Faraday Discussions Cite this: DOI: 10.1039/c4fd00056k

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Sodium molybdate – an additive of choice for enhancing the performance of AC/AC electrochemical capacitors in a salt aqueous electrolyte Q. Abbas, P. Ratajczak and F. Be´guin* Received 30th March 2014, Accepted 6th May 2014 DOI: 10.1039/c4fd00056k

Sodium molybdate (Na2MoO4) has been used as an additive to 1 mol L1 lithium sulfate electrolyte for electrochemical capacitors based on activated carbon (AC) electrodes, in order to reduce the corrosion of stainless steel current collectors. We demonstrate that the MoO42 anions improve the overall capacitance owing to pseudofaradaic processes. In a two-electrode cell, capacitance values of 121 F g1 have been achieved up to 1.6 V using 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4, as compared to 103 F g1 when 1 mol L1 Li2SO4 is used. Further, by using a two-electrode setup equipped with a reference electrode, we could demonstrate that, at 1.6 V, the positive electrode potential reaches a value of 0.96 V vs. NHE in 1 mol L1 Li2SO4, crossing the thermodynamic potential limit of oxygen evolution (Eox ¼ 0.846 V vs. NHE), and the pitting potential, Epit ¼ 0.95 V vs. NHE. By contrast, in 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4, the pseudofaradaic contribution occurring at 0.05 V vs. NHE due to MoO42 anions drives the positive electrode to reach only 0.798 V vs. NHE. Hence, the oxidation of the AC and corrosion of the stainless steel current collector at the positive electrode are unlikely in Li2SO4 + Na2MoO4 when the capacitor operates at 1.6 V. During potentiostatic floating of the capacitor at 1.6 V for 120 hours in Li2SO4 + Na2MoO4, the capacitance and resistance remain constant at 125 F g1 and 1.0 U, respectively, while the resistance increases from 1.4 U to 3.1 U in Li2SO4. Overall, the addition of MoO42 anions to Li2SO4 aqueous electrolyte allows the capacitance to be enhanced, corrosion of the positive stainless steel current collector to be inhibited and the AC/AC electrochemical capacitor to demonstrate stable performance up to 1.6 V.

1. Introduction Electrochemical energy storage systems like electrical double-layer capacitors (EDLCs), also known as supercapacitors, have received considerable attention in recent years.1–3 They are believed to be potential candidates to ll the energy gap Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland. E-mail: [email protected]

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between traditional accumulators and capacitors, and to fulll the demand for delivering peak power in various applications.4,5 One of the key research criteria for supercapacitors from an application point of view has been the enhancement of capacitance6–8 which in turn improves the energy density, E (J), as given by eqn (1): E ¼ (1/2) CU2

(1)

where C represents the capacitance (F g1) and U the voltage (V). Other driving forces have been the use of low cost, safe, environment friendly and efficient construction materials. Until recently, due to the use of acidic and basic electrolytes, it was believed that electrochemical capacitors based on activated carbon (AC) electrodes cannot work at voltages higher than 0.7–0.8 V in aqueous media.5 However, by using alkali sulphate solutions as electrolytic media, is has been demonstrated that they can operate with good cycle life at voltage values up to 1.6–2 V.9–11 Lithium sulphate (Li2SO4) stands out as the best salt among different alkali sulphates due to its high solubility and the good capacitance values of the systems. However, such good performance with these media has been achieved while using gold current collectors. For industrial viability, low cost collectors have to be used; considering the various possibilities, stainless steel seems to be the optimal choice. Our recent studies on AC/AC capacitors in lithium sulfate with stainless steel current collectors have suggested that corrosion products accumulate between the AC electrode and the positive stainless steel current collector during potentiostatic oating at voltages higher than 1.5 V, and could be partially responsible for the blockage of AC porosity, reducing the power output and lifetime of the device.12,13 Indeed, stainless steel may be corroded in an aqueous environment as a result of the production of iron oxides like Fe2O3 and Fe3O4. Such oxides are formed by gradual transformation of gFeOOH to acicular aFeOOH, and nally to sintered submicron powders of Fe2O3 and Fe3O4, resulting in the creation of new resistive elements in the equivalent electrical circuit.14,15 It has already been reported that the presence of MoO42 ions strengthens the hydrated iron oxide layer on the stainless steel surface in neutral aqueous solutions, inhibiting the corrosion provoked by aggressive anions like chlorides or sulfates.16,17 It has been proposed that in neutral aqueous media, MoO42 interacts with Fe2+ to form ferrous-molybdate, which transforms in the presence of dissolved di-oxygen to insoluble protective ferric molybdate,18 or that the nonoxidizing MoO42 forms a complex with iron which adsorbs on the metal surface.19 Therefore, the purpose of this work is to use Na2MoO4 as an additive to the salt aqueous electrolyte in order to improve the performance of AC/AC supercapacitors with stainless steel current collectors. By comparison with systems containing only 1 mol L1 Li2SO4, we show that the addition of Na2MoO4 improves the lifetime and performance of the supercapacitors by impeding the corrosion of the positive stainless steel current collector, as well as by giving higher capacitance values. A combination of electrochemical techniques, i.e. cyclic voltammetry, galvanostatic charge–discharge, chronoamperometry and electrochemical impedance spectroscopy has been used to understand the role of sodium molybdate and to analyze the performance of the supercapacitors. Faraday Discuss.

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2. Experimental The current collectors (diameter 1.2 cm) were made of a low carbon content stainless steel 316L alloy consisting of the following major elements: Fe, C (0.02%), Cr (16%), Ni (10%) and Mo (2%). Their surface was cleaned with emery paper (P1000) before the investigations. The effect of corrosion was studied by cyclic voltammetry at 10 mV s1, both in 1 mol L1 Li2SO4 and in a mixture of 1 mol L1 Li2SO4 and 0.1 mol L1 Na2MoO4, using a Teon Swagelok® type threeelectrode cell, with a stainless steel working electrode, gold counter electrode and Hg/Hg2SO4 (SME) reference electrode. The carbon electrodes for supercapacitor construction were prepared by using capacitor grade DLC Supra 30 activated carbon from Norit (SBET ¼ 2071 m2 g1 evaluated by Micromeritics ASAP 2020), named as AC from now on in this manuscript. Pellets, with a diameter of 1.0 cm and mass of 7–10 mg, were prepared by mixing AC (80 wt%), carbon black SUPER C65 (Timcal, 10 wt%) and PTFE (Sigma Aldrich, 10 wt%) as binder in 10 mL of isopropanol. The dough obtained aer mixing was rolled to a thickness of nearly 0.25 mm and was le to dry at 120  C for ten hours under vacuum. Later, disk pellets were punched-out from the rolled sheet of electrode material. Symmetric AC/AC systems were constructed using Teon Swagelok® type twoelectrode cells and stainless steel (316L) current collectors. A glassy microber lter (GF/A, Whatman™) of thickness 260 mm was used as separator. For investigating the electrochemical properties of the individual electrodes, we used a two-electrode setup equipped with a Hg/Hg2SO4 (SME) reference electrode; the same set-up was used for three-electrode investigations, one of the AC electrodes being selected as the working electrode. The experiments were performed in either 1 mol L1 Li2SO4 (pH ¼ 6.5, conductivity ¼ 64 mS cm1), or in a 1 mol L1 Li2SO4 + 0.1 mol L1 sodium molybdate (Na2MoO4, from Sigma Aldrich) mixture (pH ¼ 6.7, conductivity ¼ 72 mS cm1). The electrochemical properties of the capacitors were investigated by cyclic voltammetry (CV, 2 mV s1), galvanostatic cycling with potential limitation (GCPL, 200 mA g1) and impedance spectroscopy in the range of 1 mHz to 100 kHz, using a VMP3 multichannel potentiostat/galvanostat (Bio-Logic Instruments, France). The gravimetric capacitance C was calculated from the galvanostatic discharge of the two-electrode cells and expressed per electrode (F g1) using the formula (2): C ¼ (2)I/[(dU/dt)mam]

(2)

where I is the current (A), dU/dt is the slope of the discharge curve (V s1), and mam is the average mass of AC in one electrode (g). Accelerated ageing by potentiostatic oating has been applied to estimate the performance limits of the capacitors; in these experiments, the test cell is charged up to a certain voltage (in our experiments 1.6 V) and maintained at this value for periods of 2 hours at room temperature. Each period is followed by ve galvanostatic charge–discharge cycles at 1 A g1, in order to estimate the discharge capacitance and the equivalent series resistance (ESR) from the voltage drop between the 5th charge and discharge when reversing the sign of the current. A total of 60 such sequences were applied, corresponding to a total oating time of 120 hours. This journal is © The Royal Society of Chemistry 2014

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3. Results and discussion

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3.1. Current collector corrosion Fig. 1 represents the three-electrode cyclic voltammograms of stainless steel 316L in 1 mol L1 Li2SO4 and in 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4. For both systems, the potential scan in the positive direction shows the active dissolution region from 1.2 V to 0.65 V vs. NHE, and an anodic peak followed by a passive region. The dominant species in the active dissolution region may be ferrous hydroxide, which precipitates on the electrode surface as its concentration exceeds the solubility product.20 Further in the positive direction, Fe-hydroxide converts to different species between 0.65 V and 0.2 V vs. NHE, mainly Fe2O3 which is the major constituent of the passive layer.21 The passive layer formed on the stainless steel surface in 1 mol L1 Li2SO4 (black line) is due to oxides of iron and its alloying metals like Cr and Mo. This layer is thick in nature, protecting the stainless steel surface from corrosion caused by SO42 anion attack.22,23 Since the passive layer completely covers the electrode surface, it causes a fall in the anodic current density to a low value (jpass), leading to the passive region between 0.2 V and 0.9 V vs. NHE. A further potential increase in the positive direction shows a sudden current leap associated with the starting of pitting (Epit) at 0.95 V vs. NHE. This region is called the transpassive region, where the current suddenly rises and a further potential increase results in di-oxygen evolution.24 Adding Na2MoO4 to 1 mol L1 Li2SO4 results in a higher current for passive layer formation (red line) in the potential region between 0.65 V to 0.2 V vs. NHE; as a consequence the MoO42 anions strengthen the passive layer and its thickness increases. This is further supported by the fact that the passivation current density (jpass) in the case of Li2SO4 + Na2MoO4 is slightly lower than in the case of Li2SO4. Hence, the addition of MoO42 to Li2SO4 improves the passivation of the stainless steel, protecting it against pitting corrosion caused by the attack of SO42 anions and di-oxygen, which is produced due to water oxidation.

Three-electrode cyclic voltammograms of stainless steel 316L at 25  C (a scan rate of 10 mV s1) in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4. The dotted line (0.95 V vs. NHE) shows the onset of pitting corrosion. Fig. 1

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The explanation given above about the role played by Na2MoO4 in corrosion inhibition suggests that its addition to the 1 mol L1 Li2SO4 solution could improve the performance of supercapacitors containing stainless steel current collectors and salt aqueous electrolytes. 3.2. Electrochemical properties of the AC electrode Fig. 2 shows three-electrode cyclic voltammograms of AC with a gradual shi of the negative potential cut-off in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4. The vertical dotted lines at 0.383 V vs. NHE for Li2SO4 (pH ¼ 6.5) and 0.395 V vs. NHE for Li2SO4 + Na2MoO4 (pH ¼ 6.7) represent the thermodynamic potential values of water reduction in both media. In Li2SO4, at negative cut-off potentials higher than this value, the CVs display the typical rectangular shape related with charging the electrical double-layer. When the negative cut-off potential is lower than 0.383 V vs. NHE in Li2SO4, nascent hydrogen starts to be produced and is chemisorbed on the AC electrode until di-hydrogen evolution starts at approximately 0.8 V vs. NHE, as observed by the oscillations due to bubbling on the CVs (Fig. 2a).11,25 When the potential scan is reversed to the positive direction, reversible hydrogen desorption starts at 0.2 V vs. NHE.

Fig. 2 Three-electrode cyclic voltammograms of the AC carbon electrode recorded at 2 mV s1 in 1 mol L1 Li2SO4 (a), and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 (b). The successive loops are obtained by a stepwise decrease of the negative cut-off potential. The dotted vertical lines correspond to the thermodynamic water reduction potential. This journal is © The Royal Society of Chemistry 2014

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In Li2SO4 + Na2MoO4, the same sequence occurs with regard to electrical double-layer formation and reversible hydrogen storage, however new redox peaks are observed and, in the anodic region corresponding to hydrogen desorption (approximately 0.4 to 0.8 V vs. NHE), the hump is higher than in Li2SO4 (Fig. 2b). One also notices that hydrogen desorption is fully completed at 0.8 V in Li2SO4 + Na2MoO4, whereas at the same potential value in Li2SO4, the anodic redox current is still important. It suggests slightly different activation energies for the carbon–hydrogen bonds formed in the two electrolytic media.26 Notwithstanding, in Li2SO4 + Na2MoO4 electrolyte, the observable oscillations due to di-hydrogen evolution appear at a slightly lower potential than in Li2SO4, i.e. around 1 V vs. NHE. This is supported by the different pH values measured at the electrode/electrolyte interface, ca. 10 and 8 in Li2SO4 + Na2MoO4 and Li2SO4, respectively. Hence, according to the Nernst law, in the case of Li2SO4 + Na2MoO4, the shi in di-hydrogen formation is explained by the lower local reduction potential of water. Due to this lower potential, the oxidation peak is more pronounced in the Li2SO4 + Na2MoO4 electrolyte, suggesting that more hydrogen is stored in the AC electrode using this medium. The new peaks which appear in addition to the reversible hydrogen storage peaks in the Li2SO4 + Na2MoO4 electrolyte might be attributed to redox reactions involving the MoO42 anion (Fig. 2b). The Pourbaix diagrams of molybdenum27 show that the molybdate anions can be involved in the two redox equilibria (3) and (4): MoO42 + 4H+ + 2e 4 MoO2 + 2H2O

(3)

MoO42 + 8H+ + 6e 4 Mo + 4H2O

(4)

At pH ¼ 6.7, the equilibrium potentials are 0.215 V and 0.384 V for equilibria (3) and (4), respectively. In Fig. 2b, peaks cannot be found strictly at these positions during the negative scan, probably because of the already mentioned pH change on the negatively polarized electrode surface. Moreover, the oxidation of water at the positive electrode of the cell may also contribute to a locally decreased pH, leading, at pH < 6, to the transformation of MoO42 into HMoO4 and further into MoO3.27 Even if the origin of the new redox peaks shown by the AC electrode in the Li2SO4 + Na2MoO4 medium is not perfectly elucidated, there is clearly a splitting and an overall increase of the anodic peak, revealing an additional pseudo-capacitive contribution to the electrical double-layer (EDL) capacitance. Apart from the differences in pseudocapacitive behavior, differences in the EDL capacitance are also observable between the two electrolytes in the region from ca. 0.2 to 0.8 V vs. NHE (black curves). In Li2SO4 + Na2MoO4, the capacitance reaches 120 F g1 as compared to 98 F g1 for Li2SO4. The higher value in Li2SO4 + Na2MoO4 than in Li2SO4 is essentially attributable to the higher conductivity of the former electrolyte as compared to the latter. 3.3. Electrochemical performance of the AC/AC two-electrode cells The effects of adding Na2MoO4 to Li2SO4 were further investigated in a two electrode AC/AC cell equipped with a Hg/Hg2SO4 reference electrode. Fig. 3 shows, for the two electrolytic media, the potential limits of each electrode vs. NHE as function of cell voltage. In the case of Li2SO4 (pH ¼ 6.5), the maximum Faraday Discuss.

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Fig. 3 Potential limits of the positive and negative AC electrodes during the galvanostatic (200 mA g1) charge–discharge of the AC/AC cells up to different values of the maximum voltage in 1 mol L1 Li2SO4 (a), and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 (b) electrolytic media. E+ and E represent the maximum and minimum potentials reached by the positive and negative electrodes, respectively. The E0 values correspond to the rest potential of the electrodes after setting the cell voltage to 0 V.

potential of the positive electrode (E+) is equal to the thermodynamic oxygen evolution potential for a voltage of 1.35 V (Fig. 3a). It means that, if the supercapacitor containing Li2SO4 as an electrolyte operates at 1.6 V, oxygen evolution will denitely occur at the positive electrode, causing corrosion of the stainless steel current collector and oxidation of the AC. By contrast, for the negative electrode in the Li2SO4 electrolyte, the minimum potential (E) is well above the practical di-hydrogen evolution potential of approximately 0.8 V vs. NHE established from Fig. 2a, even if the maximum voltage applied is 1.6 V. Fig. 3(b) shows the potential limits in the presence of the Li2SO4 + Na2MoO4 electrolyte at various voltage values; at pH ¼ 6.7 of this medium, the oxygen evolution potential is 0.84 V vs. NHE. Interestingly, for a voltage of 1.6 V, the maximum potential of the positive electrode (E+) is at 0.798 V, and is shied by approximately 0.162 V as compared to Li2SO4 (0.96 V vs. NHE), lying well below the oxygen evolution potential. This should practically prevent AC oxidation and stainless steel current collector corrosion at the positive electrode. Indeed, the dotted line in Fig. 1 shows that the pitting corrosion starts at Epit  0.95 V vs. NHE, This journal is © The Royal Society of Chemistry 2014

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which is far above E+ ¼ 0.798 V in the Li2SO4 + Na2MoO4 electrolyte at a voltage of 1.6 V. By contrast, at 1.6 V in Li2SO4, the potential of the positive electrode reaches 0.96 V vs. NHE and the latter is susceptible to pitting as it operates at a potential slightly higher than Epit. Besides, the minimum potential E of the negative electrode is shied to lower values. However, even at a voltage of 1.6 V, the E value is still higher than the practical di-hydrogen evolution potential of approximately 1.0 V vs. NHE which was estimated in Fig. 2b. Table 1 shows the values of discharge capacitance for the positive and negative AC electrodes in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4. The capacitance values of the negative electrode are of the same order in the two electrolytic media; the marked increase at a voltage higher than 1.4 V is attributed to reversible hydrogen storage. The capacitance of the positive electrode is almost independent of voltage in Li2SO4, which is proof of typical EDL behavior. By contrast, for this electrode in the Li2SO4 + Na2MoO4 electrolyte, one can notice a remarkable increase of capacitance with voltage, due to pseudocapacitive contributions related to the presence of the molybdate anions. Nyquist plots measured at 0.8 V on a two-electrode AC/AC cell equipped with a reference electrode in Li2SO4 + Na2MoO4 are presented in Fig. 4a. The nearly vertical lines at low frequency indicate the good capacitance properties of the electrodes and capacitor. The AC/AC capacitor exhibits a low equivalent series resistance (ESR) of 0.34 U; the comparable ESR values of 0.17 U and 0.22 U for the positive and negative electrodes, respectively, conrm the dominant contribution of electrolyte conductivity to this parameter. The equivalent distributed resistance of the capacitor (EDR ¼ 3.75 U) is approximately the sum of the EDR values for positive (1.92 U) and negative (1.64 U) electrodes; such low values suggest a good propagation of the ionic species in the carbon pores. The charge transfer resistance Rct of the positive (1.41 U) and negative (1.36 U) electrodes is nearly the same, and low, suggesting a good contact at the electrode/current collector interfaces; the Rct measured for the capacitor (Rct ¼ 2.91 U) is approximately the sum of the values for the electrodes. The low frequency part of the capacitance vs. frequency plot at 0.8 V (Fig. 4b) is in good agreement with the data presented in Table 1, showing a much higher capacitance of the positive electrode, due to contributions of the EDL and pseudofaradic reactions associated with the molybdate anions. The capacitance of the negative electrode at 0.8 V is much lower than the positive one; as shown above, at this voltage, hydrogen storage is not involved, and only the EDL charging contributes to the capacitance value. Overall Fig. 4b shows that, as is predictable,

Capacitance values (F g1) of the positive and negative AC electrodes during the galvanostatic (200 mA g1) charge–discharge of the AC/AC cells, up to different values of the maximum voltage in 1 mol L1 Li2SO4 (a), and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 (b) Table 1

Li2SO4 + Na2MoO4 Li2SO4

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C+ (F g1) C (F g1) C+ (F g1) C (F g1)

0.8 V

1.0 V

1.2 V

1.4 V

1.6 V

124 76 96 77

131 83 98 82

145 90 99 86

158 96 104 91

165 107 102 102

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Fig. 4 Impedance spectroscopy data of an AC/AC cell in 1 mol L1 Li2SO4 + 0.1 mol L1

Na2MoO4 and of the individual electrodes at 0.8 V: (a) Nyquist plots; (b) capacitance vs. frequency.

the capacitance of the AC/AC capacitor is controlled mainly by the negative electrode. The cyclic voltammograms of the AC/AC capacitors in 1 mol L1 Li2SO4 and in 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 are compared at 0.8 V and 1.6 V in Fig. 5. For both voltage ranges, the capacitance is higher in Li2SO4 + Na2MoO4 than in Li2SO4; the enhanced capacitance at 0.8 V seems solely due to the redox processes involving the molybdate ions, whereas, at 1.6 V, reversible hydrogen storage also contributes. The slightly worse charge propagation of the system in Li2SO4 + Na2MoO4 up to 0.8 V as compared to Li2SO4 is attributed to the pseudo-faradic contribution of the molybdate anions. Up to 1.6 V, the charge propagation is comparable in both electrolytic systems, due to the dominant effect of reversible hydrogen storage.11 Fig. 6 shows plots of capacitance and efficiency (discharge time/charge time) vs. voltage obtained from galvanostatic charge–discharge of the AC/AC capacitors in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4. Whatever the maximum voltage, the capacitance is higher in Li2SO4 + Na2MoO4 than in Li2SO4; the capacitance difference between the two systems is around 10 F g1 at 0.8 V and it reaches 20 F g1 at 1.6 V, conrming a larger pseudo-capacitive contribution related to the presence of molybdate anions at high voltage. Overall, at 1.6 V, the This journal is © The Royal Society of Chemistry 2014

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Cyclic voltammograms (2 mV s1) of AC/AC capacitors in 1 mol L1 Li2SO4 and 1 mol L Li2SO4 + 0.1 mol L1 Na2MoO4 electrolytes up to 0.8 V (a), and 1.6 V (b). Fig. 5

1

capacitor in Li2SO4 + Na2MoO4 displays a capacitance of 121 F g1 as compared to 103 F g1 in Li2SO4. An efficiency higher than 98% is observed over the entire voltage range for the AC/AC capacitor in Li2SO4, and only up to 1.2 V in the

Fig. 6 Plots of capacitance and efficiency vs. maximum voltage from galvanostatic charge–discharge of the AC/AC capacitors in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4. Faraday Discuss.

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Li2SO4 + Na2MoO4 medium. At a voltage higher than 1.2 V, the efficiency in the later electrolyte decreases to reach 95% at 1.6 V. This decay in efficiency could be attributed to irreversible redox processes occurring due to the presence of MoO42 anions. Additionally, as shown in Fig. 2, the nature of the carbon–hydrogen bonds on the negative AC electrode slightly differs in both electrolytic media and this could also be responsible for the lower efficiency at 1.6 V. The evolution of capacitance with the value of the charging–discharging current applied is presented in Fig. 7 for the AC/AC capacitors in the 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 electrolytes. In the two media, the capacitance slightly drops up to ca. 1 A g1, and then it is almost constant up to 10 A g1. Over the entire current range, the difference between the values for the two kinds of electrolyte is around 15–20 F g1. Above 1 A g1, it is likely that the charging of the electrical double-layer is the main contribution to the capacitance.

3.4. Accelerated ageing of the capacitors by oating at 1.6 V The AC/AC capacitors in both types of electrolytes, without and with Na2MoO4, have been subjected to potentiostatic oating at 1.6 V over a total time of 120 hours. Every 2 hours, the values of capacitance and resistance were determined from a few galvanostatic cycles, and these are presented in Fig. 8. During the rst 20 hours, the capacitance slightly increases for both electrolytic systems, and then it remains almost constant over the next 100 hours for the Li2SO4 + Na2MoO4 electrolyte, while it slightly decreases aer a total oating time of 90 hours in the case of Li2SO4. The initial increase is attributed to the enhanced penetration of the ionic species into small micropores, provoked by the prolonged polarization at an elevated voltage.12,13 A similar trend in oating conditions has been also observed in ref. 28 for a capacitor in [EMIM][BF4] ionic liquid. The benecial effect of the Na2MoO4 inhibitor is more visible when considering the resistance (Fig. 8b). For the AC/AC capacitor in Li2SO4, the resistance rst decreases at the beginning of the oating, is then constant, and dramatically increases aer 90 hours of oating, which could be due to an increased electrode resistance, as described elsewhere.29 Both the oxidation of the AC at the positive

Fig. 7 Plots of capacitance vs. specific current (A g1) for the AC/AC capacitors in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 charged up to 1.6 V. This journal is © The Royal Society of Chemistry 2014

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Fig. 8 The evolution of capacitance (a), and resistance (b), during the floating of the AC/ AC supercapacitors in the 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 electrolytes at 1.6 V.

electrode and the accumulation of corrosion products between the AC disk pellet and the stainless steel current collector contribute to the increased resistance.13 By contrast, in the Li2SO4 + Na2MoO4 electrolyte, the resistance is constant over the whole oating time and is always smaller than in Li2SO4. This conrms that the presence of sodium molybdate prevents stainless steel corrosion and the accumulation of corrosion products at the positive electrode/current collector interface. As shown in Fig. 3b, the maximum potential of the positive electrode is dramatically reduced by the addition of sodium molybdate and these undesirable side effects no longer occur when the AC/AC capacitor operates up to 1.6 V. Cyclic voltammograms (CVs) have been recorded on the AC/AC capacitors in the two kinds of electrolytes aer prolonged oating at 1.6 V, and are shown in Fig. 9. A comparison with the CV obtained before oating in Fig. 5b reveals, in the case of Li2SO4, a drop of capacitive current at voltages higher than 0.8–1 V. Such narrowing of the CVs has been already observed for capacitors in organic electrolytes, and it was demonstrated that this is due to porosity saturation by the ions when the carbon material has a relatively moderate pore volume.30 As shown in Faraday Discuss.

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ref. 12 and 13 and discussed above, when the capacitor is charged up to or above 1.6 V, the positive electrode potential is so high that the AC is oxidized, leading to a dramatic decrease of pore volume due to the blockage of pore entrances by oxygenated groups; at the same time, the stainless steel collector is corroded, which causes a low charge propagation resulting in a lowering of the capacitance. In the case of the Li2SO4 + Na2MoO4 electrolyte, the CV aer prolonged oating (Fig. 9) nearly retains the same shape as for the fresh cell (see Fig. 5b), with good charge propagation up to 1.6 V. Once more, this is an indication that the capacitor with Li2SO4 + Na2MoO4 undergoes lesser ageing than the one with Li2SO4. As shown in Fig. 3b, at a voltage of 1.6 V in Li2SO4 + Na2MoO4, the potential of the positive electrode is well below the thermodynamic potential of water oxidation; consequently, under these conditions, the AC is not oxidized and the corrosion of the stainless steel does not occur. Nyquist plots of the AC/AC capacitors in Li2SO4 and Li2SO4 + Na2MoO4, before and aer 120 hours oating at 1.6 V, are shown in Fig. 10, and the equivalent series resistance (ESR), equivalent distributed resistance (EDR) and charge transfer resistance (Rct) values calculated from these plots are given in Table 2. In Li2SO4, the three parameters dramatically increase aer oating, whereas in Li2SO4 + Na2MoO4 they remain quasi-identical to the values before oating. The signicant increase of Rct resulting from the uneven current distribution aer oating in Li2SO4 is due to the corrosion products generated on the positive stainless steel current collector and/or oxidation of the AC at the positive electrode. By contrast, no signicant increase of Rct is observed aer oating in Li2SO4 + Na2MoO4, suggesting that the presence of molybdate anions prevents the formation of a low conductivity layer of corrosion products and the oxidation of the AC positive electrode. The performance of the capacitor in the Li2SO4 + Na2MoO4 electrolyte remains almost unaltered aer prolonged oating at 1.6 V. However, the capacitance vs. frequency plot (inset of Fig. 10b) displays a pseudocapacitive contribution at low frequency, probably due to (i) the generation of new functionalities on the AC surface during oating and/or (ii) the deposition of conductive MoO2 or Mo on the negative AC surface, as given by equilibria (3) and (4).

Fig. 9 Cyclic voltammograms (2 mV s1) of the AC/AC capacitors in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 after floating at 1.6 V for 120 h. This journal is © The Royal Society of Chemistry 2014

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Fig. 10 Nyquist plots of the AC/AC capacitors at 0 V before and after floating at 1.6 V for 120 hours in 1 mol L1 Li2SO4 (a), and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4 (b). The inset in (b) shows the frequency dependence of capacitance before and after floating.

From the preceding discussion, it is now clearly proved that the resistance decreases and the capacitance increases when sodium molybdate is added to lithium sulfate in AC/AC capacitors. Additionally, the capacitor using Li2SO4 +

Table 2 ESR, EDR and Rct values obtained from the Nyquist plots of the AC/AC capacitors in 1 mol L1 Li2SO4 and 1 mol L1 Li2SO4 + 0.1 mol L1 Na2MoO4, before and after floating at 1.6 V for 120 h

ESR (U) EDR (U) Rct (U)

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Before oating Aer oating Before oating Aer oating Before oating Aer oating

Li2SO4 + Na2MoO4

Li2SO4

0.43 0.51 1.6 1.7 0.72 0.77

0.45 1.1 3.3 10.2 1.25 8.02

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Na2MoO4 has the ability to withstand potentiostatic oating at 1.6 V for 120 hours without observable changes in resistance and capacitance. The effects of oating on the AC and the nature of the corrosion products generated during the ageing of the capacitors in the Li2SO4 + Na2MoO4 electrolyte are currently being investigated.

4. Conclusions Previous to this work, it was established that AC/AC electrochemical capacitors in alkali sulfate electrolytes operate with a good cycle life at voltages up to around 1.6–2 V. However, when stainless steel collectors were implemented in these systems, detrimental corrosion appeared on the positive collector, limiting the voltage to 1.5 V. In this paper, we have clearly demonstrated a dramatic improvement of cycle life at voltages up to 1.6 V when sodium molybdate (0.1 mol L1) is added to 1 mol L1 Li2SO4 aqueous electrolyte. Besides their known corrosion inhibition role, the MoO42 anions enhance the capacitance of the positive electrode through pseudo-faradic contributions. As a consequence, adding Na2MoO4 to 1 mol L1 Li2SO4 results in shiing the maximum operating potential of the positive AC electrode from 0.96 V vs. NHE (in Li2SO4) to 0.798 V vs. NHE (in Li2SO4 + Na2MoO4), preventing it from oxidation and pitting corrosion of the stainless steel collector when the capacitor is polarized up to 1.6 V. Electrochemical investigations on AC/AC capacitors in Li2SO4 + Na2MoO4 subjected to potentiostatic oating at 1.6 V for 120 hours prove that the performance remains nearly unaffected with almost no change in Rct. Besides, the capacitance of the AC/AC system reaches 121 F g1 in Li2SO4 + Na2MoO4 as compared to 103 F g1 for Li2SO4. Hence, sodium molybdate (Na2MoO4) is an additive of choice for supercapacitors using salt aqueous electrolytes and low cost stainless steel collectors. Further optimization of the electrolyte formulation allows energy density values comparable to those reached with organic electrolytes to be forecasted.

Acknowledgements The authors are grateful to the Foundation for Polish Science for supporting the ECOLCAP project realized within the WELCOME program, co-nanced from the European Union Regional Development Fund. Norit and Timcal are acknowledged for kindly providing the activated carbon and the carbon black, respectively.

Notes and references 1 B. E. Conway, Electrochemical Supercapacitors: Scientic Fundamentals and Technological Applications, Kluwer-Plenum, New York, 1999. 2 F. B´ eguin and E. Frackowiak, Supercapacitors: Materials, Systems and Applications, Wiley-VCH, Weinheim, 2013. 3 R. J. Miller and P. Simon, Science, 2008, 321, 651. 4 E. Frackowiak and F. B´ eguin, Carbon, 2001, 39, 937. 5 R. K¨ otz and M. Carlen, Electrochim. Acta, 2000, 45, 2483. 6 M. Ramani, B. Haran, R. White, B. Popove and L. Arsov, J. Power Sources, 2001, 93, 209. This journal is © The Royal Society of Chemistry 2014

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AC electrochemical capacitors in a salt aqueous electrolyte.

Sodium molybdate (Na2MoO4) has been used as an additive to 1 mol L(-1) lithium sulfate electrolyte for electrochemical capacitors based on activated c...
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