高活性位点密度Fe-N共掺杂碳纳米片的制备及氧还原性能

从三聚氰胺和均苯四甲酸酐单体出发, 通过熔融盐法合成了三嗪结构聚酰亚胺纳米片, 借助类石墨相氮化碳(g-C₃N₄)与铁离子的配位作用, 经高温热处理形成了高效掺杂的Fe-N/C催化剂. 研究结果表明, 该催化剂为表面粗糙的纳米片结构, 比表面积高达1794 m²/g. 通过g-C₃N₄的引入和含量的调控, 催化剂中铁元素的掺杂量最高可达1.13%(摩尔分数), 为未引入g-C₃N₄的3.3倍, 其原因可归结于g-C₃N₄配位锚定了铁离子, 其较强的配位作用可以避免高温热处理时铁元素的迁移和聚集. 该催化剂在酸性条件下氧还原反应半波电位为0.79 V, 10000周加速测试后的半波电位衰减了30 mV, 表现出较好的氧还原活性.     

Effect of microstructure of nitrogen-doped graphene on oxygen reduction activity in fuel cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.     

Effect of doping order on metal-free heteroatoms dual-doped carbon as oxygen reduction electrocatalyst

Metal-free heteroatoms dual-doped carbon has been recognized as one of the most promising Pt/C-substitutes for oxygen reduction reaction(ORR).Herein,we optimize the preparation process by doping order of metal-free heteroatoms to obtain the best electrocatalytic performance through three types of dual-doped carbon,including XC-N(first X doping then N doping),NC-X(first N doping then X doping) and NXC(N and X doping)(X=P,S and F).XC-N has more defect than the other two indicated by Raman spectra.X-ray photoelectron spectrom(XPS) measurements indicate that N and X have been dual-doped into the carbon matrix with different doping contents and modes,Electrocatalytic results,including the potential of ORR peak(E_p),the half-wave potential,the diffusion-limiting current density mainly follows the order of XC-NNC-X NXC,Furthermore,the synergistic effect of second atom doping are also compared with the single doped carbon(NC,PC,SC and FC).The differences in electronegativity and atomic radius of these metal-free heteroatoms can affect the defect degree,the doping content and mode of hete roatoms on carbon matrix,induce polarization effect and space effect to affect O_2 adsorption and product desorption,ultimately to the ORR electrocatalytic performance.     

Facile preparation of nitrogen-doped graphene as a metal-free catalyst for oxygen reduction reaction

Nitrogen-doped graphene (nG) is a promising metal-free catalyst for oxygen reduction reaction (ORR) on the cathode of fuel cells. Here we report a facile preparation of nG via pyrolysis of graphene oxide with melamine. The morphology of the nG is revealed using scanning electron microscopy and transmission electron microscopy while the successful N doping is confirmed by electron energy loss spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The resulting nG shows high electrocatalytic activity toward ORR in an alkaline solution with an onset potential of -0.10 V vs. Ag/AgCl reference electrode. The nG catalyzed oxygen reduction exhibits a favorable formation of water via a four-electron pathway. Good stability and anti-crossover property are also observed, which are advantageous over the Pt/C catalyst. Furthermore, the effect of pyrolysis temperature on the structure and activity of nG is systematically studied to gain some insights into the chemical reactions during pyrolysis.     

Polymer-Assisted Electrophoretic Synthesis of N-Doped Graphene-Polypyrrole Demonstrating Oxygen Reduction with Excellent Methanol Crossover Impact and Durability

The design and synthesis of metal-free catalysts with superior electrocatalytic activity, high durability, low cost, and under mild conditions is extremely desirable but remains challenging. To address this problem, a polymer-assisted electrochemical exfoliation technique of graphite in the presence of an aqueous acidic medium is reported. This simple, cost-effective, and mass-scale production approach could open the possibility for the synthesis of high-quality nitrogen-doped graphene–polypyrrole (NG-PPy). The NG-PPy catalyst displays an improved half wave potential (E_1/2=0.77 V) in alkaline medium compared with G-PPy (E_1/2=0.66 V). Most importantly, this catalyst demonstrates excellent stability with high methanol tolerance, and it outperforms the commercial Pt/C catalyst and other previously reported metal-free catalysts. The content of graphitic nitrogen atoms is the key factor for the enhancement of electrocatalytic activity towards oxygen reduction reactions (ORR). Interestingly, the NG-PPy catalyst can be used as a cathode material in a zinc–air battery, which demonstrates a higher peak power density (59 mW cm⁻²) than G-PPy (36.6 mW cm⁻²), highlighting the importance of the low-cost material synthesis approach towards the development of metal-free efficient ORR catalysts for fuel cell and metal–air battery applications. Remarkably, the polymer-assisted electrophoretic exfoliation of graphite with a high yield (≈88 wt %) of few-layer graphene flakes could pave the way towards the mass production of high-quality graphene for a variety of applications.     

Atomically Dispersed Fe on Nanosheet-linked,Defect-rich,Highly N-Doped 3D Porous Carbon for Efficient Oxygen Reduction

Exploring cost-effective and high-performance oxygen reduction reaction(ORR) electrocatalysts to replace precious platinum-based materials is crucial for developing electrochemical energy conversion devices but remains a great challenge. Herein, Fe single atoms anchored on nanosheet-linked, defect-rich, highly N-doped 3D porous carbon(Fe-SAs/NLPC) electrocatalysts were obtained by pyrolyzing salt-sealed Fe-doped zeolitic imidazolate frameworks(ZIFs). NaCl functions both as pore-forming agent and closed nanoreactor, which can not only lead to the formation of defects-rich three-dimensional interconnected structures with high N-doping content to expose abundant active sites, promote mass transfer and electron transfer, but also facilitate the effective incorporation of Fe to form Fe-N_x active sites without aggregation. These unique characteristics render Fe-SAs/NLPC outstanding electrocatalytic activity for ORR, with one-set potential of 0.96 V and high kinetic current density(j_K) of 33.32 mA/cm² in alkaline medium, which surpass the values of most nonprecious-metal catalysts and even commercial Pt/C.     

Cu,Ni, Co掺杂对Fe碳纳米管的结构及电催化性能的影响

在制备含铁碳纳米管(Fe@NCNTs)的过程中分别加入铜(Cu)、 镍(Ni)和钴(Co)盐, 得到3种双金属碳纳米管材料(CuFe@NCNTs, NiFe@NCNTs和CoFe@NCNTs). 通过扫描电子显微镜(SEM)、 N₂气吸附-脱附曲线测试、 X射线衍射(XRD)、 X射线光电子能谱(XPS)和拉曼光谱(Raman)对3种双金属碳纳米管的结构和组成进行了 表征, 证明3种双金属均可得到碳纳米管结构. 3种材料均表现出双功能电催化活性[氧还原反应(ORR)和氧 析出反应(OER)]; 锌-空气电池(ZABs)性能测试结果表明, CuFe@NCNTs基ZABs具有最大的峰值功率密度 (53 mW/cm²), NiFe@NCNTs和CoFe@NCNTs基ZABs具有更好的倍率性能, 3种双金属碳纳米管ZABs的循环稳定性均优于Pt/C-IrO₂ ZABs.     

S掺杂促进Fe/N/C催化剂氧还原活性的实验与理论研究

向Fe/N/C非贵金属催化剂中再引入S掺杂是进一步提高其氧还原催化活性的有效方法。为了探究活性提高的原因,本文以三聚氰胺-甲醛树脂为前驱体,氯化钙为模板,氯化铁为铁源,通过添加硫氰化钾(KSCN)来控制热解催化剂的S掺杂量。通过对比分析催化剂的物化性质,结合密度泛函理论(DFT)计算,分析S掺杂促进Fe/N/C催化剂氧还原活性的原因。透射电子显微镜(TEM)和N_2吸脱附等温线测试结果表明,S元素可抑制含铁纳米粒子的形成,促使形成多孔碳结构,提高比表面积。X射线光电子能谱(XPS)结果表明,适量S前驱体可实现较高的S掺杂含量,得到最优的活性,过量的S反而会导致Fe和S的掺杂量同时降低,影响活性。DFT计算结果表明在Fe-N_4大环中引入S掺杂,可增强O_2分子和中间体OOH与Fe-N_4结构中的Fe的相互作用,促进形成Fe―O键,从而导致O―O键的键能显著降低,为后续反应O―O键的断裂提供可能,促进ORR反应的进行。     

高活性和高稳定性的核壳结构PtPx@Pt/C催化氧还原

在质子交换膜燃料电池中,金属铂是最高效的阴极氧还原催化剂之一,但是铂昂贵的价格严重阻碍了其在燃料电池领域中的大规模商业化应用.通过铂与3d过渡金属(Fe、Co和Ni)合金化可以有效提高催化剂的氧还原活性,然而在实际的高腐蚀性、高电压和高温的燃料电池运行环境中,铂合金纳米粒子易发生溶解、迁移和团聚,从而导致催化剂耐久性差.同时过渡金属离子的溶出会影响质子交换膜的质子传导,并且一些过渡金属离子会催化芬顿反应,产生高腐蚀性?OH自由基,加快Nafion和催化剂的劣化.与过渡金属掺杂相比,非金属掺杂具有明显优势:一方面,非金属溶出产生的阴离子不会取代Nafion中的质子,也不会催化芬顿反应;另一方面,与3d过渡金属相比,非金属具有更高的电负性,其掺杂很容易调节Pt的电子结构.因此,本文通过非金属磷掺杂合成具有优异稳定性的核壳结构PtPx@Pt/C氧还原催化剂.通过热处理磷化商业碳载铂形成磷化铂(PtP2),经由酸洗处理产生富铂壳层,即PtPx@Pt/C.X射线粉末多晶衍射结果证明了PtP2相的存在,并且进一步通过电子能量损失谱对纳米粒子进行微区面扫描分析以及X射线光电子能谱分析证实了富铂壳层的存在,壳层厚度约1 nm.得益于核壳结构及磷掺杂引起的电子结构效应,PtP1.4@Pt/C催化剂在0.90 V(RHE)时的面积活性(0.62 mA cm–2)与质量活性(0.31 mAμgPt–1)分别是商业Pt/C的2.8倍和2.1倍.更重要的是,在加速耐久性测试中,PtP1.4@Pt/C催化剂在30000圈电位循环后质量活性仅衰减6％,在90000圈电位循环后仅衰减25％;而商业Pt/C催化剂在30000圈电位循环后就衰减46％.PtP1.4@Pt/C催化剂高活性与高稳定性主要归功于核壳结构、磷掺杂引起的电子结构效应以及磷掺杂增加了碳载体对催化剂粒子的锚定作用进而阻止了其迁移团聚.综上所述,本文为设计同时具有优异活性与稳定性非金属掺杂Pt基氧还原催化剂提供新的思路.     

A Chemical Dealloying Approach for Pt Surface-enriched Pt3Co Alloy Nanoparticles as Oxygen Reduction Reaction Electrocatalysts

Pt-based alloys are the optimal electrocatalysts for oxygen reduction reaction(ORR) currently. Dealloying of Pt-based alloys has shown to be an effective approach to improving ORR activity. Electrochemical dealloying is controllable for morphology by changing electrochemical parameters but is difficult to scale up due to complex operation and energy consumption. Chemical dealloying is suitable for a large scale but it is not easy to control the morphology because highly corrosive acids(HNO₃ or H₂SO₄) are commonly used. In this work, a facile chemical dealloying method for Pt₃Co/C has been employed to synthesize elec-trocatalysts for ORR using weak acids and buffer solutions of different pH, which could slow down the dissolution rate for Co atoms and increase the diffusion time for Pt atoms to improve ORR activity. It can be observed that the mass activities(MA) of the Pt₃Co/C alloy after dealloying with H₃PO₄ and NaH₂PO₄/Na₂HPO₄ buffer solution of pH=6 are close to that after electrochemical dealloying process, and are more than two times that of commercial Pt/C. In addition, Pt₃Co/C after dealloying with a buffer solution of pH=6 only showed a slight degradation in the half-wave potential and electrochemical surface area(ECSA) after stability test for 5000 cycles, which is more stable than commercial Pt/C. It shows that by controlling pH of the solvent, the ORR activity can be further increased. This facile approach provides a new strategy to control morphology of Pt-based electrocatalysts by chemical dealloying, which can contribute to promising application for cathodic electrocatalysts design of proton exchange membrane fuel cells (PEMFCs).     

吡啶掺杂碳载钴酞菁催化氧还原的电化学性能及在燃料电池中的应用

以碳黑(Vulcan XC-72R)为载体, 吡啶(Py)和钴酞菁(CoPc)为催化剂前驱体, 经溶剂分散法制备了Py掺杂碳负载纳米钴酞菁复合催化剂(Py-CoPc/C). 通过扫描电镜-能谱分析(SEM-EDS)、X射线光电子能谱(XPS)分析和X射线衍射(XRD)分析技术对催化剂的组成和微观结构进行了表征, 并运用线性扫描循环伏安法(LSV)和旋转圆盘电极(RDE)技术考察了不同Py掺杂含量对碳载钴酞菁(CoPc/C)催化氧还原反应(ORR)活性的影响及稳定性. 结果显示: Py掺杂可以明显改善CoPc/C 对ORR的电催化性能, 其中掺杂20%Py下所制备的20%Py-20%CoPc/C 催化剂对ORR表现出最佳的催化活性, 以其制备的气体扩散电极在O₂气氛饱和的0.1 mol·L^-1 KOH 电解质溶液中, 0.2 V (相对于标准氢电极)即可产生明显的氧还原电流, 半波电位为-0.03 V. 相比于40%Py/C 和未掺杂的40%CoPc/C, 20%Py-20%CoPc/C催化剂的半波电位分别正移了160 和15 mV. 进一步运用RDE理论研究表明, 在Py-CoPc/C 电极上ORR的电子转移总数为2.38, 高于CoPc/C电极上的电子转移总数1.96, 从而使ORR的选择性明显提高. SEM-EDS和XRD分析表明Py掺杂提高了CoPc/C催化剂的分散性和N含量, 更利于O₂的吸附. XPS分析表明: 吡啶结构的N与石墨结构的N均存在于Py-CoPc/C 催化剂中,与催化剂表面的Co离子配位可能是促使ORR活性提高的原因. 最后以20%Py-20%CoPc/C制备了膜电极组装(MEA)电极, 应用于H₂/O₂ 燃料电池单电池发电, 室温下获得最大发电功率密度为21 mW·cm^-2, 相对于CoPc/C提高至2.4倍.     

Polyelectrolyte functionalized carbon nanotubes as efficient metal-free electrocatalysts for oxygen reduction

Having a strong electron-withdrawing ability, poly(diallyldimethylammonium chloride) (PDDA) was used to create net positive charge for carbon atoms in the nanotube carbon plane via intermolecular charge transfer. The resultant PDDA functionalized/adsorbed carbon nanotubes (CNTs), either in an aligned or nonaligned form, were demonstrated to act as metal-free catalysts for oxygen reduction reaction (ORR) in fuel cells with similar performance as Pt catalysts. The adsorption-induced intermolecular charge-transfer should provide a general approach to various carbon-based efficient metal-free ORR catalysts for oxygen reduction in fuel cells, and even new catalytic materials for applications beyond fuel cells.     

Regulating the Oxygen Affinity of Single Atom Catalysts by Dual-atom Design for Enhanced Oxygen Reduction Reaction Activity

This work chooses Cu/Fe single-atom catalysts(SACs) with weak/strong oxygen affinity to clarify the effect of dual-atom configuration on oxygen reduction reaction(ORR) performance based on density functional theory(DFT) calculations. The stability and ORR activity of single or dual Cu/Fe atomic sites anchored on nitrogen-doped graphene sheets(Cu-N₄-C, Cu₂-N₆-C, Fe-N₄-C, and Fe₂-N₆-C) are investigated, and the results indicate the dual-atom catalysts(Cu₂-N₆-C and Fe₂-N₆-C) are thermodynamically stable enough to avoid sintering and aggregation. Compared with single-atom active sites of Cu-N₄-C, which show weak oxygen affinity and poor ORR performance with a limiting potential of 0.58 V, the dual-Cu active sites of Cu₂-N₆-C exhibit enhanced ORR activity with a limiting potential up to 0.87 V due to strengthened oxygen affinity. Interestingly, for Fe SACs with strong oxygen affinity, the DFT results show that the dual-Fe sites stabilize the two OH^* ligands structure[Fe₂(OH)₂-N₆-C], which act as the active sites during ORR process, resulting in greatly improved ORR performance with a limiting potential of 0.90 V. This study suggests that the dual-atom design is a potential strategy to improve the ORR performance of SACs, in which the activity of the single atom active sites is limited with weak or strong oxygen affinity.     

Noble metal-free catalysts for oxygen reduction reaction

Developing noble metal-free catalysts with low cost, high performance and stability for oxygen reduction reaction(ORR) in fuel cells is of great interest to promote sustainable energy devices. In this review, we summarized noble metal-free catalysts for ORR,including non-noble metal-based and heteroatom-doped carbon nanomaterials. Mesoporous structure, homogeneous distribution of nanocrystals and synergistic effect of carbon base and nanocrystals/doped heteroatoms have great effect on the ORR property.The noble metal-free nanomaterials showed comparable catalytic property, better stability and methanol tolerance than commercial platinum(Pt)-based catalysts, showing great potential as substitutes for noble metal-based catalysts. In addition, the challenges and chances of developing noble metal-free ORR catalysts are also discussed.     

Scalable Synthesis and Electrocatalytic Performance of Highly Fluorinated Covalent Organic Frameworks for Oxygen Reduction

In this study, we present a novel approach for the synthesis of covalent organic frameworks (COFs) that overcomes the common limitations of non-scalable solvothermal procedures. Our method allows for the room-temperature and scalable synthesis of a highly fluorinated DFTAPB-TFTA-COF, which exhibits intrinsic hydrophobicity. We used DFT-based calculations to elucidate the role of the fluorine atoms in enhancing the crystallinity of the material through corrugation effects, resulting in maximized interlayer interactions, as disclosed both from PXRD structural resolution and theoretical simulations. We further investigated the electrocatalytic properties of this material towards the oxygen reduction reaction (ORR). Our results show that the fluorinated COF produces hydrogen peroxide selectively with low overpotential (0.062 V) and high turnover frequency (0.0757 s⁻¹) without the addition of any conductive additives. These values are among the best reported for non-pyrolyzed and metal-free electrocatalysts. Finally, we employed DFT-based calculations to analyse the reaction mechanism, highlighting the crucial role of the fluorine atom in the active site assembly. Our findings shed light on the potential of fluorinated COFs as promising electrocatalysts for the ORR, as well as their potential applications in other fields.     

碳基非贵金属氧还原电催化剂的活性位结构研究进展

以热解型Fe/N/C为代表的碳基非贵金属材料被认为是当前最具潜力替代铂的非贵金属氧还原催化剂,其综合性能的进一步突破,对于推动质子交换膜燃料电池商业化应用具有重要意义。对热解型Fe/N/C催化剂活性位结构的深入认识是实现催化剂高活性位密度和高稳定性理性设计的关键。本文总结了热解型Fe/N/C活性位的研究进展,重点介绍了非晶态铁氮配位活性中心、氮掺杂和碳缺陷三类活性位构型。由于热解型Fe/N/C是非均相的,结构非常复杂,导致在活性位认识上还存在诸多争议,本文总结阐述了活性位结构的不同观点。最后,我们展望了Fe/N/C催化剂活性位研究的未来方向。     

Self-sacrificial template synthesis of Fe,N co-doped porous carbon as efficient oxygen reduction electrocatalysts towards Zn-air battery application

Designing highly efficient non-precious based electrocatalysts for oxygen reduction reaction（ORR） is of significance for the rapid development of metal-air batteries.Herein,a hydrothermal-pyrolysis method is employed to fabricate Fe,N co-doped porous carbon materials as effective ORR electrocatalyst through adopting graphitic carbon nitride（g-C₃ N₄） as both the self-sacrificial templates and N sources.The gC₃ N₄ provides a high concentration of unsatur...     

甘氨酸修饰的Pt(111)电极上的氧还原

利用单晶旋转圆盘电极技术（Hanging Meniscus Rotating Disk Electrode,　HMRD）在硫酸和高氯酸溶液中分别研究了甘氨酸修饰的Pt(111)电极表面氧分子的电催化还原反应. 实验发现：在硫酸溶液中,经甘氨酸修饰的Pt(111)电极表面的氧还原活性明显提高,其中氧还原的半波电位与Pt(111)电极的相比正移约0.1 V,而在高氯酸溶液中,甘氨酸修饰的Pt(111)电极的活性几乎没有发生变化. 该实验结果表明：甘氨酸修饰的Pt(111)电极一方面抑制了SO₄^2-在电极表面的吸附,另一方面又能在电极表面提供相邻的空位供氧分子吸附. 通过与文献中报道的CN^-修饰的Pt(111)电极上的氧还原结果的对比,可以推测甘氨酸修饰的Pt(111)电极表面氧还原活性提高是由于甘氨酸在Pt(111)表面可能先被氧化成CN^-后吸附在电极表面,进而促进了氧分子的电催化还原反应.     

Biomass-derived nitrogen self-doped porous activation carbon as an effective bifunctional electrocatalysts

The strategy of adopting cheap precursors or abundant resources,which can be obtained directly from nature,is a simple and excellent method of introducing accessible research into environmentally friendly development.Moreover,this is also an urgent requirement for the sustainable development of green technology.Herein,we introduce a simplistic and expandable method to prepare metal-free biomassderived nitrogen self-doped porous activation carbon(N-PAC) with large specific surface area(S_BET=1300.58 m²/g).Moreover,the manufactural electrocatalysts exhibit prominent oxygen reduction reaction(ORR) performance in all PH values.As compared with the commercial Pt/C catalyst,the N-PAC/800 with a positive onset potential at 10 mA/cm²(0.93 V),half-wave potential(0.87 V),and limiting current(6.34 mA/cm²) bring to light excellent catalytic stability,selectivity,and much-enhanced methanol tolerance.Furthermore,the prepared electrocatalysts possess considerable hydrogen evolution reaction(HER) performance with a less onset potential of 0.218 V(acidic medium) and0.271 V(alkaline medium) respectively,which can show similar catalytic activity across the whole pH range.Such bifunctional electrocatalyst,with excellent electrocatalytic properties,resource-rich,low cost,and environmental-friendly,hold a promising application in energy conversion and reserve.     

钛改性对γ-Fe2O3选择催化还原脱硝性能强化机制的分子模拟研究

采用密度泛函理论（DFT）研究了典型过渡金属Ti掺杂改性对γ-Fe₂O₃选择催化还原（NH₃-SCR）脱硝性能强化影响的作用机制。构建了单Ti和双Ti在γ-Fe₂O₃（001）表面的不同Fe位置的掺杂模型，计算了表面掺杂形成能，探讨了O₂、NO和NH₃分子在Ti掺杂前后的γ-Fe₂O₃（001）表面的吸附特性，并进行了反应机理分析。结果表明，单Ti倾向于掺杂在八面体Fe_oct位，双Ti倾向于两个Fe_oct位。Ti的掺杂增强了催化剂表面对O₂的吸附能力，吸附性能随Ti掺杂量增加而增强。单Ti和双Ti的掺杂都抑制了NO以N端在催化剂表面的吸附。Ti能够强化NH₃的吸附，增强了Lewis酸位，有利于SCR反应。Ti的掺杂增大了NO₂生成的反应能垒，降低了γ-Fe₂O₃低温区的SCR反应。Ti的掺杂抑制了NH和N的形成，避免了NH₃的过度氧化，提高NH₃的利用率，有利于SCR反应，并且抑制了通过E-R机理产生的N₂O，具有良好的N₂选择性。Ti的掺杂能够改善γ-Fe₂O₃在NH₃-SCR中还原NO的性能。