RO131917A2 - Procedure for making nanostructured electrodes based on metal oxides, for electrochemical systems of producing electric energy - Google Patents

Abstract

The invention relates to a process for making nanostructured electrodes for electrochemical systems of producing electric energy. According to the invention, the process consists in that, on a rotary electrode of glassy graphite, niobium is deposited by magnetron sputtering, in direct current, under Ar atmosphere, at pressures of 6, 9, 12, 15 and 18 mbar, employing a usual deposition system, to result in niobium oxide/niobium metal composite films of 30 and 80 nm, respectively, with no oxidic impurities.

Description

The invention relates to a process for obtaining nanostructured electrodes for electrochemical systems for the production of electricity. The process according to the invention consists in that on a rotating glass graphite electrode, niobium is deposited by sputtering in magnetron regime, in direct current, in Ar atmosphere at pressures of 6, 9, 12, 15 and 18 mbar, using a deposition system usually, resulting in composite films of 30 and 80 nm, respectively, of metal niobium oxide / niobium, which do not exhibit oxide impurities.

Claims: 2

Figures: 10

Starting from the date of publication of the patent application, the application provides, provisionally, to the applicant, the protection granted according to the provisions of art: 32 of Law no: 64/1991, except in cases where the patent application has been rejected, withdrawn or considered as withdrawn, the extent of the protection conferred by the patent application is determined by the claims contained in the application published in accordance with article 23 paragraphs) - (3).

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Procedure for making nanostructured electrodes based on metallic oxides for electrochemical systems for electricity production

Description The growing concerns about the impact of greenhouse gases, but also about the drastic reduction of the available fossil fuel resources, have made it a priority for scientists to seek new solutions for the development of a new energy structure. In addition to the ongoing effort to identify new, cleaner and more efficient renewable energy sources, defining a new energy carrier that will completely remove carbon from this chain is vital to avoiding the main problems facing the world economy. 1].

The transition to a "clean" energy structure cannot currently be conceived without, to a certain extent, hydrogen and, implicitly, a "green" technology for its production, electrolysis. Creating a sustainable energy chain, starting from the energy source to the consumer, in which hydrogen is the central element of energy transport and storage, with a higher efficiency and having characteristics of costs and reliability comparable to the current ones, is one of the priorities at the moment in this field.

The scientific development of hydrogen technologies in recent years has been driven by the opportunity to use hydrogen as the main fuel in the future, which would lead to the rapid spread of the use of renewable energy sources, as well as the growing need for development of technological solutions for energy storage becomes acute in Europe, including Romania, where many wind or photovoltaic power plants already generate hundreds of gigawatts of electricity, often in total

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11/11/2015 inconsistency with the requirements of the power network. Moreover, renewable hydrogen, given its relevance for the implementation of the concept "power-to-gas" through the possibility of injection into the natural gas network, is required as a basic element in the strategy of integrating the two energy infrastructures of electricity and heat.

The need to develop technological solutions for energy storage becomes acute in Europe, including Romania, where many wind or photovoltaic power plants already generate hundreds of gigawatt of electricity, often in total non-compliance with the requirements of the power grid. The PEM electrolysis technology represents, from several points of view, a system perfectly adapted to the application requirements of renewable energy sources. This is considered, at present, the most flexible and sustainable solution for storing renewable energy in the long term and at different scales, using the surplus of electricity generated to produce hydrogen, which is subsequently stored and converted into electrical / thermal energy by various technologies. .

In electrochemical systems, the reaction rate (power density / current density) is determined by the working temperature and the nature of the electrocatalysts.

Electrocatalysts are substances (metals, metal oxides, non-metals, organo-metallic compounds) that favor the speed of reactions (ionization, deionization) on the given surface.

By definition, an electrocatalyst is a substance that accelerates the transfer of charge forward and backward through the redox system without disturbing the chemical balance. This means that the presence of the catalyst does not affect the system from a thermodynamic point of view. An electrocatalyst actually has the role of improving the charge transfer mechanism by decreasing the activation energy of the electrochemical reaction that takes place at the electrode level.

In the case of electrolysers and fuel cells, heterogeneous electrocatalysts can be found in the form of sheets, thin films, individual particles or deposited on a support.

The process of oxygen evolution at the anode of a PEM electrolysis cell involves four electrons and is much more complex compared to the evolution of hydrogen at the cathode where two electrons are used as reducing agents. The reaction of oxygen evolution or oxidation of water (OER) is a multi-step process that involves the formation of oxygenated intermediates and surface oxides. Trasatti suggested that in the case

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11/11/2015 OER reaction, the mechanism of oxygen production is in the form of a parabola (fig. 2.3) and is the result between the applied superpower and the strength of the metal-oxygen bond from the oxide surface [5]. The oxide catalysts of RuO2 and lrC> 2 have the lowest suprapotential and the most suitable metal-oxygen bonds as a force that allows the adsorption-desorption process of oxygen to be easily carried out on the surface of the catalyst. The materials to the left of the parabola maxima have weak connections between metal and oxygen, while the right metaria (CO3O4 and FesCh) are easily oxidized and form a strong bond between metal and oxygen. Following is a difficult gas oxygen desorption.

Moreover, Trasatti emphasized that the evolution of oxygen can only occur when the potential of the electrode (anode) is higher than that of the metal-oxygen couple [2-4].

Electrocatalysts used in the production of oxygen are usually of the type of noble metals (Pt, Au, Ir, Rh, Ru, Ag), but their metal oxides are more active than the corresponding metals [5,6].

The oxides of transition metals of Ni and Co type have a high catalytic activity towards the oxygen production reaction, but their use is restricted by corrosion problems. Oxide compounds based on Co, Ni, Mn and Fe have a complex crystalline structure and are considered to be able to improve the catalytic activity and stability of the OER process in alkaline environments [7,8].

Nanostructured catalysts are currently considered a viable solution for obtaining electrodes for the OER reaction. The size and shape of the catalyst particles can determine the location and area of the active surface corresponding to the oxygen release reaction. Krtil et al. studied the effect that the size and shape of oxide nanoparticles of the RuO2 type, Rui-xCo _x O2 and Rui- _x NixO2- _y have in acidic medium on the reaction of OER.

On the other hand, the catalysts used alone use only part of the surface for catalytic and as a result, to improve their performance different types of catalytic supports are used. A catalytic support provides a physical surface used for the dispersion of small particles, which results in a high catalytic surface and high electrical conductivity.

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Therefore, catalytic supports can lead to improved catalytic efficiency and better use of the catalyst by reducing catalyst charges and thus lowering the cost of production. However, the catalytic supports used in the accepted OER reaction are difficult to use in water electrolysis due to the high corrosive environment.

From the point of view of electrocatalytic efficiency, it is of great importance that stable electrochemical materials for catalytic supports be studied and developed to be applied in the process of oxygen generation.

The main objective of this patent is the realization of a new type of catalyst for PEM type electrolysers using the DC spray method in magnetic field and the electrochemical characterization and stability of the nanostructured niobium oxide deposited at room temperature by the DC spraying method in magnetic strip in atmosphere. It would, without making use of oxygen. This procedure is different from those used to date for the synthesis of NbOx catalysts and allows the control and average size of Nb crystallites below the 10 nm limit by varying the Ar flow and aims to increase the specific surface exposed to residual oxygen contamination by obtaining a layer of oxygen. NbOx oxide with improved catalytic activity. This deposition procedure gives the composite films niobium / niobium oxide.

This patent presents the procedure for making nanostructured electrodes based on metallic oxides for electrochemical systems for electricity production. In this respect, several niobium samples were made by direct current (DC) magnetron sputtering. DC deposition in the magnetic field was performed in the Ar atmosphere at different pressures (6, 9, 12, 15 18 rribar) on the optical glass using a DC BOC EDWARDS FL 400 deposition system (figure 1). The samples were structurally analyzed using the following investigation techniques: X-ray diffraction (XRD) and ultraviolet (Uv-Vis) spectroscopy. XRD characterization was performed using a Brueker AXS D8 X-ray diffractometer, and UV-Vis tests using the WVASE 32 spectrometer (Woollhman).

In order to functionally characterize the behavior of the niobium for its use as a catalyst for PEM electrolysis, a film of approximately 250 nm was deposited on a rotary electrode of 5 mm diameter glass graphite produced by ORIGATROD. Electrochemical tests have constant cyclic voltammetry curves performed with a

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11/11/2015 Princeton applied research pointer / galvanostat PARSTAT 2273. Table 1. shows the samples of thin films of noisiness and the conditions of submission.

Figure 1. DC Boc Ewards FL 400 spraying system

Your belul 1. Cond experimental casting depictions for neo movies Aim substrate Name sample Film thickness Nb (nm) Working pressure (mTorr) It Power (W) The duck deposit (I would) Nb 1 mm thick optical glass. Nb 30 nm6 mbar 30 6 375 0.45 Nb 30 nm-9 mbar 30 9 388 0.43 Nb 30 nm12 mbar 30 12 355 0.41 Nb 30 nm15 mbar 30 15 341 0.38 Nb 30 nm18 mbar 30 18 330 0.37 Nb 30 nm22 mbar 30 22 307 0.31 Nb 80 nm-9 mbar 80 9 380 0.45 Nb 80 nm12 mbar 80 12 365 0.40 Nb 80 nm15 mbar 80 15 316 0.37 Nb 80 nm18 mbar 80 18 305 0.34

X-ray diffraction characterization

One of the methods of structural characterization of the samples was X-ray diffraction. The experiments were performed in the diffraction angle range 2Θ between 10-60 °.

Figure 2 shows the diffraction spectra of samples Nb 80 nm-9 mbar (a); Nb 80 nm-12 mbar (b); Nb 80 nm-15 mbar (c) and respectively Nb 80 nm-18 mb (d). From the analysis of the spectra one can observe the presence of the bcc phase of the nephew at different angles 2Θ located in the range 36-38 °. It should be noted that in the case of the sample Nb 80 nm-9 mbar appears an additional peak at 2Θ = 54.76 ° corresponding to the phase (200). Data provided by

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11/11/2015 XRD analyzes show us that following the deposition by DC spray in magnetic field, Nb metallic films were obtained, which do not have oxide impurities.

Figure 2. Diffraction spectra of Nb samples 80 nm-9 mbar (a); Nb 80 nm-12 mbar (b); Nb 80 nm-15 mbar (c) and respectively Nb 80 nm-18 mbar (d).

From the figure 3 where the previous superposed spectra are shown, a decrease of the bandwidth calculated at the half of the peak height [110] is observed with the increase of the working pressure of Arși a decrease of the particle size (grain size). The width of the peak is inversely proportional to the size of the particles so that it can be said that an increase in the width of the peak translates into a decrease in the true size of the crystallites as the working pressure increases. From Figure 5.3, one can also observe a displacement of the line corresponding to the phase [110] to smaller values of the angle 2φ with increasing pressure, which means an increase of the interplanar distance between the planes bcc [110].

Figure 3. Nb 80 nm superimposed spectra

The same considerations apply to Nb 30 nm samples (Figure 4).

Figure 4 Nb 30 nm superimposed spectra

Characterization by UV-Vis spectroscopy

Characterization of Nb film samples was performed by UV-Vis spectroscopy using the WVASE 32 spectroscope (Woollhman).

From the analysis of the overlapping Uv-Vis spectrometers of the 30 nm Nb samples (Figure 5), an increase of the transmission was observed with the increase of the pressure.

Figure 5. Uv-Vis superimposed spectra of 30 nm Nb samples

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The same considerations apply to Nb 80 samples (Figure 6). C's

Figure 6. Uv-Vis superposed spectra of 80 nm Nb samples

Figure 7. Superposed spectra of Nb 30 and 80 rim samples deposited in the same pressure indentations (15 mbar).

Figure 7 shows the overlapping spectra of samples of Nb 30 and 80 nm deposited in the same pressure indices (15 mbar). From the analysis of the spectra, a decrease of the transmission is observed with the increase of the film thickness, which is in full agreement with our expectations.

Also from Figures 5.6 and 5.7, it can be observed that the tangent to the transmission graph intersects the x axis corresponding to the energy at values above 3 eV, which makes us believe that in fact niobium oxide / metallic niobium oxide films were obtained below core-shell shape.

Functional characterization by cyclic voltammetry

Functional characterization by cyclic voltammetry aimed at highlighting the stability of Nb in aqueous and acidic medium characteristic of an electrolyser with proton changing polymer membrane. In such a cell, the acidic environment is constituted by the sulfonic groups in the membrane structure of NaFion.

In this respect, the experiments were carried out using a rotary electrode of 5 mm diameter glass graphite produced by the company ORIGATROD on which a layer of approximately 250 nm Nb was deposited by spraying DC in magnetic field. Electrochemical tests have constant cyclic voltammetry curves performed with a PARSTAT 2273 poentiostat / galvanostat from Princeton applied Research and 0.5M sulfuric acid was used as electrolyte [9].

The cyclic voltammetry curves were plotted in the range -0.75-1.8 V, at different scan speeds and different rotations of the rotating electrode.

Figure 8 shows the cyclic voltammetry curve obtained at a scan speed of 50 mV / s, in the potential range -0.75-1.8 V towards Ag / AgCI.

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Figure 8. Cyclic voltammetry curve obtained at a scan speed of 50 mV / s.

Analyzing the vooltamogram above, we find the presence of 3 different peaks:

The anodic offshore peak located around a, 7 V.

Well defined cathode peaks Ic1 and Ic2 located at 0.06 and 0.26 V. respectively.

The anodic peak at is associated with the formation of neobium oxide, Nb2O5 and the cathodic peaks Ic1 and Ic2 correspond to the adsorption of hydrogen.

The effect of the scan speed on the voltammograms drawn in the potential range -0.75-1.8 V towards Ag / AgCI was also studied. Figure 9 shows the overlapping spectra of the samples obtained at different scanning speeds (20, 50, 100, 200, 500, 750 and 1000 mV / s).

Figure 9. The superimposed spectra of the samples obtained at different scan speeds (20, 50, 100, 200, 500, 750 and 1000 mV / s).

Following the analysis of the spectra, it is observed that the peaks Ic1 and Ic2 are transformed into one and increases with the scanning speed and becomes more negative. This means that we have a change in the crystal structure of the Nb film by hydrogen adsorption.

Also in this paper the effect of the rotational speed of the rotating electrode on the evolution of the oxidation and re-process of a Nb film was studied. Thus, voltamograms were drawn (Figure 10) in which the scan speed was 100 mV / s at different rotations of the rotary electrode (500, 2000, 4500 and 9000 rot / min).

It was thus found that the cathode peak Ic1 decreases as the rotational speed increases, while Ic2 increases with the rotational speed and becomes more negative.

Figure 10. Effect of rotation speed of the rotating electrode on the evolution of the oxidation and reduction process

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The experimental results presented in this patent are particularly important si ^ and certify that the choice of niobium as catalyst for PEM polymer membrane electrolyzers represents an important step in the development of nanostructured catalysts with high catalytic activity with low content of precious metals.

Claims (2)

1. Procedure for filing niobium oxide / metallic niobium oxide films in core-shell form. nanostructured electrodes based on metal oxides for electrochemical systems for electricity generation. In this respect, several niobium samples were made by direct current (DC) magnetron sputtering. Magnetic field DC deposition was performed in the Ar atmosphere at different pressures (6, 9, 12, 15 18 mbar) on the optical glass using a DC BOC EDWARDS FL 400 deposition system. The samples were structurally analyzed using the following investigation techniques: X-ray diffraction (XRD) and ultraviolet (Uv-Vis) spectroscopy. In order to functionally characterize the behavior of the niobium for its use as a catalyst for PEM electrolysis, niobium film deposited on a rotating glass graphite electrode were drawn with cyclic voltammetry curves. 2. Procedure for making nanostructured electrodes based on metallic oxides for electrochemical systems for electricity production using core-shell composite films of metal / niobium oxide obtained according to claim 2. The experimental results presented in this patent certify the fact that the choice of niobium as catalyst for PEM polymer membrane electrolysers is an important step in the development of nanostructured catalysts with high catalytic activity and low content of precious metals.