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

• Bilirubin oxidase (BOx) based air breathing cathode has been studied for 45 days.
• Electrocatalytic performances have been studied in pristine and contaminated conditions.
• Enzymes lost the majority of their electrocatalytic activity in the first days.
• Pollutants seem to be responsible for an immediate deactivation.
• OCP of BOx was over 500 mV vs Ag/AgCl for over 1 month.

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 activated 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.