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Choreographing water molecules to speed up hydrogen production

Electrocatalysis, which accelerates chemical reactions driven by an electric potential at a solid–liquid interface, could be a key contributor to a sustainable global economy because it can convert electrical energy from renewable power sources into green fuels such as hydrogen gas1. Writing in Nature, Wang and colleagues2 describe an important advance in the molecular understanding of how the rate of an electrocatalytic process is rooted in the structure of water at the interface between a solid electrode and an aqueous salt solution (an electrolyte). Their findings could help to improve the reaction selectivity and energy efficiency of electrocatalytic interfaces.

Many of the unique physical–chemical properties of liquid water can be ascribed to the network of weak attractive forces of hydrogen bonding between molecules3. Water molecules at an interface formed by liquid water and a gas, solid or another liquid necessarily have fewer hydrogen-bonding partners. Those within one or a few molecular layers from the interface therefore often adopt different structural motifs (geometric arrangements of molecules) from those in the bulk liquid4,5.

Spectroscopic techniques that probe the stretching of bonds between oxygen and hydrogen, or the vibrations of ‘bending’ water molecules, have contributed to our understanding of the structure of interfacial water5,6. Analysis of these vibrations by infrared or Raman spectroscopy of interfacial water reveals the strength of the hydrogen-bonding network and the presence of specific structural motifs at the interface. Such experiments are technically challenging because they require sophisticated methods capable of selectively probing only a few layers of water at the interface. Furthermore, the interpretation of vibrational spectra is complicated by coupling effects between vibrations in the water molecules themselves7.

Water molecules at electrolyte–electrode interfaces undergo a reorientation when the surface charge on the electrode changes from positive to negative, or vice versa8,9. This reorientation is due to the interaction of electric dipoles of the molecules with the interfacial electric field that arises from the electrode surface charge, which is controlled by the electrode potential. At high potentials, positive or negative ions (cations or anions, respectively) in the electrolyte, whose charge is opposite to that of the net surface charge on the electrode, concentrate in the vicinity of the electrode. This increased concentration of ions is expected to affect the interfacial water structure. However, under potentials at which the electrocatalytic conversion of water to hydrogen occurs, the formation of bubbles of hydrogen gas interferes with spectroscopic measurements, making spectroscopy under reaction conditions highly challenging.

Wang et al. have neatly circumvented these difficulties by devising an innovative experimental approach in conjunction with Raman spectroscopy. They used a metal electrode comprising palladium atoms to act as a catalyst that dissociates water into molecular hydrogen (H2) and hydroxide (OH; Fig. 1). As the electrode potential became more negative, they found that the water structure gradually shifted from a relatively disordered to a more ordered state.

Figure 1

Figure 1 | Catalysis of water dissociation at a palladium electrode. Wang et al.2 studied the structure (geometric arrangement) of water molecules at the surface of an electrode. a, At low concentrations of dissolved sodium ions (Na+, not shown) and at relatively positive electrode potentials, a network of water molecules connected by intermolecular hydrogen bonds surrounds the electrode, which comprises a layer of palladium (Pd) atoms on a gold substrate. b, At higher concentrations of sodium ions and at relatively negative electrode potentials, hydrated sodium ions (those with associated water molecules) are electrostatically attracted to the electrode, narrowing the distance between Pd atoms and hydrogen atoms in the water molecules. This accelerates electron transfer between the electrode and the water molecules, causing the water to dissociate into hydroxide ions and hydrogen atoms; the hydrogen atoms initially adsorb to palladium atoms, and then combine to form hydrogen molecules (H2).

The spectroscopic evidence indicates that this transition could be due to weakening of the hydrogen-bonding network, with a loss of water molecules that originally had four hydrogen-bonding partners and a concurrent gain of water molecules associated with sodium cations (Na+) from the electrolyte; such water molecules form a ‘hydration shell’ around the ions. Hydrated sodium cations are electrostatically attracted to the negatively charged electrode surface and accumulate at the interface. Intriguingly, the authors found that the population of interfacial water molecules in the hydration shell of sodium ions tracks the rate of hydrogen formation across different single-crystal surfaces of palladium.

For a molecular picture of the more ordered state of interfacial water and its role in the rate of hydrogen formation, Wang et al. turned to ab initio molecular-dynamics modelling. They found that water in the hydration shell of the sodium ion can more closely approach the electrode surface than can other water molecules. This manifests in a more pronounced shift of the O–H stretching vibration of water associated with a sodium ion when the electrode potential decreases. Their theoretical modelling indicates that narrowing the physical gap between the hydration shell and the palladium surface aids electron transfer from the electrode to the water, enhancing the rate of water dissociation into molecular hydrogen and hydroxide ions.

The insights derived from this work are relevant to other technologically useful electrocatalytic processes that involve water dissociation, such as the hydrogenation of carbon dioxide to hydrocarbons or of nitrogen gas to ammonia. In those reactions, the dissociation of water needs to be carefully controlled to achieve the desired product selectivity. Wang et al. have shown that the structure of interfacial water, and, consequently, the dissociation of water, can be systematically tuned by appropriate choice of the electrode’s crystal facet and the electrolyte’s cation concentration and identity.

Inevitably, some questions remain. For example, how do the dynamics of interfacial water change with decreasing potential? These dynamics could be crucial in the formation of hydrogen and in electron transfer10,11. Investigations are now required to study the extent to which the dynamics of interfacial water at electrocatalytic interfaces can be altered with decreasing electrode potential.

Nature 600, 43-44 (2021)

doi: https://doi.org/10.1038/d41586-021-03511-5

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The author declares no competing interests.

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