applied sciences Article Evaluation of the Microstructure and the Electrochemical Properties of Ce 0.8(1-x) Gd 0.2(1-x) Cu x O [1.9(1-x)+x] Electrolytes for IT-SOFCs Grazia Accardo * , Jae Kwan Bae and Sung Pil Yoon Center of Hydrogen-Fuel Cell Research, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Korea; jaekwan.bae@kist.re.kr (J.K.B.); spyoon@kist.re.kr (S.P.Y.) * Correspondence: d16605@kist.re.kr Received: 3 June 2020; Accepted: 30 June 2020; Published: 1 July 2020   Abstract: The influence of copper addition (0.5–2 mol%) on the crystal structure, densification microstructure, and electrochemical properties of Ce 0.8 Gd 0.2 O 1.9 synthesized in a one-step sol–gel combustion synthesis route has been studied. It has been found that Cu is very active as sintering aids, with a significative reduction of GDC firing temperature. A reduction of 500 ◦ C with a small amount of copper (0.5 mol%) was observed achieving dense bodies with considerable ionic conductivities. Rietveld refined was used to investigate the crystal structure while relative density and microstructural examination were performed in the sintering temperature range of 1000–1200 ◦ C after dilatometer analysis. High dense bodies were fabricated at the lowest sintering temperature, which promotes the formation of Ce 0.8(1−x) Gd 0.2(1−x) Cu x O [1.9(1−x)+x] solid solution and the absence of secondary phase Cu-rich or the segregation or copper at the grain boundary. As compared to the pure GDC an improvement of total conductivity was achieved with a maximum for the highest copper content of 2.23·10 −3 –9.19·10 −2 S cm −1 in the temperature range of 200–800 ◦ C. Keywords: copper; gadolinium doped ceria; sintering aid; sol–gel combustion synthesis; SOFC 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 ◦ 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 [3–5], 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 Appl. Sci. 2020, 10, 4573; doi:10.3390/app10134573 www.mdpi.com/journal/applsci