Stainless Steel Mesh Modified By Nanoparticulate TiO2- and TiO2/C-Based Composites for Photoassisted Electrochemical Reduction of Aqueous CO2
Authors: Manríquez Juan , Valdez-Nava Jesus Israel , Cárdenas Jesús , García-Melo José Alberto , Bustos Erika , León Itzel , Chávez-Martínez Diana Laura , Ángeles-Garduño Itzia Yuneli 
Published: February 2020

CO2 electrochemical reduction (CO2-ER) to generate low molecular weight organic molecules (e.g. HCOH, HCOOH, CH3OH) and the H2 evolution reaction (HER) are competitive in aqueous medium, due to the second process is much fast than the first one. Consequently, RE-CO2 shows low faradaic efficiency and poor selectivity regarding the reaction products [1]. Nevertheless, it has been observed that the photoassisted CO2-ER is favorable on TiO2 (bandgap = 3.2 eV) [2], because this material is capable to generate TiIIIOOH sites under UV light (3.2 eV, 385 nm [3]) or Visible illumination (2.48 eV, 500 nm) [4]. This phenomenon occurs after a significantly number of electrons have been promoted from the valence band (bv) or from the oxygen vacancies (TiIII-H2O), respectively, up to the conduction band (cb). On the other hand, it has been reported that C-modified electrodes can inhibit the electron-transfer kinetics for the RHE in CO2-saturated aqueous media, especially when C materials were functionalized to have organic groups such as C-OH, C=O and COOH [5]. In this investigation, nanoparticulate TiO2 (P25 Degussa) and C (Vulcan XC-72R Cabot) were electrophoretically deposited (EPD) on the surface of stainless steel (SS) mesh AISI 304, looking for preparing modified electrodes containing TiO2/C nanocomposites (SS//TiO2/C) able to carry out photoassisted CO2-ER in aqueous medium [2,6]. On the contrast, nanoparticulate TiO2 was EPD on stainless steel (SS) mesh (SS//TiO2) for comparison purposes. The SS//TiO2/C and SS//TiO2 electrodes showed roughness factors of 416±35 and 171±10, respectively, thus indicating that the TiO2 electroactive area was increased in the presence of the TiO2|C junction [7]. Electrochemical behaviors of illuminated (UV light at 385 nm) SS//TiO2/C and SS//TiO2 electrodes were evaluated in aqueous pH 2 sulfates buffer solution (SBS, previously deoxygenated and CO2-saturated). Experimental results indicated that when the junction TiO2|C junction is established, polarization of CO2 molecules (d
+CO2
d
-) was highly favored by C=O groups located on the nanoparticulated C surface [5], thus driving to the formation of CO2
·
-
radicals via oxidation of TiIIIOOH sites. Sets of reactions 1-4 and 5-7 represent the photoactivation mechanisms that can be reasonably proposed to explain the RE-CO2 on SS//TiO2/C and SS//TiO2 electrodes, respectively.

TiIVO2|C=O + H+ + hv --> TiIIIOOHbc|C=O (1)

TiIIIOOHbc|C=O + H+ --> TiIVO2|C-OH (2)

TiIVO2|C-OH + CO2
⇄ TiIVO2|(C-OH)(d
+CO2
d
-)ads slow (3)

TiIVO2|(C-OH)(d
+CO2
d
-)ads --> (CO2
·
-)adsTiIVO2|C=O + H+ (4)

TiIVO2 + H+ + hv --> TiIIIOOHbc (5)

TiIIIOOHbc + CO2
⇄ (d
+CO2
d
-)adsTiIIIOOHbc slow (6)

(d
+CO2
d
-)adsTiIIIOOHbc --> (CO2
·
-)adsTiIVO2 + H+ (7)

CO2 electrolysis for 30 min were carried out at -0.7 V and -0.9 V (both vs. Ag|AgCl 3M NaCl) using SS//TiO2/C and SS//TiO2 electrodes, respectively. These potentials were choosen due to the generation of TiIIIOOH was found to be maximal for each electrode material. Consequently, the oxygen chemical demand (OCD) for the pH 2 SBS saturated with CO2 increased from 0 to 33±3 ppm (at SS//TiO2/C electrodes) and from 0 to 32±4 ppm (at SS//TiO2 electrodes), thus confirming that the photoassisted CO2-ER to organic molecules is equally viable on both electrode materials. However, the applied potential to the SS//TiO2/C system was 0.2 V less cathodic than that applied to the SS//TiO2 system, thus strongly suggesting that the free energy associated to the CO2 activation was significantly decreased in the presence of the TiO2|C junction.

References

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