Breaking OER and CER scaling relations via strain and its relaxation in RuO2 (101)

Green hydrogen production from abundant water sources is an important component of renewable energy storage. Water oxidation catalysts are typically considered bound by adsorbate scaling relations, limiting their activity for the oxygen evolution reaction (OER) as well as selectivity between OER and the chlorine evolution reaction (CER) that compete in saline water streams. RuO2 is highly active for both reactions, and recent measurements have shown that the OER activity is greater on undercoordinated, high index facets compared to the lowest energy (110) facet often studied. The growth of such orientations as epitaxial films, however, can result in appreciable strain and potential surface faceting via its relaxation. We find the activity and selectivity toward OER and CER vary with thickness in epitaxial (101) RuO2 thin films: OER activity decreases four times as film thickness increases from 8 nm to 48 nm, while CER activity is comparable. Thus, strain and its relaxation can be used to break scaling relationships between OER and CER, highlighting the important role that defects play in selective oxidation processes on RuO2 in chloride-containing media.