posted on 2023-11-15, 12:20authored byMarianne van der Merwe, Raul Garcia-Diez, Leopold Lahn, R. Enggar Wibowo, Johannes Frisch, Mihaela Gorgoi, Wanli Yang, Shigenori Ueda, Regan G. Wilks, Olga Kasian, Marcus Bär
Iridium has emerged as the leading catalyst material
for the anodic
oxygen evolution reaction (OER) in acidic media. Often, iridium is
mixed with more stable materials such as titanium. For these materials,
the electronic structure of titanium plays a crucial role since with
varying degrees of oxidation titanium transforms to semiconducting
or even insulating phases. Yet, the electronic properties of mixed
Ir-TiOx catalysts have never been systematically
studied. In this study, we correlate the catalytic performance of
mixed Ir-TiOx-based OER catalysts with
the electronic structure of the surface layers. For this, a thin film
material library with a 20–70 at. % Ir (Ir/[Ir + Ti]) compositional
gradient was prepared. We used inductively coupled plasma mass spectrometry
to test the OER activity and stability of the set of mixed Ir-TiOx catalyst candidate materials. Complementary,
Ti L2,3- and O K-edge X-ray absorption spectroscopy and
depth-dependent X-ray photoelectron spectroscopy measurements were
performed to correlate the catalytic performance with the composition
and electronic property profiles of these mixed Ir-TiOx OER anode catalysts. The spectroscopic analysis
reveals that titanium is present as an intermixed matrix of semiconductive
but stable TiO2, conductive but less stable titanium-suboxides
(TiOx), and highly conductive but highly
unstable metallic Ti(0). The extent of the titanium oxidation strongly
depends on the titanium content, with a lower degree of oxidation
observed for lower titanium (and thus higher iridium) contents. For
an iridium loading of 70 at. %, the respective mixed Ir-TiOx catalyst showed a similar OER activity to that of
the pure metallic iridium (1.74 vs 1.59 VRHE, respectively)
but with a 71% lower iridium dissolution rate relative to the pure
metallic iridium. This demonstrates the stabilization effect of titanium
addition while maintaining high OER activity.