Recent advances in electrochemical performance of Mg-based electrochemical energy storage materials in supercapacitors: Enhancement and mechanism

Yuntao Xiao,Xinfang Zhang,Can Wang,Jinsong Rao ,Yuxin Zhang

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

Abstract The application of Mg-based electrochemical energy storage materials in high performance supercapacitors is an essential step to promote the exploitation and utilization of magnesium resources in the field of energy storage.Unfortunately,the inherent chemical properties of magnesium lead to poor cycling stability and electrochemical reactivity,which seriously limit the application of Mg-based materials in supercapacitors.Herein,in this review,more than 70 research papers published in recent 10 years were collected and analyzed.Some representative research works were selected,and the results of various regulative strategies to improve the electrochemical performance of Mg-based materials were discussed.The effects of various regulative strategies (such as constructing nanostructures,synthesizing composites,defect engineering,and binder-free synthesis,etc.) on the electrochemical performance and their mechanism are demonstrated using spinelstructured MgX2O4 and layered structured Mg-X-LDHs as examples.In addition,the application of magnesium oxide and magnesium hydroxide in electrode materials,MXene’s solid spacers and hard templates are introduced.Finally,the challenges and outlooks of Mg-based electrochemical energy storage materials in high performance supercapacitors are also discussed.

Keywords: Supercapacitor;Electrochemical energy storage;Mg-based materials.

1.Introduction

As the tension between the exhaustion of fossil fuels and the growing market for fossil energy intensifies,research is exploring for green energy sources while creating an effective energy storage system to storage the energy generated from renewable energy resources [1–4].There have been many different energy storage devises proposed up,including supercapacitors,metal-ion batteries,metal-air batteries,fuel cells,solar cells,etc.[5–12].Among them,supercapacitors(SCs) have attracted a lot of attention in the field of electrochemical energy storage because of its promising properties such as superior lifetimes,higher power densities,ultrafast charge/discharge rates,and a broad range of working temperatures [13–15].

According to the energy storage process,there are two subcategories of electrode materials utilized in SCs: i) Electric double-layer capacitors (EDLCs),that capitalize on the adsorption and desorption of electrolyte ions on the surface of electrode materials to store charges through non-Faradaic mechanism.Although EDLCs have strong reversibility and excellent cycling stability,their applicability is limited because of their low energy density (5∼10 W h kg-1) [16–18].ii) Charge storage of pseudo-capacitors (PCs) relies on highly reversible,quick redox process that take place on the surface or quasi-surface of the electrode material;the specific capacitance of PCs is typically larger than that of EDLCs [19–22].Moreover,SCs can be categorized into three groups based on the various energy storage processes of the cathode materials and anode materials utilized in them: symmetric type,asymmetric type,and hybrid type [16].

The two important properties of an energy storage device are cycling stability and energy density.Enhancement of the former on can improve the structural stability of the electrode material,reduce the tendency of structure collapse during the cycle,or improve the electrical conductivity of the material,thereby reducing the Joule heat generated during the reaction[20,23].For energy density,according to formula (1),the energy density of SCs can be enhanced in two ways: improving specific capacitance and voltage window.

To improve the voltage window,ionic liquids or organic electrolytes are ordinarily utilized owing to their larger potential windows than aqueous electrolytes.And another approach is developing aqueous asymmetric supercapacitors [24,25].And for improving the specific capacitance of the electrode materials,developing new materials,or regulating the electrochemical performance of existing materials through rational strategies are attainable thoughts [26,27].

Cathode material is an important part of SCs,and its selection has great influence on the performance of SCs.Mgbased electrochemical energy storage materials have attracted much attention because of the superior properties of low toxicity,environmental friendliness,good electrical conductivity,and natural abundance of magnesium resources[28,29].However,due to the single valence state of Mg ion,it’s hard to participate in the surface Faradaic process,resulting in poor reaction kinetics,so there are few up-to-date studies on the application of Mg-based materials as SCs.Herein,it is essential to find appropriate regulative strategies to modify the electrochemical performance of Mg-based materials.In this review,we mainly focus on the enhancement of electrochemical performance of Mg-based materials through various regulative strategies and corresponding regulative mechanism.Taking spinel structured MgX2O4and layered structured Mg-X-LDHs as examples,various regulative strategies including constructing nanostructures,synthesizing composites (such as porous carbon material composites,transition metal-based material composites,conductive polymer composites),defect engineering,and binder-free synthesis,etc.are discussed,and their regulative mechanism are also introduced.Meanwhile,the application of magnesium oxide and magnesium hydroxide in supercapacitor is introduced,including electrode materials,solid spacers for MXene [30],and hard templates [31].Finally,the challenges and outlooks of Mgbased materials in high performance supercapacitors are also discussed.

2.Spinel structured MgX2O4 for supercapacitor

Spinel structure MgX2O4(X=Co,Mn,Fe,Al,etc.) materials have relatively stable structure,diversity and richness of components,so it has become one of the materials with great potential [32–35].For example,MgCo2O4,as a spinel structured Co-based material,its structural characteristics(Fig.1) can be summarized as magnesium cation fill tetrahedral interstice,cobalt cation fill octahedral interstice,and oxygen atoms are stacked in a face-centered cubic structure,belonging to the cubic crystal system [24].Due to its special structure,MgCo2O4has attracted much attention in magnesium-ion battery [36],supercapacitor [37],water oxidation[38],microwave absorption[39],etc.MgCo2O4has advantages of low toxicity,electrochemical response ability,environmental friendly,rich natural resources [40].In addition,compared with bare Co3O4,the electrical conductivity,ion diffusion,and electron transfer of MgCo2O4are improved by substituting a cobalt atom with a magnesium atom in Co3O4[41–43].MgCo2O4has a theoretical specific capacitance of up to 3122 F g-1which has been widely studied in supercapacitor [24,40].Similarly,MgMn2O4,MgFe2O4and MgAl2O4with spinel structures similar to MgCo2O4have also received a lot of attention in the field of energy storage.

Fig.1.The unit cell structure and cation distribution of MgCo2O4.Reproduced with permission from [18],Copyright (2020),with permission from Elsevier.

However,the shortcomings of MgX2O4materials gradually being exposed during the research.For example,the practical capacitance is substantially lower than the theoretical value,the cycling performance and the rate capability are inferior[40,44].Until now,scientists have made great efforts to optimize the electrochemical performance of MgX2O4,including designing special nanostructures [41,45,46],synthesizing MgX2O4-porous carbon materials composites [44,47,48],MgX2O4-transition metal-based materials composites [49–52],and MgX2O4-conductive polymers composites [53].Other regulative strategies,including defect engineering (vacancies [54],doping [55]),and binder-free synthesis [50],etc.are also widely adopted.In this section,the research progress obtained in the regulation of the electrochemical performance of MgX2O4are systematically introduced,and the relevant performance parameters and corresponding mechanisms are shown in Table 1.

2.1. Special nanostructured MgX2O4 for supercapacitor

Designing special nanostructures including onedimensional nanostructures (1D nanostructures: nanoneedles[56,57],nanowires [58,59],nanofibers [60,61],etc.),twodimensional nanostructures (2D nanostructures,including nanosheets [62,63],nanoflakes [64,65],nanoplates [66,67],etc.),three-dimensional nanostructures (3D nanostructures,including nanoflowers [68,69],nanospheres [70,71]) is advantageous to expand the contact area between the electrode material and the electrolyte,and provide more electroactive sites to facilitate redox reactions,and finally,excellent energy storage capacity and high rate performance can be achieved [42,46,72-74].According to Fig.2a,Bhagwan et al.manufactured MgMn2O4nanofibers using electrospinning method and obtained products with various morphologies by adjusting the ratio of metal precursor to PVP (metal:PVP=1,2,and 3).These products were named MGM11,MGM12,and MGM13,respectively.Among them,MGM11 had the best morphology due to the greater metal particle dispersion in a suitable concentration of PVP,which was conducive to the growth of MgMn2O4nanofibers,and the formation mechanism is shown in Fig.2b.A symmetric all-solid-state supercapacitor (ASSSC) was constructed employing PVA-H2SO4gel electrolyte and MGM11 as the cathode and anode,respectively.The ASSSC exhibited an ultra-high voltage window (0∼2 V) as shown in Fig.2c,and its energy density was∼30 W h kg-1at ∼510 W kg-1[75].Employing electrospinning method to prepare nanofibers,and broadening the voltage window through fabricating ASSSC,which were proved effective to enhance energy density by Bhagwan et al.

Fig.2.(a) Schematic diagram of MgMn2O4 nanofiber synthesis procedure.(b) FESEM micrograph of MGM11,MGM12,MGM13,and their schematic representation of the formation of nanofibers,dense nanofibers,and nanorods.(c) GCD curves of ASSSC measured at different current densities.(d) Schematic diagram of synthesis procedure MgCo2O4 microspheres and MgCo2O4 nanoflakes,and their SEM images.(e) TEM images of MgCo2O4 nanosheets.(a,b,c) Reproduced from Ref.[75],Copyright (2022),with permission from Elsevier.(d) Reproduced with permission from Ref.[76]Copyright (2022) Elsevier.(e) Reproduced with permission from Ref.[46].Copyright (2020) Elsevier.

In order to more directly demonstrate the impact of morphological differences on electrochemical performance,solvothermal method was used by Bao et al.for synthesizing MgCo2O4microspheres (MSs) and MgCo2O4nanoflakes(NFs),and their synthesis procedures are shown in Fig.2d.Because of the nanostructure of microspheres and nanoflakes,materials have larger specific surface areas and more mesopores,which is favorable for the surface redox reaction.MgCo2O4MSs and MgCo2O4NFs delivered capacities of 276.3 C g-1and 375.5 C g-1at 1A g-1,respectively.The possible reasons for the former higher than the latter might be the lower internal resistance and NF-based 2D structure[76].

Inspired by the morphology of graphene,Zhao et al.directly produced ultrathin MgCo2O4nanosheets via hydrothermal decomposition using a mixture aqueous solution of MgCl2,CoCl2,and FA at 200 °C for 6 h.The thickness is around 15 nm as can be shown in Fig.2e.The assembled MgCo2O4nanosheets//AC ASC devices had long-term cycling performance with an energy density of 32 W h kg-1at 800 W kg-1(approximately 6.3% loss after 10,000 cycles at 10 A g-1) [46].

In addition,designing multiplex nanostructures such as hierarchical structure or core-shell structure is beneficial to improve the structural stability,while also enhancing the rate at which ions diffuse to the surface of electrode materials from the electrolyte and the efficiency of charge accumulation [77–80].Xu et al.prepared porous double urchin-like MgCo2O4through hydrothermal method.SEM and TEM images shown in Fig.3a,b showed that the single nanoneedles forming the urchin-like structure were constructed of nanoparticles with a diameter of ∼50 nm.This structure facilitated mass diffusion transportation,increased the number of active sites,and enhanced the kinetics of redox reaction.The electrochemical performance of double urchin-like MgCo2O4was assessed in a three-electrode configuration and delivered a specific capacitance of 508 F g-1at 2 A g-1[45].Similarly,Wang et al.synthesized flower-like MgCo2O4(MgCo2O4MFs) composed of porous nanosheets.By reducing the length of the ion/electron diffusion channel and ensuring sufficient contact between electrolyte and electrode materials,such flower-like nanostructure (Fig.3c) facilitated Faradic reactions.The cycling stability and rate performance of an asymmetric supercapacitor (ASC) using active carbon as the anode and MgCo2O4MFs as the cathode were remarkable(as shown in Fig.3d and 3e) [81].As mentioned above,complex nanostructures are indeed effective in improving the specific surface area of the material,but the effect of structural stability on the cycling performance should also be considered during the preparation process.

Fig.3.(a)High magnification SEM images of the MgCo2O4 precursors,scale bars=1 μm and(b)TEM images of the double-urchin-like MgCo2O4 hierarchical architectures after the calcinations,scale bars=50 nm.(c) SEM images of the MgCo2O4 MFs,(d∼e) The Coulombic efficiency of the MgCo2O4 MFs/AC ASC with the last 10 GCD curves inset,as well as the specific capacitance of the MgCo2O4 MFs//AC ASC at various current densities and continuous 5000 GCD curves.(f) Low-magnification and high-magnification FE-TEM image of SiCF/MgCo2O4,(g) Nitrogen adsorption-desorption isotherm of SiCF and SiCF/MgCo2O4,and (h) CV curves for SiCF,MgCo2O4,and SiCF/MgCo2O4 electrodes at a scan rate of 5 mV s-1.(a∼b) Reproduced with permission from [45]Copyright (2016) Elsevier.(c∼e) Reproduced from Ref.[81],Copyright (2020),with permission from Elsevier.(f∼h) Reproduced with permission from [82]Copyright (2018) Elsevier.

Meanwhile,sacrificial templates are often used to prepare multiple nanostructures,for example,Kim et al.first preparedβ-polytype micro &mesoporous SiCF through carbonizing comminuted waste Si wafer and then used SiCF as template to grow MgCo2O4nanoneedles on the surface of SiCF by one-step hydrothermal method,and the structure of SiCF and MgCo2O4/SiCF observed from TEM images are shown in Fig.3f.Observed from Fig.3g,the surface area of MgCo2O4/SiCF was up to 1376 m2g-1while the SiCF reached 1069 m2g-1.It could be seen from CV curves in Fig.3h that MgCo2O4/SiCF had both the EDLC characteristics of SiCF and the pseudo-capacitance characteristics of MgCo2O4.According to the area enclosed by CV curve,the specific capacitance of MgCo2O4/SiCF was calculated as 310.02 C g-1at 5 mV s-1.Then,a solid-state hybrid supercapacitor with MgCo2O4/SiCF electrode and SiCF electrode was constructed using polymer gel electrolyte PVA-KOH-KI as the electrolyte,which exhibited an improved energy density of 41.308 W h kg-1at 464.72 W kg-1[82].Although it’s easier to prepare materials with high specific surface area through sacrificial template method,the cumbersome preparation process needs to be considered.

2.2. Composites of MgX2O4 and porous carbon materials for supercapacitor

Porous carbon materials,such as carbon nanofibers [83],carbon nanotubes[84],graphene/graphene oxide[85],reduced graphene oxide [86],are usually used to prepare composites owing to their attractive advantages including high electrical conductivity,low cost and ecofriendly,and can effectively prevent the agglomeration of nanomaterials [87].For example,Xu et al.prepared nanobrush-structured MgCo2O4/CF using carbon fiber as the template for the growth of MgCo2O4,and the responding schematic diagram was shown in Fig.5a.According to SEM images (Fig.5b,c),MgCo2O4nanorods were observed to grow uniformly and vertically on the carbon fiber.Such morphology makes it easier to expose more active sites and encourage the Faraday reaction through expanding the contact surfaces between the electrode material and electrolyte.Moreover,as shown in Fig.5d,MgCo2O4/CF demonstrated an improved rate performance with a specific capacitance reaching up to 854 mF cm-2at 2.5 mA cm-2[88].

According to Krishnan et al.,developments in MgCo2O4/graphene composites have been achieved [44].This group proposed that the practically achieved capacitance and cycling stability of MgCo2O4were inferior than other MCo2O4(M means other divalent metal ion),which can be attributed to two aspects: i) di-valence of Mg,ii) the dissolution of MgCo2O4under high alkaline conditions,which caused by the weak bond between Mg and Co ions in MgCo2O4crystals.Herein,rGO modified MgCo2O4(MgCo2O4/rGO) was used to prove above assumptions.The successful combination of rGO and MgCo2O4was proved by the Raman spectrum (Fig.4b).Observed from the high-resolution XPS spectrum of Mg 1s in Fig.4a,it found that the splitting at ∼1307 and ∼1305 eV of Mg 1s in the pristine MgCo2O4sample had been merged in the MgCo2O4/rGO,indicating that the binding of Mg and Co atoms was stronger in the MgCo2O4/rGO.Therefore,it’s considered that rGO improved the combination of Mg ion and Co ion in the MgCo2O4lattice,and the corresponding model is shown in Fig.4d.To further prove the difference after adding rGO,SEM (Fig.4c) was employed.The average size of MgCo2O4and MgCo2O4/rGO nanoflowers measured through Image J software is 400 nm and 250 nm,indicating that the size of nanoflowers decreased after adding rGO.

Fig.4.(a) High resolution XPS data of Mg 1s.(b) Raman spectra of MgCo2O4/rGO.(c) FESEM micrographs of MgCo2O4/rGO.(d) A cartoon showing the role of rGO in stabilizing the surface Mg atom.GCD rate performance (e),cycling performance (f),and Nyquist plot of MgCo2O4/rGO (g).(a∼g) Reproduced with permission from [44]Copyright (2017) Elsevier.

Compared with pristine MgCo2O4(∼98.6 mAh g-1),the specific capacitance of MgCo2O4/rGO reached∼158 mAh g-1(at 2.5 mA cm-2).However,the rate performance in Fig.4e decreased slightly after rGO modification,which can be ascribed to the decrease of average pore size after rGO modification,and thus affecting the ion exchange rate between electrode materials and electrolyte.The specific capacitance of MgCo2O4/rGO maintained 104% at 25 mA g-1after 5000 cycles (Fig.4f).The author used ICPMS technology to analyze the concentration of Mg ions and Co ions in 3 M LiOH solution after 3000 cycles.The electrolyte of pristine MgCo2O4electrode had 52 ppm of Mg ions and 2 ppb of Co ions,while the concentration in electrolyte of MgCo2O4/rGO electrode was hard to detected.Moreover,in Fig.4g,the total electrode resistance of MgCo2O4/rGO was lower than pristine MgCo2O4.Above work explains the reasons for the poor cycling performance of MgCo2O4and provides a solution to the problem.

Analogously,Guan et al.prepared graphene-coated nickel foam through CVD method,and then a solvothermal method was employed to fabricate the MgCo2O4nanowires.MgCo2O4nanowires exhibited excellent high-rate performance,that is the specific capacitance remained constant at around 76% when the current density grew from 1 to 50 A g-1.The reasons for good rate performance may be the use of graphene-coated nickel foam as template could increase the growth of MgCo2O4nanowires in a directional manner,improving the contact between the electrode materials and electrolyte and enabling more active sites to participate in the reaction under high current density[47].Through a hydrothermal process,Gao et al.synthesized MgAl2O4/GO,then GO was reduced to rGO when reacted with methanol at a high temperature (MgAl2O4/rGO).The SEM and HRTEM images,as shown in Fig.5e,f,revealed that MgAl2O4had consistent layered structure and extremely thin rGO nanosheets (diameters of about 5 nm) were adhered to the surface of MgAl2O4.The MgAl2O4/rGO electrode exhibited a specific capacitance of 536.6 F g-1at 1.0 A g-1which retained about 96.9% after 10000 cycles at 5 A g-1(Fig.5g).The reasons for better cycling stability (compared with single MgAl2O4) may be attributed to the mutual support of the layered MgAl2O4and rGO nanosheets.Thus,improvements in the structure stability allowed for the avoidance of MgAl2O4re-agglomeration and pulverization throughout the cycling process [89].

Fig.5.(a) The fabrication of MgCo2O4 supported on the carbon fiber nanobrush is depicted in a schematic.(b∼c) SEM micrograph of MgCo2O4 nanobrush-CF.(d) Dependence of the MgCo2O4 nanobrush-CF’s specific capacitance on current density.SEM (e) and HRTEM (f) images of structural characterization of MgAl2O4/rGO.(g) Cycling performance of MgAl2O4/rGO and MgAl2O4 at 5 A g-1.SEM images of MFO (h) and g-CN-MFO (i).(j) FTIR analysis plots of g-C3N4,MFO,and g-CN-MFO electrodes.(a∼d) Reproduced with permission from [88]Copyright (2017) Elsevier.(e∼g) Reproduced from Ref.[89],Copyright (2019),with permission from Elsevier.

In addition to graphene,g-C3N4is a typical twodimensional material,which has the characteristic of rapid charge separation and relatively slow charge recombination during electron transfer [90].Safa Polat et al.studied the impact of g-C3N4on the electrochemical performance of MgFe2O4.Firstly,MgFe2O4@g-C3N4was prepared using a one-step solvothermal process,and the successful combination of MgFe2O4and g-C3N4was proved by the FTIR method.The product had a spongy structure observed from SEM images.In a three-electrode configuration,the specific capacitance of MgFe2O4@g-C3N4was 600 mF cm-2at 1 mA cm-2,greater than 238 mF cm-2of unloaded MgFe2O4electrode.However,the rate performance of MgFe2O4was not improved after the introduction of g-C3N4,which needed further study [91].

2.3. Composites of MgX2O4 and transition metal-based materials for supercapacitor

Transition metal elements have complex and variable valence states,which are conducive to providing more electroactive sites and promoting Faradic reactions.Magnesium spinel compounds combined with transition metal-based compounds can help to overcome the defect that Mg ions do not participate in the electrochemical reaction,but also construct heterogeneous interfaces to realize the synergetic effect between different materials and improve the electrochemical performance[49,92].For instance,Liu et al.prepared MgCo2O4nanowires which were evenly dispersed and strongly orientated on nickel foam (designated as MC@NF).The SEM image of MC@NF has been given in Fig.6a.Then,α-Co(OH)2nanosheets were grown on the nanowires through chemical bath deposition method for 20 min (labeled as MC@COH/NF-2).As Fig.6b,the core-shell structure was consisted of MgCo2O4nanowires (core) andα-Co(OH)2nanosheets (shell),which were evenly dispersed on the nickel foam.The BET surface area of MC@COH/NF-2 was 92.17 m2g-1,and the average pore size was about 4.68 nm.The specific capacitance of MC@COH/ NF-2 was as high as 1681.2 F g-1at 1 A g-1.When the current density increased to 20 A g-1,the specific capacitance still preserved 77.3%,exhibiting excellent rate performance as shown in Fig.6c.Moreover,the morphology of MC@COH/NF-2 did not change significantly after 10 000 cycles at 20 A g-1,and the specific capacitance was preserved by 89.4% (Fig.6d).The equivalent series resistance (Rs) of the material observed from the EIS was only 0.507Ω,which means the lower interfacial resistance caused by hydroxyl and water molecules inα-Co(OH)2[49].

Fig.6.SEM micrographs of MgCo2O4/NF (a) and MgCo2O4@α-Co(OH)2/NF (b).(c) The prepared electrode rate capacity line chart.(d) Retention of capacitance in the manufactured electrode at 20 A g-1 after long cycles.(e) Schematic showing operation of a ZnCo2S4@ MgCo2O4//AC ASC with different electrolyte ions and the charge movement (the numbers are their sizes in pm).(f) Specific capacitance versus current density and (g) cycling stability at 1.25 A g-1.(h) Ionic conductivity/viscosity data and (i) transport numbers of cation/anion at 25 ◦C.SEM micrographs of MgCo2O4/NF (j),MgCo2O4@NiCo-LDH/NF-2 (k),and NiCo-LDH/NF (l) The ASC device’s cycling performance after 5000 cycles at a current density of 10 A g-1.(a∼d) Reproduced with permission from [49]Copyright (2021) Elsevier.(e∼i) Reproduced from Ref.[98],Copyright (2022),with permission from Elsevier.(j∼m) Reproduced with permission from [99]Copyright (2021) Elsevier.

Metal ions of binary metal compounds,as opposed to single-metal materials like Co(OH)2,have a variety of oxidation states,high redox performance,and may be employed as electrode materials for supercapacitors [93,94].Teng et al.had grown MgCo2O4@MMoO4(M=Co,Ni) nanosheet arrays on nickel foam via hydrothermal method.When tested in the three-electrode configuration,both MgCo2O4@CoMoO4(labeled as MCCO) and MgCo2O4@NiMoO4(labeled as MCNO) showed excellent charge storage capacity,in which the specific capacitance of MCCO was up to 1089.94 C g-1(at 1 mA cm-2),and that of MCNO was 1111.57 C g-1.The abundant active sites provided by MgCo2O4nanosheets may be responsible for the good electrochemical performance,and MMoO4grown on MgCo2O4nanosheet arrays boosted the electron transport and ion diffusion while lowering internal resistance.The energy density of the MCCO//AC ASC and MCNO//AC ASC devices was up to 22.78 W h kg-1and 23.46 W h kg-1,respectively [51].

Compared to TMOs,TMSs exhibit better electrochemical activity and higher conductivity due to the extension of chemical bonds,resulting in improved capacity and wider applications in electrochemical fields such as flexible SCs [95–97].The hybrid nanostructured MgCo2O4@ZnCo2S4was prepared by Naskar and co-workers.With the highest energy density of 217 Wh kg-1at 1000 W kg-1,an ASC assembled with MgCo2O4@ZnCo2S4electrode and AC electrode demonstrated exceptionally excellent electrochemical performance (superior to the similar oxides/chalcogenides reported so far).The ASC also displayed relatively high rate performance (Fig.6f) and excellent cycling stability,maintaining 98% of its initial specific capacitance at 1.25 A g-1after 10,000 cycles (Fig.6g).Combined with Fig.6e,the reasons for the excellent electrochemical performance were also explained: firstly,ZnCo2S4improved the electrochemical conductivity,and the spinel structure of MgCo2O4and ZnCo2S4could provide more channels for the intercalation of ions.Moreover,the hybrid nanostructure of ZnCo2S4nanoflakes and MgCo2O4nanorods increased the specific surface area which exposed more electrochemical active sites,facilitating Faradic reactions.At the same time,the role of electrolytes in controlling redox behavior was also studied.As shown in Fig.6h&i,the results showed that the KOH electrolyte had the highest ionic conductivity (189 mS cm-1) and cation transport number (0.85) compared with the other three electrolytes(Mg(OH)2,MgCl2,ZnCl2),i.e.,it had a better electrochemical performance [98].

In addition to the above work,Liu et al.grew NiCo-LDH nanosheet arrays on MgCo2O4nanowires to form a core-shell structure (as shown in Fig.6j,k,l).By adjusting the hydrothermal reaction time of NiCo-LDH growth,MC@NCLDH/NF-1,MC@NC-LDH/NF-2,and MC@NC-LDH/NF-3 were obtained.MC@NCLDH/NF-2 demonstrated a superior specific capacitance of 5701.2 F g-1at 1 A g-1.An ASC device assembled with MC@NC-LDH/NF-2 electrode and AC electrode delivered a high specific capacitance of 252.53 F g-1at 1 A g-1,which maintained 89.03% of this value after 5000 cycles (Fig.6m) [99].

Many studies have improved the electrochemical performance by synthesizing composites with transition metal compounds,but there is still a lack of in-depth discussion on the mechanism of improved performance,such as the heterogeneous interface and the substance of the active site,etc.

2.4. MgX2O4 regulated by defect engineering for supercapacitor

Defect engineering including heteroatom doping and anionic vacancy has been proved to be an effective way to modify electrochemical properties.Anionic vacancy could improve reaction kinetics and regulate the state density around the Femi level.[100–103].Wang et al.synthesized MgCo2O4through one-step hydrothermal method,and then MgCo2O4with abundant oxygen vacancies (labeled as OV-MgCo2O4)was synthesized by chemical reduction method(The synthesis schematic is depicted in Fig.7a).According to the SEM images in Fig.7b&c,the morphology of OV-MgCo2O4was 3D porous spheres stacked by nanosheets,which had no significant change compared with pristine MgCo2O4before NaBH4treatment.Furthermore,XPS was applied to prove the the increasing concentration of oxygen vacancies after NaBH4treatment.The XPS spectra of O 1s showed the intensity of the peak around 531.3 eV which attributed to oxygen vacancy was enhanced after NaBH4treatment,which indicated higher concentration of defects sites.The specific capacitance of OV-MgCo2O4was 460 F g-1at 1 A g-1,higher than the pristine MgCo2O4(340 F g-1),and 76% of the specific capacitance was preserved when the current density increased to 20 A g-1.The improved charge storage capacity can be attributed to the improvement of the conductivity of MgCo2O4by the negatively charged centers formed after O vacancies generated [54].

Fig.7.(a) Typical synthesis schematic of OV-MgCo2O4.SEM images of pristine MgCo2O4 (b) and OV-MgCo2O4 (c).(d) SEM image,(e) XPS full spectra,and (f) cycling performance of NZMCO.(g) Specific capacity as a function of current density.(h) EIS spectra of pure and Cr3+ (5 mol %) doped MgAl2O4 NPs.XRD spectra (i) and TEM images (j∼l) of Mg1-xCaxFe2O4 (x=0.1,0.3 &0.5) NPs.(a∼c) Reproduced with permission from [54]Copyright (2021)Elsevier.(d∼f) Reproduced from Ref.[55]Copyright (2019),with permission from Elsevier.(g∼h) Reproduced with permission from [104]Copyright (2022)Elsevier.(i∼l) Reproduced from Ref.[105],Copyright (2022),with permission from Elsevier.

Heteroatom doping could increase carrier concentration,thereby enhancing the electrical conductivity.Using Co foam as cobalt source and template,Zhu et al.in situ grown Ni-Zn co-doped MgCo2O4(labeled as NZMCO) on Co foam by one-step hydrothermal method.From SEM images in Fig.7d,it could see that NZMCO was highly dense and ordered array structure formed by the tightly connected upright nanosheets,and the nanosheets thickness was in the range of 50–200 nm.Many channels were formed between the nanosheets,which were conducive to ion diffusion,thus improving the contact between electrolytes and NZMCO.The successful doping of Ni and Co heteroatoms was proved by XPS (Fig.7e) and EDS.NZMCO electrode had a specific capacitance of up to 14.43 F cm-2and had outstanding cycling stability (89%of the specific capacitance was still preserved at 200 mA cm-2after 20,000 cycles,as illustrated in Fig.7f).Using NZMCO electrode and AC electrode,an asymmetric supercapacitor was assembled that had superior cycling stability and a 46.9 W h kg-1energy density at 132.8 W kg-1.Besides,the NZMCO//Zn battery demonstrated a high energy density of 120.4 W h kg-1and remarkable cycling stability(85% of the original capacitance was preserved after 30,000 cycles at 150 mA cm-2) [55].

Mir Waqas Alam et al.reported Chromium (Cr3+)-doped MgAl2O4nanoparticles (labeled as Cr-MgAl2O4NPs) for supercapacitor.The impact of Cr doping on electrical conductivity and charge storage capacity was investigated through GCD and EIS measurements.The GCD results (Fig.7g) exhibited that,compared with the undoped MgAl2O4(∼830 F g-1),the Cr-MgAl2O4NPs’ specific capacitance increased to 955.4 F g-1at 1 A g-1,demonstrating a boost in charge storage capacity.From the EIS plots in Fig.7h that the addition of Cr dopants decreased the charge transfer resistance.However,compared with undoped MgAl2O4,the cyclic stability of Cr-MgAl2O4NPs decreased [104].

Compared with the above research works,although defect engineering can improve the electrical conductivity of materials,the influence on the cycling performance and rate performance and relative mechanism have not been thoroughly studied.

The effect of the proportion of heteroatom in compounds on electrochemical properties has been investigated.The Mg1-xCaxFe2O4(x=0.1,0.3 &0.5) nanoparticles(Mg1-xCaxFe2O4NPs) were prepared by Manohar et al.through a solvothermal reflux approach.Observed from the XRD and TEM images (Fig.7i∼l),it could be inferred that the Mg1-xCaxFe2O4NPs had good crystallinity with a cubic spinel crystal structure,and the average size of nanoparticles was 12∼13 nm.According to the GCD curves,Mg0.9Ca0.1Fe2O4electrode had the highest specific capacitance of 221.57 F g-1at 0.33 A g-1(compared with Mg0.7Ca0.3Fe2O4,Mg0.5Ca0.5Fe2O4),which may be because the impurity phase in the crystal would affect the charge storage and electrolyte ion exchange at the electrode/electrolyte interface,thus affecting the electrochemical performance[105].This study shows that the proportion of doped elements involved has a significant impact on electrochemical performance,but the mechanism in it requires more characterization methods and the application of computational simulation.Meanwhile,there are few relevant works on the influence of element doping ratio on electrochemical performance,which is not conducive to the application of machine learning and other methods to find generalities.

2.5. MgX2O4 regulated by other methods for supercapacitor

Powdered electrode materials often exhibit poorer electrochemical performance due to the dead volume generated by conducting additives and binders impedes electron transportation.Therefore,electrode materials in-situ grown directly on the collector (called binder-free synthesis) become an effective way to avoid the above phenomenon.Porous MgCo2O4nanoflakes had been prepared through binder-free hydrothermal method and annealing process by Chen et al.The authors investigated how these MgCo2O4NFs’ morphologies evolved during the in-situ growth process by adjusting reaction time and temperature.The SEM images in Fig.8a∼d showed that,with the increase in reaction time,the morphology changed from urchin-like microstructure to nanoflakes.Moreover,nanoflakes tended to accumulate and form loose layered structures as the reaction time increased to 15 hours,which would reduce the overall surface free energy.This experimental result also followed the typical Ostwald ripening mechanism.As shown in Fig.8e∼g,the nanoflakes’ structure transformed from nanoflakes to nanotubes with rising temperature when the reaction time controlled at 10 h.This phenomenon was caused by an increase in temperature that provides more energy for chemical reactions.When the temperature reached a certain level,due to the rapid growth of the crystal,a small anisotropic microstructure formed,and the anisotropy of the final synthesized material became smaller.The energy density of the MgCo2O4NFs//AC ASC was 33.0 W h kg-1at 859.6 W kg-1.At the same time,the ASC also showed good cycling performance,with the capacitance remaining about 110.4%after 4000 cycles at 5 A g-1[41].This study of the influence of reaction time and temperature on the morphology of materials also provides ideas for researchers in binder-free synthesis.

Fig.8.SEM images of the MgCo2O4 samples produced after (a) 2,(b) 6,(c) 15,and (d) 24 h of hydrothermal reaction,respectively.SEM image of the MgCo2O4 samples prepared at (e) 105,(f) 135,and (g) 150 ◦C,respectively.(h) The coulombic efficiency and consecutive 4000 cycle GCD curves at 5 A g-1.(i) Ragone plots of the MgCo2O4 NFs//AC ASC.Reproduced (a-i) from [41]Copyright The Royal Society of Chemistry 2020.

In addition,conductive polymers,which combine the unique electronic properties of conventional polymers with metals or semiconductors,are extensively utilized in the energy storage field.Because of its advantages like high conductivity,excellent stability,etc.,PPy,a typical conductive polymer,is frequently employed as an electrode material [106].Gao et al.first in situ grew MgCo2O4nanowires on Ni foam by hydrothermal method.Then,a layer of PPy was covered on the nanowires through in situ chemical oxidative polymerization method to obtain urchin-like MgCo2O4@PPy with core-shell structure.Meanwhile,the effects of chemical oxidation time on morphology and electrochemical performance were studied.The product with a chemical oxidation time of 2 h,4 h,and 6 h was named MCP-2,MCP-4,and MCP-6,respectively.MCP-4 had an urchin-like structure composed of nanoneedles,each consisting of MgCo2O4nanowires covered with PPy.MCP-4 had the best crystallinity and crystalline form according to its XRD pattern.The specific capacitance of the MCP-4 electrode was 1079.6 F g-1at 1 A g-1,and the cycling performance of MCP-4 electrode was improved compared with MgCo2O4electrode and PPy electrode.To further prove the improved conductivity,they employed EIS measurement which also indicated the lower resistance of MgCo2O4after coating PPy.As shown in Fig.8h&i,MCP-4//AC ASC device was further assembled,which had a high energy density of 33.42 W h kg-1at 320 W kg-1and superior cycling stability [53].Gao et al.successfully boosted the conductivity of MgCo2O4through coating PPy,however,there are also some studies that show deterioration in rate performance after coating PPy which needs to further study [107].

3.Layered structured Mg-X-LDHs for supercapacitor

LDHs or layered double hydroxides have a wide range of applications,such as catalysts and catalyst support,anion exchanger,electrical and optical functional materials,flame retardants and nano-additives,etc.which have been reported in previous works [115–120].As a typical layered structured materials,LDHs have unique 2D ionic layer structure which can be symbolized by the general formula[M2+1–nM3+n(OH)2]n+[Az–n/z]n-·mH2O,where M2+denotes metallic bivalent cation,M3+denotes metallic trivalent cation and Az–represents interlayer anion.The value of ‘n’ is the molar ratio of M3+/(M2++M3+),usually ranges from 0.2∼0.33.By changing the ratio of the two metallic cation,type of interlayer anion,different physicochemical properties can be attained[121–124].

Due to their stable layered structure and abundant channels,LDHs have been widely explored in supercapacitors and LIBs,which are conducive to the intercalation/deintercalation of electrolyte ions and transition metal elements with various redox states [122,125-127].Such as Ni-Co LDHs [128,129],Co-Al LDHs [130,131],Ni-Mn LDHs [132,133],etc.However,due to the poor reaction kinetics of Mg ion,Mg-X-LDHs(X stands for metal cations) have been less reported [125].Herein,we collected previous research progress on Mg-XLDHs,as listed in Table 2.

Some works focus on the insertion of another electroactive cation into the binary component LDHs to prepare the ternary component LDHs which aim at improving the ion exchange capability and pseudocapacitance performance[134,135].Jing et al.employed one-step hydrothermal method to prepare ternary Ni-Mg-Al LDH,and schematic diagram of formation mechanism was shown in Fig.9a.Authors studied the effect of molar ratios of Ni2+/Mg2+and hydrothermal reaction time on the product.XRD patterns in Fig.9b demonstrated the intensity of diffraction peak increased with the increase of the Ni2+/Mg2+ratio and the extension of reaction time,indicating the better crystallinity.Observed from SEM images,all the synthesized Ni-Mg-Al LDH showed nanosheet structure with porous surface.According to N2adsorption–desorption isotherms,Ni2MgAl LDH-24 (The hydrothermal time controlled as 24 h,and the molar ratio of Ni2+/Mg2+was 2) had the highest specific surface area of 97.70 m2g-1.Ni2MgAl LDH-24’s specific capacitance was 219.2 mA h g-1at 1 A g-1,and the specific capacitance still retained 86.1%after 5000 cycles at 5 A g-1.However,the rate performance(Fig.9c),which was unsatisfactory,attributed to only Ni2+participates in the pseudocapacitive process.Also the poor conductivity of Ni-Mg-Al LDH led to greater diffusion resistance during intercalation process [125].

Fig.9.(a) Schematic drawing of formation mechanism of NiMgAl LDH by the hydrothermal method.(b) XRD patterns of NiMgAl LDHs.(c) Specific capacitances of NiMgAl LDH at different reaction time.(d) Schematic representation of the synthesis of ZMA-LDH@Fe2O3 on 3DHPCNF.FE-SEM images of 3DHPCNF (e1),Fe2O3@3DHPCNF(e2),ZMA-LDH/Fe2O3@3DHPCNF(e3).(f) The cycling response of the ZMA-LDH/Fe2O3@3DHPCNF at continuously varied currents.(g) Cycling performance of the ZMA-LDH/Fe2O3@3DHPCNF electrode.(h) Schematic diagram of synthesis strategy for oxygen-rich vacancy CoNiMg-LDH.The atomic structure of oxygen vacancies abundant LDHs.(i1) CoNi-LDH,(i2) CoNiMg-LDH (near Mg2+),(i3) CoNiMg-LDH (away from Mg2+.Density functional theory (DFT) calculation total band structures of (j1) CoNi-LDH and (j2) Oxygen vacancies abundant CoNiMg-LDH.SEM images of LDH-24(k),HPC-24(l).(m) N2 adsorption-desorption isotherm of HPCs.(n) Cycling performance of of HPC-24//HPC-24.Reproduced (a-c) from Ref.[125]Copyright The Royal Society of Chemistry 2019.Reproduced (d∼g) from Ref.[134],Copyright (2022),with permission from Elsevier.Reproduced(h∼j) from Ref.[137]Copyright (2022) Elsevier.Reproduced (k∼n) from Ref.[138],Copyright (2022),with permission from Elsevier.

As mentioned above,heterostructure engineering is effective to improve electrochemical performance.Poudel et al.adopted the “bottom-up” synthetic technology (as shown in Fig.9d) to grow Fe2O3nanorods and Zn-Mg-Al LDH nanosheets on 3D hollow and porous carbon nanofibers in turn,and the products labeled as ZMALDH@Fe2O3/3DHPCNF.From the SEM images (Fig.9e),ZMA-LDH@Fe2O3/3DHPCNF had a hierarchical structure which indicates larger specific surface area and better structural stability.The area ratio capacitance of ZMALDH@Fe2O3/3DHPCNF was 3437 mF cm-2at 1 mA cm-2(Fig.9f),and demonstrated excellent cycling stability(Fig.9g).ZMA-LDH@Fe2O3/3DHPCNF had excellent electrochemical properties for the following reasons: (i) 3DHPCNF provides more ion/electron transport channels and promotes charge transfer kinetics.(ii) The constructed Fe2O3/Zn-Mg-Al LDH heterointerface was conducive to providing active sites and promoting electronic transfer.(iii) The insertion of Zn2+in the binary component LDH was beneficial for ion exchange ability [134].

Graphene was used to produce Mg-Al LDH/GO with interlaced nanosheet structure by Gao et al.Researchers prepared carbon fiber clothes (CFC) supported 1T phase MoS2as the anode,and an asymmetric supercapacitor,CFC-MoS2//MgAl LDH-GO,was assembled.This device achieved a maximum energy density of 326.54 W h kg-1at a relatively high power density of 1500 W kg-1and displayed superior electrochemical behavior [136].

CoNiMg-LDH electrode regulated by defect engineering was reported by Zhou et al.Their group prepared CoNiMg-LDH nanosheets with oxygen vacancies through replacement reaction using magnesium powder as reductant,and its synthetic process was shown in Fig.9h.The vacancies concentration could be adjusted by controlling the amount of magnesium powder.According to density functional theory(DFT) calculations,in the three oxygen vacancy structures listed in Fig.9i,structures (i2) and (i3) have the lower formation energy,indicating the introduction of magnesium ions promoted the formation of oxygen vacancies.In addition,as shown in Fig.9j,compared with CoNi-LDH,the band gaps of CoNiMg-LDH appeared more obvious impurity bands near the Fermi level,which confirmed that the electrical conductivity effectively improved after introducing oxygen vacancies and Mg2+.Using a CoNiMg-LDH electrode and AC electrode,an asymmetric supercapacitor was able to achieve a superior energy density of 73.9 W h kg-1at 0.8 kW kg-1and 13% capacity degradation at 20 A g-1after 5000 cycles[137].Using magnesium powder as reducing agent,the introduction of Mg element and oxygen vacancies was achieved simultaneously,but the deterioration in cycling performance was not explained.

Using MgAl-LDHs as hard template is a simple,controllable and promising scale-up synthesis strategy for twodimensional materials.The researchers,Zhang and colleagues,used Mg-Al LDH as template to prepare hierarchical porous carbon materials (HPCs) through a fast-carbonization process.The specific surface area and pore size of the MgAl-LDH templates were controlled by adjusting the recrystallization time in NaOH aqueous solution to 0,12,and 24 hours.The HPCs were named HPC-0,HPC-12,and HPC-24,based on the recrystallization time.The morphologies of the LDHs and HPCs were illustrated in Fig.9k&l,showing a nanosheet structure.From the N2adsorption-desorption isotherm in Fig.9m,HPC-12 exhibited the highest specific surface area of 1210.24 m2g-1.The HPC-12 electrode had a volumetric capacitance of 273.76 F g-1at a scan rate of 2 mV s-1,with a capacitance retention of around 83.91% at 100 mV s-1.In addition,the HPC-24//HPC-24 symmetrical two electrode system achieved the highest volumetric energy density of 15.11 W h L-1at 373.2 W L-1,while retaining 98.8% of the initial capacitance over 10 000 CV cycles at 500 mV s-1,as shown in Fig.9n [138].

4.Magnesium oxide and magnesium hydroxide for supercapacitor and other applications

Magnesium hydroxide,as an alkaline metal hydroxide,has attracted much attention in the electrochemical storage fields for its excellent properties such as high negative standard potential (-2.375 V vs RHE),relative abundance and low toxicity.However,Mg ion does not participate in electrochemical redox reaction,chemical displacement reaction or electrochemical displacement reaction is commonly used to replace Mg ion with cations in transition metal hydroxide.Herein,the cycling stability and conductivity of electrode materials can be improved [30,142–144].

The basic thought of the chemical reduction method is shown in Fig.10a,and the related research works are summarized as follows.NiMg hydroxides with different crystalline phases were synthesized at room temperature by Yin et al.through adding Mg powders in different nickel salt aqueous solutions,and the diagrams of the synthetic process for theα(β)-NiMg-OH nanosheets were shown in Fig.10a.The XPS results in Fig.10b indicated the content of Mg content in theα-NiMg-OH andβ-NiMg-OH was 10.4%,and 6.6% (mol%),respectively.Observing from SEM images,theα-NiMg-OH was composed of ultra-thin nanoplates,and theβ-NiMg-OH had a graphene-like structure,and their specific surface areas were 291 m2g-1,and 71 m2g-1,respectively.The specific capacitance of theα-NiMg-OH electrode,which was greater thanβ-NiMg-OH electrode at 1942 F g-1,was 2602 F g-1at 1 A g-1,as shown in Fig.10c.However,the cycling stability ofα-NiMg-OH was worse than that ofβ-NiMg-OH,which due to the loss capacitance caused by the conversion ofα-NiMg-OH toβ-NiMg-OH during the cycling.An asymmetric supercapacitor assembled with theα-NiMg-OH electrode and the AC YP-80F electrode delivered an energy density of 31.9 W h kg-1at 800 W kg-1,and 84.6% of the capacity was retained after 4000 cycles at 5 A g-1[144].The results of Yin et al.show that magnesium powder as a reducing agent can influence the morphology,crystal structure and electrochemical performance of the material,but the mechanism have not been explained in detail.

Fig.10.(a) The diagram of the fabrication process for the α-NiMg-OH nanosheets.(b) XPS spectra of the α-NiMg-OH and β-NiMg-OH.(c) GSC vs.current density.(d) Schematic illustration for the growth of Ni(OH)2 on Ni foam by ion-exchange reaction method.(e) N2 sorption isotherms and pore size distributions (PSD) curves (insert).(f) Capacitance retention for the two nickel hydroxides after 10 000 and 3000 cycles at 10 A g-1,respectively;inset is CV curve of Mg-Ni(OH)2 at the 1st,5000th and 10 000th cycle.(g) Synthesis scheme of the preparation of various forms of MXene and expanded MXene.(h)XRD patterns of Ti3AlC2 MAX phase (a),MXene (b),expanded MXene.The specific capacitance of L-MCH electrode with different current densities.SEM images of MXene (i1) and expanded MXene (i2).Reproduced (a∼c) from Ref.[144]Copyright (2019) MDPI.Reproduced (d∼g) from Ref.[143],Copyright 2016 American Chemical Society.Reproduced (g∼i) from Ref.[30],Copyright (2020) Elsevier.

The mechanism of the electrochemical displacement reaction is shown in Fig.10d,which was explained by Xie et al.Their group prepared Mg(OH)2with nanosheet morphology as sacrificial substrate and effective dopants.The synthesis of Mg-doped Ni(OH)2grown on Ni foam through in situ ion exchange reaction (IER) was shown in Fig.10d.The BET (Fig.10e) surface area of Mg-doped Ni(OH)2was 5 times larger than dopant-free Ni(OH)2,up to 220 m2g-1.Mg-doped Ni(OH)2electrode tested in a three-electrode system had a greater specific capacitance (1931 F g-1at 1 A g-1),compared to dopant-free Ni(OH)2(1389 F g-1at 1 A g-1).As shown in Fig.10f,the capacity retention of Mgdoped Ni(OH)2after 10 000 cycles was 95%,while the capacity retention of dopant-free Ni(OH)2after 3000 cycles was only 51% [143].Analogously,A.Nanwani et al.developed layered magnesium-cobalt double hydroxide nanosheets (LMCH) with a crochet structure on nickel foam using a simple and cost-effective electrodeposition technique.An asymmetric supercapacitor assembled with activated carbon cloth as negative electrode and L-MCH positive electrode exhibited an energy density of 55.75 Wh kg-1at 1000 W kg-1and capacity retention of 97% after 10 000 cycles [142].

MXenes,similar to other 2D materials,also faces agglomeration problem during the preparation process [145].At present,the method of modifying the morphology of the layer of MXenes is often used to prevent agglomeration and to improve the through-plane ionic conductivities.The application of MgO or Mg(OH)2as solid spacer is reported by Zhu et al.[30].Authors prepared layered expanded MXenes with MgO nanoparticles as solid spacers and removed the MgO with acetic acid,and synthesis scheme of the preparation is shown in Fig.10g.Compared with pure MXenes,the diffraction peaks of the expanded MXenes were weakened in the XRD pattern (Fig.10h),and the 2θposition of of(002)diffraction peak was shifted to a lower angle,indicating successful expansion of the layer spacing.SEM images in Fig.10i also further confirmed that the interlayer spacing was expanded after the intercalation of MgO.The expanded MXenes electrode exhibited a capacitance of 112 F g-1,which was higher than pure MXenes of 80 F g-1.Compared with other solid spacers,MgO nanoparticles with adjustable size have less impact on the environment during the introduction and removal process.

MgO or Mg(OH)2can also be used to synthesize templated carbon.Y.Kado and colleagues employed hydrothermal method and calcination to synthesize MgO with a nanoplate structure,and were able to control the mesoporous structure of MgO by adjusting the temperature during the hydrothermal process.They used MgO as a template and PVA as a carbon source to prepare xHTMgO-C,where x=80,100,120,160,and 200 corresponds to the temperature of hydrothermal process.Physical mixing and high-temperature calcination were utilized in the preparation of xHTMgOC.The specific surface areas of 80HTMgO-C,100HTMgOC,120HTMgO-C,160HTMgO-C,and 200HTMgO-C were 1600,1470,1510,1560,and 1560 m2g-1,respectively.The energy storage capacity was measured using the GCD method.The 120HTMgO-C electrode delivered 42 F g-1at 1 A g-1in the voltage range of 2.5 V,while retaining 93% of the initial capacitance after 10 000 cycles [31].Similarity,Lu et al.synthesized porous carbon-supported Fe-Nxsites by a facile synthesis strategy using zinc and MgO template,which exhibited an excellent ORR activity,surpassing Pt/C [146].Using MgCO3·3H2O fibers as the template,Cui et al.prepared 1D mesoporous graphene nanofibers (GNFs) with high electrical conductivity through chemical vapor deposition.The capacitance performance of the prepared GNFs reached up to 15 μF/cm2when tested at a sufficiently wide voltage range (0∼4 V) [147].

5.Summary and outlook

In general,owning to advantages of low cost,environmental friendliness,and natural abundance of magnesium,a lot of research has focused on the development of magnesium-based energy storage devices,and much progress has been made in Mg batteries,hydrogen storage,and heat energy storage,and other fields.In view of the drawbacks of Mg-base materials used as electrode materials for supercapacitors,such as poor cycling stability and poor electrochemical reactivity,many reports have been published to improve these intrinsic problems.In this review,various regulative strategies used to improve the electrochemical performance and relative mechanism of Mg-based materials are discussed,including designing nanostructures (1D,2D,3D,core-shell structure,hierarchical structure,etc.),synthesizing composites of porous carbon materials(CNTs,graphene,rGO,g-C3N4,etc.),synthesizing composites of transition metal-based materials(TMOs,TMSs,TMHs,etc.),defect engineering (vacancies,doping),binder-free synthesis,and synthesizing composites of conductive polymer(PPy).Besides,other applications in supercapacitors are also collected such as hard templates,solid spacers,etc.

These advances are an indication that the electrochemical performance of Mg-base materials has been improved significantly in recent years.However,the following challenges remain: (1) hydrothermal methods are often used in many researches to synthesize elaborate nanostructures.However,hydrothermal method is difficult to achieve large-scale and repetitious preparation,and the influence of reaction parameters such as solution PH,reaction temperature and time on the morphology of the products and their mechanism have not been thoroughly studied.(2) Mg ions in Mg-base materials do not have variable valence states,their contribution to the overall pseudocapacitance of the material is relatively low,that is,the poor electrochemical reactivity.(3) In some Mg-base materials,such as MgCo2O4,Mg substances tend to dissolute in the alkaline solution (KOH,LiOH,etc.) during the cycling process,causing irreversible changes in the crystal structure and morphology,and thus reducing the cycling stability.(4) Less studies have focused on the design and optimization of electrolytes,collectors,and anode materials and their impact on the electrochemical reactions.

Encouragingly,previous reports have suggested some solutions to these problems: (i) development of simpler,cheaper and more repetitious preparation routes that are less sensitive to reaction conditions,such as molten salt method(MSM),physical crushing method,ball milling method and flame aerosol synthesis,etc.Moreover,the influence of reaction parameters on the morphology and structure of products during hydrothermal synthesis needs to be further investigated.(ii)Hybridization of Mg-based materials with transition metal-based materials to construct composites/heterostructures is a feasible way to improve electrochemical reactivity and electrochemical performance.In addition,defect engineering(construction of vacancies or doped transition metal elements)would have the similar effect.(iii) The problem of Mg dissolved in an alkaline solution can be alleviated by improving the binding of Mg ions to other ions,including synthesizing rGO composites and designing &optimizing suitable electrolytes.(iv) Some of the computational simulation steps(such as the First-principles Calculations) can be employed for a deeper explanation of the regulative mechanism.

Although there are still many difficulties and challenges,we still hope Mg-based materials for high performance supercapacitors will be an essential part in the development and utilization of magnesium resources in the field of energy storage.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the financial support provided by Projects (no.2020CDJXZ001) supported by the Fundamental Research Funds for the Central Universities,the Technology Innovation and Application Development Special Project of Chongqing(Z20211350 and Z20211351),Scientific Research Project of Chongqing Ecological Environment Bureau (no.CQEE2022-STHBZZ118).The authors also thank the Electron Microscopy Center of Chongqing University for materials characterizations.