Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

C.C. acknowledges financial support from ECoProbe (DFF – 4005-00129) funded by the Danish Independent Research Council. C.G. and M.B.M. acknowledge financial support from Energinet.dk through the ForskEL programme Solid Oxide Fuel Cells for the Renewable Energy Transition contract no. 2014-1-12231. J.T.S.I., M.C.V. and D.N. acknowledge support from EPSRC Platform Grant EP/K015540/1, EPSRC Tailoring of microstructural evolution in impregnated SOFC electrodes EP/M014304/1 and Royal Society Wolfson Merit Award WRMA 2012/R2.The critical region determining the performance and lifetime of solid oxide electrochemical systems is normally at the electrode side of the electrode/electrolyte interface. Typically this electrochemically active region only extends a few micrometres and for best performance involves intricate structures and nanocomposites. Much of the most exciting recent research involves understanding processes occurring at this interface and in developing new means of controlling the structure at this interface on the nanoscale. Here we consider in detail the diverse range of materials architectures that may be involved, describe the evolution of these interface structures and finally explore the new chemistries that allow control and manipulation of these architectures to optimize both performance and durability.PostprintPeer reviewe

Cation Ordering and Exsolution in Copper-Containing Forms of the Flexible Zeolite Rho (Cu,M-Rho; M=H, Na) and Their Consequences for CO₂ Adsorption

Funding: UK Engineering and Physical Sciences Research Council. Grant Numbers: EP/N024613/1, EP/N032942/1, EP/L017008/1.The flexibility of the zeolite Rho framework offers great potential for tunable molecular sieving. The fully copper-exchanged form of Rho and mixed Cu,H- and Cu,Na-forms have been prepared. EPR spectroscopy reveals that Cu2+ ions are present in the dehydrated forms and Rietveld refinement shows these prefer S6R sites, away from the d8r windows that control diffusion. Fully exchanged Cu-Rho remains in an open form upon dehydration, the d8r windows remain nearly circular and the occupancy of window sites is low, so that it adsorbs CO2 rapidly at room temperature. Breakthrough tests with 10 % CO2/40 % CH4 mixtures show that Cu4.9-Rho is able to produce pure methane, albeit with a relatively low capacity at this pCO2 due to the weak interaction of CO2 with Cu cations. This is in strong contrast to Na-Rho, where cations in narrow elliptical window sites enable CO2 to be adsorbed with high selectivity and uptake but too slowly to enable the production of pure methane in similar breakthrough experiments. A series of Cu,Na-Rho materials was prepared to improve uptake and selectivity compared to Cu-Rho, and kinetics compared to Na-Rho. Remarkably, Cu,Na-Rho with >2 Cu cations per unit cell exhibited exsolution, due to the preference of Na cations for narrow S8R sites in distorted Rho and of Cu cations for S6R sites in the centric, open form of Rho. The exsolved Cu,Na-Rho showed improved performance in CO2/CH4 breakthrough tests, producing pure CH4 with improved uptake and CO2/CH4 selectivity compared to that of Cu4.9-Rho.Publisher PDFPeer reviewe

Understanding CO2 adsorption in a flexible zeolite through a combination of structural, kinetic and modelling techniques

In this study we show how non-trivial equilibrium and kinetic adsorption behaviour in a flexible zeolite can be understood through a combination of experimental characterisation and modelling. Flexible zeolites, such as those in the RHO-family, can exhibit unusual stepped isotherms in the presence of CO2, but their structural complexity makes it hard to attribute a clear mechanism. Here we present a structural and kinetic study on (Na,TEA)-ZSM-25, an extended member of the RHO-family, and show that by combining diffraction data, lattice fluid modelling and dynamic column experiments, we obtain a plausible mechanism for CO2 adsorption and transport in this material. It is evident that by using any single technique, the behaviour is too complex to be readily understood. This is to our knowledge the first study to measure and model the changing kinetics due to adsorption induced framework flexibility.11Nsciescopu

Scaling up aqueous processing of A-site deficient strontium titanate for SOFC anode supports

The authors gratefully acknowledge funding from the Fuel Cells and Hydrogen Joint Undertaking under grant agreement n° 256730.All ceramic anode supported half cells of technically relevant scale were fabricated in this study, using a novel strontium titanate anode material. The use of this material would be highly advantageous in solid oxide fuel cells due to its redox tolerance and resistance to coking and sulphur poisoning. Successful fabrication was possible through aqueous tape casting of both anode support and electrolyte layers and subsequent lamination. Screen printing of electrolyte layers onto green anode tapes was also attempted but resulted in cracked electrolyte layers upon firing. Microstructural, electrical and mechanical properties of anode supports and half cells will be discussed. The use of two different commercial titanate powders with nominal identical, but in reality different stoichiometries, strongly affect electrical and mechanical properties. Careful consideration of such variations between powder suppliers, and batches of the same supplier, is critical for the successful implementation of ceramic anode supported solid oxide fuel cells.PostprintPeer reviewe

Oxygen deficient layered double perovskite as an active cathode for CO2 electrolysis using a solid oxide conductor

A-site ordered PrBaMn2O5+?? was investigated as a potential cathode for CO2 electrolysis using a La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte. The A-site ordered layered double perovskite, PrBaMn2O5+??, was found to enhance electrocatalytic activity for CO2 reduction on the cathode side since it supports mixed valent transition metal cations such as Mn, which could provide high electrical conductivity and maintain a large oxygen vacancy content, contributing to fast oxygen ion diffusion. It was found that during the oxidation of the reduced PrBaMn2O5+?? (O5 phase) to PrBaMn2O6-?? (O6 phase), a reversible oxygen switchover in the lattice takes place. In addition, here the successful CO2 electrolysis was measured in LSGM electrolyte with this novel oxide electrode. It was found that this PrBaMn2O5+??, layered perovskite cathode exhibits a performance with a current density of 0.85 A cm-2 at 1.5 V and 850 ??C and the electrochemical properties were also evaluated by impedance spectroscopy.open0

Doped alkaline earth (nitride) hydrides

The work in this thesis relates to the preparation and structural and electrical characterisation of calcium and strontium hydrides, imides and nitride hydrides. Conventional solid state methods in controlled atmospheres were used to synthesise these materials. High temperature neutron diffraction, thermal analysis and conductivity studies performed on calcium and strontium hydride suggest an order – disorder transition in these materials at 350 – 450°C. Disordering is believed to involve rapid exchange of hydride ions across two crystallographic sites. This manifests itself in a lowering of the activation energy for bulk hydride ion conduction. The hydride ion conduction is good in these undoped materials: σ[total]subscript = 0.01 S/cm for CaH₂ at 1000K; for SrH₂, σ[total]subscript = 0.01 S/cm at 830K. Doping of SrH₂ with NaH causes a significant increase in the low temperature conductivity, due to presence of extrinsic defects. The high temperature conductivity is negatively affected by NaH doping. Calcium nitride hydride (Ca₂NH) was obtained as a single phase material by reacting either calcium metal or calcium hydride (CaH₂) in an argon atmosphere containing 5 – 7% H₂ and 1 – 7% N₂. Imide ions substituting for hydride and nitride ions constitute a major chemical defect in this material. Long range ordering of the nitride and hydride ions occurs, giving rise to a double cubic crystal symmetry. This order breaks down at 600 – 650°C. Applying the same reaction conditions to strontium metal results in a mixed phase of strontium nitride hydride and imide. No long range order in the nitride hydride phase could be observed. Doping Ca₂NH with lithium hydride (LiH) causes the appearance of a second calcium imide phase, whereas doping with sodium hydride (NaH) increases the amount of imide ions as a defect in the nitride hydride structure, thereby decreasing the long range ordering of nitride and hydride ions

High H- ionic conductivity in barium hydride

The authors thank EPSRC for support through a Platform Grant and the Royal Society for a Wolfson Merit award.With hydrogen being seen as a key renewable energy vector, the search for materials exhibiting fast hydrogen transport becomes ever more important. Not only do hydrogen storage materials require high mobility of hydrogen in the solid state, but the efficiency of electrochemical devices is also largely determined by fast ionic transport. Although the heavy alkaline-earth hydrides are of limited interest for their hydrogen storage potential, owing to low gravimetric densities, their ionic nature may prove useful in new electrochemical applications, especially as an ionically conducting electrolyte material. Here we show that barium hydride shows fast pure ionic transport of hydride ions (H−) in the high-temperature, high-symmetry phase. Although some conductivity studies have been reported on related materials previously, the nature of the charge carriers has not been determined. BaH2 gives rise to hydride ion conductivity of 0.2 S cm−1 at 630 °C. This is an order of magnitude larger than that of state-of-the-art proton-conducting perovskites or oxide ion conductors at this temperature. These results suggest that the alkaline-earth hydrides form an important new family of materials, with potential use in a number of applications, such as separation membranes, electrochemical reactors and so on.PostprintPeer reviewe