Fabrication And Investigation Of Proton Conductive Metal-organic Frameworks And Composite Membranes

The continuous consumption of traditional fossil energy has caused a global energy crisis and environmental pollution,so the rapid development of renewable clean energy is imminent.Proton exchange membrane fuel cells(PEMFC),converting hydrogen energy into electricity,are one of the key technology vehicles for replacing fossil fuels and producing renewable and clean energy.Multifunctional metal-organic frameworks(MOFs)materials with well-defined crystal structures and abundant structural tunability show great potential and special advantages in the application of solid-state proton conductors and proton exchange membranes for fuel cells.Based on the components and structural characteristics of MOFs,the design strategy to obtain high proton conductivity revolves around two main aspects:increasing the proton carriers concentration and construction of high-density hydrogen-bonding networks.The main sources of proton carriers are guest molecules(water molecules,inorganic or organic acids,organic molecules such as imidazole,histamine,etc.),counter cations in the framework(H₃O⁺,NH₄⁺,and Me₂NH₂⁺,etc.),and functionalized organic ligands(–SO₃H,–COOH,–OH,etc.).Depending on the proton carrier,protons hop and diffuse along with hydrogen bonds in the pores.Therefore,an effective and continuous hydrogen bonding network between the proton carrier and the pore channel is the key to ensuring fast proton transport.On this basis,in this paper,a Cr-based MOF with good stability was selected to introduce a polar proton carrier to construct a stable hydrogen bond network in the pores.Based on the MOFs with high proton conductivity,MOF-polymer composite proton exchange membranes were further prepared and evaluated for their efficiency under a fuel cell environment.Cr-based MOFs(PMNS1,PTNS1)with extreme acid-base stability and high-density sulfonic acid functionalization were prepared by ligand insertion and post-modification synthesis.Unlike the rigid structure of PTNS1,PMNS1 integrates the multivariate flexibility of the intrinsic main framework and sub-structure fragments.Benefiting from the synergistic self-adaptive effect between multivariate flexibility,PMNS1 form a continuous hydrogen bonding network in a wide humidity range,exhibiting high proton conductivity.The same functional groups exhibit dramatic differences in the performance of rigid and flexible MOFs,demonstrating that the construction of continuous proton transport channels through flexible structures is an effective strategy for designing MOFs with high proton conduction performance.The construction of continuous proton transport channels through flexible structures is an effective strategy for the design of MOFs with high proton conductivity.Based on the structural characteristics of multivariate flexibility MOFs,different sultones were introduced to discuss the effect of the sub-structure fragment's flexibility on proton conductivity 1,4-butane sultone and 1,3-propane sultone functionalized PMNS2 and PMNS1 have similar structures and multivariate flexibility.The difference in the chain length of the alkyl sulfonic acid will lead to a huge difference in polarity,resulting in an approximately three times difference in proton conductivity.Based on the excellent stability and proton conductivity exhibited by PMNS1 and PMNS2,we fabricated a series of MOF-PAEK composite membranes with good mechanical properties and thermodynamic stability using Poly(arylene ether ketone)(PAEK)as the polymeric matrices.Among them,the membrane electrode of PMNS1-40 exhibits a power density of 34.76 m W/cm² in methanol fuel cells,expanding the application of MOF proton conductors in a practical fuel cell environment.To improve the poor proton conductivity of rigid MOFs at low humidity,we used ion-dipole and ion-ion interactions to orderly assemble ionic liquids into the pore channel of PTNS1.The obtained product PTNS1-IL forms a continuous hydrogen bonding network over a wide humidity range,which enhances the proton conductivity.To further transform PTNS1-IL from proton conductor to proton exchange membrane,we compounded sulfonic acid and carboxylic acid functionalized SPVA-CBA polymer with PTNS1-IL by hydrogen bonding and prepared the first MOF-polymer composite membrane with self-healing function.Meanwhile,the composite membrane is enriched with multiple coupled proton transport channels,enabling an order of magnitude improvement of proton conductivity.