N2 Reduction Reaction Research Articles

Photoelectrochemical Reduction of CO2 to Syngas by Reduced Ag Catalysts on Si Photocathodes

The photoelectrochemical reduction of CO2 to syngas that is used for many practical applications has been emerging as a promising technique to relieve the increase of CO2 in the atmosphere. Si has been considered to be one of the most promising materials for photoelectrodes, but the integration of electrocatalysts is essential for the photoelectrochemical reduction of CO2 using Si. We report an enhancement of catalytic activity for CO2 reduction reaction by Ag catalysts of tuned morphology, active sites, and electronic structure through reducing anodic treatment. Our proposed photocathode structure, a SiO2 patterned p-Si photocathode with these reduced Ag catalysts, that was fabricated using electron-beam deposition and electrodeposition methods, provides a low onset-potential of −0.16 V vs. the reversible hydrogen electrode (RHE), a large saturated photocurrent density of −9 mA/cm2 at −1.23 V vs. RHE, and faradaic efficiency for CO of 47% at −0.6 V vs. RHE. This photocathode can produce syngas in the ratio from 1:1 to 1:3, which is an appropriate proportion for practical application. This work presents a new approach for designing photocathodes with a balanced catalytic activity and light absorption to improve the photoelectrochemical application for not only CO2 reduction reaction, but also water splitting or N2 reduction reaction.

Direct Synthesis of Ammonia from N2 and H2O on Different Iron Species Supported on Carbon Nanotubes using a Gas‐Phase Electrocatalytic Flow Reactor

AbstractGreen NH3 production by direct electrocatalytic synthesis from N2 and H2O is still a challenging reaction, which requires us to better understand the nature of the active materials. We show here that iron oxide (Fe2O3) nanoparticles (supported over carbon nanotubes, CNTs) become more active than the corresponding samples after reduction to form Fe‐ or Fe2N‐supported nanoparticles, both indicated as active species in the thermal catalytic reduction of N2 to ammonia. Characterization data, however, indicate that even for these Fe‐ and Fe2N−CNT samples, obtained from Fe2O3−CNT by reduction in H2 or NH3 at 500 °C, the active species responsible for N2 reduction reaction (NRR) at low applied potential (−0.5 V vs RHE) are the same, that is, small (<1–2 nm) iron oxide nanoparticles that are not detected by XRD, but evidenced by XPS and which amount could be correlated to the rate of ammonia formation. This species is stable for at least 24 h of electrocatalytic flow tests. However, at higher applied potentials, sintering/transformation of this species occurs, with loss of the electrocatalytic activity, and Fe2N nanoparticles may also be reduced in situ, forming ammonia, but with irreversible deactivation.

Ambient Ammonia Electrosynthesis: Current Status, Challenges, and Perspectives.

Ammonia (NH3 ) electrosynthesis from atmospheric nitrogen (N2 ) and water is emerging as a promising alternative to the energy-intensive Haber-Bosch process; however, such a process is difficult to perform due to the inherent inertness of N2 molecules together with low solubility in aqueous solutions. Although many active electrocatalysts have been used to electrocatalyze the N2 reduction reaction (NRR), unsatisfactory NH3 yields and lower Faraday efficiency are still far from practical industrial production, and thus, considerable research efforts are being devoted to address these problems. Nevertheless, most reports still mainly focus on the preparation of electrocatalysts and largely ignore a summary of optimization-modification strategies for the NRR. In this review, a general introduction to the NRR mechanism is presented to provide a reasonable guide for the design of highly active catalysts. Then, four categories of NRR electrocatalysts, according to chemical compositions, are surveyed, as well as several strategies for promoting the catalytic activity and efficiency. Later, strategies for developing efficient N2 fixation systems are discussed. Finally, current challenges and future perspectives in the context of the NRR are highlighted. This review sheds some light on the development of highly efficient catalytic systems for NH3 synthesis and stimulates research interests in the unexplored, but promising, research field of the NRR.

A Janus Fe-SnO2 Catalyst that Enables Bifunctional Electrochemical Nitrogen Fixation.

Electrochemical N2 reduction reactions (NRR) and the N2 oxidation reaction (NOR), using H2 O and N2 , are a sustainable approach to N2 fixation. To date, owing to the chemical inertness of nitrogen, emerging electrocatalysts for the electrochemical NRR and NOR at room temperature and atmospheric pressure remain largely underexplored. Herein, a new-type Fe-SnO2 was designed as a Janus electrocatalyst for achieving highly efficient NRR and NOR catalysis. A high NH3 yield of 82.7 μg h-1 mgcat. -1 and a Faraday efficiency (FE) of 20.4 % were obtained for NRR. This catalyst can also serve as an excellent NOR electrocatalyst with a NO3 - yields of 42.9 μg h-1 mgcat. -1 and a FE of 0.84 %. By means of experiments and DFT calculations, it is revealed that the oxygen vacancy-anchored single-atom Fe can effectively adsorb and activate chemical inert N2 molecules, lowering the energy barrier for the vital breakage of N≡N and resulting in the enhanced N2 fixation performance.

A Janus Fe‐SnO2Catalyst that Enables Bifunctional Electrochemical Nitrogen Fixation

AbstractElectrochemical N2reduction reactions (NRR) and the N2oxidation reaction (NOR), using H2O and N2, are a sustainable approach to N2fixation. To date, owing to the chemical inertness of nitrogen, emerging electrocatalysts for the electrochemical NRR and NOR at room temperature and atmospheric pressure remain largely underexplored. Herein, a new‐type Fe‐SnO2was designed as a Janus electrocatalyst for achieving highly efficient NRR and NOR catalysis. A high NH3yield of 82.7 μg h−1 mgcat.−1and a Faraday efficiency (FE) of 20.4 % were obtained for NRR. This catalyst can also serve as an excellent NOR electrocatalyst with a NO3−yields of 42.9 μg h−1 mgcat.−1and a FE of 0.84 %. By means of experiments and DFT calculations, it is revealed that the oxygen vacancy‐anchored single‐atom Fe can effectively adsorb and activate chemical inert N2molecules, lowering the energy barrier for the vital breakage of N≡N and resulting in the enhanced N2fixation performance.

NH2–MIL-88B–Fe for electrocatalytic N2 fixation to NH3 with high Faradaic efficiency under ambient conditions in neutral electrolyte

The global NH3 production is dominated by Haber–Bosch process, requiring high temperature and pressure. Electrochemical N2 reduction reaction (NRR) under ambient conditions is a greener path for artificial N2 fixation to NH3 but calling for efficient catalyst to increase activity and selectivity. Herein, we report the iron-based metal–organic frameworks (MOFs), i.e., MIL-88B–Fe and amine-functionalized MIL-88B–Fe (NH2–MIL-88B–Fe) as efficient catalysts for electrochemical NRR under ambient temperature and pressure in neutral electrolyte. NH2–MIL-88B–Fe shows higher NH3 yield rate of 1.205 × 10–10 mol s−1 cm−2 than MIL-88B–Fe (3.575 × 10–11 mol s−1 cm−2). Furthermore, NH2–MIL-88B–Fe exhibits the highest Faradaic efficiency of 12.45% at 0.05 V versus RHE. The control experiments prove that NH3 is produced through electrocatalytic NRR. This work may trigger the interest of using MOFs as highly efficient catalysts for electrochemical NH3 production.

Model Operational Matrix for the Betterment of Ruthenium As a Catalyst for the Electrochemical Nitrogen Reduction Reaction to Ammonia in Aqueous Electrolytes

Ammonia (NH3), as a green energy carrier, potential transportation fuel and chemical for fertilizer synthesis, plays an indispensable role in the agricultural, plastic, pharmaceutical and textile industries1. Industrially, NH3 manufacturing is dominated by the Haber–Bosch (HB) process, which consumes more than 2% of the global energy supply, and releases 1.87 tons of greenhouse gas, carbon dioxide (CO2), per 1 ton of NH3 2. This energy-intensive process is also inefficient and relatively low conversion ratio are achieved due to unfavorable chemical equilibrium 3. Hence, it is of great significance to develop alternative routes for more efficient N2 fixation under milder conditions. Recently, a worldwide gold rush has been triggered, and many pioneering methods are being investigated to convert N2 to NH3, including biological catalysis 4, photocatalysis 5,6and electrocatalysis 2,7. Particularly, electrochemical reduction of N2 to NH3 is thermodynamically predicted to be more energy efficient than the HB process by about 20% 7,8 . An electrochemical process could also provide the benefit of reducing greenhouse gas emission as the source of H2 is the electrolysis of water molecules instead of natural gas. With this scenario, ammonia would be synthesized in a carbon-neutral manner if renewable electrical energy is used.In the electrochemical N2 reduction reaction (NRR) system, though electrocatalysts are the paramount components, a rational cell design, synthesize and operation conditions are very vital 4. Most of recent studies have looked on a single parameter (or two) such as catalyst morphology, catalyst deposition and loading, nitrogen reduction potential or type, temperature, pressure and components of cell and electrolytes. However, the fact that NRR and catalyst deposition is a multistep process, sampled parameters study might not provide sufficient information about the actual electrocatalytic process. Therefore, our group tried to determine the best working conditions and ways of designing NRR experiments in order to draw conclusions on the process efficiency in terms of charge used and NH3 yield. In the present study, electrochemically deposited Ru metal catalysts have been investigated. It is found that in ambient reaction conditions and in highly concentrated electrolytes, a Faradic Efficiency as high as 1.2 % can be reached by optimizing the Ru deposition morphology and deposition time (loading), as well as the NRR potential, nature of cation/anion exchange membranes and size of the counter cations in the electrolyte. This is a 4-fold improvement compared to the maximum efficiency reported 4 with the same catalyst (< 0.3 %).

(Invited) Reduced Graphene Oxides with Engineered Defects for Efficient Electrochemical Reduction of Dinitrogen to Ammonia in Wide pH Range

Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic ovepotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at -0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at -0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg·h-1 mg-1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.

Oxidation‐etching induced morphology regulation of Cu catalysts for high‐performance electrochemical N2 reduction

AbstractRenewable‐electricity‐driven N2 reduction is an attractive approach for ambient NH3 synthesis, but active electrocatalysts are needed to enable the N2 reduction reaction. Monolithic electrodes with active components anchored on conductive supports provide many advantages like structural stability, large surface area, and low electrical resistance. Here, a novel “oxidation‐etching” strategy is proposed to carve the surface of Cu foam into structures of particles, cubes, and sheets for N2 reduction electrocatalysis. The optimal catalyst achieves a Faradic efficiency as high as 18% at −0.35 V vs reversible hydrogen electrode (RHE) and a large NH3 yield of 2.45 × 10−10 mol s−1 cm−1 at −0.40 V vs RHE in 0.1 M HCl. Notably, it also shows superior long‐term electrochemical durability, with the preservation of electro‐activity for at least 20 hours.image

Rational Design of Hydroxyl‐Rich Ti3C2Tx MXene Quantum Dots for High‐Performance Electrochemical N2 Reduction

AbstractTo enable an efficient and cost‐effective electrocatalytic N2 reduction reaction (NRR) the development of an electrocatalyst with a high NH3 yield and good selectivity is required. In this work, Ti3C2Tx MXene‐derived quantum dots (Ti3C2Tx QDs) with abundant active sites enable the development of efficient NRR electrocatalysts. Given surface functional groups play a key role on the electrocatalytic performance, density functional theory calculations are first conducted, clarifying that hydroxyl groups on Ti3C2Tx offer excellent NRR activity. Accordingly, hydroxyl‐rich Ti3C2Tx QDs (Ti3C2OH QDs) are synthesized as NRR catalysts by alkalization and intercalation. This material offers an NH3 yield and Faradaic efficiency of 62.94 µg h−1 mg−1cat. and 13.30% at −0.50 V, respectively, remarkably higher than reported MXene catalysts. This work demonstrates that MXene catalysts can be mediated through the optimization of both QDs sizes and functional groups for efficient ammonia production at room temperature.

Self-supporting CoMoC nanoporous catalysts for N2 reduction reaction under ambient conditions

Electrochemical nitrogen reduction is expected to replace the industrial Haber-Bosch process due to its environment-friendly and sustainable development route, while the main challenge is to develop active and selective N2 reduction reaction (NRR) electrocatalysts. Here, we prepare the self-supporting CoMoC nanoporous catalyst by electrochemical dealloying method. Due to the nanoporous structure and MoC compositions, the CoMoC catalyst exhibits high catalytic activity and outstanding selectivity to NRR. The intimate between Co and MoC favors in electron transport, resulting in promoting the activity of the whole nitrogen reduction reaction process. In 0.1 M PBS (PH = 7), the NH3 yield rate and faradaic efficiency (FE) at 0.1 V (vs. RHE) are 18.9 ug cm−2 h−1 and 8.9%, respectively. This work opens up new opportunities to develop self-supporting CoMoC nanoporous catalysts with good conductivity and activity for NRR.

Porous LaFeO3 nanofiber with oxygen vacancies as an efficient electrocatalyst for N2 conversion to NH3 under ambient conditions

Electrocatalytic N 2 reduction to NH 3 under ambient conditions is an eco-friendly and sustainable alternative to the traditional Haber-Bosch process. However, inhibited by the high activation barrier of N 2 , this process needs efficient electrocatalysts to adsorb and activate the N 2 , enabling the N 2 reduction reaction (NRR). Herein, we report that porous LaFeO 3 nanofiber with oxygen vacancies acts as an efficient NRR electrocatalyst with abundant active sites to enhance the adsorption and activation of N 2 . When tested in 0.1 M HCl, such electrocatalyst achieves a high Faradaic efficiency of 8.77% and a large NH 3 yield rate of 18.59 µg h –1 mg cat. –1 at −0.55 V versus reversible hydrogen electrode. This catalyst also shows high long-term electrochemical stability and excellent selectivity for NH 3 formation. Density functional theory calculations reveal that, by introducing oxygen vacancy on LaFeO 3 , the subsurface metallic ions are exposed with newly localized electronic states near the Fermi level, which facilitates the adsorption and activation of N 2 molecules as well as the subsequent hydrogenation reactions. As a N 2 reduction reaction electrocatalyst, porous LaFeO 3 nanofiber with oxygen vacancies (Vo-LaFeO 3 NF) achieves a large NH 3 yield of 18.59 μg h −1 mg cat. −1 and a high Faradic efficiency of 8.77% at –0.55 V vs. RHE in 0.1 M HCl.

Computational Design of Transition Metal Single-Atom Electrocatalysts on PtS2 for Efficient Nitrogen Reduction.

Electrocatalytic nitrogen reduction is promising to serve as a sustainable and environmentally friendly strategy to achieve ammonia production. Single-atom catalysts (SACs) hold great promise to convert N2 into NH3 because of the unique molecular catalysis property and ultrahigh atomic utilization ratio. Here, we demonstrate a universal computational design principle to assess the N2 reduction reaction (NRR) performance of SACs anchored on a monolayer PtS2 substrate (SACs-PtS2). Our density functional theory simulations unveil that the barriers of the NRR limiting potential step on different SAC centers are observed to be linearly correlated to the integral of unoccupied d states (UDSs) of SACs. As a result, the Ru SAC-PtS2 catalyst with the largest number of UDSs exhibits a much lower barrier of the limiting step than those of other SACs-PtS2 catalysts and the Ru(0001) benchmark. Our work bridges the apparent NRR activity and intrinsic electronic structure of SAC centers and offers effective guidance to screen and design efficient SACs for the electrochemical NRR process.

Recent Progress on Catalysts towards Electrocatalytic Nitrogen Reduction Reaction

The energy-intensive Haber-Bosch process currently dominants the production of ammonia (NH3), an indispensable chemical for humans. For the sustainable development of society, highly efficient and green strategies to convert nitrogen (N2) to NH3 are urgently required. Electrocatalytic N2 reduction reaction (eNRR) is universally regarded as a promising strategy owing to the mild operating conditions and renewable energy supply. The key for eNRR is the high-performance catalysts, which activate the inert N-N triple bond and thus decrease the energy barrier. Herein, the recent theoretical and experimental progress on eNRR catalysts at room temperature and ambient pressure is summarized, aiming to provide a reference for future design of high-performance eNRR catalysts.

Relating N-H Bond Strengths to the Overpotential for Catalytic Nitrogen Fixation.

Nitrogen (N2) fixation to produce bio-available ammonia (NH3) is essential to all life but is a challenging transformation to catalyse owing to the chemical inertness of N2. Transition metals can, however, bind N2 and activate it for functionalization. Significant opportunities remain in developing robust and efficient transition metal catalysts for the N2 reduction reaction (N2RR). One opportunity to target in catalyst design concerns the stabilization of transition metal diazenido species (M-NNH) that result from the first N2 functionalization step. Well-characterized M-NNH species remain very rare, likely a consequence of their low N-H bond dissociation free energies (BDFEs). In this essay, we discuss the relationship between the BDFEN-H of a given M-NNH species to the observed overpotential and selectivity for N2RR catalysis with that catalyst system. We note that developing strategies to either increase the N-H BDFEs of M-NNH species, or to avoid M-NNH intermediates altogether, are potential routes to improved N2RR efficiency.

Highly Efficient Photoelectrochemical Synthesis of Ammonia Using Plasmon-Enhanced Black Silicon under Ambient Conditions.

Although photoelectrochemical synthesis of NH3 is considered as an eco-friendly and sustainable process under ambient conditions, stable and highly efficient catalysts for the N2 reduction reaction are still lacking because of the chemically inert nature of the triple bonds in elemental nitrogen and the competitive reaction of water reduction. In this paper, a photoelectrochemical N2 reduction reaction route is proposed through combining black silicon and Ag nanoparticles using a simple deposition method. The synergetic effect of Ag nanoparticles and black Si significantly enhances the activity for the ammonia evolution reaction. The obtained Ag/bSi photocathode reaches a high Faraday efficiency of 40.6% and an NH3 yield of 2.87 μmol h-1 cm-2 at -0.2 V versus the reversible hydrogen electrode in 0.1 M Na2SO4.

Si-Doped Fe Catalyst for Ammonia Synthesis at Dramatically Decreased Pressures and Temperatures.

The Haber-Bosch (HB) process combining nitrogen (N2) and hydrogen (H2) into ammonia (NH3) gas plays an essential role in the synthesis of fertilizers for food production and many other commodities. However, HB requires enormous energy resources (2% of world energy production), and the high pressures and temperatures make NH3 production facilities very expensive. Recent advances in improving HB catalysts have been incremental and slow. To accelerate the development of improved HB catalysts, we developed a hierarchical high-throughput catalyst screening (HHTCS) approach based on the recently developed complete reaction mechanism to identify non-transition-metal (NTM) elements from a total set of 18 candidates that can significantly improve the efficiency of the most active Fe surface, Fe-bcc(111), through surface and subsurface doping. Surprisingly, we found a very promising subsurface dopant, Si, that had not been identified or suggested previously, showing the importance of the subsurface Fe atoms in N2 reduction reactions. Then we derived the full reaction path of the HB process for the Si doped Fe-bcc(111) from QM simulations, which we combined with kinetic Monte Carlo (kMC) simulations to predict a ∼13-fold increase in turnover frequency (TOF) under typical extreme HB conditions (200 atm reactant pressure and 500 °C) and a ∼43-fold increase in TOF under ideal HB conditions (20 atm reactant pressure and 400 °C) for the Si-doped Fe catalyst, in comparison to pure Fe catalyst. Importantly, the Si-doped Fe catalyst can achieve the same TOF of pure Fe at 200 atm/500 °C under much milder conditions, e.g. at a much decreased reactant pressure of 20 atm at 500 °C, or alternatively at temperature and reactant pressure decreased to 400 °C and 60 atm, respectively. Production plants using the new catalysts that operate under such milder conditions could be much less expensive, allowing production at local sites needing fertilizer.

Bioinspired Mo tape-porphyrin as an efficient and selective electrocatalyst for ammonia synthesis

The efficient N2 fixation to NH3 in the biological system benefits from the electron transfer and catalysis of nitrogenase under ambient conditions, where N2 reduction occurs at Mo and Fe sites. Herein, the electrochemical N2 reduction reaction (NRR) on bioinspired Mo tape-porphyrin (denoted as MoP) has been systematically investigated on the basis of first-principles calculations. The anchoring of Mo atom to the biomolecular porphyrin changes the charge distribution and activates more delocalized π electrons in the fully conjugated π-system. The results reveal that N2 favourably coordinated with Mo atom in end-on configuration and the electronic and magnetic properties of MoP are changed. The most promising reaction pathway for NRR on MoP is via the distal route with overpotential of 0.22 V. The excellent conductivity, highly localized and spin-polarized Mo-4d states of MoP endow it preeminent NRR activity at room temperature. Furthermore, the competitive 2-electron hydrogen evolution reaction (HER) is sufficiently mitigated and circumvented during the NRR progress. The high symmetry of MoP enables a simple synthetic pathway. Our findings provide a ingenious strategy for catalytic N2 reduction and a practical manufacturing method for designing efficient and selective electrocatalyst.

Co3(hexahydroxytriphenylene)2: A conductive metal—organic framework for ambient electrocatalytic N2 reduction to NH3

As a carbon-neutral alternative to the Haber-Bosch process, electrochemical N2 reduction enables environment-friendly NH3 synthesis at ambient conditions but needs active electrocatalysts for the N2 reduction reaction. Here, we report that conductive metal—organic framework Co3(hexahydroxytriphenylene)2 (Co3HHTP2) nanoparticles act as an efficient catalyst for ambient electrochemical N2-to-NH3 fixation. When tested in 0.5 M LiClO4, such Co3HHTP2 achieves a large NH3 yield of 22.14 µg·h−1mg−1cat. with a faradaic efficiency of 3.34% at −0.40 V versus the reversible hydrogen electrode. This catalyst also shows high electrochemical stability and excellent selectivity toward NH3 synthesis.

Low-Coordinate Step Atoms via Plasma-Assisted Calcinations to Enhance Electrochemical Reduction of Nitrogen to Ammonia.

The electrochemical N2 reduction reaction (NRR) is emerging as a promising alternative to the industrial Haber-Bosch process for distributed and modular production of NH3 . Nevertheless, developing high-efficiency catalysts to simultaneously realize both high activity and selectivity for the development of a sustainable NRR is very critical but extremely challenging. Here, a unique plasma-assisted strategy is developed to synthesize iridium diphosphide nanocrystals with abundant surface step atoms anchored in P,N-codoped porous carbon nanofilms (IrP2 @PNPC-NF), where the edges of the IrP2 nanocrystals are extremely irregular, and the ultrathin PNPC-NF possesses a honeycomb-like macroporous structure. These characteristics ensure that IrP2 @PNPC-NF delivers superior NRR performance with an NH3 yield rate of 94.0 µg h-1 mg-1 cat. and a faradaic efficiency (FE) of 17.8%. Density functional theory calculations reveal that the unique NRR performance originates from the low-coordinate step atoms on the edges of IrP2 nanocrystals, which can lower the reaction barrier to improve the NRR activity and simultaneously inhibit hydrogen evolution to achieve a high FE for NH3 formation. More importantly, such a plasma-assisted strategy is general and can be extended to the synthesis of other high-melting-point noble-metal phosphides (OsP2 @PNPC-NF, Re3 P4 @PNPC-NF, etc.) with abundant step atoms at lower temperatures.