Keywords

1 Introduction

Electro-Fenton (EF) process is based on the continuous in situ production of hydrogen peroxide (H2O2) and the addition of Fe2+ ion as a catalyst to generate hydroxyl radical (•OH) at the solution through Fenton’s reaction in acidic condition as the following reaction:

$$ {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{H}}^{+}\to {\mathrm{Fe}}^{3+}{+}^{\bullet}\mathrm{OH}+{\mathrm{H}}_2\mathrm{O} $$
(1)

H2O2 can be continuously produced in an electrolytic cell from the two-electron reduction of oxygen gas at the cathode electrode by reaction (2) (E° = 0.695 V/SHE), which occurs more easily than its four-electron reduction to water from reaction (3) (E° = 1.23 V/SHE) [1]:

$$ {\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2 $$
(2)
$$ {\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O} $$
(3)

In EF process, Fe2+ can be regenerated via cathodic reduction (reaction (4)), which accelerates the generation of •OH from Fenton’s reaction (1):

$$ {\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+} $$
(4)

Cathode material has prominent effects on the oxidation power of the EF process and H2O2 electro-generation efficiency. Carbonaceous materials are subject of great interest as cathode electrodes for the two-electron reduction of O2 to H2O2 and the favorable options for electrocatalyst support in virtue of being nontoxic and stable and having high overpotential for H2 evolution and relatively good chemical resistance and conductivity [2]. In the 1970s, Oloman and Watkinson [3, 4] firstly investigated the application of graphite particles in the trickle-bed electrochemical reactors for the cathodic reduction of O2 to H2O2. Especially worth noting are the researches reporting the use of planar (2D) cathodes such as graphite [5,6,7,8,9], gas diffusion electrodes (GDEs) [10,11,12,13], three-dimensional (3D) electrodes such as activated carbon fiber (ACF) [14], carbon felt (CF) [15,16,17,18,19], carbon sponge [20, 21], reticulated vitreous carbon (RVC) [22,23,24], O2-fed carbon polytetrafluoroethylene (PTFE) [25, 26], and boron-doped diamond (BDD) [27, 28].

Due to the poor solubility of O2 in aqueous solution (about 40 or 8 mg L−1 in contact with pure O2 or air, respectively, at 1 atm and 25°C), GDEs and 3D electrodes of high specific surface area are favored as cathodes to supply reasonable current densities for practical applications. GDEs have a thin and porous structure preferring the percolation of the injected gas across its pores to contact the solution at the carbon surface. These electrodes have a great amount of active surface sites leading to a very fast O2 reduction and large production of H2O2 [1]. Figure 1 provides a schematic diagram of structure and function of GDE.

Fig. 1
figure 1

Schematic diagram of structure and function of GDE

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In the last three decades, carbon-based nanomaterials have attracted substantial attention due to their superior electronic, photonic, electrocatalytic, chemical, and mechanical features that remarkably depend on their nanoscale properties [29]. Carbon-based nanomaterials can be classified into two groups: nanosized and nanostructured carbons [30]. Many more types of carbon materials, including graphene family (e.g., graphene, graphene oxide (GO), and reduced graphene oxide (rGO)), carbon nanotubes (CNTs), nanofibers, nanodiamonds, nanocoils, nanoribbon, and fullerene belonging to nanosized class, because the shell size and thickness of these carbon materials are on the nanometer scale [29]. New carbon materials such as carbon fibers and ordered mesoporous carbons are classified as nanostructured carbons, because their nanostructure is controlled in their construction through various processes [30]. Figure 2 provides a schematic illustration of some nanocarbons. Carbon blacks are constructed of nanosized particles, but they do not usually belong to nanocarbons due to their various applications as a mass and not in their distinctive form of nanosized particles [31].

Fig. 2
figure 2

Schematic illustration of some carbon-based nanomaterials

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In addition, doping carbon nanomaterials with heteroatoms, especially nitrogen, can enhance the performance of oxygen reduction activity by improving the surface chemical reactivity, conductivity, catalytic sites, and stability [32]. Among different possible dopants, nitrogen doping could either enhance the current of oxygen reduction or diminish the onset overpotential through (1) increasing chemically active sites, (2) improving the O2 chemisorption, and (3) enhancing the hydrophilicity of surface [33].

Therefore, there are many investigations focused on the modification of cathode electrode by carbon-based nanomaterials [5, 34,35,36]. In these studies, the performance of EF process has been enhanced through improving the mass transfer characteristics of cathode. The novel EF electrode materials should possess several properties as follows: high selectivity for two-electron reduction of oxygen, good mass transfer performance, high electrochemical active reaction area, and high electrical conductivity.

The purpose of this chapter is to review the attempts in surface modification of cathode electrodes with carbon-based nanomaterials, e.g., CNTs, graphene family, and mesoporous carbons for EF process.

2 Modification of Cathodes with Carbon-Based Nanomaterials for EF Process

2.1 Carbon Nanotubes

The discovery of CNTs by Iijima in 1991 [37] has created a revolution in nanotechnology and material science. CNTs have attracted substantial consideration from the scientific community as one of the main members of carbon nanomaterials with unique optoelectronic, electrochemical, and electronic features [38]. The carbon atoms in CNTs are ordered in hexagons with sp2 hybridization (one-dimensional (1D) system) [29]. A single-walled CNT (SWCNT) is produced by the rolling of a graphite layer into a nanoscale tube form which has an approximate diameter of 1 nm. Multiwalled CNTs (MWCNTs) can be constituted of two or more numbers of coaxial SWCNTs with expanding diameters that are separated from each other by a distance of around 0.34 nm (see Fig. 3) [33].

Fig. 3
figure 3

The structure of SWCNT and MWCNT

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CNTs can be semiconducting or metallic in their electronic properties with an electrical conductivity up to 5,000 S cm−1 [38]. Their conductivity is highly dependent on their chirality of the graphitic hexagonal array and diameter. The highly conductive nature of the CNTs confirms their high charge transport ability [29]. Experimental specific surface area of SWCNTs is in the range between 370 and 1,587 m2 g−1 with micropore volume of 0.15–0.3 cm3 g−1 [39]. The MWCNT has a specific surface area between 180.9 and 507 m2 g−1 with mesopore volume of 0.5–2 cm3 g−1 [39]. The tensile modulus and strength of SWCNTs are usually in the range of 320–1,740 GPa and 13–52 GPa, respectively, while being 270–950 GPa and 11–63 GPa in MWCNTs [29, 38]. Besides the huge specific surface area and electrical conductivity, CNTs also have a great thermal conductivity of 6,000 W mK−1 [38]. Due to these interesting properties, CNTs are promising nanomaterials for different applications such as in hydrogen-storage systems, sensors, organic photovoltaic cells, supercapacitors, fuel cells, batteries, and solar cells [29, 38, 39]. The applications of CNTs and their derivatives as electrocatalysts for two-electron reduction of O2 in EF system will be discussed.

During the last years, a number of researches have been focused on the modification of cathode electrode with CNTs to improve its performance for in situ H2O2 generation in EF oxidation process. Table 1 summarizes some of the recent reported that modified cathode with CNTs and their derivatives in EF process.

Table 1 Selected results reported for modified cathodes with carbon nanotubes
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Zarei et al. [52,53,54] coated the surface of carbon paper as a GDE cathode with CNTs and compared its efficiency for in situ H2O2 generation with activated carbon/GDE. PTFE was used to bind the carbon materials into a cohesive layer and convey some hydrophobic feature to the electrode surface. The scanning electron microscopy (SEM) images of the uncoated GDE and CNTs/GDE are shown in Fig. 4. As it can be seen from SEM images, coating of CNTs on GDE electrode improves the specific surface area of the cathode. The results demonstrated that the amount of produced H2O2 on the CNTs/GDE electrode (14.3 mmol L−1) was approximately three times higher than that of activated carbon/GDE electrode (5.9 mmol L−1) (Fig. 4c). The degradation efficiency of Basic Yellow 2 (BY2) in peroxi-coagulation process reached 62% and 96% in the first 10 min using activated carbon/GDE and CNTs/GDE electrodes at 100 mA, respectively [52]. The different abilities of H2O2 electro-generation of activated carbon/GDE and CNTs/GDE electrodes are attributed to the huge surface area and good electrical conductivity of CNTs [52,53,54].

Fig. 4
figure 4

SEM image of surface of cathodes: (a) uncoated GDE, (b) CNTs/GDE, and (c) the amount of electro-generated H2O2 at the uncoated GDE and CNTs/GDE cathodes after 300 min electrolysis (Adapted from [52] with permission from publisher, Elsevier. License Number: 4054120708166)

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In another study, graphite electrode was modified by CNTs for treatment of Acid Yellow 36 (AY36) by photo-EF process [5]. The electro-generated H2O2 concentration using the CNTs/graphite cathode was approximately seven times greater than that of bare graphite cathode. The decolorization efficiency of AY36 was 31.07 and 70.98% after 120 min of photo-EF treatment for bare graphite and CNTs/graphite, respectively [5]. Also, graphite electrode was modified with MWCNTs accompanied by a cationic surfactant (cetyl trimethyl ammonium bromide (CTAB)) and used as a cathode to degrade two acid dyes by homogeneous and heterogeneous EF processes [46, 47]. The electrodeposition method was used to modify the graphite electrode surface, which was performed by applying the DC voltage to the MWCNTs and CTAB solution. High dye removal efficiency was achieved when MWCNT/graphite was as the cathode compared to the graphite electrode (92% against 64% for 50 mg L−1 of dyes), due to the higher electro-generation of H2O2 on the surface of the MWCNT/graphite cathode [46, 47].

Recently, some studies revealed that the introduction of nitrogen atoms to the pristine CNT structure can lead to promote the chemical and electrochemical reactivity of surface for oxygen reduction reaction by the generation of extra electron density in the graphite lattice [33, 38]. Zhang et al. [51] prepared the nitrogen functionalized CNT (N-CNT) electrode as a GDE cathode in EF process. In this study, pulsed high voltage discharge was applied to functionalize MWCNTs in a liquid-gas reactor. The results showed that among three electrodes including graphite, CNTs, and N-CNTs, the N-CNT electrode indicated the highest yield of H2O2 formation and faster color removal in EF process. The amount of generated H2O2 on the graphite, CNT, and N-CNT electrodes were 2.72, 3.06, and 4.28 mmol L−1, respectively. Furthermore, the N-CNT electrode had the greater current efficiency compared to that of CNT electrode. The results confirmed that the nitrogen functionalization did facilitate the electron transfer to improve the production of H2O2.

Nitrogen-doped MWCNTs (N-CNTs) was also used as the catalyst layer on the GDE cathode, which was prepared by immobilizing MWCNTs as the diffusion layer on the surface of nickel foam (NF) as the supporting material [49]. Results showed that the N-CNT/NF/CNT GDE exhibited higher H2O2 production amount and greater current efficiency in comparison with the CNT/NF/CNT GDE, consequently, the EF degradation level and total organic carbon (TOC) removal efficiency were higher.

2.2 Graphene Family

Graphene and its derivatives, such as GO, rGO, and few-layer GO, have been thoroughly investigated since their discovery because of their special physical-chemical properties [55]. Graphene, GO, and rGO have different morphological and chemical characteristics as shown in Fig. 5. Pristine graphene consists of a carbon monoatomic layer, 2D planar sheet of carbon atoms in the sp2 hybridization state, which are densely organized into a honeycomb array (Fig. 5a) [56]. It was first achieved in 2004 by Novoselov and Geim [57], who prepared graphene sheets by micro-mechanical splitting of oriented pyrolytic graphite and definitively recognized using microscopy. In recognition of the enormous significance of graphene for different applications, its discovery was awarded the 2010 Nobel Prize in Physics. Theoretical and experimental investigations have evidenced that graphene has numerous outstanding properties, comprising a huge specific area (around 2,630 m2 g−1) [55], exceptional mechanical strength (tensile strength of 130 GPa and Young’s modulus of 1,000 GPa) [58], high thermal conductivity (in the range of 4,840–5,300 W m−1 K−1) [59], high electrical conductivity (up to 6,000 S cm−1) [60], great charge-carrier mobility at room temperature (2 × 105 cm2 V−1 s−1) [61], and chemical inertness [62]. Consequently, it is not surprising that graphene has attracted great interest for using in a plethora of various applications, such as supercapacitors, batteries, solar cells, fuel cells, etc. [33, 38].

Fig. 5
figure 5

Schematic illustrating the chemical structure of a single sheet of (a) graphene, (b) GO, and (c) rGO

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In general, graphene can be produced either by bottom-up or top-down techniques. The bottom-up method comprises epitaxial growth and chemical vapor deposition (CVD), including the direct preparation of defect-free graphene from hydrocarbon precursors on solid substrates (Ni or Cu) [38, 63]. Top-down methods, such as electrochemical exfoliation and reduction of GO, refer to the mechanical cleaving of graphite layers for the mass fabrication of graphene sheets. Top-down methodologies present the opportunity to economically synthesize graphene, but it is difficult to obtain high-purity graphene sheets because of the introduction of defects through exfoliation process [29, 38].

The GO is another member of the graphene family, which is an oxygen-functionalized graphene that is fabricated by exfoliation of graphite oxide [64]. The GO is viewed mainly as the precursor to generate graphene [38]. On the GO surface, there are plentiful oxygen-based groups, including epoxy (1,2-ether) (C-O-C) and hydroxyl (−OH) groups, located on the hexagonal array of carbon plane, and carbonyl (−C = O) and carboxyl (−COOH) groups, located at the sheet edges (see Fig. 5b) [56].

The rGO, graphene-like, can be prepared via top-down methods including thermal, chemical, and electrochemical reduction of GO to decrease its oxygen content, with the ratio of C/O rising from 2:1 to up to 246:1 (Fig. 5c) [65]. Although the rGO possesses more defects and thus has less conductivity than pristine graphene, it is enough conductive for use as the electrode material for numerous applications [66]. As graphene, the rGO has also received great attention for different applications in electrochemical devices due to its high specific surface area, functional groups containing oxygen, and hydrophilicity [38]. The oxygen functionalities are opening an adjustable bandgap which is responsible for particular electronic and optical properties [56].

According to the mentioned properties, graphene and its derivatives are alternative candidates for potential use as carbon-based nanomaterials for improving the efficiency of cathode materials employed in EF system. Various scientific reports on applications of graphene family for modification of the cathodes in EF process is summarized in Table 2.

Table 2 Results reported for modified cathodes with graphene family in EF process
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Recently, Mousset and co-workers [76] studied the efficiency of pristine graphene (in the forms of monolayer (Gmono), multilayer (Gmulti), and foam (Gfoam)) as the cathode material in EF process for phenol treatment. It was found that the generated H2O2 concentration on the Gfoam (0.250 mmol L−1) cathode was 5–50 times more than that on the Gmulti (0.055 mmol L−1) and Gmono (0.005 mmol L−1), respectively. The degradation efficiency of 1 mmol L−1 phenol was 10.1%, 20.1%, and 62.7% for Gmono, Gmulti, and Gfoam electrodes, respectively. Therefore, the higher performance of Gfoam cathode was attributed to its greater electroactive surface area and its higher electrical conductivity than other forms of pristine graphene. Therefore, Gfoam cathode showed higher phenol degradation and mineralization efficiency than other graphene-based cathodes due to greater rates of •OH formation over Fenton’s reaction. Furthermore, less energy consumption and higher mineralization efficiency were achieved by using Gfoam cathode in comparison with carbon felt cathode, because of the higher electrical conductivity of Gfoam. The Gfoam cathode displayed excellent stability as degradation occurred after 10 EF runs.

In another study by this group [34], high purity of graphene was prepared by electrochemical exfoliation. Synthesized graphene was combined with Nafion as a binder to make a conductive ink which was then employed to modify the carbon cloth electrode [34]. The optimal amounts of graphene and Nafion in the ink were found to be 1.0 mg mL−1 and 0.025% (w/v), respectively, with a graphene mass loading of 0.27 mg cm−2 on the carbon cloth surface. A graphical illustration of preparation of graphene-modified carbon cloth electrode is depicted in Fig. 6. The results showed that the graphene-modified carbon cloth cathode improves electrochemical properties, such as the 97% decline of the charge transfer resistance and an 11.5-fold increment of the electroactive surface area compared with raw carbon cloth [34]. As illustrated in Fig. 6, the maximum electro-generated H2O2 concentrations were 1.01 mmol L−1 and 1.99 mmol L−1 for the uncoated and graphene-coated carbon cloth cathodes, respectively [34]. The superior electrochemical behaviors of the graphene-coated carbon cloth cathode were further proved by the improved performance in EF process for degradation of phenol. Thus, the pseudo-first-order kinetic rate constant (kapp) values of phenol degradation on the uncoated and graphene-coated carbon cloth cathodes were 0.0051 and 0.0157 min−1, respectively, a 3.08-fold increase.

Fig. 6
figure 6

Schematic steps of preparation of graphene-coated carbon cloth cathode and H2O2 accumulation yield of uncoated and graphene-coated carbon cloth cathodes (SEM images and H2O2 accumulation yield curves adapted from [34], with permission from Elsevier. License Number: 4047601289247)

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Le et al. [68, 69] modified CF electrode with rGO, which was prepared by an electrophoretic deposition of GO and was reduced with the different methods including electrochemical, chemical, and thermal. Among the used reduction methods, the electrochemical reduction of GO under a constant potential (−0.45 V vs. SCE) without addition of any binder or reductant demonstrated remarkable advantages. The schematic of preparation of electrochemically reduced GO (ErGO)/CF electrode and SEM images of ERGO/CF and raw CF were presented in Fig. 7. The ErGO/CF cathode demonstrated significant electrochemical behaviors, such as the enhancement of electroactive surface area and the decline in charge transfer resistance compared to the raw CF cathode. This improvement accelerated the O2 reduction rate on the cathode surface, which significantly increased the H2O2 accumulation in the solution. Consequently, the destruction rate of Acid Orange 7 (AO7) by the EF process was two times greater on the ErGO/CF cathode compared to uncoated CF. TOC removal after 2 h degradation was 73.9% on the ErGO/CF electrode, and this was 18.3% greater than on the unmodified CF (Fig. 7c). Moreover, the ErGO/CF cathode presented good stability over ten runs of EF process for mineralization of AO7.

Fig. 7
figure 7

Schematic steps of preparation of ErGO/CF cathode, SEM images of (a) raw CF, (b) ErGO/CF, and (c) TOC removal after 8 h EF process using raw CF and ErGO/CF cathodes. (Adapted from [68], with permission from Elsevier. License Number: 4036640134966)

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Chen et al. [36] modified the glassy carbon electrode and studied the effect of annealing temperature of GO (250 and 1,000°C) on the electro-generated H2O2 efficiency in EF process. The results indicated that the thermally reduced GO annealed at 250°C (G250) was more efficient for mineralization of methylene blue (MB) by the EF method. The oxygen functionalities in G250 were responsible for the high two-electron oxygen reduction selectivity and highest formation rate of H2O2 [36].

The results of studies obviously indicated that modification of carbon-based electrode surface with quinone functional groups could remarkably improve the redox activity of the electrode and facilitate the two-electron reduction of O2 to H2O2 reaction on the cathode [75, 80,81,82]. Zhang and co-worker [75] studied the electro-generation of H2O2 on anthraquinone@ErGO (AQ@ErGO) coated on nickel screen surface cathode and its performance for degradation of Rh B by FeOOH-catalyzed heterogeneous EF process. The strong interfacial connections of ErGO and AQ molecules led to the efficient production of H2O2 at the cathode. The AQ@ErGO cathode can efficiently catalyze the two-electron reduction of O2 to produce H2O2 (reactions (5) and (6)) on the cathode/bulk solution interface:

$$ \equiv \mathrm{AQ}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to \equiv {\mathrm{H}}_2\mathrm{AQ} $$
(5)
$$ \equiv {\mathrm{H}}_2\mathrm{AQ}+{\mathrm{O}}_2\to \equiv \mathrm{AQ}+{\mathrm{H}}_2{\mathrm{O}}_2 $$
(6)

The accumulated concentration of H2O2 was obtained at 4.01 and 4.86 mmol L−1 in 0.5 mol L−1 MgSO4 and Na2SO4 electrolyte, respectively, after 120 min of electrolysis. Then, electro-generated H2O2 molecules are catalytically converted into •OH by the FeOOH nanoparticles, and the dissolved iron ions in MgSO4 catholyte. Since, no dissolved iron ions were detected in Na2SO4 catholyte, the high yield of the hetero-EF process is ascribed generally to the H2O2 activation through the surface of FeOOH nanoparticles to form •OH and HO2 • (O2 •−).

Zhao et al. [70] synthesized the graphene/polypyrrole (PPy) modified conductive cathode membrane for the EF filtration treatment of MB as a model pollutant. The better performance of membrane cathode for treatment of MB was obtained by doping with anthraquinone monosulfonate (AQS). The observed performance enhancement can be attributed to the electrical conductivity improvement, resulted by doping with AQS [70].

In recent years, researchers studied the several carbon nanocomposites with metal/metal oxide for modification of electrodes in EF process. Magnetite (Fe3O4) seems to be promising candidate for this purpose owing to its reversible redox nature and stability. These modified electrodes revealed extraordinary mechanical stability, making them noteworthy as stable materials for in situ generation of H2O2 and •OH, diminishing the iron sludge formation, exhibiting much higher activity than homogenous EF systems under a neutral pH.

Shen et al. [72] synthesized graphene-Fe3O4 (G-FeO) hollow hybrid microspheres by a simple aerosolized spray drying method by using ferric ion and GO with various contents (e.g., 0, 5, 15, 30 wt%) as the precursor materials. Subsequently, the obtained composites were coated on the surface of Ni foam cathode. The results of electrochemical studies obviously indicated that the G-FeO composite with graphene content of 30 wt% (30G–FeO) exhibited higher conductivity and lower charge transfer resistance. Also, the two-electron pathway was the dominated process for O2 reduction on the 30G–FeO electrode. The yield of H2O2 generation notably increased when 30G–FeO was applied as the cathode in EF process. The MB degradation rate constant value of 30G–FeO coated Ni foam cathode at pH 2 was 0.140 min−1, which was nearly 8.75 times greater than that for the uncoated Ni foam cathode (0.016 min−1). Figure 8 shows the schematic illustration of EF system and mechanism for MB degradation process on the 30G–FeO cathode.

Fig. 8
figure 8

Schematic illustration of EF system and mechanism for MB degradation process on the 30G–FeO cathode (Reprinted from [72], with permission from Elsevier. License Number: 4037580393197)

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Researches revealed that palladium (Pd) nanoparticles could interact with graphene-based materials and exhibited extraordinary electrocatalytic ability. Zhang et al. [74] modified CF cathode with Pd@rGO composite and Nafion as a binder. Pd@rGO/CF cathode exhibited high electrocatalytic activity and stability for the elimination of ethylenediaminetetraacetic acid (EDTA)-Ni complex solution by the EF method.

Govindaraj et al. [80] synthesized a quinone-functionalized graphene by the electrochemical exfoliation approach (QEEG) followed by prepared QEEG@Fe3O4 nanocomposite. Then, QEEG and prepared nanocomposite were used for modifying the surface of the noncatalyzed carbon cloth (NCC) electrode. The SEM images of the NCC and the modified NCC are shown in Fig. 9a. The obtained results demonstrated that the produced H2O2 concentration at the QEEG electrode was approximately nine times higher than that at the NCC electrode at pH 3.0 and four times greater at natural pH (see Fig. 9b), which can be attributed to the presence of the quinone functional group and high electroactive surface area in the QEEG structure. Substantial improvement in the electro-generation of •OH radicals was observed with QEEG@Fe3O4 modified cathodes. Complete degradation of Bisphenol A (BPA) by EF process was achieved using the QEEG@Fe3O4 modified electrode in 90 min at pH 3. Also, 98% degradation yield was obtained at neutral condition with less than 1% of iron leaching. Schematic illustration of the overall mechanisms relating to QEEG@Fe3O4 modified cathode in the EF treatment of BPA is shown in Fig. 9c.

Fig. 9
figure 9

(a) SEM images of NCC and QEEG coated carbon cloth, (b) difference in H2O2 formation with NCC and QEEG modified cathodes, and (c) schematic illustration of the overall mechanisms relating to QEEG/Fe3O4 modified cathode in the EF treatment of BPA (Reprinted from [80], with permission from Elsevier. License Number: 4047001318719)

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2.3 Mesoporous Carbons

In the past two decades, mesoporous carbons (with pore size distribution in the range 2–50 nm) have attracted great consideration for use as electrode materials in various applications [29]. These carbon-based nanomaterials have delivered noteworthy advantages such as high specific surface areas for a huge number of surface-active sites, good electrical conductivity for facile electron transport, large accessible space for fast mass transport, high mechanical and chemical durability for powerful electrode longevity, and low density [83]. The synthetic approaches comprising hard and soft templates have established to be the most effective methods for the construction of mesoporous carbons with distinct pore structures and narrow distribution of pore sizes [29]. In these preparation methods, mesoporous carbon structures can be obtained after curing of carbonaceous precursor, elimination of template, and carbonization. In the hard templating method, inorganic templates (hard templates), including metal-organic frameworks (MOFs), zeolites, silicas, and MgO, were employed to synthesize ordered mesoporous carbons (OMC) [29, 83]. Silica templates with ordered mesoporous framework were prepared by templating self-formation of surfactants, such as SBA-15, MCM-48, and MCM-41 [83]. Schematic graphic of the preparation of OMC by silica hard templates is shown in Fig. 10. On the other hand, in the soft templating technique, phenolic resin and some block copolymer surfactants were mainly used as organic templates to produce highly OMC through organic-organic assembly of surfactants and phenolic resins [29]. Additionally, by incorporating soft and hard templating approaches, hierarchically porous carbon (HPC), sometimes described as carbon nanoarchitecture, with organized porosity on multiple levels can be achieved [29, 84].

Fig. 10
figure 10

Schematic graphic of the preparation of OMC by silica hard templates

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Recently, mesoporous carbons have been considered to be exceptional candidates for modification of cathode electrode in EF process, which can facilitate the diffusion and transformation of O2 at the cathode surface and enhance the electro-generation yield of H2O2 [85,86,87,88]. Table 3 summarizes the main reported modified cathode with CNTs and their derivatives in EF process. Hu et al. [85] grafted the surface of activated carbon fiber (ACF) cathode with OMC, which was prepared by soft templating method. For comparison, ACF was also modified with a layer of disordered mesoporous carbon (DMC). The results demonstrated that the production rate of •OH radicals pursued the order of OMC/ACF > DMC/ACF > ACF, which was in accordance with the H2O2 generation rate and Brilliant Red X3B (X3B) degradation rate. A graphical illustration of preparation of OMC modified ACF cathode is depicted in Fig. 11.

Table 3 Results reported for modified cathodes with mesoporous carbons in EF process
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Fig. 11
figure 11

A graphical illustration of preparation of OMC modified ACF cathode

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As previously mentioned, heteroatom (e.g., sulfur and nitrogen) doping of carbon materials can improve their surface attributes, specifically the electrical conductivity and the polarity of surface. For this aim, nitrogen-doped mesoporous carbons were prepared by nitrogenous precursors. For instance, nitrogen-doped OMC (N-OMC) was prepared by dicyandiamide (C2H4N4) and was coated onto the surface of ACF cathode (N-OMC/ACF), which showed more electrocatalytic activity and lower overpotential for O2 reduction compared to OMC/ACF cathode in the EF process [86].

Perazzolo et al. [91, 92] synthesized nitrogen- and sulfur-doped or co-doped mesoporous carbons (N-MC, S-MC, and N,S-MC) by means of a hard template method and used them for modifying glassy carbon electrode for the in situ formation of H2O2 and degradation of MO by the EF system. The N-MC modified electrode showed higher performance in EF process compared with S-MC and N,S-MC modified electrodes.

The correlation between mesoporous structure and efficiency of cathode materials in the EF method was investigated [87]. In this research, OMCs with average pore size of 2.6, 3.6, and 5.4 nm were prepared by means of boric acid as the expanding agent and coated on the surface of ACF. Figure 12a, b show TEM images of OMC-3.7/ACF and OMC-5.4/ACF. H2O2 accumulation and degradation profiles of Rh B in EF system in the as-prepared cathodes is illustrated in Fig. 12c, d, respectively. It was found that the large pore size (5.4 nm) promotes the mass transfer of O2 on the surface of the modified cathode, which then results in high generation of H2O2 and consequently enhances the degradation efficiency. After ten consecutive EF runs, the reactivity of OMC-5.4/ACF cathode remained approximately unchanged.

Fig. 12
figure 12

TEM images of (a) OMC-3.7/ACF, (b) OMC-5.4/ACF, (c) H2O2 accumulation, and (d) degradation profiles of Rh B in EF system in the as-prepared cathodes (Reprinted with the permission from [87], Copyright 2015 American Chemical Society)

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In another research, rGO was employed to fabricate rGO@OMC/ACF cathode with lower impedance and better electroactive surface area compared with OMC/ACF, which improved the H2O2 production and current efficiency of the EF process. The observed electrochemical performance enhancement can be attributed to the electrical conductivity improvement, resulted by coating of rGO.

Wang and co-workers [89] synthesized CMK-3-type OMC with a pore size of around 4.3 nm by applying the SBA-15 as a hard template. Then, carbon paper was covered by as-prepared CMK-3 to fabricate the GDE cathode with high porosity and large surface area. Using this electrode, the side reaction of H2 evolution is minimized at a low cathodic potential; thus the H2O2 formation is increased to rapidly degrade organic pollutant such as dimethyl phthalate (DMP) by EF process.

Recently, Liu et al. [88, 93] coated the carbon paper surface with HPC which was prepared by hydrothermal synthesis of MOF-5 as a hard template, and then its carbonization resulted HPC to exhibit high amount of sp3 carbon hybridization and defects, huge surface area (2,130 m2 g−1), and rapid O2 mass transport. The modified carbon paper presented a high selectivity for the O2 reduction to H2O2 in a broad range of pH (1–7). Perfluorooctanoate (PFOA) was efficiently treated by using HPC modified cathode at low potential (−0.4 V). The superior efficiency of this EF process can be ascribed to high H2O2 generation at the modified cathode at low energy consumption, demonstrating their promising application for efficient treatment of recalcitrant pollutants in wastewater.

3 Conclusion

The main concern with the EF process is to improve the generation of H2O2 and enhance the reduction rate of ferric ions on the cathode for effective destruction of pollutants. Thus, it is worthwhile to further develop the performance of cathode with its surface modification. Recently, carbon-based nanomaterials have attracted substantial attention due to their superior physicochemical properties including high specific surface area, good electronic conductivity, chemical inertness, and facile surface modification capability. This chapter discussed the modified cathodes with carbon-based nanomaterials, e.g., CNTs, graphene family, and mesoporous carbons, for EF system. Progress in the modification of cathodes with these nanomaterials for performance development of EF process has been tremendous in recent years, opening novel alternatives in the degradation of recalcitrant pollutants in wastewater.

Despite the extensive research on the modification of cathodes in EF processes, several challenges still need to be addressed to optimize the design of these cathodes for industrial applications at a large scale. First, a technique for better coating or condensing of carbon nanomaterials needs to be further explored. Due to the fact that nanomaterials may be leached from the coated bed, the efficient coating approaches should be developed. Second, carbon nanomaterials generally have a strong tendency to agglomerate owing to their nanosize and high surface energy. Therefore, their applications are limited due to the difficulty in dispersing them in a solvent (water or organic agent) for coating on the electrode. Improved dispersion of carbon nanomaterials could be achieved by modifying their surfaces or optimizing the coating process. Also, this matter could be resolved by preparing of spongelike or aerogel structure of carbon nanomaterials as an electrode and in situ synthesis of nanomaterials on the electrode surface. In this case, the durability of modified cathode electrodes could be improved. Third, considering the potential effects of leached carbon nanomaterials to the environment, nanomaterial leakage and its environmental toxicity also need to be systematically evaluated. Finally, there are many laboratory-scale researches on the application of modified cathodes with carbon nanomaterials in EF processes, but the industrial application of these cathodes is still not developed. More studies are needed to investigate the cost-effectiveness of large-scale modified cathode fabrication including the supply of carbon nanomaterials and to monitor the long-term stability of modified cathodes under practical application conditions.