Impact of Load Cycling at High Current Densities on the Degradation Behavior of Membrane-Electrode-Assemblies

For the establishment in the field of renewable energy and energy converters, High Temperature Polymer Electrolyte Membrane fuel cells (HT-PEMFC) need, next to an increase of performance, an improvement in their lifetime expectancy. The amendment of both, durability and performance, requires a better understanding of degradation effects and – processes within the fuel cell.  The term degradation can be defined as a decrease of performance as a function of operation time. The degeneration of fuel cells can be caused by thermal, chemical and mechanical degradation. The debasement due to mechanical stress caused by severe operational conditions lies in the focus of this work. Accelerated stress tests (AST) at high current densities will be carried out with membrane electrode assemblies (MEA) from two different providers.

AST are a good instrument to determine the durability of a MEA and its components in a shorter time span. A standard AST is an operation procedure with a load cycling between moderate current densities (0.3 – 0.6 A/cm²) and open circuit voltage (OCV). In our case, we will concentrate on load cycling treatment with higher current densities between 0.6 and 1.0 A/cm². The operation mode is presented in figure 1. The highest current (1.0 A/cm²) density will be kept for 16 min and the holding time of the lower value (0.6 A/cm²) will be 4 min. One cycle lasts 6 hours and ends with a 10 min OCV phase. The development of the OCV is a good indicator of the state of the membrane (1). After 17 hours of load cycling, the MEA will be characterized with electrochemical measurements like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) (2). After 7 hours of characterization, the load cycling takes again place in the operation procedure. This method will be repeated day by day for one week.

Next to the electrochemical characterization, selected MEAs will be also analyzed ex-situ by imaging procedures like Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and micro-computed tomography (µ-CT). The combination of electrochemical and imaging investigations allows a more detailed and holistic research of degradation effects and defects within the MEA.

With support of the µ-CT, structural changes within the MEA due to the high current densities can be observed. Expected defects like for example pin holes and fiber intrusion can be visualized. These errors (and their influences) on the fuel cell performance can be analyzed and confirmed by LSV measurements (3).

The electron microcopy facilitates the examination of microstructure changes, for example cracking of the catalyst layer or delamination (4). TEM measurements permit the ante and post mortem investigations of the catalyst-particle-formation (5) and the influence of the advanced AST on this development.

Figure 1: Overview of load cycling testing procedure at high current densities

References:

1.            F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Fuel Cell 08, 1, 3-22 (2008).

2.            A. Diedrichs, M. Rastedt, F.J. Pinar, P. Wagner, J. Appl. Electrochem, 43, 1079-1099 (2013).

3.            M. Rastedt, F.J. Pinar, N. Bruns, A. Diedrichs, P. Wagner, ECS Transactions, 58, 443-452 (2013).

4.            F. Rong, C. Huang, Z.-S. Liu, D. Song, Q. Wang, Journal of Power Sources, 175, 699-711 (2008).

5.            J. Wu, X.Z. Yuan, J.J. Martin, H. Wang, J. Zhang, J.Shen, S. Wu, W. Merida, Journal of Power Sources, 184, 104-119 (2008).