Single-entity Electrochemistry Unveils Dynamic Transformation during Tandem Catalysis of Cu2 O and Co3 O4 for Converting NO3 - to NH3

Performance enhancement of mid-infrared NH3 sensor using 9.06 μm QCL based on spectral optimization and NGO-LSTM model

A detection sensor for mid-infrared ammonia (NH3) has been developed according to wavelength modulation spectroscopy-tunable diode laser absorption spectroscopy (WMS-TDLAS) technology, which can be applied in the chemical and aquaculture industries. A 9.06 µm quantum cascade laser (QCL) and a 41.5 m multipass gas cell (MPGC) were used to increase the detection limit of NH3. Spectral optimization and the NGO-LSTM (northern goshawk optimization-long short-term memory) model applied to gas detection are designed to improve the accuracy of sensor. Among them, the design of the temperature compensation and spectral drift correction reduces the effect of temperature and other environmental factors. The original second harmonic signal was denoised using the CEEMDAN-WPD (complete ensemble empirical mode decomposition with adaptive noise-wavelet packet decomposition) algorithm. And the NGO-LSTM algorithm was applied to NH3 concentration inversion, adaptively optimizing the weight parameters. The experiment reflects that the measured value of the sensor has an excellent linear relationship with the set value (R2 0.9992). The long-term stability of the sensor was verified based on 400 ppb NH3, with an RMSE (root mean square error) of 4.754 ppb. Allan-Werle bias analysis shows that the detection limit (LoD) is approximately 792 ppt at an integration time of 232 s. Subsequent response time and atmospheric environment simulation experiments have proven that this sensor provides an efficient approach for real-time monitoring of NH3.

Effective Nitrate Electroconversion to Ammonia Using an Entangled Co3O4/Graphene Nanoribbon Catalyst

There has been huge interest among chemical scientists in the electrochemical reduction of nitrate (NO3-) to ammonia (NH4+) due to the useful application of NH4+ in nitrogen fertilizers and fuel. To conduct such a complex reduction reaction, which involves eight electrons and eight protons, one needs to develop high-performance (and stable) electrocatalysts that favor the formation of reaction intermediates that are selective toward ammonia production. In the present study, we developed and applied Co3O4/graphene nanoribbon (GNR) electrocatalysts with excellent properties for the effective reduction of NO3- to NH4+, where NH4+ yield rate of 42.11 mg h-1 mgcat-1, FE of 98.7%, NO3- conversion efficiency of 14.71%, and NH4+ selectivity of 100% were obtained, with the application of only 37.5 μg cm-2 of the catalysts (for the best catalyst ─Co3O4(Cowt %55)GNR, only 20.6 μg cm-2 of Co was applied), confirmed by loadings ranging from 19-150 μg cm-2. The highly satisfactory results obtained from the application of the proposed catalysts were favored by high average values of electrochemically active surface area (ECSA) and low Rct values, along with the presence of several planes in Co3O4 entangled with GNR and the occurrence of a kind of "(Co3(Co(CN)6)2(H2O)12)1.333 complex" structure on the catalyst surface, in addition to the effective migration of NO3- from the cell cathodic branch to the anodic branch, which was confirmed by the experiment conducted using a H-cell separated by a Nafion 117 membrane. The in situ FTIR and Raman spectroscopy results helped identify the adsorbed intermediates, namely, NO3-, NO2-, NO, and NH2OH, and the final product NH4+, which are compatible with the proposed NO3- electroreduction mechanism. The Density Functional Theory (DFT) calculations helped confirm that the Co3O4(Cowt %55)GNR catalyst exhibited a better performance in terms of nitrate electroreduction in comparison with Co3O4(Cowt %75), considering the intermediates identified by the in situ FTIR and Raman spectroscopy results and the rate-determining step (RDS) observed for the transition of *NO to *NHO (0.43 eV).

Electrochemical Selective Nitrate Reduction: Pathways to Nitrogen and Ammonia Production

Nitrate (NO3 -) contamination from industrial, agricultural, and anthropogenic activities poses significant risks to human health and ecosystems. While traditional NO3 - remediation methods are effective, they often generate secondary pollutants and incur high costs. Electrochemical NO3 -reduction (ECNR) offers a sustainable alternative, converting NO3 - into environmentally benign nitrogen (N2) or valuable ammonia (NH3). This review explores recent advancements in selective ECNR pathways for NO3 --to-N2and NO3 --to-NH3 conversion, focusing on mechanistic insights, electrocatalyst development, and optimization strategies. Key factors influencing ECNR performance, such as electrode materials, electrolyte composition, and hydrogen evolution inhibition, are discussed. Additionally, the review highlights the role of single-atom, bimetallic, and nanostructured catalysts in enhancing faradaic efficiency, total N2 removal, and selectivity, with particular attention to Pd-Cu systems. Strategies to address challenges like low selectivity and catalyst degradation are also explored. This review underscores the potential of ECNR as a viable alternative to the energy-intensive Haber-Bosch process for NH3 production, aligning with global sustainability goals. Finally, we identify research gaps and propose future directions for improving the efficiency, stability, and scalability of ECNR technologies.

Boosting Electrochemical Nitrate Reduction to Ammonia by Fe Doped CuO/Co3O4 Nanosheet/Nanowire Heterostructures

The electrochemical nitrate reduction reaction (NO3 -RR) is a novel green method for ammonia synthesis. The development of outstanding NO3 -RR performance is based on reasonable catalyst. Metal oxides have garnered significant attention due to their exceptional electrical conductivity and catalytic properties. Doping serves as an effective strategy for enhancing catalyst performance due to its ability to change the electron cloud distribution and energy levels. In this study, we develop a heterojunction catalyst Fe doped copper oxide nanosheet and cobalt tetroxide nanowire growing on carbon cloth simultaneously (Fe-CuO@Co3O4/CC) via hydrothermal method. The well-designed Fe-CuO@Co3O4/CC has excellent NH3 yield (470.9 μmol h-1 cm-2) and Faraday efficiency (FE: 84.4 %) at -1.2 V versus reversible hydrogen electrode (vs. RHE). The heterostructure increases the specific surface area of the catalyst, and the possibility of contact between the catalyst and NO3 - ions, enhances the catalytic efficiency. In addition, the catalyst has excellent stability and can stably carry out the electrocatalytic nitrate reduction reaction (NO3 -RR), which provides a way for further research on the high-efficiency reduction of nitrate.

Dual Zn5-NiS4 Sites in a Redox-Active Metal-Organic Framework Enables Efficient Cascade Catalysis for Nitrate-to-Ammonia Conversion

Electrocatalytic Nitrate Reduction to Ammonia (NO3RR) offers a promising solution to both environmental pollution and the sustainable energy conversion. Here we propose an efficient cascade catalytic mechanism based on a dual Zn5-NiS4 sites, orderly assembled in a redox-active metal-organic framework structure, which separately promotes the reaction kinetics of nitrate-to-nitrite and nitrite-to-ammonia conversions. Specifically, the Zn5 clusters adsorb and selectively reduce the NO3 - to NO2 -, whereas [NiS4] acts as an analogue to the ferredoxins, subsequently boosts the reduction of NO2 - to produce NH3. To this end, the bimetallic Zn5-NiS4TP MOF was synthesized based on the redox-active ligand [Ni(C2S2(TPCOOH)2)2]. A maximum ammonia production rate of 23477.59 μg ⋅ h-1 ⋅ mg-1 cat. and faradaic efficiency 92.87 % was achived by Zn5-NiS4TP MOF under neutral conditions. To validate the critical role of dual Zn5-NiS4 sites, Mn5-NiS4TP and Cd2-NiS4TP were synthesized as control samples, together with Zn-TTFTB, Zn-NiS4Ph and other Zn5-cluster-based MOFs applied for the investigation of electrocatalytic nitrate reduction. Our results indicated that substitution by -thienyl instead of -phenyl group increases the S-heteroatom content, improves the conductivity and facilitates electron transfer. Furthermore, Density Functional Theory (DFT) calculations of the energy changes for the reduction of each species could rationalize experimental results.

Correlating Thickness and Phase of Single Co(OH)2 Micro-Platelets to the Intrinsic Activity of Oxygen Evolution Electrocatalysis

Morphology, crystal phase, and its transformation are important structures that frequently determine electrocatalytic activity, but the correlations of intrinsic activity with them are not completely understood. Herein, using Co(OH)2 micro-platelets with well-defined structures (phase, thickness, area, and volume) as model electrocatalysts of oxygen evolution reaction, multiple in situ microscopy is combined to correlate the electrocatalytic activity with morphology, phase, and its transformation. Single-entity morphology and electrochemistry characterized by atomic force microscopy and scanning electrochemical cell microscopy reveal a thickness-dependent turnover frequency (TOF) of α-Co(OH)2. The TOF (≈9.5 s-1) of α-Co(OH)2 with ≈14 nm thickness is ≈95-fold higher than that (≈0.1 s-1) with ≈80 nm. Moreover, this thickness-dependent activity has a critical thickness of ≈30 nm, above which no thickness-dependence is observed. Contrarily, β-Co(OH)2 reveals a lower TOF (≈0.1 s-1) having no significant correlation with thickness. Combining single-entity electrochemistry with in situ Raman microspectroscopy, this thickness-dependent activity is explained by more reversible Co3+/Co2+ kinetics and larger ratio of active Co sites of thinner α-Co(OH)2, accompanied with faster phase transformation and more extensive surface restructuration. The findings highlight the interactions among thickness, ratio of active sites, kinetics of active sites, and phase transformation, and offer new insights into structure-activity relationships at single-entity level.

Spiers Memorial Lecture: New horizons in nanoelectrochemistry

This introductory lecture prefaces the 2024 New Horizons in Nanoelectrochemistry Faraday Discussion. A broad view of the previous Discussions related to nanoelectrochemistry is taken. Big ideas or concepts discussed at these previous meetings are identified, along with specific examples in each area. Closing comments aimed at a high level and related to where we are today and what is needed to continue to drive nanoelectrochemistry towards the horizon are considered.

Interfacial Electronic Interactions Promoted Activation for Nitrate Electroreduction to Ammonia over Ag-Modified Co3O4

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NRA) offers a promising pathway for ammonia synthesis. The interfacial electronic interactions (IEIs) can regulate the physicochemical capabilities of catalysts in electrochemical applications, while the impact of IEIs on electrocatalytic NRA remains largely unexplored in current literature. In this study, the high-efficiency electrode Ag-modified Co3O4 (Ag1.5Co/CC) is prepared for NRA in neutral media, exhibiting an impressive nitrate conversion rate of 96.86 %, ammonia Faradaic efficiency of 96.11 %, and ammonia selectivity of ~100 %. Notably, the intrinsic activity of Ag1.5Co/CC is ~81 times that of Ag nanoparticles (Ag/CC). Multiple characterizations and theoretical computations confirm the presence of IEIs between Ag and Co3O4, which stabilize the CoO6 octahedrons within Co3O4 and significantly promote the adsorption of reactants (NO3 -) as well as intermediates (NO2 - and NO), while suppressing the Heyrovsky step, thereby improving nitrate electroreduction efficiency. Furthermore, our findings reveal a synergistic effect between different active sites that enables tandem catalysis for NRA: NO3 - reduction to NO2 - predominantly occurs at Ag sites while NO2 - tends to hydrogenate to ammonia at Co sites. This study offers valuable insights for the development of high-performance NRA electrocatalysts.

Simultaneously Promoted Water Resistance and CO2 Selectivity in Methanol Oxidation Over Pd/CoOOH: Synergy of Co-OH and the Pd-Olatt-Co Interface

Catalytic purification of industrial oxygenated volatile organic compounds (OVOCs) is hindered by the presence of water vapor that attacks the active sites of conventional noble metal-based catalysts and the insufficient mineralization that leads to the generation of hazardous intermediates. Developing catalysts simultaneously with excellent water resistance and a high intermediate suppression ability is still a great challenge. Herein, we proposed a simple strategy to synthesize a Pd/CoOOH catalyst that contains abundant hydroxyl groups and lattice oxygen species, over which a negligible effect was observed on CH3OH conversion with 3 vol % water vapor, while a remarkable conversion reduction of 24% was observed over Pd/Co3O4. Moreover, the low-temperature CO2 selectivity over Pd/CoOOH is significantly enhanced in comparison with Pd/Co(OH)2. The high concentration of surface hydroxyl groups on Pd/CoOOH enhances the water resistance owing to the accelerated activation of H2O to generate Co-OH, which replaces the consumed hydroxyl and facilitates the quick dissociation of surface H2O through timely desorption. Additionally, the presence of Pd-Olatt-Co promotes electron transport from Co to Pd, leading to improved metal-support interactions and weakened metal-O bonds. This in turn enhances the catalyst's capacity to efficaciously convert intermediates. This study sheds new insights into designing multifunctional catalytic platforms for efficient industrial OVOC purification as well as other heterogeneous oxidation reactions.

A Theoretical Perspective for Ammonia Synthesis: Nitric Oxide or Nitrate Electroreduction?

Ammonia is an important raw material for agricultural production, playing a key role in global food production. However, conventional ammonia synthesis resulted in extensive greenhouse gas emissions and huge energy consumption. Recently, researchers have proposed electrocatalytic reverse artificial nitrogen cycle (eRANC) routes to circumvent these issues, which can be driven by electrocatalysis and sustainable electricity. Here, a theoretical and computational perspective on the challenges and opportunities with the comparison with experimental results: electrochemical reduction of nitrate (eNO3RR) and nitrite (eNO2RR), electrochemical reduction of nitric oxide (eNORR) combined with oxidative nitrogen fixation are presented. By comparison, the N2→NO→NH3 route is proposed as the most promising in case the NO solubility can be solved well in reactor design. Its high efficiency of ammonia production is demonstrated. Instead, the eNO3RR can be another choice because it is non-toxic and the solid-liquid interface is usually efficient for electrochemical reactions, while its low selectivity at low overpotentials is an issue. These fundamentals highlight the potential and key factors of eRANC as an efficient and sustainable route for ammonia production.

Homogeneously Mixed Cu-Co Bimetallic Catalyst Derived from Hydroxy Double Salt for Industrial-Level High-Rate Nitrate-to-Ammonia Electrosynthesis

Electrocatalytic nitrate reduction reaction (NO3RR) presents an innovative approach for sustainable NH3 production. However, selective NH3 production is hindered by the multiple intermediates involved in the NO3RR process and the competitive hydrogen evolution reaction. Hence, the development of highly efficient NO3RR catalysts is paramount. Herein, we report highly efficient bimetallic catalysts derived from hydroxy double salt (HDS). Under NO3RR conditions, Cu1Co1-HDS undergoes in situ reconstruction, forming nanocomposites of homogeneously distributed metallic Cu0 and Co(OH)2. Reconstruction-induced Cu0 rapidly converts NO3- to NO2-, which is further hydrogenated to NH3 by Co(OH)2. Homogeneously mixed Cu and Co species maximize this synergistic effect, achieving outstanding NO3RR performance including the highest NH3 yield rate (4.625 mmol h-1 cm-2) reported for powder-type NO3RR catalysts. Integration of Cu1Co1-HDS with a commercial Si solar cell attained 4.53% solar-to-ammonia efficiency from industrial wastewater-level concentrations of NO3- (2000 ppm), demonstrating practical application potential for solar-driven NH3 production. This study provides a strategy for enhancing the NH3 yield rate by optimizing the compositions and distributions of Cu and Co.

Gas Diffusion Electrodes for Electrocatalytic Oxidation of Gaseous Ammonia: Stepping Over the Nitrogen Energy Canyon

As ammonia continues to gain more and more interest as a promising hydrogen carrier compound, so does the electrochemical ammonia oxidation reaction (AmOR). To avoid the liberation of H2 in a reverse Haber-Bosch reaction under release of the energetically more favorable N2, we propose the oxidation of ammonia to value-added nitrite (NO2 -), which is usually obtained during the Ostwald process. We investigated the anodic oxidation of gaseous ammonia directly supplied to a gas diffusion electrode (GDE) using a variety of compositionally different multi-metal catalysts coated on Ni foam under the simultaneous formation of H2 at the cathode. This will double the amount of H2 per ammonia molecule while applying a lower overpotential than that required for water electrolysis (1.4-1.8 V vs. RHE at 50 mA ⋅ cm-2). A selectivity study demonstrated that some of the catalyst compositions were able to produce significant amounts of NO2 -, and further investigations using the most promising catalyst composition Nif_AlCoCrCuFe integrated within a GDE demonstrated up to 88 % Faradaic efficiency for NO2 - at the anode coupled to close to 100 % Faradaic efficiency for the cathodic H2 production.

In-situ electrochemical reconstruction and modulation of adsorbed hydrogen coverage in cobalt/ruthenium-based catalyst boost electroreduction of nitrate to ammonia

The electroreduction of nitrate offers a promising, sustainable, and decentralized route to generate valuable ammonia. However, a key challenge in the nitrate reduction reaction is the energy efficiency of the reaction, which requires both a high ammonia yield rate and a high Faradaic efficiency of ammonia at a low working potential (≥-0.2 V versus reversible hydrogen electrode). We propose a bimetallic Co-B/Ru12 electrocatalyst which utilizes complementary effects of Co-B and Ru to modulate the quantity of adsorbed hydrogen and to favor the specific hydrogenation for initiating nitrate reduction reaction at a low overpotential. This effect enables the catalyst to achieve a Faradaic efficiency for ammonia of 90.4 ± 9.2% and a remarkable half-cell energy efficiency of 40.9 ± 4% at 0 V versus reversible hydrogen electrode. The in-situ electrochemical reconstruction of the catalyst contributes to boosting the ammonia yield rate to a high level of 15.0 ± 0.7 mg h-1 cm-2 at -0.2 V versus reversible hydrogen electrode. More importantly, by employing single-entity electrochemistry coupled with identical location transmission electron microscopy, we gain systematic insights into the correlation between the increase in the catalyst's active sites and its structural transformations during the nitrate reduction reaction.

Sensitive colorimetric detection of Vibrio vulnificus based on target-induced shielding against the peroxidase-mimicking activity of CeO2@PtRu nanozyme

Vibrio vulnificus infection caused by contaminated aquatic products and seawater can lead to severe disease and high mortality. The development of a rapid and sensitive detection method for Vibrio vulnificus is vital to effectively prevent infection in advance. In this study, CeO2@PtRu with high peroxidase activity was used to construct a colorimetric immunoassay for Vibrio vulnificus detection by conjugating polyclonal antibodies via the biotin-streptavidin system. The developed colorimetric biosensor for Vibrio vulnificus demonstrated rapid operability and good sensitivity with a detection range from 104 CFU/mL to 109 CFU/mL, and the limit of detection (LOD) is 193 CFU/mL. Moreover, the colorimetric biosensor showed excellent specificity and good recoveries from 98.70% to 102.10% with RSD < 7.45% for spiked real samples. This novel CeO2@PtRu-based colorimetric biosensor has great application potential for the sensitive detection of Vibrio vulnificus in seafood.

Designing water resistant high entropy oxide materials

The ubiquitous presence of moisture usually shows adverse effects on industrial catalysis. Herein, a concept of engineering entropy to design water-resistant oxide catalysts is proposed. The C3H6 oxidation by spinel ACr2O4 (A=Ni, Mg, Cu, Zn, Co) catalysts is selected as a model. Through DFT calculation, the adsorption energy of C3H6, the dissociation energy of molecular H2O on the oxide surface, and the formation energy of oxygen vacancy all suggest better performance induced by higher configurational entropy. Indeed, (Ni0.2Mg0.2Cu0.2Zn0.2Co0.2)Cr2O4 experimentally show excellent water resistance (>100 h) in C3H6 oxidation, while in sharp contrast binary oxides (e.g., NiCr2O4, CoCr2O4) are deactivated in 20 h. H2O-TPD, in-situ Raman, and in-situ FTIR all confirm the low H2O adsorption energy and strong hydrothermal stability of high entropy oxide, which is attributed to their lower Gibbs free energy. This work may inspire the rational design of water-resistant 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.

Facet-Dependent Evolution of Active Components on Spinel Co3O4 for Electrochemical Ammonia Synthesis

Spinel cobalt oxides (Co3O4) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO3RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co3O4 nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO3RR activity on the crystallographic facets. Among the catalysts tested, Co3O4 {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h-1 cm-2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co3O4 to Co3O4-x with oxygen vacancy (Ov), followed by a Co3O4-x-Ov/Co(OH)2 hybrid, and finally Co(OH)2. This process is observed for all facets, but the formation of Ov and Co(OH)2 is the most rapid on the (111) surface. The presence of Ov significantly reduces the free energy of the *NH2 intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH)2 make Co3O4 {111} an ideal catalyst for ammonia synthesis via eNO3RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO3RR.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using a Heterobimetallic Covalent Organic Framework Composed of Cu-Porphyrin and Co-Bipyridine

The electrocatalytic reduction of nitrate (NO3-) to ammonia (NH3) not only offers an effective solution to environmental problems caused by the accumulation of NO3- but also provides a sustainable alternative to the Haber-Bosch process. However, the conversion of NO3- to NH3 is a complicated process involving multiple steps, leading to a low Faradaic efficiency (FE) for NH3 production. The structural designability of covalent organic frameworks (COFs) renders feasible and precise modulation at the molecular level, facilitating the incorporation of multiple well-defined catalytic sites with different reactivities into a cohesive entity. This promotes the efficiency of the overall reaction through the coupling of multistep reactions. Herein, heterobimetallic CuP-CoBpy was prepared by postmodification, involving the anchoring of cobalt ions to the CuP-Bpy structure. As a result of the cascade effect of the bimetallic sites, CuP-CoBpy achieved an outstanding NH3 yield of 13.9 mg h-1 mgcat.-1 with a high FE of 96.7% at -0.70 V versus the reversible hydrogen electrode and exhibited excellent stability during catalysis. A series of experimental and theoretical studies revealed that the CuP unit facilitates the conversion of NO3- to NO2-, while the CoBpy moiety significantly prompts the reduction of NO2- to NH3. This study demonstrates that tailoring the structural units for the construction of COFs based on each step in the multistep reaction can enhance both the catalytic activity and product selectivity of the overall process.

A Wetting and Capture Strategy Overcoming Electrostatic Repulsion for Electroreduction of Nitrate to Ammonia from Low-Concentration Sewage

Ammonia production by electrocatalytic nitrate reduction reaction (NO3RR) in water streams is anticipated as a zero-carbon route. Limited by dilute nitrate in natural sewage and the electrostatic repulsion between NO3 - and cathode, NO3RR can hardly be achieved energy-efficiently. The hydrophilic Cu@CuCoO2 nano-island dispersed on support can enrich NO3 - and produce a sensitive current response, followed by electrosynthesis of ammonia through atomic hydrogen (*H) is reported. The accumulated NO3 - can be partially converted to NO2 - without external electric field input, confirming that the Cu@CuCoO2 nano-island can strongly bind NO3 - and then trigger the reduction via dynamic evolution between Cu-Co redox sites. Through the identification of intermediates and theoretical computation. it is found that the N-side hydrogenation of *NO is the optimal reaction step, and the formation of N─N dimer may be prevented. An NH3 product selectivity of 93.5%, a nitrate conversion of 96.1%, and an energy consumption of 0.079 kWh gNH3 -1 is obtained in 48.9 mg-N L-1 naturally nitrate-polluted streams, which outperforms many works using such dilute nitrate influent. Conclusively, the electrocatalytic system provides a platform to guarantee the self-sufficiency of dispersed ammonia production in agricultural regions.

Construction of Cobalt Porphyrin-Modified Cu2O Nanowire Array as a Tandem Electrocatalyst for Enhanced CO2 Reduction to C2 Products

Here, the molecule-modified Cu-based array is first constructed as the self-supporting tandem catalyst for electrocatalytic CO2 reduction reaction (CO2RR) to C2 products. The modification of cuprous oxide nanowire array on copper mesh (Cu2O@CM) with cobalt(II) tetraphenylporphyrin (CoTPP) molecules is achieved via a simple liquid phase method. The systematical characterizations confirm that the formation of axial coordinated Co-O-Cu bond between Cu2O and CoTPP can significantly promote the dispersion of CoTPP molecules on Cu2O and the electrical properties of CoTPP-Cu2O@CM heterojunction array. Consequently, as compared to Cu2O@CM array, the optimized CoTPP-Cu2O@CM sample as electrocatalyst can realize the 2.08-fold C2 Faraday efficiency (73.2% vs 35.2%) and the 2.54-fold current density (‒52.9 vs ‒20.8 mA cm-2) at ‒1.1 V versus RHE in an H-cell. The comprehensive performance is superior to most of the reported Cu-based materials in the H-cell. Further study reveals that the CoTPP adsorption on Cu2O can restrain the hydrogen evolution reaction, improve the coverage of *CO intermediate, and maintain the existence of Cu(I) at low potential.

Cascade Electrocatalytic Nitrate Reduction Reaching 100% Nitrate-N to Ammonia-N Conversion over Cu2O@CoO Yolk-Shell Nanocubes

The electroreduction of nitrate to ammonia via a selective eight-electron transfer nitrate reduction reaction offers a promising, low energy consumption, pollution-free, green NH3 synthesis strategy alternative to the Haber-Bosch method. However, it remains a great challenge to achieve high NH4+ selectivity and complete conversion from NO3--N to NH4+-N. Herein, we report ingredients adjustable Cu2O@CoO yolk-shell nanocubes featured with tunable inner void spaces and diverse activity centers, favoring the rapid cascade conversion of NO3- into NO2- on Cu2O and NO2- into NH4+ on CoO. Cu2O@CoO yolk-shell nanocubes exhibit super NH4+ Faradaic efficiencies (>99%) over a wide potential window (-0.2 V to -0.9 V versus RHE) with a considerable NH4+ yield rate of 15.27 mg h-1 cm-2 and fantastic cycling stability and long-term chronoamperometric durability. Cu2O@CoO yolk-shell nanocubes exhibited glorious NO3--N to NH4+-N conversion efficiency in both dilute (500 ppm) and highly concentrated (0.1 and 1 M) NO3- electrolytes, respectively. The nitrate electrolysis membrane electrode assembly system equipped with Cu2O@CoO yolk-shell nanocubes delivers over 99.8% NH4+ Faradaic efficiency at cell voltages of 1.9-2.3 V.

Copper-based electro-catalytic nitrate reduction to ammonia from water: Mechanism, preparation, and research directions

Global water bodies are increasingly imperiled by nitrate pollution, primarily originating from industrial waste, agricultural runoffs, and urban sewage. This escalating environmental crisis challenges traditional water treatment paradigms and necessitates innovative solutions. Electro-catalysis, especially utilizing copper-based catalysts, known for their efficiency, cost-effectiveness, and eco-friendliness, offer a promising avenue for the electro-catalytic reduction of nitrate to ammonia. In this review, we systematically consolidate current research on diverse copper-based catalysts, including pure Cu, Cu alloys, oxides, single-atom entities, and composites. Furthermore, we assess their catalytic performance, operational mechanisms, and future research directions to find effective, long-term solutions to water purification and ammonia synthesis. Electro-catalysis technology shows the potential in mitigating nitrate pollution and has strategic importance in sustainable environmental management. As to the application, challenges regarding complexity of the real water, the scale-up of the commerical catalysts, and the efficient collection of produced NH3 are still exist. Following reseraches of catalyst specially on long term stability and in situ mechanisms are proposed.

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.

Fast and In-Depth Reconstruction of Two-Dimension Cobalt-Based Zeolitic Imidazolate Framework in Glucose Oxidation Processes

Currently, metal-organic frameworks (MOFs) have emerged as viable candidates for enduring electrode materials in nonenzyme glucose sensing. However, given the inherent water susceptibility of MOFs and their complete self-reconstruction during the process of electrochemical oxygen evolution in alkaline conditions, we are motivated to explore the truth of MOFs catalyzing glucose oxidation. In this work, we fabricated a two-dimensional cobalt-based zeolitic imidazolate framework (ZIF-L) as the electrode material for catalyzing glucose oxidation in alkaline conditions. Our explorations revealed that while the initial glucose catalytic response varied among ZIF-L samples with differing thicknesses, the ultimate steady-state catalytic performance remained largely consistent. This phenomenon arose from the transformation of ZIF-L with distinct thicknesses into CoOOH with uniform morphological and structural characteristics during the glucose catalysis process. And in situ Raman spectroscopy elucidated the sustained equilibrium within the glucose catalytic system, wherein the dynamic interconversion between CoOOH and Co(OH)2 governs the overall process. This study contributes to an enhanced understanding of the glucose catalytic mechanism aspects of nonenzymatic glucose sensor electrode materials, offering insights that serve as inspiration for the development of advanced glucose electrode materials.

General Synthesis of High-Entropy Oxide Nanofibers

The discovery of high-entropy oxides (HEOs) in 2015 has provided a family of potential solid catalysts, due to their tunable components, abundant defects or lattice distorts, excellent thermal stability (ΔG↓ = ΔH - TΔS↑), and so on. When facing the heterogeneous catalysis by HEOs, the micrometer bulky morphology and low surface areas (e.g., <10 m2 g-1) by traditional synthesis methods obstructed their way. In this work, an electrospinning method to fabricate HEO nanofibers with diameters of 50-100 nm was demonstrated. The key point lay in the formation of one-dimensional filamentous precursors, during which the uniform dispersion of five metal species with disordered configuration would help to crystallize into single-phase HEOs at lower temperatures: inverse spinel (Cr0.2Mn0.2Co0.2Ni0.2Fe0.2)3O4 (400 °C), perovskite La(Mn0.2Cu0.2Co0.2Ni0.2Fe0.2)O3 (500 °C), spinel Ni0.2Mg0.2Cu0.2Mn0.2Co0.2)Al2O4 (550 °C), and cubic Ni0.2Mg0.2Cu0.2Zn0.2Co0.2O (750 °C). As a proof-of-concept, (Ni3MoCoZn)Al12O24 nanofiber exhibited good activity (CH4 Conv. > 96%, CO2 Conv. > 99%, H2/CO ≈ 0.98), long-time stability (>100 h) for the dry reforming of methane (DRM) at 700 °C without coke deposition, better than control samples (Ni3MoCoZn)Al12O24-Coprecipitation-700 (CH4 Conv. < 3%, CO2 Conv. < 7%). The reaction mechanism of DRM was studied by in situ infrared spectroscopy, CO2-TPD, and CO2/CH4-TPSR. This electrospinning method provides a synthetic route for HEO nanofibers for target applications.

Multiscale Analysis of Electrocatalytic Particle Activities: Linking Nanoscale Measurements and Ensemble Behavior

Nanostructured electrocatalysts exhibit variations in electrochemical properties across different length scales, and the intrinsic catalytic characteristics measured at the nanoscale often differ from those at the macro-level due to complexity in electrode structure and/or composition. This aspect of electrocatalysis is addressed herein, where the oxygen evolution reaction (OER) activity of β-Co(OH)2 platelet particles of well-defined structure is investigated in alkaline media using multiscale scanning electrochemical cell microscopy (SECCM). Microscale SECCM probes of ∼50 μm diameter provide voltammograms from small particle ensembles (ca. 40-250 particles) and reveal increasing dispersion in the OER rates for samples of the same size as the particle population within the sample decreases. This suggests the underlying significance of heterogeneous activity at the single-particle level that is confirmed through single-particle measurements with SECCM probes of ∼5 μm diameter. These measurements of multiple individual particles directly reveal significant variability in the OER activity at the single-particle level that do not simply correlate with the particle size, basal plane roughness, or exposed edge plane area. In combination, these measurements demarcate a transition from an "individual particle" to an "ensemble average" response at a population size of ca. 130 particles, above which the OER current density closely reflects that measured in bulk at conventional macroscopic particle-modified electrodes. Nanoscale SECCM probes (ca. 120 and 440 nm in diameter) enable measurements at the subparticle level, revealing that there is selective OER activity at the edges of particles and highlighting the importance of the three-phase boundary where the catalyst, electrolyte, and supporting carbon electrode meet, for efficient electrocatalysis. Furthermore, subparticle measurements unveil heterogeneity in the OER activity among particles that appear superficially similar, attributable to differences in defect density within the individual particles, as well as to variations in electrical and physical contact with the support material. Overall this study provides a roadmap for the multiscale analysis of nanostructured electrocatalysts, directly demonstrating the importance of multilength scale factors, including particle structure, particle-support interaction, presence of defects, etc., in governing the electrochemical activities of β-Co(OH)2 platelet particles and ultimately guiding the rational design and optimization of these materials for alkaline water electrolysis.

Ag-Co3 O4 -CoOOH-Nanowires Tandem Catalyst for Efficient Electrocatalytic Conversion of Nitrate to Ammonia at Low Overpotential via Triple Reactions

The electrocatalytic conversion of nitrate (NO3 ‾) to NH3  (NO3 RR) offers a promising alternative to the Haber-Bosch process. However, the overall kinetic rate of NO3 RR is plagued by the complex proton-assisted multiple-electron transfer process. Herein, Ag/Co3 O4 /CoOOH nanowires (i-Ag/Co3 O4  NWs) tandem catalyst is designed to optimize the kinetic rate of intermediate reaction for NO3 RR simultaneously. The authors proved that NO3 ‾ ions are reduced to NO2 ‾ preferentially on Ag phases and then NO2 ‾ to NO on Co3 O4  phases. The CoOOH phases catalyze NO reduction to NH3  via NH2 OH intermediate. This unique catalyst efficiently converts NO3 ‾ to NH3  through a triple reaction with a high Faradaic efficiency (FE) of 94.3% and a high NH3  yield rate of 253.7 μmol h-1  cm-2  in 1 M KOH and 0.1 M KNO3  solution at -0.25 V versus RHE. The kinetic studies demonstrate that converting NH2 OH into NH3  is the rate-determining step (RDS) with an energy barrier of 0.151 eV over i-Ag/Co3 O4  NWs. Further applying i-Ag/Co3 O4  NWs as the cathode material, a novel Zn-nitrate battery exhibits a power density of 2.56 mW cm-2  and an FE of 91.4% for NH3  production.

Tuning Oxygen Vacancies in Oxides by Configurational Entropy

Tuning surface oxygen vacancies is important for oxide catalysts. Doping elements with different chemical valence states or different atomic radii into host oxides is a common method to create oxygen vacancies. However, the concentration of oxygen vacancies in oxide catalysts is still limited to the amount of foreign dopants that can be tolerated (generally less than 10% atoms). Herein, a principle of engineering the configurational entropy to tune oxygen vacancies was proposed. First, the positive relationship between the configuration entropy and the formation energy of oxygen vacancies (Eov) in 16 model oxides was estimated by a DFT calculation. To verify this, single binary oxides and high-entropy quinary oxides (HEOs) were prepared. Indeed, the concentration of oxygen vacancies in HEOs (Oβ/α = 3.66) was higher compared to those of single or binary oxides (Oβ/α = 0.22-0.75) by O1s XPS, O2-TPD, and EPR. Interestingly, the reduction temperatures of transition metal ions in HEOs were generally lower than that in single-metal oxides by H2-TPR. The lower Eov of HEOs may contribute to this feature, which was confirmed by in situ XPS and in situ XRD. Moreover, with catalytic CO/C3H6 oxidation as a model, the high-entropy (MnCuCo3NiFe)xOy catalyst showed higher catalytic activity than single and binary oxides, which experimentally verified the hypothesis of the DFT calculation. This work may inspire more oxide catalysts with preferred oxygen vacancies.

Enhanced Nitrate-to-Ammonia Efficiency over Linear Assemblies of Copper-Cobalt Nanophases Stabilized by Redox Polymers

Renewable electricity-powered nitrate (NO3 - ) reduction reaction (NO3 RR) offers a net-zero carbon route to the realization of high ammonia (NH3 ) productivity. However, this route suffers from low energy efficiency (EE, with a half-cell EE commonly <36%), since high overpotentials are required to overcome the weak NO3 - binding affinity and sluggish NO3 RR kinetics. To alleviate this, a rational catalyst design strategy that involves the linear assembly of sub-5 nm Cu/Co nanophases into sub-20 nm thick nanoribbons is suggested. The theoretical and experimental studies show that the Cu-Co nanoribbons, similar to enzymes, enable strong NO3 - adsorption and rapid tandem catalysis of NO3 - to NH3 , owing to their richly exposed binary phase boundaries and adjacent Cu-Co sites at sub-5 nm distance. In situ Raman spectroscopy further reveals that at low applied overpotentials, the Cu/Co nanophases are rapidly activated and subsequently stabilized by a specifically designed redox polymer that in situ scavenges intermediately formed highly oxidative nitrogen dioxide (NO2 ). As a result, a stable NO3 RR with a current density of ≈450 mA cm-2 is achieved, a Faradaic efficiency of >97% for the formation of NH3 , and an unprecedented half-cell EE of ≈42%.