The Horizon 2020 eCOCO2 project is developing an electrochemical membrane reactor for the conversion of CO2 from industrial sources using renewable electricity and steam, to carbon-neutral synthetic liquid fuels for use as transport fuels. A first step of this work consists of defining suitable materials' formulation and processing routes for engineering a tubular cell integrating a proton conducting ceramic electrolyte with adequate conductivity in the temperature range 400-550°C. In this work, we develop thin dense electrolytes of BZCYYb based oxides (various dopant concentrations are screened) on Ni-BZCY tubular supports, as well as BZCYYb based oxides sintered as dense disks using the solid-state reactive sintering method for both architectures. We compare the conductivity of these architectures to discriminate the effects of the processing route and the use of Ni sintering aid. The results of this study are corroborated with hydration studies and conductivity measurements performed on these compositions as function of the amount of sintering aid and the annealing profile. This systematic analysis is used to guide the design of the cell's architecture and its processing. The most promising compositions are produced as short tubular cells capped and sealed on alumina risers for dual atmosphere testing. Preliminary results on electrode screening in terms of adhesion and compatibility with the electrolyte layer will be presented. This study has received European Union’s Horizon 2020 Research and Innovation funding under grant agreement Nº 838077.