Solid-Solid Interfaces in Protonic Ceramic Devices: A Critical Review

The literature concerning protonic ceramic devices is critically reviewed focusing the reader's attention on the structure, composition, and phenomena taking place at solid-solid interfaces. These interfaces play a crucial role in the overall device performance, and the relevance of understanding the phenomena taking place at the interfaces for the further improvement of electrochemical protonic ceramic devices is therefore stressed. The grain boundaries and heterostructures in electrolytic membranes, the electrode-electrolyte contacts, and the interfaces within composite anode and cathode materials are all considered, with specific concern to advanced techniques of characterization and to computational modeling by ab initio approaches. An outlook about future developments and improvements highlights the necessity of a deeper insight into the advanced analysis of what happens at the solid-solid interfaces and of in situ/operando investigations that are presently sporadic in the literature on protonic ceramic devices

Chromium poisoning in cathodes of solid oxide fuel cells: the role of current density, humidity, and cathode composition, and strategies for mitigation

Power generation systems based on solid oxide fuel cells (SOFCs) offer a pathway to a highly efficient and pollution free energy economy. Operation of SOFCs at intermediate temperatures allow the use of metallic interconnects. However, the chromium oxide scale that forms on the metallic interconnect can volatilize and transport and deposit on the cathode, leading to cell performance degradation. The objectives of this dissertation are to understand the role of current density, cathode overpotential, humidity, and cathode composition on Cr-poisoning and suggest mitigation strategies based on this understanding. Conventional (La,Sr)MnO3 (LSM) cathode half-cells have been electrochemically studied to understand the mechanism of Cr-poisoning. The cells have been tested as a function of current density (and thus cathode overpotential), and humidity levels in the oxidant gas. The half-cell measurements have revealed that Cr-poisoning accelerated with cathode overpotential (i.e. current density) and humidity. Microstructural characterization of tested cells found evidence of Cr-rich species at the cathode/ electrolyte interface at high cathode overpotential and humidity. Based on the experimental results, a mechanism of Cr-poisoning has been proposed. With the objective of mitigating Cr-poisoning observed in LSM cathodes, lanthanum nickelate, La2NiO4+δ (LNO) has been studied as an alternative cathode material. Both half-cells and full single SOFCs featuring LNO as the working electrode/cathode, and ferritic stainless steel current collectors have been fabricated. The cells have been tested under the same conditions as the LSM cells. The chromium deposition at the cathode/ electrolyte interface was much reduced for LNO compared to LSM, and the cell performance of cell featuring LNO cathode continually improved with time in contrast to the LSM cell which started to degrade during cathodic current application. Based on the deconvolution of the polarization losses, it was concluded that the higher tolerance of the LNO cathode to Cr-poisoning compared to LSM, can be attributed to maintaining a low cathode activation polarization. The differences in the mechanisms of Cr-poisoning between LSM and LNO has been clarified. A two-pronged strategy combining chromium-tolerant cathodes and interconnect protective coatings is suggested to mitigate long-term performance degradation arising from chromium poisoning

Systematic Characterization and Analysis of Resistance from Conductors and Electrodes for Solid Oxide Fuel Cells

Solid oxide fuel cells are all ceramic devices that generate electricity by direct electrochemical reactions of a fuel and oxidizer. Recent efforts are underway to reduce the operating temperature of solid oxide fuel cells which allow these devices to become more economically competitive. However, at decreased temperatures the resistance from key electrochemical processes greatly increases. The presented work encompasses the characterization and analysis of resistances from conductors and electrodes in solid oxide fuel cells. Ionic conductivity is a thermally activated process; therefore, the conductivity of the ion conducting phase must be improved for suitable operation at lower temperatures. Ionic transport along and across grain boundaries differ distinctly between polycrystalline solids with convention and nanometers sized grains. Ionic conductivity is often greater in the grain boundaries than compared to the grain bulk due to an accumulation of charge carriers. The Van der Pauw technique was leveraged in this worked to measure the conductivity of thin films with thicknesses on the order of nanometers. The results showed that ionic conduction within nanostructured thin films exceeds that of conventional polycrystalline materials. Furthermore, there is a need to identify the resistance that arises from individual electrochemical processes. Electrochemical impedance spectroscopy (EIS) is a technique regularly employed to analyze the resistance from electrochemical processes in the electrodes. Distribution of relaxation times has been applied to the impedance spectrum obtained through EIS. This high resolution plot allowed for the identification of resistances from individual electrochemical impedance processes. The resistances from gas diffusion in the anode and cathode, electrical charge transfer, and transport of ions through the ionic phase have been identified through distribution of relaxation times

Three-dimensional Inkjet Printed Solid Oxide Electrochemical Reactors. I. Yttria-stabilized zirconia Electrolyte

Solid oxide fuel cell (SOFC) and electrolyser (SOE) performances can be enhanced significantly by increasing the densities of (electrode | electrolyte | pore) triple phase boundaries and improving geometric reproducibility and control over composite electrode | electrolyte microstructures, thereby also aiding predictive performance modelling. We developed stable aqueous colloidal dispersions of yttria-stabilized zirconia (YSZ), a common SOFC electrolyte material, and used them to fabricate 2D planar and highly-customisable 3D microstructures by inkjet printing. The effects of solids fraction, particle size, and binder concentration on structures were investigated, and crack-free, non-porous electrolyte planes were obtained by tailoring particle size and minimising binder concentration. Micro-pillar arrays and square lattices were printed with the optimised ink composition, and a minimum feature size of 35 μm was achieved in sintered structures, the smallest published to-date. YSZ particles were printed and sintered to a 23 μm thick planar electrolyte in a Ni-YSZ|YSZ|YSZ-LSM|LSM electrolyser for CO2 splitting; a feed of 9:1 CO2:CO mixture at 1.5 V and 809 °C produced a current density of −0.78 A cm−2 even without more complex 3D electrode | electrolyte geometries

Progress in proton-conducting oxides as electrolytes for low-temperature solid oxide fuel cells: From materials to devices

Among various types of alternative energy devices, solid oxide fuel cells (SOFCs) operating at low temperatures (300-600°C) show the advantages for both stationary and mobile electricity production. Proton-conducting oxides as electrolyte materials play a critical role in the low-temperature SOFCs (LT-SOFCs). This review summarizes progress in proton-conducting solid oxide electrolytes for LT-SOFCs from materials to devices, with emphases on (1) strategies that have been proposed to tune the structures and properties of proton-conducting oxides and ceramics, (2) techniques that have been employed for improving the performance of the protonic ceramic-based SOFCs (known as PCFCs), and (3) challenges and opportunities in the development of proton-conducting electrolyte-based PCFCs

Electrochemical properties of composite cathodes using Sm doped layered perovskite for intermediate temperature-operating solid oxide fuel cell

The authors are grateful for the support of the Basic Science Research Program, part of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (No. 2014R1A1A1004163).SmBaCo2O5+d (SBCO) showed the lowest observed Area Specific Resistance (ASR) value in the LnBaCo2O5+d (Ln: Pr, Nd, Sm, and Gd) oxide system for the overall temperature ranges tested. The ASR of a composite cathode (mixture of SBCO and Ce0.9Gd0.1O2−d) on a Ce0.9Gd0.1O2−d (CGO91) electrolyte decreased with respect to the CGO91 content; the percolation limit was also achieved for a 50 wt% SBCO and 50 wt% CGO91 (SBCO50) composite cathode. The ASRs of SBCO50 on the dense CGO91 electrolyte in the overall temperature range of 500 to 750 °C were relatively lower than those of SBCO50 on the CGO91 coated dense 8 mol % yttria-stabilized zirconia (8YSZ) electrolyte for the same temperature range. From 750 °C and for all higher temperatures tested, however, the ASRs of SBCO50 on the CGO91 coated dense 8YSZ electrolyte were lower than those of the CGO91 electrolyte. The maximum power densities of SBCO50 on the Ni-8YSZ/8YSZ/CGO91 buffer layer were 1.034 W cm−2 and 0.611 W cm−2 at 800 °C and 700 °C.PostprintPeer reviewe

Double perovskite cathodes for proton-conducting ceramic fuel cells: are they triple mixed ionic electronic conductors?

Published by National Institute for Materials Science in partnership with Taylor & Francis. 18 O and 2 H diffusion has been investigated at a temperature of 300 °C in the double perovskite material PrBaCo 2 O 5+δ (PBCO) in flowing air containing 200 mbar of 2 H 2 16 O. Secondary ion mass spectrometry (SIMS) depth profiling of exchanged ceramics has shown PBCO still retains significant oxygen diffusivity (~1.3 × 10 −11 cm 2 s −1 ) at this temperature and that the presence of water ( 2 H 2 16 O), gives rise to an enhancement of the surface exchange rate over that in pure oxygen by a factor of ~3. The 2 H distribution, as inferred from the 2 H 2 16 O − SIMS signal, shows an apparent depth profile which could be interpreted as 2 H diffusion. However, examination of the 3-D distribution of the signal shows it to be nonhomogeneous and probably related to the presence of hydrated layers in the interior walls of pores and is not due to proton diffusion. This suggests that PBCO acts mainly as an oxygen ion mixed conductor when used in PCFC devices, although the presence of a small amount of protonic conductivity cannot be discounted in these materials

Nanostructured Gd0.8Sr0.2Fe0.8M0.2O3 (M=Cr, Ga) materials for solid oxide fuel cell cathodes

AbstractPolycrystalline samples of Gd0.8Sr0.2Fe0.8M0.2O3 (M=Cr, Ga) are prepared by combustion route and pore wetting technique in order to compare the influence of the morphology in the performance of two cathodes for Solid Oxide Fuel Cells. When polycarbonate membranes are used as templates nanowire arrays with a diameter of 50–70 nm are obtained. Comparing the results obtained by Electrochemical Impedance Spectroscopy (EIS) measurements, it is clearly observed that the cathodic resistance considerably decreases when optimized synthesis parameters are used, obtaining a better performance for the Gd0.8Sr0.2Fe0.8Ga0.2O3 nanowires with an area specific resistance (ASR) value at 850∘C of 0.195 Ω cm2