Understanding the (de-)lithiation mechanism of nano-sized LiMn2O4 allows achieving long-term cycling stability
We report an in-depth investigation of the local atomic geometry, electronic and crystallographic structure evolution of nano-sized LiMn2O4 using operando XAS and XRD to shed light on (de-)lithiation mechanism when cycled in wide voltage range of 2.0 to 4.3 V vs Li+/Li. Leveraging on these findings, a novel electrochemical cycling protocol, with periodic deep discharge, yields superior electrochemical performance cycled in the range of 3.3 to 4.3 V exhibiting an excellent structure cyclability and an unprecedented increase in the specific capacity upon long cycling.
Electrochemical energy conversion devices, such as fuel cells and electrolyzers, using an anion exchange membrane (AEM) operating in the alkaline regime offer the prospect of the use of non-noble metal electrocatalysts and lower-cost cell construction materials. The wide-spread application of electrochemical cells with AEMs has been largely limited by the low chemical stability of the material. AEM degradation is triggered by i) nucleophilic attack by OH−, and ii) by reaction with free radicals formed during cell operation. Whereas the alkaline stability of AEMs has been greatly increased over the last 10 years, the understanding of mechanisms of radical induced degradation is limited. In this study, we have addressed this topic for the first time.
Our findings disclose that P-functionalization successfully enhances the oxygen evolution reaction (OER) activity of different cobalt-based catalysts (namely, La0.2Sr0.8CoO3–δ, La0.2Sr0.8Co0.8Fe0.2O3–δ, and CoOx) at near-neutral pHs and that both phosphate ion assistance in the OER mechanism and catalyst Co oxidation state can play a role in the enhanced OER activity.
The mass loading of Si-graphite electrodes is often considered as a parameter of secondary importance when testing their performance. However, if a sacrificial additive is present in the electrolyte, the electrode loading becomes the battery cycle-life-determining factor. A lower loading was obtained by keeping slurry preparation steps unchanged from binder to binder and resulted in a longer lifetime for some of the binders. When the final loading was kept constant instead, the performance became independent of the binder used.
Polymer electrolyte water electrolysis: Understanding the microstructure of a core-shell based anode catalyst layer
Reducing precious metal loading in the anodic catalyst layer (CL) is indispensable for lowering capital costs and enabling the widespread adoption of polymer electrolyte water electrolysis (PEWE). This work presents the first three-dimensional reconstruction of a TiO2-supported IrO2 based core shell catalyst layer, using high-resolution X-ray ptychographic tomography at cryogenic temperature of 90 K. The high data quality and phase sensitivity of the technique have allowed the reconstruction of all four phases namely pore space, IrO2, TiO2 support matrix and the ionomer network, the latter of which has proven to be a challenge in the past.
The evolution of O2 occurring in polymer electrolyte water electrolyzer anodes is a very slow reaction that must be catalyzed using iridium (Ir-) based materials. However, Ir is an extremely scarce metal, and thus the extended commercialization of these electrolyzers will only be possible if the amount of Ir implemented in their anodes is drastically reduced. This requires an improved understanding of the individual steps through which these Ir-based materials catalyze the evolution of O2. To shed light on this matter, in this work we studied four different Ir-based catalysts under operative conditions using time resolved X-ray absorption spectroscopy. Our results show for the first time that, despite the differences between these materials, their surfaces must systematically be completely oxidized to a +5 state in order for the evolution of O2 to proceed on them.
The conversion efficiency for green hydrogen production in a polymer electrolyte water electrolyzer (PEWE) is strongly influenced by the ohmic cell resistance and therefore the thickness of the membrane. The use of thin membranes (~50 micron or below) is limited by gas crossover of H2 and O2, which can lead to the formation of an explosive gas mixture. The incorporation of a Pt recombination catalyst provides remedy and allows a more dynamic operating mode (cf. Highlight 03/2022). However, the presence of Pt nanoparticles leads to an increase in the rate of membrane degradation. Therefore, we have additionally doped the membrane with cerium-zirconium-oxide (CZO) nanoparticles, which act as radical scavenger. The rate of membrane degradation can thus be reduced.
Integration of Li4Ti5O12 crystalline films on silicon towards high-rate performance lithionic devices
The growth of crystalline Li-based oxide thin films on silicon substrates is essential for the integration of next-generation solid-state lithionic and electronic devices. In this work, we employ a 2 nm γ-Al2O3 buffer layer on Si substrates in order to grow high quality crystalline thin films Li4Ti5O12 (LTO). Long-term galvanostatic cycling of 50 nm LTO demonstrates exceptional electrochemical performance, specific capacity of 175 mAh g-1 and 56 mAh g-1 at 100C and 5000C respectively, with a capacity retention of 91% after 5000 cycles.
Lithium metal negative electrodes are often used as counter electrodes while testing other electrochemically active materials, and are considered to be equivalent, independently of their thickness, supplier and production processes used. Here, we clearly demonstrate, using Electrochemical Impedance spectroscopy (EIS) that it is not the case, as well as the often-used symmetric cells are actually not so symmetric, when EIS spectra are disentangled using Thee-electrode cells.