Jolanta Swiatowska

Interface Stability of a TiO2/3‐Methoxypropionitrile‐Based Electrolyte: First Evidence for Solid Electrolyte Interphase Formation and Implications

We report an in-depth study focusing on the stability of a benchmark electrolyte composition based on a low-volatile 3-methoxypropionitrile (MPN) solvent employed in dye-sensitized solar cells. In the presence of TiO2, the semi-conductor surface plays a catalytic role in the thermal degradation of the electrolyte, which induces, among other effects, the nucleation and growth of a uniform solid electrolyte interphase (SEI) layer that wraps TiO2. On the basis of our actual understanding, we argue that SEI formation is responsible for triiodide depletion in the electrolyte during ageing and also has a simultaneous impact on TiO2 optoelectronic properties through the onset of a visible-light absorption tail, energy modification of intraband trap states, and the induction of an increase in both electron lifetime and transport time in TiO2. In-depth characterization of this layer by using XPS and ToF-SIMS indicates that the chemical composition of this SEI results from solvent and additive degradation, that is, iodide, sulfur, cyano, nitrogen, carbon, and imidazolium rings. The SEI thickness, its content, and the concentration profile strongly vary depending on the ageing conditions. The outcome of this new finding is discussed in comparison with literature observations and stresses the difficulties in reaching long-term stability at 85 °C by using MPN-based electrolytes unless new interfacial engineering is accomplished to impede pinholes between dye molecules on TiO2.

Diaminoethane adsorption and water substitution on hydrated TiO2: a thermochemical study based on first-principles calculations

Epoxy-amines are used as structural adhesives deposited on Ti. The amine adhesion to a Ti surface depends highly on the surface state (oxidation, hydroxylation). Amines may adsorb above preadsorbed water molecules or substitute them to bind directly to surface Ti(4+) Lewis acid sites. The adsorption of a model amine molecule, diaminoethane (DAE), on a model surface, hydrated TiO2-anatase (101) surface, is investigated using Density Functional Theory including Dispersive forces (DFT-D) calculations. DAE adsorption and water substitution by DAE are exothermic processes and turn nearly isoenergetic at high coverage with adsorption-substitution energies around -0.3 eV (including dispersion forces and ZPE). Complementary ab initio molecular dynamics studies also suggest that the formation of an amine-water interaction induces water desorption from the surface at room temperature, a preliminary step towards the amine-Ti bond formation. An atomistic thermodynamic approach is developed to evaluate the interfacial free energy balance of both processes (adsorption and substitution). The main contributions to the energetic balance are dispersive interactions between molecules and the surface on the exergonic side, translational and rotational entropic contributions on the endergonic one. The substitution process is stabilized by 0.55 eV versus the adsorption one when free solvation, rotational and vibrational energies are considered. The main contribution to this free energy gain is due to water solvation. The calculations suggest that in toluene solvent with a water concentration of 10(-4) M or less, a full DAE layer replaces a preadsorbed water layer for a threshold concentration of DAE ≥ 0.1 M.

Electrolyte and Solid-Electrolyte Interphase Layer in Lithium-Ion Batteries

The supply and the management of the energy are particularly at the centre of our daily concerns and represent a socio-economic priority. Indeed, while cars use fossil fuel as the main source of energy for over a century, the depletion of the oil reserves and the necessity to reduce the carbon dioxide emissions, stimulate the development of electric vehicles. Therefore, one of the main challenges for the coming decades is the development of new technologies for the storage of electrochemical energy.