Статья поступила в редакцию 07.05.10. Ред. per. № 774 УДК 541.44+541.124
The article has entered in publishing office 07.05.10. Ed. reg. No. 774
МЕХАНОХИМИЧЕСКИ ПРИГОТОВЛЕННЫЕ МАТЕРИАЛЫ НА ОСНОВЕ МАГНИЯ ДЛЯ ХРАНЕНИЯ ВОДОРОДА
И. Констанчук1, К. Герасимов1, Ж.-Л. Бобе2
'Институт химии твердого тела и механохимии СО РАН 630128 Новосибирск, ул. Кутателадзе, д. 18 Тел.: +7 (383) 3399347, факс: +7 (383) 3322847, e-mail: irina@solid.nsc.ru 2Ииститут химии твердого состояния Бордо, НЦНИ, Франция Франция, F-33608 Пессак, ул. Швайзер, д. 87 Тел.: +33-(0)5-4000-2653, факс: +33-(0)5-4000-2761, e-mail: bobet@icmcb-bordeaux.cnrs.fr
Заключение совета рецензентов: 17.05.10 Заключение совета экспертов: 22.05.10 Принято к публикации: 25.05.10
Обсуждаются различные механохимические подходы к усовершенствованию сорбционных свойств аккумулирующих водород материалов на основе магния, и некоторые экспериментальные результаты иллюстрируют возможности каждого подхода. Демонстрируется, что механохимические методы эффективны как для улучшения сорбционных характеристик известных водород-адсорбирующих фаз, так и для создания новых материалов для хранения водорода.
Ключевые слова: хранение водорода, механохимические подходы, сорбционные свойства, водород-адсорбирующие фазы.
MECHANOCHEMICALLY PREPARED MAGNESIUM-BASED MATERIALS
FOR HYDROGEN STORAGE
I. Konstanchuk, K. Gerasimov1, J.-L. Bobet2
'Institute of Sold State Chemistry and Mechanochemistry, Siberian Branch of RAS 18 Kutateladze, Novosibirsk, 630128, Russia Tel.: +7 (383) 3399347, fax: +7 (383) 3322847, e-mail: irina@solid.nsc.ru 2Institut de Chimie de la Matière Condensée de Bordeaux ICMCB-CNRS, Université Bordeaux 1 87 Av. Schweizer, F-33608 Pessac, France Tel.: +33-(0)5-4000-2653, Fax: +33-(0)5-4000-2761, e-mail: bobet@icmcb-bordeaux.cnrs.fr
Referred: 17.05.10 Expertise: 22.05.10 Accepted: 25.05.10
Various mechanochemical approaches to improvement of hydrogen storage properties of Mg-based materials are discussed and some experimental results illustrate the possibility of each approach. It is demonstrated that mechanochemical methods are effective both for improving hydriding characteristics of known hydrogen absorbing phases and for search for new materials for hydrogen storage.
Keywords: hydrogen storage, mechanochemical approaches, hydrogen absorbing phases.
Introduction
One of the main problems in development of "hydrogen economy" is a problem of effective and safe storage and transportation of hydrogen.
In comparison to other methods, hydrogen storage in metal hydrides has a number of advantages such as high density of stored energy, high purity of evolved
hydrogen, relatively safety of operating and so on. The materials for hydrogen storage have to satisfy a set of criteria. The most important of them are reversible hydrogen capacity, operating pressure/temperature range, reaction kinetics, minimal degradation after cycling of repeated hydriding and dehydriding and cost. The material that would excellently meet all these requirements is not found so far.
Международный научный журнал «Альтернативная энергетика и экология» № 5 (85) 2010 © Научно-технический центр «TATA», 2010
Magnesium and magnesium-based alloys are very attractive from hydrogen capacity point of view (the theoretical hydrogen capacity of MgH2 is 7.6 wt.%), but MgH2 is relatively stable (the equilibrium hydrogen pressure of 0.1 MPa is achieved at temperature ~550 K), magnesium requires long activation, the kinetics of hydrogen absorption by magnesium and decomposition of MgH2 are not sufficiently fast even at temperatures as high as 573-623 K. Moreover, the theoretical hydrogen capacity practically is never achieved.
A large number of researches are directed at the search for opportunities to improve these characteristics.
Mechanochemical methods are very promising techniques for fabrication of hydrogen storage materials possessing good hydriding properties. The fine materials with various microstructure, composition and components content may be obtained by means of mechanochemical treatment. The use of mechano-chemical methods permits to solve a problem of activation of the hydrogen absorbing material before hydriding, to accelerate hydriding and dehydridig reactions and to increase the hydrogen capacity. They are almost unique methods for fabrication of composites from immiscible components. This is especially important for magnesium-based materials because Mg is thermodynamically immiscible with a large number of elements of Periodic table.
Two principal approaches may be developed on the basis of these methods: (1) improving the hydrogen storage properties of known hydrogen absorbing materials by affecting their structure, morphology, surface properties and so on, using mechanical activation and mechanical alloying with various types of additives and (2) searching for new hydrogen absorbing materials with good hydriding properties. The mechanochemical methods are especially important for the second approach due to the possibility of preparation of metastable composites of components very different in nature, including thermodynamically immiscible ones. These composites may possess very interesting hydrogen storage properties and serve as precursors for synthesis of the new phases promising for hydrogen storage.
In this work, some experimental results on hydrogen storage properties of mechanochemically prepared magnesium-based materials illustrate possibilities of various approaches developed on the basis of mechanochemical methods for fabrication of materials for hydrogen storage.
Improvement of hydrogen storage properties of magnesium by mechanochemical treatment with various additives
One of the main drawbacks of magnesium as hydrogen storage material is slow rate of hydrogen absorption and desorption under the conditions appropriate to wide practical use. The investigations directed at improvement of kinetics of these reactions are now being developed very intensively.
Some peculiarities have been shown to be inherent in the magnesium-hydrogen interaction. Kinetics of hydriding at the first and subsequent cycles differ very much from each other. A compact oxide layer usually covers the magnesium particles and prohibits hydrogen chemisorption on their surface. According to one of the most reliable models proposed in [1], the overall kinetics of first hydriding of magnesium is determined by the statistical cracking of an oxide layer (owing to different coefficients of thermal expansion of Mg and MgO) and hydride nucleation on the metal sites formed. This leads to a long induction period and sigmoid shape of the kinetic curve. As a rule several hydriding-dehydriding processes are required for achieving the highest reaction rate (the so-called activation procedure).
The fragmentation of the material along with the formation of an oxide-free magnesium surface is the result of hydriding and dehydriding at the first cycle. According to the present day concept the rate of magnesium hydriding at the initial stages of second and subsequent cycles and the rate of decomposition of MgH2 are limited by the dissociative adsorption (recombination and desorption) of hydrogen on the metal surface. This is quite a common feature for interaction between metals and hydrogen [2]. The nuclei of magnesium hydride are formed on the metal surface, with the interface propagating along the metal surface [3]. The overlap of nuclei leads to formation of a "surface shell" of magnesium hydride which blocks further hydrogen absorption.
Catalyst addition accelerates the hydride formation but decreases the hydrogen capacity not only because of additional weight of the catalyst phase but also due to the earlier formation of a hydride layer.
A possible way of overcoming this problem may be found by decreasing the particle size of magnesium and magnesium-based alloys, by modifying their surface with catalytic additives and/or other chemical reagents which can change the nucleation conditions of the magnesium hydride, the morphology of the hydride layer and the hydrogen permeation through it.
The use of mechanochemical methods appreciably promotes a solution of this problem.
Mechanical alloying under inert gas atmosphere Mechanical alloying (mechanochemical treatment of powdery mixture of two or several components) have been applied for the first time for the fabrication of hydrogen storage materials in the works [4-7]. Mechanical alloying of magnesium powder with addition of transition metal (Ni, Fe, Co and other) was carried out under argon atmosphere. It has been shown that the composites with a large interface between components (mechanical alloys) are formed already at the first stages of mechanical alloying. Typical appearance and lamellar microstructure of mechanical alloys are shown in Fig. 1.
International Scientific Journal for Alternative Energy and Ecology № 5 (85) 2010
© Scientific Technical Centre «TATA», 2010
b
Рис. 1. Типичный вид (а) и слоистая микроструктура (b) механических сплавов магния с металлом-катализатором Fig. 1. Typical appearance (a) and lamellar microstructure (b) of magnesium mechanical alloy with metal-catalyst
Magnesium, being very soft and ductile, is quite difficult to be disintegrated by mechanical milling. The special surface-active additives capable of impeding aggregation processes are desired for obtaining fine magnesium powder. Some organic compounds and graphite have been used for this purpose [18, 27-35].
Interesting results have been obtained when inorganic salts were used as additives to magnesium in the course of mechanical alloying [36-38]. Salts have been shown to promote comminution of metal during mechanical alloying. Surprisingly even salts containing non-transition metals (NaF, NaCl and MgF2, which hardly can show catalytic activity in the hydrogen chemisorption processes) have a positive influence on the hydriding properties of magnesium due to modifi
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