【Research Background】
Magnesium (Mg), one of the alkaline earth metals, has high richness and low development cost, is the fourth most common element on earth after iron, oxygen and silicon, while Mg is the eleventh largest element in the human body, essential for maintaining biological systems, Mg is also the third richest element in seawater after sodium and chlorine. At present, humans can obtain magnesium metal and various magnesium-based materials through hydration, chlorination and electrolysis of various Mg compounds.
The application of MG in traditional industries is manifested in various aspects, such as aircraft and rocket components in aviation, high-strength casting and welding, solid hydrogen storage agents, steel sulfur removal reagents, photogenerated plates and flash phases in the printing industry, etc. In recent years, there has been interest in the application of magnesium based in the field of nanomaterials. In the field of catalysis, Mg alloy can be used as an electrocatalytic material for fuel cells and water electrolyzers; In the field of optoelectronics, Mg-based materials have been used in applications such as photocatalysts, solar cells and various gas sensors; In the field of energy storage, rechargeable magnesium batteries have high safety and large volume capacity and have been identified as a potential alternative to lithium metal batteries. Although magnesium-based nanomaterials have good application prospects, it is still a challenge to synthesize magnesium alloy nanoparticles with uniform dispersion at 3-10 nm. Therefore, in-depth understanding and reasonable regulation of the physical and chemical properties of Mg and the preparation of small particle nanomaterials are the key issues to improve the performance of magnesium-based materials in various energy applications.
【Article Introduction】
Recently, Professor Jong-Sung Yu of Kyungbuk College of Science and Technology in Daegu published a review article entitled "Magnesium: properties and rich chemistry for new material synthesis and energy applications" at Chemical Society Reviews. This review systematically introduces the physical and chemical properties of Mg and its research progress in material synthesis and modification, summarizes its application in energy fields such as electrocatalysis, photocatalysis, and secondary batteries, and discusses the development prospects and future challenges of Mg-based materials, which is of great significance for researchers to design new Mg-based nanomaterials with unique properties.
【Introduction】
1. Physical and chemical properties of Mg
In 1808, Humphry Davy first isolated the metal Mg by electrolyzing a mixture of moist magnesium oxide (oxide) and mercury oxide. Since its discovery, Mg has been widely used, such as to make aircraft and cars, and even to make explosives, and in the past two decades, the research of magnesium-based materials has gradually shifted to the field of energy.
1.1 Physical and chemical properties of magnesium
Mg is a white solid with atomic number 12, belongs to group IIA of the periodic table, and is an alkaline earth metal. Mg is one of the lightest elements, with an atomic weight of 24.312 g mol-1 and a density of 1.740 g cm-3, which belongs to a hexagonal densely arranged crystal structure with relatively low melting and boiling points, 650 C 1090 C, respectively, which is conducive to the use of molten Mg low-temperature synthetic materials. Magnesium metal has excellent reducing ability, easy to form oxides, nitrides, halides and other salts, etc., MgO can protect the internal magnesium metal from being reacted. Magnesium reacts with almost all non-metals and acids, but rarely with alkalis and organic solvents, and is soluble in hot water. Mg exists as a stable compound, mostly showing +2 valence and a small amount of +1 valence, and Mg can establish non-covalent interactions (magnesium bonds) with organic matter. The above unique physicochemical properties make Mg can be used as catalysts, dopants and alloy materials in most energy fields, and has potential application value.
Figure 1. Mg element
1.2 mg of related chemical reactions
(1) Magnesium thermal reaction. Refers to the process by which reactants and Mg are heated in an inert atmosphere for more than 650 C, the reactants are reduced and Mg is oxidized, which is a simple, low-cost and scalable reduction method. For example, Si can be prepared by Mg and SiO2 at high temperatures, and high-quality graphene can also be produced by burning Mg in CO2. In nanomaterials, this method can be used to construct pore structures, and Sinhamahapatra et al. successfully prepared surface-defective TiO2 by changing the molar ratio of anatase TiO2 to Mg (Figure 2). Mg is also highly reactive with other heteroatoms such as nitrogen, for example, Mg-assisted carbonization from g-C3N4 can prepare highly graphitized nitrogen-doped carbon, while the interaction of Mg and N produces Mg3N2 particles that can be embedded between carbon layers, which can act as pore generation templates (Figure 3). Currently, the only challenge with magnesium thermal reactions is its high exothermic nature, which leads to structural loss, so achieving effective temperature control is critical.
Figure 2. Magnesium thermal reaction to synthesize pore TiO2
Figure 3. G-C3N4 magnesium thermal reaction
(2) Alloying or doping of Mg
Mg can not only form alloys with several elements, but also act as a dopant for different materials. In this section, the authors focus on various magnesium alloys and magnesium-doped materials for energy applications.
Mg readily forms alloys with most transition metals (TMs) as well as group I and Group II metals. In general, magnesium alloys are obtained by mixing quantitative metals heated to a specified temperature under an inert atmosphere, but the bulk materials obtained by these preparation methods cannot be used in the energy field. At present, researchers have developed different methods to prepare Mg alloyed nanoparticles: Tetteh et al. synthesized a uniform PtMg nanocatalyst for fuel cells by combining Pt nanoparticles with molten Mg; Itahara et al. used low eutectic salt mixtures to reduce various PtMg catalysts. However, the particle size of the existing method is relatively large and the size distribution is uneven, in order to obtain better catalytic performance, it is necessary to develop more effective chemical synthesis strategies to obtain Mg-based nanoalloy particles with small size and high dispersion.
Mg doping is a simple and unique method used to adjust the properties of materials. At present, a variety of technologies have been developed, including spray pyrolysis, drop casting technology, metal-organic chemical vapor deposition (MOCVD), hydrothermal synthesis and spin coating, for doping Mg into different materials, and doping Mg2+ in metal oxides can help regulate grain size.
(3) Mg pore forming agent
The pore structure provides rich active sites for catalytic materials, which helps to obtain high catalytic performance. Commonly used hard template methods (such as zeolite, SO2) have problems such as complex methods and environmental pollution, especially to remove SO2, which requires corrosive HF or NaOH. Due to its unique chemical properties, MgO has been recognized as an environmentally friendly template for synthesizing highly porous carbon materials, which can be removed by non-corrosive acid etching.
2. The safety of magnesium
Metal Mg and its alloy have an explosion hazard, although the thin layer on the surface of its MgO can partially inhibit the reactivity, so that it can only ignite when it reaches the melting point temperature in the air, but when Mg is in the molten state, powder state, etc. There is still an explosion hazard. In terms of environmental safety, molten Mg spontaneously ignites when exposed to air; Molten Mg can react with iron oxide thermite to increase the reaction temperature to more than 2473 K. Molten Mg also has a high affinity for water, when exposed to H2O, it can expand to 1000 times the original volume, while being able to reduce water, releasing highly flammable and explosive H2 gas, so water cannot extinguish magnesium fire. In terms of storage, the degree of danger depends on the form of presence of Mg, when the powder reaches its ignition temperature, the self-heating reaction of Mg powder can cause fire and explosion. In terms of personal safety, excessive exposure to magnesium may irritate the upper respiratory tract, while excessive intake of Mg may lead to muscle weakness, drowsiness and confusion. For the above safety considerations, UV goggles should be worn when using Mg powder, because the ultraviolet rays produced by Mg combustion will permanently damage the retina of the human eye; The use of molten Mg requires ensuring that all reaction vessels are free of moisture, metal oxides, etc., to avoid explosions, etc.
3. Catalytic application
Mg is widely used as an alloying element and dopant to improve the catalytic activity and durability of transition metal-based catalysts, and Mg itself is also active in some reactions or as an additive material to improve chemical adsorption, Figure 4 shows the related electrocatalytic and photocatalytic applications using Mg and its derived materials.
Figure 4. Classification of applications of MG in the field of catalysis
3.1 Oxygen reduction reaction (ORR)
The efficiency of ORR is directly related to the performance of fuel cells and metal-air batteries, and slow dynamics and high cost and instability of precious metals are major issues limiting the commercialization of ORR. Mg has high positive electricity, which can change the surface electronic properties of metal catalysts through direct electron transfer, and the electron-rich surface reduces oxygen affinity, thereby enhancing ORR activity, and the strong interaction between Mg and metals will also improve the electrochemical stability of metal catalysts.
(1) Mg as a catalyst
Figure 5. Mg as an application example of ORR catalyst
Compared with Al and Ca, Mg has the best adsorption power and ORR catalytic activity, and theoretical calculations predict that Mg-N2-C has ORR activity similar to Fe-N4-C. Liu et al. synthesized the Mg-N-C catalyst by pyrolysis of Mg-based metal-organic frameworks (Figure 5) and found that the half-wave potential of Mg-N-C (910 mV vs. RHE) was higher than that of commercial platinum carbon (860 mV) in 0.1 M KOH solution, and retained its initial activity after 5000 cycles, indicating excellent ORR performance.
(2) Mg as a dopant
Figure 6. Application examples of Mg-doped ORR catalysts
Mg doping can improve ORR catalytic performance. Roche et al. demonstrated that doping with Mg improved the ORR selectivity of MnOx materials while helping to inhibit the formation of H2O2 in MnOx catalysts. In addition, Mg doping can often also improve the charge-discharge performance of metal-air batteries, Zhang et al. introduced Mg into a three-dimensional ordered mesoporous Co3O4 synthesized with a polystyrene spherical template (Figure 6), assisting in the formation of more Co activity centers, reducing the reaction barrier, thereby enhancing ORR reactivity.
(3) Mg as an alloying catalyst
Figure 7. Application examples of PtMg alloy catalysts
Pt-group metal alloying is an important strategy to reduce catalyst cost, and common Pt-TM alloys will reduce ORR performance due to TM metal precipitation. Mg is a good alternative metal, the ligand effect caused by the electronegativity difference between Mg and Pt can change the electronic properties of Pt, while high alloy formation can improve alloy stability and prevent dissolution. Tetteh et al. prepared a PtMg alloy for the first time in 2020 (Figure 7) with an average particle size of 5.22 nm, which showed excellent durability in practical applications, and after 30,000 cycles, the area specific activity decreased by only 13%, far better than traditional platinum-carbon catalysts (60%).
3.2 Water Splitting
(1) Electrolyzed water application
Electrocatalytic water splitting, consisting of cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), is considered one of the most promising sustainable hydrogen production energy technologies. Studies have found that positively charged Mg can play a key role in changing the electronic structure of water splitting catalysts. For the HER reaction, TM-Mg alloys with hydrogen storage have been used as HER catalysts, and Sadeghi et al. recently reported a TM-doped MgB2 catalyst (Figure 8), in which the Mg layer is placed between the honeycomb structure of B, and TM doping inhibits the transition of MgB2 to Mg(OH)2, hinders the diffusion of the catalytic active center, and exhibits ultra-small overpotential. For OER reactions, Mg doping with noble metals can reduce catalyst cost, Liu et al. reported a highly active and stable Mg-doped RuO2 catalyst (Figure 9), and found that all Mg-RuO2 samples prepared at different temperatures showed higher electrocatalytic activity than RuO2 catalysts, and the optimal sample had an overpotential of 228 mV at 10 mA·cm-1, and cycled 10,000 turns, and the overpotential increased by only 43 mV.
Figure 8. Application example of TM-doped MgB2 HER catalyst
Figure 9. Application example of Mg-RuO2 OER catalyst.
(2) Photolysis of water applications
Among the various methods for modifying photocatalytic water splitting catalysts, magnesium thermally reduced semiconductors have proven to be one of the simplest and least costly methods for designing band gaps. Sinhamahapatra et al. synthesized black TiO2 (BT-x, x is Mg content, Figure 10) for photolysis of water by heating a mixture of commercial anatase TiO2 (CT) and Mg powder mixture at 650 C for 5 hours in a tube furnace, and the band gap was reduced to 2.02 eV, and abundant oxygen vacancies and surface defects were formed, among which the hydrogen production rate of BT-0.5 was 43.2 mmol·h-1 g-1. It is 4.2 times higher than CT and maintains initial photocatalytic activity for 30 days without any attenuation.
Figure 10. Black TiO2-Mg photolyzed water application example.
3.3 CO2 reduction reaction
(1) Electrocatalytic CO2 reduction
Electrocatalytic CO2 reduction reactions can produce valuable chemicals that contribute to carbon neutrality goals, and the products of CO2 reduction reactions depend mainly on electrode composition. MgO has excellent chemical adsorption of CO2, which can promote the CO2 activation process. Li et al. prepared a hollow carbon sphere (HCS)-anchored MgO catalyst with high CO selectivity, and the synergy between MgO and HCS improved the catalytic performance. Wang et al. prepared a single-atom Mg-C3N4/CNT catalyst (Figure 11) for reducing CO2 to CO, the Mg site was conducive to CO2 adsorption and activation, and the interaction between the Mg activity center and the reaction intermediate enhanced CO2RR activity.
Figure 11. Mg-C3N4/CNT catalyzed CO2RR application example
(2) Photocatalytic CO2 reduction
As with other photocatalytic applications, reducing the bandgap of semiconductors to absorb a wider range of light is fundamental to improving CO2 conversion efficiency. Razzaq et al. used Mg to assist the preparation of TiO2-x(RT) photocatalyst (Figure 12), using Pt as a cocatalyst for the conversion of CO2 to CH4, and found that the more Mg content, the smaller the band gap, and the photocatalytic CO2 experimental results showed that the CH4 yield of Pt-1.0-RT was 1640.58 ppm g-1 h-1, which was 3 times higher than the catalytic activity of traditional Pt-1.0-CT.
Figure 12. Application example of Mg-assisted TiO2-x catalyst CO2RR
3.4 N2 reduction reaction
N2 and H2O can synthesize NH3 and are key compounds in the manufacture of various forms of fertilizers, so nitrogen reduction reactions are important for the chemical industry, especially for the large-scale synthesis of nitrogenous organic compounds. Ileperuma et al. reported a Mg-doped TiO2 catalyst that could be used for N2 photocatalytic reduction to NH3, and Mg2+ doping helped improve electron transfer with a maximum NH3 yield of 12 mol L-1. Hu et al. proposed the use of molten MgCl2 for the electrosynthesis of NH3 by electrochemical N2, with an efficiency of up to 92%.
3.5 Organic reduction reaction
In addition, Mg-based catalysts also play an important role in organic matter reduction reactions, such as photocatalytic degradation of organic matter and electrochemical alcohol oxidation (AOR). It was found that MgO, as a cocatalyst, can promote Pt and Pd catalysts to improve the oxidative activity of methanol and ethanol, similar to the role of Ru in PtRu alloy, MgO adsorbed OH can react with CO on the surface of Pt, resulting in more active sites can be exposed to facilitate further reactions, this property makes it one of the best candidates for reducing the amount of precious metals in OR.
4. Application in the field of energy storage
Rechargeable magnesium batteries (RMB) have become one of the most attractive energy storage technologies. Compared with LIBs, RMBs have many advantages, including abundant Mg resources, high theoretical volume capacity (3833 mAh cm-3), low standard reduction potential (2.37 V vs. SHE) and small ion radius (0.72 Å), and high safety. The working principle of RMBs is similar to that of LIBs, and both belong to the "rocking chair" energy storage mechanism. However, there are many problems with Mg metal anodes, such as interface passivation and corrosion of magnesium, magnesium dendrite problems, severe volume expansion, etc., which will lead to significant degradation of battery performance.
4.1 Magnesium-ion battery (negative electrode)
At present, there are two common Mg anode modification strategies: designing artificial solid electrolyte interface (SEI) on the surface of Mg anode and using alloy anode materials.
Figure 13. Mg negative electrode artificial SEI application example
(1) Construct an artificial electrolyte interface. In general, artificial SEI can be formed on the Mg negative electrode by using electrolyte additives, immersing the Mg electrode in a multifunctional solution, or modifying the Mg electrode with a special process. The artificial SEI film has good ionic conductivity but poor electronic conductivity, which prevents the electrolyte from decomposing on the electrode surface, allowing Mg2+ migration through the SEI without deposition. In addition, SEI acts as a passivation layer, protecting the Mg anode below from the continuous corrosion of the liquid electrolyte. Therefore, designing artificial SEI membranes is an effective way to improve the stability and electrochemical performance of Mg anode. Li et al. designed a stable heterogeneous SEI with a low surface diffusion barrier (Figure 13) on the Mg anode, consisting of MgCl2 and silicone, which effectively improved the interface passivation problem of Mg anode in electrolyte.
Figure 14. Application example of Bi-Mg alloy anode
(2) Mg-based alloy anode material. The anode of MG alloy has a high redox potential and exhibits the characteristics of high voltage and high capacity, but the research of high-performance alloy anode is still a major challenge. Bi is the most ideal element for Mg alloy anodes, which can provide theoretical specific capacity and fast Mg2+ transmission rate that exceeds commercial graphite anodes. Shao et al. prepared high-performance Bi nanotubes (bi-NT, Figure 14) by hydrothermal reaction, showing excellent rate performance and excellent cycle stability, and its excellent electrochemical performance can be attributed to the fact that the nanoporous structure generated in situ can effectively suppress volume changes and reduce the transmission distance of Mg2+.
Figure 15. Example of a Mg-regulated grain orientation application
(3) Physical modification strategy. In addition, Mg anode can also be modified by changing the physical properties, for example, Mg nanoparticles can reduce the thickness of the surface passivation film and improve the ion transport efficiency in the Mg anode; Higher energy density can be obtained by manufacturing ultra-thin Mg alloy anodes; Controlling the grain orientation and size of Mg alloys can also enhance the electrochemical performance of Mg anodes (Figure 15), which has the lowest overpotential compared to pure Mg anodes, while Mg-Bi and Mg-Zn alloys have relatively high overpotentials.
4.2 Magnesium-sulfur batteries
Magnesium-sulfur battery (Mg-S) has the advantages of high energy density, high safety and low cost, and the two-electron transfer between the Mg negative electrode and the S positive electrode produces a theoretical voltage of 1.77 V, and the theoretical energy density is as high as 3221 Wh L-1, even surpassing the traditional lithium-sulfur battery. However, the commercialization of Mg-S batteries must also overcome several obstacles: (1) adapted electrolyte; (2) Serious polysulfide shuttle problems. At present, in the study of Mg-S batteries, Mg anode has received less attention, and the focus has been on finding effective electrolytes.
4.3 Capacitors
Magnesium-ion hybrid capacitors (MIHCs) can transfer more electrons due to their multivalent metal ions, allowing higher energy densities to be obtained. Tian et al. reported a printable magnesium ion quasi-solid hybrid capacitor, the capacitor with vanadium nitride nanowire (VN) as the positive electrode, nanoflower-like MnO2 as the negative electrode, MgSO4-PAM as the gel electrolyte, because VN nanowires have a large number of connected pore structure, which is conducive to Mg2+ transmission, so it has excellent rate performance, which exhibits a high energy density of 13.1 mWh·cm-1, output power of 72 mW cm-3, potential range of 0~2.2 V. 5. Other applications
In addition to the above-mentioned excellent performance in the fields of catalysis and energy storage, Mg-based materials also show great potential in the fields of solar cells and hydrogen storage materials.
5.1 Solar cells
Solar cells are photovoltaic devices that generate electricity through the absorbed sunlight. According to the type of basic semiconductor material selected, it can be divided into dye-sensitized solar cells (DSSC), quantum dot solar cells (QDSC), organic solar cells (OSC), silicon solar cells, and perovskite solar cells (PSCs).
Figure 16. Mg-doped materials as charge selective layer application examples
(1) Mg-doped materials are used as charge selection layers (ESL). Doping Mg2+ in ESL materials can improve charge transport performance because photogenerated electrons easily tunnel through Mg and MgO-doped films, helping to inhibit electron-hole recombination. In addition, doping Mg2+ into metal oxides can shift the conduction band (CB) minimum and Fermi levels upwards closer to the lowest unoccupied molecular orbital (LUMO) of the lower substrate, facilitating the injection of electrons from the photoanode material in the lower layer. Kardarian et al. doped Cu2O with different proportions of Mg2+ (Figure 16) and found that the carrier mobility and concentration of Cu2O were significantly improved at the optimal doping concentration.
Figure 17. Mg oxide as a substrate application example for silicon solar cells
(2) Mg oxide as a substrate for silicon solar cells. Silicon solar cells dominate the photovoltaic field, with high conversion efficiency, simple structure and low operating temperature, and the maximum theoretical energy conversion efficiency of crystalline silicon solar cells is 30%. Wan et al. studied the thermal evaporation MgOx/Al electron band structure and conduction characteristics (Figure 17) and applied it for the first time to the back of an n-type silicon solar cell, measuring its practicality and finding that all photoelectric parameters were significantly enhanced, with a cell efficiency of 20% for MgOx containing 1 nm. Li et al. designed a transparent hybrid film for single-sided and double-sided dopant-free silicon solar cells, consisting of Mg, MgFx and MgOy mixed phases, the interaction between Mg, F and O leads to a metal phase transition that increases film transparency, which holds the record for the highest efficiency of double-sided solar cells.
5.2 Hydrogen storage applications
Hydrogen is a green energy source that can be stored as a pressurized gas, cryogenic liquid, or as a solid fuel. Among them, solid-state storage has the highest safety and lowest transportation costs, while MgH2 has the highest energy density (9 MJ kg-1Mg) of all reversible metal hydrides, making it ideal for energy storage applications.
(1) MgH2 efficient hydrogen storage. MgH2 is a transparent dielectric material with a band gap of 5.6 eV. There are three challenges to the MgH2 system: (1) thermodynamic instability; (2) High reaction temperature; (3) High reactivity with oxygen. Additives are often used to modify materials, most additives do not react with Mg/MgH2, and are generally considered to be hydrogenation/dehydrogenation catalysts, and the addition of excessive additives leads to alloy formation. At present, a series of materials including transition metals, transition metal oxides and intermetallic compounds have been reported as effective catalysts, such as Lan et al. adding (Ni-V2O3) @C nanocomposites to MgH2 (Figure 18), so that hydrogenation can occur at room temperature, and the dehydrogenation temperature can be reduced to 190 C, and the hydrogen storage performance is significantly improved.
(2) Magnesium-based hydrogen storage materials. Magnesium-based alloy materials are also good hydrogen storage materials, for example, in Mg-Pd alloys, it has been found that MgH2 grows at the Mg-Pd interface during hydrogenation, and metal Mg grows near the interface during dehydrogenation, this phenomenon is also called the blocking effect, which limits the diffusion of H2 in the MgH2 layer during the dehydrogenation process and prevents Mg rehydrogenation, thereby improving the hydrogenation and dehydrogenation cycle efficiency.
Figure 18. Application example of hydrogen storage in MG-based materials
5.3 Magnesium sensor
Figure 19. MgO sensor application examples.
Due to their unique physicochemical properties, Mg and its compounds have been used as sensors in humidity, alcohol, gas, trinitrotoluene (TNT), hydrogen sulfide (H2S), hydrogen (H2), carbon monoxide (CO) detection, and in aluminum processing. The choice of sensor material requires high selectivity, fast recovery time, long-term stability, high response, low operating temperature, and low cost. Taking the H2S gas sensor as an example, H2S can seize oxygen from the surface of MgO and at the same time supply electrons back to MgO, opening the electron transport path, resulting in a decrease in resistance and an increase in MgO current. El-shamy et al. reported a carbon dot modified magnesium oxide particle for H2S detection (CDots@MgO, Figure 19) with high sensitivity and responsiveness at low operating temperatures.
5.4 Other Applications
In addition, magnesium-based materials can also be used in thermal catalysis and plasma. Taking the plasma effect as an example, unlike traditional metals such as Au and Ag, Mg shows a very strong plasma response in the ultraviolet wavelength range (3.8 to 1.3 eV). Biggins et al. synthesized Mg and MgO nanoparticles using a mixture of lithium naphthalene and di-n-butyl magnesium in anhydrous THF, and found that the far-field scattering spectra and scattering spectra of individual nanoparticles obtained in the wavelength range of 450-850 nm were at 600 and 800 nm, respectively, indicating that Mg has a strong plasma response in the ultraviolet wavelength range.
【Full text summary】
In summary, this paper systematically introduces the application of magnesium-based materials in the fields of energy conversion and storage. Taking the basic physicochemical properties of magnesium as the starting point, this paper reviews its advantages and future application prospects in photocatalysis, electrocatalysis, energy storage, solar energy harvesting, hydrogen storage, plasmon and sensing. This review will help researchers in related fields to fully understand the design concept and research progress of magnesium-based materials, and has important guiding significance for the multi-field development of magnesium-based materials.
【Literature Information】
Magnesium: properties and rich chemistry for new material synthesis and energy applications. Chem. Soc. Rev., 2023. (DOI: 10.1039/d2cs00810f) https:// doi.org/: 10.1039/d2cs00810f