A review of Pt-based electrocatalysts for oxygen ... - Springer Link

Front. Energy DOI 10.1007/s11708-017-0466-6

REVIEW ARTICLE

Changlin ZHANG, Xiaochen SHEN, Yanbo PAN, Zhenmeng PENG

A review of Pt-based electrocatalysts for oxygen reduction reaction

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2017

Abstract Development of active and durable electrocatalyst for oxygen reduction reaction (ORR) remains one challenge for the polymer electrolyte membrane fuel cell (PEMFC) technology. Pt-based nanomaterials show the greatest promise as electrocatalyst for this reaction among all current catalytic structures. This review focuses on Ptbased ORR catalyst material development and covers the past achievements, current research status and perspectives in this research field. In particular, several important categories of Pt-based catalytic structures and the research advances are summarized. Key factors affecting the catalyst activity and durability are discussed. An outlook of future research direction of ORR catalyst research is provided. Keywords oxygen reduction reaction (ORR), electrocatalysis, platinum catalyst, activity, durability

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Introduction

The properties of catalyst materials are largely dependent on their crystalline and electronic structures. By well tuning the catalyst parameters such as facet, size, composition, and phase, the catalytic properties can be largely altered [1–4]. Platinum group metals (PGM) are among the most studied catalysts for their outstanding properties in many chemical reactions, for instance oxidative amination, electrophilic cyclization, chemoselective reduction, A3 coupling, asymmetric hydrogenation, and C-H activation[5–9]. In terms of oxygen reduction reaction (ORR) electrocatalysis, the effects of facet, structure and composition control on designed PGM Received January 16, 2017; accepted March 26, 2017 Changlin ZHANG, Xiaochen SHEN, Yanbo PAN, Zhenmeng PENG

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( ) Department of Chemical and Biomolecular Engineering, University of Akron, Akron, OH 44325, United States E-mail: [email protected]

nanomaterials are among the topics of intensive study [10–14]. Since the discovery of fuel cells by German scientist Christian Friedrich Schöenbein and Welsh scientist Sir William Robert Grove, many types of fuel cells have been developed. Nowadays, the major types of fuel cells include phosphoric acid fuel cell (PAFC), polymer electrolyte membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC) [15]. PEMFC has been recognized as one attractive solution to the growing concerns on environmental and energy-related issues such as climate change and depletion of fossil fuels [16–20]. Indeed, the PEMFC stacks have already been applied in many transportation systems including trucks, buses, and forklifts in airports and large warehousing and logistics communities [21,22]. The essential component of PEMFCs is the embedded membrane electrode assembly (MEA) where the hydrogen oxidation reaction (HOR) occurs on the anodic side and ORR performs on the cathodic side (Fig. 1) [23]. The most commonly used electrocatalyst for HOR and ORR is Platinum (Pt). Due to the sluggish reaction kinetics of ORR, a substantial amount of Pt would be required to accelerate the slow energy conversion process from oxygen to electricity and meet the specific technical requirements on both performance and durability of PEMFC. However, as one noble metal, Pt is even scarcer than gold and silver and extremely pushes up the cost of PEMFC technique [24–27]. On the other hand, the broadly used Pt-based electrocatalysts suffer from lack of durability under the startup/shutdown conditions [23]. Thus, considerable improvement in developing newly promising ORR electrocatalysts is highly desirable with respect to reduce the Pt metal usage and enhance their durability. According to the United States Department of Energy’s (DOE) 2020 technological target, the total amount of PGMs at both anodic and cathodicelectrodes should be less than 0.125 mg/cm2and the ir-free mass activity should be greater than 0.44 A/mg Pt at 0.9 V verse reversible hydrogen electrode (RHE), and in terms of durability, the

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nanofibers were broadly prepared and studied. Unsupported PGM metal/alloy nanostructures such as nanowhiskers, nanodendrites, nanotube, nanowires and nanorods were also explored. To date, the shape/sizecontrolled Pt-alloy nanoparticles supported by carbon and thin films on nanostructured crystalline organic whisker supports (NSTF catalysts) demonstrated the highest ORR activities [23].The recent discovery that Pt3Ni {111} crystal surface can exhibit an exceptionally high ORR activity of 18 mA/cm2 Pt has motivated the synthesis of octahedral Pt-M (M = 3d transition metals) alloy nanoparticles, which are enclosed by the {111} planes and have a large specific active area (Fig. 2) [29].

2 Past achievements in ORR catalyst development 2.1 Fig. 1 Unit cell cross-section of the Nth unit cell in a fuel-cell stack, showing the components of an expanded MEA (Adapted with permission from Ref. [23], copyright 2012 Nature Publishing Group)

loss in mass activity at 0.9V should not exceed 40% [28]. Great efforts have been devoted to searching new ORR electrocatalysts. Among them, PGM nanostructures with extended surface area such as de-alloyed skeletal structures, Pt-skin structures, porous metal films and thin films on high-aspect-ratio supports have been investigated. In addition, PGM alloy nanoparticles supported by carbon black, oxides, single-walled carbon nanotubes and carbon

Composition-controlled Pt-based ORR electrocatalysts

One long-standing challenge in the ORR electrocatalysis research is to reduce the noble metal usage by replacing Pt with other metal elements. Since the discovery that Pt3Ni {111} crystal surface can exhibit an exceptionally high ORR activity of 18 mA/cm2, great effects have been devoted to investigating the ORR properties of Pt-based alloys, PtM (M = Ni, Co, Fe, Cu, Pd, Rh, Ti, V, Cr, Mo, W and so on) [30,31]. From a theoretical point of view, alloying Pt with a carefully selected metal element would have great potential in changing the surficial electronic structure of PtM alloy and correspondingly boosting the electrocatalytic activity (Fig. 3) [30].

Fig. 2 Influence of the surface morphology and electronic surface properties on the kinetics of ORR. RRDE measurements for ORR in HClO4 (0.1 M) at 333 K with 1600 revolutions per minute on Pt3Ni (hkl) surfaces as compared to the corresponding Pt (hkl) surfaces (a horizontal dashed gray line marks specific activity of polycrystalline Pt) are shown. Specific activity is given as a kinetic current density ik, measured at 0.9 V versus RHE. Values of d-band center position obtained from UPS spectra are listed for each surface morphology and compared between corresponding Pt3Ni(hkl) and Pt(hkl) surfaces (modified with permission from Ref. [29], copyright 2007 American Association for the Advancement of Science)

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

Fig. 3

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Volcano plots and free-energy diagrams for the oxygen reduction reaction on Pt-based transition metal alloys

(a) Measured kinetic current density as reported in the literature for a range of alloy electrocatalysts with Pt ‘skins’ plotted as a function of the calculated oxygen adsorption energy (The sources of the experimental data are marked by: circles (Pt monolayers supported on single-crystal metal electrodes), diamonds (polycrystalline alloys annealed in ultrahigh vacuum before immersion in the electrochemical cell) and crosses (bulk Pt3Ni {111} alloys annealed in ultrahigh vacuum before immersion)); (b) output of computational screening procedure, showing the oxygen binding energy, relative to that of Pt, on a Pt or Pd skin surface, as a function of alloying energy (modified with permission from Ref. [30], copyright 2009 Nature Publishing Group)

By changing the stoichiometric Pt-Fe precursor ratio, a series of Pt-Fe alloy nanoparticles with different particle composition (Pt0.52Fe0.48, Pt0.48Fe0.52, and Pt0.30Fe0.70) were prepared using Pt(acac)2 and Fe(CO)5 as precursors by Sun and his colleagues [32]. XRD patterns show that the diffraction peaks of Pt-Fe nanoparticles position between the reference peaks of Pt and Fe, confirming the alloy formation. Also, their diffraction peaks monotonically shift to higher angles as the ratio of Fe increases, indicating the lattice constant is strongly affected by the alloy composition. Composition-controlled PtxNi1 – x alloy nanoparticles were also prepared using solvothermal and

solid-state chemistry methods [33]. Strasser et al. synthesized octahedral Pt1.5Ni, PtNi and PtNi1.5 nanoparticles in dimethyfomamide (DMF) solvent by changing the ratio between Pt(acac)2 and Ni(acac)2 shown in Fig. 4. EELS line scan and STEM elemental mapping show the prepared octahedral Pt-Ni nanoparticles have non-uniform elemental distribution, with Pt-rich frame (corners and edges) and Ni-rich facets. After an activation process, the Ni content in these Pt-Ni nanoparticles dramatically decreases and the ORR activities follow the trend PtNi>Pt1.5Ni>PtNi1.5> Pt/C (The mass and specific activities of commercial Pt/C at 0.9 VRHE are ~0.15 A/mg Pt and ~0.23 mA/cm2. The

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Fig. 4 Octahedral Pt1.5Ni, PtNi and PtNi1.5 nanoparticles in dimethyfomamide (DMF) solvent (a–c) Z-contrast STEM images of Pt1.5Ni (a), PtNi (b) and PtNi1.5 (c) octahedral nanparticles close to the < 110> zone axis; (d–f) EELS line scan analysis of Pt1.5Ni, PtNi and PtNi1.5 octahedral nanoparticles close to the < 100>zone axis, respectively; (g) EELS element map of PtNi1.5; (h) composite image of a HAADF image showing mainly Pt (red) and an EELS map showing Ni (green); (i) ball schematic sketch; (j) cyclic voltammograms (CV) of PtxNi1-x octahedra in 0.1 M HClO4 solution at 25°C (100 mV/s; top); (k) specific activity; (l) mass activity (modified with permission from Ref. [33], copyright 2013 Nature Publishing Group)

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

specific activity of polycrystalline Pt is ~1.2 mA/cm2 Pt). Our group also investigated the compositional effect of PtxNi1 – x alloy nanoparticles prepared by a solid-state chemistry method, in which the ORR activities of Pt4Ni, Pt3Ni, Pt2Ni, Pt1.5Ni and PtNi were examined and compared with commercial Pt/C [34]. Slightly different from the results from Strasser’s group, our findings show that the Pt1.5Ni has the highest ORR activity (Fig. 5). The discrepancy on the optimal composition most likely comes from the different size and surface composition resulting from the different preparation methods, which we would elaborate in more details in later content. Another example of compositional control of PtM alloy is Pt-Co alloy system reported by Choi et al. In that work, they synthesized PtxCo alloy nanoparticles with controlled composition (x = 2, 3, 5, 7, and 9) and studied the effect on the ORR activity [35]. Electrochemical measurements reveal that there is a strong correlation between ORR activity and Co composition and the Pt3Co nanoparticles appear as the most active catalyst. Pt-Cu alloy nanoparticles with various composition (Pt3Cu, PtCu and PtCu3) were also synthesized and studied [36]. The results show the ORR mass activity increases as Pt3Cu < PtCu < PtCu3. Other studies on the compositional effect of PtM alloy on the ORR activity are reported with Pt-Cr, Pt-Mn and Pt-W bimetallic systems [37–40]. All of them illustrate the

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promising capacity in replacing the noble metal Pt without sacrificing the ORR activities. In addition to the Pt-M bimetallic systems, some ternary Pt-based alloys were also considered as ORR electrocatalyst. Among them, adding element N (N = Fe, Cu, Ni, Co) into Pt-M alloy system has been investigated aiming at further enhancement in ORR activity and durability [41]. Freshly prepared Pt-Fe/Co/Ni nanoparticles are observed with high initial ORR activity, while the dissolution of non-noble elements during the electrochemical cycling and the concurrent atomic restructuring and surficial electronic structure changes would lead to a dramatic activity loss. On the other hand, adding a third element can provide a possible synergetic effect, which might outperform their binary alloy system in terms of both activity and durability. Thus, development of ORR ternary alloy electrocatalyst such as Pt2CuNi, Pt3CoNi, Pt3FeNi and Pt3FeCo, and soon have been conducted [42,43]. For the Pt2CuNi ternary alloy electrocatalyst, it was observed possessing both outstanding ORR activity far exceeding the DOE 2020 target and the durability comparable to that of state-of-theart Pt/C. For the Pt3CoNi, Pt3FeNi and Pt3FeCo alloy, Pt3CoNi exhibited a higher ORR activity than Pt3Co with an improvement factor of ~4 compared to the commercial Pt/C. Transition metal-doped octahedral Pt3Ni catalysts were

Fig. 5 Schematic illustration of the formation of octahedral Pt–Ni nanoparticles on C support in CO and H2 mixture, and (a) ORR polarization curves; (b, c) active area and mass-specified ORR current densities (jarea and jmass) of PtNi/C, Pt1.5Ni/C, Pt2Ni/C, Pt3Ni/C, Pt4Ni/C, and commercial Pt/C; (d) cyclic voltammograms; (e) ORR; (f) jarea and jmass of Pt1.5Ni/C and commercial Pt/C after accelerated stability tests (modified with permission from Ref. [34], copyright 2014 American Chemical Society)

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also studied and reported with greatly improved ORR activity by Huang et al [41]. By doping 0.5%–2% transition metal with Pt-Ni octahedral nanoparticles, an

enhancement in factor of 1–80 can be obtained regarding the area specific activity and mass activity (Fig. 6). Among the sequences of metal doping using V, Cr, Mn, Fe, Co,

Fig. 6 Representative HAADF-STEM images of the (a) Pt3Ni/C and (b) Mo-Pt3Ni/C catalysts; (c, d) HRTEM images on individual octahedral (c) Pt3Ni/C and (d) Mo-Pt3Ni/C nanocrystals; (e, f) EDS line-scanning profile across individual (e) Pt3Ni/C and (f) Mo-Pt3Ni/C octahedral nanocrystals; (g) Pt, Ni, and Mo XPS spectra for the octahedral Mo‐Pt3Ni/C catalyst; (h) cyclic voltammograms of octahedral Mo-Pt3Ni/C, octahedral Pt3Ni/C, and commercial Pt/C catalysts; (i) ORR polarization curves; (j) the electrochemically active surface area (ECSA, top), specific activity (middle), and mass activity (bottom) at 0.9 V versus RHE for these transition metal–doped Pt3Ni/C catalysts. (modified with permission from Ref. [41], copyright 2015 American Association for the Advancement of Science)

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

Mo, Re, and W, Mo-doped Pt-Ni and Cr-doped Pt-Ni show the greatest activity enhancements. The Mo and Cr doping were attributed to the enhanced Pt-Metal bond strength, increased dissolution and diffusion energy barrier for Pt, and formed intact Pt-skin layer during the potential cycling. Recently, Escudero-Escribano et al. demonstrated that the ORR activity of Pt alloy electrocatalysts could also be tuned by means of lanthanide contractions (Fig. 7) [44]. The Pt-lanthanide (lanthanum, cerium, samarium, gadolinium, terbium, dysprosium, thulium, or calcium) alloys prepared by physical vapor deposition (PVD) were observed with a shorter Pt-Pt bond length and a largely relaxed overlayer, leaving a pure Pt layer after acid leaching which boosted the electrochemical stability. It was discovered that the strain efforts can weaken the binding of H and OH species on the catalyst surface, and enthalpy contributions can help to stabilize these Pt alloys under operating conditions. A liquid half-cell using the prepared PtxGd alloy catalyst exhibited an outstanding activity of 3.6 A/mg Pt at 0.9 V vs. RHE, which was only suppressed by the Pt3Ni nanoframe catalyst and Mo-doped Pt3Ni octahedral nanoparticles. This newly found strategy also sheds light on designing the next generation of ORR electrocatalysts and suggests that ORR catalysts can be

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further improved and made more affordable by more appreciable destabilization of reaction intermediates. 2.2

Size-controlled Pt-based ORR electrocatalysts

Although an exact relationship between the Pt nanoparticle size and the ORR properties has not yet been established and more systematic studies need to be conducted, a tentative conclusion that the size of electrocatalyst would affect the ORR activity and durability could be drawn [45– 52]. Peles et al. measured the ORR activity of Pt nanoparticles with size range from 1 to 5 nm (Fig. 8) [45]. To achieve size control of Pt nanoparticles, the CuUPD-Pt-replacement method was applied through a layerby-layer growth. The results show that the mass activity of Pt nanoparticles increased by 2 times from 1.3 to 2.2 nm and then decreased with an further increase in Pt nanoparticle size. On the other hand, the area specific activity of Pt nanoparticles increased firstly rapidly by 4 times as the Pt size reached 2.2 nm and then slowly as particle size further increased. The size-dependent ORR activity of Pt nanoparticles was ascribed to the increasing ratio of {111} and {100} terrace sites and corresponding weakening averaged oxygen binding energy as the size of Pt nanoparticles increases.

Fig. 7 Schematic views and electrochemical properties of polycrystalline Pt5M (M = lanthanide or alkaline earth metal) electrocatalysts and structure of Pt5M. Three-dimensional view of the Pt5M structure (a) during sputter-cleaning and (b) after electrochemistry; (c) Kinetic current density before and after a stability test consisting of 10000 cycles; (d and e) schematic view of the bulk structure of a Pt5M (illustrated for Pt5Tb), showing Pt5Tb terminated by (d) a Pt and Tb intermixed layer and (e) a Pt kagome layer. Purple spheres represent Tb atoms, and gray spheres represent Pt atoms; (f) relation between the lattice parameter a of Pt5M measured by XRD and the covalent radius of the lanthanide atoms (modified with permission from Ref. [44], copyright 2016 American Association for the Advancement of Science)

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Fig. 8 Size effect of Pt nanoparticles on the ORR properties (a) Size dependence of specific activity (blue diamond) and mass activity (red square) of Pt/C for oxygen reduction reaction at 0.93 V. The specific activity (open blue diamond) and mass activity (open red square) of state-of-the-art Pt/C from TKK (TEC10E50E, 46.7%(wt)) with an average particle size of 2.5 nm were also included for comparison. The specific and mass activities were calculated by normalizing the kinetic current to the electrochemical active surface and the Pt weight on the electrode, respectively. The electrochemical active surface was derived by integrating the charge in the hydrogen adsorption region assuming 210 mC/cm2. The Pt weight was calculated by the Cu UPD charge. (b) averaged oxygen binding energy for fcc and all surface sites as a function of particle size. Reprinted with permission from Ref. [45]. Copyright 2011 American Chemical Society

Nesselberger et al. deemed that the ORR activity of Pt nanoparticles with size from 1 to 5 nm were not mainly affected by the Pt size but rather the catalyst dispersion and interparticle distance [46]. A suggesting explanation is that the oxygen binding energy on the surface of Pt particles dramatically changed when the size of Pt nanoparticles varied, especially when the size of Pt nanoparticles was below 2.3 nm, which might alter the ORR reaction pathway by changing the rate-limiting step from the first proton and electron transfer to the O-O bond breaking. Other studies implied that as the interparticle distance decreased, an overlap in the electric double layers increased, causing potential drop in the compact layer and lower adsorption energy of reaction species on Pt surfaces. A higher H2O2 yield has been detected and used to support their conclusion when the interparticle distance increased from well extended layer to isolated nanoparticles. Pt nanoclusters with size less than 1 nm are not commonly used in fuel cell technology and we would not discuss in details on them. For Pt-based alloys, the size effect on the ORR properties was also studied by synthesizing the nanoparticles with size control. In our recent work, we reported strong dependence of ORR properties on the size of octahedral Pt–Ni nanoparticles, including Pt3Ni/C and Pt1.5Ni/C, with particle size ranging from around 4 to 8 nm synthesized using the solid-state chemistry method we innovated (Fig. 9) [53]. The correlations between the Pt–Ni size and the ORR properties were investigated and attributed to alterations in the particle electronic/geometric structure and the Ni leaching behavior. The octahedral Pt3Ni nanoparticles demonstrated a monotonous increase

in the ORR activity with an increase in particle size. The area specific activity was measured as 2.47 mA/cm2 Pt for the 4.5 nm-Pt3Ni/C and 4.06 mA/cm2 Pt for the 8.1 nm-Pt3Ni/C at 0.9 V vs. RHE. The significantly sizedependent ORR activity was also attributed to the varying fractions of {111} terraces on the surface of Pt-Ni nanoparticle. For octahedral Pt1.5Ni nanoparticles, they exhibited a different size dependency of the ORR activity, with 5.8 nm-Pt1.5Ni/C performing the best among all four catalysts and exhibiting the highest initial activity of 4.83 mA/cm2 Pt. The volcano relationship between the ORR activity and the Pt1.5Ni size could be caused by the interplay between less stability and a higher fraction of {111} terraces of the larger octahedral Pt1.5Ni particles. 2.3 Nanoscale morphology-controlled Pt-based ORR electrocatalysts

Since the discovery that Pt3Ni {111} crystal surface can exhibit an exceptionally high ORR activity of 18 mA/ cm2Pt, large amounts of attempts have been made to directly translate properties of single crystals to nanoparticles. While there is still huge gap between bulk crystals and nanoparticles and challenges remain in many aspects, one strategy to bridge the gap is to introduce thin film structures with controlled surface and thickness. Arenz et al. studied the ORR on Pt overlayers deposited onto Pt-Au film and explained the effects originating from ligand, strain and ensemble [54]. Using an electrochemical layerby-layer deposition method, Pt overlayers with different thickness were deposited onto the Pt-Au substrates

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

Fig. 9

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Size effect of octahedral Pt-Ni nanoparticles on the ORR activities

(a, c) area-specific current density, iarea,0, and (b, d) mass-specific current density, imass,0, at 0.9 V vs. RHE of the octahedral Pt-Ni nanoparticles (Left: Pt3Ni, Right: Pt1.5Ni) with different particle size. All tests were repeated three times in O2-saturated 0.1 M HClO4 solution, with a standard measurement error of 10%. Reprinted with permission from Ref. [53]. Copyright 2014 Royal Society of Chemistry

showing that (sub)monolayer amounts of Pt exhibit higher activity than Pt. Schmidt et al. investigated the role of strain on the ORR activity of Pt thin film catalyst with high crystalline quality fabricated by pulsed laser deposition on single-crystal substrate like {111} SrTiO3. By tuning the interatomic distance of Pt atoms in the thin film, a decrease in the adsorption energy of oxygenated species and an improved ORR kinetics can be obtained. To reduce the noble metal usage, preparing Pt layer structures on various substrates has also received great attention. Atomic Pt layers on transition metal nitride were prepared by a pulsed electrochemical deposition method, and the ORR measurement shows a more than 4 times increase in mass activity and 2 times increase in area specific activity compared to commercial Pt/C (Fig. 10) [17]. It should be noted that the prepared TiNiN@Pt catalysts with several Pt atomic layers demonstrated a well-maintained intact core shell structures and extremely good durability with only a slight activity loss after 10000 potential cycles. Zeng et al. reported the synthesis of octahedral [email protected] nanocrystals with an ultrathin PtNi alloy shell composed of approximately four atomic layers, which also show higher ORR activity and durability compared to commercial Pt/C [55]. Nanoscale morphology-controlled Pt-based nanostructures show a good promise in addressing the electrochemical durability issue. Under potential cycling in acidic aqueous solutions, non-noble metal component in the electrocatalyst would be oxidized and dissolved, leading to severe lattice restructuring and consequently losing their initial catalytic properties. MgO was used to prevent nanoparticles from sintering during high temperature

annealing process. The ordered intermetallic Pt alloy nanostructures were found with superior long-term stability under the ORR operating conditions. The Sun group reported the preparation of ordered face-centered tetragonal (fct) PtFe nanostructure in which the iron exhibited superior antidissolution properties (Fig. 11) [56]. The fct-PtFe nanoparticles demonstrated greatly enhanced ORR activity and durability compared to the disordered face-centered cubic (fcc) PtFe nanoparticles and commercial Pt/C. The fct-PtFe nanoparticles showed no obvious iron dissolution and nanoparticle degradation after even 20000 cycles between 0.6 and 1.0 V (vs.RHE) in 0.1 M HClO4. One other example is preparation of fct-PtCo nanoparticles by Abruna’s group, with Pt-rich surface and strong Pt-Co bonding in the core being observed and accounted for the high ORR activity and long-term stability. Another example of nanoscale morphology control on Pt-based nanostructures is the preparation of highly crystalline Pt3Ni nanoframes with 3D elelctrocatalytic surfaces (Fig. 12) [57]. Chen et al. recently reported this approach that Ni-rich PtNi3 rhombic dodecahedron nanoparticles with size of around 20 nm were firstly made in oleylamine, followed by Ni etching and concurrent exfoliation of inner Pt onto surface of edge, forming the {111}-like Pt skin structure. The open architecture of the Pt3Ni nanoframe allows the reactants to reach both the internal and external surfaces, giving rise to a 22 times enhancement in mass activity versus Pt/C catalyst. The mass activity was calculated as 5.7 A/mg Pt at 0.9 V, which is more than 10 times larger than the U.S. Department of Energy’s 2020 target (0.44 A/mg Pt). By

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Fig. 10 ORR properties of atomic Pt layers on transition metal nitride (a–c) Evolutions of CV curves after various numbers of cycles; (d) comparison of ECSA loss; (e) polarization curves of before (solid curves) and after (dashed curves) the ADT test; (f) mass and specific activities of Pt/C, TiN@Pt, and TiNiN@Pt catalysts before and after the durability test (modified with permission from Ref. [17], copyright 2016 ACS)

applying the ionic liquid [MTBD][NTf2] that has an O2 solubility approximately twice that of the common HClO4 electrolyte, Pt3Ni nanoframes exhibited a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity relative to Pt/C. The ionic liquid–encapsulated Pt3Ni nanoframes also showed sustained superior activity upon 10000 potential cycling without noticeable decay. One more representative work in the field of nanoscale morphology control on Pt-based nanostructures for improved ORR mass activity was reported very recently

by Li et al [58]. In their work, a Pt/NiO core/shell nanowires was firstly synthesized using wet chemistry method followed by thermal annealing and then electrochemical dealloying, which generated jagged Pt nanowires with an electrochemical active surface area (ECSA) as high as 118 m2/g Pt and an area specific ORR activity of 11.5 mA/cm2and mass ORR activity of 13.6 A/mg Pt at 0.9 V shown in Fig. 13. The highly stressed, under-coordinated rhombohedral-rich surface configurations of the jagged nanowires were thought to greatly contribute to the enhanced ORR activity.

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

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Fig. 11 Intermetallic FePt nanoparticles and the electrocatalytic properties in ORR (a) HAADF-STEM image of a representative fully ordered fct-FePt NP; (b) ORR polarization curves of C-Pt, C-fcc-FePt, partially and fully ordered Cfct-FePt NPs in 0.1 M HClO4 (rotating speed, 1600 r/min; scan rate, 10 mV/s) (modified with permission from Ref. [55], copyright 2015 American Chemical Society)

2.4

Facet-controlled Pt-based ORR electrocatalysts

Since the first report that described the synthesis of cubic and tetrahedral Pt nanoparticles by El-Sayed and his colleagues, there have been a number of papers published in the field that tried to explore the physical/chemical properties of materials on nanoscale [59–70]. An important subarea of them is the investigation on ORR activity of shape-controlled PGM metal/alloy. To mimic the Pt3Ni {111}single crystal that shows extremely high ORR activity, octahedral, icosahedral Pt-based nanostructures enclosed by {111} facets and other high-indexed Pt nanoparticles have been prepared. Choi et al. reported the synthesis of uniform 9 nm Pt-Ni octahedral nanoparticles using oleylamine and oleic acid as surfactants, W(CO)6 as a source of CO for {111} facet control and benzyl ether as a solvent for reducing the surfactant coverage (Fig. 14) [71]. After a further acetic acid treatment aiming at removing the surfactant, the Pt-Ni octahedral nanoparticles exhibited an ORR area specific activity as 51 times higher than that of the commercial Pt/C at 0.93 V, and mass activity of 3.3 A/mg Pt at 0.9 V. TEM, EDS, XPS, and XRD analyses demonstrated that the introduction of proper solvent could effectively eliminate the formation of Ni particles and significantly reduce the surface adsorption of surfactants leaving a clean, wellpreserved {111} surface of the octahedral Pt-Ni nanoparticles, which delivered a great enhancement on ORR activity. More recently, they fabricated octahedral Pt nanocages by depositing a few atomic layers of Pt as conformal shells on palladium (Pd) nanocrystals with welldefined {111} facets followed by etching away the Pd templates, which also exhibited high ORR activity and greatly improved stability benefiting from the shape control effect. Wu et al. reported the synthesis of Pt-M (M = Au, Ni,

Pd) icosahedral nanocrystals in which CO gas and organic surface capping agents play critical roles in stabilizing the {111} surfaces enclosed icosahedral nanoparticles (Fig. 15) [72]. The icosahedral Pt3Ni had ORR area specific activity of 1.83 mA/cm2 Pt and 0.62 A/mg Pt. Their results also show that the area specific activity of icosahedral Pt3Ni catalysts was 50% higher than that of the octahedral Pt3Ni catalysts (1.26 mA/cm2 Pt). The great improvement may arise from strain-induced electronic effects explained by the density functional theory calculations and molecular dynamics simulations. 2.5

Core-shell structured Pt-based ORR electrocatalysts

Another way to improve ORR activity and stability of Ptbased catalysts is to design and prepare core-shell structures, in which the ligand effects and geometric effects are expected to be tuned separately orsimultaneously. Recently, a theoretical study comprehensively screened both the ORR activity and stability trends for materials in the following matrix in Fig. 16. Using the density functional theory calculations, 700 core-shell 2 nm transition metal nanoparticles including various Pt-based nanostructures have been identified based on the bonding energies and segregation energies for ORR [73]. Many bimetallic core-shell Pt-based catalysts revealed higher predicted ORR activities with much reduced Pt metal loadings. In terms of synthesis Pt-based core-shell nanoparticles, a map of preparation approaches can be summarized and pictured in Fig. 17 [74]. Among them, electrochemical dealloying, (electro)chemical leaching, absorbate/thermalinduced segregation, sequential deposition and galvanic displacement using Copper under potential deposition (CuUPD) methods have been intensively studied, with majority of them presenting different Pt mass/specific

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Fig. 12 Pt3Ni nanoframes and the electrocatalytic properties in ORR (a–d) Preparation of Pt3Ni nanoframes with Pt {111}-skin–like surfaces; (e) cyclic voltammograms; (f) ORR polarization curves; (g) Tafel plots; (h) HER activities; (i, j) specific activities (i) and mass activities (j) measured at 0.95 V (modified with permission from Ref. [57], copyright 2014 American Association for the Advancement of Science)

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

Fig. 13

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Electrocatalytic properties of jagged Pt nanowires in ORR

(a) CV curves; (b) the evolution of ECSA; (c and d) CV and ORR polarization curves; (e and f) specific activity (SA) and mass activity (MA) Tafel plot; (g) the comparison of specific activities and mass activities of the J-PtNWs, the R-PtNWs, and the Pt/C at 0.9 V versus RHE, showing that the J-PtNWs deliver 33 times higher specific activity or 52 times higher mass activity than Pt/C; (h) ORR polarization curves and mass activity Tafel plot (inset) for the J-PtNWs before and after 6000 CV cycles between 0.6 and 1.0 V versus RHE; (i) high-resolution HAADF-STEM image of the J-PtNWs after ADT test (modified with permission from Ref. [58], copyright 2016 American Association for the Advancement of Science)

surface area based ORR activity improvement factors. Active and stable Ir@Pt core-shell catalyst developed by Jaramillo’s group has demonstrated a 2.6 times of specific and 1.8 times of mass activities compared to that of commercial Pt/C (TKK) even after 10000 stability cycles [75]. Xu and his colleagues showed that a core-shell nanostructured Au@NimPt2 catalysts can maintain excellent electrochemical durability of the Au@NimPt2 NPs even after 20 000 potential cycles between 0.6 and 1.1 V (vs.RHE) in an O2-saturated 0.1 M HClO4. Compared with the commercial E-TEK Pt/C catalyst, the most-active Au@Ni2Pt2 NPs can exhibit 3–4- and 4–6-times higher Pt

activity at 0.9 V before and after the 20000 potential cycles [76]. The stability and reactivity of Pt-based core-shell nanoparticle catalysts can be further improved by careful tailoring the Pt shell thickness, core composition, and particle size [77,78]. Mechanistic insights on the degradation of core–shell nanoparticles such as Ostwald ripening, particle coalescence, shell thickness evolution, and coreshell elements redistribution are worthy to be explored in the future. The synchrotron X-ray-based spectroscopic and scattering analytics and advanced in situ (environmental) microscopic technique would be able to provide more

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Fig. 14 HRTEM image of octahedral Pt2.5Ni nanoparticles and its ORR area specific activity and mass activity of icosahedral and octahedral Pt3Ni compared to commercial Pt/C (modified with permission from Ref. [71], copyright 2013 ACS)

Fig. 15 HRTEM image of Pt–Ni icosahedral nanocrystals, atomic structures of Pt icosahedral clusters, area specific activity and mass activity of icosahedral and octahedral Pt3Ni and corresponding surface strain fields (modified with permission from Ref. [72], copyright 2012ACS)

insightful information and straightforward evidences to stress these problems.

3 Challenges and outlooks in ORR electrocatalyst development Current challenge for the ORR research community is still to develop electrocatalyst with high activity, promising durability, and cost-effeteness via scalable synthesis. Many efforts have been made to address these issues in the last decades. To reduce (or even eliminate) noble metal usage and improve the electrochemical stability of a catalyst without sacrificing its electrocatalytic performance, the preparation of a highly stable single atom noble metal catalyst on electronic conductive support would be one possible direction. By further tuning the surface chemistry of elegantly selected support, a stronger metal-support interaction can be expected, and the local catalytic environment can speed up the adsorption/desorption of

reaction intermediates. Design and preparation of a coreshell catalyst with a stable and intact atomic layers of noble metals and non-noble metal cores is another possible solution to the long-standing activity and durability issues, in which the stability and activity of synthesized catalysts are dominated by the mismatch and electronic interaction between top layer and core materials. To advance the ORR catalyst development process, another common concern should also be addressed in the near future, that is how to mediate the gap in ORR performance between RDE and MEA measurements. To get a reasonably matched results, the testing procedures and local environments between them should be set as closer as possible. On the other hand, highly sophisticated modeling on the heat/mass transfer in the MEA devices and other operando/in-situ synchrotron X-ray based techniques could potentially provide foreseeable explains to those phenomena we still do not know the exact reasons yet. As one of the final steps before implementing those

Changlin ZHANG et al. Pt-based electrocatalysts for oxygen reduction reaction

Fig. 16 Design principle for ORR core-shell catalyst (a) Activity of core@shell NPs for the ORR. Higher activity corresponds to darker shades of color. NPs that did not converge or converged to deformed structures (NC/D) are colored white; (b) stability of core@shell NPs (NPs colored orange are stable with respect to swapping a core atom with a shell atom in vacuum; yellow are stable in oxidizing conditions, with O bound at the shell swap site; green are stable in both; and red in neither condition) (modified with permission from Ref. [73], copyright 2016AIP Publishing LLC)

Fig. 17 Summaries on synthesis approaches for the preparation of core–shell nanoparticle catalysts. Electrochemical (acid) dealloying/ leaching results in (a) dealloyed Pt bimetallic core–shell nanoparticles and (b) Pt-skeleton core–shell nanoparticles, respectively. Reaction process routes generate segregated Pt skin core–shell nanoparticles induced either by (c) strong binding to adsorbates or (d) thermal annealing. The preparation of (e) heterogeneous colloidal core–shell nanoparticles and (f) Pt monolayer core-shell nanoparticles is via heterogeneous nucleation and UPD followed by galvanic displacement, respectively (modified with permission from Ref. [74], copyright 2013 ACS)

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techniques, mass production of active and durable catalysts, also requires much more attention. Usually, the preparation of catalysts in industry uses the impregnation method, in which metal precursors are impregnated onto supports followed by reduction in H2 at elevated temperature. Catalysts prepared by these methods exhibit problems such as a broad size distribution and poor control on crystallographic facets/composition/morphology. In the laboratory, a combination of colloidal, CVD, PVD, UPD methods are involved. For the mostly reported wet chemistry methods, there is a large amount of organic solvent and surfactant being used in the synthesis system, and labor-intensive product separation is required. Generally, the synthesis system is on the order of tens of milligrams, which is far less than the industrial requirements. Additionally, once the synthesis system is scaled up to even hundreds of milligrams, the product faces issues such as poor batch-to-batch reproducibility and lack of high quality control. For the CVD, PVD, and UPD methods, all of them are limited by the high cost and strict operation requirements which render them unsuitable for implementation at this time. For the solid-state method we developed, further strategies aiming to improve the catalysts stability such as the formation of phase ordered intermetallic alloys or partially bonded with a transition metal nitride support/framework might be suggestions. At this moment, noble metals are still the most stable catalysts among those with considerable activities. By further tuning the geometric/composition/size factors and concomitant electronic structure and studying the degradation mechanism of current active catalysts by timeresolved in situ/operando techniques at the solid-liquid interface, next-generation ORR electrocatalyst would be developed. To further explore on ORR reaction mechanism is also worthy since it will greatly help us to obtain a better understanding of the dominating factors involved in ratelimiting steps.

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