Ni migration in solid oxide cell electrodes: Review and revised hypothesis

Abstract

Severe degradation of Ni-YSZ (yttria stabilized zirconia) electrodes of solid oxide cells (SOCs) due to Ni migration is well known, but the literature contains apparent contradictions. The mechanisms are still under debate. Fine structured Ni-YSZ composite electrodes often degrade at operation temperature (700-950 • C), because Ni particles lose electrical contact with each other as larger Ni-particles grow on the expense of smaller ones. Another type of Ni migration, which may be very damaging, is the relocation of Ni in the most active part of the Ni-YSZ cermet electrode next to the dense YSZ electrolyte. Emphasis is put on the migration of Ni away from the YSZ electrolyte in solid oxide electrolysis cells (SOECs). This is seen as an important obstacle to the commercialization of SOC systems. Apart from temperature, degradation of Ni-YSZ electrodes in SOCs is related to Ni-YSZ electrode overpotential and the local redox potential of the gas mixture inside the porous Ni-YSZ electrode. A unifying Ni migration mechanism is proposed, and methods of alleviating the electrode degradation are discussed. The hypothesis is that Ni migrates via surface diffusion of Ni(OH) x species below ca. 800 • C and via Ni(OH) x species in gas phase above ca. 900 • C.

Functional thin films as cathode/electrolyte interlayers: a strategy to enhance the performance and durability of solid oxide fuel cells

“…Degradation of the anode was not clearly observed on the microstructural characterization but phenomena such as Ni agglomeration resulting in a loss of TPB close to the electrolyte cannot be excluded. 48,49 At the light of those observations, one can conclude that the nanometric bilayer does not seem to be the main source of the observed cell degradation, but the inhomogeneity of the high current density introduced when operated at 1 A cm À2 .…”

Multiscale modelling potentialities for solid oxide fuel cell performance and degradation analysis

“…12 Moreover, Ni migrates away from the electrolyte interface further varying the effective active area. 13 At the cathode, perovskites are commonly applied as a pure layer or mixed with high ionic conductor materials in order to increase their performance by extending TPB sites and enhancing ion migration. LSCF–CGO (Lanthanum Strontium Cobalt Ferrite–Cerium Gadolinium Oxide) or LSM–YSZ (Lanthanum Strontium Manganite–Yttria Stabilized Zirconia) are the main available solutions at the industrial level.…”

Unravelling the role of dopants in the electrocatalytic activity of ceria towards CO2 reduction in solid oxide electrolysis cells

“…Ni dusting from carbon deposition and sulphur poisoning. [10][11][12][13][14][15][16] In more recent times, ceria has also started to draw attention as an electrocatalyst material for H 2 O splitting and CO 2 reduction, with an increasing amount of studies investigating its electrocatalytic properties and unravelling its potential as single-phase fuel electrode for (co)-electrolysis. [17][18][19][20][21][22] The mixed ionic and electronic conducting (MIEC) nature of ceria under reducing conditions presents in fact the remarkable advantage of expanding the reaction zone from the typical three phase boundary (3PB) of a Ni-YSZ cermet to a two phase boundary (2PB) on the entirety of the electrode surface.…”