Efficient Synthesis of Hydroxylamine via Nitrate Electroreduction on Sulfate/Oxide‐Derived Lead Catalysts

Electrocatalytic nitrate (NO3−) reduction to ammonia (NRA) has emerged as an alternative strategy for effluent treatment and ammonia production. Despite significant advancements that have been achieved in this field, the efficient conversion of low-concentration nitrate to ammonia at low overpotential remains a formidable challenge. This challenge stems from the sluggish reaction kinetics caused by the limited distribution of negatively charged NO3− in the vicinity of the working electrode and the competing side reactions. Here, a pulsed potential approach is introduced to overcome these issues. A good NRA performance (Faradaic efficiency: 97.6%, yield rate: 2.7 mmol−1 h−1 mgRu−1, conversion rate: 96.4%) is achieved for low-concentration (≤10 mM) nitrate reduction, obviously exceeding the potentiostatic test (Faradaic efficiency: 65.8%, yield rate: 1.1 mmol−1 h−1 mgRu−1, conversion rate: 54.1%). The combined results of in situ characterizations and finite element analysis unveil the performance enhancement mechanism that the periodic appearance of anodic potential can significantly optimize the adsorption configuration of the key *NO intermediate and increase the local NO3− concentration. Furthermore, our research implies an effective approach for the rational design and precise manipulation of reaction processes, potentially extending its applicability to a broader range of catalytic applications.

Electrochemical nitrate reduction to ammonia (eNO3RR) in neutral conditions is an effective and sustainable method for ammonia production while treating nitrate pollution. Nevertheless, it still faces a great challenge due to the complicated multiple proton‐coupled‐electron process in eNO3RR. Herein, an efficient heterostructure electrocatalyst Cu‐Cu2O/Ni2P has been successfully fabricated for neutral ammonia electrosynthesis from nitrate reduction. Cu‐Cu2O/Ni2P exhibits outstanding eNO3RR performance with a Faradaic efficiency (FE) as high as 96.4% and ammonia yield rate of 14636 µg·h−1·cm−2 at −1.0 V (vs RHE) in 0.1 m PBS solution, which outperforms most of the reported eNO3RR electrocatalysts in neutral media. More importantly, the catalyst demonstrates an exceptional stability for 30 consecutive electrolysis cycles and outstanding durability even at large current density of 440 mA cm−2 in a flow cell. The tandem catalysis mechanism of NO3− → NO2− → NH3 by the synergism of two components of Cu‐Cu2O and Ni2P in Cu‐Cu2O/Ni2P is verified by the experimental and theoretical calculations. Cu‐Cu2O has strong adsorption capacity for NO3− and which can be reduced into NO2−, while Ni2P can facilitate the water splitting, which promotes the abundant *H production, thus boosting the subsequent hydrogenation process during eNO3RR process whilst inhibiting the competitive hydrogen evolution reaction (HER).

Tailored covalent organic frameworks with metal sites enabling high-efficiency ammonia generation through nitrate electroreduction.

The electroreduction of nitrate (NO3-) for sustainable ammonia (NH3) production has recently emerged as a green process to solve water contamination and produce valuable chemicals. In this study, we developed Ni6@CuFe-LDH composites comprising tiara Ni6(SC2H4COOH)12 (Ni6) clusters anchored on the edges of 2D CuFe-LDH (LDH: layered double hydroxides) nanosheets via electrostatic interactions. The Ni6@CuFe-LDH catalyst exhibits high electrochemical performance in nitrate reduction reaction (NO3RR). Specifically, the Ni6@CuFe-LDH gives rise to an excellent faradaic efficiency of ∼97%, significantly surpassing the ∼73% FE of the pristine CuFe-LDH, with the NH3 productivity (0.91 mmol mg-1 h-1) being similar to that of the CuFe-LDH. Mechanistic studies reveal that the superior electrocatalysis of Ni6-based catalysts is primarily due to the synergistic interaction between Ni6 clusters and CuFe-LDH, which alters the rate-determining step (RDS) of the desorption of *NH3 species (for CuFe-LDH) to the *NO3 → *NO2 step (for Ni6@CuFe-LDH); this is corroborated by the control experiments of NO2RR, in situ Raman and infrared spectroscopies, and computational approaches. In all, these efforts push forward the NO3RR research to study the structure-property relationships from the micro/nano-level to the precise atomic-level.

The artificial ammonia synthesis via electrochemical nitrate reduction has met increasing research interest, but it is still necessary to develop advanced catalysts with high nitrate-to-ammonia capability. Herein, we propose and demonstrate a one-step method to construct binder-free Cu foam-supported oxygen vacancy-rich Cu/CuOx in-plane heterostructured nanosheet arrays (Cu/CuOx/CF). In addition to exposing ample active sites, the two-dimensional nanosheet morphology greatly facilitates the mass/charge-transfer process during electrocatalysis. Besides, the in-plane heterojunctions and rich oxygen vacancies induced synergistic effect can modulate the electronic structure of active sites and thus tune the adsorption properties of the reactant intermediates and inhibit the formation of undesirable byproducts, which is conducive to the further improvement of nitrate reduction activity. As a result, these advantages endow the Cu/CuOx/CF with superior performance for ammonia synthesis via nitrate electroreduction, achieving high ammonia selectivity (95.00%) and Faradaic efficiency (93.58%).

The electroreduction of nitrate and nitrite on iridium‐deposited carbon fiber electrode in aqueous sodium perchlorate solutions of pH 7 was studied using cyclic voltammetric techniques. Results obtained on the modified electrode support a two‐step nitrate reduction mechanism in which nitrate is first reduced to nitrite at during the negative scan and further reduced to ammonia at during the anodic scan. An oxidation peak for the unreduced nitrite was observed at −300 mV (Ag/AgCl). A Tafel slope of approximately 178 mV/dec and a pseudo‐first‐order rate constant of were determined for the cathodic reduction peak while Tafel slope of 18.9 mV/dec and were obtained for the anodic reduction peak. The Tafel slopes obtained also support the two‐step mechanism. The results obtained strongly suggest a minimal effect of pH on nitrate reduction in the pH range of 2.0–13.0 in solution at the surface of iridium‐coated carbon fiber electrode. © 2000 The Electrochemical Society. All rights reserved.

An Extrinsic Faradaic Layer on CuSn for High-Performance Electrocatalytic CO ₂ Reduction

Electroreduction of nitrate (NO3 -) to ammonia (NH3) is an environmentally friendly route for NH3 production, serving as an appealing alternative to the Haber-Bosch process. Recently, various noble metal-based electrocatalysts have been reported for electroreduction of NO3 -. However, the application of pure metal electrocatalysts is still limited by unsatisfactory performance, owing to the weak adsorption of nitrogen-containing intermediates on the surface of pure metal electrocatalysts. In this work, we report thiol ligand-modified Au nanoparticles as the effective electrocatalysts toward electroreduction of NO3 -. Specifically, three mercaptobenzoic acid (MBA) isomers, thiosalicylic acid (ortho-MBA), 3-mercaptobenzoic acid (meta-MBA), and 4-mercaptobenzoic acid (para-MBA), were employed to modify the surface of the Au nanocatalyst. During the NO3 - electroreduction, para-MBA modified Au (denoted as para-Au/C) displayed the highest catalytic activity among these Au-based catalysts. At -1.0 V versus reversible hydrogen electrode (vs RHE), para-Au/C exhibited a partial current density for NH3 of 472.2 mA cm-2, which was 1.7 times that of the pristine Au catalyst. Meanwhile, the Faradaic efficiency (FE) for NH3 reached 98.7% at -1.0 V vs RHE for para-Au/C. The modification of para-MBA significantly improved the intrinsic activity of the Au/C catalyst, thus accelerating the kinetics of NO3 - reduction and giving rise to a high NH3 yield rate of para-Au/C.

Alternative route for electrochemical ammonia synthesis by reduction of nitrate on copper nanosheets

The development of stable, Earth-abundant, and high-activity molecular water oxidation catalysts under acidic and neutral conditions remains a great challenge. Here, the use of N-heterocyclic carbene (NHC)-based iron(III) complex 1 {[phenyl(tris(3-methylimidazol-1-ylidene))borate]2Fe(III)}+ as a catalyst for water oxidation under acidic and neutral conditions was investigated. Two iron(II) carbene complexes, 2 {[2,6-bis(3-methylimidazolium-1-yl)pyridine]2Fe}2+ and 3 {[2,6-bis(3-methylimidazolium-1-yl)pyridine-4-carboxylic acid]2Fe}2+, were also used for comparison. A series of experiments demonstrate that complex 1 has excellent performance in terms of both catalytic activity and stability. In addition, the faradaic efficiency and turnover frequency (TOF) reach 95.0% and 2.8 s-1, respectively. An overpotential of ca. 490 mV is obtained at pH 1.5. Density functional theory (DFT) calculations indicate that dehydrogenation is the potential-determining step (PDS) in water oxidation. Complex 1 has a lower free energy barrier in this process than 2 and 3. High-valent Fe species are further proven in 1 by spectroelectrochemical measurements, which are crucial in promoting water oxidation. This study is expected to contribute to the development of homogeneous water oxidation catalysis under acidic and neutral conditions.

Ammonia (NH₃) is the second highest-produced chemical by mass after sulfuric acid.¹,² It is widely used in fertilizers, refrigerants, explosives, textiles, and pharmaceuticals, with growing importance as a green energy carrier and fuel. Commercial ammonia production relies on the thermocatalytic Haber-Bosch process, developed in the early 20th century, which accounts for 1.8% of global energy consumption and emits 0.5 gigatons of CO₂ annually, accounting for 1% of global CO₂ emissions and 15-20% of the chemical sector. Moreover, the rise of processed food and fertilizer industries has disrupted the global nitrogen cycle, causing nitrate accumulation in groundwater and water bodies leading to eutrophication, oxygen depletion, and dead zones.³,⁴ Ammonia electrosynthesis from nitrites under ambient conditions offers a sustainable alternative to simultaneously reduce CO₂ emissions and address nitrate contamination in water bodies. Copper-based catalysts are highly efficient for nitrate reduction reactions (NO₃RR) as cathode electrocatalysts.⁵ However, their slower kinetic and high overpotential requirement of nitrate reduction tends to accumulate nitrite during NO₃RR, leading to a low Faradaic efficiency (FE) for ammonia production.⁶ A series of alloy and spinel oxide electrocatalysts have been synthesized for NO₃RR with a general tradeoff between partial current density and FE which breaks the adsorption-energy scaling relation of copper leading to higher Faradaic efficiency for ammonia production.⁷,⁸ However, there is little to no emphasis on the understanding of electrode kinetics and the role of the spinel structure in NO₃RR remains obfuscated. Most reported metal and spinel oxides excel in alkaline media but face severe facility corrosion; therefore, it is crucial to evaluate NO₃RR in neutral media, considering the presence of nitrate in aquatic environments with a neutral pH. Therefore, this work presents the electrocatalytic nitrate reduction on CuFe₂O₄ in neutral media showing a better yield rate and FE of ammonia formation (98.6% at -0.9 V vs. RHE) compared to that of Cu₂O (78.7% at -0.9 V vs. RHE) with 60% lower accumulation of nitrite in the spent electrolyte after electrolysis, Figure 1 a and b. DFT calculations indicate that the enhanced performance of CuFe₂O₄ compared to Cu₂O arises from several synergistic factors. These include a more favorable surface structure, enabling optimal adsorption energies for crucial intermediates, and a reduced energy barrier for nitrite reduction, Figure 1c and d. Furthermore, the unique spinel architecture of CuFe₂O₄ promotes efficient electron transfer and establishes a distinct local environment around its active sites, collectively driving its superior catalytic activity. By comparing the computational results with experimental outcomes, we evince the role of the internal magnetic field of spinel structure, and tandem electrocatalysis mechanism in the electrochemical ammonia synthesis.References International Renewable Energy Agency (IRENA). (2022). Innovation outlook: Renewable ammonia. https://www.irena.org/publications/2022/May/InnovationOutlook-Renewable-AmmoniaR. MacFarlane, P. V. Cherepanov, J. Choi, B. H. R. Suryanto, R. Y. Hodgetts, J. M. Bakker, F. M. Ferrero Vallana, A. N. Simonov, Joule 2020, 4, 1186–1205.Bird, A. Clarke, P. Davies, E. Surkovic, Ammonia: Zero-Carbon Fertiliser, Fuel and Energy Store. Policy Briefing., 2020.Fernández-López, J.A.; Alacid, M.; Obón, J.M.; Martínez-Vives, R.; Angosto, J.M. Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance. Sci. 2023, 13, 4154.Niu, Z. Zhang, X. Wang, X. Wan, C. Shao, Y. Guo, Adv. Funct.Mater. 2021, 31, 2008533;A. Cerrón-Calle, A. S. Fajardo, C. M. Sánchez-Sánchez, S. GarciaSegura, Appl. Catal.,B 2022, 302,120844.Wensheng Gao, Kefeng Xie, Jin Xie, Xiaomei Wang et al. Mater. 2023, 35, 2202952.Zhijuan Liu, Jinyu Guo, Lu-yu Liu et al. Nano Research, 2024, 17(5): 3547−3570 Figure 1

Electrochemical nitrate reduction in alkaline media offers a sustainable route for ammonia synthesis at rates rivaling those of the Haber-Bosch process. However, its energy efficiency is limited by sluggish nitrate deoxidation and hydrogenation, compounded by challenges in proton supply via H2O dissociation. Here, we develop an enzyme-like substrate transport channel through a hierarchical arrangement of metallic Ag and Ru nanophases, to enable cascade nitrate-to-ammonia conversion and optimize underpotential-deposited hydrogen utilization. Operando characterization and theoretical calculations reveal that Ag-Ru electronic synergy regulates underpotential-deposited hydrogen coverage at Ru-centered active sites by coupling facilitated H2O dissociation with *OH-mediated site regeneration, thereby promoting nitrite relay conversion. Our underpotential-deposited hydrogen-assisted nitrate electroreduction system delivers a half-cell ammonia energy efficiency of 53.7% at 0.2 V versus RHE with near-unity Faradaic efficiency across a wide nitrate concentration range, and an ammonia partial current density of 2.2 A cm-2 at 0 V versus RHE. Pairing cathodic nitrate reduction with anodic H2 oxidation enables ammonia production costs below $1.15 kg-1 while maintaining sustained energy efficiency over 100 h at 200 mA cm-2.

Open AccessCCS ChemistryMINI REVIEWS28 Dec 2022Electrocatalytic CO2 Reduction over Bimetallic Bi-Based Catalysts: A Review Wei Chen, Yating Wang, Yuhang Li and Chunzhong Li Wei Chen Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yating Wang Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yuhang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 and Chunzhong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 https://doi.org/10.31635/ccschem.022.202202357 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Electrocatalytic reduction of carbon dioxide (CO2) to valuable fuels is an up-and-coming approach. Owing to the low cost, environmental friendliness, and high selectivity to formate single product at low overpotentials, bismuth (Bi)-based catalysts have attracted extensive research attention. In this review, the reaction mechanisms of Bi-based catalysts are first introduced, and the bimetallic Bi-based catalysts synthesized by alloying, doping, and loading strategies are reviewed from the aspects of catalyst component, morphology, synthesis procedure, and performance optimization for electrocatalytic CO2 reduction. We provide an in-depth discussion of the existing challenges and an outlook for this highly promising kind of electrocatalysis. Download figure Download PowerPoint Introduction The energy crisis and environmental pollution have been ongoing significant issues and the focus of the international community. Over the past few decades, the strong dependence and overuse of fossil fuels has resulted in a rapid increase in the concentration of carbon dioxide (CO2) in the atmosphere.1,2 When the World Meteorological Organization released its latest Greenhouse Gas Bulletin, they pointed out that the current CO2 concentration in the atmosphere is 149% of preindustrial levels.3 The Bulletin additionally noted that greenhouse gases have increased by 47% through radiative forcing, with CO2 accounting for 80% of this increase from 1990 to 2020. The emission of an oversized quantity of CO2 and other harmful gases has resulted in global warming and environmental pollution, documented in numerous studies related to the energy and environment around the world. CO2 emission reduction is urgently needed. Electrocatalytic CO2 reduction reaction In order to curtail the global warming trend, scientists conduct in-depth discussions and research on the issue of reducing CO2 emissions. There are four primary strategies: (a) developing new clean energy technologies; (b) upgrading existing processes to eliminate and replace the low-efficiency sectors and equipment in traditional technologies;1 (c) afforestation and forestation; and (d) carbon capture, utilization and storage.4,5 Research on carbon capture, carbon storage, and carbon utilization has produced many advances and breakthroughs in CO2 storage and conversion. Among them, carbon capture and storage technology have certain limitations. First of all, the technology is expensive. Second, storage equipment may leak and cause a series of other hidden safety problems, such as local seawater acidification. In contrast, carbon capture and utilization technology, which reduce CO2 and convert it into usable chemical value-added fuels, possesses greater development prospects.6–8 Not only can this technology reduce high CO2 concentrations in the atmosphere, but it also produces renewable fuels to combat the energy crisis. In recent years, numerous catalytic conversion methods have been developed successively. The methods used for CO2 reduction chiefly include biological (enzyme) catalysis, photocatalysis, thermocatalysis, and electrocatalysis.9–12 At present, electrocatalysis as an emerging energy technology for CO2 emission reduction and production of value-added fuels receives a great deal of research attention.7 Compared with traditional industrial processes, electrocatalytic CO2 reduction reaction (CO2RR) can be carried out under milder environmental conditions, improving electrochemical stability and selectivity in CO2RR via selecting appropriate electrocatalysts to manipulate reaction-tailored products.13 In this regard, reports in the literature demonstrate the exploration of various electrocatalysts and electrode reaction mechanisms for CO2RR. CO2RR also utilizes renewable energy for catalytic reactions to achieve large-scale energy storage and production of high-energy products.14 Generally, CO2RR is in a position to produce a variety of reduction products through electrocatalysis. The corresponding reduction products are different as electron-transferred numbers change. Electrocatalytic CO2RR products are categorized as formic acid (HCOOH),12 carbon monoxide (CO), methane (CH4), methanol (CH3OH) as C1, and ethanoic acid (CH3COOH), ethanol (C2H5OH)15 as C2.16,17 The above reduction products are obtained by various electron-transfer mechanisms and half-reactions, as shown in Table 1.18,19 Table 1 | Electrochemical Potentials of Several CO2 Reduction Reactions CO2 Reduction Half-Reactions Electrode PotentialV (vs SHE) Electrode PotentialV (vs RHE) CO2 + 2H+ + 2e− → CO + H2O −0.52 −0.106 CO2 + 2H+ + 2e− → HCOOH −0.61 −0.250 CO2 + 4H+ + 4e− → HCHO + H2O −0.51 −0.070 CO2 + 6H+ + 6e− → CH3OH + H2O −0.38 0.016 CO2 + 8H+ + 8e− → CH4 + 2H2O −0.24 0.169 2CO2 + 12H+ + 12e− → C2H4 + 4H2O −0.34 0.064 2CO2 + 12H+ + 12e− → C2H5OH + 3H2O −0.33 0.084 During the electrocatalytic reduction process, not solely CO2RR but also other side reactions will occur, resulting in the complexity of the reaction. For instance, the hydrogen evolution reaction (HER) competes with CO2RR to generate H2, which decreases the performance of CO2RR20–22 in order to efficiently overcome the energy barrier of electron-transfer proton coupling, accelerate the catalytic reaction rate. and suppress the occurrence of side reaction processes. The development of ideal catalysts with remarkable CO2RR selectivity and activity is the focus of current research. Several parameters of electrocatalytic performance for CO2RR catalysts can primarily be evaluated, including Faradaic efficiency (FE), overpotential, current density, Tafel slope, stability, and so on.7,12,14 FE is the charge required as a percentage of the initial charge to cross the working electrode and facilitate the electrochemical reaction. In simple terms, FE is a significant indicator to measure the selectivity of electrocatalytic CO2RR products.23 Due to the complex reaction mechanism and sluggish kinetics, the actual reduction reaction working potential is more negative than the theoretical reduction potential. High reduction overpotentials lead to wasted energy and significant HER reactions. Therefore, overpotential is an important indicator in evaluating the electrocatalytic activity of CO2RR catalysts. Studies confirm that when current density goes above 300 mA cm−2, the production cost will be reduced as much as possible.24 The equation of Tafel (η = blgj + a) is able to directly reflect the rate of reaction dominated by kinetics. The smaller the Tafel slope of b in the equation, the faster the electrochemical reaction rate and the higher the catalytic activity, which is more favorable for the electrocatalytic reaction.25 Stability is an indicator of whether an excellent electrocatalyst possesses long-term stability and efficiency.26 The stability of electrocatalysts is usually effectively assessed with potentiostatic electrolysis or cyclic voltammetry. Bismuth-based electrocatalysts Recently, P-block electrocatalysts consisting of bismuth (Bi),27–30 tin (Sn),31–33 lead (Pb),34–36 and indium (In)37,38 have inarguably facilitated electrocatalytic CO2 reduction with remarkable selectivity for C1 products, especially formic acid or formate. The advantage of electrocatalytic generation of formate lies in the high selectivity and current density achieved by prohibiting competing side reactions. Compared with other value-added products from CO2 reduction, which are difficult to solely generate and low in yield, the FE of Bi-based CO2 toward formate can reach nearly 100%.39 In addition, formate is a liquid product with excellent chemical stability at room temperature for storage and transportation compared to gas-phase products such as CO. Experts evaluate various chemicals with economic viability in CO2RR and discovered that formate has considerable marketability.13 Moreover, converting CO2 toward formate is a 2-electron transfer process, leading to a low production cost of 1$US/0.59 kg, suggesting that CO2 electrolysis of formate is more cost-competitive than the C2 product production process.40 It may be difficult to attain widespread application of Pb and In metals on a marketable scale because of toxicity or low availability. However, Bi is a dramatic and promising electrocatalyst due to its low cost, nontoxicity, environmental safety, relatively single reduction product, and high formate generation activity. Bi-based catalysts can be traced back to the Bi electrocatalyst synthesized by Komatsu's team in 1995.41 In the following decades, research on Bi-based catalysts continued to deepen. Monometallic catalysts, including metallic Bi, are currently a hot topic in the field of electrocatalysis. Recently, with in-depth exploration and development of synthesis techniques, various nanostructured monometallic Bi catalysts have been designed, such as nanoparticles, nanowires, nanotubes, nanosheets, nanodendrites, and so on, in multidimensional aspects. However, monometallic Bi catalysts may be undesirable for the breakthrough of formate electrosynthesis, owing to the limited active sites on the catalyst surface. This means that monometallic Bi catalysts usually require high overpotential to achieve high formate FE and partial current density. Compared with the monometallic Bi catalysts that use advanced synthetic strategies or tedious structural optimization to enhance their performance, bimetallic Bi-based catalysts will further involve synergistic effects. The synergistic interaction between bimetals gives the bimetallic electrocatalyst a superior catalytic performance.42 The synergistic effects in bimetallic Bi-based catalysts can be broadly viewed as Bi acting as the active site and the other metals mainly playing three roles: (1) tailoring the electronic structures of Bi sites, (2) regulating the adsorption states of the key intermediates, and (3) generating interfacial active sites to further enhance performance. Through the electronic structure modulations by the second metal, the bimetallic Bi-based catalysts will boost the formation of the key intermediate OCHO*, thus improving the performance of CO2 electroreduction to formate. The activity, selectivity, and stability will be further improved via preparing bimetallic Bi-based materials through strategies such as alloying, surface doping, defect introduction, and nanoengineering. Here, in this review, the representative reaction pathways of Bi-based electrocatalysts are first introduced, and then the reaction mechanisms of bimetallic Bi-based heterogeneous CO2RR electrocatalysts are summarized with examples from the perspective of reaction pathways. Afterward, based on the Bi-based electrocatalysts in recent development, we divide bimetallic Bi-based catalysts into three categories: (1) alloyed Bi, (2) doped Bi, and (3) supported Bi. For each category, we describe in detail its performance-enhancing strategies and provide examples of catalysts, including descriptions of their preparation process, composition, morphology, catalytic activity, and product properties. Finally, we provide an in-depth analysis of the existing challenges and the current outlook for this field. Reaction Mechanisms of the Bi-Based Electrocatalysts Bi-based catalysts have high efficiency, selectivity, and stability to form formate via electrocatalytic CO2RR in aqueous solutions. They also possess the capacity to generate CO, according to some reports.43 The pathways of the formate and CO products are comparatively simple compared to that of other CO2RR products, and the essential difference is the intermediate products. An in-depth study of the electrocatalytic CO2RR process on the surface of Bi-based catalysts is required for a good understanding of the catalytic mechanism of Bi metals. In general, there are three types of steps involving the generation of products theoretically, consisting of: (1) reactant adsorption on the electrocatalytic surface, (2) transfer of electrons and protons to the reactant, and (3) the products desorption from the electrocatalyst surface.44 Research demonstrates that the first proton coupling determines the selectivity for a catalyst, which takes place at the C or O in CO2*− radical anion. Three reaction pathways for electrocatalytic CO2RR over Bi-based catalysts are displayed in Figure 1: (a) Generally, CO2 comes into contact with the catalyst via carbon or oxygen atoms. If the carbon atom binds to the catalyst electrode surface first, *COOH intermediate will be formed, which is the first intermediate for CO2 activation in this pathway. However, *COOH intermediate will have multiple pathways in the second proton coupling electron transfer (PCET) process, which is not conducive to promoting highly selective formate production. *COOH can lose H2O to form CO, or be reduced to form HCOOH. (b) Compared to pathway a, pathway b is different in that the oxygen is bound to the electrode surface, by which CO2* − hydrogenation forms the HCOO* intermediate.19 Based on the reaction mechanism of Bi-based catalysts, the second PCET process of the HCOO* intermediate can only generate HCOOH.(c) In the pathway c, CO2 forms the OCHO* intermediate during the first PCET when only one oxygen molecule is bound to the surface electrode. Formate is the only product obtained via a subsequent second PCET process. Theoretical analyses indicate that the formation energy barrier of OCHO* intermediate is lower than that of *COOH and HCOO* intermediates, leading to the importance of OCHO* intermediates in the Bi-based electroreduction process.45 Figure 1 | Possible electrochemical reaction pathways of CO2 over Bi-based catalysts. Download figure Download PowerPoint Theoretical calculations confirm that the CO2RR process of Bi-based catalysts follows the key intermediate of OCHO* from a PCET mechanism, facilitating highly selective formate production. The specific mechanism equations (1–5) for reducing CO2 pathway in solution to HCOOH are summarized as follows:46 CO 2 ( g ) → CO 2 * (1) CO 2 * + e − → CO 2 * − (2) CO 2 * − + e − + H + → OCHO * − (3) OCHO * − + e − + H + → HCOOH * (4) HCOOH * → HCOOH ( aq ) + * (5)where * denotes the catalytic surface or adsorption site, initially the CO2 molecules are dissolved in the solution and contact the electrode surface to form adsorbed CO2*. Afterwards, single electron is transferred to CO2* that forms the CO2* − radical anion. According to HCO3 − ↔ H+ + CO32−, electron transfer and proton coupling form OCHO* − intermediates. Ultimately, OCHO* − forms formic acid solution by PCET. Since the slow kinetics of the electrocatalytic CO2RR process, HER side reactions inevitably generate H2, and some reduction reactions are accompanied by CO generation, which negatively affects the highly selective production of formate from Bi-based materials. For the purpose of obtaining more formate, the production of H2 and CO is reduced as much as possible. The catalytic mechanism may be diametrically different in the same Bi-based alloy, depending on the content of two metal elements. Therefore, the reaction mechanism is manipulated via controlling different proportions of the two metals in bimetallic Bi-based electrocatalysts. For instance, Zhang et al.47 have developed CuO/Bi(OH)3 decorated on carbon nanotubes for CO2 electroreduction. CuBi#8 and CuBi#4 are bimetallic nanoparticles obtained via a two-step hydrolysis method and adjusted the Cu/Bi ratio. CuBi#8 and CuBi#4 exhibit CO and formate FE of 96% and 60% at −0.99 V versus reversible hydrogen electrode (RHE) (VRHE), respectively. It is found that with Bi content increasing, HER is well suppressed, and the products are mainly CO. By further increasing Bi content, FEHCOOH rapidly increases accompanied by the rapid decrease of FECO. FEHCOOH reaches the maximum of 96% at 12.5 mA cm−2. The conversion of intermediates from *COOH to OCHO* via increasing Bi content further illustrates the high selectivity of OCHO* to formate. Adjusting the optimal Cu/Bi ratio suppresses the production of CO and H2, leading to efficient production of formate. In addition, defects such as oxygen vacancies or doped atoms significantly improve the catalytic performance.12 Li et al.48 have prepared Sn atom-doped Bi2O3 nanosheet (NS) electrocatalysts by constant electrolysis. Three products can be detected during the electrolysis, including H2, CO, and HCOOH. The Sn-doped Bi2O3 NSs current density is significantly increased compared to the undoped Bi2O3 NSs. The 2.5% Sn-doped Bi2O3 NSs exhibit high selectivity for formate, obtaining a supreme FE of 93.4% at the potential of −0.97 V. The HER inhibition effect is significantly enhanced compared to the undoped Bi2O3 NSs. Moreover, the catalytic capacity is optimized by coping with significant HER and expanding its specific surface area. For the first time, metallic aerogel is a three-dimensional (3D) material that has attracted enormous attention due to its abundant specific surface area, contributing to the generation of more catalytic centers.49 In addition, adjusting the partial pH can also improve the selectivity of CO2RR, and proper control of pH into acidity facilitates the formation of formate.50 Advanced Bi-Based Electrocatalysts for CO2 Reduction In recent decades, various Bi-based CO2 reduction electrocatalysts have been exhaustively studied, mainly with formate as the end product. Especially, the preparation of bimetallic catalysts via different synthetic methods is the focus of most current studies. Bimetallic Bi-based catalysts can mainly be classified into three types: (1) alloyed Bi, (2) doped Bi, and (3) supported Bi. Detailed CO2RR performances of bimetallic Bi-based electrocatalysts are summarized in Table 2. Table 2 | Performance of Bimetallic Bi-Based Catalysts in Electrocatalytic CO2RR Catalyst Electrolyte Major Products FE (%) Potential at FEMax (V) Current Density (mA cm−2) Stability (h) References Bi5Sn60 0.1 M KHCO3 Formate 94.8 −1.0 (vs RHE) 34 20 52 BixSny/Cu 0.1 M KHCO3 Formate 90.4 −0.84 (vs RHE) 30 12 53 Bi-Sn aerogel 0.1 M KHCO3 Formate 93.9 −1.0 (vs RHE) 9.3 10 57 Cu-Bi 0.1 M KHCO3 Formate 90 −0.8 (vs RHE) >2 — 60 CuBi-100 0.5 M KHCO3 Formate 94.7 −1.0 (vs RHE) 12.8 8 61 CuBi 0.5 M KHCO3 Formate 94.4 −0.97 (vs RHE) 38.5 — 62 CuBi 0.5 M KHCO3 Formate 98.3 −1.07 (vs RHE) 56.6 — 62 Bi/Cu 0.5 M KHCO3 Formate 95 −0.9 (vs RHE) 59.7 12 64 CuBi75 0.5 M KHCO3 Formate 100 −0.77 (vs RHE) 33.65 24 66 Cu1-Bi/Bi2O3@C 0.5 M KHCO3 Formate 93.4 −0.94 (vs RHE) 10.1 10 67 Pd3Bi-IMA 0.1 M KHCO3 Formate >90 −0.35 (vs RHE) 3 8 68 a-NPSB 0.1 M KHCO3 Formate 88.4 −1.15 (vs RHE) 21.2 18 70 Bi–Pt complex 0.1 M TBAPF6/THF CO 82 −1.25 (vs NHE) 0.125 — 71 Mo-Bi BMC/CP 0.5 M [Bmim]BF4 CH3OH 71.2 −0.7 (vs SHE) 12.1 — 72 Sn-doped Bi2O3 NSs 0.5 M KHCO3 Formate 93.4 −0.97 (vs RHE) 24.3 8 48 Bi/Bi(Sn)Ox NWs 1 M KOH Formate ∼100 −0.7 (vs RHE) 301.4 20 77 Cu-Bi2Se3 0.5 M NaHCO3 Formate 65.31 −1.3 (vs RHE) 24.1 24 78 Ce–[email protected]x/C 0.5 M KHCO3 Formate 96 −1.7 (vs SHE) 15.2 10 79 BiIn5[email protected] 0.5 M KHCO3 Formate 97.5 −0.86 (vs RHE) 13.5 15 81 Bi-Sn/CF 0.5 M KHCO3 Formate 96 −1.1 (vs RHE) 45 100 82 Sn0.80Bi0.20@Bi-SnOx 0.5 M KHCO3 Formate 95.8 −0.88 (vs RHE) 74.6 — 83 Bi-SnO/Cu 0.1 M KHCO3 Formate 93 −1.7 (vs Ag/AgCl) — 30 84 [email protected] 0.5 M NaHCO3 Formate 95 (vs RHE) 15 12 45 NSs 0.5 M KHCO3 Formate −0.86 (vs RHE) 8 90 0.5 M KHCO3 Formate −0.8 (vs RHE) 0.5 M KHCO3 Formate −0.8 (vs RHE) 10 [email protected] 0.5 M KHCO3 Formate (vs RHE) 10 M Formate (vs RHE) 34 Bi The is one of the methods to enhance the electrocatalytic performance of Bi-based effects facilitate the catalytic reaction by a between two different metal elements. P-block catalysts have CO2 conversion that is highly selective for Among them, Bi-Sn bimetallic is the most is a and method for the preparation of The method a of two metallic elements. Bimetallic obtained via are or and possess such as high density and can improve the catalytic activity of the catalyst Li et have two Bi and on the an In this different electrode The at different of This structure is to the surface and more catalytic When the of metal Bi and the of metal Sn 60 the catalytic performance the as the of −1.0 with mA partial current density, the FE of formate it excellent formate yield, which superior to most electrocatalysts. The of bimetallic is to the OCHO* intermediate and suppress the HER process. Li et have also electrocatalysts on and discovered that the FE of formate enhanced by the Bi content in the BixSny/Cu electrode. is an emerging in electrocatalytic due to their structure and the of the catalyst, contributing to the generation of more catalytic Bimetallic prepared by the method it to compared with methods such as or solution et have prepared Bi-Sn bimetallic under with and abundant Bi and Sn the of via controlling the ratio. with with a electron as shown in Figure abundant of that favorable for and abundant active sites during electrocatalysis. The of the Bi-Sn aerogel well with of Sn and Bi suggesting that the obtained aerogel is a Bi-Sn The that the are and corresponding to the and of Bi and This that the Sn and Bi are and more reaction sites are during the catalytic process. The the of Sn and Bi in the The electrochemical performance displayed compared with Sn and Bi catalysts, Bi-Sn aerogel excellent performance for formate with FE as high as Density that electronic between Bi and Sn the energy barrier for formate, the electrocatalytic performance In can indicate the reaction pathway of Bi-Sn aerogel bimetallic catalyst in CO2RR. shown in Figure the at at V and more as the potential increased from to V. This is to the that in the formate intermediate HCOO* is in suggesting that the synergistic effects between Bi-Sn bimetals can the generation of Figure 2 | (a) of the synthesis of Bi-Sn (b) of (c) of (d) and the corresponding of the catalytic mechanism of the of Bi-Sn with various with from Download figure Download PowerPoint is a and catalyst in the field of electrocatalysis. The product selectivity of catalysts is C1 products such as formate and CH4 and value-added fuels such as The of the Bi and to form a Cu-Bi usually formate conversion and the HER The reaction intermediates with of the between bimetallic Cu-Bi facilitate the catalytic reaction. et have synthesized bimetallic Cu-Bi electrocatalysts with Owing to the difference between Bi and the of bimetallic defect sites with high density, leading to the formation of more catalytic Cu-Bi exhibit lower current for HER and CO evolution thus the higher selectivity of Cu-Bi for formate. At −0.8 the FE of the formate product In to materials obtained by have numerous and catalytic activity will be Through the of bimetallic catalysts, the of reaction sites of electrocatalysts are increased as much as and the electrochemical of product selectivity is For instance, et have synthesized bimetallic Cu-Bi electrocatalysts by at room temperature In the by the of CuBi-100 the electrochemical performance At the potential from −0.8 V to CuBi-100 excellent selectivity for formate and FE of more than The supreme at the potential of −1.0 electron that CuBi-100 on carbon and a The of CuBi-100 has

Progress and perspectives in the electroreduction of low-concentration nitrate for wastewater management.

Conducting polypyrrole (Ppy) film was synthesized electrochemically onto a copper electrode in oxalate, oxalic acid and salicylic acid solutions. The electrochemical oxidation of pyrrole to form polypyrrole film and the electroreduction of nitrate and nitrite ions at synthesized PPy modified copper electrodes (Ppy/Cu) in potassium chloride aqueous solutions were studied by cyclic voltammetry. Polypyrrole film formation and the activity of the modified Ppy/Cu electrode for nitrate and nitrite reduction were found to be dependent on synthesis medium and conditions: pH, content and concentrations of the electrolytes; pyrrole concentration; electrode potential; electrolysis duration. The finishing and storage conditions of the prepared Ppy/Cu modified electrodes, such as drying duration and temperature; soaking duration in water before use, also affect their catalytic activity for nitrate and nitrite reduction. The highest reduction currents of nitrate and nitrite were obtained for the nanoporous structured Ppy film prepared in salicylic acid solution. Nitrate and nitrite reduction occurred at cathodic potentials of ca. -0.70 V and -0.85 V (Ag/AgCl) respectively, and the peak potentials ranged from -1.00 V to -1.15 V (Ag/AgCl). In the potassium chloride aqueous solution the reduction of nitrate and nitrite ions on the Ppy/Cu electrode was found irreversible.