Beyond the Water Electrolysis Potential—A Systematic Study for Different Ionic Carriers on the Electrolyte Performance for Free-Standing Carbon Nanotube Supercapacitors

To eliminate electrolyte leakage, the development of safe and flexible supercapacitors necessitates solid-state electrolytes that integrate high mechanical and electrochemical capabilities. <i>Quasi</i>-solid-state electrolytes, which constitute a polymer matrix along with an aqueous electrolytic phase, are a viable answer to this problem. Recently, gel electrolytes have gained a lot of attention in flexible and wearable electronic devices due to their remarkable advancements. However, the limitation in the high-performance of such gels hinders the practical usage of such devices. On the electrochemical perspective, the gel electrolyte performances strictly rely on the type of ionic carrier (acidic, alkaline, or salt-based), size of the ion, solvent concentration, type of polymer, as well as the interaction between the polymer and other components. Moreover, the performance of the electrolyte differs with the electrode-electrolyte interface and thus is highly dependent on the electrode material. For this reason, it is vital to carry a parametric study to evaluate the effect of the above stated.<br/><br/>Most studies in literature focus on limited voltage windows, up to 0.8 V or 1 V, below the electrolysis potential of water. However, it is important to study the efficacy of these electrolytes for larger voltage windows for employment in broader applications. The aim of this study is to investigate the effect of several ionic carriers (namely H₃PO4, KOH and LiCl) for a cell voltage exceeding the water electrolysis potential (1.2 V), that is, up to 1.5 V. The solvent concentration of the architecturally engineered PVA-based electrolytes’ performance in free-standing CNT supercapacitor is also evaluated. In addition, the dependency of the electrolyte’s mechanical structure for long-term stability is further studied by using the optimized concentration of each (H₃PO4, KOH and LiCl) by freezing and de-freezing the gel to form membrane-like films because of the creation of increased physical cross-linking with this cycle. The supercapacitors are studied for their capacitance, charge/discharge capabilities as well their long-term stability to determine the optimum electrolyte.