Ilenia Rossetti
Dept. Chemistry, Universita' degli Studi di Milano

Solar energy storage: catalytic and photocatalytic processes for the production of H2

H2 is considered a promising energy vector to be used either as fuel in internal combustion engines, or in fuel cells, with overall higher efficiency. More generally, it can be seen as a way to store solar energy, to be used as support for intermittent renewable sources (e.g. photovoltaics or wind). At the moment it is predominantly produced through thermochemical processes based on fossil sources (i.e. exploiting the solar energy stored in such raw materials in the ancient past. Increasing efforts are put in place to adapt such thermocatalytic processes to the conversion of biomass, leading to a virtuous cycle, which exploits the energy sored in biomass during its growth in a shorter time cycle. Examples will be given on the steam reforming of bioethanol, which is a process in very advanced engineering stage.
On the other hand, the direct use of solar energy is intriguing for H2 production. The direct photocatalytic water splitting is thermodynamically limited by the high Gibbs free energy (237 kJ/mol) and very low efficiency is reported for direct WS also for kinetic reasons. Sacrificial reagents, such as methanol or EDTA, can improve hydrogen productivity, but they are non renewable. Compared to thermochemical processes, photocatalytic reforming (PR) is a valid approach to produce H2 under ambient conditions and using sunlight, the cheapest energy source available on earth. PR is also thermodynamically more feasible than WS. Thus, the attention is here focused on the use of waste organic compounds to be used as sacrificial agents, such as organic compounds obtained through the photoreduction of CO2 or the photoreforming of waste organic solutions.