Activating of molecular Nitrogen is an extremely important process as it supplies in the form of fertilizer the nitrogen that it is a prerequisite for building all amino acids and nucleic acids essential for life. After brief review of the history of activating nitrogen[1] I shall concentrate on the ammonia synthesis and motivate why an alternative route to the current commercial Haber-Bosch could be attractive Recently we found that a method published by Tsuneto et al. 25 years ago worked and proved that by simultaneous depositing Li in an N2 atmosphere it was indeed possible to activate N2 to synthesize ammonia[2]. We have subsequently followed up on this process and a very simple model for the synthesis has been proposed. Based on this insight we devised experiments that significantly improved the Faradaic and energy efficiency[3]  by oscillating the potential. Further improvements have been gained by controlling the oxygen content[4]  and by synthesizing of high area electrodes[5] and design of the SEI layer leading to Faradaic efficiency of ~80% and current densities towards 1A/cm2 [6]. Despite excellent recent progress there are still substantial outstanding problems. All data so far were relying on using the electrolyte as a sacrificial proton conductor. This issue was solved in a fuel cell type setup, but now we are back to 6 mA/cm2 and 60 % Faradaic efficiency on the other hand we can now talk about energy efficiencies of some 15% [7]. Further progress is under way and will be discussed

Over only few years of intense research, the lithium redox-mediated nitrogen reduction reaction (NRR) has proven to enable electrosynthesis of ammonia at practical yield rates and faradaic efficiencies approaching 100 %. Today, it remains the one and only practical approach towards sustainable electrosynthesis of ammonia. However, further significant research and development efforts are required to translate the redox-mediated NRR into a technology of applied significance, in the first place by addressing the two current major challenges: (i) insufficient stability in operation, and (ii) energy efficiency that is thermodynamically limited to approximately 33% (under standard conditions).

Both of these limitations stem from the need for the operation under strongly reductive conditions defined by the reversible potential of the Li⁺ + N₂ / Li₃N redox couple. Under unoptimised conditions, continuous and uncontrollable degradation of the electrolyte solution components results in progressive poisoning of the N₂-reducing cathode surface, rapid loss in the performance, and impossibility of passing charge more than few hundred C cm^-2 in typically reported experiments. Our research aims to resolve this through understanding the mechanism of operation of the NRR cathodes and design of the electrolyte solutions that prevent reductive degradation pathways while sustaining high rates and faradaic efficiencies of the ammonia production.

The talk will present some of our recent findings in this domain, in particular outcomes of the studies exploring the effects of the chemical nature of the electrolyte, proton carriers and mass-transport on the NRR performance. Feasibility of increasing the energy efficiency of the process will be briefly highlighted.

Electrochemical reduction of NO_x, CO₂, N_2, and combinations hold the promise to be a cornerstone for the sustainable production of fuels and chemicals. Importantly, all reactions share a direct competition with hydrogen, and furthermore, several products are formed from each reactant of these reactants.

For electrochemical N₂ reduction to ammonia (NH₃) the interest at ambient conditions is burgeoning [1-3]. Most interesting for the direct electrochemical N₂ reduction in aqueous, there is no working catalyst similar to “copper” for CO₂ reduction catalyst [4-6]. Instead, the electrochemical reaction is today limited to the non-aqueous lithium-mediated system.

In this talk, I will give a unified approach to these reduction reactions versus hydrogen in aqueous and discuss what it takes to reduce nitrogen electrochemically.

[1] Andersen, Čolić, Yang, Schwalbe, Nielander, McEnaney, Enemark-Rasmussen, Baker, Singh, Rohr, Statt, Blair, Mezzavilla, Kibsgaard, Vesborg, Cargnello, Bent, Jaramillo, Stephens, Nørskov, Chorkendorff, Nature, 2019, 570, 504-508.

[2] H.-L. Du, M. Chatti, R. Y. Hodgetts, P. V. Cherepanov, C. K. Nguyen, K. Matuszek, D- R. MacFarlane, A. N. Simonov. Nature, 2022, 609, 722–727

[3] M. Spry, O. Westhead, R. Tort, B. Moss, Y. Katayama, M.-M. Titirici, I. E. L. Stephens, and A. Bagger, ACS Energy Letters, 2023, 8 (2), 1230-1235

[4] Y. Hori et. al., Journal of the Chemical Society, Faraday Transactions, 1989, 85, 2309-2326.

[5] A. Bagger, W. Ju, AS Varela, P. Strasser, J. Rossmeisl., ChemPhysChem, 2017, 18, 3266–3273.

[6] A. Bagger, H. Wan, I.E.L. Stephens, J. Rossmeisl, ACS Catalysis, 2021, 11 (11), 6596-6601

While the fixation of dinitrogen to ammonia at ambient conditions has undergone remarkable advances in recent years, the direct use of dinitrogen for the synthesis of other nitrogenous products, such as amines, nitriles or nitro compounds, is a challenging task. Furthemore, catalytic protocols remain unknown.

The reductive splitting of dinitrogen into transition metal nitride complexes represents a potential entry towards N₂ fixation beyond ammonia. Over the past years, we have examined the mechanisms of chemically, photochemically, and electrochemically driven reductive N₂ splitting by group 6 and 7 metal complexes as a path for the direct conversion of N₂ into several products beyond ammonia, such as nitriles, amides and N₂O. In this contribution, the reductive splitting of dinitrogen will be presented. Electro- and photochemically driven pathways are discussed that are based on detailed mechanistic studies and electronic structure/reactivity relationships of key intermediates. Synthetic strategies for selectivity control that are guided by N–H proton-coupled electron transfer thermochemistry of intermediates are presented.

Energy-intensive thermochemical processes within chemical manufacturing are a major contributor to global CO₂ emissions. With the increasing push for sustainability, the scientific community is striving to develop renewable energy-powered electrochemical technologies in lieu of CO₂-emitting fossil-fuel-driven methods. However, to fully electrify chemical manufacturing, it is imperative to expand the scope of electrosynthetic technologies, particularly through the innovation of reactions involving nitrogen-based reactants as products from water/CO₂ electrolysis do not cover the full scope of industrial needs.

To this end, this talk focuses on my lab’s efforts nitrogen fixation to ammonia through the use of innovative electrochemical interfaces. I will proceed to detail new efforts in nitrogen fixation through the co-electrolysis of CO₂ with N-reactants (N₂, NO₃^2-…) in generating products with C-N bonds like amides, urea that are important as commodity and fine chemicals in the chemical industry.  In particular, I will discuss several new reaction pathways discovered towards C-N products and the application of operando techniques to understand the key C-N coupling steps in the reaction process.

Nitrogen reduction to ammonia stands amongst the hardest processes to decarbonise, with the Haber-Bosch process dominating the market. In that regard, electrochemical ammonia synthesis can meet this unmet need, towards sustainable and decentralised production of fertilisers and carbon-neutral fuel.

So far, the lithium-mediated synthesis is the sole process to unambiguously generate ammonia from nitrogen on a solid electrode.^1,2 Tremendous progress has been made in just a few years, both in terms of the underpinning fundamentals,^3,4 and in terms of device engineering.^5,6 However, all these improvements are burdened with the need to operate at lithium plating potential. This results in enormous overpotentials (>3.2 V) and energy losses (>70% due to lithium plating).^7,8 A solution is to identify new catalytic systems with lower intrinsic overpotential, which is highly challenging.⁹

In this talk, I will walk us through the periodic table of elements, bringing together physical descriptors obtained from theoretical calculations and literature meta-analysis. This exploration will aim at unifying the comparison of electrochemical systems for nitrogen reduction to ammonia, and pinpoint candidate elements which I will investigate experimentally. Electrochemical testing, ex situ / operando characterisation and model experiments will be performed to explore promising catalytic systems. Learning from those, I will provide fundamental explanations to the unique features of lithium and suggest pathways to breach the bottleneck of the lithium chemistry, opening the discussion for more energy efficient and sustainable chemistries.

Ammonia (NH₃), being the basis of all nitrogen fertilizers, constitutes one of the pillars of modern society, without which nearly half of the population would not be present on Earth.¹ Furthermore, thanks to its properties (high energy content and density), it is recently emerging as a backbone fuel towards decarbonisation.² Up-to-date, NH₃ is mainly produced via the energy-intensive Haber-Bosch process and, to meet the future NH₃ demand, new green synthesis techniques need to be developed.³

To this end, electrochemical nitrogen and nitrate reduction reactions (E-NRR and E-NO₃RR) have received considerable attention since, among other advantages, they permit the utilization of electricity from renewables. Also, taking into account that molybdenum and sulphur play key roles in nitrogenase-based nitrogen fixation, molybdenum disulphide (MoS₂) is expected to be active toward E-NRR, even if it has already demonstrated good catalytic activity toward hydrogen evolution reaction (HER, the main competitor of E-NRR).⁴

In this work, MoS₂ has been tested for E-NRR and E-NO₃RR in combination with different formulated electrolytes (LiSO₄ and K₂SO₄ at different concentrations and pH values). A flow cell with a gas diffusion electrode, on which the catalyst is immobilized through air-brushing technique, has been used to perform all the experiments.  Regarding E-NRR, encouraging results were obtained when employing Li⁺ as an additive in the electrolyte, leading to Faradaic efficiencies between 5 and 10%, as well as 85-301 μmol g^-1 h^-1 yield at -0.6 V vs. RHE. Further, since NH₃ was detected not only in the cathodic site, but also in the anodic one, as well as absorbed within the cationic membrane, the interactions between the Nafion membrane and NH₃ were also studied, observing that NH₃ adsorption decreased when the size of the cation raised (around 60% of NH₃ was absorbed when employing Li₂SO₄, ca. 20% when employing K₂SO₄).

Finally, in the case of NO₃RR, design of the experiment and surface response methodology (DoE/RSM) were employed to gain further insight onto the influence of operational conditions (potential, catalyst loading and salt concentration in the electrolyte) on the Faradaic efficiency and NH₃ yield of the NO₃RR, as well as the possible interaction between those parameters.

The Haber-Bosch (HB) reaction is definitely one of the most important reaction of the 20th century with tremendous impact on significant, for mankind, sectors, such as fertilizers, chemical industry, medicine, bio-fuels and ammonia production [1]. It is now well accepted that the relative activity of metallic HB catalysts can be correlated to their binding energies with N-containing species in terms of a volcano-shaped relationship. Metals that bind nitrogen too strongly or too weakly are on either side of the volcano [2]. Thus, it is important to devise a knowledge-based catalyst design that could potentially replace the Haber-Bosch method, while achieving high efficiency and energy saving towards sustainable NH3 production under ambient conditions. In this regard, herein we present two approaches for understanding the N2 acivation and conversion: (a) density functional theory (DFT) calculations are used in order to screen the N2 activation capacity and the accompanied electronic interactions for a portfolio of bimetallic alloys based on Mo, Ru and Fe metals, while using those metals as reference ones. (b) the role of defects in a new family of ceria-based oxide catalysts is explored from both experimental and computational point of view. These new type of oxide systems quickly enhance the ability to form and eliminate oxygen through redox reactions. 

Over the last decade there has been a growing interest in generating ammonia from nitrate with the aim of reducing eutrophication and acidication of water sources that has been exacerbated by anthropogenic activities.[1] In nature, the reduction of nitrate and nitrite ions is governed by Mo- and Fe-containing enzymes as a part of the nitrogen cycle. The key steps in the reduction of nitrate to nitrite are driven by nitrate reductase (Nars) enzymes involving the parent Mo(VI) oxo active site.[2] Subsequently, reduction of the nitrite to ammonium can be catalyzed by cytochrome c nitrite reductase (NrfA) enzymes, which features an Fe heme active site.[3] Inspired by this process, in the present work we introduce an Fe-substituted, two-dimensional molybdenum carbide of the MXene family (Mo₂CT_x:Fe, where T_x = O, OH, or F surface termination groups) for the electrochemical reduction of nitrate to ammonia.

Recent studies have demonstrated that MXenes efficiently enable various electrocatalytic reactions due to a high degree of control over the structure of their surface sites.[4] In this case, Mo₂CT_x:Fe contains isolated Fe sites in Mo positions of the host MXene (Mo₂CT_x), and as discussed above, the combination of these metals is known to facilitate the nitrate reduction reaction in naturally occurring reductase enzymes. Mo₂CT_x:Fe is shown here to outperform monometallic Mo₂CT_x for the electrochemical reduction of nitrate in both acidic and neutral electrolytes. Close examination of the electronic and local structure via operando X-ray absorption spectroscopy (Mo K-edge) reveals that substitution of Mo with Fe in the Mo₂CT_x lattice facilitates the defunctionalization of surface T_x groups upon reduction. This is shown to generate T_x vacancies, which then bind nitrate ions that subsequently fill the vacancies with O* via oxygen transfer. In accordance with experimental observations, density functional theory calculations provide further evidence that Fe sites promote the formation of T_x vacancies, which are thus identified as active sites and function in NO₃RR in a close analogy to the mechanism of the natural Mo-based nitrate reductase enzymes.

The intermittent availability of renewable energy poses a significant challenge that must be overcome to fully harness its potential. One promising solution is the storage of excess renewable energy in the form of carbon-free chemical fuels like hydrogen (H₂) and ammonia (NH₃). These energy-dense alternatives provide the means to transport renewable energy independently, without relying on traditional power grids. However, the current production of NH₃ primarily relies on the costly Haber-Bosch process, limiting its use as a chemical fuel. To address this limitation, there is a need for a sustainable production method, such as the electrochemical reduction of nitrogen gas (N₂) into NH₃. Among various electrochemical processes, the lithium-mediated nitrogen reduction reaction (Li-NRR) currently stands out as the sole method capable of achieving this conversion.¹

At Monash University, we have made significant advancements in Li-NRR, achieving close to 100% faradaic efficiency for NH₃ selectivity.^2,3 However, our focus now lies on reducing the energy demands and enhancing the overall sustainability of the Li-NRR process. This calls for the intelligent design of catalysts and electrolytes, which can be facilitated by expanding our knowledge of the solid electrolyte interface (SEI) which forms atop the reactive lithium deposit. Similar SEI layers have been extensively demonstrated to play a critical role in enhancing cell efficiency and lifespan in lithium-ion batteries. However, in the context of the lithium-mediated nitrogen reduction reaction (Li-NRR) system, the SEI may also hold the key to achieving NH₃ selectivity and preventing the undesired hydrogen evolution reaction.

This presentation covers the preliminary efforts of our ongoing work using in situ neutron reflectometry to gain insight into the Li-NRR mechanism occurring at the electrode interface and characterize the SEI environment that contributes to achieving exceptional faradaic efficiency.

Using NR, we have been able to observe dynamic surface changes in the scattering length density (SLD) profile of the catalytic deposit during Li-NRR, operating at a modest 10% faradaic efficiency, limited by the experimental constraints of the NR cell. To further deepen our understanding, we are utilizing operando galvanostatic electrochemical impedance spectroscopy to establish correlations between these surface changes and the electrochemical response of the cell. This multidimensional approach allows us to unlock crucial insights into the intricate processes taking place at the electrode interface, paving the way for more efficient and sustainable ammonia production through Li-mediated electrochemical reduction of nitrogen gas.

With 170 MT produced in 2020, ammonia is one of the most produced chemicals in the world [1]. However, it is currently produced using the Haber-Bosch process which contributes >1 % of global CO2 emissions. Continuous electrochemical nitrogen Reduction Reaction may not be able to completely replace the Haber-Bosch process but it has numerous advantages which may secure its place in green ammonia production. Continuous electrochemical nitrogen reduction reaction can be done at ambient pressures and temperatures, run directly from (renewable) electricity and enables ammonia production in smaller decentralised facilities. However, research into this field is still relatively new, performance is unoptimised and many important scientific questions remain unanswered. In particular, efficiency and current density are too low for commercial application and the key mechanisms by which the reaction occurs are still unknown. Recently, there have been significant advances which have focused on electrolyte optimisation [2-3] and cell design [4]. 

Optimising performance through the use of dynamic operating conditions is attractive because it can be done at little extra cost while achieving substantial improvements. Dynamic operating conditions have already been demonstrated to increase activity and selectivity towards desired products in the electrochemical CO2 Reduction Reaction [5-6] and increase electrode stability in electrochemical  [7]. 

In this work, dynamic operating conditions are investigated in the continuous electrochemical nitrogen reduction system using Electrochemistry-Mass Spectrometry, a technique which allows subsecond detection of gases and volatile species in real-time. This work builds on Krempl et al.’s work by improving detection times reducing noise which allows dynamic conditions to be investigated [8]. By monitoring H2 evolution (a key parasitic product) at different voltages, the point at which it is minimised was found. However, operating at this new optimal led to the accumulation of a reaction intermediate (lithium) which was not reacting quickly enough to form ammonia. Dynamic operating conditions were utilised to alternate between this H2 evolution minimum and an ammonia forming maximum. With this combination, ammonia selectivity was found to increase substantially from 7.9 % to 18 %. 

Lithium-mediated nitrogen reduction reaction (Li-NRR) is recognised as a high rate and selectivity pathway for green ammonia synthesis.^[1] However, practical implementation requires further advancements in yield rate and stability.^[1-3] Our previous study established a stable phosphonium cation/ylide proton shuttling cycle in a LiBF₄ electrolyte, achieving a faradaic efficiency (FE) of 69 ± 1%.^[4] Recently, the phosphonium-based proton shuttle in our robust high-concentration LiNTf₂ electrolyte outperformed LiBF₄, with doubled yield rate and improved FE of 75 ± 4%.^[5],[6] Despite the performance of the ethanol proton shuttle is still better, which is near 100 % FE and ca. 0.5 μmol s^-1 cm^-2 yield rate,^[5] the phosphonium cation is beneficial over the ethanol for no side products through tetrahydrofuranylation.^[6] This motivated us to deeply investigate the paramteres affecting the phosphonium cation shuttle’s performance in the high-concentration LiNTf₂ electrolyte.

Herein, we explored the relationship between the mass-transport characteristics of the electrolyte solution and the Li-NRR performance with three smaller relatives of the originally studied phosphonium cation, [P_6,6,6,14]⁺, specifically  [P_1,2,2,2]⁺, [P_1,4,4,4]⁺, [P_4,4,4,8]⁺ where the subscripted numbers represent the number of carbons in each of the four alkyl chains. An increase in the phosphonium alkyl chain length results in a noticeable decrease in the ionic conductivity and increase in the viscosity of the electrolyte solution. Furthermore, the diffusion coefficient of phosphonium cations measured by pulsed field gradient NMR techniques decreases as the alkyl chain length increases, with [P_6,6,6,14]⁺ having half the diffusion coefficient (0.93 × 10^-12 m² s^-1) of [P_1,2,2,2]⁺. The Li⁺ diffusion coefficient follows a similar, but less pronounced trend as it is moderately reduced from 0.74 × 10^-12 m² s^-1 in the [P_1,2,2,2]⁺ containing solution to 0.67 × 10^-12 m² s^-1 when using [P_6,6,6,14]⁺ shuttle. Interestingly, the type of phosphonium cations has minimal impact on the NTf₂^- diffusion coefficient. Based on these transport behaviours, one might expect that the longer phosphonium alkyl chain would adversely affect the Li-NRR performance because of the lower lithium diffusion. However, contrary to this expectation, the ammonia yield rate and the FE of the process increases steadly in the order [P_1,2,2,2]⁺ < [P_1,4,4,4]⁺ < [P_4,4,4,8]⁺ < [P_6,6,6,14]⁺. This suggests that the size of phosphonium cation directly affects a delicate balance of transport rates of the Li⁺, the proton shuttle and the N₂ transport at the electrolyte|electrode interface. Moreover, differential capacitance analysis demonstrated that the longer phosphonium alkyl chain, the more stable and compact ionic layer on the electrode surface. This well-structured ionic assembly acts as a protective layer that defines the structure of the solid electrolyte interphase and controls the Li⁺ electroreduction.

Optimising the Li-NRR performance with the best-performing [P_6,6,6,14]⁺ shuttle requires a decrease in the LiNTf₂ concentration from 2 to 1.5 M, which leads to an increase in the Li⁺ diffusion coefficient equal to that of the [P_1,2,2,2]⁺ system. Under this condition, the process is remarkably stable over 68 h with an invariable 80 ± 1% FE and an average yield rate of 300 ± 10 μmol s^-1 cm^-2.

Our study emphasises the complexity of the Li-NRR relationship to the mass-transport processes through correlations with bulk electrolyte properties, and to the ordering of the ionic species at the electrode surface. To achieve a 100 % FE target  with the robust phosphonium cation proton shuttles, future investigations will utilise hydrodynamic and computational modeling techniques.

Electrocatalytic NO_x reduction is a promising way to help restore the natural balance of the nitrogen cycle [1]. Various works agree that nitrate reduction to NH₄⁺/NH₃ involves NO as an intermediate, and NO hydrogenation is the potential-limiting step of NO reduction. However, debates on whether *NO hydrogenation produces *NHO or *NOH hamper catalyst optimization and rational design for NO_x electroreduction [2,3].

In this contribution, I will provide a simple classification of late transition metals for *NO hydrogenation: Cu, Ag, and Au (group 11 of the periodic table) primarily yield *NHO; Ni, Pd, and Pt (group 10) prefer to form *NOH or a combination of *NOH and *NHO. In turn, Co, Rh, and Ir (group 9) predominantly produced *NHO, particularly on adsorption sites with undercoordinated and/or square symmetry. Additionally, Cu and other group 11 elements prove very active for *NO hydrogenation and NO reduction to NH₄⁺/NH₃, while the (100) terraces of various transition metals should display reasonable activities. Statistical analysis enabled qualitative and quantitative conclusions through the use of “catalytic matrices”. These matrices identify that active catalysts for *NO electroreduction statistically favor *NHO over *NOH and possess undercoordinated sites [4].

To conclude, the matrices help uncover that highly active materials for NO reduction should typically contain undercoordinated Cu sites and be mediated by *NHO. The power of the matrices lies in their ability to rapidly extract information and use it to identify active sites with enhanced NO_x electroreduction.