(547h) 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)

*js.mcewen@wsu.edu

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 [4]. 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 [11] and can potentially be used to alter the Ni/YSZ system so as to directly
modify its electrocatalytic performance.

Our
computational results [12] 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 [13]. 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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