Fig. 4.

Specific oxygen reduction reaction activities and hydration energies of cations. Logarithm of the specific oxygen reduction reaction (ORR) activities versus hydration energies of alkali metal cations¹⁹, tetramethylammonium cation (TMA⁺), tetraethylammonium cation (TEA⁺), tetra-n-butylammonium cation (TBA⁺), and tetra-n-hexylammonium cation (THA⁺). The hydration energies of tetraalkylammonium cations were calculated using a following equation: Δ_hydrH°([H(CH₂)_n]₄N⁺) = −172–42.4n + 10n^{2 25}