Polyindole batteries and supercapacitors
Graphical abstract
Introduction
The first attempt to observe electrical conductivity from organic polymers dates back in the 1970s when Shirakawa, Heeger and MacDiarmid group doped polyacetylene with halogen vapors of chlorine, bromine, or iodine [1]. This was an incredible discovery as it meant that the widely known insulating materials (the polymers) could be made conductive. A Nobel Prize in chemistry was later awarded to them in 2000 for their discovery [2]. To this date, conducting polymers (CPs) and their constituent composites have attracted enormous research attention for use in various applications such as energy [3], [4], [5], [6], biomedicine [5,7,8], sensors [9], [10], [11], [12], corrosion protection [13], [14], [15], [16] and many others. The widely anticipated applications have led to the discovery of various other CPs including; polypyrrole (Ppy) [17], [18], [19], [20], polyaniline (Pani) [21], [22], [23], polyindole (Pind) [24], polythiophene [25,26], poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [27,28], and many others together with their substituted derivatives. CPs exhibit tuneable conductivity, excellent capacitive properties, have relatively simple components (such as C, H, N or S), and are very easy to synthesize. These properties make CPs reliable material sources for their voted applications, especially in electrochemical energy storage.
The most commonly studied electrochemical energy storage devices include batteries and supercapacitors [29], [30], [31], [32], [33], [34], [35]. The fundamental difference between the two devices is in their performance [36]. Batteries show relatively high energy density (150–200 W h/kg) but low power density (less than 1000 W/kg), while supercapacitors exhibit a high power density (greater than 10 kW/kg) and an increased lifespan (104–105 cycles), but reduced energy density (less than 10 W h/ kg) [37]. Therefore, the applications of the two storage devices tend to vary due to the amount of charge, which can be possibly delivered by each, in a specific time period. The common electrode materials for batteries and supercapacitors include; carbon-based materials [38], [39], [40] (such as carbon nanotubes (CNTs), graphene, carbon black (CB), activated carbon, etc.), electroactive 2D materials [41,42] and transition metal hydro (oxides) [43,44] (such as MXene, RuO2, MnO2, Co3O4, NiO, Co(OH)2, Ni(OH)2), metal–organic frameworks (MOFs) [45,46], etc.), ion-exchange, conducting, and redox-active polymers [47](such as Pani, Ppy, Pind, PEDOT:PSS, and their derivatives, etc.). Carbon-based electrodes have high conductivity, excellent chemical and cycling stability and high-power density [48], [49], [50], [51], [52]. However, the energy density of carbon-based electrodes is usually low as compared to CPs and some transition metal hydro (oxides), which have a high energy density but limited power density [53], [54], [55]. This is due to the carbon materials’ energy storage mechanism which depends much on the material's surface area [56], [57], [58]. Moreover, transition metal hydro (oxides) have problems like low conductivity, high cost and limited natural abundance [59], [60], [61], [62], [63], [64]. In contrast, CPs demonstrate increased charge storage efficiencies as active electrode materials, with competitive charge-discharge abilities and increased lifecycles due to their ability to utilize the reversible redox reactions at the electrode surfaces [29,65]. In addition, CPs show high conductivity, high charge storage ability, wide voltage window, ease of synthesis, low cost, and environmental friendliness. These properties make CPs suitable materials for the fabrication of electrodes for electrochemical energy storage applications [[29], [30], [31], [32], [33], [34],66].
The commonly studied CPs for energy storage applications are Pani [67], [68], [69] and Ppy [67,70] with Pind quickly starting to catch research attention in this field (Fig. 1A). Pind is interesting because of many factors. The properties of poly (para-phenylene) and Ppy can all be self-possessed by Pind [71]. This is because indole has a fused aromatic molecular structure which gives Pind a structure similar to that of Ppy and poly (para-phenylene) (Fig. 1B) [71], [72], [73], [74], [75]. Ppy has good electrical conductivity, environmental stability, good cycling stability [76,77]. In addition, Poly (para-phenylene) on the other hand, has the advantage of excellent thermal stability [73]. When employed in energy storage applications, Pind shows a slow hydrolytic degradation (as compared to Pani), competitive redox potential (as compared to Ppy) [78], high cycling and thermal stability [24,79] making it superior for energy storage for long run [80]. In addition, Pind does not involve the formation of any salt during complete charged or discharges phase, such as leucoemeraldine and pernigraniline as it is a common case with Pani [81]. This reduces the resistance which would have been offered by these salts and results in increased internal conductivity of Pind [82]. However, the conductivity of Pind is less by two orders of magnitude compared with that of Pani and Ppy, though as good as the two in both environmental and electrochemical stability [83]. Other Pind properties include high stable redox activity [84], good photoluminescent properties [85], fast switchable electrochromic ability [84,86,87] and air-stable electrical conductivity in the doped state [88]. With these properties, Pind is emerging as a good candidate for applications in various domains, such as organic electronics [79], electrocatalysis/catalysis [89,90], batteries and supercapacitors [91,92], anti-corrosion coatings [93] and sensors [94].
Having provided a brief introduction about CPs, batteries, supercapacitors and Pind, we now focus strictly on Pind in our next sections with its application in batteries and supercapacitors as an active electrode material. The next sections are as follows; Section 2 describes the synthesis (polymerization) procedures of Pind, Section 3 discusses the characterization of Pind, Section 4 reviews the applications of Pind and Pind-based composites in supercapacitors and batteries. Section 5 provides the outlook and conclusions with regards to Pind and its applications for future energy storage.
We explicitly note that only two articles have tried to summarize Pind-related literature. The article from Zhou et al. [24] attempted to review the overall progress of Pind research with reference to its synthesis, polymerization mechanisms, properties, and applications. In their review, the applications of Pind in electrochemical energy storage was made brief, since they attempted to cover many different topics. Another article is from Mudila et al. [92] which summarized only some of the Pind supercapacitors. The article was made brief, did not highlight most of the interesting recent works and lacked depth with regards to articulating the direction of innovation of Pind supercapacitors. This article is different from those two in that it focuses on batteries and supercapacitor electrodes of Pind and in a much detailed approach.
Section snippets
Synthesis of Pind
The synthesis of Pind is carried out via various polymerization routes including; chemical oxidative polymerization [71,74,[95], [96], [97], [98]], electrochemical polymerization [79,94,99,100] and emulsion polymerization [101], [102], [103]. These polymerization methods are capable of giving Pind with different morphologies including; nanowires [104], [105], [106], nanorods [103,107], nano- and micro-fiber [85,108], nano- and micro-spheres [96,98,101,103,109], and nanobelts [105]. Table 1
Morphology
The characteristic SEM morphologies of Pind CP is provided in Table 1. It can range from nanowires, nanorods, nano- and micro-fibers, nano- and micro-spheres, to crystalline nanobelts, all depending on the synthesis technique employed. The named physical structures can exist in agglomerated morphologies with smooth surfaces [85,96,98,101,[103], [104], [105], [106], [107], [108], [109]]. The agglomerated morphologies are due to the involved solution stages of Pind synthesis which usually involve
Characterization and metrics
To study the capacitive properties of the Pind electrode materials, cyclic voltammetry (CV) and galvanostatic charging-discharging (GCD) measurement techniques are commonly employed. With these techniques, one can quantify the amount of charge the electrodes can store and the charge-discharge cycles they can withstand during active use. The recorded electrochemical results from CV and GCD at a particular scan rate and the selected number of cycles are calculated from the equations below as
Summary and future directions
The article has systematically reviewed Pind CP with respect to its synthesis, characterization and application as active electrode material in batteries and supercapacitors. In comparison to Pani and PPy CPs, Pind is in an excellent position to compete well primarily in terms of capacitance, capacity and excellent retention across thousands of cycles when employed in these electrochemical energy devices. The Ragone plot of Pind and its composites is shown in Fig. 8, comparing them with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by the National Natural Science Foundation of China (61705258). I. M is grateful the QUT PhD scholarship support. M.T acknowledges the research support from the Australian government research training program (AGRTP).
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