Characterization of the Ionic Liquid/Electrode Interfacial Relaxation Processes Under Potential Polarization for Ionic Liquid Amperometric Gas Sensor Method Development

Abstract

Electrochemical amperometric sensors require a constant or varying potential at the working electrode that drives redox reactions of the analyte for detection. The interfacial redox reaction(s) can result in the formation of new chemical products that could change the initial condition of the electrode/electrolyte interface. If the products are not inert and/or cannot be removed from the system such that the initial condition of the electrode/electrolyte interface cannot be restored, the sensor signal baseline would consequently drift, which is problematic for the continuous and real-time sensors. By setting the electrode potential with the periodical ON-OFF mode, electrolysis can be forestalled during the off mode which can minimize the sensor signal baseline drift and reduce the power consumption of the sensor. However, it is known that the relaxation of the structure in the electrical double layer at the ionic liquid/electrode interface to the steps of the electrode potential is slow. This work characterized the electrode/electrolyte interfacial relaxation process of an ionic liquid based electrochemical gas (IL-EG) sensor by performing multiple potential step experiments in which the potential is stepped from an open circuit potential (OCP) to the amperometric sensing potential at various frequencies with different time periods. Our results showed that by shortening the sensing period as well as extending the idle period (i.e., enlarge the ratio of idle period versus sensing period) of the potential step experiments, the electrode/electrolyte interface is prone to relax to its original state, and thus reduces the baseline drift. Additionally, the high viscosity of the ionic liquids is beneficial for electrochemical regeneration via the implementation of a conditioning step at zero volts at the electrode/electrolyte. By setting the working electrode at zero volts instead of OCP, our results showed that it could further minimize the baseline drift, enhance the sensing signal stability, and extend the functioning lifetime of a continuous IL-EG oxygen sensor.

Keywords: amperometric sensor; electric double layer; interfacial relaxation; ionic liquids; oxygen sensor.

Figure 1.

(a) Cross section of the modified Clark-type electrochemical cell. (b) Snapshot of the KWJ electrochemical cell. Material of working, reference, and counter electrodes is platinum black. (c) Potential–time function of a typical “measurement cycle”. (d) Potential–time function when varying the t_i period and maintaining a specific t_s period.

Figure 2.

(a) Current–time responses of two separate measurements depicting the difference of the peak capacitive current and the transient time in non-faradaic condition, and the response time in faradaic condition, showing the influences from the initial condition of the IL/electrode interface. The peak capacitive current and transient time of measurement #1 and #2 are marked in black and blue, respectively. (b) Illustration of the transient time definition at constant potential polarization.

Figure 3.

Plots of the ΔI_avg/I_ti=0 as a function of the idle period, when the sensing period varied as 30 min, 200 s, and 1 s. Measurements were performed in nitrogen condition.

Figure 4.

Plots of the ΔI_avg/I_ti=0 as a function of the idle period, when the sensing period varied as 30 min, 200 s, and 1 s. Measurements were performed in 5% oxygen/nitrogen mixture gas condition.

Figure 5.

Plot of the RSD of the measurements at three different t_i/t_s ratios in (a) inert nitrogen condition and (b) 5% oxygen/nitrogen mixture gas condition. The three analyzed t_i/t_s ratios are (1) idle:sensing = 0.5000 (t_i = 15 min, t_s = 30 min), (2) idle:sensing = 4.50 (t_i = 15 min, t_s = 200 s), and (3) idle:sensing = 900.9 (t_i = 15 min, t_s = 1 s).

Figure 6.

Stepped potential voltammetry current–time responses for a total 80-cycle intermittent idle and sensing period, incorporating the IL/electrode interface conditioning step (set the conditioning potential E_cond as 0 V) during the idle period. Two t_i/t_s ratios: (a) t_i = 59 s and t_s = 1 s, and (b) t_i = 159 s and t_s = 1 s. Trends of the current–time signals at t_s period were marked by cyan lines. The time presented in the x-axis only reflected the total sensing period. The data acquisition from the Princeton AMETEK instrument did not record signals during the t_i periods. Data acquisition rate: 1 data point per 0.02 s.

Scheme 1.. Hypothesized Ion Arrangement Conditions at the IL/Electrode Interface under Different Potentialsa^a

^aThe recorded OCP of our IL-EG sensor was −38.07 μV; thus, we assume that more anions will aggregate at the IL/electrode interface when a 0 V conditioning potential was applied.