Ceramic mixed ionic–electronic conducting (MIEC) membranes enable very selective oxygen separation from air at high temperatures. Two major potential applications of oxygen‐transport membranes are: i) oxygen production for oxyfuel power plants, and, ii) integration within high‐temperature catalytic membrane reactors for methane or alkane upgrading by selective oxidative conversions. However, these applications involve contact with carbon‐bearing atmospheres and most state‐of‐the‐art highly permeable MIEC membranes do not tolerate operation under CO2‐rich environments due to carbonation processes. The present contribution shows our first attempts in the development of ceria‐based protective thin layers on monolithic LSCF membranes. Gd‐doped ceria (CGO) deposition is carried out by air blast spray pyrolysis on mirror‐polished LSCF disc membranes. The layer thickness is maintained below 0.4 μm in order to prevent the formation of cracks during thermal cycling and minimize limitations caused by the reduced oxygen permeability through the ceria layer. After optimization of the spraying process, smooth crack‐free dense coatings are obtained with high crystallinity in the as‐deposited state. The layers are characterized by XRD, SEM, AFM, DC‐conductivity measurements, interferometry and optical microscopy. Oxygen separation is studied on coated LSCF using air as the feed and argon/CO2 mixtures as the sweep gas in the temperature range 650–1000°C. The protected membrane exhibits a higher stability than the uncoated LSCF membrane, although the nominal oxygen flux is slightly reduced at temperatures below 850°C due to the limited ambipolar conductivity of doped ceria in the range of oxygen partial pressures investigated. Moreover, the protective layer (250 nm thickness) remains stable after the permeation testing.

Hematite (α-Fe 2 O 3 ) has been widely investigated for photoelectrochemical (PEC) water splitting, but questions remain regarding the nature of improvements induced by different dopants. We report on facile annealing treatments to dope hematite with Ti and Sn, and we provide insight into the effects of the dopant concentration profiles on two key steps of PEC water oxidation: charge separation and interfacial hole transfer. Hematite thin films were deposited by successive ionic layer adsorption and reaction (SILAR), with and without the presence of a TiO 2 underlayer on the F:SnO 2 substrate, and annealed to drive diffusion of Ti and Sn from the underlying layers into the hematite. PEC measurements showed that Ti and Sn at the hematite surface increase hole injection efficiency from nearly zero to above 80%. Ti and Sn also slightly improve charge separation efficiency, although separation efficiency remains below 20% due to low hole mobility and high recombination rate. To overcome the small hole transport length, extremely thin hematite coatings were deposited on Sb:SnO 2 monolayer inverse opal scaffolds. Photocurrent increased proportionately to the surface area of the scaffold. This study provides insight into the use of dopants and nanostructured architectures to improve PEC performance of hematite photoanodes.

The role that the α-Fe2O3/NiFeOOH interface plays in dictating the oxygen evolution reaction (OER) mechanism on hematite has been a source of intense debate for decades, but the chemical characteristics of this interface and its function are still ambiguous and subject to speculation. In this study, we employed operando X-ray absorption spectroscopy to investigate the interfacial dynamics at the α-Fe2O3/NiFeOOH interface. We uncovered the spontaneous formation of a FeOOH interfacial layer under (photo)­electrochemical conditions. This FeOOH interfacial layer plays a role in the surface passivation of hematite and in accumulating the (photo)­generated holes upon external potential application. This hole-accumulation process leads to the extraction of more (photo)­generated holes from hematite before releasing them to NiFeOOH to carry out the water-splitting reaction, and it also explains the reason for the delay in the nickel oxidation process. Based on these observations, we propose a model where NiFeOOH acts mainly as an OER catalyst and a facilitator of holes extraction from hematite, while the interfacial FeOOH layer acts as a surface passivation and hole-accumulation overlayer.