Ultra-Sensitive Electrochemical Methane Sensors based on Solid

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Energy, Environmental, and Catalysis Applications

Ultra-Sensitive Electrochemical Methane Sensors based on Solid Polymer Electrolyte-Infused Laser-Induced Graphene Manan Dosi, Irene Lau, Yichen Zhuang, David S. A. Simakov, Michael W. Fowler, and Michael A Pope ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22310 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Ultra-Sensitive Electrochemical Methane Sensors based on Solid Polymer Electrolyte-Infused LaserInduced Graphene Manan Dosi, †, Irene Lau, †‡ Yichen Zhuang, † David S. A. Simakov, † Michael W. Fowler, †

Michael A. Pope†‡*

†Department

of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada.

‡Waterloo

KEYWORDS:

Institute of Nanotechnology, University of Waterloo, ON, Canada

electrochemical sensor, room temperature ionic liquid, solid polymer

electrolyte, laser-induced graphene, methane detection, greenhouse gas mitigation.

Abstract: Methane is a potent greenhouse gas with large emissions occurring across gas distribution networks and mining/extraction infrastructure. The development of inexpensive, lowpower electrochemical sensors could provide a cost-effective means to carry out distributed sensing to identify leaks for rapid mitigation. In this work, we demonstrate a simple and costeffective strategy to rapidly prototype ultra-sensitive electrochemical gas sensors. A room temperature methane sensor is evaluated which demonstrates the highest reported sensitivity (0.55 A/ppm/cm2) with a rapid response time (40 s) enabling sub-ppm detection. Porous, laserinduced graphene (LIG) electrodes are patterned directly into commercial polymer films and Dosi et al.

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imbibed with a palladium nanoparticle dispersion to distribute the electrocatalyst within the high surface area support. A pseudo-solid state ionic liquid/polyvinylidene fluoride electrolyte was painted onto the flexible cell yielding a porous electrolyte, within the porous LIG electrode, simultaneously facilitating rapid gas transport and enabling the room temperature electrooxidation pathway for methane. The performance of the amperometric sensor is evaluated as a function of methane concentration, relative humidity and tested against interfering gases. Introduction Methane is the primary component of natural gas. As a result of mining and defects in the distribution network, methane emissions occur throughout the oil and gas industry. This corresponds to the largest anthropogenic source of the potent greenhouse gas (25 times more potent than CO2) and leads to a significant loss in revenue for the oil and gas industries: 60% of methane loss is due to fugitive release, out of which a major percentage is due to transmission lines (pipelines).1 Due to the expansive nature of the distribution network, these leaks are currently challenging to detect using the currently available optical, semiconductor or combustion based sensors. Such sensors are expensive and not viable for detecting minor leaks.2 Apart from natural gas pipelines, methane sensors also find applications in agriculture, waste management, landfill monitoring, lab, hospitals and also at home for maintaining health and safety. The widespread use and production of methane also increases its magnitude of being a hazard and creating safety issues. Methane sensors become vital in these situations. The development of inexpensive and low-power electrochemical sensors could provide a costeffective means to carry out distributed sensing over the entire network. While high temperature solid-state electrochemical methane sensors have been demonstrated in the past using Yttria stabilized zirconia as a solid-state electrolyte,3 the requirement of high temperature (> 500°C) makes them impractical for use. More recently, room temperature ionic liquids (RTILs) have gained traction for use in gas sensor design due to their ease of processing and, more importantly, their effective non-volatility;4,5 Furthermore, the structured ionic phase can solvate or complex with a variety of solutes including methane – opening up a room temperature oxidation pathway for methane – as discussed by Wang et al.6,7 While promising results have been demonstrated using RTILs, previous designs have run into challenges with slow diffusion of gases through the fully dense liquid films formed8 or have been carried out by sparging the gases through a bulk ionic

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liquid.5 To overcome this problem, to some extent, Mason’s group has designed a cell where gold electrodes are patterned by photolithography onto a porous polytetrafluoroethylene (PTFE) substrate.9 In this case, the methane could penetrate through the membrane and to the RTIL/gold interface instead of diffusing through the thin RTIL film covering the electrodes. Their sensor achieved a sensitivity of 0.31 A/% methane (31 pA/ppm) with a 0.28% (2,800 ppm) detection limit for a ~1 mm diameter working electrode. This miniaturized system demonstrated improve performance over a previously reported organic solvent/Pt-black based system with a sensitivity of 0.5 A/% methane (50 pA/ppm) with a detection limit of 0.5% (5,000 ppm). While promising, the detection limits of these electrochemical sensors is far from the sub-ppm values desired by industry and achievable by more advanced but expensive spectroscopic methods.10 One way to improve the sensitivity and detection limits of electrochemical methane sensors is to create a higher surface area electrode/electrolyte interface which is accessible by the gas phase analyte. It has been shown possible to create high surface area electrodes by techniques such as screen or ink-jet printing of high surface area carbonaceous materials like graphene11 or carbon nanotubes.12 More recently, a simpler approach to create patterned high surface area carbonaceous arrays, compatible with rapid prototyping, has been demonstrated by Tour’s group. Using a CO2 laser cutting/etching system it is possible to locally carbonize polyimide (specifically KaptonTM) films into a material which they referred to as laser-induced graphene.13 These laser systems are inexpensive and used industrially for various laser etching and cutting applications. Furthermore, each pass of the laser is capable of rapidly carbonizing ~100 m width lines and is thus amenable to medium throughput manufacturing – creating a ~cm2 device within seconds. Over just the last few years, this technique has been used to create a variety of devices including flexible supercapacitors,14 water-splitting cells,15,16,17 biosensors,18,19piezo-resistive sensors,20 and robust superhydrophobic surfaces and membranes.21 While electrochemical sensors for liquid-phase analytes have been demonstrated,18 gas sensors pose a challenge because liquid electrolytes typically fill the pore space and inhibit gas diffusion to

the

electrode

(and

catalyst,

if

one

is

required).

Since

RTILs

with

bis(trifluoromethylsulfonyl)imide (TFSI) anions have been shown to facilitate the room temperature electro-oxidation reaction at Pt electrodes,18 we strived to develop a porous and pseudo-solid state system from such RTILs. Kubersky et al., demonstrated that a porous, solid polymer-electrolyte (SPE) could be formed by casting a solution of polyvinylidene fluoride (PVDF) Dosi et al.


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with 1-ethyl, 3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) in Nmethylpyrrolidone (NMP) onto planar, patterned Pt electrode substrates. The morphology of the SPE layer could be tuned from micron-sized fractal aggregates of SPE to larger, connected spheroidal structures caused by differences in the nucleation and crystallization rates during drying.22 Aerosol-jet printing of a CNT film as a working electrode (WE) on top of the porous SPE layer resulted in an NO2 sensor with high sensitivity (0.66 A/ppm) and a low detection limit for NO2 gas. Building off of these advances in materials development and sensor design, herein, we demonstrate an electrochemical methane sensor based on LIG electrodes decorated with a palladium (Pd) nanoparticle catalyst with the meso/microporous network formed by the LIG imbibed with the SPE based on the PVDF/EMImTFSI system. As illustrated in Figure 1, the porous electrolyte network that forms within the porous electrode structure leads to fast response times of less than a minute and enables a high contact area between electrode/electrolyte/methane. As we show below, the flexible, planar device achieves a sensitivity of 1.1 A/ppm of methane for a small area device (~2 cm2). This is a four order of magnitude improvement over previous approaches and enables a detection limit of ~ 9 ppm. The cell operates at only 0.6 V which minimizes power output and improves selectivity by avoiding side reactions that might occur at higher voltages. We found that the sensor response is affected by relative humidity and similar gases like ethane and propane but with a lower sensitivity. This is likely due to water being a reaction product, along with CO2, as it was verified by off-gas analysis, confirming the RTILmediated oxidation pathway proposed by Wang et al.23 The fabricated sensor is inexpensive and amenable to medium throughput manufacturing with a sensitivity and selectivity that might allow the technology to compete with laser-based spectroscopic systems and making distributed pipeline/infrastructure monitoring an economically feasible option.

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Figure 1: Schematic of the fabrication process of the electrochemical methane sensor. A CO2 laser is used to convert the polyimide sheet into a patterned, interdigitated laser-induced graphene design. Pd nanoparticles are imbibed into and decorate the high surface area LIG structure after which a porous PVDF/RTIL layer is painted onto the electrodes. This provides a high number of three-phase contacts between gas, Pd and RTIL facilitating high sensitivity and rapid response.

Results and discussion: Device Fabrication and Characterization As shown in Figure 2(a), Raman spectroscopy was used to confirm the graphene-like structure of the material. The D, G and 2D peaks characteristic of graphitic carbon materials are observed at 1349 cm-1, 1583 cm-1 and 2699 cm-1, respectively and are consistent with the results of similar LIG work in published literature.13,14,24,25 The full-width-at-half-maximum (FWHM) of the 2D peak (56.45 cm-1) and the intensity ratio of 2D to G peaks (0.51), suggest that the porous material is composed of multi-layer graphene. The D peak is associated with aromatic domains but for lateral crystallite sizes (La ) larger than ~ 2 nm, the vibrations are dampened and thus the presence of a D peak and its relative intensity is often taken as a measure of the defect density.26 Based on this assumption, an ID/IG = 0.61 leads to an estimate of La ~22 nm by the Tuinstra-Koenig empirical relation.27 These observations all suggest that the LIG is composed of a polycrystalline multilayer graphene. Figure 2(b) shows a schematic of the dimensions of the laser-scribed device and indicates where the electrolyte is applied. The conductivity of the traces was found to be to be ~24 S/cm by two-point probe which is similar to what has been reported by others (20 – 26 S/cm).13 A cross-sectional SEM image of two adjacent electrodes shows that the electrode structure is not completely embedded within the polyimide, but the porous LIG rises about 100 m above the

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plane of the unexposed polyimide film. This is a result of the rapid decomposition of the polyimide into gaseous products upon laser exposure which causes the material to expand while carbonizing.

Figure 2: Characterization of LIG electrodes: (a) Raman spectra of LIG electrode material, inset: photograph of electrodes prior to applying the electrolyte; (b) Sketch of the interdigitated electrodes with dimensions; (c) SEM image of the cross-section of the LIG electrodes.

Pd nanoparticles (Pd NPs) were chosen as the electrocatalyst for the methane oxidation reaction. As shown in Figure 3(a), dynamic light scattering (DLS) was used to measure the approximately hydrodynamic diameter of the Pd NPs to be approximately 50 nm prior to imbibing the Pd solution into the porous LIG electrodes. As shown in Figure 3(b), after imbibing the dispersion, corresponding to 25 mM of palladium nitrate, into the electrode by a solvent exchange approach, the Pd NPs were found by SEM (Figure 3(c)) to uniformly distribute within the porous network. However, some aggregates are clearly visible as bright contrast in the backscattered electron (BSE) image, and likely an artifact of the drying procedure. Many particles are sub-micron and likely composed of single Pd particles supported by the carbon of the LIG. Energy-dispersive X-ray spectroscopy (EDS) was used to confirm that these particles were indeed Pd and EDS

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mapping over the width of a finger was used to demonstrate the uniform distribution over larger length scales as shown in Figure 3(c). The mapping results indicated that the electrodes are composed of ~2 at% of Pd. The complete mapping results that includes the EDS elemental spectra along with the weight and atomic composition of the individual elements has been shown in Figure S2. Based on the total amount of Pd dispersion imbibed into the fingers, a loading of about 0.11 mg/cm2 is estimated. Based on the current market price for Pd (~$30/g), this corresponds to less than 0.01 cents/sensor and should thus constitute a negligible cost to the device. Tour’s group has recently demonstrated a method to incorporate Pt nanoparticles into LIG by preparing polyimide films polymerized in the presence of Pt nanoparticles.15 While their approach yields a better dispersion, much of the Pt loaded into the polymer is wasted as only a small fraction is carbonized to form the Pt-decorated LIG. Our simple solvent-exchange approach enables the utilization of much less catalyst material and also allows for commercial polymers films to continue to be used as a substrate. As illustrated in Supporting Information, Figure S1, higher loadings of Pd NPs within the electrodes did not significantly improve the electrochemical results and thus the 25 mM concentration was taken as optimal.

Figure 3: Characterization of Pd nanoparticles before and after loading into LIG structure. (a) Particle size distribution measured by DLS of freshly made Pd nanoparticle dispersion. (b) Backscattered electron contrast SEM image illustrating the distribution of Pd nanoparticles on/within the porous LIG electrodes. Pd nanoparticles are brighter due to their higher molecular weight and tendency to backscatter electrons. (c) Elemental mapping of Pd over the width of a finger indicating Pd is distributed uniformly on the macroscale (inset shows secondary electron contrast image of the mapping region).

After depositing the Pd nanoparticles, the next step of the design was to develop an electrolyte system that is sufficiently ionically conducting at room temperature, non-volatile and permeable Dosi et al.


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to methane – while enabling the room temperature oxidation pathway. In previous work, a PVDF/IL system has been used to create high performance sensors for NO2 detection by casting solutions onto monolithic electrodes.22 Instead, we hypothesized that this system may also work to create a porous electrolyte network inside of a porous carbon material like LIG. This is akin to how Nafion is used in polymer electrolyte membrane-based fuel cells to allow the diffusion of oxygen to and water from the Pt catalyst. The surface area of the electrode material accessible to nitrogen was found to be ~150 m2/g. As illustrated in the Supporting Information, Figure S4, nitrogen adsorption exhibited a Type II isotherm typical of multilayer adsorption and pore filling in micro and mesoporous materials. The pore-size distribution estimated from this data indicates that the majority of the pores are micropores which are ~1-2 nm wide with a significant fraction of mesopores ~3 nm in width with a tail extending to larger pore sizes. The SEM images shown in Figure 4(a) also reveal larger macropores on the order of 10 m in size which is beyond the range in which gas adsorption measurements can measure (typically limited to < 50 nm). This porosity likely evolves due to the rapid evolution of decomposition gases upon photothermal heating of the Kapton by the laser. This expansion due to gas evolution is also evident in the cross-sectional SEM image in Figure 2(c) where the carbonized material is found to expand several tens of micrometers beyond the surface plane of the uncarbonized Kapton. From the SEM images, line patterns within each electrode trace can be observed which correspond to the scanning of the laser over the electrode area in multiple passes as it was operated in scan mode with each raster of the laser separated by ~100 m. A second scan over the entire pattern was also carried out to ensure uniformity of the electrode lines and also contributes to a slight annealing affect which slightly increases the conductivity of the lines. Figure 4(a) and (b) compares the top view of the LIG electrodes with and without the added SPE. In Fig 4(b) the electrode lines are uniformly covered with the SPE but the raised sections caused by the underlying LIG can clearly be seen in the inset of Fig. 4(b). The SPE is found to form porous, globular-like structure on top of the entire film. The effective pore size, as determined from these images is as large as 5 to 20 m. The porous network allows methane to pass through the SPE more rapidly than a liquid film. Figure 4(c) and (d) compares the cross-sectional view of electrode before and after application of the SPE, respectively. From these images, it is clear that the SPE is imbibed into and penetrates through the porous LIG material providing ionic contact to Dosi et al.


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the Pd dispersed over the high surface area LIG network. Figure 4(e) shows a magnified view of the cross-sectional where an intimate network of SPE/LIG can be seen.

Figure 4: Morphology of the porous SPE layer coated on and infiltrated within the LIG electrodes. (a) SEM image of the top view of the LIG electrodes showcasing the porous surface, inset: large area overview of the electrodes; (b) SEM image of top view of the SPE covered LIG electrodes showcasing the porous nature of SPE, inset: large area overview of the SPE-covered LIG electrodes; (c) Cross-sectional view of the LIG electrode; (d) Cross-sectional view of the SPE covered LIG electrode; (e) Magnified view of the SPE covered LIG electrode cross-section.

Electrochemical Testing In order to determine the operating voltage of the electrochemical cell, as shown in Figure 5(a), cyclic voltammetry was performed with and without 50 ppm methane between a cell voltage of 0 and 1 V. In the absence of methane, the CVs are rectangular and indicate capacitive charging only. In the presence of methane, a clear oxidation peak between 0.8 and 0.9 V becomes apparent on the forward scan. Figure 5(b) shows that the peak position does not change significantly between 20 and 100 mV/s scan rate. The onset of the reaction Dosi et al.


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corresponding to this oxidation peak, which is a result of diffusion limitations, would proceed at a lower potential than the peak potential. These results motivated us to study the current response of the sensors under applied voltages between 0.4 and 0.7 V. As shown in Figure 5(c), at low potentials (< 0.5 V) only a spike in current which exponentially decays to zero is observed which is indicative of capacitive charging and results from the high surface area of the LIG electrodes in contact with the SPE. At 0.6 V, the current no longer decays to zero but achieves a stable current of more than 0.1 mA for over 15 min. However, after about 17 min, the current response decays to zero. When the potential is further increased, a slightly higher current is observed and remains constant for about 7 min with the current decaying more rapidly over the next few minutes. To further understand what is happening in the system, we carried out testing of one sensor under different gas environments as shown in Figure 5(d). The sensor is found to be unresponsive to methane if high purity nitrogen is used as the carrier gas and only responds in the presence of oxygen suggesting that oxygen is required in the sensing mechanism. Furthermore, as shown in Figure 5(e), the SPE changes from white to slightly translucent after the sensor response decays. The electrolyte initially appears white due the presence of the porous micron-sized domains of the PVDF/RTIL (surrounded by gas) which scatters light. The translucency suggests that these domains become surrounded by liquid which causes a lower refractive index mismatch and less scattering of light. Off-gas analysis by inline Fourier-transform infrared spectroscopy (FTIR), shown in Figure 5(f), demonstrates that water and carbon dioxide are the main reaction products and provides further support for the hypothesis that it is liquid water that condenses within the porous SPE when the water generation rate is high (i.e., the sensor current). These observations suggest that our sensor carries out the electro-oxidation of methane by the following reaction: 𝐶𝐻4 + 𝑂2

𝑃𝑑/𝑇𝐹𝑆𝐼

𝐶𝑂2 +2𝐻2𝑂,

(1)

While we cannot comment specifically on the half-cell reactions occurring, our results provide support for the mechanism proposed by Wang & Zeng, 29 who were the first to demonstrate that the TFSI anion forms a catalytic complex with platinum-group metals which is able to oxidize methane in the presence of O2 to CO2 and water at the working electrode. They later demonstrated that superoxide is likely responsible for converting any carbon monoxide (CO) produced by incomplete oxidation into CO2.6 Due to the possibility of the reaction also producing CO, we also monitored for CO in the off-gas but could not observe any response beyond the noise level of the FTIR (10-100 ppb is reported by the manufacturer). This lack of detection suggests that it is only Dosi et al.


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water flooding that is responsible for rapid current drop after prolonged constant current operation. Liquid water likely blocks methane and oxygen transport to the three-phase boundary (Pd/LIG/RTIL). The RTIL used is a so-called hydrophobic one and, as such, is not water miscible nor does it have much solubility for water. Thus the PVDF/IL domains should not be significantly affected by the presence of water, only the pore space surrounding these domains will become blocked.

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Figure 5: (a) Cyclic voltammetry performed with and without 50 ppm methane. (b) Cyclic voltammetry with 50 ppm methane at different scan rates. (c) Current response of sensor at constant methane concentration (50 ppm) at various applied potentials. (d) Current measured at 0.6 V in the presence of various gases. (e) Photographs of the same sensor before and after testing at constant potential. The sensor was operated until the current dropped as shown around 12 min in (c). (f) Concentration of methane, water, carbon dioxide and carbon monoxide over time as measured by inline FTIR. At time zero, the sensor was already under an applied potential and the gas controller was switched from 20 to 30 ppm.

To demonstrate that these changes are indeed reversible, we tested the system after drying the water saturated sensors for various times as shown in Figure 6(a). The current resulting from 5 ppm methane was stable for ~18 min after which the current drops rapidly. Once the current output decayed to 5 A the sensor was allowed to rest under ambient conditions for ~16 min after which the sensor was retested and found to yield 99.7% of the initial output current. A similar trend was observed for the 25 and 50 ppm cases, but the drying time required for higher concentrations increased to 20 and 23 min respectively to reach approximately 99% of the initial current response. Figure 6(b) shows an example of the current recovery observed upon cycling a sensor between 25ppm methane and a dry air environment. After about 18min of sensor operation, the current drops. After the sensor is allowed to dry for about 18 min the current upon exposure to methane again regains 98% of its original performance. Increasing the drying time to 20 min, leads to effectively 100% current recovery. The increase in drying time can be attributed to a higher rate of reaction with increasing concentration of methane leading to increase in product formation. The saturation time likely corresponds to the timescale associated with the competition between the generation of water vapor, the diffusion of water vapor out of the porous matrix resulting in accumulation and eventual condensation as the partial pressure approaches the level of super-saturation required for heterogeneous nucleation of liquid water droplets on some inner surface of the porous network.

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Figure 6 Electrochemical performance of methane sensors operated at 0.6 V: (a) Current recovery as a function of drying time for sensors exposed to different concentrations of methane to saturation; (b) Effect of saturation and drying time on the performance of the sensor(c) Performance of multiple sensor devices under similar conditions; (d) Performance of sensor over extended period of time.

Another important aspect of sensor performance is its ability to provide consistent results between devices and also to perform consistently over an extended period of time. In order to test these aspects of the sensor, six similar devices (18 electrode lines) produced over the span of several months were tested as a function of methane concentration. As can be seen in Figure 6(c), the variation amongst the six devices at different concentrations is low. The average of the standard deviations at each methane concentration was found to be 0.0026 mA which corresponds to a relative uncertainty of 4% when the lowest concentration (10 ppm) corresponding to the lowest current (~0.065mA) is considered. Figure 6(d) demonstrates the performance of the sensor over extended cycling. The sensor is tested for a consecutive period of 30 days for the response to 25 ppm methane. The current response varies by only about 1 A over this time period or by only about 1.2% of the average current recorded over that period.

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Figure 7 Electrochemical performance of methane sensors operated at 0.6 V: (a) Current response of the sensor (14 electrode lines) to varying methane concentrations measured by FTIR; (b) Effect of interfering gases on current response of the sensor.

Based on the information obtained from the drying time experiments with 50 ppm methane, a stable current could be observed for about 16 – 18 minutes. Thus to avoid water flooding effects, this time range was used to expose the same cell held at 0.6 V to different methane concentrations in steps of 10 ppm up to 50 ppm to assess the linearity and response time (Figure 7(a)). The sensor current reached its steady state value after only 30 s when the potential was switched from open circuit to 0.6 V after exposure to 10 ppm of methane. Subsequent steps took between 30s and 60s to move from one steady state current to the next. However, this is likely influenced by the time required for mixing and transport of the gases through the tubing upon switching the concentration. An offset of 15-20 s is observed between when the methane concentration was switched and when the current of the sensor began to respond (see Fig. 5(f) which illustrates the time required for the methane concentration to switch from 20 to 30 ppm). This can be explained by the residence time of the gas in the tubing which was calculated to be about 12 s. The current response over this range is approximately linear and indicates that this sensor has a sensitivity of approximately 1.5 A/ppm (Figure 7(b)). In order to estimate the lower detection limit of the sensor, the current response from a blank cell is required which depends on time due to the large double-layer capacitance of the sensor. An example of the charging time was already shown in Figure 5(d). Thus taking the current at 5 min after applying 0.6 V to the blank, the charging current is 4.6 A and for a signal to noise ratio of 3, a lower detection limit of 9.2 ppm can be estimated. However,

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this could be reduced by either waiting for the charging current to decay further, or in practice, using a method like differential pulse voltammetry. Due to the ease of electrode fabrication and the rapid prototyping capabilities of the laser, we could easily change the design of the sensor to investigate the impact of further miniaturization. This was achieved by both lowering the number of electrode lines while keeping all other aspects constant as well as by simply shrinking the design proportionally in all dimensions as illustrated in the figure present in supporting information (S3). In all designs, the response scaled almost linearly with sensor size enabling good design flexibility. Next, we tested the sensor response to other gases present in natural gas including ethane, propane and atmospheric water. As shown in Figure 7 (b), the sensor does respond to ethane and propane but the sensitivity is lower than for methane. Furthermore, the sensitivity for ethane is greater than for propane. The sensor was also tested with methane but under simulated humid conditions (RH ~100%). As shown in Figure 7(b), the sensitivity was reduced in the presence of water vapor. This can be explained by the fact that water is a reaction product and shifts equilibrium to the reactant side of Equation 1 according to le Chatelier’s principle. This would reduce the driving force for oxidation and thus the rate. Thus, while the high sensitivity of the sensor might be appropriate for detecting leaks, it would require a parallel humidity sensor and information about other small hydrocarbon contaminants to provide an accurate estimate of methane concentrations in practice. As illustrated in Figure 8, in comparison to other room temperature electrochemical sensors, our system is four orders of magnitude more sensitive while being produced by a technique by a scalable and inexpensive method – capable of rapid prototyping in a laboratory environment. However, the performance is somewhat lower than high temperature solid-state systems utilizing electrodes made of indium-tin oxide (ITO) and platinum mixed with yttria-stabilized zirconia as the solid-state electrolyte. Several aspects of our design could be improved to enhance the performance such as achieving improved dispersion of Pd nanoparticles, enhancing the surface area of LIG electrodes and by improving the amount of 3-phase contact in the IL/PVDF system. Furthermore, while these sensors are promising for monitoring and detection of natural gas, the combination of porous, high surface area LIG with a porous SPE and dispersed catalyst layer could be promising as a platform for a variety of different gas sensing technologies.

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Figure 8: Comparison of sensitivity and detection limit of the reported sensors with other published work.

Conclusions The fabrication of a solid state electrochemical sensor using laser induced graphene has been reported. The sensor was made by laser induced graphene interdigitated electrodes with deposition of palladium nanoparticle and a porous solid polymer electrolyte. The staircase response of the sensor indicates responsiveness to different concentration of methane with a sensitivity of 0.55 A/ppm/cm2, response time of 40 s and an experimental detection limit of 9 ppm. A sharp decrease in sensitivity was observed on testing the sensor at 0.6 V constant potential supply after around 18 min. This decrease was attributed to the formation of water causing the pores in the electrode to block. A simple remedy of air drying allowed normal resumption of the performance of the sensor. With the usage of solid polymer electrolyte, issues such as loss of electrolyte were avoided. Moreover, the SPE allowed a porous network on top of the porous electrode leading to the easy diffusion of methane through this pores avoiding the use of polymer membrane substrate for gaselectrode-electrolyte interaction. Methods Sensor fabrication An interdigitated electrode design was written into a 127 m thick Kapton film (McMasterCarr) using a CO2 laser engraver (Bosslaser 50W LS-1416) and accompanying software. An 18 x Dosi et al.


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21 mm area was patterned at 25 mm/s and using 10 % power using scan mode in the X-swing direction (with the X-direction defined along the length of the anode and cathode). The pattern consisted of two busbars, each with dimensions of 21  1 mm. Each busbar is connected to 14 anode/cathode fingers, each with a dimension of 13  0.4 mm with a spacing of 0.5 mm between them. The overlap area between the cathode/anode pairs for which the electrolyte was covered was ~2 cm2. Overall the pattern takes 80 s to write. Measurements were also carried out for various miniaturized designs with different dimensions and a smaller amount of anode/cathode lines as indicated in the text and figures. The miniaturization was conducted via two different ways, the first technique involved altering the number of electrode lines only. Four sets; 18, 9, 6 and 3 lines of electrodes were tested. In an alternative design, the interdigitated design was shrunk in all dimensions with the same number of lines to electrode areas of 2.39 1.47 and 1.99 cm2. Pd NPs were used as the methane electro-oxidation catalyst. These were synthesized from a palladium nitrate salt using a technique followed by Azhari et al. 30 A 100 mL solution of 5 mM sodium citrate and 0.025 mM tannic acid were prepared in deionized water. The temperature of this solution was brought to 70 C and 8 mL of a 25 mM palladium nitrate solution was rapidly injected and the temperature maintained for 15 min. In order to introduce the Pd NPs into the porous, high surface area of the LIG electrodes, these hydrophobic electrodes were first immersed in isopropyl alcohol (IPA) to displace the air from the pore space. The film was immediately transferred from the IPA to the aqueous 25 mM Pd NP dispersion for 2 to 3 min to allow for solvent exchange. The dispersion was allowed to dry overnight (12 hours) to deposit the Pd NPs onto the LIG. The samples were then baked on a hotplate held at 60 C for a duration of 10 min to remove any remaining water prior to casting the electrolyte. A

pseudo-solid

state

electrolyte

composed

of

1-ethyl-3-methyl

imidazolium

bis(trifluoromethanesulfyl)imide (EMImTFSI) and polyvinylidene fluoride (PVDF) was used as the non-volatile and porous electrolyte. As shown in Supporting Information, Figure S1, the weight ratio of two parts PVDF and 3 parts EMImTFSI were dissolved in 3 parts N-methyl-2-pyrrolidone (NMP) by heating at 75C.22 The resulting viscous mixture was then applied onto the interdigitated array using a glass rod. The NMP was evaporated at room temperature for 24 h leaving behind a thin, white layer of pseudo-solid and porous PVDF/EMImTFSI.

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Characterization The resulting sensor was placed in a custom, sealed acrylic cell with inlets and outlets for flowing gas and the anode and cathode busbars contacted with copper strips which were press fit against the LIG with stainless steel screws. After connecting the cell to the gas lines, mass flow controllers (Omega Engineering) were used to control the relative flow rates of dry air (Praxair) and diluted methane (500 ppm in N2, Praxair) which were mixed via a T-joint connection prior to entering the acrylic set-up. The system was first flushed with nitrogen using a flow rate of 200 mL/min, followed by flushing with dry air with flow rate of 200 mL/min. After 5 minutes, the air flow rate and methane flow rate were adjusted using Labview 2015 software to provide the required concentration in the chamber. Concentrations ranging from 10 ppm to 100 ppm were studied. After each test at a set concentration, the system was flushed with nitrogen for 2 min and the procedure repeated for the next concentration. The response from interfering gases ethane (500 ppm in N2, Praxair) and propane (500 ppm in N2, Praxair) were tested in the same way. To simulate humid conditions, porous paper soaked with water was placed within the chamber to create a saturated environment. Where indicated, preliminary testing was carried out in an acrylic batch chamber (20 cm x 15 cm x 10 cm) which was evacuated and backfilled with dry air or nitrogen. A 60 mL syringe was filled with pure methane gas (99.5 % methane with 0.5 % nitrogen, developed by Praxair) and used to inject a known volume of methane, while another syringe was used to withdraw some volume to maintain a constant pressure. The hydrodynamic diameter of Pd NPs was examined by dynamic light scattering (DLS, Zetasizer Nano-ZS90, Malvern) after diluting the dispersion to 0.1 mg/mL with deionized water. The nanoparticles and electrodes were imaged by scanning electronic microscopy (SEM) using a Zeiss FESEM Leo 1530 operated at an acceleration voltage of 5 kV. Energy dispersive X-ray spectroscopy (EDS) was carried out using the same instrument as well. Raman spectroscopy was conducted by a Horiba Raman Division Olympus BX 41 instrument with a 532.06 nm laser and using 10% power. For the batch measurements, electrochemical analysis, cyclic voltammetry and potentiostatic measurements, was carried out using a BioLogic VMP3B-100 potentiostat while a portable pocketSTAT (Ivium Technologies) was used for the flow cell measurements. Off-gas analysis from the flow cell (total flow rate of 700 ml/min) was carried out by FTIR (MKS, MultiGas 2030) which has a reported lower detection limit of 10-100 ppb.

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Surface area and porosity measurements were conducted using a surface area analyzer (Gemini VII 2390, Micromeritics Instrument Corporation), between partial pressures of P/P0 = 00.99 using nitrogen as the adsorbate. A large area of polyimide was scribed in order to generate sufficient mass (~10mg) for the measurement. The LIG was scraped off the PI substrate and the resulting powder degassed at 300 °C for 1 h. The pore-size distribution was calculated according to the model of Dollimore and Heal31 using the adsorption branch of the isotherm. The sensor’s longevity was tested by operating a single sensor everyday between 10 and 11 am for about 10 min at 0.6 V, injecting 25 ppm methane and measuring the stable current response using pocketSTAT system for a period of 30 days. The sensor was dried and stored in an isolated air tight container after each daily use. Acknowledgments This work was supported by Pro-Flange Ltd, our industrial partner via the NSERC Engage and the Ontario Centers for Excellence Voucher for Innovation and Productivity I programs. Supporting Information Available: Further information and discussion regarding optimizing sensor performance, EDS spectra and elemental analysis and sensor miniaturization is provided in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org. References (1)

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Table of Contents Image

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Ultra-Sensitive Electrochemical Methane Sensors based on Solid

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