Toward Tailoring of Electrolyte Additives for Efficient Alkaline Water

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Towards tailoring of electrolyte additives for efficient alkaline water electrolysis: salicylate-based ionic liquids Luís Amaral, Justyna Minkiewicz, Biljana Šljuki#, Diogo M.F. Santos, César A.C. Sequeira, Milan Vranes, and Slobodan Gadzuric ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00858 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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ACS Applied Energy Materials

Towards tailoring of electrolyte additives for efficient alkaline water electrolysis: salicylate-based ionic liquids

Luís Amaral,†,* Justyna Minkiewicz,† Biljana Šljukić,† Diogo M.F. Santos,† César A.C. Sequeira,† Milan Vraneš,‡ Slobodan Gadžurić‡

†

Center of Physics and Engineering of Advanced Materials (CeFEMA), Instituto Superior Técnico,

Universidade de Lisboa, 1049-001 Lisbon, Portugal ‡

Faculty of Science, Department of Chemistry, Biochemistry and Environmental Protection,

University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia *

Corresponding author email: [email protected]

Abstract The effect on the hydrogen evolution reaction (HER) of adding small amounts of three home-made

salicylate

([Sal])-based

ionic

liquids

(ILs),

specifically

1-butyl-3-

methylimidazolium salicylate ([Bmim][Sal]), 1-(3-hydroxylpropyl)-3-methylimidazolium salicylate ([C3OHmim][Sal]) and imidazolium salicylate ([Im][Sal]), to KOH electrolyte solutions is investigated. Density, viscosity and electrical conductivity of these non-toxic ILs are determined, further enabling determination of their ionicity. Electrochemical characterization is performed using platinum electrodes in 8 M KOH solution and after addition of 1 vol.% of each ionic liquid. The measurements in the IL-added electrolytes showed higher currents and a small effect on the activation energy. Tafel analysis and electrochemical impedance spectroscopy measurements show the Volmer reaction as the rate-determining step of the HER, as well as a strong decrease of the overall impedance in 1 ACS Paragon Plus Environment

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the ILs-added electrolytes, which allow stable operation. The charge transfer and the polarization resistances are substantially reduced in the IL-added solutions, particularly after [Bmim][Sal] addition, with the effect of the additives becoming less important as the temperature increases. A Langmuir-type monolayer adsorption of Hads to the electrode surface is observed. This work shows that adding small amounts of selected ILs to the KOH electrolyte may enhance the HER in alkaline media, supporting the goal of designing efficient ionic liquid additives for water electrolysis.

Keywords: ionic liquids; electrolyte additives; alkaline water electrolysis; hydrogen evolution reaction; impedance spectroscopy.

Introduction Current growing global energy demands and the need for sustainability drive the world to an energy paradigm shift. One possible direction is towards the hydrogen economy1 and water electrolysis is certainly the cleanest method for hydrogen production.2-4 Hydrogen produced by water electrolysis is seen as the energy vector of a truly green and efficient energy system if the electricity used in the process comes from renewable sources.2,5 However, the overall efficiency of water electrolysis is still low, meaning that high amounts of energy input are needed to reduce the electrode overpotentials and ohmic drop in the electrolyte.2,6 Corrosion of the electrodes,7 as well as the removal of gas bubbles from the electrodes surface,8 thus avoiding the decrease of active surface area, are also issues of concern. Several approaches have been envisaged to tackle these issues, mainly focusing on

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the development of novel electrocatalysts,9-24 improvement of electrolyte properties25-30 and optimization of the operation conditions.31,32 The use of ionic liquids (ILs) as replacement of conventional aqueous electrolytes or as a mixture with those has been receiving increasing attention25-28,33 due to their effects on the intermolecular interactions and consequently on the electrode-electrolyte interface.26,34 IL is a common designation for semi-organic salts composed of organic cations and organic or inorganic anions, often liquid around room temperature, generally characterized by a wide range of fluidity, high ionic conductivity, excellent thermal and chemical stability, high heat capacity and high cohesive energy density.33-35 Additionally, one interesting feature is that their properties may be tailored by combining appropriate cations and anions from a wide variety of species.36,37 Making use of these characteristics, ILs can form stable, ionconductive suspensions used in many electrochemical applications such as solar cells,38,39 electrochemical sensors,40-42 fuel cells,43,44 double-layer capacitors,45-48 lithium batteries,4851

CO2 electroreduction,52-56 synthesis and functionalization of electrocatalysts,44,54,57 or as

electrolytes for water electrolysis.25-28,58 Thus, 1-butyl-3-methylimidazolium tetrafluoroborate, [Bmim][BF4], and 1-butyl-3methylimidazolium hexafluorophosphate, [Bmim][PF6], have been investigated as electrolytes for water electrolysis using different electrodes.25,28,58 The highest H2 production rates were obtained in solutions containing 10 vol.% of the ILs, which was correlated with the increased ionic conductivity of the electrolyte (as well as increased viscosity of the electrolyte and the formation of bubbles on the electrode surface for high IL concentrations),25 or with the enhancement of proton-donating properties on the IL preadsorbed surface along with decreased hydrogen evolution reaction (HER) activation energy.28,29 Improvements in the HER were also observed using [TEA-PS][BF4] (33 ACS Paragon Plus Environment


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triethylammonium-propanesulfonic acid tetrafluoroborate),59,60 attributed to the formation of an electroactive IL layer on the cathode and to high conductivity of the aqueous IL media and correspondent facilitated proton transport. Enhancement of the HER in [NTf2]based ILs was also reported, which was not observed in [C6mim][Cl] or [Bmim][NO3], confirming the important role of anion protonation in the HER in ILs electrolytes.61,62 Moreover, small amounts of [Emim]-based ILs were recently tested as electrolyte additives for the HER.26,27 The conductivity of the electrolyte was increased after the addition of the [Emim]-containing ILs. Increased currents were observed, as well as a significant decrease of the overall impedance in the ILs-added solutions, particularly after addition of [Emim][MeSO3]. The positive effects on the HER were rationalized as a result of surface pre-adsorption of the IL additives, stabilizing the intermediate hydrogen atoms and modifying adsorption and charge transfer processes at the metal-electrolyte interface.26,27 In the present work, salicylate-based ILs are prepared and investigated as electrolyte additives for hydrogen evolution in alkaline media. The studied ILs, 1-butyl-3methylimidazolium salicylate ([Bmim][Sal]), 1-(3-hydroxylpropyl)-3-methylimidazolium salicylate ([C3OHmim][Sal]) and imidazolium salicylate ([Im][Sal]), share the same anion and have different cations in their compositions. This allows the evaluation of the effect of the salicylate anion, as well as of the several cations and their combined effects, on the HER kinetics, which were not previously reported. Indeed, [Bmim]+ cation was successfully used in electrolytes for water electrolysis,25,28,43,58 but in combination with anions different than those used in this work. Regarding the other two cations, [C3OHmim]+ and [Im]+, as well as the salicylate anion, reports on their effect on the HER are scarce and their effects unknown. Linear scan voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) measurements were performed in 4 ACS Paragon Plus Environment

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the 25 – 80 ºC temperature range, using a Pt foil electrode in 8 M KOH electrolyte, with and without the addition of 1 vol.% of salicylate-based ILs.

Experimental section Preparation and characterization of the ILs All chemicals for ILs synthesis were used without purification as purchased from the manufacturer: 1-methylimidazole (Sigma Aldrich, ω ≥ 0.99), 3-chloro-1-propanol (Sigma Aldrich, ω ≥ 0.98), ethyl acetate (Sigma Aldrich, ω ≥ 0.998), sodium salicylate (Reanal, ω ≥ 0.995), imidazole (Sigma Aldrich, ω ≥ 0.99), hydrochloric acid (Sigma Aldrich) and acetone (Lachner). Three different salicylate-based ionic liquids (Figure 1) were synthesized, 1-butyl-3-methylimidazolium salicylate, [Bmim][Sal], 1-(3-hydroxylpropyl)3-methylimidazolium salicylate, [C3OHmim][Sal], and imidazolium salicylate, [Im][Sal], starting from the corresponding chloride salts as it is described elsewhere.63,64 Obtained chloride ionic liquids were converted into salicylates by the addition of the equimolar amount of sodium salicylate using acetone as a solvent. Resulting solution was stirred and refluxed at room temperature for 12 h. After that, the white precipitate (NaCl) was removed and the acetone solution of the ionic liquid was obtained. Acetone was removed by evaporation under vacuum at 70 ºC for 1 h achieving the constant mass. Water content was determined by Karl-Fischer titration and chloride content by ion chromatography. Physicochemical characterization of pure ionic liquids (density, viscosity and electrical conductivity) was performed in order to get more information about ionicity of the studied ILs and their behavior as media for hydrogen evolution and water electrolysis. Pure



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[Im][Sal] has a high melting point and therefore its characterization by the same methods as the other two ionic liquids could not be properly performed.

Figure 1: Chemical structure of the studied ILs sharing the same anion: A) 1-butyl-3methylimidazolium

salicylate

([Bmim][Sal]),

B)

1-(3-hydroxylpropyl)-3-

methylimidazolium salicylate ([C3OHmim][Sal]) and C) imidazolium salicylate ([Im][Sal]).

Density. The vibrating tube Rudolph Research Analytical DDM 2911 densimeter with precision of ±0.00001 g cm–3 was used for density measurements. The instrument was thermostated (Peltier-type) within ±0.01 ºC and viscosity was automatically corrected. Before each series of measurements, calibration of the instrument was performed at the atmospheric pressure. Each experimental density value is the average of at least three


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measurements at temperatures from 20 to 50 ºC. Standard uncertainty of determining the density is less than 5 × 10–4 g cm–3.

Viscosity. The viscosity of the investigated ionic liquids was measured using a Brookfield Viscometer DV II + Pro thermostated within ±0.01 K and filled with about 16 cm3 of pure ionic liquid. The spindle type (LV-4) was immersed and rate per minute (RPM) was set to obtain a suitable torque. A viscometer cell was protected from the moisture with the compartment made by the manufacturer and calibrated using the liquids of different viscosities purchased from the manufacturer. The viscosity of investigated ionic liquids was measured in the temperature range from 293.15 to 323.15 K. Presented experimental values are the mean of three measurements and the measurement uncertainty was found to be about 1%. Dynamic viscosity was calculated using Eq. 1,

η = ( Kt − L / t ) d

(1)

where K and L are constants of the viscometer, t is the flow time and d is the experimental density of the liquid.

Electrical conductivity. Measurements of pure ILs’ electrical conductivity in the 20 to 50 ºC temperature range were carried out in a Pyrex-cell with platinum electrodes using a Jenco 3107 conductivity meter with DC signal. The cell constant amounted to 1.0353 cm-1. The measurements were repeated three times.



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Electrochemical measurements The electrolyte solutions were prepared by dissolving 1 vol.% of each IL in 8 M KOH (AnalaR NORMAPUR, 87 %). Millipore water was used to prepare the solutions. The experiments were carried out at temperatures ranging from 25 ºC up to 80 ºC, controlled by water circulation using an Ultraterm 6000383 P-Selecta bath. The electrochemical measurements were performed in a 125 mL cell using a PAR 273A potentiostat/galvanostat (Princeton Applied Research, Inc.) controlled by PowerSuite software. The standard threeelectrode setup was used, composed of a Pt foil (Metrohm 6.0305.100) with 1 cm2 of surface area as working electrode, a 50 cm2 Pt mesh (Johnson Matthey) as counter electrode and a saturated calomel electrode (SCE, Hannah instruments, HI 5412) as the reference. All electrode potentials are presented relatively to SCE reference. The HER performance in the KOH solutions with and without the addition of ILs using a Pt electrode was studied by linear scan voltammetry (LSV), scanning from the open circuit potential (OCP, ca. -1.1 V) to -1.5 V at 2 mV s-1, chronoamperometry (CA) for 1 h at -1.4 V, and electrochemical impedance spectroscopy (EIS). EIS measurements were carried out with the potentiostat coupled to a frequency response analyzer (Schlumberger SI 1255), at different potentials (from OCP up to -1.5 V), in the 10-2 to 105 Hz frequency range, and with an AC potential amplitude of 5 mV. The EIS data were modelled using an equivalent circuit with ZView software (Scribner Associates, Inc.). These three techniques, LSV, CA, and EIS, are commonly used to collect data on electrolytes or electrocatalysts behavior for HER (and in electrochemistry in general), complementing each other in terms of the information they provide about a system. Thus, LSV data were used for determination of Tafel slope and exchange current density, CA for stability evaluation, while EIS data were used for the investigation of the overall impedance response of the 8 ACS Paragon Plus Environment

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systems, further allowing for the determination of the several resistances and capacitances involved in the HER processes.

Results and discussion ILs characterization Water content was found to be less than 200 ppm and chloride content less than 8.3 ppm in all synthesized ILs. Table 1 presents the mean values of [Bmim][Sal] and [C3OHmim][Sal] ILs’ density (with relative standard uncertainty, ur, of 0.01 %), viscosity (ur < 1 %) and electrical conductivity (ur < 1.5 %). In a general way, [Bmim][Sal] shows higher conductivity, lower viscosity and slightly lower density than [C3OHmim][Sal] in the entire temperature range. Furthermore, [Bmim][Sal] shows significantly higher ionicity than [C3OHmim][Sal] in the studied temperature range. Namely, Walden plot was constructed based on experimental values of viscosity and electrical conductivity, in order to examine ionicity of studied ILs. The relation between molar conductivity and viscosity can be demonstrated by Eq. 2, log Λm= log C + A log η–1

(2)

where Λm is the molar conductivity, η–1 is fluidity, A is the slope of the Walden plot that reflects the decoupling of the ions, and C is Walden product of fractional Walden rule. In order to quantify ionicity, Angell method65 was applied by measuring the vertical distance from ionic liquids line to the KCl line. The Walden plot is presented in Figure 2 and shows that herein prepared ILs are below ideal KCl line.



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Figure 2: Walden plot in the temperature range from 20 to 50 ºC for [Bmim][Sal] and [C3OHmim][Sal].

Obtained results for ionicity of [Bmim][Sal] ranging between 50 and 65 % are much higher than those obtained for [C3OHmim][Sal] ranging between 26 and 38 %. Strong cation–anion interactions, which cause poor IL ionicity,66 are indeed expected in the case of [C3OHmim][Sal] due to the formation of a strong intermolecular H-bond between the OH group in the side chain of the cation and the OH group (as well as the COO-) group in the anion.


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Table 1: Experimental density, d, electrical conductivity, κ, viscosity, η, and ionicity for [Bmim][Sal] and [C3OHmim][Sal] at several temperatures, T, and atmospheric pressure.

T / ºC

κ /

mPa s

d/ g cm–3

ionicity / %

η / –1

mS cm [Bmim][Sal] 20

0.182

1194.1

1.1523

52.12

25

0.281

777.63

1.1491

52.55

30

0.447

501.42

1.1457

54.06

35

0.692

338.14

1.1422

56.61

40

0.965

240.00

1.1388

56.20

45

1.560

167.97

1.1352

63.78

50

2.103

127.06

1.1318

65.24

[C3OHmim][Sal] 20

0.050

2374.1

1.2379

26.69

25

0.098

1419.2

1.2345

31.35

30

0.168

876.52

1.2310

33.29

35

0.281

570.55

1.2274

36.35

40

0.424

392.75

1.2238

37.87

45

0.596

277.74

1.2202

37.76

50

0.802

208.71

1.2165

38.30

HER studies Figure 3 presents the cathodic polarization curves of the Pt electrode obtained at 25 ºC at 2 mV s-1, between the OCP (ca. -1.1 V) and -1.5 V, in the IL-free 8 M KOH and the 1



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vol.% IL-added electrolytes. It may be seen that at 25 ºC the ILs addition leads to higher current densities, as compared to those obtained in the IL-free electrolyte. The improvement occurs for any given potential value above -1.3 V for [Bmim][Sal] and [C3OHmim][Sal] and above ca. -1.15 V regarding [Im][Sal].

Figure 3: Polarization curves recorded at 2 mV s-1 and 25 ºC in the IL-free KOH electrolyte and after addition of 1 vol.% of the studied ILs. The inset shows detail at high overpotential.

Using the data from the polarization curves, Tafel plots were constructed (Figure 4), which were analyzed considering the Tafel expression in Eq. 3, η = a + b log j =

2.3RT αF

log j0 +

2.3RT αF

log j

(3)


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where η is the overpotential, a is the intercept related with the exchange current density, j0, corresponding to the electron transfer intrinsic rate. The Tafel slope, b, reflects the rate of change of j with η, α stands for the charge transfer coefficient, R is the ideal gas constant (8.314 J mol-1 K-1), T the temperature in Kelvin and F is Faraday’s constant (96485 C mol-1). Figure 4 shows excellent adjustment of the data to Eq. 3 in the selected regions, regarding all the studied electrolytes, with correlation coefficients being always higher than 0.99. From the Tafel analysis, kinetic parameters for the HER were calculated for the studied electrolyte solutions, which are shown in Table 2.



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Figure 4: Effect of the temperature on the Tafel regions for the HER in the electrolytes containing (A) [Bmim][Sal], (B) [C3OHmim][Sal] and (C) [Im][Sal], and (D) in the IL-free 8 M KOH.

Table 2: Kinetic parameters of HER at Pt electrode after addition of studied ILs and for the IL-free KOH electrolyte at several temperatures. T / ºC b / mV dec-1 Α j0 / mA cm-2 8 M KOH + 1 vol.% [Bmim][Sal] 230 0.25 0.95 25 266 0.23 2.45 40 217 0.30 1.13 50 250 0.26 2.10 60 319 0.21 6.11 70 306 0.23 6.39 80 8 M KOH + 1 vol.% [C3OHmim][Sal] 196 0.30 0.49 25 186 0.34 0.34 40 207 0.31 0.53 50 210 0.32 0.66 60 273 0.25 2.69 70 346 0.20 3.22 80 8 M KOH + 1 vol.% [Im][Sal] 221 0.26 0.72 25 192 0.32 0.37 40 185 0.35 0.29 50 196 0.34 0.83 60 286 0.27 3.86 70 244 0.29 3.22 80 8 M KOH 203 0.29 0.49 25 170 0.37 0.31 40 188 0.34 0.71 50 191 0.35 0.86 60 223 0.31 2.05 70 224 0.31 2.49 80 14 ACS Paragon Plus Environment

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The lines in Figure 4 are generally shifted to higher current densities with increasing temperature, as expected. In other words, with increasing temperature, the same current density is observed at lower overpotentials. The parameters presented in Table 2 show the effect of temperature on the HER kinetics in the studied electrolytes. The Tafel slope and the exchange current density generally increase with increasing temperature, whereas the charge transfer coefficient shows small sensitivity to the temperature variations. When comparing to the IL-free KOH solution, the IL-added electrolytes generally show higher Tafel slopes, slightly higher exchange current densities and lower charge transfer coefficients, as previously observed for [Emim]-based ILs.26,27 This is accompanied by higher current densities, as shown in the polarization curves of Figure 3. Considering that HER in alkaline solution occurs in multiple steps described by Eqs. 46,24,67 the Tafel analysis may be used to determine the reaction rate-determining step (RDS). The first step, the so-called Volmer step, is a primary discharge that occurs by covering the metal surface with adsorbed protons as described by Eq. 4. This step is then followed by a catalytic recombination of the adsorbed intermediates (MHads) according to Eq. 5, via Tafel step, or by an electrodesorption of the adsorbed species via the Heyrovsky step, as in Eq. 6. H2O + M + e- → MHads + OHMHads + MHads → H2 + 2 M H2O + MHads + e- → H2 + M + OH-

(Volmer step) (Tafel step) (Heyrovsky step)

(4) (5) (6)



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For α value of 0.5 at 25 ºC, the Tafel slope for Volmer, Heyrovsky and Tafel step being the RDS is 120, 40 and 30 mV dec-1, respectively.17,67 Table 2 shows Tafel slopes higher than 120 mV dec-1, therefore suggesting the Volmer step as the RDS of HER in the studied electrolytes. Experimentally observed Tafel slopes higher than the theoretically predicted are often found, and usually correlated with a potential dependence of the surface coverage of adsorbed intermediates, changes in the RDS with potential, and aspects related to the electrode surface, such as surface roughness and real surface area.13,15 Regarding the charge transfer coefficient, enthalpic and entropic components may be considered in kinetic studies, with α being given by Eq. 7,68-70 α = αH + TαS

(7)

where αH is related to the change of electrochemical enthalpy of activation with electrode potential and αS is related to the change of electrochemical entropy of activation with electrode potential. Therefore, according to Eqs. 3 and 7, the Tafel slope can be written as Eq. 8. b = α

2.3RT

H +TαS F

(8)

The thermodynamic parameters may then be determined using the Conway plot,71 the reciprocal of Tafel slope versus the reciprocal of the temperature, as presented in Figure 5. The obtained values are presented in Table 3.


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Figure 5: Conway plots regarding the plain 8 M KOH electrolyte and the IL-added electrolytes.

Table 3: Thermodynamic components of the charge transfer coefficients obtained by Conway plots. αH

αS / K-1

0.42

-5.33 × 10-4

[C3OHmim][Sal] 0.85

-1.73 × 10-3

[Im][Sal]

0.36

-1.88 × 10-4

8 M KOH

0.31

3.64 × 10-5

[Bmim][Sal]

It can be seen that the enthalpic component increases in the IL-added electrolytes, diminishing the enthalpy of activation; on the contrary, the entropic component decreases and changes to negative values in the presence of ILs, with the corresponding increase of



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the entropy of activation. Moreover, the entropic component is always smaller than the enthalpic component. By considering the data in Table 3 it easy to note that the electrochemical standard Gibbs energy of activation for the hydrogen charge transfer process at the cathode diminishes, which confirms that the studied ILs improve the water electrolyzer performance. Further characterization of the HER was performed by considering the variation of j0 with the temperature, allowing the calculation of activation energies, Ea. A simple Arrhenius relation, Eq. 9, where Ai is the Arrhenius pre-exponential factor, was applied, with the resulting plots being presented in Figure 6. log j0 = 2.3 log Ai

2.3 Ea RT

(9)

Figure 6: Arrhenius plots for the IL-free KOH solution and for the IL-added electrolytes.

The obtained HER Ea values are 30, 15, 33 and 31 kJ mol-1 in the presence of


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[Bmim][Sal], [C3OHmim][Sal], [Im][Sal] and in IL-free KOH, respectively. The addition of ILs seems to have little effect on the HER activation energy, with the exception of the case of [C3OHmim][Sal] that showed lower Ea values than the other electrolytes. These values are comparable to or lower than those previously reported for the HER at Pt-based materials, such as Pt-Dy (41 kJ mol-1)19 or Pt-Sm alloys (39 kJ mol-1) in IL-free KOH electrolyte.18 Stability of the IL additives was assessed by running CA measurements at an applied potential of -1.4 V for 1 h. Figure 7 shows stable performance in all IL-added electrolytes, with small decreases in the measured current density along the experiment of similar magnitude to those in the IL-free KOH solution. This result supports the stability of the ILadded solutions for long time operation water electrolysis. Moreover, higher currents were observed after addition of [Bmim][Sal], whereas [Im][Sal] originated similar currents to those obtained with the IL-free KOH solution, and [C3OHmim][Sal] lead to a decrease of the currents at -1.4 V.



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Figure 7: Stability curves for all the studied electrolytes obtained by CA at 25 ºC for 1 h under an applied potential of -1.4 V.

The HER in the studied electrolytes was also characterized by EIS. Figure 8 presents the corresponding Nyquist (A,B) and Bode (C,D) plots for the IL-free KOH electrolyte and the IL-added electrolyte compositions at applied potentials of -1.2 V (A,C) and -1.4 V (B,D). The figures show a 30-fold decrease of the impedance at -1.4 V, as compared to the lower applied potential of -1.2 V, which is associated with the easier occurrence of the hydrogen evolution at the higher potential. Moreover, at both applied potentials, a clear decrease of the impedance is observed after addition of the ionic liquids, particularly in the case of [Bmim][Sal]. Besides the analysis of the overall impedance of the system, EIS measurements allows the separation of the several contributions. In the Nyquist plots in Figure 8A and B, the intersection of the spectra with the Z’ axis at high frequency consists in the sum of the


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resistances of the wiring, the electrode material and the electrolyte.22,26 The impedance spectra revealed two other main contributions, which are present in all cases. The semicircle regarding the high frequency contribution is particularly well resolved in the case of [Im][Sal], as can be seen in the inset in Figure 8A and in the corresponding peaks in the Bode plot of Figure 8C. The real part of this semi-circle is assigned to the resistance of the charge transfer process, while the imaginary part is attributed to the capacitance of the double layer.26 In the impedance spectra of the other electrolytes, the semi-circle regarding the high frequency contribution is hidden by the largest one at intermediate frequencies, with the corresponding peaks in the phase angle Bode plots (Figure 8C) being merged. In fact, the largest contribution to the impedance of the system is found as a well resolved semi-circle at intermediate frequencies, which is attributed to the mass transfer resistance of the adsorbed intermediate Hads.26,27 In some cases, a low frequency relaxation, detected below 0.1 Hz, may be assigned to the oscillation of the H2 concentration at the electrode/electrolyte interface.26,27,72 However, the semi-circles regarding this low frequency contribution were not well resolved in most cases, related to high instability of the measurements for frequencies below 0.1 Hz, due to the presence of increased amounts of gas bubbles in the vicinity of the electrode.26,27 No inductive loop was observed at the low frequency end of the impedance spectra, as it was expected considering that Tafel analysis suggested the Volmer step as the RDS.12 Regarding the Bode plots at both applied potentials (Figure 8C and D), another effect of the ILs addition to the electrolyte solution is the shifting of the peaks of the phase angle to higher frequencies.



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Figure 8: Nyquist (A, B) and Bode (C, D) plots at -1.2 V (A, C) and -1.4 V (B, D) at 25 ºC for all studied electrolytes.

The impedance data of all herein studied systems was interpreted using the well-known equivalent electric circuit of Armstrong and Henderson14,67,73 presented in Figure 9. In this circuit, Rs is the ohmic resistance of the solution, Rct is the charge transfer resistance related with the reaction at the working electrode, Cdl is the double layer capacitance of the working electrode, Rp is the mass transfer resistance of the adsorbed intermediate Hads, also known as pseudo-resistance, and Cp is the pseudo-capacitance of the working electrode.


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Since the low frequency relaxation was not observable in most of the obtained spectra, it was not considered in the fitting of the impedance data using the Armstrong and Henderson equivalent circuit. The obtained values for the fitting parameters of the data in Figure 8 are presented in Table 4.

Figure 9: Armstrong and Henderson equivalent electric circuit used to fit the data. Rs is the ohmic resistance of the solution, Rct is the charge transfer resistance, Cdl, the double layer capacitance, Rp the pseudo-resistance and Cp the pseudo-capacitance.

From the analysis of Table 4, it may be seen that the resistance of the electrolyte solution at 25 ºC actually increased with the addition of the salicylate-based ionic liquids. This is in contrast with previous observations with highly conductive [Emim]-based ionic liquids, which originated a decrease of the electrolyte resistance.26 Nevertheless, the increase in Rs after addition of [Bmim][Sal] is smaller than in the case of [C3OHmim][Sal], which is consistent with the higher conductivity of the former (Table 1).



Page 24 of 47

Table 4: Parameters obtained from the impedance data at applied potentials of -1.2 V and -1.4 V for the IL-free KOH electrolyte and after addition of the several ILs at 25 ºC. Applied potential Rs /

Rct /

Cdl /

Rp /

Cp /

Ω cm2

Ω cm2

µF cm-2

Ω cm2

µF cm-2

[Bmim][Sal]

0.53

12.7

67

8.35

264

[C3OHmim][Sal]

0.63

45.0

329

122.7

1550

[Im][Sal]

0.59

5.59

41

57.31

403

8 M KOH

0.48

40.1

194

165.6

363

[Bmim][Sal]

0.55

2.99

95

1.60

573

[C3OHmim][Sal]

0.65

3.56

222

3.69

656

[Im][Sal]

0.62

4.43

116

3.38

407

8 M KOH

0.51

4.93

133

4.13

402

/V -1.2 V

-1.4 V

On the other hand, despite the modest effect of [C3OHmim][Sal] at -1.2 V, Rct and Rp are generally substantially reduced in the IL-added solutions at both applied potentials, suggesting that, in agreement with the higher currents registered in the polarization curves of Figure 3, the ionic liquid additives indeed enhance the charge and mass transfer processes for the HER. Thus, at -1.2 V, the charge transfer resistance massively decreased from 40.14 Ω cm2 to 12.74 and 5.59 Ω cm2 after addition of [Bmim][Sal] and [Im][Sal], respectively. Similar decreases of Rct were verified at -1.4 V, particularly with [Bmim][Sal] addition, which led to a decrease of 39 %. The capacitance of the double layer, Cdl, in the range of 41 – 329 µF cm-2, close to previous reported values,14,26,74 showed an undefined effect of the ILs addition, as well as a small dependence on the applied potential.


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Regarding the mass transfer processes, the effect of ionic liquids addition is similar to that observed for the charge transfer. All the additives originated lower resistances, at both applied potentials. The decrease in Rp is small with [C3OHmim][Sal] but more intense in the other IL-containing solutions, with [Bmim][Sal] showing the highest effect at both applied potentials. The pseudo-capacitance values observed were of the order of 10-4 F cm2

, similar to those observed for this contribution when using [Emim]-based ILs added

electrolytes.26 The pseudo-capacitive characteristics of an electrode are important for its performance for HER, as hydrogen is typically adsorbed on the electrode surface prior to its oxidation.75 Higher pseudo-capacitance indicates more active sites that can participate in the adsorption processes. The highest pseudo-capacitance values were observed after [C3OHmim][Sal] addition at both applied potentials, which may be related to the lower value of HER activation energy evaluated from the temperature dependence of exchange current density in Figure 6. Figure 10 presents the variation with the temperature in the 25-80 ºC range of the impedance spectra at an applied potential of -1.4 V, in the IL-added electrolytes ([Bmim][Sal] (A), [C3OHmim][Sal] (B) and [Im][Sal] (C)) and in the IL-free KOH electrolyte (D)). The parameters resulting from the fitting of these data using the Armstrong and Henderson equivalent circuit in Figure 9 are presented in Table 5. In all cases, and as expected, the general tendency is the systematic decrease with increasing temperature of the overall impedance of the system, as well as of all the resistances of the several processes.



Page 26 of 47

Figure 10: Nyquist plots at -1.4 V and several temperatures of the electrolytes containing (A) [Bmim][Sal], (B) [C3OHmim][Sal] and (C) [Im][Sal], and (D) of the IL-free 8 M KOH.

Table 5: Parameters obtained from the impedance data after addition of the several ILs and for the IL-free KOH electrolyte at several temperatures. Rs / Rct / Cdl / 2 2 Ω cm Ω cm µF cm-2 8 M KOH + 1 vol.% [Bmim][Sal] 0.55 2.99 95 25 0.57 1.61 326 40 0.53 1.50 290 50 0.52 1.39 219 60 0.46 1.17 311 70 0.44 0.97 341 80 8 M KOH + 1 vol.% [C3OHmim][Sal] 0.65 3.56 222 25 0.50 3.01 214 40 0.45 2.61 215 50 0.41 2.17 212 60 0.39 1.43 283 70 0.39 0.91 405 80 8 M KOH + 1 vol.% [Im][Sal]

T / ºC

Rp / Ω cm2

Cp / µF cm-2

1.60 2.54 2.44 2.13 2.03 1.61

573 869 717 728 540 642

3.69 4.32 3.53 4.17 2.39 1.55

656 542 406 389 553 969


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0.62 25 0.50 40 0.45 50 0.41 60 0.38 70 0.33 80 8 M KOH 0.51 25 0.46 40 0.44 50 0.34 60 0.32 70 0.31 80

4.43 3.20 2.64 1.76 1.23 0.96

116 81 165 207 245 258

3.38 2.58 2.44 1.68 1.26 1.03

407 283 425 472 508 435

4.93 3.66 1.91 1.27 0.81 0.76

133 112 89 286 280 349

4.13 2.70 1.89 1.54 1.22 1.00

402 382 422 528 579 619

It may be concluded that the addition of 1 vol.% of the salicylate-based ILs increases the resistance of the electrolyte in the whole temperature range. Regarding the other relaxations, the values of Cdl and Cp at -1.4 V are again of the order of 10-4 F cm-2, Cdl roughly in the range of 100-400 µF cm-2, slightly increasing with the temperature, and Cp in the range of 400-1000 µF cm-2, showing less sensitivity to the temperature increase. The charge transfer and mass transfer resistances, Rct and Rp, are systematically decreased with increasing temperature in all electrolytes. However, the effect of the ILs addition on these relaxations seems to be dependent on the temperature. In fact, Rct and Rp are lower in the IL-added electrolytes than in the IL-free KOH solution at low temperatures, but the effect of the additives is less important as the temperature increases. Rct tends to similar values in all electrolytes as the temperature approaches 80 ºC and Rp actually becomes slightly higher in the IL-containing solutions at temperatures above 50 ºC. This effect of the ILs addition at high temperatures may be related to their effect on the resistance of the electrolyte, increasing it, which is, however, observed in the whole 27 ACS Paragon Plus Environment


Page 28 of 47

temperature range. Furthermore, the highest Rct and Rp values are generally observed after addition of [C3OHmim][Sal], which did not show the highest Rs increase, suggesting that other factors are present. Moreover, some degree of decomposition of the ILs as the temperature increases must also not be ruled out and deserves further investigation. At a given applied potential, the sum of Rct and Rp is related with the HER kinetics and therefore its reciprocal value is directly related to the Faradaic current density.14,26,27,74 Therefore, from the variation of the reciprocal of Rct+Rp with the reciprocal temperature (values in Table 5), apparent activation energy may be calculated by using the Arrhenius relation in Eq. 9. The resulting linear relations are presented in Figure 11, yielding values of 9, 16, 22 and 28 kJ mol-1, regarding [Bmim][Sal], [C3OHmim][Sal], [Im][Sal] and IL-free KOH electrolyte, respectively. These values are lower but comparable to those obtained from the temperature dependence of j0 (Figure 6). It may then be concluded that the ILadded solutions lead to lower apparent activation energies than the IL-free KOH, particularly the [Bmim][Sal] containing electrolyte, which originated a 3-fold decrease.


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Figure 11: Arrhenius plots of the reciprocal of (Rct + Rp) at -1.4 V regarding the IL-free and the several IL-added electrolytes.

The variation of the impedance with the applied potential is shown in Figure 12, in the Nyquist (A,B) and Bode (C,D) plots of the [Bmim][Sal]–added and the IL-free KOH electrolytes. The fitting results are shown in Table 6. The resistance of the electrolyte seems to increase with the applied potential in both cases. Furthermore, Rs is higher in the ILadded solution than in the IL-free KOH in the entire potential range. On the other hand, it is clear that the overall impedance decreases with the increasing applied potential (or overpotential) in both solutions and that the [Bmim][Sal]–added electrolyte shows lower resistance at any given potential, regarding all the HER-related relaxations. Cdl is of 10-4 F cm-2 order of magnitude, being not much affected by the applied potential, whereas Cp is slightly higher but generally of the same order of magnitude and increasing with the increasing potential.



Page 30 of 47

Figure 12: Nyquist (A, B) and Bode (C, D) plots obtained in the [Bmim][Sal]-added electrolyte (A, C) and IL-free KOH (B, D) at 25 ºC and different applied potentials.

Table 6: Evolution with the applied potential of the parameters obtained from the impedance data for the electrolyte with addition of 1 vol.% [Bmim][Sal] and for the IL-free KOH electrolyte. Applied

Rs /

Rct /

Cdl /

Rp /

Cp /

potential / V

Ω cm2

Ω cm2

µF cm-2

Ω cm2

µF cm-2

8 M KOH + 1 vol.% [Bmim][Sal]


Page 31 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.2

0.53

12.74

67

8.35

264

-1.3

0.50

6.76

116

7.58

360

-1.4

0.55

2.99

95

1.60

573

-1.5

0.78

0.74

127

0.13

4160

-1.2

0.48

40.14

194

165.6

363

-1.3

0.53

13.94

176

29.11

420

-1.4

0.51

4.93

133

4.13

402

-1.5

0.68

1.13

117

0.15

2310

8 M KOH

As previously mentioned, at any given potential, the reciprocal value of (Rct+Rp) is directly related to the Faradaic current density, and therefore the simulated η vs. log (Rct+Rp)-1 plot should be linear with a slope equivalent to the Tafel slope.14,26,27,74 Figure 13 compares the Tafel plot (from Figure 4) and the η vs. log (Rct+Rp)-1 plots regarding the 8 M KOH + 1 vol.% of [Bmim][Sal] electrolyte. It may be seen that the plots have indeed relatively similar slopes of 230 and 201 mV dec-1 for the Tafel and simulated plots, respectively. Additionally, the separation between the two lines is 1.16, consistent with a Langmuir-type monolayer adsorption of the Hads to the surface of the electrode, which has a theoretical value of 1.29,74 as demonstrated by Conway et al. The equivalent slopes of the Tafel and simulated plots and the agreement with the predicted separation between the two lines for the Langmuir-type adsorption74 support the applicability of the Armstrong and Henderson equivalent circuit used in the present analysis of EIS experimental data for the HER in IL-added alkaline media.



Page 32 of 47

Figure 13: Comparison between the experimental Tafel (η vs. log j) and the simulated η vs. log (Rct+Rp)-1 plots regarding the 1 vol.% of [Bmim][Sal]-containing electrolyte, at 25 ºC.

The particularly favorable effects of [Bmim][Sal] may be related with its physicochemical properties, namely higher conductivity, lower viscosity, slightly lower density and significantly higher ionicity, as compared with [C3OHmim][Sal]. Moreover, the ILs used in this work have the same anion, salicylate, and different cations in their composition. Therefore, the herein reported effects on the HER must be related with the salicylate anion, as well as with the several cations, [Bmim], [C3OHmim] and [Im], and the cation-anion interaction. It has been reported that the anion has effects on the wetting of the electrode surface, as well as on the double layer capacitance of several ILs.46 In fact, the double layer capacitance of a quaternary ammonium salt with a methoxyalkyl group on the nitrogen atom was shown to depend on the nature of the anion rather than on the cation.76 Additionally, the role of the anion is very important regarding the nature and strength of


Page 33 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


anion-cation interactions,77 which was observed, for example, in different orientation of [Bmim] cations at a Pt electrode with changing potential depending on the anion ([BF4] or [PF6]).78 On the other hand, it is known that the IL’s cathodic stability and potential window are affected by the cation in the IL’s composition.46,79 Moreover, cation size plays a role in the electrolyte viscosity and conductivity, as well as the capacitance. ILs with different cations, such as imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium, were compared and imidazolium- and pyridinium-based ILs showed the highest capacitance.80,81 Herein, the highest capacitance at different potentials at 25 ºC has been evaluated in the presence of [C3OHmim][Sal], possibly contributing to the lowest HER activation energy evaluated from the temperature dependence of the exchange current density. Additionally, the adsorption of cations and cation-metal intermediate complexes to the electrode surface was observed for [Emim]-based ILs,52 as was the formation of an electroactive [TEAPS][BF4] layer at the cathode surface, enhancing the HER.59 Therefore, the adsorption of ions from the ILs additives onto the surface of the electrode, as well as the formation of intermediate complexes and their adsorption to the electrode,82 must be considered and related to the present results on the effect of the ILs on the HER. In this way, with results particularly interesting at temperatures below 50 ºC, addition of the three studied ILs affects the electrode/electrolyte double layer, with impact on the Pt electrocatalytic activity towards the HER. Surface pre-adsorption of these three additives may then affect the intermediate hydrogen atoms formed in Volmer step and the adsorption and charge transfer processes at the electrode/electrolyte interface.



Page 34 of 47

Conclusions Three non-toxic salicylate ([Sal])-based ILs with varying cation were prepared and characterized by density, viscosity and electrical conductivity measurements, followed by ionicity determination. The effect on the HER of adding small amounts of these ILs to the KOH electrolyte is investigated. When comparing to IL-free KOH solution, higher currents were observed in the IL-added electrolytes. With the exception of the case of [C3OHmim][Sal], the addition of RTILs seems to have small effect on the activation energy, with values of 30, 15, 33 and 31 kJ mol-1 being obtained for [Bmim][Sal], [C3OHmim][Sal], [Im][Sal] and IL-free KOH, respectively. Additionally, stability experiments were performed at 25 ºC and very stable behavior was observed in all the ILadded electrolytes. Tafel analysis and EIS characterization show the Volmer reaction as the RDS of the HER. At 25 ºC, despite that the resistance of the electrolyte solution was increased, a strong decrease of the overall impedance was observed in the EIS measurements with the addition of the salicylate-based ionic liquids, independently of the applied potential. In fact, the charge transfer and the polarization resistances were generally substantially reduced in the IL-added solutions, particularly after [Bmim][Sal] addition, showing that the ionic liquid additives indeed enhance the charge and mass transfer processes for the HER. The particularly favorable effects of [Bmim][Sal] may be related with its physicochemical properties, i.e., higher conductivity, lower viscosity, somewhat lower density and notably higher ionicity, in comparison with those of [C3OHmim][Sal]. The charge and mass transfer resistances are systematically decreased with increasing temperature in all electrolytes. The effect of the additives becomes less important as the


Page 35 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature increases, with the charge transfer and the polarization resistances tending to similar values in all electrolytes as the temperature approaches 80 ºC. The polarization resistance actually becomes slightly higher in the IL-containing solutions at temperatures above 50 ºC. The separation between the experimental Tafel (η vs. log j) and the simulated η vs. log (Rct+Rp)-1 plots is consistent with a Langmuir-type monolayer adsorption of Hads to the electrode surface. Surface pre-adsorption of the IL additives enhances the adsorption and charge transfer processes at the electrode/electrolyte interface. It is shown that adding small amounts of selected environmental-friendly ILs to the KOH electrolyte may be beneficial for the HER, particularly at low temperatures, supporting the design of efficient ionic liquid based additives that enable increased performance of industrial alkaline electrolyzers.

Acknowledgments The authors would like to thank Fundação para a Ciência e Tecnologia (FCT, Portugal) for postdoctoral

research

grants

SFRH/BPD/97453/2013

(L.

Amaral)

and

SFRH/BPD/77768/2011 (B. Šljukić) and for contract no. IF/01084/2014/CP1214/CT0003 under IF2014 Programme (D.M.F. Santos).

References 1.

Penner, S. S. Steps toward the hydrogen economy. Energy 2006, 31, 33-43.

2.

Santos, D. M. F.; Sequeira, C. A. C.; Figueiredo, J.L. Hydrogen production by alkaline water electrolysis. Quim. Nova 2013, 36, 1176-1193. 35 ACS Paragon Plus Environment


3.

Page 36 of 47

Momirlan, M.; Veziroglu, T. N. Current status of hydrogen energy. Renew. Sustain. Energy Rev. 2002, 6, 141-179.

4.

Turner, J. A. Sustainable hydrogen production. Science 2004, 305, 972-974.

5.

Bhattacharyya, R.; Misra, A.; Sandeep, K. C. Photovoltaic solar energy conversion for hydrogen production by alkaline water electrolysis, Conceptual design and analysis. Energy Convers. Manag. 2017, 133, 1-13.

6.

Kaninski, M. P. M.; Nikolic, V. M.; Tasic, G. S.; Rakocevic, Z. L. Electrocatalytic activation of Ni electrode for hydrogen production by electrodeposition of Co and V species. Int. J. Hydrogen Energy 2009, 34, 703-709.

7.

Sequeira, C. A. C.; Cardoso, D. S. P.; Amaral, L.; Šljukić, B.; Santos, D. M. F. On the performance of commercially available corrosion-resistant nickel alloys: a review. Corros. Rev. 2016, 34, 187-200.

8.

Sequeira, C. A. C.; Santos, D. M. F.; Šljukić, B.; Amaral, L. Physics of electrolytic gas evolution. Braz. J. Phys. 2013, 43, 199-208.

9.

Cardoso, D. S. P., Amaral, L.; Santos, D. M. F.; Šljukić, B.; Sequeira, C. A. C.; Maccio, D.; Saccone, A. Enhancement of hydrogen evolution in alkaline water electrolysis by using nickel-rare earth alloys. Int. J. Hydrogen Energy 2015, 40, 4295-4302.

10.

Cardoso, D. S. P.; Eugenio, S.; Silva, T. M.; Santos, D. M. F.; Sequeira, C. A. C.; Montemor, M. F. Hydrogen evolution on nanostructured Ni-Cu foams. RSC Adv. 2015, 5, 43456-43461.

11.

Cardoso, J. A. S. B.; Amaral, L.; Metin, O.; Cardoso, D. S. P.; Sevim, M.; Şener, T.; Sequeira, C. A. C.; Santos, D. M. F. Reduced graphene oxide assembled Pd-based


Page 37 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanoalloys for hydrogen evolution reaction. Int. J. Hydrogen Energy 2017, 42, 3916-3925. 12.

Delgado, D.; Minakshi, M.; Kim, D. J. Electrochemical impedance spectroscopy studies on hydrogen evolution from porous Raney cobalt in alkaline solution. Int. J. Electrochem. Sci. 2015, 10, 9379-9394.

13.

Dominguez-Crespo, M. A.; Torres-Huerta, A. M.; Brachetti-Sibaja, B.; Flores-Vela, A. Electrochemical performance of Ni-RE (RE = rare earth) as electrode material for hydrogen evolution reaction in alkaline medium. Int. J. Hydrogen Energy 2011, 36, 135-151.

14.

Franceschini, E. A.; Lacconi, G. I.; Corti, H. R. Kinetics of the hydrogen evolution on nickel in alkaline solution, new insight from rotating disk electrode and impedance spectroscopy analysis. Electrochim. Acta 2015, 159, 210-218.

15.

Kibria, M. F.; Mridha, M. S.; Khan, A. H. Electrochemical studies of a nickel electrode for the hydrogen evolution reaction. Int. J. Hydrogen Energy 1995, 20, 435-440.

16.

Santos, D. M. F.; Amaral, L.; Šljukić, B.; Maccio, D.; Saccone, A.; Sequeira, C. A. C. Nickel-cerium electrodes for hydrogen evolution in alkaline water electrolysis. ECS Trans. 2013, 58, 113-121.

17.

Santos, D. M. F.; Amaral, L.; Šljukić, B.; Maccio, D.; Saccone, A.; Sequeira, C. A. C. Electrocatalytic activity of nickel-cerium alloys for hydrogen evolution in alkaline water electrolysis. J. Electrochem. Soc. 2014, 161, F386-F390.

18.

Santos, D. M. F.; Sequeira, C. A. C.; Maccio, D.; Saccone, A.; Figueiredo, J. L. Platinum-rare earth electrodes for hydrogen evolution in alkaline water electrolysis. Int. J. Hydrogen Energy 2013, 38, 3137-3145. 37 ACS Paragon Plus Environment


19.

Page 38 of 47

Santos, D. M. F.; Šljukić, B.; Sequeira, C. A. C.; Maccio, D.; Saccone, A.; Figueiredo, J. L. Electrocatalytic approach for the efficiency increase of electrolytic hydrogen production: Proof-of-concept using platinum-dysprosium alloys. Energy 2013, 50, 486-492.

20.

Sequeira, C. A. C.; Santos, D. M. F.; Sousa, J. R.; Brito, P. S. D. Reactively deposited cobalt electrodes for hydrogen evolution in alkaline solution. Mater. Technol. 2009, 24, 119-121.

21.

Šljukić, B.; Milikic, J.; Santos, D. M. F.; Sequeira, C. A. C.; Maccio, D.; Saccone, A. Electrocatalytic performance of Pt-Dy alloys for direct borohydride fuel cells. J. Power Sources 2014, 272, 335-343.

22.

Šljukić, B.; Santos, D. M. F.; Vujkovic, M.; Amaral, L.; Rocha, R. P.; Sequeira, C. A. C.; Figueiredo, J. L. Molybdenum carbide nanoparticles on carbon nanotubes and carbon xerogel: low-cost cathodes for hydrogen production by alkaline water electrolysis. Chemsuschem 2016, 9, 1200-1208.

23.

Šljukić, B.; Vujkovic, M.; Amaral, L.; Santos, D. M. F.; Rocha, R. P.; Sequeira, C. A. C.; Figueiredo, J. L. Carbon-supported Mo2C electrocatalysts for hydrogen evolution reaction. J. Mater. Chem. A 2015, 3, 15505-15512.

24.

Safizadeh, F., Ghali, E.; Houlachi, G. Electrocatalysis developments for hydrogen evolution reaction in alkaline solutions - A review. Int. J. Hydrogen Energy 2015, 40, 256-274.

25.

de Souza, R. F.; Padilha, J. C.; Gonçalves, R. S.; de Souza, M. O.; Rault-Berthelot, J. Electrochemical hydrogen production from water electrolysis using ionic liquid as electrolytes: Towards the best device. J. Power Sources 2007, 164, 792-798.


Page 39 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26.

Amaral, L.; Cardoso, D. S. P.; Šljukić, B.; Santos, D. M. F.; Sequeira, C. A. C. Room temperature ionic liquids as electrolyte additives for the HER in alkaline media. J. Electrochem. Soc. 2017, 164, F427-F432.

27.

Amaral, L; Cardoso, D. S. P.; Šljukić, B.; Santos, D. M. F.; Sequeira, C. A. C. Electrochemistry of hydrogen evolution in RTILs aqueous mixtures. Mater. Res. Bull. 2018, in press, https://doi.org/10.1016/j.materresbull.2018.04.041

28.

de Souza, R.F.; Loget, G.; Padilha, J. C.; Martini, E. M. A.; de Souza, M. O. Molybdenum electrodes for hydrogen production by water electrolysis using ionic liquid electrolytes. Electrochem. Commun. 2008, 10, 1673-1675.

29.

Loget, G.; Padilha, J. C.; Martini, E. A.; de Souza, M. O.; de Souza, R. F. Efficiency and stability of transition metal electrocatalysts for the hydrogen evolution reaction using ionic liquids as electrolytes. Int. J. Hydrogen Energy, 2009, 34, 84-90.

30.

Zeng, K; Zhang, D. K. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog. Energy Combust. Sci. 2010, 36, 307-326.

31.

Balabel, A.; Zaky, M.S.; Sakr, I. Optimum operating conditions for alkaline water electrolysis coupled with solar PV energy system. Arabian J. Sci. Eng. 2014, 39, 4211-4220.

32.

Milewski, J.; Guandalini, G.; Campanari, S. Modeling an alkaline electrolysis cell through reduced-order and loss-estimate approaches. J. Power Sources, 2014, 269, 203-211.

33.

Lagrost, C.; Carrie, D.; Vaultier, M.; Hapiot, P. Reactivities of some electrogenerated organic cation radicals in room-temperature ionic liquids: Toward an alternative to volatile organic solvents? J. Phys. Chem. A, 2003, 107, 745-752.



34.

Page 40 of 47

Zhang, G.R.; Etzold, B .J. M. Ionic liquids in electrocatalysis. J. Energy Chem. 2016, 25, 199-207.

35.

Opallo, M.; Lesniewski, A. A review on electrodes modified with ionic liquids. J. Electroanal. Chem. 2011, 656, 2-16.

36.

Armakovic, S.; Armakovic, S. J.; Vranes, M.; Tot, A.; Gadzuric, S. DFT study of 1butyl-3-methylimidazolium salicylate, a third-generation ionic liquid. J. Mol. Model. 2015, 21, 246-255.

37.

Shmukler, L.E.; Gruzdev, M.S.; Kudryakova, N.O.; Fadeeva, Y.A.; Safonova, L.P.; Triethylammonium-based protic ionic liquids with sulfonic acids: Phase behavior and electrochemistry, J Mol. Liq. 2018, 266, 139-146.

38.

Ku, S. Y.; Lu, S. Y. Inexpensive room temperature ionic liquids for low volatility electrolytes of dye-sensitized solar cells. Int. J. Electrochem. Sci. 2011, 6, 52195227.

39.

Wang, Z. Y.; Wang, L.; Zhang, Y.; Guo, J. N.; Li, H.; Yan, F. Dye-sensitized solar cells based on cobalt-containing room temperature ionic liquid redox shuttles. RSC Adv. 2017, 7, 13689-13695.

40.

Li, H. T.; Mu, X. Y.; Wang, Z.; Guo, M.; Zeng, X. Q.; Mason, A. J. Room temperature ionic-liquid electrochemical gas sensor array system for real-time mine safety monitoring. 2013 IEEE Sensors, 2013, 1-4.

41.

Mu, X. Y.; Wang, Z.; Zeng, X. Q.; Mason, A. J. A robust flexible electrochemical gas sensor using room temperature ionic liquid. IEEE Sens. J. 2013, 13, 3976-3981.

42.

Wan, H.; Yin, H.Y.; Mason, A. J. Room temperature ionic liquid electrochemical gas sensor for rapid oxygen detection with transient double potential amperometry. 2016 IEEE Sensors, 2016. DOI: 10.1109/ICSENS.2016.7808787 40 ACS Paragon Plus Environment

Page 41 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


43.

de Souza, R. F.; Padilha, J. C.; Goncalves, R. S.; Dupont, J. Room temperature dialkylimidazolium ionic liquid-based fuel cells. Electrochem. Commun. 2003, 5, 728-731.

44.

Zhang, G. R.; Munoz, M.; Etzold, B. J. M. Accelerating oxygen-reduction catalysts through preventing poisoning with non-reactive species by using hydrophobic ionic liquids. Angew. Chem., Int. Ed. 2016, 55, 2257-2261.

45.

Yuyama, K.; Masuda, G.; Yoshida, H.; Sato, T. Ionic liquids containing the tetrafluoroborate anion have the best performance and stability for electric double layer capacitor applications. J. Power Sources 2006, 162, 1401-1408.

46.

Mousavi, M. P. S.; Wilson, B. E.; Kashefolgheta, S.; Anderson, E. L.; He, S. Y.; Buhlmann, P.; Stein, A. Ionic liquids as electrolytes for electrochemical doublelayer capacitors: Structures that optimize specific energy. ACS Appl. Mater. Interfaces 2016, 8, 3396-3406.

47.

Kasprzak, D.; Stępniak, I.; Galiński, M.; Acetate- and lactate-based ionic liquids: Synthesis, characterisation and electrochemical properties, J. Mol. Liq. 2018, 264, 233-241.

48.

Martins, V.L.; Torresi, R.M.; Ionic liquids in electrochemical energy storage, Curr. Opin. Electrochem. 2018, 9, 26-32.

49.

Safa, M.; Chamaani, A.; Chawla, N.; El-Zahab, B. Polymeric ionic liquid gel electrolyte for room temperature lithium battery applications. Electrochim. Acta 2016, 213, 587-593.

50.

Yong, T. Q.; Zhang, L. Z.; Wang, J. L.; Mai, Y. J.; Yan, X. D.; Zhao, X. Y. Novel choline-based ionic liquids as safe electrolytes for high-voltage lithium-ion batteries. J. Power Sources 2016, 328, 397-404. 41 ACS Paragon Plus Environment


51.

Page 42 of 47

Eftekhari, A.; Liu, Y.; Chen, P. Different roles of ionic liquids in lithium batteries. J. Power Sources 2016, 334, 221-239.

52.

Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I. Ionic liquid-mediated selective conversion of CO2 to CO at low overpotentials. Science 2011, 334, 643-644.

53.

Rosen, B. A.; Zhu, W.; Kaul, G.; Salehi-Khojin, A.; Masel, R. I. Water enhancement of CO2 conversion on silver in 1-ethyl-3-methylimidazolium tetrafluoroborate. J. Electrochem. Soc. 2013, 160, H138-H141.

54.

Hanc-Scherer, F. A.; Montiel, M. A.; Montiel, V.; Herrero, E.; Sanchez-Sanchez, C. M. Surface structured platinum electrodes for the electrochemical reduction of carbon dioxide in imidazolium based ionic liquids. Phys. Chem. Chem. Phys. 2015, 17, 23909-23916.

55.

Lu, Q.; Jiao, F. Electrochemical CO2 reduction: Electrocatalyst, reaction mechanism, and process engineering. Nano Energy 2016, 29, 439-456.

56.

Zhou, F.; Liu, S. M.; Yang, B. Q.; Wang, P. X.; Alshammari, A. S.; Deng, Y. Q. Highly selective and stable electro-catalytic system with ionic liquids for the reduction of carbon dioxide to carbon monoxide. Electrochem. Commun. 2015, 55, 43-46.

57.

Kathiresan, M.; Velayutham, D. Ionic liquids as an electrolyte for the electro synthesis of organic compounds. Chem. Commun. 2015, 51, 17499-17516.

58.

de Souza, R. F.; Padilha, J. C.; Goncalves, R. S.; Rault-Berthelot, J. L. Dialkylimidazolium ionic liquids as electrolytes for hydrogen production from water electrolysis. Electrochem. Commun. 2006, 8, 211-216.


Page 43 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


59.

Fiegenbaum, F.; de Souza, M. O.; Becker, M. R.; Martini, E. M. A.; de Souza, R. F. Electrocatalytic activities of cathode electrodes for water electrolysis using tetraalkyl-ammonium-sulfonic acid ionic liquid as electrolyte. J. Power Sources 2015, 280, 12-17.

60.

Fiegenbaum, F.; Martini, E. M.; de Souza, M. O.; Becker, M. R.; de Souza, R. F. Hydrogen production by water electrolysis using tetra-alkyl-ammonium-sulfonic acid ionic liquid electrolytes. J. Power Sources 2013, 243, 822-825.

61.

Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. An electrochemical study of the oxidation of hydrogen at platinum electrodes in several room temperature ionic liquids. J. Phys. Chem. B, 2007, 111, 5000-5007.

62.

Silvester, D. S.; Ward, K. R.; Aldous, L.; Hardacre, C.; Compton, R. G. The electrochemical oxidation of hydrogen at activated platinum electrodes in room temperature ionic liquids as solvents. J. Electroanal. Chem. 2008, 618, 53-60.

63.

Vranes, M.; Armakovic, S.; Tot, A.; Papovic, S.; Zec, N.; Armakovic, S.; Banic, N.; Abramovic, B.; Gadzuric, S. Structuring of water in the new generation ionic liquid - Comparative experimental and theoretical study. J. Chem. Thermodyn. 2016, 93, 164-171.

64.

Vranes, M.; Tot, A.; Jovanovic-Santa, S.; Karaman, M.; Dozic, S.; Tesanovic, K.; Kojic, V.; Gadzuric, S. Toxicity reduction of imidazolium-based ionic liquids by the oxygenation of the alkyl substituent. RSC Adv. 2016, 6, 96289-96295.

65.

MacFarlane, D. R.; Forsyth, M.; Izgorodina, E. I.; Abbott, A. P.; Annat, G.; Fraser, K. On the concept of ionicity in ionic liquids. Phys. Chem. Chem. Phys. 2009, 11, 4962-4967.



66.

Page 44 of 47

Ueno, K., Tokuda, H.; Watanabe, M. Ionicity in ionic liquids: correlation with ionic structure and physicochemical properties. Phys. Chem. Chem. Phys. 2010, 12, 1649– 1658.

67.

Krstajic, N.; Popovic, M.; Grgur, B.; Vojnovic, M.; Sepa, D. On the kinetics of the hydrogen evolution reaction on nickel in alkaline solution - Part I: The mechanism. J. Electroanal. Chem. 2001, 512, 16-26.

68.

Conway, B. E., in Modern Aspects of Electrochemistry, Conway, B. E.; White, R. E.; Bockris, J. O. M., Editors, Plenum Press: New York. 1985.

69.

Damjanovic, A., Temperature-dependence of symmetry factors and the significance of experimental activation-energies. J. Electroanal. Chem. 1993, 355, 57-77.

70.

Gonzalez-Huerta, R. G.; Gonzalez-Cruz, R.; Citalan-Cigarroa, S.; MonteroOcampo,

C.;

Chavez-Carvayar,

J.;

Soorza-Feria,

O.

Development

and

electrochemical studies of ruthenium nanoparticles as cathode in a PEMFC. J. New Mater. Electrochem. Syst. 2005, 8, 15-23. 71.

Conway, B. E.; Tessier, D. F.; Wilkinson, D. P. Temperature-dependence of the Tafel slope and electrochemical barrier symmetry factor, β, in electrode-kinetics. J. Electrochem. Soc. 1989, 136, 2486-2493.

72.

Barber, J.; Morin, S.; Conway, B.E.; Specificity of the kinetics of H2, evolution to the structure of single-crystal Pt surfaces, and the relation between opd and upd H. J. Electroanal. Chem. 1998, 446, 125-138.

73.

Armstrong, R. D.; Henderson, M. Impedance plane display of a reaction with an adsorbed intermediate. J. Electroanal. Chem. 1972, 39, 81-90.

74.

Bai, L.; Harrington, D. A.; Conway, B. E. Behavior of overpotential deposited species in Faradaic reactions. 2: AC impedance measurements on H2 evolution 44 ACS Paragon Plus Environment

Page 45 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kinetics at activated and unactivated Pt cathodes. Electrochim. Acta 1987, 32, 17131731. 75.

Eftekhari, A.; Electrocatalysts for hydrogen evolution reaction, Int. J. Hydrogen

Energy 2017, 42, 11053 – 11077. 76.

Galinski, M.; Lewandowski, A.; Stepniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567-5580.

77.

Castner, E. W.; Wishart, J. F. Spotlight on ionic liquids. J. Chem. Phys. 2010, 132, 120901.

78.

Rivera-Rubero, S.; Baldelli, S. Surface spectroscopy of room-temperature ionic liquids on a platinum electrode: A sum frequency generation study. J. Phys. Chem. B 2004, 108, 15133-15140.

79.

Mousavi, M. P. S.; Kashefolgheta, S.; Stein, A.; Buhlmann, P. Electrochemical stability of quaternary ammonium cations: An experimental and computational study. J. Electrochem. Soc. 2016, 163, H74-H80.

80.

Lewandowski, A.; Galinski, M. Carbon-ionic liquid double-layer capacitors. J. Phys. Chem. Solids 2004, 65, 281-286.

81.

Sato, T.; Masuda, G.; Takagi, K. Electrochemical properties of novel ionic liquids for electric double layer capacitor applications. Electrochim. Acta 2004, 49, 36033611.

82.

Motobayashi, K.; Osawa, M.; Recent advances in spectroscopic investigations on ionic liquid/electrode interfaces, Curr. Opin. Electrochem. 2018, 8, 147-155.



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TOC/abstract graphic


Toward Tailoring of Electrolyte Additives for Efficient Alkaline Water

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