In-Situ/Operando Electrochemical Investigation of Reduced Graphene Oxide in Aqueous Solution

The development of new carbon nano-porous materials with increased capacitance, high conductivity and electrochemical stability is of high interest for a variety of applications. It is reported an anomalous increase of the specific capacitance in nano-porous materials with a pore size below of 1 nm¹. However, the fundamental understanding of the mechanisms that involve these phenomena is still under study. Reduced graphene oxide (rGO) has gained significant attention due to its remarkable physicochemical properties such a high structural stability, large specific surface area, low cost, and availability. In the case of healthcare applications, highly porous rGO films have been widely studied because of their biocompatibility and biochemical sensing capabilities². A proper understanding of electrokinetic phenomena such as potential-controlled ionic diffusion within the nano-porous and reduction of functional groups during electrochemical operation is necessary to improve the ultimate performance of rGO-based electrodes. Also, for these applications, it is key to investigate the chemical and structural changes happening in the rGO electrodes during their electrochemical operation.<br/><br/>In this work, we combine different <i>in-situ</i>/operando spectroscopic techniques to understand the dynamics and irreversible/reversible chemical and structural changes within rGO-nanoporous electrodes. The high electrochemical performance of the nanoporous rGO films can be explained by different phenomena such as ionic diffusion, ionic adsorption/desorption processes, protonation mechanisms and chemical and structural changes like those induced by defects or vacancies. The electrochemical performance of the rGO-nanoporous material, in terms of specific capacitance, potential windows and impedance, is studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). To have a deeper understanding of the fundamental phenomena that boost the electrochemical performance, we focus on the structural and chemical changes induced by the electrode-electrolyte interaction in the nano-porous electrode. To study the structural changes that take place due to the electrode-electrolyte interaction, we have developed different custom-made electrochemical cells to couple to spectroscopic techniques like X-ray diffraction (XRD) and Raman spectroscopy. <i>In-situ </i>XRD measurements experiments give information about the impact on the rGO structure of the electrochemical operation, such irreversible/reversible changes on the interlayer distance induced by the applied potential. Operando Raman spectroscopy is used to study the structural properties of such defects, disorder and reduction degree induced by the applied potential. In this study we are able to evaluate the nature of the defects generated during electrochemical operation that impacts on the electrochemical performance³. Combined with analysis of CV coupled with electrochemical quartz crystal microbalance (EQCM), our results provide detailed information about ionic adsorption and charge transfer processes at the electrode-electrolyte. This work aims at expanding the current understanding of the properties of rGO-based nanoporous electrodes for their use as electrode material in healthcare applications.<br/><br/>This work has received funding from the<i> Project PID2020-113663RB-I00 (Neuro2Dtec) funded by MCIN/ AEI /10.13039/501100011033 </i>(Neuro2Dtech)<br/><br/>1. Chmiola, J. <i>et al.</i> Anomalous Increase in Carbon Capacitance at Pore Sizes Less Than 1 Nanometer. <i>Science</i> <b>313</b>, 1760–1763 (2006).<br/>2. Apollo, N. V. <i>et al.</i> Soft, Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide. <i>Adv Funct Materials</i> <b>25</b>, 3551–3559 (2015).<br/>3. Eckmann, A. <i>et al.</i> Probing the Nature of Defects in Graphene by Raman Spectroscopy. <i>Nano Lett.</i> <b>12</b>, 3925–3930 (2012).