In this paper, poly(diallyldimethylammonium chloride)(PDDA)/poly(sodium styrene sulfonate)(PSS) nanomembranes were deposited on the surface of long-period fiber gratings (LPFG) using the electrostatic layer-by-layer (LBL) assembly method, and the effect of NaCl on the modulation of LPFG double peaks by PDDA/PSS nanomembranes was investigated. The principle behind the emergence of double peaks was first explored using coupled mode theory, revealing that changes in the mode effective refractive index(RI) occur as the number of nanomembrane layers increases. The experimental results showed that under the conditions of PDDA with NaCl/PSS without NaCl and PDDA without NaCl/PSS with NaCl, double peaks do not appear in the spectra of LPFG as the number of thin film layers increases. In contrast, highly sensitive double peaks appear in the spectra of LPFG when both PDDA and PSS contain NaCl. Additionally, the range of these double peaks was explored. It was shown that the sensitivity of LPFG to the nanomembrane reached a maximum at 79-81 bilayers, with a sensitivity of 37.8 nm/bilayer, and the appearance of double peaks was consistent with the theoretical simulation. The findings of this study will be applied to the modification of LPFG surface biofunctional membranes and the highly sensitive detection of biochemical substances.
Yufei Wang,Shuanjin Wang,Min Xiao,Dongmei Han,Michael A. Hickner,Yuezhong Meng
Sulfonated poly(fluorenyl ether ketone) (SPFEK) membranes have been first modified by layer-by-layer (LbL) self-assembly of positively charged polyelectrolyte PDDA (poly(diallyldimethylammonium chloride)) and negatively charged PSS (poly(sodium styrene sulfonate)). The membranes were investigated as an ion exchange membrane for vanadium redox flow batteries (VRBs). The permeability of the vanadium ions in VRBs was effectively suppressed by depositing the LbL thin film on the SPFEK membrane. The permeability decreased with increasing the number of PDDA/PSS bilayers. For the membrane with two self-assembly bilayers of PDDA/PSS, 50% and 10% of vanadium ion permeability of a pristine SPFEK and Nafion 117 membranes can be afforded, respectively. Moreover, the oxidative stability of the PDDA/PSS-SPFEK composite membrane is improved remarkably compared with the pristine one. Consequently, the performance of VRBs using the PDDA/PSS-SPFEK composite membrane exhibits the highest coulombic efficiency (CE) of 82.1% at 30 mA cm−2 and the longest duration stability in the self-discharge test.
Teng Xiangguo,Yu Cong,Wu Xiufen,Dong Yichao,Gao Peng,Hu Huili,Zhu Yongming,Dai Jicui
How to solve the crossover of vanadium ions through ion exchange membrane is a key issue in vanadium redox flow battery (VRB), especially for ultra-thin membranes used for VRB to obtain a lower cell resistance. Herein, an ultra-thin (~ 30 μm) PTFE/SPEEK [polytetrafluoroethylene/sulfonated poly(ether ether ketone), P/S] membrane is successfully prepared and modified by using layer-by-layer (LBL) self-assembly technique with polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate) (PSS). P/S membranes are alternatively immersed in positively and negatively charged polyelectrolyte to form 2 to 8 bilayers onto its surface. Consequently, a series of P/S-[PDDA/PSS]
n
( n is the number of multilayers) membranes are fabricated. Both the physicochemical properties and VRB performances of the P/S-[PDDA/PSS]
n
membranes are then investigated in detail. Results show that the ion selectivity of the P/S-[PDDA/PSS]
n
membranes is much higher than that of pristine P/S membrane, especially for P/S-[PDDA/PSS] 6 membrane. As a result, the VRB with the P/S-[PDDA/PSS] 6 membrane exhibits the highest coulombic efficiency (CE) of 96.5% at 80 mA cm −2 , the highest voltage efficiency of 94.7% at 40 mA cm −2 and the highest energy efficiency of 87.7% at both 40 and 50 mA cm −2 , respectively. In addition, 80 times charge–discharge test proves that the P/S-[PDDA/PSS] 6 membrane possesses high stability and no obvious CE decay after running. All the results show that the LBL technique is an effective way to prepare ultra-thin membrane with high ion selectivity for VRB application.
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