High‐Conductive Protonated Layered Oxides from H₂O Vapor‐Annealed Brownmillerites

Abstract Protonated 3d transition‐metal oxides often display low electronic conduction, which hampers their application in electric, magnetic, thermoelectric, and catalytic fields. Electronic conduction can be enhanced by co‐inserting oxygen acceptors simultaneously. However, the currently used redox approaches hinder protons and oxygen ions co‐insertion due to the selective switching issues. Here, a thermal hydration strategy for systematically exploring the synthesis of conductive protonated oxides from 3d transition‐metal oxides is introduced. This strategy is illustrated by synthesizing a novel layered‐oxide SrCoO₃H from the brownmillerite SrCoO_2.5. Compared to the insulating SrCoO_2.5, SrCoO₃H exhibits an unprecedented high electronic conductivity above room temperature, water uptake at 250 °C, and a thermoelectric power factor of up to 1.2 mW K⁻² m⁻¹ at 300 K. These findings open up opportunities for creating high‐conductive protonated layered oxides by protons and oxygen ions co‐doping.