Direct Determination of Equilibrium Potentials for Hydrogen Oxidation/Production by Open Circuit Potential Measurements in Acetonitrile

Open circuit potentials were measured for acetonitrile
solutions of a variety of acids and their conjugate bases under 1
atm H2. Acids examined were triethylammonium, dimethylformamidium,
2,6-dichloroanilinium, 4-cyanoanilinium, 4-bromoanilinium, and 4-anisidinium
salts. These potentials, along with the pKa values of the acids, establish the value of the standard hydrogen
electrode (SHE) potential in acetonitrile as −0.028(4) V vs
the ferrocenium/ferrocene couple. Dimethylformamidium forms homoconjugates
and other aggregates with dimethylformamide; open circuit potentials
(OCPs) were used to quantify the extent of these reactions. Overpotentials
for electrocatalytic hydrogen production and oxidation were determined
from open circuit potentials and voltammograms of acidic or basic
catalyst solutions under H2. For these solutions, agreement
between OCP values and potentials calculated using the Nernst equation
is within 12 mV. Use of the measured equilibrium potential allows
direct comparison of catalytic systems in different media; it requires
neither pKa values, homoconjugation constants,
nor the SHE potential.