| The proton exchange membrane(PEM),a crucial component in many electrochemical devices including fuel cells,vanadium flow batteries(VFBs),and reverse electrodialysis(RED),plays a role of conducting protons while separating reactants.An ideal PEM should exhibit high proton conductivity,and in VFBs and RED,it should have high ion selectivity as well.Notably,the proton conductivity is highly dependent on the water content of the PEM,and it may decrease significantly under conditions of high temperature and low humidity,due to the severe water loss.Additionally,because protons,and other ions as well,transport in PEMs along the hydrophilic domains,achieving a balance between conductivity and selectivity can be challenging,as higher proton conductivity is typically associated with lower ion selectivity.To address these challenges,organic-inorganic nanocomposite membranes can leverage the advantages of both inorganic nanofillers and polymer matrices,making them a promising replacement for conventional membranes.Hydrophilic nanofillers can increase the hydrophilic domains for the PEM,while the hollow nanofillers with specific pore sizes can sieve the ions efficiently.Most nanofillers,such as cellulose nanocrystal(CNC),graphite carbon nitride(g-C3N4),covalent organic frameworks(COFs),and metal organic frameworks(MOFs)have high aspect ratios or ordered internal pore networks that can provide additional proton transfer pathways or pores with specific size.However,the lack of protonconducting groups in these nanofillers makes them suffer from insufficient proton conductivity.Additionally,although some proton conductors like phosphotungstic acid(HPW)and polystyrene sulfonic acid(PSSA)exhibit high proton conductivity,the high water solubility has limited their application in proton exchange membranes(PEMs).Based on the aforementioned considerations,the following studies have been undertaken:(1)The narrow and serpentine ion pathways in PEMs are one of the main causes for low proton conductivity.One-dimensional nano-fillers have the potential to construct long-range ordered proton pathways so that the proton transfer can be facilitated in PEMs.However,it is still a challenge to fabricate a highly proton conductive water-insoluble nanofiller.In this work,we coated a COF layer on the surface of polydopamine-decorated CNC via the surfaceinitiated polymerization method.The imine groups on COF layer,as anchor sites,can covalently bind the CNC with phosphotungstic acid(HPW)after a hydrothermal treatment.The HPW/CNC nanohybrids(PCC)effectively prevented HPW leakage,and their linear structure facilitated the construction of long-range ordered proton pathways,which enhanced the proton transfer of Nafion/PCC nanocomposite membranes in low relative humidity(RH)conditions.At 25℃under 25%RH,the proton conductivity of Nafion/PCC-8 nanocomposite membrane improved to 85×10-4 S cm-1,more than one order of magnitude larger than that of the recast Nafion.The increased proton conductivity of the Nafion/PCC-8 nanocomposite membrane under low RH conditions resulted in an outstanding performance in the H2/O2 PEMFC single cell test.At 80℃ under 50%RH,the membrane electrode assembly(MEA)based on Nafion/PCC-8 membrane achieved the peak power density of 904 mW cm-2,197%higher than that based on the recast Nafion(304 mW cm-2).(2)A hydrothermal method was applied to synthesize the HPW functionalized COF nanohybrids(P-COF),in which the HPW was immobilized within the cavities of COF through the chemical bonds.The strong acidity of HPW and the ordered structure of P-COF facilitated the fast and efficient protonconduction in the Nafion/P-COF nanocomposite membranes,resulting in superior proton conductivity along with outstanding performance in H2/O2 PEMFCs under low humidity.At 25℃ under 55%RH,the proton conductivity of Nafion nanocomposite membrane with 4 wt%P-COF(Nafion/P-COF-4)reached a proton conductivity of 0.061 S cm-1,which is 307%higher than that of the recast Nafion.The membrane electrode assembly(MEA)based on Nafion/P-COF-4 membrane exhibited excellent performance in a H2/O2 fuel cell,achieving the peak power density of 724 mW cm-2 at 80℃ and 50%RH,138%higher than the MEA based on the recast Nafion.(3)Water-insoluble P-C3N4 nanohybrids was prepared via the hydrothermal reactions between HPW and another 2D material,g-C3N4 nanosheets,in which HPW was chemically bonded with g-C3N4.The introduction of P-C3N4 nanohybrids promoted the proton transport by providing the strong acidity of HPW and the continuous 2D proton transport pathways,resulting in the proton conductivity reaching up to 0.086 S cm-1 in liquid water and 0.91×10-3 S cm-1 at 45%RH at 20℃ for the sulfonated poly(ether ether ketone)(SPEEK)nanocomposite membrane,which was 50%and 6000%higher than the SPEEK control membrane(0.056 S cm-1 and 1.5×10-5 S cm-1),respectively.The fuel cell performance for the MEA based on the SPEEK/P-C3N4 nanocomposite membrane was significantly improved,achieving a 67.8%increase in the maximum power density.(4)For 3D material,the HPW-MOF(MIL-101-NH2)nanohybrids(HMN)was prepared.The formation of chemical bonding between HPW and MIL-101NH2 in HMN during the sintering process renders HMN water insolubility and prevents the HPW leakage in water.More importantly,HPW occupies the pores of MIL-101-NH2 to block the permeation of vanadium ions and creates additional continuous ordered pathways for proton transport,allowing the improvement of proton conductivity and the reduction of the vanadium permeability simultaneously for the SPEEK composite membranes filled with HMN.The SPEEK composite membrane with 6 wt%HMN(SPEEK/HMN-6)exhibits proton conductivity of 0.070 S cm-1 and vanadium permeability of 1.4×10-7 cm2 min-1 at 25℃,63%higher and 80%lower than those of the SPEEK control membrane(0.043 S cm-1 and 6.9×10-7 cm2 min-1),respectively.The VFB assembled with the SPEEK/HMN-6 composite membrane demonstrated an energy efficiency of 82.1%at 120 mA cm-2,much higher than that with the SPEEK control membrane(76.9%).(5)The UiO-66-NH2 threaded with PSSA(S-UiO)was prepared by in situ polymerization of the sulfonated monomer impregnated in the pores of UiO-66NH2.The incorporation of S-UiO into the SPEEK increased the size of hydrophilic domains and the extent of phase separation for the SPEEK/S-UiO composite membranes,resulting in the significant enhancement of proton conductivity.Furthermore,S-UiO also acted as the barriers for the vanadium ion permeation.As a result,compared to that of the SPEEK control membrane,the SPEEK/S-UiO membrane with 15 wt%S-UiO exhibited 63%higher proton conductivity and 83%lower vanadium permeability.This led to much improved VFB performance for the composite membrane with an excellent EE of 83.9%under 120 mA/cm2,much higher than that of the SPEEK control membrane(77.3%).(6)The uniaxial-aligned nanofibers was fabricated by electrospinning polyacrylonitrile with UiO-66-SO3H(N/P/US).The dense N/P/US nanofiber membranes were prepared by the impregnation of uniaxially-aligned nanofibers with Nafion dispersion.The highly aligned nanofibers,on which the proton conductors were attached,were expected to construct long-range ordered ion pathways in the N/P/US nanofiber membranes,thus facilitating ion transfer,especially along the nanofibers.As a result,in-plane proton conductivity and mechanical strength of the N/P/US nanofiber membranes from a parallel direction were highly promoted compared to those from a perpendicular direction.Meanwhile,through-plane area resistance and permselectivity of the N/P/US nanofiber membranes were found to be isotropic and similar to those of the randomly-oriented nanofibers(R-N/P/US)nanofiber membranes,which are based on the random-aligned nanofibers.Moreover,the area resistance of both nanofiber membranes was only 35.3%of recast Nafion,while remaining high permselectivity.The performances of the RED stack with the R-N/P/US nanofiber membranes were tested under 5 and 50-fold salinity gradients,showing 81.8%and 97.2%improvement of the peak power density compared to that with recast Nafion,respectively. |
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