Electrode wettability and capacitance of electrical double layer capacitor: a classical density functional theory study

Abstract

We suggest a coarse-grained water model for use in classical density functional theory (cDFT) to describe aqueous inorganic salt solutions that act as working electrolytes in electrical double-layer capacitors (EDLCs) with electrodes comprising two face-to-face doped carbon monolayers. Focus of the cDFT calculations lies on the influence of solvent electrode wettability (SEW) on capacitance and energy storage behaviors, while also considering its interaction with factors like electrolyte bulk concentration, pore size, electrode voltage, and temperature. New phenomena are disclosed theoretically. Remarkably, this study challenges the traditional notion that energy storage is consistently boosted by enhancing the electrode’s ionophobicity. Contrarily, the SEW effect reduces energy storage below the standard aqueous electrochemical window voltage (around 1.2 V) and only enhances the energy storage as the voltage surpasses a certain threshold up to the optimal window voltage (2 V–2.5 V). Furthermore, a non-monotonic SEW effect on energy storage is demonstrated under appropriate conditions, shedding new light on the complex relationship between ionophobicity and energy storage. Moreover, the present coarse-grained water model enables the prediction of the experimentally observed inverse relationship between temperature and capacitance. In contrast, the widely used electrolyte primitive model predicts the existence of a maximum value. The decisive factor for the impact of SEW on capacitance and energy storage is identified as congestion within the electrode pore, while other factors contribute by affecting this congestion. The present research offers valuable insights, highlighting the significance of SEW in the innovative and strategic design of aqueous inorganic EDLC devices.