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

Abstract

Active chlorine species-mediated electrocatalytic oxidation is a promising strategy for ammonia removal in decentralized wastewater treatment. Flow-through electrodes (FTEs) provide an ideal platform for this strategy because of enhanced mass transport and sufficient electrochemically accessible sites. However, limited insight into spatial distribution of electrochemically accessible sites within FTEs inhibits the improvement of reactor efficiency and the reduction of FTE costs. Herein, a microfluidic-based electrochemical system is developed for the operando observation of microspatial reactions within pore channels, which reveals that reactions occur only in the surface layer of the electrode thickness. To further quantify the spatial distribution, finite element simulations demonstrate that over 75.0% of the current is accumulated in the 20.0% thickness of the electrode surface. Based on these findings, a gradient-coated method for the active layer was proposed and applied to a Ti/RuO2 porous electrode with an optimized pore diameter of ∼25 μm, whose electrochemically accessible surface area was 381.7 times that of the planar electrode while alleviating bubble entrapment. The optimized reactor enables complete ammonia removal with an energy consumption of 60.4 kWh kg–1 N, which was 24.2% and 39.9% less than those with pore diameters of ∼3 μm and ∼90 μm, respectively.

A Bipolar Membrane-Integrated Electrochlorination Process for Highly Efficient Ammonium Removal in Mature Landfill Leachate: The Importance of ClO• Generation

“…Previous studies have shown that the reactions between FC and Cl • also contribute to the formation of ClO • (eqs and ). , Its high reactivity and selectivity toward ammonium oxidation to nitrogen even at acidic pH values is noteworthy (eq ). ,, Small amounts of nitrate resulting from HClO-mediated ammonium oxidation (eq ) are directed to the cathode via internal recycling and expected to be reduced by the electrons and/or adsorbed H (H*) from the cathode (eqs and ). , H 2 O → H + + OH − OH − + MO x → ≡MO x false( normalHO • false) + e − Cl − + ≡MO x false( normalHO • false) → Cl • + OH − + MO x …”

“…29,53 Its high reactivity and selectivity toward ammonium oxidation to nitrogen even at acidic pH values is noteworthy (eq 13). 16,26,28 Small amounts of nitrate resulting from HClOmediated ammonium oxidation (eq 14) 11 are directed to the cathode via internal recycling and expected to be reduced by the electrons and/or adsorbed H (H*) from the cathode (eqs 15 and 16). 54,55 H O H OH…”

“…Traditional breakpoint chlorination is conducive to ammonium conversion into nitrogen but suffers from limitations associated with the transport, storage, and handling of free chlorine (FC; i.e., HOCl/OCl – /Cl 2 ) and difficulties in controlling the dosage. Given the ubiquitous presence of chloride ion (Cl – ) in landfill leachates, electrochlorination (EC) has emerged as a promising approach to ammonium oxidation, with advantages including in situ generation of active chlorine, ease of automation control, and good versatility and safety. − The EC of ammonium into nitrogen proceeds through the conventional breakpoint chlorination mechanism with the help of reactive chlorine species (RCS) produced from the metal oxide anode. In collaboration with our industry partners, our research group has started up a full-scale electrochemical reactor for mature landfill leachate treatment, which exhibited good carbon removal performance.…”

Enhancement of pore confinement caused by the mosaic structure on Ru nanoparticles for pH-universal hydrogen evolution reaction

“…However, flaky Ru/HMCs-1000 are different from the other spherical Ru/HMCs- x , because this structure is not beneficial for the transport of substances and the diffusion of reactants. The HER becomes the mass transfer limitation at large polarizations, 42 so the activity is affected by the volumetrically averaged mass transfer coefficient. Therefore, the current density in the LSV polarization curve of Ru/HMCs-1000 is lower than that of Ru/HMCs-500 although Ru/HMCs-1000 have a larger electrochemically accessible surface.…”

Key Role of NiCoP Arrays as an Electron Reservoir in a Membrane-Free Electrochemical Reactor for Energy-Saving Hydrogen Recovery and Ammonia Decomposition

“…Electrochemical oxidation is a very promising technology for ammonia wastewater treatment, − but it also has the prominent disadvantage of high energy consumption. , In recent years, many studies have been conducted to solve this problem, including research on a catalytic electrode with high efficiency, , enhancement of the reaction kinetic process, − and optimization of experimental conditions. , However, enhancing the recovery of H 2 via the exploitation of electron acceptors at the cathode is often overlooked, which is meaningful to reduce the energy consumption but increase the wastewater resource utilization. Hence, an advanced electrochemical reactor with simultaneous hydrogen recovery and ammonia remains a vital problem for wastewater treatment.…”