|Schematic of a microbial electrolysis cell. Click to enlarge. Source: OSU|
Researchers at Oregon State University (OSU) have developed a new membrane-free microbial electrolysis cell (MEC) for the production of hydrogen gas from several types of biowaste—including ordinary municipal sewage. The findings are reported in the journal Water Research.
Microbial electrohydrogenesis is a similar process to water electrolysis, except that microbes at the anode decompose organic matter in CO2, electrons, and protons. A distinct advantage of the microbial system is the lower energy consumption compared to water electrolysis. Other studies have shown that as little as 0.2 V is needed to produce hydrogen in microbial electrohydrogenesis, while a theoretically minimum applied voltage of 1.23 V is required for water electrolysis.
Other benefits include eliminating the requirement for expensive catalysts on the anode and the potential achievement of simultaneous waste reduction—e.g., the MEC process can produce hydrogen and clean sewage water at the same time. Compared to fermentative biohydrogen production, the MEC process also offers higher rates of hydrogen recovery (more than 90% of hydrogen can be harvested, versus 33% by fermentation) and the ability to use more diverse substrates.
So far, however, most reported microbial electrolysis cells (MECs) use membranes to separate the anode and cathode to avoid hydrogen losses due to bacterial consumption of the product gas. Membranes are expensive, however, and can increase the internal resistance especially at neutral pH. Other studies have reported considerable potential losses associated with cation exchange membranes and anion exchange membranes (AEM, 0.26 V), which increased the optimal value of the applied voltage and significantly decreased the energy recovery.
In March, Dr. Bruce Logan at Penn State, one of the pioneers in the area of microbial fuel cells and microbial electrolysis cells, reported developing a membrane-free MEC used with acetic acid, demonstrating that high hydrogen recovery and production rates are possible in a single chamber MEC without a membrane. (Earlier post.)
The OSU team devised single-chamber, membrane-free MECs to investigate hydrogen production by one mixed culture and one pure culture: Shewanella oneidensis MR-1.
The removal of membrane not only can simplify the construction, operation and maintenance of MECs, but also can decrease the internal resistance, thus increase the hydrogen production rate.—Dr. Hong Liu, Oregon State University assistant professor of biological and ecological engineering, lead author
|Biogas production and current densities at applied voltages of (a) 0.6 V. Click to enlarge. Credit: Elsevier|
At an applied voltage of 0.6 V, the system with a mixed culture achieved a hydrogen production rate of 0.53 m3/day/m3 (0.11 m3/day/m2) with a current density of 9.3 A/m2 at pH 7 and up to 0.69 m3/day/m3 (0.15 m3/day/m2) with a current density of 14 A/m2 at pH 5.8. Although methane produced during the hydrogen production negatively affected the hydrogen production rate, the team suppressed the hydrogentrophic methanogens via a variety of “suitable approaches”, such as exposure of cathodes to air.
These values are 76% and 130% higher than that (0.3 m3/day/m3) of using a gas cathode-chamber MEC at applied voltage of 1.0 V (Rozendal et al., 2007), but lower than the 1.1 m3 H2/day/m3 reported by Cheng and Logan (2007) using a two-chamber MEC.
The current density (9.3 A/m2 at pH 7.0 and 14 A/m2 at pH 5.8) generated in this study, however, is over three times higher than that reported in a two-chamber system (3 A/m2) reported by Liu et al. (2005) and 18 times higher than that of the gas cathode-chamber MEC (less than 0.5 A/m2) (Rozendal et al., 2007) at same applied voltage of 0.6 V. Furthermore, it is also 69% higher than the current density of 5.5 A/m2 (calculated based on the hydrogen production rate and energy efficiency) reported by Cheng and Logan (2007).
The high current densities achieved here was possibly resulted from the removal of membrane, which could contribute 38% of total resistance using CEM and 26% using AEM at an applied voltage of 1.0 V (Rozendal et al., 2007). While high current density should result in high hydrogen production rate, the relatively low hydrogen production rate reported here was mainly due to the large MEC volume/electrode ratio employed in this study.
...The current density and volumetric hydrogen production rate of this system have potential to increase significantly by further reducing the electrode spacing and increasing the ratio of electrode surface area/cell volume.—Hu et al. (2008)
In terms of cost, Liu said that in the laboratory there were already quite close to the Department of Energy (DOE) hydrogen cost goal of $2 to $3 per gasoline gallon equivalent (GGE).
Hu, H., Fan, Y., Liu H. (2008) Hydrogen Production Using Single-chamber Membrane-free Microbial Electrolysis Cells Water Research, 42(15):4172-4178 doi: doi:10.1016/j.watres.2008.06.015