Effects of Current Collector Materials on Performances of Micro-Tubular Solid Oxide Electrolysers for Splitting CO2

As renewable energy sources are inherently intermittent, their increasing contributions to electricity grids will necessitate large-scale energy storage to balance supply and demand. Storing electrical energy in chemical bonds by electrolysis of CO₂ and / or H₂O is one option to achieve this. We chose solid oxide reactors for this application, inter alia, because in principle they can be operated cyclically in electrolyser (SOE) and fuel cell (SOFC) modes.

Results will be presented of measurements of ohmic and electrode polarization resistances and global performances for CO₂ splitting to CO and O₂ in micro-tubular solid oxide electrolysers (SOEs), with different types of current collectors; electrolysis of steam and (CO₂ + steam) exhibited similar behaviour.

Silver or nickel wire current collectors, with and without silver or nickel paste, were used to contact the 220 μm thick inner lumen Ni-YSZ electrode (SOE cathode) of micro-tubular solid oxide electrolysers ^[1] (SOEs: Ni-YSZ|YSZ|YSZ-LSM|LSM) incorporating 34 μm thick yttria-stabilized zirconia (YSZ) electrolyte, and 40 μm thick lanthanum strontium manganite (LSM, SOE anode), as described previously for CO₂ reduction ^[2]. Silver wire and paste was used as the current collector for the LSM electrode.

As shown in Fig. 1, the best performance (ca. 1 A cm^-2 at 1.5 V (thermo-neutral potential difference) at 800 °C) was achieved for reactors with nickel wire current collectors alone, implying metallic pastes had no net beneficial effects on area-specific ohmic resistances. Furthermore, applying silver paste had a detrimental effect on the Ni cathode polarization resistance, for reasons that will be presented. Fig. 2 shows polarization and ohmic resistance data at open circuit; similar behaviour was exhibited with potential differences applied to reactors. Ohmic resistances were expected to decrease by using silver wire, due to its higher electronic conductivity compared to nickel, but no stable cell performance was achieved.

References

[1]    L. Kleiminger, Ph.D. Thesis, Imperial College London, to be published (2015).

[2]    L. Kleiminger, T. Li, K. Li and G. H. Kelsall, RSC Adv., 4 (2014) 50003-50016.