During the lifetime of a polymer electrolyte fuel cell, the pore structure of the Pt/C catalyst layer may change as a result of carbon corrosion. Three-dimensional visualization of porosity changes is important to understand the origin of fuel cell performance deterioration. A focused ion beam/scanning electron microscopy (FIB/SEM) approach was adopted together with electron tomographic studies to visualize the three-dimensional pore structure of a Pt/C catalyst. In the case of pristine catalyst layers, the pores form an interconnected network. After 1000 start-up/shut-down cycles, severe carbon corrosion leads to a collapse of the support structure. The porosity of the degraded catalyst layer shrinks drastically, resulting in a structure of predominantly isolated pores. These porosity changes hinder the mass transport in the catalyst layer, consequently leading to a substantial loss of fuel cell performance. FIB/SEM serial sectioning and electron tomography allows three-dimensional imaging of the catalyst pore structure, which is a prerequisite for modeling and optimizing mass transport in catalyst layers.
The level of Pt loadings in polymer electrolyte fuel cells (PEFC) is still one of the main hindrances for implementation of PEFCs into the market. Therefore, new catalyst and electrode preparation methods such as sputtering are of current interest, because they allow thin film production and have many cost saving advantages for electrode preparation. This paper summarises some of the most important studies done for sputtered PEFCs, including non carbon supported electrodes. Furthermore, it will be shown that an understanding of the main morphological differences between sputtered and ink-based electrodes is crucial for a better understanding of the resulting fuel cell performance. Especially, the electrochemical surface area (ECSA) plays a key role for a further increase in PEFC performance of sputtered electrodes. The higher surface specific activities i(k,spec) of sputtered compared to ink-based electrodes will be discussed as advantage of the thin film formation. The so- called particle size effect, known in literature for several years, will be discussed as reason for the higher i(k,spec) of sputtered electrodes. Therefore, a model system on a rotating disc electrode (RDE) was studied. For sputtered PEFC cathodes Pt loadings were lowered to 100 μg(Pt)/cm(2), yet with severe performance losses compared to ink-based electrodes. Still, for Pt sputtered electrodes on a carbon support structure remarkably high current densities of 0.46 A/cm(2) at 0.6 V could be achieved.
Anodes for polymer electrolyte fuel cells (PEFC) with ultra-low platinum loadings (3, 15 and 25 μg Pt /cm 2 ) were prepared by sputter deposition of platinum onto carbon cloth covered with a microporous carbon layer. Platinum nanoparticles as well as agglomerates were observed by TEM for all investigated Pt loadings. Sputtered anodes with Pt loadings of 15 and 25 μg Pt /cm 2 can be operated at currents up to 2 A/cm 2 without voltage losses compared with a commercial anode with 500 μg Pt /cm 2 . Anode loadings of 3 μg Pt /cm 2 yield a similar PEFC performance only at low current densities (<0.5 A/cm 2 ). Longevity tests at constant current density (0.5 A/cm 2 ) show nearly constant cell voltages for up to 1,000 h for an anode loading of 25 μg Pt /cm 2 . Lower Pt loadings (3 and 15 μg Pt /cm 2 ) lead to more distinct voltage losses, probably caused by CO traces in the hydrogen reactant gas. The CO poisoning is in part reversible by (electro)chemical oxidation of CO.
The degradation of Polymer Electrolyte Fuel Cell (PEFC) electrodes was examined by electrochemical and 3D imaging methods. A potential cycling test was conducted with a commercial catalyst coated membrane (CCM) where the cathode side of the cell was subjected to a potential square wave of 0.6V and open circuit voltage. After 24000 cycles, the PEFC performance remains nearly unchanged except for minor current losses in the low current regime. According to hydrogen under potential desorption measurements the electrochemically active Pt area decreases to 25% of the initial value. Synchrotron-based X-Ray tomography of the CCMs yields thickness distributions of catalyst layers and the membrane. A bubble like structure of the cathode layer is observed. Focused Ion Beam/Scanning Electron Microscopy-serial sectioning was employed to determine the pore structure of the cathode. The porosity before and after degradation was almost unaltered by the cycling test. TEM lamellas of the pristine and degraded cathode catalyst layers show an increase of the Pt particle size from 2.2nm to 4.6nm. Under the given conditions, an increase of hydrogen permeability across the membrane and a loss of Pt surface area are the predominant degradation mechanisms.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.