2009-Catal Comm-Wei Fei-SAPO-34 supported Pt–Sn-based novel catalyst for propane

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的速率快 速 增长 ＿ １ ］ 。 目前 ， 丙 烯供 应 主要 来 自 蒸 汽 裂 解 制 乙 烯 和 石 油 催 化 裂 化 过 程 的 副 产 品… ２ ］ 。自２ ０世 纪 ９ ０年 代 以 来 ， 随 着 丙 烯 需 求 量 的不 断增 加 ， 传 统 的 丙 烯 生 产 工 艺 已 不 能 满
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下 的标 准 平 衡 常 数 和 不 同 条 件 下 的 平 衡 转 化 率， 并 研 究 不 同 工 艺 条 件 对 丙 烷 催 化 脱 氢 反 应
收 稿 日期 ：２ ０ １ ２ — ０ ８ — ２ ７ ；修 改 稿 收 到 日期 ： ２ ０ １ ２ — １ ０ — ２ ３ 。 作 者 简 介 ：刘 昌呈 （ １ ９ ８ ２ 一） ， 男， 工程 师， 博 士， 主 要 从 事 重 整

A highly ordered nanostructured carbon–sulphur cathode for lithium–sulphur batteriesXiulei Ji,Kyu Tae Lee and Linda F.Nazar *The Li–S battery has been under intense scrutiny for over two decades,as it offers the possibility of high gravimetric capacities and theoretical energy densities ranging up to a factor of five beyond conventional Li-ion systems.Herein,we report the feasibility to approach such capacities by creating highly ordered interwoven composites.The conductive mesoporous carbon framework precisely constrains sulphur nanofiller growth within its channels and generates essential electrical contact to the insulating sulphur.The structure provides access to Li +ingress/egress for reactivity with the sulphur,and we speculate that the kinetic inhibition to diffusion within the framework and the sorption properties of the carbon aid in trapping the polysulphides formed during redox.Polymer modification of the carbon surface further provides a chemical gradient that retards diffusion of these large anions out of the electrode,thus facilitating more complete reaction.Reversible capacities up to 1,320mA h g −1are attained.The assembly process is simple and broadly applicable,conceptually providing new opportunities for materials scientists for tailored design that can be extended to many different electrode materials.Safe,low-cost,high-energy-density and long-lasting recharge-able batteries are in high demand to address pressing environmental needs for energy storage systems that can be coupled to renewable sources 1,2.These include wind,wave and solar energy,as well as regenerative braking from vehicular transport.With production of oil predicted to decline,and the number of vehicles and their pollution impact increasing globally,a transformation in transportation economy is inevitable given that we live in a carbon-constrained world.One of the most promising candidates for storage devices is the lithium–sulphur cell.Under intense scrutiny for well over two decades,the cell in its simplest configuration consists of sulphur as the positive electrode and lithium as the negative electrode 3,4.It differs from conventional lithium-ion cells,which operate on the basis of topotactic inter-calation reactions:reversible uptake of Li ions and electrons in a solid with minimal change to the structure.They typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode)that de/re-intercalates Li +at a high potential with respect to the carbon negative electrode (anode).As the reaction is topotac-tic at both electrodes,the charge storage capability is inherently limited to about 300mA h g −1for any prospective system,and maximum capacities observed so far are 180mA h g −1with high power characteristics having been reported 5.The lithium–sulphur cell operates quite differently.The redox couple,described by the reaction S 8+16Li ↔8Li 2S lies near 2.2V with respect to Li +/Li o ,a potential about 2/3of that exhibited by conventional positive electrodes 6.However,this is offset by the very high theoretical capacity afforded by the non-topotactic ‘assimilation’process,of 1,675mA h g −1.Thus,compared with intercalation batteries,Li–S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage).Values can approach 2,500W h kg −1or 2,800W h l −1on a weight or volume basis respec-tively,assuming complete reaction to Li 2S (refs 7,8).Despite its considerable advantages,the Li–S cell is plagued with problems that have hindered its widespread practical realization.These arise from the fact that all components of the cell must be addressed as a whole,including the interfaces betweenUniversity of Waterloo,Department of Chemistry,Waterloo,Ontario N2L 3G1,Canada.*e-mail:lfnazar@uwaterloo.ca.them.Sulphur or sulphur-containing organic compounds are highly electrically and ionically insulating 9.To enable a reversible electrochemical reaction at high current rates,the sulphur must maintain intimate contact with an electrically conductive additive.Various carbon–sulphur composites have been used for this purpose,but they have limitations owing to the scale of the contact area.Typical reported capacities are between 300and 550mA h g −1at moderate rates 10.To make a sulphur-containing cathode ionically conductive,liquid electrolytes are used that act not only as a charge transport medium but also as ionic conductors within the sulphur-containing cathode 11.This presents difficulties of electrolyte access.Another major hurdle is capacity degradation on repeated discharge–charge of the cell.This is mainly due to the high solubility of the polysulphide anions formed as reaction intermediates in both discharge and charge processes in the polar organic solvents used in electrolytes 12.During cycling,the polysulphide anions can migrate through the separator to the Li negative electrode whereupon they are reduced to solid precipitates (Li 2S 2and/or Li 2S),causing active mass loss.In addition,the solid product that extensively precipitates on the surface of the positive electrode during discharge becomes electrochemically irreversible,which also contributes to active mass loss 13.In response to these considerable challenges,novel advances in materials design such as new electrolytes 14–17and protective films for the lithium anode have been developed 18–binations of electrolyte modification,additives and anode protection have resulted in some promising results,although rates are not given 21.Much of the difficulty still remains at the cathode,where the lack of breakthroughs has led to some cell configurations in which all of the sulphides are solubilized (so-called ‘catholyte’cells)22.In the opposite approach,that is,to contain the sulphides,some interesting cathode developments have been reported recently 23–26.However,they still fall short of the mark for practical electrochemical performance.They include,for example,the fabrication of disordered mesoporous carbon/sulphur 50:50composites in conjunction with ionic liquid electrolytes;systems that achieve high initial capacity,but suffer extensive capacity0.20.40.60.81.01.21.4x in Li x S CMK¬3+S mixtureCMK¬3/S 155 °CV o l t a g e (V ) v e r s u s L i /L i +V o l t a g e (V ) v e r s u s L i /L i +Specific capacity (mA h g ¬1)2004006008001,0001,200Specific capacity (mA h g ¬1)02004006008001,0001,2001.52.02.53.01.52.02.53.0Sab cd2 µm2 µmFigure 1|SEM images of CMK-3/sulphur,and its electrochemical characterization.a ,Mixture of CMK-3and elemental sulphur before heating.b ,CMK-3/S heated at 155◦C,showing the disappearance of the sulphur mass indicated by the red rectangle in a .c ,d ,Comparison of the galvanostatic discharge–charge profiles of the first cycles of the carbon–sulphur composites shown in a ,b ,at a current rate of 168mAg −1.The marked increase in capacity in d is due to the encapsulation effect.fading posites with sulphur embedded in conducting polymers have shown some promising results 27.However,a large polarization was observed,resulting in a very low operating voltage that reduces the energy density of cells.The loading of active mass in the S-polymer composite is also limited (less than 55wt%)owing to the low surface area of the conducting polymer.Here,we demonstrate that cathodes based on nanostructured sulphur/mesoporous carbon materials can overcome these challenges to a large degree,and exhibit stable,high,reversible capacities (up to 1,320mA h g −1)with good rate properties and cycling efficiency.Our proof-of-concept studies are based on CMK-3,the most well-known member of the mesoporous carbon family,although they are not limited to this material.Highly ordered mesoporous carbons exhibit a uniform pore diameter,very high pore volume,interconnected porous structure and can exhibit high conductivity 28,29.They,and their oxide analogues 30,31,have attracted much attention recently as nanoscale electrode materials in Li batteries 32,33,as supercapacitors and as supports for proton-exchange-membrane fuel-cell catalysts 34.CMK-3was synthesized by a nanocasting method that uses silaceous SBA-15as a hard template.The resulting replica comprises an assembly of hollow 6.5-nm-thick carbon rods separated by empty 3–4-nm-wide channel voids 35.The channel space is spanned by carbon microfibres that prevent the collapse of the nano-architecture of the two-dimensional hexagonally ordered carbon rods.We tuned the synthesis of the CMK-3to produce a short rod-like morphology,to optimize access to the mesoporous channels 36.The CMK-3/sulphur composite was prepared following a simple melt-diffusion strategy.A 3:7weight ratio mixture of CMK-3and sulphur was heated just above the melting point of sulphur,where the viscosity is lowest 37.The melt is imbibed into the channels by capillary forces,whereupon it solidifies and shrinks to form sulphur nanofibres that are in intimate contact with the conductive carbon walls.The scanning electron microscopy (SEM)images in Fig.1reveal the changes in the mixture of CMK-3and sulphur before and after heating.The bulk sulphur evident in the SEM image of the composite on initial mixing (Fig.1a)largely disappears at 145◦C (see Supplementary Fig.S1),and completely disappears after heat treatment at 155◦C (Fig.1b).Full incorporation of sulphur into the channels of CMK-3occurs at this latter temperature.CMK-3and sulphur are both hydrophobic materials,which accounts for the ready absorption of sulphur into the channel structure.The filling of the carbon channels with sulphur is corroborated by the transmission electron microscopy (TEM)image shown in Fig.2a,along with the magnified image shown in Fig.2b.The fibres have a similar diameter to that of the channels of the mesoporous carbon (3.3nm),and a comparable diameter to the carbon nanorods that enclose them (6–7nm).The filling of the pores with sulphur,of similar density to carbon,is also evident from the decrease in contrast in relation to CMK-3itself (shown in the inset in Fig.2b).The sulphur and carbon elemental maps (Fig.2c,d)clearly demonstrate that sulphur is homogeneously distributed in the framework of the mesoporous carbon,with no significant fraction on the external surface.The marked diminution3 nm6.5 nmC Ka1_2S Ka1S meltS xtalx abc def30 nmFigure 2|TEM image and elemental maps of a CMK-3/S-155composite particle and schematic diagrams of the structure and redox processes.a ,CMK-3/S-155composite particle.b ,Image expansion corresponding to the area outlined by the red square in a ,where the inset shows the TEM image for pristine CMK-3at the same magnification.c ,d ,Corresponding carbon and sulphur elemental maps showing the homogeneous distribution of sulphur.e ,A schematic diagram of the sulphur (yellow)confined in the interconnected pore structure of mesoporous carbon,CMK-3,formed from carbon tubes that are propped apart by carbon nanofibres.f ,Schematic diagram of composite synthesis by impregnation of molten sulphur,followed by its densification on crystallization.The lower diagram represents subsequent discharging–charging with Li,illustrating the strategy of pore-filling to tune for volume expansion/contraction.of the X-ray diffraction (XRD)peak (low-angle diffraction pattern,Fig.3a)due to long-range order in CMK-3is further proof of pore-filling,which is the result of the decrease in the scattering contrast (Fig.3a)paring the wide-angle XRD patterns in Fig.3b,the well-resolved peaks corresponding to bulk crystalline sulphur completely disappear after sulphur impregnation,and thermogravimetric analysis (TGA;Supplementary Fig.S2)shows the composites range up to 70wt%sulphur.A schematic diagram illustrating the impregnation of the CMK-3with sulphur is shown in Fig.2e,showing the alignment of the channels in comparison with the inset of Fig.2b.Note that most of the sulphur is contained within the interior of the pore structure,as the particles span hundreds of carbon channels in width.The average CMK-3particle size is of the order of 1µm (Fig.1b).Table 1summarizes the physical characteristics of the CMK-3and the CMK-3/S composite derived from Brunauer–Emmett–Teller (BET)and conductivity measurements.After imbibition of the sulphur in the channels,the pore size of the CMK-3/S composite decreases markedly,indicating that the channels of CMK-3are partially filled.Along with the presence of residual micropores in the carbon wall structure 39,this allows ingress of electrolyte within the structure.Empty volume within the pores is also necessary to accommodate the uptake of Liions,I n t e n s i t y2 (°)I n t e n s i t yi abθ2 (°)θFigure 3|XRD patterns of CMK-3/S before and after heating.a ,Low-angle XRD patterns of a mixture of CMK-3and sulphur before heating (i)and after heating at 155◦C (ii).The disappearance of the first peak is due to the loss of contrast on sulphur imbibition.b ,Wide-angle XRD patterns of a mixture of CMK-3and sulphur before heating (i)and after heating at 155◦C (ii),showing the complete incorporation of crystalline sulphur within the framework.given by the reaction S +2Li →Li 2S,because of the lower density of Li 2S (1.66g cm −3)compared with sulphur.Note that the 70wt%sulphur/composite ratio is less than the theoretical limit of 79wt%sulphur/composite based on the pore volume of CMK-3(2.1cm 3g −1)and the density of liquidized sulphur (1.82g cm −3),and is precisely tuned for the volume expansion (see the Methods section).Using even lower S/carbon ratios provides less ‘stuffed’structures and extra porosity,but at the expense of reduced active mass.Most importantly,the electrical conductivity of the composites (∼0.2S cm −1for 70wt%sulphur/composite)is the same as its mesoporous carbon counterpart.The insulating sulphur merely occupies the empty channels in the mesoporous carbon and does not block the electrical current transporting paths.Three-dimensional,multiple electronic contacts are provided by the numerous carbon interconnects that span the channels,as illustrated schematically in Fig.2e,f (ref.35).Coin cells using a metallic Li anode were assembled to evaluate the materials.All of the capacity values in this article are calculated on the basis of sulphur mass.The first discharge–charge curve for a typical nanostructured CMK-3/S cathode is shown in Fig.1d alongside its SEM image,and is compared with a simple physical (unheated)mixture of 7:3weight ratio of sulphur and CMK-3in Fig.1c.The nanostructured composite exhibits an impressive capacity of 1,005mA h g −1.In contrast,the ‘macro-mixture’exhibited a reversible capacity of 390mA h g −1(on average between 300and 420mA h g −1),similar to that reported in the literature for C–S composites 10.The capacity of CMK-3/S was3.02.52.01.5V o l t a g e (V ) v e r s u s L i +/L i3.02.52.01.5V o l t a g e (V ) v e r s u s L i +/L i3006009001,2001,500Specific capacity (mA h g ¬1)S p e c i f i c c a p a c i t y (m A h g ¬1)CMK¬3/S at 55 °C with C/10 + C/10000.20.40.60.81.01.21.41.6x in Li x S Cycle numberCycle number200406080100S u l p h u r i n e l e c t r o l y t e /t o t a l s u l p h u r (%)1,0001,400abcFigure 4|Electrochemical characterization of PEG-coated CMK-3/S and comparison to reference materials.a ,Lower panel:galvanostaticdischarge–charge profile of PEG-modified CMK-3/S-155recorded at room temperature at 168mA g −1.The reversible capacity of 1,320mA h g −1at room temperature is very close to that obtained for unmodified CMK-3/S obtained at elevated temperature under ‘quasi-equilibrium’conditions shown in the upper panel (CMK-3/S-155recorded at 55◦C at 168mA g −1on discharge to 1.0V followed by quasi-equilibrium discharge at 16.8mA g −1).The slight overcharge in the latter case is due to dissolution of some polysulphide,which is minor even at these conditions.This also indicates that storage of the cell at partial or full discharge does not lead to significant capacity loss.b ,Cycling stability comparison of CMK-3/S-PEG (upper points,in black)versus CMK-3/S (lower points,in red)at 168mA g −1at room temperature.c ,Percentage of sulphur dissolution into the electrolyte from:the CMK-3/S-PEGcomposite cathode (black curve);from the CMK-3/S composite cathode (blue curve);a cathode made of a mixture of acetylene black carbon and sulphur with the exact same C /S ratio (red curve).highly reproducible over many cells.The coulombic efficiency for CMK-3/S in the first discharge–charge cycle is 99.94%without any overcharge,with virtually no irreversibility.This indicates that a very low fraction of polysulphide anions diffuse into the electrolyte.The polarization was decreased by more than a factor of three,owing to the greatly enhanced electrical contact achieved in the nanostructure.Further unequivocal proof of the effectiveness of the contact arises from experiments in which the degree of S incorporation was varied.Nanostructured composites (CMK-3/S-145)with the same S/C ratio,but heated at 145◦C instead of 155◦C result in less complete diffusion of sulphur into the channels because of the higher viscosity at the lower temperature.These composites showed less utilization of sulphur (capacity of 780mA h g −1)in the first discharge sweep (see Supplementary Fig.S3),and an irreversible capacity of 50mA h g −1on plete imbibition prevents sulphur agglomerates on the externalsurface of the mesoporous framework that would have poorer electrical wiring of the conductive carbon phase.These results are superior to those reported for sulphur in contact with multi-walled carbon nanotubes.Such composites exhibit lower capacities and a large electrochemical hysteresis 23.Although the sulphur is apparently confined in the carbon,the contact is limited owing to the relatively large diameter (∼50nm)of the multi-walled carbon nanotubes,and hence of the sulphur fibres within them.Thus,the efficiency of electron transfer to the sulphur mass and accessibility to the Li +electrolyte has a vitally important role in determining the electrochemical behaviour.As seen in Fig.1d,there are two plateaux in the discharge process.The first,which contributes a minor part to the overall capacity from 2.4to 2.0V,corresponds to the conversion from elemental sulphur (S 8)to Li polysulphide anions (Li 2S x ;where x is typically 4–5).The kinetics of this reaction is fast 40.The second plateau atHeat flow (W g ¬1)W e i g h t (%)Temperature (°C)Figure 5|TGA of PEG-modified CMK-3.TGA and differential scanning calorimetry curves recorded in air with a heating rate of 20◦C min −1,for PEG-CMK-3(solid lines),compared with PEG itself (dashed lines),showing the shift to higher temperature of the PEG release on bonding to the CMK-3framework.around 2.0V is due to the conversion of polysulphides to Li 2S 2and then to Li 2S,which occurs at a much slower rate.As we achieve a nominal reversible capacity of Li 1.2S in the nanostructured composite,we wanted to explore the limitations to full conversion.To gain a measure of the reversible capacity under conditions where the kinetics should be a minimal concern,we carried out discharge of the CMK-3/S cathode at 55◦C at 168mA g −1to a cutoff of 1.0V,and allowed the voltage to relax to equilibrium.We then switched the discharge current to a rate of 16.8mA g −1to the end of discharge,and completed charge at 168mA g −1.The electrochemical profile is presented in Fig.4a (upper panel).Under these close-to-equilibrium conditions of full discharge,we achieve a reversible capacity of 1,400mA h g −1—84%of the theoretical capacity (1,675mA h g −1)—indicating that indeed,the kinetics of the last reaction step has a role in capacity limitation.The other factor could be a transport problem.There is progressively more limited accessibility of Li +ions and electrolyte to the sulphur mass towards the end of discharge because the pores become filled with insoluble Li x S (x =1–2)—even though at 70wt%sulphur loading,there is sufficient space for the volume expansion based on the conversion of S to Li 2S.However,we observed that in doubling the rate from 168to 336mA g −1(equivalent to C/5rate),the capacity is reduced by only a small amount to 930mA h g −1(see Supplementary Fig.S4).The mesoporous carbon clearly performs very well as a sulphur container.This is apparent from the small degree of overcharge even under rigorous (55◦C;C/100discharge)conditions as shown in Fig.4a.The complete lack of a sharp minimum in the discharge curve between the two plateaux,as observed by others and ascribed to supersaturation of the electrolyte with S 2−(refs 21,41),is also indicative of the strong extent of sulphide containment in our case.Experiments were carried out to evaluate the degree of self-discharge,by taking the cell to a voltage of 2.1V,holding it at the open-circuit voltage for 24h and then completing discharge.The discharge capacity after relaxation was 5%less than the cell taken to full discharge without the open-circuit voltage step.However,this suggests that the framework still allows for some egress of dissolved sulphur species.We propose that the complex inner pathway and porous,absorptive carbon greatly retard the diffusion of the bulky polysulphide anions out from the channels into the electrolyte,butcannot entirelyprevent it.This is evident by the very slow capacity fading shown in Fig.4b(upperred points).To further trapthe highly polar polysulphide species,we adjusted the hydrophilicity of the carbon external surface afterabcd300 nmFigure 6|Changes in surface morphology of CMK-3/S-155versusPEG-modified CMK-3/S-155on cycling.a ,b ,SEM images of CMK-3/S-155before (a )and after (b )the 15th charge.c ,d ,SEM images of PEG-modified CMK-3/S before (c )and after (d )the 15th charge.Images show the effects of ‘polymer protection’in inhibiting surface deposition.sulphur imbibition by functionalizing the surface with polyethylene glycol (PEG)chains of varying molecular weight.The attachment of the PEG to CMK-3is evident by TGA (Fig.5).The release of the PEG tethered to the CMK-3occurs at 50◦C higher than in PEG itself owing to the ester bonds.The discharge–charge profile of CMK-3/S-PEG is shown in Fig.4a (lower panel).Not only is the initial discharge capacity increased to 1,320mA h g −1(approaching the ‘equilibrium’limit for CMK-3/S of 1,400mA h g −1),and the polarization decreased to low values,but no fading is observed in the second 10cycles and the capacity is stabilized at 1,100mA h g −1on cycling (Fig.4b,upper black points).The entrapment of sulphur active mass on cycling in the polymer-modified CMK-3/S composite is demonstrated in Fig.4c.To measure the degree of sulphur retention in the cathode,a 1.0M LiPF 6solution in a sulphur-free solvent,tetra(ethylene glycol)dimethyl ether (TEGDME),was used as the electrolyte.Glyme solvents are known for their excellent ability to dissolve polysulphides,and hence represent an ‘aggressive’pared with the cathode made of a mixture of sulphur and acetylene black that loses 96%of the total active mass into the electrolyte after 30cycles,the polymer-modified composite shows significant retention of sulphur.Only 25%of the total active mass is solubilized in the electrolyte after 30cycles.The polysulphide retention is also improved in relation to CMK-3/S.We believe that the effect of the PEG-functionalized surface is twofold.First,it serves to trap the polysulphide species by providing a highly hydrophilic surface chemical gradient that preferentially solubilizes them in relation to the electrolyte.Second,by limiting the concentration of the polysulphide anions in the electrolyte,the redox shuttle mechanism is curtailed to a large degree.Deposition of insoluble sulphur species on the surface of the Li electrode and formation of irreversible Li 2S on the cathode surface are strongly inhibited.The last point is clearly demonstrated in SEM images of the PEG-functionalized CMK-3/S cathode before and after cycling,which exhibit very little change in surface morphology (Fig.6),compared with CMK-3/S,which clearly shows precipitation of insoluble products on the surface of the mesoporous carbon particles.In summary,we demonstrate that the strategy illustrated here provides a versatile route to nanostructured polymer-modified mesoporous carbon–sulphur composites that display all of the benefits of confinement effects at a small length scale.Intimate contact of the insulating sulphur and discharge-product sulphides with the retaining conductive carbon framework at nanoscaledimensions affords excellent accessibility of the active material. The carbon framework not only acts as an electronic conduit to the active mass encapsulated within,but also serves as a mini-electrochemical reaction chamber.The entrapment ensures that a more complete redox process takes place,and results in enhanced utilization of the active sulphur material.This is vital to the success of all conversion reactions to ensure full reversibility of the back-reaction.The polymer coating on the external surface of the composite further helps retard diffusion of polysulphide out of the cathode structure,minimize the loss of the active mass in the cathode and improve the cycling stability.The composite materials reported here can supply up to nearly80%of the theoretical capacity of sulphur(1,320mA h g−1),representing more than three times the energy density of lithium transition-metal oxide cathodes,at reasonable rates with good cycling stability.In our laboratory,mesoporous carbon frameworks with various wall thicknesses,conductivities and connectivities have recently been prepared to take advantage of structural and electronic variation of the constraining support.The three-dimensional variants such as CMK-1and CMK-8are particularly promising in this respect42. We will report those results in a forthcoming paper.Owing to the flexibility of the method,the high capacity of the carbon for active material incorporation and facile functionalization of the surface,we believe that a wide variety of nanostructured‘imbibed’composites could find broad application in many areas of materials science,not only as advanced electrode materials that rely on assimilation and conversion reactions.MethodsSynthesis.For the synthesis of SBA-15with controlled morphology43,2g of Pluronic P123(EO20PPO70EO20)was dissolved in60ml of2M HCl at38◦C. Tetraethylorthosilicate(4.2g)was added to the above solution with vigorous stirring.The mixture was stirred for only6min and remained quiescent for24h at38◦C.The mixture was subsequently heated at100◦C for another24h in an autoclave.The as-synthesized SBA-15with short-rod morphology was collected by filtration,dried and calcined at550◦C in air.A nanocasting method was used to fabricate CMK-3from SBA-15as a hard template44.Sucrose(1.25g)was dissolved in5.0ml of water containing0.14g H2SO4.Surfactant-free SBA-15(1.0g)was then dispersed in the above solution and the mixture was sonicated for1h;heated at100◦C for12h and at160◦C for another12h.The impregnation process was repeated once with another5.0ml aqueous solution containing0.8g sucrose and 0.09g H2SO4.The composite was completely carbonized at900◦C for5h in an argon atmosphere.To remove the SBA-15silica template,the composite was stirred in a5%HF solution at room temperature for4h,although NaOH can also be used to dissolve the silica.The CMK-3/S nanocomposite was prepared following a melt-diffusion strategy.CMK-3(1.0g)and sulphur(2.33g)were ground together,and heatedto155◦C.The weight ratio of sulphur/carbon was adjusted to be equal to or less than7:3,to allow for expansion of the pore content on full lithiation to Li2S.For example,1.0g of CMK-3can accommodate3.486g of Li2S(1.66g cm−3(density of Li2S)×2.1cm3g−1,the pore volume of the CMK-3),which corresponds to a maximum of2.425g of sulphur.To prepare the CMK-3/S-PEG composite,CMK-3was first functionalized with carboxylic groups by oxidization treatment in concentrated HNO3solution for half an hour at80◦C,before incorporation of the sulphur.To tether the PEG chains to the surface of the CMK-3/S composite,the composite was dispersedin a PEG aqueous solution and the solution was heated at58◦C and stirred continuously overnight to ensure complete reaction of the carboxylic groups on the carbon particles with the hydroxyl groups on the PEG.The mixture was sonicated for20min to completely remove physically absorbed PEG on the composite,and the CMK-3/S-PEG composite was collected by filtration and dried. Characterization.X-ray diffraction patterns at low-angle(0.75◦to4◦2θ)and wide-angle(from10◦to80◦2θ)were collected on a D8-ADVANCE powderX-ray diffractometer operating at40kV and30mA and using Cu-Kαradiation (λ=0.15406nm).Nitrogen adsorption and desorption isotherms were obtained using a Micromeritics Gemini2735system at−196◦C.Before measurement of CMK-3,the sample was degassed at150◦C on a vacuum line following a standard protocol.It was not possible to carry this out for CMK-3/S owing to the volatility of the sulphur,and so no pretreatment was used.The BET method was usedto calculate the surface area45.The total pore volumes were calculated fromthe amount adsorbed at a relative pressure of0.99.The pore size distributions were calculated by means of the Barrett–Joyner–Halenda method applied to the desorption branch46.As the mesopores of CMK-3/S are decreased to micropores on(partial)filling with sulphur,the possibility of water entrapment,and/or pore blockage means that the values represent lower estimates.The morphology of the sulphur/CMK-3composites were examined by SEM using a LEO1530field-emission SEM instrument or a Hitachi S-5200 instrument.TEM was carried out on a Hitachi HD-2000STEM.Conductivity measurements were carried out at room temperature using the four-point method.Sample bars for the measurement were cut from the pellets and then cold pressed using a force of45kN.Elemental analyses were carried out at M-H-W Laboratories,Phoenix,USA.Electrochemistry.Positive electrodes were comprised84wt%CMK-3/S composite,8wt%Super-S carbon and8wt%poly(vinylidene fluoride)binder. The cathode materials were slurry-cast from cyclopentanone onto a carbon-coated aluminium current collector(Intelicoat).The electrolyte is composed of a1.2M LiPF6solution in ethyl methyl sulphone47.Lithium metal foil was used as the counter electrode.The equivalent current density for the168mA g−1rate is0.19 and0.37mA cm−2for the336mA g−1rate.To measure the degree of sulphur retention in the cathode,a1.0M LiPF6solution in TEGDME was used as the electrolyte.Cathodes comprising CMK-3/S-PEG were compared with simple mixtures of sulphur and acetylene black at the exact same S/C ratio.We used large Swagelok-type cells that accommodate a sufficient excess of the electrolyte to dissolve sulphur species.Swagelok cells were disassembled and immersed into TEGDME to completely extract sulphur species from the electrolyte.Sulphur analysis was carried out by Galbraith Laboratories(Tennessee,USA).Received10September2008;accepted17April2009; published online17May2009References1.Winter,M.&Brodd,R.Batteries,fuel cells and supercapacitors.Chem.Rev.104,4245–4269(2004).2.Bruce,P.G.Energy storage beyond the horizon:Rechargeable lithium batteries.Solid State Ion.179,752–760(2008).3.Rauh,R.D.,Abraham,K.M.,Pearson,G.F.,Surprenant,J.K.&Brummer,S.B.A lithium/dissolved sulfur battery with an organic electrolyte.J.Electrochem.Soc.126,523–527(1979).4.Shim,J.,Striebel,K.A.&Cairns,E.J.The lithium/sulfur rechargeable cell.J.Electrochem.Soc.149,A1321–A1325(2002).5.Kang,K.,Meng,Y.S.,Bréger,J.,Grey,C.P.&Ceder,G.Electrodes withhigh power and high capacity for rechargeable lithium batteries.Science311, 977–980(2006).6.Peled,E.&Yamin,H.Lithium/sulfur organic battery.Prog.Batteries Sol.Cells5,56–58(1984).7.Chu,M.-Y.Rechargeable positive Patent US5686201(1997).8.Peramunage,D.&Licht,S.A solid sulfur cathode for aqueous batteries.Science261,1029–1032(1993).9.Dean,J.A.(ed.)Lange’s Handbook of Chemistry3rd edn,3–5(McGraw-Hill,1985).10.Cunningham,P.T.,Johnson,S.A.&Cairns,E.J.Phase equilibria inlithium–chalcogen systems:Lithium–sulfur.J.Electrochem.Soc.119,1448–1450(1972).11.Choi,J.-W.et al.Rechargeable lithium/sulfur battery with suitable mixedliquid electrolytes.Electrochim.Acta52,2075–2082(2007).12.Rauh,R.D.,Shuker,F.S.,Marston,J.M.&Brummer,S.B.Formationof lithium polysulfides in aprotic media.J.Inorg.Nucl.Chem.39,1761–1766(1977).13.Cheon,S.-E.et al.Rechargeable lithium sulfur battery II.Rate capability andcycle characteristics.J.Electrochem.Soc.150,A800–A805(2003).14.Shin,J.H.&Cairns,E.J.Characterization of N-methyl-N-butylpyrrolidiniumbis(trifluoromethanesulfonyl)imide-LiTFSI-tetra(ethylene glycol)dimethyl ether mixtures as a Li metal cell electrolyte.J.Electrochem.Soc.155,A368–A373(2008).15.Yuan,L.X.et al.Improved dischargeability and reversibility of sulfur cathodein a novel ionic liquid mun.8,610–614(2006).16.Ryu,H.-S.et al.Discharge behavior of lithium/sulfur cell with TEGDME basedelectrolyte at low temperature.J.Power Sources163,201–206(2006).17.Wang,J.et al.Sulfur-mesoporous carbon composites in conjunction with anovel ionic liquid electrolyte for lithium rechargeable batteries.Carbon46, 229–235(2008).18.Chung,K.-I.,Kim,W.-S.&Choi,Y.-K.Lithium phosphorous oxynitride as apassive layer for anodes in lithium secondary batteries.J.Electroanal.Chem.566,263–267(2004).19.Visco,S.J.,Nimon,Y.S.&Katz,B.D.Ionically conductive composites forprotection of active metal Patent7,282,296,October16(2007). 20.Skotheim,T.A.,Sheehan,C.J.,Mikhaylik,Y.V.&Affinito,J.Lithium anodesfor electrochemical patent7247,408,July24(2007).21.Akridge,J.R.,Mikhaylik,Y.V.&White,N.Li/S fundamental chemistry andapplication to high-performance rechargeable batteries.Solid State Ion.175, 243–245(2004).。

[Article]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.-Chim.Sin.2013,29(X),0001-0009Month Received:July 11,2012;Revised:September 26,2012;Published on Web:September 27,2012.∗Corresponding authors.LIN Rui,Email:ruilin@;Tel:+86-21-69583837.MA Jian-Xin,jxma@;Tel:+86-21-69583837.The project was supported by the National Natural Science Foundation of China (21276199),Program for Young Excellent Talents in TongjiUniversity,China (2006KJ022),Shanghai Leading Academic Discipline Project,China (B303),and Energy Saving and Environmental Protection Automobile Innovation “111Project ”(B08019).国家自然科学基金(21276199),同济大学青年优秀人才基金(2006KJ022),上海市重点学科建设项目(B303)和节能与环保汽车创新引智基地“111计划”(B08019)资助ⒸEditorial office of Acta Physico-Chimica Sinicadoi:10.3866/PKU.WHXB 201209272用于燃料电池Co@Pt/C 核壳结构催化剂的制备及表征曹春晖林瑞*赵天天黄真马建新*(同济大学新能源汽车工程中心,上海201804;2同济大学汽车学院,上海201804)摘要:采用两步化学还原法制备了Co@Pt/C 电催化剂,并在还原气氛下对催化剂进行热处理.通过高分辨透射电镜(HR-TEM)和X 射线光电子能谱(XPS)等技术对催化剂的微观结构和形貌进行表征.结果表明:形成的Co@Pt/C 催化剂具有核壳结构,金属纳米颗粒均匀负载于碳上,其粒径分布范围较窄;热处理对催化剂的结构和形貌有较大影响.利用循环伏安(CV)法和线性伏安扫描(LSV)法表征催化剂的电化学活性、氧还原反应(ORR)动力学特性及耐久性.制备的Co@Pt/C 催化剂在电解质溶液中表现出良好的电化学性能,核壳结构的形成有助于提高Pt 的利用率.动力学性能测试表明催化剂的ORR 反应以四电子路线进行.相比于合金催化剂,核壳结构催化剂的耐久性和稳定性有很大程度的改善.关键词:燃料电池;核壳结构Co@Pt/C 催化剂;电化学性能;反应动力学;耐久性中图分类号:O643Preparation and Characterization of Core-ShellCo@Pt/C Catalysts for Fuel CellCAO Chun-HuiLIN Rui *ZHAO Tian-TianHUANG ZhenMA Jian-Xin *(Clean Energy Automotive Engineering Center,Tongji University,Shanghai 201804,P .R.China ;School of Automotive Studies,Tongji University,Shanghai 201804,P .R.China )Abstract:Co@Pt/C core-shell catalysts have been synthesized by a two-step chemical reduction method,followed by heat treatment in a H 2and N 2mixture.High resolution transmission electron microscopy (HR-TEM)and X-ray photoelectron spectroscopy (XPS)techniques were used to characterize the catalyst microstructure and morphology.The results indicate that the core-shell structure of Co rich in core and Pt rich in shell is formed and the nano-particles are highly dispersed on the surface of the carbon support.Heat treatment affects the structure and morphology of the catalysts.The electrocatalytic performance,kinetic characteristics of O 2reduction reaction (ORR),and durability of the catalysts were measured by cyclic voltammetry (CV)and linear sweep voltammetry (LSV)techniques.It was found that the formation of the core-shell structure is favorable for improving the performance and utilization of Pt.The Co@Pt/C catalyst mechanism proceeds by an approximately four-electron pathway in acid solution,through which molecular oxygen is directly reduced to pared with alloy catalysts,the formation of the core-shell structure obviously improves the catalyst durability.Key Words:Fuel cell;Core-shell structure Co@Pt/C catalyst;Electrochemical performance;Reaction kinetic;Durability0001Acta Phys.⁃Chim.Sin.2013V ol.291引言质子交换膜燃料电池(PEMFC)因具备能量转换效率高、可靠性高、无污染、组装和维修方便等优点而成为交通工具的理想替代动力系统之一.1但制约燃料电池大规模应用的两大障碍,即成本和耐久性问题尚未完全解决.质子交换膜燃料电池常用的催化剂是负载于炭黑的铂(Pt),它具有活性高、化学稳定性好的特点.2而Pt的储量却极为有限,稀缺性决定了其成本会随着燃料电池汽车的推广而急剧上升.因此,提高铂的利用率、降低铂的载量显得尤为重要.在纳米尺度上对金属催化剂颗粒的结构进行理性设计和化学裁剪可显著改变金属催化剂的物理化学性质,获得性能更好的催化剂.其中核壳结构(记为“核@壳”)纳米金属颗粒具有特殊的电子结构及表面性质,因而在催化等领域的应用日益受到重视.3研究证明,以Pt为壳层的M@Pt(M为Co,Ni, Cu等过渡金属)核壳型催化剂对氧还原具有更高的催化活性.一方面,Pt壳层可防止过渡金属的溶解,防止电催化活性的衰减,增加Pt的比表面积,提高Pt的利用率;另一方面,核壳催化剂的核/壳之间可产生相互作用,添加过渡金属引起Pt氧还原反应(ORR)催化性能提高的机理主要是:减小了Pt-Pt之间的距离(几何效应),形成了具有更多5d轨道空位的电子结构,导致氧的π电子向Pt表面转移的增加(电子效应).4核壳结构燃料电池催化剂的研究和应用已有大量的报道,5-14其中Co@Pt催化剂表现出较高ORR活性.有报道9通过置换和沉积方法成功地制备出负载碳的Pt-Co核壳结构催化剂,发现该催化剂的ORR活性是Pt/C催化剂的2-4倍.Wu等13用还原法制备了Co@Pt催化剂,通过X射线衍射(XRD)、透射电子显微镜(TEM)和电化学测试技术证明了核壳结构的形成.在电化学测试中该催化剂表现出优于Pt/ C催化剂的ORR活性.Lee等14用热分解和后续还原方法制备了负载碳的Co@Pt催化剂,并考察了酸洗对催化剂性能的影响,发现Co@Pt具有良好的ORR 催化剂性能,且经过适当酸处理的Co@Pt催化剂表现出更好的ORR活性.但目前对Co@Pt催化剂应用于燃料电池并且其耐久性的考察较少,因此深入地研究其结构、性能及耐久性显得很有必要.本研究采用两步化学还原法制备了系列Co@Pt/C催化剂,用透射电镜(TEM)、X射线光电子能谱(XPS)分析以及电化学技术对其进行形貌和电化学性能、ORR动力学特性进行表征.考察了催化剂在电解质溶液中的稳定性及耐久性.2实验部分2.1Co@Pt/C催化剂的制备实验所用的碳载体为XC-72碳黑(美国Cabot 公司,比表面积为235m2·g-1).氯化钴(CoCl2)、氯铂酸(H2PtCl6)、氢氧化钠(NaOH)、乙醇(C2H5OH)、乙二醇(EG)、聚乙烯吡咯烷酮(PVP)、盐酸(HCl)、硝酸(HNO3)和异丙醇(C3H8O)均为分析纯,国药集团化学试剂有限公司;质量分数为5%的Nafion溶液购自美国Dupont公司.碳黑的预处理:XC-72碳黑用2.0mol·L-1盐酸在120°C回流处理4h,以去除硫等无机杂质;用5.0mol·L-1硝酸在120°C回流进行表面氧化处理4 h,以增加表面的官能团;过滤,用去离子水洗涤, 100°C真空干燥12h.本研究采用两步还原法制备Co@Pt/C催化剂.第一步,通过还原CoCl2·6H2O制备Co粒子.15在0.1 mol·L-1CoCl2乙醇溶液中加入10mL PVP乙醇溶液作为稳定剂,搅拌条件下缓慢加入新制备的0.05 mol·L-1NaBH4(97%)乙醇溶液到上述混合溶液中.在制备过程中通入氮气,以防止还原出的Co纳米粒子被氧化.加入NaBH4乙醇溶液反应30min后,对产物用去离子水离心水洗4-5次,得到Co纳米粒子备用.第二步,采用乙二醇还原H2PtCl6和Co的混合悬浮液制备Co@Pt/C催化剂.将一定量的XC-72碳黑载体分散在30mL乙二醇中,超声振荡2h备用.上一步制备的Co粒子分散在20mL乙二醇中,溶液移入三颈烧瓶中,搅拌条件下加入一定量PVP作为稳定剂,搅拌30min后逐滴加入0.03862mol·L-1 H2PtCl6乙二醇溶液.混合均匀后,用2mol·L-1 NaOH乙二醇溶液将混合液pH值调为12.升温到120°C,回流搅拌反应2h后加入制备好的碳浆,再于120°C下搅拌1h后,让其自然冷却.上述过程中均以氮气为保护气.待悬浮液温度降至室温后,加入5mol·L-1的盐酸调节pH值到3.过滤所得产物,用大量去离子水清洗,于80°C真空干燥8h,即制得Co@Pt/C催化剂.以同样的方法制备了Co/C粒子作为对比.为进一步还原Pt和Co及去除残留的稳定剂,对制得的催化剂在氢气气氛(5%H2+95%N2,400002曹春晖等:用于燃料电池Co@Pt/C核壳结构催化剂的制备及表征No.XmL·s-1)、300°C条件下进行2h还原处理.文中凡经过还原处理的催化剂标记为Co@Pt/C(reduced),未经还原处理的催化剂标记为Co@Pt/C.在制备的催化剂中,金属(Pt+Co)质量占催化剂总质量的40%. Pt和Co的原子比分别为3:1和1:1.2.2催化剂结构表征采用JEM-2010透射电子显微镜(日本电子光学公司)对催化剂的金属粒子晶体大小、颗粒度和颗粒分布进行高分辨透射电子显微镜观察.加速电压为200kV,采用铜网微栅样品架,样品在无水乙醇中超声分散,然后负载到样品架上制成电镜样品.催化剂表面元素通过PHI Model5700型X光电子能谱仪(美国PHI公司)进行分析.辐射源为Mg Kα(hν=1253.6eV),电子结合能用碳(C1s=284.6eV)校正.2.3催化剂电化学表征催化剂的电催化性能评价在上海辰华CHI104P电化学工作站上进行,采用美国PINE公司的AFMS-LXF型旋转圆盘电极(RDE)及装置.测试条件为0.1mol·L-1HClO4的电解质,温度25°C.参比电极为可逆氢电极(RHE),采用铂丝作为对电极,涂有催化剂层的玻碳电极为工作电极.电极催化层的制备过程如下:将2.0mg催化剂加入5mL 称量瓶中,然后加入1mL质量比为1:30的5% Nafion溶液和甲醇溶液,混合后超声30min.分散均匀后用微量进样器抽取混合液10μL,滴加到玻碳电极表面,干燥后进行测试.在电化学测试中,涂覆在电极表面上的催化剂质量相等.循环伏安(CV)测试的扫描范围为0.05-1.15V (vs RHE,下同),扫描速率为50mV·s-1.在进行CV 测试之前,HClO4溶液中先通N2半小时,以驱赶出溶液中的O2.计算CV曲线中氢吸脱附峰的面积,原子量级平滑的Pt催化剂表面的吸收电荷为210μC·cm-2,据此可以推算出被测催化剂中Pt的电化学活性面积(ECSA).16为测试催化剂的耐久性,对催化剂进行500次CV循环扫描,实验条件同上.线性伏安测试(LSV)的扫描范围为0.05-1.2V (vs RHE,下同),扫描速率为5mV·s-1,转速为1600 r·min-1.LSV扫描之前,HClO4溶液中通O2半小时,以使溶液中达到O2饱和.为计算ORR的动力学参数,分别在300、600、800、1200、1600、2000和2400 r·min-1下进行LSV扫描,扫描范围为0.05-1.2V,扫描速率为5mV·s-1.3结果与讨论3.1HR-TEM测试图1所示是未经热还原和经过热还原的Co@Pt(1:3)/C催化剂的高分辨TEM(HR-TEM)照片.可以看出,催化剂颗粒在碳载体上分布均匀,这说明该制备方法可以得到分散均匀、粒径较小的催化剂颗粒.图中显示明显的亮度对比,表明在碳载体上附有不同的元素.可在图片中辨别出Co@Pt(1: 3)/C催化剂中的Pt和Co.此外,从图可以清楚地看到,经过热还原处理后催化剂的粒径有所增大,晶格更加明显.未经热处理的催化剂粒径在2.5nm左右,而经过300°C还原热处理后的催化剂粒径有所增大,至3.0nm左右.3.2XPS测试为了分析制备样品中元素价态分布,对制备的样品做了XPS测试,结果如图2所示.从图中可以清晰地观察到分别对应于Co2p,Pt4f,C1s和O1s的四个光电子峰.图2(a)是Co@Pt(1:1)/C(reduced)和Co@Pt(1: 3)/C(reduced)催化剂的XPS全谱,图2(b)、图2(c)、图2(d)和图2(e)分别是催化剂的Pt(4f)和Co(2p)的XPS谱图.4f亚层电子自旋轨道耦合引起能级分裂而产生两个不同的峰,分别反映出4f5/2和4f7/2的简并度,峰的相对强度比为0.75.17通过XPS图谱,计算Co@Pt(1:1)/C(reduced)和Co@Pt(3:1)/C(reduced)催化剂的4f5/2和4f7/2峰相对强度比分别为0.84和0.92.4f5/2和4f7/2峰的相对强度比偏离标准值的原因可能是由于有其他物质的存在,如添加的Co元素、载体碳,使其自旋耦合效应受到影响.根据标准XPS谱,17Pt(0)和Co(0)的结合能分别图1Co@Pt(1:3)/C(reduced)和Co@Pt(1:3)/C催化剂的HR-TEM图Fig.1HR-TEM images of Co@Pt(1:3)/C(reduced)andCo@Pt(1:3)/C electrocatalyst(a)Co@Pt(1:3)/C(reduced);(b)Co@Pt(1:3)/C.The ratio in bracket is atomic ratio of Co and Pt.0003Acta Phys.⁃Chim.Sin.2013V ol.29是71.2、74.5eV 和778.3、793.7eV .实验所制备的催化剂样品中Pt(0)所对应的结合能的值有所增大,表明其中部分Pt 以Pt 氧化物的形态存在.可能是空气中的氧气对Pt 纳米粒子氧化所致.对XPS 结果的Pt 4f 峰进行拟合,其中的最强峰可归属于零价Pt,第二强峰可以归属为2价Pt 的存在(Pt(II),PtO 和Pt(OH)2).可以看出Pt 4f 7/2和Pt 4f 5/2的峰最为明显,表明在这两种催化剂上的Pt 主要以还原态形式存在.在XPS 全谱图中也发现了对应于Co(0)2p 的较弱的峰.通过XPS 数据,可以计算得到Co@Pt(1:1)/C (reduced)催化剂中Pt 和Co 的含量分别为1.63%和0.24%,而在Co@Pt(1:3)/C (reduced)催化剂中该比例分别为1.75%和0.19%.同时也发现Co(0)2p 峰变弱的同时,Pt (0)4f 的峰增强.这些结果表明,催化剂表面的Co 大部分被制备过程中沉积的Pt 所包覆.3.3电化学性能测试Co@Pt/C 和商业Pt/C 催化剂的电化学活性通过CV 和LSV 技术表征.催化剂的CV测试在氮气气图2Co@Pt (1:1)/C (reduced)和Co@Pt (1:3)/C (reduced)系列催化剂的XPS 图谱Fig.2XPS spectra for as-prepared Co@Pt(1:1)/C (reduced)and Co@Pt(1:3)/C (reduced)catalysts(a)full spectrum,(b)Pt 4f of Co@Pt(1:1)/C (reduced),(c)Co 2p of Co@Pt(1:1)/C (reduced),(d)Pb 4f of Co@Pt(1:3)/C (reduced),and(e)Co 2p of Co@Pt(1:3)/C (reduced)catalysts.The horizontal axe represents the binding energy corrected by that of C 1s .0004曹春晖等:用于燃料电池Co@Pt/C 核壳结构催化剂的制备及表征No.X氛、0.1mol ·L -1HClO 4中进行,同时也进行了Co/C 粒子的CV 测试.测试结果如图3所示.氢吸脱峰出现在0-0.3V ,氧的还原氧化峰位于0.7-0.8V 之间,氢吸附峰的形状与商业Pt/C 催化剂类似.而对于Co/C 粒子,基本没有氢吸脱附峰出现.通过氢吸脱附峰,可以计算得到Pt/C 、Co@Pt(1:3)/C (reduced),Co@Pt(1:1)/C (reduced),Co@Pt(1:3)/C 和Co@Pt(1:1)/C 催化剂的ECSA 分别是52.2、53.1、50.4、45.2和29.6m 2·g -1.其中Co@Pt(1:3)/C(reduced)催化剂的ECSA 稍高于商用催化剂Pt/C (JM),可见Co@Pt 核壳结构的形成有助于Pt 利用率的提高.从计算结果中也可以看出,催化剂中Co 和Pt 的比例对其性能有很大影响,通过调节该比例能使性能得到明显的提高.Co:Pt 为1:1时,热还原前后其ECSA 分别为29.6和45.2m 2·g -1,当该比例变为1:3时,热还原前后催化剂ECSA 分别提高至为50.4和53.1m 2·g -1.为研究催化剂的ORR 性能,对其进行LSV 测试,结果如图4所示.Co@Pt/C 催化剂的ORR 性能很大程度上受Co 和Pt 比例的影响,Co@Pt(1:3)/C(reduced)催化剂表现出最好的ORR 性能,甚至要优于Pt/C (JM)催化剂.相比于Pt/C 催化剂,Co@Pt (1:3)/C (reduced)催化剂LSV 曲线半波电位有8mV 的正移.Co@Pt(1:3)/C(reduced)催化剂ORR 性能的提升可能是因为形成的核壳结构诱导产生了几何效应,这使得在颗粒表面出现更多的Pt 催化活性位.此外,催化剂Pt -Pt 原子间距也可能有所变化,这也导致其吸附和还原氧的能力.18有文献通过XRD 数据计算得到的Pt-Co 催化剂晶格常数要小于纯Pt 的,Pt -Pt 相邻原子之间的距离的减小被认为有助于提升催化剂的ORR 性能.19,20Co@Pt(1:3)/C 催化剂LSV 曲线的半波电位为0.803V ,经过热还原处理后,Co@Pt(1:3)/C(reduced)催化剂的半波电位有所提升,达到0.842V .通过LSV 曲线可以发现,热处理之后的催化剂具有更好的ORR 性能.制备的系列催化剂的ORR 性能的排序为Co@Pt(1:3)/C (reduced)>Pt/C (JM)>Co@Pt (1:3)/C.Co@Pt/C 催化剂的ORR 动力学性质的测试结果如图5所示.将Co@Pt(1:3)/C (reduced)涂覆在RDE 上,在O 2饱和的0.1mol ·L -1HClO 4溶液中,进行LSV 测试.氧还原的动力学性质用Koutecký-Levich 关系式分析,分别以图5中0.45、0.60和0.75图4制备的Co@Pt/C 和Pt/C (JM)催化剂的线性伏安扫描(LSV)曲线Fig.4LSV curves of O 2reduction on Co@Pt/C andcommercial Pt/C (JM)catalystsin O 2-saturated 0.1mol ·L -1HClO 4at room temperature;sweep rate:5mV ·s -1,rotation speed:1600r ·min -1图3商用Pt/C (JM)和Co@Pt/C 催化剂的CV 曲线Fig.3Cyclic voltammogram (CV)curves of as-preparedCo@Pt/C and commercial Pt/C (JM)catalystsin N 2-saturated 0.1mol ·L -1HClO 4;room temperature;sweep rate:50mV ·s -1图5不同转速下Co@Pt(1:3)/C (reduced)在0.1mol ·L -1HClO 4溶液中氧化还原性质的RDE 测试结果Fig.5Rotation disk electrade (RDE)measurements for ORR on Co@Pt(1:3)/C (reduced)in 0.1mol ·L -1HClO 4with different speedssweep rate:5mV ·s -10005Acta Phys.⁃Chim.Sin.2013V ol.29V 下得到的电流倒数和转速平方根的倒数为对应关系作图,结果如图6所示,得到一组斜率相近的平行直线.以下方程中,21斜率(1/B )直接与氧还原反应的总电子数相关联:1j =1j k +1j dj d =0.62nAFC O D O 2/3v -1/6ω1/2=B ω1/2式中,j 是实验观察到的电流,j k 是动力学电流,n 是每摩尔O 2被还原时所转移的电子数目,F 是法拉第常数,A 是电极面积,D O (1.97×10-5cm 2·s -1)是氧气的扩散系数,C O (1.2×10-6mol ·cm -3)是氧气的溶解度,v (1.0×10-2cm 2·s -1)是溶液的动力学粘度.在电压0.45-0.75V 之间,计算得到n 值为4.09-4.19(表1),表明Co@Pt/C 催化剂表面发生的ORR 反应是以四电子路线进行的.催化剂的稳定性与耐久性也是本实验研究的一个重要方面.将Pt/C (JM)催化剂和Co@Pt(1:3)/C (reduced)催化剂按实验部分描述的方法涂覆于工作电极,在0.1mol ·L -1HClO 4溶液中进行500个循环的CV 测试以考察其耐久性.实验结果如图7所示.在开始的100个CV 循环后,催化剂性能几乎没有下降;但此后催化剂的性能有明显的下降.经过500个循环后,Co@Pt(1:3)/C (reduced)催化剂的ECSA 下降了30.8%.而对于Pt/C 催化剂,在开始的100个循环结束后,催化剂也基本保持了较高的性能,完成300个循环之后出现了一定的衰减,但与自制催化剂相比其衰减速率较小.500个循环之后,Pt/C 催化剂的ECSA 的衰减率为15.5%.可见自制催化剂的稳定性不如Pt/C (JM)催化剂,但是与文献22中报道的Pt-Co 合金催化剂相比,稳定性有很大的提高.这可能是由于Pt 包覆在过渡金属Co 的外层,有效地阻止了Co 的溶解和流失,从而提高了催化剂的稳定性和耐久性.可见,核壳结构是提高催化剂稳定性和耐久性的有效途径之一.4结论通过HR-TEM 表征结果可以看出本研究制备的Co@Pt/C 催化剂形成了核壳结构,催化剂的粒径主要分布在3-5nm 范围内,热处理对催化剂的形貌图6Co@Pt(1:3)/C (reduced)催化剂氧还原反应的Koutecký-Levich 图线Fig.6Koutecký-Levich plots for ORR onCo@Pt (1:3)/C (reduced)catalystin 0.1mol ·L -1HClO 4,derived from the data inFig.5图7室温氮饱和气氛下Co@Pt(1:3)/C (reduced)催化剂在0.1mol ·L -1HClO 4电解质溶液中的耐久性Fig.7Durability of as-prepared Co@Pt/C in N 2-saturated0.1mol ·L -1HClO 4at room temperature(a)Co@Pt(1:3)/C (reduced)catalyst;(b)Pt/C (JM)catalyst,sweep rate:50mV ·s -1表1在0.1mol ·L -1HClO 4中不同电位下Co@Pt(1:3)/C(reduced)催化剂ORR 动力学电流(j k )和电子转移数(n i )Table 1Kinetic current (j k )and average number of (n i )electrons transferred for O 2reduction reaction (ORR)onCo@Pt(1:3)/C (reduced)at different potentials in-10006曹春晖等:用于燃料电池Co@Pt/C核壳结构催化剂的制备及表征No.X也有较大的影响.通过XPS分析发现,纳米颗粒表现出Co2p、Pt4f、C1s和O1s四个明显的光电子峰, Pt主要是以金属态存在.催化剂电化学性能测试结果表明所形成的核壳结构在降低Pt用量的同时也提高了Pt的利用率.核壳结构的形成同时使得催化剂的稳定性和耐久性得到提升,制备得到的Co@Pt/ C催化剂经溶液中500个循环的CV测试之后性能虽有所下降,但衰减程度远小于Pt-Co合金催化剂.此外,本研究进行了催化剂的动力学性质研究,发现催化剂ORR遵循四电子路线进行.References(1)Wang,Y.;Chen,K.S.;Mishler,J.;Cho,S.C.;Adroher,X.C.Appl.Energy2011,88,981.doi:10.1016/j.apenergy.2010.09.030 (2)Wee,J.H.;Lee,K.Y.;Kim,S.H.J.Power 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化工进展Chemical Industry and Engineering Progress2023 年第 42 卷第 3 期PtSn/MgAl 2O 4-sheet 催化剂的制备及其PDH 反应性能张孟旭，王红琴，李金，安霓虹，戴云生，钱颖，沈亚峰（昆明贵金属研究所，稀贵金属综合利用新技术国家重点实验室，云南 昆明 650106）摘要：近年来，因为页岩气大规模开采的成功可以为丙烷脱氢制丙烯（PDH ）工艺提供大量廉价的丙烷，丙烷脱氢制丙烯已成为最有前途和最具吸引力的丙烯生产技术。

目前工业上丙烷脱氢主要采用的是负载型PtSn/Al 2O 3催化剂。

然而在丙烷脱氢高温反应中，PtSn 纳米粒子易烧结和积炭使催化剂遭受严重的失活。

为了解决上述问题，本文合成了片状的MgAl 2O 4尖晶石载体负载PtSn 金属纳米粒子，制备了PtSn/MgAl 2O 4-sheet 催化剂。

该催化剂具有较大的孔径，有利于PDH 反应中反应物的吸附和产物的脱附，提高了催化剂的活性同时降低了积炭含量。

同时片状的MgAl 2O 4尖晶石载体的（111）面与PtSn 纳米颗粒存在着强的相互作用，阻止了PtSn 纳米颗粒在高温反应中的烧结。

在丙烷脱氢反应中，丙烷的转化率达到了43.2%，丙烯的选择性达到了95.0%，失活速率仅为0.008h −1，其性能优于商用的PtSn/Al 2O 3催化剂。

关键词：丙烷脱氢；铂锡催化剂；形貌控制；丙烯；镁铝尖晶石中图分类号：TQ426.82 文献标志码：A 文章编号：1000-6613（2023）03-1365-08Preparation of PtSn/MgAl 2O 4-sheet catalyst and its PDH reactionperformanceZHANG Mengxu ，WANG Hongqin ，LI Jin ，AN Nihong ，DAI Yunsheng ，QIAN Yin ，SHEN Yafeng(State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute ofPrecious Metals, Kunming 650106, Yunnan, China)Abstract: In recent years, propane dehydrogenation to propylene (PDH) has become the most promising and attractive propylene production technology because the successful large-scale exploitation of shale gas can provide a large amount of cheap propane. Currently, supported PtSn/Al 2O 3 catalysts are mainly used for propane dehydrogenation in industry. In the high temperature reaction of propane dehydrogenation, PtSn nanoparticles are prone to sintering and coke deposition, which causes severe catalyst deactivation. To solve the above problems, we synthesized PtSn/MgAl 2O 4-Sheet catalyst by using MgAl 2O 4-Sheet spinel as support. The catalyst had a large pore size, which was favorable for the adsorption of reactants and the desorption of products in the PDH reaction, and thus improves the activity of the catalyst and reduced the content of coke. Meanwhile, the (111) plane of the MgAl 2O 4 spinel support had a strong interaction with the PtSn nanoparticles, which prevented the sintering of PtSn nanoparticles in the high temperature reaction. In the propane dehydrogenation reaction with the prepared catalyst, the conversion of propane reached 43.2%, the selectivity of propylene reached 95.0%, and the deactivation研究开发DOI ：10.16085/j.issn.1000-6613.2022-0919收稿日期：2022-05-17；修改稿日期：2022-07-19。