Water-based supercapacitors with amino acid electrolytes: a green perspective for capacitance enhancement

State-of-the art Electrochemical Double-Layer Capacitors (EDLCs) usually extend their operating electrochemical stability window (ESW) by means of organic electrolytes, or highly concentrated aqueous (water-in-salt) electrolytes hindering parasitic water splitting reactions. Unfortunately, organic solvents and high concentrations of ions penalize the dielectric constant of the electrolyte, hence the capacitive performance. We suggest here a new concept of cost-effective and sustainable aqueous electrolytes based on concentrated amino acid water solutions with a dielectric permittivity much higher than pure water, unlocking the capacitive performance of aqueous EDLC references. Amino acids are natural zwitterionic molecules with a large separation between the positive and negative moiety, leading to huge dipoles with excellent dielectric properties. Some of them (e. g., lysine and proline), have a solubility >= ${\ge }$10 m at ambient temperature. With an experimental characterization we prove that aqueous EDLCs based on electrolytes obtained with L- lysine or L-proline added to 2 M NaNO3 solution have +50 % of gravimetric capacitance enhancement at low specific currents (0.1 A/g) compared to a reference device based on 2 M NaNO3 electrolyte without amino acids. A theoretical model suggests that this performance may be further enhanced by increasing the ionic accessibility of commercially available active materials, with porosity optimized to the size of amino acid ions. Electrolytes based on amino acids solutions are shown to be a valuable option for green water-based supercapacitors, with an experimental characterization and a theoretical model. Amino acids in their zwitterionic neutral state have a much higher dipolar moment than water, thus they provide enhanced dielectric permittivity and specific capacitance.image