Co-electrolysis of carbon dioxide and steam has been shown to be an efficient way to produce syngas, however further optimisation requires detailed understanding of the complex reactions, transport processes and degradation mechanisms occurring in the solid oxide cell (SOC) during operation. Whilst electrochemical measurements are currently conducted in situ, many analytical techniques can only be used ex situ and may even be destructive to the cell (e.g. SEM imaging of the microstructure). In order to fully understand and characterise co-electrolysis, in situ monitoring of the reactants, products and SOC is necessary. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is ideal for in situ monitoring of co-electrolysis as both gaseous and adsorbed CO and CO2 species can be detected, however it has previously not been used for this purpose. The challenges of designing an experimental rig which allows optical access alongside electrochemical measurements at high temperature and operates in a dual atmosphere are discussed. The rig developed has thus far been used for symmetric cell testing at temperatures from 450 °C to 600 °C. Under a CO atmosphere, significant changes in spectra were observed even over a simple Au|10Sc1CeSZ|Au SOC. The changes relate to a combination of CO oxidation, the water gas shift reaction, carbonate formation and decomposition processes, with the dominant process being both potential and temperature dependent.
Syngas production through co-electrolysis of steam and carbon dioxide has been shown as an effective method of CO2 utilisation, however little is known about the complex surface reaction mechanisms involved. Further understanding is needed in order to optimise SOC operation parameters and inform materials development. Currently many investigative techniques are ex-situ, some of which are even destructive: e.g. SEM imagining of the microstructure. To fully understand reaction intermediates, characterise electrochemical performance, and develop novel materials and microstructures for co-electrolysis SOCs; in-situ analysis methods are required. The EPSRC funded research programme, 4CU aims to enhance the fundamental understanding of CO2 and its role in co-electrolysis by developing a suite of in-situ investigative methods for high temperature SOC operation. Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) we can monitor both adsorbed and gas phase species in-situ. Variation of potential and temperature conditions allow for visualization of surface mechanisms associated with single atmosphere fuel cell operation. This, along with dual atmosphere measurements, will be discussed. This work illustrates the applicability of the new technique for SOCs operated in a range of atmospheres using Au, Pt and Ag electrodes. The comparison of potential dependent DRIFTS combined with electrochemical measurements provides an important insight into the roles of oxygen ion and proton transport within the systems studied. Figure 1
CO2 emissions are a significant environmental problem which has garnered global interest from both the scientific and political community. One method to reduce emissions is to capture CO2 and convert it to useful products, such as chemical feed-stocks. A method to achieve this is via co-electrolysis of CO2 and H2O, using a high temperature solid oxide cell, to produce synthesis gas – an important intermediate to the production of synthetic liquid fuels. The potential to convert CO2 into synthetic fuel forms a major pathway for global CO2 utilization and could address the issues of depleting fossil fuel reserves and global warming, however, the reaction mechanisms occurring during cell operation for a co-electrolysis system are currently not well understood. This information is necessary to improve performance by engineering materials and operating conditions which are ideal for high gas conversion rates and long term cell performance. Previous studies have suggested that carbonate species may be an important intermediate to CO2 reduction during electrolysis and shows dependencies with temperature and applied potential.[1] This study investigates the electrochemical impedance of electrolysis cells to determine potential rate limiting mechanisms occurring during cell operation. Results correlating cell performance to surface species detected in-situ using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) obtained under varying fuel and temperature conditions will be discussed. [1] D.J. Cumming, C. Tumilson, R. Taylor, S. Chansai, A.V. Call, J. Jacquemin, C. Hardacre, R.H. Elder, Faraday Discuss., 2015, 182, 97-111
Na0.5Bi0.5TiO3 (NBT) is a well-known lead-free piezoelectric material with potential to replace lead zirconate titanate (PZT),1 however high leakage conductivity for the material has been widely reported.2 Through a combination of Impedance Spectroscopy (IS), O2- ion transference (EMF) number experiments and O18 tracer diffusion measurements, combined with Time-of-flight Secondary Ion Mass Spectrometry (TOFSIMS), it was identified that this leakage conductivity was due to oxygen ion conductivity. The volatilization of bismuth during synthesis, causing oxygen vacancies, is believed to be responsible for the leakage conductivity.3 The oxide-ion conductivity, when doped with magnesium, exceeds that of yttria-stabilized zirconia (YSZ) at ~500 °C,3 making it a potential electrolyte material for Intermediate Temperature Solid Oxide Cells (ITSOCs). Figure 1 shows the comparison of bulk oxide ion conductivity between 2 at.% Mg-doped NBT and other known oxide ion conductors. As part of the UK wide £5.7m 4CU project, research has concentrated on trying to develop NBT for use in Intermediate Temperature Solid Oxide Cells (ITSOCS). With the aim of achieving mixed ionic and electronic conduction, transition metals were chemically doped on to the Ti-site. A range of experimental techniques was used to characterize the materials aimed at investigating both conductivity and material structure (Scanning Electron Microscopy (SEM), IS, X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS)). The potential for NBT as an ITSOC material, as well as the challenges of developing the material, will be discussed. (1) Takenaka T. et al. Jpn. J. Appl. Phys 1999, 30, 2236. (2) Hiruma Y. et al. J. Appl. Phys 2009, 105, 084112. (3) Li. M. et al. Nature Materials 2013, 13, 31. Figure 1
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