Lewis Hsu

Scale up considerations for sediment microbial fuel cells

Scale-up of sediment microbial fuel cells (SMFCs) is important to generating practical levels of power for undersea devices. Sustained operation of many sensors and communications systems require power in the range of 0.6 mW to 20 W. Small scale SMFC systems evaluated primarily in the laboratory indicate power densities for typical graphite plate anodes on the order of 10–50 mW m−2. However, previous work also suggests that SMFC power production may not scale directly with size. Here, we describe a combination of lab and field studies to evaluate scale up for carbon fabric anodes with a projected surface area ranging from 25 cm2 to 12 m2. The results indicate that power generation scales almost linearly with anode size up to about 1–2 m2 of projected surface area. Our model suggests that anodes larger than this can experience significant reduction in power density, confirming laboratory observations. These results suggest that the majority of losses along the anode surface occur closest to the electronics, where the amount of current passing along an anode is the greatest. A multi-anode approach is discussed for SMFCs, suggesting that scale-up can be achieved using segmented anode arrays.

Multiple Cathodic Reaction Mechanisms in Seawater Cathodic Biofilms Operating in Sediment Microbial Fuel Cells

In this study, multiple reaction mechanisms in cathodes of sediment microbial fuel cells (SMFCs) were characterized by using cyclic voltammetry and microelectrode measurements of dissolved oxygen and pH. The cathodes were acclimated in SMFCs with sediment and seawater from San Diego Bay. Two limiting current regions were observed with onset potentials of approximately +400 mVAg/AgCl for limiting current I and -120 mVAg/AgCl for limiting current II. The appearance of two catalytic waves suggests that multiple cathodic reaction mechanisms influence cathodic performance. Microscale oxygen concentration measurements showed a zero surface concentration at the electrode surface for limiting current II but not for limiting current I, which allowed us to distinguish limiting current II as the conventional oxygen reduction reaction and limiting current I as a currently unidentified cathodic reaction mechanism. Microscale pH measurements further confirmed these results.

Chip-on-mud: Ultra-low power ARM-based oceanic sensing system powered by small-scale benthic microbial fuel cells

An ARM-based sensing platform powered entirely by small-scale benthic microbial fuel cells (MFCs) for oceanic sensing applications is presented. The ultra-low power chip featuring an ARM Cortex-M0 processor, 3kB of SRAM, and power management unit (PMU) with energy harvesting from MFCs is designed to consume 11nW in sleep mode for perpetual sensing operation. A small-scale micro-MFC with 21.3cm2 anode surface area was connected to the on-chip PMU to charge a thin film battery of 1mAh capacity. A 49.3-hour long-term experiment with 8-min sleep interval and 1 sec wake-up time demonstrated the sustainability of chip-on-mud concept. During sleep mode, the system charges the 4V battery at 380nA from the micro-MFC generating 5.4μW of power, which can support up to 20mA of active mode current.

Pumping microbial fuel cells

Experimental data is presented comparing microbial fuel cell (MFC) power from buried (control) and chambered anodes exposed to slow flow pumping (2 mL/min). Results show that upon initial pumping (3 hrs), a robust upturn in MFC power from the chambered anodes was stimulated over several days, while a second pumping (4 hrs) appeared to resuscitate and sustain increased power for five more days. Analysis of energy gained (G) in the test setup vs. energy input (I) required for a commercial low power pump revealed a potential G/I ratio of 2.4. A pier side test was also conducted to demonstrate how tide-induced hydrostatic pressure changes could be used to pump an MFC chamber.

Design and performance considerations for benthic microbial fuel cells

Benthic Microbial Fuel Cells (BMFCs) ability to provide long-lasting power is an attractive trait for powering oceanographic equipment requiring long duration deployments. Recent work to scale up BMFC power output with large surface areas (greater than 1 meter squared) has revealed a need to improve power output to make BMFCs viable power sources for a wider breadth of undersea equipment. To improve power output of large surface area BMFCs, experiments were conducted to (1) evaluate different carbon based electrodes; (2) examine alternative materials as current collectors; and (3) determine effects of sediment chemistry and grain size on BMFC power production by testing various mixtures of sand and silt laden sediments. Experiments showed that (1) despite more specific surface area, carbon felt did not generate more power; (2) gold plated copper and titanium current collectors were most capable of reliable connections; and (3) achieving the highest power appears to be a balanced mixture of both fine and coarse sediment. This research provides several important design considerations that demonstrating the importance of the sediment quality at deployment sites for sustaining power output levels capable of powering oceanographic instruments.

Improving power production in linear forms of microbial fuel cells

Benthic microbial fuel cells (BMFCs) are devices that generate persistent energy by coupling bioanodes and biocathodes through an external energy harvester. Advances in BMFC system design have increased feasibility for its use in ocean monitoring. Previous iterations of BMFCs designed by this same group have relied upon carpet like anodes to harvest energy from electrochemically active bacteria in the sediment at power densities of 10 - 20 mW/m-2. Although successful, these 2D anodes are difficult to deploy. In this paper, we evaluate linear cable anodes as an alternative scaling strategy. Commercially manufactured, these cable anodes are wound around insulated underwater cables and can be handled similar to linear hydrophone arrays. Used as delivered, cable anodes performed poorly and generated ~0.3 mW/m. To improve power production, we added a washing step to remove any chemical binders used in manufacturing and frayed the cable, exposing short strands of carbon yarn. Treated cable anodes generated 1-2 mW/m, which was comparable to and slightly improved over the performance of carpet like anodes.

Development and deployment of a surface based benthic micorbial fuel cell

Benthic microbial fuel cells (BMFCs) have the potential to provide long-term, sustainable power for undersea devices. BMFCs operate by harnessing organic matter in the sediment as a fuel source. Field systems have demonstrated power densities in the range of 5 to 20 mW/m2 depending on size and operating conditions. Operation of anodes under anaerobic conditions is critical to the functionality of BMFCs because the presence of oxygen will negatively affect the microbiology and chemistry driving power production. In order to provide an anaerobic environment the BMFC anodes are usually buried at least 10 cm under the sediment which generally ensures a completely anaerobic environment in reducing coastal sediments. Furthermore, to increase a systems power output, large surface areas for the anode and cathodes are needed. Installation of these large-scale anodes is difficult, time consuming, and in the case of very deep water environments, almost impossible. In order to solve the issue of complex burial techniques for large anodes, a unique design of BMFCs has been created which allows the anode to be placed on the sediment surface without being buried and still operate under anaerobic conditions. Carbon cloth anodes were covered on one side with metalized plastic film, specifically biaxially-oriented polyethylene terephthalate (BoPET), and sealed at the edges. This type of metal film has very low oxygen permeability which prevents oxygen in overlying water from diffusing through the film and interfering with current production at the surface of the anode. Weights were added to the perimeter of the film in order to ensure a seal between the anode edge and the sediment to prevent aerobic seawater from contacting the anode thus ensuring an anaerobic environment for the anode. The concept was tested at various scales in the lab environment and a prototype deployment system was developed and tested for future scale-up and field applications.

Novel nutrient extraction to increase power for microbial fuel cells

We present experimental data showing how the power output of a sediment microbial fuel cell (MFC) can be dramatically increased by recirculating a dilute 1% mud solution between a settling tank and anode chamber with and without carbon granules. This enables both greater power output and MFC operation out of the mud. With carbon granules added to the bottom of the 125 ml anode chamber, power outputs of 1.3 mW were achieved with total anode surface areas of 77 cm2 (graphite chips) and an anode displaced volume of 31 cc. This equates to a volumetric energy density of 1.2 W per ft3 of anode volume.