Preparation And Electrocatalytic Water Oxidation Performance Of A Dinuclear Copper Complex Featuring A Flexible Linker

Nowadays the limited reserves of fossil fuels on Earth cannot meet the quick growing demand of energy by human beings.Hydrogen is a carbon-free energy carrier of high energy density,which is considered to be a good substitute for fossil fuels.Electrocatalytic water splitting to oxygen and hydrogen by using the electric power generated from renewable energy such as wind and solar energy is a promising approach to indirectly utilizing renewable energy to produce hydrogen.Water splitting includes two half reactions,namely O₂-evolution reaction?OER?and H₂-evolution reaction?HER?.The OER is a complicated process involving the transfer of 4 electrons and 4 protons.Therefore it requires a large overpotential and has a sluggish reaction rate.The development of highly active,stable,and inexpensive water oxidation molecular catalysts?WOCs?is one of the key problems for eventually realizing artificial photosynthesis.Inspired by the synergistic catalysis effect of metal ions in many metalloenzymes,we designed a Cu-based dinuclear molecular WOCs,in which the two Cu?II?ions were located close to each other bridged by a flexible but stable CH₂CH₂ chain and a?-OH group.It was expected that the latent synergy of two adjacent copper ions in the molecule could enhance the catalytic activity of the catalyst for water oxidation.In this thesis,two dinuclear copper complexes,[L1Cu₂??-OH?]?BF₄?₃?1,L1=N,N?-dimethyl-N,N?-bis{2-[bis?2-pyridinylmethyl?amino]ethyl}ethane-1,2-diamine?and[L1Cu₂?CH₃CN?₂]?BF₄?₄?1??,together with their mononuclear counterpart[L2Cu?OH₂?]?BF₄?₂?2,L2=N,N-dimethyl-N?,N?-bis?2-pyridylmethyl?ethane-1,2-diamine?and[L2Cu?CH₃COCH₃?]?BF₄?₂?2??,were prepared as WOCs.The structures of these complexes were characterized by mass spectrum?MS?and elemental analysis,and the molecular structures of 1,1?,and 2?were determined by single crystal X-ray diffraction analysis.Besides,the existing state and stability of the copper complexes in basic solutions were studied by MS,UV-vis,and EPR spectroscopy.On the basis of above results,the electrochemical properties and the catalytic performances of1 and 2 for electrochemical water oxidation were comparatively studied.The results showed that the overpotential required by 1 for catalyzing OER was lower than that reqired by 2,while the catalytic activity of 1 was much higher than that of 2.In phosphate buffer solution at pH 12,the k_cat1 value of 1 was 144 s^-1,which was about 30 times that of 2.The k_cat1 of 1 is on a par with those of the state-of-the-art earth-abundant molecular catalysts reported to date under similar test conditions.Moreover,the long-time control potential electrolysis experiments showed that 1 had good stability under water oxidation conditions,and the faradaic efficiency of OER was 97%for 1 and 87%for 2.Experimental studies and DFT calculations suggest that the water oxidation catalyzed by 1 proceeds via a unimolecular two-site mechanism with the synergy of two copper centers.As such,the formation of unstable high-valent copper intermediates is avoided during the process of OER,which greatly reduces the energy barrier for the O-O bond formation in the rate-determining step and thus enhances both the activity and stability of the catalyst.The energy barrier for mononuclear copper catalyst 2 to form an O-O bond is much higher than that that for dinuclear copper catalyst 1.