Effect of Additives on Electrochemical and Corrosion Behavior of Gel

Article pubs.acs.org/IECR

Effect of Additives on Electrochemical and Corrosion Behavior of Gel Type Electrodes for Zn-Air System Yong Nam Jo,† Hong Shin Kim,† K. Prasanna,† P. Robert Ilango,† Won Jong Lee,† Seung Wook Eom,‡ and Chang Woo Lee*,† †

Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1732 Deogyeong-daero, Gihung, Yongin, Gyeonggi 446-701, South Korea ‡ Battery Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 641-120, South Korea ABSTRACT: The corrosion behavior of the Zn anode is yet another major issue to be taken under consideration in the Zn-air system. In order to enhance electrochemical performance, a new aim is to find a possible delay in corrosion rates using 1 and 3 wt % of Bi or Ni as additives when compared with pure Zn anodes as gel type electrodes. Through Tafel extrapolation, the pure and 3 wt % Bi added Zn anodes show a 1.2718 mA and 0.8490 mA average corrosion current, respectively. Furthermore, the Zn anode with 3 wt % Bi shows increased 1328.9 Ω charge transfer resistance and lowered hydrogen evolution, meaning Bi is an effective additive for controlling the hydrogen evolution reaction as well as corrosion reaction in gel type electrodes.

1. INTRODUCTION The Zn-air system is an attractive system for energy generation or storage due to its high specific energy, low cost, and environmental friendliness.1,2 The Zn-air system has been classified as a fuel cell, primary battery, or secondary battery, according to the technological concept or reaction mechanism of the energy system. Among them, electrically recharged Znair batteries suffer from several problems. One of the main problems is performance limitation due to zinc dendrite formation and/or shape change.3,4 Various researchers have attempted to overcome those disadvantages in a battery system. One of the methods is a mechanical replacement of the Zn electrode after each discharge, followed by a short recharge time. However, this type of system requires service and maintenance after each discharge step. On the other hand, a hydraulic recharging system is another technique to overcome the problem of the electrical recharging system. In this system, the negative Zn electrodes are continuously supplied with fresh anode materials in flowing electrolytes2,5 and so maintain a fully charged state. Even though fresh metals are supplied, zinc has a greater negative reduction potential than hydrogen, which evolves hydrogen gas on the zinc particle surface and creates a problem of corrosion. An aqueous system has the disadvantage of low voltages leading to lower energies. However, this drawback is compensated by higher specific energy.1,5,6 The discharge reaction can be expressed as follows:6,7

In these reactions, Eoverall is theoretically 1.65 V, but it decreases to 1.45 V in the open circuit state. Many attempts to suppress hydrogen evolution and the corrosion reaction have been, so far, focused on using pellet type electrodes in an aqueous electrolyte system. However, this research area is still in the frontier stage to find applicable energy materials as anodes to overcome corrosion issues and satisfy electrochemical performances. In this study, we have tried to find proper additives for suppressing the corrosion reaction of the negative Zn electrode in a gel electrolyte system. The Zn electrodes have been named, in this paper, as a gel type electrode because Zn particles have shown gel-like behavior effectively mixing with a gel electrolyte. Additionally, our main interests like hydrogen evolution and corrosion reaction could be applicable out of specific classification of the Zn-air system. In order to study the electrochemical behavior, alkaline Zn-air cells were assembled using Zn anodes with additives and subjected to electrochemical discharge analysis, electrochemical impedance spectroscopy (EIS), Tafel extrapolation at room temperature, and hydrogen gas evolution characterization at 60 °C.

2. EXPERIMENTAL SECTION Pure Zn powder was used as a reference electrode, and 0.5, 1, and 3 wt % Ni or Bi powders were chosen as additives. It has been reported that certain amounts of metallic materials had an impact on corrosion rate and discharge time.8−11 To fabricate the proper electrolyte for the Zn-air system, 7, 8, and 9 M KOH solutions were prepared to produce an electrolyte mixed with poly acrylic acid (PAA) as a gelling agent. The ratio of mixed solution was 98 wt % of a KOH solution and 2 wt % of a poly acrylic acid. This gel electrolyte was mixed with Zn powder

O2 + 2H 2O + 4e− → 4OH−, Ecathodic = 0.40V

at the cathode and Zn 2 + + 2OH− → ZnO + H 2O + 2e−, Eanodic = 1.25V

at the anode. The overall reaction can be expressed as follows:

Received: July 9, 2014 Revised: September 19, 2014 Accepted: October 17, 2014

2Zn + O2 → 2ZnO, Eoverall = 1.65V © XXXX American Chemical Society

A

dx.doi.org/10.1021/ie5027315 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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electrode system, we characterized the electrochemical discharge performance for pure Zn electrodes in 7, 8, and 9 M KOH solutions. Figure 2 shows the electrochemical

and/or additives, and we named the mixed paste the gel type electrode. To characterize the electrochemical and corrosion performances, we manufactured a prototype Zn-air system as shown in Figure 1. The cell was composed of a top cover with

Figure 1. Schematic illustration of a prototype Zn-air system.

Figure 2. Electrochemical potential profiles for pure Zn electrodes with different concentrations of KOH solution.

small holes, a conventional air cathode (ADE-75, MEET), a polypropylene-based hydrophilic separator (Celgard 3401), an electrode container, a Cu current collector, and a bottom cover. The prepared gel type electrode was located in an electrode container 2.5 cm in length, 2 cm in width, and 0.1 cm in height. The Zn gel anode was composed of 0.6 g of Zn and 0.6 mL of gel electrolyte with a specified amount of additives. Electrochemical discharge performance at room temperature was characterized by a cycler (BaTester 05001, HTC) with 60 min rest for gelled electrolyte soaking into active materials, a 100 mA current, and a 0.2 V cutoff potential. Another electrochemical characterization is as in the following. The EIS measurement and Tafel extrapolation were performed with an electrochemical analyzer (COMPACTSTAT, IVIUM technologies). EIS measurements were carried out in the frequency range of 100 kHz to 0.01 Hz with an amplitude of 0.01 V. The electrochemical cell for Tafel extrapolation was formed using a normal hydrogen reference electrode (NHE), a platinum wire as a counter electrode, and the working electrode. All electrochemical characterizations were carried out at room temperature. The corrosion rate has been examined by volumetric measurement of hydrogen evolution as a function of time with aqueous electrolyte. The gas collecting apparatus consisted of a glass vial with rubber septa and a syringe.6,12 To accelerate the hydrogen evolution reaction, the volumetric measurement was performed at 60 °C. The 1 g of active materials with or without additives was placed into a glass vial filled with 7 M KOH solution. The vial was sealed by rubber septa and connected to a needle of the syringe. Levels of the piston were monitored for 12 h.

potential profiles for pure Zn with different concentrations of KOH solutions. A 7 M KOH solution with pure Zn showed a longer discharge time than the 8 and 9 M KOH solutions with pure Zn. A 7 M KOH solution is suitable for the gel type electrode. Therefore, we used a 7 M KOH solution as an electrolyte for all other electrochemical experiments. 3.2. Discharge Performances of Zn Anodes with Additives. Many research groups have reported the effect of Bi and Ni as additives and/or alloying with Zn for a pellet type electrode.8,9,11,15 However, in this study, we tried to apply Bi and Ni as additives for the gel type electrode system. Figure 3 shows the electrochemical potential profiles of (a) Zn with 1 and 3 wt % of Bi and (b) Zn with 1 and 3 wt % of Ni. In all of these experiments, discharging was performed using CC mode up to 0.2 V. Zn with 1 wt % Bi showed a discharge time of 10 320 s. This value was higher than that of Zn with 3 wt % Bi. Zn with 1 and 3 wt % Ni as additives showed poor discharge times compared with Zn with Bi as an additive. With the increasing amount of additives, the discharge performance was deteriorated. When comparing Bi with Ni, Bi is a more effective additive than Ni in terms of increasing discharge performance. Comparatively, Bi showed better electrochemical behavior than Ni as an additive. It is attributed to better electronic conductivity and lower polarization of Bi. Bi as an additive provides an electronic conduction path, and it improves electronic Zn electrodes. Since Bi is a high hydrogen overpotential material, the addition of Bi leads to decreasing or delaying the corrosion rate.9 3.3. Corrosion Behavior of Zn Anodes with Additives. As shown in Figure 3, Zn with 1 wt % Bi showed a longer discharge time than others. However, pure Zn has better discharge performance than that of Zn with additives in our gel type electrode system. However, other research groups reported better electrochemical discharge performance using Bi or Ni as additives and/or coating agents than that of pure Zn in pellet type electrodes.8,9,11 The difference between pellet type and gel type electrode is surface contact area between electrode and electrolyte. The gel type electrode is composed of active material powders and gelled electrolyte. Its surface area is

3. RESULTS AND DISCUSSION 3.1. Suitable Concentrations of Electrolyte. Increasing concentration of KOH lead to reducing the resistance of the electrolyte, but too high of a concentration of KOH can lead to increased viscosity of the electrolyte.13 Previous research groups have reported the relationship between conductivity and concentration of KOH solution with a pellet type electrode.4,14 To find the optimized concentration of electrolyte in the gel B



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Figure 4. Nyquist diagrams of pure Zn and Zn electrodes with 1 and 3 wt % of Bi or Ni as additives.

materials was measured using the volumetric method. Figure 5 shows the results of the volumetric amount of hydrogen evolved from Zn with two kinds of additives at 60 °C. Zn with 1 wt % Bi has the lowest hydrogen evolution rate, and Zn with

Figure 3. Electrochemical potential profiles: (a) Zn with 1 and 3 wt % of Bi; (b) Zn with 1 and 3 wt % of Ni.

larger than that of a pellet type electrode. Hence, the active materials in the gel type electrode have more chances to contact the aqueous electrolyte. For this reason, a spontaneous corrosion reaction may easily occur in the gel type electrode. It is the reason for the discharge performance difference between the pellet type electrode and the gel type electrode. The authors think that the electrochemical discharge performance is closely related to corrosion behavior. Thus, we tried to clarify the effect of Bi and Ni as additives on the relationship between the corrosion behavior and electrochemical discharge performance. The value of charge transfer resistance relates to the hydrogen evolution reaction. Figure 4 shows Nyquist diagrams of pure Zn and Zn electrodes with 1 and 3 wt % of Bi or Ni as additives. A semicircle indicates that the hydrogen evolution reaction is taking place on the electrode, and it is controlled by charge transfer. The pure Zn and Zn with 1 wt % Ni had charge transfer resistance values of 47.5 and 8.0 Ω, respectively. They showed significantly lower charge transfer resistance compared with Zn with Bi as additives. Zn with 3 wt % Bi showed a charge transfer resistance value of 1328.9 Ω, which was a higher value than that of other attempted materials. So, the hydrogen evolution reaction on pure Zn and Zn with Ni occurred far easier than that of Zn with Bi as an additive. In other words, a high value of charge transfer resistance is associated with a slow corrosion rate,17,18 suggesting that corrosion can be delayed by using Bi as an additive in a gel type electrode. In order to study the additive effect on the hydrogen evolution reaction, the hydrogen evolved from the attempted

Figure 5. Volumetric amount of hydrogen evolved: (a) pure Zn and Zn with 1 wt % of Bi or Ni for 12 h; (b) pure Zn and Zn with 1 wt % of Bi initially for 1 h. C



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Figure 6. Anodic and cathodic Tafel curves: (a) pure Zn, (b) Zn with 0.5 wt % of Bi, (c) Zn with 1 wt % of Bi, (d) Zn with 3 wt % of Bi as an additive in a 7 M KOH solution.

Table 1. Value of Corrosion Potential and Corrosion Current for Pure Zn Electrode and Zn Electrodes with Certain Amounts of Bi

Zn

Zn + 0.5 wt % of Bi Zn + 1 wt % of Bi

Zn + 3 wt % of Bi

Ecorr/V Icorr/ mA Ecorr/V Icorr/ mA Ecorr/V Icorr/ mA Ecorr/V Icorr/ mA

first measurement

second measurement

third measurement

fourth measurement

fifth measurement

average

standard deviation

standard error

−1.4012 1.1040

−1.4070 1.1470

−1.4052 1.5570

−1.4061 1.3700

−1.4055 1.1810

−1.4050 1.2718

0.0020 0.0017

0.0010 0.0008

−1.3868 0.8938

−1.4095 0.8609

−1.3933 0.9572

−1.3891 0.9450

−1.3943 0.8415

−1.3946 0.8997

0.0079 0.0005

0.0040 0.0002

−1.3948 0.8657

−1.4009 0.7987

−1.4166 0.9172

−1.3949 0.9249

−1.3966 0.8826

−1.40076 0.8778

0.0082 0.0005

0.0041 0.0002

−1.3915 0.08106

−1.3937 0.8246

−1.4002 0.8709

−1.4011 0.8431

−1.3975 0.8958

−1.3968 0.8490

0.0037 0.0003

0.0018 0.0002

1 wt % Ni has the highest hydrogen evolution rate compared with pure Zn. These results corresponded well to the results of the EIS. Additionally, it is obvious that Bi, in itself, plays a role in retarding the spontaneous corrosion reaction of a pure Zn gel anode. Based on the reaction Zn + 2H2O → Zn(OH)2 + H2↑,6 the spontaneous corrosion reaction occurs. That is, Zn is consumed to produce zinc hydroxide during the spontaneous corrosion reaction, and this causes a loss of energy generation. As shown in Figure 5, Zn with Ni has the highest hydrogen evolution rate comparing with pure Zn as well as Zn with Bi. It implies that the spontaneous corrosion reaction actively occurs in Zn with Ni and causes a lower discharge capacity of Zn with Ni as shown in Figure 3. Zn with Bi maintained a slightly lesser

amount of hydrogen evolution than pure Zn for 12 h, whereas both Zn and Zn with Bi evolve almost the same amount of hydrogen initially for 1 h. In particular, the evolved hydrogen amount of Zn with Bi is 4 times lower than Zn with Ni at 12 h. For more precise analysis initially for 1 h, an additional experiment was conducted by using 2 g of active material and syringes with a smaller volume. We allowed 1 h of rest time for electrolyte soaking into active materials. As shown in Figure 5b, at 1 h, a lesser amount of hydrogen evolved at pure Zn. Thus, we found that a spontaneous corrosion reaction easily occurred at Zn with Bi initially for 1 h. For this reason, pure Zn showed a higher discharge capacity than others, as shown in Figures 2 and 3. D



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extrapolation, the corrosion current value of Zn with 3 wt % Bi was 0.8490 mA lower than that of the pure Zn anode. Thus, these results were in accordance with volumetric hydrogen evolution behavior. Even though Ni effectively contributed as a corrosion inhibitor in pellet type electrode, it was not noticeable in gel type electrode, whereas Bi was an effective additive to delay the corrosion reaction in a gel type electrode for the Zn-air system.

Tafel extrapolation was conducted using a three electrode system to evaluate the corrosion current and potential for pure Zn and Zn with a certain amount of Bi. Figure 6 shows the anodic and cathodic Tafel curves of the pure Zn electrode and Zn electrodes with 0.5, 1, and 3 wt % Bi as an additive in a 7 M KOH solution. To determine corrosion potential and current, an intersection of the tangent line with the cathodic and anodic slope curves was found. The Tafel extrapolation method for the determination of corrosion rate is valid with well-defined anodic or cathodic Tafel regions. However, the cathodic curve generally produces a better defined Tafel region.16 As shown in Figure 6, the anodic polarization curves did not show extensive Tafel regions, while the cathodic curve had a well-defined Tafel region for all samples. So, five tangent lines were drawn on the anodic curve and the intersection with a fixed tangent line, and the cathodic curve was identified. The values of corrosion potential and corrosion current for Zn and Zn with certain amounts of Bi are derived from Figure 6 and are summarized in Table 1. Figure 7 shows the relationship between a measurable

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the next generation secondary battery R&D program of MOTIE/KEIT [10042575, Development of 5 kW Zn-air battery for EV and 3.3 V-1,000F pouch type high-power supercapacitor].

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REFERENCES

(1) Garche, J.; Dyer, C.; Moseley, P.; et al. Encyclopedia of Electrochemical Power Sources; Elsevier: Amsterdam, 2009; Vol. 4, p 238. (2) Zhang, J. G.; Bruce, P. G.; Zhang, X. G. Hand Book of Battery Materials; Wiley-VCH: Weinheim, 2011; Vol. 2, chap. 22. (3) Lee, C. W.; Sathiyanarayanan, K.; Eom, S. W.; et al. Effect of additives on the electrochemical behaviour of zinc anodes for zinc/air fuel cells. J. Power Sources 2006, 160, 161−164. (4) Lee, J.-S.; Kim, S. T.; Cao, R.; et al. Metal-air batteries with high energy density: Li-air versus Zn-air. Adv. Energy Mater. 2011, 1, 34−50. (5) Smedley, S. I.; Zhang, X. G. A regenerative zinc-air fuel cell. J. Power Sources 2007, 165, 897−904. (6) Lee, S.-M.; Kim, Y.-J.; Eom, S.-W.; et al. Improvement in selfdischarge of Zn anode by applying surface modification for Zn-air batteries with high energy density. J. Power Sources 2013, 227, 177− 184. (7) Yap, C. K.; Tan, W. C.; Alias, S. S.; et al. Synthesis of zinc oxide by zinc-air system. J. Alloy. Compd. 2009, 484, 934−938. (8) Lee, C. W.; Sathiyanarayanan, K.; Eom, S. W.; et al. Novel alloys to improve the electrochemical behavior of zinc anodes for zinc/air battery. J. Power Sources 2006, 160, 1436−1441. (9) Zhang, C.; Wang, J. M.; Zhang, L.; et al. Study of the performance of secondary alkaline pasted zinc electrodes. J. Appl. Electrochem. 2001, 31, 1049−1054. (10) Wilcox, G. D.; Mitchell, P. J. Electrolyte additives for zincanoded secondary cells I. Brighteners, levellers and complexants. J. Power Sources 1989, 28, 345−359. (11) McBreen, J.; Gannon, E. Bismuth oxide as an additive in pasted zinc electrodes. J. Power Sources 1985, 15, 169−177. (12) Cho, Y.-D.; Fey, G. T.-K. Surface treatment of zinc anodes to improve discharge capacity and suppress hydrogen gas evolution. J. Power Sources 2008, 184, 610−616. (13) Sapkota, P.; Kim, H. An experimental study on the performance of a zinc air fuel cell with inexpensive metal oxide catalysts and porous organic polymer separators. J. Ind. Eng. Chem. 2010, 16, 39−44. (14) Gilliam, R. J.; Graydon, J. W.; Kirk, D. W.; et al. A review of specific conductivities of potassium hydroxide solutions for various concentrations and temperatures. Int. J. Hydrog. Energy 2007, 32, 359− 364. (15) Yano, M.; Fugitani, S.; Nishio, K.; et al. Effect of additives in zinc alloy powder on suppressing hydrogen evolution. J. Power Sources 1998, 74, 129−134. (16) McCafferty, E. Validation of corrosion rates measured by the Tafel extrapolation method. Corros. Sci. 2005, 47, 3202−3215.

Figure 7. Relationship between measurement number of extrapolation and corrosion current for pure Zn electrode and Zn electrodes with various amounts of Bi.

amount of extrapolation and corrosion current for pure Zn and Zn with certain amounts of Bi. As shown in Table 1, pure Zn had a −1.4050 V average corrosion potential and a 1.2718 mA average corrosion current, while Zn with 3 wt % Bi had a −1.3968 V average corrosion potential and a 0.8490 mA average corrosion current. The values of average corrosion potential were similar among tested materials. However, the material with more Bi showed a lower corrosion current value, whereas pure Zn had a higher corrosion current value than the other samples involving Bi as an additive. Therefore, it indicates that Bi effectively acted as a corrosion inhibitor in gel electrode system.

4. CONCLUSION A 7 M KOH solution was suitable for the gel type electrode system. The Zn anode with Bi showed comparatively better electrochemical discharge performance than that of the Zn anode with Ni as additives. The application of the EIS, Tafel extrapolation, and volumetric hydrogen evolution method has analytically revealed the corrosion rate of the gel type electrodes with and without additives. Through the EIS, Zn electrodes with Bi showed the biggest charge transfer resistance as 1328.9 Ω among the attempted materials. From Tafel E



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(17) Barranco, V.; Feliu, S., Jr.; Feliu, S. EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed coatings. Corros. Sci. 2004, 46, 2203− 2220. (18) Khaled, K. F. The inhibition of benzimidazole derivatives on corrosion of iron in 1 M HCl solutions. Electrochim. Acta 2003, 48, 2493−2503.

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Effect of Additives on Electrochemical and Corrosion Behavior of Gel

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