Sulfur-doped carbon spheres as efficient metal-free electrocatalysts for oxygen reduction reaction

Highly Selective Oxygen Electroreduction to Hydrogen Peroxide on Sulfur-Doped Mesoporous Carbon

As a paradigm-shifting material platform in energy catalysis, precisely engineered ordered mesoporous carbon spheres emerge as supreme metal-free electrocatalysts, outperforming conventional carbon-based counterparts through synergistic structural and electronic innovations. Herein, we architecturally design vertically aligned cylindrical mesoporous carbon spheres with atomic-level sulfur doping (S-mC) that establish unprecedented performance benchmarks in the two-electron oxygen reduction reaction (2e--ORR) to hydrogen peroxide. Systematic comparative studies reveal that the S-mC catalysts achieve exceptional H2O2 selectivity (>99%) and activity at current density of -3.5 mA cm-2, surpassing state-of-the-art metal-free catalysts in current density. Impressively, the optimized S-mC electrocatalyst in a flow cell device achieves an exceptional H2O2 yield of 25 mol gcatalyst -1 h-1. The carbon matrix's unique sp2/sp3 hybrid network coupled with S-induced charge redistribution generates electron-deficient hotspots that selectively stabilize *OOH intermediates, as evidenced by in situ spectroscopic characterization and DFT calculations. This structural-electronic synergy endows the carbon framework with metal-like catalytic efficiency while maintaining inherent advantages of chemical robustness and cost-effectiveness. The marriage of S-doping engineering with mesoscopic pore architecture control opens a new way for developing efficient carbon-based electrocatalysts for oxygen selective reduction to H2O2.

Size-, shape-, facet- and support-dependent selectivity of Cu nanoparticles in CO2 reduction through multiparameter optimization

This study investigates the limited selectivity of the Cu111 surface for C-C bond formation during CO2 reduction and explores the factors influencing selectivity using Cu nanoparticles smaller than 2 nm. The optimal nanoparticle size for C-C bond formation on the 111 facet with minimal overpotential is determined using density functional theory. A suitable supporting surface to enhance the stability and catalytic performance of the Cu-based nanoparticles is identified. Various Cu catalyst geometries, including planar surfaces and cuboctahedral, icosahedral, and truncated octahedral Cu nanoparticles, are considered. Size-dependent effects on the binding energies of reaction intermediates and hydrogen atoms are examined. Carbon-based surfaces, particularly 2SO2-doped graphene nanoribbons, are stable hosts for the Cu nanoparticles and help in retaining the activity for CO2 reduction. Scaling relations between the binding energies of the intermediates suggest COOH binding energy as an energy descriptor. Through multiparameter optimization and with the help of parity line and graphical construction, Cu38 and Cu79 are found to be the most promising surface for C2 product generation. This study provides insights into the factors influencing the selectivity and catalytic performance of Cu nanoparticles, aiding the development of efficient catalysts for CO2 reduction.

Defect- and oxygen-rich nanocarbon derived from solution plasma for bifunctional catalytic activity of oxygen reduction and evolution reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are key for renewable energy systems, including metal-air batteries, fuel cells, and water electrolysis. In particular, metal-air batteries require multiple catalysts for the ORR and OER. Thus, bifunctional catalysts are required to improve efficiency and simplify catalytic systems. Hence, we developed defect- and oxygen-rich nanocarbons as bifunctional catalysts through a one-pot formation by applying plasma discharge in mixed solvents of benzene with crown ether. Raman and X-ray photoelectron spectroscopy results confirmed that oxygen was embedded and functionalized into the carbon matrix and abundant defects were formed, which highly affected the catalytic activity of the ORR and OER. The obtained CNP-CEs revealed a tuned electron transfer trend to a rapid four-electron pathway (n = 3.5) for the ORR, as well as a decreased onset potential and Tafel slope for the OER. Consequently, CNP-CE-50 exhibited an improved bifunctional catalytic characteristic with the narrowest potential gap between the ORR and OER. We believe that our findings suggest new models for carbon-based bifunctional catalysts and provide a prospective approach for a synthetic procedure of carbon nanomaterials.

Iron/iron carbide coupled with S, N co-doped porous carbon as effective oxygen reduction reaction catalyst for microbial fuel cells

As a novel energy device, microbial fuel cells (MFCs) have attracted much attention for their dual functions of electricity generation and sewage treatment. However, the sluggish oxygen reduction reaction (ORR) kinetic on the cathode have hindered the practical application of MFCs. In this work, metallic organic framework derived carbon framework co-doped by Fe, S, N tri-elements was used as alternative electrocatalyst to the conventional Pt/C cathode catalyst in pH-universal electrolytes. The amount of thiosemicarbazide from 0.3 to 3 g determined the surface chemical property, and therefore the ORR activity of FeSNC catalysts. The sulfur/nitrogen doping and Fe/Fe₃C embedded in carbon shell was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The synergy of iron salt and thiosemicarbazide contributed to the improvement of nitrogen and sulfur doping. Sulfur atoms were successfully doped into the carbon matrix and formed a certain amount of thiophene- and oxidized-sulfur. The optimal FeSNC-3 catalyst synthesized with 1.5 g of thiosemicarbazide exhibited the highest ORR activity with a positive half wave potential of 0.866 V in alkaline and 0.691 V (vs. Reversible Hydrogen Electrode) in neutral electrolyte, which both outperformed the commercial Pt/C catalyst. However, as the amount of thiosemicarbazide surpassed 1.5 g, the catalytic performance of FeSNC-4 was lowered, and this could be assigned to the decreased defects and low specific surface area. The excellent ORR performance in neutral medium urged FeSNC-3 as good cathode catalyst in single chambered MFC (SCMFC). It showed the highest maximum power density of 2126 ± 100 mW m^-2, excellent output stability of 8.14% decline in 550 h, chemical oxygen demand removal of 90.7 ± 1.6% and coulombic efficiency of 12.5 ± 1.1%, all superior to those of benchmark SCMFC-Pt/C (1637 ± 35 mW m^-2, 15.4%, 88.9 ± 0.9%, and 10.2 ± 1.1%). These outstanding results were associated to the large specific surface area and synergistic interaction of multiple active sites, like Fe/Fe₃C, Fe-N₄, pyridinic N, graphite N and thiophene-S.

Progress of Nonmetallic Electrocatalysts for Oxygen Reduction Reactions

As a key role in hindering the large-scale application of fuel cells, oxygen reduction reaction has always been a hot issue and nodus. Aiming to explore state-of-art electrocatalysts, this paper reviews the latest development of nonmetallic catalysts in oxygen reduction reactions, including single atoms doped with carbon materials such as N, B, P or S and multi-doped carbon materials. Afterward, the remaining challenges and research directions of carbon-based nonmetallic catalysts are prospected.

Fe Powder Catalytically Synthesized C3N3 toward High-Performance Anode Materials of Lithium-Ion Batteries

Recently, carbon nitrides and their carbon-based derivatives have been widely studied as anode materials of lithium-ion batteries due to their graphite-like structure and abundant nitrogen active sites. In this paper, a layered carbon nitride material C₃N₃ consisting of triazine rings with an ultrahigh theoretical specific capacity was designed and synthesized by an innovative method based on Fe powder-catalyzed carbon-carbon coupling polymerization of cyanuric chloride at 260 °C, with reference to the Ullmann reaction. The structural characterizations indicated that the as-synthesized material had a C/N ratio close to 1:1 and a layered structure and only contained one type of nitrogen, suggesting the successful synthesis of C₃N₃. When used as a lithium-ion battery anode, the C₃N₃ material showed a high reversible specific capacity up to 842.39 mAh g^-1 at 0.1 A g^-1, good rate capability, and excellent cycling stability attributed to abundant pyridine nitrogen active sites, large specific surface area, and good structure stability. Ex situ XPS results indicated that Li⁺ storage relies on the reversible transformation of -C=N- and -C-N- groups as well as the formation of bridge-connected -C=C- bonds. To further optimize the performance, the reaction temperature was further increased to synthesize a series of C₃N₃ derivatives for the enhanced specific surface area and conductivity. The resulting derivative prepared at 550 °C showed the best electrochemical performance, with an initial specific capacity close to 900 mAh g^-1 at 0.1 A g^-1 and good cycling stability (94.3% capacity retention after 500 cycles at 1 A g^-1). This work will undoubtedly inspire the further study of high-capacity carbon nitride-based electrode materials for energy storage.

Molten-Salt-Assisted Synthesis of Nitrogen-Doped Carbon Nanosheets Derived from Biomass Waste of Gingko Shells as Efficient Catalyst for Oxygen Reduction Reaction

Developing superior efficient and durable oxygen reduction reaction (ORR) catalysts is critical for high-performance fuel cells and metal–air batteries. Herein, we successfully prepared a 3D, high-level nitrogen-doped, metal-free (N–pC) electrocatalyst employing urea as a single nitrogen source, NaCl as a fully sealed nanoreactor and gingko shells, a biomass waste, as carbon precursor. Due to the high content of active nitrogen groups, large surface area (1133.8 m2 g−1), and 3D hierarchical porous network structure, the as-prepared N–pC has better ORR electrocatalytic performance than the commercial Pt/C and most metal-free carbon materials in alkaline media. Additionally, when N–pC was used as a catalyst for an air electrode, the Zn–air battery (ZAB) had higher peak power density (223 mW cm−2), larger specific-capacity (755 mAh g−1) and better rate-capability than the commercial Pt/C-based one, displaying a good application prospect in metal-air batteries.

Hollow nitrogen-doped carbon nanospheres as cathode catalysts to enhance oxygen reduction reaction in microbial fuel cells treating wastewater

Hollow nanospheres play a pivotal role in the electro-catalytic oxygen reduction reaction (ORR), which is a crucial step in microbial fuel cell (MFC) device. Herein, the hollow nitrogen-doped carbon nanospheres (HNCNS) were synthesized with the sacrifice of silica coated carbon nanospheres (CNS@SiO₂) as template. HNCNS remarkably enhanced the ORR activity compared to the solid carbon and solid silica spheres. By tuning calcination temperature (800-1100 °C), the surface chemistry properties of HNCNS were effectively regulated. The optimal HNCNS-1000 catalyst which was calcined at 1000 °C exhibited the highest ORR activity in neutral media with the onset potential of 0.255 V and half-wave potential of -0.006 V (vs. Ag/AgCl). Single chamber MFC (SCMFC) assembled with HNCNS-1000 cathode unveiled comparable activity to a conventional Pt/C reference. It showed the highest maximum power density of 1307 ± 26 mW/m², excellent output stability of 5.8% decline within 680 h, chemical oxygen demand (COD) removal of 94.0 ± 0.3% and coulombic efficiency (CE) of 7.9 ± 0.9%. These excellent results were attributed to a cooperative effect of the optimized surface properties (e.g., structural defects, relative content of pyrrolic nitrogen and specific surface area) and the formation of hollow nanosphere structure. Furthermore, the positive linear relationship of the structural defects and pyrrolic nitrogen species with the maximum power generation in SCMFC were clearly elucidated. This study demonstrated that the cost effective HNCNS-1000 was a promising alternative to commercial Pt/C catalyst for practical application in MFCs treating wastewater. Our result revealed the effectiveness of MFC fabricated with HNCNS-1000 cathode catalyst in terms of power generation and wastewater treatment.

Fabrication of Two-Dimensional Functional Covalent Organic Frameworks via the Thiol-Ene "Click" Reaction as Lubricant Additives for Antiwear and Friction Reduction

To address the energy wastage problem caused by friction, novel lubricant additives other than the traditional and basic used additives with outstanding performance are urgently needed. A facile and efficient postsynthetic strategy for modification of two-dimensional (2D) covalent organic frameworks (COFs) was proposed to obtain dialkyl dithiophosphate (DDP)-functionalized COFs (DDP@TD-COF) as lubricant additives. The DDP@TD-COF was prepared by amine-aldehyde condensation reaction of the triazine compound and vinyl-functionalized monomers through a solvothermal process to form a vinyl-functionalized 2D COF (TD-COF), followed by covalent bonding of commercial lubricating molecules (DDP) via the UV-induced thiol-ene "click" reaction. The as-obtained DDP@TD-COF with homogeneous distribution of N, P, and S elements exhibits exceptional dispersion stability in the 500SN base oil, which remains stable for over 6 days. With a trace amount addition of 0.05 wt %, superior friction and wear reduction of DDP@TD-COF are observed with the friction coefficient lessened to 0.096 from 0.19, wear volume loss declined by 94.9%, and load carrying ability increased from 150 to 650 N simultaneously. The mechanism studies show that the shear force can induce interlayer slipping during the friction process, and the stripped DDP@TD-COF can get involved in the contacting interface inducing tribo-chemical reactions via N, P, and S elements forming a protective layer on the surfaces. Consequently, the DDP@TD-COF demonstrated remarkable friction diminution and abrasion resistance abilities even with a trace amount addition, and this work provides a dependable and valid route for the design and preparation of functional COF-based nanoadditives.

Electrochemical Reduction of CO2 to CO by N,S Dual-Doped Carbon Nanoweb Catalysts

Converting CO₂ into useful chemicals through an electrocatalytic process is an attractive solution to reduce CO₂ in the atmosphere. However, the process suffers from high overpotential, low activity, or poor product selectivity. In this study, N,S dual-doped carbon nanoweb (NSCNW) materials were proposed as an efficient nonmetallic electrocatalyst for CO₂ reduction. The NSCNW catalysts preferentially and rapidly converted CO₂ into CO with a high Faradaic efficiency of 93.4 % and a partial current density of -5.93 mA cm^-2 at a low overpotential of 490 mV. A small Tafel slope value (93 mV dec^-1 ) was obtained, demonstrating a high rate for CO₂ reduction. Moreover, the catalysts also exhibited a quite stable current-density profile during 20 h with a high CO Faradaic efficiency above 90 % throughout the electrolysis reaction. The high catalytic performance of the catalysts for CO₂ reduction could be attributed to synergistic effects associated with the structural advantages of 3 D carbon nanoweb structures and effective S doping of the carbon materials with the highest ratio of thiophene-like S to oxidized S species.

Nanocomposite Titania-Carbon Spheres as CO2 and CH4 Sorbents

Photocatalysis can offer solutions for the transformation of greenhouse gases, such as methane and carbon dioxide. In the paper, a candidate for such a photocatalyst is presented, based on a composite of titania with carbon spheres. The material was obtained using microwave assisted solvothermal synthesis, enabling good dispersion of titania. The studies of carbon dioxide and methane adsorption were performed under ambient pressure and temperatures of 40, 60, and 80 °C. The effect of temperature increase was less favorable for carbon dioxide than for methane. Satisfying values of carbon dioxide and methane uptake were obtained-3.94 mmol CO₂/g and 2.77 mmol CH₄/g at 40 °C.

Pd nanoparticles confined in mesoporous N-doped carbon silica supports: a synergistic effect between catalyst and support

Palladium nanoparticles of similar size were deposited on different supports, layers of carbon materials (with and without nitrogen doping) on the surface of a MCF (mesocellular foam) silica.

Enhancement mechanism of sulfur dopants on the catalytic activity of N and P co-doped three-dimensional hierarchically porous carbon as a metal-free oxygen reduction electrocatalyst

A sulfur, nitrogen and phosphorus ternary-doped cattle-bone-derived hierarchically porous carbon metal-free electrocatalyst was synthesized, exhibiting superior oxygen reduction performance compared to Pt/C.

Advanced Biomass-Derived Electrocatalysts for the Oxygen Reduction Reaction

Recent progress in advanced nanostructures synthesized from biomass resources for the oxygen reduction reaction (ORR) is reviewed. The ORR plays a significant role in the performance of numerous energy-conversion devices, including low-temperature hydrogen and alcohol fuel cells, microbial fuel cells, as well as metal-air batteries. The viability of such fuel cells is strongly related to the cost of the electrodes, especially the cathodic ORR electrocatalyst. Hence, inexpensive and abundant plant and animal biomass have become attractive options to obtain electrocatalysts upon conversion into active carbon. Bioresource selection and processing criteria are discussed in light of their influence on the physicochemical properties of the ORR nanostructures. The resulting electrocatalytic activity and durability are introduced and compared to those from conventional Pt/C-based electrocatalysts. These ORR catalysts are also active for oxygen or hydrogen evolution reactions.

Facile synthesis of microporous sulfur-doped carbon spheres as electrodes for ultrasensitive detection of ascorbic acid in food and pharmaceutical products

The active interfacial surface of S-doped microporous carbon spheres strongly binds with ascorbic acid in food and pharmaceutical products.

Bifunctional hydrous RuO2 nanocluster electrocatalyst embedded in carbon matrix for efficient and durable operation of rechargeable zinc-air batteries

Ruthenium oxide (RuO₂) is the best oxygen evolution reaction (OER) electrocatalyst. Herein, we demonstrated that RuO₂ can be also efficiently used as an oxygen reduction reaction (ORR) electrocatalyst, thereby serving as a bifunctional material for rechargeable Zn–air batteries. We found two forms of RuO₂ (i.e. hydrous and anhydrous, respectively h-RuO₂ and ah-RuO₂) to show different ORR and OER electrocatalytic characteristics. Thus, h-RuO₂ required large ORR overpotentials, although it completed the ORR via a 4e process. In contrast, h-RuO₂ triggered the OER at lower overpotentials at the expense of showing very unstable electrocatalytic activity. To capitalize on the advantages of h-RuO₂ while improving its drawbacks, we designed a unique structure (RuO₂@C) where h-RuO₂ nanoparticles were embedded in a carbon matrix. A double hydrophilic block copolymer-templated ruthenium precursor was transformed into RuO₂ nanoparticles upon formation of the carbon matrix via annealing. The carbon matrix allowed overcoming the limitations of h-RuO₂ by improving its poor conductivity and protecting the catalyst from dissolution during OER. The bifunctional RuO₂@C catalyst demonstrated a very low potential gap (ΔE_OER-ORR = ca. 1.0 V) at 20 mA cm⁻². The Zn||RuO₂@C cell showed an excellent stability (i.e. no overpotential was observed after more than 40 h).

Iron phosphide nanocrystals decorated in situ on heteroatom-doped mesoporous carbon nanosheets used for an efficient oxygen reduction reaction in both alkaline and acidic media

Oxygen reduction catalysts based on heteroatom-doped mesoporous carbon nanosheets loaded with highly crystalline FeP nanoparticles (FeP@FePNCs) were fabricated using a simple, one-step carbonization–phosphization methodology.

Cu,N-codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction

It remains a huge challenge to develop nonprecious electrocatalysts with high activity to substitute commercial Pt catalysts for oxygen reduction reactions (ORR). Here, the Cu,N-codoped hierarchical porous carbon (Cu-N-C) with a high content of pyridinic N was synthesized by carbonizing Cu-containing ZIF-8. Results indicate that Cu-N-C shows excellent ORR electrocatalyst properties. First of all, it nearly follows the four-electron route, and its electron transfer number reaches 3.92 at -0.4 V. Second, both the onset potential and limited current density of Cu-N-C are almost equal to those of a commercial Pt/C catalyst. Third, it exhibits a better half-wave potential (∼16 mV) than a commercial Pt/C catalyst. More importantly, the Cu-N-C displays better stability and methanol tolerance than the Pt/C catalyst. All of these good properties are attributed to hierarchical structure, high pyridinic N content, and the synergism of Cu and N dopants. The metal-N codoping strategy can significantly enhance the activity of electrocatalysts, and it will provide reference for the design of novel N-doped porous carbon ORR catalysts.

In situ growth of polyphosphazene nanoparticles on graphene sheets as a highly stable nanocomposite for metal-free lithium anodes

A polyphosphazene/GN nanocomposite was readily synthesized by thermal polymerization of hexachlorocyclotriphosphazene with graphene oxide, which exhibits a stable and uniform nanostructure.

Synthesis of hybrid carbon spheres@nitrogen-doped graphene/carbon nanotubes and their oxygen reduction activity performance

The hybrid architecture of carbon spheres@nitrogen-doped graphene/carbon nanotubes (CS@N-G/CNT) was synthesized by a hydrothermal and ultrasonic-assisted method.

Synthesis of hierarchically porous carbon spheres by an emulsification-crosslinking method and their application in supercapacitors

Porous carbon sphere materials with hierarchical pore structure have greatly developed as promising electrode materials for electric double-layer capacitors (EDLCs).