Progress towards sustainable energy storage: A concise review

In this study, the benefits and challenges of existing energy storage systems are presented. The environmental threats and the apparent unreliability of fossil fuel energy sources necessitate the need for alternative sources of electrical power. Energy storage has been sourced from mechanical, electrical, thermal, chemical, and electrochemical systems. Perhaps, an electrochemical energy storage system, is a better option toward achieving a net zero carbon‐dioxide emission by 2050. Energy storage sources, such as: batteries and supercapacitors, can be reliably fabricated from the hybrid of polymers and two‐dimensional materials for electric vehicles, aviation, and grid load balancing applications. The performances of lithium‐ion batteries are yet to meet the requirements for high‐duty machines and devices. Graphite, the anode of lithium‐ion batteries, suffers from low capacity and high volume‐expansion, resulting in cracking and fracture of the electrodes. Herein, based on evidence from literature, this study suggests substituting graphite with polymer nanocomposite or metal‐oxide nanocomposites such as: conducting polymers, copper oxide, and graphene. This approach, giving attention to these materials' excellent electrochemical, thermal, electrical, mechanical, and redox properties, offers possible ways to overcome the limited limitations confronting lithium‐ion battery technology.

perform voltage regulation, load, and frequency balancing on a grid system.Wind-plus-storage, photovoltaic-plus-storage, tidal-plus-storage, small-hydro-plus-storage and so on can change the face of global power and transportation systems.An energy storage system stores energy when it is available and used when it is not.Charging is the period when energy sources (e.g., sun, wind, etc.) are available and stored in the storage system, while when the storage system is in use, the process is called discharging.Charging and discharging energy storage systems do not only apply to electrochemical energy devices; it cuts across almost all energy conversion processes.Compressed air, flywheel, superconducting magnetic, pump-hydro, and many others have been used to store energy.Amongst others, Table 1 shows the classifications of energy storage technology.Compressed air, pump-hydro, and flywheel energy storage technologies are matured energy storage systems that have been used to improve the power efficiency of grid systems and renewable energy penetration. 5lobally, the pump-hydro energy storage system contributes the most to grid storage. 6However, geographical location determines the general viability of pumped hydropower energy storage.Compressed air energy storage system has long life-span, low cost of maintenance, and efficient operational flexibility.Notwithstanding, compressed air energy storage techno-economic efficiency, is hitherto, under debate. 7Battery energy storage system is another advanced and matured energy storage system.The installation flexibility, compactness, and global availability, are some of the advantages of battery energy storage systems. 8It is good to mention that battery and supercapacitor energy storage systems are still used for short-term storage.Electric vehicles' powering and electronic devices have also been made possible by battery and supercapacitor storage systems.It is evident, however, that great possibility abounds for greater efficiency and long-term storage duration, only if the electrode challenges, such as: energy and power density problems, production cost, cycling, and lifespan problems, are solved.Some other challenges confronting the present-day battery and supercapacitor include: safety, environmental pollution, and low efficiency.
As an energy support system, energy storage also finds its advantage in grid systems due to the variability of power demand at different times of the day.In other words, the energy storage technology quickly responds when there is a high demand to provide additional power required to meet the demand.In addition, storage systems store excess energy produced at every period.Wind and solar energy fluctuate, so they cannot produce continuous power.The power from such renewable needs to be stored and used when they are not available. 9As power electronics advance, energy storage technology is also gaining popularity, and its viability is guaranteed to tackle fossil fuel menaces.Renewable and nonrenewable energy wastage is curbed by energy storage.When needed, green technologies for sustainable development capture and store renewable energy for efficient utilization (power converters transform this energy into a useful form).
Figure 1 presents the graph of the total installed world energy sources between 1800 and 2021.The figure shows that energy from waste, wood, and charcoal were the earliest energy sources.However, technological advancement and population growth brought about a spiral increase in oil, gas, and coal use. 10 Moreover, the figure reveals the exponential growth of wind and solar energy sources (in 2021, wind and solar energy contributed about 5171 and 11,627 TWh, globally).Without any ado, oil, coal, and gas remain the most prominent energy production sources, worldwide.Considering the spontaneous reliance on fossil fuel energy sources, the question which requires an answer is, "Can the United Nations call for a net zero CO 2 emission by 2050 possible, since the energy sector contributes about 75% of greenhouse gas emissions?"It is also astonishing that the world's carbon emission as of 2021 via fossil fuel burning is about 37.12 billion tonnes, while land use contributed about 3.94 billion tonnes, totaling 41.02 billion tonnes. 10esides the environmental obnoxious of fossil fuels, the generation of electricity using the traditional power grid methods is burdened with many challenges (Figure 2).Overloading the grid system will create instability in the system

Mechanical
Electrical Electrochemical Chemical Thermal

Pumped hydro Superconducting magnetic
Primary cell: zinc carbon, lithium-iron, lithium carbon, zinc air, lithium silver and so on.

Hydrogen and fuel cell Adsorption and absorption system
Compressed air Capacitor Supercapacitor Secondary cell: lead acid, nickel metal, lithium-ion, nickel cadmium and so on.

Flywheel and liquid air
Reserve cell Fuel cell: alkaline, direct methanol, polymer electrolyte membrane fuel cells.

Synthetic natural gas Biofuel Thermos chemical
Sensible heat system

F I G U R E 2
Power network general problems and invariably results in voltage and frequency fluctuations.Population growth is proportional to power demand, and when the generated power is less than the power demanded, the grid will experience a frequency fall.Mechanical failure, poor maintenance culture, managerial incompetence, and technical faults are daily problems confronting the present-day grid system.The benefits of an electrical energy storage system at the grid level include the following 11 : 1. Load following 2. Peaking power and standby reserve 3. Increase in net efficiency of thermal plants 4. Reduction in harmful emissions 5.It promotes distributed generated system.Distributed generation is a system that involves the generation of power close to the distribution stations to cater for energy loss and extra-power demand at the distribution level.This method is considered to be efficient, reliable, environmentally benign, and less costly.Nevertheless, distributed generation is susceptible to capacity fade and line fault, resulting in voltage drops and load fluctuation.
In order to harness the abundance of renewable energy sources, the production/fabrication of high-energy and power-density electrochemical electrodes cannot be overemphasized.Such electrodes must be free of dendrite formation, have long cycle life and withstand short-circuiting, have high electrical conductivity, excellent electrochemical stability, be cheap, safe, have a high charging rate, and low self-discharge. 12,13The energy capacity of electrodes determines the amounts of ions the material can accommodate per gram of the material.Graphene, an excellent electrically conductive material, can host twice as many lithium-ions as compared to the conventional graphite electrode.If a suitable method is employed to fabricate graphene and graphene-based materials, a possible revolutionary energy storage electrode can emerge. 14With high energy/power density, electrical conductivity, and safe and environmentally friendly materials used in making energy storage electrodes, the likelihood of a new positive revolutionized power sector abounds.That is, the contribution of energy to the grid from energy storage devices is more than the energy produced via the spinning of generators, and electric cars no longer depend on the grid to charge.The impact of electric power on the economy advantage of any nation requires stability and an efficient grid system.The amelioration of carbon emissions is also a global concern.Therefore, developing an electrochemical energy storage system for sustainable, reliable, and efficient electricity utilization should be a priority of research and innovation sectors.
This study reviewed pumped hydro energy storage, compressed air energy storage, superconducting magnetic energy storage, and some existing electrochemical energy storage systems.Special attention is paid to the fabrication of polymer and polymer nanocomposite electrochemical electrodes.As mentioned earlier, energy storage is divided into electrical, mechanical, electrochemical, chemical, and thermal energy storage.Amongst these, polymer and polymer-based energy storage systems belong to electrochemical energy storage systems; these materials have the potential for efficient, reliable, and sustainable batteries and supercapacitors.The Section 2 of this study discusses the existing energy storage systems; Sections 3 and 4 provides the trends in battery technology and nanomaterial energy storage.The summary, conclusion, and perspective are provided in Section 5.

EXISTING ENERGY STORAGE SYSTEMS
According to the United States Department of Energy Information Administration, the contribution of fossil fuel to the global-installed power capacity will remain substantive beyond the year 2050. 15As shown in Table 2, in 2050, fossil fuel is expected to contribute about 32.2%, while renewables will contribute 59.6% to the total world energy demand.Though the contribution of fossil fuel, as predicted, will experience a continuous decrease as renewables experience incremental contribution, the impact of battery contribution is still unsatisfactory.Trahey et al. mentioned that the deployment of battery storage is the focal point of decarbonizing the world and making renewable energy relevant. 16Therefore, the pivot of solar and wind energies is how effectively they are electrochemically stored during their availability.Hence, it is an urgent need to improve battery and supercapacitor technology for sustainable, total carbon emission eradication, sufficient power accessibility, and availability.Battery and supercapacitors remain the most optimistic solution to energy production problems.Because, as good as renewable energies are, their direct utilization or integration into the power grid often causes concerns such as: instability, unreliability, and frequency mismatch. 17urthermore, the possibility of reducing carbon emission by 95% requires electric batteries and hydrogen storage with capacities up to 1.4 and 19.4 times the mean hourly power demand. 18Pumped hydro-system is the most significant energy storage contributor to the global energy need.However, the sustainability of a pumped hydro energy storage system will rely on how actively the abundance of environmental energies is stored.Therefore, the electrochemical energy storage system must be improved in order to bridge the present and future energy gap between demand and generation and also to dampen the impact of fossil fuel emissions.Figure 3 shows the comparison between the existing energy storage technologies.The categories of some of the existing energy storage systems, as grouped in Figure 3, revealed that lithium-ion batteries, with all their attendant advantages, is promising to become bulk energy storage device if their present challenges can be adequately addressed.The current storage capacity of lithium battery, is, of course, far from the enormous energy demand of the present-day technology.Purposefully fabricating batteries and supercapacitors that can meet the ever-growing diversity of energy demand in sectors such as: aviation, rail, marine, automobile, and power grid, will require innovative research and resilience.The succeeding subsections briefly discuss present-day energy storage technologies, their shortcomings, and their benefits.

Pumped hydro-energy storage
Pumped-hydro energy storage technology is a technically matured energy storage system, contributing about 96% to the global energy storage pool. 19The system contains two reservoirs at different locations (upper-elevation and lower-elevation reservoirs).During off-peak, that is, when the electricity demand is low, the upper-elevation reservoir is charged to store water and otherwise, during peak period, the upper-elevation reservoir is discharged to supply power to the turbine which in turn spins the alternator to generate electric power for balancing energy demand at the grid and also controls the grid frequency. 20During low demand from the grid, the excess power supply is sourced for generating power to pump water from the lower reservoir to the upper reservoir.Other sources of pumping could be solar energy, coal, and wind.The advantage of a pumped-hydro system is to serve as load balancing and power factor correction for grid power systems.The disadvantages include the cost of water treatment, environmental pollution, uncertainty in many regions of the world due to landscape topology, water problems, geological faults, and small-scale power production. 21igure 4  system is to utilize the abundance of energy from natural-flowing rivers to harness energy from reliable renewables (wind and solar) to pump water to the upper reservoir during low-demand periods.During high-demand periods of electrical power, the water is released through some mechanical means to turn a turbine which drives a generator that produce more power to the grid. 9Pumped-hydro energy system is an open loop system when the water storage is naturally sourced from lakes, rivers and so on.However, it is a closed-loop system when the reservoirs are constructed independently on naturally flowing rivers or lakes. 23,24Though the construction of a pumped-hydro energy system is expensive, however, the benefits are enormous.Nevertheless, the charging and discharging of pumped-hydro energy storage must be autonomous of fossil fuel grid-generated power in order to maximize the system's green advantages.The continuous dependence of supply from the grid to charge pumped hydro-system will increase and encourage the continuous emission of CO 2 from fossil-fuel-driven turbines.Hence, the use of an electrochemical storage system to store energy for the significant functionality of pumped hydro systems, is essential.

Compressed-air energy storage
Compared to a pumped-hydro energy storage system, compressed air storage system releases compressed air from an air storage tank to rotate a turbine during high electric power demand.The system basically consists of an air compressor and air tank.The compressor compresses the air during low electricity demand and releases it at peak or high demand point of electric power.Compressed air energy storage is used in conjunction with wind energy generators to cover the lapses of wind energy feasibility.Therefore, unlocking the potential in compressed air energy storage when used in conjunction with renewable energy, especially wind energy, is a green method toward sustainable and reliable energy production.This, of course, will reduce the wastage experience as a result of the variability of wind and its undesirable effect on the grid power system. 25,26From this point of view, compressed air energy storage serves as peak shaving, load balancing, static VAR compensator, and power factor correction device for grid power systems.Solar and wind energies and excess supply from the grid are the sources of charging the compressed air energy storage system.High energy wastage and cost, the unpredictability of air, and environmental pollutions are the disadvantages of compressed air energy storage. 25,27,28Figure 5 gives the comprehensive technology of compressed air energy storage.The renewable energies shown in the figure are wind and solar.The wind and solar act as sources that provide energy to compress air in the cavern when available.However, the compressed air energy storage system has a very low efficiency.During the energy conversion process, the release of exhaust into the environment often results in heat loss.That is, the pressure and temperature from the exhaust are higher than the atmospheric pressure and temperature, hence, a heat loss due to the release of the exhaust into the atmospheric environment. 29More so, heat loss also occurs in the system when the compressed air is cooled to a lower temperature in order to minimize the compressor's power consumption.The compressed air heating at the expander inlet contributes to carbon-emission.Though, in the adiabatic compressed air energy storage system shown in Figure 6, the heat from the compressor is stored for the heating of the compressed air at the expander. 30Nevertheless, low efficiency in the compressed air energy storage system renders it unreliable, unsustainable, and inefficient.

Superconducting magnet energy storage
Superconducting magnetic energy storage is a cryogenic-based operated energy storage system.Because of the zero resistance of the magnetic coils, a superconducting magnetic energy storage system can store a large amount of power.
Nonetheless, superconducting magnetic energy storage is the most expensive storage technology.The enormous magnetic field produced by the dc signals is stored if the magnetic coils are kept below the coils' critical temperatures.A superconducting magnetic energy storage system consists of superconducting coils, a cryogenic refrigerator, protecting devices that monitor the coils' temperature, and rectifiers. 31The advantages of a superconducting magnetic energy storage system include a static VAR compensator and 10 of megawatts worth of power. 32Figure 7 shows the basic superconducting magnetic energy storage system.The superconducting magnets store the induced magnetic field energy through signal conditioning.The stored energy is converted to a form of energy that can be supplied to the grid.Superconducting magnetic energy storage systems are power fluctuation suppressors, and they are used to improve grid's power transient stability. 33However, during the power transfer between the signal conditioning system and the grid, the superconducting magnetic energy storage induces high-frequency over-voltage on the supermagnetic energy storage system.The high-frequency over-voltage will lead to the magnetic coils' aging and short life span.In addition, the superconducting magnetic energy storage system cryogenic requirement is not economical for large electrical power production.A superconducting magnetic energy storage system has low energy density.Therefore, it may not be suitable for large-scale power production. 34

Electrochemical energy storage
Generally, an electrochemical energy system converts chemical energy into electrical energy, namely, redox reaction.
During the oxidation process, the chemical substance loses electrons; the reduction process occurs when the chemical substance gains electrons.The oxidizer reduces to gain electrons while the reducer is oxidized to give off electrons. 35The essential components of any electrochemical energy storage system include: an anode, current collector, electrolyte, and cathode.Electrochemical cells connected in series or parallel determine the battery voltage and energy capacity. 36As shown in Figure 8, electromotive force is developed at the terminals of the anode and cathode when they are completely immersed in liquid or solid conducting electrolyte.Current will flow when an external circuit is applied across the two terminals of the electrodes.The electron flows from the anode (negative electrode) to the cathode (positive electrode), that is, discharging process.In other words, the electrolyte, which is ionically conducting media, separates the reduction and oxidation processes and causes the electrons flow from the anode to the cathode, to produce electrical work.The typical (mercury battery) chemical equations which occur in the transfer of electrons during oxidation and reduction in an electrochemical storage system are given in Equations ( 1) and (2).
F I G U R E 7 Superconducting magnetic energy storage system

F I G U R E 8 Battery elements: showing ion diffusion and current flow during discharging
Zn The current flow depends on the effective resistances of the entire circuit.The problem of electrochemical energy storage is the power and energy densities, rate of charge and discharge, and safety.Electrochemical energy storage can further be classified as: standard battery (lead acid, Ni-Cd), modern battery (Li-ion, Li-polymer, Ni-MH), special battery (Ag-Zn, Ni-H 2 ), flow battery (Br 2 -Zn, vanadium redox), and high-temperature battery (Na-S, Na-metal chloride).
In a quest to develop high-capacity electrochemical energy storage, numerous challenges encountered include: cathode surface reaction with electrolyte, capacity loss or fade during the charge-discharge process, safety hazard, cracking-fracture, ion-electron mobility, solid-electrolyte interphase and so on. 37Therefore, to find a sustainable energy source for the ever-growing modern world, researchers must work more to find proper solutions to the challenges confronting electrochemical energy storage systems.The key areas researchers need to concentrate on are how to achieve: According to Poizot et al., the present-day electrochemical electrodes, which operate based on redox-active inorganic components, are subject to the high cost of production, shortage of metal resources, and ecological footprint problems. 38n another view, Abu et al. identified and categorized the challenges battling battery energy storage technology as: materials selection problems, solid-electrolyte stability problems, charging/discharging problems, thermal stability problems, recycling, and safety problems. 39he possible solution to opt out of these challenges is probably the recognition and adoption of organic or organic/inorganic-based electrodes as efficient substitutes for the existing electrochemical electrodes.Electric vehicles, flexible electronic devices, and grid energy delivery are amongst the areas where electrochemical energy storage systems will find significant advantages.The aviation sector can also be decarbonized by using an electrochemical energy storage system.

Supercapacitors
A supercapacitor is an electrochemical energy storage system with a high power density, short response time, and long-life expectancy. 40Therefore, supercapacitors are power-saving energy storage devices with a short operating time, while batteries are energy-saving energy storage devices with a long duration of time.
A non-Faradaic supercapacitor comprises current collectors, electrodes, and an electrolyte permeable separator.During the charging process, an accumulation of electrons exits in the surrounding electrodes.That is, the application of an electric field to the current collectors, causes ions in the electrolyte to transit or diffuse over the electrolyte permeable separator onto the pores of the active electrodes. 41Hence, these supercapacitors are called electric-double layer capacitors.The adsorption/desorption of the ions in the electrolyte at the electrode interface occurs due to the formation of the electric-double layers.A peculiar characteristic of electric-double-layer capacitors is their high-power density. 40The electrodes are made from carbonaceous materials, such as: graphene, activated carbon, and carbon nanotube.Conductive polymers also find excellent application in manufacturing supercapacitor electrodes due to their large surface area, porosity, and electrochemical stability.The current collector is the contact between the supercapacitor and the external source or energy-consuming devices. 42s shown in Figure 9, the pseudocapacitor is another type of supercapacitor.It operates by redox reaction (i.e., the Faradaic process produces electrons at the electrode surfaces).The Faradaic process involves the transfer of electrons between the electrode and electrolyte interface.In this case, reduction and oxidation occur when an electric field is applied to the supercapacitor. 43In aqueous electrolytes, pseudocapacitor is characterized by high power density, long cycle life, F I G U R E 9 Supercapacitor classification with electrode types and fast reversible redox reaction. 44Pseudocapacitor electrodes are made from conductive polymers, metal oxides, and nanocomposites.
Another classification of supercapacitors is the hybrid supercapacitor.Hybrid supercapacitors combine the electric-double layer and pseudocapacitor properties to store energy for efficient and better performances.Some advantages of hybrid supercapacitors over electric-double layer and pseudocapacitors include: high energy and power densities, cycling stability, and economic benefits. 45Asymmetric, composite, and battery-type are the types of hybrid supercapacitors.

TRENDS IN BATTERY TECHNOLOGY
An electric vehicle performance centers on the quality of the battery in use.In other words, the cost and endurance mileage of electric vehicles, rely on the prospective battery in use.A lithium-ion battery is one of the leading batteries for electric cars due to its low weight/volume ratio, high energy/power density, and long cycle life. 46Lithium-ion battery technology generally has fascinating characteristics regarding scalability, modularity, and fast response time. 47Batteries are classified as: primary and secondary batteries.It is primarily categorized when cell contents cannot be recharged; secondary classifications are called rechargeable batteries.Electrodes and electrolytes used in the manufacturing of rechargeable batteries further determine their types and applications.Briefly, the following subsections discuss the secondary electrochemical storage.

Lead acid battery
Lead acid battery is the most aged electrochemical energy storage battery.It is relatively cheap and simple in construction with a long-life span.However, the lead acid battery is made up of very heavy components; therefore, its utilization is limited to some applications that do not conform to the present technological need and future advantages. 48Specifically, lead-acid battery has cheap production cost, and suitable for vehicle-ignition system, and has quality charge retention and scalability.Nevertheless, poor energy density, different categories of acids, environmental hazards, poor stationary performances, poor temperature stability, corrosion, and safety problems due to acid leakage are inimical to the battery energy storage. 49In a lead acid battery, lead is the anode, lead-dioxide is the cathode, and sulfuric acid is the electrolyte, which involves internal cell reactions.A prominent advantage of lead acid is its 90% recyclability. 50The basic chemical reactions at the anode and cathode are presented in Equations ( 3) and (4).

Nickel battery
A nickel battery is a secondary electrochemical energy storage system.The battery structure includes: Ni-Cd, Ni-Fe, Ni-H 2 , Ni-MH, and Ni-Zn.Nickel cadmium (Ni-Cd) is the most popular and frequently used nickel battery structural type.The structural technology of nickel batteries is the same; the difference could only be traced to their electrodes.Nickel cadmium is more valuable because of its higher efficiency than others.Alkaline is used as the electrolyte, the anode is nickel hydroxide, and the cathode is cadmium.A separator is used to present electrical contact between the two electrodes.The disadvantage of this battery can be seen in its cost, weight, toxicity, and low energy densities. 44Besides the aforementioned dissatisfied performances, nickel batteries are prone to volume expansion and generation of molecular oxygen.The chemical reactions at the anode and cathode, are presented in Equation ( 5). (5)

Sodium sulfur
In a sodium-sulfur battery, sodium is the anode; sulfur is the cathode, sodium-beta-alumina (Al 2 O 3 ) is the electrolyte, and ceramic is used as the separator.Sodium sulfur battery possesses high energy and power density, low material cost, and long-life cycle.Sodium-polysulfide shuttling, dendrite formation, low discharge capacity, undesirable cycling, and loss of sodium when reacted with electrolytes are detrimental to the perceived performances of sodium-sulfur.Moreover, the advantages of sodium sulfur battery are short-lived as a result of the parasitic effect of the temperature at which it must operate. 51Hence, a large portion of the electrical energy generated by the sodium-sulfur battery is used to maintain its temperature at above or exactly 300 • C so as to keep the electrode in a molten conducting state. 52Although, researchers are working on fabricating room-temperature sodium-sulfur battery electrodes, expensive operating costs, corrosion, and safety problems are still hindering the battery's promising efficient and sustainable usage.The chemical reaction of sodium sulfur is presented in Equation (6).
The energy storage of redox battery technology is not in electrodes but in electrolytes stored in two different external tanks.A typical redox battery has the ability to store energy in large quantities and dissipate the energy in different entities. 53The redox battery technology is for large-scale stationary application, which requires several kilowatts of power.
The flexibility in controlling redox batteries' power and energy densities stands it out in power system applications such as power quality regulation, load balancing, and green energy storage. 54 chemical stability, ionic exchange capacity and conductivity, and must be produced at low cost. 55The practical application of redox batteries may not be feasible in electric vehicles due to their construction size, weight, and cost of refilling reactants.Flow batteries have large ohmic resistance in graphite anode, high cost of active materials, and their application is limited to stationary use. 56

Fuel cell
A fuel cell is a special type of energy storage system, which involves the conversion of chemical fuels, such as hydrogen and methane, to electrical energy.A fuel cell essentially consists of an anode (e.g., platinum) and cathode (e.g., flake graphite) electrodes and conducting electrolytes.The cathode takes oxygen from the air, and the chemical fuel passes through the anode; the positively charged hydrogen ions transit through the electrolyte-the reaction of the positively charged hydrogen ions with the oxygen at the cathode results in water formation.And the flow of electrons through the external electrical circuit produces electrical power.The chemical fuel at the anode is oxidized to produce electrons and protons; the protons permeate through the electrolyte membrane to the cathode to react with the oxygen and electrons to produce an electric current in the connected load. 57While the energy of a redox battery is stored in the electrolyte, fuel cells' energy is stored in the chemical fuel.Some advantages of fuel cells are excellent fuel conversion efficiency, environmental benign, high reliability, high energy/power densities, continuous power generation when chemical fuel is present, and simple operation. 58,59Polymer electrolyte membranes, molten carbonate, solid oxide, direct methanol, alkaline, and phosphoric acid are fuel cells energy storage systems types. 60Fuel cell energy generator is usually identified with a high cost of catalysts and maintenance, durability, and reliability problems.Wang et al. envisaged that the improvement in the reliability and durability of fuel cells would reduce their operational cost and end-user feasibility. 61

Lithium battery
Lithium batteries have the same chemistry employed by every other battery technology; however, they are the present-day leading electrochemical energy storage.The most advanced battery technology is associated with lithium-ion batteries, as they are the engine room of modern digital electronic and electric vehicles.Basically, lithium-ion battery anodes are made of carbonaceous and conductive polymer materials.The cathode can be lithium-cobalt-oxide, lithium-iron-manganese-oxide, or lithium-iron-phosphate.Lithium-cobalt-oxide is temperature selective within the range of −40 • C to 70 • C. 62 The temperature effect limits the advantages of lithium-cobalt-oxide.
The separator isolates the active materials from the anode-they are porous materials inserted into the electrolyte to provide ionic conduction between the electrode/electrolyte. 63To mention but a few, the lithium-ion battery is the engine of the present-day cell phones, computer devices, small-range electric cars, airplanes, and home accessories. 64ithium's high electro-positivity and flexibility make it gain a high advantage over hydrogen.Lithium-ion batteries may be very useful in power grid frequency regulation and for short-term storage.Nonetheless, the economic aspect of lithium production and scarcity of the mineral is not beneficial for grid power energy generation. 65Lithium tetrafluoroborate (LiBF 4 ), lithium hexafluoro-arsenate (LiAsF 6 ), lithium percolate (LiClO 4 ), lithium trifluoromethanesulfonate (LiTf), lithium Bis(trifluoromethanesulphonyl)imide (Lilm), lithium hexafluorophosphate (LiPF 6 ), are the electrolytes for lithium-ion batteries. 66Table 3 presents some lithium-ion positive electrodes technology and their drawbacks.Both lithium metal and graphite anode have shortcomings of safety hazards.Even though lithium metal anode has a very high specific capacity of about 3884 mAh/g and low redox potential, their dendrite formation (this effect often leads to cell failure) hampered their effective performances. 74Solid-electrolyte interface, which forms on the graphite anode during electron transfers, reduces the electrolyte on the graphite anode.Generally, low energy density, high cost of active materials, low voltage, loss of power, a dramatic increase in resistance, and sharp response to external stimulus, are the major critical problems of lithium-ion batteries technology.

Comparison of the energy storage systems
Based on the data collected by Das et al., Figure 11 presents the energy capacity (MW) and the efficiencies of lead-acid, lithium-ion, and vanadium redox flow batteries in comparison with pumped hydro and compressed air energy storage

F I G U R E 11
Comparison of energy storage systems efficiencies systems. 8From the figure, it is optimistic that lithium-ion storage systems can deliver high megawatts of electricity and sustainably supply energy to electric vehicles.However, this may only be possible if the lithium-ion graphite electrode is substituted with more efficient anode electrodes.Moreover, shown in Table 4 is the comparative analysis of the discussed energy storage systems in this study, based on their efficiency, environmental impact, application, location, cost, energy, and power density.
In summary of this section, it can be concluded that the search for an electrochemical energy storage system for the energy demand of the present and future generations must be able to possess the following characteristics:

NANOMATERIALS ENERGY STORAGE SYSTEMS
Through research and innovation discoveries, the potentials of extrinsic and intrinsic polymers, graphene, and other nanomaterials have been ascertained as promising energy storage materials for grid and electric vehicle applications.By considering the excellent properties of nanocomposites, then, there is hope for the aspirations of material, chemical, and electrical engineers of a better world.That is, transportation made easy, sustainable electric power delivery with less carbon-dioxide emission, smart-home, smart devices, and so on.Ionically and electronically conductive polymers have the potentials to replace the electrolyte and the electrodes of the present-day battery technologies. 80Amongst others, the advantages of polymer and its nanocomposites are high-range conductivity, easy fabrication methods, light weight, mechanical and chemical stability, large scale manufacturing, and application flexibility.Hitherto, the general problems of electrochemical and chemical energy storage systems remain the same: energy/power density, cycling, charging rate, the weight of the material, durability, safety, and cost of materials are the various challenges electrochemical energy storage faces.Conducting polymers and polymer nanocomposites stand out to substitute the conventional materials for battery and supercapacitor electrodes.Therefore, polymer nanocomposites for sustainable energy storage must be given proper attention and dedicated research in order to harness the potential of the hybrid of various polymers and two-dimensional nanocomposites.
The lithium-ion battery is the most adaptable electrochemical energy storage system for electric vehicles due to its light weight and energy depth.Therefore, by considering the prospect of lithium-ion batteries, the graphite anode must be replaced in order to manufacture more sustainable and reliable lithium-ion batteries for electric vehicles.Lithium-ion battery charging process involves the deintercalation of lithium from the cathode electrode to the anode electrode via the electrolyte.Hence, the energy/power density, cycling, and other battery performances depend on the negative electrode.Anode electrode made from graphite has low capacity and charge/discharge problems. 81In addition, relatively large lateral sizes are disadvantages to lithium-ions graphite electrode electrochemical performances.Materials or composites of small lateral size will provide an easy pathway for diffusing lithium-ions. 82wo-dimensional materials like graphene and borophene are nanomaterials with small lateral sizes.Graphene is a carbonaceous material with excellent electrical conductivity, large surface area, high thermal and chemical stabilities, and mechanical strength.The efficient electron mobility between the graphene electrode active material and electrolyte can be attributed to the electrode's large specific surface area and porosity. 83,84Graphene oxide, reduced graphene oxide, graphene nanoplatelets, and many others are the various terminologies for graphene.Conductive polymers, on the other hand, are conjugated polymers having exceptional electrical conductivity and redox reaction behavior, low toxicity, and less pollution properties.Polypyrrole, polythiophene, polyaniline, and polyacetylene are conductive polymers.The tunability of conductive polymers in order to tailor their properties to the desired applications makes them choice materials for diverse applications.Therefore, the nanocomposites of two-dimensional materials with conducting polymers for electrochemical energy are appealing and envisaged as promising storage materials for electric vehicles and electrical power generators for the grid.For instance, Xing et al. presented a report on the development of the nanocomposite of graphene-oxide/polypyrrole containing silver and manganese oxide. 85The fabricated micro-porous electrode exhibited highly stable cyclability and efficiency of about 10,000 cycles and 105.30%.The facile methods of preparing the superconductive electrode, and the cheap source of materials, are benefits of nanocomposite electrodes.

Nanomaterials batteries and supercapacitors
Nanomaterials are organic, inorganic, and composites-based materials with particle sizes ranging between 1 and 100 nm.The nanomaterials are useful in the field of energy storage, sensors, telecommunication, biomedical engineering, and electronics due to their exceptional properties such as large surface area, porosity, tuneable size, excellent electrical conductivity, high sensitivity, thermal and chemical stabilities, low toxicity, and biocompatibility. 85The most fascinating issue about nanomaterials is the ability to control and manipulate their properties at atomic and molecular levels.In energy storage applications, nanomaterials evolve for efficiency of energy storage and charge/discharge process improvement.
The design and performances of the electrochemical or chemical storage systems also benefit from nanomaterials' unique properties. 86n an investigation conducted by Zhang et al., a ternary system of nanocomposite consisting of graphene nanosheet, polypyrrole, and sulfur was prepared using a facile in-situ chemical polymerization method. 87The fabricated graphene/polypyrrole/sulfur nanocomposite lithium-ion electrode showed an initial capacity of about 1416 and 642 mAh/g after 40-cycles at 0.1 C rate.A nanocomposite of reduced graphene oxide, polypyrrole, and polyoxomolybdate prepared by one-pot hydrothermal synthesis method, yielded a lithium-ion storage capacity of about 1000 mAh/g at 100 mAh/g after 50-cylce and 372.4 mAh/g at 2 A/g after 400-cycles. 88As shown in Figure 12, the reduced graphene oxide/polypyrrole/polyoxomolybdate lithium-ion anode electrode involved a simple synthesis method (Figure 12A).The formation of the solid electrolyte interface occurred at 0.6 V (Figure 12B), which indicates the cyclic stability; excellent electrochemical reversibility (Figure 12C), and the unique performances of the electrode in lithium-ion battery, (Figure 12D,E).
Graphene/polyaniline nanocomposite has also been demonstrated for chloride-ion energy storage. 89Acetylene black, Super P, and graphene carbonaceous materials were individually composited with polyetherimide (PEI) and zirconium oxide (ZrO 2 ) by the synthesis method shown in Figure 13. 90The carbon materials/polyetherimide/zirconium oxide nanocomposite was used as interlayers for lithium anode in the lithium-sulfur battery.Lithium-sulfur batteries suffer from the shuttling effect of polysulfide during the charge and discharge processes of the battery.The dissolvement of polysulfide in electrolyte during the active work of the battery usually results in the electrode volume expansion and loss of active material.The polysulfide shuttling effect is a threat to the anode electrode capacity-the impact can lead to damage of the electrode, capacity reduction, and corrosion. 91Therefore, a porous carbon interlayer inserted between the cathode (sulfur) and the separator has been shown to reduce the lithium-sulfur shutting effect. 92In the comparison of carbon materials reinforced polyetherimide and zirconium oxide, Liu et al. noted that the nanocomposite of graphene/zirconium oxide/polyetherimide improved the physiochemical stability and electrochemical performance of the lithium-sulfur battery. 90The excellent electrical conductivity of graphene alleviated the loss of active-sulfur and blocked lithium polysulfide, while polyetherimide enhances the structural stability of the interlayer.Graphene has also been investigated to improve the electrical conductivity of lithium-ion battery active materials in order to decrease the electrode's internal resistance and polarization. 93The electrical conductivity of electrochemical electrodes, the cycle life, wettability of electrode by electrolytes, enhances the ionic transition, and ultimately improve the capacity of the battery or supercapacitor.Carbonaceous material in the form of activated carbon has large surface areas, a property that is essential for electrochemical electrodes' excellent performances.In Bigdeloo et al. investigation, an activated carbon obtained from canola waste was composited with functionalized graphene oxide/poly (ortho aminophenol) nanocomposite to produce a supercapacitor electrode. 94The specific capacitance of the activated carbon/graphene/polymer nanocomposite was 1351 F/g, 57.3 Wh/kg energy density, and 830.24W/kg power density at 2 A/g current density.After 5000 cycles, the capacity retention was 96.2% (Figure 14A); the electrochemical stability of the electrode showed excellent electrochemical performance of the electrode.The capacitance of the pseudocapacitor, according to Figure 14B, is that capacitance is inversely proportional to current density.The nanocomposite electrode also has a large surface area and good redox reaction (Figure 14C).The charge/discharge ability of the electrode also depends on the current density (Figure 14D).A graphene/polyaniline nanocomposite prepared by the oxidative polymerization method was co-precipitated with iron oxide (Fe 2 O 4 -iron (II-III) oxide) to fabricate a supercapacitor electrode. 95When subjected to 6 M KOH electrolyte, the electrode showed a gravimetric capacitance of about 300 F/g, 61.1 Wh/kg energy density, 60% capacity retention at 250-cycles, and at 2000-cycles, 18% of the total capacitance was retained.The effect of the iron oxide was evident in the thermal stability of the electrode.Alsulami et al. combined graphene oxide, polyaniline, and spinel zinc ferrite nanoparticles to fabricate a high-performing supercapacitor electrode. 96The facile synthesis method produced an electrode with a specific capacitance of about 2169.7 F/g at 90% capacity retention.Spinel ferrites are metal oxides nanoparticles usually denoted as MFe 2 O 4 , where M represents iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), magnesium (Mg), copper (Cu), Zinc (Zn), gold (Au), and many others.Spinel ferrites are environmentally friendly, cheap, excellent catalytic, and tuneable bandgap.Polyaniline on the other hand, has excellent electrical conductivity, and considerable surface area.The nanocomposite of these nanoparticles as prepared by Alsulami et al. was reported to be an excellent superconductor cathode electrode due to its desirable porosity, cost-effective, facile preparation method, and electrochemical stability.
Metal oxide, such as cobalt disulfide, has many advantages for developing supercapacitors.For instance, cobalt disulfide is inexpensive, stable under acidic conditions, and abundant on earth.For this reason, Hasanzadeh et al. proposed a one-pot hydrothermal preparation of cobalt/polypyrrole nanocomposites for supercapacitor energy storage electrode. 97he specific capacitance of the cobalt/polypyrrole nanocomposite is 605.2C/g at 1.0 A/g.The energy and power densities are 88.07 Wh/kg and 0.83 kW/kg, with a remarkable cycling stability after 5000 cycles.
Polyaniline/manganese ferrites nanocomposites electrode was recently prepared by Somasundaram et al. 98 The fabricated electrode using the in-situ oxidative polymerization method yielded 287.1 F/g higher than the capacitance of the manganese ferrites electrode.The polyaniline/manganese nanocomposite electrode displayed a longer discharge rate at 1 A/g (Figure 15A).However, as the current density increases, the discharge time decreases (Figure 15B).The current density also showed inverse behavior to the capacitance of the electrode (Figure 15C).The Nyquist plot (Figure 15D) also showed the excellent electrochemical performance of manganese ferrites reinforced polyaniline conductive polymer.The electroactive properties of polyaniline/reduced graphene oxide and graphene oxide were investigated by Gandara et al. 99 Gandara et al. mentioned that graphene oxide and reduced graphene oxide maintained structural and electroactive stabilities in polyaniline conductive polymer for a 500 cycles galvanostatic charge/discharge performances.
Metal selenide is a transition metal-chalcogenide and a nanomaterial with properties such as large surface area, good electrochemical, thermal, and mechanical stabilities, and excellent electrical conductivity. 100The synergy between the transition metals' cations and the selenides' anions is beneficial for manufacturing high-energy and power-density electrochemical electrodes.In Safdar et al. experimentation, the nanocomposite of aluminum-selenide and reduced graphene oxide for supercapacitor electrodes were presented. 101The aluminum-selenide and reduced graphene oxide prepared by the solvothermal synthesis method yielded a specific capacitance of about 1138.10 F/g, 39.57Wh/kg energy density, and 0.006 kW/kg power density at 2.5 A/g current density.
Apart from conductive polymers, metal oxides, metal ferrites, or metal selenides, geopolymers like metakaolin can be modified to fabricate efficient electrochemical electrodes.Metakaolin hardened by Revathi et al. using phosphoric acid and alkali, was reinforced with reduced graphene oxide to fabricate supercapacitor nanocomposite electrode. 102The specific capacitances of the electrodes are: 53.2 and 42.5 F/g.The electrochemical stability of the electrode, proved that graphene and graphene-based nanomaterials have enormous potential to tailor organic and inorganic materials for energy storage applications.More so, the gravimetric charge/discharge curves of the nanocomposite, evident the effective ionic conduction and electron transition during the charge/discharge behavior of the supercapacitor.
Electrostatic self-assembly of graphene oxide, hydrotalcite, and polyamide was performed by Zhu et al. 103 Hydrotalcite is a positively charged two-dimensional insulating sheet, which can interact with graphene oxide, electrostatically.The electrostatically self-assembled graphene oxide and hydrotalcite were incorporated into the polyamide polymer to improve the nanocomposite's thermal stability.The unique supercapacitor performances of the electrode were attributed to the synthesis method, the excellent properties of graphene, and hydrotalcite leakage current resistance.
Two-dimensional MXene is analogous to graphene, with properties such as: excellent electrical conductivity, and hydrophilicity. 104Considering the electrochemical stability of MXene and polyaniline, Seenath and Biswal performed an experimentation to investigate the performances of MXene/polyaniline nanocomposite supercapacitor electrodes in solid-state and ionic-liquid-based electrolytes. 105The MXene-based electrode in polyvinyl alcohol (PVA)-H 2 SO 4 delivered 28.6 Wh/kg energy density at 200 W/kg power density.When the electrode was subjected to an ionic-liquid electrolyte, the electrode delivered 139 Wh/kg energy density at 1660 W/kg power density.

SUMMARY, CONCLUSION, AND PERSPECTIVE
In summary, the performances of nanomaterials and nanocomposites for energy storage, such as inexpensive cost of production, abundance materials in nature, environmental benignity, excellent thermal and electrochemical stability, high energy/power density, excellent discharge/charge ability, and safety, their consideration for the manufacturing of electrodes for lithium-ion batteries and others, would assist in curbing the various menaces of fossil fuel.According to Figure 16, Kumar et al. discussed some possible methods to optimize the electrodes and electrolytes of supercapacitors. 106The discussion proposed nanomaterials as viable materials that can increase the supercapacitor electrode's surface area and cycle life.The supercapacitor electrolyte must be pure; the purity of supercapacitor electrolyte decreases the negative impacts of impurities on the supercapacitor's working voltage and life-cycle.In order to overcome the electrochemical resistance between the current collector and the electrode interface, the electrode must be porous, possesses large surface areas, and be electrically conductive.In addition, the thermal stability of the electrodes must be able to withstand the continuous operating temperature of the electrodes.The synthesis method plays a significant role in any type of electrochemical electrode performance.For instance, the active sites of pseudocapacitor electrodes are determined by the uniform porosity of the electrode at either nano, atomic, or micro-scale. 107Realistically, the nanoelectrodes' porosity and specific surface area depend primarily on the synthesis methods and the single or composited materials.
In order to achieve sustainable green energy, more attention must be paid to the fabrication of high energy/power density electrodes, possessing high electrical conductivity, mechanical and chemical stabilities, and versatility for both F I G U R E 16 Supercapacitor performance optimization approach.Reproduced with permission: Copyright 2022, MDPI 106 industrial and transport applications.Lithium-ion batteries' low capacity can be eradicated by using polymer-graphene and graphene-based nanocomposites.Polymer nanocomposites, for example, can ensure the mediation of the electrode materials by modifying their morphology, size, and shape in order to improve their electrochemical, electronic, thermal, and mechanical performances.The reinforcement of graphene and related materials with conductive polymers, metal oxides, and the likes, will aid the conductivity and wettability of the electrodes, thereby reducing capacity loss during charge and discharge cycles. 108 lithium-ion battery, for example, undergoes solid electrolyte interfacing during the initial cycle of the battery-the solid electrolyte interface, which passivates the negative electrode during the decomposition of electrolyte (i.e., when the electrolyte is subjected to a potential operating window outside its electrochemical stability), hitherto remains a questionable occurrence in lithium-ion batteries graphite electrodes.The solid electrolyte interface plays a vital influence on the cyclability and electrochemical stability of the battery.According to Berrueta et al., the electrolyte's decomposition not only results in solid electrolyte interface formation, but also impedes electron transition reaction between the graphite anode and the electrolyte. 109The breaking and continuous reformation of the solid electrolyte interface reduces the battery efficiency and lifetime.Fast charge time is important to electric vehicle battery usage: to achieve this, ionic charge transportation via solid electrolyte interface resistance must be eradicated.Hence, this enigmatic challenge is envisaged to be solved by polymer-graphene and graphene-based fabricated anode electrodes for stable solid electrolyte interfaces.
Considering silicon's storage capability (about 3570 mAh/g), it can be categorized as excellent material for lithium-ion battery anodes.Nevertheless, the interfacial instability, low electrical conductivity, and mechanical instability of silicon, renders it unsuitable for electrochemical batteries.However, coating silicon with graphene has been proved a solution to silicon electrochemical instability. 110Kim et al. presented silicon-coated graphene as a means to abrogate silicon's poor mechanical, and electrical conductivity and cycling stability.In another study, silicon nanoparticles of doped-nitrogen, were admixture to reduced graphene oxide/carbon-nanofiber nanocomposite by self-assembling synthesis method for lithium-ion battery anode electrode. 111The silicon-coated reduced graphene oxide/carbon nanofiber nanocomposite, reduced silicon anode electrode volume expansion and contraction while maintaining excellent electrochemical performances of the lithium-ion battery.Graphene and the graphene-based nanocomposite tend to effectively accommodate the volume change and still ensures the conductivity of the active materials.The electrochemical benefits of graphene are versatile.Lithium-vanadium-oxide anode electrode for lithium-ion battery is characterized by poor electrical conductivity.
Nonetheless, the composite of graphene with lithium-vanadium-oxide prepared by sol-gel method, has been presented as a high-performing anode electrode for lithium-ion batteries. 112The following points must be pertinently considered in order to develop polymer-graphene and graphene-based electrochemical energy storage devices: 1. Synthesis methods must be suitable for proper control of morphology, thermal, electronic, and mechanical properties.2. Conductive polymers and metal oxides are beneficial for electrical conductivity enhancement and electrochemical stability (charge and discharge) of the nanocomposite electrodes.3. The electrically insulating but ionically conducting interface (solid electrolyte interface), which passivates the negative electrode is a necessary phenomenon in lithium-ion batteries.However, it can also be very unpleasant to the battery's performance.Accordingly, solid electrolyte interface impedance must be small, insulate the contact between electrolyte and electrode, and be mechanically stable. 113Nevertheless, when it becomes thick or condensed, the battery life is shortened as a result of the large impedance to lithium-ions transport.Therefore, to avoid the continuous building up of solid electrolyte interface to the point of impeding lithium-ions transport, the thermal stability of negative electrodes must be improved in order to reduce the thermal expansion of the electrodes during cycling.4. Electric vehicles will demand energy at high-voltage; high-voltage may overheat the battery, which may likely lead to an explosion.Hence, electrode modification using graphene/polymer nanocomposites is proposed.5. Polymer-graphene and graphene-based materials can correct the low electrical conductivity, slow lithium-ion transport, volume expansion, and mechanical and thermal instabilities of lithium-ion battery electrodes.
Be that as it may, most energy storage systems have excellent economic and industrial impact, especially pump-hydro, supercapacitors, fuel cells, and batteries.A fuel cell is gaining popularity in transport systems 114 ; uninterrupted power supply and electric vehicles feasibility have been achieved by batteries and supercapacitors; pump-hydro is still relevant in supplying several megawatts of power to industries, residential buildings, and community electrification. 115owever, pumped hydro energy storage must be independent of fossil fuel in order to reduce carbon-emission.Hence, the production of an efficient and sustainable electrochemical energy storage is the most optimistic storage system to achieve global warming reduction and better standard of living.Battery and supercapacitor electrodes manufactured from polymer-graphene and graphene-based nanocomposites, have potentials for the industrial, commercial, and domestic applications.The life-span, cost versus capacity and efficiency, recyclability, and thermal stability of these electrodes, are areas which must be thoroughly investigated.

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Global energy status (1800-2050) (Data sourced fromReference 10) provides brief science behind pump hydro energy storage.The highlight importance of a pumped-hydro energy F I G U R E 3 Existing energy storage systems and their positioning F I G U R E 4 Pumped-hydro energy storage system.Reproduced with permission: Copyright 2016, Elsevier Ltd 22

F I G U R E 5 F I G U R E 6
Compressed air energy storage technology Diabatic and adiabatic compression air energy storage systems.Reproduced with permission: Copyright 2021, Elsevier Ltd 30

Figure 10
is a simple sketch of a redox battery showing the ion exchange membrane, the electrolyte tanks (analyte and catholyte), the electrodes, and the load or source to charge the battery when the reactants are depleted.The membrane of the vanadium flow battery, shown in Figure 10, is the most crucial part of the battery system.Redox or flow battery membrane generally determines the cyclability and the economic affordability of the battery.According to Shi et al., flow battery membranes should possess excellent F I G U R E 10 Vanadium redox battery configurations

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I G U R E 14 Activated carbon/graphene oxide/poly (ortho aminophenol) nanocomposite supercapacitor electrode (A) capacity retention, (B) specific capacitance in relation to current density, (C) energy density information, (D) cycling dependency on current density.Reproduced with permission: Copyright 2022, Elsevier Ltd 94 Polyaniline/manganese ferrite nanocomposite electrode (A), (B) galvanostatic charge/discharge curves, (C) capacitance with respect to current density, (D) Nyquist plot.Reproduced with permission: Copyright 2023, Elsevier Ltd 98 15e comparison of the world energy sources and projections between 2020 and 205015 TA B L E 2Abbreviations: Geoth, geothermal; GW, installed capacity in Gigawatts; %X, the percent contribution of each energy technology.
Cathode lithium-ion electrodes and their drawbacks TA B L E 3 Comparative characteristics of energy storage systems TA B L E 4