The lack of chemical understanding and efficient catalysts impedes the development of electrocatalytic nitrogen reduction reaction (eNRR) for ammonia production. In this work, we employed density functional theory calculations to build up a picture (activity trends, electronic origins, and design strategies) of single-atom catalysts (SACs) supported on nitrogen-doped carbons as eNRR electrocatalysts. To construct such a picture, this work presents systematic studies of the eNRR activity of SACs covering 20 different transition metal (TM) centers coordinated by nitrogen atoms contained in three types of nitrogen-doped carbon substrates, which gives 60 SACs. Our study shows that the intrinsic activity trends could be established on the basis of the nitrogen adatom adsorption energy (ΔE N* ). Furthermore, the influence of metal and support (ligands) on ΔE N* proved to be related to the bonding/antibonding orbital population and regulating the scaling relations for adsorption of intermediates, respectively. Accordingly, a two-step strategy is proposed for improving the eNNR activity of TM-SACs, which involves the following: (i) selection of the most promising family of SACs (g-C 3 N 4 supported SACs as predicted in this work) and (ii) further improvement of the activity of the best candidate in the aforementioned family via tuning the adsorption strength of the key intermediates. Also, the stability of Ndoped carbon supports and their selectivity in comparison to the competing hydrogen evolution need to be taken into consideration for screening the durable and efficient candidates. Finally, an effective strategy for designing active, stable, and selective SACs based on the mechanistic insights is elaborated to guide future eNRR studies.

also aggravate the greenhouse effect. [8] Considering the huge energy consumption of this process and the great potential of NH 3 in future energy systems, protonassisted electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is becoming increasingly important in realizing low-cost artificial nitrogen fixation. [10][11][12] More importantly, the NRR system can be easily integrated with renewable solar and wind energy into an environmentally benign process for NH 3 production. [2,7] Even though various materials have shown NRR activity in aqueous system, their performance are still hindered by the sluggish reaction kinetics and competitive hydrogen evolution reaction (HER) resulting in poor activity and unsatisfactory selectivity. [13][14][15] The active site for electrochemical NRR needs twofold properties: for one thing, it can accept the lone-pair electrons for effective N 2 adsorption and for the other thing it can donate electrons to antibonding orbitals of dinitrogen molecules for the NN triple-bond activation. [9,13,[16][17][18][19][20] From this perspective, vacancy-engineering materials are ideal for practical applications. [9,16,20] By virtue of the electron redistribution and special chemical properties, the vacancies can provide unique active sites for nitrogen adsorption and activation. [9,[21][22][23][24] Up to now, the most common vacancy that has been studied for NRR is oxygen vacancy. For example, oxygen vacancies on transition metal oxides are active for NRR by enhancing the dinitrogen molecule adsorption. [9,25] However, oxygen vacancy can also improve the HER performance of the host matrix, which may result in poor NRR selectivity. [26][27][28] Alternatively, nitrogen vacancies on transition metal nitride (TMN) are considered as an ideal active site for NRR because of its unique vacancy properties for dinitrogen molecule adsorption and poor HER activity. [29,30] However, the nitrogen vacancy on TMN would be deactivated during NRR process thus results in poor stability. [29] In addition, researchers claim that some TMNs are inactive toward NRR in aqueous system, which is in contrary to theoretical calculations. [31] To gain insightful understanding of NRR mechanism, it is necessary to design TMNs with simplex structure and stable surface vacancies for the linkage of theoretical models and real-world catalysts. 2D material has onefold and fully exposed crystal surface, which is widely used as platform for both theoretical calculations and Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon-free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new ...

Transition metal nitrides (TMNs) have great potential for energy-related electrocatalysis because of their inherent electronic properties. However, incorporating nitrogen into a transition metal lattice is thermodynamically unfavorable, and therefore most of the developed TMNs are deficient in nitrogen. Consequently, these TMNs exhibit poor structural stability and unsatisfactory performance for electrocatalytic applications. In this work, we design and synthesize an atomically thin nitrogen-rich nanosheets, Mo5N6, with the help of a Ni-inducing growth method. The as-prepared single-crystal electrocatalyst with abundant metal–nitrogen electroactive sites displays outstanding activity for the hydrogen evolution reaction (HER) in a wide range of electrolytes (pH 0–14). Further, the two-dimensional Mo5N6 nanosheets exhibit high HER activity and stability in natural seawater that are superior to other TMNs and even the Pt benchmark. By combining synchrotron-based spectroscopy and the calculations of electron density of state, we find that the enhanced properties of these nitrogen-rich Mo5N6 nanosheets originates from its Pt-like electronic structure and the high valence state of its Mo atoms.

Developing efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) is important for clean energy systems. Non-noble transition metals are the most promising candidates for replacement of conventional Pt group catalysts for the HER. However, most non-noble metals show poor HER activity due to their intrinsic electronic structures. Herein, we use a multifaceted heteroatom doping method (nitrogen, sulfur, and phosphorus) to directly and continuously fine-tune the electronic structure and HER activity of non-noble metals without changing their chemical composition. As a proof-of-concept, a nitrogen and phosphorus dual-doped Ni catalyst is explored by precisely manipulating doping modes, revealing the best HER performance among all doped Ni catalysts tested. The doping-induced charge redistribution in the Ni metal significantly influences its catalytic performance for the HER in alkaline media, which is confirmed by merging theoretical calculation with synchrotron-based spectroscopy. The principle that can bridge the doping modes and HER activity of the doped catalysts is built with a volcano relationship.

Exploring electrocatalysts with high activity is essential for the production of ammonia via an electrochemical routine. By employing density functional theory calculations, we investigated the electrochemical nitrogen reduction reaction (eNRR) activity on binary metal borides, a model system of metal borides. To elaborate the mechanisms, molybdenum borides (Mo 2 B, α-MoB, and MoB 2 ) were first modeled; the results indicate that the crystal structures greatly impact the N 2 adsorption and therefore the electrocatalytic activity. Our electronic structure investigation suggests that boron p-orbital hybrids with dinitrogen π*-orbital, and the population on p−π*-orbital determine the N 2 adsorption strength. Therefore, the isolated boron site of Mo 2 B with less filled p z -orbital benefits the activation of N 2 and weaken the triple bond of dinitrogen. This isolated boron sites concept was successfully extended to other metal borides in the form of M 2 B (M stands for Ti, Cr, Mn, Fe, Co, Ni, Ta, W). Mo 2 B, Fe 2 B, and Co 2 B were discovered as the most promising candidates with low limiting potentials due to appropriate adsorption strength of reaction intermediates led by moderate p z filling. This work provides insights for designing metal borides as promising eNRR electrocatalysts.

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The dissociative chemisorption energy of water was proposed to address both thermodynamics and kinetics of alkaline hydrogen evolution.

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