(677e) Improvement of Sn Electrocatalytic Reaction for CO2 Conversion into Formate

Carbon capture and utilization is an interesting strategy to contribute to reduce CO₂ emissions and mitigate climate change, since it may involve the conversion of CO₂ into added-value products, such as liquid fuels. Among the different possibilities for CO₂ conversion, its electrochemical reduction appears as an interesting option because it could be used to store the intermittent and unpredictable energy from renewable sources, such solar or wind power, in form of chemical products. One of the most interesting products obtained from CO₂ electroreduction is formate. This product is used as raw material in several industrial processes with growing demand and it has also been proposed as a suitable fuel for fuel-cells and as a renewable energy hydrogen carrier.

One of the electrocatalysts that allow CO₂ conversion into formate on aqueous solutions is Sn [1]. This metal could be used in electrodes with different configurations, such as plate electrodes or in the form of particles of different sizes deposited on porous supports. The aim of this work is to analyze the performance of Sn electrodes with different configuration and to remark the improvements achieved using Sn nanoparticles in gas diffusion electrodes (GDEs) with respect to our previous studies, taking account the faradaic efficiency, formate concentration in the product, rate of formate production and the current density of the process.

Starting with conventional Sn plate electrodes for CO₂ electrochemical reduction, a maximum formate rate of 4.4 ·10^-4 mol m^-2 s^-1 was achieved with a 70% faradaic efficiency and formate concentration of 140 ppm at current densities around 12 mA cm^-2 [2]. This low performance is due to the small electrocatalytic surface of the plate electrode and the mass transport limitations. To increase the specific surface and mass transport on the electrode, Sn particles were deposited on carbon paper to manufacture particulated electrodes. In these electrodes, gaseous CO₂ is directly fed to the cell and flows through the electrode. Sn commercial particles of 150 µm [3] achieved a formate concentration of 1.3 g L^-1 with 70% faradaic efficiency working at 40 mA cm^-2. When the size of particle was reduced to 150 nm [4], it was possible to keep a 70% faradaic efficiency with a current density of 90 mA cm^-2, increasing the concentration to 1.5 g L^-1 with a formate rate of 3.21 ·10^-3 mol m^-2 s^-1.

Recent research efforts are focused on the development of Sn Gas Diffusion Electrodes (Sn-GDEs) which allow working at high current density with high faradaic efficiencies . Sn-GDE electrodes consist of a carbon paper as carbonaceous support, a microporous layer and the catalytic layer formed by the Sn carbon supported nanoparticles. Using GDEs we were able to achieve a 70% faradaic efficiency to formate at a current density of 150 mA cm^-2, obtaining a formate concentration of 2.5 g L^-1, which outperforms our results obtained with particulated electrodes. When the current density is increased and with a low catholyte flow, concentrations over 16 g L^-1 were achieved, but at the expense of a 42% faradaic efficiency. However, despite the promising results obtained with Sn-GDEs, more research efforts are still required in terms of stability of the electrodes and optimization of the process.

[1] D. Kopljar, A. Inan, P. Vindayer, N. Wagner, E. Klemm., J. Appl. Electrochem (2014) 44, 1107-1116

[2] Alvarez-Guerra M, Del Castillo A, Irabien A., Chem. Eng. Res. Des. (2014) 92, 692â??701

[3] A. Del Castillo, M. Alvarez-Guerra, A. Irabien, AIChE J., (2014) 60, 3557-3564.

[4] A. Del Castillo, M. Alvarez-Guerra, J. Solla-Gullón, A. Sáez, V. Montiel, A. Irabien., Appl. Energy (2015) 157, 165-173