Electrode kinetics of porous Ni-3YSZ cermet operated in fuel cell and electrolysis modes for solid oxide cell application

Abstract

The electrochemical reactions of hydrogen oxidation and steam reduction have been investigated for a porous cermet electrode made of Ni and 3YSZ. The electrochemical characterizations have been performed over a large range of gas compositions at 700°C. It has been shown that the fuel electrode response is activated by the potential under anodic current while a limiting current density appears under cathodic polarization. Moreover, the impedance diagrams exhibit a shape representative of a kind of finite-length Gerischer element with a low-frequency contribution sensitive to the steam content. To interpret these experimental results, a continuous dynamic model has been developed by describing the mass and charge transfers within the electrode. The reaction has been divided into a sequence of elementary steps considering two scenarios of charge transfer based on the oxygen and hydrogen spillover mechanisms. Threedimensional reconstructions obtained by synchrotron X-ray nano-holotomography have been used to provide the cermet structural properties for the simulations. The numerical computations have shown that the hydrogen spillover scenario is the most appropriate mechanism to reproduce correctly the experiments. Besides, the electrode response is controlled by the charge transfer at triple phase boundary lengths, the oxygen vacancies migration in the 3YSZ network and a pure chemical surface process depending on the polarization. In fuel cell mode, desorption of water molecules from 3YSZ would co-limit the electrode response, while in electrolysis mode, the steam adsorption on Ni would become one of the rate determining steps. Finally, a sensitivity analysis has shown that surface diffusion would also play a key role in the electrode response.

A multiscale model validated on local current measurements for understanding the solid oxide cells performances

“…on the reaction mechanisms and electrode performances. Two models were developed considering the state-of-the-art materials with the LSCF-CGO [28], [47] composite and Ni-YSZ [48] cermet for the air and fuel electrodes, respectively. Both models were built up considering a slice of porous electrode described as a homogenous medium.…”

“…All the kinetic constants are thermally activated and expressed with an Arrhenius law. Activations energies were already calculated and published in [28] and [48] for the air and fuel electrodes, respectively.…”

“…For the hydrogen electrode, the model is based on a hydrogen spillover mechanism proposed and validated in [49]. In this case, the reactions mechanism is co-limited by the charge transfer at Triple Phase Boundary lines (TPBls), the oxygen vacancies migration in the YSZ network and a pure chemical surface process depending on the polarization [48]. According to this model, steam can interact with the surface of both Ni and YSZ in two parallel pathways.…”

“…They are extracted from the digital twins obtained with the truncated random field, establishing a link between the two-length scales. Finally, it is worth noting that all the equations and the details for this module of the multiscale model have been already published in [48] and [28] for the fuel and the air electrodes, respectively. In these previous studies, a special attention was paid to validate the mechanisms using experimental results obtained on symmetrical cells tested in a three-electrode set-up.…”

“…• First, the kinetic constants for the reactions implemented in the validated microscale models have been scaled up to reach the performances required at the cell level, using the polarization resistance measured by EIS at OCV and 750 °C as reference. This step consisted to multiply all the kinetic constants, previously determined on symmetrical cells [28], [48], by a correcting factor. Indeed, this correction is necessary since the electrode performances measured in a symmetrical setup are usually lower than those determined for the complete cell.…”