Hydrogen evolution behavior of electrochemically active carbon modified with indium and its effects on the cycle performance of valve-regulated lead-acid batteries

Abstract

Electrochemically active carbon (EAC) was modified with In and In(OH)₃ respectively. Its properties were characterized by TEM, EDS and XRD, and its hydrogen evolution behaviour and effects on the cycle life of valve-regulated lead-acid (VRLA) batteries were investigated. The study found that the modification of EAC with In or In(OH)₃ did not significantly influence the crystal structure and surface morphology of EAC, but it can effectively increase the overpotential of hydrogen evolution and decrease the evolution rate of hydrogen on EAC. It is also observed that the addition of EAC modified with an appropriate amount of In or In(OH)₃ in the negative plates of VRLA batteries can remarkably decrease the evolution rate of hydrogen and prolong the cycle life of batteries under high-rate partial-state-of-charge conditions. Moreover, in comparison with EAC modified with In, EAC modified with In(OH)₃ showed better performance in terms of the improved cycle life of batteries.

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1. Introduction

Lowering the energy consumption of automobiles has encouraged the use of various types of hybrid electric vehicles (HEVs). HEVs are generally defined to be those that use batteries only to provide an energy boost or storage for short periods such as during acceleration, engine start-up, idle-stop, and regenerative braking.¹ Discharge and charge events typically involve only a small fraction of the battery's capacity, but they occur continuously and frequently. At present, energy-storage candidate systems for HEV applications mainly include valve-regulated lead-acid (VRLA) battery, nickel–metal hydride (Ni–MH) battery, rechargeable lithium battery and supercapacitor. It should be stated that flooded-electrolyte lead-acid battery is also considered to be used in micro-HEVs and mild-HEVs.² As a prospective candidate, VRLA battery has some advantages including low initial cost, well-established manufacturing base, and high recycling efficiency compared to other competitive batteries.

Operation of VRLA battery under high-rate partial-state-of-charge (HRPSoC) conditions leads to progressive accumulation of PbSO₄ on the negative plates, which limits the cycle life of the battery. Recent works have shown that this problem can be solved by addition of the higher levels of carbon to the negative active material.^3–9 Incorporation of carbon into the negative plates of the lead-acid battery can overcome the accumulation of lead sulfate, which allows the production of prototype lead-acid battery operated successfully in the HEV mode.¹⁰ Lam and co-authors have made the ultra battery with a conventional PbO₂ positive plate and a negative plate comprising two parts: half of it is a carbon electrode and another half is a regular negative plate.^2,11,12 Carbon reduces negative plate sulfation and inhibits capacity loss by competing with lead to react with O₂ during charge. Carbon also increases the conductivity of the negative plate, and it reduces cell heating on overcharge.¹³ The carbon capacitor electrode acts as a buffer to share the discharge and charge currents with the lead-acid negative plate, and thus prevents it being discharged and charged at the full rates required by the HEV duty.¹⁴ All these effects contribute to increase the cycle life for VRLA batteries in high-rate, partial state-of-charge cycling applications, such as HEVs.¹⁵ In order to facilitate the electrochemical reactions of charge, the carbon material added to the negative plate should have the following characteristics: (a) high electroconductivity, (b) high specific surface area, and (c) strong adhesion to the lead surface. The carbon with the above characteristics is called as “electrochemically active carbon” (labelled as EAC).¹⁶

Although the addition of carbon materials in the negative plates can restrain lead sulfate crystal's growth and share the charge–discharge currents with the negative plates, it increases the evolution rate of hydrogen on the negative plates during charging process. At the end of charge, the carbon materials in the negative plates significantly evolves more hydrogen gas than the lead negative plates because its potential shifts to a more negative value than it should be,¹⁷ which badly influences the cycle performance of battery. Therefore, it is necessary to reduce the hydrogen evolution rate on carbon materials during charging process. Surface modification is considered to be an effective way to change the characterization of carbon materials.^18–22 In this work, EAC (provided by Tianneng Group) was modified with In(OH)₃ and In respectively with the aim of inhibiting the hydrogen evolution on the negative plates and prolonging the cycle life of VRLA batteries under HRPSoC conditions.

2. Experimental

2.1 Modification of EAC with In(OH)₃

Firstly, required amount of In₂O₃ was dissolved in 20 mL H₂SO₄ solution with a concentration of 1 mol L⁻¹ to prepare In₂(SO₄)₃ solution, and then 2 g EAC was dispersed in the prepared In₂(SO₄)₃ solution. After being stirred for 7 h, the solution was evaporated to dryness and then the product was dispersed in ammonia water to form the precipitate of In(OH)₃ on EAC. The reaction system was kept at 25 °C and stirred constantly for 20 minutes. Finally, EAC was filtered, rinsed with distilled water, and oven-dried at 60 °C. In this paper, the mass ratio of In(OH)₃ to EAC were controlled at 0, 2.9%, 5.8%, 8.7% and 11.6%, respectively.

2.2 Modification of EAC with In

A certain amount of EAC was impregnated with an aqueous solution of In₂(SO₄)₃. After being stirred for 7 h, the solution was evaporated to dryness and then the product EAC was dispersed in alcoholic solution. Excessive NaBH₄ solution was dropped into the alcoholic solution in iced bath conditions. Then, the sample was filtered, washed with distilled water several times, and dried at 80 °C for 10 h. In our study, the mass ratio of In to EAC were controlled at 0, 2%, 4%, 6% and 8%, respectively.

2.3 Materials characterization

Phase structures of EAC were characterized by powder X-ray diffraction (XRD), Rigaku D/MAX-rB, Cu Kα radiation at 45 kV. The surface morphologies and microstructures of EAC were observed at a voltage of 100 kV using a TEM-1200EX transmission electron microscope (TEM).

2.4 Electrochemical measurements

In our investigation, EAC negative electrodes and the negative plates of VRLA batteries were prepared, respectively. EAC negative electrodes were prepared by mixing modified EAC and acetylene black with the mass ratio of 9 : 1 in CMC binder to form a paste. As-prepared paste was filled into 1 cm × 1 cm Pb–Ca grids, dried in air. Negative plates of VRLA batteries were prepared with lead oxide powder, barium sulfate, humic acids, and a certain amount of modified EAC by following a common industrial method.

The effects of surface modification on the evolution rate of hydrogen were studied by using linear sweep voltammetric method. A prepared negative electrode was used as working electrode, a commercial PbO₂ positive plate and a Hg/Hg₂SO₄ electrode were used as counter electrode and reference electrode, respectively. H₂SO₄ solution with a density of 1.35 g cm⁻³ was used as electrolyte. The measuring equipment was a CHI430A electrochemical workstation made by CH Instruments Inc.

Galvanostatic charge–discharge cycling was performed on a BTS series battery testing system (made by Neware Technology Limited) in the range of 1.60–2.90 V to investigate the effects of modified EAC on the cycle performance of VRLA batteries under HRPSoC conditions. Test cells were assembled from two commercial positive plates and one prepared negative plate, and its capacity was decided by the capacity of the negative plate. H₂SO₄ solution with a density of 1.35 g cm⁻³ was used as the electrolyte. After being activated, the test cells were firstly discharged to 50% of their initial capacity at 1 C and then cycled as follows: charge at 2.5 C rate for 60 s, stand for 60 s, discharge at 2.5 C rate for 60 s, stand for 60 s. The test process was terminated automatically when discharging voltage fell down to 1.60 V or charging voltage increased to 2.90 V.

3. Results and discussion

3.1 Characterization of modified EAC

Fig. 1 shows the XRD patterns of EAC modified with 8% In and 11.6% In(OH)₃. It can be seen from Fig. 1 that there were a strong diffraction peak at 22.2° and two weak diffraction peaks at 31.7° and 56.5° for EAC modified with 11.6% In(OH)₃. These three peaks are the characteristic peaks of In(OH)₃, that is to say that EAC can be modified with In(OH)₃ by precipitation method. There was only one obvious diffraction peak of metallic indium at 32.8° for EAC modified with 8% In because other diffraction peaks were too weak. In order to verify whether there is metallic indium on the modified EAC, energy dispersive spectrometer (EDS) was used to qualitatively analyze chemical element. Fig. 2 shows the EDS spectra of the modified EAC. It can be seen from Fig. 2 that indium element has been modified on EAC through liquid phase reaction methods.

Fig. 1 XRD diffraction patterns of EAC modified with In and In(OH)₃.

Fig. 2 EDS spectra of EAC modified with 5.8% In(OH)₃ (a), 11.6% In(OH)₃ (b), 4% In (c), 8% In (d).

Fig. 3 shows the TEM images of the modified EAC. It can be seen that some nanoparticles were decorated on the surface of EAC, and with increasing In or In(OH)₃ content on EAC, the amount of nanoparticles increased obviously. It can be concluded from above measurements that the nanoparticles decorated on the surface of EAC was In or In(OH)₃ particles, so the surface of EAC can be modified with In or In(OH)₃ particles.

Fig. 3 TEM patterns of EAC (a), EAC modified with 5.8% In(OH)₃ (b), 11.6% In(OH)₃ (c), 4% In (d), 8% In (e).

3.2 Electrochemical performance of the modified EAC

In order to investigate the effects of surface modification on hydrogen evolution rate of EAC, linear sweep voltammetry technique was used in this study. Fig. 4 shows the linear sweep curves of EAC modified with indium or indium hydroxide. It is obviously found that the modification of In(OH)₃ or In on EAC can influence the evolution rate of hydrogen on EAC. The current corresponding to certain potential consists of two parts: the current of hydrogen evolution and the one of double layer charge. Because the scan rate was 10 mV s⁻¹ for each sample, the current of double layer charge was the same. Therefore the different currents among the samples reflect the different hydrogen evolution rates. With increasing In(OH)₃ content on EAC, the hydrogen evolution rate decreased initially, and then increased; 8.7% In(OH)₃ resulted in a minimum hydrogen evolution rate. For EAC modified with In, increasing In content decreased the hydrogen evolution rate.

Fig. 4 Linear sweep curves of EAC modified with In(OH)₃ (a), In (b) at a scan rate of 10 mV s⁻¹.

Although the effects of modification of EAC with In(OH)₃ or In on the hydrogen evolution rate can be observed in Fig. 4, the effective degree is not very clear because of the disturbance of the charge current of double layer. Steady-state method can eliminate the disturbance of double layer charge, so the steady hydrogen evolution curves of the modified EAC were measured as given in Fig. 5. It can be seen from Fig. 5a that the modification of EAC with In(OH)₃ can influence obviously the steady hydrogen evolution rate: with increasing the content of In(OH)₃ on EAC, the hydrogen evolution rate decreased initially, and then increased; 8.7% In(OH)₃ resulted in a minimum steady hydrogen evolution rate. This trend is well consistent with the finding showed in Fig. 4a. Fig. 5b shows the effect of In modification on the steady hydrogen evolution rate of EAC. It is obviously found that the steady hydrogen evolution rate on EAC decreases gradually as the content of indium element on EAC increases, which is in good agreement with Fig. 4b. Therefore, it can be proposed that the modification of In(OH)₃ or In can decrease the hydrogen evolution rate on EAC effectively. By comparing Fig. 5 to 4, it can be found that the current in Fig. 4 is bigger than that of in Fig. 5, which is due to the current of capacitor in Fig. 4. Therefore, it can be deduced that the modified EAC still has super capacitor property, which is beneficial to improving the performance of the electrodes in batteries.

Fig. 5 Steady hydrogen evolution curves of EAC modified with different amounts of In(OH)₃ (a), In (b).

3.3 Effects of the modified EAC on the cathodic process of the negative plates of VRLA battery

According to the above observations, EAC modified with 8.7% In(OH)₃ and 8% In showed lower hydrogen evolution rates, so these two samples were used as the additives for the negative plates of VRLA battery. Fig. 6 shows the linear scanning curves of the negative plates of VRLA battery. It can be seen from Fig. 6 that, for all negative plates, there was a very weak reduction peak at the potential of −1.25 V, which corresponds to the reduction of PbSO₄ to Pb. Hydrogen started to evolve as the potential was lower than −1.25 V. In the potential range of hydrogen evolution, the response currents of the electrodes added with modified EAC were between that of Pb–C electrode and Pb electrode. That indicates that the addition of EAC in the negative plates will increase the evolution rate of hydrogen on the negative plates, while the modification of EAC with 8% In or 8.7% In(OH)₃ can decrease the evolution rate of hydrogen on the negative plates. It is well known that In or In(OH)₃ is a kind of high hydrogen evolution overpotential material.²³ In or In(OH)₃ adsorbed on EAC can restrain the formation of atomic hydrogen in H₂SO₄ solution, and then suppress hydrogen evolution on EAC during cathodic polarization process. So, in order to inhibit the hydrogen evolution on the negative plates, EAC added in negative plates should be modified with In or In(OH)₃.

Fig. 6 Linear scanning curves of the negative plates at a scan rate of 10 mV s⁻¹.

3.4 Influences of the modified EAC on HRPSoC performance of VRLA battery

The cycle performance of VRLA batteries added with the modified EAC in the negative plates under HRPSoC conditions was tested. Fig. 7 shows the cut-off charging–discharging voltages of the prepared VRLA batteries during HRPSoC cycles. It can be seen from Fig. 7a that the batteries added with the modified EAC in the negative plates showed a lower stable cut-off charging voltage and a higher cut-off discharging voltage than the batteries added with un-modified EAC and without EAC. The charging–discharging curves in Fig. 7b indicate that the addition of the modified EAC in the negative plates can improve discharging voltage, and decrease charging voltage. According to the test mode of cycle life, the electric quantity of charge and discharge is equivalent for a battery in a single cycle. So the battery with a higher charge acceptance could be charged more effective electric quantity, showing a higher discharging voltage and a higher cut-off discharging voltage at the end of the discharge process. In our test system, the major factor influencing the charge acceptance of the battery is the evolution rate of hydrogen on the negative plates. A lower hydrogen evolution rate will lead to a higher charge acceptance and a higher discharging voltage. So, it can be concluded that the battery with modified EAC addition in negative plates had a higher charge acceptance and lower hydrogen evolution rate (see also Fig. 6). It also can be found from Fig. 7 that, the addition of EAC in negative plates improved the HRPSoC cycle performance of VRLA battery, and the modification of EAC can furthermore improve the HRPSoC cycle performance of VRLA battery under HRPSoC conditions. The battery containing In(OH)₃ modified EAC in negative plates reached the longest cycle life (nearly 950 cycles), which is longer than 240 cycles of the battery added with In modified EAC and 400 cycles with un-modified EAC in negative plates. Therefore it can be concluded that the addition of EAC modified with 8.7% In(OH)₃ in negative plates can effectively inhibit evolution of hydrogen during the charge process, and enhance the charge acceptance and cycle performance of batteries under HRPSoC conditions.

Fig. 7 Cut-off charging–discharging voltages curves (a), charging–discharging curves (b) of batteries contained EAC in negative plates during cycle test process.

4. Conclusions

Surface modification of EAC with In or In(OH)₃ by liquid phase reaction method has a unconspicuous influence on the crystal structure and surface morphology of EAC, but it can obviously increase the overpotential of hydrogen evolution on EAC electrodes, and decrease the evolution rate of hydrogen. The addition of EAC modified with an appropriate amount of In or In(OH)₃ in negative plates of VRLA batteries can remarkably decrease the evolution rate of hydrogen on negative plates and prolong the cycle life of VRLA batteries under HRPSoC conditions. The EAC modified with In(OH)₃ showed the better performance of improving the cycle life of VRLA batteries under HRPSoC conditions. The battery added with the EAC modified with 8.7% In(OH)₃ in negative plates exhibited nearly twice longer cycle life than that without In(OH)₃.

Acknowledgements

We acknowledge financial supports from Postdoctoral Science-Research Developmental Foundation of Heilongjiang (LBH-Q11129) and Tianneng Group.

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