Pyrolysis Strategy Research Articles

Constructed surface-enriched atomically dispersed Co–N4 active sites via stepwise pyrolysis strategy for oxygen reduction

Atomically-dispersed transition-metal nitrogen–carbon catalysts with specific coordination structures are promising cheap candidates for efficient oxygen reduction reaction (ORR) in metal–air and fuel cells. Nevertheless, a large part of internal active sites in this class of catalysts cannot really take effect, and the density of surface active sites is seriously insufficient. Here, a reasonable surface-host stepwise method involving adsorption and pyrolysis was designed to prepare Co–Nx/CNC electrocatalysts with highly-dispersed surface active sites, and to stabilize the isolated atomically dispersed Co–NX sites on the surface of graphitized N-doped carbon (NC). The new catalysts achieved admirable ORR activity with half-wave potential (E1/2) of 0.88 V under alkalinity. Moreover, Co–Nx/CNC showed stronger four-electron transfer ability and much higher stability. The main reason was that the coordination environment and density of active sites were effectively optimized, enabling the improvement of surface active sites and reactivity. This work provides a general approach to design and prepare highly-stabilized and abundant atomically-dispersed surface active sites for production in electrochemical energy devices.

Defect-Engineered Mesoporous Undoped Carbon Nanoribbons for Benchmark Oxygen Reduction Reaction.

For large-scale fuel cell applications, it is significant to replace expensive Pt-based oxygenreductionreaction (ORR) electrocatalysts with nonprecious metal- or metal-free carbon-based catalysts with high activity. However, it is still challenging to deeply understand the role of intrinsic defects and the origin of ORR activity in pure nanocarbon. Therefore, a novel self-assembly and a pyrolysis strategy to fabricate defect-rich mesoporous carbon nanoribbons are presented. Due to the effective regulation of nanoarchitecture, a vast number of defective catalytic sites (edge defects and holes) are exposed, which thereby enhances the electron transfer kinetics and catalytic activity. Such undoped nanoribbons display a large half-wave potential of 0.837V, excellent long-term stability, and exceptional methanol tolerance, surpassing the most undoped ORR catalysts and the commercial Pt/C (20wt.%) catalyst. Structural characterizations and density functional theory (DFT) calculations confirm that the zigzag edge defects and the armchair pentagon at the hole defect are responsible for outstanding ORR performance.

Lignosulfonate-assisted in situ synthesis of Co9S8–Ni3S2 heterojunctions encapsulated by S/N co-doped biochar for efficient water oxidation

The development of highly active and stable earth-rich electrocatalysts remains a major challenge to release the reliance on noble metal catalysts in sustainable (electro)chemical processes. In this work, metal sulfides encapsulated with S/N co-doped carbon were synthesized with a one-step pyrolysis strategy, where S was introduced during the self-assembly process of sodium lignosulfonate. Due to the precise coordination of Ni and Co ions with lignosulfonate, an intense-interacted Co9S8–Ni3S2 heterojunction was formed inside the carbon shell, causing the redistribution of electrons. An overpotential as low as 200 mV was obtained over Co9S8–Ni3S2@SNC to reach a current density of 10 mA cm−2. Only a slight increase of 14.4 mV was observed in a 50 h chronoamperometric stability test. Density functional theory (DFT) calculations showed that Co9S8–Ni3S2 heterojunctions encapsulated with S/N co-doped carbon can optimize the electronic structure, lower the reaction energy barrier, and improve the OER reaction activity. This work provides a novel strategy for constructing highly efficient and sustainable metal sulfide heterojunction catalysts with the assistance of lignosulfonate biomass.

Single-Atom Fe Nanozyme with Enhanced Oxidase-like Activity for the Colorimetric Detection of Ascorbic Acid and Glutathione

Single-atom nanozymes (SAzymes) have drawn ever-increasing attention due to their maximum atom utilization efficiency and enhanced enzyme-like activity. Herein, a facile pyrolysis strategy is reported for the synthesis of the iron-nitrogen-carbon (Fe-N-C) SAzyme using ferrocene trapped within porous zeolitic imidazolate framework-8 (ZIF-8@Fc) as a precursor. The as-prepared Fe-N-C SAzyme exhibited exceptional oxidase-mimicking activity, catalytically oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) with high affinity (Km) and fast reaction rate (Vmax). Taking advantage of this property, we designed two colorimetric sensing assays based on different interaction modes between small molecules and Fe active sites. Firstly, utilizing the reduction activity of ascorbic acid (AA) toward oxidized TMB (TMBox), a colorimetric bioassay for AA detection was established, which exhibited a good linear range of detection from 0.1 to 2 μM and a detection limit as low as 0.1 μM. Additionally, based on the inhibition of nanozyme activity by the thiols of glutathione (GSH), a colorimetric biosensor for GSH detection was constructed, showing a linear response over a concentration range of 1-10 μM, with a detection limit of 1.3 μM. This work provides a promising strategy for rationally designing oxidase-like SAzymes and broadening their application in biosensing.

Controllable synthesis of Co/MnO heterointerfaces embedded in graphitic carbon for rechargeable Zn–air battery

Owing to the unique geometric and electronic structure, design and synthesis of electrocatalysts with well defined heterointerfaces are essential for clean energy technologies, for instance water-splitting and Zn-air batteries. Herein, a bifunctional electrocatalyst assembled by Co/MnO nanoparticles and nitrogen doping double-sphere carbon (denoted as Co/MnO@N-DSC), was fabricated via a solvothermal and pyrolysis strategy. The Co/MnO@N-DSC catalysts exhibit an enhanced bifunctional oxygen electrocatalytic performance for Oxygen reduction reaction (ORR) (E1/2, 0.84 V vs. RHE) and oxygen evolution reaction (OER) (Eonset, 1.54 V vs. RHE). As an air cathode catalyst, the Co/MnO@N-DSC-based Zn-air battery can afford prime performance, over the commercial noble-metal-based Zn-air battery. Theoretical calculation results indicate that the synergism of Co (111)/MnO (200) heterointerfaces can enhance charge transfer and provide extra electrons for the reaction processes. This work provides a promising manoeuvre to uplift bifunctional catalytic activity by increasing the synergy from heterointerfaces of transition-metal/metal oxide in oxygen electrocatalysis.

Salt–template assisted fabrication of Co3O4 nanodots anchored into 2D N–doped carbon nanosheets as an advanced anode for lithium–ion batteries

The design and exploitation of advanced transition metal oxides (TMO)/carbon hybrid anodes remain significant but challenging for the evolution of lithium–ion batteries (LIBs). In this work, Co3O4 nanodots anchored into 2D N-doped carbon nanosheets (Co3O4 NDs/NCN) hybrid is successfully fabricated by a facile salt-template assisted pyrolysis strategy using glucose, Co(NO3)2·6H2O and NaCl salt template as reaction precursors. The morphology, structure and composition of the resultant Co3O4 NDs/NCN hybrid are systematically investigated, and the effect of salt template on the preparation of products is also deeply explored. It is surprisingly found that the introduction of NaCl template promotes the formation of Co3O4, which is different from what has been reported previously. In addition, the added NaCl template can be used to regulate the microstructure of Co3O4 NDs/NCN hybrid, which displays unique micromorphology (Co3O4 NDs anchored into 2D NCN), successful N doping in carbon matrix, Co-N-C bond between two components and much increased specific surface area. As a result, this hybrid shows excellent lithium storage performance when used as the anode for LIBs, which can be comparable to those previously reported Co3O4/C hybrid anodes. Clearly, this work offers a good reference for the construction of low-cost and high-performance TMO/carbon LIBs anodes.

MoO3-MoC/Co@NC coupled heterostructures with strong connections and abundant interfaces for electromagnetic wave absorption

Improving the electromagnetic wave (EMW) absorption capability at the thin thickness of heterostructures is critical but remains a great challenge. Here, we propose a MoO3 @ZIF-67 core-shell heterostructure derived molybdenum trioxide- molybdenum carbide/cobalt@N-doped carbon (MoO3-MoC/Co@NC) coupled heterostructure with a one-dimensional (1D) short rod-like structure for EMW absorption. A strong connection is constructed between Co@NC nanoparticles (NPs) and MoO3-MoC specie due to the pyrolysis strategy and in situ reaction between MoO3 nanobelts and ZIF-67 rhombic dodecahedrons. The abundant heterointerfaces between N-doped carbon (NC) and metallic Co, Co@NC NPs and MoC species, Co@NC NPs and MoO3 nanobelts, as well as MoC and MoO3 nanobelts improve the interfacial polarization. Abundant defects, such as N heteroatoms and oxygen vacancies, are also established in the MoO3-MoC/Co@NC coupled heterostructure. As a result, the MoO3-MoC/Co@NC coupled heterostructure inherits excellent EMW absorption performance with a reflection loss (RL) value of − 64.3 dB at a thickness of 3.875 mm, outperforming many advanced EMW absorbers. This work shows the great potential of strong connections and abundant interfaces in heterostructure for improvements of EMW absorbers.

Sulfur and phosphorus co-doped hard carbon for potassium ion storage

Due to the low cost, good chemical stability and structural diversity, hard carbon has been considered as an important anode material for potassium-ion batteries (PIBs). However, due to the large diameter of K+, PIBs with both excellent rate performance and long-life is still challenging. Herein, sulfur (S), phosphorus (P) co-doped hard carbon anode are synthesized via polymerization of thiophene and phytic acid and the following concise pyrolysis strategy. S in hard carbon can used as reactive sites for K+ storage and P doping will effectively improve wettability of electrolyte. After temperature regulation, the fabricated SP-700 with dual and abundant heteroatom doping exhibits high initial reversible capacity (412 mAh g−1 at 0.05 A g−1), excellent rate performance (130 mAh g−1 at 5 A g−1) and stable cyclic performance (94 mAh g−1 after 1500 cycles at 2 A g−1).

A Versatile Sulfur‐Assisted Pyrolysis Strategy for High‐Atom‐Economy Upcycling of Waste Plastics into High‐Value Carbon Materials

With the overconsumption of disposable plastics, there is a considerable emphasis on the recycling of waste plastics to relieve the environmental, economic, and health-related consequences. Here, a sulfur-assisted pyrolysis strategy is demonstrated for versatile upcycling of plastics into high-value carbons with an ultrahigh carbon-atom recovery (up to 85%). During the pyrolysis process, the inexpensive elemental sulfur molecules are covalently bonded with polymer chains, and then thermally stable intermediates are produced via dehydrogenation and crosslinking, thereby inhibiting the decomposition of plastics into volatile small hydrocarbons. In this manner, the carbon products obtained from real-world waste plastics exhibit sulfur-rich skeletons with an enlarged interlayer distance, and demonstrate superior sodium storage performance. It is believed that the present results offer a new solution to alleviate plastic pollution and reduce the carbon footprint of plastic industry.

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 VRHE , ultrahigh limiting current density of 6.5mA 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 1100h 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.

Preferential Pyrolysis Construction of Carbon Anodes with 8400 h Lifespan for High-Energy-Density K-ion Batteries.

Carbonaceous materials are promising anodes for practical potassium-ion batteries, but fail to meet the requirements for durability and high capacities at low potentials. Herein, we constructed a durable carbon anode for high-energy-density K-ion full cells by a preferential pyrolysis strategy. Utilizing S and N volatilization from a π-π stacked supermolecule, the preferential pyrolysis process introduces low-potential active sites of sp2 hybridized carbon and carbon vacancies, endowing a low-potential "vacancy-adsorption/intercalation" mechanism. The as-prepared carbon anode exhibits a high capacity of 384.2 mAh g-1 (90 % capacity locates below 1 V vs. K/K+ ), which contributes to a high energy density of 163 Wh kg-1 of K-ion full battery. Moreover, abundant vacancies of carbon alleviate volume variation, boosting the cycling stability over 14 000 cycles (8400 h). Our work provides a new synthesis approach for durable carbon anodes of K-ion full cells with high energy densities.

Preferential Pyrolysis Construction of Carbon Anodes with 8400 h Lifespan for High‐Energy‐Density K‐ion Batteries

AbstractCarbonaceous materials are promising anodes for practical potassium‐ion batteries, but fail to meet the requirements for durability and high capacities at low potentials. Herein, we constructed a durable carbon anode for high‐energy‐density K‐ion full cells by a preferential pyrolysis strategy. Utilizing S and N volatilization from a π–π stacked supermolecule, the preferential pyrolysis process introduces low‐potential active sites of sp2 hybridized carbon and carbon vacancies, endowing a low‐potential “vacancy‐adsorption/intercalation” mechanism. The as‐prepared carbon anode exhibits a high capacity of 384.2 mAh g−1 (90 % capacity locates below 1 V vs. K/K+), which contributes to a high energy density of 163 Wh kg−1 of K‐ion full battery. Moreover, abundant vacancies of carbon alleviate volume variation, boosting the cycling stability over 14 000 cycles (8400 h). Our work provides a new synthesis approach for durable carbon anodes of K‐ion full cells with high energy densities.

Activating Octahedral Center in Co-Doped NiFe2 O4 via Bridging Amorphous MoSx for Electrocatalytic Water Oxidation: A Case for eg Orbital Regulation in Spinel Oxide.

Moderate eg filling for octahedral metal cations (MOh ) is strongly correlated with the electrocatalytic water oxidation performance in the oxides system. Here, the eg fillings of NiOh and FeOh in NiFe2 O4 -based spinel are controllably regulated by introducing an external radical of catalytically inactive MoSx as an electron acceptor via a novel ultrasonic anchored pyrolysis strategy. The electron occupied in eg orbit of MOh emigrates with the amount of MoS hanging on the apical of octahedral sites, and results in a salutary transition from high to medium eg occupancy state, as confirmed by the X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. In addition, benefiting from the abundant unsaturated S atoms in amorphous MoSx , the MOh at the surface furthest activates and consequently shows a superior water oxidation performance. Density functional theory also reveals that the eg fillings of Ni and Fe decrease to 1.4 and 1.2 after MoSx modification, which can effectively reduce the free energy of the OOH* intermediates in the oxygen evolution reaction process. This work opens an avenue for further releasing the electrocatalytic activity of octahedral sites through bridging external phases with rational electron-capturing/donating capability.

A systematic study on Equisetum ramosissimum Desf. derived honeycomb porous carbon for supercapacitors: Insight into the preparation-structure-performance relationship

It is always of considerable significance for the development of high-performance carbon-based supercapacitors to regulate the porosity and understand the preparation-structure-performance relationship. In this work, a KOH-assisted hydrothermal pre-treatment combined with pyrolysis strategy is successfully employed to fabricate N-doped honeycomb-like three-dimensional porous carbon from Equisetum ramosissimum Desf. weeds. The pore structure is well-regulated by tuning the KOH concentration and carbonization temperature, thus optimizing the electrochemical properties. Three types of pore structures are proposed, and their differences and connections in terms of preparation and electrochemical performance are systematically explored. The carbon sample with the optimized pore structure delivers a high specific capacitance of 427 F g−1 at 0.2 A g−1 in the aqueous KOH electrolyte. When assembled into an aqueous symmetric supercapacitor device, the energy density reaches up to 30.9 Wh kg−1 at a power density of 500 W kg−1. The results indicate that a moderate Vmirco/Vmeso should play the key role in the excellent rate capability, while the co-contribution of double-layer capacitance and pseudo-capacitance as well as the rational pore size regulation should be responsible for the high specific capacitance.

Regulated adsorption sites using atomically single cluster over biochar for efficient elemental mercury uptake

Carbon-based materials have been widely used in gaseous pollutant removal because of their sufficient surface functional groups; however, its removal efficiency for elemental mercury (Hg0) is low. In this study, we fabricated biomass using a chelated coupled pyrolysis strategy and further constructed the regulated adsorption sites for gaseous Hg0 uptake. A series of Mnδ-N2O2/BC with different manganese cluster sizes demonstrated that manganese clusters anchored on biochar acted as highly active and durable adsorbents for Hg0 immobilization, which increased the adsorption efficiency of Hg0 by up to 50%. Shrimp- and crab-based biochar adsorbents exhibited excellent Hg0 removal because of their chitosan-like structure. In particular, small Mn clusters and oxygen species around the defect led to a boost in the Hg0 adsorption by carbon. The results of density functional theory calculation revealed that the presence of oxygen in the carbon skeleton can tune the electrons of small-sized Mn clusters, thereby promoting the affinity of mercury atoms. The newly developed Mnδ-N2O2/BCshrimp had an adsorption capacity of 7.98–11.52 mg g−1 over a broad temperature range (50–200 °C) and showed a high tolerance to different industrial flue gases (H2O, NO, HCl, and SO2). These results provide novel green and low-carbon disposal methods for biomass resource utilization and industrial Hg0 emission control.Graphical

Direct Observation of Transition Metal Ions Evolving into Single Atoms: Formation and Transformation of Nanoparticle Intermediates.

Understanding the dynamical evolution from metal ions to single atoms is of great importance to the rational development of synthesis strategies for single atom catalysts (SACs) against metal sintering during pyrolysis. Herein, an in situ observation is disclosed that the formation of SACs is ascertained as a two-step process. There is initially metal sintering into nanoparticles (NPs) (500-600°C), followed by the conversion of NPs into metal single atoms (Fe, Co, Ni, Cu SAs) at higher temperature (700-800°C). Theoretical calculations together with control experiments based on Cu unveil that the ion-to-NP conversion can arise from the carbon reduction, and NP-to-SA conversion being steered by generating more thermodynamically stable Cu-N4 configuration instead of Cu NPs. Based on the evidenced mechanism, a two-step pyrolysis strategy to access Cu SACs is developed, which exhibits excellent ORR performance.

Molten NaCl assisted pyrolysis of ZIF-8/PAN electrospun fibers to synthesis 1D cross-linked mesoporous N-rich carbon as oxygen reduction electrocatalysts

Controllable preparation of cross-linked mesoporous carbon materials on a large scale remains challenging. In the present work, a simple NaCl-assisted pyrolysis strategy is proposed here to efficiently convert closed pores into open pores in the pyrolysis products of 2-methylimidazole zinc salt nanoparticles/polyacrylonitrile (ZIF-8/PAN) electrospun fibers to prepare 1D cross-linked mesoporous N-rich carbon-based oxygen reduction reaction (ORR) electrocatalyst. The total specific surface area of micropores and mesopores was significantly increased from 162 m2/g to 1069 m2/g (mesopores: 704 m2/g). The ORR catalytic activity of open pore was superior to that of closed pore with more positive onset potential (0.96 vs. 0.85 V) and half-wave potential (0.79 vs. 0.72 V), larger limiting current density (5.23 vs. 3.62 mA cm−2) and electron transfer number (3.89 vs. 3.15). Such enhanced performance was attributed to the transformation from closed to open pores which greatly improved the mass transfer performance and the number of active sites.

Coupling Hydrazine Oxidation with Seawater Electrolysis for Energy-Saving Hydrogen Production over Bifunctional CoNC Nanoarray Electrocatalysts.

Seawater electrolysis is a promising method to produce H2 without relying on scarce freshwater resource, but its high energy consumption and inevitable accompany of competitive chlorine oxidation reaction (ClOR) are still great technological challenges. Herein, a metal-organic framework(MOF)-templated pyrolysis strategy to prepare uniform cobalt/nitrogen-codoped carbon nanosheet arrays on carbon cloth (CC@CoNC) as highly-efficient but low-cost bifunctional electrocatalysts for hydrazine-assisted seawater electrolysis is explored. The optimized CoNC nanosheet arrays can be used as an efficient bifunctional electrocatalyst to catalyze hydrazine oxidation reaction and hydrogen evolution reaction, remarkably reducing the energy consumption and nicely overcome the undesired anodic corrosion problems caused by ClOR. Impressively, a hydrazine-assisted water electrolysis system is successfully assembled by using the optimized CC@CoNC as both cathode and anode, which only needs an ultra-low cell voltage of 0.557V and an electricity consumption of 1.22kW h per cubic meter of H2 to achieve 200mA cm-2 . Furthermore, the optimized CC@CoNC can also show greatly improved stability in the hydrazine-assisted seawater electrolysis system for H2 production, which can work steadily for above 40h at ≈10mA cm-2 . This study may offer great opportunities for obtaining hydrogen energy from infinite ocean resource by an eco-friendly method.

Rapid laser synthesis of surfactantless tantalum‐based nanomaterials as bifunctional catalysts for direct peroxide–peroxide fuel cells

AbstractEfficient and durable electrocatalysts are instrumental in enabling next‐generation fuel cell technologies. At present, expensive precious metals are used as state‐of‐the‐art catalysts. In this report, cost‐effective nanosized tantalum‐based alternatives are synthesized for the first time via a green and scalable laser pyrolysis method as bifunctional catalysts for direct peroxide–peroxide fuel cells. This rapid laser pyrolysis strategy allows for the production of nanoparticles at a laboratory scale of grams per hour, compatible with a detailed exploration of the functional properties of as‐synthesized nanoparticles. By varying the precursor ratio between ammonia and tantalum ethanolate, five tantalum‐based nanomaterials (TaNOC) are prepared with crystalline phases of Ta2O5, Ta4N5, Ta3N5, and TaN in tunable ratios. Electrochemical studies in neutral and alkaline conditions demonstrate that Ta4N5 is the active component for both H2O2 oxidation and reduction. Kinetic isotope effect studies show that protons are involved at or before the rate‐determining step. Long‐term stability studies indicate that Ta3N5 grants surfactant‐free TaNOC‐enhanced longevity during electrocatalytic operations. Taken together, bifunctional TaNOC can act as active and robust electrocatalysts for H2O2 reduction and oxidation. Laser pyrolysis is envisioned to produce refractory metal nanomaterials with boosted corrosion resistance for energy catalysis.

Nitrogen-Doped Carbon Nanoflowers Decorated with PtNi Nanoparticles for Colorimetric Detection of Total Antioxidant Capacity

A total antioxidant capacity (TAC) assay plays an essential role in evaluating antioxidant behaviors. It mainly involves examining the dosages of antioxidants such as ascorbic acid (AA), while TAC can be measured by nanozyme with peroxidase-like activity. Herein, nitrogen-doped carbon nanoflowers decorated with uniform PtNi nanoparticles (PtNi/NCFs) were fabricated by a simple pyrolysis strategy. The obtained PtNi/NCFs showed high peroxidase-mimicking activity to catalyze 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation by generating rich hydroxyl radicals (·OH) in the H2O2 system. By virtue of the PtNi/NCFs nanozyme, a visual colorimetric sensor was developed for assays of H2O2 and AA. The limits of detection were as low as 45.3 and 0.94 μM for determination of H2O2 and AA with linear ranges of 100–1000 and 1.0–20.0 μM, respectively. Moreover, such sensor was also successfully employed to detect AA and H2O2 in real tablets and juices. The PtNi/NCFs nanozyme holds great potential for practical applications.