Batteries 2023, 9, 108. https://doi.org/10.3390/batteries9020108 www.mdpi.com/journal/batteries Article High Performance H2−Mn Regenerative Fuel Cells through an Improved Positive Electrode Morphology Javier Rubio-Garcia 1,2, *, Anthony Kucernak 2,3, *, Barun Kumar Chakrabarti 4 , Dong Zhao 2 , Danlei Li 2 , Yuchen Tang 2 , Mengzheng Ouyang 5 , Chee Tong John Low 4 and Nigel Brandon 3,5 1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 1TN, UK 2 Department of Chemistry, Faculty of Science, Imperial College London, South Kensington, London SW7 2AZ, UK 3 RFC Power Ltd., London SW7 2PG, UK 4 WMG, Warwick Electrochemical Engineering Group, Energy Innovation Centre, University of Warwick, Coventry CV4 7AL, UK 5 Department of Earth Science and Engineering, Imperial College London, South Kensington, London SW7 2AZ, UK * Correspondence: jr873@cam.ac.uk (J.R.-G.); anthony@imperial.ac.uk (A.K.) Abstract: The effective scaling-up of redox flow batteries (RFBs) can be facilitated upon lowering the capital costs. The application of ubiquitous manganese along with hydrogen (known as H2−Mn regenerative fuel cells (RFC)) is seen as an effective solution for this purpose. Here, we aim to eval- uate different positive electrodes so as to improve the key performance metrics of the H2/Mn RFC, namely electrolyte utilization, energy efficiency, and peak power densities. Commercially available carbon paper and graphite felt are used to show that the latter provides better key performance indicators (KPIs), which is consistent with the results reported for standard all-vanadium RFBs in the literature. Even better KPIs are obtained when an in-house carbon catalyst layer (CCL) is em- ployed in combination with graphite felt electrodes (e.g., more than 80% energy efficiency, >0.5 W cm −2 peak power density and electrolyte utilization of 20 Ah L −1 for felt and carbon metal fabric (CMF), prepared by means of electrospinning and carbonization, in comparison with about 75% energy efficiency 0.45 W cm −2 peak power density and 11 Ah L −1 electrolyte utilization for felt on its own). It is envisaged that if the electrochemical performance of CCLs can be optimized then it could open up new opportunities for the commercial exploitation of H2−Mn systems. Keywords: redox flow battery; regenerative fuel cell; hydrogen; manganese; electrodes; electrospinning 1. Introduction The deployment of grid-scale energy storage is predominantly constrained by cost, which is a function of the number of hours of storage and, to a lesser extent, the power output. Redox flow batteries (RFBs) can offer exceptional engineering flexibility due to their ability to independently scale power and their energy capacity [1,2]. The former is directly affected by the dimensions of the electrode stack, while the latter is a function of the electrolyte volume. Investigations into improving electrode performance are essential because improving the operational current density while maintaining high electrolyte uti- lization means lowering the levelized cost of storage [3,4]. For cost-effectiveness and max- imizing the benefits of RFBs, electrodes are engineered to tune their microstructural fea- tures and chemical stability, so they are suitable for operation under extreme pH condi- tions [5]. Most RFB redox reactions are outer-sphere processes that can be catalyzed on carbon- based materials. As a result, the performance of the electrode is dictated by its Citation: Rubio-Garcia, J.; Kucernak, A.; Chakrabarti, B.K.; Zhao, D.; Li, D.; Tang, Y.; Ouyang, M.; Low, C.T.J.; Brandon, N. High Performance H2−Mn Regenerative Fuel Cells through an Improved Positive Electrode Morphology. Batteries 2023, 9, 108. https://doi.org/10.3390/ batteries9020108 Academic Editors: Pascal Venet and Carlos Ziebert Received: 5 December 2022 Revised: 19 January 2023 Accepted: 25 January 2023 Published: 3 February 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/license s/by/4.0/).