The development of new microelectrode gas sensors: an odyssey. Part 1. O2 and CO2 reduction at unshielded gold microdisc electrodes

CO2 Gas Sensing at Microelectrodes in Nonaqueous Solvents

“…Linear potential sweeps at a gold microelectrode in 0.1 M TEAP, DMSO produced a current plateau similar to that obtained by Hahn et al [30], with a range of about 250 mV between the beginning of the plateau and the onset of solvent reduction (Fig. 1).…”

“…The lower value recorded at the gold microelectrode may relate to a loss of active surface area as indicated by tests with ferrocene in acetonitrile, while the larger value at platinum may also be due to a two-electron process for CO 2 reduction. We can also note that the limiting currents obtained by Hahn et al [30] (15 nA at a 10 mm microelectrode), are a little lower than the above 20. On the other hand, the additional current of 0.23 mA cm À2 at a 3 mm diameter gold rotating disk electrode in this work observed for each extra 1 % of CO 2 in the gas¯ow is in good agreement with the theoretical calculation, con®rming the consistency of the above values of C, D and n. At the same time the response time of the system to a new level of CO 2 was 1±2 minutes when the gas was purged through the solution, but of the order of 20±30 minutes when the atmosphere was circulated above the solution.…”

“…In the present example, the test gas was passed over the top of the solution rather than purged through it, to give an indication of the time required for equilibrium to be established between CO 2 in the surrounding atmosphere and dissolved in DMSO. The background currents due to residual O 2 were also larger, and appeared as two reduction waves, as observed in earlier studies [30]. Additional currents of about 0.23 mA cm À2 were obtained at À2.0 V (AgaAgCl) for each 1 % step increase in CO 2 content, which required 20±30 minutes to reach a steady value, re¯ecting the need for CO 2(g) to dissolve in the solution.…”

“…The instability of the potential expected at the bare Ag wire, particularly with changes in dissolved O 2 and during the passage of current, creates uncertainty in the recorded potential values, but is quite adequate for the purposes of applying a large negative potential. The microelectrodes were tested by the oxidation of ferrocene in acetonitrile [30], which con®rmed the 10 mm diameter of the platinum microelectrode. However, the limiting currents on the gold microelectrode were only half as intense, possibly due to pieces of diamond grit which remained following use of the BAS cleaning procedure, even after sonication.…”

“…However, their calculation of a limiting current of 800 nA, using the saturated CO 2 concentration of Albery of 84 mM [26] and n 2, uses a diffusion coef®cient of 6.15610 À6 cm 2 s À1 which is probably too low, given that it is calculated from the Sand equation using a saturated concentration of 128 mM [27] which is higher than values reported in later studies [24,26]. Hahn et al, took fast potential scans at 3.4 mm and 10 mm diameter gold microdisk electrodes [30,31], to produce current plateaus proportional to the concentrations of both 10±30 vol. % O 2 (monitored at À1.0 V (Ag wire quasireference)) and 3±15 vol.…”