Carbon and Related Composites for Sensors and Energy Storage: Synthesis, Properties, and Application

In recent years, mankind’s energy needs have been increasing; therefore, current research is focused on the collection and storage of energy [1]. Thus, there is a need to develop new materials with improved characteristics, such as improved collection and conversion efficiencies as well as improved energy storage properties [2]. This Special Issue, ‘Carbon and Related Composites for Sensors and Energy Storage: Synthesis, Properties, and Application’ of C—Journal of Carbon Research presents state-of-the-art contributions to the preparation and characterization techniques of carbon-related materials in the field of energy storage and sensor applications. Specifically, three review papers and sixteen research papers were included in this Special Issue.

It is well known that carbon and carbon nanomaterials, including 0D quantum dots, fullerenes, 1D carbon nanotubes, 2D graphene, reduced graphene oxide, and other carbon-related nanostructures, have shown unique morphological, electrical, thermal, mechanical, electromechanical, and electromagnetic properties for a wide range of applications [3]. One of the carbon-related materials—carbon fibers—is detailly described in the review paper written by Bisheh and Abdin entitled ‘Carbon Fibers: From PAN to Asphaltene Precursors; A State-of-Art Review’ [contribution 1]. This paper reviews the material properties and use of carbon fibers in various applications and industries and compares them with other existing fillers and reinforcing fibers. In addition, the processing of the carbon fibers and the main challenges in their fabrication were examined. This review demonstrates that low-cost asphaltene-based carbon fibers can be a substitute for expensive polyacrylonitrile/pitch-based carbon fibers for functional applications. The value proposition, performance/cost advantages, potential markets, market size, processing challenges, and methods for overcoming these issues are discussed.

Carbon-related materials are widely used in sensors [4], and this Special Issue contains a review article by Moheimani et al. which discusses current progress in capacitive sensor design approaches [contribution 2]. Such capacitive sensors can have different applications such as electrical capacitance tomography, capacitive voltage sensors, capacitive humidity sensors, capacitive gas sensors, displacement detection, and muscle action for interaction. Contribution 2 presents a the detailed comparison of the properties of flexible capacitance-type proximity sensors fabricated from different nanomaterials, including graphene oxide (GO), carbon microcoils, and carbon nanotubes (CNT).

Deeper state-of-the-art stain sensors for harsh environments were developed by Wang et al. [Contribution 3]. The change in capacitance with pressure or humidity and the problem of dispersion of nanofillers such as graphene and CNT in the polymer matrix are discussed, and some improvements have been proposed to overcome this problem.

Several studies have been published on the use of carbon-based materials in special sensing applications. Nantes et al. developed H₂O₂ sensor based on electrochemical Prussian blue (PB) synthesized from the acid suspension of δ-FeOOH and K₃[Fe(CN)₆] using cyclic voltammetry (CV) and anchored on carbon felt (CF) [contribution 4]. The CF/PB-FeOOH electrode exhibits excellent selectivity for H₂O₂ in the presence of dopamine (DA), uric acid (UA), and ascorbic acid (AA). Thus, the proposed electrodes could be used in electrochemical sensing technologies for various biological and environmental applications.

Ghaffari et al. demonstrated a sensor that could effectively quantify gallium (Ga3+) and indium (In3+) in tap water using square-wave anodic stripping voltammetry was demonstrated by [contribution 5]. This novel electrochemical sensor combines reduced graphene oxide sheets with a bismuth–mercury (Bi/Hg) film electroplated onto pencil graphite electrodes for the high-sensitivity detection of trace amounts of gallium and indium in water samples.

The introduction of the properties of nitrogen vacancy centers in diamonds was proposed by Toural et al. [contribution 6]. Quantitative measurement of external magnetic fields with high sensitivity is possible under optical and radiofrequency excitations. In this paper, a step-by-step process for developing and testing a simple magnetic quantum sensor based on color centers with significant potential for the development of highly compact multisensor systems is described.

Four ferrocene derivatives, 4-ferrocenyl-3-methyl aniline (FMA), 3-Chloro-4-ferrocenyl aniline (CFA), 4-ferrocenyl aniline (FA), and ferrocenyl benzoic acid (FBA), were studied by Ayaz et al. to understand their biochemical actions [contribution 7]. It has been reported that the redox behavior of these compounds is sensitive to pH, concentration, scan number, and scan rate and can be used as electrochemical sensors based on ferrocene as a mediator.

The optical spectroscopic properties and photoinduced antimicrobial activities of carbon dots (CDots) and nanocarbon/organic hybrids were compared in a study by Adcock et al. [Contribution 8]. The samples were obtained by thermal processing citric acid (CA) with an oligomeric polyethylenimine (PEI) precursor mixture under different conditions, including hydrothermal and heating by microwave irradiation, and were compared with PEI–CDots (classically defined CDots of PEI-functionalized pre-existing small-carbon nanoparticles). The optical spectroscopic properties of the samples showed significant similarities; however, significant differences in their visible light-activatedd antibacterial activities were detected.

The use of carbon-based materials as sorbents to remove dyes or ibuprofen was reported by Vashchynskyi et al. [contribution 9] and Ngernyen et al. [contribution 10], respectively. Vashchynskyi et al. proposed the use of apricot-pit-derived carbon sorbents prepared using two-step carbonization and acid treatment [contribution 9]. The obtained sorbents exhibited adsorption activities of 190–235 mg g^-1 and 210–260 mg g^-1 for methylene blue and methyl orange, respectively. Ngernyen et al. used vinasse to synthesize carbon–silica composite with a low-cost silica source available in Thailand (sodium silicate, Na₂SiO₃) [contribution 10]. Tetraethyl orthosilicate (TEOS) was used as a silica source for comparison. A one-step sol–gel process was used to prepare these samples by varying weight ratio of vinasse (carbon source) to silica source (Na₂SiO₃ or TEOS). The maximum adsorption capacities of the Na₂SiO₃-based composites were 406 mg g^-1 and 418 mg g^-1, respectively.

In addition to the aforementioned applications of carbon family materials for sensors, these materials have been widely studied for energy storage in various electrical energy storage devices, such as supercapacitors [5,6], batteries [7,8], and fuel cells [9].

In the current Special Issue, Darabian et al. [contribution 11] and Vieira et al. [contribution 12] reported on the preparation of reduced graphene oxide paper and graphene quantum dots, respectively, using graphite from spent Li-ion batteries. Darabian et al. proposed a simple and low-cost synthesis of graphene quantum dots starting from an alcoholic aqueous suspension of graphene oxide using a hydrothermal route [contribution 11]. In contrast, Vieira et al. used mixtures of sulfuric and phosphoric acids (with different H₂SO₄/H₃PO₄ ratios), leading to the production of materials with significant S and P contents, to obtain GO via the oxidation of graphite recycled from spent Li-ion batteries [contribution 12]. This material reached 155 and 110 F g⁻¹ at 1 and 8 A g⁻¹, respectively, and showed good capacitance retention (~95%) after 1000 cycles.

Daniele et al. proposed a flexible supercapacitor developed from a polyethylene terephthalate substrate, reused from beverage bottles, and conductive ink based on carbon black (CB) and cellulose acetate (CA) [contribution 13]. This supercapacitor was fabricated using carbonaceous and eco-friendly materials composed of 2.45 Wh kg⁻¹ and a specific energy of approximately 1000 W kg⁻¹ for specific power.

The combination of nitrogen-containing graphene and MnO₂ with varying Mn contents was used by Chiu and Cho as electrode materials for flexible asymmetric solid-state supercapacitors [contribution 14]. It has been reported that excessive MnO₂ reduces the conductivity, hindering ion diffusion and charge transfer. However, overloading the electrode with active materials also negatively affects conductivity.

Zallouz et al. studied the effects of aging, thermal treatments, and activation on the structural and other properties of carbon/FeS₂ formation for a supercapacitor electrode [contribution 15]. The aging of the FeS₂ nanoparticles was investigated in air, and a progressive transformation of nano-FeS₂ into hydrated iron hydroxy sulfate with significant morphological modification was observed, resulting in a drastic decrease in capacitance (70%) and retention. In contrast, aging nano-FeS₂ during cycling led to the formation of a supplementary iron oxyhydroxide phase which contributed to the enhanced capacitance (57%) and long-term cycling (132% up to 10,000 cycles) of the device.

The critical roles of the dispersion medium and temperature during the solvothermal synthesis of nitrogen-doped reduced graphene oxide as an active material in supercapacitor electrodes were discussed by Yadav et al. [contribution 16]. Among the different solvents (THF, ethanol, acetonitrile, water, N, N-dimethylformamide, ethylene glycol, or N-Methyl-2-pyrrolidone) as the dispersive medium and different temperatures (60, 75, 95, 120, 150, 180, and 195 °C) used for the preparation of N-doped reduced graphene oxide, the highest 514 F g^-1 at 0.5 Ag^-1 was reported for the material synthesized with N,N-Dimethylformamide at 150 °C.

Chang et al. used silicon carbide microspheres/graphite composites to grow Ni–Co–O nanosheets for energy-storage applications [contribution 17]. Ni–Co–O prepared by electrodeposition on the silicon carbide (SiC) microspheres/graphite composite presented a larger cyclic voltammetry (CV) curve area than that of Ni–Co–O grown on graphite only, which can be attributed to the synergistic effects between the Ni–Co–O nanosheets and silicon carbide microspheres.

The utilization of porous carbon in Li-S batteries was reported by Shu et al. [contribution 18]. In this study, nitrogen/oxygen dual-doped porous carbon (N/O-PC) was synthesized by annealing a precursor of zeolitic imidazolate framework-8 grown in situ on MWCNTs (ZIF-8/MWCNTs). The interconnected porous carbon-based structure facilitates electron and ion transfer, and the S/N/O-PC cathode exhibits high cycling stability in the LiCF₃SO₃ electrolyte (a stable capacity of 685.9 mA h g⁻¹ at 0.2 C after 100 cycles).

In addition to the use of carbon materials in supercapacitors and batteries for energy storage, Ohta et al. proposed the use of carbon nanowalls as catalyst support materials in polymer electrolyte fuel cells (PEFC) [Contribution 19]. It was concluded that voltage losses could be mitigated by increasing the height of the carbon nanowalls, which could improve the power generation of the PEFC.

In summary, this Special Issue, ‘Carbon and Related Composites for Sensors and Energy Storage: Synthesis, Properties, and Application’ of C—Journal of Carbon Research, compiles a series of 19 original research articles and review papers that provide new insights into the preparation of carbon-related materials and their possible sensing applications as well as energy storage applications. We hope that the articles published in this Special Issue will improve the readers’ general understanding of the research in this rapidly developing scientific field.