Ambient electrosynthesis of NH3 from N2 using Bi-doped CeO2 cube as electrocatalyst

The synthesis of ammonia (NH₃) from electrochemical nitrogen reduction reaction (NRR) under environmental conditions is a promising technology. Compared with the traditional artificial nitrogen fixation process by the Haber-Bosch process, electrochemical nitrogen reduction reaction (NRR) requires no harsh reaction conditions. In this work, we report that Bi-doped CeO₂ nanocubes show high NRR activity as electrocatalysts. The NH₃ yield of 17.83 μgh⁻¹ mg⁻¹_cat. and the Faradaic Efficiency (FE) of 1.61% at −0.9 V are achieved in 0.1 M Na₂SO₄. The performance is much higher than that for the traditional CeO₂ nanoparticles. The detailed analysis indicates that both the Bi doping and the cube morphology are critical for this encouraging NRR performance. The mechanism for improving NRR is further explored with first-principle calculations, demonstrating the importance of Bi-doping for performance enhancement.     

Facile electrochemical fabrication of magnetic Fe3O4 for electrocatalytic synthesis of ammonia used for hydrogen storage application

Ammonia (NH₃) offers extensive applications in industrial production; moreover, it is a potential carrier for hydrogen energy and an eco-friendly fuel. Electrocatalytic synthesis of NH₃ has drawn increasing research attention, wherein an excellent electrocatalyst plays a vital role. Iron (Fe) oxide nanomaterials with their high activity and cost effectiveness of its raw material Fe, have received significant attention in electrocatalytic N₂ reduction reaction (NRR) to synthesize NH₃. This study reports a rapid and cost-effective electrochemical method for synthesizing magnetic Fe₃O₄ nanoparticles, achieving gram-level production under ambient conditions. The synthesized magnetic Fe₃O₄ nanoparticles as electrocatalyst for NRR, achieved excellent faradaic efficiency of 16.9% and an optimal NH₃ yield of 12.09 μg h^?1 mg^?1_cat. at ?0.15 V (versus the reversible hydrogen electrode (RHE)) in 0.1 M Na₂SO₄. Besides, density functional theory (DFT) calculations indicate that the N≡N bond was fully activated, and the NRR proceeds mainly along the alternating hydrogenation pathway.     

Ultrasmall size FeNi Prussian blue analogue on rGO with accurate heteronuclear adsorption sites toward efficient electrochemical nitrogen fixation

The electrochemical NRR to produce ammonia under ambient conditions represents an attractive prospect in nitrogen fixation which can greatly ease the energy crisis and environmental pollution. However, suffering from awful N₂ activation results in low ammonia yield rate and Faradaic efficiency in electrochemical NRR. In this work, we devote to construct FeNi heteronuclear sites in FeNi-PBA with accurate coordination structure to facilitate N₂ activation employing a side-on configuration. Control experiments have verified N₂ activation on FeNi heteronuclear sites was enhanced compared with that on FeFe sites. Meanwhile, the particle size of PBA was decreased to boost active sites. FeNi-PBA nanocrystals with ultrasmall size (10 ± 2.8 nm) through microwave-assisted synthesis achieve an ammonia yield rate of 17.7 mg_NH3 g_cat.^?1 h^?1 and Faradaic efficiency of 27.5% at a bias of ?0.2 V (V vs. RHE) in 0.05 M H₂SO₄ solution. Faradaic efficiency achieved in this work well exceeds than that of most Fe based transition metal electrocatalysts.     

B (boron), O (oxygen) dual-doped carbon spheres as a high-efficiency electrocatalyst for nitrogen reduction

Recently, electrochemical nitrogen reduction (NRR) has attracted significant interest due to its green synthesis process. However, the slow kinetics caused by the difficulty of NN bond activation and the competitive hydrogen evolution reaction hinder the development and application of electrochemical nitrogen reduction. Herein, we synthesized a metal-free electrocatalyst of B, O-decorated carbon microspheres (B,O-CMS) by a hydrothermal method using nitrogen-free raw materials. The doping of B increases the specific surface area, induces abundant microporous structure and adjusts the electronic structure of the materials. As a result, in 0.1 M HCl, B,O-CMS achieves a high V_NH3 of 19.2 μg h^?1 mg^?1_cat. and a high FE of 5.57% at ?0.25 V vs. RHE, along with outstanding selectivity for NH₃ production and stability. This work provides an effective approach to rationally design high-performance metal-free catalysts for NRR by doping heteroatoms into carbon materials.     

Post-synthetic Ti exchanged UiO-66-NH2 metal-organic frameworks with high faradaic efficiency for electrochemical nitrogen reduction reaction

Electrochemical nitrogen reduction reaction (NRR) is a promising approach for NH₃ production to take place of the traditional Haber-Bosch process, which is still limited by the low NH₃ yield rate and low Faradaic efficiency. Herein, Ti was post-synthetic exchanged into Zr-based metal organic frameworks to synthesize UiO-Zr-Ti as NRR electrocatalysts. The incorporated Ti is found to function as active sites for NRR and benefit the improved charge-transfer efficiency, which has a positive effect on the high NH₃ yield rate. Moreover, the existence of Zr and Ti species can effectively suppress the competing HER, thus leading to high Faradaic efficiency. Therefore, the modified UiO-Zr-Ti-5d shows the highest NH₃ yield rate of 1.16 × 10⁻¹⁰ mol cm⁻² s⁻¹ and the highest Faradaic efficiency of 80.36%, which is comparable to recently reported NRR electrocatalysts.     

Facile SILAR preparation of Fe(OH)3/Ag/TiO2 nanotube arrays ternary hybrid for supercapacitor negative electrode

The ternary hybrid composite electrode of Fe(OH)₃/Ag/TNTA (where TNTA stands for TiO₂ nanotube arrays) was prepared by a simple successive ionic layer adsorption and reaction method. The effects of calcination temperature of Ag/TNTA, drying temperature of Fe(OH)₃/Ag/TNTA, and deposition amount of Ag and Fe(OH)₃ on the supercapacitor performance of the composite electrode were investigated, and the related reasons were discussed in detail. The results show that Ag modification can obviously improve the performance of Fe(OH)₃/TNTA composite electrode. Both the calcination temperature of Ag/TNTA and the deposition amount of Ag affect the particle size of Ag and the reaction resistance of the electrode. The deposition amount of Fe(OH)₃ also has influence on the reaction resistance of the electrode. Under the optimized conditions, the capacitance value of the Fe(OH)₃/Ag/TNTA composite electrode is as high as 84.67 mF cm^?2@5 mV s^?1（596.30 F g^?1@5 mV s^?1）, and the electrode has high rate performance and good cycle stability. The asymmetric supercapacitor assembled with Fe(OH)₃/Ag/TNTA as the negative electrode and activated carbon as the positive electrode can store energy stably under the potential window of 0–1.5 V. When the power density is 2.77 kW kg^?1 (50 mW cm^?3), the energy density can reach 18.34 Wh kg^?1 (0.33 mWh cm^?3).     

Controlling hydrogenation pathway by metal hydride enable efficient electrocatalytic N2 fixation

Electrocatalytic nitrogen fixation under ambient conditions represents an energy-saving sustainable alternative strategy to the energy-consuming traditional Haber–Bosch process toward ammonia synthesis. However, the traditional electrocatalysts for nitrogen reduction reaction (NRR) often suffer low selectivity and low activity. From quantum-mechanical calculations, we obtain a benefit clue that the Ni atom adsorbed by the first hydrogen ion in the catalyst exhibits selectivity for the adsorption of N₂ and other H atoms, and it preferentially adsorbs N₂ molecules. Thus, we propose an interfacial engineering strategy to simultaneously accelerate selectivity and activity using metal/metal hydroxide. The remarkable activity of metal/metal hydroxide originates from its synergized water dissociation and unique hydrogenation pathway of metal hydride. The priority absorption of the N₂ suppresses the competitive hydrogen evolution reaction and accelerates the kinetics to generate 1N₂H: 1H + N₂ → 1N₂H, which a is rate-limiting step for NH₃ synthesis. Using Ni/NiFe–OH as prototypes, here we show that selectivity and catalytic activity are simultaneously enhanced, surprisingly, in simple inorganic hybrid and confers exceptionally Faradaic efficiency of 23.34% and NH₃ yield 19.74 μg h^?1 cm^?2 at ?0.15 V versus reversible hydrogen electrode (RHE) in 0.5 M KOH electrolyte under ambient conditions. The long-term durability is also excellent. This work provides a possibility for the rational design of efficient electrocatalysts for N₂ electrochemical reduction with a large-scale production.     

Single transition metal atom anchored on g-C3N4 as an electrocatalyst for nitrogen fixation: A computational study

Electrocatalytic nitrogen reduction reaction (NRR) provides a green and sustainable way to produce ammonia at ambient conditions. The key to realize highly efficient NRR is the catalysts. To design highly active electrocatalysts for NRR, the multistep mechanism involved in NRR must be clearly unraveled. Herein, single V atoms anchored on g-C₃N₄ is identified to be an efficient electrocatalyst for NRR by screening single 3d transition metal (TM = Sc to Zn) atoms anchored by g-C₃N₄ (TM@g-C₃N₄) through density functional theory calculations. NRR takes place on V@g-C₃N₄ preferentially through distal path with a relatively low limiting potential of ?0.55 V. The outstanding NRR performance of V@g-C₃N₄ is found from the peculiar electronic structure of V after anchored in the six-fold cavity of g-C₃N₄ and the good transmitter role of V for electron transfer between N_xH_y species and g-C₃N₄. Moreover, the formation energy and dissolution potential indicate that V@g-C₃N₄ is thermodynamically and electrochemically stable and the aggregation of V atoms is unfavorable thermodynamically, signifying that the synthesis of V@g-C₃N₄ is feasible in experiments. Our work screens out a superior noble metal-free NRR electrocatalyst and will be helpful for the development of ambient artificial nitrogen fixation.     

Electrochemical N2 reduction at ambient condition – Overcoming the selectivity issue via control of reactants’ availabilities

Ammonia production via the electrochemical N₂ reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N₂) on NRR using a FeO_x-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton- and high N₂ availabilities, which are achieved using 0.12 vol% water in LiClO₄-ethyl acetate electrolyte and gaseous N₂ supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 ± 19 pmol_NH3 s⁻¹ cm⁻² at a Faradaic efficiency of 70 ± 24% at the applied potential of −0.1 V vs. NHE. Other conditions (high proton-, or low N₂-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.     

Preparation of Cu atom doped LaFeO3/activated porous carbon composites and their electrocatalytic nitrogen reduction performance

The electrocatalytic N₂ reduction reaction (NRR) under ambient conditions is a green and sustainable method for ammonia (NH₃) synthesis, and the development of efficient electrocatalysts for NRR is a top priority. In recent years, LaFeO₃ has been widely used in the field of catalysis because of its high stability, low cost, and green advantages. Through strategies such as heteroatom doping and carbon loading, we can effectively increase the content of oxygen vacancies and improve the electrical conductivity of the material to produce composites with unique electronic structures and excellent catalytic properties. In the present work, we prepared single-atom doped LaFeO₃/activated porous carbon composites (LFC/AC) for electrocatalytic NRR. The NH₃ yield and Faraday efficiency of LFC/AC were the highest at 23.876 μg h⁻¹ mg⁻¹ and 6.53% in 0.1 M Na₂SO₄ electrolyte solution, both of which were higher than those of LFC. A series of characterizations and tests have shown that LFC/AC has excellent stability, electrical conductivity, and electrocatalytic properties. The density flooding theory (DFT) simulations were performed to explore the main mechanisms to improve the NRR performance of the materials.     

Evaluation of the composition of continuously-cultivated Arthrospira (Spirulina) platensis using ammonium chloride as nitrogen source

This work is focused on the influence of dilution rate (0.08 ≤ D ≤ 0.32 d^?1) on the continuous cultivation and biomass composition of Arthrospira (Spirulina) platensis using three different concentrations of ammonium chloride (c_No = 1.0, 5.0 and 10 mol m^?3) as nitrogen source. At c_No = 1.0 and 5.0 mol m^?3 the biomass protein content was an increasing function of D, whereas, when using c_No = 10 mol m^?3, the highest protein content (72.5%) was obtained at D = 0.12 d^?1. An overall evaluation of the process showed that biomass protein content increased with the rate of nitrogen supply (D c_No) up to 72.5% at D c_No = 1.20 mol m^?3 d^?1. Biomass lipid content was an increasing function of D only when the nitrogen source was the limiting factor for the growth (D c_No ≤ 0.32 mol m^?3 d^?1), which occurred solely with c_No = 1.0 mol m^?3. Under such conditions, A. platensis reduced its nitrogen reserve in the form of proteins, while maintaining almost unvaried its lipid content. The latter was affected only when the concentration of nitrogen was extremely low (c_No = 1.0 mol m^?3). The most abundant fatty acids were the palmitic (45.8 ± 5.20%) and the γ-linolenic (20.1 ± 2.00%) ones. No significant alteration in the profiles either of saturated or unsaturated fatty acids was observed with c_No ≤ 5.0 mol m^?3, prevailing those with 16 and 18 carbons.     

CuCo2S4 integrated multiwalled carbon nanotube as high-performance electrocatalyst for electroreduction of nitrogen to ammonia

Ammonia synthesis based on electrocatalytic nitrogen reduction reaction (NRR) by using renewable sources of energy under ambient conditions has attracted wide research attentions. Herein, we report that the noble-metal-free CuCo₂S₄/multiwalled carbon nanotube nanocomposite, which is synthesized via a facile one-step hydrothermal and sulfuration approach, can work as the high active and durable catalyst for electrocatalytic NRR. This nanocomposite achieves a high NH₃ yield of 137.5 μg h⁻¹ mg_cat⁻¹ and a high Faradaic efficiency of 8.7% at −0.5 V vs reversible hydrogen electrode (RHE) in 0.1 M Na₂SO₄ solution, which outperforms CuCo₂S₄ counterpart and most reported NRR catalysts. These results reveal that the MWCNT in nanocomposite not only suppresses the aggregation of CuCo₂S₄ nanoparticles and maximizes the exposure of active sites, but also contributes to the synergistic effect between CuCo₂S₄ nanoparticles and MWCNT, and facilitates the interfacial reaction kinetics.     

Zinc doped Fe2O3 for boosting Electrocatalytic Nitrogen Fixation to ammonia under mild conditions

Artificial nitrogen fixation is emerging as a promising approach for synthesis of ammonia at mild conditions. Inspired by biological nitrogen fixation based on bacteria containing iron, zinc doped Fe₂O₃ nanoparticles are proposed as an efficient and earth abundant electrocatalyst for converting N₂ to NH₃. In neutral media, it achieves a maximum Faradaic efficiency (FE) of 10.4% and a large NH₃ yield rate of 15.1 μg h^?1 mg^?1_cat. at ?0.5 V vs. reversible hydrogen electrode. This catalyst also exhibits excellent selectivity and stability. Theoretical calculations suggest the reaction follows the associative enzymatic mechanism and it has a barrier of as low as 0.68 eV.     

Ni3Mo3N coupled with nitrogen-rich carbon microspheres as an efficient hydrogen evolution reaction catalyst and electrochemical sensor for H2O2 detection

Hydrogen evolution reaction (HER) and electrochemical analysis are two important fields of electrochemical research at present. We found that both HER and some electrochemical analytical reactions relied on the concentration of hydrogen ions (H⁺) in solution, so we intended to develop an electrode material that is sensitive to H⁺ and can be used for both HER and some electrochemical analyses. In this work, we synthesized Ni₃Mo₃N coupled with nitrogen-rich carbon microspheres (Ni₃Mo₃N@NC MSs) as highly efficient electrode material for HER and detection of Hydrogen peroxide (H₂O₂), which plays an important role in physiological processes. Here the aniline was used as the nitrogen and carbon sources to synthesize Ni₃Mo₃N@NC. The Ni₃Mo₃N@NC MSs showed high performance for HER in 1 M KOH solution with a small overpotential of 51 mV at 10 mA cm^?2 and superior stability. For H₂O₂ detection, a detection limit of 1 μM (S/N = 3), sensitivity of 120.3 μA·mM^?1 cm^?2 and linear range of 5 μM–40 mM can be achieved, respectively. This work will open up a low-cost and easy avenue to synthesize transition metal nitrides coupled with N-doped carbon as bifunctional electrode material for HER and electrochemical detection.     

Sputtered Fe1·5CoNi0.5 coating: An improved protective coating for SOFC interconnect applications

In an attempt to optimize the properties of FeCoNi coating for planar solid oxide fuel cell (SOFC) interconnect application, the coating composition is modified by increasing the ratio of Fe/Ni. An Fe_1·5CoNi_0.5 (Fe:Co:Ni = 1.5:1:0.5, atomic ratio) metallic coating is fabricated on SUS 430 stainless steel by magnetron sputtering, followed by oxidation in air at 800°C. The Fe_1·5CoNi_0.5 coating is thermally converted to (Fe,Co,Ni)₃O₄ and (Fe,Co,Mn,Ni)₃O₄ without (Ni,Co)O particles. After oxidation for 1680 h, no further migration of Cr is detected in the thermally converted coating region. A low oxidation rate of 5.9 × 10^?14 g² cm^?4 s^?1 and area specific resistance of 12.64 mΩ·cm² is obtained for Fe_1·5CoNi_0.5 coated steels.     

Boron doping induced electronic reconfiguration of Fe-Nx sites in N-doped carbon matrix for efficient oxygen reduction reaction in both alkaline and acidic media

Developing non-precious metal-based catalysts as the substitution of precious catalysts (Pt/C) in oxygen reduction reaction (ORR) is crucial for energy devices. Herein, a template and organic solvent-free method was adopted to synthesize Fe, B, and N doped nanoflake-like carbon materials (Fe/B/N–C) by pyrolysis of monoclinic ZIF-8 coated with iron precursors and boric acid. Benefiting from introducing B into Fe–N–C, the regulated electron cloud density of Fe-N_x sites enhance the charge transfer and promotes the ORR process. The as-synthesized Fe/B/N–C electrocatalyst shows excellent ORR activity of a half-wave potential (0.90 V vs 0.87 V of Pt/C), together with superior long-term stability (95.5% current density retention after 27 h) in alkaline media and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.74 V vs 0.82 V of Pt/C) in an acidic electrolyte. A Zn-air battery assembled with Fe/B/N–C as ORR catalyst delivers a higher open-circuit potential (1.47 V), specific capacity (759.9 mA h g^?1_Zn at 10 mA cm^?2), peak power density (62 mW cm^?2), as well as excellent durability (5 mA cm^?2 for more than 160 h) compared to those with commercial Pt/C. This work provides an effective strategy to construct B doped Fe–N–C materials as nonprecious ORR catalyst. Theoretical calculations indicate that introduction of B could induce Fe-N_x species electronic configuration and is favorable for activation of OH1 intermediates to promote ORR process.     

Efficient overall water splitting over a Mo(IV)-doped Co3O4/NC electrocatalyst

To meet the demand of producing hydrogen at low cost, a molybdenum (Mo)-doped cobalt oxide (Co₃O₄) supported on nitrogen (N)-doped carbon (x%Mo–Co₃O₄/NC, where x% represents Mo/Co molar ratio) is developed as an efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This defect engineering strategy is realized by a facile urea oxidation method in nitrogen atmosphere. Through X-ray diffraction (XRD) refinement and other detailed characterizations, molybdenum ion (Mo⁴⁺) is found to be doped into Co₃O₄ by substituting cobalt ion (Co²⁺) at tetrahedron site, while N is doped into carbon matrix simultaneously. 4%Mo–Co₃O₄/NC is the optimized sample to show the lowest overpotentials of 91 and 276 mV to deliver 10 mA cm^?2 for HER and OER in 1 M potassium hydroxide solution (KOH), respectively. The overall water splitting cell 4%Mo–Co₃O₄/NC||4%Mo–Co₃O₄/NC displays a voltage of 1.62 V to deliver 10 mA cm^?2 in 1 M KOH. The Mo⁴⁺ dopant modulates the electronic structure of active cobalt ion (Co³⁺) and boosts the water dissociation process during HER, while the increased amount of lattice oxygen and formation of pyridinic nitrogen due to Mo doping benefits the OER activity. Besides, the smaller grain size owing to Mo doping leads to higher electrochemically active surface area (ECSA) on 4%Mo–Co₃O₄/NC, resulting in its superior bifunctional catalytic activity.     

Ru-doped 3D porous Ni3N sphere as efficient Bi-functional electrocatalysts toward urea assisted water-splitting

Developing efficient, stable and ideal urea oxide (UOR) electrocatalyst is key to produce green hydrogen in an economical way. Herein, Ru doped three dimensional (3D) porous Ni₃N spheres, with tannic acid (TA) and urea as the carbon and nitrogen resources, is synthesized via hydrothermal and low-temperature treated process (Ru–Ni₃N@NC). The porous nanostructure of Ni₃N and the nickel foam provide abundant active sites and channel during catalytic process. Moreover, Ru doping and rich defects favor to boost the reaction kinetics by optimizing the adsorption/desorption or dissociation of intermediates and reactants. The above advantages enable Ru–Ni₃N@NC to have good bifunctional catalytic performance in alkaline media. Only 43 and 270 mV overpotentials are required for hydrogen evolution (HER) and oxygen evolution (OER) reactions to drive a current of 10 mA cm^?2. Moreover, it also showed good electrocatalytic performance in neutral and alkaline seawater electrolytes for HER with 134 mV to drive 10 mA cm^?2 and 83 mV to drive 100 mA cm^?2, respectively. Remarkably, the as-designed Ru–Ni₃N@NC also owns extraordinary catalytic activity and stability toward UOR. Moreover, using the synthesized Ru–Ni₃N@NC nanomaterial as the anode and cathode of urea assisted water decomposition, a small potential of 1.41 V was required to reach 10 mA cm^?2. It can also be powered by sustainable energy sources such as wind, solar and thermal energies. In order to make better use of the earth's abundant resources, this work provides a new way to develop multi-functional green electrocatalysts.     

Transition metal atoms anchored on nitrogen-doped α-arsenene as efficient electrocatalysts for nitrogen electroreduction reaction

Electrocatalytic reduction of N₂ to NH₃ under ambient conditions, inspired by biological nitrogen fixation, is a new approach to address the current energy shortage crisis. As a result, developing efficient and low-cost catalysts is critical. The catalytic activity, catalytic mechanism, and selectivity of α-arsenene (α-Ars) catalysts anchored with various transition metal atoms and doped with different numbers of N atom were investigated for N₂ reduction reaction (NRR) in this paper. Results reveal that compared with WN₃-α-Ars which is coordinated with three N atoms, asym-WN₂As-α-Ars that coordinated with two N atoms not only exhibits high catalytic activity (U_L = ?0.36 V), but can also successfully suppress the hydrogen evolution reaction (HER). It is manifested that reducing the number of coordination atoms can promote the selectivity of the transition metal (TM) loaded N-doped arsenene catalysts. Furthermore, activity origin analyses show both the charge on 1N–NH and φ form volcano-type relationship with the limiting potential. The active center of the catalyst, which acts as the charge transporter and has the moderate ability to retrieve charges, is the most efficient in NRR. Overall, this research creates high performance NRR catalysts by varying the number of coordinating N atoms, which provides a novel idea for the development of new NRR catalysts.