Ion Transport Through Nanochannels In Two-Dimensional-Material Membranes

Two-dimensional(2D)materials have provided an ideal platform for the investigation of novel physical and chemical phenomenon due to the unique structure and physical properties.Self-assembled 2D-material nanosheets membranes,which not only inherit many advantages of pristine 2D materials but also show different characteristics and broad applications in the area of heat dissipation,filtration,desalination,energy storage and conversion,optoelectronic devices,and catalysis.Up to now,although the field of 2D material has developed from single-element material,such as graphene,to multiple-element materials,there are many new systems and unique properties that have not been fully explored.However,the present self-assembled 2D-material nanosheets membranes remained many challenges.For example,the proton conductivity performance of 2D materials membranes,like graphene based membranes,is much lower than that of commercial Nafion proton exchange membrane.The low content and uniformly distributed functional groups in the 2D material membranes caused low surface charge density and low ion selectivity.In addition,2D anion exchange membrane based on intrinsic 2D materials with high density of positively charges has not been found yet.In this dissertation,we systematically studied the fabrication of 2D material membranes based on vacancy resided transition metal phosphorus trichalcogenide(TMPTC),layered double hydroxide(LDH),and vermiculite,and explored their applications in proton exchange membrane,anion exchange membrane,and static pressure difference driven energy conversion.The main results are shown as follows:We synthesized high-quality CdPS3 crystals by chemical vapor transport method.The Cd vacancy resided monolayer Cd0.85PS3Li0.3 nanosheets aqueous colloids were produced by alanine intercalation and exchange method and the Cd0.85PS3Li0.15H0.15 nanosheets aqueous colloids were obtained by further acid treatment.The Cd vacancies on nanosheets,which make them negatively charged and electrostatically repulsed,enable the formation of well-dispersed stable colloids with long term stability and the use of conventional solution-phase processing techniques to fabricate various macrostructures.By using the electrochemical Nyquist plots,we measured the proton and Li ion conductivity of Cd0.85PS3Li0.15H0.15 and Cd0.85PS3Li0.3 membranes made from their dispersions by vacuum filtration at different temperatures and relative humidity(RH).With increased temperatures and RHs,the proton conductivity of Cd0.85PS3Li0.15H0.15 membrane increases and reaches 0.952 S cm-1 at 98%RH and 90?.It is important to note that in the whole temperature range from 30 to 90?,the proton conductivity of our Cd0.85PS3Li0.15H0.15 membranes is several times higher than the polymeric materials,including Nafion,the benchmark for PEM,and 1-4 orders of magnitude higher than those of the graphene oxide(GO)-based materials,metal organic frameworks(MOFs),and bio-derived materials.Based on the theoretical calculation,kinetic isotope effect and activation energy experiments,the proton conduction mechanism were fully understood.In addition,the Cd0.85PS3Li0.3 membranes show high Li ion conductivity of 0.805 S cm-1 at 98%RH and 90?,which value is highest among the Li ion conducting materials in the presence of water.To prove the universality of vacancies resided 2D TMPTC membranes achieving superhigh ion conductivity,we fabricated Mn vacancies resided Mn0.77PS3Li0.46 membrane,which Li ion conductivity is 0.752 S cm-1 at 98%RH and 90?.We obtained monolayer CoAl-LDH nanosheets formamide colloids exfoliated through anion-exchanged and formamide intercalated CoAl-LDH crystals prepared by co-precipitation process.The CoAl-LDH nanosheets show high density of positively charges?3.93 mC m-2,and stabilize in formamide solution for long time without precipitation.The CoAl-LDH membrane,which were fabricated from the CoAl-LDH nanosheets formamide colloids via vacuum filtration method,possess multiple positively charged nanochannels with interlayer distance of?0.75 nm and surface-charge-governed ion transport behavior.In dilute salt solution,the transmembrane ion conductance becomes constant and independent of salt concentration,which is influenced by the charge density in nanochannels.Further investigation found that the CoAl-LDH membranes show high selectivity to monovalent and divalent anions,in which cation transference number(t+)is smaller than 0.05 and 0.08,respectively.In addition,the absolute value of open circuit voltage(Voc)increases from?50 mV to?140 mV and short circuit current(Isc)increases from 2.6 ?A to 8.9 ?A with the increase of concentration gradient ratio from 10 to 1000 for CoAl-LDH anion-exchange membrane.By stacking a pair of anion-exchange CoAl-LDH and cation-exchange vermiculite membrane in a three-chamber electrochemical cell sequentially filling NaCl solution,we constructed intact reverse electro dialysis system which net current density and diffusion potential reached 18.5 A m-2 and 176 mV,with output power density of 0.7 W m-2 and energy conversion rate of 45.4%.The CoAl-LDH anion-exchange membrane with thousands of negatively charged subnanometer channels was used to convert the hydrostatic pressure differences into positive streaming currents and prove the universality of uniformly distributed charges to streaming current.We have fabricated 2D vermiculite cation-exchange membrane,which was used for pressure-driven devices for static pressure difference energy conversion.We systematically investigate the cations transporation through vermiculite membrane under applied pressure difference.The generated streaming currents in this device increased linearly with applied pressure gradient increased from 5 kPa to 30kPa and reached 50 nA when the pressure gradient reach 30 kPa in 0.1 M KCl solution with good stability.When the streaming current driven by pressure gradient pass through an external resistor,electrical energy is harvested from the device with maximum output powder density?4.2?Wm-2.