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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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Cyclic voltammograms of the catalysts taken in N2-purged 0.1 M HClO4 at a scan rate of 20 mV·s–1 (left); ORR voltammograms of the catalysts taken in O2-purged 0.1 M HClO4 at a scan rate of 5 mV·s–1 (right).
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fig1: Cyclic voltammograms of the catalysts taken in N2-purged 0.1 M HClO4 at a scan rate of 20 mV·s–1 (left); ORR voltammograms of the catalysts taken in O2-purged 0.1 M HClO4 at a scan rate of 5 mV·s–1 (right).

Mentions: The mass activity, specific activity, and electrochemical surface area (ECSA) of the PtxCo/C and Pt/C catalysts are summarized in Table 2. The ECSAs of the catalysts were obtained from hydrogen-adsorption/desorption (HAD) analysis by integrating hydrogen adsorption area from 0.08 to 0.4 V in the cyclic voltammograms (CVs) at 20 mV/s scan rate (Figure 1). Each of the PtxCo/C catalysts exhibit marked enhancement in SA and MA over pure Pt/C, and the highest activity enhancements (6.2 times and 4.5 times in MA and SA) are comparable to those of the dealloyed Pt1Co1/C and PtCo3/C NP catalysts,33,38 as well as other M-rich dealloyed PtxM (x < 1) NPs (4–8 times higher in MA and SA),26,31 albeit with low initial Co content. Although the activity in general increases with increasing initial Co content, a strictly monotonic activity–composition correlation is not observed as the Pt2Co/C is not more active than the Pt3Co/C. Thus, the activity trends cannot be fully accounted for by the monotonic lattice contraction trend of these catalysts as expected from compressive–strain effects.17,18,31,32 To elucidate the origin of the increasing activity trends with Co content, as well as the cause of the breakdown, in situ XAS measurements were performed to identify the atomic structure of the catalysts under in situ 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)

Cyclic voltammograms of the catalysts taken in N2-purged 0.1 M HClO4 at a scan rate of 20 mV·s–1 (left); ORR voltammograms of the catalysts taken in O2-purged 0.1 M HClO4 at a scan rate of 5 mV·s–1 (right).
© Copyright Policy
Related In: Results  -  Collection

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

fig1: Cyclic voltammograms of the catalysts taken in N2-purged 0.1 M HClO4 at a scan rate of 20 mV·s–1 (left); ORR voltammograms of the catalysts taken in O2-purged 0.1 M HClO4 at a scan rate of 5 mV·s–1 (right).
Mentions: The mass activity, specific activity, and electrochemical surface area (ECSA) of the PtxCo/C and Pt/C catalysts are summarized in Table 2. The ECSAs of the catalysts were obtained from hydrogen-adsorption/desorption (HAD) analysis by integrating hydrogen adsorption area from 0.08 to 0.4 V in the cyclic voltammograms (CVs) at 20 mV/s scan rate (Figure 1). Each of the PtxCo/C catalysts exhibit marked enhancement in SA and MA over pure Pt/C, and the highest activity enhancements (6.2 times and 4.5 times in MA and SA) are comparable to those of the dealloyed Pt1Co1/C and PtCo3/C NP catalysts,33,38 as well as other M-rich dealloyed PtxM (x < 1) NPs (4–8 times higher in MA and SA),26,31 albeit with low initial Co content. Although the activity in general increases with increasing initial Co content, a strictly monotonic activity–composition correlation is not observed as the Pt2Co/C is not more active than the Pt3Co/C. Thus, the activity trends cannot be fully accounted for by the monotonic lattice contraction trend of these catalysts as expected from compressive–strain effects.17,18,31,32 To elucidate the origin of the increasing activity trends with Co content, as well as the cause of the breakdown, in situ XAS measurements were performed to identify the atomic structure of the catalysts under in situ 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