Nitrogen-Doped Graphene with Pyridinic Dominance as a Highly Active and Stable Electrocatalyst for Oxygen Reduction

The nitrogen-doped graphene (NG) with dominance of the pyridinic-N configuration is synthesized via a straightforward process including chemical vapor deposition (CVD) growth of graphene and postdoping with a solid nitrogen precursor of graphitic C₃N₄ at elevated temperature. The NG fabricated from CVD-grown graphene contains a high N content up to 6.5 at. % when postdoped at 800 °C but maintains high crystalline quality of graphene. The obtained NG exhibits high activity, long-standing stability, and outstanding crossover resistance for electrocatalysis of oxygen reduction reaction (ORR) in alkaline medium. The NG treated at 800 °C shows the best ORR performance. Further study of the dependence of ORR activity on different N functional groups in these metal-free NG electrodes provides deeper insights into the origin of ORR activity. Our results reveal that the pyridinic-N tends to be the most active N functional group to facilitate ORR at low overpotential via a four-electron pathway

Carbon Nitrogen Nanotubes as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions

Oxygen reduction and evolution reactions are essential for broad range of renewable energy technologies such as fuel cells, metal-air batteries and hydrogen production through water splitting, therefore, tremendous effort has been taken to develop excellent catalysts for these reactions. However, the development of cost-effective and efficient bifunctional catalysts for both reactions still remained a grand challenge. Herein, we report the electrocatalytic investigations of bamboo-shaped carbon nitrogen nanotubes (CNNTs) having different diameter distribution synthesized by liquid chemical vapor deposition technique using different nitrogen containing precursors. These CNNTs are found to be efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions. The electrocatalytic activity strongly depends on the nanotube diameter as well as nitrogen functionality type. The higher diameter CNNTs are more favorable for these reactions. The increase in nanotube diameter itself enhances the catalytic activity by lowering the oxygen adsorption energy, better conductivity, and further facilitates the reaction by increasing the percentage of catalytically active nitrogen moieties in CNNTs

Liquid Phase Exfoliation of Two-Dimensional Materials by Directly Probing and Matching Surface Tension Components

Exfoliation of two-dimensional (2D) materials into mono- or few layers is of significance for both fundamental studies and potential applications. In this report, for the first time surface tension components were directly probed and matched to predict solvents with effective liquid phase exfoliation (LPE) capability for 2D materials such as graphene, h-BN, WS₂, MoS₂, MoSe₂, Bi₂Se₃, TaS₂, and SnS₂. Exfoliation efficiency is enhanced when the ratios of the surface tension components of the applied solvent is close to that of the 2D material in question. We enlarged the library of low-toxic and common solvents for LPE. Our study provides distinctive insight into LPE and has pioneered a rational strategy for LPE of 2D materials with high yield

Achieving Highly Efficient, Selective, and Stable CO₂ Reduction on Nitrogen-Doped Carbon Nanotubes

The challenge in the electrosynthesis of fuels from CO₂ is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO₂ to CO. The unprecedented overpotential (−0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO₂ activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed COOH and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode

Dynamic Hosts for High-Performance Li–S Batteries Studied by Cryogenic Transmission Electron Microscopy and in Situ X‑ray Diffraction

Developing a high-performance sulfur host is central to the commercialization and general development of lithium–sulfur batteries. Here, for the first time, we propose the concept of dynamic hosts for lithium–sulfur batteries and elucidate the mechanism through which TiS₂ acts in such a fashion, using in situ X-ray diffraction and cryogenic scanning transmission electron microscopy (cryo-STEM). A TiS₂–S composite electrode delivered a reversible capacity of 1120 mAh g^–1 at 0.3 C after 200 cycles with a capacity retention of 97.0% and capacities of 886 and 613 mAh g^–1 at 1.0 C up to 200 and 1000 cycles, respectively. Our results indicate that it is Li_<i>x</i>TiS₂ (0 < <i>x</i> ≤ 1), rather than TiS₂, that effectively traps polysulfides and catalytically decomposes Li₂S

Carbon Dioxide Hydrogenation over a Metal-Free Carbon-Based Catalyst

The hydrogenation of CO₂ into useful chemicals provides an industrial-scale pathway for CO₂ recycling. The lack of effective thermochemical catalysts currently precludes this process, since it is challenging to identify structures that can simultaneously exhibit high activity and selectivity for this reaction. Here, we report, for the first time, the use of nitrogen-doped graphene quantum dots (NGQDs) as metal-free catalysts for CO₂ hydrogenation. The nitrogen dopants, located at the edge sites, play a key role in inducing thermocatalytic activity in carbon nanostructures. Furthermore, the thermocatalytic activity and selectivity of NGQDs are governed by the doped N configurations and their corresponding defect density. The increase of pydinic N concentration at the edge site of NGQDs leads to lower initial reaction temperature for CO₂ reduction and also higher CO₂ conversion and selectivity toward CH₄ over CO