Microstructural evolution of a Solid Oxide Electrolyser Cell (SOEC) Ni-YSZ cermet cathode is investigated using three dimensional electrode characterisations. 3D reconstructions are obtained on a reference and two long-term tested cells, which were maintained at -0.5 and -0.8 A cm(-2) for 1000 h at 800 degrees C. During the long term tests, air was fed at the anode and a mixture of 10% H-2-90% H2O was fed at the cathode. In this framework, reconstructions have been obtained from synchrotron X-ray nano-tomography technique. Microstructural properties extracted from the 3D reconstructions exhibit an evolution during the tests. Triple Phase Boundary length is decreasing from 10.49 +/- 1.18 mu m(-2) for the reference cell to 6.18 +/- 0.6 mu m(-2) for the long term tested cell at -0.8 A cm(-2). Evolutions of morphological parameters were introduced in an in-house multi-scale model to evaluate their impacts on the electrode degradation, and hence, on the global SOEC performance. (C) 2014 Elsevier B.V. All rights reserved.