Worldwide energy requirements and conservation efforts have grown since the industrial age, highlighting a pressing need to address social and environmental issues over the last 5 decades (Yang et al., 2011; Chu and Majumdar, 2012). As a result, the reliance on fossil fuels has declined since the 1970s, whereas the demand for renewable and sustainable energy sources has surged (Jiang et al., 2020; Gür, 2018). For renewable energy sources like solar, wind, and geothermal to stabilize their intermittent nature, energy-storage systems (ESSs) are essential. When compared to fossil fuels, electrochemical conversion and storage of renewable energy have a significant effect (Noori A. et al., 2019; Blanco and Faaij, 2018). Supercapacitors (SCs) have exceptional energy storage capabilities, including low cost, long cycle life, high power density, specific capacity, rapid charge/discharge time, and operational stability (Castro-Gutiérrez et al., 2025; Huang et al., 2017). SCs are classified into two types according to their energy storage methods: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. Currently, pseudocapacitors are gained greater attention since, in comparison to EDLCs, the energy density related to the faradaic process is significantly larger. Therefore, significant efforts are made to create novel designs and prepare electrode materials based on pseudocapacitors with the goal of improving the electrochemical characteristics for energy storage. Pseudocapacitors are increasingly being used as a remedy for these issues (Zhang et al., 2018; Simon and Gogotsi, 2008). Recently designed electrochemical capacitors have demonstrated superior energy density in comparison to conventional carbon-based electrical double-layer capacitors (EDLCs) (Kavaliauskas et al., 2011). The advantages are larger voltage window and increased capacity of the Faradaic electrode. On the other hand, pseudocapacitors to perform faster reversible redox reactions, they have a higher energy density, unlocking the door to potential large-scale uses. Transition metal-based hydroxides and oxides have been thoroughly studied as potential electrode materials for high-performance pseudocapacitors (Wang et al., 2015; Peng and Zhang, 2023). For instance, research has been done on electrodes like NiO (Burke, 2000), RuO₂ (Anderson et al., 2010), Co₃O₄ (Frackowiak and Beguin, 2001), MnO₂ (Wang et al., 2012), Ni(OH)₂ (Arbizzani et al., 1996), and Co(OH)₂ (Mala et al., 2023), as well as their compounds. The Rich redox reaction sites associated with bimetallic nickel-based hydroxides have recently been demonstrated to have substantial theoretical specific capacitance values, indicating that these materials have great potential as batteries (Jayakumar et al., 2024). Despite their rapid charging and discharging times, supercapacitors have lower energy densities (5 Wh kg⁻¹) than batteries; nevertheless, they can achieve a far higher power density (10 kW kg⁻¹) in the same amount of time (Liu et al., 2023; Yan et al., 2023). SCs are therefore viewed as a viable alternative to batteries, especially in the area of energy appliance load lifting and various types of storage batteries electrochemical performances as shown in Figure 1. However, meeting the growing demand for future arrangements necessitates substantial advancements in supercapacitor performance as new materials are developed and the scope of nanoscale electrochemical alliances is expanded (Shoeb et al., 2023; Hou et al., 2023).

Over the last 10 years, major improvements have been achieved an experimental and theoretical study of SCs, including the development of various SCs material categories, device fabrication types, and electrolytic compounds. The production of next-generation energy storage materials requires a special method for creating novel materials. Developing novel materials with unique morphologies and stable crystal structures essential for SCs, batteries, hydrogen storage, and other applications aim of contemporary synthetic chemistry. Therefore, electrode materials with suitable properties are needed for high-performance ESS systems. Reviews on these subjects are quite good (Hu and Wang, 2002). Among the most promising candidates for advanced SCs are transition metal oxides and 2D materials, such as MXenes, metal–organic frameworks (MOFs), and covalent organic frameworks (COFs). These emerging materials exhibit exceptional potential for a wide range of energy storage applications (Li and Zhitomirsky, 2013). Pseudocapacitive materials derived from these systems offer enhanced energy storage capabilities due to their multiple accessible oxidation states. Additionally, their tunable porosity and hierarchical architectures can be strategically engineered to improve ion transport and achieve high energy densities. However, several challenges hinder their widespread application in supercapacitor systems, including limited electrical conductivity, structural instability during cycling, and complex synthesis routes. Overcoming these limitations is critical for the development of high-performance, durable, and scalable energy storage devices.

This review focusses on current research into pseudocapacitive electrode materials for SC applications. We begin by discussing the origins of pseudocapacitance and the fundamental principles of SCs, followed by characteristics of pseudocapacitive materials and pseudocapacitive electrodes. Next, we examine the mechanism of intercalation, redox and surface adsorption of pseudocapacitance electrode materials. Particularly, we discuss the following categories: 1) intercalation-type materials such as Nb₂O₅, TiO₂, and V₂O₅, highlighting their structural features and electrochemical performance; 2) redox-active materials, including metal oxides (MnO₂, Fe₃O₄, and V₂O₅) as well as MXenes and metal-organic frameworks (MOFs), which play vital roles in enhancing charge storage via reversible redox reactions; and 3) surface adsorption mechanism-based materials, involving functionalized 2D materials and porous organic frameworks (POFs), contribute to high-rate capability and excellent cycling stability through hydrogen bonding at the electrode–electrolyte interface. Furthermore, Ni-based materials are highlighted as highly promising electrode material for SCs. They offer several unique characteristics, like cost-effective cost, friendly to the environment, the availability in nature, and high theoretical capacitance. Ni-based electrodes also exhibit high specific capacitance, reversible redox activity, and good electrochemical stability, making them suitable for high-performance energy storage systems. This review aims to provide insights into strategies for achieving optimal material properties for pseudocapacitive applications. We hope it will guide future research toward leveraging the intrinsic characteristics of advanced materials and identifying the most effective systems for next-generation energy storage technologies.

The first carbon-based (NEC) Supercapacitor (SC) electrode, known as “Supercap,” entered the market after 10 years of arduous work by various businesses, including NEC and Sohio, to create commercial products beginning in the 1970s. Research on the alleged “pseudocapacitors” started around 1980. These devices are entirely new and operate in a different manner (Wu et al., 2007; Zhang et al., 2009). The simultaneous “double layer” and faradaic reactions, which combine the functions of electrochemical batteries and capacitors, allow for electrical charge storage. Thin layers of transitional metal oxide active materials were deposited on metallic substrates to create these devices.

More recently, enhanced activated carbon-based SCs were used to create high-voltage electrolytes (such as organic and ionic liquids) (Garche, 2024), which allowed the market to meet demands for high-power devices. It is important to note that, since then, the number of categories and definitions has increased steadily, resulting in a state of near-anarchy in the field of SCs (Ganesh et al., 2006). The electrolyte is impregnated into the two electrodes, which are separated by a membrane separator that promotes ion mobility and inhibits electric contact. These devices have a high degree of recyclability due to the usual electrostatic charge transfers (Joseph and Thomas, 2023). New porous and conductive electrode materials have high specific surface areas, which contribute to the increased capacitances of SCs.

In fact, the EDLC is only responsible for a small percentage of the charge; faradaic mechanisms (electrosorption, redox reactions, and intercalation) are used to transport and store a much greater quantity of charge. pseudocapacitors store an electrical charge (and thus energy) using a different process. Their electrodes are made of redox active materials. A pseudocapacitor’s electrode undergoes quick and reversible redox reactions involving the transfer of charges between the electrode and electrolyte when an external voltage is applied. A pseudocapacitor’s charge and discharge mechanism is comparable to that of batteries (Mishra et al., 2022; Wu et al., 2024). The reaction at the cathode during the charging phase could be explained by Equations 1, 2 as follows: E 1 + A ‐ →  charge  E 1 δ +   / /   A ‐ + δ e ‐ (1)

Moreover, the reaction at the anode is represented by: E 2 + C + + δ e ‐ →  charge  E 2 δ ‐ / /   C + (2)

The cathode and anode are denoted by E1 and E2, the anion and cation by A⁻ and C⁺, respectively, and the electrolyte–electrode interface by//. The electrosorption valence, which is associated with oxidation-reduction reactions, is represented by the value δe⁻.

Pseudocapacitors offer significantly higher energy density nearly twice that of EDLCs, due to their Faradaic charge storage mechanism, which utilizes both the surface and the bulk of electrode materials. Initially, ceramic compounds like IrO₂, RuO₂, and NiO were explored, but in recent years, attention has expanded to include conducting polymers and alternative metal oxides (Conway and Gileadi, 1962; Conway and Angerstein-Kozlowska, 1981), aiming to enhance performance while reducing production costs. Although materials such as conducting polymers and metal oxides deliver high Faradaic capacitance, they often suffer from limited stability over long-term cycling. (Bhojane, 2022). Identified three key Faradaic mechanisms responsible for improved capacitance: underpotential deposition, redox pseudocapacitance (e.g., in RuO₂·nH₂O), and intercalation-based pseudocapacitance. Figure 2 shows an overall growth and understanding of mechanism, and theoretical aspects of pseudocapacitance for SCs applications.

Advantages of supercapacitors must be acknowledged in order to mitigate the drawbacks and improve mechanical strength and energy in supercapacitors applications. In these applications where integrated structures are not regularly replaced, supercapacitors are advantageous due to their long longevity and noticeably greater cycle life than batteries. First, the limitation for supercapacitor materials is compromising between specific capacitance and mechanical strength. The electrodes were treated with chemical activation processes at high temperatures to increase the electric double layer capacitance, but the treatment decreased structural strength because of damaged electrodes in nickel-based batteries. Second, a supercapacitor (SC) is one of the better energy storage alternatives that the research community is currently investigating due to the drawbacks of certain battery types materials. SCs provide energy rapidly and effectively, making them suitable for a wide range of applications. These are suitable for large-scale generate and used for low-cost and simple design in energy storage applications.

Pseudocapacitive materials are notable for their potential in designing innovative electrode systems for energy storage technologies. The field of materials chemistry is central to enhancing charge storage performance by developing such advanced materials. Electrochemical energy storage (EES) involves the conversion of chemical energy into electrical energy through redox reactions. In these systems, energy is stored within the chemical bonds of electrode materials used in batteries and pseudocapacitors (Trasatti and Buzzanca, 1971) Effective energy storage methods depend on electrodes that offer excellent electrical and ionic conductivity, along with high power and energy densities. The following problems will be addressed by the material’s chemistry design for the EES: 1) Intercalation materials such as Nb₂O₅, TiO₂, and V₂O₅ enhance electrochemical performance by enabling fast and reversible ion insertion without compromising structural stability. 2) Redox mechanisms in 2D materials include Mxenes and metal-organic frameworks (MOFs), which offer high conductivity and rich surface chemistry for efficient charge storage. 3) Surface adsorption in pseudocapacitors involves functionalized 2D materials that provide abundant active sites for enhanced ion interaction and charge storage. Thus, the structure-property materials are helping to improve properties and electrochemical performance. When it comes to element selection, metal cations that can transmit multiple electrons per metal cation can be chosen to create high energy density electrode materials. The structure’s design and selection should provide high stability and high-rate capability; to accomplish this, the crystal structure must be compact and strong, and the chemical bonding must be strong enough to withstand the severe and demanding testing that occurs during electrochemical reactions (Gogotsi and Penner, 2018).

The faradic law, which follows surface redox and intercalation, is caused by the charge transformation reaction involved in the energy storage mechanism. Surface absorption, surface redox, and intercalation all involve the thermodynamic and kinetic behaviour of electrosorption/desorption, resulting in pseudocapacitance (Costentin et al., 2017; Dubal et al., 2015). Electrode surface interactions with reactive species typically occur in two modes: one involves repulsive interactions, leading to peak broadening in cyclic voltammetry, often associated with Frumkin-type electrosorption (characterized by a broad peak); the other entails attractive interactions that result in redox activity and follow Langmuir-type electrosorption, evident as sharp peaks in the voltammetric curve (Devaraj and Munichandraiah, 2008). Ruthenium dioxide (RuO₂) was the first compound recognized for exhibiting pseudocapacitive charge storage behavior (Liu and Anderson, 1996). In neutral electrolytes, hydrated manganese dioxide (MnO₂·H₂O) demonstrated capacitive characteristics (Kamila et al., 2019). Unlike electric double-layer capacitors (EDLCs), pseudocapacitors utilize redox reactions that allow access to multiple oxidation states, thereby achieving higher energy densities (Gao et al., 2010; Zhou J. Ye et al., 2020). Asymmetric supercapacitor cells can enhance energy density more effectively when the battery-type electrode facilitates faster electron transport, while the capacitive electrode stores charge electrostatically. This synergy leads to improved charge transfer kinetics, particularly under high current conditions (Wang et al., 2005). Extensive research is ongoing into transition metal oxide-based materials, such as nickel oxide (NiO), vanadium pentoxide (V₂O₅), cobalt oxide in spinel form (Co₃O₄), iron oxide (Fe₂O₃), and binary spinel compounds like NiCo₂O₄ for their potential in pseudocapacitor applications (Samantara et al., 2018; Huai et al., 2023). However, transition metal oxide-related structural instability and performance degradation concerns prompt research into innovative framework structures for improved structural stability and increased surface charge storage (Mishra et al., 2021; Zhang et al., 2015).

Metal-organic frameworks (MOFs) are formed by coordinating metal ions with organic ligands, resulting in fascinating two- or three-dimensional networks that exhibit permanent porosity (Saraf et al., 2016). These porous frameworks often display faradaic pseudocapacitive behavior, particularly when oxalate linkers are involved, as they can actively engage in redox reactions with metal centers (Han et al., 2018; Yang et al., 2014). However, many transition metal oxalate-based structures tend to retain water molecules within their framework due to their open architecture, which facilitates water absorption. To address this, we propose a strategy involving the controlled elimination of these structural water molecules. This approach aims to produce a tailored porous network capable of supporting enhanced charge storage by enabling efficient anion intercalation and deintercalation, alongside electric double-layer capacitance. Maintaining the intrinsic porosity during water removal is crucial, as it supports rapid ion diffusion and charge transport, ultimately leading to superior capacitive performance. Pseudocapacitors, a subclass of electrochemical capacitors, differ significantly from (EDLC) in their charge storage mechanism. In contrast to electric double-layer capacitors (EDLCs), which store energy via electrostatic charge buildup at the interface between the electrode and electrolyte, pseudocapacitors utilize rapid and reversible Faradaic redox reactions near or on the electrode surface for energy storage. These reactions typically involve electron exchange between the electrode and electrolyte, often coupled with ion adsorption or insertion into the electrode material. As a result, pseudocapacitors can deliver higher specific capacitance and energy density compared to EDLCs, while retaining relatively fast charge–discharge capabilities. The overall charge storage behavior in pseudocapacitors can be categorized into three main mechanisms: redox pseudocapacitance, intercalation pseudocapacitance, and surface adsorption (electrosorption).

In supercapacitors, pseudocapacitance performs excellent electrochemical charge storage mechanism in various metal oxides, 2D materials like Mxene, metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) to overcomes the electrostatic ion accumulation seen in EDLCs. Unlike electric EDLCs, which rely solely on electrostatic ion accumulation, pseudocapacitors overcome this limitation by enabling faradaic processes.

Pseudocapacitive behavior involves intercalation, redox reactions, and electrosorption, with rapid and reversible redox processes occurring at or near the electrode surface. The development of emerging pseudocapacitive materials can be broadly classified into three categories based on their charge storage mechanisms (as shown in Figure 3): 1) Intercalation-based pseudocapacitance, which combines the benefits of both capacitive and battery-like behaviour, enabling high-performance energy storage; 2) Redox-actice pseudocapacitance, wherein materials facilitate rapid and reversible redox reactions at or near their surfaces, contributing to efficient charge storage; 3) Electrosorption-driven pseudocapacitance, which has recently seen advancements through the use of functionalized carbon materials that enhance ion adsorption and surface interaction.

Pseudocapacitive batteries represent an approach to battery design that utilizes the fast, surface-based redox reactions of materials to enhance power density and charging rates, aiming to bridge the performance gap between supercapacitors and batteries. Ni-based metal oxides have shown exceptionally high pseudocapacitive properties, ranking among the most promising transition metal oxides for energy storage applications. Ni-based electrodes have garnered significant attention due to several favorable properties, including high theoretical capacitance, environmental compatibility, chemical stability, and abundant electroactive sites. From a practical standpoint, the suitability of nickel in large-scale energy storage systems is not solely determined by electrochemical performance it must also consider factors such as material abundance, cost, and supply chain resilience. Nickel is relatively abundant in the Earth’s crust compared to critical elements like cobalt or ruthenium and is already widely used in stainless steel, coinage, and battery industries. However, the recent surge in global demand driven by electric vehicle (EV) battery production and renewable energy systems has caused fluctuations in nickel prices and raised concerns about long-term availability and sustainability. Despite this, low-to-medium-grade nickel resources are widespread and economically viable, and ongoing efforts in nickel recycling and recovery technologies are helping to stabilize supply and reduce environmental impact.

Therefore, nickel remains a cost-effective and scalable option, especially for supercapacitor and pseudocapacitive battery applications where its loading per device is relatively low compared to high-energy lithium-ion batteries. Moreover, Ni-based materials, particularly in nanostructured or composite forms, offer an ideal balance of cost-performance, positioning them as competitive candidates for future energy storage technologies especially in grid-level systems and portable electronics. As a result, achieving high electrochemical values in the device requires a large electrode surface area. Various metal oxide electrode materials are deposited with nanostructured compounds to increase the electrode’s surface area, as shown in Figure 4. Furthermore, Ni-based compounds are becoming more popular because of their outstanding electrical performance and unique advantages over oxide/hydroxide-based materials. NiO-based core-shell structures have generally been successfully used in supercapacitors, showing major improvements in terms of increasing their specific capacity and cyclic stability. To achieve high pseudocapacitive performance in Ni-based materials, a primary goal is to synthesize nanostructured materials with a high specific surface area, enabling fast ion diffusion, efficient charge transfer, and high rate capability.

Furthermore, the vast majority of NiO particles generated in experiments show an ability to form. Investigators have used a number of methods to improve NiO’s electrochemical performance in reaction to the aforementioned problem. In materials synthesized process have various groups, Ni-based scanning electron microscopy exhibit unique surface morphology. The development of NiO nanosheets contributes to a more uniform distribution within the sample, effectively increasing the available surface area for electrochemical reactions. Among various transition metal oxides, NiCo₂O₄ stands out over NiO and Co₃O₄ due to its broader range of redox-active valence states and significantly higher electrical conductivity, making it a highly promising candidate for advanced supercapacitor electrodes (Kim et al., 2015). Structurally, NiCo₂O₄ exhibits a standard spinel configuration, where divalent nickel ions occupy tetrahedral positions, while cobalt ions existing in both +2 and +3 oxidation states are distributed across both tetrahedral and octahedral sites. This mixed-valence arrangement enhances electron transport efficiency, enabling single-crystal NiCo₂O₄ nanoplates to achieve electrical conductivities as high as 62 S/cm at room temperature (300 K), attributed to the low activation energy for charge movement facilitated by the coexistence of multiple oxidation states. At a current density of 0.05 A g⁻¹, it provided a specific capacitance of 1125 F/g (Zhang et al., 2021; Zhu et al., 2011). Ni-based metal oxides achieving high surface area, due to their inherent advantages of electrochemical properties. Due to low cost, eco-friendliness and high theoretical capacitance further performance their capability in energy storage systems.

Pseudocapacitive materials represent a promising class of advanced electrode materials for supercapacitors (SCs), utilizing mechanisms such as ion intercalation, surface redox reactions, and adsorption-based charge storage. Over the past decade, significant progress has been made in engineering high-performance pseudocapacitive electrodes, despite persistent challenges like material degradation, poor conductivity, and active layer detachment during prolonged cycling. Nickel-based compounds, with their multiple oxidation states and high theoretical capacitance, have emerged as cost-effective and high-performing candidates. Likewise, newer materials like Mxenes and MOFs have enhanced interfacial contact and charge transfer through structural engineering. Traditional materials such as transition metal oxides (e.g., RuO₂, MnO₂) and conductive polymers (e.g., polyaniline, polypyrrole) continue to serve as key platforms for understanding and improving pseudocapacitive behavior. Looking ahead, pseudocapacitive materials are expected to play pivotal roles in a range of practical energy storage scenarios, including: Fast-charging power tools and electric vehicles, where high-rate capability is essential, Grid-level energy buffering for renewable energy integration, Wearable and flexible electronics, which require lightweight, high-capacity storage, Low-temperature or harsh-environment electronics, due to the robust charge/discharge behavior of pseudocapacitors.

(1) Material Synthesis and Fabrication. Future advancements will hinge on designing hierarchical nanostructures and binder-free electrodes that optimize electroactive surface area, conductivity, and mechanical stability. Tailoring the chemical composition and surface functionality of pseudocapacitive materials will be crucial for improving their scalability and real-world applicability.

In conclusion, continued interdisciplinary research combining materials science, electrochemistry, computational modeling, and AI will be key to unlocking the full potential of pseudocapacitive materials. Their unique balance of power and energy performance makes them strong contenders in a future dominated by rapid, efficient, and sustainable energy storage solutions.