(Invited) In Situ Optical and Electrochemical Studies of SOFC Carbon Tolerance

High operational temperatures needed for solid oxide fuel cells (SOFCs) are required by the large activation energy of the oxide diffusion through the electrolyte. These high temperatures, in the range of 800-1000 °C, accelerate cell degradation and severely reduce the number of materials suitable for construction.  Employing newly developed, more highly conducting electrolytes such as scandia-stabilized zirconia (ScSZ) enable SOFCs to operate at lower temperatures without sacrificing power (1). Intermediate temperature (IT) solid oxide fuel cells, operating in the 500-750 °C range, permit non-ceramic and non-metallic components to be used in the fuel cell stack, reducing costs and enabling a larger variety of stack geometries (2). However, the high catalytic activity of nickel still causes unwanted carbon deposition on the anode surface (3). This carbon buildup can cause premature cell degradation as well as reduce the power density of the fuel cell.

             Chronopotentiometry, a technique that couples real time  electrochemical monitoring of cell voltage with synchronous in situRaman spectroscopy, provides insight into the mechanisms of carbon deposition and removal as well as the effect deposited carbon has on fuel cell performance (3). This technique can quantify the carbon growth with < 2 second temporal resolution and 1 μm spatial resolution across the anode surface.

Figure 1: Raman “G” peak growth at 675 °C observed in situfrom a cell operating under methane with a ScSZ electrolyte supported membrane electrode assembly.

In this work, we examine qualitative differences in carbon buildup (Fig. 1) between yittria-stabilized zirconia (YSZ) and ScSZ supported solid oxide fuel cells with a Ni-YSZ cermet anode exposed to methane at 675 °C and 725 °C. We also discuss the differences in the linear sweep voltammetry (LSV) between cells running under methane and hydrogen. (Fig. 2)

Figure 2: (Top) Comparison of carbon growth at different polarizations of YSZ and ScSZ SOFCs operating at 725 °C under methane fuel. (a) ScSZ OCV (b) ScSZ 75% I_Max (c) YSZ OCV (d) YSZ 75% I_Max (Bottom) Comparison of LSV traces running under H₂ and CH₄. The “swing” in the CH₄ trace, indicating a change in carbon removal mechanism can be seen at 150 mA (4).

Observed differences between the amounts of carbon formed for YSZ and ScSZ fuel cells at high and low temperatures and different polarizations (Fig. 2) implicate a role played by the electrolyte in the formation of carbon on the anode. Different carbon removal mechanisms have fundamentally different Nernst potentials, and when coupled with Raman spectroscopy, allow us to identify not only the relative amounts, but also the type of carbon formed.

References

1. S.P.S. Badwal, F.T. Ciacchi and D. Milosevic, Solid State Ionics, 136-137 91 (2000)

2. Brett et al., Chemical Society Reviews, 1568-1578 37 (2008)

3. Kirtley et al., Anal. Chem., 9745-53 84 (2012)

4. A.J. Bard, and L.R. Faulkner. Electrochemical Methods, p. 226-47 John Wiley & Sons, Inc., New York (2000).