ZIF-8/ZIF-67 Derived Sea Urchin-Like Co,N Co-Modified Carbon Dodecahe-drons as Efficient Counter Electrode Catalysts for Photovoltaics
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 I3-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 carbon-based 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(AEp)and the highest peak current density as compared with Co@NCD and Pt,indicating the highest catalytic activity of Co@NCD/CNT for I3 reduction.EIS results showed that Co@NCD/CNT had a lower interface electron transfer resistance(Rc=0.79 Ω·cm-2),than those of Co@NCD(0.91 Ω·cm2)and Pt(1.32 Ω.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(J0)and limiting diffusion current density(Jlim)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(Jsc=15.53 mA·cm-2),large open circuit voltage(Voc=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％,Jsc=14.86 mA·cm-2,Voc=0.75 V,FF=65％),and Pt(PCE=6.95％,Jsc=14.47 mA·cm-2,Voc=0.76 V,FF=63％)electrodes.Finally,after continuing 50 cycles of CV scanning,Co@NCD/CNT exhibited higher stability than the Pt in I3-/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,CO2 reduction and oxygen reduction.
counter electrodedye-sensitized solar cellzeolitic imidazolate frameworkscarbon material
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