Ligand-Based Regulatory Strategy Impact On Electrochemical Performance

The tension between energy supply and economic progress is reshaping the global landscape.Seeking a green,clean and recyclable energy conversion mechanism is the common goal of the current sustainable development strategy.Electrocatalytic technology is preferred due to its low carbon,environmental,high efficiency,and energy saving characteristics.It plays a unique role in energy storage,conversion and environmental mitigation,such as the oxygen evolution reaction（OER）of electrolyzed water and the electrocatalytic reduction of CO₂（CO₂RR）.However,the large-scale and commercial application of electrocatalytic technology must face the problem of catalyst selection.Synthesis of cost-effective,long-lasting and production-friendly catalysts that are selectively controllable is a significant barrier to their development.It is essential to choose a structurally stable and customizable material as a research basis for designing and screening substrates that meet specific requirements in various environments and conditions.Based on the periodic arrangement of the structure of metal-organic framework materials（MOFs）and the stability of porphyrin molecules,a series of catalysts with different functions were designed and synthesized from three aspects:functional group regulation of ligands,global introduction of organic ligands,and ligand deletion.This progressive relationship can not only reveal the structure-performance relationship,but also study the electrocatalytic reaction mechanism to achieve the goal of functional customization and enhanced activity of catalytic materials.The research content and innovations of this paper mainly include:（1）Using the most common ZIF materials as templates,a series of catalytic materials controllable by ligands and metal centers（M[C₆H₆N₄]X,M=metal,X=functional group）were synthesized through a simple room temperature reaction.The mechanism of MOF ligands regulating the electrocatalytic performance of OER was established,and the regulation of electron absorption and electron donor groups on the electron cloud density of metal nodes was explored.The change in physicochemical properties was investigated by characterization and identification of the morphological structure and experimental tests,and it was found that the electron cloud of the Co site is more favorable for the adsorption of OH^-and the reduction of the outer electron layer,which leads to the lowering of its energy barriers and promotes the production and desorption of the reactive substances*OH or*OOH.The overpotential of Co[C₆H₆N₄]NO₂ at 10 m A·cm^-2 was only 280 m V,and the Tafel slope was only 151m V·dec^-1.On the other hand,the electronic conductivity was reduced due to the wrapping of the active site by the layer of electron-donating groups,which led to an increase in the energy barrier of the OER reaction and hindered the active intermediate formation,which in turn severely affects the progress of the electrocatalytic reaction.Through this work,a new strategy for tuning the electron density of the metal sites to improve the OER performance is offered and provides a valuable reference for the development of the design and synthesis of efficient molecular catalysts for use in electrocatalytic processes.（2）The successful application of regulating functional group ligand structure can be extended and utilized for the introduction of organic ligands such as 2-hydroxybenzaldehyde.The spatial effect of the porphyrin molecule itself can well prevent the C-C coupling of*CO intermediates.Meanwhile,the atropisomer o-Cu-Por Sa（αβαβ）was able to immobilize or adsorb H₂O and CO₂ molecules through the hydrogen bonding of the ligand on the column,which improved the CO₂ coverage and accelerated the CO₂RR process.The comprehensive characterization experiments of the catalyst’s physicochemical properties underscore its exceptional hydrophobicity and robust CO₂ adsorption capacity.When operated at-1.4 V vs.RHE and 0.1 M KHCO₃,the catalyst exhibited significant catalytic activity towards CH₄,with an FE（CH₄）value of 84%and a partial current density j（CH₄）of 33 m A·cm^-2,demonstrating excellent stability.The DRIFTS spectra and isotopic calibration confirmed that the hydroxyl group of the second coordination layer of o-Cu-Por Sa（αβαβ）can bind to CO₂and H₂O molecules,and the subsequent exothermic reaction generates*CO₃H₂,which significantly reduces the adsorption energy of CO₂.Furthermore,the interaction between the intermediate*CO and H bond,as well as the steric hindrance effect,impeded the occurrence of C-C coupling reaction,thereby enhancing the selectivity of CH₄.The DFT calculations corroborated the advantages of the aforementioned catalysts.The incorporation of organic ligands not only optimizes the desorption of reaction intermediates but also suppresses HER competition reaction and boosts electrocatalytic activity.This work highlights the positive impact of organic ligand regulation strategies on CO₂RR electrocatalysis,providing valuable insights for the rational design of novel and efficient catalysts.（3）Regulation of functional groups and the deliberate introduction of ligands are essential components in the design of effective catalysts.Therefore,we use reverse thinking to enhance electrocatalytic performance through defects and unsaturated check points caused by ligand deletion.By mixing the molar ratio of MOF ligands,an asymmetric ferrocenecarboxylic acid（Fc）was used as a substitute to synthesize a catalyst with changed CO₂ reduction selectivity.After ligand regulation,oxygen vacancies increased,hydrophobicity was enhanced,and MOF material changed from Cu-O₄ to Cu-O₃ or Cu-O₂.After the regulation of asymmetric ligands,the chemical valence state changed from Cu²⁺to Cu⁺,which provided favorable conditions for the formation and adsorption of intermediates in subsequent electrocatalytic reduction reactions.In the characterization experiments of physical and chemical properties,it was found that the morphology of Cu-TDC-Fc_0.1 was unchanged from that of the original MOF material,and both presented a stable spherical structure with a rough and concave surface.In the electrocatalytic experiment,it showed good selectivity for C2products.The FE of C₂H₄ in-1.1 V vs.RHE,0.1 M KHCO₃ solution reached 70%,and the partial current density j（C₂H₄）was 31 m A·cm^-2.It could maintain a good condition in the long-term stability test,and its morphology and structure changed little.DRIFTS spectroscopy confirmed that surface defects and oxygen vacancies in Cu-TDC-Fc_0.1could reduce the CO₂ adsorption energy barrier and promote the formation of intermediates.The adjacent Cu⁺unsaturated metal check point can effectively fix*CO and accelerate the C-C coupling reaction process.The introduction of Fc has a positive effect on the electron conduction rate.In addition,the periodicity of the MOF structure can further strengthen these characteristics,thus promoting the CO₂RR process.This work opens up a new path for the design of ligands for catalysts,and has certain reference value for the selection of other electrocatalytic reaction materials.