5 research outputs found
Graphene coating of Nafion membranes for enhanced fuel cell performance
Electrochemically exfoliated graphene (e-G) thin films on Nafion
membranes exhibit a selective barrier effect against undesirable fuel
crossover. The approach combines the high proton conductivity of
state-of-the-art Nafion and the ability of e-G layers to effectively
block the transport of methanol and hydrogen. Nafion membranes are
coated with aqueous dispersions of e-G on the anode side, making use of a
facile and scalable spray process. Scanning transmission electron microscopy
(STEM) and electron energy-loss spectroscopy (EELS) confirm the formation of a
dense percolated graphene flake network which acts as diffusion barrier. The
maximum power density in direct methanol fuel cell (DMFC) operation with e-G
coated Nafion N115 is 3.9 times higher than the Nafion N115
reference (39 vs. 10 mW cm @ 0.3 V) at 5M methanol feed concentration.
This suggests the application of e-G coated Nafion membranes for
portable DMFCs, where the use of highly concentrated methanol is desirable
On the State and Stability of Fuel Cell Catalyst Inks
Catalyst layers (CL), as an active component of the catalyst coated membrane (CCM), form the heart of the proton electrolyte membrane fuel cell (PEMFC). For optimum performance of the fuel cell, obtaining suitable structural and functional characteristics for the CL is crucial. Direct tuning of the microstructure and morphology of the CL is non-trivial; hence catalyst inks as CL precursors need to be modulated, which are then applied onto a membrane to form the CCM. Obtaining favorable dispersion characteristics forms an important prerequisite in engineering catalyst inks for large scale manufacturing. In order to facilitate a knowledge-based approach for developing fuel cell inks, this work introduces new tools and methods to study both the dispersion state and stability characteristics, simultaneously. Catalyst inks were prepared using different processing methods, which include stirring and ultrasonication. The proposed tools are used to characterize and elucidate the effects of the processing method. Structural characterization of the dispersed particles and their assemblages was carried out by means of transmission electron microscopy. Analytical centrifugation (AC) was used to study the state and stability of the inks. Herein, we introduce new concepts, S score, and stability trajectory, for a time-resolved assessment of inks in their native state using AC. The findings were validated and rationalized using transmittograms as a direct visualization technique. The flowability of inks was investigated by rheological measurements. It was found that probe sonication only up to an optimum amplitude leads to a highly stable colloidal ink.</p
The Impact of Antimony on the Performance of Antimony Doped Tin Oxide Supported Platinum for the Oxygen Reduction Reaction
Daniel Jalalpoor, Daniel Göhl, Paul Paciok, Marc Heggen, Johannes Knossalla, Ivan Radev, Volker Peinecke, Claudia Weidenthaler, Karl J. J. Mayrhofer, âThe Impact of Antimony on the Performance of Antimony Doped Tin Oxide Supported Platinum for the Oxygen Reduction Reactionâ, J. Electrochem. Soc. 168, (2021) 024502 https://doi.org/10.1149/1945-7111/abd830 Abstract:Antimony doped tin oxide (ATO) supported platinum nanoparticles are considered a more stable replacement for conventional carbon supported platinum materials for the oxygen reduction reaction. However, the interplay of antimony, tin and platinum and its impact on the catalytic activity and durability has only received minor attention. This is partly due to difficulties in the preparation of morphology- and surface-area-controlled antimony-doped tin oxide materials. The presented study sheds light onto catalystâsupport interaction on a fundamental level, specifically between platinum as a catalyst and ATO as a support material. By using a previously described hard-templating method, a series of morphology controlled ATO support materials for platinum nanoparticles with different antimony doping concentrations were prepared. Compositional and morphological changes before and during accelerated stress tests are monitored, and underlying principles of deactivation, dissolution and catalytic performance are elaborated. We demonstrate that mobilized antimony species and strong metal support interactions lead to Pt/Sb alloy formation as well as partially blocking of active sites. This has adverse consequences on the accessible platinum surface area, and affects negatively the catalytic performance of platinum. Operando time-resolved dissolution experiments uncover the potential boundary conditions at which antimony dissolution can be effectively suppressed and how platinum influences the dissolution behavior of the support
PEM Fuel Cell Degradation Analysis Based on Joint Experimental and Simulation Techniques
The research presented in this paper is aimed at the analysis and quantification of degradation processes in the"br" membrane-electrode-assembly (MEA) of low-temperature PEM fuel cells based on a joint experimental /"br" simulation based approach."br" For this purpose the PEM fuel cell catalyst layer model available in a multi-physics simulation environment is"br" extended from a zero-dimensional interface treatment to a three-dimensional agglomerate approach. The threedimensional"br" agglomerate catalyst layer model serves as the basis for modelling the effects of degradation on MEA"br" performance and durability by taking into account the fundamental aspects of chemical kinetics, mechanics and"br" physics. Model development and verification is supported by experimental studies of degradation in laboratory"br" cells under well-defined accelerated-stress-test conditions."br" Catalyst layer and degradation modeling details are presented together with results of the experimental / simulation"br" based analysis of cells with idealized and industrial flow fields under degradation relevant conditions
Toward Highly Stable Electrocatalysts via Nanoparticle Pore Confinement
The durability of electrode materials is a limiting parameter
for
many electrochemical energy conversion systems. In particular, electrocatalysts
for the essential oxygen reduction reaction (ORR) present some of
the most challenging instability issues shortening their practical
lifetime. Here, we report a mesostructured graphitic carbon support,
Hollow Graphitic Spheres (HGS) with a specific surface area exceeding
1000 m<sup>2</sup> g<sup>â1</sup> and precisely controlled
pore structure, that was specifically developed to overcome the long-term
catalyst degradation, while still sustaining high activity. The synthetic
pathway leads to platinum nanoparticles of approximately 3 to 4 nm
size encapsulated in the HGS pore structure that are stable at 850
°C and, more importantly, during simulated accelerated electrochemical
aging. Moreover, the high stability of the cathode electrocatalyst
is also retained in a fully assembled polymer electrolyte membrane
fuel cell (PEMFC). Identical location scanning and scanning transmission
electron microscopy (IL-SEM and IL-STEM) conclusively proved that
during electrochemical cycling the encapsulation significantly suppresses
detachment and agglomeration of Pt nanoparticles, two of the major
degradation mechanisms in fuel cell catalysts of this particle size.
Thus, beyond providing an improved electrocatalyst, this study describes
the blueprint for targeted improvement of fuel cell catalysts by design
of the carbon support