Design & Characterization of Pb Electrode for Sensing Trichloroethylene in Ionic Liquid/Acetonitrile Co-Solvents
Authors: Chen Tongtong , Zeng Xiangqun 
Published: July 2021

Volatile organic compounds (VOC) contamination through vapor intrusion is an emerging threat to public health that has garnered intense attention. For example, superfund sites contain a number of VOCs including trichloroethylene (TCE), tetrachloroethylene (PERC), and volatile gasoline derivatives such as benzene, toluene, ethylbenzene and xylenes (BTEX). These compounds can contaminate shallow groundwater aquifers and soils beneath the built environment and, through vapor intrusion, negatively affect human health in exposed individuals and trans-generationally. Currently it is very challenging to assess and mitigate vapor intrusion due to the complexity and high cost of conventional off-line sampling methods. Gas sensors allow monitoring and measuring of gaseous and volatile chemical molecules under various conditions in the local environment. There are many gas monitoring systems on the market, but none of them are capable of real-time, continuous measurement of gas concentrations in real world. An ideal gas sensor would be compact, inexpensive, portable, and have low power requirements – but also provide continuous monitoring of the analytes with high sensitivity, specificity and accuracy.

Electrochemical gas sensors possess an impressive capacity to achieve the analytical requirements of high sensitivity and selectivity as well as continuous monitoring of analytes within a small footprint, are constructed with low-cost materials and fabrication techniques, and are controlled by compact low-power microelectronic circuits. However, current electrochemical sensors typically use noble metal and carbon as electrodes. They suffer from low sensitivity and specificity for VOC detection. Those use aqueous electrolytes or traditional non-aqueous electrolytes suffer from dry-out problems of the electrolytes and/or narrow potential windows due to decomposition of the electrolytes (electrolytes can be oxidized or reduced before the analytes redox processes).

In this work, several innovations are incorporated into the design of electrode catalysts and electrolyte materials in developing next generation electrochemical gas sensors to address the limitations in current electrochemical gas sensor for detecting VOCs. We selected TCE as our target analyte. Trichloroethylene (TCE) is a colorless, volatile liquid that evaporates quickly into the air. Due to the widely use of TCE as a solvent to remove grease from metal parts and as a chemical to make other chemicals, especially the refrigerant, HFC-134a, there is strong evidence of significant TCE exposure in many neighborhood. TCE is frequently found at Superfund sites as a contaminant in soil and groundwater. Research shows that lifetime exposure to TCE resulted in increased liver cancer in mice and increased kidney cancer in rats at relatively high exposure levels and TCE poses potential cancer and non-cancer human health hazards. The concentrations of TCE are very low in the urban ambient conditions (ppm to ppb level) with many interfering compounds in much higher concentration making their detection very challenging. Instead of noble metals which are expensive and also provide limited reactivity, we fabricated lead (Pb) thin film electrode as our sensing electrode. Instead of using aqueous or traditional non-aqueous electrolytes that have high volatility, we tested an ionic liquid (IL) BmimBF4 and acetonitrile (AN) co-solvents as the electrolytes. ILs are organic salts that are nonvolatile, electrically conductive, and have wide stability. Acetonitrile is widely used in battery applications because of its relatively high dielectric constant (38.8) with a dipole moment (3.92 D) and its ability to dissolve common electrolytes. Moreover, acetonitrile has a free electron pair at the nitrogen atom, which can form many transition metal nitrile complexes. Being weakly basic, it is an easily displaceable ligand. We select BmimBF4 since BF4
- is a non-coordinating anion that interacts weakly with cation Bmim+. We expect some interesting chemistry could occur between Pb electrode and/or the BmimBF4/Acetonitrile co-solvent that could provide additional sensitivity and selectivity for detecting VOC.

In this work, We fabricated Pb thin film on glassy carbon electrode substrate using lead tetrafluoroborate salt that dissolved in HBO3 and HBF4 aqueous electrolyte with added trace Poly(ethylene glycol) as the electrolyte and an applied reduction potential of -0.4 V (vs Ag/Ag+ reference electrode) for Pb electrochemical deposition. This electrodeposition process results in a smooth Pb film on the GC working electrode.

We thoroughly characterized the Pb thin film modified electrode by multiple cycles cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) in BmimBF4/ Acetonitrile co-solvents. We vary the volume percentage (10%, 30%, 50% and 100%) of BmimBF4 in the co-solvents to study the effects of ILs and optimize the sensing conditions for TCE detection. Our results show that in 30% BmimBF4
+ AN electrolyte, the Pb electrode shows a higher sensitivity than those other co-solvent systems tested. Additionally, 30% BmimBF4/Acetonitrile electrolyte also shows better stability for TCE sensing. Finally the TCE sensing was characterized by chronoamperometry at constant potential E = -2.1 V. We obtain quantitative detection of TCE at 100 ppm to 600 ppm range. Several potential interfering species such as dichloroethane and acetaldehyde were tested, Our results show good sensitivity and selectivity of the fabricated Pb electrode for TCE detection in our IL-AN co-solvent. IR and UV-vis spectroscopy experiments were used to characterize the electrolyte after TCE electrolysis to study the mechanism of the TCE electrolysis on the Pb electrode in IL-AN electrolyte. This work shows that the Pb electrode together with an ionic liquid based non-aqueous aprotic co-solvent provide unique interfacial electrochemistry, allowing the synergistic integrating of Pb electrode catalysts, with IL based electrolytes that open up new opportunities to develop powerful, low-cost and miniaturized electrochemical VOC sensors for highly sensitive and selective detection of environmental vapors for a range of applications.