Facile and scalable synthesis of Cu2O-SnO2 catalyst for the photoelectrochemical CO2 conversion

The conversion of the atmospheric CO2 to value-added compounds is more and more attractive to the scientific community, since natural sink cannot keep up with the constant anthropogenic emission and amplification processes. Recently, CO2 concentration in the atmosphere exceeds 410 ppm, and its growth has remained constant since the 50s[1]. Renewable and green approaches to CO2 recovery are aimed to minimize the worrying impact of its emission to the environment, and to drive the transition to a new circular economy approach in chemistry and energy production. Within the depicted scenario, electrochemical and photoelectrochemical CO2 reduction are being widely investigated as promising methods to transform CO2, under mild reaction conditions, into useful chemicals or fuels. For instance, alcohols, CO and HCOOH that can be exploited as renewable energy sources or as key intermediates for the chemical industry. Among the non-precious metal oxides, Cu2O is a cheap, abundant and intrinsically p-type semiconductor. Due to its narrow band gap (~ 2 eV) and the suitable positioning of conduction and valence bands, Cu2O is an ideal photocatalyst for CO2RR. Simultaneously, SnO2 is an n-type direct band-gap semiconductor with noticeable electron mobility together with an intrinsic stability. It openly transpires the dual role of Cu2O, as photoabsorber and forming a p-n junction with Tin Oxide. In this work, the synthesis of photoactive copper-tin-oxide-based catalyst was optimized by a co-precipitation method[2], employing Cu(NO3)2·3H2O and SnCl4∙5H2O into a stirred and heated reactor. A solution of Na2CO3 was added as a precipitant agent, while NaBH4 as the reducing one[3], in order to promote the Cu2O formation. The work-up protocol, based on copious MilliQ water washings, was implemented and finally optimized. The characterization step included Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Analysis (EDX) X-rays Diffraction Analysis(XRD), UV-Visible Spectroscopy analysis, among others, and allowed the morphological assessment, the porosity value estimation and the crystalline phase evaluation. With the latter, it has been found a correspondence to the cubic crystalline phase (cuprite) of Cu2O and consequently confirmation that the reduction process has been successfully carried out. The so obtained catalyst was then deposited it onto a GDL (Gas Diffusion Layer) and FTO-based substrates by spray coating of an ink containing: the catalysts, Vulcan carbon (to increase the electrode conductivity), Nafion as binder and Isopropanol as carrier. The photo-electrochemical activity for the CO2 reduction reaction was tested in the dark and under sunlight simulated conditions by means of Linear Sweep Voltammetry (LSV) and Chrono-Potentiometry (CP) analyses. Relevant current density (j) values of up to 40 mA/cm2 were observed, and from the products analysis during the CP a high Faradaic Efficiency to CO was obtained. The influence of the ink composition was accurately investigated in terms of interaction among all the components and with respect to the employed substrate, taking into account sun-light activity and stability of the prepared electrodes towards their future utilization in a device for the sun-driven CO2 conversion to high-added value products. AKCNOWLEDGMENT This work has received funding from the European Union’s Horizon 2020 Research and Innovation Action programme under the Project SunCoChem (Grant Agreement No 862192). [1] X. Lan, B. D. Hall, G. Dutton, J. Mühle, and J. W. Elkins. (2020). Atmospheric composition [in State of the Climate in 2018, Chapter 2: Global Climate]. [2] Schuth F., et al., Journal of Catalysis, (2008), 258 [3] Angew. Chem. Int. Ed. 10.1002/anie.201808964