1. What are the challenges in multivalent ion transport in nanochannels?
Multivalent ion transport in nanochannels is more difficult than monovalent ion transport due to steric effects and stronger interactions with channel walls. The ion mobility decreases significantly as temperature decreases. This results in multivalent ion transport having conductivities at least one order of magnitude smaller than monovalent ion transport, typically lower than 10^-3 S cm^-1 at room temperature. Most existing solid ionic conductors (SICs) cannot attain practically useful conductivities below 0°C for multivalent ions. However, the recent development of monolayer CdPS 3 nanosheets-based membranes intercalated with diverse cations has shown high conductivities for both monovalent and multivalent ions in a wide temperature range, opening new possibilities for designing superionic conductors that can conduct various cations and discovering unusual nanofluidic phenomena in nanocapillaries.
read more
2. What is the effect of incorporating Li-containing salts on the stability of CdPS 3 -Li membrane in aqueous solutions?
The stability of CdPS 3 -Li membrane in aqueous solutions can be significantly improved by the incorporation of Li-containing salts. This is evident from the X-ray photoelectron spectroscopy (XPS) measurements and inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements, which confirm the complete exchange of Li + by other cations while maintaining the valence states of Cd, P, and S in the CdPS 3 -Z membranes. The presence of abundant negatively charged vacancies in the CdPS 3 -Z membranes makes them highly hydrophilic, enabling easy adsorption of water molecules in the nanochannels. The incorporation of Li-containing salts enhances the stability of the CdPS 3 -Li membrane in aqueous solutions, as observed in the XRD measurements and contact angle measurements under different relative humidity (RH) conditions.
read more
3. What are the ion conductivities of CdPS 3 -Z membranes at different temperatures?
The ion conductivities of CdPS 3 -Z membranes are in the range of 0.43-0.78 and 0.17-0.32 S cm -1 at 90 and 30 degC, respectively. These conductivities are dozens of times higher than the practically useful values (0.01 S cm -1). The ion conductivities remain high even at temperatures as low as -30 degC for K+, Li+, and Ca2+ and -20 degC for Na+, Mg2+, and Al3+. This is in sharp contrast to other ionic conductors, where the conductivities of multivalent ions are suppressed by over ten times compared to monovalent ions. The CdPS 3 -Z membranes maintain practically useful conductivities even at low RHs (from 30% to 75% RH) at 60 degC, with conductivities ranging from about 0.24 to 0.02 S cm -1.
read more
4. How do high-density cations in CdPS 3 -Z membranes enhance ion migration?
The high-density cations inside the well-ordered 2D nanochannels of CdPS 3 -Z membranes enable concerted ion migrations. These cations overcome steric effects, strong interactions with channel walls, and freezing of aqueous solutions that limit cation migration in nanochannels. As a result, the membranes exhibit superhigh conductivities for both monovalent and multivalent ions over a broad temperature range (-30 to 90 degrees Celsius). These conductivities are over one order of magnitude higher than those of the corresponding best solid-ionic conductors (SICs). These findings provide insights into nanofluidic phenomena in confined nanocapillaries and open avenues for designing superionic conductors that can conduct various cations under extreme temperatures. This is crucial for the development of diverse electrochemical devices with expanded applications.
read more