Hydrogen Production in a Microbial Electrolysis Cell Lacking a Membrane

Open Access
Author:
Call, Douglas
Graduate Program:
Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 10, 2008
Committee Members:
  • Bruce Ernest Logan, Thesis Advisor
Keywords:
  • hydrogen production
  • bioelectrochemical cell
  • wastewater
  • bioenergy
Abstract:
Microbial electrolysis is a recently developed technology for generating hydrogen gas from organic matter that relies on two sources of energy: bacterial oxidization of organic matter, and electricity. The reactors used for this process, called microbial electrolysis cells (MECs), have always included a membrane to prevent bacterial consumption of the produced hydrogen and to ensure high hydrogen recoveries. However, substantial voltage losses in system performance have been attributed to the inclusion of a membrane. It is shown here that high hydrogen recoveries and production rates are possible without the presence of a membrane. Performance of an MEC lacking a membrane was investigated in batch operation at various applied voltages (0.2 V < Eap < 0.8 V) using a mixed culture and acetate as a substrate at two different solution conductivities (7.5 and 20 mS/cm). Overall energy recoveries using the 7.5 mS/cm solution averaged 78 ± 4% with a maximum of 84 ± 2% at an applied voltage of 0.4 V. The efficiency relative to only the electrical energy input decreased with applied voltage from 406 ± 6% (Eap = 0.3 V) to 194 ± 2% (Eap = 0.8 V). The maximum production rate was 3.12 ± 0.02 m3-H2/m3-reactor per day (m3-H2/m3-d) at Eap = 0.8 V (7.5 mS/cm), and increasing the solution conductivity increased the production rate for 0.3 V < Eap < 0.6 V. Reactors with membrane electrode assemblies (MEAs) were also tested to investigate their usefulness for hydrogen production. Two MEAs using Nafion membranes (N-117 and NRE-212) were examined with a platinum catalyst on either one or both sides of the MEA using a mixed bacteria culture and acetate as the substrate. Current densities as high as 3.3 ± 0.8 A/m2 (two sided catalyst MEA, Eap = 0.6 V) were obtained, but the highest overall energy recoveries were obtained with the MEA with the catalyst on only one side (17.2 – 55.9 %, 0.4 V < Eap < 1.0 V). The MEAs were limited by their high surface electrical conductivity, and a reactor design that caused decreased system performance.