Performance of CO2 electrochemical reduction with surface modified self-growing SnO2 on carbon cloth electrode prepared by hydrothermal method

Over the past 100 years, the consumption of a large amount of fossil energy has led to an increase in CO2 in the atmosphere, triggering a serious greenhouse effect. In response to the energy crisis and global climate change, some strategies have been proposed to reduce CO2 emissions and convert CO2, such as biotransformation, photochemical process, thermochemical and electrochemical reduction. Among these technologies, electrocatalytic reduction of CO2 is an effective method for the conversion and utilization of CO2 and mitigation of greenhouse effect. Sn-based catalysts are widely used in electrocatalytic reduction of CO2 due to its non-toxicity, abundant reserves, low cost and high selectivity. However, the low solubility of CO2 in electrolyte solution (only 0.33 mol/L at room temperature and ordinary pressure) limits the electrochemical reduction performance. Herein, the SnO2 nanosheet catalyst was prepared on the carbon cloth substrate by hydrothermal self-growing method. Then, trimethoxysilane was used to modify the wettability of the electrode surface to make it hydrophobic, increasing the contact surface of the electrode-solution-CO2 three-phase reaction. The surface modified electrode was denoted as SnO2/CC-HB and the unmodified one was SnO2/CC-HL. The micromorphology and element valence states of hydrophobic and hydrophilic electrodes were studied by physical characterization. The results show that SnO2 nanosheets grow successfully on the carbon cloth for both SnO2/CC-HB and SnO2/CC-HL electrodes. By measuring the contact angle, it is found that SnO2/CC-HB electrode shows hydrophobicity while SnO2/CC-HL electrode is completely hydrophilic. The electrochemical reduction performance of SnO2/CC-HB and SnO2/CC-HL electrodes, including electrochemical active surface area, Tafel slope and electrochemical impedance were measured in an H-cell with three-electrode system. The electrolyte was 0.5 mol/L potassium bicarbonate CO2 saturated solution (pH 7.3). The results show that although the electrochemical activity of SnO2/CC-HB electrode is lower than that of SnO2/CC-HL electrode, the CO2 transfer and the three-phase reaction interface are increased because of the hydrophobicity. The Tafel slope of both electrodes is greater than 120 mV/dec, indicating that the restricted process of the electrochemical reduction of CO2 reaction is the process of CO2 gaining electrons. Moreover, the Tafel slope of SnO2/CCHB electrode is smaller, indicating that SnO2/CC-HB electrode has a better reaction kinetics. In addition, although the charge transfer resistance of SnO2/CC-HB electrode is higher, the mass transfer resistance is lower, suggesting enhanced mass transfer for SnO2/CC-HB electrode. Compared with SnO2/CC-HL electrode, SnO2/CC-HB electrode exhibits a better electrochemical reduction performance. The current density is (28.0 +/- 0.6) mA cm(-2) and Faradaic efficiency is 77.2%+/- 1.9% at -1.8 V (vs. Ag/AgCl) electrolytic potential, which is 14.7% higher than that of SnO2/CC-HL electrode. After 12 hours of electrochemical reaction, the SnO2/CC-HB electrode still shows good hydrophobicity, and the Faradaic efficiency remains 72.6%, which proves that the electrode has a good durability. In summary, tuning the wettability of the electrode can improve the mass transfer of CO2 and effectively enhance the performance of CO2 electrochemical reduction.