ZIF-8/ZIF-67衍生钴氮共修饰碳的合成及电催化性能研究

ZIF-8/ZIF-67 Derived Sea Urchin-Like Co,N Co-Modified Carbon Dodecahedrons as Efficient Counter Electrode Catalysts for Photovoltaics

开发并利用高效廉价的电催化剂来代替传统的贵金属铂(Pt)基催化剂,对新能源材料与器件的开发和利用意义重大。以锌/钴基沸石咪唑酯骨架材料(ZIF-8/ZIF-67)为前驱体,通过不同的热解方式,合成表面原位生长碳纳米管的钴氮共修饰碳十二面体(Co@NCD/CNT)和表面光滑的钴氮共修饰碳十二面体(Co@NCD)。采用X射线衍射仪(XRD)、拉曼光谱仪(Raman)、场发射扫描电镜(FESEM)对合成催化剂的组份和形貌进行表征。XRD图谱显示Co@NCD和Co@NCD/CNT为无定型碳结构,并存在Co金属物种。Raman图谱显示Co@NCD/CNT比Co@NCD具有更高的石墨化程度。FESEM图像显示Co@NCD呈表面光滑的十二面体状,Co@NCD/CNT呈十二面体状且表面长有碳纳米管。电化学测试结果显示,Co@NCD/CNT和Co@NCD催化剂在碘电解液体系中的的传荷电阻分别为0.79和0.91Ω·cm^(2),还原峰值电流密度分别为-6.43和-4.46 mA·cm^(2)。由光伏测试结果可知,Co@NCD/CNT对电极催化剂组装的染料敏化太阳能电池获得了8.05%的光电能量转化效率(PCE),优于Co@NCD(7.20%)和Pt(6.95%)对电极组装电池的PCE。50次连续循环伏安测试表明Co@NCD/CNT具有较好的电化学稳定性,可作为潜在类Pt催化剂使用。

Dye-sensitized solar cells(DSSCs)are promising photoelectric conversion devices that have attracted considerable atten-tion in new energy fields because of their low-cost,simple fabrication process,and high theoretical power conversion efficiency(PCE).The counter electrode(CE)is an indispensable component in DSSCs and plays a major role in collecting electrons from the ex-ternal circuit and catalyzing the reduction of I_(3)^(-)to I^(-)in the electrolyte.Platinum(Pt)is the preferred CE material in DSSCs due to its excellent electrocatalytic activity.However,the high cost and scarcity of Pt CE limit their large-scale commercial application.There-fore,developing a low-cost catalyst to replace the Pt catalyst used in DSSCs is urgent.To date,a variety of possible replacements have been developed,including conductive polymers,metal and alloy,nano hybrids,transition metal compounds,and carbon materials.Carbon materials,particularly zeolitic imidazolate framework(ZIF)-derived carbon,have sparked considerable interest due to their low cost,adjustable pore structure,and excellent electrochemical corrosion resistance.It has been shown that a porous N-containing carbon dodecahedron(NCD)complex with a high surface area,abundant pore defects,and uniform N-group functional active sites can be acquired through direct pyrolysis of Zn-based ZIF(ZIF-8).However,NCDs still have the disadvantages of being a monotonous catalytically active species and limited electronic conductivity.These shortcomings lead to poor catalytic performance of porous N-con-taining carbon dodecahedrons,thus preventing them from offering an improved catalytic function in electrocatalytic fields.Many efforts have suggested that the introduction of transition metals(such as Cu,Ni,Co and so on)as a doped element in N-coordinated metal modified carbon can effectively optimize catalytic properties.In addition to the metal component,the carbon nanostructure plays a key role in enhancing catalytic performance.Particularly,one-dimension carbon structures,such as carbon nanotubes,which are endowed with well-defined channels,high crystallinity,and large surface areas,can serve as an electronic transmission channel to boost the catalytic process efficiently.To this end,in this work,two carbon-based catalysts with different morphologies were controllably synthe-sized by direct pyrolysis,in which ZIF-8/ZIF-67 and methanol were used as precursors and solvent,respectively.The resultant carbonbased catalysts could be divided into two categories,that was,Co,N modified carbon dodecahedrons with a smooth surface(Co@NCD)and Co,N modified carbon dodecahedrons with in-situ growth of carbon nanotubes(Co@NCD/CNT).The structures and morphologies of the as-synthesized catalysts were characterized using X-ray diffraction(XRD),Raman spectroscopy,and field emis-sion scanning electron microscopy(FESEM).XRD results showed that Co@NCD and Co@NCD/CNT exhibited amorphous carbon structures,and Co metal nanoparticles were present in the carbon skeletons.Raman spectroscopy demonstrated that Co@NCD/CNT possessed a higher degree of graphitization than Co@NCD,which would be beneficial to enhancing the conductivity of catalyst.FES-EM images showed that Co@NCD/CNT exhibited rhomboid dodecahedron morphology with a rough surface,and carbon nanotubes were in-suit grown on the catalyst surface.The existing carbon nanotubes could provide a convenient pathway for ion transmission,thus enhancing in the catalytic activity of the catalyst.In addition,the electrocatalytic properties of the as-prepared catalysts were in-vestigated using electrochemical-based cyclic voltammetry(CV),Tafel polarization,and electrochemical impedance(EIS)tests.CV curves showed that Co@NCD/CNT had the smallest peak-to-peak separation(ΔE_p)and the highest peak current density as compared with Co@NCD and Pt,indicating the highest catalytic activity of Co@NCD/CNT for I_(3)^(-)reduction.EIS results showed that Co@NCD/CNT had a lower interface electron transfer resistance(R_(ct)=0.79Ω·cm^(-2)),than those of Co@NCD(0.91Ω·cm^(-2))and Pt(1.32Ω·cm^(-2)),thus indicating an easier electron transfer from Co@NCD/CNT interface to the iodine electrolyte.Tafel curves showed that Co@NCD/CNT had higher exchange current density(J_0)and limiting diffusion current density(J_(lim))values than those of Co@NCD and Pt.These CV,EIS and Tafel results demonstrated that Co@NCD/CNT possessed higher catalytic activity than Co@NCD and Pt.The high catalyt-ic activity of Co@NCD/CNT could be attributed to the higher degree of graphitization,special morphology,and a one-dimensional car-bon nanotube structure.As a result,the solar cell based on Co@NCD catalyst yielded a high short-circuit current(J_(sc)=15.53 mA·cm^(-2)),large open circuit voltage(V_(oc)=0.78 V),higher fill factor(FF=66%),and a high PCE of 8.05%,as compared with DSSC with Co@NCD(PCE=7.20%,J_(sc)=14.86 mA·cm^(-2),V_(oc)=0.75 V,FF=65%),and Pt(PCE=6.95%,J_(sc)=14.47 mA·cm^(-2),V_(oc)=0.76 V,FF=63%)electrodes.Finally,after continuing 50 cycles of CV scanning,Co@NCD/CNT exhibited higher stability than the Pt in I_(3)^(-)/I^(-)electro-lyte,indicating the tremendous potential of Co@NCD/CNT for industrialized applications.This work designed a highly-efficient electro-catalyst for DSSCs and provided guidance on the controllable synthesis of ZIF-derived carbon catalyst,which could be well used in new energy fields involving hydrogen evolution reaction,CO_(2) reduction and oxygen reduction.