Solid Oxide Fuel Cell (SOFC) devices convert chemical energy into electrical energy by oxidizing fuels such as hydrogen, natural gas, ethanol or hydrocarbon mixtures, which highlights the capability of SOFCs as candidates for applications in stationary power generation. The main species used as fuel in the SOFC are H₂ and CO which are oxidized at the anode [1]. Instead of pure H₂ or CO, H₂/CO mixtures can be used leading to more complicated mechanisms since other processes such as the water-gas shift reaction (CO + H₂O ↔ CO₂ + H₂) are involved.

Robust materials are needed for SOFC since the working temperature is approximately equal to 800˚C. For example, a suitable anode is Ni/YSZ (nickel – yttria stabilized zirconia) since it has a high catalytic activity, mechanical and chemical stability and compatibility with the electrolyte under working conditions [2]. It is well known that the performance of the Ni/YSZ cermet depends on the microstructure and the distribution of Ni and YSZ phases in the cermet [3]. Additionally, this performance depends on the key reactions occurring at the triple phase boundary where the gas phase, Ni particles and YSZ surface meet. Thus, it is relevant to understand, at the atomic scale, the structure of Ni/YSZ and its interaction with the gas phase, and in particular CO₂ and H₂.

To simulate the interaction of CO₂ and H₂ with Ni-YSZ, we have used electronic structure calculations based on the Density Functional Theory using the Vienna Ab-initio Simulation Package (VASP). We have studied the deposition of Ni_n (n=4-10) clusters on the YSZ(111) followed by the adsorption of molecules on the optimised Ni_n/YSZ(111) structures and calculated the geometric and electronic structure at the most stable adsorption site. We have also analysed the reaction path of the reverse water-gas shift reaction (CO₂ + H₂ ↔ CO + H₂O) on three different Ni_n/YSZ(111) (n = 5, 6 and 10) interfaces, considering two possible pathways intermediates: hydrocarboxyl or formate.

Finally, we have observed that both CO₂ and H₂ prefer to adsorb on the Ni_n cluster rather than on the surface. On the metal cluster the CO₂ molecule is activated and the H₂ is dissociated upon adsorption, whereas charge is transferred from the cluster to the molecules. We have determined two intermediate states and three intermediate states for the carboxylic acid and formate intermediate pathway, respectively, on the three Ni_n/YSZ(111) (n = 5, 6 and 10) interfaces.

References

1. J. Hanna, W. Y. Lee, Y. Shi and A. F. Ghoniem, Prog. Energy Combust. Sci., 2014, 40, 74–111.

2. Kim, S. et al. Solid State Ionics, (2007), 178, 1304–1309.

3. Kim, S. et al. Solid State Ionics, (2006), 177, 931–938.