Solid State Ionics 136–137 (2000) 139–148 www.elsevier.com / locate / ssi Mixed hydrogen ion–electronic conductors for hydrogen permeable membranes * Truls Norby , Yngve Larring Department of Chemistry, University of Oslo, Centre for Materials Science, Gaustadalleen 21, N-0349 Oslo, Norway Abstract The potential uses for hydrogen permeable gas separation membranes are introduced, and a simple mixed protonic– electronic conductor theory for calculation of flux rates is given. Example permeations are derived as a function of pressure gradients for different defect models, and the results of the very few available experimental studies of hydrogen permeation through oxides are discussed briefly. Strategies for new candidate materials are outlined. Indications of hydride ion transport in oxides under reducing conditions are introduced, and the behaviour and possible role of hydride defects in oxides is discussed.  2000 Elsevier Science B.V. All rights reserved. Keywords: Protons; Proton conductivity; Hydride ions; Hydride ion conductivity; Mixed proton conduction; Hydrogen permeation; Gas separation Materials: SrTiO ; Nd O ; BaCeO ; CeO 3 2 3 3 2 1. Introduction may be used together in a combined process of partial oxidation and hydrogen extraction. After the recent advances in the development of Oxygen permeable membranes operate in a large oxygen permeable membranes for production of chemical potential gradient (e.g. syngas production) oxygen from air or production of syngas (CO 1 H) or in a merely hydrostatic pressure difference under 2 from methane, there is emerging interest in the oxidising conditions (oxygen production). Hydrogen possibility of developing mixed protonic–electronic permeable membranes, on the other hand, may be conductors for hydrogen separation [1]. Potential foreseen to work mainly in a hydrostatic pressure applications comprise extraction of pure hydrogen difference of hydrogen, and thus only under reducing from syngas or from other oxidation or dehydrogena- conditions. This has important consequences for the tion steps, as well as hydrogen purification in strategies of materials development. general. Fig. 1 illustrates how the oxygen permeable A palladium membrane or a two-phase system of and hydrogen permeable membranes in principle one proton conductor and one electronically conduct- ing (e.g. metallic) phase are alternatives to the mixed conductor. Hydrogen is also well suited for filtering through (nano)porous materials. However, here the *Corresponding author. Fax: 147-22-958-749. E-mail address: truls.norby@fys.uio.no (T. Norby). topic is that of hydrogen permeability based on 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(00)00300-3