Preparation Of Acid-base Composite Lamellar Membranes And Intensification Of Proton Conduction

Proton exchange membrane is a key component of the proton exchange membrane fuel cell.The proton conductivity and structural stability of the membrane respectively determine the performance and service life of the full cell.Currently,the proton exchange membranes that have been developed are polymer membranes.However,at high temperature and low humidity,evaporation and water loss of polymer membranes shrink or even collapse the transfer nanochannels,leading to sharply decreased proton conductivity.Meantime,the mobility of the molecular segment is accelerated,causing poor structural stability of the membrane.Compared with polymer membranes,the two-dimensional（2D）lamellar membranes prepared by self-stacking of 2D nanosheets show unique advantages:the continuous and stable interlayered nanochannels.Graphene oxide（GO）laminar membranes with highly ordered nanochannels display ultra-high mass transfer ability and separation selectivity and have attracted much attention in the fields of gas separation,nanofiltration or water treatment.However,they are rarely utilized as proton exchange membranes in fuel cells due to the lack of sufficient functional groups capable of proton transfer and poor structural stability.This paper focuses on conquering two major problems of GO lamellar membranes:inferior proton conduction and structural stability.Acid-base composite lamellar membranes were prepared by filtration-assembly of 1D acidic nanofiber/2D nanosheets and 2D basic nanosheets.The construction of rich and continuous acid-base pairs in interlayers of membrane efficiently enhances proton conduction and structural stability.The details and main conclusions are as follows:（1）The preparation of phosphorylated nanofibers intercalated GO lamellar membrane and study on performance intensification.Acid-base lamellar composite membranes were prepared by“line-plane”filtration-assembly of 1D acidic nanocellulose with high surface area and 2D basic nanosheets for high proton conduction and structural stability.DGO@PBC_nlamellar composite membrane was fabricated by assembling of bacterial cellulose（PBC_n）with different phosphorylated degrees and dopamine-treated GO nanosheets（DGO）via acid-base electrostatic interactions.The construed acid-base pairs between DGO（containinggroups）and PBC_n（containing–P=O（OH）₂groups）create a large number of proton defects,and thus paving low-energy-barrier transfer pathways for proton hopping.As a result,the proton conductivity is highly enhanced.The resultant membrane obtains in-plane conductivity of 215 m S cm^-1,244.9%improvement over GO membrane.Moreover,it acquires a more obvious augment in through-plane conductivity by almost 10 times of that of GO membrane.As a consequence,the transfer anisotropy coefficient remarkably decreases,and hydrogen fuel cell performances are enhanced by over 120%.Meantime,the presence of acid-base pairs endows lamellar composite membrane no breakdown or delamination in water,acid,and basic solutions even after 14 d.Besides,the mechanical properties of the lamellar composite membrane are improved（the tensile strength achieves a value of 203.5 MPa）.（2）The preparation of sulfonated nanosheet intercalated GO lamellar membrane and study on performance intensification.Acid-base lamellar composite membranes were prepared by“plane-plane”filtration-assembly of 2D acidic nanosheets with high surface area and aspect ratio and 2D basic nanosheets for high proton conduction and structural stability.S_nGO@DGO or NGO@DGO lamellar membrane was prepared by assembling acidic S_nGO/NGO nanosheets and basic DGO nanosheets.Acidic S_nGO and NGO nanosheets were gained by modification of DGO nanosheets using sulfonated poly（ether ether ketone）（SP_n）with different sulfonated degrees and Nafion,respectively.The more and continuous acid-base pairs are formed between the–SO₃H groups on S_nGO/NGO nanosheets and–NH/–NH₂groups on DGO nanosheets.The in-plane proton conductivities of DGO@S₂GO and NGO@DGO lamellar membrane achieve the value of 258.5 and 306.2 m S cm^-1,3.9 and 4.6 times over GO membrane,respectively.Compared with GO membrane(1.1 m S cm^-1),more obvious augment in through-plane conductivities of DGO@S₂GO and NGO@DGO membrane are observed,achieving 42.6 and 48.7 m S cm^-1,respectively.Therefore,the transfer anisotropy coefficient is significantly decreased from 48.7 for GO membrane to 5.4 for lamellar composite membrane.Meanwhile,the mechanical properties and structural stability of the membrane are improved.The tensile strength of lamellar composite membrane is about 7.1 times that of GO membrane（211.4MPa）.And the membrane keeps intact after being immersed in water for 1 month.In this paper,a acid-base composite lamellar membrane was prepared by pre-assembly of acidic 1D/2D nanomaterials and basic GO nanosheets.The formation of abundant rich and continuous low-barrier transfer sites（acid-base pairs）in interlayered nanochannels efficiently enhances proton conduction;Meantime,the acid-base electrostatic interaction intensifies structural stability of membranes.This strategy may pave a way to designing advanced proton exchange membrane with highly enhanced performances.