Life cycle assessment of hydrogen produced through a liquid-phase photocatalytic process

Main Presenter:    Tabita González Zubieta 

Co-Authors:   Vafa Feyzi     Javier Dufour      Diego Iribarren                                    

Hydrogen can play a crucial role in the energy transition towards clean and renewable sources, and therefore in global climate change mitigation. However, nowadays only 4% of the worldwide hydrogen production comes from renewable sources [1]. Hence, the deployment of a hydrogen economy requires the development of diverse methods to obtain green hydrogen, such as renewable electrolysis and light-driven photocatalytic processes. In particular, photo-activated processes for hydrogen production are novel techniques for water splitting on the surface of a photocatalyst, excited by light irradiation. Different kinds of photocatalysts have been tested for hydrogen production in gas and liquid phases in the presence of different sacrificial agents. Unlike the large number of studies on laboratory-scale synthesis and testing of photocatalysts for hydrogen production, considering photocatalytic hydrogen production in a large-scale process is a research gap. In this regard, the assessment of
market-scale photocatalytic processes for hydrogen production is necessary to check their sustainability and pave the way for industrialisation.
In this study, the life cycle assessment (LCA) methodology was applied to identify the environmental hotspots of a photocatalytic process producing 5 tonnes of hydrogen per day in the presence of ethanol as the sacrificial agent. The process was simulated using Aspen Plus®️ to provide inventory data for the LCA study. A cradle-to-gate approach was followed, and a functional unit of 1 kg of pure hydrogen was defined. The hydrogen production rate was determined by an intrinsic kinetic model for a Pt/TiO2 photocatalyst suspended in ethanol aqueous phase [2], while annual production was determined based on the annual radiation in Móstoles (Madrid, Spain) considering absorption of UV spectrum by the photocatalyst. This photocatalytic system yields hydrogen as the primary product, and methane as the main by-product. According to the photocatalyst deactivation rate, it must be replaced in two-day cycles in order to maintain the hydrogen production rate.
The results for the carbon footprint indicator show that the analysed process exceeds the established target for qualification as renewable hydrogen. The high rate of catalyst deactivation and sluggish reaction kinetics were identified as hotspots of the system. Nevertheless, this situation could be reversed by enhancing the durability of the photocatalyst via cyclic regeneration, sensibly selecting the origin of the sacrificial agent, and implementing by-product use strategies such as reforming the produced methane for increased hydrogen production.

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