The CVD process assists in the synthesis of hybrid MOFs with components in vapour form producing high-porosity materials with uniform structural morphological outcomes [61]. Owing to extraordinary properties such as large crystal structures with one-layer thickness graphene was manipulated using CVD for exfoliation of graphite structure, reducing rGO and graphite oxidation that helped in the charge transfer process [62]. Combining graphene with multi-walled CNTs manipulating CVD, supercapacitors were fabricated showing excellent electrochemical properties by Kalam et al. [63]. Hybrid structures of CNTs and graphene were also produced using this process where magnesium oxide was used as a catalyst thus providing conductive paths for electron charge transfer [64]. Preparation and growth of single crystal MOF of Mo₂(INA)₄ has been reported by Claire et al. [65] with excellent performance. Cao et al. [66] also used the CVD method for the preparation of ZIF-67 MOF and fabricated an ASC device of Ni@Z67. Legenstein et al. reported Cu-INA [67] synthesis using CVD while 3D graphene MOFs [68] and Ni@ MOF [69] have also been prepared using this method.

This environmentally friendly process is best utilized for close system diffusivity. For the preparation of designer precipitates such as high-purity particles, and particles with controlled chemical and physical characteristics, this process is widely used. It requires less amount of energy with low temperatures along with the advantage of easy implementation with scaling [70]. MOFs involving Ni-MOF [89], Co & Cd MOFs [71–75], Ni-Co MOF [76], Ni-Co-Mn MOF [119], Ni-Mn@C [107], Zr-Ce-Ni MOF [109], Ni-MOF with CNTs [111], Mn-MOF with CNTs [112], etc. have been reported by the researchers prepared using this facile method.

Microwave-assisted processes can be used to create small metal and oxide particles. The solution is exposed to microwaves and allows precise control over the size and shape of MOFs being synthesized [77,78]. Synthesis of ZIF-8 MOF has been reported by Xu et al. [79] thus obtaining significant outcomes by noting a reaction time reduction from 48 hrs. upto 10 minutes. A report from Wang et al. [80] also described the synthesis of MIL-100 (chromium). The valuable findings revealed a time reduction in comparison to the solvothermal approach with improved yield. Weiyang et al. [81] also made use of this method to prepare SnO@Zr hybrid MOF structures with a reported capacity of 994 mAh g⁻¹ for energy storage applications in batteries. Ni-MOF [88] with a reported energy density of 33 Wh kg⁻¹ has also been manipulated by using this method. The fastest MOF synthesis of MIL-53 [82] is also one of the beneficial results of this technique. Two studies reported MIL-101 chromium synthesis also using this method [83,84].

It is a powerful technique for the preparation of MOFs, which involves the use of high-frequency ultrasound to promote the synthesis of MOFs. In this method, the metal precursors are mixed and subjected to ultrasonic waves that assist in nucleation and crystal growth under cavitation and temperature effects. Various MOFs have been prepared for energy storage applications such as supercapacitors that involve Cu-MOF [85] with H₆TDPAT as a linker in DMF solvent. Along with this other MOFs such as Co-MOF [86], Mn-MOF [87] and Ni-MOF have also been prepared using an ultrasound-assisted method.

For the preparation of huge amounts of powder samples, this method can be very useful as it provides basis for the formation of precipitates which are formed due to an excess quantity of solute particles in a solvent breaching the solubility limit. The fast rate of precipitation due to high temperature puts limitations against the morphology regulation of particles to be prepared. Using this facile technique preparation of magnetic nanoparticles of cobalt-iron oxide [88], nickel phosphate deposited on GO [89] has been reported exhibiting (approx.) 1330 F g⁻¹ at a small current density of 0.5 A g⁻¹. Zn-MOF was prepared by Liu et al. [90] using this method reporting an energy density of 22.4 Wh kg⁻¹. Also, MOF-5 [91], CeO₂ from Ce-MOF [92] and ZIF-8 MOF in Na-ion batteries are few reported researches in a recent era using the precipitation method.

For achieving highly crystalline MOFs slow diffusion process is often employed. The steady and gradual mixing of precursors is achieved in this process with elevated electrochemical performance often employed due to slow diffusion species in MOF preparation process. Better electrochemical performance of MOF can be attributed to efficient diffusion of precursor ions. Preparation of Cu based MOFs have been reported by Rajak et al. using amalgamation of ligand approach while MOF derived cobalt iron oxide with MXene have also been reported in the literature by Xie et al. recently [93,94]. Moreover, using concurrent approach of using mix ligand, polymer reinforced Zn based MOF have been reported by manipulating slow diffusion process [95].

Reaction of Sulphur containing compounds or the introduction of Sulphur in MOF structures can be used to enhance the electrochemical properties of MOF derived materials. Sulphur being advantageous for having more energy density and highly active oxidation/reduction material can be of more value. While enhancing the stability and conductivity of MOF derived structures a heat treatment process of MOF sulfides brings efficient results to the whole scenario of energy storage. Treatment of ZIF-67 with sulfur caused the conversion of Co-MOF into CoS through sulfidation reactions [96] while the reaction of titanium oxide with ZIF-67 MOF produced CoS structures with hierarchical morphology [97]. Production of porous carbon based nanostructures have been achieved by carbonization treatment of iron carbide nanocrystals with a large surface area for efficient charge storage capability [98].

Monometallic MOFs with satisfactory surface areas and small pore sizes can be beneficial for energy storage applications. Mofokeng et al. synthesized Ni-MOF using a microwave-assisted technique reporting an energy density (33 Wh kg⁻¹), power density (983 W kg⁻¹) and a specific capacity of 199 mAh g⁻¹. Carbonizing along with chemical etching was carried out on prepared Ni-MOF for a reduction in nanoparticle size [104]. Excellent cycling stability with only a 12% loss was observed in a 2-electrode assembly with less series resistance. The better performance of Ni-MOF was attributed to flake-like morphology revealed by SEM along with the induction of oxygen vacancies. Li et al. fabricated Ni-MOF by solvothermal approach with BTC (1,3,5-benzene tricarboxylic acid) as a ligand. They reported an excellent specific capacitance of 977.04 F g⁻¹ with only an 8% cycling stability loss. Exposure of (001) crystallographic plane provides the shortest ions transportation path that contributed to the remarkable performance of electrode material [105]. Lee et al. [106] synthesized Co-MOF with pseudocapacitive nature that revealed an energy density (7.18 Wh kg⁻¹) with a specific capacitance value of 206.76 F g⁻¹. SEM images showed a porous uniformly distributed structure that provided easy pathways for ions movement from which the relationship between molecular linker length and pore size along with the surface area was explored by the same group [107]. Their findings produced that longer-length molecular morphologies provide a large surface area for ions to settle thus providing better electrochemical properties. Benzene dicarboxylic acid (BDC), 2,6-naphthalene dicarboxylic acid (NDC) and 4,4-biphenyl dicarboxylic acid (BPDC) with varying lengths were used as organic linkers to link cobalt atoms. Indium-based MOF was synthesized by Du et al. [108] using indium nitrate (In (NO₃)₃) with a reduced pore size of 3 nm along with excellent stability. A specific capacitance of 150.2 F g⁻¹ was reported at 0.2 A g⁻¹ against 437-MOF which was credited to a large surface area supported by an ordered porous structure.

Considering the inferior performance of monometallic MOFs, Pan et al. [109] reported the doping of manganese (Mn) in Ni for making an attempt to reduce the conduction path between Ni and Mn ions using a binary metal approach. 1.89 mAh cm⁻² was reported by Pan and his co-workers at 1 mA cm⁻². Two-electrode assembly of Ni-Mn-MOF//EDLC (AC) yielded an energy density of 67.5 Wh kg⁻¹ along with a remarkable 84.5 W kg⁻¹ power density bearing 6% capacity retention loss. Diaz et al. [110] synthesized Co8-MOF-5 with varying amounts of MOF that exhibited pseudo-capacitive behavior generally shown by EDLC materials.

Rajak et al. [111] took a step to synthesize heterometallic Na-Co-MOF using a facile slow diffusion process with an approach of using mixed ligands namely, Azopy (4,4′-Azopyridine) and SDCA (2,5′-thiophene carboxylic acid). The specific capacitance obtained after electrochemical analysis was 321.8 F g⁻¹ at a current density of 4 A g⁻¹ with 21% capacitance loss. The better performance of MOF was attributed to the block-like morphological features. Hussain et al. fabricated Ni-Co ZIF (Fig. 1(a)) with CuO using the dry oxidation method achieving 43 Wh kg⁻¹ of energy density. Decoration on CuO surface with forest-like Ni-Co ZIF provided accessibility to pores providing highly active redox sites showing incredible electrochemical performance [112].

A comparison was made between the performance parameters of Cu-MOF and Ni@Cu MOF by Wang et al. [113]. Ni@Cu-MOF showed a very smaller pore size in contrast to Cu-MOF found by BET which formed the basis of marvelous performance. A way much higher specific capacitance value of 526 F g⁻¹ was calculated than pristine Cu-MOF (126 F g⁻¹). The energy density shown by the asymmetric supercapacitor (ASC) device was 17.3 Wh kg⁻¹ at an incredible power density of 798.5 W kg⁻¹. Fu et al. [114] successfully synthesized a Ni-Fe MOF using precursors of iron chloride (FeCl₃) and nickel chloride (NiCl₂) by carbonizing polymer linkers. Better morphological features with large surface area contributed to charge transportation (1049.51 m² g⁻¹) revealing an energy density of 18.3 Wh kg⁻¹ along with a maximum power density of 350 W kg⁻¹. The synergistic effect of Ni and Fe ions showed coral cross-linked structure with small pore size observed from SEM image (Fig. 1(b)). Making use of the solvothermal process followed by calcination with precursors of Ni-Zn MOF, the formation of seaweed-like morphologies of ZnO/NiO was attempted [115]. Investigation have been carried out on changing the molar ratios with an effect on characteristic morphology of specified MOF. SEM images (Fig. 1(c)) revealed the seaweed-like structure that assisted in ion transport efficiently thus exhibiting a specific capacitance of 435.1 F g⁻¹ at 1 A g⁻¹.

Li et al. [116] used a solvothermal method to prepare a yolk-shell hollow structure comprising of bimetallic MOFs, NiO/ZnO by manipulating a template-based strategy using H₂BDC as a linker. 497 F g⁻¹ of capacitance value was reported by their team at 1.3 A g⁻¹ with an excellent stability. SEM analysis showed the formation of uniform spherical structures having a core inside with a shell covering while BET analysis was an indicative of mesoporous structure. The better electrochemical performance was attributed to the hollow volume structure that provides ease in accommodating more ions inside it. Highly porous-natured zirconia (Zr) MOF was synthesized by Gao et al. [117] from Zr-Zn bimetallic precursor. Zinc chloride and zirconia nitrate were used as precursors in the presence of BDC linker which were subsequently soaked in acid to remove Zn-MOF from synthesized Zr-Zn MOF. SEM images showed a lot of porous sites with sponge-like morphology assisting in ion transport. Ultimately, better electrochemical performance was reported by researchers including 849 F g⁻¹ of capacitance with energy density (32 Wh kg⁻¹) and power density (240 W kg⁻¹). Rajpurohit et al. [118] investigated Cu-Fe MOF nanocomposite with sulphur-doped graphene using BTC as a ligand aiming to achieve a super capacitive behavior of the synthesized material. Rod cube-like structures (Cu-MOF) with some agglomerated particles (Fe-MOF) showing a vast number of porous sites in SEM analysis was revealed. Electrochemical results exhibited 1164.3 F g⁻¹ of specific capacitance with an extra ordinary 96.5 Wh kg⁻¹ and 1595 W kg⁻¹. Such remarkable electrochemical performance was attributed to the porous structures of nanocomposite with high electrical conductivity of sulphur doped graphene as shown in Fig. 1(d1,d2).

Keeping in view the compromised performance of mono as well as bi-metallic MOFs, Han et al. [119] put an effort to upgrade the electrochemical performance of MOFs by working on ternary MOFs. Ni-Co-Mn-MOF was synthesized (Fig. 2(a)) via a solvothermal technique using BTC as an organic linker. The synthesized MOF exhibited a typical battery-type supercapacitive behavior producing 655 F g⁻¹ specific capacitance at 1 A g⁻¹. Due to the excellent layered structure of the as prepared MOF, there was reported approximately 8% capacity retention loss after 10,000 cycles. The ASC device produced a remarkable energy density of 61 Wh kg⁻¹ at 844 W kg⁻¹ due to the availability of more active redox sites confirmed from SEM analysis as shown in Fig. 2(b). Aghazadeh et al. [120] synthesized TM-MOF via cathodic electrodeposition methodology using lanthanum, gadolinium and thulium as metallic precursors. A specific capacity of 412 C g⁻¹ was reported at 1 A g⁻¹ low current density achieving a rate capability of 52.9%. The smooth surface of Ni-foam was observed covered with TM-MOF providing a rough surface morphology showing a thin film growth over it using SEM image. A simple scalable template-based approach was used to prepare ternary MOFs comprising of nickel-cobalt-zinc. The prepared material was subjected to various analysis techniques including SEM, EDX, TEM and XRD. Interconnected porous shell morphology played a crucial role in its charge storage capability. Moreover, in terms of electrochemical performance, a specific capacitance of 247 F g⁻¹ was reported in a coin-shaped assembly. 99% coulombic efficiency retention with an energy density of approximately 28 Wh kg⁻¹ was also reported by Ezeigwe et al. in this studies [121].

A revolutionary tri-metallic MOF NiMn@C was fabricated (Fig. 2(c)) using the combined hydrothermal and carbonization methods [122]. Electrochemical measurements revealed 574 F g⁻¹ of capacitance values showing an elevated energy density (0.027 mWh cm⁻³) and power density (0.672 mW cm⁻³). A capacitance retention loss of 13% was reported after subjecting the material to 10,000 cycles. Characteristic nanosheet structures (Fig. 2(d)) with a combined effect of Ni and Mn decorated carbon proved the basis for good electrochemical results. Nanosheets of Mo-Se-Co were prepared on active carbon and used as a MOF precursor material. Excellent supercapacitive properties have been exhibited by ASC device with a specific capacity of 221.7 mAh g⁻¹, and excellent stability with only 5% loss in rate capability after 8000 cycles. Its real application has been tested when it ran a watch for 70 minutes thus showing the capability of prepared electrode material [123]. The synthesis and SEM micrographs can be seen in Fig. 2(e,f). Ni-Ce-Zr trimetallic MOF was synthesized by Meshram et al. [124] using a facile solvothermal technique illustrating an excellent thermal stability of trimetallic MOF when compared to mono and bimetallic MOF combinations.