The Rate-limiting Step in a Glucose/Oxygen Biofuel cell

Abstract

<p> In this thesis, the rate-limiting step is determined in a biofuel cell with a bio-anode, a Nation membrane and a conventional, platinum-based cathode using reference electrode method. It was discovered by surprise that the cathode overpotential dominated the cell overpotential. Na + in the membrane was found to hinder the W transport. The cathode overpotential increased due to the presence of Na + in the membrane and at the cathode. The limited H+ transport causes the increase of the cathode overpotential. H+ transport is the rate-limiting step in our biofuel cell, rather than commonly believed electron transport. Moreover, the cell power output degradation is not due to the conventionally believed depletion of the fuel substrate, inter-penetration of the fuel and oxidizer and the degradation of the biocatalysts, but the limited W transport in our biofuel cell. </p> <p> The existing oxygen reduction mechanism at the cathode was questioned and revised. When Na+ occupies all sulfonate groups in the membrane, only the Na+ from the buffer can pass through the membrane. The oxygen reacts with the water transported with Na+ and electrons to produce OH", which balances with the transported Na+ to keep electroneutrality at the cathode. </p> <p> Tris buffer without Na + was utilized as alternative anolyte in the biofuel cell. It was found that the cell with Tris buffer had a poorer performance in comparison with sodium phosphate buffer due to the increases of the anode and cathode overpotentials. Tris buffer does not constitute a solution to the problem. </p> <p> This work represents a step toward a more complete understanding of the properties of biofuel cells. To improve biofuel cell output, the herein identified H+ transport limitation in Na + contained Nation needs to be overcome. </p>