How potentials and electric fields together control reactions on catalyst surfaces

Key aspects

• Two effects are observed: the applied overpotential tends to lower the activation energy, while interfacial electric fields raise it.
• The chemical potential of the electrons is linked to the potential of zero total charge, which describes the potential where the surface has no excess charge. The difference between the applied potential and the potential of zero total charge can serve as proxy for the degree of interfacial polarization.
• For the Pt(111) surface, the CO-coverage directly modulates the chemical potential of the electrons due to local charge rearrangement in the Pt-CO bonds. This provides the opportunity to control the potential of zero total charge via the CO coverage.
• Through their potential- and coverage-dependent Arrhenius analysis, the researchers reveal a multi-dimensional kinetic landscape.

Why this matters?

Electrochemical reactions power technologies central to the energy transition, including fuel cells and water electrolyzers. The energetic cost of driving these reactions forward is closely related to the overpotential required. Ideally, this overpotential should be reduced as much as possible to improve the energy conversion efficiency and help these new technologies to become cost-competitive with fossil fuels. However, despite decades of research, the nature and origin of their kinetic limitations remain poorly understood.

For a very narrow range of reactions across static interfaces, the applied potential is directly translated into a thermodynamic driving force along the reaction coordinate that primarily reduces the activation energy. However, for most reactions, the overpotential at least partially modulates the structural and chemical composition of the solid-solution interface.

Dr. Sebastian Oener, group leader of the Interfacial Ionics team explains further: ”Previously, our group discovered the widespread existence of overpotential-dependent compensation effects and correlations between the Arrhenius pre-exponential factor and the apparent activation energy across many electrocatalytic reactions and catalysts. Until then, such effects were thought to be important for a much narrower range of conditions or disregarded altogether by drawing unwarranted parallels to other fields. However, in general, the overpotential can induce electric polarization, change the coverage of intermediates or induce structural and chemical changes on the electrode surface. Therefore, we decided to study a test-reaction for which we can control the coverage experimentally, while keeping the parallel structural transformations to a minimum. Together with Dr. Francisco Sarabia, we quickly concluded that the CO oxidation on Pt(111) might be an excellent test reaction for this task.”

What the researchers did

PhD student Caroline Aschendorf investigated carbon monoxide (CO) stripping over a Pt(111) single crystal surface. First, a well-known amount of carbon monoxide (CO) was adsorbed onto the surface and then oxidatively stripped at an applied step potential to calculate the amount of remaining CO coverage on the Pt(111) surface. This way, it was possible to extract a continuous current vs. coverage dependence at constant step potentials. Repeating these transients for different temperatures and performing electrochemical Arrhenius analysis, not only as a function of the step potential, but also as a function of the CO coverage allowed the researchers to extract the apparent activation parameters for a Faradaic reaction as a function of two independent experimental control variables.

Dr. Sarabia adds: “Another uniqe angle of this study comes from the fact that the coverage can give direct access to the local chemical potentials of the electrons. This way, we can change the degree of interfacial polarization independently from the applied potential and study how it impacts the activation parameters.”

Caroline explains: ”When the CO coverage on the Pt(111) surface is modified, different CO binding motifs on the surface can lead to a local charge rearrangement in the chemical bonds that can directly impact the work function and chemical potential of the electrons. This was already previously been observed by others, including Prof. Gerhard Ertl, in ultrahigh vacuum conditions. We wanted to understand whether such a local charge rearrangement in the CO-Pt bond, might modulate the electrochemical potential of zero total charge at the surface, and how this impacts the electrochemical kinetics.”

To test this hypothesis, the researchers determined the CO coverage-dependent potential of zero total charge and discovered a CO coverage dependence that qualitatively matches the work function changes reported by Prof. Ertl.

Bringing it all together

When the applied potential was increased, the apparent activation energy decreased—consistent with a stronger driving force helping the reaction to proceed. In contrast, when the applied potential was held constant, but the difference between this potential and the potential of zero total charge increased, the apparent activation energy increased. The researchers further found indications that these changes are related to water reorientation in the interfacial double layer. More generally, the opposing trends show that the reaction speed depends not only on how much potential is applied but also on how that potential redistributes charge and alters the chemical potentials in the extended environment, including the interfacial double layer.

Outlook

The results are a substantial advance in the fundamental understanding of the kinetics of reactions over polarizable interfaces. In fact, the link between the potential of zero charge and the kinetics of a Faradaic reaction has been a long-standing quest in electrochemistry. Whereas more studies are needed to dissect other kinetically-relevant effects at the interface, the team has demonstrated a new way to dissect such interplays in detail.

Finally, with the new understanding, future catalysts and electrodes could be designed to intentionally increase or decrease electric fields to tune not only the activity, but also selectivity for complicated multi-step reactions. In the long term, this can help lower the cost of green hydrogen and improve the use of renewable energy.