The hydrogen production rate reached 2.2±0.2 mL L^-1 d^-1, the cathodic hydrogen recovery (RH₂) and overall systemic Coulombic efficiency (CEsys) were 88~96% and 28~33%, respectively, and the overall systemic hydrogen yield (Y_sysH2) peaked at 1.2 L mol-H₂ mol-acetate^-1. The hydrogen production was elevated by increasing the phosphate buffer concentration, and the highest hydrogen production rate of 14.9±0.4 mL L^-1 d^-1 and Y_sysH2 of 1.60± 0.08 mol-H₂ mol-acetate^-1 were achieved at 100 mM of phosphate buffer. The performance of the MEC and the MFC was influenced by each other.

An MEC consists of anode and cathode chambers. The bacteria (exoelectrogens) in the anode chamber catalyze the oxidation of organic substances to carbon dioxide via several metabolic reactions. Electrons from these reactions travel through an external circuit and combine with protons migrating through the proton exchange membrane (PEM) to form hydrogen on cathode. Other researchers have successfully produced hydrogen from cellulose, glucose, acetate, butyrate, lactate, propionate, and valerate in MECs.

Theoretically, an applied voltage of 0.14 V is required for hydrogen production through the microbial electrolysis of acetate. Because of overpotentials at the electrodes, a voltage of around 0.22 V should be applied (4). Usually, a voltage of 0.6 V or more was used for a high efficiency hydrogen production [earlier post]. Although the electricity supply in MECs is lower than that for alkaline electrolysis, the energy consumption is still high. Thus, reducing electricity supply is one of the key issues for the development and application of efficient and cost-effective MECs.

Here, we report a modified microbial electrolysis system for biohydrogen production, on the base of the conventional microbial electrolysis proposed by Liu et al. This system was composed of a coupled MEC and MFC: the electrolysis was performed in the MEC designed according to Liu et al., whereas the extra electricity for the electrolysis was supplied by the MFC with an air cathode. This MEC-MFC coupled biohydrogen-producing system was developed through taking in situ utilization of the power generated from an MFC into account.

Since the open circuit voltage of an MFC could reach as high as 0.80 V, the extra energy needed for an MEC can be supplied by an MFC. In such an MEC-MFC-coupled system, hydrogen was entirely harvested from substrate in the microbial cells, and the external power supply was saved. Our experimental results demonstrate that this MEC-MFC-coupled system has a potential to produce biohydrogen from wastes.

—Sun et al. (2008)

To become a mature biohydrogen-producing system, the researchers wrote, further efforts need made to improve the coupled system.

• Hydrogen production is operated under unstable conditions, resulting in the fluctuation of the circuit current and hydrogen production in the process. There are many factors influencing the stability of an MFC, such as substrate type and concentration, catalytic activity of the anode microorganisms, and internal resistance, etc. In the conventional MEC, the extra voltage was fixed, thus the system was relatively stable. In the coupled system, the MEC and MFC have an influence on each other, thus, even a slight change of these related factors may cause instability of the system. Therefore, efforts should be pursued to increase the long-term stability of MFCs/MECs.

• Any measures aimed at increasing the anodic performance of MEC/MFC, such as new designs of configurations and modification of electrodes and membrane are expected to enhance the system performance.

• Optimization of the process parameters should be made to elevate the hydrogen production.

In summary, the MEC-MFC-coupled system described in this paper was able to save the extra electricity supply for biohydrogen production. Our experimental results demonstrate that, for the first time, the power generated from an MFC could be utilized for hydrogen production in an MEC in situ. Extensive studies are required for make this technique more effective and applicable.

—Sun et al. (2008)

Resources

• Min Sun, Guo-Ping Sheng, Lei Zhang, Chang-Rong Xia, Zhe-Xuan Mu, Xian-Wei Liu, Hua-Lin Wang, Han-Qing Yu, Rong Qi, Tao Yu, and Min Yang (2008) An MEC-MFC-Coupled System for Biohydrogen Production from Acetate. ASAP Environ. Sci. Technol., doi: 10.1021/es801513c