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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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Measured specific (red) and mass (blue) activities (left) and the Δμ amplitude derived from Figure 3 (/Δμ/ = /μ(1.0 V) – μ(0.54 V)/) (right) of the PtxCo/C and Pt/C as a function of RPt–Pt or the strain (upper x-axis). The dashed lines are the linear fitting results, and the corresponding equations and R values are given in the figure.
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fig6: Measured specific (red) and mass (blue) activities (left) and the Δμ amplitude derived from Figure 3 (/Δμ/ = /μ(1.0 V) – μ(0.54 V)/) (right) of the PtxCo/C and Pt/C as a function of RPt–Pt or the strain (upper x-axis). The dashed lines are the linear fitting results, and the corresponding equations and R values are given in the figure.

Mentions: As shown in Figure 6 left, the intrinsic SAs of the PtxCo/C and Pt/C increase nearly linearly with decreasing RPt–Pt or increasing strain. As these catalysts possess a narrow range of ECSA (Table 2), a linear correlation between their MA and RPt–Pt is also observed. These results demonstrate that the compressive strain plays a dominant role in determining the ORR activity of the PtxCo/C NP catalysts and can be represented by the bulk RPt–Pt. This implies that the bulk and surface RPt–Pt are strongly correlated despite the surface relaxation. This is likely because the surface strain induced by the core–shell lattice mismatch only gets gradually released for more than 5 Pt layers19 and, thus, remains largely intact for small NPs with limited atomic layers. In addition, for PtxCo/C NPs with low Co content, the surface relaxation is mild due to the relatively weak surface strain. For PtxCo/C NPs with high Co content, the strong surface relaxation tendency to relieve some of the intensive strain is however suppressed by the populated Co in the near-surface region.41 Thus, we conjecture that the PtxCo/C NPs with different Co content are subjected to a comparable level of surface relaxation, which may also account for the strong correlations between the bulk and the surface RPt–Pt. Nevertheless, the linear correlation between the bulk RPt–Pt and the ORR activity of PtxCo/C NPs found in this work shows that the bulk RPt–Pt is a valid strain descriptor as the alternative to the surface RPt–Pt, which makes it possible to correlate the catalytic performance of PtM NP catalysts to a property that can be measured under ORR operating 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)

Measured specific (red) and mass (blue) activities (left) and the Δμ amplitude derived from Figure 3 (/Δμ/ = /μ(1.0 V) – μ(0.54 V)/) (right) of the PtxCo/C and Pt/C as a function of RPt–Pt or the strain (upper x-axis). The dashed lines are the linear fitting results, and the corresponding equations and R values are given in the figure.
© Copyright Policy
Related In: Results  -  Collection

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

fig6: Measured specific (red) and mass (blue) activities (left) and the Δμ amplitude derived from Figure 3 (/Δμ/ = /μ(1.0 V) – μ(0.54 V)/) (right) of the PtxCo/C and Pt/C as a function of RPt–Pt or the strain (upper x-axis). The dashed lines are the linear fitting results, and the corresponding equations and R values are given in the figure.
Mentions: As shown in Figure 6 left, the intrinsic SAs of the PtxCo/C and Pt/C increase nearly linearly with decreasing RPt–Pt or increasing strain. As these catalysts possess a narrow range of ECSA (Table 2), a linear correlation between their MA and RPt–Pt is also observed. These results demonstrate that the compressive strain plays a dominant role in determining the ORR activity of the PtxCo/C NP catalysts and can be represented by the bulk RPt–Pt. This implies that the bulk and surface RPt–Pt are strongly correlated despite the surface relaxation. This is likely because the surface strain induced by the core–shell lattice mismatch only gets gradually released for more than 5 Pt layers19 and, thus, remains largely intact for small NPs with limited atomic layers. In addition, for PtxCo/C NPs with low Co content, the surface relaxation is mild due to the relatively weak surface strain. For PtxCo/C NPs with high Co content, the strong surface relaxation tendency to relieve some of the intensive strain is however suppressed by the populated Co in the near-surface region.41 Thus, we conjecture that the PtxCo/C NPs with different Co content are subjected to a comparable level of surface relaxation, which may also account for the strong correlations between the bulk and the surface RPt–Pt. Nevertheless, the linear correlation between the bulk RPt–Pt and the ORR activity of PtxCo/C NPs found in this work shows that the bulk RPt–Pt is a valid strain descriptor as the alternative to the surface RPt–Pt, which makes it possible to correlate the catalytic performance of PtM NP catalysts to a property that can be measured under ORR operating 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