Phosphoric Acid Diethylmethylammonium Trifluoromethanesulfonate

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Phosphoric acid diethylmethylammonium trifluoromethanesulfonate based electrolytes for non-humidified intermediate temperature fuel cells Jie Yu, Shojiro Kikuchi, Hasna Puthen Peediyakkal, Hirokazu Munakata, and Kiyoshi Kanamura ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21231 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Materials & Interfaces

Phosphoric acid diethylmethylammonium trifluoromethanesulfonate - based electrolytes for non-humidified intermediate temperature fuel cells Jie Yu, Shojiro Kikuchi, Hasna Puthen Peediyakkal, Hirokazu Munakata, Kiyoshi Kanamura* Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, 192-0397 Tokyo, Japan

*Corresponding author's email address: [email protected]

Keywords：intermediate temperature fuel cell, PA_diethylmethylammonium trifluoromethanesulfonate mixed electrolyte, interaction, oxygen reduction reaction activity.

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Abstract: The present study reports a new series of electrolytes for non-humidified intermediate temperature fuel cells. This series of new mixed electrolytes, composed

of

phosphoric

acid

(PA)

and

diethylmethylammonium

trifluoromethanesulfonate ([Dema][TfO]), were designed as non-humidified intermediate temperature fuel cell electrolytes. The mixed electrolytes show a higher thermal stability than pure PA, which is dehydrated at intermediate temperatures. The thermal stability of the mixed electrolytes could be explained by the interaction between the triflate group in [Dema][TfO] and phosphoric acid, as indicated by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) spectroscopies. On the other hand, the ionic conductivity and proton transference number of the mixed electrolytes were similar to those of the pure PA. However, the oxygen reduction reaction activity of a platinum catalyst is significantly enhanced in the mixed electrolytes, which was due to the several orders of magnitude increase in oxygen solubility by the addition of [Dema][TfO] to PA. Specifically, for the equimolar fraction mixed electrolyte, the diffusion coefficient and the solubility of oxygen were ca. 1.47 × 10−5 cm2 s−1 and

ca. 1.28 mmol dm−3 at 150 ℃, respectively. The addition of [Dema][TfO] to PA could significantly enhance the oxygen reduction reaction activity. Therefore, the PA_[dema][TfO] mixed electrolyte can be one of the solutions to develop nonhumidified intermediate fuel cell electrolytes.　


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1. Introduction

Due to the environmental problems and energy crisis, many countries in the world have intensified their research on new energy sources. Fuel cells are regarded as one of the most promising new energy devices1, which can convert electrochemical energy to electricity and heat with almost zero emissions. Especially, polymer electrolyte fuel cells (PEFCs) are ideal for Combined Heat and Power (CHP) generation2-7, which have been used as clean power generation systems for households and vehicles because of their high energy conversion efficiency and low emissions. For decades, perfluorosulfonic acid (PFSA) polymers, such as DuPont’s Nafion®, have been used as electrolyte membranes in the PEFCs for decades due to their high proton conductivity, excellent chemical stability and superior durability8. However, the electrolyte membranes require water, which limits the operating temperature to below 100 °C. As a result, a large amount of heat is unutilized. Also, in order to maintain the high humidity, a large humidification system is necessary9-11. In addition, this kind of PEFC has a very low tolerance for carbon monoxide in the fuel, thus 99.99999% pure hydrogen is needed, which is very costly12-13. Moreover, the low operating



temperature leads to low electrochemical reaction activity of oxygen on the gas diffusion electrode (GDE)14-20. The intermediate temperature (100 °C ~ 200 °C) FCs (IT-FCs) are the solution to all these previously mentioned problems. They could be operated without humidification, with a high tolerance to CO (3% at 160 °C), and high catalytic activities. In addition, the waste heat can be more easily utilized21-23. For instance, the generated heat energy can be used for reforming methanol to hydrogen. These advantages result in an increased efficiency, which is expected to be close to 80% and a simplification of the system24-27. To realize such an FC system, new electrolytes applicable to intermediate temperatures (ITs) above 100 ºC without humidification are required. Meanwhile, phosphoric acid (PA) has been investigated and used as an electrolyte for the IT-FCs28-29. However, the release of PA from the membrane, low oxygen reduction reaction (ORR) activity and dehydration of PA are the main issues to be addressed30-34. On the other hand, protic ionic liquids (PILs) have unique properties such as a high ionic conductivity, high thermal stability, wide electrochemical potential window and non-volatility35-36. Several groups have already found that excellent ORR activities can be obtained using these ionic liquids37-38. However, their proton conductivity and proton transference number require further improvement for practical use39. Thus, in this study, we focused


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on mixing a PIL with PA to form a new mixed electrolyte, and investigated its thermal stability and electrochemical properties. 2. Experimental section 2.1.

Preparation of PA_[Dema][TfO] mixed electrolytes Diethylmethylamine ([Dema] 98% Tokyo Kasei, Ltd.), trifluoromethanesulfonic acid ([TfOH] 98% Tokyo Kasei, Ltd.) and ortho-Phosphoric acid (PA 99.999% Aldrich Corp.) were used as received without any pretreatment. The [dema][TfO] was prepared by direct neutralization of the Brønsted acid ([TfOH]) and base ([Dema]) in an equimolar ratio, and then vacuum dried at 100 ºC for 48 h38. Consequently, the PA was mixed in different mole ratios with [Dema][TfO] at 80 ºC in an argon glove box, stirring for 20 minutes.

2.2.

Characterization Thermogravimetry (TA-60WS, Shimadzu) was conducted in a nitrogen atmosphere to investigate the thermal stability of the PA_[Dema][TfO] mixed electrolytes. Each sample was heated from room temperature to 600 ℃ at the heating rate of 5 ℃ min-1. The 1H-NMR spectrum was collected at 25 °C using a JEOL JMN-ECS300 spectrometer. The frequency of the spectrometer was no-lock during data acquisition because there was no deuterated solvent. All the 1H chemical shifts were externally referenced to the tetramethylsilane (TMS) peak of a mixture of 1% TMS in CDCl3 shortly before the spectra were collected.



Fourier transform infrared spectroscopy (FT-IR) was carried out using a 670 Plus spectrometer (JASCO Co.) and a Triglycine sulfate (TGS) detector for the samples under a nitrogen atmosphere. The measurement range was 400 cm-1 to 4000 cm-1, the resolution was 8.0 cm-1, and the number of integrations was 128. The PA_[Dema][TfO] mixed electrolytes were placed in a glass cell with two platinum tablets fixed at a constant electrode distance. The cell instrument setup was shown in Supporting Information (Figure S1). The conductivities of the PA_[Dema][TfO] mixed electrolytes were determined employing the A.C. impedance method at different temperatures from 100 to 200 °C under a nitrogen atmosphere using an ALS-660B(CH Instruments). The frequency range was from 100 Hz - 100 kHz and the A.C. amplitude was 5 mV. The cell constant was determined using a 0.1 M KCl standard aqueous solution and was 1.476 cm-1. 2.3.

Electrochemical Properties The electromotive force measurement was conducted by the method of concentration cell40. Two reversible hydrogen electrodes (RHEs) were placed in a PFA cell containing different mole ratios of the PA_[Dema][TfO] mixed electrolytes, and the cell setup is shown in the Supporting Information (Figure S2). One RHE was employed with 0.1 MPa hydrogen gas. The other RHE was employed with 0.1 MPa mixed gas of hydrogen and nitrogen for controlling the partial pressure of the hydrogen. The electromotive force ΔE was obtained between the two RHEs, by measuring the open circuit potential using an


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ALS-660B (CH Instruments) at 180 °C. The proton transference number tH+ was calculated by the Nernst equation (1) as follows: ΔE = [(2 tH+-1) RT / F] ln (P2 / P1)

(1)

where R is the molar gas constant; T is the measuring temperature; F is the faraday constant; P1 is the partial pressure of hydrogen in the hydrogen and nitrogen mixed gas; and P2 is the pressure of the hydrogen gas. Electrochemical measurements were conducted using a three electrodes system (Figure S3). A Pt disk (φ= 5 mm, BAS) embedded in poly ether ether ketone (PEEK) and a Pt microelectrode (φ= 25 μm, BAS) were used as the working electrode, respectively. A Pt mesh and reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. Before the electrochemical measurements, the Pt disk electrode was polished using 0.05 μm alumina powder on a polishing felt pad, then ultrasonically washed in deionized water for 10 minutes. The deoxygenation of the PA_[Dema][TfO] mixed electrolyte was conducted by nitrogen gas bubbling. The potential sweep measurements were conducted using an electrochemical analyzer (ALS-660A, CH Instruments) under an oxygen atmosphere at 100 °C, 120 °C, 150 °C and 180 °C. The oxygen concentration c and diffusion coefficient D in PA_[Dema][TfO] mixed electrolyte was calculated by the method as follows. The diffusion-controlled voltammetric peak current for an n-electron (n = 4) reaction at a regular disk electrode (φ= 5 mm) was



expressed as jp = 0.446 Fnc (FDv/RT) 1/2

41,

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then the slope of jp / v1/2 should be related to

c and D as follows: slope = jp / v1/2 = 0.446 Fnc (FD / RT) 1/2

(2)

In order to calculate the slope, the scan rate was measured at 10 mV s-1, 30 mV s-1, 50 mV s-1, 70 mV s-1 and 100 mV s-1. These measurements were conducted using the regular working electrode (φ= 5 mm) at the temperatures given above. The steady-state limiting current at the microelectrode was given by IL= 4FncDa

(3)

where a is the radius of the microelectrode. IL was confirmed as the intercept of the current measured by the microelectrode related to the square root of the scan rates (2 mV s-1, 4 mV s-1, 6 mV s-1, 8 mV s-1 and 10 mV s-1). 3. Results and discussion 3.1 Thermal properties


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Fig. 1 Thermal properties for different mole ratios of the PA_[dema][TfO] mixed electrolyte, (a) Thermogravimetric analysis and (b) Decomposition temperatures analysis.

Figure 1(a) shows the thermogravimetric (TG) curves for the different mole ratios of the PA_[dema][TfO] mixed electrolytes. For comparison, the TG curves of [dema][TfO] and PA were also provided. It was confirmed that [dema][TfO] was thermally stable at 200 ℃ while the loss weight ratio of PA at 200 ℃ was as high as 6.4% (Figure S4). When [dema][TfO] in the mixed electrolyte was more than 25 mol. %, the weight loss sharply decreased, which could be understood by plotting the decomposition temperature vs. mol. % of [dema][TfO] in the mixed electrolytes as shown in Figure 1(b). The weight loss of PA started at ~107 ℃, while that of [dema][TfO] started at ~350 ℃. The decomposition temperature of the mixed electrolytes increased with the addition of [dema][TfO]. When the content of [dema][TfO] exceeded 25 mol. %, the decomposition temperature was higher than 200 ℃. On the other hand, to check the ion exchange reaction between PA and [dema][TfO] occurred or not, the protic ionic liquid of [dema][H2PO4] was synthesized by the neutralization reaction of [dema] and PA. From the Figure S5, the decomposition temperature of [dema][H2PO4] was 148 ℃ which indicated there was no [dema][H2PO4] existed in the PA_[dema][TfO] mixed electrolytes. In order to explain why the dehydration reaction of PA was suppressed by the addition of [dema][TfO], a more detailed characterization based on FT-IR and 1H-NMR spectroscopies was carried out.



3.2 FT-IR analyses It is known that the dehydration of PA occurs by heating, two hydroxyl groups of the neighboring PA molecules react with each other to form pyrophosphoric acid and water42. In order to suppress the dehydration, the triflate anion, which has a similar tetrahedral structure, was chosen as the anion of the protic ionic liquid for interaction with the PA molecules. The molecule’s structure was illustrated by using J-mol Gaussian (Figure S6). The O-H bond length was stretched from 0.099 nm to 0.11 nm when the [dema][TfO] anion approached the PA. This result indicated that there was an interaction between the hydroxyl group of PA and the triflate group of [dema][TfO]. In order to check the interaction of the [dema][TfO] and PA, an FT-IR experiment was employed. The FT-IR spectra comparing the wavenumbers from 600 cm-1 to 1400 cm-1 of the mixed electrolyte with that of the pure [dema][TfO] and pure PA are shown in Figure 2(a). The S=O symmetric stretching vibration frequency peak in the FT-IR spectrum for the pure [dema][TfO] was located at 1028 cm-1

43-45. The peak around 1220 cm-1 could be assigned

to δ C-F45. Compared with the PA_[dema][TfO] mixed electrolyte, only the absorbance changed with the different mole ratios of [dema][TfO] in the mixed electrolyte. However, the peak at 1150 cm-1, which was assigned to vs SO2, showed a blue shift when the concentration of [dema][TfO] was reduced in the mixed electrolyte43-45. On the other hand, for the pure PA, the peak around 935 cm-1 could be assigned to the P(OH)2 antisymmetric stretching vibration46-49. The vas [P(OH)2] peak also showed a blue shift compared to that


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of the PA_[dema][TfO] mixed electrolyte. This result indicated that there was an interaction existing between the phosphate groups and triflate groups. The blue shift of the peaks showed that the bond energy of mixed electrolyte became higher than that of the pure IL or PA. Thus the interaction could be assigned to the triflate groups in the [dema][TfO] with PA.

Fig. 2 Characterization of the PA_[dema][TfO] mixed electrolytes (a) FT-IR spectra with different mole ratios at room temperature in the region of 600-1400 cm-1; (b) 1H-NMR spectra with different mole ratios at room temperature.

3.3 1H-NMR analyses 1H-NMR

studies also confirmed the interaction of [dema][TfO] and PA, as shown in

Figure 2(b). The resonances below 4 ppm labeled with “a, b and c” can be associated with the C-H bonds of the [dema] cation. The peaks labeled with “d” are resonances associated with the N-H bond for the [dema] cation. The peak “e” which is associated with the O-H bond for PA which is associated with the O-H bond for PA. The peaks “d” undergo a



chemical shift (upfield) when the PA ratio increased (Figure S7(b)). This result of N-H bond in [dema]+ became stronger indicated [TfO]- must be an attractive interaction with PA. On the other hand, the peak “e” had a maximum chemical shift of 0.06 ppm for a 2:3 mixture of PA to [dema][TfO] (Figure S7(a)). The maximum chemical shift close to equal mole ratio of PA and [dema][TfO] showed strongest interaction among different ratios of mixed electrolyte. With the addition of [dema][TfO], the O-H bond then exhibited an upfield chemical shift, but this did not mean a weakening of the interaction because the continuous upfield chemical shift of the N-H bond was confirmed. When more than one triflate existed around one PA molecule for the interaction, the O-H bond upfield chemical shift occurred, which can be explained by the Cage Effect50. Both the weakened O-H bond in PA and enhanced N-H bond in the ionic liquid can be explained by interactions existing between the phosphate groups and sulfonic groups in the mixed electrolyte, consistent with the FT-IR results.

3.4 Ionic conductivity The ionic conductivity of the PA_[dema][TfO] mixed electrolytes was investigated in the temperature range from 100 to 200 ℃ under non-humidified conditions. Figure 3(a) shows the Arrhenius plots of the ionic conductivities. All the electrolytes with different mole ratios of the PA_[dema][TfO] mixed electrolytes exhibited increasing conductivities with temperature. The lowest conductivity was found in the mixed


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electrolyte having an equimolar ratio of the mixed electrolyte, which agreed with the 1HNMR results. The interaction inhibited the dissociation of protons from PA and increased the size of anions in the mixed electrolytes. As well know, low viscosity is favorable as it enhances the ionic conductivity in the electrolytes 37. The anion size effects the viscosity, so the conductivity of mixed electrolytes were lower than PA or [dema][TfO], could be explain as the mixed electrolytes showed higher viscosities than either PA or [dema][TfO] (Figure S8). However even for the equimolar ratio mixed electrolyte, the ionic conductivity was still as high as 60.6 mS cm-1 at 180 ℃. The activation energies of the different ratios were calculated in the range of 9.8 kJ mol-1 to 21.4 kJ mol-1 as shown in Figure 3(b). When the types of cation were considered, the imidazolium cation based ionic liquids, such as 1-ethyl-3-methylimidazolium ([EMIm]), have higher ionic conductivities as Watanabe et al. reported

37.

But in addition, the proton transference

number (tH+) was an important parameter for fuel cell electrolytes. The [EMIm] based ionic liquids were belonged to aprotic ionic liquids. For the protic ionic liquid, such as [dema][TfO], the N-H bond of the cation could provide a proton for transference by both the Grotthuss mechanism and Vehicle mechanism, even though the low tH+ values limit the application of protic ionic liquid on non-humidified fuel cells39. On the other hand, the tH+ of PA was high in the range of 0.98 - 0.91 from 42 ℃ to 150 ℃51-52.The tH+ was measured for different mole ratios of the PA_[dema][TfO] mixed electrolyte as shown in Figure 3(c) at 180 ℃. With the addition of [dema][TfO], the tH+ value gradually decreased with the increasing [dema][TfO] ratio to 40 mol. % in the mixed electrolyte, because the



tH+ value was mainly supplied by dissociation of the rich PA in the bulk. The tH+ value rapidly decreased around the [dema][TfO] ratio of 40 mol. % to 60 mol. %, then became a constant value at ca. 0.62. This result could be explained by the lack of free H+ by equimolar mixing of [dema][TfO] and PA, namely a 50 mol. % addition of [dema][TfO] to the mixed electrolyte; when the amount of [dema][TfO] was more than 50 mol. %, the tH+ value was decided by the rich N-H bond provided by [dema][TfO].

Fig. 3 (a) The Arrhenius curve of PA_[dema][TfO] mixed electrolytes with temperature range from 100 ºC to 200 ºC; (b) Activation energy (Ea) as a function of molar ratio of PA_[dema][TfO] mixed electrolytes; (c) Proton transference number, tH+, for different mole ratios of PA_[dema][TfO] mixed electrolytes at 180 ℃, by measurement of using a concentration cell.

3.5 ORR activities At 180 ℃, with the addition of [dema][TfO], the increase in the ORR activity was confirmed by the cyclic voltammograms in Figure 4. The equimolar fraction mixed electrolyte showed the high open circuit potential (OCP) of 1.05 V (vs. RHE), in which


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the values were 1.03 V35 and 1.06 V for the pure [dema][TfO] and PA, respectively, under similar conditions. It was found that with the increasing temperatures, the ORR activities increased as shown in Figures 5(a) and (b). A linear relationship between jp and v1/2 was found to go through the origin of the coordinates. This suggests that the oxygen reduction reaction on the Pt disk electrode is mainly diffusion controlled. On the other hand, by using a microelectrode of Pt in Figures 5(c) and (d), a leaner relationship between jp and v1/2 was also found. The limiting current value IL should be the intercept of the y-axis41. As a result, by using equations (2) and (3) in the experimental section, the c and D values of oxygen in the PA_[dema][TfO] mixed electrolyte were calculated.

Fig. 4 Cyclic voltammograms of Pt disk electrode in pure PA and PA_[dema][TfO] mixed electrolytes, measured at 180 ºC under an O2 atmosphere at 50 mV s-1.



Fig. 5 Variations of jp with v1/2 for anodic voltammograms of (a) PA_0.5[dema][TfO] mixed electrolyte, (b) PA_[dema][TfO] mixed electrolyte; IL with v1/2 for anodic voltammograms of (c) PA_0.5[dema][TfO] mixed electrolyte, and (d) PA_[dema][TfO] mixed electrolyte, respectively.

Figure 6(a) shows the temperature dependence of the oxygen solubility in the PA_[dema][TfO] mixed electrolytes. In general, the oxygen solubility in the mixed electrolyte decreased with the increasing temperature which is a similar phenomena in aqueous solutions and ionic liquids39. In addition, the low ORR activity of Pt in PA has been well known53. We noticed that the solubility had increased one order of magnitude with the addition of [dema][TfO] to PA. The solubility of oxygen in PA was 0.11 mmol dm-3, while the value in the equimolar fraction mixed electrolyte was 1.28 mmol dm-3. This improvement can be explained by the high solubility of oxygen due to the C-F chains of the protic ionic liquids54-58. Figure 6(b) shows the Arrhenius plots of the oxygen diffusion coefficient in the PA_[dema][TfO] mixed electrolytes. The oxygen diffusion


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coefficient generally increased with the increasing temperature, while the oxygen solubility decreased with the increasing temperature in the PA_[dema][TfO] mixed electrolyte. The order of 10-5 cm2 s-1 D values in both the PA and PA_[dema][TfO] mixed electrolytes were found in the intermediate temperatures domain. This can be explained by the Stokes-Einstein equation with the similar viscosities of PA and [dema][TfO].

Fig. 6 (a) Temperature dependences of the O2 solubility; (b) Arrhenius plots of O2 diffusion coefficient; (c) Temperature dependences of the transmission coefficient c*D value in PA and PA_[dema][TfO] mixed electrolytes.

The product of c and D as the transmission coefficient of oxygen is shown in Figure 6(c). For a liquid based electrolyte, a high value means that a high crossover of oxygen occurs in the fuel cells. At 150 ℃, it was ca. 5 times higher than that in PA. However, the value was still lower than 18.7 × 10-12 mol cm-1 s-1 of the Nafion ®117 membrane electrolyte at 80 ℃59. The replacement of PA with the mixed electrolytes is expected to improve the electrochemical performance of the currently used PA-based fuel cells. However, a new design of fuel cells may be required to utilize the advantageous properties of the mixed



electrolytes since the direct use of liquid type electrolytes is basically difficult. The combination with porous scaffolds is one of prospective ways. 4. Conclusions In this study, the characteristics of a PA_[dema][TfO] mixed electrolyte as a proton conductor for intermediate temperature fuel cell electrolytes were described. The thermal stability was confirmed by a thermogravimetric analysis. Characterization and explained by the results from FT-IR and 1H-NMR analyses. For the electrochemical properties, even the PA_[dema][TfO] mixed electrolyte showed slightly lower conductivities and proton transference number than those in PA. On the other hand, with the addition of [dema][TfO], the higher ORR activities were confirmed by the cyclic voltammograms. The calculated solubility of oxygen was an increased one order of magnitude with the addition of [dema][TfO]. The oxygen solubility in PA was 0.11 mmol dm-3, while the value in the equimolar fraction mixed electrolyte was 1.28 mmol dm-3 at 150 ℃. The high thermal stability and high ORR activity of the mixed electrolytes encouraged us to construct intermediate non-humidified fuel cells as electrolyte materials. Acknowledgement This study was supported by “Platform for Technology and Industry” of the Tokyo Metropolitan Government. We thank Prof. Sato Kiyoshi for assistance with the 1H-NMR analyses.


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Supporting information Experimental instruments, TGA analysis, molecular simulation, 1H-NMR analysis, viscosities and cyclic voltammogams.



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Phosphoric Acid Diethylmethylammonium Trifluoromethanesulfonate

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