Synthesis of self-renewing Fe(0)-dispersed ordered mesoporous carbon for electrocatalytic reduction of nitrates to nitrogen

Nickel-based dual single atom electrocatalysts for the nitrate reduction reaction

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) to ammonium (NH4+) or nitrogen (N2) provides a green route for nitrate remediation. However, nitrite generation and hydrogen evolution reactions hinder the feasibility of the process. Herein, dual single atom catalysts were rationally designed by introducing Ag/Bi/Mo atoms to atomically dispersed NiNC moieties supported by nitrogen-doped carbon nanosheet (NCNS) for the NO3-RR. Ni single atoms loaded on NCNS (Ni/NCNS) tend to reduce NO3- to valuable NH4+ with a high selectivity of 77.8 %. In contrast, the main product of NO3-RR catalyzing by NiAg/NCNS, NiBi/NCNS, and NiMo/NCNS was changed to N2, giving rise to N2 selectivity of 48.4, 47.1 and 47.5 %, respectively. Encouragingly, Ni/NCNS, NiBi/NCNS, and NiAg/NCNS showed excellent durability in acidic electrolytes, leading to nitrate conversion rates of 70.3, 91.1, and 93.2 % after a 10-h reaction. Simulated wastewater experiments showed that NiAg/NCNS could remove NO3- up to 97.8 % at -0.62 V after 9-h electrolysis. This work afforded a new strategy to regulate the reaction pathway and improve the conversion efficiency of the NO3-RR via engineering the dual atomic sites of the catalysts.

Enhancing Efficiency of Nitrate Reduction to Ammonia by Fe and Co Nanoparticle-Based Bimetallic Electrocatalyst

The green and sustainable electrocatalytic conversion of nitrogen-containing compounds to ammonia is currently in high demand in order to replace the eco-unfriendly Haber-Bosch process. Model catalysts for the nitrate reduction reaction were obtained by electrodeposition of metal Co, Fe, and bimetallic Fe/Co nanoparticles from aqueous solutions onto a graphite substrate. The samples were characterized by the following methods: SEM, XRD, XPS, UV-vis spectroscopy, cyclic (and linear) voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. In addition, the determination of the electrochemically active surface was also performed for all electrocatalysts. The best electrocatalyst was a sample containing Fe-nanoparticles on the layer of Co-nanoparticles, which showed a Faradaic efficiency of 58.2% (E = -0.785 V vs. RHE) at an ammonia yield rate of 14.6 μmol h-1 cm-2 (at ambient condition). An opinion was expressed to elucidate the mechanism of coordinated electrocatalytic action of a bimetallic electrocatalyst. This work can serve primarily as a starting point for future investigations on electrocatalytic conversion reactions to ammonia using model catalysts of the proposed type.

Platinum-ruthenium-iron embedded in nitrogen-doped ordered mesoporous carbon for adrenaline electrochemical sensing study

A novel nitrogen-doped ordered mesoporous carbon (OMC) pore-embedded growth Pt-Ru-Fe nanoparticles (Pt1-Ru7.5-Fex@N-OMCs) composite was designed and synthesized for the first time. SBA-15 was used as a template, and dopamine was used as a carbon and nitrogen source and metal linking reagent. The oxidative self-polymerization reaction of dopamine was utilized to polymerize dopamine into two-dimensional ordered SBA-15 template pores. Iron porphyrin was introduced as an iron source at the same time as polymerization of dopamine, which was introduced inside and outside the pores using dopamine-metal linkage. Carbonization of polydopamine, nitrogen doping and iron nanoparticle formation were achieved by one-step calcination. Then the templates were etched to form Fex@N-OMCs, and finally the Pt1-Ru7.5-Fex@N-OMCs composites were stabilized by the successful introduction of platinum-ruthenium nanoparticles through the substitution reaction. The composite uniformly embeds the transition metal nanoparticles inside the OMC pores with high specific surface area, which limits the size of the metal nanoparticles inside the pores. At the same time, the metal nanoparticles are also loaded onto the surface of the OMCs, realizing the uniform loading of metal nanoparticles both inside and outside the pores. This enhances the active sites of the composite, promotes the mass transfer process inside and outside the pores, and greatly enhances the electrocatalytic performance of the catalyst. The material shows high electrocatalytic performance for adrenaline, which is characterized by a wide linear range, high sensitivity and low detection limit, and can realize the detection of actual samples.

Advancing mesoporous carbon synthesis for supercapacitors: a systematic investigation of cross-linking agent effects on pore structure and functionality

Soft-templating synthesis provides an effective route to prepare ordered mesoporous carbons (MCs) that can be used for supercapacitors. During this process, the cross-linking of carbon precursors is critical to obtain tailored pore structural MCs, thus careful selection of appropriate cross-linking agents is required. Despite the shift from the prevailing cross-linker formaldehyde to its more environmentally friendly alternatives, detailed understanding on the influence of different cross-linking agents on templating synthesis is still lacking. Therefore, it remains challenging to draw a conclusion regarding which cross-linker can effectively enable an ideal cross-linking and a robust templating synthesis of ordered MCs. This work presents a systematic study, by comparing three typical cross-linkers (formaldehyde, glyoxal, and glyoxylic acid), on the pore architecture, surface functionality, and electrochemical performance of resulting MCs. Both the type of cross-linker and its ratio with precursor monomer were found to be crucial for the pore architecture and electrochemical performance of resulting MCs. Glyoxal showed to be a promising cross-linker for easily generating ordered mesopores between 3.3-6.1 nm when the molar ratio between cross-linker and carbon precursor ranged from 1 to 2, whereas glyoxylic acid and formaldehyde induced interrupted or disordered mesopores. When the resulting MCs were used as supercapacitor electrodes, those cross-linked with glyoxal also led to overall higher capacitance in both 6 M KOH aqueous and ionic liquid [N2220][NTf2]/acetonitrile electrolytes thanks to the dominance of ordered mesopore channels, especially MC prepared at glyoxal/precursor molar ratio of 1.5. These findings on the effect of cross-linking on templating synthesis can be used to guide the customisation of MCs for supercapacitors and other applications by smartly choosing a suitable cross-linking agent and its ratio with the precursor.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Iron Nanoparticles Protected by Chainmail-structured Graphene for Durable Electrocatalytic Nitrate Reduction to Nitrogen

The electrochemical nitrate reduction reaction (NO₃ RR) is an appealing technology for regulating the nitrogen cycle. Metallic iron is one of the well-known electrocatalysts for NO₃ RR, but it suffers from poor durability due to leaching and oxidation of iron during the electrocatalytic process. In this work, a graphene-nanochainmail-protected iron nanoparticle (Fe@Gnc) electrocatalyst is reported. It displays superior nitrate removal efficiency and high nitrogen selectivity. Notably, the catalyst delivers exceptional stability and durability, with the nitrate removal rate and nitrogen selectivity remained ≈96 % of that of the first time after up to 40 cycles (24 h for one cycle). As expected, the conductive graphene nanochainmail provides robust protection for the internal iron active sites, allowing Fe@Gnc to maintain its long-lasting electrochemical nitrate catalytic activity. This research proposes a workable solution for the scientific challenge of poor lasting ability of iron-based electrocatalysts in large-scale industrialization.

Iron-Based Nanocatalysts for Electrochemical Nitrate Reduction

Nitrate has a high level of stability and persistence in water, endangering human health and aquatic ecosystems. Due to its high reliability and efficiency, the electrochemical nitrate reduction reaction (NO₃ RR) is regarded as the best available option for mitigating excess nitrate in water and wastewater, especially for the removal of trace levels of nitrate. One of the most critical factors in the electrochemical reduction are the catalysts, which directly affect the reaction efficiency of nitrate removal. Iron-based nanocatalysts, which have the advantages of nontoxicity, wide availability, and low cost, have emerged as a promising electrochemical NO₃ RR material in recent years. This review covers major aspects of iron-based nanocatalysts for electrochemical NO₃ RR, including synthetic methods, structural design, performance enhancement, electrocatalytic nitrate reduction test, and reduction mechanism. The recent progress of iron-based nanocatalysts for electrochemical NO₃ RR and the mechanism of functional advantages for modified structures are reviewed from the perspectives of loading, doping, and assembly strategies, in order to realize the conversion from pollutant nitrate to harmless nitrogen or ammonia and other sustainable products. Finally, challenges and future directions for the development of low-cost and highly-efficient iron-based nanocatalysts are explored.