Co3O4, which is of mixed valences Co2+ and Co3+, has been extensively investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). The proper control of Co2+/Co3+ ratio in Co3O4 could lead to modifications on its electronic and thus catalytic properties. Herein, we designed an efficient Co3O4-based OER electrocatalyst by a plasma-engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co3O4 surface with more Co2+ formed. The increased surface area ensures the Co3O4 has more sites for OER, and generated oxygen vacancies on Co3O4 surface improve the electronic conductivity and create more active defects for OER. Compared to pristine Co3O4, the engraved Co3O4 exhibits a much higher current density and a lower onset potential. The specific activity of the plasma-engraved Co3O4 nanosheets (0.055 mA cm−2BET at 1.6 V) is 10 times higher than that of pristine Co3O4, which is contributed by the surface oxygen vacancies.

Plasma-Engraved Co3 O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction.

Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, ...

Emerging Two-Dimensional Nanomaterials for Electrocatalysis

Oxygen electrocatalysis, including the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER), is a critical process for metal-air batteries Therefore, the development of electrocatalysts for the OER and the ORR is of essential importance Indeed, various advanced electrocatalysts have been designed for the ORR or the OER; however, the origin of the advanced activity of oxygen electrocatalysts is still somewhat controversial The enhanced activity is usually attributed to the high surface areas, the unique facet structures, the enhanced conductivities, or even to unclear synergistic effects, but the importance of the defects, especially the intrinsic defects, is often neglected More recently, the important role of defects in oxygen electrocatalysis has been demonstrated by several groups To make the defect effect clearer, the recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed An overview of the defects in carbon-based, metal-free electrocatalysts for ORR and various defects in metal oxides/selenides for OER is also provided The types of defects and controllable strategies to generate defects in electrocatalysts are presented, along with techniques to identify the defects The defect-activity relationship is also explored by theoretical methods

Defect Chemistry of Nonprecious-Metal Electrocatalysts for Oxygen Reactions

As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium-sulfur (Li-S) batteries. In this paper, a mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite is reported as a novel cathode for advanced Li-S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verifi ed by X-ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC-sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm −2 with a high sulfur loading (4.2 mg S cm −2 ) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm −2 ) is demonstrated by using the novel cathode, which is crucial for the practical application of Li-S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon-sulfur composite cathodes for Li-S batteries.

Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries

Chemical doping has been demonstrated to be an effective way to realize new functions of graphene as metal-free catalyst in energy-related electrochemical reactions. Although efficient catalysis for the oxygen reduction reaction (ORR) has been achieved with doped graphene, its performance in the hydrogen evolution reaction (HER) is rather poor. In this study we report that nitrogen and sulfur co-doping leads to high catalytic activity of nanoporous graphene in HER at low operating potential, comparable to the best Pt-free HER catalyst, 2D MoS2 . The interplay between the chemical dopants and geometric lattice defects of the nanoporous graphene plays the fundamental role in the superior HER catalysis.

High Catalytic Activity of Nitrogen and Sulfur Co-Doped Nanoporous Graphene in the Hydrogen Evolution Reaction†

One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction

MoS2 with a similar layered structure to graphene has been widely applied in various areas including lithium ion batteries. However, low conductivity, capacity fading and poor rate performance are still the main challenges for MoS2 anode materials. In this work, for the first time, we prepared nitrogen-doped MoS2 (N-MoS2) nanosheets through a simple two-step method involving the preparation of MoS2 with defects by the hydrothermal method, followed by sintering in a NH3 atmosphere. Our electrochemical characterizations and density functional theory calculations demonstrated that nitrogen doping could enhance the electron conductivity and showed higher specific capacity than pristine MoS2 as anode materials of lithium ion batteries, which can be attributed to the faster transportation of electrons and ions because of nitrogen doping. This work helps us understand the origin of the enhanced performance of N-doped MoS2 in lithium ion batteries.

https://iopscience.iop.org/article/10.1088/0957-4484/27/17/175402/pdf

The origin of the enhanced performance of nitrogen-doped MoS2 in lithium ion batteries

One-pot synthesis of nitrogen and sulfur co-doped graphene supported MoS2 as high performance anode materials for lithium-ion batteries

To reduce the shuttle effect of lithium sulfur (Li–S) batteries and improve the cycling stability, the surface of the commercial separator in an Li–S battery was modified by the O2 plasma treatment, which generated lots of electronegative oxygenic functional groups such as –COOH and –OH on the surface of the separator. In order to confirm the existence and the effect of the electronegative oxygenic functional groups on the modified separator, the contact angle measurement and Fourier Transform Infrared Spectrometry (FTIR) were investigated. The charge/discharge and electrochemical impedance spectroscopy (EIS) tests of the Li–S battery assembled with the normal and the O2 plasma treated separator were analyzed. The surface characterization demonstrated that the oxygenic functional groups on the surface of the separator by the plasma modification played a critical role in improving the wettability and increasing electrical insulating properties. The cycling performance of Li–S batteries with the plasma treated separator had an obvious improvement owing to the electrostatic repulsion between electronegative oxygenic functional groups on the surface of O2 plasma treated separator and electronegative polysulfide. The battery assembled with O2 plasma treated separator had a higher capacity retention (48.53%) than that of the normal separator (24.51%).

Qiuhong Liu

Papers

One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction

The origin of the enhanced performance of nitrogen-doped MoS2 in lithium ion batteries

One-pot synthesis of nitrogen and sulfur co-doped graphene supported MoS2 as high performance anode materials for lithium-ion batteries

Oxygen plasma modified separator for lithium sulfur battery

In-situ Formation of Ni3S2 Interlayer between MoS2 and Ni Foam for High-rate and Highly-durable Lithium Ion Batteries