Facile, Controllable, and Ultrathin NiFe-LDH In Situ Grown on a Ni Foam by Ultrasonic Self-Etching for Highly Efficient Urine Conversion

Vacancy-rich NiFe-LDH/carbon paper as a novel self-supporting electrode for the electro-Fenton degradation of polyvinyl chloride microplastics

Electrochemically upcycling polyvinyl chloride (PVC) into high-value small molecules represents a sustainable strategy for mitigating plastic pollution. Herein, a cost-effective self-supporting electrode with abundant vacancies, i.e., NiFe-layered double hydroxide nanoarrays in-situ grown on the surface of carbon paper (denoted as NiFeV-LDH/CP), is developed for the electro-Fenton degradation of PVC microplastics (MPs). The NiFeV-LDH catalyst shows a high selectivity of 76 % towards H2O2 production via two-electron oxygen reduction reaction (2e- ORR). Density functional theory (DFT) calculations reveal that the energy barrier of rate-determining step (*H2O2 desorption) decreases over the vacancy-enriched NiFeV-LDH related to the pristine NiFeZn-LDH. The influence of vacancy concentration, reaction temperature and initial concentration of PVC MPs were systematically investigated. Under optimized conditions, the NiFeV-LDH/CP electrode exhibits an outstanding degradation performance of PVC MPs via direct cathodic reduction and oxidation by hydroxyl radicals. This work demonstrates that the electro-Fenton technology using LDH-based self-supporting electrodes is a promising and environmentally-friendly approach for waste plastic treatment.

Synergistic etching of nickel foam by Fe3+ and Cl- ions to synthesize nickel-iron-layered double hydroxide nanolayers with abundant oxygen vacancies for superior urea oxidation

Urea electrolysis is an appealing topic for hydrogen production due to its ability to extract hydrogen at a lower potential. However, it is plagued by sluggish kinetics and noble-metal catalyst requirements. Herein, we developed nickel-iron-layered double hydroxide (NiFe-LDH) nanolayers with abundant oxygen vacancies (OV) via synergistically etching nickel foam with Fe3+ and Cl- ions, enabling the efficient conversion of urea into H2 and N2. The synthesized OV-NiFe-LDH exhibits a lower potential (1.30 vs. reversible hydrogen electrode, RHE) for achieving 10 mA cm-2 in the urea oxidation reaction (UOR), surpassing most recently reported Ni-based electrodes. OV provides favorable conductivity and a large surface area, which results in a 4.1-fold in electron transport and a 5.1-fold increase in catalyst reactive sites. Density Functional Theory (DFT) calculations indicate that OV can lower the adsorption energy of urea, and enhance the bonding strength of *CONHNH, giving rise to improved UOR. This study provides a viable path toward economical and efficient production of high-purity hydrogen.

Highly selective urea electrooxidation coupled with efficient hydrogen evolution

Electrochemical urea oxidation offers a sustainable avenue for H2 production and wastewater denitrification within the water-energy nexus; however, its wide application is limited by detrimental cyanate or nitrite production instead of innocuous N2. Herein we demonstrate that atomically isolated asymmetric Ni-O-Ti sites on Ti foam anode achieve a N2 selectivity of 99%, surpassing the connected symmetric Ni-O-Ni counterparts in documented Ni-based electrocatalysts with N2 selectivity below 55%, and also deliver a H2 evolution rate of 22.0 mL h-1 when coupled to a Pt counter cathode under 213 mA cm-2 at 1.40 VRHE. These asymmetric sites, featuring oxygenophilic Ti adjacent to Ni, favor interaction with the carbonyl over amino groups in urea, thus preventing premature resonant C⎓N bond breakage before intramolecular N-N coupling towards N2 evolution. A prototype device powered by a commercial Si photovoltaic cell is further developed for solar-powered on-site urine processing and decentralized H2 production.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

A moderate method for in situ growing Fe-based LDHs on Ni foam for catalyzing the oxygen evolution reaction

Fe-based LDHs have been proven to be an excellent class of catalysts for the oxygen evolution reaction (OER). To achieve industrial applications of water splitting, it is critical to develop a cost-effective and simple strategy to achieve large-area catalytic electrodes. Herein, we present a moderate in situ method for growing Fe-based layered double hydroxide nanosheets on a Ni foam (LDH@NF) substrate at room temperature. Through systematic experimental design characterization, it is found that this in situ growth process is mainly driven by moderate oxidation of Fe2+ in an O2-dissolved solution, the consequent local alkaline environment, and abundant TM2+ ions (Ni2+, Co2+, Ni2+/Co2+). Compared with other in situ methods, this method is not accompanied by violent redox reactions and is favorable for the uniform growth of LDHs, and the composition of the catalyst can be easily regulated. Specifically, the optimized NiFe-LDH@NF catalyst demonstrates excellent catalytic performance in the alkaline water oxidation reaction with a low overpotential of 206/239 mV at a current density of 10/100 mA cm-2, respectively.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.