Among the electrolyzers under development for CO2 electroreduction at practical reaction rates, gas-fed approaches that use gas diffusion electrodes (GDEs) as cathodes are the most promising. However, the insufficient long-term stability of these technologies precludes their commercial deployment. The structural deterioration of the catalyst material is one possible source of device durability issues. Unfortunately, this issue has been insufficiently studied in systems using actual technical electrodes. Herein, we make use of a morphologically tailored Ag-based model nanocatalyst [Ag nanocubes (NCs)] assembled on a zero-gap GDE electrolyzer to establish correlations between catalyst structures, experimental environments, electrocatalytic performances, and morphological degradation mechanisms in highly alkaline media. The morphological evolution of the Ag–NCs on the GDEs induced by the CO2 electrochemical reduction reaction (CO2RR), as well as the direct mechanical contact between the catalyst layer and anion-exchange membrane, is analyzed by identical location and post-electrolysis scanning electron microscopy investigations. We find that at low and mild potentials positive of −1.8 V versus Ag/AgCl, the Ag–NCs undergo no apparent morphological alteration induced by the CO2RR, and the device performance remains stable. At more stringent cathodic conditions, device failure commences within minutes, and catalyst corrosion leads to slightly truncated cube morphologies and the appearance of smaller Ag nanoparticles. However, comparison with complementary CO2RR experiments performed in H-cell configurations in a neutral environment clearly proves that the system failure typically encountered in the gas-fed approaches does not stem solely from the catalyst morphological degradation. Instead, the observed CO2RR performance deterioration is mainly due to the local high alkalinity that inevitably develops at high current densities in the zero-gap approach and leads to the massive precipitation of carbonates which is not observed in the aqueous environment (H-cell configuration).

Gas diffusion electrode (GDE) setups have very recently received increasing attention as a fast and straightforward tool for testing the oxygen reduction reaction (ORR) activity of surface area proton exchange membrane fuel cell (PEMFC) catalysts under more realistic reaction conditions. In the work presented here, we demonstrate that our recently introduced GDE setup is suitable for benchmarking the stability of PEMFC catalysts as well. Based on the obtained results, it is argued that the GDE setup offers inherent advantages for accelerated degradation tests (ADT) over classical three-electrode setups using liquid electrolytes. Instead of the solid-liquid electrolyte interface in classical electrochemical cells, in the GDE setup a realistic three-phase boundary of (humidified) reactant gas, proton exchange polymer (e.g. Nafion) and the electrocatalyst is formed. Therefore, the GDE setup not only allows accurate potential control but also independent control over the reactant atmosphere, humidity and temperature. In addition, the identical location transmission electron microscopy (IL-TEM) technique can easily be adopted into the setup, enabling a combination of benchmarking with mechanistic studies.

A new approach for efficiently investigating the degradation of fuel cell catalysts under realistic conditions is presented combining accelerated stress tests (ASTs) in a gas diffusion electrode (GDE) setup with small angle X-ray scattering (SAXS). GDE setups were recently introduced as a novel testing tool combining the advantages of classical electrochemical cells with a three-electrode setup and membrane electrode assemblies (MEAs). SAXS characterization of the catalyst layer enables an evaluation of the particle size distribution of the catalyst and its changes upon applying an AST. The straight-forward approach not only enables stability testing of fuel cell catalysts in a comparative and reproducible manner, it also allows mechanistic insights into the degradation mechanism. Typical metal loadings for proton exchange membrane fuel cells (PEMFCs), i.e. 0.2 mgPt cm−2 geo, are applied in the GDE and the degradation of the overall (whole) catalyst layer is probed. For the first time, realistic degradation tests can be performed comparing a set of catalysts with several repeats within reasonable time. It is demonstrated that independent of the initial particle size in the pristine catalyst, for ASTs simulating load cycle conditions in a PEMFC, all catalysts degrade to a similar particle size distribution.

In recent years, extensive research has been performed concerning the stability of fuel cell catalysts in an acidic environment. By comparison, only few studies address the degradation mechanism(s) of fuel cell catalysts in alkaline media. In this work, we investigate the stability of four different types of Pt/C fuel cell catalysts upon applying accelerated degradation tests in a gas diffusion electrode (GDE) setup equipped with an anion exchange membrane. In contrast to previous investigations exposing the catalysts to a liquid electrolyte, the GDE setup provides a realistic three-phase boundary of the reactant gas, catalyst, and ionomer which enables reactant transport rates close to real fuel cells. Therefore, the GDE setup mimics the degradation of the catalyst under more realistic reaction conditions as compared to conventional electrochemical cells. Combining the determination of the loss in the electrochemically active surface area of the Pt/C catalysts via CO stripping measurements with the change in particle size distribution determined by small-angle X-ray scattering measurements, we demonstrate that (i) the degradation mechanism depends on the investigated Pt/C catalyst and might indeed be different from the one observed in conventional electrochemical cells, (ii) degradation is increased in an oxygen gas atmosphere (as compared to an inert atmosphere), and (iii) the observed degradation mechanism depends on the mesoscopic environment of the active phase. The measurements indicate an increased particle growth if small and large particles are immobilized next to each other on the same carbon support flakes as compared to a simple mix of two catalysts with small and large particles, respectively.

In this work, we discuss the application of a gas diffusion electrode (GDE) setup for benchmarking electrocatalysts for the reductive conversion of CO2 (CO2 RR: CO2 reduction reaction). Applying a silver nanowire (Ag-NW) based catalyst, it is demonstrated that in the GDE setup conditions can be reached, which are relevant for the industrial conversion of CO2 to CO. This reaction is part of the so-called 'Rheticus' process that uses the CO for the subsequent production of butanol and hexanol based on a fermentation approach. In contrast to conventional half-cell measurements using a liquid electrolyte, in the GDE setup CO2 RR current densities comparable to technical cells (>100 mA cm–2) are reached without suffering from mass transport limitations of the CO2 reactant gas. The results are of particular importance for designing CO2 RR catalysts exhibiting high faradaic efficiencies towards CO at technological reaction rates.

Based on H-cell measurements, gold (Au) is one of the most selective catalysts for the CO 2 reduction reaction (CO 2 RR) to CO. To ensure a high dispersion, typically small Au nanoparticles (NPs) are used as a catalyst. However, the preparation of small Au NPs based on conventional synthesis methods often requires the use of surfactants such as polyvinylpyrrolidone (PVP). Here, a systematic evaluation of the performance of lasergenerated, surfactant-free Au NPs for the CO 2 RR in a gas diffusion electrode (GDE) setup was presented and the results were compared to investigations in an H-cell configuration. The GDE setup supplied a continuous CO 2 stream at the electrodeelectrolyte interface to circumvent CO 2 mass transport limita-tions encountered in conventional H-cells. The influence of the catalyst loading and the effect of PVP were investigated. By comparing the two screening methods, that is GDE and H-cell measurements, it was shown that the performance of the same catalyst could be substantially different in the two environments. In the GDE setup without liquid electrolyte-catalyst interface a higher reaction rate, but lower faradaic efficiency was determined. Independent of the setup, the presence of PVP favoured the hydrogen evolution reaction (HER). However, in the GDE setup PVP was more detrimental for the performance than in the H-cell.

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