Ammonia synthesis via the direct nitrogen reduction reaction mechanism has the potential to be more flexible in production level and scale of operation and may enable cost reductions compared to alternative technologies for green NH3 synthesis. For the research field to advance to higher Technology Readiness Levels selective electrocatalysts that promote the reaction over the competing hydrogen evolution reaction are needed. The aim of this work was to build tools that enable the development of selective electrocatalysts for NRR with high NH3 production rate. We have identified two limitations in the workflow which is typically used to test promising materials for NRR activity, that hindered the development of selective electrocatalysts thus far: 1) NRR activity measurements are found to be unreliable due to NH3 contaminations and 2) the experimental throughput of the workflow is too slow to enable rapid progress, due to single catalyst studies that require elaborate ammonia detection and calibration methods. In Chapter 2 and 3 we systematically analyzed the steps involved in an NRR activity measurement to develop alternative methods that overcome these limitations. For even more effective NRR catalyst development, we explored in Chapters 4 and 5 how to accelerate the experimental workflow even further by enabling combinatorial catalyst screenings and by carrying out experiments under thermodynamically more favourable conditions for NRR, respectively..... @en

The electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the current greenhouse gas emission intensive process to produce ammonia (NH₃) from nitrogen (N₂). However, finding an electrocatalyst that promotes NRR over the competing hydrogen evolution reaction (HER) has proven to be difficult. This difficulty could potentially be addressed by accelerating the electrocatalyst development for NRR by orders of magnitude using high-throughput (HTP) workflows. In this work, we developed a HTP gas diffusion electrode (GDE) cell to screen up to 16 electrocatalysts in parallel. The key innovation of the cell is the use of expanded Polytetrafluoroethylene (ePTFE) gas diffusion layers (GDL) which simplifies the handling of catalyst arrays compared to carbon fabrics and enables sufficient N₂ mass transport. We demonstrate the robustness of the HTP workflow by screening 528 bimetallic catalysts of composition AB (A,B = Ag, Al, Au, Co, Cu, Fe, Mn, Mo, Ni, Pd, Re, Ru, W) for NRR activity. None of the materials produced ammonia significantly over background level which emphasizes the difficulty of finding active electrocatalysts for NRR and narrows down the search space for future studies.

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Continued advancements in the electrochemical reduction of CO 2 (CO 2RR) have emphasized that reactivity,selectivity, and stability are not explicit material properties butcombined effects of the catalyst, double-layer, reaction environ-
ment, 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 CO2 RR (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 andalkaline 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 CO 2RR experiments for the first time.@en

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.@en

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 Hcell 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 Hcells to commercial device level can be avoided by testing catalysts under conditions close to those in commercial devices from the start.@en

Hydrogen permeable electrodes can be utilized for electrolytic ammonia synthesis from dinitrogen, water, and renewable electricity under ambient conditions, providing a promising route toward sustainable ammonia. The understanding of the interactions of adsorbing N and permeating H at the catalytic interface is a critical step toward the optimization of this NH3 synthesis process. In this study, we conducted a unique in situ near ambient pressure X-ray photoelectron spectroscopy experiment to investigate the solid-gas interface of a Ni hydrogen permeable electrode under conditions relevant for ammonia synthesis. Here, we show that the formation of a Ni oxide surface layer blocks the chemisorption of gaseous dinitrogen. However, the Ni 2p and O 1s XPS spectra reveal that electrochemically driven permeating atomic hydrogen effectively reduces the Ni surface at ambient temperature, while H2 does not. Nitrogen gas chemisorbs on the generated metallic sites, followed by hydrogenation via permeating H, as adsorbed N and NH3 are found on the Ni surface. Our findings suggest that the first hydrogenation step to NH and the NH3 desorption might be limiting under the operating conditions. The study was then extended to Fe and Ru surfaces. The formation of surface oxide and nitride species on iron blocks the H permeation and prevents the reaction to advance; while on ruthenium, the stronger Ru-N bond might favor the recombination of permeating hydrogen to H2 over the hydrogenation of adsorbed nitrogen. This work provides insightful results to aid the rational design of efficient electrolytic NH3 synthesis processes based on but not limited to hydrogen permeable electrodes.

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Direct electrolytic N2 reduction to ammonia (NH3) is a renewable alternative to the Haber-Bosch process. The activity and selectivity of electrocatalysts are evaluated by measuring the amount of NH3 in the electrolyte. Quantitative 1H nuclear magnetic resonance (qNMR) detection reduces the bench time to analyze samples of NH3 (present in the assay as NH4+) compared to conventional spectrophotometric methods. However, many groups do not have access to an NMR spectrometer with sufficiently high sensitivity. We report that by adding 1 mM paramagnetic Gd3+ ions to the NMR sample, the required analysis time can be reduced by an order of magnitude such that fast NH4+ detection becomes accessible with a standard NMR spectrometer. Accurate, internally calibrated quantification is possible over a wide pH range.

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