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Fig. 5. Sensor response current vs. elapsed time for step change of methanol concentration from 0 to 1.5 M operated in pulsed mode in comparison with potentiostatic mode (solid line) at 50°C. (The inlet is the partially enlargement of pulsed result.)

Figure 5 Sensor response current vs. elapsed time for step change of methanol concentration from 0 to 1.5 M operated in pulsed mode in comparison with potentiostatic mode (solid line) at 50°C. (The inlet is the partially enlargement of pulsed result.)

Related Figures (8)

Fig. 1. Schematic diagram showing sensor configuration: (a, a’, e, e’) gas diffusion layers; (b, b’) catalyst layers of cathodes; (c, c’) Nafion membrane; (d, d’) catalys' layers of anodes; (f) gasket with vent and (g) meshy collector of current. In the present work, pulsed amperometric detection (PAD) method was employed to this sensor to alleviate the anodic satu- ration and improve the stability. It is well known that PAD tech- nique have been extensively utilized in detecting carbohydrates in anion-exchange chromatography to enhance its sensitivity and long-time stability [14,15]. PAD works by applying a repeat- ing potential versus time waveform (normally a triple-potential waveform) [16] to the working electrode in a flowthrough detec- tor cell. The first portion of the waveform is the detection potential Eger at which analyte oxidation current is measured. The detection portion is followed by steps to anodic oxidation potentials (Eoxa) and cathodic activation potential (E;eq). The a CHI 760B electrochemical workstation (Austin, Texas) together with supporting software (Version 4.23) was used to control the potential. Cyclic voltammetry was measured at a scan rate of 20 mV s~! in the range from 0 to 0.9 V. The poten-
Fig. 1. Schematic diagram showing sensor configuration: (a, a’, e, e’) gas diffusion layers; (b, b’) catalyst layers of cathodes; (c, c’) Nafion membrane; (d, d’) catalys' layers of anodes; (f) gasket with vent and (g) meshy collector of current. In the present work, pulsed amperometric detection (PAD) method was employed to this sensor to alleviate the anodic satu- ration and improve the stability. It is well known that PAD tech- nique have been extensively utilized in detecting carbohydrates in anion-exchange chromatography to enhance its sensitivity and long-time stability [14,15]. PAD works by applying a repeat- ing potential versus time waveform (normally a triple-potential waveform) [16] to the working electrode in a flowthrough detec- tor cell. The first portion of the waveform is the detection potential Eger at which analyte oxidation current is measured. The detection portion is followed by steps to anodic oxidation potentials (Eoxa) and cathodic activation potential (E;eq). The a CHI 760B electrochemical workstation (Austin, Texas) together with supporting software (Version 4.23) was used to control the potential. Cyclic voltammetry was measured at a scan rate of 20 mV s~! in the range from 0 to 0.9 V. The poten-
Fig. 2. Cyclic voltammograms of the sensor anode at a scan rate of 20mV s~!. Flow rate of methanol solution: 4.2 ml min7!. Fig. 3 shows the working curves of the sensor at differ- ent temperatures operated in potentiostatic mode (0.8 V versus
Fig. 2. Cyclic voltammograms of the sensor anode at a scan rate of 20mV s~!. Flow rate of methanol solution: 4.2 ml min7!. Fig. 3 shows the working curves of the sensor at differ- ent temperatures operated in potentiostatic mode (0.8 V versus
Fig. 3. Response current of the sensor operated in potentiostatic mode vs. methanol concentration at various temperatures (dot: experimental result; solid line: fitting result).
Fig. 3. Response current of the sensor operated in potentiostatic mode vs. methanol concentration at various temperatures (dot: experimental result; solid line: fitting result).
Fig. 4. Effect of the Eyeq (a) and Epre-oxa (b) applied in the pulsed amperometric detection mode to the sensor anode on the amperometric response. The methanc solution is 2M methanol at a flow rate of 2.6 ml min7!.
Fig. 4. Effect of the Eyeq (a) and Epre-oxa (b) applied in the pulsed amperometric detection mode to the sensor anode on the amperometric response. The methanc solution is 2M methanol at a flow rate of 2.6 ml min7!.
Fig. 6. Two periods of PAD response for methanol concentration (0; 1; 2; 3M at 50°C. Fig. 6 shows two periods of the signal for methanol solu- tion with 0, 1, 2 and 3M in pulsed mode. As observed the amperometric response increases significantly with increasing concentration. The current at the end of detection step is pro- portional to concentration. It is interesting that not only the detection current but also the shape of curve change with concen- tration. At the initial part of detection step, the charging current decreases with increasing concentration and a dull peak appears
Fig. 6. Two periods of PAD response for methanol concentration (0; 1; 2; 3M at 50°C. Fig. 6 shows two periods of the signal for methanol solu- tion with 0, 1, 2 and 3M in pulsed mode. As observed the amperometric response increases significantly with increasing concentration. The current at the end of detection step is pro- portional to concentration. It is interesting that not only the detection current but also the shape of curve change with concen- tration. At the initial part of detection step, the charging current decreases with increasing concentration and a dull peak appears
Fig. 7. Response current of the sensor operated in pulsed mode (sampled at the end of Eger) vs. methanol concentration at various temperatures. (The inlet graph: current sampled at the end of Epre-oxa (0.6 V) vs. methanol concentration at different temperatures). The current at the end of detection step was chosen to be the amperometric response to minimize the charging effect. Fig. 7 shows the working curves of the sensor at different temperatures obtained in pulse mode. The sensitivity was increased in pulse mode. As for 50 °C the sensitivity of 73.2 mA M7! is about twice as high as that of potentiostatic mode. It is evident that a cluster of curves of the current versus concentration is different from the potentiostatic result [5]. The increase in amplitude with ele-
Fig. 7. Response current of the sensor operated in pulsed mode (sampled at the end of Eger) vs. methanol concentration at various temperatures. (The inlet graph: current sampled at the end of Epre-oxa (0.6 V) vs. methanol concentration at different temperatures). The current at the end of detection step was chosen to be the amperometric response to minimize the charging effect. Fig. 7 shows the working curves of the sensor at different temperatures obtained in pulse mode. The sensitivity was increased in pulse mode. As for 50 °C the sensitivity of 73.2 mA M7! is about twice as high as that of potentiostatic mode. It is evident that a cluster of curves of the current versus concentration is different from the potentiostatic result [5]. The increase in amplitude with ele-
Fig. 8. The addition result of detection current and pre-oxidation current vs. concentration at different temperatures in pulsed mode. Furthermore, on increasing temperature, the amplitude of pre-oxidation current (shown by arrows) increases obviously with increasing temperature. The increasing of amplitude seems to compensate for the decreasing in detection step. In order to analyze the relationship of compensation, the addition of detec- tion current and pre-oxidation current is shown in Fig. 8. Under the same condition, the current is about 70% larger than in poten- tiostatic mode which is the consequence of transient potential pulse. The concentration dependence with a non-linear trend and temperature dependence with a linear trend agree with working curves of potentiostatic mode. Thus, the pre-oxidation step can calibrate the bias and alleviate the saturation by partially oxi- dizes the poison material accumulated during detection steps. This calibration effect was enhanced by increasing temperature and the linearity range was expanded as a result.
Fig. 8. The addition result of detection current and pre-oxidation current vs. concentration at different temperatures in pulsed mode. Furthermore, on increasing temperature, the amplitude of pre-oxidation current (shown by arrows) increases obviously with increasing temperature. The increasing of amplitude seems to compensate for the decreasing in detection step. In order to analyze the relationship of compensation, the addition of detec- tion current and pre-oxidation current is shown in Fig. 8. Under the same condition, the current is about 70% larger than in poten- tiostatic mode which is the consequence of transient potential pulse. The concentration dependence with a non-linear trend and temperature dependence with a linear trend agree with working curves of potentiostatic mode. Thus, the pre-oxidation step can calibrate the bias and alleviate the saturation by partially oxi- dizes the poison material accumulated during detection steps. This calibration effect was enhanced by increasing temperature and the linearity range was expanded as a result.
Fig. 9. The stability test of sensor at 50 °C in pulsed mode (a) and in potentio- static mode (b). Methanol solution: 2 M at flow rate of 2.6 ml min7!.
Fig. 9. The stability test of sensor at 50 °C in pulsed mode (a) and in potentio- static mode (b). Methanol solution: 2 M at flow rate of 2.6 ml min7!.
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