On the Nature of Electro-Osmotic Drag

Torsten Berning

doi:10.3390/en13184726

Abstract

Electro-osmotic drag (EOD) is usually thought of as a transport mechanism of water inside and through the polymer electrolyte membrane (PEM) in electrochemical devices. However, it has already been shown that the transport of dissolved water in the PEM occurs exclusively via diffusion, provided that the EOD coefficient nd is constant. Consequently, EOD is not a water transport mechanism inside the electrolyte membrane, and this means that its nature is not yet understood. This work proposes a theory that suggests that the root of the EOD is located in the catalyst layers of the electrochemical device where the electric current is generated, and consequently could be linked to one or more of the elementary reaction steps. It is therefore also conceivable that EOD exists at one electrode in an electrochemical device, but not in the other. Moreover, the EOD coefficient nd may depend on the current density as well as the oxidization level of the catalyst. The last consequence, if EOD is linked to an elementary reactions step, it could also be part of the rate-determining elementary step, and this could open pathways to increase the reaction kinetics by finding ways of enhancing the water/hydronium ion transport out of or into the polymer phase.

Highlights

IntroductionAs exchange membrane membrane fuel fuel cells cells (PEMFCs) (PEMFCs)and andAs we we are are entering entering the the hydrogen hydrogen age, proton exchange electrolyzer because of of their their high high efficiencies efficienciesand andtheir theirnoiseless noiseless electrolyzer cells cells (PEMECs)(PEMECs) play a central role because and can be be ideally ideally paired paired with with intermittent intermittentpower powersources sources andclean cleanoperation operation [1].[1]
Springer et al [14] developed a model for water transport in a PEMFC membrane where they assumed that the membrane is always in equilibrium with the neighboring gas/liquid phase, λw,equil = λw, and the nonequilibrium sorption (NES) term would be identical to zero
It should be stated that the current view and hypotheses apply to water transport in the membrane and are not in conflict with the typical view on proton transport through the membrane via two competing mechanisms, the vehicle transport mechanism and the Grotthuss “hopping” mechanism (e.g., [27]). It can be concluded from this work that the Electro-osmotic drag (EOD) is not a transport mechanism for membrane water

Summary

Introduction

As we we are are entering entering the the hydrogen hydrogen age, proton exchange electrolyzer because of of their their high high efficiencies efficienciesand andtheir theirnoiseless noiseless electrolyzer cells cells (PEMECs). Oxygen and hydrogen are transported through the porous gas diffusion layers towards the catalyst layers where the electrochemical reactions occur, driven by the electromotive force of the fuel cell and local overpotentials that are the main source of waste heat (e.g., [2]). In a PEM electrolyzer cell, the opposite reactions take place, and electricity must be supplied to the cells. For the low-temperature PEM technology, the membrane must be kept in a hydrated state while at the same time cell flooding must be avoided [3,4], which requires a careful adjustment of the operating conditions [5,6]. The water management of the fuel cell or electrolyzer cell in general and of the membrane in particular need to be fundamentally understood and the various water transport mechanisms carefully studied

Water Transport Inside a Polymer Electrolyte Membrane

The EODunderstanding

Hypotheses on the Nature of EOD

Discussion

Conclusions