A modular device for large area integrated photo-electrochemical water-splitting

Solar water splitting is a promising route for hydrogen production from renewable sources. It can be readily achieved by combining two commercial technologies: photovoltaics and electrolysis. However, photovoltaic powered electrolysis is still too expensive for large-scale solar hydrogen production. In such a device, a solar cell serves as a power source with sufficient output voltage to initiate an electro-chemical reaction, which may be the splitting of water into molecular oxygen and hydrogen.

The solar cell is required to have multiple junctions in order to provide adequate output voltage at a practical solar-to-hydrogen efficiency. A vertical set up of an integrated multi-junction solar cell and an electrochemical cell is of particular interest. In the setup, both components are in direct contact in a way that the electrical rear contact of the solar cell also functions as an electrode of the electrolyzer. The setup has the advantage of a compact design and the avoidance of the absorption of the incident light by the semi-transparent catalyst or the electrolyte. However, the close contact of the electrolyte and the semiconducting materials put high demands on the chemical stability of the contact.

A number of reports on the feasibility of solar water splitting devices with a device area of less or equal to 1 cm² have been published. However, for commercial realization of such technology, the area will have to be scaled up significantly. Unfortunately, current collection of a large area solar cell via the front electrode with substantial sheat resistance will lead to considerable ohmic losses. In addition, protecting the solar cell against corrosion using protective coatings is challenging. Therefore, the solar cell should be either inherently stable in the electrolyte or physically separated from the electrolyte.

Jan-Philipp Becker and colleagues at Forschungszentrum Juelich GmbH in Germany demonstrated a scalable integrated photovoltaic-biased electro-synthetic cell module based on multi-junction thin film silicon solar cells with an area of 64 cm² and a compact design, applying a metal sheet to ensure the stability of the cell. Their research work is published in Journal of Materials Chemistry A.

The upscaling of the photo-electrode entailed the fabrication of multi-junction silicon solar cells utilizing suitable grid contact patterns. The authors demonstrated two successful routes for the technical implementation of the latter: thermal evaporation of silver grids and an innovative laser scribing process. Laser scribing was applied in an integrated photovoltaic electro-chemical module with an active area of 64 cm². Furthermore, sheet metal electrodes were applied to protect the solar cell from corrosion. The authors employed two sets of catalysts in alkaline and acidic electrolytes. Finally, membranes were used to separate the half cells and to avoid oxygen and hydrogen mixing.

The study demonstrated a successful realization of an up-scaled, vertically integrated photovoltaic-biased electro-synthetic cell for solar water splitting based on thin film silicon multi-junction solar cells. The setup provides a tool for the long-term investigation and optimization of all elements on the electrolyzer part of the photovoltaic-biased electro-synthetic cell module, such as the catalysts and the reactor geometry, by overcoming chemical stability concerns at the semiconductor/electrolyte interface. The modular design allows for the exchange of each element individually and is compatible with electrolysis in acidic and alkaline electrolytes.