Low Humidity Performance Enhancement Of Self-humidified Ultrathin PEM And Self-organized PEM

With the advantages of low operating temperature, quick starting and high energy density, proton exchange membrane fuel cell (PEMFC) is of great potential in fuel cell electric vehicles, power plants and portable devices. As fuel/oxidant separator and proton conductor, proton exchange membrane (PEM) is the key and performance limiting component of PEMFC. Unfortunately, its performance is strongly dependent on the water condition. Inadequate proton conductivity at low humidity is one of the most radical challenges for PEM. To increase their proton conductivity at low humidity, serf-hum idification and self-organization have been used to improve the cluster morphology. Wide and connected proton conductive channels (PCCs) have been developed, achieving PEM with excellent performance, such as high proton conductivity, good mechanical stability and depressed gas crossover.Ultra-thin PEM benefits from better water management and low proton conducting resistance. However, the consequent huge anode water requirement, massive gas crossover and poor mechanical performance challenge its application. Incorporation of2D platelet Pt-LDHs nanocatalyst could solve such problems. Exfoliation of LDHs platelets has been successfully achieved by depositing of Pt nano-particles via CVD method. Pt nanoparticles are deposited on LDHs platelets by chemical adsorption, which provides superior stability of Pt nanocatalysts against leaching and agglomeration. The concentration of chemical adsorption is decreased along reduction reaction. Pt-LDs16h shows the best performance. In addition, derived from vaporized Pt precursor, nano-scale Pt particles (2to4nm) are distributed on LDHs.To solve the humidification and gas crossover issues of ultra-thin PEM, self-humidified ultra-thin membrane has been prepared by incorporating Pt-LDHs nanocatalyst in Nafion polymer matrix. Benefitting from low resistance of ultra-thin PEM, gas barrier, self-humidifying and reinforcing properties of Pt-LDHs, high performance ultra-thin Nafion/Pt-LDHs composite membrane has been achieved successfully. The composite membrane was fabricated by solution casting process by doctor-blade. The solution was20wt%Nafion with1wt%Pt-LDHs. Two-dimensional Pt-LDHs platelets are distributed in Nafion matrix in nano-scale without agglomeration. The composite membrane shows perfect self-humidifying effect, gas barrier property and mechanical performance. In dry condition, it exhibits a maximum power density of1174mW cm-2, which is3times of Nafion211.Proton conductivity is strongly dependent on the PCCs. However, dead-ends and poor connectivity of PCCs usually occurs at low humidity. To improve PCCs morphology of Nafion at low humidity,3,4-dimethylbenzaldehyde (DMBA) was used to induce the self-organization of Nafion through hydrogen bonding and hydrophobic interaction. The relationship between PCCs and the conductivity has been developed as well. Molecular dynamic simulation (MD) reveals that the hydrogen bonding and hydrophobic interaction accelerate the mobility of Nafion segments, resulting in a higher degree of phase separation. The obtained proton conductivity increases from0.068S cm-1to0.139S cm-1while the PCCs increase from1.72nm to5.15nm. Compared with pristine Nafion, DMBA-i-Nafion presents34%(RH=40%) enhancement on proton conductivity. Finally, without the application of a humidifier, DMBA-i-Nafion shows a maximum power density of1018mW cm-2, yielding an improvement of39%on pristine Nafion.To prepare high performance non-fluorosulfonated PEMs in low humidity, self-organization inducer was firstly introduced and3nm PCCs has been developed, resulting in extremely high proton conductivity at low humidity. Hydrogen-bonding and hydrophobic interaction between a series of amphiphilic alcohols and SPEEK have been revealed by MD. Such interaction is dominated by their amphiphilicity and molecular length. The molecular length of n-BuOH is5A, perfectly matching the distance between hydrogen of sulfonic acid group and benzene ring. The PCCs of n-BuOH-i-SPEEK have been increased from1nm to3nm, yeilding a proton conductivity as high as0.314S cm-1in fully hydrated state. To the best of our knowledge, this is higher than the reported proton conductivities of SPEEK-based PEMs. In addition, with improved phase separation morphology, n-BuOH-i-SPEEK shows excellent conductivity at RH=40%, which is25time of pristine SPEEK,28%higher than Nafion.