In Situ Derived Co2B Nanosheet Array: A High-Efficiency Electrocatalyst for Ambient Ammonia Synthesis via Nitrate Reduction

Electrodegradation of nitrogenous pollutants in sewage: from reaction fundamentals to energy valorization applications

The excessive accumulation of nitrogen pollutants (mainly nitrate, nitrite, ammonia nitrogen, hydrazine, and urea) in water bodies seriously disrupts the natural nitrogen cycle and poses a significant threat to human life and health. Electrolysis is considered a promising method to degrade these nitrogenous pollutants in sewage, with the advantages of high efficiency, wide generality, easy operability, retrievability, and environmental friendliness. For particular energy devices, including metal-nitrate batteries, direct fuel cells, and hybrid water electrolyzers, the realization of energy valorization from sewage purification processes (e.g., valuable chemical generation, electricity output, and hydrogen production) becomes feasible. Despite the progress in the research on pollutant electrodegradation, the development of electrocatalysts with high activity, stability, and selectivity for pollutant removal, coupled with corresponding energy devices, remains a challenge. This review comprehensively provides advanced insights into the electrodegradation processes of nitrogenous pollutants and relevant energy valorization strategies, focusing on the reaction mechanisms, activity descriptors, electrocatalyst design, and actuated electrodes and operation parameters of tailored energy conversion devices. A feasibility analysis of electrodegradation on real wastewater samples from the perspective of pollutant concentration, pollutant accumulation, and electrolyte effects is provided. Challenges and prospects for the future development of electrodegradation systems are also discussed in detail to bridge the gap between experimental trials and commercial applications.

Nanocluster-agminated amorphous cobalt nanofilms for highly selective electroreduction of nitrate to ammonia

Developing highly-efficient electrocatalysts for the nitrate reduction reaction (NITRR) is a persistent challenge. Here, we present the successful synthesis of 14 amorphous/low crystallinity metal nanofilms on three-dimensional carbon fibers (M-NFs/CP), including Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, In, Sn, Pb, Au, or Bi, using rapid thermal evaporation. Among these samples, our study identifies the amorphous Co nanofilm with fine agglomerated Co clusters as the optimal electrocatalyst for NITRR in a neutral medium. The resulting Co-NFs/CP exhibits a remarkable Faradaic efficiency (FENH3) of 91.15 % at - 0.9 V vs RHE, surpassing commercial Co foil (39 %) and Co powder (20 %), despite sharing the same metal composition. Furthermore, during the electrochemical NITRR, the key intermediates on the surface of the Co-NFs/CP catalyst were detected by in situ Fourier-transform infrared (FTIR) spectroscopy, and the possible reaction ways were probed by Density functional theory (DFT) calculations. Theoretical calculations illustrate that the abundant low-coordinate Co atoms of Co-NFs/CP could enhances the adsorption of *NO3 intermediates compared to crystalline Co. Additionally, the amorphous Co structure lowers the energy barrier for the rate-determining step (*NH2→*NH3). This work opens a new avenue for the controllable synthesis of amorphous/low crystallinity metal nano-catalysts for various electrocatalysis reaction applications.

Electrocatalytic Nitrate Reduction on Metallic CoNi-Terminated Catalyst with Industrial-Level Current Density in Neutral Medium

Green ammonia synthesis through electrocatalytic nitrate reduction reaction (eNO3RR) can serve as an effective alternative to the traditional energy-intensive Haber-Bosch process. However, achieving high Faradaic efficiency (FE) at industrially relevant current density in neutral medium poses significant challenges in eNO3RR. Herein, with the guidance of theoretical calculation, a metallic CoNi-terminated catalyst is successfully designed and constructed on copper foam, which achieves an ammonia FE of up to 100% under industrial-level current density and very low overpotential (-0.15 V versus reversible hydrogen electrode) in a neutral medium. Multiple characterization results have confirmed that the maintained metal atom-terminated surface through interaction with copper atoms plays a crucial role in reducing overpotential and achieving high current density. By constructing a homemade gas stripping and absorption device, the complete conversion process for high-purity ammonium nitrate products is demonstrated, displaying the potential for practical application. This work suggests a sustainable and promising process toward directly converting nitrate-containing pollutant solutions into practical nitrogen fertilizers.

Ultrathin High-Entropy Fe-Based Spinel Oxide Nanosheets with Metalloid Band Structures for Efficient Nitrate Reduction toward Ammonia

Spinel oxides with tunable chemical compositions have emerged as versatile electrocatalysts, however their performance is greatly limited by small surface area and low electron conductivity. Here, ultrathin high-entropy Fe-based spinel oxides nanosheets are rationally designed (i.e., (Co0.2Ni0.2Zn0.2Mg0.2Cu0.2)Fe2O4; denotes A5Fe2O4) in thickness of ≈4.3 nm with large surface area and highly exposed active sites via a modified sol-gel method. Theoretic and experimental results confirm that the bandgap of A5Fe2O4 nanosheets is significantly smaller than that of ordinary Fe-based spinel oxides, realizing the transformation of binary spinel oxide from semiconductors to metalloids. As a result, such A5Fe2O4 nanosheets manifest excellent performance for the nitrate reduction reaction (NO3 -RR) to ammonia (NH3), with a NH3 yield rate of ≈2.1 mmol h-1 cm-2 at -0.5 V versus Reversible hydrogen electrode, outperforming other spinel-based electrocatalysts. Systematic mechanism investigations reveal that the NO3 -RR is mainly occurred on Fe sites, and introducing high-entropy compositions in tetrahedral sites regulates the adsorption strength of N and O-related intermediates on Fe for boosting the NO3 -RR. The above findings offer a high-entropy platform to regulate the bandgap and enhance the electrocatalytic performance of spinel oxides.

Regulation of the Fe/Ni ratio on the morphology of FexNiyO4 and the performance of nitrate reduction in ammonia synthesis

The combination of theoretical calculations and experimental synthesis provides valuable insights into the performance of FexNiyO4 as a catalyst for ammonia (NH3) synthesis through the electrocatalytic nitrate reduction reaction (eNO3-RR). Here, an observation of a volcano-shaped trend in the theoretical calculations reveals that the catalytic activity of FexNiyO4 for NH3 synthesis varies with the Fe/Ni ratio. The subsequent experimental syntheses of FexNiyO4 with different Fe/Ni ratios validate this trend and demonstrate the morphological changes associated with the varying Fe/Ni ratios. The evolution of the FexNiyO4 morphology from nanosheets to sea urchin-like structures, nanowires and nanoflowers composed of rotated nanosheets as the Fe/Ni ratio increases further supports the influence of the composition on the resulting morphology. This morphological diversity can be attributed to the specific growth conditions and self-assembly processes involved in the synthesis. The correlation between the Fe/Ni ratio, morphology and NH3 yield reinforces the theoretical calculations. The observed volcanic trend in the NH3 yield, consistent with the theoretical predictions, indicates that there is an optimal Fe/Ni ratio (Fe2NiO4) with the highest NH3 yield of 12.51 mg h-1 cm-2 at -1.1 V. The excellent Faradaic efficiency of 95.97 % in neutral solution further highlights the suitability of Fe2NiO4 as a catalyst for NH3 synthesis through eNO3-RR. Moreover, the remarkable stability of FexNiyO4, regardless of the Fe/Ni ratio, is an important finding. The consistent performance of FexNiyO4 indicates its potential for long-term and practical applications in NH3 synthesis. Furthermore, the observed morphological changes, volcano-shaped trend in the NH3 yield and remarkable stability of FexNiyO4 highlight its potential as a promising catalyst.

Two-dimensional Cu Plates with Steady Fluid Fields for High-rate Nitrate Electroreduction to Ammonia and Efficient Zn-Nitrate Batteries

Nitrate electroreduction reaction (eNO3 -RR) to ammonia (NH3) provides a promising strategy for nitrogen utilization, while achieving high selectivity and durability at an industrial scale has remained challenging. Herein, we demonstrated that the performance of eNO3 -RR could be significantly boosted by introducing two-dimensional Cu plates as electrocatalysts and eliminating the general carrier gas to construct a steady fluid field. The developed eNO3 -RR setup provided superior NH3 Faradaic efficiency (FE) of 99 %, exceptional long-term electrolysis for 120 h at 200 mA cm-2, and a record-high yield rate of 3.14 mmol cm-2 h-1. Furthermore, the proposed strategy was successfully extended to the Zn-nitrate battery system, providing a power density of 12.09 mW cm-2 and NH3 FE of 85.4 %, outperforming the state-of-the-art eNO3 -RR catalysts. Coupled with the COMSOL multiphysics simulations and in situ infrared spectroscopy, the main contributor for the high-efficiency NH3 production could be the steady fluid field to timely rejuvenate the electrocatalyst surface during the electrocatalysis.

Phosphorus-rich CoP4@N-C nanoarrays for efficient nitrate-to-ammonia electroreduction

The electrochemical nitrate reduction reaction (NO3-RR) is a novel green method for ammonia synthesis. However, the lack of sufficient catalysts has hindered the development of the NO3-RR. This research develops a transformation of porous CoP@N-C/CC into porous phosphorus-rich CoP4@N-C/CC through high-temperature calcination. Due to its unique phosphating-rich structure, CoP4@N-C/CC exhibits an excellent Faraday efficiency (FE: 92.3%) and NH3 yield (610.2 μmol h-1 cm-2). Such a catalyst with more P-P bonds can provide more active sites, effectively enhancing the adsorption and reaction processes of reactant molecules. In addition, the catalyst has good durability and catalytic stability, which provides a possibility for the future application of electrocatalytic ammonia production.

Designing Efficient Nitrate Reduction Electrocatalysts by Identifying and Optimizing Active Sites of Co-Based Spinels

Cobalt-based spinel oxides (i.e., Co3O4) are emerging as low-cost and selective electrocatalysts for the electrochemical nitrate reduction reaction (NO3-RR) to ammonia (NH3), although their activity is still unsatisfactory and the genuine active site is unclear. Here, we discover that the NO3-RR activity of Co3O4 is highly dependent on the geometric location of the Co site, and the NO3-RR prefers to occur at octahedral Co (CoOh) rather than tetrahedral Co (CoTd) sites. Moreover, CoOhO6 is electrochemically transformed to CoOhO5 along with the formation of O vacancies (Ov) during the process of NO3-RR. Both experimental and theoretic results reveal that in situ generated CoOhO5-Ov configuration is the genuine active site for the NO3-RR. To further enhance the activity of CoOh sites, we replace inert CoTd with different contents of Cu2+ cations, and a volcano-shape correlation between NO3-RR activity and electronic structures of CoOh is observed. Impressively, in 1.0 M KOH, (Cu0.6Co0.4)Co2O4 with optimized CoOh sites achieves a maximum NH3 Faradaic efficiency of 96.5% with an ultrahigh NH3 rate of 1.09 mmol h-1 cm-2 at -0.45 V vs reversible hydrogen electrode, outperforming most of other reported nonprecious metal-based electrocatalysts. Clearly, this work paves new pathways for boosting the NO3-RR activity of Co-based spinels by tuning local electronic structures of CoOh sites.

Coordinative Stabilization of Single Bismuth Sites in a Carbon-Nitrogen Matrix to Generate Atom-Efficient Catalysts for Electrochemical Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia powered by renewable electricity is not only a promising alternative to the established energy-intense and non-ecofriendly Haber-Bosch reaction for ammonia generation but also a future contributor to the ever-more important denitrification schemes. Nevertheless, this reaction is still impeded by the lack of understanding for the underlying reaction mechanism on the molecular scale which is necessary for the rational design of active, selective, and stable electrocatalysts. Herein, a novel single-site bismuth catalyst (Bi-N-C) for nitrate electroreduction is reported to produce ammonia with maximum Faradaic efficiency of 88.7% and at a high rate of 1.38 mg h-1 mgcat -1 at -0.35 V versus reversible hydrogen electrode (RHE). The active center (described as BiN2 C2 ) is uncovered by detailed structural analysis. Coupled density functional theory calculations are applied to analyze the reaction mechanism and potential rate-limiting steps for nitrate reduction based on the BiN2 C2 model. The findings highlight the importance of model catalysts to utilize the potential of nitrate reduction as a new-generation nitrogen-management technology based on the construction of efficient active sites.

A brush-like Cu2O-CoO core-shell nanoarray: an efficient bifunctional electrocatalyst for overall seawater splitting

Herein, a brush-like Cu2O-CoO core-shell nanoarray on copper foam (Cu2O-CoO/CF) can achieve efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance in alkaline seawater electrolyte. This Cu2O-CoO/CF shows overpotentials as low as 315 and 295 mV at 100 mA cm-2 for the OER and HER, respectively. Moreover, it could also be operated at 1.82 V with 100 mA cm-2 in a two-electrode electrolyzer and exhibits strong stability for at least 50 hours of electrolysis. The excellent performance and hierarchical structure advantages of Cu2O-CoO/CF provide new ideas for designing efficient seawater splitting electrocatalysts.

NiMoO4 nanorods with oxygen vacancies self-supported on Ni foam towards high-efficiency electrocatalytic conversion of nitrite to ammonia

As an eco-friendly and sustainable strategy, the electrochemical reduction of nitrite (NO₂^-) can simultaneous generation of NH₃ and treatment of NO₂^- contamination in the environment. Herein, monoclinic NiMoO₄ nanorods with abundant oxygen vacancies self-supported on Ni foam (NiMoO₄/NF) are considered high-performance electrocatalysts for ambient NH₃ synthesis by reduction of NO₂^-, which can deliver an outstanding yield of 18089.39 ± 227.98 μg h^-1 cm^-2 and a preferable FE of 94.49 ± 0.42% at -0.8 V. Additionally, its performance remains relatively stable during long-term operation as well as cycling tests. Furthermore, density functional theory calculations unveil the vital role of oxygen vacancies in promoting nitrite adsorption and activation, ensuring efficient NO₂^-RR towards NH₃. A Zn-NO₂^- battery with NiMoO₄/NF as the cathode shows high battery performance as well.