Drastic Enhancement of H₂O₂ Electro-generation by Pulsed Current for Ibuprofen Degradation: Strategy Based on Decoupling Study on H₂O₂ Decomposition Pathways

Abstract

Efficient H₂O₂ electrogeneration from 2-electron oxygen reduction reaction (ORR) represents an important challenge for environmental remediation application. H₂O₂ production is determined by 2-electron ORR as well as H₂O₂ decomposition. In this work, a novel strategy based on the systematical investigation on H₂O₂ decomposition pathways was reported, presenting a drastically improved bulk H₂O₂ concentration. Results showed that bulk phase disproportion, cathodic reduction, and anodic oxidation all contributed to H₂O₂ depletion. To decrease the extent of H₂O₂ cathodic reduction, the pulsed current was applied and proved to be highly effective to lower the extent of H₂O₂ electroreduction. A systematic study of various pulsed current parameters showed that H₂O₂ concentration was significantly enhanced by 61.6% under pulsed current of “2s ON + 2s OFF” than constant current. A mechanism was proposed that under pulsed current, less H₂O₂ molecules were electroreduced when they diffused from the porous cathode to the bulk electrolyte. Further results demonstrated that a proper pulse frequency was necessary to achieve a higher H₂O₂ production. Finally, this strategy was applied to Electro-Fenton (EF) process with ibuprofen as model pollutant. 75.0% and 34.1% ibuprofen were removed under pulsed and constant current at 10 min, respectively. The result was in consistent with the higher H₂O₂ and ·OH production in EF under pulsed current. This work poses a potential approach to drastically enhance H₂O₂ production for improved EF performance on organic pollutants degradation without making any changes to the system except for power mode.

1. Introduction

The Electro-Fenton (EF) process is an advanced oxidation process commonly used for water remediation. It was developed by Brilla’s group and Oturan’s group since the 2000s [1][2]. One of the key features of EF process is that H₂O₂ is electrogenerated on the cathode by O₂ reduction reaction. The H₂O₂ generation rate is one of the most crucial parameters of process efficiency since the rate of Fenton’s reaction is predominantly determined by this parameter [3][4]. Therefore, many research has been focusing on the design of cathode materials [5][6][7] and its modification [8][9][10][11], cell configuration [12][13], as well as operation parameters optimization [12][14], to achieve a higher concentration of H₂O₂ in bulk electrolyte. However, few works are focused on H₂O₂ decomposition pathways. As many studies reported [6][7][11][12], H₂O₂ decomposition can result in yield loss and lower current efficiency, it is therefore essential to enhance the understanding of different decomposition pathways and factors affecting these reactions, which may provide a new strategy for enhanced H₂O₂ electrogeneration and improved EF performance.

The H₂O₂ concentration in bulk electrolyte measured by various techniques (e.g. titration, spectrophotometry) is accumulative concentration, which is the total H₂O₂ generated minus the decomposition of H₂O₂ (Eq. 1). Oturan et al. [15] suggested that some H₂O₂ is electrochemically reduced at the cathode surface and to a much lesser extent, disproportion in bulk. H₂O₂ is also oxidized to O₂ at the anode via HO₂• as an intermediate. Consequently, the accumulation of H₂O₂ is lower than its electrogeneration. Another work from Oturan et al. [3] reported that H₂O₂ could be destroyed by parasitic chemical decomposition, cathodic reduction (divided cell), or anodic oxidation (undivided cell), resulting in a slower accumulation in the bulk and current efficiency of H₂O₂ production. Bard et al. [16] reported that H₂O₂ is formed by “2-electron” pathway, can thereafter be reduced to H₂O. It can also be re-oxidized to oxygen or be transported to the bulk. Moreover, it can disproportionate into H₂O and O₂ in a non-electrochemical bimolecular reaction.

$${\mathrm{H}}_2{\mathrm{O}}_2\text{accumulative concentration}=\text{total}{\mathrm{H}}_2{\mathrm{O}}_2\text{generation}-{\mathrm{H}}_2{\mathrm{O}}_2\text{invalid decomposition}$$ (1)

Based on aforementioned published works, we summarized the generation and decomposition pathways of H₂O₂ in Tab. 1. In EF process, H₂O₂ is generated by 2-electron ORR pathway on cathode [17][18]. Then H₂O₂ partially decompose via electroreduction pathway, disproportion pathway, and anodic oxidation pathway. The most important feature of this process is that there is a significant step in which the electrogenerated H₂O₂ molecules diffuse from the cathode (both surface and inside) to bulk electrolyte [19][20].

Tab. 1.

Proposed H₂O₂ reaction pathways based on literature.

Reaction and Position		Equation	References	No.
H2O2 generation (2 electron O2 reduction)	Cathode surface and inside	O2(g) + 2 H+ + 2 e− → H2O2	Bard et al., 2009[16] Oturan et al., 2014[15] Oturan et al., 2014[3]	(2)
Disproportion of H2O2*	Cathode surface and inside	2 H2O2 → O2(g) + 2 H2O	Oturan et al., 2014[3] Bard et al., 2009[16]	(3)
Cathodic reduction of H2O2	Cathode surface and inside	H2O2 + 2 H+ + 2 e− → 2 H2O	Oturan et al., 2014[3] Bard et al., 2009[16]	(4)
Anodic oxidation of H2O2	Anode surface	H2O2 → HO2· + H+ + e− HO2· → O2(g) + H+ + e−	Oturan et al., 2014[15] Oturan et al., 2014[3]	(5) (6)
Disproportion of H2O2	Bulk electrolyte	2 H2O2 → O2(g) + 2 H2O	Oturan et al., 2014[15]	(7)

Note:

^*H₂O₂ disproportion occurred at cathode surface and inside would produce O₂, which was used to generate H₂O₂ again. This process is different from disproportion take place in bulk electrolyte.

However, electrogeneration and decomposition of H₂O₂ occur at several sites (cathode surface and inside, bulk electrolyte, anode vicinity) simultaneously, making it difficult to study. Many previous works that enhance H₂O₂ generation [5][10][21] inevitably couple the two processes, in which H₂O₂ decomposition varies under different operating parameters or by different electrodes. Hence, two important questions arose: (1) How to decouple different decomposition pathways and investigate the influence of key parameters on these reactions; (2) how to reduce the extent of decomposition pathways and obtain a higher H₂O₂ concentration in bulk electrolyte?

2-electron ORR is closely related to cathode materials as well as applied cathodic voltage [22]. Many porous carbonaceous materials and the properly applied current have been implemented since this reaction is the prerequisite to achieve a high yield of H₂O₂. H₂O₂ electroreduction co-occurs with 2-electron ORR; the extent of H₂O₂ electroreduction is lower so that H₂O₂ molecules generated by 2-electron ORR can partially diffuse to bulk before being electroreduced to H₂O. The key feature of H₂O₂ electroreduction pathway is that it is closely relevant to diffusion length and electricity, making it possible to reduce the extent of H₂O₂ electroreduction pathway by decreasing the thickness of porous cathode or applying pulsed current. Once H₂O₂ generated from 2-electron ORR has some time to diffuse to bulk without current during a particular time, it is reasonable to hypothesize that more H₂O₂ molecules could be obtained in bulk. H₂O₂ disproportion in cathode vicinity is a non-electrochemical bimolecular reaction [3][16], we assume that its extent can also be reduced by applying pulsed current as H₂O₂ concentration inside porous cathode decreased. H₂O₂ disproportion in bulk electrolytes is influenced by current crossing bulk electrolyte and concentration of the electrolyte. H₂O₂ anodic oxidation is majorly determined by applied current and cell configuration [23]. Therefore, we assume that the extent of H₂O₂ disproportion in bulk electrolytes and H₂O₂ anodic oxidation pathway cannot be reduced in a certain system.

Ibuprofen, a nonsteroidal anti-inflammatory drug, was frequently detected in the aquatic environment. It can pose a considerable threat to human health and potential long-term adverse effects on ecosystem [24][25][26]. Thus, it was chosen as the target contaminant. The aims of this study were to (1) investigate several H₂O₂ decomposition pathways in a decoupling way; (2) verify the feasibility of pulsed current on decreased extent of H₂O₂ electroreduction pathway; (3) systematically study the pulsed current parameters on improved H₂O₂ production; and (4) compare the ibuprofen degradation performance by EF process enabled by pulsed and constant current.

2. Materials and Methods

2.1. Materials and chemicals

All chemicals used in this study were analytical grade. H₂O₂ (30% solution), ferrous sulfate and ibuprofen (2-(4-(2-methyl propyl)phenyl)propanoic acid, C₁₃H₁₈O₂) was obtained from Fisher Scientific. Sodium sulfate (anhydrous, ≥99%) and titanium sulfate (99.9%) were purchased from Sigma-Aldrich. Deionized water (18.2 MΩ cm) obtained from a Millipore Milli-Q system was used in all the experiments. Solution pH was adjusted by sulfuric acid (98%, JT Baker) [27].

Graphite plate (Shanxi Kaida Chemical Ltd.) with a size of 2 cm × 4 cm × 2 mm was used as anode and cathode for the study of H₂O₂ decomposition. Reticulated vitreous carbon foam (RVC foam, 45 pores per inch (PPI), thickness of 2.5 mm, KR Reynolds Ltd.) with an average pore diameter of 508 μm was cut into 2 cm × 4 cm as cathode. The specific surface area is 31 cm²/cm³. RVC foam was used for H₂O₂ production as it is one of the commonly used carbonaceous materials in EF process [20][28][29]. It has a 3D porous structure [30], which can support our hypothesis that H₂O₂ molecules generated inside the porous cathode, are partially electroreduced to H₂O before it diffuse into the bulk electrolyte. Ti/mixed metal oxide (MMO, 3N International) mesh was used as anode, which produces O₂ effectively and support the H₂O₂ generation on cathode. The Ti/MMO electrode consists of IrO₂ and Ta₂O₅ coating on titanium mesh with dimensions of 3.6 cm diameter by 1.8 mm thickness [24]. Digital photograph of electrodes are shown in Fig. 1.

Fig. 1.

Digital photos of electrodes employed in this study. (a) RVC foam with 45PPI; (b) graphite plate; (c) Ti/MMO mesh.

A constant current was provided by an Agilent E3612A DC power supply. A programmable DC power supply (MODEL 62000P Series, Chroma Ate Inc.) was used to provide pulsed current with various parameters.

2.2. Experimental design

Invalid H₂O₂ decomposition results from several reaction pathways. To propose a method that can decrease the invalid H₂O₂ decomposition, decoupling study on these pathways should be conducted firstly. Herein, a series of experiments were designed to study each pathway and the method to reduce them (Tab. 2).

Tab. 2.

Parameters associated with decoupling study of H₂O₂ decomposition pathways.

Target Reaction	No.	H2O2 Concentration (mmol L−1)	Current (mA)	Na2SO4 concontration (mol L−1)	Electrodea
Anodic Oxidation	A1	2	50	0.05	G+G
	A2	2	100	0.05	G+G
	A3	2	200	0.05	G+G
Cathodic Reductionb	C1	0.5	100	0.05	G+G
	C2	2	50	0.05	G+G
	C3	2	100	0.05	G+G
	C4	2	200	0.05	G+G
	C5	10	100	0.05	G+G
	C6	2	50	0.05	G+R
	C7	2	100	0.05	G+R
	C8	2	200	0.05	G+R
Disproportion in Bulkc	D1	0.5	200	0.01	G+G
	D2	0.5	200	0.05	G+G
	D3	0.5	200	0.25	G+G
	D4	2	200	0.01	G+G
	D5	2	200	0.05	G+G
	D6	2	200	0.25	G+G
	D7	10	200	0.01	G+G
	D8	10	200	0.05	G+G
	D9	10	200	0.25	G+G

Note:

^aG means graphite plate, and R means RVC foam.

^bpH in the cathodic and anodic compartment was monitored, results were shown in part 3.3.

^cpH was adjusted to 3.0 for D1~D9, which is the best pH for the EF process [33].

To study H₂O₂ disproportion pathway in bulk electrolyte, an undivided electrochemical cell with a long distance between two electrodes (20.5 cm) was designed. Two graphite plate electrodes were employed to make electricity passing through the H₂O₂ solution. The solution was not stirred, and samples were taken from six different points to represent the average concentration of H₂O₂ (Fig. 2(a)).

Fig. 2.

Schematic diagram of (a) undivided electrochemical cell for investigation of H₂O₂ disproportion in bulk and (b) divided cell for investigation of H₂O₂ cathodic reduction and anodic oxidation.

Fig. 2(b) shows the system we designed for the study of anodic oxidation and cathodic reduction of H₂O₂. Two glass beakers (180 mL) served as compartment 1 and 2. The two compartments were connected by a salt bridge. Samples were taken from point B and C. In several previous reports, a salt bridge was used to connect anodic and cathodic compartments for the EF process [31][32].

3D porous carbonaceous electrodes were widely used as cathode in the EF process as it has abundant reactive sites for O₂ reduction. However, H₂O₂ may also be electroreduced to H₂O on these sites when H₂O₂ diffuse into the bulk electrolyte from porous structure. Therefore, RVC foam was used to represent the porous cathode and confirm our hypothesis that porous structure causes severe H₂O₂ electroreduction than non-porous cathode (e.g. graphite).

2.3. Analytical methods

Digital photos of RVC foam with 45 PPI were taken by Nikon D7000. Solution pH and dissolved oxygen (DO) were measured by pH meter and DO meter with corresponding microprobes (Microelectro, USA). The microprobes allow the measurement on these parameters using a small amount of solution (≈0.2 mL). H₂O₂ was measured at 405 nm on a Shimazu UV-Vis spectrometer after coloration with TiSO₄ [31]. Ibuprofen was measured by a 1200 Infinity Series HPLC (Agilent) equipped with a 1260 diode array detector (DAD), a 1260 fluorescence detector (FLD) and an Agilent Eclipse AAA C18 column (4.6×150 mm). The mobile phase was a mixture of methanol and water (68:32, v/v) at the flow rate of 0.5 mL/min. The detection wavelengths for DAD was set at 282 nm, and the column temperature at 40 °C [24][26]. Unless otherwise specified, for different pulse parameters, the sampling time was calculated to ensure the electrochemical reaction with electricity is same. The abscissa axis thus means the reaction time with electricity.

The current efficiency (CE) of H₂O₂ generation, defined as the ratio of the electricity consumed by the electrode reaction over the total electricity passed through the circuit, was calculated by Eq. 8 [12]:

$$\mathit{CE}=\frac{{\mathit{nFC}}_{H_2{O}_2}V}{\underset{0}{\overset{t}{\int }}\mathit{Idt}}\times 100\%$$ (8)

Where n is the number of electrons transferred for O₂ reduction to H₂O₂, F is the Faraday constant (96, 486 C mol⁻¹), C_H2O2 is the concentration of H₂O₂ (mol L⁻¹), V is the electrolyte volume (L), I is the applied current intensity (A), and t is the reaction time (s).

3. Results and Discussion

3.1. Visualization of the generation and diffusion process of H₂O₂

H₂O₂ molecules are generated in cathode vicinity (inside and surface) and accumulated in bulk electrolyte. It is well recognized that H₂O₂ generation is closely correlated with cathode materials and structures, H₂O₂ decomposition is related with where it exists in electrochemical cell. Cathode vicinity, bulk electrolyte, and anode vicinity all causes the decomposition of H₂O₂. To visualize the diffusion process of H₂O₂ molecules, an undivided electrochemical cell with addition of litmus reagents to the electrolyte was used.

Water electrolysis at the anode generates O₂ and H⁺ (Eq. 9), and at the cathode, it produces H₂ and OH⁻ (Eq. 10). Meanwhile, H₂O₂ is produced via Eq. 2 at the cathode. Therefore, it is reasonable to visualize the diffusion process of H₂O₂ molecule by tracing the diffusion process of OH⁻. In Fig. 3, once OH⁻ was generated, it diffused to bulk and anode vicinity because of the concentration gradient and electrical field. The pink color shows the diffusion process of OH⁻ generated from graphite rod cathode vicinity. By this method, we can conclude that H₂O₂ molecules also follow this diffusion path, and more importantly, cathode vicinity, bulk electrolyte, and anode vicinity all contribute to the decomposition of H₂O₂. In part 3.2 and 3.3, their contributions on H₂O₂ decomposition were systematically investigated.

Fig. 3.

The pink color shows the diffusion process of OH⁻ generated from graphite rod cathode vicinity, which also shows the diffusion process of H₂O₂ molecules generated from cathode vicinity.

$$2{\mathrm{H}}_2\mathrm{O}-4{\mathrm{e}}^{-}\to {\mathrm{O}}_2(\mathrm{g})+4{\mathrm{H}}^{+}$$ (9)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2(\mathrm{g})+2{\mathrm{OH}}^{-}$$ (10)

3.2. H₂O₂ disproportion in bulk solution

Many factors induce H₂O₂ decomposition, such as UV light, heat, activated carbon, inorganic salts, metal oxides, electric field [34][35]. Best to our knowledge, a decoupling investigation on H₂O₂ disproportion in bulk is difficult. Herein, we used a static electrochemical cell with a long distance between anode and cathode (20.5 cm, shown in Fig. 2(a)) to minimize the impact of anode and cathode on H₂O₂ decomposition. The solution was not stirred, 3 mL samples were taken from 6 points (0.5 mL from each point) to accurately represent the H₂O₂ concentration in bulk electrolyte. A constant current intensity of 200 mA was applied, the influence of different initial H₂O₂ concentration and Na₂SO₄ concentration on residual H₂O₂ concentration in bulk were shown in Fig. 4.

Fig. 4.

Profiles of H₂O₂ concentration in 0.01, 0.05 and 0.25 mol/L Na₂SO₄ electrolyte with different initial concentration: (a) 0.5 mmol/L; (b) 2 mmol/L; (c) 10 mmol/L. (d) Oxygen bubbles on the bottom of the static cell. Conditions: 450 mL, without stirring, current of 200 mA, initial pH of 3.

It is obvious that electric field cause H₂O₂ disproportion in bulk electrolyte. On the bottom of the static cell, we observed many small oxygen bubbles (Fig. 4(d)) which can not be formed just by water electrolysis without H₂O₂. This phenomenon illustrated that H₂O₂ partially decomposed to H₂O and O₂ in bulk electrolyte. H₂O₂ disproportion ratio under different initial H₂O₂ concentration followed a similar sequence: 0.25 mol/L > 0.05 mol/L > 0.01 mol/L. For an initial H₂O₂ concentration of 0.5 mmol/L, 64.6% of total H₂O₂ decomposed under Na₂SO₄ concentration of 0.25 mol/L. Even at a low concentration of Na₂SO₄ (0.05 mol/L and 0.01 mol/L), severe disproportion occurred (46.7% and 41.7%, respectively). Interestingly, for the initial H₂O₂ concentration of 2 mmol/L and 10 mmol/L, the extent of H₂O₂ disproportion is lower than 0.5 mmol/L. Usually, the concentration of H₂O₂ in EF process is several or tens of mmol/L, our results show that severe H₂O₂ disproportion occurs in bulk electrolyte at a wide range of electrolyte concentration.

3.3. Anodic oxidation and cathodic reduction of H₂O₂

The setup shown in Fig. 2(b) was used to investigate anodic oxidation and cathodic reduction of H₂O₂ in a decoupling way. By using a divided electrochemical cell connected by a salt bridge, H₂O₂ anodic oxidation and cathodic reduction can be achieved in the anodic compartment and cathodic compartment, respectively.

3.3.1. Effect of current intensity

Effect of the current intensity of 50 mA, 100 mA, and 200 mA on H₂O₂ concentration in anodic and cathodic compartment were shown in Fig. 5. Current intensity plays a significant role on H₂O₂ decomposition. In the cathodic compartment, H₂O₂ was electroreduced to H₂O via Eq. 4. While in anodic compartment, H₂O₂ was oxidized to O₂ via Eq. 5~6. At the low current intensity of 50 mA, 30.0% and 33.7% of total H₂O₂ was decomposed via cathodic reduction and anodic oxidation after 60 min, respectively. At higher current intensity, the profile of H₂O₂ concentration in two compartments is notably different. At the current intensity of 100 mA and 200 mA, more than 90% H₂O₂ decomposed in anodic compartment at 40 min, while more than 50% H₂O₂ (50.1% and 56.3%, respectively) was electroreduced in cathodic compartment at 60 min.

Fig. 5.

Profiles of H₂O₂ concentration in compartment 1 and 2 with different current intensity: (a) 50 mA; (b) 100 mA; (c) 200 mA, and (d) Profiles of DO and pH in compartment 1 and 2. Conditions: initial H₂O₂ concentration of 2 mmol/L, the total volume of 180 mL in each compartment, initial pH of 7.

The results indicate that high current can boost H₂O₂ generation via Eq. 2. Meanwhile, it also results in severe H₂O₂ electroreduction. During operation, the profile of DO and pH of the two compartments were recorded and shown in Fig. 5(d). H⁺ and O₂ were produced on anode, and OH⁻ and H₂ was provided on cathode. These species was separated by a salt bridge. Hence pH in cathodic compartment decreased while increased in anodic compartment. pH has no noticeable effect on H₂O₂ decomposition within 60 min for 2 mmol/L H₂O₂ (Fig. S1).

3.3.2. Effect of initial H₂O₂ concentration on H₂O₂ cathodic reduction

Based on our analysis, anodic oxidation cannot be inhibited because H₂O₂ generated in cathode vicinity will inevitably transfer to anode vicinity especially under stirring conditions. However, the extent of cathodic reduction can be possibly reduced by applying pulsed current. Here, we further investigate the influence of initial H₂O₂ concentration on H₂O₂ cathodic reduction. The current intensity of 100 mA was applied. Residual H₂O₂ concentration in the cathodic compartment was measured and plotted in Fig. 6. For an initial concentration of 0.5 mmol/L, 81.2% of total H₂O₂ was electroreduced at 60 min, while for an initial concentration of 2 mmol/L and 10 mmol/L, 56.1% and 52.2% of total H₂O₂ was electroreduced at 60 min, respectively. This result suggests that if the electrogenerated H₂O₂ concentration is around 0.5 mmol/L, the electroreduction of H₂O₂ should be considered since a large proportion of H₂O₂ was wasted by this pathway.

Fig. 6.

Influence of initial H₂O₂ concentration on H₂O₂ cathodic reduction. Conditions: 180 mL, current of 100 mA, initial pH of 7.

3.3.3. Effect of cathode material on H₂O₂ cathodic reduction

As results from 3.3.1 suggested, cathodic reduction cause severe depletion of H₂O₂. This reaction occurred simultaneously with the electrogeneration of H₂O₂. Therefore, it is reasonable to assume that this process is closely related to cathode structure, that is, the porous or non-porous cathode will show entirely different results on H₂O₂ electroreduction. Herein, RVC foam and graphite plate were selected as a porous and non-porous cathode, 2 mmol/L H₂O₂ was added to cathodic compartment in advance. Influence of current intensity on this pathway was shown in Fig. 7. It is evident that under all current intensity, porous cathode (RVC foam) causes severe H₂O₂ electroreduction than non-porous cathode (graphite plate).

Fig. 7.

Comparison of H₂O₂ cathodic reduction by graphite plate and RVC foam cathode under different current intensity. Conditions: total volume of 180 mL, 0.05 mol/L Na₂SO₄, initial pH of 7, the initial H₂O₂ concentration of 2 mmol/L.

However, the exact process by which the H₂O₂ formed on one site will be reduced to H₂O on another site. This process may occur either through electroreduction (Eq. 4) or disproportion reaction (Eq. 3). In the latter case, the O₂ originating from the disproportion reaction could be electrochemically reduced to H₂O₂ once again [20].

3.4. Reduced H₂O₂ electroreduction pathway by applying pulsed current

3.4.1. Mechanism

Our efforts were then aimed to put forward strategies to reduce H₂O₂ electroreduction, thus increase the yield of H₂O₂ in bulk electrolyte. Fig. 8 illustrates the behavior of H₂O₂ molecules before it was transported to the bulk electrolyte. Principally, after produced on active sites, the H₂O₂ molecules will diffuse through porous structure before reaching the bulk, in which the electroreduction occurred at other active sites. Therefore, if pulsed current is applied, the diffusion process of H₂O₂ molecules will be facilitated, and thus, higher H₂O₂ concentration can be obtained in the bulk electrolyte.

Fig. 8.

The behavior of H₂O₂ molecules in porous RVC foam cathode, including electrogeneration, disproportion, electroreduction, and its diffusion to the bulk electrolyte.

3.4.2. Effect of pulsed current parameters

The key feature of electrogeneration and electroreduction of H₂O₂ is that these two processes occur within the porous cathode. They take place simultaneously, which means the diffusion of H₂O₂ molecules to bulk will cause its partially electroreduction. This section is dedicated to explain in detail the impact of pulsed current parameters on H₂O₂ concentration in bulk and the extent of electroreduction.

As Fig. 9 shows, different ON and OFF time scale was set to explore the best combination. With power on, the reaction of H₂O₂ electrogeneration occurred. However, the reaction of H₂O₂ electroreduction also occurred. With power off, the two reactions stopped, and the generated H₂O₂ has some time to diffuse to bulk electrolyte. A “volcano-shape” curve (Fig. 9(e)) was plotted based on our results, which showed the relationship between H₂O₂ concentration and different “ON+OFF” combinations. “2s ON + 2s OFF” showed the highest yield of H₂O₂ in bulk. Long time scale such like “5s ON + 5s OFF”, “10s ON + 10s OFF”, “30s ON + 30s OFF”, and “60s ON + 60s OFF” also showed the enhanced effect on H₂O₂ yield compared with control experiment. However, short time scale such as “0.5s ON+0.5s OFF” and “0.1s ON + 0.1s OFF” showed a decreased yield of H₂O₂, especially for “0.1s ON+0.1s OFF”.

Fig. 9.

Influence of different pulse parameters on H₂O₂ production. (a) Parameters of pulsed current; (b) low frequency (60s+60s, 30s+30s, 10s+10s, 5s+5s); (c) high frequency (2s+2s, 0.5s+0.5s, 0.1s+0.1s); (d) with same ON time (2s) and different OFF time; (e) “volcano-shape” curve. Conditions: total volume of 180 mL, Ti/MMO anode, RVC Foam cathode, 0.05M Na₂SO₄, initial pH of 7, 350 rpm, constant or pulsed current intensity of 100 mA. Error bars represent the standard error of the mean.

Little is known about how pulsed current influence EF performance. Based on our analysis above, two reasons may account for this phenomenon. For short time scale such like 0.1 s, even anodic O₂ cannot be efficiently generated, which significantly influence the 2-electron oxygen reduction on RVC foam cathode. For long time scale such like 60 s, anodic O₂ generation was not affected by such a long ON time. However, on the other hand, it is so long that most H₂O₂ was electroreduced within this period, although some H₂O₂ molecules can diffuse to bulk during power off. Therefore, it had nearly the same result as control experiments. “2s ON + 2s OFF” is, therefore, the best combination for an enhanced yield of H₂O₂ bulk electrolyte, which achieved the best compromise between anodic O₂ generation and H₂O₂ diffusion within the porous cathode.

Moreover, we investigated whether OFF time influence the H₂O₂ yield when ON time is 2s. Results were shown in Fig. 9(d). It is evident that OFF time of 0.5 s and 0.1 s is not as effective as 2 s. This was possibly because the time scale is so short that the generated H₂O₂ molecules cannot transfer to bulk during this period. A longer OFF time of 2.5 s, 3 s, and 10 s doesn’t necessarily increase the H₂O₂ yield, indicating that OFF time of 2 s is sufficient for the diffusion of H₂O₂ molecules. The H₂O₂ production rate was calculated and compared with literature in Tab. 3. Interestingly, we also found pulsed current is not effective on non-porous cathode such as graphite electrode (Fig. S2). We believe this finding is significant because it can be applied to many porous carbonaceous cathodes, such as RVC foam, graphite felt, carbon felt, carbon fibers. We also showed our strategy worked on graphite felt in Fig. S3.

Tab. 3.

Comparison of H₂O₂ production rate with literature.

Cathode	pH	Current density (A/m2)	t (min)	O2 flow rate (L/min)	[H2O2] (mg/h/cm2)	Ref.
Graphite felt	3	132	300	0.14	0.11	[36][37]
Carbon felt	3	161	180	0.1	0.62	[38]
GDEa	3	204	300	0.14	1.94	[37]
Graphite felt	6.4	−0.65b	120	0.4	0.44	[39]
Graphite felt	3	50	60	0	0.58	[40]
ACF	3	250	180	0.1	0.55	[41]
Graphite	3	−0.65c	120	0.33	1.00	[42]
RVC foam	7	125	40	0	0.67	This work

Note:

^a-GDE means gas diffusion electrodes;

^b-the bias potential of the cathode (volts) (vs the saturated calomel electrode (SCE));

^c-the bias potential of the cathode (vs. SCE).

3.5. Performance on ibuprofen degradation in EF process

To test the feasibility of EF process under pulsed current for organic pollutants degradation, ibuprofen, which is frequently detected in the aquatic environment and poses a threat to human health, was selected as a model pollutant. Results of the comparative study were shown in Fig. 10. Fig. 10(a) and (b) demonstrates that EF under constant and pulsed current are both effective for ibuprofen removal with an initial concentration of 40 mg/L. However, it is obvious that EF under pulsed current is a lot more efficient than EF under constant current. At 10 min, the ibuprofen removal efficiency in EF under constant and pulsed current is 34.1% and 75.0%, respectively. At 60 min, nearly all the ibuprofen was removed in EF under pulsed current, while it is 100 min in EF under constant current. The pseudo first-order kinetics are 0.032 min⁻¹ and 0.065 min⁻¹ (Fig. 10(c)) for ibuprofen removal in EF under constant and pulsed current, respectively. TOC measurement results are shown in Fig. 10(c) also implies that EF under pulsed current is more efficient for TOC removal, in which 59.5% and 89.3% TOC was removed at 120 min in EF under constant and pulsed current, respectively. Results were compared with literature and shown in Tab. S1. Moreover, based on the aromatic intermediates and carboxylic acids detected (Tab. S2), a possible degradation pathway was proposed in Fig. S4.

Fig. 10.

HPLC analysis of ibuprofen samples taken at different time under constant (a) and pulsed current of “2s ON + 2s OFF” (b); (c) profile of normalized concentration of ibuprofen and TOC in EF process, and (d) profile of fluorescence intensity at different emission wavelength in EF process with externally added H₂O₂ at 22.0 and 12.3 mg/L. Conditions: total volume of 600 mL, Ti/MMO anode and RVC foam cathode, 0.05 M Na₂SO₄, initial pH of 3, 350 rpm, current of 100 mA, ibuprofen of 40 mg/L, Fe²⁺ of 70 mg/L, benzoic acid of 250 mg/L. The sampling time of EF with pulsed current and constant current was same. Error bars represent the standard error of the mean.

The enhanced ibuprofen removal efficiency verified our hypothesis that enhanced H₂O₂ production in bulk could accelerate EF process for organic pollutants degradation. This result is consistent with results of enhanced H₂O₂ production by pulsed current shown in Fig. 9(c), where 19.9 mg/L and 12.3 mg/L H₂O₂ was obtained in bulk at 40 min under a constant current of 100 mA and pulsed current of “2s ON + 2s OFF” (intensity: 100 mA). Furthermore, the production of hydroxyl radicals, which is responsible for ibuprofen removal and TOC removal, are compared and shown in Fig. 10(d) (fluorescence spectra of the solution under constant current was shown in Fig. S5). Hydroxyl radicals were captured by benzoic acid and form hydroxybenzoic acid with fluorescence properties [43]. Thus, the higher spectral peak value in Fig. 10(d) implies that more hydroxyl radicals are generated in EF process under pulsed current.

4. Conclusions

In EF process, the concentration of H₂O₂ in bulk electrolyte is determined by H₂O₂ electrogeneration via 2-electron ORR as well as H₂O₂ decomposition. In this work, we designed several experiments to investigate H₂O₂ disproportion in bulk, H₂O₂ anodic oxidation, and H₂O₂ cathodic reduction in a decoupled way. H₂O₂ Electroreduction is the only pathway that can be weakened based on our analysis. When applying pulsed current, H₂O₂ concentration in bulk was dramatically enhanced by 61.6% (“2s ON + 2s OFF”) than under constant current. A summary of different parameters showed a “volcano-shape” curve, indicating that a proper pulse frequency is necessary to achieve the enhanced H₂O₂ production. Finally, EF process enabled by pulsed current was proved to be more efficient for ibuprofen degradation than constant current, proving potential practical application for organic wastewater treatment.