New investigation method enables fundamental insights into electrocatalytic water splitting under real conditions
Whether as fuel or energy storage: hydrogen will be traded as the energy source of the future. How exactly the chemical process of the decomposition of water to hydrogen and oxygen takes place on a molecular level on a catalyst surface could not be observed adequately by current methods. Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz have now developed a new method to investigate such processes in the nanometer range “live”. With the help of this detailed insight into the splitting of water on gold surfaces, the design of energy-efficient catalysts could be made significantly easier in the future.
It is a well-known school experiment: If a voltage is applied between two electrodes in water, hydrogen and oxygen are produced. For industrial use of this process, it is important to make water splitting as energy-efficient as possible. In addition to the material of the electrode, the surface quality is also decisive for the splitting efficiency. Primarily rough spots with a size of only a few nanometers, i.e. millionths of a millimeter – so-called reactive centers – give electrodes their special electrochemical reactivity.
Previous investigation methods were not precise enough to follow the chemical reactions taking place at these reactive centers of the electrode surface with sufficient spatial resolution under real conditions, ie in electrolyte solution at room temperature and with the application of voltage. A team of scientists led by Dr. Katrin Domke, independent Boehringer Ingelheim “Plus 3” group leader at the MPI-P, has now developed a new method for this, with which the process of water splitting on a gold surface could be examined for the first time with a spatial resolution of less than 10 nanometers.
“We were able to prove experimentally that smooth surfaces split water less energy-efficiently than surfaces with roughness in the nanometer range can,” says Katrin Domke. “With our pictures, we track the catalytic activity of the reactive centers during the first steps of water splitting.”
They combined different techniques for their method: In Raman scattering, molecules are illuminated with light. The light spectrum reflected by them contains information that contains a kind of chemical fingerprint of the molecule – in other words, a statement about the type of molecule. However, Raman spectroscopy is typically a technique that generates only very weak and, above all, not spatially resolved signals.
For this reason, the researchers combined Raman technology with scanning tunneling microscopy: by scanning a nanometer-thin gold tip illuminated with laser light over the surface to be examined, the Raman signal is amplified by a power of ten by a kind of antenna effect directly at the tip . On the one hand, this enables the measurement of only a few molecules. On the other hand, the strong focus of the light through the tip leads to a spatial resolution of less than ten nanometers. The peculiarity of the equipment is that it can be operated under real conditions.
“We were able to show that when water splitting occurs at such a rough location – ie a reactive center – two different gold oxides are formed, which could be the important intermediates in the separation of the oxygen atom from the hydrogen atoms,” said Domke. With their investigations, it is now possible to get a more precise insight into the processes on a nanometer scale on reactive surfaces and thus to design more efficient electrocatalysts in the future, in which less energy has to be used to split water into hydrogen and oxygen.
The scientists published their results in the renowned journal “Nature Communications”.
Source: MPI for Polymer Research