Surfactant-tolerant Cathodes for Electrochemical Generation of Hydrogen Peroxide for Wastewater Treatment

doi:10.21203/rs.3.rs-4124588/v1

Download PDF

Research Article

Surfactant-tolerant Cathodes for Electrochemical Generation of Hydrogen Peroxide for Wastewater Treatment

https://doi.org/10.21203/rs.3.rs-4124588/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

Cathode materials based on carbon substrates are of high interest for the electrochemical generation of hydrogen peroxide (H₂O₂) for wastewater treatment because of their low cost, chemical stability and high selectivity. However, the H₂O₂ selectivity of carbon materials can be significantly reduced in presence of surfactants, which are frequent contaminants in wastewater. Therefore, the development of surfactant-tolerant cathode materials is highly important. In this paper, composite electrodes comprising of polytetrafluoroethylene and carbon black on a carbon felt substrate were prepared. The effect of sodium dodecyl sulphate on the electrode activity was investigated. It was found that the electrodes prepared with high bulk density carbon black featured a high H₂O₂ Faradaic efficiency of 95% in surfactant-free solutions. These electrodes also showed significant surfactant tolerance having a 70% Faradaic efficiency in the presence of 1mM sodium dodecyl sulphate. The enhanced surfactant tolerance is attributed to the hydrophobic properties of the electrode surface.

Hydrogen peroxide

Faradaic efficiency

surfactants

carbon black

wastewater treatment

Electrochemical generation of hydrogen peroxide (H₂O₂) attracts increased attention as a critical step in many advanced oxidation processes (AOPs) which are used for treatment of wastewater [1]. In contrast to the currently industrialized anthraquinone route for H₂O₂ production, the electrochemical method is less energy-intensive, does not require complex infrastructure and expensive hydrogenation catalysts, and does not produce a large volume of byproducts [2]. Generated on the cathode through a two-electron pathway oxygen reduction reaction (ORR), H₂O₂can be further converted to a hydroxyl radical (^•OH). This is a powerful oxidant and reacts with anodically generated Fe²⁺ in the electro-Fenton AOP which is an emerging technology for wastewater treatment [3].

The two-electron pathway ORR can take place over a wide range of pH values. In acidic media, the process involves oxygen and protons as reactants [4]:

O₂ + 2H⁺ + 2e^- = H₂O₂ E^o = 0.695 V vs SHE (1)

In moderately basic media (pH=7 - 12), the reactants are oxygen and water [5]:

O₂ + 2H₂O + 2e^- = H₂O₂ + 2OH^- E^o = 0.682V vs SHE (2)

With respect to wastewater treatment applications, the electrochemical generation of H₂O₂is often performed by a direct electrolysis of aerated wastewater, which has a neutral or near-neutral pH value. In neutral media, having low concentrations of protons, oxygen and water participate as reactants as in the alkaline milieu. However, multiple reactions can occur, and the entire reaction set can be written as [5]:

(main reaction) O₂ + 2H₂O + 2e^- = H₂O₂ + 2OH^- E^o = 0.682 V vs SHE (2)

(further reduction of H₂O₂) H₂O₂ + 2e^- = 2OH^- E^o = 1.776 V vs SHE (3)

(side reaction 1) O₂ + 2H₂O + 4e^- = 4OH^- E^o = 1.228 V vs SHE (4)

(side reaction 2) 2H₂O +2e^- = H₂ + 2OH^- E^o = 0.000 V vs SHE (5)

The rate of the H₂O₂ production is determined by the amount of electrical charge that passed between the electrodes, the Faradaic efficiency (FE) of main reaction (2), and the further conversion of H₂O₂ to other products. For the latter, the further reduction reaction (3) of H₂O₂ at the cathode is particularly eminent [4]. Achieving high FE values is a challenging task since the two-electron pathway ORR competes with the four-electron ORR (4) [6]. Furthermore, when more negative potentials are applied to achieve high cathodic current densities (> 50 A/m²), the increasing relevance of the hydrogen evolution reaction (5) can reduce the FE of the two-electron ORR [7,8].

Several classes of cathode materials were proposed to achieve high FE values in the two-electron ORR, including platinum-group metal alloys [9], single metal atoms [10], and metal oxides [11]. Likewise, various carbon materials, such as graphene [12], carbon fiber [13], carbon felt [14, 15], hierarchically porous carbon [16], boron-doped diamond [17], and carbon black (CB) are known for their efficiency with respect to two-electron ORR [6, 18-21]. The latter gained particular interest for wastewater treatment applications because of the low cost and capability to support H₂O₂ production with FE high values at relatively high current densities in the mA/cm² range [6, 7, 19-21]. Carbon materials have high overpotential for hydrogen evolution reaction and low catalytic activity for H₂O₂ decomposition [3], which explains the high FE of the two-electron pathway ORR on carbon cathodes.

Yu et al. [6] developed cathodes by modifying graphite felt with a mixture of CB and polytetrafluoroethylene (PTFE) and studied their activity for both H₂O₂and electro-Fenton reaction. The authors demonstrated that the concentration of H₂O₂ reached a plateau when these materials are used in an oxygen aerated Na₂SO₄electrolyte. A FE above 95% was achieved at a current density of 5 mA/cm² when the mass ratio of CB to PTFE was 1:5. Pérez et al. [18] adopted the Yu et al. electrode preparation method and used the 1:5 CB-to-PTFE mass ratio cathodes for generating H₂O₂in a jet-aerated cell. The maximum production rate was observed at 15 mA/cm² with an FE of around 90%. Zhang et al. [19] used CB and PTFE mixtures to make superhydrophobic natural air diffusion electrodes, achieving FE of more than 90% at current densities below 40 mA/cm² and more than 60% at current densities as high as 200 mA/cm². The authors suggested that a PTFE/CB ratio of 0.6 results in the best performance for H₂O₂ production. Cordeiro-Junior et al. [20] suggested that CB-PTFE cathodes are flooded with electrolyte if the PTFE loading is below 20%, resulting in limited oxygen access to the reaction sites. However, PTFE loadings above 40% restricted the access of the electrolyte to the electrode bulk, reducing the active sites available for the reaction. Karatas et al. [7] used CB-PTFE electrodes and investigated the effect of pH and applied voltage on H₂O₂generation and electro-Fenton process. The authors demonstrated a very high FE of 99.80% as well as a high rate of pollutant removal in neutral solutions (pH = 7) at an applied voltage of 0.8 V vs SCE.

Another critical quality of cathodes for wastewater treatment is the tolerance to the contaminants that might be present in wastewater and adversely influence the two-electron pathway ORR. It was suggested that the presence of surfactants can suppress ORR and drastically reduce H₂O₂ production by forming surface structures that block access of oxygen to the reaction sites [22, 23]. Gyenge and Oloman [22] demonstrated that anionic surfactants like Sodium dodecyl sulfate (SDS) reduces the H₂O₂ formation on a reticulated vitreous carbon electrode by a factor of two, even at very low concentrations of SDS (< 1 mM). Surfactants are frequently present in wastewater, especially in so-called grey water – the wastewater that is produced in bathtubs, showers, laundry machines, dishwashers and kitchen sinks [24-26].

Treatment and reuse of grey water becomes increasingly important since it constitutes a large portion of household wastewater [25, 27]. Application of electro-Fenton AOP for grey water treatment was intensively studied but remains challenging due to the decreased FE of the two-electron pathway ORR in presence of surfactants [27- 29].

For example, Panizza et al. [30] observed that the rate constant of the electro-Fenton degradation of anionic surfactants does not increase proportionally with current, which the authors explained by progressive enhancement of parasitic reactions like hydrogen evolution and four-electron oxygen reduction. Hence, the development of “surfactant-tolerant” cathode materials for electrochemical treatment of wastewater relying on the two-electron pathway ORR remains very challenging.

The goal of this work is to identify the physical characteristics of electrode materials that influence their tolerance to surfactants and to develop cathodes at low cost and with high activity for H₂O₂ generation that can be used for wastewater treatment applications. Hence, we investigated the performance of carbon felt cathodes modified with PTFE and two different types of carbon black having high (Black Pearls L) or low bulk density (Carbon Black Acetylene). The performance of pristine and hydrophobized carbon felt electrodes was also studied for the sake of comparison. It was found that the hydrophobic electrodes result in very high FE (up to 95%) values for the two-electron pathway ORR in surfactant-free solution; the performance is on the same level with other CB-PTFE electrodes reported in literature [7,8,19-21]. In the presence of 1 mM SDS, the electrodes still provide high FE values (up to 70% FE). This is a considerable improvement compared to reticulated vitreous carbon electrodes, on which the ORR is significantly inhibited in presence of SDS [22].

2.1 Materials

Black Pearls L GP-3822 carbon black (further referred as BPL) was obtained from Cabot Corp., USA. Carbon black (acetylene), 50% compressed, ≥99.9% purity (further referred as CBA) was obtained from Alfa Aesar, Thermo Fisher Scientific, USA. Carbon felt (AVCarb G475A) was purchased from Fuel Cell Store, USA. Sodium Dodecyl Sulfate was obtained from MP Biomedicals, LLC, France. Potassium Sulfate (K₂SO₄, ≥99.0%), Cerium (IV) Sulfate Tetrahydrate (Ce(SO₄)₂·4H₂O, ≥98.0%), PTFE powder (1 mm particle size), 30% hydrogen peroxide (H₂O₂) and 60% PTFE emulsion were purchased from Sigma Aldrich, Canada. Sulfuric acid (H₂SO₄, 18M, ACS Reagent) was purchased from Ward’s Science, USA. Ethyl Alcohol (95%) was obtained from Commercial Alcohols Inc, Greenfield Global, Canada. Electrolysis solutions were prepared using ultrapure (Type 1) water from Millipore Synergy system, MilliporeSigma, USA. Cylinders with compressed air of industrial grade and argon were obtained from Praxair Inc., Canada.

2.2 Fabrication of electrodes

Electrode bases were made by depositing a layer of a dispersion comprising CB and PTFE onto the surface of the carbon felt substrates. The substrates with nominal area 2 x 1 cm² were cleaned by sonication in acetone and then in water. The cleaned electrode bases were hydrophobized by sonication in a 2 wt.% PTFE aqueous emulsion for 30 min, and then dried for 1 hour at 80 ^oC in air.

Catalyst inks were prepared by mixing CB, PTFE powder and ethyl alcohol, followed by sonication of the mixture for 30 min. Typical ink for preparation of 3 electrodes (6 cm² total area) consisted of 60 mg of CB, 36 mg of PTFE powder and 3 mL of ethyl alcohol. After sonication, the catalyst ink was immediately pipetted onto the active area of the hydrophobized electrode bases and dried at 80 ^oC in air for 1 hour. Thermal treatment of the so-made electrodes was a three-stage procedure: heating to 180 ^oC with a rate of 3 ^oC /min and holding at this temperature for 2 hours; heating to 280 ^oC with a rate of 2^oC/min and holding for 1 hour; and heating to 340 ^oC with a rate of 1 ^oC/min and holding for 30 min. After the thermal treatment, the heating was turned off and the electrodes cooled down overnight inside the furnace. The same thermal treatment procedure was applied to electrode bases (hydrophobized carbon felts) without the deposition of catalyst ink.

2.3 Characterization methods

Electrochemical characterization of the electrodes was performed in a 3-electrode undivided glass cell with 120 mL of 0.1M K₂SO₄ solution with or without addition of 1 mM SDS. Silver-silver chloride reference electrode (Ag/AgCl) in 3M KCl (Metrohm AG, Switzerland) and 5 mm diameter graphite rod served as a reference and counter electrode, respectively. The recorded potentials were converted to the scale of reversible hydrogen electrode (RHE) for improved comparability with literature. A SP-150 potentiostat (BioLogic, France) was used for all electrochemical experiments.

Cyclic voltammetry was conducted in argon saturated solution. The electrode potential was scanned with 50 mV/s scan rate for 10 times to achieve a repeatable cyclic voltammogram.

Electrochemical impedance spectroscopy (EIS) was performed at open circuit voltage with a 10 mV sinusoidal potential perturbation over a frequency range of 50 kHz to 0.1 Hz.

In the electrolysis experiments, the solution was continuously purged with air and agitated with a magnetic stirring bar before and during the reaction. Electrolysis was run in a galvanostatic mode at constant cathodic current of 10 mA. Solution samples (0.2 mL) were taken before and during the reaction with a 10-minute interval. The samples were immediately added to vials containing 2 mL of 0.7 mM Ce(SO₄)₂ in 0.5M H₂SO₄ solution and vigorously shaken. The produced hydrogen peroxide was consumed according to the following reaction:

H₂O₂ + 2Ce(SO₄)₂ = Ce₂(SO₄)₃ + H₂SO₄ + O₂ (6)

The remaining concentration of Ce⁴⁺ ion was analysed with UV-visible spectroscopy in the 300 - 500 nm wavelength range using an Evolution 300 spectrometer (Thermo Scientific, USA). The intensity of absorbance at 318 nm (A₃₁₈) was used for quantification of the H₂O₂ concentration in the electrolysis solution. For this purpose, a calibration curve (dependence of A₃₁₈ on concentration of H₂O₂) was obtained from standard solutions of 0.7 mM Ce(SO₄)₂ with 0.1M K₂SO₄ and different concentrations of H₂O₂ in the 0 - 100 ppm range. Then, the A₃₁₈in the so-prepared standard solutions was measured. The slope of the calibration curve S_cal expressed in arbitrary units per unit of H₂O₂ concentration [au/ppm] was used to calculate the concentration of hydrogen peroxide produced in the electrolysis reaction according to:

C_H2O2 = (A^o₃₁₈ – A₃₁₈)/S_cal (7)

where A^o₃₁₈ and A₃₁₈ are absorbances at 318 nm measured in mixtures of Ce⁴⁺ solution and samples of electrolysis solution before and after the reaction, respectively.

Scanning Electron Microscopy (SEM) was performed using Quanta 250 (ThermoFisher, United States) scanning electron microscope.

In this section, the surface morphology of electrodes and the interaction of their surfaces with water was investigated. Then, the performance of the electrodes for the H₂O₂ reaction was studied. Finally, the effect of SDS on the electrode performance was examined.

Figure 1 shows SEM images of the surface of pristine (a) and hydrophobized (b) carbon felts, as well as carbon felts with BPL-PTFE (c) and CBA-PTFE (d) coatings. The untreated felt in (a) consists of randomly oriented carbon fibers with diameters in the range of 5 - 10 mm. The formations of solid PTFE throughout the fiber structure as can be seen in the image of the hydrophobized felt (b).

Similar formations of PTFE, which form a partial film with pores having several micrometers of diameter, can also be found on the surface of the BPL-PTFE electrode (c). This is notably different from the CBA-PTFE substrate (d), where a more uniform porous surface structure can be observed. The contrast in the surface structure can be explained by the difference in bulk density of these two types of CB; that is, 106 g/L for CBA and 464 g/L for BPL as reported by the manufacturers. The same mass load of CBA forms a much thicker coating with a higher porosity in comparison with BPL. Such a thick and porous layer can better absorb PTFE, which is then more uniformly distributed within the bulk of the carbon layer. Unlike CBA, the denser BPL provides a less porous structure to absorb PTFE, which results in partial film formation on the surface of the layer.

Figure 2 shows the image of three electrodes, after 10 cycles from 0.00 to 1.23 V vs. RHE, placed under a layer of water: a pristine carbon felt (a), a carbon felt which was hydrophobized by impregnating with PTFE (b), and a hydrophobized carbon felt coated with a BPL-PTFE layer (c). Cycling the potential of a pristine carbon felt increased its hydrophilicity. When immersed in water, the surface features a uniform appearance with a deep black colour and without attached gas bubbles. In contrast, the carbon felt impregnated with PTFE (b), as well as all CB-PTFE substrate (c), remained their hydrophobicity after cycling which is demonstrated by the layer of gas bubbles when immersed into water, as can be clearly seen in Figure 2 locations (b,c) at the vertical surfaces of the substrates.

Cyclic voltammograms for carbon felt (pristine and hydrophobized) and for CBA-PTFE and BPL-PTFE electrodes are shown in Figure 3. The voltammograms reveal wide redox peaks between 0.6 V and 0.8 V for the pristine felt and CBA-PTFE electrodes. These peaks are frequently observed on different carbon materials and can be assigned to a quinone–hydroquinone redox reaction of the respective surface functional groups [31]. The peaks disappeared and the double layer charging (capacitive) currents were decreased when the carbon felt was hydrophobized with PTFE as shown in Figure 3a, suggesting a reduction in the solid/solution interface area. In contrast, the currents increased when the PTFE impregnated felt was coated with CB material as can be seen in Figure 3b. The current increase was more apparent for the electrodes with CBA, which also was accompanied with the re-appearance of quinone–hydroquinone peaks. This suggests that CBA possesses respective surface groups and that a larger area of carbon was in contact with the solution. The differences between CBA and BPL in the capacitive behaviour correlates with the bulk density of these materials. It is more than 4 times higher for BPL as discussed earlier. The difference in bulk density suggests that the specific surface is much larger for CBA than for BPL. Since the loading of CB was the same for both types of electrodes, we can anticipate that the carbon area that is available for electrochemical reactions is much larger in CBA-PTFE than in BPL-PTFE electrodes.

Figure 4 shows Nyquist plots for the BPL-PTFE and CBA-PTFE electrodes. The plots reveal a linear region at frequencies below about 1 kHz and a semicircle in the 1-50 kHz range. The observed high frequency resistance was around 3 Ω for both electrodes indicating good electronic conductivity of both substrates.

Figure 5a shows the change in H₂O₂ concentration during electrolysis for pristine carbon felt, BPL-PTFE and CBA-PTFE cathodes. All three types of electrodes showed activity for the reaction, and the concentration of H₂O₂ increased with time. The pristine carbon felt had the production rate of H₂O₂, resulting in concentration a concentration of less than 10 ppm after 40 minutes of electrolysis. The Faradaic efficiency over the first 10 min for this electrode was 27 ± 6%.

Both types of CB electrodes showed an improved H₂O₂ production rate and higher FE than the pristine carbon felt electrode as can also be seen in Figure 5a. Despite having a lower surface area, the BPL-PTFE electrode produces H₂O₂ at a faster rate than the CBA-PTFE electrode. After 40 minutes of electrolysis, the concentration of H₂O₂ was 1.5 – 2 times larger when BPL was used as compared to the electrodes made with CBA. The initial FE of the BPL-PTFE cathodes was about 95%, while for the CBA-PTFE it was only around 50%. However, the FE dropped more significantly over time for the BPL-PTFE electrode, but it still produced more H₂O₂ than the other electrodes.

The higher activity of the BPL-PTFE electrodes for the H₂O₂ reaction is also demonstrated by the electrode potentials. Figure 5b shows the cathode potentials that were measured during the experiments reported in Figure 5a. The figure shows that the potential changed somewhat from its initial value of around 190 mV over the first 10-20 minutes, after which it reached a plateau located around 220 mV. There is a significant difference of about 600 mV between the potentials of the BPL-PTFE and the CBA-PTFE electrode. One factor that can be responsible for such a contrast is that the ohmic resistance can be significantly different between these two electrodes. A larger resistance in case of CBA-PTFE would lead to a much higher IR component of the measured potential. However, the high frequency resistance (HFR) was about the same for both electrodes as seen from the EIS measurements in Figure 4. Furthermore, the HFR value of around 3 Ω should only result in 30 mV of ohmic drop for a 10 mA cathodic current, which is an order of magnitude less than the 600 mV difference between the BPL-PTFE and CBA-PTFE electrode potentials. Therefore, we conclude that a higher catalytic activity of the BPL towards the ORR results in lower overvoltage, which is a plausible explanation for the observed potential difference.

The overvoltage can be estimated as a difference between IR-corrected electrode potential under the applied current (about 30 mV more positive than the potentials reported in Figure 5b) and the equilibrium potential that can be calculated from the concentration of hydrogen peroxide using Nernst equation. The concentration of H₂O₂, achieved after 40 min of electrolysis with BPL-PTFE and CBA-PTFE electrodes, was in the range 10-30 ppm (or mg/L) as seen in Figure 5a, which is on the order of 10^-4 M assuming a molar mass of 34 g/mol. According to Nernst equation, the equilibrium potential for the 2-electron ORR is then +0.81 V vs RHE while the standard potential is +0.695 V vs RHE [4]. This implies that the reaction overvoltage of the BPL-PTFE electrodes was about 550 mV at the end of electrolysis. The electrode potential remained at positive values (vs. RHE); i.e., above the potentials of the hydrogen evolution reaction, which explains the high FE observed on this type of electrodes. It is noteworthy that the BPL-PTFE electrodes potentials and FE observed in this work are in agreement with those reported by Karatas et al. [7]. The authors reported about 95% FE of a CB-PTFE electrode in a neutral solution at a potential of 0.25 V vs. RHE (-0.4 V vs. SCE at pH = 7), which is in good agreement with our data reported in Figures 5a and 5b respectively.

The electrode potentials of the BPL-PTFE electrodes are remarkably different to that of the CBA-PTFE electrodes. The electrode potentials were well in the negative values (cf. Figure 5b) suggesting more than 1000 mV overvoltage. These electrode potentials and the respective overvoltages explain the lower FE values for the CBA-PTFE electrodes. At negative potentials of the RHE scale, the ORR competes with the hydrogen evolution reaction and a fraction of the electrical current is spent on producing hydrogen. This is also the case for the pristine carbon felt. We observe that the potential and the FE is even lower than that of the CBA-PTFE electrode, cf. Figures 5a and 5b.

Figure 6 shows the changes in H₂O₂ concentration during a stability test with both BPL-PTFE and CBA-PTFE electrodes. Here, the electrodes were tested for a longer electrolysis time of 160 min and the solution was replaced every 40 minutes. It can be seen that the difference in the activity of these two types of electrodes remains unchanged. That is, the BPL-PTFE electrode shows higher activity than the CBA-PTFE electrode and no apparent degradation was observed over this operation duration. Considering that the BPL-PTFE electrodes are more active in generating H₂O₂, we continued our studies with testing the influence of surfactant on the ORR with this electrode.

Figure 7 depicts the effect of SDS on the H₂O₂ production using BPL-PTFE electrodes and, for the sake of comparison, the pristine carbon felt. In case of the pristine carbon felt, the production of H₂O₂ was drastically reduced and a very low amount of H₂O₂, if any, was generated when SDS was added to the solution. Such a strong “poisoning” effect was also observed for a reticulated vitreous carbon cathode when the electrolyte solution contained SDS concentrations in the mM range [22]. This effect was explained by strong adsorption of anionic surfactants resulting in the loss of reaction centers.

In the case of the BPL-PTFE electrodes, the H₂O₂ concentrations are just moderately lower in the presence of SDS than in the surfactant-free solution, which is remarkably different from the performance of the pristine carbon felt electrode. The initial FE value decreased from 95% to 70% for the electrolyte without SDS while the FE is constantly around 70% in presence of SDS. Hence, the BPL-PTFE electrodes has a relatively better degradation performance in present of the surfactant. Considering the apparent difference in hydrophobicity between the BPL-PTFE electrodes and pristine carbon felt, we hypothesize that this surface property is responsible for the tolerance of PTFE-containing electrodes towards the poisoning effect of SDS on the two-electron ORR.

In order to verify this hypothesis, we compared the H₂O₂ generating activity of pristine carbon felt and PTFE impregnated (hydrophobized) carbon felt electrodes. Figure 8 shows H₂O₂ concentration over time for the pristine and the hydrophobized carbon felt electrodes with and without SDS in the solution. The figure reveals that the hydrophobized felt produces considerable amounts of H₂O₂ even in the presence of SDS. The difference at the end of the reaction time is around 25%. This is notably different from the pristine (hydrophilic) carbon felt which generally produced lower H₂O₂ concentrations. In the presence of SDS, the hydrophilic carbon felt hardly produces H₂O_2.The differences in SDS tolerance between the hydrophobic and hydrophilic carbon felt electrodes supports our hypothesis. Hydrophobicity of the surface is responsible for the electrode tolerance towards the poisoning effect of SDS on the two-electron ORR.

The observed difference in ORR Faradaic efficiency on hydrophobic and hydrophilic surfaces can be explained by the different structure of the adsorption layers that surfactants form. Kronberg et al. [32] suggested that adsorption of ionic surfactants on hydrophilic and hydrophobic surfaces have different driving forces and result in remarkably different adsorption layers.

Adsorption of SDS molecules on hydrophobic and hydrophilic surfaces is schematically shown in Figure 9. In case of hydrophobic surfaces, the adsorption process is driven by the tendency of hydrocarbon chains in the surfactant molecules to escape the aqueous environment. With such driving force, the mechanism of surfactant layer formation on the hydrophobic surface is similar to the one for a gas-liquid interface. That is, the formation of a monolayer of surfactant species with tails pointing away from the liquid phase (towards hydrophobic surface in case of solid-liquid interface) as illustrated in Figure 9a. Kronberg et al. [32] noticed that in case of SDS the packing density of the surfactant species in such layers is only about 1/2 of the theoretical close packing density of carbon chains, which suggests that the adsorption layers are not densely packed and are permeable for relatively small molecules like oxygen.

In case of a hydrophilic surface, it does not provide an entity where hydrocarbon chains can escape the aqueous environment. Therefore, arranging the molecules into a monolayer with hydrocarbon chain pointing towards the surface is no longer thermodynamically favourable. Instead, the surfactants form surface-induced self-assemblies. That is, aggregates of entangled molecules with tails pointing towards the internal space of the assemblies and heads towards the hydrophilic surface and the surrounding aqueous solution as depicted in Figure 9b. Such surface aggregates form a much thicker monolayer on the hydrophilic surface which can block the access of oxygen molecules. A comparable mechanism was also proposed to be responsible for the sharp decrease in the ORR activity of reticulated vitreous carbon electrodes in the presence of anionic surfactants like SDS [22]. However, the results of our work suggest that by inducing hydrophobicity on the cathode surface, the adverse effect of surfactants can be mitigated and high Faradaic efficiencies for the H₂O₂ generation can be achieved.

Hydrophobic electrodes comprising carbon black and PTFE deposited onto carbon felt substrates showed high Faradaic efficiency for the two-electron pathway oxygen reduction reaction (up to 95%) and high stability over 140 minutes. The Faradaic efficiency depended on the type of carbon black. Electrodes made with Black Pearls L had an about two-times higher efficiency than the electrodes made with carbon black acetylene and were about four-times higher than pristine carbon felt. Furthermore, the carbon black and PTFE electrodes had high tolerance to the anionic surfactant SDS, which can be attributed to the hydrophobicity of the electrode surface. The surfactant tolerance resulted in a good Faradaic efficiency of about 70% for hydrogen peroxide production through the reduction of oxygen. This potentially makes these electrodes cost-effective candidates for the electrochemical treatment of wastewater, in particular of grey water that frequently contains surfactants.

Author Contribution

D.M performed majority of experiments and results interpretation and wrote draft manuscript. S.M. and S.G.R performed some experiments and edited draft manuscript. C.-T.D. and D.P.J.B. acquired funding, supervised research and edited draft manuscript. All authors reviewed the manuscript.

Acknowledgments

The authors gratefully acknowledge the financial support from the Ontario Centre for Innovation VIP – Natural Sciences and Engineering Research Council Alliance Program and from Grafoid Inc.

Statements and Declarations

The authors declare that they have no competing financial or non-financial interests that are relevant to the content of this paper.

Xie J, Jing J Gu J, Guo J, Li Y, Zhou M (2022) Hydrogen peroxide generation from gas diffusion electrode for electrochemical degradation of organic pollutants in water: A review. J Environ Chem Eng 10:107882. https://doi.org/10.1016/j.jece.2022.107882
Wang N, Ma S, Zuo P, Duan J, Hou B (2021) Recent progress of electrochemical production of hydrogen peroxide by two-electron oxygen reduction reaction. Adv. Sci. 8:2100076. https://doi.org/10.1002/advs.202100076
Brillas E, Sirés I, Oturan MA (2009) Electro-Fenton process and related electrochemical technologies based on Fenton’s reaction chemistry. Chem Rev 109:6570–6631. https://doi.org/10.1021/cr900136g
Katsounaros I, Schneider WB, Meier JC, Benedikt U, Biedermann PU, Auer AA, Mayrhofer KJJ (2012) Hydrogen peroxide electrochemistry on platinum: towards understanding the oxygen reduction reaction mechanism. Phys Chem Chem Phys 14:7384–7391. https://doi.org/10.1039/c2cp40616k
Blank M, Findl, E. (1987). Mechanistic Approaches to Interactions of Electric and Electromagnetic Fields with Living Systems. Springer US.
Zhou Y, Chen G, Zhang J (2020) A review of advanced metal-free carbon catalysts for oxygen reduction reactions towards the selective generation of hydrogen peroxide. J Mater Chem A 8:20849-20869. https://doi.org/10.1039/d0ta07900f
Yu F, Zhou M, Yu X (2015) Cost-effective electro-Fenton using modified graphite felt that dramatically enhanced on H₂O₂ electro-generation without external aeration. Electrochim Acta 163:182-189. https://doi.org/10.1016/j.electacta.2015.02.166
Karatas O, Gengec NA, Gengec E, Khataee A, Kobya M (2022) High-performance carbon black electrode for oxygen reduction reaction and oxidation of atrazine by electro-Fenton process. Chemosphere 287:132370. https://doi.org/10.1016/j.chemosphere.2021.132370
Siahrostami S, Verdaguer-Casadevall A, Karamad M, Deiana D, Malacrida P, Wickman B, Escudero-Escribano M, Paoli EA, Frydendal R, Hansen TW, Chorkendorff I, Stephens IEL, Rossmeisl J (2013) Enabling direct H₂O₂ production through rational electrocatalyst design. Nat Mat 12:1137-1143. https://doi.org/10.1038/NMAT3795
Song X, Li N, Zhang H, Wang L, Yan Y, Wang H, Wang L, Bian Z (2020) Graphene-supported single nickel atom catalyst for highly selective and efficient hydrogen peroxide production. ACS Appl Mater Interfaces 12:17519−17527. https://doi.org/10.1021/acsami.0c01278
Carneiro JF, Rocha RS, Hammer P, Bertazzolic R, Lanza MRV (2016) Hydrogen peroxide electrogeneration in gas diffusion electrode nanostructured with Ta₂O₅. Appl Catal A 517:161-167. https://doi.org/10.1016/j.apcata.2016.03.013
Yang W, Zhou M, Cai J, Liang L, Ren G, Jiang L (2017) Ultrahigh yield of hydrogen peroxide on graphite felt cathode modified with electrochemically exfoliated graphene. J Mater Chem A 5:8070-8080. https://doi.org/10.1039/c7ta01534h
Jiao Y, Ma L, Tian Y, Zhou M (2020) A flow-through electro-Fenton process using modified activated carbon fiber cathode for orange II removal. Chemosphere 252: 126483. https://doi.org/10.1016/j.chemosphere.2020.126483
Pan Z, Wang K, Wang Y, Tsiakaras P, Song S (2018) In-situ electrosynthesis of hydrogen peroxide and wastewater treatment application: A novel strategy for graphite felt activation. Appl Catal B 237:392–400. https://doi.org/10.1016/j.apcatb.2018.05.079
Covarrubias-Del-Toro R, Huerta-Rocha M, Lezama L, García EXM, Estrada-Vargas A (2023) Hydrogen peroxide formation in carbon clothes for enhancement of an electro-oxidation tertiary treatment for tequila vinasse wastewater. Front Environ Sci 11:1059259. https://doi.org/10.3389/fenvs.2023.1059259
Liu Y, Quan X, Fan X, Wang H, Chen S (2015) High-yield electrosynthesis of hydrogen peroxide from oxygen reduction by hierarchically porous carbon. Angew Chem Int Ed Engl 54:6837. https://doi.org/10.1002/anie.201502396
Cai J, Xie J, Zhang Q, Zhou M (2022) Enhanced degradation of 2,4-dichlorophenoxyacetic acid by electro-Fenton in flow-through system using B, Co-TNT anode. Chemosphere 292:133470. https://doi.org/10.1016/j.chemosphere.2021.133470
Pérez JF, Llanos J, Sáez C, López C, Cañizares P, Rodrigo MA (2017) The jet aerator as oxygen supplier for the electrochemical generation of H₂O₂. Electrochim Acta 246:466–474. https://doi.org/10.1016/j.electacta.2017.06.085
Zhang Q, Zhou M, Ren G, Li Y, Li Y, Du X (2020) Highly efficient electrosynthesis of hydrogen peroxide on a superhydrophobic three-phase interface by natural air diffusion. Nat Commun 11:1731. https://doi.org/10.1038/s41467-020-15597-y
Cordeiro-Junior PJM, Lobato Bajo J, Lanza MRV, Rodrigo Rodrigo MA (2022) Highly efficient electrochemical production of hydrogen peroxide using the GDE technology. Ind Eng Chem Res 61(30):10660-10669. https://doi.org/10.1021/acs.iecr.2c01669
Muddemann T, Haupt DR, Sievers M, Kunz U (2020) Improved operating parameters for hydrogen peroxide-generating gas diffusion electrodes. Chem Ing Tech 92:505–512. https://doi.org/10.1002/cite.201900137
Gyenge EL, Oloman CW (2001) Influence of surfactants on the electroreduction of oxygen to hydrogen peroxide in acid and alkaline electrolytes. J Appl Electrochem 31:233-243. https://doi.org/10.1016/j.cej.2022.136665
Ma Y, Zhao E, Xia G, Zhan J, Yu G, Wang Y (2023) Effects of water constituents on the stability of gas diffusion electrode during electrochemical hydrogen peroxide production for water and wastewater treatment. Water Res 229:119503. https://doi.org/10.1016/j.watres.2022.119503
Wiel-Shafran A, Ronen Z, Weisbrod N, Adar EM, Gross A (2006) Potential changes in soil properties following irrigation with surfactant-rich greywater. Ecol Eng 26:348-354. https://doi.org/10.1016/j.ecoleng.2005.12.008
Ghaitidak DM, Yadav KD (2013) Characteristics and treatment of greywater—A review. Environ Sci Pollut Res 20:2795–2809. https://doi.org/10.1007/s11356-013-1533-0
Eriksson E, Auffarth K, Henze M, Ledin A (2002) Characteristics of grey wastewater. Urban Water 4:85–104. https://doi.org/10.1016/S1462-0758(01)00064-4
Tony MA, Parker HL, Clark JH (2016) Treatment of laundrette wastewater using Starbon and Fenton's reagent. J Environ Sci Health A 51:974-979, https://doi.org/10.1080/10934529.2016.1191817
Hassanshahi N, Karimi-Jashni A (2018) Comparison of photo-Fenton, O3/H2O2/UV and photocatalytic processes for the treatment of gray water. Ecotoxicol Environ Saf 161:683-690. https://doi.org/10.1016/j.ecoenv.2018.06.039
Faggiano A, Ricciardi M, Fiorentino A, Cucciniello R, Motta O, Rizzo L, Proto A (2022) Combination of foam fractionation and photo-Fenton like processes for greywater treatment. Sep. Purif. Technol. 293:121114. https://doi.org/10.1016/j.seppur.2022.121114
Panizza M, Barbucci A, Delucchi M, Carpanese MP, Giuliano A, Cataldo-Hernández M, Cerisola G (2013) Electro-Fenton degradation of anionic surfactants. Sep Purif Technol. 118:394–398. https://doi.org/10.1016/j.seppur.2013.07.023
Choo H-S, Kinumoto T, Nose M, Miyazaki K, Abe T, Ogumi Z (2008) Electrochemical oxidation of highly oriented pyrolytic graphite during potential cycling in sulfuric acid solution. J Power Sources 185:740–746. https://doi.org/10.1016/j.jpowsour.2008.07.086
Kronberg B, Holmberg K, Lindman B (2014) Surface Chemistry of Surfactants and Polymers, 1st edn. Wiley, pp 153-173. https://doi.org/10.1002/9781118695968

No competing interests reported.

GraphicalAbstract.png

Download PDF

Editorial decision: Revision requested
18 May, 2024
Reviews received at journal
12 May, 2024
Reviews received at journal
11 May, 2024
Reviewers agreed at journal
23 Apr, 2024
Reviewers agreed at journal
21 Apr, 2024
Reviewers invited by journal
21 Apr, 2024
Editor assigned by journal
23 Mar, 2024
Submission checks completed at journal
19 Mar, 2024
First submitted to journal
18 Mar, 2024

You are reading this latest preprint version

Surfactant-tolerant Cathodes for Electrochemical Generation of Hydrogen Peroxide for Wastewater Treatment

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials and Methods

3 Results and discussions

4 Conclusions

Declarations

Author Contribution

References

Additional Declarations

Supplementary Files

Status:

Version 1