A parameter study of solid oxide electrolysis cell degradation: Microstructural changes of the fuel electrode

Abstract

A parameter study of 20 solid oxide electrolysis cells was carried out to systematically investigate long-term degradation each over 1,000 h under variation of temperature, humidity and current density. The influence of operating temperature was investigated between 750 and 850 °C, the humidity ranged from 40 % to 80 % H 2 O, and the current density varied between open circuit voltage (OCV) and 1.5 A•cm-2. The progress of degradation was monitored insitu by electrochemical impedance spectroscopy. Five different contributions to the spectra were identified by calculating the distribution of relaxation times and separated via a complex non-linear square fitting routine. The present work focuses on the degradation of the fuel electrode. From SEM analysis, Ni depletion and an increased pore fraction close to the electrode/electrolyte interface was derived, which is correlated with an increased ohmic resistance of the cells. This unidirectional transport of Ni away from the fuel electrode/electrolyte interface leads to an effective electrolyte extension and is the main source of degradation. Ni depletion is shown to be driven by current density and its extent is shown to be dependent on the complex interplay between the operating parameters current density, anode overpotential, humidity and temperature. It is particularly pronounced for pH 2 O larger than 0.8 atm and temperatures above 800 °C. Furthermore, the fuel electrode electrochemistry also exhibits degradation in the high-frequency region around 10 4 Hz.

Evolution of the Morphology Near Triple-Phase Boundaries in Ni–Yttria Stabilized Zirconia Electrodes Upon Cathodic Polarization

“…The observation suggests that the transport regime that requires detachment to trigger gas-phase transport and re-deposition of Ni volatile species on connected Ni near active TPBs may not necessarily dominate in the conditions considered in this study. Loss of contact is often observed in 2D micrographs together with Ni depletion [6,7] but not reported systematically [8]. They may therefore not be a necessary condition in all cases, except if they are not observable after extended periods, because the formed Ni/pore interface is rapidly modified to minimize energy.…”

“…Contact losses between Ni and YSZ are however not systematically observed along with Ni depletion [8]. Another potential significant consequence of polarization is a change in surface tensions because of electrowetting and/or the known dependence of the work of adhesion at the metal/oxide interface on oxygen activity [6,17].…”

“…It is typically enhanced by cathodic polarization at high current density during extended periods. Twodimensional (2D) electron microscopy and representations of the heterogeneous Ni-YSZ microstructure as a packing of Ni and YSZ particles with close to spherical shapes significantly contributed to the current understanding [6][7][8]. Contact losses between Ni and YSZ are often observed near the electrolyte, along with accumulation of Ni further away.…”

Investigation of the Long‐term Stability of Solid Oxide Electrolysis Stacks under Pressurized Conditions in Exothermic Steam and Co‐electrolysis Mode

“…On cell level several experimental studies were carried out and degradation mechanisms were already proposed [10][11][12][13][14][15]. However, the reported degradation data are mostly hard to compare since cell concepts, materials and experimental conditions such as reactant flows, gas composition, conversion, voltage or current density strongly differ from one study to the other.…”

High‐Temperature CO2 Electrolysis in Solid Oxide Electrolysis Cells: Developments, Challenges, and Prospects

“…Ni depletion (Figure B) mainly occurs in steam electrolysis. After 1000 h of SOEC operation, Ni migrates away from the cathode–electrolyte interface evidently, resulting in the increase of ohmic resistance . Physicochemical and electrochemical characterizations suggest that Ni depletion is related to the current density, humidity, temperature, and operation time.…”