Kinetics of interfacial ion-transfer reactions studied using the indirect laser-induced temperature jump technique: Theory

Kinetic Isotope Effects on Electron Transfer Across Self-Assembled Monolayers on Gold

Reactions requiring controlled delivery of protons and electrons are important in storage of energy in small molecules. While control over proton transfer can be achieved by installing appropriate chemical functionality in the catalyst, control of electron-transfer (ET) rates can be achieved by utilizing self-assembled monolayers (SAMs) on electrodes. Thus, a deeper understanding of the ET through SAM to an immobilized or covalently attached redox-active species is desirable. Long-range ET across several SAM-covered Au electrodes to covalently attached ferrocene is investigated using protonated and deuterated thiols (R-SH/R-SD). The rate of tunneling is measured using both chronoamperometry and cyclic voltammetry, and it shows a prominent kinetic isotope effect (KIE). The KIE is ∼2 (normal) for medium-chain-length thiols but ∼0.47 (inverse) for long-chain thiols. These results imply substantial contribution from the classical modes at the Au-(H)SR interface, which shifts substantially upon deuteration of the thiols, to the ET process. The underlying H/D KIE of these exchangeable thiol protons should be considered when analyzing solvent isotope effects in catalysis utilizing SAM.

What Do Laser-Induced Transient Techniques Reveal for Batteries? Na- and K-Intercalation from Aqueous Electrolytes as an Example

Technological advancement has been revolutionized by rechargeable batteries, without which the use of various modern devices would not be possible. Aqueous Na ion batteries have lately garnered much attention, being recognized as a promising alternative to the commonly used Li ion batteries for the large-scale energy storage systems. However, further improvement and optimization of such systems require a more detailed understanding of intercalation mechanisms. In this work, we for the first time demonstrate the implementation of the laser-induced current transient (LICT) technique for in situ characterization of battery systems and investigate the interface between Na₂Ni[Fe(CN)₆] model battery electrodes and aqueous electrolytes in contact with aqueous electrolytes. Quite counterintuitively, the LICT method revealed that at the quasi-steady-state the electrode surface stays positively charged within the potential range where the intercalation/deintercalation of sodium as well as of potassium is possible, evidencing that the intercalation mechanism of the alkali-metal cations should be rather complex. Furthermore, the specific shape of the observed current transients indicates that the interfacial processes of intercalation/deintercalation have at least three different relaxation time constants. The relaxation behavior is highly influenced by the nature of the alkali-metal cations-most likely through their different solvation energy. In addition, we outline how the laser-based experiments can intensify detailed in situ investigations of battery systems.