Tuning the Oxygen Reduction Reactivity of Layered Perovskites Using the Jahn-Teller Effect.

Compositional tuning of layered perovskite oxides provides a means of systematically studying how local distortions affect fundamental aspects of electrochemical reaction pathways. Structural analysis of a family of samples La 1.2 Sr 0.8 Ni 1- y Co y O 4 shows that Ni-rich compositions have an expanded crystalline c axis, which is anisotropically compressed by systematic Co incorporation. Raman spectra reveal the strong growth of a symmetry forbidden mode, which suggests that Co acts through localized distortions. Crystallographic and spectroscopic parameters describing this structural distortion correlate to the measured Tafel slopes for the oxygen reduction reaction for all Ni-containing samples, which is attributed to the distortion of potential energy surfaces by the Jahn-Teller expansion of d 7 Ni(III) cations. Incorporation of Co not only minimizes the distortion but alters the apparent selectivity of the oxygen reduction reaction away from H 2 O 2 and toward H 2 O. Rotating ring-disk electrochemical measurements, however, indicate that the apparent change in selectivity is due to activation of a first-order chemical disproportionation of H 2 O 2 that is activated by Co in the lattice. These outcomes will support efforts to design electrocatalysts and reactors for the electrochemical synthesis of H 2 O 2 .