Spin-polarised density functional theory (DFT) calculations were carried out using the Vienna Ab initio Simulation (VASP) code. The ionic cores were described by PAW potentials (O pseudopotential was used for oxygen) and the wave functions were expanded in plane waves with an energy cut off at 520 eV. The PBE-generalized gradient approximation (GGA) was used. To describe the Ce 4f and Fe 3d electrons, DFT+U was implemented using the Dudarev treatment. For Ce 4f electrons we used U eff = 5eV following the wokr of Castleton et al., and for the Fe 3d electrons we used U eff = 3eV following the work of Grau-Crespo et al.. The surfaces were modelled as a symmetric slabs with a thickness of 12 atomic layers and 3 × 3 cell expansion in the lateral directions. The bottom three atomic layers were fixed during geometry optimisations. The periodic images of the slab were separated along the c-direction by a vacuum region of about 15Å. The convergence parameters for electronic and ionic relaxation were set to 10 -7 eV / and 10 -4 eV , respectively, to guarantee a sufficient accuracy of the calculated forces. The dipole correction was used to decouple the electrostatic interaction between the periodic images. The calculations were performed with a 4 × 4 × 1 Monkhorst Pack grid. For gases, electronic calculations were carried out in a 13 × 14 × 15Å box. The standard chemical potential was calculated as µ o = E el + ZP E -T S where the entropy of the gas and surface was calculated using the ASE thermochemistry package. Electric fields were implements using the EFIELD tag. H 2 O reduction on CeO 2 : The CeO 2 (111) termination was studied as it was previously reported to be the most stable termination under solid oxide cell operational conditions. Additionally, Feng et al. speculated that the SDC thin film grown on the current collector was (111)-oriented. The rate limiting step of H 2 O reduction on CeO 2 was reported to be ET (Table ), where we calculate the free energy of the pristine slab and a slab with a singly charged OH • O adsorbate (Fig. ). termination. The rate limiting step was determined to be CIET, where we calculated the free energy of the pristine slab and a neutral CO x 3 O state on a reduced CeO 2 (111) slab (Fig. ).