Magnesium-Based Materials for Energy Conversion and Storage

The development of materials for energy conversion and storage systems is essential for the realization of the sustainable and low-carbon society for which China pledged the long-term strategic target to reach peak carbon dioxide(CO2)emissions before 2030 and achieve carbon neutrality by 2060.Because the progress in this field can improve the utilization of the intermittent harvested renewable energy,to level out the fluctuations in production and consumption,to distribute and transport the energy,to elaborate highly efficient technologies for electric vehicles,portable electronics and other applications,and to reduce the overall consumption of fossil fuels.Magnesium(Mg)-based materials are one of the most significant,promising,and rapidly developing materials in the field of energy conversion and storage systems due to their environmental benignity,abundant geological reserves,and high energy density in both hydrogen storage and battery fields [1–10].

In recent years,significant efforts have been made on Mg-based H2storage materials and Mg-based batteries so that the hydrogen storage properties,including thermodynamic,kinetic properties and the cycling properties of Mgbased materials have been much improved and the Mgbased cell with an anodic utilization efficiency of 82% is achieved [11–21].For their application,future progress must be made on the low cost preparation technology,stable performance and long service life of Mg-based materials.So that significant technical innovation and chanllenges always need to be addressed proactively,such as(1)combination of Mg-base materials with nanostructure design and emerging technologies;(2)thermodynamics and dynamics and their effects on the performance of Mg-based materials;(3)highly efficient catalysts and their mechanism;(4)the compatibility between the Mg-based materials and the electrolyte.

Considering the tremendous progress made in recent years,the editors will bring a selection of the latest innovative results on Mg-based materials related to H2storage and batteries in the present special issue.Ten peer-reviewed articles have been selected to highlight the research trends,discuss the advanced preparation methods for the state-of-the-art materials,and summarize the related comprehensive characterization technologies.

Mg-based hydride is one of the most promising candidates,which can meet the capacity requirements for onboard vehicles [22–24].Dornheim et al.provided a broad overview of the most appealing systems and of their hydrogenation/dehydrogenation properties.Especially,they emphasized the efforts made by the community on improving the material’s thermodynamic and kinetic properties while maintaining a high H2storage capacity.Hu et al.detailedly investigated the effect of Co@C catalyst on the H2storage behavior of Mg90Ce5Y5alloy by TEM,SEM,and PCI techniques.The positive impact attributes to the defects on carbon nanosheets and Co nanoparticles,which effectively reduce the energy barriers for the nucleation of Mg/MgH2phase and accelerate the recombination of the hydrogen molecule.Zhang et al.discussed the H2storage properties of La7Sm3Mg80Ni10composite.The reaction kinetics of the composite was improved by TiO2and La2O3doping due to the decreased activation energy of H2desorption.Furthermore,carbon-based materials have excellent catalytic effects on H2storage reaction of MgH2.Herein,the Tan group reported a carbon-supported Ni3S2catalyst synthesized by facile chemical route using ion exchange resin and nickel acetate tetrahydrate as precursors.The resulting catalyst was used to improve the H2storage properties of MgH2.Apart from H2storage,Ouyang et al.studied the hydrolysis of MgLi-graphite composites as potential candidates for safe and convenient hydrogen release.Both desirable H2conversion efficiency and fast reaction kinetics were found in the MgLi-10 wt% graphite composite.Similar work was presented by Bobet group that the hydrolysis reaction and corrosion behavior of AZ91 with graphite or AlCl3was investigated by polarization curve and electrochemical impedance spectroscopy.To date,the main-hurdle for the commercialization of Mg-based H2storage materials is the high operating temperature and slow dehydrogenation rate,which is caused by the sluggish kinetics.

Aiming to ultimately achieve carbon neutrality,ongoing efforts being attempted include investigating batteries beyond Li-ion that guarantee even higher energy density,such as Mgion batteries.Moreover,the occurrence of Mg dendrites is not common owing to its less reactive nature,leading to improvable safety.Notwithstanding,despite landmark breakthrough that proved the feasible pathway of rechargeable magnesium batteries [25,26],their potential merit are still not fully exploited.There is a lack of practical electrolytes and electrodes that would sufficiently replace Li-ion batteries.Rising to these challenges,mainstream research has focused on overcoming three significant Mg-centric issues:(1)Mg metal as negative electrode is readily passivated,which is not conducive to Mg ion transportation.(2)The compatibility between high voltage cathodes and electrolytes severely restricts the reversible insertion or reaction of Mg-ions in the cathodic electrode materials.(3)poor diffusion kinetics of Mg2+caused by strong electrostatic interactions with anions in electrolyte structures owing to its divalent.

In this special issue,Mg-based materials are also investigated as the next-generation battery materials(the anode for Mg-ion,and Mg-Air batteries),due to the merits of high Coulombic efficiency(close to 100%),high specific volumetric capacity(3833 mA h cm?3vs.Li metal:2046 mA h cm?3),good capacity retention(95% after 1000 cycles)[27,28].It is expected that these in-depth studies can inspire the solution to the above issues of Mg-ion batteries.Gao et al.systematically summarized the effect of alloying elements on the battery discharge properties of Mg alloy anode.The current challenges and perspectives are also discussed,aiming to provide a comprehensive understanding of high-performance Mg-air batteries from the aspects of theoretical and practical considerations.Han et al.studied the phase transformation of their synthesized rare–earth–Mg–Nibased alloy and clarified the formation mechanism process of the AB4-type phase.The discharge properties and electrochemical behaviors of as-cast AZ80-La-Gd anode for Mg-air battery have been investigated by Le et al.The results indicate that the modified AZ80-La-Gd is an ideal anode material for Mg-air batteries in terms of good electrochemical stability and catalytic efficiency.Zhang et al.found that the selfsupporting nanoporous Bi electrodes deliver satisfactory Mg storage performance.Furthermore,the alloy-typed anodes are well-compatible with conventional electrolytes.

This special issue provides deep insights into the forefront of Mg research,highlighting the improvement in the H2storage and the application in the Mg-based batteries.We would like to thank Professor Fusheng Pan,Editor-in-Chief of Journal of Magnesium and Alloys,Professor Xianhua Chen,the Guest Editor of the special issue for Mg functional materials of Journal of Magnesium and Alloys,the publisher,and all of the staff supported our initiative and made this topical issue possible.Also,the editors thank all of the contributors of selected papers presented herein.