Mogg L, Hao G-P, Zhang S, Bacaksiz C, Zou Y-C, Haigh S J, Peeters F M, Geim A K, Lozada-Hidalgo M
National Graphene Institute, The University of Manchester, Manchester, UK.
School of Physics and Astronomy, The University of Manchester, Manchester, UK.
Nat Nanotechnol. 2019 Oct;14(10):962-966. doi: 10.1038/s41565-019-0536-5. Epub 2019 Sep 2.
Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons. For thicker two-dimensional (2D) materials, proton conductivity diminishes exponentially, so that, for example, monolayer MoS that is just three atoms thick is completely impermeable to protons. This seemed to suggest that only one-atom-thick crystals could be used as proton-conducting membranes. Here, we show that few-layer micas that are rather thick on the atomic scale become excellent proton conductors if native cations are ion-exchanged for protons. Their areal conductivity exceeds that of graphene and hBN by one to two orders of magnitude. Importantly, ion-exchanged 2D micas exhibit this high conductivity inside the infamous gap for proton-conducting materials, which extends from ∼100 °C to 500 °C. Areal conductivity of proton-exchanged monolayer micas can reach above 100 S cm at 500 °C, well above the current requirements for the industry roadmap. We attribute the fast proton permeation to ~5-Å-wide tubular channels that perforate micas' crystal structure, which, after ion exchange, contain only hydroxyl groups inside. Our work indicates that there could be other 2D crystals with similar nanometre-scale channels, which could help close the materials gap in proton-conducting applications.
石墨烯和六方氮化硼（hBN）的单层对热质子具有高度渗透性。对于更厚的二维（2D）材料，质子传导率呈指数下降，例如，仅三个原子厚的单层MoS对质子是完全不可渗透的。这似乎表明只有单原子厚的晶体才能用作质子传导膜。在这里，我们表明，如果将天然阳离子离子交换为质子，那么在原子尺度上相当厚的少层云母会成为优异的质子导体。它们的面电导率比石墨烯和hBN高出一到两个数量级。重要的是，离子交换的二维云母在质子传导材料臭名昭著的间隙（从约100°C到500°C）内表现出这种高电导率。质子交换单层云母在500°C时的面电导率可以达到100 S/cm以上，远高于行业路线图的当前要求。我们将快速质子渗透归因于贯穿云母晶体结构的约5埃宽的管状通道，离子交换后，这些通道内部仅含有羟基。我们的工作表明，可能存在其他具有类似纳米级通道的二维晶体，这有助于缩小质子传导应用中的材料差距。
