Research progress of anionic vacancies in electrocatalysts for oxygen evolution reaction

Renewable energy conversion as well as water electrolysis technologies are constrained by the fact that kinetics are always slow in the electrocatalytic oxygen evolution reaction(OER).There are numerous means and strategies for the enhancement of OER activity.In this paper,we systematically review the important role of anionic vacancies in enhancing the OER activity of catalysts:increasing catalyst conductivity,improving electrical conductivity,and enhancing intermediate adsorption.In order to better detect the presence of vacancies in the samples,the principle of vacancy detection is reviewed in detail in terms of both spectroscopic and microscopic characterization,and the methods of vacancy formation as well as the factors influencing the concentration of vacancies are summarized in detail.In addition,the challenges and new directions for the study of anionic vacancies are provided.Lei Wang was awarded a Ph.D.in chemistry from Jilin University in 2006 under the supervision of Prof.Shouhua Feng.He worked as a Postdoctoral Scholar in Shandong University,the State Key Laboratory of Crystal Materials from 2008 to 2010.He is currently a professor at Qingdao University of Science and Technology.His research interests mainly focus on the design and synthesis of functional organic-inorganic hybrids and porous MOFs materials,as well as their applications in photocatalysis,electrocatalysis,lithium-ion battery,etc.Jingqi Chi received her B.S.degree and Ph.D.degree from the State Key Laboratory of Heavy Oil Processing,China University of Petroleum(East China).She is currently an associate professor at Qing dao University of Science and Technology.Her research interests focus on the design and synthesis of transition metal-based nanostructures and porous MOFs materials for electrochemical applications.

Achieving photocatalytic water oxidation on LaNbON2 under visible light irradiation

LaNbON2 has narrow bandgap and wide visible-light absorption band, yet no photocatalytic water oxidation on LaNbON2 has been reported. By a post-annealing treatment in Ar, anion vacancies were brought into LaNbON2 as shown by EPR spectroscopy. These could act as donors in the semiconductor. And consequently the oxidative power of holes was enhanced as indicated by the difference between fermi level and valence band maximum（EF-EVBM） evaluated from valence band XPS. The annealed LaNbON2 photocatalyst acquired water oxidation ability for the first time, which was improved by combining CoOx as cocatalyst. Annealed LaNbON2 derived from La3NbO7 had smaller particle size, higher concentration of anion vacancies, bigger EF-EVBM and better performance for photocatalytic oxygen evolution reaction than LaNbON2 derived from LaNbO4.

A novel non-stoichiometric medium-entropy carbide stabilized by anion vacancies

Recently,high-entropy ceramics have attracted considerable attentions because of comprehensive physical and chemical properties of high hardness,fracture toughness,and conductivity.However,as a newly emerging class of materials,the synthesis,performance and applications of high-entropy ceramics are subject to further development.Here,we reported a new non-stoichiometric TiC0.4/WC/0.5Mo2C medium-entropy carbide(MEC)with a rock-salt structure.Attributed to the solid solution strengthening and twinning strengthening,the TiCO0.4/WC/0.5Mo2C sintered at 1900℃by spark plasma sintering(SPS)shows superior mechanical behaviors of microhardness(21.7 GPa),which exceeds that expected from the rule of mixture(ROM)of three individual metal carbides(19.1 GPa)and good fracture toughness(5.3 MPa m1/2).Significantly,the bulk synthesized via high-pressure and high-temperature(HPHT)sintering possesses smaller grain size and shows better comprehensive mechanical properties of microhardness(23.7 GPa)and fracture toughness(6.2 MPa m1/2).In addition,the effect of anion vacancies on the thermodynamic stability and synthesizability of TiC0.4/WC/0.5Mo2C was analyzed via quantitatively calculated entropy.Vacancies could significantly enhance the configuratio nal entropy of mixing of the solid phase.The introduction of vacancy defects may expand synthetic path for entropy-stabilized ceramics,especially for multi-component high tempe rature refractory ceramics.

Abnormal n-type doping effect in nitrogen-doped tungsten diselenide prepared by moderate ammonia plasma treatment

To facilitate potential applications of tungsten diselenide （WSe2） in electronics, controllable doping is of great importance. As an industrially compatible technology, plasma treatment has been used to dope two-dimensional （2D） materials. However, owing to the strong etching effect in transition metal dichalcogenides （TMDCs）, it is difficult to controllably dope 2D WSe2 crystals by plasma. Herein, we develop a moderate ammonia plasma treatment method to prepare nitrogen-doped WSe2 with controlled nitrogen content. Interestingly, Raman, photoluminescence, X-ray photoelectron spectroscopy, and electrical Lts reveal abnormal n-doping behavior of nitrogen-doped WSe2, which is attributed to selenium anion vacancy introduced by hydrogen species in ammonia plasma. Nitrogen-doped WSe2 with abnormal n-doping behavior has potential applications in future TMDCs-based electronics.

Integrating molybdenum sulfide selenide-based cathode with C-O-Mo heterointerface design and atomic engineering for superior aqueous Zn-ion batteries

Transition metal dichalcogenides(TMDs)have been regarded as promising cathodes for aqueous zinc-ion batteries(AZIBs)but suffer from sluggish reaction kinetics due to their poor conductivity and the strong electrostatic interaction between Zn-ion and cathode materials.Herein,a well-defined structure with MoSSe nanosheets vertically anchored on graphene is used as the cathode for AZIBs.The dissolution of Se into MoS2 lattice together with heterointerface design via developing C-O-Mo bonds improves the inherent conductivity,enlarges interlayer spacing,and generates abundant anionic vacancies.As a result,the Zn2+intercalation/deintercalation process is greatly improved,which is confirmed by theoretical modeling and ex-situ experimental results.Remarkably,the assembled AZIBs exhibit high-rate capability(124.2 mAh·g^(−1)at 5 A·g^(−1))and long cycling life(83%capacity retention after 1,200 cycles at 2 A·g^(−1)).Moreover,the assembled quasi-solid-state Zn-ion batteries demonstrate a stable cycling performance over 100 cycles and high capacity retention over 94%after 2,500 bending cycles.This study provides a new strategy to unlock the electrochemical activity of TMDs via interface design and atomic engineering,which can also be applied to other TMDs for multivalent batteries.

Intrinsic defects of nonprecious metal electrocatalysts for energy conversion: Synthesis, advanced characterization, and fundamentals

With the depletion of fossil fuels and environmental pollution, energy storage and conversion have become the focus of current research. Water splitting and fuel cell technologies have made outstanding contributions to energy conversion. However, the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have slow kinetics, which limit the capacity of fuel cells. It is of great significance to develop catalysts for the OER and ORR and continuously improve their catalytic performance. Many studies have shown that intrinsic defects, especially vacancies (anion and cation vacancies), can effectively improve the efficiency of electrochemical energy storage and conversion. The introduction of intrinsic defects can generally expose more active sites, enhance conductivity, adjust the electronic state, and promote ion diffusion, thereby enhancing the catalytic performance. This review comprehensively summarizes the latest developments regarding the effects of intrinsic defects on the performance of non-noble metal electrocatalysts. According to the type of intrinsic defect, this article reviews in detail the regulation mechanism, preparation methods and advanced characterization techniques of intrinsic defects in different materials (oxides, non-oxides, etc.). Then, the current difficulties and future development of intrinsic defect regulation are analyzed and discussed thoroughly. Finally, the prospect of intrinsic defects in the field of electrochemical energy storage is further explored.