On the Coupling of Electron Transfer to Proton Transfer at Electrified Interfaces

Abstract

Many electrochemical processes are governed by the transfer of protons to the surface, which can be coupled with electron transfer; this electron transfer is in general non-integer and unknown a priori, but is required to hold the potential constant. In this study, we employ a combination of surface spectroscopic techniques and grandcanonical electronic-structure calculations in order to rigorously understand the thermodynamics of this process. Specifically, we explore the protonation/deprotonation of 4-mercaptobenzoic acid as a function of the applied potential. Using grand-canonical electronic-structure calculations, we directly infer the coupled electron transfer, which we find to be on the order of 0.1 electron per proton; experimentally, we also access this quantity via the potential-dependence of the pK a . We show a striking agreement between the potential-dependence of the measured pK a and that calculated with electronic-structure calculations. We further employ a simple electrostatics-based model to show that this slope can equivalently be interpreted to provide information on the degree of coupled electron transfer or the potential change at the point of the charged species.

Revealing the activity of Co3Mo3N and Co3Mo3N0.5 as electrocatalysts for the hydrogen evolution reaction

“…It has been proposed that the strongly adsorbing sites with negative DG H act as spectators rather than participating in the HER while the weakly bound hydrogens (enabled on sites with the DG H > 0) drive the HER forward. 21,22,45 Therefore, we modelled the impact of the presence of strong adsorption sites (H3 > H1 > B1 > B4) on the reactive ability of the H2-OUT site (Tables S17 and S18 † for the DG H of the sites and Fig. S34 † for the algorithm how the model was built).…”

Single-Molecule Sensing of Interfacial Acid–Base Chemistry

“…where θ is the fractional surface coverage and pK a surf is the pH when half of the groups are ionized (θ = 0.5) at the interface. The conductance peak intensity that refers to the formation probability of molecular junctions is proportional to the fractional surface coverage of -COO − , and it could be described as follows as in previous reports 41,42…”

“…This also accounts for the conductance peak intensity (I) reaching a maximum after a certain amount of surface covered by the -COO − form, which is not at a θ surf COO − of 100%. Because molecules are also adsorbed on the tip in our case, 2πr 2 /ρ surf is used as follows 1,8,41,42…”

“…For an interfacial acid–base reaction at equilibrium, the degree of dissociation of surface-immobilized -COOH groups could be expressed by a Henderson–Hasselbalch type equation ,, where θ is the fractional surface coverage and p K a surf is the pH when half of the groups are ionized (θ = 0.5) at the interface. The conductance peak intensity that refers to the formation probability of molecular junctions is proportional to the fractional surface coverage of -COO – , and it could be described as follows as in previous reports , where I is the normalized intensity of the conductance peak at a pH and I min and I max are the minimum and maximum at in the current pH range, respectively. During the break junction experiment, the deprotonated molecules under the tip area can randomly bind to the Au tip to form molecular junctions.…”

“…This also accounts for the conductance peak intensity ( I ) reaching a maximum after a certain amount of surface covered by the -COO – form, which is not at a θ surf COO – of 100%. Because molecules are also adsorbed on the tip in our case, 2π r 2 /ρ surf is used as follows ,,, With a reciprocal transformation of eq , we obtain eq Thus, we plot log­[( I – I min )/( I – I max )] versus pH, and a linear functional relation is observed in Figure b with an R 2 value of 0.99 for both 4-MTBA and TPA. Similar behavior is also found with MPA (Figure S8), which demonstrates the universality of such a method for pH measurement.…”