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Time-Resolved Local pH Measurements during CO2 Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip
Mariana C O Monteiro1, Alex Mirabal2, Leon Jacobse3
1Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
JACS Au
|December 1, 2021
Summary
This study measures local pH during electrochemical CO2 reduction using a novel voltammetric sensor with Scanning Electrochemical Microscopy (SECM). Results reveal pH buffering in CO2 atmospheres, contrasting with hydrogen evolution, and quantify the impact of the SECM tip on measurements.
Area of Science:
- Electrochemistry
- Surface Science
- Chemical Engineering
Background:
- Electrochemical reduction of carbon dioxide (CO2) is a key area for sustainable fuel and chemical production.
- Electrolyte pH significantly influences CO2 reduction reaction (CO2RR) activity and selectivity.
- Understanding local pH gradients at the electrode-electrolyte interface is crucial for optimizing CO2RR.
Purpose of the Study:
- To quantify local pH changes in the diffusion layer during CO2 reduction using Scanning Electrochemical Microscopy (SECM).
- To compare pH behavior under CO2 reduction versus hydrogen evolution conditions.
- To investigate the buffering effect of bicarbonate formation and the influence of the SECM tip on pH measurements.
Main Methods:
- Development and application of a novel voltammetric pH sensor for high time-resolution measurements.
- Utilized SECM with a functionalized gold ultramicroelectrode (4-hydroxylaminothiophenol/4-nitrosothiophenol) on a gold substrate.
- Performed finite element method (FEM) simulations to model fluid dynamics, reaction kinetics, and tip-sample interactions.
Main Results:
- Observed a gradual pH increase during hydrogen evolution in an argon atmosphere.
- Identified a pH plateau in a CO2 atmosphere, indicative of bicarbonate (HCO3-) buffering.
- FEM simulations revealed that the SECM tip causes localized diffusion hindrance, leading to more acidic conditions without the tip.
Conclusions:
- The developed voltammetric pH sensor enables effective in-situ local pH monitoring during CO2 reduction.
- Bicarbonate formation provides significant buffering at the electrode interface during CO2 electroreduction.
- Accurate local pH determination requires accounting for the SECM tip's influence on diffusion dynamics.
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