The energy available in 1 kg of molecular hydrogen gas is the same as in almost 3 kg of gasoline. Hydrogen’s superior energy density has attracted the attention of scientists and engineers working to develop more efficient and sustainable energy systems to replace fossil fuels (see the article by Joan Ogden, Physics Today, April 2002, page 69).

One way to boost H2 production is electrocatalysis, which drives chemical reactions with an electric potential at a liquid–solid interface. In the hydrogen-evolution reaction, two protons and two electrons combine at a cathode to yield H2. The reaction rate strongly depends on interfacial water—that is, the water in the immediate vicinity of a solid electrode’s surface. It behaves differently from the bulk and forms a layered, ordered structure.

Chemists have used various spectroscopic methods over the years to try to better understand how the specific structure interacts with...

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