Impedance characteristics and polarization behavior of a microbial fuel cell in response to short-term changes in medium pH.

pH oppositely influences anode and cathode performance in microbial fuel cells. The differential electrochemical effects at each electrode and the resultant full-cell performance were analyzed in medium pH from 6.0 to 8.0. Potentials changed -60 mV/pH for the anode and -68 mV/pH for the cathode, coincident with thermodynamic estimations. Open circuit voltage reached a maximum (741 mV) at pH 7, and maximum power density was highest (712 mW/m²) at pH 6.5 as the cathode performance improved at lower pH. Maximum current density increased and apparent half-saturation potential (E(KA)) decreased with increasing medium pH due to improved anode performance. An equivalent circuit model composed of two time constant processes accurately fit bioanode impedance data. One of these processes was consistently the rate-limiting step for acetate-oxidizing exoelectrogenesis, with its pH-varying charge transfer resistance R₂ ranging from 2- to 321-fold higher than the pH-independent charge transfer resistance R₁. The associated capacitance C₂ was 2-3 orders of magnitude larger than C₁. R₂ was lowest near E(KA) and increased by several orders of magnitude at anode potentials above E(KA), while R₁ was nearly stable. However, fits deviated slightly at potentials above E(KA) due to emerging impedance possibly associated with diffusion and excessive potential.