Donor-Dependent Kinetics of Interfacial Proton-Coupled Electron Transfer

Abstract

The effect of the proton donor on the kinetics of interfacial concerted proton-electron transfer (CPET) to polycrystalline Au was probed indirectly by studying the rate of hydrogen evolution from trialkylammonium donors with different steric profiles, but the same pKa. Detailed kinetic studies point to a mechanism for HER catalysis that involves rate-limiting CPET from the proton donor to the electrode surface, allowing this catalytic reaction to serve as a proxy for the rate of interfacial CPET. In acetonitrile electrolyte, triethylammonium (TEAH(+)) displays up to 20-fold faster CPET kinetics than diisopropylethylammonium (DIPEAH(+)) at all measured potentials. In aqueous electrolyte, this steric constraint is largely lifted, suggesting a key role for water in mediating interfacial CPET. In acetonitrile, TEAH(+) also displays a much larger transfer coefficient (β = 0.7) than DIPEAH(+) (β = 0.4), and TEAH(+) displays a potential-dependent H/D kinetic isotope effect that is not observed for DIPEAH(+). These results demonstrate that proton donor structure strongly impacts the free energy landscape for CPET to extended solid surfaces and highlight the crucial role of the proton donor in the kinetics of electrocatalytic energy conversion reactions.

Strong Electronic Coupling of Molecular Sites to Graphitic Electrodes via Pyrazine Conjugation

“…Instead, we attribute this broadness to the known sluggishness of interfacial PCET in acetonitrile versus water. 15,16 Together, these data indicate that in protic electrolytes, both surface-bound GCC-phenazine units and dissolved molecular phenazines undergo chemically reversible proton-coupled ET reactions. While phenazine and GCC-phenazine display similar redox behavior in protic electrolytes, they display radically different electrochemical behavior in the absence of a proton donor.…”

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

“…The efficient interconversion of electrical and chemical energy requires functional interfaces capable of catalyzing complex multi-proton-coupled electron transfer (PCET) reactions of diverse small-molecule substrates. [1][2][3][4][5] Consequently,i nterfacial electrocatalysis is strongly dependent on the proton activity at the electrode surface.I ndeed, there is ag rowing appreciation that the interfacial pH and proton donor environment play ac entral role in determining the kinetics,t hermodynamics,a nd mechanism of elementary PCET steps, [3,4] thereby defining the selectivity and efficiency of key interfacial reactions ranging from CO 2 and O 2 reduction to the oxidation of small-molecule fuels,s uch as MeOH and HCO 2 H. [6][7][8][9][10][11][12] More broadly,t he interfacial pH value has been shown to impact aw ide variety of electrochemical processes,i ncluding metal finishing,c orrosion chemistry,e lectrodeposition, and electro-organic synthesis. [13,14] Clearly,k nowledge of the pH value at the electrode surface and its variation under electrochemical polarization is an essential prerequisite for attaining molecular-level understanding of interfacial reactivity and catalysis.…”

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

“…[1][2][3][4][5] Consequently,i nterfacial electrocatalysis is strongly dependent on the proton activity at the electrode surface.I ndeed, there is ag rowing appreciation that the interfacial pH and proton donor environment play ac entral role in determining the kinetics,t hermodynamics,a nd mechanism of elementary PCET steps, [3,4] thereby defining the selectivity and efficiency of key interfacial reactions ranging from CO 2 and O 2 reduction to the oxidation of small-molecule fuels,s uch as MeOH and HCO 2 H. [6][7][8][9][10][11][12] More broadly,t he interfacial pH value has been shown to impact aw ide variety of electrochemical processes,i ncluding metal finishing,c orrosion chemistry,e lectrodeposition, and electro-organic synthesis. In the absence of electrocatalysis,n anostructured Pt/C surfaces mediate the reaction of H 2 with cis-2-butene-1,4-diolt of orm am ixture of 1,4-butanediol and n-butanol with selectivity that is linearly dependent on the bulk solution pH value.Weshow that kinetic branching occurs from acommon surface-bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface.W eu sed the pH-dependent selectivity of this reaction to trackchanges in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (> 3units) relative to the bulk value even at modest current densities in well-buffered electrolytes.T his study highlights the key role of interfacial pH variation in modulating inner-sphere electrocatalysis.…”

“…[1][2][3][4][5] Consequently,i nterfacial electrocatalysis is strongly dependent on the proton activity at the electrode surface.I ndeed, there is ag rowing appreciation that the interfacial pH and proton donor environment play ac entral role in determining the kinetics,t hermodynamics,a nd mechanism of elementary PCET steps, [3,4] thereby defining the selectivity and efficiency of key interfacial reactions ranging from CO 2 and O 2 reduction to the oxidation of small-molecule fuels,s uch as MeOH and HCO 2 H. [6][7][8][9][10][11][12] More broadly,t he interfacial pH value has been shown to impact aw ide variety of electrochemical processes,i ncluding metal finishing,c orrosion chemistry,e lectrodeposition, and electro-organic synthesis. [1][2][3][4][5] Consequently,i nterfacial electrocatalysis is strongly dependent on the proton activity at the electrode surface.I ndeed, there is ag rowing appreciation that the interfacial pH and proton donor environment play ac entral role in determining the kinetics,t hermodynamics,a nd mechanism of elementary PCET steps, [3,4] thereby defining the selectivity and efficiency of key interfacial reactions ranging from CO 2 and O 2 reduction to the oxidation of small-molecule fuels,s uch as MeOH and HCO 2 H. [6][7][8][9][10][11][12] More broadly,t he interfacial pH value has been shown to impact aw ide variety of electrochemical processes,i ncluding metal finishing,c orrosion chemistry,e lectrodeposition, and electro-organic synthesis.…”