Bilayer Porous Electrocatalysts for Stable and Selective Electrochemical Reduction of CO2 to Formate in the Presence of Flue Gas Containing NO and SO2.

The development of stable and selective electrocatalysts for converting CO2 to value-added chemicals or fuels has gained much interest in terms of their potential to mitigate anthropogenic carbon emissions. Most of the electrocatalysts are tested under pure CO2; however, industrial outlet flue gas contains numerous impurities, such as NO and SO2, which poison the electrocatalysts and alter the product selectivity. Developing electrocatalysts that are resistant to such impurities is essential for commercial implementation. Herein, we prepared bilayer porous electrocatalysts, namely, Sn, Bi, and In, on porous Cu foam mesh (Sn/Cu-f, Bi/Cu-f, and In/Cu-f) by a two-step electrodeposition process and employed these electrodes for the electrochemical reduction of CO2 to formate. It was observed that the bilayer porous electrocatalysts exhibited high CO2 reduction activity compared to catalysts coated on a Cu mesh. Among bilayer porous electrocatalysts, Sn/Cu-f and Bi/Cu-f electrocatalysts showed more than 80% faradaic efficiency (FE) toward formate production, with a formate partial current density of around -16 and -10.4 mA cm-2, respectively, at -1.02 V vs RHE. In/Cu-f electrocatalyst showed nearly 40% formate FE with formate partial current density of -15 mA cm-2 at -1.22 V vs RHE. We investigated the effect of NO and SO2 impurities (500 ppm of NO, 800 ppm of SO2, and 500 ppm of NO + 800 ppm of SO2) on these electrocatalysts' selectivity and stability toward formate. It was observed that the Bi/Cu-f electrocatalyst showed 50 h stability with 80 ± 5% formate FE, and Sn/Cu-f showed 18 h stability with above 80 ± 5% efficiency in the presence of NO and SO2 mixed with CO2. Furthermore, we studied the effect of CO2 concentration with Sn/Cu-f and Bi/Cu-f catalysts in the range of 15-100% CO2, for which formate FEs of 45-80% were observed.

Tubular Sediment-Water Electrolytic Fuel Cell for Dual-Phase Hexavalent Chromium Reduction.

A novel tubular sediment-water electrolytic fuel cell (SWEFC) was fabricated for the reduction of Cr(VI) in a dual-phase system. The approach simulates a standing water body with Cr(VI)-contaminated overlying water (electrolyte) and bottom sediment phase with electrodes placed in both the phases, supplemented with urea as a potential electron donor. Cr(VI) reduction efficiency of 93.2 ± 1.3% from electrolyte (in 1.5 h) and 81.2 ± 1.3% from the sediment phase (in 8 h) with an initial Cr(VI) concentration of 1,000 mg/L was observed in a single-cell configuration. The effect of initial Cr(VI) concentration, variation in sediment salinity and pH, and different electron donors on the SWEFC performance were systematically investigated. SWEFC showed enhanced performance with 2.4-fold higher current (193.9 mA) at 400 mg/L Cr(VI) concentration when cow dung was used as a low-cost alternative to urea as an electron donor. Furthermore, reactor scalability studies were carried out with nine-anode and nine-cathode configuration (3 L electrolyte and 2 kg sediment), and reduction efficiencies of 98.9 ± 0.9% (in 1 h) and 97.6 ± 2.2% (in 8 h) were observed from the electrolyte and sediment phases, respectively. The proposed sediment-water electrolytic fuel cell can be an advanced and environmentally benign strategy for Cr(VI) remediation from contaminated sediment-water interfaces along with electricity generation.

Xerogel based catalyst for improved cathode performance in microbial fuel cells.

In a microbial fuel cell (MFC) the reduction reaction at cathode has been a limiting factor in achieving maximum power density, and numerous strategies have been implemented in an attempt to overcome this. Herein, we demonstrate that carbon xerogel (CX) doped with iron (Fe) and nitrogen (N) followed by modification with graphene oxide (GO) is an efficient catalyst for MFCs. The CXFeNGO catalyst was characterized using a scanning electron microscope, and X-ray diffraction, and the catalytic activity was confirmed using cyclic voltammetry studies. At the anode, colonization of bacterial cells on the electrode surface, forming a biofilm, was observed. When the CXFeNGO-modified electrode was used at the cathode in the MFC, a maximum power density of 176.5 ± 6 mW m-2 was obtained, compared to that of plain graphite electrode, which produced 139.1 ± 4 mW m-2. The power density of the modified electrode is thus 26.8% higher. The power density further increased to 48.6% when the pH of the catholyte was increased to 12, producing a power density of 207 ± 4 mW m-2.

Hexavalent chromium reduction through redox electrolytic cell with urea and cow urine as anolyte.

The present study demonstrates the potential utilization of urea/cow urine as anolyte for Cr(VI) reduction via a simple three-chambered electrolytic cell. The inherent chemical energy in the dual-waste stream (Cr(VI)-urea/urine) is employed for its self-oxidation-reduction without the need for any external energy supply. Ni foam as electroactive anode and catalyst-free carbon felt as cathode, along with the appropriate positioning of ion-selective separators, indirectly improved the cell performance by impeding electrolyte crossover. A fundamental study involving five different membrane configurations was conducted herein to improve Cr(VI) reduction efficiency. The Cr(VI) reduction efficiencies were 11.84 ±â€¯0.27%, 10.55 ±â€¯0.17%, 77.24 ±â€¯0.38% at 24 h, 13.57 ±â€¯0.25% at 72 h with glass frit, cation exchange membrane (CEM), sandwiched membrane, and anion exchange membrane (AEM) as separators in a dual-chambered H-cell, respectively, with an initial Cr(VI) concentration of 100 mg/L. The fifth configuration, consisting of a middle chamber between the anode and cathode with the CEM close to the anode and the AEM close to the cathode resulted in a reduction efficiency of 79.98 ±â€¯2.24% within 45 min for an initial Cr(VI) concentration of 400 mg/L. The first order rate constants were determined to be 0.024, 0.018, and 0.013 min-1 for Cr(VI) concentrations of 100, 200, and 400 mg/L, respectively. Moreover, when urea was replaced with cow urine as anolyte, a reduction efficiency of 98.94 ±â€¯1.28% was achieved at pH 2 in 45 min with 400 mg/L as initial Cr(VI) concentration. Furthermore, the XPS spectra of reduced Cr corresponding to binding energies of 579.4 eV and 589.3 eV, respectively, confirmed the presence of low-toxic Cr(III). The effect of applied load, initial Cr(VI) and urea concentration, Cr(VI) reduction under different initial H2SO4 concentrations were succinctly investigated to evaluate the performance of the electrolytic cell. The redox electrolytic cell can thus be an alternative to the conventional chemical or energy intensive processes for the reduction of hexavalent chromium.

Effect of reduction temperature on the preparation and characterization of Pt-Ru nanoparticles on multiwalled carbon nanotubes.

Carbon nanotubes (CNT) supported platinum-ruthenium (Pt-Ru) catalysts were prepared by impregnation-reduction using an ethanolic solution of H2PtCl6 and RuCl3. The effect of reduction temperatures on particle size, surface area and their relationship to the electrocatalytic activity for methanol oxidation were investigated. Thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopy (XPS) were used for the catalyst characterization. XRD analysis showed that the PtRu/ CNT catalysts possibly consist of separate Pt and Ru phases. XPS analysis showed that the catalysts contain hydrous ruthenium oxide in addition to Pt and Ru metal and oxide species. The electrocatalytic activities of the catalysts were investigated in half-cell experiments using cyclic voltammetry, CO stripping voltammetry, chronoamperometry, and impedance spectroscopy. The results showed that the catalyst reduced at a temperature of 350 degrees C had the largest electrochemical surface area, lowest charge transfer resistance and the highest electrocatalytic activity for methanol oxidation. The superior catalytic activity is discussed based on the presence of appropriate amount of hydrated Ru oxide.

In situ FTIR studies on the electrochemical reduction of halogenated phenols.

Electrochemical reduction of a variety of mono- and di-chloro- and bromo- phenols at a palladised titanium electrode afforded phenolate in all cases according to in situ FTIR studies, with the same intermediate species being observed in some cases.