SFB 1073 Atomic scale control of energy conversion
The publications presented here prominently represent the overarching goal of the CRC 1073 which is to understand and control the elementary steps of energy conversion in materials with tunable excitations and interactions:
Viewing the atomic-scale motion and energy dissipation pathways involved in forming a covalent bond is a longstanding challenge for chemistry. We performed scattering experiments of H atoms from graphene and observed a bimodal translational energy loss distribution. Using accurate first-principles dynamics simulations, we show that the quasi-elastic channel involves scattering through the physisorption well where collision sites are near the centers of the six-membered C-rings. The second channel results from transient C-H bond formation, where H atoms lose 1 to 2 electron volts of energy within a 10-femtosecond interaction time. This remarkably rapid form of intramolecular vibrational relaxation results from the C atom's rehybridization during bond formation and is responsible for an unexpectedly high sticking probability of H on graphene. (see under Publications no. 132)
The study reports on environmental transmission electron microscopy (ETEM) of the highly correlated perovskite Pr1–xCaxMnO3 and the Ruddlesden–Popper Pr0.5Ca1.5MnO4 used as model electrodes to understand and control the elementary steps of energy conversion during electrochemical reactions in H2O vapor. The structural changes in these materials observed in the ETEM investigations depend on the amount of doping (x) and environmental conditions, and correlate with the catalytical activity revealed by ex situ experiments.
P91-PCMO single crystal observed at 〈110〉 zone axis after a) 4 min and b) 7 min of the ETEM experiment in 0.5 Pa H2O. FFT shown as inset.
We study LiMn2O4 nanoparticles as a model system for the oxygen evolution reaction, which is an important reaction of natural photosynthesis in Photosystem II. Our nanoparticles locally share the arrangement of the manganese atoms with the active site of the natural photosystem and also the average orbital occupancy of the resting state of photosynthesis. These commonalities make LiMn2O4 an ideal model system to investigate the mechanism of oxygen evolution by water oxidation, which is best understood for the natural catalyst. We asked ourselves as a thought experiment: what would be the mechanistic parameters if the natural catalyst operated by an electrochemical mechanism? The measured mechanistic parameters of the slowest mechanistic step on LiMn2O4 matched one of the steps in the thought experiment, yet it was a different step as the slowest step in natural photosynthesis. Thus, this insight provided a guideline for the development of electrocatalysts that may exhibit mechanisms even closer to that of the very efficient process of natural photosynthesis. Moreover, the insight also contributes to the overarching goal of the CRC 1073 to understand the elementary steps of energy conversion in materials.