Highly efficient H2O2 electrogeneration from O2 reduction by pulsed current: Facilitated release of H2O2 from porous cathode to bulk

Cation Effects on the Acidic Oxygen Reduction Reaction at Carbon Surfaces

Hydrogen peroxide (H2O2) is a widely used green oxidant. Until now, research has focused on the development of efficient catalysts for the two-electron oxygen reduction reaction (2e- ORR). However, electrolyte effects on the 2e- ORR have remained little understood. We report a significant effect of alkali metal cations (AMCs) on carbons in acidic environments. The presence of AMCs at a glassy carbon electrode shifts the half wave potential from -0.48 to -0.22 VRHE. This cation-induced enhancement effect exhibits a uniquely sensitive on/off switching behavior depending on the voltammetric protocol. Voltammetric and in situ X-ray photoemission spectroscopic evidence is presented, supporting a controlling role of the potential of zero charge of the catalytic enhancement. Density functional theory calculations associate the enhancement with stabilization of the *OOH key intermediate as a result of locally induced field effects from the AMCs. Finally, we developed a refined reaction mechanism for the H2O2 production in the presence of AMCs.

Electrochemical degradation of nanoplastics in water: Analysis of the role of reactive oxygen species

Microplastics and nanoplastics (NPs) are emerging water contaminants which have recently gained lots of attention because of their effects on the aquatic systems and human life. Most of the previous works on the treatment of plastic pollution in water have been focused on microplastics and a very limited study has been performed on the NPs treatment. In this work, the role of main reactive oxygen species (ROSs) in the electrooxidation (EO) and electro-peroxidation (EO-H₂O₂) of NPs in water is investigated. In-situ generation of hydroxyl radicals (^•OH), persulfates (S₂O₈^2-), and hydrogen peroxide (H₂O₂) were performed using boron-doped diamond (BDD) as the anode, whereas titanium (in EO process) and carbon felt (CF, in EO-H₂O₂ process) were used as cathode. In the EO process, NPs were mainly oxidized by two types of ROSs on the BDD surface: (i) ^•OH from water discharge and (ii) SO₄^•- via S₂O₈^2- reaction with ^•OH. In EO-H₂O₂ process, NPs were additionally degraded by ^•OH formed from H₂O₂ decomposition as well as SO₄^•- generated from direct or indirect reactions with H₂O₂. Analysis of the degradation of NPs showed that EO-H₂O₂ process was around 2.6 times more effective than EO process. The optimum amount of NPs degradation efficiency of 86.8% was obtained using EO-H₂O₂ process at the current density of 36 mA·cm^-2, 0.03 M Na₂SO₄, pH of 2, and 40 min reaction time. In addition, 3D EEM fluorescence analysis confirmed the degradation of NPs. Finally, the economic analysis showed the treatment of NPs using EO-H₂O₂ process had an operating cost of 2.3 $US.m^-3, which was around 10 times less than the EO process. This study demonstrated that the in-situ generation of ROSs can significantly enhance the degradation of NPs in water.

Electrosynthesis of >20 g/L H2O2 from Air

Hydrogen peroxide (HP) production via electrochemical oxygen reduction reaction (ORR-HP) is a critical reaction for energy storage and environmental remediation. The onsite production of high-concentration H₂O₂ using gas diffusion electrodes (GDEs) fed by air is especially attractive. However, many studies indicate that the air-GDE combination could not produce concentrated H₂O₂, as the [H₂O₂] leveled off or even decreased with the increasing reaction time. This study proves that the limiting factors are not the oxygen concentration in the air but the anodic and cathodic depletion of the as-formed H₂O₂. We proved that the anodic depletion could be excluded by adopting a divided electrolytic cell. Furthermore, we demonstrated that applying poly(tetrafluoroethylene) (PTFE) as an overcoating rather than a catalyst binder could effectively mitigate the cathodic decomposition pathways. Beyond that, we further developed a composite electrospun PTFE (E-PTFE)/carbon black (CB)/GDE electrode featuring the electrospun PTFE (E-PTFE) nanofibrous overcoating. The E-PTFE coating provides abundant triphase active sites and excludes the cathodic depletion reaction, enabling the production of >20 g/L H₂O₂ at a current efficiency of 86.6%. Finally, we demonstrated the efficacy of the ORR-HP device in lake water remediation. Cyanobacteria and microcystin-LR were readily removed along with the onsite production of H₂O₂.

Janus graphite felt cathode dramatically enhance the H2O2 yield from O2 electroreduction by the hydrophilicity-hydrophobicity regulation

Hydrogen peroxide (H₂O₂) electrosynthesis from 2-electron O₂ reduction reaction (2eORR) is widely regarded as a promising alternative to the current industry-dominant anthraquinone process. Design and fabrication of effective, low-cost carbon-based electrodes is one of the priorities. Many previous work well confirmed that hydrophilic carbon-based electrodes are preferable for 2eORR. Here, we proposed a strategy of hydrophilicity-hydrophobicity regulation. By using commercially available graphite felt (GF) as electrodes, we showed that both hydrophilic GF, hydrophobic GF, and Janus GF yielded significantly higher H₂O₂ production, which is 7.3 times, 7.6 times, and 7.7 times higher than the original GF, respectively. Results showed that currents and stirring rates affect the H₂O₂ yields. The enhancement of hydrophilic GF is due to the incorporation of oxygen-containing functional groups, while the hydrophobic and Janus GF comes from the locally confined O₂ bubbles, which built a gas-liquid-solid interface inside GF and thus enhance the H₂O₂ formation kinetics. Finally, the effectiveness of the hydrophilicity-hydrophobicity regulation concept was tested in Electro-Fenton process by removing typical dyes and antibiotics. This work supply an effective but facile strategy to enhance the performance of carbon-based electrodes towards 2eORR by regulating the micro-environment of electrodes.

Power Management and Reaction Optimization for a Self-Powered Electrochemical System Driven by a Triboelectric Nanogenerator

Harvesting distributed and low-quality mechanical energies by triboelectric nanogenerators to power electrochemical reactions is beneficial to electric energy saving and certain applications. However, the conventional self-powered electrochemical process is awkward about the reaction rate, energy conversion efficiency, high-operation frequency, and mismatched impedance. Here we demonstrate an advanced self-powered electrochemical system. In comparison with the conventional system that is inert in activity, the superior power management and electrochemical reaction regulation in tandem make the novel system outstanding for hydrogen peroxide production. In addition to the visible product, an internal current efficiency of 24.6% in the system was achieved. The developed system provides an optimization strategy toward electric energy saving for electrochemical reactions as well as enabling their applications in remote areas by converting environmental mechanical vibrational energy for ecological improvement or recyclable chemical fuel generation.

Facile Preparation of Granular Copper Films as Cathode for Enhanced Electrochemical Degradation of Methyl Orange

In this paper, granular copper films (GCFs) were prepared through electrodeposition in CuSO₄ solution containing triethanolamine, and the films were used as electro-Fenton-like cathodes for degradation of methyl orange (MO). The effects of triethanolamine concentration, pH value, current intensity and temperature on the morphology of the films, as well as the MO decolorization ratio (DR), were investigated in detail. Results show that when the concentration of triethanolamine is 0.2 wt%, the prepared GCF exhibits the best performance. Under room temperature and neutral conditions, no external O₂ or catalyst, MO is completely decolorized after 240 min. Compared with the commonly used carbon cathode, the GCF cathode can increase the MO decolorization rate by approximately 70.9%. The kinetics of the electrochemical degradation reaction is also discussed.

O-doped Graphitic Granular Biochar Enables Pollutants Removal via Simultaneous H2O2 Generation and Activation in Neutral Fe-free Electro-Fenton Process

H₂O₂ generation by 2-electron oxygen electroreduction reaction (2eORR) has attracted great attention as an alternative to the industry-dominant anthraquinone process. Electro-Fenton (EF) process, which relies on the H₂O₂ electrogeneration, is regarded as an important environmental application of H₂O₂ generation by 2eORR. However, its application is hindered by the relatively expensive electrode materials. Proposing cathode materials with low cost and facile synthetic procedures are the priority to advance the EF process. In this work, a composite cathode structure that uses graphitic granular bamboo-based biochar (GB) and stainless steel (SS) mesh (GBSS) is proposed, where SS mesh functions as current distributor and GB supports synergistic H₂O₂ electrogeneration and activation. The graphitic carbon makes GB conductive and the oxygen-containing groups serve as active sites for H₂O₂ production. 11.3 mg/L H₂O₂ was produced from 2.0 g GB at 50 mA after 50 min under neutral pH without external O₂/air supply. The O-doped biochar further increased the H₂O₂ yield to 18.3 mg/L under same conditions. The GBSS electrode is also effective for H₂O₂ activation to generate ·OH, especially under neutral pH. Ultimately, a neutral Fe-free EF process enabled by GBSS cathode is effective for removal of various model organic pollutants (reactive blue 19, orange II, 4-nitrophenol) within 120 min, and for their partial mineralization (48.4% to 63.5%). Long-term stability of the GBSS electrode for H₂O₂ electrogeneration, H₂O₂ activation, and pollutants degradation were also examined and analyzed. This work offers a promising application for biomass waste for removals of organic pollutants in neutral Fe-free EF process.

Hydrogen peroxide generation from O2 electroreduction for environmental remediation: A state-of-the-art review

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

Efficient H2O2 electrogeneration at graphite felt modified via electrode polarity reversal: Utilization for organic pollutants degradation

Electrochemical synthesis of H2O2 offers a great potential for water treatment. However, a significant challenge is the development of efficient cathode materials for the process. Herein, we implement a practical electrochemical cathode modification to support efficient H2O2 electrogeneration via the reduction of dissolved anodic O2. Graphite felt (GF) is in situ anodically modified by electrode polarity reversal technique in an acid-free, low-conductivity electrolyte. The modified GF exhibits a significantly higher activity towards O2 reduction. Up to 183.3% higher H2O2 yield is obtained by the anodized GF due to the increased concentrations of oxygen-containing groups and the hydrophilicity of the surface, which facilitates electron and mass transfer between GF and the electrolyte. Another significant finding is the ability to produce H2O2 at a high yield under neutral pH and low current intensity by the modified GF (35% of the charge need to produce the same amount by unmodified GF). Long-term stability testing of the modified GF showed a decay in the electrode's activity for H2O2 production after 30 consecutive applications. However, the electrode regained its optimal activity for H2O2 production after a secondary modification by electrode polarity reversal. Finally, in situ electrochemically modified GF is more effective for removal of reactive blue 19 (RB19, 20 mg/L) and ibuprofen (IBP, 10 mg/L) by the electro-Fenton process. The modified GF removed 62.7% of RB19 compared to only 28.1% by the unmodified GF in batch reactors after 50 min. Similarly, 75.3% IBP is removed by the modified GF compared to 57.6% by the unmodified GF in a flow-through reactor after 100 min.

Activated carbon as effective cathode material in iron-free Electro-Fenton process: Integrated H2O2 electrogeneration, activation, and pollutants adsorption

Major challenges for effective implementation of the Electro-Fenton (EF) water treatment process are that conventional efficient cathodes are relatively expensive, and H2O2 activation by Fe2+ may cause secondary pollution. Herein, we propose a low-cost activated carbon/stainless steel mesh (ACSS) composite cathode, where the SS mesh distributes the current and the AC simultaneously supports H2O2 electrogeneration, H2O2 activation, and organic compounds (OCs) adsorption. The oxygen-containing groups on the AC function as oxygen reduction reaction (ORR) sites for H2O2 electrogeneration; while the porous configuration supply sufficient reactive surface area for ORR. 8.9 mg/L H2O2 was obtained with 1.5 g AC at 100 mA under neutral pH without external O2 supply. The ACSS electrode is also effective for H2O2 activation to generate ‧OH, especially under neutral pH. Adsorption shows limited influence on both H2O2 electrogeneration and activation. The iron-free EF process enabled by the ACSS cathode is effective for reactive blue 19 (RB19) degradation. 61.5% RB19 was removed after 90 min and 74.3% TOC was removed after 720 min. Moreover, long-term stability test proved its relatively stable performance. Thus, the ACSS electrode configuration is promising for practical and cost-effective EF process for transformation of OCs in water.

"Floating" cathode for efficient H2O2 electrogeneration applied to degradation of ibuprofen as a model pollutant

The performance of the Electro-Fenton (EF) process for contaminant degradation depends on the rate of H2O2 production at the cathode via 2-electron dissolved O2 reduction. However, the low solubility of O2 (≈1×10-3 mol dm-3) limits H2O2 production. Herein, a novel and practical strategy that enables the synergistic utilization of O2 from the bulk electrolyte and ambient air for efficient H2O2 production is proposed. Compared with a conventional "submerged" cathode, the H2O2 concentration obtained using the "floating" cathode is 4.3 and 1.5 times higher using porous graphite felt (GF) and reticulated vitreous carbon (RVC) foam electrodes, respectively. This surprising enhancement results from the formation of a three-phase interface inside the porous cathode, where the O2 from ambient air is also utilized for H2O2 production. The contribution of O2 from ambient air varies depending on the cathode material and is calculated to be 76.7% for the GF cathode and 35.6% for the RVC foam cathode. The effects of pH, current, and mixing on H2O2 production are evaluated. Finally, the EF process enhanced by the "floating" cathode degraded 78.3% of the anti-inflammatory drug ibuprofen after 120 min compared to only 25.4% using a conventional "submerged" electrode, without any increase in the cost.