A Biofuel Cell with Electrochemically Switchable and Tunable Power Output

Abstract

An electroswitchable and tunable biofuel cell based on the biocatalyzed oxidation of glucose is described. The anode consists of a Cu²⁺-poly(acrylic acid) film on which the redox-relay pyrroloquinoline quinone (PQQ) and the flavin adenine dinucleotide (FAD) cofactor are covalently linked. Apo-glucose oxidase is reconstituted on the FAD sites to yield the glucose oxidase (GOx)-functionalized electrode. The cathode consists of a Cu²⁺-poly(acrylic acid) film that provides the functional interface for the covalent linkage of cytochrome c (Cyt c) that is further linked to cytochrome oxidase (COx). Electrochemical reduction of the Cu²⁺-poly(acrylic acid) films (applied potential −0.5 V vs SCE) associated with the anode and cathode yields the conductive Cu⁰-poly(acrylic acid) matrixes that electrically contact the GOx-electrode and the COx/Cyt c-electrode, respectively. The short-circuit current and open-circuit voltage of the biofuel cell correspond to 105 μA (current density ca. 550 μA cm^-2) and 120 mV, respectively, and the maximum extracted power from the cell is 4.3 μW at an external loading resistance of 1 kΩ. The electrochemical oxidation of the polymer films associated with the electrodes (applied potential 0.5 V) yields the nonconductive Cu²⁺-poly(acrylic acid) films that completely block the biofuel cell operation. By the cyclic electrochemical reduction and oxidation of the polymer films associated with the anode and cathode between the Cu⁰-poly(acrylic acid) and Cu²⁺-poly(acrylic acid) states, the biofuel cell performance is reversibly switched between “ON” and “OFF” states, respectively. The electrochemical reduction of the Cu²⁺-polymer film to the Cu⁰-polymer film is a slow process (ca. 1000 s) because the formation and aggregation of the Cu⁰-clusters requires the migration of Cu²⁺ ions in the polymer film and their reduction at conductive sites. The slow reduction of the Cu²⁺-polymer films allows for the controlling of the content of conductive domains in the films and the tuning of the output power of the biofuel cell. The electron-transfer resistances of the cathodic and anodic processes were characterized by impedance spectroscopy. Also, the overall resistances of the biofuel cell generated by the time-dependent electrochemical reduction process were followed by impedance spectroscopy and correlated with the internal resistances of the cell upon its operation.