by Allison Kerley (Photo above of Hanyu Wang, first author of this paper at the University of California, Santa Cruz.)
Most hydrogen generating devices require an external addition of a 0.2 to 1.0 V electric potential in order to sustain the hydrogen generation. Wang et al. (2013) explored the feasibility of a self-powering photoelectrochemical-microbial fuel cell (PEC-MFC) hybrid device to generate hydrogen. The PEC-MFC was a combination of a photoelectrochemical fuel cell and a microbial fuel cell. The Hydrogen production of the device was tested when powered by a ferricyanide solution inoculated with a pure strain of Shewanellla oneidensis MR-1 and when powered by microorganisms found naturally occurring in the municipal wastewater. In both scenarios, given replenishments of fuel, the device produced enough voltage to be self-sustaining. However, when the device was powered by wastewater it produced both a lower current and a smaller hydrogen production than when powered by ferricyanide solution.
This study primarily examined whether the PEC-MFC device could produce enough power to be self-sufficient and self-sustaining. The PEC device used rutile TiO2 as the photoanode material, and when run independent of the MFC at zero external bias (no outside input of energy) yielded a photocurrent density of 0.0013 mA cm–2. No gas bubbles were observed in the PEC device when run individually, hypothesized by the authors to be the result of the low current. When the MFC fueled by the ferricyanide solution was run independent of the PEC, generated a current of 0.1 to 0.6 mA, with a spike consistently returning the current generation to peak efficiency immediately following the injection of new fuel and slowly decreasing as the nutrients became depleted. It is unclear whether or not the MFC unit was run without an external source of energy, and no mention of gas accumulation was made by the authors
When run in conjunction, the PEC-MFC generated a reproducible photocurrent current density of approximately 1.25 mA cm–2 at zero external bias, with gas bubbles containing H2 continuously forming in the device. However, the authors state they were unable to accurately measure the H2/O2 ratio due to the limitation of the instruments available. The authors proposed the MFC device as serving as a battery for the hydrogen producing PEC device. In addition, they confirmed that the overall current generation is determined by the MFC performance, which varies depending on level of activity of the microorganisms/bacteria (which were found to be dependent on the amount of nutrients in the surrounding solution).
In addition to using a ferricyanide solution as the catholyte for the MFC device, they also tested the MFC device using air as the cathode with S. oneidensis as the anolyte, as well as air as the cathode and microorganisms found in municipal wastewater collected from the Livermore Water Reclamation Plant (Livermore, California, USA) as the anolyte. A 1:1 ratio of anaerobic and aerobic sludge collected from the Livermore Water Reclamation Plant was used to inoculate the MFC until the electrochemically active bacteria were enriched and generated an electrical current. The air/wastewater PEC-MFC device had a current density between 0.0 and 1.4 mA cm–2, whereas the air/ferricyanide PEC-MFC device shows the current density to reside between 0.2 and 1.6 mA cm–2.
Wang, H., Qian, F., Wang, G., Jiao, Y., He, Z., Li, Y., 2013.Self-biased solar-microbial device for sustainable hydrogen generation. ACSNANO 7, 8728–8735.