ELECTROCHEMICAL THIN FILMS AND INTERFACES
Group leader: Dr. Chris Baeumer
Challenge: store intermittent renewable energy
We perform fundamental materials research in the light of today’s grand challenges: climate change, energy and resource efficiency. Renewable energy sources must replace current fossil fuel based technologies as soon as possible. This is difficult because of the intermittency of most renewable energy sources.
Green hydrogen for energy storage
Therefore, energy transformation and storage options such as conversion to chemical fuel are necessary. The simplest and most attractive candidate for climate-neutral fuel and sustainable chemical synthesis is hydrogen produced by water splitting. Excess electricity from renewable energy can be used to split water into hydrogen and oxygen gas, and the energy stored in the hydrogen gas can be used later on or in a different process, effectively storing energy and coupling different sectors.
Materials Science: Designing efficient catalysts
For this water-splitting reaction, catalyst materials are required, which reduce the amount of energy needed to generate a given amount of gas. These materials must be made of earth-abundant and safe materials, and must be efficient at catalysing the reaction to increase efficiency and stable under reaction conditions. We approach this fundamental research question through a special materials-by-design approach and through novel characterization tools.
Atomically-defined thin film catalysts
We utilize atomically defined surfaces of epitaxial catalyst thin films. This class of materials allows selective tuning of the stoichiometry, crystallographic orientation, strain state and surface termination of the catalysts. Thus, we can study the effects of the chemical, crystallographic, electronic, and magnetic structure of the catalysts. In addition, we can stimulate new properties by combining different materials on a nanometre-length scale. The structure-property-function relationships that we identify and the new materials we create will help us identify design rules for future catalysts that are efficient and stable.
CURRENT RESEARCH PROJECTS:
Characterization during the reaction (“operando” science)
When we apply a voltage to catalyst materials in contact with water, multiple reactions occur simultaneously: While we begin to split water into oxygen and hydrogen, the surface of the catalyst changes the chemical composition, the structure and the electronic properties. The catalyst surface is especially important, because here the changes are biggest and here is where the reaction happens. Some details are described in our recent publication . All of these properties determine if the material can be a good catalyst. Therefore, we need to understand and engineer the true active state of the catalyst surface during the reaction.
We achieve this understanding using new characterization tools that can probe the catalyst surface during the water splitting reaction, including X-ray spectroscopy. These experiments are performed at the MESA+ Institute at the University of Twente and at international synchrotron facilities. The available techniques are summarised here.
The role of magnetic properties
State-of-the-art electrocatalyst still have an activity below the conceptual maximum. One reason can lie in the spin state of the reactants and products, for example in water splitting. The reactants are singlet, diamagnetic molecules, while the product oxygen molecule has a triplet, paramagnetic ground state with parallel spin alignment (↑O=O↑). It is thus proposed that using magnetic catalysts can enhance these reactions by enhancing the formation of the ground state as shown in the image below.
The focus of this work is to experimentally demonstrate the effects of intrinsic magnetic order in catalysts on their electrochemical activity and develop a deeper understanding of these effects. We try to find atomic structure-magnetic order-electrocatalytic activity relationships by investigating the efficiency of the oxygen evolution reaction during water splitting on epitaxial catalysts with a well-defined intrinsic magnetic order.
Bilayer electrocatalyst materials
Many perovskite materials are active for the oxygen evolution reaction, but a fundamental challenge remains: the most active electrocatalysts are chemically unstable under reaction conditions. This limits the applicability of highly active electrocatalysts. Instead, high activity is often compromised by shorter lifetime. In our projects, we try to overcome this challenge by the use of multilayered perovskite materials. The focus lies on activation of ultrathin surface layers through the introduction of different subsurface layers. This enables new pathways to maximize the activity of chemically stable electrocatalysts.
New “high entropy” materials for electrocatalysis
High entropy oxides (HEOs) are a novel category of materials that usually contains five or more cation components, which leads to the formation of a single-phase structure. Because of the coexistence of multiple potential active sites, HEOs offer a compelling opportunity for efficient electrochemical energy storage. My research focuses on understanding the effect of entropy, a thermodynamic parameter describing the disorder, on the activity and stability of electrocatalysts.
The figure shows an Illustration of HEOS for water oxidation and an illustration of PLD process for the synthesis of HEO thin films and heterostructures.
Toward the water splitting application of high-activity catalyst oxide perovskites
High-quality epitaxial oxide perovskite thin films as models systems have shown their high performance in terms of activity and stability in oxygen evolution reaction. But so far, they were confined to lab scale due to the use of single crystal substrates, which are size-limited, expensive and have low surface area. High surface area of technically relevant substrates are highly desired for the water splitting application. The combination of highly active oxide perovskite thin films and high surface area substrates will offer the opportunity to realize the efficient water splitting for application.
In the project, oxide perovskite thin films will be deposited on technically relevant substrates. Furthermore, the use of buffer layers on these substrates will promote the growth of high-quality oxide perovskite thin films.
Students in the group