Abstract
Water-fed photo-electrolysis cells equipped with perfluorosulfonic acid (Nafion® 115) and quaternary ammonium-based (Fumatech® FAA3) ion exchange membranes as separator for hydrogen and oxygen evolution reactions were investigated. Protonic or anionic ionomer dispersions were deposited on the electrodes to extend the interface with the electrolyte. The photo-anode consisted of a large band-gap Ti-oxide semiconductor. The effect of membrane characteristics on the photo-electrochemical conversion of solar energy was investigated for photo-voltage-driven electrolysis cells. Photo-electrolysis cells were also studied for operation under electrical bias-assisted mode. The pH of the membrane/ionomer had a paramount effect on the photo-electrolytic conversion. The anionic membrane showed enhanced performance compared to the Nafion®-based cell when just TiO2 anatase was used as photo-anode. This was associated with better oxygen evolution kinetics in alkaline conditions compared to acidic environment. However, oxygen evolution kinetics in acidic conditions were significantly enhanced by using a Ti sub-oxide as surface promoter in order to facilitate the adsorption of OH species as precursors of oxygen evolution. However, the same surface promoter appeared to inhibit oxygen evolution in an alkaline environment probably as a consequence of the strong adsorption of OH species on the surface under such conditions. These results show that a proper combination of photo-anode and polymer electrolyte membrane is essential to maximize photo-electrolytic conversion.
Highlights
The continuous increase of energy consumption and the associated increase of greenhouse gas emissions causing global warming have produced significant concerns
A sketch of the photo-electrolysis cell fed with pure deaerated water is represented in Figure 1 together with the energy diagram related to the water splitting process for this device
The information gained from this study, using TiO2 as model system, and the selected membranes can be useful in developing photo-electrolysis cells based on smaller energy gap semiconductors than TiO2, providing a relevant increase of water splitting efficiency
Summary
The continuous increase of energy consumption and the associated increase of greenhouse gas emissions causing global warming have produced significant concerns. It is widely recognised that there is an urgent need to increase the level of utilisation renewable sources in order to address the environmental issues. Hydrogen is an alternative fuel and, according to its high energy density and clean combustion, can represent a suitable energy vector mediating between renewable sources and sustainable mobility [1,2,3,4]. The energy associated with solar radiation striking the earth is several orders of magnitude the global energy consumption. An efficient and wide-scale use of sunlight can occur through a decentralised conversion into hydrogen that can be stored and used as fuel in several applications including highly efficient fuel cell devices for stationary generation, transportation and portable power [4].
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