Characterization and improvement of oxygen reducing cathodes in undersea power systems (2017)

Power sources for marine systems are becoming more versatile, taking advantage several different electrochemical couples to provide energy in locations that are not serviced by conventional electrical loads. Renewable power from wind, solar, and tidal power continues to dominate power generation at the ocean surface. In sub-surface locations, primary batteries and sealed secondary battery systems are often used. More recently, microbial fuel cells are being testing to support undersea sensors and electronics. Both battery and fuel cell systems must take advantage of electrochemical differences in order to generate voltage and current. Typically, these systems operate with an oxygen reducing cathode that is paired with an oxidation reaction at the anode. Both aluminum and magnesium have been used in primary battery systems in underwater systems. Under standard conditions, theoretical cell voltages provided by aluminum and magnesium can be calculated to be 1.26V and 1.97V respectively (comparable to a typical alkaline battery cell at 1.5V). Similarly, theoretical voltages for sediment microbial fuel cells have been estimated on the order of 1V based on standard conditions. These theoretical voltages do not account for losses or differences due to environmental conditions, transport, electrodes, or cell design. Engineering these systems to minimize losses can focus on many different aspects of system design. We focus here on the cathode, in part because improvements in oxygen reduction could be applicable to both abiotic metal battery systems as well as the biological fuel cell system. Our approach here is to analyze the performance of the cathode using electrochemical and molecular biology techniques. Employing these techniques is important to obtain a more complete understanding of the cathode because of the typical biomass growth that must be accounted for on exposed electrode surfaces. In most prototype systems, cathode materials use inert materials such as carbon or stainless steel. In this study, we look at three common-place carbon materials, graphite plates, graphite fiber brushes, and carbon cloth, and their performance as cathode substrates in undersea power systems. A major driving force behind this study was the availability of these materials for large scale applications in the future. Additional consideration was given to past use in published field studies with primary batteries (aluminum, magnesium) or in microbial fuel cell systems. For this reason, we report here the performance of undersea cathodes through operation and electrochemical testing in a sediment microbial fuel cell system. The effect of oxygen concentration on performance was evaluated and a minimum dissolved oxygen concentration of about 3 mg/L was determined for proper function at cathodes. Electrochemical studies indicate two distinct processes taking place at the cathode surface. We also take an exploratory look at the microbial community supported on each of these materials. An examination of typical cathode microbial communities offers some mechanisms that could be used to improve abiotic cathodes through use of metal catalysts based on iron or manganese. Preliminary tests of manganese coatings indicated possible improvements in operating voltage and sustainable current density.