Poly(neutral red) as a NAD+ reduction catalyst and a NADH oxidation catalyst: Towards the development of a rechargeable biobattery

In this paper, we have established that poly(neutral red), PNR, functions as an electrocatalyst for the reduction and oxidation of NAD+/NADH in a rechargeable biobattery environment. The reversibility of this catalyst was possible only with the addition of Zn2+ for complexation to the redox polymer. The zinc ion complexation with the polymer facilitates electron and proton transfer to/from the substrate and the NAD+/NADH coenzyme without forming covalent bonds between the nicotinamide and the substrate surface. This research presents use of this reversible catalyst in a rechargeable biobattery. The rechargeable battery includes a Prussian blue cathode and a bioanode including NAD+-dependent alcohol dehydrogenase and zinc complexed PNR. This bioanode was coupled to the cathode with Nafion® 212 acting as the ion exchange membrane separator between the two compartments. The biobattery has an open circuit potential of 0.545(±0.009) V when first assembled and 0.053(±0.005) V when fully discharged. However, when fully charged, the biobattery has an open circuit potential of 1.263(±0.051) V, a maximum power density of 16.3(±4.03) μW cm−3 and a maximum current density of 221(±13.2) μA cm−3. The efficiency and stability of the biobattery were studied by cycling continuously at a discharging rate of 1 C and the results obtained showed reasonable stability over 50 cycles.