Hydrogen evolution inhibition by L-serine at the negative electrode of a lead–acid battery

Abstract

The inhibition effect of L-serine on the hydrogen evolution at the negative electrode of a lead–acid battery (Pb) in 5.0 M H₂SO₄ has been studied by hydrogen evolution and electrochemical methods. The surface of Pb is analyzed with a scanning electron microscope (SEM) and energy-dispersive X-ray (EDX). The results demonstrate that L-serine is an adequate inhibitor, retarding the hydrogen evolution reaction. Polarization curves denote that L-serine performs as the cathodic inhibitor. The inhibition efficiency increases with an increase in L-serine concentration but decreases with an upturn in temperature. The most efficient inhibitor concentration is 10 mM. Adsorption of L-serine on the Pb surface is unprompted and complies with Langmuir's isotherm. L-serine induces an energy barrier for the hydrogen evolution reaction and this barrier increases with increasing L-serine concentration.

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1. Introduction

The lead–acid battery is the oldest type of rechargeable battery.^1,2 It consists of PbO₂ as a positive electrode, Pb as a negative electrode and an electrolyte of aqueous H₂SO₄. The concentration of H₂SO₄ in a fully charged auto battery measures a specific gravity of 1.265–1.285.² This is equivalent to a molar concentration of 4.5–6.0 M. As the battery discharges, PbO₂ and Pb react with an aqueous solution of H₂SO₄ to form PbSO₄ and water. On recharge, PbSO₄ converts back to PbO₂ and Pb. At the same time, SO₄²⁻ ions are driven back into the electrolyte solution to form H₂SO₄. The reactions can be presented as the following:

At the negative electrode:³

Pb + H₂SO₄ = PbSO₄ + 2H⁺+ 2e (1)

2H⁺ + 2e = H₂ (2)

At the positive electrode:

PbO₂ + H₂SO₄ + 2H⁺ + 2e = PbSO₄ + 2H₂O (3)

H₂O = 1/2O₂ + 2H⁺+ 2e (4)

Under normal conditions, gases vented from a lead–acid battery are mainly oxygen and hydrogen gas.^4,5

Hydrogen gas can collect at the top of a battery. If this gas is exposed to a flame or spark, it can explode. Hydrogen gas evolution harm the valve regulated lead acid battery during charging and discharging processes.⁶ In addition to the primary focus on human and system safety, hydrogen evolution also affects on the battery life and maintenance economics. Therefore the inhibition of hydrogen evolution during the charging of a lead–acid battery is an important part of the engineering for any battery system.^7–10

The aim of this work is to explore the use of L-serine to inhibit the hydrogen evolution during the reaction of Pb with 5.0 M H₂SO₄.

L-Serine is a non essential amino acid. It is manufactured by fermentation from carbohydrate sources.¹¹ In contrast to most commercial acid corrosion inhibitors which are highly toxic, L-serine is an environmentally friendly corrosion inhibitor. However, there is no literature to date about the corrosion inhibitory effect of L-serine in acid solution.

2. Experimental

2.1. Materials

The working electrodes were made from pure lead (Pb) 99.99%. Specimens used for hydrogen evolution measurements were mechanically cut into 2.0 × 1.0 × 0.2 cm dimensions. For electrochemical measurements, the cylindrical rod of Pb with exposed surface area of 0.452 cm² was allowed to contact the test solutions. The working electrodes were abraded with different grads of emery papers up to 1200 grad. Then they were cleaned by using acetone and distilled water before the tests.

Experiments were done in 5.0 M H₂SO₄ in the absence and presence of L-serine. L-Serine was obtained commercially from Sigma-Aldrich Co.

All solutions were prepared from analytical grade chemicals and distilled water. The temperature of the solutions was controlled by a thermostat.

2.2. Hydrogen evolution rate measurements

The schematic diagram of the hydrogen evolution measurements was reported in previous paper.¹² The volume of H₂ gas evolved (ΔV) during the reaction of Pb specimens in 100 ml of 5.0 M H₂SO₄ was measured at specified time intervals (t). The hydrogen evolution rate (H_R) was expressed using the following correlation:¹³

H_R= (ΔV)/(t) (5)

2.3. Electrochemical measurements

Electrochemical experiments (potentiodynamic polarization) were performed in a regular three-electrode cell. A platinum coil was used as the counter electrode and potentials were measured relative to Hg/Hg₂SO₄ reference electrode. Electrochemical experiments were conducted using potentiostat/galvanostat (Gill AC model no. 947, ACM instruments). Before electrochemical measurements, the Pb electrode was immersed in freshly prepared solutions at open circuit potential (OCP) for 2 h, to attain a steady case. The potential of polarization curves was done from −200 mV to +200 mV versus OCP with 1.0 mV s⁻¹ sweep rate.

2.4. SEM-EDX analysis

The surface morphology of the metal specimen was evaluated by SEM-EDX analysis using JOEL-JEM-1200 EX II ELETRON MICROSCOPE and a Traktor TN-2000 energy dispersive spectrometer.

3. Results and discussion

3.1. Effect of L-serine concentration on the hydrogen evolution

The effect of the addition of various concentrations of L-serine (0.1–20 mM) on the H_R during the reaction of Pb in 5.0 M H₂SO₄ at 303 K is shown in Fig. 1. It should be noted that the presence of L-serine in 5.0 M H₂SO₄ has a considerable influence on the inhibition of the hydrogen evolution rates H_R. Furthermore, increasing the concentration of L-serine from 0.1 to 20 mM causes a noticeable decrease in hydrogen evolution rates H_R.

Fig. 1 Variation of hydrogen evolution rate H_R with L-serine concentration for Pb in 5.0 M H₂SO₄ at 303 K.

The efficiency of L-serine (I_H%) to inhibit the hydrogen evolution is calculated according to:¹³

where H_R0 and H_R are the hydrogen evolution rates without and with L-serine, respectively.

Fig. 2 shows variations of the efficiency of L-serine I_H% as a function of the logarithmic of L-serine concentration. The plot of Fig. 2 exhibits S-shaped adsorption isotherm. This suggests, but does not prove, that L-serine inhibits the hydrogen evolution during the reaction of Pb in 5.0 M H₂SO₄ by adsorption at the Pb/H₂SO₄ solution interface.¹⁴ It is evident from Fig. 2 that upon increasing L-serine concentration to 10 mM, I_H% values significantly increases reaching a maximum value (I_H% = 88). However, a further increase in L-serine concentration to 20 mM does not lead to a change in I_H% values. This may be due to the adsorption of L-serine on the surface of Pb have reached the state of equilibrium at 10 mM and consequently, any further addition will not yield any increase in the inhibition efficiency¹⁵

Fig. 2 Variation of the efficiency of L-serine I_H% as a function of the logarithmic concentration of L-serine.

3.2. Effect of temperature and activation energy calculation

The rate of the chemical reactions inside the lead–acid battery depends on the temperature. In general, the increase of the temperature causes the acceleration in the reaction rate of the internal processes, e.g. corrosion (aging), self discharge and the corresponding hydrogen and oxygen gas evolution rates.^16,17

To assess the influence of temperature on the hydrogen evolution rate and the hydrogen evolution inhibition process, the hydrogen evolution rates H_R are performed at different temperatures (303 and 323 K) without and with different concentrations of L-serine. The results are recorded in Table 1. The results obtained indicate that the rates of the hydrogen evolution rate in all cases increased with temperature while I_H% decreased. The increase in the H_R in the absence of L-serine with the rise of temperature may be arises from an increase in PbSO₄ solubility with temperature.¹⁸ In the same time, the raise in temperature accelerates both of the diffusion and migration rates for the reactant and product species. This drives to an increase in the rate of hydrogen evolution reaction. On the other hand, the increase in the H_R and the decrease in the I_H% in the presence of L-serine with the rise of temperature may be assign to the partial desorption of L-serine molecules from Pb surface with temperature.¹⁹

Table 1 The values of hydrogen evolution rates (H_R) at 303 and 323 K, and activation energy (E_a) for Pb in 5.0 M H₂SO₄ in the absence and presence of various concentrations of L-serine

L-Serine (mM)	HR1 (ml h^−1) 303 K	HR2 (ml h^−1) 323 K	Ea (kJ mol^−1)
Blank	25	38	16.92
0.1	23	36	18.09
0.5	17	27	18.68
1	7	14	27.99
5	4	9	32.75
10	3	8	39.61
15	3	7	34.22
20	3	7	34.22

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The activation energy (E_a) for the process responsible for hydrogen generation (diffusion of both protons and uncharged hydrogen in solution) was computed using the Arrhenius relation:¹³

where R is the ideal gas constant, H_R1 and H_R2 are the hydrogen evolution rates at the absolute temperatures T₁ and T₂, respectively.

The calculated values of E_a are recorded in Table 1.

The data examination shows that the activation energy is higher in the presence of L-serine than in its absence. The higher E_a value in the presence of L-serine, implies that physical adsorption exists between L-serine molecules and the charged surface. Moreover, the values of E_a increase with increasing L-serine concentration. This means that, the presence of L-serine induces an energy barrier for the hydrogen evolution reaction and this barrier increases with increasing concentration of L-serine.²⁰

3.3. Electrochemical measurements

Fig. 3 shows anodic and cathodic polarization plots (Tafel plots) recorded on Pb electrode in 5.0 M H₂SO₄ containing different concentrations of L-serine at 303 K.

Fig. 3 Tafel polarization curves for Pb electrode in 5.0 M H₂SO₄ containing different concentrations of L-serine at 303 K.

It can be noticed that the addition of L-serine causes a remarkable decrease in the corrosion rate.

It is clear that, the presence of L-serine shifts cathodic curves to lower values of current densities. Whereas the anodic curves are slightly retarded by L-serine. This result suggests that the addition of L-serine retards the hydrogen evolution reaction.²¹

Table 2 displays the kinetic parameters for corrosion process, i.e., corrosion potential (E_corr) and corrosion current density (j_corr).²²

Table 2 Polarization parameters for Pb in 5.0 M H₂SO₄ without and with various concentrations of L-serine

L-Serine (mM)	Ecorr (mV vs. Hg/Hg2SO4)	jcorr (mA cm^−2)	ηj%
Blank	−940	5.12	—
0.1	−1030	4.60	10.15
0.5	−1045	3.35	34.57
1	−1071	1.25	75.58
5	−1087	0.696	86.40
10	−1112	0.460	91.01
15	−1123	0.452	91.17
20	−1130	0.450	91.21

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Comparing with blank solution, E_corr shifts to negative side more than 85 mV in the presence of L-serine; this elucidates that L-serine works as cathodic-type inhibitor.²³ Evidently, j_corr decreases frequently in the presence of L-serine and decreases with L-serine concentration. The corrosion current density j_corr values can be used to calculate the inhibition performances of L-serine (η_j%) from the following equation:²⁴

where j_corr(0) and j_corr are uninhibited and inhibited current densities, respectively.

From results given in Table 2, an increase of η_j% with L-serine concentration, reaching a maximum value (91%) at 10 mM, was observed. A slight a change in η_j% values was found above the 10 mM. This phenomenon is consistent with results obtained form hydrogen evolution measurements.

3.4. Study of the adsorption phenomenon

The adsorption isotherms can supply helpful information about the mechanism of corrosion inhibition.^25,26

The degree of surface coverage (θ) of L-serine from hydrogen evolution and electrochemical measurements can be calculated using the following equations:²⁷

The surface coverage values θ were fitted to different adsorption isotherm models and best results judged by the correlation coefficient (R²) were obtained with Langmuir adsorption isotherm as follows:²⁸

where C_inh is L-serine concentration and K_ads is Langmuir isotherm constant.

The plot of C_inh/θ versus C_inh (Fig. 4) yields a straight lines with a correlation coefficients (R²) are close to one (Table 3). This supports the assumption that the adsorption of L-serine from 5.0 M H₂SO₄ solution on the Pb surface at the studied temperatures comply with the Langmuir adsorption isotherm.²⁹ The high values of K_ads (Table 3) reflect the high adsorption efficiency of L-serine on Pb surface.³⁰ K_ads can be used to determine the free Gibbs energy of adsorption (ΔG⁰_ads) using the following equation:³¹

ΔG⁰_ads = −RT ln(55.5K_ads) (12)

Fig. 4 Langmuir adsorption isotherm plot for adsorption of L-serine on the Pb surface at 303 K.

Table 3 Adsorption parameters for L-serine adsorption on Pb surface in 5.0H₂SO₄ at 303 K

Method	R^2	Kads (M^−1)	ΔG^0ads (kJ mol^−1)
Hydrogen evolution	0.9985	1428	−28.36
Electrochemical	0.9991	1666	−28.75

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The calculated ΔG⁰_ads values obtained from hydrogen evolution and electrochemical data were given in Table 3. The data clearly show that ΔG⁰_ads values were negative. In essence, this means that the adsorption of L-serine molecules on Pb surface will be favored and will release energy. Furthermore, the values ΔG⁰_ads of less than −40 kJ mol⁻¹ indicate physical adsorption.³²

The heat of adsorption (Q_ads) of L-serine on Pb surface has been determined as a function of the surface coverage (θ) and temperature as follows:³³

where θ₁ and θ₂ are the degrees of surface coverage (θ₁ and θ₂ values were calculated using eqn (9)) at T₁ and T₂, respectively.

The calculated values for Q_ads are presented in Table 4. The negative value of Q_ads assumes that the adsorption process is exothermic and physisorption.³³

Table 4 The heat of adsorption (Q_ads) of L-serine on Pb surface in 5.0 M H₂SO₄ at various concentrations of L-serine

L-Serine (mM)	θ1 303 K	θ2 323 K	Qads (kJ mol^−1)
0.1	0.080	0.052	−18.64
0.5	0.320	0.289	−6.51
1	0.720	0.631	−16.57
5	0.840	0.763	−19.93
10	0.880	0.789	−27.43
15	0.880	0.815	−20.67
20	0.880	0.815	−20.67

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3.5. SEM-EDX examinations

The aim of SEM-EDX examinations was to confirm the results obtained from the hydrogen evolution and electrochemical measurements that a protective surface film of L-serine molecules is formed on the Pb surface. To attain this aim, SEM-EDX examinations of the Pb surface were performed in 5.0 M H₂SO₄ in the absence and presence of 20 mM of L-serine.

As it is shown in Fig. 5a, the Pb surface was strongly damaged in the absence of L-serine due to Pb corrosion in 5.0 M H₂SO₄. However, less corrosion attack was found for the sample exposed to H₂SO₄ solution containing L-serine (Fig. 5b). A smoother surface is seen in the presence of 20 mM L-serine in comparison to that observed in Fig. 5a.

Fig. 5 SEM images of Pb exposed to 5.0 M H₂SO₄ in the absence (a) and presence of 20 mM of L-serine (b).

Fig. 6 presents EDX survey spectra recorded for Pb surface exposed for 5.0 M H₂SO₄ in the absence and presence of 20 mM L-serine. In H₂SO₄ solution free solution, the EDX spectra (Fig. 6a) show the characteristics peaks of Pb element. In addition, PbSO₄ is present, as indicated by the Pb, S and O signals. In solution contain 20 mM L-serine (Fig. 6b), the EDX spectra showed an additional line characteristic for the existence of N and C. In addition, the intensity of S signal decreased. The appearance of the N and C signals is due to the N and C atoms of the adsorbed L-serine molecules.

Fig. 6 The EDX spectra of Pb in 5.0 M H₂SO₄ in the absence (a) and presence of 20 mM of L-serine (b).

These data confirm the adsorption of L-serine molecules on the Pb surface and formulation a protective surface film.

3.6. Explanation for inhibition

The effectiveness of organic inhibitor may be due to the adsorption of inhibitor molecules on the metal surface.^34,35 Adsorption process occurs through the electrostatic attractive forces between dipoles and/or ionic charges on the adsorbed molecules and the electric charge on the metal surface. The presence of heteroatoms with loosely bound electrons or π-electron systems or aromatic rings in molecular structure of the inhibitor enhance the adsorption efficiency process.^36,37

In acidic solutions the anodic reaction produces metal ions, and the principal cathodic reaction produces hydrogen gas. An inhibitor may decrease the metal dissolution, hydrogen gas evolution, or both processes. The shift of the corrosion potential to the positive direction indicates mainly inhibition of the metal dissolution process (anodic inhibitor), whereas the shift to the negative direction indicates mainly inhibition of the hydrogen gas evolution (cathodic inhibitor).³⁸

In the present work, the essential step in the inhibition of hydrogen evolution during the reaction of Pb in 5.0 M H₂SO₄ is the adsorption of L-serine on cathodic sites on the Pb surface. L-serine has three polar groups, namely, NH₂, COOH, and OH groups. It can coordinate with Pb surface through the nitrogen atom and oxygen atom of the polar groups.³⁹ In neutral solutions, L-serine molecules are presented usually as zwitter ions. Whereas in acidic solutions, L-serine molecules are presented in protonated form as the following:

This means that in acidic medium, L-serine molecules are adsorbed through the ⁺NH₃ on the electrode surface (cathodic sites) and decrease the rate of the cathodic reaction, thus the rate of hydrogen evolution will be decreased.

4. Conclusion

The inhibition performance of L-serine on the hydrogen evolution at the negative electrode of a lead–acid battery (Pb) in 5.0 M H₂SO₄ solution was evaluated using hydrogen evolution and electrochemical methods. The results show that L-serine works as an adequate inhibitor for the hydrogen evolution at Pb electrode in 5.0 M H₂SO₄. The inhibition efficiency of L-serine depends on its concentration and solution temperature. The results of electrochemical measurements demonstrate that L-serine behaves as a cathodic-type inhibitor. The adsorption of L-serine on Pb surface complies with Langmuir's isotherm. SEM and EDX analysis clearly show that the inhibitor molecules form a good protective film on the Pb surface.

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