Invited: Ammonia Generation at Algae Modified Electrodes: Some Mechanistic Considerations Drawn from Redox Potentials

Nature designs eloquent systems to effect electrochemical changes that pose daunting objectives for address by simple electrodes. In a sequence of electrochemical steps in a highly structured, bio-matrix, biological systems provide energy and kinetic control that yields important products from oxidized reactants. Examples are CO2 and water to form O2 and sugars and dinitrogen to form ammonia. By capturing these biological matrices on electrodes, the biomachines can generate products important in energy and materials. Minteer and coworkers have demonstrated many systems of this type, including capture of the entire Krebs cycle on an electrode where the various components of the cycle are captured in a modified Nafion membrane. By analogy, we have captured algae on electrodes to generate ammonia. Cyanobacteria generates ammonia from dinitrogen with nitrogenase enzyme and from fixed nitrogen of nitrate and nitrite with nitrate and nitrite reductase. Thermodynamically, the conversion of dinitrogen to ammonia is not energetically taxing, but the energetic cost to break the nitrogen nitrogen triple bond is high. In review of the kinetics for ammonia generation from N2, NO3 -, and NO2 - reactants, the thermodynamics were reviewed. From a potential axis and speciation diagram, some reactions are identified with sufficient energy to break the nitrogen triple bond. These reactions and other mechanistic steps in ammonia generation are presented and the role of local pH is considered. The protocol is anticipated applicable to identifying energy generating reactions important in other bioelectrochemical systems.