The Ni-YSZ interface

Jensen Karin Vels87-550-3042-4, 87-550-3043-2

The anode/electrolyte interface ш solid oxide fuel cells (SOFC) is known to cause electrical losses. Geometrically simple Ni yttria-stabilised zirconia (YSZ) interfaces were examined to gain information on the structural and chemical changes occurring during experiments at 1000°C in an atmosphere of 97% H2/3% H20. Electrochemical impedance spectroscopy at open circuit voltage (OCV) and at anodic and cathodic polarisations (100 mV) was performed. A correlation of the electrical data with the structure development and the chemical composition was attempted. Nickel wires with different impurity content (99.8% Ni and 99.995% Ni) were used to examine the impact of impurities on the polarisation resistance and contact area morphology. The electropolished nickel wires were pressed against a polished 8 mol% YSZ surface.Extensive structural changes from a flat interface to a lull and valley structure were found to occur ш the contact area with the impure nickel wire, and a ridge of impurities was built along the rim of the contact area. Impurity particles in the interfacial region were also observed. The impurity phase was described as an alkali silicate glassy phase. No differences were found between polarised and non-polarised samples. With pure nickel wires, however, the microstructures depended on the polarisation /non-polarisation conditions. At non-polarised conditions a hill and valley type structure was found. Anodic polarisation produced an up to 1 um thick interface layer consisting of nano-sized YSZ particles with some Ni present. At cathodic polarisation both a granulated structure and a lull and valley structure resembling the structure of non-polarised samples were found. Small impurity ridges were surrounding the contact areas on non-polarised and cathodically polarised samples. TOF-SIMS and XPS analyses showed the presence of impurities in both the impure and pure contact areas. The impedance spectroscopy revealed that depending on the impurity content of the nickel, different developments of the polarisation resistance with time took place. At open circuit voltage the samples with impure nickel electrodes showed an initial increase toward a high constant polarisation resistance, whereas the samples with pure nickel electrodes showed a considerable decrease to a low constant polarisation resistance with time. For both types of nickel the polarisation resistance dropped upon polarisation. The area specific polarisation resistances for the samples with pure electrodes were approximately 10 times lower than for samples with impure electrodes. This was mainly ascribed to the impurity content and distribution, both in the three phase boundary zone and as a more or less continuous film covering the interfacial region. The drop in the Rp upon polarisation may be ascribed to changes ш the distribution of the impurity phase in the interfacial region.

---

Table of contents :
SOFC principle of operation……Page 9
Electrolyte……Page 10
Electrical losses/electrochemical efficiency……Page 11
Previous studies on Ni point electrodes……Page 12
Thesis layout……Page 13
Nickel wires……Page 14
Geometry of the set-up……Page 15
Electrochemical measurements……Page 16
Other experiments/analyses……Page 18
SEM/EDS……Page 19
TOF-SIMS……Page 20
Samples with impure nickel wires as electrodes……Page 21
Contact areas on the YSZ pellets……Page 22
Structure with holes in the grains (type c)……Page 23
Hill and valley/pyramidal tips transition structure (type d)……Page 24
Hill and valley structure/ pyramidal tips (type b)……Page 25
The rim ridge……Page 26
Other features concerning the YSZ contact area……Page 27
The nickel wires……Page 29
Contact areas on the YSZ pellet……Page 30
Non-polarised samples……Page 31
Anodically polarised samples……Page 34
Thermally annealed sample……Page 37
Impurity influence on the formation of hill and valley structures……Page 40
The rim ridge……Page 41
Influence of current……Page 42
Conclusion on morphology……Page 43
Results of XPS analysis of YSZ……Page 44
Impure nickel electrodes……Page 46
Contact areas on YSZ pellets……Page 47
Pure nickel electrodes……Page 49
Contact areas on YSZ pellets……Page 50
Overview scans……Page 51
The contact area interior……Page 52
The contact area border……Page 55
XPS analyses……Page 56
Impurity segregation from the Ni to the interface……Page 58
Impurities in the pure interface……Page 59
Rim ridge formation……Page 60
Conclusion on impurities……Page 63
Series resistance……Page 64
Impure nickel wires as electrodes……Page 65
Pure nickel wires as electrodes……Page 66
Electrical contact areas……Page 67
Impure nickel wires as electrodes……Page 69
Multiple polarisations……Page 71
Pure nickel wires as electrodes……Page 72
Spectra from samples with impure electrodes……Page 73
Spectra from samples with pure electrodes……Page 74
Capacitance……Page 79
Chronoamperometry……Page 81
Potential sweeps……Page 83
Trends for OCV conditions……Page 84
Polarisation resistance values……Page 85
Conclusion on electrical measurements……Page 86
Impurities and resistance……Page 87
Structures and polarisation……Page 90
Final Conclusions……Page 91
Outlook……Page 92
References……Page 93
List of publications……Page 97