Oxygen reduction in alkaline solutions - a theoretical investigation.

WOLFGANG SCHMICKLER; ALEKSEJ GODULJAN; ANNA IGNACZAK; PAOLA QUAINO; ELIZABETH SANTOS; RENAT NAZMUTDINOV

Luebeck

Simposio; 13th International Fischer Symposium. A meeting on nanoscale electrochemistry.; 2015

International Fischer Symposium.

In general, oxygen reduction is faster in alkaline than in acid solutions; it does not require an expensive transition-metal catalyst, and is fast on coinage metals like gold and silver.In the following, we focus on Au(100).The first and rate-determining step is:O2 + e- = O-2 (1)We have investigated this step by a combination of DFT and our own theory.  On Au(100) this takes place in a near outer sphere mode, with a low energy of activation of about 0.33 V at the equilibrium potential [1].For this mechanism to be effective, the subsequent reactions must be fast. From the various mechanisms proposed in the extensive literature, we believe that the following reaction is the most favorable candidate for the second step:O-2 + H2O + e- = HO2- +  OH- (2)At pH 14 this has a very suitable standard equilibrium potential of 0.2 V SHE, only about 0.2 V below the standard potential for the overall reaction. Since no adsorbates are involved, this should occur in the outer sphere mode. DFT calculation showed that the transfer of an electron to a hydrated O2- ion leads to an elongation of the O-O bond, and favors the breakup into OH- and HO-2 with a small activation energy.As the final step we consider:HO-2 + H2O = OH- + 2OHad (3)In the outer sphere mode this reaction would be uphill by about 1.3 eV; however, adsorption of the two OH radicals makes this reaction highly exothermic, and according to our calculations, based on the Evans-Polanyi principle, the reaction proceeds on Au(100) without activation.Finally, we discuss why oxygen reduction requires a d-band catalyst in acid solutions.