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.

The synthesis and characterization of highly stable and conductive F:SnO2 (FTO) nanopyramid arrays are investigated, and their use as scaffolds for water splitting is demonstrated. Current densities during the oxygen evolution reaction with a NiFeOx catalyst at 2 V vs reversible hydrogen electrode were increased 5-fold when substituting commercial FTO (TEC 15) by nanostructured FTO scaffolds. In addition, thin α-Fe2O3 films (∼50 nm thick) were employed as a proof of concept to show the effect of our nanostructured scaffolds during photoelectrochemical water splitting. Double-layer capacitance measurements showed a drastic increase of the relative electrochemically active surface area for the nanostructured samples, in agreement with the observed photocurrent enhancement, whereas UV–vis spectroscopy indicates full absorption of visible light at wavelengths below 600 nm.

NiFeO x thin films prepared by successive ionic layer adsorption and reaction (SILAR) were deposited onto nanostructured hematite (Fe 2 O 3 ) photoanodes and their effect on the current density and photo-onset potential was studied. After optimization of bath concentration, immersion times, and number of SILAR cycles, very conformal and active NiFeO x films with controlled Fe/Ni content ratios were obtained. Upon the incorporation of Fe 2 + species in the NiCl 2 solution bath, a cathodic shift in the overpotential required for the oxygen evolution reaction (OER) by more than 200 mV with no decrease in current density was observed after 40 SILAR cycles. We demonstrate that by fine-tuning the film composition and thickness, NiFeO x can be employed as an efficient OER catalyst with very low absorbance in the visible spectrum. By doing so, we demonstrate that this material has great potential for incorporation in semiconductor photoelectrodes for direct solar-driven water electrolysis. Being a simple water-based layer-by-layer growth method, SILAR offers promise for the synthesis of catalyst coatings in nano-structured and high surface area electrodes.

Introducing hierarchical porosity to zeolites is vital for providing molecular access to microporous domains. Yet, the dynamics of meso-and macropore formation has remained elusive and pore space ill-characterized by a lack of (in situ) microscopic tools sensitive to nanoporosity. Here, we probe hierarchical porosity formation within a zeolite ZSM-5 crystal in real-time by in situ fluorescence microscopy during desilication. In addition, we introduce small-angle X-ray scattering microscopy as novel characterization tool to map intracrystal meso-and macropore properties. It is shown that hierarchical porosity formation initiates at the crystal surface and propagates to the crystal core via a pore front with decreasing rate. Also, hierarchical porosity only establishes in specific (segments of) subunits which constitute ZSM-5. Such spacedependent meso-and macroporosity implies local discrepancies in diffusion, performance and deactivation behaviors even within a zeolite crystal.Zeolite catalysts are microporous crystalline aluminosilicates, displaying unique shape selectivity through their steric pore space. [1,2] However, such selectivity requires molecu-

Delafossite CuFeO 2 photocathodes have recently attracted attention for water splitting due to their suitable band gap (1 .5 eV) and high stability in aqueous media. The preparation of CuFeO 2 usually requires long and energy-intense treatments in an inert atmosphere for the full conversion of spinel CuFe 2 O 4 to delafossite CuFeO 2 . Herein, we report the preparation and characterization of highly uniform and stable CuFeO 2 thin films obtained via a combination of inexpensive ultrasonic spray pyrolysis followed by a short hybrid microwave treatment (4 min). The resulting films show good stability in alkaline media and produce a photocurrent of~650 μA/cm 2 under 1.5 AM simulated sunlight and with oxygen bubbling. The effect of the rapid transformation from the spinel to the delafossite phase induced by hybrid microwave annealing was investigated with synchrotron-based X-ray absorption spectroscopy (XAS) and Xray photoelectron spectroscopy (XPS).[a] I.

Non‐noble metal electro‐catalysts for water splitting are highly desired when we are moving towards a society where green electrons are becoming abundantly available, offering clear prospects to make our society more sustainable. In this work, Ni−Fe−S is reported as a high performing anode material for the water splitting reaction, operating at low overpotentials and showing high apparent stability. Furthermore, Ni−Mo electrodes are developed on metallic foam substrates and optimized in terms of their performance. The Ni−Fe−S material as anode, combined and integrated with Ni−Mo as cathode in a cell configuration, splits water at 10 mA cm−2 and a potential of 1.55 V. Similar to previous reports, we confirm that Mo leaches from Ni−Mo/Ni foam electrodes. Cycling tests and ICP‐AES measurements show that the stability of Ni−Fe−S is apparent, and that in reality S is leaching from the material as was already suggested in literature. We expand on this knowledge and show that the leaching of S is dependent on both pH and the cation used during electrocatalysis. Furthermore, we find that applying an oxidative potential is in truth stabilizing towards S and that the alkalinity causes leaching. S was furthermore mobile and found to segregate towards the surface. Finally, using too low pH values (11 and lower) result in the passivating hydroxide metal layers being destroyed and the Ni−Fe−S dissolving completely.