Green hydrogen: Improving iridium catalysts with titanium oxides

Die Iridium-Atome (rot) sind in unterschiedliche Titanoxide eingebettet, die für mehr Stabilität sorgen. 

Die Iridium-Atome (rot) sind in unterschiedliche Titanoxide eingebettet, die für mehr Stabilität sorgen.  © Marianne van der Merwe

Anodes for the electrolytic splitting of water are usually iridium-based materials. In order to increase the stability of the iridium catalyst, a team at HZB and a group at HI-ERN have now produced a so-called material library: a sample in which the concentration of iridium and titanium oxides is systematically varied. Analyses of the individual sample segments at BESSY II in the EMIL laboratory showed that the presence of titanium oxides can increase the stability of the iridium catalyst significantly.

One option for storing energy from sun or wind is the production of “green” hydrogen by electrolysis. Hydrogen stores energy in chemical form and releases it again when burnt, producing no exhaust gases, only water. Today, iridium is the state-of-the-art catalyst for this reaction. However, iridium increasingly dissolves in the acidic environment of the electrolysis cell, so that the catalytic effect quickly wanes.

“We wanted to investigate whether the stability of the catalyst can be improved by adding different proportions of titanium oxide,” says Prof Dr Marcus Bär (HZB). Although titanium oxide is not catalytically active, it is very stable. “We had some indications that the presence of titanium oxide would have a positive effect on stability without influencing the catalytic effect of the iridium. But we also wanted to find out whether there is an ideal mixing ratio.”

The sample as a materials library

The sample was produced at the Helmholtz Institute Erlangen-Nuremberg for Renewable Energies (HI-ERN) in Prof Dr Olga Kasian’s team by sputtering titanium and iridium with locally varying compositions. It is a so-called thin-film materials library on which the iridium content varies from 20% to 70%

At BESSY II, the team used X-ray spectroscopic methods to analyse how the chemical structure changes depending on the iridium content of the mixed iridium-titanium oxide samples. Several effects played a role here: for instance, the presence of titanium suboxides (such as TiO and TiOx) improved the conductivity of the material. Another exciting result was that some of the titanium oxides dissolve faster in the aqueous electrolyte than iridium, creating micropores on the surface. This promoted the oxygen evolution reaction because more iridium atoms from the lower layers come into contact with the electrolyte.

The main effect, however, is that titanium oxides (TiO2, as well as TiO and TiOx) significantly reduce the dissolution of iridium. “In the sample with 30 % titanium added compared to a pure iridium electrode material, we saw an iridium resolution that was approximately 70 % lower,” says Marianne van der Merwe, who carried out the measurements as part of her doctorate with Marcus Bär.

High relevance for practical use

But how relevant are such results from laboratory research for industry? “If there are already established technologies, it’s always difficult to change anything at first,” says Marcus Bär. “But here we show how the stability of the anodes can be significantly increased with a manageable amount of effort.”

arö


You might also be interested in

  • BESSY II: How pulsed charging enhances the service time of batteries
    Science Highlight
    08.04.2024
    BESSY II: How pulsed charging enhances the service time of batteries
    An improved charging protocol might help lithium-ion batteries to last much longer. Charging with a high-frequency pulsed current reduces ageing effects, an international team demonstrated. The study was led by Philipp Adelhelm (HZB and Humboldt University) in collaboration with teams from the Technical University of Berlin and Aalborg University in Denmark. Experiments at the X-ray source BESSY II were particularly revealing.
  • Fuel Cells: Oxidation processes of phosphoric acid revealed by tender X-rays
    Science Highlight
    03.04.2024
    Fuel Cells: Oxidation processes of phosphoric acid revealed by tender X-rays
    The interactions between phosphoric acid and the platinum catalyst in high-temperature PEM fuel cells are more complex than previously assumed. Experiments at BESSY II with tender X-rays have decoded the multiple oxidation processes at the platinum-electrolyte interface. The results indicate that variations in humidity can influence some of these processes in order to increase the lifetime and efficiency of fuel cells. 
  • Fertilisation under the X-ray beam
    Science Highlight
    19.03.2024
    Fertilisation under the X-ray beam
    After the egg has been fertilized by a sperm, the surrounding egg coat tightens, mechanically preventing the entry of additional sperm and the ensuing death of the embryo. A team from the Karolinska Institutet has now gained this new insight through measurements at the X-ray light sources BESSY II, DLS and ESRF.