Effect of the Oxygen Vacancies and Structural Order on the Oxygen Evolution Activity: A Case Study of SrMnO3-δ Featuring Four Different Structure Types.

We report the facile synthesis methods of four materials, with the general formula SrMnO_3-δ, which have previously been synthesized in multiple steps, involving switching between different oxidizing and reducing gases, quenching, the use of zirconium metal as a reductant, etc. However, we have shown that it is possible to synthesize all of these materials by facile processes without unnecessary complications. In fact, we have found methods of synthesizing the oxygen-deficient phases in only one step. Given the diverse range of structures that are formed for SrMnO_3-δ, we have investigated the correlations between the structural order and electrocatalytic activity for the oxygen evolution reaction (OER) of water splitting. We have uncovered a systematic trend in the OER activity, where the most oxygen-deficient compound, SrMnO_2.5, which features square-pyramidal coordination geometry around manganese, shows the highest OER performance. The next OER activity belongs to SrMnO_2.6, which contains both MnO₅ trigonal bipyramids and MnO₆ octahedra. SrMnO₃(cubic), containing only corner-sharing MnO₆ units, shows the third best OER performance. The least activity is observed in SrMnO₃(hexagonal), featuring both face- and corner-sharing MnO₆ octahedra. We have also studied the electrochemically active surface area, as well as the kinetics of OER for all four materials, and found that the trend in these properties is the same as the trend in the OER activity. These findings indicate that the electrocatalytic activity is correlated with the degree of oxygen deficiency, as well as the polyhedral connectivity.