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Pulsed Electrochemical Mass Spectrometry for Operando Tracking of Interfacial Processes in Small-Time-Constant Electrochemical Devices, Such as Supercapacitors Nicolas Batisse, and Encarnacion Raymundo-Piñero ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12068 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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ACS Applied Materials & Interfaces

Pulsed Electrochemical Mass Spectrometry for Operando Tracking of Interfacial Processes in Small-Time-Constant Electrochemical Devices, Such as Supercapacitors Nicolas Batisse, Encarnación Raymundo-Piñero*

Dr. N. Batisse Université Clermont Auvergne, CNRS, Institut de Chimie de Clermont-Ferrand, F-63000 Clermont-Ferrand, France E-mail: [email protected] Dr. E. Raymundo-Piñero CNRS, CEMHTI UPR3079, Univ. Orléans, F-45071 Orléans, France Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS 3459, France E-mail: [email protected]

Keywords: supercapacitors, interface electrode/electrolyte, cell voltage, DEMS, PEMS

Abstract A more detailed understanding of the electrode/electrolyte interface degradation during the charging cycle in supercapacitors is of great interest for exploring the voltage stability range and therefore the extractable energy. The evaluation of the gas evolution during the charging, discharging, and aging processes is a powerful tool toward determining the stability and energy capacity of supercapacitors. Here, we attempt to fit the gas analysis resolution to the time response of a low-gas-generation power device by adopting a modified pulsed electrochemical mass spectrometry (PEMS) method. The pertinence of the method is shown using a symmetric carbon/carbon supercapacitor operating in different aqueous electrolytes. The differences observed in the gas levels and compositions as a function of the cell voltage correlate to the evolution of the physicochemical characteristics of the carbon electrodes and to the electrochemical performance, giving a complete picture of the processes taking place at the electrode/electrolyte interface.

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INTRODUCTION In recent decades, the production of electrical energy from renewable resources has rapidly grown, increasing the demand for practical electrical energy storage solutions. Supercapacitors have attracted interest due to their energy and power densities, filling the gap between batteries and electrolytic capacitors.1,2 Furthermore, their outstanding cycling capabilities, completing millions of cycles in industrial devices, have promoted their use in a large range of applications from the automotive field to wearable electronics. Meanwhile, extensive efforts have been focused on understanding key phenomena in supercapacitors regarding electrolyte ion fluxes and the double-layer formation mechanism through theoretical or experimental studies.3-9 However, fewer investigations have been devoted to the complex performance degradation pathways during device aging. During aging, the main issue is the voltage limit of the cell, which is affected by irreversible reactions at the electrode/electrolyte interface, influencing the selection of the materials employed. Carbonbased materials, especially nanoporous carbon-based materials, are currently the most adopted electrode materials due to their favorable cost-to-performance ratio, which is highlighted by their widespread use in the majority of commercialized devices.10-11 Furthermore, the adoption of organic acetonitrile-based electrolytes allows cell voltages to reach 2.8-2.9 V, whereas equivalent systems in aqueous electrolytes, such as H2SO4 or KOH, do not exceed 0.8-1.0 V.9-12 Nevertheless, organic electrolytes present safety issues and cell-handling requirements that are not compatible with a number of applications. Among the various alternatives,13-16 neutral aqueous electrolytes, such as Na2SO4 or Li2SO4, have been highlighted as reliable ecofriendly alternatives for carbon/carbon supercapacitors because they can attain voltage windows up to 1.8 V-2.0 V.17,18 The maximum operating voltage in an aqueous medium is highly dependent on the potentials at which oxygen and hydrogen gas evolutions appear when overcoming the thermodynamic values for water oxidation and reduction. The Nernst equation theoretically predicts these potentials, but in practice, the pH 2 ACS Paragon Plus Environment

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of the electrolyte is not sufficient to predict the operating potential window. Indeed, it has been shown that under negative polarization the sorption of nascent hydrogen generated by water splitting at the carbon electrode-electrolyte interface induces a local pH increase of the electrolyte resulting in an important overpotential for water reduction.19,20 This effect is more noticeable in neutral electrolytes than in acidic or alkaline electrolytes resulting in a larger potential window.21 Thus, a more detailed understanding of the gas evolution upon charging in supercapacitors is important to assess the reactions at the electrode/electrolyte interface and to explain the voltage stability window in the different aqueous electrolytes. In recent years, the development of the differential electrochemical mass spectrometry technique (DEMS) has attracted interest for monitoring gas evolution during the operation of electrochemical devices, especially Li-ion and Li-O2 batteries and recently supercapacitors.2225

In fact, understanding the gas generation by studying the electrolyte decomposition or the

reactions at the electrode/electrolyte interface provides a more precise and direct evaluation of the potential polarization limits than can be determined by cyclic voltammetry. Moreover, gas evolution from the cell is one of the main concerns of industrial devices because it results in an overpressure in large cell designs. However, in DEMS experiments, there is a delay between gas production and evolution/detection reaching as high as tens of seconds.24 Although those delays are not important to consider for devices such as Li-ion or Li-O2 batteries, they are not compatible with the time response of a power device such as a supercapacitor, which requires a range of few seconds. Here, we present a different approach for allowing operando quantitative gas measurements under real supercapacitor operation conditions using pulsed electrochemical mass spectrometry (PEMS). This technique uses fast accumulative and fully synchronized gas sampling sequences sent from the electrochemical cell to the MS. Defined time-resolved pulses are obtained in each m/z ratio signal and are adapted to the time scale of the electrochemical measurement. Moreover, the design of a custom airtight electrochemical cell 3 ACS Paragon Plus Environment


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with a reduced dead volume (tested against 3 bars of He pressure with no detectable leaks) using large surface area electrodes (30 cm2) provides maximum sensitivity. Low gas production can be quantitatively measured using an operando method from a charge/discharge cycle in a few seconds or during supercapacitor aging. For aging experiments, this information aids in understanding the processes taking place at the electrode/electrolyte interface that determine the supercapacitor operation limits.

EXPERIMENTAL SECTION Electrode preparation. Commercial activated carbon powder (83 wt%, MeadWestvaco, USA), PVDF (12 wt%, Solef 5130, Solvay, Belgium) and carbon black (5 wt%, C65, Timcal, Switzerland) were dispersed and mixed in N-methyl-2-pyrrolidone to form the electrode slurry. The slurry was then coated on 316L stainless steel current collector with an automatic applicator (Elcometer 3430, France) at 90 °C and densified by a rolling process. The electrodes were then outgassed for 12 h at 60 °C under vacuum. The obtained electrodes had a surface area of 30 cm2 and a thickness of 120±5 µm including collector (collector thickness: 20 µm).

Operando electrochemical mass spectrometry. The specially designed system for in operando electrochemical mass spectrometry and the subsequent custom electrochemical cell have been previously described [WO2015059281A1, 2013]. Briefly, the electrochemical cell is an airtight and compact rectangular cell that can house 30 cm2 electrodes having the possibility of adjusting the width depending on the electrodes thickness. The cell is particularly designed to minimize dead space. Special channels inside the cell allow for the flow of a carrier gas (He, 20 ml min-1) to sweep all the gases produced during the cell operation on a time-scale of few seconds. Different valves and flowmeters are synchronized to the potentiostat/galvanostat (SP-50, Biologic, France) using 4 ACS Paragon Plus Environment

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software developed in-house for transporting the gases to the mass spectrometer (QMS 403C, Aeolos, Netzsch, Germany) at the desired moment during the cell operation (a picture of the device is included as Figure S1 in supporting information).

Porous texture characterization. The porous texture of the materials was analyzed by N2 adsorption at 77 K (Autosorb-1, Quantachrome, USA) and by CO2 sorption at 273 K (Quadrasorb, Quantachrome, USA). Prior to the measurements, the samples were outgassed at 200 °C for 12 h under vacuum. The specific surface area was determined from the N2 adsorption isotherm using the Brunauer– Emmett–Teller (BET) equation. The microporous volumes were determined by applying the Dubinin–Radushkevich method to the CO2 adsorption data. The mesopore volume and the pore size distribution were determined by the 2D-non local density functional theory (2DNLDFT) method with a slit-shaped pore model applied to the N2 adsorption data.26

Surface functionality characterization. The surface functionality of the carbons was analyzed by temperature-programmed desorption (TPD) under an inert He atmosphere. The sample (10 mg) was placed in an STA 449C thermobalance (Netzsch, Germany) and kept at room temperature for 2 h under a helium flow of 150 mL min-1. Then, the temperature was increased at a constant rate of 10 °C min-1 until 1100 °C. The decomposition products were analyzed by online mass spectrometry (QMS 403C, Aeolos, Netzsch, Germany) connected to the thermobalance by a heated silica capillary. To investigate the evolution of the surface chemistry upon aging, the electrodes were removed from the supercapacitor cell at a given aging time, washed for 8 h by gentle stirring with distilled water to remove the electrolyte ions, vacuum filtered five times with distilled water and finally dried in an oven at 110 °C for 12 h. Fresh electrodes were also subjected to the same treatment for comparison. 5 ACS Paragon Plus Environment


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RESULTS AND DISCUSSION

Figure 1 illustrates the differences in using DEMS and PEMS (see Figure S2 in supporting information for more details in sampling methodology). In the conventional DEMS mode (Figure 1a), a carrier gas continuously flushes the gases released inside the electrochemical cell during operation to the MS. In the PEMS mode, the original approach for minimizing gases dilution and increasing sensitivity involved accumulating the gases produced inside the supercapacitor cell and later introducing short pulses of a carrier gas through the cell to the MS (Figure 1b). Therefore, the partial pressure of the gas mixture released from the cell can be modulated to obtain time-resolved MS peaks for each m/z signal. Figure 1c shows the electrochemical response and the gas evolution detected by DEMS during cyclic voltammetry at 2 mV s-1 of a carbon electrode in a three-electrode cell configuration in different aqueous electrolytes (H2SO4, Na2SO4 and Li2SO4 at a concentration of 1 mol L-1). At a positive polarization, overcoming or approaching the thermodynamic potential for H2O oxidation results in the evolution of carbon oxides, such as CO2 and CO (not shown), before the release of O2. The results show that the phenomena arising at the electrode/electrolyte interphase are pH dependent. In sulfuric acid, no oxygen gas evolution is detected at high potentials, but more CO2 is released compared to that in neutral electrolytes. Therefore, in the case of sulfuric acid, the O2 produced during water electrolysis at the positive electrode is fully consumed to form oxygen functional groups on the carbon surface, whereas in neutral electrolytes, only a small fraction of the overall generated O2 is involved in the oxidation of carbon. In the negative polarization range, H2 is the only gas detected. The potential at which H2 is released is approximately -0.2 V vs NHE from H2SO4 and approximately -1.2 V vs NHE for both Na2SO4 and Li2SO4, whereas the Nernst equation predicts -0.06 V vs NHE and -0.36 V 6 ACS Paragon Plus Environment

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vs NHE, respectively. The results confirm that the electrochemical stability window of neutral electrolytes is extended compared to acidic (and alkaline) electrolytes due to the larger hydrogen electrosorption effect of the carbon nanopores. The results in Figure 1c indicate that symmetric carbon/carbon capacitors could operate at a cell voltage of 2.1 V in Na2SO4 and as high as 2.4 V in Li2SO4. However, such values are far away to the maximum cell voltage determined by galvanostatic charge/discharge cycling previously obtained using the same carbon material being 1.6 V in Na2SO4 and 1.8 V in Li2SO4.17,21 The results confirm that in such differential DEMS experiments, there is a delay between gas production and evolution/detection.24 Moreover, Figure 1c shows a residual detection of all gases after reversing the potential sweep, highlighting the slow desorption of these gases from the electrolyte and/or electrode surface; this induces significant delays in MS measurements, revealing another limitation when using DEMS. These limitations are amplified in electrochemical systems with low amounts of released gases and fast time constants such as supercapacitors, leading to the inaccurate identification of the processes occurring at the electrode/electrolyte interface.



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Figure 1. Sampling strategies and MS signals for m/z=44 during a galvanostatic cycle for (a) conventional DEMS mode and (b) PEMS mode. (c) The electrochemical response and qualitative CO2, O2 and H2 gas evolutions in DEMS mode during cyclic voltammetry at a scan rate of 2 mV s-1 for a 3-electrode cell in 1 mol L-1 H2SO4, 1 mol L-1 Na2SO4, and 1 mol L-1 Li2SO4. The arrows indicate the voltage sweep direction. The gas evolution levels are expressed as the current at the mass spectrometer detector. (d) Integrated CO2 amounts in PEMS and DEMS mode during galvanostatic charge/discharge at 200 mA g-1 with gradual voltage increase of a two-electrode cell in 1 mol L-1 H2SO4, 1 mol L-1 Na2SO4, and 1 mol L-1 Li2SO4. (e) Integrated CO2 amounts from PEMS mode during galvanostatic charging and discharging (gas measurements at the maximum and 0.0 V voltages values) of a 2-electrodes cell in 1 mol L-1 H2SO4, 1 mol L-1 Na2SO4, and 1 mol L-1 Li2SO4. Such limitations can be overcome by adapting the gas sampling methodology. Actually, the gases produced during the irreversible reactions at the electrode/electrolyte interphase can reach the cell free space or being adsorbed in the electrolyte, separator or carbon electrode porosity. In the DEMS mode (Figure 1a) the carrier gas is continuously flowing and sweeping the gases inside the cell. Figure 1a shows that some amount of gas is exhausted during the charge/discharge of a carbon/carbon symmetric supercapacitor with a delay of tens of seconds but for desorbing the gases strongly adsorbed i.e. mainly in the electrode porosity, the cell must be swept during tens of minutes. Contrastingly, in the PEMS mode gases are cumulated inside of the cell during supercapacitor operation driving to a slight overpressure inside the cell. Therefore the ΔP inside the cell (associated with a synchronized He flushing) makes the gases instantly exhaust and go to the MS when opening the cell as shown in Figure 1b. In 8 ACS Paragon Plus Environment

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such configuration, a gas pulse of few seconds will be enough for desorbing the strongly adsorbed species and to renew the cell atmosphere. By performing gas sweeping and sampling only at the end of each galvanostatic charge/discharge cycle avoids residual signals (as found in DEMS) when following the gas evolution. Figure 1b illustrates that the detection system is fast enough so that the MS analysis time can be ignored in the calculation of the total charge or discharge time. Therefore, Figure 1d shows that, with PEMS, we can mimic the results obtained by conventional DEMS by increasing the voltage in a symmetric carbon/carbon supercapacitor, but the results are more precise and quantitative at a lower detection threshold. CO2 gas evolution has been chosen as a precise indicator of supercapacitor operation limits because it reflects surface degradation at the positive electrode, which is shown later. The results obtained using PEMS coincided with the cell voltages limits obtained by long-term galvanostatic charge/discharge cycling. However, when using DEMS, CO2 is detectable at significantly higher cell voltages independently of the electrolyte. Therefore, the cell voltage limit obtained from the release of CO2 during oxidation at the positive electrode is artificially higher. Moreover, more detailed information of the gas produced independently during the charge or the discharge at a given cell voltage (Figure 1e) can be obtained if gas sweeping and sampling are performed at the maximum cell voltage and after reaching 0 V (as shown in Figure 1b). Figure 1e shows that carbon oxidation occurs more extensively during the charge cycle. The above results demonstrate that PEMS is better adapted to measure gas evolution in supercapacitors than DEMS. The advantages of the PEMS technique are highlighted when gases are monitored in an electrochemical cell at a given voltage over a long period of time (Figure 2) (see sampling information in Figure S2c). Operating voltages ranging from 1.4 V to 2.0 V using 1 mol L-1 Na2SO4 and 1 mol L-1 Li2SO4 electrolytes were explored for each voltage in a newly assembled supercapacitor. 9 ACS Paragon Plus Environment


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Figure 2. Potentiostatic PEMS in a two-electrodes cell. Quantitative gas evolution of (a,e) CO2, (b,f) CO, (c,g) H2, and (d,h) O2 at different constant voltages and accumulation periods when using 1 mol L-1 Na2SO4 (a,b,c,d) or 1 mol L-1 Li2SO4 (e,f,g,h) as electrolyte. First, it can be observed in Figure 2 that the amounts of the released gases (H2, CO, CO2 and O2) follow a linear variation versus aging time for accumulation periods less than one hour for any cell voltage. The order of magnitude of the gas levels varies depending on the product. For gases involved in the solvent degradation process, i.e., H2 and O2 due to water reduction and oxidation, respectively, levels are on the order of tens of µmol g-1 h-1 for both electrolytes, whereas for gases ascribed more specifically to the carbon electrodes degradation, i.e., CO2 and CO, the released quantities are on the order of 10-2 µmol g-1 h-1. Hence, it is important to note that an increase in the pressure in a supercapacitor cell is most likely governed by gases that do not directly indicate supercapacitor aging. Therefore, measurements of pressure changes inside the cell do not provide a suitable standalone strategy to track supercapacitor aging, and more precise gas measurements via MS, for example, are required.27 Figure 2 shows that the variation of the amount of gases evolved with the voltage depends on the nature of the gas. In the case of H2, which is the only detectable gas related to phenomena at the negative electrode/electrolyte interface (only non-quantifiable traces of CH4 are observed for voltage values beyond 1.8 V in Na2SO4), higher voltages induce greater releases 10 ACS Paragon Plus Environment

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in H2 for a given accumulation period. The H2 evolution potential at the negative electrode is not attained if the cell voltage does not reach 1.6 V for Na2SO4 and 1.8 V for Li2SO 4. The CO, CO2 and O2 gases ascribed to phenomena at the positive electrode can be detected even for low cell voltages if the voltage is held for a long time. CO 2 levels continuously increase with the cell voltage for the same accumulation period, whereas CO and O 2

quantiti quantities es demonstrate the opposite behavior. Such results clearly indicate that the electro electro--oxidation oxidation pathways at the positive carbon electrode depend on the applied cell voltage voltage,, i.e., the maximum potential reached at the positive electrode. For low voltage val values, ues, the oxidation mechanism at the positive electrode

involves oxygen oxygen functional functional groups that further decompose into CO gas gas rather rather than than in in CO CO2 . At higher cell voltages, the oxidation mechanism causes more O O-rich rich functionalities to desorb

preferentially as CO2. Therefore, the decrease in the O2 levels with cell voltage for a given aging time can be ascribed to the formation of more oxygenoxygen-rich rich groups in addition to more extensive oxidation of the positive electrode (Schema 1) 1)..

1. Carbon oxidati oxidation on mechanism depending on cell voltage. *C represents an active Schema 1. carbon site for oxidation.

It is interesting to note that in both neutral electrolytes, CO2 presents a linear dependence dependenc e on time,, as observed for all the gases (Figure S3 ). That observation suggests a voltage and on time S 3). correlation between V, t (respectively defined as the fixed voltage cell and the accumulation time at this fixed voltage) and the CO2 evolution for a defined state of aging (E (Equation quation 1) (K 1 11 ACS Paragon Plus Environment


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and K2 represent empirical constants, which could depend on parameters that remain unresolved, such as cell characteristics, electrode materials characteristics and further aging state). CO2 evolution = (K1V + K2) t

(1)

This behavior explains the exponential variations observed in Figure 1d for galvanostatic PEMS and confirms that the CO2 levels determined at a given condition of V or t can be used to predict positive electrode aging and the maximum operating cell voltage. Therefore, PEMS provides evidence of phenomena arising at the electrode/electrolyte interface, highlighting the dependence of gas evolution on the cell voltage. That information is crucial when extending the PEMS approach to long-term aging to target real systems analysis and performances. The main factors on supercapacitors aging are temperature and voltage28 but considering the sensitivity of the PEMS technique, aging experiments can be performed at room temperature by holding the cell voltage at a slightly higher value than the highest stable voltage in a so-called floating test. Therefore, aging can be accelerated without inducing interface reactions that do not occur under normal operating conditions. Figure 3 depicts the gases evolved while aging carbon/carbon supercapacitors using Li2SO4 or Na2SO4 electrolytes. The sampling procedure is described in supporting information S2d. Figure 3a shows that aging a Li2SO4-based supercapacitor at a cell voltage of 1.8 V does not cause extensive reactions generating gases, whereas at the same cell voltage, different gases are detected in the Na2SO4 electrolyte (Figure 3b). In Li2SO4, a cell voltage of 2.0 V is needed to accelerate capacitor aging in a floating experiment.


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Figure 3. PEMS during long-term accelerated aging. Quantitative evolutions of CO2 (m/z=44), CO (m/z=28), H2 (m/z=2) and O2 (m/z=28) obtained from a carbon/carbon supercapacitor operating at (a) 1.8 V (grey) and 2.0 V (red) in 1 mol L-1 Li2SO4 (b) 1.8 V in 1 mol L-1 Na2SO4. Such results are in agreement with the potentials reached at the positive and the negative electrodes during the charge cycle of a supercapacitor in the different electrolytes, as shown in Figure 4. In fact, for Li2SO4 the larger size of the hydrated cations causes the potentials to shift to values lower than those in Na2SO4 due to the smaller capacitance of the negative electrode. Therefore, the oxidation of carbon at the positive electrode arises at higher cell voltages for Li2SO4 than for Na2SO4. At cell voltages of 1.8 V for Na2SO4 and 2.0 V for Li2SO4, the potential for the negative electrode does not yet reach that required for H2 evolution, which explains the absence of m/z 2 during the first hours of aging in Figure 3.



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Figure 4. Operation of a symmetric AC/AC supercapacitor. The potential limits of the positive and negative electrodes during galvanostatic (200 mA g-1) charging of the cell up to different maximum voltages. The E0V values correspond to the potential of both electrodes after discharging the cell at 0 V before the following charge up to a higher maximum voltage. The lower horizontal line represents the negative potential limit related to a noticeable H2 evolution estimated in a three-electrode cell. The upper horizontal line corresponds to the thermodynamic limit for water oxidation. Electrolyte: dashed lines, Na2SO4; full lines, Li2SO4. However, Figure 3 shows that after the first hour of voltage floating, the gas evolution profiles do not increase linearly with time, as demonstrated in Figure 2. During the floating experiments, the potential of electrodes is not constant, as shown in Figure 5a for Na2SO4 at 1.8 V and in Figure S4 for Li2SO4 at 2.0 V. Especially for Na2SO4, for the first 60 hours of aging in Figure 5a show a downward drifting potential for the positive and the negative electrodes. Consequently, the O2 gas evolution rate (m/z=32) decreases as H2 progressively increases during that time, as shown if Figure 3b. This potential drift is ascribed to modifications in the electrode materials during the first stages of floating. During the first hours of aging, the evolution of CO and CO2 related to the release of oxygen surface functionalities mainly occurs via carbon oxidation at the positive electrode (Figure 3). Degradation of the positive electrode induces a decrease in its capacitance, leading to asymmetric behavior of the cell, which impacts the operating potential of each electrode. 14 ACS Paragon Plus Environment

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Therefore, the supercapacitor shows a sharp decay in the specific capacitance together to a pronounced increase on resistance during the first hours of floating (Figure 5b). The CO and CO2 levels decrease in parallel with the positive electrode potential and O2 production. After approximately 20 hours of aging at a cell voltage of 1.8 V, the CO and CO2 levels no longer decrease because the positive electrode potential is now low enough that electrode oxidation is decreased. Beyond that point, the supercapacitor capacitance and resistance variations are less pronounced and mostly relate to the oxidation of carbon at the positive electrode releasing CO2 gas.

Figure 5. Electrochemical characterization of a C/C supercapacitor during accelerated aging. (a) Evolution of the electrodes potential during first 40 hours. (b) evolution of specific capacitance (squares) and resistance (circles) at a constant voltage of 1.8 V in 1 mol L-1 Na2SO4. Therefore, PEMS provides the key to understand the sharp decrease in the capacitance at the first stages of aging (for comparison, the capacitance retention in Li2SO4 at 2.0 V is presented in the supporting information, Figure S5), which is a general feature observed for supercapacitors operating in aqueous electrolytes.17,21 Furthermore, the oxidation of the carbon material by the positive electrode at the beginning of aging causes asymmetry in the cell, as the capacitance of both electrodes is no longer the same. Once the electrode potentials are stabilized accordingly, the capacitance of the cell varies only if irreversible reactions continue to occur at the electrode/electrolyte interface. 15 ACS Paragon Plus Environment


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The above results strongly suggest that the PEMS data must be examined along with the ex situ analysis of the surface functionality and the porous texture of the electrodes to gain a more complete understanding of the overall pathways of the irreversible reactions at the electrode/electrolyte interface that are responsible of the supercapacitor performance degradation. Figure 6 depicts the amount of oxygen obtained by temperature programmed desorption (TPD; Figure 6a) and the specific surface areas obtained by N2 and CO2 adsorption (SBET and SDR, respectively; Figure 6b) for the positive and negative electrodes after 0, 40, 80 and 120 h of floating at 1.8 V for a C/C supercapacitor in 1 mol L-1 Na2SO4.

Figure 6. Physicochemical characterization of electrodes. (a) Weight percentage of oxygen by TPD analysis for the positive and negative electrodes at different aging times. Textural properties of the aged electrodes by N2 and CO2 gas sorption at 77K and 273K, respectively: (b) specific surface area, (c) pore volume and (d) 2D-NLDFT pore size distribution for the positive electrode by 77K N2 sorption. Mass is reported as the total electrode material including the binder.

The results in Figure 6a confirm that accelerated aging induces the electro-oxidation of the positive electrode, whereas the surface functionality of the negative electrode is mostly 16 ACS Paragon Plus Environment

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unmodified. Grafting of heteroatoms at the positive electrode (preferentially oxygen atoms in this case) induces structural defects in the graphene sheets that define the porous network of carbon resulting in collapsing pores. The specific surface area drops continuously from 1580 m2 g-1 to 1070 m2 g-1 (ascribed to an SSA decrease of 32%) after 120 h of aging at 1.8 V (Figure 6b). Figures 6c and 6d show that those severe modifications of the textural properties primarily affect the micropores (pore widths smaller than 2.0 nm) and also the supermicropores (pore widths ranging from 0.7 nm to 2.0 nm). The largest variation in the oxygen content is obtained from the first 40 h of aging starting with 3.5 wt% of the fresh electrode and increasing to 8.2 wt%. This amount of oxidation results in a pronounced decrease in the micropore SSA and in the volume related to pores smaller than 0.7-0.8 nm (SDR in Figure 6b and VDR in Figure 6c, respectively). The collapse of the most efficient pores for the formation of the double layer at the positive electrode will be the main reason for the fast capacitance drop of the supercapacitor at the first hours of floating, as observed in Figure 5b and S529,30. To explore the electro-oxidation pathways at the positive electrode, a detailed analysis of the data obtained by TPD at various states of aging has been carried out (see example of TPD results over the positive electrode after 40 h aging in Figure S6).



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Figure 7. Electrode oxygenated surface functionalities. TPD desorption profiles for positive electrodes at different states of aging at 1,8 V in 1 mol L-1 Na2SO4 for CO2 (a) and CO (b). Evolution of the number of oxygen functional groups obtained from a deconvolution of the TPD profiles of CO2 (c) and CO (d). The TPD analysis allows the characterization of either the acidic (carboxylic, peroxide, lactone) or basic (ether, phenol, carbonyl, hydroxyl, pyrone, quinone) oxygen functional groups at the electrode surface, which are released as CO2 and CO, respectively, by thermal desorption.31,32 Figures 7a and 7b show an increase of both groups of oxygen functionalities with the aging time. It is important to note here that the CO2 and CO obtained by ex situ characterization of the electrodes are not necessarily related with the carbon oxides detected during operando ECMS. In fact, thermal treatment does not involve the same reaction pathways than desorption from electro-oxidation. The deconvolution of the TPD profiles (Figures 7c and d; see procedure in supporting information Figure S7) reveals that carbon electro-oxidation preferentially generates phenol, carbonyl and carboxylic functional groups. Notably, multiple pathways of electro-oxidation 18 ACS Paragon Plus Environment

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could occur ranging from the dihydroxylation of carboxylic to carboxylic anhydride groups to the conversion of phenol into carbonyl, which is further oxidized to carboxylic groups. The number of phenol oxygen functionalities relating to the first electro-oxidation stages increases linearly with the aging time since carbonyl and carboxylic functionalities follow similar profiles to the CO2 gas evolution from PEMS, i.e., an increase from the first hours of aging before reaching a plateau (Figure 3). Therefore, a fraction of carbonyl and carboxylic functionalities will rapidly evolve as CO2 gas leaving new sites for the formation of phenol groups (see schema of the above oxidation pathway in supporting information Figure S8). Therefore, a combined gas evolution/physicochemical electrode characterization approach elucidates the electrode reaction pathways as a function of cell voltage or during aging capacitors at a constant cell voltage. Notwithstanding, there is a two-fold order of magnitude difference in the amount of oxygen functionalities detected on the carbon surface by ex situ TPD compared to the levels of CO2 and CO released as gases during the aging of the supercapacitor as determined by PEMS (see supporting information Table S1). Indeed, a majority of the oxygen functional groups generated during aging does not further evolve into gaseous products. Again, these results provide evidence that the PEMS technique is sensitive enough to detect low amounts of gases for monitoring the irreversible reactions arising at the electrode/electrolyte interface under real operating conditions. Therefore, such technique would come to complement other insitu/operando techniques33,34 for reinforcing the understanding of the dynamic electrochemical processes taking place during the operation of an electrochemical device.

CONCLUSIONS A pulsed electrochemical mass spectrometry (PEMS) method has been introduced to quantitatively analyze the gases produced at the electrode/electrolyte interface during supercapacitor operation. PEMS allows a precise and quantitative way to track gases with a 19 ACS Paragon Plus Environment


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low detection threshold. Therefore, it is sensitive enough to suitably characterize low-gasgeneration systems such as supercapacitors. Moreover, gas evolution can be monitored without residual signals to obtain a time-resolved response adapted to the time constant of a power device as a supercapacitor. PEMS allows for more precise calculations of maximum operating cell voltages by providing direct evidence of the electrode potential limits. Moreover, combining the information of gas evolution with the electrochemical performance and physicochemical electrode characteristics increases understanding of the processes taking place at the electrode/electrolyte interface. This approach applied for supercapacitors operating in aqueous electrolytes introduces new insight on the degradation pathways of electrode/electrolyte interfaces, opening the way towards new strategies for improving system operations. Therefore, PEMS broadens the field of study of low-gas-generation electrochemical storage systems for a more precise understanding of the irreversible processes that limit operation.

ACKNOWLEDGEMENTS This work was financially supported by the French National Agency for Research (ANR): PROGELEC project FLEXCAP (ANR-2011-PRGE-008)

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Graphical Abstract Pulsed Electrochemical Mass Spectrometry for Operando Tracking of Interfacial Processes in Small-Time-Constant Electrochemical Devices, Such as Supercapacitors Nicolas Batisse, Encarnación Raymundo-Piñero*


Pulsed Electrochemical Mass Spectrometry for ... - ACS Publications

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