title: Degradation Pathways of Zero-gap Electrolyzer Systems for Electrochemical CO₂ Reduction
Creators: name: Kong, Ying
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: Electrochemical CO₂ conversion to chemical feedstock is of environmental and economic benefit as it makes use of excess CO₂ waste and stores (temporarily) renewable energy in the form of value-added chemicals at the same time. High performance catalysts, including nanomaterials, have already been developed in the past to optimize product selectivity of the CO₂ electrolysis. To overcome CO₂ mass transfer limitations, which are intrinsic to classical aqueous reaction environments, more advanced CO₂ electrolyzer devices based on gas diffusion electrodes (GDEs) are currently being developed which in principle allow achieving industrially relevant current densities. What, however, still prevents their implementation into real industrial applications is the actual electrolyzer stability which can be compromised at these high reaction rates by so-called “electrolyte flooding” phenomena and related salt precipitation occurring inside the GDEs during their operation and causing severe and irreversible deterioration in the overall electrolysis performance.
Despite the growing interest in this particular kind of electrode degradation scenario in recent years, the physical origin of the experimentally observed electrolysis performance losses remained unclear. To develop tailored concepts towards the long-term stabilization of the CO₂ electrolyzers it is, however, crucial to identify possible GDE degradation pathways based on flooding phenomena which presumably depend on the structural and chemical characteristics of the applied GDE.
This PhD project particularly aimed at elucidating the interrelation between the GDL “crackness” (defectivity) and its stability in a so-called zero-gap GDE-membrane configuration. Furthermore, effects related to the catalyst layer (CL) formation (e.g., the chemical nature of the ionomer binder) were systematically addressed in this work using silver nanoparticles (Ag-NPs) as model catalysts for the CO₂-to-CO conversion. To this end dedicated GDE stressing protocols were developed which allowed for accelerated but controlled electrode degradation. Electrolysis-time dependent phenomena of electrolyte precipitation could be made visible and quantified by a new experimental approach based on combining cross-sectional potassium (EDX) mapping with post-electrolysis inductively coupled plasma mass spectrometry (ICP-MS). Using these novel analytical approaches it was possible to unambiguously correlate the GDE stability in the zero-gap configuration with an increasing crack abundance of the gas diffusion (support) layer (GDL) which facilitates electrolyte transport. These findings demonstrate that the effective electrolyte transport through the GDE during operation, a phenomenon called “electrolyte perspiration”, is vital for keeping the GDE stable at high reaction rates (“electrolyte transport matters”). In the absence of an effective electrolyte perspiration pathway, electrolyte may accumulate in the CL thereby physically blocking the active sites for CO₂ reduction and causing performance losses through hindered CO₂ mass transport.
The absence/presence of polymeric capping agents, typically applied for the synthesis of the nanoparticulate Ag catalysts, and the chemical nature of the ionomeric binder, used to improve the adherence of the NP catalyst to the carbon support, were identified as further contributors either hindering or facilitating the electrolyte perspiration and therefore crucially affecting the system stability.
Results presented herein pave the way for the tailored design of high-performance gas diffusion electrodes used in (quasi) zero-gap electrode/membrane configurations.