Unsupported Pt₃Ni Aerogels as Corrosion Resistant PEFC Anode Catalysts under Gross Fuel Starvation Conditions

Abstract

Mitigating catalyst corrosion is crucial for the commercial success of polymer electrolyte fuel cells (PEFCs). Novel catalysts that can withstand the harsh conditions in case of gross fuel (i.e. H 2 ) starvation events at the PEFC anode are needed to increase the fuel cell stack's service life and to meet the durability targets set for automotive applications. To make progress in this respect, we have tested an unsupported, bimetallic Pt 3 Ni alloy (aerogel) catalyst at the PEFC anode and subjected it to a stress test that mimics the high potentials (≥ 1.5 V vs. the reversible hydrogen electrode) encountered upon fuel starvation. In contrast to commercial carbon-supported platinum catalysts (Pt/C), the Pt 3 Ni aerogel displays excellent durability and performance retention in end-oflife fuel cell polarization curves. Additionally, the aerogel catalyst shows ≈35% higher surface-specific activity for the hydrogen oxidation/evolution reaction than Pt/C. These results highlight the great potential of using novel unsupported catalysts at the anode of PEFCs. Polymer electrolyte fuel cells (PEFCs) customarily rely on platinum nanoparticles supported on carbon (Pt/C) to catalyze the anodic hydrogen oxidation and the cathodic oxygen reduction reactions (HOR and ORR, respectively, leading to anode and cathode sometimes being referred to as hydrogen-and oxygen-electrodes).1,2 Among the latter, the ORR is catalytically more demanding, and thus higher Pt loadings are implemented in PEFC cathodes vs. anodes (up to ≈0.4 vs. ≈0.05 mg Pt /cm 2 geom ).3 Irrespectively of this activity difference, both anodic and cathodic Pt/C catalysts suffer from significant corrosion of the carbon support during the high potential excursions (> 1 V vs. the reversible hydrogen electrode, V RHE ) concomitant to PEFC operation, which limits the device's service life.4 Potentials ≥ 1.5 V RHE on the cathode side can for instance be caused by PEFC start-up/shut-down and local fuel starvation. 5,6 To minimize the damage of these start-up/shut-down events, technical solutions like a highflow air purge of the anode compartment that minimizes the residence time of the H 2 /air anode front have been developed. 7 On the other hand, complete absence of fuel in the anode compartment under load leads to the so called gross hydrogen starvation. This is typically triggered by the blockage of hydrogen gas inlets in an individual anode flow field by liquid water which, in order to sustain the cell current, leads to an anode potential increase up to a value at which water oxidation and carbon support corrosion can occur.8 Most importantly, unlike in the start-up/shut-down degradation discussed above, these hydrogen starvation issues are difficult to overcome by engineering solutions. Alternatively, material-based mitigation strategies include composite anode electrodes exhibiting high HOR and oxygen evolution reaction (OER) activity, as well as corrosion-resistant, non-carbon supported or even completely support-free catalysts. 9Inspired by the latter approach, an...

Fe-Based O2-Reduction Catalysts Synthesized Using Na2CO3 as a Pore-Inducing Agent

“…Despite this remarkable initial ORR-activity at low current densities, significant deviations from this kinetically controlled behavior (see the 70 mV·dec –1 slope dashed line in Figure b) are observed at current densities >15 A·g catalyt –1 (corresponding to >20 mA cm –2 in Figure a). Such performance losses are often related to the mass transport properties of the catalyst upon processing into a catalyst layer (CL), which are determined by its agglomerate morphology and pore size distribution and can be studied using focused ion beam scanning electron microscopy (FIB-SEM) . The CL cross-section image obtained using this technique and displayed in Figure c reveals low open porosity (i.e., low content of pores >50 nm in width), which causes poor O 2 - and H 2 O-transport.…”

Aerogels: promising nanostructured materials for energy conversion and storage applications

“…Therefore, Henning et al prepared an unsupported, bimetallic Pt 3 Ni aerogel and employed as anode catalyst for polymer electrolyte fuel cell (PEFC) application. The Pt 3 Ni electrocatalyst exhibits excellent durability and performance retention under stringent conditions and 35% higher mass activity towards hydrogen oxidation reaction compared to commercial Pt/C catalyst [81]. These results suggest that these metallic aerogel nanostructures are the sustainable electrocatalytic materials for fuel cell applications.…”

Simulative Investigation on Local Hydrogen Starvation in PEMFCs: Influence of Water Transport and Humidity Conditions

“…[9][10][11] Besides, mass transfer in the PEMFC worsens due to porosity decay, ionomer redistribution and decreasing hydrophobicity within the CLs. 6,[12][13][14] Possible collapse of CL porous structure further exacerbates the issue. 1,15,16 As a result, small extent of carbon weight loss (in the order of 5%-10%) can already lead to deterioration of the fuel cell performance and the failure in meeting the PEMFC durability target.…”