Chatterjee Shyambo, Zamani Ehsan, Farzin Seefat, Evazzade Iman, Obewhere Oghenetega Allen, Johnson Tyler James, Alexandrov Vitaly, Dishari Shudipto Konika
Department of Chemical and Biomolecular Engineering, University of Nebraska-Lincoln, Lincoln 68588, Nebraska, United States.
JACS Au. 2022 May 11;2(5):1144-1159. doi: 10.1021/jacsau.2c00143. eCollection 2022 May 23.
Poor ionic conductivity of the catalyst-binding, sub-micrometer-thick ionomer layers in energy conversion and storage devices is a huge challenge. However, ionomers are rarely designed keeping in mind the specific issues associated with nanoconfinement. Here, we designed nature-inspired ionomers (calix-2) having hollow, macrocyclic, calix[4]arene-based repeat units with precise, sub-nanometer diameter. In ≤100 nm-thick films, the in-plane proton conductivity of calix-2 was up to 8 times higher than the current benchmark ionomer Nafion at 85% relative humidity (RH), while it was 1-2 orders of magnitude higher than Nafion at 20-25% RH. Confocal laser scanning microscopy and other synthetic techniques allowed us to demonstrate the role of macrocyclic cavities in boosting the proton conductivity. The systematic self-assembly of calix-2 chains into ellipsoids in thin films was evidenced from atomic force microscopy and grazing incidence small-angle X-ray scattering measurements. Moreover, the likelihood of alignment and stacking of macrocyclic units, the presence of one-dimensional water wires across this macrocycle stacks, and thus the formation of long-range proton conduction pathways were suggested by atomistic simulations. We not only did see an unprecedented improvement in thin-film proton conductivity but also saw an improvement in proton conductivity of bulk membranes when calix-2 was added to the Nafion matrices. Nafion-calix-2 composite membranes also took advantage of the asymmetric charge distribution across calix[4]arene repeat units collectively and exhibited voltage-gating behavior. The inclusion of molecular macrocyclic cavities into the ionomer chemical structure can thus emerge as a promising design concept for highly efficient ion-conducting and ion-permselective materials for sustainable energy applications.
在能量转换和存储设备中，催化剂结合的亚微米厚离聚物层的离子电导率较差，这是一个巨大的挑战。然而，离聚物在设计时很少考虑与纳米限域相关的具体问题。在此，我们设计了受自然启发的离聚物（杯芳烃-2），其具有中空的、大环的、基于杯[4]芳烃的重复单元，直径精确到亚纳米级。在厚度≤100 nm的薄膜中，杯芳烃-2在85%相对湿度（RH）下的面内质子电导率比当前的基准离聚物Nafion高8倍，而在20 - 25% RH下比Nafion高1 - 2个数量级。共聚焦激光扫描显微镜和其他合成技术使我们能够证明大环腔在提高质子电导率方面的作用。原子力显微镜和掠入射小角X射线散射测量证明了杯芳烃-2链在薄膜中系统地自组装成椭球体。此外，原子模拟表明了大环单元排列和堆叠的可能性、跨该大环堆叠存在一维水线，从而形成了长程质子传导途径。当将杯芳烃-2添加到Nafion基质中时，我们不仅看到薄膜质子电导率有前所未有的提高，而且还看到本体膜的质子电导率也有所提高。Nafion - 杯芳烃-2复合膜还共同利用了杯[4]芳烃重复单元上不对称的电荷分布，并表现出电压门控行为。因此，将分子大环腔纳入离聚物化学结构可成为用于可持续能源应用的高效离子传导和离子选择性渗透材料的一个有前景的设计概念。
