Use of Biocompatible Buffers to Reduce the Concentration Overpotential for Hydrogen Evolution

The hydrogen evolution reaction (HER) at Pt-cathodes of microbial electrolysis cells (MEC) has been associated with overpotentials of several hundred millivolts. The high overpotentials challenge the sustainability of an MEC. This paper shows that the HER overpotential at MEC relevant pH values is reduced if buffer is present. At 15 A/m2 and 50 mM buffer, the lowest overpotential for phosphate was -0.05 V at pH 6.2, for ammonia was -0.05 V at pH 9.0, for carbonate was -0.09 V at pH 9.3, for Tris(hydroxymethyl)aminomethane was -0.07 V at pH 7.8, and for N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid was -0.08 V at pH 7.2. It was shown that the effect of buffer on the overpotential is strongly pH dependent. Furthermore, experimental data and a mass transport equation showed that by increasing the buffer concentration or linear flow speed (i.e., pump speed), or decreasing the current density (i) the overpotential reduces and (ii) the minimum overpotential is reached at a pH that approaches the buffer dissociation constant (pKa). Thus, to reduce the HER overpotential of an MEC, buffer (i.e., pKa), buffer concentration, linear flow speed, and current density must be well balanced with the expected operational pH