Journal of Power Sources 569 (2023) 233026 Available online 2 April 2023 0378-7753/© 2023 Elsevier B.V. All rights reserved. A microfuidic proton fow reactor system: In-situ visualisation of hydrogen evolution and storage in carbon-based slurry electrodes Alireza Heidarian a, c , Malte Wehner c , Maria Padligur c , Robert Keller c , Sherman C.P. Cheung a , Ewan W. Blanch b , Matthias Wessling c, d , Gary Rosengarten a, * a School of Engineering, RMIT University, Melbourne, Victoria, Australia b School of Science, STEM College, RMIT University, Melbourne, Victoria, Australia c RWTH Aachen University, AVT.CVT - Department of Chemical Engineering, Chemical Process Engineering, Forckenbeckstrasse 51, Aachen, 52074, Germany d DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074, Aachen, Germany HIGHLIGHTS • Hydrogen storage in carbon slurries is shown using a proton fow reactor (PFR). • Key processes in hydrogen storage were observed in a microfuidic PFR. • Hydronium ions were detected in the electrolyte using fuorescence microscopy. • Hydrogen was identifed on the particles after charging using Raman spectroscopy. A R T I C L E INFO Keywords: Proton fow reactor Carbon particles Slurry electrode Hydrogen storage Fluorescence microscopy Raman spectroscopy ABSTRACT A Proton fow reactor (PFR) system stores energy in the form of hydrogen in porous carbon particles in slurry electrodes. However, the hydrogen storage mechanisms and the reactions in this system are not well understood. In this study, we design and fabricate a microfuidic proton fow reactor (MPFR) as a small-scale PFR to enable in-situ visualisation of the key processes underlying PFR operations. We observe the behaviour of slurries and water in both hydrogen and oxygen sides with emphasis on processes in the vicinity of membranes. We use fuorescence microscopy with quinine to visualize hydronium transport from the oxygen to the hydrogen side and employ in-situ Raman spectroscopy to analyze surface structural changes in carbon particles before and after charging. Fluorescence microscopy demonstrates the formation of hydronium ions on the oxygen side and their subsequent migration to the hydrogen side, proving that the oxygen evolution reaction occurs on the oxygen side. Raman heat maps prove the formation of carbon–hydrogen bonding in particles after they are charged with PFR. Although the MPFR is operated at non-optimal slurry concentrations to allow optical access, we demon- strate that it provides maximum hydrogen storage capacity of 0.64 wt%. 1. Introduction Hydrogen and hydrogen-containing compounds can generate energy for different practical purposes with high effciency, zero carbon emis- sions, and competitive economics. Hydrogen storage offers a wide va- riety of potential applications in both fxed and portable renewable energy systems [1–3]. In recent years, research interest has increased in electrochemical hydrogen storage in porous carbon-based materials [4, 5]. Conventional hydrogen storage systems are subject to numerous limitations. In contrast to a battery, which allows electricity to fow directly into and out of a single device, a fuel cell to store hydrogen has multiple parts, including an electrolyser, storage containers, a fuel cell, and a control system [6]. In addition, a conventional hydrogen storage system has a relatively low roundtrip energy effciency (45%). These limitations can be circumvented by devices such as proton batteries which store hydrogen electrochemically in carbon-based materials [7, 8]. Activated carbon (aC) particles are used as the conductive material in slurry electrodes to store charge and hydrogen, and they are regarded as an excellent material for electrochemical hydrogen storage [9–12]. * Corresponding author. E-mail address: Gary.rosengarten@rmit.edu.au (G. Rosengarten). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour https://doi.org/10.1016/j.jpowsour.2023.233026 Received 15 December 2022; Received in revised form 25 February 2023; Accepted 29 March 2023