Xiao-Dong Zhou

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.

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.

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.

Three-dimensional nanoarchitecture of Sn-Sb-Co alloy as an anode of lithium-ion batteries with excellent lithium storage performance

A novel nanoarchitectured Sn–Sb–Co alloy electrode is reported, which was prepared by direct electrodeposition on a Cu nanoribbon array in order to target the rapidly fading capacity and the poor rate-capability issues of Sn based materials for Li-ion batteries. The SEM images indicate a three-dimensional (3D) nanoarchitectured Sn–Sb–Co alloy with an array structure. Electrochemical measurements show that the 3D nanoarchitectured Sn54Sb41Co5 alloy electrode exhibits a reversible capacity as high as 512.8 mA h g−1 at 0.2 C (1 C = 650 mA g−1) after 150 cycles. Furthermore, the 3D nanoarchitectured Sn54Sb41Co5 anode can deliver a high reversible capacity (275 mA h g−1) up to the 80th cycle at a high discharge–charge rate of 23 C (∼15 A g−1). These outstanding electrochemical properties are attributed to the unique nanoarchitectures of the Sn54Sb41Co5 electrodes, making them an excellent anode material.

Electrochemical CO2 reduction to formic acid on crystalline SnO2 nanosphere catalyst with high selectivity and stability

A novel catalyst for CO 2 electroreduction based on nanostructured SnO 2 was synthesized using a facile hydrothermal self-assembly method. The electrochemical activity showed that the catalyst gave outstanding catalytic activity and selectivity in CO 2 electroreduction. The catalytic activity and formate selectivity depended strongly on the electrolyte conditions. A high faradaic efficiency, i.e., 56%, was achieved for formate formation in KHCO 3 (0.5 mol/L). This is attributed to control of formate production by mass and charge transfer processes. Electrolysis experiments using SnO 2 -50/GDE (an SnO 2 -based gas-diffusion electrode, where 50 indicates the 50% ethanol content of the electrolyte) as the catalyst, showed that the electrolyte pH also affected CO 2 reduction. The optimum electrolyte pH for obtaining a high faradaic efficiency for formate production was 8.3. This is mainly because a neutral or mildly alkaline environment maintains the oxide stability. The faradaic efficiency for formate production declined with time. X-ray photoelectron spectroscopy showed that this is the result of deposition of trace amounts of fluoride ions on the SnO 2 -50/GDE surface, which hinders reduction of CO 2 to formate.

Cationic Metallo‐Polyelectrolytes for Robust Alkaline Anion‐Exchange Membranes

Chemically inert, mechanically tough, cationic metallo-polyelectrolytes were conceptualized and designed as durable anion-exchange membranes (AEMs). Ring-opening metathesis polymerization (ROMP) of cobaltocenium-containing cyclooctene with triazole as the only linker group, followed by backbone hydrogenation, led to a new class of AEMs with a polyethylene-like framework and alkaline-stable cobaltocenium cation for ion transport. These AEMs exhibited excellent thermal, chemical and mechanical stability, as well as high ion conductivity.

A Natural Biopolymer Film as a Robust Protective Layer to Effectively Stabilize Lithium-Metal Anodes

Li metal is considered as an ideal anode for Li-based batteries. Unfortunately, the growth of Li dendrites during cycling leads to an unstable interface, a low coulombic efficiency, and a limited cycling life. Here, a novel approach is proposed to protect the Li-metal anode by using a uniform agarose film. This natural biopolymer film exhibits a high ionic conductivity, high elasticity, and chemical stability. These properties enable a fast Li-ion transfer and feasiblity to accomodate the volume change of Li metal, resulting in a dendrite-free anode and a stable interface. Morphology characterization shows that Li ions migrate through the agarose film and then deposit underneath it. A full cell with the cathode of LiFPO4 and an anode contaning the agarose film exhibits a capacity retention of 87.1% after 500 cycles, much better than that with Li foil anode (70.9%) and Li-deposited Cu anode (5%). This study provides a promising strategy to eliminate dendrites and enhance the cycling ability of lithium-metal batteries through coating a robust artificial film of natural biopolymer on lithium-metal anode.

Enabling Lithium Metal Anode Encapsulated in a 3D Carbon Skeleton with a Superior Rate Performance and Capacity Retention in Full Cells

Suppressing the formation of lithium (Li) dendrites is central to implementing Li-metal anode, which has gained growing attention due to its ultrahigh specific capacity and low redox potential. Here, a novel approach is adopted to deposit Li-metal within a rigid three-dimensional (3D) carbon paper (3DCP) network, which consists of a cross-link framework of carbon fibers and graphene nanosheets (GNs). This unique structure yields a uniform distribution of Li-nuclei during the preliminary stage of Li-plating and the formation of a stable solid-electrolyte interface. The as-obtained anode can deliver a high areal capacity of 10 mAh cm-2 without the dendritic formation after 1000 cycles in a Li@3DCP/LiFePO4 full cell at 4 C. In addition, the Li@3DCP anode displays low voltage platform (<20 mV at 1 mA cm-2), high plating/stripping efficiency (99.0%), and long lifespan (>1000 h). When coupled with LiNi0.8Co0.15Al0.05O2 cathode, the Li@3DCP electrode exhibits a superior rate capability up to 10 C and high temperature performance (60 °C). The unprecedented performance is attributed to the desirable combination of micro/nanostructures in 3DCP, in which carbon fiber framework provides the mechanical stability for volume change, whereas numerous lithiophilicity sites on GNs enable the suppression of Li-dendrite growth.