Aliovalent Metal Substitution Strategy For Improvement Of Proton-conductive MOFs Performance

The development of excellent proton-conducting materials for proton exchange membrane fuel cells（PEMFCs）is one of the valid methods to advance renewable new energy,improve energy shortage and solve environment pollution.Metal-organic frameworks（MOFs）,as a new class of porous materials,have been considered as the best candidate materials for the next generation of proton conductors due to their periodic structural units,designable backbone structures and tunable pore surfaces,but remain a challenge since the difficulty in simultaneous fulfillment of rich proton carriers,fast mobility of protons,long-term durable proton conduction and high stability.In order to explore a simple,efficient and general method towards high proton conductivity and to understand transport mechanism,we propose an aliovalent substitution metal strategy and implement in isostructural proton conductive MOFs.Meanwhile,a series of expansive works have been carried out,including the incorporation of protonic carriers into the pores,the grafting of bridging ligands or metal nodes with acid moieties on the frameworks and the modification of open metal sites by post synthesis.In addition,mole cular dynamics（MD）simulation of the structure model was carried out based on the first theoretical calculation principle,and the possible conductive mechanism was proposed combining with experimental data.The dissertation is divided into the following six parts:1.In the introduction,we firstly summarize the research background of PEMFCs including the advantages and disadvantages of the commercial Nafion materials.Subsequently,the development of proton-conductive MOFs under water,no-water and two-in-one conditions is introduced.Then,the current research strategies for improving MOFs performance are classified and listed in detail.Finally,the topic selection and work progress of this paper are summarized.2.In order to improve the performance of proton-conductive MOFs,we creatively propose an aliovalent substitution metal strategy.This idea is verified by 100-fold enhanced conductivity in compounds（Me₂NH₂）₂[Cd（mdhbqdc）₂]（Cd-BQ）and（Me₂NH₂）（Me₂NH）[In（mdhbqdc）₂]（In-BQ）（H₂mdhbqdc=dimethyl3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylic acid）.Accompanied by in situ formation of anilicate ligand,a great number of-OH groups are grafted on the inner wall of the pores,which are interacted with neutral Me₂NH and/or protonated Me₂NH₂⁺cations via N-H···O hydrogen bonds.These structural characteristics enable In-BQ to exhibit moderate conductivity(2.10×10^-4 S/cm at 303 K and 95%RH)with activation energy of 0.73 eV（95%RH）.With a simple substitution of Cd（Ⅱ）for In（Ⅲ）,the original Me₂NH molecules and Me₂NH₂⁺cations in In-BQ are protonated into the doubling of Me₂NH₂⁺cations,indicating more frequent hopping and multiple proton-transfer pathways.This indication is supported by a very high protonic conductivity of 2.30×10^-2 S/cm and reduced activation energy of 0.48eV at the same condition.MD simulation visually elucidates that compared with In-BQ,aliovalent substituted Cd-BQ have shorter proton migration distances,which in combination with more proton numbers results in more frequent hopping and sliding of protons,in agreement with the experimental results.This aliovalent metal substitution strategy provides a simple,efficient and general method for in situ enhancing carrier density towards high proton conductive MOFs.3.Based on the above work,we further extend the aliovalent metal substitution strategy by ligand post-modification to obtain maximum proton sources.In-BQ is a 3D diamond framework constructed by-COOCH₃ functionalized anilicate linkers and its 2D intersected channel filled with Me₂NH molecule and Me₂NH₂⁺cation,which prompts us to sequentially modify by the cooperation of aliovalent Cd（Ⅱ）substitution and post-synthetic ligand esterolysis.The modified framework Cd-BQ-COOH has three kinds of proton sources:the first are the attached-OH groups,which could dissociate H⁺during the coordinate progress;the second are the filled Me₂NH molecule and Me₂NH₂⁺cation,which could protonate into the doubling of Me₂NH₂⁺cations through aliovalent replacement of In（Ⅲ）by Cd（Ⅱ）;the third are the functionalized-COOH groups,which are potentially converted from the post-synthetic esterolysis of-COOCH₃ on anilicate ligands.Compared with pristine In-BQ,the modified Cd-BQ-COOH exhibits 300-fold enhancement with the value up to 6.06×10^-2 S/cm（303 K and 95%RH）.This work not only confirms the effectiveness and feasibility of the aliovalent metal substitution strategy extended on ligand modification,but also provides a promising route to maximize the proton conduction performance of MOFs materials.4.The previous works was carried out from the perspectives of carrier loading and ligand functionalization,it is necessary to further develop this strategy on anchoring metal nodes.After browsing and screening,we found a chiral 3D framework InOF-1[In₂（OH）₂（BPTC）]·6H₂O(BPTC^4-=3,3’,5,5’-biphenyl tetracarboxylic acid).It has a typical1D nanochannel with a large number of helically distributed-OH groups grafting onto its inner wall.This distribution facilitates the infinite migration of protons along the c-axis.With Ni（Ⅱ）replacing In（Ⅲ）,we successfully prepared isomorphic NiOF-1（[Ni₂（BPTC）（HCOOH）₂]·3H₂O）and formic acid replaced the original-OH groups of InOF-1,which was in situ anchored on the inner wall and pointed to the interior of the pore.In addition,the cross section distance inside the channel decreased from 11.38×11.38（?）in InOF-1 to10.87×10.87（?）in NiOF-1.The large number of-OH groups and the migration of lattice water molecules in the pores have caused InOF-1 showing moderate conductivity(7.86×10^-3S/cm at 328 K and 95%RH).Notably,adsorption experiments and Monte Carlo simulations indicated that more water molecules can enter the NiOF-1 channel at 95%RH.By simple Ni（Ⅱ）substitution and formic acid modification,the proton conductivity of NiOF-1 exhibited five-fold than InOF-1,reaching 3.41×10^-2 S/cm at the same condition.This study provides a new idea for the functionalization of metal nodes in proton-conducting MOFs.5.Compared with single metal MOFs,cluster-based MOFs have larger pore volume and higher stability.In order to develop MOFs materials with high proton conductivity and high stability,we screened three cluster-based soc-topology structures,In-ABTC,In/Mn-ABTC and Mn-ABTC and studied their proton conductivities by aliovalent metal substitution strategy.With the gradual replacement of In（Ⅲ）by Mn（Ⅱ）,the cubic cage volumes of In-ABTC,In/Mn-ABTC and Mn-ABTC decrease in sequence,and the counterions transform from NO₃^-anion to H₂O molecule and then to Me₂NH₂⁺cation,which plays a key role in improving carrier density and proton mobility.However,compared with In-ABTC,In/Mn-ABTC,the coordinated DMA solvent molecules on Mn-ABTC occupies the unsaturated metal sites,and hinders the effective proton migration in some extent.Mn-ABTC-H₂O was successfully prepared by removing DMA molecules and anchoring H₂O molecules.Compared with Mn-ABTC,the proton conductivity of Mn-ABTC-H₂O increased by 5 times,reaching 1.14×10^-2 S/cm（328 K,95%RH）.In this study,the aliovalent metal substitution strategy combined with open metal sites to synergistically promote the proton conductivity of cluster-based MOFs.It not only forms strong interaction with guest molecules,but also plays a key role in adsorbing and exchanging proton carriers.6.We systematically summary the overall work of this paper and further prospect the future work of this project.