Simultaneous neutron imaging of six operating PEFCs: Experimental set-up and study of the MPL effect

The commercial success of polymer electrolyte fuel cells (PEFCs) depends on the development of Pt-based oxygen reduction reaction (ORR) catalysts with greater activity and stability to reduce the amount of expensive noble metal per device. To advance toward this goal, we have tested a novel class of unsupported bimetallic alloy catalysts (aerogels) as the cathode material in PEFCs under two accelerated stress test conditions and compared it to a state-of-the-art carbon-supported benchmark (Pt/C). The investigated Pt 3 Ni aerogel shows little degradation under high potential conditions (> 1.0 V) which can occur during fuel starvation and start-up/shutdown of the cell. If tested under the same conditions, the Pt/C benchmark displays significant losses of electrochemical surface area and ORR activity due to carbon support corrosion as observed in cross section and transmission electron microscopy analysis. When testing the durability upon extended load cycling (0.6-1.0 V), Pt 3 Ni aerogel demonstrates less stability than Pt/C which is related to the severe Ni leaching from the alloy under such conditions. These findings highlight the advantages of using unsupported ORR catalysts in PEFCs and point to the reduction of non-noble metal dissolution as the next development step. Polymer electrolyte fuel cells (PEFCs) currently rely on large amounts of carbon-supported platinum (Pt/C) catalysts (≈0.4 mg Pt /cm 2 electrode ) to reduce the voltage losses due to the sluggish kinetics of the cathodic oxygen reduction reaction (ORR).1 Recent advancements in minimizing the Pt loading and associated costs were achieved by alloying platinum with other metals like Ni, Cu and Co which increases the Pt mass-specific ORR activity.2 On the other hand, these catalysts suffer from significant corrosion of the carbon support and Pt nanoparticles during PEFC operation which compromises their long-term efficiency and reliability.3 To specifically mitigate the issue of support stability, researchers have developed alternative corrosionresistant supports (e.g. conductive metal oxides 4-7 ), extended metal surfaces (e.g. 3 M nanostructured thin film catalysts 8 ) and unsupported materials (e.g. Pt-coated Ni, Co or Cu nanowires 9-11 ). Pursuing this last strategy, unsupported bimetallic Pt-Ni electrocatalysts with high specific surface area (≈30 m 2 /g Pt ) and nanochain network structure, referred to as aerogels, were synthesized in a previous work. 12These materials reach the U.S. Department of Energy target (DOE; i.e. 440 A/g Pt at 0.9 V vs. the reversible hydrogen electrode (V RHE )) for automotive PEFC application when tested as thin films by the rotating disk electrode (RDE) technique, 13 which is the standard tool employed by the majority of researchers in this field for initial assessment of catalyst activities. Considering that performance figures derived from such RDE experiments, often do not translate fully to the technical system, it is fundamental to also assess activity and durability in PEFCs to evaluate the real application p...

Section: Methodsmentioning

confidence: 99%

Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes

Henning

Herranz

Ishikawa

et al. 2017

“…A more detailed description can be found in Ref. 45. In order to study cell-to-cell variations, 5 identical cells were studied.…”

Section: Experimental and Modellingmentioning

confidence: 99%

When Size Matters: Active Area Dependence of PEFC Cold Start Capability

Biesdorf

Stahl

Siegwart

et al. 2015

Understanding the freezing mechanism of liquid water within polymer electrolyte fuel cells (PEFC) is crucial for its commercialization for mass market. Although various publications investigated the cold start capability of PEFCs, contradictory behaviors during cold starts at freezing temperatures have been published in literature. Interestingly, differences can be mainly identified between large and small size fuel cells: the latter have a significantly higher cold start capability than large size fuel cells. In order to understand the influence of different active areas, we present measurements of cold starts performed with cells sizes of 1 and 50 cm2 using the same materials, including a large count of experiments (>200) on the 1 cm2 cells to perform statistical analysis. The cold start capability is strongly reduced with an increasing size of the active area, and the operating times changes from stochastic values for small cells to reproducible values for large cells. Both effects can be explained by the higher probability of the aggregate state transition from super-cooled water to ice in large cells. Consequently this paper highlights the implications of downscaling PEFCs to draw conclusions for technical relevant cold-starts.

“…For this purpose, we applied the anisotropic imaging technique which was established for fuel cells, [7][8][9][10][11][12][13] including the tilted-detector setup, 8 adapted for application to a small scale PEM electrolysis cell. * Electrochemical Society Member.…”

mentioning

confidence: 99%

Communication—Neutron Radiography of the Water/Gas Distribution in the Porous Layers of an Operating Electrolyser

Seweryn

Biesdorf

Schmidt

et al. 2016

Neutron radiography was applied to image the water-gas distribution in an operating small scale PEM electrolyser, building up on the anisotropic resolution improvements previously developed for fuel cell imaging. The ability of neutrons to image this distribution across porous materials made of titanium was demonstrated for the first time. This paper presents the procedure's description and limitations, focusing on water content in the porous layers as a function of distance from electrode. A surprising pattern of water content in anodic porous material was observed, which sheds light on the topic of mass transport limitations in PEM electrolysers.