The Environmental Feasibility of Decentralised Solar Ammonia.

Intense efforts have been devoted to developing green and blue centralised Haber-Bosch processes (gHB and bHB, respectively), but the feasibility of a decentralised and sustainable scheme has yet to be assessed. Here we reveal the conditions under which small-scale systems based on the electrocatalytic reduction of nitrogen (eN2R) powered by photovoltaic energy (NH3-leaf) could become a competitive technology in terms of environmental criteria. To this end, we calculated energy efficiency targets based on solar irradiation atlases to guide research in the incipient eN2R field. Even under this germinal state, the NH3-leaf technology would compete favourably in sunny locations relative to the business-as-usual production scenario. The disclosed sustainability potential of NH3-leaf makes it a strong ally of gHB toward a non-fossil ammonia production.

A General Strategy toward Metal Sulfide Nanoparticles Confined in a Sulfur-Doped Ti3 C2 Tx MXene 3D Porous Aerogel for Efficient Ambient N2 Electroreduction.

Three-dimensional (3D) porous MXene-based aerogel architectures have attracted great interest for many applications, despite limits in renewable energy conversion owing to the lack of multifunctionality in their components. Herein, a simple and general strategy for constructing a novel functional 3D MXene-based composite heterojunction aerogel (MS@S-MAs) is presented via divalent metal-ion assembly and subsequent thermal sulfidation, and its application in electrochemical nitrogen reduction reaction (NRR) is studied. The as-prepared MS@S-MAs comprises metal sulfide nanoparticles uniformly confined in 3D interconnected conductive S-doped MXene sheets with intimate interfacial interaction. Benefiting from the unique properties and an interfacial interaction, MS@S-MAs exhibit significantly improved NRR catalytic performance and excellent stability because of the higher exposure of electrochemically active sites coupled with easier accessibility, faster mass diffusion, and quicker carrier transport at the interface. Remarkably, CoS@S-MAs show an NH3 yield rate and a Faradaic efficiency of 12.4 µg h-1 mg-1 cat and 27.05% at the lower potential of -0.15 V versus a reversible hydrogen electrode in 0.1 m Na2 SO4 solution under ambient conditions, which rivals or exceeds most of the previously reported MXene-based and Co-based catalysts. This work will open avenues to construct 3D MXene-based materials with rich functionalities for energy storage and conversion, catalysis, and other applications.

Boosting Electroreduction Kinetics of Nitrogen to Ammonia via Tuning Electron Distribution of Single-Atomic Iron Sites.

Electrocatalytic nitrogen reduction reaction (NRR) plays a vital role for next-generation electrochemical energy conversion technologies. However, the NRR kinetics is still limited by the sluggish hydrogenation process on noble-metal-free electrocatalyst. Herein, we report the rational design and synthesis of a hybrid catalyst with atomic iron sites anchored on a N,O-doped porous carbon (FeSA -NO-C) matrix of an inverse opal structure, leading to a remarkably high NH3 yield rate of 31.9 µg NH 3 h-1 mg-1 cat. and Faradaic efficiency of 11.8 % at -0.4 V for NRR electrocatalysis, outperformed almost all previously reported atomically dispersed metal-nitrogen-carbon catalysts. Theoretical calculations revealed that the observed high NRR catalytic activity for the FeSA -NO-C catalyst stemmed mainly from the optimized charge-transfer between the adjacent O and Fe atoms homogenously distributed on the porous carbon support, which could not only significantly facilitate the transportation of N2 and ions but also effectively decrease the binding energy between the isolated Fe atom and *N2 intermediate and the thermodynamic Gibbs free energy of the rate-determining step (*N2 → *NNH).

Amorphous Oxide Nanostructures for Advanced Electrocatalysis.

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO2 electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.

Enhancing Green Ammonia Electrosynthesis Through Tuning Sn Vacancies in Sn-Based MXene/MAX Hybrids.

Renewable energy driven N2 electroreduction with air as nitrogen source holds great promise for realizing scalable green ammonia production. However, relevant out-lab research is still in its infancy. Herein, a novel Sn-based MXene/MAX hybrid with abundant Sn vacancies, Sn@Ti2CTX/Ti2SnC-V, was synthesized by controlled etching Sn@Ti2SnC MAX phase and demonstrated as an efficient electrocatalyst for electrocatalytic N2 reduction. Due to the synergistic effect of MXene/MAX heterostructure, the existence of Sn vacancies and the highly dispersed Sn active sites, the obtained Sn@Ti2CTX/Ti2SnC-V exhibits an optimal NH3 yield of 28.4 µg h-1 mgcat-1 with an excellent FE of 15.57% at - 0.4 V versus reversible hydrogen electrode in 0.1 M Na2SO4, as well as an ultra-long durability. Noticeably, this catalyst represents a satisfactory NH3 yield rate of 10.53 µg h-1 mg-1 in the home-made simulation device, where commercial electrochemical photovoltaic cell was employed as power source, air and ultrapure water as feed stock. The as-proposed strategy represents great potential toward ammonia production in terms of financial cost according to the systematic technical economic analysis. This work is of significance for large-scale green ammonia production.

Delicate Tuning of the Ni/Co Ratio in Bimetal Layered Double Hydroxides for Efficient N2 Electroreduction.

Electroreduction of N2 to NH3 at ambient conditions using renewable electricity is promising, but developing efficient electrocatalysts is still challenging due to the inertness of N≡N bonds. Layer double hydroxides (LDHs) composed of first-row transition metals with empty d-orbitals are theoretically promising for N2 electroreduction (NRR) but rarely reported. Herein, hollow NiCo-LDH nanocages with different Ni/Co ratios were prepared, and their electronic structures and atomic arrangements were critical. The synergetic mechanisms of Ni and Co ions were revealed, and the optimized catalytic sites were proposed. Besides, in-situ Raman spectroscopy and 15 N2 isotopic labeling studies were applied to detect reaction intermediates and confirm the origin of NH3 . As a result, high NH3 yield of 52.8 µg h-1 mgcat -1 and faradaic efficiency of 11.5 % were obtained at -0.7 V, which are top-ranking among Co/Ni-based NRR electrocatalysts. This work elucidates the structure-activity relationship between LDHs and NRR and is instructive for rational design of LDH-based electrocatalysts.

Actinide-uranium single-atom catalysis for electrochemical nitrogen fixation.

Actinide-based catalysts have been regarded as promising candidates for N2 fixation owing to their unique 5f orbital with flexible oxidation states. Herein, we report for the first time the dispersion of uranium (U) single atoms on TiO2 nanosheets via oxygen vacancy confinement for N2 electroreduction. The single-atom U catalyst exhibited a high NH3 yield of 40.57 µg h-1 mg-1, with a reasonably high Faraday efficiency of 25.77%, ranking first among the reported nitrogen-free catalysts. Isotope-labeling operando synchrotron infrared spectroscopy verifies that the key *N2Hy intermediate species was derived from the N2 gas of the feed. By using operando X-ray absorption spectroscopy, we found enhanced metal-support interaction between U single atoms and TiO2 lattice with more U-Olatt coordination under working conditions. Theoretical simulations suggest that the evolved 1Oads-U-4Olatt moieties act as a critical electron-feedback center, lowering the thermodynamic energy barrier for the N2 dissociation and the first hydrogenation step. This work provides the possibility of tailoring the interaction between metal active sites and supports for designing high-performance actinide-based single-atom catalysts.

Enhanced catalytic activity of MXene for nitrogen electoreduction reaction by carbon doping.

The electrocatalytic performance of nitrogen reduction reaction (NRR) is seriously hindered by the lack of cost-effective electrocatalysts with high-efficiency and high-selectivity. In this work, the NRR catalytic activity of single carbon (C) atom embedded into two-dimensional (2D) transition metal carbides (M2CO2, M = Ti, Zr, Hf, Nb, Ta, Mo, and W) with oxygen vacancy was systematically evaluated by means of comprehensive density functional theory (DFT) computations. Our results revealed that the embedded single C atom possesses good durability due to its strong interaction with metal atoms around vacancy in these MXenes. Interestingly, through high-throughput screening, the single C atoms anchored on Nb2CO2, Mo2CO2, and W2CO2 nanosheets are identified as promise NRR catalysts with high-activity due to their low limiting potentials (-0.14 to  -0.38 V) via a distal mechanism and outstanding selectivity again the competing hydrogen evolution reaction. Remarkably, the intrinsic activity of the C single atom supported by these MXenes mainly originates from the activation degree of the adsorbed N2 molecule, which is greatly dependent on the electron filling degree of pz orbital in C atom. Thus, by carefully choosing suitable substrates, the single C catalyst can be utilized as ideal NRR catalysts for NH3 synthesis.

A Highly Efficient Metal-Free Electrocatalyst of F-Doped Porous Carbon toward N2 Electroreduction.

N2 electroreduction into NH3 represents an attractive prospect for N2 utilization. Nevertheless, this process suffers from low Faraday efficiency (FE) and yield rate for NH3 . In this work, a highly efficient metal-free catalyst is developed by introducing F atoms into a 3D porous carbon framework (F-doped carbon) toward N2 electroreduction. At -0.2 V versus reversible hydrogen electrode (RHE), the F-doped carbon achieves the highest FE of 54.8% for NH3 , which is 3.0 times as high as that (18.3%) of pristine carbon frameworks. Notably, at -0.3 V versus RHE, the yield rate of F-doped carbon for NH3 reaches 197.7 µgNH3 mg-1 cat. h-1 . Such a value is more than one order of magnitude higher than those of other metal-free electrocatalysts under the near-ambient conditions for NH3 product to date. Mechanistic studies reveal that the improved performance in N2 electroreduction for F-doped carbon originates from the enhanced binding strength of N2 and the facilitated dissociation of N2 into *N2 H. F bonding to C atom creates a Lewis acid site due to the different electronegativity between the F and C atoms. As such, the repulsive interaction between the Lewis acid site and proton H suppresses the activity of H2 evolution reaction, thus enhancing the selectivity of N2 electroreduction into NH3 .