Aqueous/non-aqueous electrolyte tradeoffs in charge transfer and electrochromics of pseudocapacitive oxide films

Environmental sustainability, safety, cost, and performance are the driving metrics for modern technological developments. Progress in these realms has been made for electrochromic (EC) devices by optimizing anode/cathode electrode materials. Yet, by these standards, the role of the electrolyte has remained unexplored. This investigation on charge transfer mechanisms at the electrolyte/electrode interface facilitates a contrast of the aqueous and non-aqueous electrolytes studied. A classic EC, high-performing, non-aqueous, lithium chlorine oxide in propylene carbonate (PC-LiClO₄) is examined against a non-flammable, low reactive, cost-effective, aqueous, potassium hydroxide (KOH) electrolyte; to strengthen the understanding of electrochromics the electrolytes are referenced against the anodic EC nickel oxide (NiO) thin films. The KOH presents as a diffusion dominant response, supported by the findings of the cyclic voltammetry and electrochemistry impedance data (b = 0.56, 45°∠), respectively, compared to the more surface capacitive PC-LiClO₄ (b = 0.68, 60°∠). Interestingly, despite the KOH full redox potential window being half the PC-LiClO₄, the KOH system's current density reached more than 3 times higher than PC-LiClO₄. Additionally, realizing the same current density (2 mA cm^-2) in multi-step chronoamperometry, the required potential is ∼5 times lower for KOH than for PC-LiClO₄ electrolyte, albeit the KOH has a longer response time. Inherent tradeoffs in the systems are considered for theoretical analysis of these phenomena, i.e., molar mass, ionization energy, viscosity, etc. The chemical nature of the electrolyte shows a profound effect on electrochemical kinetics at the NiO/electrolyte interface, pointing to the significance of all aspects in an electrochemical cell. The coupled effect of the electrolyte composition/electrode material pairing dictates the charge-storage mechanisms (and subsequently, EC properties). Furthermore, knowledge of contrasts in electrolyte type is of great interest to the scientific community for the modern metric-based optimizations of many other clean energy systems.