Mixed Protonic and Electronic Conduction in N-Type Oxide Semiconductor MgIn2O4 Under Hydrogen Atmosphere

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There is an emerging interest in mixed protonic-electronic conductors for use as hydrogen-permeable membranes and electrodes of fuel cell and water electrolyzer working at 300–500℃. Up to now, the research for mixed protonic-electronic conductors has been limited to a common approach: an acceptor cation is doped to a perovskite-type oxide, creating positively charged compensating defects, notably oxygen vacancy, then hydrate these1;H2O(g) + VO •• + OO × → 2OHO •.Since the doped acceptor has an effective negative charge relative to the species they replace, the introduced protons are plausibly trapped surrounding the acceptors (proton trapping). Therefore, to achieve mixed protonic-electronic conductors with high proton conductivity at intermediate temperatures. In n-type oxide semiconductors, such as In2O3, SnO2, and ZnO, hydrogen is a ubiquitous impurity that acts as an electron donor. Hydrogen is incorporated into interstitial sites and forms a shallow donor state2, according toH2(g) → 2H i • + 2e’. Here, we reveal that the interstitial proton introduced via this mechanism conducts through n-type conducting MgIn2O4 3 at intermediate temperatures. We show that hydrogen is incorporated into MgIn2O4 in a hydrogen atmosphere, leading to an increase in the electronic conductivity. To assess the partial proton conductivity of hydrogen-incorporated MgIn2O4, we prepared an electron-blocking cell. A MgIn2O4 thin film was deposited by RF magnetron sputtering on a proton-conducting phosphate glass4. We conducted chronoamperometry and electrochemical impedance spectroscopy for the electron-blocking cell. The proton partial conductivity of MgIn2O4 was determined as 1.1×10−6 Scm−1 at 250℃. In the presentation, the proton mobility of MgIn2O4 will be discussed in comparison with perovskite-type proton conductors, showing that n-type conducting oxides are promising for mixed protonic-electronic conductors. References Kreuer, K.-D. Proton Conductivity: Materials and Applications. Chem. Mater. 8, 610–641 (1996).Kılıç, Ç. & Zunger, A. n-type doping of oxides by hydrogen. Appl. Phys. Lett. 81, 73–75 (2002).Ueda, N. et al. New oxide phase with wide band gap and high electroconductivity, MgIn2O4. Appl. Phys. Lett. 61, 1954–1955 (1992).Ishiyama, T. et al. Transport properties of proton conducting phosphate glass: An electrochemical hydrogen pump enabling the formation of dry hydrogen gas. Int. J. Hydrog. Energy 44, 24977–24984 (2019).