Electrochemical NOx Research Articles

Rapid Fabrication of N-, Cu-, and Co-Doped Electrodes with Strong Electronic Coupling by Cold Plasma for Electrocatalytic Reduction of Nitrate to Ammonia.

Electrochemical NOx- reduction (NOxRR) is a sustainable technology for ammonia synthesis. The development of a simple, fast, and economical catalytic electrode preparation technique is crucial for large-scale ammonia synthesis. Herein, we propose a plate-to-plate DBD plasma strategy to synthesize the catalytic electrodes M-N/CP (M = Cu, Co, Ni, CuCo, and CuNi), achieving in suit codoping of N and bimetals on carbon paper (CP) at room temperature within 3 h. N-doping improves the metal loading and the NOxRR performance by forming N-M bonds. The electronic coupling and synergistic effect of Cu and Co sites facilitate the relay conversion of NO3- to NO2- to NH3. Notably, the NO3- removal efficiency of CuCo-N/CP reaches 82%, with NH3 yield rate of 346 μg h-1 cm-2, and the faradaic efficiency is as high as 86% at -0.58 V vs RHE, which remains competitive in terms of NOxRR performance. Importantly, wet flue gas denitrification and desulfurization coupled with electrocatalytic reduction can convert NOx and SO2 to (NH4)2SO4, Additionally, the maneuverability of plasma technology offers the potential for batch preparation of CuCo-N/CP electrodes for electrochemically driven wet denitrification wastewater valorization.

Performance improvement of NiO/YSZ sensitive electrode for high temperature electrochemical NOx gas sensors

In the mixed-potential gas sensor, the electrode type, particle size, microstructural morphology, as well as the interface state between the oxide-sensitive electrode and the solid electrolyte, are the key factors for determining the magnitude of the mixed potential. NiO-based material with the addition of Y2O3-stabilized ZrO2 (YSZ) particles is considered as one of the promising sensitive electrode materials for preparing the mixed-potential NOx sensors due to its excellent gas sensitivity. In this paper, the preparation technology of NiO-sensitive electrodes is developed and optimized for improving NOx (NO and NO2) gas-sensitivity properties of the NOx sensors. The experimental results show that the addition of YSZ can effectively improve the agglomeration morphology of NiO particles in the NOx-YSZ-based electrode material during high temperature sintering (i.e. 1200 °C) and enhance the bonding strength with the solid electrolyte. The sensor which was prepared by using the developed NOx-YSZ-based electrode material with the YSZ addition of 20 wt% has the most sensitive response characteristic to NO gas, and its response potential value increases with NO gas concentration. Elevating electrode sintering temperature can increase the size of NiO particles and shorten the length of the three-phase boundary (TPB) in the sensitive electrode. The average length of TPB is the longest in the sensitive electrode sintered at 1000 °C which corresponds to the best sensitive response to NO gas. The sensitive electrode's thickness also affects the NO response sensitivity, and the sensor with an electrode thickness of 14 μm has the most NO response sensitivity. In addition, the NO/NO2 gas mixture test results indicate that the current developed sensor is more sensitive to NO2 gas than that of NO gas.

Boosting Electrochemical Ammonia Synthesis via NOx Reduction over Sulfur‐Doped Copper Oxide Nanoneedle Arrays

AbstractThe electrochemical NOx reduction reactions, involving nitrate and nitrite reduction reactions (NO3−RR and NO2−RR), have emerged as promising approaches for both NO3− and NO2− removal, and ammonium (NH3) synthesis under ambient conditions. However, the incorporation and stabilization of sulfur dopants in the catalysts for efficient NOx reduction are rarely explored, leading to an unclear effect of sulfur on the NOx reduction mechanism. Herein, sulfur‐doped Cu2O (S‐Cu2O) nanoneedle arrays via in situ electrochemical treatment are synthesized. The S‐Cu2O catalyst possesses excellent durability and selectivity for NH3 over a wide range of potentials in NO3−RR, attaining a maximum NH3 Faradaic efficiency of 94% at −0.6 VRHE and a maximum NH3 yield as high as 1.06 mmol h−1 cm−2. In NO3−RR, the sulfur dopant can accelerate the step from NO2− to NH3, contributing superior performance in NO2−RR and assembled Zn−NO2− battery device. Density functional theory (DFT) calculations reveal that the presence of sulfur can enhance the initial step of *NO3 adsorption, lower the reaction barriers for the formation of *NHO intermediate, and activate the H2O dissociation process. The work sheds light on the role of sulfur in enhancing electrocatalytic performance and provides a unique perspective for understanding the NOx reduction mechanism.

Efficient ammonia synthesis from the air using tandem non-thermal plasma and electrocatalysis at ambient conditions

While electrochemical N2 reduction presents a sustainable approach to NH3 synthesis, addressing the emission- and energy-intensive limitations of the Haber-Bosch process, it grapples with challenges in N2 activation and competing with pronounced hydrogen evolution reaction. Here we present a tandem air-NOx-NOx−-NH3 system that combines non-thermal plasma-enabled N2 oxidation with Ni(OH)x/Cu-catalyzed electrochemical NOx− reduction. It delivers a high NH3 yield rate of 3 mmol h−1 cm−2 and a corresponding Faradaic efficiency of 92% at −0.25 V versus reversible hydrogen electrode in batch experiments, outperforming previously reported ones. Furthermore, in a flow mode concurrently operating the non-thermal plasma and the NOx− electrolyzer, a stable NH3 yield rate of approximately 1.25 mmol h−1 cm−2 is sustained over 100 h using pure air as the intake. Mechanistic studies indicate that amorphous Ni(OH)x on Cu interacts with hydrated K+ in the double layer through noncovalent interactions and accelerates the activation of water, enriching adsorbed hydrogen species that can readily react with N-containing intermediates. In situ spectroscopies and density functional theory (DFT) results reveal that NOx− adsorption and their hydrogenation process are optimized over the Ni(OH)x/Cu surface. This work provides new insights into electricity-driven distributed NH3 production using natural air at ambient conditions.

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

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

Redox mediators boost NOx reduction via trade-off electron charges using a cube-shaped (cMn@rGO) catalyst; mechanism and electrochemical study

Gaseous pollutants like sulfur dioxide and nitrogen oxide(s) (SO2, NOx) have been increasing exponentially for the last two decades, which have had adverse effects on human health, aquatic life, and the environment. Recently, for air pollution taming, manganese/oxide (Mn/MnO) has become a very promising heterogeneous catalyst due to its environment-friendly, low-price, and remarkable catalytic abilities for toxic gases. In this work, cube-shaped Mn nanoparticles (cMn NPs) were decorated on the surface of reduced graphene oxide (rGO) by the solvothermal method. The resulting cMn@rGO composite was employed for electrochemical NOx reduction. However, the microscopic (TEM/HRTEM) and structural analysis were utilised to investigate the morphology and characteristics of the cMn@rGO composite. This electrochemical-based treatment for NOx reduction is employed by using electron shuttle or redox mediators. Here, four distinct redox mediators are used to address electrochemical obstacles, which effectively facilitate electron transportation and promoted NOx reduction on the electrode surface. These mediators not only significantly enhanced the NOx conversion into valuable products, i.e., N2 and N2O, but also made the process smooth with high performance. Among these mediators, neutral red (N.R) exhibited extraordinary potential in enhancing NOx reduction. The obtained results indicated that the remarkable catalytic performance (∼93%) of the cMn@rGO can be attributed to several factors, including the catalyst's three-dimensional architecture structure and abundant active sites. The designed catalyst (cMn@rGO) is not only cost-effective and sustainable but also exhibits excellent potential in effectively reducing NOx, which could be beneficial for large-scale NOx abatement.

Protection of NOx Sensors from Sulfur Poisoning in Glass Furnaces by the Optimization of a "SO2 Trap".

Electrochemical NOx sensors based on yttria-stabilized zirconia (YSZ) provide a reliable onboard way to control NOx emissions from glass-melting furnaces. The main limitation is the poisoning of this sensor by sulfur oxides (SOx) contained in the stream. To overcome this drawback, an "SO2 trap" with high SOx storage capacity and low affinity to NOx is required. Two CuO/BaO/SBA-15 traps with the same CuO loading (6.5 wt.%) and different BaO loadings (5 and 24.5 wt.%, respectively) were synthetized, thoroughly characterized and evaluated as SO2 traps. The results show that the 6.5%CuO/5%BaO/SBA-15 trap displays the highest SO2 adsorption capacity and can fully adsorb SO2 for a specific period of time, while additionally displaying a very low NO adsorption capacity. A suitable quantity of this material located upstream of the sensor could provide total protection of the NOx sensor against sulfur poisoning in glass-furnace exhausts.

Synthesis and characterization of Ba(Fe,Zr,Ni)O3−δ perovskites for potential electrochemical applications

Phase relationships in the BaFe1-x-yZryNixO3-δ system were evaluated in the search for Ba-rich electrode materials with potential application in solid electrolyte cells for electrochemical NOx reduction. Co-substitutions by zirconium and nickel into the iron sublattice of barium ferrite stabilize the cubic perovskite structure with a solid solution formation range limited to (x + y) ⪅ 0.3. Characterization of BaFe0.7Zr0.3-xNixO3-δ (x = 0.10–0.20) perovskites included the determination of oxygen nonstoichiometry variations by TGA, measurements of electrical conductivity as a function of temperature and oxygen partial pressure, studies of thermal expansion, and assessment of chemical compatibility with selected solid electrolytes. BaFe0.7Zr0.3-xNixO3-δ perovskites exhibit substantial oxygen losses from the lattice above 350 °C, reaching Δδ ∼ 0.2 on heating to 1000 °C in air, with effects on electrical properties and thermochemical expansion. Electrical conductivity is p-type under oxidizing conditions, increases with Ni content, and reaches 3.5 S/cm at 400 °C. The perovskite phase is stable down to p(O2) ∼ 10‐17 at 900 °C. The average thermal expansion coefficients in air increase from (15−18)× 10‐6 K‐1 at lower temperatures to ∼ 30 × 10‐6 K‐1 above 400 °C. The optimum co-doping level was found to be x = y = 0.15 as this composition exhibits the best chemical compatibility with yttria-stabilized zirconia and BaZr0.85Y0.15O3-δ solid electrolytes.

Characterization of Ruddlesden-Popper La2−xBaxNiO4±δ Nickelates as Potential Electrocatalysts for Solid Oxide Cells

Ruddlesden-Popper La2-xBaxNiO4±δ (x = 0-1.1) nickelates were prepared by a glycine-nitrate combustion route combined with high-temperature processing and evaluated for potential application as electrocatalysts for solid oxide cells and electrochemical NOx elimination. The characterization included structural, microstructural and dilatometric studies, determination of oxygen nonstoichiometry, measurements of electrical conductivity and oxygen permeability, and assessment of chemical compatibility with other materials. The formation range of phase-pure solid solutions was found to be limited to x = 0.5. Exceeding this limit leads to the co-existence of the main nickelate phase with low-melting Ba- and Ni-based secondary phases responsible for a strong reactivity with Pt components in experimental cells. Acceptor-type substitution of lanthanum by barium in La2-xBaxNiO4+δ is charge-compensated by decreasing oxygen excess, from δ ≈ 0.1 for x = 0 to nearly oxygen-stoichiometric state for x = 0.5 at 800 °C in air, and generation of electron-holes (formation of Ni3+). This leads to an increase in p-type electronic conductivity (up to ~80 S/cm for highly porous La1.5Ba0.5NiO4+δ ceramics at 450-900 °C) and a decline of oxygen-ionic transport. La2-xBaxNiO4+δ (x = 0-0.5) ceramics exhibit moderate thermal expansion coefficients, 13.8-14.3 ppm/K at 25-1000 °C in air. These ceramic materials react with yttria-stabilized zirconia at 700 °C with the formation of an insulating La2Zr2O7 phase but show good chemical compatibility with BaZr0.85Y0.15O3-δ solid electrolyte.

Synergistic Au passivation and prolonged aging optimization enhance the long-term catalytic stability of porous YSZ/Pt electrodes

The catalytic activity of an oxygen-pumping electrode is very important to the performance of amperometric NOx (NO and NO2) sensors. To enhance the stability of electrochemical NOx sensors based on Pt electrodes, we fabricated sensitive electrodes of Pt-based materials doped with small and different Au contents on 8% mol Y2O3-stabilized ZrO2 (8YSZ) solid electrolyte substrates by screen printing and high-temperature co-firing. The effects of Au on the porous Pt electrode’s catalytic activity and long-term stability were investigated, and the results are as follows. First, the catalytic capacity of the Pt–Au alloy electrode decreases with increasing Au content due to the inhibition of the oxygen adsorption on Au, which decreases the amount of oxygen participating in the reaction at triple-phase boundaries (TPBs). Second, the catalytic ability of the alloy electrode decreases exponentially with aging time due to the reduction in the length of TPBs caused by the significant reduction in the number of holes in the electrode, and the oxidation of Pt. In addition, the conductivity decay rate of the electrode decreases with increasing Au doping. This indicates that Pt-based electrodes doped with varying Au contents, long-term, and high-temperature aging have notably regular effects on the catalytic activity of oxygen-pumping electrodes, for which we have developed a model of Au passivation and the synergistic effect of aging time on the activity of Pt-based oxygen-pumping electrodes.

Support vector machine for a diesel engine performance and NOx emission control-oriented model

A control oriented diesel engine NOx emission and Break Mean Effective Pressure (BMEP) model is developed using Support Vector Machine (SVM). Steady state experimental data from a medium duty diesel engine is used to develop BMEP and NOx emission model using Support Vector Machine (SVM). The engine speed, the amount of injected fuel and the injection rail pressure are used as input variables to predict the steady state engine NOx emission and BMEP. The steady state model results were then implemented in the control oriented model. A fast response electrochemical NOx sensor is used to experimentally study the engine transient NOx emission and to verify the transient response of the control oriented model. The results show that the SVM algorithm is capable of accurately learning the engine BMEP and NOx which improves the accuracy of the control oriented model compared to a conventional regression algorithm (trust-region) used in the literature. The control oriented model results closely match the experiments in both transient and steady state conditions with a root mean square error of 0.26 (bar) and 10 (ppm) for BMEP and NOx respectively.

Electrochemical decomposition of NOx and oxidation of HC and CO emissions by developing electrochemical cells for diesel engine emission control.

Diesel engines are the most extensively used power source in automobiles and stationary power generation. The main drawback of using diesel engines is that it liberates a significant amount of NOx and PM emissions in the exhaust. NOx emission has a serious effect on the environment, and it has to be controlled effectively. SCR is the most widely used after-treatment technology to control NOx emission, but it has various disadvantages like ammonia slip and degradation of the catalyst. In this study, electrochemical decomposition of NOx is proposed for the simultaneous control of NOx, HC, and CO emissions in a diesel engine. In this work, ionically conducting ceramic electrochemical cells are investigated for control of diesel exhaust emissions. The electrochemical cell consisting of yttrium stabilized zirconia (YSZ) substrate plates as electrolyte and Ag-YSZ and NiO-YZS as an electrode material. The decomposition of NOx in an electrochemical cell is attained by passing electric current. A 2V supply of power was sufficient for effective operation of the electrochemical cell in all load conditions. All the experiments were conducted in a single-cylinder diesel engine. It is observed that the electrochemical cell shows high NOx decomposition rate of 80% at the exhaust temperatures between 350 and 400°C. The HC reduction up to 65% and CO reduction up to 45% was observed with this technique. The power required to operate the electrochemical cell was low. The electrochemical NOx reduction is relatively simple technology with reduced complexity. From the experiment, it is observed that this concept works efficiently in the oxygen-rich diesel exhaust.

Electrochemical reduction of NOx species at the interface of nanostructured Pd and PdCu catalysts in alkaline conditions

Electrochemical reduction of NOx species such as nitrates (NO3−), nitrites (NO2−), nitric oxide (NO), nitrogen dioxide (NO2) and their mixtures, was studied at the interface of palladium (Pd) and palladium-copper (PdCu) nanoparticles supported on carbon Vulcan (C). The electro-catalysts were synthesized by impregnation route with a low noble metal content of 5% wt. Pd and 2.5% wt. Pd for the mono and bi-metallic electrocatalyst, respectively. It was found by XRD analysis the formation of a solid solution in the bi-metallic catalyst and the TEM analysis suggest that the incorporation of copper decreases the particle size from 12 to 3 nm in comparison with its counterpart free of copper. Also, XPS technique verify the presence of Pd and Cu species in their metallic-oxidation states. Linear sweep and cyclic voltammetry techniques were used for the evaluation of the electrochemical NOx reduction, using alkaline solutions of NO2− or NO3− saturated with NO2 (synthesized in-situ) and NO (from commercial source). The results showed that the catalytic-activity at the current versus potential (i-E) characteristics improves significantly due to the presence of copper (as also demonstrated by CO-stripping-electrochemical active surface area calculations), inhibiting the process associated with the hydrogen evolution reaction. It is also noted in this work that the reduction faradic-current is c.a. 6 times higher at saturated solutions with NO and NO2. The NOx species were reduced mainly to nitrogen, ammonia and hydrazine as confirmed using on-line differential electrochemical mass spectrometry (DEMS) during steady-state experiments.

운행 경유차용 전기화학식 질소산화물(NO, NO2)센서 개발 및 효율성에 관한 연구

운행 경유차용 전기화학식 질소산화물(NO, NO2)센서 개발 및 효율성에 관한 연구

A New Low Temperature Electrochemical Hydrocarbon and NOx Sensor

In this article, a new investigation on a low temperature electrochemical hydrocarbon and NOx sensor is presented. Based on the mixed potential based sensing scheme, the sensor is constructed using platinum and metal oxide electrodes, along with Yttria-Stabilized Zirconia (YSZ)/Strontium Titanate (SrTiO3) thin film electrolyte. Unlike traditional mixed potential sensors which operate at higher temperatures (> 4000 C), this potentiometric sensor operates at 2000 C with selective hydrocarbon (HC) and NOx response in the open-circuit and biased mode respectively. The possible low temperature operation of the sensor is speculated primarily due to the enhanced oxygen ion conductivity of the electrolyte, which may be attributed to the space charge effect, epitaxial strain, and atomic reconstruction at the interface of YSZ/STO thin film. The response and recovery time for the NOx sensor are found to be 7 seconds and 8 seconds respectively. The sensor exhibited stable response even after 60 days of testing, with 9.4% decrease in HC response and a 1.9% decrease in NOx response.

Electrochemical NOx reduction and oxidation of HC and PM emissions from biodiesel fuelled diesel engines using electrochemically activated cell

ABSTRACTNOx emission is produced during combustion of fuels at high temperature. Excessive release of NOx causes several effects on living organisms and environment. In this work, the efforts to reduce NOx emission by developing electrochemically activated cells (EACs) for a diesel engine fuelled with diesel and biodiesel fuel are discussed. EAC technique is vital after treatment technology attempted in this work to simultaneous control of NOx, HC, and PM emissions. In this method, two types of EACs were developed. The CuO–YSZ electrolyte and CuO–YSZ electrolyte with BaO coating were developed and tested with diesel and biodiesel exhaust. Compared with diesel fuel, use of biodiesel fuel increased NOx emission by 11% and PM emission was slightly reduced with biodiesel, which was due to the presence of fuel bond oxygen content in biodiesel. The investigation has demonstrated low-temperature activation of the EACs at 250–350°C which was due to the addition of CuO to YSZ. In this work, maximum NOx reduction was achieved for CuO–YSZ cells with BaO NOx storage and the simultaneous control of HC and PM emission also was observed in this technique. NOx reduction by EAC is a vital technique and can be retrofitted with any diesel engine for emission reduction.

A New Low-Temperature Electrochemical Hydrocarbon and NOx Sensor.

In this article, a new investigation on a low-temperature electrochemical hydrocarbon and NOx sensor is presented. Based on the mixed-potential-based sensing scheme, the sensor is constructed using platinum and metal oxide electrodes, along with an Yttria-Stabilized Zirconia (YSZ)/Strontium Titanate (SrTiO3) thin-film electrolyte. Unlike traditional mixed-potential sensors which operate at higher temperatures (>400 °C), this potentiometric sensor operates at 200 °C with dominant hydrocarbon (HC) and NOx response in the open-circuit and biased modes, respectively. The possible low-temperature operation of the sensor is speculated to be primarily due to the enhanced oxygen ion conductivity of the electrolyte, which may be attributed to the space charge effect, epitaxial strain, and atomic reconstruction at the interface of the YSZ/STO thin film. The response and recovery time for the NOx sensor are found to be 7 s and 8 s, respectively. The sensor exhibited stable response even after 120 days of testing, with an 11.4% decrease in HC response and a 3.3% decrease in NOx response.

In-situ electrochemical NOx removal process for the lean-burn engine exhaust based on carbon black gas diffusion electrode

With the aim to remove NOx from the lean-burn engine exhaust, a novel in-situ electrochemical device was designed using carbon black-based gas diffusion electrode (GDE) as cathode. Taking advantage of the excessive O2 from the exhaust, in-situ electro-generation of H2O2 from oxygen reduction reaction (ORR) under catalysis of the GDE could oxidize NOx to NO3−. The highest NOx removal efficiency was 77.7% for 1000 ppm NOx with energy consumption of 1.327 W cm−2 cm−3. The electrochemical NOx removal pathway was proposed that, as NO and O2 were adsorbed on different sites in the catalyst layer (CL), electro-generated H2O2 attacked the C―N adsorption bonds mostly in the nitro compounds to oxidize adsorbed NO to NO3−, while NO2 was directly oxidized by the produced H2O2 in the electrolyte. The NOx removal efficiency is mainly controlled by the NO adsorption rate as well as the H2O2 oxidation rate, and the former is the dominant factor. This in-situ electrochemical NOx removal system exhibited properties of energy-saving, low–cost and environmental compatibility, indicating great potential for industrial application.

Review—Electrochemical NOx Gas Sensors Based on Stabilized Zirconia

Nitrogen oxides originated from combustion-exhausts are not only harmful to human health, but they also form acid rain and photochemical smog, both of which inflict enormous environmental damage. Therefore, monitoring and controlling NOx are an urgent requirement. Electrochemical sensors based on solid electrolyte are a promising method for detection of NOx. We review the working principles and the recent developments in the area of ZrO2-based electrochemical NOx sensors, including equilibrium potentiometric, mixed potential, pulsed polarization, amperometric, and impedancemetric NOx sensors. The improvement of triple-phase boundary (TPB), the limiting factors in measurement, and the future perspectives are also discussed in this review.

NO2-Selective Electrochemical Sensors for Diesel Exhausts

Electrochemical NOx sensors based on yttria-stabilized zirconia (YSZ) are commercialized to detect overall NOx concentrations in Diesel exhausts without distinction between NO and NO2. To overcome this inconvenient, a high-sensitive sensor based on selective electrochemical reduction of NO2 is proposed. An electrochemical cell consisting of three metallic electrodes on solid-state electrolyte (YSZ) operating under electric polarization between a gold working electrode and a platinum counter electrode allows selective NO2 detection at 400-550°C with high sensitivity, and without any significant interferences to other gases like hydrocarbons, carbon monoxide and also ammonia and nitrogen monoxide. A mechanism based on cathodic overpotential is proposed.