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

Abstract

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.

Visualization of Electrochemically Accessible Sites in Flow-through Mode for Maximizing Available Active Area toward Superior Electrocatalytic Ammonia Oxidation

“…The production of nitrite and nitrate increased with increasing initial Cl – ion concentration (Figure f). Nevertheless, the total production of nitrite and nitrate was less than 2.5% of the initial NH 4 + -N, which was much lower than that of other processes such as direct electrochemical oxidation and photoelectrocatalysis, indicating the superiority of the reaction system. , …”

“…Nevertheless, the total production of nitrite and nitrate was less than 2.5% of the initial NH 4 + -N, which was much lower than that of other processes such as direct electrochemical oxidation and photoelectrocatalysis, indicating the superiority of the reaction system. 3,39 Environmental Implications. Because of the ubiquity of Cl − ions in a wastewater matrix, the active chlorine speciesmediated pathway has been extensively investigated for ammonia oxidation into dinitrogen.…”

“…In recent decades, the removal of ammonia has been a major challenge in wastewater treatment due to offensive odor, aquatic biota toxicity, and eutrophication of receiving waters . Notably, indirect electrocatalytic ammonia oxidation mediated by in situ produced active chlorine species has a promising application in medium-concentration ammonia-containing wastewater (e.g., 500–5000 mg L –1 NH 4 + –N) treatment, due to its easily automated operation, versatility for various pollutants, a lack of need for additional chemicals, and fewer nitrogen-containing byproducts. , However, one of the main obstacles that inhibit its widespread application is the high energy consumption of the electrochemical systems at typical wastewater treatment-relevant Cl – ion concentration and pH . Fundamentally, overall energy consumption of electrocatalytic oxidation is dictated by the intrinsic and extrinsic factors .…”

“…Using a flow-through configuration to overcome these limitations is regarded as a feasible solution and has been demonstrated in electrocatalytic wastewater treatment, such as direct ammonia oxidation, heavy metal detoxication, electro-Fenton, and electrochemical disinfection . A flow-through configuration comprises two parallel three-dimensional porous electrodes perpendicularly permeated by flowing electrolyte.…”

“…8 On the one hand, the insufficient surface area of the planar electrode limits the exposure of electrochemically accessible sites; 9 on the other hand, the mass transport of Cl − ions is severely limited by diffusion through a thick (50− 200 μm) stagnant boundary layer. 10 Using a flow-through configuration to overcome these limitations is regarded as a feasible solution and has been demonstrated in electrocatalytic wastewater treatment, such as direct ammonia oxidation, 3 electro-Fenton, 9 and electrochemical disinfection. 12 A flowthrough configuration comprises two parallel three-dimensional porous electrodes perpendicularly permeated by flowing electrolyte.…”