Design And Synthesis,Structure,and Property Study Of Crystalline Proton-conducting Materials Based On Carboxylic Or Sulfonic Ligands

The increasing energy consumption and environmental pollution all over the world has prompted the pursuit of new clean energy sources.Proton exchange membrane fuel cell technology seems to be regarded as one of the most efficient and environment-friendly methods.Nowadays,the widely used perfluorosulfonate Nafion membrane exhibits a high proton conductivity of 0.13 S cm^-1 at 75°C under100%relative humidity（RH）.However,it suffers from a number of short comings such as low proton conductivity above 100°C,complicated synthetic process as well as expensive price,which limits the development of fuel cells.However,the complex microstructure and phase problem cause difficulty in clearly understanding the relationship between the structure and proton conductivity.Thus,it is significant and challenging for developing a series of molecularly adjustable crystalline materials with high proton conductivity to make a direct connection between the structure and activity.In this paper,a series of new crystalline materials based on carboxylic and sulfonic ligands with high proton conductivities were constructed via different synthetic methods and structurally characterized by IR,elemental analyses,TGA and single cryatal X-ray diffractions.The proton-conducting properties of these crystalline materials had been studied.We revealed the relationships between structural features and proton-conducting properties by adjusting their structures.The main contents are as follows:1.We employed the HCl steam-assisted conversion method and conventional hydrothermal method based on H₃sfpip ligand（H₃sfpip=2-（2,4-disulfophenyl）imidazo（4,5-f）（1,10）-phenanthroline）to synthesize three nonporous coordination complexes（compounds 1-3）.[Cu（Hsfpip）（H₂O）₂]·H₂O（1）[Cu H₂（Hsfpip）₂（H₂O）]（2）[Cu H（Hsfpip）Cl（H₂O）]（3）We changed experimental methods to adjust proton conductivity by stepwisely protonating sulfonated ligands and introduction of halide ions,and we revealed the relationship between the nature of proton conduction and structural features.The protonated sulfonate group together with the halide Cl^-group plays a positive role in increasing proton conductivity.Three resulting coordination compounds 1-3 showed high proton conductivity with a maximum value of 1.43 m S cm^-1 for 1,2.58 m S cm^-1 for 2 and 15 m S cm^-1for 3 at 95°C and 97%RH,respectively,and meanwhile,we proved their proton conduction nature and electron resistors using D ₂O-exchanged experiments and the Hebb-Wagner direct current polarization method.We believe that these nonporous solid electrolytes intrinsically posses s proton carriers and may avoid the fuel crossover,which makes them good candidates for PEMFCs in real-life applications.2.We utilized the HCl steam-assisted conversion method and conventional hydrothermal method based on H₂spip ligand（H₂spip=2-（2-sulfophenyl）imidazo（4,5-f）（1,10）-phenanthroline）to synthesize three new coordination complexes（compounds 4-6）.Cu₂H₂（Hspip）₂Cl₄·H₂O（4）Cu（H₂spip）Cl₂·H₂O（5）CuH（Hspip）（HPO₄）·H₂O（6）By using multi-functional-group method,the sulfonate group and the Cl^-or HPO₄^2-group were introduced into framework,which could adjust proton conductivity and further disclose the relationship between structure characteristics and the nature of proton conductivity.The protonated sulfonate group together with the halide Cl^-group plays a positive role in increasing proton conductivity,meanwhile the packing mode of the structure is also an important factor influencing proton conduction.The compounds 4-6 exhibited high proton conductivity values in the range of 10^-4-10^-2 S cm^-1 under 95°C and 97%RH,in which compound 5 exhibited the highest value of 1.09×10^-2S cm^-1 at 97%RH and 95°C.This work provides some new ideas for the design and preparation of new proton conductors.3.We synthesized two new HOFs materials via traditional aqueous solution synthesis or environmentally-friendly mechanochemical grinding synthetic method by using sulfonate and N-donor ligands（compounds 7,7S,8and 8S）.（CH₂SO₃H）（C₃N₆H₆）·H₂O（7）（CH₂SO₃H）（C₃N₆H₆）·H₂O（7S）（C₅H₃SO₃H）（CH₂CH₂NH）（8）（C₅H₃SO₃H）（CH₂CH₂NH）（8S）Water contact angles measurements showed that compounds 7 and 8were more hydrophilic than compounds 7S and 8S,indicating that compounds7 and 8 were easier to form hydrophilic proton-conducting pathways,which caused higher proton conductivities.The field-emission scanning electron microscopy were performed on the samples after impedance measurements with grinding,which certified the average particle sizes of the samples obtained by aqueous solution synthesis were larger than that synthesized by mechanochemical grinding method,indicating that grain boundaries played a crucial role in proton conduction.The formed HOFs via traditional aqueous solution synthesis exhibited high proton conductivity value up to 10^-2 S cm^-1through acid-base pairs.The research used the strategy of adjusting particle sizes to change proton conductivity,which may provide some ideas for developing novel proton conducting materials.4.We used tartaric acid,Cu²⁺ion,K⁺ion,acetic acid and Ln³⁺ion(Ln=Nd³⁺and Gd³⁺)via traditional aqueous solution method to obtain four isostructural high-nuclearity 3d-4f metal-oxygen clusters（compound 9S,9R,10S and 10R）.KNa₁₀H₃[Nd₄Cu₂₂（L-（+）-tart）₁₆（CH₃COO）₈]·14H₂O（9S）KNa₁₀H₃[Nd₄Cu₂₂（D-（–）-tart）₁₆（CH₃COO）₈]·14H₂O（9R）KNa₁₀H₃[Gd₄Cu₂₂（L-（+）-tart）₁₆（CH₃COO）₈]·14H₂O（10S）KNa₁₀H₃[Gd₄Cu₂₂（D-（–）-tart）₁₆（CH₃COO）₈]·14H₂O（10R）High-nuclearity metal-oxygen clusters own high-density hydrophilic oxo/hydroxo groups,which can also act as either proton donors or acceptors for hydrogen bonding with guest water molecules,which is crucial to build proton-conducting pathways.Although Ln³⁺ions were different in metal-oxygen clusters,compounds 9S and 10S exhibited similar proton-conducting behaviours,they had the highest proton conductivity values of 1.18×10^-2 and 8.03×10^-3 S cm^-1 at 97%RH and 25°C,which were comparable to commercial Nafion.The work may provide experimental guidance and basis for the design and functional application of high-nuclearity metal-oxygen clusters.