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Interplay of the Open Circuit Potential-Relaxation and the Dissolution Behavior of a Single H2 Bubble Generated at a Pt Microelectrode Franziska Karnbach, Xuegeng Yang, Gerd Mutschke, Jochen Froehlich, Jurgen Eckert, Annett Gebert, Kristina Tschulik, Kerstin Eckert, and Margitta Uhlemann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02305 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016
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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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The Journal of Physical Chemistry
Interplay of the Open Circuit Potential-Relaxation and the Dissolution Behavior of a Single H2 Bubble generated at a Pt Microelectrode a,b
c
c,d
c
Franziska Karnbach , Xuegeng Yang , Gerd Mutschke , Jochen Fröhlich , a,**
Jürgen Eckert
a
e
c
, Annett Gebert , Kristina Tschulik , Kerstin Eckert , ,a
Margitta Uhlemann* a
b
IFW Dresden, Institute for Complex Materials, D-01171 Dresden, Germany TU Dresden, Faculty of Mechanical Science and Engineering, D-01062 Dresden, Germany
c
TU Dresden, Institute of Fluid Mechanics, D-01062 Dresden, Germany
d
Helmholtz-Zentrum Dresden-Rossendorf, Institute of Fluid Dynamics, D-01328 Dresden, Germany
e
Ruhr-University Bochum, Micro/Nano-Electrochemistry and Center for Electrochemical Sciences, D-44780
Bochum, Germany
*Corresponding authors: Phone +49 351 4659-717, fax +49 351 4659-452 (MU+FK), E-mail:
[email protected] ** Present address: Montanuniversität Leoben, Department Materials Physics and Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria ACS Paragon Plus Environment
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Abstract The dissolution behavior of a single H2 bubble electrochemically generated at a Pt microelectrode in 1 M H2SO4, was studied. The open circuit potential (OCP) relaxation after the polarization end was recorded and correlated with the dissolved H2 concentration at the interface electrode/electrolyte/gas. Simultaneously, the shrinking of the bubble was followed optically by means of a high speed camera. In addition, analytical modelling and numerical simulations for the bubble dissolution were performed. Three characteristic regions are identified in the OCP and the bubble radius transients: (i) slow relaxation and shrinking, (ii) transition region and (iii) a long-term slowed down dissolution process. The high supersaturation after polarisation remains longer than theoretically predicted and feeds the bubble in region (i). This reduces the dissolution rate of the bubble which differs significantly from that of non-electrochemically produced bubbles. Numerical multi-species simulations prove that oxygen and nitrogen dissolved in the electrolyte additionally influence the bubble dissolution and slow down its shrinkage compared to pure hydrogen diffusion. In region (iii), a complete exchange of hydrogen gas with nitrogen and oxygen has occurred in the gas bubble.
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1. Introduction The search for novel and effective solutions to store energy, nowadays especially from renewable sources, has been in the focus of energy research for several decades. A promising and widely investigated method is the H2 production via water electrolysis
1-3
. For a further
increase of process efficiency and fundamentals, a detailed understanding of the single H2 bubble behavior at an electrode surface is essential. It comprises three main processes: (i) the kinetic processes of the hydrogen evolution reaction (HER), (ii) the bubble nucleation, growth and detachment, and (iii) the chemical behavior including the dissolution of a single gas bubble after the polarization process. Until now these processes were only investigated separately from each other and for different experimental scenarios. Thus, despite decades of research in this field, no model exists to usefully describe this seemingly simply and industrially extremely important process. Based on the fundamental work of Vetter on the kinetics of HER
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Harrington and Conway proposed a kinetic approach to analyse
numerically the open circuit potential (OCP) relaxation for HER at flat Pt electrodes in acidic solution 5. The work aimed at understanding the coverage behavior of ad- and absorbed intermediates generated during overpotential electrolysis, a fundamental part that even today is intensively studied 6. Therefore, Harrington and Conway 5, analysed the known HER steps: electrosorption, electrodesorption and recombination. Representative OCP decay transients showed different regions which are determined by the rate constants for the three HER steps. A fast initial decay to an overpotential of about 100 mV proceeds within one microsecond and at constant coverage. The final relaxation of the remaining 100 mV may last for several seconds. This was attributed to an anomalously high amount of adsorbed H species of more than 8 monolayers (the characteristic value of charge density for 1 monolayer of H2 is 220 µC/cm² which is used to determine the microscopic area of Pt surfaces), calculated numerically from the pseudocapacitance and measured experimentally by Conway and Bai 3 ACS Paragon Plus Environment
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and Zolfaghari et al. 8. Similar investigations were performed by Bai
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at rotating Pt disc
electrodes to avoid the influence of supersaturation at the electrode. Fast decays (
