SOFC anode modelling using TPB based kinetics

Solid oxide fuel cell exhibits its advantages such as fuel variability. Capable of running on renewable fuels, SOFCs are regarded as potential solution to carbon-neutral energy conversion path. SOFCs operate on fuel supplied to the anode and oxidant supplied to the cathode. The anode should facilitate the oxidation of fuel and transport of electrons from the reaction site to the current collector. It should also enable the diffusion of gaseous fuel to the reaction sites and reaction products away from the reaction sites. Meeting abovementioned requirements, Ni/YSZ cermets are competent SOFC anodes, where Ni is an electronic conductor, YSZ is an ionic conductor, and pores in between Ni and YSZ facilitate the fuel supply. Fuel oxidation occurs in the vicinity of triple phase boundaries (TPBs), where Ni particles, YSZ particles and pore interact. Thus, TPB length quantifies the electrochemically active sites within the anode. The fuel oxidation involves physical processes and multi-elementary reaction steps in reality, among which exists a rate-limiting step. The overall reaction rate can be represented by the rate-limiting step whose kinetics are proposed as a function of the TPB length. In many past computational studies of the cermet anode, a global electrochemical reaction expressed by Butler-Volmer equation is put forward, simplifying electrode reaction to a single step process and probably inadequately to present insights into intrinsic processes occurring within the anode. In this work, TPB-based kinetics, derived from the pattern anode experiment, are implemented in a CFD model to evaluate the performance of Ni/YSZ cermet-based cells. For model validation, simulated polarization curves are compared with the experimental ones. TPB length as a fitting parameter is determined to ensure the agreement between simulated polarization curves and experimental ones. The fitted TPB length is found to be several orders of magnitudes lower than physical TPB length of typical Ni-YSZ cermet anode, which might imply that only a small fraction of the physical TPB length actually participates in the electrochemical reactions. CFD behavior like species distribution and electrochemical behavior like overpotential breakdown are investigated. The gradient of species molar fraction in anode turns to be larger than that in fuel channel, agreeing with the fact that fluid flow is much slower in anode where species transport in porous electrode is dominated by diffusion rather than convection. The cathode activation overpotential makes the most significant contribution to the overall overpotential. The anode activation overpotential at high current density region in the case of low molar fraction of inlet hydrogen yields rapid increase, which could be explained as a consequence of rapid decrease of exchange current density in the vicinity of anode/electrolyte interface. Parametric study of operating temperature is conducted later to see its effect on the cell performance. The higher the operating temperature is, the slower drop of cell voltage with respect to cell current is observed. At a fixed cell voltage, an elevated temperature will cause intensified fuel consumption/vapor generation within anode, leading to a larger gradient of species molar fraction and accordingly a larger concentration overpotential.