Jingjie Wu

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.

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.

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.

Nitrogen‐Doped Carbon Nanotube Arrays for High‐Efficiency Electrochemical Reduction of CO2: On the Understanding of Defects, Defect Density, and Selectivity

Nitrogen-doped carbon nanotubes (NCNTs) have been considered as a promising electrocatalyst for carbon-dioxide-reduction reactions, but two fundamental chemistry questions remain obscure: 1) What are the active centers with respect to various defect species and 2) what is the role of defect density on the selectivity of NCNTs? The aim of this work is to address these questions. The catalytic activity of NCNTs depends on the structural nature of nitrogen in CNTs and defect density. Comparing with pristine CNTs, the presence of graphitic and pyridinic nitrogen significantly decreases the overpotential (ca. -0.18 V) and increases the selectivity (ca. 80%) towards the formation of CO. The experimental results are in congruent with DFT calculations, which show that pyridinic defects retain a lone pair of electrons that are capable of binding CO2. However, for graphitic-like nitrogen, electrons are located in the π* antibonding orbital, making them less accessible for CO2 binding.

Incorporation of Nitrogen Defects for Efficient Reduction of CO2 via Two-Electron Pathway on Three-Dimensional Graphene Foam.

The practical recycling of carbon dioxide (CO2) by the electrochemical reduction route requires an active, stable, and affordable catalyst system. Although noble metals such as gold and silver have been demonstrated to reduce CO2 into carbon monoxide (CO) efficiently, they suffer from poor durability and scarcity. Here we report three-dimensional (3D) graphene foam incorporated with nitrogen defects as a metal-free catalyst for CO2 reduction. The nitrogen-doped 3D graphene foam requires negligible onset overpotential (-0.19 V) for CO formation, and it exhibits superior activity over Au and Ag, achieving similar maximum Faradaic efficiency for CO production (∼85%) at a lower overpotential (-0.47 V) and better stability for at least 5 h. The dependence of catalytic activity on N-defect structures is unraveled by systematic experimental investigations. Indeed, the density functional theory calculations confirm pyridinic N as the most active site for CO2 reduction, consistent with experimental results.

Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction

A facile chemical vapor deposition method to prepare single-crystalline VS2 nanosheets for the hydrogen evolution reaction is reported. The electrocatalytic hydrogen evolution reaction (HER) activities of VS2 show an extremely low overpotential of -68 mV at 10 mA cm(-2), small Tafel slopes of ≈34 mV decade(-1), as well as high stability, demonstrating its potential as a candidate non-noble-metal catalyst for the HER.

A metal-free electrocatalyst for carbon dioxide reduction to multi-carbon hydrocarbons and oxygenates

Electroreduction of carbon dioxide into higher-energy liquid fuels and chemicals is a promising but challenging renewable energy conversion technology. Among the electrocatalysts screened so far for carbon dioxide reduction, which includes metals, alloys, organometallics, layered materials and carbon nanostructures, only copper exhibits selectivity towards formation of hydrocarbons and multi-carbon oxygenates at fairly high efficiencies, whereas most others favour production of carbon monoxide or formate. Here we report that nanometre-size N-doped graphene quantum dots (NGQDs) catalyse the electrochemical reduction of carbon dioxide into multi-carbon hydrocarbons and oxygenates at high Faradaic efficiencies, high current densities and low overpotentials. The NGQDs show a high total Faradaic efficiency of carbon dioxide reduction of up to 90%, with selectivity for ethylene and ethanol conversions reaching 45%. The C2 and C3 product distribution and production rate for NGQD-catalysed carbon dioxide reduction is comparable to those obtained with copper nanoparticle-based electrocatalysts.

Electrochemical reduction of carbon dioxide III. The role of oxide layer thickness on the performance of Sn electrode in a full electrochemical cell

A full electrochemical cell was employed to investigate the role of the surface oxide thickness on the activity of Sn-based electrodes for the electrochemical conversion of CO2. The current density showed a negligible dependence on the thickness of the surface SnOx layer of Sn nanoparticles (100 nm), while the selectivity towards the formation of CO and formate exhibited a strong relationship with the initial SnOx thickness. Electrodes with a native SnOx layer of ∼3.5 nm exhibited the highest Faradaic efficiency (64%) towards formate formation at -1.2 V. The Faradaic efficiency towards CO production reached a maximum (35%) for the electrode with an oxide thickness of 7.0 nm, formed by annealing the Sn nanoparticles at 180 °C for 6 hours. The electrodes with a native SnOx layer displayed the highest overall selectivity towards CO2 reduction. The decrease of the selectivity towards CO2 reduction with increasing the thickness of the SnOx layer can be attributed to the enhancement of hydrogen evolution on the Sn clusters with a low-coordination number derived from the reduction of SnOx. The Faradaic efficiency towards hydrogen production was observed to increase with increasing the thickness of the SnOx layer. Our results suggest the importance of the underlying surface structure on the selectivity and activity of the Sn electrode for CO2 reduction and provide an insight into the development of efficient catalysts.

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.

Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution

Hydrogen is a promising energy carrier and key agent for many industrial chemical processes 1 . One method for generating hydrogen sustainably is via the hydrogen evolution reaction (HER), in which electrochemical reduction of protons is mediated by an appropriate catalyst—traditionally, an expensive platinum-group metal. Scalable production requires catalyst alternatives that can lower materials or processing costs while retaining the highest possible activity. Strategies have included dilute alloying of Pt 2 or employing less expensive transition metal alloys, compounds or heterostructures (e.g., NiMo, metal phosphides, pyrite sulfides, encapsulated metal nanoparticles) 3-5 . Recently, low-cost, layered transition-metal dichalcogenides (MX2) 6 based on molybdenum and tungsten have attracted substantial interest as alternative HER catalysts 7-11 . These materials have high intrinsic per-site HER activity; however, a significant challenge is the limited density of active sites, which are concentrated at the layer edges. 8,10,11 . Here we use theory to unravel electronic factors underlying catalytic activity on MX2 surfaces, and leverage the understanding to report group-5 MX2 (H-TaS2 and H-NbS2) electrocatalysts whose performance instead derives from highly active basal-plane sites. Beyond excellent catalytic activity, they are found to exhibit an unusual ability to optimize their morphology for enhanced charge transfer and accessibility of active sites as the HER proceeds. This leads to long cycle life and practical advantages for scalable processing. The resulting performance is comparable to Pt and exceeds all reported MX2 candidates.