Hydrogen power!

How water is split into hydrogen and oxygen at the nanoscale

March 02, 2023

The splitting of water into hydrogen and oxygen at an electrochemical electrode lies at the heart of renewable energies. Knowledge of the molecular structure and orientation of water molecules next to an electrode surface is of fundamental technological importance and an enigma in surface science. This can be traced to the challenge of probing the buried electrode-water interface. Scientists at the Max Planck Institute for Polymer Research have overcome this challenge using their developed ultrafast laser spectroscopy. They have clarified the chemistry and physics occurring at the electrode-water interface.

Fuel cells or cars which take hydrogen as a fuel: the splitting of water into oxygen and hydrogen – the so-called “electrochemical water splitting”- could be an important building block in a future energy economy. However, the chemistry and physics of water splitting, while part of the standard high school curriculum, is only poorly understood on a microscopic level.

Therefore, the research group around Yuki Nagata and Mischa Bonn has now looked at the water molecules in contact with an electrochemical graphene electrode using state-of-the-art laser spectroscopy. The water molecules (H2O), each consisting of two hydrogen atoms and one oxygen atom arranged in a triangular shape, can be converted to oxygen and hydrogen molecules when a voltage is applied to the electrode.

Insights into the microscopic structure and orientation of water molecules near the electrode are essential for understanding and improving water splitting. Yet probing the few water molecules directly at the electrode constitutes a major challenge. To selectively explore the water molecule near the electrode, the researchers used sum-frequency generation (SFG) spectroscopy. Two laser beams with different colors are focused onto the water near the electrode. The reflected light - with yet a third color - contains information about the structure of water molecules near the electrode. The scientists used atomically thin graphene as a transparent electrode to have the light reach the electrode-water interface. Using this scheme, the team clarified the chemistry and physics occurring at the electrode-water interface.

Remarkably, the researchers found specific OH groups pointing towards the bulk water phase, that, however, were not interacting with the water. Moreover, these OH groups were observed under conditions where one would expect water OH groups to point away from the bulk water phase. This counterintuitive observation holds the key to the mystery occurring in the 1/10000000 cm thickness of water near the electrode.

The solution to the puzzle is that electrons on the graphene electrode are released into the water, generating hydrogen and thereby changing the pH. This pH change is very tiny. It occurs directly at the electrode interface, and does not affect the pH change in the bulk. However, the pH in the 1/10000000 cm thickness of water near the electrode is greatly affected by such a release of the electron from the electrode. Specifically, the increase in pH gives rise to hydroxyl (OH-) groups that can attach to the substrate supporting the graphene. These OH groups point towards the bulk water, but cannot interact directly with it due to the presence of graphene.

The work directly reveals the reorganization of water on an electrochemical electrode surface and highlights the molecular-level effects of chemical reaction (pseudocapacitive process) at the electrode/aqueous electrolyte interface. Their findings are relevant for a wide range of scientific and technological systems, such as water desalination, biosensing, energy storage, and catalysis.

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