Proton-Relaying Adsorbates Induce Non-Nernstian Behavior in Oxygen Reduction

Area of Science:

• Electrochemistry
• Surface Science
• Computational Chemistry

Background:

• Proton-coupled electron transfer (PCET) is crucial for energy conversion.
• Nernst equation predicts a -60 mV/pH shift for PCET equilibrium potentials.
• Deviations from Nernstian behavior are observed in specific electrochemical reactions.

Purpose of the Study:

• Investigate deviations from Nernstian behavior in the oxygen reduction reaction (ORR) at Pt(111).
• Elucidate the role of anions and cations in pH-dependent electrocatalysis.
• Understand the influence of the local reaction environment (LRE) on PCET.

Main Methods:

• Density functional theory (DFT) calculations.
• Multistep microkinetic modeling.
• Local reaction environment (LRE) model incorporating mass transport and electrical double layer effects.

Main Results:

• Observed significant deviations from Nernstian behavior in H2SO4/M2SO4 solutions, dependent on cation identity (Li, Cs).
• Identified adsorbed sulfate anions as key mediators of proton transfer, forming bisulfate species.
• Determined reduced proton reaction orders (0.5-0.75) in sulfate solutions compared to perchlorate (1).

Conclusions:

• Adsorbed sulfate anions alter the proton donor mechanism in ORR, explaining non-Nernstian pH dependence.
• Anion and cation identity significantly modulate LRE effects in electrocatalysis.
• Combined theoretical and computational methods can unravel complex electrochemical reaction mechanisms.