Carbon Active Sites Research Articles

Monomicelle-Directed Engineering of Strained Carbon Nanoribbons as Oxygen Reduction Catalyst.

To date, precisely tailoring local active sites of well-defined earth-abundant metal-free carbon-based electrocatalysts for attractive electrocatalytic oxygen reduction reaction (ORR), remains challenging. Herein, the authorssuccessfully introduce a strain effect on active C-C bonds adjacent to edged graphitic nitrogen (N), which raises appropriate spin-polarization and charge density of carbon active sites and kinetically favor the facilitation of O2 adsorption and the activation of O-containing intermediates. Thus, the constructed metal-free carbon nanoribbons (CNRs-C) with high-curved edges exhibit outstanding ORR activity with half-wave potentials of 0.78 and 0.9V in 0.5m H2 SO4 and 0.1m KOH, respectively, overwhelming the planar one (0.52 and 0.81V) and the N-doped carbon sheet (0.41 and 0.71V). Especially in acidic media, the kinetic current density (Jk ) is 18 times higher than that of the planar one and the N-doped carbon sheet. Notably, thesefindings show the spin polarization of the asymmetric structure by introducing a strain effect on the C-C bonds for boosting ORR.

Theoretical insights into the methane catalytic decomposition on graphene nanoribbons edges.

Catalytic methane decomposition (CMD) is receiving much attention as a promising application for hydrogen production. Due to the high energy required for breaking the C-H bonds of methane, the choice of catalyst is crucial to the viability of this process. However, atomistic insights for the CMD mechanism on carbon-based materials are still limited. Here, we investigate the viability of CMD under reaction conditions on the zigzag (12-ZGNR) and armchair (AGRN) edges of graphene nanoribbons employing dispersion-corrected density functional theory (DFT). First, we investigated the desorption of H and H2 at 1200K on the passivated 12-ZGNR and 12-AGNR edges. The diffusion of hydrogen atom on the passivated edges is the rate determinant step for the most favourable H2 desorption pathway, with a activation free energy of 4.17eV and 3.45eV on 12-ZGNR and 12-AGNR, respectively. The most favourable H2 desorption occurs on the 12-AGNR edges with a free energy barrier of 1.56eV, reflecting the availability of bare carbon active sites on the catalytic application. The direct dissociative chemisorption of CH4 is the preferred pathway on the non-passivated 12-ZGNR edges, with an activation free energy of 0.56eV. We also present the reaction steps for the complete catalytic dehydrogenation of methane on 12-ZGNR and 12-AGNR edges, proposing a mechanism in which the solid carbon formed on the edges act as new active sites. The active sites on the 12-AGNR edges show more propensity to be regenerated due lower free energy barrier of 2.71eV for the H2 desorption from the newly grown active site. Comparison is made between the results obtained here and experimental and computational data available in the literature. We provide fundamental insights for the engineering of carbon-based catalysts for the CMD, showing that the bare carbon edges of graphene nanoribbons have performance comparable to commonly used metallic and bi-metallic catalysts for methane decomposition.

Removal of NO at low concentrations from polluted air in semi-closed environments by activated biochars from renewables feedstocks

Efficient measures are urgently required in large cities for nitric oxide (NO) elimination from air in urban semi-closed environments (parking lots and tunnels), characterized by low NO concentrations (<10 ppmv) and temperatures. One of the most promising abatement alternatives is the NO oxidation to NO2, which can be further easily captured in an alkali solution or over a porous solid. However, most of the research devoted to this topic is focused on the elimination of NO from fuel exhaust gases, with high NO concentrations (400–2000 ppmv). In this work, sustainable and low-cost activated biochars of different origin and having very different ash contents were employed in NO removal at very low concentrations. Thus, low ash content forestry (oak woodchips, OAK) and high ash content from agriculture (oilseed rape straw, OSR) biochars were subjected to physical activation with CO2 at 900 °C (OAK550-A900CO2 and OSR700-A900CO2, respectively). The NO removal performance tests of such activated carbons were carried out at different experimental conditions: i.e., temperature, relative humidity (0–50 vol% RH), NO-containing gas (N2 or air), amount of activated carbon, and NO concentration, to assess how the activated biochar properties influence their NO removal capacity. The sample OSR700-A900CO2 contained a higher population of oxygen surface functionalities, which might play an important role in the NO removal efficiency in dry conditions since they could assist NO oxidation on carbon active sites when used above room temperature (50–75 °C). However, at room temperature (25 °C), the presence of narrow micropore size distribution at 6 Å became a more relevant property, since it facilitates an intimate contact between NO and O2. Accordingly, the activated biochar from OAK was much more efficient, achieving complete removal of NO from air flow (dry or with 50 vol% RH) at 25 °C during 400 min of testing, making it an ideal candidate as biofilter for purifying air streams of semi-closed spaces contaminated with low concentrations of NO.

Spectroscopic Identification of Active Sites of Oxygen-Doped Carbon for Selective Oxygen Reduction to Hydrogen Peroxide.

The electrochemical synthesis of hydrogen peroxide (H2 O2 ) via a two-electron (2 e- ) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst for H2 O2 electrochemical production. The optimized PCC900 material exhibits remarkable activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H2 O2 selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2 e- ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.

Paradox of thiourea: A false-positive and promoter for electrochemical nitrogen reduction on nickel sulfide catalysts

Electrochemical nitrogen reduction reaction (NRR) is an environmentally friendly process for ammonia synthesis. An unusually high NH3 production rate (86.91 ± 9.29 μg h−1 cm−2) and Faradaic efficiency (F.E.) (57.17 ± 4.14 %) was observed in electrochemical tests on NiS catalysts synthesized using thiourea (TU). Through joint experiment-theory work, we demonstrate the paradoxical catalytic effect of TU as a false positive from TU-derived thiocyanate reduction and a promoter via the generation of catalytically active carbon sites after TU reduction on the NiS surface (with an effective NH3 rate of 52 ± 5.57 μg h−1 cm−2). TU reduction leads to the substitution of an S atom to a reactive C atom with fewer valence electrons, facilitating a more facile NRR pathway. This study presents a new possibility for NRR catalysts based on metal-nonmetal compounds (e.g., sulfides, chalcogenides, or nitrides) by nonmetal dopants with fewer valence electrons to create new catalytically active sites.

Recent advancements in hydrogen storage - Comparative review on methods, operating conditions and challenges

Green hydrogen, proposed as a sustainable alternative for conventional fuels, has gained utmost importance due to its reduced carbon footprint and potential application in maintaining balance between energy generation and demand. Utilization of hydrogen-based systems are challenged by cost effective storage methods. Bio-hydrogen storage technologies using cryogenic and adsorptive methods are discussed in this review paper, along with their operating conditions and storage capacities. An analysis of operational challenges and recent advancements in hydrogen storage techniques is presented. With a storage pressure of 70 MP, cryogenic hydrogen is almost twice as dense as compressed hydrogen. Technical challenges such as material cost and explosion risk can be addressed by hydrogen adsorption on activated carbon active sites. Maximum hydrogen storage capacity are reported by activated carbon obtained from Chitosan (6.77 wt%), Bamboo (6.6 wt%) and African palm shell (6.5 wt%) at 77 K and 4–20 Mpa. Further, nanotechnology approaches are presented in detail for increasing hydrogen storage capacity in pristine Si and Mg. The future perspectives in terms of technological, and economic aspects are discussed.

Electrocatalytic performance of cobalt/nickel nanoparticles encapsulated by N-doped carbon nanotubes toward the oxygen reduction reaction

Electrocatalysts based on nanosized cobalt or nickel particles encapsulated by nitrogen-doped carbon nanotubes (M@NCNTs (M = Co or Ni)) were synthesized using the calcination method. They were served as the catalyst toward an oxygen reduction reaction (ORR). The results implied that the M@NCNT electrocatalysts had ORR catalytic activity in an alkaline electrolyte. The half-wave potential (E1/2) on the Co@NCNT catalyst reached 0.881 V, which was higher than for commercial Pt/C (0.856 V). The M@NCNT electrocatalysts exhibited good stability in the ORR. After 3000 cycles, the E1/2 for the Co@NCNTs was almost completely unchanged, whereas for the Ni@NCNTs, shifted negatively to 19 mV, was less than for commercial Pt/C (34 mV). These promising electrochemical properties were due to abundant active carbon sites and the synergistic reaction between Co/Ni and NCNTs. Density functional theory calculations indicated that the OOH had strong absorption on the C atoms (adjacent to pyridinic N) in the Co@NCNT and Ni@NCNT catalysts.

Ultrastable Fe-N-C Fuel Cell Electrocatalysts by Eliminating Non-Coordinating Nitrogen and Regulating Coordination Structures at High Temperatures.

Pyrolyzed Fe-N-C materials have attracted considerable interest as one of the most active noble-metal-free electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Despite significant progress is made in improving their catalytic activity during past decades, the Fe-N-C catalysts still suffer from fairly poor electrochemical and storage stability, which greatly hurdles their practical application. Here, an effective strategy is developed to greatly improve their catalytic stability in PEMFCs and storage stability by virtue of previously unexplored high-temperature synthetic chemistry between 1100 and 1200°C. Pyrolysis at this rarely adopted temperature range not only enables the elimination of less active nitrogen-doped carbon sites that generate detrimental peroxide byproduct but also regulates the coordination structure of Fe-N-C from less stable D1 (O-FeN4 C12 ) to a more stable D2 structure (FeN4 C10 ). The optimized Fe-N-C catalyst exhibits excellent stability in PEMFCs (>80% performance retention after 30h under H2 /O2 condition) and no activity loss after 35 day storage while maintaining a competitive ORR activity and PEMFC performance.

Investigation on the influence of inherent AAEMs on gasification reactivity of solid digestate char

Char reactivity usually determines the overall efficiency of the entire gasification process, while the presence of the alkali and alkaline earth metals (AAEMs) has a catalytic effect on char gasification. In this study, the influence of inherent AAEMs in the gasification behaviour of char was investigated. The char sample was prepared through the pyrolysis of solid digestate derived from anaerobic co-digestion of silage and dairy cattle slurry. The raw char and HCl-washed char were characterized by elemental analyzer, ICP-OES, SEM and XRD to explore their structural changes. HCl-char loses weight in the temperature range of 440–620 °C, and the weight loss percentage is significantly higher than that of char. Ash content of char is reduced by half after a pickling process. AAEMs are largely removed after char acid pickling, resulting in an increase in activation energy of the gasification reaction and a decrease in gasification reactivity, resulting in the gasification reaction time of HCl-char longer than char. Water-soluble AAEM and ion-exchange AAEM affect the evolution of carbon structure, which directly lead to char reactivity during the gasification reaction. As the carbon is consumed, the carbon microcrystalline structure of the residual carbon tends to be ordered, resulting in fewer active free carbon sites for the gasification reaction. Kinetic analysis showed that the loss and deactivation of AAEMs after char acid washing increased the average activation energy Ea by 115.76 KJ/mol compared with the original char.

Using molecular dynamics simulations to study the non-thermal effects of microwave radiation on the mechanism of char gasification

Studying the chemical effects of alternating electric fields on char gasification is key to exploring the non-thermal effects of microwaves on the reaction mechanism. The gasification mechanism of char in the presence of an electric field was explored through molecular dynamics (MD) simulations and quantum chemical calculations. The results show that the promotion effect of 0.02 V/Å alternating electric field on the gasification reaction rate of char is about 8 times that of non-electric field, and the square wave electric field and static electric field are 6.79 times and 5.39 times respectively, this effect increases exponentially as the field intensity increases. Interestingly, the use of alternating electric fields showed, for the first time, that the changes in the electric field intensity and direction also have a significant effect. In terms of the adsorption process during gasification, the electric field promotes the formation of O and OH free radicals (about 5 times of no electric field) and increases the polarity of the char edge, both of which are conducive to adsorption. The electric field can weaken the curling of char macromolecules and expose more active carbon sites, thus increasing the overall reaction rate. For the desorption process during gasification, the presence of the electric field promotes the deformation and tear of char, which destroys its aromaticity, loosening the carbon-oxygen compounds at the carbon edge, which facilitates the desorption of CO, the desorption rate is about 10 times that of non-electric field system. Further, the change in direction of the electric field also promotes the desorption process. The results from this study suggest that the remarkable effects of alternating electric fields on char molecules can have an impact on the field of solid fuel gasification.

Dispersed Mn2Co2C nanoparticles in interconnected nitrogen-doped carbon framework as cathode catalysts for efficient and long-life Li-CO2 batteries

The Li-CO2 batteries are concerned as a potential energy conversion and storage system which can not only effectively capture CO2 gas but also offer a high energy density. Therefore, they can be widely used in CO2-enriched environments such as submarine operations. However, the development of Li-CO2 batteries is still restricted by the high polarization and poor cycle performance mainly due to the insulating discharge product, e.g., Li2CO3. Herein, the Mn2Co2C nanoparticles dispersed in interconnected nitrogen-doped carbon frameworks (Mn2Co2C@NC), obtained by pyrolyzing MnCo Prussian blue analogous (MnCo-PBA), are first utilized as the cathode materials in Li-CO2 batteries. The batteries deliver a high discharge capacity (18962 mAh g−1 at 100 mA g−1), the lowest overpotential (1.47 V), and the best cycle performance (330 cycles, close to 1800 h, with a limited capacity of 500 mAh g−1 at 200 mA g−1). The good cycling stability under 200 mA g−1 is more prominent for current Li-CO2 batteries. The superior performance of Mn2Co2C@NC is ascribed to abundant catalytic sites of Mn/Co-nitrogen-carbon (Mn/Co-NC) and nitrogen-doped carbon, which have outstanding adsorption and activation effects on CO2. Meantime, the robust interconnected carbon framework provides fast electron/electrolyte transport paths and a large deposition space of Li2CO3. The density functional theory calculations reveal the strong adsorption of Li+ and CO2, strong catalytic performance for CO2 reduction, and excellent electrical conductivity of the Mn/Co-NC and nitrogen-doped carbon active sites. Finally, when combining a gel electrolyte, to the best of our knowledge, for the first time the fabricated quasi-solid-state Li-CO2 batteries demonstrate good cycling stability with high deliverable specific discharge capacity, highlighting its great potential for the applications of flexible and wearable batteries.

Activity Quantification of Fuel Cell Catalysts via Sequential Poisoning by Multiple Reaction Inhibitors.

The development of non-Pt or carbon-based catalysts for anion exchange membrane fuel cells (AEMFCs) requires identification of the active sites of the catalyst. Since not only metals but also carbon materials exhibit oxygen reduction reaction (ORR) activity in alkaline conditions, the contribution of carbon-based materials to ORR performance should also be thoroughly analyzed. However, the conventional CN- poisoning experiments, which are mainly used to explain the main active site of M-N-C catalysts, are limited to only qualitative discussions, having the potential to make fundamental errors. Here, we report a modified electrochemical analysis to quantitatively investigate the contribution of the metal and carbon active sites to ORR currents at a fixed potential by sequentially performing chronoamperometry with two reaction inhibitors, CN- and benzyl trimethylammonium (BTMA+). As a result, we discover how to quantify the individual contributions of two active sites (Pt nanoparticles and carbon support) of carbon-supported Pt (Pt/C) nanoparticles as a model catalyst. This study is expected to provide important clues for the active site analysis of carbon-supported non-Pt catalysts, such as M-N-C catalysts composed of heterogeneous elements.

Structural characteristics and thermal conversion performance of ash and slag from circulating fluidized bed gasifier

Based on the circulating fluidized bed (CFB), Xinjiang Zhundong coal (ZDC) gasification ash (FA: fly ash; BA: bottom slag) was analyzed by industrial analysis, ultimate analysis and Fourier infrared spectroscopy to determine the basic properties and functional group species. The results show that BA contains up to 99.30% in ash, while FA shows high fixed carbon and C content of 69.30% and 73.78% respectively. Furthermore, the carbonaceous forms and surface morphology of ZDC and FA were characterized by Raman, XRPES and SEM, and the pyrolysis, combustion and gasification characteristics of ZDC and FA were studied with TG-DTG methods. XRPES show that C content of the FA surface is 89.42%, and exists primarily as >C–C< and >C–H, while O was in the form of >C=O. Alkaline earth metals Ca bound to the above-mentioned carbon-involved functional groups cause high disorder in FA. SEM observed that the rough and porous FA surface occurs due to spherical particles of molten mineral attached and embedded surface and pore channels. The thermal conversion characteristics show that the maximum weight loss rate peak temperature of pyrolysis and combustion of FA is significantly higher than that of ZDC, indicating that the pyrolysis and combustion performance of FA is reduced. However, 100% carbon conversion of FA uses about half the time compared with ZDC and the gasification performance has improved significantly since it has well-developed pore structures, more amorphous carbon and abundant active sites, enhancing diffusion of CO2 from gasifiers. Briefly, FA has the potential and ability to be recycled for direct utilization in CFB as gasification feed.

Investigation into kinetic compensation effects for the production of hydrogen-rich gas during gasification of sub-bituminous coal char in varying gas environments

This study has focused on the evolution of kinetic parameters, i.e., Eapp and lnAapp and kinetic compensation effects (KCE) of char consumption and CO, CO₂, and H₂ formation during gasification of Collie coal char in varying gas environments (15%H₂O–Ar, 8%H₂O–Ar, and 0.4%O₂+8%H₂O–Ar). The instantaneous rate was obtained continuously during experiments by measuring the product gas composition using a quadrupole mass spectrometer. Observed variations of Eapp, lnAapp, and the extent of KCE suggest that the decrease in steam partial pressure from 15%H₂O to 8%H₂O surrounding the coal char particles appears to affect the path of CO₂ and H₂ formation from surface water-gas shift (WGS) reaction in 15%H₂O–Ar to char-H₂O heterogeneous reaction in 8%H₂O–Ar, at higher conversion (conversion >0.5). However, H₂ formation displays strong KCE and increased H₂ molar flux in 0.4%O₂+8%H₂O–Ar when compared to gasification of char with steam only (8%H₂O–Ar and 15%H₂O–Ar). In contrast, the introduction of 0.4%O₂ to 8%H₂O–Ar dominates the char gasification with only consumption of carbon active sites present on the char surface. That resulted in weak KCE for char consumption and decreased CO/CO₂ (molar flux ratio) compared to the gasification of char in 15%H₂O–Ar and 8%H₂O–Ar. Furthermore, the results indicate that the residual char obtained from the faster diffusion-controlled char-O₂ heterogeneous reaction in the 0.4%O₂+8%H₂O–Ar acts as an activated char precursor for subsequent kinetics-controlled char-H₂O heterogeneous reactions. This enhances the migration of H radicals generated from the H₂O molecule to the inner char matrix in 0.4%O₂+8%H₂O–Ar with an increased H₂ formation rate than gasification of char with steam only.

Identification of ORR activity of random graphene-based systems using the general descriptor and predictive model equation

Carbon based electrocatalysts are well known promising candidates for the oxygen reduction reaction (ORR), but the random approach to find the best catalyst using experimental method delayed the screening process and it leads to a huge cost with less proficiency. Using Quantum Mechanics followed by Machine Learning (QM/ML) approach, we can predict the best catalyst faster way and the origin of the cause can be identified for further development of carbon-based catalyst. Using the π electronic descriptor unveiled using density functional theory, we applied the analytical simple fit method and six different machine learning algorithms to develop a highly effective predictive model to estimate ΔGOH. Furthermore, structural relations of ZGNR and AGNR are demonstrated to estimate the Dπ(EF), R-Oπ, and ΔGOH of different widths of ribbon that reduces additional DFT calculations. By applying both SVR predictive model and structural relations, we predicted the ORR performance of 2500 sites of GNRs and listed a few most ideal active carbon sites with lower overpotential (η < 0.5V). To validate our study, we predicted the ORR performance of different sites in 0D, 1D, 2D doped graphene systems using SVR model and confirmed the values with the DFT computed results.

Density functional theory study on catalytic reduction of NO on semi-coke containing N and Fe: The effect of CO

NOx is a common air pollutant, and its removal research is imminent. In this paper, the semi-coke segment was used as the catalyst carrier to study the three important factors of nitrogen doping, iron doping and CO introduction, and the NO heterogeneous reduction was analyzed by the DFT calculation method. And at the range of 300 to 1000 K, the reaction kinetics was quantitatively analyzed by the conventional transition state theory (TST). Only one NO molecule was needed to finish the whole reaction path by doping the nitrogen, as suggested the nitrogen doping improved the NO removal rate at low NO concentrations. Through the analysis of the adsorption energy, it was found that the adsorption capacity of carbon active sites for NO and CO was significantly enhanced with iron doping. Among them, the appearance of FeO groups also reduced the activation energy of the stage reaction. In addition, the CO introduction significantly promoted the production of N2, CO2 and N2O, which was manifested in the reduction of activation energy of NO catalytic reaction and the increase of the N2 and N2O reaction rate constant. This theoretical research provided a theoretical foundation for the subsequent experimental research on NO removal.

A bifunctional nitrogen doped carbon network as the interlayer for dendrite-free Zn anode

Aqueous rechargeable zinc-ion batteries promise attractive advantages including safety, high energy density, and low cost. However, the uncontrollable dendrite growth and intricate side-reactions impede the practical application. Herein, we report a bifunctional nitrogen doped carbon network as interlayer (NCL) in cells to suppress the dendrite growth. In physical effect, NCL can balance the interfacial electrostatic field of Zn surface and guide Zn ions to deposit uniformly. In chemical effect, the nitrogen doped carbon active sites can reconstruct the typical solvation structure of [Zn(H2O)6]2+, and further promote the de-solvation process. Benefiting from the synergetic bifunctional interlayer both physically and chemically, dendrite and side-reactions are obviously restrained during repeated Zn plating/stripping procedures. Finally, the NCL-Zn/NCL-Zn symmetric cell acquires a cycle life of over 4000 h and persists for over 125 h even at a depth of discharge (DOD) of 38%.

Synthesis of nitrogen-rich porous carbon nanotubes coated Co nanomaterials as efficient ORR electrocatalysts via MOFs as precursor

Transition metal-based nitrogen-doped carbon materials have been proved to be a promising low-cost electrocatalyst for oxygen reduction reaction (ORR), which is a critical step in systems such as fuel batteries and zinc-air batteries. In this paper, transition metal (cobalt) N-doped carbon nanomaterials derived from metal-organic framework (MOFs) were synthesized by calcination of ZIF-67 precursors at high temperature (Co@NCNTs). Nitrogen atoms were introduced into the carbon matrix by capturing volatile CNx from dicyandiamide, and porous carbon nanotube structures with N-doped and uniformly dispersed active sites were obtained. The effects of different carbonization temperatures and molar ratio of Co/2-MI on the catalytic activity of materials for ORR reaction were studied. It was found that the Co@NCNTs prepared at 700 ℃ with the ratio of cobalt metal to dimethylimidazole of 1: 8 shows excellent ORR activity in both acidic and alkaline electrolytes. In alkaline conditions, the half-wave potential E 1/2 = 0.81 V, initial potential E onset = 0.89 V, and limiting current density even reaching 7.4 mA cm −2 . And in acidic electrolytes, E 1/2 = 0.66 V, while the limiting current density is 6.4 mA cm −2 , which is still better than that of Pt/C. Its excellent ORR performance is due to the synergistic effect of the porous carbon nanotube structure, N-doped carbon and Co-Nx active sites. Meanwhile, the optimum catalyst also shows better stability and methanol resistance than Pt/C in both acidic and alkaline electrolytes. • N-doped porous carbon nanotubes coated Co material was used to enhance ORR activity in alkaline and acidic conditions. • Nanotube structure provides sufficient effective contact area and active sites. • Porous structure is conducive to mass diffusion and reduces electron transmission resistance. • N-doped carbon tube, Co-Nx and pyridine nitrogen jointly promote ORR activity.

The nitrogen-doped carbon coated Na4MnV(PO4)3 as a high electrochemical performance cathode material for sodium-ion batteries

As a promising cathode material for sodium-ion batteries, the NASICON-structured Na4MnV(PO4)3 (NMVP) possesses a high working voltage with two discharge potential plateaus located at ∼3.4 V and 3.6 V from the V4+/V3+ and Mn3+/Mn2+ redox couples, respectively. However, the rate capability and cycling stability of NMVP is unsatisfactory because of the low electronic conductivity and dissolution of manganese. In this work, a series of NMVP@C-xN (x = 10, 20 and 30) was synthesized using a sol-gel method with the generation of a nitrogen-doped carbon on the surface of NMVP particles to reveal the effect of heteroatom-doped carbon coating on improving the electrochemical performance of the cathode material. Among the prepared samples, NMVP@C–20N exhibited a high initial reversible capacity of 103.8 mAh g−1 at 0.1 C and an excellent cycling stability with a capacity retention of 86.1 % after 2000 cycles at 5 C. The excellent performance of the cathode material should be ascribed to the generation of defects and active sites of the nitrogen-doped carbon coating, leading to increased electronic conductivity and diffusion rate of sodium ions.

A DFT Study of the VRFB Positive Electrode: Carbon Active Sites for the VO2 +/VO2+ Reaction

The vanadium redox flow battery (VRFB) is a promising candidate for use in grid-scale battery applications to support a deeper reliance on intermittent renewable energy. However, it suffers from a sluggish redox reaction at the positive electrode. Previous studies indicate that its performance can be improved by oxidizing a graphene-based carbon electrode and thus increasing the concentration of its oxygen-containing surface sites. While such performance enhancement does not appear to be controversial, the underlying mechanisms is far from being clear. Some authors suggest that this procedure increases the number of electrocatalytically active sites; others simply attribute it to improved wettability. In this ongoing study, we use density functional theory (DFT) to examine the role of hydroxyl and carboxyl groups, as well as free edge carbon atoms, in the oxygen transfer process at the positive electrode. Preliminary results using carefully selected model compounds (with appropriate charge C and spin multiplicity M) indicate that oxygen functional groups are unlikely to be active sites in the electrochemical redox process, which in many respects is analogous to the oxidation reduction reaction (ORR). Both the thermodynamics (less favorable adsorption energy of vanadium oxide) and the kinetics (problematic transition states) are less plausible than in analogous mechanistic steps involving carbene-type active sites. The carbene site was probed through two possible reaction mechanisms: V-down and O-down (see graph below). The former adsorption step is essentially a ‘dead-end’ process. The latter is not only thermodynamically and kinetically more feasible; it is consistent with other oxygen-mediated electron transfer processes occurring on the surface of graphene-based materials, be they catalysts or reactants. Furthermore, it has been shown in such processes that the presence of oxygen functional groups can stabilize and enhance the concentration of free carbene-type sites at graphene edges. Figure 1