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Ammonia, as an important carbon-free energy carrier and also an important chemical for producing fertilisers, is mainly synthesized by a traditional Haber–Bosch process with high ...energy consumption and large amounts of greenhouse gas emissions. Recently, electrocatalytic nitrogen reduction reaction (NRR) has attracted worldwide research attentions as a promising route for achieving green and sustainable ammonia synthesis at ambient conditions. Although exciting advances have been made in the NRR field, the development of electrochemical nitrogen-to-ammonia conversion is still challenging because of the low ammonia yield and unsatisfactory Faradaic efficiency mainly deriving from the poor catalytic activity of catalysts. Herein, various catalyst design strategies for increasing the exposed active sites or altering the electronic structure aiming at improving the apparent activity or intrinsic activity are summarized in this review article. On the basis of effective design strategies, a range of recently reported NRR electrocatalysts, including noble metal-based materials, non-noble metal-based materials, single-metal-atom catalysts, and metal-free materials, are summarized, and the mechanisms of tuning the catalytic activity by applying the design strategies are emphasized based on the combination of theoretical calculations and experimental investigations. It is anticipated that the established correlation between physicochemical properties of catalysts and NRR performance can provide guidance for designing heterogeneous NRR electrocatalysts with high activity, good selectivity, and high stability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The electrocatalytic nitrogen reduction reaction (NRR) has been regarded as a promising strategy for producing ammonia (NH3) at ambient conditions. However, the development of the NRR is severely ...hindered by the difficult adsorption and activation of N2 on the catalyst surface and the competitive hydrogen evolution reaction (HER) due to the lack of efficient NRR electrocatalysts. Herein, Mo2C-MoO2 heterostructure quantum dots embedded in reduced graphene oxide (RGO) are proposed as efficient catalysts for the electrocatalytic NRR. The ultrasmall size of the quantum dot is beneficial for exposing the active sites for the NRR, and the synergetic effect of Mo2C and MoO2 can promote N2 adsorption and activation and suppress the competitive HER simultaneously. As a result, a well-balanced NRR performance is achieved with a high NH3 yield rate of 13.94 ± 0.39 μg h–1 mg–1 at −0.15 V vs RHE and a high Faradaic efficiency of 12.72 ± 0.58% at −0.1 V vs RHE. Density functional theory (DFT) calculations reveal that the Mo2C (001) surface has a strong N2 adsorption energy of −1.47 eV with the side-on configuration, and the NN bond length is elongated to 1.254 Å, indicating the enhanced N2 adsorption and activation on the Mo2C surface. On the other hand, the low ΔG H* for HER over the MoO2 (−111) surface demonstrates the impeded HER process for MoO2. This work may provide effective catalyst-design strategies for enhancing the electrocatalytic NRR performance of Mo-based materials.
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IJS, KILJ, NUK, PNG, UL, UM
Electrocatalytic nitrogen reduction reaction (NRR) is promising for achieving clean ammonia (NH
3
) production under mild conditions but suffers from the difficult adsorption/activation of nitrogen ...molecules and the severe hydrogen evolution reaction (HER). Herein, crystalline-amorphous interfaces between crystalline Bi and amorphous MoO
x
anchored on reduced graphene oxide (RGO) (Bi-MoO
x
@RGO) are constructed for achieving the electrochemical NRR. The Bi-MoO
x
@RGO electrocatalysts show excellent NRR performance with an NH
3
yield rate of 19.93 ± 0.47 μg h
−1
mg
−1
at −0.4 V
vs.
reversible hydrogen electrode (RHE) and a faradaic efficiency of 17.17 ± 0.81% at −0.3 V
vs.
RHE. The combination of experimental results and theoretical calculations reveals that the boosted NRR performance is due to the crystalline Bi-amorphous MoO
x
interfaces which facilitate the adsorption/activation of N
2
while suppressing the competitive HER, thus achieving the simultaneous enhancement of NH
3
yield rate and the faradaic efficiency of the NRR. The utilization of the gas diffusion electrode in the flow cell further increased the NH
3
yield rate to 35.29 ± 1.08 μg h
−1
mg
−1
at −0.3 V
vs.
RHE by virtue of enhanced N
2
transportation. This work paves the way for the rational design of electrocatalysts by phase engineering and interface modulation for the efficient electrocatalytic NRR.
Crystalline Bi-amorphous MoO
x
interfaces anchored on RGO exhibit superior electrocatalytic NRR performance by enhancing the N
2
adsorption/activation while suppressing the competitive HER process.
Abstract
Electrocatalytic C‐N coupling reaction is regarded as a promising strategy for achieving clean and sustainable urea production by coreducing CO
2
and nitrogen species, thus contributing to ...carbon neutrality and the artificial nitrogen cycle. However, restricted by the sluggish adsorption of reactants, competitive side reactions, and multistep reaction pathways, the electrochemical urea production suffers from a low urea yield rate and low selectivity so far. In order to comprehensively improve urea synthesis performance, it is crucial to develop highly efficient catalysts for electrochemical C‐N coupling. In this article, the catalyst‐designing strategies, C‐N coupling mechanisms, and fundamental research methods are reviewed. For the coreduction of CO
2
and different nitrogen species, several prevailing reaction mechanisms are discussed. With the aim of establishing the standard research system, the fundamentals of electrocatalytic urea synthesis research are introduced. The most important catalyst‐designing strategies for boosting the electrocatalytic urea production are discussed, including heteroatom doping, vacancy engineering, crystal facet regulation, atom‐scale modulation, alloying and heterostructure construction. Finally, the challenges and perspectives are proposed for future industrial applications of electrochemical urea production by C‐N coupling.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Electrocatalytic nitrogen reduction reaction (NRR) is a promising strategy for ammonia (NH3) production under ambient conditions. However, it is severely impeded by the challenging activation of the ...NN bond and the competing hydrogen evolution reaction (HER), which makes it crucial to design electrocatalysts rationally for efficient NRR. Herein, the rational design of bismuth (Bi) nanoparticles with different oxidation states embedded in carbon nanosheets (Bi@C) as efficient NRR electrocatalysts is reported. The NRR performance of Bi@C improves with the increase of Bi0/Bi3+ atomic ratios, indicating that the oxidation state of Bi plays a significant role in electrochemical ammonia synthesis. As a result, the Bi@C nanosheets annealed at 900 °C with the optimal oxidation state of Bi demonstrate the best NRR performance with a high NH3 yield rate and remarkable Faradaic efficiency of 15.10 ± 0.43% at −0.4 V versus RHE. Density functional theory calculations reveal that the effective modulation of the oxidation state of Bi can tune the p‐filling of active Bi sites and strengthen adsorption of *NNH, which boost the potential‐determining step and facilitate the electrocatalytic NRR under ambient conditions. This work may offer valuable insights into the rational material design by modulating oxidation states for efficient electrocatalysis.
An oxidation state modulation strategy is proposed to boost nitrogen reduction to ammonia. As a proof‐of‐concept, the surface oxidation of Bi species is tuned with the less occupied p orbital, which leads to stronger adsorption of *NNH and lower ΔG of the potential‐determining step. By optimizing Bi surface oxidation, superior nitrogen reduction reaction performance of Faradaic efficiency of 15.10 ± 0.43% is achieved.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Electrocatalytic nitrogen reduction reaction (NRR) is promising for achieving ammonia (NH3) synthesis under ambient conditions, but restricted by the lack of efficient catalysts. Herein, 3D ...hierarchical architectures of Mo2C nanosheets supported on porous carbon matrix (Mo2C@C) are rationally designed for boosting the electrocatalytic NRR. The morphology engineering endows Mo2C@C electrocatalysts with hierarchically porous architectures, which can sufficiently expose the surface-active sites and enhance the mass diffusion and electron transfer. Moreover, the composition modulation strategy can not only retain the adequate adsorption of N2, but also restrict the hydrogen evolution reaction (HER). Accordingly, Mo2C@C electrocatalysts exhibit a high NH3 yield rate of 12.55 ± 1.04 μg/h/mg at −0.20 V vs. RHE and a well-balanced Faradaic efficiency. Theoretical calculations further confirm the strong N2 adsorption/activation on the Mo2C catalyst and the NRR mechanism of the preferred distal pathway. The morphology engineering and composition modulation strategies can be extended to other materials, guiding the rational design of efficient NRR catalysts.
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•3D hierarchically porous Mo2C@C was constructed via a salt-template method.•The hierarchically porous structures can effectively increase the surface-active sites.•Modulation of the Mo2C content can balance the competitive HER process and the N2 adsorption/activation.•DFTcalculations reveal the NRR mechanism on the Mo2C catalyst.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Vanadium carbide with periodic carbon vacancies (□C-VC) was synthesized for efficient catalyzing N2 to NH3. The as-prepared □C-VC with high density of carbon vacancies and mesoporous structure has ...displayed superior activity as well as durability towards electrocatalytic nitrogen reduction reaction.
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Although electrocatalytic nitrogen reduction reaction (NRR) has been considered as an emerging pathway to produce ammonia (NH3) under ambient conditions owing to its low energy consumption, it still lacks efficient the electrocatalysts to dissociate inert NN bonds. Here, we develop an efficient approach to produce vanadium carbide with abundant periodic carbon vacancies (12.5 at. %) and mesoporous structure as electrocatalysts for NRR via a carbothermic reaction. The typical synthesis protocol involves the use of zinc vanadate decorated vanadium pentoxide nanosheets to homogeneously guide the nucleation and growth of metal organic frameworks (MOFs) on their surface, thus facilitating the in-situ formation of unique vanadium carbide during the subsequent carbothermic reaction. Owing to the optimized substrate-adsorbate binding strength, the intrinsic periodic carbon vacancies of the resultant vanadium carbide could act as coordinatively unsaturated sites to adsorb and activate nitrogen through π-back-donation process, thus promoting the reduction of N2 to NH3. As a consequence, a high yield rate and high Faradaic efficiency with good stabilities are achieved for producing NH3 under ambient conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Hydrogen (H
2
) is an important clean energy carrier due to the merits of high combustion value and zero-carbon emission. Water electrolysis has been regarded as a promising technology for achieving ...green and sustainable production of H
2
. However, most of the electrocatalysts for water splitting can only work at low current density with poor long-term durability, and it is difficult to meet the extensive requirements of industrial-scale applications. In this article, challenges including the charge transfer, mass diffusion, and catalyst stability during high-current-density water electrolysis are discussed. With the aim of addressing these issues, various electrocatalyst design strategies including morphology engineering, electronic structure modulation, and surface/interface engineering are summarized in detail. For the purpose of promoting practical applications, recent achievements of practical anion exchange membrane water electrolysis (AEMWE) and proton exchange membrane water electrolysis (PEMWE) technologies are discussed. Finally, outlooks toward future investigations on high-current-density water electrolysis are presented. It is believed that this review will guide the rational design of catalysts with both high activity and high stability for high-current-density water electrolysis, and promote the development of industrial-scale green H
2
production.
Challenges and design strategies of electrocatalysts for high-current-density water electrolysis.
Mesoporous NiCo2O4 fibers were synthesized by the coordination co-precipitation-decomposition method using ammonia as morphology-controller. The as-prepared NiCo2O4 powders were characterized by ...X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis absorption spectra and photoluminescence (PL) analysis. The as-prepared NiCo2O4 materials showed typical intertwined porous nanofibrous structures with specific surface area of 74.6 m2/g and average pore size of 11.79 nm. The mesoporous NiCo2O4 fibers showed remarkably enhanced photocatalytic performance towards methyl red (MR) degradation under visible-light irradiation compared to the Co3O4 and NiO. The degradation efficiency of MR over the NiCo2O4 reached up to 95.1% after irradiation for 120 min. Superior photocatalytic activity of 1D NiCo2O4 fibers can be attributed to the higher light harvesting capacity resulted from large surface area and unique mesoporous structure. Finally, a possible mechanism for the photodegradation of MR over the NiCo2O4 was proposed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
