Electrocatalytic Reduction Reaction Research Articles

Confining intermediates within a catalytic nanoreactor facilitates nitrate-to-ammonia electrosynthesis

Electrocatalytic nitrate reduction reaction (NitRR) has gained attention because of its potential to mitigate environmental nitrogen pollution and recycle artificial nutrients. NitRR produces ammonia through complicated pathways that involve multistep electron transfer, while the formation of byproducts, e.g. toxic nitrite (NO2ˉ), leads to lower energy efficiency and recontamination. In this work, we report the incorporation of CuOx active species into a TiO2-nanotube reactor (TiO2 NTs/CuOx) for the highly selective NitRR. In particular, the NO2ˉ intermediate is diminished due to the hindered diffusion within the nanoconfined space. Thus, the CuOx modified nanoreactor performs a maximum faradaic efficiency of 92.23% and a yield rate of 1241.81 μg h−1 cm−2 for nitrate-to-ammonia conversion. Theoretical insights further support a fundamental understanding of a cross-scale interaction over the surface and interface. The findings suggest a promising approach for enhancing reaction activity and selectivity enabled by rationally designed active sites coupled with geometrically regulated structures.

Copper single-atom catalyst as a high-performance electrocatalyst for nitrate-ammonium conversion

Electrocatalytic nitrate reduction reaction (NO3RR), as a promising alternative to the Haber-Bosh process, provides new opportunities for ammonia (NH3) production from the environmental and energy viewpoint. However, the NH3 yield rate and selectivity for NO3RR are still limited due to the lack of efficient electrocatalysts. Herein, we demonstrate an active and selective copper single-atom catalyst (Cu-N-C) for nitrate reduction to NH3. The complete conversion of nitrate (50 mg L−1 NO-3-N) was achieved at −1.5 V vs. SCE with a high NH3 yield rate (9.23 mg h−1 mg−1cat.) and selectivity (94%). Remarkably, Cu-N-C dramatically inhibited the formation of toxic nitrite and double-nitrogen products due to the enhanced nitrite adsorption and restrained N-N coupling that led to nitrate deep reduction to NH3. The remaining nitrate (0.06 mg L−1) and nitrite (1 mg L−1) fully meet the drinking-water standards. Density functional theory simulations reveal that the single-site nature of Cu-N-C facilitated the reduction of HNO*3 to NO*2 and NH*2 to NH3, thus leading to the selective nitrate reduction to NH3.

Prognostication of two-dimensional transition-metal atoms embedded rectangular tetrafluorotetracyanoquinodimethane single-atom catalysts for high-efficiency electrochemical nitrogen reduction

Extensive investigations on the electrocatalytic nitrogen reduction reactions (eNRR) and the high-efficiency single-atom catalysts (SACs) have increasingly given us confidence in intensive arrival of nitrogen (N2) fixation into ammonia (NH3) under ambient conditions in the future, which prompts us to speed up the exploration for highly active SACs for eNRR. Excellent SACs in eNRR should have three advantages: high selectivity, low overpotential, and high stability. Based on these aspects, we employed high-throughput screening method and first-principles calculations to study the catalytic performance of 30 transition-metal atoms (TMs) embedded rectangular tetrafluorotetracyanoquinodimethane (denoted as TM-rF4TCNQ) monolayers (TM = 3d, 4d, and 5d series transition metal atoms) for the eNRR process, and four potential catalysts, i.e., Ti-, Mo-, Nb-, and Tc-rF4TCNQ, were obtained. Among them, Ti-rF4TCNQ catalyzing the N2 reduction to NH3 through an enzymatic mechanism needs a theoretical onset potential of only −0.41 V. When Mo-rF4TCNQ catalyzes eNRR through a distal mechanism, the theoretical onset potential is as low as −0.43 V. The band structures show that these materials are all metallic, ensuring good charge transport during the eNRR process. Analyzing the projected density of states (PDOSs) before and after N2 adsorption, the differential charge density, and the spin density reveals that the Ti-, Mo-, Nb-, and Tc-rF4TCNQ monolayers all can effectively adsorb and activate inert N2, which may be mainly attributed to the “acceptance-donation” interaction between TM and N2.

Selective electrocatalytic reduction of nitrate to dinitrogen by Cu2O nanowires with mixed oxidation-state

• CuO/Cu 2 O NWs contains switchable oxidation states of Cu (I/II to 0/I). • N 2 selectivity is 99.8% over Cu/Cu 2 O NWs, the highest reported so far. • Cu/Cu 2 O NWs provides active sites for hydrogen/oxygen-adsorbing simultaneously. • Cu/Cu 2 O NWs yields sufficient H* and limit hydrogen bubbles negative effect. • N 2 production based on NO 3 RR is proceeded by H*-mediated route. Selective electrocatalytic nitrate reduction reaction to dinitrogen is a promising water treatment technology to manage nitrate. While, a lack of efficient electrode remains the primary hurdle for achieving the goal. For non-noble electrode, the strong adsorption of oxygen atoms on Cu resulted in low selectivity and inducing nitrite accumulation during the long-term operation. Herein, it is found that Cu 2 O nanowires with mixed oxidation-state can selective reduction of nitrate to dinitrogen. CuO-coated Cu 2 O nanowires (CuO/Cu 2 O NWs) was electrochemically converted into Cu/Cu 2 O NWs for hydrogen/oxygen-adsorbing which was verified by In-situ infrared spectro-electrochemical experiments. H 2 split and H*-adsorbing of Cu/Cu 2 O are desired to yield sufficient H* and limit hydrogen bubbles negative effect. Online differential electrochemical mass spectrometry confirmed dinitrogen generation over Cu/Cu 2 O NWs with over 99.8% selectivity. Computational studies suggest that active Cu/Cu 2 O NWs reduces the free energy changes associated with both nitrate activation and N − N coupling. Energetically favorable pathways for dinitrogen production based on nitrate reduction are proposed herein and the origin of this selective dinitrogen formation is clarified.

A Review on Advances in Electrocatalytic N2 Reduction to Ammonia

Ammonia is one of the most important industrial chemicals which is prepared by the traditional Haber–Bosch process. Haber–Bosch process is an expensive process requiring higher temperature, pressure, and a lot of energy. Electrocatalytic nitrogen reduction reactions (NRR) have gained a lot of interest as the alternate method for the production of ammonia. For obtaining a higher yield, higher Faradic efficiency (PE), inhibition of side reactions e.g., hydrogen evolution reaction (HER), and reducing the production cost for ammonia by NRR, selection of appropriate catalysts, cell layout, and electrolyte selection are important controlling parameters for the NRR. Non-metallic catalysts are preferred over noble metallic catalysts due to their lower cost, more resources, and characteristic dorbital electron but have the issue of low selectivity due to the higher HER. The selectivity for the NRR can be improved by employing the catalysts with higher absorption of N2. PEM-type cells and the back-to-back cells are used to inhibit the HER. Apart from these factors, NRR is also dependent on other factors to obtain better experimental results e.g., no nitrides, removing the ammonia from contaminants sites, and controlling the experiment time.

Strong Correlation between the Dynamic Chemical State and Product Profile of Carbon Dioxide Electroreduction.

Utilizing renewable electricity energy to convert CO2 into fuels and chemicals, namely, CO2 electrocatalytic reduction reaction (CO2RR), is becoming increasingly significant yet challenged by low activity and selectivity. Recently, a growing number of studies have demonstrated that oxidized species can surprisingly survive on the catalyst surface under highly cathodic CO2RR conditions and play crucial roles in affecting the product selectivity. However, dynamic evolutions of the surface chemical state together with its real correlation to the product selectivity are still unclear, which is one of the most controversial topics for CO2RR. Herein, we particularly resurvey recent CO2RR researches that are all based on advanced in situ/operando methodologies, aiming to clearly reveal the realistic variations in surface chemical state under the working conditions. Then, recent progress in the regulation of the surface chemical state toward specific CO2RR products in current state-of-the-art catalysts with varying metal centers is systematically summarized, which shows an impressive relation between the dynamic chemical state and product profile. Next, we further highlight the developed strategies to regulate the surface chemical state in catalysts and discuss the debates over the effects of chemical state on product profile during CO2RR. Finally, on the basis of previous achievements, we present major challenges and some perspectives for the exploration of the imperative chemical state sensitivity to product profile during CO2RR.

Efficient Ammonia Electrosynthesis and Energy Conversion through a Zn‐Nitrate Battery by Iron Doping Engineered Nickel Phosphide Catalyst

AbstractThe electrocatalytic nitrate reduction reaction (NO3‐RR) to ammonia (NH3) offers a promising alternative approach for NH3 production and nitrate‐based voltaic cells which can deliver both electricity and NH3 as products, are also highly attractive. However, nitrate‐to‐NH3 conversion involves a proton‐assisted multiple‐electron transfer process with considerable kinetic barrier, underlying the need for efficient catalysts for the NO3–RR. A Zn‐nitrate battery is reported to enable a “killing three birds with one stone” strategy for energy supply, ammonia production and removal of pollutants with the iron doped nickel phosphide (Fe/Ni2P) as a NO3–RR catalyst electrode. Iron doping induces a downshift of the d‐band center of Ni atoms to the Fermi level, allowing the optimization of Gibbs free energies for reaction intermediates. The Fe/Ni2P catalyst exhibits 94.3% NH3 Faradaic efficiency (FE) and nearly 100% nitrate conversion efficiency at –0.4 V vs. reversible hydrogen electrode (RHE). Further applying this highly efficient NO3–RR electrocatalyst as the cathode material, a novel Zn‐nitrate battery exhibits a power density of 3.25 mW cm–2 and a FE of 85.0% for NH3 production. This work enriches the application of Zn‐based batteries in the field of electrocatalysis and highlights the promise of bimetal phosphide for the NO3–RR.

Fabrication and enzyme-free electrochemical sensing response of polyaniline/silver/clove modified composite electrode

The sensitive detection of Hg(II) is a huge consequence for safe environment monitoring and industrial processing. Herein, a sensitive and environment-friendly electrochemical sensor was fabricated using Ag nanoparticles anchored Pani-based bio-nanocomposite. In a simple one-step process AgNPs were prepared using Syzygium aromaticum extract followed by incorporating polyaniline nanostructural array (Pani/Ag/CL) at room temperature through in-situ oxidative polymerization using K2S2O8. The glassy carbon electrode (GCE) was modified with Pani/Ag/CL for the selective electrochemical determination of trace amounts of Hg(II) from different aqueous samples. The electrochemical sensing behavior of the Pani/Ag/CL modified GCE was examined through cyclic voltammetry. The electrochemical impedance analysis of Pani/Ag/CL modified GCE displays favorable kinetic charges for transport. The best reactivity performance for electro-catalytic reduction reactions was observed in a phosphate buffer solution of pH = 5.7. The proposed PANI/Ag/CL modified GCE attains a high sensitivity of 256 μA μM−1 cm−2, a very low limit of detection (0.382 nM), and the limit of quantification (1.27 nM). The practical feasibility and selectivity of the PANI/Ag/CL/GCE were investigated in the sea, tap and wastewater samples with the adequate recovery of Hg(II) added. In addition, the proposed Pani/Ag/CL modified GCE also displayed attractive features such as fast response, simple preparation, repeatability, good stability, as well as detection of the Hg(II) in the occurrence of potential interferences metal cations.

Electrocatalytic nitrate reduction on bimetallic Palladium-Copper Nanowires: Key surface structure for selective dinitrogen formation

Selective electrocatalytic nitrate reduction reaction (NO3RR) to dinitrogen (N2) is essential to nitrogen-cycle management and environmental sustainability. Palladium-based nanomaterials were extensively studied as NO3RR catalysts, but only delivered limited N2 selectivity (<50%, NH3 dominated the product). Here we report that, by tuning bimetallic composition of CuPd alloy nanowires, the Cu28Pd72 nanowires deliver a near-unit conversion of NO3–-N and an unprecedented N2 product selectivity of 75.9 ± 5.2% in treating 22.5 mg L−1 NO3–-N solution at −0.80 V vs. Ag/AgCl. Combining in-situ spectrometric measurements and theoretical calculations, we demonstrate that the contiguous Pd ensemble with a specific size at catalyst surface is essential to the N2 formation via a kinetically-favored pathway of NO2* → NO* → N* → N2*. Alloying Pd with Cu is indispensable as Cu contributes to the N*-N* pairing by initiating the NO3––NO2* conversion and lowering the Hads coverage on catalyst. This work discloses the key surface atomic structure for N2 formation, and provides a guideline for the design of efficient NO3RR catalyst that enables selective NO3––N2 conversion.

Electron-deficient Cuδ+ stabilized by interfacial Cu–O-Al bonding for accelerating electrocatalytic nitrate conversion

Electrocatalytic nitrate reduction to NH3 not only enables the recycling of nutrients from nitrate wastewater but can also lead to complete nitrogen elimination when paired with an NH3 oxidation reaction to achieve innocuous N2. Albeit electron-deficient copper (Cuδ+) is an active catalytic site for an electrocatalytic nitrate reduction reaction (ENRR), its stabilization under the reductive polarization potential of ENRR is challenging. Herein, we report the in-situ electroreduction of CuAl-mixed oxide to construct a Cu-Al2O3 interface with strong metal-support interactions (CuAl-LDO-r), which can form and stabilize Cuδ+via Cu–O-Al bonds. The batch ENRR tests revealed that Cu52Al48-LDO-r delivers an NH3-N selectivity of 97.4%, stable specific activity of 661.6 mg-N m-2h−1 for NH3-N production (faradaic current efficiency of 70.4%), and minimal Cu leaching when reducing 22.5 mg L-1 NO3–-N at −1.10 V vs. Ag/AgCl, outperforming most of the reported catalysts under similar reaction conditions. Combined in-situ spectrometric analyses and theoretical calculations demonstrate that the robust performance of Cuδ+ originates from its substantial affinity toward the N-intermediates, which promotes molecule activation and prevents intercoupling to dinitrogen species. For nitrate elimination, the ENRR was coupled with an anode-driven breakpoint chlorination reaction, and the designed system enabled complete conversion of NO3–-N (44.5 mg L-1) to N2 with an electrical consumption of 1.82 kwh molN-1.

Electrochemical Approaches for CO2 Conversion to Chemicals: A Journey toward Practical Applications.

Carbon capture, utilization, and sequestration play an essential role to address CO2 emissions. Among all carbon utilization technologies, CO2 electroreduction has gained immense interest due to its potential for directly converting CO2 to a variety of valuable commodity chemicals using clean, renewable electricity as the sole energy source. The research community has witnessed rapid advances in CO2 electrolysis technology in recent years, including highly selective catalysts, larger-scale reactors, specific process modeling, as well as a mechanistic understanding of the CO2 reduction reaction. The rapid advances in the field brings promise to the commercial application of the technology and the rapid rollout of the CO2 electroreduction for chemical manufacturing.This Account focuses on our contributions in both fundamental and applied research in the field of electrocatalytic CO2 and CO reduction reactions. We first discuss (1) the development of novel electrocatalysts for CO2/CO electroreduction to enhance the product selectivity and lower the energy consumption. Specifically, we synthesized nanoporous Ag and homogeneously mixed Cu-based bimetallic catalysts for the enhanced production of CO from CO2 and multicarbon products from CO, respectively. Then, we review our efforts in (2) the field of reactor engineering, including a dissolved CO2 H-type cell, vapor-fed CO2 three-compartment flow cell, and vapor-fed CO2 membrane electrode assembly, for enhancing reaction rates and scalability. Next, we describe (3) the investigation of reaction mechanisms using in situ and operando techniques, such as surface-enhanced vibrational spectroscopies and electrochemical mass spectroscopy. We revealed the participation of bicarbonate in CO2 electroreduction on Au using attenuated total-reflectance surface-enhanced infrared absorption spectroscopy, the presence of an "oxygenated" surface of Cu under CO electroreduction conditions using surface-enhanced Raman spectroscopy, and the origin of oxygen in acetaldehyde and other CO electroreduction products on Cu using flow electrolyzer mass spectrometry. Lastly, we examine (4) the commercial potential of the CO2 electrolysis technology, such as understanding pollutant effects in CO2 electroreduction and developing techno-economic analysis. Specifically, we discuss the effects of SO2 and NOx in CO2 electroreduction using Cu, Ag, and Sn catalysts. We also identify technical barriers that need to be overcome and offer our perspective on accelerating the commercial deployment of the CO2 electrolysis technology.

Iridium‐based electrocatalysts toward sustainable energy conversion

AbstractElectrocatalysis is an important branch in electrochemistry and also has a significant position in energy storage/conversion. Iridium (Ir)‐based electrocatalysts show promise because of the inherent redox property. Considering the abundance and expensive cost of such noble metal, significant efforts have been made to rationally improve the electrochemical performance of Ir‐based electrocatalysts during these years from the viewpoints of structural design and kinetic analysis. Most of the reports may aim to achieve high Ir atomic utilization with more exposed active sites, and both high electrochemical activity and durable stability are expected. In this case, Ir‐based single atoms, nanostructures, and composites are reported with various electrocatalytic applications of water splitting (involving hydrogen evolution and oxygen evolution reaction) and other electrocatalytic oxidation reactions (involving hydrogen oxidation reaction, methanol oxidation reaction, ammonia oxidation reaction, and formic acid oxidation reaction) as well as other electrocatalytic reduction reactions (involving oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction). These works deserve to be discussed and this review article comprehensively summarized the most significant reports. We analyzed Ir‐based electrocatalysts from the viewpoints of synthesis, structure, electrochemical property, and reaction mechanism. The challenges and outlooks in the future were also provided from the aspects of fundamental studies and industrial applications, which may attract more attention and provide new insight into the design of advanced Ir‐based electrocatalysts with high performance.image

Effect of Dealloying Time and Post-Annealing on the Surface Morphology and Electrocatalytic Behavior of Nanoporous Copper Films for CO2 Reduction Reaction

Electrocatalytic reduction reaction of CO2 (CO2RR) is one of the promising routes to mitigate global warming via transforming greenhouse gas into valuable chemical feedstocks. By adding proper electrocatalysts, such as nanoporous copper (NPC) with an average ligament size of 37 ± 6 nm, hydrocarbons could be produced at a relatively low overpotential. As the dealloying time increased to 156 h, the NPC was transformed into CuO nanosheet structure, which yielded larger electrochemical surface area (ECSA) and current density than the as-prepared NPC films. However, the Faraday efficiency (FE) of the major conversion product, formic acid (HCOOH), decreased from 29 to 8% when the nanosheet structure was used as electrocatalyst. On the other hand, the surface morphology of the NPC films remained similar while the average ligament size increased from 37 to 63 nm after a post-annealing treatment at 500 °C for 4 h. Both the current density and ECSA of this post-annealed NPC film were nearly 3 times higher than those of as-prepared NPC film, and the FE toward HCOOH increased from 29 to 45%. X-ray photoelectron spectroscopy and Raman spectroscopy revealed that Cu2O were present on the nanoporous structure, which enhanced the selectivity and FE toward HCOOH in CO2RR.

Accessing Organonitrogen Compounds via C-N Coupling in Electrocatalytic CO2 Reduction.

Given the limited product variety of electrocatalytic CO2 reduction reactions solely from CO2 and H2O as the reactants, it is desirable to expand the product scope by introducing additional reactants that provide elemental diversity. The integration of inorganic heteroatom-containing reactants into electrocatalytic CO2 reduction could, in principle, enable the sustainable synthesis of valuable products, such as organonitrogen compounds, which have widespread applications but typically rely on NH3 derived from the energy-intensive and fossil-fuel-dependent Haber-Bosch process for their industrial-scale production. In this Perspective, research progress toward building C-N bonds in N-integrated electrocatalytic CO2 reduction is highlighted, and the electrosyntheses of urea, acetamides, and amines are examined from the standpoints of reactivity, catalyst structure, and, most fundamentally, mechanism. Mechanistic discussions of C-N coupling in these advances are emphasized and critically evaluated, with the aim of directing future investigations on improving the product yield and broadening the product scope of N-integrated electrocatalytic CO2 reduction.

Advancement of Bismuth‐Based Materials for Electrocatalytic and Photo(electro)catalytic Ammonia Synthesis

AbstractAmmonia (NH3) plays a vital role in the fertilizer industry, nitrogen‐containing chemical production, and hydrogen storage. The development of electrocatalytic and photo(electro)catalytic nitrogen reduction reaction (NRR) to synthesize NH3 are desirable, which are more environmentally friendly than the conventional Haber–Bosch process. Due to the strong nonpolar bonding of the NN bond, the discovery of efficient catalysts is essential to overcome the high kinetic barrier. Here, bismuth‐based materials that show proven activities for NRR are highlighted. The fundamental knowledge, including thermodynamics, mechanisms, reaction systems, evaluation aspects of NRR, and product quantification, is introduced. Together with scientific reasoning and exhibited activities, the strategies for improving the performance are discussed in detail. Perspective and outlook, as well as the opportunities to further develop bismuth‐based materials for NH3 synthesis, are provided. This review aims to provide a comprehensive understanding of the advancement in this field and serves as a guide for the future design of highly efficient NRR catalysts.

Heterogeneous Two-Atom Single-Cluster Catalysts for the Nitrogen Electroreduction Reaction

The electrocatalytic dinitrogen reduction reaction (NRR) is promising to realize the decentralized production of ammonia by using renewable energies, which contrasts with the energy-intensive Haber–Bosch process. The key to achieve it is to find stable, efficient and selective catalysts. Recently, the heterogeneous single-cluster catalysts (SCCs) have emerged as a promising class of catalysts for electrochemical reactions due to their atomically precise active site, abundant active atoms and atomic level controllability. Herein, the NRR catalyzed by the two-atom SCCs consisting of homonuclear 3d transition metal (TM) dimers over the N-doped graphene, denoted as M2-N6G, is systematically investigated by using density functional theory (DFT). Our results indicate that the ability of metal dimer to capture N2 is related to the reducibility of the catalyst and the orbital interaction between the N-2p states and the TM-3d states. Subsequently, comparing with those metals which overbind N2 through side-on configurations, the M2-N6G SCCs with end-on adsorption of N2 work better. Furthermore, we obtain a linear relationship between the adsorption free energies of *N2H (ΔadsG*NH2) and that of *NH2 (ΔadsG*NH2). Based on this scaling relationship, we propose a compromised strategy for screening efficient two-atom SCCs for NRR. Finally, by comparing the stability, activity and selectivity of various M2-N6G SCCs, the Cr2-N6G and Mn2-N6G are predicted to be most active for NRR with low limiting potential and high suppression to hydrogen evolution reaction (HER). The present work not only provides experimentally synthesizable electrocatalyst candidates for NRR, but also gives insight into the development of the two-atom SCCs.

Comparison between layered Pt3Te4 and PtTe2 for electrocatalytic reduction reactions

Among 2D layered chalcogenides materials, platinum tellurides can form various stoichiometries with a layered structure that has attracted significant attention due to their yield of low overpotentials for hydrogen evolution reaction. In this work, layered Pt3Te4 and PtTe2 were synthesized and characterized, including their fundamental structural and electrochemical properties. We present a direct synthesis of Pt3Te4 and PtTe2 from the respective elements, resulting in a monoclinic and trigonal crystal structure, respectively. Their electrocatalytic properties were tested and compared with each other for the hydrogen evolution and oxygen reduction reactions, showing promising results for future catalytic applications.

Regulating Electronic Spin Moments of Single-Atom Catalyst Sites via Single-Atom Promoter Tuning on S-Vacancy MoS2 for Efficient Nitrogen Fixation.

The electrocatalytic activity of transition-metal (TM)-based catalysts is correlated with the spin states of metal atoms. However, developing a way to manipulate spin remains a great challenge. Using first-principles calculations, we first report the crucial role of the spin of exposed Mo atoms around an S-vacancy in the electrocatalytic dinitrogen reduction reaction on defective MoS2 nanosheets and propose a novel strategy for regulating the electronic spin moments by tuning a single-atom promoter (SAP). Single TM atoms adsorbed on a defective MoS2 basal plane serve as SAPs via a noncontact interaction with an exposed Mo active site, inducing a significant spin polarization that promotes N2 adsorption and activation. Interestingly, by changing only the adsorption site of the TM atom, we are able to change the spin moments of the Mo atom, over a wide range of tunable values. The spin moments can be tuned to largely improve the catalytic activity of MoS2 toward the reduction of N2 to NH3.

Understanding potential-dependent competition between electrocatalytic dinitrogen and proton reduction reactions

A key challenge to realizing practical electrochemical N2 reduction reaction (NRR) is the decrease in the NRR activity before reaching the mass-transfer limit as overpotential increases. While the hydrogen evolution reaction (HER) has been suggested to be responsible for this phenomenon, the mechanistic origin has not been clearly explained. Herein, we investigate the potential-dependent competition between NRR and HER using the constant electrode potential model and microkinetic modeling. We find that the H coverage and N2 coverage crossover leads to the premature decrease of NRR activity. The coverage crossover originates from the larger charge transfer in H+ adsorption than N2 adsorption. The larger charge transfer in H+ adsorption, which potentially leads to the coverage crossover, is a general phenomenon seen in various heterogeneous catalysts, posing a fundamental challenge to realize practical electrochemical NRR. We suggest several strategies to overcome the challenge based on the present understandings.

First-principles mechanistic study on nitrate reduction reactions on copper surfaces: Effects of crystal facets and pH

As a beneficial technology for nitrate removal and ammonia synthesis, electrocatalytic nitrate reduction reaction (NO3RR) has received much attention in experimental and theoretical studies. Previous work suggests that Cu is one of the most active electrocatalysts for NO3RR. However, the fundamental reaction mechanism, governing the activity and selectivity of Cu towards NO3RR with various facets and pH, remains elusive. Herein, a hybrid model is developed by combining the explicit water layer and the implicit solvation scheme to simulate the water/Cu interface based on theoretical density functional theory simulations. The hybrid model is used to elucidate the effects of crystal facets and pH on NO3RR mechanism over Cu. Our simulation results illustrate that NO3RR in acidic media proceeds via the formation of *NOH to produce NH3 on both (100) and (111) facets. While in alkaline media, Cu(100) prefers the formation of NH2OH via *NHO at low overpotentials, and Cu(111) kinetically favors the formation of NH3. In general, the (100) facet is more active towards NO3RR than the (111) facet, especially at higher pH, due to lower thermodynamic and kinetic barriers. Our results provide deeper mechanistic insights into the effects of both crystal facets and pH on the electrocatalytic activity and product selectivity of Cu towards NO3RR. The hybrid model developed here can be applied to other electrochemical reactions at water/metal interfaces.