RU2781376C1 - Application of a hybrid material based on a metal-free electrocatalyst for generating hydrogen from water - Google Patents

Abstract

FIELD: electrochemistry.

SUBSTANCE: invention relates to the field of electrochemistry and electrocatalysis, in particular to materials used as electrocatalysts for the production of molecular hydrogen in the presence of heteropolycyclic organic compounds. It is proposed to use a hybrid material based on a metal-free electrocatalyst, which is organic heterocyclic compounds 4,4'-bipyridine, 2,2'-bipyridine and 1,10-phenanthroline, immobilized on the surface of a carbon-containing cathode, to generate molecular hydrogen from water.

EFFECT: use of an electrocatalyst based on heterocyclic compounds capable of physical adsorption on a carbon surface by the π-π stacking mechanism, which provides high process speeds and record stability in the process of electrochemical production of molecular hydrogen from water.

1 cl, 7 dwg, 1 tbl

Description

The invention relates to the field of electrochemistry and electrocatalysis, in particular to materials used as electrocatalysts for the production of molecular hydrogen in the presence of heteropolycyclic organic compounds.

In connection with the rapid transition of direct methods of hydrogen production by electrolysis, which require the use of catalysts based on expensive platinum and other platinum group metals, the issue of replacing them with more affordable, but not inferior in efficiency systems is undoubtedly relevant.

In the course of long-term studies, electrocatalytic systems based on transition metal complexes with sufficient catalytic activity were proposed, but they do not meet the main requirements for them, namely, high chemical and thermodynamic stability, no need to create special conditions for the process, as well as commercial competitiveness relative to currently existing catalysts. In addition, a priority task is a partial or complete transition from metal catalysts to synthetic analogues, which will allow, with the same efficiency, to significantly reduce the cost of the catalytic system.

Active developments in this area contributed to the identification of the main approaches to the production of electrocatalysts based on organic compounds. The bottom line is the step-by-step synthesis of molecules, which are then immobilized on the surface of a carbon carrier - the cathode.

Known catalyst for producing molecular hydrogen. The solution describes the use of an organic stable electron-excess radical as a catalyst for the production of molecular hydrogen (RU 2480283, IPC B01J 31/00, C01B 3/16, C25B 1/44, publ. 27.04.2013).

The disadvantage of the known solutions is the low catalytic activity and the impossibility of immobilization on the working electrode.

It is known from the prior art to use a material based on a metalless electrocatalyst to produce molecular hydrogen from water in the presence of organic salts. The invention relates to the use of a material based on metal-free electrocatalyst, which is acridine, 9-phenyl-acridine or N-methyl-9-phenylacridine, adsorbed on carbon material, to obtain molecular hydrogen from water in the presence of organic salts (RU 2706117, IPC B01J 31 /02, C25B 1/04, published 11/14/2019).

The material used is synthetically available and thermodynamically stable. The disadvantage of the known solution is the low time of continuous operation. It is being deactivated.

The technical result, when using the claimed invention, is the use of an electrocatalyst based on heterocyclic compounds capable of physical adsorption on the carbon surface by the π-π-staking mechanism, which ensures high process speeds and record stability in the process of electrochemical production of molecular hydrogen from water.

The essence of the invention lies in the fact that it is proposed to use a hybrid material based on a metal-free electrocatalyst, which is organic heterocyclic compounds 4,4'-bipyridine, 2,2'-bipyridine and 1,10-phenanthroline, immobilized on the surface of a carbon-containing cathode, for generating molecular hydrogen from water.

The essence of the invention is illustrated by drawings, where:

in FIG. 1 shows the structural formulas of 2,2'-bipyridine (I), 4,4'-bipyridine (II) and 1,10-phenanthroline (III);

in FIG. 2 shows the sorption of compounds I (a), II (b) and III (c) on the VulcanXC-72 carbon support in acetonitrile;

in FIG. 3 shows the equilibrium states obtained using the DFT method on the B3LIP/6-31+G(2d,p) basis for the adsorption of compounds I-a, II-b and III-c on the surface of a carbon support;

in FIG. 4 shows adsorption isotherms of compounds of compounds I-a, II-b and III- with a carbon-containing sorbent in acentonitrile in the coordinates of the linear form of the Langmuir equation;

in FIG. Figure 5 shows cyclic voltammograms for compounds I, II, and III immobilized on a VulcanXC-72 carbon support. Recording conditions: acetonitrile, v = 100 mV/s, Ag/AgCl/KCl;

in FIG. Figure 6 shows the polarization curves obtained for compounds 1, 2, and 3 immobilized on a VulcanXC-72 carbon carrier in the presence of chlorosulfonic acid (0.1 mol/l). Shooting conditions: methylene chloride, v = 5 mV/s;

in FIG. 7 shows the results of preparative electrolysis based on compounds I, II and III at the cathode, and also based on a similar carbon material activated with platinum metal, used as a control sample (4).

Implementation of the invention

The manufacture and use of a metal-free electrocatalyst based on organic heterocyclic compounds immobilized on the surface of a carbon carrier is carried out as follows. The crushed sorbent, previously washed with distilled water, filtered and dried at a temperature of 440-450°C, is alternately mixed with acetonitrile solutions of 2,2'-bipyridine, 4,4'-bipyridine and 1,10'-phenanthroline (10 ^-3 mol/ l). Sorption is carried out by a static method, which is implemented in a mixing device. The solutions are continuously stirred at room temperature for 30 minutes.

Studies of the luminescent properties of heterocyclic polyaromatic compounds were carried out using the method of fluorescent optical spectrometry. Luminescence spectra were recorded on a Shimadzu RF-5301PC spectrofluorimeter. The radiation source in the lamp is a 150 W xenon lamp with a deozone generator. Optical range: excitation 220-900 nm, emission 220-900 nm. Spectrofluorimeter RF-5301PC is designed for quantitative chemical analysis of organic and inorganic substances.

The following materials were used:

1. Carbon carrier VulcanXC-72 manufactured by CabotCorporation-Additives.

2. 2,2'-bipyridine (chemically pure). Molecular formula: C ₁₀ H ₈ N ₂ . Molecular weight: 156.188.

3. 4,4'-bipyridine (chemically pure) Molecular formula: C ₁₀ H ₈ N ₂ . Molecular weight: 156.188.

4. 1,10-phenanthroline (chemically pure) Molecular formula: C ₁₂ H ₈ N ₂ . Molecular weight: 180.21.

The claimed invention shows the results of studies of the sorption of 2,2'-bipyridine, 4,4'-bipyridine and 1,10-phenanthroline (Fig. 1), used as adsorbates, on a carbon carrier, as well as the electrochemical properties of the obtained electrocatalytic systems.

The aim of the study was to identify the optimal conditions for the sorption process, as well as to identify its specific mechanism, which in the future will allow more detailed regulation of the process of creating metalless electrocatalysts based on other, more complex systems.

The time for establishing equilibrium sorption conditions is the shortest for compound I, then for compound III, and for compound II, the establishment of equilibrium is the slowest. Compound III has a flat structure, which should be perfectly suitable for adsorption on the surface of carbon carriers due to π-π - staking, which is responsible for its rather high sorption rate. Such a geometry of the molecule will be more conducive to immobilization on the support surface. To confirm the results obtained, quantum-chemical calculations of the adsorption process of compounds I, II, III were carried out on a carbon support model using the DFT method on the B3LIP/6-31+G(2d,p) basis. For calculations, a model of a graphene surface was used, consisting of 28 conjugated cycles (Fig. 3).

Presumably, the structure of 1,10-phenanthroline is a flat, structurally rigid molecule, which is sorbed exclusively by the π-π-staking mechanism. This compound cannot be adsorbed by pores because the molecule is large and rigid. Energetically, this type of sorption is not particularly favorable, so the process takes a long time. In the case of the 4,4'-bipyridine molecule, the planar configuration is energetically more favorable, in contrast to 2,2'-bipyridine, for sorption by the π-π staking mechanism, since in the case of 2,2'-bipyridine, the unshared electron pairs of nitrogen in the planar configuration will repel each other to a greater extent, therefore, energetically this situation is not favorable. Therefore, it can be assumed that 2,2'-bipyridine will be sorbed due to the structural type of sorption, that is, it will be sorbed only by the pores of the carbon material. Whereas in the case of the 4,4'-bipyridine molecule, sorption can proceed by a mixed mechanism: by π-π - staking, and by the structural type of interaction.

To verify the validity of the results obtained, Langmuir isotherms were constructed and the adsorption constant (K), adsorption limit value (A _∞ ) and free energy of the adsorption process (ΔG° _ads ) were calculated. In table. Figure 1 shows the parameters of sorption of acetonitrile solutions of organic compounds on a carbon-containing carrier VulcanXC-72. The nature of the surface of the sorbent can be judged by the forms of adsorption isotherms (Fig. 2). It can be noted that all isotherms are characterized by a convex shape, which corresponds to the monomolecular type of adsorption. A similar type of equilibrium is described by the Langmuir equation:

where A is the adsorption value, mol/kg;

A _∞ - limiting monomolecular adsorption;

K is the adsorption equilibrium constant;

C is the concentration of the adsorbate in the solution, kmol/m ³ .

The experimental results of adsorption are processed using the Langmuir equation written in linear form:

выражается прямой, пересекающей ось ординат. Отрезок, отсекаемый на оси ординат, определяет величину, обратную емкости монослоя

. Тангенс угла наклона прямой равен

, что позволяет найти константу адсорбционного равновесия К_равн.Graphic dependency

is expressed by a straight line intersecting the y-axis. The segment cut off on the y-axis determines the reciprocal of the monolayer capacitance

. The tangent of the slope of the straight line is

, which makes it possible to find the adsorption equilibrium constant K _equal .

The isotherms are reduced to a linear form (Fig. 4), for which the following parameters are calculated: tgα (tangent of the slope of the straight line), K _equals (adsorption equilibrium constant), ΔG° _ads (change in the Gibbs energy of adsorption).

;For I connection

;

;for II

;

.for III

.

The tangent of the slope of the straight line is:

For I: tan α = 0.1159;

II: tan α = 0.1134;

III: tgα = 0.0531;

and allows you to find the constant of adsorption equilibrium K _equal. :

;For I: _Equal. =

;

;II: _Equal. =

;

.III: _Equal. =

.

Knowing the adsorption equilibrium constant, we can calculate the change in the Gibbs energy of adsorption:

For I: ΔG° _ads = - 8.31 ⋅ 298 ⋅ ln 7874 = -22216.41 J

II: ΔG° _ads = - 8.31 ⋅ 298 ⋅ ln 10000 = -22808.3 J

III: ΔG° _ads = - 8.31 ⋅ 298 ⋅ ln 25582 = -25134.37 J

As can be seen from the data given in Table 1, parameter A_∞, which characterizes the efficiency of the process, the largest for compound I, then for II and decreases for III. An increase in the size and volume of the molecule of compound III, in comparison with compound I, leads to the fact that the maximum adsorption of A decreases by almost one and a half times._∞, which agrees well with the conclusion that the limiting type of adsorption is the structural type.

It is interesting to note that a change in the position of the nitrogen in the 4,4'-bipyridine molecule leads to a slight decrease in the А _{∞ parameter,} while the presence of an additional condensed ring leads to a decrease in the А _∞ parameter by almost 1.5 times compared to compound I. It is also important to note that at the lowest A _∞ values for compound III, the K value is almost three times greater than the K values for compounds I and II, and the ΔG° _{ads value is} also significantly higher.

The values of the free energy of adsorption are approximately the same in all cases, which indicates a similar adsorption process. Also, all values of ΔG° _{ads are} negative, that is, the process proceeds spontaneously. For 1,10-phenanthroline, the change in the Gibbs energy of adsorption ΔG° _ads is higher than for the other two compounds; therefore, the analytical sorption process is most advantageous. Due to the low adsorption rate, the number of sites capable of sorption is insignificant, and the process itself is much more profitable due to π-π interaction.

Thus, based on the data obtained, it can be concluded that the I molecule is adsorbed in pores on a carbon support not by the π-π-staking mechanism, but due to physical adsorption. While the III molecule is sorbed exclusively by π-π - interaction on the areas of the carbon material, which has a suitable geometry of the carbon web. And the molecule II, in turn, can be sorbed by two parallel mechanisms: due to π-π-staking, and due to the mechanism of physical sorption by the pores of the sorbent.

To study the electrochemical properties of the presented systems, the method of cyclic voltammetry (CV) was used. All compounds immobilized on the surface of the carbon carrier are electrochemically active in the cathodic region at potentials close to the potentials of the initial compounds (Fig. 5). In addition, the lgIp-lgv dependence was studied for all systems, resulting in a curve slope close to 1. This suggests that the limiting stage of the process is the rapid migration of an electron from the electrode surface to the catalytic center. The obtained values testify to the high efficiency of the studied electrocatalytic systems.

In addition, polarization curves were constructed (Fig. 6), on the basis of which we note that in all cases there is a slope equal to 57 mV/dec, which corresponds to the process proceeding according to the Heyrovsky mechanism.

Next, for immobilized compounds I, II, and III, current-voltage characteristics were obtained using preparative electrolysis. Volt-ampere characteristics of a water electrolyzer with membrane-electrode blocks based on samples I, II and III on the cathode, as well as on the basis of a similar carbon material activated by metallic platinum deposited on a Vulcan XC-72 carbon carrier and used as a control sample (Fig. 7) . It can be seen that for all compounds are close to each other.

Thus, the obtained synthetic systems are promising for use in water electrolyzers, although the activity of these electrocatalysts is inferior to that of platinum-containing materials.

Table 1 Options 2,2'-bipyridine 4,4'-bipyridine 1,10-phenanthroline A _∞ 0.00109 0.000882 0.000736 tga 0.1159 0.1134 0.0531 K 7874 10000 22582 ΔG° _, kJ -22.22 -22.81 -25.13

Claims (1)

Application of a hybrid material based on a metal-free electrocatalyst, which is organic heterocyclic compounds 4,4'-bipyridine, 2,2'-bipyridine and 1,10-phenanthroline, immobilized on the surface of a carbon-containing cathode, for generating molecular hydrogen from water.

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