Finding efficient ways to store and deliver electrical energy is urgently needed for the large-scale development of renewable energy sources. The use of pseudocapacitive materials, such as 2D transition metal carbides and nitrides, so-called MXenes, is an extremely promising solution to achieve electrochemical energy storage with high power and energy densities, benefiting from fast redox reactions on transition metal oxides. Nevertheless, local electrochemical processes occurring at the solid-liquid interface of pseudocapacitors are currently largely unexplored. The goal of this project is to image for the first time electrochemical processes occurring during pseudocapacitive electrochemical storage on MXenes at the nanoscale with operando Scanning Transmission X-ray microscopy (STXM). Using synchrotron X-ray light, STXM will allow element-selective chemical mapping with 30 kHz). Redox and intercalation pseudocapacitive charging processes will be investigated directly in acidic or alkali cations-containing electrolytes, respectively. By offering unprecedented chemical sensitivity, spatial and temporal resolutions in liquid simultaneously, NANOMXM will provide a radically new method to probe pseudocapacitive electrochemical storage in MXene. Achieving operando imaging of fast electrochemical reactions at the nanoscale would be a major breakthrough that could open new perspectives to investigate further electrochemical processes on metal oxide-based materials.

Materials that are both electrically conductive and optically transparent are an essential element in important light conversion applications, such as solar cells, solar fuels, displays, and illumination. Their high conductivity is achieved either through electrons (n-type) or through positively charge holes (p-type). However, the figure of merit of state-of-the-art p-type materials is more than 100 times lower than that of the best n-type materials. Therefore current devices must be designed to have electrons as the main charge carriers at the transparent electrode. If this constraint was removed, new design possibilities could be explored, and even new types of devices (e.g. see-through electronic transistors) could be fabricated. Thus, the goal of this project is to synthesize a p-type transparent conductor with a figure of merit twice as high as that of the current state-of-the-art hole conductive material. I will focus on phosphide materials, as recent theoretical work points to their favorable hole-conducting properties. Among phosphides, I have prioritized one specific material and selected two other promising materials as back-ups. I will learn and apply a high-throughput combinatorial approach championed by my host institution (NREL, USA) in order to accelerate the development of optimal synthesis conditions and dopants. This knowledge will be transferred to my European host (HZB, Germany), which is currently building a full combinatorial research lab. I will use HZB’s combinatorial tools to fabricate simple diode structures on top of the material developed at NREL, using an n-type sulfide semiconductor. Electrical analysis of the diodes will indicate the practical applicability of the new hole conductor in a real device. In parallel, I will be trained in advanced defect spectroscopy techniques at HZB. They will reveal the nature of defects that compensate the dominant p-type character of the hole conductor, thus defining a roadmap for further improvement.

Ferroelectric materials have been known, and their fascinating properties exploited for more than 100 years. Still, in recent years, exotic polar textures resembling those found in magnetic materials have been unraveled in nanoscale ferroelectric perovskite oxides, such as flux closure domains, curling polarization, vortices, and the groundbreaking discoveries of polar skyrmions and merons in 2019/2020. Polar textures hold huge promise for novel robust topological electronic devices such as ultra-compact (> Tbit/in2) memories. However, so far, nanoscale polar domains have only been studied on special oxide substrates (e.g. SrTiO3), which precludes practical applications. I propose to take a revolutionary approach to the field of topological polar textures by bringing them to the realm of the Si platform. Not only is this necessary to implement energy-efficient nanodevices, but Si material and advanced CMOS semiconductor technologies offer multiple unexplored ways to engineer mechanical and electrical boundary conditions in ultrathin films and nanostructures of transition metal oxides. With LUCIOLE, I will push the limits in the exploration and future harnessing of emergent states in ferroics with the following objectives: - Create monolithically-integrated topological polar textures on silicon by molecular beam epitaxy and atomic layer deposition of epitaxial and glass-composite nanoscale ferroelectrics using strain, confinement and frustration engineering. - Understand the polar textures created on silicon by combining advanced state-of-the-art correlative microscopies and spectroscopies at the nanoscale, including operando time-resolved methods. - Integrate ferroelectrics at the nanoscale at the front-end- and back-end-of-line of Si chips in sub-500 nm two- and three-terminal devices with tunable stress and study their manipulation under electric field by statistically screening their properties at the wafer scale to unveil potentially rich behavioral patterns.

Photocatalytic water splitting using transition metal oxides (TMOs) has the potential to play a key role in the sustainable large-scale production of hydrogen. Due to their activity, cost-effectiveness, and stability TMOs are viewed as attractive materials to catalyze water splitting by harnessing solar energy. A major challenge is effectively preventing the recombination of electrons and holes in the TMOs produced upon (solar) light absorption. While these charge recombination processes occur on the pico-to-nanosecond timescale, the whole water splitting process is almost 12 orders of magnitude slower! This huge difference urgently demands a better understanding of the underlying mechanisms and charge-driven chemical reactions involving electron transfer (reduction reaction) or hole transfer (oxidation reaction) that take place at the TMO semiconductor–liquid interface. In my WATER-X project I will investigate these sub-10-picoseconds processes at the interface of TMO nanoparticles in bulk water by using time-resolved femtosecond laser photoelectron spectroscopy by applying liquid microjet setup. The objective is to measure the early-time molecular intermediates and their associated electronic-structures, their lifetimes, energetics, photoelectron angular distributions, and decay mechanisms of the short-lived molecular intermediates. With this knowledge we can determine the exact mechanisms of light-induced water dissociation and will pave the way to manipulating light-induced interactions to the solid-aqueous interface for improving the efficiency of light-to-energy conversion. These novel experiments will be performed for four nanoparticle photocatalysts, hematite, titanium dioxide, cerium oxide, and nickel-iron-oxyhydroxide with manifold electronic-structure properties (bandgap, charge carrier dynamics, and energetics), which make them attractive for future applications.

I propose innovative strategies to elucidate and engineer the electrocatalytic mechanism of earth-abundant transition metal oxides with the aim of enhancing the low efficiency of the oxygen evolution reaction (OER). Mastering multi-electron reactions such as the OER is critical for the transition from dwindling fossil fuels to ecologically and economically sustainable fuels based on renewable energy. Water is the most abundant source of hydrogen bonds on earth and fuels based on these bonds have the highest energy densities, which makes water an attractive resource for sustainable fuels production. However, the production of any hydrogen-based fuel from water is currently thwarted by the low efficiency of the OER. Improved catalysts are presently designed by optimizing a single step in the reaction sequence. In contrast, I target the low efficiency of the OER by engineering multiple steps of the mechanism to (i) control the number of electron transfers before the limiting step; and (ii) enforce a reaction path close to the thermodynamic limit. Combining these two strategies increases the catalytic current of transition metal oxides at typical overpotentials by a factor of 100,000. Rational design of the mechanism on this fundamental level calls for unprecedented insight into the active state of electrocatalysts. My team will achieve this firstly by novel approaches to prepare catalytically limiting states for their elucidation by synchrotron-based X-ray spectroscopy and secondly by studying transitions between these states in pioneering time-resolved experiments. Both the required breakthroughs in method development and the innovative scientific strategies are generalizable to other multi-electron reactions, which opens the door for industrial catalysts that store energy sustainably in hydrogen-based fuels on a global scale.