A Guide to Concentration Alternating Frequency Response Analysis of Fuel Cells
Summary
We present a protocol for concentration alternating frequency response analysis of fuel cells, a promising new method of studying fuel cell dynamics.
Abstract
An experimental setup capable of generating a periodic concentration input perturbation of oxygen was used to perform concentration-alternating frequency response analysis (cFRA) on proton-exchange membrane (PEM) fuel cells. During cFRA experiments, the modulated concentration feed was sent to the cathode of the cell at different frequencies. The electric response, which can be cell potential or current depending on the control applied on the cell, was registered in order to formulate a frequency response transfer function. Unlike traditional electrochemical impedance spectroscopy (EIS), the novel cFRA methodology makes it possible to separate the contribution of different mass transport phenomena from the kinetic charge transfer processes in the frequency response spectra of the cell. Moreover, cFRA is able to differentiate between varying humidification states of the cathode. In this protocol, the focus is on the detailed description of the procedure to perform cFRA experiments. The most critical steps of the measurements and future improvements to the technique are discussed.
Introduction
Characterizing the dynamic behavior of a PEM fuel cell is important in order to understand which mechanisms dominate the transient operational states lowering the performance of the cell. Electrochemical impedance spectroscopy (EIS) is the most commonly used methodology for studying PEM fuel cell dynamics, due to its ability to separate different process contributions to the overall dynamic performance1,2. However, transient processes with similar time constants are often coupled in the EIS spectra, making it difficult to interpret them. For this reason, in the past transient diagnostic tools based on the application of non-electrical inputs with the aim of detecting the impact of a few or individual dynamics have been developed and proposed3,4,5,6,7.
A novel frequency response technique based on concentration perturbation input and electrical outputs named concentration-alternating frequency response analysis (cFRA) has been developed in our group. The potential of cFRA as a selective diagnostic tool has been investigated theoretically and experimentally6,7. It was found that cFRA can separate different kinds of mass transport phenomena and discriminate between the different states of operation of the cell. In this protocol, we focus on the step-by-step description of the procedure for performing cFRA experiments. The assembling of the cell, its conditioning and the experimental setup for creating a feed with periodic concentration perturbation, as well as the data analysis will be shown and discussed in detail. Finally, the most critical points of the procedure will be highlighted and several strategies for improving the quality and selectivity of cFRA spectra will be pinpointed.
Protocol
Representative Results
Discussion
In contrast to classical EIS, cFRA is a diagnostic tool focused on the characterization of dynamics related to the different mass transport phenomena occurring in the fuel cell. It is not able to detect any transients having a time constant below the oxygen diffusion in the electrode, as for example the charging/discharging of the double layer6. Therefore, unlike EIS where several phenomena are coupled, cFRA can help to identify patterns related to specific dynamics more clearly. This would decrea…
Divulgations
The authors have nothing to disclose.
Acknowledgements
Max Planck Institute for Dynamics of Complex Technical Systems assisted in meeting the publication costs of this article.
Materials
| Membrane Electrode Assemby N115 25,8 cm2 | QuinTech | EC-NM-115 | cathode/anode loding: 1mg Pt/cm2 |
| Potentiostat | Metrhohm | PGSTAT302N | |
| Booster | Metrohm | BOOSTER20A | |
| Retractable fiber oxygen sensor | Pyro Science | OXR430-UHS | |
| Dew Point and Temperature Meter | VAISALA | DMT340 | |
| Software process control system | Siemens | Simatic PCS 7 | |
| Software MATLAB2012a | Mathworks | ||
| Hydrogen | Linde | Hydrogen 6.0 | |
| Nitrogen | Linde | Nitrogen 5.0 | |
| Oxygen | Linde | Oxygen 5.0 |
References
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