Electrocatalytic ammonia oxidation over a nickel foam electrode: Role of Ni(OH)2(s)-NiOOH(s) nanocatalysts

In situ electro-generated Ni(OH)2 synergistic with Cu cathode to promote direct ammonia oxidation to nitrogen

To solve the problem of low removal rate and poor N2 selectivity in direct electrochemical ammonia oxidation (EAO), commercial Ni foam and Cu foam were used as anode and cathode of the EAO system, respectively. The coupling effect between the cathode and anode promoted nitrogen cycling during the reaction process, which improved N2 selectivity of the reaction system and promoted it to achieve a high ammonia removal rate. This study showed that the thin Ni(OH)2 with oxygen vacancy formed on the surface of Ni foam anode played an effective role in the dimerization of intermediate products in ammonia oxidation to form N2. This electrochemical system was used to treat real goose wastewater containing 422.5 mg/L NH4+-N and 94.5 mg/L total organic carbon (TOC). After treatment, this electrochemical system achieved good performance with an ammonia removal rate of 87%, N2 selectivity of 77%, and TOC removal rate of 72%. Therefore, this simple and efficient system with Ni foam anode and Cu foam cathode is a promising method for treating ammonia nitrogen wastewater.

Suppressing Redox Reactions at the Perovskite-Nickel Oxide Interface with Zinc Nitride to Improve the Performance of Perovskite Solar Cells

For p-i-n perovskite solar cells (PSCs), nickel oxide (NiOx) hole transport layers (HTLs) are the preferred interfacial layer due to their low cost, high mobility, high transmittance, and stability. However, the redox reaction between the Ni≥3+ and hydroxyl groups in the NiOx and perovskite layer leads to oxidized CH3NH3 + and reacts with PbI in the perovskite, resulting in a large number of non-radiative recombination sites. Among various transition metals, an ultra-thin zinc nitride (Zn3N2) layer on the NiOx surface is chosen to prevent these redox reactions and interfacial issues using a simple solution process at low temperatures. The redox reaction and non-radiative recombination at the interface of the perovskite and NiOx reduce chemically by using interface modifier Zn3N2 to reduce hydroxyl group and defects on the surface of NiOx. A thin layer of Zn3N2 at the NiOx/perovskite interface results in a high Ni3+/Ni2+ ratio and a significant work function (WF), which inhibits the redox reaction and provides a highly aligned energy level with perovskite crystal and rigorous trap-passivation ability. Consequently, Zn3N2-modified NiOx-based PSCs achieve a champion PCE of 21.61%, over the NiOx-based PSCs. After Zn3N2 modification, the PSC can improve stability under several conditions.

The Enhanced Performance of NiCuOOH/NiCu(OH)2 Electrode Using Pre-Conversion Treatment for the Electrochemical Oxidation of Ammonia

Electrochemical oxidation of ammonia is an attractive process for wastewater treatment, hydrogen production, and ammonia fuel cells. However, the sluggish kinetics of the anode reaction has limited its applications, leading to a high demand for novel electrocatalysts. Herein, the electrode with the in situ growth of NiCu(OH)2 was partially transformed into the NiCuOOH phase by a pre-treatment using highly oxidative solutions. As revealed by SEM, XPS, and electrochemical analysis, such a strategy maintained the 3D structure, while inducing more active sites before the in situ generation of oxyhydroxide sites during the electrochemical reaction. The optimized NiCuOOH-1 sample exhibited the current density of 6.06 mA cm-2 at 0.5 V, which is 1.67 times higher than that of NiCu(OH)2 (3.63 mA cm-2). Moreover, the sample with a higher crystalline degree of the NiCuOOH phase exhibited lower performance, demonstrating the importance of a moderate treatment condition. In addition, the NiCuOOH-1 sample presented low selectivity (<20%) towards NO2- and stable activity during the long-term operation. The findings of this study would provide valuable insights into the development of transition metal electrocatalysts for ammonia oxidation.

Potassium-Hydrogen Hybrid Ion Alkaline Battery: A New Rechargeable Aqueous Battery Combined a K+ Storage Cathode and an Electrochemical Hydrogen Storage Anode

A rechargeable aqueous hybrid ion alkaline battery, using a proton and a potassium ion as charge carriers for the anode and cathode, respectively, is proposed in this study by using well-developed potassium nickel hexacyanoferrate as the cathode material and mesoporous carbon sheets as the anode material, respectively. The constructed battery operates in a concentrated KOH solution, in which the energy storage mechanism for potassium nickel hexacyanoferrate involves the redox reaction of Fe2+/Fe3+ associated with potassium ion insertion/extraction and the redox reaction of Ni(OH)2/NiOOH. The mechanism for the carbon anode is electrochemical hydrogen storage. The cathode made of potassium nickel hexacyanoferrate exhibits both an ultrahigh capacity of 232.7 mAh g-1 under 100 mA g-1 and a consistent performance of 214 mAh g-1 at 2000 mA g-1 (with a capacity retention of 92.8% after 200 cycles). The mesoporous carbon sheet anode exhibits a capacity of 87.6 mAh·g-1 at 100 mA g-1 with a good rate and cyclic performance. The full cell provides an operational voltage of 1.55 V, a capacity of 93.6 mAh g-1 at 100 mA g-1, and 82.4% capacity retention after 1000 cycles at 2000 mA g-1 along with a low self-discharge rate. The investigation and discussion about the energy storage mechanisms for both electrode materials are also provided.

Highly Selective Ammonia Oxidation on BiVO4 Photoanodes Co-catalyzed by Trace Amounts of Copper Ions

High-efficient photoelectrocatalytic direct ammonia oxidation reaction (AOR) conducted on semiconductor photoanodes remains a substantial challenge. Herein, we develop a strategy of simply introducing ppm levels of Cu ions (0.5-10 mg/L) into NH3 solutions to significantly improve the AOR photocurrent of bare BiVO4 photoanodes from 3.4 to 6.3 mA cm-2 at 1.23 VRHE , being close to the theoretical maximum photocurrent of BiVO4 (7.5 mA cm-2 ). The surface charge-separation efficiency has reached 90 % under a low bias of 0.8 VRHE . This AOR exhibits a high Faradaic efficiency (FE) of 93.8 % with the water oxidation reaction (WOR) being greatly suppressed. N2 is the main AOR product with FEs of 71.1 % in aqueous solutions and FEs of 100 % in non-aqueous solutions. Through mechanistic studies, we find that the formation of Cu-NH3 complexes possesses preferential adsorption on BiVO4 surfaces and efficiently competes with WOR. Meanwhile, the cooperation of BiVO4 surface effect and Cu-induced coordination effect activates N-H bonds and accelerates the first rate-limiting proton-coupled electron transfer for AOR. This simple strategy is further extended to other photoanodes and electrocatalysts.

Nickel-based catalysts for electrolytic decomposition of ammonia towards hydrogen production

Nickel is an attractive metal for electrochemical applications because it is abundant, cheap, chemically resilient, and catalytically active towards many reactions. Nickel-based materials (metallic nickel, its alloys, oxides, hydroxides, and composites) have been also considered as promising electrocatalysts for ammonia oxidation. The electrolysis of ammonia aqueous solution results in evolution of gaseous hydrogen and nitrogen. Up to date studies showed that metallic Ni and Ni (hydro)oxides are not catalytically active unless they are electrochemically converted to NiOOH at ~1.3 V vs. RHE. Then, dehydrogenation of NH3 begins with electron coupled proton transfer to NiOOH resulting in a would-be reversible reduction of the latter to Ni(OH)2. Unlike the water electrolysis process, in which solely oxygen is obtained at the anode, during ammonia electrooxidation apart from release of N2, many undesired oxygenated nitrogen moieties may also turn up. These products appear after at least partial dehydrogenation of ammonia. Studies on NiOOH activity have been conducted for systems containing various modifiers, e.g., Cu, Co, S, P, however, their particular role in catalytic activity has not yet been elucidated. Nowadays research is being conducted in the direction of increasing the activity, selectivity, and stability of NiOOH. In this review, the electroactivity of Ni is analyzed and discussed in accordance with its oxidation states along with the ammonia oxidation mechanism. The main research problems to be solved and challenges for the future industrial use of ammonia are presented.

Facile route of fluorine incorporation into nickel (oxy)hydroxide for improving the oxygen evolution reaction

Despite extensive development, oxygen evolution reaction (OER) catalysts still require significant overpotentials to function. In this study, we show that the overpotential of a nickel (Ni) electrode for the OER can decrease by about 100 mV with fluorine (F) incorporation, particularly by a facile electrochemical approach at room temperature.

Tailored Bimetallic Ni-Sn Catalyst for Electrochemical Ammonia Oxidation to Dinitrogen with High Selectivity

Direct electrochemical ammonia oxidation reaction (eAOR) is an efficient and sustainable strategy to process wastewater containing ammonia, and it endures overoxidation and severely competitive oxygen evolution reaction (OER). Herein, we synthesized a Ni(OH)₂/SnO₂ composite catalyst by a multistep strategy and applied it to the eAOR process. Ni(OH)₂/SnO₂ exhibited a N₂-N Faradaic efficiency (FE_N2-N) of 84.2%, with a N₂ partial current density (j_N2-N) of 2.7 mA cm^-2 at 1.55 V vs reversible hydrogen electrode (RHE) in 0.5 M K₂SO₄ with 10 mM NH₃-N (pH 11). The oxophilic Sn promoted NH₃ absorption on Ni sites while suppressing the OER. As the active species, NiOOH accelerated the dimerization of intermediates (*NH₂ or *NH) to form N₂.

Facile and Rapid Electrochemical Conversion of Ni into Ni(OH)2 Thin Film as the Catalyst for Direct Growth of Carbon Nanotubes on Ni Foam for Supercapacitors

In this paper, a facile and rapid aqueous-based electrochemical technique was used for the phase conversion of Ni into Ni(OH)₂ thin film. The Ni(OH)₂ thin film was directly converted and coated onto the network surface of Ni foam (NF) via the self-hydroxylation process under alkaline conditions using a simple cyclic voltammetry (CV) strategy. The as-formed and coated Ni(OH)₂ thin film on the NF was used as the catalyst layer for the direct growth of carbon nanotubes (CNTs). The self-converted Ni(OH)₂ thin film is a good catalytic layer for the growth of CNTs due to the fact that the OH^- of the Ni(OH)₂ can be reduced to H₂O to promote the growth of CNTs during the CVD process, and therefore enabling the dense and uniform CNTs growth on the NF substrate. This binder-free CNTs/NF electrode displayed outstanding behavior as an electric double-layer capacitor (EDLC) due to the large surface area of the CNTs, showing excellent specific capacitance values of 737.4 mF cm^-2 in the three-electrode configuration and 319.1 mF cm^-2 in the two-electrode configuration, at the current density of 1 mA cm^-2 in a 6 M KOH electrolyte. The CNTs/NF electrode also displayed good cycling stability, with a capacitance retention of 96.41% after 10,000 cycles, and this the excellent cycling performance can be attributed to the stable structure of the direct growth of CNTs with a strong attachment to the NF current collector, ensuring a good mechanical and electrical connection between the NF collector and the CNTs.

A Survey of Catalytic Materials for Ammonia Electrooxidation to Nitrite and Nitrate

Studies of the ammonia oxidation reaction (AOR) for the synthesis of nitrite and nitrate (NO_2/3 ^- ) have been limited to a small number of catalytic materials, majorly Pt based. As the demand for nitrate-based products such as fertilisers continues to grow, exploration of alternative catalysts is needed. Herein, 19 metals immobilised as particles on carbon fibre electrodes were tested for their catalytic activity for the ammonia electrooxidation to NO_2/3 ^- under alkaline conditions (0.1 m KOH). Nickel-based electrodes showed the highest overall NO_2/3 ^- yield with a rate of 5.0±1.0 nmol s^-1  cm^-2 , to which nitrate contributed 62±8 %. Cu was the only catalyst that enabled formation of nitrate, at a rate of 1.0±0.4 nmol s^-1  cm^-2 , with undetectable amounts of nitrite produced. Previously unexplored in this context, Fe and Ag also showed promise and provided new insights into the mechanisms of the process. Ag-based electrodes showed strong indications of activity towards NH₃ oxidation in electrochemical measurements but produced relatively low NO_2/3 ^- yields, suggesting the formation of alternate oxidation products. NO_2/3 ^- production over Fe-based electrodes required the presence of dissolved O₂ and was more efficient than with Ni on longer timescales. These results highlight the complexity of the AOR mechanism and provide a broad set of catalytic activity and nitrate versus nitrite yield data, which might guide future development of a practical process for the distributed sustainable production of nitrates and nitrites at low and medium scales.

In situ engineering of highly conductive TiO2/carbon heterostructure fibers for enhanced electrocatalytic degradation of water pollutants

Rational design of nanocomposite electrode materials with high conductivity, activity, and mechanical strength is critical in electrocatalysis. Herein, freestanding, flexible heteronanocomposites were fabricated in situ by carbonizing electrospun fibers with TiO₂ nanoparticles on the surface for electrocatalytic degradation of water pollutants. The carbonization temperature was observed as a dominant parameter affecting the characteristics of the electrodes. As the carbonization temperature increased to 1000 °C, the conductivity of the electrode was significantly enhanced due to the high degree of graphitization (I_D/I_G ratio 1.10) and the dominant rutile phase. Additionally, the formation of TiO₂ protrusions and the C-Ti heterostructure were observed at 1000 °C, which contributed to increasing the electrocatalytic activity. When 1.5 V (vs. Ag/AgCl) was employed, electrocatalytic experiments using the electrode achieved 90% degradation of crystal violet and 10.9-87.5% for an array of micropollutants. The electrical energy-per-order (EEO) for the removal of crystal violet was 0.7 kWh/m³/order, indicative of low-energy requirement. The efficient electrocatalytic activity can be ascribed to the fast electron transfer and the strong ability to generate hydroxyl radicals. Our findings expand efforts for the design of highly conductive heteronanocomposites in a facile in situ approach, providing a promising perspective for the energy-efficient electrocatalytic degradation of water pollutants.

Process Optimization of Electrochemical Treatment of COD and Total Nitrogen Containing Wastewater

In this work, an electrochemical method for chemical oxygen demand (COD) and total nitrogen (TN, including ammonia, nitrate, and nitrite) removal from wastewater using a divided electrolysis cell was developed, and its process optimization was investigated. This process could effectively relieve the common issue of NO₃^-/NO₂^- over-reduction or NH₄⁺ over-oxidation by combining cathodic NO₃^-/NO₂^- reduction with anodic COD/NH₄⁺ oxidation. The activity and selectivity performances toward pollutant removal of the electrode materials were investigated by electrochemical measurements and constant potential electrolysis, suggesting that Ti electrode exhibited the best NO₃^-/NO₂^- reduction and N₂ production efficiencies. In-situ Fourier transform infrared spectroscopy was used to study the in-situ electrochemical information of pollutants conversion on electrode surfaces and propose their reaction pathways. The effects of main operating parameters (i.e., initial pH value, Cl^- concentration, and current density) on the removal efficiencies of COD and TN were studied. Under optimal conditions, COD and TN removal efficiencies from simulated wastewater reached 92.7% and 82.0%, respectively. Additionally, reaction kinetics were investigated to describe the COD and TN removal. Results indicated that COD removal followed pseudo-first-order model; meanwhile, TN removal followed zero-order kinetics with a presence of NH₄⁺ and then followed pseudo-first-order kinetics when NH₄⁺ was completely removed. For actual pharmaceutical wastewater treatment, 79.1% COD and 87.0% TN were removed after 120 min electrolysis; and no NH₄⁺ or NO₂^- was detected.

Catalytic oxidation and deionization of nitrite and nitrate ions using mesoporous carbon-supported nano-flaky cobalt and nickel oxyhydroxides

The composite electrode of NiCo oxide supported by porous carbon was synthesized for nitrite oxidation and nitrate electro-sorption. The crystal structure and chemical state of the Co and Ni oxyhydroxides which were precipitated on loofah-derived activated carbon (AC) using hypochlorite were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and BET surface area. The voltammetry showed that the redox couple of Co(II)/Co(III) and Ni(II)/Ni(III) as the mediator catalytically transferred the electrons of NO₂^-/NO₃^-; the Ni site had a relatively high transfer coefficient and diffusive current, while the Co site was better in the capacitive removal of the nitrite and nitrate compounds. A batch electrolysis of nitrite ions was operated under constant anodic potential mode (0 to + 1.5 V vs. Ag/AgCl) to assess the performance of the composite electrodes. The adsorption capacity of NiCo/AC (Ni = 5% and Co = 5% on AC by weight) was 23.5 mg-N g^-1, which was twice that of AC substrate (7.5 mg-N g^-1), based on a multilayer adsorption model. The steady-state kinetics of the consecutive reaction were derived to determine the rate steps of the electrochemical oxidation of NO₂^- and adsorption of NO₃^-.

Copper-Catalyzed Electrosynthesis of Nitrite and Nitrate from Ammonia: Tuning the Selectivity via an Interplay Between Homogeneous and Heterogeneous Catalysis

Electrocatalytic oxidation of ammonia is an appealing, low-temperature process for the sustainable production of nitrites and nitrates that avoids the formation of pernicious N₂ O and can be fully powered by renewable electricity. Currently, however, the number of known efficient catalysts for such a reaction is limited. The present work demonstrates that copper-based electrodes exhibit high electrocatalytic activity and selectivity for the NH₃ oxidation to NO₂ ^- and NO₃ ^- in alkaline solutions. Systematic investigation of the effects of pH and potential on the kinetics of the reaction using voltammetric analysis andin situ Raman spectroscopy suggest that ammonia electrooxidation on copper occurrs via two primary catalytic mechanisms. In the first pathway, NH₃ is converted to NO₂ ^- via a homogeneous electrocatalytic process mediated by redox transformations of aqueous [Cu(OH)₄ ]^-/2- species, which dissolve from the electrode. The second pathway is the heterogeneous catalytic oxidation of NH₃ on the electrode surface favoring the formation of NO₃ ^- . By virtue of its nature, the homogeneous-mediated pathway enables higher selectivity and was less affected by electrode poisoning with the strongly adsorbed "N" intermediates that have plagued the electrocatalytic ammonia oxidation field. Thus, the selectivity of the Cu-catalyzed NH₃ oxidation towards either nitrite or nitrate can be achieved through balancing the kinetics of the two mechanisms by adjusting the pH of the electrolyte medium and potential.

Defect-Engineered β-MnO2-δ Precursors Control the Structure-Property Relationships in High-Voltage Spinel LiMn1.5Ni0.5O4-δ

This study examines the role of defects in structure-property relationships in spinel LiMn_1.5Ni_0.5O₄ (LMNO) cathode materials, especially in terms of Mn³⁺ content, degree of disorder, and impurity phase, without the use of the traditional high-temperature annealing (≥700 °C used for making disordered LMNO). Two different phases of LMNO (i.e., highly P4₃32-ordered and highly Fd3̅m-disordered) have been prepared from two different β-MnO_2-δ precursors obtained from an argon-rich atmosphere (β-MnO_2-δ (Ar)) and a hydrogen-rich atmosphere [β-MnO_2-δ (H₂)]. The LMNO samples and their corresponding β-MnO_2-δ precursors are thoroughly characterized using different techniques including high-resolution transmission electron microscopy, field-emission scanning electron microscopy, Raman spectroscopy, powder neutron diffraction, X-ray photoelectron spectroscopy, synchrotron X-ray diffraction, X-ray absorption near-edge spectroscopy, and electrochemistry. LMNO from β-MnO_2-δ (H₂) exhibits higher defects (oxygen vacancy content) than the one from the β-MnO_2-δ (Ar). For the first time, defective β-MnO_2-δ has been adopted as precursors for LMNO cathode materials with controlled oxygen vacancy, disordered phase, Mn³⁺ content, and impurity contents without the need for conventional methods of doping with metal ions, high synthetic temperature, use of organic compounds, postannealing, microwave, or modification of the temperature-cooling profiles. The results show that the oxygen vacancy changes concurrently with the degree of disorder and Mn³⁺ content, and the best electrochemical performance is only obtained at 850 °C for LMNO-(Ar). The findings in this work present unique opportunities that allow the use of β-MnO_2-δ as viable precursors for manipulating the structure-property relationships in LMNO spinel materials for potential development of high-performance high-voltage lithium-ion batteries.

Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis

Urea electrolysis is a prospective technology for simultaneous H₂ production and nitrogen suppression in the process of water being used for energy production. Its sustainability is currently founded on innocuous N₂ products; however, we discovered that prevalent nickel-based catalysts could generally over-oxidize urea into NO₂ ^- products with ≈80 % Faradaic efficiencies, posing potential secondary hazards to the environment. Trace amounts of over-oxidized NO₃ ^- and N₂ O were also detected. Using ¹⁵ N isotopes and urea analogues, we derived a nitrogen-fate network involving a NO₂ ^- -formation pathway via OH^- -assisted C-N cleavage and two N₂ -formation pathways via intra- and intermolecular coupling. DFT calculations confirmed that C-N cleavage is energetically more favorable. Inspired by the mechanism, a polyaniline-coating strategy was developed to locally enrich urea for increasing N₂ production by a factor of two. These findings provide complementary insights into the nitrogen fate in water-energy nexus systems.

Optimization of a Hierarchical Porous-Structured Reactor to Mitigate Mass Transport Limitations for Efficient Electrocatalytic Ammonia Oxidation through a Three-Electron-Transfer Pathway

Regulation of fast three-electron-transfer processes for electrocatalytic oxidation of ammonia to nitrogen by achieving efficient generation and utilization of active sites is the optimal strategy in ammonia-containing wastewater treatment. However, the limited number of accessible active sites and sluggish interfacial mass transfer are two main bottlenecks restricting conventional ammonia oxidation configurations. Herein, we develop a macroporous Ni foam electrode integrated with vertically aligned two-dimensional mesoporous Ni₂P nanosheets to create sufficient exposure of active centers. A novel ammonia oxidation reactor with the developed hierarchical porous-structured electrodes was assembled to construct an intensified microfluidic process with flow-through operation to mitigate macroscopic mass transport limitations. The confined microreaction space in the hierarchical porous reactor further promotes spontaneous nanoscale diffusion/convection of the target contaminant to high-valence Ni sites and enhances the microscopic mass transfer. The combined results of electrochemical measurements and in situ Raman spectra showed that the ammonia degradation mechanism results from direct oxidation by the high-valence Ni, significantly different from the conventional indirect active-chlorine-species-mediated oxidation. The optimized reactor achieves high-efficiency three-electron-transfer ammonia conversion with an ammonia removal efficiency of ∼70% from an initial concentration of ∼1400 mg/L and byproduct production of ∼4%, significantly superior to a conversion unit comprising a featureless Ni-based electrode in the immersed configuration, which had >50% byproduct yield. 20 days of continuous operation under variable conditions achieved >90% ammonia degradation performance and an energy consumption of 25.42 kW h kg^-1 N (1 order of magnitude lower than the active-chlorine-mediated process), showing the potential of the reactor in medium-concentration ammonia-containing wastewater treatment.

Ni- and Cu-co-Intercalated Layered Manganese Oxide for Highly Efficient Electro-Oxidation of Ammonia Selective to Nitrogen

We fabricated a thin film of layered MnO₂ whose interlayer space was occupied by hydrated Ni²⁺ and Cu²⁺ ions. The process consisted of electrodeposition of layered MnO₂ intercalated with tetrabutylammonium cations (TBA⁺) by anodic oxidation of aqueous Mn²⁺ ions in the presence of TBA⁺, followed by ion exchange of the initially incorporated bulkier TBA⁺ with the denser transition metals in solution. The resulting layered MnO₂ co-intercalated with Ni²⁺ and Cu²⁺ ions (NiCu/MnO₂) catalyzed the ammonia oxidation reaction (AOR) in an alkaline electrolyte with a much lower overpotential than its Ni²⁺- and Cu²⁺-intercalated single-cation counterparts. Surprisingly, the NiCu/MnO₂ electrode achieved a faradic efficiency as high as nearly 100% (97.4%) for nitrogen evolution at a constant potential of +0.6 V vs Hg/HgO. This can be ascribed to the occurrence of the AOR in the potential region where water is stable and dimerization of the partially dehydrogenated ammonia species is preferred, thereby forming an N-N bond, rather than to be further oxidized into NO_x species.

A highly activated iron phosphate over-layer for enhancing photoelectrochemical ammonia decomposition

Environmentally friendly ammonia (NH₃) decomposition has attracted a lot of interests in recent years to resolve the issue of water eutrophication from a wastewater and achieve a clean H₂ storage. Here, we report a novel strategy for solar-driven ammonia decomposition by introducing a highly-activated iron phosphate (FePi) over-layer on the surface of α-Fe₂O₃ nanorods photoanode (FePi/Fe₂O₃), and innovatively propose a photoelectrochemical (PEC) ammonia degradation system with enhanced performance. After a facile electrochemical (EC) activation, the FePi over-layer is converted into FeOOH. The EC-activated over-layer provides the efficient active sites for the ammonia adsorption process, which promotes the high catalytic kinetics for ammonia oxidation reaction (AOR). Due to the synergistic effect of the electrocatalytic and the photocatalytic process, the FePi/Fe₂O₃ exhibits the enhanced PEC AOR performance, which competes with water oxidation reaction (WOR). Comparing to the initial concentration of ammonia, the FePi/Fe₂O₃ achieves a 54.4% ammonia degradation rate within 3 h at 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, which demonstrates the reliable ammonia decomposition performance. This study confirms that it is feasible to achieve PEC ammonia decomposition in an aqueous solution without chloride mediators and provides a promising strategy for the harmless treatment of ammonia wastewater.

Mitigating Bubble Traffic in Gas-Evolving Electrodes via Spinodally Derived Architectures

Porous electrodes are widely used in the industry because of their high surface area to volume ratio. However, the stochastic morphology of most commercially available porous electrodes results in poor electrical connections in the solid phase and inefficient mass transport through the pore phase. This can be especially detrimental for gas-evolving processes such as water electrolysis for hydrogen and oxygen generation. Bicontinuous interfacially jammed emulsion gels (bijels) offer templates from which to create porous electrodes with robust solid-state interconnectivity and a uniform pore structure that facilitate improved electron and mass transport. In this study, gas release rates and electrochemical experiments are utilized to study the effects of powder- and bijel-derived microstructures on hydrogen generation by water electrolysis. The bijel-derived electrodes are shown to expel product gas faster and require up to 25% less overpotential to drive water electrolysis over the range of current densities tested (-5 to -40 mA/cm²) than their powder-derived analogs. Our findings suggest that the uniform and bicontinuous domains of bijel-derived porous electrodes can mitigate the limited current distribution and deleterious bubble effect found in stochastic electrodes, in turn improving the overall performance of electrolytic processes requiring transport of gaseous species.

Structure and electrochemical activity of nickel aluminium fluoride nanosheets during urea electro-oxidation in an alkaline solution

An electrocatalyst of potassium nickel aluminium hexafluoride (KNiAlF₆) nanosheets has been prepared using solid-phase synthesis at 900 °C. X-ray diffraction, scanning electron microscopy, and conductivity studies confirmed the formation of KNiAlF₆ nanosheets having a cubic defect pyrochlore structure with an average thickness of 60–70 nm and conductivity of 1.297 × 10³ S m⁻¹. The electrochemical catalytic activity of the KNiAlF₆ nanosheets was investigated for urea oxidation in alkaline solution. The results show that the KNiAlF₆ nanosheets exhibit a mass activity of ∼395 mA cm⁻² mg⁻¹ at 1.65 V vs. HRE, a reaction activation energy of 4.02 kJ mol⁻¹, Tafel slope of 22 mV dec⁻¹ and an oxidation onset potential of ∼1.35 V vs. HRE which is a significant enhancement for urea oxidation when compared with both bulk Ni(OH)₂ and nickel hydroxide-based catalysts published in the literature. Chronoamperometry and impedance analysis of the KNiAlF₆ nanosheets reveal lower charge transfer resistance and long-term stability during the prolonged urea electro-oxidation process, particularly at 60 °C. After an extended urea electrolysis process, the structure and morphology of the KNiAlF₆ nanosheets were significantly changed due to partial transformation to Ni(OH)₂ but the electrochemical activity was sustained. The enhanced electrochemical surface area and the replacement of nickel in the lattice by aluminium make KNiAlF₆ nanosheets highly active electrocatalysts for urea oxidation in alkaline solution.

An electrocatalyst of potassium nickel aluminium hexafluoride (KNiAlF₆) nanosheets for enhanced urea oxidation reaction.

One-step synthesis of carbon nanospheres with an encapsulated iron-nickel nanoalloy and its potential use as an electrocatalyst

For many years, in electrochemical processes, carbon nanostructures with metal support have been employed as electrodes due to their high surface area, chemical stability, and excellent performance as catalyst support by allowing a better electronic transfer. Nevertheless, on the surface, metallic nanoparticles are susceptible to corrosion. Instead, by encapsulating individual nanoparticles, they are protected. Among the carbon nanostructures, the most common are graphene, carbon nanotubes (CNTs), and carbon nanospheres (CNSs). Unlike CNTs and CNSs, graphene is difficult to obtain in mass production, limiting their applications. Regarding CNTs and CNSs, the latter presents better catalytic activity. Nonetheless, the process of synthesis of CNSs with metal inside is commonly made by time-consuming autoclave processes, some involving more than 43 h, and hence are expensive. Here, we suggest an advantageous synthesis of CNSs with an iron-nickel alloy encapsulated inside, by using a one-step chemical vapor deposition (CVD) process in less than 3 h. This material has potential applications for environmental and energy processes. According to the authors, the uses of iron-nickel alloys as an electrocatalyst for the ammonia oxidation reaction has not been proved. Thus, we evaluate the composite as an electrocatalyst for the ammonia oxidation reaction, an electrochemical process that offers environmental remediation and hydrogen as a fuel. The electrochemical characterization shows that the use of a bimetallic electrode improves the catalytic activity. In this case, nickel is the active specie and iron is the metal added which reduces the reaction potential. Besides, the composite presents high specific capacitance, better than other materials proposed such as graphene decorated with FeNi alloys. This behavior can be related to the variation of the catalyst morphology (supported vs. encapsulated) by improving the catalyst dispersion and particle size stabilization.

In-situ membrane fouling control by electrooxidation and microbial community in membrane electro-bioreactor treating aquaculture seawater

Ammonia and nitrite in aquaculture recirculating seawater need to be strictly controlled to avoid deleterious effects on aquatic organisms. However, traditional biological approach can hardly meet the standard due to the short hydraulic retention time (HRT) and nitrite accumulation. A Membrane Electro-Bioreactor (MEBR) was developed for ammonia removal enhancement and in-situ electrochemical membrane fouling mitigation. The fouling mechanism was first found to proceed via the standard filtration model. The flux decrease was mainly caused by an internal pore clogging phenomenon. Membrane fouling resistance was enhanced by increasing anode potential from 0 to 1.4 V vs. SCE (Saturated Calomel Electrode). The ammonia removal rate in the MEBR was above 95% (HRT: 2 h, after day-13) and membrane fouling was mitigated that operation duration was extended by 71.4%. Higher total proportion of Proteobacteria, Bacteroidetes, Planctomycetes and Actinobacteria was obtained in the MEBR, suggesting higher nitrification and nitrogen removal potentials.

Anodic hydrazine electrooxidation boosted overall water electrolysis by bifunctional porous nickel phosphide nanotubes on nickel foam

Water electrolysis is an environmentally friendly and sustainable technique for ultra-pure hydrogen production, while expensive electrode materials and high driving voltage have seriously hindered its commercialization process. Here, Earth-abundant bifunctional porous Ni2P hollow nanotubes on nickel foam (Ni2P-HNTs/NF) electrocatalysts are synthesized through a facile self-template method and a phosphating process, which are perfectly combined with the hydrazine electrooxidation reaction (HzOR) boosted water electrolysis. Benefiting from the unique structural characteristic of open-framework and abundant step atoms, Ni2P-HNTs/NF achieves 10 mA cm-2 at 91 mV (vs. RHE) for the cathodic hydrogen evolution reaction and 18 mV (vs. RHE) for the anodic HzOR in a three electrode system, respectively. The corresponding two-electrode hydrazine electrolyzer produces 10 mA cm-2 with a total voltage of only 152 mV for ultra-pure hydrogen production, highlighting a cost-effective and energy-saving water electrolysis mode.

Nickel supported on carbon nanotubes and carbon nanospheres for ammonia oxidation reaction

Here it is proposed the use of nickel supported on carbon nanotubes and carbon nanospheres as a cheaper catalyst alternative to the platinum electrodes for the oxidation process of ammonia. Using scanning electron microscopy and Raman spectroscopy confirmed the presence of the Ni on the surface of the carbon nanostructures and based on the electrochemical results is established that the redox process between Ni⁺² and Ni⁺³ plays the role of intermediate in the ammonia oxidation reaction. This process causes an increment in the anodic current density that is dependent on the pH and ammonia concentration. Even more, using the rotating disk electrode technique, the ammonia oxidation, is defined as a diffusion-controlled process that follows first-order kinetics in respect of the concentration of ammonia and suggests the formation of nitrogen as the principal reaction product, where the carbon nanospheres present the best results.

Loofah-derived activated carbon supported on nickel foam (AC/Ni) electrodes for the electro-sorption of ammonium ion from aqueous solutions

Activated carbon (AC), prepared from dried loofah sponge, was supported on nickel foam to fabricate AC/Ni electrodes. The characteristics of ammonium electrosorption on AC/Ni electrodes was studied. Results showed that AC prepared in one-step activation (without pre-pyrolysis), i.e., OAC, had relatively low crystallinity, high mesoporosity, and high specific capacitance compared to those made in two-step carbonation followed by activation. Adsorption and desorption density of NH₄⁺ were measured at constant potential of -1.0 V (vs. Hg/HgO) and +0.1 V (vs. Hg/HgO), respectively. Non-faradaic charging contributed to the electrochemical storage and adsorption of ammonium ions on the AC surface with a maximal charge efficiency of 80%, at an applied potential of -1.0 V (vs. Hg/HgO). Multiple-layer adsorption isotherm better described the electrosorption of ammonium ion on OAC/Ni electrodes yielding a maximum adsorption capacity of 6 mg-N g^-1, which was comparable with other similar systems. Overall, results clearly demonstrated the effect of synthesis strategy on the capacitive charging behaviors of AC/Ni electrodes and its relationship to NH₄⁺ electrosorption.

Process Optimization of Electrochemical Oxidation of Ammonia to Nitrogen for Actual Dyeing Wastewater Treatment

To mitigate the potential environmental risks caused by nitrogen compounds from industrial wastewater, residual ammonia after conventional wastewater treatment should be further eliminated. In this work, an electrochemical oxidation process for converting ammonia to nitrogen in actual dyeing wastewater was investigated. The effects of the main operating parameters, including initial pH value, applied current density, NaCl concentration, and flow, were investigated on ammonia removal and products distribution. Experimental results indicated that, under optimal conditions of an initial pH value of 8.3, applied current density of 20 mA cm-2, NaCl concentration of 1.0 g L-1, and flow of 300 mL min-1, the ammonia could be completely removed with N2 selectivity of 88.3% in 60 min electrolysis. A kinetics investigation using a pseudo-first-order model provided a precise description of ammonia removal during the electro-oxidation process. Experimental functions for describing the relationships between kinetic constants of ammonia removal and main operating parameters were also discussed. Additionally, the mechanisms and economic evaluation of ammonia oxidation were conducted. All these results clearly proved that this electro-oxidation process could efficiently remove ammonia and achieve high N2 selectivity.

NiFe Alloy Nanotube Arrays as Highly Efficient Bifunctional Electrocatalysts for Overall Water Splitting at High Current Densities

A bubble-releasing assisted pulse electrodeposition method was developed to create metallic alloy, NiFe, nanotube arrays in one step. The NiFe alloy nanotube array exhibited excellent bifunctional electrolytic activities, achieving low overpotentials of 100 mV for the hydrogen evolution reaction and 236 mV for the oxygen evolution reaction at 10 mA cm^-2, both in 1 M KOH at room temperature. For overall water splitting, the NiFe alloy nanotube array delivered 10 mA cm^-2 at an ultralow cell voltage of 1.58 V, among the top tier of the state-of-the-art bifunctional electrocatalysts. The NiFe alloy nanotube array also exhibited ultrastability at high current densities, experiencing only a minor chronoamperometric decay of 6.5% after a 24 h operation at 400 mA cm^-2. The success of the present binder-free nanotube array-based electrode can be attributed to the much enlarged reaction surface area, one-dimensionally guided charge transport and mass transfer offered by the nanotube structure, and improved product crystallinity provided by the pulse current electrodeposition. The nanotube array structure proves to be a promising new architecture design for electrocatalysts.

Monitoring compositional changes in Ni(OH)2 electrocatalysts employed in the oxygen evolution reaction

Relating morphology and compositional changes spatially across a catalyst is important for understanding the active site involved in a reaction which is studied here for the OER at Ni(OH)₂.