Studies On Electrocatalytic Properties Of Co、Ni Coordination Polymers Based On N-pyridylbenzimidazole Straight-chain Ether Ligands

Electrocatalysts play an important role in the synthesis of chemicals,energy conversion,sensing and other fields.Unfortunately,electrocatalysts with excellent performance are dependent on precious metals,which is naturally less abundant,and the high cost of preparation limits their industrialisation.Therefore,the study and development of highly efficient and low-cost electrochemical catalysts are crucial.On top of that,we have systematically investigated the design strategies and research progress of various novel electrode materials.In comparison,ferrous series transition metal coordination polymers（CPs）show potential applications in electrocatalysis due to their low cost of preparation,high purity,controllable structure and adjustable performance,and using them instead of precious metals is a good choice to promote the industrialisation of electrochemical catalysts.However,the electrocatalytic activity of the currently reported iron-based transition metal coordination polymers is still low and has not yet met the requirements of industrial applications.Hence,in this thesis,multi dentate N-pyridylbenzimidazole straight-chain ether ligands were designed and self-assembled with cobalt/nickel ions to synthesise novel coordination polymers,and their electrocatalytic properties（HER,OER and sensing）were investigated by preparing coordination polymer/acetylene black/Nafion modified glassy carbon electrodes,and the"structure-activity"relationship between their structure and electrocatalytic activity was explored.Details of the studies are as follows.（1）Synthesis and characterization of coordination polymersFirstly,twoN-pyridylbenzimidazolestraight-chainetherligands,bis[1-（pyridin-3-ylmethyl）-benzimidazol-2-ylmethyl]ether（3py-bbo for short）and bis[1-（pyridin-4-ylmethyl）-benzimidazol-2-ylmethyl]ether（namely 4py-bbo）,were designed and synthesized.Next,seven structurally novel coordination polymers were synthesized by a hydrothermal-solventothermal method using 3py-bbo/4py-bbo as the primary ligand,terephthalic acid（H₂BDC）,isophthalic acid（H₂ip）or homophthalic acid（H₃BTC）as secondary ligands and cobalt/nickel salts as metal sources.[Co₂（4py-bbo）（ip）₂（H₂O）₂]_n（1）{[Co₂（4py-bbo）（ip）₂（H₂O）₂]·（H₂O）₄}_n（2）{[Co₃（4py-bbo）₂（BTC）₂（H₂O）₄]·（H₂O）₂}_n（3）{[Co₃（3py-bbo）₂（BTC）₂（H₂O）₆]·（H₂O）₂}_n（4）[Co（3py-bbo）₂（BDC）（H₂O）₂]_n（5）{[Ni₂（3py-bbo）（BDC）₂（H₂O）₃]·（H₂O）₃}_n（6）{[Ni（3py-bbo）₂（BDC）（H₂O）₂]·（H₂O）₈}_n（7）Coordination polymers 1 and 6 are three-dimensional framework structures,2 and 3 are two-dimensional lamellar structures,while 4,5 and 7 are one-dimensional chain structures,according to structural study.Cobalt ions are found in cobalt coordination polymers 1 to 3 in two different coordination geometries:a six-coordinated twisted octahedron and a tetra-coordinated twisted tetrahedron.Cobalt ions present in cobalt coordination polymers 4and 5,both of which have a six-coordinated twisted octahedron geometry.Nickel coordination polymers 6 and 7 contain nickel ions in a six-coordinated twisted octahedron geometry.The 4py-bbo has a bridging mode ofμ₃-（κ²N,κ¹N,κ¹N）in both coordination polymers 1 and 2 andμ₂-（κ²N,κ¹N）in 3;3py-bbo has a bridging mode ofμ₂-（κ¹O,κ²N,κ¹N）andμ₃-（κ¹O,κ²N,κ¹N,κ¹N）in coordination polymers 4 and 6,respectively,whereas in 5 and7 are mono-dentate ligands.The bridging pattern of the auxiliary ligands in coordination polymers 1 to 7 are allμ₂-（κ¹O,κ¹O）.（2）Electrocatalytic performance for hydrogen evolution reaction（HER）The electrocatalytic performance of CPs/AB/Nafion modified glassy carbon electrodes（1～7/GCE）for HER were investigated in 0.5 M sulphuric acid solution using cyclic voltammetry（CV）,linear scanning voltammetry（LSV）and electrochemical impedance spectroscopy（EIS）.The results showed that theη₁₀values of 1～7/GCE were positively shifted by 311～467 m V and the Tafel slope was 137.3～201.0 m A dec^-1compared to AB/Nafion modified glassy carbon electrodes（bare/GCE）at room temperature,indicating that all the coordination polymers 1～7 have good electrocatalytic performance for HER and the order of activity was 2>4>3>6>1>7>5.Based on the determined structure analysis the electrocatalytic activity of CPs 1～7 are generally influenced by the number of coordination water molecules and their mutual position.Specifically,for the geometric isomeric CPs 1 and 2,the electrocatalytic activity of trans-2 is superior to that of cis-1,which is attributed to the spatial site resistance formed by the relative positions of the two coordination water molecules in the coordination polymers,while for the heterogeneous homocrystalline CPs 5 and 7,the nickel ion has a smaller radius than the cobalt ion and is more strongly polarised,making the coordination water molecules more susceptible to activation,leading to a higher activity of the nickel CP 7.（3）Electrocatalytic performance for oxygen evolution reaction（OER）The whole process of electrolysis of water can be divided into two half-reactions,HER and OER,where OER is a kinetically slow four electron process that severely affects the efficiency of hydrogen production from electrolytic water.Therefore,in this paper,the electrocatalytic performances of CPs 1～7 for OER were further investigated by CV,LSV and EIS in 0.5 M sulphuric acid solution.Theη₁₀values of 1～7/GCE were negatively shifted by252～341 m V relative to bare/GCE at room temperature,with a Tafel slope of 239.3～293.3m V dec^-1;in particular,2/GCE exhibited the most efficient electrocatalytic OER activity with a negative shift of 341 m V in itsη₁₀value;the Tafel slope was 239.3 m V dec^-1.The results indicate that all 1 to 7/GCE were effective in mitigating the effect of OER on the efficiency of hydrogen production from electrolytic water,and that the order of the electrocatalytic activities for OER and OER was consistent.（4）Electrochemical sensing performanceThe electrochemical sensing performance of 1～7/GCE was investigated by chronoamperometry in 0.2 M phosphate buffered salt solution（PBS）.It was found that the cobalt coordination polymer modified electrodes（1～5/GCE）were capable of quantifying identified ascorbic acid over a linear range from 0.5 to 4000μM,with 2/GCE showing the best electrochemical sensing performance with a theoretical detection limit of 0.074μM(sensitivity of 42.642μA m M^-1).The nickel coordination polymer modified electrodes（6～7/GCE）were able to quantify nitrite over a linear range from 0.5 to 4000μM,with the6/GCE in particular showing a theoretical detection limit as low as 0.047μM(sensitivity82.954μA m M^-1).The above experimental data suggest that differences in the metal centres of the coordination polymers may lead to different targets being specifically identified.In conclusion,in this paper,seven structurally novel coordination polymers were successfully constructed by selecting different metal ions,primary and secondary ligands,and adjusting the p H of the solvent system.The synthesized coordination polymer molecular electrocatalysts all exhibited good electrocatalytic properties（HER,OER and sensing）.By analysing the structure activity relationships of electrocatalysts,the effects of the number and relative position of the ligand water and the radius of the metal ions on the electrocatalytic activity were investigated.