Electrochemical Processes in Thin Films

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...

Section: Resultsmentioning

confidence: 97%

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

Majima

Matsushima

Fukunaka

et al. 2014

“…Since this result is symmetrical with respect to the mid-plane where the shear stress vanishes, Eq. [6] applies locally in the present system for small angles and thin films. …”

Section: Quantitative Description Of the Evaporating Meniscusmentioning

confidence: 99%

“…Using a linearized momentum equation, they obtained Eq. [6] for the velocity in a converging plane-walled passage with the half-taper angle, ~i. Since this result is symmetrical with respect to the mid-plane where the shear stress vanishes, Eq.…”

Section: Quantitative Description Of the Evaporating Meniscusmentioning

confidence: 99%

The Effect of Viscous Shear on a Meniscus in an Electrochemical System

Wayner

1972

Fluid flow in the evaporating meniscus of an electrode-gas-electrolyte reacting system has been studied to examine the effect of viscous shear on the meniscus profile. The evaporating meniscus profile, the pressure derivative profile, and the curvature profile for various assumed evaporation profiles were calculated for 1N H2804. Due to the thinness of the meniscus profile in the interline region, large pressure gradients were needed to balance the viscous shear stress. The resulting evaporating meniscus profile was significantly different from the static profile. Depending on the evaporation rate the interline radius of curvature could be micron size for a large meniscus. Heat and mass transfer rates would also be significantly effected by these changes during an electrochemical reaction. Therefore, the effect of viscous shear should not be neglected in analyzing electrochemical processes in a "finite contact angle" meniscus.The triple interline region of a meniscus formed on an immersed solid surface has been the subject of considerable research. In part, this derives from interest in the potentially high heat and mass transfer rates associated with a large surface area covered by a thin liquid film in the form of a myriad of menisci (porous fuel cell electrodes are of particular interest herein). The study of electrochemical processes on wetted metallic surfaces partially immersed in an electrolyte could be classified according to whether or not a thin film extends above the intrinsic meniscus. The obvious and significant characteristic of a meniscus without a thin film ["finite contact angle meniscus" (1)] is that the diffusion path length across the liquid film approaches zero at the interline. This study is particularly concerned with the outstanding work of Bockris and Cahan (1) which discusses the effect of a finite contact angle meniscus on kinetics in porous electrode systems. Conversely, various other authors (2-4) considered models and systems that included a thin film above the intrinsic meniscus. Due to the large number of experimental variables, the complete set of conditions that insure the presence of a particular type of meniscus is still questionable.The following three of many conclusions concerning electrochemical kinetics in a meniscus presented by Bockris and Cahan (1) are of particular interest herein: (i) "most of the current is produced in the first 1% of the meniscus;" (ii) "the extreme concentration of current density into this small area can produce local heating and create a dynamic situation in the physical location of the meniscus;" and (iii) "the local geometry of the meniscus, particularly in the three-phase region, dominates the behavior of porous electrodes." They also state that some of the heat is readily removed by evaporation. In their analysis of this extremely complex system, the geometry of the meniscus is represented by the equation for a cylindrical surface, thereby neglecting the effect of viscous flow of the evaporating liquid on the meniscus profile. The major effec...

“…Most of previous reports using these systems have dealt with acid electrolytes, which were utilized for proton-exchange membrane fuel cells, phosphoric acid fuel cells, etc. [3][4][5][6][7][8][9][10][11] There are a few reports that have dealt with alkaline electrolytes 13,14 , but there is little information on the influences of various conditions (such as electrolyte concentrations, oxygen partial pressures, and electrode wettability) on TPB regions. In addition, OER, that is the charging process for air electrodes, has not been examined by the model electrode.…”

Section: Introductionmentioning

confidence: 99%

“…2). [3][4][5][6][7][8][9][10][11][12][13][14] Mass transports of the ions and gases in the catalyst layers can be simulated in the partially immersed electrode system. In this system, ion transportation can be limited within thin liquid films, which cover the electrode surface.…”

mentioning

confidence: 99%

Investigations of Electrochemically Active Regions in Bifunctional Air Electrodes Using Partially Immersed Platinum Electrodes

Ikezawa

Miyazaki

Fukutsuka

et al. 2015

Oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) on glassy-carbon-supported platinum electrodes (Pt/GCs), which are partially immersed in alkaline electrolytes, are investigated as a model of the triple phase boundary (TPB) in air electrodes for metal-air secondary batteries. ORR currents are measured with changing the vertical position of Pt/GCs, and OER currents are measured by linear sweep voltammetry. Based on the electrochemical results, it is found that thin liquid film on Pt/GCs effectively serves to expand TPB regions for ORR, but the liquid film hardly increases OER currents. Therefore, we conclude that the most effective TPB form are determined by the electrode reactions (ORR or OER), which are corresponding to discharge and charge processes for metal-air secondary batteries. In practice, it is strongly necessary to control the wettability of electrode inside, in order to construct high-performance bifunctional air electrodes.