On the Importance of the Volcano Slope and Mechanistic Pathways to Comprehend Activity and Selectivity Trends in Electrocatalysis
Authors: Exner Kai S. 
Published: February 2024

Electrochemical processes for energy storage or conversion require efficient electrocatalysts to enhance intrinsic activity as well as to minimize energy losses in terms of applied overpotential. Since the initial works of Nørskov and coworkers, it is believed that the understanding of correlations between materials in a homologous series contributes to the rational design of electrocatalysts [1]. In this context, volcano plots have been largely used to capture the trends of electrocatalytic processes, thereby relying on scaling relations and thermodynamic considerations at the equilibrium potential by connecting the thermodynamics to the kinetics via the Sabatier principle and Brønsted−Evans−Polanyi (BEP) relation [2].
The thermodynamic theory of volcano plots identifies the limiting reaction step in the thermodynamic picture, which is reconciled with the activity descriptor U
L (limiting potential) and denoted as the potential-determining step (PDS) [3]. In recent years, several works reported that the concept of the PDS is too simplistic to determine the trends of electrocatalytic processes correctly. To overcome this shortcoming, the author has introduced the notion of the free-energy span model in terms of the descriptor G
max(η), which allows for a dedicated analysis of the elementary steps to comprehend activity and selectivity trends of proton-coupled electron processes at electrified solid-liquid interfaces [4,5].
In my lecture, I am going to focus on the construction of volcano plots by the descriptor G
max(η) for the oxygen evolution and reduction reactions, corresponding to the bottleneck in electrolyzers and fuel cells, respectively. In this context, I will highlight important factors that, hitherto, were overlooked in trend studies in that
a) the volcano slope is prone to change even at the leg of the volcano whereas, so far, it was considered to be altered at the volcano apex only [6,7];
b) the importance of various mechanistic pathways for volcano analyses is stressed, demonstrating that the consideration of traditional mechanisms only is insufficient for a proper description of highly active catalyst [8].
The derived methodology for the construction of volcano plots by the activity measure G
max(η) is of universal nature [9], and its application even goes beyond the oxygen electrocatalysis, indicating that it may also contribute to the identification of next-generation battery materials [10].
References:

[1] Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892.

[2] Ooka, H.; Huang, J.; Exner, K. S. The Sabatier principle in electrocatalysis: Basics, limitations, and extensions. Front. Energy Res. 2021, 9, 654460.

[3] Koper, M. T. M. Theory of Multiple Proton-Electron Transfer Reactions and its Implications for Electrocatalysis. Chem. Sci. 2013, 4, 2710−2723.

[4] Exner, K. S. A Universal Descriptor for the Screening of Electrode Materials for Multiple-Electron Processes: Beyond the Thermodynamic Overpotential, ACS Catal. 2020, 10, 12607-12617.

[5] Razzaq, S.; Exner, K. S. Materials Screening by the Descriptor G
max(η): The Free-Energy Span Model in Electrocatalysis. ACS Catal. 2023, 13, 1740-1758.

[6] Exner, K. S. Rapid Screening of Mechanistic Pathways for Oxygen-Reduction Catalysts. ChemCatChem 2023, 15, e202201222.

[7] Exner, K. S. Steering Selectivity in the Four-Electron and Two-Electron Oxygen-Reduction Reactions: On the Importance of the Volcano Slope. ACS Phys. Chem. Au 2023, 3, 190-198.

[8] Exner, K. S. On the mechanistic complexity of oxygen evolution: potential-dependent switching of the mechanism at the volcano apex. Mater. Horiz. 2023, 57, DOI: 10.1039/D3MH00047H.

[9] Exner, K. S. Toward data- and mechanistic-driven volcano plots in electrocatalysis. Electrochem. Sci. Adv. 2023, 355, DOI: 10.1002/elsa.202200014.

[10] Exner, K. S. On the concept of metal–hydrogen peroxide batteries: improvement over metal–air batteries?. Energy Adv. 2023, 381, DOI: 10.1039/D3YA00002H.

Figure 1