Figure 1

Model of the $\mathrm{Ni}/\mathrm{YSZ}$ three phase boundary. Blue (dark gray), red (medium gray), gray, and green (medium-light gray) spheres represent Ni, O, Zr and Y atoms, respectively. Yellow (light gray) spheres depict the positions of constitutional vacancies in YSZ. The interface and bulk O atoms labeled ${\mathrm{O}}_{\mathrm{int}}$ and ${\mathrm{O}}_{\mathrm{bulk}}$ are involved in the oxidation pathways discussed in the text. The white circle indicates the interface area where H oxidation occurs. A side view of the slab is shown in the inset.Reuse & Permissions

Figure 2

Potential energy profile (eV) along the minimum energy path for H oxidation. The energy of the slab plus ${\mathrm{H}}_2$ in gas phase is set to zero. The insets represent the intermediate states along the oxidation pathway. (1) ${\mathrm{H}}_2$ adsorbed on Ni (not shown in the insets); (2) O migrated on top of Zr (${\mathrm{O}}_{\mathrm{int}}$ of Fig. 1); (2’) interface vacancy refilled by a bulk O; (3) H moved to the TPB for the first H spillover; (4) first H spillover forming ${\mathrm{OH}}^{-}$; (5) H moved to the TPB for the second H spillover; (6) second H spillover leading to an adsorbed ${\mathrm{H}}_2\mathrm{O}$; (7) desorbed ${\mathrm{H}}_2\mathrm{O}$ where the change in entropy of the gas phase ${\mathrm{H}}_2$ and ${\mathrm{H}}_2\mathrm{O}$ molecules has been added leading to a reaction free energy $\Delta F$. This last free energy level is 0.16 eV lower than that of the reactants. The color code is the same as in Fig. 1. The red dashed line indicates the energy gain due to the refilling of the interfacial O vacancy by a bulk O. Red horizontal lines are TS energies with activation energies (with respect to neighboring local configuration) given by red numbers on the left. 1st ${\mathrm{H}}_{\mathrm{spill}}$ and 2nd ${\mathrm{H}}_{\mathrm{spill}}$ represent the TS energy of the first and second H spillover, 1st ${\mathrm{H}}_{\mathrm{spill}\mathrm{\text{−}}\mathrm{bis}}$ represent the TS energy of the first H spillover along an alternative reaction pathway, involving the direct jump of H from Ni to the interfacial O (see text). ${\mathrm{O}}_{\mathrm{spill}}$ and ${\mathrm{OH}}_{\mathrm{spill}}$ represent the TS energies for ${\mathrm{O}}^{2-}$ and ${\mathrm{OH}}^{-}$ spillovers from YSZ to Ni.Reuse & Permissions

Figure 3

Sketch of the process leading to the formation of water on the wet YSZ surface. In the initial configuration (left panel) there are two surface hydroxyls ideally coming from 2 H spillover from Ni and a hydroxyl adsorbed on Zr coming from dissociative chemisorption of an additional water molecule which also generates a third surface hydroxyl simply acting as a spectator (not shown). The blue arrows depict the movement of a surface hydroxyl on top of a neighboring Zr site and the ${\mathrm{H}}^{+}$ transfer from the second surface hydroxyl. The resulting surface vacancy—represented by a transparent red sphere in the right panel—is filled by O transfer from the bulk. The newly formed water molecule (right panel) is H bonded with the neighboring adsorbed hydroxyl. See also Fig. S6 in 15.Reuse & Permissions