Thorsten Plaggenborg

Manganese oxide phases and morphologies: A study on calcination temperature and atmospheric dependence.

Summary Manganese oxides are one of the most important groups of materials in energy storage science. In order to fully leverage their application potential, precise control of their properties such as particle size, surface area and Mnx + oxidation state is required. Here, Mn3O4 and Mn5O8 nanoparticles as well as mesoporous α-Mn2O3 particles were synthesized by calcination of Mn(II) glycolate nanoparticles obtained through an economical route based on a polyol synthesis. The preparation of the different manganese oxides via one route facilitates assigning actual structure–property relationships. The oxidation process related to the different MnOx species was observed by in situ X-ray diffraction (XRD) measurements showing time- and temperature-dependent phase transformations occurring during oxidation of the Mn(II) glycolate precursor to α-Mn2O3 via Mn3O4 and Mn5O8 in O2 atmosphere. Detailed structural and morphological investigations using transmission electron microscopy (TEM) and powder XRD revealed the dependence of the lattice constants and particle sizes of the MnOx species on the calcination temperature and the presence of an oxidizing or neutral atmosphere. Furthermore, to demonstrate the application potential of the synthesized MnOx species, we studied their catalytic activity for the oxygen reduction reaction in aprotic media. Linear sweep voltammetry revealed the best performance for the mesoporous α-Mn2O3 species.

Synthesis and electrochemical performance of surface-modified nano-sized core/shell tin particles for lithium ion batteries

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.

In situ X-ray diffraction study on the formation of α-Sn in nanocrystalline Sn-based electrodes for lithium-ion batteries

In situ X-ray diffraction (XRD) was performed to study the formation of the α-Sn structure in nanocrystalline Sn-based electrodes during electrochemical lithium insertion and extraction at room temperature. Therefore, pure β-Sn nanoparticles were synthesised and further processed into electrodes. The lithiation and de-lithiation process of the β-Sn nanoparticles follows the formation of discrete lithium–tin phases which perfectly fits the voltage plateaus in the charge/discharge diagram. However, unlike bulk electrodes, where no α-Sn is formed, we observed the formation of the semiconducting α-modification at 870 mV vs. Li within the first de-lithiation process. This observation explains earlier reports of an increasing internal resistance of such an electrode. Additionally, our study supports earlier suggestions that predominantly small tin crystallites are transformed from the β-Sn phase into the α-Sn phase, while larger crystallites retain their metallic β-Sn structure.

Critical size for the β- to α-transformation in tin nanoparticles after lithium insertion and extraction

Tin nanoparticles can be transformed from the metallic β-Sn structure to the semiconducting α-Sn structure after electrochemical lithiation and delithiation at room temperature. Here, we studied the influence of the size of the crystallites on the β- to α-transformation in Sn nanoparticles. Differently sized Sn/SnOx nanoparticles were synthesized, processed in electrodes and cycled ten times in a lithium-ion cell at room temperature. X-ray diffraction (XRD) patterns before and after electrochemical lithium insertion/extraction reveal that samples with small particles contain the α-Sn structure. The critical size for this transformation is 17(4) nm. Smaller particles were transformed into the α-Sn structure while particles larger than 17 nm retain the β-Sn structure. Temperature dependent XRD measurements show that this α-Sn structure is stable up to 220 °C before its reflections disappear. The formation of the α-Sn structure at room temperature in small particles and the unexpected high transition point can be explained by the substantial contribution of the surface energy (facilitating formation of alloys not observed in the bulk), lithium impurities in the α-Sn structure and the Li2O shell which is formed during lithium insertion.

Influence of Vanadium Ions on the Degradation Behavior of Platinum Catalysts for Oxygen Reduction Reaction

The vanadium air redox flow battery is a combination of a redox flow battery and a reversible fuel cell. For the oxygen reduction during discharge, platinum (Pt) catalysts are common. During operation, vanadium (V) cations can penetrate through a proton exchange membrane into the water/air half-cell. The aim of the present work is to study whether V compounds are deposited on the Pt surface under operation conditions or whether the V ions influence the stability of Pt in any other way. Thereby, bulk platinum electrodes are compared as a simple model system to carbon-supported Pt nanoparticles via cyclic voltammetry. In the case of bulk platinum, electrochemical quartz crystal microbalance measurements showed no deposition of vanadium compounds but indicated the decrease of the (hydr)oxide layer on Pt above V3+ and VO2+ redox potentials. Cycling 100 times between oxygen reduction and oxygen evolution potentials with and without a heavy V contamination did not lead to significant degradation of the model catalyst and shows no influence of V ions. On the contrary, the nanoparticle-based catalyst significantly degraded during the same stability protocol. The V contamination lowered the degradation in this case.

Synthesis of facetted Pt nanoparticles on SnO2 as an oxygen reduction catalyst

Fuel cells are an important technology to match the volatile energy production with the energy demand in a sustainable energy system. One crucial task is to raise the Pt mass specific activity of Pt oxygen reduction catalysts in fuel cells and to increase their stability. Therefore, different approaches are investigated like the facetting of Pt nanoparticles and the use of supporting particles like SnO2, which stabilize Pt. For the first time, the growth of facetted Pt nanoparticles on SnO2 nanoparticles is reported which combines both approaches. The synthesis is based on a polyol process and employs Ag ions to influence the shape evolution of the Pt nanoparticles. The samples were studied by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and cyclic voltammetry (CV). Since SnO2 is etched during the original synthesis, KOH is added to compensate the protons released. The fraction of the facetted Pt nanoparticles is lowered in the presence of SnO2 likely due to adsorption of Ag on the SnO2 surface and, thus, a decrease of the Ag concentration in the bulk solution. Consequently, the Ag concentration was increased leading to the generation of a fair amount of facetted Pt nanoparticles. To avoid the occupation of Ag redeposited on the Pt catalysts surface and, thus, lowering of its catalytic activity, a cleaning procedure consisting of potentiostatic oxidation of Ag and replacement of the electrolyte was applied.