2 research outputs found
Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during an Accelerated Stress Test
The long-term durability
of the catalyst layers of a low-working
temperature fuel cell such as a polymer electrolyte membrane fuel
cell (PEMFC) is of significant scientific interest because of their
operation criteria and high initial cost. Identification of degradation
mechanisms quantitatively during an accelerated stress test (AST)
is essential for assessing and improving the durability of such catalyst
layers. In this study, we present a quantitative analysis of the degradation
mechanisms such as (i) electronic connectivity loss due to carbon
support corrosion, (ii) proton connectivity loss due to ionomer/catalyst
interface loss, (iii) catalyst loss due to dissolution or detachment,
and (iv) physical surface area loss due to particle growth that is
responsible for the electrochemical surface area (ECSA) loss in Pt-based
catalyst layers for PEMFCs during an AST performed through potential
cycling (linear sweep cyclic voltammetry) between 0.4 and 1.6 V for
7000 cycles in Ar-saturated 1 M H<sub>2</sub>SO<sub>4</sub>. Using
a half-membrane electrode assembly (half-MEA), where a gas diffusion
electrode with genuine three-phase boundaries is used as a working
electrode through a solid electrolyte, we have observed the ECSA loss
due to ionomer/catalyst interface loss and identified a catalyst heterogeneous
degradation pattern during an AST. Results suggest a significant ECSA
loss due to catalyst isolation (∼64% of ECSA loss) from loss
of electron and proton connectivities by catalyst support corrosion
(∼45%) and ionomer/catalyst interface loss (∼19%), followed
by particle growth (∼30%) and dissolution/detachment (6%).
Such knowledge and methodology can effectively contribute to catalyst
material screening and electrode structure development to advance
the PEMFC technology
Adsorption Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly Graphitized Carbon Nanofibers and Their Thermal Stabilities
A systematic
adsorption study of perfluorinated sulfonic acid Nafion ionomer on
ribbon-type highly graphitized carbon nanofibers (CNFs) was carried
out using fluorine-19 nuclear magnetic resonance spectroscopy. On
the basis of the values obtained for the equilibrium constant (<i>K</i><sub>eq </sub>, derived from Langmuir isotherm), the
ionomer has varying affinities for CNFs (<i>K</i><sub>eq </sub>between 5 and 22) as compared to Vulcan (<i>K</i><sub>eq </sub> = 18), depending on surface treatments. However, the interactions
are most likely governed by different adsorption mechanisms depending
on hydrophilicity/hydrophobicity of the adsorbent carbon. The ionomer
is probably adsorbed via the polar sulfonic group on hydrophilic Vulcan,
whereas it is adsorbed primarily via hydrophobic −CF<sub>2</sub>– backbone on the highly hydrophobic pristine CNFs. Ionomer
adsorption behavior is gradually altered from apolar to polar group
adsorption for the acid-modified CNFs of decreasing hydrophobicity.
This is indicated by the initial decrease and then increase in the
value of <i>K</i><sub>eq </sub>with the increasing
strength of the acid treatment. The corresponding carbon–ionomer
composite also showed varying thermal stability depending on Nafion
orientation. The specific maximum surface coverage (Γ<sub>Smax</sub>) of the CNFs is 1 order of magnitude higher than that of Vulcan.
The large discrepancy is due to the fact that the ionomers are inaccessible
to the internal surface area of Vulcan with high microporosity