Electrolysis, the splitting of water into hydrogen and oxygen using electricity, is a tried and tested process, but is not yet fully utilised in terms of energy technology. Although the theoretical voltage required is 1.23 volts, in practice 1.5 to 1.6 volts are often necessary. This discrepancy is costly and slows down economic utilisation.
A research team led by Franz Geiger has now identified a central cause. Before oxygen is released, the water molecules must rotate on their axis in order to align their oxygen atoms with the electrode. Only then can the oxygen evolution reaction take place. This rotation requires a considerable amount of energy comparable to that which holds water molecules together in a liquid state.
Visualisation using laser technology
This insight was made possible by a new type of analysis method, phase-resolved second harmonic generation. Using this laser technology, the researchers were able to observe in real time when and how many molecules change their orientation. This data provides a precise energetic quantification of the rotation for the first time. A milestone for the further development of more efficient electrolysis processes.
Particular attention was paid to the haematite electrode, an inexpensive iron oxide that, despite its promising properties, has so far suffered from low efficiency. The new analysis now reveals where there is potential for optimisation.
Basic pH value as a lever for increasing efficiency
Another key factor is the pH value of the solution. The study shows that an alkaline environment, i.e. a pH value above 9, significantly reduces the energy required for molecule rotation. This significantly increases the efficiency of the oxygen evolution reaction. Electrolysis hardly takes place below this threshold.
This realisation opens up new perspectives for industrial hydrogen production. In combination with targeted catalysts and advanced cell materials, electrolysis plants can be operated more economically and with fewer resources in future.
