CET Volume 86 DOI: 10.3303/CET2186235 Paper Received: 19 September 2020; Revised: 11 March 2021; Accepted: 20 April 2021 Please cite this article as: Giusi D., Tavella F., Miceli M., Ampelli C., Centi G., Cosio D., Genovese C., Perathoner S., 2021, Synergetic Electrocatalytic Effects of Cu2O-Tio2 Heterostructures in a Solar Driven Pec Device for CO2 Reduction to >c1 Chemicals, Chemical Engineering Transactions, 86, 1405-1410 DOI:10.3303/CET2186235 CHEMICAL ENGINEERING TRANSACTIONS VOL. 86, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-84-6; ISSN 2283-9216 Synergetic Electrocatalytic Effects of Cu2O-TiO2 Heterostructures in a Solar Driven PEC Device for CO2 Reduction to >C1 Chemicals Daniele Giusia, Francesco Tavellaa,*, Matteo Micelia, Claudio Ampellia,*, Gabriele Centia, Daniele Cosiob, Chiara Genovesea, Siglinda Perathonera a Department of Chemical, Biological, Pharmaceutical and Environmental Sciences (ChiBioFarAm) – University of Messina, ERIC aisbl and CASPE/INSTM, v.le F. Stagno d'Alcontres, 31 – 98166 Messina, Italy b Department of Mathematical, Computer, Physical and Earth Sciences (MIFT) – University of Messina, v.le F. Stagno d'Alcontres, 31 – 98166 Messina, Italy ftavella@unime.it, ampellic@unime.it A solar driven photo-electro-catalytic (PEC) device for CO2 reduction/water oxidation is here presented. The electrodes are engineered to be used in the n-type configuration, without using precious metals or rare/expensive raw materials. Specifically, the photoanode for water oxidation is made of TiO2 nanotube arrays prepared via controlled anodic oxidation, while Cu2O-TiO2 heterostructures, synthesised by ultra- sonicated assisted co-precipitation and then spray-coated over a carbon-based gas diffusion layer (GDL), are used as electrocathode materials for CO2 reduction. All the tests are performed without applying external bias or adding sacrificial donors, in a compact homemade PEC reactor in comparison with a conventional slurry photoreactor. The catalytic performances are evaluated in terms of formic acid and acetic acid production, the latter involving the formation of C-C bond. The effect of the presence of TiO2 is investigated in comparison with a bare cuprous oxide (Cu2O) film. Results for the bare Cu2O/GDL electrode (no TiO2) show 31.8 and 80.6 µmol h-1 gCu -1 as formic acid and acetic acid production rates, respectively. Cu2O-TiO2/GDL electrode, instead, shows more than one order of magnitude higher productivity (0.69 and 2.59 mmol h-1 gCu -1, respectively). The Faradaic efficiency (FE) to acetic acid is much higher for Cu2O-TiO2/GDL electrodes (61.9 %) considering the higher number of electrons involved for acetic acid with respect to formic acid (8 vs. 2). This behaviour can be ascribed to the synergetic electrocatalytic effects of the as-formed Cu2O-TiO2 heterostructures, favouring the formation of C-C bond for the sustainable production of >C1 chemicals and fuels. 1. Introduction Carbon dioxide (CO2) reduction by using sunlight has attracted more and more attention in the scientific community in the last years (Nitopi et al., 2019). In general, “solar energy exploitation” in its wide meaning is not only related to the direct exposure to solar radiation, but includes a wide range of renewable energy sources like wind, photovoltaic, hydropower, etc. However, a direct use of solar energy by photo-electro- catalytic (PEC) approach represents a more attractive long-term solution (Iqbal et al., 2018). A device capable of directly converting sunlight and CO2 into chemicals in one-step process may reduce the loss of energy, thus resulting to an increase of overall efficiency (Ding et al., 2020). The development of a device for the conversion of CO2 and H2O into >C1 valuable products by using solar energy represents a great challenge for the next future (Giusi et al., 2020). However, the realization of an efficient PEC system must go through the preparation of robust and low-cost electrodes and devices, because a rapid and efficient electrocatalytic reaction requires not only an active catalytic material, but also a proper electrode and cell design (Ampelli, 2020). Many aspects should be taken into account in electrode and cell design for CO2 reduction: i) light harvesting and charge separation, ii) electron transfer and mass diffusion, iii) yield and type of products formed (Wang Y. et al., 2020). The correct choice of materials for the electrode preparation, and their assembling in a proper 1405 reactor configuration are the key to enhance the photo-conversion efficiency (Genovese et al., 2013). For these reasons, a PEC device should have a compact design to decrease the volume of the electrolytes, two separate chambers for anodic and cathodic half-reactions, and high robustness for continuous flow operation (Ampelli et al., 2019). Regarding the kind of materials, the photoanode should be resistant to photo-corrosion and have a proper nanoarchitecture to improve the electron transport and minimize electron-hole charge recombination (Tavella et al., 2017). The electrocathode, instead, should be designed to minimize side-reactions (i.e., hydrogen evolution) and to address CO2 reduction process towards the formation of the more desired >C1 products (Marepally et al., 2017a). Among the electrodes investigated so far in literature for CO2 reduction, copper- based materials have been the most active (Mais et al., 2019). In this context, the combination of titania (TiO2) nanotube arrays as the photoanode with Cu2O nanoparticles (also mixed with TiO2 P25) coated on a gas diffusion layer as the electrocathode, was here investigated in a compact homemade PEC device for CO2 reduction, in comparison with conventional slurry reactors. 2. Experimental 2.1 TiO2 nanotube arrays Titania nanotube arrays (TiO2 NTs) were prepared by controlled anodic oxidation technique, starting from metallic Ti discs (99.98% purity, 0.025 mm thickness, supplied by Alfa-Aesar) (Ampelli et al., 2017). The Ti discs were anodized in a two-electrode cell under the application of a constant voltage (50 V) for 45 min. The electrolyte consists of ethylene glycol with 2 % distilled H2O and 0.33 % NH4F. After the synthesis, the samples were rinsed with water and annealed at 450 °C in air for 3 h in order to obtain crystallization into the anatase phase (Perini et al., 2021). The main advantage of adopting this technique is related to the possibility of tailoring the morphology of TiO2 as “a priori” design for light-harvesting and catalytic applications (Tesler et al., 2020). 2.2 Cu2O nanocubes A facile wet precipitation process was used to synthesize cuprous oxide (Cu2O) nanocubes (Liu et al. 2016). In brief, CuCl2·2H2O (0.85 g) was dissolved in distilled water (500 mL) under stirring at 60°C. After dissolution, 2 M NaOH (50 mL) was added and the resulting solution was stirred for 30 min. Then, 0.6 M ascorbic acid (50 mL) was slowly added and the temperature set to 60° C. The formation of an orange Cu2O precipitate was obtained in 3 h. The as-obtained precipitate, after cooling at room temperature, was collected by centrifugation and dried in a vacuum oven at 60°C overnight. 2.3 Cu2O-TiO2 P25 Cuprous oxide / titania (Cu2O/TiO2, Cu loading ≃5) catalyst was prepared by an ultra-sonication assisted precipitation method. Copper acetate (0.3 g) was dissolved in 25 mL of ethanol forming a blue solution. Then, an appropriate amount of commercial TiO2 (Degussa P25) was added under ultrasonication. The resulting suspension was heated up to 60 °C; then, 0.4 M glucose solution (25 mL) and 0.3 M NaOH (30 mL) were added. The solution was stirred for 30 min at 60 °C, and then it was cooled naturally to room temperature, collected by centrifugation, and washed with distilled water to remove the impurity. The resulting powder was dried under vacuum at 60 °C overnight. 2.4 Electrocathode preparation The electrocathode materials (bare Cu2O nanocubes or Cu2O/TiO2) were deposited in the form of a stable ink over a carbon-based gas diffusion layer (GDL, Sigracet® 29BCE, supplied by Ion Power) pre-heated to allow the evaporation of the solvent, by spray-coating technique. The ink (an organic solvent suspension of the catalytic powder) was prepared as follows: the powder was mixed with isopropanol (50 mL) and Nafion 5 wt. % perfluorinated solution (50 µL) under stirring for 20-30 min. The as-obtained stable suspension was sprayed on the GDL with a loading of 1 mg cm-2. 2.5 Membrane-electrode assembly (MEA) The electrodes were compacted by hot-pressing a proton exchange membrane (Nafion® NR212) to form a membrane-electrode assembly (MEA). This type of assembly is widely used in fuel cells and allows efficient transport of protons (H+) between the two catalytic layers, also physically separating the two half-cell compartments. 1406 2.6 PEC device The experimental apparatus for the photo-electrocatalytic tests consists of a solar simulator and a photoreactor. A Xe-arc lamp (Lot Oriel, 300 W) equipped with a set of lenses for light collection and focusing, and a water filter to eliminate the infrared radiation, was used as the light source. The solar-driven reactor is made of Plexiglas and equipped with a quartz window. It has a compact two-electrode configuration with separate compartments for the two half-reactions of CO2 reduction and water oxidation. In comparison with the conventional photocatalytic reactors, this photo-device was designed to be more compact and better suited for a delocalized energy production (de Brito et al., 2019). 1 M NaOH aqueous solution (40 mL) was used as the anolyte, while 40 mL of 0.1 M KHCO3 aqueous solution (saturated with a continuous flow of CO2) were used as the catholyte. The irradiated area is about 5 cm 2. The liquid catholyte containing the products was analysed by Gas Chromatography-Mass Spectrometry (GC–MS, column Stabilwax, He carrier gas, detection limit around 1–5 ppm) to check the formation of methanol, ethanol, and other alcohols, and by Ion Chromatography (IC, column organic acids, detection limit around 0.1– 1 ppm) to detect formic acid and acetic acid. The presence of products in the gas reactor outlet stream (H2, CH4, CO, C2H4, C2H6) was checked directly on-line by a Gas Chromatograph (MicroGC GCX Pollution Analytic Equipment), having a sensitivity of 1–2 ppm. 3. Results and Discussion 3.1 X-Ray diffraction crystallography The crystallographic structure and the purity of the electrocathode materials (Cu2O and Cu2O-TiO2) were investigated by X-ray Diffraction (XRD), as shown in Figure 1. The well-defined peaks prove the crystallinity of the two samples. The following peaks belong to cuprous oxide: 29.7, 36.6, 42.5, 52.5, 61.6, 73.7, and 77.5 ° referring to the planes (110), (111), (200), (211), (220), (311), and (222), respectively. XRD spectrum of Cu2O- TiO2 sample (black line) evidences the presence of the characteristic peaks of the two main components (Cu2O and TiO2). The TiO2 phase distribution in the sample also reflects the P25 Degussa distribution, consisting in anatase and rutile in 3:1 ratio. Figure 1: XRD patterns of Cu2O (red line) and Cu2O-TiO2 (black line). Anatase and rutile TiO2, and Cu2O phases are highlighted. 3.2 Morphology analysis Figures 2a and b show the Scanning Electron Microscopy (SEM) images of Cu2O and Cu2O-TiO2 samples synthesized by wet precipitation and ultra-sonication assisted precipitation methods, respectively. Particularly, Cu2O prepared by wet precipitation evidences a nanocube-shape morphology, with aggregates having a size of about 170-200 nm, while for Cu2O-TiO2 prepared by ultra-sonication assisted precipitation, the particle size 1407 of the aggregates is larger, ranging from 400 to 700 nm. More in detail, a structure composed by spheroidal Cu2O aggregates, deriving from the intergrowth of many nanocube crystals along different directions, in combination with a porous TiO2 substrate, can be observed. Figure 2. SEM images of a) Cu2O nanocubes synthesized by wet-precipitation method and b) Cu2O/TiO2 obtained through ultra-sonication assisted wet precipitation method. 3.3 CO2 reduction tests The performances of Cu2O/GDL and Cu2O-TiO2/GDL electrodes in the process of CO2 reduction were evaluated using the solar-driven compact PEC device described in the experimental part. The tests were carried out using an n-type photo-active thin film of ordered arrays of titania nanotubes (grown by anodic oxidation on a perforated Ti foil) in the anodic part. The procedure of working of the PEC system was the following: the electrons generated by irradiation of the TiO2 nanotubes were collected through the Ti layer remained not-oxidized after the anodization, and transported to the cathodic part of the cell through an external circuit; the protons, which were generated from water oxidation in the anode, first diffused along the TiO2 nanotubes and then migrated through the Nafion membrane towards the cathodic part of the PEC cell to reduce CO2 over Cu2O or Cu2O-TiO2 electrocatalysts. The two electrolyte aqueous solutions (NaOH and KHCO3) circulated continuously through each cell compartment and an outer reservoir. Before each test, the aqueous KHCO3 solution was saturated with CO2 for 30 min in order to stabilize the pH, which corresponds to a value of 6.8 for 0.1 M KHCO3 concentration. A continuous CO2 flow was maintained during the irradiation tests (20 mL min-1). This operation allowed to create a sort of buffer CO2/KHCO3, guaranteeing a relative high concentration of CO2 in solution. However, the diffusion of CO2 through the Helmholtz double layer remains a great issue for CO2 reduction in liquid systems, and a different gas-phase approach (without using an electrolyte in the cathode compartment) would be highly desirable (Marepally et al., 2017b). Note that the reaction was driven only by light irradiation, using a solar simulator and without adding any external current or electrical bias. The behaviour of the electrodes was investigated in the photo- electrocatalytic device for 4 h of irradiation, monitoring the formation of both gas and liquid products. The main products were formic acid, acetic acid and hydrogen. Other products of CO2 reduction, such as methanol, ethanol, isopropanol, methyl formate, acetone, methane and carbon monoxide, were analysed, but they were absent or formed only in traces in all tests. The production rates of formic acid and acetic acid obtained with Cu2O/GDL and Cu2O-TiO2/GDL electrodes are reported in Table 1. The bare Cu2O/GDL electrode (no TiO2) showed 31.8 and 80.6 µmol h-1 gCu -1 of formic acid and acetic acid production rates, respectively. Cu2O- TiO2/GDL electrode, instead, showed more than one order of magnitude higher productivity (0.69 and 2.59 mmol h-1 gCu -1 as formic acid and acetic acid production rates, respectively). This can be ascribed to the synergetic electrocatalytic effects of the as-formed Cu2O-TiO2 heterostructures. 1408 Table 1: Production rates obtained with Cu2O/GDL and Cu2O-TiO2/GDL as electrocathodes and TiO2 nanotube arrays as photoanode. Sample Formic acid mmol gCu -1 h-1 Acetic acid mmol gCu -1 h-1 Cu2O/GDL 0.0318 0.0806 Cu2O-TiO2/GDL 0.69 2.59 As the GDL support is not inert, a blank experiment without spraying Cu2O or Cu2O-TiO2 on the GDL was also performed. In this case, only very low amounts of formic acid and acetic acid were detected (about 20 times less than the electrode containing Cu2O). A further blank experiment without CO2 in the supporting electrolyte was also made, evidencing the formation of only formic acid as carbon product, but with a much lower productivity (a thousand times lower) than in presence of CO2, which is to ascribe to the conversion of small amounts CO2 released from KHCO3 solution. A low amount of carbon products was also detected in the anode part, likely due to cross-over of anions (formate and acetate) through the Nafion membrane, evidencing that the behaviour of this kind of membrane is far from ideal. Figure 3 shows a plot of Faradaic Efficiency (FE) to formic acid and acetic acid. The values of current density are also plotted in the same graph. A much higher FE to acetic acid was observed for Cu2O-TiO2/GDL electrode (61.9 %) with respect to the bare Cu2O/GDL (18.9 %), indicating that the presence of TiO2 favoured the formation of C-C bond, as also recently demonstrated for the photocatalytic reduction of CO2 in a gas flow- through reactor (Giusi et al., 2020). Note that a higher number of electrons is needed for the formation of acetic acid than formic acid (8 vs. 2), Moreover, a stable current density of 142.1 μA cm-2 was obtained using Cu2O/GDL electrode, while the current density was lower for Cu2O-TiO2/GDL electrode (70.1 μA cm -2). The cathode performances were stable for at least 24 h. Figure 3: Faradaic efficiency (FE) and current density for CO2 reduction tests with Cu2O/GDL and Cu2O- TiO2/GDL electrodes. Table 2 shows instead a comparison of productivity obtained in the solar-driven PEC device (using Cu2O/GDL electrode) with results obtained using a conventional photo-reactor working in liquid phase (with Cu2O powder in slurry) irradiated by the same solar simulator. The productivity was much higher (of about one order of magnitude) in the solar-driven PEC reactor, as well as the selectivity to acetic acid, highlighting the importance of electrode and cell design to improve performances of CO2 reduction towards the sustainable production of >C1 chemicals and fuels. Table 2: Production rates in the solar-driven PEC device in comparison with a conventional slurry photoreactor, with Cu2O as electrocatalyst. Reactor type Acetic acid μmol gCu -1 h-1 Formic acid μmol gCu -1 h-1 PEC 81 32 Slurry 3.1 3.7 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0 10 20 30 40 50 60 70 Cu2O Cu2O-TiO2 Cu rr en t d en si ty / m A cm -2 Fa ra da ic e ff ic ie nc y / % Acetic acid FE% Formic acid FE% Current density 1409 4. Conclusions This study has analysed the synthesis, characterization, and application of Cu2O/GDL and Cu2O-TiO2/GDL electrodes in the photo-electro-catalytic reduction of CO2 (in combination with TiO2 nanotube arrays as photoanode for water oxidation) in a compact-design solar-driven PEC cell without application of any external potential/current and without sacrificial agents. Both electrodes were made using only earth-abundant materials and low-cost preparation procedures suitable for scale up. High Faradaic efficiency (FE) to acetic acid was obtained especially for Cu2O-TiO2/GDL electrodes (61.9 %), evidencing that the synergetic catalytic effects of the as-formed Cu2O-TiO2 heterostructure favoured the formation of C-C bond, thus improving the production of the more desired >C1 products. Results also showed that a rapid and efficient electrocatalytic reaction requires not only an active catalytic material, but also a proper electrode and cell design. Acknowledgments This work was funded by the European Union through the A-LEAF project (732840-A-LEAF) and by the MIUR (Italy) through the PRIN Project CO2 ONLY (No. 2017WR2LRS), which are gratefully acknowledged. References Ampelli C., 2020, Electrode design for ammonia synthesis, Nature Catalysis, 3, 420–421. Ampelli C., Genovese C., Cosio D., Perathoner S., Centi G., 2019, Effect of Current Density on Product Distribution for the Electrocatalytic Reduction of CO2, Chemical Engineering Transactions, 74, 1285–1290. Ampelli C., Tavella F., Genovese C., Perathoner S., Favaro M., Centi G., 2017, Analysis of the factors controlling performances of Au-modified TiO2 nanotube array based photoanode in photo-electrocatalytic (PECa) cells, Journal of. Energy Chemistry, 26, 284–294. de Brito J.F., Genovese C., Tavella F., Ampelli C., Boldrin Zanoni M.V., Centi G., Perathoner S., 2019, CO2 Reduction of Hybrid Cu2O–Cu/Gas Diffusion Layer Electrodes and their Integration in a Cu‐based Photoelectrocatalytic Cell, ChemSusChem, 12, 4274–4284. Ding P., Jiang T., Han N., Li Y., 2020, Photocathode engineering for efficient photoelectrochemical CO2 reduction, Materials Today Nano, 10, 100077. Genovese C., Ampelli C., Perathoner S., Centi G., 2013, A Gas-phase Electrochemical Reactor for Carbon Dioxide Reduction back to Liquid Fuels, Chemical Engineering Transactions, 32, 289–294. Giusi D., Ampelli C., Genovese C., Perathoner S., Centi G., 2021, A novel gas flow-through photocatalytic reactor based on copper-functionalized nanomembranes for the photoreduction of CO2 to C1-C2 carboxylic acids and C1-C3 alcohols, Chemical Engineering Journal, 408, 127250. Iqbal M.Z., Siddique S., 2018, Recent progress in efficiency of hydrogen evolution process based photoelectrochemical cell, International Journal of Hydrogen Energy, 43, 21502–21523. Liu H., Hu Z., Hu R., Liu B., Ruan H., Zhang L., Xiao W., 2016, Large-scale synthesis of Cu2O nanocubes and their electrochemical properties, International Journal of Electrochemical Science, 11, 2756–2761. Mais L., Palmas S., Vacca A., Mascia M., Ferrara F., Pettinau, A., 2019, Catalytic activity of Cu and Cu/Sn electrodes during CO2 reduction from aqueous media, Chemical Engineering Transactions, 73, 97–102. Marepally B.C., Ampelli C., Genovese C., Tavella F., Veyre L., Thieuleux C., Quadrelli E.A., Perathoner S., Centi G., 2017a, Role of small Cu nanoparticles in the behaviour of nanocarbon-based electrodes for the electrocatalytic reduction of CO2, Journal of CO2 Utilization, 21, 534–542. Marepally B.C., Ampelli C., Genovese C., Saboo T., Perathoner S., Wisser F.M., Veyre L., Canivet J., Quadrelli E.A., Centi G., 2017b, Enhanced formation of >C1 Products in the Electroreduction of CO2 by Adding a CO2 Adsorption Component to a Gas-Diffusion Layer-Type Catalytic Electrode, ChemSusChem, 10, 4442–4446. Nitopi S., Bertheussen E., Scott S.B., Liu X., Engstfeld A.K., Horch S., Seger B., Stephens I.E.L., Chan K., Hahn C., Nørskov J.K., Jaramillo T.F., Chorkendorff I., 2019, Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte, Chemical Reviews, 119, 7610–7672. Perini J.A.L., Tavella F., Ferreira Neto E.P., Zanoni M.V.B., Ribeiro S.J.L., Giusi D., Centi G., Perathoner S., Ampelli C., 2021, Role of nanostructure in the behaviour of BiVO4-TiO2 nanotube photoanodes for solar water splitting in relation to operational conditions, Solar Energy Materials & Solar Cells, 223, 110980. Tavella F., Ampelli C., Frusteri L., Frusteri F., Perathoner S., Centi G., 2018, Development of photoanodes for photoelectrocatalytic solar cells based on copper nanoparticles on titania thin films of vertically aligned nanotubes, Catalysis Today, 304, 190–198. Tesler A.B., Altomare M., Schmuki P., 2020, Morphology and Optical Properties of Highly Ordered TiO2 Nanotubes Grown in NH4F/o-H3PO4 Electrolytes in View of Light-Harvesting and Catalytic Applications, ACS Applied Nano Materials, 3, 10646–10658. Wang Y., Zu M., Zhou X., Lin H., Peng F., Zhang S., 2020, Designing efficient TiO2-based photoelectrocatalysis systems for chemical engineering and sensing, Chemical Engineering Journal, 381, 122605. 1410