Towards a Harmonized Accelerated Stress Test Protocol for Fuel Starvation Induced Cell Reversal Events in PEM Fuel Cells: The Effect of Pulse Duration

Global fuel starvation is an undesired event during fuel cell operation that results in serious degradations at the anode catalyst layer caused by the concomitant reversal of the cell potentials. Several groups have therefore intensified their research efforts towards the implementation of suitable diagnostic tools and accelerated stress test (AST) protocols that mimic cell reversal events. However, the current number of different test protocols requires consolidation and harmonization to define durability targets towards cell reversal tolerance and to benchmark newly developed materials. To create a basis for harmonization, this study examines the difference between pulsed and quasi-continuous AST protocols at the catalyst-coated membrane level. Utilizing a single-cell setup combined with an on-line mass spectrometer, a 2.5-fold increase in the carbon corrosion rates were found for short-pulsed compared to long-lasting cell reversal events. The enhanced corrosion was associated with a 2.2-fold higher loss of electrochemically active surface area and a 15% higher reduction in anode catalyst layer thickness. By contrast, the overall cell performance decreased additionally by 40–50 mV for samples under long-lasting cell reversal events. The decay is mainly driven by an increased ohmic resistance, presumably originating from a more pronounced surface oxide formation on the carbon support

Shape-selected bimetallic nanoparticle electrocatalysts: evolution of their atomic-scale structure, chemical composition, and electrochemical reactivity under various chemical environments

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Solid surfaces generally respond sensitively to their environment. Gas phase or liquid phase species may adsorb and react with individual surface atoms altering the solid-gas and solid-liquid electronic and chemical properties of the interface. A comprehensive understanding of chemical and electrochemical interfaces with respect to their responses to external stimuli is still missing. The evolution of the structure and composition of shape-selected octahedral PtNi nanoparticles (NPs) in response to chemical (gas-phase) and electrochemical (liquid-phase) environments was studied, and contrasted to that of pure Pt and spherical PtNi NPs. The NPs were exposed to thermal annealing in hydrogen, oxygen, and vacuum, and the resulting NP surface composition was analyzed using X-ray photoelectron spectroscopy (XPS). In gaseous environments, the presence of O2 during annealing (300 °C) lead to a strong segregation of Ni species to the NP surface, the formation of NiO, and a Pt-rich NP core, while a similar treatment in H2 lead to a more homogenous Pt-Ni alloy core, and a thinner NiO shell. Further, the initial presence of NiO species on the as-prepared samples was found to influence the atomic segregation trends upon low temperature annealing (300 °C). This is due to the fact that at this temperature nickel is only partially reduced, and NiO favors surface segregation. The effect of electrochemical cycling in acid and alkaline electrolytes on the structure and composition of the octahedral PtNi NPs was monitored using image-corrected high resolution transmission electron microscopy (TEM) and high-angle annular dark field scanning TEM (HAADF-STEM). Sample pretreatments in surface active oxygenates, such as oxygen and hydroxide anions, resulted in oxygen-enriched Ni surfaces (Ni oxides and/or hydroxides). Acid treatments were found to strongly reduce the content of Ni species on the NP surface, via its dissolution in the electrolyte, leading to a Pt-skeleton structure, with a thick Pt shell and a Pt-Ni core. The presence of Ni hydroxides on the NP surface was shown to improve the kinetics of the electrooxidation of CO and the electrocatalytic hydrogen evolution reactions. The affinity to water and the oxophilicity of Ni hydroxides are proposed as likely origin of the observed effects.DFG, EXC 314, Unifying Concepts in Catalysi

Block copolymer templated synthesis of PtIr bimetallic nanocatalysts for the formic acid oxidation reaction

Arrays of PtIr alloy nanoparticle (NP) clusters are synthesized from a method using block copolymer templates, which allows for relatively narrow NP diameter distributions (~4–13 nm) and uniform intercluster spacing (~ 60 or ~100 nm). Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer micelles were used to create thin film templates of NPs with periodic pyridinium-rich domains that are capable of electrostatically loading PtCl62- and IrCl62- anion precursors for the preparation of NP arrays. The composition of PtIr NPs was specified by the ratio of metal anions in a low-pH immersion bath. Formic acid oxidation, studied by cyclic voltammetry, shows that the arrays of clusters of PtIr alloy NPs are highly active catalysts, with mass activity values on par or exceeding current industrial standard catalysts. The uniformity in the NP population in a cluster and the small diameter range established by the block copolymer template permit an estimate of the optimal Pt:Ir ratio for the direct oxidation of formic acid, where, ~10 nm Pt16Ir84 alloy NPs were the most active with a mass activity of 37 A/g. &nbsp