Impact of Interfacial Proton Accumulation on Protonation in a Brownmillerite Oxide

Abstract Electrochemical protonation provides ways to control physical properties and even explore unprecedented phases of solid‐state materials. While how proton accumulation changes materials’ properties is investigated, how protonation of solids can be controlled and promoted remains an enigmatic puzzle. In the work reported here, the influence of electrochemical proton injection duration (tVg) is investigated on the protonation of SrCoO2.5 (SCO) films in electric‐field‐effect transistor structures with gate layers of the proton‐conducting electrolyte Nafion. The proton concentration accumulated in SCO films varies depending on the duration of the proton injection. When protons are injected in a relatively short tVg (≤ 600 s), the hydrogen concentration accumulated in SCO film increases with increasing tVg, reaching the maximum proton concentration of ≈1.9 per formula unit of SCO for the tVg = 600 s case. On the other hand, when tVg is longer than 900 s, the proton concentration decreases with tVg, implying the occurrence of counterreactions that extract protons from protonated SCO and oxidize the channel. These observations indicate that protons accumulated at the Nafion/SCO interface play a role in the protonation of SCO films and that suppressing the interfacial proton accumulation is the key to maximizing the proton concentration accumulated in SCO films.