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 Ti^IIIOOH 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 (Ti^III-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 Ti^IIIOOH 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.

Ti^IVO₂|C=O + H⁺ + hv --> Ti^IIIOOH_bc|C=O (1)

Ti^IIIOOH_bc|C=O + H⁺ --> Ti^IVO₂|C-OH (2)

Ti^IVO₂|C-OH + CO₂ ⇄ Ti^IVO₂|(C-OH)(^d+CO₂^d-)_ads slow (3)

Ti^IVO₂|(C-OH)(^d+CO₂^d-)_ads --> (CO₂^·-)_adsTi^IVO₂|C=O + H⁺ (4)

Ti^IVO₂ + H⁺ + hv --> Ti^IIIOOH_bc (5)

Ti^IIIOOH_bc + CO₂ ⇄ (^d+CO₂^d-)_adsTi^IIIOOH_bc slow (6)

(^d+CO₂^d-)_adsTi^IIIOOH_bc --> (CO₂^·-)_adsTi^IVO₂ + 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 Ti^IIIOOH 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

[1] J. Wu, Y. Huang, W. Ye, Y. Li, Adv. Sci., 4 (2017) 1-29.

[2] G.K. Ramesha, J.F. Brennecke, P.V. Kamat, ACS Catal., 4 (2014) 3249-3254.

[3] J. Manríquez, L.A. Godínez, Thin Solid Films, 515 (2007) 3402-3413.

[4] H. Kozuka, Y. Takahashi, G. Zhao, T. Yoko, Thin Solid Films, 358 (2000) 172-179.

[5] S. Pérez-Rodríguez, E. Pastor, M.J. Lázaro, Int. J. Hydrogen Energy, 43 (2018) 7911-7922.

[6] J.M. Peralta-Hernández, J. Manríquez, Y. Meas-Vong, F.J. Rodríguez, T.W. Chapman, M.I. Maldonado, Luis A. Godínez, J. Hazard. Mater., 147 (2007) 588-593.

[7] S.H. Kang, J.-Y. Kim, Y.-E. Sung, Electrochim. Acta, 52 (2007) 5242-5250.