Water drives sequential breakdown of dynamic nanodomains in deep eutectic electrolytes

Abstract

The rational design of functional materials hinges on understanding molecular interactions in complex hydrogen-bonded liquids like deep eutectic electrolytes (DEEs), where molecular structure governs ionic conductivity and electrochemical stability. Hydration levels critically influence these properties, yet the underlying mechanisms remain elusive, hindering systematic design. Using multidimensional NMR, 2D infrared spectroscopy, and molecular dynamics simulations, we studied choline chloride-malic acid DEEs at varying hydration levels. We show a sequential, component-specific breakdown of molecular nanodomains that overturns dilution models. This process proceeds in distinct stages: water first disrupts ionic domains at moderate hydration levels, while hydrogen-bonded organic networks persist until higher water content. These insights enabled us to design a DEE with enhanced electrolyte performance, achieving ionic conductivity of 13.0 mS cm^-1 and stable cycling over 1000 cycles while suppressing parasitic reactions. This work demonstrates how fundamental molecular insights can resolve critical bottlenecks in sustainable technology development, enabling systematic engineering of nanostructured liquids for energy storage, catalysis, and sustainable chemistry applications.

Fig. 1. (a) ¹H NMR spectra recorded on a 500 MHz spectrometer at 298 K, showing progressive line narrowing with increasing water content, providing direct evidence for hydrogen-bond network disruption and enhanced molecular dynamics. (b) 2D NOESY cross-peak evolution in the aliphatic region demonstrating hierarchical breakdown: choline network disruption at 5w (cross-peak inversion from positive to negative) while MA domains persist (positive cross-peaks remain). Spectra are shown for neat DES, 0.6w, 5w, and 10w hydration levels. Positive and negative contours are indicated in red and blue, respectively. (c) MD simulation snapshot of neat DES: heterogeneous nanostructure is dominated by choline chloride-rich (green with chloride ions shown in magenta) and malic acid-rich domains (pink) (d) MD-calculated hydrogen bond populations revealing component-specific breakdown: Ch–Ch interactions decrease steadily, while MA–MA bonds remain almost constant till 12w with a nominal fluctuation and decrease after that. Ch–Ch data are multiplied by a factor of 10 for clarity. (e) Experimental PFG-NMR and estimated diffusion coefficients (inset) showing component-specific mobility enhancement. Clear divergence emerges at 5w and above: choline diffuses faster than malic acid despite similar molecular weights, indicating sequential liberation from the hydrogen-bond network. (f) MD-calculated diffusion coefficients corroborate the experimental trend.

Fig. 2. (a) ³⁵Cl NMR spectra of ChCl-MA at 298 K, showing dramatic line narrowing from 5095 Hz (5w) to 426 Hz (15w), indicating enhanced Cl⁻ mobility due to progressive hydration shell formation. Peak positions and line widths are displayed in bar plots. Below 5w, resonances remain extremely broad due to strong quadrupolar interactions in asymmetric environments. (b) MD-calculated water-Cl⁻ coordination numbers increasing systematically from 1.99 (2w) to 4.35 (10w), demonstrating progressive formation of well-defined chloride hydration shells. (c) Spatial distribution functions reveal preferential water positioning around chloride anions rather than random distribution throughout the DES matrix, providing direct computational evidence for a selective solvation mechanism. (d) Radial distribution functions of water oxygen (Ow) and chloride ions showing decreasing peak intensities with concomitant increases in coordination number. This peak height reduction reflects increased mobility of water molecules in chloride solvation shells rather than decreased water density, corroborating the ³⁵Cl NMR line narrowing observations.

Fig. 3. (a) 2D IR spectra of SCN⁻ vibrational reporter in ChCl-MA DES showing evolution with waiting time (T_w) at key hydration levels. Spectra recorded at 1 ps and 5 ps waiting times, with T_w increasing from top to bottom. (b) Experimental frequency–frequency correlation function (FFCF, details in SI, Section 1) decay curves demonstrating dramatic hydration-dependent acceleration, reflecting transition from a highly viscous, extensively hydrogen-bonded network to a fluid, dynamically exchanging system. (c) Three distinct regimes were identified based on the timescales: minimal disruption (≤2w, r₁), rapid breakdown (2w–10w, r₂), and water-like dynamics (>10w, r₃). (d) MD-derived intermittent hydrogen bond autocorrelation functions for MA–MA, MA–Ch, and Ch–Ch interactions showing accelerated decay with increasing hydration. Calculated correlation times exhibit excellent qualitative agreement with experimental FFCF measurements. (e) Anisotropy decay measurements of SCN⁻ at 5w and 10w hydration levels. Faster anisotropy decay at 10w confirms that preferential solvation of the anion facilitates anion rotation within the evolving network structure.

Fig. 4. Structure–property-function correlations. (a) Glass transition temperature evolution showing distinct structural reorganization regimes: initial increase up to 5w reflecting network reorganization rather than simple dilution, followed by T_g disappearance beyond 5w signalling major structural transformation coinciding with observed nanodomain breakdown. (b) Macroscopic property evolution demonstrating direct consequences of network disruption: monotonic changes in conductivity and viscosity with diminishing rates of change per water increment, reflecting transition from structured, heterogeneous liquid to homogeneous, water-dominated fluid. (c) MD cluster analysis providing quantitative insight into network fragmentation: probability of largest Ch–Ch clusters and MA–MA clusters drops sharply beyond 10w, representing percolation-like breakdown where networks fragment into isolated clusters rather than gradual erosion. (d) Molecular snapshot of 10w showing the breakdown of DES nano-domains and formation of water-bridged molecular conformations. (e) Molecular snapshot of 10w showing the formation of water channels. (f) Electrochemical impedance spectroscopy in SS‖SS showing ionic conductivity maximum of 13.0 mS cm⁻¹ at 10w hydration, linear polarization resistance (LPR) analysis in Zn‖Zn symmetric cells shows that the corrosion current density is minimized at the 10w hydration level. (g) Linear sweep voltammetry profile demonstrating hydrogen evolution reaction suppression: onset potential shifts negatively by 50 mV compared to aqueous electrolytes, with current density at −2.0 V reduced by 43% due to a unique water-sequestering nanostructure at optimal hydration. (h) Galvanostatic Zn plating/stripping analysis in Zn‖Zn symmetric cells showing stable cycling over 1000 cycles at 1.0 mA cm⁻² with low overpotential, directly correlating superior performance with rationally identified optimal nanostructure at 10w hydration level.

Fig. 5. (a) The cyclic voltammograms (CV) at 1 mV s⁻¹ and (b) galvanostatic charge discharge (GCD) profiles of the Zn‖MnO₂ full cells employing deep eutectic electrolytes (DEEs) with varying hydration levels at 0.1 A g⁻¹.