Photoelectrochemical Splitting of Water Using Nanostructured Photoelectrodes of p-Cu₂o and n-Fe₂O₃

Abstract

The quasi-unidimensional (Q1D) nanostructured semiconductors materials have certain advantages, such as, have a higher surface area/volume ratio [1-3], better crystallinity and present a decrease of the recombination process, due to the less distance that the photogenerated charge carriers must travel to attain the interface [4]. Thus, these nanomaterials are being applied in photovoltaic and photoelectrochemical devices. One of these cases is the manufacture of nanostructures for energy storage and production of fuels with a high relation between produced energy/CO2 emissions. An example of the latter is hydrogen production from water splitting. In normal water electrolysis process, i.e. with platinum electrodes, a bias potential of approximately 1.2 V is required. This value is equivalent to 463.2 kJ / mol H2, for a current value of 0.03 mAcm-2. For this reason, devices are required that reduce the energy consumption, in this way the metallic oxides semiconductors appear as good candidates. Among them, iron (III) oxide (n-Fe2O3) and copper (I) oxide (p-Cu2O) can be highlighted. These oxides have band gap values of 2.0 eV and 2.1 eV, respectively. Therefore, they can cover a wide area of the solar spectrum. On the other hand, ultrasound irradiation coupled with chemical methods had shown to be a convenient way for manipulate the size and shape of nanostructured materials (Q1D structures like nanowires, nanoneedles, nanorods, nanobelts and nanotubes). These techniques are called Sonochemical and Sonoelectrochemical. In this work the p-Cu2O were prepared by ultrasound-assisted anodization of copper foils (Merck, 99.7%, 0.1 mm) at two different potential values (75 V and 100 V) in ethylene glycol (EG; 99.8%, anhydrous), 5wt% and 10wt% of H2O, 0.5wt% NH4Cl and 75°C. The anodizations were carried out using ultrasonic waves (37 kHz, 60 W) and for 720 s < t < 900 s. The above process was carried out using a two electrode system: flag shaped 1.0 cm2 Cu foil as anode and carbon plate, 22.55 cm2 as cathode; the distance between cathode and anode was kept at 3 cm. The anodized samples are properly washed with distilled water and dried with Argon flow. Synthesis of n-Fe2O3 was performed following the methodology previously published by R. Schrebler et al [5]. Preliminary result obtained with Fe|nanotubes n-Fe2O3|0.05 M Na2SO4 (pH 10)|nanostructures p-Cu2O|Cu system, under illumination conditions of both photoelectrodes, requires a bias potential of ~ 0.200 V. This value equals 77.3 kJ / mol H2, for the same current density condition as in the previous case. Therefore, a decrease in the energy cost by using photoelectrochemical cells to obtain H2 from the splitting of the water from an aqueous alkaline solution is evidenced. Acknowledgment The financial support of Fondecyt-Chile (Project N° 1140963) and DI-PUCV (Projects N° 125.789/2014) is gratefully acknowledged by the authors. References R. Vijaya Kumar, R. Elgamiel, Y. Diamant, A. Gedanken, Langmuir, 17 (2001) 1406. V. Sáez, T.J. Mason, Molecules, 14 (2009) 4284. Bisquert, Phys. Chem. Chem. Phys., 10 (2008) 49. T. Zhang, Z.U. Rahman, N. Wei, Y. Liu, J. Liang, and D. Wang, Nano Res., 10 (2017) 1021. R. Schrebler, L.A. Ballesteros, H. Gómez, P. Grez, R. Córdova, E. Muñoz, R. Schrebler, J.R. Ramos-Barrado, and E.A. Dalchiele, J. Electrochem. Soc., 161 (2014) H903.