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Article

Fomes fomentarius as a Bio-Template for Heteroatom-Doped Carbon Fibers for Symmetrical Supercapacitors

by
Daria Chernysheva
1,*,
Maksim Konstantinov
1,
Ekaterina Sidash
1,
Tatiana Baranova
1,
Victor Klushin
1,
Denis Tokarev
1,
Veronica Andreeva
1,
Evgeny Kolesnikov
2,
Vasily Kaichev
3,
Mikhail Gorshenkov
2 and
Nina Smirnova
1,*
1
Research Institute “Nanotechnologies and New Materials”, Platov South-Russian State Polytechnic University (NPI), 346428 Novocherkassk, Russia
2
Department of Functional Nanosystems and High-Temperature Materials, National University of Science and Technology MISiS, 119049 Moscow, Russia
3
Department of Catalysis Research, Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
*
Authors to whom correspondence should be addressed.
Symmetry 2023, 15(4), 846; https://0-doi-org.brum.beds.ac.uk/10.3390/sym15040846
Submission received: 23 February 2023 / Revised: 24 March 2023 / Accepted: 28 March 2023 / Published: 1 April 2023
(This article belongs to the Special Issue Electrochemical Energy Conversion and Storage Device Based on Symmetrical Electrode Materials)
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Abstract

:
Nowadays, commercial electric double-layer supercapacitors mainly use porous activated carbons due to their high specific surface area, electrical conductivity, and chemical stability. A feature of carbon materials is the possibility of obtaining them from renewable plant biomass. In this study, fungi (Fomes fomentarius) were used as a bio-template for the preparation of carbon fibers via a combination of thermochemical conversion approaches, including a general hydrothermal pre-carbonization step, as well as subsequent carbonization, physical, or chemical activation. The relationships between the preparation conditions and the structural and electrochemical properties of the obtained carbon materials were determined using SEM, TEM, EDAX, XPS, cyclic voltammetry, galvanostatic measurements, and EIS. It was shown that hydrothermal pretreatment in the presence of phosphoric acid ensured the complete removal of inorganic impurities of raw fungus hyphae, but at the same time, saved some heteroatoms, such as O, N, and P. Chemical activation using H3PO4 increased the amount of phosphorus in the carbon material and saved the natural fungus’s structure. The combination of a hierarchical pore structure with O, N, and P heteroatom doping made it possible to achieve good electrochemical properties (specific capacitance values of 220 F/g) and excellent stability after 25,000 charge/discharge cycles in a three-electrode cell. The electrochemical performance in both three- and two-electrode cells exceeded or was comparable to other biomass-derived porous carbons, making it a prospective candidate as an electrode material in symmetrical supercapacitors.

Graphical Abstract

1. Introduction

Due to their fast charge/discharge, long cycle life, and safe operation, supercapacitors (SCs) are used in short-term energy storage systems, for example, for balancing power surges in an electrical network, recuperation of braking energy, or as a pulsed energy supply when starting cars [1,2,3,4]. The predicted SCs’ market will reach the value of USD 3.5 billion by the year 2025 [5]. The selection of electrode materials is significant to ensure good SC performance [3,6,7,8]. Nowadays, commercial electric double-layer supercapacitors (EDLC), which are based on the mechanism of electrostatic charge accumulation at the electrode/electrolyte interface, mainly use highly porous activated carbons due to their high specific surface area (SSA), electrical conductivity, and chemical stability [5,9]. A feature of carbon materials (CMs) is the possibility of obtaining them from various raw materials, including plant biomass [10,11,12,13,14]. Biomass is not only a sustainable resource but could also be a unique organic framework to provide a bio-template to introduce excellent porous structures. Therefore, when developing methods for obtaining CMs, it is important to try to preserve the morphology of the original natural raw materials, as well as to convert the original organic and inorganic components into doping heteroatoms [15]. Numerous studies showed that doping heteroatoms or surface functional groups (SFGs) can significantly change the functional properties of carbon materials used as electrode materials for electrochemical energy conversion and storage devices. For example, they play an important role in the surface-adsorption processes and reversible surface redox reactions, which facilitate more storage sites, faster ion diffusion [16], and can increase conductivity [10,11,17,18]. Heteroatoms can be associated with carbon alone, as well as with each other (for example, H, N, S, or P together with O in the form of oxygenated functional groups) [19]. One of the most common heteroatoms is oxygen, which provides acidic properties and hydrophilicity of carbon materials. The inclusion of nitrogen in the structure results in enhanced redox reactions [19,20]. In comparison with O and N, phosphorus has received much less attention; however, P doping produces some oxygen-containing SFGs, which improve the wettability of the electrode materials by the electrolyte [19]. Phosphorus can change the charge and spin densities of carbon materials because its electronegativity is lower than that of carbon. The bigger size of a P atom than a C atom contributes to the formation of structural defects in the carbon framework. Such defects can act as active sites in electrochemical reactions [19]. Moreover, co-doping can apparently improve the efficiency of a single heteroatom doping via synergetic effects [16].
One of the sources of renewable biomass is fungi. Polypore fungi (tinder fungi, bracket fungi, lat. Fomes fomentarius) is a group of basidial wood-destroying macromycetes, which are found everywhere on deciduous trees in the forests of Russia and Europe. It most often appears on dead trees and stumps, but can also affect weakened living trees, which causes issues for forestry and park management. Fungi are rich in chitin, carbohydrates, proteins, and organic acids. Many carboxyl, hydroxyl, and amino groups make them attractive materials for obtaining CMs doped with nitrogen and oxygen. Fungi are used to obtain carbon materials for efficient VOC adsorption [21,22], natural lightweight construction materials [23], safer fire-resistant alternatives to synthetic polymers for binding matrices [24], fillers in high-density polyethylene (HDPE)/wood composites [25], capacitive deionization [26], electrocatalysts for oxygen reduction reaction [27,28,29], lithium–sulfur batteries [30], lithium-ion batteries [31], and supercapacitors [32,33,34,35,36]. However, in most of the previously described carbon materials for different electrochemical applications [27,29,30,32,34,35], the unique natural fibrous frameworks of the raw fungi were not saved, which, for some of them [27,35], negatively affected important characteristics of electrode materials, such as the SSA and the porosity.
Hydrothermal carbonization (HTC) is widely used to convert renewable plant biomass into porous carbon materials [13,37,38,39]. The important advantage of HTC is the ability to transform wet raw materials since, in this method, water is used simultaneously as a reagent, solvent, and catalyst [38]. The carbon materials obtained via HTC usually do not have a highly developed porous structure but are convenient high-carbon intermediates for further processing using various methods (carbonization, chemical activation, etc.) to obtain carbon materials with different porous structures [38]. The process is carried out by heating the carbonaceous material with water within a pressure range of 2–10 MPa and low temperature (180–350 °C) to upgrade the diverse biomass feedstock into biocarbon [38,40]. One of the advantages of this method is also the possibility of obtaining heteroatom-doped carbon materials by using solutions of doping reagents, such as phosphoric acid [36], sulfuric acids, or ammonium salts [41,42]. In this regard, HTC with the presence of phosphoric acid was used as an effective method for pretreatment in mild conditions to obtain a series of heteroatom-doped carbon materials in this study. The CMs of this series further differed from each other in terms of thermochemical conversion.
One of the ways to obtain carbon materials from biomass is carbonization (pyrolysis). The process of carbonization is based on various high-temperature reactions providing the self-activated reactions of carbonaceous material due to the release of H2O and CO2, which promotes the formation of the porous structure [43]. It should be noted that a similar two-step method (HTC + carbonization) was already used to obtain CMs from the auricularia fungus, but with water as a solution at the HTC step, which negatively affected the SSA (80 m2/g) and electrochemical properties of the obtained material [35]. Carbonization of X. ylaria fungi strains pre-soaked in organic dyes was used to obtain electrode materials for SCs [33]. This method made it possible to preserve the original structure of fungal hyphae; however, it required a three-day soaking in toxic dyes in order to introduce N and S heteroatoms, but the final material was not sufficiently resistant to long-term cycling (83.4% of initial capacity after 3000 charge/discharge cycles) in the three-electrode configuration [33].
More efficient methods for obtaining CMs with a developed surface are physical and chemical activation [44]. Physical activation of carbon is a cost-effective and chemical-free method for carbon material preparation because it can be carried out using two gasifying agents: CO2 and water vapor, together or alone [45]. These agents “extract” carbon atoms from the carbon framework through the formation and release of carbon monoxide. A more widely used method is chemical activation due to several advantages, such as the possibility of obtaining CMs with a high specific surface area, as well as the possibility of introducing different functional groups on the CM surface [17]. Chemical activation involves the use of activating agents, such as hydroxides and carbonates of alkali and alkaline earth metals, inorganic acids, and molten salts [46,47]. The KOH as an activator promotes the formation of a well-developed porosity and high surface area, but almost completely destroys the original structure of the natural biomass [32,34]. For example, chemical activation with alkaline was used to obtain nitrogen-doped CMs from aniline-modified fungus [34], bamboo fungus [32], and pleurotus ostreatus fungus [36] as raw materials. Despite the huge SSA (2339 m2/g [34] and 2149 m2/g [36]), the resulting materials did not show outstanding electrochemical properties as electrode materials for SC. Unlike alkali, H3PO4 provides chemical activation in mild conditions, wherein the activated carbons not only acquire a three-dimensional network structure but also phosphorous doping [19,48,49], but not in all cases [36].
Thus, based on our previous results [43,50,51], as well as the literature data [13,19,37,38,39,44,45,46,47], after using three post-processing methods on natural raw materials, we compared their effects on the structural and functional properties of the obtained CMs. Unlike [32,33,34,35,36], we aimed to find conditions that would allow us to use the uniqueness of the natural structure of the fungus Fomes fomentarius as a ready bio-template for the preparation of carbon fibers with a three-dimensional network structure and heteroatom doping. We also demonstrated the ability to use the resulting carbon materials as electrodes in symmetrical supercapacitors.

2. Materials and Methods

2.1. Carbon Material Preparation

A series of CMs based on natural biomass (Fomes fomentarius) was obtained using various thermochemical conversion approaches, including a common-to-all-materials pre-treatment step, namely, hydrothermal carbonization with a phosphoric acid solution, as well as subsequent carbonization, physical, or chemical activation.
Hydrothermal pretreatment was carried out in a steel autoclave with a Teflon insert with a volume of 100 cm3. Preliminary ground natural raw materials weighing 8 g were placed in the autoclave, 52 g of water was poured in, and 3 mL of H3PO4 was added to obtain materials of the HT-P-T series. The autoclave was heated up to 250 °C and kept for 8 h. The pressure in the autoclave was 4.3 MPa. The material yield after this pretreatment was 45.6%.
The materials that underwent hydrothermal pre-treatment were divided into 3 groups for further thermochemical conversion via carbonization in an inert atmosphere, physical activation under the action of CO2, or chemical activation in the presence of phosphoric acid.
The carbonization of the HT-P-T materials was carried out in a horizontal tube furnace in a nitrogen flow (50 mL/min) in accordance with the described regime [50]. The samples obtained in this way were named HT-P-C-T.
Physical activation took place under the same conditions as carbonization, except that an oxidizing medium was used instead of an inert one due to the supply of carbon dioxide during calcination at 900 °C [43,45] to create conditions conducive to an increase in the area surface of the resulting CM. The samples obtained in this way were named HT-P-CO2-T.
For chemical activation, dried HT-P-T material was mixed in a 1:4 weight ratio with H3PO4 (conc.) (ρ = 1.685 g/cm3). The suspension was placed in a Pt crucible and pyrolyzed in a tube furnace in an inert N2 atmosphere in accordance with the following temperature regime [50]: 200 °C with exposure for 30 min, 500 °C with exposure for 30 min, and finally, 700 °C with exposure for 1 h. The heat treatment temperature is very important because phosphorus species are eliminated at temperatures higher than 800–830 °C [19,49]. This is why, in this study, for the chemical activation using H3PO4 simultaneously as an activator and doping agent, the maximum temperature was 700 °C. The materials obtained in this way were referred to as HT-P-ChP-T.

2.2. Morphology and Surface Chemistry of Carbon Materials

The morphology, elemental composition, and distribution of elements over the surface of raw materials and carbon materials obtained under different thermochemical conversion conditions were investigated via scanning electron microscopy (SEM) using a Quanta 200 microscope combined with an EDAX Genesis XVS 30 X-ray microanalysis system.
The microstructure of the powders was also investigated via transmission electron microscopy (TEM) using a JEM-2100 microscope (JEOL, Japan) operated at an acceleration voltage of 200 kV. For the investigation, the fine powder was mixed with alcohol and then ultrasonically treated until a slurry was reached. Then, a drop of the slurry was deposited onto a copper grid coated with an electron-transparent carbon film.
The specific surface area (SSA) of the obtained materials was determined by using a TOP-200 surface area and porosity analyzer (Altamira instruments, Cumming, GA, USA) and calculated using N2 adsorption/desorption isotherms according to the Brunauer–Emmett–Teller method (see Supplementary Materials).
The Raman spectroscopy results of the carbon materials were obtained with a Thermo Scientific DXR Raman Microscope in the range of 50–3500 cm−1 using a solid-state green laser (λ = 532 nm).
The chemical composition of the HT-P-ChP-T carbon material was studied using X-ray photoelectron spectroscopy (XPS). XPS measurements were performed on an X-ray photoelectron spectrometer equipped with an XR-50 X-ray radiation source and a PHOIBOS-150 hemispherical analyzer (SPECS Surface Nano Analysis GmbH: Berlin, Germany). The core-level spectra were obtained using Mg Kα radiation. Data processing was carried out using the CasaXPS software as described in detail elsewhere [46,47]. The charge correction was performed by setting the most intense C1s peak at 284.8 eV.

2.3. Electrochemical Properties of Carbon Materials

Electrochemical studies were carried out in a three-electrode cell using a P-45X potentiostat/galvanostat (Elins). In this three-electrode configuration, a platinum wire was used as the counter electrode and the Ag/AgCl saturated electrode served as the reference electrode. All potentials were given relative to Ag/AgCl. Working electrodes were made by applying them to a rectangular glassy carbon plate “ink” suspension (120 mg of the investigated carbon material, 0.45 mL of isopropanol, and 0.12 mL of an aqueous solution of Nafion® (10 wt%)). The weight of the dried ink on the electrode was 1.4 mg. In more detail, the parameters of electrochemical measurements (cyclic voltammetry, galvanostatic studies in the charge/discharge mode, and electrochemical impedance spectroscopy) are given in the Supplementary Materials.
The specific capacitances (Cs) of the carbon materials were calculated using the discharge curves according to the following equation [51,52]:
C s = I   ·   Δ t   m   ·   Δ V  
where Cs is the specific capacitance in a three-electrode system (F/g), I is the discharge current (A), Δt is the discharge time (s), m is the mass of the carbon material (g), and ΔV is the potential window (V).
A multichannel potentiostat/galvanostat (Elins) was used to conduct studies of the symmetrical two-electrode coin cells (CR2025) with an aqueous electrolyte (6 M KOH) and polypropylene membrane (PORP A1) as a separator. The “ink” prepared based on investigated carbon material, as described earlier, was applied in two current collector disks (D = 14 mm) made of nickel foil. The mass of dried “ink” on each electrode was approximately 10 ± 1 mg.
The specific capacitance of the electrode material in the symmetrical coin cell was calculated according to the following equation [51,52]:
C s = 4   ·   I   ·   Δ t   M   ·   Δ V  
where Cs is the specific capacitance (F/g), I is the discharging current (A), Δt is the discharge time (s), M is the total mass of the investigated material on both electrodes (g), and ΔV is the potential window (V).

3. Results and Discussion

3.1. Raw Material Characterization

The fruit bodies (mycelia) of tinder fungus (Fomes fomentarius) are formed as long filaments (hyphae) and are hierarchically structured foams (Figure 1a–d). The mycelium has a transversely isotropic microstructure with open porosity at the nano- and micro-length scales [23]. Such a structure could be a natural bio-template for the preparation of carbon bio-fibers with a hierarchical porous structure.
Various forms of protein, nitrogen, and phosphorus were investigated using laboratory tests developed by the International Organization for Standardization [53,54,55] to estimate the heteroatom content in raw Fomes fomentarius (Table 1). According to the chemical analysis, the total protein content of the raw material was approximately 3.3%. The content of the true protein was approximately 1.6%. The phosphorus and nonprotein nitrogen contents were not very high, but under appropriate conditions may influence the production of N- or P-doped carbon materials.
According to the EDAX data (Figure 1e), the fresh air-dried tinder fungus contained only 33.7% carbon, almost twice as much oxygen (62.6%), and small amounts of alkali metals (Mg, Al, K, and Ca), silicon, sulfur, nitrogen, and phosphorus (Table 2).

3.2. Preparation and Structural Properties of Carbon Materials

For the preparation of carbon fibers with a hierarchical porous structure and heteroatoms, the possibility of using a type of natural biomass, namely, the fungus Fomes fomentarius, as a bio-template was investigated. To do this, various approaches for thermochemical conversion were used, including those that were general for all materials, namely, the stage of hydrothermal carbonization in the presence of phosphoric acid, as well as the subsequent carbonization, physical, or chemical activation (Figure 2).
Figure 3 shows SEM and TEM images and the surface distribution of elements (EDAX data) of the obtained carbon materials. The HTC pretreatment contributed to the preservation of the original structure of raw natural materials (Fomes fomentarius), which was found to be an excellent natural bio-template. Thus, all three carbon materials (HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T) obtained under different post-treatment conditions became carbon bio-fibers.
It was shown that hydrothermal pretreatment in the presence of phosphoric acid also helped to ensure the complete removal of impurities (Mg, Al, Si, K, and Ca) presented in the hyphae of multicellular fungi before the treatment (Figure 3, Table 2). In addition, the HTC conditions used in this research allowed for saving some heteroatoms (N and P) that existed in the raw material (Table 1).
Due to the subsequent stage of chemical activation in the presence of phosphoric acid, it was possible to increase the amount of phosphorus in the resulting HT-P-ChP-T material by 12 times (Table 2).
In the Raman spectra of the HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T materials, there were two peaks ascribed to the D-band and G-band of carbons (Figure 4a). The intensity ratio of the D and G Raman peaks (ID/IG) indicated the degree of the defective nature and graphitization of the samples after carbonization and activation of the HTC pretreated fungus (Table 2). The highest degree of graphitization (lowest ID/IG ratio of 0.65) was observed in the Raman spectra of the carbonized HT-P-C-T material.
Figure 4b shows the nitrogen adsorption–desorption isotherms for the obtained carbon materials. For all samples of the series, the nature of adsorption isotherms demonstrated a combination of type I and IV isotherms, with a sharp increase at low relative pressures (P/P0 < 0.10) and an H4 hysteresis loop according to the IUPAC classification. For the HT-P-C-T sample, the adsorption isotherm had a small H4-type hysteresis loop at relative pressures in the range of 0.50–0.95. The sample was characterized by a smaller specific surface of 239 m2/g and lower porosity compared with other samples obtained in this research (Figure 4, Table 2). However, the SSA of this material, which was pretreated using HTC with the presence of phosphoric acid, turned out to be three times larger than the SSA of the CM obtained from the fungus auricularia using a similar two-step method but with water in the HTC step [35].
The adsorption isotherm of the HT-P-CO2-T sample, as in the previous case, had the features of type I and IV isotherms. A steeper plateau at low relative pressures (P/P0 < 0.10) indicated many micropores, which is typical for carbon materials prepared using the physical activation methods [15]. The type IV hysteresis loop existed in a wide range of relative pressures (P/P0 = 0.45–0.90) and was more pronounced compared with the HT-P-C-T sample, which indicated a greater number of mesopores. The specific surface area of the sample was more than twice as high compared with the HT-P-CO2-T sample and was 530 m2/g (Figure 4, Table 2).
In the case of the HT-P-ChP-T sample, as well as for HT-P-C-T and HT-P-CO2-T, the adsorption isotherm had a sharp jump at low relative pressures, but then continued to grow rather than reach saturation, which was associated with the filling of the available volume of micropores and subsequently affected the process of capillary condensation in the mesopores.
This HT-P-ChP-T sample had a high specific surface area of 1491 m2/g and a wide range of pore size distribution (Figure 4, Table 2). The H3PO4 activation was widely used for CM fabrication with a micromesoporous structure [18]. As has long been known, the combination of both micropores and mesopores is ideal for energy storage because mesopores provide ion transportation channels, whereas micropores offer the main ion storage location [56]. Thus, the hierarchical pore structure in HT-P-ChP-T, together with a large percentage of heteroatoms (Table 2), should allow for better electrochemical behavior compared with the other two obtained materials (HT-P-C-T and HT-P-CO2-T).
For the HT-P-ChP-T material, which was characterized by the highest SSA (Figure 4), XPS analysis was used (Figure 5). The XPS survey spectrum showed peaks corresponding to C, O, N, and P. No impurities of other elements were detected. The relative concentrations of the corresponding elements in the near-surface layer, which were determined on the basis of the XPS data, are given in Table 2.
Figure 5a shows the C1s spectrum of the HT-P-ChP-T sample. The spectrum contained several peaks corresponding to carbon in different chemical environments. Thus, the peak at 284.8 eV corresponded to amorphous carbon. According to the literature data, the peaks at 285.7, 286.7, 288.0, and 289.3 eV can be attributed to carbon forming C–N, C–O, C=O, and O–C=O bonds, respectively [57,58]. The low-intensity peak at 290.8 eV refers to π-π* excitation in graphite-like carbon. It was shown that the oxygen-containing SFGs in carbon materials increase surface wettability [18]. CO-type oxygen-containing groups have a positive contribution to the Faradic pseudocapacitance of porous carbon. The higher concentration of phenol and quinone groups makes carbon surfaces hydrophilic [59,60].
The N1s spectrum of the studied sample contained three peaks at 398.4, 400.2, and 401.7 eV (Figure 5b). According to the literature data, the peak at 398.4 eV can be attributed to nitrogen in the imine and/or pyridine groups –N=, the peak at 400.2 eV was due to nitrogen in the composition of imide groups, and the peak around 401.7 eV corresponded to terminal pyrrole groups -NH [61]. The negatively charged pyridine and pyrrolic -N can increase electrode wettability, improve charge transfer kinetics, and introduce pseudocapacitance into carbon electrode materials [18].
Figure 5c shows the P2p spectrum of the studied sample. The spectrum was an unresolved P2p3/2-P2p1/2 doublet; the spin–orbit splitting was 0.84 eV. The P2p3/2 binding energy is 133.3 eV, which corresponds to phosphorus in the composition of phosphates [19,62]. During the chemical activation, H3PO4 can be hydrolyzed and penetrates the lignocellulosic material, causing the lignocellulose to depolymerize into sugar monomers, and form a new polymeric phosphate structure through the phosphorylation of the lignocellulosic material [18]. Phosphorus atoms in carbon materials can enhance the wettability of the interface between the electrode and the electrolyte to improve carbon’s electrochemical performance. The beneficial effect of the phosphorous existence in the oxygen-containing groups can be assumed to have increased the acidity of the electrode surface and hence facilitated the adsorption of more ions [63].

3.3. Electrochemical Properties of the Obtained Materials

The electrochemical properties of a series of CMs obtained using various approaches of thermochemical conversion of Fomes fomentarius were studied under the model conditions of a three-electrode cell with an aqueous alkaline electrolyte (Figure 6 and Figure 7) in order to assess the possibility of their use as electrodes in a coin-cell-type symmetrical supercapacitor.
According to the summary graphs (Figure 6) and Table 2, it can be concluded that the method of CMs preparation affects their electrochemical properties. The shape of the cyclic voltammograms (CV) of all three samples deviates from the rectangular shape characteristic of a double-layer capacitor. This deviation is associated with the presence of oxygen-, nitrogen- and phosphorus-containing SFGs, which are involved in the redox processes taking place on the surface of the electrode during cycling and are expressed as the corresponding wide Faradaic humps (Figure 6a,b). The main EDLC capacitance and part of pseudocapacitance coexist in these heteroatom-doped carbon electrodes and contribute to charge storage together [32]. Figure 6c shows the charge–discharge curves. At the same current density, the discharge time increased in the order of HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T, indicating the optimized capacitance was achieved at HT-P-ChP-T and is in accordance with the results of CV curves (Figure 6a,b). According to Figure 6d, for all samples, the capacitance increased with decreasing current density. This could be explained by an increase in the contribution of the pseudocapacitance of slower redox reactions that take place in SFG in the low-current region [51]. Further, we explain the electrochemical performance of each sample in detail below.
Because of microstructural features (lowest SSA and porosity, Table 2), the specific capacity of the carbonized material (HT-P-C-T) did not exceed 25 F/g, even at a low current density.
The physical activation of the hydrothermally treated feedstock (HT-P-T) led to a significant increase in SSA (Figure 4, Table 2), which made it possible to increase the specific capacity by a factor of 8.5 (HT-P-CO2-T) compared with carbonized material (Figure 6d). However, such an increase was observed only at low current densities, where the electrode charging process took a relatively long time, during which the electrolyte could penetrate hard-to-reach pores, and slower redox processes involving SFG could take place. At high current densities (10 A/g), only 30% of the specific capacitance achieved at 0.2 A/g was retained for the electrode based on HT-P-CO2-T (Figure 6d). Since the content of heteroatoms in both samples (HT-P-C-T and HT-P-CO2-T) was practically the same, it can be said that the difference in capacitance was associated with the different porosity structures of these materials (Figure 4, Table 2).
The situation is different with the chemically activated in the presence of phosphoric acid material (HT-P-ChP-T). This method of thermochemical conversion made it possible to increase the amount of phosphorus by 12 times (according to EDAX data) and the oxygen and nitrogen by almost 2 times compared with the two materials described above (Table 2). As is known, various SFGs in the carbon material, which includes oxygen, as well as groups with other heteroatoms, such as N and P, contribute to an increase in the wettability of the electrode, due to which an increase in specific capacitance via electrolyte access to pores and an increase in the surface area on which EDL is formed [20]. In addition, such SFGs also participate in electrochemical processes, increasing the total capacitance of the electrode of a double-layer supercapacitor due to the Faraday pseudocapacitance that appears [20]. The presence of SFG in combination with a developed surface made it possible to obtain a material with the best electrochemical properties in this series (the specific capacity was 220 F/g). Unlike HT-P-C-T and HT-P-CO2-T materials, the difference between the specific capacitance obtained at high and low current densities was significantly smaller for the HT-P-ChP-T sample. Thus, even at a high current density of 10 A/g, 62% of the capacitance obtained at a low current density of 0.2 A/g was retained (Figure 6d).
Electrochemical impedance spectroscopy (EIS) of the obtained electrode materials HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T was conducted in a three-electrode configuration in the frequency range of 0.01–100,000 Hz (at the OCV with an amplitude of 10 mV). The Nyquist plots of three samples are shown in Figure 6e,f. In the high-frequency region, the point of intersection of the Nyquist plots with the horizontal axis Z’ is associated with the equivalent series resistance (Rs). It was shown that the lowest Rs value was observed for the HT-P-ChP-T sample (0.18 Ohm), for the other two materials, it was higher and amounted to 0.26 Ohm and 0.33 Ohm for HT-P-CO2-T and HT-P-C-T, respectively (Figure 6f). The straight line in the low-frequency region indicates the capacitive behavior of the samples. The lowest values of the impedance modulus were also registered for the HT-P-ChP-T material, and thus, in the low-frequency region, the impedance modulus was below 180 Ω, and in the high-frequency region, it was below 1 Ω. In the mid-frequency region, the graph shows a Warburg line (45° slope), which describes the diffusion efficiency of the electrolyte ions.
Figure 7 shows a more detailed study of the electrochemical properties of the HT-P-ChP-T material, which demonstrated the best electrochemical activity. Figure 7a illustrates the CVs in the potential scan rate from 5 to 100 mV/s and Figure 7b shows the charge–discharge curves obtained for various current densities (0.2–10 A/g). The curves were linear with some slight curvature because of the pseudocapacitive behavior of the material. There were minor potential drops (IR drops) at the beginning of the discharge curves, which indicates low contact resistances and rapid ion transport.
From the EIS curves in Figure 7d, it can be seen that the total resistance values (Rs) of the HT-P-ChP-T material were quite low (only 0.18 Ohm), which was much lower than other carbon materials based on plant biomass, such as fungi strains (0.34 Ohm) [33], poplar wood (0.5 Ohm) [64], sewage sludge (0.9 Ohm) [56], potato (1.22 Ohm) [65], and oak nutshell (2.4 Ohm) [66]. This indicated a high rate of ion diffusion and high electronic conductivity because of a 3D hierarchical porous structure (Figure 4) with transport channels, as well as through the presence of SFGs of various compositions (Figure 5), providing an increase in wettability and electronic conductivity [3].
Figure 7e shows the relationship between the phase angle and frequency. At a phase angle of 45°, the corresponding time is defined as τ0 (τ0 = 1/f). In the case of the HT-P-ChP-T material at a frequency of 0.4 Hz, τ0 was 2.4 s, indicating that 50% of the full capacity could be reached in a short time at low frequency, demonstrating high power characteristics. The Bode phase in the low-frequency region is often used to understand the capacitive nature of electrode materials. The Bode phase of the investigated material was 81° (Figure 7e), which is close to an ideal supercapacitor (90°) and indicates charge accumulation, mainly via the EDL and, to a lesser extent, via the pseudo capacitance caused by redox processes involving SFGs [3,67].
The HT-P-ChP-T-based electrode showed excellent long-term cycling stability in a concentrated alkali solution over a selected potential range compared with previously described biomass-based carbon materials (Table S1). After 25,000 charge/discharge cycles at a high current density of 10 A/g, 92% of its initial specific capacity remained (Figure 7f).
The development of efficient symmetrical electrode materials plays an important role in advancing the further application of electrochemical energy storage devices. Such devices use symmetrical electrode materials for both cathodes and anodes, which simplifies the manufacturing process and reduces the final cost. Therefore, symmetrical coin-cell-type supercapacitors (CR2025) with the HT-P-ChP-T carbon material and an aqueous electrolyte were assembled and tested (Figure 8).
In EDLC supercapacitors, charges are physically accumulated due to electrostatic adsorption at the interface between the electrode and electrolyte [68]; however, for HT-P-ChP-T//HT-P-ChP-T, in addition to the capacitance of the EDL, which was predominant, there was also a small share of pseudocapacitance provided by SFGs (Figure 5, Table 2), which was reflected in the CV shape (Figure 8a).
The charge–discharge curves are presented in Figure 8b. There is an evident internal resistance (IR) drop was observed on the initiation point of the discharge curves. The IR drop in this case was more related to the resistance of the stainless-steel cases and spacers used in the coin cell increasing the electronic resistivity of the device than with the conductivity of the material, which showed low resistance values when tested in a three-electrode cell (Figure 7). The maximum specific capacity of HT-P-ChP-T in the symmetric coin cell was 158 F/g at 2 mA/g (Figure 8b). The Ragone plot of the symmetric supercapacitor, which shows the relationship between the energy and power densities, is shown in the Supplementary Materials (Figure S1).
Despite the fact that aqueous electrolytes are highly conductive, have low viscosity, and contribute to high specific capacitance, the voltage output from a single two-electrode aqueous electrolyte cell was limited to 1 V due to the water decomposition process at higher voltages. Since this is not enough to power small devices, at least several cells should be connected in series to expand the potential window, depending on the operating voltage of the connected device. As a demonstration, such an assembled tandem device powered a red LED (whose rated voltage is 2.1 V) well in a fully charged state (Figure 8c,d).
The electrochemical performance of obtained HT-P-ChP-T carbon fibers in both three- and two-electrode cells exceeded [34,35,36,48,56,60,64,69,70] or was comparable [32,33,52,71] with biomass-derived porous carbons (Table S1), which makes it a prospective candidate as an electrode material for symmetrical supercapacitors.

4. Conclusions

Carbon fibers based on a renewable biomass source (tinder fungus Fomes fomentarius) as a bio-template were obtained using a combination of various thermochemical conversion approaches, including a hydrothermal pre-carbonization step that was general for all procedures, as well as subsequent carbonization, physical, or chemical activation. The relationship between the structural and functional properties of the obtained carbon materials was investigated.
It was shown that HTC pretreatment in the presence of phosphoric acid ensured the complete removal of inorganic impurities (Mg, Al, Si, K, and Ca) present in the raw fungus hyphae, but at the same time, saved O, N, and P heteroatoms. Chemical activation using H3PO4 further increased the amount of phosphorus in the HT-P-ChP-T carbon material and saved the natural fungus structure. These carbon bio-fibers had the largest SSA of 1491 m2/g and a pore volume of 0.255 cm3/g.
The combination of a three-dimensional network structure with O, N, and P doping made it possible to achieve good electrochemical characteristics in a three-electrode cell, such as specific capacitance values of 220 F/g, a low total resistance (0.18 Ohm), and excellent stability after 25,000 charge/discharge cycles at a high current density (10 A/g). Thus, the electrochemical properties of HT-P-ChP-T in both three- and two-electrode setups exceeded or were comparable to previously reported biomass-derived carbons, which makes it a prospective candidate as an electrode for symmetrical supercapacitors.

Supplementary Materials

The following supporting information can be downloaded from https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/sym15040846/s1: Description of the structural and electrochemical measurements of obtained carbon materials. Table S1: Comparing the electrochemical performances of carbon material from this work with other electrode materials from biomass. Figure S1: Ragone plot for a symmetric coin cell with HT-P-ChP-T as the electrode material

Author Contributions

Conceptualization, D.C. and V.K. (Victor Klushin); material preparation, V.A., V.K. (Victor Klushin), and D.T.; electrochemical measurements, M.K., E.S., and T.B.; SSA measurements, E.K.; XPS measurements, V.K. (Vasily Kaichev); TEM investigation, M.G.; writing—original draft preparation, D.C., E.K., and V.K. (Vasily Kaichev); writing—review and editing, N.S. and D.C.; visualization, D.C. and V.A.; supervision, N.S.; project administration, D.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Russian Science Foundation, project no. 21-13-00177.

Data Availability Statement

All data generated or analysed during this study are included in this article.

Acknowledgments

The X-ray photoelectron spectroscopy measurements were conducted using the equipment of the Shared Research Center “National Center of Catalyst Research” at the Boreskov Institute of Catalysis. The authors also thank the Shared Research Center “Nanotechnologies” of Platov South-Russian State Polytechnic University for the scanning electron microscopy and elemental analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Symmetry 15 00846 g001 550
Figure 1. Photos of the fungus Fomes fomentarius: (a) in the natural environment, (b) part of the fungus; (c) ground fungus; (d) SEM image of ground air-dried fungus, and (e) distribution of elements (using EDAX).
Figure 1. Photos of the fungus Fomes fomentarius: (a) in the natural environment, (b) part of the fungus; (c) ground fungus; (d) SEM image of ground air-dried fungus, and (e) distribution of elements (using EDAX).
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Figure 2. Scheme of carbon fibers (HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T) preparation using various approaches of thermochemical conversion of the fungus Fomes fomentarius as a bio-template.
Figure 2. Scheme of carbon fibers (HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T) preparation using various approaches of thermochemical conversion of the fungus Fomes fomentarius as a bio-template.
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Figure 3. SEM and TEM images and surface distribution of elements (EDAX data) of the heteroatom-doped carbon bio-fibers HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T based on natural fungus biomass (Fomes fomentarius) obtained using various thermochemical conversion approaches.
Figure 3. SEM and TEM images and surface distribution of elements (EDAX data) of the heteroatom-doped carbon bio-fibers HT-P-C-T, HT-P-CO2-T, and HT-P-ChP-T based on natural fungus biomass (Fomes fomentarius) obtained using various thermochemical conversion approaches.
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Figure 4. Structural properties of carbon materials obtained under different conditions of thermochemical conversion of Fomes fomentarius: (a) Raman spectra, (b) adsorption–desorption isotherms, (c) BJH pore size distributions, and (d) DFT pore size distributions.
Figure 4. Structural properties of carbon materials obtained under different conditions of thermochemical conversion of Fomes fomentarius: (a) Raman spectra, (b) adsorption–desorption isotherms, (c) BJH pore size distributions, and (d) DFT pore size distributions.
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Figure 5. C1s (a), N1s (b), and P2p (c) core-level spectra of the HT-P-ChP-T sample. All spectra were normalized to the integrated intensity of the C1s spectrum.
Figure 5. C1s (a), N1s (b), and P2p (c) core-level spectra of the HT-P-ChP-T sample. All spectra were normalized to the integrated intensity of the C1s spectrum.
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Symmetry 15 00846 g006 550
Figure 6. Electrochemical properties of carbon materials based on Fomes fomentarius in a three-electrode cell with a 6 M KOH solution: CV at potential scan rates of 5 mV/s (a) and 50 mV/s (b); (c) charge–discharge curves at a current density of 0.5 A/g; (d) specific capacitance versus current density; (e) Nyquist plot (insert: enlarged graph in the high-frequency region); (f) Bode modulus plots.
Figure 6. Electrochemical properties of carbon materials based on Fomes fomentarius in a three-electrode cell with a 6 M KOH solution: CV at potential scan rates of 5 mV/s (a) and 50 mV/s (b); (c) charge–discharge curves at a current density of 0.5 A/g; (d) specific capacitance versus current density; (e) Nyquist plot (insert: enlarged graph in the high-frequency region); (f) Bode modulus plots.
Symmetry 15 00846 g006
Symmetry 15 00846 g007 550
Figure 7. Electrochemical properties of HT-P-ChP-T in a three-electrode cell with a 6 M KOH aqueous solution: (a) CV at different potential scan rates; (b) charge–discharge curves at different current densities; (c) specific capacitance depending on current density; (d) Nyquist plot (insert: enlarged graph in the high-frequency region and equivalent circuit model); (e) Bode plot of phase angle versus frequency; (f) results of long cycling in charge/discharge mode at a current density of 10 A/g for 25,000 cycles.
Figure 7. Electrochemical properties of HT-P-ChP-T in a three-electrode cell with a 6 M KOH aqueous solution: (a) CV at different potential scan rates; (b) charge–discharge curves at different current densities; (c) specific capacitance depending on current density; (d) Nyquist plot (insert: enlarged graph in the high-frequency region and equivalent circuit model); (e) Bode plot of phase angle versus frequency; (f) results of long cycling in charge/discharge mode at a current density of 10 A/g for 25,000 cycles.
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Symmetry 15 00846 g008 550
Figure 8. Electrochemical properties of HT-P-ChP-T in a symmetric coin cell with a 6 M KOH aqueous solution: (a) CV at different cycling rates; (b) charge–discharge curves at various current densities; (c) specific capacitance as a function of current density; (d) optical picture of a red LED (rated voltage 2.1 V) powered by three HT-P-ChP-T//HT-P-ChP-T supercapacitor cells connected in series; enlarged images of powered (e) and unconnected (f) red LEDs.
Figure 8. Electrochemical properties of HT-P-ChP-T in a symmetric coin cell with a 6 M KOH aqueous solution: (a) CV at different cycling rates; (b) charge–discharge curves at various current densities; (c) specific capacitance as a function of current density; (d) optical picture of a red LED (rated voltage 2.1 V) powered by three HT-P-ChP-T//HT-P-ChP-T supercapacitor cells connected in series; enlarged images of powered (e) and unconnected (f) red LEDs.
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Table
Table 1. Results of the chemical analysis Fomes fomentarius for protein, nitrogen, and phosphorus.
Table 1. Results of the chemical analysis Fomes fomentarius for protein, nitrogen, and phosphorus.
Laboratory TestValue, %
Mass fraction of crude protein3.29 ± 0.41
Mass fraction of protein according to the Dumas1.66 ± 0.37
Mass fraction of non-protein nitrogen0.10
Mass fraction of phosphorus0.76 ± 0.13
Table
Table 2. Preparation conditions, elemental composition (according to EDAX), parameters of the Raman spectra, and adsorption–desorption data of materials obtained based on natural fungi biomass using various thermochemical conversion approaches.
Table 2. Preparation conditions, elemental composition (according to EDAX), parameters of the Raman spectra, and adsorption–desorption data of materials obtained based on natural fungi biomass using various thermochemical conversion approaches.
Raw
Material		Thermochemical ConversionElemental Composition according to the
EDAX Data, at.%		Ratio of D and G Raman Peaks, ID/IGSSA, m2/gPore
Volume b, cm3/g		Average Pore Diameter, nm
PretreatmentActivationSample CONPMgAlSiSKCa
Fomes
fomentarius		--T33.762.61.70.20.40.30.20.10.50.4 ---
HTC
with H3PO4
(8 h)
240 °C 		N2
(700 °C)		HT-P-C-T90.57.81.40.3------0.652390.1022.0
CO2
(900 °C)		HT-P-CO2-T93.94.41.40.3------0.805300.1682.8
H3PO4 (700 °C)HT-P-ChP-T85.9
(82.6) a		9.6
(15.1) a		2.1
(0.8)a		2.4
(1.5) a		------0.7514910.2553.0
a XPS data, b calculated using the t-plot micropore volume.
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MDPI and ACS Style

Chernysheva, D.; Konstantinov, M.; Sidash, E.; Baranova, T.; Klushin, V.; Tokarev, D.; Andreeva, V.; Kolesnikov, E.; Kaichev, V.; Gorshenkov, M.; et al. Fomes fomentarius as a Bio-Template for Heteroatom-Doped Carbon Fibers for Symmetrical Supercapacitors. Symmetry 2023, 15, 846. https://0-doi-org.brum.beds.ac.uk/10.3390/sym15040846

AMA Style

Chernysheva D, Konstantinov M, Sidash E, Baranova T, Klushin V, Tokarev D, Andreeva V, Kolesnikov E, Kaichev V, Gorshenkov M, et al. Fomes fomentarius as a Bio-Template for Heteroatom-Doped Carbon Fibers for Symmetrical Supercapacitors. Symmetry. 2023; 15(4):846. https://0-doi-org.brum.beds.ac.uk/10.3390/sym15040846

Chicago/Turabian Style

Chernysheva, Daria, Maksim Konstantinov, Ekaterina Sidash, Tatiana Baranova, Victor Klushin, Denis Tokarev, Veronica Andreeva, Evgeny Kolesnikov, Vasily Kaichev, Mikhail Gorshenkov, and et al. 2023. "Fomes fomentarius as a Bio-Template for Heteroatom-Doped Carbon Fibers for Symmetrical Supercapacitors" Symmetry 15, no. 4: 846. https://0-doi-org.brum.beds.ac.uk/10.3390/sym15040846

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