Highly Efficient Nb₂C MXene Cathode Catalyst with Uniform O‐Terminated Surface for Lithium–Oxygen Batteries

Abstract

are still severe technological challenges in the wide practical applications for LOBs, such as poor round-trip efficiency, high overpotential, inferior cycle stability, and terrible rate capability. The major factor constraining the performance advantage of LOBs is the sluggish kinetics of the oxygen reduction reaction (ORR, discharge process) and the oxygen evolution reaction (OER, charge process) on the cathode. In the discharge process, the surface of cathode is gradually shrouded by Li 2 O 2 as the insulative discharging product, which reduces the conductivity of electrode system and increased the decomposed energy barrier of surface products, eventually resulting in excessive overpotential and inferior cycle performance. Furthermore, the generated superoxide radicals would attack the electrolyte and the cathode materials in the discharge/ charge process, causing the side reactions and formation of various by-products on the electrode. The development of highefficient cathode catalyst is one of the most important strategies to solve the problems of LOBs.Carbon materials were first studied as cathode materials because of the low cost and good conductivity characteristics. However, due to the superoxide radicals generated during the ORR/OER process, a large amount of side products (Li 2 CO 3 ) is produced, which contributes to the poor cycle stability. [3] Researchers have been working on the development of novel catalysts with high efficiency and their electrochemical reaction mechanisms to reduce by-products and improve cycle life and efficiency, such as precious metals (Pt, [4][5][6][7] Au, [8] Pd, [4] and Ru [9,10] ), metal oxides (CeO 2 , [11] RuO 2 , [12] NiFeO, [13] NiCo 2 O 4 , [14] etc.), metal carbides/nitrides (TiC, [15] Mo 2 C, [16] CoN, [17] etc.). High-efficient cathode catalysts could dominate the nucleation and growth kinetics, morphology, crystal states, and chemical composition of discharge products. During the ORR/OER process, LiO 2 is considered to be an important intermediate, which greatly affects the stability of catalyst and composition of discharge products. [18][19][20][21][22][23][24] The formation of discharge products could proceed through surface or solution pathway with LiO 2 species at the interface of cathode and electrolyte. Moreover, the morphology of the discharge products formed under the assistant of the cathode catalysts is strongly related to the cycle stability. Toroid like discharge products consisting of Li 2−x O 2 and Li 2 O 2 could be obtained on carbon-based material Highly-efficient cathode catalysts are the key to improve high rate cycle stability, avoid side reactions, and lower the overpotential of lithium-oxygen batteries (LOBs). MXenes are predicted to be one of the most impressive materials for energy applications. In this work, the catalytic capability of Nb 2 C MXene is demonstrated with a uniform O-terminated surface as a cathode material for LOBs. The easily fabricated uniform O-terminated surface, high catalytic activity of Nb 2 CO 2 sites, and unique re...

MoSe2@CNT Core–Shell Nanostructures as Grain Promoters Featuring a Direct Li2O2 Formation/Decomposition Catalytic Capability in Lithium‐Oxygen Batteries

“…Finally, at stage IV, when discharged to voltage limitation of 2.35 V with a large capacity of over 20 000 mAh g −1 , only sheet‐like Li 2 O 2 aggregation and large particles can be observed on the electrode, which is consistent with the previous reports that the plate‐shaped morphology on the cathode and the considerably high capacity can be attributed to the large size of Li 2 O 2 . [ 39–41 ] Unlike typical thermodynamic formation process from platelet‐shaped particle to fully formed toroid in electrolyte especially with a high DN number (DMSO) [ 11 ] or surface growth path accumulating quasi‐amorphous film Li 2 O 2 right on the electrode surface, [ 10,12 ] the plate cluster of Li 2 O 2 is eventually observed which directly grow from tiny seeds in particular on the MoSe 2 surface. In contrast, the pasty‐shaped discharge product forms on the CNT surface when discharged to the capacity limitation of 2000 mAh g −1 as shown in Figure S11, Supporting Information.…”

“…[ 9,11 ] On the other hand, the decomposition kinetics of superoxide radicals in charge highly depends on the electrolyte, catalyst species, current rate, or potential and the feature of discharge products including size, morphology, defects, and crystallinity. [ 9,12 ] Notably, the strong oxidant intermediate LiO 2 during the formation/decomposition of discharge products contributes to the degradation of LOBs through the corrosion of the carbon electrode and the instability of the electrolyte. [ 11 ] The accumulation of side products for both, eventually results in cell deterioration and capacity fading.…”

“…[ 11 ] The accumulation of side products for both, eventually results in cell deterioration and capacity fading. [ 11,12 ] Searching for a highly efficient cathode catalyst with an intrinsic catalyst capability for direct formation/decomposition of Li 2 O 2 without the existence of intermediates at low overpotentials is important and dramatic for the development of LOBs.…”

Atomic Heterointerface Boosts the Catalytic Activity toward Oxygen Reduction/Evolution Reaction

“…DFT calculation was also conducted to study the intrinsic information of thin film catalysts at the atomic scale for Li-O 2 batteries. Three elementary reactions and three intermediates and terminal products (LiO 2 , Li 2 O 2 and Li 2 O) were chosen for theoretical calculation: i [67,68] The symbol of * indicates that the species are absorbed on the surface of the catalysts. A phase diagram of different discharge products and intermediates was found to analyze the possible discharge processes on Pd (111)/Ni (111) heterostructure (Figure S20, Supporting Information).…”

Surface plasmon resonance aptasensor based on niobium carbide MXene quantum dots for nucleocapsid of SARS-CoV-2 detection

“…MXenes, which are known as two-dimensional materials, have attracted extensive attention due to their similar structure and analogous performances to graphene. The versatile chemical structure, compositions, and tunable surface functionalization of MXenes facilitate the diverse applications of MXene, such as in solar cells [42], electronic devices [43], catalysts [44,45], gas sensors or biosensors [46,47], and cancer therapy [48]. MXenes have nanosheet-like structure, unique surface chemistry, high conductive properties, and excellent biocompatibility.…”