Investigating the electrowetting of silver‐based gas‐diffusion electrodes during oxygen reduction reaction with electrochemical and optical methods

Spatio-Temporal Electrowetting and Reaction Monitoring in Microfluidic Gas Diffusion Electrode Elucidates Mass Transport Limitations

The use of gas diffusion electrodes (GDEs) enables efficient electrochemical CO2 reduction and may be a viable technology in CO2 utilization after carbon capture. Understanding the spatio-temporal phenomena at the triple-phase boundary formed inside GDEs remains a challenge; yet it is critical to design and optimize industrial electrodes for gas-fed electrolyzers. Thus far, transport and reaction phenomena are not yet fully understood at the microscale, among other factors, due to a lack of experimental analysis methods for porous electrodes under operating conditions. In this work, a realistic microfluidic GDE surrogate is presented. Combined with fluorescence lifetime imaging microscopy (FLIM), the methodology allows monitoring of wetting and local pH, representing the dynamic (in)stability of the triple phase boundary in operando. Upon charging the electrode, immediate wetting leads to an initial flooding of the catalyst layer, followed by spatially oscillating pH changes. The micromodel presented gives an experimental insight into transport phenomena within porous electrodes, which is so far difficult to achieve. The methodology and proof of the spatio-temporal pH and wetting oscillations open new opportunities to further comprehend the relationship between gas diffusion electrode properties and electrical currents originating at a given surface potential.

Nonconductive Metal Oxide Gas Diffusion Layer for Mitigating Electrowetting during CO2 Electrolysis

Gas diffusion electrodes (GDEs) are extensively used for high current density electrochemical CO2 electrolysis (ECO2R), enabled by significantly reducing mass transfer resistance of CO2 to the catalyst layer. Conventionally, these GDEs are based upon hydrophobic carbon-based gas-diffusion layers (GDLs) that facilitate the gas transport; however, these supports are prone to flood with electrolyte during electrolysis. This potential-induced flooding, known as electrowetting, is related to the inherent conductivity of carbon and limits the activity of ECO2R. To investigate the effect of electrical conductivity more carefully, a GDE is constructed based on a Cu mesh with a nonconductive microporous GDL applied to this substrate, the latter composed of a mixture of metal oxide and polytetrafluoroethylene. With alumina as the metal oxide, a stable operation is obtained at -200 mA cm-2 with 70% selectivity for ECO2R (with over half toward C2+ products) without flooding as observed by in situ microscopy. On the contrary, with a Vulcan carbon-based GDL, the initial activity is rapidly lost as severe flooding ensues. It is reasoned that electrowetting is averted by virtue of the nonconductive nature of alumina, providing a new perspective on alternative GDL compositions and their influence on ECO2R performance.

Non-invasive current collectors for improved current-density distribution during CO2 electrolysis on super-hydrophobic electrodes

Electrochemical reduction of CO2 presents an attractive way to store renewable energy in chemical bonds in a potentially carbon-neutral way. However, the available electrolyzers suffer from intrinsic problems, like flooding and salt accumulation, that must be overcome to industrialize the technology. To mitigate flooding and salt precipitation issues, researchers have used super-hydrophobic electrodes based on either expanded polytetrafluoroethylene (ePTFE) gas-diffusion layers (GDL's), or carbon-based GDL's with added PTFE. While the PTFE backbone is highly resistant to flooding, the non-conductive nature of PTFE means that without additional current collection the catalyst layer itself is responsible for electron-dispersion, which penalizes system efficiency and stability. In this work, we present operando results that illustrate that the current distribution and electrical potential distribution is far from a uniform distribution in thin catalyst layers (~50 nm) deposited onto ePTFE GDL's. We then compare the effects of thicker catalyst layers (~500 nm) and a newly developed non-invasive current collector (NICC). The NICC can maintain more uniform current distributions with 10-fold thinner catalyst layers while improving stability towards ethylene (≥ 30%) by approximately two-fold.

Developing Catalysts Integrated in Gas-Diffusion Electrodes for CO2 Electrolyzers

ConspectusAs the demand for a carbon-neutral society grows rapidly, research on CO2 electrolysis has become very active. Many catalysts are reported for converting CO2 into value-added products by electrochemical reactions, which have to perform at high Faradaic and energy efficiency to become commercially viable. Various types of CO2 electrolyzers have been used in this effort, such as the H-cell, flow cell, and zero-gap membrane-electrode assembly (MEA) cell. H-cell studies are conducted with electrodes immersed in CO2-saturated electrolyte and have been used to elucidate reaction pathways and kinetic parameters of electrochemical CO2 reduction on many types of catalytic surfaces. From a transport phenomenological perspective, the low solubility and diffusion of CO2 to the electrode surface severely limit the maximum attainable current density, and this metric has been shown to have significant influence on the product spectrum. Flow and MEA cells provide a solution in the form of gas-diffusion electrodes (GDEs) that enable gaseous CO2 to closely reach the catalyst layer and yield record-high current densities. Because GDEs involve a complicated interface consisting of the catalyst surface, gaseous CO2, polymer overlayers, and liquid electrolyte, catalysts with high intrinsic activity might not show high performance in these GDE-based electrolyzers. Catalysts showing low overpotentials at low current densities may suffer from poor electron conductivity and mass transfer limitations at high current densities. Furthermore, the stability of the GDE-based catalysts is often compromised as CO2 electrolysis is pursued with high activity, most notoriously by electrolyte flooding.In this Account, we introduce recent examples where the electrocatalysts were integrated in GDEs, achieving high production rates. The performance of such systems is contingent on both GDE and cell design, and various parameters that affect the cell performance are discussed. Gaseous products, such as carbon monoxide, methane, and ethylene, and liquid products, such as formate and ethanol, have been mainly reported with high partial current density using the flow or MEA cells. Different strategies to this end are described, such as controlling microenvironments by the use of polymers mixed within the catalyst layer or the functionalization of catalyst surfaces with ligands to increase local concentrations of intermediates. Unique CO2 electrolyzer designs are also treated, including the incorporation of light-responsive plasmonic catalysts in the GDE, and combining the electrolyzer with a fermenter utilizing a microbial biocatalyst to synthesize complex multicarbon products. Basic conditions which the catalyst should satisfy to be adapted in the GDEs are listed, and our perspective is provided.

When Flooding Is Not Catastrophic-Woven Gas Diffusion Electrodes Enable Stable CO2 Electrolysis

Electrochemical CO₂ reduction has the potential to use excess renewable electricity to produce hydrocarbon chemicals and fuels. Gas diffusion electrodes (GDEs) allow overcoming the limitations of CO₂ mass transfer but are sensitive to flooding from (hydrostatic) pressure differences, which inhibits upscaling. We investigate the effect of the flooding behavior on the CO₂ reduction performance. Our study includes six commercial gas diffusion layer materials with different microstructures (carbon cloth and carbon paper) and thicknesses coated with a Ag catalyst and exposed to differential pressures corresponding to different flow regimes (gas breakthrough, flow-by, and liquid breakthrough). We show that physical electrowetting further limits the flow-by regime at commercially relevant current densities (≥200 mA cm^-2), which reduces the Faradaic efficiency for CO (FE_CO) for most carbon papers. However, the carbon cloth GDE maintains its high CO₂ reduction performance despite being flooded with the electrolyte due to its bimodal pore structure. Exposed to pressure differences equivalent to 100 cm height, the carbon cloth is able to sustain an average FE_CO of 69% at 200 mA cm^-2 even when the liquid continuously breaks through. CO₂ electrolyzers with carbon cloth GDEs are therefore promising for scale-up because they enable high CO₂ reduction efficiency while tolerating a broad range of flow regimes.

Micromodel of a Gas Diffusion Electrode Tracks In-Operando Pore-Scale Wetting Phenomena

Utilizing carbon dioxide (CO₂ ) as a resource for carbon monoxide (CO) production using renewable energy requires electrochemical reactors with gas diffusion electrodes that maintain a stable and highly reactive gas/liquid/solid interface. Very little is known about the reasons why gas diffusion electrodes suffer from unstable long-term operation. Often, this is associated with flooding of the gas diffusion electrode (GDE) within a few hours of operation. A better understanding of parameters influencing the phase behavior at the electrolyte/electrode/gas interface is necessary to increase the durability of GDEs. In this work, a microfluidic structure with multi-scale porosity featuring heterogeneous surface wettability to realistically represent the behavior of conventional GDEs is presented. A gas/liquid/solid phase boundary was established within a conductive, highly porous structure comprising a silver catalyst and Nafion binder. Inoperando visualization of wetting phenomena was performed using confocal laser scanning microscopy (CLSM). Non-reversible wetting, wetting of hierarchically porous structures and electrowetting were observed and analyzed. Fluorescence lifetime imaging microscopy (FLIM) enabled the observation of reactions on the model electrode surface. The presented methodology enables the systematic evaluation of spatio-temporally evolving wetting phenomena as well as species characterization for novel catalyst materials under realistic GDE configurations and process parameters.