An alkaline direct ethylene glycol fuel cell with an alkali-doped polybenzimidazole membrane

Transcription

1 international journal of hydrogen energy 38 (213) 162e166 Available online at journal homepage: An alkaline direct ethylene glycol fuel cell with an alkali-doped polybenzimidazole membrane L. An, L. Zeng, T.S. Zhao* Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Special Administrative Region, China article info Article history: Received 18 May 213 Received in revised form 9 June 213 Accepted 1 June 213 Available online 9 July 213 Keywords: Fuel cell Direct ethylene glycol fuel cell Ethylene glycol Alkali-doped polybenzimidazole membrane abstract An alkaline direct ethylene glycol fuel cell (DEGFC) with an alkali-doped polybenzimidazole membrane (APM) is developed and tested. It is demonstrated that the use of APMs enables the present fuel cell to operate at high temperatures. The fuel cell results in the peak power densities of 8 mw cm 2 at 6 C and 112 mw cm 2 at 9 C, respectively. The power output at 6 C is found to be 67% higher than that by DEGFCs with proton exchange membranes, which is mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation and oxygen reduction reactions in alkaline media. Copyright ª 213, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Alkaline direct oxidation fuel cells (DOFC) running on various fuels have received ever-increasing attention, mainly due to increased performance as a result of fast electrochemical kinetics at both the anode and cathode in alkaline media [1]. Among the fuels used, ethanol has been recognized as the most suitable fuel as it is a sustainable and carbon-neutral transportation fuel [2e4]. However, a critical issue associated with an alkaline DOFC running on ethanol (DEFC) is that with state-of-the-art catalysts, the CeC bond in ethanol is difficult to break at low temperatures (<1 C), and the product of ethanol oxidation reaction (EOR) in alkaline media is predominated by acetic acid (CH 3 COOH), rather than carbon dioxide (CO 2 ) [5]. Hence, the actual and maximum electron transfer numbers per ethanol molecule are 4 (partial oxidation to acetic acid) and 12 (complete oxidation to carbon dioxide), respectively. Under this circumstance, the electron transfer rate (ETR) of the ethanol oxidation reaction (EOR) is only 33%, lowering the Faraday efficiency [6]. For this reason, finding alternative liquid fuels with a high ETR to replace ethanol becomes essential. Ethylene glycol (EG) is another choice for alkaline DOFCs; as the main product of the EG oxidation reaction is oxalic acid (HOOCeCOOH) [7,8], the ETR of the EG oxidation reaction reaches 8% [9]. For this reason, alkaline DOFCs running on EG (DEGFC) have received ever-increasing attention [1e12]. In principle, an anion exchange membrane (AEM) is used to conduct OH ions in alkaline DEGFCs (AEM-DEGFC), as shown in Fig. 1a. However, a barrier that limits the performance of AEM-DEGFCs is that AEMs cannot withstand high operating temperatures (typically lower than 6 C) due to the poor thermal stability associated with the use of instable functional groups (typically quaternary ammonium) in this type of * Corresponding author. Tel.: þ address: metzhao@ust.hk (T.S. Zhao) /$ e see front matter Copyright ª 213, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

2 international journal of hydrogen energy 38 (213) 162e Fig. 1 e Schematic of various types of direct ethylene glycol fuel cell (DEGFC). membrane [13]. Polybenzimidazole (PBI), with well-known good thermal stability, has been recognized as an alternative for high-temperature proton exchange membrane fuel cells (PEMFC) after doping with an acid (typically phosphoric acid) [14]. Similarly, doping with an alkali also enables PBI membranes to conduct OH ions [15]. For this reason, alkali-doped PBI membranes (APM) have been recently applied to alkaline DOFCs (APM-DOFC) [16,17]. Hou et al. [16] developed an APM- DMFC and demonstrated that the peak power density could reach 31 mw cm 2 at 9 C. The same group also reported an APM-DEFC and demonstrated that the APM-DEFC could yield a peak power density of 6 mw cm 2 at 9 C [17]. In this work, we developed and tested an APM-DEGFC, as shown in Fig. 1b. The present fuel cell not only can withstand a relatively high operating temperature (up to 1 C), but also can yield a peak power density of as high as 112 mw cm 2 at 9 C. 2. Experimental 2.1. Pre-treatment of the PBI membrane The PBI membrane, with a thickness of 3 mm, was provided by Yick-Vic. As the conductivity of the pure PBI membranes is extremely low, i.e.: 1 12 Scm 1 [18], the PBI membranes were pre-treated by immersing them in 6.-M KOH solution for 7 days to form APMs [19]. The APMs were then washed in DI water for several times to remove free alkali remained in the membrane matrix, and kept in DI water before use Measurement of the ionic conductivity The ionic conductivity of the APM in the through-plane direction was measured by the electrochemical impedance spectra (EIS). The detailed measurement of the ionic conductivity can be found elsewhere [15]. The ionic conductivity was determined by: s ¼ d (1) R,S where s is the ionic conductivity (S cm 1 ), d is the thickness of the membrane (cm), R is the membrane resistance obtained from the real axis intercept of the impedance Nyquist plot (U), and S is the effective area of the membrane (cm 2 ) MEA fabrication and measurement instrumentation A membrane electrode assembly (MEA), with an active area of cm cm, was comprised of a pre-treated APM sandwiched between an anode and a cathode electrode. The anode electrode was made by brushing the catalyst ink ( mg cm 2 PdNi/C and 5 wt.% PTFE) onto a piece of nickel foam [2]. The cathode electrode was made by attaching the commercial HYPERMECä catalyst ( mg cm 2 ) to a carbon cloth [21]. The prepared MEA was clamped between an anode and a cathode flow field (316L stainless steel). A fuel solution containing EG and KOH was fed into the anode flow channel by a peristaltic pump with a flow rate of 2. ml min 1, while pure oxygen was fed to the cathode with a flow rate of 1 standard cubic centimeters per minute (sccm). Polarization curves were measured by an electric load (Arbin BT2). The internal resistance of the cell was measured by the built-in function of Arbin BT2. 3. Results and discussion 3.1. General performance Fig. 2 presents the polarization and power density curves of the present APM-DEGFC. The experiment was conducted at 6 C with an aqueous solution of M EG þ 7. M KOH at a flow rate of 2. ml min 1 and with O 2 at a flow rate of 1 sccm. As seen in Fig. 2, a peak power density of 8 mw cm 2 was achieved at a current density of 25 ma cm 2 at 6 C. It was reported earlier that a PEM-DEGFC (Fig. 1c) that consisted of a proton conducting membrane and the Pt catalyst exhibited the peak power densities of 48 mw cm 2 and 8 mw cm 2 at 65 C and 8 C, respectively [22]. The substantially improved performance achieved with the present fuel cell is mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation reaction and oxygen reduction reaction in the alkaline medium. It is also worth mentioning that an AEM-DEGFC (Fig. 1a) was recently developed and the performance tests

3 164 international journal of hydrogen energy 38 (213) 162e showed that the peak power density was 67 mw cm 2 at 6 C [9], which is lower than that by the present fuel cell. More importantly, the main advantage of the present fuel cell is the capability of running at high temperatures (typically 9 C); while the AEM-DEGFC can work stably at temperatures below 6 C Effect of the EG concentration 6 o C Fig. 2 e Polarization and power density curves of the APM- DEGFC. Fig. 3 shows the cell performance with different EG concentrations ranging from.5 M to 2. M when the KOH concentration was fixed at M. It can be seen that the cell voltage first increased with the EG concentration and then decreased over the whole current density range; -M operation exhibited the highest peak power density. This trend in the present APM-DEGFC is similar to that in an AEM-DEGFC [9]. The reason leading to this phenomenon is explained as follows. For the fuel-electrolyte-fed APM-DEGFC, there are two reactants in the anode solution, i.e.: EG and OH ions. Hence, a competitive adsorption on the active surface exists between two reactants. It can be seen that when the EG concentration was increased from.5 M to M, the cell voltage increased. The increased voltage is attributed to the increased EG concentration that enables a faster transport of EG to active sites, decreasing the concentration loss of EG and thus increasing the cell When the EG concentration was further increased to 2. M, the cell voltage decreased. This is because when the EG concentration exceeds M, the EG concentration in the anode CL is sufficient for the EG oxidation reaction [9]. On the other hand, too high EG concentration will decrease the active sites occupied by OH ions, not only lowering the electrochemical kinetics, but also increasing the concentration loss of OH ions. Therefore, the cell voltage declined when the EG concentration was higher than M, resulting in an optimal EG concentration that can yield the highest 3.3. Effect of the KOH concentration We also investigated the effect of the KOH concentration on the cell performance and the results are shown in Fig. 4. Itis seen that 7.-M operation resulted in the highest peak power density (8 mw cm 2 ), but a concentration lower or higher than 7. M would cause the performance to decline. Generally, the alkalinity of the anode environment affects not only the electrochemical kinetics, but also the transfer rate of species to the anode [23,24]. Specifically, increasing the KOH concentration can enhance the kinetics of the EG oxidation reaction, resulting in an increase in the cell voltage, as shown in Table 1, but too high KOH concentration can dramatically decrease the active sites available for the EG adsorption in the anode CL, causing the cell voltage to decline as a result of the inadequate EG concentration even at low current densities. On the other hand, it is the fact that the net transport of OH ions in an APM-DEGFC is from the cathode to anode [19]. Increasing the KOH concentration results in a higher OH concentration in the anode CL, impeding the OH transport from the cathode to anode and thereby increasing the ionic transport resistance, as shown in Fig. 5. Also, the increased viscosity as a result of increasing KOH concentration can cause an increase in the mass/charge transport resistance (concentration loss and Temperature: 6 o C Anode: EG + 1M KOH, 2mL min -1, 1sccm.5 M M 2. M Temperature: 6 o C Anode: 1M EG + KOH, 2mL min -1, 1sccm M 3. M 5. M 7. M 9. M Fig. 3 e Effect of the EG concentration on the cell Fig. 4 e Effect of the KOH concentration on the cell

4 international journal of hydrogen energy 38 (213) 162e Table 1 e Effect of the KOH concentration on the opencircuit voltage (OCV). C KOH OCV (V) ohmic loss). Therefore, too high KOH concentration will take up much more active sites (concentration loss: EG) and resist the OH transport from the cathode to anode (ohmic loss), leading to the poor performance; too low KOH concentration will lower the kinetics of the EG oxidation reaction (activation loss) and lessen the active sites for OH adsorption (concentration loss: OH ions), causing the decreased cell voltage. Consequently, the competition between the favorable effect of the faster EG oxidation reaction kinetics and the adverse effect of the increased internal resistance and the existed competitive adsorption results in an optimal KOH concentration (7. M) that gives the best performance (8 mw cm 2 ). Anode: 1M EG + 7M KOH, 2 ml min -1, 1 sccm o C 9 o C Fig. 6 e Effect of the operating temperature on the cell Effect of the operating temperature This section presents the effect of the operating temperature on the performance of the present APM-DEGFC. We are particularly interested in whether the APM can withstand high operating temperatures (above 6 C). The cell performances at different operating temperatures are shown in Fig. 6. It is seen that the cell voltage increases with the operating temperature over the whole current density region. Specifically, the peak power density reaches 8 mw cm 2 at 6 C, and then it increases to 112 mw cm 2 at 9 C. In addition, the maximum current density increases from 45 ma cm 2 to 52 ma cm 2 when the operating temperature is raised from 6 Cto9 C. The increased performance as a result of increasing the operating temperature can be explained as follows. On one hand, increasing the operating temperature enhances the electrochemical kinetics on the both electrodes [25,26]. On the other hand, an increase in the operating temperature improves the ionic conductivity of the membrane (33.71 ms cm 1 at 6 C [15], and ms cm 1 at 9 C), and enhances the mass/charge transport, reducing the ohmic loss and concentration loss, respectively [25]. Therefore, the cell performance got improved with the operating temperature over the whole current density region. It should be pointed out that the performance of this type of fuel cell running on EG (112 mw cm 2 at 9 C) is much higher than that fueled with methanol (31 mw cm 2 at 9 C) [16] and ethanol (6 mw cm 2 at 9 C) [17]. As a comparison, DEFC with AEMs could not stably discharge at 9 C [27]. To further confirm the operating stability of the present APM-DEGFC at 9 C, the discharging behavior at a constant current density (1 ma cm 2 ) was investigated and the result is presented in Fig. 7. No fluctuations in the cell voltage were found during the 7-h continuous operation. In addition to oxygen, air was also tested as the oxidant of the present APM-DEGFC; the results are shown in Fig. 8. It can be seen that the peak power density is 92 mw cm 2 at 9 C, which is 18% lower than that using pure oxygen as the oxidant (112 mw cm 2 ) o C Internal Resistance, m ma cm -2 Anode: 1M EG + 7M KOH, 2mL min -1, 1sccm KOH Concentration, M Fig. 5 e Effect of the KOH concentration on the internal resistance Time, Hour Fig. 7 e Constant-current discharging behavior.

5 166 international journal of hydrogen energy 38 (213) 162e Concluding remarks In this work, we have developed and tested an APM-DEGFC. The effects of operating conditions, including the EG concentration, KOH concentration, the oxygen concentration, and the operating temperature on the cell performance were investigated. The experimental results show that the present fuel cell not only can withstand high operating temperatures (up to 1 C), but also yields the peak power densities of 8 mw cm 2 at 6 C and 112 mw cm 2 at 9 C, respectively. This DEGFC shows better performance than other types of cell running on EG do. In addition, the transient discharging behavior further confirms the good operating stability of the present APM-DEGFC at 9 C. Future attention should be paid to the development of the highly active electrocatalysts for the ethylene glycol oxidation reaction and the durability test for the present fuel cell system. 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