Enzymatic Gas-Diffusion Cathodes with Increased Performance and Stability
Authors: Garcia Sergio Omar , Villarrubia Claudia Narvaez , Babanova Sofia , Atanassov Plamen 
Published: February 2020

The design of a viable enzymatic gas-diffusional cathode has to satisfy the following main requirements: 1) consist of a porous hydrophobic layer that allows the flow of oxygen to feed the catalytic layer; 2) have a porous thin catalytic layer with enzyme-air-electrolyte three-phase interface, and 3) preserve enzymatic activity. Additionally, a direct electron transfer mechanism is desired for enhanced catalytic performance at the electrode surface. Among those requirements, the stability of the enzymatic cathodes is the biggest challenge and the main drawback of the technology due to instability and short lifetime of the enzymatic component.
This research introduces the design of gas-diffusional cathode employing bilirubin oxidase (BOx, a multicopper oxidase proven to have a direct electron transfer mechanism [1-8]) immobilized on a complex matrix composed of carbon nanotube(CNT) modified Toray paper (TP) and encapsulated in silica-gel. The developed enzymatic cathode consists of two layers, hydrophobic gas-diffusional layer (GDL) and hydrophilic catalytic layer (CL), pressed together at 1000 psi for five minutes. The GDL (35 wt% teflonized Vulcan carbon powder (XC35)), exposed to air, has hydrophobic and porous properties that allows facilitated oxygen diffusion. The CL (CNTs modified TP) is thin with high surface area providing a 3D CNT/silica-gel matrix for enzyme immobilization and preserved stability. The design of the catalytic layer was improved by growing a CNT “forest”, which was possible by pulse chronoamperometry electrodeposited [11-12] Ni seeds that in turn go through chemical vapor deposition (CVD) mechanism providing highly conductive and high surface area nano-structured layer (Fig. 1). This CNTs-enzyme composite is then utilized as the catalytic layer of the dual layered electrode.
The assembly of the bio-cathode on a capillary driven microfluidic system provides a constant flow of electrolyte and facilitates the electrolytic solution transport to the electrode. Therefore this set up design was used to study the performance of the cathode at several different pHs (pH 5 to pH 8) of the electrolytic solution (Fig. 2). All tested cathodes demonstrated an open circuit potential of ~ 0.52V. The performed potentiostatic polarization curves show that the CNT modified TP cathode demonstrates highest performance of approximately 660 μA.cm-2 at 0.0 V and ~350 μA.cm-2at 0.3 V for pH 5.5 (Fig. 2). After this optimal pH, the performance of the cathode decreased demonstrating lowest output at pH 8.
To extend the lifetime of the attached enzyme, the advantages of silica encapsulation were explored. Entrapment of BOx is realized using tetramethyl orthosilicate (TMOS) in a CVD process [9-10]. CVD reaction occurs in a chamber of predetermined geometry to ensure sufficient mass transport of TMOS to the electrode surface. TMOS (volatile at room temp.) and water are introduced to the chamber with the modified cathode, having the enzyme deposited on it, causing a reaction with water to take place and a polymerization process to occur. The BOx is entrapped by the bulk silica-gel or silica matrix on the CNT modified TP. This entrapment procedure prolongs the enzymes life up to 6 months (Fig. 3). At day 1 the recorded current density at 0.25 V was 160 μA.cm-2 and at day 180 the current density obtained was 210 μA.cm-2(Fig. 3B). All values of the current densities obtained during the 6 months testing period fall into the confidence limits of the initial output showing preserved enzymatic activity and cathode stability during that time interval.
The design and stability demonstrated for the enzymatic electrode can be considered a significant achievement that opens the avenue to new practical applications of biofuel cells. This research demonstrates that increased stability of those systems is a reality.

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