Advanced characterization of alkaline water electrolysis through electrochemical impedance spectroscopy and polarization curves

Highlights

• EIS enables distinction between anodic and cathodic processes.
• Tafel slopes can be derived from charge transfer resistances.
• Capacitances can potentially provide information on the number of active sites.

Abstract

Improved electrolyzer components are needed to make alkaline water electrolyzers more flexible and durable. The performance of these new components can be assessed through in situ electrochemical characterization in the form of polarization curves and electrochemical impedance spectroscopy (EIS). Presently, EIS is still mostly used for the IR-correction of the polarization curve, but more valuable information can be extracted. In this work we show how EIS data can be used to determine the dependence of ohmic resistance on current density, to derive anodic and cathodic Tafel slopes and exchange current densities from fitted charge transfer resistances, and to derive anodic and cathodic capacitances from fitted constant phase elements. We do this for both a two electrode alkaline electrolysis flow cell setup as well as for a three electrode beaker type setup with two-dimensional nickel electrodes. The presented tools can be used in performance studies of new and existing electrodes and membranes in alkaline water electrolysis.

Introduction

Green hydrogen is expected to play a critical role in the energy transition [1], since it is one of the few viable options to decarbonize hard-to-abate sectors such as the chemical and the steel industry. Projections for green hydrogen demand amount to ∼ 500 Million tons in 2050 [2], which will require a water electrolysis capacity of a couple of terawatts. Of the different water electrolysis technologies alkaline water electrolysis is well positioned to fulfill a significant part of this demand, since contrary to other technologies it does not depend on the use of noble or rare earth metals and therefore is less likely to run into material scarcity challenges [3].

The development of improved water electrolysis systems still occurs in a primarily empirical manner by making newly formulated electrodes, membranes and membrane electrode assemblies. These components are then combined in an electrochemical cell and their performance is assessed by measuring polarization curves. A challenge of the use of polarization curves is that it only gives the cell potential for the complete cell. This makes it difficult to assign a good or poor performance of the cell to a particular component and makes it more challenging to focus development efforts.

One way to derive information on the contributions of different cell components from polarization curves is by analyzing the dependence of the cell potential on the current density: ohmic contributions to the cell potential such as membrane or electrolyte resistance have a linear dependance on current density, whereas overpotentials have a logarithmic dependance according to the Tafel equation. By analyzing the polarization curve it becomes possible to split the ohmic contributions from the overpotentials [4]. However, this method only works if the ohmic resistance is independent of current density and if the electrodes display constant Tafel behavior over the complete current density range. This is not always the case, since bubbles can increase ohmic resistance, electrodes can display deviating Tafel behavior and also mass transfer limitations can increase cell potential.

More valuable information can be obtained with Electrochemical Impedance Spectroscopy (EIS), which is a technique in which sinusoidal perturbations of different frequencies are applied to the current or voltage applied to an electrochemical system and the resulting current or voltage is measured and analyzed [5]. It is a powerful technique that can provide significantly more information than the analysis of polarization curves, since it is not only able to provide information on the ohmic resistance, but also on charge transfer resistances, capacitance and mass transfer limitations. Moreover, EIS can already provide this information at a single current density, whereas a polarization curve always requires measurements over a range of current densities.

In the field of water electrolysis the use of EIS is currently still mostly limited to the determination of the ohmic resistance of the electrochemical cell, which can be derived from the resistance as measured at high frequencies [6], [7]. Yet, more valuable information can be derived from the EIS experiments. One possible reason for the fact that EIS is not used to its full potential in water electrolysis is probably the complexity of data interpretation in EIS. It is not helpful that reviews on EIS tend to have a strong focus on mathematical derivations [8], [9], which makes them less inviting for more experimentally oriented electrochemists. Moreover, in these reviews the link between the EIS equations and physical electrochemical parameters such as Tafel slopes and exchange current densities receives limited attention.

Goal of this work is to show the combined potential of EIS and polarization curves in alkaline water electrolysis with two-dimensional (2D) electrodes. We do this by analyzing experimental EIS and polarization data and focus on the practical translation to common electrochemical parameters such as ohmic resistance, capacitance, Tafel slope and exchange current density. In this way we demonstrate how experimental electrochemists can effectively use EIS and polarization curves for assessing the performance of water electrolysis cell components.

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Credit authorship contribution statement

Matheus T. de Groot: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. 

Paul Vermeulen: Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Conceptualization.