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C cathode catalyst for passive air-breathing alkaline anion exchange membrane μ-direct methanol fuel cell (AEM-μDMFC)
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Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC) R.W. Verjulio a, J. Santander a,*, J. Ma b, N. Alonso-Vante b a b
Instituto de Microelectronica de Barcelona, IMB-CNM (CSIC), Campus UAB, 08193, Bellaterra, Barcelona, Spain IC2MP, UMR-CNRS 7285, Universite de Poitiers, 4 rue Michel Brunet, 86022, Poitiers, France
article info
abstract
Article history:
In this work, carbon-supported CoSe2 is used as a Pt-free cathode catalyst in a passive, air-
Received 23 December 2015
breathing, alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC).
Accepted 23 January 2016
The obtained results demonstrate the improvement in the performance, when the device
Available online xxx
is operated at high fuel concentrations, if the proposed cathode catalyst is used instead of the standard Pt/C catalyst, due to the better tolerance to methanol crossover even in
Keywords:
alkaline medium. This result reinforces the suitability of AEM-DMFC's as a promising op-
Alkaline anion exchange membrane
tion for mobile devices powering.
Micro-fuel cells
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Direct methanol fuel cell Non-noble metal catalysts Tolerance to methanol crossover
Introduction Micro-fuel cells are considered as a promising alternative to batteries to power small portable devices both at the mini(mobile phones, laptops, …) and at the micro-scales (microelectromechanical systems (MEMS), wearables, …) [1e3]. In order to be implemented in the micro-scale, fuel cells must be designed in the simplest way, simplifying the added components usually found in standard size fuel cells to improve the performance of this electrochemical device, such as pumps or temperature control [4e7]. This principle guides research towards innovative new device architectures involving new materials which outperform traditional fuel cell components [8e15]. In this way, typical architectures for micro-fuel cells are based in the use of micro-channels driving liquid fuels by
capillarity instead of pumping it through traditional bipolar plates. In this context, membrane-less micro-fluidic fuel cells could be considered as an alternative architecture which permits omitting the ion conducting membrane at the cost of maintaining anode and cathode electrolytes in continuous movement in laminar flow regime, which prevents mixing of both electrolytes. The fluidic condition and the small amount of produced energy limit, in this case, the application of these devices to specialized environments (lab-on-a-chip, in-vivo, …) [16e18]. For general purpose micro-scale applications alkaline anion exchange membrane micro-direct methanol fuel cells (AEM-mDMFC) are being considered as promising devices for further research as integrated power devices for the microsystems field (PowerMEMS) [19e22]. The main advantage of this type of micro-fuel cell is related with the improved
* Corresponding author. E-mail address: [email protected] (J. Santander). http://dx.doi.org/10.1016/j.ijhydene.2016.01.132 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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catalysis in the alkaline media, which makes possible the use of non-Pt-based catalysts, obtaining a good performance at a lower cost. Moreover, the cross-over of fuel to the cathode is reduced due to the opposite movement of the hydroxide anions. This effect, together with the use of a tolerant cathode catalyst, will permit the use of high concentration methanol solutions [23e25], reinforcing the high energy density of this fuel in order to reduce the fuel reservoir volume of the microdevice. In this paper, a further improvement on our previous work, on AEM-mDMFC [26,27] focused on the cathode catalyst, is presented. Oxygen reduction reaction (ORR) essentially studied on Ptbased catalysts has been extended on transition metal chalcogenides [28,29]. Furthermore, an alternative non-precious metal chalcogenide electrocatalyst based on Cobalt for oxygen reduction reaction (ORR) in alkaline medium was reported [30] as well as macrocycles. Both families of materials are considered as potential alternatives to Pt-based materials due to their comparable ORR activity, abundance and remarkable tolerance to the presence of small organic molecules [24,31]. It was assessed that carbon-supported CoSe2 catalysts shows an efficient electrocatalytic activity for the ORR in acid and alkaline medium [30,31]. In this work, this non-precious Pt-free CoSe2/C cathodic catalyst has been tested in a passive air-breathing AEMmDMFC, which micro-device architecture is based on microfabricated silicon current collectors, with methanol concentrations up to 7 mol L1.
commercial Pt/C (20 wt% of Pt, E-TEK) or the synthesized CoSe2/C (20 wt% of metal) were used as cathodes catalysts. The electrocatalytic inks were prepared according to the following procedure: For each 5 mg of catalyst powder, 125 mL of alkaline ionomer AS-4 (5 wt%, Tokuyama) and 625 mL of ultrapure deionized water (18 MU cm) were mixed. The inks were then homogenized in an ultrasonic bath for 2 h. The airbrush technique was used to deposit the ink on the substrates. PTFE treated carbon cloth (ELAT, E-TEK) and nonteflonized carbon cloth (E-TEK) were used as substrates to deposition of the cathode and the anode inks, respectively. The metal loadings were 6 mg cm2 for the PtRu/C anodes and 1.5 mg cm2 for the (Pt/C or CoSe2/C) cathodes. The electrodes and the pretreated alkaline membranes were hot-pressed at 2 kN and 110 C for 3 min, in order to form the MEAs. The geometric active area of each electrode of the MEAs was 0.25 cm2.
Experimental
Catalysts characterization techniques
Synthesis of CoSe2/C electrocatalyst
Powder X-ray diffraction (XRD) was performed on a Bruker D5005 diffractometer under the following conditions: 40 kV, 40 mA using Cu Ka (l ¼ 1.5418 A) radiation source. The samples were step-scanned in steps of 0.05 (2q) in the range of 20 e90 using a counter time of 2 s per step. Elemental composition of the catalysts was determined using an inductively coupled plasma optical emission spectrometry (ICP-OES, spectrometer Optima 2000 DV, Perkin Elmer). The electrochemical measurements were conducted at 25 C in a thermostated three compartment electrochemical cell using a potentiostat (m-Autolab Type II). A plate of glassy carbon and a reversible hydrogen electrode (RHE) with a Luggin capillary were used as counter and reference electrode, respectively. A rotating disk electrode (RDE) of glassy carbon disk with a 3.0 mm diameter and geometric area of 0.07 cm2 served as working electrode. The working electrodes were prepared using the ultra-thin porous coating technique, by which the electrocatalytic ink was prepared by dispersing 10 mg of the catalyst powder in a mixture of 250 mL AS-4 anionic ionomer solution (5 wt%,Tokuyama) and 1250 mL Milli-Q ultrapure deionized water (18 MU cm) in an ultrasonic bath for 2 h. Thereafter, a 3 mL aliquot of the dispersed suspension (catalyst loading ca. 56 mg cm2) was pipetted on the top of the glassy carbon electrode previously polished (until a mirror-finished surface) with alumina powder (3A, Escil). Then it was dried in a stream of inert gas at room temperature for 30 min. The electrodes were subjected to 20 cyclic
The synthesis of 20 wt% CoSe2/C electrocatalyst was earlier reported [31,32]. In short, 0.395 mmol Co2(CO)8 (SigmaeAldrich) and 0.68 g Vulcan XC-72 (Cabot) were dispersed in 10 mL p-xylene (SigmaeAldrich) with stirring under N2 atmosphere at RT for 30 min. The suspension was heated to the refluxing temperature, and then cooled to RT. Thereafter, 1.58 mmol selenium powder (Alfa Aesar) was dispersed in 8 mL p-xylene by ultrasound for 30 min. This dispersion was mixed to the above suspension at RT for 30 min. The final black powder was collected by filtration, and washed with ethanol and dried under vacuum at RT. The CoSe2/C nanoparticle catalyst was heat-treated at 400 C under N2 atmosphere for 3 h.
Preparation of the alkaline MEAs Two different MEAs were prepared, one including the CoSe2/C based cathode and the other including a standard commercially available Pt/C based cathode for comparison. To prepare the MEAs, square pieces of 1 cm2 of alkaline anion exchange membrane A201 (Tokuyama, thickness ~ 28 mm) were cut. They were then submitted to a 24 h pre-treatment in 1 mol L1 KOH solution at room temperature followed by 2 h in 1 mol L1 KOH solution at 50 C and finally washed in ultrapure water (MilliQ). Commercial Pt:Ru/C (1:1, 60 wt% of metal, E-TEK) was used as anodes while
AEM-mDMFC device mounting The architecture and the fabrication of the whole device used in this work have been described in detail in previous works [26,27]. It is based on silicon micro-machined small chips working as current collectors, mounted together with the alkaline MEA by means of PMMA custom-made pieces and screws. The cell is assembled with the anode side up, in which the liquid fuel is added and the cathode is facing down and open to the ambient air. The external apertures of both electrodes have a geometric surface area of 0.25 cm2.
Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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voltammetry cycles (CVs) from 0.05 V to 0.90 V vs. RHE, at a scan rate of 50 mV s1, under a high purity nitrogen saturated 0.1 mol L1 KOH electrolyte solution to clean the surface. Following this cleaning, the ORR studies were performed with linear sweep voltammetry (LSV) techniques in O2-saturated 0.1 mol L1 KOH, at 900 rpm at a scan rate of 5 mV s1.
Alkaline AEM-mDMFC measurements The IeV polarization curves of the micro-fuel cell systems were obtained using a Keithley 2400 source meter acting as an electronic load and an in-house computer program built on LabVIEW [27] used as a data acquisition system interface. The program allows the control and recording of voltage and current in the micro fuel cell. This air-breathing AEM-mDMFC was tested near to real application conditions at room temperature (22 ± 2 C) and under totally passive mode, without using any external pump or pressurizer. The cathode, which is facing down, is fed with oxygen from ambient air by natural convection. The anode reservoir, which is facing up, is filled with 15 mL of fuel containing-solution. This solution permeates through the micro-channels of the anode current collector to reach the catalytic layer of the MEA. The stabilization of the open-circuit voltage (OCV) was performed before each measurement. After each measurement, the micro-fuel cell system was washed with deionized water and finally dried with N2 at room temperature. In this work we studied the AEM-mDMFC increasing concentrations of methanol (1, 3 and 7 mol L1) in 1 mol L1 KOH to compare the performance and methanol-tolerance of the synthesized CoSe2/C cathode catalyst and the commercial Pt/ C.
Results and discussion
Fig. 1 e Powder X-ray diffraction patterns of commercial Pt/ C and the synthesized CoSe2/C catalysts. (Bruker D5005 radiation, diffractometer: 40 kV, 40 mA Cu Ka (l ¼ 1.5418 A) step-scans of 0.05 (2q) in the range from 10 to 70 /2q for cabon and CoSe2/C catalyst and 10e90 /2q for Pt/C catalyst using a counter time of 2 s per step). Asterisk refers to the presence of carbon.
Table 1 e ICP-OES elemental composition of the anodic (PtRu/C) and cathodic (Pt/C and CoSe2/C) electrocatalysts. Catalyst Nominal metal loading (wt.%) PtRu/C Pt/C CoSe2/C
60 20 20
ICP-OES metal loading (wt.%) 50.0% (Pt 33.5% þ Ru 16.5%) 18.9% (Pt) 16.0% (Co 5.1% þ Se 10.9%)
Catalysts characterization results Although the CoSe2/C catalyst has been characterized in detail in previous work [30e33], some additional measurements have been done previous to its use in the passive alkaline micro-fuel cell presented in this work. Fig. 1 shows powder X-ray diffraction patterns of carbon support, carbon supported 20 wt% CoSe2 and commercial carbon supported 20 wt% Pt catalysts. XRD patterns of both samples exhibit the diffraction characteristics of carbon with a broad shoulder in the range from 20 to 30 /2q and 40 e50 / 2q. Pt/C electrocatalyst diffractogram showed Bragg reflections peaks centred at approximately 2q ¼ 39.7 , 46.2 , 67.5 , 81.2 and 85.7 , which are associated with the (111), (200), (220), (311) and (222) planes, respectively, under the face centred cubic (f.c.c.) crystal structure characteristic with Fm -3 m space group of platinum [34]. A nearly pure-phase CoSe2 was confirmed by the powder XRD pattern for CoSe2/C catalyst, under well-indexed peaks for the cubic crystal structure with Pa -3 space group (JCPDS No. 9-234) [31]. The analysis of the prepared catalysts by ICP-OES revealed that the bulk metal loadings were close to their nominal values (Table 1).
Fig. 2 e Linear down sweep potential scan curves of 20 wt.% CoSe2/C, and 20 wt% Pt/C obtained with a rotating disk electrode for the ORR in 0 and 0.1 mol L¡1 methanol electrolyte containing 0.1 mol L¡1 KOH saturated with oxygen at 25 C, respectively. Electrode rotate speed: 900 rpm; sweep rate: 5 mV s¡1 with a catalyst loading of 56 mg cm¡2.
Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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Fig. 2 shows the measured LSV ORR curves of CoSe2/C and commercial Pt/C in O2-saturated 0.1 mol L1 KOH containing 0 and 0.1 mol L1 methanol at the electrode rotating speed of 900 rpm at 25 C. With an increase in methanol concentration from 0 to 0.1 mol L1, for 20 wt.% Pt/C, the onset potential shifts from ca. 1.00 V to ca. 0.65 V; for CoSe2/C, the onset potential remains unchanged and no oxidation peak is observed testifying immunity to methanol 0.1 mol L1 solution. These results are consistent with work previously reported in the literature in acidic [31] and alkaline medium [30], where, although the prepared catalyst CoSe2/C doesn't show a complete tolerance to methanol, it has a higher selectivity towards the ORR in the presence of methanol than the Pt/C catalyst in 0.1 mol L1 KOH. This demonstrates that the presented Pt-free catalyst can be considered as a highly promising to be used for ORR in alkaline fuel cells. Indeed, as recently reported its use in micro-laminar flow fuel cells [35].
Alkaline AEM-mDMFC performance The AEM-mDMFC performance operation has been measured for both the CoSe2/C and commercial Pt/C catalysts as cathodes for high concentrations of methanol. Three different methanol concentrations (1, 3 and 7 mol L1) in 1 mol L1 KOH solution were tested. After each concentration experiment the cell was washed and dried with N2. All the polarization curves measurements were done at room temperature and ambient air and at the same scan rate. Before measurements some cycles of MEA stabilization were performed. Each cycle is tested with a volume of 15 mL of fuel solution. Fig. 3 shows the performance of the standard alkaline MEA, using a Pt/C cathode. The maximum power density peak increased when using methanol concentrations higher than 1 mol L1 in 1 mol L1 KOH. However, the maximum power density peak was obtained for the intermediate value of methanol concentration of 3 mol L1 in 1 mol L1 KOH
Fig. 3 e Polarization curves (full symbols) and power density curves (open symbols) obtained during the operation of the passive air-breathing AEM-mDMFC with Pt/C cathode using different methanol concentrations (1, 3 and 7 mol L¡1) in 1 mol L¡1 KOH. Electrodes geometric surface areas are 0.25 cm2. Fuel cell tests were performed at room temperature (≈22 C).
(~0.6 mW cm2). The open-circuit voltages (OCV) values decreased slightly from ~0.45 V to 0.35 V when increasing the methanol concentration from 1 to 7 mol L1, respectively. Fig. 4 shows the performance of the alkaline MEA with the CoSe2/C cathode catalyst. It can be observed that the behavior in response to different fuel concentration is different than in the case of the standard Pt/C cathode catalyst. The maximum power density peaks in the passive air-breathing AEM-mDMFC increases with the concentration increase of methanol (power density at 1 < 3 < 7 mol L1). The highest value of maximum power density was found for the methanol concentration of 7 mol L1 in 1 mol L1 KOH (~1.2 mW cm2). Furthermore, the observed OCV increased from ~0.6 V to ~0.7 V with the methanol concentration from 1 to 7 mol L1. Such behavior suggests that the CoSe2/C catalyst is more methanol-tolerant than the standard Pt/C. In fact, the improvement in the methanol tolerance makes the performance, at high fuel concentration of the device with the CoSe2/C cathode catalyst, better than the one with the standard Pt/C catalyst, as can be seen in the direct comparison shown in Fig. 5.
Conclusions Compared to Pt/C, carbon-supported CoSe2 cathode catalyst showed higher tolerance towards methanol even at high methanol concentrations up to 7 mol L1 on the AEM-mDMFC. This effect is responsible to outperform the electrical performance of the micro-fuel cells reported herein. So, this Pt-free cathode catalyst applied to an alkaline micro-fuel cell contributes to reinforce the suitability of AEM-DMFCs as a promising option to power mobile devices or AEM-mDMFC as PowerMEMS.
Fig. 4 e Polarization curves (full symbols) and power density curves (open symbols) obtained during the operation of the passive air-breathing AEM-mDMFC with CoSe2/C cathode using different methanol concentrations (1, 3 and 7 mol L¡1) in 1 mol L¡1 KOH. Geometric surface areas of the electrodes are 0.25 cm2. Fuel cell tests were performed at room temperature (≈22 C).
Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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Fig. 5 e AEM-mDMFC's performance comparison between Pt/C and CoSe2/C cathodes with 7 mol L¡1 methanol in 1 mol L¡1 KOH. Geometric surface areas of the electrodes are 0.25 cm2. Fuel cell tests were performed at room temperature (≈22 C).
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Acknowledgments [13]
The authors thank the financial support of the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER (DADDi2 project TEC2013-48506-C3), and the CNRS/ CSIC agreement program (2012). R. W. Verjulio thanks MINECO for the fellowship. The authors would also like to acknowledge Tokuyama Corporation for supplying membrane materials.
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Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
Available online at www.sciencedirect.com
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Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC) R.W. Verjulio a, J. Santander a,*, J. Ma b, N. Alonso-Vante b a b
Instituto de Microelectronica de Barcelona, IMB-CNM (CSIC), Campus UAB, 08193, Bellaterra, Barcelona, Spain IC2MP, UMR-CNRS 7285, Universite de Poitiers, 4 rue Michel Brunet, 86022, Poitiers, France
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abstract
Article history:
In this work, carbon-supported CoSe2 is used as a Pt-free cathode catalyst in a passive, air-
Received 23 December 2015
breathing, alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC).
Accepted 23 January 2016
The obtained results demonstrate the improvement in the performance, when the device
Available online xxx
is operated at high fuel concentrations, if the proposed cathode catalyst is used instead of the standard Pt/C catalyst, due to the better tolerance to methanol crossover even in
Keywords:
alkaline medium. This result reinforces the suitability of AEM-DMFC's as a promising op-
Alkaline anion exchange membrane
tion for mobile devices powering.
Micro-fuel cells
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Direct methanol fuel cell Non-noble metal catalysts Tolerance to methanol crossover
Introduction Micro-fuel cells are considered as a promising alternative to batteries to power small portable devices both at the mini(mobile phones, laptops, …) and at the micro-scales (microelectromechanical systems (MEMS), wearables, …) [1e3]. In order to be implemented in the micro-scale, fuel cells must be designed in the simplest way, simplifying the added components usually found in standard size fuel cells to improve the performance of this electrochemical device, such as pumps or temperature control [4e7]. This principle guides research towards innovative new device architectures involving new materials which outperform traditional fuel cell components [8e15]. In this way, typical architectures for micro-fuel cells are based in the use of micro-channels driving liquid fuels by
capillarity instead of pumping it through traditional bipolar plates. In this context, membrane-less micro-fluidic fuel cells could be considered as an alternative architecture which permits omitting the ion conducting membrane at the cost of maintaining anode and cathode electrolytes in continuous movement in laminar flow regime, which prevents mixing of both electrolytes. The fluidic condition and the small amount of produced energy limit, in this case, the application of these devices to specialized environments (lab-on-a-chip, in-vivo, …) [16e18]. For general purpose micro-scale applications alkaline anion exchange membrane micro-direct methanol fuel cells (AEM-mDMFC) are being considered as promising devices for further research as integrated power devices for the microsystems field (PowerMEMS) [19e22]. The main advantage of this type of micro-fuel cell is related with the improved
* Corresponding author. E-mail address: [email protected] (J. Santander). http://dx.doi.org/10.1016/j.ijhydene.2016.01.132 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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catalysis in the alkaline media, which makes possible the use of non-Pt-based catalysts, obtaining a good performance at a lower cost. Moreover, the cross-over of fuel to the cathode is reduced due to the opposite movement of the hydroxide anions. This effect, together with the use of a tolerant cathode catalyst, will permit the use of high concentration methanol solutions [23e25], reinforcing the high energy density of this fuel in order to reduce the fuel reservoir volume of the microdevice. In this paper, a further improvement on our previous work, on AEM-mDMFC [26,27] focused on the cathode catalyst, is presented. Oxygen reduction reaction (ORR) essentially studied on Ptbased catalysts has been extended on transition metal chalcogenides [28,29]. Furthermore, an alternative non-precious metal chalcogenide electrocatalyst based on Cobalt for oxygen reduction reaction (ORR) in alkaline medium was reported [30] as well as macrocycles. Both families of materials are considered as potential alternatives to Pt-based materials due to their comparable ORR activity, abundance and remarkable tolerance to the presence of small organic molecules [24,31]. It was assessed that carbon-supported CoSe2 catalysts shows an efficient electrocatalytic activity for the ORR in acid and alkaline medium [30,31]. In this work, this non-precious Pt-free CoSe2/C cathodic catalyst has been tested in a passive air-breathing AEMmDMFC, which micro-device architecture is based on microfabricated silicon current collectors, with methanol concentrations up to 7 mol L1.
commercial Pt/C (20 wt% of Pt, E-TEK) or the synthesized CoSe2/C (20 wt% of metal) were used as cathodes catalysts. The electrocatalytic inks were prepared according to the following procedure: For each 5 mg of catalyst powder, 125 mL of alkaline ionomer AS-4 (5 wt%, Tokuyama) and 625 mL of ultrapure deionized water (18 MU cm) were mixed. The inks were then homogenized in an ultrasonic bath for 2 h. The airbrush technique was used to deposit the ink on the substrates. PTFE treated carbon cloth (ELAT, E-TEK) and nonteflonized carbon cloth (E-TEK) were used as substrates to deposition of the cathode and the anode inks, respectively. The metal loadings were 6 mg cm2 for the PtRu/C anodes and 1.5 mg cm2 for the (Pt/C or CoSe2/C) cathodes. The electrodes and the pretreated alkaline membranes were hot-pressed at 2 kN and 110 C for 3 min, in order to form the MEAs. The geometric active area of each electrode of the MEAs was 0.25 cm2.
Experimental
Catalysts characterization techniques
Synthesis of CoSe2/C electrocatalyst
Powder X-ray diffraction (XRD) was performed on a Bruker D5005 diffractometer under the following conditions: 40 kV, 40 mA using Cu Ka (l ¼ 1.5418 A) radiation source. The samples were step-scanned in steps of 0.05 (2q) in the range of 20 e90 using a counter time of 2 s per step. Elemental composition of the catalysts was determined using an inductively coupled plasma optical emission spectrometry (ICP-OES, spectrometer Optima 2000 DV, Perkin Elmer). The electrochemical measurements were conducted at 25 C in a thermostated three compartment electrochemical cell using a potentiostat (m-Autolab Type II). A plate of glassy carbon and a reversible hydrogen electrode (RHE) with a Luggin capillary were used as counter and reference electrode, respectively. A rotating disk electrode (RDE) of glassy carbon disk with a 3.0 mm diameter and geometric area of 0.07 cm2 served as working electrode. The working electrodes were prepared using the ultra-thin porous coating technique, by which the electrocatalytic ink was prepared by dispersing 10 mg of the catalyst powder in a mixture of 250 mL AS-4 anionic ionomer solution (5 wt%,Tokuyama) and 1250 mL Milli-Q ultrapure deionized water (18 MU cm) in an ultrasonic bath for 2 h. Thereafter, a 3 mL aliquot of the dispersed suspension (catalyst loading ca. 56 mg cm2) was pipetted on the top of the glassy carbon electrode previously polished (until a mirror-finished surface) with alumina powder (3A, Escil). Then it was dried in a stream of inert gas at room temperature for 30 min. The electrodes were subjected to 20 cyclic
The synthesis of 20 wt% CoSe2/C electrocatalyst was earlier reported [31,32]. In short, 0.395 mmol Co2(CO)8 (SigmaeAldrich) and 0.68 g Vulcan XC-72 (Cabot) were dispersed in 10 mL p-xylene (SigmaeAldrich) with stirring under N2 atmosphere at RT for 30 min. The suspension was heated to the refluxing temperature, and then cooled to RT. Thereafter, 1.58 mmol selenium powder (Alfa Aesar) was dispersed in 8 mL p-xylene by ultrasound for 30 min. This dispersion was mixed to the above suspension at RT for 30 min. The final black powder was collected by filtration, and washed with ethanol and dried under vacuum at RT. The CoSe2/C nanoparticle catalyst was heat-treated at 400 C under N2 atmosphere for 3 h.
Preparation of the alkaline MEAs Two different MEAs were prepared, one including the CoSe2/C based cathode and the other including a standard commercially available Pt/C based cathode for comparison. To prepare the MEAs, square pieces of 1 cm2 of alkaline anion exchange membrane A201 (Tokuyama, thickness ~ 28 mm) were cut. They were then submitted to a 24 h pre-treatment in 1 mol L1 KOH solution at room temperature followed by 2 h in 1 mol L1 KOH solution at 50 C and finally washed in ultrapure water (MilliQ). Commercial Pt:Ru/C (1:1, 60 wt% of metal, E-TEK) was used as anodes while
AEM-mDMFC device mounting The architecture and the fabrication of the whole device used in this work have been described in detail in previous works [26,27]. It is based on silicon micro-machined small chips working as current collectors, mounted together with the alkaline MEA by means of PMMA custom-made pieces and screws. The cell is assembled with the anode side up, in which the liquid fuel is added and the cathode is facing down and open to the ambient air. The external apertures of both electrodes have a geometric surface area of 0.25 cm2.
Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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voltammetry cycles (CVs) from 0.05 V to 0.90 V vs. RHE, at a scan rate of 50 mV s1, under a high purity nitrogen saturated 0.1 mol L1 KOH electrolyte solution to clean the surface. Following this cleaning, the ORR studies were performed with linear sweep voltammetry (LSV) techniques in O2-saturated 0.1 mol L1 KOH, at 900 rpm at a scan rate of 5 mV s1.
Alkaline AEM-mDMFC measurements The IeV polarization curves of the micro-fuel cell systems were obtained using a Keithley 2400 source meter acting as an electronic load and an in-house computer program built on LabVIEW [27] used as a data acquisition system interface. The program allows the control and recording of voltage and current in the micro fuel cell. This air-breathing AEM-mDMFC was tested near to real application conditions at room temperature (22 ± 2 C) and under totally passive mode, without using any external pump or pressurizer. The cathode, which is facing down, is fed with oxygen from ambient air by natural convection. The anode reservoir, which is facing up, is filled with 15 mL of fuel containing-solution. This solution permeates through the micro-channels of the anode current collector to reach the catalytic layer of the MEA. The stabilization of the open-circuit voltage (OCV) was performed before each measurement. After each measurement, the micro-fuel cell system was washed with deionized water and finally dried with N2 at room temperature. In this work we studied the AEM-mDMFC increasing concentrations of methanol (1, 3 and 7 mol L1) in 1 mol L1 KOH to compare the performance and methanol-tolerance of the synthesized CoSe2/C cathode catalyst and the commercial Pt/ C.
Results and discussion
Fig. 1 e Powder X-ray diffraction patterns of commercial Pt/ C and the synthesized CoSe2/C catalysts. (Bruker D5005 radiation, diffractometer: 40 kV, 40 mA Cu Ka (l ¼ 1.5418 A) step-scans of 0.05 (2q) in the range from 10 to 70 /2q for cabon and CoSe2/C catalyst and 10e90 /2q for Pt/C catalyst using a counter time of 2 s per step). Asterisk refers to the presence of carbon.
Table 1 e ICP-OES elemental composition of the anodic (PtRu/C) and cathodic (Pt/C and CoSe2/C) electrocatalysts. Catalyst Nominal metal loading (wt.%) PtRu/C Pt/C CoSe2/C
60 20 20
ICP-OES metal loading (wt.%) 50.0% (Pt 33.5% þ Ru 16.5%) 18.9% (Pt) 16.0% (Co 5.1% þ Se 10.9%)
Catalysts characterization results Although the CoSe2/C catalyst has been characterized in detail in previous work [30e33], some additional measurements have been done previous to its use in the passive alkaline micro-fuel cell presented in this work. Fig. 1 shows powder X-ray diffraction patterns of carbon support, carbon supported 20 wt% CoSe2 and commercial carbon supported 20 wt% Pt catalysts. XRD patterns of both samples exhibit the diffraction characteristics of carbon with a broad shoulder in the range from 20 to 30 /2q and 40 e50 / 2q. Pt/C electrocatalyst diffractogram showed Bragg reflections peaks centred at approximately 2q ¼ 39.7 , 46.2 , 67.5 , 81.2 and 85.7 , which are associated with the (111), (200), (220), (311) and (222) planes, respectively, under the face centred cubic (f.c.c.) crystal structure characteristic with Fm -3 m space group of platinum [34]. A nearly pure-phase CoSe2 was confirmed by the powder XRD pattern for CoSe2/C catalyst, under well-indexed peaks for the cubic crystal structure with Pa -3 space group (JCPDS No. 9-234) [31]. The analysis of the prepared catalysts by ICP-OES revealed that the bulk metal loadings were close to their nominal values (Table 1).
Fig. 2 e Linear down sweep potential scan curves of 20 wt.% CoSe2/C, and 20 wt% Pt/C obtained with a rotating disk electrode for the ORR in 0 and 0.1 mol L¡1 methanol electrolyte containing 0.1 mol L¡1 KOH saturated with oxygen at 25 C, respectively. Electrode rotate speed: 900 rpm; sweep rate: 5 mV s¡1 with a catalyst loading of 56 mg cm¡2.
Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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Fig. 2 shows the measured LSV ORR curves of CoSe2/C and commercial Pt/C in O2-saturated 0.1 mol L1 KOH containing 0 and 0.1 mol L1 methanol at the electrode rotating speed of 900 rpm at 25 C. With an increase in methanol concentration from 0 to 0.1 mol L1, for 20 wt.% Pt/C, the onset potential shifts from ca. 1.00 V to ca. 0.65 V; for CoSe2/C, the onset potential remains unchanged and no oxidation peak is observed testifying immunity to methanol 0.1 mol L1 solution. These results are consistent with work previously reported in the literature in acidic [31] and alkaline medium [30], where, although the prepared catalyst CoSe2/C doesn't show a complete tolerance to methanol, it has a higher selectivity towards the ORR in the presence of methanol than the Pt/C catalyst in 0.1 mol L1 KOH. This demonstrates that the presented Pt-free catalyst can be considered as a highly promising to be used for ORR in alkaline fuel cells. Indeed, as recently reported its use in micro-laminar flow fuel cells [35].
Alkaline AEM-mDMFC performance The AEM-mDMFC performance operation has been measured for both the CoSe2/C and commercial Pt/C catalysts as cathodes for high concentrations of methanol. Three different methanol concentrations (1, 3 and 7 mol L1) in 1 mol L1 KOH solution were tested. After each concentration experiment the cell was washed and dried with N2. All the polarization curves measurements were done at room temperature and ambient air and at the same scan rate. Before measurements some cycles of MEA stabilization were performed. Each cycle is tested with a volume of 15 mL of fuel solution. Fig. 3 shows the performance of the standard alkaline MEA, using a Pt/C cathode. The maximum power density peak increased when using methanol concentrations higher than 1 mol L1 in 1 mol L1 KOH. However, the maximum power density peak was obtained for the intermediate value of methanol concentration of 3 mol L1 in 1 mol L1 KOH
Fig. 3 e Polarization curves (full symbols) and power density curves (open symbols) obtained during the operation of the passive air-breathing AEM-mDMFC with Pt/C cathode using different methanol concentrations (1, 3 and 7 mol L¡1) in 1 mol L¡1 KOH. Electrodes geometric surface areas are 0.25 cm2. Fuel cell tests were performed at room temperature (≈22 C).
(~0.6 mW cm2). The open-circuit voltages (OCV) values decreased slightly from ~0.45 V to 0.35 V when increasing the methanol concentration from 1 to 7 mol L1, respectively. Fig. 4 shows the performance of the alkaline MEA with the CoSe2/C cathode catalyst. It can be observed that the behavior in response to different fuel concentration is different than in the case of the standard Pt/C cathode catalyst. The maximum power density peaks in the passive air-breathing AEM-mDMFC increases with the concentration increase of methanol (power density at 1 < 3 < 7 mol L1). The highest value of maximum power density was found for the methanol concentration of 7 mol L1 in 1 mol L1 KOH (~1.2 mW cm2). Furthermore, the observed OCV increased from ~0.6 V to ~0.7 V with the methanol concentration from 1 to 7 mol L1. Such behavior suggests that the CoSe2/C catalyst is more methanol-tolerant than the standard Pt/C. In fact, the improvement in the methanol tolerance makes the performance, at high fuel concentration of the device with the CoSe2/C cathode catalyst, better than the one with the standard Pt/C catalyst, as can be seen in the direct comparison shown in Fig. 5.
Conclusions Compared to Pt/C, carbon-supported CoSe2 cathode catalyst showed higher tolerance towards methanol even at high methanol concentrations up to 7 mol L1 on the AEM-mDMFC. This effect is responsible to outperform the electrical performance of the micro-fuel cells reported herein. So, this Pt-free cathode catalyst applied to an alkaline micro-fuel cell contributes to reinforce the suitability of AEM-DMFCs as a promising option to power mobile devices or AEM-mDMFC as PowerMEMS.
Fig. 4 e Polarization curves (full symbols) and power density curves (open symbols) obtained during the operation of the passive air-breathing AEM-mDMFC with CoSe2/C cathode using different methanol concentrations (1, 3 and 7 mol L¡1) in 1 mol L¡1 KOH. Geometric surface areas of the electrodes are 0.25 cm2. Fuel cell tests were performed at room temperature (≈22 C).
Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132
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Fig. 5 e AEM-mDMFC's performance comparison between Pt/C and CoSe2/C cathodes with 7 mol L¡1 methanol in 1 mol L¡1 KOH. Geometric surface areas of the electrodes are 0.25 cm2. Fuel cell tests were performed at room temperature (≈22 C).
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Acknowledgments [13]
The authors thank the financial support of the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER (DADDi2 project TEC2013-48506-C3), and the CNRS/ CSIC agreement program (2012). R. W. Verjulio thanks MINECO for the fellowship. The authors would also like to acknowledge Tokuyama Corporation for supplying membrane materials.
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Please cite this article in press as: Verjulio RW, et al., Selective CoSe2/C cathode catalyst for passive air-breathing alkaline anion exchange membrane m-direct methanol fuel cell (AEM-mDMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.132