Electrosynthesis of >20 g/L H2O2 from Air

High H2O2 production in membrane-free electrolyzer via anodic bubble shielding towards robust rural disinfection

Hydrogen peroxide (H2O2) can be sustainably synthesized through the electrochemical oxygen reduction reaction in a dual-chamber water electrolyzer separated by expensive ion exchange (IX) membranes. The development of an IX membrane-free electrolyzer has been limited by direct anodic degradation of the produced H2O2. Here, we devise a bubble shielding strategy by using a low-cost polytetrafluoroethylene hydrophobic porous layer (HPL) on the anode that enables numerous sites for anodically generated oxygen bubbles and significantly suppresses H2O2 degradation in the electrolyte. The H2O2 production increases by ~600% compared to that using non-bubble shielded anode. A high H2O2 concentration of 10.05 ± 0.05 g L-1 at 40 mA cm-2 can be obtained with both HPL-coated anode and cathode. A solar-driven disinfection device equipped with HPL-coated electrodes achieves >99.9% E. coli inactivation within 60 min. This innovative approach for achieving high electrochemical H2O2 concentrations in IX membrane-free electrolyzers more generally provides insights for fine tuning three-phase interfaces and could be applicable to other reactions pathways in electrochemical applications.

Electrochemical production of HO2- and O2 for sulfide removal from sewage

In this study, a comparative analysis of two electrochemical methods for sulfide control in sewer networks was performed for the first time. In addition, the mechanism of sulfide control by HO2- was elucidated, and an analysis of the device operation and electrolyte selection was performed. The two-electron oxygen reduction reaction (2e--ORR) using untreated gas diffusion electrode (GDE) was superior to the hydrogen evolution reaction (HER) using stainless-steel mesh in terms of cell voltage, product formation, and sulfide suppression. The GDE maintained a stable HO₂⁻ production capacity, achieving a concentration of 4566.6 ± 173.3 mg L⁻¹ with a current efficiency (CE) of 84.13 ± 3.5 %. During the electrolysis period, a stable dissolved oxygen (DO) level in sewage was consistently observed due to continuous in-situ oxygen production in anode. HO2- exhibited a notable increase in sewage pH (10.20 ± 0.01), effectively inhibiting the release of 99.93 % of sulfides. Moreover, the combined treatment of HO2- and DO significantly surpassed that of individual treatments. Seawater treated with cation exchange resin (CER) emerged as the most promising alternative to freshwater as the electrolyte. Overall, this study demonstrates that in-situ generation of HO₂⁻ and oxygen is a more effective strategy for sulfide control in sewer systems.

Domestic wastewater treatment towards reuse by "self-supplied" microbial electrochemical system assisted UV/H2O2 process

Domestic wastewater is a potential source of water for non-potable reuse that may help address the global water, energy, and resource challenges. Herein, a "self-supplied" process through integrating microbial electrochemical system (MES) with UV/H2O2 was developed and investigated for wastewater treatment. H2O2 was "self-supplied" from MES while the MES catholyte was "self-supplied" from the final effluent of UV/H2O2. It was found that the MES accomplished > 80 % degradation of chemical oxygen demand (COD) through bioanode degradation, and produced 18 - 20 mg L-1 H2O2 via oxygen reduction reaction in the gas diffusion cathode. The MES effluent was further treated by the UV/H2O2 process, which achieved the complete removal of recalcitrant diclofenac and > 6 log inactivation of Escherichia coli. The enhanced treatment performance of UV/H2O2 was demonstrated via a comparison with the control experiments (UV or H2O2 treatment) and benefited from ·OH generation and sulfide removal. When treating the actual wastewater, the proposed system exhibited consistent treatment performance for the organic compounds and recalcitrant contaminants, and the quality of the treated water would meet the non-potable water reuse guidelines. The results of this study encourage the further exploration of emerging contaminant removal, system coordination, and use of renewable energy by the cooperation between MES and UV/H2O2.

Integrated electrocatalytic synthesis of ammonium nitrate from dilute NO gas on metal organic frameworks-modified gas diffusion electrodes

The electrocatalytic conversion of NO offers a promising technology for not only removing the air pollutant but also synthesizing valuable chemicals. We design an integrated-electrocatalysis cell featuring metal organic framework (MOF)-modified gas diffusion electrodes for simultaneous capture of NO and generation of NH4NO3 under low-concentration NO flow conditions. Using 2% NO gas, the modified cathode exhibits a higher NH4+ yield and Faradaic efficiency than an unmodified cathode. Notably, the modified cathode shows a twofold increase in NH4+ production with 20 ppm NO gas supply. Theoretical calculations predict favorable transfer of adsorbed NO from the adsorption layer to the catalyst layer, which is experimentally confirmed by enhanced NO mass transfer from gas to electrolyte across the modified electrode. The adsorption layer-modified anode also exhibits a higher NO3- yield for NO electro-oxidation compared to the unmodified electrode under low NO concentration flow. Among various integrated-cell configurations, a single-chamber setup produces a higher NH4NO3 yield than a double-chamber setup. Furthermore, a higher NO utilization efficiency is obtained with a single-gasline operation mode, where the NO-containing gas flows sequentially from the cathode to the anode.

Trans-electrode pressure of gas-diffusion electrodes significantly influencing the electrochemical hydrogen peroxide production

Hydrogen peroxide (H2O2) synthesis by electrochemical two-electron oxygen reduction has garnered increasing interest as a wide range of potential applications. Gas diffusion electrodes (GDEs) can effectively promote the H2O2 production efficiency by overcoming the oxygen mass transfer limitations but strongly influenced by the electrowetting process along the long-term operation. In this study, the effect of trans-electrode pressure (TEP) of GDE cathode on the electrowetting process was further elucidated. We controlled the TEP values of four types of GDEs: two Ni-based GDEs and two carbon cloth GDEs prepared by hot-pressing or brushing carbon black. SBA-15 was further used to regulate the microstructure of one Ni-based GDE. It was found that an optimal range of TEP occurred for all tested GDEs in terms of the max. concentration, the yield efficiency, the energy consumption, and the stability because TEP may change the triple-phase interface and influence the anti-electrowetting effect. The porosity of hot-pressed Ni GDE can maintain the TEP window and thus enhance the production of H2O2, likely via creating oxygen-containing functional groups and a bimodal pore structure on the electrode, revealed with several characterization techniques including SEM, CA, XPS, Raman spectra, CV and EIS. The porous Ni GDE presented an efficient and stable production of H2O2 for 10 cycles: yielding H2O2 at 4393.2-4602.2 mmol m-2 h-1 with current efficiencies of 94.2-98.7%. The best accumulated H2O2 concentration can reach up to 3.58 ωt% H2O2 at 10 h. The results provide an important reference for the industrial scaleup of electro-production of H2O2 with GDEs.

Electrochemical Generation of Hydroxide and Hydrogen Peroxide for Hydrolysis of Sulfuryl Fluoride Fumigant

The post-harvest fumigant, sulfuryl fluoride (SO2F2), is a >1000-fold more potent greenhouse gas than carbon dioxide and methane. Pilot studies have shown that SO2F2 fumes vented from fumigation chambers can be captured and hydrolyzed by hydroxide (OH-) and hydrogen peroxide (H2O2) at pH ∼ 12 in a scrubber, producing SO42- and F- as waste salts. To reduce the costs and challenges associated with purchasing and mixing these reagents onsite, this study evaluates the electrochemical generation of OH- and H2O2 within spent scrubbing solution, taking advantage of the waste SO42- and F- as free sources of electrolyte. The study used a gas diffusion electrode constructed from carbon paper coated with carbon black as a catalyst selective for the reduction of O2 to H2O2. Under galvanostatic conditions, the study evaluated the effect of electrochemical conditions, including applied cathodic current density and electrolyte strength. Within an electrolyte containing 200 mM SO42- and 400 mM F-, comparable to the waste salts generated by a SO2F2 scrubbing event, the system produced 250 mM H2O2 at pH 12.6 within 4 h with a Faradaic efficiency of 98.8% for O2 reduction to H2O2. In a scrubbing-water sample from lab-scale fumigation, the system generated ∼200 mM H2O2 at pH 13.5 within 4 h with a Faradaic efficiency of 75.6%. A comparison of the costs to purchase NaOH and H2O2 against the electricity costs for electrochemical treatment indicated that the electrochemical approach could be 38-71% lower, depending on the local cost of electricity.

Efficient electrosynthesis of HO2- from air for sulfide control in sewers

Electrochemically in-situ generation of oxygen and caustic soda is promising for sulfide management while suffers from scaling, poor inactivating capacity, hydrogen release and ammonia escape. In this study, the four-compartment electrochemical cell efficiently captured oxygen molecules from the air chamber to produce HO2- without generating toxic by-products. Meanwhile, the catalyst layer surface of PTFE/CB-GDE maintained a relatively balanced gas-liquid micro-environment, enabling the formation of enduring solid-liquid-gas interfaces for efficient HO2- electrosynthesis. A dramatic increase in HO2- generation rate from 453.3 mg L-1 h-1 to 575.4 mg L-1 h-1 was attained by advancement in operation parameters design (flow channels, electrolyte types, flow rates and circulation types). Stability testing resulted in the HO2- generation rate over 15 g L-1 and the current efficiency (CE) exceeding 85%, indicating a robust stable operational capacity. Furthermore, after 120 mg L-1 HO2- treatment, an increase of 11.1% in necrotic and apoptotic cells in the sewer biofilm was observed, higher than that achieved with the addition of NaOH, H2O2 method. The in-situ electrosynthesis strategy for HO2- represents a significance toward the practical implementation of sulfide abatement in sewers, holding the potential to treat various sulfide-containing wastewater.

Electro-Fenton and Induced Electro-Fenton as Versatile Wastewater Treatment Processes for Decontamination and Nutrient Removal without Byproduct Formation

It is a long-pursued goal to develop electrified water treatment technology that can remove contaminants without byproduct formation. This study unveiled the overlooked multifunctionality of electro-Fenton (EF) and induced EF (I-EF) processes to remove organics, pathogens, and phosphate in one step without halogenated byproduct formation. The EF and I-EF processes used a sacrificial anode or an induced electrode to generate Fe2+ to activate H2O2 produced from a gas diffusion cathode fed by naturally diffused air. We used experimental and kinetic modeling approaches to illustrate that the •OH generation and radical speciation during EF were not impacted by chloride. More importantly, reactive chlorine species were quenched by H2O2, which eliminated the formation of halogenated byproducts. When applied in treating septic wastewater, the EF process removed >80% COD, >50% carbamazepine (as representative trace organics), and >99% phosphate at a low energy consumption of 0.37 Wh/L. The EF process also demonstrated broad-spectrum disinfection activities in removing and inactivating Escherichia coli, Enterococcus durans, and model viruses MS2 and Phi6. In contrast to electrochemical oxidation (EO) that yielded mg/L level byproducts to achieve the same degree of treatment, EF did not generate byproducts (chlorate, perchlorate, trihalomethanes, and haloacetic acids). The I-EF carried over all the advantages of EF and exhibited even faster kinetics in disinfection and carbamazepine removal with 50-80% less sludge production. Last, using septic wastewater treatment as a technical niche, we demonstrated that iron sludge formation is predictable and manageable, clearing roadblocks toward on-site water treatment applications.

Recycle graphite from spent lithium-ion batteries for H2O2 electrosynthesis

On-site H2O2 synthesis via the two-electron route oxygen reduction reaction for environmental remediation is attractive. This work offers a novel strategy for both spent graphite recovery and H2O2 electrosynthesis catalyst preparation. The graphite is directly recycled from spent lithium-ion batteries to an H2O2 electrosynthesis catalyst. From the view of sustainable development and environmental protection, the H2O2 electrosynthesis catalyst prepared using spent graphite is eco-friendly and cost-efficient. The surface functional groups of the recycled graphite are finely tuned by the HNO3 medium to induce -COOH and C-O-C groups. The activated graphite exhibits high H2O2 activity and selectivity, compared to the raw spent graphite. The activated graphite can achieve an H2O2 Faradic efficiency of about 80%. The activated graphite has a good prospect for T-acid wastewater treatment as the H2O2 generation catalyst. Almost 92% of chemical oxygen demand can be removed.

Resolving the "health vs environment" dilemma with sustainable disinfection during the COVID-19 pandemic

The overuse of disinfection during the COVID-19 pandemic leads to an emerging "health versus environment" dilemma that humans have to face. Irresponsible and unnecessary disinfection should be avoided, while comprehensive evaluation of the health and environmental impacts of different disinfectants is urgently needed. From this discussion, we reach a tentative conclusion that hydrogen peroxide is a green disinfectant. Its on-demand production enables a circular economy model to solve the storage issues. Water, oxygen, and electrons are the only feedstock to generate H2O2. Upon completion of disinfection, H2O2 is rapidly converted back into water and oxygen. This model adopts several principles of green chemistry to ensure overall sustainability along the three stages of its whole life cycle, i.e., production, disinfection, and decomposition. Physical methods, particularly UV irradiation, also provide sustainable disinfection with minimal health and environmental impacts.

Kinetics and mechanism of the reaction of hydrogen peroxide with hypochlorous acid: Implication on electrochemical water treatment

Reduction of HOCl to Cl^- by in-situ electrochemical synthesis or ex-situ addition of H₂O₂ is a feasible method to minimize Cl-DBPs and ClO_x^- (x = 2, 3, and 4) formation in electrochemical oxidative water treatment systems. This work has investigated the kinetics and mechanism of the reaction between H₂O₂ and HOCl. The kinetics study showed the species-specific second order rate constants for HOCl with H₂O₂ (k₁), HOCl with HO₂^- (k₂) and OCl^- with H₂O₂ (k₃) are 195.5 ± 3.3 M^-1s^-1, 4.0 × 10⁷ M^-1s^-1 and 3.5 × 10³ M^-1s^-1, respectively. The density functional theory calculation showed k₂ is the most advantageous thermodynamically pathway because it does not need to overcome a high energy barrier. The yields of ¹O₂ generation from the reaction of H₂O₂ with HOCl were reinvestigated by using furfuryl alcohol (FFA) as a probe, and an average of 92.3% of ¹O₂ yields was obtained at pH 7-12. The second order rate constants of the reaction of ¹O₂ with 13 phenolates were determined by using the H₂O₂/HOCl system as a quantitative ¹O₂ production source. To establish a quantitative structure activity relationship, quantum chemical descriptors were more satisfactory than empirical Hammett constants. The potential implications in electrochemical oxidative water treatment were discussed at the end.

Catalysis for e-Chemistry: Need and Gaps for a Future De-Fossilized Chemical Production, with Focus on the Role of Complex (Direct) Syntheses by Electrocatalysis

The prospects, needs and limits in current approaches in catalysis to accelerate the transition to e-chemistry, where this term indicates a fossil fuel-free chemical production, are discussed. It is suggested that e-chemistry is a necessary element of the transformation to meet the targets of net zero emissions by year 2050 and that this conversion from the current petrochemistry is feasible. However, the acceleration of the development of catalytic technologies based on the use of renewable energy sources (indicated as reactive catalysis) is necessary, evidencing that these are part of a system of changes and thus should be assessed from this perspective. However, it is perceived that the current studies in the area are not properly addressing the needs to develop the catalytic technologies required for e-chemistry, presenting a series of relevant aspects and directions in which research should be focused to develop the framework system transformation necessary to implement e-chemistry.