Carbon felt based-electrodes for energy and environmental applications: A review

TL;DR: In this paper, the carbon felt (CF) based-electrodes are discussed in a holistic manner, and various methods and equations used to identify physical values of CF material are supplied.

About: This article is published in Carbon.The article was published on 2017-10-01 and is currently open access. It has received 251 citations till now. The article focuses on the topics: Microbial fuel cell.

Summary

1 Introduction

• Carbon Felt (CF) is commonly used as electrodes due to their good electronic conduction.
• They have high surface area and porosity able to provide abundant redox reaction sites, excellent electrolytic efficiency and mechanical stability at relatively low cost [1-4].
• Other carbon-based materials like vitreous carbon, carbon sponge, carbon fiber or carbon paper have been also applied widely for electrode application.
• Obviously, reviews dedicated to CF based-electrodes have concentrated only a few of their characteristic features or modification methods towards individual applications.
• First, the authors present the definition about EF process and the studies using non-modified CF cathode.

2.1 Manufacturing method of CF material

• Generally, there are two types of precursors which are most commonly used for graphite/carbon felts manufacture, including polyacrylonitrile (PAN) and rayon which is a regenerated cellulose [15].
• The manufacturing process for felts material is usually done via needle-punching processing and subsequently graphitization [16].
• Figure 2-1 describes a needle punching process where the barbs on the needles hook the fibers and insert them vertically.
• Their outward appearances are almost exactly alike Figure 2-2.
• A study of Liu et al. showed that CF (Shanghai Q-Carbon Material Co., Ltd., China) exhibited a much higher electrocatalytic activity than GF (GFD4.6, SGL Group, Germany), due to its higher amount of COH and quaternary nitrogen groups and the presence of higher amounts of defect sites [20].

3.1 Plasma treatment

• The purpose of plasma treatment is to improve the wettability of commercial CF due to the growth of oxygen-containing functional groups or/and nitrogen doping on the surface of fibers, and both of which enhanced the electrochemical reactivity.
• In particularly, oxygen plasma treatment was carried out in a radiofrequency (rf) plasma setup and felts were putted into the plasma chamber which was filled by oxygen.
• The X-ray photoelectron spectroscopy (XPS) measurement revealed that this method rather favored the formation of phenolic (C-O) groups than carboxyl (C=O) groups [24].
• The plasma treatment process only often increases the amount of functional groups on felts and not remarkably the surface area.
• Consequence, both the basicity of carbon and the electrical conductivity of nitrogen doped carbon will be increased [44].

3.2 Thermal treatment

• The thermal treatment is a simple way for felts modification where samples are annealed in a furnace feeding by gas flow containing oxygen and/or nitrogen.
• In view of the fact that the microcrystalline structure of the rayon felts made it more extensive oxygen interaction.
• The enhanced electrochemical properties of the modified electrodes are attributed to the increased electrical conductivity, the increase of active sites amount, and the improved wettability provided by the introduction of the nitrogenous groups on the surface of GF [16, 47].
• Interestingly, the heat treatment can also improve the surface area of the pristine electrodes.

3.3 Chemical treatment

• Chemical treatment of CF mainly involves the use of chemicals like nitric acid and/or sulfuric acid in order to activate the surface of felt materials.
• The combination between thermal and chemical treatments sometimes is necessary to improve the electrochemical behavior of the felts [51].
• Additionally, the oxygen functional groups like COOH, CH=O, OH, etc. and micropores could be formed on the surface of the CF electrodes by KOH activation at around 800 °C.
• The modification following the electrochemical oxidation way was successfully applied to improve the properties (i.e. the hydrophilicity, electrocatalytic activity) of different felts like graphite (Shanghai Energy Carbon Limited Co., China) [53] or GF (Sanye Carbon Co., Ltd.) [54].

3.4 Metallic modification

• The metallic modification is towards enhancement of the electro-conductivity of felts electrode materials.
• Coating metal on the felts fibres can be performed through a simple way of impregnation with solutions containing metallic ions like Pt4+, Pd2+, Au4+, Mn2+, Te4+, In3+, Ir3+, etc.
• Alternatively, thermal deposition of Fe(CO)5 was utilized to load Fe directly on GF (Sanye Carbon Co., Ltd., Beijing) in a sealed reactor made of glass .

3.5 Graphene based modification

• In recent years, graphene has emerged as an exciting topic of research in materials science and condensed matter physics research.
• It has received extensive attention due to its remarkable electrical, physical, thermal, optical, high specific surface area and mechanical properties [64, 65].
• Therefore, graphene was widely applicable for electrochemical activity of electrodes [66].
• Different methods as dip-coating, constant potential technique and electrophoretic deposition (EPD) were used separately or combined together for the coating of graphene based materials on felts electrodes.
• By comparing the response of CVs curves in 0.5 M Na2SO4 solution, the rGO/CF electrode has a higher current density than the bare CF in the voltage range from -0.6 to 0.6 V, suggesting a larger electrode surface area and better conductivity after modification [62].

3.6 Carbon nanotubes based modification

• CNTs have been attractive for the modification of felts electrodes because of their mechanical flexibility, excellent electrical and thermal conductivities, and significantly large surface area [69].
• Thanks to the small size (∼30 nm) of CNTs, they created a significant increase of the electrochemical surface area of the felt materials.
• The CNTs/CF electrode was also obtained by growing CNTs via CVD of methanol on cobalt and manganese metallic particles deposited on CF.
• Beside CVD, EPD shows noticeable advantages as a low-cost and simple method [75, 76].
• Similarly, the COOH-MWCNTs were ultrasonically dispersed in dimethyl formamide and then the CF was immersed in this solution.

3.7 Carbon nanofiber based modification

• Among modifications performed by carbon nanomaterials, carbon nanofibers (CNFs) appear to be a promising candidate, owing to their unique electrical, physico-chemical and mechanical properties [79, 80].
• The calcination and reduction was necessary afterward to change nickel salt to nickel metal.
• This method was utilized for the CNFs modification of both GF provided by Shanghai Qijie Carbon Material Co., Ltd. [82] or GF provided by Carbone Lorraine Co [83].
• The synthesis approaches are similar to the CVD method for synthesis of CNFs, but it can decrease the processing times remarkably [85, 86].
• The full power (1400 W) of the microwave was applied for 5 to 10 min and repeated two more times to enable further CNFs growth.

3.8 Polymer based modification

• Polyaniline (PANi) and polypyrrole (PPy) are the most common conducting polymers for electrode modification because of their high electrical conductivity, ease of preparation, and environmental stability [88, 89].
• The modified electrode pointed out a larger current response in comparison with the unmodified electrode in the scan range between -1V to 0.6 V on CVs .
• In order to improve the physicochemical and electrochemical properties of the conducting polymers, many copolymers were prepared and investigated.
• A poly(aniline-co-o-aminophenol) film with average mass of 1.17±0.1 g was deposited on CF by Cui et al. through electrochemical synthesis in solution containing simultaneously aniline and oaminophenol [96].
• The composite of PPy/GO was grown up on the surface of GF electrode by one-step electro-synthesis way where the electropolymerization of PPy was carried out in a solution including simultaneously GO and pyrrole monomers .

3.9 Zeolites based modification

• Zeolites are porous crystalline aluminosilicates of SiO4 4− and AlO4 5− tetrahedra connected by oxygen bridges.
• Electrochromic [106], electrochemical [107], photophysical [108], as well as molecular magnetic properties [109] were shown some years later.
• CF was recently modified by Zeolitic Imidazolate Framework (ZIF-8) starting from an Atomic Layer Deposition (ALD) of Zinc Oxide (ZnO), its subsequent solvothermal conversion to ZIF-8 and heat treatment under control atmosphere.

4.1 Introduction

• The application of CF– based electrodes in energy field will be discussed, consisting of vanadium redox flow batteries, BFCs, MFCs, capacitors, electrochemical solar energy and Li-ion batteries.
• The operability of the modified electrodes is evident through the comparison with bare electrodes in each issue.
• Additionally, the felts electrodes produced by different companies or modification methods will be collated and collected for a multidimensional overview about this material.

4.2 Vanadium redox flow batteries

• Vanadium redox flow batteries (VRFB) have been known as one of the most promising candidates for energy storage applications.
• VRFB consists of two half-cells separated by a membrane and equipped of carbon electrodes.
• Carbon and graphite felts-based materials were selected as the most suitable electrodes for both the positive and negative half-cells in the VRFB because of their three-dimensional network structures and specific surface area, as well as high conductivity and chemical and electrochemical stability [10, 113, 114].
• Besides that, the energy efficiency of a VRFB with flow fields was improved in comparison with the battery without flow fields using GF (SGL GmbH, Germany, 3 mm thickness).

4.3 Biofuel cells

• Biofuel cells (BFCs) is a device based on the use of biocatalyst (enzymes or living cells) to convert chemical energy of organic and inorganic fuels into electrical energy [120, 121] .
• Moreover, the CF was employed as electrodes for a sucrose/O2 BFC.
• The bio-cathode for the oxygen reduction was prepared by coating enzyme BOD (multi-copper oxidase enzyme) with entrapment of 2,2'-azino-bis(3ethylbenzothiazoline-6-sulphonic acid) (ABTS), an efficient mediator, on CF.
• Polymer-modified felts were also employed in BFCs.
• The bio-anode could operate continuously for more than 30 days, proving the good capacity of felts material for loading of enzyme to produce the energy in the BFCs system [129].

4.4 Microbial fuel cells

• Microbial fuel cells (MFCs) have been considered as a renewable energy source where the chemical energy can be transferred directly into electrical energy though oxidization of organic substrates by electroactive bacteria.
• With the help of conductive polymer coating acting as a barrier, the cathodic biofilm became less sensitive to the dissolved oxygen as well as pH change in cathodic compartment, and provided better growing condition for the catalyst leading to higher power output [93].
• With the aim to improve electron transfer between the microorganisms and the anodes, felts materials owning good biocompatibility have been investigated.
• This new anode (PPy/GO-GF) proved its high stability when the MFC performance was kept nearly steadily after 120 cycles of lactate feeding [97].
• The MFCs operated with this modified anode showed the maximum power density of 1303 mW m−2, which was 13 times larger than that obtained from the MFCs equipped with an unmodified anode [138].

4.5 Capacitors - supercapacitors

• The excellent electronic performance of the modified CF makes them interesting for capacitor and supercapacitor.
• An electric double-layer capacitor was fabricated from growth nanocrystalline diamond (NCD) film on CF using a hot-filament CVD method.
• Consequence, this modification facilitated the easy access of electrolyte to the composites and leading to enhancement of the capacitive performance [141].
• Another flexible supercapacitor electrode was produced through the synthesis of tubular MnOOH on GF.

4.6 Electrochemical Solar cells

• Solar energy plays a vital type of renewable energy because of the power conversion efficiency of devices [143, 144].
• Some photosynthetic microorganisms like cyanobacteria have been used to convert directly solar energy into electric energy in an electrochemical cell [145- 147].
• After that, the electron was transferred to a CF (CF, Toray Co., Tokyo, Japan, thickness 1 mm) anode through the mediator, 2,6-dimethyl-1,4-benzoquinone (DMBQ) or diaminodurene (DAD).
• Water was regenerated from dioxygen reduction through ABTS2- (2,2′-azinobis(3-ethylbenzothiazolin-6-sulfonate)) as a mediator at the CF cathode and bilirubin oxidase (BOD) as a biocatalyst.
• W m-2 as the maximum electric power were generated with an apparent efficiency of the light energy conversion of 2–2.5% [148].

4.7 Lithium ion batteries

• Owing to their high power and energy density, Li-ion batteries (LIBs) discovered in 1990s by Sony Corporation, have become the leading energy storage systems for portable applications[149].
• The behavior of CF electrode modified by CNTs (CNT/CF) for energy storage was analyzed on the basis of lithium intercalation.
• The CNT/CF was determined as a threedimensional web of electronic conductive carbon fiber which owned excellent mechanical properties.
• Over 50 cycles of discharge/charge, the reversible specific capacity kept stably [32], proving that the modified felts electrodes will be promising material for electrochemical devices.
• In addition, the CF cathode after borax treatment was used in lithium-air batteries [150].

5.1 Introduction

• For environmental application, CF-based electrodes were used for different purposes like electrochemical reduction of heavy metals [151, 152], electrically swift ion exchange (ESIX) [96], electrosorption desalination [153], etc.
• The CF-based electrodes once again proved their benefits when they were applied as cathodes during EF process to eliminate POPs in aqueous medium.
• First of all, an overview about EF process was understood to highlight the advantages of this technology for wastewater treatment.
• After that, the felts cathodes were applied to remove different kinds of POPs.
• Towards these targets, the authors focused both on modified cathodes to upgrade the hydrogen peroxide production and on new catalysts.

5.2 Electro-Fenton (EF) process

• EAOPs based on Fenton’s reaction chemistry are emerging technologies for water remediation.
• Nevertheless for Fenton reaction large amount of chemicals are requested (hydrogen peroxide), ferric hydroxide sludges formation and slow regeneration rate of the ferrous ions catalysis are limiting factors.
• Over the past decade, EAOPs have experienced important advancements, proving its high efficiency for the treatment of wastewater contaminated with toxic and persistent pesticides, organic synthetic dyes, pharmaceuticals and personal care products, and a great deal of industrial pollutants.
• Nowadays the EF process is definitely the most popular EAOP .
• Cathodic air feeding improves the oxygen reduction reaction (ORR) into H2O2 (Eq. (5-1).).

5.3 CF for EF process

• (i) commercial availability with good mechanical properties and high specific surface area [56, 156]; (ii) good adaptability to different shapes and surfaces from small (2 cm2) [23, 25] to large size (60 cm2) [154, 157]; and (iii) high stability to decline significantly the cost for the EF technology [158], also known as exhibiting outstanding properties like.
• One of their first papers in 2000s described the EF process in divided cell.
• From that, a series of studies using EF technology for water treatment on felts cathodes have been conducted to eliminate many different kinds of POPs in aqueous medium, including: (1) Dye pollutants: 95% TOC of the anthraquinone dye Alizarin Red S was removed in 210 min of electrolysis on GF (Carbone-Loraine, thickness 0.5 cm)/BDD [169].

5.4.1 Modified felts cathodes for homogeneous EF

• The production of hydrogen peroxide and its reaction with catalyst (i.e. iron salt) in solution is a crucial factor for the effective destruction of POPs by homogeneous EF process.
• Aiming to improve the in situ generation of H2O2, various attempts have been made to upgrade the electrocatalytic characteristic of CF cathodes.
• As discussed in section 3.3, chemical modification is a simple and efficient way to ameliorate the electrochemical activity of the felt electrodes by changing their surface functional groups.
• The pnitrophenol mineralizationwas clearly improved with the modified cathode compared to the pristine one: TOC removal ratio moves from 22.2%, to 51.4% from CF to CF-B.
• The AQDS/PPy composite film was grown on graphite electrodes by electropolymerization of the pyrrole monomer in the presence of anthraquinone-2,6-disulfonic acid.

5.4.2 Modified felts cathodes for heterogeneous EF

• To overcome the drawbacks of basic EF treatments (i.e. pH regulation at 3, loss of soluble iron catalyst [190, 191], post-treatment requirements prior to discharge [192]) many attempts have been performed on the use of iron based heterogeneous catalyst.
• The self-regulation of iron ions and the possibility to work at near neutral pH is definitely the main advantages of heterogeneous catalyst [193].
• In addition, the recovery of pyrite can be carried out easily by the filtration from the treated solution to reuse for following times.
• Versatility of iron alginate beads was proved with decolorisation of two typical dyes, Lissamine Green B and Azure B (87% and 98% respectively after 30 min) maintaining FeAB particle shapes throughout the oxidation process [170].
• In very recent study, Özcan et al. prepared a new iron containing Fe2O3 modified kaolin (Fe2O3-KLN) catalyst, to develop a heterogeneous EF process with three-dimensional CF cathode for the electrochemical oxidation of ENXN.

5.4.3 Hybrid EF system using CF cathodes

• To boost the degradation efficiency and reduce the treatment cost, many attempts have been made to change the EF reactor.
• A beginning concentration of tartrazine at 100 mg/L could then reach near 100% removal.
• The degradation of methyl orange (MO) by the EF process was conducted in a hemisphere-shaped quartz reactor using dual rotating GF disks (Shanghai Qijie Carbon Material Co., Ltd) cathode to supply oxygen.
• In order to avoid the use of expensive membranes in two-chamber MFCs and to increase the generated power densities, more efficient dual reactor systems were advanced by using a single-chamber in a modified EF/MFC system .

5.4.4 Pilot-scale

• To assess industrial applications, the EF pilot program was set up to treat large volume of contaminated solutions.
• An organic micropollutant, diclofenac (DCF), from drinking water was removed by a novel EF filter pilot .
• The EF laboratory-scale pilot plant with a capacity of 200 L shows satisfactory stability regarding both cathode integrity and removal efficiency.
• Voltage regulator, 8: powered agitator, 9: feed tank.
• Besides, a solar photoelectro-Fenton (SPEF) process in a lab-scale pilot plant (8.0 L) was proposed for textile dye solutions, (mainly cid yellow 42) treatment.

6 Conclusion

• The CF is potential materials which have been widely applied as electrodes in energy and environmental field.
• Pristine felts have excellent properties with respect to electronic conductivity, chemical stability, light weight, and low cost.
• Briefly, CF electrodes will become more useful with interesting properties depending on various modification methods which can be applied in a wide panel of applications.
• The aim of these studies was to lower the cost of VRFB, so cheaper modification methods like chemical treatments were often chosen.
• Wastewater treatment by EF process, using the CF-based cathodes is a wise and very highly efficient choice.

Figures

Figure 2-3. SEM images of (a, b) CF (Johnson Matthey Co., Germany, thickness 1.27 cm) [23] and (c, d) GF (GFD 2.5, SGL Group, thickness 2.8 mm) at various magnifications. Reprinted from Ref. [1]. Copyright (2015), with permission from Elsevier.

Table 2-2. Textural and electrical properties of carbon and graphite felts materials.

Figure 3-5. SEM images of rGO coated on (a) CF (Shanghai Qijie Carbon Co., Ltd.); (b,c) GF (RVG-2000, Carbon-Lorraine company); (d) CVs curves of the CF in 0.5M Na2SO4 solution at a scan rate of 10 mV s-1 and (e) CVs of the various GF electrodes recorded in solution (1.0 M H2SO4) containing 0.05 M VOSO4 (Scan rate of 1 mV s −1). Reprinted from Ref. [62, 67]. Copyright (2016), with permission from Elsevier.

Figure 3-6. EPD of graphene on CF electrodes. Reprinted from Ref. [68]. Copyright (2013), with permission from ACS Publications.

Figure 2-4. Shape of water droplet on (a) GF (SGL GFD2.5, thickness 2.8 mm) and (b) CF (Johnson Matthey Co., Germany, thickness 1.27 cm). Reprinted from Ref. [1] and [25]. Copyright (2015, 2016), with permission from Elsevier.

Figure 3-2. SEM images and contact angles of GF (a, d), GF-ethanol (b, e) and GFethanol/hydrazine (c, f). Reprinted from Ref. [56]. Copyright (2014), with permission from Elsevier.

Figure 5-2. Basic laboratory experimental set-up of the EF process using CF cathode. (1) beaker containing the polluted solution, (2) magnetic stir bar, (3) CF cathode, (4) anode (Pt, Ti, etc), (5) compressed air diffuser, (6) air drying solution and (7) power supply. Reprinted from Ref. [159]. Copyright (2008), with permission from Elsevier.

Figure 5-5. Two hybrid EF systems using CF cathode: (a) vertical-flow EF reactor; and (b) Continuous bubble EF reactor: (1) speed controller, (2) motor, (3) electrolytic cell, (4) rotating GF disk cathode, (5) Pt anode, and (6) carbon brush. Reprinted from Ref. [201] and [114]. Copyright (2016, 2014), with permission from Elsevier.

Figure 5-10. An EF pilot plant tested to treat DCF solutions; 1: feed pump, 2: cartridge filter 60 mm, 3: EF ‘filter’, 4: control valve, 5: flowmeter, 6: PLC (programmable logic controller), 7:

Figure 5-9. Schematic diagram of the Fuel Cell-Fenton system. Reprinted from Ref. [110]. Copyright (2016), with permission from RSC Publishing.

Figure 5-8. A single-chamber MFC powering the EF system. Reprinted from Ref. [207]. Copyright (2013), with permission from Elsevier.

Figure 3-8. (a, b) SEM images and (c) high-resolution TEM (HR-TEM) of CNFs on CF. Reprinted from Ref. [41]. Copyright (2011), with permission from Elsevier.

Figure 3-3. Electrochemical impedance spectroscopy of CF, CF-Bi, GF and GF-Bi in solution of 0.05 M V2+ and 0.05 M V3+ + 3 M H2SO4. Reprinted from Ref. [20]. Copyright (2015), with permission from Elsevier.

Figure 5-3. Reaction sequence for the mineralization of methyl parathion by the EF process. Reprinted from Ref. [163]. Copyright (2007), with permission from Elsevier.

Table 2-3. Textural characteristics of rayon and PAN based GF [19].

Figure 4-3. MFCs configuration. Reprinted from Ref. [137]. Copyright (2015), with permission from Elsevier.

Figure 3-11. SEM images of (a) HNO3-NaX/GF, (b) PB@Pt/GF, (c) TEM image of PB nanoparticles, (d) CVs of the three GF electrodes at the end of the ex-situ acclimatization (vs. Ag/AgCl, scan rate of 5 mV/s ranging from −700 mV to +700 mV), and (e) Nyquist plots of GF, Pt/GF-50 and PB@Pt/GF electrodes in 0.5 M KCl aqueous solution. Reprinted from Ref. [4, 101]. Copyright (2013, 2016), with permission from Elsevier.

Figure 4-1: Components of a VRFB stack using CF. Reprinted from Ref. [115]. Copyright (2006), with permission from Elsevier.

Figure 5-1. Schematic presentation of the electrocatalytic production of hydroxyl radicals by the EF process. Reprinted from Ref. [155]. Copyright (2001), with permission from Elsevier.

Figure 5-7. BEF system in batch mode using non-catalyzed CF for medicinal herbs wastewater treatment. Reprinted from Ref. [195]. Copyright (2016), with permission from Elsevier.

Figure 5-4. Evolution of the inhibition of marine bacteria during the EF process (Pt ( ) and

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Carbon felt based-electrodes for energy and environmental applications: a review" ?

In this review, the carbon felt ( CF ) basedelectrodes are discussed in a holistic manner. First of all, the study centers on the issues relevant to pristine CF materials such as manufacturing method and specific properties. For environmental applications, the authors focus their study on the wastewater treatment containing biorefractory pollutants by electro-Fenton ( EF ) process. 

Q2. What have the authors stated for future works in "Carbon felt based-electrodes for energy and environmental applications: a review" ?

Therefore, the choice of suitable methods added to several techniques for calculation of these structural and physical parameters of CF electrodes will still be a fascinating story in the future studies. Therefore, these new fields will be a fertile ground for future studies to develop new applications of CF-based electrodes. The modification and the use of carbon based-electrodes for energy and environment applications will be a very interesting topic in the future. For this special application, CNTs have displayed great potential owing to their novel structural, electrical and mechanical properties [ 2 ]. 

Q3. What are the main advantages of heterogeneous catalyst?

The self-regulation of iron ions and the possibility to work at near neutral pH is definitely the main advantages of heterogeneous catalyst [193]. 

Q4. What is the effect of noble metals on the surface of felts?

The electrocatalyst introduction of noble metals on the surface of felts enhances the electroconductivity of electrode materials, which reduces the reaction over potential of vanadium ion redox couples. 

Q5. What is the effect of iron-doping on graphite felt?

In particular, when it was applied as cathode for microbial fuel cells (MFCs), iron-doping increased the electron transfer efficiency, because iron coated on graphite felt was oxidized rapidly to iron oxide when it comes in contact with the solution in MFC and oxygen from the air. 

Q6. What is the main step in the manufacturing of graphite felts?

The needle punching is animportant step which decides the internal structure, textile structures or thickness homogeneity of produced felts. 

Q7. What methods were used for the coating of graphene based materials on felts electrodes?

Different methods as dip-coating, constant potential technique and electrophoretic deposition (EPD) were used separately or combined together for the coating of graphene based materials on felts electrodes. 

Q8. What is the main reason for the use of felts electrodes in energy applications?

For application of felts electrodes in these fields, the choice of modification is highly important, because these applications require nanostructured electrodes with considerable conductivity. 

Q9. What are the main methods for identifying specific surface areas of felts?

Three main methods for identification of specific surface areas of felts, including: physical methods (the permeametry mercury porosimetry [27], physical adsorption of gases [28]); structural methods (the filamentary analog procedure [29]); and electrochemical methods [30]. 

Q10. How much surface area of the modified felts increased after the heat treatment?

After the heat treatment in air at 400 °C, the surface area of the modified felts increased by more than ten times in comparison to the pristine one based on rayon (SGL, thickness 3 mm) [24]. 

Q11. What is the reversible specific capacity of the modified felts electrodes?

Over 50 cycles of discharge/charge, the reversible specific capacity kept stably [32], proving that the modified felts electrodes will be promising material for electrochemical devices. 

Q12. What is the effect of the GF modification on the electrical performance of the composites?

this modification facilitated the easy access of electrolyte to the composites and leading to enhancement of the capacitive performance [141]. 

Q13. What is the effect of the bi-modified felt on the electrolytes?

Despite the low metal content (1 at.%) on the surface of the fibers, the Bi-modified felt showed an excellent electrochemical improvement when an electrode was applied for vanadium redox flow battery [6]. 

Q14. What are the main applications of felts electrodes?

for capacitors, supercapacitors, electrochemical solar cells or Li-ion batteries, felts electrodes have seen little exploitation yet. 

Q15. What could be the main reason for the improvement of the energy efficiency of a VRFB?

This could be pointed out that the inclusion of flow fields in a VRFB could be an effective approach for improving the system efficiency [119]. 

Q16. How was the electrochemical activity of poly(aniline-co-o-amino?

The electrochemical activity of poly(aniline-co-o-aminophenol) was about four times as high as that of PANi in 0.3M Na2SO4 solution of pH 5. 

Q17. How was the coating of graphene oxide on feltselectrode performed?

The coating of single-walled carbon nanotube (SWCNT) was performed by simple way where CF was immersed into the SWCNT suspension. 

Q18. What is the effect of the AQDS/PPy composite film on pnitrophenol?

The AQDS/PPy composite film was grown on graphite electrodes by electropolymerization of the pyrrole monomer in the presence of anthraquinone-2,6-disulfonic acid. 

Q19. What was the performance of the new electrode?

The new electrode exhibited improved performance compared with PPy alone when it could increase significantly the power density of MFCs [97]. 

Q20. What are the two methods used to calculate the porosity of CF?

Tocalculate this value, the helium density and mercury porosimetry (density methods) are usually applied because these methods seem suitable for compressible materials such as CF. 

Q21. How many times higher was the active surface of the modified electrode?

By the in-situ growth of CNFs, the active surface identified by the electrochemical double layer capacities of the modified electrode was 50 times higher than non-modified one [87].