Buffer pK_a and Transport Govern the Concentration Overpotential in Electrochemical Oxygen Reduction at Neutral pH

Microbial fuel cells (MFCs) show large cathodic overpotentials for the O₂ 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 NH₄⁺ buffer results in the best performance at current densities >10Am^-2, owing to buffering of the local pH close to its pK_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.