(659b) Invited Talk: Solar Water Splitting: A Decoupled Photoelectrochemical Benchtop Demonstration System

Photoelectrochemical (PEC) water splitting is an elegant route for direct conversion of solar energy to green hydrogen. Theoretically, photovoltaic-photoelectrochemical (PV-PEC) tandem cells with ideal photoelectrodes could surpass the efficiency of PV-electrolysis systems that use the same PV cells, owing to the enhanced solar energy conversion efficiency of multi-junction tandem cells.

Due to the diffuse nature of natural sunlight, an immense area must be covered by PEC cells to produce hydrogen at a commercial scale. However, in the current architecture, PEC cells are unsuitable for application at such a large scale. Namely, the current architecture is that of a single, hermetically sealed cell, that produces hydrogen and oxygen simultaneously in the same cell. Accordingly, a PEC hydrogen production plant would require hermetic sealing of the entire solar array, construction and maintenance of an immense hydrogen piping infrastructure, and constant monitoring in accordance with hydrogen safety guidelines. The safety hazards associated with conventional electrolyzers are also exacerbated in this case due to the low-current densities and intermittent power supply characteristic of PEC cells.

To overcome these challenges, we proposed a disruptive approach of decoupling of the hydrogen and oxygen evolution reactions using solid redox mediating electrodes based on nickel hydroxide[1]. This strategy enables oxygen evolution alone to take place at the PEC cell, while hydrogen evolution takes place in a separate electrolytic cell that does not require exposure to light. This eliminates the need to hermetically seal PEC cell. Furthermore, there is no need for a membrane or hydrogen collection from the solar (PEC) site, as hydrogen can be generated in a separate compact cell.

Thus far, only a handful of studies have proposed scalable PEC-PV tandem devices, all of which are based on the conventional coupled architecture. In this work, we present a first of its kind prototype of a benchtop-scale decoupled and membrane-free PEC device based on a 100 cm² hematite photoanode stack and commercial Si PV modules.[2] Nickel hydroxide electrodes are used as hydroxide-mediating auxiliary electrodes, enabling cell separation over > 10 m distance. We address the issues of material selection, stability and light harvesting, as well as designing, building and optimizing the system for continuous hydrogen production. Finally, we demonstrate successful operation of our prototype device under solar simulated sunlight and under natural sunlight.

[1] Landman, Avigail et al. “Photoelectrochemical water splitting in separate oxygen and hydrogen cells.” Nature Materials 16.6 (2017): 646-651.

[2] Landman, Avigail and Halabi, Rawan, et al. “Decoupled photoelectrochemical water splitting system for centralized hydrogen production.” Joule (2020)