Conceptual Design of an industrial-scale artificial leaf device

Abstract

Hydrogen, if produced from clean and abundant sources, has the potential for solving the concerns on energy supply security, climate change and local air pollution. Photoelectrochemical (PEC) water-splitting is a promising technology under development for the production of hydrogen from water by using sunlight. This design project aims to investigate the practical implementation of this innovative technology by developing aninitial conceptual design of a modular PEC water-splitting device that could be on the market by 2020.
An analysis of the state-of-the-art of the so-called artificial leaf technology was used to identify the main design challenges: (a) the need of finding efficient, durable, low-cost, earth-abundant semiconductors and catalysts, (b) the separation of the evolved gases in a reliable way to ensure the safety of the device, (c) the optimization of the components size and relative positioning to minimize internal losses and enhance light absorption, and (d) the optimum operating conditions.
To facilitate the design process of a device that could overcome the identified challenges, a step-wise methodology was applied. In each level, various design alternatives were investigated and evaluated according to technical, economical, safety and sustainability criteria. A device consisting of one photoelectrode and a counter metal electrode facing each other was selected, since this configuration offers low Ohmic losses.
Moreover, the photoelectrode is illuminated from the back to minimize the light losses. Low cost and earthabundant materials were selected for the main components: (i) multifunction a-Silicon for the photoelectrode, (ii) Nickel Molybdenum protection layer for the photoelectrode and (iii) Nickel counter electrode. For these materials to be stable and efficient, the device should operate under alkaline conditions. Moreover, to ensure the separation of the gases, an anion exchange membrane is placed in between the electrodes. Nevertheless, the design offer flexibility to implement material developments.
The economic feasibility of a hydrogen production plant utilising the designed device has been investigated, leading to potential hydrogen cost below 6 $/kg. This device could be manufactured with commercially available components and manufacturing process, with an estimated cost of ~70 $/m2. Moreover, a sustainability life cycle assessment (LCA) showed the potential environmental benefits of this technology,
with an energy payback time lower than 2 years, and savings of 2.5 ton CO2 eq. emissions per m2 of device during its full lifetime (15 years). It was concluded that the developed conceptual design could succeed in the market, providing a safe and environmental friendly process for hydrogen production.
Nevertheless, some practical issues were identified that need to be resolved before this PEC technology is marketable, and therefore it is recommended that laboratory research focuses on the further development of (a) protection layers to improve the stability of the semiconductor photoelectrodes and (b) anion exchange membranes to minimize the gas crossover and ensure the safety of the device. With respect to engineering development of the device it is recommended to initiate a detailed design project that focuses on the optimization of the operating conditions and the flow management to minimize the internal losses and the gas crossover.