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@PHDTHESIS{Wolf:1050639,
      author       = {Wolf, Stephanie},
      title        = {{I}nvestigation of solid oxide electrolysis cells’
                      degradation mechanisms with electrochemical impedance
                      spectroscopy},
      school       = {RWTH Aachen},
      type         = {Dissertation},
      publisher    = {RWTH Aachen University},
      reportid     = {FZJ-2026-00391},
      pages        = {130},
      year         = {2025},
      note         = {Dissertation, RWTH Aachen, 2025},
      abstract     = {This work focuses on the analysis of the performance and
                      degradation behavior of Ni-cermet and Ni-free perovskite
                      materials for the fuel gas electrode under steam, CO$_2$,
                      and co-electrolysis conditions. Degradation tests of
                      commercial solid oxide cells with a Ni-8YSZ fuel electrode
                      were carried under a constant current density of $-1$ A
                      $\cdot$ cm$^{-2}$ for 1000 h. Subsequent microstructural
                      analysis showed nickel particle migration and agglomeration
                      at the interface between the active and support layer. These
                      significant microstructural changes led to an increased cell
                      potential during the degradation tests under steam
                      electrolysis conditions. As the main degradation process of
                      Ni-cermet electrodes was identified as Ni depletion, Ni-free
                      Sr$_2$FeMoO$_{6-\delta}$ electrode materials were
                      synthesized and tested. Electron microscopy measurements
                      showed that nanoparticles are dissolved from the SFM
                      perovskite matrix in a hydrogen atmosphere, forming a new
                      perovskite-metal heterointerface that increases the reactive
                      surface area. Conductivity studies showed that
                      Sr$_2$FeMoO$_{6-\delta}$-based materials exhibit higher
                      conductivities in reducing atmospheres than other perovskite
                      materials considered as alternatives to Ni-cermet
                      electrodes. The materials Sr$_2$FeMoO$_{6-\delta}$ (SFM) and
                      Sr$_2$FeMoO$_{6-\delta}$-GDC (SFM-GDC) were
                      electrochemically investigated and achieved higher current
                      densities at 1.5 V than Ni-8YSZ in steam and
                      co-electrolysis, but lower current densities than Ni-GDC.
                      Long-term testing under steam electrolysis conditions showed
                      significantly higher degradation for SFM compared to
                      SFM-GDC, which was attributed to microstructural changes in
                      the SFM electrode. The effect of doping was investigated by
                      synthesizing the double perovskite
                      Sr$_2$FeMo$_\mathrm{0.65}$M$_\mathrm{0.35}$O$_{6-\delta}$
                      with M = Ti, Co, Cu, Mn, and Ni. B-site doping showed an
                      increased conductivity in oxidizing and reducing atmospheres
                      up to 152 S $\cdot$ cm$^{-1}$ for
                      Sr$_2$FeMo$_\mathrm{0.65}$Cu$_\mathrm{0.35}$O$_{6-\delta}$-GDC
                      due to the exsolution of bimetallic Fe-Cu nanoparticles. The
                      highest current density of all materials studied, compared
                      to Ni-GDC and Ni-8YSZ, was found for
                      Sr$_2$FeMo$_\mathrm{0.65}$Ni$_\mathrm{0.35}$O$_{6-\delta}$
                      (SFM-Ni). After 500~h under steam electrolysis conditions,
                      however, the SFM-Ni microstructure showed particle
                      agglomeration and electrode densification. To investigate
                      the effect of the fuel gas, SFM-based electrode materials
                      were tested for CO$_2$ electrolysis in an atmosphere of 80\%
                      CO$_2$ + 20\% CO. The highest current density and very good
                      long-term stability over 1000 h was shown by
                      Sr$_2$FeMo$_\mathrm{0.65}$Ni$_\mathrm{0.35}$O$_{6-\delta}$.},
      keywords     = {H2 (Other) / SOEC (Other) / co-electrolysis (Other) /
                      hydrogen (Other) / impedance spectroscopy (Other) /
                      performance analysis (Other) / solid oxide electrolysis
                      (Other)},
      cin          = {IET-1},
      ddc          = {540},
      cid          = {I:(DE-Juel1)IET-1-20110218},
      pnm          = {1231 - Electrochemistry for Hydrogen (POF4-123) / HITEC -
                      Helmholtz Interdisciplinary Doctoral Training in Energy and
                      Climate Research (HITEC) (HITEC-20170406)},
      pid          = {G:(DE-HGF)POF4-1231 / G:(DE-Juel1)HITEC-20170406},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2025-01207},
      url          = {https://juser.fz-juelich.de/record/1050639},
}