Barnes Austin M, Buratto Steven K
Department of Chemistry and Biochemistry, University of California , Santa Barbara, California 93106-9510, United States.
J Phys Chem B. 2018 Jan 25;122(3):1289-1295. doi: 10.1021/acs.jpcb.7b08230. Epub 2018 Jan 11.
Channel connectivity is an important material property that is considered in making higher-performance proton-exchange membranes. Our group has previously demonstrated that nearly 50% of the aqueous surface domains in Nafion films do not have a connected path to the opposite side of the membrane. These so-called "dead-end" channels lead to a loss in the conductance efficiency of the membrane. Understanding the structure of these dead-end channels is an important step in improving the conductance of the membrane. Although conductive atomic force microscopy is able to provide insight into the connected channels, it does directly report on the dead-end channels. To address this, we use electrostatic force microscopy (EFM) to probe channel connectivity in a Nafion thin film (100-300 nm) under ambient conditions. EFM provided an image of the capacitive phase shift, which is influenced by surface charge, dielectric permittivity, and tip-sample geometry. We studied several individual channels and measured the quadratic dependence of the EFM signal with the bias voltage. Applying a simple parallel plate model allowed us to assign differences in the EFM signal to particular channel shapes: connected cylindrical channels, dead-end cylinder channels, and bottleneck channels.
通道连通性是制造高性能质子交换膜时需要考虑的一项重要材料特性。我们团队之前已经证明，Nafion膜中近50%的水相表面区域没有与膜的另一侧相连的路径。这些所谓的“死端”通道会导致膜的传导效率损失。了解这些死端通道的结构是提高膜传导性的重要一步。虽然导电原子力显微镜能够洞察连通通道，但它并不能直接报告死端通道的情况。为了解决这个问题，我们使用静电力显微镜（EFM）在环境条件下探测Nafion薄膜（100 - 300纳米）中的通道连通性。EFM提供了一个电容相移图像，该图像受表面电荷、介电常数和针尖 - 样品几何形状的影响。我们研究了几个单独的通道，并测量了EFM信号与偏置电压的二次相关性。应用一个简单的平行板模型使我们能够将EFM信号的差异归因于特定的通道形状：连通圆柱形通道、死端圆柱形通道和瓶颈形通道。
