Ammonia is not only an essential component in fertilisers for agriculture but is also emerging as a promising carbon-free fuel and energy carrier for the future. However, its production remains heavily reliant on the Haber-Bosch process, which is environmentally damaging and only works well in large scale centralsied facilities. Despite extensive research, a viable alternative process has yet to be realised. An electrochemical approach, operating under ambient conditions, holds great promise by enabling decentralised, on-demand ammonia production.

To date, among solid electrodes, only lithium- and calcium-based systems in organic electrolytes have been unequivocally demonstrated to reduce nitrogen to ammonia. However, the current performance of these systems leaves considerable room for improvement.

To address this challenge, our research employs a multidisciplinary approach combining electrochemical methods, cryo-electron microscopy, infrared spectroscopy, electrochemical mass spectrometry, time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). We aim to construct a comprehensive understanding of the reaction mechanisms and factors influencing electrochemical nitrogen reduction.

We focus on elucidating the roles of various components in the process, including the choice of cations, salts, electrolytes, and proton donors. Building on this understanding, we propose pathways to achieve higher reaction rates and improved efficiencies for nitrogen reduction, paving the way for scalable and environmentally friendly ammonia production.

Ammonia is a key fertilizer ingredient and, if synthesized in a sustainable way, a carbon-free alternative to liquid fuel. An alluring prospect is the production of ammonia via the electrochemical reduction of atmospheric nitrogen and water, driven by renewable electricity. However, a catalyst that facilitates the formation of ammonia while simultaneously hindering the parasitic side reaction of hydrogen evolution in aqueous electrolytes, has yet to be identified. Moreover, the persistent disagreement between theoretical predictions and experimental results hinders the further development of the field. Herein, we scrutinize the methodology and assumptions used in the bulk of theoretical studies on nitrogen reduction, suggesting improvements where applicable.

Over the last decade we have been searching, using density functional theory (DFT) calculations, for alternative materials that can catalyze nitrogen reduction reaction (NRR) while suppressing hydrogen evolution reaction (HER). The class of materials we have investigated are transition metal ceramics of e.g. nitrides, oxides, sulfides, carbides, oxynitrides and carbonitrides. Several promising candidates are predicted within each class of materials and we have tested several of them experimentally. There, we grow the catalysts in thin-films using magnetron sputtering, which are then tested in a micro reactor for electrocatalytic performance. The electrochemical micro reactor is connected in-line with the ammonia detection unit, preventing any possible contamination which makes the results reliable and robust. Experiments are done both in N2 saturated electrolyte and in Ar saturated electrolyte and isotope labelled 15N2 is used to proof catalysis. In this presentation, we discuss both the theoretical predictions and the experimental performance of several candidates for NRR. Finally, we explore the extent to which deep neural networks can be used to aid in the search for a novel catalyst, highlighting the pitfalls that must be avoided.

Ammonium nitrate is a key chemical feedstock which is produced by two energy intense processes, that is nitrogen reduction (Haber-Bosch Process) and catalytic ammonia oxidation (Ostwald Process). To-date, in industry, both processes use fossil fuels as energy input and raw materials. In this presentation, we propose the development of (photo-)electrochemically driven reactors capable of coupling the nitrogen-to-ammonium reduction reaction (NRR) and the nitrogen-to nitrate oxidation reaction (NOR), leading to autonomous flow photoreactors for ammonium nitrate production from nitrogen, water, sunlight, and sustainable electricity.

Specifically, we weill report on the development of noble-metal-free single atom catalysts (SACs, 1 metal reaction center) and single-site catalysts (SSCs 2 metal reaction centers) anchored to electrically conductive carbon supports.[1,2] Variation of the SAC/SSC metal sites will allow control of NRR/NOR performance, while metal stabilization in a well-defined all-oxo coordination environment will allow us to tune reactivity. Deposition of these molecular precursors on high-porosity is used to facilitate stable mechanical and electrical linkage between catalyst and electrode. In situ/operando (photo-)electrochemical studies and theoretical modelling will be reported to gain insights into the system performance, selectivity and stability. This will provide insights from the atomic to the reactor-level on the catalytic performance, its limitations, and enable us to identify key optimization parameters.

The development of the Haber-Bosch process in the early 20th century paved the way for a significant increase in ammonia production, primarily for agricultural and industrial applications. The traditional Haber-Bosch process operates under harsh conditions (high temperature and pressure) with the use of Ru- or Fe-containing catalysts [1]. Despite of the high importance of this process, its harmful impact on the environment requires to find greener alternatives.

The 2D-structures, particularly graphene-based derivatives, attract much attention in the electrochemical catalysis. It was demonstrated that nitrogen-doped graphene C₂N materials exhibits remarkable catalytic activity for N₂ reduction to NH₃ under ambient conditions attributed to their high surface area and tunable electronic structure [2]. Additionally, the presence of the nitrogen-containing functional groups in C₂N materials may serve as active sites for catalyzing nitrogen reduction and hydrogenation reactions [3]. Despite these advances, a deep theoretical understanding of the underlying catalytic mechanisms in C₂N for ammonia synthesis remains underdeveloped, highlighting the need for further exploration in this area.

This work aims to elucidate the most favorable reaction pathway for NH₃ synthesis from N₂ on C₂N catalysts using quantum-chemical calculations. Both pristine and defective C₂N structures are investigated as isolated molecule model systems (in both vacuum and solvent environments) and as extended materials using periodic boundary conditions. We have analyzed associative distal and associative alternative pathways with the different adsorption sides of N₂ molecule with the implementation of computational hydrogen electrode schema [4]. Our findings indicate that the distal reaction pathway is energetically favorable on defected C₂N, requiring only an applied potential of < 1 eV. We believe that our results provide valuable insights into the catalytic mechanisms of C₂N and contribute to the development of efficient and sustainable strategies for ammonia synthesis.

The electrochemical conversion of nitrogen (N₂) to ammonia (NH₃) represents a crucial advancement toward sustainable ammonia synthesis. With ammonia being essential for fertilizers and numerous industrial processes, the development of efficient and environmentally friendly production methods is of global importance. Traditional Haber-Bosch synthesis is energy-intensive and carbon-emitting, highlighting the need for green alternatives such as electrochemical methods.

Efficient nitroconversion requires a carefully optimized interplay between the electrocatalyst, electrode design, and electrochemical reactor configuration. This study introduces an innovative approach that integrates solid proton-conducting materials into a temperature-controlled electrochemical cell, designed to operate under moderate thermal conditions for enhanced reaction kinetics and stability. The cell features heated end plates for precise temperature regulation, supporting a variety of catalytic processes.

The modular design of the cell allows flexibility and adaptability across different configurations, enabling its use not only for electrochemical nitroconversion but also for other electrocatalysis applications involving gaseous substrates, such as CO₂ electrolysis. The cell can accommodate a range of membranes and solid electrolytes with varying thicknesses and can be operated in both catalyst-coated membrane (CCM) and catalyst-coated substrate (CCS) modes.

The performance of the cell was evaluated in both N2R and CO₂ reduction (CO2R) modes. This benchmarking approach was used to verify the functionality of the anode and assess overall cell performance. Ammonia formation was observed in small quantities during nitroconversion using metallic and organic catalyst systems, confirming the system’s capabilities. This work establishes a versatile platform for advancing N₂R research, with insights for optimizing cell design and operational strategies.

Electrocatalytic systems are essential for various renewable energy conversion and storage technologies, serving as a cornerstone for a sustainable future. Unlocking their full potential requires advancements in catalytic materials to enhance efficiency, stability, and cost-effectiveness. Achieving these goals demands an atomic-level understanding of electrocatalytic systems, particularly the complex interface between the electrocatalyst and electrolyte, which involves numerous interacting components and processes.

The properties of this interface can vary significantly depending on factors such as the solvent and electrode potential, and these variations can directly influence electrocatalytic behavior. Theoretical and computational methods play a pivotal role in unraveling these complexities, as they provide atomic-level insights into interface chemistry under realistic reaction conditions. However, further development of these methods is crucial to fully address the challenges.

Constant potentialm grand canonical ensemble (GCE) DFT calculations [1] offer a powerful framework for modeling electrochemical interfaces and reactions at the atomic level while maintaining fixed electrode potentials. In my presentation, I will discuss recent advancements in GCE-DFT [2], which extend its applicability to systems beyond the capabilities of standard approaches.

As an example, I will present our work on the electrocatalytic nitrogen reduction reaction (NRR) to ammonia using a graphene-based single-atom catalyst. This includes an exploration of the potential-dependent reaction thermodynamics and kinetics, as well as the critical role of explicit water molecules in the calculations [3]. The competition between the hydrogen evolution reaction (HER) and NRR is analyzed in detail, with a focus on the effects of co-adsorbates and innocent ligands.

Finally, I will outline the advantages and limitations of this method, comparing it with standard DFT calculations to highlight its potential and areas for further improvement.

Aerogels are a unique class of 3-dimensional porous materials with outstanding properties like ultra-low density, a high surface area, and excellent thermal and acoustic insulation. Over the last century, aerogels have evolved significantly, with advancements in synthesis, aging, drying, and post-synthesis treatment techniques, enabling the production of aerogels with various properties and from various materials, including silica, carbon, metal oxides, and (bio-)polymers.

Within this work we will present the fascinating properties and possible applications of various aerogels in aviation, transportation and energy. We will then lay a focus on the design of semiconducting aerogels to be used as photocatalysts. Their open porous structure of interconnected particles provides interesting advantages compared to nanoparticle-based systems, such as advanced charge carrier transport, larger surface area and not being prone to agglomeration.

We will present a novel sol-gel based synthesis routes for (semi-)crystalline Titania aerogels and show that depending on the synthesis procedure the material parameters such as surface area, the polymorph present in the sample and the degree of crystallinity can be controlled and tuned. We thereby provide an alternative synthesis route for highly porous, highly crystalline Titania samples with specific surface areas of up to 600 m² g^-1 can be obtained without calcination steps and thus obtain the open porous 3D structure.

The aerogel properties can be selectively controlled by the different synthesis parameters. Especially, the direct influence of acid concentration was investigated. Due to the dependence on the steric hindrance of the alkoxy ligands, the applied chloride ions coordinate in different ways to the used Titania precursor which changes the hydrolysis and condensation reaction kinetics. Also, the usage of different solvents leads to a ligand exchange of the Titania precursor which causes the formation of different intermediate complexes with coordinated chloride ions that act as templates for the formation of mixtures of anatase and brookite polymorphs with tunable ratios. Furthermore, the addition of denaturing agents typically used for lab-purpose ethanol significantly alters the material properties.

Ammonia, a widely produced chemical, contributes to more than 1% of fossil energy consumption and plays a significant role in global greenhouse gas emissions. Therefore, exploring alternative processes to the Haber-Bosch process is of considerable scientific interest.^[1] This contribution presents a two-reactor system designed for the systematic investigation of process parameters in photocatalytic ammonia synthesis, using TiO₂ aerogels. TiO₂ aerogels are able to store large amounts of photogenerated electrons in surface trap states upon illumination in water−methanol mixtures.^[2] The designed setup splits the processes of photocharging and nitrogen reduction into two different reactors. The first reactor is dedicated to photocharging TiO₂ aerogels, preparing it for N₂ reduction. In this reactor, TiO₂ aerogels are exposed to controlled UV-LED irradiation, focusing on maximizing the storage of photogenerated charge carriers within the TiO₂ structure. The second reactor is dedicated to reducing nitrogen with the electrons in the aerogel stored by utilizing Taylor-flow conditions for enhanced mass transfer and nitrogen reduction efficiency. This reactor is engineered to maximize the contact between nitrogen and the photocharged aerogels, ensuring a more efficient and controlled reduction of nitrogen to ammonia. This study includes characterization of the photocharging reactor and an analysis of the operational parameters of both reactors. The effects of photocharging conditions and flow rates on the overall efficiency of ammonia production were investigated. Concluding this contribution, the potential of this two-reactor system in ammonia production will be discussed. The separation of photocharging and nitrogen reduction processes, combined with the use of Taylor flow, presents a significant advancement in photocatalytic ammonia synthesis. This system offers a more flexible approach to ammonia production, and fundamental understanding of both photocharging and N₂ reduction process.

Quantification of ammonium at very low concentrations represents an enormous challenge and generally conventional methods are time-consuming. The indophenol method that utilizes salicylic acid, sodium hypochlorite and sodium nitroferricyanide, usually requires about 80 to 120 minutes per sample which is not only time-consuming but can be limiting for studies with a large number of samples. To minimize the effort of ammonia detection, an automated yet simple and cheap setup was established utilizing syringe pumps and a temperature-controlled reactor, finally providing the possibility for online monitoring of experiments.^[3] This setup will be presented as well.

Ammonia is the most important component in fertilizers, as 80% of the worldwide production goes into fertilizers. Currently, the energetically expensive Haber–Bosch process is used to break the triple bond of N₂. In the last years, the photoelectrochemical (PEC) reduction of nitrogen has gained attention, providing an alternative route to produce ammonia more affordable and sustainable. Despite beneficial properties of binary copper oxides as photocathodes for catalytic reactions such as the nitrogen reduction reaction (NRR), binary copper oxides show degradation processes, limiting their stability as electrode materials.[1,2]

To this end, the ternary copper oxides CuFeO2 and CuBi2O4 are investigated as materials for photocathodes as promising alternatives for the conversion of solar energy into chemical fuels.[3] In this work we performed Density Functional Theory calculations (DFT+U) to analyze their electronic properties, thermodynamic stability of different surfaces and adsorption energy trends for NRR intermediates. Beyond that, we investigated diverse defects in thermodynamically stable surfaces for both materials and their impact on the NRR mechanism. Moreover, the influence of an aqueous surrounding on the stability of the surfaces and reaction intermediates thereon will be investigated using the hybrid QMMM simulation approach (SAFIRES [4]) implemented in ASE and GPAW, which to this end will be extended to be compatible with periodic surface models.

Electrochemical and photoelectrochemical reduction of nitrogenous small molecules, including N₂ and NO₃^-, represent appealing routes for the production of NH₃ under ambient conditions. However, major challenges include the activation of highly stable reactants, avoidance of spurious artifacts introduced by contamination, control of product selectivity, and suppression of the competing H₂ evolution reaction (HER) in aqueous environments. In this work, we comparatively investigate the activities of Cu-based oxide semiconductors, CuO, Cu₂O, and CuBi₂O₄, for the photo-assisted N₂ and NO₃ reduction reactions (N₂RR and NO₃RR, respectively), taking advantage of their p-type characters, moderate bandgaps, and suitable band edge positions. The ternary compound CuBi₂O₄ is specifically selected for its improved photoelectrochemical stability compared to the binaries. The Cu-oxide electrodes were synthesized via electrodeposition methods and the photoelectrochemical investigations were carried out in alkaline aqueous electrolytes. Notably, all photoelectrochemical investigations were performed in an optimized workstation, where background contamination (NH₃, N₂H₄, and NO_x) are carefully evaluated and high purity is ensured. Although all three Cu-based semiconductors were found to be photoactive, we observed no production of NH₃ when tested for N₂RR. Moreover, during photoelectrochemical NO₃RR, Cu-oxide photocathodes showed major selectivity for NO₂^- rather than NH₃. Although the selectivity for NH₃ increased for Cu₂O at decreased positive potentials, NO₂^- remained the major product of this reaction. Furthermore, photocorrosion leads to a rapid loss of photoactivity for the Cu-oxide photoelectrodes. While CuBi₂O₄ showed improved stability, photoelectrochemical activity was found to be reduced. However, complementary electrochemical measurements of Cu single-atom catalysts hosted on carbon (Cu@C) show appreciable activity and stability for NO₃RR, yielding the two major products NH₃ and NO₂^-, fully suppressed HER, and an NH₃:NO₂^- ratio of 3:1 at an applied potential of -0.53 V vs. RHE. Overall, these results indicate that Cu-based oxides are poorly suited for NH₃ synthesis via N₂RR and NO₃RR under aqueous photoelectrochemical conditions but that Cu@C may provide promise for dark electrochemical NO₃RR under otherwise similar conditions.

Photoelectrochemical (PEC) energy storage is a promising technology for our sustainable future. Many photoelectrodes have been developed to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Optimizing photoelectrodes that are highly active and selective to the reactions is a challenging task, as well as ensuring their stability during operation in harsh PEC environments.

In this talk, we present an illuminated scanning flow cell (iSFC) setup that enables the performance of operando PEC characterization on the activity of photoelectrodes, and simultaneously on their stability against aqueous dissolution. Selected examples of the operando setup will be presented, for instance the photo-corrosion of BiVO₄ photoanodes [1], [2], and the photo-stability range of ZnFe₂O₄ photoanodes [3].

Then, studies on a promising photocathode material, CuBi₂O₄, will be presented. The photocathode has been investigated not only for HER, but also nitrogen reduction reaction (NRR) as a PEC route to fixate N₂. Dissolution of Bi has been observed in an alkaline environment, and the PEC corrosion was further revealed using scanning transmission electron microscopy (STEM).

The synthetic tunability of electronic structure and surface chemistry of semiconductor nanocrystals make them attractive light absorbers for light-driven chemistry. In this talk, I will describe our collaborative work on coupling nanocrystals with the MoFe protein of nitrogenase for light-driven N₂ reduction to ammonia.  The focus will be on our efforts to understand how nanocrystal properties determine the binding interactions with the enzyme, how electron transfer from the nanocrystals to the enzyme to drive ammonia formation, and how this platform can be used for studies of MoFe catalysis mechanisms.

We have functionalized nanocrystals to guide electrostatic attachment to the MoFe protein. Studies of experimental determinants of photochemical formation of H₂ and NH₃ suggest that the high excitation rates achieved with nanocrystals allow for accumulation of sufficient electrons on the enzyme to drive NH3 production. Transient absorption studies of electron transfer kinetics suggest that electron transfer efficiency is limited at single particle level by the competition between relatively slower electron transfer and relatively faster electron-hole recombination. At the ensemble level, electron transfer efficiency is also limited by the strength of the binding interactions. Microscale thermophoresis studies reveal that the nanocrystal-enzyme binding is highly sensitive to the nanocrystal dimensions. Light-driven injection of electrons into MoFe protein has enabled electron paramagnetic studies of the enzyme catalytic cycle in these systems and the key results of those studies will be highlighted.

Inspired by the active sites of the nitrate and nitrite reductase enzymes, we will introduce in this talk bio_‑inspired strategies to design heterogeneous catalysts for NO₃RR with enhanced selectivity, activity, and stability.

Thanks to the development of an Fe-substituted molybdenum carbide MXene catalyst, Mo₂CT_x:Fe, we demonstrated via a combination of in situ X-ray absorption spectroscopy and density functional theory calculations that the Fe sites facilitates the formation of oxygen vacancies, which served as active sites for nitrate reduction, mimicking the mechanism of enzymatic nitrate reductase.[1] Building on this result, which highlights the importance of oxygen vacancies to provide binding sites for nitrate, we developed a reduced molybdenum oxide shell grown on dendritic nickel foam (MoOx/NiNF).[2] This catalyst displays a substantial presence of oxygen vacancies, as confirmed by X-ray photoelectron spectroscopy and in situ Raman spectroscopy, and exhibits very high NO₃RR performance, achieving an FE of 99% and an ammonia yield rate of 4.29 mmol h^-1 cm^-2 at −1.0 V vs. RHE in neutral media, while maintaining operational stability for over 3100 hours at high current densities.

Finally, we will present novel strategies for ammonia recovery in nitrate and NO_x reduction, enhancing the efficiency and practicality of NO₃RR and paving the way for more scalable and applicable ammonia production methods.[3]

The Haber-Bosch (HB) process continues to dominate the ammonia (NH₃) production market despite its significant carbon footprint. This process heavily relies on the steam reforming of methane (CH₄) to generate hydrogen (H₂) and requires substantial energy to allow the reaction between nitrogen (N₂) and H₂. Additionally, HB plants are unevenly distributed globally, meaning the transportation of NH₃ and its derivatives to end users constitutes a considerable share of the overall environmental impact. [1]

In recent years, research has increasingly explored alternative methods for NH₃ production. One approach is the direct electrochemical reduction of N₂ (NRR) in aqueous electrolytes under ambient conditions, utilizing renewable energy. However, this method faces challenges such as low selectivity at high current densities and limited productivity. These issues stem from the high dissociation energy of the N₂ triple bond and the competing H₂ evolution reaction (HER). [2] Instead, the possibility to exploit NO₃⁻ present in groundwaters and wastewaters and electrochemically convert them into NH₃ results more feasible. [3] Indeed, this process has lower activation energy, which makes the reaction thermodynamically favoured compared to NRR. [4]

Our work aims to find the best operational parameters using a commercial MoS₂ catalyst deposited on a gas-diffusion electrode. The tests are carried out in a flow cell of 10 cm² geometrical area. Such a setup has the advantage of guaranteeing a better mass transport of the active species and of being scaled up. Design of experiments and surface response methodology (DoE/RSM) have been chosen to gain further insight into the influence of some operating conditions (i.e., potential, catalyst loading, and supporting salt concentration) on the Faradaic efficiency (FE) and NH₃ production rate. To assess wastewater treatment, it has been decided to maintain NO₃⁻ concentration at 500 mg L^­−1. The model suggests the presence of two optimal conditions: one for the FE (76.9%) at -1.2 V vs Ag/AgCl potential and K₂SO₄ 0.3 M as supporting salt and one for the NH₃ production rate at -1.6 V vs Ag/AgCl and K₂SO₄ 0.36 M (77.67 µg h^-1 cm⁻²). Catalyst loading did not show any effect on the system responses. Setup stability has been tested for over 100 h. Additionally, other parameters such as carbon-based electrode support, flow rate, cations in the electrolyte and type of binder have been explored, at the optimal conditions obtained in the DoE, to determine if further maximization of the response is possible. Moreover, the reaction mechanism is elucidated using pulsed chronoamperometry.

The growing risk of a potential energy crisis and escalating environmental challenges highlight the inminet need for research focused on alternative, sustainable energy sources capable of substituting fossil fuels. Ammonia (NH₃) has been extensively produced and utilised as a fertiliser for over a century. Currently, due to its significant hydrogen content (17.6% wt), high energy density (4.32 kW h L−1 for liquid NH₃), easy liquefaction for handling, storage and transportation, it is gaining momentum as a promising alternative renewable energy carrier and storage intermediate for global use in the future. [1-3] NH₃ is produced via the well-established Haber-Bosch process (HBP) [1], which is well known for its high energy demands that consume 1-2% of fossil fuels worldwide, contributing to the greenhouse effect by releasing the equivalent of ca. 2% of total CO₂ global emissions. [4-6] Therefore, the development of greener and more sustainable technologies to replace the century-old Haber-Bosch process is imperative. The electrochemical reduction reaction of nitrogenous spices (E-NRR) is a suitable technology widely recognised as an alternative option to the traditional HBP. E-NRR offers several key benefits:: (i) it is thermodynamically predicted to be more energy efficient than the HBP by 20%; (ii) it permits the elimination of fossil fuels as H₂ source; (iii) it can be integrated with renewables energy resources (e.g. solar panel, windmill, etc.); (4) it is scalable and can lead to on-demand & on-site NH₃ production. [4] Thus far, research on E-NRR has mostly focused on the electrocatalyst; however, it is well known that the electrolyte plays a crucial role, potentially having an impact of several orders of magnitude on the process outcome, particularly affecting selectivity and efficiency. In E-NRR the optimal electrolyte should enhance N₂ solubility, limit the H⁺ to minimize the parasitic hydrogen evolution reaction (HER), maintain the catalyst stability, and thus improve the Faradic efficiency (FE). The types of proton donors, solvents and additives, such as alkali metal ions, are critical in enhancing the selectivity of the E-NRR process.

In this context, the present work aims to explore the use of glycerol-water mixture in different proportions as a solvent in the electrolyte to study the potential beneficial effect of glycerol in terms of N₂ solubility increment and HER mitigation. Furthermore, it has been demonstrated that Nafion membranes, usually used as separators on E-NRR systems, represent an important source of NH₃ impurities, leading to false positives or overestimation of the NH₃ production. Taking that into account, a set of experiments has been performed to elucidate the membrane-ammonia interactions and quantify the impurities in our system.

To realize the electrocatalytic nitrogen reduction reaction (NRR) numerous material systems have been experimentally investigated but their NRR activity and selectivity are far from satisfactory and thus very challenging. There is potential to approach this challenge with catalyst nanoengineering including facet-controlled growth, vacancy engineering, heteroatom doping and the fabrication of composite catalyst structures.^[1] Gas phase synthesis routes like chemical vapor deposition (CVD) are promising as they allow the fabrication different materials including oxides, carbides, nitrides and sulfides on the nanoscale, while also offering the potential to tune the material properties by variation of the deposition parameters.

ZrN has been identified as a promising catalyst material for NRR,^[2] as it owns electronic properties comparable to noble metals, while being cheap and abundant.

Since theoretical investigations suggest the highest selectivity and stability under typical electrochemical conditions of the (100) facets of rock salt structured ZrN,^[3] we followed a metal organic chemical vapor deposition (MOCVD) approach to synthesize facet-controlled ZrN along the (100) plane. Through variation of the process parameters including the precursor choice, the substrate and the deposition temperature, crystalline ZrN (100) layers were grown on Si and glassy carbon substrates. Complementary analysis of the films by X-ray diffraction (XRD) , Rutherford backscattering spectrometry in combination with nuclear reaction analysis (RBS/NRA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to gain more insights into the catalyst material. Interestingly, ZrN rapidly oxidizes when exposed to ambient conditions and forms amorphous ZrO_xN_y species on the surface. This was further substantiated by ab initio molecular dynamics simulations (aiMD), while first proof of principle electrochemical experiments hint towards a potential activity of this material.^[4] While further electrochemical experiments need to be conducted in an attempt to prove the genuine NRR of this material, alternative composite catalyst materials based on the reported MoS₂/Ru^[5] structure are currently developed by MOCVD approach.

The direct electrochemical ammonia synthesis (EAS) from water and nitrogen using renewable energy shows great potential for future decentralized production of indispensable green ammonia [1]. However, identification of catalyst materials that genuinely catalyze the conversion of nitrogen to ammonia via the nitrogen reduction reaction (NRR) in aqueous electrolyte is the bottleneck for the successful development of EAS technology. To date, only Lithium-mediated NRR has been demonstrated to show sufficient production rate and faradaic efficiency for a prospective application [2], while the reaction in aqueous electrolyte, independent of the studied material class, suffers from false-positive results due to low productions rates, low Faradaic efficiencies and complex contamination issues [3].

Several catalyst systems are currently investigated for NRR in aqueous electrolytes. Transition metal nitrides (TMN) may offer an energetic advantage as electrocatalyst, because the NRR is expected to be catalyzed via the Mars-van-Krevelen mechanism (MvK) on TMNs. Here the protonation of a lattice nitrogen atom forms the first ammonia molecule. The resulting N-vacancy is filled by molecular nitrogen where one N adatom is subsequently protonated to form the second ammonia molecule. Thus, the N₂ activation step occurs at the N-vacancy site and avoids the surface adsorption and dissociation of N₂. Specific facets of Zr, Cr, V and Nb were described as stable, active and selective TMN catalysts for the NRR based on theoretical catalyst screening [4]. However, current literature studies state contradictory results and require the differentiation of genuine activity and non-catalytic decomposition of TMN catalysts [5], which is currently lacking.

In our work, we focus on the electrochemical evaluation of the NRR dependent on different electrode morphologies of Zr-based TMNs with trace analysis of ammonia established by ion chromatography technique [6]. Comparison of nanoparticulate ZrN catalysts [7] processed as gas diffusion electrode (GDE) to surface engineered model thin film catalyst [8] as well as insights from first principles simulation delivers a comprehensive analysis of material properties and experimental conditions for the electrosynthesis of ammonia. Detailed structural characterization after NRR experiments by x-ray photoelectron spectroscopy and dedicated chemical dissolution experiments elucidate the stability of the material as an important prerequisite of electrocatalytic NRR.

The conversion of dinitrogen into nitrous compounds like amino acids is essential for sustaining life on Earth. In addition to natural nitrogen fixation, industrial methods are indispensable to fulfil the immense global requirements, particularly for fertilizer manufacturing, mostly dependent on ammonia. Presently, ammonia synthesis relies on the energy-intensive Haber-Bosch process. In consideration of the current climate and energy challenges, investigating alternative methods for ammonia production is imperative. Our focus lies in investigating the photocatalytic reduction of nitrogen to ammonia using various bismuth oxyhalides as catalysts.

The main challenge for the photocatalytic nitrogen reduction is the low quantum yield due to the difficulty of breaking the very strong N-N triple bond. Enhancements in this process can be pursued through two approaches: identifying more suitable catalysts and refining the reaction conditions to promote the desired outcomes. Bismuth based catalysts showed the most promising results in the last years of research into photocatalytic ammonia generation. Especially bismuth oxyhalides (BiOX, X=Cl, Br, I) are an interesting material class due to their well-fitting band positions and high variability in composition, which allows to tune their reactivity. Therefore, different bismuth oxyhalides were synthesized, which we have been tested for reactivity, as well as ammonia yields. Preliminary findings have revealed that BiOBr demonstrated the highest ammonia yields. Consequently, we have shifted our focus towards the modification and enhancement of this catalyst.

Furthermore, we investigated the influence of temperature, pressure, and light intensity on the reaction. Temperature holds particular interest due to its well-documented enhancement of reaction kinetics, while pressure augments nitrogen availability, and light intensity accelerates the generation of electron-hole pairs. For the photoreactions, we employed closed vessels under a nitrogen atmosphere containing ultrapure water and the corresponding photocatalyst. To control temperature and light intensity, we positioned the samples in an oil bath and illuminated them with a high-power 365 nm LED. To assess ammonia yield, we utilized two distinct methods: an optical assay based on the common indophenol blue method, and gradient flow ion chromatography.

To define reaction parameters for the comparison, we will show results of using different temperatures, pressure, light intensity, and reaction time by using the example of titanium dioxide. Endowed with these defined parameters, we will then proceed to compare the performance of different bismuth oxyhalides alongside titanium dioxide.

Transition metal nitrides and oxynitrides were tested as electrochemical nitrogen reduction reaction (eNRR) catalysts for ammonia synthesis under ambient conditions. There have been several promising experiments on thin film electrodes in μ-reactor (Antec Scientific) (e.g.[1]). We have synthesized these catalysts as nanoparticles and incorporated to gas diffusion electrodes (GDE) to optimize the catalytic environment. One of the optimization steps needed is to understand the effect of pressure on the system. Gas pressure has been demonstrated to significantly affect the triple phase boundary and catalytic activity in CO₂ reduction and is therefore anticipated also to have an effect in NRR [2]^, [3]^, [4]^, [5]. To increase the pressure in the flow cell and study the effect of the gas pressure on eNRR we have constructed a system for adjustable backpressure. To this end, a pressure-relief valve is used at the gas outlet of the flowcell.  We will present studies focusing on the effect of the pressure increase on the catalytic behavior in a flow electrocell with gas diffusion electrodes for eNRR.

[1] F. Hanifpour, et al., Electrochimica Acta 2022, 403, 139551

[2] K. Hara et al., Journal of the Electrochemical Society 1995, 142, L57

[3] N. Sonoyama et al., Electrochemistry Communications 1999, 1, 213

[4] E.J. Dufek et al., Journal of The Electrochemical Society 2012, 159, F514

[5] B. Sahin, Energy Technology 2022, 10, 2200972

Ammonia, produced via the Haber-Bosch process for more than 150 10⁶ t per year, is responsible of around 1.5% of the global greenhouse gas emissions. [1] It is a fundamental building-block for fertilizers and could represent a future H₂ carrier, but it is still dependent from fossil fuels. To find a delocalized electrochemical process complementary to Haber-Bosch could be a key solution to move towards a renewable-driven NH₃ production.

Nowadays, the lithium-mediated pathway represents the most promising solution in the N₂ reduction reaction challenging field, achieving the highest Faradaic efficiency and NH₃ production rate. [2], [3] Different strategies are under evaluation in literature, all exploiting the ability of this metal to bind N₂ even in standard conditions. They could be divided into continuous or step-by-step strategies. In the first, Li⁺ ions from the aprotic electrolyte are electrodeposited on the cathode, where N₂ is reduced and protonated into NH_3, directly and in the same environment. [4] In the second case, the electroreduction of N₂ at the cathode has been proposed, and the formation of an intermediate product containing fixated nitrogen is the key step. Only in a second separate step, this intermediate should be protonated into NH₃. To achieve the first step of N₂ activation, the exploitation of a Li-N₂ galvanic cell, inspired by lithium-air batteries, has been proposed in literature to optimize the process efficiency and allow the direct protonation with H₂O [5].

In both cases, is crucial to study the phenomena at the cathodic electrode-electrolyte interphase, as the aprotic electrolyte inevitably reacts on the active sites, forming a solid electrolyte interphase. This component of the system determines both the selectivity towards NH₃ formation and the stability of the process. Due to the dynamicity of this layer, a critical eye should be adopted for a correct electrochemical characterization and NH₃ quantification of the systems. Our laboratory is currently addressing these challenges within the SuN₂rise project, applying also statistical methods as the design of experiment to optimize the studied factor with a restricted number of experiments.

An overview of electrochemical and engineering-related crucial aspects will be presented in this contribution. In particular, the challenges related to the NH₃ quantification in highly concentrated aprotic electrolyte will be discussed, as well as the reliability of electrochemical characterization aimed to prove the N₂ electrochemical reduction at the cathode. The advantages and issue related to the strategy based on the Li-N₂ cell concept will be critically addressed, highlighting the precautions needed to avoid misleading result in presence of a metallic lithium anode.

Multiphase electrochemical reactions at the gas/liquid/solid phase boundary are often limited by the low solubility of the gaseous component in the electrolyte, leading to slow reaction rates. A particularly striking example is the electrochemical synthesis of ammonia from atmospheric dinitrogen and a proton source, which occurs at 0.148 V vs. NHE and hence strongly competes with the hydrogen evolution reaction (HER) occurring at 0 V vs. NHE.[1] Since the kinetics of the nitrogen reduction reaction (NRR) are intrinsically sluggish as compared to that of the HER, high overpotentials must be applied to yield ammonia in measurable quantities so that achieving a selectivity towards NRR becomes a key challenge in the field.[2] On top, solubility of nitrogen in the commonly used aqueous electrolytes is low, while protons are ubiquitous, rendering the HER the dominating reaction in such systems. On the other hand, due to their unique structure, ionic liquids exhibit high gas solubilities, typically by an order of magnitude higher than water.[3] Hence, the utilization of composites between microporous carbons and room-temperature ionic liquids (RTILs) as electrocatalysts is an efficient way to tailor the electrode/electrolyte interface in a way that protons are prevented from accessing the electrode surface and at the same time increased amounts of nitrogen can accumulate near the catalytically active centers to enable electrochemical NRR.

This approach was performed using a literature-known iron oxide-based catalyst, deposited on activated ultramicroporous carbon via a wet impregnation technique and employing it on a gas-diffusion electrode cell. This setup ensures the direct contact between gaseous N₂ molecules, solid electrocatalyst and the protons in the electrolyte, creating an ideal environment for e-NRR under ambient conditions. Ion Chromatography was used to measure the amount of produced ammonia. The resulting NH₃ yield rate and Faradaic Efficiency (FE) confirm the compatibility and the synergetic effect of nanoporous AC and iron oxide towards e-NRR, paving the way for further investigation for example of the effects of pore size, hydrophilicity of the RTIL.

References:

[1] F. Qu et al., J. Mater. Chem. A, 2019, 7, 3531,

[2] J. K. Nørskov et al., ACS Catal. 2017, 7 (1), 706–709.

[3] D. R. MacFarlane et al., Energy Environ. Sci. 2017, 10 (12), 2516–2520.

Ammonia (NH₃) is an important chemical raw material, playing an integral role in the agricultural, industrial and pharmaceutical fields. Industrially, NH₃ is still produced using the conventional Haber-Bosch process (developed at the start of the 20^th century): N₂ is reduced to NH₃ by H₂ using a metal-based catalyst, temperatures between 400 and 500°C and pressures between 150-250 atm. These harsh conditions make the process energetically expensive and environmentally polluting: H₂ mainly comes from the reforming of hydrocarbons (usually supplied through natural gas), and it produces large emissions of CO₂.[1,2]

Considering the global energy crisis and the devastating effect large emissions of greenhouse gasses have on the environment, it is of utmost importance to replace this production method with one that is greener and more sustainable, in both materials and conditions. Among many catalytic methods, photocatalytic nitrogen fixation (PNF) has been considered one of the best alternative candidates, even if it is less efficient than other photocatalytic process, because it’s powered by solar energy and both nitrogen fixation reactions (NRR and NOR) are carried out in mild conditions. [1,2]

In the search of novel and efficient photocatalysts, we investigated the potential use of heterojunctions based on graphitic carbon nitride nanosheets (gC₃N₄) coupled with BiOX (X=Br and I). By mapping the photocatalytic behavior of the heterojunction as a function of the weight relation between the two compounds, we found that the composite with a ratio equal to gCN(90)-BiOBr(10) produces the higher yield of NH₃ (~19 μmol/g/h).

We decided to further investigate these types of heterojunctions by keeping similar ratios between the two semiconductors, but changing the components: in tandem, we mapped the behavior of the same heterojunctions, swapping gCN with rice-husk Biochar (rBC) to explore sustainable materials. In this case too, we found that the higher yield (~10 μmol/g/h) is produced by rBC(90)-BiOBr(10) and that the heterojunction maintains its structure after it has been used in the PNF reaction.

The catalytic activation of small molecules/ions such as N₂, CO₂ or nitrate on the surface of porous electrocatalysts at their interface with electrolytes belongs to the most fundamental phenomena on the way to a more sustainable and decentralized production of commodity chemicals such as ammonia or methanol. Nanoporous carbon materials play an ever increasing role in this regards because adsorption phenomena on the carbon surface are crucial for the working principles of the respective devices. It is well known that the adsorption properties of such materials are a function of their pore architecture. Pore size, pore geometry, pore connectivity, and pore hierarchy determine important factors like mass transport and the strength of interaction with different guest species. Another (and possibly even more powerful) “regulation screw” to control the adsorption properties of nanoporous carbon materials is their atomic construction. The controlled integration of heteroatoms (most often nonmetallic group III or group V and VI elements with nitrogen being the most widely studied heteroatom) into porous sp²-based carbon networks can significantly change their physicochemical properties [1]. This includes but is not limited to their acidity/basicity, oxidation resistance, electric conductivity, and surface polarity. In order to make use of these effects it is important that the heteroatoms are significant in number, that they are uniformly distributed over the bulk of the material, and that the local atomic construction motives are as defined as possible. The synthesis of nitrogen-rich carbon materials by controlled condensation of well-defined nitrogen-rich molecular precursors is a particularly elegant way to synthesize porous carbon materials with large concentrations and precisely incorporated heteroatoms [2].

My presentation will give an overview of current attempts to develop synthetic methods for the precise tailoring of the chemical architecture and pore structure of functional nanoporous carbon materials [2],[3]. Special focus will be on the fabrication of all-carbon hybrid materials which combine a rather heteroatom-rich carbon phase and a pristine porous carbon on the nanoscale to combine, for instance, a demanded chemical property with high electrical conductivity. The structure-property-relationships of these materials in the electrochemical conversion of N₂ and nitrate will also be presented [3],[4]. In addition, attempts to modify the adsorption properties of the carbon pores by immobilization of ionic liquids will be presented.

We recently summarized the strategies to improve the selectivity and faradaic efficiency of the photochemical and electrochemical reduction of nitrogen to ammonia (NRR).^[1] One of the possibilities is to follow a Mars-van-Krevelen mechanism (MvK) on transition metal nitrides (TMN).^[2] Moreover, TMNs possess a good chemical stability, high conductivity, flexible electronic structure and avoid the use of precious metals.^[3] Skúlason and co-workers have published theoretical, density functional theory (DFT) based catalyst screening of TMNs as novel catalysts for NRR; vanadium nitride (VN) was found among the most promising active, selective, and vacancy regenerating mononitrides; for VN films especially the (100) facet is stable.^[4] Also experimentally, VN has been reported to be an active electrochemical NRR catalyst.^[5] However, commercial VN suffers from severe instability, both chemical and electrochemical experiments lead to significant vanadium and nitrogen leaching.^[6] Thus, Pan and coworkers established an alternative NRR pathway over vanadium oxynitride; they proposed that mixed anion arrangement improve the adsorption of N₂, the stability of active surface states, and their activity and selectivity over the unwanted hydrogen evolution.^[7]

We have synthesized VN powders, VN nanotubes and VN sheets starting from various vanadium precursors (VOCl₃, V₂O₅, VO_x nanotubes, NH₄VO₃) and comparing different routes (microwave assisted hydrothermal, urea glass, carbothermal). Pure VN phases were obtained with the urea glass route operating at 800°C and the carbothermal route heated up to 1200°C. Best morphology stability of nanotubes and nanosheets was found with the urea glass route. Linear sweep analysis in N₂ atmosphere indicated NRR activity somewhat preferred over hydrogen evolution, most pronounced for VN nanotubes prepared by the urea glass route.

Ongoing work includes ammonolysis to obtain stable VN morphologies and to combine VN with titanium nitride (TiN) as a stabilizing substrate. Based on DFT calculations enhanced catalytic performance due to optimized proton adsorption was proposed for VN/TiN alloys.^[8] The synthesis of such alloys with TiN or other TMNs should be possible with slight modifications of our VN synthesis routes. Actual synthesis results and electrochemical NRR data employing indophenol test and ¹⁵N experiments will be presented in the talk.

The interest in covalent materials has been rising recently, and structures with specifically designed properties and chemical features have widened the application spectrum of this class of materials.^[1] Exemplarily, the
introduction of lightweight heteroatoms, such as nitrogen, into a sp2-bonded carbon expands the spectrum of possibilities and enables tailoring the electronic and optical properties of carbon materials. Among those, carbon nitride materials, a class of 2D covalent semiconductor with ideal formula C₃N₄, have recently attracted much attention especially in photocatalysis. However, up to now, their application as thin films was hindered due to the low homogeneity of the coatings available. Recently, we developed an innovative method to produce carbon nitride thin films with tunable thickness by means of chemical vapor deposition. The CVD method enables to deposit carbon nitride thin film with tunable thickness over large substrates and regardless their shape, from flat silicon wafers to fibers and even bulky irregular materials.The as prepared thin films are highly stable, homogeneous, and flat with a very high refractive index, even in the range of diamond.^[2,3] The high homogeneity and conformal deposition of the carbon nitride thin films prepared enabled to use them to develop innovative batch and microfluidic photoreactors, by coating the reactors’ walls, achieving high selectivity and conversion in shorter time, and enabling overcoming problems typically associated with the use of bulk materials.^[4,5] Furthermore, the utilization of carbon nitride thin films in photocatalysis enabled the development of in-operando spectroscopic techniques, such as atmospheric pressure-XPS and -XAS, combined with TOF-MS under controlled precursor vapor pressure and using a solar simulator as light stimulus. This revealed fundamental mechanistic insights and the critical role of surface  interactions in key reactions, such as water splitting. The utilization of covalent semiconductor thin films, such as carbon nitrides, is still in its infancy, however, it sets the premises for significant improvements in energy conversion, energy storage,  selective ion separation, and beyond.

Two principal pathways exist for the photocatalytic valorization of nitrogen. The reductive route targets the formation of ammonia and the replacement of the energy-intensive Haber-Bosch process. As an alternative, nitrogen can be oxidized to nitrous oxide and nitrate, as precursors to nitric acid. Since the traditional synthesis of nitric acid via the Ostwald process is based on ammonia as feedstock, photocatalytic nitric acid formation “out of thin air” is clearly also a more sustainable solution. It has already been demonstrated that titania is able to oxidize molecular nitrogen, but the yields of nitrate were low [1]. Since titania is known to allow much higher quantum efficiencies in other challenging photosynthetic reactions [2], we attempted to improve the catalytic functionality of titania for nitrogen and oxygen activation.

In previous works we have shown for the cases of methanol and isopropanol oxidation that well-known active sites from classical catalysis are capable to carry out the same reaction light induced [3,4]. Based on this concept, we modified TiO₂ with well-known nitrogen activation catalysts, based on Fe and Mo, or sites for oxygen activation, based on V and W. As a third category, based on the fundamental rule that catalysts always facilitate both forward and reverse reaction, active sites from DeNOx catalysis, based on Pd, were added to TiO₂. Using an extensive set of synthesis methods, such as grafting organometallic precursors or impregnation procedures, the size range of the active sites was varied, studying both single sites and nanoparticles.

Photocatalytic activity studies revealed that Pd, in particular, can improve significantly the formation of nitrogen oxides on TiO₂. Ongoing studies using in situ vibrational spectroscopy attempt to clarify the influence of the deposited active sites on the formation of reactive intermediates on the photocatalyst surface, to eventually understand the reaction pathway and to identify persistent kinetic barriers.

Gas-solid photocatalytic oxidation of dinitrogen to nitrogen oxides

Abdelkarim Zaim,^a Jonathan Z. Bloh^a

^a DECHEMA Research Institute, Frankfurt am Main, Germany

In recent years, there has been a notable increase in the demand for sustainable fertilizer production, driven primarily by rising energy costs and concerns about climate change. Additionally, as the global population continues to expand, there is a greater demand for agricultural commodities. While nitrogen is the most abundant element in the atmosphere, only a tiny fraction is fixed in the soil as bioavailable nitrogen, highlighting the necessity of nitrogen-based fertilizers in modern agriculture [1].

In the present era, nitric acid utilized in fertilizer manufacturing predominantly originates from the Ostwald process, wherein ammonia is oxidized. In this highly exothermic process, most of the energy contained in ammonia is converted to heat. The primary source of ammonia is derived from the Haber-Bosch process, which is responsible for approximately 1-2 percent of global CO₂ emissions and is regarded as one of the largest global energy consumers [2]. A more efficient solution to this problem is the direct oxidation of nitrogen to nitric acid, as this process does not require the enormous energy input associated with the formation of NH₃ (Δ_RH = 383 kJ mol^-1), but instead only necessitates the energy required for the formation of NO (Δ_RH = 91 kJ mol^-1) from the elements [3].

Photocatalysis represents a viable method for the direct generation of nitric acid from its elemental constituents. Under mild conditions, the process requires only light, water, and air. As a preliminary demonstration, the production of nitrates was shown in a gas phase reaction from water and air with the use of titanium dioxide (TiO₂) as a photocatalyst. It was determined that water is an essential component of the reaction, and the primary product is NO₂; this can be converted to nitric acid in water [4].

Although the mechanism of this reaction remains to be fully elucidated, we present an initial investigation of the influences of the reaction conditions on the kinetics and product composition. For example, the effects of the reactant ratio, temperature, and light intensity are considered, and all products of the reaction are analyzed both qualitatively and quantitatively. This contributes to a more comprehensive understanding of the mechanism and kinetics of the reaction.

In conclusion, a methodology for enhancing the photocatalytic oxidation of nitrogen will be presented, with the objective of achieving higher product concentrations and selectivities. This will contribute to a comprehensive understanding of the photocatalytic synthesis of nitric acid from dinitrogen.

Transition metal nitrides have been discussed and tested for the purpose as catalysts for electrochemical nitrogen reduction reaction and demonstrated to be promising candidates for this reaction in aqueous electrolytes and ambient environment [1]^, [2]. In order to take the step from the sub-optimal environment for eNRR using thin film catalysts on solid electrodes to a more optimal environment with gas diffusion electrodes (GDE) for enhanced triple phase boundary and using nanoparticles for increased surface area aspect ratio, we have synthesized the selected transition metal nitrides using the urea gel route [3]^, [4]^, [5].

A significant amount of carbon is added to the catalysts through the catalyst synthesis method and due to sintering in N₂(g) environment it may not all be removed as gaseous compounds. Therefore, the total catalyst (transition metal nitride) proportion in the powder needs to be known. We have used TGA-MS to identify the carbon and quantify the total metal proportion in the catalyst powder material. We have used XRD to analyze the crystal structure, SEM-EDS to analyse the particle morphology and C&N analysis to quantify N and C, this way allowing for a complete understanding of the powder chemical composition.