One-Pot Etching Pyrolysis to Defect-Rich Carbon Nanosheets to Construct Multiheteroatom-Coordinated Iron Sites for Efficient Oxygen Reduction

Constructing multiheteroatom coordination structure in carbonaceous substrates demonstrates an effective method to accelerate the oxygen reduction reaction (ORR) of supported single-atom catalyst. Herein, the novel etching route assisted by potassium thiocyanate (KCNS) is developed to convert metal-organic framework to 2D defect-rich porous N,S-co-doped carbon nanosheets for anchoring atomically dispersed iron sites as the high-performance ORR catalysts (Fe-SACs). The well-designed KCNS-assisted etching route can generate spatial confinement template to direct the carbon nanosheet formation, etching condition to form defect-rich structure, and additional sulfur atoms to coordinate iron species. Spectral and microscopy analysis reveals that the iron element in Fe-SACs is highly isolated on carbon nanosheet and anchored by nitrogen and sulfur atoms in unsymmetrical Fe-S1N3 structure. The optimized Fe-SACs with large specific surface area could show remarkable alkaline ORR performances with a high half-wave potential of 0.920 V versus RHE and excellent durability. The rechargeable zinc-air battery assembled with Fe-SACs air electrodes delivers a large power density of 350 mW cm-2 and a stable voltage platform during charge and discharge over more than 1300 h. This work proposes a novel strategy for the preparation of single-atom catalysts with multiheteroatom coordination structure and highly exposed active sites for efficient ORR.

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.

Effective Low-Powered Photocatalytic Disinfection via Synchronous Introduction of Oxygen Dopants and Carbon Defects in Carbon Nitride

Establishing an effective metal-free photocatalyst for sustainable applications remains a huge challenge. Herein, we developed ultrathin oxygen-doped g-C3N4 nanosheets with carbon defects (OCvN) photocatalyst via a facile gas bubble template-assisted thermal copolymerization method. A series of OCvN with different dopant amounts ranging from 0 to 10% were synthesized and used as photocatalysts under illumination of low-power (2 × 18 W, 0.18 mW/cm2) and commercially available energy-saving light bulbs. Upon testing for photocatalytic Escherichia coli inactivation, the best-performing sample, OCvN-3, demonstrated an astonishing disinfection activity of over 7-log reduction after 3 h of illumination, boasting an 18-fold improvement in its antibacterial activity compared to that of pristine g-C3N4. The enhanced performance was attributed to the synergistic effects of increased surface area, extended visible light harvesting, improved electronic conductivity, and ultralow resistance to charge transfer. This study successfully introduced a green photocatalyst that demonstrates the most effective disinfection performance ever recorded among metal-free g-C3N4 materials. Its disinfection capabilities are comparable to those of metal-based photocatalysts when they are exposed to low-power light.

Tuning Carbon Defect in Copper Single-Atom Catalysts for Efficient Oxygen Reduction

Defect chemistry in carbon matrix shows great potential for promoting the oxygen reduction reaction (ORR) of metal single-atom catalysts. Herein, a modified pyrolysis strategy is proposed to tune carbon defects in copper single-atom catalysts (Cu-SACs) to fully understand their positive effect on the ORR activity. The optimized Cu-SACs with controllable carbon defect degree and enhanced active specific surface area can exhibit improved ORR activity with a half-wave potential of 0.897 V_RHE , ultrahigh limiting current density of 6.5 mA cm^-2 , and superior turnover frequency of 2.23 e site^-1 s^-1 . The assembled Zn-air batteries based on Cu-SACs can also show well-retained reversibility and voltage platform over 1100 h charge/discharge period. Density functional theory calculations reveal that suitable carbon defects can redistribute charge density of Cu-N4 active sites to weaken the O-O bond in adsorbed OOH* intermediate and thus reduce its dissociation energy. This discovery offers a universal strategy for fabricating superior single-atom catalysts with high-efficiency active sites toward energy-directed applications.

Carbon Surface Chemistry: New Insight into the Old Story

The enormous complexity of the carbon material family has provoked a phenomenological approach to develop its potential in different applications. Although the electronic, chemical, mechanical, and magnetic properties of carbon materials have been widely discussed based on defect control engineering, there is still a lack of fundamental understanding of the carbon surface chemistry, which leads to many controversial conclusions. Here, by analyzing various defects on carbon surface, some commonly neglected aspects and misunderstandings in this field are pointed out, clarifying how surface chemistry affects the chemical behaviors of carbon in some specific chemical reactions. With this full-scale consideration of the carbon surface chemistry, the behaviors of carbon materials with various functions can be well defined, which is indispensable for their scalable applications. Perspectives on future developments of carbon surface chemistry are also provided to enable practically accessible design of advanced carbon in those applications.

Intrinsic Carbon Defects Induced Reversible Antimony Chemistry for High-Energy Aqueous Alkaline Batteries

Developing high-capacity, dendrite-free, and stable anode materials for robust aqueous alkaline batteries (AABs) is an ongoing challenge. Antimony (Sb) is predicated as an attractive anode material, but it still suffers from low capacity and poor stability caused by the obstructed kinetic behavior and uncontrollable nucleation for SbO₂ ^- . Herein, designing a new defect-modified carbon skeleton (D-CS), a highly reversible Sb anode with ultralong cycling stability is realized at practical levels of capacity and high depth of discharge (DOD). The abundant intrinsic carbon defects can effectively form positive charge centers to weaken electrostatic repulsion between SbO₂ ^- and electrode surface, facilitating the fast ion kinetics and provide generous controllable nucleation sites. In addition, the uniform electric field distribution of the D-CS induces manageable plating and stripping of the Sb metal, which effectively boosts its electrochemical reversibility and restrains adverse reactions. Accordingly, the Sb/D-CS electrode achieves a long cycle life of over 500 h with a capacity of 2 mAh cm^-2 . Even at an ultrahigh capacity of 10 mAh cm^-2 , it can still work stably up to 40 h. Furthermore, its feasibility as advanced anode in AABs is also confirmed by assembled Ni//Sb/D-CS full batteries with an ultrahigh capacity of 13.5 mAh cm^-2 and a considerable stability of 4500 cycles.

Pyrogenic Carbon Initiated the Generation of Hydroxyl Radicals from the Oxidation of Sulfide

Sulfide is one of the most abundant reductants in the subsurface environment, while pyrogenic carbon is a redox medium that widely exists in sulfide environment. Previous studies have found pyrogenic carbon can mediate the reductive degradation of organic pollutants under anoxic sulfide conditions; however, the scenario under oxic sulfide conditions has rarely been reported. In this study, we found that pyrogenic carbon can mediate hydroxyl radicals (^•OH) generation from sulfide oxidation under dark oxic conditions. The accumulated ^•OH ranged from 2.07 to 101.90 μM in the presence of 5 mM Na₂S and 100 mg L^-1 pyrogenic carbon at pH 7.0 within 240 min. The Raman spectra and electrochemical cell experiments revealed that the carbon defects were the possible chemisorption sites for oxygen, while the graphite crystallites were responsible for the electron transfer from sulfide to O₂ to generate H₂O₂ and ^•OH. Quenching experiments and degradation product identification showed that As(III) and sulfanilamide can be oxidized by the generated ^•OH. This research provides a new insight into the important role of pyrogenic carbon in redox reactions and dark ^•OH production.

Metal-Free Catalyst with Large Carbon Defects for Efficient Direct Overall Water Splitting in Air at Room Pressure

Hydrogen can be used as a conventional clean energy resource, only by obtaining it in a cheap, simple, and feasible way. From the perspective of solar energy utilization, photocatalytic water splitting is essentially limited by high electric energy and intermittent solar energy. Here, we demonstrate that the carbon defects could decompose water directly by using low-value mechanical energy in the daily environment without using the sacrificial agent. The prepared metal-free carbon catalyst (OPC-900) with rich graphene topological defects shows several redox states in acetonitrile, which can accumulate redox equivalents and respond for overall water splitting in air at room pressure. Under stirring or light conditions, charges can be created and accumulated on the catalyst surface and split water into H₂ and O₂ in low conductivity medium. The mechanical energy conversion efficiency of stirring in ultrapure water is 7.01%. Our work combines visible light with low-value mechanical energy for widespread decomposition of water to produce hydrogen in air under normal pressure.

B, N Codoped and Defect-Rich Nanocarbon Material as a Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions

The development of highly active, inexpensive, and stable bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts to replace noble metal Pt and RuO2 catalysts remains a considerable challenge for highly demanded reversible fuel cells and metal-air batteries. Here, a simple approach for the facile construction of a defective nanocarbon material is reported with B and N dopants (B,N-carbon) as a superior bifunctional metal-free catalyst for both ORR and OER. The catalyst is prepared by pyrolyzing the composites of ethyl cellulose and high-boiling point 4-(1-naphthyl)benzeneboronic acid in NH3 atmosphere with an inexpensive Zn-based template. The obtained porous B,N-carbon with rich carbon defects exhibits excellent ORR and OER performances, including high activity and stability. In alkaline medium, B,N-carbon material shows high ORR activity with an onset potential (Eonset) reaching 0.98 V versus reversible hydrogen electrode (RHE), very close to that of Pt/C, a high electron transfer number and excellent stability. This catalyst also presents the admirable ORR activity in acidic medium with a high Eonset of 0.81 V versus RHE and a four-electron process. The OER activity of B,N-carbon is superior to that of the precious metal RuO2 and Pt/C catalysts. A Zn-air battery using B,N-carbon as the air cathode exhibits a low voltage gap between charge and discharge and long-term stability. The excellent electrocatalytic performance of this porous nanocarbon material is attributed to the combined positive effects of the abundant carbon defects and the heteroatom codopants.