31.07.2024
Green hydrogen: ‘Artificial leaf’ becomes better under pressure

Hydrogen can be produced via the electrolytic splitting of water. One option here is the use of photoelectrodes that convert sunlight into voltage for electrolysis in so called photoelectrochemical cells (PEC cells). A research team at HZB has now shown that the efficiency of PEC cells can be significantly increased under pressure.

Some call it an ‘artificial leaf’: instead of the natural Photosystem II complex that green leaves in nature use to split water with sunlight, photoelectrochemical cells, or PEC cells for short, use artificial, inorganic photoelectrodes to generate the voltage required for the electrolytic splitting of water from sunlight.

Minimising losses
The best performing devices already achieve impressive energy conversion efficiencies of up to 19 per cent. At such high efficiencies, losses due to bubble formation start to play an important role. This is because bubbles scatter light, preventing optimal illumination of the electrode. Moreover, bubbles may block the electrolyte from contacting the electrode surface and thus cause electrochemical deactivation. To minimize these losses, it would help to reduce the bubble sizes by operating the device at higher pressure. However, all PEC devices reported thus far have been operating at atmospheric pressure (1 bar).

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Energy losses halved
The analysis shows that increasing the operating pressure to 8 bar halves the total energy loss, which could lead to a relative increase of 5-10 percent in the overall efficiency. “The optical scattering losses can be almost completely avoided at this pressure," explains Liang. “We also saw a significant reduction in product cross-over, especially the transfer of oxygen to the counter electrode”.

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