Toward decentralized wastewater treatment: A flow-through module using microtubular gas diffusion electrodes for micropollutants removal.

Electro-Fenton (EF) represents an eco-friendly and cost-effective advanced oxidation process that can remove highly persistent and hazardous pharmaceuticals, e.g., contrast media agents, from water bodies. However, up to date, EF modules incorporate a planar carbonaceous gas diffusion electrode (GDE) cathode containing fluorinated compounds as polymeric binders. Here, we introduce a novel flow-through module that deploys freestanding carbon microtubes (CMT) as microtubular GDEs, omitting any risks of secondary pollution by highly-persistent fluorinated compounds (e.g., Nafion). The flow-through module was characterized for electrochemical hydrogen peroxide (H2O2) generation and micropollutant removal via EF. H2O2 electro-generation experiments illustrated high production rates (1.1 ± 0.1-2.7 ± 0.1 mg cm-2 h-1) at an applied cathodic potential of - 0.6 V vs. SHE, depending on the porosity of CMTs. Diatrizoate (DTZ), as the model pollutant, with a high initial concentration of 100 mg L-1 was successfully oxidized (95-100 %), reaching mineralization (TOC-total organic carbon removal) efficiencies up to 69 %. Additionally, Electro-adsorption experiments demonstrated the capability of positively charged CMTs to remove negatively charged DTZ with a capacity of 11 mg g-1 from a 10 mg L-1 DTZ solution. These results reveal the potential of the as-designed module to serve as an oxidation unit coupled with other separation techniques, e.g., electro-adsorption or membrane processes.

Titanium-Based Static Mixer Electrodes to Improve the Current Density of Slurry Electrodes.

Complex geometries for electrodes are a great challenge in electrochemical applications. Slurry electrodes have been one example, which use complex flow distributors to improve the charge transfer between the current collector and the slurry particles. Here we use titanium-based flow distributors produced by indirect 3D-printing to improve further the electron transfer from highly conductive flow distributors to the slurry particles for a vanadium redox flow application. The titanium static mixers are directly coated with graphite to increase the activity for vanadium redox reactions. Increasing layers of graphite have shown an optimum for the positive and negative electrolytes. The application of heat treatment on the electrodes improves the anodic and cathodic current peaks drastically. Testing the highly conductive static mixers in a self-made redox flow cell results in 110 mA cm-2 discharge polarization.

A Tubular Electrochemical Reactor for Slurry Electrodes.

The research on electrochemical reactors is mostly limited to planarly designed modules. In this study, we compare a tubular and a planar electrochemical reactor for the utilization of the slurry electrodes. Cylindrical formed geometries demonstrate a higher surface-to-volume ratio, which may be favorable in terms of current density and volumetric power density. A tubular shaped electrochemical reactor is designed with conductive static mixers to promote the slurry particle mixing, and the vanadium redox flow battery is selected as a showcase application. The new tubular design presents similar cell resistances to the previously designed planar battery and shows increased discharge polarization behavior up to 100 mA cm-2. The volumetric power density reaches up to 30 mW cm-3, which is two times higher than that of the planar one. The battery performance is further investigated and 85 % coulombic, 70 % voltage and 60 % energy efficiency is found at 15 mA cm-2 with 15 wt.% slurry content.

On the Resistances of a Slurry Electrode Vanadium Redox Flow Battery.

We studied the half-cell performance of a slurry-based vanadium redox flow battery via the polarization and electrochemical impedance spectroscopy methods. First, the conductive static mixers are examined and lower ohmic and diffusion resistances are shown. Further analyses of the slurry electrodes for the catholyte (VO2+-VO2 +) and anolyte (V3+-V2+) are presented for the graphite powder slurry containing up to 15.0 wt.% particle content. Overall, the anolyte persists as the more resistive half-cell, while ohmic and diffusion-related limitations are the dominating resistances for both electrolytes. The battery is further improved by the addition of Ketjen black nanoparticles, which results in lower cell resistances. The best results are achieved when 0.5 wt.% Ketjen black nanoparticles are dispersed with graphite powder since the addition of nanoparticles reduces ohmic, charge transfer and mass diffusion resistances by improving particle-particle dynamics. The results prove the importance of understanding resistances in a slurry electrode system.

Atomic layer deposition for efficient oxygen evolution reaction at Pt/Ir catalyst layers.

We provide a direct comparison of two distinct methods of Ti felt surface treatment and Pt/Ir electrocatalyst deposition for the positive electrode of regenerative fuel cells and vanadium-air redox flow batteries. Each method is well documented in the literature, and this paper provides a direct comparison under identical experimental conditions of electrochemical measurements and in identical units. In the first method, based on classical engineering, the bimetallic catalyst is deposited by dip-coating in a precursor solution of the salts followed by their thermal decomposition. In the alternative method, more academic in nature, atomic layer deposition (ALD) is applied to the felts after anodization. ALD allows for a controlled coating with ultralow noble-metal loadings in narrow pores. In acidic electrolyte, the ALD approach yields improved mass activity (557 A·g-1 as compared to 80 A·g-1 at 0.39 V overpotential) on the basis of the noble-metal loading, as well as improved stability.