In typical porous Ni/yttria-stabilized zirconia (YSZ) anode of solid oxide fuel cells (SOFCs), hydrogen oxidation reaction (HOR) occurs at the pore/nickel/zirconia triple-phase boundaries (TPBs). This anode compound i) allows gaseous hydrogen molecules to move in pores to the reaction sites, ii) makes current flow possible from the TPBs to current collector via nickel matrix, iii) allows oxide ions to transfer to the reaction sites by means of oxygen ion diffusion in zirconia. The last point is possible only when zirconia is doped by cations with oxidation states different from that of Zr⁴⁺. For instance, doping by Y³⁺implies creation of an oxygen vacancy for a pair of dopant cations. So, it is actually presence of the oxygen vacancies that eventually makes the oxygen diffusion possible in SOFC anode. Moreover, oxygen diffusion provides an oxygen-ion flow from cathode to anode through the solid electrolyte made of doped-zirconia. Considering high importance of the dopants in zirconia for SOFC performance, there have been intensive studies of their effect on various phenomena: oxygen diffusion, nickel sintering, phase stability, etc. However, as for details of HOR on atomic scale, all up-to-date theoretical studies considered only Ni/YSZ systems [1-3]. In the present work we perform atomistic modeling study based on density-functional theory (DFT) to see how qualitative and quantitative characteristics of HOR can be affected by selection of dopant species.

The TPB structure for our study is based on the (111) zirconia and (111) Ni slabs that are attached to each other as shown in Figure 1 [4]. Two dopants are introduced instead of two zirconium cations near the TPB (sites A and B). We also introduce oxygen vacancies into zirconia slab, whose number is appropriate to formal oxidation state of the dopants and to the fact that we consider one-oxygen rich zirconia slab in reactant structures (product structures are stoichiometric). In this work we consider three trivalent (Y, Sc, Al) and two divalent dopants (Ba, Ca). Three reaction mechanisms are considered: interface reaction, O spillover, and H spillover (terms are used according to notation by Shishkin et al. [5]). All calculations were performed with Vienna Ab initio Simulation Package (VASP) [6], which can perform electronic structure calculations based on the projector-augmented wave method and the generalized gradient approximation. To evaluate transition states, we performed nudge elastic band calculations as implemented in VASP by Henkelman et al. [7]. Obtained DFT results for the total energy of the structures were processed with open-circuit voltage (OCV) correction in a similar concept given by Nørskov et al. [8].

Energy profiles built for the HOR mechanisms allow to determine the most energetically favorable mechanism for each dopant. This judgement is based on energy barriers of the rate-limiting steps (activation energy). Obtained results for Y-doped TPB agree well with literature data that predicts interface reaction mechanism to be dominant with the rate-limiting step being H transfer from the nickel surface to oxygen ion at the TPB (as indicated in Figure 1 by blue line) [2-3,5]. Qualitatively similar results were obtained for Sc and Ba. In contrast, Al and Ca dopants make O spillover mechanism favorable with the rate-limiting step being O transfer from TPB region onto the nickel surface (as indicated in Figure 1 by red line). Considering dopants promoting the same HOR mechanism, activation energies for trivalent dopants are lower, which makes them better dopant candidates. Specifically, low activation energy of O spillover mechanism in Al-doped system makes Al best candidate to promote this mechanism. Background behind switching of the reaction mechanism with respect to dopant species is analyzed and discussed in this paper.

References

[1] Shishkin, M.; Ziegler, T. Phys. Chem. Chem. Phys. 2014, 16, 1798

[2] Cucinotta, C. S.; Bernasconi, M.; Parrinello, M. Phys. Rev. B 2011, 107, 206103

[3] Liu, S.; Ishimoto, T.; Monder, D. S.; Koyama, M. J. Phys. Chem. C 2015, 119, 27603

[4] Tada T. ECS Transactions 2015, 68, 2875

[5] Shishkin, M.; Ziegler, T. J. Phys. Chem. C 2010, 114, 11209

[6] Kresse, G.; Hafner, J. Phys. Rev. B. 1993, 47, 558

[7] Henkelman, G.; Jónsson, H. J. Chem. Phys. 2000, 113, 9901

[8] Nørskov, J.K; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. J. Phys. Chem. B 2004, 108, 17886

Caption:

Figure 1. TPB model consisting of nickel and zirconia slabs. Nickel atoms are shown in black, oxygen - in red, zirconium - in white. Positions of cation dopants A and B are shown in green. Red line indicates O transfer step of O spillover mechanism, blue line - H transfer step of interface reaction mechanism.