Carbon-based Metal-free Catalysts Research Articles

A metal-free cascaded process for efficient H2O2 photoproduction using conjugated carbonyl sites

Carbon-based metal-free catalysts are promising green catalysts for photocatalysis and electrocatalysis due to their low cost and environmental friendliness. A key challenge in utilizing these catalysts is identifying their active sites, given their poor crystallinity and complex structures. Here we demonstrate the key structure of the double-bonded conjugated carbon group as a metal-free active site, enabling efficient O2 photoreduction to H2O2 through a cascaded water oxidation − O2 reduction process. Using ethylenediaminetetraacetic acid as a precursor, we synthesized various carbon-based photocatalysts and analyzed their structural evolution. Under the polymerization conditions of 260 °C to 400 °C, an N-ethyl-2-piperazinone-like structure was formed on the surface of the catalyst, resulting in high photocatalytic H2O2 photoproduction (2884.7 μmol g−1h−1) under visible light. A series of control experiments and theoretical calculations further confirm that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts.

Nitrogen-doped vertical graphene for highly efficient hydrogen peroxide electrosynthesis in acidic environment

Electrochemical oxygen reduction (ORR) through a two-electron (2e−) pathway offers a green route for on-demand production of hydrogen peroxide (H2O2), which is an industrially valuable chemical as well as a renewable energy carrier. Metal-free carbon-based materials have been exploited as promising catalysts for H2O2 generation; however, they often exhibit poor performance with low productivity under acidic conditions. Herein, a nitrogen-doped vertical graphene catalyst is developed with a stable hydrophobic solid–liquid-gas three-phase interface (TPI) enabled by polydimethylsiloxane (PDMS) coating. The catalyst exhibits a high Faradaic efficiency (FE) of ∼ 80 % across a wide potential range from 0 to −0.6 V vs. reversible hydrogen electrode (RHE) at a maximum current density of 140 mA cm−2, resulting in a remarkably high H2O2 productivity of 21 mol gcatalyst−1h−1 in acid. Furthermore, a high concentration of H2O2 up to 10,500 ppm is obtained with the FE maintained at ∼ 75 %. Confocal laser scanning microscopy (CLSM) and fluorescence lifetime analysis reveal that the hydrophobic TPI increases gas diffusion and local pH value to enhance both activity and selectivity towards H2O2. This work provides valuable insights into the rational design of metal-free carbon-based catalysts for efficient H2O2 generation.

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.

Synthesis of nitrogen and sulfur bi-doped carbon obtained from spent sapelli wood sawdust and polyhydric alcohol with improved catalytic activity for hydrogen evolution reaction

Developing an efficient water electrolysis system is an important solution for on the development of highly active, cost-effective, and stable electrocatalysts. Carbon-based metal-free catalysts are one of the promising candidates for this purpose. Herein, we present a facile strategy to synthesize and characterize nitrogen and sulfur co-doped porous carbon (NS-PC) from spent sapelli wood sawdust and polyhydric alcohol (xylitol) for efficient hydrogen evolution reaction (HER) in alkaline electrolyte. The resultant NS-PC metal-free catalyst exhibits porous structure, high surface areas and enhanced HER performance with lowest Tafel slope of 68.1 mV dec−1. The enhanced performance is due to the high porosity of carbonaceous material and co-existence of active species (N in the forms of graphitic-N, pyridinic-N, pyrrolic-N, and S in the forms of C–S–C) induced by the maximum carbon-carbon bond polarization, charge re-distribution in the carbon matrices with a synergistically HER activity. In addition, this work provides a versatile and effective strategy for synthetizing excellent carbon-based metal free catalysts from the low cost biowaste for large-scale production of hydrogen through HER.

Boron functionalized carbon-based metal-free catalyst for efficient photoelectrocatalytic degradation of pollutants

Boron functionalized carbon-based metal-free catalyst for efficient photoelectrocatalytic degradation of pollutants

Robust and efficient hydrogen production performance using nitrogen- and phosphorus-doped microalgal carbon-based metal-free carbon composite

Carbon-based metal-free catalysts are attracting attention in different applications. However, these metal-free carbon materials do not have sufficient active sites, resulting in low catalytic activity. Therefore, modification of metal-free carbon materials with appropriate approaches is important. The simultaneous doping of different electronegative heteroatoms has a positive effect on catalytic performance due to the synergistic effect caused by the electronic interactions between different atoms compared to mono-doping. In this study, Spirulina platensis, a microalgae species that is easy to culture, was used as the carbon source. First, ammonia (NH3) and phosphoric acid (H3PO4) activation were used to modify the surface of activated carbon obtained from Spirulina platensis by KOH activation (SPAC). Thus, the SPAC surface is enriched with N(SPAC-N) and P atoms whose electronegativity is different from carbon. These modified carbon particles (SPAC-N-P) were used as a metal-free catalyst to increase the H2 production rate from NaBH4 methanolysis. XRD, TEM, TG/DTG, FTIR and XPS analyses were used for the characterization of the obtained catalysts. FTIR and XPS analyses showed successful incorporation of N, P and O atoms. HGR values of 5737, 6104 and 13,052 mL min−1gcat−1 by SPAC, SPAC-N and SPAC-N-P catalysts were obtained, respectively. There is a significant increase in the catalytic performance of the catalysts obtained with both NH3 and H3PO4 activations.

Combined Experimental and Density Functional Theory Study on the Mechanism of the Selective Catalytic Reduction of NO with NH3 over Metal-Free Carbon-Based Catalysts.

Metal-free carbon-based catalysts are attracting much attention in the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR). However, the mechanism of the NH3-SCR reaction on carbon-based catalysts is still controversial, which severely limits the development of carbon-based SCR catalysts. Herein, we successfully reconstructed carbon-based catalysts through oxidation treatment with nitric acid, thereby enhancing their low-temperature activity in NH3-SCR. Combining experimental results and density functional theory (DFT) calculations, we proposed a previously unreported NH3-SCR reaction mechanism over carbon-based catalysts. We demonstrated that C-OH and C-O-C groups not only effectively activate NH3 but also remarkedly promote the decomposition of intermediate NH2NO. This study enhances the understanding of the NH3-SCR mechanism on carbon-based catalysts and paves the way to develop low-temperature metal-free SCR catalysts.

Carbon-based metal-free nanomaterials for the electrosynthesis of small-molecule chemicals: A review

Electrocatalysis is a key component of many clean energy technologies that has the potential to store renewable electricity in chemical form. Currently, noble metal-based catalysts are most widely used for improving the conversion efficiency of reactants during the electrocatalytic process. However, drawbacks such as high cost and poor stability seriously hinder their large-scale use in this process and in sustainable energy devices. Carbon-based metal-free catalysts (CMFCs) have received growing attention due to their enormous potential for improving the catalytic performance. This review gives a concise comprehensive overview of recent developments in CMFCs for electrosynthesis. First, the fundamental catalytic mechanisms and design strategies of CMFCs are presented and discussed. Then, a brief overview of various electrosynthesis processes, including the synthesis of hydrogen peroxide, ammonia, chlorine, as well as various carbon- and nitrogen-based compounds is given. Finally, current challenges and prospects for CMFCs are highlighted.

Electrochemical Performance of Metal-Free Carbon-Based Catalysts from Different Hydrothermal Carbonization Treatments for Oxygen Reduction Reaction.

This research investigates the difference between products obtained through two hydrothermal carbonization treatments. Our aim is to synthesize metal-free, carbon-based catalysts for the oxygen reduction reaction (ORR) to serve as efficient and cost-effective alternatives to platinum-based catalysts. Catalysts synthesized using the traditional hydrothermal approach exhibit a higher electrocatalytic activity for ORR in alkaline media, despite their more energy-intensive production process. The superior performance is attributed to differences in the particle morphology and the chemical composition of the particle surfaces. The presence of functional groups on the surfaces of catalysts obtained via a traditional approach significantly enhances ORR activity by facilitating deprotonation reactions in an alkaline environment. Our research aims to provide a reference for future investigations, shifting the focus to the fine-tuning of surface chemical compositions and morphologies of metal-free catalysts to enhance ORR activity.

Defect-rich porous carbon as a metal-free catalyst for high-performance Li-CO2 batteries

Li-CO2 batteries have been considered as next-generation high-energy-density batteries due to their high theoretical energy density and efficient CO2 fixation. However, the slow reaction kinetics of CO2 reduction and evolution reactions during discharge and charge process severely limits its practical application. Hence, a MOF-5 derived defect-rich porous carbon (c-p-MOF-5) is developed as a durable metal-free catalyst to accelerate CO2 conversion for high-performance Li-CO2 batteries. Benefiting from plentiful pores with large surface area for facilitating CO2 and Li+ diffusion, and rich defect for improving CO2 conversion reactions kinetics, the resultant Li-CO2 battery with c-p-MOF-5 shows excellent electrochemical performance with a high discharge capacity of 22,000 mA h g−1, good rate capability with a low potential gap of 1.7 V at 1000 mA g−1, and high stability over 250 cycles at a current density of 500 mA g−1. This work not only develops an efficient cathode catalyst for Li-CO2 batteries, but also provides a facile method to prepare defect-rich metal-free carbon-based catalysts for Li-CO2 batteries and beyond.

Application and Mechanism Study of Carbon-Based Metal-Free Catalysts in Organic Synthesis

Application and Mechanism Study of Carbon-Based Metal-Free Catalysts in Organic Synthesis

Revision of the oxygen reduction reaction on N-doped graphenes by grand-canonical DFT.

Nitrogen-doped graphenes were among the first promising metal-free carbon-based catalysts for the oxygen reduction reaction (ORR). However, data on the most efficient catalytic centers and their catalytic mechanisms are still under debate. In this work, we study the associative mechanism of the ORR in an alkaline medium on graphene containing various types of nitrogen doping. The free energy profile of the reaction is constructed using grand-canonical DFT at a constant electrode potential in combination with an implicit electrolyte model. It is shown that the reaction mechanism differs from the generally accepted one and depends on the surface potential and doping type. In particular, as the potential decreases, coupled electron-proton transfer changes to sequential electron and proton transfer, and the potential at which this occurs depends on the doping type. It has been shown that oxygen chemisorption is the limiting step. The electrocatalytic mechanism of the nitrogen dopants involves reducing the oxygen chemisorption energy. Calculations predict that, at different potentials, different types of nitrogen impurities most effectively catalyze the ORR.

N-doped defect-rich porous carbon nanosheets framework from renewable biomass as efficient metal-free bifunctional electrocatalysts for HER and OER application

In response to the demand for clean, renewable energy sources and storage technologies, innovative materials and combinations have been developed at reasonable prices as effective electrode materials. Current research and development of carbon-based metal-free catalysts have opened up new research areas for bifunctional electrocatalysts for overall water splitting. Herein, we developed N-self-doped defect-rich porous carbon nanosheets derived from platycladus orientalis tree-cone biomass for overall water splitting. The developed N-self-doped defect-rich porous carbon nanosheets have large surface areas (3369 m2/g), high pore volumes (2.1 cm3 g−1), and high electrical conductivities (12.69 S/cm). N-self-doped defect-rich porous carbon nanosheet framework was employed in water splitting as a dual function in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Electrodes fabricated from the KOH-II ABC nanosheets show excellent OER and HER performances in the 0.5 M H2SO4 solution. The current density of 10 mA cm−2 is reached at a low overpotential of 90 mV for OER and 188 mV for HER. In addition, KOH-II ABC nanosheets also show a remarkable performance on overall water splitting, which only required a cell voltage of 1.49 V to reach current densities of 10 mA cm−2 in 0.5 M H2SO4 solution. The presence of defects in the carbon nanosheets, higher specific surface area, excellent electrochemical active surface area and N doping effectively enhance the reaction kinetics for attaining efficient OER and HER performances. This facile approach to preparing biomass-derived N-doped defect-rich porous carbon materials empowers next-generation green fuel conversion technologies for carbon neutrality.

Metal-Free Covalent Organic Frameworks for the Oxygen Reduction Reaction.

The oxygen reduction reaction (ORR) is the key reaction in metal air and fuel cells. Among the catalysts that promote ORR, carbon-based metal-free catalysts are getting more attention because of their maximum atom utilization, effective active sites and satisfactory catalytic activity and stability. However, the pyrolysis synthesis of these carbons resulted in disordered porosities and uncontrolled catalytic sites, which hindered us in realizing the catalysts' design, the optimization of catalyst performance and the elucidation of structure-property relationship at the molecular level. Covalent organic frameworks (COFs) constructed with designable building blocks have been employed as metal-free electrocatalysts for the ORR due to their controlled skeletons, tailored pores size and environments, as well as well-defined location and kinds of catalytic sites. In this Concept article, the development of metal-free COFs for the ORR is summarized, and different strategies including skeletons regulation, linkages engineering and edge-sites modulation to improve the catalytic selectivity and activity are discussed. Furthermore, this Concept provides prospectives for designing and constructing powerful electrocatalysts based on the catalytic COFs.

Regulating the reactive site density of two-dimensional N self-doped carbon for boosted Fenton-like catalytic ability

The mediocre catalytic ability of metal-free carbon-based catalysts limits its practical application in wastewater treatment. Herein, a facile “steam-etching” method is utilized to boost the catalytic ability of N self-doped carbon (NSDC) catalysts. Compared to the original NSDC catalyst (NSDC-N2), the NSDC-H2O-50 catalyst modified with the water vapor content of 50 % exhibits much better catalytic performance. The pseudo-first-order kinetic rate constant k of NSDC-H2O-50 is 4.88 times that of NSDC-N2. The presence of water vapor can cause a massive vacancy-like defects in the carbon structure, which is conducive to form more edge reactive groups. Pyrrolic N and CN groups are proved to be the reactive sites for the catalytic reaction. Reactive site density can be easily adjusted by changing the water vapor content. The prepared NSDC-H2O catalysts can effectively activate peroxydisulfate (PDS) to degrade various organic pollutants with high efficiency in the pH range of 2.02–10.28. In addition, the NSDC-H2O/PDS system exhibits strong anti-interference ability to common anions and humic acid. The catalytic ability has not been affected in actual water sources. By a simple calcination treatment, most of the catalytic ability can be restored. Therefore, the prepared NSDC-H2O catalysts show strong application potential in organic wastewater treatment. Furthermore, singlet oxygen is proved to be the main active species in the NSDC-H2O/PDS system, and sulfate radical and superoxide radical play an auxiliary role.

Hybrid scheme of DFT and machine learning to accelerate the design of graphyne nanoribbons as electrocatalysts for the ORR and HER

Recently, graphyne and its family members (GYFs) have emerged as promising carbon-based metal-free catalysts (CMFCs) for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Herein, we performed density functional theory simulations to explore the ORR and HER performances of β-graphyne/graphdiyne nanoribbons (βGyNRs/βGDyNRs), which have received scant attention. Our results reveal that βGyNRs/βGDyNRs are excellent bifunctional CMFCs after functionalization by manipulating the edge shape or N-doping. Moreover, we verified the role of the binding strength of the H atom (ΔEH*) as a universal descriptor for predicting the bifunctional activities for the ORR and HER on GYFs. We also proposed a machine learning model based on an extreme gradient boosting algorithm for predicting the bifunctional activities of GYFs. Feature importance analysis indicated that the atomic charge of the catalytic site (Q) played a determining role in the ORR activity of the GYFs. This study not only identifies promising GYFs, but also accelerates the search for highly active GYFs for the ORR and HER.

The mechanism of B, N co-doping for enhancing graphene catalytic performance: CO oxidation and O2 dissociation as model reactions

Carbon-based metal-free catalysts are becoming increasingly attractive for the low cost and environmental friendliness. With one more valence electron than C, N doping has been extensively studied for the preparation feasibility. While the main group element B is comparable to transition metals in many reactions for co-existence of lone pair electrons and vacant orbitals. The synergistic effect of co-doping B and N has been explored both experimentally and theoretically. However, the in-depth understanding of the structure–property relationships of B, N co-doping in carbon materials remains to be uncovered. Herein, through extensive density functional theory (DFT) calculations, the doping sites (meta vs ortho) and number of N atoms around B on the synergetic promotion for O2 activation was investigated. Our results reveal that ortho-N captured more electrons from B compared with meta-N. Moreover, O2 activation effect is directly related to the energy level of empty sp3 orbital of B. Subsequently, O2 dissociation and CO oxidation were used as probe reactions to evaluate the catalytic performance of all B, N co-doped graphene structures. It is found that the introduced N atoms, especially meta-N creates another electrophilic center to accommodate the dissociated O atom, thus O2 dissociation process was promoted kinetically. Besides, CO oxidation proceeds easily on meta- and meta, ortho-N co-doped structures, while the strong interaction between O2 and ortho-N doped substrates hampers the reaction, which perfectly obeys the Sabatier Principle. This work not only uncovers the synergetic effect of BN co-doping in carbon-based materials, but also shed new light on developing metal-free catalysts.

Lignin-derived sulfonate base metal-free N, S co-doped carbon microspheres doped with different nitrogen sources as catalysts for oxygen reduction reactions

The oxygen reduction reaction (ORR) is an important step in the widespread application of metal-air batteries, so it is necessary to study and develop low-cost and efficient metal-free carbon-based catalysts to catalyze the ORR reaction. Heteroatomic doping, especially N and S co-doped carbon materials, has received much focus as a promising ORR catalyst. Meanwhile, the lignin material has high carbon content, wide source, and low price, and has wide application prospects for the preparation of carbon material catalysts. Here we report a hydrothermal‑carbonation preparation method for the synthesis of carbon microspheres by utilizing lignin derivatives as carbon precursors. And a variety of N, S co-doped carbon microsphere materials were prepared by adding different nitrogen sources (urea, melamine, NH4Cl) to the microspheres. The N, S co-doped carbon microspheres (NSCMS-MLSN) catalysts achieved with NH4Cl as the nitrogen source displayed superior RR catalytic activity with high half-wave potential (E1/2 = 0.83 V vs. RHE) and current density (JL = 4.78 mA cm−2). This work provides some references on the method of preparing carbon materials co-doped with N and S and the choice of nitrogen sources.

Defect-Enriched Hollow Porous Carbon Nanocages Enable Highly Efficient Chlorine Evolution Reaction.

Metal-free carbon-based catalysts are crucial for the electrocatalytic chlorine evolution reaction (CER) to reduce the usage of noble metals and industrial cost. However, the corresponding catalytic activity of high overpotential and low durability hinders their wide application. Here, a hollow porous carbon (HPC) nanocage with a controlled oxygen electronic state around designed carbon defects for CER activity is reported. Alkali etching can bring defects in zeolite with a hollow structure. In a hard template strategy, the type of carbon defects is directly related to etching degree of the zeolite template. More importantly, the oxygen atoms can be "borrowed" from the zeolite framework by the defective carbon. The electron density around unsaturated O atoms can be decreased on the minor defects in carbon compared with that on large defects which is favorable for the adsorption of Cl- . Consequently, the as-synthesized HPC nanocages with minor defects show excellent electrocatalytic performance for CER with a low overpotential of 94mV at current density of 10mA cm-2 with good stability, which is superior to the commercial precious metal catalyst of dimensionally stable anode (DSA), and the best in the reported carbon materials. The designed carbon materials provide an option for metal-free industrial catalysts with significant CER activities.

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.