## 1 Introduction  
The development of ceramic ceria-based powders doped with rare earth elements is extensively regarded as favorable electrolytes type-component for intermediate and low-temperature solid oxide fuel cells (SOFCs). The selection of these materials as electrolytes gives rise in their high ionic conductivity and good compatibility with high performing Cu-content anode or cathode [1; 2]. Despite the enhanced ionic properties, compared to the standard yttria-stabilized zirconia (YSZ), ceria-based ceramic materials needs processing temperatures higher than 1400 \({}^{\circ}\)C to achieve full densification. However, during sintering at high temperatures, many drawbacks can occur. For example, the formation of microcracks due to the O\({}^{2-}\) migration, as a consequence of the reduction of Ce\({}^{4+}\) to Ce\({}^{3+}\), or the development of undesirable interfacial reactions during co-sintering can influence the fabrication procedure and the electrolyte durability. If a dense sintered body could be fabricated at a lower temperature, the overall fabrication process can be simplified and the electrolytes can be fully exploited its potential with an additional reduction of production costs. To improve the sinterability of doped ceria ceramics, several methods are proposed in literature which can be classified into two classes: (i) use of ultrafine powders and (ii) addition of sintering aid in a moderate amount according to their solubility in ceria solid solution. The first method implies the utilization of extremely sensitive particles derived from wet synthesis as sol-gel [3; 4; 5], hydrothermal [6], co-precipitation [7], polymeric precursor methods [8], etc. These syntheses can produce nanometer and submicron powders which facilitate the growth of dense bodies even if the sintering temperature is lower than 1500 \({}^{\circ}\)C. With the second methodology instead, ceria-based electrolytes can be sintered at lower temperatures by the direct addition of sintering promoters into commercial or synthesized powders with mechanochemical methods combined to heat treatments. The use of low melting point agents increases the atom mobility at the grain boundaries and encourages the liquid phase sintering. In fact, the aids influence the atomic diffusivity of the grain boundary inducing a closed packing. Combining the two approaches, it is possible to reduce the heating temperature until 950-1000 \({}^{\circ}\)C as reported in our previous works [4; 9] where lithium precursor was added to gadolinium doped ceria. The direct addition of metal precursors during synthesis is a simple process to fabricate high homogenous powders with a faithful composition inducted by the molecular and not mechanical mixing of precursors. This promotes the microstructure and electrochemical properties at the same time. Among the many types of sintering aids investigated for doped ceria, precursors of bismuth, cobalt, copper, iron, lithium, and praseodymium showed a good compromising between electrochemical properties and sintering temperature [10; 11; 12; 13]. CuO-doped SDC, for example, is a promising anode for LT-SOFCs and the influence of copper precursors on the sintering and morphology of ceria based materials is interesting to understand the compatibility between anode and electrolyte. As known from the literature [14; 15], CuO can form a Gd\({}_{2}\)O\({}_{3}\)-CeO\({}_{2}\)-CuO in the grain boundary. This ternary phase, can melt at a definite temperature and promote liquid phase sintering. As a consequence, dense bodies can be formed quickly due to the diffusion of liquid phase under capillary action, along with grain rearrangement with an effect on total conductivity. In a very recent work, [PERSON] and [PERSON] [16] achieved a value of 0.0642 S cm\({}^{-1}\) at 800 \({}^{\circ}\)C by adding the 0.5 mol% to SDC. The authors found that the continuous addition of copper, imply the segregation of a CuO on the surface and grain boundaries of SDC with a consequent decrease of total conductivity. [PERSON] et al.