Bilirubin oxidase based enzymatic air-breathing cathode: Operation under pristine and contaminated conditions

The performance of bilirubin oxidase (BOx) based air breathing cathode was constantly monitored over 45 days. The effect of electrolyte composition on the cathode oxygen reduction reaction (ORR) output was investigated. Particularly, deactivation of the electrocatalytic activity of the enzyme in phosphate buffer saline (PBS) solution and in ac- tivated sludge (AS) was evaluated. The greatest drop in current density was observed during the first 3 days of constant operation with a decrease of ~60 μAcm−2 day−1. The rate of decrease slowed to ~10 μAcm−2 day−1 (day 3 to 9) and then to ~1.5 μAcm−2 day−1 thereafter (day 9 to 45). Despite the constant decrease in output, the BOx cathode generated residual current after 45 days operations with an open circuit potential (OCP) of 475 mV vs. Ag/AgCl. Enzyme deactivation was also studied in AS to simulate an environment close to the real waste operation with pollutants, solid particles and bacteria. The presence of low-molecular weight soluble contaminants was identified as the main reason for an immediate enzymatic deactivation within few hours of cathode operation. The presence of solid particles and bacteria does not affect the natural degradation of the enzyme.