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.

Effect of Temperature and H Flux on the NH3 Synthesis via Electrochemical Hydrogen Permeation

Invited for this month's cover is the group of Prof. Fokko M. Mulder at the Delft University of Technology. The image on the cover shows how in the NH₃ synthesis via hydrogen-permeable electrode the N, H species on the catalyst surface can be controlled, using the analogy of a traffic controller. The Research Article itself is available at 10.1002/cssc.202300460.

Effect of temperature and H flux on the NH3 synthesis via electrochemical hydrogen permeation

Ammonia is an indispensable commodity and a potential carbon free energy carrier. The use of H permeable electrodes to synthesize ammonia from N2, water and electricity, provides a promising alternative to the fossil fuel based Haber-Bosch process. Here, H permeable Ni electrodes are investigated in the operating temperature range 25-120°C, and varying the rate of electrochemical atomic hydrogen permeation. At 120°C, a steady reaction is achieved for over 12h with 10 times higher cumulative NH3 production and almost 40-fold increase in faradaic efficiency compared to room temperature experiments. NH3 is formed with a cell potential of 1.4V, corresponding to a minimum electrical energy investment of 6.6kWh∙ [[EQUATION]] . The stable operation is attributed to a balanced control over the population of N, NHx and H species at the catalyst surface. This findings extend the understanding on the mechanisms involved in the nitrogen reduction reaction and may facilitate the development of an efficient green ammonia synthesis process.

Revisiting the Electrochemical Nitrogen Reduction on Molybdenum and Iron Carbides: Promising Catalysts or False Positives?

The electrochemical dinitrogen reduction reaction (NRR) has recently gained much interest as it can potentially produce ammonia from renewable intermittent electricity and replace the Haber-Bosch process. Previous literature studies report Fe- and Mo-carbides as promising electrocatalysts for the NRR with activities higher than other metals. However, recent understanding of extraneous ammonia and nitrogen oxide contaminations have challenged previously published results. Here, we critically assess the NRR performance of several Fe- and Mo-carbides reported as promising by implementing a strict experimental protocol to minimize the effect of impurities. The successful synthesis of α-Mo₂C decorated carbon nanosheets, α-Mo₂C nanoparticles, θ-Fe₃C nanoparticles, and χ-Fe₅C₂ nanoparticles was confirmed by X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron and Mössbauer spectroscopy. After performing NRR chronoamperometric tests with the synthesized materials, the ammonia concentrations varied between 37 and 124 ppb and are in close proximity with the estimated ammonia background level. Notwithstanding the impracticality of these extremely low ammonia yields, the observed ammonia did not originate from the electrochemical nitrogen reduction but from unavoidable extraneous ammonia and NO _x impurities. These findings are in contradiction with earlier literature studies and show that these carbide materials are not active for the NRR under the employed conditions. This further emphasizes the importance of a strict protocol in order to distinguish between a promising NRR catalyst and a false positive.

CO2 Electrolysis via Surface-Engineering Electrografted Pyridines on Silver Catalysts

The electrochemical reduction of carbon dioxide (CO₂) to value-added materials has received considerable attention. Both bulk transition-metal catalysts and molecular catalysts affixed to conductive noncatalytic solid supports represent a promising approach toward the electroreduction of CO₂. Here, we report a combined silver (Ag) and pyridine catalyst through a one-pot and irreversible electrografting process, which demonstrates the enhanced CO₂ conversion versus individual counterparts. We find that by tailoring the pyridine carbon chain length, a 200 mV shift in the onset potential is obtainable compared to the bare silver electrode. A 10-fold activity enhancement at −0.7 V vs reversible hydrogen electrode (RHE) is then observed with demonstratable higher partial current densities for CO, indicating that a cocatalytic effect is attainable through the integration of the two different catalytic structures. We extended the performance to a flow cell operating at 150 mA/cm², demonstrating the approach’s potential for substantial adaptation with various transition metals as supports and electrografted molecular cocatalysts.

Characterizing CO2 Reduction Catalysts on Gas Diffusion Electrodes: Comparing Activity, Selectivity, and Stability of Transition Metal Catalysts

Continued advancements in the electrochemical reduction of CO₂ (CO₂RR) have emphasized that reactivity, selectivity, and stability are not explicit material properties but combined effects of the catalyst, double-layer, reaction environment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm^-2, which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO₂RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and 50 mA cm^-2, respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral and alkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm^-2. We hope the collective work provides a resource for researchers setting up CO₂RR experiments for the first time.

Overcoming Nitrogen Reduction to Ammonia Detection Challenges: The Case for Leapfrogging to Gas Diffusion Electrode Platforms

The nitrogen reduction reaction (NRR) is a promising pathway toward the decarbonization of ammonia (NH₃) production. However, unless practical challenges related to the detection of NH₃ are removed, confidence in published data and experimental throughput will remain low for experiments in aqueous electrolyte. In this perspective, we analyze these challenges from a system and instrumentation perspective. Through our analysis we show that detection challenges can be strongly reduced by switching from an H-cell to a gas diffusion electrode (GDE) cell design as a catalyst testing platform. Specifically, a GDE cell design is anticipated to allow for a reduction in the cost of crucial ¹⁵N₂ control experiments from €100-2000 to less than €10. A major driver is the possibility to reduce the ¹⁵N₂ flow rate to less than 1 mL/min, which is prohibited by an inevitable drop in mass-transport at low flow rates in H-cells. Higher active surface areas and improved mass transport can further circumvent losses of NRR selectivity to competing reactions. Additionally, obstacles often encountered when trying to transfer activity and selectivity data recorded at low current density in H-cells to commercial device level can be avoided by testing catalysts under conditions close to those in commercial devices from the start.

Operando isotope selective ammonia quantification in nitrogen reduction studies via gas chromatography-mass spectrometry

Rapid advances in electrocatalytic ammonia synthesis are impeded by laborious detection methods commonly used in the field and by constant risk of external contaminations, which generates misleading false positives. We developed a facile real-time GC-MS method for sensitive isotope NH₃ quantification, requiring no external sample manipulations. This method ensures high detection reliability paramount to accelerate (electro-)catalyst screening.

Ammonia Synthesis at Ambient Conditions via Electrochemical Atomic Hydrogen Permeation

Direct electrochemical nitrogen reduction holds the promise of enabling the production of carbon emission-free ammonia, which is an important intermediate in the fertilizer industry and a potential green energy carrier. Here we show a strategy for ambient condition ammonia synthesis using a hydrogen permeable nickel membrane/electrode that spatially separates the electrolyte and hydrogen reduction side from the dinitrogen activation and hydrogenation sites. Gaseous ammonia is produced catalytically in the absence of electrolyte via hydrogenation of adsorbed nitrogen by electrochemically permeating atomic hydrogen from water reduction. Dinitrogen activation at the polycrystalline nickel surface is confirmed with ¹⁵N₂ isotope labeling experiments, and it is attributed to a Mars-van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. We further show that gaseous hydrogen does not hydrogenate the adsorbed nitrogen, strengthening the benefit of having an atomic hydrogen permeable electrode. The proposed approach opens new directions toward green ammonia.