Analysis of the hydrogen electrode reaction mechanism in thin-layer cells. 1. Theory

Voltammetric and Scanning Electrochemical Microscopy Investigations of the Hydrogen Evolution Reaction in Acid at Nanostructured Ensembles of Ultramicroelectrode Dimensions: Theory and Experiment

“…40 SSVs calculated for UME-NEEs with disk NEs and v e V = 2 × 10 −5 mol cm −2 s −1 , v e H = 6 × 10 −10 mol cm −2 s −1 , v e T = 1 × 10 −6 mol cm −2 s −1 , and θ e = 0.2, parameters close to those measured on Pt, 40 for different f values (and fixed R NE ) are shown in Figure 1I. The SSV corresponding to the HER operating at equilibrium (reversible), 47 given by eq 11 (which results from the Nernst equation, as demonstrated in Supporting Information S4), was included as a reference for an infinitely fast reaction (dashed lines). (11) SSVs in Figure 1I show that as f decreases, the kinetically controlled response extends to more negative potentials.…”

“…For example, it is possible to identify a kinetic limiting current ( i L,k ) insensitive to the potential at η more negative than −0.2 V, which decreases from i L,UME‑NEE as f decreases. This kinetically controlled limiting process is caused by the prevalence of the Volmer–Tafel route at low overpotentials, which sets a constant current when H ad coverage reaches its limiting value before exhausting the surface proton concentration . As η becomes very negative (η < −0.4 V), the rate of the Volmer–Heyrovsky route predominates over the Volmer-Tafel rate (which decreases and becomes negligible).…”

“…This kinetically controlled limiting process is caused by the prevalence of the Volmer−Tafel route at low overpotentials, which sets a constant current when H ad coverage reaches its limiting value before exhausting the surface proton concentration. 47 As η becomes very negative (η < −0.4 V), the rate of the Volmer−Heyrovsky route predominates over the Volmer-Tafel rate (which decreases and becomes negligible). As seen in the dotted lines of Figure 1I(A), under these very cathodic conditions where θ is constant, the SSV follows a simple function for an irreversible reaction given by eq 12, where κ VH is defined by eq 13…”

“…These equations involve the equilibrium rate of each elementary step ( v i e ) normalized according to eq , the adsorbed-H coverage evaluated at η (i.e., θ) and at the conditions of the RHE (i.e., θ e ) and the term γ that groups the proton and dissolved hydrogen concentrations in the RHE (i.e., c * H+ and c * H2 , respectively) and diffusion coefficients (γ = D H+ c * H+ /2 D H2 c * H2 ). Moreover, α is the symmetry factor (considered equal to 0.5 for both electrochemical steps), F is the Faraday constant, R is the universal gas constant, and T is temperature.…”

Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy

“…In this report, we employ scanning electrochemical microscopy (SECM) equipped with a platinum (Pt) ultramicroelectrode (UME) to determine the kinetic information on both unstrained and strained S vacancies on the basal plane of MoS 2 monolayers. SECM has been previously applied to study the HER kinetics of metallic catalysts, such as Pt UME, manganese (Mn) electrodes, Pt and gold (Au) thin layer cells, and metal nanoparticles. − These studies successfully determined the apparent rate constants and electron-transfer coefficients of the HER on Pt, palladium (Pd), and Mn electrodes. ,,, More importantly, these studies established the methodology to quantify the HER kinetics, including simplifying the HER to be a quasi-reversible one-step reaction, using a thin layer cell formalism of the HER in the SECM theoretical equations and describing the rates of the elementary steps and coverage of adsorbed hydrogen atoms on metallic electrodes based on the Volmer–Heyrovsky–Tafel mechanism . Herein, we leverage the existing framework of the HER study (using SECM to investigate heterogeneous electrochemical reactions in the energy materials) − and adapt it to study the HER kinetics of the two-dimensional (2-D) catalyst of MoS 2 monolayers.…”