RU2518466C1 - Iron-based material for electrode for electrochemical production of hydrogen and method of its manufacturing - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: powdered composite Fe-C is applied to the electrode surface and synthesis is carried out for nanocrystal elements Fe-C with an average size within the range of 10-15 nm by treatment with laser pulses with wave length of 1-1.5 mcm at radiant density of 10⁷-10⁹ W/cm², laser scanning rate of 8-15 cm/s, frequency of pulses of 33-60 kHz in vacuum or in argon, at that in the process melting and formation of ferric carbide Fe₃C is not reached. The invention is also related to the iron-based cathode material for electrochemical production of hydrogen.

EFFECT: modification of iron surface that allows improvement of electrocatalytic activity for such material.

2 cl, 2 tbl

Description

Iron-based electrode material for the electrochemical production of hydrogen and method for its manufacture

The invention relates to materials for the manufacture of electrodes used as cathodes for the production of hydrogen by electrolysis of aqueous solutions of alkalis.

In industrial electrolyzers, iron is used as a cathode to produce hydrogen by electrolysis of aqueous solutions of alkalis [1]. Iron, like other metals of this subgroup, has an average hydrogen overvoltage, which leads to significant energy consumption during electrolysis. It would be ideal to use platinum group metals with low hydrogen overvoltage as electrodes. However, these metals are expensive and therefore are not used in water electrolysis technology.

As an alternative to metallic materials for electrodes, carbides of a number of metals were studied [2]. One of the most promising was tungsten carbide. However, the scarcity of this material and tungsten in general, the difficulty in manufacturing electrodes with a large surface, did not allow the use of tungsten carbide in industrial electrolyzers.

Cheaper and affordable carbide is iron carbide Fe ₃ C (cementite). According to studies [2, 3], the hydrogen overvoltage on such carbide is lower than on iron. However, cementite can only be obtained metallurgically at high temperature. Due to its high hardness and brittleness, it is impossible to make electrodes with a large surface from this material, i.e. flat electrodes.

As a result of this, the electrolysis of aqueous solutions of alkalis with the aim of producing hydrogen mainly uses electrodes made of iron (unalloyed steels) [1], the disadvantages of which were noted above. Thus, iron electrodes were selected as a prototype.

The task was to create a technology for modifying the surface of iron, which would make it possible to increase the electrocatalytic activity of such a material, i.e. to reduce the overstrain of the hydrogen evolution reaction during the electrolysis of an aqueous alkali solution.

As a result of research, it was found that using laser technology it is possible to obtain electrodes based on iron (steel) enriched with carbon. So, in the method of manufacturing a coating on an article by the method of layer-by-layer laser synthesis [4], selected as the prototype, laser sintering of a mechanically activated metal powder and a metal-metal powder mixture was performed using a pulsed laser with a pulse generation frequency from 20,000 to 100,000 Hz and a pulse duration of 100 nanoseconds.

The problem was solved in that a Fe-C powder composition was deposited on the electrode surface and nanocrystalline Fe-C elements were synthesized under the action of laser irradiation, without leading to the melting of the surface and the stage of synthesis of iron carbide. In this case, the synthesis of Fe-C nanocrystalline elements with an average size in the range of 10-15 nm was carried out by laser pulses with a wavelength of 1-1.5 μm, a radiation density of 10 ⁷ -10 ⁹ W / cm ² , and a laser scanning speed of 8-15 cm / s, a pulse frequency of 33-60 kHz in vacuum or in argon, without leading to the appearance of iron carbide Fe ₃ C. The resulting composite electrode material retained the nanoscale volume and surface elements and had a higher electrocatalytic activity than iron and even nickel, which enables and prepare therefrom a cathode for electrolysis of aqueous solutions of alkalis in order to obtain hydrogen.

Note that the technology used allowed us to obtain flat electrodes corresponding to the sizes of electrodes for industrial electrolyzers.

An example of a specific implementation of the invention

Cooking images. A composition composed of a mixture of dispersed materials based on carbonyl iron with the addition of 0.5 wt.% Carbon in the form of graphite was applied to a steel substrate 40. The applied mixture of powdered substances was a paste-like coating, in which carbon tetrachloride was used as a binder. A coating of 0.07 mm thickness was applied to the surface of the substrate with a roller.

To process the obtained layer, we used a fiber-optic ytterbium pulsed laser with a radiation wavelength of ~ 1 μm at a radiation density of ~ 10 ⁸ W / cm ² . The scanning speed of the laser beam in the processing zone was 10 cm / s, and the pulse repetition rate was 33 kHz. Processing was carried out in a protective environment of argon. After laser treatment, the thickness of one sintered layer was 0.05 mm. Further, the coating process and laser treatment was repeated several times (up to 16) to obtain a given thickness of the sintered layer. During laser sintering, surface coatings with nanostructured properties were obtained.

The choice of optimal processing modes was carried out using mathematical modeling using the two-phase zone method. This method, widely used in foundry technologies, was extended to the case of high-intensity heat treatment of powders with a laser. A computer model [5], developed using this approach, was used for the quantitative analysis of heating and cooling rates, penetration depth and temperature gradients in the processing zone. Based on these data, the granularity of the crystal structure and the degree of segregation of chemical components in the sintered layer were evaluated. The selected modes ensured the formation of nanocrystalline structures on the surface of materials.

Samples No. 1-3 after obtaining coatings by laser sintering were subjected to grinding to a depth of 0.1 mm to remove surface roughness. The samples differed in the depth of the remaining sintered layer: sample No. 1 — 0.2 mm; No. 2 - 0.4 mm; No. 3 - 0.6 mm.

X-ray diffraction analysis of the samples was performed on a DRON-6M diffractometer in Co-Kα radiation using the constant-time method, gaining intensity at each point over an angle of 2θ for 5 s.

Samples for investigation by x-ray photoelectron spectroscopy (XPS) were fixed on an indium substrate. Preliminarily, the samples were washed in rectified alcohol. In order to study the distribution of the concentration of elements over the depth, etching by argon ions with an energy of 0.9 kV and a current density of 12 μA / cm ² was carried out. The etching rate was ~ 1.0-1.2 nm / min. X-ray electron spectra (RE spectra) of all samples were obtained upon MgKα excitation (1253.6 eV). The vacuum in the chamber of the spectrometer is 10 ^-6 Pa. Resolution (width at half height) along the line Au4f _7/2 -1.2 eV. The relative error in determining the concentration of elements is 5% of the measured value (in the range of average concentrations). The accuracy of determining the binding energy was determined by a scanning step of 0.1 eV.

Samples for electrochemical measurements were cylinders with an end surface area of 0.6 cm ² . The layer deposited on the end of the cylinder was the working surface of the electrode. Non-working surfaces were insulated with epoxy resin.

Polarization measurements of the samples under study in the anode region of potentials were carried out in the potentiodynamic mode at a potential sweep speed of 1 mV / s. Used potentiostat IPC-Pro L, cell YSE-2. Temperature 22 ± 2 ° C, background electrolyte - 0.1 M aqueous NaOH solution. Aeration of the solution is natural.

Before electrochemical tests, the electrodes were cleaned with finely divided alumina deposited on a damp cloth. After that, the samples were washed with distilled water and degreased with alcohol. Next, the samples were placed in an electrochemical cell with a solution of NaOH.

At the same time, the potentiostat was turned on and the potential was set to the electrodes φ = -1500 mV (potentials were measured relative to the saturated silver chloride electrode). At this potential, the electrodes were held for 15 minutes, which contributed to the removal of oxide layers present on the surface. Then, anodic polarization was turned on and the potential was brought to a value of 1100 mV, i.e., slightly higher than the potential of a reversible hydrogen electrode in a given medium (the value of this potential was taken equal to -970 mV). Then, the cathodic polarization was turned on again at the same speed and the potential was brought to its initial value, i.e., φ = -1500 mV. Note that the forward and backward polarization curves almost coincide, which may indicate the reversibility of the processes associated with the cathodic evolution of hydrogen from an alkaline medium according to the following equation:

H ₂ O + e ^- → _^1/2 H ₂ + OH ^-

The current density was converted to the area of the geometric surface of the electrode. For comparison, under the same conditions, the cathodic polarization curves of electrodes made of steel 40, iron-armco, and nickel (grade H0) were taken.

On the diffraction patterns of the samples studied, only lines of two phases can be distinguished: α-Fe and γ-Fe. The carbide phase is absent, which indicates the absence of Fe ₃ C carbide in the coating composition. At the same time, according to the XPS data, the formation of Fe – C bonds with energy not corresponding to the formation of carbides, but corresponding to a solid solution of carbon in iron, is detected.

Thus, the surface of the studied samples obtained by high-speed laser sintering of Fe-C powders is a complex system of surface and adsorption compounds. As noted above, the performed mathematical modeling of the development of thermal fields during laser processing of the powder layer made it possible to evaluate the conditions under which sintering occurs with the possibility of the formation of nanoscale structures. It is the presence of a high degree of heating and cooling (up to 10 ¹¹ K / s) during processing by pulsed laser radiation that leads to the appearance of significant (up to 10 ⁸ K / m) temperature gradients and a high crystallization rate, which has a major effect on the forming structure. According to the calculations, the maximum velocity of the crystallization front motion leads to the suppression of microsegregation of components and the formation of chemically homogeneous structures. From the results of X-ray diffraction analysis (see Table 1), it can be seen that the average size of crystallites formed during laser sintering is in the range of 10-15 nm.

Table 1 The results of x-ray diffraction analysis of sample No. 3: phase content; lattice parameters a; average size <L> of coherent scattering regions; relative strain e of the crystal lattice Phase The content of the phase, wt.% a, nm <L>, nm ε,% α-Fe (A2, bcc) 37 0.2866 13 <0.1 γ-Fe (0.5 wt.% C) (A1, fcc) 63 0.3621 fifteen <0.05

From an analysis of the cathodic potentiodynamic curves of the samples studied, as well as from a comparison of iron-Armco, steel 40 (substrate material), nickel (grade H0), it follows that nickel has the lowest hydrogen overvoltage from all metals in the iron subgroup. The initial sections of the curves (up to approximately φ = -1200 mV) are linear, that is, they obey the Tafel equation. The slope of the linear sections dφ / dlgi in this potential range is 110 ± 10 mV for the materials studied, which is in good agreement with the known data for Fe and Ni. At potentials less than -1200 mV, the polarization curves increase the slope, which is probably due to the alkalization of the solution due to its low buffer capacity.

To characterize the cathodic activity in the hydrogen evolution reaction, the current density of this reaction can be used, i.e., the cathodic reaction rate at a given cathodic potential, i.e. at a given overvoltage of hydrogen. Table 2 shows the values of the density of cathodic currents on the studied samples at an overvoltage of 200 mV. It can be seen that all laser-treated samples are more cathode-active than Fe, and sample No. 3 is more active than Ni.

To characterize the electrocatalytic activity of metals, one can measure the overvoltage of the cathodic reaction at a given current density. Moreover, the lower the overvoltage, the more active the cathode. It can be seen that this indicator also favors a higher activity of laser-treated electrodes compared to Fe. Sample No. 3, in turn, is even more efficient than a nickel electrode.

The presence on the surface of laser-treated electrodes of structures detected by the XPS method in conjunction with the data in Table 1 confirms their nanoscale.

Thus, the surface layer to a depth of 20-30 nm consists of nanosized elements that have a strong bond between themselves in the tangential direction and a strong bond with the surface of the starting iron (steel), making the electrodes more active in the cathodic reaction.

Table 2. The current density of the cathodic evolution of hydrogen (i, mA / cm ² ) in 0.1 M NaOH at an overvoltage of η = 200 mV and the overvoltage of hydrogen evolution at a density of the cathodic current i = 1 mA / cm ² Sample No. Sample material i, mA / cm ² η, mV one Fe-C; 0.2 mm 2.82 183 2 Fe-C; 0.4 mm 3.10 158 3 Fe-C; 0.6 mm 7.15 103 four Fe Armco 0.81 263 5 Art.40 2,31 203 6 Ni 4.10 143

List of references

1. Yakimenko L.M. Electrochemical processes in the chemical industry: the production of hydrogen, oxygen, chlorine and alkalis. M .: Chemistry, 1981. p. 52-60.

2. Tsirlina G.A., Petriy O.A. Electrochemistry of carbides. In the book: Results of science and technology. Series "Electrochemistry". M .: VNITI, 1987.V. 24, p. 154-206.

3. Korostyleva TK, Podobaev NI, Devatkina TS Protection of metals, 1983. T. 18, No. 4, p.531-556.

4. RF patent No. 2443506. A method of manufacturing a coating on the product by the method of layered laser synthesis.

5. Computer program No. 20100614748. The software package "Computer optimization of the processes of laser processing of powders."

Claims (2)

1. A method of manufacturing an electrode material for the electrochemical production of hydrogen, characterized in that a powder composition Fe-C is applied to the electrode surface and the synthesis of Fe-C nanocrystalline elements with an average size of 10-15 nm is processed by laser pulses with a wavelength of 1-1 , 5 μm at a radiation density of 10 ⁷ -10 ⁹ W / cm ² , a laser scanning speed of 8-15 cm / s, a pulse frequency of 33-60 kHz in vacuum or in argon, without bringing the process to melting and the appearance of iron carbide Fe ₃ C. 2. The material of the electrode based on iron as a cathode material for the electrochemical production of hydrogen, obtained by the method according to claim 1.