Electrocatalytic Hydrogen Evolution By Cobalt A₂B Corroles Bearing Amide Pyridyl Triaryl Groups

Due to the continuous consumption of fossil fuels,the demand for sustainable and environmentally friendly energy is increasing.Hydrogen,the cleanest fuel,has a huge application potential.The electrolysis of water to hydrogen is one of the most environmentally friendly methods.Finding catalysts that can efficiently catalyze the electrolysis of water is a significant research direction.At present,the precious metal platinum has the best catalytic performance.However,due to its limited reserves on the earth,it is necessary to seek transition metal catalysts based on Co,Fe,Ni,etc.Therefore,this paper synthesized four kinds of cobalt corrole complexes based on the idea of molecular design and explored the performance of aldehyde groups and amide pyridyl groups differences in electrocatalytic reduction of protons and the decomposition of water into hydrogen.Since the amide pyridyl group contains a side chain amine structure,it may bind to protons and act as a proton relay to accelerate the proton transfer rate and improve the catalytic hydrogen production performance.Based on this idea,an amide pyridyl group was attached to the aldehyde phenyl group through aldehyde and amine condensation at the 10-meso position of corrole.In this paper,5,15-bis(pentafluorophenyl)-10-(4-formylphenyl)corrole(4-BPFC),5,15-bis(pentafluorophenyl)-10-(3-formylphenyl)corrole(3-BPFC),5,15-bis(pentafluoroph-enyl)-10-(4-schiff-phenyl)corrole(4-BPSC),5,15-bis(pentafluorophenyl)-10-(3-schiff-phenyl)corrole(3-BPSC)and their corresponding cobalt complexes(1-4)had been synthesized.Using ultraviolet spectroscopy(UV-Vis),nuclear magnetic(NMR),high resolution mass spectrometry(HRMS),XPS,X-single crystal diffraction to characterize these complexes.CV,Tafel,R_ctand other electrochemical test methods were used to study the different mechanisms when three different acids were used as the proton sources.It was found that under weak acid(Ac OH)condition,the catalytic path was EECC(E:electron,C:charge),and under strong acid(TFA/Ts OH)condition,the catalytic path depended on the acid concentration.At low acid concentration,the catalytic path was EECC,while at high acid concentration,the catalytic path passed through EECEC.The formation of Co^?-H active specie was a key step in the process of catalytic hydrogen evolution.The presence of Co^?-H was detected by nuclear magnetism,which provided a reference for the catalytic mechanism.The p K_a of Co^?-H under different proton source condition was measured by UV-Vis kinetic experiments.Tafel and impedance test indicated that 4 had a higher charge transfer rate than 3,which may be due to the meta-substituted amide pyridine group was closer to the active metal center.In the mixed acetonitrile and water neutral phosphoric acid buffer,cobalt corrole complex was added for continuous electrolysis for one hour.The gas after the electrolysis was analyzed by GC.It was found that the hydrogen production after adding the catalyst was much greater than that produced by the electrolysis of the bare glass carbon electrode.The TOFs of four kinds of cobalt corrode were measured by constant potential electrolysis(CPE),and they were 184.3 h^-1,171.0 h^-1,113.6 h^-1,129.5 h^-1.Interestingly,the TOF of corrole 1 is higher than that of the other three cobalt corroles.The reasons for this performance need to be further studied.Whether in organic phase or mixed phase solution,the TOF of corrole 4 is higher than that of 3,which may be due to the fact that the amide pyridyl group was closer to the cobalt center,which promoted the catalytic activity of HER.