Significance of Electrolyte Films for Performance of Porous Hydrogen Electrodes

Will, Fritz G.; BenDaniel, David J.

doi:10.1149/1.2412173

Cited by 15 publications

(8 citation statements)

References 15 publications

(32 reference statements)

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“…Typical values for the thin-film thicknesses: 25-300 nm. [38] shows a significant mass transport limitation at about j � 1.3 kA m À 2 at E Ext = 0.1 V vs. RHE, no mass transport limitation proceed for the system with electrolyte convection at j = 3.8 kA m À 2 at the same potential. The ORR can proceed at locations within the flooded agglomerates where the reactants oxygen and water both are present.…”

Section: Steady State Analysis Of Odc Performancementioning

confidence: 87%

The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes

et al. 2019

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Advanced chlor‐alkali electrolysis with oxygen depolarized cathodes (ODC) requires 30 % less electrical energy than conventional hydrogen‐evolution‐based technology. Herein, we confirm that the activities of hydroxide and water govern the ODC performance and its dynamics. Experimental characterization of ODC under varying mass transfer conditions on the liquid side reveals large differences in the polarization curves as well as in potential step responses of the electrodes. Under convective transport in the liquid electrolyte, the ODC is not limited by mass transfer in its current density at j>3.9 kA m−2, whereas transport limitations are already reached at j≈1.3 kA m−2 with a stagnant electrolyte. Since gas phase conditions do not differ significantly between the measurements, these results are in contrast the common assumption that oxygen supply determines ODC performance. A dynamic model reveals the strong influence of the electrolyte mass transfer conditions on oxygen availability and thus performance. Dynamic responses of the current density to step‐wise potential changes are dominated by the mass transport of water and hydroxide ions, which is by orders of magnitude faster with convective electrolyte flow. Without convective liquid electrolyte transport, a high accumulation of hydroxide ions significantly lowers the oxygen solubility. Thus, a fast mass transport of water and hydroxide is essential for high ODC performance and needs to be ensured for technical applications. The predicted accumulation of ions is furthermore validated experimentally by means of scanning electrochemical microscopy. We also show how the outlined processes can explain the distinctively different potential step responses with and without electrolyte convection.

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Section: Steady State Analysis Of Odc Performancementioning

confidence: 87%

The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes

et al. 2019

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“…As shown in Figure 1, three of them are located within the porous ODC:apure gas phase;athin film;and aflooded porous structure, which is denoted as flooded agglomerates hereafter.T he thin film represents the interface of liquid and gas phases within the electrode;t his approach is ac ommon methodf or modeling phase boundaries with ah igh specific interface. [21] The fourth section is al iquid boundary layer beyond the electrode. [21] The fourth section is al iquid boundary layer beyond the electrode.…”

Section: Model Concept and Assumptionsmentioning

confidence: 99%

“…The ORR causes al ocal change of water and hydroxide concentration, whichc onsequently leads to ac hange in liquid density (cf. (21) and (22) in which r is the reactionr ate, n ORR H 2 O is the stoichiometric factor of water,a nd n ORR OH À is the stoichiometric factor of hydroxide ions in the ORR. Although the molecules of water and hydroxide per volumec hange depend on the reaction, the total concentration, c total ,s tays almostt otally constant (cf.…”

Section: Mass Transporti Nthe Thin Filmmentioning

confidence: 99%

“…[20] Typical values for the thin-film thicknesses are between 25 and 300 nm. [21] The fourth section is al iquid boundary layer beyond the electrode. For the thin film, the flooded agglomerates and liquid diffusion layer,hereafter,t he collective term liquid phase is used.…”

Section: Model Concept and Assumptionsmentioning

confidence: 99%

“…Consequently, the additional convection based on the density change between the reaction area and the electrolyte bulk phase can be described by the removal of additionally produced ions[Eqs. (21) and (22) in which r is the reactionr ate, n ORR H 2 O is the stoichiometric factor of water,a nd n ORR OH À is the stoichiometric factor of hydroxide ions in the ORR.…”

Section: Mass Transporti Nthe Thin Filmmentioning

confidence: 99%

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Processes and Their Limitations in Oxygen Depolarized Cathodes: A Dynamic Model‐Based Analysis

2019

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Oxygen depolarized cathodes (ODCs) are key components of alkaline fuel cells and metal–air batteries or of chlor‐alkaline electrolysis, but suffer from limited oxygen availability at the reaction zone. Dynamic analysis is a highly suitable approach to identify the underlying causes, especially the limiting steps and process interactions in such gas diffusion electrodes. Herein, a one‐dimensional, dynamic, three‐phase model for analyzing the oxygen reduction reaction in silver‐based ODCs is presented. It allows for a detailed evaluation of the electrochemical reaction, the mass transport processes, and their interaction. The model also reveals that the depletion of reactant oxygen in the liquid electrolyte is caused by the current‐dependent change of the gas–liquid equilibrium as the limiting subprocess. The phase equilibrium, in turn, depends on the slow mass transport of water and hydroxide ions in the liquid phase. Key parameters are the location or size of the gas–liquid interface within the electrode. Profiles of local concentrations and partial pressures of different species reveal a steep gradient of oxygen in the liquid phase, but no limitation in oxygen mass transport in the gas phase. Dynamic simulations with potential steps allowed the identification of different time constants to separate overlapping processes. Accordingly, the mass transport of water and hydroxide ions was identified as the slowest process that strongly influenced the dynamic response of all species, including oxygen, and of the current. The characteristic time constant of the mass transport of water and hydroxide ions across the liquid phase within the ODC is determined to be τmt,0.166667emnormalH2normalO ≈0.176 s, whereas the time constant of oxygen mass transport into the reaction zone is several magnitudes smaller: τmt,0.166667emnormalO2 ≈1.70×10−6 s. Finally, a sensitivity analysis confirms that the overall performance is best improved by adjusting mass transport properties in the liquid phase.

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Porous Electrodes

Chizmadzhev

Chirkov

1983

Comprehensive Treatise of Electrochemistry

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Significance of Electrolyte Films for Performance of Porous Hydrogen Electrodes

Cited by 15 publications

References 15 publications

The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes

The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes

Processes and Their Limitations in Oxygen Depolarized Cathodes: A Dynamic Model‐Based Analysis

Porous Electrodes

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