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Proton transfer is a key step of the lithium-mediated electrochemical reduction of nitrogen to ammonia. Herein, we investigate the impact of the chemical nature of the proton carrier on the performance and highlight shuttles that enable effective ammonia synthesis.

Shuttling protons in ammonia synthesis An electrochemical route to ammonia could substantially lower the greenhouse gas emissions associated with the current thermal Haber-Bosch process. One relatively promising option under study involves reductive formation of lithium nitride, which can be protonated to ammonia. However, the ethanol used to date as a local proton source in these studies may degrade under the reaction conditions. Suryanto et al. report the use of a tetraalkyl phosphonium salt in place of ethanol (see the Perspective by Westhead et al. ). This cation can stably undergo deprotonation–reprotonation cycles and, as an added benefit, it enhances the ionic conductivity of the medium. Science , abg2371, this issue p. 1187 ; see also abi8329, p. 1149

As research on sustainable ammonia synthesis via electrochemical and photochemical N2 reduction progresses to include a wider variety of aqueous and aprotic electrolytes, 1H NMR spectroscopy is inc...

In addition to its use in the fertilizer and chemical industries

Apart from its role as a main precursor of the synthetic fertiliser production, ammonia is likely to become an even more important chemical in the future due to its prospects as an energy carrier.(1) To meet the associated growing demand while meeting the CO2 emission reductions targets, recent efforts have aimed to translate the conventional polluting NH3 synthesis to low CO2 emissions technology by supplying the Haber-Bosch reactors with H2 derived from renewable-powered water electrolysis rather than steam methane reforming.(2) This solution will likely to be implemented in many NH3 synthesis plants soon, but the most preferred future technology for green ammonia synthesis is through a 100% renewable-powered, one step electrochemical process involving nitrogen reduction at the cathode and water oxidation at the anode. However, the possibility of direct dinitrogen reduction with traditional heterogeneous catalysts in both aqueous and aprotic electrolyte solutions at practical rates and faradaic efficiencies is yet to be proven,(3, 4) which calls for investigations of alternative pathways. One prominent possibility, with clear practical prospects, is the indirect lithium redox mediated process in organic electrolytes, which has been evidenced as genuine and by far the most efficient process to electrochemically convert N2 to NH3.(5) However, several key challenges, including insufficiently high yield rate and faradaic efficiency as well as instability of the system, are yet to be resolved before the lithium-mediated nitrogen reduction reaction (Li-NRR) can be considered a process of applied significance. Our first step towards this aim was to introduce a stable phosphonium cation/ylide proton shuttle that delivers protons to the cathode to support rapid and controllable conversion of lithium nitride into ammonia.(6) It was demonstrated that phosphonium cation provides favourable proton activity in the system, which enables high Li-NRR faradaic efficiency of 69 ± 1 %. Moreover, the phosphonium shuttle exhibits high stability under the reaction conditions, i.e. was genuinely able to cycle between anode and cathode without being consumed. Our further efforts have focussed on understanding the effects of the electrolyte-electrode environment on the Li-NRR kinetics. As a result of this investigation, a system configuration that enables the electroreduction of N2 to ammonia at a faradaic efficiency closely approaching 100% was discovered. Moreover, the synthesis can run in both uninterrupted and interrupted regimes on a timescale of days. The rate of ammonia electrosynthesis in the Li-NRR with the optimised electrode-electrolyte interface can achieve as high as ca 500 nmol s-1 cm-2 (per geometric electrode surface area) with electrodes of cm2 scale. Finally, we examined the degradation of the electrolyte solution components during the high-performance Li-NRR. It was found that the major source of these undesired processes are the anode processes rather than the cathode. This highlights the urgent need for the development of effective H2- or H2O-feed anodes that can be integrated with the lithium mediated process. This and other future challenges on our way towards achieving sustainable and stabile NH3 electrosynthesis system to support the development of the Ammonia Economy will be highlighted. D. R. MacFarlane, J. Choi, B. H. R. Suryanto, R. Jalili, M. Chatti, L. M. Azofra and A. N. Simonov, Adv. Mater., 32, 1904804 (2020). D. R. MacFarlane, P. V. Cherepanov, J. Choi, B. H. R. Suryanto, R. Y. Hodgetts, J. M. Bakker, F. M. Ferrero Vallana and A. N. Simonov, Joule, 4, 1186 (2020). H.-L. Du, T. R. Gengenbach, R. Hodgetts, D. R. MacFarlane and A. N. Simonov, ACS Sustainable Chem. Eng., 7, 6839 (2019). H.-L. Du, R. Y. Hodgetts, M. Chatti, C. K. Nguyen, D. R. Macfarlane and A. N. Simonov, J. Electrochem. Soc., 167, 146507 (2020). B. H. R. Suryanto, H.-L. Du, D. Wang, J. Chen, A. N. Simonov and D. R. MacFarlane, Nat. Catal., 2, 290 (2019). B. H. Suryanto, K. Matuszek, J. Choi, R. Y. Hodgetts, H.-L. Du, J. M. Bakker, C. S. Kang, P. V. Cherepanov, A. N. Simonov and D. R. MacFarlane, Science, 372, 1187 (2021).

The results presented herein unambiguously demonstrate that neither nanoparticulate carbon-supported gold, nor bismuth powder are active catalysts for the electrocatalytic dinitrogen reduction, but...

Ammonia is a widely produced chemical that is the basis of most fertilisers. However, it is currently derived from fossil fuels and there is an urgent need to develop sustainable approaches to its production. Ammonia is also being considered as a renewable energy carrier, allowing efficient storage and transportation of renewables. For these reasons, the electrochemical nitrogen reduction reaction (NRR) is currently being intensely investigated as the basis for future mass production of ammonia from renewables. This Perspective critiques current steps and miss-steps towards this important goal in terms of experimental methodology and catalyst selection, proposing a protocol for rigorous experimentation. We discuss the issue of catalyst selectivity and the approaches to promoting the NRR over H2 production. Finally, we translate these mechanistic discussions, and the key metrics being pursued in the field, into the bigger picture of ammonia production by other sustainable processes, discussing benchmarks by which NRR progress can be assessed. The electrochemical reduction of nitrogen is being intensely investigated as the basis for future ammonia production. This Perspective critiques current steps and missteps towards this goal in terms of experimental methodology and catalyst selection, proposing a protocol for rigorous experimentation.

Previous theoretical work has predicted vanadium and niobium nitrides to be catalytically active toward the electrochemical reduction of dinitrogen to ammonia and inactive for the hydrogen evolutio...

In this study, we synthesize a carbon-free anatase/bronze TiO2 microsphere (TiO2(A/B)-MS) via the solvothermal method and demonstrate its potential for use as a high-performance anode material for sodium-ion batteries. The highly compact structure of the microsphere constructed from nanoprimary particles not only enhances the structural stability and subsequently leads to good cycling performance but also enables the transport pathways for Na+ ions and electrons to be shortened, ensuring fast Na-storage performance. In addition, an anatase/bronze interfacial structure of the material further improves fast Na+-ion diffusion kinetics. Benefiting from these merits, the proposed TiO2(A/B)-MS demonstrates a high specific capacity of 221 mAh g–1 at 0.1 C, fast charge–discharge capability up to 50 C, and long-term cycling stability over 1000 cycles at 1 and 10 C without using a conductive carbon matrix. A combination of various experiments and theoretical studies is used to verify the outstanding Na-storage perf...

Ammonia is of emerging interest as a liquefied, renewable-energy-sourced energy carrier for global use in the future. Electrochemical reduction of N2 (NRR) is widely recognised as an alternative to the traditional Haber–Bosch production process for ammonia. However, though the challenges of NRR experiments have become better understood, the reported rates are often too low to be convincing that reduction of the highly unreactive N2 molecule has actually been achieved. This perspective critically reassesses a wide range of the NRR reports, describes experimental case studies of potential origins of false-positives, and presents an updated, simplified experimental protocol dealing with the recently emerging issues., Discovering a sustainable route to ammonia as a fertiliser and as an energy carrier is critically important, but many recent reports on the electrochemical nitrogen reduction are false positives. Here the authors uncover the emerging experimental traps and detail protocols to reliably avoid them.

We demonstrate that bismuth exhibits no measurable electrocatalytic activity for the nitrogen reduction reaction to ammonia in aqueous electrolyte solutions, contrary to several recent reports on the highly impressive rates of Bi-catalysed electrosynthesis of NH3 from N2.

Despite its highest theoretical capacity, the practical applications of the silicon anode are still limited by severe capacity fading, which is due to pulverization of the Si particles through volume change during charge and discharge. In this study, silicon nanoparticles are embedded in micron-sized porous carbon spheres (Si-MCS) via a facile hydrothermal process in order to provide a stiff carbon framework that functions as a cage to hold the pulverized silicon pieces. The carbon framework subsequently allows these silicon pieces to rearrange themselves in restricted domains within the sphere. Unlike current carbon coating methods, the Si-MCS electrode is immune to delamination. Hence, it demonstrates unprecedented excellent cyclability (capacity retention: 93.5% after 500 cycles at 0.8 A g–1), high rate capability (with a specific capacity of 880 mAh g–1 at the high discharge current density of 40 A g–1), and high volumetric capacity (814.8 mAh cm–3) on account of increased tap density. The lithium-ion...

A boron-doped carbon layer (BC) was coated on porous micron-sized Li4Ti5O12 (LTO) via a facile wet-chemical method for use as a promising anode material for sodium-ion batteries. As determined by X-ray photoemission spectroscopy and Raman spectroscopy, three different species (BC3, BC2O, and BCO2) were doped in the carbon layer on the surface of LTO. These heteroatoms lead to abundant extrinsic defects in the carbon layer that result in improved Na-ion diffusivity across the active material. Using electrochemical impedance spectroscopy, the diffusion coefficient of the boron-doped carbon-coated LTO (BC-LTO) was found to be 2.241 × 10−15 cm2 s−1. The BC-LTO exhibited 85% and 60% higher Na-ion diffusivity than pristine LTO and carbon-coated LTO (C-LTO), respectively. Moreover, electron-deficient boron in BC3 enhances the electric conductivity through positively charged holes that carry electrons through the carbon structure. Accordingly, the BC-LTO electrode showed a high specific capacity of 148 mA h g−1 at 0.5C and 98.4 mA h g−1 at 10C rate. In addition, the BC-LTO anode demonstrated a great capacity retention of about 90% at 1 and 10C after 300 cycles. These findings demonstrate the promising potential of LTO as a high-performance anode material for sodium-ion batteries.

Inspired by the previously published theoretical findings, the present work aims to assess the electrocatalytic activity of molybdenum(IV) sulfide modified with metallic molybdenum for the nitrogen reduction reaction in aqueous electrolyte solution (0.1 M Li2SO4; pH 3) and in aprotic [C4mpyr][eFAP] ionic liquid electrolyte at ambient temperature. The material of interest was synthesized via a high-temperature partial reduction of MoS2, while electrocatalytic tests followed a previously established robust protocol, which in particular involves strict control over any NH3 and NO3 −/NO2 − contamination at every key step. As expected, no activity was found in aqueous solutions. In aprotic medium, the formation of small amounts of ammonia at low rates was observed and was found to strongly depend on the water concentration and applied potential. However, the amount of electrochemically generated NH3 always reached a particular limit and did not increase further, even when the N2 pressure was increased from 1 to 16 bar. The results suggest rapid blockage of the surface of the investigated electromaterial with NH3, which prevents its operation as a catalyst for the ammonia electrosynthesis.

An effective approach to generate nitrogen-doped carbon coating layer on porous silicon (CN@P-Si), one of most promising anode materials for lithium-ion batteries, was addressed in this study to minimize their intrinsic drawbacks of low electrical conductivity and large volume expansion. The resulting enhanced electrochemical performance of the cell using the prepared CN@P-Si materials is attributed to the suppression of volume expansion and formation of the stable solid electrolyte interface by the combination of the porous structure and nitrogen-doped carbon coating layer during the repeated lithiation and delithiation process. After 100 cycles at 0.8 A g −1 , the capacity retention is 82% in contrast to 69% for the non-coated samples. Even at the increased discharge current of 20 A g −1 , the cell with CN@P-Si electrode delivers a high specific capacity of 1904 mAh g −1 . After 100 cycles, the P-Si electrode with pores shows huge pulverization; in contrast the CN@P-Si electrode remains intact with reasonably low volume expansion. Nitrogen-doped carbon coating layer on porous Si surface successfully suppress the pulverization of CN@P-Si electrode owing to its shielding capability. These results suggest that CN@P-Si is an attractive candidate for a high-capacity anode for lithium-ion batteries.

As an ideal alternative to the Haber-Bosch process, the renewable energy-powered electrochemical N2 reduction to NH3 is a promising approach since the electrochemical reduction can occur under mild conditions; only water and N2 are consumed during overall nitrogen reduction reaction.[1] Recently, there have been many efforts devoted to developing electrocatalysts for energy efficient NH3 synthesis from N2; however, it remains very challenging due to the thermodynamic inertness of the dinitrogen triple bond. The electrochemical reduction of N2 to NH3 (the nitrogen reduction reaction “NRR”) requires the consecutive six-electron/proton transfer reactions to proceed and this leads to the sluggish kinetics. In addition, the competitive hydrogen evolution reaction (HER, Eº = 0 V vs. RHE) can concomitantly occur at similar potentials to the NRR (Eº = 0.092 V vs. RHE), resulting in both low faradaic efficiency (< 10 %) and low yield rates (< 10-10 mole cm-2 s-1) for NH3 synthesis. Such poor conversion efficiency and yield rates also make it more difficult to confirm the origins of the NH3 production, ie whether it genuinely comes from electrocatalytic NRR, as opposed to some other readily reducible N-containing contaminants (NO, NO2, N2O and doped N atoms in the materials) under reducing potentials. Herein, we investigate the catalytic nature of nitrogen reduction reaction on three different types of preeminent electrocatalysts from the literature (bismuth, gold and N-doped carbon)[2, 3] using a rigorous experimental protocol developed by our group.[4] It is demonstrated that all of the catalysts are essentially inactive (below LOD) towards dinitrogen reduction to NH3. We also systematically unravel the origins of the reported activity, showing that other N-containing species, particularly ionic/gaseous NOx or doped N atoms in the materials, are strongly active reactants towards NH3 production. Our presentation will conclude with a summary of the critical contaminants leading to false-positive NRR and also provide further protocol recommendations to avoid this outcome. [1] S.L. Foster, S.I.P. Bakovic, R.D. Duda, S. Maheshwari, R.D. Milton, S.D. Minteer, M.J. Janik, J.N. Renner, L.F. Greenlee, Nature Catalysis, 1 (2018) 490-500. [2] Y.-C. Hao, Y. Guo, L.-W. Chen, M. Shu, X.-Y. Wang, T.-A. Bu, W.-Y. Gao, N. Zhang, X. Su, X. Feng, Nature Catalysis, 2 (2019) 448. [3] Y. Liu, Y. Su, X. Quan, X. Fan, S. Chen, H. Yu, H. Zhao, Y. Zhang, J. Zhao, ACS Catalysis, 8 (2018) 1186-1191. [4] B.H.R. Suryanto, H.-L. Du, D. Wang, J. Chen, A.N. Simonov, D.R. MacFarlane, Nature Catalysis, 2 (2019) 290-296.

Water oxidation, a kinetically demanding half-reaction, plays a vital role in hydrogen production through electrochemical water-splitting. Thus, the development of efficient and stable oxygen evolution reaction electrocatalysts has become a central theme for basic research in the field of renewable energy. Substantial progress has been made in developing robust and inexpensive noble metal-free heterogeneous electrocatalysts for water oxidation operating under alkaline and neutral pH conditions. There is, however, a scarcity of active and stable electrocatalysts for acidic water oxidation, which is an ideal pH condition for high purity hydrogen production and offers several other technological advantages. Low performance and poor stability of existing water oxidation electrocatalysts based on non-noble metals are mainly attributed to the dissolution of respective elements in acidic solution. Considering these challenging aspects, we constructed the water oxidations catalysts based on thermodynamically and acid-stable oxides, which are incorporated with catalytically active species (transition metal-based water oxidation catalysts). Specifically, the highly-disordered mixed metal oxide, based on cobalt, iron, and lead, produced in situ exhibits excellent OER performance with unprecedented stability, through self-healing mechanism, under harsh operating conditions of pH 0 and temperature 80 °C. Importantly, the Cobalt-iron-lead oxide demonstrates the capability of operating at industrially relevant rates of 0.5 A cm-2 at low OER overpotentials of 0.7 V. These findings highlight the new strategy to design highly active and acid-stable, noble metal-free, OER electrocatalysts that can essentially operate indefinitely. Our most recent developments focused on replacement of lead with alternative, less toxic components. The presentation will also highlight these new systems, some of which demonstrate outstanding stability even without a self-healing mechanism.

Electrochemical nitrogen reduction reaction (NRR) that can be powered by renewable energy is currently broadly investigated as a sustainable alternative method for the industrial ammonia synthesis to replace, at least partially, the Haber-Bosch technology. Inspired by recent theoretical reports advocating several types of materials as active NRR catalysts, our experimental work aimed to assess the validity of some of these predictions. Vanadium and niobium nitrides were suggested by the previous computational work to be catalytically active towards the electrochemical reduction of dinitrogen to ammonia occurring via the Mars-van Krevenlen (MvK) mechanism [1]. The present experimental study thoroughly investigates the electrocatalytic activity of cubic vanadium(III) nitride, niobium(III) nitride and tetragonal Nb4N5 for the nitrogen reduction reaction in aqueous electrolyte solutions of different pH under ambient conditions [2]. VN and Nb4N5 (supported on carbon cloth) were synthesised by annealing of hydrothermally produced hydroxide precursors in NH3 atmosphere at 600-1100 °C; NbN was obtained by solid state reaction between niobium(V) chloride and urea at 1000 °C. Comprehensive testing of the materials under a wide range of aqueous conditions unambiguously demonstrates their inability to catalyse the electrosynthesis of ammonia from dinitrogen, as well as the propensity of VN synthesised at 600 °C and Nb4N5 to release lattice nitride in a non-catalytic process, which produces ammonia under reductive conditions. Thus, polycrystalline nitrides of vanadium and niobium are concluded to be catalytically inactive towards the ammonia electrosynthesis from N2 dissolved in water. When tested in non-aqueous aprotic electrolyte, Nb4N5 sustained slow ammonia electrosynthesis (4 pmol s-1 cm-2) at low faradaic efficiency (20 %). The present work additionally emphasises the compulsory requirement for the implementation of reliable testing and analysis procedures for the assessment of the catalytic properties of materials for the nitrogen reduction reaction. Another type of materials suggested by DFT to be catalytically active for the NRR at low overpotentials is molybdenum disulphide promoted with Mo metal particles (Mo/MoS2), which was suggested to provide energetically favourable binding with N2 molecule [3]. However, MoS2 is a well-known hydrogen evolution catalyst rendering any reasonable selectivity for the N2 reduction with Mo/MoS2 in aqueous medium essentially impossible, as confirmed in our experiments. Therefore, we applied two aprotic ionic liquids providing high N2 solubility [4], viz. [C4mpyr][eFAP] and [P66614][eFAP], as electrolyte media for testing the electrocatalytic properties of molybdenum-based materials. Molybdenum metal and Mo/MoS2 catalysts were synthesised by reducing 2H-MoS2 with H2 at 950 and 850 °C, respectively. In [C4mpyr][eFAP], metal catalyst did not demonstrate significant rates of the NRR, but Mo/MoS2 enabled the electrosynthesis of ammonia at the yield rate of 17 pmol cm-2 s-1 and faradaic efficiency of ca 50% at an applied potential of -0.97 V vs. NHE, when using a mixture of 90% dry N2 and 10% H2O-saturated N2 as a feed gas. However, there no ammonia was detected when Mo/MoS2 was tested in [P66614][eFAP] under the same as well as other potential and humidity conditions. Juxtaposition of cyclic voltammograms of Mo/MoS2 recorded in two ionic liquids reveals very different electrochemical behaviour of the material in [C4mpyr][eFAP] and [P66614][eFAP], which is likely to explain the lack of activity in the latter medium. Further tests in tetrahydrofuran as a solvent and using a conventional lithium triflate electrolyte have further supported the hypothesis on the catalytic activity of Mo/MoS2 for the NRR, although the ammonia yield rate (3 pmol s-1 cm-2) and faradaic efficiency (ca 3%) were very low. However, further control experiments are needed before the final conclusion on the genuine NRR catalytic activity of this material can be achieved. Overall, the present study demonstrates that theoretical predictions on the catalytic activity of different materials for the NRR should be considered with caution, and that robust NRR testing and ammonia analysis protocols must be implemented to unambiguously prove genuine ammonia electrosynthesis from N2 [5]. References 1. Abghoui et al. ACS Cat. 2015, 6 (2), 635-646. 2. Du et al. ACS Sustainable Chem. Eng. 2019, 7 (7), 6839-6850. 3. Zhao et al. Phys. Chem. Chem. Phys. 2018, 20 (14), 9248-9255. 4. Zhou et al. Energy Environ. Sci. 2017, 10 (12), 2516-2520. 5. Suryanto et al. Nat. Cat. 2019, 2 (4), 290-296.

Since early 2018, the nitrogen reduction reaction to ammonia (NRR) has become a focus of active research as an approach to sustainable production of ammonia to support and eventually replace the century-old yet highly robust Haber-Bosch catalytic technology. More than one hundred reports on the successful NRR in aqueous electrolyte solutions catalysed by a comparatively wide range of materials have been published by the end of 2019, though the reported ammonia yield rates (-1 cm-2, per geometric surface area of the electrode) and faradaic efficiencies (< 20%) are typically low. In fact, the observed amounts of NH3 produced in aqueous media are most often comparable to the level of adventitious nitrogen-based contaminants, thereby questioning the genuine nature of the reported NRR. The problems of the aqueous NRR, in the first place low faradaic efficiency, can be effectively addressed by employing aprotic electrolyte media for the electrochemical reduction of dinitrogen.1-2 Under such conditions, the prevalence of the NRR over the competing and undesirable in this context hydrogen evolution reaction is suppressed due to the significantly higher solubility of N2 than in water and controlled supply of the proton source. Ammonia electrosynthesis in organic media can be realised in at least two ways — either via direct electrocatalytic reaction,3-4 or through a lithium-mediated process.5-6 Both approaches have their pros and cons, and both are currently investigated in our groups. The talk will focus on some of the experimental challenges and pitfalls relevant to the non-aqueous NRR and on our recent progress in this area. References 1. Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonov, A. N.; MacFarlane, D. R., Challenges and prospects in the catalysis of electroreduction of nitrogen to ammonia. Nature Catal. 2019, 2 (4), 290-296. 2. Andersen, S. Z.; Čolić, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Nørskov, J. K.; Chorkendorff, I., A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504-508. 3. Zhou, F.; Azofra, L. M.; Ali, M.; Kar, M.; Simonov, A. N.; McDonnell-Worth, C.; Sun, C.; Zhang, X.; MacFarlane, D. R., Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 2017, 10 (12), 2516-2520. 4. Suryanto, B. H. R.; Kang, C. S. M.; Wang, D.; Xiao, C.; Zhou, F.; Azofra, L. M.; Cavallo, L.; Zhang, X.; MacFarlane, D. R., Rational Electrode–Electrolyte Design for Efficient Ammonia Electrosynthesis under Ambient Conditions. ACS Energy Lett. 2018, 3 (6), 1219-1224. 5. Tsuneto, A.; Kudo, A.; Sakata, T., Lithium-mediated electrochemical reduction of high pressure N2 to NH3. J. Electroanal. Chem. 1994, 367 (1–2), 183-188. 6. McEnaney, J. M.; Singh, A. R.; Schwalbe, J. A.; Kibsgaard, J.; Lin, J. C.; Cargnello, M.; Jaramillo, T. F.; Nørskov, J. K., Ammonia synthesis from N2 and H2O using a lithium cycling electrification strategy at atmospheric pressure. Energy Environ. Sci. 2017, 10 (7), 1621-1630.