Effect of Sulfate Acid on Electrolysis Process an Oxygen...

Sensors & Transducers, Vol. 169, Issue 4, April 2014, pp. 296-301

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© 2014 by IFSA Publishing, S. L. http://www.sensorsportal.com

Effect of Sulfate Acid on Electrolysis Process for Preparing EMD Using

an Oxygen Depolarized Cathode

1 Hui ZHANG, 2 Huimin MENG 1 Corrosion and Protection Center, University of Science and Technology Beijing,

30 Xueyuan Road, Beijing 100083, China 2 Beijing Key Laboratory for Corrosion, Erosion and Surface Technology,

University of Science and Technology Beijing, Beijing 100083, China 1 Tel.: 8618611420432, fax: 8601062332567

1 E-mail: [email protected]

Received: 7 April 2014 Accepted: 28 April 2014 Published: 30 April 2014 Abstract: Electrolytic manganese dioxide (EMD) was prepared by using gas diffusion electrode (GDE) instead of traditional hydrogen evolution cathode. Sulfate acid’s effects on the cell voltage, the current efficiency, the electro-catalytic activity and the lifetime of GDE are studied. The results show that as the concentration of sulfuric acid increases, the current efficiency increases, while the cell voltage and the lifetime of GDE decreases. When the concentration of sulfuric acid is 20-40 g·dm-3, the cell voltage is 1.1-1.2 V and the current efficiency is more than 99 %. When the concentration of sulfuric acid is 30 g·dm-3, the GDE has the best electro-catalytic activity, meanwhile the cell voltage maintains at 1.6 V after EMD is deposited on anode, and the lifetime of GDE reaches 400 h. XRD patterns show that all of the electrolytic products belong to γ-MnO2 in different sulfuric acid concentration. Copyright © 2014 IFSA Publishing, S. L. Keywords: Gas diffusion electrode, Electrolytic manganese dioxide, Sulfuric acid, Electro-catalytic activity, Stability. 1. Introduction

We employed a new production method of electrolytic manganese dioxide (EMD) [1, 2]. This approach has advantages including energy saving, environmental protection and safety. It has been demonstrated that the use of gas diffusion electrode (GDE) improves the cathode potential, thereby reduces the cell voltage. It also avoids the acid mist so that reduces environmental pollution and equipment corrosion. In the traditional preparation method of EMD [3-9], there mainly occur two kinds of reaction, oxygen evolution reaction and precipitated manganese dioxide reaction.

2 24 4 2++ + →O H e H O

(Po2=100 Pa), (1)

60

80

100

=1.200-0.066pH

=1.1834-0.07005pH

=1.167-0.074pH

φφφ

22 24 2 2+ ++ + → +MnO H e Mn H O

(C[Mn2+]=1 mol·dm-3), (2)

60

80

100

=1.206-0.132pH

=1.1943-0.1401pH

=1.1824-0.148pH

φφφ

Article number P_RP_0101

http://www.sensorsportal.com/


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From the equation (1) and (2), it demonstrates that the effect of pH value on the differences between the φ of two reactions is quite notable [10]. The pH value is higher, and it is more beneficial to preferentially precipitate EMD. But when the pH value is too high, MnOOH will be precipitated, so the quality and discharge performance of EMD are affected. Meanwhile higher pH also leads to higher solution resistance, poor conductivity, resulting in the cell voltage increasing and the current efficiency decreasing [11].

At present, GDE is mainly used in chlor-alkali industry, the fuel cell and synthesis of hydrogen peroxide. However, the electrolyte is in alkaline, neutral or weak acid condition [12-22], strong acid condition remains comparatively rare. The sulfuric acid concentration of traditional industrial is 20-50 g·dm-3 for EMD [10].

It is still unclear what effect it can cause to electro-catalytic activity and stability of the GDE by a large number of adsorption of H+ on cathode due to the role of electric force. In this paper, the effect of the different sulfate concentrations of the electrolyte on the cell voltage, current efficiency, the structure of EMD, the electrocatalytic activity and the life of the GDE is investigated.

2. Experimental The GDE was composed of a catalyst layer (ca. 3

mm thickness), a gas-diffusion layer (ca. 4 mm thickness) and a current collector (nickel mesh with a 0.1-1 mm mesh and 10 mm thick), as shown in Fig. 1(a). [1] The current collector was placed between the gas-diffusion layers to avoid direct contact with aqueous acidic solution. The catalyst layer contained Pt/C (40 %), activated carbon (AR), carbon black (50 %), and polytetrafluoroethylene [PTFE (60 wt. %)] in the mass ratio of 1:12:3:4 and the diffusion layer contained sodium sulfate anhydrous (AR), activated carbon, carbon black, and PTFE (60 wt. %) in the mass ratio of 1:1:1:2. To prepare the catalyst layer and diffusion layer, the constituent materials were first dispersed in ethanol. Then, the comprising materials were rolled into sheets. Finally, the layers were rolled together and compressed at 1000 kg·cm-2 for 5 min.

The self-made Pt/C type GDE (5 cm2) as a cathode, and the titanium plate with Ti-Mn alloy coating (7 cm2) as an anode, constitute an air-liquid cell, applying for EMD preparetion, with electrode spacing of 5 cm2 [2], as shown in Fig. 1. The electrochemical tests are carried out with 120 g·dm-3 manganese sulfate and 0-40 g·dm-3 sulfuric acid aqueous solution at the current density of 100 A·m-2 and the temperature of 70 °C.

Shanghai Chenhua CHI660B electrochemical workstation and the three-electrode system testing are used in this experiment. In the testing system, Hg/Hg2SO4 electrode (MSE) works as a reference electrode. After immersed in the electrolyte for

4 hours, the measured electrode is then tested at the scanning speed of 5 mV·s-1.

X-ray powder diffraction (XRD) is carried out on a Rigaku (Japan) D/max-RB X-ray diffractometer with Cu Ka radiation (λ = 0.15406 nm) at a scanning rate of 10°/min. The surface morphology and composition are analyzed by SEM (Cambridge-S360) and EDX (Trace Northen), respectively.

Fig. 1. Schematic of (a) gas diffusion electrode and (b) electrolysis cell [1, 2].

3. Results and Discussion 3.1. Effect of Sulfate Acid Concentration

on the Performance of Electrolytic Products

All peaks of XRD patterns of high quality EMD

are a typical γ-type crystal phase: a consecutive sequence between γ-MnO2 and ε-MnO2 phase sequence, called γ-ε-MnO2 crystal phase. The peaks of 22.2° and 37.1° respectively represents γ-MnO2

and ε-MnO2, and Q is the ratio of peak of 22.2° and 37.1° (Q=Q22.2°/Q37.1°). When its value ranges from 0.5 to 0.7, EMD has better discharge performance [23].

Fig. 2(a) displays the X-ray diffraction (XRD) patterns of the as-synthesized products at the different concentration of sulfate acid. The EMD pattern matches well with the standard pattern of γ-MnO2. The characteristic broad diffraction peaks at 2θ values of 21.8° and the sharp major peak at 37.2°, 42.9°, 56.5° and 67.8° can be ascribed to the reflection of (1 2 0), (1 3 1), (3 0 0), (1 6 0), and (0 0 3) planes of the γ-MnO2 (JCPDS file no. 14- 0644), respectively. None of EMD contains low activity α and β phase characteristic peaks. In addition all of the characteristic peaks show low intensity and broad peak, indicating that the samples are low crystallinity and small grain.

Q value changes with sulfate acid concentration as shown in Fig. 2(b). The results show that, within a certain range of sulfate acid concentration


298

(0-40 g·dm-3), the enhancement of Q value is positively related to the increasing of sulfate acid concentration. The Q values are between 0.5-0.7, when the concentration of sulfate acid is 20-40 g·dm-3. From the literature [23], it can be speculated that the discharge performance of EMD with sulfate acid concentration above 20 g·dm-3 is better than it below 20 g·dm-3.

Fig. 2. (a) XRD pattern and (b) peak strength ratio of EMD at different concentration of sulfate acid,

C(MnSO4·H2O) =120 g·dm-3, I=100 A·m-2, T=70 °C

3.2. Effect of Sulfate Acid Concentration on the Cell Voltage and the Current Efficiency of GDE System

In order to meet the lower cell voltage and higher

current efficiency, meanwhile, the optimum concentration of sulfate acid is evaluated by the change of the cell voltage and current efficiency at 100 A·m-2, as shown in Fig. 3. The current efficiency (CE) is calculated by Eq. 3 [24],

2

2

(%) 100%MnO

MnO

W FzCE

M Q

×=

×,

(3)

Q It= , (4)

where F is Faraday’s number, z the differential atomic charge (for Mn2+/Mn4+, z equals 2), MMnO2 the molar weight of EMD, WMnO2 the total weight of deposited EMD, I the deposition current, t the deposition time (t equals 2 hours).

Fig. 3. The cell voltage and the current efficiency vs. the concentration of sulfate acid (vs. Hg/Hg2SO4)

C(MnSO4·H2O)=120 g·dm-3, I=100 A·m-2, T=70 °C.

At the electrolysis temperature of 70 °C, current density of 100 A·m-2, MnSO4·H2O electrolyte concentration of 120 g·dm-3, sulfate acid concentration show a great impact on cell voltage and current efficiency. As the sulfate acid concentration increases, the cell voltage decreases greatly by 1 V. When sulfate acid concentration is over 20 g·dm-3, the cell voltage come up to a steady state. While current efficiency curve show a reverse curve trend that the current efficiency increases rapidly with sulfate acid concentration increasing. When sulfate acid concentration is over 20 g·dm-3, the current efficiency is over 99 %. From the viewpoint of cell voltage and current efficiency, the optimum concentration of sulfate acid is at 30-40 g·dm-3.

3.3. Optimum Concentration of Sulfate Acid

In order to determine the optimum concentration of sulfate acid of the best electrocatalytic activity of EMD, the polarization curves of the GDE at different sulfate acid concentration are shown in Fig. 4, when the current density, the concentration of manganese sulfate and temperature are 100 A·m-2, 120 g·dm-3 and 70°C, respectively. As shown in Fig. 4, at the same potential, the current density increases first then decreases slightly with the sulfate acid concentration increasing. When the sulfate acid concentration is 30 g·dm-3, the GDE has the best electro-catalytic activity. The initial potential of oxygen reduction reaction and the oxygen reduction current of dynamics control area (the area of small current density) are not changed with sulfate acid concentration increasing. It is speculated that the cathode process of GDE is controlled by oxygen ionization reaction speed at low current density. Oxygen electrode process is a complex four-electron reaction, O2+4H++4e→H2O in an acidic solution, H+ and ionized oxygen combine to generate H2O. The increasing of H+ promotes ionization diffusion of oxygen, thereby the cathode polarization decrease. Since the activity of the oxygen reduction reaction on Pt/C is affected by surface oxygen coverage on GDE,


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it is affected by acidic condition change. From the viewpoint of electrocatalytic activity, the optimum concentration of sulfate acid is 30 g·dm-3.

Fig. 4. The polarization curves of GDE in different sulfate acid (vs. Hg/Hg2SO4) C(MnSO4·H2O)=120 g·dm-3,

I=100 A·m-2, T=70 °C. 3.4. Effect of Sulfate Acid Concentration

on the Stability and the Lifetime of GDE

To study the effect of sulfate acid concentration on the stability of GDE, the changes of cathode potential with time at different concentration of sulfate acid are shown in Fig. 5. During the preliminary electrodeposition stage, the cathode potential slightly increases by no more than 0.1 V. The reason of cathode potential increasing is mainly that the solution gradually penetrates into the catalyst layer and then fully contacts with the catalyst layer, thus the cathode potential is stabilized after the formation of a sufficient number of the three-phase boundaries. In the later stage of electrodeposition, the cathode potential shows a sharp drop and GDE becomes invalid. As sulfate acid concentration increasing, the cathode potential increases greatly and then tends to be stable, and the electro-catalytic activity of GDE increases first then tends to be stable, in accord with the conclusion for Fig. 4. From the viewpoint of the stability of GDE, the optimum concentration of sulfate acid is 30 g·dm-3. The cathode potential keeps a long period of stability, and then rapidly declines, and the period of stability shorten with the concentration of sulfate acid increasing. The lifetime of GDE increases first and then decreases with increasing concentration of sulfate acid.

As Fig. 5 shows, cathode potential suddenly decreased after a certain time. This result is not caused by catalyst aggregation, because catalyst aggregation is a slow process [25], which cannot result in the sudden raise of cathode over potential. In order to find the effect of electrolyte on the structure and stability of GDE, the contact angle of catalyst layer and the SEM of cross section of GDE are tested as shown in Fig. 6 and Fig. 7, respectively.

Fig. 5. The cathode potential vs. time in different concentration of sulfate acid, C(MnSO4·H2O)=120 g·dm-3,

I=100 A·m-2, T=70 °C.

Fig. 6. The contact angle of GDE of catalyst layer: (a) 0 h, (b) 25 h, (c) 50 h, (d) 400 h.

Fig. 6 is the test chart of catalyst layer contact

angle with different performance period of GDE. Fig. 6(a) shows that unused electrode behaved as hydrophobicity. Fig. 6(b) shows that after 25 h electrolyzation, contact angle decreases, while the catalyst layer also behaved as hydrophobicity. Fig. 6(c) shows that after 50 h, contact angle continues to decrease. However, after 400 h electrolyzation, catalyst layer turned out to be hydrophilic, as shown in Fig. 6(d).

When the reaction begins, the contact angle between the catalyst layer and the electrolyte decreases, due to the electrolyte gradually soaks the catalyst layer, so that the solid-liquid-gas interface and the active sites of the catalytic increases. On this account, the cathode potential is appreciably going up at the electrolyzation beginning. As the reaction continues, a stable number of three-phase interface are formed, at this time the cathode potential is relatively stable, as shown in Fig. 5. This phenomenon is called as electrolyte migration [26-27], namely, as the time of electrolyzation increases, the contact angle reduces, so that the electrode turns from hydrophobicity to hydrophily. The main reason for this phenomenon is when electrode is powered on, the contact angle decreases under the influence of micro electric-field.


300

Fig. 7. The morphology and the Mn2+ line distribution of cross section of GDE after 300 h, and the concentration

of sulfate acid are (a) 0 g·dm-3, (b) 10 g·dm-3, (c) 20 g·dm-3, (d) 30 g·dm-3 and (e) 40 g·dm-3.

During final electrolyzation term, the cell voltage suddenly increases, and the surface of electrode separates gas out, at this time it can be observed visually droplets deposited on the electrode surface. It is speculated that inner part of the catalytic layer changes from hydrophobic to hydrophilic, and form a new liquid passage, so that the electrolyte can pass through the catalytic layer and gradually immerses in the nickel mesh and the gas diffusion layer. It is noted that nickel mesh is very unstable in strongly acidic electrolyte, can easily be corrodible and dissolved, this conclusion is verified by Fig. 7. The electrolyte passes through catalyst layer and gets into the gas diffusion layer, which not only changes the internal micro-structure of the catalyst layer, but also changes the micro-structure of the nickel mesh and the gas diffusion layer. Finally, the GDE structure is changed, and the cathode potential increases.

In order to find the effect of sulfate acid concentration on the stability of GDE, SEM images and the Mn2+ line scan analysis of cross section of GDE after 300 h at different concentration of sulfate acid are shown in Fig. 7. As can be seen, the degree of damage of GDE increases with increasing the concentration of sulfate acid. When sulfate acid concentration is 0-10 g·dm-3, the complete structure of GDE has no obvious ruptures, and the Mn2+ show a descending trend in the catalyst layer with increasing depth, as shown in Fig. 7(a) and (b). When sulfate acid concentration is 20 g·dm-3, the catalyst layer occur bump cracking, and the Mn2+ increase in

the nickel mesh layer, as shown in Fig. 7(c). When sulfate acid concentration is 30 g·dm-3, nickel mesh in GDE has been partially dissolved, producing a lot of voids, which result in the catalyst layer separated from the gas diffusion layer, as shown in Fig. 7(d). When sulfate acid concentration is 40 g·dm-3, nickel mesh in GDE has been completely dissolved, and GDE has been completely saturated by the electrolyte, as shown in Fig. 7(e). The stability of GDE decreases with increasing the concentration of sulfate acid, because that nickel mesh has poor stability in acidic electrolyte, and the increasing H+ accelerates the dissolution of nickel mesh, which verifies the conclusion of the previous. It also indicates the reason for shortening period of stability, as shown in Fig. 5, that with increasing the concentration of sulfate acid, the dissolution rate of nickel mesh increase, then nickel mesh is partially dissolved result in the catalyst layer separated from the gas diffusion layer, and the area of three-phase boundary decrease. Consequently, the cathode begins to hydrogen evolution reaction, hydrogen in order to leave the GDE surface rupture the catalyst layer, result in accelerating dissolution rate of nickel mesh, at last, the oxygen cathode failures and becomes completely hydrogen evolution cathode.

4. Conclusions

1) Prepared EMD is the typical crystal structure of γ -MnO2. The linear relation of Q value was excellent within the concentration range of sulfate acid was 0-40 g·dm-3 and Q value is between 0.5 and 0.7 at 20-30 g·dm-3.

2) The cell voltage decreased with the concentration of sulfate increasing and tend to be stable when the concentration of sulfate acid reached 30 g·dm-3 and more. The cell voltage decreases by about 1.0 V. The current efficiency was improved to 99 % as sulfate acid content more than 20 g·dm-3.

3) Polarization curves show that the current density increases first and then decreases with increasing sulfate acid at the same overpotential. The catalytic activity of GDE was the highest at 30 g·dm-3.

4) Sulfate acid greatly affects the stability and life of GDE. Since the electro-capillary phenomenon, the water channel is changed gradually by the electrolyte catalyst layer, so that it changes from hydrophobicity to hydrophilicity. Finally the electrolyte immerses into the nickel mesh layer. With increasing acid concentration, the dissolution rate of nickel net is accelerated and the life of GDE is reduced.

5) Comprehensive consideration suggests that the best concentration of sulfate acid was 30 g·dm-3, at which the life of GDE can reach up to 400 h.

Acknowledgements

The project was supported by the National Natural Science Foundation of China (No.51274027).


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Effect of Sulfate Acid on Electrolysis Process an Oxygen...

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