Metalloporphyrins Catalyzed Water Splitting And Oxygen Reduction

Development and utilization of clean and renewable energy technologies,such as water splitting,fuel cells,metal-air batteries,etc.,is expected to solve the current energy crisis and environmental problems.Hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)play a pivotal role in clean and renewable energy technologies.It is very important to design and prepare highly efficient catalysts for water splitting and ORR.In catalytic reactions,the regulation of molecular electronic structure and proton transfer has a great influence on catalytic activity.Metalloporphyrin-based molecular catalysts are widely used in catalytic reactions due to their clear structure,excellent designability,and good stability.Therefore,the catalytic activity of molecule can be improved by precisely regulating the electronic structure of metalloporphyrin-based molecules and adjusting the proton transfer rate during the reaction.At the same time,rationally design molecular catalysts based on the theory of molecular catalysis to realize the heterogeneity of metalloporphyrin-based molecular catalysts is of great significance for improving molecular catalytic activity and developing molecular catalysts with practical application value.The main research results are as follows:(1)Inspired by enzymes,an Fe porphyrin with axial coordination imidazole group was synthesized by regulating the electronic structure of molecular.This Fe porphyrin can act as an efficient catalyst for both ORR and OER in alkaline solutions.It significantly beats its imidazole-free analogue for electrocatalytic ORR and OER.For ORR,the axial imidazole can increase electron density on Fe to improve O2 binding and assist the O-O bond cleavage.For OER,the imidazole group is likely to allow hydroxide attack to[Fev=O]+for the O-O bond formation by protecting the trans axial site of[Fev=O]+.As a practical demonstration,Zn-air battery assembled with this Fe porphyrin shows equal performance to batteries with Pt/Ir-based materials.This work represents the first example of molecular porphyrin-based Zn-air battery and underlines unique benefits and promising applications of molecular electrocataly sis in new energy technologies.(2)We have designed three water-soluble Co porphyrin polymers with different HER activities by regulating the proton transfer rate.Different side chain groups were introduced around the Co porphyrin as the second coordination layer structure,which affected the proton transfer rate in the reaction process,so as to regulate the HER activity of Co porphyrin.At the same time,the polymer was used to improve the catalytic stability of Co porphyrin.In neutral media,Co porphyrin polymers can achieve high hydrogen evolution efficiency for electro-(TOF>23000 s-1)and photo-catalysis(TON>27000).The advantages of this design are that porphyrin units are surrounded and protected by polymer chains and the activity can be tuned by different side-chain groups.Therefore,by molecular catalyst design strategies,the solubility and stability of molecules in water can be changed by polymers,and the second coordination layer structure can be introduced to regulate the proton transfer rate to improve catalytic activity,instead of revising molecular structures that is difficult from both design and synthesis points of view.(3)Co corroles were covalently grafted onto Fe3O4 nanoarrays by azide-alkyne cycloaddition reaction,to synthesize a molecular modified heterogeneous catalyst.The resulted catalyst shows significantly improved electron transfer rate,catalytic activity and durability for OER and ORR in neutral media as compared to Fe3O4 alone and with directly adsorbed Co corroles.It also displays higher atom efficiency—at least two magnitudes larger turnover frequency—than reported catalysts.Using this catalyst in neutral Zn-air battery,small charge-discharge voltage gap of 1.19 V,large peak power density of 90.4 mW/cm2,and high rechargeable stability for>100 h are achieved,opening a promising avenue of molecular electrocatalysis in metal-air battery.This work represents a molecule-engineered self-supported catalyst electrode for electrocatalysis and demonstrates their potential applications in energy conversion and storage.