High Temperature Solid Oxide Cells for Power-to-X applications: Status, Materials, Challenges and Prospects

Abstract

In the context of the energy transition, all sectors have the need to achieve the CO2 reduction goals and to use renewable energy as basis of their activity. In particular, a corner stone is to radically transform the chemical industry by integrating highly efficient electrochemical processes based on renewable power. This is a possibility to decarbonize the synthesis of chemicals, which was one aspect of the Power-to-X concept.Due to the fast kinetic at high temperature (~ 800°C), Solid Oxide Cells (SOC) enable not only efficient conversion of steam into Hydrogen, but also simultaneous electrolysis of H2O and CO2 to produce in one step syngas (H2 + CO), which is one of the main feedstock for the production of valuable chemicals, e.g. via methanol.In this contribution we report on state-of-the-art electrolyte supported cells operated in H2O-CO2 co-electrolysis mode in a range of temperature between 770°C and 860°C with different steam/carbon ratios for syngas production.The electrochemical behavior of these cells is presented and discussed with regards of the specific thermodynamic of CO2 reduction. Additionally, the thermodynamic boundaries for carbon deposition are shown in order to identify safe operating regimes for cells and SOC stacks [1]. In this regard also ten-layer stacks with electrolyte supported cells were tested for 2000 h in exothermic steam and co-electrolysis mode at elevated pressures of 1.4 and 8 bar. The predominant increase of the ohmic resistance during operation was identified to be mainly responsible for the observed degradation of all three stacks whereas the increase of the polarization resistances played a subordinate role. Within the post-test analysis noticeably high Nickel depletion was observed for the stack operated at the highest pressure in steam electrolysis mode. When operated at high conversion for long time, i.e. typically 1000 hours, it is shown that the fuel electrode is affected by significant irreversible morphological changes due to silicon species originating very likely from the feed water, highlighting the needs in purified stream in order to keep degradation rates sufficiently low to enable sufficient lifetime of the systems. The important contributions by Mogens Mogensen with regard to steam electrolysis, co-electrolysis and degradation phenomena will be acknowledged.Aiming at enhancing durability and flexibility in SOC operation, we report as well on the potential use of an alternative perovskite electrocatalyst La0.65Sr0.3Cr0.85Ni0.15O3-δ (LSCrN) as fuel electrode for reversible operation in which performance in fuel cell steam electrolysis and co-electrolysis operating modes are evaluated.[1] Amaya Dueñas D. M, Riedel M., Riegraf M., Costa R., Friedrich K. A. (2020) High Temperature Co-electrolysis for Power-to-X, Chemie Ingenieur Technik 2020, 92, No. 1–2, 45–52, doi: 10.1002/cite.201900119