Carbon‐Based Metal‐Free Catalysts for Electrocatalytic Reduction of Nitrogen for Synthesis of Ammonia at Ambient Conditions

The electrocatalytic nitrogen reduction reaction (NRR) is a promising catalytic system for N₂ fixation in ambient conditions. Currently, metal‐based catalysts are the most widely studied catalysts for electrocatalytic NRR. Unfortunately, the low selectivity and poor resistance to acids and bases, and the low Faradaic efficiency, production rate, and stability of metal‐based catalysts for NRR make them uncompetitive for the synthesis of ammonia in comparison to the industrial Haber–Bosch process. Inspired by applications of carbon‐based metal‐free catalysts (CMFCs) for the oxygen reduction reaction (ORR) and CO₂ reduction reaction (CO₂RR), the studies of these CMFCs in electrocatalytic NRR have attracted great attention in the past year. However, due to the differences in electrocatalytic NRR, there are several critical issues that need to be addressed in order to achieve rational design of advanced carbon‐based metal‐free electrocatalysts to improve activity, selectivity, and stability for NRR. Herein, the recent developments in the field of carbon‐based metal‐free NRR catalysts are presented, along with critical issues, challenges, and perspectives concerning metal‐free catalysts for electrocatalytic reduction of nitrogen for synthesis of ammonia at ambient conditions.     

Design and Synthesis of Noble Metal-Based Alloy Electrocatalysts and Their Application in Hydrogen Evolution Reaction

Hydrogen energy is regarded as the ultimate energy source for future human society, and the preparation of hydrogen from water electrolysis is recognized as the most ideal way. One of the key factors to achieve large-scale hydrogen production by water splitting is the availability of highly active and stable electrocatalysts. Although non-precious metal electrocatalysts have made great strides in recent years, the best hydrogen evolution reaction (HER) electrocatalysts are still based on noble metals. Therefore, it is particularly important to improve the overall activity of the electrocatalysts while reducing the noble metals load. Alloying strategies can shoulder the burden of optimizing electrocatalysts cost and improving electrocatalysts performance. With this in mind, recent work on the application of noble metal-based alloy electrocatalysts in the field of hydrogen production from water electrolysis is summarized. In this review, first, the mechanism of HER is described; then, the current development of synthesis methods for alloy electrocatalysts is presented; finally, an example analysis of practical application studies on alloy electrocatalysts in hydrogen production is presented. In addition, at the end of this review, the prospects, opportunities, and challenges facing noble metal-based alloy electrocatalysts are tried to discuss.     

Defect Engineering for Fuel-Cell Electrocatalysts

The commercialization of fuel cells, such as proton exchange membrane fuel cells and direct methanol/formic acid fuel cells, is hampered by their poor stability, high cost, fuel crossover, and the sluggish kinetics of platinum (Pt) and Pt-based electrocatalysts for both the cathodic oxygen reduction reaction (ORR) and the anodic hydrogen oxidation reaction (HOR) or small molecule oxidation reaction (SMOR). Thus far, the exploitation of active and stable electrocatalysts has been the most promising strategy to improve the performance of fuel cells. Accordingly, increasing attention is being devoted to modulating the surface/interface electronic structure of electrocatalysts and optimizing the adsorption energy of intermediate species by defect engineering to enhance their catalytic performance. Defect engineering is introduced in terms of defect definition, classification, characterization, construction, and understanding. Subsequently, the latest advances in defective electrocatalysts for ORR and HOR/SMOR in fuel cells are scientifically and systematically summarized. Furthermore, the structure–activity relationships between defect engineering and electrocatalytic ability are further illustrated by coupling experimental results and theoretical calculations. With a deeper understanding of these complex relationships, the integration of defective electrocatalysts into single fuel-cell systems is also discussed. Finally, the potential challenges and prospects of defective electrocatalysts are further proposed, covering controllable preparation, in situ characterization, and commercial applications.     

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism,Synthesis, Characterization,Application, and Prospects

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO₃RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO₃RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO₃RR. However, the real active sites of TM-SACs applied to NO₃RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO₃RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO₃RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO₃RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.     

Interfacially Engineered Nanoporous Cu/MnOx Hybrids for Highly Efficient Electrochemical Ammonia Synthesis via Nitrate Reduction

Electrochemical reduction of nitrate to ammonia (NH₃) not only offers a promising strategy for green NH₃ synthesis, but also addresses the environmental issues and balances the perturbed nitrogen cycle. However, current electrocatalytic nitrate reduction processes are still inefficient due to the lack of effective electrocatalysts. Here 3D nanoporous Cu/MnO_x hybrids are reported as efficient and durable electrocatalysts for nitrate reduction reaction, achieving the NH₃ yield rates of 5.53 and 29.3 mg h⁻¹ mg_cat.⁻¹ with 98.2% and 86.2% Faradic efficiency in 0.1 m Na₂SO₄ solution with 10 and 100 mm KNO₃, respectively, which are higher than those obtained for most of the reported catalysts under similar conditions. Both the experimental results and density functional theory calculations reveal that the interface effect between Cu/MnO_x interface could reduce the free energy of rate determining step and suppress the hydrogen evolution reaction, leading to the enhanced catalytic activity and selectivity. This work provides an approach to design advanced materials for NH₃ production via electrochemical nitrate reduction.     

Nitrogen Vacancies on 2D Layered W2N3: A Stable and Efficient Active Site for Nitrogen Reduction Reaction

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 2D layered W₂N₃ nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h^?1 mg_cata^?1 (3.80 ± 0.32 × 10^?11 mol cm^?2 s^?1) and Faradaic efficiency of 11.67 ± 0.93% at ?0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron‐based characterizations prove that the nitrogen vacancies on 2D W₂N₃ are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W₂N₃ can provide an electron‐deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR.     

Electrochemical Fabrication of Porous Au Film on Ni Foam for Nitrogen Reduction to Ammonia

Electrochemical reduction of N₂ to NH₃ provides an alternative strategy to replace the industrial Haber–Bosch process for facile and sustainable production of NH₃. The development of efficient electrocatalysts for the nitrogen reduction reaction (NRR) is highly desired. Herein, a micelle‐assisted electrodeposition method is presented for the direct fabrication of porous Au film on Ni foam (pAu/NF). Benefiting from its interconnected porous architectonics, the pAu/NF exhibits superior NRR performance with a high NH₃ yield rate of 9.42 µg h^?1 cm^?2 and a superior Faradaic efficiency of 13.36% at ?0.2 V versus reversible hydrogen electrode under the neutral electrolyte (0.1 m Na₂SO₄). The proposed micelle‐assisted electrodeposition strategy is highly valuable for future design of active NRR catalysts with desired compositions toward various electrocatalysis fields.     

Electrochemical Ammonia Synthesis via Nitrogen Reduction Reaction on a MoS2 Catalyst: Theoretical and Experimental Studies

The discovery of stable and noble‐metal‐free catalysts toward efficient electrochemical reduction of nitrogen (N₂) to ammonia (NH₃) is highly desired and significantly critical for the earth nitrogen cycle. Here, based on the theoretical predictions, MoS₂ is first utilized to catalyze the N₂ reduction reaction (NRR) under room temperature and atmospheric pressure. Electrochemical tests reveal that such catalyst achieves a high Faradaic efficiency (1.17%) and NH₃ yield (8.08 × 10^?11 mol s^?1 cm^?1) at ?0.5 V versus reversible hydrogen electrode in 0.1 m Na₂SO₄. Even in acidic conditions, where strong hydrogen evolution reaction occurs, MoS₂ is still active for the NRR. This work represents an important addition to the growing family of transition‐metal‐based catalysts with advanced performance in NRR.     

Synergizing Mo Single Atoms and Mo2C Nanoparticles on CNTs Synchronizes Selectivity and Activity of Electrocatalytic N2 Reduction to Ammonia

Previous research of molybdenum-based electrocatalysts for nitrogen reduction reaction (NRR) has been largely considered on either isolated Mo single atoms (MoSAs) or Mo carbide particles (e.g., Mo₂C) separately, while an integrated synergy (MoSAs-Mo₂C) of the two has never been considered. The theoretical calculations show that the Mo single atoms and Mo₂C nanoparticles exhibit, respectively, different catalytic hydrogen evolution reaction and NRR selectivity. Therefore, a new role-playing synergistic mechanism can be well enabled for the multistep NRR, when the two are combined on the same N-doped carbon nanotubes (NCNTs). This hypothesis is confirmed experimentally, where the MoSAs-Mo₂C assembled on NCNTs (MoSAs-Mo₂C/NCNTs) yields an ammonia formation rate of 16.1 µg h⁻¹ cm_cat⁻² at −0.25 V versus reversible hydrogen electrode, which is about four times that by the Mo₂C alone (Mo₂C/NCNTs) and 4.5 times that by the MoSAs alone (MoSAs/NCNTs). Moreover, the Faradic efficiency of the MoSAs-Mo₂C/NCNTs is raised up to twofold and sevenfold of the Mo₂C/NCNTs and MoSAs/NCNTs, respectively. The MoSAs-Mo₂C/NCNTs also demonstrate outstanding stability by the almost unchanged catalytic performance over 10 h of the chronoamperometric test. The present study provides a promising new prototype of synchronizing the selectivity and activity for the multistep catalytic reactions.     

Efficient Nitrate Synthesis via Ambient Nitrogen Oxidation with Ru-Doped TiO2/RuO2 Electrocatalysts

A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to nitrate is proposed, and Ru-doped TiO₂/RuO₂ (abbreviated as Ru/TiO₂) as a proof-of-concept catalyst is employed accordingly. Density functional theory (DFT) calculations suggest that Ru^δ+ can function as the main active center for the NOR process. Remarkably doping Ru into the TiO₂ lattice can induce an upshift of the d-band center of the Ru site, resulting in enhanced activity for accelerating electrochemical conversion of inert N₂ to active NO*. Overdoping of Ru ions will lead to the formation of additional RuO₂ on the TiO₂ surface, which provides oxygen evolution reaction (OER) active sites for promoting the redox transformation of the NO* intermediate to nitrate. However, too much RuO₂ in the catalyst is detrimental to both the selectivity of the NOR and the Faradaic efficiency due to the dominant OER process. Experimentally, a considerable nitrate yield rate of 161.9 µmol h⁻¹ g_cat⁻¹ (besides, a total nitrate yield of 47.9 µg during 50 h) and a highest nitrate Faradaic efficiency of 26.1% are achieved by the Ru/TiO₂ catalyst (with the hybrid composition of Ru_xTi_yO₂ and extra RuO₂ by 2.79 wt% Ru addition amount) in 0.1 m Na₂SO₄ electrolyte.     

Photoactive Earth‐Abundant Iron Pyrite Catalysts for Electrocatalytic Nitrogen Reduction Reaction

The generation of ammonia, hydrogen production, and nitrogen purification are considered as energy intensive processes accompanied with large amounts of CO₂ emission. An electrochemical method assisted by photoenergy is widely utilized for the chemical energy conversion. In this work, earth‐abundant iron pyrite (FeS₂) nanocrystals grown on carbon fiber paper (FeS₂/CFP) are found to be an electrochemical and photoactive catalyst for nitrogen reduction reaction under ambient temperature and pressure. The electrochemical results reveal that FeS₂/CFP achieves a high Faradaic efficiency (FE) of ≈14.14% and NH₃ yield rate of ≈0.096 µg min^?1 at ?0.6 V versus RHE electrode in 0.25 m LiClO₄. During the electrochemical catalytic reaction, the crystal structure of FeS₂/CFP remains in the cubic pyrite phase, as analyzed by in situ X‐ray diffraction measurements. With near‐infrared laser irradiation (808 nm), the NH₃ yield rate of the FeS₂/CFP catalyst can be slightly improved to 0.1 µg min^?1 with high FE of 14.57%. Furthermore, density functional theory calculations demonstrate that the N₂ molecule has strong chemical adsorption energy on the iron atom of FeS₂. Overall, iron pyrite‐based materials have proven to be a potential electrocatalyst with photoactive behavior for ammonia production in practical applications.     

Untangling Cooperative Effects of Pyridinic and Graphitic Nitrogen Sites at Metal‐Free N‐Doped Carbon Electrocatalysts for the Oxygen Reduction Reaction

Metal‐free carbon electrodes with well‐defined composition and smooth topography are prepared via sputter deposition followed by thermal treatment with inert and reactive gases. X‐ray photoelectron spectroscopy (XPS) and Raman spectroscopy show that three carbons of similar N/C content that differ in N‐site composition are thus prepared: an electrode consisting of almost exclusively graphitic‐N (N_G), an electrode with predominantly pyridinic‐N (N_P), and one with ≈1:1 N_G:N_P composition. These materials are used as model systems to investigate the activity of N‐doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity toward 4e‐reduction of O₂ is strongly influenced by the N_G/N_P site composition, with the material possessing nearly uniform N_G/N_P composition being the only one yielding a 4e‐reduction. Computational studies on model graphene clusters are carried out to elucidate the effect of N‐site homogeneity on the reaction pathway. Calculations show that for pure N_G‐doping or N_P‐doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak, thus favoring desorption prior to complete 4e‐reduction to hydroxide. Clusters with mixed N_G/N_P sites display synergistic effects, suggesting that co‐presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.     

Biomass Derived N‐Doped Porous Carbon Supported Single Fe Atoms as Superior Electrocatalysts for Oxygen Reduction

Exploring sustainable and high‐performance electrocatalysts for the oxygen reduction reaction (ORR) is the crucial issue for the large‐scale application of fuel cell technology. A new strategy is demonstrated to utilize the biomass resource for the synthesis of N‐doped hierarchically porous carbon supported single‐atomic Fe (SA‐Fe/NHPC) electrocatalyst toward the ORR. Based on the confinement effect of porous carbon and high‐coordination natural iron source, SA‐Fe/NHPC, derived from the hemin‐adsorbed bio‐porphyra‐carbon by rapid heat‐treatment up to 800 °C, presents the atomic dispersion of Fe atoms in the N‐doped porous carbon. Compared with the molecular hemin and nanoparticle Fe samples, the as‐prepared SA‐Fe/NHPC exhibits a superior catalytic activity (E _1/2 = 0.87 V and J _k = 4.1 mA cm^?2, at 0.88 V), remarkable catalytic stability (≈1 mV negative shift of E _1/2, after 3000 potential cycles), and outstanding methanol‐tolerance, even much better than the state‐of‐the‐art Pt/C catalyst. The sustainable and effective strategy for utilizing biomass to achieve high‐performance single‐atom catalysts can also provide an opportunity for other catalytic applications in the atomic scale.     

Dendritic Cell‐Inspired Designed Architectures toward Highly Efficient Electrocatalysts for Nitrate Reduction Reaction

Electrocatalysis for nitrate reduction reaction (NRR) has recently been recognized as a promising technology to convert nitrate to nitrogen. Catalyst support plays an important role in electrocatalytic process. Although porous carbon and metal oxides are considered as common supports for metal‐based catalysts, fabrication of such architecture with high electric conductivity, uniform dispersion of nanoparticles, and long‐term catalytic stability through a simple and feasible approach still remains a significant challenge. Herein, inspired by the signal transfer mode of dendritic cell, an all‐carbon dendritic cell‐like (DCL) architecture comprising mesoporous carbon spheres (MCS) connected by tethered carbon nanotubes (CNTs) with CuPd nanoparticles dispersed throughout (CuPd@DCL‐MCS/CNTs) is reported. An impressive removal capacity as high as 22 500 mg N g^?1 CuPd (≈12 times superior to Fe‐based catalysts), high nitrate conversion (>95%) and nitrogen selectivity (>95%) are achieved under a low initial concentration of nitrate (100 mg L^?1) when using an optimized‐NRR electrocatalyst (4CuPd@DCL‐MCS/CNTs). Remarkably, nitrate conversion and nitrogen selectivity are both close to 100% in an ultralow concentration of 10 mg L^?1, meeting drinking water standard. The present work not only provides high electrocatalytic performance for NRR but also introduces new inspiration for the preparation of other DCL‐based architectures.