Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide

Abstract

A variety of next-generation energy processes utilize the electrochemical interconversions of dioxygen and water as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Reported here are the first estimates of the standard reduction potential of the O2 + 4e(-) + 4H(+) ⇋ 2H2O couple in organic solvents. The values are +1.21 V in acetonitrile (MeCN) and +0.60 V in N,N-dimethylformamide (DMF), each versus the ferrocenium/ferrocene couple (Fc(+/0)) in the respective solvent (as are all of the potentials reported here). The potentials have been determined using a thermochemical cycle that combines the free energy for transferring water from aqueous solution to organic solvent, -0.43 kcal mol(-1) for MeCN and -1.47 kcal mol(-1) for DMF, and the potential of the H(+)/H2 couple, - 0.028 V in MeCN and -0.662 V in DMF. The H(+)/H2 couple in DMF has been directly measured electrochemically using the previously reported procedure for the MeCN value. The thermochemical approach used for the O2/H2O couple has been extended to the CO2/CO and CO2/CH4 couples to give values of -0.12 and +0.15 V in MeCN and -0.73 and -0.48 V in DMF, respectively. Extensions to other reduction potentials are discussed. Additionally, the free energy for transfer of protons from water to organic solvent is estimated as +14 kcal mol(-1) for acetonitrile and +0.6 kcal mol(-1) for DMF.

Homogenous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions

“…For each of the reaction conditions used here, there is a characteristic equilibrium potential for O 2 reduction ( E O2/H2O ), depending on the solvent, proton activity, etc. 11 Similarly, each of the molecular catalysts has a characteristic redox potential under these conditions ( E Fe(III/II) ), which we approximate as the measured E 1/2 of the catalyst (see Supporting Information, page 5). We therefore define a characteristic overpotential under these conditions, η Fe/ORR = E O2/H2O – E Fe(III/II) .…”

Mechanisms of Two‐Electron versus Four‐Electron Reduction of Dioxygen Catalyzed by Earth‐Abundant Metal Complexes

“…The two‐electron/two‐proton reduction of O 2 by 1,1′‐dibromoferrocene (Br 2 Fc: E ox =0.72 V vs. SCE) was catalyzed by Co II (Ch), whereas virtually no reduction of O 2 by Br 2 Fc occurred with Co II (OEP) in the presence of HClO 4 (25 m m ) in PhCN at 298 K . Because the standard reduction potential of O 2 to H 2 O in MeCN is reported to be 1.21 V vs. Fc + /Fc, the standard reduction potential of O 2 to H 2 O 2 in MeCN is evaluated to be 0.56 V vs. Fc + /Fc, based on the difference in the standard reduction potential of O 2 to H 2 O (1.23 V vs. SHE) and that of O 2 to H 2 O 2 (0.68 V vs. SHE) . Then, the reduction potential of O 2 to H 2 O 2 in the presence of HClO 4 (25 m m ) in PhCN is estimated to be 0.47 V vs. Fc + /Fc in MeCN, because the difference in concentration of H + (25 m m and 1.0 m ) results in a difference in the reduction potential of 0.09 V. In such case, the reduction of O 2 to H 2 O 2 ( E 0 ′=0.47 V vs. Fc + /Fc) with Br 2 Fc (0.72 V vs. SCE=0.35 V vs. Fc + /Fc) is thermodynamically feasible with a small overpotential (0.12 V).…”

“…The standard redox potential of the four‐electron reduction of O 2 with four protons to water under the standard conditions of aqueous acid with unit activity (pH 0), an oxygen pressure of 1 atm and at 25 °C is 1.229 V referenced to the standard hydrogen electrode (SHE) [Eq. ] …”

Critical Aspects of Heme–Peroxo–Cu Complex Structure and Nature of Proton Source Dictate Metal–Operoxo Breakage versus Reductive O–O Cleavage Chemistry

“…For C c O, or for a given efficient molecular catalyst able to promote four-electron, four-proton O 2 -reduction to water ( E ° MeCN = 1.21 V vs Fc +/0 ), 47 these particular nuances relevant to the promotion of the O–O cleavage process need to be delineated and understood within the framework of reduction/protonation chemistry. The breadth of these PCET studies involves probing different (or perhaps simultaneous) orderings of the events of (i) electron transfer (from Fe, Cu, or the Tyr phenol moiety), and (ii) proton transfer derived from different sources.…”