481293 Enhancing the Ni/YSZ Catalytic Performance in the Activation of Methane: The Influence of the Electric Field and Interface Carbon
Enhancing the Ni/YSZ Catalytic Performance in the Activation of Methane: The Influence of the Electric Field and Interface Carbon
Fanglin Che1, Su Ha1, and Jean-Sabin McEwen1*
1Washington State University, Pullman, WA 99163 (United States)
Understanding the oxygen vacancy formation at the triple phase boundary (TPB) of a yttrium-stabilized zirconia supported Ni nanocluster (Ni/YSZ) is of importance since such vacancies are the active sites for coke formation [1-3] and sulfur poisoning . In this way, one can make a step toward designing catalysts that decrease the deactivation of the Ni/YSZ electrode via the formation of coke in a solid oxide fuel cell (SOFC) and sulfur poisoning in a solid oxide electrolysis cell (SOEC). Our previous field-dependent microkinetic model and the corresponding experimental evidence have shown that a positive electric field can enhance the methane conversion and reduce coke formation in important ways during the Ni-based methane steam reforming (MSR) process [5-10]. Therefore, considering the field effects on the oxygen vacancy formation is necessary as such fields exist in both cells  and can potentially be used to alter the Ni/YSZ system so as to directly modify its electrocatalytic performance.
Our computational results  show that a positive electric field (less than 0.6 V/Å) can significantly increase the TPB oxygen vacancy formation energy in the Ni/YSZ+O system (Figure 1(a)). This suggests that the presence of such a positive electric field can reduce the oxygen vacancy concentration at the TPB region, and subsequently suppress the sulfur poisoning at the TPB region of the Ni/YSZ+O electrode in a SOEC. A negative electric field in a SOFC could lead to more active TPB vacancies by reducing the vacancy formation energies in the Ni/YSZ+O model. Both charge distribution and effective dipole moments verify the qualitative findings with regard to how the field influences the formation of an oxygen vacancy in Ni/YSZ. Overall, this investigation provides guidance for designing a Ni/YSZ electrode with an improved electrocatalytic performance via a simulated positive electric field.
In addition, there has been some debate over the past decades on the role of low concentrations of interface carbon complexes in the activation of hydrocarbons over transition metal surfaces . It is also a mystery as to whether or not electric fields can enhance the conversion of methane in a fuel cell or during electro-reforming at temperatures that are lower than the normal reforming conditions [14, 15]. To provide a 'bottom-up' fundamental understanding, we present the first C-H cleavage in methane over the Ni/YSZ catalysts as a case study since such reaction is the rate-limiting step during the MSR process. Our theoretical results show that the presence of carbon or carbide-like species at the interface between the Ni cluster and its metal oxide support, as well as the application of an external positive electric field, can significantly increase the local oxidation states of the Ni nanocluster. Furthermore, the first C-H cleavage in methane is thermodynamically and kinetically favored when the oxidation state of Ni is increased (Figure 1(b)). As a result, the presence of a low concentration of surface carbon, interfacial carbide species, or the addition of a positive electric field will facilitate the methane activation process.
Figure 1. (a) Optimized structures used for the oxygen enriched Ni/YSZ (Ni/YSZ+O) model. Atoms with the light green, dark green, silver, and red colors represent the Zr, Y, Ni, and O species. (b) DFT results of the equilibrium constants at 873 K of the first C-H cleavage over Ni/YSZ as a function of the Ni oxidation state, which can be altered by the applied electric fields or altered by the presence of carbon species.
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