Using this value for 𝑎 𝑂 2 •-we obtain a formal reduction voltage of 𝐸 𝑂 2 𝑂 2 •- ⁄ 0′ = -0.16 V vs. SHE, which defines the reversible voltage of dissolved oxygen reduction to superoxide in aqueous electrolytes. We note this value corresponds well with commonly observed halfwave voltages (E1/2) on carbon-based electrodes in aqueous electrolytes. In a fully outer-sphere reaction pathway, the second electron reduction yields hydrogen peroxide, H2O2, at pH ≤ pKa = 11.7, or hydroperoxide, HO2 -, at pH > pKa and may proceed through the following reactions depending on pH: