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Buffer p K a and Transport Govern the Concentration Overpotential in Electrochemical Oxygen Reduction at Neutral pH
Author(s) -
Popat Sudeep C.,
Ki Dongwon,
Young Michelle N.,
Rittmann Bruce E.,
Torres César I.
Publication year - 2014
Publication title -
chemelectrochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.182
H-Index - 59
ISSN - 2196-0216
DOI - 10.1002/celc.201402058
Subject(s) - overpotential , cathode , electrochemistry , chemistry , inorganic chemistry , diffusion , polarization (electrochemistry) , analytical chemistry (journal) , electrode , chemical engineering , thermodynamics , chromatography , physics , engineering
Microbial fuel cells (MFCs) show large cathodic overpotentials for the O 2 reduction reaction. The local cathode pH is important, as OH − ions accumulate on the catalyst surface, resulting in a Nernstian concentration overpotential. Herein, we explore how various relevant buffers affect cathode performance at neutral pH, by reducing the concentration overpotential. We show that NH 4 + buffer results in the best performance at current densities >10 A m −2 , owing to buffering of the local pH close to its p K a (ca. 9.2). With other buffers, the local pH increases to >12 at similar current densities. We also show the importance of diffusion in buffer transport. Increasing the buffer concentration or improving the hydrodynamic conditions leads to lower overpotentials. We also present a mathematical model that includes buffer diffusion, for use in predicting cathode polarization curves. Overall, our results point to the promise of reducing cathode overpotentials in MFCs using naturally available buffers.