pH effect on oxygen reduction reaction at Pt(1 1 1) electrode

Abstract pH effect on oxygen reduction reaction (ORR) at Pt(1 1 1) electrode has been investigated in 0.1 M HClO 4 , 0.1 M NaOH, 0.5 M NaClO 4  +  x  mM HClO 4 ( x  = 0.1–35 mM) and 0.5 M NaClO 4  +  y  mM NaOH ( y  = 2–20 mM) solutions using hanging meniscus rotating disk electrode system (HMRDE). We found that (i) during ORR in solutions with 2.5  3 O + is not fast enough to maintain O 2  + 4H +  + 4e −  → 2H 2 O (reaction (1) ), O 2  + 2H 2 O + 4e −  → 4OH − (reaction (2) ) will set in, which is accompanied with sensitive change of the pH at electrode/electrolyte interface (pH s ); (ii) at the same overpotential, ORR current density ( j ORR ) is approximately the same for both reactions (1) , (2) in all solutions irrespective of pH s ; (iii) the kinetic and kinetic-mass transport mixed-control regions for ORR locate in the same potential region for OH ad  + H +  + e −  ⇄ H 2 O or OH ad  + e −  ⇄ OH − in O 2 free solution, both ORR and OH ad adsorption/desorption display similar pH independence; (iv) During ORR, OH ad adsorption/desorption occurs fast in both the forward and backward direction in parallel with ORR. The results reveal that the high activation overpotential and the slow kinetics for ORR at Pt(1 1 1) are limited by the thermo-electrochemistry of OH ad adsorption/desorption.