Batteries Nickel-hydride metal having powerful electrodes and connections of low-resistance electrodes.

Conventional storage methods (in cylinders) of compressed or liquefied hydrogen are a sufficiently dangerous occupation. In addition, hydrogen penetrates very actively through most metals and alloys, which makes the shut-off and transport reinforcement very expensive.

The hydrogen property is known to dissolve in metals from the 19th century, but only now the prospects for the use of metal hydrides and intermetallic compounds are visible as compact hydrogen warehouses.

Types of hydrides

The hydrides are divided into three types (some hydrides may have several properties of bonds, for example, being metal-covalent): metal, ion and covalent.

Ionic hydrides -as a rule, it is created at high pressures (~ 100 atm.) and at temperatures more than 100 ° C. Typical representatives - alkali metal hydrides. An interesting feature of ionic hydrides is a large degree of density of atoms than in the starting matter.

Covalent hydrides - practically do not find applications due to low stability and high toxicity of metals used and intermetallic. A typical representative - hydride beryllium, obtained by the "wet chemistry" by the dimethylberyl radiation with lithium-fluid cride in the diethyl ether solution.

Metal hydrides - It can be considered as alloys of metallic hydrogen, these compounds are characterized by high electrical conductivity as both maternal metals. Metal hydrides form almost all transition metals. Depending on the types of bonds, metal hydrides may be covalent (for example, magnesium hydride) and ionic. Almost all metalhydrides require high temperatures for dehydrogenation (hydrogen return response).

Typical metal hydrides

• Lead hydride - PBH4 - binary inorganic chemical switches with hydrogen. Very active, in the presence of oxygen (in air) is self-propagated.

• Zinc hydroxide - Zn (OH) 2 - amphoteric hydroxide. Widespread as a reagent in many chemical production.

• Palladium hydride is a metal in which hydrogen is between palladium atoms.
• Nickel hydride - Nih - is often used with Lantane additives LANI5 for batteries electrodes.

Metal hydrides can form the following metals:
Ni, Fe, Ni, Co, Cu, PD, PT, RH, PD-PT, PD-RH, MO-FE, AG-CU, AU-CU, CU-NI, CU-PT, CU-SN.

Metals and record holders for the volume of hydrogen

The best metal for hydrogen storage is palladium (PD). In one volume of palladium, almost 850 hydrogen volumes can be "packaged". But the prospect of such a repository causes strong doubts in view of the high cost of this metal platinum group.
In contrast, some metals (for example, copper Cu) dissolve only 0.6 of hydrogen volume per volume of copper.

Magnesium hydride (MGH2) can store up to 7.6% of the mass fractions of hydrogen in the crystal lattice. Despite the tempting values \u200b\u200band the small share of such systems, the obvious obstacle are high temperatures of the direct and reverse reaction charge-discharge and high endothermic losses in dehydrogenation of the compound (about a third of the energy of stained hydrogen).
Crystal structure of the β-phase hydride MGH2 (drawing)

Accumulation of hydrogen in metals

The reaction of the absorption of hydrogen with metals and intermetallices occurs with greater pressure than its selection. This is determined by the residual plastic deformations of the crystal lattice during the transition from the saturated α-solution (incense) to the β-hydride (substance with stored hydrogen).

Metals not dissolving hydrogen

The following metals are not absorbed by hydrogen:
AG, AU, CD, PB, SN, ZN
Some of them are used as shut-off valves for storing compressed and liquefied hydrogen.

Low-temperature metallic hydrides are one of the most promising hydrides. They have small loss values \u200b\u200bfor dehydrogenation, high speed of charge-discharge cycles are almost completely safe and low toxic. The restriction is a relatively small specific density of hydrogen storage. Theoretical maximum is storage of 3%, and in reality 1-2% of the mass fractions of hydrogen.

The use of powdered metal hydrides imposes restrictions on the rate of cycles "charge-discharge" due to low thermal conductivity of powders and require a special approach to the construction of containers for storage. Typical is the introduction into the storage capacity of areas that contribute to the transfer of heat and the manufacture of thin and flat cylinders. Some increase in the rate of discharge-charge cycles can be achieved by introducing an inert binder to the metal hydride, having a large thermal conductivity and a high threshold of inertia to hydrogen and the base substance.

Intermetallic hydrides

In addition to metals, the storage of hydrogen in the so-called "intermetallic connections" is promising. Such hydrogen storage facilities were widely used in household metal hydride batteries. The advantage of such systems is a sufficiently low cost of reagents and a small harm to the environment. Currently, metal hydride batteries are almost universally repulsed by lithium energy accumulation systems. The maximum basic energy of industrial samples in nickel-metal-hydride batteries (Ni-MH) is 75 W · h / kg.

An important property of some intermetallic is high resistance to impurities contained in hydrogen. This property allows you to exploit similar compounds in polluted media and in the presence of moisture. The multiple "charge-discharge" cycles in the presence of contamination and water in hydrogen do not poison the working substance, but reduce the capacity of subsequent cycles. The decrease in the useful tank occurs due to the contamination by oxides of the base substance metals.

Separation of intermetallic hydrides

Intermetallic hydrides are divided into high-temperature (dehydrating at room temperature) and high-temperature (more than 100 ° C). The pressure at which the hydride phase is placed) is usually not more than 1 atm.
In real practice, complex intermetallic hydrides consisting of three or more elements are applied.

Typical intermetallic hydrides

Lanthan Nickel hydride - LANI5 - hydride, in which one unit LANI5 contains more than 6 atoms N. The desorption of hydrogen from lanthanum nickel is possible at room temperatures. However, elements included in this intermetallide are also very underlying.
In a unit, the volume of lanthana-nickel is one and a half times more hydrogen than in liquid H2.

Features of intermetal hydrogen systems:

• high hydrogen content in hydride (wt.%);
• exo (Endo) -Tellic absorption reaction (desorption) of hydrogen isotopes;
• change in the volume of the metal matrix in the process of absorption - desorption of hydrogen;
• revolving and selective hydrogen absorption.

Areas of practical use of intermetallic hydrides:

• stationary hydrogen storage;
• mobility of repository and transportation of hydrogen;
• compressors;
• department (cleaning) of hydrogen;
• heat pumps and air conditioners.

Examples of applying metal hydrogen systems:

• thin hydrogen purification, all sorts of hydrogen filters;
• reagents for powder metallurgy;
• moderators and reflectors in nuclear fission systems (nuclear reactors);
• separation of isotopes;
• thermonuclear reactors;
• water dissociation settings (electrolyzers, vortex chambers of producing hydrogen gas);
• electrodes for batteries based on tungsten-hydrogen systems;
• metal hydride batteries;
• air conditioners (thermal pumps);
• transducers for power plants (nuclear reactors, CHP);
• transportation of hydrogen.

The article mentions metals:

Let's start with the composition of the implementation compounds. Consider this question on the example of hydrides of transitional elements. If, in the formation of the phase of implementation, the hydrogen atoms fall into tetrahedral voids in the metal grille, the limit content of hydrogen in such a compound should correspond to the MEN 2 formula (where the metal, the atoms of which form dense packaging). After all, tetrahedral voids in the lattice are twice as many than atoms forming a dense packaging. If the hydrogen atoms also fall into octahedral emptiness, then from the same considerations it follows that the limit content of hydrogen must comply with the formula of men, - octahedral voids in a dense packaging as much as the categories of this packaging of atoms.

Usually, in the formation of compounds of transition metals with hydrogen, either octahedral or tetrahedral voids is filled. Depending on the nature of the initial substances and the conditions of the process, the complete or only partial filling can occur. In the latter case, the composition of the compound will deviate from the integer formula, it will be undefined, for example, men 1-x; Men 2-x. Connections of implementation, therefore, by its very nature should be variable compounds,i.e., those whose composition, depending on the conditions for their production and further processing, changes in fairly wide limits.

Consider some typical properties of the implementation phases on the example of compounds with hydrogen. To do this, compare the hydrides of some transition elements with an alkali metal hydride (lithium).

When lithium compounds with hydrogen, a substance of a certain composition of LIH is formed. In physical properties, it has nothing to do with the starting metal. Lithium conducts electric current, has a metal glitter, plasticity, a word, a whole complex of metallic properties. The hydride of lithium does not have any of these properties. This is a colorless salt-like substance, not at all similar to the metal. Like other alkaline and alkaline earth metal hydrides, lithium hydride is a typical ionic compound, where lithium atom has a significant positive charge, and a hydrogen atom is the same negative charge. Lithium density is 0.53 g / cm 3, and lithium hydride density 0.82 g / cm 3 - occurs noticeable increasing density. (The same is observed in the formation of hydrides of other alkaline and alkaline earth metals).

Completely different transformations undergo palladium (typical transitional element) when interacting with hydrogen. A demonstration experience is well known in which the palladium plate coated with a gas-tight varnish bended when hydrogen is bleed.

This is because the density of the resulting hydride palladium decreases. Such a phenomenon may occur only if the distance between the metal atoms increases. The atoms of the deployed hydrogen "sweep" metal atoms, changing the characteristics of the crystal lattice.

An increase in the volume of metals when the hydrogen is absorbed to form the phase of the introduction occurs so much noticeably that the density of a metal saturated with hydrogen is significantly lower, the density of the original metal (see Table 2)

Strictly speaking, the grill formed by the metal atoms usually does not remain completely unchanged after absorbing this metal of hydrogen. Like a small hydrogen atom, it still makes a distortion in the lattice. In this case, it is usually not just a proportional increase in distances between atoms in the lattice, but also a change in its symmetry. Therefore, often only for ease, it is said that hydrogen atoms are introduced in voids in dense packaging - the dense packaging of metal atoms in itself is still violated during the introduction of hydrogen atoms.

Table 2 Changes in the density of some transition metals in the formation of phases of introduction with hydrogen.

This is far from the only difference between the hydrides of typical and transition metals.

In the formation of hydrides of the introduction, such typical properties of metals are preserved as metal shine, electrical conductivity. True, they can be less pronounced than in the starting metal. Thus, the introduction hydrides are much more similar to the original metals than alkaline and alkaline earth metal hydrides.

Such a property is much stronger than the plasticity - hydrogen-saturated metals are made fragile, often the starting metals are difficult to turn into a powder, and with the hydrides of the same metals it is much easier.

Finally, it is necessary to note a very important property of hydrides of the introduction. In the interaction of transition metals with hydrogen, the metal sample is not destroyed. Moreover, it retains its original form. This occurs during the reverse process - decomposition of hydrides (loss of hydrogen).

A natural question may occur: is it possible to consider the process of formation of phases of introduction of chemical in the full sense of the word? Is the formation of aqueous solutions - a process having much more "chemistry"?

To respond, you must attract chemical thermodynamics.

It is known that the formation of chemical compounds from simple substances (as well as other chemical processes) is usually accompanied by noticeable energy effects. Most often, these effects are exothermic, and, the more energy it is allocated, the stronger the obtained connection.

Thermal effects are one of the most important signs that there is no simple mixing of substances, but the chemical reaction proceeds. Once the internal energy of the system changes, therefore, the formation of new links.

Let's see now, what energy effects cause the formation of hydrides of the introduction. It turns out that the scatter is large here is quite large. At metals of side subgroups of III, IV and V groups of the periodic system, the formation of hydrides of the introduction is accompanied by significant heat release, about 30--50 kcal / mol (in the formation of lithium hydride from simple substances, about 21 kcal / mol is released). It can be recognized that the hydrides of the introduction, at least the elements of the specified subgroups, are quite "real" chemical compounds. However, it should be noted that for many metals located in the second half of each transition range (for example, for iron, nickel, copper), the energy effects of the formation of hydrides of the introduction of smallness. For example, for hydride of the approximate composition of the FEH 2 thermal effect is only 0.2 kcal / mol .

The small size of the training of such hydrides dictates the methods of obtaining them - not a direct interaction of the metal with hydrogen, but an indirect path.

Consider several examples.

Nickel hydride, the composition of which is close to Nih 2, can be obtained by acting on the ethereal solution of nickel chloride with phenylmagniybromide in the current N 2:

The hydride obtained as a result of this reaction is a black powder, easily deriving hydrogen (which is generally characteristic of the hydrides of the introduction), with a slight heating in the oxygen atmosphere, it flammifies.

In the same way, hydrides of nickel neighbors on the periodic system - cobalt and iron can be obtained.

At the heart of another method of obtaining transition hydrides is the use of lithium alanate LIALH in the interaction of the corresponding metal chloride with LiAlh 4 in the ethereal solution, the alanate of this metal is formed:

Mecl 2 + LiAlh 4 \u003e ME (Alh 4 ) 2 + LICL(5)

For many metals, alanates are fragile compounds that are disintegrating with increasing temperature.

ME (Alh. 4 ) 2 \u003e Meh. 2 + Al + H 2 (6)

But for some metals, the side subgroups flows a different process:

ME (Alh. 4 ) 2 \u003e Meh. 2 + Alh. 3 (7)

In this case, instead of a mixture of hydrogen and aluminum, aluminum hydride is formed, which is soluble on the air. Washing the reaction product by ether, it is possible to obtain a clean hydride of the transition metal in the residue. In this way, such as low-resistant zinc hydrides, cadmium and mercury were obtained.

It can be concluded that the preparation of hydrides of elements of side subgroups is based on typical methods of inorganic synthesis: exchange reactions, thermal decomposition of fragile compounds under certain conditions, etc. The hydrides of almost all transition elements were obtained, even very fragile. The composition of the hydride obtained is usually close to stoichiometric: FEH 2, Coh 2, Nih 2 ZnH 2, CDH 2, HGH 2. Apparently, the achievement of stoichiometry contributes to a low temperature at which these reactions are carried out.

We will analyze, now the influence of the reaction conditions to the composition of the hydrides of the introduction. It flows directly from the principle of leschatel. The higher the pressure of hydrogen and below the temperature, the closer to the maximum value of the metal saturation with hydrogen. In other words, each specific temperature and each magnitude of the pressure corresponds to a certain degree of metallium saturation with hydrogen. Conversely, each temperature corresponds to a certain equilibrium hydrogen pressure above the metal surface.

From here, it stems one of the possible applications of transitional elements. Suppose, in some system you need to create a strictly defined pressure of hydrogen. A metal saturated with hydrogen is placed in such a system (titanium was used in experiments). It can be heated to a certain temperature, you can create in the system the necessary pressure of gaseous hydrogen.

Any class of compounds is interesting for its chemical nature, the composition and structure of particles of which consists of the nature of the relationship between these particles. This chemists are dedicated to their theoretical and experimental work. Are no exception to the implementation phase.

The final point of view on the nature of the hydride introduction is not yet. Often different, sometimes the opposite points of view successfully explain the same facts. In other words, there are no single theoretical views on the structure and properties of implementation compounds.

Consider some experimental facts.

The process of absorbing hydrogen palladium was studied in the most detailed. For this transition metal, it is characteristic that the concentration of hydrogen dissolved in it at a constant temperature is proportional to the square root from the external pressure of hydrogen.

At any temperature, hydrogen is to some extent, dissociates to free atoms, so there is a balance:

Constant of this balance:

where r N. - pressure (concentration) of atomic hydrogen.

From here (11)

It can be seen that the concentration of atomic hydrogen in the gas phase is proportional to the root square from the pressure (concentration) of molecular hydrogen. But the same magnitude is proportional to the concentration of hydrogen in palladium.

From here we can conclude that palladium dissolves hydrogen as individual atoms.

What, in this case, the nature of communication in the palladium hydride? A number of experiments were done to answer this question.

It was found that when the electric current is passed through the palladium saturated palladium, the nonmetal atoms are moved to the cathode. It should be assumed that the hydrogen-produced metal in the lattice is completely or partially dissociated on protons (i.e. ions H +) and electrons.

Data on the electron structure of the palladium hydride was obtained in the study of magnetic properties. The change in the magnetic properties of the hydride from the amount of hydrogen, which entered the structure was investigated. Based on the study of magnetic properties of the substance, it is possible to estimate which the number of unpaired electrons is contained in particles from which this substance consists. On average, one palladium atom accounts for about 0.55 unpaired electrons. When saturated palladium hydrogen, the number of unpaired electrons decreases. And in the substance of the composition PDH 0, 55, unpaired electrons are practically absent.

Based on these data, it can be concluded: the unpaired palladium electrons form pairs with unpaired electrons of hydrogen atoms.

However, the properties of hydrides of the introduction (in particular, electrical and magnetic) can be explained on the basis of the opposite hypothesis. It can be assumed that in the hydrides of the introduction there are ions H - formed by capturing the hydrogen atoms of a part of the semi-free electrons existing in the metal lattice. In this case, the electrons obtained from the metal would also form pairs with electrons existing on hydrogen atoms. This approach also explains the results of magnetic measurements.

Perhaps, both types of ions coexist in hydrides. Metal electrons and hydrogen electrons form pairs and, therefore, a covalent connection occurs. These electronic pairs can be shifted to one degree or another to one of the atoms - metal or hydrogen.

The electron pair is shifted stronger to the metal atom in the hydrides of those metals, which are less inclined to give electrons, for example in palladium hydrides or nickel. But in the hydrides of Scandia and Uranus, apparently, the electronic pair is strongly shifted towards hydrogen. Therefore, hydrides of lanthanides and actinoids are largely similar to hydrides of alkaline earth metals. By the way, the lanthanium hydride reaches the composition of Lah 3. For typical hydrides of the introduction of hydrogen content, as we now know not higher than the corresponding formulas of men or men 2.

Another experimental fact shows the difficulties of determining the nature of communication in the hydrides of implementation.

If at low temperatures remove hydrodium from hydride hydrogen, then it is possible to maintain a distorted ("expanded") grid, which was in hydrogen saturated palladium. Magnetic properties (tick it), electrical conductivity and hardness in such palladium generally the same as the hydride was.

It follows that in the formation of the introduction hydrides, the change in the properties is caused not only by the presence of hydrogen in them, but also simply by changing the interatomic distances in the lattice.

We have to admit that the question of the nature of the introduction hydrides is very complex and far from the final permit.

Humanity has always been famous for, not even knowing all the aspects of any phenomena, it skillfully practically use these phenomena. This fully applies to the hydrides of the introduction.

The formation of hydrides of the introduction in some cases is deliberately used in practice, in other cases, on the contrary, they are trying to avoid.

The introduction hydrides relatively easily give hydrogen when heated, and sometimes at low temperatures. Where can I use this property? Of course in rediscovering processes. Moreover, hydride-given hydrodes at some stage of the process is in atomic state. This is probably associated with the chemical activity of the introduction hydrides.

It is known that good catalysts for reactions in which hydrogen is attached to any substance are metals of the eighth group (iron, nickel, platinum). Perhaps their catalytic role is associated with the intermediate formation of fragile implanting hydrides. Dissociating further, hydrides provide a reaction system with a certain amount of atomic hydrogen.

For example, fine-dispersed platinum (the so-called platinum black) catalyzes the hydrogen oxidation reaction - in its presence, this reaction comes at a noticeable speed even at room temperature. This property of platinum mobile is used in fuel cells - devices where chemical reactions are used to directly produce electrical energy, bypassing the heat (burning stage). On the same property of fine platinum, the so-called hydrogen electrode is based - an important tool for studying the electrochemical properties of solutions.

The formation of introduction hydrides are used to obtain particularly clean metal powders. Metal uranium and other actinides, as well as very clean titanium and vanadium plastic, and therefore it is almost impossible to prepare powders from them by the method of rubbing metal. To deprive the metal of plasticity, it is saturated with hydrogen (this operation is called "the embrittlement" of metal). The hydride formed is easily triturated into powder. Some metals already when saturated with hydrogen themselves go to the powder state (uranium). Then, when heated in vacuo, hydrogen is removed and the pure metal powder remains.

Thermal decomposition of some hydrides (UH 3, TIH 2) can be used to obtain pure hydrogen.

The most interesting areas of the use of titanium hydride are most interesting. It is used for the production of foametal (for example, foaming). For this, the hydride is injected into molten aluminum. At high temperature, it decomposes, and hydrogen bubbles formed foaming liquid aluminum.

Titanium hydride can be used as a reducing agent of oxides of some metals. It can serve as a solder for connecting metal parts, and a substance accelerating the process of sintering metal particles in powder metallurgy. In the latter two cases, the reduction properties of the hydride are also used. On the surface of the metal particles, and the metal parts are usually formed by a layer of oxides. It prevents the adhesion of neighboring plots of metal. Titanium hydride during heating restores these oxides, thereby cleaning the metal surface.

Titanium hydride is used to obtain some special alloys. If you decide it on the surface of the copper product, the thin layer of copper alloy with titanium is formed. This layer gives the surface of the product special mechanical properties. Thus, several important properties (electrical conductivity, strength, hardness, abrasion resistance, etc. can be combined in one product, etc.).

Finally, titanium hydride is a very effective means to protect against neutrons, gamma rays and other hard radiation.

Sometimes it is necessary to deal with the formation of hydrides of introduction. In metallurgy, in chemical, oil and other industries, hydrogen or its compounds are under pressure and at high temperatures. In such conditions, hydrogen may diffusely diffuse through the heated metal, simply "leave" from the equipment. In addition (and this is perhaps the most important thing!), Due to the formation of hydrides of the introduction, the strength of metal equipment can strongly decrease. And this is already taking a serious danger when working with high pressures.

Hydride Nickel Describes alloy made, combining nickel and hydrogen. The hydrogen content in the nickel hydride is up to 0.002% per milk.

Hydrogen acts as a strengthened agent, preventing dislocations in the nickel atom crystal lattice from slipping by each other. The change in the amount of hydrogen alloy and the form of its presence in the hydride of nickel (accelerated phase) manages the qualities such as hardness, fuel and strength of the resulting nickel hydride. Nickel hydride with increased hydrogen content can be made more solid and stronger than nickel, but such a nickel hydride is also less complicated than nickel. The loss of adequacy occurs due to cracks that support sharp points due to the suppression of the elastic deformation by hydrogen and voids formed under tension due to the decomposition of the hydride. Hydrogen Embrittlement may be a problem in nickel to use in turbines at high temperatures.

In a narrow assortment of concentrations that make up the hydride of nickel, a mixture of hydrogen and nickel can only form several different structures with completely different properties. Understanding such properties is important for the creation of high-quality nickel hydride. At room temperature, the most stable shape of the nickel is the structure of the cubic (FCC) α-Nickel structure focused on the face. This is a rather mild metal material that can only terminate a very small concentration of hydrogen, no more than 0.002% of the weight in, and only 0.00005% in. The phase of the solid solution with dissolved hydrogen, which supports the same crystalline structure as the original nickel, is called α-phase. 25 ° C 6 of the hydrogen pressure kbar is needed to get enough into B \u003d Nickel, but hydrogen returns from the solution if the pressure drops below 3.4 kbar.

Surface

Hydrogen bond atoms strongly with nickel surface, with hydrogen molecules separating to do so.

Dihydrogen disconnection requires a sufficient amount of energy to cross the barrier. On nor (111), the crystalline surface of the barrier is 46 kJ / molecular weights, whereas on nor (100) the barrier is 52 kJ / molecular weights. In no (110), the crystalline surface of the aircraft is the lowest activation energy to break the hydrogen molecule in 36 kJ / molecular weights. The surface layer of hydrogen on the nickel can be released, warming up. Nor (111) lost hydrogen between 320 and 380 K. Nor (100) lost hydrogen between 220 and 360 K. Nor (110) crystalline surfaces lost hydrogen between 230 and 430 K.

To get enough sleep in nickel, hydrogen must migrate from the surface through the face of the nickel crystal. This does not occur in vacuo, but may occur when other molecules affect the coated surface of the nickel of hydrogen. Molecules should not be hydrogen, but they seem to work as hammers that hit the fist hydrogen atoms through the nickel surface to the depths. The activation energy of 100 kJ / molecular masses is required to penetrate the surface.

High pressure phases

The true crystallographic manner is an excellent phase of nickel hydride can be produced with a high-pressure hydrogen gas of 600 MPa. Alternatively, this can be produced electrolytic manner. Crystal shape is a concentrated cubic or β-nickel s hydride. Hydrogen to nickel atomic relations up to one with hydrogen, which occupies an eight-marginal place. The density of β-hydride is 7.74 g / cm. It is painted in gray. In the current density of 1 amp per square decimeter in 0.5 molecular weights / liter of sulfuric acid and thiourea, the surface layer of nickel will be converted to the hydride of nickel. This surface is overflowing, praises to millimeters long. The direction of hacker is located in (001) the aircraft of the original nickel crystals. The lattice constant from the nickel hydride is 3.731 Å, which is 5.7% more than that of nickel.