Roman Schmack

An efficient bifunctional two-component catalyst for oxygen reduction and oxygen evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes

We report on a non-precious, two-phase bifunctional oxygen reduction and evolution (ORR and OER) electrocatalyst with previously unachieved combined roundtrip catalytic reactivity and stability for use in oxygen electrodes of unitized reversible fuel cell/electrolyzers or rechargeable metal–air batteries. The combined OER and ORR overpotential, total, at 10 mA cm−2 was a record low value of 0.747 V. Rotating Ring Disk Electrode (RRDE) measurements revealed a high faradaic selectivity for the 4 electron pathways, while subsequent continuous MEA tests in reversible electrolyzer cells confirmed the excellent catalyst reactivity rivaling the state-of-the-art combination of iridium (OER) and platinum (ORR).

Time-resolved in situ studies on the formation mechanism of iron oxide nanoparticles using combined fast-XANES and SAXS

The reaction of iron chlorides with an alkaline reagent is one of the most prominent methods for the synthesis of iron oxide nanoparticles. We studied the particle formation mechanism using triethanolamine as reactant and stabilizing agent. In situ fast-X-ray absorption near edge spectroscopy and small-angle X-ray scattering provide information on the oxidation state and the structural information at the same time. In situ data were complemented by ex situ transmission electron microscopy, wide-angle X-ray scattering and Raman analysis of the formed nanoparticles. The formation of maghemite nanoparticles (γ-Fe2O3) from ferric and ferrous chloride was investigated. Prior to the formation of these nanoparticles, the formation and conversion of intermediate phases (akaganeite, iron(II, III) hydroxides) was observed which undergoes a morphological and structural collapse. The thus formed small magnetite nanoparticles (Fe3O4) grow further and convert to maghemite with increasing reaction time.

Oxygen Evolution Catalysts Based on Ir-Ti Mixed Oxides with Templated Mesopore Structure: Impact of Ir on Activity and Conductivity

The efficient generation of hydrogen via water electrolysis requires highly active oxygen evolution catalysts. Among the active metals, iridium oxide provides the best compromise in terms of activity and stability. The limited availability and usage in other applications demands an efficient utilization of this precious metal. Forming mixed oxides with titania promises improved Ir utilization, but often at the cost of a low catalyst surface area. Moreover, the role of Ir in establishing a sufficiently conductive mixed oxide has not been elucidated so far. We report a new approach for the synthesis of Ir/TiOx mixed-oxide catalysts with defined template-controlled mesoporous structure, low crystallinity, and superior oxygen evolution reaction (OER) activity. The highly accessible pore system provides excellent Ir dispersion and avoids transport limitations. A controlled variation of the oxides Ir content reveals the importance of the catalysts electrical conductivity: at least 0.1 S m-1 are required to avoid limitations owing to slow electron transport. For sufficiently conductive oxides a clear linear correlation between Ir surface sites and OER currents can be established, where all accessible Ir sites equally contribute to the reaction. The optimized catalysts outperform Ir/TiOx materials reported in literature by about a factor of at least four.