115,99 €
Mehr erfahren.
- Herausgeber: Wiley-VCH GmbH
- Kategorie: Fachliteratur
- Sprache: Englisch
Understand the energy storage technologies of the future with this groundbreaking guide
Magnesium-based materials have revolutionary potential within the field of clean and renewable energy. Their suitability to act as battery and hydrogen storage materials has placed them at the forefront of the world’s most significant research and technological initiatives. It has never been more essential that professionals working in energy storage and energy systems understand these materials and their extraordinary potential applications.
Magnesium-Based Energy Storage Materials and Systems provides a thorough introduction to advanced Magnesium (Mg)-based materials, including both Mg-based hydrogen storage and Mg-based batteries. Offering both foundational knowledge and practical applications, including step-by-step device design processes, it also highlights interactions between Mg-based and other materials. The result is an indispensable guide to a groundbreaking set of renewable energy resources.
Magnesium-Based Energy Storage Materials and Systems readers will also find:
- In-depth analysis of the effects of employing catalysts, nano-structuring Magnesium-based materials, and many more subjects
- Detailed discussion of electrolyte, cathode, and anode materials for Magnesium batteries
- Snapshots of in-progress areas of research and development
Magnesium-Based Energy Storage Materials and Systems is ideal for materials scientists, inorganic chemists, solid state chemists, electrochemists, and chemical engineers.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 351
Veröffentlichungsjahr: 2024
Ähnliche
Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
Acknowledgments
1 Overview
1.1 Introduction to Mg‐based Hydrogen and Electric Energy Storage Materials
1.2 Overview of Mg‐based Hydrogen Storage Materials and Systems
1.3 Overview of Mg‐ion Batteries
2 Hydrogen Absorption/Desorption in Mg‐based Materials and Their Applications
2.1 The Characterizations of Mg‐based Hydrogen Storage Materials
2.2 Methods for Improving the Hydrogen Storage Performance of Mg‐based Materials
2.3 Synthesis Technologies for Mg‐based Hydrogen Storage Materials
2.4 Advanced Characterization Techniques
2.5 Fundamentals and Applications of Mg‐based Hydrogen Storage Tanks
3 Hydrolysis of Mg‐based Hydrogen Storage Materials
3.1 Hydrolysis Processes of Mg/MgH
2
3.2 Control of Hydrolysis Processes
3.3 Controllable Hydrolysis Systems
4 Electrolytes for Mg Batteries
4.1 Liquid Electrolytes
4.2 Solid and Quasi‐solid State Electrolytes
5 Cathodes and Anodes for Mg Batteries
5.1 Intercalation‐type Cathode Materials
5.2 Conversion‐type Cathode Materials
5.3 Organic Cathodes
5.4 Anodes for Mg Batteries
6 Conclusions and Outlook
List of Abbreviations
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 The kinetic models used for hydrogen absorption and desorption rea...
List of Illustrations
Chapter 1
Figure 1.1 The role of Mg‐based materials in hydrogen storage and batteries....
Figure 1.2 An overview of essential metal hydrides for hydrogen storage appl...
Figure 1.3 Illustration of rechargeable Mg‐ion batteries (RMBs).
Chapter 2
Figure 2.1 (a) Pressure‐composition isotherm plot of Mg + H
2
⇄ MgH
2
transiti...
Figure 2.2 Schematic illustration of the kinetic steps in the hydrogen stora...
Figure 2.3 Enhancing hydrogen storage properties of Mg
2
NiH
4
via nanosizing, ...
Figure 2.4 Synthesis of various Mg‐based hydrogen storage alloys. (a1) The D...
Figure 2.5 Synthesis of nanostructured Mg‐based composites via mechanical mi...
Figure 2.6 Synthesis of nanostructured Mg‐based composites via mechanical mi...
Figure 2.7 Synthesis of the free‐standing nano‐sized Mg/MgH
2
. (a1) TEM image...
Figure 2.8 Synthesis of nanostructured Mg‐based composites through the nano‐...
Figure 2.9 Synthesis of nanostructured Mg‐based composites through nano‐conf...
Figure 2.10 (a) The XRD patterns of 2LiNH
2
+ 3MgH
2
under different states; (...
Figure 2.11 Hydrogenation/dehydrogenation process of a Li–Mg–N–H system.
Figure 2.12 (a) The XRD pattern of Mg‐10 wt% NaBH
4
before and after dehydrog...
Figure 2.13 (a) The rehydrogenation kinetics of
n
MgH
2
–LiAlH
4
at different mo...
Figure 2.14 (a) Illustration of equipment used in BMAS process; (b) Program ...
Figure 2.15 The first five (a) absorption curves and (b) desorption curves o...
Figure 2.16 The illustration of HCS method and conventional process of prepa...
Figure 2.17 (a) The bright‐field and (b) dark‐field TEM images of 2MgH
2
–Fe p...
Figure 2.18 Different SPD methods for Mg‐based hydrogen storage alloy prepar...
Figure 2.19 Schematic diagram of the reaction process of HCVD.
Figure 2.20 The MEM(maximum entropy method) charge density maps of MgH
2
at r...
Figure 2.21 (a) The sample holder of in‐situ TEM. (b) Nanoreactor of the in‐...
Figure 2.22 SAED patterns of a few Pd particles exposed to 1.2 bar hydrogen....
Figure 2.23 The eight dehydrogenation processes of the MgH
2
(110) surface. T...
Figure 2.24 The migration and dehydrogenation energy barriers of H atom of t...
Figure 2.25 Schematic diagram of a typical HST.
Figure 2.26 The Mg‐based HSTs of McPhy.
Figure 2.27 The large‐scale oil‐heating type Mg‐based HST and the ton‐level ...
Chapter 3
Figure 3.1 Relationship of magnesium ion and pH in the solution after hydrol...
Figure 3.2 Released hydrogen as a function of time at different pH.
Figure 3.3 Schematic illustration of the mechanism of the Mg–H
2
O reaction, w...
Figure 3.4 Schematic illustration of the mechanism for the hydrolysis reacti...
Figure 3.5 Mechanism of hydrolysis reaction highlighting the role of chlorid...
Figure 3.6 SEM images of (a) the Mg nanoparticles, (b) the MgH
2
nanoparticle...
Figure 3.7 Hydrolysis performance of commercial MgH
2
, hydrogenated pure Mg a...
Figure 3.8 The SEM image of (a) Pure Mg and (b) Mg‐graphite composites after...
Figure 3.9 (A) Commercially available MgH
2
(a) and magnesium hydride after b...
Figure 3.10 Illustration of the mechanism of the hydrolysis process of Mg–Mg
Figure 3.11 Hydrolysis profiles of hydrogenated pure MgH
2
, MgH
2
@MgAg, and Mg...
Figure 3.12 Schematic diagram of MgH
2
hydrolysis reactor.
Figure 3.13 Schematic diagram of a hydrolysis reactor for Mg‐based hydrogen ...
Figure 3.14 Schematic of the typical MgH
2
hydrolysis – hydrogen fuel cell sy...
Chapter 5
Figure 5.1 Typical electrochemical behavior and the basic structure of the M...
Figure 5.2 The mechanism diagram of Mg desolvation and intercalation at the ...
Figure 5.3 The role of Cu in Mo
6
S
8
mixture cathodes.
Figure 5.4 Crystal structure model of V
2
O
5
. (a) The structural model on the ...
Figure 5.5 The mechanism diagram of phase conversion during Mg
2+
inserti...
Figure 5.6 The possible equilibrium phases under different electrolyte condi...
Figure 5.7 Synchrotron X‐ray diffraction, scanning electron microscopy, and ...
Figure 5.8 Characterization of the as‐synthesized V
2
O
5
samples. (a) Schemati...
Figure 5.9 Schematic diagrams of rhombohedral structure (a, c) and monoclini...
Figure 5.10 Typical structures of (a) graphite, (b) graphene, (c) carbon nan...
Figure 5.11 A schematic diagram of structural evolution of TiS
2
at different...
Figure 5.12 The typical preparation process of a Mg battery pouch cell.
Figure 5.13 The real images of magnesium pouch cells for lighting LEDs with ...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Acknowledgments
Begin Reading
List of Abbreviations
References
Index
End User License Agreement
Pages
iii
iv
ix
x
xi
1
2
3
4
5
6
7
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
Magnesium‐Based Energy Storage Materials and Systems
Jianxin ZouYanna NuLiZhigang HuXi LinQiuyu Zhang
Authors
Prof. Jianxin ZouShanghai Jiao Tong UniversityDongchuan Road 800Minxing DistrictShanghaiCH, 200240
Prof. Yanna NuLiShanghai Jiao Tong UniversityDongchuan Road 800Minxing DistrictShanghaiCH, 200240
Prof. Zhigang HuShanghai Jiao Tong UniversityDongchuan Road 800Minxing DistrictShanghaiCH, 200240
Dr. Xi LinShanghai Jiao Tong UniversityDongchuan Road 800Minxing DistrictShanghaiCH, 200240
Dr. Qiuyu ZhangShanghai Jiao Tong UniversityDongchuan Road 800Minxing DistrictShanghaiCH, 200240
Cover: © Roman Samokhin/Shutterstock; © Andriy Onufriyenko/Getty Images
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2024 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐35226‐5ePDF ISBN: 978‐3‐527‐84259‐9ePub ISBN: 978‐3‐527‐84260‐5oBook ISBN: 978‐3‐527‐84261‐2
Preface
In recent years, the importance of renewable energy sources has become increasingly evident as the whole world is facing the challenges of climate change, severe environmental pollution, and the urgent demand for sustainable development. While renewable energy offers great potential for a cleaner and greener future, it also presents certain challenges, particularly in terms of its intermittency, low energy intensity, and the need for efficient energy storage systems. One of the most significant challenges in renewable energy is its fluctuating nature. Solar and wind power, for example, are highly dependent on weather conditions and can vary in their availability. To mitigate this issue, large‐scale energy storage techniques have to be developed, which involve storing surplus energy during periods of high production and releasing it during times of low production for renewable energies. Two promising candidates for such energy storage are electricity and hydrogen, as they are clean, sustainable, and independent of geological conditions.
On one hand, compressed hydrogen storage technology and liquid‐state storage suffer from high costs, high energy consumption from compression/liquefication, and safety issues. In contrast, hydrogen storage in solid‐state form has been regarded as a viable alternative since it is possible to store more hydrogen per unit volume than that of liquid or high‐pressure hydrogen gas while maintaining high safety of operation. Among different hydrogen storage materials, magnesium‐based materials have shown significant advantages in this regard. For instance, it possesses high hydrogen storage capacity (up to ∼7.6 wt% and 110 g l−1 for MgH2), abundant resources, and low cost, making it a promising option for hydrogen storage and transportation.
On the other hand, rechargeable magnesium‐ion batteries (RMBs) are also emerging as a promising alternative for high‐density energy storage systems beyond lithium‐ion batteries (LIBs), because of their high volumetric capacity and dendrite‐free metal anodes.
Nevertheless, there is no such book available till now that links fundamental knowledge in magnesium‐based hydrogen storage materials and magnesium batteries to the basic applications in energy storage devices. While there is such an abundance of research papers, reviews, perspectives, and monographs published in relation to magnesium‐based energy storage materials, these publications are almost exclusively for senior researchers in the field of energy materials, with few providing an introductory ramp to readers of a wide range of interests.
This book aims to fill the gap mentioned above. It provides a comprehensive understanding of magnesium‐based energy storage materials and their systems, linking the fundamental concepts to the actual challenges encountered in real‐life applications.
Shanghai, ChinaFebruary 2024
Jianxin Zou
Acknowledgments
The authors would like to express their gratitude for the support received from the National Key Research and Development Program of China (Grant No. 2022YFB3803700), the National Natural Science Foundation (Grant No. 52171186), and the Center of Hydrogen Science, Shanghai Jiao Tong University.
Shanghai, China
Jianxin Zou
1Overview
1.1 Introduction to Mg‐based Hydrogen and Electric Energy Storage Materials
The heavy reliance on fossil fuels has incurred serious environmental consequences because of the resultant carbon dioxide (CO2) emissions into the atmosphere, which are, however, driven by the accelerating energy demands due to global civilization and economic development [1]. The access to abundant, cheap, and clean energy has become our most essential foundation for economic prosperity and human civilization. Among all other new alternative clean energy sources, such as solar, biomass, and nuclear sources, hydrogen has been widely recognized as a clean, renewable, and high‐density energy carrier [2]. Although it is believed to be the long‐term solution for the world energy supply, the current hydrogen storage and transportation technologies remain the bottleneck challenge to be tackled [3]. Therefore, developing safe, effective, and economical technologies to store and transport hydrogen is an essential step to make it more competitive with respect to other fuels.
Nowadays, hydrogen is mainly stored in three forms: compressed gas storage, liquid‐state storage, and solid‐state storage [4]. Compressed hydrogen storage technology, as the most mature and widely implemented storage method, suffers from difficult‐to‐produce and expensive carbon‐fiber tanks, low volumetric energy density, and large energy consumption for hydrogen compression. Meanwhile, the liquefied hydrogen storage method requires an energetically unfavorable deep cooling to −253 °C, and up to 30% energy is required for liquefaction in real applications. Beside gas and liquid storage, hydrogen storage in a solid‐state form has been regarded as a viable alternative since it is possible to contain more hydrogen per unit volume than liquid or high‐pressure hydrogen gas while maintaining high safety of operation.
Among different energy storage materials, magnesium and magnesium‐based materials may play an important role in high‐density energy storage systems (Figure 1.1) [6]. On the one hand, they have been already intensively investigated in hydrogen storage and transportation technologies because of their natural abundance and availability, as well as their extraordinary high gravimetric (7.6 wt%) and volumetric (110 g l−1) storage densities [7]. Moreover, magnesium hydrides have been also used as a one‐time hydrogen carrier, where their water hydrolysis can give a doubled gravimetric capacity up to 15.2 wt% and a high volumetric capacity of 150 g l−1. On the other hand, rechargeable Mg‐ion batteries (RMBs) can also act as a promising alternative for high‐density energy storage systems beyond Li ion batteries (LIBs), because of their high volumetric capacity (3833 mA h cm−3) and dendrite‐free metal anodes [8].
Figure 1.1 The role of Mg‐based materials in hydrogen storage and batteries.
Source: Reproduced with permission from Sun et al. [5] Copyright 2018, Elsevier.
1.2 Overview of Mg‐based Hydrogen Storage Materials and Systems
Hydrogen has been considered a potential clean energy vector because of its high gravimetric energy density of 33.3 kWh kg−1, as compared to that of gasoline (12.4 kWh kg−1) and natural gas (13.9 kWh kg−1) [4]. Although highly appealing, the employment of hydrogen as an energy carrier is largely hindered by the lack of appropriate and economical storage and transportation solutions. In general, ideal hydrogen storage technologies should possess the following characteristics: (i) high volumetric and gravimetric hydrogen density; (ii) adequate recyclability; (iii) high safety; and (iv) best operated under ambient conditions [9]. Nowadays, hydrogen is mainly stored in three different forms: (i) compressed gas storage (e.g. 20, 35, and 70 MPa); (ii) liquid storage (−253 °C); and (iii) solid state in hydrides (e.g. metal hydrides and complex metal hydrides) [10]. It is worth noting that compressed hydrogen storage technology is currently the most mature and widely implemented storage method; however, it suffers from several major drawbacks: (i) difficult‐to‐produce and expensive carbon‐fiber tanks; (ii) poor volumetric energy density (e.g. 5.6 MJ l−1 at 70 MPa compared to gasoline of 32.0 MJ l−1); and (iii) a large energy consumption for the compression work (13–18% of hydrogen when compressed to 70 MPa) [11]. Meanwhile, the liquefied hydrogen storage method requires an energetically unfavorable deep cooling to −253 °C, and up to 30% energy is required for liquefaction in real applications [12]. Moreover, due to the boiling‐off phenomenon, a daily hydrogen loss of 1–2% has been considered. Therefore, the solid‐state storage method has been considered an alternative and promising method (e.g. metal hydrides) for hydrogen storage and transportation due to its high achievable volumetric hydrogen density and high safety. Such metal hydrides have been discovered since 1866, when Graham affirmed the high affinity of hydrogen for Pd [13]. However, metal hydrides have been considered for hydrogen storage purposes since the 1960s.
In the past three decades, magnesium and magnesium‐based materials have been intensively investigated as potential hydrogen storage carriers due to their natural abundance and availability, as well as their extraordinary high gravimetric and volumetric storage densities [5]. Among several high potential hydride systems, magnesium hydrides exert a high volumetric and gravimetric hydrogen density (110 kg m−3 and 7.6 wt%), making it one of the most widely studied hydrogen storage materials (Figure 1.2). It is worth noting that these values are much higher than those of compressed hydrogen, i.e. 23 kg m−3 at 35 MPa and 38 kg m−3 at 70 MPa, and 71 kg m−3 of liquid hydrogen (−253 °C). In 1951, Wiberg first synthesized MgH2 by heating Mg at 570 °C and 20 MPa H2 using MgI2 catalysts directly [6]. Once MgH2 is formed, the reversible reaction between magnesium and hydrogen can be described by the following equation: MgH2(s) → Mg(s) + H2(g). For this reaction, the measured changes of enthalpy (ΔH) and entropy (ΔS) are 74.1 ± 0.4 kJ mol−1 and 133.4 ± 0.7 J K−1 mol−1, which entails an equilibrium pressure of 1 bar at 283 °C. Therefore, when selecting hydrogen storage materials, the ΔH and the ΔS of hydrogenation and dehydrogenation are among the most important parameters. Such values can be easily derived from the PCT isotherms using the van't Hoff plot. Beside thermodynamic considerations, the kinetic properties of MgH2 are also pivotal when selecting a suitable hydrogen storage system. In fact, achieving the fastest hydrogenation/dehydrogenation kinetics is an indispensable goal for MgH2. Alloying, catalyzing, nano‐structuring, and combining with complex hydrides, are among the most effective strategies to improve the hydrogen storage kinetics and thermodynamics of MgH2[8].
Figure 1.2 An overview of essential metal hydrides for hydrogen storage applications.
Source: Reproduced with permission from Sun et al. [8]. Copyright 2018 Elsevier.
In practice, Mg‐based materials must be processed and placed in a hydrogen storage tank (HST) for efficient storage and transportation of hydrogen. However, when processed into pellets and packed in HSTs, Mg‐based alloys suffer from sluggish hydrogen desorption kinetics, which largely impedes their practical applications, due to the large enthalpy change of hydrogen desorption reaction and the low powder thermal conductivity of Mg‐based alloys [14]. And this phenomenon unfortunately becomes more significant in large‐scale Mg‐based HSTs. To enhance the hydrogen desorption kinetics of large‐scale Mg‐based HSTs, appropriate types of heating are indispensable to provide a high heating efficiency [15, 16]. Currently, direct electrical heating is one conventional way to heat Mg‐based HSTs; however, it becomes strenuous to apply due to the limited heat‐exchange surfaces and accumulated heat during the hydrogen absorption process with the increase of the HST scale. Heat transfer fluid (HTF) tube is another more efficient way to heat Mg‐based HSTs with more uniform control of the local temperature due to the increased heat exchange areas [17–19]. Before carrying out real experiments, numerical simulations are always applied to assist the design of high‐efficiency HSTs.
Magnesium hydride can also be used to produce hydrogen through hydrolysis with water, offering a doubled gravimetric capacity of 15.2 wt% and a high volumetric capacity of 150 g l−1[20]. The hydrolysis of magnesium materials refers to the reactions of Mg and MgH2 with aqueous solution to produce more hydrogen and insoluble magnesium hydroxide at room temperature under standard atmospheric environment. Beside instant high gravimetric capacity and volumetric capacity, the hydrolysis of Mg and MgH2 shows distinct advantages, including low material cost, abundant Mg resources, non‐pollution oxidation by‐products, moderate reaction temperature, and mature Mg recycling technology [21]. Therefore, hydrolysis of Mg‐based materials is a promising technology for the development of portable hydrogen fuel cells. Nevertheless, the insoluble and dense Mg(OH)2 passivation layer deposited on the surface of Mg or MgH2 could minimize the diffusion of water molecules into inner particles for continuous hydrogen production, thus resulting in undesirable and uncontrollable hydrolysis performance [22]. To address such issues, the addition of catalysts can help to improve hydrogen generation. In addition, control of temperature, pH values, salt solution, etc. can also be effective.
1.3 Overview of Mg‐ion Batteries
The current indispensability of economical and sustainable energy storage prospers battery research nowadays. Since their invention in 1991, Li ion batteries (LIBs) have become the state‐of‐the‐art energy storage technology, which exerts widespread applications in portable devices, electrical vehicles, and grid energy storage. However, LIBs are limited by their capacity and energy density, approaching the theoretical limits of intercalation chemistry, as well as the ever‐increasing consumption of expensive Li resources. Beyond Li‐ion batteries, because of owning a higher volumetric capacity (3833 mA h cm−3 for Mg versus 2046 mA h cm−3 for Li) and dendrite‐free metal anode, rechargeable Mg‐ion batteries are a promising alternative for high‐density energy storage applications (Figure 1.3) [24]. However, RMBs remain underdeveloped due to the absence of high‐performance electrolytes, cathodes, and anodes for any practical applications.
Figure 1.3 Illustration of rechargeable Mg‐ion batteries (RMBs).
Source: Reproduced with permission from Wang et al. [24]. Copyright 2017 American Chemical Society.
First of all, the electrolytes for RMBs play a crucial role in bridging the electrodes and transferring electroactive species via ionic transport, and the ideal electrolytes should possess not only compatibility with the electrodes, but also electrochemical and thermodynamic stability and safety in the energy storage system [7]. In general, the RMB electrolytes can be classified into liquid‐ and solid‐state (quasi‐solid‐state) electrolytes. Liquid electrolytes can be classified into aqueous and organic electrolytes. Aqueous electrolytes include alkaline solutions (e.g. magnesium perchlorate with magnesium hydroxide), neutral saline solutions (NaCl solution), and seawater and seawater with acid (H2SO4). Organic electrolytes include Grignard‐based, boron‐based, (HMDS)2Mg‐based, and boron‐centered electrolytes. However, one key challenge with MIBs in organic liquid electrolytes is their strong chemical reactivity toward electrolyte components (e.g. salt, solvent, additives) due to their low electronegativity [25]. Solid and quasi‐solid electrolytes have been considered one of the most promising alternatives, because of their distinct advantageous properties over liquid electrolytes, such as good safety, excellent non‐volatility, and superior flexibility. In general, solid electrolytes can be classified into inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and composite polymer electrolytes (CPEs). Quasi‐solid electrolytes (QSSEs) are often confused with SPEs, also known as gel polymerized electrolytes (GPEs), such as PEO/SiO2 and poly(tetrahydrofuran)‐based GPE (PTHF‐based GPE). However, solid and quasi‐solid electrolytes remain in a nascent stage and suffer from low ionic conductivity.
Secondly, due to its bivalent nature, the diffusion of Mg2+ in solid‐state cathode materials is more sluggish than monovalent cations, such as Li+ and Na+, resulting in a large voltage hysteresis and low magnesiation degree [26]. As such, finding suitable cathode materials with fast kinetics has become the significant challenge for developing RMBs. As already commercialized in LIBs, intercalation‐type cathode materials have been attempted for magnesium batteries because of their potential to achieve high cell voltage, high energy density, and cycling stability [7]. Common intercalation‐type cathode materials include Chevrel phase CP (Mo6T8; T = S, Se, Te) materials, V2O5‐Mg2+ insertion‐type materials, molybdenum oxide (MoO3), uranium oxide (α‐U3O8)‐Mg2+ insertion‐type materials, layered structure (e.g. V2O5, MnO2, Mo6S8, MXene) materials, spinel structure (e.g. Mn2O4, Cr2S4, Ti2S4, Mn2S4) materials, olivine structure [MgMSiO4 (M = Mn, Co, Fe)] materials, NASICON structure materials [NaM2IV(PO4)3 (MIV = Ge, Ti, Zr)], carbon‐based (e.g. graphite, hard carbon, soft carbon, graphene) materials, and MT2 (M = metal, T = S, Se) type (TiS2, TiSe2, MoS2) intercalation materials. However, conventional oxide‐based cathodes face challenges with Mg intercalation chemistry due to strong electrostatic interactions within the host structures [25], which lead to polarization and low reversibility [27–32]. Beside intercalation compounds, conversion‐type cathode materials are also widely investigated for RMBs, as they have the potential to achieve higher energy density and lower cost in theory [26]. Common conversion‐type cathode materials include chalcogenides (e.g. CuS, Cu2MoS4, CuS1−xSex, Ag2S, Cu2−xSe, Sb2Se3). Meanwhile, Mg–O2 batteries, Mg–S batteries, Mg–Se batteries, Mg–Te batteries, and Mg–I2 batteries, are also within the regime of conversion‐type cathode materials. These materials take advantage of the large capacity released by structural rearrangement and chemical bond rupture during the charge and discharge process, and the resulting phase structure changes are electrochemically reversible [30, 32, 33]. In addition, redox‐active organic materials can also be used for cathodes, which can be classified as n‐type, p‐type, or bipolar‐type in terms of their capabilities to release electrons (oxidation) or receive electrons (reduction) in their neutral state during the electrochemical reaction. Common organic cathodes include carbonyl compounds (e.g. 2,5‐dimethoxybenzoquinone [DMBQ], polyanthraquinone sulfides [PAQS], pyrene‐4,5,9,10‐tetraone [PTO], organosulfur compounds (e.g. 2,5‐dimercapto‐1,3,4‐thiadiazole [DMcT], poly‐2,2′‐dithiodianiline [PDTDA]), and nitrogen‐based compounds (e.g. imines [C=N], nitriles [C≡N], azo compounds (N=N)].
Lastly, direct Mg metal anodes in RMBs are also considered a major obstacle to the construction of practical RMBs because of the incompatibility of Mg metal anodes with electrolyte solutions [23]. Conventional electrolyte solutions include Mg‐based electrolytes [Mg(ClO4)2, Mg(BF4)2, Mg(PF6)2, Mg(AsF6)2, etc.] and organic solvents (carbonates, nitriles, lactones, esters, etc.), which show very low compatibilities with Mg metal anodes. Therefore, Mg metal anodes cannot work reversibly in these conventional electrolyte solutions, while high‐voltage/high‐capacity Mg insertion‐type cathodes can work because of the passivation phenomena that fully block them. In addition, magnesium is highly reactive and can react with many organic solvents and electrolyte components, leading to the formation of passivation layers and other side reactions that can degrade battery performance [34]. Replacing Mg metal anodes with alternative anodes that can work reversibly in conventional electrolyte solutions provides a promising way to develop high‐performance RMBs. Indeed, a series of insertion‐type materials (Li4Ti5O12, FeVO4, Li3VO4, TiO2‐B, layered Na2Ti3O7, etc.) have been probed as anodes for MIBs but show poor diffusion kinetics because of the strong electrostatic interactions between Mg2+ and their surrounding anions/cations. Moreover, the relatively high reaction potential and low specific capacity of insertion anodes greatly compromise the energy density of expected high‐voltage Mg batteries. In addition, MgxM anodes using Group IIIA, IVA, and VA elements (M) alloying with Mg could theoretically deliver high specific capacities at low alloying potentials but need to be verified.
In general, finding suitable electrode materials that can more effectively catalyze the electrode reaction and overcome the limitations of the passivation film remains the key challenge for developing high‐performance RMBs. It is also worth noting that the electrodeposition of Mg metal is not fully free of dendrite formation under all conditions. Nevertheless, these attractive characteristics and distinct advantages have already made RMBs one of the most promising energy storage technologies beyond LIBs.
2Hydrogen Absorption/Desorption in Mg‐based Materials and Their Applications
2.1 The Characterizations of Mg‐based Hydrogen Storage Materials
2.1.1 An Introduction to the Crystal Structure of Mg and MgH2
Pure Mg possesses a hexagonal crystal structure and is reversibly reacted with hydrogen to form the hydrogenated phase MgH2. The hydrogen absorption/desorption process of hydrides is a dynamic equilibrium of three phases: hydrogen, metal, and the corresponding hydride (Figure 2.1). As shown in Figure 2.1a, hydrogen pressure, composition, and temperature are the crucial factors determining the phase equilibrium [35]. During the process of isothermal hydrogenation, a solid solution (α‐phase) is first formed. With the increase in hydrogen pressure, the solid solution starts to transform into hydride (β‐phase), the β‐phase nucleates and grows, and the hydrogen pressure remains unchanged as the phase transformation proceeds. Until the α‐phase completely transforms into the β‐phase, the hydrogenation reaction is complete. By calibrating the pressure‐composition‐temperature equilibrium point in the process of hydrogen absorption/desorption, the PCT curve can be obtained (Figure 2.1b).
2.1.2 Thermodynamic Mechanisms for the Hydrogen Absorption/Desorption of Mg/MgH2
The relationship between plateau pressure (P) and temperature (T) in the PCT curve can be described by the van't Hoff equation:
In this formula, P0 is the atmospheric pressure (1.01 × 105 Pa); ΔH and ΔS are the enthalpy and entropy of the hydrogen absorption/desorption, respectively; and T is the absolute temperature; R is the gas constant (R = 8.314 J mol−1 K−1). According to the linear fitting between lnP and 1000/T, ΔH and ΔS can be calculated. Notably, the value of the re/de‐hydrogenation enthalpy (ΔH) is an important indicator to measure the strength of the Mg—H bond. The larger the absolute value of ΔH is, the stronger the Mg—H bond will be.
Figure 2.1 (a) Pressure‐composition isotherm plot of Mg + H2 ⇄ MgH2 transition. (b) van't Hoff plot related to the phase transition of Mg + H2 ⇄ MgH2. The enthalpy and entropy of hydrogenation and dehydrogenation could be obtained from the slope and intercept, respectively. Schematic representations of the α‐phase (left) and β‐phase (right) of metal hydride are also presented.
Source: Reproduced with permission from Ren et al. [36] Copyright 2023 Springer Nature.
The thermodynamic stability of Mg/MgH2 system is mainly determined by the feature of the Mg—H bond. The Mg–H bond in the Mg/MgH2 system is covalent‐ionic mixed [37], having a relatively high bonding energy of around 3.35 eV. The nature of Mg—H bond results in a high enthalpy for the decomposition of MgH2, which is around 75 kJ for releasing 1 mol of H2[38], leading to a high temperature for hydrogen desorption. Under atmospheric pressure, MgH2 starts to release H2 at a temperature of 280 °C, which is far from the requirements of practical applications [39]. Many efforts have been made to thermodynamically destabilize the Mg/MgH2 systems, such as alloying of Mg with other elements, inducing the formation of metastable γ‐MgH2 phase, and nano‐structuring [40]. Cheung et al. [41] simulated the relationship between grain size reduction and decreased structural stability. They concluded that significant changes in the thermodynamics of hydrogen desorption are observed only if the grain size is reduced to below 2 nm. Furthermore, ΔS is another important thermodynamic value for hydrogen desorption and is considered to be a constant value of about 130 J mol−1 K−1. Nevertheless, some studies have shown that the ΔS of the desorption hydrogen process is variable [42, 43]. Therefore, if ΔH does not change significantly, increasing entropy would be an effective way to reduce the operating temperature of MgH2.
2.1.3 Kinetic Mechanisms for the Hydrogen Absorption/Desorption of Mg/MgH2
The hydrogen absorption processes of Mg‐based materials can be described as follows [14, 44]: (i) physisorption: H2 is physisorbed on the surface of the particle at a certain pressure through intermolecular forces or Mg—H bonds formation; (ii) chemisorption: are dissociated to hydrogen atoms and chemisorbed on the surface of the Mg‐based materials; (iii) penetration and diffusion: The chemisorbed hydrogen atoms diffuse from the surface into the Mg‐based materials (α phase); (iv) nucleation and growth: the Mg hydride (β phase) nucleates and grows when the hydrogen concentration exceeds the solid solubility limit. Accordingly, the hydrogen desorption process is regarded as their inverse process. The rate‐controlling step (RCS) may vary for different Mg‐based hydrogen storage materials.
The hydrogen absorption/desorption process is a typical gas–solid multi‐phase reaction. During the hydrogen absorption process, the kinetics may change because of the following issues [14] (Figure 2.2): (i) MgH2 can form on the surface of bulk Mg and impede hydrogen diffusion, which is due to the fact that the hydrogen diffusion rate in MgH2 is much lower than that in Mg. (ii) The formation of Mg metal on MgH2 surface is thermodynamically unfavored. (iii) The hydrogen dissociation/re‐combination on Mg surface typically needs to overcome high activation barriers, when impurity phases, such as MgO and Mg(OH)2, are formed on Mg surface. (iv) The surface states and low diffusion distance of Mg/MgH2 nanocrystallization will also affect the RCS and improve the kinetics. Therefore to improve the kinetics of hydrogen absorption and desorption of Mg‐based material, it is required to optimize the hydrogen dissociation and diffusion routes.
Figure 2.2 Schematic illustration of the kinetic steps in the hydrogen storage process (up) and Lennard‐Jones potential diagram describing continuous energy barriers in the process of hydrogen absorption and desorption (down).
Source: Reproduced with permission from Li et al. [14] Copyright 2021 Elsevier.
The hydrogen adsorption process can be described by the Langmuir theory. The monolayer adsorption is first formed on the Mg‐based materials. After the monolayer adsorption saturates, the multi‐layer adsorption randomly occurs on the first layer. The uppermost molecule layer is always in equilibrium with the gas phase H2. The reaction rate of this process can generally be described by Eq. (2.2):
where and are the coefficients of physical ab‐/desorption, respectively. and are the activation energies of physical ab‐/desorption. θph is the surface coverage ratio, P is the hydrogen gas pressure, T is the Kelvin temperature, and R is the gas constant. If the physical absorption does not require activation energy, and the absorption rate in the process is much faster than the desorption rate, Eq. (2.3) can be expressed as:
The chemisorption and dissociation of H2 occur on the surface of the Mg‐based materials. H2 can be transformed into hydrogen atoms, and then diffuse into metals and hydrides. Generally, the process of chemisorption needs activation energy. With the increase in the percentage of the absorbed hydrogen on the surface (θch), the activation energy will increase, and the rate of chemisorption will decrease. The rate of chemisorption vch can be calculated by the Elovich equation [45], as follows:
where and are the coefficients of chemisorption for the hydrogen absorption and desorption processes, respectively. a and b are the related coefficients. θch is generally between 0 and 1.
Aside from the Elovich equation, the chemisorption rate can also be expressed as follows [46]:
where r0 is the initial particle radius, Peq is the MH equilibrium pressure, and kch is the rate constant of chemisorption. The effects of particle radius and equilibrium pressure on the chemisorption rate are considered. Then, combined with the Arrhenius equation, Eq. (2.6) can be expressed as [46]:
where ξ is the reaction fraction and t is the time, kch,0 is the factor for the rate constant of chemisorption.
As the unavoidable oxidation, the surface of Mg‐based material is always covered by an oxide passivation layer. Due to the low diffusion rate of hydrogen atoms in the oxide layer, hydrogen atoms will be inclined to transport to certain locations without the passivation layer. Then, the high concentration of hydrogen in this region is created, favoring the nucleation and growth of hydrides. Then, due to the volume difference between α and β phases, the microscopic cracks on the surface will be generated by the action of stress. These cracks will provide a faster channel for hydrogen transport. The absorbed hydrogen on the metal surface can contact with the β phase directly, causing hydrogen penetration. The surface penetration process refers to the chemically absorbed hydrogen atoms on the surface passing through several atomic layers inside the alloy. The rate of surface penetration can be expressed as follows:
where and are the coefficients of surface penetration for the hydrogen absorption and desorption processes, and Cβ is the hydrogen concentration in β phase. When surface penetration is the RCS, other steps of the hydrogen absorption or desorption process can reach a quasi‐equilibrium state. Then, the surface penetration rate can be given as [46, 47]:
where ksp is the rate constant of surface penetration. Then, Eq. (2.8) can be changed to the form of reaction fraction with time:
where ksp,0 is the factor for the rate constant of surface penetration.
When the hydrogen atoms penetrate the inside of β phase, the transport in β phase follows the classical diffusion law. However, with the complexity of the actual material, the diffusion coefficient is affected by many factors, such as temperature, phase composition, particle shape, and grain boundaries/bulk regions. Many analytical models are derived from the diffusion equation at isothermal conditions and shown in Table 2.1, including the Jander [48], Ginstling–Brounshtein (G–B) [49], Valensi–Carter (V–C) [50], and Chou [47] models. These kinetic models are geometrical contraction models, which assume that the hydrogen ab‐/desorption occurs uniformly on the surface. However, the geometrical disorder, surface roughness and heterogeneity explicitly, etc. are not taken into account [14].
Table 2.1 The kinetic models used for hydrogen absorption and desorption reactions [45, 52].
Kinetic models
Equations
Geometrical contraction
Jander
[1 − (1 −
ξ
)
1/
d
]
2
=
kt
a)
G‐B
(1 −
ξ
) ln(1 −
ξ
) +
ξ
=
kt
(for cylinder particles) (for sphere particles)
V‐C
b)
Chou
Nucleation and growth
JMAK
[− ln(1 −
ξ
)]
1/
n
=
k
1/
n
t
c)
a)d is the dimensionality.
b)z is the volumetric expansion ratio.
c)n is the Avrami exponent.
The nucleation and growth of the β phase is the essential step in hydrogen absorption. This step is realized by the generation and migration of the interface between α phase and β phase. As the β phase grows, the hydrogen reaches the α‐β interface by diffusion. Generally, the generation and migration of the α‐β interface are mainly affected by three factors: the local chemical driving force, the stress caused by volume expansion, and the local Laplace pressure [14]. The classical JMAK model was established to describe the nucleation and growth of β phase, which is suitable for isothermal conditions.
Although there are many kinetic models of hydrogen absorption and desorption reactions, most of them are based on three factors of kinetics: rate constant, activation energy, and the corresponding kinetic equation according to the RCS. Generally, the kinetic models can be used to fit the experimental data of the hydrogen absorption and desorption reactions according to different kinetic measurement methods. The RCS at different pressures, temperatures, and particle radii can be analyzed using the fitted results, as well as the values of activation energy and rate constant.
2.2 Methods for Improving the Hydrogen Storage Performance of Mg‐based Materials
2.2.1 Alloying
Alloying is an effective strategy to improve the hydrogen storage kinetics and thermodynamics of MgH2. Multitudinous works reported that the thermodynamic destabilization can be achieved by alloying Mg with other elements, including Co [52], Cd [53], Al [54], Si [55], Fe [56], Ni [57], and Cu [58], to form various intermetallic compounds. However, it's worth noting that the hydrogen content of these alloy systems notably degrades along with the change in crystal structures.
Among numerous Mg‐based alloys, Mg2Ni is extensively studied. The corresponding hydride Mg2NiH4 possesses enthalpy and entropy of formation of −64 kJ mol−1 H2 and −122 J K−1 mol−1 H2, which are lower than that of pure Mg (−74 kJ mol−1 H2 and −130 J K−1 mol−1 H2) [59]. It implies that the dehydrogenation storage properties of Mg2NiH4 are improved compared to MgH2. Hydriding combustion synthesis (HCS) [60] is one of the most efficient technologies for synthesizing Mg–Ni‐based hydrides. Compared to the traditional smelting process, HCS can be used to prepare alloys of higher purity and improved kinetics under conditions that are easy to obtain. Subsequently, further enhancement of the hydrogen storage properties of Mg–Ni‐based hydrides has been achieved by combining nanosizing, solid‐solution, surface modification, and catalysis on the basis of the HCS technology.
Zhang and coworkers [61] synthesized monodispersed Mg2NiH4 single crystal nanoparticles with the well‐defined MgO coating layer (∼3 nm) that are in situ encapsulated on the surface of graphene sheets through a gas–solid‐based methodology called hydriding chemical vapor deposition (HCVD), which possess ultrahigh structural stability and superior desorption kinetics (Figure 2.3a). It is worth mentioning that the MgO layer shows superior gas‐selective permeability to prevent further oxidation of Mg2NiH4 nanoparticles, meanwhile accessible for hydrogenation and dehydrogenation. Ma et al. [62] prepared ultrafine Mg(In) solid‐solution particles via the arc plasma method to significantly enhance the hydrogen storage kinetics and thermodynamics of Mg‐based hydrides (Figure 2.3b). They have clearly confirmed for the first time that the indium diffusion‐dominated phase transition is the driving force for the evolution from a mixture of MgD2 and Mg–In intermetallic to Mg‐based solid solution during the deuterium release by means of in‐situ synchrotron X‐ray diffraction (ISXRD), neutron powder diffraction (), and density functional theory (DFT) calculations. Shi et al. [63] proposed a special mechanism for “vacancy‐mediated hydrogen spillover” (VMHS) in highly reactive and air‐stable bulk Mg–Ni‐based hydrides by a simple and efficient surface modification, water‐activated, which presented superior hydrogen desorption properties at relatively low temperatures as depicted in Figure 2.3c. DFT calculations suggested the VMHS mechanism for the dehydrogenation process (Figure 2.3c3): the Ni@Mg(OH)2
