Controllable Ions Transportation Through Functionalized Metal-Organic Framework Membranes

Metal-organic frameworks（MOF）,as a kind of novel porous materials,have drove much attention and shown excellent performance for various applications,such as separation,catalysis,energy conversion and storage.It has been one of research hotspots nowadays.And the preparation of MOF membranes has further expanded its application field.We noticed that the selective ion transportation process exhibited obvious influence on the performance of energy conversion and storage devices.Therefore,the battery separator is required not only to maintain rapid ion transportation but also to block active material diffusion.However,the widely used polymer-based separator cannot possess both the aforementioned properties.Attributed to the unique porous structure,MOF exhibited excellent separation property.The inter-connected channels of MOF acted as the ion transporting pathway.It is promising for MOF to realize effective selective ion transportation after decorating its channels.Since the first proton conductive MOF reported by Kitagawa,there were hundreds of MOF structure with high ionic conductivity of 10-2～10-3 S cm-1 after decades’development.However,the recent research on ionic conductive MOF mainly based on MOF powders,which limited the practical application in membrane-based device.In order to further promote MOF applications,we employed metal hydroxide nanostrands as precursor to prepare continuous and well-intergrown MOF membranes through self-confined conversion method and decorated them with functional molecules to endow them with selective ionic transportation.（1）We employed copper hydroxide nanostrands as precursor to prepare continuous polystyrene sulfonate threaded HKUST-1 membrane（PSS@HKUST-1）through self-confined conversion method.The PSS in HKSUT-1 channels constructed 3D sulfonate groups network as ions transportation pathway.Thus,the PSS@HKSUT-1 membrane achieved high Li+ conductivity.Meanwhile,PSS@HKUST-1 membranes exhibited excellent Li+selectivity and high Li+flux over Na+,K+,Mg2+,attributed to the size sieving and ionic affinity to sulfonate group.（2）Based on the above work,we prepared PSS@HKUST-1 membrane on the surface of Celgard separator and we assembled it into Li-S batteries.PSS@HKUST-1 membrane not only supported rapid Li+ transportation in batteries but also effective depressed polysulfide shuttle effect due to the size sieving,electrostatic repulse and adsorption.Therefore,the Li-S batteries with PSS@HKUST-1 membrane demonstrated improved cycle performance.The capacity fading was only 0.05%/cycle after 500 cycles at 0.5 C.The areal capacity was as high as 7 mA h cm^-2 with sulfur loading of 12.27 mg cm^-2 at 0.5 C.（3）We employed zinc hydroxide nanostrands as precursor to prepare single strand DNA threaded ZIF-8 membranes（DNA@ZIF-8）.The DNA in MOF channels improved the water adsorption of DNA@ZIF-8 membranes,and built hydrogen-bond network for proton transportation.In the meantime,the DNA@ZIF-8 membrane was able to block methanol diffusion through membrane since the diameter of methanol was large than the window size of ZIF-8.The proton conductivity of DNA@ZIF-8 membrane was as high as 0.17 S cm-1 under 75℃,95%RH.But,the methanol permeability was as low as 1.25×10-8 cm2 s-1.Further,we assembled the DNA@ZIF-8 membrane into direct methanol fuel cells（DMFC）,the power density of which was 9.87 mW cm^-2.（4）We employed zinc hydroxide nanostrands to prepare sulfobetaine methacrylate threaded ZIF-8 membranes（SBMA@ZIF-8）.Zwitterion SBMA possesses positive and negative segments in its molecule structure,which could facilitate anions and cations transportation,respectively.Therefore,the SBMA@ZIF-8 membrane achieved both proton and hydroxide ion conductivity.On the other hand,the continuous and intergrown membrane can effectively block methanol diffusion due to the size sieving.The proton conductivity of SBMA@ZIF-8 reached up to 3.62×10-2 S cm-1,and the hydroxide ion conductivity was 2.52×10-3 S cm-1 under 75℃,97%RH.The methanol permeability was decreased to 7.66×10-9 cm2 s-1.The direct methanol fuel cells with SBMA@ZIF-8 membranes can operate in both acid and alkaline system.The power density of corresponding acid and alkaline direct methanol fuel cells was 113.77 mW cm^-2 and 12.65 mW cm^-2,respectively.（5）We employed zinc hydroxide nanostrands to prepare ultrathin heparin threaded ZIF-8 membranes（heaprin@ZIF-8）through chemical vapor assisted self-confined conversion.Since the microporous structure and plenty of heparin in channels,the heparin@ZIF-8 exhibited high cation selectivity and depressed anion diffusion thorough membrane.Meanwhile,the heparin@ZIF-8 membrane possessed higher channel density than traditional polymer membranes and the thickness was only about 200 nm,which could allow rapid cation transportation and decrease ionic resistance.Then we assembled the as-prepared heparin@ZIF-8 membranes into salinity-gradient-driven power generator with porous Ag/AgCl membrane electrodes and LiCl electrolyte to reduce internal resistance.The effective area was enlarged to 0.785 cm^-2.As a result,the output power of the power generator was 9.03 μW.