Citation: Sahu, S.K.; Panthi, D.; Soliman, I.; Feng, H.; Du, Y. Fabrication and Performance of Micro-Tubular Solid Oxide Cells. Energies 2022, 15, 3536. https:// doi.org/10.3390/en15103536 Academic Editor: Bahman Amini Horri Received: 31 March 2022 Accepted: 10 May 2022 Published: 12 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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 Article Fabrication and Performance of Micro-Tubular Solid Oxide Cells Sulata K. Sahu 1 , Dhruba Panthi 2 , Ibrahim Soliman 1 , Hai Feng 1 and Yanhai Du 1, * 1 College of Aeronautics and Engineering, Kent State University, Kent, OH 44242, USA; ssahu2@kent.edu (S.K.S.); isoliman@kent.edu (I.S.); hfeng2@kent.edu (H.F.) 2 Department of Engineering Technology, Kent State University at Tuscarawas, New Philadelphia, OH 44663, USA; dpanthi@kent.edu * Correspondence: ydu5@kent.edu Abstract: Solid Oxide Cells (SOC) are the kind of electrochemical devices that provide reversible, dual mode operation, where electricity is generated in a fuel cell mode and fuel is produced in an electrolysis mode. Our current work encompasses the design, fabrication, and performance analysis of a micro-tubular reversible SOC that is prepared through a single dip-coating technique with multiple dips using conventional materials. Electrochemical impedance and current-voltage responses were monitored from 700 to 800 ◦ C. Maximum power densities of the cell achieved at 800, 750, and 700 ◦ C, was 690, 546, and 418 mW cm −2 , respectively. The reversible, dual mode operation of the SOC was evaluated by operating the cell using 50% H 2 O/H 2 and ambient air. Accordingly, when the SOC was operated in the electrolysis mode at 1.3 V (the thermo-neutral voltage for steam electrolysis), current densities of −311, −487 and −684 mA cm −2 at 700, 750 and 800 ◦ C, respectively, were observed. Hydrogen production rate was determined based on the current developed in the cell during the electrolysis operation. The stability of the cell was further evaluated by performing multiple transitions between fuel cell mode and electrolysis mode at 700 ◦ C for a period of 500 h. In the stability test, the cell current decreased from 353 mA cm −2 to 243 mA cm −2 in the fuel cell mode operation at 0.7 V, while the same decreased from −250 mA cm −2 to −115 mA cm −2 in the electrolysis operation at 1.3 V. Keywords: micro-tubular; reversible solid oxide cells; light weight; dip-coating; fuel cell operation; electrolysis operation; hydrogen generation; durability 1. Introduction Solid Oxide Cells (SOCs) are the kind of electrochemical devices that have reversible, dual modes of operation, such that, in one mode, the SOC operates in a solid oxide fuel cell (SOFC) mode, and, in another mode, the SOC operates in a solid oxide electrolysis cell (SOEC) mode. That is, the SOFC mode converts fuel into electricity and heat, whereas the SOEC mode generates fuel from the electrolysis of water by utilizing electricity and heat. From a thermodynamic view, the energy demand for the endothermic water splitting reaction can be partially obtained from the heat generated within the cell. While the dual mode operation of SOCs provides several advantages, the technology enabling the reversible operation of the SOC still remains at the research and development stage and the stability over the long term is still a challenge [1–3]. Since a compact and lightweight fuel cell design is highly sought after for the facilitation of large-scale commercialization of SOC systems for use in portable and transportation applications, considerable efforts have been made to develop novel and cost-effective methods of creating lightweight and compact reversible SOCs [4–9]. Among the SOC designs, micro tubular solid oxide cells (MT-SOCs) have attracted strong interest due to their high tolerance to thermal-cycling, quick start-up ability, high power density over unit volume and robust portable characteristics [10–12]. However, Energies 2022, 15, 3536. https://doi.org/10.3390/en15103536 https://www.mdpi.com/journal/energies