1 Introduction

Energy storage has emerged as a significant research area in recent years. The growing demand for fossil fuels exacerbates the depletion of nonrenewable resources and causes environmental pollution and increasingly frequent natural disasters, imposing severe impacts on human health and ecosystems [1, 2]. Therefore, there is an urgent demand for clean and sustainable energy supply technologies, which have continuously stimulated research and investment in many cutting-edge technologies in related fields worldwide [3,4,5,6]. Supercapacitors (SCs) offer notable energy storage benefits compared to traditional capacitors. However, they exhibit a maximum energy density that is markedly lower than that of rechargeable secondary batteries [7]. SCs can achieve rapid charge–discharge within seconds to minutes, it takes minutes to several hours for secondary batteries to complete the charge–discharge process [8]. In addition, SCs have advantages such as long cycle stability, high-power density, high safety factor, high energy conversion efficiency, good ultralow-temperature characteristics, convenient detection and environmentally friendly production [9,10,11].

Generally, SCs were only classified according to the nature of material: carbon-based and metal-based materials. These materials are well-categorized accordingly into their storage mechanism: electrochemical double layer capacitors (EDLCs) and pseudocapacitors (PCs), respectively. EDLCs were first introduced by Helmholtz et al. based on the electric double layer theory, which indicated that energy storage in EDLCs was facilitated by the creation of a double electric layer at the interface between the electrolyte and various electrode materials, including activated carbon, carbon fibers and graphene [12]. During charging, the positive and negative charges on the electrode, under the influence of an electric field, draw anions and cations present in the electrolyte, respectively. After reaching the surfaces of the positive and negative electrodes, the electrolyte ions are transferred onto the electrode material surface via Coulomb, Van der Waals and other forces. At the same time, opposite charges appear on the electrode and a stable double layer exists at the interface between the electrode material and the electrolyte, completing electric energy storage. During the operation of EDLCs, anions and cations in the electrolyte undergo only electromigration and diffusion, which are physical phenomena that do not occur during the electrochemical reaction with the electrode. PCs mainly utilize the fast reversible redox reactions of metal oxides to achieve charge storage and release. The theory of pseudocapacitance was first proposed by Conway [13] based on the concept of underpotential deposition involving electroactive substances. This process occurs either on the electrode surface or within the two-dimensional (2D) or quasi-2D spaces of the bulk phase. Subsequently, the capacitance associated with the charging potential of the electrode arises from highly reversible chemical processes, specifically, adsorption/desorption or redox reactions. The charge–discharge reaction process is as follows: MOx-1 + H+ − e− ↔ MOx-1(OH) (acidic conditions); MOx + OH − e− ↔ MOx(OH) (alkaline conditions) [14, 15]. Generally, energy storage methods for PCs include double layer energy storage and charge storage, in which the active materials in the electrode react chemically. PCs exhibit greater capacitance and energy density than EDLCs due to the occurrence of redox reactions within the material, not limited to their surfaces.

However, with the rapid development of nanoscience and nanotechnology in recent years, nanomaterial-based electrodes are playing increasingly important roles in electrochemical energy storage. Nanomaterials are small with large surface areas, meaning that their “surface” and “bulk” cannot be clearly distinguished. Accordingly, some Faradaic electrode materials that typically show strong redox reactions in bulk exhibit behaviors such as pseudocapacitance when reduced in size to the nanoscale, characteristic of the disappearance of the redox peaks in cyclic voltammetry (CV) and plateaus in galvanostatic charge–discharge (GCD) curves. An increasing number of studies have demonstrated that crystal phase, surface or interfacial effects may introduce pseudocapacitive contribution in the charge storage process of battery electrodes. Consequently, the boundary between battery and pseudocapacitive materials has blurred in recent years. Terms such as “intercalation pseudocapacitance” and “extrinsic pseudocapacitance” as well as kinetic analysis toward CV have been proposed to better understand the charge storage mechanisms of emerging electrode materials. Indeed, these definitions and methods help distinguish between batteries and pseudocapacitive materials; however, many researchers misunderstand or misuse them, reducing their utility in the electrochemistry community. To accurately distinguish between pseudocapacitive and battery materials, a method was proposed that considered both the electrochemical signatures (CV and GCD) and quantitative kinetics analysis as a supplement (calculation of b in formula i(V) = avb). EDLC materials have a rectangular CV curve, a potential of GCD linear with respect to time, and a value of b always equal to 1. In contrast, pseudocapacitive materials have an approximately rectangular CV and a GCD curve with an almost linear relationship between potential and time. In addition, compared with EDLC, there should be some inflection points in the GCD curve but no obvious plateaus. The calculated b-value is close to 1. For battery-type materials, the typical CV has obvious redox peaks and the GCD shows detectable plateaus (regardless of whether the peaks/plateaus disappear at high rates). The b value should be 0.5, which is generally obtained in traditional bulk battery electrode materials; however, for nanomaterial battery electrodes or those with specific electrode engineering and structural design, the b value may be > 0.5, provided that the redox process is no longer limited by ion diffusion. Researchers have demonstrated differences among symmetric, asymmetric and hybrid devices based on the three electrode materials (EDLC, pseudocapacitive and battery). Symmetric devices use identical materials as the positive and negative electrodes, such as EDLC-type AC//EDLC-type AC, battery-type Ni(OH)2//battery-type Ni(OH)2 or pseudocapacitive-type MnO2//pseudocapacitive-type MnO2. Asymmetric devices include EDLC and/or pseudocapacitive electrodes of two different materials. The configuration of asymmetric devices can be classified as pseudocapacitive-type Nb2O5//EDCL-type AC, pseudocapacitive-type Nb2O5//pseudocapacitive-type MnO2 or EDLC-type AC//EDCL-type carbon nanotube (CNT). Hybrid devices combine the advantages of EDLC, batteries and pseudocapacitive materials. The hybrid device configuration can be battery-type Ni(OH)2//EDCL-type AC or battery-type Ni(OH)2//pseudocapacitive-type Nb2O5. Combining these advantages improves the specific capacitance cycle stability, and energy density of the material and broadens the working voltage range. The large capacity, high-power charge and discharge of SCs are derived from these two principles, according to which high-performance SCs can be prepared.

Among the components of a SC, the electrode material is of paramount significance for effective charge storage. The electrode material must exhibit good chemical stability. Otherwise, the performance of the SCs is diminished. At the same time, ions and electrons are required to have good electrical conductivity. Only when the electrode materials meet the above requirements, SCs can exhibit good performance and better application [16,17,18,19]. Therefore, the advancement of electrode materials with superior performance has emerged as a crucial factor in enhancing the efficiency of SCs and represents the most direct avenue to achieve their practical implementation. Currently, the electrode materials of SCs predominantly involve carbon-based materials, conductive polymers, metal compounds and MXenes, as proposed by Gogotsi et al. [20,21,22,23,24]. In the context of SCs, metal compounds serve as electrode materials, offering higher energy density than carbon materials, and exhibit superior electrochemical stability compared to conductive polymers [25]. Metal compounds can be divided into single- and multi-metallic compounds based on the number of metal atoms. Single-metal compounds have been studied as electrode materials, including Mn-based single-metal compounds [26], Co-based single-metal compounds [27] and Ni-based monometallic compounds [28]. To compensate for the shortcomings of various single-metal compounds and fully exploit the advantages of various metal-atom electrochemical performances, researchers have found that metal ternary materials exhibit better conductivities and cycle performances than binary materials. To date, multimetal compounds have been applied as active electrode materials for energy storage devices and have achieved excellent electrochemical performance [29,30,31,32]. Currently, multimetal oxides and hydroxides [33, 34] are the main metal compounds used as electrode materials for SCs, and a small number of multimetal sulfides [35] and phosphides [36, 37] have been reported (Fig. 1).

Fig. 1
figure 1

Schematic diagram of transition metal oxide/hydroxide, sulfide, phosphide and selenide

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In this paper, the research status and existing issues of transition metal compound electrode materials for SCs are comprehensively reviewed. Moreover, we focus on the synthetic methods and regulation strategies of transition metal compound electrode materials to address these issues. Finally, we discuss the future development directions of transition metal compound electrode materials for SCs.

2 Research status and existing problems

To attain remarkable performance in SCs, extensive research has been conducted on a variety of metal compounds, including transition metal oxides/hydroxides, transition metal sulfides, transition metal phosphides and transition metal selenides [38,39,40]. However, there are still great challenges in ensuring their stable and efficient use in SCs to their disadvantages.

2.1 Transition metal oxide/hydroxide

Transition metal oxides/hydroxides have the advantages of high specific capacitance, high conductivity, vast natural abundance, rich valence states and easy design and manufacture [41]. To date, extensive research efforts have been dedicated to investigating a wide range of metal oxides/hydroxides, including RuO2 [42], MnO2 [26], NiO [43], SnO2 [44], Co3O4 [45], Ni(OH)2 [46] and Co(OH)2 [47]. In particular, RuO2 is one of the most classical pseudocapacitance (PC) materials due to its exceptional theoretical specific capacitance (1450 F·g−1), excellent electrochemical reversibility, high proton conductivity and high rate performance. In addition, RuO2 has a wide potential window and abundant valence states [48, 49]. When the RuO2 electrode is charged in an alkaline solution, three oxidation states (RuO42−, RuO4− and RuO4) are formed. These high-valence intermediates are completely reduced to the initial valence state during the discharge process, thus facilitating Faraday charge transfer. Jow et al. believed that the energy storage process of RuO2 mainly involved the following reactions [49]:

$$ {\text{RuO}}_{{2}} +_{{}} x{\text{H}}^{ + } + x{\text{e}}^{ - } \leftrightarrow {\text{RuO}}_{{{2} - x}} \left( {{\text{OH}}} \right)_{x} (0 \le x \le {2}) $$
(1)

Kim et al. successfully synthesized carbon nanofibres with RuO2 electrode materials loaded on the surface using precipitation and recrystallization (Fig. 2a, b), which exhibited a wide voltage range of − 0.2–0.8 V in 6 mol·L−1 KOH electrolyte (Fig. 2c) and specific capacitance of 188 F·g−1 at 1 mA·cm−2 (Fig. 2d) [50]. Although RuO2 has excellent electrochemical properties, its poor long-term cycle performance and high manufacturing costs hinder its application.

Fig. 2
figure 2

Reproduced with permission from Ref. [50]. Copyright 2020, ACS Publications. e, f SEM images, g GCD curves and h cycle retention of ultrafine MnO2 nanowires@CC electrode. Reproduced with permission from Ref. [53]. Copyright 2018, Elsevier. i TEM and j HRTEM images and k rate performance curves of CoO nanomaterials. Reproduced with permission from Ref. [55]. Copyright 2019, Elsevier

Characterization and electrochemical performance of RuO2–CNF nanorods and ultrafine MnO2 nanowires@CC electrode. a SEM, b TEM, c CV curves and d rate performance of RuO2–CNF nanorods.

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MnO2 is another classic PC material after RuO2 and is a base metal oxide with good electrochemical performance in neutral environments (Na2SO4 and K2SO4). Notably, MnO2 exhibits high theoretical capacitance (1390 F·g−1), cost-effectiveness, minimal toxicity and environmentally friendly attributes. Researchers have extensively investigated its properties and harnessed its potential for various applications [51]. The MnO2 electrode has good pseudocapacitance characteristics owing to the changes in various Mn valence states (Mn2+/Mn3+/Mn4+Mn5+) during charging and discharging. Cheng et al. [52] suggested that MnO2 may be involved in the following reversible energy storage processes:

$$ {\text{MnO}}_{{2}} + {\text{H}}^{ + } + {\text{e}}^{ - } \leftrightarrow {\text{MnOOH}} $$
(2)
$$ {\text{MnO}}_{{2}} + {\text{M}}^{ + } + {\text{e}}^{ - } \leftrightarrow {\text{MnOOH}} $$
(3)
$$ {\text{2MnOOH}} \leftrightarrow {\text{Mn}}^{{{2} + }} + {\text{ MnO}}_{{2}} + {\text{ 2OH}} - $$
(4)

where M is an alkali metal ion (Na+ or K+). For example, Xu et al. [53] fabricated an ultrafine electrode composed of MnO2 nanowires grown on carbon cloth (CC) without binders using a simple hydrothermal technique. The ultrafine MnO2 nanowires developed on the CC exhibited a weed-like morphology, forming a complex and interconnected network (Fig. 2e, f). The distinctive pore structure facilitates the formation of a substantial surface area at the interfaces, allowing the rapid diffusion of electrolytes and efficient electron transfer. The material displayed a remarkable specific capacitance (1174.3 F·g−1 at 2 A·g−1), exceptional rate performance and impressive cycling stability (Fig. 2g, h). Nevertheless, their application in SCs faces significant limitations owing to their inadequate conductivity and sluggish ion transfer rates [26].

In addition, CoO and Co3O4, well-regarded metal oxides, have become focal points in SC research due to their ability to adapt to various structural configurations, impressive theoretical specific capacitance (3560 F·g−1), and chemical stability [54]. Cheng et al. suggested that CoO may be involved in the following reversible reactions in the energy storage process [27]:

$$ {\text{CoO }} + {\text{ OH}}^{ - } \leftrightarrow {\text{CoOOH }} + {\text{ e}}^{ - } $$
(5)
$$ {\text{CoOOH }} + {\text{ OH}}^{ - } \leftrightarrow {\text{CoO}}_{{2}} + {\text{ H}}_{{2}} {\text{O }} + {\text{ e}}^{ - } $$
(6)
$$ {\text{CoOOH}} + {\text{H}}_{{2}} {\text{O}} + {\text{e}}^{ - } \leftrightarrow {\text{Co}}\left( {{\text{OH}}} \right)_{{2}} + {\text{OH}}^{ - } $$
(7)

For example, Xiao et al. [55] successfully synthesized one-dimensional (1D) CoO nanobelts and 2D CoO nanoplates with porous structures. These materials were obtained by the calcination of single-crystal cobalt hydroxide nanobelts and nanoplates using a solvothermal approach (Fig. 2i, j). The porous CoO nanobelts exhibit a high specific capacitance of 1178 F·g−1 at 1 A·g−1. Although Co3O4 material has an excellent theoretical specific capacitance, the actual electrode test results are often less than 1,000 F·g −1 (Fig. 2k). This is due to that the inherent problems of low conductivity, small specific surface area, slow dynamic process and volume expansion/contraction of the powdered Co3O4 material in electrochemical tests seriously limit its specific capacitance and cycle stability [56].

Ni(OH)2-based electrodes have advantages such as outstanding theoretical capacity, strong chemical stability, prominent rate performance, low cost, definite redox transitions and environmental friendliness. In recent years, the synthesis of such electrode materials has increased sharply. Typically, Ni(OH)2 exhibits a hexagonal-layered structure and can exist in two different pseudopolymorphs referred to as the α and β phases. The distinct phases were first described by Bode et al. [57] in 1966. The hydrotalcite-like structured α-Ni(OH)2 is composed of multiple layers of positively charged Ni(OH)2. These layers are enriched with anions such as carbonate, nitrate, chloride, sulfonate and water molecules, resulting in the expansion of the spacing between the layers to 0.72 nm. This arrangement contributes to its superior electrochemical activity compared to β-Ni(OH)2. Additionally, the oxidation state of Ni in α-Ni(OH)2 is + 4, its average oxidation state is 3.3–3.7, which exhibits excellent electrochemical properties compared with β-Ni(OH)2. Lokhande et al. [58] prepared ultrathin α-Ni(OH)2 nanosheets by adding nonionic surfactant Triton X-100 through a simple chemical deposition method. A three-dimensional (3D) mesoporous Ni foam was used as the substrate. The thin nanosheets are skillfully interconnected, creating a unique flower-like structure with pores. This intricate structure played a crucial role in enabling the electrolytes to easily enter the active substance and diminish the diffusion resistance (Fig. 3a, b), resulting in a high specific capacitance of 539 F·g−1 at 10 mV·s−1 (Fig. 3c, d). It is important to highlight that the metastable phase of α-Ni(OH)2 can rapidly transition into β-Ni(OH)2 during synthesis and charge–discharge at low discharge current densities. This transformation significantly affects the capacity of SCs, leading to a considerable loss in capacity and a decline in overall cycle performance.

Fig. 3
figure 3

Reproduced with permission from Ref. [58]. Copyright 2018, Elsevier. Characterization and electrochemical performance of electrode materials. e SEM image, f GCD curves and g cycling stability of NiMoO4@C nanowires. Reproduced with permission from Ref. [66]. Copyright 2017, ACS Publications. h SEM and i TEM images of NiCo–LDHs; j GCD and k rate performance curves of NiCo–LDHs. Reproduced with permission from Ref. [63]. Copyright 2023, Elsevier

a, b SEM images, c CV and d rate performance curves of α-Ni(OH)2 nanosheets deposited on Ni foam.

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Binary transition metal oxide/hydroxide electrode materials, such as NiCo2O4 [59], MnCo2O4 [60], ZnFe2O4 [61], MnMoO4 [62] and NiCo-hydroxides [63], have attracted research attention. The two metal components of binary transition metal oxide/hydroxide electrode materials undergo redox reactions and contribute to their specific capacitance; thus, binary materials have a higher specific capacitance than single-metal oxides. More importantly, the conductivity of binary metal oxide electrode materials is several times or even dozens of times those of single-metal oxides, thus improving their electronic transmission and cycling performance compared to single-metal materials [64, 65]. Wang et al. [66] prepared a freestanding carbon-coated NiMoO4 nanowire electrode on Ni foam for use in SCs (Fig. 3e). The carbon-coated NiMoO4 nanowire electrode showed a high areal capacitance and remarkable stability during cycling (Fig. 3f, g)). Wang et al. [63] introduced a novel hollow NiCo–LDH electrode through straightforward morphological manipulation (Fig. 3h, i). After optimization, the NiCo–LDH/M–FC electrode demonstrated exceptional specific capacitance and impressive rate performance (Fig. 3j, k). However, its disadvantages are evident in practical applications, such as a short cycle life and a large gap between the actual capacitance and theoretical capacitance.

2.2 Transition metal sulfide

Transition metal sulfides have been widely investigated as a new type of PC material due to their good conductivity, low cost, low electronegativity, good semiconductivity and good redox activity. As the electronegativity of sulfur atoms is weaker than that of oxygen atoms, transition metal sulfides exhibit higher conductivity, thermal stability, mechanical stability and electrochemical performance than their corresponding metal oxides under the same conditions [67, 68]. Nickel sulfide has been explored as an electrode material for SCs due to its abundant phases (e.g., NiS, NiS2 and Ni3S4) and morphological characteristics [69]. Wu et al. [70] introduced an innovative "inner-outer synergistic strategy" for the development of advanced electrode materials. This strategy involves the integration of NiS nanoflakes within electrospun carbon fibers that incorporate NiS nanoparticles (NPs), resulting in a composite material referred to as NiSNF/CF@NiSNP (Fig. 4a–c). Due to its ingenious structure, the NiSNF/CF@NiSNP−3 electrode yielded impressive results, with a remarkable specific capacitance of 1691.1 F·g−1 at 1 A·g−1, highlighting its exceptional ability to store electrical charge efficiently. Furthermore, it demonstrated a remarkable Coulombic efficiency of 98.5% (Fig. 4d, e). In addition, the cobalt sulfide electrode displayed excellent electrochemical performance in SCs. Peng et al. synthesized CoS2 hollow nanospheres with adjustable internal structures using a one-step hydrothermal method [47]. Through the precise manipulation of the quantity of carbon disulfide, various types of CoS2 (including solid, core–shell and double-shell) can be obtained. The results from the electrochemical tests unveiled the specific capacitance of the mesoporous CoS2 hollow nanosphere (2–5 nm), which reached an impressive 1301 F·g−1 at 1 A·g−1. Remarkably, even after 2,000 cycles, the capacity retention remained as high as 90.1%.

Fig. 4
figure 4

Reproduced with permission from Ref. [70]. Copyright 2022, Elsevier. f SEM and EDS mapping images of NCS4–0.5/NCC; g specific capacitance and h cycling stability of NCS/CC, NCS/NCC and NCS4–0.5/NCC electrodes. Reproduced with permission from Ref. [71]. Copyright 2016, Wiley–VCH

Characterization and electrochemical evaluation of innovative electrode materials. a–c TEM images, d GCD curves and e cycle life curves of NiSNF/CF@NiSNP−3.

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Compared to individual metal sulfides, binary mixed-metal sulfides exhibit superior conductivity and provide a greater number of redox reaction sites. The synergistic effect between different elements has a significant influence on the electrochemical performance. Fully utilizing the advantages of the two metal elements can improve the electrochemical performance of electrode materials. For example, the presence of Ni increases the cell parameters c and a while reducing c/a, which helps improve the capacity. However, excessive Ni content can lead to the deterioration of cycling and rate performance, and the high pH value of high-nickel materials affects their actual use. Co can effectively stabilize the structure of electrode materials, suppress cation mixing, improve the electronic conductivity of materials, and improve the cycling performance. Therefore, utilizing the synergistic effect of Ni and Co will maximize the advantages of electrode materials for SCs, ensuring high-energy density, high magnification performance and long cycle life [71, 72]. Liu et al. [73] presented a dual-regulation strategy for developing high-performance electrode materials. This approach involved nitrogen doping of the CC (NCC) current collector and the introduction of sulfur vacancies in NiCo2S4 (NCS) (Fig. 4f). The NCS4-0.5/NCC electrode benefited from the combined effect of NCC and sulfur vacancies, resulting in a remarkable specific capacitance of 4075.89 mF·cm−2 at 1 mA·cm−2 and exceptional cycling stability (Fig. 4g, h).

Transition metal sulfides are commonly found as nanoflowers, nanospheres, nanoflakes and nanorods. During frequent charge and discharge processes, NPs agglomerate easily, resulting in reduced capacity. In addition, fluctuation, expansion, thermodynamic instability, acid oxidation and volatility of metal sulfides are common problems in practical market applications. Therefore, efficient electrode structures must be developed.

2.3 Transition metal phosphide

Transition metal phosphides are n-type semiconductor materials with metal-like properties, rich redox reaction sites and excellent conductivities [74, 75]. Owing to the large radius of the phosphorus atoms, phosphides exhibit various crystal structures [76]. In addition, the low electronegativity of phosphorus ensures fast charge transfer [77]. Compared to transition metal compounds, transition metal phosphides are conducive to fast electron transport in SCs with a high-power density [78]. Transition metal phosphides include Co, Ni and Mo. Other transition metals, such as W, Mn and Cu, have been used to synthesize a variety of phosphides and have received increasing attention [37]. Typical single-transition metal phosphides include Co2P, Ni2P, Ni5P4 and Cu3P. Zhou et al. [79] uniformly grew interconnected Ni2P nanosheets on a Ni-foam surface, providing sufficient space for ion diffusion (Fig. 5a). The Ni2P electrode achieved a specific capacitance of 3496 F·g−1 at 2.5 A·g−1 and retained a capacitance of 1109 F·g−1 at 83.3 A·g−1 (Fig. 5b). When cycled at a high current density of 10 A·g−1, the Ni2P electrode exhibited rapid capacitance fading before 2,000 cycles, which may be caused by the reaction of the electrolyte and Ni2P nanosheets to generate Ni(OH)2. However, after 2,000 cycles, Ni(OH)2 preserved the entire structure of Ni2P and guaranteed good cycling stability (Fig. 5c).

Fig. 5
figure 5

Reproduced with permission from Ref. [79]. Copyright 2015, Wiley–VCH. d–g TEM and EDS mapping images for Ni8–Co1–P materials; h XRD patterns for Ni8–Co1–P, Ni–P and Co–P electrode; i GCD curves of Ni8–Co1–P electrode; j rate performance of Ni–P, Co–P and Ni–Co–P (Ni/Co ratio = 16:1, 8:1, 4:1, 2:1) electrodes. Reproduced with permission from Ref. [80]. Copyright 2017, Elsevier

a SEM image, b GCD curves and c cycling stability of Ni2P.

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Although most single-transition metal phosphides have good electrochemical capacity properties, their poor structural stability leads to lower rate performance and poor cycle stability, which is challenging for practical applications. Similar to transition metal oxides and sulfides, bimetallic phosphide materials play an important role. Binary metal phosphides usually have faster electronic conductivity, higher electrochemical reactivity and more abundant redox reaction sites than single transition metal phosphides, which are attributed to the synergy of the two transition metal elements. The two metal cations in the binary metal phosphide participate in the redox reaction, which has the advantage of producing a variety of oxidation states that will help store the charge and provide a higher capacity than a single-metal phosphide. Moreover, one metal cation in some binary metal phosphides participates in the redox reaction to contribute to the capacity, whereas another metal cation unrelated to the redox reaction facilitates electrochemical activity and enhances structural stability during the cycle, which considerably fosters the electrochemical performance of the entire material. Ding et al. [80] synthesized micro/nanostructured ternary Ni–Co–P materials for SCs using a one-pot solvothermal approach (Fig. 5d–g). Analysis via X-ray diffraction (XRD) revealed that the Ni8–Co1–P materials displayed distinct phases, including M12P5 and M2P (M = Ni, Co) (Fig. 5h). Ni8–Co1–P outperformed the Ni–Co–P electrode in terms of the specific capacitance, rate capability and charge transfer kinetics. This superior performance can be attributed to the abundance of electroactive sites from Ni and Co on the electrode surface as well as the synergistic interaction between the Ni and Co redox species (Fig. 5i, j). Although transition metal phosphides have the above advantages, the phosphorus skeleton in a transition metal phosphide has high flexibility and various bonding modes, and there are often many phase components in its preparation process, making it challenging to synthesize pure phase transition metal phosphides.

2.4 Transition metal selenide

Selenium, oxygen and sulfur are elements found in group VI-A of the periodic table. They shared the same number of valence electrons. Therefore, transition metal selenides have similar chemical properties to transition metal oxides and sulfides, suggesting that they have broad application prospects in SCs [81]. Moreover, selenium has better conductivity (1 × 10–3 S·m−1) than O (1 × 10–5 S·m−1) and S (1 × 10–28 S·m−1) due to that e it has a larger radius and a smaller ionization energy, which ensure fast charge transfer. Transition metal selenides usually exhibit a 2D layered sandwich structure similar to that of graphite, with metal atoms located between the two selenide layers. This layered structure allows the simple reversible intercalation of other atoms in the interchain space so that energy can be easily stored without structural deformation, indicating its high rate performance and enhanced cycling stability. Recently, various single, binary, multinary and composite/hybrid/integrated metal selenides have been explored for SCs [82,83,84,85,86,87,88,89,90]. Among the various types of selenides, Ni3Se2 exhibits excellent electrical conductivity, a stable phase and robust physicochemical properties. These characteristics are closely linked to the composition, phase structure and morphology, as dictated by the phase diagram. Notably, in Ni3Se2, three Ni atoms are involved in a three-electron reaction, leading to an increased capacity for charge storage. Therefore, Ni3Se2 is a promising candidate for an advanced electrode material [91]. Zhao et al. [92] synthesized a nanodendrite array of Ni3Se2 on nickel foam without a binder using a one-step hydrothermal method as a cathode material for SCs (Fig. 6a, b). The nanodendrite array Ni3Se2 on nickel foam exhibits a specific capacitance of 1234 F·g−1 at 1 A·g−1 (Fig. 6c, d). In comparison to single-metal selenides, binary- and multinary-metal selenides have more Faraday reaction sites and abundant structural defects owing to the coexistence and synergistic action of multiple metal ions and show better electrochemical performance. Qu et al. [93] synthesized a nano-polyhedral Ni–Co–Se electrode material with a hollow structure (Fig. 6e), which exhibited 1668 F·g−1 specific capacitance at 1 A·g−1 and high rate performance (Fig. 6f). Furthermore, Hosseini et al. [88] constructed a self-supporting porous Zn–Ni–Co/Cu selenide microsphere array on CC. The Zn–Ni–Co/Cu microball selenide exhibited a remarkable specific capacitance of 884.6 C·g−1 at 1 A·g−1 and an exceptional rate performance (Fig. 6g, h). However, the shortcoming of the volume effect (volume expansion or collapse) of transition metal selenides during the charging and discharging processes still needs to be overcome to improve their electrochemical performance and structural stability.

Fig. 6
figure 6

Reproduced with permission from Ref. [92]. Copyright 2020, Elsevier. e TEM image and f specific capacitances of Ni–Co–Se electrode. Reproduced with permission from Ref. [93]. Copyright 2020, Elsevier. g SEM image and h specific capacitances of Zn–Ni–Co/Cu microballs selenide. Reproduced with permission from Ref. [88]. Copyright 2019, Elsevier

Electrode characterization and specific capacitance analysis. a HRTEM and b mapping images, c GCD curves and d corresponding specific capacitance of Ni3Se2 electrode.

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The data based on transition metal compounds as electrode materials for SC are shown in Table 1 [50, 53, 55, 58, 63, 66, 70, 79, and 80].

Table 1 Comparative data of transition metal compounds as electrode materials
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3 Synthetic methods and regulation strategies

3.1 Synthetic methods

The microstructure and properties of SC electrode materials are significantly affected by the thoughtful design, comprehensive research and precise control of the preparation process. Transition metal compounds can be synthesized using a range of preparation techniques, including hydrothermal/solvothermal, precipitation, electrodeposition, chemical vapor deposition (CVD) and sol–gel methods.

3.1.1 Hydrothermal/solvothermal method

The hydrothermal method involves synthesizing the precursor by dissolving and recrystallizing it in a closed-pressure reactor using water as the solvent under high-temperature and high-pressure conditions. The solvothermal method is a new material preparation method developed from the hydrothermal method that uses organic, amine or mixed solvents. The hydrothermal/solvothermal method involves transferring a prepared uniformly mixed solution to an autoclave, which is then placed in an oven to set the heating temperature and time. The microstructure of the material changed with changes in the hydrothermal time and temperature during the experiment. Zhang et al. [94] successfully used a hydrothermal method to synthesize 3D SnO2 nanosheets on a CC substrate. As the hydrothermal process progressed, the micromorphology of the material gradually transitioned from NPs to a nanosheet structure. Hydrothermal synthesis is used to produce nanomaterials of superior purity characterized by well-developed crystal structures. This approach effectively eliminates impurities and prevents structural defects that commonly arise during post-processing techniques such as high-temperature calcination or ball milling. However, this approach has limitations, such as the inability to directly observe the growth of crystals and synthesis of materials. In addition, this approach requires high-temperature and high-pressure equipment, corrosion-resistant linings, meticulous temperature control, substantial costs and safety concerns.

3.1.2 Precipitation method

The precipitation method precipitates insoluble compounds from water-soluble substances through a precipitation reaction, followed by subsequent treatment of the sediment, including direct precipitation, coprecipitation and uniform precipitation. The coprecipitation method involves homogeneously dissolving several kinds of cations in a solution and adding an appropriate amount of precipitator to induce the reaction and produce precipitation products with uniform composition. This is an effective method for synthesizing ultrafine powders containing several metal elements of composite oxides and hydroxides. This method has the advantages of mild reaction conditions, high product purity, easy industrial production, and easy nucleation control during the reaction process, and low cost. Xu et al. [95] synthesized Ni0.75Co0.15Al0.1(OH)2 using the coprecipitation method. The resulting Ni0.75Co0.15Al0.1(OH)2 electrode exhibited exceptional performance, even under an ultrahigh mass loading of 20.24 mg·cm−2. Furthermore, it displayed an excellent specific capacitance of 23.85 F·cm−2 at 10 mA·cm−2, along with a high rate capability (19.27 F·cm−2 at 40 mA·cm−2), indicating remarkable suitability for demanding applications. The electrode also exhibited outstanding cycling stability, highlighting its potential for long-term use. The precipitation method offers several benefits, including a straightforward preparation process, cost-effectiveness, ease of control over the synthetic conditions and a relatively short synthesis cycle. However, the addition of precipitation agents can potentially lead to excessively high local concentrations, which, in turn, may cause issues such as material agglomeration and uneven compositions during material synthesis.

3.1.3 Electrodeposition method

Electrodeposition involves a redox process in which positive and negative ions in an electrolyte solution migrate under the influence of an external electric field, leading to the formation of a coating on the electrode. The preparation of transition metal compounds by this method mainly involves introducing the desired metal cations and anions, preparing a certain proportion of electrolytes in the electrolytic cell, and connecting the electrodes on a chemical workstation to carry out the electrodeposition process according to the different deposition processes. Simultaneously, nanomaterials with different properties can be prepared by changing the experimental conditions. Xu et al. [96] implemented an electrodeposition technique to grow Ni–Co–S nanosheets on an electrode composed of C–NiCo2O4/C nanowires. The resulting electrode demonstrated exceptional performance, including a high specific capacitance of 2396 F·g−1 at 3 A·g−1, impressive rate performance, and remarkable cycling stability of 92%. Electrodeposition can be used to obtain nanomaterials with various grain sizes, controllable material thicknesses, low cost and high efficiency. However, it is difficult to coat large workpiece areas.

3.1.4 CVD method

CVD technique involves the formation of solid deposits through the reaction of gaseous or vapor substances, either in the gas phase or at the gas–solid interface. This method mainly involves mixing two types of reactants (precursor and sulfur source, phosphorus source or selenium source) in a certain proportion in a tube furnace filled with inert gas or placing the sulfur source (phosphorus source or selenium source) upstream of the gas flow in the tube furnace and placing the precursor downstream to prepare transition metal sulfides (phosphide or selenide) at a constant temperature. Cheng et al. [97] phosphorylated CoNi2S4 into CoNi2S4/Ni3P electrode material by CVD method using NaH2PO2 as phosphorus source, which showed a high area-specific capacitance of 8.86 F·cm−2 at 3 A·g−1. CVD method does not have a deposition line of sight shadow and can deposit coatings on complex 3D workpieces at high deposition speeds. Thick coatings (sometimes up to a centimeter thick) can be obtained and the deposited coatings have good adhesion to the substrate. In addition, it has a good vacuum sealing performance and can be coated with high-melting-point materials at relatively low temperatures. It also has the advantage of controlling grain size and microstructure. However, this method also has a series of drawbacks, such as the need for volatile compounds in the reaction, which are unsuitable for commonly electroplated metals owing to the lack of suitable reactants such as tin, zinc and gold. Due to the release of highly toxic substances, corrosive exhaust gases and deposition reactions must be properly controlled and a closed system is required. The reactant utilization rate is low and the reaction is often limited by the equilibrium constant of the deposition reaction.

3.1.5 Sol–gel method

The sol–gel method employs precursors containing potent chemical compounds to initiate the process. This method uniformly mixes the active ingredients in a liquid phase and allows for chemical reactions, including hydrolysis and condensation. Consequently, a stable and transparent sol system is formed in the solution. This method offers several benefits, including simplicity of operation, affordability, the production of high-purity products, excellent uniformity and the ability to create abundant pore structures. Wei et al. [98] obtained a Co3O4 aerogel using a sol–gel method with subsequent drying and heat treatment. The isothermal adsorption–desorption test results showed that the Co3O4 aerogels obtained at different heat treatment temperatures contained mesopores. When the heat treatment temperature increases from 200 to 400 °C, the specific surface area of Co3O4 decreases from 235 to 69 m2·g−1 and its specific capacitance decreases with the decrease of the specific surface area. Compared with alternative approaches, the sol–gel method exhibits an array of distinctive benefits. In this method, raw materials are dispersed and dissolved in a solvent, resulting in a low-viscosity solution. This characteristic enables the rapid achievement of molecular-level uniformity, ensuring efficient and thorough mixing of the reactants when the gel forms. Moreover, the requirement for solution reaction steps in the sol–gel method presents an opportunity to introduce trace elements easily and precisely. This method enables accurate and uniform doping at the molecular level, thereby facilitating an enhanced control of the process. In contrast to solid-state reactions, chemical reactions are more manageable and can be conducted at lower synthesis temperatures using the sol–gel method. However, it is essential to acknowledge that this technique has several challenges. First, the current selection of raw materials for the sol–gel process can be financially burdensome, with some containing organic substances that pose health risks. Moreover, the sol–gel procedure typically requires considerable time, often over several days or even weeks. Additionally, the presence of numerous micropores in the gel can trigger the release of substantial quantities of gases and organic substances during the drying phase, leading to subsequent contraction.

3.1.6 Other methods

In addition to the aforementioned conventional techniques, alternative approaches such as the microwave-assisted method, chemical bath deposition method and ion exchange method have been utilized to fabricate transition metal compounds. For example, Younas et al. [99] successfully synthesized Co–Se nanosheets using a microwave-assisted method. The results showed that the Co–Se electrode had a high specific capacitance of 1580 F·g−1 at 1 A·g−1. Patil et al. [100] prepared La2Se3 nanoflakes using chemical bath deposition. The maximum specific capacitance of the La2Se3 nanoflake electrode was 331 F·g−1 at 5 mV·s−1 and the specific capacitance remained 84% after 1,000 cycles. Chen et al. [101] prepared the precursor NiCo2(CO3)1.5(OH)3 on foam nickel by a hydrothermal method and then carried out a secondary hydrothermal ion exchange reaction with Na2S to obtain NiCo2S4 nanotubes with a hollow structure. NiCo2S4 nanotubes electrode shows a very high area-specific capacitance (14.39 F·cm−2).

The advantages and disadvantages of the synthesis method are shown in Table 2.

Table 2 Advantages and disadvantages of synthesis methods
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3.2 Regulation strategies

Based on the above problems with transition compounds, three strategies have been proposed to improve their electrochemical performance: (1) regulation of nanostructures, (2) interface engineering and (3) design of composite electrodes.

3.2.1 Regulation of nanostructures

The morphology and nanostructure of electrode materials significantly affect the practicability area, diffusion path of the electrolyte ions and structural stability. There are many ways to construct nanomaterials with special structures and morphologies: 1D nanorods or nanowires, which increase the conductivity of ions or electrons, 2D nanosheets which increase the active area; and design 3D hierarchical nanomaterials [102, 103]. They also include nanospheres, octahedral and cubes. All types of special structures (such as porous structures, micron/nano hierarchical structures, hollow structures and core–shell structures) can significantly increase the specific surface area of the material, facilitate the penetration of electrolytes, shorten the propagation path of ions and electrons, and alleviate the volume change caused during the charge–discharge process [104,105,106,107].

Owing to the good conductivity of 1D nanowires, Rauf et al. [108] prepared ZnS nanowires with a large aspect ratio using a one-step hydrothermal method (Fig. 7a–c). The ZnS nanowires interconnected to form a 3D network structure that facilitated electrolyte diffusion toward the material surface and improved the utilization of the active materials. In addition, the fine nanowire characteristics of ZnS promotes the diffusion of electrolytes in the internal region of the electrode, thereby reducing the internal resistance. The contact area between the electrodes and electrolyte can also increase the number of ion adsorption sites for charge transfer. The obtained ZnS nanowire electrode showed a specific capacitance of 781 F·g−1 at 0.5 A·g−1. 2D nanosheets with nanoscale thicknesses and microscale lateral dimensions displayed extraordinary morphological anisotropy, leading to an increased surface area. 2D nanomaterials have no interlayer contacts owing to electronic confinement and display better electronic properties than other nanomaterials [109]. Liu et al. [110] reported a nanosheet array of nickel–cobalt selenide ((Ni,Co)Se2) with a hierarchical NP/nanosheet structure, which was grown on an acidified CC substrate via hydrothermal and selenization processes (Fig. 7d–f). The nanosheet array (Ni,Co)Se2 had a rough and porous surface, which increased the contact area between the electrode and electrolyte and supplied more active sites. The (Ni,Co)Se2/CC electrode showed a high specific capacitance of 2054 F·g−1 at 1 A·g−1 and excellent rate performance of 90.3% retention at 10 A·g−1. A 3D structure can incorporate many nano-components into microscale structures, such as 3D micro-nanostructures interwoven by nanosheet structures and nanowire structures, while sustaining the unique performance of the components at the nanoscale. Han et al. [111] reported a CoNi2S4 electrode consisting of 2D microsheets and 1D nanowires, which were ornamented by 2D ultrathin nanosheets homogeneously covering the surface of the Ni foam, forming an architecture with a 3D porous structure using a two-step hydrothermal method (Fig. 7g–i). The CoNi2S4 electrode displayed an enhanced specific capacitance (3784.6 F·g−1 at 3 A·g−1) and extraordinary rate capability (2932.3 F·g−1 at 20 A·g−1). The exceptional electrochemical performance can be attributed to the benefits derived from the multidimensional structure formed through the interconnection of the 2D microsheets, 1D nanowires and 2D ultrathin nanosheets. This structural arrangement offers abundant active sites and provides a substantial contact area at the electrode/electrolyte interface, thereby significantly enhancing the transport kinetics of electrons and ions.

Fig. 7
figure 7

Reproduced with permission from Ref. [108]. Copyright 2022, Elsevier. d Schematic diagram for synthesis process of (Ni,Co)Se2/CC nanoarrays; e SEM image and f GCD curves of (Ni,Co)Se2/CC. Reproduced with permission from Ref. [110]. Copyright 2023, Elsevier. g Schematic illustration of Co–Ni–S by hydrothermal process; h SEM images and i GCD curves of CoNi2S4. Reproduced with permission from Ref. [111]. Copyright 2020, The Royal Society of Chemistry

Synthesis and characterization of nanowires and nanoarrays. a Schematic diagram for hydrothermal synthesis, b SEM image, and c GCD curves of ZnS nanowires.

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3.2.2 Interface engineering

The interface between the current collector and active materials is important for effective electron transmission; as the structure without a binder has better electrical contact, an array of active materials can be directly grown on the substrate. The active material with an array structure is grown in situ on the current collector, which can avoid the “dead surface” owing to that it introduces polymer binder and conductive agent, reduces the contact resistance, maximizes the use of the active materials, and improves the interface of the active materials/current collector to achieve fast and effective electronic transmission. Compared with traditional powder samples, the capacitance performance was greatly improved by designing NP, nanowire, nanotube and nanosheet structures. Cao et al. [112] prepared MnMoO4 nanoplates grown directly on Ni foam using a hydrothermal method (Fig. 8a–d). Figure 8e shows CV curves of the pure MnMoO4 powder and MnMoO4 NPs on Ni foam at 50 mV·s−1, demonstrating that the MnMoO4 NP electrode on Ni foam had a higher specific capacitance than the pure MnMoO4 powder electrode (Fig. 8f). As integrated electrodes for SCs, the MnMoO4 nanoplate electrode supported directly on Ni foam exhibited good electrochemical performances with a high specific capacitance of 1.15 F·cm−2 (2300 F·g−1 at 4 mA·cm−2 and a good cyclic stability (92% capacity retention rate after 3,000 cycles) (Fig. 8g, h). This remarkable electrochemical performance can be ascribed to the porous structure formed by the interconnected MnMoO4 nanoplates. This structure enhances the efficiency of electrolyte diffusion and promotes electron transport, thereby contributing to an overall superior electrochemical performance.

Fig. 8
figure 8

Reproduced with permission from Ref. [112]. Copyright 2014, The Royal Society of Chemistry. i TEM and j HRTEM images of C–NiCo2O4/C nanowires; k TEM images, l GCD curves of C–NiCo2O4/C@Ni–Co–S electrode; m rate capacitance of C–NiCo2O4/C, C–NiCo2O4/C@Ni–Co–S–10, C–NiCo2O4/C@Ni–Co–S, and C–NiCo2O4/C@Ni–Co–S–20 electrode; n cycling retention of C–NiCo2O4/C@Ni–Co–S electrode. Reproduced with permission from Ref. [96]. Copyright 2021, ACS Publications

Morphological characterization and electrochemical performance of MnMoO4 NPs on Ni Foam and C–NiCo2O4/C@Ni–Co–S electrode. SEM images of a bare Ni foam and b–d MnMoO4 NPs grown directly on Ni foam; e CV curves, f rate performance, g GCD curves and h cycling performance of MnMoO4 NPs grown on Ni foam.

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In addition, the introduction of a buffer layer between the active materials and substrate can facilitate rapid electron collection and transport, improving the electrochemical performance of the electrode materials. Xu et al. [96] designed a C–NiCo2O4/C@Ni–Co–S electrode by coupling zero-dimensional (0D)/2D carbon-modified NiCo2O4 in the interior with Ni–Co–S nanosheets in the external sheath (Fig. 8i–k). By incorporating 0D carbon doping, the agglomeration of NiCo2O4 NPs was effectively inhibited, leading to enhanced electronic conductivity. A C–NiCo2O4 nanowire array was used as the skeleton and scaffold to prevent the aggregation of the Ni–Co–S nanosheets, thus exposing more active sites. The ultrathin 2D carbon intermediate layer on the surface of the C–NiCo2O4 nanowire can supply fast electronic channels for charge storage and transportation, and the perfect Ni–Co–S nanosheets can coat on the surface of C–NiCo2O4 nanowire array as a “protection” layer, thus improving the stability of the nanowire structure. C–NiCo2O4/C@Ni–Co–S electrode delivers a high specific capacitance of 2396 F·g−1 at 3 A·g−1 and good cycling stability (Fig. 8l–n).

Another remarkable interface engineering technique is the direct growth of a core–shell array on the current collector, which has an interconnected network structure that provides efficient ion/electron transportation and alleviates volume variation. Moreover, the core–shell nanostructure can integrate two active materials and inherit the advantages of “core” material and “shell” material. Zhang et al. [113] successfully synthesized hierarchical core–shell CoMoO4@NiMoO4 nanosheet arrays on Ni foam using a two-step hydrothermal method and calcination (Fig. 9a–c). The CoMoO4@NiMoO4 composite electrode displayed an excellent specific capacitance of 1639.8 F·g−1, which was higher than those of the CoMoO4 and NiMoO4 electrodes (Fig. 9d, e) due to its larger specific surface area and increased number of active sites. Adhikari et al. [114] successfully prepared a binder-free 2D/2D FeCo2O4@Ni(OH)2 material with a core–shell structure grown on the Ni foam, in which FeCo2O4 microsheets are as “core” and Ni(OH)2 nanosheets are as “shell” (Fig. 9f). SEM image shows that FeCo2O4 has a microsheet structure and the high-magnification image shows that the thickness of the microsheet is 289 nm (Fig. 9g, h). After coating with Ni(OH)2, the thickness of FeCo2O4@Ni(OH)2 increased to 310 nm (Fig. 9i, j). The FeCo2O4@Ni(OH)2 electrode displayed an excellent specific capacitance of 1944 F·g−1 at 5 A·g−1, which was higher than the specific capacitance of FeCo2O4 and Ni(OH)2 electrodes (Fig. 9k, l).

Fig. 9
figure 9

Reproduced with permission from Ref. [113]. Copyright 2021, The Royal Society of Chemistry. f Diagrammatic sketch of FeCo2O4@Ni(OH)2 electrode preparation; SEM images of g, h FeCo2O4 and i,j FeCo2O4@Ni(OH)2 core–shell arrays; GCD curves of k FeCo2O4, Ni(OH)2, FeCo2O4@Ni(OH)2 and l FeCo2O4@Ni(OH)2 electrode. Reproduced with permission from Ref. [114]. Copyright 2023, Elsevier

a Diagrammatic sketch of hierarchical core–shell CoMoO4@NiMoO4 nanosheet array synthesis process; b SEM and c TEM images of CoMoO4@NiMoO4; GCD curves of d CoMoO4, NiMoO4, CoMoO4@NiMoO4 and e CoMoO4@NiMoO4 electrode.

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3.2.3 Design of composite electrode materials

It is difficult for a single material to simultaneously balance the high specific capacity, good cycling stability and high energy/power densities. Therefore, researchers have shifted their focus to composite electrode materials and have used the synergy of composite electrode materials to improve the comprehensive performance of electrode materials. Due to the low conductivity and cycle stability of transition metal compounds, carbon materials with high conductivity and perfect material stability are used for composites. The excellent performance of these composite materials is mainly due to that carbon can inhibit the aggregation of transition metal compounds, offer a large accessible surface area, and reduce the volume change of the composite material during the charging-discharging process to improve the stability of the electrode material. In addition, the good conductivity of carbon compensates for the influence of the low conductivity of transition metal compounds. Various transition metal compounds with different morphologies have been combined with carbon materials to form composite electrode materials. For example, Sun et al. [115] prepared α-Ni(OH)2 nanosheet arrays modified by carbon quantum dots using a one-step hydrothermal method (CQDs/α-Ni(OH)2). SEM images in Fig. 10a, b show that the morphology of Ni(OH)2 is similar to that of CQDs/α-Ni(OH)2. Notably, the surface of CQDs/α-Ni(OH)2 is rougher, suggesting that the addition of CQDs can raise the specific surface area, leading to exposure of the active site. Transmission electron microscopy (TEM) images of CQDs/α-Ni(OH)2 in Fig. 10c, d show the introduction of CQDs into α-Ni(OH)2. The optimized electrode has a specific capacitance of 1724.0 F·g−1 at 3 A·g−1, which is 2.3 times that of bare α-Ni(OH)2. (Fig. 10e, f) The hierarchical structure of α-Ni(OH)2 and the aid of CQDs enhanced the reaction sites of CQDs/α-Ni(OH)2 and increased conductivity, thus obtaining high electrochemical performances.

Fig. 10
figure 10

Reproduced with permission from Ref. [115]. Copyright 2023, Elsevier. SEM images of g NiCo2O4 nanowires and h NiCo2O4@MnMoO4 nanocolumns; TEM images of i, j NiCo2O4 nanowires and k, l NiCo2O4@MnMoO4 nanocolumns; GCD curves of m MnMoO4, n NiCo2O4 and o NiCo2O4@MnMoO4. Reproduced with permission from Ref. [116]. Copyright 2017, ACS Publications

SEM images of a Ni(OH)2 and b CQDs/Ni(OH)2; c, d TEM images of CQDs/Ni(OH)2; e CV curves and f GCD curves of CQDs/Ni(OH)2 electrode.

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In addition, composite electrode materials consisting of two different transition metal compounds, in which one compound acts as a skeleton for the growth of the other compounds to generate the array structure, have been extensively studied. This composite structure effectively prevents the accumulation of electroactive materials. This is particularly instructive for practical applications in which a high-energy density can be achieved even when high loads normally require the use of SCs. Yuan et al. [116] successfully combined a highly electronically conductive NiCo2O4 material with a high-capacitance MnMoO4 material. SEM images in Fig. 10g, h show the morphology and structure of the as-prepared NiCo2O4 and NiCo2O4@MnMoO4 electrode materials, respectively. NiCo2O4 nanowires were densely coated on foam nickel in a multitudinous space. Each NiCo2O4 nanowire is about 100 nm in diameter and over 1 μm in length. TEM results revealed the detailed structure of the NiCo2O4 nanowires and NiCo2O4@MnMoO4 nanocolumns (Fig. 10i–l). NiCo2O4 nanowires consist of numerous interconnected NPs with diameters of ~ 50 nm. TEM images of the NiCo2O4@MnMoO4 nanocolumn showed that the NiCo2O4 nanowires were covered with a large number of MnMoO4 nanosheets. The NiCo2O4@MnMoO4 nanoarray electrode exhibited an excellent capacitance of 1169 F·g−1 at 2.5 mA·cm−2, obviously higher than those of the pristine NiCo2O4 and MnMoO4 electrodes (Fig. 10m–o). The excellent electrochemical performance of the NiCo2O4@MnMoO4 electrode is mainly due to the increase in its specific surface area, abundance of electroactive sites, fast electron and ion transport paths, and synergistic effect of the NiCo2O4 and MnMnO4 electrode materials.

The advantages and disadvantages of the regulation strategies are shown in Table 3.

Table 3 Advantages and disadvantages of regulation strategies
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4 Conclusions and prospects

This study reviewed the research progress, existing problems and repair strategies of transition metal compounds as electrode materials for SCs. Currently, the main transition metal compounds used as electrode materials for SCs are oxides/hydroxides, sulfides, phosphides and selenides. Although progress has been made in using transition metal compounds as electrode materials for SCs, their development has been hindered by shortcomings. For example, pure-phase electrode materials are expensive to manufacture, complex to synthesize, and exhibit poor conductivity. Powder electrode materials easily agglomerate during electrochemical tests, resulting in a small specific surface area. The specific capacitance and cycle stability of nanomaterials are severely limited by their volume expansion/contraction during frequent charging and discharging processes. Thus, to prepare transition metal compound electrode materials with excellent electrochemical properties, researchers have proposed many strategies, including the improvement of synthetic methods, regulation of nanostructures, interface engineering and synthesis via the synergistic effect of nanomaterials.

Despite considerable progress in the use of transition metal compounds as electrode materials for SCs through various improvement strategies, there is still a large gap between laboratory research results and commercial applications and many challenges faced by SCs are yet to be solved. 1) Although many metals can form transition metal compounds, their practical applications are limited and mainly concentrated in Ni– and Co-based materials. Material properties can be predicted theoretically using computer science. Therefore, more efficient transition metal compounds will be synthesized and show great prospects in the future. 2) Elemental doping can adjust the electronic state of transition metal compounds and the adsorption properties of the reactants, thereby altering the activity and selectivity of the reaction and promoting the conductivity of transition metal compounds. However, the doping elements are limited to a few elements such as O and N. Apart from cation doping, anion doping also needs to be further explored. 3) A more in-depth exploration of the energy storage mechanism is needed to provide theoretical support for the more targeted and accurate design of electrode materials. 4) It is necessary to develop more advanced characterization techniques to determine the intrinsic causal relationships of the material composition, structure and properties from the macro to the micro level and the molecular to the atomic level. 5) The construction of high-mass-loading electrodes and structures should be further studied as it is an important step in bridging the gap between laboratory and commercialization.