A NEW APPROACH TO THE MECHANISM OF HYDROGEN EVOLUTION REACTION ON RHENIUM: 3D-RECOMBINATION Текст научной статьи по специальности «Химические науки»

УДК 54

Vladimir L. Krasikov1

A NEW APPROACH TO THE MECHANISM OF HYDROGEN EVOLUTION REACTION ON RHENIUM: 3D-RECOMBINATION

JSC Russian Institute of Radionavigation and Time, pl. Ras-trelli, 2, St. Petersburg, 191124, Russia. e-mail kras257@km.ru

In this analytical paper we have viewed the characteristics of ca-thodic hydrogen evolution reaction on electrodeposited rhenium in acid media with hydrogen atoms recombination as a limiting stage. We have also compared the results obtained by different authors. We came to the conclusion that a part of forming adsorbed hydrogen atoms penetrates into the metal subsurface layer thus releasing the rhenium surface for the following hydroxonium ions discharge on the same spot. Thereafter the recombination of atoms adsorbed on the surface and atoms absorbed in the bulk of metal proceeds with hydrogen molecule formation. According to the presented model of the three-dimensional recombination the rhenium surface is never filled with adsorbed hydrogen entirely which explains why the recombination limiting current is not observed experimentally. A similar process scheme may be applied for the explanation of cathodic hydrogen evolution on many metals and alloys able to absorb hydrogen. We showed that in order to explain the regularities of cathodic hydrogen evolution on metals it is necessary to take into account the atomic hydrogen mass transfer on the surface and in the bulk of metals.

Keywords: rhenium, electrodeposition, hydrogen overvoltage, hydrogen diffusion in metal, 3D-recombination.

.651.23

В.Л. Красиков

О МЕХАНИЗМЕ КАТОДНОГО ВОССТАНОВЛЕНИЯ ВОДОРОДА НА РЕНИИ: 3D- РЕКОМБИНАЦИЯ

АО «Российский институт радионавигации и времени», пл. Растрелли, 2, Санкт-Петербург,191124, Россия e-mail: kras257@km.ru

В аналитической работе рассмотрены особенности процесса катодного выделения водорода на электролитическом рении в кислых растворах, лимитирующей стадией которого является рекомбинация атомов водорода. Сопоставлены результаты, полученные разными авторами. Сделан вывод, что часть образующихся адсорбированных атомов водорода проникает в подповерхностный слой, освобождая таким образом поверхность рения для разряда следующих ионов гидроксония на том же месте, после чего происходит рекомбинация адсорбированных на поверхности и абсорбированных в металле атомов с образованием молекулы водорода. В соответствии с представленной моделью трёхмерной рекомбинации поверхность рения полностью не заполняется адсорбированным водородом, и это объясняет то, почему предельный ток рекомбинации экспериментально не наблюдается. Аналогичная схема процесса может быть применена для объяснения катодного выделения водорода на многих металлах и сплавах, сорбирующих водород. Показано, что для объяснения закономерностей катодного выделения водорода на металлах необходимо учитывать массоперенос атомарного водорода по поверхности и в объёме металла.

Ключевые слова: рений, электроосаждение, перенапряжение выделения водорода, диффузия водорода в металле, 3D-рекомбинаци.

DOI 10.15217/issn1998984-9.2016.34.24

Introduction

Recently we observe an increased interest in rhenium, its alloys and compounds behavior that is, in particular, cathodic hydrogen evolution on rhenium. Though we have already discussed this issue [1], it is necessary to touch upon some features of this process once again.

Cathodic hydrogen evolution on electrodeposited rhenium is peculiar in the following way.

First, rhenium electrodeposition from, for instance, the electrolyte of KReO4 - 12 g/l, H2SO4 - 35 g/l at the temperature of 60"C and the current density of 400 mA/cm2 is accompanied by intensive hydrogen evolution, rhenium current efficiency amounts to only 7.4% [1], that is the largest part of the current is spent on hydrogen evolution. After rhenium electrodeposition and current cut-off we can still observe hydrogen evolving on electrode as if the current was not switched off. Such hydrogen evolution can last for several tens of seconds but still it is determined not by rhenium dissolution in acid deposition electrolyte, but by hydrogen desorption from the metal. This fact indicates that the electrolytic rhenium is oversaturated with hydrogen in compliance with the data [2-4]. The volume of the dissolved hydrogen exceeds by many times the volume of rhenium, and

the rate of hydrogen diffusion in rhenium is obviously quite high. The special kind of electrodeposited rhenium structure allows accumulating reversibly a great amount of hydrogen, which may be interesting for hydrogen energetics.

Second, if after rhenium electrodeposition from acid electrolyte, followed by cathodic polarization in 0.25 M H2SO4 solution at current density 400 mA/cm2 within 2 minutes to reduce possible intermediate oxides according to the technique [1] and fast intensive rinsing the electrode with bidistillate, exposed to air, we take this electrode to a three-compartment cell for polarization measurements into electrolyte, also exposed to air, then in several seconds the electrode potential becomes stable and equal to the potential of reversible hydrogen electrode in the same solution despite the electrolyte's pH [1]. Only after several minutes the potential starts to move slowly to the anode direction. A similar phenomenon was reported in [5]. To make a comparison, it is necessary to note that if the same manipulations were performed with electrodeposited palladium, its potential immediately sets in much more positive, than reversible hydrogen electrode because of fast hydrogen desorption from the surface and oxygen adsorption from the electrolyte [6]. Thus, there is quite enough hydrogen accumulated in rhenium to saturate the electrolyte near the electrode with hydrogen and within several

1 Vladimir L. Krasikov, Ph.D. (Chem.), deputy chief of department, e-mail: kras257@km.ru Красиков Владимир Леонидович, канд. хим. наук, зам. начальника отд., e-mail: kras257@km.ru

Received April, 05 2016

tens of seconds to provide conditions under which rhenium electrode exhibits the functions of reversible hydrogen electrode. Electrodeposited rhenium is very inclined to oxidation determined by its non-equilibrium nanocrystalline or amorphous structure [2, 7-11], but till there is adsorbed hydrogen on its surface diffusing out of the bulk of the metal, rhenium is protected from oxidation. Exactly this fact was taken into account in our papers [1, 12-14] when developing the technique of preparation and execution of the experiment. It allowed us to get the data about the properties of clear, active and unoxidized rhenium surface. Hydrogen may exist in electrodeposited rhenium in a form of atomic state as interstitial solid solution, as well as it can be sorbed in a molecular form at the grain boundaries, in pores, cracks and other non-continuities [15-18]. One may also suppose that a part of hydrogen in rhenium is dissociated and is presented in a form of protons. The authors [11] discovered two forms of adsorbed hydrogen in rhenium, which can be removed while annealing under proper mode, and according to our preliminary data the annealing mode significantly influences the kinetics of hydrogen evolution. Rhenium is chemically inert in regard to hydrogen; these elements do not interact till the temperature of red heat, and the powder of rhenium only sorbs hydrogen [19-21]. Under high pressure also only physical adsorption of hydrogen by rhenium may take place [22]. The papers sometimes view the possibility of rhenium hydride and rhenide ions Re- formation [23-25], though their occurrence in solution can be observed experimentally only at very high ca-thodic potentials, for example, while reducing perrhenates by zinc amalgam in acid media [23], but while electrodep-ositing rhenium from acid perrhenates solutions, there is no rhenide-ions formation [23-24]. An important observation made in [23] should be taken into account: gaseous hydrogen evolution on rhenium in acid electrolyte, saturated with nitrogen, starts not immediately after switching on cathode current, but with some delay, so we can presume that in the initial period of time all atomic hydrogen evolving is absorbed by the subsurface layer of the metal in case if it did not contain hydrogen before. The same phenomenon can be observed not only on vacuum-melted rhenium, but also on iron [26] and palladium [27]. We can presume, that on any other vacuum-annealed metals under cathodic polarization the gaseous hydrogen evolution starts only after the subsurface layer having been saturated with atomic hydrogen. This presumption requires experimental check.

Third, after immersion in electrolyte, previously saturated with hydrogen, rhenium electrode without current at once has time-stable potential equal to the potential of reversible hydrogen electrode that is determined by significant exchange current density io, amounting to (2,1±0,6)-10-5 A/cm2 in solution 0,25 M H2SO4 at 25 °C, according to our data [1]. The exact equality of rhenium electrode potential without current in hydrogen saturated electrolyte and the potential of reversible hydrogen electrode as well as its long-term stability is the important indicator of rhenium surface purity. According to the most reliable data, on metallurgical rhenium in 0,145 M HCl solution at 25 °C the hydrogen exchange current density equals to 0,73-10-5 A/cm2 [28], which is close to our data [1] considering the difference in electrolytes' composition. Hydrogen exchange current 0,8-10-5 A/cm2 in 0,5 M H2SO4 solution at 20 °C is also exhibited in [29]. Rhenium no-current potential does not depend on the surface-active substances additions [1, 12]. If there are oxides on the surface of the electrode hydrogen exchange current density increases significantly [12]. Some references show unusually high value of the hydrogen exchange current on the order of 10-3 A/cm2 [30], though there can be doubts whether such significant exchange current characterizes the equilibrium between molecular hydrogen and hydroxonium ions on the chemically inert metallic surface, or rhenium oxides take part in establishing the equilibrium. We have not managed

to create such conditions of electrodeposited and metallurgical rhenium preparation that we were able to obtain such significant value of exchange current density. Necessary to note that the value of hydrogen exchange current density io on rhenium 10-3 A/cm2, more characteristic for platinum group metals [31-33], was used in the paper of S. Tras-atti for illustrating io dependence on metal-hydrogen binding energy EMe-H [34], then replicated in the other papers. Later on, the same value of hydrogen exchange current on rhenium was repeated in the often quoted paper [35], but with all this it can be clearly seen on the plot lgi0-EMe-H that the point, characterizing rhenium, obviously does not correspond with Sabatier principle [36]. The authors of the papers [24-25] paid attention to this discrepancy. Dependence lgi0-EMe-H may be corrected completely in case of using the data of reference [1] (or [28-29]).

Fourthly, according to our data hydrogen evolution overvoltage on electrodeposited rhenium in acid solutions obeys Tafel equation with slope coefficient b equal to 0,03 V (at 25 °C) [1]. Hydrogen overvoltage does not depend on the solution pH and agitation, but increases with the addition of the surface-active substances in electrolyte, despite their nature. And the more current density and SAS concentration is [12], the more is the extent of the increase. All this allows us to conclude that cathodic hydrogen evolution is limited with the stage of adsorbed hydrogen atoms recombination [1]. The same conclusion is deduced in papers [2, 28]. At the same time in paper [30] they obtained Tafel slope of 0,12-0,14 V, that corresponds better to slow discharge, and in papers [5, 29] they cited Tafel slope 0,055-0,065 V. According to the data of [5] hydrogen overvoltage does not depend on pH; the authors make a conclusion that the limiting stage of hydrogen evolution on electrodeposited rhenium is barrierless discharge. It is known that rhenium oxides are hydrogen evolution catalysts [37], therefore we can presume that in the presence of tracer amounts of oxides cathodic hydrogen evolution follows the scheme:

ReOx + e" ReO~ (1)

ReO~ + H30+ -> ReOx + V2 H2 + H20, (2)

or similar. If two electrons transfer simultaneously in the limiting stage, one may expect the value of Tafel slope about 0,06 V, and the participation of hydroxonium ions in the limiting stage leads to the hydrogen overvoltage dependence on pH. Though such data characterize not hydrogen evolution on rhenium but describe the behaviour of the second kind oxide electrode. For clear and active rhenium surface it is more reliable to observe Tafel slope about 0.03 V [1, 2, 28] and to follow the function of reversible hydrogen electrode.

Fifthly, the decrease in layer thickness of rhenium electrodeposited on copper (Figure 1), graphite, gold and nickel [1] in the range from 20 to 0,35 jm, practically does not influence the hydrogen overvoltage [1, 12]; further decrease in layer thickness down to 3,5 nm results simultaneously both in increase in cathodic potential and Tafel slope of polarization curves [1]. The influence of the substrate material becomes significant with the thickness of rhenium less than 350 nm, though the reasons of such influence are various and still not quite comprehensible. They can be determined by the breaking of the coating continuity, change in structure and electrophysical behavior [38], as well as by the change in the chemical composition of rhenium coating in thickness. The properties of mirror-smooth rhenium layer remain practically constant with rhenium thickness of 0.35 |jm and more, therefore hydrogen evolution kinetics does not depend on thickness.

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V*

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У

А

у

JO

• 1

о 2

'3 о 5

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-5

-3 -2

£gL, b/cm

Figure 1. Galvanostatic polarization curves of hydrogen evolution on mirrorlike rhenium layer electrodeposited on electropolished copper. Rhenium thickness (microns): 1-0; 2-0,0035; 3-0,035; 4-0,35; 5-3,5. Electrolyte -0,25 M H2SO4, 25oC, hydrogen media. Data of the paper [1].

As for chemical composition of the coatings, one should note that rhenium is deposited through the stage of intermediate oxides formation. [3, 39-41]. Metal layer sprightly close to the substrate material surface involves oxygen-containing impurities, but their content decreases in layers more distant from the substrate [3]. According to [42-43] oxides in electrodeposited rhenium are, vice versa, present only at the outer boundary of the metal and electrolyte. Obviously both cases are possible depending on the specific terms of deposition. Without the data about oxides content the author of the present paper has once discovered experimentally an electrode preparation mode including mandatory cathodic polarization in H2SO4 solution immediately after rhenium electrodeposition, reducing oxides and saturating the electrode with hydrogen [1]. Well-reproducible results became possible. Still we think that even such a preparation does not allow to completely remove oxygen-containing impurities in depth of the metal. Most likely these are not oxides, but stable rhenium-hydrogen bronzes HxReOy [44-45] which are impossible to be reduced electrochemically to metal and, in our opinion, their presence is the main reason of the difference in the properties of electrodeposited and metallurgical (fused) rhenium. We investigated the possibility of oxygen-containing metal compounds formation and the mechanism of their inclusion into cathodic coating composition in paper [46] concerning cobalt electrodeposition. In comparison with cobalt cations electrochemical reduction, the cathodic reduction of metals, which exist in the solution in form of anions of high oxidation state, proceeds through several sequential stages of intermediate oxides formation, therefore the possibility of oxygen-containing impurities formation and their inclusion in metal coating is especially high.

Sixthly, the ionization of hydrogen adsorbed by electrodeposited rhenium in 0.25 M H2SO4 solution takes place in the area of potentials from equilibrium hydrogen potential -0,04 V to +0,22 V (SHE, hydrogen atmosphere). More positive potentials provoke rhenium oxidation (Figure 2) [13]. Hydrogen ionization current is proportional to rhenium layer thickness in the range of 0,0035-3,5 |jm and does not depend on electrolyte agitation. At the same time under the same conditions on electrodeposited palladium hydrogen ionization rate increases by more than an order in intensive agitation electrolyte compared with unagitated electrolyte [6]. That is, on palladium there takes place the ionization of hydrogen dissolved in electrolyte while on rhenium there takes place ionization of hydrogen dissolved in metal. This difference is determined by the fact that hydrogen adsorption rate is low on rhenium in comparison with palladium (exchange currents densities differ by two orders, according to our data), and solubility of hydrogen in electrodeposited rhenium is significant. According to [2], hydrogen content in electrodeposited rhenium may reach 20 at. %.

E,V($HB)

Figure 2. Anodic polarization curves on hydrogen ionization, rhenium dissolution and oxygen evolution of rhenium-carbon electrodes.

Rhenium thickness (microns): 1-0; 2-0,0035; 3-0,035; 4-0,35; 5-3,5. Electrolyte - 0,25 M H2SO4, 25oC, hydrogen media. Data of the paper [12].

Seventhly, Tafel dependence E-lgik, describing the process of cathodic hydrogen evolution on electrodeposited rhenium in H2SO4 solution, intensively agitated for preventing any miserable hydrogen bubbles adhesion, remains linear at high current densities till at least 1 A/cm2 [12], which is quite uncommon. In compliance with the recombination theory, the first stage of hydrogen evolution process - hydroxonium ion discharge

H.O+ + e

Hads + H20

(3)

proceeds on the free surface rhenium atom (Figure 3a) forming adsorbed hydrogen atom (Figure 3b), thereafter follows discharge of the next hydroxonium ion on the neighbour metal atom (Figure 3c, 3d), then follows the chemical interaction of two neighbour adsorbed hydrogen atoms (Figure 3e).

Hads +

ads

H,

(4)

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3a

3b

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3c

V

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3d

3e

Figure 3. Volmer-Tafel scheme of hydroxonium ions discharge and hydrogen molecule formation

each of them is formed as a result of the same reaction (3). The reaction product - hydrogen molecule - removes into electrolyte or joins growing hydrogen bubble. Hydroxonium ions discharge takes place in quasi-equilibrium conditions. The rate of H3Û+ ions reduction is described by the conventional equation:

i, —K.a.

.(\-0„)exp(-aFE/RT).

(5)

Binary chemical recombination reaction (4) controls the whole process of cathodic hydrogen evolution on rhenium. Its rate grows along with the increase in hydrogen surface coverage Qh.

Hydrogen overvoltage n and the rate of process are bound via Tafel equation:

2,3RT^ . 2,3RT ^ . Л = lg«o + lSh

2 ßF

2ßF

(7)

where p is a coefficient including metal surface irregularity and repulsive forces among neighbour adsorbed hydrogen atoms. Using the data from the article [1] for cathodic hydrogen evolution on electrodeposited rhenium in 0,25 M H2SO4 solution at 25 °C the last equation can be presented as follows:

П = 0,14 + 0,03 lg ik,

(8)

where the current ik is expressed in A/cm2, overvoltage n in volts. Obviously, coefficient ß equals to one. However, if only

two processes (3) and (4) proceed, then the increase in current density inevitably would cause the increase in hydrogen surface coverage 9H of rhenium surface till complete occupation of the active centres that would result in limiting recombination current density. Nevertheless, neither in our works [1, 12], nor in other authors' papers there is no limiting current on rhenium or on similar metals adsorbing hydrogen. There is no explanation for this fact. The task of the present work is an attempt to find out the reason for such phenomenon.

The discussion of results

Basing on our experimental data [1, 12-14] and considering the electrochemical behaviour of rhenium we can suppose that cathodic hydrogen evolution on rhenium proceeds according to the following mechanism.

Hydroxonium ion discharge (reaction 3) takes place on rhenium surface atom or, more likely, on cluster of certain configuration, consisting of several rhenium atoms (Figure 4a). As a result of Volmer reaction (3) atomic hydrogen Hads forms on the metal surface (Figure 4b). There Hads atoms partially participate in surface recombination reaction (4) and partially penetrate into the metal subsurface layer (Figure 4c):

Hads Hads (9)

Under the outer rhenium atoms layer hydrogen atom Habs loses chemical bond with the solvent molecules. The rhenium cluster, on which the hydroxonium ion discharge has taken place, becomes free from hydrogen again and new hydroxonium ion discharge is possible on it (Figure 4d) according to the reaction (3). The electron transfer is also possible with the particles configuration, exhibited in Figure 4e. Hydroxonium ion discharge results in atom Hads, connected with a certain surface rhenium atom (Figure 4f). Then an absorbed hydrogen atom rises from the subsurface rhenium layer (Figure 4g):

Hads > HadS

(10),

and two hydrogen atoms recombination proceeds with hydrogen molecule formation (Figure 4h):

(11)

Formally two last reactions can be united in one:

Hads + Hads ^ H2 ^ (12)

4a

4b

V

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4c

4d

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4e

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4h

Fugure 4. Scheme of cathodic hydroxonium ion reduction on rhenium surface in accordance with 3-D recombination. Explanations in the text.

The reaction (10) of hydrogen atom transfer from the bulk metal to the metal surface, as well as the reverse reaction (9), are connected with interatomic bond breakage and bond formation, and require to overcome a certain potential barrier. Adsorbed hydrogen and absorbed hydrogen are in dynamic equilibrium, relationship between them depends on many factors and is determined by hydrogen adsorption, desorption, diffusion rates constants in metal and is considered in details in a number of papers, for example [27, 47-60].

Recombination reaction (11) proceeds on rhenium surface at the boundary metal-electrolyte with adsorbed hydrogen atoms participation, formed not only as a result of hydroxonium ions discharge in compliance with the reaction (3) and then merging into hydrogen molecule according to Tafel reaction, but also coming from the bulk of the metal according to reactions (9, 10). It means that not two-dimensional, but three-dimensional recombination of hydrogen atoms takes place. Basic propositions of Tafel theory are applicable to such scheme of 3D-recombination, though high degree of the metal surface occupation with adsorbed hydrogen is not reached due to the excessive outflow of Hads into the metal, hence, hydroxonium ions discharge is not hampered. As a result, linear Tafel dependence remains even at relatively high hydrogen evolution rates. Bulk of the metal is a high-capacity hydrogen accumulator.

The presented scheme is the simplest one of hydrogen reduction process in acid electrolyte, when two hydroxonium ions discharge sequentially on the same point of surface without interfering each other.

We should add that hydrogen reduction scheme, illustrated in Figure 4, will not change in principle, if hydrogen molecules formation takes place as a result of electrochemical desorption, not recombination. In this case we should presume that first H3O+ interacts with the absorbed hydrogen atom (see Figure 4d) forming H2+---OH2 complex, and then it discharges forming water and hydrogen molecules:

H30+ + Hads^H2+-OH2

Hn

•OH, + e~ ->//„ +H.0

(14)

(15)

Coming back to the recombination, it becomes obvios that more complicated situation arises in multi-component electrolyte in the presence of firmly adsorbed surface-active substances blocking significant part of the surface. When hydrogen atoms are formed at a distance from each other and reciprocal surface diffusion and recombination are impeded, the simplest way to transport hydrogen atom to another atom is its diffusion via the bulk of metallic phase. Possible transport scheme for Hads is presented in Figure 5.

0

© ёоОоО '0\Р О О/О'б

© о о о ою о/о о о

Figure 5. The possible scheme of hydrogen atom transfer along the surface and inside the bulk of the metal. Explanations in the text.

In general case hydrogen atom can diffuse along the surface from the point of formation to the point of penetration to the subsurface metal layer, then penetrate from the metal surface to the subsurface layer, diffuse in the bulk of the metal under the surface, occupied with firmly adsorbed SAS, then move to the metal surface at the rhenium-electrolyte boundary, diffuse along the surface again

and, having met another atom, form hydrogen molecule. The way of hydrogen atom moving in Figure 5 may look rather exotic but it is quite real. It is obvious from the papers [27, 47-60] analysis, concerning cathodic hydrogen evolution and diffusion in iron and palladium, the point on the electrode surface at which hydroxonium ions discharge, the point of hydrogen atom penetration into metal, the point of hydrogen desorption from subsurface layer to the boundary of metal-electrolyte and the point of recombination (or electrochemical desorption) - these are different points, in general case. Hence, the change in experimental conditions - chemical composition of the solution, SAS presence, structure and the electrode prehistory, change in temperature, etc. - would affect diversely different stages of hydrogen atoms mass transfer, complicating significantly the interpretation of electrochemical experimental data, whether it concerns cathodic hydrogen evolution, hydrogen diffusion through metallic membrane or metal corrosion research. The references report about the fact of significant dependence of hydrogen evolution kinetics on various metals and alloys, exposed to mechanical deformation or heat treatment [61-62], that is quite difficult to explain from electrochemical point of view. According to our data, mechanical, radiation, magnetic, heat etc. exposure on metal exerts primary impact just on the metal structure and by means of it - on atomic hydrogen mass transfer rate across the metal surface, in the bulk of metal or through the metal surface layer, and in the total, on kinetics and mechanism of hydrogen evolution. The points of hydrogen atoms formation and the point of their recombination can be at whatever far distance from each other, the duration of hydrogen presence in atomic state inside metal is not limited.

Atomic hydrogen sorption can change the metal structure and metal-hydrogen binding energy, resulting in the fact that along with the increase in cathodic current density and the increase in hydrogen content in metal a constant of the Tafel equation stops to be constant. Tafel slope of polarization curves will increase with the increase in current densities, but its change will be determined not by the change in constant b or transfer coefficient, but with the increase in a constant. Tafel equation changes:

E = a(ik) + b\gik (16)

where a(ik) is a Tafel equation constant, depending on current density, on hydrogen content in metal.

Comparing the galvanostatic polarization curves in H2SO4 solution of hydrogen evolution on rhenium [1] and palladium [63], electrodeposited on graphite, it can be seen (Figure 6), that Tafel slope on palladium gradually increases with the increase in hydrogen evolution rate [63], that can be connected with the formation of palladium-hydrogen two phases (a and в phases) with significantly different parameters of crystal lattice, equilibrium hydrogen content and hydrogen evolution overvoltage [27]. With the increase of cathodic current density palladium is significantly saturated with hydrogen, a and в phases ratio changes, that is in fact each time with the change of hydrogen evolution rate we deal with another metal with higher hydrogen overvoltage, characterized with higher a "constant". On the contrary, there are no phase transfers in rhenium. Hence, despite hydrogen exchange current density on palladium is by two orders higher than on rhenium, at small current densities hydrogen overvoltage is lower on palladium, and at higher current densities is lower on rhenium (Figure 6).

Comparing two metals, we can note that the equilibrium hydrogen surface coverage of palladium surface is close to one [27, 64-65], and hydrogen-palladium ratio (in the bulk of metal) in 0,5 M H2SO4 at 30C at pressure 1 atm. amounts to 0.691at. % [66] and increases with the increase of cathodic polarization.

Figure 6. Polarization curves of hydrogen evolution reaction on rhenium (1) and palladium electrodeposited on carbon in 0,25 M H2SO4.

Potential E versus standard hydrogen electrode SHE.

According to our preliminary data obtained by the method of pulse galvanostatic current switching from cathodic to anodic and with the electrode potential control it was discovered that the amount of electricity spent for ionization of adsorbed hydrogen on rhenium surface does not exceed 1/10 of theoretical amount of electricity necessary for hydrogen ionization provided monolayer filling of polycrystalline rhenium, which amounts to 2,6710-4 C/ cm2. At the same time the electrode capacity calculated from the potential decay curves in the time range of 0,03-30 s, determined by desorption of hydrogen dissolved in rhenium, exceeds the capacity with monolayer filling (covering) of the surface with hydrogen by 20 or more times. That is hydrogen is predominantly located in the bulk of rhenium, not on the surface. Based on these data we can present preliminary equivalent electric circuit of rhenium electrode consisting of resistors and two different capacitors, charged to different voltages (Figure 7), where C1 is a small capacity determined by the hydrogen adsorbed on rhenium surface, C2 is a significantly larger capacity determined by atomic hydrogen accumulated in the metal bulk.

R1 ,,ci

R2 R3

,C2

Figure 7. Preliminary equivalent circuit for rhenium electrode under cathodic polarization

Undoubtedly it is reliable to fulfil impedance measurements on electrodeposited rhenium in order to elucidate the electric equivalent circuit and to compare them with the data obtained by the authors [24-25]. It should be noted that the scheme represented in Figure 7 most exactly describes the properties of iron electrode in acid solution [55].

The difference in hydrogen distribution in palladium and rhenium can be illustrated in Figure 8. Palladium surface is almost completely filled with adsorbed hydrogen, and each adsorbed hydrogen atom is connected with each individual surface palladium atom (Figure 8a). On the contrary, rhenium surface is mostly free from hydrogen that in particular determines the exchange current density by two orders lower than on palladium, and each surface hydrogen atom is likely to be connected with the rhenium cluster of a certain configuration (Figure 8b). Equilibrium hydrogen content in the bulk of rhenium reaches the amount of 20 at. % [2]. Hydrogen can move easily to rhenium surface and got ionized under anodic polarization (Figure 2) and can easily be adsorbed by the bulk of metal as well.

ably within the certain range of current densities, Tefel equation may be written as:

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o®o®cfcfo

8a

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8b

Figure 8. Scheme of hydrogen distribution in palladium (a)

and rhenium (b) in the electrolyte saturated with hydrogen

There is a common opinion, that cathodic hydrogen evolution reaction is quite simple and is performed in compliance with one of two schemes, each of them consists of two stages: discharge-recombination or discharge-electrochemical desorption [56, 60, 67]. According to our data after hydroxonium ion discharge and hydrogen atom formation there are several stages of surface and/ or bulk mass transfer, and only after that hydrogen atoms recombination or electrochemical desorption proceeds. We consider that we should somewhat change an approach to metal-hydrogen system study taking into account atomic hydrogen mass transfer in the electrode material. Such an approach can improve significantly the understanding of metal-hydrogen system, hydrogen saturation of metals, hydrogen reduction and ionization kinetics, metals corrosion, kinetic isotope effect, electrocatalysis processes, hydrogen-induced phase transformations in metals and alloys, metals and alloys electrodeposition and many other processes with atomic hydrogen participation. We should note that the difference in experimental data obtained by different authors on the same metal under the same conditions is often connected namely with the difference in atomic hydrogen mass transfer, determined by hardly distinctive difference in the state of surface and subsurface layer of the metal, as well as the scarcely noticeable deformation of polycrystalline semiconducting material causes considerable change of its electroconductivity.

The alteration of rhenium structure, and hence, change of atomic hydrogen mass transfer conditions effects considerably the kinetics of hydrogen evolution. Some authors observed the Tafel slope of hydrogen evolution polarization curves of about 0,06 V [5, 29]. Taking into consideration the other data obtained by the authors, that value of Tafel slope may be explained that the limiting stage of hydrogen evolution is 3D-recombination reaction in which two hydrogen atoms take part, one atom is adsorbed on the rhenium surface, another atom transfers from the subsurface layer on the metal surface (reaction 12). The reaction rate

h = K>aHabs@L

(17)

2,3RT л . 2,3RT л . Л = —7— !g lo + —— lg h

(18)

is proportional to hydrogen surface coverage Qh of rhenium surface in the first order, and if the hydrogen content in subsurface layer is large enough and does not change consider-

If 2D and 3D-recombination take place simultaneously on different spots of the electrode surface, Tafel slope may be equal to some intermediate value derived from equations (7) and (18). At last, if Tafel reaction takes place with a great rate, but the hydrogen atoms mass transfer is limited, then the overall cathodic reaction is limited with hydroxonium ions discharge and recombination reaction with approximately equal rates.

The author cannot carry out necessary research to specify the items presented here. Still it would be very desirable to study the cathodic hydrogen evolution reaction on rhenium thin films obtained by electron-beam evaporation on isotropic glass carbon's freshly formed surface, repeating experimental conditions of paper [1], exercising control over the character of potential's change in time when switching the current. Besides, it would be interesting to fulfil electrochemical measurements of the kinetics of hydrogen, deuterium and tritium diffusion through rhenium membrane obtained via rhenium deposition on glass carbon plate with the following removal of glass carbon in oxygen plasma and rhenium cleaning with delicate ion etching. It is known, that rhenium is the only refractory metal not able to form carbides, so carbon-base materials are the best substrate for it with high hydrogen overvoltage and high chemical stability as well. Such works are deemed to be important not only from theoretical viewpoint, but also in terms of practice in order to obtain new alloys and composite materials and use them in a range of special areas of contemporary engineering.

Conclusions

We discussed the phenomena of rhenium electro-deposition and cathodic hydrogen evolution on rhenium in acid solutions. We compared the experimental data obtained by different authors and our own research results. It was shown that hydrogen exchange current density of 2,110-5 А/ см2 found out long ago is in a good accordance with Sabati-er's principle. We offered a new approach to the study of hydrogen evolution mechanism including the stage of hydroxonium ions discharge, the stage of evolved hydrogen atoms mass transport through the metal subsurface layer and the stage of recombination (or electrochemical desorption) for hydrogen atoms diffusing from the volume to the metal surface. We showed that taking into account the stage of atomic hydrogen mass transfer in the bulk of metal it is possible to explain some complicated electrochemical processes.

References

1 Krasikov V.L. Mechanism of Hydrogen Liberation on Rhenium // Elektrokhimiya. 1981. V. 17. No. 10. P. 15181522.

2. Суворова О.А. Электроосаждение рения из водных растворов: дис. ... д-pa хим. наук. М. 1962. 502 с.

3. Salakhova E., Majidzade V., Novruzova F., Kalantarova P., Huseynova R. The Electrodeposition of Rhenium in Alkaline and Acidic Electrolytes // J. Chem. Chem. Eng. 2012. V. 6. P. 489-494.

4. Salakhova E. The Electrochemical Deposition of Rhenium Chalcogenides from Different Electrolytes // J. Chem. Eng. Chem. Res. 2014. V. 1. No. 3. 2014. P. 185-198.

5. Гаврикова А.Е., Ротинян А.Л., Варыпаев В.Н. Закономерности катодного выделения водорода на электролитическом рении из сернокислых и сульфаматных растворов // Электрохимия. 1979. Т. 15. № 10. С. 15191522.

6. Красиков В.Л, Варыпаев В.Н. Коррозионное поведение палладий-графитовых электродов // Ж. прикл. химии. 1982. Т. 55. № 4. С. 905-907.

7. Суров Ю.И., Вартанова Г.А. Исследование метастабильной структуры электролитического рения // Электрохимия. 1976. Т. 12. № 9. С. 1378-1382.

8. Вартанова Г.А., Суров Ю.И. Исследование тонкой кристаллической структуры электроосаждённого рения // Ж. физ. химии. 1973. Т. 47. № 1. С. 121-123.

9. Вартанова Г.А., Суров Ю.И., Поволоцкий Е.Г. О структуре электролитически осаждённого рения // Ж. физ. химии. 1974. Т. 48. № 3. С. 740-742.

10. Суров Ю.И., Вартанова Г.А. Поволоцкий Е.Г. Рентгенографическое изучение дефектов упаковки в электролитических осадках рения // Электрохимия. 1975. Т. 11. № 5. С. 806-809.

11. Вартанова Г.А., Суров Ю.И., Ловпаче К.Ю., Белоусов Е.Б., Финкельштейн С.Х. О роли водорода в формировании структуры электролитических осадков рения: в сб. Рений. Химия, технология, анализ. М.: Наука. 1976. С. 97-99.

12. Krasikov V.L. Mechanism of Hydrogen Evolution on Rhenium. Effect of Surfactants and Electrode Preparation Conditions // Elektrokhimiya. 1983. V. 19. No. 2. P. 209-212.

13. Варыпаев В.Н., Красиков В.Л. Электрохимическое поведение рений-графитового электрода // Журн. прикл. химии. 1980. Т. 53. № 3. С. 586-590.

14. Красиков В.Л., Варыпаев В.Н. Электрохимическое поведение рений-графитового электрода // Журн. прикл. химии. 1980. Т. 53. № 5. С. 1061-1064.

15. Гаркунов Д.Н., Суранов Г.И., Хрусталев Ю.А. Водородное изнашивание деталей машин. Ухта: УГТУ. 2003. 199 с.

16. Максимчук В.П., Половников С.П. Водородное растрескивание высокопрочных сталей после нанесения гальванохимических покрытий. М.: Энергоатомиз-дат, 2002. 320 с.

17. Кузнецов В.В., Халдеев Г.В., Кичигин В.И. На-водороживание металлов в электролитах. М.: Машиностроение, 1993. 244 с.

18. Чернов И.П., Черданцев Ю.П., Тюрин Ю.И. Методы исследования систем металл-водород. М.: Энер-гоатомиздат, 2004. 270 с.

19. Рипан Р., Четяну И. Неорганическая химия. М.: Мир, 1972. Т. 2. 872 с.

20. Коровин С.С., Букин В.И., Фёдоров П.И., Резник А.М. Редкие и рассеянные элементы. Химия и технология. М.: МИСИС. 2003. 404 с.

21. Ежовска-Требятовска Б., Копач С., Микульский Т. Редкие элементы. М.: Мир. 1979. 367 с.

22. Антонов В.Е., Белаш И.Т., Малышев В.Ю., Понятовский Е.Г., Тулина Е.А. Растворимость водорода в рении при давлениях до 90 кбар // Докл АН СССР. 1983. Т. 269. № 3. С. 617-619.

23. Костромин А.И., Евгеньев М.И. Исследование катодного поведения рениевого электрода в растворах хлорной кислоты: в сб. аспирантских работ. Естественные науки. Биология. Химия. Казань: Изд-во Казанск. ун-та. 1975. С. 259-263.

24. Garcia-Garcia R., Ortega-Zarzosa G., Rincón M. E., Orozco G. The Hydrogen Evolution Reaction on Rhenium Metallic Electrodes: A Selected Review and New Experimental Evidence // Electrocatalysis. 2015. V. 6. P. 263-273.

25. Garcia-Garcia R., Rivera J.G., Antaño-Lopez R., Castañeda-Olivares F., Orozco G.. Impedance Spectra of the Cathodic Hydrogen Evolution Reaction on Polycrystalline Rhenium // Int. J. Hydrogen Energy. 2016. V. 41. P. 46604669.

26. Tsuru Yutaka, Kawamoto Mika. Evaluation of Internal Stress in Deposits and the Amount of Hydrogen Evolved at the Beginning of Iron Electroplating // J. of the Surface Finishing Society of Japan. 2004. V. 55. No. 1 P. 56-59.

27. Фрумкин А.Н., Аладжалова Н. К теории водородного перенапряжения на палладии // Журн. физ. химии. 1944. Т. 23. № 11-12. С. 493-517.

28. Joncich M.J., Stewart L.S., Posey F.A. Hydrogen Overvoltage on Rhenium and Niobium Electrodes // J. Elec-trochem. Soc. 1965. V. 112. No. 7. P. 717-721.

29. Макарян Э.М., Кудряшов И.В., Камыщенко С.Д. Исследование кинетики реакции катодного выделения водорода на катодах, изготовленных из вольфрама, рения и их сплава // Электрохимия. 1974. Т. 10. № 1. С. 65-67.

30. Печёрская А.Г., Стендер В.В. Потенциалы выделение водорода в кислых растворах // Журн. физ. химии. 1950. Т. 24. № 7. С. 856-859.

31. Smale S. A Study of the Hydrogen Evolution Reaction on Platinum and Platinum Group Metal Surfaces: Thesis... Ph.D.. Cardiff. 2008. 233 p.

32. Daoping Tang, Juntao Lu, Lin Zhuang, Peifang Liu. Calculations of the Exchange Current Density for Hydrogen Electrode Reactions: A Short Review and a New Equation.// J. Electroanal. Chem. 2010. V. 644. P. 144-149.

33. Wenchao Sheng, Hubert A. Gasteiger, Yang Shao-Horn. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes // J. Electrochem. Soc. 2010. V. 157 (11). P. B1529-B1536.

34. Trasatti S. Electronegativity, Work Function, and Heat of Adsorption of Hydrogen on Metals // J. Chem. Soc. Faraday Trans. 1972. V. 68. P. 229-236.

35. N0rskov J.K., Bligaard T., Logadottir A., Kitchin J.R., Chen J.G., Pandelov S., Stimming U. Trends in the Exchange Current for Hydrogen Evolution // J. Electrochem. Soc. 2005. V. 152(3). P. J23-J26.

36. Laursen A.B., Varela A.S., Dionigi F., Fanchiu H., Miller C., Trinhammer O.L., Rossmeisl J., Dahl S. Electrochemical Hydrogen Evolution: Sabatier's Principle and the Volcano Plot // J. Chem. Educ. 2012. V. 89, 1595-1599.

37. Vargas-Uscategui A., Mosquera E., Chornik B., Cifuentes L. Electrocatalysis of the Hydrogen Evolution Reaction by Rhenium Oxides Electrodeposited by Pulsed-Current // Electrochim. Acta. 2015. V. 178. P. 739-747.

38. Мансуров Г.Н., Петрий О.А. Электрохимия тонких металлических плёнок. М.: МГОУ. 2011. 351 с.

39. Синякова Г.С., Пурин Б.А., Арш Д.Р. Влияние рН электролита на химический состав осадков рения, полученных при катодном восстановлении ионов рения из сернокислых растворов: в сб. Рений. Химия, технология, анализ. М.: Наука, 1976. С. 92-94.

40. Vargas-Uscategui A., Mosquera E., Cifuentes L.. Analysis of the Electrodeposition Process of Rhenium and Rhenium Oxides in Alkaline Aqueous Electrolyte // Electro-chimica Acta. 2013. V.109. P. 283-290.

41. Гершов В.М., Пурин Б.А. Образование соединений рения низшей валентности при восстановлении перренат-ионов в сернокислом электролите // Изв. АН Лит. ССР. 1970. № 5. С. 527-559.

42. Schrebler R., Cury P., Suárez C., Muñoz E., Vera F., Córdova R., Gómez H., Ramos-Barrado J.R., Leinen D., Dalchiele E.A. Study of the Electrodeposition of Rhenium Thin Films by Electrochemical Quartz Microbalance and X-ray Photoelectron Spectroscopy // Thin Solid Films. 2005. V. 483. P. 50-59.

43. Schrebler R., Cury P., Orellana M., Gómez H., Córdova R., Dalchiele E.A.. Electrochemical and Nanoelec-trogravimetric Studies of the Nucleation and Growth Mechanisms of Rhenium on Polycrystalline Gold Electrode // Elec-trochimica Acta. 2001. V. 46. P. 4309-4318.

44. Dickens, P.G.; Weller, M.T. The Structure of a Cubic Hydrogen Rhenium Bronze, H136ReO3 // J. Solid State Chem. 1983. V. 48. No. 3. P. 407-411.

45. Weller M.T., Dickens P.G. Studies of Some Hydrogen Rhenium Bronzes // Solid State Ionics. 1983. V. 9-10. Part 2. P. 1081-1085.

46. Krasikov V.L. The Role of Electrochemical Cobalt Reduction Intermediates in the Formation of Oxygen-Containing Admixtures // Izvestia SPbGTI(TU).. 2015. No. 31(57). P. 40-46.

47. Маршаков А.И., Рыбкина А.А., Ненашева Т.А., Малеева М.А. Влияние атомарного водорода на кинетику активного растворения железа. I. Расчёт степени заполнения поверхности железа атомами водорода и их концентрации в поверхностном слое металла // Конденсированные среды и межфазные границы. 2012. Т. 14. № 2. С. 208-216.

48. Маршаков А.И., Рыбкина А.А., Ненашева Т.А., Малеева М.А. Влияние атомарного водорода на кинетику активного растворения железа. II. Методы циклического импульса потенциала и биполярного электрода-мембраны // Конденсированные среды и межфазные границы. 2012. Т. 14. № 3. С. 349-357.

49. Маршаков А.И., Рыбкина А.А., Ненашева Т.А., Малеева М.А. Влияние атомарного водорода на кинетику активного растворения железа. III. Изучение механизма растворения наводороженного железа методом электрохимической импедансной спектроскопии // Конденсированные среды и межфазные границы. 2012. Т. 14. № 4. С. 438-447.

50. Вигдорович В.И., Цыганкова Л.Е., Балыбин Д.В., Крыльский Д.В., Яковлева В.А. Влияние гуаниди-на на выделение и диффузию водорода // Вестник ТГТУ. 2010. Том 16. № 1. С. 94-107.

51. Вигдорович В.И., Цыганкова Л.Е., Копылова Е.Ю. Влияние анодной поляризации на диффузию водорода через стальную мембрану в этиленгликолевых растворах HCl, содержащих Na2AsO4. // Электрохимия. 2005. Т. 41. № 2. С. 175-182.

52. Tsygankova L.E., Vigdorovich V.I., Vigdorovich M.V. The Connection between Hydrogen Diffusion through a Steel Membrane and Mechanism of the Hydrogen Evolution Reaction // Surface and Interface Analysis. 2004. V. 36. No. 8. P. 1083-1088.

53. Vigdorovich M.V., Vigdorovich V.I., Tsyganko-va L.E. Hydrogen Diffusion through the Steel Membrane in Conditions of its Anodic Polarization // J. Electroanal. Chem. 2006. V. 596. No. 1. P. 1-6.

54. Tsygankova L.E., Vigdorovich V.I., Zarapina I.V. Hydrogen Diffusion through a Steel Membrane in the Acidic Water-Ethylene Glycol Solution Containing Pyridine in Conditions of the Cathodic Polarization // J. Electroanal. Chem. 2009. V. 636. No. 1. P. 24-29.

55. Кичигин В.И. О механизме катодного выделения водорода на железе в растворе серной кислоты // Вестник Удмуртского ун-та. 2006. № 8. С. 21-36.

56. Кичигин В.И. Определение кинетических параметров реакции выделения водорода из спектров импе-

данса // Вестник Пермского ун-та. 2013. № 3 (11). С. 4-14.

57. Amokrane N., Gabrielli C., Ostermann E., Perrot H. Investigation of Hydrogen Adsorption-Absorption on Iron by EIS // Electrochimica Acta. 2007. V. 53. No. 2. P. 700-709.

58. Цыганкова Л.Е., Косьяненко Е.С., Протасов А.С. Закономерности катодного выделения и диффузии водорода в сталь в кислых сульфатных растворах. Влияние роданида калия // Вестник ТГТУ. 2005. Том 11. № 4. C. 910-922.

59. Введенский А. В., Гуторов И. А., Морозова Н. Б. Кинетика катодного выделения водорода на переходных металлах. I. Теоретический анализ // Конденсированные среды и межфазные границы. 2010. Т. 12. № 3. С. 288-300.

60. Введенский А. В., Гуторов И. А., Морозова Н. Б. Кинетика катодного выделения водорода на переходных металлах. II. Экспериментальные данные // Конденсированные среды и межфазные границы. 2010. Т. 12. № 4. С. 337-347.

61. Лавренко В.А. Об анодном электролитическом окислении рения // Ж. физ. химии. 1961. Т. 35. № 1. С. 198-200.

62. Варес П.М., Тамм Ю.К. Влияние чистоты металла и способа подготовки поверхности железа на перенапряжение водорода. Двойной слой и адсорбция на твёрдых электродах //. Тезисы докл. Всесоюзного симпозиума 29 июня - 01 июля 1981 г. Тарту. Т. 6. 1981. С. 45 - 47.

63. Красиков В.Л., Иванищева Т.Ф., Силантьева Т.Е., Варыпаев В.Н. Влияние условий палладирования на электрохимические свойства палладий-графитовых электродов // Журн. прикл. химии. 1982. Т. 55. № 5. С. 10941097.

64. Hubler G.K. Anomalous Efects in Hydrogen-Charged Palladium — A Review // Surface and Coatings Technology. 2007. V. 201. P. 8568-8573.

65. Бурштейн Р.Х, Виленская В.С., Тарасевич М.Р. Исследование адсорбции водорода и кислорода на дисперсном палладии // Электрохимия. 1967. Т. 3. № 3. С. 349-355.

66. Maoka T., Enio M. Hydrogen Adsorption by Palladium Electrode Polarised in Sulfuric Acid Solution Containing Surface Active Substances. II. The Anodic Region // Electro-chem. Acta. 1981. V. 26. No. 5. P. 615-619.

67. Балыбин Д.В., Корякина Е.А., Калинушки-на Е.Ю., Попова Е.Д. Кинетика и природа замедленной стадии реакции выделения водорода на железе в кислых средах // Вестник ТГУ. 2014. Т.19. № 1. С. 145-148.