Green hydrogen with PEC electrolysers: New insights into transport processes

One method of storing solar energy is to use PEC electrolysers to produce hydrogen. However, scaling up this technology remains challenging. Now, a team at the HZB Institute for Solar Fuels has used 2D fluorescence imaging and particle velocimetry to observe the movement of ions and dissolved gases within the electrolyte during electrolysis. These new insights may prove useful in the development of larger PEC electrolysers.

Green hydrogen plays a major role in achieving a climate-neutral future. One option for producing green hydrogen involves photoelectrochemical (PEC) electrolysers with special photoelectrodes to harness the energy of sunlight. In recent years, scientists have already achieved significant improvements in photoelectrodes, which can even catalytically accelerate the desired reactions – albeit only on a laboratory scale. However, when scaled up, efficiency drops dramatically. This is due to various factors, such as the larger volumes allowing for convective flows and pH gradients, which contribute to the degradation of the electrode materials.

Analysing transport processes 

‘In order to scale up, we need a better understanding of how ions and dissolved gases move within the electrolyte during these processes. To this end, we have developed several visualisation techniques at HZB to map these processes,’ explains Professor Roel van de Krol.

Surprising new insights

By 2D fluorescence imaging, it is possible to track the local pH levels and how they change over time; also the concentration of dissolved oxygen gas can be analysed by this technique. The movement of individual particles in the electrolyte can be determined using particle image velocimetry (PIV).

Both methods have now provided new insights. The experiments showed that a continuously flowing electrolyte in a pH-neutral electrolyte solution is not enough to prevent the formation of large pH gradients. ‘That surprised us,’ says van de Krol. ‘To avoid such gradients, we also had to increase the concentration of buffer ions.’

Experiment remains gold standard

Furthermore, the experimental results showed that the large pH gradients do not form at the surfaces, but rather within the volume. This was an unexpected finding, as computer simulations by other teams had predicted that such gradients would occur very close to surfaces. ‘This shows that computer simulations are not (yet) capable of truly capturing all physical relationships; the experiment remains the gold standard,’ van de Krol emphasises.

Outlook on upscaling

The work was carried out by Dr Feng Liang at HZB, who has since taken up a professorship in China. Co-author Dr Fatwa Abdi had established this field of research at HZB and is now conducting research at City University of Hong Kong.

These findings are valuable for driving forward the upscaling of efficient PEC electrolysers capable of producing green hydrogen on a large scale.