To restore the natural nitrogen cycle (N-cycle), artificial N-cycle electrocatalysis with flexibility, sustainability, and compatibility can convert intermittent renewable energy (e.g., wind) to harmful or value-added chemicals with minimal carbon emissions. The background of such N-cycles, such as nitrogen fixation, ammonia oxidation, and nitrate reduction, is briefly introduced here. The discussion of emerging nanostructures in various conversion reactions is focused on the architecture/compositional design, electrochemical performances, reaction mechanisms, and instructive tests. Energy device advancements for achieving more functions as well as in situ/operando characterizations toward understanding key steps are also highlighted. Furthermore, some recently proposed reactions as well as less discussed C–N coupling reactions are also summarized. We classify inorganic nitrogen sources that convert to each other under an applied voltage into three types, namely, abundant nitrogen, toxic nitrate (nitrite), and nitrogen oxides, and useful compounds such as ammonia, hydrazine, and hydroxylamine, with the goal of providing more critical insights into strategies to facilitate the development of our circular nitrogen economy.

https://www.sciopen.com/article_pdf/1531201054523838466.pdf

Recent advances in nanostructured heterogeneous catalysts for N-cycle electrocatalysis

Natural nitrogen cycle has been severely disrupted by anthropogenic activities. The overuse of N-containing fertilizers induces the increase of nitrate level in surface and ground waters, and substantial emission of nitrogen oxides causes heavy air pollution. Nitrogen gas, as the main component of air, has been used for mass ammonia production for over a century, providing enough nutrition for agriculture to support world population increase. In the last decade, researchers have made great efforts to develop ammonia processes under ambient conditions to combat the intensive energy consumption and high carbon emission associated with the Haber-Bosch process. Among different techniques, electrochemical nitrate reduction reaction (NO3 RR) can achieve nitrate removal and ammonia generation simultaneously using renewable electricity as the power, and there is an exponential growth of studies in this research direction. Here we provide a timely and comprehensive review on the important progresses of electrochemical NO3 RR, covering the rational design of electrocatalysts, emerging C-N coupling reactions, and advanced energy conversion and storage systems. Moreover, future perspectives are proposed to accelerate the industrialized NH3 production and green synthesis of chemicals, leading to a sustainable nitrogen cycle via prosperous N-based electrochemistry. This article is protected by copyright. All rights reserved.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond.

Transition metal-based oxyhydroxides (MOOH) derived from the irreversible structural reconstruction of precatalysts are often acknowledged as the real catalytic species for the oxygen evolution reaction (OER). Typically, the reconstruction-derived MOOH would exhibit superior OER activity compared to their directly synthesized counterparts, despite being fundamentally similar in chemistry. As such, structural reconstruction has emerged as a promising strategy to boost the catalytic activity of electrocatalysts. However, the in-depth understanding of the origin of the superior OER activity of reconstructed materials still remains ambiguous, which significantly hinders the further developments of highly efficient electrocatalysts based on structural reconstruction chemistry. In this review, a comprehensive overview of the structural reconstruction behaviors in the reported reconstruction-derived electrocatalysts is provided and the intrinsic chemical and structural origins of their high efficiency toward OER are unveiled. The fundamentals of structural reconstruction mechanisms, along with the recommended characterization techniques for the understanding of the dynamic structural reconstruction process and analyzing the structure of real catalytic species are also interpreted. Finally, in view of structural reconstruction chemistry, the potential perspectives to facilitate the design and synthesis of highly efficient and durable OER electrocatalyst are presented. 

Fundamental Understanding of Structural Reconstruction Behaviors in Oxygen Evolution Reaction Electrocatalysts

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting.

Ammonia as an irreplaceable chemical has been widely demanded to keep the sustainable development of the modern society. However, its industrial production consumes huge energy and releases extraordinary green-house gases, leading to various environmental issues. To achieve the green production of ammonia is a great challenge that has been extensively pursued recently. In the review, a most promising strategy, electrochemical nitrate reduction reaction (e-NO3RR) for the purpose is comprehensively investigated to give a full understanding of its development and mechanism and provide guidance for future directions. Particularly, the development of electrocatalysts is focused to realize the high ammonia yield rate and Faraday efficiency for industrial applications. The recent-developed catalysts, including noble metallic materials, alloys, metal compounds, single-metal-atom catalysts, and metal-free materials, are systematically discussed to review the effects of various factors on the catalytic performance in e-NO3RR. Accordingly, the strategies, including defects engineering, coordination environment modulating, surface controlling, and hybridization, are carefully discussed to improve the catalytic performance, such as the intrinsic activity and selectivity. Finally, perspectives and challenges are given out. This review shall provide insightful guidance on the development of advanced catalytic systems for the production of green ammonia efficiently in the industry. 

Recent Advances in Electrocatalysts for Efficient Nitrate Reduction to Ammonia

For nickel-based catalysts, in-situ formed nickel oxyhydroxide has been generally believed as the origin for anodic biomass electro-oxidations. However, rationally understanding the catalytic mechanism still remains challenging. In this work, we demonstrate that NiMn hydroxide as the anodic catalyst can enable methanol-to-formate electro-oxidation reaction (MOR) with a low cell-potential of 1.33/1.41 V at 10/100 mA cm-2, a Faradaic efficiency of nearly 100% and good durability in alkaline media, remarkably outperforming NiFe hydroxide. Based on a combined experimental and computational study, we propose a cyclic pathway that consists of reversible redox transitions of NiII-(OH)2/NiIII-OOH and a concomitant MOR. More importantly, it is proved that the NiIII-OOH provides combined active sites including NiIII and nearby electrophilic oxygen species, which work in a cooperative manner to promote either spontaneous or non-spontaneous MOR process. Such a bifunctional mechanism can well account for not only the highly selective formate formation but also the transient presence of NiIII-OOH. The different catalytic activities of NiMn and NiFe hydroxides can be attributed to their different oxidation behaviors. Thus, our work provides a clear and rational understanding of the overall MOR mechanism on nickel-based hydroxides, which is beneficial for advanced catalyst design. 

/pdf/unraveling-a-bifunctional-mechanism-for-methanol-to-formate-3itvwa2h.pdf

Unraveling a bifunctional mechanism for methanol-to-formate electro-oxidation on nickel-based hydroxides

Coupling methanol-to-formate oxidation (MFO) with hydrogen evolution reaction (HER) may promote hydrogen economics by introducing value-added anodic product. Herein, we develop a ternary catalyst Ru&Fe-WOx for boosting both HER and MFO in alkaline media. An electrode pair based on Ru&Fe-WOx delivers 10/100/500 mA/cm2 current density at a low cell-potential of 1.38/1.50/1.62 V with excellent durability and nearly 100 % Faradaic efficiency (FE), outperforming the benchmark pair of Pt/C||RuO2. By using integrated experimental and theoretical approaches, we verify synergy between WOx host and Ru&Fe-based dopants, which results in optimized H* adsorption on lattice oxygen (HER) as well as enhanced HCOO* desorption on ruthenium site (MFO). In addition, the unique nanostructure of Ru&Fe-WOx may further benefit to the mass transfer and catalyst robustness at large current density. This work thus demonstrates an integrated catalyst design, which is highly efficient for boosting concurrent, cost-effective and scalable production of hydrogen and formate.

Synergistic modulation of nanostructure and active sites: Ternary Ru&Fe-WOx electrocatalyst for boosting concurrent generations of hydrogen and formate over 500 mA cm-2

Electro-reduction of nitrate to ammonia (e-NRA) has been considered as a facile and promising approach to eliminate nitrate pollution and produce ammonia (NH3) under ambient conditions. Nevertheless, in recent state-of-the-art...

In-Situ Reconstruction Enhanced Dual-Site Catalysis towards Nitrate Electroreduction to Ammonia

Ru/NC heterointerfaces boost energy-efficient production of green H2 over a wide pH range

Electrochemical reduction reaction of nitrate (NITRR) provides a sustainable route toward the green synthesis of ammonia. Nevertheless, it remains challenging to achieve high-performance electrocatalysts for NITRR especially at low overpotentials. In this work, hierarchical nanospheres consisting of polycrystalline Iridium&copper (Ir&Cu) and amorphous Cu2 O (Cux Iry Oz NS) have been fabricated. The optimal species Cu0.86 Ir0.14 Oz delivers excellent catalytic performance with a desirable NH3 yield rate (YR) up to 0.423 mmol h-1  cm-2 (or 4.8 mg h-1  mgcat -1 ) and a high NH3 Faradaic efficiency (FE) over 90% at a low overpotential of 0.69 V (or 0 VRHE ), where hydrogen evolution reaction (HER) is almost negligible. The electrolyzer toward NITRR and hydrazine oxidation (HzOR) is constructed for the first time with an electrode pair of Cu0.86 Ir0.14 Oz //Cu0.86 Ir0.14 Oz , yielding a high energy efficiency (EE) up to 87%. Density functional theory (DFT) calculations demonstrate that the dispersed Ir atom provides active site that not only promotes the NO3 - adsorption but also modulates the H adsorption/desorption to facilitate the proton supply for the hydrogenation of *N, hence boosting the NITRR. This work thus points to the importance of both morphological/structural and compositional engineering for achieving the highly efficient catalysts toward NITRR.

Hierarchical Nanospheres with Polycrystalline Ir&Cu and Amorphous Cu2 O toward Energy-Efficient Nitrate Electrolysis to Ammonia.

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