How Electrolyte pH Affects the Oxygen Reduction Reaction

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Electrolyte pH is a critical factor influencing the oxygen reduction reaction (ORR) kinetics and selectivity.
• The precise mechanisms by which pH affects ORR rates differently across various catalysts remain unclear.
• Understanding these pH-dependent effects is crucial for designing efficient electrocatalysts.

Purpose of the Study:

• To elucidate the fundamental reasons behind the varying pH sensitivity of ORR rates on different metal catalysts.
• To investigate the role of electric fields at the catalyst-electrolyte interface in mediating pH effects.
• To correlate changes in intermediate binding energies with observed ORR rate variations.

Main Methods:

• Experimental kinetic studies of the ORR on various metal electrodes (Pt, Ir, Ru, Pd, Au, Ag).
• Atomistic simulations to model electric field effects and intermediate binding energies.
• Analysis of proton-coupled electron transfer (PCET) and non-PCET steps in the ORR mechanism.

Main Results:

• ORR rates on strongly binding metals (Pt, Ir, Ru, Pd) show weak pH dependence, as intermediate binding energies are minimally affected by electric fields.
• ORR rates on weakly binding metals (Au, Ag) increase significantly in alkaline electrolytes due to stabilization of adsorbed O2 by negative electric fields.
• The rate-determining step of the ORR is not altered by pH, but the activation barrier for O2 adsorption decreases on weakly binding catalysts.

Conclusions:

• pH effects on ORR are primarily driven by electric field-induced changes in intermediate binding energies.
• Strongly binding catalysts exhibit low pH sensitivity, while weakly binding catalysts show high sensitivity.
• Tailoring electrolyte pH can tune ORR kinetics by modifying adsorption barriers, particularly for weakly binding materials.