Dongxiao Ji

Cobalt nanoparticles encapsulated in carbon nanotube-grafted nitrogen and sulfur co-doped multichannel carbon fibers as efficient bifunctional oxygen electrocatalysts

Developing flexible, efficient, and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of paramount importance for designing fuel cells, metal–air batteries and water splitting units. Herein we present an economical approach for the synthesis of self-standing cobalt nanoparticles (NPs) anchored on carbon nanotube-grafted multichannel carbon fibers, co-doped with nitrogen and sulfur (Co@NS/CNT-MCFs), which exhibit comparable ORR (OER) activity to that of commercial Pt/C (RuO2) catalysts in terms of a half-wave potential of 0.837 V for the ORR, and a mere 362 mV overpotential at a current density of 10 mA cm−2 for the OER, as well as remarkable stability in an alkaline medium. The excellent electrocatalytic properties can be attributed to the hierarchically porous network structure and multiple heteroatom dopants introduced, which favor efficient reagent/product mass transport, in addition to providing a great number of active sites. As a proof of concept, the designed flexible catalysts are also employed as air cathodes in an assembled lithium–oxygen (Li–O2) cell with high specific capacity and outstanding operational durability. These results demonstrate the potential of this novel approach in developing suitable catalysts to enable the next generation of metal–air batteries.

Design and synthesis of porous channel-rich carbon nanofibers for self-standing oxygen reduction reaction and hydrogen evolution reaction bifunctional catalysts in alkaline medium

Carbon-nanofiber-based (CNF-based) nonprecious catalysts and electrodes are essential components in next generation energy conversion and storage technologies. Moreover, porous architectures are highly desirable for active material embedded CNFs. Despite recent progress, controllable synthesis of porous CNFs with favorable mechanical properties is still challenging. Herein, we present a general and novel approach to prepare porous and channel-rich CNFs on a large scale through a free-surface electrospinning technique and subsequent carbonization of polyacrylonitrile (PAN)/cellulose acetate (CA) nanofibers. The resultant free-standing and flexible PAN/CA CNFs (CACNFs) possess abundant porous and channel-rich structures, which can be easily controlled by adjusting the weight ratio of PAN and CA. Based on the porous CACNFs, binder-free Fe3C embedded Fe/N doped CACNF films are successfully prepared. Combining the porous channel-rich structures and the high electrical conductivity of the carbon fibers, abundant accessible active sites and fast mass transport pathways are generated in the carbon fibers, leading to favorable catalytic activity and superior stability for ORR (half-wave potential 12 mV more positive than that of Pt/C) and HER (overpotential 440 mV@80 mV cm−2 and more than 100 000 s catalytic stability) in alkaline medium, demonstrating their promising potential for application in fuel cells, metal–air batteries and water splitting devices.

Engineering Co9S8/WS2 array films as bifunctional electrocatalysts for efficient water splitting

An elaborate design of highly active, low-cost and durable bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in water splitting is undoubtedly vital but challenging to develop the hydrogen economy. Herein, we report a novel kind of three-dimensional Co9S8/WS2 electrocatalyst with array structures as efficient bifunctional electrocatalysts for overall water splitting. The resulting hierarchically Co9S8/WS2, as a robust integrated 3D hydrogen-evolving electrode, can deliver a current density of 10 mA cmgeometric area−2 at an overpotential of 138 mV in alkaline media, together with high stability. Combined with density functional theory calculations, the superior electrocatalytic activity is attributed to the unique structures and synergistic effect of chemical and electronic couplings between Co9S8 and WS2. Furthermore, this bifunctional electrode enables a high-performance alkaline water electrolyzer with 10 mA cm−2 at a cell voltage of 1.65 V. Therefore, this unique 3D architecture opens an exciting new avenue to explore the use of composite sulfides toward full water splitting.

Thin MoS2 nanosheets grafted MOFs-derived porous Co–N–C flakes grown on electrospun carbon nanofibers as self-supported bifunctional catalysts for overall water splitting

Active and stable non-precious metal electrocatalysts are critical for the large-scale production of hydrogen/oxygen. Herein, a facile strategy for the in situ growth of MOFs combined with carbonization and subsequent solvothermal treatment for the rational design of thin MoS2 nanosheets grafted Co–N–C flakes (CoNC@MoS2) and grown on electrospun carbon nanofibers (CNFs) as bifunctional electrocatalysts for both hydrogen and oxygen evolution reactions (HER/OER) is reported. Binder-free CoNC@MoS2/CNF films exhibited unique hierarchical architectures with interconnected vine-like CNFs, which imparted favorable flexibility and satisfactory electrical conductivity to the self-supported electrocatalysts for electrochemical reactions. Due to the synergistic effect of the CoNC@MoS2 hybrid nanostructures and fast mass-transport properties of porous carbons, the resultant CoNC@MoS2/CNFs exhibited high catalytic activities and favorable stabilities for the HER and OER in a basic medium. When acting as electrocatalytic electrodes for overall water splitting, CoNC@MoS2/CNF films displayed a low overpotential of 1.62 V to generate a current density of 10 mA cm−2 with remarkable stability at different voltages for 200 000 s, and even outperformed Pt/C–RuO2 electrode in high current density water electrolysis. This study highlights the rational design of hybrid nanostructures based on MOFs and CNFs as efficient self-supported electrocatalysts, opening new possibilities for the fabrication of functional free-standing materials in energy chemistry.

In Situ Fabrication of Hierarchically Branched TiO2 Nanostructures: Enhanced Performance in Photocatalytic H2 Evolution and Li-Ion Batteries

1D branched TiO2 nanomaterials play a significant role in efficient photocatalysis and high-performance lithium ion batteries. In contrast to the typical methods which generally have to employ epitaxial growth, the direct in situ growth of hierarchically branched TiO2 nanofibers by a combination of the electrospinning technique and the alkali-hydrothermal process is presented in this work. Such the branched nanofibers exhibit improvement in terms of photocatalytic hydrogen evolution (0.41 mmol g-1 h-1 ), in comparison to the conventional TiO2 nanofibers (0.11 mmol g-1 h-1 ) and P25 (0.082 mmol g-1 h-1 ). Furthermore, these nanofibers also deliver higher lithium specific capacity at different current densities, and the specific capacity at the rate of 2 C is as high as 201. 0 mAh g-1 , roughly two times higher than that of the pristine TiO2 nanofibers.