Operando Characterization of CO2 Reduction Interfaces by Electrochemical Atomic Force Microscopy
Authors: Kelly Maria , Hamilton Sara T , Brimley Paige N , Nesbitt Nathan T , Park Ah-Hyung Alissa , Smith Wilson A 
Published: December 2024

The electrode-electrolyte interface is perhaps the most important interface of an electrochemical device. At this interface, the electrochemical reactions of interest occur, and the local microenvironment (e.g. pH, reactant concentrations, water activity) can differ significantly from bulk electrolyte conditions due to double layer formation, migration, mass transport effects, catalyst restructuring, and more. In addition, common electrode materials such as polycrystalline metal foils and nanoparticle catalyst layers are heterogeneous with regions of differing activity, making the interface vary across the surface of the electrode both spatially and temporally. Understanding the electrode-electrolyte interface, its local microenvironment, and the evolution of the heterogeneous electrode surface is crucial to the development of high performing electrochemical devices. However, traditional electrochemical methods average current and potential measurements across the electrode surface. Thus, combining electrochemistry with advanced in situ and operando is necessary for a holistic view of the electrode-electrolyte interface.

Scanning probe techniques are well-suited for operando studies of electrode-electrolyte interfaces due to their compatibility with liquid environments and suitable spatial (µm to nm scale) and temporal (minutes to seconds) resolutions. One popular operando technique for the study of electrode-electrolyte interfaces is electrochemical atomic force microscopy (EC-AFM). EC-AFM measures the interaction forces between a sharp physical probe and sample surface to map the surface topography. These measurements can also extract information about the sample’s mechanical properties by generating force-distance curves across the surface. EC-AFM measurements are readily performed in a three-electrode cell under potential bias, thereby allowing spatially and temporally resolved imaging of the electrode-electrolyte interface in operando.

Here, we applied EC-AFM to two studies of the electrode-electrolyte interface during the electrochemical CO2 reduction reaction (CO2RR). First, we present EC-AFM measurements of a common degradation mode for CO2RR electrolyzers: salt precipitation. CO2RR produces OH-, which accumulates at the electrode-electrode interface and causes the local pH to shift more alkaline. However, this alkaline environment also promotes the formation and precipitation of (bi)carbonate salts by the reaction of OH- and CO2. Herein, we conducted EC-AFM measurements on a Ag electrode in calcium-containing bicarbonate electrolytes. CO2RR was catalyzed at the Ag surface to cause a local pH shift at the electrode surface and result in the formation of CaCO3 crystallites on the electrode surface. Through EC-AFM measurements, we investigated the effect of applied current and bulk electrolyte pH on this deleterious mineralization process.

Second, we studied the interactions between polyethylenimine (PEI) and a Ag catalyst. PEI is a polymer that can capture CO2 from the atmosphere by forming a carbamate. Recent work has shown that CO2 bound in PEI can be directly converted to CO without an intermediate CO2 release step. However, very little information is known about the interaction of the polymer with the electrode surface during CO2RR. We studied the morphology and mechanical properties of a Ag electrode surface in PEI-containing electrolytes as a function of applied potential and electrolyte composition for the conversion of CO2 to CO. Both variables significantly affected the mechanical properties (i.e. modulus) of the electrode surface, suggesting polymer chain reorganization at the interface due to interactions with electrolyte species and charged electrode surfaces. Overall, these examples demonstrate that EC-AFM is a powerful operando technique for building fundamental understanding of the electrode-electrolyte interface.