Poly(arylene alkylene piperidinium)s show greatly improved alkaline stability and ion conductivity in comparison to current state of the art poly(arylene piperidinium)s.
To develop anion exchange membranes (AEMs) that combine high chemical stability and hydroxide conductivity, we have designed and prepared poly(arylene piperidine)s carrying tunable mono- or dicationic side chains. Poly(biphenyl piperidine) and poly(biphenyl N-methylpiperidine), respectively, were first synthesized by superacid-catalyzed polyhydroxylalkylations. Subsequently, the piperidine rings of these polymers were reacted with bromoalkylated N,N-dimethylpiperidinium (DMP) and 6-azonia-spiro[5.5]undecane (ASU) cations, respectively. This gave two series of AEMs in which the polymer backbone contained tertiary and quaternary piperidine rings, respectively, resulting in mono- and dicationic side chains in series 1 and 2, respectively. In series 1, both the piperidine rings in the backbone and the pendant cations in the side chains showed excellent alkaline stability, resulting in AEMs, which retained more than 92% of the cations after storage in 2 M NaOH at 90 °C during 30 days. In addition, these AEMs reached a hydroxide conductivity up to 131 mS cm–1 at 80 °C. Benefiting from a high local ionic concentration through the dicationic configuration, the AEMs in series 2 reached a higher conductivity, almost 170 mS cm–1 at 80 °C at moderate water uptake and swelling. Still, these AEMs were more vulnerable to hydroxide attack than the ones in series 1 because of the quaternary piperidinium groups placed in the polymer backbone. In conclusion, the AEMs in series 1 can be employed in electrochemical devices that operate under harsh alkaline conditions, while those in series 2 should be preserved for less aggressive alkaline conditions.
We have demonstrated a facile and low-cost approach for the fabrication of binary "island" shaped arrays (BISA) with high-density hot spots as reproducible surface-enhanced Raman scattering (SERS) substrates by depositing a self-assembled monolayer Au nanoparticle (AuNP) film with small gaps onto a two-dimensional (2D) silica microsphere opal structure. By varying the size of silica spheres, the SERS performance of the BISA substrate with an enhancement factor (EF) of 3.74 × 10 magnitude and the corresponding signal intensity deviation of below 8% using 770 nm silica sphere arrays were achieved. Compared with the assembled monolayer AuNP film on a planar substrate, the BISA enabled the installation of more AuNPs as a source of hot spots due to the undulation of morphology on the nanoscale within the designated laser-illumination area. In addition, a finite-difference time-domain (FDTD) simulation suggested that the BISA structure provided geometric conditions for increasing the intensity of the formed hot spots, and the strong periodic electric fields on the BISA are located not only in the gap between adjacent AuNPs, but also along the boundary of the neighboring island of silica spheres. Surface plasmon-decayed hot carriers (hot electrons and hot holes) from AuNPs can be applied in the field of energy conversion (i.e., photocatalysis), integrated with the SERS as a sensitive optical indicator to accurately monitor the catalytic reaction process. Furthermore, we examined the catalytic reaction process of the dimerization of 4-ATP into DMAB and found that photocatalytic activity could be tuned by changing the size of silica spheres. This study provides a new design route for the fabrication of the SERS platform with high sensitivity and reproducibility to detect molecules or improve catalyst efficiency.
Anion exchange membranes (AEMs) functionalized with certain alicyclic quaternary ammonium cations show high alkaline stability, which is necessary for use in fuel cells and water electrolyzers. Here, we have prepared and studied a series of poly(fluorene alkylene)s incorporating N,N-dimethylpiperidinium (DMP) and 6-azoniaspiro[5.5]undecane (ASU) cations in spirocyclic and bis-spirocyclic arrangements, respectively. First, a new fluorene monomer carrying a piperidine ring attached in the C9 position was prepared in a cycloalkylation reaction with N-Boc-N,N-bis(2-chloroethyl)amine. After deprotection, this monomer was employed in superacid-mediated polyhydroxyalkylations with 2,2,2-trifluoroacetophenone (Ap) and 1,1,1-trifluoroacetone (Ac), respectively, to prepare piperidine-functional precursor polymers. Finally, the DMP and ASU cations were formed in quaternizations and cycloquaternizations of the piperidine groups in the polymers by using methyl iodide and 1,5-dibromopentane, respectively. Solvent-cast AEMs showed high thermal stability in combination with good mechanical properties and restricted water uptake as a result of the stiff polymer backbone and bulky cations. Still, the AEM prepared with Ac and carrying DMP cations reached high OH– conductivity, exceeding 80 mS cm–1 at 80 °C. At the same temperature, this AEM showed a high alkaline stability and retained more than 96% of the DMP cations after storage during 30 days in 1 M NaOH. Detailed NMR analysis revealed that ionic loss via Hofmann elimination dominated, but evidence of ring-opening and methyl substitution reactions was also found. Degradation of the ASU cation in the bis-spirocyclic arrangement occurred mainly through ring-opening reactions in the ring directly attached to the fluorene unit in the polymer backbone. These results provide valuable insights when it comes to molecular design of piperidinium-based cations in polycyclic arrangements toward alkali-stable and highly conductive AEMs.
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