Generator−Collector Experiments at a Single Electrode: Exploring the General Applicability of This Approach by Comparing the Performance of Surface Immobilized versus Solution Phase Sensing Molecules

Abstract

We demonstrate proof-of-concept that generator-collector experiments can be performed at a single macroelectrode and used to determine mechanistic information. The practical advantages of such a system over conventional generator-collector techniques are also outlined. The single-electrode generator-collector technique is applied to study the known mechanism of oxygen reduction in aqueous conditions as a model system. We seek to demonstrate that the single-electrode generator-collector approach is capable of detecting local pH changes, immediately adjacent to the electrode surface during a redox reaction. Experiments are performed using a molecular pH probe attached to the electrode surface. Comparison of experimental data with numerical simulations verifies that the reduction of oxygen at pH 6.8 proceeds via a two-electron, two-proton mechanism. Experiments were also performed with a molecular pH probe dissolved in the electrolyte solution in order to explore the feasibility of this approach, which is potentially applicable to a much wider range of electrochemical systems.

Electrochemical Hydrogen Evolution: H+or H2O Reduction? A Rotating Disk Electrode Study

“…to the results of digital simulation), x 1. Thus, by using the asymptotic approximation 44 that exp(x) s (x) ≈ x s−1 in Equation 24, we arrive to the following approximative closed-form expression defining λ:…”

In Situ Observation of the pH Gradient near the Gas Diffusion Electrode of CO2 Reduction in Alkaline Electrolyte

“…In addition to mathematic models and simulations, , possible experimental methods to study the local pH include pH-sensitive microelectrodes, rotating disk electrode (RDE) measurements, and spectroscopic tools. Microelectrodes need to be positioned near the catalytic electrode surface and thus inevitably invade the local environment and disrupt the species fluxes there. − RDE experiments could circumvent this issue and measure the electrode surface pH during hydrogen oxidation/evolution reactions, , but whether this method would be applicable to CO 2 reduction reactions is not clear. Nondestructive Raman or IR spectroscopy could directly probe pH-sensitive electrolyte species such as carbonate (CO 3 2– ) and bicarbonate (HCO 3 – ) at the electrode/electrolyte interface, but almost all of these studies are conducted on gas-impermeable electrodes, some of which require the catalyst to be coated on a prism and hence are not even compatible with the GDE and flow cell configuration. − …”

Examination of Near-Electrode Concentration Gradients and Kinetic Impacts on the Electrochemical Reduction of CO2 using Surface-Enhanced Infrared Spectroscopy

“…The phenomenon of localized changes in electrolyte concentrations near electrode surfaces relative to those in the bulk during electrolysis is well-understood and has been proven to have significant impacts on a wide variety of electrochemical processes. − During electrodeposition of Ni and Co for example, increases in [OH – ] due to the accompanying hydrogen evolution reaction (HER) can lead to precipitation of undesired, insoluble metal hydroxides, which can cause undesired changes in the physical or chemical properties of the electrodeposited layer. ,,, Similarly, during the anodic passivation of Zn, decreases in [OH – ] can induce a positive shift in the equilibrium potential of the oxidation, thereby requiring additional overpotential for the formation of ZnO rather than soluble hydroxides . Aside from changes to the chemical environment, concentration gradients in the diffuse layer can lead to mass-transport limitations, thereby influencing electrokinetics as described by the Nernst–Planck equation .…”

“…Despite the theoretical and demonstrated impact of η c changes due to the near-electrode accumulation/depletion of [H + ] or [OH – ], the effects are generally ignored due to the difficulty of reliably quantifying η c . Although various experimental methods have been employed to measure and account for the concentration gradients that drive η c , they generally suffer from poor precision, require destructive techniques, or are limited to specific electrochemical systems. ,,,,, …”