Development of Hybrid Bio-Inorganic Catalyst for Oxygen Reduction Reaction in Enzymatic Fuel Cells

The search for alternative power sources able to generate electricity out of a fuel rather than just storing electrical current, like the battery, have turn into the development of fuel cells. Some of these fuel cells mimic the way the human body extracts energy from sugars or other organic compounds and these are called enzymatic biofuel cells (EFC). EFCs utilize enzymes to convert the chemical energy into electrical current. An important part of their design is the development of cathodes that can carry out oxygen reduction reaction (ORR) and explore the benefits of the abundant, cheap and easily available oxygen. The enzymes capable of ORR belong to the family of multi-copper oxidases, very well known in the area of EFCs. One particular enzyme, bilirubin oxidase (BOx), from that family is gaining more and more attention in the last years due to its superior performance and resistivity to halide ions. The main advantage of the enzymatic cathodes for ORR is the low overpotential of the reaction and thus the high open circuit potential (OCP) of the electrodes. At the same time, the maximum current generated from those types of cathodes is significantly lower in comparison to the traditional inorganic electrodes, which start the ORR at high overpotentials, but in contrast producing higher currents in this low potential region.

In order to increase the performance of BOx cathodes for ORR, we combined the advantages of enzymatically catalyzed oxygen reduction with the advantages of ORR catalyzed by inorganic catalyst, such as non-platinum metal group catalyst (NPGM)¹.

Several NPGM catalysts were incorporated with BOx in ink type composite cathodes and tested using rotating disk electrode (RDE) technique in electrolyte with pH 7.5. Those catalysts were synthesized by sacrificial support method, as it was previously demonstrated¹. Based on the performed screening study, two NPGM catalysts (Fe-DANM and Fe-AAPyr) demonstrated higher compatibility with the enzyme and as a result, higher performance was achieved. Fe-DANM catalyst used diaminomaleonitrile as a precursor and Fe-AAPyr proceeds from aminoantipyrine.

Due to the high hydrophobicity of the NPGM catalyst, the interaction between the enzyme and the inorganic catalyst is hindered. Therefore, to provide more hydrophilic environment and enhance the enzyme immobilization, carbon nanotubes were introduced in the NPGM-BOx composite along with 1-pyrenebutanoic acid succinimidyl ester (PBSE) as a tethering agent. The ink composition was optimized in terms of: (1) NPGM:CNTs ratio, (2) amount of enzyme, (3) linker and (4) time of immobilization, showing that 1:1 NPGM:CNTs ratio with 10 mg/ml BOx, 10 mM PBSE and 16-18 hours of immobilization is the optimum ink formula.

The aim of these NPGM-CNTs-BOx composites was to increase the cathodes final output in the broad potential region, combining the pronounced advantages of the enzymatic catalysis at high potentials and the ones of the inorganic catalysis at low potentials. Figure 1 clearly demonstrates that the simultaneous utilization of bio- and inorganic catalysis leads to improvement of the generated current densities in the whole range of potentials tested.

After the positive effect of the combined NPGM-CNTs-BOx catalyst was demonstrated, a step forward was taken to improve the NPGM-CNTs interactions. NPGM catalyst (Fe-AAPyr) was in situ synthesized on multi-walled carbon nanotubes creating “fused” NPGM-CNTs composite. This composite was used for the development of the catalytic layer (CL) of gas-diffusion “hybrid” cathode. The gas-diffusion layer (GDL) of this cathode consisted on teflonized carbon black, on which the CL was pressed. BOx was physically adsorbed on the NPGM-CNTs “fused” catalyst and the performance of the cathodes was studied by polarization measurements (Fig.2). The comparison of the output of two identical cathodes differing in the preparation of the NPGM-CNTs composition shows the benefits of the in situ preparation technique.

The introduction of NPGM catalysts into biological electrochemical systems, such as an enzymatic electrode for ORR, showed a notable improvement of the cathode’s performance, proving the described herein principal. This opens the venue for new applications of enzymatic biofuel cell, from biosensors to energy production.

[1] Brocato, S., A. Serov, P. Atanassov Electrochimica Acta 2013, 87, 361– 365

[2] Brocato, S., C. Lau, P. Atanassov Electrochimica Acta 2012, 61, 44– 49