Development of Fuel Electrode Supported Solid Oxide Cell with Ni/CGO Active Layer

Abstract

Increasing the current density, at which substantial lifetime can be achieved, is one way to lower the cost of solid oxide electrolysis cell (SOEC) technology. Many state-of-the-art fuel electrode supported SOECs employ Ni/8YSZ fuel electrodes. These degrade at high current density because the increased fuel electrode overpotential accelerates Ni depletion/migration and loss of percolation in the Ni/8YSZ structure. This limits maximum current density. The stability for Ni/CGO (Ce0.9Gd0.1O1.95) fuel electrodes is not documented quite as well, but several reports show very stable fuel electrode performance for Ni/CGO in electrolyte supported cells. Good fuel electrode stability with Ni/CGO was also found in our tests at 750°C above -1.1 A/cm2 as shown in Figure b). To exploit this high current density performance stability at intermediate SOEC operating temperatures (~750ºC) we are working to incorporate Ni/CGO in a co-sintered thin electrolyte cell.This contribution will give select results from our ongoing efforts to incorporate a Ni/CGO active layer into a fuel electrode supported SOEC consisting of a Ni/3YSZ support and a thin stabilized zirconia electrolyte. Half cells consisting of Ni/3YSZ support, Ni/CGO fuel electrode, zirconia electrolyte and CGO barrier layer have been successfully co-sintered, as shown in Figure a). Results from full cell tests based on these architectures will be reported.To evaluate the general feasibility of the cell concept in parallel with cell manufacturing, supporting studies have been carried out. The chemical expansion of CGO caused by strongly reducing conditions at the fuel electrode is a potential mechanical threat to the cell concept. Initial modelling has shown a technologically relevant operating range with respect to gas composition, temperature and current density where the cell is predicted by modelling to stay intact. As shown in Figure c) thermoneutral operation falls well within the boundaries of mechanically safe operation. Subsequent experimental testing has even showed that the model gave a conservative estimate of the safe range, as we were unable to enforce a mechanical failure by testing under conditions of very strong chemical expansion (predicted expansion of 2%). In-situ XRD studies revealing strain levels and phases present have been performed to elucidate why mechanical failure does not occur, even under very strong reduction.Beyond that, we are also testing the durability of Ni/CGO electrodes on commercial electrolyte supported cells to verify that stable electrode performance can be achieved under conditions relevant to fuel electrode supported SOECs with a thin electrolyte. So far these tests show no change in the polarization resistance contribution from the charge transfer reaction denoted Rp,lf, after operation at 0.5-1.3 A/cm2 regardless of current density. Figure 1