Chiara Genovese

Electrocatalytic conversion of CO2 to liquid fuels using nanocarbon-based electrodes

Abstract Recent advances on the use of nanocarbon-based electrodes for the electrocatalytic conversion of gaseous streams of CO2 to liquid fuels are discussed in this perspective paper. A novel gas-phase electrocatalytic cell, different from the typical electrochemical systems working in liquid phase, was developed. There are several advantages to work in gas phase, e.g. no need to recover the products from a liquid phase and no problems of CO2 solubility, etc. Operating under these conditions and using electrodes based on metal nanoparticles supported over carbon nanotube (CNT) type materials, long C-chain products (in particular isopropanol under optimized conditions, but also hydrocarbons up to C8–C9) were obtained from the reduction of CO2. Pt-CNT are more stable and give in some cases a higher productivity, but Fe-CNT, particular using N-doped carbon nanotubes, give excellent properties and are preferable to noble-metal-based electrocatalysts for the lower cost. The control of the localization of metal particles at the inner or outer surface of CNT is an importact factor for the product distribution. The nature of the nanocarbon substrate also plays a relevant role in enhancing the productivity and tuning the selectivity towards long C-chain products. The electrodes for the electrocatalytic conversion of CO2 are part of a photoelectrocatalytic (PEC) solar cell concept, aimed to develop knowledge for the new generation artificial leaf-type solar cells which can use sunlight and water to convert CO2 to fuels and chemicals. The CO2 reduction to liquid fuels by solar energy is a good attempt to introduce renewables into the existing energy and chemical infrastructures, having a higher energy density and easier transport/storage than other competing solutions (i.e. H2).

H2 production by selective photo-dehydrogenation of ethanol in gas and liquid phase on CuOx/TiO2 nanocomposites

CuOx/TiO2 nanocomposites prepared by copper photodeposition (1.0 and 2.5 wt% copper loading) on TiO2 (synthesized by three different routes) are studied in the ethanol photo-dehydrogenation in gas- and liquid-phase operations, and characterized in terms of surface area, phase composition by XRD, morphology and copper-oxide nanoparticle size distribution, and copper species by UV-visible diffuse reflectance spectroscopy. Cu2+ ions partially enter into the titania structure leading to the creation of oxygen vacancies responsible for the shift in the band gap, but also the creation of traps for photogenerated holes and electrons. While the band gap shifts to lower energies with the copper content, a maximum photocatalytic activity is shown for the intermediate copper loading. Gas-phase operations allow a higher H2 productivity with respect to liquid-phase operations, and especially a higher selectivity (about 92–93%) to acetaldehyde. It is remarked that the route of photo-dehydrogenation of ethanol to H2 and acetaldehyde has an economic value about 3.0–3.5 times higher than the alternative route of photoreforming to produce H2. Gas-phase operations would be preferable for the photo-dehydrogenation of ethanol.

The use of a solar photoelectrochemical reactor for sustainable production of energy

The conversion of solar energy into H2 via water splitting process is one of the most attractive ways to obtain clean and renewable energy. Unfortunately, the fast back reaction of recombination and high band gap needed to activate the photo-catalytic materials, strongly limit the performances in conventional slurry photo-reactors. In this context we present a new photoelectrochemical approach with a double-chamber reactor configuration for H2 production by water photo-electrolysis. The core of the photo-system is a membrane electrode assembly consisting of different layers which hold distinct two areas of the reactor where the generation of O2 and H2 occurs separately. Particular attention is given to the development, on a nano-scale level, of the materials to be used as photoanode and electrocathode: nanostructured TiO2 arrays and carbon nanotubes are used respectively in the form of thin films separated by a proton conductive membrane. Results showed 3.2 mmol h−1 g−1 of H2 evolution that is about one order of magnitude higher with respect to the activity obtained with conventional slurry photoreactors. Moreover, we present the opportunity to recycle CO2 back to liquid fuels by using the same photoelectrochemical approach.

Mechanism of C–C bond formation in the electrocatalytic reduction of CO2 to acetic acid. A challenging reaction to use renewable energy with chemistry

Copper nanoparticles on carbon nanotubes are used in the reduction of CO2 to acetic acid (with simultaneous water electrolysis) in a flow electrocatalytic reactor operating at room temperature and atmospheric pressure. A turnover frequency of about 7000 h−1 and a carbon-based Faradaic selectivity to acetic acid of about 56% were observed, indicating potential interest in this approach for using renewable energy. The only other products of reaction detected were formic acid and methanol (the latter in some cases), besides H2. The reaction mechanism, particularly the critical step of C–C bond formation, was studied by comparing the reactivity in tests with CO2 or CO, where formic acid or formaldehyde where initially added. The results indicate the need for having dissolved CO2 to form acetic acid, likely via the reaction of CO2˙− with surface adsorbed –CH3 like species. The pathway towards formic acid is instead different from the route of the formation of acetic acid.

Nanoscale Engineering in the Development of Photoelectrocatalytic Cells for Producing Solar Fuels

Engineering at the nanoscale level is a key aspect for the design of novel devices for sustainable energy to address the changeover from fossil fuels to renewable energy sources. This perspective paper, after introducing this topic, analyses the design and development of photoelectrocatalytic cells for producing solar fuels. To overcome limitations in the design of photoelectrocatalytic cells, a different one is proposed which eliminates the need of having a liquid electrolyte, where the electrodes are immersed. This cell design requires specific characteristics in the related electrodes/materials, which in combination with the different operation conditions, determine the need to investigate new fundamental aspects in the area. Some of the aspects analyzed regard (i) the role of nanostructure for visible light absorption of the semiconductor used, (ii) the need to use catalytic concepts (photoelectro-catalysis rather than photoelectro-chemistry), (iii) the mobility of charge carriers and relation with electrode characteristics, and (iv) space charge and Helmholtz layer.

Operando spectroscopy study of the carbon dioxide electro-reduction by iron species on nitrogen-doped carbon

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.Trapping carbon dioxide within usable chemicals is a promising means to mitigate climate change, yet electrochemical C–C couplings are challenging to perform. Here, the authors prepared iron oxyhydroxides on nitrogen-doped carbon that efficiently convert carbon dioxide to acetic acid.

Enhanced formation of >C1 Products in Electroreduction of CO2 by Adding a CO2 Adsorption Component to a Gas-Diffusion Layer-Type Catalytic Electrode

The addition of a CO2 -adsorption component (substituted imidazolate-based SIM-1 crystals) to a gas-diffusion layer-type catalytic electrode enhances the activity and especially the selectivity towards >C1 carbon chain products (ethanol, acetone, and isopropanol) of a Pt-based electrocatalyst that is not able to form products of CO2 reduction involving C-C bond formation under conventional (liquid-phase) conditions. This indicates that the increase of the effective CO2 concentration at the electrode active surface is the factor controlling the formation of >C1 products rather than only the intrinsic properties of the electrocatalyst.

Monitoring of glucose in fermentation processes by using Au/TiO2 composites as novel modified electrodes

This paper demonstrates an effective method of monitoring glucose in fermentation processes based on the development of enzyme-free glucose electrochemical sensors. The sensing electrodes were manufactured by preparing size-controlled Au nanoparticles (NPs), in the form of colloidal solutions, which were dispersed on TiO2 substrates, and then deposited on commercial carbon screen printed electrodes. The as-synthesized samples were fully characterized by transmission electron microscopy, X-ray diffraction, atomic absorption spectroscopy, and UV–visible diffuse reflectance spectroscopy to obtain information about their morphological, structural, and electronic properties. Particularly, the ability to control the size of Au NPs in the colloidal solution by using different reducing agents and stabilizers is presented here. The Au/TiO2-based modified sensors were assembled and tested for glucose monitoring in an alkaline solution. Results of cyclic voltammetry showed the high electrocatalytic activity of these sensors toward glucose oxidation, whereas no response was detected toward ethanol. This suggests the possibility of using this type of sensor for glucose monitoring in the fermentation processes without ethanol interference. The efficient sensing properties of Au NP-embedded TiO2 composites may be ascribed to the higher electrocatalytic activity of smaller Au NPs stabilized on TiO2.

Cu-MOF: a new highly active catalyst for WHPCO of waste water from agro-food production

The catalytic activity of copper-based metal organic framework compound (Cu-MOF) in the wet hydrogen peroxide catalytic oxidation (WHPCO), using p-coumaric acid as model phenolic compound, have been studied and compared with that of a copper-based pillared clays (Cu-PILC). Cu-MOF shows a high activity, but a structural and morphological change is observed during the reaction, even if the activity of the reused catalyst is higher in consecutive cycles with respect to that of the fresh sample.