Hydrogen Production in a Single Chamber Microbial Electrolysis Cell Lacking a Membrane

TLDR

Hydrogen gas can be produced by electrohydrogenesis in microbial electrolysis cells at higher yields and energy efficiencies than fermentation or water electrolysis, yet it has been assumed that a membrane is required to prevent bacterial consumption of the product gas. The authors used a membrane‑less MEC with a graphite fiber brush anode and close electrode spacing, achieving a maximum hydrogen production rate of 3.12 m³ H₂ m⁻³ day (292 A m⁻³) at 0.8 V. Despite lacking a membrane, the system achieved 78–96 % cathodic hydrogen recoveries, more than double the rates of previous studies, with a peak production of 3.12 m³ H₂ m⁻³ day at 0.8 V, overall energy efficiencies up to 86 % (average 78 %), and demonstrated that high recovery and production rates are attainable in a single‑chamber MEC, potentially lowering costs and simplifying designs.

Abstract

Hydrogen gas can be produced by electrohydrogenesis in microbial electrolysis cells (MECs) at greater yields than fermentation and at greater energy efficiencies than water electrolysis. It has been assumed that a membrane is needed in an MEC to avoid hydrogen losses due to bacterial consumption of the product gas. However, high cathodic hydrogen recoveries (78 +/- 1% to 96 +/- 1%) were achieved in an MEC despite the absence of a membrane between the electrodes (applied voltages of 0.3 < E(ap) < 0.8 V; 7.5 mS/cm solution conductivity). Through the use of a membrane-less system, a graphite fiber brush anode, and close electrode spacing, hydrogen production rates reached a maximum of 3.12 +/- 0.02 m3 H2/m3 reactor per day (292 +/- 1 A/m3) at an applied voltage of E(ap) = 0.8 V. This production rate is more than double that obtained in previous MEC studies. The energy efficiency relative to the electrical input decreased with applied voltage from 406 +/- 6% (E(ap) = 0.3 V) to 194 +/- 2% (E(ap) = 0.8 V). Overall energy efficiency relative to both E(ap) and energy of the substrate averaged 78 +/- 4%, with a maximum of 86 +/- 2% (1.02 +/- 0.05 m3 H2/m3 day, E(ap) = 0.4 V). At E(ap) = 0.2 V, the hydrogen recovery substantially decreased, and methane concentrations increased from an average of 1.9 +/- 1.3% (E(ap) = 0.3-0.8 V) to 28 +/- 0% of the gas, due to the long cycle time of the reactor. Increasing the solution conductivity to 20 mS/ cm increased hydrogen production rates for E(ap) = 0.3-0.6 V, but consistent reactor performance could not be obtained in the high conductivity solution at E(ap) > 0.6 V. These results demonstrate that high hydrogen recovery and production rates are possible in a single chamber MEC without a membrane, potentially reducing the costs of these systems and allowing for new and simpler designs.