Electrochemical Ammonia Synthesis Research Articles

NbC Nanoparticles Decorated Carbon Nanofibers as Highly Active and Robust Heterostructural Electrocatalysts for Ammonia Synthesis

AbstractTransition‐metal carbides with metallic properties have been extensively used as electrocatalysts due to their excellent conductivity and unique electronic structures. Herein, NbC nanoparticles decorated carbon nanofibers (NbC@CNFs) are proposed as an efficient and robust catalyst for electrochemical synthesis of ammonia from nitrate/nitrite reduction, which achieves a high Faradaic efficiency (FE) of 94.4 % and a large ammonia yield of 30.9 mg h−1 mg−1cat.. In situ electrochemical tests reveal the nitrite reduction at the catalyst surface follows the *NO pathway and theoretical calculations reveal the formation of NbC@CNFs heterostructure significantly broadens density of states nearby the Fermi energy. Finite element simulations unveil that the current and electric field converge on the NbC nanoparticles along the fiber, suggesting the dispersed carbides are highly active for nitrite reduction.

NbC Nanoparticles Decorated Carbon Nanofibers as Highly Active and Robust Heterostructural Electrocatalysts for Ammonia Synthesis.

Transition-metal carbides with metallic properties have been extensively used as electrocatalysts due to their excellent conductivity and unique electronic structures. Herein, NbC nanoparticles decorated carbon nanofibers (NbC@CNFs) are proposed as an efficient and robust catalyst for electrochemical synthesis of ammonia from nitrate/nitrite reduction, which achieves a high Faradaic efficiency (FE) of 94.4 % and a large ammonia yield of 30.9 mg h-1 mg-1 cat.. In situ electrochemical tests reveal the nitrite reduction at the catalyst surface follows the *NO pathway and theoretical calculations reveal the formation of NbC@CNFs heterostructure significantly broadens density of states nearby the Fermi energy. Finite element simulations unveil that the current and electric field converge on the NbC nanoparticles along the fiber, suggesting the dispersed carbides are highly active for nitrite reduction.

Regulating Interfacial Microenvironment in Aqueous Electrolyte via a N2 Filtering Membrane for Efficient Electrochemical Ammonia Synthesis.

Electrochemical synthesis of ammonia (NH3) in aqueous electrolyte has long been suffered from poor nitrogen (N2) supply owing to its low solubility and sluggish diffusion kinetics. Therefore, creating a N2 rich microenvironment around catalyst surface may potentially improve the efficiency of nitrogen reduction reaction (NRR). Herein, a delicately designed N2 filtering membrane consisted of polydimethylsiloxane is covered on catalyst surface via superspreading. Because this membrane let the dissolved N2 molecules be accessible to the catalyst but block excess water, the designed N2 rich microenvironment over catalyst leads to an optimized Faradaic efficiency of 39.4% and an NH3 yield rate of 109.2µgh-1mg-1, which is superior to those of the most report metal-based catalysts for electrochemical NRR. This study offers alternative strategy for enhancing NRR performance.

Active Hydrogen for Electrochemical Ammonia Synthesis

AbstractElectrochemical ammonia synthesis (EAS) presents an attractive alternative to the Haber–Bosch process due to the benefits of energy saving, low carbon emission, environmental friendliness, and so on. However, the competing hydrogen evolution reaction (HER) severely limits the yield, selectivity, and current efficiency of NH3. Although the accumulation and self‐aggregation of active hydrogen (H*) are the primary causes of the competing HER, it also serves as the critical active species and intermediate for the multistep hydrogenation and deoxygenation processes. Therefore, the sensible regulation of the H* generation and consumption are essential for enhancing EAS performance. And it is significant to thoroughly review the current strategies for H* control. Herein, a comprehensive introduction of H* to provide a fundamental understanding of its role in electrochemical reactions, including generation, conversion, identification, and quantification protocols is first proposed. In addition, the role and control strategies of H* in EAS are carefully summarized with a particular focus on regulating H* generation and consumption to enhance the activity, selectivity, and Faradaic efficiency. Finally, the remaining challenges and perspectives are discussed. This critical review is intended to offer a profound understanding of the regulation of H* in electrochemical reactions and the development of EAS technology.

Rapid plasma preparation of CuO nanowires for efficient ammonia synthesis

The conversion of nitrate into high-value ammonia (NH3) by electrochemical means can significantly reduce environmental pollution and alleviate the energy crisis. The development of easily prepared, high-performance electrocatalysts and understanding the origins of their catalytic activity are crucial. In this study, we rapidly prepared an electrocatalyst (P-CuO-10/CF) using plasma technology, subsequently employed under neutral and low nitrate concentration conditions for electrochemical ammonia synthesis. In situ Raman spectroscopy revealed that the as-prepared material underwent an in situ phase transformation to CuO/Cu, its actual active phase, and the transformed CuO exists in an amorphous structure. This heterogeneous structure demonstrated superior nitrate conversion performance, achieving an ammonia yield of 3.57 mg h–1 cm−2 at -0.6 V vs RHE and a Faradaic efficiency of 95.7 %. This strategy expands the application scope of copper-based materials in electrocatalysis and highlights the promising uses of plasma-prepared materials for electrocatalytic nitrate reduction to generate NH3.

A parallel line probe for spatially selective electrochemical NMR spectroscopy

In situ NMR is a valuable tool for studying electrochemical devices, including redox flow batteries and electrocatalytic reactors, capable of detecting reaction intermediates, metastable states, time evolution of processes or monitoring stability as a function of electrochemical conditions. Here we report a parallel line detector for spatially selective in situ electrochemical NMR spectroscopy. The detector consists of 17 copper wires and is doubly tuned to 1H/19F and X nuclei ranging from 63Cu (106.1 MHz) to 7Li (155.5 MHz). The flat geometry of the parallel line detector allows its insertion into a high electrode surface-to-volume electrochemical flow reactor, enabling a detector-in-a-reactor design. This integrated device is named “eReactor NMR probe”. Combined with B1-selective pulse sequences, selective detection of the nuclei at the electrode-electrolyte interface, that is within a distance of 800 μm from the electrode surface, has been achieved. The selective detection of 7Li and 19F nuclei is demonstrated using two electrolytes, LiCl and LiBF4 solutions, respectively. A good B1 homogeneity with an 810° to 90° pulse intensity ratio of 68–72 % was achieved. Using electrochemical plating of lithium metal as a model reaction, we further demonstrated the operando functionality of the probe. The new eReactor NMR probe offers a general method for studying flow electrochemistry, and we envision applications in a wide range of environmentally relevant energy systems, for example, Li metal batteries, electrochemical ammonia synthesis, carbon dioxide capture and reduction, redox flow batteries, fuel cells, water desalination, lignin oxidation etc.

Phosphomolybdic acid regulated the defective metal-organic framework UiO-66 for electrochemical nitrogen reduction reaction

In recent years, electrochemical ammonia synthesis has been considered a green process with excellent application prospects. The nitrogen reduction process (NRR) needs more activation energy when NN bonds are broken, which decreases the ammonia yield rate and Faraday efficiency (FE) for ammonia synthesis. Herein, UiO-66-xHPMo electrocatalysts are effectively synthesized by regulating the defects of the metal-organic framework (UiO-66) using phosphomolybdic acid (HPMo) as a defect regulator. XRD, FT-IR, TEM, XPS, TPD, etc., characterized the catalysts. As a result, the reasons for the increase in NH3 yield rate (RNH3) are catalysts losing the ligand of terephthalic acid, exposing more Zr6 nodes as active sites, and providing numerous active sites for NN cleavage. However, introducing HPMo makes catalysts more alkaline and have lower charge transfer resistance, which are more inclined to adsorb H+ and enhance hydrogen evolution reaction (HER), reducing FEs. Thus, with a voltage of −0.3 V (vs. RHE) and a neutral electrolyte (0.1 mol L−1 Na2SO4), UiO-66–2HPMo has excellent NRR performance (RNH3: 36.61 μg h−1 mg−1, FE: 31.09%).

Vacancy enhanced proton preference in bimetallic phosphide catalysts for electrochemical ammonia synthesis and energy supply in Zn-NO3− battery

Electrochemical ammonia synthesis via nitrate reduction reaction (NO3RR) under ambient condition is increasingly recognized as a sustainable alternative to the conventional Haber-Bosch process. Transition metal phosphides have attracted extensive attention as promising electrocatalysts owing to their unique proton provision. However, a challenge persists as the protons tend to dimerize to form H2 through the hydrogen evolution reaction (HER). In this work, we manipulate the proton preference in a Ni-Co bimetallic phosphide through vacancy engineering, leading to a notable enhancement in NH3 production. The developed NiCoP-VP with electron-rich surface demonstrates high selectivity, faradaic efficiency, and yield rate for ammonia, achieving a maximum yield rate of 16.63 mgNH3 h−1 cm−2 (ca. 83.17 mgNH3 h−1 mgcat.−1) at − 0.7 V vs. RHE. Density functional theory calculation reveal an increased energy barrier for proton dimerization and a more favorable rate-determining protonation step of *NO in NO3RR. Furthermore, a Zn-NO3− battery system was constructed based on NiCoP-VP, which showed dual functionality of generating electricity and producing NH3. With an open circuit voltage of 1.39 V and a peak-power density of 1.14 mW cm−2, the battery system presents a promising application for sustainable energy supply and green NH3 synthesis.

Supervised AI and Deep Neural Networks to Evaluate High-Entropy Alloys as Reduction Catalysts in Aqueous Environments.

Competitive surface adsorption energies on catalytic surfaces constitute a fundamental aspect of modeling electrochemical reactions in aqueous environments. The conventional approach to this task relies on applying density functional theory, albeit with computationally intensive demands, particularly when dealing with intricate surfaces. In this study, we present a methodological exposition of quantifying competitive relationships within complex systems. Our methodology leverages quantum-mechanical-guided deep neural networks, deployed in the investigation of quinary high-entropy alloys composed of Mo-Cr-Mn-Fe-Co-Ni-Cu-Zn. These alloys are under examination as prospective electrocatalysts, facilitating the electrochemical synthesis of ammonia in aqueous media. Even in the most favorable scenario for nitrogen fixation identified in this study, at the transition from O and OH coverage to surface hydrogenation, the probability of N2 coverage remains low. This underscores the fact that catalyst optimization alone is insufficient for achieving efficient nitrogen reduction. In particular, these insights illuminate that system consideration with oxygen- and hydrogen-repelling approaches or high-pressure solutions would be necessary for improved nitrogen reduction within an aqueous environment.

Coupling Au with BOx matrix induced by Closo-boron cluster for electrochemical synthesis of ammonia

Au is considered as one of the most promising catalysts for nitrogen reduction reaction (NRR), however maximizing the activity utilization rate of Au and understanding the synergistic effects between Au and carriers pose ongoing challenges. Herein, we systematically explore the synergistic catalytic effect of incorporating Au with boron clusters for accelerating NRR kinetics. An in-situ abinitio strategy is employed to construct B-doped Au nanoparticles (2–6 nm in diameter) loaded on BOx substrates (Au–BOx), in which B not only modulates the surface electronic structure of Au but also forms strong coupling interactions to stabilize the nanoparticles. The electrochemical results show that Au–BOx possesses excellent NRR activity (NH3 yield of 48.52 μg h−1 mgcat.−1, Faraday efficiency of 56.18%), and exhibits high stability and reproducibility throughout the electrocatalytic NRR process. Theoretical calculations reveal that the introduction of B induces the formation of both Au dangling bond and Au–B coupling bond, which considerably facilitates the hydrogenation of *N2–*NH3. The present work provides a new avenue for the preparation of metal-boron materials achieved by one-step reduction and doping process, utilizing boron clusters as reducing and stabilizing agents.

Two-dimensional peak-valley alternating self-supporting electrode accelerating nitrate electrocatalytic reduction: Ammonia synthesis and wastewater treatment

Electrochemical nitrate reduction synthesis of ammonia can use clean energy to convert low-value nitrate pollutants into high value-added ammonia yield. The development of eNitRR catalyst with high activity, high selectivity and stability is the key to achieve distributed small-scale production of nitrate. In this study, a self-supported catalyst (Cu/PTS) with high catalytic activity (for eNitRR) was constructed by anodic oxidation combined with hydrothermal/pyrolysis strategy to realize the embedding of Cu species on the substrate of porous titanium sheet. The ammonia production rate of Cu/PTS catalyzed eNitRR is as high as 7292.43 μg h−1 cm−2 (−1.0 V vs. RHE), the Faradaic efficiency is close to 100 %, reaching 97.34 %, and isotope labeling and Operando ATR-FTIRAS verified and revealed the fact that NO3− to NH3, respectively. Density functional theory calculations well reveal the roles of various components in Cu/PTS and reveal the factors for the enhancement of eNitRR performance. Thanks to the self-supporting characteristics of porous titanium sheet and the surface embedding of copper species, Cu/PTS not only have good electrochemical stability when catalyzing eNitRR, but also can drive eNitRR to operate efficiently in complex water environment. While realizing the purification of nitrate wastewater and the synthesis of ammonia, Cu/PTS can be used as a positive electrode to construct a nitrate–zinc battery to realize the dual functions of self-driven ammonia production and external power supply.

Ambient Electrochemical Ammonia Synthesis: From Theoretical Guidance to Catalyst Design.

Ammonia, a vital component in the synthesis of fertilizers, plastics, and explosives, is traditionally produced via the energy-intensive and environmentally detrimental Haber-Bosch process. Given its considerable energy consumption and significant greenhouse gas emissions, there is a growing shift toward electrocatalytic ammonia synthesis as an eco-friendly alternative. However, developing efficient electrocatalysts capable of achieving high selectivity, Faraday efficiency, and yield under ambient conditions remains a significant challenge. This review delves into the decades-long research into electrocatalytic ammonia synthesis, highlighting the evolution of fundamental principles, theoretical descriptors, and reaction mechanisms. An in-depth analysis of the nitrogen reduction reaction (NRR) and nitrate reduction reaction (NitRR) is provided, with a focus on their electrocatalysts. Additionally, the theories behind electrocatalyst design for ammonia synthesis are examined, including the Gibbs free energy approach, Sabatier principle, d-band center theory, and orbital spin states. The review culminates in a comprehensive overview of the current challenges and prospective future directions in electrocatalyst development for NRR and NitRR, paving the way for more sustainable methods of ammonia production.

Study on ammonia transport and separation in Aquivion® perfluoro sulfonated acid membranes

The present study reports the results of a series of sorption and permeation tests of pure ammonia as well as nitrogen and hydrogen carried out on Aquivion C87-05 (short-side chain perfluoro sulfonic acid ionomer). Such material is indeed of interest for possible applications in sustainable processes for ammonia production, either as base material for polymer electrolyte membranes in low-temperature electrochemical ammonia synthesis or as a membrane for effective product separation.NH3, N2, and H2 permeation tests are performed at different temperatures (20, 35, and 50 °C) and both in dry and humid conditions (R.H. up to 80%), aiming to assess the influence of these parameters on the resulting permeabilities, while ammonia sorption is inspected at the same temperatures, and pressures up to near saturation conditions.Pure ammonia permeability reached outstanding values around 7000 Barrer in dry Aquivion membranes, revealing an increasing trend with upstream pressure, while it decreased with temperature. The same behavior is recorded for NH3 solubility, indicating that sorption drives the ammonia transport through the membrane.The obtained separation performances are found to be significantly better than those of other polymeric membranes proposed for the same separations, as compared to a permeability-selectivity plot.

Boosting the Faraday Efficiency of Electrochemical Ammonia Synthesis via the Strain Effect Induced by Interfacial Hybrid Formation between BN and Carbon Nanotubes.

The electrochemical nitrogen reduction reaction (eNRR) is a highly promising alternative to the Haber-Bosch (H-B) process, but its commercial development is limited by the high bond energy of N2 molecules and the presence of the competitive hydrogen evolution reaction (HER). Here, a metal-free composite electrocatalyst of boron nitride (h-BNNs) and carbon nanotubes (CNTs) was explored through the interfacial hybridization of h-BNNs and CNTs, which showed a highly improved eNRR Faraday efficiency (FE) of 63.9% and an NH3 yield rate of 36.5 μg h-1 mgcat.-1 at -0.691 V (vs RHE). New chemical bonds of C-B and C-N were observed, indicating a strong interaction between CNTs and h-BNNs. According to the Raman spectra and the optimized model of h-BNNs/CNTs, an obvious strain effect between h-BNNs and CNTs was supposed to play a significant role in the highly improved FE, compared with the FE of h-BNNs alone (4.7%). Density functional theory (DFT) calculations further showed that h-BNNs/CNTs had lower energy barriers in eNRR, giving them higher N2 to NH3 selectivity, while h-BNNs have lower energy barriers in the HER. This work shows the important role of the strain effect in boosting the selectivity in the eNRR process.

Highly efficient electrochemical ammonia synthesis via nitrate reduction over metallic Cu phase coupling sulfion oxidation.

Electrochemical nitrate reduction reaction (NO3RR) is a promising technology for ammonia production and denitrification of wastewater. Its application is seriously restricted by the development of the highly active and selective electrocatalyst and a rational electrolysis system. Here, we constructed an efficient electrochemical ammonia production process via nitrate reduction on the metallic Cu electrocatalyst when coupled with anodic sulfion oxidation reaction (SOR). The synthesized Cu catalyst delivers an excellent NH3 Faradaic efficiency of 96.0 % and a NH3 yield of 0.391 mmol h-1 cm-2 at -0.2 V vs. reversible hydrogen electrode, which mainly stem from the more favorable conversion of NO2 - to NH3 on Cu0. Importantly, the well-designed electrolysis system with cathodic NO3RR and anodic SOR achieves a dramatically reduced cell voltage of 0.8 V at 50 mA cm-2 in comparison with the one with anodic oxygen evolution reaction (OER) of 1.9 V. This work presents an effective strategy for the energy-saving ammonia production via constructing effective nitrate reduction catalyst and replacing the OER with SOR while removing the pollutants including nitrate and sulfion.

Electrochemical Synthesis of Ammonia via Nitrogen Reduction and Oxygen Evolution Reactions—A Comprehensive Review on Electrolyte-Supported Cells

The application of protonic ceramic electrolysis cells (PCECs) for ammonia (NH3) synthesis has been evaluated over the past 14 years. While nitrogen (N2) is the conventional fuel on the cathode side, various fuels such as methane (CH4), hydrogen (H2), and steam (H2O) have been investigated for the oxygen evolution reaction (OER) on the anode side. Because H2 is predominantly produced through CO2-emitting methane reforming, H2O has been the conventional carbon-free option thus far. Although the potential of utilizing H2O and N2 as fuels is considerable, studies exploring this specific combination remain limited. PCEC fabrication technologies are being developed extensively, thus necessitating a comprehensive review. Several strategies for electrode fabrication, deposition, and electrolyte design are discussed herein. The progress in electrode development for PCECs has also been delineated. Finally, the existing challenges and prospective outlook of PCEC for NH3 synthesis are analyzed and discussed. The most significant finding is the lack of past research involving PCEC with H2O and N2 as fuel configurations and the diversity of nitrogen reduction reaction catalysts. This review indicates that the maximum NH3 synthesis rate is 14 × 10−9 mol cm−2 s−1, and the maximum current density for the OER catalyst is 1.241 A cm−2. Moreover, the pellet electrolyte thickness must be maintained at approximately 0.8–1.5 mm, and the stability of thin-film electrolytes must be improved.

Carbon Nanocage Confining CuCo Bimetallic Interface with Low Nitrate Adsorption Energy for Highly Efficient Electrochemical Ammonia Synthesis

Carbon Nanocage Confining CuCo Bimetallic Interface with Low Nitrate Adsorption Energy for Highly Efficient Electrochemical Ammonia Synthesis

Membranes Matter: Preventing Ammonia Crossover during Electrochemical Ammonia Synthesis.

The electrochemical nitrogen and nitrate reduction reactions (E-NRR and E-NO3RR) promise to provide decentralized and fossil-fuel-free ammonia synthesis, and as a result, E-NRR and E-NO3RR research has surged in recent years. Membrane NH3/NH4+ crossover during E-NRR and E-NO3RR decreases Faradaic efficiency and thus the overall yield. During catalyst evaluation, such unaccounted-for crossover results in measurement error. Herein, several commercially available membranes were screened and evaluated for use in ammonia-generating electrolyzers. NH3/NH4+ crossover of the commonly used cation-exchange membrane (CEM) Nafion 212 was measured in an H-cell architecture and found to be significant. Interestingly, some anion exchange membranes (AEMs) show negligible NH4+ crossover, addressing the problem of measurement error due to NH4+ crossover. Further investigation of select membranes in a zero-gap gas diffusion electrode (GDE)-cell determines that most membranes show significant NH3 crossover when the cell is in an open circuit. However, uptake and crossover of NH3 are mitigated when -1.6 V is applied across the GDE-cell. The results of this study present AEMs as a useful alternative to CEMs for H-cell E-NRR and E-NO3RR electrolyzer studies and present critical insight into membrane crossover in zero-gap GDE-cell E-NRR and E-NO3RR electrolyzers.

In situ growth of copper oxide on MXene by combustion method for electrochemical ammonia production from nitrate.

The elimination of the nitrogen pollutant nitrate ions through the electrochemical synthesis of ammonia is an important and environment friendly strategy. Electrochemical nitrate reduction requires highly efficient, selective, and stable catalysts to convert nitrate to ammonia. In this work, a composite of copper oxide and MXene was synthesized using a combustion technique. As reported, nitrate ions are effectively adsorbed by CuxO (CuO & Cu2O) nanoparticles. Herein, MXene is an excellent assembly for anchoring CuxO on its layered surface because it has a strong support structure. Powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses show the presence of oxidation states of metal ions and the formation of CuxO nanofoam anchors on the surface of MXene (Ti3C2Tx). The optimized CuxO/Ti3C2Tx composite exhibits an improved nitrate reduction reaction. The electrochemical studies of CuxO/Ti3C2Tx show an interesting nitrate reduction reaction (NO3RR) with a current density of 162 mA cm-2. Further, CuxO/Ti3C2Tx shows an electrocatalytic activity with an ammonia production of 41 982 μg h-1 mcat-1 and its faradaic efficiency is 48% at -0.7 V vs. RHE. Thus, such performance by CuxO/Ti3C2Tx indicates a well-suitable candidate for nitrate ion conversion to ammonia.

Real-time screening of NixBy bifunctional electrocatalysts for overall NH3 synthesis via SG-TC SECM.

Electrochemical ammonia synthesis, which couples oxygen evolution at the anode with nitrogen reduction at the cathode, holds great significance for future food and energy needs. Both of these half-cell reactions determine the overall cell potential and efficiency of the process. However, the employment of different catalysts on either side, due to discrete mechanisms, increases the complexity and material processing costs of the system, where the designing of a bifunctional catalyst active towards both the NRR and OER is of huge significance. Unfortunately, the initial screening of the designed catalysts via physical characterizations, optical methods and other techniques, does not provide details about the electrochemical activity. The scanning electrochemical microscopy (SECM) technique can be useful to screen multi-catalysts at the same time for their electrochemical activities. Herein, we employed the sample generation-tip collection (SG-TC) mode of SECM to screen the designed NixBy catalysts before half-cell investigations, which suggested that the catalyst synthesized via sonochemical reduction (SR), i.e. NixBy (SR), was a better catalyst. This inference was in accordance with the half-cell NRR and OER measurements (FE: 49% for NH3 production, OER overpotential: 300 mV). By virtue of this remarkable bifunctional activity, the NRR-OER coupled full cell was assembled, which initiated the NH3 production at just 1.7 V and produced NH3 (1.08 mg h-1 mgcat-1) at the cathode and O2 (0.81 mg h-1 mgcat-1) at the anode after 2 h of electrolysis at 1.9 V.