Molecular Electrocatalysts For Hydrogen Evolution And Oxygen Reduction Of Cobalt Porphyrins

Since the industrial revolution in the 1760s,with the missive liberation of productivity and rapid development of economy,mankind realized that the ensuing energy and environmental crisis was getting worse.Based on the world's fossil energy reserves and consumption levels,the total amount of energy on Earth can only be used for about two hundred years.Moreover,the use of fossil energy will cause greenhouse effect and environmental pollution at the same time,so it is urgent to find resource-saving and environment-friendly renewable energy.Because the only exhaust of hydrogen combustion is water,the entire process of the reaction meets the requirements of green,clean and sustainable entirely,and hydrogen is considered as one of the new energy sources with the greatest development potential in response to the energy and environmental crisis.Hydrogen fuel cells generate water by combustion of hydrogen,and hydrogen requires light energy or electrical energy to split water.In this ideal sustainable cycle that constitute these two processes,the steps that affect energy conversion efficiency are the hydrogen evolution reaction(HER),oxygen evolution reaction(OER),oxygen reduction reaction(ORR)and hydrogen oxidation reaction(HOR).The catalyst plays a vital role in these processes by improving the kinetics of these reactions,so it is also required to be cheap,efficient and clean.Inspired by nature,it has been discovered that porphyrin-based compounds coordinated with inexpensive metal have excellent performance in catalyzing the four reactions mentioned above.At the same time,if porphyrin-based compounds is combined with a suitable inorganic material,the advantages of the two materials can be exploited to the full by improving the conductivity and stability in the aqueous solution.According to the actual situation,the first content of our study is the synthesis of covalent cobalt porphyrin framework(FCoP@CNT)on the surface of carbon nanotubes using cobalt(?)tetrakis(2,3,5,6-tetrafluoro-4-ethynylphenyl)porphyrin(FCoP-H)as monomer.The resulting FCoP@CNT has the obvious advantages to catalyze HER in aqueous media compared to the covalent cobalt porphyrin framework using non-fluorinated cobalt porphyrin analogues(CoP@CNT),in terms of both catalytic current and onset overpotential.There are obvious advantages in both catalytic current and peak overpotential.At the same time,compared with the materials made by simply adsorbing two kinds of cobalt porphyrin monomers on carbon nanotubes respectively(FCoP/CNT,CoP/CNT),the former combination has significantly improved the performance for HER.We first studied the electrochemical behavior and performance for HER of the cobalt porphyrin molecule in the organic solvent media.Then the successful synthesis of the material was proved by various characterization methods.FCoP@CNT has excellent catalytic performance for HER in a 0.5 M sulfuric acid aqueous solution.Besides,FCoP@CNT also has good stability for HER,and its Faraday efficiency can reach 94%.However,it is well known that when a large number of fluorine atoms are introduced in molecule,the hydrophilicity of the material will also be changed,which will affect the catalytic performance of the catalyst in aqueous media.Therefore,the second content of our study is to synthesize 5,10,15,20-tetraphenyl porphyrin cobalt(?)(CoP-H),5,10,15,20-tetra(perfluorophenyl)porphyrin cobalt(?)(CoP-F)and 5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphyrin cobalt(?)(CoP-OMe),respectively.These three porphyrins are introduced with different hydrophilic group.the molecular synthesis was first determined by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry,and then the three porphyrins were simply adsorbed on the surface of carbon nanotubes,the contact angles between the three samples and water were tested.The experimental data show that the hydrophobic relationship between the three was:CoP-F>CoP-H>CoP-OMe.Finally,the performance for HER and ORR in aqueous media was tested,and the results of the experiment showed that hydrophilicity increases the performance of electrocatalytic HER and ORR.