Improving SOEC cell and stack performance via post-firing/post-assembly infiltration

Four to five layer cell architectures based on a structurally supporting Ni/YSZ component, a thin YSZ electrolyte, a CGO “barrier” layer and mixed conducting oxygen electrode (typically LSC or LSCF) represents today the state of the art in terms of electrical performance for solid oxide electrolysis cells. The durability at the current densities typically achieved operating at the thermo-neutral potential is however not satisfactory. Here, we report on new routes to improve durability by infiltration. Recently, well performing oxygen electrodes prepared by infiltrating CGO and LCN precursor solutions in a YSZ backbone anchored to the electrolyte were reported [1]. Also recently we have found [2] that the durability of the Ni/YSZ electrode can be improved by infiltrating CGO pre-cursors in the Ni/YSZ electrode. Practical use of this approach is complicated by the fact that only when the fuel electrode is in reduced state the porosity is sufficient for efficient infiltration. This means that either infiltration must be carried out at stack level after reduction, which has indeed been demonstrated to be feasible [3], or the cells must be prepared in reduced state. The latter approach entails a number of challenges. First of all it requires that also a good oxygen electrode can be prepared by post reduction infiltration. Secondly, there are a number of mechanical challenges to be considered; 1) the conditions of reduction must be carefully chosen to ensure that the expansion on reduction of the CGO barrier layer does not lead to delamination or cell fracture, and 2) the implications of the reduction to Ni for the residual stress state in the cell and thus its handling strength must be considered. We have recently reported that when reduction occurs at high temperature (T >800 oC) effectively residual stresses in the cell are zeroed [4]. Also, we have found that though cell strengths are higher for NIO/YSZ than for Ni/YSZ, for the failure strain it is vice versa [5] indicating that good handling stability is achievable also in reduced state. In the paper we will present a detailed analysis of the mechanical implications of a low temperature reduction of the above described four layer cells and report on achievable electrochemical performance with this type of cell.