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Controlled Surface Elemental Distribution Enhances Catalytic Activity and Stability
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Controlled Surface Elemental Distribution Enhances Catalytic Activity and Stability Zhenhai Xia,1,* Chang Ming Li,2 and Liming Dai3,* Alloying Pt with transition metals is widely recognized as a promising approach to synthesize high-performance and low-cost electrocatalysts, but binary Pt alloys such as PtNi usually have poor stability in spite of high ORR catalytic activities. Although the introduction of a third element to form a ternary alloy (e.g., PtNiCu) has been demonstrated to enhance stability and/or activity, its mechanism remains unclear. Researchers now clearly reveal the catalytic mechanism via a combined approach of innovative synthesis, advanced characterization, and theoretical simulation.
The overuse of fossil fuel has posed serious energy and environmental challenges. One of the promising solutions is to develop clean-energy-conversion technologies. Instead of burning fuel to create energy, fuel cells convert chemical energy directly into electricity by electrochemically oxidizing renewable fuel (e.g., hydrogen) at the anode and reducing oxygen into water at the cathode. Owing to its high-energy conversion efficiency, virtually no pollution, and potential large-scale applications, the fuel cell technology currently receives intensive research and development focus.1 However, platinum is required to promote the oxygen reduction reaction (ORR) in fuel cells, and its high cost and scarcity have hindered the widespread application of fuel cells.2 Therefore, the large-scale practical application of fuel cells will not be realized if low-cost, highly active and durable non-noble-metal catalysts are not developed. Among various innovative approaches to numerous new ORR catalysts, including nonprecious metal,3 metal-free (e.g., heteroatom-doped carbon nanomaterials)4,5 and MOF-/COF-based catalysts,6 alloying Pt with transition metals is widely recognized as a promising
method for reducing the amount of platinum, and hence the catalyst cost, needed for the desired catalytic effect toward ORR in fuel cells.3,4 Compared with commercial Pt/C catalysts, octahedral PtNi catalysts generated by introducing Ni into Pt nanoparticles can achieve drastically improved ORR catalytic activity but have poor stability.7 Introducing a third element to form a ternary alloy (e.g., PtNiCu) has been demonstrated to enhance stability and/or activity.8 However, it remains unclear how the third alloying element contributes to the enhanced activity and stability. Writing in Matter, Yu Huang and co-workers now report the first solution-phase synthesis of octahedral PtNiCu nanoparticles with well-controlled octahedral morphology, as well as the use of an innovative combined experimental and theoretical approach to vividly elucidate the role that the third alloying element (i.e., Cu) plays to enhance the catalytic activity and stability.9 Using a solution-phase synthesis approach, Huang and co-workers have for the first time synthesized octahedral PtNiCu nanoparticles with wellcontrolled octahedral morphology
and uniform dispersity on carbon support in solution.9 As shown in Figures 1A–1C, these newly-developed PtNiCu octahedral nanocatalysts (Figure 1D) exhibited significantly improved ORR activity and stability compared with the commercial Pt/C catalyst, and are even superior to PtNi of similar size and similar Pt-composition. The stability of PtNiCu/C exceeded the U.S. DOE target10 of fuel cell catalyst stability (less than 40% mass activity (MA) loss after 30000 CV cycles). To understand the underlying catalytic mechanism, Huang and Mueller et al., tracked the growth of the nanoparticle from six to sixty h and calculated the initial layer-by-layer compositions for PtNi and PtNiCu nanoparticles in order to examine the structures of the catalysts that were trapped at metastable state due to the low growth temperature. With the aid of energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), the composition evolution during the particle growth was unveiled. The tracking of the composition showed that Cu played a critical role at the early stage of the nanoparticle nucleation and growth and significantly reduced Ni dissolution from the catalysts during the ORR.9 To gain further insight into the mechanism on the catalytic activity and stability enhancement of PtNiCu nanoparticles, the authors also developed the density functional theory
1Department
of Materials Science and Engineering, and Department of Chemistry, University of North Texas, Denton, TX 76203, USA
2Qingdao
University, 308 Ningxia Road, Qingdao, Shangdong, China
3Center
of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
*Correspondence: [email protected] (Z.X.), [email protected] (L.D.) https://doi.org/10.1016/j.matt.2019.11.009
Matter 1, 1445–1455, December 4, 2019 ª 2019 Elsevier Inc. 1447
Figure 1. The Structures and Catalytic Activities of PtNiCu Catalysts (A) ORR polarization curves. (B) Specific activity (SA) and mass activity (MA). (C) Comparison of MA retention between PtNi/C and PtNiCu/C. (D—G) (D) Atomic resolution STEM images of octahedral PtNiCu nanoparticles, and the initial layers of atoms in PtNiCu that ended up in the (E) first, (F) second, and (G) third layers of representative snapshots after KMC runs. The black spheres mean that the atoms were created through growth events. 9
(DFT) trained kinetic Monte Carlo (KMC) models to determine the atomic-scale structures of the particles after electrochemical cyclic voltammetry (CV) activation, in conjunction with the time-tracking experiments, to determine the initial composition profile of the particles. The simulations provided detailed atom distributions in different layers in PtNiCu (Figures 1E– 1G). The tracked movement of individual atoms in KMC suggests that the enhanced stability can be attributed to increased surface Pt composition in the as-synthesized catalysts, which reduces the generation of surface vacancies and suppresses the surface migration and subsequent dissolution of subsurface Cu and Ni atoms. This provides a guideline for future efforts in rationally designing highly active and durable catalysts for ORR in fuel cells and beyond.
1448 Matter 1, 1445–1455, December 4, 2019
The work of Huang and co-workers9 represents a major breakthrough in the research and development of PtNibased catalysts. While the innovative solution phase synthesis paves the way for preparing octahedral PtNiCu nanoparticles with well-controlled octahedral morphology, the combined approach of the controlled synthesis, advanced characterization and computational simulation tracks the surface composition change of the octahedron catalyst nanoparticles at the atomic scale and correlates it with the catalyst activity and stability changes induced by the introduction of the third element (i.e., Cu) in the PtNiCu catalyst. This work can not only expand the controlled synthesis of various well-defined, highlyactive and durable catalysts but also lead to further breakthroughs in screening of novel catalysts with improved catalytic activity and stability for clean energy conversion.
1. Cano, Z.P., Banham, D., Ye, S., Hintennach, A., Lu, J., Fowler, M., and Chen, Z. (2018). Batteries and fuel cells for emerging electric vehicle markets. Nat. Energy 3, 279–289. 2. Sealy, C. (2008). The problem with platinum. Mater. Today 11, 65–68. 3. Wu, G., More, K.L., Johnston, C.M., and Zelenay, P. (2011). High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 332, 443–447. 4. Dai, L., Xue, Y., Qu, L., Choi, H.J., and Baek, J.-B. (2015). Metal-free catalysts for oxygen reduction reaction. Chem. Rev. 115, 4823– 4892. 5. Gong, K., Du, F., Xia, Z., Durstock, M., and Dai, L. (2009). Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 323, 760–764. 6. Lin, C.-Y., Zhang, D., Zhao, Z., and Xia, Z. (2018). Covalent organic framework electrocatalysts for clean energy conversion. Adv. Mater. 30, 1703646. 7. Choi, S.-I., Xie, S., Shao, M., Odell, J.H., Lu, N., Peng, H.C., Protsailo, L., Guerrero, S., Park, J., Xia, X., et al. (2013). Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high
activity of 3.3 A/mg(Pt) for the oxygen reduction reaction. Nano Lett. 13, 3420– 3425. 8. Zhang, C., Sandorf, W., and Peng, Z. (2015). Octahedral Pt2CuNi uniform alloy nanoparticle catalyst with high activity and promising stability for oxygen
reduction reaction. ACS Catal. 5, 2296– 2300. 9. Cao, L., Zhao, Z., Liu, Z., Gao, W., Dai, S., Gha, J., Xue, W., Sun, H., Duan, X., Pan, X., et al. (2019). Differential surface elemental distribution leads to significantly enhanced stability of PtNi-
based ORR catalysts. Matter 1, this issue, 1567–1580. 10. 3.4 Fuel Cells, 2016 (Updated May 2017) (Department of Energy https://www.energy.gov/sites/prod/ files/2017/05/f34/ fcto_myrdd_fuel_cells.pdf).
Preview
Nature-Inspired Color beyond Pigments
polydopamine and graphene oxide encapsulated silica nanoparticles were obtained.
Lianxin Shi1 and Shutao Wang1,2,* Natural beauty is hard to be exhibited by the pale colors of man-made pigments. In this issue of Matter, Zhao and co-workers used graphene oxide-encapsulated silica nanoparticles to assemble polydopamine-adhered microsphere and provide brighter structural color with angle independence and optimal stability. Colors are regarded as the God-given gifts for human beings. As early as Middle Stone Age, Ochre—a natural red pigment—has been possibly used for decoration in Africa.1 Diverse colorful mineral, flowers, whelk, and even beetles were also triturated and refined for the scarce pigments. Bright colors became symbols of fames and status, such as the grand church murals and the royal gorgeous costumes, but these limited natural pigments were still insufficient for people’s colorful life. With the development of fossil industry and nanotechnology, man-made organic and inorganic pigments were well developed to enrich the color palette.2 However, those colors depend on the light with specific wavelengths that pigments absorb and reflect. Nature also provides another alternative strategy—pigment-independent structural color—to display its colorful beauty, like peacock feathers, butterfly wings, and beetle shells.3,4 People have attempted to mimic these structural colors by assembling colloidal particles,5 but the particles are limited in their insufficient color saturation and
weak adhesion particles.
between
colloidal
By mimicking the specific microstructure and the existence of melanosomes in bird feathers, Zhao and coworkers from China have recently developed a bio-inspired non-iridescent structural-color pigments with excellent brightness, robustness, and self-adhesivity (Figure 1).6 The unique pigments were fabricated by assembling the specially designed multi-shell silica nanoparticles. The original unmodified silica nanoparticles were first encapsulated by a polydopamine layer. Then, it was coated by a graphene oxide layer with the assistance of the polydopamine’s adhesivity. Here, graphene oxide was served as a dark element to mimic the melanosomes in birds’ feathers for enhancing the color saturation and brightness of the material, while the existence of polydopamine was aimed to improve the adhesivity of pigment components so as to increase the robustness of the resultant pigments. By repeating these two coating processes, the multi-shell
To achieve the generation of the specially-designed non-iridescent pigments, these multi-shell nanoparticles were served as elementary units of the pigments. These particles were first dispersed in absolute ethanol and then assembled into microspheres with different sizes after being sprayed under the near-infrared radiation. These formed microspheres exhibited amorphous structure, thus exhibiting non-iridescent properties, scilicet the angle-independent characteristic. It was interesting that the bio-inspired multi-shell nanoparticle-derived pigments showed much brighter structural colors and more robust stability after various destroying methods. Moreover, the research team also expanded the advanced functions of the pigments through the special features of graphene oxide. They generated inverse opal structured hydrogel patterns through negatively replicating the template pigments. Owing to the existence of graphene oxide in the resultant hydrogels, these hydrogel patterns were endowed with photothermal
1CAS
Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
2School
of Future Technology, University of Chinese Academy of Sciences, Beijing, P. R. China *Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.11.007
Matter 1, 1445–1455, December 4, 2019 ª 2019 Elsevier Inc. 1449
Controlled Surface Elemental Distribution Enhances Catalytic Activity and Stability Zhenhai Xia,1,* Chang Ming Li,2 and Liming Dai3,* Alloying Pt with transition metals is widely recognized as a promising approach to synthesize high-performance and low-cost electrocatalysts, but binary Pt alloys such as PtNi usually have poor stability in spite of high ORR catalytic activities. Although the introduction of a third element to form a ternary alloy (e.g., PtNiCu) has been demonstrated to enhance stability and/or activity, its mechanism remains unclear. Researchers now clearly reveal the catalytic mechanism via a combined approach of innovative synthesis, advanced characterization, and theoretical simulation.
The overuse of fossil fuel has posed serious energy and environmental challenges. One of the promising solutions is to develop clean-energy-conversion technologies. Instead of burning fuel to create energy, fuel cells convert chemical energy directly into electricity by electrochemically oxidizing renewable fuel (e.g., hydrogen) at the anode and reducing oxygen into water at the cathode. Owing to its high-energy conversion efficiency, virtually no pollution, and potential large-scale applications, the fuel cell technology currently receives intensive research and development focus.1 However, platinum is required to promote the oxygen reduction reaction (ORR) in fuel cells, and its high cost and scarcity have hindered the widespread application of fuel cells.2 Therefore, the large-scale practical application of fuel cells will not be realized if low-cost, highly active and durable non-noble-metal catalysts are not developed. Among various innovative approaches to numerous new ORR catalysts, including nonprecious metal,3 metal-free (e.g., heteroatom-doped carbon nanomaterials)4,5 and MOF-/COF-based catalysts,6 alloying Pt with transition metals is widely recognized as a promising
method for reducing the amount of platinum, and hence the catalyst cost, needed for the desired catalytic effect toward ORR in fuel cells.3,4 Compared with commercial Pt/C catalysts, octahedral PtNi catalysts generated by introducing Ni into Pt nanoparticles can achieve drastically improved ORR catalytic activity but have poor stability.7 Introducing a third element to form a ternary alloy (e.g., PtNiCu) has been demonstrated to enhance stability and/or activity.8 However, it remains unclear how the third alloying element contributes to the enhanced activity and stability. Writing in Matter, Yu Huang and co-workers now report the first solution-phase synthesis of octahedral PtNiCu nanoparticles with well-controlled octahedral morphology, as well as the use of an innovative combined experimental and theoretical approach to vividly elucidate the role that the third alloying element (i.e., Cu) plays to enhance the catalytic activity and stability.9 Using a solution-phase synthesis approach, Huang and co-workers have for the first time synthesized octahedral PtNiCu nanoparticles with wellcontrolled octahedral morphology
and uniform dispersity on carbon support in solution.9 As shown in Figures 1A–1C, these newly-developed PtNiCu octahedral nanocatalysts (Figure 1D) exhibited significantly improved ORR activity and stability compared with the commercial Pt/C catalyst, and are even superior to PtNi of similar size and similar Pt-composition. The stability of PtNiCu/C exceeded the U.S. DOE target10 of fuel cell catalyst stability (less than 40% mass activity (MA) loss after 30000 CV cycles). To understand the underlying catalytic mechanism, Huang and Mueller et al., tracked the growth of the nanoparticle from six to sixty h and calculated the initial layer-by-layer compositions for PtNi and PtNiCu nanoparticles in order to examine the structures of the catalysts that were trapped at metastable state due to the low growth temperature. With the aid of energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), the composition evolution during the particle growth was unveiled. The tracking of the composition showed that Cu played a critical role at the early stage of the nanoparticle nucleation and growth and significantly reduced Ni dissolution from the catalysts during the ORR.9 To gain further insight into the mechanism on the catalytic activity and stability enhancement of PtNiCu nanoparticles, the authors also developed the density functional theory
1Department
of Materials Science and Engineering, and Department of Chemistry, University of North Texas, Denton, TX 76203, USA
2Qingdao
University, 308 Ningxia Road, Qingdao, Shangdong, China
3Center
of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
*Correspondence: [email protected] (Z.X.), [email protected] (L.D.) https://doi.org/10.1016/j.matt.2019.11.009
Matter 1, 1445–1455, December 4, 2019 ª 2019 Elsevier Inc. 1447
Figure 1. The Structures and Catalytic Activities of PtNiCu Catalysts (A) ORR polarization curves. (B) Specific activity (SA) and mass activity (MA). (C) Comparison of MA retention between PtNi/C and PtNiCu/C. (D—G) (D) Atomic resolution STEM images of octahedral PtNiCu nanoparticles, and the initial layers of atoms in PtNiCu that ended up in the (E) first, (F) second, and (G) third layers of representative snapshots after KMC runs. The black spheres mean that the atoms were created through growth events. 9
(DFT) trained kinetic Monte Carlo (KMC) models to determine the atomic-scale structures of the particles after electrochemical cyclic voltammetry (CV) activation, in conjunction with the time-tracking experiments, to determine the initial composition profile of the particles. The simulations provided detailed atom distributions in different layers in PtNiCu (Figures 1E– 1G). The tracked movement of individual atoms in KMC suggests that the enhanced stability can be attributed to increased surface Pt composition in the as-synthesized catalysts, which reduces the generation of surface vacancies and suppresses the surface migration and subsequent dissolution of subsurface Cu and Ni atoms. This provides a guideline for future efforts in rationally designing highly active and durable catalysts for ORR in fuel cells and beyond.
1448 Matter 1, 1445–1455, December 4, 2019
The work of Huang and co-workers9 represents a major breakthrough in the research and development of PtNibased catalysts. While the innovative solution phase synthesis paves the way for preparing octahedral PtNiCu nanoparticles with well-controlled octahedral morphology, the combined approach of the controlled synthesis, advanced characterization and computational simulation tracks the surface composition change of the octahedron catalyst nanoparticles at the atomic scale and correlates it with the catalyst activity and stability changes induced by the introduction of the third element (i.e., Cu) in the PtNiCu catalyst. This work can not only expand the controlled synthesis of various well-defined, highlyactive and durable catalysts but also lead to further breakthroughs in screening of novel catalysts with improved catalytic activity and stability for clean energy conversion.
1. Cano, Z.P., Banham, D., Ye, S., Hintennach, A., Lu, J., Fowler, M., and Chen, Z. (2018). Batteries and fuel cells for emerging electric vehicle markets. Nat. Energy 3, 279–289. 2. Sealy, C. (2008). The problem with platinum. Mater. Today 11, 65–68. 3. Wu, G., More, K.L., Johnston, C.M., and Zelenay, P. (2011). High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 332, 443–447. 4. Dai, L., Xue, Y., Qu, L., Choi, H.J., and Baek, J.-B. (2015). Metal-free catalysts for oxygen reduction reaction. Chem. Rev. 115, 4823– 4892. 5. Gong, K., Du, F., Xia, Z., Durstock, M., and Dai, L. (2009). Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 323, 760–764. 6. Lin, C.-Y., Zhang, D., Zhao, Z., and Xia, Z. (2018). Covalent organic framework electrocatalysts for clean energy conversion. Adv. Mater. 30, 1703646. 7. Choi, S.-I., Xie, S., Shao, M., Odell, J.H., Lu, N., Peng, H.C., Protsailo, L., Guerrero, S., Park, J., Xia, X., et al. (2013). Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high
activity of 3.3 A/mg(Pt) for the oxygen reduction reaction. Nano Lett. 13, 3420– 3425. 8. Zhang, C., Sandorf, W., and Peng, Z. (2015). Octahedral Pt2CuNi uniform alloy nanoparticle catalyst with high activity and promising stability for oxygen
reduction reaction. ACS Catal. 5, 2296– 2300. 9. Cao, L., Zhao, Z., Liu, Z., Gao, W., Dai, S., Gha, J., Xue, W., Sun, H., Duan, X., Pan, X., et al. (2019). Differential surface elemental distribution leads to significantly enhanced stability of PtNi-
based ORR catalysts. Matter 1, this issue, 1567–1580. 10. 3.4 Fuel Cells, 2016 (Updated May 2017) (Department of Energy https://www.energy.gov/sites/prod/ files/2017/05/f34/ fcto_myrdd_fuel_cells.pdf).
Preview
Nature-Inspired Color beyond Pigments
polydopamine and graphene oxide encapsulated silica nanoparticles were obtained.
Lianxin Shi1 and Shutao Wang1,2,* Natural beauty is hard to be exhibited by the pale colors of man-made pigments. In this issue of Matter, Zhao and co-workers used graphene oxide-encapsulated silica nanoparticles to assemble polydopamine-adhered microsphere and provide brighter structural color with angle independence and optimal stability. Colors are regarded as the God-given gifts for human beings. As early as Middle Stone Age, Ochre—a natural red pigment—has been possibly used for decoration in Africa.1 Diverse colorful mineral, flowers, whelk, and even beetles were also triturated and refined for the scarce pigments. Bright colors became symbols of fames and status, such as the grand church murals and the royal gorgeous costumes, but these limited natural pigments were still insufficient for people’s colorful life. With the development of fossil industry and nanotechnology, man-made organic and inorganic pigments were well developed to enrich the color palette.2 However, those colors depend on the light with specific wavelengths that pigments absorb and reflect. Nature also provides another alternative strategy—pigment-independent structural color—to display its colorful beauty, like peacock feathers, butterfly wings, and beetle shells.3,4 People have attempted to mimic these structural colors by assembling colloidal particles,5 but the particles are limited in their insufficient color saturation and
weak adhesion particles.
between
colloidal
By mimicking the specific microstructure and the existence of melanosomes in bird feathers, Zhao and coworkers from China have recently developed a bio-inspired non-iridescent structural-color pigments with excellent brightness, robustness, and self-adhesivity (Figure 1).6 The unique pigments were fabricated by assembling the specially designed multi-shell silica nanoparticles. The original unmodified silica nanoparticles were first encapsulated by a polydopamine layer. Then, it was coated by a graphene oxide layer with the assistance of the polydopamine’s adhesivity. Here, graphene oxide was served as a dark element to mimic the melanosomes in birds’ feathers for enhancing the color saturation and brightness of the material, while the existence of polydopamine was aimed to improve the adhesivity of pigment components so as to increase the robustness of the resultant pigments. By repeating these two coating processes, the multi-shell
To achieve the generation of the specially-designed non-iridescent pigments, these multi-shell nanoparticles were served as elementary units of the pigments. These particles were first dispersed in absolute ethanol and then assembled into microspheres with different sizes after being sprayed under the near-infrared radiation. These formed microspheres exhibited amorphous structure, thus exhibiting non-iridescent properties, scilicet the angle-independent characteristic. It was interesting that the bio-inspired multi-shell nanoparticle-derived pigments showed much brighter structural colors and more robust stability after various destroying methods. Moreover, the research team also expanded the advanced functions of the pigments through the special features of graphene oxide. They generated inverse opal structured hydrogel patterns through negatively replicating the template pigments. Owing to the existence of graphene oxide in the resultant hydrogels, these hydrogel patterns were endowed with photothermal
1CAS
Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
2School
of Future Technology, University of Chinese Academy of Sciences, Beijing, P. R. China *Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.11.007
Matter 1, 1445–1455, December 4, 2019 ª 2019 Elsevier Inc. 1449