FeOOH-Catalyzed Heterogeneous Electro-Fenton System upon Anthraquinone@Graphene Nanohybrid Cathode in a Divided Electrolytic Cell: Catholyte-Regulated Catalytic Oxidation Performance and Mechanism

The aim of the present work was to illuminate the catholyte-regulated catalytic oxidation performance and mechanism of the FeOOH-catalyzed heterogeneous electro-Fenton (Hetero-EF) system operating in a divided electrolytic cell. Depending on pH evolution with electrolysis time, the effect of the catholyte type on the H2O2 yield and current efficiency was investigated on an anthraquinone@electrochemically reduced grapheme oxide nanohybrid cathode. Based on the physicochemical characterization of the supported FeOOH nanoparticles, it was found that the Hetero-EF system exhibits the synchronous role of coupled adsorption and electrocatalytic oxidation for rhodamine B (RhB) degradation, with a higher apparent rate constant in MgSO4 catholyte and a higher mineralization rate in Na2SO4 catholyte. The catholyte-regulated catalytic oxidation mechanism was proposed according to radical scavenging experiments. In MgSO4 catholyte, the Hetero-EF process follows a classic Haber-Weiss mechanism mediated by

In recent years, electrochemical advanced oxidation processes (EAOPs) have received increasing attention due their high effectiveness in the degradation of refractory organic pollutants via hydroxyl radicals (•OH) and superoxide radicals (O •− 2 ). [1][2][3][4][5][6][7][8][9] Brillas and coworkers have extensively reviewed the applications of EAOPs for the efficient degradation and mineralization of organic contaminants. [1][2][3][4] Among EAOPs, the electro-Fenton (EF) processes generally operate in two types of reactors, undivided cells and divided cells. The majority of studies have been performed in undivided electrochemical cells, 2 where the electrolysis requires a lower cell voltage because of the low voltage loss in the absence of separator. However, it is more difficult to discuss the oxidation mechanism of the EF process in an undivided cell due to the disturbance of reactive oxygen species and other weaker oxidants generated at the anodes. 2 In comparison, divided cells are favorable for investigating the reaction mechanism of the EF process. Moreover, the current efficiency and H 2 O 2 yield are universally higher than those obtained in an undivided cell, 2,10 where the simultaneous anodic oxidation of the generated H 2 O 2 takes place.
Here, the in situ electrogeneration of H 2 O 2 was investigated first on anthraquinone@reduced graphene oxide nanohybrid cathode (AQ@ERGO-NC), which was fabricated through a facile and controllable electrophoretic deposition-electrochemical reduction method. Then, a Hetero-EF system was constructed in a divided electrolytic cell on this cathode and the home-made α-(goethite) and γ-(lepidocrocite) mixed-phase FeOOH catalysts for degrading the probe pollutant, rhodamine B (RhB). To the best of our knowledge, the preparation and application of the large-area and functionalized graphene nanohybrid electrodes has not yet been studied in EAOPs for organic contaminant degradation. The resulting AQ@ERGO nanohybrids exhibited the inherent advantages of quinonoid compounds and ERGO nanosheets, such as high selectivity for 2e − oxygen reduction reaction (ORR) and a large electrochemical active reaction area. The present work aimed at (i) evaluating the effects of catholyte type and pH evolution on the in situ electrogeneration of H 2 O 2 on AQ@ERGO-NC, (ii) evaluating the synergetic adsorption-catalytic oxidation performance induced by the FeOOH nanoparticles toward RhB degradation, and (iii) clarifying the catholyte-regulated homogeneous/heterogeneous catalytic oxidation mechanism of the Hetero-EF system operating in a divided electrolytic cell and evaluating its stability after multiple reutilization.

Experimental
Materials and preparations.-For detailed procedures used in the preparation of AQ/ERGO-NC and FeOOH/γ-Al 2 O 3 heterogeneous catalyst, see the supplementary material S1 and S2.
Characterization.-The resulting samples were characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) for element mapping, Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N 2 adsorption-desorption, and electrochemical impedance spectra (EIS). For the full description see the supplementary materials S3.  Electrolytic systems.-All of the electrolytic trials, including the H 2 O 2 electrogeneration and RhB degradation, were conducted on a REXDJS-292 potentiostat/galvanostat electrochemical workstation (Cany Precision Instrument Co., Ltd., China) in constant potential mode and carried out in an open and divided cylindrical glass cell (Fig. 1). The reactor was externally connected to circulating water to keep the reaction at a constant temperature of 25 • C. A spiral platinum wire was used as anode and a cation exchange membrane (Nafion 117, DuPont) was used to separate the two compartments. The anolyte is 0.01 M NaClO 4 . A saturated Ag/AgCl electrode placed in the cathodic chamber was used as the reference electrode. In the case of H 2 O 2 electrogeneration, 200 ml 0.5 M Na 2 SO 4 or MgSO 4 was used as the catholyte. To in situ electrogenerate H 2 O 2 effectively, highly pure oxygen at 0.6 L min -1 was insufflated into the cathodic chamber through a porous diffuser placed under the AQ/ERGO-NC. In the RhB degradation experiment, the front, back, and sides of AQ@ERGO-NC was buried beneath the accumulational 100 g L -1 FeOOH/γ-Al 2 O 3 heterogeneous catalyst. The concentration of RhB was 10.0 mg L -1 , and 200 ml, 0.5 M Na 2 SO 4 or MgSO 4 as supporting electrolyte to guarantee the conductivity.

Results and Discussion
Structure of AQ@ERGO nanohybrid.-The surface modification and functionalization of ERGO/g-PTFE/Ni-screen by the electrochemical reduction of Fast Red AL Salt is described in Scheme 1. It is currently well understood that the organic free radicals are the  most attractive species reacting with sp 2 carbons (C=C bonds) of graphene. 33 In the present work, the highly reactive AQ radicals (AQ • ) were produced through the electroreduction of Fast Red AL Salt (AQN + 2 ) by cyclical scanning potential between 0.65 and −0.3 V. The electrogenerated AQ • will not be further reduced even at more negative potential than 0.32 V (see Fig. S2d), which can effectively attack the sp 2 carbon atoms of ERGO, forming a strong covalent bond between AQ groups and ERGO surface, i.e. the covalent grafting of AQ groups on ERGO substrate.
Effect of catholyte type on H 2 O 2 accumulation and current efficiency.-Considering that the Hetero-EF system was carried out with the constant potential method, the determination of the applied cathode potential was quite important to guarantee the effective electrogeneration of H 2 O 2 upon AQ@ERGO-NC. For this purpose, CV curves of AQ@ERGO-NC were recorded in N 2 -and O 2 -saturated 0.5 M Na 2 SO 4 and MgSO 4 solutions without adding FeOOH/γ-Al 2 O 3 catalysts, and the results are shown in Fig. 2a. In N 2 -or O 2 -saturated Na 2 SO 4 and MgSO 4 solutions, the shape of CV curve is the same. A pair of quasi-reversible redox peak at E 1/2 [(E pa +E pc )/2] = −0.49 V was observed in N 2 -saturated solution, attributing to the redox process of AQ molecule. While in O 2 -saturated solutions, the corresponding reduction current increased sharply until an irreversible cathodic peak appeared at −0.5 V, which demonstrates that AQ@ERGO-NC can efficiently catalyze ORR in neutral media, in accordance with the previous reports on the AQ/GE nanocomposite modified electrodes. 34 As shown in Fig. 2b, the concentration of electrogenerated H 2 O 2 increases continuously to 136.3 and 165.2 mg L −1 in MgSO 4 and Na 2 SO 4 catholyte, respectively, after 120 min electrolysis. In the meanwhile, the catholyte pH (inset of Fig. 2b) drops quickly from the initial neutral to about 3.4 for MgSO 4 catholyte, whereas an obvious increase in catholyte pH (ca. 11.3) is observed in Na 2 SO 4 catholyte. This result is mostly related to the fact that (i) in Na 2 SO 4 catholyte, the electrogenerated H + ions from water oxidation in 0.01 M NaClO 4 anolyte is insufficient to compensate the consumption of H + ions caused by ORR, and (ii) the precipitation of magnesium hydroxide and the transmission of H + ions from anolyte to catholyte can adequately offset the deficit of H + ions resulted from ORR, and the excess H + ions lead to a notable drop of pH value of MgSO 4 catholyte.
In Na 2 SO 4 catholyte, the current efficiency for the electrogeneration of H 2 O 2 decreased from 100% to 83.4% within 120 min. This current efficiency decline is even lower than that observed in MgSO 4 catholyte, which decreased from 100% to 67.5%. Therefore, H 2 O 2 yield and current efficiency are favored in Na 2 SO 4 catholyte, whose pH value develops gradually to alkalinity within 120 min of electrolysis. However, Fig. 2a displays a much higher cathodic current response under acidic conditions. Theoretically, the increasing current response commonly results in the higher H 2 O 2 yield. Nevertheless, the acidic media can simultaneously accelerate H 2 O 2 decomposition and H 2 evolution at AQ@ERGO-NC, which are known to be competing electrons with ORR. As a result, a much higher current response restrains H 2 O 2 yield and generates a decreasing current efficiency in MgSO 4 catholyte within 2 h of electrolysis. On the other hand, the decomposition of the accumulated H 2 O 2 at the cathode-solution interface takes place either on the cathode surface or in the aqueous medium, but H 2 O 2 destruction at cathode or in an alkaline medium proceeds at lower rate in comparison to its anodic oxidation in an undivided cell. 10,36 Consequently, a higher H 2 O 2 production and current efficiency are achieved in Na 2 SO 4 catholyte after 120 min of electrolysis, which is in accordance with the previous findings that H 2 O 2 accumulation and current efficiency in alkaline media, especially at pH 10-11, are higher than those in acidic media. 10,[36][37][38] Characterization of FeOOH/γ-Al 2 O 3 catalyst.-The phase constitution of the blank γ-Al 2 O 3 and FeOOH/γ-Al 2 O 3 samples were determined by XRD analysis (Fig. 3d). The XRD pattern of γ-Al 2 O 3 shows three typical peaks at 2θ = 37.4 0 , 46.3 0 , and 67.3 0 . 14 The most intense peaks located at 2θ = 26.7, 37.2, 47.3, and 53.2 • in FeOOH and FeOOH/γ-Al 2 O 3 samples are assigned to lepidocrocite (γ-FeOOH) phase, 32,39,40 and other several weaker diffraction peaks loaded at 2θ = 21.7, 33.4, 34.6, 37.5, 41.5, 58.3, and 61.6 • are attributed to goethite (α-FeOOH) phase. 40 The similar XRD results were also obtained in the synthesis of nano-FeOOH using the air oxidation of ferrous hydroxide suspension method. 41 The SEM-EDX (Fig. 3a-3c) and TEM (Fig. 3e,3f) images of FeOOH and FeOOH/γ-Al 2 O 3 samples exhibit abundant pine-like or acicular structures with average widths of 20−30 nm and lengths up to 150 nm. Also observed was a small quantity of rhombohedral or flake-like micrographs. Combined with the XRD patterns, it can be concluded that the home-made nano-FeOOH are composed of a mixed-phase of flake-like lepidocrocite (γ-FeOOH) and needle-like goethite (α-FeOOH) particles.   The degradation dynamics of RhB through the Hetero-EF process are depicted in Fig. 4b. In both cases, the complete decolorization of RhB is achieved within 60 min, indicating the superior catalytic ability of FeOOH nanoparticles for the decomposition of H 2 O 2 or HO − 2 . Even so, the contribution of the adsorption of FeOOH/γ-Al 2 O 3 catalysts on RhB decolorization should be taken into account. Thus, the additional adsorption experiments were carried out. As seen from Fig. 4b, almost no appreciable effect of catholyte type on RhB removal was observed and the adsorption of RhB on FeOOH/γ-Al 2 O 3 catalyst approached equilibrium within 60 min. About 27.8% of RhB was adsorbed in both cases. Therefore, the adsorption of RhB by FeOOH/γ-Al 2 O 3 catalyst cannot be neglected.
From the N 2 adsorption-desorption results depicted in Fig. 5 and Table I, it is clear that after loading FeOOH on γ-Al 2 O 3 supports, its surface area decreased from ∼84.1 to ∼66.7 m 2 g -1 , while the total pore volume increased from ∼0.47 to ∼0.85 cc g -1 . The pore size was distributed mainly over the range of 2-100 nm with the maximum   Fig. 4, it can be concluded that RhB removal is achieved mainly through FeOOH nanoparticles catalyzed Hetero-EF oxidation process. The Hetero-EF degradation of RhB fits well with the persudo-first-order kinetics (inset of Fig. 4b), and a higher apparent rate constant (k = 0.1055 min -1 ) is achieved in MgSO 4 catholyte compared to that in Na 2 SO 4 catholyte (k = 0.0556 min -1 ). This suggests that catholyte type plays a significant role in the Hetero-EF process in divided electrolysis cell, and acidic medium is favorable to the enhancement in catalytic oxidation performance.
The rapid degradation of RhB by the Hetero-EF process in MgSO 4 catholyte can be clearly observed from the temporal UV-vis adsorption spectra (Fig. 6a). The absorption peak at λ max = 554 nm in the visible region diminished gradually with increasing reaction time and disappeared completely for about 45 min, indicating a rapid RhB degradation via the cleavage of ethyl containing -N = N-bond. 16,26,42 The absorption intensity at λ = 350 nm also decreased gradually during the Hetero-EF treatment, which implies the destruction of the xanthene rings bonded -C = C-group, i.e. the cleavage of the naphthalene rings. 16 FTIR spectra changes of RhB during the Hetero-EF process in Na 2 SO 4 catholyte are presented in Fig. 6b. The bands for original RhB consist of the peaks at 1597, 1476, 1413, and 1334 cm -1 , which are attributed to the stretching vibrations of C=C and −CH 2 − bonds in the aromatic ring, while the peaks at 1261, 1181, and 798 cm -1 were caused by alkyl halide, aryl−N stretching, and C−H deformation. [43][44][45] These characteristic peaks disappear after a 45 min reaction. The accompanying disappearance of the peaks at about 1413 and 1334 cm -1 indicates the total destruction of aromatic links, 44,45 and the new peaks observed at 1633 and 1394 cm −1 are due to the stretching vibrations of C=O and C−N bonds, respectively. Moreover, the new peaks at 3403, 898, and 809 cm −1 are assigned to the bending vibration of N−H bonds. 44 The above results indicate that the large conjugated chromophore structure of Ar−N(C 2 H 5 ) 2 as well as −C−O−C− bands are cleaved, generating primary amines and smaller carboxylic acid during the oxidative degradation of RhB. 43,46 Potential contribution of the Homo-EF process.-As discussed above, the catholyte pH decreased quickly to ca. 3.4 for 0.5 M MgSO 4 and increased quickly to ca. 11.3 for 0.5 M Na 2 SO 4 . With regard to the heterogeneous Fenton-like process, it is reported that the homogeneous catalysis is dominant at 3.0 ≤ pH ≤ 5.5, because of the chemical reductive dissolution of the supported catalysts. 12,29 In this context, it is necessary to investigate whether the observed catalytic activity is caused by the catalyst leaching. For this purpose, the potentiostatic electrolysis was conducted in 200 ml N 2 -saturated solutions at pH 3.4 and 11.3 containing 100 g L -1 FeOOH/γ-Al 2 O 3 catalysts at −0.5 V for 120 min. Then, the solid catalysts were separated by filtration and centrifugation to obtain leaching solutions. The results showed that no dissolved iron ions was detected at pH 11.3, while the concentration of leached iron ions was 6.89 mg L -1 at pH 3.4, which corresponds to about 0.25% of the added Fe in the form of FeOOH/γ-Al 2 O 3 . Despite FeOOH is a low soluble ferric oxyhydroxide, the previous research reported that when the Hetero-EF process is conducted at pH below 4, iron elements were released from iron oxides into the bulk solution through proton promoted iron dissolution reaction (Eq. 1 and 2), electron-donor-involved reduction reaction (Eq. 3) and a direct cathode electro-reduction reaction (Eq. 4). 11,12,29,30,47,48 Fe III aq +e − ⇔ Fe II aq [4] where ≡Fe III represents for Fe(III) sites on the catalyst surface. Therefore, it can be safely concluded that it is due to the negligible dissolved iron ions in catholyte, which prevents the formation of Fe III aq -carboxylic acid (especially short-linear organic diacids, such as oxalic acid) complexes, resulting in a higher mineralization rate of RhB in Na 2 SO 4 catholyte compared to that obtained in MgSO 4 catholyte, as shown in Fig. 7.
By adding 10.0 mg L -1 RhB, as seen in Fig. 8, only about 10.9% RhB is removed in the leaching solutions at pH 11.3 within 120 min, which agrees with the result shown in Fig. 4a. However, the leaching solution at pH 3.4 causes a typical "platform behavior" and about 30.8% of RhB removal rate, which is much less than the complete decolorization of RhB by the Hetero-EF process in the same time region. Therefore, the degradation of RhB is atributed primarily to the Hetero-EF oxidation process in Na 2 SO 4 catholyte, while the Homo-EF plays an increasingly role in MgSO 4 catholyte, due to the reductive dissolution of FeOOH nanoparticles.
Possible catalytic oxidation mechanism of the Hetero-EF process.-The heterogeneous Fenton-like reaction is a surfacecontrolled process depending on H 2 O 2 concentration, solid catalyst property, and solution pH. Until now, its reaction mechanism was not agreed upon. Nevertheless, it is commonly accepted that the generation of •OH from the decomposition of H 2 O 2 catalyzed by metal oxides is the critical step in the entire oxidation process, which fits the Harber-Weiss circle mechanism similar to that which occurs in the classic Fenton system. 49,50 Previous research has pointed out that in acidic media, •OH formation involves the initial adsorption of the free H 2 O 2 on the surfacebound Fe III generating ≡Fe III · H 2 O 2 complex (Eq. 5) which can be first converted to ≡Fe II species and HO • 2 (Eq. 6). 24,25 Then, the further reaction between the generated HO • 2 and ≡Fe III produces ≡Fe II species (Eq. 7). 19 As opposed to the cases in acidic media, a surface-based catalytic mechanism was proposed in basic solutions, 24 which involves the interaction of H 2 O 2 with the surface iron species, such as ≡Fe III -OH and its one-electron reduzate, ≡Fe II -OH (Eq. 9). 25,29 In addition, H 2 O 2 favors the more negatively charged oxide surface and can form strong ≡Fe III -OH(H 2 O 2 ) (s) complexes with alkalescent ≡Fe III -OH (Eq. 10). 30 The ≡Fe III -OH(H 2 O 2 ) (s) can be converted into ≡Fe II -OH and HO • 2 finally via the ground-state electron transfer (Eq. 11) and deactivation (Eq. 12). 24,[47][48][49][50] Doubtlessly, the reaction rates of Eq. 11 and 12 can be significantly promoted in basic solution, due to the rapid consumption of the formed H + . With regard to ≡Fe II -OH, it catalyzes H 2 O 2 decomposition directly forming •OH (Eq. 15) and achieving the regeneration of ≡Fe III -OH 25 Because no dissolved iron ions were detected in Na 2 SO 4 catholyte, the high efficiency of the Hetero-EF process is attributed mainly to the outstanding H 2 O 2 activation via surface iron species to generate •OH and HO • 2 (O •− 2 )(Eq. 9-14). Therefore, •OH and/or HO • 2 (O •− 2 ) should provide major contributions for the oxidative degradation of RhB in Na 2 SO 4 catholyte, although O •− 2 is less reactive than •OH. 29,49 To confirm the dominant oxidizing species in the Hetero-EF process, the radical scavengers of t-butanol and benzoquinone (BQ) were employed to investigate their quenching effects for validating the  24,51 As shown in Fig. 9, the Hetero-EF process was suppressed when 20 mM t-butanol was added into the cathode chamber, which resulted in apparent declines in RhB decolorization efficiency by about 48% and 16% for MgSO 4 and Na 2 SO 4 catholyte, respectively. However, adding 20 mM BQ caused a significant increase in RhB removal in MgSO 4 catholyte and ca. 30.4% decline in the decolorization efficiency in Na 2 SO 4 catholyte. It has been reported that the addition of hydroquinone/quinone analogues can significantly enhance the homogeneous Fenton oxidation ability through accelerating the Fe 3+ /Fe 2+ cycle. 47,48,52 In the present work, HO • 2 generated from Eq. 6, 13, and 19 reacted quickly with BQ to form corresponding hydroquinone radicals (Eq. 20). Then, the resulting hydroquinone reacted with Fe III aq (Eq. 21) achieving the quinone/hydroquinone cycle and the rapider regeneration of Fe II aq compared to the slow step of Eq. 19, which caused a beneficial Fenton chain reaction the in MgSO 4 catholyte. This is undoubtedly evidence that, in MgSO 4 catholyte, the predominant oxidizing species generated from the Hetero-EF process are •OH.
[20] [21] It is now known that in alkaline solutions, the homogeneous disproportionation rate of HO • 2 is much faster than that of O •− 2 . 28,49 Therefore, the added BQ in Na 2 SO 4 catholyte is mainly consumed for quenching O •− 2 , due to the absence of Fe III aq in bulk solution. Compared to t-butanol, the more apparent inhibitory effect of BQ on RhB decolorization indicates that the dominant oxidizing species generated in It is difficult to distinguish the role of ≡Fe IV = O complex, but it has been shown that the oxidative activity of •OH is much higher than for solution phase Fe(IV)=O species, and the latter cannot oxidize aromatic compounds satisfactorily. 26 According to the above analysis, the possible oxidation mechanism of the Hetero-EF process operating in MgSO 4 and Na 2 SO 4 catholytes are described in Scheme 2.
Stability and reusability of the proposed Hetero-EF process.- Figure 10 shows the decolorization performance of RhB over the reused AQ@ERGO-NC cathode and FeOOH/γ-Al 2 O 3 catalysts in MgSO 4 and Na 2 SO 4 catholytes. A slight difference after five consecutive cyclic utilizations was observed compared to the first oxidation cycle, indicating the good stability and reusability of the Hetero-EF process both in acidic and alkaline media. Herein, we think that the satisfying and promising performance of the Hetero-EF process is attributed mainly to the superior structural stability of FeOOH/γ-Al 2 O 3 catalysts and AQ@ERGO-NC cathode as well as the less active component leaching.
XPS with high sensitivity to the iron chemical state was used to further confirm the surface chemical properties of FeOOH nanoparticles after consecutive cyclic utilizations in the Hetero-EF process. As seen in Fig. 11a, all the high-resolution Fe 2p spectra can be divided into three peaks at ca. 711.4, 719.6, and 724.8 eV, which are assigned to Fe 2p 3/2 , the satellite peak (approximately 8.2 eV higher than the Fe 2p 3/2 main peak), and Fe 2p 1/2 in Fe 2 O 3 or FeOOH, respectively. 24,25,27 Due to the iron component leaching, the Fe 2p spectrum of the catalyst used in MgSO 4 catholyte is slight weaker than the other two, but the peak shape is quite similar to the fresh catalyst. In all of the cases, the Fe 2p 3/2 peaks are fitted into only one peak component at ca. 711.4 eV, which indicates the absence of Fe 2+ (at ca. 710 eV) on the outside catalyst surface due to its quick oxidation into Fe 3+ during Fenton reactions. The O 1s region of fresh catalyst shown in Fig. 11b can also be decomposed into three peaks at ca. 530.0, 532.2, and 533.4 eV, attributing to metal oxide (M-O), -OH species in FeOOH or chemical bonded -OH group (M-OH), and chemically or physically adsorbed H 2 O, respectively. 49,50,53 The existence of surface -OH groups further suggests that Fe species are in the form of FeOOH (Fe III ) in the fresh catalyst. 54 After being used in the RhB degradation, the M-OH component at 532.2 eV is still the dominating region as the fresh catalyst. But the height and area of M-O component peak at 530.0 eV decrease obviously after used in MgSO 4 catholyte, which indicates that the M-O groups on catalyst surface participate in RhB removal, and FeOOH surface is strongly hydroxylated during the Hetero-EF process. 24 Furthermore, it is noteworthy that the binding energy at ca. 534.5 eV, which is characteristic of carbonyl oxygen (C-O) groups in phenolic, etheric, and alcoholic organics, 55 exhibits an increase in peak height and area for the catalyst after used in Na 2 SO 4 catholyte. This result further confirms the higher mineralization rate of RhB in Na 2 SO 4 catholyte than that of in MgSO 4 catholyte, as described in Fig. 7.
Generally, the excellent electrocatalytic activity and stability of the cathode material are crucial factors limiting practical application of an EF system. The electron transfer capabilities of the fresh and used AQ@ERGO-NC were investigated by EIS. As seen from Fig. 12, all of the Nyquist curves present the suppressing semicircle arcs corresponding to the electron-transfer resistance (the diameter provides R ct ) and the double-layer capacity (C dl ) nature in the high-frequency region, and almost straight lines reflecting the diffusion-controlled process (Warburg impedance, Z w ) in the low-frequency region. 56 The slight increase in R ct after five cycles in Hetero-EF process suggests that the AQ@ERGO-NC cathodes still maintain good electrical conductivity, and can provide superior electron transport pathways and fast electron transfer kinetics as well toward ORR for the in situ electrogeneration of H 2 O 2 .

Conclusions
By combining the EPD process with the in situ electrochemical reduction of GO and Fast Red AL Salt, AQ@ERGO nanohybrids H365 were successfully synthesized on the nickel screen surface. Based on AQ@ERGO-NC and home-made mixed-phase FeOOH nanoparticles catalysts, a Hetero-EF system was then constructed in a divided electrolytic cell with 0.01 M NaClO 4 serving as anolyte for RhB degradation. The strong interfacial interactions of ERGO nanosheets and AQ molecules assured the efficient cathodic electrogeneration of H 2 O 2 . After 120 min of electrolysis, the accumulated concentration of H 2 O 2 achieved at ca. 136.3 and 165.2 mg L −1 in 0.5 M MgSO 4 and Na 2 SO 4 catholytes, respectively. Meanwhile, the corresponding catholyte pH developed to 3.4 and 11.3. The FeOOH nanoparticles induced a significant synergetic effect of adsorption-electrocatalytic oxidation on RhB degradation. The high activity of FeOOH nanoparticles is related to the capability of the material surface to convert the electrogenerated H 2 O 2 molecules into oxidative radicals. In the MgSO 4 catholyte, the dissolved iron ions and surface iron species react with H 2 O 2 to generate •OH via the Haber-Weiss mechanism, while in the Na 2 SO 4 catholyte, the catalytic decomposition of H 2 O 2 with surface Fe II -OH sites and the deactivation of Fe III -OH · H 2 O 2 complex produced HO • 2 (O •− 2 ), •OH and ferryl species (≡Fe IV =O). XPS and EIS measurements indicated that FeOOH/γ-Al 2 O 3 catalyst and AQ@ERGO-NC maintain their structures well after successive reutilization in the Hetero-EF process. The new insight into this catholyte-regulated homogeneous/heterogeneous catalytic oxidation mechanism may promote the development of more efficient solid catalysts for practical wastewater treatment.