Optimizing the Performance of Low-Loaded Electrodes for CO2-to-CO Conversion Directly from Capture Medium: A Comprehensive Parameter Analysis

Gas-fed reactors for CO2 reduction processes are a solid technology to mitigate CO2 accumulation in the atmosphere. However, since it is necessary to feed them with a pure CO2 stream, a highly energy-demanding process is required to separate CO2 from the flue gasses. Recently introduced bicarbonate zero-gap flow reactors are a valid solution to integrate carbon capture and valorization, with them being able to convert the CO2 capture medium (i.e., the bicarbonate solution) into added-value chemicals, such as CO, thus avoiding this expensive separation process. We report here a study on the influence of the electrode structure on the performance of a bicarbonate reactor in terms of Faradaic efficiency, activity, and CO2 utilization. In particular, the effect of catalyst mass loading and electrode permeability on bicarbonate electrolysis was investigated by exploiting three commercial carbon supports, and the results obtained were deepened via electrochemical impedance spectroscopy, which is introduced for the first time in the field of bicarbonate electrolyzers. As an outcome of the study, a novel low-loaded silver-based electrode fabricated via the sputtering deposition technique is proposed. The silver mass loading was optimized by increasing it from 116 μg/cm2 to 565 μg/cm2, thereby obtaining an important enhancement in selectivity (from 55% to 77%) and activity, while a further rise to 1.13 mg/cm2 did not provide significant improvements. The tremendous effect of the electrode permeability on activity and proficiency in releasing CO2 from the bicarbonate solution was shown. Hence, an increase in electrode permeability doubled the activity and boosted the production of in situ CO2 by 40%. The optimized Ag-electrode provided Faradaic efficiencies for CO close to 80% at a cell voltage of 3 V and under ambient conditions, with silver loading of 565 μg/cm2, the lowest value ever reported in the literature so far

Engineering copper nanoparticle electrodes for tunable electrochemical reduction of carbon dioxide

The electrochemical conversion of CO2 catalyzed by copper (Cu)-based materials is widely reported to produce different valuable molecules, and the selectivity for a specific product can be achieved by tuning the characteristics of catalytic materials. Differing from these studies on materials, the present work focuses on the engineering of gas diffusion electrodes in order to properly modify the selectivity, particularly by changing the Cu nanoparticle catalyst loading of the electrodes. Low catalyst loadings (≤ 0.25 mg cm−2) favor CH4 production, and intermediate (∼ 1.0 mg cm−2) loadings shift the selectivity toward C2H4. Eventually, larger values (≥ 2.0 mg cm−2) promote CO production. Detailed analyses reveal that both bulk and local CO generation rates, and charge transfer mechanism are responsible for the observed loading-dependent selectivity. The present work provides a new strategy for steering the CO2RR selectivity by simple electrode engineering beyond material development

Tin sulfide supported on cellulose nanocrystals-derived carbon as a green and effective catalyst for CO2 electroreduction to formate

This work reports a whole green two-step approach for the synthesis of novel catalysts for efficient CO2 conversion. A conductive carbon support was firstly obtained via pyrolysis of cellulose nanocrystals (CNCs), and the carbon surface was successively decorated with tin sulfide (SnS) through a microwave-assisted hydrothermal process. The morphology and carbon structure were characterized by field emission scanning electron microscopy and Raman spectroscopy, and the presence of SnS decoration was confirmed by X-ray photoelectron spectroscopy and X-ray diffraction analyses. The SnS supported on CNC-derived carbon shows enhanced catalytic activity for the CO2 conversion to formate (HCOO-). Good selectivity of 86% and high partial current density of 55 mA cm(-2) are reached at - 1.0 V vs. reversible hydrogen electrode in KHCO3 electrolyte. Additionally, the mass activity of the composite catalyst achieves a value as high as 262.9 mA mgSn(-1) for HCOO- formation, demonstrating good utilization efficiency of Sn metal. In this work, the low-cost CNC-derived carbon is evidenced to be easily decorated with metal species and thus shows high versatility and tailorability. Incorporating metal species with conductive high-surface carbon supports represents an effective strategy to realize active and stable electrocatalysts, allowing efficient utilization of metals especially the raw and precious ones

Microwave-assisted synthesis of N/S-doped CNC/SnO2 nanocomposite as a promising catalyst for oxygen reduction in alkaline media

In this study, we report an all-green approach for the synthesis of novel catalysts for oxygen reduction reaction (ORR) via a simple two-step procedure. In particular, conductive cellulose nanocrystals (CNCs) were obtained via pyrolysis, and a successive microwave-assisted hydrothermal process was employed to activate the carbon lattice by introducing sulfur (S) and nitrogen (N) dopants, and to decorate the surface with tin oxide (SnO2) nanocrystals. The successful synthesis of N/S-doped CNC/SnO2 nanocomposite was confirmed by X-ray Photoelectron Spectroscopy analysis, Energy Dispersive X-ray microanalysis, X-ray Diffraction and Field Emission Scanning Electron Microscopy. The synergistic effects of the dopants and SnO2 nanocrystals in modifying the catalytic performance were proved by various electrochemical characterizations. Particularly, the nanocomposite material reaches remarkable catalytic performance towards the ORR, close to the Pt/C benchmark, in alkaline environviment, showing promising potential to be implemented in alkaline fuel cell and metal-air battery applications

Thermally evaporated Cu-Co top spin valve with random exchange bias

A cobalt-copper top spin valve was prepared by thermal evaporation of a stack of ferromagnetic thin films separated by thin layers of the diamagnetic metal, with a cap layer containing an antiferromagnetic AFM exchange-biasing material. A nonconventional top AFM layer was used, in order to optimize the multilayer roughness and to avoid electrical interference with metallic layers; it consists of a composite material easily processed by means of optical lithography, basically a polymeric matrix composite with a dispersion of nickel oxide microparticles. Magnetization and magnetoresistance measurements were performed from 4 to 300 K. The measurements of both quantities indicate random pinning action of the top AFM layer, resulting in a small exchange-bias field and in asymmetric magnetization and magnetoresistance curves. A simple model explains the observed physical effect

Electrochemical Reduction of {CO}2 With Good Efficiency on a Nanostructured Cu-Al Catalyst

Carbon monoxide (CO) and formic acid (HCOOH) are suggested to be the most convenient products from electrochemical reduction of CO2 according to techno-economic analysis. To date, tremendous advances have been achieved in the development of catalysts and processes, which make this research topic even more interesting to both academic and industrial sectors. In this work, we report nanostructured Cu-Al materials that are able to convert CO2 to CO and HCOOH with good efficiency. The catalysts are synthesized via a green microwave-assisted solvothermal route, and are composed of Cu2O crystals modified by Al. In KHCO3 electrolyte, these catalysts can selectively convert CO2 to HCOOH and syngas with H-2/CO ratios between 1 and 2 approaching one unit faradaic efficiency in a wide potential range. Good current densities of 67 and 130 mA cm(-2) are obtained at -1.0 V and -1.3 V vs. reversible hydrogen electrode (RHE), respectively. When switching the electrolyte to KOH, a significant selectivity up to 20% is observed for C2H4 formation, and the current densities achieve 146 and 222 mA cm(-2) at -1.0 V and -1.3 V vs. RHE, respectively. Hence, the choice of electrolyte is critically important as that of catalyst in order to obtain targeted products at industrially relevant current densities

Hyperbolic Metamaterials via Hierarchical Block Copolymer Nanostructures

Hyperbolic metamaterials (HMMs) offer unconventional properties in the field of optics, enabling opportunities for confinement and propagation of light at the nanoscale. In‐plane orientation of the optical axis, in the direction coinciding with the anisotropy of the HMMs, is desirable for a variety of novel applications in nanophotonics and imaging. Here, a method for creating localized HMMs with in‐plane optical axis, based on block copolymer (BCP) blend instability, is introduced. The dewetting of BCP thin film over topographically defined substrates generates droplets composed of highly ordered lamellar nanostructures in hierarchical configuration. The hierarchical nanostructures represent a valuable platform for the subsequent pattern transfer into a Au/air HMM, exhibiting hyperbolic behavior in a broad wavelength range in the visible spectrum. A computed Purcell factor as high as 32 at 580 nm supports the strong reduction in the fluorescence lifetime of defects in nanodiamonds placed on top of the HMM

High rejection stacked single-layer graphene membranes for water treatment

Nowadays, the production of pure water from saltwater and wastewater is one of the most challenging issues. Polymeric materials represent, at the moment, the best solution for membranes technology but new materials with improved functionalities are desirable to overcome the typical limitations of polymers. In this work, graphene membranes with superior filtration properties are fabricated by stacking up to three graphene layers on a porous support and exploiting the intrinsic nanopores of graphene to filter diclofenac (drug), and methylene blue (dye). The rejection improves increasing the number of the stacked graphene layers, with the best results obtained with three graphene layers. Mass diffusion properties depend on the size of the probe molecule, consistently with the existence of intrinsic nanometer-sized pores within graphene. From the results of an in depth transmission electron microscopy analysis and molecular dynamics simulations it is inferred that graphene stacking results in a decrease of effective membrane pore sizes to about 13 Å diameter which corresponds to 97% rejection for diclofenac and methylene blue after one hour filtration

Effect of Thermal Stabilization on PAN-Derived Electrospun Carbon Nanofibers for CO2 Capture

Carbon capture is amongst the key emerging technologies for the mitigation of greenhouse gases (GHG) pollution. Several materials as adsorbents for CO2 and other gases are being developed,which often involve using complex and expensive fabrication techniques. In this work, we suggest a sound, easy and cheap route for the production of nitrogen-doped carbon materials for CO2 capture by pyrolysis of electrospun poly(acrylonitrile) (PAN) fibers. PAN fibers are generally processed following specific heat treatments involving up to three steps (to get complete graphitization), one of these being stabilization, during which PAN fibers are oxidized and stretched in the 200–300 °C temperature range. The effect of stabilization temperature on the chemical structure of the carbon nanofibers is investigated herein to ascertain the possible implication of incomplete conversion/condensation of nitrile groups to form pyridine moieties on the CO2 adsorption capacity. The materials were tested in the pure CO2 atmosphere at 20 °C achieving 18.3% of maximum weight increase (equivalent to an uptake of 4.16 mmol g-1), proving the effectiveness of a high stabilization temperature as route for the improvement of CO2 uptake