Design And Triiodide Reduction Properties Of Carbon Derived From Zinc-Based Metal-Organic Frame

Green awareness continues to grow in popularity with the development of the times.Scientists have developed a series of products with low carbon emissions to replace traditional energy with the power of nature,and DSSCs have emerged,which are fully favored for their ease of manufacture,low cost and superior stability.As a key link of DSSCs,the counter electrode（CEs）plays a connecting role.Firstly,it collects and transmits the external circuit electrons,and secondly it is responsible for the reduction of I₃^-.The traditional CEs is platinum,but its poor stability,rarity and high cost result in the slow commercial application of DSSCs.Therefore,it is inevitable to seek low price,high efficiency and stable non-platinum materials.For the past few years,carbon materials have been widespreadly used in DSSCs with characteristics of multi-dimension,great stability and high conductivity.Zinc-based organic metal framework（Zn-MOF）derived carbon materials have excellent physical and chemical properties such as good morphology,high conductivity,large porosity and good stability,and are expected to replace expensive platinum counterelectrode.In this paper,in situ nitrogen-doped porous carbon（NPC）and in situ nitrogen-doped carbon sheet（NCS）materials were constructed using Zn-MOF as the precursor material,which were used as counter electrodes to investigate the redox reactions occurring on the cathode and the photovoltaic properties.The major tasks are as follows:A graphene oriented assembly synthesis route is developed,using GO supported ZIF-8 nanocrystalline arrays（ZIF-8/GO）as precursor for reducing triiodide anions.After carbonization and pickling process,multistage nanocrystalline materials（NPC）with macroporous/mesoporous/microporous structures,high specific surface area and excellent electrical conductivity are synthesized.By changing the carbonization temperature to optimize the performance,NPC-900 showed the best I₃^-reduction catalytic performance.The device achieves a photoelectric conversion efficiency（PCE）of 8.25%.It is better than Pt reference electrode（7.75%）and single nitro-doped carbon（NC-900,6.23%）and r GO-900 counterelectrode（7.20%）.This is mainly attributed to the synergistic effect between NC-900 and r GO-900.NC-900 has high catalytic activity and its three-dimensional mesh pore structure improves the conductivity and diffusion ability of I₃^-,while r GO,as a conductive substrate and structural guide agent,provides a conductive network,enormously enhancing the electron mobility of the NPC materials.Using zinc nitrate hexahydrate as zinc source and benzimidazole as organic ligand,two-dimensional sheet MOF with high yield is prepared by a simple and low-cost bottom-up method.After calcination and pickling,NCS are obtained.This process retains the original morphology of MOF,and porous structure and in situ heteroatomic nitrogen doping were formed.By changing the carbonation temperature to adjust the performance.Finally,it was found that when the carbonation temperature was 900℃,the NCS-900electrode material showed the highest PCE of 8.63%than that of 7.75%for Pt CE.This is because the porous structure and high specific surface area of the NCS-900 facilitate the touch between the active component and I₃^-,which reduces the loss of electrons,and N-doped carbon in situ provides an active site for the reduction of triiodide.In conclusion,the photovoltaic device composed of NCS-900 achieves the highest photoelectric conversion efficiency.