(Invited) Fuel Electrode Degradation for Solid Oxide Electrolysis Cells – How to Characterize It and What to Do about It
Authors: Hauch Anne , Chen Ming , Frandsen Henrik Lund , Hendriksen Peter Vang , Jensen Søren Højgaard , Skafte Theis Løye , Sun Xiufu , Tong Xiaofeng , Traulsen Marie Lund , Trini Martina , Mogensen Mogens Bjerg 
Published: May 2020

Solid oxide cells (SOC) are reversible electrochemical cells that can be operated as solid oxide fuel cells (SOFC) or as solid oxide electrolysis cells (SOEC). SOFC convert fuels such as H2 and natural gas into electric power and heat; while SOEC can be used e.g. for storing surplus renewable electrical energy via electrolysis of H2O and/or CO2 to produce fuels like H2, CO or synthesis gas (CO+H2). Synthesis gas can subsequently be catalyzed into a variety of synthetic fuels.
In addition to the fact that SOC are made of abundant materials (no precious metals, expensive IrO2 or the like), the SOEC are also highlighted as superior when it comes to flexibility in comparison to well-known electrolysis technologies such as alkaline and PEM. The flexibility of the SOCs lies in: 1) reversible operation mode (rSOC) 2) high efficiency in both SOFC and SOEC mode, 3) operating at high current density with lower internal resistance than other electrolysis technologies and 4) high fuel flexibility e.g. the ability to operate the cell in CO2 electrolysis or co-electrolysis mode or with direct reforming of ammonia in SOFC mode. However; operating an SOC as rSOC is not necessarily simple and for the operation to match fluctuating supply from renewable energy sources, harsh rSOC operating conditions may be required [1,2]. Operating the SOEC at high current density is possible, but can lead to severe and irreversible degradation caused by migration of Ni in the innermost part of the fuel electrode as previously reported [3,4] and illustrated in Figure 1. Lastly, even though SOC offer large fuel flexibility, special attention should be paid to the effect of impurities in the gasses. For CO2 electrolysis operation significant degradation has been observed to be caused by traces (down to ppb level) of impurities. The effect becomes increasingly critical at operation conditions close to the carbon deposition threshold [5]. This talk will therefore touch upon the three issues: 1) Load cycling operation of SOC, 2) Ni/YSZ electrode degradation caused by Ni migration and 3) the puzzling interplay between impurities in the fuel and carbon deposition during CO2 electrolysis.
In the context of load cycling operation of SOC results from a European project, REFLEX, will be presented. This project aims to develop an innovative renewable energy storage solution based on rSOC (“Smart Energy Hub”) for decentralized storage of electrical energy and to produce electrical energy and heat locally when needed. Tests in this project have included both single cells and stacks. Moreover; test operation schemes have been designed to investigate harsh load cycling operation.
In relation to Ni migration; this talk will discuss and seek to answer questions such as: what is the critical parameter for onset of Ni migration away from the electrolyte/electrode interface and what is the driving mechanism for the transport? Can we use phase-field modelling to describe not only Ni coarsening but also Ni migration? Will Ni migration only occur for electrolysis when steam is present? Or will it also take place for CO2 electrolysis? And can we find means to minimize or fully hinder Ni migration?
With respect to sensitivity towards impurities in the fuel and the interplay with carbon deposition, this talk will exemplify the effect of impurities – even in the ppb range - and seek to provide solutions for handling of gas stream impurities. Furthermore, this topic provides an excellent opportunity to showcase complementary characterization techniques, such as impedance spectroscopy, scanning electron microscopy (SEM) and Raman spectroscopy for investigation of the degrading Ni/YSZ fuel electrode.

References

[1] X. Sun, B.R. Sudireddy, X. Tong, M. Chen, K. Brodersen, A. Hauch, ECS Trans. . 91 (2019) 2631–2639.
[2] A. Ploner, A. Hauch, S. Pylypko, S. Di Iorio, G. Cubizolles, J. Mougin, ECS Trans. 91 (2019) 2517–2526.
[3] M.B. Mogensen, A. Hauch, X. Sun, M. Chen, Y. Tao, S.D. Ebbesen, K. V. Hansen, P. V. Hendriksen, Fuel Cells, 17(4), (2017), 434-441.
[4] A. Hauch, K. Brodersen, M. Chen, M.B. Mogensen, Solid State Ionics. 293 (2016).
[5] Hauch, A.; Skafte, T.L.; Küngas, R.; Traulsen, M.L.; Jensen, S.H. in: A. Weber (Ed.), 13th Eur. SOFC SOE Forum 2018, Lucerne, Switzerland, 2018: p. B1501.

Figure 1: Low-voltage in-lens SEM image of a cross section of a fuel inlet piece of a long-term (½ year) tested SOEC. Bright appearing particles are Ni particles constituting a percolating network.

Figure 1