A Review on Nano-/Microstructured Materials Constructed by Electrochemical Technologies for Supercapacitors

Highlights Recent progress of active materials in supercapacitors synthesized by electrochemical techniques is reviewed. Electrochemically synthesized nanostructures of various dimensions, compositions, and electrochemical properties are discussed. The advantages and challenges of electrochemical technologies in preparing nano-/microstructured materials for electrochemical energy storage devices are summarized.


Introduction
The rapidly expanding markets of mobile electronics, electrified transportation, wireless networks (e.g., the Internet of Things), and sustainable energy utilization have substantially fueled the development of electrochemical energy storage systems [1][2][3][4]. Supercapacitors, including electrochemical capacitors and pseudocapacitors, stand out among diverse arrays of energy storage devices due to their ultrahigh power density and ultralong life spans [5][6][7][8][9]. Since the first introduction in 1957 by Howard Becker of American General Electric [10], supercapacitors have pervaded in applications demanding energy input and output with high power, e.g., the power source to the emergency doors of Airbus A380 aircraft [11,12]. Therefore, elevating the capacitance and energy density of supercapacitors at ultrafast charging rates has remained a central topic [13][14][15][16]. Extensive research efforts have been devoted to developing high-performance electrode materials [13,[15][16][17], since electrodes primarily determine the capacitance, energy density, and power density of supercapacitors.
Supercapacitors are classified into two categories: electric double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store charges through adsorption and desorption of ions in electrolytes at the electrolyte/electrode interfaces. Carbon materials are conventional electrode materials in EDLCs. Pseudocapacitors store charges through kinetically fast Faradaic reactions. Surface redox pseudocapacitance, intercalation pseudocapacitance, and underpotential deposition are examples of pseudocapacitance [13]. Redox pseudocapacitance occurs when electrolyte ions adsorbed on or near the electrode surfaces and involves interfacial charge transfer. Intercalation pseudocapacitance comes from reversible insertion and desertion of electrolyte ions in layered or tunneled electrode materials without phase transitions. Underpotential deposition will be elaborated in Sect. 2.2, but it is not used for charge storage due to its limited capacity. Pseudocapacitance has its electrochemical features, such as quasi-rectangular cyclic voltammetry curves, linear galvanostatic charge/discharge profiles, as well as a near-linear response between current and scan rate in cyclic voltammograms. These characteristics must be clearly distinguished from battery-type behaviors (Sect. 5.6).
Electrochemical techniques are a group of synthesis methods of versatile active materials in supercapacitors. These methods have a history of more than 150 years [18], and their first application is to plate metals for jewelry decoration, surface protection, and electronic-circuit manufacturing [19]. Along with the boom of electrochemical energy storage, the role of electrochemical techniques has been enriched by synthesizing nano-/microstructured materials for electrochemical energy storage [2,20]. The syntheses involve electrochemical processes including reduction, oxidation, gas evolution, ion intercalation, and combinations of these methods thereof. Chemical reactions, such as the electrolysis of electrolytes (e.g., H 2 and O 2 evolution in aqueous electrolytes), redox reactions of electrolyte ions, as well as modifications of the structures and compositions of electrode materials, are typically accompanying phenomena.
Compared to other synthesis strategies, electrochemical techniques have their unique advantages. First, they are facile and mild. Room temperature, ambient pressure, and aqueous solutions are sufficient for performing electrochemical techniques. Second, the experimental setups, such as electrolytic cells and electrochemical workstations, are often readily available in electrochemistry laboratories. This availability allows electrochemists to prepare active materials without delicate instruments and sophisticated protocols. Perhaps the most striking feature of electrochemical techniques is their high tunability toward the structure, composition, property, and morphology of products [2,20]. Magnitudes of applied current or voltage, types and concentrations of salts in electrolytes, reaction durations, solution temperatures, as well as substrate morphologies are all tunable parameters that result in versatile materials. Benefited from these merits, electrochemical techniques have been extensively studied and rapidly developed in the past decades. The materials that have been prepared electrochemically include exfoliated graphene [21], metal oxides [22][23][24], conducting polymers [25,26], and their composites [27][28][29].
This review article presents a thorough survey of electrochemically synthesized nano-/microstructured materials for supercapacitors. It starts with introduction of the operating mechanisms, characteristics of input and output signals, strengths, and weaknesses of cyclic voltammetry, potentiostatic (constant voltage) and galvanostatic (constant current) depositions, pulse electrodeposition, and electrophoretic deposition. Afterward, the article reviews the recent progress of the active materials synthesized by electrochemical techniques. This part is segmented based on the compositions of 1 3 materials, including carbon-based materials, metal oxides, conducting polymers, composites, and other materials. Each subsection starts with typical synthesis mechanisms and is exemplified with one or more representative examples in the literature. At last, the article comments on the merits, challenges, and opportunities of electrochemical technologies in terms of synthesizing nano-/microstructured materials for electrochemical energy storage.

Fundamentals of Electrochemical Synthesis Techniques
Cyclic voltammetry, galvanostatic deposition, potentiostatic deposition, pulse deposition, and electrophoretic deposition constitute the most widely and extensively investigated and practiced electrochemical synthesis techniques for nano-/ microstructured materials as supercapacitor electrodes. These processes are typically carried out in electrolytic cells powered by electrochemical workstations. Based on the number of electrodes involved, the setup for electrochemical synthesis is categorized into two types: two-electrode (Fig. 1a) and three-electrode (Fig. 1b) configurations [30]. The two-electrode configuration contains a positive electrode and a negative electrode that are both immersed in electrolytes. An electrochemical workstation or power source provides voltage across the two electrodes. Therefore, the measured voltage in this scenario is the overall cell voltage. The three-electrode system comprises a working electrode (WE), a counter electrode (CE), and a reference electrode (RE). Ideally, current flows only between WE and CE, and the voltage of WE is referenced to that of RE. Saturated calomel electrode (SCE), Ag/AgCl, and Hg/HgO electrodes with their nearly constant half-reaction potentials are common REs. REs are placed in vicinity to WEs to minimize iR drop and voltage fluctuation due to electrolyte resistance [31]. The measured voltage in three-electrode configurations is the real-time potential of WEs.

Cyclic Voltammetry (CV)
Besides a conventional electrochemical technique for probing electrochemically redox activities [32], CV serves as a synthetic tool. It linearly scans potential within a range, termed potential window, and simultaneously records current as a response [33]. Increasing the applied potential, the forward scan oxidizes species in electrolytes or on electrodes and produces anodic current. Conversely, the backward scan decreases the applied potential, reduces active components, and generates a cathodic current. CV has three main advantages as a synthetic approach. First, it allows for determining the onset potential of an electrodeposition reaction. Oxidation or reduction reactions involving charge transfer across the electrolyte-electrode interfaces will display sharp increases or well-defined peaks in the current. Since the onset potential is the minimum voltage needed to initiate electrodeposition reactions, CV is useful for developing experimental protocols. Second, the potential linear scan of CV is beneficial for growing uniform and conformal films. This characteristic provides a gradient driving force for deposition: Deposition will only begin until the potential is scanned above the onset potential, and the driving force of deposition scales linearly with the potential gradually elevated away from the onset potential. This gradient driving force of CV adjusts the deposition rate and avoids consistently high deposition voltages that can lead to overgrowth of materials, rapid clogging of pores, and/or uneven deposition of films. Third, CV is suitable for synthesizing materials with multiple valence states, e.g., transition metal oxides. For example, deposited by CV within a potential range between − 1.5 and 1.5 V vs. SCE in a 0.1 M VOSO 4 aqueous electrolyte, vanadium oxide nanorods contained ~ 50% V 5 O 12 (a mixture of V 5+ and V 4+ ) and ~ 50% VO 2 [34]. The high valence state, V 5+ , formed during the anodic or forward scan, while the as-deposited V 5 O 12 was partially reduced to VO 2 in the subsequent cathodic or backward scan. These redox processes were documented by the broad peaks in the corresponding CV curve (Fig. 2). Compounds with multivalent species reportedly possess augmented capacitance [35], improved rate capability [34,36], as well as enhanced cycling stability [37,38] compared to their monovalent counterparts.

Potentiostatic (Constant Voltage) Deposition
Potentiostatic deposition synthesizes materials by applying a constant potential across the positive and negative electrodes (two-electrode system) or between the working and counter electrodes (three-electrode system). The deposition potential is maintained constant by an electrochemical workstation ( Fig. 3a), and the current is recorded as a function of time ( Fig. 3b) [39]. Based on the difference between applied and thermodynamic equilibrium potentials, potentiostatic deposition is categorized into underpotential deposition (UPD) and overpotential deposition (OPD) [40,41].
UPD happens at potentials below thermodynamic equilibrium potentials. For example, metal deposition can initiate at potentials smaller than the corresponding equilibrium reduction potentials, due to the lower work function of the deposited metal than that of the substrate metal, as described by the Kolb-Gerischer equation [42]: where ΔE is the downshift of deposition potential (in V) and Δ is the difference in the work functions between the deposited and substrate metals (in eV). The coefficient 0.5 (in V eV −1 ) comes from a linear fitting involving 21 metal-metal couples [42]. UPD involves adsorption, nucleation, and growth processes determined by surface characteristics of substrates (e.g., chemical composition, crystal structure, morphology, and electrolyte wettability) and ion-substrate interactions. Besides, the types of cations in electrolytes and anions strongly influence the structure and properties of the deposited materials, as well as deposition kinetics [43][44][45]. One example is the UPD of Cu on Au (111) facets in aqueous sulfuric acid solutions [46]. The deposition was much slower under pH = 2 than pH = 4. This discrepancy in the deposition rate was correlated with the different anions under different pH values. Increasing the solution acidity converted bisulfate ions to sulfate ions. The latter adsorbed much more strongly than the former on the gold substrate, which blocked some active sites for deposition and hence decelerated the UPD.
OPD occurs in potentials above thermodynamic equilibrium potentials [39,47]. The structure and properties of the OPD deposits highly depend on various factors, including overpotential (the difference between applied and equilibrium potentials), electrolyte concentration, growth mechanism and deposit-substrate interactions [41]. Notably, diffusion-controlled nucleation is often the rate-determining step of OPD, while that of UPD is the deposit lattice incorporation into substrate [46].

Galvanostatic (Constant Current) Deposition
Galvanostatic deposition refers to electrodeposition with constant currents between the positive and negative electrodes in a two-electrode system, or between the working electrode and counter electrode in a three-electrode setup (Fig. 4a) [39]. The recorded response is the time-dependent potential of the cell (two-electrode) or the working electrode (threeelectrode). The V − t curves (Fig. 4b) sometimes are called polarization curves. Unlike potentiostatic deposition that can start the moment when potentials are applied, galvanostatic deposition needs a short period to begin [48,49]. It is because that some applied current needs to charge electrical double layers (EDLs) first. After potential reaches certain thresholds (usually equilibrium potentials plus overpotentials), electrochemical reactions occur. Therefore, the applied constant current (I) is contributed from two components: (2) I = I DL + I ct where I DL is capacitive current for charging EDLs and I ct is charge transfer current for electrodeposition. I DL rapidly approaches zero when electrodeposition starts.
V − t curves of galvanostatic deposition contain essential information on electrodeposition chemistries. Since EDL charging time is on the order of milliseconds, V − t curves collected on the timescale of minutes or hours are almost contributed from electrodeposition. For example, the V − t curves of manganese dioxide (MnO 2 ) deposition processes at different temperatures qualitatively elucidate the nucleation kinetics (Fig. 4d) [50]. The increased potentials at the beginning of the electrodeposition corresponded to the nucleation of MnO 2 as nucleation demanded more energy than its growth to surmount the activation energy barrier. Increasing temperature from 25 to 60 °C offered the additional electrodeposition energy to enable the MnO 2 deposition at the reduced deposition potentials. The prolonged nucleation process at 25 °C results in dense manganese oxide nanosheets.

Pulse Electrodeposition
Pulse electrodeposition technique deposits materials by applying pulses of potential or current, i.e., a series of pulses with equal polarization, amplitude, and duration, separated by periodic zero current or open-circuit potentials (Fig. 5) [39,51]. Each pulse has "on" periods when the current or potential is applied and "off" periods with no current or potential (Fig. 5) [52][53][54]. During the "off" periods, ions in electrolytes diffuse into electric double layers along the surfaces of the deposition substrates, which is beneficial to obtain the uniform deposition of fine-grained deposits during "on" periods [55][56][57].
Potential, E Current, I  [57]. The "off" period allowed pyrrole monomers to diffuse into the intersheet spaces of graphene and then electropolymerized into uniform polypyrrole coatings over the "on" periods. By contrast, the continuous, unpulsed electrodeposition triggered the fast polymerization of pyrrole near the graphene sheets. Since there are no "off" periods to replenish pyrrole monomers near graphene surfaces, the lowered reactant concentration led to scattered polypyrrole particles on the graphene surface. This work highlights the suitability of pulse electrodeposition in coating uniform films onto irregularly shaped substrates.

Electrophoretic Deposition
Electrophoretic deposition (EPD) differs from all the abovediscussed techniques. First, the charge carriers in EPD are suspending, charged colloidal particles, not ions. Second, EPD involves electrostatic attractions between the particles and substrates, but no charge transfer. Third, unlike electrodeposition that demands electrolytes to conduct ions, EPD can perform in poorly conductive media, e.g., water [58,59].
Depending on charges carried by the colloidal particles, EPD is classified into cathodic and anodic EPD. The  cathodic EPD refers to the deposition of positively charged particles onto negatively charged substrates (Fig. 6a), whereas anodic EPD proceeds in a reverse manner (Fig. 6b) [58]. The structures of the deposits are tailorable by varying parameters of applied potential, particle concentration, and deposition duration [60][61][62][63][64][65]. Notably, the stoichiometry of the electrosorbed particles directly determines the stoichiometry of the deposit [59].

Electrochemically Synthesized Carbon Materials
Carbon materials, including activated carbon, carbon fibers, carbon aerogels, carbon nanotubes, and graphene, are conventional materials in electrical double-layer capacitors [17]. Their high electrical conductivity, large surface areas, cost-effectiveness, chemical inertness, and tailorable porous structures make them suitable electrode candidates [66,67].
The two outstanding advantages of carbon electrodes are their exceptional rate capability and ultralong lifetimes. Electrochemical approaches have been widely used to produce carbon materials [68][69][70]. The following sections first present the recent progress in syntheses of graphene (Sect. 3.1) and 3D carbons (Sect. 3.2), the two carbon materials that have been prepared by electrochemical technologies. Their applications in supercapacitors are highlighted separately in Sect. 3.3.

Anodic Exfoliation
Anodic exfoliation separates graphite into graphene by anion intercalation [73,76]. It is the most used electrochemical approach for graphene production due to its high exfoliation efficiency. With inorganic acids (e.g., H 2 SO 4 [78,84], HNO 3 [85,86], and H 3 PO 4 [87]) or salts (e.g., KNO 3 [88] and (NH 4 ) 2 SO 4 [89]) as supporting electrolytes, a high exfoliation potential (e.g., 3-10 V) can generate single-layer or multilayer graphene sheets from graphite. The synthesis procedures involved multiple steps (Fig. 7a). First, a high potential of 10 V was applied across a graphite positive electrode and a Pt negative electrode (Fig. 7b), splitting water into hydroxyl (OH·) and oxygen radicals (O·). These radicals preferentially oxidized the boundaries and defects of graphite, opening its edges. Second, driven by the applied electrical field, sulfate ions (SO 4 2− ), together with water molecules, intercalated into graphite layers through the open edges and expanded graphite layers. Meanwhile, oxygen gas evolution in between graphite layers further torn apart graphite sheets and dispersed exfoliated graphene layers into electrolytes ( Fig. 7c- calation that significantly expanded graphite into graphene nanoribbons. Increasing the water/IL ratio increased the populations of OH· and O·, which substantially oxidized and broke graphite into hydroxylated carbon particles. In this case, the oxygen-containing radicals acted as electrochemical "scissors" that cut the graphite plates into nanoribbons or nanoparticles. Graphite exfoliation in aqueous solutions often yields oxygenated graphene with reduced electrical conductivity, a property unfavorable for rapid charge storage [36,80,[90][91][92][93][94]. To circumvent this shortcoming, a variety of additives, including reducing agents [95] and oxygen radical scavengers [76], have been introduced into electrolytes to prevent overoxidation of exfoliated graphene. For example, Yang et al. demonstrated that (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), ascorbic acid, and sodium borohydride could consume radicals. This characteristic kept oxygen content at low levels (3.8 atom% O of TEMPO-added exfoliated graphene vs. 11 atom% O of TEMPO-free exfoliated graphene) [95]. Besides, Ejigu et al. reported that transition metal ions (e.g., Co 2+ , Ni 2+ , Fe 3+ , Mn 2+ , Ru 3+ , Ir 3+ , and V 3+ ) as electrolyte additives were conducive to acquiring high-quality graphene because they scavenged oxygen radicals [76]. Among all these ions, Co 2+ was the most promising one because it converted to an oxygen evolution reaction catalyst. During anodic exfoliation, Co 2+ was first oxidized to Co 4+ , an active species for oxygen evolution. The Co 4+ adsorbed on graphite facilitated oxygen evolution from water, bypassing the formation of oxygen radicals that oxidized exfoliated graphene. The lowest oxygen content was 2.6%.

Cathodic Exfoliation
Cathodic exfoliation, which produces graphene by applying negative biases to graphite, is a method free of oxidation concerns. Though this technique is not as developed as anodic exfoliation, it has successes in graphite exfoliation in organic-based electrolytes [21,67,81,83] and molten salts [82]. For example, Wang et al. deployed a cathodic exfoliation method that acquired highly conductive, less than five layers of graphene nanosheets from graphite with yields of > 70% [21]. At a high cathodic potential of − 15 ± 5 V, Li + -propylene carbonate (PC, a solvent) complexes intercalated into graphite and expanded graphite layers. Subsequently, the reduction of PC molecules liberated gas bubbles in between graphite layers, eventually exfoliating graphite into graphene sheets (Fig. 8a-c). Because the exfoliation involved no oxidation, the resultant graphene contained little defects, as evidenced by the small I D /I G ratio of 0.1 revealed by Raman spectroscopy (Fig. 8d).
In addition to liquid electrolytes, quasi-solid molten salts were potent electrolytes for graphite exfoliation. For instance, Huang et al. successfully exfoliated graphite in molten LiOH with a high cathodic current of 15 A [82]. This process was based on intercalation, expansion, and micro-explosion. First, Li + ions in the molten LiOH intercalated into graphite layers and widened the interlayer gap of graphite, forming graphite intercalation compound (Li x C y ). Afterward, the Li + -intercalated graphite was soaked in water. Li x C y and metallic Li reacted violently with water (micro-explosion), creating hydrogen gas bubbles that further exfoliated graphite into graphene. The conversion efficiency was ~ 80%.

Three-Dimensional Carbons
As supercapacitor electrodes, three-dimensional (3D) carbon materials have advantages over other conventional carbon powders (e.g., activated carbon) and graphene. First, their self-standing nature requires no binder blending for preparing electrodes, which eases fabrication and reduces the negative impact of the binders on electron transport. Second, their tailorable structures offer opportunities to achieve both high surface areas and hierarchical porous networks known to facilitate ion diffusion [4,[96][97][98][99][100][101][102].
Electrochemical partial exfoliation of graphitic materials is the most common synthesis method of electrochemically synthesizing 3D carbon materials. As indicated by its name, electrochemical partial exfoliation only partially exfoliates graphitic precursors, e.g., carbon fibers [37,85,97,99], graphene aerogels [99], and graphite foils [88,103,104], leading to graphene sheets anchored on the exposed surfaces. For example, Song et al. demonstrated a two-step electrochemical partial exfoliation method to prepare oxygenfunctionalized, partially exfoliated graphite foils (Fig. 9a) [88]. The authors first scanned a piece of graphite foil (EG in Fig. 9b) in aqueous K 2 CO 3 electrolytes by cyclic voltammetry. This step partially exfoliated the graphite layers on and near the outer surface of EG through vigorous gas evolution from water splitting (Fig. 9c). The secondary exfoliation process intercalated NO 3 − into EG through the open edges and defects formed during the first step, forming graphite intercalation compounds (C-NO 3 ). These compounds, when placed in water, hydrolyzed and released oxygen gas. The gas evolution further exfoliated and oxygenated the top layers of EG (Fig. 9d). The introduced oxygen functionalities rendered the foil superhydrophilicity as reflected from the zero contact angle (Fig. 9d inset). The potential applied in the second step controlled the degrees of the exfoliation and oxygenation. The functionalized, exfoliated EG possessed a 3D network consisting of oxygenated graphene sheets integrated onto graphite foil. The seamless integration between the top layer and the graphite bottom ensured fast electron conduction pathways. Besides improving electrolyte wettability, the oxygen moieties served as anchoring sites for depositing guest materials for polyaniline [27], polypyrrole [105], manganese oxides [86], vanadium oxides [37], iron oxides [35,106], nickel-cobalt double hydroxides [106], and molybdenum-based materials [28]. In addition to graphite, carbon fibers can also be partially electroexfoliated. For example, Wang et al. synthesized electrochemically activated carbon fiber cloth electrodes by applying a voltage of 3 V in HNO 3 /H 2 SO 4 mixed aqueous electrolytes [85]. The high voltage, together with the highly oxidative and corrosive acids, roughened the carbon fiber surfaces and introduced functionalities of C-OH, -C=O, and -COOH. This activated carbon cloth electrode exhibited a high areal capacitance of 756 mF cm −2 at 6 mA cm −2 . Song et al. demonstrated an acid-free method to exfoliate carbon fiber cloth partially [37]. First, NO 3 − anions intercalated into carbon fibers, exfoliating and oxidizing the outer surface (Fig. 9e). To recover the electrical conductivity of carbon fibers, the researchers immersed the oxidized carbon   [88] and e-g Ref. [37] with permission cloth in a 0.1% hydrazine hydrate aqueous solution to reduce the oxygen content. This reduction process removed most of the oxygen functionalities ( Fig. 9f) and resulted in enhanced capacitive performance (Fig. 9g). The partially exfoliated carbon cloth electrode exhibited a high areal capacitance of ~ 500 mF cm −2 at 20 mA cm −2 .

Application for Supercapacitors
Owing to the enhanced surface areas and high electrical conductivity, electrochemically exfoliated graphene-based materials or activated carbon fibers have functioned as either electrode materials or current collectors in supercapacitors. Liu et al. fabricated an in-plane micro-supercapacitor through directly printing electrochemically exfoliated graphene on patterned microelectrodes. This micro-supercapacitor delivered an areal capacitance of 800 μF cm −2 at 1 mV s −1 [107]. Wu et al. used electrochemically exfoliated graphene to prepare graphene paper and 3D graphene foams as supercapacitor electrodes [75]. The specific capacitance of the 3D graphene electrode reached 113.2 F g −1 and 58.9 F g −1 at 0.5 A g −1 in 6 M KOH aqueous and 1 M triethylmethylammonium tetrafluoroborate acetonitrile (TEMABF 4 /AN), respectively.
Besides reporting the outstanding electrochemical performance, some researchers devoted to revealing the interplays between exfoliation conditions and electrochemical properties. Ambrosi et al. compared the capacitive performances of electrochemically exfoliated graphene synthesized in different aqueous electrolytes, i.e., 0.5 M H 2 SO 4 , Na 2 SO 4 , and LiClO 4 [71]. The results showed that the graphene prepared in H 2 SO 4 and Na 2 SO 4 exhibited relatively high specific capacitance of 78 and 106 F g −1 , respectively. LiClO 4 introduced a large amount of oxygen functional groups on the exfoliated graphene, which could be anchoring sites for growing other materials to form graphene-based composites. Notably, the specific capacitances of the electrochemically exfoliated graphene often fall in the range of 50-100 F g −1 , which are slightly lower than those of graphene obtained through chemical oxidation. The underlying reason was revealed by Xia et al., who studied the different graphite exfoliation routes via chemical oxidation and electrochemical exfoliation [66]. The authors discovered that the surface area of the electrochemically exfoliated graphite was only 6.6 m 2 g −1 , which was three orders of magnitude lower than that of the theoretical value (2600 m 2 g −1 ) of a graphene monolayer achieved by chemical exfoliation. Therefore, electrochemical exfoliation is challenging to fully separate graphite into monolayer graphene, resulting in the relatively low gravimetric capacitance due to the small surface area (Table 1). This limitation has motivated the introduction of pseudocapacitive functional groups to boost capacitance [85,99,101].
Compared with fully exfoliated graphene sheets, partial exfoliated graphite and carbon electrodes have the main advantage that the whole electrodes remain structurally and electrically connected, enabling them to function as current collectors for loading pseudocapacitive materials. The resultant composite materials possess both high capacitance from the incorporated pseudocapacitive materials and the excellent rate capability characteristic of carbon-based materials. In this regard, the mass loadings of the pseudocapacitive materials must be meticulously tuned to ensure the highest specific capacitance without significantly compromising the rate capability.
The polymerization of another conducting polymer, polyaniline, follows a similar path as that of PPy (Fig. 10b) [135]. Its monomers are first oxidized to radical cations, then coupled, and deprotonated to dimers. Unlike PPy, whose chain propagation is driven by continuous dimer combination, aniline dimers undergo further oxidation and couple with one aniline radical cation at a time. The coupling extends to polyaniline. For both cases, anions (or counterions in general) will dope into the as-formed conducting polymers to balance the charges carried by the positively charged sites on the polymer backbones. Note that the concept of doping in the context of conducting polymers is fundamentally different from that in conventional solid-state semiconductors. Doping semiconductors means to incorporate dopants into the crystal lattices of the host materials [136].
This counterion doping process maintains the electroneutrality of conducting polymer and affects the electrical conductivity of conducting polymers [137]: where Pol* is the positively charged sites in a conducting polymer, A − stands for counterion, and S represents a solvent molecule. When electrons enter in conducting polymers, doping of A − and concurrently co-insertion of solvent molecules lead to volumetric expansion of the host polymers. Conversely, electron extraction causes de-doping of A − and de-solvation of conducting polymers, resulting in volumetric contraction. The irreversible volumetric deformation is a typical culprit for the structural instability that causes unsatisfactory cycling stability of conducting polymers [138,139].

Films
Conformal films are the most common morphologies of electrochemically polymerized conducting polymer electrodes [97,140,141]. Parameters associated with  Fig. 10 a Electrochemical polymerization mechanism of pyrrole. Adapted from Ref. [133] with permission. b Electrochemical polymerization mechanism of polyaniline. Adapted from Ref. [135] with permission electropolymerization have profound influences on the chemical compositions, morphologies, and electrochemical properties of the deposited polymer films [130,142]. The surface properties of the substrates influence the adhesion strength and chemical compositions of the polymer films. For example, Feng et al. electrochemically deposited a thin PPy layer on oxygen-functionalized carbon cloth (FCC) [97]. Compared with PPy deposited on pristine carbon cloth (CC), PPy/FCC exhibited enhanced dopant concentrations and electrical conductivity, because the oxygen functional groups on FCC could dope into PPy and reinforced the adhesion of PPy onto FCC. Consequently, PPy/FCC displayed an areal capacitance of 341 mF cm −2 at 1 mA cm −2 , about 40 mF cm −2 higher than that of PPy/CC at the same current density.
Electrochemically polymerized conducting polymer films have different molecular structures from those prepared by chemical polymerization. Huang et al. reported that PPy film deposited via galvanostatic electrodeposition exhibited higher molecular order than that made by chemical oxidation (Fig. 11a) [143]. During electrochemical polymerization, the α-α coupled PPy chains stacked layer by layer with an interlayer spacing of 3.45 Å, as evident from the pronounced X-ray diffraction peak (Fig. 11b, c). This layered molecular structure facilitated ion transport within the electrode and induced a homogeneous stress distribution in the polymer films, both of which improved the cycling stability of PPy.
Coating conducting polymer films onto flexible substrates is a strategy of making flexible electrodes for wearable supercapacitors. Huang et al. demonstrated a stretchable stainless steel mesh as an electrically conductive substrate to endow the deposited PPy film excellent stretchability (Fig. 11d) [132]. The PPy-coated stainless steel mesh delivered a specific capacitance of 170 F g −1 at 0.5 A g −1 , and the capacitance augmented to 214 F g −1 when the electrode was applied a 20% strain. The strain improved the contact between PPy and stainless steel and reduced the contact resistance, which augmented the specific capacitance (Fig. 11e).
It should be noted that thin films are usually obtained at the early stage of electrode polymerization, and prolonging deposition time may modify the film morphology due to overgrowth. For instance, Song et al. observed that instead of thin films, PPy cauliflowers formed (Fig. 11f) after 10 cycles scanning from 0 to 0.8 V vs. SCE at 50 mV s −1 [105].
Soft templates (e.g., surfactant) can introduce porosity in electrodeposited conducting polymer films. Kurra et al. used a potentiostatic method to deposit a thin layer of PEDOT on an Au-coated, interdigitated electrode (Fig. 12a,   Fig. 11 a SEM image of electrodeposited PPy film on an oxygen-functionalized carbon fiber. b XRD patterns of electrochemically and chemically deposited PPy. c A possible molecular structure of electrodeposited PPy. d PPy film electrochemically deposited on a flexible stainless steel mesh. e Schematic illustration of the stretched mesh structure. f Cauliflower-like PPy film. Adapted from a-c Ref. [143], d, e Ref. [132], f Ref. [105] with permission b) [144]. Sodium dodecyl sulfate, an anionic surfactant, was used to increase the solubility of 3,4-ethylenedioxythiophene (EDOT) in water and thus decreased the polymerization potential of EDOT. The surfactant molecules also served as soft templates that created cracks in the PEDOT film (Fig. 12c, d). These cracks provided electrolyte ion percolation pathways and benefited rate capability at high frequencies. A symmetric micro-supercapacitor consisting of two identical interdigitated electrodes displayed a typical capacitive behavior as reflected from the plateau-free galvanostatic charge-discharge profiles (Fig. 12e). This micro-supercapacitor exhibited a positive trend between its areal capacitance and the electropolymerization time, but the volumetric capacitance peaked after 15 min polymerization (Fig. 12f). The drop in the volumetric capacitance was attributed to the increased PEDOT thickness that impeded ion diffusion.
In the absence of any structure-directing agents, polyaniline preferentially forms randomly intertwined nanofibers [151]. Liu et al. electrodeposited polyaniline (PANI) nanowires on carbon cloth using cyclic voltammetry within a potential window between − 0.2 and 0.8 V in aqueous electrolytes containing 0.1 M aniline and 1 M H 2 SO 4 [25]. PANI nanowires were uniformly grown on carbon cloth fibers (Fig. 13a). To address the intrinsic cycling instability of PANI, the researchers conformally coated the deposited PANI nanowires with 5-nm-thick carbonaceous shells by hydrothermally decomposing glucose. The coated PANI electrode exhibited a high theoretical areal capacitance of 787.4 mF cm −2 (estimated by the Trasatti method) and excellent cycling stability of ~ 95% after 10,000 charge-discharge cycles. SEM revealed that the carbonaceous shell mitigated the volumetric deformation-induced structural pulverization of PANI. Confining the growth of conducting polymers from current collector surfaces is a prerequisite to obtaining binderfree supercapacitor electrodes. To suppress electropolymerization of monomers in bulk electrolytes, small current, low potential, and dilute monomer solutions are preferred. Once polymer nucleates on substrate surfaces, they minimize the energy barrier for the subsequent growth of conducting polymer nanostructures [152,153]. For example, PANI nanorod arrays were grown on a gold plate using a galvanostatic method with a small current density of 0.01 mA cm −2 (Fig. 13b) [153]. The formation mechanism followed the nucleation-initiated growth process. The electrodeposited PANI nanorod array electrode exhibited a high specific capacitance of 950 F g −1 at 1 A g −1 . Following the same protocol, PANI nanorod arrays were grown on other conductive substrates, such as carbon nanotubes (Fig. 13c) [145], exfoliated graphene sheets [154], as well as graphene papers [155,156]. The generality of substrates indicated that the nucleation growth process is independent of substrate properties.
In addition to PANI, electrochemical technology also produces polypyrrole (PPy) nanorod arrays on conductive substrates. Huang et al. fabricated PPy nanorod arrays via a one-step galvanostatic deposition at 1 mA cm −2 with p-toluenesulfonate acid (TsOH) as a soft template (Fig. 13d) [149]. The TsOH anions prevented the as-formed PPy oligomers from growing in random directions, promoting the growth of PPy nanorods on carbon cloth. Significantly, the PPy nanorods exhibited capacitance of 699 F g −1 at 1 A g −1 .
When the current density increased from 1 to 20 A g −1 , 81.5% capacitance retained, indicating its excellent rate capability.
The use of hard templates enables the growth of sophisticated 1D nanostructures, such as nanotubes. Using nickel nanotube array (NiNTA) hard templates, Chen et al. made perchlorate-doped PPy nanotubes (Fig. 14) [157]. First, Ni Fig. 13 Electrochemically deposited 1D polymer structures. a PANI nanofibers on carbon cloth; b PANI nanorod arrays on Au plates; c PANI nanorod arrays on carbon nanotube paper; d PPy nanorod arrays on carbon cloth. Adapted from a Ref. [25], b Ref. [153], c Ref. [145], d Ref. [149] with permission nanoparticles were dispersed on ZnO nanorod arrays to form ZnO@NiNRAs, followed by dissolving the ZnO templates to produce NiNTAs. Second, the electropolymerization of PPy on NiNTAs generated NiNTAs@PPy (Fig. 14a, b). Highresolution TEM showed abundant mesopores throughout the PPy layer (Fig. 14c, d). Possibly, the mesopores formed during the polymerization process when the anions were inserted into PPy, and the cations were extracted. The highly porous hollow nanotube arrays in NiNTAs@PPy acted as ion reservoirs that shortened ion diffusion distance.  [157] with permission Therefore, NiNTAs@PPy electrode displayed a high specific capacitance of 474.4 F g −1 at 5 mV s −1 . The PPy nanotubes also had excellent electrochemical stability with 75.3% capacitance retention after 10,000 charge-discharge cycles. The nanotube morphology and many pores and voids in PPy buffered the volumetric change of PPy and facilitated ion diffusion (Fig. 14e), which ensured excellent cycling stability.

Nanoplates
Electrochemically synthesized two-dimensional (2D) conducting polymers are rare because conducting polymers intrinsically prefer to grow into fibers or films. One typical example of 2D conducting polymer made by electropolymerization is pyrene nanosheets (Fig. 15a). They were grown in a mixed electrolyte containing boron trifluoride diethyl etherate (BFEE), trifluoroacetic acid (TFA), and polyethylene glycol (PEG), using a potentiostatic technique (1.2 V vs. SCE) [158]. Spectroscopy revealed that the formation of nanosheets was due to oligomer growth via α-α coupling of pyrene rings. Besides pyrene, PPy nanosheets (Fig. 15b) were synthesized using cyclic voltammetry at a high scan rate of 200 mV s −1 in an aqueous electrolyte containing 0.05 M pyrrole and 0.1 M KNO 3 [159]. These PPy nanosheets interconnected with each other and assembled into a macroporous structure. It had a specific surface area of 37.1 m 2 g −1 and a specific capacitance of 584 F g −1 at 5 mA cm −2 .

3D Networks
Three-dimensional (3D) conductive structures provide large ion-accessible surface areas and abundant pores compared to 2D architectures and, thus, are increasingly popular morphologies of electrochemically deposited conducting polymers. High surface area can effectively reduce the local current density and polarization in bulk electrodes and thus improve the electrodes' charge storage kinetics. Unfortunately, ideal 3D structures composed merely of conducting polymers are challenging to acquire, due to their preferably random growth into films or fibers during electropolymerizations.
To circumvent this challenge, researchers adopt templates to construct 3D conducting polymers. Demonstrated templates, including carbon nanotube foam [163] (Fig. 16a, b), graphene foam [164] (Fig. 18c), partial exfoliated graphite [26] (Fig. 16d), as well as Ni foam [165] (Fig. 16e-g), have been used to construct 3D polymer-based electrodes. For example, Park et al. deposited a PPy film on graphene foam (Fig. 16c) [164]. The high surface area of graphene foam and the pseudocapacitance of PPy synergistically improved the performance of the electrode. Wang et al. reported a PPy foam using a sacrificial Ni foam template (Fig. 16e) [165]. PPy was first electrodeposited on a Ni foam (Fig. 16f), and subsequently, the Ni foam was etched away, leaving a freestanding 3D PPy foam (Fig. 16g). This as-prepared 3D PPy foam was mechanically strong and highly flexible, making it a multifunctional 3D material in sensors, supercapacitors, and supports for graphene. Moreover, the freestanding 3D PPy exhibited a capacitance of 316.2 F g −1 at 2 mV s −1 , and the graphene-coated 3D PPy achieved a higher capacitance of 702.9 F g −1 at the same scan rate. The incorporation of graphene created highly conductive surface coatings as well as increasing specific surface area from 72 to 113.4 m 2 g −1 .

Manganese Oxides
Manganese oxides, particularly manganese dioxide (MnO 2 ), have attracted immense interest as one of the most commercially promising pseudocapacitive materials, due to its high theoretical capacitance (~ 1000 F g −1 ), cost efficiency, source abundance, and environmental friendliness [166,167]. A variety of chemical and electrochemical techniques  [163], c Ref. [164], d Ref. [26], e-g Ref. [165] with permission have synthesized manganese oxides. Among them, anodic electrodeposition is the most time-efficient. This technique deposited MnO x by consecutive oxidation of Mn 2+ , as illustrated in the following equations [168,169]: The nanostructures of electrodeposited MnO x are tunable by varying the electrolyte composition, temperature, potential, and current density. For example, Feng et al. demonstrated that complexing agents such as CH 3 COO − and NH 4 + significantly reduced the charge transfer resistance of the electrooxidation of Mn 2+ [166], changing the morphology of MnO x from 2D nanosheets to 1D nanoneedles. These observations indicate that diminishing charge transfer resistance of MnO x electrodeposition impedes its lateral growth.
Wei et al. proposed a theory of the supersaturation ratio of Mn 2+ to rationalize the diverse morphologies of anodically electrodeposited MnO x (Fig. 17a) [170]. Supersaturation ratio is defined as the ratio of the actual concentrations (or more vigorously speaking, activities) of all the ions associated with electrodeposition to the equilibrium concentrations of the same set of ions. The authors observed that high concentrations of Mn(NO 3 ) 2 and large current densities induced high supersaturation ratios that led to uniform coatings. In contrast, low concentrations of Mn(NO 3 ) 2 and small current densities favored epitaxial growth into interconnected nanosheets (Fig. 17b-f). These different morphologies were associated with the number of nucleates formed at the beginning of electrodeposition. High supersaturation ratios yielded abundant nucleation sites that suppressed epitaxial growth. Parameters that lowered the supersaturation ratio decreased the number of nucleation sites and favored the formation of nanostructures.

Nanorods and Nanotubes
Templating is a typical strategy to prepare 1D MnO x nanostructures [171,172]. ZnO nanorods [173], anodized alumina [174,175], hydrogenated TiO 2 nanorods [176], and silicon square pillars [177] are reported templates for electrodepositing MnO x nanorod or nanotube arrays. For example, Li and coworkers synthesized double-walled carbon/ MnO 2 nanotube arrays using ZnO nanorods as sacrificial templates [173] (Fig. 18a). First, ZnO nanorod arrays were grown on Ti plates via electrodeposition, and thin layers of carbon were coated on the nanorods to render the ZnO nanorods electrically conductive. Afterward, a uniform MnO 2 film was electrodeposited on the carbon-coated ZnO nanorod arrays (Fig. 18b). Finally, dissolving the ZnO nanorod arrays using 0.5 M NaOH solution generated the double-walled carbon/MnO 2 nanotube arrays (Fig. 18c). These double-walled nanotubes displayed a high specific capacitance of 793 F g −1 at 1.5 A g −1 , and a rate capability of 83% when the scan rate increased from 5 to 50 mV s −1 . The excellent electrochemical performances were ascribed to factors including: (1) the hollow structure of the nanotube arrays exposed plentiful active sites of MnO 2 and provided ions with fast diffusion pathways; (2) the conformal carbon coating served as electron transport expressways, minimizing capacitance loss at elevated scan rates; and (3) the high weight fraction of MnO 2 (~ 98.94 wt%) in the electrodes was beneficial to achieve high specific capacitance and energy density.
Besides templating, template-free methods could also synthesize 1D nanostructured MnO x . These methods are time-efficient and can synthesize products of high purity because they lift the needs for template incorporation and removal [178,179]. For example, Lu and coworkers demonstrated that adding dimethyl sulfoxide (DMSO) in the deposition solution of MnO 2 led to MnO 2 nanorod arrays without any templates [180]. They applied a constant anodic current of 0.2 mA cm −2 at 70 °C and used aqueous electrolytes containing 0.01 M manganese(II) acetate, 0.02 M ammonium acetate, and 10 wt% DMSO. The resultant MnO 2 nanorods had diameters between 70 and 100 nm, and lengths up to ~ 1.5 μm (Fig. 18d). Electrodeposition without DMSO only yielded MnO 2 herringbones (Fig. 18e). Though the authors did not justify how DMSO changed the deposit morphology, we hypothesized that the addition of DMSO reduced the supersaturation ratio of Mn 2+ , and thus promoted epitaxial growth of MnO 2 into nanorods. The specific capacitance of the MnO 2 nanorod array was 660.7 F g −1 at 10 mV s −1 , which was ~ 100 F g −1 higher than that of the herringbone structured MnO 2 (564.3 F g −1 ). This capacitance discrepancy was associated with the morphology: The ordered vertically aligned nanorods, compared with the herringbones, reduced the tortuosity and distances for electron transport, which boosted capacitance (Fig. 18f, g).

Nanosheets and Nanoplates
Ultrathin 2D MnO x nanosheets were other common morphologies of electrodeposited MnO x [181][182][183][184][185]. Anodic deposition is widely demonstrated to prepare MnO x nanosheets [186][187][188][189][190][191][192][193][194][195]. For example, Yao et al. deposited MnO 2 nanosheets onto 3D printed graphene aerogel lattices through an anodic galvanostatic deposition at 10 mA cm −2 (Fig. 19a) [196]. The outstanding property of these 3D printed MnO 2 /graphene composite electrodes was their uncompromised electrochemical performance at MnO 2 mass loadings as high as 182.2 mg cm −2 . The areal capacitance scaled linearly with the thickness of the electrode, reaching 44.13 F cm −2 at 0.5 mA cm −2 in 3 M LiCl aqueous electrolytes at a thickness of 4.0 mm (MnO 2 mass loading 182.2 mg cm −2 ). This linear relationship indicated that the charge storage process of the electrode was not under diffusion control or limited by ion percolation even at ultrahigh mass loadings and thicknesses. This merit was attributed to the 3D-printed graphene lattices with macropores of 5-50 μm pores (Fig. 19b, c), which promoted the uniform Fig. 19 a Schemes of the synthesis procedures of MnO 2 nanosheets deposited on 3D printed graphene aerogel lattices. b A top-view SEM image of MnO 2 -coated 3D printed graphene aerogel. c, d SEM images of the deposited MnO 2 nanosheets at two magnifications. Adapted from Ref. [196] with permission deposition of MnO 2 and opened up wide ion diffusion pathways throughout the entire electrodes.
In addition to anodic electrodeposition, cathodic electrodeposition also synthesizes 2D MnO x nanosheets. For example, Beyazay et al. used a chronoamperometry technique to deposit Mn 3 O 4 hexagonal nanosheets on graphene paper. This electrode delivered a maximal specific capacitance of 546 F g −1 at 0.5 A g −1 . Interestingly, the capacitance increased about 1.5 times after being charged and discharged for 10,000 times. XPS analysis after the stability test found that the average valence of Mn raised from 2.7 to 3.2, indicating that part of Mn 3 O 4 was oxidized to MnO 2 . Besides the valence change, some hexagonal nanosheets transformed into particles and needles. These results suggested that the hexagonal Mn 3 O 4 nanosheets were both chemically and structurally unstable during long-term cycling tests.

Hierarchical Structures
Hierarchical MnO x integrates nanostructures of different dimensions, e.g., 1D nanorod, 2D nanosheet, and 2D nanoplate. Electrodepositing hierarchical structures often begins with one specific structure. For example, Jabeen et al. synthesized Mn 3 O 4 nanosheet-on-nanowall arrays via a cathodic potentiostatic method (− 1.8 V vs. Ag/AgCl) in an aqueous solution containing 0.1 M manganese acetate and 0.1 M sodium sulfate. High-resolution scanning electron microscopy revealed that these nanowalls were composed of interconnected nanoparticles (Fig. 20a) [14]. After 500 cycles of electrochemical oxidation in 10 M sodium sulfate aqueous solutions, the nanoparticles disappeared, and nanosheets appeared on the surface of the nanowalls, assembling the nanosheet-on-nanowall hierarchical structure (Fig. 20b, c). Meanwhile, the composition of the electrode changed from Mn 3 O 4 to Na 0.5 MnO 2 . The hierarchically structured Na 0.5 MnO 2 exhibited a specific capacitance of 366 F g −1 at 1 A g −1 . Besides, the redox peak of Na 0.5 MnO 2 at ~ 0.96 V vs. Ag/AgCl extended the upper limit potential to approximately 1.3 V vs. Ag/AgCl, enabling the development of aqueous-based supercapacitors with high voltages and energy densities.
The most significant characteristic of hierarchical structures is their capability to maintain excellent electrochemical performance at MnO x mass loadings exceeding 10 mg cm −2 . Increasing the mass loadings of MnO x (and other pseudocapacitive materials) has become a trend in recent years, due to the consideration of practicality. Unfortunately, the capacitances of poorly conductive pseudocapacitive materials, including MnO x , are greatly compromised when enhancing their mass loadings, particularly under fast charge and discharge rates. Hierarchical structures could resolve this challenge. For example, Song et al. developed an Ostwald ripening strategy that improved the rate capability of electrodeposited MnO x thick films with a high mass loading of ~ 10 mg cm −2 (Fig. 20d-f) [197]. The authors first coated carbon fibers with MnO x films of ~ 4.5 µm thick using a constant current of 10 mA cm −2 in 0.1 M manganese acetate aqueous solutions (Fig. 20d). They then hydrothermally treated the electrodeposited MnO x at 90 °C, which appreciably altered the morphology of the MnO x films. First, many crystalline MnO x nanosheets formed on the surface. Second, the porosity of the MnO x core increased (Fig. 20e, f). The porous MnO x core and oxide shell together constituted a core-shell hierarchical structure. The crystalline surface ensured good electrical conductivity, and the porous MnO x core sped up ion diffusion. Therefore, the electrode exhibited improved rate performance even at a high mass loading of 10 mg cm −2 . Recently, Huang et al. have demonstrated a facile electrochemical technology that synthesized a nanorodon-nanosheet hierarchical structure. The structure consisted of primary two-dimensional ε-MnO 2 nanosheets and secondary one-dimensional α-MnO 2 nanorod arrays ( Fig. 20g-i) [50]. Morphology studies indicated that elevating the deposition temperature to 60 °C and 80 °C added nucleation sites on the as-formed nanorods, which favored the secondary growth of nanorods. This hierarchical electrode had a high MnO 2 mass loading of 10 mg cm −2 and delivered a high areal capacitance of 3.04 F cm −2 at 3 mA cm −2 . Significantly, the areal capacitance maintained at 1.9 F cm −2 at 30 mA cm −2 . The authors ascribed this excellent rate capability performance to two factors: First, the multiple connections between the nanorods and nanosheets created fast avenues for electron transport. Second, the voids among the nanorods and nanosheets throughout the hierarchical structure facilitated electrolyte ion percolation and ion diffusion.

Vanadium Oxides
Vanadium oxides (VO x ), mainly vanadium pentoxide (V 2 O 5 ), have the advantages of high specific capacitance (multiple electron reaction, e.g., from + 3 to + 5), low cost, ease of fabrication, as well as wide potential windows [37,[198][199][200][201][202]. Electrochemical technologies are particularly suitable for synthesizing VO x of diverse morphologies, crystal structures, and valence states [203][204][205][206][207][208][209]. In aqueous electrolytes, vanadium oxide is typically synthesized from the oxidation of vanadium-containing ions with the aid of water molecules. For example, oxovanadium(IV) cations, VO 2+ , are electrooxidized to high-valence vanadium oxides (e.g., V 2 O 5 ) through the following equation [208]: For example, Xie et al. demonstrated that the pH value and composition of acetate salts (CH 3 COONa, CH 3 COOLi, CH 3 COOK) were critical in tuning the deposition rate, crystal structure, and morphology of VO x [209]. Drosos et al. studied the effects of the deposition current density on the morphology and electrochemical performance of V 2 O 5 coatings on indium-doped tin oxide glass substrates in 1 M LiClO 4 polypropylene carbonate electrolytes. The V 2 O 5 film deposited at 1 mA cm −2 exhibited the highest capacitance owing to its roughest surface [210].
Electrodeposited VO x with mixed V valences are platforms for studying the interplays between V valence and cycling stability. Recent studies indicated that the  [14]; d-f Ref. [197]; g-i Ref. [50] with permission performance degradation was linked to dissolution, structural pulverization, and irreversible phase transition of VO x [34,37,199]. Investigating the cycling behavior of electrodeposited VO x electrodes in various aqueous environments, Engstrom and Doyle concluded that the formation of water-soluble V-containing species, including H 2 VO 4 − , HVO 4 2− , HV 2 O 5 − , VO 2+ , HVO 2+ , and VO + (Fig. 21a), was the primary cause of capacitive decay of VO x during extensive cycling tests [199]. Though chemical strategies such as surface coating [211,212] and electrolyte pH value tuning [199] minimized dissolution of VO x , the altered electrode kinetics usually compromised capacitance. To circumvent these limitations, Song et al. utilized a potentiostatic electrochemical method to tune the V valence in VO x and achieved record-high cycling stability without capacitive decay over 100,000 cycles ( Fig. 21b-d) [37]. VO x nanorods were first electrodeposited on electrochemically exfoliated carbon cloth fibers using cyclic voltammetry between − 1.5 and 1.4 V vs. SCE (Fig. 21c). The authors then reduced the asdeposited VO x at a constant potential of − 1.5 V vs. SCE for 1 min. This reduction raised the V 4+ /V 5+ ratio from 0.4 to an optimal value of around 0.5. The optimized V 4+ /V 5+ ratio in VO x effectively suppressed the chemical dissolution of VO x . Meanwhile, the firm anchoring of the reduced VO x nanorods on oxygenated, exfoliated carbon cloth fibers via C-O-V bonds retained the structural integrity of VO x . Both factors contributed to the excellent cycling stability (ECC/ RVO x in Fig. 21d).
The easy valence tuning of VO x by electrochemical techniques allowed the synthesis of heterojunctions between VO x of two valences. These configurations help facilitate electron transfer within VO x electrodes having large thicknesses or high mass loadings [34]. For instance, Dong et al. performed a density functional theory (DFT) calculation and discovered that a built-in electric field formed at the V 5 O 12 / VO 2 heterojunction. The charge redistribution between the two oxides led to an electric field pointing from VO 2 to V 5 O 12 (Fig. 22a) [34]. This built-in electrical filed facilitated electron transfer and modulated ion absorption during charge storage processes, which improved electrochemical performance. Inspired by this calculation result, the authors adopted cyclic voltammetry (− 1.5 to 1.5 V vs. SCE) to electrodeposit V 5 O 12 /VO 2 nanorods on an exfoliated graphite substrate. V 5 O 12 first formed during the positive scan, and it was partially reduced to VO 2 in the subsequent negative scan (Fig. 22b). V 5 O 12 /VO 2 with a high mass loading of about 10.8 mg cm −2 delivered a high areal capacitance of 5.03 F cm −2 (465 F g −1 ) at 1 mA cm −2 , outperforming pure V 5 O 12 and VO 2 (Fig. 22c). Significantly, V 5 O 12 /VO 2 also exhibited enhanced cycling stability compared to V 5 O 12 and VO 2 alone (Fig. 22d). Two reasons could account for this stability enhancement. First, EIS indicated that V 5 O 12 / VO 2 exhibited reduced charge transfer resistance due to the heterojunction (Fig. 22e). Second, ex situ XRD confirmed that V 5 O 12 /VO 2 underwent no phase transitions after charging and discharging, while the phases of V 5 O 12 and VO 2 changed dramatically (Fig. 22f-h). The reduced resistance and suppressed phase change of V 5 O 12 /VO 2 both enhanced cycling stability.
Nanostructured substrates with large ion-accessible surface areas are preferred electrodeposition scaffolds for MoO x . For example, a ~ 18-nm-thick layer of MoO x (3 mg cm −2 ) was deposited on tungsten oxide nanowires, forming a WO 3-x /MoO 3-x core/shell structure (Fig. 23a-d) [226]. This core/shell electrode delivered an areal capacitance of 500 mF cm −2 . Li et al. electrodeposited a 40-nmthick MoO 3 layer (2.43 mg cm −2 ) on ZnO nanorod arrays (Fig. 23e, f). The ZnO-supported MoO 3 displayed a specific capacitance of 241 F g −1 at 5 mV s −1 and 198 F g −1 at 100 mV s −1 [227]. Liu et al. demonstrated that functionalized, partially exfoliated graphite foil substrates could support MoO x films with high mass loadings (18.4 mg cm −2 ) (Fig. 23g) [228]. The exfoliated graphene sheets and the laminar structure of the graphite base addressed the negative impact of the poor electrical conductivity of the atop MoO x . This highly conductive carbon-based structure permitted efficient ion diffusion and fast electron transport. Besides, the O-functional groups on the exfoliated graphite foil formed covalent C-O-Mo bonds with MoO 3 , which served as bridges that permitted fast charge transport from MoO x to the substrate. All the above factors led to excellent rate capability: The optimized electrode with a high MoO x mass loading of 15.4 mg cm −2 delivered an areal capacitance of 4.34 F cm −2 at 1 mA cm −2 and retained 67.8% of the initial capacitance at 20 mA cm −2 .
Films deposited onto fibers can develop unique morphologies, such as helical cracks demonstrated by Lu et al. They utilized a combined electrochemistry-annealing strategy and deposited hierarchical porous MoO 2 films composed of mesoporous nanoparticles on carbon cloth fibers (Fig. 23h,  i)  Subsequently, the as-formed MoO x was annealed in NH 3 at 700 °C to obtain MoO 2 films. Scanning electron microscopy revealed that the MoO 2 films exhibited helical cracks of ~ 100-200 nm wide. Owing to the helical openings that reduced the dead volume of MoO 2 , the electrode delivered a high areal capacitance of 175 mF cm −2 at 1.43 mA cm −2 in Na 2 SO 4 aqueous electrolytes. The formation mechanism of the helical cracks remained unclear, but we hypothesized that it might be associated with dehydration of the electrodeposited films during the thermal treatment in NH 3 . The cracks were initiated by volume shrinkage during annealing and propagated around carbon fibers following a helical path.
Like other pseudocapacitive materials, electrodeposited MoO x exhibits high capacitance, but unsatisfactory cycling instability. To extend the life span of electrodeposited MoO x , Cai et al. reported a potential window tuning strategy for MoO x [38]. They discovered that the potential window within − 1 to − 0.4 V vs. SCE permitted the redox reaction associated with Mo 4+ and Mo 5+ and prevented the formation of Mo 6+ . This feature resulted in no capacitance decay in 30,000 cycles. In contrast, the same electrode scanned between − 1.0 and 0 V vs. SCE irreversibly generated Mo 6+ and its capacitance decayed by more than 25% within 500 charge-discharge cycles. Electrochemical impedance spectroscopy revealed that the accumulation of Mo 6+ increased the combined series resistance of MoO x , which made the electrode electrically insulating. Besides, optimizing the composition of electrodeposited MoO x materials via electrochemical technologies could provide new opportunities for enhancing their durability, as already proved to be successful in stabilizing VO x [34].
Changing electrolyte composition to extend the lower potential limit of WO x electrodes is necessary to increase the capacitance and energy density of WO x -based (9) supercapacitors. As a hydrogen evolution reaction (HER) catalyst, WO x has a low potential of around − 0.5 V vs. SCE [233]. Reducing pH values of electrolytes could push this limit to − 1 V vs. SCE, but at the cost of cycling stability (Fig. 24e) (Fig. 24f) [234]. XRD indicated that a new crystal phase (Na 6 WO 6 ) formed after cycling in the Na 2 SO 4 electrolyte (Fig. 24g), creating internal stress and detaching the active material from the current collector (carbon cloth fibers). While cycled in the mixed electrolyte, the structure of H x WO 3 was well maintained. It was because, in proton-rich electrolytes, H + insertion and de-insertion became the dominant charge storage mechanism, which was less destructive for the structure of H x WO 3 . Moreover, the mixed electrolyte enhanced the capacitance of H x WO 3 , as illustrated by the expanded area enclosed by the CV curve (Fig. 24h). Specifically, when tested in the mixed electrolyte, the electrode exhibited a high areal capacitance Adapted from a-d Ref. [226], e, f Ref. [227], g Ref. [228], h Ref.
[229] with permission of 860 mF cm −2 , corresponding to 143 F g −1 , at 5 mV s −1 (Fig. 24i). The rational design of electrolyte composition enabled electrodeposited WO x electrodes with mutually high capacitance and excellent cycling stability.

Iron Oxides and Hydroxides
Iron oxides and hydroxides are one of the most attractive, lowcost negative electrode active materials for supercapacitors [242][243][244]. Electrodeposition is a simple strategy to prepare nanostructured iron oxide and hydroxide [245]. Typically, Fe 2+ (e.g., Fe(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O) is the iron source [35,36,246,247]. During electrodeposition, Fe 2+ is first oxidized to Fe 3+ on positive electrodes, which then combines with OH − that was present in weakly alkaline electrolytes (pH ~ 8) or dissociated from water reduction to form Fe(OH) 3 deposits. The as-deposited Fe(OH) 3 further dehydrates to FeOOH in air. The associated chemical reactions are [248]: Intensity (a.u.) 10 Fig. 24 a, b SEM images of a electrodeposited tungsten oxide and b tungsten bronze films on carbon cloth. c XRD patterns of tungsten oxide, tungsten bronze, and carbon cloth. d N 2 adsorption-desorption isotherms of tungsten oxide and tungsten bronze. Inset: pore size distributions. e, f Cycling stability of tungsten bronze in e 1 M Na 2 SO 4 and f Na 2 SO 4 /H 2 SO 4 (1 M/0.01 M) mixed aqueous electrolytes. g XRD patterns of tungsten bronze after cycled in Na 2 SO 4 and Na 2 SO 4 /H 2 SO 4 . h CV curves of tungsten bronze in Na 2 SO 4 (green), Na 2 SO 4 /H 2 SO 4 (red), and H 2 SO 4 (blue) aqueous electrolytes. i Rate capability of tungsten bronze in Na 2 SO 4 /H 2 SO 4 . Adapted from Ref. [234] with permission To date, iron oxides of different dimensionalities, including 0D nanoparticles (Fig. 25a) [249], 1D nanorods (Fig. 25b) [250], 2D nanosheets (Fig. 25c) [251], and 3D hierarchical structures (Fig. 25d) [35], have been synthesized electrochemically. For example, Mai et al. reported a cyclic voltammetry method that transformed highly crystalline Fe 2 O 3 nanoparticles into low-crystalline FeOOH nanoparticles (Fig. 25a) [249]. The resultant FeOOH electrode, with a high mass loading of 9.1 mg cm −2 , exhibited an outstanding specific capacitance of 716 F g −1 at 1 A g −1 in 2 M KOH electrolyte. The specific capacitance was attributed to the facilitated ion diffusion kinetics in the FeOOH electrode, but further studies were needed to unveil the mechanism fully.
As electrodeposited iron species are typically Fe(OH) 3 and FeOOH, thermal conversion to Fe 2 O 3 with enhanced electrochemical activity is usually performed for processing supercapacitor electrodes [252]. For example, FeOOH nanoneedles were first deposited on Ni-plated ZnO nanorods using a constant potential of 1.5 V vs. Ag/AgCl [253]. During the electrodeposition, the acidic environment created by Fe 3+ hydrolysis corroded the ZnO nanorods, forming Ni@ FeOOH. Subsequent annealing Ni@FeOOH in Ar at 450 °C dehydrated FeOOH to Ni@Fe 2 O 3 nanoneedles. The Ni nanotubes swiftly conducted electrons between Fe 2 O 3 and the substrate, while the ultrathin thickness of Fe 2 O 3 nanosheets endowed fast charge transfer kinetics. Benefiting from these merits, Ni@Fe 2 O 3 achieved a high specific capacitance of 418 F g −1 at 10 mV s −1 in the potential window from − 0.8 to 0 V vs. Ag/AgCl, excellent rate capability (215 F g −1 at 64 A g −1 ), and cycling stability (93% capacitance retention after 5000 charge-discharge cycles).
In addition to thermal annealing, chemical conversion in hot alkaline solutions represents another means to convert electrodeposited FeOOH into iron oxides. Chemical conversion avoids high-temperature annealing that may trigger structural deformation and particle agglomeration. One excellent example was demonstrated by Sun et al. [35]. They utilized a post-chemical transition method to obtain composite mesoporous iron oxides (Fe 3 O 4 and Fe 2 O 3 ) from electrodeposited FeOOH nanosheets on a 3D exfoliated graphite substrate (EG). First, FeOOH nanosheets were deposited on EG using a potentiostatic method (− 0.5 V vs. SCE for 80 min in an aqueous electrolyte containing 0.05 M Fe(NH 4 ) 2 (SO 4 ) 2 and 0.05 M (NH 4 ) 2 SO 4 ). Afterward, the asdeposited nanosheets were immersed in a 1 M NaOH aqueous solution for 1 h at 70 °C to convert iron oxy-hydroxides to iron oxide (containing Fe 2 O 3 and Fe 3 O 4 ) nanosheets interconnected into a honeycomb-like structure (Fig. 25d). The conversion mechanism is unclear, but we hypothesize that it is associated with dehydration of iron oxy-hydroxides during the treatment. High-resolution SEM image presented that each oxide nanosheet was composed of nanoparticles (Fig. 25d inset). The EG@Fe 3 O 4 /Fe 2 O 3 electrode delivered an ultrahigh areal capacitance of 1.57 F cm −2 at 5 mA cm −2 , corresponding to 165 F g −1 based on the total mass of the electrode. The excellent capacitive performance of the EG@ Fe 3 O 4 /Fe 2 O 3 electrode was ascribed to several factors. First, the hierarchically porous structure with mesopores and macropores in the electrode shortened ion diffusion distance. Second, the heterojunction between Fe 3 O 4 /Fe 2 O 3 introduced a built-in electric field that facilitated charge transfer between different oxide particles. Third, the interconnected graphene sheets on EG constructed highly conductive electron transport networks that minimized capacitance loss at fast discharging rates.

Nickel/Cobalt Oxides and Hydroxides
A critical issue that we would like to highlight first in this section is the concepts of intrinsic and extrinsic pseudocapacitive materials. Intrinsic pseudocapacitive materials refer to materials that always show pseudocapacitive characteristics (CV curves with broad or no redox peaks and plateaufree charge-discharge profiles) irrespective of size. Extrinsic pseudocapacitive materials exhibit pseudocapacitive signatures only at nanoscales. This transition of the electrochemical behaviors of extrinsic pseudocapacitive materials was due to the reduced ion diffusion distance that accelerated charge storage kinetics and, in some cases, suppressed phase transformations [13].

and deeply oxidized O-CoOOH
Introducing a secondary cation, such as Fe 3+ and Al 3+ , in Ni/Co hydroxides to form double hydroxides have been proved to enhance capacitive performance due to charge hopping or valence interchange between different cations [96,266,269,270]. Double hydroxides usually possess layered structures with large interlayer spacings (e.g., ~ 1 nm), which resulted in their high specific capacitance (e.g., > 2000 F g −1 ) [96,106,271]. Unfortunately, their limited potential windows in alkaline electrolytes (~ 0.5 V) and poor rate performance still hinder their large-scale applications in aqueous batteries and supercapacitors. Neutral aqueous electrolytes can suppress appreciable oxygen evolution in alkaline solutions and thus extend the potential window of layered double hydroxides (LDHs) to ~ 1 V.
Since anions residing between the interlayer space and the electrostatic repulsion from positively charge LDH laminate, cation intercalation into LDHs in neutral electrolytes remains thermodynamically unfavorable, resulting in unsatisfactory electrochemical performance. Recently, Li et al. have demonstrated an electrochemical strategy to ease the cation intercalation into LDHs in neutral electrolytes [272]. Co-Fe LDH nanoplates were first electrodeposited on Ni foam using a cathodic electrodeposition method (Fig. 28a). Subsequently, electrochemical activation (EA) of the Co-Fe LDH nanosheets was conducted by CV between 0 and 0.6 V vs. SCE in KOH or NaOH electrolyte (denoted as EA-Co-Fe LDH). The obtained electrode exhibited pronounced electrochemical activity in various aqueous electrolytes, including NaNO 3 , KNO 3 , Ca(NO 3 ) 2 , Mg(NO 3 ) 2 , Zn(NO 3 ) 2 between 0 and 1 V vs. SCE (Fig. 28b, c). The specific capacitance of EA-Co-Fe LDH in NaNO 3 reached 417 F g −1 , which was 27 times higher than that of as-deposited Co-Fe LDH nanoplates. XRD, XPS, FTIR, and X-ray absorption nearedge structure (XANES) all indicated that during the electrochemical activation, Co(OH) 2 in CoFe-LDH was oxidized to CoOOH, resulting in hydrogen vacancies and removal of carbonate anions residing within the interlayer space of Ref. [255], c, d Ref. [267] with permission CoFe-LDH (Fig. 28d). Density functional theory calculations elucidated the adsorption sites of metal ions in LDH and EA-LDH materials (Fig. 28e). The results suggested that the adsorption of metal ions (Li + , Na + , K + , Ca 2+ , Mg 2+ , and Zn 2+ ) on terminal H was unfavorable, whereas adsorptions of the same set of ions on O termination were thermodynamically stable, as reflected from the negative adsorption energy (Fig. 28e, blue bars). The enhanced adsorption tendency with O terminals was ascribed to the H vacancies formed during the activation step. Taking together, the formation of H vacancies and extraction of interlayer anions after the electrochemical activation together imparted the ion intercalation capability of CoFe-LDH in neutral electrolytes.
Not all the active materials are co-electrodepositable in one step. One prerequisite is that the materials to be compounded must be synthesizable under similar electrochemical conditions, including pH value, polarization potential, current density, and temperature. For example, Zou et al. electrodeposited a tungsten oxide and polyaniline composite via one-step cyclic voltammetry in a mixed acidic electrolyte containing tungstic acid and aniline monomers [289]. The success of the co-deposition was owing to the acidic environment needed for both tungsten oxide and polyaniline. In contrast to the densely stacked tungsten oxide particles, the composite possessed widened pores and interconnected particles. This difference in the morphology was ascribed to the incorporated polyaniline, which acted as a structural scaffold that prevented the aggregation of tungsten oxide particles. The composite electrode exhibited enhanced pseudocapacitive performance over a wide potential window from − 0.5 to 0.7 vs. SCE, which combined the potential windows of polyaniline (− 0.1-0.7 V vs. SCE) and tungsten oxide (− 0.5-0.3 V vs. SCE). The synergistic effects between polyaniline and tungsten oxide improved the capacitive performance of the composite. Tungsten oxide offered high pseudocapacitance. Polyaniline created interparticle pores that increased surface area and conducted electrons to maintain superior rate capability to those of bare tungsten oxide or polyaniline.
Another example is a series of molybdenum-tungsten mixed oxide composites co-electrodeposited into titanium dioxide nanotube arrays using a one-step galvanostatic plating [276]. The plating electrolyte was an aqueous mixture of different concentrations of sodium molybdate dihydrate, sodium tungstate dihydrate, ethylenediamine tetra-acetic acid disodium, and ammonium acetate. The as-electrodeposited amorphous Mo-W mixed oxide film was thermally annealed at 450 °C in air to improve its crystallinity. The similarities in the valence state, ionic radius, and electronegativity of W 6+ and Mo 6+ were the keys to the successful co-deposition. XRD and Raman spectroscopy indicated that the fabricated Mo-W mixed oxides were monoclinic, and the crystal structure transited from m-WO 3 -like to β-MoO 3like when increasing the Mo/W ratio (Fig. 29a). Though pure MoO 3 exhibited higher capacitance than the composite electrodes at 20 mV s −1 (Fig. 29b), its rate capability  (Fig. 30a) [285]. MoO x layer was first deposited on graphite foil during the cathodic scan, while the PPy layer was subsequently grown on MoO x in the following anodic scan. Therefore, a layer-by-layer PPy/MoO x structure was obtained after multiple depositions (Fig. 30b-d). In addition, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy detected the reduction in Mo valence and the enhancement of protonation level of PPy, both of which resulted from the strong coupling between PPy and MoO x (Fig. 30h) that are beneficial for enhancing the electrochemical performance. The composite electrode exhibited a specific capacitance of 398 F g −1 at 1 A g −1 , higher than those of PPy (160 F g −1 ) and MoO x (320 F g −1 ) at identical current density (Fig. 30i-k).
Besides one-step co-electrodeposition, composites have been prepared using multistep electrodeposition methods [290][291][292][293][294]. For example, Wang and Cai et al. synthesized a VO x @MoO 3 composite through a two-step electrochemical deposition. VO x nanorods were first grown on carbon cloth by cyclic voltammetry (Fig. 31a) [295]. Subsequently, a thin layer of MoO 3 was coated on the VO x nanorods by a constant current deposition for 9 s (Fig. 31b, c). The composite electrode VO x @MoO 3 displayed an areal capacitance of 1980 mF cm −2 at 2 mA cm −2 , against 1309 mF cm −2 of VO x and 233 mF cm −2 of MoO 3 under identical testing conditions. Fourier transform infrared spectroscopy revealed that the V-O-V peaks of VO x @MoO 3 blueshifted in comparison with that of VO x (Fig. 31d) [285] with permission showed that the binding energies of both V 5+ and V 4+ of VO x @MoO 3 downshifted by 0.2 eV. Both results indicated that the electronic structure and chemical environment of VO x in VO x @MoO 3 were modified by the strong interaction between VO x and MoO 3 , accounting for the improved capacitive performance. Specifically, VO x @MoO 3 reached a high areal capacitance of 1980 mF cm −2 at 2 mA cm −2 , exceeding 1309 mF cm −2 of VO x (Fig. 31f). Strong interaction among the incorporated materials is indispensable in realizing the synergy of the composite electrodes. Poor interactions will not only interrupt electron transport but also induce structural instability of composite [296].

Other Materials
Electrodeposition has occasionally been used to synthesize nano-/microstructured sulfides [297][298][299][300], polyanionic compounds [28], and selenides [301]. These materials typically exhibit higher capacitance than their corresponding oxides due to the different chemical environments created by substituting oxygen near the redox sites, which enhanced electrical conductivity or led to crystal structures favorable for fast ion diffusion, e.g., channels, slits, and pores [28, 297,302,303]. Electrochemical syntheses of these materials usually proceed in electrolytes containing precursors of sulfide, phosphate, or selenide.
Using a one-step electrochemical co-deposition method, Chen et al. electrodeposited ternary nickel-cobalt sulfide nanosheets on carbon cloth [302]. The electrodeposition was conducted in a three-electrode electrolytic cell containing 5 mM CoCl 2 with different concentrations of NiCl 2 (1, 2.5, 5, 7.5, and 10 mM) and 0.75 M thiourea. The deposition technique was cyclic voltammetry within a potential range between − 1.2 and 0.2 V vs. Ag/AgCl at 5 mV s −1 for 15 cycles. SEM displayed that the electrodeposited materials formed a dense array of highly porous nanosheets. Highangel annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) confirmed the uniform distributions of Co, Ni, and S in the deposited nanosheets. The as-deposited sulfide exhibited extrinsic pseudocapacitive characteristics Fig. 31 a, b SEM images of a VO x and b VO x @MoO 3 . c Elemental mappings of VO x @MoO 3 . d FTIR spectra of VO x and VO x @MoO 3 . e V 2p 3/2 XPS spectra of VO x and VO x @MoO 3 . f Areal capacitances of VO x and VO x @MoO 3 as a function of current density. Reproduced from Ref. [295] with permission of symmetric and plateau-free charge-discharge profiles. The sulfide with an optimal Ni-to-Co ratio (Ni-Co-S-4) showed the highest specific capacitance of 1418 F g −1 at 5 A g −1 among all Ni-Co-S compounds. It retained 90.6% of its initial capacitance when the current density increased to 100 A g −1 . Falola et al. synthesized MoS 2 film on a glass carbon electrode using an electrochemical deposition coupled with thermal annealing [300]. The electrochemical deposition was conducted via a CV scan from − 1.2 to 1 V vs. Ag/AgCl in an aqueous electrolyte containing 10 mM (NH 4 ) 2 MoS 4 and 0.2 M KCl at pH = 6.8, following the reaction [304]: The as-deposited MoS 2 was thermally annealed under Ar at 600 °C to increase its crystallinity. The resultant MoS 2 film exhibited a gravimetric capacitance of ~ 500 F g −1 at 0.75 A g −1 . Increasing the film thickness from 50 to 200 nm decreased the capacitance to ~ 100 F g −1 , which could be due to the sluggish ion diffusion kinetics throughout the thick films.
Molybdenophosphate (A-Mo-O-P, A = Na, K, etc.) materials are a family of polyanionic phosphate compounds with open frameworks, which have recently aroused great interest in rechargeable batteries [305][306][307][308]. Similar promising performance is also expected in capacitive applications, albeit seldomly reported. Recently, Song (Fig. 32a). EDS indicated the even contributions of Mo, O, P, and K elements in A-MoPO/EG (Fig. 32b- Fig. 32f, g). Meanwhile, the content of O vacancy increased, as indicated by the enhanced intensity in the electron spin resonance (EPR) spectra (Fig. 32h). Additionally, all Na + in MoPO/EG was fully exchanged by K + and additional K + incorporated, both expanding the lattice spacing of molybdophosphate. Benefiting from the widened layers, as well as the enhanced electrical conductivity brought by the O vacancy, A-MoPO/ EG showed better charge transfer kinetics than MoPO/EG (Fig. 32i). A-MoPO/EG exhibited quasi-rectangular CV curves, even at scan rates up to 200 mV s −1 (Fig. 32j). CV (Fig. 32k), XPS, and inductively coupled plasma mass spectroscopy (ICP-MS) indicated that cation (e.g., Li + , Na + , and K + ) intercalation was the primary charge storage mechanism of A-MoPO/EG. This polyanionic negative electrode exhibited a high specific capacitance of 556 F g −1 at 4.5 A g −1 , a low cutoff potential window limit of − 1.5 V vs. SCE, as well as high electrochemical durability without capacitance decay after 100,000 charge-discharge cycles (Fig. 32l). The authors speculated that the stable potential window of A-MoPO/EG down to − 1.5 V vs. SCE could be due to the reversible K + -intercalation (around − 1.4 V vs. SCE) near the water splitting potential, which suppressed hydrogen gas evolution [309]. Besides electrodeposition, electrochemical exfoliation is applicable to synthesize high-quality two-dimensional (2D) nanomaterials, e.g., MoS 2 , boron nitride, and MXene, from their corresponding bulk materials [310]. One example is 2D titanium carbide, Ti 3 C 2 T x (T = O and OH), belonging to the MXene family [311]. Yang et al. applied a constant potential of 5 V for 5 h that delaminated bulk TiAlC 2 into Ti 3 C 2 T x . The experimental setup was a two-electrode system with the first exfoliation step in an aqueous electrolyte composed of 1.0 M ammonium chloride (NH 4 Cl) and 0.2 M tetramethylammonium hydroxide (TMA·OH). The subsequent delamination in 25 wt % TMA·OH yielded single-or double-layer Ti 3 C 2 T x flakes with sizes up to 18.6 μm. During the electrochemical etching, Cl − ions etched Al and broke the Ti-Al bonds. The intercalation of ammonium hydroxide subsequently opened the edges of the etched materials and triggered the etching. This fluoride-free electrochemical exfoliation process provided a safe and scalable way to synthesize MXenes. As a supercapacitor electrode, the Ti 3 C 2 T x film exhibited an areal capacitance of 220 mF cm −2 (volumetric capacitance 439 F cm −3 ) at 10 mV s −1 , comparable or even superior to those made from traditional wet chemical etching methods.

Summary and Outlook
Electrochemical methods constitute a family of facile, economic, and versatile synthesis technology for a plethora of nano-/microstructured materials as active materials in supercapacitors. It has its unique advantages, challenges, and opportunities as a synthesis tool (Fig. 33). In this review article, we have demonstrated that electrodeposition strategies can synthesize active materials consisting of zero-dimensional, one-dimensional, two-dimensional, and three-dimensional nano-/micromaterials, including particles, rods, tubes, wires, plates, sheets, and hierarchical structures.  To sum up, we have listed below the advantages of electrochemical technologies in preparing nano-/microstructured materials for supercapacitors or, broadly speaking, electrochemical energy storage devices.
(1) Their synthesis conditions are often mild (e.g., under room temperature) without elevated temperatures or ultrahigh pressures that might damage the structural integrity or alter the composition of the deposits and substrates. Additionally, electrochemical synthesis requires no advanced instruments and sophisticated operations, making it highly attainable and readily achievable. (2) Electrodeposition directly and seamlessly incorporates active materials onto current collectors, a feature eliminating the need for polymer binders and conductive additives and easing electrode preparations. (3) Electrodeposition offers facile tunability over the composition, crystal phase, and morphology of the deposited materials via changing depositing conditions, electrolyte compositions, current, voltage, and temperature. (4) Electrodeposition is possible to direct the growth of active components onto user-designed patterns, as it only deposits materials in electrically conductive regions. This deposition selectivity makes electrodeposition particularly suitable to coat active materials on supercapacitors with delicate electrode architectures, such as micro-supercapacitors with interdigitated electrodes [196,312,313]. Additionally, the preferred deposition at ion-accessible locations ensures the high utilization efficiency of the active materials. (5) Electrochemically synthesized materials are usually poorly crystalline or completely amorphous. The amorphous nature and abundant defects sometimes are beneficial for enhancing capacitance.
Despite the above strengths, electrochemical techniques are facing many challenges and difficulties, including: (1) Post-treatments after electrodeposition, which are usually needed to improve the crystallinities of deposits, might alter the mechanical strength, functionality concentrations, crystal structures, or porous structure. For example, electrochemically exfoliated graphene sheets are usually stacks of 2-20 nm thick, instead of few-or single-layer graphene. Therefore, ultrasonication is needed to delaminate the exfoliated graphene into fewlayer graphene sheets. This process, however, will inevitably break the resultant graphene sheets into pieces that will increase sheet-to-sheet contact resistance.
(2) As discussed in Sect. 7, some active materials, e.g., metal sulfides, have not been extensively synthesized directly by electrodeposition. Converting metal oxides into the corresponding sulfides through sulfurization is one possible way. Still, this post-conversion usually involves high temperatures that might encounter problems with thermally unstable compounds or loss of structural water that are critical for charge transport [314]. (3) Controlling the uniformity of deposits remains a grand hurdle for electrodeposition, as edges of the deposition substrates are usually deposited first due to the strong local electrical field.
Projecting forward, we believe the following issues, if adequately addressed, could considerably enhance the electrochemical performance of electrodeposited materials and the practicality of electrodeposition.
(1) Rational design and realization of hierarchical structures with one-step electrodepositions is highly attractive to make high-performance supercapacitor electrodes with high mass loadings. Alternatively, combining electrodeposition strategies with other established materials synthesis methods (e.g., hydrothermal reactions to induce Ostwald ripening [197]) could also be explored to achieve hierarchical structures. (2) Developing substrates with mutually high surface areas and excellent electrical conductivity is preferred to improve the ion diffusion kinetics in supercapacitors, but one must be meticulous about the deposition time and rate to avoid pore clogging. In this respect, selflimiting electrodeposition to control the deposit thickness is necessary [315]. (3) Diversifying the materials synthesized by electrodeposition. For example, metal sulfides have gained increasing attention as a new generation of electrochemical energy storage materials [316][317][318], but it is a pity that electrochemical methods can hardly synthesize them without post-treatments. Besides metal sulfides, electrodeposition can grow electrochemically active, 2D materials beyond graphene (e.g., layered metal oxides/hydroxides, boron nitride, and g-C 3 N 4 ) [319], porous polymers and organic compounds, or pseudocapacitive materials compatible with nonaqueous electrolytes. The room for the development of electrochemical synthesis technologies is undoubtedly immense. (4) The purity of electrodeposited materials is often subpar to those made by vapor phase depositions. The multi-valence could benefit charge storage, but might pose challenges for studies on charge storage mechanisms because differentiating outcomes resulting from collective behaviors of all the active components is nontrivial. (5) Deepening the understandings of the electrodepositions mechanism, including electrochemical nucleation and growth processes, will be highly rewarding to propel the development of electrochemical synthesis. For example, understanding the microscopic, complicated, and transient nucleation processes is imperative for controlling the morphology and uniformity of deposits. Mechanistic studies of electrodeposition, especially those developed recently, are limited. With the aid of in situ imaging technologies practiced in battery communities, the gain of insights into the early stages of electrodeposition will accumulate steadily.
Finally, we would like to stress that electrochemical synthesis technologies by no means can replace any existing materials synthesis methods. On the contrary, electrochemical strategies need to cooperate with other techniques to facilitate the explorations and development of high-performance electrodes in supercapacitors. Considering that electrodes play a central role in charge storage performances, we envision that the advancement and diversification of electrochemical synthesis technologies will directly push the development of electrochemical energy storage devices within and beyond electrochemical energy storage fields.