3D Imaging of Fuel Cell Electrode Structure Degraded under Cell Voltage Reversal Conditions Using Nanoscale X-Ray Computed Tomography

Polymer electrolyte fuel cells (PEFCs) have gained significant attention as highly efficient and clean power sources for fuel cell electric vehicles (FCEVs). One challenge faced by PEFCs is hydrogen starvation at the anode that can arise from either malfunction of the hydrogen supply system or hydrogen passage blockage by water flooding, ice formation in wintertime, or foreign impurities [1,2]. In particular, hydrogen starvation could be exacerbated when FCEVs are operated under transient conditions such as start-up and rapid load change. When the anode is starved of hydrogen, the anode requires an additional source of protons and its potential increases until the cell potential reverses and water electrolysis occurs, typically followed by carbon corrosion at higher potentials. These cell voltage reversal events eventually lead to catastrophic cell failure [2]. In order to develop highly robust FCEVs, it is important to understand the detailed morphological change in fuel cell electrodes under cell voltage reversal conditions. Over the last few years, nano-scale X-ray computed tomography (nano-CT) with the ability to image within samples in a non-invasive and non-destructive nature has been adopted to investigate the three-dimensional (3D) structure of fuel cell electrodes [3,4]. However, visualization of the 3D structure of degraded electrodes using nano-CT has been lacking, with a key challenge being the extraction of undisturbed samples from end-of-life (EOL) cells. Here we present the morphological analysis of fuel cell electrodes degraded under cell voltage reversal conditions by nano-CT using high precision laser micro-milling for sample preparation. The value in nano-CT imaging is the ability to image internal 3D structure without surface sample preparation artifacts.

A standard membrane-electrode assembly (MEA) consisting of typical membrane (Nafion^® NRE211CS (H⁺), DuPont) and electrodes (Pt/C catalyst (HISPEC4000^®, Johnson Matthey) and ionomer binder (Nafion^® D2021, DuPont)) was used throughout the study. An in-house 4 cm² MEA was produced through a decal transfer process using a polyimide (Kapton^® HN, DuPont) film. Under hydrogen (anode) and air (cathode) conditions, a single fuel cell was fully activated and then the beginning-of-life (BOL) polarizations were measured at 65^oC and 100 % relative humidity. Cell voltage reversal tests were performed by operating the fuel cell at 0.2 A cm^-2 under nitrogen (anode) and air (cathode) conditions until the cell voltage reached -2 V using a potentiostat. Cell polarization curves were measured after each reversal test. When the cell voltage at 1.2 A cm^-2 was reduced to 34% of its BOL value, the cell status was considered to be at EOL. The EOL MEA sample was taken out of the cell for post-mortem analysis. A nano-scale resolution X-ray computed tomography microscope (UltraXRM-L200, Carl Zeiss X-ray Microscopy) was used for resolving the electrode structures as described in literature [3,4]. For sample preparation, a razor blade was used to cut out a small area of MEA (ca. 1.0 x 0.7 mm). The anode side of the MEA was ablated using a high-precision (~ 1 μm) laser mill (QuickLaze 50ST2, ESI^®), leaving only the pillar-shaped anode specimen of approximately 65 μm in diameter on the membrane surface.

Figure 1(a) shows the polarization curves of the MEA measured after cell voltage reversal tests. The cell performance decreases significantly as the voltage reversal tests were repeated, which may be attributed to combined degradation due to carbon corrosion at anode and an increased Ohmic resistance as characterized by electrochemical impedance spectroscopy and in-situ cyclic voltammetry. Figure 1(b) represents the nano-CT image of the EOL anode in the MEA. It is found that the thickness of the EOL anode is reduced by approximately 50%, from about 4 μm to 2 μm (EOL), implying a severe carbon corrosion at anode during the cell voltage reversal tests. The interface between the membrane and electrode appears mostly intact over the interface area, a feature difficult to observe from 2D SEM cross-sections, indicating that the increased Ohmic resistance could be mainly related with the structural collapse of anode, but not with a significant delamination at the interface. A variety of post-mortem analysis data will be presented to further elucidate the fundamental aspects of the cell voltage reversal degradation.

References

[1] A. Taniguchi, T. Akita, K. Yasuda, Y. Miyazaki, J. Power Sources 130 (2004) 42-49.

[2] T.P. Ralph, M. P. Hogarth, Platinum Met. Rev. 46 (2002) 117-135.

[3] W.K. Epting, J. Gelb, S. Litster, Adv. Funct. Mater. 22 (2012) 555-560.

[4] P. Mandal, S. Litster, ECS Trans. 58 (2013) 481-488.