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Three-phase interfacial phenomena in alkaline unitized regenerative fuel cell
Electrochimica Acta 114 (2013) 509–513
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Three-phase interfacial phenomena in alkaline unitized regenerative fuel cell Hisayoshi Matsushima a,∗ , Wataru Majima b , Yasuhiro Fukunaka b a b
Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu 400-8511, Japan Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 4 September 2013 Received in revised form 17 October 2013 Accepted 18 October 2013 Available online 29 October 2013 Keywords: Unitized regenerative fuel cell (URFC) Oxygen reduction reaction (ORR) Oxygen evolution reaction (OER) Wettability Contact angle
a b s t r a c t The dynamic behavior of the three-phase interface on a platinum electrode during oxygen reduction (ORR) and oxygen evolution reactions (OER) in a potassium hydroxide droplet was studied by a chargecoupled device (CCD) and confocal laser microscopy. Contact angle measurements reveal a spreading interface during ORR, whereas the droplet figure remains unchanged during OER. The microscopy results demonstrated the formation of many fine droplets in the vicinity of the meniscus boundary during ORR, which was attributed to vapor condensation on the surface. Correlation of these observations with electrochemical data and differences in the results between ORR and OER suggest that the motion of the three-phase interface is induced by local pH and temperature gradients in the meniscus, presumably caused by ORR uniformity under the limitation of dissolved oxygen. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction A considerable amount of electric energy is generated by using fossil fuels in thermoelectric power stations. However, fossil fuel reserves are limited and will be exhausted within the century, especially if oil continues to be used at the current rapid pace. In addition, when fossil fuels are burned, enormous amounts of toxic gases (for example, CO2 , NOx , SOx ) are generated. These gases cause many environmental problems such as global warming and acid rain. To prevent energy shortage and environmental problems, clean sources of alternative energy should be developed. Such sources today include solar, wind, geothermal, and tidal energy. Hydrogen is a particularly attractive energy source as an alternative to fossil fuels [1]. One reason for this is that when hydrogen is burned, only water is generated and no toxic gases are produced. Therefore, by using a fuel cell system, hydrogen is directly converted into electric energy with water as the sole product. Hydrogen is produced by electrolysis of water, which is obtained easily and can be replenished [2,3]. If electricity could be generated efficiently from natural energy sources, we could produce hydrogen from water and use the hydrogen in fuel cells. No toxic gases would be generated, and thus, energy shortages and environmental problems could be prevented.
∗ Corresponding author. Tel.: +81 552208076. E-mail address: [email protected] (H. Matsushima). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.10.121
The combination of a fuel cell and water electrolysis system is called a regenerative fuel cell (RFC) [4–8]. Although RFCs have advantages in terms of long-term energy storage and higher energy density compared with secondary batteries, they are rather costly because they use two expensive electrochemical devices: a fuel cell and a water electrolyzer. This problem could be overcome by developing a unitized regenerative fuel cell (URFC) [9–12] because the fuel cell and water electrolyzer have similar structures. The potential advantages of the URFC system are that its cost, weight, and volume are lower than those of the RFC, and that the room separating the water electrolyzer and fuel cell subsystems is smaller. In particular, an alkaline URFC has been considered as an attractive candidate energy system for use in the aerospace industry and the space station owing to its high output and compactness. However, the round-trip energy conversion efficiency of the URFC is still lower than that of batteries. Therefore, the efficiency of the URFC must be improved to make it practically useful. To improve the efficiency of a URFC, the development of a catalyst that is highly active for both the fuel cell reaction and water electrolysis reaction is important. Furthermore, the wettability of the catalyst is also crucial because the reactions that occur on the catalyst involve gas reactions [13–17]. Although the development of active catalyst materials and catalyst layer structures has been described in several reports [18–21], the wettability of the catalyst and water management of the URFC have not been discussed [22,23]. The main purpose of this study is to investigate the behavior of the gas/electrolyte/electrode three-phase interface during the fuel
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B Fig. 1. Schematic cross-sectional drawing of experimental setup. (A) Working electrode (Pt, Ø 42 mm × 0.1 mm); (B) reference electrode (Pd, Ø 0.5 mm); (C) counter electrode (Pd, Ø 4 mm × 0.1 mm); (D) electrolyte droplet; (E) CCD camera; and (F) confocal laser microscope.
cell reaction and water electrolysis reaction in an alkaline solution. The transient current and contact angle at the three-phase interface during the reactions were measured. 2. Experimental Here we simplify the geometry of the three-phase interface by observing a droplet. Fig. 1 shows a schematic cross-sectional drawing of the experimental cell. The working electrode was a diskshaped platinum sheet ( 42 mm × 0.1 mm), and the counter was a palladium electrode ( 4 mm × 0.1 mm). The center of the platinum disk was perforated, and palladium wire was attached at the perforation. The platinum electrode was polished with 1.0-, 0.3-, and 0.05-m alumina powder. The reference electrode was a palladium wire ( 0.5 mm, 99.99%, Nilaco Corp.). All subsequent potential values mentioned in this paper were referenced to this wire. Prior to the measurements, all electrodes were ultrasonically cleaned in ethanol and purified water for 5 min each. Hydrogen gas was fully absorbed into the palladium electrode by the electrolysis method. Aqueous potassium hydroxide (Wako Pure Chemical Industries) was employed as the electrolyte. The solution was degassed with oxygen for 2 h. A droplet from the syringe needle was placed over the center of the electrodes. Oxygen gas was flown at the rate of 100 ml min−1 to prevent the entry of carbon dioxide gas during the measurements. Electrochemical measurements (HZ-3000, Hokuto Denko Corp.) were performed in the potentiostatic mode for 600 s at 298 K. During the measurements, the side view of the droplet was recorded by a CCD camera, and the overhead view was imaged by confocal laser scanning microscopy (VK-X100, Keyence) with a helium neon laser (633 nm). 3. Results and discussion First, the time variation of the contact angle at open circuit potential (OCP) was observed to investigate the effect of hydroxide concentration on the wettability. An electrolyte solution varying in KOH concentration (0.001 M, 0.01 M, 0.36 M, or 4.46 M) was dropped onto the platinum plate under oxygen atmosphere and the contact angle was plotted (Fig. 2). The spreading motion of the meniscus was recorded for a few minutes. A large reduction in the contact angle occurred in the high concentration solution. The electrode surface became more hydrophilic (smaller in contact angle) with increasing hydroxide concentration, presumably because of the surface tension. 3.1. Fuel cell mode An electrolyte solution of 0.36 M KOH was employed and a cathodic potential was applied to the working electrode. Oxygen was reduced on the working electrode (O2 + 2H2 O + 4e− → 4OH− ),
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t/s Fig. 2. Time-dependent changes in the contact angle of a droplet varying in KOH concentration ((a) 0.001 M, (b) 0.01 M, (c) 0.36 M, and (d) 4.46 M) on a platinum electrode at open circuit potential under oxygen atmosphere.
and absorbed hydrogen was discharged from the counter electrode. We refer to this mode of operation as the fuel cell mode. A side view of the entire droplet was recorded by CCD camera. Simultaneously, a top view of the meniscus rim was imaged by a confocal microscope. Fig. 3 shows sequential images of (a) the side and (b) top views of the droplet when the electrode potential was set at −400 mV. As soon as the reaction began, the meniscus rim, indicated by the reference arrows in Fig. 3(a), rapidly crept across the metal surface and the elliptic droplet became flat during ORR. The thickness of the droplet had decreased by 1/3 at the end of the experiment. The image of the top view demonstrates the formation of many fine drops in front of the meniscus rim at 60 s. These drops continuously nucleated and grew there, after which they were swallowed into the meniscus by the spreading motion. This unique phenomenon clearly suggests condensation from the vapor phase. The condensation is likely caused by the difference in water vapor pressure associated with the temperature or concentration gradient. The ORR occurs heterogeneously on the electrode and preferentially at the meniscus under the limitation of the dissolved oxygen. That is, the current is focused around the meniscus region and generates Joule heat to evaporate the liquid. The water gas espousing to the cold metal formed the droplet. In contrast, Tobias proposed that the concentration difference between bulk and meniscus causes water vaporization during the ORR [24]. The water in the bulk could be easily vaporized by the lower KOH concentration. Thus, vapor diffusing toward the meniscus condensed on the metal surface. Fig. 4 shows the time variation of the contact angle during ORR obtained at several cathodic potentials. The angle decreased with increasing cathodic potential. For example, at 600 s, it was much smaller than that of the OCP experiments (Fig. 2), being 52◦ at 200 mV and 28◦ at −400 mV. In correspondence with the large dynamic depression of the droplet in Fig. 3(b), the angle sharply decreased during the first 100 s, after which it stabilized by around 300 s, independently of the electrode potential. Fig. 5 shows the time variation of the ORR current. The current density is not shown here because the droplet motion made it difficult to determine the exact wetting area. The current markedly decreased after the operation began. Although the wetting area increased at high cathodic potentials, the ORR current decreased. This might be explained by the mass transfer limitation. The oxygen molecules dissolved in the droplet are quickly consumed by the slow diffusion [25].
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Fig. 3. Sequential images of (a) the side view by CCD and (b) front view by confocal laser microscopy showing the dynamic three-phase interface under oxygen atmosphere when the electrode was set at −400 mV.
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t/s Fig. 5. Time-dependent changes of the electrical current in a 0.36 M KOH droplet during ORR obtained at several electrode potentials: (a) 200 mV, (b) 0 mV, (c) −200 mV, and (d) −400 mV.
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In contrast, the current conversely increased in the range of 40–60 s, after which it reached a steady-state value at 200–300 s depending on the potential. The ORR current acceleration could be explained mainly by the spreading area. In particular, the rapid increase in the rate of ORR during the initial operation agreed well with the behavior of the droplet. Moreover, oxygen could pass a shorter distance at low contact angles owing to the thinness of the film. This would help oxygen diffuse toward the electrode surface and regions distant from the meniscus to become the reaction sites. 3.2. Water electrolysis mode The same experimental conditions as those for the fuel cell mode were employed, with the exception that an anodic potential was applied to the working electrode. This led to the evolution of oxygen gas by the oxidation of hydroxide ions (4OH− → O2 + 2H2 O + 4e− ). We refer to this mode of operation as the water electrolysis mode. Fig. 6 demonstrates that fine gas bubbles uniformly evolved on the working electrode when the anodic potential (>1000 mV) was applied. The homogenous gas evolution suggested that the electrochemical reaction occurred not only at the meniscus but also over the whole electrode surface. This suggests fast diffusion of the hydroxide ions. The bubbles remained for a few seconds and then ascended by buoyancy. They adhered to the liquid/air
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Fig. 6. Sequential images of the side view by CCD showing the gas evolution in a 0.36 KOH droplet when the electrode was set at 1600 mV.
interface and the droplet turned to a milky color (Fig. 6). The CCD images demonstrated that the shape of the droplet did not substantially change during the OER. The top view by laser microscopy was not clearly recorded because the evolution of bubbles caused irregular reflections. Fig. 7 shows the time variation of the contact angle during the OER. The contact angle was around 65◦ at every anodic potential regardless of the OER. As in the case of OCP, the angle slightly decreased over the course of 600 s. The scatter in the data of Fig. 7 is likely attributed to experimental error, probably because the physical liquid motion induced by the bubble evolution changed the interface configuration. Unlike the case in the fuel cell mode, the wettability under the water electrolysis mode seemed to be independent of the electrode potential and electrolysis time. Fig. 8 shows the time variation of the OER current. The bubbles evolved actively and the electrolysis current increased with increasing potential (>900 mV). Moreover, although a few bubbles were seen, the current was very low at less than 900 mV. The current reached a stable value without showing any of the concave polarization curves seen in Fig. 5. This behavior corresponded well to the stability of the contact angle.
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t/s Fig. 8. Time-dependent changes of the electrical current in a 0.36 M KOH droplet during ORR obtained at several electrode potentials: (a) 900 mV, (b) 1200 mV, (c) 1400 mV, and (d) 1600 mV.
H. Matsushima et al. / Electrochimica Acta 114 (2013) 509–513
The present results suggest that the mechanism of the spreading droplet is determined by factors such as the concentration of hydroxide ions or temperature. Under the fuel cell mode, the ORR occurring locally could increase the pH and temperature at the meniscus, and lead to low surface tension [26]. In contrast, the OER consumes hydroxide ions and makes the droplet more acidic. From the analogy of pH dependency on the contact angle, the wettability at the meniscus should become hydrophobic. However, the experimental observations showed unexpected results. The uniform reaction could not induce large changes in the pH and temperature at the meniscus. Moreover, the microscopic convection by rising gas bubbles maybe moderate the local change of pH and temperature by the pumping effect [27–31]. In terrestrial environments, the weight of the droplet is another factor that helps the droplet to spread but makes it difficult to lift up the droplet. Therefore, microgravity is one of the ideal environments to reveal the mechanism and problems of URFC in the space station [5,32,33]. 4. Conclusion In situ observations of the droplet presented here clearly demonstrate the dynamic motion of potassium hydroxide, which is well correlated with the ORR and OER on this electrode surface. Comparisons of the contact angle behavior during the ORR and OER described in the present study suggest a role for hydroxide ions in reducing the surface tension, followed by a spread of the threephase interface. Consequently, the ORR is enhanced by the short diffusion path of dissolved oxygen gas and apparent enlargement of reaction area. In contrast, the OER is promoted not by a geometrical factor but by the convectional factor of gas evolution. In summary, our observations illustrate the importance of clarifying the three-phase interface under reaction conditions for evaluating wettability–reactivity relationships in alkaline URFC reactions. References [1] N. Armaroli, V. Balzani, The hydrogen issue, ChemSusChem 4 (2011) 21. [2] S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy, Int. J. Hydrogen Energy 31 (2006) 171. [3] D.L. Stojic, M.P. Marceta, S.P. Sovilj, S.S. Miljanic, Hydrogen generation from water electrolysis, J. Power Source 118 (2003) 315. [4] X. Wu, K. Scott, A non-precious metal bifunctional oxygen electrode for alkaline anion exchange membrane cells, J. Power Source 206 (2012) 14. [5] S. Markgraft, M. Horenz, T. Schmiel, W. Jehle, J. Lucas, N. Henn, Alkaline fuel cells running at elevated temperature for regenerative fuel cell system applications in spacecrafts, J. Power Source 201 (2012) 236. [6] A. Bergen, T. Schmeister, L. Pitt, A. Rowe, N. Djilali, P. Wild, Development of a dynamic regenerative fuel cell system, J. Power Source 164 (2007) 624. [7] S. Busquet, C.E. Hubert, J. Labbe, D. Mayer, R. Metkemeijer, A new approach to empirical electrical modelling of a fuel cell, an electrolyser or a regenerative fuel cell, J. Power Source 134 (2004) 41. [8] A. Verma, S. Basu, Feasibility study of a simple unitized regenerative fuel cell, J. Power Source 135 (2004) 62. [9] S. Altmann, T. Kaz, A. Friedrich, Bifunctional electrodes for unitised regenerative fuel cells, Electrochim. Acta 56 (2011) 4287.
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[10] J.D. Maclay, J. Brouwer, G.S. Samuelsen, Dynamic analyses of regenerative fuel cell power for potential use in renewable residential applications, Int. J. Hydrogen Energy 31 (2006) 994. [11] J. Pettersson, B. Ramsey, D. Harrison, A review of the latest developments in electrodes for unitised regenerative polymer electrolyte fuel cells, J. Power Source 157 (2006) 28. [12] S.D. Yim, W.Y. Lee, Y.G. Yoon, Y.J. Sohn, G.G. Park, T.H. Yang, C.S. Kim, Optimization of bifunctional electrocatalyst for PEM unitized regenerative fuel cell, Electrochim. Acta 50 (2004) 713. [13] F.G.B. San, I. Isik-Gulsac, Effect of surface wettability of polymer composite bipolar plates on polymer electrolyte membrane fuel cell performances, Int. J. Hydrogen Energy 38 (2013) 4089. [14] J.R. Selmann, P.H. Hsieh, Wetting of polarized materials by molten carbonate, Int. J. Hydrogen Energy 37 (2012) 19270. [15] V. Gurau, M.J. Bluemle, E.S. DeCastro, Y.M. Tsou, J.A. Mann, T.A. Zawodzinski, Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells – 1. Wettability (internal contact angle to water and surface energy of GDL fibers), J. Power Source 160 (2006) 1156. [16] H.M. Yu, C. Ziegler, M. Oszcipok, M. Zobel, C. Hebling, Hydrophilicity, Hydrophobicity study of catalyst layers in proton exchange membrane fuel cells, Electrochim. Acta 51 (2006) 1199. [17] Y. Mugikura, J.R. Selman, Meniscus behavior of metals and oxides in molten carbonate under oxidant and reducing atmospheres, J. Electrochem. Soc. 143 (1996) 2442. [18] X.Y. Ding, S. Didari, T.F. Fuller, T.A.L. Harris, Membrane electrode assembly fabrication process for directly coating catalyzed gas diffusion layers, J. Electrochem. Soc. 159 (2012) B746. [19] W. Bi, Q. Sun, W. Deng, T.F. Fuller, The effect of humidity and oxygen partial pressure on degradation of Pt/C catalyst in PEM fuel cell, Electrochim. Acta 54 (2009) 1826. [20] H.Y. Jung, S. Park, B.N. Popov, Electrochemical studies of an unsupported PtIr electrocatalyst as a bifunctional oxygen electrode in a unitized regenerative fuel cell, J. Power Source 191 (2009) 357. [21] G.B. Chen, H.M. Zhang, H.P. Ma, H.X. Zhong, Effect of fabrication methods of bifunctional catalyst layers on unitized regenerative fuel cell performance, Electrochim. Acta 54 (2009) 5454. [22] M. Matsumura, J.R. Selman, Polarization effects on meniscus characteristics in molten carbonate, J. Electrochem. Soc. 139 (1992) 1255. [23] R.H. Muller, Optical studies of electrolyte films on gas electrodes, J. Electrochem. Soc. 113 (1966) 943. [24] D.N. Bennion, C.W. Tobias, Current, Distribution at a gas-electrode–electrolyte interface, J. Electrochem. Soc. 113 (1966) 589. [25] N. Ramaswamy, S. Mukerjee, Influence of inner- and outer-sphere electron transfer mechanisms during electrocatalysis of oxygen reduction in alkaline media, J. Phys. Chem. C 115 (2011) 18015. [26] E.N. Lightfoot, V. Ludviksson, Electrochemical processes in thin films, J. Electrochem. Soc. 113 (1966) 1325. [27] Ph. Mandin, R. Wuthrich, H. Roustan, Polarization curves for an alkaline water electrolysis at a small pin vertical electrode to produce hydrogen, AIChE J. 56 (2010) 2446. [28] Ph. Mandin, A.A. Aissa, H. Roustan, J. Hamburger, G. Picard, Two-phase electrolysis process: from the bubble to the electrochemical cell properties, Chem. Eng. Process. 47 (2008) 1926. [29] H. Matsushima, D. Kiuchi, Y. Fukunaka, Measurement of dissolved hydrogen supersaturation during water electrolysis in a magnetic field, Electrochim. Acta 54 (2009) 5858. [30] H. Matsushima, D. Kiuchi, Y. Fukunaka, K. Kuribayashi, Single bubble growth during water electrolysis under microgravity, Electrochem. Commun. 11 (2009) 1721. [31] H. Vogt, The role of single-phase free-convection in mass transfer at gas evolving electrodes, Electrochim. Acta 38 (1993) 1421. [32] M. Sakurai, Y. Sone, T. Nishida, H. Matsushima, Y. Fukunaka, Fundamental study of water electrolysis for life support system in space, Electrochim. Acta 100 (2013) 350. [33] H. Matsushima, T. Nishida, Y. Konishi, Y. Fukunaka, Y. Ito, K. Kuribayashi, Water electrolysis under microgravity, Electrochim. Acta 48 (2003) 4119.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Three-phase interfacial phenomena in alkaline unitized regenerative fuel cell Hisayoshi Matsushima a,∗ , Wataru Majima b , Yasuhiro Fukunaka b a b
Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu 400-8511, Japan Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 4 September 2013 Received in revised form 17 October 2013 Accepted 18 October 2013 Available online 29 October 2013 Keywords: Unitized regenerative fuel cell (URFC) Oxygen reduction reaction (ORR) Oxygen evolution reaction (OER) Wettability Contact angle
a b s t r a c t The dynamic behavior of the three-phase interface on a platinum electrode during oxygen reduction (ORR) and oxygen evolution reactions (OER) in a potassium hydroxide droplet was studied by a chargecoupled device (CCD) and confocal laser microscopy. Contact angle measurements reveal a spreading interface during ORR, whereas the droplet figure remains unchanged during OER. The microscopy results demonstrated the formation of many fine droplets in the vicinity of the meniscus boundary during ORR, which was attributed to vapor condensation on the surface. Correlation of these observations with electrochemical data and differences in the results between ORR and OER suggest that the motion of the three-phase interface is induced by local pH and temperature gradients in the meniscus, presumably caused by ORR uniformity under the limitation of dissolved oxygen. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction A considerable amount of electric energy is generated by using fossil fuels in thermoelectric power stations. However, fossil fuel reserves are limited and will be exhausted within the century, especially if oil continues to be used at the current rapid pace. In addition, when fossil fuels are burned, enormous amounts of toxic gases (for example, CO2 , NOx , SOx ) are generated. These gases cause many environmental problems such as global warming and acid rain. To prevent energy shortage and environmental problems, clean sources of alternative energy should be developed. Such sources today include solar, wind, geothermal, and tidal energy. Hydrogen is a particularly attractive energy source as an alternative to fossil fuels [1]. One reason for this is that when hydrogen is burned, only water is generated and no toxic gases are produced. Therefore, by using a fuel cell system, hydrogen is directly converted into electric energy with water as the sole product. Hydrogen is produced by electrolysis of water, which is obtained easily and can be replenished [2,3]. If electricity could be generated efficiently from natural energy sources, we could produce hydrogen from water and use the hydrogen in fuel cells. No toxic gases would be generated, and thus, energy shortages and environmental problems could be prevented.
∗ Corresponding author. Tel.: +81 552208076. E-mail address: [email protected] (H. Matsushima). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.10.121
The combination of a fuel cell and water electrolysis system is called a regenerative fuel cell (RFC) [4–8]. Although RFCs have advantages in terms of long-term energy storage and higher energy density compared with secondary batteries, they are rather costly because they use two expensive electrochemical devices: a fuel cell and a water electrolyzer. This problem could be overcome by developing a unitized regenerative fuel cell (URFC) [9–12] because the fuel cell and water electrolyzer have similar structures. The potential advantages of the URFC system are that its cost, weight, and volume are lower than those of the RFC, and that the room separating the water electrolyzer and fuel cell subsystems is smaller. In particular, an alkaline URFC has been considered as an attractive candidate energy system for use in the aerospace industry and the space station owing to its high output and compactness. However, the round-trip energy conversion efficiency of the URFC is still lower than that of batteries. Therefore, the efficiency of the URFC must be improved to make it practically useful. To improve the efficiency of a URFC, the development of a catalyst that is highly active for both the fuel cell reaction and water electrolysis reaction is important. Furthermore, the wettability of the catalyst is also crucial because the reactions that occur on the catalyst involve gas reactions [13–17]. Although the development of active catalyst materials and catalyst layer structures has been described in several reports [18–21], the wettability of the catalyst and water management of the URFC have not been discussed [22,23]. The main purpose of this study is to investigate the behavior of the gas/electrolyte/electrode three-phase interface during the fuel
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B Fig. 1. Schematic cross-sectional drawing of experimental setup. (A) Working electrode (Pt, Ø 42 mm × 0.1 mm); (B) reference electrode (Pd, Ø 0.5 mm); (C) counter electrode (Pd, Ø 4 mm × 0.1 mm); (D) electrolyte droplet; (E) CCD camera; and (F) confocal laser microscope.
cell reaction and water electrolysis reaction in an alkaline solution. The transient current and contact angle at the three-phase interface during the reactions were measured. 2. Experimental Here we simplify the geometry of the three-phase interface by observing a droplet. Fig. 1 shows a schematic cross-sectional drawing of the experimental cell. The working electrode was a diskshaped platinum sheet ( 42 mm × 0.1 mm), and the counter was a palladium electrode ( 4 mm × 0.1 mm). The center of the platinum disk was perforated, and palladium wire was attached at the perforation. The platinum electrode was polished with 1.0-, 0.3-, and 0.05-m alumina powder. The reference electrode was a palladium wire ( 0.5 mm, 99.99%, Nilaco Corp.). All subsequent potential values mentioned in this paper were referenced to this wire. Prior to the measurements, all electrodes were ultrasonically cleaned in ethanol and purified water for 5 min each. Hydrogen gas was fully absorbed into the palladium electrode by the electrolysis method. Aqueous potassium hydroxide (Wako Pure Chemical Industries) was employed as the electrolyte. The solution was degassed with oxygen for 2 h. A droplet from the syringe needle was placed over the center of the electrodes. Oxygen gas was flown at the rate of 100 ml min−1 to prevent the entry of carbon dioxide gas during the measurements. Electrochemical measurements (HZ-3000, Hokuto Denko Corp.) were performed in the potentiostatic mode for 600 s at 298 K. During the measurements, the side view of the droplet was recorded by a CCD camera, and the overhead view was imaged by confocal laser scanning microscopy (VK-X100, Keyence) with a helium neon laser (633 nm). 3. Results and discussion First, the time variation of the contact angle at open circuit potential (OCP) was observed to investigate the effect of hydroxide concentration on the wettability. An electrolyte solution varying in KOH concentration (0.001 M, 0.01 M, 0.36 M, or 4.46 M) was dropped onto the platinum plate under oxygen atmosphere and the contact angle was plotted (Fig. 2). The spreading motion of the meniscus was recorded for a few minutes. A large reduction in the contact angle occurred in the high concentration solution. The electrode surface became more hydrophilic (smaller in contact angle) with increasing hydroxide concentration, presumably because of the surface tension. 3.1. Fuel cell mode An electrolyte solution of 0.36 M KOH was employed and a cathodic potential was applied to the working electrode. Oxygen was reduced on the working electrode (O2 + 2H2 O + 4e− → 4OH− ),
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t/s Fig. 2. Time-dependent changes in the contact angle of a droplet varying in KOH concentration ((a) 0.001 M, (b) 0.01 M, (c) 0.36 M, and (d) 4.46 M) on a platinum electrode at open circuit potential under oxygen atmosphere.
and absorbed hydrogen was discharged from the counter electrode. We refer to this mode of operation as the fuel cell mode. A side view of the entire droplet was recorded by CCD camera. Simultaneously, a top view of the meniscus rim was imaged by a confocal microscope. Fig. 3 shows sequential images of (a) the side and (b) top views of the droplet when the electrode potential was set at −400 mV. As soon as the reaction began, the meniscus rim, indicated by the reference arrows in Fig. 3(a), rapidly crept across the metal surface and the elliptic droplet became flat during ORR. The thickness of the droplet had decreased by 1/3 at the end of the experiment. The image of the top view demonstrates the formation of many fine drops in front of the meniscus rim at 60 s. These drops continuously nucleated and grew there, after which they were swallowed into the meniscus by the spreading motion. This unique phenomenon clearly suggests condensation from the vapor phase. The condensation is likely caused by the difference in water vapor pressure associated with the temperature or concentration gradient. The ORR occurs heterogeneously on the electrode and preferentially at the meniscus under the limitation of the dissolved oxygen. That is, the current is focused around the meniscus region and generates Joule heat to evaporate the liquid. The water gas espousing to the cold metal formed the droplet. In contrast, Tobias proposed that the concentration difference between bulk and meniscus causes water vaporization during the ORR [24]. The water in the bulk could be easily vaporized by the lower KOH concentration. Thus, vapor diffusing toward the meniscus condensed on the metal surface. Fig. 4 shows the time variation of the contact angle during ORR obtained at several cathodic potentials. The angle decreased with increasing cathodic potential. For example, at 600 s, it was much smaller than that of the OCP experiments (Fig. 2), being 52◦ at 200 mV and 28◦ at −400 mV. In correspondence with the large dynamic depression of the droplet in Fig. 3(b), the angle sharply decreased during the first 100 s, after which it stabilized by around 300 s, independently of the electrode potential. Fig. 5 shows the time variation of the ORR current. The current density is not shown here because the droplet motion made it difficult to determine the exact wetting area. The current markedly decreased after the operation began. Although the wetting area increased at high cathodic potentials, the ORR current decreased. This might be explained by the mass transfer limitation. The oxygen molecules dissolved in the droplet are quickly consumed by the slow diffusion [25].
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Fig. 3. Sequential images of (a) the side view by CCD and (b) front view by confocal laser microscopy showing the dynamic three-phase interface under oxygen atmosphere when the electrode was set at −400 mV.
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In contrast, the current conversely increased in the range of 40–60 s, after which it reached a steady-state value at 200–300 s depending on the potential. The ORR current acceleration could be explained mainly by the spreading area. In particular, the rapid increase in the rate of ORR during the initial operation agreed well with the behavior of the droplet. Moreover, oxygen could pass a shorter distance at low contact angles owing to the thinness of the film. This would help oxygen diffuse toward the electrode surface and regions distant from the meniscus to become the reaction sites. 3.2. Water electrolysis mode The same experimental conditions as those for the fuel cell mode were employed, with the exception that an anodic potential was applied to the working electrode. This led to the evolution of oxygen gas by the oxidation of hydroxide ions (4OH− → O2 + 2H2 O + 4e− ). We refer to this mode of operation as the water electrolysis mode. Fig. 6 demonstrates that fine gas bubbles uniformly evolved on the working electrode when the anodic potential (>1000 mV) was applied. The homogenous gas evolution suggested that the electrochemical reaction occurred not only at the meniscus but also over the whole electrode surface. This suggests fast diffusion of the hydroxide ions. The bubbles remained for a few seconds and then ascended by buoyancy. They adhered to the liquid/air
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interface and the droplet turned to a milky color (Fig. 6). The CCD images demonstrated that the shape of the droplet did not substantially change during the OER. The top view by laser microscopy was not clearly recorded because the evolution of bubbles caused irregular reflections. Fig. 7 shows the time variation of the contact angle during the OER. The contact angle was around 65◦ at every anodic potential regardless of the OER. As in the case of OCP, the angle slightly decreased over the course of 600 s. The scatter in the data of Fig. 7 is likely attributed to experimental error, probably because the physical liquid motion induced by the bubble evolution changed the interface configuration. Unlike the case in the fuel cell mode, the wettability under the water electrolysis mode seemed to be independent of the electrode potential and electrolysis time. Fig. 8 shows the time variation of the OER current. The bubbles evolved actively and the electrolysis current increased with increasing potential (>900 mV). Moreover, although a few bubbles were seen, the current was very low at less than 900 mV. The current reached a stable value without showing any of the concave polarization curves seen in Fig. 5. This behavior corresponded well to the stability of the contact angle.
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The present results suggest that the mechanism of the spreading droplet is determined by factors such as the concentration of hydroxide ions or temperature. Under the fuel cell mode, the ORR occurring locally could increase the pH and temperature at the meniscus, and lead to low surface tension [26]. In contrast, the OER consumes hydroxide ions and makes the droplet more acidic. From the analogy of pH dependency on the contact angle, the wettability at the meniscus should become hydrophobic. However, the experimental observations showed unexpected results. The uniform reaction could not induce large changes in the pH and temperature at the meniscus. Moreover, the microscopic convection by rising gas bubbles maybe moderate the local change of pH and temperature by the pumping effect [27–31]. In terrestrial environments, the weight of the droplet is another factor that helps the droplet to spread but makes it difficult to lift up the droplet. Therefore, microgravity is one of the ideal environments to reveal the mechanism and problems of URFC in the space station [5,32,33]. 4. Conclusion In situ observations of the droplet presented here clearly demonstrate the dynamic motion of potassium hydroxide, which is well correlated with the ORR and OER on this electrode surface. Comparisons of the contact angle behavior during the ORR and OER described in the present study suggest a role for hydroxide ions in reducing the surface tension, followed by a spread of the threephase interface. Consequently, the ORR is enhanced by the short diffusion path of dissolved oxygen gas and apparent enlargement of reaction area. In contrast, the OER is promoted not by a geometrical factor but by the convectional factor of gas evolution. In summary, our observations illustrate the importance of clarifying the three-phase interface under reaction conditions for evaluating wettability–reactivity relationships in alkaline URFC reactions. References [1] N. Armaroli, V. Balzani, The hydrogen issue, ChemSusChem 4 (2011) 21. [2] S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy, Int. J. Hydrogen Energy 31 (2006) 171. [3] D.L. Stojic, M.P. Marceta, S.P. Sovilj, S.S. Miljanic, Hydrogen generation from water electrolysis, J. Power Source 118 (2003) 315. [4] X. Wu, K. Scott, A non-precious metal bifunctional oxygen electrode for alkaline anion exchange membrane cells, J. Power Source 206 (2012) 14. [5] S. Markgraft, M. Horenz, T. Schmiel, W. Jehle, J. Lucas, N. Henn, Alkaline fuel cells running at elevated temperature for regenerative fuel cell system applications in spacecrafts, J. Power Source 201 (2012) 236. [6] A. Bergen, T. Schmeister, L. Pitt, A. Rowe, N. Djilali, P. Wild, Development of a dynamic regenerative fuel cell system, J. Power Source 164 (2007) 624. [7] S. Busquet, C.E. Hubert, J. Labbe, D. Mayer, R. Metkemeijer, A new approach to empirical electrical modelling of a fuel cell, an electrolyser or a regenerative fuel cell, J. Power Source 134 (2004) 41. [8] A. Verma, S. Basu, Feasibility study of a simple unitized regenerative fuel cell, J. Power Source 135 (2004) 62. [9] S. Altmann, T. Kaz, A. Friedrich, Bifunctional electrodes for unitised regenerative fuel cells, Electrochim. Acta 56 (2011) 4287.
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