Construction Of Metal Porphyrin Polymers-Based Cathode And Bismuth Vanadate-Based Photoanode And Their Research On (Photo) Electrocatalytic Performance

Artificial photosynthesis imitates the photosynthesis of green plants,which can utilize the solar energy to convert the greenhouse gas CO₂ into high value-added fuels or chemicals.Among various artificial photosynthesis systems,constructing a highly efficient and stable photoelectrochemical cell（PEC）is a promosing strategy to realize the CO₂ reduction reaction（CO₂RR）and store solar energy with high-energy chemical bonds.The PEC system for CO₂RR is fabricated with the photoanode for water oxidation reaction（OER）and the（photo）electric cathode for CO₂RR,in which the photoelectrode has been employed as the light absorbing material and electrocatalysts for water oxidation and CO₂RR have been used to reduce the related reaction energy barrier.However,low utilization of photo-induced hole for photoanode and poor selectivity for CO₂RR have impeded the development and application of high-efficient PEC CO₂ RR system.Therefore,the investigation is dedicated to constructing a high-efficiency PEC CO₂RR system,where CO₂RR and OER process occur stimutaneously and a complete photoelectrochemical cycle can been achieved.The optimization of the photoanode and electrocatalysts for CO₂RR by various reguglating strategies has been expolored.The specific research content and experimental results are summarized as follows:1.The ethynyl-linked cobalt（II）porphyrin conjugated polymer（PCP）modified the carbon nanotubes（CNTs）（Co Co PCP/CNTs）has been designed as an energetic electrocatalysts for CO₂RR by in-situ polymerization of cobalt（II）5,10,15,20-tetrakis（4’-bromophenyl）porphyrin（Co Por-Br）and cobalt（II）5,15-di（4-ethynylphenyl）-10,20-diphenylporphyrin（Co Por-Ethynyl）on the surface of CNTs.Structural and electrtocatalytic experiments have releved that the in-situ polymerization of Co Co PCP on CNTs not only endows an intimate contact between the organic polymer and CNTs,but also can get the facilitated charge transfers and enlarged active area.The covalent coupling of the monomer by ethynyl bond and the coexistence of two kinds of Co sites in different porphyrin monomers are favorable for the intermolecular electron transfer between the two different Co centers.The mechanism for CO₂RR has been studied using DFT calculation and demonstrates that the introduction of bi-metal center reduces the reaction energy barrier.Benefitted from the in-situ polymerization,the intruduction of CNTs and the coexistence of bi-metal center,the Co Co PCP/CNTs exhibits a high CO faradic efficiency of 94%at an extremely low overpotential of 0.44 V.2.Developing highly-efficient photoanodes that can match with the porphyrin-based electrocatalysts for CO₂RR is critical for constructing a two-electrode PEC CO₂RR system,which can simultaneously achieve CO₂ reduction and conversion along with the water oxidation.The low-cost and stable Bi VO₄ photoanode,featured Cu Pc and Ni Co layered double metal hydroxide（Ni Co-LDH）have been employed as light absorber,a highway for hole transfer and the oxygen evolution catalysts（OECs）,respectively.The in-situ growth of Ni Co-LDH provides an excellent water oxidation electrocatalysts to facilitate the kinetics of water oxidation,reducing the recombination of the photo-induced hole.Introducing an ultrathin Cu Pc layer on the interface of Bi VO₄ and Ni Co-LDH accelerates hole transfer from Bi VO₄ to Ni Co-LDH,which further improves the utilization of photo-induced holes.The resulting Ni Co-LDH/Cu Pc/Bi VO₄ photoanode can deliver a photocurrent density of 4.03 m A cm^-2 at 1.23 V vs.RHE,which provides a possibility for constructing a two-electrode PEC CO₂ reduction system.Additionally,the introduction of the high hole transfer capacity of Cu Pc provides a promising strategy for constructing an efficient photoanode.3.To further improve the electrocatalytic activity of the cobalt porphyrin-based polymers for CO₂RR and construct a two-electrode PEC CO₂RR system,the electron-rich N’NN’-pincer ligand（2,6-bis（5-amino-1H-benzimidazol-2-yl）pyridine,N₃）has been employed as a novel bridging group to fabricate a N₃-pincer ligand-coupled cobalt porphyrin polymer（Co Por-N₃）for the first time.The electrocatalytic and photo-assisted electrocatalytic activity of Co Por-N₃have been investigated.Combined the optimized Ni Co-LDH/Cu Pc/Bi VO₄ photoanode with Co Por-N₃ electrocatalysts,a two-electrode system（Co Por-N₃|Ni Co-LDH/Cu Pc/Bi VO₄）for PEC CO₂RR has been fabricated.Electrochemical measurements demonstrate that the introduction of electron-rich N₃ enhances the conductivity and instrinic electrocatalytic activity for CO₂RR,the Co Por-N₃ delivers good long-term durability and high FE（CO）of 96%at an ultralow potential of-0.50 V vs.RHE（an overpotential of 0.39 V）.Moreover,the external light has been introduced to regulate the electronic structure of Co Por-N₃ due to its photosensitive property.A FE（CO）over 90%can be achieved for Co Por-N₃ at a reduced overpotential（0.34V）under illumination,which is benefitted from the promoted CO₂ activation,facilitated electron transfer and enriched electron at Co centers by introducing the external light.The Co Por-N₃|Ni Co-LDH/Cu Pc/Bi VO₄ delivers a FE（CO）of 87%at 1.0 V.Experimental results demonstrate that the external bias only accelerates the transfer of photogenerated electrons from photoanode to the Co Por-N₃,while the photogenerated holes quickly transfer to Ni Co-LDH by the high-hole mobility Cu Pc and the water is oxidized to O₂.The constructed Co Por-N₃|Ni Co-LDH/Cu Pc/Bi VO₄ system can simultaneously realize CO₂RR and water oxidation under a relatively low bias voltage and exhibits a potential for large-scale applications.