Interpreting the Electrocatalytic Voltammetry of Homogeneous Catalysts by the Foot of the Wave Analysis and Its Wider Implications

Abstract

Mechanistic studies of electrocatalytic reactions play a crucial role in developing efficient electrocatalysts and solar-fuel devices. The foot of the wave analysis (FOWA) for cyclic voltammetry, recently developed by Saveánt and Costentin, provides a powerful means to evaluate the performance of molecular electrocatalysts. However, there is a considerable amount of confusion in the community on how to interpret FOWA in multielectron electrochemical reactions. Herein, we further expand their earlier models from the Nernstian region to all scenarios (i.e., including non-Nernstian behavior) and systematically examine individual parameters, such as formal potentials and reaction rate constants, to explore deeper insights and limitations. Detailed analysis from in silico voltammograms based on different mechanistic models reveals characteristic features of FOWA traces for different kinetic phenomena, which is useful to diagnose kinetic profiles and elucidate the limits of FOWA. The lessons learned from these analyses are further used to reconcile the discrepancy of rate constants determined by FOWA versus other methods, such as time-resolved spectroscopy, for molecular electrocatalysts that catalyze proton reduction or the reduction of CO 2 to CO. Such reconciliation demonstrates that electrochemical methods along with FOWA can serve as an alternative tool to determine kinetic information and probe mechanistic insights, which otherwise may be challenging and complicated to achieve by conventional methods. In addition, general guidelines and warnings are also presented to avoid potential errors or mishandling when using FOWA.

An Iron Porphyrin Complex with Pendant Pyridine Substituents Facilitates Electrocatalytic CO2Reduction via Second Coordination Sphere Effects

“…When unwanted side reactions hinder the possibility of observing an S‐shaped voltammogram, application of Foot‐of‐the‐wave analysis (FOWA) proposed by Savéant [4a] and described in more detail by Dempsey [13] and Wang [14] has been shown to produce good approximate values for an apparent rate constant ( k app‐FOWA ) of the electrocatalytic reaction, assuming that no other electrochemical process takes place in this foot‐of‐the‐wave regime and thus catalytic onset occurs under pure kinetic conditions.…”

Nitrate Radical Facilitates Indirect Benzyl Alcohol Oxidation on Bismuth(III) Vanadate Photoelectrodes

“…The photocurrent dependence on the scan rate in Figure also supports an EC i mechanism; the peak current density depends on the square root of scan rate ( j p α ν 0.5 ), as predicted by the Randles‐Sevcik equation . The anodic peaks observed for fast scan rates (ν≥50 mV s −1 ) correspond to depletion of NO 3 − at the photoanode surface that would not occur under catalytic regeneration . The shifting peak potential with increasing scan rate is also characteristic of irreversible NO 3 − oxidation.…”

Reversible catalysis

“…Moreover, if the two-electron catalytic cycle includes more than one bidirectional chemical step, then the two catalytic potentials in eq 7 necessarily depart from the reduction potentials of the catalysts measured in the absence of substrate 3 . This forbids using the popular "foot of the wave analysis" 103,104 , which assumes that the catalytic sigmoidal wave is centered on the potential of the catalyst measured under non-catalytic conditions.…”