Nitrite Electroreduction to Ammonia Promoted by Molecular Carbon Dioxide with Near-unity Faradaic Efficiency

Toward Durable CO2 Electroreduction with Cu-Based Catalysts via Understanding Their Deactivation Modes

The technology of CO2 electrochemical reduction (CO2ER) provides a means to convert CO2, a waste greenhouse gas, into value-added chemicals. Copper is the most studied element that is capable of catalyzing CO2ER to obtain multicarbon products, such as ethylene, ethanol, acetate, etc., at an appreciable rate. Under the operating condition of CO2ER, the catalytic performance of Cu decays because of several factors that alters the surface properties of Cu. In this review, these factors that cause the degradation of Cu-based CO2ER catalysts are categorized into generalized deactivation modes, that are applicable to all electrocatalytic systems. The fundamental principles of each deactivation mode and the associated effects of each on Cu-based catalysts are discussed in detail. Structure- and composition-activity relationship developed from recent in situ/operando characterization studies are presented as evidence of related deactivation modes in operation. With the aim to address these deactivation modes, catalyst design and reaction environment engineering rationales are suggested. Finally, perspectives and remarks built upon the recent advances in CO2ER are provided in attempts to improve the durability of CO2ER catalysts.

P-Block Antimony-Copper Single-Atom Alloys for Selective Nitrite Electroreduction to Ammonia

Electrocatalytic reduction of NO2- to NH3 (NO2RR) offers an effective method for alleviating NO2- pollution and generating valuable NH3. Herein, a p-block single-atom alloy, namely, isolated Sb alloyed in a Cu substrate (Sb1Cu), is explored as a durable and high-current-density NO2RR catalyst. As revealed by the theoretical calculations and operando spectroscopic measurements, we demonstrate that Sb1 incorporation can not only hamper the competing hydrogen evolution reaction but also optimize the d-band center of Sb1Cu and intermediate adsorption energies to boost the protonation energetics of NO2--to-NH3 conversion. Consequently, Sb1Cu integrated in a flow cell achieves an outstanding NH3 yield rate of 2529.4 μmol h-1 cm-2 and FENH3 of 95.9% at a high current density of 424.2 mA cm-2, as well as a high durability for 100 h of electrolysis.

Unveiling the Reaction Mechanism of Nitrate Reduction to Ammonia Over Cobalt-Based Electrocatalysts

Electrocatalytic reduction of nitrate to ammonia (NRA) has emerged as an alternative strategy for sewage treatment and ammonia generation. Despite excellent performances having been achieved over cobalt-based electrocatalysts, the reaction mechanism as well as veritable active species across a wide potential range are still full of controversy. Here, we adopt CoP, Co, and Co3O4 as model materials to solve these issues. CoP evolves into a core@shell structured CoP@Co before NRA. For CoP@Co and Co catalysts, a three-step relay mechanism is carried out over superficial dynamical Coδ+ active species under low overpotential, while a continuous hydrogenation mechanism from nitrate to ammonia is unveiled over superficial Co species under high overpotential. In comparison, Co3O4 species are stable and steadily catalyze nitrate hydrogenation to ammonia across a wide potential range. As a result, CoP@Co and Co exhibit much higher NRA activity than Co3O4 especially under a low overpotential. Moreover, the NRA performance of CoP@Co is higher than Co although they experience the same reaction mechanism. A series of characterizations clarify the reason for performance enhancement highlighting that CoP core donates abundant electrons to superficial active species, leading to the generation of more active hydrogen for the reduction of nitrogen-containing intermediates.

Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value-added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high-value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value-added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin-film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value-added products and other pharmaceuticals.

Polyoxometalate-Intercalated Tremella-Like CoNi-LDH Nanocomposites for Electrocatalytic Nitrite-Ammonia Conversion

The electrocatalytic reduction of NO2- (NO2RR) holds promise as a sustainable pathway to both promoting the development of emerging NH3 economies and allowing the closing of the NOx loop. Highly efficient electrocatalysts that could facilitate this complex six-electron transfer process are urgently desired. Herein, tremella-like CoNi-LDH intercalated by cyclic polyoxometalate (POM) anion P8W48 (P8W48/CoNi-LDH) prepared by a simple two-step hydrothermal-exfoliation assembly method is proposed as an effective electrocatalyst for NO2- to NH3 conversion. The introduction of POM with excellent redox ability tremendously increased the electrocatalytic performance of CoNi-LDH in the NO2RR process, causing P8W48/CoNi-LDH to exhibit large NH3 yield of 0.369 mmol h-1 mgcat-1 and exceptionally high Faradic efficiency of 97.0% at -1.3 V vs the Ag/AgCl reference electrode in 0.1 M phosphate buffer saline (PBS, pH = 7) containing 0.1 M NO2-. Furthermore, P8W48/CoNi-LDH demonstrated excellent durability during cyclic electrolysis. This work provides a new reference for the application of POM-based nanocomposites in the electrochemical reduction of NO2- to obtain value-added NH3.

Dynamic Reconstruction of Two-Dimensional Defective Bi Nanosheets for Efficient Electrocatalytic Urea Synthesis

Catalyst surface dynamics drive the generation of active species for electrocatalytic reactions. Yet, the understanding of dominant site formation and reaction mechanisms is limited. In this study, we thoroughly investigate the dynamic reconstruction of two-dimensional defective Bi nanosheets from exfoliated Bi2Se3 nanosheets under electrochemical CO2 and nitrate (NO3 -) reduction conditions. The ultrathin Bi2Se3 nanosheets obtained by NaBH4-assisted cryo-mediated liquid-phase exfoliation are more easily reduced and reconstructed to Bi nanosheets with high-density grain boundaries (GBs; GB-rich Bi). The reconstructed GB-rich Bi catalyst affords a remarkable yield rate of 4.6 mmol h-1 mgcat. -1 and Faradaic efficiency of 32 % for urea production at -0.40 V vs. RHE. Notably, this yield rate is 2 and 8.2 times higher than those of the low-GB Bi and bulk Bi catalysts, respectively. Theoretical analysis demonstrates that the GB sites significantly reduce the *CO and *NH2 intermediate formation energy and C-N coupling energy barrier, enabling selective urea electrosynthesis on the GB-rich Bi catalyst. This work will trigger further research into the structure-activity interplay in dynamic processes using in situ techniques.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

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 (NO3RR) 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, a timely and comprehensive review on the important progresses of electrochemical NO3RR, covering the rational design of electrocatalysts, emerging CN coupling reactions, and advanced energy conversion and storage systems is provided. 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.

Recent Advances in Electrocatalytic Hydrogenation Reactions on Copper-Based Catalysts

Hydrogenation reactions play a critical role in the synthesis of value-added products within the chemical industry. Electrocatalytic hydrogenation (ECH) using water as the hydrogen source has emerged as an alternative to conventional thermocatalytic processes for sustainable and decentralized chemical synthesis under mild conditions. Among the various ECH catalysts, copper-based (Cu-based) nanomaterials are promising candidates due to their earth-abundance, unique electronic structure, versatility, and high activity/selectivity. Herein, recent advances in the application of Cu-based catalysts in ECH reactions for the upgrading of valuable chemicals are systematically analyzed. The unique properties of Cu-based catalysts in ECH are initially introduced, followed by design strategies to enhance their activity and selectivity. Then, typical ECH reactions on Cu-based catalysts are presented in detail, including carbon dioxide reduction for multicarbon generation, alkyne-to-alkene conversion, selective aldehyde conversion, ammonia production from nitrogen-containing substances, and amine production from organic nitrogen compounds. In these catalysts, the role of catalyst composition and nanostructures toward different products is focused. The co-hydrogenation of two substrates (e.g., CO2 and NOx n, SO3 2-, etc.) via C─N, C─S, and C─C cross-coupling reactions are also highlighted. Finally, the critical issues and future perspectives of Cu-catalyzed ECH are proposed to accelerate the rational development of next-generation catalysts.

Electrocatalytic reduction of nitrite to ammonia on undercoordinated Cu

Electrocatalytic NO2--to-NH3 reduction (NO2RR) has emerged as an intriguing route for simultaneous mitigation of harmful nitrites and production of valuable NH3. Herein, we design for the first time undercoordinated Cu nanowires (u-Cu) as an efficient and selective NO2RR electrocatalyst, delivering the maximum NO2--to-NH3 faradaic efficiency of 94.7% and an ammonia production rate of 494.5 μmol h-1 cm-2 at -0.7 V vs. RHE. Theoretical calculations reveal that the created undercoordinated Cu sites on u-Cu can enhance NO2- adsorption, boost NO2--to-NH3 energetics and restrict competitive hydrogen evolution, thereby enabling the active and selective NO2RR.

Tandem Nitrate-to-Ammonia Conversion on Atomically Precise Silver Nanocluster/MXene Electrocatalyst

Electrocatalytic reduction of nitrate (NO3 RR) to synthesize ammonia (NH3 ) provides a competitive manner for carbon neutrality and decentralized NH3 synthesis. Atomically precise nanoclusters, as an advantageous platform for investigating the NO3 RR mechanisms and actual active sites, remain largely underexplored due to the poor stability. Herein, we report a (NH4 )9 [Ag9 (mba)9 ] nanoclusters (Ag9 NCs) loaded on Ti3 C2 MXene (Ag9 /MXene) for highly efficient NO3 RR performance towards ambient NH3 synthesis with improved stability in neutral medium. The composite structure of MXene and Ag9 NCs enables a tandem catalysis process for nitrate reduction, significantly increasing the selectivity and FE of NH3 . Besides, compared with individual Ag9 NCs, Ag9 /MXene has better stability with the current density performed no decay after 108 hours of reaction. This work provides a strategy for improving the catalytic activity and stability of atomically precise metal NCs, expanding the mechanism research and application of metal NCs.

Recent research progress on building C-N bonds via electrochemical NOx reduction

The release of NOx species (such as nitrate, nitrite and nitric oxide) into water and the atmosphere due to human being's agricultural and industrial activities has caused a series of environmental problems, including accumulation of toxic pollutants that are dangerous to humans and animals, acid rain, the greenhouse effect and disturbance of the global nitrogen cycle balance. Electrosynthesis of organonitrogen compounds with NOx species as the nitrogen source offers a sustainable strategy to upgrade the waste NOx into value-added organic products under ambient conditions. The electrochemical reduction of NOx species can generate surface-adsorbed intermediates such as hydroxylamine, which are usually strong nucleophiles and can undergo nucleophilic attack to carbonyl groups to build C-N bonds and generate organonitrogen compounds such as amine, oxime, amide and amino acid. This mini-review summarizes the most recent progress in building C-N bonds via the in situ generation of nucleophilic intermediates from electrochemical NOx reduction, and highlights some important strategies in facilitating the reaction rates and selectivities towards the C-N coupling products. In particular, the preparation of high-performance electrocatalysts (e.g., nano-/atomic-scale catalysts, single-atom catalysts, alloy catalysts), selection of nucleophilic intermediates, novel design of reactors and understanding the surface adsorption process are highlighted. A few key challenges and knowledge gaps are discussed, and some promising research directions are also proposed for future advances in electrochemical C-N coupling.

Electrocatalysis of nitrogen pollution: transforming nitrogen waste into high-value chemicals

On 16 June 2023, the United Nations Environment Programme highlighted the severity of nitrogen pollution faced by humans and called for joint action for sustainable nitrogen use. Excess nitrogenous waste (NW: NO, NO2, NO2-, NO3-, etc.) mainly arises from the use of synthetic fertilisers, wastewater discharge, and fossil fuel combustion. Although the amount of NW produced can be minimised by reducing the use of nitrogen fertilisers and fossil fuels, the necessity to feed seven billion people on Earth limits the utility of this approach. Compared to current industrial processes, electrocatalytic NW reduction or CO2-NW co-reduction offers a potentially greener alternative for recycling NW and producing high-value chemicals. However, upgrading this technology to connect upstream and downstream industrial chains is challenging. This viewpoint focuses on electrocatalytic NW reduction, a cutting-edge technology, and highlights the challenges in its practical application. It also discusses future directions to meet the requirements of upstream and downstream industries by optimising production processes, including the pretreatment and supply of nitrogenous raw materials (e.g. flue gas and sewage), design and macroscopic preparation of electrocatalysts, and upscaling of reactors and other auxiliary equipment.

Bridging Together Theoretical and Experimental Perspectives in Single-Atom Alloys for Electrochemical Ammonia Production

Ammonia is an essential commodity in the food and chemical industry. Despite the energy-intensive nature, the Haber-Bosch process is the only player in ammonia production at large scales. Developing other strategies is highly desirable, as sustainable and decentralized ammonia production is crucial. Electrochemical ammonia production by directly reducing nitrogen and nitrogen-based moieties powered by renewable energy sources holds great potential. However, low ammonia production and selectivity rates hamper its utilization as a large-scale ammonia production process. Creating effective and selective catalysts for the electrochemical generation of ammonia is critical for long-term nitrogen fixation. Single-atom alloys (SAAs) have become a new class of materials with distinctive features that may be able to solve some of the problems with conventional heterogeneous catalysts. The design and optimization of SAAs for electrochemical ammonia generation have recently been significantly advanced. This comprehensive review discusses these advancements from theoretical and experimental research perspectives, offering a fundamental understanding of the development of SAAs for ammonia production.

Electrochemical reduction of nitrite to ammonia on amorphous MoO3 nanosheets

Electrocatalytic NO2- reduction to NH3 (NO2RR) is an appealing approach for mitigating NO2- pollution and for the synthesis of valuable NH3, and so the exploration for high-performance NO2RR catalysts is pivotal yet remains challenging. Herein, amorphous MoO3 nanosheets (am-MoO3) were designed as a high-performance NO2RR electrocatalyst, delivering a maximum NO2--to-NH3 faradaic efficiency of 94.8% and NH3 yield rate of 480.4 μmol h-1 cm-2 at -0.6 V vs. RHE. Theoretical computations revealed that the largely enhanced NO2RR activity of am-MoO3 originated from the amorphization-induced O-vacancies, which could enhance the NO2--to-NH3 reaction energetics and hamper the competitive hydrogen evolution.

Promoting Nitrite-to-Ammonia Electroreduction over Amorphous CoS2 Nanorods

Electrocatalytic nitrite reduction to ammonia (NO2RR) emerges as a promising route to simultaneously attain harmful NO2- removal and green NH3 synthesis. In this study, amorphous CoS2 nanorods (a-CoS2) are first demonstrated as an effective NO2RR catalyst, which exhibits the maximum FENH3 of 88.7% and NH3 yield rate of 438.1 μmol h-1 cm-2 at -0.6 V vs RHE. Detailed experimental and computational investigations reveal that the high NO2RR performance of a-CoS2 originates from the amorphization-induced S vacancies to facilitate NO2- activation and hydrogenation, boost the electron transport kinetics, and inhibit the competitive hydrogen evolution.

Understanding the Activity Trends in Electrocatalytic Nitrate Reduction to Ammonia on Cu Catalysts

Cu-based catalysts possess great potential in the electrocatalytic nitrate (NO3-) reduction reaction for ammonia (NH3) synthesis. However, the low atomic economy limits their further application. Here we report a Cu single-atom (SA) incorporated in nitrogen-doped carbon (Cu SA/NC) with high atomic economy, which exhibits superior NH3 Faradaic efficiency (FE) of 100% along with an impressive NH3 yield rate of 7480 μg h-1 mgcat.-1. As counterparts, Cus+n/NC, with mixed SA and nanoparticles (NPs), shows decreasing NH3 FE with decreasing SA content, but the production of N2 and N2O increases gradually, which reaches the maximum on pure Cu NPs. In situ characterizations and theoretical calculations reveal that a higher NH3 FE of Cu SA/NC is ascribed to a lower free energy of the rate-limiting step (HNO* → N*) and effective inhibition for the N-N coupled process. This work provides the intuitive activity trends of Cu-based catalysts, opening an avenue for subsequent catalysts design.

Electrocatalytic nitrite-to-ammonia reduction on isolated Cu sites

We report that isolated Cu atoms anchored on MnO2 nanowires (Cu1/MnO2) can be an effective catalyst towards the electrocatalytic NO2--to-NH3 reduction (NO2RR). A combination of experiments and theoretical calculations reveals that isolated Cu sites can effectively activate NO2-, lower the energy barrier of *NO→*NOH rate-determining step and suppress the competitive hydrogen evolution, thus facilitating both activity and selectivity towards the NO2RR. As a result, Cu1/MnO2 shows the maximum NH3-Faradaic efficiency of 93.3% with a corresponding NH3 yield rate of 439.8 μmol h-1 cm-2 at -0.7 V vs. RHE, together with an excellent electrocatalytic durability.

High-Efficiency Electroreduction of Nitrite to Ammonia on Ni Nanoparticles Strutted 3D Honeycomb-Like Porous Carbon Framework

Electroreduction of nitrite (NO2 - ) to ammonia (NH3 ) provides a sustainable approach to yield NH3 , whilst eliminating NO2 - contaminants. In this study, Ni nanoparticles strutted 3D honeycomb-like porous carbon framework (Ni@HPCF) is fabricated as a high-efficiency electrocatalyst for selective reduction of NO2 - to NH3 . In 0.1 M NaOH with NO2 - , such Ni@HPCF electrode obtains a significant NH3 yield of 12.04 mg h-1  mgcat. -1 and a Faradaic efficiency of 95.1 %. Furthermore, it exhibits good long-term electrolysis stability.

Single-atom Cu anchored on Mo2C boosts nitrite electroreduction to ammonia

We design single-atom Cu anchored on Mo2C (Cu1/Mo2C) as an effective electrocatalyst towards electrochemical nitrite reduction to ammonia (NO2RR), exhibiting an NH3-faradaic efficiency of 91.5% with a corresponding NH3 yield rate of 472.9 μmol h-1 cm-2 at -0.6 V vs. RHE. Theoretical computations unravel that single-atomic Cu couples with the surface Mo atom of Mo2C to enable the construction of Cu-Mo dual-active centers, which can synergistically activate NO2- and minimize the NO2--to-NH3 reaction energy barrier, whilst suppressing the competing hydrogen evolution reaction.