Technical considerations　Hydrogen storage alloy

Hydrogen storage alloy
 
Hydrogen has the property of easily penetrating into metal bonds with free electrons. As a result, all metals are known to have problems such as bubbles being mixed in during solidification and strength deterioration (hydrogen embrittlement) due to hydrogen dissolution or intrusion. In response to this phenomenon, metal processing manufacturers are taking measures to remove hydrogen. Taking advantage of the tendency for hydrogen to penetrate and accumulate in metals, a hydrogen absorbing alloy was developed for the purpose of storing hydrogen by optimizing the composition of the alloy. Also called hydrogen storage alloy. *(Hydrogen storage alloys are not easy to put in and take out like tanks, and many of the alloys developed are difficult to extract hydrogen or can only be partially extracted, and the weight of the metal is involved, so there are many problems in practical application. exists.)
 
history
The phenomenon of hydrogen uptake by metals has been known for a long time [1]. For example, steel in an acidic solution may suddenly crack, but this is due to hydrogen ions in the solution penetrating into the steel and making the steel brittle (hydrogen embrittlement).
Research into actively using this phenomenon for hydrogen storage began in the 1960s by J.J. Reilly and his colleagues at Oak Ridge National Laboratory in the United States. He demonstrated through experiments that magnesium-based alloys and vanadium-based alloys, which are the basis of current hydrogen-absorbing alloys, can absorb and release hydrogen, and that their properties can be changed by controlling the alloy composition.
Since Reilly, the development of hydrogen storage alloys has continued from the perspective of gaseous hydrogen storage, heat pumps, high-efficiency batteries, etc., and in Japan in particular, the Ministry of International Trade and Industry (currently the Ministry of Economy, Trade and Industry) and its affiliated organization NEDO Development has progressed through the development projects led by the Sunshine Project and WE-NET, and currently maintains one of the highest development standards in the world.
 
reaction
MH alloy (M) undergoes the following reversible reaction with hydrogen gas (H2) to form metal hydride (MHx).

In the equilibrium state of the reaction in the equation, Van't Hoff's equation approximately holds between the hydrogen pressure (P) and the temperature (T) of the MH alloy within a certain temperature range [5].
: Standard enthalpy change
: Standard entropy change
: Gas constant
When , it becomes.
This reaction is extremely fast and requires the removal and supply of the heat of reaction. In reality, by repeating the reaction, the MH alloy is pulverized down to several microns, so the heat conduction rate decreases.
 
principle
The principles of hydrogen storage alloys can be broadly divided into two: solid solution phenomena and chemical bonding.
In order to achieve both absorption and desorption of hydrogen, first of all, there is a void in the crystal structure into which hydrogen can enter, and if the hydrogen atom can exist stably to some extent in that position, and if hydrogen cannot move from that position (it cannot be released). (must be done) In order to optimize the crystal structure and electronic state of alloys from these viewpoints, various alloys have been developed that have crystal structures with relatively many voids and also contain elements that have catalytic activity.
 
solid solution phenomenon
The solid solution phenomenon refers to the incorporation of other elements into a solid crystal and occupying a stable position between the atoms that make up the crystal or replacing the atoms that make up the crystal. In particular, the former is called an interstitial solid solution, and the latter is called a substitutional solid solution. In the case of a hydrogen storage alloy, the former interstitial solid solution is formed with hydrogen and the alloy.
 
chemical bond
Chemical bonding means that the elements in the alloy actually combine with hydrogen. For example, magnesium forms a compound called MgH2 with hydrogen. Once this reaction is complete, magnesium will absorb 7.6% of its weight in hydrogen. However, since chemical bonds are more stable than solid solutions, catalysts or crystal structures are required to break the bonds under appropriate conditions [6].
 
kinds
Currently known hydrogen storage alloys include the following.
AB2 type
Based on alloys of transition elements such as titanium, manganese, zirconium, and nickel. The crystal has a hexagonal-based structure called the Laves phase. Although it is possible to increase the capacity due to the high hydrogen density, the disadvantage is that the higher the capacity of the alloy, the more difficult it is to activate it.
AB5 type
Those based on alloys containing transition elements (nickel, cobalt, aluminum, etc.)5 that have a catalytic effect on rare earth elements, niobium, and zirconium1 (typified by LaNi5, ReNi5, etc.). It is easy to hydrogenate from the initial stage, but it is expensive because it contains rare earth elements and cobalt. However, research is underway to avoid this problem by using unrefined rare earth elements (misch metals).
Ti-Fe type
This system is based on a body-centered cubic intermetallic compound with relatively many voids.
V series
Vanadium is known to react efficiently with hydrogen, and various body-centered cubic alloys based on vanadium, which have relatively many voids, are being studied.
Mg alloy
Magnesium absorbs as much as 7.6 wt% of hydrogen, but since magnesium hydride is relatively stable, various alloys with catalytic elements that destabilize it are being studied.
Pd series
Palladium can absorb 935 times its own volume of hydrogen, but its drawback is that it is expensive.
Ca-based alloy
Mainly alloys of calcium and transition elements (such as nickel), which have a strong affinity for hydrogen.
 
advantages and disadvantages
advantage
In hydrogen storage alloys, hydrogen is arranged regularly following the crystal structure. Therefore, it is possible to achieve an extremely high hydrogen filling density compared to gas. Furthermore, since hydrogen is released relatively gently, accidents caused by sudden hydrogen leakage can be prevented. Furthermore, by taking advantage of the fact that electrochemical hydrogen storage occurs in solution, it can also be used as an electrode for high-efficiency secondary batteries.
 
Disadvantage
All alloys other than V-based alloys and Mg-based alloys are heavy and unsuitable for automotive applications. Additionally, during the process of hydrogen storage and desorption, there is an exchange of heat associated with the reaction, and although there are examples of actively utilizing this (heat pumps, etc.), there is still an issue of improving heat transfer efficiency during hydrogen storage and desorption.[ 8]. Furthermore, there are other problems such as the rare earth elements and catalyst elements used in the alloy are expensive and scarce in resources, recycling is not easy, and repeated hydrogen storage and release causes embrittlement (hydrogen embrittlement), which reduces the storage rate. There is.
 
Application example
•Nickel-metal hydride rechargeable battery
-Fuel tanks for hydrogen vehicles and fuel cell vehicles
•Neutron shield - Mainly used in areas where water or concrete cannot be used. It blocks neutrons by absorbing and scattering them in absorbed hydrogen molecules.
•Heat pump - Utilizes heat absorption and heat generation when hydrogen is pumped in and out.
• Base metal for enamel - If the enamel is made of a metal that cannot absorb hydrogen, defects are likely to occur, so a hydrogen-absorbing alloy is used as the base before glazing is applied.
•Actuator - Drives the piston by absorbing and releasing hydrogen. It has been put into practical use in the welfare field as a power source for wheelchair seat lift units and transfer assistance devices.
•Hydrogen purification - When obtaining hydrogen by reforming hydrocarbon fuels such as methanol, in order to prevent the catalyst from being poisoned by by-product carbon monoxide and water vapor and reducing its activity, hydrogen, water vapor, and monoxide are purified. It is used to increase the purity of hydrogen by permeating and separating only hydrogen from a mixed gas of carbon.
In addition, since the optical properties of some hydrogen storage alloys change when hydrogen is absorbed and when they are released, we have developed a "switchable" system that changes the reflectance by depositing these alloys on glass and supplying hydrogen with an aqueous solution. ・Mirrors" etc. are also being researched.

储氢合金

氢具有容易渗入带有自由电子的金属键的特性。 因此，众所周知，所有金属都存在诸如在凝固过程中混入气泡以及由于氢溶解或侵入而导致强度劣化（氢脆）等问题。 针对这一现象，金属加工制造商正在采取除氢措施。 利用氢在金属中渗透和积聚的趋势，通过优化合金成分，开发出储氢合金。 又称储氢合金。 *（储氢合金不像储氢罐那样方便放入和取出，而且许多开发的合金很难提取氢气或只能部分提取，并且涉及到金属的重量，因此在使用中存在很多问题 实际应用。存在。）

历史
金属吸氢现象早已为人所知[1]。 例如，钢在酸性溶液中可能会突然破裂，但这是由于溶液中的氢离子渗入钢中，使钢变脆（氢脆）。
J.J. 于 20 世纪 60 年代开始积极研究利用这种现象进行储氢。 赖利和他在美国橡树岭国家实验室的同事。 他通过实验证明，目前吸氢合金的基础镁基合金和钒基合金可以吸收和释放氢，并且可以通过控制合金成分来改变其性能。
自Reilly以来，储氢合金的开发不断从气态储氢、热泵、高效电池等角度进行，特别是在日本，通产省（现经济产业省） 贸易和工业）及其附属组织NEDO Development通过Sunshine Project和WE-NET主导的开发项目取得了进展，目前保持着世界上最高的开发标准之一。

反应
MH合金（M）与氢气（H2）发生以下可逆反应，形成金属氢化物（MHx）。

在方程中的反应平衡状态下，在一定的温度范围内，Van't Hoff方程在MH合金的氢压力（P）和温度（T）之间近似成立[5]。
：标准焓变
：标准熵变
：气体常数
当 时，就变成了。
该反应非常快并且需要除去和供应反应热。 实际上，通过重复反应，MH合金被粉碎至几微米，因此热传导率降低。

原则
储氢合金的原理大致可分为两种：固溶现象和化学键合。
为了实现氢的吸附和脱附，首先晶体结构中存在氢可以进入的空隙，并且氢原子能否在该位置上一定程度地稳定存在，并且氢不能从晶体结构中移出。 该位置（无法释放）。 （必须做） 为了从这些角度优化合金的晶体结构和电子状态，已经开发了具有相对较多空隙的晶体结构并且还含有具有催化活性的元素的各种合金。

固溶现象
固溶现象是指其他元素掺入固体晶体中并在构成晶体的原子之间占据稳定的位置或取代构成晶体的原子。 特别地，前者称为间隙固溶体，后者称为置换固溶体。 在储氢合金的情况下，前一种填隙固溶体由氢和合金形成。

化学键
化学键合意味着合金中的元素实际上与氢结合。 例如，镁与氢形成一种称为 MgH2 的化合物。 一旦该反应完成，镁将吸收其重量的 7.6% 的氢气。 然而，由于化学键比固溶体更稳定，因此需要催化剂或晶体结构在适当的条件下打破化学键[6]。

种类
目前已知的储氢合金包括以下几种。
AB2型
基于钛、锰、锆和镍等过渡元素的合金。 该晶体具有称为拉夫斯相的六方结构。 虽然由于氢密度高可以提高容量，但缺点是合金容量越高，激活就越困难。
AB5型
那些基于含有过渡元素（镍、钴、铝等）5 的合金，对稀土元素、铌、