Maximilian Kutter

Molecular catalysts offer tunable active and peripheral sites, rendering them ideal model systems to explore fundamental concepts in catalysis. However, hydrophobic designs are often regarded as detrimental for dissolution in aqueous electrolytes. Here we show that established cobalt terpyridine catalysts modified with hydrophobic perfluorinated alkyl side chains can assemble at the gas–liquid–solid interfaces on a gas diffusion electrode. We find that the self-assembly of these perfluorinated units on the electrode surface results in a catalytic system selective for electrochemical CO₂ reduction to CH₄, whereas every other cobalt terpyridine catalyst reported previously was only selective for CO or formate. Mechanistic investigations suggest that the pyridine units function as proton shuttles that deliver protons to the dynamic hydrophobic pocket in which CO₂ reduction takes place. Finally, integration with fluorinated carbon nanotubes as a hydrophobic conductive scaffold leads to a Faradaic efficiency for CH₄ production above 80% at rates above 10 mA cm⁻²—impressive activities for a molecular electrocatalytic system.

The manuscript presents a novel approach for polymer-based Ag-DHBT-GDE fabrication, enabling highly efficient CO₂ electrolysis to CO with remarkable performance and selectivity.

The commercialization of PEM fuel cells offers a promising opportunity to overcome our dependence on fossil fuels and provides an attractive alternative for automotive and stationary power applications.[1] However, PEMFCs are still costly in production, with the main cost factor due to the fabrication of the perfluorosulfonic acid (PFSA)-based proton exchange membrane and the noble metal catalysts.[2,3] Recycling of these components could increase the economic attractiveness of PEMFCs. However, while the high resistance of the ionomer membrane to thermal and chemical degradation is favorable for their use in fuel cells, it also makes membrane recycling a major challenge. Here, we present a newly developed recycling strategy to recover PFSA-based membranes after operation by a hydrothermal treatment. Both short side chain (SSC) and long side chain (LSC) ionomers with EWs of 800 and 1000 were investigated. In contrast to previously published methods, we were able to recover the PFSA ionomer by using water instead of high-boiling organic solvents, such as DMF or DMSO.[4,5] The recycled membranes were obtained from the recycled PFSA ionomer solution by doctor-blading. They were investigated by in-situ Raman techniques for studying the water uptake and analyzed by IR and small-angle X-ray scattering (SAXS) techniques to get a more detailed insight into their morphology. In single cell tests at standard (80 °C) and enhanced (up to 130 °C) PEMFC operating temperatures, the recycled membranes showed an increased cell performance as compared to the pristine 3M-800EW membrane. Solid-state NMR measurements confirmed that no chemical changes in the membrane occurred due to the heat-treatment process. We assume that the observed performance boost can be attributed to an increased water uptake and retention. Furthermore, a change in the polymer matrix formation may be responsible for the observed behavior, which led to an expanded water filled channel structure, where water can accumulate to facilitate proton diffusion as H₃O⁺ through the channels. To confirm this assumption, the morphology of the recycled membranes will be investigated by in situ SAXS measurements at different temperatures and degrees of hydration. [1] I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward, Energy Environ. Sci. 2019, 12, 463–491. [2] J. Fan, M. Chen, Z. Zhao, Z. Zhang, S. Ye, S. Xu, H. Wang, H. Li, Nat. Energy 2021, 6, 475–486. [3] D. Papageorgopoulos, Fuel Cell R&D Overview. 2019 Annual Merit Review and Peer Evaluation Meeting, 2019. [4] Q. Xu, X. Chen, S. Wang, C. Guo, Y. Niu, R. Zuo, Z. Yang, Y. Zhou, C. Xu, Energies 2022, 15, 8717. [5] H. F. Xu, X. Wang, Z. G. Shao, I. M. Hsing, J. Appl. Electrochem. 2002, 32, 1337–1340.

Energy-conversion technologies like the electrocatalytic nitrogen reduction reaction (N₂RR), carbon monoxide reduction reaction (CORR) and in particular the electrochemical carbon dioxide reduction reaction (CO₂RR) can contribute decisively to overcome the challenge of reducing greenhouse gas emissions. By reducing CO₂ electrochemically, the carbon-cycle can be closed and industrially desired chemicals like syngas (CO + H₂), formic and acetic acid, or hydrocarbons, such as methanol and ethylene, can be produced fossil-fuel free, depending on the catalyst material applied. Ag-based catalysts are very promising in the field of CO₂RR, since they show high selectivities towards the reduction of CO₂ to CO and current efficiencies of up to 100 %. In aqueous-fed systems, a significant drawback is the limited CO₂ solubility, resulting in long diffusion pathways of CO₂ within the bulk electrolyte ^{1, 2}. Consequently, industrial conditions cannot be reached for CO₂ electrolysis in aqueous solution in H-cell configurations due to the CO₂ depletion within the porous electrodes that limit lab scale testing of CO₂RR to current densities of approx. 35 mAcm^-². One way to achieve commercially relevant current densities (> 200 mAcm^-²) is the utilization of gas diffusion electrodes (GDEs) in flow cell configuration. These enable a decrease in CO₂ diffusion lengths to approx. 50 nm, enhancing the mass transport to the catalyst ³. In our contribution, we will demonstrate a facile and fast production process of Ag-based GDEs by combining pulsed electrochemical deposition of the Ag catalyst with ionomer infiltration of the electrode. Using the dynamic hydrogen bubble templation method (DHBT), we utilized the parasitic hydrogen evolution reaction (HER) to aid in a solvent free structuring of the 3D catalyst network. In this work, we focused on the deposition parameters by varying different pulse-to-pause ratios in order to increase the amount of deposited catalyst and thus, successfully reduced the overpotential during CO₂RR operation (Fig. 1, left). To inhibit electrode flooding and decrease CO₂ mass transport limitations during CO₂RR, we infiltrated the electrode with a perfluorinated ionomer. SEM and EDS analyses showed a uniform Ag/F distribution along the cross section of the electrodes (Fig. 1, right), which resulted in the conversion of CO₂ to CO at industrially viable current densities. References S. Hernandez-Aldave and E. Andreoli, Catalysts, 10(6), 713 (2020). Z. Sun, T. Ma, H. Tao, Q. Fan and B. Han, Chem, 3(4), 560–587 (2017). T. Burdyny and W. A. Smith, Energy Environ. Sci., 12(5), 1442–1453 (2019). Figure 1

The modularity of molecular catalysts enables the tuning of both active site and peripheral units to maximize functionality, thus rendering them as ideal model systems to explore fundamental concepts in catalysis. Hydrophobicity is often regarded as an undesirable aspect that hinders their dissolution in aqueous electrolytes. In contrast, we modified established Co terpyridine catalysts with hydrophobic perfluorinated alkyl side chains and took advantage of their hydrophobic character by utilizing them not as dissolved species in an aqueous electrolyte but at the gas-liquid-solid interfaces on a gas diffusion electrode (GDE) applied towards the electrochemical reduction of CO2. We found that the self-assembly of these perfluorinated units on the GDE surface results in a catalytic system selective for CH4 production, whereas every other Co terpyridine catalyst reported before was only selective for CO or formate. An array of mechanistic and operando spectroscopic investigations suggests a mechanism in which the pyridine units function as proton shuttles that deliver protons to the dynamic hydrophobic pocket in which CO2 reduction takes place. Finally, optimizing the system by integrating fluorinated carbon nanotubes as a hydrophobic conductive scaffold leads to a Faradaic efficiency for CH4 production above 80% at rates above 10 mA/cm2, thus far unprecedented for a molecular electrocatalytic system.

Fuel cells, as a local, emission-free and versatile system, promise to overcome our dependence on fossil fuels.^[1] Polymer electrolyte membrane fuel cells (PEMFCs) are considered one of the most promising technologies among the various kinds of existing fuel cells and offer an attractive alternative for automotive and stationary energy applications. Especially PEMFCs operating at an increased temperature range (MT-PEM) offer an enhanced performance. Operating temperatures between 100 – 130 °C lead to better reaction kinetics, higher tolerance to fuel impurities and to an improved heat, water and power management of the system.^[2] However, some issues regarding durability and performance, such as low proton conductivity of PFSA-based membranes, higher membrane degradation and lower long-term stability at increased temperatures, are still unsolved.^[3] In previous works we have shown that various metal fluorides implemented into perfluorosulfonic acid (PFSA)-membranes have a positive impact on performance and mechanical stability at operating temperatures above 100 °C.^[4] In this work, we describe the modification of lithium fluoride nanoparticles, their influence on membrane durability in single cell tests at enhanced PEMFC operating temperatures and the morphology of the composite membranes, at different temperatures and degree of hydration, by in-situ small-angle X-ray scattering (SAXS). The lithium fluoride modified membrane showed increased cell performance under both standard and harsher cell conditions as well as in various long-term stability tests, such as accelerated OCV tests, load cycles and on-off cycles.One explanation for the performance boost, in addition to the increased mechanical stability of the membrane, would be an increased water uptake and storage capability, especially at low humidity levels during cell operation. We assume that the nanoparticles adsorb water molecules by hydrogen bond formation, which leads to an enhanced proton conductivity even at high temperatures. To confirm this assumption, we applied in-situ SAXS to analyze the water uptake of the modified membranes at various relative humidity and temperatures to understand the structural changes. In addition, we hope to connect different nanoparticle shapes to their influence on water uptake and retention. [1] I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward, Energy Environ. Sci. 2019, 12, 463–491. [2] R. E. Rosli, A. B. Sulong, W. R. W. Daud, M. A. Zulkifley, T. Husaini, M. I. Rosli, E. H. Majlan, M. A. Haque, Int. J. Hydrogen Energy 2017, 42, 9293–9314. [3] L. Mazzapioda, S. Panero, M. A. Navarra, Polymers 2019, 11, 914. [4] A. Moszczynska, H. Wolf, M. A. Willert-Porada, Patent WO/2009/014930, 2009.

Metal-based gas diffusion electrodes are utilized in chlor-alkali electrolysis or electrochemical reduction of carbon dioxide, allowing the reaction to proceed at high current densities. In contrast to planar electrodes and predominantly 2D designs, the industrially required high current densities can be achieved by intense contact between the gas and liquid phase with the catalytically active surfaces. An essential asset for the knowledge-based design of tailored electrodes is therefore in-depth information on electrolyte distribution and intrusion into the electrode’s porous structure. Lab-based and synchrotron radiography allow for monitoring this process operando . Herein, we describe the development of a cell design that can be modularly adapted and successfully used to monitor both the oxygen reduction reaction and the electrochemical reduction of CO ₂ as exemplary and currently very relevant examples of gas-liquid reactions by only minor modifications to the cell set-up. With the reported cell design, we were able to observe the electrolyte distribution within the gas diffusion electrode during cell operation in realistic conditions.