To decrease Pt utilization and further improve catalytic performance, including both activity and stability, the introduction of a foreign metal is the most promising strategy. Bifunctional and electronic effects (ligand effect) are proposed to account for enhanced bimetallic/trimetallic electrocatalytic performance. In terms of bifunctional effects, such as in the MOR on PtM, the foreign metal is employed for water dissociation at lower potentials, thereby promoting the oxidation of CO on Pt sites and enhancing the durability. Regarding electronic effects, foreign metals can alter the electronic properties of Pt and lower the adsorption energy of CO, thereby facilitating the oxidation of CO at lower potentials. It is difficult to define the exact mechanism for a bimetallic system. In most cases, bifunctional and electronic effects would simultaneously contribute to an electrochemical process. For example, Ataee-Esfahani et al.19h prepared hollow mesoporous PtRu alloy particles as highly CO-tolerable catalysts for the MOR. They indicated that such superiority originated from two aspects, a downshifted d-band center (electronic effect) and adsorbed oxygen-containing species on Ru atoms (bifunctional effect).
In general, there are three possible types of mixing patterns between two or three metals:24, 25 1) Homogeneously mixed alloys, in which the two metals are mixed in either an atomically ordered (Figure 3 a, top) or a statistically random manner (Figure 3 a, bottom). Ordered intermetallic alloys have long-range atomic order and are less-commonly reported than random alloys. 2) Heterostructures (also called sub-cluster-segregated nanoalloys), in which the two metal components share the same interface (Figure 3 b). 3) Core–shell structures, which consist of a shell (single shell) of one type of atom that surrounds a core of another type of atom (Figure 3 c<xfigr3, top). Multiple-shell structures with layered or onion-like shells have also been experimentally produced (Figure 3 c, bottom). With the development of various theoretical and synthetic techniques, an abundance of composites structures have been experimentally prepared, such as a core–shell structures that consists of an alloyed core or shell (e.g., Pt3Fe2@Pt core–shell structures with an alloyed core26 and Au@FePt3 core–shell structures with an alloyed shell27). Such composite structures derive good qualities from its constituents and, hence, are of great interest and importance for various applications.
The alloys of Pt with various base metals, such as V,28 Cr,29 Co,30 Ti,31 Pb,17d, 32 Hg,33 Cu,34 and Ni,16b and of some noble metals, such as Pd,18a, 35 Ru,19i, 36 Au,37 and Ag,38 have been found to exhibit significantly higher electrocatalytic stabilities and better activities towards the ORR in PEMFCs than Pt alone. Such enhancement can be ascribed to different structural changes that are caused by alloying, such as geometrical factors (decreased PtPt bond length),39 dissolution of the more oxidizable alloying component,40 changes in surface structure,41 or electronic factors (increased Pt d-band electrochemical vacancy of the Pt skin layer, which originates from the bulk alloy).28, 42
The catalytic stability and activity of PtM alloy catalysts not only depends the nature of Pt, but also on the second metal. In terms of catalytic activity for the ORR and the tolerance towards chemical corrosion, He et al.43 classified the PtM (M=base metal) alloy cathodic catalysts into four categories: 1) Highly corrosive and highly active (M=Fe, Co, V, Mn); 2) corrosive and highly active (M=Zn, Cu, Mo, Ni); 3) stable but less active (M=Zr, Cr, Ta); and 4) stable and active (M=W, Ti). However, the leaching of the base metals from the alloys should be carefully handled. On one hand, it is thought that the loss of the base metal from the PtM catalysts can increase their stability/activity through surface roughening and, hence, increase the Pt surface area, or by modification of the electronic structure of the Pt skin layer. For example, Yu′s group19b utilized the dissolution of the base metal to reconstruct more stable catalysts. They found that the ECSA of PtNi disordered alloyed nanotubes (PNPT) increased about 100-fold after 250 potential cycles, owing to the dissolution of Ni from the PtNi random alloy (Figure 4 c). The experimental data further indicated that such dissolution of Ni during potential cycling should be responsible for the persistent surface restructuring, increased surface roughness and lattice strain, and, hence, high catalytic activity, stability, and resistance to poisoning. As shown in Figure 4 a, b, the activity and CO tolerance of PNPT was much higher than that of Pt/C. More importantly, after the degradation test, the ECSA of PNPT only decreased by 10 % (Figure 4 d), whereas Pt black and Pt/C lost 35 % and 70 % of their initial ECSAs, respectively.
Figure 4. a) CV and b) CA curves of Pt/C and PNPT catalysts for the MOR. c) Change in the CV of the PNPT catalyst with repeated cycling; the 1st and 250th cycles are indicated. d) ECSA of the PNPT catalyst after each 2000 s CA test in MeOH solution and 250 cycles in Ar-saturated HClO4. (Reproduced with permission from Ref. 19b. Copyright 2011, The Royal Society of Chemistry).
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However, in most case, the loss of the base metal will result in decreased stability/activity because of the change in surface structure and contamination of the electrochemical environment. To obtain highly active and stable catalysts, a promising synergistic effect has been proposed, based on the four categories mentioned above. Active metals in categories 1 or 2 are introduced into stable and less-active binary alloys to enhance the activities. Similarly, stable metals in categories 3 or 4 are added to stabilize or passivate the active and corrosive binary alloys. As such, there has been a boom in the creation of multicomponent alloys, thus confirming that such synergistic effects are effective. For example, Jeon et al.44 introduced Cr into PtCo/C to form PtCoCr/C alloy catalysts. Over a 20 h chronoamperometric (CA) test, the MOR activity of a Pt20Co30Cr40 powdered alloy catalyst showed 160 % higher mass activity than that of a PtRu/C catalyst, thus suggesting greatly increased durability and activity. Sun and co-workers45 demonstrated that the ternary FePtCu alloy nanorods exhibited much better ORR activity and stability than commercial Pt/C catalysts and binary FePt nanorod catalysts. The CV polarization curves of the Fe10Pt75Cu15 nanorods before and after 5000 cycles almost overlapped, thus indicating significantly enhanced stability.
High-temperature thermal annealing28, 46 and the formation of ordered intermetallic alloys31, 47 have also been shown to minimize the base-metal contamination and to offer enhanced stability in PEMFCs. In general, high-temperature thermochemical treatment can give rise to the formation of atomic-scale, chemically ordered nanoalloys. For example, Wang and co-workers34 prepared ordered intermetallic Cu3Pt/C catalysts with a Pt-rich skin after annealing at 1273 K under a H2 atmosphere, whereas a random CuxPt/C alloy was obtained at 573 K. Wanjiala et al.46 observed that PtVCo catalysts went through a phase-transition on treatment at elevated temperatures, from a chemically disordered fcc-type structure to a long-range-ordered fct-type structure with an enhanced degree of alloying. An ordered alloy structure generally displays enhanced stability and activity. Specifically with regard to Pt alloys with first-row transition metals, Gonzalez and co-workers1a proposed that PtCr and PtCo were generally more stable than PtV, PtNi, and PtFe in acidic media, because Cr and Co presented a higher degree of alloying with Pt than with V, Ni, and Fe. Quite recently, Botton′s group26 demonstrated a new intermetallic core–shell (IMCSs) regime in PtFe systems, which comprised an ordered Pt3Fe2 core that was encapsulated within a bilayer Pt-rich shell (about two atomic layers thick, induced by heat treatment; Figure 5 a–c). A minimal decrease (9 %) in catalytic activity, even after 6000 cycles, showed extended stability. Both the STEM-HAADFA image (particle center, Figure 5 e) and the XRD pattern (Figure 5 f) of the non-cycled Pt3Fe2 and after cycling about 10 000 times indicated that, despite a constant enrichment of the Pt shell over 10 000 electrochemical cycles, the particle still retained core-ordering. The changing tendency in the core and shell is directly reflected by the plots shown in Figure 5 g, h. Such endurance in the chemical order of the core is the major reason for the enhanced durability.
Figure 5. a, b) STEM-HAADF images of Pt3Fe2 nanocatalysts, which show alternating bright and dark areas that correspond to Pt and Fe atomic columns, respectively. c) 3D model of a typical IMCS nanocatalyst (color code: Pt gray, Fe yellow). (d, e) Atomic-resolution STEM-HAADF images of the non-cycled (d) and cycled catalysts (e). f) XRD data of the bulk sample. g, h) Plot of ordered core volume (g) and shell volume (h) versus particle size. Particles within the size range 3.2–4.2 nm retained comparable core volumes, even after 10 000 cycles, albeit with an increase in their shell volume (Reproduced with permission from Ref. 26. Copyright 2013, American Chemical Society).
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2.2.2. Heterogeneous structures
Pt-based heterogeneous structures, such as 1D heterojunction nanowires/dimers and 3D dendrites, are usually prepared by growing a small amount of Pt NPs on a less-expensive metal-NP substrate. Such structures can greatly lower the Pt consumption and broadly expand the spatially distribution of Pt sites, thereby exhibiting good resistance towards aggregation. In addition, owing to the third-body effect,48 electronic effects,49 increasing interface area,50 etc., heterogeneous bimetallic nanomaterials are highly stable and active towards fuel-cell reactions.
A classic example is PdPt bimetallic nanodendrites that were prepared through seeded-mediated growth by Xia′s group,17a which consisted of a dense array of Pt branches on a truncated octahedral Pd core. The Pt branches that were supported on faceted Pd NPs exhibited relatively large surface areas and particularly active facets toward the ORR and, hence, were much more active than commercial Pt catalysts. In addition, the stability of the nanodendrites was higher than that of commercial Pd catalysts. Yang′s50a group described a different approach for simultaneously addressing the activity and stability issues of Pt-on-Pd heterostructures. The Pd seeds were synthesized through an oleylamine-based procedure and, thus, were fairly monodispersed. The as-made catalysts exhibited superior performance for the ORR, especially long-term stability. After 30 000 cycles, the catalysts only lost about 12 % of the initial ECSA and showed a small degradation (9 mV) in the half-wave potential. The authors attributed the improved stability to the favorable interfacial structures between Pt and the Pd support. In addition, the heteronanostructures were larger than the typical overall particle size of Pt-on-Pd nanostructures, thus preventing the small Pt particles from dissolving during the ORR. Later, Gou et al.17b fabricated hybrid catalysts that consisted of 3D Pt-on-Pd nanodendrites that were loaded onto 2D graphene nanosheets that were decorated by PVP. Owing to the porous and high-surface-area properties of dendrite structures, the strong coordinative interactions between PVP and the nanodendrites, as well as the strong ππ interactions between graphene and PVP, the as-prepared catalysts exhibited much higher activity and stability toward the MOR than platinum black and commercial E-TEK Pt/C catalysts. Recently, Yamauchi′s51 group reported a facile one-pot formation of Pt-on-Pd nanodendrites by using Pluronic P123 as an additive, which exhibited high activity and durability for the MOR. Such an open dendritic structure of the Pt-on-Pd nanodendrites was highly beneficial for their superior resistance towards agglomeration of the active sites.
Recently, Zhang et al.49b demonstrated that Au clusters could be used to stabilize Pt ORR catalysts against dissolution (Figure 6 a). More importantly, there were no leakage issues on Au. The decoration of Au clusters on Pt greatly enhanced the stability of Pt, as shown in Figure 6 b, c; the Pt surface area changed little (only 4 % in ECSA) after a durability test (30 000 scans) in an O2-satrurated HClO4 solution. In situ X-ray absorption near-edge spectroscopy and voltammetry data suggested that the Au clusters conferred stability by raising the Pt-oxidation potential. Liu et al.48 modified tetrahexahedral Pt nanoparticles with Au adatoms. The Au decoration promoted the direct mechanism, thereby leading to highly active, stable, and poison-resistant anodic electrocatalysts for the FAOR.
Figure 6. a) Electron micrographs of a Au/Pt/C catalyst. The different structure in the area indicated by the arrow is ascribed to Au clusters. Inset shows an STM (125 nm×125 nm) image. b, c) Polarization curves (b) and CVs (c) of the Au/Pt/C catalyst before and after 30 000 cycles. d) Comparison of the change in the absorption-edge peaks of the XANES spectra for Au/Pt/C and Pt/C catalysts as a function of potential, as obtained at four different potentials in 1 M HClO4 (Reproduced with permission from Ref. 49b. Copyright 2007, AAAS).
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Trimetallic heterostructures, such as Pt-on-PdBi nanowires,52 with enhanced stability during electrocatalytic processes have also recently been reported.
2.2.3. Core-shell structures
Various methods have been developed for the preparation of core–shell NPs, such as galvanic replacement,53 seed-mediated growth,54 surface dealloying,34, 55 heat treatment,26, 34 and combinations of these.56 Core–shell structures represent an effective structural model, in which a monolayer or a few layers of Pt are arranged on less-expensive core materials.13d Such structure can greatly decrease the Pt utilization, because the majority of Pt atoms are distributed at the electrochemical reaction interface.3 More importantly, the structure and properties of a Pt shell can be effectively tailored by the core material, owing to structure-induced strain (geometry) and electronic effects (alloying). For example, the existence of subsurface Au atoms would make an additional contribution to the durability enhancement of the Pt skin layer by modifying the well-known core-hindered place-exchange mechanism.3, 57 Such a mechanism has been well-demonstrated in Au@FePt3 NPs,27 which were synthesized by mixing Au particles (7 nm) with [Fe(CO)5] and [Pt(acac)2] in a non-organic system (Figure 7 a, b). In addition to greatly enhanced activity, the Au@FePt3 NPs possessed superior durability and showed negligible morphology changes after a stability test of 60 000 cycles (Figure 7 c–e), much better than Pt/C and FePt3 NPs catalysts. The existence of thermodynamically stable Au in the subsurface layer makes the formation of subsurface oxide energetically harder and, thus, prevents dissolution.
Figure 7. a) Schematic representation of the synthesis of core–shell Au@FePt3 NPs. b) HAADF-STEM elemental mapping of Au, Pt, and a single Au@FePt3 core–shell NP. c) Summary of the mass activities of different NPs before and after the cycling tests. d, e) TEM images of Au@FePt3 NP catalysts before (d) and after (e) stability tests over 60 000 cycles (Reproduced with permission from Ref. 27. Copyright 2011, American Chemical Society).
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In recent years, the concept of Pt-monolayer catalysts for addressing the future challenges of limited Pt resources has been established. However, the lifetime that is required by the US Department of Energy (DOE) for these Pt catalysts requires the use of thick Pt layers or larger particles. To seek a balance between cost and long-term activity, a promising solution is controlling the core′s composition, size, and shape. Recently, Adzic′s group54b synthesized a NiN@Pt core–shell catalysts by encapsulating NiN core NPs inside 2–4-monolayer-thick Pt shells. The NiN@Pt catalysts had very good stability for the ORR, which only lost 11 mV in its half-wave potential after 35 000 cycles. Nitride caused a bifunctional effect, which facilitated the formation of the core–shell structures and improved the performance of the Pt shell through both geometric and electronic effects. Sasaki et al.58 demonstrated that a PtML/Pd/C catalyst (ML=monolayer) showed a small loss of ECSA after 100 000 potential cycles between 0.7–0.9 V during an accelerated durability test (ADT). On alloying the Pd core with Au to obtain a Pd9Au1 alloy core, the PtML/Pd9Au1/C catalyst showed a small loss of ECSA, even after 200 000 cycles, further showing the superiority of the composited structure. These experimental data and density functional theory (DFT) calculations indicated that the core could increase the stability of the Pt shell by positively shifting its oxidation potential and by preventing the cathode potential from reaching values at which Pt dissolution takes place. Further work on the PdAu alloy@PtML catalyst was performed.58b Although more severe ADT conditions (0.6–1.4 V) were employed in this case, there was no marked loss of Pt and Au, despite the dissolution of Pd. Such work clearly demonstrates the significant role of Au in the stability enhancement of the Pt monolayer.
Multi-shell-structured catalysts are also stable catalysts in an electrocatalytic environment. For example, Yamauchi′s group prepared a series of multilayered Au@Pd@Pt NPs with the assistance of polyvinylpyrrolidone (PVP)54a or Pluronic F12759 in a one-step process at room temperature. These trimetallic multilayered NPs showed higher activities and better durabilities than Au@Pt bimetallic core–shell structures and commercial Pt/C and Pt-black catalysts. Fang et al.60 studied the electrocatalytic performance for the FAOR on Au@Pd@Pt NPs that consisted of a gold core covered by a Pd shell, on which Pt clusters were deposited. By carefully investigating the synergistic effects between the three different nanostructure components (sphere, shell, and islands), they indicated that the optimized structure should only have two atomic layers of Pd and a half-monolayer equivalent of Pt. A further increase in the loading of Pd or Pt would actually decrease the catalytic performance.