Due to their increased surface area to volume ratio and molecular accessibility, microporous and mesoporous materials are a promising strategy to electrocatalyze the cathodic oxygen reduction reaction (ORR), the key reaction in proton-exchange membrane fuel cells (PEMFC). Here, we synthesized and provided atomically resolved pictures of porous hollow PtNi/C nanocatalysts, investigated the elemental distribution of Ni and Pt atoms, measured the Pt lattice contraction, and correlated these observations to their ORR activity. The best porous hollow PtNi/C nanocatalyst achieved 6 and 9-fold enhancement in mass and specific activity for the ORR, respectively over standard solid Pt/C nanocrystallites of the same size. The catalytic enhancement was 4 and 3-fold in mass and specific activity, respectively, over solid PtNi/C nanocrystallites with similar chemical composition, Pt lattice contraction, and crystallite size. Furthermore, 100% of the initial mass activity at E = 0.90 V vs RHE (0.56 A mg −1 Pt) of the best electrocatalyst was retained after an accelerated stress test composed of 30 000 potential cycles between 0.60 and 1.00 V vs RHE (0.1 M HClO 4 T = 298 K), therefore meeting the American Department of Energy targets for 2017−2020 both in terms of mass activity and durability (0.44 A mg −1 Pt, mass activity losses < 40%). The better catalytic activity for the ORR of hollow PtNi/C nanocatalysts is ascribed to (i) their opened porosity, (ii) their preferential crystallographic orientation ("ensemble effect"), and (iii) the weakened oxygen binding energy induced by the contracted Pt lattice parameter ("strain effect").
different anode materials with high theoretical capacities have been investigated. [1,2] Among them, silicon has drawn a lot of attention due to its gravimetric capacity ten times higher than graphite (3579 mAh g −1 for Li 15 Si 4 vs 372 mAh g −1 for LiC 6 ). Si is also earth abundant, low cost, and nontoxic. However, its use in commercial Li-ion batteries is challenging because the capacity retention and coulombic efficiency of Si electrodes are much lower than for graphite electrodes. [3,4] This is related to the important volume change of Si during its lithiation (up to 280% for Li 15 Si 4 vs ≈10% for LiC 6 ), which deteriorates the electrode architecture and destabilizes the solid electrolyte interphase (SEI). The optimization of the mechanical properties of Si-based electrodes is thus a key issue to improve their electrochemical performance. This optimization must be done at the Si particle scale as well as at the composite electrode scale.The use of nanosized Si materials (nanoparticles, nanowires, nanopillars, nanocomposites) has demonstrated a great efficiency for limiting the pulverization of the Si active material upon cycling as its fracturing resistance is size-dependent. [5] One must, however, note that the synthesis of such nanosized materials at industrial scale and competitive cost remains an issue, in addition to their low tap density and high surface reactivity. It thus appears more relevant to use micrometric particles, for example, ball-milled nanocrystalline/amorphous Si [6] or nanostructured Si alloys [1,7] able to prevent the formation of crystalline Li 15 Si 4 , which is known to promote particle fracture due to its large phase boundary stress with amorphous Li x Si during delithiation.The mechanical properties (compliance) of the SEI layer must also be optimized to be able to tolerate the large volume variation of the Si particles. To date, the use of appropriate electrolytes or electrolyte additives such as fluoroethylene carbonate (FEC) [8] appears as the most judicious strategy. [9] However, this interfacial issue is still not fully resolved and is probably the most important obstacle for the implementation of Si-rich based anodes in commercial Li-ion batteries.At a larger spatial scale, the volume change of the electrode is likely to induce its macrocracking, collapsing, and/or delamination from the current collector, leading to electrical disconnection of some Si particles. [10] In order to limit these morphological degradations, the cohesion and adhesion strength of the Si electrodes needs to be improved. These have The alloying reaction of silicon with lithium in negative electrodes for lithiumion batteries causes brutal morphological changes that severely degrade their cyclability. In this study, the dynamics of their expansion and contraction, of their cracking in the bulk and of their debonding at the interface with the current collector are visualized by in situ synchrotron X-ray computed tomography and quantified from appropriate 3D imaging analyses. Two electrodes made with s...
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