An aqueous proton battery under alkaline electrolyte

Aqueous proton batteries (APBs) have attracted much attention owing to their fast kinetics, low cost and sustainability. However, the most reported APBs operate under acidic conditions, which will lead to serious hydrogen evolution reaction (HER) and corrosion problem, inevitably. Here, a small molecule organic compound, azobenzene (AB) containing an azo group (N = N), is explored as an anode material for APBs under alkaline electrolyte with a low redox potential of around -0.6 V (vs. Hg/HgO). AB anode achieves high-rate capacity of 151 mAh g^-1 at a high current density of 10 A g^-1 and excellent cycling stability over 12,500 cycles under 2 M KOH electrolyte. The experimental results and theoretical calculations confirm that AB performs a two-electron redox reaction involving a central N = N group with two protons from H₂O molecules. This work reveals the protonation in azo-materials and promotes new insights on design small organic molecules with multi-electron redox reactions in advanced proton secondary batteries.