Electrode materials and oxygen sensing

In order to improve the sensitivity of the Clark type electrode, various methods of testing catalysts, methods of applying catalysts to electrodes, and carbon rods were examined. A systematic approach for screening catalysts for oxygen reduction, determining background and charging current, overpotential lowering and net current at a given applied potential is presented.;Cyclic-voltammetry (CV) was a very effective technique for evaluating the efficacy of a particular catalytic matrix by measuring oxygen reduction reaction (ORR) peak current enhancements. Some electrodes showed good catalytic response while others prepared in the "same" way did not. Electrode-to-electrode variations, initially as much as 1700%, so large it was impossible to determine if the improved response was due to addition of the catalyst or electrode-to-electrode variations. Improved reproducibility between electrodes was achieved by the development of a protocol of careful preparation of the electrodes, prior testing of background levels, thus obtaining a characterization of the electrodes indicating minimal variances before testing the carbon/catalysts response. Reproducibility in the packing of carbon paste electrodes (CPE) was found to be a critical factor in comparing CPEs, as was the aging of the electrode prior to characterization. The protocol developed to prepare CPEs evolved to only accepting CV data from samples characterized with a minimum of five different electrodes and five replicates per electrode. Once variances for responses of a carbon/catalyst mixture were lowered, valid comparisons between catalytic mixtures were made.;Using cyclic voltammetry and the construction protocol, it was possible to optimize the carbon/catalyst mixture by pre-testing before pursuing oxygen sensitivity testing. For the first time, CPEs composed of electro catalytic carbons are evaluated. Here, CPE paste concentrations of 5% to 100% Etek C13 that provide significant ORR current enhancements are reported. A concentration of 15% Etek C13 gave the maximum ORR current enhancement. In the case of electro catalytic carbons, the charging current observed was so large that faradaic current was difficult to observe or not observable at all under casual inspection.;Constructing an electrical equivalent circuit modeling the CPE background signal that included a "leakage" or charge transfer resistance term provided insight into low level ORR affects the background currents. Results indicate that the packing of the CPE has a profound effect on the magnitude of the background current and the overall performance as an oxygen electrode. An evaluation of an electrical model using equations derived from simple electrical equivalent circuits using Ohms law and Kirchhoff's law with derivations by Thevenin's theorem and Laplace transforms is presented. An evaluation of current observed at different times during the electrochemical experiment compares the currents predicted from these equivalent circuits. The model predicted currents within 25% of the experimental data observed for Etek C13. The construction of a theoretical model using an equivalent circuit allowed modeling the electrical response from the CPEs.;In a separate work, casting different carbon catalysts in polymer mixtures onto gold electrodes found a mixture that would remain intact upon drying, without shrinkage, cracking or separation from the electrode. Formulations were examined by obtaining currents for each electrode at varying DO concentrations, and obtaining sensitivity curves of peak current density per area versus the DO concentration. Formulations of 10 mg of 100% Etek C13 in 1 mL of 5% Nafion polymer and 10 mg of a 50% mixture of Etek C13 and Vulcan XC72R carbon in 1 mL of 5% Nafion polymer were cast onto the gold cathode of the probe of a commercial Yellow Springs Inst. (YSI) DO meter. These modifications to the YSI DO probe showed an increase in sensitivity approaching 76% and 137%, respectively.;Graphite rod electrodes were tested for sensitivity to oxygen and sensitivity curves constructed. They were found to show significantly higher sensitivity than that of a typical gold electrode for dissolved oxygen. The sensitivity of solid 0.5 mm graphite rods were determined to be ten times more sensitive than the gold electrodes, yielding an average sensitivity slope of 227 (+/-90) uA·(cm2)-1·(ppm O2 )-1 compared to 22.7 (+/-2.1) uA·(cm 2)-1·(ppm O2)-1 for the gold electrodes (n=6 for graphite rods, n=10 for gold electrodes).