It is of great importance to design and develop electrocatalysts that are both long-lasting and efficient for seawater oxidation. Herein, a three-dimensional porous cauliflower-like Ni3S2 foam on Ni foam (Ni3S2 foam/NF) is proposed as a high-performance electrocatalyst for the oxygen evolution reaction in alkaline seawater. The as-synthesis Ni3S2 foam/NF achieves exceptional efficacy, achieving a current density of 100 mA·cm−2 at mere overpotential of 369 mV. Notably, its electrocatalytic stability extends up to 1000 h at 500 mA·cm−2.
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Seawater electrolysis, especially in coastlines, is widely considered as a sustainable way of making clean and high-purity H2 from renewable energy; however, the practical viability is challenged severely by the limited anode durability resulting from side reactions of chlorine species. Herein, we report an effective Cl− blocking barrier of NiFe-layer double hydroxide (NiFe-LDH) to harmful chlorine chemistry during alkaline seawater oxidation (ASO), a pre-formed surface-derived NiFe-phosphate (Pi) outer-layer. Specifically, the PO43−-enriched outer-layer is capable of physically and electrostatically inhibiting Cl− adsorption, which protects active Ni3+ sites during ASO. The NiFe-LDH with the NiFe-Pi outer-layer (NiFe-LDH@NiFe-Pi) exhibits higher current densities (j) and lower overpotentials to afford 1 A·cm−2 (η1000 of 370 mV versus η1000 of 420 mV) than the NiFe-LDH in 1 M KOH + seawater. Notably, the NiFe-LDH@NiFe-Pi also demonstrates longer-term electrochemical durability than NiFe-LDH, attaining 100-h duration at the j of 1 A·cm−2. Additionally, the importance of surface-derived PO43−-enriched outer-layer in protecting the active centers, γ-NiOOH, is explained by ex situ characterizations and in situ electrochemical spectroscopic studies.
Advancing efficient and affordable electrocatalysts to boost the oxygen evolution reaction (OER) is pivotal for sustainable green hydrogen production. Herein, we propose the fabrication of nickel-iron alloy nanoparticles-encapsulated on N-doped vertically aligned graphene array on carbon cloth (NiFe@NVG/CC) as a highly active three-dimensional (3D) catalyst electrode for OER. In 1 M KOH, such NiFe@NVG/CC demonstrates outstanding catalytic performance, necessitating merely overpotential of 245 mV for achieving a current density of 10 mA·cm−2, a remarkably low Tafel slope of 36.2 mV·dec−1. Furthermore, density functional theory calculations validate that the incorporate of N species into graphene can reinforce the electrocatalytic activity though reducing the reaction energy barrier during the conversion of *O to *OOH intermediates. The outstanding performance and structural benefits of NiFe@NVG/CC offer valuable insights for the development of innovative and efficient electrocatalysts for water oxidation.
Nitrate (NO3−), a nitrogen-containing pollutant, is prevalent in aqueous solutions, contributing to a range of environmental and health-related issues. The electrocatalytic reduction of NO3− holds promise as a sustainable approach to both eliminating NO3− and generating valuable ammonia (NH3). Nevertheless, the reduction reaction of NO3− (NO3−RR), involving 8-electron transfer process, is intricate, necessitating highly efficient electrocatalysts to facilitate the conversion of NO3− to NH3. In this study, Fe-doped Co3O4 nanowire strutted three-dimensional (3D) pinewood-derived carbon (Fe-Co3O4/PC) is proposed as a high-efficiency NO3−RR electrocatalyst for NH3 production. Operating within 0.1 M NaOH containing NO3−, Fe-Co3O4/PC demonstrates exceptional performance, obtain an impressively large NH3 yield of 0.55 mmol·h−1·cm−2 and an exceptionally high Faradaic efficiency of 96.5% at −0.5 V, superior to its Co3O4/PC counterpart (0.2 mmol·h−1·cm−2, 73.3%). Furthermore, the study delves into the reaction mechanism of Fe-Co3O4 for NO3−RR through theoretical calculations.
Nitrate (NO3−) removal by photochemical-reduction has received extensive attention. However, the low selectivity of NO3− reduction to N2 hinders the application of this technology. In this study, a novel Cu@Fe-Cu-CuFe2O4−x photocatalyst was prepared by modifying CuFe2O4 with KBH4 and Cu(II), and used to selectively reduce NO3− to N2 with a two-step reduction process. In step (1), with Cu@Fe-Cu-CuFe2O4−x/ultraviolet (UV) system, 91.0% NO3− was reduced to 52.3% NO2− and 39.4% N2 within 60 min. The rapid removal of NO3− was due to the synergistic effect of oxygen vacancies, Fe-Cu corrosion cell, and CuFe2O4 photocatalysis. In step (2), H2C2O4 and H2O2 were introduced into the effluent of step (1) to promote CO2·− formation via Fe(II) and Fe(III) catalysis and UV radiation, which boosted the selective reduction of NO2− to N2. When H2C2O4 and H2O2 dosages were both 4.0 mmol·L−1 and the reaction time was 30 min, the removal efficiency of NO2− achieved 100% and the selectivity of N2 was 83.0%. Overall, the two-step reduction process achieved 95.0% NO3− removal efficiency and 90.1% N2 selectivity with initial NO3− concentration of 30 mg·N·L−1. In addition, the denitrification mechanism of the two-step reduction process was tentatively proposed.
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Seawater electrolysis is an extremely attractive approach for harvesting clean hydrogen energy, but detrimental chlorine species (i.e., chloride and hypochlorite) cause severe corrosion at the anode. Here, we report our recent finding that benzoate anions-intercalated NiFe-layered double hydroxide nanosheet on carbon cloth (BZ-NiFe-LDH/CC) behaves as a highly efficient and durable monolithic catalyst for alkaline seawater oxidation, affords enlarged interlayer spacing of LDH, inhibits chlorine (electro)chemistry, and alleviates local pH drop of the electrode. It only needs an overpotential of 320 mV to reach a current density of 500 mA·cm–2 in 1 M KOH. In contrast to the fast activity decay of NiFe-LDH/CC counterpart during long-term electrolysis, BZ-NiFe-LDH/CC achieves stable 100-h electrolysis at an industrial-level current density of 500 mA·cm–2 in alkaline seawater. Operando Raman spectroscopy studies further identify structural changes of disordered δ (NiIII-O) during the seawater oxidation process.
Seawater electrolysis is the most promising technology for large scale hydrogen production due to the abundance and low cost of seawater in nature. However, compared with the traditional freshwater electrolysis, the issues of electrode poisoning and corrosion will occur during the seawater electrolysis process, and active and stable electrocatalysts for the hydrogen evolution reaction (HER) are thus highly desired. In this work, N, O-doped carbon foam in-situ derived from commercial melamine foam is proposed as a high-active metal-free HER electrocatalyst for seawater splitting. In acidic seawater, our catalyst shows high hydrogen generation performance with small overpotential of 161 mV at 10 mA·cm−2, a low Tafel slop of 97.5 mV·dec−1, and outstanding stability.
Electrocatalytic nitrate reduction reaction (NO3−RR) emerges as a highly efficient approach toward ammonia synthesis and degrading NO3− contaminant. In our study, CeO2 nanoparticles with oxygen vacancies (VO) decorated N-doped carbon nanorods on graphite paper (CeO2−x@NC/GP) were demonstrated as a highly efficient NO3−RR electrocatalyst. The CeO2−x@NC/GP catalyst manifests a significant NH3 yield up to 712.75 μmol·h−1·cm−2 at −0.8 V vs. reversible hydrogen electrode (RHE) and remarkable Faradaic efficiency of 92.93% at −0.5 V vs. RHE under alkaline conditions, with excellent durability. Additionally, an assembled Zn-NO3− battery with CeO2−x@NC/GP as cathode accomplishes a high-power density of 3.44 mW·cm−2 and a large NH3 yield of 145.08 μmol·h−1·cm−2. Density functional theory results further expose the NO3− reduction mechanism on CeO2 (111) surface with VO.
Ambient electroreduction of nitrogen (N2) is considered as a green and feasible approach for ammonia (NH3) synthesis, which urgently demands for efficient electrocatalyst. Morphology has close relationship with catalytic activity of heterogeneous catalysts. Nanoribbon is attractive nanostructure, which possesses the flexibility of one-dimensional nanomaterials, the large surface area of two-dimensional nanomaterials, and lateral size confinement effects. In this work, Cu3P nanoribbon is proposed as a highly efficient electrocatalyst for N2-to-NH3 conversion under benign conditions. When measured in N2-saturated 0.1 M HCl, such Cu3P nanoribbon achieves high performance with an excellent Faradaic efficiency as high as 37.8% and a large yield of 18.9 µg·h−1·mgcat.−1 at −0.2 V. It also demonstrates outstanding stability in long-term electrolysis test at least for 45 h.
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To restore the natural nitrogen cycle (N-cycle), artificial N-cycle electrocatalysis with flexibility, sustainability, and compatibility can convert intermittent renewable energy (e.g., wind) to harmful or value-added chemicals with minimal carbon emissions. The background of such N-cycles, such as nitrogen fixation, ammonia oxidation, and nitrate reduction, is briefly introduced here. The discussion of emerging nanostructures in various conversion reactions is focused on the architecture/compositional design, electrochemical performances, reaction mechanisms, and instructive tests. Energy device advancements for achieving more functions as well as in situ/operando characterizations toward understanding key steps are also highlighted. Furthermore, some recently proposed reactions as well as less discussed C–N coupling reactions are also summarized. We classify inorganic nitrogen sources that convert to each other under an applied voltage into three types, namely, abundant nitrogen, toxic nitrate (nitrite), and nitrogen oxides, and useful compounds such as ammonia, hydrazine, and hydroxylamine, with the goal of providing more critical insights into strategies to facilitate the development of our circular nitrogen economy.