Toward Practical Powering of Oceanographic Sensors By Benthic Microbial Fuel Cells
Authors: Book Jeffrey , Golden Joel , Martens Ian , Quaid Andrew , Tender Leonard M 
Published: February 2020

Described here are the latest results from field tests of the Benthic Microbial Fuel Cell (BMFC). The BMFC sits on the sediment/water (benthic) interface of marine environments where it utilizes organic matter naturally residing in marine sediments as its fuel and oxygen in overlying water as its oxidant to generate electrical power. Both fuel and oxidant are used as found in the environment. The BMFC is expected to generate power indefinitely due to constant replenishment of organic matter and oxygen by naturally occurring mass transport (i.e., diffusion, advection, and tidal pumping); durability of its electrode catalysts consisting of self-forming biofilms comprised of microorganisms naturally inhabiting the benthic interface; and lack of moving, degrading, or depleting components.

The BMFC is being developed to persistently power battery-powered oceanographic sensors presently limited in operational lifetime by battery depletion. During the past 10 years, we have successfully deployed small scale (< 0.1 Watt continuous output) BMFCs in coastal waters world wide ranging in depth from < 1 m to > 1000 m over for up to 2 years before being retrieved without any depletion in power output; and have powered a riverine metrological buoy with a radio transceiver link, a hydrophone with a radio transceiver link, and a surveillance camera with a cellular link. The highest power density we have thus far achieved is 0.39 Watt continuous power per square meter footprint area of sediment surface occupied by an early prototype that required extensive in-water manipulation by divers to deploy. This BMFC had a 29-liter volume, a 0.09-square meter footprint area (area of sediment surface occupied), and sustained 0.036 Watt continuously. Subsequently, by scaling up this BMFC we achieved the highest sustained power by a BMFC of 0.47 Watt continuous power (370 liter, 1.2 square meter footprint area, 0.39 Watt continuous power per square meter footprint area). Based on the 3.1 square meter footprint area of the oceanographic moorings we presently use to house battery-powered sensors, our long-term goal of a BMFC-equipped mooring capable of sustaining > 1 Watt is thought achievable. This amount of power, the equivalent of 440 alkaline D cells per year of operation based on optimal battery performance, is more than sufficient to operate complicated oceanographic instruments (e.g., an acoustic Doppler current profiler), including processing and transmitting data at least once a day either by acoustic telemetry or by radio to satellites. Practical considerations that limit the size of oceanographic battery packs (weight, cost, and size) compounded by poor battery performance at low temperatures encountered at the bottom of most marine environments limit the duration of most battery-powered sensors to 1 year. In contrast, the putative unlimited duration of BMFCs make them appealing for long-duration applications if BMFCs can be reliably deployed without in-water manipulation.

Our current efforts to be reported on here center on iterative design and evaluation of full scale (targeting > 1 Watt continuous power) by BMFCS that do not require in-water manipulation by divers and/or remotely operated vehicles to deploy (i.e., drop and go). The challenge for deployment is the requirement that the anode be in contact with anoxic sediment and isolated from over lying oxic water. Our most recent design, to be described here, sustained 0.03 W continuous power when configured with a 1-m diameter bottom mount sensor mooring and was deployed without in-water manipulation. The power is intended to increase dramatically for each design-evaluation cycle and is projected to reach > 1 Watt continuous power as the mass-transport accessible surface area of the anode is increased. At this time 2 BMFC-equipped moorings are undergoing field testing. In addition to describing the results of these field tests, results of earlier deployments will be presented in which the dependency of BMFC power output vs sediment properties and environmental conditions was evaluated.