🪫 Electrode poisoning - how to turn hydrogen into electricity?

Dihydrogen H₂ is a very useful energy vector for converting and storing renewable energy. Its production and use rely on devices, electrolyzers and fuel cells, at the heart of which electrochemical reactions involving hydrogen and oxygen take place, carried by electrodes typically made of platinum.

The efficiency and lifespan of these devices require perfect control of the physicochemical mechanisms occurring at the electrode surface. Their performance indeed strongly depends on the events that occur at the interface between the electrode and the electrolyte, a nanometric zone where water molecules, ions, and electric charges coexist.


Unfortunately, these interfaces remain difficult to observe in operando mode, i.e., directly during use, and many fundamental mechanisms remain poorly understood.

As part of the EMOSEINE project, scientists from the Institute of Physical Chemistry (CNRS/Université Paris-Saclay) and the Laboratory of Waves and Matter in Aquitaine (CNRS/Université de Bordeaux) tracked in real time the phenomena occurring on the surface of a platinum electrode immersed in an aqueous solution containing phosphates.

These phosphate ions (H_nPO₄^(3-n)- where n = 0,1,2: H₂PO4^-, HPO₄^2-, PO₄^3-), derived from phosphoric acid (H₃PO₄) in which the platinum electrocatalyst is immersed, are necessary to stabilize the pH and ensure ionic conduction.

Chemically and thermally stable at high temperature, this acid can be optimally used in fuel cells. However, knowledge of the adsorption/desorption mechanisms of phosphate ions in contact with electrodes, promoting or limiting their efficiency, remains very partial, given the complexity and variety of ionic species present.

To advance the understanding of these phenomena, the scientists developed an electrochemical cell coupled with nonlinear optical spectroscopy by second harmonic generation (SHG). This spectro-electrochemical cell allowed them to specifically probe in real time the chemical processes (reaction kinetics, nature and organization of species adsorbed on the platinum electrode surface, and final products) occurring on the surface of electrocatalytic materials immersed in an aqueous medium, regardless of the electrical voltage applied to them.

Their work shows that phosphate ions do not all interact with the platinum surface in the same way. The dihydrogen phosphate ion (H₂PO₄⁻) behaves as a weak adsorbent. It remains mostly at the periphery of the interface without directly occupying the metal's active sites and acts as a spectator ion.

Conversely, the less protonated forms of phosphates (HPO₄²⁻ and PO₄³⁻) exhibit a stronger affinity for the surface. They can compete with other species, such as hydroxide ions (OH⁻), to occupy catalytic sites. This adsorption can profoundly modify the electronic structure of the interface and slow down electrochemical reactions.

The scientists also show that these phenomena strongly depend on the pH of the solution. In an acidic medium, adsorption remains limited and relatively reversible. However, when the pH increases, interactions become stronger and more complex, with notable effects on reaction kinetics and electrode stability.

These results provide a better understanding of what is called "electrode poisoning," i.e., the blocking of the catalyst's active sites on its surface by adsorbed species, which reduces reaction efficiency. By precisely identifying which species are responsible for this phenomenon and under what conditions, this study constitutes an important step forward for the development of more efficient and durable fuel cells.

Author: Christophe CARTIER DIT MOULIN