frequency range of 100 kHz to 100 mHz with an applied potential of 0.70 V and an alternating current amplitude of 10 mV, when using Nio.ssSe / NaCI as the electrocatalyst.

[0076] DETAILED DESCRIPTION OF THE INVENTION

[0077] The present invention relates to an Nio.ssSe / NaCI composite nanomaterial, useful as a multifunctional catalyst for hydrogen and oxygen evolution from water and for the electrochemical urea oxidation reaction and purification of aqueous solutions contaminated by organic pollutants, both in an electrochemical context and in a photoelectrochemical context.

[0078] The Nio.ssSe / NaCI composite nanomaterial is obtained with a method of synthesis that uses mild reaction conditions and economically affordable materials, thus representing a valid alternative to the current commercially available catalysts.

[0079] The non-stoichiometric nickel-based Nio.ssSe / NaCI composite nanomaterial of the invention has a nanostructure characterised by the integration of sodium chloride into the crystal lattice of Nio.ssSe: XRD analysis confirmed, in fact, the crystalline structure and phase of the nanomaterial of the invention, in which Nio.ssSe and NaCI coexist, as demonstrated by the X-ray diffraction signal pattern shown in figure 1A (obtained by means of X-ray diffraction models collected using an X-ray diffractometer (PANalytical Empyrean XRD) with Cu Ka radiation in the 10°-80° 20 range, as indicated in example 2).

[0080] XRD analysis suggested that the NaCI fills the voids in the non- stoichiometric Nio.ssSe structure, becoming integrated within the crystal lattice and resulting in a composite nanostructure comprising two phases: Nio.ssSe and NaCI.

[0081] According to a preferred aspect, the Nio.ssSe / NaCI composite nanomaterial is characterised by an X-ray diffraction spectrum with the following ±0.20 degrees (2 theta) peaks: 33.40°, 50.50°, 60.40°, and 62.00° with reference to the Nio.ssSe crystalline phase; 27.37°, 31.86°, 66.20°, and 75.35° with reference to the NaCI crystalline phase.

[0082] The coexistence of Nio.ssSe and NaCI in the composite nanomaterial of the invention was also confirmed by the Raman spectrum of the Nio.ssSe / NaCI composite nanomaterial shown in figure 1 B (obtained using an unpolarised laser at 532 nm, as indicated in example 2).

[0083] The morphology and elemental composition of the Nio.ssSe / NaCI composite nanomaterial were characterised using scanning electron microscopy (SEM) (field-emission scanning electron microscopy (FESEM), Magellan XHR 400L, with an acceleration voltage of 5 kV and equipped with an EDS detector) (Figure 1C) and energy-dispersive X-ray spectroscopy (EDX) (Figure 1 D) (carried out as indicated in example 2). The SEM analysis revealed the formation of nanostructures on a nanometric scale, whose average lateral dimension is comprised between 50 and 500 nm, preferably between 100 and 300 nm, more preferably between 150 and 250 nm, and even more preferably it is 200 nm (the determination of the size of the nanostructures was achieved by extrapolation, using Imaged software, of micrographs of the Nio.ssSe / NaCI composite nanomaterial).

[0084] Therefore, according to a further preferred aspect, the Nio.ssSe / NaCI composite nanomaterial of the invention has an average lateral dimension comprised between 50 and 500 nm, preferably between 100 and 300 nm, more preferably between 150 and 250 nm, and even more preferably it is 200 nm, as measured by SEM analysis.

[0085] The EDX analysis, together with elementary mapping, confirmed the uniform distribution of Ni and Se within the composite nanomaterial of the invention and provided an estimate of the elemental composition as shown in Table 1 below:

[0086] Table 1 - table summarising the EDX analysis of the Nio.ssSe / NaCI composite nanomaterial and showing the atomic percentages of selenium, nickel, oxygen, carbon, chlorine and sodium.

[0087] (*) the oxygen detected in the analysis can be attributed to residual traces in the atmosphere of the microscope chamber.

[0088] (°) the carbon detected in the analysis comes from the conductive tape used to fix the sample onto the sample holder. According to another preferred aspect, the Nio.ssSe / NaCI composite nanomaterial is characterised by a uniform distribution of Ni and Se within the structure.

[0089] The Nio.ssSe / NaCI composite nanomaterial of the invention was tested for its performances in hydrogen evolution, oxygen evolution and urea oxidation reactions.

[0090] In HER, the Nio.ssSe / NaCI composite nanomaterial as a catalyst is distinguished by a low electric overpotential, a small Tafel slope, and high current densities, reaching up to 327 mA cm-2during hydrogen evolution. Generally, an electric overpotential is considered “low” in the literature if it stands at a few millivolts (100-150 mV) for a given current density (for example, 10 mA / cm2for HER). Higher values are usually considered suboptimal.

[0091] A Tafel slope is described as “small” in the literature when it is around or below 30-70 mV / decade, indicating favourable electrochemical kinetics. Higher values (>120 mV / decade) suggest less efficient kinetics.

[0092] Current density is considered “high” in the literature when it exceeds values of about one hundred mA / cm2in practical applications, based on the type of reaction and the electrochemical system analysed. For specific reactions such as HER or OER, a density greater than 200 mA / cm2could be indicative of high performance.

[0093] The overpotential values of the Nio.ssSe / NaCI electrocatalyst of the present invention represent a significant improvement over previously reported Nio.ssSe-based catalysts such as NiSe-Nio.ssSe / CP (101 mV) (C. Jiang et al., Chem. Soc. Rev. 2017, 46(15), 4645-4660), Nio.ssSe / CP (195 mV) (C. Jiang et al. (2017) already cited above), Nio.ssSe (190 mV) (W. Lei et al., Mater. Today. 2022, 52, 133-160), Nio.8sSe / Ni3S2 (145 mV) (W. Zhao et al. J. Zhang, ACS Appl. Mater. Interfaces. 2018, 10, 40491 ) and iron-doped Nio.ssSe (246 mV) (D. Shi et al., Int. J. Hydrogen. Energy. 2022, 47, 305).

[0094] The overpotential of the noble catalyst Pt / C is 1 10 mV and the results reported herein for an electrocatalyst such as the composite nanomaterial of the invention, whose constituent elements are not precious but rather abundant on Earth, are particularly relevant for the development of novel nanomaterials for splitting water. These results indicate a superior performance for the Nio.ssSe / NaCI composite nanomaterial compared to similar compounds.

[0095] In fact, the Tafel slope value for Nio.ssSe / NaCI is relatively small, comparable to the best Nio.ssSe-based catalysts reported previously, such as pure Nio.ssSe: 81 mV dec1(M. Gong et al., H. Dai, Nat. Commun. 2014, 5, 1 ), 65 mV dec1(X. Wu etal., Int. J. Hydrogen. Energy. 2016, 41, 10688), and 101 mV deci (B. Yu et al., Electrochim. Acta. 2017, 242, 25)) and Nio.ssSe / rGO (91 mV dec1) (M. Zhu et al., Int. J. Hydrogen Energy. 2020, 45, 10486).

[0096] According to a preferred aspect, the Nio.ssSe / NaCI composite nanomaterial of the invention is used in HER at a pH comprised between 0.5 and 1.5, more preferably at a pH of 1 .

[0097] In addition to its exceptional activity in HER, as an electrocatalyst the Nio.ssSe / NaCI composite nanomaterial demonstrated an effective urea oxidation, while simultaneously evolving oxygen.

[0098] In fact, as a catalyst the Nio.ssSe / NaCI composite nanomaterial effectively oxidates urea at optimal concentrations of 10-50 mM, producing oxygen and consequently reducing the energy necessary to initiate the reaction to 220 mV, a low value for the Tafel slope (67.9 mV dec1), among the best reported in the prior art for similar catalysts.

[0099] Moreover, the material demonstrated a significant OER activity, reaching a current density of 10 mA cm-2with an overpotential of 330 mV in the absence of urea in the electrolyte, though the performance is not so high as for UOR. It may thus be concluded that the presence of small amounts of urea in the electrolyte leads to a lower energy demand (equal to 220 mV) to initiate oxygen evolution, thus aligning this material with principles of sustainability and energy savings. According to a preferred aspect, the Nio.ssSe / NaCI composite nanomaterial of the invention is used in OER and UOR at a pH comprised between 13.0 and 14.0, more preferably at a pH of 13.5.

[0100] These results confirm the superior catalytic activity and stability of the Nio.ssSe / NaCI composite nanomaterial in different reactions.

[0101] In fact, the incorporation of NaCI into non-stoichiometric nickel selenide, Nio.ssSe, optimises the properties of the latter by improving the charge transfer and providing additional active sites for catalysis, i.e. stabilising it and increasing the lifetime thereof, thus assuring lasting electrocatalytic performances with improved reaction kinetics. For this reason, the Nio.ssSe / NaCI composite nanomaterial of the invention demonstrated a high efficiency in reactions evolving hydrogen and oxygen from water and urea oxidation reactions, demonstrating it to be a multifunctional electrocatalyst of great interest.

[0102] Thus, the invention relates to the use of the Nio.ssSe / NaCI composite nanomaterial as a multifunctional electrocatalyst for hydrogen evolution reactions (HER), oxygen evolution reactions (OER) and / or urea oxidation reactions (UOR).

[0103] Moreover, the light-absorbing ability of the Nio.ssSe / NaCI composite nanomaterial was investigated in detail and its optical properties were examined using UV-Vis-NIR diffuse reflectance spectroscopy (carried out as indicated in example 3). The resulting spectrum and the extrapolated Tauc plot are shown in Figure 2. The Tauc plot indicates that the synthesised nanomaterial exhibits a semiconductor behaviour with a band gap energy (Eg) of 1 .38 eV. Such a low band gap value is advantageous for applications requiring an efficient light absorption in the visible to nearinfrared regions. The incorporation of NaCI into the crystalline structure of Nio.ssSe significantly improves its optical properties. Specifically, the presence of NaCI brings about a reduction in the band gap energy compared to the values typically reported for the material pure Nio.ssSe (G. Han et al., Angew. Chem. Int. Ed. 2016, 55, 6433). This decrease in energy can be attributed to modifications in the electron structure and an increase in disorder introduced by the incorporation of NaCI, which facilities the movement of charge carriers.

[0104] The observed value of 1.38 eV positions Nio.ssSe / NaCI as a promising candidate for applications such as photoelectrochemical (PEC) water splitting, a sustainable approach for hydrogen production which exploits solar energy to make water electrolysis occur (Z. Yang et al., Int. J. Hydrogen. Energy. 2019, 44, 26109).

[0105] In this context, the efficiency of PEC water splitting greatly depends on the light absorption properties of the semiconductor materials used as photoelectrodes. A band gap of 1.38 eV is close to the ideal value for maximising the absorption of sunlight and enabling an efficient exploitation of the solar spectrum. This assures that a significant portion of the sun’s energy can be converted into chemical energy, thus improving the overall efficiency of the PEC process. Moreover, the reduction in the band gap energy due to the incorporation of NaCI not only improves the absorption of light, but it also favours the separation and transport of charge carriers. A favourable dynamic is crucial in PEC water splitting to minimise the losses due to the recombination of charge carriers and to maximise the generation of hydrogen and oxygen.

[0106] The Nio.ssSe / NaCI composite nanomaterial of the invention can thus be advantageously used as a catalyst both in the electrochemical field and in the photoelectrochemical field.

[0107] The invention thus relates to the use of the Nio.ssSe / NaCI composite nanomaterial as a multifunctional photoelectrocatalyst for hydrogen evolution reactions (HER), oxygen evolution reactions (OER) and / or urea oxidation reactions (UOR).

[0108] The Nio.ssSe / NaCI composite nanomaterial of the invention was obtained with a co-precipitation process using a supersaturated aqueous NaCI solution as the solvent. The invention thus relates to a method of synthesis for obtaining the Nio.ssSe / NaCI composite nanomaterial, comprising a step of co-precipitating a nickel precursor and a selenium precursor in a supersaturated aqueous NaCI solution, wherein the nickel precursor is selected from nickel acetate, nickel chloride and nickel nitrate, optionally hydrated, and the selenium precursor is selected from H2Se, H2SeO3 and H2SeC and NaSeH (sodium hydroselenide), and wherein the co-precipitation is carried out in the presence of a temperature comprised between 80° C and 150° C and / or after a time comprised between 1 and 4 hours.

[0109] According to a preferred aspect, the nickel precursor is nickel nitrate, optionally hydrated. According to a particularly preferred aspect, the nickel precursor is nickel nitrate hexahydrate (Ni(NO3)2.6H2O).

[0110] According to another preferred aspect, the selenium precursor is NaSeH.

[0111] The sodium hydroselenide is obtained by reducing active selenium powder with sodium borohydride (NaBH4).

[0112] The nickel precursor is dissolved in supersaturated saltwater (NaCI), to which the selenium precursor is added, and, by co-precipitation, the Nio.ssSe / NaCI composite nanomaterial of the invention is formed.

[0113] According to a preferred aspect, the co-precipitation of the nickel precursor and selenium precursor as Nio.ssSe / NaCI takes place at a temperature comprised between 90° C and 1 10° C, more preferably comprised between 95°C and 105° C and / or after a time preferably comprised between 1 .5 and 3.5 hours.

[0114] According to another preferred aspect, the method of synthesis of the invention has a high yield: in fact, between 2 and 3 grams of nanomaterial are produced using only 150 mL of reaction solution, with an optimal volume yield.

[0115] According to a preferred aspect the % yield by weight of the method of synthesis of the invention is comprised between 40% and 95%, preferably between 60% and 80%; more preferably it is comprised between 65% and 75%. Nickel selenide compounds are often very difficult to obtain and require extreme conditions. Contrary to what is known, however, the reaction conditions of the method of synthesis of the invention are mild compared to the extreme conditions and long times typical of conventional synthesis.

[0116] Thanks to the use of a supersaturated sodium chloride solution, one obtains a stable composite nanomaterial with an optimised morphology for use as a multifunctional electrocatalyst or photoelectrocatalyst, both for the production of hydrogen and oxygen from water and for urea oxidation.

[0117] This stable composite nanomaterial with an optimised morphology is not obtainable in the absence of sodium chloride. In fact, comparative tests were performed with other salts, e.g. potassium chloride, or other compounds, e.g. the surfactant sodium dodecyl sulphate (SDS), but none showed the same advantageous effect as sodium chloride in controlling morphology and stabilising the structure of the composite nanomaterial. Sodium chloride revealed to be crucial for obtaining the desired properties, whereas other salts or other compounds did not produce comparable results.

[0118] The strategy of synthesis used is not only simple but also scalable, making it highly advantageous for large-scale production of the multifunctional nanomaterial. The absence of impurities in the final product underscores the efficacy of the co-precipitation method in producing composite nanomaterials of high purity.

[0119] Moreover, the method of synthesis of the invention has an extremely low production cost, making the process economically advantageous.

[0120] The evident advantages of the method for synthesising the Nio.ssSe / NaCI composite nanomaterial and its performances as a multifunctional electrocatalyst, both in the electrochemical field and in the photoelectrochemical field, make it suitable for the production of multifunctional electrodes and photoelectrodes.

[0121] The invention thus also relates to the use of the Nio.ssSe / NaCI composite nanomaterial of the invention for obtaining a multifunctional electrode and / or a multifunctional photoelectrode for hydrogen evolution, oxygen evolution, and / or urea oxidation reactions.

[0122] According to a preferred aspect, the Nio.ssSe / NaCI composite nanomaterial of the invention is used to obtain a multifunctional electrode which can be the cathode, the anode or both.

[0123] According to another preferred aspect, the Nio.ssSe / NaCI composite nanomaterial of the invention is used to obtain a multifunctional photoelectrode which can be the photocathode, the photoanode or both. Moreover, the invention thus also relates to a multifunctional electrode and / or a multifunctional photoelectrode comprising the Nio.ssSe / NaCI composite nanomaterial of the invention, useful both in the electrochemical field and in the photoelectrochemical field.

[0124] According to a preferred aspect, the invention thus relates to a multifunctional electrode comprising the Nio.ssSe / NaCI composite nanomaterial of the invention, wherein said multifunctional electrode can be the cathode, the anode or both.

[0125] According to another preferred aspect, the invention thus relates to a multifunctional photoelectrode comprising the Nio.ssSe / NaCI composite nanomaterial of the invention, wherein said multifunctional photoelectrode can be the photocathode, the photoanode or both.

[0126] The multifunctional electrode and / or photoelectrode comprising the Nio.ssSe / NaCI composite nanomaterial of the invention can each further comprise any substrate of a metallic, non-metallic and / or semi-conductive nature, preferably selected from a substrate made of nickel, carbon, platinum, iridium and / or palladium, more preferably selected from carbon fibre fabric, nickel foam and glassy carbon, even more preferably said substrate being glassy carbon.

[0127] The excellent performance metrics, such as a higher current density, smaller Tafel slope and reliability in the three reactions (HER, UOR and OER) qualify the Nio.ssSe / NaCI composite nanomaterial as a catalyst that represents a valid alternative to precious metal-based catalysts. Its versatility and high efficiency suggest broader implications and new potential applications besides water splitting, including the integration of the Nio.ssSe / NaCI composite nanomaterial into fuel cells that exploit solar energy, thanks to its band gap.

[0128] In conclusion, the development of the Nio.ssSe / NaCI composite nanomaterial represents significant progress in the field of electrocatalysis and photoelectrocatalysis. The exceptional catalytic performance, purity and scalability of the compound include it as a possible candidate for various catalytic applications, both in an industrial context and in an environmental context, in relation to non-precious catalysts in sustainable energy technologies.

[0129] The invention is illustrated below by means of experimental examples, which are not to be considered limiting for the scope of the invention.

[0130] EXAMPLES nanomaterial.

[0131] Materials

[0132] The following materials were used to prepare the samples: nickel(ll) nitrate hexahydrate Ni(NO3)2-6H2O (99.99%, Sigma-Aldrich), sodium chloride NaCI (>99.0%, Sigma-Aldrich), sodium borohydride NaBH4 (99%, Sigma- Aldrich), selenium Se (99.99%, Sigma-Aldrich), absolute ethanol (99%, Sigma-Aldrich) and deionised water. All the products were used without further purifications.

[0133] Synthesis by co-precipitation from a supersaturated NaCI solution

[0134] 969 mg of Ni(NO3)2'6H2O were dissolved in 50 mL of supersaturated NaCI solution. In the meantime, an NaHSe solution was prepared by dissolving 263 mg of selenium powder and 252 mg of NaBH4 in 50 mL of deionised water according to the following reaction:

[0135] 4NaBH4+ 2Se + 7H2O 2NaHSe + Na2B4O7 + 14H2(?) Eq.5 The reducing agent obtained was added dropwise to the solution containing the Ni precursor. After the addition, the reaction process was maintained at 100°C for 3 hours. The precipitate was then centrifuged (5000 rpm for 10 minutes), rinsed with deionised water and ethanol, and finally dried overnight in an oven at 65°C.

[0136] The crystallographic structure, morphology, composition, and optical and electrochemical properties of the prepared samples, whose preparation is summed up in Table 2, were determined by means of different analytic techniques as described below. Table 2: prepared samples

[0137] Experimental Yield (g)

[0138] Nio.ssSe / NaCI Precursors conditions (%)

[0139] Ni(NO3)2-6H20 (969 mg

[0140] - 290.79 g / mol); NaBH4Co-

[0141] 2.945

[0142] Sample 1 (252 mg - 37.83 g / mol); precipitation,

[0143] Se (263 mg - 78.96 100 °C for 3 h (72-4 / °) g / mol)

[0144] Ni(NO3)2'6H20 (969 mg

[0145] - 290.79 g / mol); NaBH4Co-

[0146] 3.010

[0147] Sample 2 (252 mg - 37.83 g / mol); precipitation,

[0148] Se (263 mg - 78.96 100 °C for 3 h (74-%) g / mol)

[0149] Ni(Nb3)2;6H20 (969 mg

[0150] - 290.79 g / mol); NaBH4Co-

[0151] 2.535

[0152] Sample 3 (252 mg - 37.83 g / mol); precipitation,

[0153] (62.3%)

[0154] Se (263 mg - 78.96 100 °C for 3 h g / mol)

[0155] Ni(NO3)2-6H20 (969 mg Co-

[0156] Sample 4 - 290.79 g / mol); NaBH4precipitation,

[0157] (69.3%)

[0158] (252 mg - 37.83 g / mol); 100 °C for 3 h Se (263 mg - 78.96 g / mol)

[0159] Ni(NO3)2-6H20 (969 mg

[0160] - 290.79 g / mol); NaBH4Co-

[0161] 3.221

[0162] Sample 5 (252 mg - 37.83 g / mol); precipitation,

[0163] (79.1 %)

[0164] Se (263 mg - 78.96 100 °C for 3 h g / mol)

[0165] Ni(NO3)2-6H20 (969 mg

[0166] - 290.79 g / mol); NaBH4Co-

[0167] 2.910

[0168] Sample 6 (252 mg - 37.83 g / mol); precipitation,

[0169] Se (263 mg - 78.96 100 °C for 3 h (71-2 / °) g / mol)

[0170] Ni(NO3)2'6H20 (969 mg

[0171] - 290.79 g / mol); NaBH4Co-

[0172] 3.065

[0173] Sample 7 (252 mg - 37.83 g / mol); precipitation,

[0174] (75.3%)

[0175] Se (263 mg - 78.96 100 °C for 3 h g / mol)

[0176] The analyses described in the following examples were conducted on all the samples in Table 2 and all the basic characterisations were used to verify the reproducibility of the method in all the examples.

[0177] Example 2

[0178] Characterisation of the Nio.ssSe / NaCI composite nanomaterial.

[0179] X-ray diffraction (XRD analysis)

[0180] A characterisation of the Nio.ssSe / NaCI composite nanomaterial obtained in example 1 was carried out by XRD analysis.

[0181] The two diffractograms shown in figure 1 A relate to samples 1 and 2.

[0182] The crystallographic structure of the prepared samples was determined by X-ray diffraction using an X-ray diffractometer (PANalytical Empyrean XRD) with Cu Ka radiation in the 10°-80° 29 range. The analysis confirmed the crystalline structure and phase of the nanomaterial of the invention in which Nio.ssSe and NaCI coexist, as demonstrated by the X-ray diffraction signal pattern shown in figure 1A. The characteristic peaks corresponding to the (1 0 1 ), (1 0 2), (1 1 0), (1 0 3), (2 0 1 ), (2 0 2) and (0 0 4) planes confirmed the presence of Nio.ssSe, whilst the peaks corresponding to the (1 1 1 ), (2 0 0), (2 2 0), (3 1 1 ), (2 2 2), (4 0 0), (3 3 1 ) and (4 2 0) planes confirmed the presence of NaCI.

[0183] The X-ray diffraction spectrum obtained shows the following ±0.20 degree (2 theta) peaks: 33.40°, 50.50°, 60.40°, and 62.00° with reference to the Nio.ssSe crystalline phase; 27.37°, 31 .86°, 66.20°, and 75.35° with reference to the NaCI crystalline phase.

[0184] Raman Spectroscopy

[0185] The Nio.ssSe / NaCI composite nanomaterial obtained in example 1 was also analysed by Raman spectroscopy.

[0186] The coexistence of Nio.ssSe and NaCI in the Nio.ssSe / NaCI composite nanomaterial was also confirmed by the Raman spectrum shown in figure 1 B.

[0187] Raman spectroscopy was carried out using an unpolarised laser at 532 nm, which enabled the identification of the peaks associated with the polarised vibrational modes. The Raman spectrum showed two predominant peaks for Nio.ssSe: the D band centred at 131 1 cm’1and the G band centred at 1599 cm’1. Moreover, a peak at 198 cm’1was attributed to the Ni-Se vibrational mode.

[0188] The Raman spectroscopy studies were carried out using a Thermo Scientific DXR Raman microscope. An Nd:YAG laser beam (780 nm, 14 mW - maximum power) was used with a 10x / 0.25 BD microscope objective on the surface of the sample. The Raman spectra were acquired in the 50-3400 cm’1range with a spectral resolution of 2 cm’1, a photobleaching time of 2 minutes, 100 scans, a 400 lines / mm grating, a 50 pm pinhole opening and a spot size of 3.1 pm. An FESEM analysis was carried out on the Nio.ssSe / NaCI composite nanomaterial obtained in example 1 .

[0189] The morphology, composition and elementary mapping were obtained by field-emission scanning electron microscopy (FESEM) using a Magellan XHR 400L with an acceleration voltage of 5 kV and equipped with an EDS detector.

[0190] The FESEM analysis revealed the formation of nanostructures on a nanometric scale, whose average lateral dimension is 200 nm (Figure 1C). EDX Analysis

[0191] An EDX analysis was carried out on the Nio.ssSe / NaCI composite nanomaterial obtained in example 1 .

[0192] The EDX analysis was carried out in the mode coupled to scanning electron microscopy (SEM), using an integrated EDX detector. The sample was fixed onto a conductive substrate and subjected to an electron beam that induced the emission of characteristic X-rays by the elements present. The acquired data were subsequently processed by the instrument to determine the surface elemental composition of the sample and attribute the correct element to each value obtained.

[0193] The EDX analysis, together with the elementary mapping (Figure 1 D) confirmed the uniform distribution of Ni and Se within the composite nanomaterial of the invention and provided an estimate of the elemental composition as shown in Table 3 below:

[0194] Table 3 - table summarising the EDX analysis of the Nio.ssSe / NaCI composite nanomaterial and showing the atomic percentages of selenium, nickel, oxygen, carbon, chlorine and sodium.

[0195] (*) the oxygen detected in the analysis can be attributed to residual traces in the atmosphere of the microscope chamber.

[0196] (°) the carbon detected in the analysis comes from the conductive tape used to fix the sample onto the sample holder.

[0197] Example 3

[0198] UV-Vis-NIR diffuse reflectance spectroscopy.

[0199] UV-Vis-NIR diffuse reflectance spectroscopy was carried out on the Nio.ssSe / NaCI composite nanomaterial obtained in example 1 .

[0200] The absorption spectra and reflectance were recorded in the UV-Vis-NIR range using a Perkin-Elmer UV-Vis-NIR Lambda 1050+ spectrophotometer, covering wavelengths from 200 to 1200 nm. The Kubelka-Munk transform was used to quantitatively analyse the band gap of the samples.

[0201] Example 4

[0202] Electrochemical characterisations: hydrogen evolution reaction (HER) Electrochemical characterisations were conducted using an SP-300 bipotentiostat (Biologic) with a built-in EIS module.

[0203] Method of performing the experiments

[0204] Linear sweep voltammetry (LSV)

[0205] The performances of the Nio.ssSe / NaCI composite nanomaterials synthesised as per example 1 as catalysts for HER were evaluated in a three-electrode electrochemical cell at room temperature. Electrodes made of glassy carbon (GCEs, AMETEK) with a diameter of 3.0 mm, Ag / AgCI (3 M KCI), and Pt were used, respectively, as the working electrode, reference electrode and counter electrode. Linear sweep voltammetry (LSV) was carried out using 0.5 M H2SO4 as the electrolyte for HER, with a scan rate of 25 mV s’1. Figure 3 shows the Tafel plot obtained from the LSV curves at a scan rate of 25 mVs 1in a 0.5 M aqueous solution of H2SO4.

[0206] Rotating disk electrode linear sweep voltammetry (LSV-RDE)

[0207] Rotating disk electrode linear sweep voltammetry (LSV-RDE) was carried out by means of an RDE-2 BASi rotator system using a Biologic SP-300 bipotentiostat at 200, 500, 1000, 1500, 2000, 3000, 4000 and 6000 rpm. The potential of the reference electrode was converted into a reversible hydrogen electrode (RHE) by means of the formula:

[0208] E(RHE) = E(Ag / Agci) + 0.059-pH + 0.1976 Eq. 6 at a pH of 1 .0. The catalyst (5 mg) was dissolved in 20 pL of Nation (5% by weight of aliphatic alcohols and water, containing 15-20% water, Sigma- Aldrich) and 200 pL of isopropyl alcohol (99.5%, Sigma-Aldrich), and the solution thus obtained was stirred for 1 hour. The working electrode was subsequently modified by depositing 10 pL of catalyst ink onto the GCE. In order to determine the onset potential in the LSV curves for the hydrogen evolution reaction, the baseline was first identified in the non-Faradaic region, where the absence of current or a minimal presence thereof was observed. Subsequently, a tangent was drawn to the LSV curve in the point where the current density began to grow abruptly. The onset potential was then determined as the potential at the intersection between this tangent and the baseline.

[0209] Results of electrochemical tests

[0210] Linear sweep voltammetry (LSV)

[0211] Figure 3A shows the polarisation curves obtained by linear sweep voltammetry (LSV), with a scan rate of 25 mV s’1in a 0.5 M solution of H2SO4. The Nio.ssSe / NaCI composite nanomaterial exhibits HER activity at low dynamic potentials: in fact, it reaches a current density of 10 mA cm’2at an overpotential of 180 mV and provides a current density of up to 327 mA cm’2. Even after 10 cycles, the Nio.ssSe / NaCI composite nanomaterial is capable of providing at least 250 mA cm’2, indicating reproducibility in terms of current. This behaviour is attributable to the synergistic effect of the NaCI integrated into Nio.ssSe, which improves the charge transfer already usually guaranteed by non-stoichiometric nickel selenides.

[0212] Examining the HER process further, the HER kinetics can be assessed from the linear regions of the Tafel graph in figure 3B, by adapting the LSV curves with the Tafel equation (Y. Liu et al., Cryst Eng Comm. 2022, 24, 1704):

[0213] T] = b • log(j) + a Eq. 7 where a is correlated to the exchange current density, j, and b represents the Tafel slope. In HER, Tafel slope values of 30, 40 and 120 mV dec1are usually associated with the Tafel, Heyrovsky and Volmer mechanisms as limiting processes (Z. H. Ibupoto et al., Adv. Funct. Mater. 2019, 29, 1807562). The Tafel slope of 58.3 mV dec1suggests that the step represented by the Heyrovsky mechanism is the determining one in 0.5 M H2SO4 for the Nio.ssSe / NaCI nanostructures.

[0214] Considering the current delivered, the overpotential and the Tafel slope, the electrocatalyst of the present invention shows to be one of the best performing nickel-based electrocatalysts for HER, a result that is relevant for industrial applications.

[0215] The samples of example 1 were then studied before and after the hydrogen evolution process (figure 3C) in order to understand how the electrocatalytic process influenced the nanomaterial itself. The peaks typical of Nio.ssSe, centred at 1311 cm’1, 1599 cm’1and 198 cm’1, are still clearly visible, even though the signal is more intense. Moreover, the peak at 198 cm’1shifts to 232 cm’1and increases its intensity after the HER process. This suggests that, during the process, a structural reorganisation occurred, making the Ni-Se vibrational modes in the material more evident. Rotating disk electrode linear sweep voltammetry (LSV-RDE) In order to obtain further information on the HER kinetics, the composite nanomaterials of the invention as obtained in example 1 were studied by linear sweep voltammetry using a rotating disk electrode (LV-RDE) at a scan rate of 10 mV s’1and at different rotation speeds, from 200 to 6000 rpm, in an acidic environment (figure 4A).

[0216] The number of electrons transferred, n, and jk can be obtained from the slope and the intercept of Koutecky-Levich curves, according to the following correlation: where y is the measured current density, jk and / / ware respectively the kinetic and diffusion-limited current densities, w is the rotation speed (rpm), n is the total number of electrons transferred in the process, F is the Faraday constant (F = 96485 C mol’1), the concentration of oxygen in the solution (2.5 ■ 10’4mol L’1), is the oxygen diffusion coefficient (1 .4 ■ 10’5cm2s’1), u is the kinematic viscosity of the electrolyte (0.01 cm2s’1), and k is the electron transfer constant (B. Garza-Campos etal., Electrochim. Acta. 2018, 269, 232). The number of electrons transferred, n, and jk can be obtained from the slope and the intercept of Koutecky-Levich graphs.

[0217] The results shown in figure 4B suggest that Nio.ssSe / NaCI involves a process of transferring two electrons in hydrogen evolution, with an extrapolated linear interpolation indicative of the typical behaviour of two- electron processes. Based on these results, the following mechanism was proposed for the hydrogen evolution reaction:

[0218] 2Ni( / / ) + 2e“ - 2Ni( / ) Eq. 10 suggesting that the HER mechanism for the Nio.ssSe / NaCI composite nanomaterial entails a further step in which the adsorption of H+onto the catalytic sites of Ni(l) occurs.

[0219] Electrochemical impedance spectroscopy

[0220] The ability to provide high current densities was further studied by means of electrochemical impedance spectroscopy (EIS). The information extrapolated from Nyquist and Bode plots (Figures 5A and 5B) shows a low resistance of the system to the current flow. From a geometric viewpoint, the Nyquist plot of the catalyst consists in a semicircle which reveals the charge transfer resistance, Rct, at the electrode / electrolyte interface. The resistance at high frequencies is mainly tied to the solution resistance.

[0221] Randles circuit

[0222] The experimental data were adapted to a Randles circuit, shown in Diagram 1 , which comprises two resistors and a constant phase element (CPE), with the calculated values shown in Table 4. The solution resistance, Rs, remained unchanged in several repeats of the experiment (-23.4 Q), suggesting that the electrode-electrolyte interactions did not significantly influence the system’s overall behaviour. However, the capacitance is very low, indicating a low stress of the system in initiating the current flow.

[0223] Diagram 1. Randles equivalent circuit.

[0224] Table 4. Results obtained from the adaptation of the Randles equivalent circuit[a]shown in Diagram 1 for the EIS measurements relating to the hydrogen evolution reaction, where CPE is a parameter of adaptation of the constant phase element, and a is the exponent of adaptation (when a approaches 0, the CPE acts like a pure resistor, whereas when a approaches 1 , it acts like a pure capacitor).

[0225] [a]Equivalent circuit: Rs+ CPE / Rct

[0226] Most of the studies in the literature on non-stoichiometric nickel selenide- based catalysts for HER use conductive substrates, which to some degree compromise their validity, since these, too, are active from a catalytic viewpoint. In the present invention, by contrast, the powders deposited on the GCE avoid the use of substrates, thus improving the reliability of the results.

[0227] The Nio.ssSe / CI composite nanomaterial of the present invention has some of the highest current density values and the lowest overpotentials ever reported for Nio.ssSe-based nanomaterials, indicating favourable HER kinetics.

[0228] Example 5

[0229] Electrochemical characterisations: oxygen evolution reaction (PER) and urea oxidation reaction (UOR)

[0230] Electrochemical characterisations were conducted using an SP-300 bipotentiostat (Biologic) with a built-in EIS module.

[0231] Method of performing the experiments

[0232] Linear sweep voltammetry (LSV)

[0233] The performances of the Nio.ssSe / NaCI composite nanomaterials synthesised as per example 1 as catalysts for PER were investigated in a three-electrode electrochemical cell at room temperature. Electrodes made of glassy carbon (GCE, AMETEK) with a diameter of 3.0 mm, Ag / AgCI (3 M KCI), and Pt were used, respectively, as the working electrode, reference electrode and counter electrodes. Linear sweep voltammetry (LSV) was carried out in 1 .0 M KGH for GER and UOR (with the addition of 20 mM of urea) with a scan rate of 25 mV s-1. The potential of the reference electrode was converted into a reversible hydrogen electrode (RHE) by means of the formula:

[0234] E(RHE) = E(Ag / Agci) + 0.059-pH + 0.1976 Eq. 12 considering a pH of 14.0. The catalyst (5 mg Nio.ssSe / NaCI) was dissolved in 20 pL of Nation (5% by weight of aliphatic alcohols and water, containing 15-20% water, Sigma-Aldrich) and 200 pL of isopropyl alcohol (99.5%, Sigma-Aldrich), and the solution thus obtained was stirred for 1 hour. The working electrode was subsequently modified by depositing 10 pL of catalyst ink onto the GCE. In order to determine the onset potential in the LSV curves for the oxygen evolution and urea oxidation reactions, the baseline was first identified in the non-Faradaic region, where the absence of current or a minimal presence thereof was observed. Subsequently, a tangent was drawn to the LSV curve in the point where the current density began to grow abruptly. The onset potential was then determined as the potential at the intersection between this tangent and the baseline.

[0235] Electrochemical impedance spectroscopy (EIS)

[0236] Electrochemical impedance spectroscopy (EIS) was used to investigate the interface charge transfer process in a three-electrode electrochemical cell at room temperature. The working electrode (functionalised GCE) was prepared by depositing 10 pL of an ink (obtained by dispersing 5 mg of Nio.ssSe / NaCI electrocatalyst powder in 200 pL of isopropanol and 20 pL of Nation, followed by stirring for 1 hour) onto the GCE. The analysis was conducted in the frequency range comprised between 100 kHz and 100 mHz at an applied potential of -0.20 V (for HER) and 0.70 V (for OER and UOR) with an alternating current amplitude of 10 mV.

[0237] Results of electrochemical tests

[0238] The ability of the Nio.ssSe / NaCI composite nanomaterial in catalysing the urea oxidation reaction was assessed using different concentrations of urea. Initial tests identified the optimal range of concentration for urea oxidation by examining the current density values. The optimal concentration range for obtaining the best performances was determined as between 10 mM and 50 mM (Figure 6A). Beyond this range, high concentrations lead to the generation of molecules of carbon monoxide, CO, which poison the catalytic sites and decrease overall performance.

[0239] As shown in Figure 6B, the Nio.ssSe / NaCI composite nanomaterial exhibits a current density of 10 mA cm-2with an overpotential of 110 mV at a urea concentration of 20 mM, indicating an exceptional catalytic activity. Similar performances were observed at slightly higher urea concentrations, underscoring the reproducibility and applicability of this nanomaterial for urea oxidation. However, at concentrations higher than 50 mM, the surface of the catalyst was covered by reaction products and by-products, impeding the electrolyte-catalyst interactions and leading to performance saturation. This observation is consistent with the optimal concentration range previously established for urea. Moreover, the Nio.ssSe / NaCI composite nanomaterial demonstrated a significant OER activity, reaching a current density of 10 mA cm-2with an overpotential of 330 mV in the absence of urea in the electrolyte, though the performance was not so high as for UOR. Therefore, the presence of small amounts of urea in the electrolyte leads to a lower energy demand (equal to 220 mV) in order to initiate the oxygen evolution, aligning this material with principles of sustainability and energy savings.

[0240] In order to further investigate the electrocatalytic properties of the Nio.ssSe / NaCI composite nanomaterial, the Tafel slopes for OER and UOR were derived from the measurements carried out by linear sweep voltammetry (LSV) shown in Figure 6C. The Tafel slopes were 172.9 mV dec1for OER and 67.9 mV dec1for UOR. These values are among the best reported in the prior art in relation to other Nio.ssSe-based compounds and similar electrocatalysts, such as Nio.ssSe / rGO (91 mV dec1) (G. Liu et al., New. J. Chem. 2020, 44, 17313), Nio.ssSe-NHCS (142 mV dec1), Coo.ssSe-NHCS (136 mV dec1) and Nio.ssSe / Coo.ssSe-NHCS (118 mV dec1) (L.-J. Peng et al., RSC Adv. 2021 , 11, 19406). This indicates that, as an electrocatalyst, Nio.ssSe / NaC not only demonstrates a promising activity for hydrogen evolution, but also effectively oxidates urea while it evolves oxygen.

[0241] Electrochemical impedance spectroscopy (EIS)

[0242] Electrochemical impedance spectroscopy was used to further investigate the ability of the Nio.ssSe / NaCI composite nanomaterial to provide high current densities. The data obtained from Nyquist and Bode plots (Figures 7A-C) demonstrated a low resistance within the system as regards current flow under the experimental urea oxidation conditions.

[0243] Randles circuit

[0244] The experimental data were adapted to a Randles circuit, shown in Diagram 2, made up of two resistors and a constant phase element (CPE), with the calculated values shown in Table 5. The solution resistance (Rs) remained constant across multiple experimental repetitions, with a value of 21.6 Q, indicating that the electrode-electrolyte interactions did not significantly influence the system’s overall behaviour. However, under the experimental UOR conditions, the observed charge transfer resistance was four times lower than in the oxygen evolution process, suggesting that the presence of urea improves conductivity, favours the reaction kinetics and facilitates charge transfer.

[0245] Diagram 2. Randles equivalent circuit.

[0246] Table 5. Results obtained from the adaptation of the Randles equivalent circuit[a]shown in Diagram 2 for the EIS measurements relating to OER and UOR, where CPE is a parameter of adaptation of the constant phase element, and a is the exponent of adaptation (when a approaches 0, the CPE acts like a pure resistor, whereas when a approaches 1 , it acts like a pure capacitor).

[0247] [alEquivalent circuit: Rs+ CPE / Rct

Claims

1. CLAIMS1 . Nio.ssSe / NaCI composite nanomaterial.

2. The composite nanomaterial according to claim 1 , characterised by an X- ray diffraction spectrum with the following ±0.20 degree (2 theta) peaks: 33.40°, 50.50°, 60.40°, and 62.00° with reference to the Nio.ssSe crystalline phase; 27.37°, 31.86°, 66.20°, and 75.35° with reference to the NaCI crystalline phase.

3. The composite nanomaterial according to any one of the preceding claims, characterised by an average lateral dimension comprised between 50 and 500 nm, preferably between 100 and 300 nm, more preferably between 150 and 250 nm, and even more preferably it is 200 nm, as measured by SEM analysis.

4. The composite nanomaterial according to any one of the preceding claims, characterised by a uniform distribution of Ni and Se within the structure.

5. Method for the synthesis of Nio.ssSe / NaCI, comprising a step of coprecipitating a nickel precursor and a selenium precursor in a supersaturated aqueous NaCI solution, wherein the nickel precursor is selected from nickel acetate, nickel chloride and nickel nitrate, optionally hydrated, the selenium precursor is selected from H2SeO3, H2SeC and NaSeH (sodium hydroselenide) and wherein the co-precipitation is carried out in the presence of a temperature comprised between 80° C and 150° C and / or after a time comprised between 1 and 4 hours.

6. Method of synthesis according to claim 5, wherein the nickel precursor is nickel nitrate, optionally hydrated, and preferably it is nickel nitrate hexahydrate (Ni(NO(NO3)2.6H2O).

7. Method of synthesis according to any one of claims 5 to 6, wherein the selenium precursor is NaSeH.

8. Method of synthesis according to any one of claims 5 to 7, wherein the co-precipitation is carried out in the presence of a temperature comprised between 90° C and 1 10° C, preferably comprised between 95° C and 105° C, and / or after a time comprised between 1 , 5 and 3.5 hours.

9. Method of synthesis according to any one of claims 5 to 8, wherein the % yield by weight of the product is comprised between 40% and 95%, preferably between 60% and 80%, more preferably between 65% and 75%.

10. Use of the Nio.ssSe / NaCI composite nanomaterial according to any one of claims 1 to 4, as a multifunctional electrocatalyst and / or multifunctional photoelectrocatalyst for hydrogen evolution reactions, oxygen evolution reactions and / or urea oxidation reactions.1 1 . Use of the Nio.ssSe / NaCI composite nanomaterial according to any one of claims 1 to 4, for obtaining a multifunctional electrode and / or a multifunctional photoelectrode for hydrogen evolution reactions, oxygen evolution reactions and / or urea oxidation reactions.

12. Multifunctional electrode and / or multifunctional photoelectrode comprising the Nio.ssSe / NaCI composite nanomaterial according to any one of claims 1 to 4.

13. Multifunctional electrode and / or multifunctional photoelectrode according to claim 12, wherein the electrode and / or photoelectrode may each further comprise any substrate of a metallic, non-metallic and / or semi- conductive nature, preferably selected from a substrate made of nickel,carbon, platinum, iridium and / or palladium, more preferably selected from carbon fibre fabric, nickel foam and glassy carbon, even more preferably said substrate being glassy carbon.

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