Ammonia Faradaic Efficiency Research Articles

Solar driven ammonia synthesis with Co-TiOx and Ag nanowires enhanced Cu2ZnSnS4 photocathodes

Restoring ammonia from waste nitrate stands as a promising strategy for reducing reliance on the energy-intensive Haber-Bosch process and tackling environmental pollutants. Advancing the catalytic aspects of photoelectrochemical (PEC) ammonia synthesis via waste nitrate reduction is of great importance to enhance its viability for sustainable chemical production. However, this process still suffers from low ammonia faradaic efficiency (FE) with high operational potential due to its involvement in multi-electron reactions. Herein, we integrated a cobalt-doped TiOx (Co-TiOx) cocatalyst and Ag nanowires (NWs) electron extraction layer onto TiOx/CdS/Cu2ZnSnS4 (CZTS) photocathode, achieving nearly 100 % ammonia FE and an onset potential of ∼0.49 V vs. RHE. Evidenced by the in-situ synchrotron-radiated FTIR (SR-FTIR) and theoretical calculations, the increased ratio of surface oxygen vacancy sites (Vo) induced by Co-TiOx is crucial for the key reaction intermediates adsorption (i.e. *NO3 and *NO2) for subsequent ammonia production. Moreover, the transparent Ag NWs facilitates the electron extraction from TiOx/CdS/CZTS to the surface catalytic sites. Powered by CZTS solar cells, a standalone solar-to-ammonia system has been demonstrated with outstanding activity and catalytic performance.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia.

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO3-RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe-N/P-C) as a NO3-RR catalyst electrode. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3-RR process. The Fe-N/P-C catalyst exhibits 90.3% ammonia Faradaic efficiency with a yield rate of 17980 μg h-1 mgcat-1, greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3-RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the enhanced function of heteroatom doping on single atoms for NO3-RR, and provides an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

AbstractThe electrochemical conversion of nitrate pollutants into value‐added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate‐to‐ammonia reduction reaction (NO3−RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single‐atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe−N/P−C) as a NO3−RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single‐Fe‐atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3−RR process. The Fe−N/P−C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h−1 mgcat−1, greatly outperforming the reported Fe‐based catalysts. Furthermore, operando SR‐FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3−RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic‐level precision modulation by heteroatom doping for the NO3−RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia

Restructuring is ubiquitous in thermocatalysis and of pivotal importance to identify the real active site, yet it is less explored in electrocatalysis. Herein, by using operando X-ray absorption spectroscopy in conjunction with advanced electron microscopy, we reveal the restructuring of the as-synthesized Cu-N4 single-atom site to the nanoparticles of ∼5 nm during the electrochemical reduction of nitrate to ammonia, a green ammonia production route upon combined with the plasma-assisted oxidation of nitrogen. The reduction of Cu2+ to Cu+ and Cu0 and the subsequent aggregation of Cu0 single atoms is found to occur concurrently with the enhancement of the NH3 production rate, both of them are driven by the applied potential switching from 0.00 to -1.00 V versus RHE. The maximum production rate of ammonia reaches 4.5 mg cm-2 h-1 (12.5 molNH3 gCu-1 h-1) with a Faradaic efficiency of 84.7% at -1.00 V versus RHE, outperforming most of the other Cu catalysts reported previously. After electrolysis, the aggregated Cu nanoparticles are reversibly disintegrated into single atoms and then restored to the Cu-N4 structure upon being exposed to an ambient atmosphere, which masks the potential-induced restructuring during the reaction. The synchronous changes of the Cu0 percentage and the ammonia Faradaic efficiency with the applied potential suggests that the Cu nanoparticles are the genuine active sites for nitrate reduction to ammonia, which is corroborated with both the post-deposited Cu NP catalyst and density functional theory calculations.

Boosting nitrate electroreduction to ammonia via in situ generated stacking faults in oxide-derived copper

Electrochemical reduction of nitrate is a promising approach to the production of value-added ammonia, which is meaningful in wastewater remediation and energy saving. However, the structural changes of metal oxide electrocatalysts have not fully understood under the electrocatalytic nitrate reduction reaction (NO3RR). Here, we find that the pristine Cu oxides can be transformed into Cu (oxide-derived Cu) with a high density of stacking faults during NO3RR process. The oxide-derived Cu directly grown on Cu foam produces great nitrate-N removal, ammonia-N selectivity, and ammonia Faradaic efficiency up to 93, 94, and 80%, respectively. Physical characterizations and theoretical investigations indicate that the oxide-derived Cu with in situ generated stacking faults causes tensile strain, which is favorable for nitrate adsorption and makes the metallic Cu more active. This work offers a new strategy of altering the electrocatalyst properties by in situ generating stacking faults from its oxides during nitrate reduction.

Concave-convex surface oxide layers over copper nanowires boost electrochemical nitrate-to-ammonia conversion

The room-temperature nitrate electroreduction to ammonia recycles the fixed nitrogen and offers an appealing ammonia-synthesis scenario. Electrocatalyst engineering is of vital importance to accelerate the reaction kinetics and increase the product selectivity during nitrate electroreduction to ammonia. In this work, Cu nanowires with concave-convex surface Cu2+1O layers (Cu@Cu2+1O NWs) were fabricated by a facile surface engineering strategy. Interior metallic Cu components allow for efficient electronic transmission capability along the nanowire structure, while exterior concave-convex Cu2+1O layers endow abundant catalytically active sites. Furthermore, the electronic interaction and interface effect between Cu/Cu2+1O enable tuning of the Cu d-band center and modulating the adsorption energies of intermediates. Consequently, the electroreduction ability of nitrate-to-ammonia over the Cu@Cu2+1O NWs is substantially improved, evident by the high nitrate-N conversion rate (78.57%), ammonia yield rate (576.53 µg h−1 mgcat.−1) and ammonia Faradaic efficiency (87.07%) at the optimal applied potential (-1.2 V vs. saturated calomel electrode) for 2 h. The findings in the study are worth reference to tailor surface/interface properties and atom structure towards highly efficient electrocatalysts.

Enhancing the rate of electrochemical nitrogen reduction reaction for ammonia synthesis under ambient conditions using hollow gold nanocages

Ammonia production is imperative to increase the food supply for the growing global population. Ammonia is also considered a major hydrogen energy carrier. The current industrial method for ammonia production is energy intensive and heavily relies on fossil fuels, which are responsible for environmental pollution. To meet ammonia demands, it is necessary to develop sustainable and environmentally friendly production methods that consume significantly less energy than the current methods. The use of nanocatalysis in an electrochemical system under ambient conditions can make an alternative route for fertilizer production. Here, the use of hollow gold nanocages (AuHNCs) as an effective electrocatalyst is evaluated for electrochemical nitrogen reduction reaction (NRR) under ambient conditions. The electrochemical experiments are carried out at various potentials in 0.5 M LiClO4 aqueous solution using AuHNCs, and their catalytic efficiency is determined for the conversion of nitrogen to ammonia. The highest ammonia Faradaic efficiency (30.2%) is achieved at − 0.4 V vs. RHE while the highest ammonia yield (3.9 µg cm-2 h-1) is obtained at − 0.5 V vs. RHE. These are greater than the highest values currently reported in the literature in aqueous solution under ambient conditions. Furthermore, the role of temperature on the electrochemical NRR performance is evaluated. It is found that by increasing the operating temperature from 20 °C to 50 °C at − 0.4 V vs. RHE, the ammonia Faradaic efficiency increases from 30.2% at 20 °C to 40.5% at 50 °C. The electrocatalytic activity of NRR using AuHNCs is further compared with that of solid Au nanoparticles of various shapes (i.e., rods, spheres or cubes) to elucidate the enhanced rate of the reaction resulting from the increase in surface area and confinement effects. The three-fold enhancement in ammonia Faradaic efficiency is achieved by using the AuHNCs (30.2%) compared to the solid Au nanocubes (11.4%).