Atomically thin micas as proton-conducting membranes

Abstract

Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons 1,2 . For thicker two-dimensional (2D) materials, proton conductivity diminishes exponentially so that, for example, monolayer MoS 2 that is just three atoms thick is completely impermeable to protons 1 . 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-two orders of magnitude. Importantly, ion-exchanged 2D micas exhibit this high conductivity inside the infamous gap for proton-conducting materials 3 , which extends from 100 ˚C to 500 ˚C. Areal conductivity of protonexchanged monolayer micas can reach above 100 S cm -2 at 500 ˚C, well above the current requirements for the industry roadmap 4 . 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 5 with similar nmscale channels, which could help close the materials gap in proton-conducting applications.Ion exchangers are non-soluble materials that contain ions within their crystal structure. These ions are easily substituted with other ions of the same polarity, if the material is placed in contact with suitable electrolytes 6 . In essence, ion exchangers act as sponges that can absorb and release ions. Micas are well-known ion exchangers 7-10 . They consist of aluminosilicate layers that are normally covered with cations such as, for example, K + . These native species can be exchanged for other ions such as H + , Li + or Cs + , which adsorb on the surface of aluminosilicate layers and can also absorb in between them (see Fig. 1). Ion exchange at micas' surfaces proceeds much faster than that within the interlayer space, taking seconds rather than hours 8,10 . It is particularly easy to substitute native ions with protons (H + ) as shown by surface force 11-13 , XPS 14 , X-ray reflectivity 15,16 , AFM 17,18 , NMR 19 and zeta-potential 20 experiments. Besides being proton exchangers, micas exhibit a relatively sparse crystal structure. Their basal planes contain hexagonal rings of 5.2 Å in size (Fig. 1b), which are considerably larger than the rings making up, e.g., graphene and MoS 2 (2.5 and 3.2 Å, respectively). From this perspective, micas can be considered as aluminosilicate slabs pierced by tubular channels as shown in Fig. 1a. The channels are not empty but filled with hydroxyl (OH -) groups; which resembles proton-conducting 1D chains in water 21 (Fig. 1a). In this report, we investigate whether these atomic-scale channels in micas allow for proton permeation so that few-layer micas could be

Combined Intrinsic and Extrinsic Proton Conduction in Robust Covalent Organic Frameworks for Hydrogen Fuel Cell Applications

“…Hydrogen fuel cell (HFC) technology, especially proton exchange membrane (PEM) fuel cells, offer a clean and reliable alternative energy source due to their high energy conversion efficiency, zero emissions, and mild operating conditions . Currently, exploring new proton conducting materials that can serve as solid electrolyte membranes of PEM fuel cells is a primary focus in this field . At present, few materials offer the high proton conductivity and long working life necessary for commercial application, with the main exception being Nafion, a perfluorinated sulfonated polymer.…”

Revisiting Chlor-Alkali Electrolyzers: from Materials to Devices

“…The early-aged chloralkali electrolysis cell employed PFSA-based IEMs, which suffered from low ion selectivity and current efficiency. By contrast, the PFCA-based IEM is usually used as proton exchange membranes in fuel cells [86][87][88] and has excellent current efficiency, but is only suitable for producing alkali metal hydroxides with a maximum yield of 35% by weight due to its low ion flux and high operating voltage. PFSA and PFCA are usually combined into a double-layer IEM to improve performance.…”

Graphoepitaxy of Large Scale, Highly Ordered CsPbBr3 Nanowire Array on Muscovite Mica (001) Driven by Surface Reconstructed Grooves

“…Few‐layered muscovite exhibits useful physical properties, such as excellent proton and electrical conductivity, efficient cations exchange for pollutants, etc. [ 2–5 ] Moreover, due to weak interlayer interactions, naturally cleaved muscovite (001) can be easily achieved without active surface dangling bonds. Thus, as a consequence of the atomic‐level flatness, it has become one of the most frequently used substrates in van der Waals epitaxial technologies.…”

“…[ 22 ] The surface reconstruction may not only provide a powerful platform to fabricate large scale anisotropic structures as demonstrated in organic molecules and crystals, but also can be applied to anisotropic few‐layered muscovite devices. [ 2,4,21 ]…”