Implementing Structural Disorder as a Promising Direction for Improving the Stability of PtNi/C Nanoparticles

Because of their enhanced oxygen reduction reaction (ORR) kinetics (9 and 6-fold) enhancement of specific activity and mass activity for the ORR relative to those of a commercial Pt/C catalyst, respectively), hollow PtNi/C nanoparticles are attracting a growing level of interest. This catalytic enhancement arises from the synergetic combination of strain and ligand effects, the presence of structural defects, and their hollow morphology producing a convex and concave catalytic sites. However, preventing a loss of catalytic activity under practical proton exchange membrane fuel cell (PEMFC) cathode operating conditions on alloys of platinum with other transition metals (PtM alloys, M being a transition metal) or M-rich core@Pt-rich shell nanoparticles remains highly challenging. A loss of performance is usually observed because of the dissolution of Pt and M under the harsh operating conditions of a PEMFC cathode, but the question remains unanswered for nanomaterials in which catalytic activity is not solely due to alloying effects. Herein, we have carefully investigated the changes in the ORR activity of solid and hollow PtNi/C nanoparticles with identical chemical compositions but different nanostructures during an accelerated stress test simulating PEMFC cathode operation. By combining chemical, physical, and electrochemical techniques, we show that the dissolution of Ni atoms constitutes the primary reason for the loss of ORR catalytic activity but that the initial catalytic advantage of hollow over solid PtNi/C nanoparticles is maintained in the long term. Hence, implementing structural disorder in PEMFC cathode electrocatalysts represents a promising direction for sustainably improving ORR kinetics

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Effect of the Carbon Structure, the Degradation Protocol, and the Gas Atmosphere

The impact of the carbon structure, the aging protocol, and the gas atmosphere on the degradation of Pt/C electrocatalysts were studied by electrochemical and spectroscopic methods. Pt nanocrystallites loaded onto high-surface area carbon (HSAC), Vulcan XC72, or reinforced-graphite (RG) with identical Pt weight fraction (40 wt %) were submitted to two accelerated stress test (AST) protocols from the Fuel Cell Commercialization Conference of Japan (FCCJ) mimicking load-cycling or start-up/shutdown events in a proton-exchange membrane fuel cell (PEMFC). The load-cycling protocol essentially caused dissolution/redeposition and migration/aggregation/coalescence of the Pt nanocrystallites but led to similar electrochemically active surface area (ECSA) losses for the three Pt/C electrocatalysts. This suggests that the nature of the carbon support plays a minor role in the potential range 0.60 < <i>E</i> < 1.0 V versus RHE. In contrast, the carbon support was strongly corroded under the start-up/shutdown protocol (1.0 < <i>E</i> < 1.5 V versus RHE), resulting in pronounced detachment of the Pt nanocrystallites and massive ECSA losses. Raman spectroscopy and differential electrochemical mass spectrometry were used to shed light on the underlying corrosion mechanisms of structurally ordered and disordered carbon supports in this potential region. Although for Pt/HSAC the start-up/shutdown protocol resulted into preferential oxidation of the more disorganized domains of the carbon support, new structural defects were generated at quasi-graphitic crystallites for Pt/RG. Pt/Vulcan represented an intermediate case. Finally, we show that oxygen affects the surface chemistry of the carbon supports but negligibly influences the ECSA losses for both aging protocols

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: From Model Experiments to Real-Life Operation in Membrane Electrode Assemblies

The electrochemical oxidation of carbon is a pivotal problem for low-temperature electrochemical generators, among which are proton-exchange membrane fuel cells (PEMFCs), and (non)­aqueous-electrolyte Li–air batteries. In this contribution, the structure-sensitivity of the electrochemical corrosion of high-surface area carbon (HSAC) used to support catalytic materials in PEMFC electrodes is investigated in model (liquid electrolyte, 96 h potentiostatic holds at different electrode potentials ranging from 0.40 to 1.40 V at <i>T</i> = 330 K) and real PEMFC operating conditions (solid polymer electrolyte, 12,860 h of operation at constant current). Characterizations from Raman spectroscopy demonstrate that the disordered domains of HSAC supports (amorphous carbon and defective graphite crystallites) are preferentially oxidized at voltages related to the PEMFC cathode (0.40 < <i>E</i> < 1.00 V). Excursions to high electrode potential <i>E</i> > 1.00 V, witnessed during start-up and shut-down of PEMFC systems, accelerate this phenomenon and propagate the electrochemical oxidation to the graphitic domains of the HSAC. Thanks to X-ray photoelectron spectroscopy, a better understanding of the relationships existing between structural changes and carbon surface oxides coverage is also emerging for the first time

Reversibility of Pt-Skin and Pt-Skeleton Nanostructures in Acidic Media

Following a well-defined series of acid and heat treatments on a benchmark Pt₃Co/C sample, three different nanostructures of interest for the electrocatalysis of the oxygen reduction reaction were tailored. These nanostructures could be sorted into the “Pt-skin” structure, made of one pure Pt overlayer, and the “Pt-skeleton” structure, made of 2–3 Pt overlayers surrounding the Pt–Co alloy core. Using a unique combination of high-resolution aberration-corrected STEM-EELS, XRD, EXAFS, and XANES measurements, we provide atomically resolved pictures of these different nanostructures, including measurement of the Pt-shell thickness forming in acidic media and the resulting changes of the bulk and core chemical composition. It is shown that the Pt-skin is reverted toward the Pt-skeleton upon contact with acid electrolyte. This change in structure causes strong variations of the chemical composition

Modulating the Fe–N₄ Active Site Content by Nitrogen Source in Fe–N–C Aerogel Catalysts for Proton Exchange Membrane Fuel Cell

Fe–N–C material is regarded as a promising non-precious-metal catalyst for oxygen reduction reaction (ORR) to replace Pt-based catalysts, but its activity and mass transport remain problematic before a large-scale application in proton exchange membrane fuel cells (PEMFCs). Our previous research developed an Fe–N–C aerogel catalyst by pyrolyzing resorcinol–melamine–formaldehyde (RMF) aerogel containing iron precursors. The abundance of micro- and mesopores in aerogel is known to improve the mass transport properties of Fe–N–C cathodes in PEMFC, facilitating the diffusion of O2 to the Fe–N4 sites. Herein, to further improve the ORR activity while maintaining good mass transport properties, a series of Fe–N–C aerogel catalysts were synthesized by modulating the nitrogen source (melamine) content and the texture in the RMF aerogel precursor. The Fe content in catalysts presents a positive relationship with melamine content in the aerogel, with adequate texture, indicating the important function of nitrogen source in stabilizing Fe atoms during pyrolysis to form Fe–N4 active sites. 57Fe Mössbauer spectroscopy revealed a majority of O–Fe(III)N4C12 configuration of the active sites, which is consistent with the variation of pyrrolic N content with Fe derived from X-ray photoelectron spectroscopy. As a result, the mass activity of the series of catalysts exhibits a linear relationship with Fe content and reaches 3.0 A g–1 at 0.8 V vs reversible hydrogen electrode (RHE) in 0.05 M H2SO4 and rotating disk electrode (RDE) setup. Their performance in PEMFC exhibits the same tendency as the RDE setup. In addition, the H2/air PEMFC polarization curves do not show any diffusion-limited current density effects, even at 0.7 A cm–2, with a cathode based on an Fe–N–C catalyst prepared with high melamine content. This work reveals the importance of nitrogen sources to reach a high atomically dispersed Fe content in Fe–N–C catalysts with a low yield of Fe nanoparticles, and the mass transport properties in PEMFC are not affected by low mesopore volume for aerogel-based catalysts