Performance of a direct ethylene glycol fuel cell with an anion-exchange membrane

international journal of hydrogen energy 35 (2010) 4329–4335

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Performance of a direct ethylene glycol fuel cell with an anion-exchange membrane L. An, T.S. Zhao*, S.Y. Shen, Q.X. Wu, R. Chen Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China

article info

abstract

Article history:

This paper reports on the development and performance test of an alkaline direct ethylene

Received 8 January 2010

glycol fuel cell. The fuel cell consists of an anion-exchange membrane with non-platinum

Received in revised form

electrocatalysts at both the anode and cathode. It is demonstrated that this type of fuel cell

31 January 2010

with relatively cheap membranes and catalysts can result in a maximum power density of

Accepted 1 February 2010

67 mW cm2 at 60  C, which represents the highest performance that has so far been

Available online 26 February 2010

reported in the open literature. The high performance is mainly attributed to the increased kinetics of both the ethylene glycol oxidation reaction and oxygen reduction reaction

Keywords:

rendered by the alkaline medium with the anion-exchange membrane. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Fuel cell Ethylene glycol Non-platinum catalyst Anion-exchange membrane Direct oxidation fuel cell Power density Operation conditions

1.

Introduction

Direct oxidation fuel cells (DOFCs), which generate electricity from electro-oxidation of a liquid fuel, are promising electrochemical devices for various applications. During the past decade, much attention has been paid to direct methanol fuel cells (DMFCs), due to the fact that methanol is the simplest alcohol and has faster electrochemical reaction kinetics, as well as its unique advantage, such as higher energy density, facile liquid fuel storage and simpler system [1]. Although promising, the DMFC technology is facing some challenging technical issues that are inherently associated with the nature of the fuel: methanol is toxic and volatile (boiling point (b.p.) ¼ 65  C), highly flammable and has a tendency to pass through the fuel-cell membrane. For this reason, finding

alternative liquid fuels to replace methanol becomes essential. Due to its special properties, such as high energy density, non-toxicity, and availability from renewable energy sources, ethanol has received increasing attention [2–4]. However, it has been shown that the C–C bond of ethanol is difficult to break with the existing electrocatalysts at low temperatures; as a result, the main product of ethanol oxidation reaction (EOR) is the acetic acid (CH3–COOH) rather than CO2 [5–7]. Under this circumstance, the electron transfer rate (ETR) of ethanol oxidation is rather low (typically 33%), lowering the faradic efficiency. In a search for alternative fuels that have lower toxicity, safer handling, higher energy density, and higher ETR, ethylene glycol (EG) is another choice for DOFCs. EG is much less volatile due to the high boiling point (b.p. ¼ 198  C) and less toxic than methanol; in addition, EG

* Corresponding author. Tel.: þ852 2358 8647. E-mail address: [email protected] (T.S. Zhao). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.009

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has the higher theoretical capacity than methanol (EG: 4.8 Ah mL1; methanol: 4 Ah mL1) [8–10]. It has been demonstrated that the main product of EG oxidation is oxalic acid (HOOC–COOH) [11,12]. For per mole of EG, the actual and maximum electron transfer numbers are 8 (partial oxidation to oxalic acid) and 10 (complete oxidation to carbon dioxide), hence the ETR of EG is 80%, which is much higher than that of ethanol. For the above reasons, DEGFCs are worth being investigated. Typically, DEGFCs are classified into two types in terms of the employed membrane: proton exchange membrane based DEGFCs (PEM-DEGFCs) and anion-exchange membrane based DEGFCs (AEM-DEGFCs). It has been shown that PEM-DEGFCs can achieve decent performance at high temperatures (>110  C) [9]. However, the performance of PEM-DEGFCs is usually rather low at room temperature because of the sluggish kinetics associated with acid membranes and the mixed potential caused by EG crossover. Recently, it has been demonstrated that when the acid electrolyte was changed to alkaline one, i.e: AEM-DEGFCs, the cell performance could be substantially improved [13–16]. The increased performance can be mainly attributed to the enhanced kinetics of the EG oxidation reaction and oxygen reduction reaction (ORR) in alkaline media than that in acid media [17]. Moreover, in an AEM-DEGFC the direction of the electro-osmotic drag is from the cathode to the anode, which can reduce the rate of fuel crossover from the anode to cathode, improving the cell performance. Particularly, EG shows the highest reactivity in alkaline solution and much less significant electrode poisoning by adsorbed CO upon oxidation in alkaline solution, improving the cell performance. In addition, the cost of AEMs is much lower than that of PEMs (typically Nafion). The above advantages make AEM-DEGFCs more appealing than PEMDEGFCs. Ogumi et al. [17] reported a systematic comparison of electro-oxidation of a series of polyhydric alcohols in alkaline solutions on a platinum electrode and found that EG showed the highest activity among the alcohols examined in KOH and K2CO3 solutions. Coutanceau et al. [18] developed PtPd/C electrocatalysts for the oxidation of EG. It was demonstrated that the improvement of the cell performance could be obtained if sodium hydroxide was added to the EG aqueous solution. Demarconnay et al. [19] investigated the EG electro-oxidation in alkaline media with multi-metallic Ptbased catalysts; they found that the addition of foreign atoms to platinum led to a decrease in the ability of the catalyst to break the C–C bond, but the catalyst containing Pd and Bi seems to activate the oxidation of EG in oxalate compared to platinum alone. Recently, Jin et al. [20] investigated the electrocatalytic oxidation of EG on PtAu nanocomposite catalysts in alkaline, neutral and acidic media; they found that the PtAu catalysts exhibited high electrocatalytic activity and stability in alkaline solution; the concentration of sodium hydroxide affects the oxidation peak on both potential and current density, and a higher alkalinity is favorable to the reaction. More recently, Ogumi et al. [21] investigated the performance of EG and methanol with AEM and Pt as anode catalyst at 50  C. The cell performance with EG (9.2 mW cm2) was higher than that with methanol (5.5 mW cm2), due to smaller polarization for electro-oxidation of EG than that of methanol. Matsuoka et al. [22] compared the performance among

different alcohols based on the PtRu/C as the anode catalyst in the alkaline media at 50  C and found that the cell fuelled with EG yielded the highest power density of 9.8 mW cm2. Our literature review indicates that past efforts on the development of the AEM-DEGFCs have been mainly concentrated on the EG oxidation activity and electrocatalysts. In this work, we developed a high performance AEM-DEGFC with non-platinum electrocatalysts. The developed fuel cell can yield a maximum power density of 67 mW cm2 at 60  C, which represents the highest performance that has so far been reported in the open literature. In addition, the effects of operation conditions on the cell performance were also investigated.

2.

Experimental

2.1.

Membrane electrode assembly

A membrane electrode assembly (MEA), with an active area of 1.0 cm  1.0 cm, was comprised of an AEM sandwiched between an anode and a cathode electrode. The AEM, 28 mm thick, was provided by Tokuyama. The cathode electrode was a single-side electrode consisting of a non-platinum HYPERMEC catalyst (Acta) with a loading of 1.0 mg cm2, which was attached to a backing layer made of carbon cloth (ETEK). On the anode, the catalyst layer (CL) was fabricated in-house with an anode catalyst coated substrate (CCS) method [23]. The anode catalyst ink was prepared by mixing home-made carbon supported PdNi catalyst [24] with a loading of 2.0 mg cm2, ethanol as the solvent and 5% PTFE as the binder [25]. Subsequently, the anode catalyst ink was stirred continuously in an ultrasonic bath for 20 min such that it was well dispersed. The anode catalyst ink was then brushed onto a piece of nickel foam (Hohsen Corp., Japan) that served as the backing layer to form an anode electrode.

2.2.

Fuel-cell setup and instrumentation

As shown in Fig. 1, the prepared MEA was fixed between an anode and a cathode flow field. The both flow fields were made of 316 L stainless steel plate, in which a single serpentine flow channel, 1.0 mm wide, 0.5 mm deep, and 1.0 mm wide, was grooved by the wire-cut technique. A fuel solution

Load Anode

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(CH2OH)2

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OHADL ACL

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Fig. 1 – Schematic of the AEM-DEGFC.

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containing EG and potassium hydroxide (KOH) was fed into the anode flow channel by a peristaltic pump, while pure oxygen was fed to the cathode without humidification. The flow rate of the oxygen was controlled and measured by a mass flow controller (Omega FMA-7105E). Additionally, the cell temperature was measured with a thermocouple located at the anode current collector, and two electrical heating rods were installed in the cell fixtures to control the operating temperature. An Arbin BT2000 (Arbin Instrument Inc.) was employed to measure the polarization curves. The internal resistance of the cell was measured by the built-in function of Arbin BT2000.

3.

Results and discussion

3.1.

General performance

Fig. 2 shows the polarization and power density curves of the AEM-DEGFC with the non-platinum catalysts both at the anode and cathode. The experiment was performed at 60  C with an aqueous solution of 1.0 M EG mixed with 7.0 M KOH pumped into the anode at a rate of 2.0 mL min1 and with dry pure oxygen at a flow rate of 100 standard cubic centimeter per minute (sccm) fed to the cathode. It is worth mentioning that an AEM-DEGFC with no metal hydroxide salts in the anode solution was recently developed [26]. However, the cell performance of this AEM-DEGFC was rather low (about 2.0 mW cm2 at 50  C). A maximum power density of 67 mW cm2 was achieved at a current density of 225 mA cm2, which is the highest among the other PEMDEGFCs with Pt-based catalysts [9] and AEM-DEGFCs [18,21,22]. It was reported earlier that a PEM-DEGFC that consisted of a nanoporous proton conducting membrane and the Pt catalyst could achieve a maximum power density of about 48 mW cm2 at 65  C [9]. It was also reported that an AEMDEGFC with the Pt/C as the anode and cathode catalysts could achieve a maximum power density of 19 mW cm2 at 20  C [18]. The substantially better performance achieved with the present AEM-DEGFC is attributed to the superior 1.0

electrocatalytic activity of the home-made PdNi/C catalyst for the EG oxidation reaction and the HYPERMEC catalyst for the ORR in the alkaline medium.

3.2.

Fig. 3 shows the cell performance with different EG concentrations when the KOH concentration was fixed at 1.0 M. At low current densities, it is seen that there was almost no change in the cell voltage when the EG concentration was increased from 0.25 M to 1.0 M, but the cell voltage underwent a significant drop when the EG concentration was further increased to 3.0 M. The reason leading to this phenomenon is explained as follows. In general for a given anode catalyst, the anode potential depends on the local concentrations of both EG and hydroxyl ions in the anode CL. However, a change in either of EG or hydroxyl ions’ concentrations will lead to a change in the other. For a given KOH concentration of 1.0 M, at low current densities, feeding EG with the concentrations of 0.25 M and 1.0 M can maintain the EG concentration at an appropriate level corresponding to the hydroxyl ions’ concentration at the active surfaces. Hence, at low current densities, the cell voltage remained almost the same when the EG concentration was increased from 0.25 M to 1.0 M. However, when the EG concentration was further increased to 3.0 M, the EG concentration will be too high at the active surfaces corresponding to the hydroxyl ions’ concentration rendered by 1.0-M KOH, leading to the difficulty in the adsorption of hydroxyl on the active site, hence the electrochemical kinetics is lowered and the cell performance is reduced [15]. It is interesting to notice from Fig. 3 that with an increase in the current density, the cell voltage dropped more rapidly with the 0.25-M EG concentration than with the 1.0-M EG concentration. This is because that feeding 0.25-M EG cannot furnish a sufficiently high mass transfer rate required at high current densities, causing the EG concentration in the anode CL to become insufficient. On the other hand, it is noticed that when the EG concentration was increased from 0.25 M to 1.0 M, the limiting current density increased from

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Fig. 2 – Polarization and power density curves of the AEMDEGFC. Anode: 1.0 M EG D 7.0 M KOH concentration aqueous solutions, 2.0 mL minL1. Cathode: pure oxygen, 100 sccm. Temperature: 60 8C.

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Temperature:60 C Anode:1M EG+7M KOH,2mL/min Cathode:O2,100sccm

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Fig. 3 – Effect of EG concentrations on cell performance. Anode: 1.0 M KOH containing various EG concentration aqueous solutions, 2.0 mL minL1. Cathode: pure oxygen, 100 sccm. Temperature: 60 8C.

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3.3.

Effect of KOH concentrations

We also investigated the effect of KOH concentrations on the cell performance at a fixed EG concentration of 1.0 M as presented in Fig. 5. In the low current density region (below 100 mA cm2), it can be seen that the cell voltage increases with the KOH concentration. This is because the voltage loss is predominated by the activation polarization at low current densities, so that the cell performance was high with the high 320

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Fig. 5 – Effect of KOH concentrations on cell performance. Anode: 1.0 M EG containing various KOH concentration aqueous solutions, 2.0 mL minL1. Cathode: pure oxygen, 100 sccm. Temperature: 60 8C.

KOH concentration. Although the internal cell resistance increases with the KOH concentration (170–198 mohm), as shown in Fig. 6, increasing the KOH concentration can faster the kinetics of EG oxidation, resulting in an increase in the cell voltage. This fact indicates that the cell performance monotonously increases with the KOH concentration at low current densities. At moderate and high current densities, it can be seen from Fig. 5 that the KOH concentration of 7.0 M resulted in the highest peak power density of 67 mW cm2 at the current density of 225 mA cm2, but a concentration lower or higher than 7.0 M would cause the performance to decline. The reason for this behavior is explained as follows. Generally, the alkalinity of the anode environment not only affects the electrochemical kinetics, but also the transfer of species to the anode [15,28]. The increase in KOH concentration from 1.0 M to 7.0 M can enhance the kinetics, which can be demonstrated by the OCV behavior in Fig. 6. Although the high hydroxyl

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200 mA cm2 to 330 mA cm2. However, when the EG is further increased to 3.0 M, too high EG concentration may cover the active sites in the anode CL, blocking the hydroxyl adsorption. Therefore, the anode reaction is slowed down, which is evidenced by that the open-circuit voltage (OCV) dropped from 652 mV to 628 mV, as shown in Fig. 4. The slow anode reaction results in the decreased cell performance. On the other hand, too high EG concentration may create a barrier for the transfer of the hydroxyl ions [16,27], giving rise to a significant increase in the internal cell resistance from 170 mohm to 300 mohm, as shown in Fig. 4. The increased internal cell resistance causes a reduction in the cell voltage. Hence, the lowered cell performance with too high EG concentration can be attributed to the poor kinetics and the high internal cell resistance at moderate and high current densities. In summary, for a given KOH concentration (1.0 M), the peak power density can reach 35 mW cm2 with the EG concentration of 1.0 M, as shown in Fig. 3. At low current densities, the cell voltages are almost the same when the EG concentration ranging from 0.25 M to 1.0 M, due to the fact that both EG concentration and hydroxyl concentration in the anode CL are adequate. However, a further increase in the EG concentration will lower the cell voltage resulting from the lowered kinetics of EG oxidation. In the moderate and high current density regions, the competition between the decreased concentration polarization and the increased ohmic polarization results in an optimal EG concentration for a given KOH concentration.

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Fig. 4 – Effect of EG concentrations on the open-circuit voltage and internal resistance. Anode: 1.0 M KOH containing various EG concentration aqueous solutions, 2.0 mL minL1. Cathode: pure oxygen, 100 sccm. Temperature: 60 8C.

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Fig. 6 – Effect of KOH concentrations on the open-circuit voltage and internal resistance. Anode: 1.0 M EG containing various KOH concentration aqueous solutions, 2.0 mL minL1. Cathode: pure oxygen, 100 sccm. Temperature: 60 8C.

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international journal of hydrogen energy 35 (2010) 4329–4335

Effect of the cell temperature

The effect of the cell operating temperature on the cell performance is illustrated in Fig. 9. It is seen that the cell performance increases with the temperature. In particular,

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Temperature:60 C Anode:1M EG+1M KOH Cathode:O2,100sccm

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Fig. 7 – Cell polarization curves with different anode flow rates. Anode: 1.0 M EG D 1.0 M KOH aqueous solution, 2.0–8.0 mL minL1. Cathode: pure oxygen, 100 sccm. Temperature: 60 8C.

the limiting current density increased from 150 mA cm2 to 260 mA cm2 when the temperature was increased from 23.0  C to 60.0  C. An increase in the operating temperature can enhance the kinetics of both the EG oxidation reaction and ORR, leading to the low activation polarization and thus improving the cell performance. In addition, the hydroxyl conductivity increases with the temperature, which will reduce the ohmic polarization and increase the cell performance. Furthermore, increasing the operating temperature will increase the EG and oxygen diffusivities, resulting in the low mass transport polarization, particularly at high current densities. Consequently, the improvement in the cell performance with increasing the operating temperature can be attributed to the faster electrochemical kinetics, the increased conductivity of the hydroxyl ions, and enhanced mass transfer [2]. 0.7 o

Temperature:60 C Anode:1M EG+1M KOH,2mL/min Cathode:O2

0.6

Effect of anode and cathode flow rates

The effect of the anode flow rate on the cell performance from 2.0 mL min1 to 8.0 mL min1 at 60  C is presented in Fig. 7. It is seen that the variation in the EG solution flow rate from 2.0 mL min1 to 8.0 mL min1 almost did not change the cell performance. Similarly, Fig. 8 shows that the polarization curves corresponding to the different oxygen flow rate are also almost the same. In conclusion, the effects of the anode and cathode flow rate on the cell performance are rather small for the present MEA.

3.5.

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concentration in the anode CL resists the hydroxyl ions’ transport from the cathode to the anode, leading to a slight increase from 170 mohm to 175 mohm in the cell resistance, as shown in Fig. 6; the favorable effect of the improved kinetics exceeds the negative effect of the cell resistance on the cell performance, which leads to the better cell performance with the higher KOH concentration up to 7.0 M. However, when the KOH concentration is further increased to 9.0 M, too high KOH concentration will lead to the excessive hydroxyl ions’ coverage, decreasing the number of sites available for EG adsorption in the anode CL [29], hence causing the performance to decline. Also, the consumption of the EG is increased at the moderate and high current densities. As a result, the anode reaction is slowed down due to too low EG concentration in the anode CL, decreasing the cell performance. On the other hand, the internal cell resistance increases rapidly from 175 mohm to 198 mohm when the KOH concentration is over 7.0 M, as shown in Fig. 6. The increase in the internal resistance with increasing the KOH concentration is explained as follows. In the AEM-DEGFC, the net transport of the hydroxyl ion is from the cathode to the anode. Increasing the KOH concentration at the anode causes a higher hydroxyl ion concentration in the anode CL, impeding the hydroxyl ion transport from the cathode to the anode and thereby increasing the transport resistance of hydroxyl ion. In summary, at low current densities the cell performance is predominated by the KOH concentration, due to the faster kinetics of the EG oxidation reaction with the higher KOH concentration. However, in the moderate and high current density regions, too high KOH concentration will resist the transport of hydroxyl ions from the cathode to the anode and take up much more active sites, leading to the large ohmic polarization and thus the poor performance; too low KOH concentration will lower the EG oxidation reaction kinetics, leading to the high activation polarization and thus the poor performance. Consequently, the competition between the favorable effect of the faster EG oxidation reaction kinetics and the adverse effect of the increased internal cell resistance results in an optimal KOH concentration (7.0 M) that gives the best cell performance (67 mW cm2) for a fixed EG concentration (1.0 M).

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Fig. 8 – Cell polarization curves with different cathode flow rates. Anode: 1.0 M EG D 1.0 M KOH aqueous solution, 2.0 mL minL1. Cathode: pure oxygen, 100–400 sccm. Temperature: 60 8C.

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international journal of hydrogen energy 35 (2010) 4329–4335

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Fig. 9 – Effect of temperature on cell performance. Anode: 1.0 M EG D 1.0 M KOH aqueous solution, 2.0 mL minL1. Cathode: pure oxygen, 100 sccm.

4.

Conclusion

In this work, we have developed an AEM-DEGFC with the nonplatinum electrocatalysts. The effects of operating conditions, including the EG concentration, KOH concentration, the flow rates of fuel and oxygen supply, and the operating temperature, on the cell performance were investigated. The experimental results show that this fuel cell can yield a maximum peak power density of 67 mW cm2 with the current density of 225 mA cm2 at 60  C, which represents the best performance among all the EG fuelled fuel cells that have been so far reported in the open literature. The high performance is mainly attributed to the increased kinetics of both the ethylene glycol oxidation reaction and oxygen reduction reaction rendered by the alkaline medium with the anionexchange membrane. We have also shown that the operating parameters, including the EG concentration, the KOH concentration, and the operating temperature, have significant influence on the cell performance.

Acknowledgements The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 623008).

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