|Representative schematic of hardware employed for testing of a complete biofuel cell. Credit: ACS, Ulyanova et al. Click to enlarge.|
A team from the University of Utah and CFD Research Corporation (CFDRC) reports the first bioelectrocatalysis of alkanes to produce electricity. In an paper published in the journal ACS Catalysis, they describe the use of a two-enzyme cascade in an enzymatic biofuel cell to oxidize hexane, octane and then JP-8, a jet fuel (C6-C16) comprising a mixture of alkanes.
An enzymatic biofuel cell contains many of the same components as a hydrogen/oxygen fuel cell—i.e., anode, cathode, and separator. However, instead of metallic electrocatalysts at the anode and the cathode, the enzymatic biofuel cell uses enzymes as the catalysts. The enzyme cascade reported in this new work is efficient, sulfur-tolerant, and produces power densities up to 3 mW/cm2 in a JP-8 enzymatic biofuel cell at room temperature without preprocessing of the fuel—as opposed to traditional metal catalysts which require fuel pre-processing. This output is comparable to high power density sugar and alcohol biofuel cells, the researchers said.
JP-8 is a kerosene based jet fuel that is used in military supply chains. It has been designed to operate in a variety of conditions from sub-zero temperatures to extreme deserts and is safe due to a lack of flammability. However, the current methods for converting JP-8 to energy are insufficient. The dominant mechanism for decomposing JP-8 is through a standard internal combustion engine or tactical quiet generator (TQG). There are numerous issues with this method including low efficiency (<30% at peak load and significantly lower at non-optimal loads) and large thermal and acoustic signatures. Based on these shortcomings, a solution for electrochemical oxidation of JP-8 is highly desirable.
There has been significant research into the use of solid oxide fuel cells (SOFCs) for conversion of JP-8 into electrical energy. However, the metal catalysts in a traditional SOFC are poisoned by the sulfur inherent in JP-8 which can vary from 400 to 1600 ppm depending on source. In order to use the SOFCs, a reformer needs to be placed in front of the fuel cell adding complexity and weight to the overall solution. The SOFC catalysts also require very high temperatures (>500°C). Enzymatic biofuel cells are a low temperature electrochemical alternative to SOFCs. They can operate at ambient temperature ranges (from -20°C to 60°C depending on the enzyme cascade), tolerate impurities such as sulfur, and are not easily passivated, but alkanes have never been explored as fuels for enzymatic biofuel cells.—Ulyanova et al.
Common fuels for enzymatic biofuel cells are sugars and alcohols. Most alkane-based fuels are mixtures, and requires “promiscuous” enzymes with broad substrate specificity of alkanes of different carbon length, the researchers noted. Further, to derive sufficient energy from an alkane fuel, an enzyme cascade is needed to deeply oxidize the fuel, they said.
In their work, the team used an enzyme cascade of two enzymes—alkane monooxygenase (AMO) and alcohol oxidase (AO)—to catalyze hexane, octane and JP-8. AMO is an oxidoreductase enzyme which converts alkanes to alcohols. AO, which oxidizes an alcohol to an aldehyde, is rarely used for bioelectrocatalysis, but the team found that it is a stable and promiscuous enzyme that is ideal for this application.
The researchers found that adding sulfur to their enzymatic fuel cell did not reduce power production.
The major advance in this research is the ability to use Jet Propellant-8 directly in a fuel cell without having to remove sulfur impurities or operate at very high temperature. This work shows that JP-8 and probably others can be used as fuels for low-temperature fuel cells with the right catalysts.—Professor Shelley Minteer, senior author
Solid-oxide fuel cells at temperatures above 950 ˚F have made use of JP-8, but this is the first demonstration at room temperature, Minteer says. Now that the team has shown the enzyme catalysts works, it will focus on designing the fuel cell and improving its efficiency, she said.
Minteer conducted the study with University of Utah postdoctoral researchers Michelle Rasmussen and Mary Arugula, and with Yevgenia Ulyanova, Erica Pinchon, Ulf Lindstrom and Sameer Singhal of CFD Research Corp. in Huntsville, Alabama.
This research was funded by Northrop Grumman Corp. and the National Science Foundation through the University of Utah’s Materials Research Science and Engineering Center.
Yevgenia Ulyanova, Mary A Arugula, Michelle Rasmussen, Erica Pinchon, Ulf Lindstrom, Sameer Singhal, and Shelley D. Minteer (2014) (web) “Bioelectrocatalytic Oxidation of Alkanes in a JP-8 Enzymatic Biofuel Cell,” ACS Catalysis doi: 10.1021/cs500802d
Plamen Atanassov, Chris Apblett, Scott Banta, Susan Brozik, Scott Calabrese Barton, Michael Cooney, Bor Yann Liaw, Sanjeev Mukerjee, and Shelley D. Minteer (2007) “Enzymatic Biofuel Cells” The Electrochemical Society Interface