Effects of nonionic surfactant on the parasitic corrosion of lithium anode in lithium–water battery

Abstract

The use of a nonionic surfactant, namely cocamide diethanol amine (CDA), as a parasitic inhibitor for lithium corrosion in 4.0 M LiOH aqueous solution has been evaluated in the 298–313 K temperature range. The efficiency of the CDA surfactant is evaluated by hydrogen evolution at OCP and potentiodynamic polarization measurements. Results show that the CDA surfactant is an appropriate inhibitor for parasitic corrosion. The CDA surfactant slows down the hydrogen evolution reaction during the immersion of lithium in 4.0 M LiOH aqueous solution. The efficiency of the CDA surfactant is promoted with growing surfactant concentration, reaching an extreme value (78.6%) at the critical micelle concentration (CMC). A Tafel polarization study reveals that the CDA surfactant acts as a cathodic-type inhibitor. Adsorption of the CDA surfactant on the lithium surface is unprompted and fits with Langmuir's isotherm. The associated activation energy and thermodynamic parameters are calculated to elaborate the mechanism of corrosion inhibition.

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1. Introduction

Lithium metal is attractive as a battery anode due to its high voltage, electrochemical equivalence and conductivity in addition to its light weight. Because of these marvelous natural features, the use of lithium has dominated the development of high-performance primary and secondary batteries during the last two decades.¹

Lithium–water battery is the most promising system, because of it provides reliable power for long durations of time in an aqueous environment. However, lithium–water battery has the problem of severely parasitic corrosion hydrogen evolution which reduces its efficiency and operation safety.²

The corrosion reaction of lithium with water resulting in parasitic corrosion.^3,4 The parasitic corrosion reaction is highly undesirable because it produces no useful electrical energy and consumes active lithium. The reaction is highly exothermic and can detrimentally accelerate local corrosion.⁵ Further, the produced lithium hydroxide reacts with water and eventually precipitates as a monohydrate crystal. Accordingly, a principal challenge with respect to reactive metal–water batteries is corrosion inhibitors to the electrolyte that minimize this parasitic corrosion, thereby extending battery life and improving efficiency.^6,7

Recently, surfactant inhibitors have been studied as corrosion inhibitors by researchers because of their high inhibition efficiency and low toxicity.^8–13

The intent of the present study is to inhibit the hydrogen evolution rate during the lithium corrosion in 4 M LiOH aqueous solution using cocamide diethanol amine (CDA) as a nonionic surfactant. The study comprised hydrogen evolution rate at OCP and potentiodynamic polarization measurements.

2. Experimental

Lithium (purity ∼ 99.9%) was used as a working test electrode with exposed area 0.625 cm².

The tests were performed in a traditional three electrode cell structure. It is comprised of working electrode (lithium), reference electrode (Hg/HgO) and counter electrode (platinum).

For hydrogen evolution rate measurements at open circuit potential OCP, an inverted burette was situated above the lithium electrode to determine the hydrogen gas volume evolved during the experiments.

All the measurements were carried out using ACM instruments Potentiostat/Galvanostat (Gill AC Serial no. 947). The polarization measurements were registered at a scan rate of 1.0 mV s⁻¹ in a potential scope of ±250 mV vs. open circuit potential.

The aqueous electrolyte (4.0 M LiOH aqueous solution) was prepared from lithium hydroxide monohydrate (Sigma-Aldrich Co.) and deionized water.

Cocamide diethanol amine (CDA) as a nonionic surfactant (Fig. 1), which was obtained commercially from KOA Chemicals Europe company, was added to 4.0 M LiOH aqueous solution in concentrations ranging from 200 to 1000 ppm (by weight).

Fig. 1 Molecular structure of cocamide diethanol amine (CDA) as a nonionic surfactant.

The CMC of CDA surfactant was determined by measuring the surface tension of surfactant using du Noüy ring method.

Experiments were carried out using calibrated thermostat at temperatures 298 and 313 K (±0.5 °C). All the test solutions were open to air.

3. Results and discussion

3.1. Hydrogen evolution rate measurements

The principal reactions of lithium with water are:

2Li → 2Li⁺ + 2e (anodic reaction) (1)

2H₂O + 2e → 2OH⁻ + H₂ (cathodic reaction) (2)

2Li + 2H₂O → 2LiOH + ↑H₂ (overall) (3)

From eqn (3) the hydrogen evolution rate (H_R) was calculated using the following relation:¹⁴

H_R = (V)/(S × t) (4)

where V is H₂ gas volume (mL), S is electrode surface area (cm²) and t is the experimental time (min).

The hydrogen evolution rates H_R obtained during the immersion of lithium in 4.0 M LiOH aqueous solution in the presence of different concentrations of CDA surfactant at 298 K are presented in Fig. 2.

Fig. 2 The hydrogen evolution rates during the immersion of lithium in 4.0 M LiOH aqueous solution at OCP in the absence and presence of various concentrations of CDA surfactant at 298 K.

I it is apparent that the presence of CDA surfactant in 4.0 M LiOH aqueous solution has a considerable influence on the inhibition of the hydrogen evolution rates H_R. Furthermore, increasing the concentration of CDA surfactant from 200 to 600 ppm causes a noticeable decrease in the hydrogen evolution rates H_R. A little bit of changes occur in H_R values changes when the surfactant concentration exceeds 600 ppm.

The efficiency of CDA surfactant (I_H%) to inhibit the hydrogen evolution is calculated according to:¹⁵

where H_R0 and H_R are the hydrogen evolution rates without and with CDA surfactant, respectively.

Fig. 3 shows variations of I_H% as a function of the logarithmic CDA surfactant concentration. The data evidently display that the efficiency of CDA surfactant I_H% to inhibit the hydrogen evolution rises with growing CDA concentration reaching a highest value (78.6%) at 600 ppm. However, a further increase in CDA surfactant concentration to 1000 ppm does not lead to a change in I_H% values. These results suggest that CDA surfactant acts as fairly efficient inhibitor for parasitic corrosion, i.e. it retard H₂ evolution reaction during the immersion of lithium in 4.0 M LiOH aqueous solution, likely as a consequence of its capability to adsorb and to form a covering layer on the lithium surface.¹⁶ It is obvious from CDA molecular structure (Fig. 1) that the CDA surfactant is able to adsorb on the lithium surface by using lone pair of electron of N and O atoms, as well the π-electron of C=O group.¹⁷ The possibility that the protons of OH groups in CDA surfactant can be exchanged for lithium is very weak. This is due to the negative inductive effect of the carbon atoms in the CDA molecule increases which in turn increases the electron density on the neighboring C–OH bond. This leads to difficulty in the release of proton causing lower dissociation and higher pK_a value (pK_a of CDA ≈ 14.6).

Fig. 3 The relationship between the efficiency of CDA (I_H%) and the logarithmic concentration of surfactant.

The inhibition effect of the surfactants mainly depends upon the critical micelle concentration (CMC).¹⁸ The critical micelle concentration of the CDA surfactant was specified by surface tension assessment giving value of 520 ppm.⁹ This value is in good convention with that gained in Fig. 3. This indicates that the inhibitory effect of CDA surfactant reaches a greatest value around its CMC value.¹⁹ At CMC value the CDA surfactant form one monolayer on the lithium surface and this leads to the highest efficiency. On other hand, the extra CDA molecules that adsorb at concentrations above the CMC have no affect on the inhibition effect.

The impact of temperature on the performance of CDA surfactant is investigated by the hydrogen evolution measurements during the immersion of lithium in 4 M LiOH aqueous solution in the presence 600 ppm of CDA surfactant in the temperature range 298–313 K.

Fig. 4 illustrates plot of H₂ evolution rates H_R and the efficiency of CDA surfactant I_H% versus the solution temperature. The plot shows that the rates of the H₂ evolution rate in the presence of CDA surfactant increased and the efficiency of CDA surfactant decreased with increasing solution temperature. This behavior may be due to that the partial desorption of CDA surfactant molecules from the lithium surface with rising temperature and in the same time the rising temperature activates the rate of motion of the reactants and product species. Eventually, this leads to enhance in the rate of H₂ evolution.^20,21

Fig. 4 The relationship between the hydrogen evolution rates (H_R) and the efficiency of CDA (I_H%) versus the solution temperature.

The apparent activation energy (E_a) for H₂ evolution reaction was calculated using the Arrhenius equation:²²

H_R = A exp (−E_a/RT) (6)

where R symbolize to the ideal gas constant, A symbolize to the Arrhenius constant and T symbolize to the absolute temperature. The variation of logarithm of H_R with (1/T) in the absence (Blank) and presence of 600 ppm of CDA surfactant is shown in Fig. 5. The slopes of the lines of Fig. 5 are equal to E_a/2.303R from which the E_a were calculated and their values in the absence and presence of CDA were found as 28.46 and 39.16 kJ mol⁻¹, respectively.

Fig. 5 Arrhenius plots for the hydrogen evolution rates during the immersion of lithium in 4.0 M LiOH aqueous solution in the absence and presence of 600 ppm CDA surfactant.

Inspect ion of the data shows that the values of E_a for inhibited solution are higher than that for blank solution. The higher E_a value in the presence of CDA surfactant, implies physical adsorption exist between surfactant molecules and lithium surface.²³ The increase in the activation energy in the presence of CDA surfactant may be due to the hindering of H₂ evolution by CDA adsorption on the lithium surface.²⁴

3.2. Potentiodynamic polarization measurements

Fig. 6 shows Tafel polarization curves for lithium immersed in 4.0 M LiOH aqueous solution at 298 K in the presence of several CDA concentrations. Electrochemical kinetic values (E_corr = corrosion potential and j_corr = corrosion current density) are extracted form Tafel lines and offered in Table 1.

Fig. 6 Tafel polarization curves for lithium immersed in 4.0 M LiOH aqueous solution at 298 K in the absence and presence of different concentrations of CDA surfactant.

Table 1 Electrochemical parameters and the corresponding inhibition efficiency for lithium in 4.0 M LiOH aqueous solution in the absence and presence of various concentrations of CDA surfactant at 298 K

CDA surfactant (ppm)	Ecorr (V vs. Hg/HgO)	jcorr (mA cm^−2)	ηj%
Blank	−2.897	46.66	—
200	−2.984	24.63	47.21
400	−3.020	16.53	64.57
600	−3.054	8.61	81.54
800	−3.057	8.59	81.59
1000	−3.060	8.49	81.80

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The inhibition efficiencies (η_j%) of CDA surfactant were calculated using eqn (7).²⁵ These values are also offered in Table 1.

where j_corr(0) and j_corr are the corrosion current densities without and with CDA surfactant, respectively.

From Fig. 6, it is apparent that both lithium dissolution and hydrogen evolution reactions are impeded when CDA surfactant is added to 4.0 M LiOH aqueous solution. The values of j_corr greatly decrease with increasing CDA surfactant concentration up to 600 ppm; then j_corr values begin to a little bit change at higher concentration (>600 ppm) (Table 1). The corrosion potentials E_corr (in the presence of CDA surfactant) goes towards more negative values as compared to the E_corr value (in blank solution). Therefore, a predominant control of the cathodic reaction (hydrogen evolution reaction) could be supposed.²⁶ On other hand, CDA surfactant has slightly effect on the lithium dissolution.

From results given in Table 1, an increase of η_j% with CDA surfactant concentration, reaching a maximum value (81.5%) at CMC of CDA surfactant, was observed. However, the rises of CDA concentration beyond CMC value lead to very little changes in the inhibitory effect of CDA surfactant. This phenomenon is consistent with results obtained form hydrogen evolution measurements.

3.3. Application of adsorption isotherm

It is important to elucidate the mode of CDA adsorption on the lithium surface. For this target, the values of surface coverage degree (θ) for the inhibitor were obtained from hydrogen evolution (θ = I_H%/100) and potentiodynamic polarization data (θ = η_j%/100).²⁷

Langmuir adsorption isotherm (eqn (8)) has been utilized to fit the experimental data.²⁸

where C_inh is CDA concentration and K_ads is the equilibrium constant of adsorption process.

It was found that the plots (C_inh/θ) versus C_inh (Fig. 7) yield to straight lines with a strong correlation coefficient (R² ∼ 1), confirming that the adsorption of CDA surfactant on lithium surface complies with Langmuir isotherm.²⁸

Fig. 7 Langmuir's isotherm adsorption model of CDA surfactant on the lithium surface in 4.0 M LiOH aqueous solution at 298 K.

The values of K_ads were calculated as 1.1 × 10³ M⁻¹ from hydrogen evolution data and 1.2 × 10³ M⁻¹ from potentiodynamic polarization data. The high value of K_ads reflects the strong adsorption capability of CDA surfactant on the lithium surface.²⁹

The correlation between the standard free energy for adsorption process (ΔG⁰_ads) and K_ads, as follows:³⁰

ΔG⁰_ads = −R × T ln(55.5 × K_ads) (9)

where R and T are symbolic representatives of the molar gas constant and the absolute temperature, respectively.

The average values of ΔG⁰_ads were found as −27.2 and −27.4 kJ mol⁻¹ from hydrogen evolution and potentiodynamic polarization data, respectively.

The values of ΔG⁰_ads are negative, indicating that the adsorption of CDA surfactant on the lithium surface is thermodynamically possible.³¹ In this study, the value of ΔG⁰_ads is slightly negative than −20 kJ mol⁻¹; suggesting that the adsorption of CDA surfactant on lithium surface in 4.0 M LiOH aqueous solution at 298 K is mainly the physical adsorption.³²

4. Conclusions

(1) CDA surfactant acts as inhibitor for the H₂ evolution during the immersion of lithium in LiOH aqueous solution.

(2) Inhibition efficiency of CDA surfactant enhances with increase in the CDA surfactant concentration and diminishes with rise in solution temperature.

(3) The inhibition efficiency of CDA surfactant reaches a maximum value concentration around its CMC values.

(4) The adsorption of CDA surfactant on top of lithium surface from LiOH aqueous solution complies with Langmuir isotherm.

(5) Potentiodynamic polarization curves indicated that CDA surfactant behaves as cathodic-type inhibitor by inhibiting cathodic hydrogen evolution reactions.

(6) The high value of K_ads and negative value of ΔG⁰_ads suggested that, CDA surfactant molecules strongly adsorb on the lithium surface.

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