Citation: Mah, J.C.W.; Aznam, I.; Muchtar, A.; Somalu, M.R.; Raharjo, J. Synthesis of (Cu,Mn,Co) 3 O 4 Spinel: Effects of Citrate-to-Nitrate Ratio on Its Homogeneity and Electrical Properties. Energies 2023, 16, 1382. https://doi.org/10.3390/en16031382 Academic Editors: Mohammed S. Ismail and Jin-Soo Park Received: 27 December 2022 Revised: 10 January 2023 Accepted: 24 January 2023 Published: 30 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Communication Synthesis of (Cu,Mn,Co) 3 O 4 Spinel: Effects of Citrate-to-Nitrate Ratio on Its Homogeneity and Electrical Properties Joelle C. W. Mah 1 , Isyraf Aznam 1,2 , Andanastuti Muchtar 1,3, * , Mahendra Rao Somalu 1 and Jarot Raharjo 4 1 Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor Darul Ehsan, Malaysia 2 Material Sciences Division, Gaia Science (M) Sdn Bhd, Puchong 47100, Selangor Darul Ehsan, Malaysia 3 Department of Mechanical and Manufacturing Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor Darul Ehsan, Malaysia 4 Research Center for Advanced Materials, National Research and Innovation Agency, Puspiptek, South Tangerang 15314, Banten, Indonesia * Correspondence: muchtar@ukm.edu.my Abstract: The (Cu,Mn,Co) 3 O 4 (CMC) spinel layer is useful in inhibiting Cr vaporization that dete- riorates the solid oxide fuel cell performance. The effectiveness of the spinel layer in suppressing volatile Cr species from the metallic interconnects is strongly dependent on layer density, which is influenced by particle size distributions and agglomerations of the spinel powders. Considering that the material properties were influenced by the synthesizing conditions, this study elucidated the influences of citric acid (fuel) on the structure, morphology, and electrical properties of sol–gel derived CMC spinel powders. Dual-phase CMC spinel powders, consisting of cubic CuMnCoO and tetragonal Mn 2 CoO 4 , were successfully synthesized at citrate-to-nitrate (CA/MN) ratios of 0.8, 1.0, and 1.2. An undesired CuCo 2 O 4 phase was observed in spinel powders synthesized at a low CA/MN ratio of 0.5. The CA/MN ratio has influenced not only the phase formation of CMC spinel, but also the particle size distributions. The CA/MN ratio of 1.0 yielded the finest CMC spinel with the least agglomerates, which then produced the highest electrical conductivity of 116 Scm −1 . Therefore, the CA/MN ratio of 1.0 was recommended for the synthesis of CMC spinel, which can be used in fabricating the protective coating of solid oxide fuel cell interconnects. Keywords: solid oxide fuel cells; spinels; sol–gel processes; agglomerations; electrical properties 1. Introduction Progressive efforts in developing solid oxide fuel cells (SOFCs) that operate at a temperature ranging from 500 ◦ C to 800 ◦ C have facilitated the use of metallic interconnects as substitutes for conventional ceramic interconnects [1,2]. However, excessive Cr 2 O 3 -scale growth and Cr vaporization from metallic interconnects to the cathode/electrolyte interface causes severe cell degradation [3–5]. Research is trending toward the development of protective layers that improve the performance of SOFCs [6–9]. (Cu,Mn,Co) 3 O 4 (CMC) spinel exhibits considerable potential as a barrier to suppress the outward diffusion of Cr from metallic interconnects [10]. The effectiveness of the spinel layer in suppressing inter- diffusion behavior is highly dependent on the layer density; in particular, the rate of Cr vaporization in a porous layer can be three times higher than the denser counterpart [11,12]. Given that the layer density of the deposited layer is influenced by the homogeneity of the spinel powders, particularly particle size distribution and the degree of agglomeration of the spinel powders [13,14], the determination of a suitable fuel-to-nitrate ratio that produces homogenous particle size distribution and a low degree of agglomeration is essential, before depositing the protective layer and assessing the inter-diffusion behavior. Rapid development in sol–gel synthesis produced nanoceramics with enhanced mi- crostructures and photocatalytic properties, which are crucial for energy conversion and Energies 2023, 16, 1382. https://doi.org/10.3390/en16031382 https://www.mdpi.com/journal/energies