Crystal Phase Engineering of Ultrathin Alloy Nanostructures for Highly Efficient Electroreduction of Nitrate to Ammonia

Carbon-Extraction-Triggered Phase Engineering of Rhodium Nanomaterials for Efficient Electrocatalytic Nitrate Reduction Reaction

Phase engineering plays a crucial role in tuning the physicochemical properties of noble metal nanomaterials. However, synthesis of high-purity unconventional-phase noble metal nanomaterials remains highly challenging via current wet-chemical methods. Herein, we develop a unique synthetic methodology to prepare freestanding unconventional hexagonal close-packed (2H) Rh nanoplates (NPLs) via a rationally designed two-step strategy. By extracting C from pre-synthesized rhodium carbide of different sizes and morphology, phase-controlled synthesis of Rh nanomaterials can be achieved. Impressively, the obtained parallelogram 2H Rh NPLs have high phase purity, well-defined 2H (0001)h and (101 ¯ ${\mathrm{\bar{1}}}$ 0)h facets, and good thermostability (stable up to 300 °C). In the proof-of-concept electrocatalytic nitrate reduction reaction (NO3RR), the 2H Rh NPLs achieve higher ammonia (NH3) Faradaic efficiency (91.9%) and NH3 yield rate (156.97 mg h-1 mgcat -1) with lower overpotentials compared to the conventional face-centered cubic (3C) Rh nanocubes with (100)f facets. Density functional theory calculations reveal that the unconventional (0001)h surface has energetically favored NO3RR pathway and stronger H* absorption ability compared to the (100)f surface, which may lead to the higher activity and selectivity of NH3 production on 2H Rh NPLs. This work opens new avenues to the rational synthesis of unconventional-phase metal nanomaterials and provides important guidelines to design high-performance electrocatalysts.

Enhanced p-d Orbital Coupling in Unconventional Phase RhSb Alloy Nanoflowers for Efficient Ammonia Electrosynthesis in Neutral Media

Phase control provides a promising approach for physicochemical property modulation of metal/alloy nanomaterials toward various electrocatalytic applications. However, the controlled synthesis of alloy nanomaterials with unconventional phases remains challenging, especially for those containing both p- and d-block metals. Here, we report the one-pot synthesis of ultrathin RhSb alloy nanoflowers (NFs) with an unconventional 2H phase. Using 2H RhSb NFs as an electrocatalyst for nitrite reduction reaction in neutral media, the optimal NH3 Faradaic efficiency and yield rate can reach up to 96.8% and 47.2 mg h-1 mgcat -1 at -0.3 and -0.6 V (vs. reversible hydrogen electrode), respectively. With 2H RhSb NFs as a bifunctional cathode catalyst, the as-assembled zinc-nitrite/methanol batteries deliver a high energy efficiency of 96.4% and improved rechargeability with 120-h stable running. Ex/in situ characterizations and theoretical calculations have demonstrated that the phase change of RhSb from face-centered cubic (fcc) to 2H has optimized the electronic structure through stronger interactions between Rh and Sb by p-d orbital couplings, which improves the adsorption of key intermediates and reduces the reaction barriers of nitrite reduction to guarantee the efficient electrocatalysis. This work offers a feasible strategy of boosting the electrocatalytic performance of alloy nanostructures by integrating phase control and p-d orbital coupling.

Interfacial Electronic Interactions Induced by Self-Assembled Amorphous RuCo Bimetallenes/MXene Heterostructures for Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NO3RR) is an attractive green route to generate valuable ammonia and remove nitrates in industrial processes. However, under the intense competition of hydrogen evolution reactions (HER), it is a key challenge to improve the selectivity and reduce the energy consumption of the nitrate reduction reaction. Herein, a unique amorphous RuCo Bimetallenes confined on Ti3C2Tx-MXene (RuCo/Ti3C2Tx) is reported as a highly efficient NO3RR catalyst, showing a remarkable Faradaic efficiency for ammonia of 94.7% at -0.2 V versus reversible hydrogen electrode (RHE), with the corresponding high ammonia yield rate of 98.8 mg h-1 mgcat -1 at -0.6 V versus RHE. Significantly, the RuCo/Ti3C2Tx heterostructures are able to operate stably at 1 A cm-2 for over 100 h under membrane electrode assembly (MEA) conditions with a stabilized NH3 Faraday efficiency. In-depth theoretical and operando spectroscopic investigations unveil that the in situ generation of heterojunction via interfacial Ru/Co─O bridges can induce charge redistribution through Ru/Co─O-Ti structure and modulate the electronic structure of RuCo Bimetallenes, significantly promoting *H production and the adsorption and activation of reactants/intermediates, while suppressing HER, thereby boosting NO3RR performance. This study offers a new insight the metal-support interaction for the development of high-performance electrocatalysts.

Synergistic Cu2O@Ni(OH)2 Core-Shell Electrocatalyst for High-Efficiency Nitrate Reduction to Ammonia

The electrocatalytic reduction reaction of nitrate (NO3RR) is anticipated to convert nitrogen-containing pollutants into valuable ammonia products. Copper-based catalysts have received great attention because of their good performance in the NO3RR due to the strong binding energy with *NO3 intermediates. However, the poor H2O dissociation ability of Cu is unable to provide H• in time for the hydrogenation reaction of NOx, thus hindering the electroreduction of the NO3-. Herein, we designed a shell-core nanocube electrocatalyst Cu2O@Ni(OH)2-x (x represents the molar ratio of Ni/Cu) using the liquid phase reduction combined with the etching and precipitation method for electrocatalytic NO3RR. Due to the synergistic effect between the strong nitrate activation ability of Cu and the excellent H2O dissociation ability of Ni(OH)2, Cu2O@Ni(OH)2-3.3% shows an impressive ammonia yield rate (557.9 μmol h-1 cm-2) and Faradaic efficiency (97.4%) at -0.35 V vs. RHE. Operando Raman and Auger electron spectroscopy observe the reduction of Cu2O to Cu during the NO3RR process. Density functional theory calculations combined with electron paramagnetic resonance analysis reveals that Ni(OH)2 can lower the activation energy barrier of H2O dissociation, thereby promoting the generation of H• and accelerating the hydrogenation of *NO during the NO3RR. This research provides an efficient Cu-based catalyst for reducing NO3- and may motivate the development of effective ammonia electrocatalysts for further experimentation.

Boosting nitrate removal efficiency via synergistic strategy: Oxygen vacancy engineering and morphological control

The nitrate reduction reaction (NO3-RR) is a promising strategy for groundwater remediation. However, the development of electrocatalysts with high performance and low energy consumption remains a significant challenge. Here, we developed a new method with a dual modulation strategy involving vacancy engineering and morphology modulation engineering to synthesize rough surface nanospheres (c-Fe2O3) enriched with oxygen vacancies. The c-Fe2O3 exhibits excellent catalytic performance, with a Faraday efficiency of up to 95.54 %. More importantly, this electrocatalyst simultaneously achieves high performance and low energy consumption (EC = 0.35 kWh/mol, EEO = 1.40 kWh/m3) in under low concentration. Theoretical research verify that morphology modulation enriches the reaction substrate and improves the active site utilization efficiency. Moreover, in addition to optimizing the electronic structure of Fe2O3 and improving the charge transfer efficiency, the presence of oxygen vacancies (Ov) provides active sites that enhance the adsorption and dissociation of NO3- and reduce the energy barrier of the reaction step. This study develops a new method for preparing electrode materials that exhibit low energy consumption and high performance through a dual modulation strategy involving morphological regulation and vacancy engineering. The developed strategy also provides a broader technical route for the activity enhancement of other metal oxide catalysts.

Interfacial Water Structure Modulation on Unconventional Phase Non-Precious Metal Alloy Nanostructures for Efficient Nitrate Electroreduction to Ammonia in Neutral Media

Electrocatalytic nitrate reduction reaction (NO3RR) has been recognized as a sustainable route for nitrate removal and value-added ammonia (NH3) synthesis. Regulating the surface active hydrogen (*H) behavior is crucial but remains a formidable challenge, especially in neutral electrolytes, greatly limiting the highly selective NH3 formation. Herein, we report the controlled synthesis of heterophase hcp/fcc non-precious CuNi alloy nanostructures for efficient NH3 electrosynthesis in neutral media. Significantly, hcp/fcc Cu10Ni90 exhibits excellent performance with NH3 Faradaic efficiency and yield rate of 98.1% and 57.4 mg h-1 mgcat -1, respectively. In situ studies suggest that the high proportion of interfacial K+ ion hydrated water (K+-H2O) on hcp/fcc Cu10Ni90 creates high *H coverage via boosting interfacial water dissociation, enabling the rapid hydrogenation kinetics for NH3 synthesis. Theoretical calculations reveal that the superior NO3RR performance of hcp/fcc Cu10Ni90 originates from both the existence of hcp phase to improve the electroactivity and the high Ni content to guarantee an efficient active hydrogen supply. The strong interaction between Ni and Cu also optimizes the electronic structures of Cu sites to realize fast intermediate conversions with low energy barriers. This work provides a novel strategy to optimize surface *H behavior via tuning interfacial water structure by crystal phase control.

Electron-Deficient Mo2C Nanoclusters Embedded B, N Co-Doped Hollow Carbon Fibers for Electrocatalytic Nitrate Reduction to Ammonia

Ammonia (NH3) production from electrocatalytic nitrate reduction reaction (NO3RR) is anticipated as a promising route to achieve both sustainable NH3 generation and nitrate water pollution removal. Herein, the molybdenum carbide (Mo2C) nanoclusters embedded in boron, nitrogen co-doped hollow carbon fibers (Mo2C@BNHCFs) electrocatalyst is fabricated for NO3RR by coaxial electrospinning and pyrolysis method. The uniformly dispersed Mo2C nanoclusters and the B, N doped-carbon layer provide more adsorption sites for nitrate reduction, effectively improving the activity and long-term stability of Mo2C@BNHCFs. Mo2C@BNHCFs-2 achieves a maximum NH3 yield of 6487.43 μg h-1 mgcat. -1 and Faradaic efficiency of 74.5% at -1.1 V (vs. reversible hydrogen electrode). Electrochemical in situ characterizations identify the formation of intermediates and products during the electrocatalytic NO3 - reduction process. Meanwhile, theoretical calculations indicate that electrons transfer from Mo2C nanoclusters to carbon supports can induce the creation of electron-deficient Mo2C, thus effectively activating the NO3 - and facilitating the electrochemistry process.

Controlling electrocatalytic nitrate reduction efficiency by utilizing dπ-pπ interactions in parallel stacking molecular systems

Electrochemical reduction of nitrate to ammonia using electrocatalysts is a promising alternative strategy for both wastewater treatment and production of green ammonia. Numerous tactics have been developed to increase the electrocatalyst's NO3RR activity. Herein, we report a unique molecular alignment-dependent NO3RR performance using α-CuPc and β-CuPc nanostructures as effective electrocatalysts for the ambient synthesis of ammonia. The well-aligned β-CuPc demonstrated an impressive ammonia yield rate of 62 703 μg h-1 mgcat -1 and a Faradaic efficiency of 96%. In contrast, the less well-aligned α-CuPc exhibited a yield rate of 36 889 μg h-1 mgcat -1 and a Faradaic efficiency of 61% at -1.1 V vs. RHE under the same conditions. Scanning tunneling microscopy/spectroscopy (STM/S) confirms that the well-aligned β-CuPc exhibits superior transport properties due to optimal interaction of the Cu atom with the nitrogen atom of parallel molecules (dπ-pπ) in its one-dimensional nanostructure, which is clearly reflected in the electrocatalytic performance. Furthermore, theoretical research reveals that the NO3RR is the predominant process on the β-CuPc catalyst in comparison to the hydrogen evolution reaction, which is verified by gas chromatography, with β-CuPc exhibiting weaker binding of the *NO intermediate at the copper site and a lower overpotential, hence facilitating the NO3RR relative to α-CuPc.

Metastable Phase Noble-Metal-Free Core-Shell Structure for Efficient Electrocatalytic Nitrobenzene Transfer Hydrogenation

In order to study the catalytic behavior of a metastable-phase catalyst in electrocatalytic hydrogenation, we report a new metastable-phase noble-metal-free core-shell catalyst with a metastable hexagonal closest packed (hcp) phase Ni as the shell and face-centered-cubic (fcc) phase Cu as the core (Cu@hcp Ni NPs) for electrocatalytic hydrogenation of nitrobenzene (Ph-NO2) to aniline (Ph-NH2). Using H2O as the hydrogen source, it achieves up to 99.63% Ph-NO2 conversion and ∼100% Ph-NH2 selectivity, with an improved activity turnover frequency (TOF: 6640 h-1), much higher than those of hcp Ni NPs (5183.7 h-1) and commercial Pt/C (3537.6 h-1). It can also deliver a variety of aminoarenes with outstanding selectivity and excellent functional group compatibility with several groups. Mechanistic studies have shown that the introduction of Cu enhances hcp Ni's ability to dissociate water in situ to produce H* and improves the hydrogenation rate, resulting in the rapid conversion of Ph-NO2 to the final product Ph-NH2.

Advanced Ruthenium-Based Electrocatalysts for NOx Reduction to Ammonia

Ammonia (NH3) is widely recognized as a crucial raw material for nitrogen-based fertilizer production and eco-friendly hydrogen-rich fuels. Currently, the Haber-Bosch process still dominates the worldwide industrial NH3 production, which consumes substantial energy and contributes to enormous CO2 emission. As an alternative NH3 synthesis route, electrocatalytic reduction of NOx species (NO3 -, NO2 -, and NO) to NH3 has gained considerable attention due to its advantages such as flexibility, low power consumption, sustainability, and environmental friendliness. This review timely summarizes an updated and critical survey of mechanism, design, and application of Ru-based electrocatalysts for NOx reduction. First, the reason why the Ru-based catalysts are good choice for NOx reduction to NH3 is presented. Second, the reaction mechanism of NOx over Ru-based materials is succinctly summarized. Third, several typical in situ characterization techniques, theoretical calculations, and kinetics analysis are examined. Subsequently, the construction of each classification of the Ru-based electrocatalysts according to the size of particles and compositions is critically reviewed. Apart from these, examples are given on the applications in the production of valuable chemicals and Zn-NOx batteries. Finally, this review concludes with a summary highlighting the main practical challenges relevant to selectivity and efficiency in the broad range of NOx concentrations and the high currents, as well as the critical perspectives on the fronter of this exciting research area.

Interfacial Water Regulation for Nitrate Electroreduction to Ammonia at Ultralow Overpotentials

Nitrate electroreduction is promising for achieving effluent waste-water treatment and ammonia production with respect to the global nitrogen balance. However, due to the impeded hydrogenation process, high overpotentials need to be surmounted during nitrate electroreduction, causing intensive energy consumption. Herein, a hydroxide regulation strategy is developed to optimize the interfacial H2O behavior for accelerating the hydrogenation conversion of nitrate to ammonia at ultralow overpotentials. The well-designed Ru─Ni(OH)2 electrocatalyst shows a remarkable energy efficiency of 44.6% at +0.1 V versus RHE and a nearly 100% Faradaic efficiency for NH3 synthesis at 0 V versus RHE. In situ characterizations and theoretical calculations indicate that Ni(OH)2 can regulate the interfacial H2O structure with a promoted H2O dissociation process and contribute to the spontaneous hydrogen spillover process for boosting NO3 - electroreduction to NH3 at Ru sites. Furthermore, the assembled rechargeable Zn-NO3 -/ethanol battery system exhibits an outstanding long-term cycling stability during the charge-discharge tests with the production of high-value-added ammonium acetate, showing great potential for simultaneously achieving nitrate removal, energy conversion, and chemical synthesis. This work can not only provide a guidance for interfacial H2O regulation in extensive hydrogenation reactions but also inspire the design of a novel hybrid flow battery with multiple functions.

Phase Control in Monometallic and Alloy Nanomaterials

Metal nanomaterials with unconventional phases have been recently developed with a variety of methods and exhibit novel and attractive properties such as high activities for various catalytic reactions and magnetic properties. In this review, we discuss the progress and the trends in strategies for synthesis, crystal structure, and properties of phase-controlled metal nanomaterials in terms of elements and the combination of alloys. We begin with a brief introduction of the anomalous phase behavior derived from the nanosize effect and general crystal structures observed in metal nanomaterials. Then, phase control in monometallic nanomaterials with respect to each element and alloy nanomaterials classified into three types based on their crystal structures is discussed. In the end, all the content introduced in this review is summarized, and challenges for advanced phase control are discussed.

Mo Migration-Induced Crystalline to Amorphous Conversion and Formation of RuMo/NiMoO4 Heterogeneous Nanoarray for Hydrazine-Assisted Water Splitting at Large Current Density

Manipulating the atomic structure of the catalyst and tailoring the dissociative water-hydrogen bonding network at the catalyst-electrolyte interface is essential for propelling alkaline hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR), but remains a great challenge. Herein, we constructed an advanced a-RuMo/NiMoO4/NF heterogeneous electrocatalyst with amorphous RuMo alloy nanoclusters anchored to amorphous NiMoO4 skeletons on Ni foam by a heteroatom implantation strategy. Theoretical calculations and in situ Raman tests show that the amorphous and alloying structure of a-RuMo/NiMoO4/NF not only induces the directional evolution of interfacial H2O, but also lowers the d-band center (from -0.43 to -2.22 eV) of a-RuMo/NiMoO4/NF, the Gibbs free energy of hydrogen adsorption (ΔGH*, from -1.29 to -0.06 eV), and the energy barrier of HzOR (ΔGN2(g)=1.50 eV to ΔGN2*=0.47 eV). Profiting from these favorable factors, the a-RuMo/NiMoO4/NF exhibits excellent electrocatalytic performances, especially at large current densities, with an overpotential of 13 and 129 mV to reach 10 and 1000 mA cm-2 for HER. While for HzOR, it needs only -91 and 276 mV to deliver 10 and 500 mA cm-2, respectively. Further, the constructed a-RuMo/NiMoO4/NF||a-RuMo/NiMoO4/NF electrolyzer demands only 7 and 420 mV to afford 10 and 500 mA cm-2.

Cu3P/CoP Heterostructure for Efficient Electrosynthesis of Ammonia from Nitrate Reduction Reaction

Electrocatalytic nitrate reduction (ENO3RR) for ammonia production is one of the potential alternatives to Haber-Bosch technology for the realization of artificial ammonia synthesis. However, efficient ammonia production remains challenging due to the complex electron transfer process in ENO3RR. In this study, we fabricated a Cu3P/CoP heterostructure on carbon cloth (CC) by electrodeposition and vapor deposition, which exhibits an exceptional ENO3RR performance in alkaline medium, and showcases a Faradaic efficiency of ammonia (FENH3) and an ammonia yield rate as high as 97.95% and 17,637.3 μg h-1 cm-2 at -0.9 V vs RHE. Moreover, Cu3P/CoP also has excellent catalytic activity for nitrite reduction to ammonia, with an FENH3 up to 98.31% at -0.7 V vs RHE. The experimental and theoretical calculations reveal and confirm that the formation of a heterogeneous interface between Cu3P and CoP effectively promotes the electron transfer, where Cu3P as an electron donor induces the decrease of electron density around Cu and results in an enhancement of NO2- adsorption, thereby accelerating the ENO3RR process while inhibiting the competitive hydrogen evolution reaction (HER). Moreover, the metal phosphide catalyst facilitates the water dissociation, which accelerates the abundant *H generation, thus enhancing the subsequent hydrogenation process toward ENO3RR.

Aldehyde Directed In Situ Loading of Ag Nanodots Around the Open Metal Sites of MOFs for the Tandem Catalysis of Nitrate to Ammonia

Both spatial arrangement and intrinsic activity of electrocatalysts with dual-active sites are widely designed to match the coupling reaction between nitrate and water, in which most of the reactive intermediates can be optimized to achieve a high yield rate of ammonia. Herein, by introducing the aldehyde group inside metal-organic frameworks (MOFs) in advance, an aldehyde-induced method is achieved to direct the in situ nucleation of Ag nanodots depending on the mesopores of MOFs via a simple silver mirror reaction. The key point here is that the spatial arrangement between the aldehyde group and open metal sites is fixed end to end, which makes the aldehyde group a built-in redox-active site to drive the in situ nucleation of Ag nanodots next to the open metal sites of MOFs. Accordingly, by varying the metal sites of MOFs, a group of M-MOFs@Ag (M = Fe, Co, Ni, Cu, etc.) hybrids with dual active sites are acquired. Taking Ni-MOFs@Ag as an example, the interaction between Ni2+ and Ag sites makes it available for the tandem catalysis of nitrate-to-ammonia, in which the H· and NO2 - generated on the open Ni2+ sites and Ag nanodots, respectively, can migrate to each other to evolve into ammonia.

Isolated Rhodium Atoms Activate Porous TiO2 for Enhanced Electrocatalytic Conversion of Nitrate to Ammonia

The direct electrochemical reduction of nitrate to ammonia is an efficient and environmentally friendly technology, however, developing electrocatalysts with high activity and selectivity remains a great challenge. Single-atom catalysts demonstrate unique properties and exceptional performance across a range of catalytic reactions, especially those that encompass multi-step processes. Herein, a straightforward and cost-effective approach is introduced for synthesizing single-atom dispersed Rh on porous TiO2 spheres (Rh1-TiO2), which functions as an efficient electrocatalyst for the electroreduction of NO3 - to NH3. The synthesized Rh1-TiO2 catalyst achieve a maximum NH3 Faradaic efficiency (FE) of 94.7% and an NH3 yield rate of 29.98 mg h-1 mgcat -1 at -0.5 V versus RHE in a 0.1 M KOH+0.1 M KNO3 electrolyte, significantly outperforming not only undoped TiO2 but also Ru, Pd, and Ir single-atom doped titania catalysts. Density functional theory calculations reveal that the incorporation of Rh single atom significantly enhances charge transfer between adsorbed NO3 - and the active site. The Rh atoms not only serve as the highly active site for electrochemical nitrate reduction reaction (NO3RR), but also activates the adjacent Ti sites through optimizating the electronic structure, thereby reducing the energy barrier of the rate-limiting step. Consequently, this results in a substantial enhancement in electrochemical NO3RR performance. Furthermore, this synthetic method has the potential to be extended to other single-atom catalysts and scaled up for commercial applications.

Unveiling the Dual Role of Oxophilic Cr4+ in Cr-Cu2O Nanosheet Arrays for Enhanced Nitrate Electroreduction to Ammonia

Cuprous oxide (Cu2O)-based catalysts present a promising activity for the electrochemical nitrate (NO3 -) reduction to ammonia (eNO3RA), but the electrochemical instability of Cu+ species may lead to an unsatisfactory durability, hindering the exploration of the structure-performance relationship. Herein, we propose an efficient strategy to stabilize Cu+ through the incorporation of Cr4+ into the Cu2O matrix to construct a Cr4+-O-Cu+ network structure. In situ and quasi-in situ characterizations reveal that the Cu+ species are well maintained via the strong Cr4+-O-Cu+ interaction that inhibits the leaching of lattice oxygen. Importantly, in situ generated Cr3+-O-Cu+ from Cr4+-O-Cu+ is identified as a dual-active site for eNO3RA, wherein the Cu+ sites are responsible for the activation of N-containing intermediates, while the assisting Cr3+ centers serve as the electron-proton mediators for rapid water dissociation. Theoretical investigations further demonstrated that the metastable state Cr3+-O-Cu+ favors the conversion from the endoergic hydrogenation of the key *ON intermediate to an exoergic reaction in an ONH pathway, and facilitates the subsequent NH3 desorption with a low energy barrier. The superior eNO3RA with a maximum 91.6 % Faradaic efficiency could also be coupled with anodic sulfion oxidation to achieve concurrent NH3 production and sulfur recovery with reduced energy input.

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.

Regulating the Electrochemical Nitrate Reduction Performance with Controllable Distribution of Unconventional Phase Copper on Alloy Nanostructures

Electrochemical nitrate reduction reaction (NO3RR) is emerging as a promising strategy for nitrate removal and ammonia (NH3) production using renewable electricity. Although great progresses have been achieved, the crystal phase effect of electrocatalysts on NO3RR remains rarely explored. Here, the epitaxial growth of unconventional 2H Cu on hexagonal close-packed (hcp) IrNi template, resulting in the formation of three IrNiCu@Cu nanostructures, is reported. IrNiCu@Cu-20 shows superior catalytic performance, with NH3 Faradaic efficiency (FE) of 86% at -0.1 (vs reversible hydrogen electrode [RHE]) and NH3 yield rate of 687.3 mmol gCu -1 h-1, far better than common face-centered cubic Cu. In sharp contrast, IrNiCu@Cu-30 and IrNiCu@Cu-50 covered by hcp Cu shell display high selectivity toward nitrite (NO2 -), with NO2 - FE above 60% at 0.1 (vs RHE). Theoretical calculations have demonstrated that the IrNiCu@Cu-20 has the optimal electronic structures for NO3RR due to the highest d-band center and strongest reaction trend with the lowest energy barriers. The high electroactivity of IrNiCu@Cu-20 originates from the abundant low coordination of Cu sites on the surface, which guarantees the fast electron transfer to accelerate the intermediate conversions. This work provides a feasible tactic to regulate the product distribution of NO3RR by crystal phase engineering of electrocatalysts.

Enhancing Compatibility of Two-Step Tandem Catalytic Nitrate Reduction to Ammonia Over P-Cu/Co(OH)2

Electrochemical nitrate reduction reaction (NO3RR) is a promising approach to realize ammonia generation and wastewater treatment. However, the transformation from NO3 - to NH3 involves multiple proton-coupled electron transfer processes and by-products (NO2 -, H2, etc.), making high ammonia selectivity a challenge. Herein, a two-phase nanoflower P-Cu/Co(OH)2 electrocatalyst consisting of P-Cu clusters and P-Co(OH)2 nanosheets is designed to match the two-step tandem process (NO3 - to NO2 - and NO2 - to NH3) more compatible, avoiding excessive NO2 - accumulation and optimizing the whole tandem reaction. Focusing on the initial 2e- process, the inhibited *NO2 desorption on Cu sites in P-Cu gives rise to the more appropriate NO2 - released in electrolyte. Subsequently, P-Co(OH)2 exhibits a superior capacity for trapping and transforming the desorbed NO2 - during the latter 6e- process due to the thermodynamic advantage and contributions of active hydrogen. In 1 m KOH + 0.1 m NO3 -, P-Cu/Co(OH)2 leads to superior NH3 yield rate of 42.63 mg h- 1 cm- 2 and NH3 Faradaic efficiency of 97.04% at -0.4 V versus the reversible hydrogen electrode. Such a well-matched two-step process achieves remarkable NH3 synthesis performance from the perspective of optimizing the tandem catalytic reaction, offering a novel guideline for the design of NO3RR electrocatalysts.

Crystal-Phase- and B-Content-Dependent Electrochemical Behavior of Pd─B Nanocrystals toward Oxygen Reduction Reaction

The manipulation of crystal phases in metal-nonmetal interstitial alloy nanostructures has attracted considerable attention due to the formation of unique electronic structures and surface atomic arrangements, resulting in unprecedented catalytic performances. However, achieving simultaneous control over crystal phase and nonmetal elements in metal-nonmetal interstitial alloy nanostructures has remained a formidable challenge. Here, a novel synthesis approach is presented for Pd─B interstitial alloy nanocrystals (NCs) that allows investigation of the crystal-phase- and B-content-dependent catalytic performance. Through comparison of the oxygen reduction reaction (ORR) properties of Pd─BX interstitial alloy NCs with different crystal phases and B contents, achieved by precise control of reaction temperature and time, the influences of crystal phase and B contents in the Pd─BX interstitial alloy NCs on ORR are precisely investigated. The hexagonal closed packed (hcp) PdB0.5 NCs exhibit superior catalytic activity, with mass activities reaching 2.58 A mg-1, surpassing Pd/C by 10.3 times, attributed to synergistic effects by the hcp crystal phase and relatively high B contents. This study not only provides a novel approach to fabricate interstitial alloy nanostructures with unconventional crystal phases and finely controlled nonmetal elements but also elucidates the importance of crystal phase and nonmetal element content in optimizing electrocatalytic efficiency.

Regulating RuxMoy Nanoalloys Anchored on Porous Nitrogen-Doped Carbon via Domain-Confined Etching Strategy for Neutral Efficient Ammonia Electrosynthesis

Neutral electrochemical nitrate (NO3-) reduction to ammonia involves sluggish and complex kinetics, so developing efficient electrocatalysts at low potential remains challenging. Here, we report a domain-confined etching strategy to construct RuxMoy nanoalloys on porous nitrogen-doped carbon by optimizing the Ru-to-Mo ratio, achieving efficient neutral NH3 electrosynthesis. Combining in situ spectroscopy and theoretical simulations demonstrated a rational synergic effect between Ru and Mo in nanoalloys that reinforces *H adsorption and lowers the energy barrier of NO3- hydrodeoxygenation for NH3 production. The resultant Ru5Mo5-NC surpasses 92.8% for NH3 selectivity at the potential range from -0.25 to -0.45 V vs RHE under neutral electrolyte, particularly achieving a high NH3 selectivity of 98.3% and a corresponding yield rate of 1.3 mg h-1 mgcat-1 at -0.4 V vs RHE. This work provides a synergic strategy that sheds light on a new avenue for developing efficient multicomponent heterogeneous catalysts.

Efficient Tandem Electrocatalytic Nitrate Reduction to Ammonia on Bimodal Nanoporous Ag/Ag-Co across Broad Nitrate Concentrations

Electrocatalytic nitrate (NO3-) reduction reaction (NO3-RR) represents a promising strategy for both wastewater treatment and ammonia (NH3) synthesis. However, it is difficult to achieve efficient NO3-RR on a single-component catalyst due to NO3-RR involving multiple reaction steps that rely on distinct catalyst properties. Here we report a facile alloying/dealloying-driven phase-separation strategy to construct a bimodal nanoporous Ag/Ag-Co tandem catalyst that exhibits a remarkable NO3-RR performance in a broad NO3- concentration range from 5 to 500 mM. In 10 and 50 mM NO3- electrolytes, the NH3 yield rates reach 3.4 and 25.1 mg h-1 mgcat.-1 with corresponding NH3 Faradaic efficiencies of 94.0% and 97.1%, respectively, outperforming most of the reported catalysts under the same NO3- concentration. The experimental results and density functional theory calculations demonstrate that Ag ligaments preferentially reduce NO3- to NO2-, while bimetallic Ag-Co ligaments catalyze the reduction of NO2- to NH3.

Fe2O3/ZnO heterojunction for efficient electrochemical nitrate reduction to ammonia

Electrochemical nitrate reduction to ammonia (ENO3RR) has attracted great attention owing to its characteristics of treating wastewater while producing high value-added ammonia. In this study, we successfully prepared a heterojunction electrocatalyst Fe2O3/ZnO consisting of Fe2O3 nanosheets and ZnO nanoparticles, where the construction of the Fe2O3/ZnO heterojunction not only increased the exposure of the active sites of the catalyst, accelerated the interfacial electron transfer, and improved the conductivity of the catalyst but also optimized its overall electronic structure. Thus, Fe2O3/ZnO demonstrated a high Faraday efficiency of 97.4% and an ammonia yield of 6327.2 μg h-1 cm-2 at -1.0 V (vs. RHE) in 0.1 M KNO3 and 0.1 M PBS. DFT calculations also confirmed that the constructed Fe2O3/ZnO heterojunction effectively decreased the reaction energy barrier of *NO → *NHO and accelerated the reaction kinetics, which is favourable for ENO3RR. This study provides a new and facile design strategy of catalysts for electrochemical nitrate reduction to ammonia.

Three-dimensional RuCo alloy nanosheets arrays integrated pinewood-derived porous carbon for high-efficiency electrocatalytic nitrate reduction to ammonia

Electricity-driven nitrate (NO3-) to ammonia (NH3) conversion presents a unique opportunity to simultaneously eliminate nitrate from sewage while capturing ammonia. However, the Faradaic efficiency and ammonia yield in this eight-electron process remain unsatisfactory, underscoring the critical need for more effective electrocatalysts. In this study, a RuCo alloy nanosheets electrodeposited on pinewood-derived three-dimensional porous carbon (RuCo@TDC) is introduced as a highly-efficient electrocatalyst for the nitrate reduction reaction. The RuCo@TDC catalyst exhibits superior electrocatalytic performance, achieving the highest NH3 yield of 2.02 ± 0.11 mmol h-1 cm-2 at -0.6 V versus the reversible hydrogen electrode (vs. RHE) and the highest Faradaic efficiency of 95.7 ± 0.8 % at -0.2 V vs. RHE in an electrolyte mixture of 0.1 M KOH and 0.1 M KNO3. Furthermore, the Zn-NO3- battery using RuCo@TDC as the cathode provides a maximum power density of 2.46 mW cm-2 and a satisfactory NH3 yield of 1110 μg h-1 cm-2.

Modulating the Electrolyte Microenvironment in Electrical Double Layer for Boosting Electrocatalytic Nitrate Reduction to Ammonia

Electrochemical nitrate reduction reaction (NO3RR) is a promising approach to achieve remediation of nitrate-polluted wastewater and sustainable production of ammonia. However, it is still restricted by the low activity, selectivity and Faraday efficiency for ammonia synthesis. Herein, we propose an effective strategy to modulate the electrolyte microenvironment in electrical double layer (EDL) by mediating alkali metal cations in the electrolyte to enhance the NO3RR performance. Taking bulk Cu as a model catalyst, the experimental study reveals that the NO3 --to-NH3 performance in different electrolytes follows the trend Li+ Collapse

Key Words

• alkali metal cation
• electrical double layer
• nitrate reduction
• proton transport
• reaction kinetics

Collapse

MESH Headings Collapse

Grants

• GXXT-2022-026 University Synergy Innovation Program of Anhui Province
• 2022AH050112 and 2022AH010004 Project of Anhui Provincial Department of Education
• KJ2021A0070 and KJ2021A1168 Natural Science Research Project of Universities in Anhui Province

Collapse

Affiliation(s)

Weidong Wen School of Materials Science and Engineering, Anhui University, Hefei, 230601, P. R. China
Shidong Fang Institute of Energy, Hefei Comprehensive National Science Centre (Anhui Energy Laboratory), Hefei, 230051, P. R. China Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS), Hefei, 230031, P. R. China
Yitong Zhou Institutes of Physical Science and Information Technology, Anhui University, Hefei, 230601, P. R. China
Ying Zhao School of Pharmacy, Anhui Xinhua University, Hefei, 230088, P. R. China
Peng Li School of Materials Science and Engineering, Anhui University, Hefei, 230601, P. R. China
Xin-Yao Yu School of Materials Science and Engineering, Anhui University, Hefei, 230601, P. R. China

Collapse

Collaborators Collapse

Site-Selective Growth of fcc-2H-fcc Copper on Unconventional Phase Metal Nanomaterials for Highly Efficient Tandem CO2 Electroreduction

Copper (Cu) nanomaterials are a unique kind of electrocatalysts for high-value multi-carbon production in carbon dioxide reduction reaction (CO2RR), which holds enormous potential in attaining carbon neutrality. However, phase engineering of Cu nanomaterials remains challenging, especially for the construction of unconventional phase Cu-based asymmetric heteronanostructures. Here the site-selective growth of Cu on unusual phase gold (Au) nanorods, obtaining three kinds of heterophase fcc-2H-fcc Au-Cu heteronanostructures is reported. Significantly, the resultant fcc-2H-fcc Au-Cu Janus nanostructures (JNSs) break the symmetric growth mode of Cu on Au. In electrocatalytic CO2RR, the fcc-2H-fcc Au-Cu JNSs exhibit excellent performance in both H-type and flow cells, with Faradaic efficiencies of 55.5% and 84.3% for ethylene and multi-carbon products, respectively. In situ characterizations and theoretical calculations reveal the co-exposure of 2H-Au and 2H-Cu domains in Au-Cu JNSs diversifies the CO* adsorption configurations and promotes the CO* spillover and subsequent C-C coupling toward ethylene generation with reduced energy barriers.

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.

Vacancy-Rich SnO2 Quantum Dot Stabilized by Polyoxomolybdate as Electrocatalyst for Selective NH3 Production

The pronounced conductivity of tin dioxide (SnO2) nanoparticles makes it an ideal multifunctional electrode material, while the challenge is to stabilize the quantum dot (QD) SnO2 nanocore in water. An Anderson-type polyoxomolybdate, (NH4)6[Mo7O24], is employed as an inorganic ligand to stabilize a ca. 6 nm SnO2 QD (Mox@SnO2). X-ray scattering and diffraction studies confirm the tetragonal SnO2 nanocore in Mox@SnO2. Elemental analyses are in good agreement with the mass spectrometric detection of the [Mo7O24]6- cluster present in Mox@SnO2. The ionic POMs attached to the SnO2 surface through [Mo-O-Sn] covalent linkages have been established by surface zeta potential, shift of the [Mo = O]t Raman vibration, and extended X-ray absorption fine structure (EXAFS) analyses. The presence of the [Mo7O24]6- cluster in the Mox@SnO2 is responsible for the remarkable aqueous stability of Mox@SnO2 in the pH range of 3-9. Dominant oxygen vacancy in the SnO2 core, identified by EXAFS data and the anisotropic electron paramagnetic resonance (EPR) signals (g ∼ 2.4 and 1.9), results in facile electronic conduction in Mox@SnO2 while being deposited on the electrode surface. Mox@SnO2 acts as an active catalyst for the electrocatalytic nitrate reduction (eNOR) to ammonia with 94% faradaic efficiency (FE) at -0.2 V vs RHE and a yield rate of 28.9 mg h-1 cm-2. The stability of Mox@SnO2 in acidic pH provides scope to reuse the Mox@SnO2 electrode at least four times with notable NH3 selectivity and a superior production rate (239.06 mmol g-1(cat) h-1). This study demonstrates the essential role of POM in stabilizing SnO2 QD, harnessing its electrochemical activity toward electrocatalytic ammonia production.

Strategies for Achieving Ultra-Long ORR Durability-Rh Activates Interatomic Interactions in Alloys

The stability of platinum-based alloy catalysts is crucial for the future development of proton exchange membrane fuel cells, considering the potential dissolution of transition metals under complex operating conditions. Here, we report on a Rh-doped Pt3Co alloy that exhibits strong interatomic interactions, thereby enhancing the durability of fuel cells. The Rh-Pt3Co/C catalyst demonstrates exceptional catalytic activity for oxygen reduction reactions (ORR) (1.31 A mgPt -1 at 0.9 V vs. the reversible hydrogen electrode (RHE) and maintaining 92 % of its mass activity after 170,000 potential cycles). Long-term testing has shown direct inhibition of Co dissolution in Rh-Pt3Co/C. Furthermore, tests on proton exchange membrane fuel cells (PEMFC) have shown excellent performance and long-term durability with low Pt loading. After 50,000 cycles, there was no voltage loss at 0.8 A cm-2 for Rh-Pt3Co/C, while Pt3Co/C experienced a loss of 200 mV. Theoretical calculations suggest that introducing transition metal atoms through doping creates a stronger compressive strain, which in turn leads to increased catalytic activity. Additionally, Rh doping increases the energy barrier for Co diffusion in the bulk phase, while also raising the vacancy formation energy of the surface Pt. This ensures the long-term stability of the alloy over the course of the cycle.

Electronic Structure Design of Transition Metal-Based Catalysts for Electrochemical Carbon Dioxide Reduction

With the increasingly serious greenhouse effect, the electrochemical carbon dioxide reduction reaction (CO2RR) has garnered widespread attention as it is capable of leveraging renewable energy to convert CO2 into value-added chemicals and fuels. However, the performance of CO2RR can hardly meet expectations because of the diverse intermediates and complicated reaction processes, necessitating the exploitation of highly efficient catalysts. In recent years, with advanced characterization technologies and theoretical simulations, the exploration of catalytic mechanisms has gradually deepened into the electronic structure of catalysts and their interactions with intermediates, which serve as a bridge to facilitate the deeper comprehension of structure-performance relationships. Transition metal-based catalysts (TMCs), extensively applied in electrochemical CO2RR, demonstrate substantial potential for further electronic structure modulation, given their abundance of d electrons. Herein, we discuss the representative feasible strategies to modulate the electronic structure of catalysts, including doping, vacancy, alloying, heterostructure, strain, and phase engineering. These approaches profoundly alter the inherent properties of TMCs and their interaction with intermediates, thereby greatly affecting the reaction rate and pathway of CO2RR. It is believed that the rational electronic structure design and modulation can fundamentally provide viable directions and strategies for the development of advanced catalysts toward efficient electrochemical conversion of CO2 and many other small molecules.

RhNi Bimetallenes with Lattice-Compressed Rh Skin towards Ultrastable Acidic Nitrate Electroreduction

Harvesting recyclable ammonia (NH3 ) from acidic nitrate (NO3 - )-containing wastewater requires the utilization of corrosion-resistant electrocatalytic materials with high activity and selectivity towards acidic electrochemical nitrate reduction (NO3 ER). Herein, ultrathin RhNi bimetallenes with Rh-skin-type structure (RhNi@Rh BMLs) are fabricated towards acidic NO3 ER. The Rh-skin atoms on the surface of RhNi@Rh BMLs experience the lattice compression-induced strain effect, resulting in shortened Rh-Rh bond and downshifted d-band center. Experimental and theoretical calculation results corroborate that Rh-skin atoms can inhibit NO2 */NH2 * adsorption-induced Rh dissolution, contributing to the exceptional electrocatalytic durability of RhNi@Rh BMLs (over 400 h) towards acidic NO3 ER. RhNi@Rh BMLs also reveal an excellent catalytic performance, boasting a 98.4% NH3 Faradaic efficiency and a 13.4 mg h-1 mgcat -1 NH3 yield. Theoretical calculations reveal that compressive stress tunes the electronic structure of Rh skin atoms, which facilitates the reduction of NO* to NOH* in NO3 ER. The practicality of RhNi@Rh BMLs has also been confirmed in an alkaline-acidic hybrid zinc-nitrate battery with a 1.39 V open circuit voltage and a 10.5 mW cm-2 power density. This work offers valuable insights into the nature of electrocatalyst deactivation behavior and guides the development of high-efficiency corrosion-resistant electrocatalysts for applications in energy and environment.