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The synthesis of NH 3 is mainly dominated by the traditional energy-consuming Haber−Bosch process with a mass of CO 2 emission. Electrochemical conversion of N 2 to NH 3 emerges as a carbon-free process for the sustainable artificial N 2 reduction reaction (NRR), but requires an efficient and stable electrocatalyst. Here, we report that the Mo 2 C nanorod serves as an excellent NRR electrocatalyst for artificial N 2 fixation to NH 3 with strong durability and acceptable selectivity under ambient conditions. Such a catalyst shows a high Faradaic efficiency of 8.13% and NH 3 yield of 95.1 μg h −1 mg −1 cat at −0.3 V in 0.1 M HCl, surpassing the majority of reported electrochemical conversion NRR catalysts. Density functional theory calculation was carried out to gain further insight into the catalytic mechanism involved. A s a necessary industrial chemical, NH 3 has been employed in medication, fertilizer, fuel, and explosives, etc. 1−5 Today, ever-increasing NH 3 consumption stimulates intensive research on artificial N 2 fixation technology. 6−10 However, industrial-scale NH 3 production mainly depends on the Haber−Bosch process, which is performed under rigorous conditions (350−550°C and 150−350 atm) with rather high energy consumption and CO 2 emission. 11,12 Therefore, it is urgently desired to develop an energy-saving and environmentally benign technological process for NH 3 production. Electrochemical reduction has emerged as a promising method for artificial N 2 fixation under ambient conditions. 13 However, the N 2 reduction reaction (NRR) process needs to break a rather inert molecular structure of N 2 with extremely high bond energy of about 941 kJ mol −1. 6 Thus, electro-catalysts with high activity for the NRR are a prerequisite. 13−15 In nature, N 2 fixation can be catalyzed under ambient conditions by Mo-dependent nitrogenases, via multiple proton and electron transfer steps. 16−18 Mo has also emerged as an interesting metal for homogeneous N 2 functionalization reactions, and some Mo-based molecular complexes have been designed 19,20 and synthesized for artificial N 2 fixa-tion. 21−24 However, other than stability of these catalysts, it is also challenging to effectively graft such catalysts onto electrodes for electrochemical tests and applications. Therefore , it is highly pressing to develop Mo-based heterogeneous electrocatalysts to solve these problems. Recently, (110)-oriented Mo nanofilm was reported for N 2 reduction electrocatalysis with only a Faradaic efficiency (FE) of 0.72%. 25 Our recent studies suggest that MoS 2 , 26 MoO 3 , 27 MoN, 28 and Mo 2 N 29 are effective for the NRR process with Faradaic efficiencies of 1.17%, 1.9%, 1.15%, and 4.5%, respectively. As such, to develop new Mo-based electro-catalysts for the NRR with improved activity is highly desired. Here, we present our recent study in developing the Mo 2 C nanorod as a superb NRR catalyst for artificial N 2 fixation to NH 3 with strong electrochemical durability and acceptable selectivity under ambient conditions. Such Mo 2 C achieves an FE as high as 8.13% with NH 3 yield of 95.1 μg h −1 mg −1 cat at
The Journal of Physical Chemistry C, 2018
Electrochemical N 2 reduction reaction (NRR) under ambient conditions offers us an environmentally friendly route for artificial synthesis of NH 3. However, up to now, few noble-metal-free electrocatalysts with satisfactory catalytic activities have been explored. In this Letter, we demonstrate that MoN nanosheets array on carbon cloth (MoN NA/CC) acts as a high-performance NRR electrocatalyst toward NH 3 electrosynthesis in 0.1 M HCl under ambient conditions. This catalyst achieves a large NH 3 yield of 3.01 × 10 −10 mo1 s −1 cm −2 and a Faradaic efficiency of 1.15% at −0.3 V vs reversible hydrogen electrode with strong electrochemical durability and selectivity. Density functional theory calculations reveal that MoN NA/CC catalyzes NRR via the Mars−van Krevelen mechanism.
Electrocatalytic N 2 reduction under ambient conditions is a promising alternative to the traditional Haber-Bosch process for environmentally benign and sustainable NH 3 production but requires efficient electrocatalysts for the N 2 reduction reaction (NRR). Here, we report that metal-organic framework-derived shuttle-like V 2 O 3 /C acts as an outstanding NRR electrocatalyst for N 2-to-NH 3 conversion with excellent selectivity under ambient conditions. In 0.1 M Na 2 SO 4 , such V 2 O 3 /C exhibits a remarkable NH 3 yield of 12.3 μg h −1 mg −1 cat. and a high faradaic efficiency of 7.28% at a potential of −0.6 V versus reversible hydrogen electrode, outperforming most of the reported aqueous-based NRR electrocatalysts. Notably, it also shows high electrochemical and structural stability. Ammonia (NH 3) has found wide applications in fertilizer production , pharmaceutical production, and many other industrial processes. 1,2 NH 3 is also regarded as a carbon-neutral liquid fuel with high energy density and suitability, and it plays a significant role in the future hydrogen economy. 3-5 Currently, NH 3 synthesis is heavily dependent on the traditional Haber-Bosch process at high temperature (300-500°C) and high pressure (150-300 atm), 6,7 accounting for more than 1% of the world's energy supply and responsible for 450 million metric tons of CO 2 emission annually. 8,9 This pushes the researchers to explore alternative approaches for sustainable NH 3 synthesis under ambient conditions, which can simultaneously reduce the CO 2 emission. Electrocatalytic N 2 reduction can not only be conducted in aqueous media but also tackle the H 2-and energy-intensive operations by the Haber-Bosch process, thus offering us an attractive strategy toward artificial N 2 fixation to NH 3 under ambient conditions. 10-17 However, the high bond energy of diatomic N 2 (NuN bond energy of 940.95 kJ mol −1) and its lack of a permanent dipole make running the NRR under ambient conditions extremely challenging, 18,19 which requires efficient electrocatalysts for effective N 2 activation. A series of precious metals (e.g., Au, 20 Ag, 21 Ru, 22 and Rh 23) were studied, but the scarcity and high cost limit their widespread uses, inspiring the development of non-precious alternatives. 24-31 As a transition metal element, V has been found in some organisms such as algae and fungi. 32 Meanwhile, V is directly relevant to the active center or cofactor of nitrogenase. 32,33 Theoretical 34 and experimental 17 studies also suggest that VN is active for electrocatalytic artificial N 2 fixation. Such nitride suffers from using NH 3 as a N source for catalyst preparation; however, its oxide counterpart can be more easily obtained on a large scale, holding greater promise for electrocatalytic N 2 reduction application. Meanwhile, V oxide nanomaterials have been widely used in batteries and electrolysis due to their structural flexibility and distinctive redox behavior. 35 Among various V oxides, V 2 O 3 has less toxicity 36 but its intrinsically low electrical conductivity 37 is not favourable for electro-chemical performances. Metal-organic frameworks (MOFs) comprising metal ions (or metal clusters) and organic linkers are considered as desirable self-templates for conductive metal oxide/carbon composites. 38 It is thus anticipated that the V 2 O 3 /C composite is an efficient NRR catalyst, which, however, has never been explored before. In this communication, we report our recent experimental verification that a MOF-derived shuttle-like V 2 O 3 /C hybrid is effective for electrochemical N 2-to-NH 3 fixation with excellent selectivity under ambient conditions. In 0.1 M Na 2 SO 4 , the resulting V 2 O 3 /C shuttle achieves a remarkable NH 3 yield of † Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See
Journal of The Electrochemical Society
Currently,N H 3 productionp rimarily depends on the Haber-Bosch process, which operates at elevated temperatures and pressures and leads to serious CO 2 emissions. Electrocatalytic N 2 reduction offersa ne nvironment-ally benign approachf or the sustainable synthesis of NH 3 under ambient conditions. This work reports the development of biomass-derived amorphous oxygen-doped carbon nanosheet (OÀCN) using tannin as the precursor. As am etal-free electrocatalyst for N 2-to-NH 3 conversion, such OÀCN shows high catalytic performances,a chieving al arge NH 3 yield of 20.15 mgh À1 mg À1 cat. and ah igh Farad-ic efficiencyo f4 .97 %a tÀ0.6 Vv s. reversibleh ydrogen electrode (RHE) in 0.1 m HCl at ambient conditions. Remarkably ,i ta lso exhibits highe lectrochemical selectivity and durability. As one of the extremelyc ommon industrial chemicals, NH 3 is widely used in variousf ields such as agriculture and industry. [1-3] In addition to the biosynthesis of NH 3 , [4] industrially, NH 3 is primarily generated from N 2 through the Haber-Bosch process , yieldingr oughly5 00 million tons per year with an Fe-or Ru-basedc atalyst. [5, 6] Such ap rocess, however,o perates at high temperatures (400-500 8C) and pressures (200-250 atm), [6, 7] accounting for % 2% of the world's energy consumption and releasingl arge amountso fC O 2 annually. [2, 8] Hence, as ustainable and economical route to produce NH 3 is currently of paramount importance. Electrochemical N 2 reduction is regarded as as ustainable alternative for ambient NH 3 production,b ut efficient electrocata-lysts are neededt om eet the challenge to break the strong N Nt riple bond of N 2. [9-11] Up till now,g reat efforts have been made to explore earth-abundantt ransition-metal-based catalyst materials. [12-21] Nevertheless, transition metals that bind N 2 too weakly have poor capability for activating N 2. [22, 23] Additionally ,t he inherentc orrosion and oxidation susceptibility of a wide variety of transition-metal-based materials largely restricts their application in acidic proton-exchange membrane-based electrochemical devices. [24, 25] Because of the abundance of carbon-based material and its robust tolerance to acidic conditions , it possesses unique advantages for designated cataly-sis. [22-26] Doping of non-metal heteroatoms (e.g.,O ,B, S, N, P) has been demonstrated to be af easible way to tune the electronic character and electrochemical properties of such materials , [25, 27-29] promisingi ts use in designing carbon-based metal-free catalysts for N 2 reduction reaction (NRR) applications. [30, 31] In this work, we report the first experimental evidencet hat biomass-derived amorphous oxygen-doped carbon nanosheet (OÀCN) acts as an efficient NRR electrocatalyst with excellent selectivity.I n0 .1 m HCl, such OÀCN attainsalarge NH 3 yield of 20.15 mgh À1 mg À1 cat. with aF aradic efficiency (FE) of 4.97 %a t À0.6 Vv ersus reversibleh ydrogen electrode (RHE). Remarkably, the catalyst also exhibits high electrochemical durability. OÀCN was synthesized by carbonization of tannin in Ar gas at 900 8Cf or 1h.C oncerning the carbon structure, as shown in Figure 1a,t he sample exhibits relativelyb road diffraction peaks centered at % 238 and aw eaker peak at % 438,s uggest-ing the formation of an amorphous phase. [32, 33] The transmission electron microscopy (TEM) pattern of OÀCN (Figure 1b) verifiesits sheet-like morphology.F igure 1c displays ah igh-res-olutionT EM (HRTEM) image and the corresponding selected area electron diffraction (SAED) pattern. The results further demonstrated that OÀCN is of amorphous structure.T he energyd ispersive X-ray (EDX) elemental mapping images (Fig-ure 1d)r eveal Oa nd Ce lements are well-distributed throughout OÀCN. X-ray photoelectron spectroscopy (XPS) was utilized to analyze the elementalc omposition and configurationi nO ÀCN. Figure 2a reveals the XPS survey spectrum of OÀCN, presenting the coexistence of Ca nd Oe lements with the contento f 94.76 %a nd 5.24 %, respectively.F or the C1sr egion (Fig-ure 2b), the C1ss ignal at 284.8 eV can be divided into as harp peak that is sp 2 CÀC(284.8 eV) bond and the other three ones from CÀO(285 eV), C=O(286.2 eV) and OÀC=O(289 eV) [a] Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.
Currently, industrial-scale NH 3 production almost relies on energy-intensive Haber-Bosch process from atmospheric N 2 with large amount of CO 2 emission, while low-cost and high-efficient catalysts are demanded for the N 2 reduction reaction (NRR). In this study, Mn 3 O 4 nanoparticles@reduced graphene oxide (Mn 3 O 4 @rGO) composite is reported as an efficient NRR electrocatalyst with excellent selectivity for NH 3 formation. In 0.1 M Na 2 SO 4 solution, such catalyst obtains a NH 3 yield of 17.4 µg·h −1 ·mg −1 cat. and a Faradaic efficiency of 3.52% at −0.85 V vs. reversible hydrogen electrode. Notably, it also shows high electrochemical stability during electrolysis process. Density functional theory (DFT) calculations also demonstrate that the (112) planes of Mn 3 O 4 possess superior NRR activity.
Ammonia (NH3) is a key agricultural component and a source of clean energy as a hydrogen mediator. Ammonia is produced by Haber Bosch process, resulting in massive energy consumption and severe environmental impact. It is a thriving challenge to design and develop efficient nitrogen reduction reaction (NRR) and nitrate reduction reaction (NO3RR) electrocatalysts using variable reactants sources for ammonia synthesis at STP. 2D graphene sheets wrapped cobalt phthalocyanine nanotube to obtain (1D-2D) heterostructure, which serve as an active bifunctional electrocatalyst for NRR and NO3RR. At –0.2 V vs RHE, the electrocatalyst displayed NH3 yield rate of 58.82 μg h−1 mg−1 cat and a Faradaic efficiency (FE) of 95.12 % for NO3RR and 143.38 μg h−1 mg−1 cat and 43.69 % for NRR. Isotope tracing experiment confirmed the origin of ammonia synthesis. DFT calculations through Bader charge analysis revealed that charge transfer from the RGO to the Co-N4 sites in CoPc aided in the formation of NN...
Ammonia (NH 3) is an industrially important chemical for its use in manufacturing fertilizers, carbon-free fuel and synthesis of essential biological building blocks and as energy carrier. The most widely used industrial process NH 3 production, the Haber-Bosch process, has several bottlenecks such as high operational costs and high energy consumption and is a severe detriment for environment due to its large carbon footprint. In recent decades, electrocatalysis of N 2 to produce NH 3 has emerged as a sustainable alternative and provides an efficient means for the production of NH 3 from N 2 under ambient conditions. Till date, various kinds of electrocatalyst have been developed for N 2 reduction which covers a wide range of materials that includes noble metals, transition metals, single-atom catalyst and various carbon-based metal-free composites. Also, to increase the catalytic potential, different operational strategies have been developed that generate electrocatalysts with low overpotential. Molecular dynamics simulation-based studies have enabled the development of new generation electrocatalysts and have been investigated for their thermodynamics and mechanism in nitrogen reduction reaction (NRR). The combination of the theoretical and experimental provides a promising perspective to develop efficient electrocatalyst with increased surface active site, selectivity and durability in NRR.
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