Engineering topics
Bär, Marcus
Publications and source records attributed to Bär, Marcus.
The Chemical and Electronic Properties of Stability-Enhanced, Mixed Ir-TiO x Oxygen Evolution Reaction Catalysts
Iridium has emerged as the leading catalyst material for the anodic oxygen evolution reaction (OER) in acidic media. Often, iridium is mixed with more stable materials such as titanium. For these materials, the electronic structure of titanium plays a crucial role since with varying degrees of oxidation titanium transforms to semiconducting or even insulating phases. Yet, the electronic properties of mixed Ir-TiO x catalysts have never been systematically studied. In this study, we correlate the catalytic performance of mixed Ir-TiO x -based OER catalysts with the electronic structure of the surface layers. For this, a thin film material library with a 20-70 at. % Ir (Ir/[Ir + Ti]) compositional gradient was prepared. We used inductively coupled plasma mass spectrometry to test the OER activity and stability of the set of mixed Ir-TiO x catalyst candidate materials. Complementary, Ti L 2,3 - and O K-edge X-ray absorption spectroscopy and depth-dependent X-ray photoelectron spectroscopy measurements were performed to correlate the catalytic performance with the composition and electronic property profiles of these mixed Ir-TiO x OER anode catalysts. The spectroscopic analysis reveals that titanium is present as an intermixed matrix of semiconductive but stable TiO 2 , conductive but less stable titanium-suboxides (TiO x ), and highly conductive but highly unstable metallic Ti(0). The extent of the titanium oxidation strongly depends on the titanium content, with a lower degree of oxidation observed for lower titanium (and thus higher iridium) contents. For an iridium loading of 70 at. %, the respective mixed Ir-TiO x catalyst showed a similar OER activity to that of the pure metallic iridium (1.74 vs 1.59 VRHE, respectively) but with a 71% lower iridium dissolution rate relative to the pure metallic iridium. This demonstrates the stabilization effect of titanium addition while maintaining high OER activity.
Core-Level Spectroscopy with Hard and Soft X-rays on Phosphorus-Containing Compounds for Energy Conversion and Storage
The electronic properties of nine solid phosphorus (P)-containing compounds with varying oxidation states and chemical environments, including GaP (–III) , InP (–III) , red-P (0) , H 3 P (III) O 3 , Na 2 H 2 P 2 (IV) O 6 , H 3 P (V) O 4 , KH 2 P (V) O 4 , Na 2 HP (V) O 4 , and InP (V) O 4 , are investigated using X-ray absorption near-edge structure (XANES) spectroscopy in the hard (P K-edge) and soft X-ray regime (P L 2,3 - edge). We find shifts in the absorption-edge positions and correlate them with the ligands surrounding the P atom, likely causing a different core–hole interaction screening for different compounds. Complementing the experimental analysis, ab initio many-body calculations of XANES spectra provide insights into the excitonic nature of the observed spectral features and their impact on the electronic structure of the materials. Furthermore, we report on P K-edge XANES measurements on aqueous phosphorus-containing acids, including H 3 PO 3 , H 3 PO 4 , and their mixtures. At first sight, the spectra of the aqueous acids are similar to those of their solid counterparts. However, close inspection reveals a slight red shift of the absorption edge and the presence of fewer spectral features compared with spectra of the respective solids. Mixtures of aqueous acids display spectral features corresponding to the individual components, indicating the potential for speciation and quantification through fingerprinting.
Tracking the nanoparticle exsolution/reoxidation processes of Ni-doped SrTi 0.3 Fe 0.7 O 3- δ electrodes for intermediate temperature symmetric solid oxide fuel cells
In situ NAP-XPS and XAS are used to elucidate the mechanisms of the exsolution/reoxidation process in Sr 0.93 Ti 0.3 Fe 0.63 Ni 0.07 O 3 δ (STFN) perovskite, used as an electrode for intermediate temperature-SOFC.
Utilizing the unique charge extraction properties of antimony tin oxide nanoparticles for efficient and stable organic photovoltaics
Simultaneously enhancing device performance and longevity, as well as balancing the requirements on cost, scalability, and simplification of processing, is the goal of interface engineering of organic solar cells (OSCs). In our work, we strategically introduce antimony (Sb 3+ ) cations into an efficient and generic n-type SnO 2 nanoparticles (NPs) host during the scalable flame spray pyrolysis synthesis. Accordingly, a significant switch of conduction property from an n-type character to a p-type character is observed, with a corresponding shift in the work function (WF) from 4.01 ± 0.02 eV for pristine SnO 2 NPs to 5.28 ± 0.02 eV for SnO 2 NPs with 20 mol. % Sb content (ATO). Both pristine SnO 2 and ATO NPs with fine-tuned optoelectronic properties exhibit remarkable charge carrier extraction properties, excellent UV resistance and photo-stability being compatible with various state-of-the-art OSCs systems. The reliable and scalable pristine SnO 2 and ATO NPs processed by doctor-blading in air demand no complex post-treatment. Our work offers a simple but unique approach to accelerate the development of advanced interfacial materials, which could circumvent the major existing interfacial problems in solution-processed OSCs.
Dynamic Effects and Hydrogen Bonding in Mixed-Halide Perovskite Solar Cell Absorbers
The organic component (methylammonium) of CH 3 NH 3 PbI 3- x Cl x -based perovskites shows electronic hybridization with the inorganic framework via H-bonding between N and I sites. Femtosecond dynamics induced by core excitation are shown to strongly influence the measured X-ray emission spectra and the resonant inelastic soft X-ray scattering of the organic components. The N K core excitation leads to a greatly increased N-H bond length that modifies and strengthens the interaction with the inorganic framework compared to that in the ground state. The study indicates that excited-state dynamics must be accounted for in spectroscopic studies of this perovskite solar cell material, and the organic-inorganic hybridization interaction suggests new avenues for probing the electronic structure of this class of materials. Furthermore, it is incidentally shown that beam damage to the methylamine component can be avoided by moving the sample under the soft X-ray beam to minimize exposure and that this procedure is necessary to prevent the creation of experimental artifacts.