The Dissolution Dilemma for Low Pt Loading Polymer Electrolyte Membrane Fuel Cell Catalysts

Cost and lifetime currently hinder widespread commercialization of polymer electrolyte membrane fuel cells (PEMFCs). Reduced electrode Pt loadings lower costs; however, the impact of metal loading (on the support) and its relation to degradation (lifetime) remain unclear. The limited research on these parameters stems from synthetic difficulties and lack of in situ analytics. This study addresses these challenges by synthesizing 2D and 3D Pt/C model catalyst systems via two precise routes and systematically varying the loading. Pt dissolution was monitored using on-line inductively coupled plasma mass spectrometry (on-line-ICP-MS), while X-ray spectroscopy techniques were applied to establish the oxidation states of Pt in correlation with metal loading. Dissolution trends emerge which can be explained by three particle proximity dependent mechanisms: (1) shifts in the Nernst dissolution potential, (2) redeposition, and (3) alteration of Pt oxidation states. These results identify engineering limitations, which should be considered by researchers in fuel cell development and related fields

The Carbocation Rearrangement Mechanism, Clarified

The role of protonated cyclopropane (PCP⁺) structures in carbocation rearrangement is a decades-old topic that continues to confound. Here, quantum-chemical computations (PBE molecular dynamics, PBE and CCSD optimizations, CCSD­(T) energies) are used to resolve the issue. PCP⁺ intermediates are neither edge-protonated nor corner-protonated (normally) but possess “closed” structures mesomeric between these two. An updated mechanism for hexyl ion rearrangement is presented and shown to resolve past mysteries from isotope-labeling experiments. A new table of elementary-step barrier heights is provided. The mechanism and barrier heights should be useful in understanding and predicting product distributions in organic reactions, including petroleum modification

Dissolution of Platinum Single Crystals in Acidic Medium

Platinum single crystal basal planes consisting of Pt(111), Pt(100), Pt(110) and reference polycrystalline platinum Pt(poly) were subjected to various potentiodynamic and potentiostatic electrochemical treatments in 0.1 M HClO4. Using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC‐ICP‐MS) the transient dissolution was detected on‐line. Clear trends in dissolution onset potentials and quantities emerged which can be related to the differences in the crystal plane surface structure energies and coordination. Pt(111) is observed to have a higher dissolution onset potential while the generalized trend in dissolution rates and quantities was found to be Pt(110)>P(100)≈Pt(poly)>Pt(111)

Tuning the Electrocatalytic Performance of Ionic Liquid Modified Pt Catalysts for the Oxygen Reduction Reaction via Cationic Chain Engineering

Modifying Pt catalysts using hydrophobic ionic liquids (ILs) has been demonstrated to be a facile approach for boosting the performance of Pt catalysts for the oxygen reduction reaction (ORR). This work aims to deepen the understanding and initiate a rational molecular tuning of ILs for improved activity and stability. To this end, Pt/C catalysts were modified using a variety of 1-methyl-3-alkylimidazolium bis(trifluoromethanesulfonyl)imide ([CnC1im][NTf2], n = 2–10) ILs with varying alkyl chain lengths in imidazolium cations, and the electrocatalytic properties (e.g., electrochemically active surface area, catalytic activity, and stability) of the resultant catalysts were systematically investigated. We found that ILs with long cationic chains (C6, C10) efficiently suppressed the formation of nonreactive oxygenated species on Pt; however, at the same time they blocked active Pt sites and led to a lower electrochemically active surface area. It is also disclosed that the catalytic activity strongly correlates with the alkyl chain length of cations, and a distinct dependence of intrinsic activity on the alkyl chain length was identified, with the maximum activity obtained on Pt/C-[C4C1im][NTf2]. The optimum arises from the counterbalance between more efficient suppression of oxygenated species formation on Pt surfaces and more severe passivation of Pt surfaces with elongation of the alkyl chain length in imidazolium cations. Moreover, the presence of an IL can also improve the electrochemical stability of Pt catalysts by suppressing the Pt dissolution, as revealed by combined identical-location transmission electron microscopy (TEM) and in situ inductively coupled plasma mass spectrometry (ICP-MS) analyses

Effect of Ionic Liquid Modification on the ORR Performance and Degradation Mechanism of Trimetallic PtNiMo/C Catalysts

Ionic liquids (ILs) modification, following the concept of “solid catalyst with ionic liquid layer (SCILL)”, has been demonstrated to be an effective approach to improving both activity and stability of Pt-based catalysts for the oxygen reduction reaction. In this work, the SCILL concept has been applied to a trimetallic PtNiMo/C system, which has been documented recently to be significantly advantageous over the benchmark PtNi-based catalysts for oxygen reduction. To achieve this, two hydrophobic ILs ([BMIM][NTF2] and [MTBD][BETI]) were used to modify PtNiMo/C with four IL-loading amounts between 7 and 38 wt %. We found that the Pt mass activity (@0.9 V) could be improved by up to 50% with [BMIM][NTF2] and even 70% when [MTBD][BETI] is used. Exceeding a specific IL loading amount, however, leads to a mass transport related activity drop. Moreover, it is also disclosed that both ILs can effectively suppress the formation of nonreactive oxygenated species, while at the same time imposing little effect on the electrochemical active surface area. For a deeper understanding of the degradation mechanism of pristine and IL modified PtNiMo/C, we applied identical location transmission electron microscopy and in situ scanning flow cell coupled to inductively coupled plasma mass spectrometry techniques. It is disclosed that the presence of ILs has selectively accelerated the dissolution of Mo and eventually results in a more severe degradation of PtNiMo/C. This shows that future research needs to identify ILs that prevent the Mo dissolution to leverage the potential of the IL modification of PtNiMo catalysts

Structure dependency of the atomic-scale mechanisms of platinum electro-oxidation and dissolution

Platinum dissolution and restructuring due to surface oxidation are primary degradation mechanisms that limit the lifetime of Pt-based electrocatalysts for electrochemical energy conversion. Here, we studied well-defined Pt(100) and Pt(111) electrode surfaces by in situ high-energy surface X-ray diffraction, on-line inductively coupled plasma mass spectrometry, and density functional theory calculations, to elucidate the atomic-scale mechanisms of these processes. The locations of the extracted Pt atoms after Pt(100) oxidation reveal distinct differences from the Pt(111) case, which explains the different surface stability. The evolution of a specific stripe oxide structure on Pt(100) produces unstable surface atoms which are prone to dissolution and restructuring, leading to one order of magnitude higher dissolution ratesWe acknowledge the European Synchrotron Radiation Facility for provision of SXRD facilities, and H. Isern and T. Dufrane for their help with the SXRD experiments. Funding is acknowledged from NSERC (grant RGPIN-2017-04045) and Deutsche Forschungsgemeinschaft (grants MA 1618/23 and CH 1763/5-1)

Tuning the Electrocatalytic Performance of Ionic Liquid Modified Pt Catalysts for the Oxygen Reduction Reaction via Cationic Chain Engineering

Modifying Pt catalysts using hydrophobic ionic liquids (ILs) has been demonstrated to be a facile approach for boosting the performance of Pt catalysts for the oxygen reduction reaction (ORR). This work aims to deepen the understanding and initiate a rational molecular tuning of ILs for improved activity and stability. To this end, Pt/C catalysts were modified using a variety of 1-methyl-3-alkylimidazolium bis­(trifluoromethanesulfonyl)­imide ([C_<i>n</i>C₁im]­[NTf₂], <i>n</i> = 2–10) ILs with varying alkyl chain lengths in imidazolium cations, and the electrocatalytic properties (e.g., electrochemically active surface area, catalytic activity, and stability) of the resultant catalysts were systematically investigated. We found that ILs with long cationic chains (C6, C10) efficiently suppressed the formation of nonreactive oxygenated species on Pt; however, at the same time they blocked active Pt sites and led to a lower electrochemically active surface area. It is also disclosed that the catalytic activity strongly correlates with the alkyl chain length of cations, and a distinct dependence of intrinsic activity on the alkyl chain length was identified, with the maximum activity obtained on Pt/C-[C₄C₁im]­[NTf₂]. The optimum arises from the counterbalance between more efficient suppression of oxygenated species formation on Pt surfaces and more severe passivation of Pt surfaces with elongation of the alkyl chain length in imidazolium cations. Moreover, the presence of an IL can also improve the electrochemical stability of Pt catalysts by suppressing the Pt dissolution, as revealed by combined identical-location transmission electron microscopy (TEM) and in situ inductively coupled plasma mass spectrometry (ICP-MS) analyses

Anisotropy of Pt nanoparticles on carbon- and oxide-support and their structural response to electrochemical oxidation probed by in situ techniques

Identifying the structural response of nanoparticle–support ensembles to the reaction conditions is essential to determine their structure in the catalytically active state as well as to unravel the possibledegradation pathways. In this work, we investigate the (electronic) structure of carbon- and oxide-supported Pt nanoparticles during electrochemical oxidation byin situX-ray diffraction, absorptionspectroscopy as well as the Pt dissolution rate byin situmass spectrometry. We prepared ellipsoidal Pt nanoparticles by impregnation of the carbon and titanium-based oxide support as well as spherical Pt nanoparticles on an indium-based oxide support by a surfactant-assisted synthesis route. Duringelectrochemical oxidation, we show that the oxide-supported Pt nanoparticles resist (bulk) oxideformation and Pt dissolution. The lattice of smaller Pt nanoparticles exhibits a size-induced latticecontraction in the as-prepared state with respect to bulk Pt but it expands reversibly during electrochemical oxidation. This expansion is suppressed for the Pt nanoparticles with a bulk-like relaxedlattice. We could correlate the formation of d-band vacancies in the metallic Pt with Pt lattice expansion. PtOxformation is strongest for platelet-like nanoparticles and we explain this with a higher fraction of exposed Pt(100) facets. Of all investigated nanoparticle–support ensembles, the structural response of RuO2/TiO2-supported Pt nanoparticles is the most promising with respect to their morpho-logical and structural integrity under electrochemical reaction conditions