Morphological modulation of polypyrrole thin films through oxidizing agents and their concurrent effect on supercapacitor performance

Conductive Polymer-Based Electrodes and Supercapacitors: Materials, Electrolytes, and Characterizations

New materials and the interactions between them are the basis of novel energy storage devices such as supercapacitors and batteries. In recent years, because of the increasing demand for electricity as an energy source, the development of new energy storage materials is among the most actively studied topics. Conductive polymers (CPs), because of their intrinsic electrochemical activity and electrical conductivity, have also been intensively explored. While most of the high capacitance reported in the literature comes from hybrid materials, for example, conductive polymers composed of metal oxides and carbon materials, such as graphene and carbon nanotubes, new chemistry and the 3D structure of conductive polymers remain critical. This comprehensive review focuses on the basic properties of three popular conductive polymers and their composites with carbon materials and metal oxides that have been actively explored as energy storage materials, i.e., polypyrrole (PPy), polyaniline (PANi), and polythiophene (PTh), and various types of electrolytes, including aqueous, organic, quasi-solid, and self-healing electrolytes. Important experimental parameters affecting material property and morphology are also discussed. Electrochemical and analytical techniques frequently employed in material and supercapacitor research are presented. In particular, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are discussed in detail, including how to extract data from spectra to calculate key parameters. Pros and cons of CP-based supercapacitors are discussed together with their potential applications.

In-situ composited g-C3N4/polypyrrole nanomaterial applied as energy-storing electrode with ameliorated super-capacitive performance

Graphitic carbon nitride (g-C3N4) and polypyrrole (ppy) nanocomposites are synthesized and cast off as material for electrodes intended for energy storage, where the amount of pyrrole is being kept static after optimization by altering the amount of g-C3N4 to make a series of g-C3N4/ppy (pcn) nanocomposites. These nanocomposites are successfully synthesized by employing in-situ oxidation polymerization by oxidizing pyrrole. The nanocomposites are further characterized by Fourier transform infrared spectroscopy (FT-IR) for structural investigation, thermal gravimetric analysis (TGA) for thermal stability analysis, and field emission scanning electron microscopy (FESEM) and transmission electron microscopy (HR-TEM) for surface morphological scrutiny. The electrochemical measurements of the series are inspected with the help of galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) measurements. It is detected that 0.4 pcn has the highest specific capacitance value of 555 F g-1 at 10 mV s-1 scan rate through CV and 475 F g-1 at a current density of 0.5 A g-1 through GCD in 1 M H2SO4 in contrast with neat g-C3N4 as well as ppy where both the precursors have this value below 100 F g-1. This composite exhibited good cyclic stability with high retention. The high energy density of 0.4 pcn composite is analyzed at 86 Wh/kg at a power density of 300 W/kg. Due to facile synthesis, significant specific capacitance, and excellent energy density, pcn is a promising candidate for its application in energy storage purposes.

Recent advances on surface mounted metal-organic frameworks for energy storage and conversion applications: Trends, challenges, and opportunities

Establishing green and reliable energy resources is very important to counteract the carbon footprints and negative impact of non-renewable energy resources. Metal-organic frameworks (MOFs) are a class of porous material finding numerous applications due to their exceptional qualities, such as high surface area, low density, superior structural flexibility, and stability. Recently, increased attention has been paid to surface mounted MOFs (SURMOFs), which is nothing but thin film of MOF, as a new category in nanotechnology having unique properties compared to bulk MOFs. With the advancement of material growth and synthesis technologies, the fine tunability of film thickness, consistency, size, and geometry with a wide range of MOF complexes is possible. In this review, we recapitulate various synthesis approaches of SURMOFs including epitaxial synthesis approach, direct solvothermal method, Langmuir-Blodgett LBL deposition, Inkjet printing technique and others and then correlated the synthesis-structure-property relationship in terms of energy storage and conversion applications. Further the critical assessment and current problems of SURMOFs have been briefly discussed to explore the future opportunities in SURMOFs for energy storage and conversion applications.

One-Step Accelerated Synthesis of Conducting Polymer/Silver Composites and Their Catalytic Reduction of Cr(VI) Ions and p-Nitrophenol

In this paper, silver nitrate was used as an oxidant to prepare polyaniline, polypyrrole, and poly(3,4-ethylene dioxythiophene)/silver composites through a simultaneous oxidation/reduction process. In addition, p-phenylenediamine was added with 1 mole% relative to the concentrations of the monomers to accelerate the polymerization reaction. The prepared conducting polymer/silver composites were characterized by scanning and transmission electron microscopies to study their morphologies; Fourier-transform infrared and Raman spectroscopies to confirm their molecular structures; and thermogravimetric analysis (TGA) to study their thermal stabilities. The silver content in the composites was estimated by energy-dispersive X-ray spectroscopy, ash analysis, and TGA. The conducting polymer/silver composites were utilized for the remediation of water pollutants through catalytic reduction. Hexavalent chromium ions (Cr(VI)) were photocatalytically reduced to trivalent chromium ions, and p-nitrophenol was catalytically reduced to p-aminophenol. The catalytic reduction reactions were found to follow the first-order kinetic model. Among the prepared composites, polyaniline/silver composite has shown the highest activity for the photocatalytic reduction of Cr(VI) ions with an apparent rate constant of 0.226 min^-1 and efficiency of 100% within 20 min. Additionally, poly(3,4-ethylene dioxythiophene)/silver composite showed the highest catalytic activity towards the reduction of p-nitrophenol with an apparent rate constant of 0.445 min^-1 and efficiency of 99.8% within 12 min.

Substrate-independent three-dimensional polymer nanosheets induced by solution casting

Nanosheets are important structures usually composed of inorganic materials, such as metals, metal oxides, and carbon. Their creation typically involves hydrothermal, electrochemical or microwave processes. In this study, we report a novel formation mechanism of 3D polymer nanosheets via facile solution casting using a comb copolymer consisting of poly(ethylene glycol) behenyl ether methacrylate and poly(oxyethylene) methacrylate (PEGBEM–POEM). Controlling the composition of comb copolymer yielded nanosheets with different packing density and surface coverage. Interestingly, the structure exhibits substrate independence as confirmed by glass, inorganic wafer, organic filter paper, and porous membrane. The formation of 3D nanosheets was investigated in detail using coarse-grained molecular dynamics simulations. The obtained polymer nanosheets were further utilized as templates for inorganic nanosheets, which exhibit high conductivity owing to interconnectivity, and hence have promising electronic and electrochemical applications.

Unprecedented substrate-independent polymeric 3D nanosheets were induced via simple solution casting using PEGBEM–POEM comb copolymer. A possible mechanism is the change in the polymer–solvent interactions on the surface.

Structural Tuning of a Flexible and Porous Polypyrrole Film by a Template-Assisted Method for Enhanced Capacitance for Supercapacitor Applications

Constructing a rational electrode structure for supercapacitors is critical to accelerate the electrochemical kinetics process and thus promote the capacitance. Focusing on the flexible supercapacitor electrode, we synthesized a three-dimensional (3D) porous polypyrrole (PPy) film using a modified vapor phase polymerization method with the use of a porous template (CaCO₃). The porous design provided the PPy film with an improved surface area and pore volume. The porous PPy film electrode was studied as a binder-free electrode for supercapacitors. It was found that the abundant interpenetrated pores created by the CaCO₃ templates within the 3D framework are beneficial to overcoming the diffusion-controlled limit in the overall electrochemical process. It was revealed by electrochemical investigation that a more pseudocapacitive contribution than diffusion-controlled process contribution was observed in the total charge in the redox reaction. The galvanostatic charge/discharge (GCD) measurements showed that the optimized 3D porous PPy film electrode delivered a high capacitance of 313.6 F g^-1 and an areal capacitance of 98.0 mF cm^-2 at 1.0 A g^-1 in a three-electrode configuration, which is nearly three times that of the dense counterpart electrode synthesized in the absence of the CaCO₃ template. A specific capacitance of 62.5 F g^-1 at 0.5 A g^-1 and 31.1 F g^-1 at 10 A g^-1 was obtained in a symmetric capacitor device. In addition, the porous structure provided the PPy film with the attractive capability of accommodating the volume change during the doping/dedoping process. This is essential for the PPy film to maintain a long cycling life in a practical operation for a supercapacitor. It turned out that a high capacitance retention up to 81.3% after 10,000 GCD cycles was obtained for the symmetric supercapacitor device with the 3D porous PPy electrode (57.1% capacitive retention was observed for the dense PPy electrode). The strategy and the insight analysis are expected to provide valuable guidance for the design and the synthesis of flexible and wearable film electrodes with high performance.

Electrolyte selection for supercapacitive devices: a critical review

Electrolytes are one of the vital constituents of electrochemical energy storage devices and their physical and chemical properties play an important role in these devices' performance, including capacity, power density, rate performance, cyclability and safety. This article reviews the current state of understanding of the electrode-electrolyte interaction in supercapacitors and battery-supercapacitor hybrid devices. The article discusses factors that affect the overall performance of the devices such as the ionic conductivity, mobility, diffusion coefficient, radius of bare and hydrated spheres, ion solvation, viscosity, dielectric constant, electrochemical stability, thermal stability and dispersion interaction. The requirements needed to design better electrolytes and the challenges that still need to be addressed for building better supercapacitive devices for the competitive energy storage market have also been highlighted.

Cellulose membrane modified with polypyrrole as an extraction device for the determination of emerging contaminants in river water with gas chromatography-mass spectrometry

In this study, a simple, efficient, and reusable device based on cellulose membranes modified with polypyrrole was developed to extract 14 emerging contaminants from aqueous matrices. For chemical polymerization, a low-cost cellulose membrane was immersed in 0.1 mol/L pyrrole and 0.5 mol/L ammonium persulfate for 40 min in an ice/water bath. The cellulose membranes modified with polypyrrole were accommodated in a polycarbonate holder suitable for solid-phase extraction disks. Solid-phase extraction parameters that affect extraction efficiency, such as sample volume, pH, flow rate, and desorption were optimized. Subsequently, determination of target compounds was performed by gas chromatography with mass spectrometry. The linear range for analytes ranged from 0.05 to 500 μg/L, with coefficients of determination above 0.990. The limits of quantification varied between 0.05 and 10 μg/L, with relative standard deviations lower than 17%. The performance of the proposed cellulose membranes modified with polypyrrole device for real samples was evaluated after extraction of emerging contaminants from a river water sample from the city of Curitiba, Brazil. Bisphenol A (6.39 μg/L), caffeine (17.83 μg/L), and paracetamol (19.28 μg/L) were found in these samples.

Conductivity and dielectric analysis of nanocolloidal polypyrrole particles functionalized with higher weight percentage of poly(styrene sulfonate) using the dispersion polymerization method

Nanocolloidal polypyrrole/poly(styrene sulfonate) PPy:PSS composites were prepared by dispersion polymerization of pyrrole with 0.5 wt%, 1 wt%, 2.5 wt%, 5 wt%, 10 wt% and 15 wt% of PSS. Higher doping level of PPy was confirmed with increased S/N value of elemental analysis. Morphological variations of PPy composite matrix based on PSS were analyzed in which spherical shaped PPy particles of 20–40 nm were obtained for 1:1 wt% of PPy:PSS. Presence of higher concentration of PSS within the PPy matrix substantially improved its thermal stability. Dielectric properties were investigated using complex impedance analyzer as a function of frequency (50 Hz–5 MHz) and temperature between 30°C and 120°C. PPy, with improved dispersion, showed higher dielectric constant values up to 15 wt% of anionic polyelectrolyte PSS and the dielectric loss varied between 4.7 and 7.9 for different wt% of PSS. AC conductivity (σac) enhanced up to 1:1 wt% of PPy:PSS composite, which is found to be the optimum wt% in this study. DC conductivity was found to decrease after 1:1 wt% of PPy:PSS composite, which is due to excess oxidation, leading to reduced π conjugation of PPy chains. Higher dielectric constant values of composite, with relatively low dielectric loss values, indicate their potential usage in the electric and electronic industry.

Functionalization of Polypyrrole Nanopipes with Redox-Active Polyoxometalates for High Energy Density Supercapacitors

Hybrid materials are very attractive for the fabrication of high-performance supercapacitors. Here, we have explored organic-inorganic hybrid materials based on open-end porous 1 D polypyrrole nanopipes (PPy-NPipes) and heteropolyoxometalates (phosphotungstate ([PW₁₂ O₄₀ ]^3- , PW₁₂ ) or phosphomolybdate ([PMo₁₂ O₄₀ ]^3- , PMo₁₂ )) that display excellent areal capacitances. Two different hybrid materials (PMo₁₂ @PPy and PW₁₂ @PPy) were effectively synthesized and used for symmetric supercapacitors. The anchoring of the inorganic nanoclusters onto the conducting polymer nanopipes led to electrodes that stood up to our best expectations exhibiting outstanding areal capacitances that are almost 1.5 to 2 fold higher than that of pristine PPy-NPipes. In addition, symmetric cells based on PMo₁₂ @PPy and PW₁₂ @PPy hybrid electrodes were fabricated and showed significant improvement in cell performance with very high volumetric capacitances in the range of 6.3-6.8 F cm^-3 (considering the volume of whole device). Indeed, they provide extended potential windows in acidic electrolytes (up to 1.5 V) which led to ultrahigh energy densities of 1.5 and 2.2 mWh cm^-3 for PMo₁₂ @PPy and PW₁₂ @PPy cells, respectively. Thus, these unique organic-inorganic hybrid symmetric cells displayed extraordinary electrochemical performances far exceeding those of more complex asymmetric systems.

Bioinspired Fabrication of Free-Standing Conducting Films with Hierarchical Surface Wrinkling Patterns

Mechanical instability has been shown to play an important role in the formation of wrinkle structures in biofilms, which not only can adopt instability modes as templates to regulate their 3D architectures but also can tune internal stresses to achieve stable patterns. Inspired by nature, we report a mechanical-chemical coupling method to fabricate free-standing conducting films with instability-driven hierarchical micro/nanostructured patterns. When polypyrrole (PPy) film is grown on an elastic substrate via chemical oxidation polymerization, differential growth along with in situ self-reinforcing effect induces stable wrinkle patterns with different scales of wavelengths. The self-reinforcing effect modifies the internal stresses, hence PPy films with intact wrinkles can be removed from substrates and further transferred onto target substrates for functional device fabrication. To understand the buckling mechanics, we construct a model which reveals the formation of hierarchical wrinkle patterns.

A review of electrolyte materials and compositions for electrochemical supercapacitors

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

Controlled Fabrication of Polypyrrole Surfaces with Overhang Structures by Colloidal Templating

Here we present the fabrication of polypyrrole (PPy) surfaces with a controlled overhang structure. Regularly structured PPy films were produced using interfacial polymerization around a sacrificial crystalline colloidal monolayer at the air/water interface. The morphology of the final inverse colloidal PPy film is controlled by the amount of monomer, the monomer: oxidant ratio and polymerization time. The PPy films exhibit an overhang structure due to depth of particle immersion in the water phase. As a result of the overhang structure, the PPy films are made hydrophobic, although the material itself is hydrophilic. The apparent contact angle of water on the structured surfaces is 109.5°, which is in agreement with the predicted contact angle using the Cassie-Baxter equation for air-filled cavities. This fabrication technique is scalable and can be readily extended to other systems where controlled wettability is required.

Ag-doped manganese oxide prepared by electrochemical deposition on carbon fiber for supercapacitors

An Ag-doped MnO_x electrode with high specific capacitance of 825 F g⁻¹ has been fabricated by a cathodic deposition method for supercapacitors.

Analysis of photocurrent and capacitance of TiO2 nanotube–polyaniline hybrid composites synthesized through electroreduction of an aryldiazonium salt

TiO₂ nanotube–polyaniline hybrid composites were synthesized using an aminophenyl under-layer electrochemically grafted on TiO₂ obtaining improvements in photocurrent and capacitance.