| In the past 20 years,crystalline polyoxometalates(POMs)have attracted much attention in the research of new solid proton conductive materials due to their controllable structure,high crystallinity,excellent water and thermal stability,rapid charge transfer ability,good hydrophilicity and water retention capacity.At present,many exciting achievements have been obtained in the research of crystalline POM-based proton conducting materials.However,it still faces great challenges for its further practical application in fuel cells.The main problems include:only a small number of crystalline POM-based proton conducting materials show ultra-high conductivity;the research on proton conduction path and mechanism of these materials is relatively scarce;their brittleness and crystallinity lead to poor processability,so the research on the preparation and development of their membrane materials is still very limited.Based on this,this paper plans to extend the research of POM-based proton conductive materials by pre-designing,post-modification and synthesizing blended polymer membranes.Specific research contents are as follows:1.A novel 3D pure inorganic framework based on POM clusters rich in phosphoric acid groups was obtained by hydrothermal synthesis,H7{[Na2(H2O)2Na4Fe4?(H2O)4(PO4)][Fe?0.5Mo4?Mo2?(OH)O14(PO4)4]4[Na2(H2O)4Fe4?(H2O)8]}·10.36H2O(1).To our knowledge,this is the first case of a POM compound containing a mixed valence{P4MoV4MoVI2}cluster.It has underwent a single crystal to single crystal conversion in the air,forming a new framework,H3{[Na2(H2O)2Na4Fe4?(H2O)4(PO4)][Na0.5(H2O)Fe?0.5Mo4?Mo2?(OH)O14(PO4)4]4[Fe4?(H2O)8]}·12H2O(1').The crystal structure and X-ray photoelectron spectroscopy showed that the single crystal conversion process was related to the oxygen in the air,and the oxidation states and coordination environment of some metals changed simultaneously during the conversion process.Interestingly,although the two compounds exhibit a similar 3D structure,the crystal water molecules within them are arranged in completely different ways.In compound 1,the water molecular chains formed by crystal water molecules in the 1D pathway are discrete,while a continuous band of water molecules is formed in the pathway of compound 1'.AC impedance analysis showed that compound 1'exhibited extremely high proton conductivity(1.33×10-2 S cm-1)at 95°C and 98%RH,which was comparable to that of commercial Nafion membranes.In addition,compound 1'has stable proton conductivity at higher temperatures(95°C),unlike most POM-based compounds,which have reduced proton conductivity due to the loss of water molecules at>80°C.These properties are attributed to the excellent water retention ability of compound 1'and the continuous and abundant hydrogen bond network formed between the water molecular band and the coordination water molecule on the main framework and the abundant oxygen atoms on the{P4MoV4MoVI2}cluster.In addition,compound 1'has a good stable response to humidity at room temperature,which indicates that it is expected to be used in humidity sensors.2.Constructing acid-base pairs has become one of the effective strategies for designing materials with high proton conductivity.Taking into account the characteristics of protonated organic amines that can provide protons,POM anions can accept protons,and they can form an acid-base combination through electrostatic interaction.A series of acid-base adducts consisting of different polyamine cations and the classical Stranberg-type POM anion[HxP2Mo5O23](6-x)-(x=0,1,2)(expressed as P2Mo5)have been synthesized,including[C6N4H22][H2P2Mo5O23]·H2O(P2Mo5-TETA),[C4N3H16]2[P2Mo5O23]·H2O(P2Mo5-DETA)and[C2N2H10]2[H2P2Mo5O23](P2Mo5-EN),(TETA=triethylenetetramine,DETA=diethylenetramine,EN=ethylenediamine).Crystallography analysis shows that,the nitrogen-containing sites of the organic amines in the three adducts are all protonated,and the oxygen site of the P2Mo5 anion can not only be used as a basic group that accepts protons,but also form abundant hydrogen bond networks with protonated organic amines.These characteristics contribute to the rapid proton transfer.Three adducts can be regarded as a model system based on polyamine-P2Mo5.Combined with their crystal structure analysis,this paper systematically discusses the influence of the p Ka value,proton density and size of polyamine cations on the conductivity of adducts.It is expected that the rules obtained will provide guidance for the design and optimization of conductive materials for organic amine POM-based proton conductive materials.3.Vanadium borates are a class of POM compounds with various attractive structures constructed from[VOx](x=4,5,6)and[BOx](x=3,4)polyhedron units.Recent studies have shown that the rich hydrogen bond network and counterbalance cations in the structure of vanadium borates make them a new candidate for proton conducting materials.In this paper,the 3D vanadium borate{[Cd(H2O)2]3V(12B18O54(OH)6}·4H3O·26H2O(VB)with two kinds of open channels was selected as the research object.The proton conduction properties of VB-based materials were investigated from four aspects.First,the AC impedance of the VB powder pressed tablet sample was measured,and it showed a medium level of proton conductivity at room temperature.This is the first case of proton conductivity of 3D structure vanadium borate.Second,the anisotropic conductivity tests of vanadium borate single crystal samples were carried out for the first time,and the relationship between the porous structure of vanadium borate and the anisotropic proton conductivity was deeply analyzed.Third,VB-Gl was prepared by post-modification method.The doping of polyhydroxyl Gl leads to the formation of a continuous proton transport path in the composite,which can optimize the proton conduction properties of VB powder samples.Fourth,a series of PVA mixed membranes PVA-Gl-VB-x containing VB and Gl are prepared by solution casting method.By optimizing the preparation conditions,the hybrid membrane exhibits good dispersibility,enhanced mechanical properties,increased water adsorption rate,optimized proton conductivity and low methanol permeability.This method provides a new impetus for the application of vanadium borate-based conductive materials in membrane. |
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