Nitrate Electroreduction Research Articles

One Bicopper Complex with Good Affinity to Nitrate for Highly Selective Electrocatalytic Nitrate Reduction to Ammonia

Ammonia (NH3) plays an irreplaceable role in human life as a promising energy carrier and indispensable chemical raw material. Nitrate electroreduction to ammonium (NRA) not only removes nitrate pollutants, but also can be used for efficient NH3 production under ambient conditions. However, achieving high efficiency and selectivity of electrocatalysts is still a great challenge. Herein, a complex Cu2(NO3)4(BMMB)·H2O with a bicopper core is assembled by Cu(NO3)2·3H2O and 1,4-bis{[2-(2’-pyridyl)benzimidazolyl]methyl}benzene (BMMB) for NRA under alkaline conditions. The optimal sample showed excellent nitrate reduction performance with the NO3− conversion rate of 70%, Faradaic efficiency of up to 90%, and NH3 selectivity of more than 95%. The high-catalytic activity is mainly due to the ingeniously designed copper cores with strong affinity for NO3−, which accelerates the transferring rate of adsorbed nitrate on the Cu surface and increases the efficiency of rate-determining step (NO3− → NO2−) in the whole catalytic process. Therefore, the transformation of surface-exposed nitrate can be rapidly catalyzed by the Cu active sites, facilitating the conversion efficiency of nitrate.

Engineering sulfur vacancies optimization in Ni3Co6S8 nanospheres toward extraordinarily efficient nitrate electroreduction to ammonia

Electrocatalytic nitrate reduction into recyclable ammonia (NH3) plays a critical role in maintaining the global nitrogen balance and meeting the growing energy demand. However, the research on controllable defects of traditional semiconductor electrocatalytic materials remains lacking, resulting in limiting nitrate reduction reaction (NO3RR) activity. Herein, we develop a facile electrical joule heating method to flexibly regulate the sulfur vacancies (SVs) content on the surface of Ni3Co6S8 (NCS), in which SVs can regulate the electronic structure of NCS. Therefore, more 3d electrons can be localized at Co sites that can reduce the energy barrier of the rate-determining step (HNO3*→NO2*). As a result, the NCS with reasonable SVs concentration exhibits the outstanding selectivity (98.2%), Faradaic efficiency (85.3%) and yield rate (2388.4 μg h−1 cm−2) for electrocatalytic nitrate reduction to ammonia while maintaining their structural integrity. This work provides a new insight for addressing both nitrate pollution and ammonia energy production.

Electrochemical Reduction of Nitrate with Simultaneous Ammonia Recovery Using a Flow Cathode Reactor.

The presence of excessive concentrations of nitrate in industrial wastewaters, agricultural runoff, and some groundwaters constitutes a serious issue for both environmental and human health. As a result, there is considerable interest in the possibility of converting nitrate to the valuable product ammonia by electrochemical means. In this work, we demonstrate the efficacy of a novel flow cathode system coupled with ammonia stripping for effective nitrate removal and ammonia generation and recovery. A copper-loaded activated carbon slurry (Cu@AC), made by a simple, low-cost wet impregnation method, is used as the flow cathode in this novel electrochemical reactor. Use of a 3 wt % Cu@AC suspension at an applied current density of 20 mA cm-2 resulted in almost complete nitrate removal, with 97% of the nitrate reduced to ammonia and 70% of the ammonia recovered in the acid-receiving chamber. A mathematical kinetic model was developed that satisfactorily describes the kinetics and mechanism of the overall nitrate electroreduction process. Minimal loss of Cu to solution and maintenance of nitrate removal performance over extended use of Cu@AC flow electrode augers well for long-term use of this technology. Overall, this study sheds light on an efficient, low-cost water treatment technology for simultaneous nitrate removal and ammonia generation and recovery.

Ammonia-Mediated CO2 Capture and Direct Electroreduction to Formate

Direct electrochemical conversion of CO2 capture solutions (instead of gaseous CO2) to valuable chemicals can circumvent the energy-intensive CO2 regeneration and pressurization steps, but the performance of such processes is limited by the sluggish release of CO2 and the use of energy-consuming bipolar membranes (BPMs). Herein, we discovered that an ammonium bicarbonate (NH4HCO3)-fed electrolyzer outperforms the state-of-the-art KHCO3 electrolyzers largely because of its favorable thermal decomposition property, which allows for a 3-fold increase in the in situ CO2 concentration, a maximum 23% increase in formate faradaic efficiency, and a 35% reduction in cell voltage by substituting BPM with an anion exchange membrane (AEM). We then demonstrated an integrated process by combining NH4HCO3 electrolysis with CO2 capture by on-site generated ammonia from the electroreduction of nitrate, which features a remarkable 99.8% utilization of of CO2 capturing agent. Such a multipurpose process offers a sustainable route for the simultaneous removal of N wastes and streamlined CO2 capturing and upgrading.

Nanoreactor Based on Cyclodextrin for Direct Electrocatalyzed Ammonia Synthesis.

The high-efficiency transition metal-free electrocatalytic nitrate reduction reaction (NO3-RR) for ammonia synthesis has received more attention because of its green and environmentally friendly characteristics. Here, we report an efficient electrochemical NH3 synthesis directly from purely organic macrocyclic compounds α-, β-, and γ-cyclodextrins (CDs)-catalyzed transition metal-free electroreduction of nitrate under ambient conditions. In comparison with α-, and β-CDs, parent γ-CD presented uncommon catalytic performance with a relatively higher NH3 yield that can reach up to 2.28 mg h-1 cm-2 with a Faradaic efficiency (FE) of 63.2% at -0.9 V versus a reversible hydrogen electrode in alkaline medium, and the potassium ion-coordinated γ-CD complex can achieve a maximum NH3 production rate up to 4.66 mg h-1 cm-2 with an NH3 FE of 79.3%. Further comparison with permethyl-γ-CD, d-glucose, and poly(vinyl alcohol) for the NO3-RR indicated that the typical torus-shaped cyclic conformation and edge hydroxyl groups of parent CDs play important roles in the electrocatalytic process. The K+-mediated 3D γ-CD-K+ frameworks containing six CDs as nanoreactors greatly strengthen the enrichment effect of nitrate through hydrogen-bonding interaction and electrostatic interaction and promote the mass transfer, thus leading to the efficient NO3-RR in an alkaline electrolyte. This work provides a convenient, green, and economic method for high-performance NO3-RR, which has potential applications in the fields of environment, energy, and industry.

In Situ Growth of Fe2O3 Nanorod Arrays on Carbon Cloth with Rapid Charge Transfer for Efficient Nitrate Electroreduction to Ammonia.

Electrochemical reduction of nitrate to ammonia (NH3), a green NH3 production route upon combining with renewable energy sources, is an appealing and alternative method to the Haber-Bosch process. However, this process not only involves the complicated eight-electron reduction to transform nitrate into various nitrogen products but simultaneously suffers from the competitive hydrogen evolution reaction, challenged by a lack of efficient catalysts. Herein, the in situ growth of Fe2O3 nanorod arrays on carbon cloth (Fe2O3 NRs/CC) is reported to exhibit a high NH3 yield rate of 328.17 μmol h-1 cm-2 at -0.9 V versus RHE, outperforming most of the reported Fe catalysts. An in situ growth strategy provides massive exposed active sites and a fast electron-transport channel between the carbon cloth and Fe2O3, which accelerates the charge-transport rate and facilitates the conversion of nitrate to NH3. In situ Raman spectroscopy in conjunction with attenuated total reflection Fourier transform infrared spectroscopy reveals the catalytic mechanism of nitrate to NH3. Our study provides not only an efficient catalyst for NH3 production but also useful guidelines for the pathways and mechanism of nitrate electroreduction to NH3.

Predictive Theoretical Model for the Selective Electroreduction of Nitrate to Ammonia.

The electrochemical reduction of nitrate (eNO3RR) emerges as a promising route for decentralized ammonia synthesis. However, the competitive production of nitrite at low overpotentials is a challenging issue. Herein, using the combination of density functional theory and microkinetic modeling, we show that the selectivity for NH3 surpasses that of NO2- at -0.66 VRHE, which nicely reproduced the experimental value on titania. NH2OH* → NH2* is the kinetically controlling step at a low overpotential for NH3 generation, while NO2* → HNO2 has the highest barrier to producing nitrite. Based on these mechanistic insights, we suggest that ΔG1 (NH2OH* → NH2*) - ΔG2 (NO2* → HNO2) can serve as a descriptor to predict the S(NO2-)/S(NH3) crossover potential. Such a model is verified by the experimental results on Ag, Cu, TiO2-x, Fe3O4, and Fe-MoS2 and can be extended to the Au catalyst. Thus, this work sheds light on the rational design of catalysts that are simultaneously energy-efficient and selective to NH3.

Electroreduction of nitrate to ammonia on atomically-dispersed Cu-N4 active sites with high efficiency and stability

The electroreduction of nitrate to ammonia under ambient temperature and pressure driven by renewable energy provides an attractive green alternative for ammonia synthesis. However, it is difficult to selectively convert nitrate into ammonia without other side products for long-term stable operation. In this manuscript, atomically-dispersed Cu-N4 active sites anchored on zeolitic imidazolate frameworks with porous N-doped carbon (Cu-NC) were designed to promote Faraday efficiency (FE) and operational stability for the electrocatalytic conversion of nitrate to ammonia. Density functional theory calculations clarify that the adsorption of NO3– at the Cu-N4 site is thermodynamically favorable and the conversion of *NO to *N is the limiting step. Electron microscopy and X-ray absorption spectroscopy results reveal that Cu is uniformly distributed as individual atoms on N-doped carbon surfaces, while the coordination structure of Cu in Cu-NC catalyst is Cu-N4. The Cu-NC catalyst with excellent activity and stability increases the ammonia FE to 82.1 % at −0.89 V vs reversible hydrogen electrode (RHE) and yield rate to 4731.0 μg h−1 mgcat.-1 at −1.09 V vs RHE, respectively.

Adsorbate chemical environment-based machine learning framework for heterogeneous catalysis

Heterogeneous catalytic reactions are influenced by a subtle interplay of atomic-scale factors, ranging from the catalysts’ local morphology to the presence of high adsorbate coverages. Describing such phenomena via computational models requires generation and analysis of a large space of atomic configurations. To address this challenge, we present Adsorbate Chemical Environment-based Graph Convolution Neural Network (ACE-GCN), a screening workflow that accounts for atomistic configurations comprising diverse adsorbates, binding locations, coordination environments, and substrate morphologies. Using this workflow, we develop catalyst surface models for two illustrative systems: (i) NO adsorbed on a Pt3Sn(111) alloy surface, of interest for nitrate electroreduction processes, where high adsorbate coverages combined with low symmetry of the alloy substrate produce a large configurational space, and (ii) OH* adsorbed on a stepped Pt(221) facet, of relevance to the Oxygen Reduction Reaction, where configurational complexity results from the presence of irregular crystal surfaces, high adsorbate coverages, and directionally-dependent adsorbate-adsorbate interactions. In both cases, the ACE-GCN model, trained on a fraction (~10%) of the total DFT-relaxed configurations, successfully describes trends in the relative stabilities of unrelaxed atomic configurations sampled from a large configurational space. This approach is expected to accelerate development of rigorous descriptions of catalyst surfaces under in-situ conditions.

Selective electroreduction of nitrate to ammonia via NbWO6 perovskite nanosheets with oxygen vacancy

Electrocatalytic nitrate reduction to ammonia (NRA) under ambient conditions is significant for carbon-neutral synthetic fuels. Nevertheless, the lack of efficient electrocatalysts with tunable nanostructure for NRA remains a grand challenge. Herein, NbWO6 nanosheets with oxygen vacancy (NbWO6-x) was demonstrated via thermal treatment and exfoliation with NH3 selectivity of 86.8% and Faradaic efficiency of 85.7% toward NRA. 1H nuclear magnetic resonance spectra coupled with 15N isotope labeling experiments proved that NH3 originated from NO3−. The function of oxygen vacancy was revealed by computational studies in NRA. Moreover, the reaction mechanism and pathway of NRA could be deduced based on the results of online differential electrochemical mass spectrometry (DEMS). This work provides a selective NH3 generation strategy to decarbonize the energy-chemical sector, bridging the gap between batteries and biofuels.

Oxygen-Vacancy-Rich Cu2O Hollow Nanocubes for Nitrate Electroreduction Reaction to Ammonia in a Neutral Electrolyte.

The electrocatalytic nitrate reduction reaction (NO3--ERR) to ammonia (NH3) is a promising strategy for NH3 production. Cu-based nanomaterials have been regarded as a kind of effective NO3--ERR catalysts. In this work, high-quality hollow Cu2O nanocubes (Cu2O h-NCs) are facilely synthesized by a simple one-step reduction method. The as-prepared Cu2O h-NCs reveal high selectivity and activity for NO3--ERR, which is ascribed to abundant oxygen vacancies, high surface area, hollow architecture, low mass transfer resistance, and strong adsorbing ability toward NO3-. In fact, Cu2O h-NCs can achieve a Faradic efficiency of 92.9% and an NH3 yield of 56.2 mg h-1 mgcat-1 for NH3 production at -0.85 V (vs RHE) potential, which exceeds those of other transition-metal-based NO3--ERR electrocatalysts.

Electronic Metal-Support Interaction Triggering Interfacial Charge Polarization over CuPd/N-Doped-C Nanohybrids Drives Selectively Electrocatalytic Conversion of Nitrate to Ammonia.

Selective electrocatalytic nitrate-to-ammonia conversion holds significant potential in treatment of nitrate wastewater and simultaneously produces high-value-added ammonia. However, today's development of nitrate-to-ammonia technology remains hindered by the lack of electrocatalysts with high activity and selectivity. In this work, metal-organic framework-derived CuPd bimetallic nanoparticles/nitrogen-doped carbon (CuPd/CN) hybrid nanoarrays for efficient ammonia electrosynthesis from nitrate are designed and synthesized. Systematic characterization reveals that the electronic metal-support interaction between the CuPd nanoparticles and N-doped nanocarbon matrix could trigger interfacial charge polarization over the CuPd/CN composite and make Cu sites electron deficient, which is conducive to the adsorption of nitrate ions. Moreover, the Pd atom sites separate by Cu atoms and could catalyze the dissociation of H2 O molecules to form adsorbed H species, which evolves into hydrogen radicals and behaves as the dominant reactive species in accelerating nitrate-to-ammonia electrocatalysis. These advantages endow the CuPd/CN nanoarrays with high faradaic efficiency (96.16%), selectivity (92.08%) as well as excellent catalytic stability for electroreduction of nitrate to ammonia.

PdCu/C Catalyst with Electricity Self-Generation via a Fuel Cell for Electroreduction of Nitrate

PdCu/C Catalyst with Electricity Self-Generation via a Fuel Cell for Electroreduction of Nitrate

Engineering the surface chemical microenvironment over CuO nanowire arrays by polyaniline modification for efficient ammonia electrosynthesis from nitrate

Electrochemical nitrate reduction into ammonia offers an attractive alternative to value-added ammonia (NH3) production under benign conditions. The critical to achieve satisfactory NH3 production rate from nitrate electroreduction that can compare with industrial Haber–Bosch route is the development of highly efficient electrocatalysts. In this work, we descript a post-modification strategy for synthesizing polyaniline (PANI)-modified CuO nanowire arrays (CuO@PANI) for selective electrocatalytic nitrate-to-ammonia transformation. Surface modification of CuO with PANI can not only well-retain the nanowire array structure and large specific surface area, but also modulate the surface chemical microenvironment of catalyst, which promotes nitrate enrichment and hydrogenated species accumulation, and thus facilitates selective nitrate-to-ammonia transformation. As a result, the CuO@PANI exhibits high Faradaic efficiency (93.88%) and excellent NH3 selectivity (91.38%) for NH3 electrosynthesis from nitrate. The strategy is worthy for designing high-efficiency electrocatalysts by surface modification with organic molecular or polymers for selective nitrate reduction into ammonia.

B-doped MoS2 for nitrate electroreduction to ammonia

Electrocatalytic nitrate-to-ammonia conversion (NO3RR) is a promising route to achieve both NH3 electrosynthesis and wastewater treatment. Herein, we report B-doped MoS2 nanosheet arrays as an efficient NO3RR catalyst, delivering an NH3-Faradaic efficiency of 92.3 % with the corresponding NH3 yield of 10.8 mg h−1 cm−2 at −0.7 V (RHE). Theoretical computations identify B-dopants as the pivotal active sites to enhance NO3– activation and optimize the free energies of reaction intermediates, leading to the expedited NO3RR activity. Meanwhile, the undesired hydrogen evolution can be well suppressed on B-MoS2 to render a high NO3RR selectivity.

Selenium-vacancy-rich WSe2 for nitrate electroreduction to ammonia

Electrocatalytic nitrate reduction to ammonia (NO3RR) is an attractive route for renewable NH3 electrosynthesis. Herein, we demonstrated Se-vacancy-rich WSe2 (WSe2-x) nanoplatelets as a highly efficient NO3RR catalyst, exhibiting a NH3-Faradaic efficiency of 92.7 % with the corresponding NH3 yield of 2.42 mg h−1 cm−2 at −0.8 V, substantially surpassing Se-vacancy-free WSe2 nanoplatelets. Theoretical computations by density function theory calculations and molecular dynamics simulations revealed that the introduced Se-vacancy enabled the creation of unsaturated W sites as active centers to strongly activate the NO3– and reduce the reaction barriers to boost the NO3RR process.

Electrocatalytic reduction of nitrate to nitrogen on silver nanoparticles-polypyrrole composite film grown on cellulosic paper substrate

ABSTRACT In this work, we describe a simple strategy for the preparation of a low-cost electrode material based on polypyrrole (PPy) film grown on an insulating cellulosic paper substrate via in-situ oxidative polymerisation technique and functionalised by silver nanoparticles (AgNPs) uniformly dispersed on its surface. The properties of the obtained AgNPs-PPy composites were characterised using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis. The electrocatalytic activity of the prepared electrode was investigated for the electroreduction of nitrate using cyclic voltammetry in 0.1 M KOH solution. Results demonstrated that the incorporation of AgNPs into the PPy matrix improves significantly the catalytic behaviour of this film. The effect on the electrode response of the amount of the incorporated AgNPs was also investigated. This paper highlights a simple procedure for the making of low-cost paper electrodes for the efficient reduction of nitrates.

Iron-Doped Cobalt Phosphide Nanohoops for Electroreduction of Nitrate to Ammonia.

Heteroatom doping can effectively tune the electronic structure of an electrocatalyst to accelerate the adsorption/desorption of reaction intermediates, which sharply increases their intrinsic electroactivity. Herein, we successfully prepare iron (Fe)-doped cobalt phosphide (CoP) nanohoops (Fe/CoP NHs) with different Fe/Co atomic ratios as highly active electrocatalysts for the nitrate electrocatalytic reduction reaction (NIT-ERR). Electrochemical measurements reveal that appropriate Fe doping can improve the electroactivity of cobalt phosphide nanohoops for the NIT-ERR. In a 1 M KOH electrolyte, the Fe/CoP NHs with the optimized chemical composition can achieve an efficient ammonia (NH3) generation rate of 27.6 mg h-1 mgcat-1 for the conversion of NO3- into NH3 and a Faradaic efficiency of 93.3% at a -0.25 V potential, which exceed the values of various previously reported nanomaterials in an alkaline electrolyte.

Comparing the effects of Cu-Ti/RuO2-IrO2 electrode configuration on the electro-reduction of nitrate.

Nitrate pollution is a global problem as it affects both the environment and human health. The objective of this research was to study the effect of electrode configuration on the electro-reduction of nitrate. Coaxial cylindrical (inner rod and outer tube copper cathodes) and vertical plate parallel copper cathodes paired with Ti/RuO2-IrO2 (rod, tube, and plate) configurations were studied under various current densities and initial nitrate concentrations. The efficiency of each configuration was determined based on the removal efficiency of nitrate, specific energy consumption, mass transfer coefficients, and first order rate constants. Additionally, the overall systems' resistance and geometric factors are discussed. It was found that the performances of the inner rod and outer tube copper cathodes were similar. The vertical plate parallel configuration was superior to the coaxial cylindrical electrode setup, as evident from a higher maximum nitrate removal of 74 and 56% at a current density of 7 mA/cm2 and lower energy consumption of 46.7 × 10-3 and 54.3 × 10-3 kWh/mmol NO3- at 36.4 mA/cm2, respectively. In addition, the mass transfer coefficients and first order rate constants were higher in all conditions tested for the vertical plate parallel configuration.

Coordination polymers-derived core-shell Co@N–C nanostructures as efficient dual functional catalysts for nitrate electroreduction and Fenton-like catalytic dye degradation

A new Co-coordination polymer was prepared and thermally converted into the special Co@N–C core-shell nanostructures. It was demonstrated as the efficient and stable electrocatalyst for nitrate electroreduction of (NO3−-RR). In neutral Cl−-free electrolyte, nitrate was electrochemically reduced into harmless nitrogen (N2–N) with 94% nitrate conversion and 97% N2–N selectivity. This Co@N–C material also acted as the efficient catalysts in Fenton-like catalytic degradation system. With very low catalyst amount of 0.1g ‧L−1, about 90% of Rhodamine B solution (50 ​mg/L) was quickly degraded within 2 ​min in the existence of peroxymonosulfate even without additional irradiation.