Potentiometric c2h4-selective detection on solid-state sensors activated with bifunctional catalytic nanoparticles

chemosensors Article Potentiometric C2H4-Selective Detection on Solid-State Sensors Activated with Bifunctional Catalytic Nanoparticles Fidel Toldra-Reig and Jose Manuel Serra *   Citation: Toldra-Reig, F.; Serra, J.M. Potentiometric C2H4-Selective Detection on Solid-State Sensors Activated with Bifunctional Catalytic Nanoparticles. Chemosensors 2021,9, 274. https://doi.org/10.3390/ chemosensors9100274 Academic Editors: Valerio Vignoli and Enza Panzardi Received: 31 July 2021 Accepted: 22 September 2021 Published: 27 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Instituto de Tecnología Química, Universitat Politècnica de València, Consejo Superior de Investigaciones Cientèficas, Av Naranjos s/n, E-46022 Valencia, Spain; [email protected].es *Correspondence: [email protected].es Abstract: This work presents a solid-state ionic-based device to selectively detect C 2 H 4 in car exhaust gases. The sensor consists of 8YSZ as the electrolyte and two electrodes: Fe 0.7 Cr 1.3 O 3 /8YSZ and LSM/8YSZ. The main aim of this work is to optimize the catalytic behavior of the working electrode to C 2 H 4 and reduce cross-sensitivity toward CO and H 2 O. Several catalyst nanoparticles were infiltrated to tailor C 2 H 4 adsorption and electrochemical oxidation properties while diminishing adsorption and conversion of other gas components such as CO. The infiltrated metal catalysts were selected, taking into account both adsorption and redox properties. Infiltration of Ti or Al, followed by a second infiltration of Ni, enabled the selective detection of C2H4with low cross-sensitivity toward CO and H 2 O in a moist gas environment. Further insight into potentiometric C 2 H 4 sensing is achieved by electrochemical impedance analysis of the electrodes activated with bifunctional catalysts. Keywords: hydrocarbon; ethylene; potentiometric; sensor; catalysis 1. Introduction Currently, the public health factor involving pollution in cities is a hot topic [ 1 – 3 ]. Legislation is becoming more restrictive with regard to both polluting emissions from road vehicles and car circulation in the innermost part of cities. This involves both existing and future automotive fleets and better monitoring of environmental performance (defeat devices, tampered antipollution systems, etc.). This is a concern for the European Commission, and it has been included in their work programs for smart, green integrated transport in H2020 during the 2018–2020 period. This is also tackled in the Horizon Europe work program for Climate, Energy, and Mobility for 2021–2022 [ 4 ]. Among different actions, pollution tolls and further emission controls are under consideration in cities to ban the most polluting cars within urban areas [ 5 – 9 ]. Therefore, more restrictive legislation is expected in the short term. However, legislators cannot lower emission limits because of the current lack of devices able to detect low contents of hydrocarbons selectively. Therefore, the availability of an economically attractive, reliable, and highly sensitive hydrocarbon sensor could help to establish this sensor in combustion engines but also in the monitoring of gas streams in other combustion or thermochemical processes. The elevated temperature of exhaust gases and exhaust gas content make potentiometric sensors the most appealing option. In the literature, zirconia and platinum are widely employed as electrolyte and reference electrodes, respectively [ 10 – 17 ]. As working electrodes, different oxides, from simple to more complex oxides [ 12 , 17 – 21 ] such as spinels and perovskites, are used for the detection of several elements, e.g., hydrocarbons [ 10 , 15 , 22 – 26 ], CO [ 19 , 27 – 29 ], and NH 3 [ 30 , 31 ]. The general problem is not only the use of expensive noble metals but also poor selectivity to the target gas and/or high cross-sensitivity toward other elements. Responses to single gases are compared without comparing the effect of other gases in a gas mixture as expected in real applications. In this work, a potentiometric hydrocarbon sensor is optimized by surface functionalization of its working electrode [ 32 , 33 ], avoiding the use of noble metals to enhance selectivity Chemosensors 2021,9, 274. https://doi.org/10.3390/chemosensors9100274 https://www.mdpi.com/journal/chemosensors Chemosensors 2021,9, 274 2 of 14 to ethylene. It consists of a solid ionic electrolyte (8YSZ) and two electrodes: LSM/8YSZ as the reference electrode and Fe 0.7 Cr 1.3 O 3 /8YSZ as the working electrode. Additionally, both electrodes are exposed to the same atmosphere. This simplifies the device and allows it to behave according to the mixed-potential theory, i.e., several reduction and oxidation reactions running simultaneously at each electrode, where one controls the kinetics. When equilibrium is achieved in each electrode, a difference in voltage that can be measured is generated [13,34–36]. Therefore, reference and working electrodes must be selective to an oxygen cathodic reaction and to a target gas anodic reaction, respectively. Kinetics in each electrode must be controlled by these reactions. C 2 H 4 is a major hydrocarbon in exhaust gas [ 37 – 40 ]; therefore, it is considered a target gas. Additionally, cross-sensitivity toward any elements common in exhaust gas such as CO, H 2 O, and polyaromatics (C 11 H 10 and C14H10) should be kept low during dynamic operation. This work focuses on the catalytic functionalization of the working electrode (WE) in order to provide a specific response to C 2 H 4 with low cross-sensitivity against CO and H 2 O, i.e., fostering C 2 H 4 electrochemical oxidation kinetics. This is accomplished by means of electrode infiltration with distinct nanosized catalysts. The device consists of a common reference electrode on one face of the solid electrolyte and four different working electrodes on the other face of the electrolyte. Nickel dispersion on the WE has been reported to boost electrochemical C 2 H 4 oxidation [ 41 ]. Several catalytic materials are selected in this work because of their redox activity or adsorption properties related to hydrocarbons and CO: Al [ 42 ], Ti [ 43 ], Ni [ 41 , 44 ], Ru [ 45 – 47 ], Pd [ 48 , 49 ], Nb [ 50 ], and Ba. Some of these specific elements were selected because of their Lewis acidity or basicity (for the alkali elements) of related cationic species [48]. Thus, several nanosized catalysts, including binary combinations, are incorporated in the sensor WE to selectively improve electrochemical C 2 H 4 oxidation. The open-circuit voltage generated between both electrodes is measured for concentration pulses of C 2 H 4 and CO in both dry and humidified conditions. Moreover, electrochemical impedance spectroscopy is carried out for a better understanding of the processes taking place at the activated electrodes. 2. Experimental 2.1. Sample Preparation Both La 0.9 Sr 0.1 MnO 3 perovskite (LSM) and Fe 0.7 Cr 1.3 O 3 were synthesized by means of a sol–gel chemical route. Commercial nitrates from Sigma-Aldrich were used as precursors. Citric acid (Sigma-Aldrich, St. Louis, MO, USA) was added to the stoichiometric waterbased solution to prevent partial segregation of the metallic elements. Addition of ethylene glycol generated polymerization in a 1:2:4 ratio (nitrate precursors, citric acid chelating agent, and ethylene glycol, respectively). Two-step thermal decomposition (200 ◦ C and 600 ◦C) led to the generation of nanosized crystalline phases. Such powders were ground in a ball mill and later sintered at 1350 ◦ C for 10 h to produce the desired crystalline phase (ICDD 00-035-1112 for Fe0.7Cr1.3O3). Both Fe 0.7 Cr 1.3 O 3 and LSM were mixed with 8YSZ (Tosoh) in a 1:1 vol. ratio to avoid delamination with the electrolyte and to obtain a mixed ionic–electronic material, respectively. Electronic conductivity is offered by LSM (and selective O 2 activation), while ionic conductivity is provided by 8YSZ. Then, these mixtures were ball milled, mixed with an organic binder, and passed through a triple roll mill to produce inks for screen printing. 2.2. Fabrication of the Sensor Device A four-working-electrode (WE) multidevice was constructed to measure up to four different WE compositions simultaneously, as shown in Figure 1. The electrolyte acts as a support, and it consists of a dense 50 mm diameter disk made of 8YSZ (Tosoh). The disk was uniaxially pressed and then calcined in two steps: (1) 1000 ◦ C for 4 h to machine holes for later wiring and (2) 1450 ◦C for 10 h to densify the electrolyte Chemosensors 2021,9, 274 3 of 14 Chemosensors 2021, 9, x FOR PEER REVIEW 3 of 15 2.2. Fabrication of the Sensor Device A four-working-electrode (WE) multidevice was constructed to measure up to four different WE compositions simultaneously, as shown in Figure 1. The electrolyte acts as a support, and it consists of a dense 50 mm diameter disk made of 8YSZ (Tosoh). The disk was uniaxially pressed and then calcined in two steps: (1) 1000 °C for 4 h to machine holes for later wiring and (2) 1450 °C for 10 h to densify the electrolyte Electrodes were screen printed in a rectangular shape: four Fe 0.7 Cr 1.3 O 3 /8YSZ electrodes on one side of the electrolyte and only one LSM/8SZ on the other side. This configuration is convenient because it consists of a reference electrode for oxygen, which is common, and four different working electrodes for testing different catalytic materials to functionalize the electrode by nanoparticle infiltration. Replacing Pt, which is usually employed as RE [12,13,51–57], with LSM/8SZ results in a larger triple-phase boundary (TPB). This means an increment in the contact point between electronic and ionic materials and the gas. A screen-printed gold layer was used as the current collector (900 °C for 2 h) on top of the electrodes, while silver paste was employed to assure the attachment of the lead wires to the electrodes. Figure 1. Scheme of the device that consists of a dense 8YSZ electrolyte with a common LSM/8YSZ reference electrode (right). Each of the working electrodes (Fe 0.7 Cr 1.3 O 3 ) is infiltrated using a micropipette with several elements (left). The lead wires are mechanically fixed to the device using 5 boreholes. The working electrode is infiltrated with several nanocatalysts: Ti, Al, Nb, Ba, and Pd. Nitrate precursor solutions of the aforementioned elements were dropped onto the WE. The solution filled the pores through the capillarity, ensuring full coverage of the electrode surface. The device was then exposed to a thermal treatment to eliminate the organic fraction. First, it was treated with argon at 550 °C for 4 h. Subsequently, the gas composition was changed to 5% H 2 in Ar, and the device was exposed again at 550 °C for another 4 h. The stability of the LSM electrode was confirmed after thermal treatment in these gas atmospheres. After testing each of the aforementioned elements, a second Ni infiltration was performed for each, following the same procedure described previously. Nickel has already been reported to increase device selectivity to C 2 H 4 [41]. 3. Sample Characterization The obtained crystalline phases were identified through X-ray diffraction analysis (XRD) using a PANalytical Cubix fast diffractometer (CuKα1 radiation (λ = 1.5406 Å) and an X′Celerator detector in Bragg–Brentano configuration). X′Pert Highscore Plus was employed to analyze the patterns measured in the 2θ range from 10° to 90°. The cross-sections of the devices were studied by SEM and energy-dispersive X-ray spectroscopy (EDS) using a ZEISS Ultra55 field-emission scanning electron microscope. Figure 1. Scheme of the device that consists of a dense 8YSZ electrolyte with a common LSM/8YSZ reference electrode ( right ). Each of the working electrodes (Fe 0.7 Cr 1.3 O 3 ) is infiltrated using a micropipette with several elements (left). The lead wires are mechanically fixed to the device using 5 boreholes. Electrodes were screen printed in a rectangular shape: four Fe 0.7 Cr 1.3 O 3 /8YSZ electrodes on one side of the electrolyte and only one LSM/8SZ on the other side. This configuration is convenient because it consists of a reference electrode for oxygen, which is common, and four different working electrodes for testing different catalytic materials to functionalize the electrode by nanoparticle infiltration. Replacing Pt, which is usually employed as RE [ 12 , 13 , 51 – 57 ], with LSM/8SZ results in a larger triple-phase boundary (TPB). This means an increment in the contact point between electronic and ionic materials and the gas. A screen-printed gold layer was used as the current collector (900 ◦ C for 2 h) on top of the electrodes, while silver paste was employed to assure the attachment of the lead wires to the electrodes. The working electrode is infiltrated with several nanocatalysts: Ti, Al, Nb, Ba, and Pd. Nitrate precursor solutions of the aforementioned elements were dropped onto the WE. The solution filled the pores through the capillarity, ensuring full coverage of the electrode surface. The device was then exposed to a thermal treatment to eliminate the organic fraction. First, it was treated with argon at 550 ◦ C for 4 h. Subsequently, the gas composition was changed to 5% H 2 in Ar, and the device was exposed again at 550 ◦ C for another 4 h. The stability of the LSM electrode was confirmed after thermal treatment in these gas atmospheres. After testing each of the aforementioned elements, a second Ni infiltration was performed for each, following the same procedure described previously. Nickel has already been reported to increase device selectivity to C2H4[41]. 3. Sample Characterization The obtained crystalline phases were identified through X-ray diffraction analysis (XRD) using a PANalytical Cubix fast diffractometer (CuK α 1 radiation ( λ = 1.5406 Å) and an X 0 Celerator detector in Bragg–Brentano configuration). X 0 Pert Highscore Plus was employed to analyze the patterns measured in the 2 θ range from 10 ◦ to 90 ◦ . The crosssections of the devices were studied by SEM and energy-dispersive X-ray spectroscopy (EDS) using a ZEISS Ultra55 field-emission scanning electron microscope. Regarding electrochemical characterization, the voltage was measured (Keithley 3706) as the potential difference generated between both electrodes (no current applied) at 550 ◦C and different C 2 H 4 and CO concentrations. The flow of gases was controlled by means of mass flow controllers, and the total gas flow was set to 550 mL/min (with 6% O 2 and balanced with argon). The sensor response (V cell , mV) was corrected, taking into consideration the background gas consisting of 6% O2/Ar, and it was defined as: Vcell = Vanalyte −Vbackground Chemosensors 2021,9, 274 4 of 14 When the sample was stabilized at 550 ◦ C, either C 2 H 4 or CO concentration pulses were performed from 50 ppm (used as base gas) to 100, 150, and 200 ppm. In order to determine the cross-sensitivity, this procedure was repeated at a fixed concentration of 200 ppm opposing gas. Finally, impedance sweeps from 0.03 Hz to 1 MHz were measured (Autolab PGSTAT204 with an FRA32M module) for both C2H4and CO at 200 ppm. 4. Results and Discussion 4.1. Microstructural Characterization XRD patterns of Fe 0.7 Cr 1.3 O 3 and LSM (Figure S1) confirm that the phases for both materials were formed as desired, i.e., no diffraction peaks were detected for other phases or the precursor. Postmortem SEM characterization of the device was carried out to determine the dispersion of the nanoparticles over the electrode grains. In every pore, the distribution of Fe 0.7 Cr 1.3 O 3 and 8YSZ grains is homogeneous, although the grain size is different, i.e., Fe 0.7 Cr 1.3 O 3 grains are larger (Figure 2). Layer thickness observed for the WE and RE is 34 and 13 µm, respectively. Chemosensors 2021, 9, x FOR PEER REVIEW 5 of 15 Figure 2. FESEM images of the working electrode cross-section for the bare sensor with 8YSZ in yellow and Fe 0.7 Cr 1.3 O 3 in blue (a) and several infiltrations: (b) Ni, (c) Ti, (d) Al, (e) Nb, (f), Ba and (g) Pd. Nanoparticles for each element can be observed. Additionally, different sizes of nanoparticles can be observed when nickel is added to the first element infiltrated. Fe 0.7 Cr 1.3 O 3 and 8YSZ grains are observed to be homogeneously distributed. 4.2. Electrochemical Characterization In an exhaust-gas-like atmosphere, where several pollutants such as hydrocarbons, CO, NO x , O 2 , etc., can be present, several oxidation and reduction reactions can take place on both the WE and RE. The kinetics of one of these reactions will prevail on each electrode, controlling the electrode, and the difference in voltage between both electrodes will provide the final device response. Ideally, the oxidation of the reducing agent will take place in the WE when achieving the equilibrium (Equation (1) or (2)), while O 2 is reduced Figure 2. FESEM images of the working electrode cross-section for the bare sensor with 8YSZ in yellow and Fe 0.7 Cr 1.3 O 3 in blue ( a ) and several infiltrations: ( b ) Ni, ( c ) Ti, ( d ) Al, ( e ) Nb, ( f ), Ba and ( g ) Pd. Nanoparticles for each element can be observed. Additionally, different sizes of nanoparticles can be observed when nickel is added to the first element infiltrated. Fe 0.7 Cr 1.3 O 3 and 8YSZ grains are observed to be homogeneously distributed. Chemosensors 2021,9, 274 5 of 14 A deeper look at the working electrode shows a good distribution of the nanoparticles all over the electrode for all the performed infiltrations (Figure 2). Oxide nanoparticles are equally attached to both Fe 0.7 Cr 1.3 O 3 and 8YSZ grains, and therefore, the surface active area for electrochemical sensing is enlarged. Two nanoparticle size ranges can be observed for each infiltration. Nanoparticles formed upon infiltration of Ti, Nb, Al, Ba, and Pd are smaller than Ni-based nanoparticles. Energy-dispersive X-ray spectroscopy (EDX) analysis and subsequent comparison with a device infiltrated with only nickel confirm that the largest nanoparticles are made of nickel [ 41 ]. Thus, the smaller nanoparticles must be the second element infiltrated into the electrode (EDX was limited and could not identify the main element of these small nanoparticles due to the limits of this technique). The nanoparticles are well distributed and constant along the electrode, except at a lower extent for Ba and Pd. Moreover, no presence of nanoparticles is observed in the reference electrode, confirming that the infiltration was selectively carried out in the WE. Thus, any improvement in sensor performance in comparison to the bare sensor can be attributed to the infiltration in the working electrode. The poor distribution of Ba and Pd may cause low activity of these electrodes to C2H4when compared to other elements such as Ti or Al. 4.2. Electrochemical Characterization In an exhaust-gas-like atmosphere, where several pollutants such as hydrocarbons, CO, NO x , O 2 , etc., can be present, several oxidation and reduction reactions can take place on both the WE and RE. The kinetics of one of these reactions will prevail on each electrode, controlling the electrode, and the difference in voltage between both electrodes will provide the final device response. Ideally, the oxidation of the reducing agent will take place in the WE when achieving the equilibrium (Equation (1) or (2)), while O 2 is reduced in the RE (Equation (3)). The electrode must be porous to facilitate the diffusion of the gaseous analyte to the contact points with both electronic and ionic conductors (TPB). Both anodic and cathodic reactions are coupled by the oxygen ion diffusion through the 8YSZ electrolyte. This kind of sensor follows the so-called theory of mixed potential, and therefore, the response of the device is kinetically controlled [ 13 , 35 , 58 – 60 ]. A zero current is imposed, and a mixed potential is established in each electrode (Equation (1) or (2) for the anodic reaction and Equation (3) for the cathodic reaction) when the steady state is reached. The final voltage of the cell is given by this built-up mixed-potential difference. Additionally, a heterogeneous catalytic conversion process could take place at the electrodes. The analytes could react with locally adsorbed O 2 (Equations (4)–(6)), the electrochemical reaction not being favored [ 14 , 61 – 63 ]. The reaction network on the electrodes can be described by the following reactions. CO +O2−→CO2+2e−(1) C2H4+6O2−→2CO2+2H2O+12e−(2) 1 2O2+2e−→O2−(3) CO +1 2O2→CO2(4) C2H4+3O2→2CO +2H2O (5) C2H4+1 2O2→CH3COH (6) 4.2.1. Potentiometric Characterization As previously reported, the (catalytically nonactivated) bare sensor response is not specific to C 2 H 4 [ 41 ]. The device is exposed to concentration pulses of both pure CO and C 2 H 4 from 50 to 200 ppm for 20 min. Additionally, the device response is measured for the same concentration pulses of one analyte but with a fixed concentration of 200 ppm of the other, as shown in Figure 3. This counter plot helps to summarize the sensor response to Chemosensors 2021,9, 274 6 of 14 both C 2 H 4 and CO. In these plots, the xand y-directions are CO and C 2 H 4 concentrations, respectively. The colormap indicates the voltage response offered by the sensor when exposed to a given concentration of analytes. Thus, a device selective to C2H4must show an increase within the y-direction, with its response in the x-direction remaining constant. Figures 3and 4display how the response to both C 2 H 4 and CO is similar, and therefore, the C 2 H 4 electrochemical reaction is not favored. As observed in Figure 3, the lines of constant potential are diagonal, increasing from left to right. This confirms the lack of selectivity to C 2 H 4 . Figure 4shows the transient response for different scenarios with C 2 H 4 and CO, confirming similar responses. This indicates a high cross-sensitivity toward CO, and therefore, the device is unable to measure C2H4in an exhaust-gas-like atmosphere. Chemosensors 2021, 9, x FOR PEER REVIEW 7 of 15 Figure 3. Sensor response (Vcell) to analyte concentration prior to any infiltration. The device could not detect C2H4 due to cross-sensitivity toward CO. Standard error ± 0.02 mV. Figure 4. Bare sensor transient response prior to any infiltration to improve catalytic activity: (a) pure C2H4, (b) C2H4 with 200 ppm of CO as background, (c) pure CO, and (d) CO with 200 ppm of C2H4 as background. Therefore, the working electrode should be catalytically activated to promote the electrochemical reaction of C2H4. Several catalyst nanoparticles are infiltrated into each channel to achieve this promotion of the C2H4 reaction rate. As several channels are measured, the response is normalized to enable comparison. After the infiltration of nanoparticles in dry conditions, it was found that Ti and Al led to an increase in the electrochemical reaction of C2H4 (Figure 5). Thus, this reaction is kinetically favored. It should be noted that the response to C2H4 is not affected by the addition of 200 ppm of CO. Thus, the device Figure 3. Sensor response (V cell ) to analyte concentration prior to any infiltration. The device could not detect C2H4due to cross-sensitivity toward CO. Standard error ±0.02 mV. Chemosensors 2021, 9, x FOR PEER REVIEW 7 of 15 Figure 3. Sensor response (Vcell) to analyte concentration prior to any infiltration. The device could not detect C2H4 due to cross-sensitivity toward CO. Standard error ± 0.02 mV. Figure 4. Bare sensor transient response prior to any infiltration to improve catalytic activity: (a) pure C2H4, (b) C2H4 with 200 ppm of CO as background, (c) pure CO, and (d) CO with 200 ppm of C2H4 as background. Therefore, the working electrode should be catalytically activated to promote the electrochemical reaction of C2H4. Several catalyst nanoparticles are infiltrated into each channel to achieve this promotion of the C2H4 reaction rate. As several channels are measured, the response is normalized to enable comparison. After the infiltration of nanoparticles in dry conditions, it was found that Ti and Al led to an increase in the electrochemical reaction of C2H4 (Figure 5). Thus, this reaction is kinetically favored. It should be noted that the response to C2H4 is not affected by the addition of 200 ppm of CO. Thus, the device Figure 4. Bare sensor transient response prior to any infiltration to improve catalytic activity: (a) pure C 2 H 4 , ( b ) C 2 H 4 with 200 ppm of CO as background, ( c ) pure CO, and ( d ) CO with 200 ppm of C 2 H 4 as background. Chemosensors 2021,9, 274 7 of 14 Therefore, the working electrode should be catalytically activated to promote the electrochemical reaction of C 2 H 4 . Several catalyst nanoparticles are infiltrated into each channel to achieve this promotion of the C 2 H 4 reaction rate. As several channels are measured, the response is normalized to enable comparison. After the infiltration of nanoparticles in dry conditions, it was found that Ti and Al led to an increase in the electrochemical reaction of C 2 H 4 (Figure 5). Thus, this reaction is kinetically favored. It should be noted that the response to C 2 H 4 is not affected by the addition of 200 ppm of CO. Thus, the device is able to detect C 2 H 4 even in the presence of CO. On the other hand, Nb, Ba, and Pd infiltration provide a poorer response in dry conditions. Despite improving the response in comparison to the bare sensor, the cross-sensitivity toward CO is too high, albeit for C2H4detection purposes (Figure 5). Chemosensors 2021, 9, x FOR PEER REVIEW 8 of 15 is able to detect C 2 H 4 even in the presence of CO. On the other hand, Nb, Ba, and Pd infiltration provide a poorer response in dry conditions. Despite improving the response in comparison to the bare sensor, the cross-sensitivity toward CO is too high, albeit for C 2 H 4 detection purposes (Figure 5). Figure 5. Device performance as a function of C 2 H 4 and CO concentration after infiltration of the WE. The first column shows the sensor response for the first element infiltrated in dry conditions, the second column depicts the sensor response after an additional infiltration with Ni in dry conditions, and the last column shows the response after the second infiltration with Ni in wet conditions. Each row indicates the first element infiltrated. Standard errors of sensor response for each infiltrated element are: ±0.02 mV for Ti, ±0.03 mV for Al, ±0.07 mV for Nb, ±0.07 mV for Nb, ±0.003 mV for Ba, and 0.01 mV for Pd. Figure 5. Device performance as a function of C 2 H 4 and CO concentration after infiltration of the WE. The first column shows the sensor response for the first element infiltrated in dry conditions, the second column depicts the sensor response after an additional infiltration with Ni in dry conditions, and the last column shows the response after the second infiltration with Ni in wet conditions. Each row indicates the first element infiltrated. Standard errors of sensor response for each infiltrated element are: ± 0.02 mV for Ti, ± 0.03 mV for Al, ± 0.07 mV for Nb, ± 0.07 mV for Nb, ± 0.003 mV for Ba, and 0.01 mV for Pd. Chemosensors 2021,9, 274 8 of 14 The subsequent infiltration with Ni enhanced the sensor detection of C 2 H 4 even more for Ti and Al devices (Figure 5). Additionally, this second infiltration improved the performance of the Nb-infiltrated device: the C 2 H 4 electrochemical reaction is promoted, providing a response dependent on C 2 H 4 but not on CO (Figure 5). Unfortunately, crosssensitivity toward CO of Ba and Pd devices is not improved adequately for sensing purposes (Figure 5), as previously attributed to the poor dispersion of Ba and Pd in the electrode (Figure 2). Ti and Al devices are still selective to C 2 H 4 even when the device is exposed to a wet gas stream (3% vol. H 2 O). As previously reported, H 2 O usually negatively affects the sensor response, i.e., increasing the cross-sensitivity toward CO [ 32 , 64 – 66 ]. However, the catalytically activated sensor is still selective to C 2 H 4 under humid atmospheres. The low cross-sensitivity toward CO and H 2 O makes this configuration promising for hydrocarbon detection in atmospheres with several pollutants. The improvement in sensing capability might be due to the improved electrocatalytic properties of the material itself. Nonetheless, in the case of Nb, the addition of water affects the device performance, and thus, the device is no longer selective to C2H4. 4.2.2. Electrochemical Impedance Spectroscopy Analysis Electrochemical impedance spectroscopy was performed for each one the five infiltrated elements, exposing the device to 200 ppm of pure CO and C 2 H 4 . The EIS measurement procedure is as follows: first the bare sensor, then with the addition of the first element, and finally for the first element plus Ni in dry and wet conditions. Figure 6displays Nyquist plots from 0.03 Hz to 1 MHz. The sensors infiltrated with Ti, Al, Ba, and Pd present two-arc contributions. The respective Bode plots are depicted in Figure S2. The equivalent circuit proposed consists of two parallel combinations of resistance-constant phase elements (R-CPE) connected in series. However, the Nbinfiltrated sensor has a three-arc contribution that was fitted to an equivalent circuit consisting of three parallel combinations of R-CPE. This study is focused on Ti, Al, and Nb, as they exhibited the best sensing properties. The arc shape at high frequencies in the Nyquist plot is similar in all cases, and this indicates the similar behavior of oxide ion transport through crystalline grain, as expected from the identical electrolyte and electrode backbone structure. In general terms, the first arc contribution (appearing at higher frequencies and with C = 10 5 –10 6 F as in Figure 6and Figure S3) at 550 ◦ C may be due to the electrode–8YSZ electrolyte interface [ 67 – 69 ]. When the device is exposed to C 2 H 4 and CO, this resistance remains almost constant for the bare sensors, as well as when the device is infiltrated, firstly with Ti, Al, and Nb and secondly with Ni (see Figure 6). Accordingly, the ionic mobility remains almost unaffected, i.e., ion diffusion in the bulk is not affected by the addition of the nanocatalysts. The second arc contribution appearing at lower frequencies (10 3 –10 4 F, as depicted in Figure 6) is related to electrocatalytic processes taking place at the surface of the electrode and depending on the type of infiltration performed. The effect of this lower-frequency contribution can also be observed in the Bode plots in the range from 0.03 to 145 Hz (Figure S2). In the case of the bare sensor, the polarization resistance is almost the same for both C 2 H 4 and CO, as shown for example in Figure 6. This agrees with the potentiometric characterization (Figure 5) and can explain the lack of selectivity to C2H4. Chemosensors 2021,9, 274 9 of 14 Chemosensors 2021, 9, x FOR PEER REVIEW 10 of 15 Figure 6. EIS results for the devices infiltrated with Ti, Al, and Nb for the bare sensor (dots), first infiltration (triangle), additional infiltration with Ni in dry conditions (inverse triangle), and additional infiltration with nickel in wet conditions (hexagon). C 2 H 4 response is depicted in red on the left side, and CO response is shown in green on the right side. Inset indicates the equivalent circuit fitting the response: two resistance-constant phase-parallel elements for Ti and Al, while for Nb, there is an additional resistance-constant phase-parallel element. In general terms, the first arc contribution (appearing at higher frequencies and with C = 10 5 –10 6 F as in Figures 6 and S3) at 550 °C may be due to the electrode–8YSZ electrolyte interface [67–69]. When the device is exposed to C 2 H 4 and CO, this resistance remains almost constant for the bare sensors, as well as when the device is infiltrated, firstly with Ti, Al, and Nb and secondly with Ni (see Figure 6). Accordingly, the ionic mobility remains almost unaffected, i.e., ion diffusion in the bulk is not affected by the addition of the nanocatalysts. The second arc contribution appearing at lower frequencies (10 3 –10 4 F, as depicted in Figure 6) is related to electrocatalytic processes taking place at the surface of the electrode and depending on the type of infiltration performed. The effect of this lower-frequency contribution can also be observed in the Bode plots in the range from 0.03 to 145 Hz (Figure S2). In the case of the bare sensor, the polarization resistance is almost the same for both C 2 H 4 and CO, as shown for example in Figure 6. This agrees with the potentiometric characterization (Figure 5) and can explain the lack of selectivity to C 2 H 4 . For both Al and Ti, their infiltration led to an increase in the CO contribution, while the C 2 H 4 contribution decreased (as observed in Figures 6 and 7). This agrees with the potentiometric study and can explain the better performance. Electrochemical reaction to C 2 H 4 is catalytically promoted. The second infiltration with nickel decreases both contributions, although the decrease for C 2 H 4 is higher. Therefore, the C 2 H 4 resistance is lesser than CO, as observed in Figure 7, and this explains again the better performance of the device for both configurations in dry conditions. This is also backed by the evolution of the low-frequency contribution observed in the Bode plots in Figure S2. Figure 6. EIS results for the devices infiltrated with Ti, Al, and Nb for the bare sensor (dots), first infiltration (triangle), additional infiltration with Ni in dry conditions (inverse triangle), and additional infiltration with nickel in wet conditions (hexagon). C 2 H 4 response is depicted in red on the left side, and CO response is shown in green on the right side. Inset indicates the equivalent circuit fitting the response: two resistance-constant phase-parallel elements for Ti and Al, while for Nb, there is an additional resistance-constant phase-parallel element. For both Al and Ti, their infiltration led to an increase in the CO contribution, while the C 2 H 4 contribution decreased (as observed in Figures 6and 7). This agrees with the potentiometric study and can explain the better performance. Electrochemical reaction to C 2 H 4 is catalytically promoted. The second infiltration with nickel decreases both contributions, although the decrease for C 2 H 4 is higher. Therefore, the C 2 H 4 resistance is lesser than CO, as observed in Figure 7, and this explains again the better performance of the device for both configurations in dry conditions. This is also backed by the evolution of the low-frequency contribution observed in the Bode plots in Figure S2. As aforementioned, H 2 O can have a negative influence on the sensor performance affecting the output voltage and cross-sensitivity. Therefore, it is sensible to the effect of H 2 O on the sensor response. The addition of 3% H 2 O to the gas flow causes a further decrease in both contributions. However, the C 2 H 4 contribution is smaller than that of CO. This agrees with the performed potentiometric study, and it confirms that the C 2 H 4 reaction is more favored. Thus, according to the potentiometric study and the electrochemical impedance, both Ti and Al plus Ni are strong candidates for C 2 H 4 detection, as they promote the electrochemical reaction of C2H4and even partial heterogeneous catalysis of CO.