Nanoscale View on Water Splitting

A combination of Raman spectroscopy and scanning tunnel microscopy is used to look at rough areas of a catalyst surface, where water is split into hydrogen and oxygen in a more energy efficient way than at smooth areas. (Source: MPI-P; CC-BY-SA)

When a voltage is applied between two electrodes inserted in water, molecular hydrogen and oxygen are produced. To further the industrial use of this process, it is indis­pensable to make water splitting as energy-efficient as possible. In addition to the material of the electrode, its surface quality is a crucial aspect for the splitting effi­ciency. In particular, nanometer sized reactive centers determine the electro­chemical reactivity of an electrode. Previous inves­tigation methods were not accurate enough to follow chemical reactions taking place at such reactive centers on the electrode surface with sufficient spatial reso­lution under real operating conditions, i.e. in electrolyte solution at room tempera­ture and with an applied voltage. A team of scientists led by Katrin Domke, inde­pendent Boehringer Ingelheim „Plus 3“ group leader at the Max Planck Institute for Polymer Research in Mainz, has now developed a new method with which the initial steps electro­catalytic water splitting on a gold surface could be studied for the first time with a spatial resolution of less than 10 nm under operating conditions.

“We were able to show experi­mentally that surfaces with protrusions in the nanometer range split water in a more energy effi­cient way than flat surfaces,” says Katrin Domke. “With our images, we can follow the catalytic activity of the reactive centers during the initial steps of water splitting”. For their method, they have combined different techniques: In Raman spectro­scopy, molecules are illu­minated with light that they scatter. The scattered light spectrum contains information that provides a chemical finger­print of the molecule, enabling the identification of chemical species. However, Raman spectro­scopy is typi­cally a technique that produces only very weak and, moreover, only spatially averaged signals over hundreds or thousands of nanometers.

For this reason, the researchers have combined the Raman technique with scanning tunneling micro­scopy: by scanning a nano­meter-thin gold tip illu­minated with laser light over the surface under inves­tigation, the Raman signal is amplified by many orders of magnitude directly at the tip apex that acts like an antenna. This strong enhance­ment effect enables the inves­tigation of very few molecules at a time. Further­more, the tight focusing of the light by the tip leads to a spatial optical resolution of less than ten nanometer. The distinc­tive feature of the apparatus is that it can be operated under realistic electro­catalytic operating conditions.

“We were able to show that during water splitting at nanometer rough spots – i.e. a reactive centers – two different gold oxides are formed, which could represent important inter­mediates in the separation of the oxygen atom from the hydrogen atoms,” says Domke. With their inves­tigations, it is now possible to gain a more precise insight into the processes taking place on the nanometer scale on reactive surfaces and faci­litate the design of more efficient electro­catalysts in the future, where less energy is needed to split water into hydrogen and oxygen. (Source: MPI-P)

Reference: J. H. K. Pfisterer et al.: Reactivity mapping of nanoscale defect chemistry under electrochemical reaction conditions, Nat. Commun. 10, 5702 (2019); DOI: 10.1038/s41467-019-13692-3

Link: Molecular Spectroscopy Dept., Electrochemical Surface Science Group, Max Planck Institute for Polymer Research, Mainz, Germany

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