Tailoring surface carboxyl groups of mesoporous carbon boosts electrochemical H2O2 production

Spent coffee ground-derived hydrochar: An ecologically compatible material for solar-driven H2O2 production and wastewater purification

Solar-driven H2O2 photosynthesis via dioxygen activation presents a sustainable pathway to produce energy carriers and purify wastewater, which hinges on the scale-acquired ecologically compatible materials. Herein, a spent coffee ground-based hydrochar (SHTC) photocatalyst was hydrothermally synthesized in dilute H2SO4 solution. Mechanism analysis revealed that superoxide radical (•O2-) generated via an indirect two-step one-electron involved O2 photoreduction is responsible for H2O2 production, which were systematically evidenced by reactive species trapping experiments, electron paramagnetic resonance, in-situ Fourier transform infrared spectroscopy, rotating disk electrode measurements and theoretical calculation. The sufficient protons endowed by the abundant carboxyl and hydroxy groups of SHTC promoted the successive •O2- hydrogenation towards H2O2, thus breaking the rate-determining step and realizing superior H2O2 photosynthesis. Under solar light, this subtly synthesized SHTC photocatalyst displayed an impressive H2O2 production yield (610 µmol L-1) in pure water. Impressively, the accumulated H2O2 yield within 32 h under natural sunlight irradiation reached at 760 µmol L-1. The produced H2O2 could be directly used as the oxidant of traditional Fenton process to degrade typical organic contaminants, including sulfadimidine, p-chlorophenol, and rhodamine B. This work presents the feasibility of environmentally compatible photocatalyst from waste biomass for sustainable H2O2 photosynthesis and on site wastewater purification.

Rationally Designing Efficient Biomass Carbon Electrocatalysts for H2O2 Synthesis and Near-Neutral Zn-Air Batteries with Preliminary Machine Learning Guidance

Biomass-based carbon materials are considered promising metal-free catalysts for the 2e- oxygen reduction reaction (ORR) to synthesize H2O2 and act as air electrodes in Zn-air batteries. However, optimization of the catalyst structure is a complex process due to the diversity of biomass precursors and synthesis parameters. Machine learning, a new artificial intelligence technology, has recently been used in various fields owing to its ability to rapidly analyze large amounts of data and guide material synthesis. Consequently, we constructed a machine learning model based on previously reported experimental data and guided the fabrication of a boron-doped biomass carbon catalyst for the 2e- ORR. The achieved catalytic performance exceeded most reported ORR catalysts in terms of H2O2 selectivity (90-95% in broad potentials of 0.30-0.68 V vs reversible hydrogen electrode), stability (maintaining over 90% selectivity for 12 h), yield (3450 mmol gcatalyst-1 h-1), and Faraday efficiency (over 90%). We applied the catalysts to Zn-air batteries and showed a high capacity (2856 mAh g-1) and stability twice that of traditional commercial metal catalysts. Therefore, this study proposed an effective machine learning model to guide the fabrication of biomass-based catalysts in the field of electrocatalysis.

Capacity Enhancement in Quasi-Solid-State Lithium-Oxygen Batteries via Self-Constructing Li+ Transport Channels

Lithium-oxygen batteries (LOBs) attract widespread attention due to their high energy density and safety. The morphology of the solid discharge product bears a close correlation with the battery capacity. In this work, the capacity of LOBs surprisingly increases from 790 mAh g-1 under the liquid electrolyte to 2395 mAh g-1 by using a quasi-solid-state electrolyte (QSSE). The thin film and spherical Li2O2 under the QSSE system construct new Li+ transport channels, which help to extend the solid-phase Li+ transport boundary to the entire electrode to enhance the spatial utilization efficiency of the electrode. Furthermore, a novel mechanism for the growth of Li2O2 is proposed, which is determined by the coupling of the conductivity of Li+ and electrons within the products and the electrode. The result innovatively reveals a new mechanism for the growth of discharge products and a new model of Li+ conduction in LOBs under QSSE systems.

Evaluating the synergistic effect of UVC/e-H2O2 applied for benzotriazole degradation in water matrices using catalyst-free printex L6 carbon-based gas diffusion electrode

The present work investigated the application of UVC combined with electrogenerated H2O2 (UVC/e-H2O2) for BTA degradation using a Printex L6 carbon-based (PL6C) gas diffusion electrode (GDE). The studies were carried out by analyzing the influence of the current density, pH and initial BTA concentration in the contaminant degradation process. Under optimal conditions using 0.05 mol L-1 K2SO4 as supporting electrolyte, BTA removal was evaluated in different water matrices. In all cases, 100% of BTA elimination was reached in the first 15 minutes of treatment under the UVC/e-H2O2 process, while mineralization rates ranging between 59.2 and 78.0% were obtained after 90 minutes of electrolysis. The active species O2•- and e- played an important role in the BTA removal. The toxicity test conducted on the river water sample post-treatment using the Lactuca sativa L seeds showed that the BTA by-products had low toxicity. The results obtained from the LC-ESI-MS/MS analyses showed the same BTA degradation by-products when the BTA-containing water matrices were treated using the UVC/e-H2O2 and photo-electro-Fenton processes. The PL6C-GDE developed in the study exhibited high efficiency when applied for H2O2 electrogeneration and BTA degradation. Additionally, the electrode demonstrated remarkable stability and durability, enabling the generation of reproducible data for up to 81 hours of continuous operation.

An efficient electrocatalytic in-situ hydrogen peroxide generation for ballast water treatment with oxygen groups

The in-situ electrochemical production of hydrogen peroxide (H2O2) offers a promising approach for ballast water treatment. However, further advancements are required to develop electrocatalysts capable of achieving efficient H2O2 generation in seawater environments. Herein, we synthesized two-dimensional lamellated porous carbon nanosheets enriched with oxygen functional groups, which exhibited exceptional performance in H2O2 electrosynthesis. The carbon nanosheet electrocatalysts demonstrated high selectivity for H2O2 production, reaching 90 % at 0.33 V vs. RHE under neutral conditions. Maximum yields were achieved at 2238 mmol gcat-1 h-1 at -0.5 V in an H-type electrolysis cell and 3681 mmol gcat-1 h-1 at a current density of 150 mA cm-2 in a flow cell, with Faraday efficiencies exceeding 70 %. Notably, a continuous 9-hour electrosynthesis test produced a high cumulative H2O2 concentration of 1.2 wt% at a current density of 100 mA cm-2, highlighting the stability and scalability of carbon nanosheets. The outstanding performance of carbon nanosheets is attributed to the abundant basal plane C-O-C group, which provide optimal *OOH binding energy and minimal overpotential. Additionally, the in-situ generated H2O2 from the electrocatalytic system achieved complete sterilization within 60 min against Escherichia coli and several marine bacterial strains isolated from seawater. Furthermore, treatment of real seawater with H2O2 significantly altered the bacterial population abundance at both the phylum and genus levels, highlighting its effectiveness in microbial control. This study presents a high-performance electrocatalytic system for ballast water treatment, offering both scalability and environmental sustainability.

Tuning the structure and properties of carbon cloth by FeCl3intercalation for efficient two-electron oxygen reduction catalysis

The advancement of various energy conversion and storage technologies hinges on the development of efficient and stable electrocatalysts for the oxygen reduction reaction (ORR). In this study, we report the enhancement of carbon cloth (CC) for robust ORR through an FeCl3intercalation reaction. Utilizing a thermal annealing method, FeCl3was intercalated into the graphite structure on the surface of CC, resulting in the creation of numerous defects and the incorporation of Fe species. These newly introduced defects play a pivotal role in activating the ORR via a two-electron pathway. The presence of Fe species further stabilizes the catalytic activity, leading to efficient and stable ORR performance. Our findings highlight the significance of defect engineering and Fe species incorporation in carbon-based materials for improved ORR catalysis and pave the way for the development of advanced electrocatalysts for energy-related applications.

Advanced Nanocarbons Toward two-Electron Oxygen Electrode Reactions for H2O2 Production and Integrated Energy Conversion

Hydrogen peroxide (H2O2) plays a pivotal role in advancing sustainable technologies due to its eco-friendly oxidizing capability. The electrochemical two-electron (2e-) oxygen reduction reaction and water oxidation reaction present an environmentally green method for H2O2 production. Over the past three years, significant progress is made in the field of carbon-based metal-free electrochemical catalysts (C-MFECs) for low-cost and efficient production of H2O2 (H2O2EP). This article offers a focused and comprehensive review of designing C-MFECs for H2O2EP, exploring the construction of dual-doping configurations, heteroatom-defect coupling sites, and strategic dopant positioning to enhance H2O2EP efficiency; innovative structural tuning that improves interfacial reactant concentration and promote the timely release of H2O2; modulation of electrolyte and electrode interfaces to support the 2e- pathways; and the application of C-MFECs in reactors and integrated energy systems. Finally, the current challenges and future directions in this burgeoning field are discussed.

Roles of Oxygen-Containing Functional Groups in Carbon for Electrocatalytic Two-Electron Oxygen Reduction Reaction

Recent years have witnessed great research interests in developing high-performance electrocatalysts for the two-electron (2e-) oxygen reduction reaction (ORR) that enables the sustainable and flexible synthesis of H2O2. Carbon-based electrocatalysts exhibit attractive catalytic performance for the 2e- ORR, where oxygen-containing functional groups (OFGs) play a decisive role. However, current understanding is far from adequate, and the contribution of OFGs to the catalytic performance remains controversial. Therefore, a critical overview on OFGs in carbon-based electrocatalysts toward the 2e- ORR is highly desirable. Herein, we go over the methods for constructing OFGs in carbon including chemical oxidation, electrochemical oxidation, and precursor inheritance. Then we review the roles of OFGs in activating carbon toward the 2e- ORR, focusing on the intrinsic activity of different OFGs and the interplay between OFGs and metal species or defects. At last, we discuss the reasons for inconsistencies among different studies, and personal perspectives on the future development in this field are provided. The results provide insights into the origin of high catalytic activity and selectivity of carbon-based electrocatalysts toward the 2e- ORR and would provide theoretical foundations for the future development in this field.

Effects of porous structure and oxygen functionalities on electrochemical synthesis of hydrogen peroxide on ordered mesoporous carbon

Two-electron oxygen reduction reaction (2e- ORR) is a promising alternative to energy-intensive anthraquinone process for hydrogen peroxide (H2O2) production. Metal-free nanocarbon materials have garnered intensive attention as highly prospective electrocatalysts for H2O2 production, and an in-depth understanding of their porous structure and active sites have become a critical scientific challenge. The present research investigates a range of porous carbon catalysts, including non-porous, microporous, and mesoporous structures, to elucidate the impacts of porous structures on 2e- ORR activity. The results highlighted the superiority of mesoporous carbon over other porous materials, demonstrating remarkable H2O2 selectivity. Furthermore, integration of X-ray photoelectron spectroscopy (XPS) data analysis with electrochemical assessment results unravels the moderate surface oxygen content is the key to increase 2e- ORR activity. These results not only highlight the intricate interplay between pore structure and oxygen content in determining catalytic selectivity, but also enable the design of carbon catalysts for specific electrochemical reactions.

Efficient Electrosynthesis of Hydrogen Peroxide Using Oxygen-Doped Porous Carbon Catalysts at Industrial Current Densities

Metal-free carbon catalysts (MFCCs) are one of the commonly used catalysts for electrocatalytic two-electron oxygen reduction (2e- ORR) synthesis of hydrogen peroxide (H2O2). Oxygen doping is an effective means to improve the performance of MFCCs, but the performance of oxygen-doped carbon catalysts is still not high enough, and the contribution of different oxygen functional groups (OFGs) to the catalytic performance is still inconclusive. In this paper, carbon-based catalysts with different oxygen contents and ratios of OFGs were prepared, and the high 2e- ORR activity of COOH + C-OH was demonstrated by combining the results of experiments and theoretical calculations. The prepared oxygen-doped carbon-based catalyst C-0.1M80 achieved an onset potential of 0.795 V (vs RHE), a selectivity of up to 98.2% (0.6 V vs RHE), and a H2O2 oxidation current of 1.33 mA cm-2 (0.5 V vs RHE) in a rotating ring-disk electrode test (0.1 M KOH solution), which was an outstanding performance in MFCCs. In a solid electrolyte flow cell, C-0.1M80 achieved a Faraday efficiency of 97.5% at 200 mA cm-2 with a corresponding H2O2 production rate of 123.7 mg cm-2 h-1. In addition, a flow cell stability test was performed at an industrial current density (100 mA cm-2) with an astounding 200 h of uninterrupted operation, also achieving an outstanding average Faradaic efficiency (95.8%).

Carbon Catalysts Empowering Sustainable Chemical Synthesis via Electrochemical CO2 Conversion and Two-Electron Oxygen Reduction Reaction

Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2 RR) and two-electron oxygen reduction reaction (2e- ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2 RR and 2e- ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro-nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure-activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single-atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

Selective H2O2 Electrosynthesis over Defective Carbon from Electrochemical Etching of Molybdenum Carbide

The controllable synthesis of specific defective carbon catalysts is crucial for two-electron oxygen reduction reaction (2e^- ORR) to generate H₂O₂ due to the great potential applications. Herein, the defective carbon catalysts (Mo-CDC-ns) were prepared by an electrochemical activation (ECA) method with Mo₂C/C as a parent. Electrochemical cyclic voltammetry curves, X-ray photoelectron spectroscopy, inductively coupled plasma-mass spectrometry, scanning electron microscopy, and high-resolution transmission electron microscopy confirm the evolution process of a defective carbon structure from the Mo₂C phase in which Mo species are first oxidized to Mo⁶⁺ species and then the latter are dissolved into the solution and defective carbon is simultaneously formed. Raman and electron paramagnetic resonance spectra reveal that the defect types in Mo-CDC-ns are the edge defect and vacancy defect sites. Compared with the parent Mo₂C/C, Mo-CDC-ns exhibit gradually increased kinetic current density and selectivity for H₂O₂ generation with an extension of activation cycles from 10 (Mo-CDC-10) to 30 (Mo-CDC-30). Over Mo-CDC-30, a kinetic current density of 19.4 mA cm^-2 and a selectivity close to 90% in 0.1 M KOH solution were achieved, as well as good stability for H₂O₂ production in an extended test up to 12 h in an H-cell. Graphene planes and Stone Wales 5757-carbon were constructed as basic models for density functional theory calculations. It revealed that the obtained defective structure after the removal of Mo atoms contains the double vacancy at the edge of graphene (Edge-DVC) and the topological defect on the plane of 5757-carbon (5757C-D), which show more moderate reaction free energy for forming *OOH and smaller energy barrier of 2e^- ORR.