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Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles: in situ observation of the linear composition-strain-activity relationship.

Jia Q, Liang W, Bates MK, Mani P, Lee W, Mukerjee S - ACS Nano (2015)

Bottom Line: Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization.Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt).These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology and ‡Department of Biology, Northeastern University , Boston, Massachusetts 02115, United States.

ABSTRACT
Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt-Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques. The PtxCo/C NPs with high Co content possess a Co concentration gradient such that Co is concentrated in the core and gradually depletes in the near-surface region, whereas the PtxCo/C NPs with low Co content possess a relatively uniform distribution of Co with low Co population in the near-surface region. Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt). The RPt-Pt is further shown to contract linearly with the increase in Co/Pt composition. These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

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Pt L3 edge (left) and Co K edge (right) XANES spectra of Pt2Co/C and Pt7Co/C in N2-purged 0.1 M HClO4 at various potentials.
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fig2: Pt L3 edge (left) and Co K edge (right) XANES spectra of Pt2Co/C and Pt7Co/C in N2-purged 0.1 M HClO4 at various potentials.

Mentions: For all the catalysts, the Pt white line intensity increases with applied potentials (Figure 2), which is caused by the charge transfer from Pt to the oxygenated species (such as hydroxyl intermediate (*OH)) accumulated on the surface under high potentials.7,8,23,46 The high Pt white line intensity of the ex situ electrodes indicates their surfaces are largely covered by oxygenated species. Unlike Pt, which can adsorb and desorb oxygenated species repeatedly during voltage cycling, the Co on the surface is spontaneously oxidized when exposed to air, and the soluble Co oxides are quickly dissolved during the conditioning pretreatment in acidic electrolyte, accounting for the drastic reduction of the Co white line intensity of Pt2Co/C (Figure 2, top right) and Pt3Co/C NPs. As for catalysts with low Co content (Pt4Co/C, Pt6Co/C, and Pt7Co/C (Figure 2, bottom right)), the Co X-ray absorption near-edge spectroscopy (XANES) between the ex situ and in situ electrodes are similar, indicating the low population of the exposed Co on the near-surface region. More importantly, the Co XANES of all the PtxCo/C catalysts remain unchanged with varying potentials (Figure 2 right), which shows that the majority of Co atoms are located inside the NPs without being directly exposed to the electrochemical environment. Therefore, it is clear that all the PtxCo/C NPs have Pt–Co cores covered by a Pt surface (Ptsurf@PtCocore) under in situ acidic environment. The drastic difference in XANES between the dry electrode and the electrode under in situ electrochemical control highlights the necessity of characterizing the electrode under in situ conditions.


Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles: in situ observation of the linear composition-strain-activity relationship.

Jia Q, Liang W, Bates MK, Mani P, Lee W, Mukerjee S - ACS Nano (2015)

Pt L3 edge (left) and Co K edge (right) XANES spectra of Pt2Co/C and Pt7Co/C in N2-purged 0.1 M HClO4 at various potentials.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4492796&req=5

fig2: Pt L3 edge (left) and Co K edge (right) XANES spectra of Pt2Co/C and Pt7Co/C in N2-purged 0.1 M HClO4 at various potentials.
Mentions: For all the catalysts, the Pt white line intensity increases with applied potentials (Figure 2), which is caused by the charge transfer from Pt to the oxygenated species (such as hydroxyl intermediate (*OH)) accumulated on the surface under high potentials.7,8,23,46 The high Pt white line intensity of the ex situ electrodes indicates their surfaces are largely covered by oxygenated species. Unlike Pt, which can adsorb and desorb oxygenated species repeatedly during voltage cycling, the Co on the surface is spontaneously oxidized when exposed to air, and the soluble Co oxides are quickly dissolved during the conditioning pretreatment in acidic electrolyte, accounting for the drastic reduction of the Co white line intensity of Pt2Co/C (Figure 2, top right) and Pt3Co/C NPs. As for catalysts with low Co content (Pt4Co/C, Pt6Co/C, and Pt7Co/C (Figure 2, bottom right)), the Co X-ray absorption near-edge spectroscopy (XANES) between the ex situ and in situ electrodes are similar, indicating the low population of the exposed Co on the near-surface region. More importantly, the Co XANES of all the PtxCo/C catalysts remain unchanged with varying potentials (Figure 2 right), which shows that the majority of Co atoms are located inside the NPs without being directly exposed to the electrochemical environment. Therefore, it is clear that all the PtxCo/C NPs have Pt–Co cores covered by a Pt surface (Ptsurf@PtCocore) under in situ acidic environment. The drastic difference in XANES between the dry electrode and the electrode under in situ electrochemical control highlights the necessity of characterizing the electrode under in situ conditions.

Bottom Line: Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization.Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt).These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology and ‡Department of Biology, Northeastern University , Boston, Massachusetts 02115, United States.

ABSTRACT
Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt-Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques. The PtxCo/C NPs with high Co content possess a Co concentration gradient such that Co is concentrated in the core and gradually depletes in the near-surface region, whereas the PtxCo/C NPs with low Co content possess a relatively uniform distribution of Co with low Co population in the near-surface region. Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt). The RPt-Pt is further shown to contract linearly with the increase in Co/Pt composition. These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

Show MeSH
Related in: MedlinePlus