Polypyrrole-Based Metal Nanocomposite Electrode Materials for High-Performance Supercapacitors

: Metallic nanostructures (MNs) and metal-organic frameworks (MOFs) play a pivotal role by articulating their signiﬁcance in high-performance supercapacitors along with conducting polymers (CPs). The interaction and synergistic pseudocapacitive effect of MNs with CPs have contributed to enhance the speciﬁc capacitance and cyclic stability. Among various conjugated heterocyclic CPs, polypyrrole (PPy) (prevalently knows as “synthetic metal”) is exclusively studied because of its excellent physicochemical properties, ease of preparation, ﬂexibility in surface modiﬁcations, and unique molecular structure–property relationships. Numerous researchers attempted to improve the low electronic conductivity of MNs and MOFs, by incorporating conducting PPy and/or used decoration strategy. This was succeeded by ﬁne-tuning this objective, which managed to get outstanding supercapacitive performances. This brief technical note epitomizes various PPy-based metallic hybrid materials with different nano-architectures, emphasizing its technical implications in fabricating high-performance electrode material for supercapacitor applications.


Background
Modern advancements in the field of flexible electronics have been actively involved in design strategies to replace conventional inorganic semiconductors by organic and hybrid inorganic materials [1].In this context, conjugated organic polymers, along with complementary metallic nanostructures (MNs), have been receiving much attention as promising hybrid components for flexible electronics [2].It was an exciting moment for the scientific community in December 2000, as the pioneering joint research works of three scientists, namely Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa, collectively received the Nobel Prize in Chemistry for the discovery and the development of conductive polymers (CPs), [3].Due to ease of fabrication, mechanical robustness, chemical resistance, excellent electrochemical properties, and comparatively high conductivity (>10 3 S cm −1 ), these CPs have crucial importance in emerging energy storage device applications as an active material.In particular, to construct advanced energy storage systems (ESSs), such as batteries and supercapacitors, functional CPs are directly incorporated as one of the energy storage active materials in addition to inorganic hybrid nanocomposites [4,5].
Depending on the charge transfer mechanism, supercapacitors can be technically categorized into two major types, namely (i) electric double layer supercapacitors (EDLCs) and (ii) pseudocapacitors.In the former case, there is no redox process (non-Faradaic charge transfer) involved, i.e., the charges are stored progressively in the electric double layer formed at the electrode's and electrolyte's interface, whereas in the latter type, a series of reversible and fast sequence of redox reactions (see Equation (1), note: n = integer and e − = electron) (Faradaic charge transfer) can be noticed on the electroactive surfaces: Comparatively, pseudocapacitors can accumulate greater electrochemical storage electricity and demonstrate higher energy density than EDLCs (for an illustration, see Figure 1).Accordingly, the synergistic and tunable complimenting properties of diverse nanoarchitecture metal-organic frameworks (metal oxides/phosphides/sulfides) with conjugated organic polymers, especially polypyrrole (PPy) derivatives, have found widespread application in fabricating electrochemical sensors and energy storage technologies [6][7][8][9].The pioneering research works on PPy from chemical, electrochemical routes by Bocchi and coworkers [10] and Gardini [11] was a remarkable invention, and it contributed exceptional impact to PPy chemistry.Subsequently, plentiful research works are documented on PPy derivatives.In 2006, Akar and coworkers [12] documented optimized polymerization parameters to achieve PPy and its block copolymers with conductivities up to 4000 S cm −1 .The authors claim that the properties of these block copolymers of α,ω-diamine polydimethylsiloxane (DA•PDMS) and PPy can be regulated by adopting ratio parameters to attain unique morphology with ceric ammonium nitrate as an oxidizing agent [12].Pyrrole is a sensitive organic compound known for its rapid aerial oxidation to form autoxidized red tar compounds.The rapidity of aerial oxidation is even quicker when the electronic donating groups are its substituents [13].Despite its sensitiveness and reactivity, pyrroles can skillfully be oxidized to achieve PPy.The unique electronic properties and conductivities of PPy were enough to share the title as "organic metal/synthetic metal/metallic polymer" among other prominent conducting polymers [14].
The literature survey reveals that PPy exists as conducting salt.Deprotonation reaction is feasible after treatment with base, resulting in less conducting or insulating PPy base.The researchers proposed that the molecular structure of deprotonated PPy could encompass both reduced and oxidized forms of pyrrole subunit.Although the presence of polarons and unpaired spins are identified using electron spin resonance (ESR) spectroscopy along with localized positive charges on PPy, the hypothesis about conduction mechanisms is still in contention.The molecular structures of conducting/insulating PPy and rearrangement of electrons in PPy salt for the probable creation of polarons are sketched in Figure 2 [15][16][17][18].Despite semiconducting properties, other CP families, such as polyaniline (PANi), polythiophene (PT) and poly(3,4-ethylene dioxythiophene) (PEDOT), are also well-exhibited pseudocapacitance behaviors.The controlled physicochemical/electrical properties and ease of synthesis make them an ideal material for energy storage applications.To the best of our knowledge, we are the first reporting a concise report that specifically reveals PPy-based metal nanocomposite electrode materials for high-performance supercapacitors.Concerning device construction strategies, both batteries and electrochemical capacitors follow similar prototypes but differ in energy storage mechanisms and applications.In brief, an electrical insulator (separator) often separates two electrodes (current collectors).The design strategy for electrolytic capacitors is usually represented as parallel plate capacitors; dielectric materials often separate the electrodes.The energy storage capability is because of polarization in the existence of an external electric field.Since the energy storage mechanism of electrochemical capacitors is in the capacitor of the electric double layer (an interface between an electrode and an aqueous/non-aqueous electrolyte), the capacitance and energy density of these devices are compatibly larger than electrolytic capacitors.Although electrolytic capacitors have larger cycling efficacy, they suffer from low energy density.Hence, nanostructured CP electrodes with high surface area exhibiting a pronounced pseudocapacitance behavior are widely used to construct electrochemical capacitors [19][20][21].Along with metal-organic frameworks (metal oxides/phosphides/sulfides), these nanostructured CPs display synergistic interaction, resulting in contributing superior supercapacitive properties.Figure 3 represents the device prototype of an electrochemical capacitor made of a CP electrode and its equivalent circuit illustration.Bryan et al. [22] recently briefly outlined the construction of electrochemical capacitors from various CPs and their device engineering strategies.The authors predominantly emphasized the pseudocapacitive performance of PPy with synthetic chemical doping strategies [22].Compared to EDLCs, ion transport in pseudocapacitors is indolent because of faradaic processes.Since CPs possess high electrochemical capacitance and conductivity, it must facilitate facile kinetics, charge carriers, charge mobility, and accessible counterions.By chemical reduction process (n-doping), the insertion of electrons to the conduction band will be carried out.Removal of electrons from the valence band was done by oxidation process (p-doping), which eventually upsurge the concentration of charge carriers [23,24].Consider the ionization of CPs, there will be a difference in the equilibrium geometry of the ionized state and its respective ground state.The ionized state of CPs possesses lower equilibrium geometry than its ground state.This lattice distortion causes the highest occupied molecular orbital (HOMO) to swing upwards, and the lowest unoccupied molecular orbital (LUMO) shifts downwards, forming new energy bands in the bandgap.These newly created energy bands are delocalized over the polymer chain, results in the creation of a charge "island."When the polymer chain is chemically doped, it results in ionization of polymer chains, and the overlapping and delocalization of these islands facilitates the conducting behavior of the polymers [25,26].Since the conjugated sp 2 carbons are the key frameworks of CPs, the semi-metallic properties of CPs resemble the conductivities of 2D graphene or 3D graphite materials.Conversely, a Jahn−Teller-like relaxation (or Peierls transition) will ensure the separation among unfilled and filled portions of the sp 2 band.When graphene is chemically doped, it will also exhibit a bandgap.In contrast to graphene electronic conductivity, to demonstrate increased electronic conductivity in the case of CPs, new energy levels are essential, and they must be added to the gap via doping (see Figure 4 for a schematic illustration).The charge island formation in the case of CPs (especially PPy) is a more divergent doping technique than that of conventional semiconductors [18,27,28].Over the past few decades, the research works on metal-organic frameworks (MOFs) have gained increased importance in conventional gas separation and storage, catalysis, electrochemical sensors, and rechargeable batteries due to their unique surface characteristics, tunable porosities, and electrochemical properties.However, the application of MOFs in supercapacitor's electronic components is diminutive because of its high electrical resistivity.To fix this issue, decorating MOFs with a well-known conductive polymer, PPy (prevalently knows as "synthetic metal"), can contribute favorable solicitations in the construction of electrochemical energy storage devices [29][30][31][32][33].In the chemical doping procedure of PPy, two electrons are removed from the polymer chain, creating a carbodication, or bipolaron, balanced by dopant counterions [23].As the polymer is doped to its saturation limit, progressive bipolaronic states are formed.Their energy bands overlap, creating intermediate band structures that facilitate electronic transport throughout the PPy chain [24].Adopted with permission from [22], ACS, 2016.
Depending on the peaks obtained from cyclic voltammograms, the specific capacitance (Cs) at different scan rates can be calculated by Equation (2): where I is the response current in A, v is the scan rate in V s −1 , ∆V is the potential window in V and m is the mass of two active electrode material in g.In addition, from cyclic voltammograms of the charge/discharge curves, the specific capacitance (Cs) can be calculated from Equation (3): where I is the charge/discharge current in A, ∆t is the discharge time in s, ∆V is the potential window in V and M is the total mass of active electrode material in g [34][35][36][37].

Polypyrrole-Based Hybrid Metallic Nanostructures as Electrode Materials for High Performance Supercapacitors
Hybrid metallic nanostructures embedded with PPy can exert noteworthy effects on the physico-chemical properties and the electrochemical properties due to the unique characteristics of both PPy and metallic nanostructures [38,39].Since the booming progress in fabricating supercapacitor electrode materials from carbonaceous network structures has been proposed and studied by various researchers to accomplish the performances beyond the limitation of carbonaceous materials [40][41][42], Feng et al. [43] proposed nitrogen-doped porous carbon matrix complexed with PPy.The uniformly grown PPy nanospheres on porous carbon matrix surface showed remarkably specific capacitance reaching a value of 237.5 F g −1 with 88.53% discharge after 1000 cycles.The complimenting characteristics of notable mechanical flexibility and high capacitance of PPy was utilized to develop wearable supercapacitors by growing nanotubular arrays with carbon nano-onions on fabric material [44].These PPy-based hybrid nanostructures grown on fabric materials exhibited stretchable characteristics with superior energy storage capacitance (specific capacitance of 64.0 F g −1 ).In addition, 99.0% capacitance was retained even at a strain of 50.0% after 500 stretching cycles [44].
Our main aim is to focus and provide a comprehensive inventory on PPy-based hybrid metallic nanostructures as supercapacitor electrodes; a systematic survey was carried out, and comparative metal-based PPy nanocomposite electrode parameters concerning supercapacitor applications are documented.By using electrospinning technique, Li et al. [45] successfully fabricated the hollow V 2 O 5 fibers by the emulsion of vanadyl acetylacetonate, polyvinylpyrrolidone and polystyrene in N, N-dimethyl formamide followed by sintering in air at 430 • C for 30 min.Furthermore, in order to achieve hollow, capsular PPy fibers on V 2 O 5 , two-step vapor-phase polymerization technique was adopted and the electrodes showed appreciable specific capacitance of 203.0 mV s −1 with over 90.0%capacitance retention after 11,000 cycles at 10.0 A g −1 [45].Dubal et al. [46] reported an inexpensive and straightforward electrodeposition protocol to synthesize nano-brick structures of PPy on stainless steel (SS) substrate.The deposition of PPy nano-bricks was achieved potentiostatically at +0.9 V/SCE for 2 min.These 3D nano-brick PPy structures showed appreciable electrochemical reversibility and a large specific capacitance of 476.0 F g −1 [46].Shinde et al. [47] put forward a new cost-effective chemical bath deposition (CBD) method to synthesize PPy thin film on SS substrate.These instantly grown additive-free and binderless PPy thin films showed maximum achieved specific capacitance value 329.0 F g −1 at 5.0 mV s −1 .Furthermore, the low equivalent series resistance (R s = 1.08 Ω) value reflects negligible ohmic potential drop during the discharge process [47].Since smartly tailored SS mesh shows superior stretchability, Huang et al. [48] fabricated PPy-based solid-state supercapacitors by electrochemically polymerizing pyrrole monomer.The fabricated supercapacitors showed an initial capacitance of 170.0 F g −1 at a relaxed state and 214.0 F g −1 at a 20% strain (at a specific current of 0.5 A g −1 ) [48].
The fabrication of silver nanoparticles/nanoclusters-decorated hybrid PPy (Ag@PPy) nanocomposites were done by Gan et al. [49].The hybrid Ag@PPy nanocomposites demonstrated an enhanced specific capacitance of 414 F g −1 compared to that of the pure PPy electrode (273 F g −1 ).Fine-sized (2-4 nm) silver nanoparticles were initially distributed homogeneously on PPy, which effectually improved the electron hopping system PPy, thus enhancing the capacitance of the PPy.Medium-sized silver nanoparticles (55-100 nm) adhered to the PPy surface, acting as a spacer that minimizes the restacking of PPy.Furthermore, the transport pathway for electrons was shortened by this unique morphology, leading to improved cycling stability and specific capacitance of hybrid Ag@PPy nanocomposites [49].Iqbal et al. [50] performed the oxidative chemical polymerization of pyrrole monomer in FeCl 3 as an oxidant.The authors also prepared the binary (Co 3 O 4 @PPy) and ternary (Ag/Co 3 O 4 @PPy) nanocomposites, in situ synthesis of Co 3 O 4 nanograins and silver nanoparticles along with PPy.The authors revealed spherical, tubular and globular appearances of PPy with Co 3 O 4 and silver nanoparticles (some nanoparticles were also embedded inside the PPy structures).The authors showed that the ternary nanocomposites (Ag/Co 3 O 4 @PPy) demonstrated highest specific capacitance of 355.64 C g −1 compared to binary (Co 3 O 4 @PPy) nanocomposite (280.68C g −1 ) and pure PPy (143.28C g −1 ) [50].
Although the high theoretical capacity of pseudocapacitive transition-metal oxides/hydroxides is promising for supercapacitor electrodes, they suffer severely from lower electrical conductivity and specific capacitance; this is why they are not often used in practical applications [51][52][53].In concern to this, Mao et al. [50] designed onedimensional silver nanowires (AgNWs) with hierarchical nanostructured Ni(OH) 2 archi-tecture.Thesecoaxial hierarchical core-shell structured AgNW@Ni(OH) 2 @PPy hybrid electrode materials exhibited outstanding specific capacitance of 3103.5 F g −1 at 2.6 A g −1 .
To date, this comprehensive performance has been reported to belong dedicatedly to the hybrid Ni(OH) 2 systems with PPy [54].
A thin-film electrode composed of ceramic oxide and yttrium aluminum garnet (YAG: Al 5 Y 3 O 12 ) with PPy was developed by Ehsani et al. [55].This new type of film electrode was fabricated using pulse electrochemical deposition technology, and the electrodes showed enhanced specific capacitance (254.0F g −1 ) compared to a pure Ppy electrode (109.0F g −1 ).The authors also discussed the stability advantages of these thin-film electrodes in aqueous electrolytes over commonly used ruthenium-based perovskites [55].Previous investigations by Ariyanayagamkumarappa and Zhitomirsky showed that chromotropic acid (CHR) is an auspicious dopant material for the preparation of PPy.The PPy films synthesized using CHR dopant showed the highest specific capacitance of 343.0 F g −1 at 2.0 mV s −1 [56].With these inspiring results, Zhu et al. [57] studied the influence of 2,7-Bis(2-sulfophenylazo)chromotropic acid tetrasodium salt (CHR-BS) on the supercapacitor behavior of fabricated PPy electrodes.The CHR-BS doped PPy electrodes showed elite capacitive retention of 109.9% even after 1000 cycles [57] (for the chemical structures of CHR and CHR-BS dopants, please see Figure 5).with PPy and studied its merits as supercapacitor electrodes.The Co 3 S 4 -HNCs@PPy hybrid nanocomposites showed extendable durability with an outstanding specific capacitance of 1706.0F g −1 at 1.0 A g −1 [62].
Diverse research reports have demonstrated the representative metal nanoarchitectures hybrid composites with PPy network including oxides, phosphides, sulfides etc.
The appreciable specific capacitance demonstrated by these PPy-Metal hybrid nanocomposites signifies that these electrode materials are the primary choice for supercapacitor applications.The research contribution involving a variety of hybrid nanocomposites along with PPy were succinctly tabulated in Table 1 with its electrode parameters [41,[45][46][47][48][49][50]55,.

Concluding Remarks
To curtail the environmental and economic effects instigated by the exhaustion of non-renewable energy sources, the use of the most demanding green renewable energy storage devices is intensified, expressly supercapacitors.To summarize, there are several advantages encountered experimentally by using PPy with metal nano architectures as supercapacitor electrode materials that can be helpful in practical device applications.With suitable methods (hydrothermal, in situ polymerization along with metallic nanostructures) and successful implementation of synthetic strategies, this would provide access to develop various nano architectures with diverse physico-chemical properties.In recent years, supercapacitors have attracted a great deal of interest and have emerged as an embedded system for Internet of Things (IoT) applications [123].Due to its unique high energy density and remarkable power efficiency characteristics, supercapacitors could hold a very high electrical charge, replacing the use of batteries in tiny portable devices.

Figure 2 .
Figure 2. Deprotonation reaction of conducting PPy salt under alkaline conditions and rearrangement of electrons in PPy, generation of bipolarons/polarons by delocalization over PPy chain.Note: HX = arbitrary acid, X − = corresponding counter ion, polarons = acts as charge carriers.

Figure 3 .
Figure 3. (a) Schematic illustration of a pseudocapacitor cell and (b) its equivalent circuit diagram that models the electrical behavior of the cell.Adopted with permission from [22], ACS, 2016.

Figure 4 .
Figure 4. Unlike 2D graphene or 3D graphite, polypyrrole (PPy) has a distinct band gap leading to its semiconducting properties.In the chemical doping procedure of PPy, two electrons are removed from the polymer chain, creating a carbodication, or bipolaron, balanced by dopant counterions[23].As the polymer is doped to its saturation limit, progressive bipolaronic states are formed.Their energy bands overlap, creating intermediate band structures that facilitate electronic transport throughout the PPy chain [24].Adopted with permission from[22], ACS, 2016.

Figure 5 .
Figure 5.The chemical structures of (a) chromotropic acid (CHR) disodium salt and (b) 2,7-Bis(2-sulfophenylazo)chromotropic acid tetrasodium salt (CHR-BS).By combining solvothermal reaction and electrodeposition technique, Yang et al. [58] developed hybrid nanosheet arrays having Co 3 O 4 as a core and PPy as a shell.The electrode (Co 3 O 4 @PPy) made of Co 3 O 4 and PPy showed unique synergistic effects and exhibited a large capacitance of 2.11 F cm −2 at a current density of 2.0 mA cm −2 .Furthermore, the equivalent series resistance value of the Co 3 O 4 @PPy hybrid electrode (0.238 Ω) is pointedly inferior to that of the new Co 3 O 4 electrode (0.319 Ω) [58].The ternary nanocomposites developed by Wei et al. [59] were done by electropolymerizing PPy onto flower-like cobalt oxide (f-Co 3 O 4 ) morphological structures coating uniformly on carbon paper (CP).These ternary nanocomposites demonstrated more extended stability in 2.0 M aqueous KOH with a specific capacitance of 398.4 F g −1 [59].To improve the electrochemical performances, Wang et al. [60] proposed the ternary core-shell hetero-structured composites composed of Co 3 O 4 @PPy@MnO 2 , which show a remarkable specific capacitance of 782.0 F g −1 at 0.5 A g −1 .The hybrid composites made of MWCNTs with PPy and Co 3 O 4 show high electrochemical performances, with only 2.9% loss of their initial capacitance after 5000 cycles [61].Since metal sulfides are promising electrode materials for supercapacitor applications, Cheng et al. [62] considered individual synergistic effects of Co 3 S 4 and PPy to construct a hybrid with tangled conductive networks of Co 3 S 4 hollow nanocages (HNCs)with PPy and studied its merits as supercapacitor electrodes.The Co 3 S 4 -HNCs@PPy hybrid nanocomposites showed extendable durability with an outstanding specific capacitance of 1706.0F g −1 at 1.0 A g −1[62].Diverse research reports have demonstrated the representative metal nanoarchitectures hybrid composites with PPy network including oxides, phosphides, sulfides etc.

Table 1 .
Partial list of polypyrrole (PPy)-based fabricated metal nanocomposite electrodes and its parameters for highperformance supercapacitor applications.