Improving Stability of CO2 Electroreduction by Incorporating Ag NPs in N-Doped Ordered Mesoporous Carbon Structures

The electroreduction of carbon dioxide (eCO2RR) to CO using Ag nanoparticles as an electrocatalyst is promising as an industrial carbon capture and utilization (CCU) technique to mitigate CO2 emissions. Nevertheless, the long-term stability of these Ag nanoparticles has been insufficient despite initial high Faradaic efficiencies and/or partial current densities. To improve the stability, we evaluated an up-scalable and easily tunable synthesis route to deposit low-weight percentages of Ag nanoparticles (NPs) on and into the framework of a nitrogen-doped ordered mesoporous carbon (NOMC) structure. By exploiting this so-called nanoparticle confinement strategy, the nanoparticle mobility under operation is strongly reduced. As a result, particle detachment and agglomeration, two of the most pronounced electrocatalytic degradation mechanisms, are (partially) blocked and catalyst durability is improved. Several synthesis parameters, such as the anchoring agent, the weight percentage of Ag NPs, and the type of carbonaceous support material, were modified in a controlled manner to evaluate their respective impact on the overall electrochemical performance, with a strong emphasis on operational stability. The resulting powders were evaluated through electrochemical and physicochemical characterization methods, including X-ray diffraction (XRD), N2-physisorption, Inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy (SEM-EDS), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), STEM-EDS, electron tomography, and X-ray photoelectron spectroscopy (XPS). The optimized Ag/soft-NOMC catalysts showed both a promising selectivity (∼80%) and stability compared with commercial Ag NPs while decreasing the loading of the transition metal by more than 50%. The stability of both the 5 and 10 wt % Ag/soft-NOMC catalysts showed considerable improvements by anchoring the Ag NPs on and into a NOMC framework, resulting in a 267% improvement in CO selectivity after 72 h (despite initial losses) compared to commercial Ag NPs. These results demonstrate the promising strategy of anchoring Ag NPs to improve the CO selectivity during prolonged experiments due to the reduced mobility of the Ag NPs and thus enhanced stability.

Steering Hydrocarbon Selectivity in CO2 Electroreduction over Soft-Landed CuOx Nanoparticle-Functionalized Gas Diffusion Electrodes

The use of physical vapor deposition methods in the fabrication of catalyst layers holds promise for enhancing the efficiency of future carbon capture and utilization processes such as the CO₂ reduction reaction (CO₂RR). Following that line of research, we report in this work the application of a sputter gas aggregation source (SGAS) and a multiple ion cluster source type apparatus, for the controlled synthesis of CuO_x nanoparticles (NPs) atop gas diffusion electrodes. By varying the mass loading, we achieve control over the balance between methanation and multicarbon formation in a gas-fed CO₂ electrolyzer and obtain peak CH₄ partial current densities of -143 mA cm^-2 (mass activity of 7.2 kA/g) with a Faradaic efficiency (FE) of 48% and multicarbon partial current densities of -231 mA cm^-2 at 76% FE (FE_C2H4 = 56%). Using atomic force microscopy, electron microscopy, and quasi in situ X-ray photoelectron spectroscopy, we trace back the divergence in hydrocarbon selectivity to differences in NP film morphology and rule out the influence of both the NP size (3-15 nm, >20 μg cm^-2) and in situ oxidation state. We show that the combination of the O₂ flow rate to the aggregation zone during NP growth and deposition time, which affect the NP production rate and mass loading, respectively, gives rise to the formation of either densely packed CuO_x NPs or rough three-dimensional networks made from CuO_x NP building blocks, which in turn affects the governing CO₂RR mechanism. This study highlights the potential held by SGAS-generated NP films for future CO₂RR catalyst layer optimization and upscaling, where the NPs' tunable properties, homogeneity, and the complete absence of organic capping agents may prove invaluable.

Effects of Benzyl-Functionalized Cationic Surfactants on the Inhibition of the Hydrogen Evolution Reaction in CO2 Reduction Systems

Cationic surfactants, mainly hexadecyl cetrimonium bromide (CTAB), are widely used in electrocatalysis to affect the selectivity of the reaction, specifically to inhibit the hydrogen evolution reaction (HER) in CO₂ reduction (CO₂R) systems. However, little research has been done on the modification of the functional groups present in such surfactants in order to promote this HER-inhibiting effect. In this work, the effectiveness of CTAB was promoted by substituting a methyl group of the quaternary amine for a benzyl group. This cationic surfactant, cetalkonium chloride (CKC), increased the hydrophobicity of the surface of the electrode, promoting the HER inhibition and the CO₂R when HCO₃^- is used as a carbon source, which allows combining capture and conversion in one and the same medium, making it industrially highly attractive. By performing a detailed electrochemical characterization, we proved that the benzyl group formed an enhanced hydrophobic layer on the surface of the electrode in addition to the alkyl chain of the surfactant, showing higher effectiveness compared to CTAB. In fact, the Faradaic efficiency of the CO₂R increased from 39 to 66% in saturated HCO₃^- electrolytes by using CKC instead of CTAB as the HER inhibitor. This opens up a wide range of avenues for research on the application of surfactants in the field of electrocatalysis, because, as proven, a selective modification of it can tune the selectivity of the reaction, adding a new variable in the design of an efficient carbon capture and utilization system.

High-performance membranes with full pH-stability

Following current strong demands from, among others, paper, food and mining industries, a novel type of nanofiltration membrane was developed, which displays excellent performance in terms of selectivity/flux with a unique combination of chemical stability over the full (0-14) pH-range and thermal stability up to 120 °C. The membrane consists of polyvinylidene fluoride grafted with polystyrene sulfonic acid. The optimum membrane showed water permeances of 2.4 L h-1 m-2 bar-1 while retaining NaCl, MgSO4 and Rhodamine B (479 Da) for respectively ≈60%, ≈80% and >96%.