Direct electrolysis of water. Electrolysis of water: what do we know about it?

Introduction

Over the past decades, hundreds of water electrolysis installations have been created to produce hydrogen and oxygen, equipped with electrolyzers that operate at both atmospheric and elevated pressure. Currently, about a thousand electrolyzers of various types operate at power plants alone.

To meet your needs National economy in electrolytic hydrogen in the coming years, an additional significant number of powerful electrolyzers with a capacity of 500 - 650 hydrogen and smaller electrolysers to produce small quantities of hydrogen.

In many countries, electrolysis plants have been used to produce heavy water as by-product. Subsequently, more were developed effective methods its production, however, the by-product production of by-product water at large electrolysis plants in some cases it is advisable.

1. General information about the process of water electrolysis

As is known, when an electric current passes through electrolyte solutions, ions are discharged at the electrodes and associated chemical reactions occur. The course of the electrolysis process is determined by the transfer of electric current in the liquid and the discharge conditions of the electrolyte ions present in the solution.

The process of electrolysis of water to produce hydrogen and oxygen is described by the following summary equation:

Pure water cannot be directly subjected to electrolysis, since its electrical conductivity is very low. Electrical conductivity tap water close to * very pure distilled water about 4* . Therefore, in electrolysis, aqueous solutions of electrolytes - acids, alkalis, and salts - are used.

By changing the composition, concentration and temperature of the electrolyte and selecting the conditions that determine the magnitude of the overvoltage, it is possible to change the course of electrode processes in the desired direction.

In industrial water electrolysis processes, only alkaline electrolytes are currently used - caustic potassium and caustic sled. If technical alkalis are used as electrolytes, their solutions contain ion impurities etc. It is also possible that small amounts of iron and other contaminants are present in the electrolyte.

During long-term operation of water electrolysis plants, foreign ions introduced with impurities contained in the feed water accumulate in the electrolyte solution. If any impurity, such as ions , constantly enters the electrolyte solution, then with a sufficient duration of the electrolysis process, the maximum concentration of this impurity is reached, which is determined from the equality of its arrival and consumption in the electrolyzer per unit time.

When feeding the electrolyzer with distilled water, the content of simple ions in the electrolyte is usually very small and in total does not exceed 1 - 5 g/l, excluding carbonates, the content of which in 1 liter of electrolyte solution can reach tens of grams. In electrolyzers with an open electrolyte mirror in contact with air, the concentration of carbonates can be even greater. For electrolyzers of some designs, the electrolyte is prepared in sealed tanks with a nitrogen blanket, which prevents contamination by carbonates.

During the electrolysis of water, hydrogen is released at the cathode and oxygen at the anode. Depending on the conditions of the cathodic process, there are two possible mechanisms for its occurrence. In acidic solutions with a high content of hydrogen ions, its release occurs due to the discharge of ions with the formation of atomic hydrogen, which is adsorbed on the surface of the cathode, which can be described by the expression:

Since the hydrogen ion in the solution is hydrated, the stage of its discharge can be represented as:

The next stage of the cathodic process is the recombination of atomic hydrogen into molecular hydrogen, which occurs through a catalytic mechanism.

Under certain conditions, both stages of the cathodic process are ion discharge and the release of molecular hydrogen - can occur simultaneously.

If other cations are present in the solution, having a more positive release potential compared to hydrogen, they are released at the cathode, forming a precipitate. This is observed, for example, in the presence of impurities in the electrolyte of compounds of lead, tin, zinc, iron, chromium, molybdenum and some other metals. If such a deposit forms on the cathode, the potential for hydrogen evolution and the conditions for the cathodic process may change. In industrial conditions, the electrolyte almost always contains a small amount of iron ions due to constant corrosion of the steel parts of the electrolyzers. Therefore, a deposit in the form of a metal (iron) sponge usually forms on the cathode surface.

The release of oxygen at the anode during the electrolysis of water occurs as a result of the discharge of hydroxyl ions or water molecules. Small amounts present in the electrolyte and other ions, as well as ions at a sufficiently high concentration of alkali in the solution (200 - 300 g/l or more) they cannot be discharged, since this under these conditions requires a higher potential than for the discharge of ions or water molecules. In alkaline solutions at moderate current densities, the supply of hydroxyl ions to the anode is not a limiting process and they are discharged at the anode according to the reaction:

In acidic solutions at any current density and in alkaline solutions at high current densities, ion supply is the limiting stage and a second mechanism is proposed for their discharge:

During electrolysis, all ions present in the electrolyte take part in current transfer. The proportion of their participation is determined by the relative concentration and mobility of the ions. In alkaline electrolytes, due to the very low concentration of hydrogen ions, current transfer is carried out almost exclusively by ions.

Almost only water molecules are discharged at the cathode, and ions at the anode. . In this case, for each molecule of hydrogen released at the cathode, two water molecules decay to form two molecules . Ions And , participating in the transfer of current to the cathode, as well as , and other anions involved in current transfer to the anode are not discharged at the electrodes.

Due to the fact that during the electrolysis of water gases are released on both electrodes, the electrolyte layer adjacent to the electrode is intensively mixed. Therefore, the formation of local zones with a greatly reduced concentration of KOH and, accordingly, with an increased concentration of ions, is unlikely on the anode surface etc. However, in the depths of narrow gaps between the electrode and adjacent parts or under the slurry near the surface of the electrode, a significant change in the ion concentration is possible for the previously discussed reasons. Such concentration changes apparently cause local intense electrochemical corrosion of some parts of electrolyzers.

As with other electrochemical processes, costs electrical energy during electrolysis of water are large and often determine the economics of this process. Therefore, much attention is always paid to the issues of energy consumption for electrolysis and reducing the voltage on the electrolytic cell.

. Electrochemical cells

An electrochemical cell usually consists of two half-cells, each of which is an electrode immersed in its own electrolyte. Electrodes are made of electrically conductive material (metal or carbon), or less often of a semiconductor. The charge carriers in the electrodes are electrons, and the charge carriers in the electrolyte are ions. An aqueous solution that is an electrolyte table salt(sodium chloride NaCl) contains charged particles: sodium cations Na +and chlorine anions Cl -If you place such a solution in electric field, then Na ions +will move to the negative pole, Cl ions -- to the positive. Molten salts, such as NaCl, are also electrolytes. Electrolytes can also be solid substances, for example b-alumina (sodium polyaluminate), containing mobile sodium ions, or ion-exchange polymers.

The half-cells are separated by a partition, which does not interfere with the movement of ions, but prevents mixing of electrolytes. The role of such a partition can be performed by a salt bridge, a tube with an aqueous solution closed at both ends with glass wool, an ion exchange membrane, or a porous glass plate. Both electrodes of the electrolytic cell can be immersed in the same electrolyte.

There are two types of electrochemical cells: galvanic cells and electrolytic cells (electrolysers).

The same reactions take place in the electrolytic cell as in industrial electrolyzers for producing chlorine and alkali: the conversion of brine (a concentrated aqueous solution of sodium chloride) into chlorine and sodium hydroxide NaOH:

electrolysis oxidation ion

Chloride ions on the graphite electrode are oxidized to chlorine gas, and water on the iron electrode is reduced to hydrogen and hydroxide ions. Electrolytes remain electrically neutral due to the movement of sodium ions through a partition - an ion exchange membrane. The electrode at which oxidation occurs is called the anode, and the electrode at which reduction occurs is called the cathode.

Bibliography

1. O.D. Khvolson, Course of Physics, RSFSR, Gosizdat, Berlin, 1923, vol. 4.

A.I. Levin, Theoretical basis Electrochemistry, State. Scientific and technical Publishing house, Moscow, 1963.

A.P. Sokolov, ZhRFKhO, vol. 28, p. 129, 1896.

Phys. Encycl. Slov., ed. " Soviet Encyclopedia", Moscow, 1960, vol. 1, p. 288.

L.M. Yakimenko et al., Electrolysis of water, ed. "chemistry", Moscow, 1970.

Tutoring

Need help studying a topic?

Our specialists will advise or provide tutoring services on topics that interest you.
Submit your application indicating the topic right now to find out about the possibility of obtaining a consultation.

The chemical composition of water was first determined by the French chemist Lavoisier in 1784. Lavoisier, together with the military engineer Meunier, passing water vapor over a hot sheet of iron, discovered that the water decomposed, releasing hydrogen and oxygen. Yes, of course, for their time, for the era of “ordering things,” these conclusions were of great importance. In fact, before this discovery, water was considered a completely homogeneous substance. One cannot, however, fail to note another thing: this discovery also played a very obvious negative role, since it diverted the attention of other scientists for a long time from searching in this area and established infallibility in the minds of many generations this conclusion, consecrated, moreover, by the authority of a scientist.
But the conditions under which it was carried out were so imperfect, they were “dirty”.
Just think about the presence of iron, over which water vapor was passed. It can introduce moments into experience that are even difficult to take into account in advance. Lavoisier and his partner recorded in their experiment what was most obvious: the release of two gases - hydrogen and oxygen, and what was beyond that, they did not pay attention to this at all, most likely for the reason that this “in addition” did not was as obvious as the release of two gases.
Since before this discovery the general opinion prevailing in science was that water is a homogeneous substance, the fact of the discovery of its heterogeneous composition can be called revolutionary. What more could one ask from the discoverers! Moreover, the obviousness of the results of the experiment was too captivating.
The old view of water was discarded and replaced by a new idea of ​​water as a combination of two elements - hydrogen and oxygen, which quickly established itself in science. This was greatly facilitated by the development electrochemistry.

ELECTROLYSIS according to Davy
A number of scientists (Nicholson, Cavendish, etc.) conducted an experiment on electrochemicaldecompositionwater (such a definition of this process is completely erroneous). By the word “decomposition” we must understand the electrolysis of water as a complex redox process, but by no means as a simple decomposition of water into its constituent elements.
So, during decomposition, i.e. During the electrolysis of water, hydrogen and oxygen were released, which seemed to externally confirm Lavoisier's conclusion. However, at the same time, the “black box” suddenly began to provide additional information that was not there before. During the electrolysis process, two strange phenomena were discovered: firstly, both components of water were released not together, but separately from each other - oxygen at one electrode, hydrogen at the other; secondly, the formation of acid at the oxygen pole and alkali at the hydrogen pole was observed. This"strange" decomposition of ozad waternumber of scientists; Moreover, they were more worried about the second “strangeness”, i.e. the appearance of acid and alkali.

The fact that when an electric current was passed through water, hydrogen and oxygen were released completely suited the scientists, because it seemed to confirm the already dominant opinion about the composition of water. The question of how these components were distinguished, under what accompanying circumstances, although it occupied the scientists of that time, was still not to that extent: their attention was directed mainly to the second "weirdness", because she cast a shadow of doubt on the discovered formula of water. The question inevitably arose about what causes the formation of acid and alkali during the electrolysis of water.
An outstanding English chemist took on the solution to this riddle. Humphry Davy(1778-1829). Davy, through a series of experiments, seemed to confirm the fact assumed by all scientists of that time that the formation of acid and alkali during the electrolysis of water is a random phenomenon, not related to the water itself, which consists as it was determined Lavoisier, from hydrogen and oxygen. But how did Davy manage to “prove” this?
Davy carried out numerous experiments on the “decomposition” of carefully purified water with electricity in various vessels: agate, glass, made of fluorspar, barite sulfate, etc., in order to minimize the influence of the vessel material on the results of the experiments. In all experiments without exception during the electrolysis of water, he received a strong acid at the anode and an alkali at the cathode. He connected this with the fact that clean water partially decomposed the material of the vessels, which was the reason for the formation of acid and alkali. An important consequence of the experiments, however, was that the amount of acid and alkali formed at the electrodes was directly dependent on the duration of the experiments, namely: the longer they were, the more acid and alkali were formed and the stronger their concentration.
In Davy's experiments on the electrolysis of various salt solutions, a similar picture was obtained: at the anode, acid was formed with the release of oxygen, and at the cathode, an alkali was formed with the release of hydrogen or pure ammonia. These processes themselves should have prompted, by analogy, to conclusions regarding general patterns related to the electrolysis process.
After all, it is well known that during the electrolysis of various substances at the electrodesredox processes occur, but not simple decomposition of substances. Moreover, only in the presence of a redox process can electrolysis itself occur.
In this case, the oxidation reaction occurs at one electrode, and the reduction reaction at the other. Therefore it would be the biggest mistake consider electrolysis as a simple process of decomposition of substances into their constituent elements, be it water, salt or acid. Oxidation at one pole occurs with simultaneous reduction at the other, and vice versa. These provisions are the essence of the holy of holies of electrochemical processes, completely consistent with second law of thermodynamics. Indeed, if we take examples with the electrolysis of salts, it is easy to see that a reduction reaction occurred at the anode with the release of oxygen (the product of this reaction, accumulating at the anode, in all cases was some kind of acid). An oxidation reaction occurred at the cathode with the release of hydrogen or metal (the product of this reaction, accumulating at the cathode, was always some kind of alkali).

It would seem natural to extend the same pattern to water: water, as a chemical substance possessing acidic properties in many respects, in principle cannot serve as an exception in this case and simply fall apart, like some mechanical mixture, into its constituents parts where all other substances undergo complex redox processes. Therefore, a priori one could expect the formation of acid and alkali at the corresponding electrodes during the electrolysis of water. The only question is - which acid and which alkali?
But it was precisely this completely obvious thing that was rejected. The thought of her was not allowed or she simplyneglected. Moreover, this was not done by some amateurs, but by professionals high class. For them, it seems, the fact that water consists of two elements - hydrogen and oxygen - became some kind of symbol of faith, a kind of “sacred cow”, and they directed all their remarkable abilities precisely at confirming this fact, but by no means not to verify its truth. The fact that both gases were released during electrolysis, although at different electrodes, seemed to confirm this belief, even contrary to all the laws of electrolysis and thermodynamics. At the same time, no one was at all embarrassed that water could so easily be divided into its component parts, like two glued pieces of wood dropped into water.

In order to avoid any side effects, Davy conducted a series of experiments in golden vessels with well-purified water. Over the course of fourteen hours, during which the experiment lasted, the amount of acid in the anode vessel constantly increased. Davy discovered that its properties were no different fromnitrogen acids, which was formed in exactly the same way in the experiments he had previously conducted in glass vessels. In the cathode vessel, a volatile alkali was formed, the amount of which soon reached a certain limit. She discovered the property of ammonia ( NH 3).
Davy repeated his experiment and continued it without interruption for three days. By the end of this time, as he himself testifies, the water in the vessels was decomposed and evaporated to more than half of its originalinitial volume. As a result, strong nitric acid was formed in the anode vessel, while the amount of alkali remained approximately at the same level as in the previous experiment. Davy believed that the latter was due to its constant evaporation.

Davy's water is "under torture"
Was everything really so immaculately pure and good in Davy’s experiments? Let's consider Davy's experiment on the electrolysis of water under the bell of an air pump. Why in this experiment was only a small amount of acid formed in the anode vessel and no alkali was detected at all in the cathode vessel? Was it really, as Davy thought, due to the lack of air pumped out from under the bell? In part, yes, but in a completely different sense than he expected. To begin with, Davy made a serious mistake in his initial assumption that the formation of acid and alkali was caused bywas the nitrogen of the air. The formation of acid and alkali could not have anything to do with atmospheric nitrogen for the simple reason that nitrogen under normal conditions is not chemically active, does not dissolve in water and does not react with either oxygen or hydrogen. This fact alone should prompt a search for other sources of acid and alkali formation. Later, however, it was suggested that the formation of acid and alkali in the experiments was possibly caused by the presence of a certain amount of ammonium salts in the air. We were satisfied with this explanation. However, it is hardly possible to take this explanation seriously, since, firstly, it was made after the fact and, Secondly, even if a certain amount of such salts were indeed present, it should have been so small that it could not have a constant and regular formation of acid and alkali in each experiment, the amount of which was, as stated, only in direct dependence on the duration of the experiments.

The main thing, however, is not this, but what exactly happened in the experiments under the bell and why, unlike normal conditions, only a small amount of acid was formed there and there was no alkali at all. Let us consider, first of all, the possible influence of a highly rarefied atmosphere on the results of the experiment. It is known that in a rarefied atmosphere there is a rapid release of gases dissolved in it from liquids and the process of evaporation is significantly accelerated, the latter first affecting more volatile substances, and then less volatile substances. It is natural to assume that in Davy’s experiments in a highly rarefied atmosphere, first of all, the process of releasing volatile alkali from the solution began, which is partly why it was not detected in the cathode vessel. Then, since the temperatureWhen nitric acid began to sing below the boiling point of water, the nitric acid formed in the anode vessel also began to partially evaporate.

However, the side effects on the course of the experiment were not limited to this. Since during the electrolysis of water oxygen and hydrogen are released, and the volume of hydrogen released is seven times greater than the volume of oxygen, these gases, and, above all, hydrogen, could not but have an influence on the course of the experiment. If under normal conditions, i.e. not under the bell, both ammonia and hydrogen formed during the experiment evaporated and did not affect the outcome of the experiment, then under the bell these substances collected in a confined space. In this case, ammonia could partially react with the resulting nitric acid, neutralizing some of it. In addition, and this is perhaps the most important thing, hydrogen as a strong reducing agent, collecting in significant quantities under the bell, undoubtedly influenced the entire course of the reaction, giving the results that were recorded by Davy as final.
Illustration of the reducing action of hydrogen.
E If, take two electrodes, one of which is a polished silver plate, and the other is an ordinary sewing needle, place them under the bell, and pass electricity so that the electric discharge passes from the tip of the needle to the polished plate, then opposite the tip of the needle the plate will noticeably change - it will oxidize and fade, and the more, the longer the electric current is passed. If after this the air is replaced with rarefied hydrogen, then, under all other equal and unchanged conditions, further passing of current will lead to the fact that the oxide on the plate will gradually come off, and the polishing will mostly be restored.becomes, which well illustrates the reducing properties of hydrogen.

IN Another example is from the field of living nature. Claude Bernard gives the following experiment: he mixed one volume of air with two volumes of hydrogen and placed seeds in this atmosphere. Under all other favorable conditions (moisture, heat, etc.), seed germination did not occur, although the oxygen tension was quite sufficient for life activity. Obviously, the negative result was again due to the action of hydrogen, which had a strong reducing effect, interfering with the flow of the redox process, and with it the formation of its necessary products - acid and alkali.
T Retier: It is well known from physical chemistry that nitric acid is an easily reducing substance. For example, it is reduced by hydrogen to free nitrogen:
2 N 0 3 + 12Н + 10е—> N 2 + 6Н 2 0
This property of nitric acid is specially used in some galvanic cells to prevent polarization. In these cases, nitric acid is added to the cathode compartment, where hydrogen is released.
Similar processes occurred under the bell in Davy’s experiments. When he replaced air with hydrogen in the second experiment, he thereby created a powerful reducing environment there, the effect of which did not fail to affect the results: naturally no acid was (and could not be) detected in the anode vessel, but in the cathodenom - alkali. Everything was natural and natural. But the fact remains: Davy’s experiments finally convinced everyone that water consists of two simple elements - hydrogen and oxygen.

Davy It was only possible to create conditions under which, during the electrolysis of water, neither acid nor alkali were formed, which invariably form under normal, natural conditions.
However, let us assume that water actually consists of hydrogen and oxygen. Then it would be natural to assume that since water is so easily decomposed into its constituent parts, it should just as easily be formed as a result of their synthesis. Nothing of the kind, however, happens. As is known, a mixture of two gases in a ratio of one to two (one volume of oxygen and two volumes of hydrogen) produces the so-called detonating gas, but not water at all. Attempts to form water from hydrogen and oxygen were successful only in the presence of a catalyst (by the way, iron can also act as a catalyst, the same iron over which Lavoisier passed water vapor and drew his historical conclusions).
It can be said that most experiments to determine the chemical composition of water were not aimed atas much for objective searches as for adjusting their results to an existing conclusion, which has truly become an article of faith. The “black box” provided basically the information that was expected from it and which was often deliberately predetermined by a directed action on its inputs.

So, many facts of biological, chemical and physical properties does not give grounds to recognize the existing formula of water as correct. Not only empirical facts speak against it, but also theoretical provisions and, above all, those that follow from such fundamental provisions as beginning of thermodynamics . Exactly - air and vacuum
- spontaneous generation
- electrolysis of water (part 2)

F.G.Lepekhin - Electrolysis of water.The possibility of implementing an energetically favorable method for producing hydrogen in low-voltage electrolysis of water is being considered. At the same time, the estimated amount of heat that can be obtained after the combustion of hydrogen may be even greater than the energy taken from the network to carry out the hydrogen production process. In such a process, hydrogen becomes not just a “fuel”, but is actually a working fluid heat pump, since the energy required for the dissociation of water molecules into hydrogen and oxygen is obtained by reducing internal energy environment. And this is the energy of the Sun, accumulated by the Earth over millions of years of its existence. By human standards, its reserves are limitless. It is shown that this possibility does not contradict any good established laws physics, and therefore can be technically implemented.

1. Introduction

Problems of hydrogen energy in last years are discussed in the media, and at different levels - from US President D. Bush to the Presidium of the Russian Academy of Sciences. There are cars and planes that use hydrogen as fuel. Most often, the environmental purity of hydrogen as a fuel is pointed out - during combustion, water is formed, from which it, in principle, can be obtained, and is obtained in large quantities in industrial electrolyzers. Of course, it can be obtained, for example, from methane, but you need methane, or another gas that burns without extracting hydrogen from it. And in industrial electrolyzers, the energy consumption to produce hydrogen is one and a half to two times more than the heat that can be produced by burning this hydrogen. But the electricity already obtained from the combustion of hydrocarbon fuels can be converted into either heat or work, but the heat obtained from the combustion of hydrogen cannot be completely converted into either electricity or work. Producing hydrogen as a fuel, not as a raw material chemical industry for the production of another product is not economically profitable. Expensive. This is the main problem with using hydrogen as a fuel. It cannot be said that they were not looking for a solution. But the fact is that it has not yet been resolved. Is it possible to find it at all, what prevents this from happening, and in what direction should this solution be sought - all these questions will be considered in this work.

2. Physics and electrochemistry

Since the subject of consideration is the electrolysis of water, and the discovery and its main principles were studied in physics, we will start with physics. In the fundamental “Course of Physics” by O. D. Khvolson we read: “The phenomenon that occurs in an electrolyte introduced into a closed circuit is called electrolysis.” It also defines what “electrolyte”, “anion” and “cation” are. And further, in the same place: "With outside anion and cation appear to be products of decomposition of the electrolyte, and, moreover, decomposition produced by a current passing through the electrolyte." During the electrolysis of some acids and alkalis, oxygen and hydrogen are released. We see that "the current decomposes water." So we took this for granted and obvious until the second half of the 19th century.

However, in the works of Clausius (1857), Helmholtz (1880) and Arrhenius (1894), the mechanism of electrolysis was established and the foundations of the theory were created electrolytic dissociation, which are not outdated today. Clausus already pointed out that if we proceed from the idea that electric forces “decompose” the electrolyte, overcoming the force of chemical affinity, then for each chemical compound a certain electric force would be required to overcome this affinity. “In fact, even the weakest electromotive force causes electrolysis in any electrolyte” - page 564, .

Helmholtz's main merit is that he accurately pointed out the role of electric current, found out where the energy that is obviously consumed during electrolysis comes from, and which is numerically equal to the energy released during the chemical combination of electrolysis products. In the electrolysis of water, this is the energy released when hydrogen burns and produces water. According to Helmholtz, the decomposition of water during electrolysis is carried out due to the internal energy of the electrolyte, and not at all “the current decomposes the water.” This is precisely the basis for the idea of ​​using hydrogen as the working fluid of a heat pump under certain conditions for electrolysis of water. But more about this below, but for now let’s turn to electrochemistry.

She defines electrolysis as “the process of reduction or oxidation of substances on electrodes, accompanied by the acquisition or loss of electrons by particles of the substance as a result of an electrochemical reaction” (see A.I. Levin). And this is significantly different from what physics understands by electrolysis. If the goal of physics is to understand the laws of Nature, then electrochemistry solves the problem of “intensifying the production of non-ferrous, rare, noble and trace metals.” In physics: “In a circuit in which an electrolyte is included, there cannot be a current without electrolysis, i.e., the appearance of ions on the electrodes in contact with the electrolyte. For example, Oswald and Nernst (1889) showed that when passing discharge of a Leyden jar, containing only 5 * 10 -6 coulombs, through a solution of sulfuric acid, a hydrogen bubble was obtained at the cathode, the dimensions of which turned out to be quite consistent with the first law of electrolysis." And further, in the same place - “The experiments of A.P. Sokolov, who managed to prove the existence of polarization at an EMF equal to 0.001 volts, were of decisive importance here. There is no reason to assume that this has reached the limit below which polarization stops.” And the phenomenon of electrode polarization, which will be discussed later, arises as a consequence of electrolysis. Thus, in physics, electrolysis occurs at an arbitrarily low voltage on the electrodes. This is understandable - the component of the speed of chaotic movement of ions in the electrolyte under the influence of an electric field, after applying voltage to the electrodes, is not quantized. It can change by an infinitesimal amount. Note that, in contrast, the energy required, for example, to dissociate one molecule of water into oxygen and hydrogen (about 1.228 eV) is quantized. It cannot be communicated to the molecule in parts, in one collision and then in another. This must be done immediately, in one inelastic interaction.

And in electrochemistry, where the practical result is important, for example, the decomposition voltage during the electrolysis of water is understood as the voltage at which hydrogen bubbles appear on the neutral electrodes on the cathode. This concept, of course, is important in practice, but today it “...does not have a definite physical meaning.” Since this issue is important in practical terms when producing hydrogen by electrolysis, we will consider it in more detail.

3. Hydrogen evolution overvoltage

The processes that occur when current passes through an electrolyte, both in the electrolyte itself and on both electrodes, are very complex and diverse. For this reason, the results of electrolysis are often practically not reproducible. Once electrolysis has begun, and has been going on for some time, it is no longer possible to return to the original state after it has stopped. Changes will occur both in the electrolyte and on the electrodes, which will not be restored even after an arbitrarily long wait. And the beginning of electrolysis is not reproducible - this process depends on the material and condition of the electrode surface, the presence of minor impurities in it, etc. Almost the same applies to the chemical composition of the electrolyte. Therefore, even despite the fact that, due to the widespread industrial use of electrochemical processes, studies of the phenomenon of electrolysis as electrochemistry understands it have been and are being carried out by many special institutes, there is still no complete clarity of understanding of what happens during electrolysis. All the numerous details of electrolysis are beyond the scope of fundamental science. She doesn't deal with details.

But what can we say about electrolysis, when we don’t know everything about water. Thus, “There is a point of view according to which water is a mixture various kinds associated molecules, for example, 8(H 2 O), 4 (H 2 O)... and “simple” molecules H 2 O." This is trying to explain some of the anomalous properties of water. In this light, discussions about the mechanism of movement of H ions are naive + or H 3 O + in electrolysis, about processes in the double layer between the electrode and the electrolyte. It is clear that it exists even between gas and solid body, and even more so between a liquid and a solid. Of course, its role in the electrolysis process is great. But an accurate quantitative description of this role is hardly possible, and may not be necessary. “It’s worthless” from the point of view of fundamental science, as our outstanding theorist Ya. I. Frenkel said on another occasion.

Of course, there is a potential jump between the electrode and the electrolyte even without any externally applied voltage. And when it is there, and even a weak current appears, and we do not see the evolution of hydrogen at the cathode, changes begin on the electrodes in the material of the electrode, the structure of its surface, and the composition of the electrolyte near the electrode. Everything changes over time and never comes back. According to the well-known laws of physics, all processes that begin in the first moments after voltage is applied to the electrodes will be directed against the causes that caused them, i.e., against the already ongoing electrolysis process. This is Le Chatelier's principle. Complex processes of electrode polarization will begin. This is how we describe this process of counteracting the electrolysis process. An EMF appears directed against the applied voltage. The electrolysis process that has begun will almost stop. In order for it to move stationary and at the speed we need, we need to increase the external voltage. And this is “overvoltage”. But its value is not related to the “decomposition potential” or “decomposition voltage” of water, which is 1.228 volts. It depends on the current strength, on the nature of the electrodes, the state of their surface, etc. So, for tungsten, at a current density of 5 mA per square meter. see this is 0.33 volts.

It is not difficult to find the amount of energy required to decompose a water molecule into hydrogen and oxygen, knowing how much energy is released during the combustion of one gram-mole of hydrogen. But this does not have any evidentiary force that this energy is wasted precisely by current. If electrolysis occurs at a voltage on the electrodes of more than 1.228 volts, this does not mean that it is the current that consumes the energy of 1.228 eV to destroy water molecules. Yes, nowhere, except implicitly in , is this stated. But this is not a scientific, but a “...production and technical...” monograph, as stated in its abstract. Let us consider in more detail how the internal energy of the electrolyte is spent on the decomposition of water molecules into oxygen and hydrogen during the process of electrolysis. What is the mechanism of this phenomenon.

4. Mechanism of water dissociation during electrolysis

The question of how exactly “current decomposes water” and in what elementary act this occurs is not considered in electrochemistry. A.I. Levin, for example, writes: “It can be assumed that one of the following processes will take place at the anode...”, and then three processes are given in which a neutral water molecule gives 4, or 2 of its electrons to the anode, turning into H + and OH - ions. This “one can assume” is wonderful. But like a neutral molecule, it suddenly gives up its electrons. After all, she needs a “payment” for this - 1.228, 1.776 or 2.42 eV in each of the three above processes. And all at once, and not in parts. Who has this energy near the anode and can spend it on destroying the water molecule.

Further, A.I. Levin writes: “...the decrease in water observed during electrolysis... in the anolyte indicates the occurrence of its decomposition. This can apparently occur through the reaction
2H 2 O - 4 e - = O 2 + 2H +." (1)

“Apparently” - but how? Electrochemistry does not answer these questions. Yes, in fact, she does not insist that this is actually what happens. But in physics all this is available. We read from O. D. Khvolson: “A reaction occurs at the anode
SO 4 + H 2 O = H 2 SO 4 + O..." (2)

And the neutral residue of sulfuric acid is obtained from a negative ion, which is neutralized at the anode. The resulting sulfuric acid molecule immediately breaks down into ions, replenishing their loss at the anode and cathode. According to this scenario, the concentration of water molecules in the “anolyte” actually decreases. Water decomposes. But according to a different reaction. The discharge of negative SO 4 2- ions at the anode seems quite natural. True, O. D. Khvolson lists a whole bunch of chemical reactions that take place in the electrolyte. But what is important to us is the general line, not the details.

Now where does this minimum energy of 1.228 eV come from, which still needs to be spent in one act? Physics knows the answer to this question too. At normal pressure, and a temperature of 2000 degrees, without any electrolysis, 0.081% of all water molecules are dissociated. At 5000 degrees, 95.4% of all water molecules already disintegrate. This occurs in acts of inelastic interaction between two neutral water molecules. Such processes are well known to us in particle physics.

The reaction probability is equal to the product of this phase volume and the matrix element. In the absence of particle resonances in this system, it is usually set to unity. As the energy increases above the threshold, the probability of a reaction increases sharply - the impulse part of the phase volume grows like the cube of the impulse in the SDH system. In our case, the greater the energy of two water molecules in their SCI, i.e., the greater the relative and absolute velocities of the colliding molecules, the greater the probability of one of them decaying into hydrogen and oxygen in the act of an inelastic collision of two particles. This is observed as the temperature increases. The distribution of molecular speeds is described by the Maxwell distribution. It always contains a “tail” of high-energy molecules. It is they who will be eliminated during the “self-disintegration” of water at any temperature. The same happens during electrolysis in reaction (2). The removal of molecules with high speeds from the velocity distribution leads to a decrease in the average speed of all molecules. The average speed is proportional to the temperature. Both during the “self-disintegration” of water molecules and during the electrolysis of water, the energy for the dissociation of water molecules is obtained by reducing the internal energy of the liquid, i.e. due to its cooling in these processes.

Of course, the work of the current in the electrolyte, as in any conductor, is also spent on heating it. The ions, coming into accelerated motion in the direction of the electric field, elastically interact with neutral water molecules and transfer part of their energy to them, heating the electrolyte. If this change in the internal energy of the electrolyte due to its heating by current is equal to or greater than the decrease in the internal energy of the electrolyte spent on the decomposition of water molecules, then its temperature will be constant, or it will heat up. This is what happens in industrial electrolyzers. An illusion is created: “the current decomposes the water.” If in fact this is not the case, it is not the “current decomposes the ox”, and it is not the magnitude of the “decomposition voltage” that prevents the electrolysis process at low voltage, when the electrolyte must be cooled, then how can this be accomplished? What reasons actually prevent this?

5. Heat pump

The most interesting and effective of all attempts to implement low-voltage electrolysis so far can be considered the electric-hydrogen generator (EVG) of V.V. Studennikov. His proposal is based on the work of R. Colley (1873), who discovered a new source of EMF. It was shown that if the electrodes in the electrolyzer are not placed vertically, at the same height, when the ions move horizontally, but are spaced apart in height, then due to the difference in the masses of the positive and negative ions, now moving up and down in the Earth’s gravitational field, EMF will occur. The artificial gravitational field generated by rotation gives the Tolman-Stewart effect. They have a link to the work of R. Colley. In patents, this effect is used in the design of electrolyzers with electrolyte rotation. It was patented in the USA in 1929 and 1964. A quantitative study of the effect of reducing the anode and cathode potential differences obtained by rotating the electrolyzer was published in.

As V.V. Studennikov claimed, he managed to obtain “... intense self-cooling of the solution, providing conditions for absorbing heat from the environment... i.e., operating in the mode... of a heat pump.” Unfortunately, this statement was contained in a message posted on the Internet by V.V. Studennikov himself, but its scientific publication never appeared. However, the fact of indicating the possibility of using hydrogen as the working fluid of a heat pump belongs to V.V. Studennikov. The possibility of a cheaper way to produce hydrogen as a fuel looks rather pale in comparison. Of course, the processes occurring in EVG can be even more complex than in the classical electrolysis scheme. Two facts seem important. Firstly, during rotation, the electrolyte constantly rubs against the electrodes, “renewing” them. This leads to a decrease in polarization emf. And secondly, there is no external source of EMF. Electrolysis occurs due to the internal voltage drop of the EMF source. And the electrolyte resistance is low. This means that the voltage drop is also small. Hence the self-cooling of the electrolyte. The fatal drawback of EVG is the very expensive method of generating EMF using the energy of the gravitational field. It cannot be compared in any way with the generation of EMF when a conductor moves in a magnetic field. At least, there is no evidence that in EVG the EMF is not really generated simply by rotating the electrolyte in the Earth’s magnetic field. Well, the statement that in addition to hydrogen, there is also a source of constant voltage in the external circuit looks completely strange. We need to decide - either we get hydrogen by cooling the environment, or we design new car for the production of electricity.

6. Prospects

Research in the field of hydrogen energy in Russia alone is carried out by 20 institutes of the Russian Academy of Sciences. Some of them have been doing this for 20 years. Fuel cells have been created that are used in space research. But it will most likely not come to their widespread production and introduction into our everyday life for a long time. The scientific value of the contribution of RAS institutes in this area is, to put it mildly, not great. The main problem of hydrogen energy, which was mentioned in the introduction, is not solved by them, and will not be solved. There is no customer. Improving industrial electrolyzers using traditional electrolysis is also futile.

Only unconventional methods its solutions, which are the lot of individual inventors. But among them there are quite a few dubious, and often simply illiterate, proposals and statements. An example of this is "Kazakova's Eternal Energy" from Alma-Ata. This is what a correspondent writes about this work, who perhaps simply did not understand Kazakov’s work well. Kazakov uses infrasound, and claims that “self-electrolysis of water” occurs at tremendous speed. This phenomenon is unknown in physics. In one second, 9 cubic meters of hydrogen are obtained, i.e., about 7 liters of water per second “self-disintegrates” into hydrogen and oxygen. If this is true, then the installation capacity is 95 MW. If there were about 200 liters of water in the tank, then in 2-3 seconds it should have frozen. True, the author only needed 100 thousand dollars to release an industrial design and make humanity happy. Scientific publications As a rule, there are no specialists of this kind on this topic. They often criticize conservative “official science.” Checks of such applicants always reveal that they, out of simplicity of heart or ignorance, are wishful thinking.

It is possible that from all that has been said, only Studennikov’s EVG may have some prospects if it works in tandem with a conventional compression heat pump. Then it will utilize the heat of the environment with a conventional heat pump and produce hydrogen with a conversion coefficient common to both it and the heat pump, even slightly greater than one. But all this still needs to be done and done. The main thing that I wanted to show here is that there are no fundamental obstacles, including the need to overcome the “water decomposition potential” by increasing the voltage applied to the electrodes.

Literature

1. O. D. Khvolson, Course of Physics, RSFSR, Gosizdat, Berlin, 1923, vol. 4.
2. A. I. Levin, Theoretical foundations of electrochemistry, State. Scientific and technical Publishing house, Moscow, 1963.
3. A. P. Sokolov, ZhRFKhO, vol. 28, p. 129, 1896.
4. Phys. Encycl. Slov., ed. "Soviet Encyclopedia", Moscow, 1960, vol. 1, p. 288.
5. L. M. Yakimenko et al., Electrolysis of water, ed. "chemistry", Moscow, 1970.
6. Stanley Meyer Cell
7. EVG Studennikov
8. R. Colley, Journal of the Russian Chemical Society and the Physical Society at St. Petersburg University, vol. 7, Physical Part, St. Petersburg, 1873, p. 333.
9. R. C. Tolman, T. D. Stsward, Phys. Rev. 8, 97, 1916.
10. E. Thomson, U.S. Pat. 1, 701.346(1929).
11. T. B. Hoover, U. S. Pat. 3, 119, 759(1964).
12. H. Cheng at al., Jorn. Of the Electrochemical Society, 149(11), D172-D177(2002).

Electrolysis of water- This is a well-known electrolysis process for everyone who is familiar with technology, in which water is used as an electrolyte.

However, it should be noted that water is always present during electrolysis. First, let's look at what the electrolysis process is in general.

Electrolysis

Electrolysis is an electrochemical process that is carried out by placing two electrodes in an electrolyte and connecting a direct current to them.

Electrolytes are called liquid conductors, which belong to the second type of conductors. Liquid conductors mean liquids/solutions that have electrical conductivity.

For reference, we add that the vessels into which electrolytes are poured are called galvanic baths.

During the electrolysis process, ions are exposed to electromagnetic field, formed in the electrolyte by direct electric current, begin to move towards the electrodes. Ions with a positive charge, in accordance with the laws of physics, move towards an electrode with a negative charge, which is called CATHODE, and negatively charged ions accordingly move towards another electrode, called ANODE. Electrolysis is accompanied by the release of substances on the electrodes, which indicates the movement of atoms in electrolytes. For example, metals and hydrogen are typically released at the CATHODE.

The electrolysis process is influenced by several factors:

• current strength connected to the electrodes;
• ion potential;
• electrolyte composition;
• the material from which the electrodes are made - CATHODE and ANODE.

Electrolysis of water

As we noted above, electrolysis of water involves the use of water as an electrolyte.

Typically, in the electrolysis of water, for better passage process, a little of some substance is added to the water, for example baking soda, but not necessarily, because plain water almost always already contains impurities.

As a result of the electrolysis of water, hydrogen and oxygen are released. Oxygen will be released at the ANODE, and hydrogen at the CATHODE.

Application of water electrolysis

Water electrolysis technology is used:

• in water purification systems from all kinds of impurities;
• to produce hydrogen. Hydrogen, for example, is used in very promising industry- hydrogen energy. We have already written about this in more detail in our material.

As we see, water electrolysis, despite its apparent simplicity, is used in very important areas - in areas on which the development and prosperity of our entire civilization depends.

In which a liquid, or, in other words, an electrolyte, breaks down into positive and negative ions. This happens under the influence of electric current. How does it proceed? this process?

Electrolysis of water occurs due to the fact that an electric current passing through the electrolyte causes a reaction at the electrodes, on which positive and negative ions are deposited. Cations are deposited on the negatively charged electrode (cathode), and anions are deposited on the positive (anode). The electrolyte may consist of water to which an acid has been added, or it may be a solution of salts. The decomposition of salts into metal and an acidic residue occurs after an electric current is passed through the electrolyte. A metal charged with positive electricity approaches the cathode (negatively charged electrode), and it is this metal that is called a cation. The acidic residue, negatively charged, tends to the anode (positively charged electrode), and is called an anion. Electrolysis makes it possible to obtain well-purified elements from salts, due to which it is widely used in various branches of modern industry.

Electrolysis of water is vital today, when thousands of enterprises use water for individual stages of their production. This is explained by the fact that after most of the processes that are carried out in enterprises, water after use turns into a liquid dangerous for people and wildlife. Electrolysis of water serves to purify Wastewater, which should not fall into the ground or into sources clean water. This wastewater must be treated to prevent ecological disaster, the risk of which is already quite high in many regions of Russia.

Today there are several methods of water electrolysis. These include electroextraction, electrocoagulation and electroflotation. Electrolysis of water used for wastewater treatment is carried out in electrolyzers. These are special structures in which metals, acids and other substances classified as inorganic origin are decomposed. It is especially important to carry out wastewater treatment in hazardous industries, such as chemical industry enterprises, where work is carried out with copper and lead, as well as in factories producing paints, varnishes, and enamels. Of course, this is far from cheap way water purification using electrolysis, but the costs associated with water purification cannot be compared with human health and concern for the environment.

Interesting fact, but you can carry out electrolysis of water at home. This process will not take much time and money and will provide the opportunity for hydrogen. Two electrodes are lowered into a container of water in which salt has been previously dissolved (salt must be taken at least ¼ of the volume of water). They can be made from any metal. The electrodes are connected to a power source with a current of at least 0.5 A. Bubbles form on one of the electrodes, which indicates that water electrolysis at home is successful. Using this method you can obtain sodium hydroxide, chlorine and other chemical elements, depending on what the electrolyte consists of. Plasma electrolysis of water is used in plasma heaters. This is the latest modern device that operates in modes of plasma electrolysis of water and its direct heating to certain temperatures. Plasma electrolysis of water makes it possible to obtain new types of energy, which humanity needs more and more every day. The energy that can be obtained from water will provide an opportunity to create new, safe and efficient types of energy sources. The phenomena of plasma electrolysis of water have not yet been fully studied, but they have great prospects and are therefore intensively studied by modern scientists.