All-water supercapacitor enabled by 1-nm clay channels

Abstract

Water confined to channels one nanometer thick exhibits electrochemical behavior distinct from bulk water, including enhanced protonic conductivity and large dielectric anisotropy. Here, we exploit these characteristics to design a scalable electrochemical energy-storage system ("blue capacitor") constructed entirely from naturally abundant materials. By assembling layered clays and conductive graphene, we produce 1-nm-thick channels in which confined water acts as the sole electrolyte. We systematically study different clay types, the electrode composition, and separator thickness using complementary physicochemical and electrochemical techniques. The device operates stably up to 1.6 V, achieves specific capacitances of up to 40 F/g, nearly 100% coulombic efficiency, and stable performance over more than 60,000 charge-discharge cycles. Structural and dynamic analyses validate the device architecture, water purity, and proton transport in the nanopores. These results demonstrate that nanoconfined water can function as an electrolyte in a macroscopic electrochemical device, providing a platform for exploring sustainable aqueous energy-storage systems.

Figures (6)

• Figure 1: Evolution of double-layer capacitors (DLCs). (a) Leyden jar: an early DLC based on water and nonporous electrodes, storing charge in a nanometer-wide interfacial water layer. (b) Supercapacitor: a state-of-the-art DLC with high-surface-area electrodes and a separator immersed in a bulk-like concentrated electrolyte or ionic liquid (light blue), enabling high capacitance at the cost of chemical complexity. (c) Blue capacitor of this study: a new configuration, topologically distinct from the previous two, with a continuous network of strongly confined water as a sole electrolyte (dark blue), continuously crosslinking the electrodes and the separator.
• Figure 2: Structure and dielectric properties of clays. (a) Crystal structure of the three most abundant natural clays. (b) Enlarged structure of montmorillonite (MMT) clay with an interlayer space accessible to water. (c) Synchrotron small-angle X-ray scattering (SAXS) data for wet and dry MMT (raw data see in SI Fig. S6), showing interlayer water penetration. (d) Proton conductivity of clays under wet (top) and dry (bottom) conditions (see also SI Figs. S12-15). The inset schematically shows the path of a proton through the confined water in wet clays.
• Figure 3: Fabrication of the blue capacitor. (a) Vacuum filtration-based assembly of membrane-electrode units (MEUs) from colloidal suspensions. (b) Schematic of the resulting vdW heterostructure composed of graphene-clay composite electrodes and a pure clay separator, forming aligned 1-nm water channels. (c) Photograph of a flexible MEU. (d) Conceptual illustration of the membrane-electrode unit (MEU), built from aligned graphene and clay nanosheets with interlayer water acting as the sole electrolyte. The inset illustrates the Grotthuss-type proton hopping along a confined water channel, facilitating charge separation across the device. (e) Cross-section imaged by integrated differential phase contrast (iDPC) STEM of the clay separator. The inset shows atomic-resolution imaging and the underlying lattice structure, with oxygen (red), silicon (blue), and aluminum (cyan) atoms.
• Figure 4: Morphology and chemical composition of the blue capacitor. (a) SEM image of a cross-section of the blue capacitor membrane-electrode unit (MEU). (b) Close-up of the SEM image showing a layered structure. (c, d) EDX elemental mapping for Si (red), Al (blue), O (green), and C (magenta), confirming material purity. (e) Further close-up of the SEM image near the electrode-separator contact showing no gap between them. (f) Annular dark-field (ADF) STEM image of a cross-section of the separator showing layered morphology. (g) iDPC-STEM close-up of nanosheet intercalation; blue lines are plotted contours of domains.
• Figure 5: Blue capacitor characteristics. (a) Schematic of the MEU measurements assembly. (b) Typical charge-discharge plots at various current densities. (c) Typical cycling voltammograms at various scan rates. (d) Capacity retention, coulomb, and energy efficiency at long-term cycling at 1.6 V and 10 mA. (e) Coulombic efficiency and the energy density vs. voltage window at 8 mA. The error bars show measurement variance at high voltages. (f) Electronic conductivity of electrodes (green) and MEU capacitance (black) as functions of graphene concentration in the electrodes. The yellow line shows an optimal region.