Proton‐conducting Fuel or Electrolysis Cells (PCFCs) are thought to be a promising alternative to Solid Oxide Fuel/Electrolysis Cells [1‐2]. The most interesting proton conducting materials for electrolytes include doped barium zirconate, doped barium cerate or their solid solution. Recently, an innovative approach was proposed which allowed obtaining dense ceramics with grain size of typically 2‐4 µm. This new process is based on the reactive sintering of all oxide precursors with the use of NiO as sintering aid [2‐4]. Besides being dense, the samples obtained, such as yttrium‐doped barium zirconate (BZY), show a very high proton conduction with little influence of Ni‐species on conduction properties. We tested the mechanical properties of this BZY material for application as PCFCs electrolyte and we showed that materials made from this process present a fast degradation of mechanical properties when put in hydrogen‐rich conditions. The objective of the present work was thus to understand the atomic‐scale origin of this fast degradation. For this purpose, different electron microscopy techniques have used such as Scanning Transmission Electron Microscopy (STEM‐HAADF), Energy dispersive spectroscopy (EDS), Electron Energy Loss spectroscopy (EELS). Structural and chemical microscopy analysis show that the sample failure is due to the reduction of NiO nanoparticles at grain boundaries (Figure 1, 2).