Karl Skadell

A multiscale simulation approach is presented for hydroxide transport in aqueous solutions of potassium hydroxide, combining ab initio molecular dynamics (AIMD) simulations with force field ensemble averaging and lattice Monte Carlo techniques. This method achieves near ab initio accuracy by capturing the femtosecond scale dielectric relaxation dynamics of the aqueous hydrogen bonding network, while extending the simulation capability to millisecond diffusion timescales. This extraordinary extension of the available length and time scales enables future studies of hydroxide mobility in functional materials such as nanostructured anion‐exchange membranes, where hydroxide ions migrate through nanometer‐sized channels. Remarkably, this approach demonstrates that a single AIMD trajectory is sufficient to predict hydroxide conductivity over a range of concentrations, underscoring its computational efficiency and relevance to the design of advanced energy materials.

In this study, it is revealed that carbon‐free gas diffusion electrodes (CF‐GDEs) with macropore sizes outperform the a carbon‐based GDE (GDE_ref). These CF‐GDEs exhibit notably reduced overpotentials and increased electrochemical stability. By combining three distinct macropore‐sized substrates, coated with MnO_x and hydrophobized with polytetrafluorethylen, a range of CF‐GDEs with distinct porosity gradients is designed. In the results, the pivotal role of substrate layers and their hydrophilic/hydrophobic attributes in steering the formation of the electrolyte thin film are unveiled. Specifically, one CF‐GDE shows a reduction by one‐third of the η_OER (0.24 V) compared to GDE_ref at 10 mA cm⁻². Noteworthy, this CF‐GDE also displays excellent long‐term stability without degradation, which is a common issue with carbon‐based GDEs due to carbon corrosion. Impressively, the stability measurement conditions the active catalyst sites of the CF‐GDE and leads to the formation of NiO_x, Ni₆MnO₈, and NiMn layered double hydroxides. This results in a doubling of the current densities.

Na-β″-alumina is the commercially most successful solid electrolyte due to its application in ZEBRA and NAS^® batteries. In this work, Li-stabilized Na-β″-alumina electrolytes were doped with 3d transition metal oxides, namely TiO₂, Mn₃O₄, and NiO, in order to improve their ionic conductivity and fracture strength. Due to XRD and EDX measurements, it was concluded that Mn- and Ni-ions are incorporated into the crystal lattice of Na-β″-alumina. In contrast, TiO₂ doping results in the formation of secondary phases that enable liquid-assisted sintering at temperatures as low as 1500 °C. All dopants increased the characteristic fracture strength of the electrolytes; 1.5 wt% of NiO doping proved to be most efficient and led to a maximal characteristic fracture strength of 296 MPa. Regarding the ionic conductivity, TiO₂ doping showed the uppermost value of up to 0.30 S cm⁻¹ at 300 °C. In contrast to the other dopants, TiO₂ doping lowered the sintering temperature needed to obtain a dense, stable, and highly conductive Na-β″-alumina electrolyte suitable for applications in Na based batteries.

A new preparation concept of a partially porous solid-state bilayer electrolyte (BE) for high-temperature sodium-ion batteries has been developed. The porous layer provides mechanical strength and is infiltrated with liquid and highly conductive NaAlCl₄ salt, while the dense layer prevents short circuits. Both layers consist, at least partially, of Na-β-alumina. The BEs are synthesized by a three-step procedure, including a sol-gel synthesis, the preparation of porous, calcined bulk material, and spin coating to deposit a dense layer. A detailed study is carried out to investigate the effect of polyethylene oxide (PEO) concentration on pore size and crystallization of the bulk material. The microstructure and crystallographic composition are verified for all steps via mercury intrusion, X-ray diffraction, and scanning electron microscopy. The porous bulk material exhibits an unprecedented open porosity for a Na_xAlO_y bilayer-system of ≤57% with a pore size of ≈200–300 nm and pore volume of ≤0.3 cm³∙g⁻¹. It contains high shares of crystalline α-Al₂O₃ and Na-β-alumina. The BEs are characterized by impedance spectroscopy, which proved an increase of ionic conductivity with increasing porosity and increasing Na-β-alumina phase content in the bulk material. Ion conductivity of up to 0.10 S∙cm⁻¹ at 300 °C is achieved.