Meniscus Behavior of Metals and Oxides in Molten Carbonate under Oxidant and Reducing Atmospheres: I. Contact Angle and Electrolyte Displacement

Mugikura, Yoshihiro; Selman, J. R.

doi:10.1149/1.1837028

Cited by 36 publications

(50 citation statements)

References 6 publications

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“…The meniscus plays an important role in controlling the reaction rate. [8][9][10][11][12][13] Inaba et al reported that the hydrogen oxidation reaction (HOR) was affected by the shape of the meniscus, which determines the diffusion path of the dissolved gas.14 With an alkaline electrolyte, interesting interfacial phenomena have been observed. [15][16][17][18][19][20][21] In a previous study, we observed an in situ meniscus motion during the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) on a platinum electrode.…”

mentioning

confidence: 99%

Observation of Three-Phase Interface during Hydrogen Electrode Reactions in Unitized Regenerative Fuel Cell

Majima

Matsushima

Fukunaka

et al. 2014

J. Electrochem. Soc.

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The dynamic behavior of the meniscus of a potassium hydroxide and sulfuric acid droplet on a platinum electrode was studied using a charge-coupled device (CCD) camera and confocal laser microscopy. The three-phase interface was investigated during the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER). Contact angle measurements revealed a spreading interface during the HER whereas the droplet shape remained unchanged during the HOR for both droplets. The overhead view revealed the formation of many fine droplets near the meniscus boundary during the HOR in the alkaline electrolyte, which agrees with previous results for the oxygen reduction reaction (ORR). The correlation of these observations with electrochemical data and differences in the results between the HOR and the HER suggest that the motion of the meniscus was induced by local pH and temperature gradients, presumably caused by a non-uniform reaction because of the limitations of the dissolved gas. Hydrogen is an attractive energy source as an alternative to fossil fuels. [1][2][3][4] Hydrogen can be produced from water and used in fuel cells. No toxic gases are generated and thus energy shortages and environmental problems can be prevented. Unitized regenerative fuel cells (URFCs) are ideal energy devices.5-7 They function by generating electricity as fuel cells and they produce hydrogen gas by water electrolysis. They are compact and economic because common electrodes may be used. However, URFCs require highly controlled wettability and this is dependent on the operational mode. A hydrophobic nature is desired for the fuel cell mode and a hydrophilic is required for water electrolysis. Therefore, intrinsic interfacial properties make it difficult to improve the energy conversion efficiency.The complex interface consists of an electrode (solid), an electrolyte (liquid), and a gas (air) and this is referred to as the three-phase interface, which is an essential part of the relevant electrochemical reaction. The meniscus plays an important role in controlling the reaction rate. [8][9][10][11][12][13] Inaba et al. reported that the hydrogen oxidation reaction (HOR) was affected by the shape of the meniscus, which determines the diffusion path of the dissolved gas.14 With an alkaline electrolyte, interesting interfacial phenomena have been observed. [15][16][17][18][19][20][21] In a previous study, we observed an in situ meniscus motion during the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) on a platinum electrode. 17 A spread of the three-phase interface during the ORR was observed and the role of hydroxide ions in determining the surface tension was investigated.To clarify the mechanism of this dynamic motion, in this study we measured the contact angle using different electrolytes and gas atmosphere used in a previous study. The relationship between the wettability and the electrochemical reaction of the HOR and the hydrogen evolution reaction (HER) is thus discussed in detail. ExperimentalThe experi...

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mentioning

confidence: 99%

Observation of Three-Phase Interface during Hydrogen Electrode Reactions in Unitized Regenerative Fuel Cell

Majima

Matsushima

Fukunaka

et al. 2014

J. Electrochem. Soc.

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“…This equation is derived from the condition of mechanical equilibrium between the interfacial tension (capillary) forces and the force associated with the hydrostatic pressure. This equation has been applied by Selman and co-workers [3,4,7] to analyse the meniscus height data at potentials very close to the equilibrium potential (open circuit potential, OCP), where nearequilibrium in the thermodynamic sense may be assumed.…”

Section: Resultsmentioning

confidence: 99%

“…Matsumura and Selman [3] used optical observation of the meniscus rise on vertical plate to study the effects of current passage and gas composition on wetting and spreading at a gold electrode. Mugikura and Selman [4] measured the contact angle values under oxidant and reducing atmospheres and used these results to estimate the change of electrolyte distribution in the MCFC under load. Suski et al [5,6] determined contact angles on Ni and NiO electrodes and compared the results with those on gold.…”

Section: Introductionmentioning

confidence: 99%

Electrowetting of Molten Carbonate Electrolytes

Hsieh

Selman

2011

Fuel Cells

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The wetting characteristics of various molten carbonate electrolytes were experimentally measured and the dependence of the contact angle on polarisation was thoroughly examined for various temperatures and gas atmospheres. At open circuit under an atmosphere of CO2 and/or oxidant gases, addition of alkaline earth carbonates decreases the wettability of the electrolyte. Addition of alkaline earth carbonates also tends to decrease the temperature dependence of the contact angle. Upon polarisation, the contact angle shows a complex behaviour. In an oxidising atmosphere its dependence on potential is roughly symmetric (like an electrocapillary curve) but in a reducing atmosphere this is not the case. Generally, the contact angle increases upon cathodic polarisation but decreases in the anodic direction. These changes in contact angle are interpreted in terms of interfacial tensions. The complex dependence of contact angle on polarisation can be interpreted in terms of a superimposed electrocapillary and transport effects. As such, it may be affected by specifically adsorbed species at the electrode surface (especially complex oxides), by the basicity (oxide ion activity) of the melt, and by the potential‐induced segregation of the cationic species in the carbonate melt.

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“…Also shown in Fig. 1 is the curve for the same melt we calculated from measurements of the wetting angle as a function of potential under the 3% H 2 + 40% ëé 2 + 57% N 2 atmosphere along the meniscus height at the vertical surface of the sample [6].…”

Section: Physical Chemistrymentioning

confidence: 99%

Free Energy of Formation of a Gold-Carbonate Melt Interphase under Various Atmospheres

et al. 2005

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Carbonate melts are successfully applied as supporting electrolytes in the organization of electrolytic separation of heavy metals [1] provided that the melts are minimally involved in the electrode reaction. The carbonate ion at high temperatures is known to dissociate by the reactionThe oxygen ion activity in the melt varies widely as a function of carbon dioxide pressure. The equilibrium acquired on an inert electrode in the anodic potential region [2] is (B)Under a reducing atmosphere with anodic polarization, hydroxide anions appear in the melt [3]: (C)All of the specified components of the electrochemical system can participate in adsorption and electrochemical processes, thus determining the energy of the electrode-electrolyte interphase. Here, we elucidate the role of the gas phase over the carbonate electrolyte in the energy state of the interface between a solid electrode and the salt melt.We chose the free energy, which expresses the reversible work of unit surface formation, to characterize the interphase boundary. The free energy was determined by measuring the pulling-in force acting on a solid sample in a wetting liquid [4]. Experiments were carried out on an installation assembled around a Mettler AT-20 electronic balance; the installation is described elsewhere [5]. The installation comprises a weighing device, a precise moving device, an electrochemical cell, and a heating system. The balance is enclosed in a housing that is connected to vacuum and gas systems to create the required atmosphere in the housing and the measurement cell. The cell, whichC O 2 OH -e. + + = comprised a stoppered quartz tube, a gold crucible containing the melt, working and ancillary electrodes, and a reference electrode, was mounted in a temperaturecontrolled heating furnace. The working electrode was suspended on the balance beam. The housing was rigidly mounted to a vertical mover equipped with an indicator (a KM-6 cathetometer), which provided the vertical advance of the working electrode with ∆ h = ± 0.001 mm.The test electrode was weighed first freely hanging ( P 0 ) and then at the moment when it touched the melt ( P ). The weight difference ∆ Pg = W ( g is the gravity acceleration) and the wetting parameter L (a known value) allow us to calculate the increment in the surface formation energy from ∆σ = . The use of a polarizing electrode and a reference electrode in the cell enabled us to study the interphase energy as a function of electric potential. The maximal scatter of data points in one experiment did not exceed 0.5 mJ/m 2 . The precision in a set of experiments was 3%.A high-purity (99.99%) gold cylinder served as a model electrode to record electrocapillarity curves since gold is the most electropositive metal in the test medium and is indifferent to the components of the electrolyte [2]. The electrode potential was set using a potentiostat with reference to an ancillary gold electrode and was measured against an oxygen reference electrode. The latter was a gold wire with a developed surface immersed int...

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Meniscus Behavior of Metals and Oxides in Molten Carbonate under Oxidant and Reducing Atmospheres: I. Contact Angle and Electrolyte Displacement

Cited by 36 publications

References 6 publications

Observation of Three-Phase Interface during Hydrogen Electrode Reactions in Unitized Regenerative Fuel Cell

Observation of Three-Phase Interface during Hydrogen Electrode Reactions in Unitized Regenerative Fuel Cell

Electrowetting of Molten Carbonate Electrolytes

Free Energy of Formation of a Gold-Carbonate Melt Interphase under Various Atmospheres

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