Recent advances in bimetallic electrocatalysts for oxygen reduction: design principles, structure-function relations and active phase elucidation

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Review Article Recent advances in bimetallic electrocatalysts for oxygen reduction: design principles, structure-function relations and active phase elucidation María Escudero-Escribano∗ , Kim D. Jensen and Anders W. Jensen The electrocatalytic oxygen reduction reaction (ORR) is of fundamental importance for sustainable energy conversion. Model studies are essential to elucidate the design principles as well as the structure-activity-stability relations. Knowledge from extended surfaces combined with fine control of the nanoscale structure has enabled the development of highly efficient nanoparticulate catalysts. This review discusses recent advances in bimetallic electrocatalysts for enhanced ORR, from model surfaces to nanoparticles. A special focus has been placed on novel strategies involving morphology-controlled Pt-based nanomaterials with enhanced mass activity and high electrochemically active surface area. We conclude by highlighting the importance of in situ characterisation methods in order to both elucidate the active phase and understand the catalyst degradation mechanisms. Address Nano-Science Centre, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark ∗

Corresponding author.: Escudero-Escribano, María ([email protected])

Current Opinion in Electrochemistry 2018, XX:XX–XX This review comes from a themed issue on Surface Electrochemistry Edited by Marc Koper

ture of the electrode-electrolyte interface [1,6–9]. Therefore, detailed knowledge of the interface structure is essential to develop efficient catalysts [10–13]. In order to reduce the Pt loading while increasing the ORR activity, we can modify the geometric arrangement [12,14] and/or the electronic structure [15,16] of the Pt surface atoms. The latter can be achieved by alloying Pt with other metals [5,17]; this has been widely studied on Pt-late transition metal alloys [17–20,21•• ]. Most recently, a new set of alloys of Pt and rare- and alkaline-earth metals have shown unprecedented activity enhancement over pure Pt [22,23,24•• ,25]. Moreover, some Pt-rare earth alloys have very negative enthalpy of formation [26], which may stabilise them kinetically against degradation through dealloying [27]. Inspired by the work on extended surfaces, especially by the outstanding activity of Pt3 Ni(111) [11,28• ], tremendous progress has been made on tailored Ptbased nanocatalysts with enhanced ORR activity in acid [18,21,29,30,31•• ,32–35]. The development of electrocatalysts for alkaline-based fuel cells [36] has recently received increasing attention. Beyond Pt-based materials, catalysts based on non-precious metals such as Ag can be stable under alkaline conditions [37–39].

For a complete overview see the Issue and the Editorial Available online XX XXXX 2018 https://doi.org/10.1016/j.coelec.2018.04.013 2451-9103/© 2018 Elsevier B.V. All rights reserved.

Introduction The oxygen reduction reaction (ORR) is one of the most intensively investigated electrocatalytic reactions. Its slow kinetics limits the performance of sustainable energy conversion devices such as low-temperature fuel cells [1,2]. Under acidic conditions, at which polymer electrolyte membrane fuel cells operate, only Pt-based catalysts are both active and stable for this reaction [3• ,4,5]. The ORR activity depends strongly on the strucwww.sciencedirect.com

Figure 1 shows a comparison of the specific and mass activities (normalised by mass of precious group metal, [PGM]) from rotating disk electrode (RDE) measurements [35,40] for a selection of some of the most active ORR electrocatalysts in both acidic [11,15,22,24•• ,30,31•• ,41–46,47•• ,48• ,49–53] (Figure 1a–c) and alkaline [28• ,37–39,54,55•• ,56• ] (Figure 1d–e) electrolytes. Most of these catalysts have been reported between 2014 and 2018, indicating how fast the field is moving. We note that, in alkaline media, a wide array of benchmarking potentials, electrolytes and scan rates have been reported in the literature. To further develop this field, standardised procedures should be implemented. Although the work on ORR bimetallic catalysts in alkaline is an emerging field, it is not within the scope of this review. Herein, we review the most recent advances in the development of efficient Pt-based ORR electrocatalysts. First, Current Opinion in Electrochemistry 2018, 000:1–12

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Figure 1

Specific and mass activity of a selection of the most active state-of-the-art ORR electrocatalysts in 0.1 M HClO4 acidic (a–c) and various alkaline (d and e) electrolytes at 0.90 V versus the reversible hydrogen electrode (RHE) unless otherwise stated. (a) Specific activity of ORR nanocatalysts in 0.1 M HClO4 ; the electrochemically active surface area (ECSA) was obtained from CO stripping measurements unless otherwise specified below: commercial Pt/C TKK 46% [41], PtNi3 @HGS [42], Pd@PtNi octahedral nanoparticles (ECSA from Cu underpotential deposition) [43], dealloyed PtNi nanoparticles (ECSA from H adsorption) [44], PtCu@PtCuNi/C nanoparticles [46], Pt-Co nanowires (ECSA from H adsorption) [45], Pt-Pb nanoplates (ECSA from H adsorption) [47•• ], Ptx Gd nanoparticles [48• ], Mo-doped PtNi octahedral nanoparticles [49], Pt-Ni nanowires (ECSA from H adsorption) [50], Pt3 Ni nanoframes [30], Pt2.5 Ni octahedral nanoparticles [51], Pt nanowires [31•• ], and Ptx Y nanoparticles [52]. (b) Specific activity of extended electrode surfaces in 0.1 M HClO4 : polycrystalline Pt [24•• ,35], polycrystalline Pt3 Ni and Pt3 Co [15], polycrystalline Pt5 La, Pt5 Ce, Pt5 Tm, Pt5 Dy, Pt5 Ca, Pt5 Sm, Pt5 Gd and Pt5 Tb [24•• ], polycrystalline Pt3 Y [22], Cu/Pt(111) near-surface alloy [53] and Pt3 Ni(111) [11]. (c) Mass activity of the nanocatalysts in (a) normalised to the total mass of precious group metal (PGM) [35]. (d) Specific activity of ORR catalysts in alkaline electrolyte: Ag and AgCo nanoparticles [38], Ag nanoplates [37], AgCu nanoparticles [39], Pt/C and PtCo3 nanoparticles [54], Cu/Pt(111) [55•• ] and Pt3 Ni(111) [28• ]. (e) Mass activity in alkaline electrolyte of the nanocatalysts in (d) together with Pd3 Pb nanoplates [56• ]. The mass activity for Ag-based catalysts normalised by total mass of catalyst (PGM∗ ). In alkaline, the activity values are given at 0.80, 0.85 or 0.90 V vs. RHE. In particular, the activity on Ag-based catalysts is typically given at 0.80, due to their low activity at 0.90 V vs. RHE.

we introduce key model studies on the elucidation of the design principles for the ORR. We then highlight a selection of strategies aiming at improving the mass activity of nanocatalysts. Finally, we summarise relevant in situ methods to characterise the active phase and the degradation mechanisms of the catalysts.

Design principles for enhanced oxygen reduction electrocatalysis on model Pt-alloys Elucidating the factors governing oxygen reduction is essential to design more efficient and stable catalysts [5,24,57–60]. A Sabatier volcano relationship between the ORR activity and the binding energy of the oxygencontaining reaction intermediates was established from Current Opinion in Electrochemistry 2018, 000:1–12

density functional theory calculations [59,61,62]: the optimum electrocatalyst should bind OH 0.1 eV weaker than Pt [61]. The OH binding energy, and thus the ORR activity, can be tuned by controlling other parameters such as the Pt-Pt interatomic distance [24•• ,63–65], the d-band centre [15,19,61,66] and the generalised coordination number [12]. On Pt-alloys, the OH binding energy can be weakened by means of ligand [16] and strain effects [18,67]. Ligand effects occur when the electronic structure of the surface atoms is modified due to subsurface alloying [53], and strain effects originate from the formation of a laterally compressed Pt overlayer [18,24•• ,68]. Model studies on well-defined surfaces have enabled to separate these electronic effects [24•• ,53,55,69–71]. www.sciencedirect.com

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Figure 2

Relationships on Pt-based ORR electrocatalysts: (a) Activity as a function of the potential shift in OH adsorption on Cu/Pt(111) near-surface alloys (NSAs) by Jensen et al. [55•• ] and Stephens et al. [53]; Activity as a function of (b) the nearest neighbour Pt-Pt distance and (c) the potential shift in H adsorption on polycrystalline Pt-lanthanide and Pt-alkaline earth alloys by Escudero-Escribano et al. [24•• ] and Vej-Hansen et al. [25]; (d) Specific activity as a function of the strain on mass-selected Ptx Gd, Ptx Y and Pt nanoparticles by Velázquez-Palenzuela et al. [48• ] and Hernández-Fernández et al. [52]. The strain (%) was determined through EXAFS after initial ORR activity measurement (filled squares) and after stability test (empty square) [48• ]. The specific activity in (a–d) corresponds to the kinetic current density evaluated at 0.9 V vs. RHE.

Recent works on Cu/Pt(111) near-surface alloys (NSAs) and Pt-lanthanide alloys provide a strong experimental proof of the volcano model [24•• ,53,55•• ]. Ligand effects induced by subsurface alloying were studied in the absence of strain on Cu/Pt(111) NSAs (see Figure 2a) [53,55•• ]. Cyclic voltammograms (CVs) showed a positive shift in the OH adsorption region as the amount of subsurface Cu increased, indicating destabilisation of adsorbed OH with higher amounts of Cu [53,55•• ]. As a consequence, the ORR activity as a function of the OH adsorption follows a Sabatier volcano relation (Figure 2a), the optimal catalyst exhibiting an OH binding energy ∼0.1 eV weaker than that of Pt(111) [53,55•• ]. This volcano-type relation exists both in acid www.sciencedirect.com

and alkaline, suggesting that the ORR shares the same reaction intermediates in both media. The results on Cu/Pt(111) NSAs provide the key design principle for the ORR in both electrolytes [55•• ,72]. The ORR activity and stability trends were systematically studied by controlling strain effects on polycrystalline Pt-lanthanide alloys [24•• ]. These catalysts present up to a 6-fold activity enhancement over Pt [23,24•• ,25]. The active phase consists of a thick Pt overlayer under compressive strain formed by acid leaching [24•• ,73]. This overlayer is key for the enhanced kinetic stability presented by some Pt-lanthanide alloys [23,24•• ]. The activity versus the bulk nearest-neighbour Pt-Pt distance follows a volcano relation (Figure 2b). Interestingly, Current Opinion in Electrochemistry 2018, 000:1–12

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Figure 3

(a) Mass activity at 0.9 V vs. RHE as a function of the electrochemically active surface area (ECSA) from RDE measurements for recently developed nanoparticulate catalysts (values from [18,30,31•• ,34,41,43,44,47•• ,48• ,49,51,52,77,78,79• ]). The ECSA was determined from: CO-stripping measurements where available (references [30,31•• ,48• ,49,51,52,79• ]), Cu underpotential deposition for Pd@PtNi octahedral nanoparticles [43] and H adsorption for references [18,34,41,44,47•• ,77,78]. The figure shows both activity and ECSA before (filled blue) and after stability test (hollow blue) if provided (see original references for details). The blue arrow illustrates the importance of increasing the ECS in order to obtain enhanced mass activities. We note that the number of cycles and conditions of the stability test (e.g. potential range, temperature, wave nature of the voltammetry) vary considerably for the different nanoparticulate catalysts, impeding any direct comparison. (b) Elemental mapping of 9 nm Ptx Y mass selected NPs by Hernandez-Fernandez et al. [52]; (c) Transmission electron microscopy (TEM) image of 9 nm Pt2.5 Ni octahedral NPs by Choi et al. [51]; (d) TEM image of biaxially strained Pt-Pb nanoplates by Bu et al. [47•• ]; (e) TEM image of Pt3 Ni nanoframes by Chen et al. [30]; and (f) illustration and TEM image of jagged Pt nanowires by Li et al. [31•• ].

compressed overlayers present a negative shift in the H adsorption from the CVs, the ORR activity increasing linearly with the potential shift (Figure 2c). This shift indicates destabilisation of adsorbed H, Pt5 Tb exhibiting the maximum destabilisation. Our trends strongly suggest that Pt5 Tb presents the highest compressive strain; beyond larger levels of bulk strain, the Pt overlayer likely become unstable and relaxes toward lower surface strain [24•• ]. The relaxation of the most strained overlayers suggests that there is a limit to how far we can tune the activity by controlling the compressive strain without strain relaxation. Knowledge from extended Pt-rare earth electrodes enabled the development of nanoparticulate catalysts [48• ,52,74]. Mass selected Ptx Gd and Ptx Y nanoparticles synthesised in a magnetron nanoparticle source showed Current Opinion in Electrochemistry 2018, 000:1–12

very high ORR activity [48• ,52]. The ORR activity increased exponentially with the compressive strain from extended X-ray absorption fine structure measurements (Figure 2d) [48• ], in agreement with the formation of a Pt shell under compressive strain. Moreover, the loss in specific activity after accelerated stability tests could be explained by strain relaxation.

Recent strategies for the development of efficient nanoparticulate electrocatalysts The past years have witnessed tremendous progress on the development of bimetallic nanoparticles for ORR [29,75,76]. It is technologically relevant to compare the catalyst mass activity, that is the product of the specific activity and the electrochemically active surface area (ECSA). Figure 3a shows the mass activity as a function of the ECSA for a selection of state-of-the-art www.sciencedirect.com

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nanocatalysts before and after stability test (if provided) [18,30,31•• ,34,41,43,44,47•• ,48• ,49,51,52,77,78,79• ]. Increasing the ECSA is key for enhanced activities [3,80]; the catalysts presenting the highest mass activity in Figure 3a [30,31,47•• ,49] achieve this by increasing the ECSA. We have used ECSA data determined from CO-stripping measurements where available [30,31,48• ,49,51,52,79• ]. However, many works in the literature evaluate the ECSA from other methods such as H adsorption [18,34,41,44,47•• ,77,78]. Since the latter is typically affected by electronic effects on Pt-alloys [24•• ,69,74], we recommend using CO-stripping instead. Core-shell nanoparticles, where a Pt shell surrounds a core of a foreign metal/alloy [81,82], enable high ECSAs and can achieve increased activity and stability [65,77,83,84]. Notably, core-shell nanoparticles typically present a high stability; Pd9 Au cores showed an impressive mass activity loss of only 6% after 100 000 potential cycles (0.6–1.0 V vs. RHE at 80 °C) [77]. However, most core-shell catalysts incorporate expensive core-metals, resulting in mass activities normalised by PGM comparable to Pt/C both in RDE [85,86] and membrane electrode assembly (MEA) measurements [87]. Recent works employ non-precious metals [88,89] or porous/hollow cores [90–94]. Similar to core-shell nanoparticles, dealloyed nanoparticles consist of a Pt-rich shell around a Pt-alloy core, because of dealloying of the less noble metal [18,34,44,95]. Nanoconfined dealloyed nanoparticles have recently been synthesised [42,79,96], this method enabling higher ECSA and improved stability by reducing metal leaching, particle agglomeration and carbon corrosion [42,97,98]. Strain-controlled nanoparticles provide an attractive route to enhance the ORR activity [99]. Mass-selected Ptx Y [52] (see Figure 3b) and Ptx Gd [48• ] nanoparticles exhibit very high mass activity thanks to the formation of a compressed Pt-shell. Their ECSA is limited by the thick Ptshell required for optimum strain (Figure 2d), resulting in optimal nanoparticle sizes of around 8–9 nm. The high oxygen affinity of rare earths hinders the chemical synthesis of these alloys [100]. Recently, alloyed Pt-Y nanoparticles were synthesised on a gram scale for the first time [101]; further optimisation of this method could enable to control the composition and size and the nanoparticles, thus allowing improved activity. In the search of more active catalysts, shape-controlled nanoparticles have emerged [102–104]. Their intrinsic activity can be enhanced by exposing highly active facets. In particular, octahedral PtNi catalysts [43,51,105• ] can facilitate highly active Pt3 Ni(111) facets [11]. Octahedral Modoped Pt3 Ni [49] achieved a mass activity of 7.0 A/mgPt [106,107]. This could be in part related to the high ECSA achieved by the small particle size (4.2 nm), as compared to octahedral Pi2.5 Ni (9 nm). Although stable below 1.1 V vs. RHE, Mo-doped Pt3 Ni shows a specific activity loss www.sciencedirect.com

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of 78% after only 1000 cycles between 0.6 and 1.26 V vs. RHE, suggesting a metastable configuration. Recent works have shown that doping Pt-Ni nanoparticles with Rh [108] and Au [109] also enhance the stability. However, stabilising shape-controlled nanoparticles without activity loss still remains a key challenge [110]. Morphology-controlled catalysts including 1D nanowires [31,45,50,111], 2D nanoplates [47•• ,112] and 3D nanoframes [30,113–115] provide a highly attractive approach to enhance catalytic activity while maintaining high Pt-surface utilisation. Strained Pt-Pb nanoplates [47•• ] (see Figure 3d) achieved an activity of 4.3 A/mgPt by combining strain effects with an increase in the surface-to-volume ratio. Notably, Pt3 Ni nanoframes (Figure 3e) present both an outstanding mass activity of 5.7 A/mgPt and high stability [30]. Pt-based nanowires are of special interest due to their high ECSA. Jagged Pt nanowires (see Figure 3f) yielded a record-breaking activity of 13.6 A/mgPt [31]. The cause of the activity enhancement is still uncertain; simulations suggest that it is related to the formation of a highly uncoordinated surface configuration. Furthermore, the unique 1D nature of nanowires with multiple support anchor points are believed to inhibit Ostwald ripening and particle agglomeration [31,116], a major cause for degradation [41]. In addition, because of their highly crystalline surfaces, nanoframes [30] and nanowires [31,45] have few lattice boundaries and less defective sites vulnerable to Pt dissolution [117•• ]. Addressing the stability of nanoparticulate catalysts is of fundamental importance to develop technologically relevant catalysts [21•• ]. Hence, understanding the mechanisms governing degradation of Pt-based catalysts is critical. Several key mechanisms have been identified; these include Oswald ripening, particle detachment, Pt dissolution and leaching alloying elements [41,116]. Moreover, carbon supported catalysts suffer from carbon corrosion. Some concepts such as nanostructured thin films circumvent this problem [4]; however, these are significantly limited by low ECSAs. Some recent strategies include the development of more stable support materials [118] and unsupported catalysts such as aerogels [65,119]. For a more detailed discussion on both degradation mechanisms and mitigation strategies we refer to other recent reviews [94,120]. Finally, the impressive activity enhancement from RDE experiments has not been translated to MEA measurements in fuel cells yet [29,121]. Furthermore, many ORR electrocatalysts achieve high mass activities by increasing their intrinsic activities at a cost of lower ECSA as compared to Pt/C. At low overpotentials (0.9 V vs. RHE), this strategy leads to impressive values [121]. However, when increasing the overpotential to typical fuel cell voltages (0.6–0.7 V vs. RHE), the improvement factors gradually Current Opinion in Electrochemistry 2018, 000:1–12

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Figure 4

Selection of in situ characterisation of the electrochemical interface of relevance for oxygen reduction: (a) Schematic representation of a selection of techniques that provide an in situ insight into ORR electrocatalysts: in situ optical spectroscopy, electrochemical methods combined with mass spectrometry, electrochemical scanning probe microscopy and in situ X-ray characterisation techniques. (b) Correlation of CO stretching from in situ FTIR with the ORR activity of Pt-Ni nanoparticles, adapted from Beermann et al. [105• ]. (c) Pt(111) oxidation and catalyst roughening probed by in situ GI-XRD, adapted from Ruge et al. [132]. (d) Electrochemical flow cell combined with ICP-MS monitoring the Pt/Ni dissolution during dealloying of Pt-Ni nanoparticles, adapted from Baldizzone et al. [138]. (e) In situ STM images and CVs of a Pt(111) electrode surface during oxidation-reduction cycles, adapted from Jacobse et al. [141•• ].

decrease. Since the ECSA is unaffected by such potential relationship, it is desirable to develop nanocatalysts with optimised mass activity and ECSA [3• ]. Finally, it would be beneficial to combine RDE methods with complementary tools with reaction environments and current density ranges resembling those of fuel cells [122,123]. It was recently demonstrated that gas diffusion electrodes provide similar catalytic performances to those measured in MEAs [124• ], thus allowing catalyst benchmarking under realistic conditions.

In situ elucidation of the active surface phase and degradation mechanisms In order to develop more efficient and stable electrocatalysts, it is key to identify the active phase. Traditionally, Current Opinion in Electrochemistry 2018, 000:1–12

characterising the active surface phase has relied on ex situ surface science studies before and/or after the electrochemical tests. However, during the electrocatalytic reactions, the active sites evolve and their dynamics depend on the interface structure. Thus, we need to understand this structure under reaction conditions. Recent developments in advanced in situ characterisation techniques enable probing the interface at the atomic and molecular level (see Figure 4a) [125,126]. Electrochemical methods combined with in situ spectroscopy, synchrotron-based Xray studies, mass spectrometry and scanning probe microscopy can provide key in situ insight on the structurereactivity relations as well as degradation mechanisms. Below, we briefly summarise some recent examples.

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In situ infrared spectroscopy is very powerful to examine the electrochemical interface at the molecular level. Fourier transform infrared spectroscopy (FTIR) in the attenuated total reflectance (ATR) mode has been recently used on Pt/C under ORR conditions in order to study the adsorption of anions from different electrolytes under mass controlled conditions [127]. On Pt-Ni/C bimetallic octahedral nanoparticles, the ORR activity could be correlated to the CO binding energies from FTIR–ATR (Figure 4b) [105• ]. Synchrotron-based spectroscopic techniques such as Xray absorption spectroscopy or ambient-pressure X-ray photoelectron spectroscopy are essential to directly probe the electrocatalyst surface as well as the reaction intermediates of the ORR [52,128–131]. Recently, the oxidation potentials and surface roughening were elucidated on Pt(111) by means of in situ gracing incidence X-ray diffraction (GI-XRD) and small-angle X-ray scattering (see Figure 4c) [132,133]. Similar investigations on Ptalloys may result in further insights on the formation and degradation mechanisms on strained Pt overlayers [73,134]. Understanding corrosion on bimetallic alloys remains a crucial challenge [135,136]. Combining inductivelycoupled plasma mass spectrometry with on-line electrochemical flow cells or RDE has recently provided important insights into degradation mechanisms [137,116,117•• ]. Figure 4d shows the degradation during dealloying of Pt-Ni nanoparticles [138]. Lastly, electrochemical scanning tunnelling microscopy can play an essential role in the visualisation of the active phase and the surface dissolution [139,140] down to the atomic level. Figure 4e shows potential-induced surface roughening on Pt(111) under repeated oxidation and reduction cycles [141•• ]. Combining electrochemistry with high-resolution EC-STM can enable structural information at the atomic level. Both EC-STM [141•• ] and in situ GI-XRD/SAXS [132,133] studies on Pt(111) oxidationdriven restructuring offer important insight for the development of technologically relevant Pt-alloy nanocatalysts, as Pt-alloys form Pt(111)-like surfaces after acid leaching [139]. These studies, together with new approaches on elucidating and monitoring the active site during reaction conditions [142• ] are expected to play a crucial role in the development of more efficient active sites on bimetallic ORR electrocatalysts.

Concluding remarks and outlook In the recent years, remarkable progress has been made on improving and understanding the activity enhancement of bimetallic ORR catalysts. Model studies on Ptbased electrodes have provided key insights on the design principles for the ORR. This knowledge, combined with precise control of size, shape and morphology has www.sciencedirect.com

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led to the development of nanoparticulate catalysts with intrinsic activities close to those observed for extended model surfaces. Further investigations on novel catalysts under more realistic operating conditions might provide insights that enable a reduction in the current gap between RDE and MEA measurements. Thus far, only a few novel concepts have been tested in real devices; however, the results suggest that mass activity at low overpotential as an exclusive performance indicator is insufficient. Nevertheless, significant advances have been made on the efficiency of ORR nanocatalysts; in particular, the combination of high ECSA and mass activity presented by morphology-controlled nanocatalysts is highly encouraging. Yet, to further develop more active and stable electrocatalysts, a better understanding of the active phase is critical. In situ studies are essential in order to fully unlock the complex structural and chemical changes taking place during the electrocatalytic reaction.

Acknowledgments We gratefully acknowledge the Villum Foundation V-SUSTAIN grant 9455 to the Villum Center for the Science of Sustainable Fuels and Chemicals. M.E.-E. gratefully acknowledges support from the Villum Foundation under the VILLUM Young Investigator Programme (project no. 19412). The authors also acknowledge support from the Innovation Fund Denmark.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •

Paper of special interest Paper of outstanding interest.

••

1.

Stamenkovic VR, Strmcnik D, Lopes PP, Markovic NM: Energy and fuels from electrochemical interfaces. Nat Mater 2017, 16:57–69. https://doi.org/10.1038/nmat4738.

2.

Hong WT, Risch M, Stoerzinger KA, Grimaud A, Suntivich J, Shao-Horn Y: Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ Sci 2015, 8:1404–1427. https://doi.org/10.1039/C4EE03869J.

3.

Kongkanand A, Mathias MF: The priority and challenge of high-power performance of low-platinum proton-exchange membrane fuel cells. J Phys Chem Lett 2016, 7:1127–1137. https://doi.org/10.1021/acs.jpclett.6b00216. This perspective addresses the technical importance of ECSA for high-power performance and describes the loading limitations imposed on PEMFCs at operational current-densities. •

4.

Debe MK: Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486:43–51. https://doi.org/10.1038/nature11115.

5.

Stephens IEL, Bondarenko AS, Grønbjerg U, Rossmeisl J, Chorkendorff I: Understanding the electrocatalysis of oxygen reduction on platinum and its alloys. Energy Environ Sci 2012, 5:6744. https://doi.org/10.1039/c2ee03590a.

6.

Koper MTM: Structure sensitivity and nanoscale effects in electrocatalysis. Nanoscale 2011, 3:2054. https://doi.org/10.1039/c0nr00857e.

7.

Markovic NM, Ross PN: Surface science studies of model fuel cell electrocatalysts. Surf Sci Rep 2002, 45:121–229.

8.

Sheng T, Tian N, Zhou ZY, Lin WF, Sun SG: Designing Pt-based electrocatalysts with high surface energy. ACS Energy Lett 2017, 2:1892–1900. https://doi.org/10.1021/acsenergylett.7b00385. Current Opinion in Electrochemistry 2018, 000:1–12

Please cite this article as: Escudero-Escribano, Jensen, Jensen, Recent advances in bimetallic electrocatalysts for oxygen reduction: design principles, structure-function relations and active phase elucidation, Current Opinion in Elec-

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9.

ARTICLE IN PRESS

[mNS;June 16, 2018;4:6]

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Kobayashi S, Wakisaka M, Tryk DA, Iiyama A, Uchida H: Effect of alloy composition and crystal face of Pt-Skin/Pt 100– x Co x [(111), (100), and (110)] single crystal electrodes on the oxygen reduction reaction activity. J Phys Chem C 2017, 121:11234–11240. https://doi.org/10.1021/acs.jpcc.6b12567.

10.

Strmcnik D, Kodama K, van der Vliet D, Greeley J, Stamenkovic VR, Markovic´ NM: The role of non-covalent interactions in electrocatalytic fuel-cell reactions on platinum. Nat Chem 2009, 1:466–472. https://doi.org/10.1038/nchem.330.

11.

Stamenkovic VR, Fowler B, Mun BS, Wang G, Ross PN, Lucas Ca, Markovic´ NM: Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 2007, 315:493–497. https://doi.org/10.1126/science.1135941.

12.

24.

Escudero-Escribano M, Malacrida P, Hansen MH, Vej-Hansen UG, Velázquez-Palenzuela A, Tripkovic V, Schiøtz J, Rossmeisl J, Stephens IEL, Chorkendorff I: Tuning the activity of Pt alloy electrocatalysts by means of the lanthanide contraction. Science 2016, 352:73–76. https://doi.org/10.1126/science.aad8892. This report presents a new set of highly active Pt-alloy electrocatalysts; the lattice parameter of the bulk alloys controls their electrocatalytic properties. ••

25.

Vej-Hansen UG, Escudero-Escribano M, Velázquez-Palenzuela A, Malacrida P, Rossmeisl J, Stephens IEL, Chorkendorff I, Schiøtz J: New platinum alloy catalysts for oxygen electroreduction based on alkaline earth metals. Electrocatalysis 2017, 8:594–604. https://doi.org/10.1007/s12678- 017- 0375- 9.

Calle-Vallejo F, Tymoczko J, Colic V, Huy Vu Q, Pohl MD, Morgenstern K, Loffreda D, Sautet P, Schuhmann W, Bandarenka AS: Finding optimal surface sites on heterogeneous catalysis by counting nearest neighbors. Science 2015, 350:185–190.

26.

Malacrida P, Escudero-Escribano M, Verdaguer-Casadevall A, Stephens IEL, Chorkendorff I: Enhanced activity and stability of Pt–La and Pt–Ce alloys for oxygen electroreduction: the elucidation of the active surface phase. J Mater Chem A 2014, 2:4234. https://doi.org/10.1039/c3ta14574c.

13.

Da He Z, Hanselman S, Chen YX, Koper MTM, Calle-Vallejo F: Importance of solvation for the accurate prediction of oxygen reduction activities of Pt-based electrocatalysts. J Phys Chem Lett 2017, 8:2243–2246. https://doi.org/10.1021/acs.jpclett.7b01018.

27.

14.

Strmcnik D, Escudero-Escribano M, Kodama K, Stamenkovic VR, Cuesta Á, Markovic NM: Enhanced electrocatalysis of the oxygen reduction reaction based on patterning of platinum surfaces with cyanide. Nat Chem 2010, 2:880–885. https://doi.org/10.1038/NCHEM.771.

Dubau L, Lopez-Haro M, Castanheira L, Durst J, Chatenet M, Bayle-Guillemaud P, Guétaz L, Caqué N, Rossinot E, Maillard F: Probing the structure, the composition and the ORR activity of Pt3Co/C nanocrystallites during a 3422h PEMFC ageing test. Appl Catal B Environ 2013, 142–143:801–808. https://doi.org/10.1016/j.apcatb.2013.06.011.

15.

Stamenkovic V, Mun BS, Mayrhofer KJJ, Ross PN, Markovic NM, Rossmeisl J, Greeley J, Nørskov JK: Changing the activity of electrocatalysts for oxygen reduction by tuning the surface electronic structure. Angew Chem Int Ed 2006, 45:2897– 2901.

16.

Kitchin JR, Nørskov JK, Barteau MA, Chen JG: Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals. J Chem Phys 2004, 120:10240–10246.

17.

a Gasteiger H, Kocha SS, Sompalli B, Wagner FT: Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl Catal B Environ 2005, 56:9–35. https://doi.org/10.1016/j.apcatb.2004.06.021.

18.

Strasser P, Koh S, Anniyev T, Greeley J, More K, Yu C, Liu Z, Kaya S, Nordlund D, Ogasawara H, Toney MF, Nilsson A: Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts. Nat Chem 2010, 2:454–460. https://doi.org/10.1038/nchem.623.

19.

Stamenkovic VR, Mun BS, Arenz M, Mayrhofer KJJ, Lucas CA, Wang GF, Ross PN, Markovic NM: Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces. Nat Mater 2007, 6:241–247.

20.

Toda T, Igarashi H, Uchida H, Watanabe M: Enhancement of the electroreduction of oxygen on Pt alloys with Fe, Ni, and Co. J Electrochem Soc 1999, 146:3750–3756.

Han B, Carlton CE, Kongkanand A, Kukreja RS, Theobald BRC, Gan L, O’Malley R, Strasser P, Wagner FT, Shao-Horn Y: Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells. Energy Environ. Sci. 2015, 8:258–266. https://doi.org/10.1039/C4EE02144D. This paper reports a record breaking MEA activity of dealloyed Pt-Ni/C, exceeding the 2020 target from the Department of Energy of the United States.

28.

Staszak-Jirkovský J, Subbaraman R, Strmcnik D, Harrison KL, Diesendruck CE, Assary R, Frank O, Kobr L, Wiberg GKH, Genorio B, Connell JG, Lopes PP, Stamenkovic VR, Curtiss L, Moore JS, Zavadil KR, Markovic NM: Water as a promoter and catalyst for dioxygen electrochemistry in aqueous and organic media. ACS Catal 2015, 5:6600–6607. https://doi.org/10.1021/acscatal.5b01779. This work presents interesting results for the ORR in alkaline aqueous and organic media; Pt3Ni(111) shows a record in activity of ∼170mA/cm2 in 0.1M KOH alkaline media. •

29.

Banham D, Ye S: Current status and future development of catalyst materials and catalyst layers for proton exchange membrane fuel cells: an industrial perspective. ACS Energy Lett 2017, 2:629–638. https://doi.org/10.1021/acsenergylett.6b00644.

30.

Chen C, Kang Y, Huo Z, Zhu Z, Huang W, Xin HL, Snyder JD, Li D, Herron Ja, Mavrikakis M, Chi M, More KL, Li Y, Markovic NM, Somorjai Ga, Yang P, Stamenkovic VR: Highly crystalline multimetallic nanoframes with three-dimensional electrocatalytic surfaces. Science 2014, 343:1339–1343. https://doi.org/10.1126/science.1249061.

31.

Li M, Li M, Zhao Z, Cheng T, Fortunelli A, Chen C, Yu R, Gu L, Merinov B, Lin Z, Zhu E, Yu T, Jia Q, Guo J, Zhang L, Iii WAG, Huang Y, Duan X: Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction. Science 2016, 259:1414–1419. https://doi.org/10.1126/science.aaf9050. In this report, jagged Pt-nanowires demonstrate an unprecedented mass activity of 13.6 A/mgPt, by combining high specific activity with high ECSA. ••

32.

Lv H, Li D, Strmcnik D, Paulikas AP, Markovic NM, Stamenkovic VR: Recent advances in the design of tailored nanomaterials for efficient oxygen reduction reaction. Nano Energy 2016, 29:149–165. https://doi.org/10.1016/j.nanoen.2016.04.008.

33.

Liu W, Rodriguez P, Borchardt L, Foelske A, Yuan J, Herrmann AK, Geiger D, Zheng Z, Kaskel S, Gaponik N, Kötz R, Schmidt TJ, Eychmüller A: Bimetallic aerogels: high-performance electrocatalysts for the oxygen reduction reaction. Angew Chemie Int Ed 2013, 52:9849–9852. https://doi.org/10.1002/anie.201303109.

34.

Wang D, Xin HL, Hovden R, Wang H, Yu Y, Muller Da, DiSalvo FJ, Abruña HD: Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nat Mater 2013, 12:81–87. https://doi.org/10.1038/nmat3458.

35.

Pedersen CM, Escudero-Escribano M, Velázquez-Palenzuela A, Christensen LH, Chorkendorff I, Stephens IEL: Benchmarking

21. ••

22.

23.

Greeley J, Stephens IEL, Bondarenko AS, Johansson TP, Hansen HA, Jaramillo TF, Rossmeisl J, Chorkendorff I, Nørskov JK: Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. Chem 2009, 1:552–556. https://doi.org/10.1038/nchem.367. Escudero-Escribano M, Verdaguer-Casadevall A, Malacrida P, Grønbjerg U, Knudsen BP, Jepsen AK, Rossmeisl J, Stephens IEL, Chorkendorff I: Pt5Gd as a highly active and stable catalyst for oxygen electroreduction. J Am Chem Soc 2012, 134:16476–16479. https://doi.org/10.1021/ja306348d.

Current Opinion in Electrochemistry 2018, 000:1–12

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Bimetallic electrocatalysts for oxygen reduction Escudero-Escribano, Jensen and Jensen

Pt-based electrocatalysts for low temperature fuel cell reactions with the rotating disk electrode: oxygen reduction and hydrogen oxidation in the presence of CO (review article). Electrochim Acta 2015, 179:647–657. https://doi.org/10.1016/j.electacta.2015.03.176. 36.

Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, Kucernak AR, Mustain WE, Nijmeijer K, Scott K, Xu T, Zhuang L: Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 2014, 7:3135–3191. https://doi.org/10.1039/C4EE01303D.

37.

Garlyyev B, Liang Y, Butt FK, Bandarenka AS: Engineering of highly active silver nanoparticles for oxygen electroreduction via simultaneous control over their shape and size. Adv Sustain Syst, vol 1 2017 1700117. https://doi.org/10.1002/adsu.201700117.

38.

Holewinski A, Idrobo JC, Linic S: High-performance Ag-Co alloy catalysts for electrochemical oxygen reduction. Nat Chem 2014, 6:828–834. https://doi.org/10.1038/nchem. 2032.

39.

40.

41.

Wu X, Chen F, Zhang N, Qaseem A, Johnston RL: Engineering bimetallic Ag–Cu nanoalloys for highly efficient oxygen reduction catalysts: a guideline for designing Ag-based electrocatalysts with activity comparable to Pt/C-20%. Small 2017, 13:1–13. https://doi.org/10.1002/smll.201603876. Mayrhofer KJJ, Strmcnik D, Blizanac BB, Stamenkovic V, Arenz M, Markovic NM: Measurement of oxygen reduction activities via the rotating disc electrode method: from Pt model surfaces to carbon-supported high surface area catalysts. Electrochim Acta 2008, 53:3181–3188. https://doi.org/10.1016/j.electacta.2007.11.057. Meier JC, Galeano C, Katsounaros I, Witte J, Bongard HJ, Topalov AA, Baldizzone C, Mezzavilla S, Schüth F, Mayrhofer KJJ: Design criteria for stable Pt/C fuel cell catalysts. Beilstein J Nanotechnol 2014, 5:44–67. https://doi.org/10.3762/bjnano.5.5.

42.

Pizzutilo E, Knossalla J, Geiger S, Grote JP, Polymeros G, Baldizzone C, Mezzavilla S, Ledendecker M, Mingers A, Cherevko S, Schüth F, Mayrhofer KJJ: The space confinement approach using hollow graphitic spheres to unveil activity and stability of Pt-Co nanocatalysts for PEMFC. Adv Energy Mater, vol 7 2017 1700835. https://doi.org/10.1002/aenm.201700835.

43.

Choi S, Shao M, Lu N, Ruditskiy A, Peng H, Park J, Guerrero S, Wang J, Kim MJ, Xia Y: Synthesis and characterization of Pd @ Pt-Ni core-shell octahedra with high activity toward oxygen reduction. ACS Nano 2014, 10:10363–10371. https://doi.org/10.1021/nn5036894.

44.

Cui C, Gan L, Heggen M, Rudi S, Strasser P: Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis. Nat Mater 2013, 12:765–771. https://doi.org/10.1038/nmat3668.

45.

46.

Bu L, Guo S, Zhang X, Shen X, Su D, Lu G, Zhu X, Yao J, Guo J, Huang X: Surface engineering of hierarchical platinum-cobalt nanowires for efficient electrocatalysis. Nat Commun, vol 7 2016. https://doi.org/10.1038/ncomms11850. Park J, Kanti Kabiraz M, Kwon H, Park S, Baik H, Il Choi S, Lee K: Radially phase segregated PtCu@PtCuNi Dendrite@Frame nanocatalyst for the oxygen reduction reaction. ACS Nano 2017, 11:10844–10851. https://doi.org/10.1021/acsnano.7b04097.

Bu L, Zhang N, Guo S, Zhang X, Li J, Yao J, Wu T, Lu G, Ma J-Y, Su D, Huang X: Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis. Science 2016, 354:1410–1414. https://doi.org/10.1126/science.aah6133. This work shows that the ORR activity on exposed Pt(110) facets can be enhanced by tensile strain using biaxial Pt-Pb nanoplates with a high surface-to-volume ratio.

nanoparticles, the activity increasing for larger nanoparticles presenting higher compressive strain. 49.

Huang X, Cao L, Chen Y, Zhu E, Lin Z, Li M, Yan A, Zettl A, Wang YM, Duan X, Mueller T: High-performance transition metal – doped Pt 3 Ni octahedra for oxygen reduction reaction. Science 2015, 348:1230–1234.

50.

Jiang K, Shao Q, Zhao D, Bu L, Guo J, Huang X: Phase and composition tuning of 1D platinum-nickel nanostructures for highly efficient electrocatalysis. Adv Funct Mater, vol 27 2017. https://doi.org/10.1002/adfm.201700830.

51.

Choi S-I, Xie S, Shao M, Odell JH, Lu N, Peng H-C, Protsailo L, Guerrero S, Park J, Xia X, Wang J, Kim MJ, Xia Y: Synthesis and characterization of 9nm Pt-Ni octahedra with a record high activity of 3.3 A/mg(Pt) for the oxygen reduction reaction. Nano Lett 2013, 13:3420–3425. https://doi.org/10.1021/nl401881z.

52.

Hernandez-Fernandez P, Masini F, McCarthy DN, Strebel CE, Friebel D, Deiana D, Malacrida P, Nierhoff A, Bodin A, Wise AM, Nielsen JH, Hansen TW, Nilsson A, Stephens IEL, Chorkendorff I: Mass-selected nanoparticles of PtxY as model catalysts for oxygen electroreduction. Nat Chem 2014, 6:732–738. https://doi.org/10.1038/nchem.2001.

53.

Stephens IEL, Bondarenko AS, Perez-Alonso FJ, Calle-Vallejo F, Bech L, Johansson TP, Jepsen AK, Frydendal R, Knudsen BP, Rossmeisl J, Chorkendorff I: Tuning the activity of Pt(111) for oxygen electroreduction by subsurface alloying. J Am Chem Soc 2011, 133:5485–5491. https://doi.org/10.1021/ja111690g.

54.

Oezaslan M, Hasche´ F, Strasser P: Oxygen electroreduction on PtCo3, PtCo and Pt3Co alloy nanoparticles for alkaline and acidic PEM fuel cells. J Electrochem Soc 2012, 159:B394–B405. https://doi.org/10.1149/2.075204jes.

55.

Jensen KD, Tymoczko J, Rossmeisl J, Bandarenka A, Chorkendorff I, Escudero-Escribano M, Stephens IEL: Elucidation of the oxygen reduction volcano in alkaline media using a copper-platinum(111) alloy. Angew Chemie Int Ed 2018, 57:2800–2805. https://doi.org/10.1002/anie.201711858. This work on Cu/Pt(111) alloys shows that the key descriptor controlling the oxygen reduction in alkaline media is, as in acid, the binding to the reaction intermediates. ••

56.

Wang K, Qin Y, Lv F, Li M, Liu Q, Lin F, Feng J: Intermetallic Pd3Pb nanoplates enhance oxygen reduction catalysis with excellent methanol tolerance. Small 2018, 1700331:1–8. https://doi.org/10.1002/smtd.201700331. This paper presents Pd3Pb nanoplates as highly active ORR catalysts in alkaline media with excellent MeOH tolerance for direct methanol fuel cells. •

57.

Calle-Vallejo F, Pohl MD, Reinisch D, Loffreda D, Sautet P, Bandarenka AS: Why conclusions from platinum model surfaces do not necessarily lead to enhanced nanoparticle catalysts for the oxygen reduction reaction. Chem Sci 2017, 8:2283–2289. https://doi.org/10.1039/C6SC04788B.

58.

Suntivich J, May KJ, Gasteiger HA, Goodenough JB, Shao-Horn Y: A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 2011, 334:1383–1385. https://doi.org/10.1126/science.1212858.

59.

Busch M, Halck NB, Kramm UI, Siahrostami S, Krtil P, Rossmeisl J: Beyond the top of the volcano? A unified approach to electrocatalytic oxygen reduction and oxygen evolution. Nano Energy 2016, 29:126–135.

60.

Kodama K, Jinnouchi R, Takahashi N, Murata H, Morimoto Y: Activities and stabilities of Au-modified stepped-Pt single-crystal electrodes as model cathode catalysts in polymer electrolyte fuel Cells. J Am Chem Soc 2016, 138:4194–4200. https://doi.org/10.1021/jacs.6b00359.

61.

Nørskov JK, Rossmeisl J, Logadottir A, Lindqvist L, Kitchin JR, Bligaard T, Jonsson H: Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J Phys Chem B 2004, 108:17886–17892.

62.

Kulkarni A, Siahrostami S, Patel A, Nørskov JK: Understanding catalytic activity trends in the oxygen reduction reaction. Chem Rev 2017. https://doi.org/10.1021/acs.chemrev.7b00488.

47. ••

48.

Velázquez-Palenzuela A, Masini F, Pedersen AF, • Escudero-Escribano M, Deiana D, Malacrida P, Hansen TW, Friebel D, Nilsson A, Stephens IEL, Chorkendorff I: The enhanced activity of mass-selected PtxGd nanoparticles for oxygen electroreduction. J Catal 2015, 328:297–307. This paper shows that strain can be maintained on PtxGd

www.sciencedirect.com

9

Current Opinion in Electrochemistry 2018, 000:1–12

Please cite this article as: Escudero-Escribano, Jensen, Jensen, Recent advances in bimetallic electrocatalysts for oxygen reduction: design principles, structure-function relations and active phase elucidation, Current Opinion in Elec-

JID: COELEC

10

ARTICLE IN PRESS

63.

Mukerjee S, Srinivasan S, Soriaga MP, McBreen J: Role of structural and electronic-properties of Pt and Pt alloys on electrocatalysis of oxygen reduction - an in-situ Xanes and Exafs investigation. J Electrochem Soc 1995, 142:1409–1422.

64.

Jia Q, Liang W, Bates MK, Mani P, Lee W, Mukerjee S: Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles : in situ observation of the linear relationship. ACS Nano 2015, 9:387–400.

65.

Cai B, Hübner R, Sasaki K, Rellinghaus B, Adzic RR, Eychmüller A: Core-shell structuring of pure metallic aerogels towards highly efficient Pt utilization for the oxygen reduction reaction. Angew Chemie Int Ed 2018. https://doi.org/10.1002/anie.201710997.

66.

Zhang JL, Vukmirovic MB, Xu Y, Mavrikakis M, Adzic RR: Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different substrates. Angew Chem Int Ed 2005, 44:2132–2135.

67.

Mavrikakis M, Hammer B, Nørskov JK: Effect of strain on the reactivity of metal surfaces. Phys Rev Lett 1998, 81:2819–2822.

68.

Asano M, Kawamura R, Sasakawa R, Todoroki N, Wadayama T: Oxygen reduction reaction activity for strain-controlled Pt-based model alloy catalysts: surface strains and direct electronic effects induced by alloying elements. ACS Catal 2016, 6:5285–5289. https://doi.org/10.1021/acscatal.6b01466.

69.

Hoster HE, Alves OB, Koper MTM: Tuning adsorption via strain and vertical ligand effects. Chemphyschem. 2010, 11:1518–1524.

70.

Heider EA, Jacob T, Kibler LA: Platinum overlayers on PtxRu1-x(111) electrodes: tailoring the ORR activity by lateral strain and ligand effects. J Electroanal Chem 2018. https://doi.org/10.1016/j.jelechem.2017.10.063.

71.

Deng YJ, Tripkovic V, Rossmeisl J, Arenz M: Oxygen reduction reaction on Pt overlayers deposited onto a gold film: ligand, strain, and ensemble effect. ACS Catal 2016, 6:671–676. https://doi.org/10.1021/acscatal.5b02409.

72.

Schilter D: Electrocatalysis: volcano spews out hot new catalyst. Nat Rev Chem 2018, 2:116. https://doi.org/10.1038/s41570- 018- 0116.

73.

Pedersen AF, Ulrikkeholm ET, Escudero-Escribano M, Johansson TP, Malacrida P, Pedersen CM, Hansen MH, Jensen KD, Rossmeisl J, Friebel D, Nilsson A, Chorkendorff I, Stephens IEL: Probing the nanoscale structure of the catalytically active overlayer on Pt alloys with rare earths. Nano Energy 2016, 29:249–260. https://doi.org/10.1016/j.nanoen.2016.05.026.

74.

Zamburlini E, Jensen KD, Stephens IEL, Chorkendorff I, Escudero-Escribano M: Benchmarking Pt and Pt-lanthanide sputtered thin films for oxygen electroreduction: fabrication and rotating disk electrode measurements. Electrochim Acta 2017, 247:708–721. https://doi.org/10.1016/j.electacta.2017.06.146.

75.

Strasser P, Gliech M, Kuehl S, Moeller T: Electrochemical processes on solid shaped nanoparticles with defined facets. Chem Soc Rev 2018. https://doi.org/10.1039/c7cs00759k.

76.

Tian XL, Xu YY, Zhang W, Wu T, Xia BY, Wang X: Unsupported platinum-based electrocatalysts for oxygen reduction reaction. ACS Energy Lett 2017, 2:2035–2043. https://doi.org/10.1021/acsenergylett.7b00593.

77.

78.

79. •

[mNS;June 16, 2018;4:6]

Surface Electrochemistry

Sasaki K, Naohara H, Choi Y, Cai Y, Chen W-F, Liu P, Adzic RR: Highly stable Pt monolayer on PdAu nanoparticle electrocatalysts for the oxygen reduction reaction. Nat Commun 2012, 3:1115. https://doi.org/10.1038/ncomms2124. Wang JX, Ma C, Choi Y, Su D, Zhu Y, Liu P, Si R, Vukmirovic MB, Zhang Y, Adzic RR: Kirkendall effect and lattice contraction in nanocatalysts : a new strategy to enhance sustainable activity. J Am Chem Soc 2011, 133:13551–13557. https://doi.org/10.1021/ja204518x. Mezzavilla S, Baldizzone C, Swertz AC, Hodnik N, Pizzutilo E, Polymeros G, Keeley GP, Knossalla J, Heggen M, Mayrhofer KJJ,

Current Opinion in Electrochemistry 2018, 000:1–12

Schüth F: Structure-activity-stability relationships for space-confined PtxNiy nanoparticles in the oxygen reduction reaction. ACS Catal 2016, 6:8058–8068. https://doi.org/10.1021/acscatal.6b02221. This paper shows the synthesis and structure-activity-stability relations for spaced-confined Pt-Ni nanoparticles with enhanced durability towards metal leaching and particle agglomeration. 80.

Nonoyama N, Okazaki S, Weber AZ, Ikogi Y, Yoshida T: Analysis of oxygen-transport diffusion resistance in proton-exchange-membrane fuel cells. J Electrochem Soc 2011, 158:B416. https://doi.org/10.1149/1.3546038.

81.

Adzic RR, Zhang J, Sasaki K, Vukmirovic MB, Shao M, Wang JX, Nilekar aU, Mavrikakis M, Valerio Ja, Uribe F: Platinum monolayer fuel cell electrocatalysts. Top Catal 2007, 46:249–262. https://doi.org/10.1007/s11244- 007- 9003- x.

82.

Strasser P: Free electrons to molecular bonds and back: closing the energetic oxygen reduction (ORR)-oxygen evolution (OER) cycle using core-shell nanoelectrocatalysts. Acc Chem Res 2016, 49:2658–2668. https://doi.org/10.1021/acs.accounts.6b00346.

83.

Strickler AL, Jackson A, Jaramillo TF: Active and stable Ir@Pt core–shell catalysts for electrochemical oxygen reduction. ACS Energy Lett 2016:244–249. https://doi.org/10.1021/acsenergylett.6b00585.

84.

Kuttiyiel KA, Choi YM, Sasaki K, Su D, Hwang SM, Yim SD, Yang TH, Park GG, Adzic RR: Tuning electrocatalytic activity of Pt monolayer shell by bimetallic Ir-M (M=Fe, Co, Ni or Cu) cores for the oxygen reduction reaction. Nano Energy 2016, 29:261–267. https://doi.org/10.1016/j.nanoen.2016.05.024.

85.

Wang X, Il Choi S, Roling LT, Luo M, Ma C, Zhang L, Chi M, Liu J, Xie Z, Herron JA, Mavrikakis M, Xia Y: Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction. Nat Commun, vol 6 2015. https://doi.org/10.1038/ncomms8594.

86.

Kuttiyiel KA, Sasaki K, Choi Y, Su D, Liu P, Adzic RR: Bimetallic IrNi core platinum monolayer shell electrocatalysts for the oxygen reduction reaction. Energy Environ Sci 2012, 5:5297. https://doi.org/10.1039/c1ee02067f.

87.

Kongkanand A, Subramanian NP, Yu Y, Liu Z, Igarashi H, Muller DA: Achieving high-power PEM fuel cell performance with an ultralow-Pt-content core-shell catalyst. ACS Catal 2016, 6:1578–1583. https://doi.org/10.1021/acscatal.5b02819.

88.

Tian X, Luo J, Nan H, Zou H, Chen R, Shu T, Li X, Li Y, Song H, Liao S, Adzic RR: Transition metal nitride coated with atomic layers of Pt as a low-cost, highly stable electrocatalyst for the oxygen reduction reaction. J Am Chem Soc 2016, 138:1575–1583. https://doi.org/10.1021/jacs.5b11364.

89.

Kuttiyiel KA, Choi YM, Hwang SM, Park GG, Yang TH, Su D, Sasaki K, Liu P, Adzic RR: Enhancement of the oxygen reduction on nitride stabilized pt-M (M=Fe, Co, and Ni) core-shell nanoparticle electrocatalysts. Nano Energy 2015, 13:442–449. https://doi.org/10.1016/j.nanoen.2015.03.007.

90.

Chattot R, Asset T, Drnec J, Bordet P, Nelayah J, Dubau L, Maillard F: Atomic-scale snapshots of the formation and growth of hollow PtNi/C nanocatalysts. Nano Lett 2017, 17:2447–2453. https://doi.org/10.1021/acs.nanolett.7b00119.

91.

Zhang L, Roling LT, Wang X, Vara M, Chi M, Liu J, Il Choi S, Park J, Herron JA, Xie Z, Mavrikakis M, Xia Y: Platinum-based nanocages with subnanometer-thick walls and well-defined, controllable facets. Science 2015, 349:412–416. https://doi.org/10.1126/science.aab0801.

92.

Zhang Y, Ma C, Zhu Y, Si R, Cai Y, Wang JX, Adzic RR: Hollow core supported Pt monolayer catalysts for oxygen reduction. Catal Today 2013, 202:50–54. https://doi.org/10.1016/j.cattod.2012.03.040.

93.

Wang X, Figueroa-Cosme L, Yang X, Luo M, Liu J, Xie Z, Xia Y: Pt-based icosahedral nanocages: using a combination of {111} facets, twin defects, and ultrathin walls to greatly enhance their activity toward oxygen reduction. Nano Lett 2016, 16:1467–1471. https://doi.org/10.1021/acs.nanolett.5b05140. www.sciencedirect.com

Please cite this article as: Escudero-Escribano, Jensen, Jensen, Recent advances in bimetallic electrocatalysts for oxygen reduction: design principles, structure-function relations and active phase elucidation, Current Opinion in Elec-

JID: COELEC

ARTICLE IN PRESS

[mNS;June 16, 2018;4:6]

Bimetallic electrocatalysts for oxygen reduction Escudero-Escribano, Jensen and Jensen

94.

Asset T, Chattot R, Fontana M, Mercier-Guyon B, Job N, Dubau L, Maillard F: A review on recent developments and prospects for oxygen reduction reaction on hollow Pt-alloy nanoparticles. ChemPhysChem 2018. https://doi.org/10.1002/cphc.201800153.

95.

Wang C, Chi M, Li D, Strmcnik D, Van Der Vliet D, Wang G, Komanicky V, Chang K, Paulikas AP, Tripkovic D, Pearson J, More KL, Markovic NM, Stamenkovic VR: Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces. J Am Chem Soc 2011, 133:14396–14403. https://doi.org/10.1021/ja2047655.

96.

97.

98.

99.

Baldizzone C, Mezzavilla S, Carvalho HWP, Meier JC, Schuppert AK, Heggen M, Galeano C, Grunwaldt J-D, Schueth F, Mayrhofer KJJ: Confined-space alloying of nanoparticles for the synthesis of efficient PtNi fuel-cell catalysts. Angew Chem Int Ed 2014, 53:14250–14254. https://doi.org/10.1002/anie.201406812. Galeano C, Meier JC, Soorholtz M, Bongard H, Baldizzone C, Mayrhofer KJJ, Schüth F: Nitrogen-doped hollow carbon spheres as a support for platinum-based electrocatalysts. ACS Catal 2014, 4:3856–3868. https://doi.org/10.1021/cs5003492. Polymeros G, Baldizzone C, Geiger S, Grote JP, Knossalla J, Mezzavilla S, Keeley GP, Cherevko S, Zeradjanin AR, Schüth F, Mayrhofer KJJ: High temperature stability study of carbon supported high surface area catalysts—expanding the boundaries of ex-situ diagnostics. Electrochim Acta 2016, 211:744–753. https://doi.org/10.1016/j.electacta.2016.06.105. Luo M, Guo S: Strain-controlled electrocatalysis on multimetallic nanomaterials. Nat Rev Mater, vol 2 2017. https://doi.org/10.1038/natrevmats.2017.59.

100. Brandiele R, Durante C, Gradzka E, Rizzi GA, Zheng J, Badocco D, Centomo P, Pastore P, Granozzi G, Gennaro A: One step forward to a scalable synthesis of platinum-yttrium alloyed nanoparticles on mesoporous carbon for oxygen reduction reaction. J Mater Chem A 2016, 4:12232–12240. https://doi.org/10.1039/C6TA04498K. 101. Roy C, Knudsen BP, Pedersen CM, Velázquez Palenzuela AA, Christensen LH, Damsgaard CD, Stephens IEL, Chorkendorff I: Scalable synthesis of carbon supported platinum - lanthanide and rare earth alloys for oxygen reduction. ACS Catal 2018. https://doi.org/10.1021/acscatal.7b03972. ´ 102. Vidal-iglesias FJ, Aran-Ais RM, Solla-Gullon´ J, Herrero E, Feliu JM: Electrochemical characterization of shape-controlled Pt nanoparticles in different supporting electrolytes. ACS Catal 2012, 2:901–910 dx.doi.org/10.1021/cs200681x. 103. Arán-Ais RM, Solla-Gullón J, Gocyla M, Heggen M, Dunin-Borkowski RE, Strasser P, Herrero E, Feliu JM: The effect of interfacial pH on the surface atomic elemental distribution and on the catalytic reactivity of shape-selected bimetallic nanoparticles towards oxygen reduction. Nano Energy 2016, 27:390–401. https://doi.org/10.1016/j.nanoen.2016.07.024. 104. Wang Y-J, Long W, Wang L, Yuan R, Ignaszak A, Fang B, Wilkinson DP: Unlocking the door to highly active ORR catalysts for PEMFC applications: polyhedron-engineered Pt-based nanocrystals. Energy Environ Sci 2018. https://doi.org/10.1039/C7EE02444D. 105. Beermann V, Gocyla M, Kühl S, Padgett E, Schmies H, Goerlin M, • Erini N, Shviro M, Heggen M, Dunin-Borkowski RE, Muller DA, Strasser P: Tuning the electrocatalytic oxygen reduction reaction activity and stability of shape-controlled Pt-Ni nanoparticles by thermal annealing -elucidating the surface atomic structural and compositional changes. J Am Chem Soc 2017, 139:16536–16547. https://doi.org/10.1021/jacs.7b06846. In this work, in situ FTIR spectroscopy shows a correlation between CO stretching frequency and activity for shape-controlled Pt-Ni nanoparticles. 106. Cao L, Mueller T: Theoretical insights into the effects of oxidation and mo-Doping on the structure and stability of Pt-Ni nanoparticles. Nano Lett 2016, 16:7748–7754. https://doi.org/10.1021/acs.nanolett.6b03867.

www.sciencedirect.com

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107. Jia Q, Zhao Z, Cao L, Li J, Ghoshal S, Davies V, Stavitski E, Attenkofer K, Liu Z, Li M, Duan X, Mukerjee S, Mueller T, Huang Y: Roles of Mo surface dopants in enhancing the ORR performance of octahedral PtNi nanoparticles. Nano Lett 2018 acs.nanolett.7b04007. https://doi.org/10.1021/acs.nanolett.7b04007. 108. Beermann V, Gocyla M, Willinger E, Rudi S, Heggen M, Dunin-Borkowski RE, Willinger MG, Strasser P: Rh-doped Pt-Ni octahedral nanoparticles: understanding the correlation between elemental distribution, oxygen reduction reaction, and shape stability. Nano Lett 2016, 16:1719–1725. https://doi.org/10.1021/acs.nanolett.5b04636. 109. Lu BA, Sheng T, Tian N, Zhang ZC, Xiao C, Cao ZM, Bin Ma H, Zhou ZY, Sun SG: Octahedral PtCu alloy nanocrystals with high performance for oxygen reduction reaction and their enhanced stability by trace Au. Nano Energy 2017, 33:65–71. https://doi.org/10.1016/j.nanoen.2017.01.003. 110. Li D, Wang C, Strmcnik D, Tripkovic´ DV, Sun X, Kang Y, Chi M, Snyder J, van der Vliet D, Tsai Y, Stamenkovic V, Sun S, Markovic N: Functional links between Pt single crystal morphology and nanoparticles with different size and shape: the oxygen reduction reaction case. Energy Environ Sci 2014, 7:4061–4069. https://doi.org/10.1039/C4EE01564A. 111. Huang H, Li K, Chen Z, Luo L, Gu Y, Zhang D, Ma C, Si R, Yang J, Peng Z, Zeng J: Achieving remarkable activity and durability toward oxygen reduction reaction based on ultrathin Rh-doped Pt nanowires. J Am Chem Soc 2017, 139:8152–8159. https://doi.org/10.1021/jacs.7b01036. 112. Liu H, Zhong P, Liu K, Han L, Zheng H, Yin Y, Gao C: Synthesis of ultrathin platinum nanoplates for enhanced oxygen reduction activity. Chem Sci 2018, 0:1–7. https://doi.org/10.1039/C7SC02997G. 113. Becknell N, Son Y, Kim D, Li D, Yu Y, Niu Z, Lei T, Sneed BT, More KL, Markovic NM, Stamenkovic VR, Yang P: Control of architecture in rhombic dodecahedral Pt-Ni nanoframe electrocatalysts. J Am Chem Soc 2017, 139:11678–11681. https://doi.org/10.1021/jacs.7b05584. 114. Luo S, Tang M, Shen PK, Ye S: Atomic-scale preparation of octopod nanoframes with high-index facets as highly active and stable catalysts. Adv Mater 2017:29. https://doi.org/10.1002/adma.201601687. 115. Luo S, Shen PK: Concave platinum-copper octopod nanoframes bounded with multiple high-index facets for efficient electrooxidation catalysis. ACS Nano 2017, 11:11946–11953. https://doi.org/10.1021/acsnano.6b04458. 116. Cherevko S, Kulyk N, Mayrhofer KJJ: Durability of platinum-based fuel cell electrocatalysts: dissolution of bulk and nanoscale platinum. Nano Energy 2016, 29:275–298. https://doi.org/10.1016/j.nanoen.2016.03.005. 117. Lopes PP, Strmcnik D, Tripkovic D, Connell JG, Stamenkovic V, •• Markovic NM: Relationships between atomic level surface structure and stability/activity of platinum surface atoms in aqueous environments. ACS Catal 2016, 6:2536–2544. https://doi.org/10.1021/acscatal.5b02920. This study combines in situ ICP-MS detection with RDE measurements on single-crystalline Pt surfaces, revealing the atomic-scale structure-stability relations. 118. Jackson C, Smith GT, Inwood DW, Leach AS, Whalley PS, Callisti M, Polcar T, Russell AE, Levecque P, Kramer D: Electronic metal-support interaction enhanced oxygen reduction activity and stability of boron carbide supported platinum. Nat Commun, vol 8 2017. https://doi.org/10.1038/ncomms15802. 119. Cai B, Henning S, Herranz J, Schmidt TJ, Eychmüller A: Nanostructuring noble metals as unsupported electrocatalysts for polymer electrolyte fuel cells. Adv Energy Mater, vol 7 2017 1700548. https://doi.org/10.1002/aenm.201700548. 120. Chung DY, Yoo JM, Sung YE: Highly durable and active Pt-based nanoscale design for fuel-cell oxygen-reduction electrocatalysts. Adv Mater 2018, 1704123:1–20. https://doi.org/10.1002/adma.201704123.

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121. Stephens IEL, Rossmeisl J, Chorkendorff I: Toward sustainable fuel cells. Science 2016, 354:1378–1379. https://doi.org/10.1126/science.aal3303. 122. Zalitis CM, Kramer D, Kucernak AR: Electrocatalytic performance of fuel cell reactions at low catalyst loading and high mass transport. Phys Chem Chem Phys 2013, 15:4329. https://doi.org/10.1039/c3cp44431g. 123. Pinaud BA, Bonakdarpour A, Daniel L, Sharman J, Wilkinson DP: Key considerations for high current fuel cell catalyst testing in an electrochemical half-cell. J Electrochem Soc, vol 164 2017. https://doi.org/10.1149/2.0891704jes. 124. Inaba M, Jensen AW, Sievers GW, Escudero-Escribano M, • Zana A, Arenz M: Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions. Energy Environ Sci 2018, 11:988–994. This work shows the design and application of a GDE that enable measuring the ORR activity of nanocatalysts, showing performance data comparable to MEAs. 125. Wu CH, Weatherup RS, Salmeron MB: Probing electrode/electrolyte interfaces in situ by X-ray spectroscopies: old methods, new tricks. Phys Chem Chem Phys 2015, 17:30229–30239. https://doi.org/10.1039/C5CP04058B. 126. Choi Y-W, Mistry H, Roldan Cuenya B: New insights into working nanostructured electrocatalysts through operando spectroscopy and microscopy. Curr Opin Electrochem 2017, 1:95–103. https://doi.org/10.1016/j.coelec.2017.01.004. 127. Nesselberger M, Arenz M: In situ FTIR spectroscopy: probing the electrochemical interface during the oxygen reduction reaction on a commercial platinum high-surface-area catalyst. ChemCatChem 2016, 8:1125–1131. https://doi.org/10.1002/cctc.201501193. 128. Russell AE, Rose A: X-ray absorption spectroscopy of low temperature fuel cell catalysts. Chem Rev 2004, 104:4613–4636. http://pubs3.acs.org/acs/journals/doilookup? in_doi=10.1021/cr020708r. 129. Casalongue HS, Kaya S, Viswanathan V, Miller DJ, Friebel D, Hansen Ha, Nørskov JK, Nilsson A, Ogasawara H: Direct observation of the oxygenated species during oxygen reduction on a platinum fuel cell cathode. Nat Commun 2013, 4:2817. https://doi.org/10.1038/ncomms3817. 130. Zhang J, Sasaki K, Sutter E, Adzic RR: Stabilization of platinum oxygen-reduction electrocatalysts using gold clusters. Science 2007, 315:220–222. https://doi.org/10.1126/science.1134569. 131. Ogasawara H, Kaya S, Nilsson A: Operando X-ray photoelectron spectroscopy studies of aqueous electrocatalytic systems. Top Catal 2016, 59:439–447. https://doi.org/10.1007/s11244- 015- 0525- 3.

133. Drnec J, Ruge M, Reikowski F, Rahn B, Carlà F, Felici R, Stettner J, Magnussen OM, Harrington DA: Initial stages of Pt(111) electrooxidation: dynamic and structural studies by surface X-ray diffraction. Electrochim Acta 2017, 224:220–227. https://doi.org/10.1016/j.electacta.2016.12.028. 134. Escudero-Escribano M, Pedersen AF, Ulrikkeholm ET, Jensen KD, Hansen MH, Rossmeisl J, Stephens IEL, Chorkendorff I: Active phase formation and stability of Gd/Pt(111) electrocatalysts for oxygen reduction: an in situ surface X-ray diffraction study. Under review in Chemistry - A European Journal 2018. 135. Mezzavilla S, Cherevko S, Baldizzone C, Pizzutilo E, Polymeros G, Mayrhofer KJJ: Experimental methodologies to understand degradation of nanostructured electrocatalysts for PEM fuel cells: advances and opportunities. ChemElectroChem 2016, 3:1524–1536. https://doi.org/10.1002/celc.201600170. 136. Dubau L, Maillard F: Unveiling the crucial role of temperature on the stability of oxygen reduction reaction electrocatalysts. Electrochem Commun 2016, 63:65–69. https://doi.org/10.1016/j.elecom.2015.12.011. 137. Mayrhofer KJJ, Hartl K, Juhart V, Arenz M: Degradation of carbon-supported Pt bimetallic nanoparticles by surface segregation. J Am Chem Soc 2009, 131:16348–16349. https://doi.org/10.1021/ja9074216. 138. Baldizzone C, Gan L, Hodnik N, Keeley GP, Kostka A, Heggen M, Strasser P, Mayrhofer KJJ: Stability of dealloyed porous Pt/Ni nanoparticles. ACS Catal 2015, 5:5000–5007. https://doi.org/10.1021/acscatal.5b01151. 139. Wan LJ, Moriyama T, Ito M, Uchida H, Watanabe M: In situ STM imaging of surface dissolution and rearrangement of a Pt-Fe alloy electrocatalyst in electrolyte solution. Chem Commun 2002:58–59. https://doi.org/10.1039/b107037a. 140. Kobayashi S, Aoki M, Wakisaka M, Kawamoto T, Shirasaka R, Suda K, Tryk DA, Inukai J, Kondo T, Uchida H: Atomically flat Pt skin and striking enrichment of Co in underlying alloy at Pt3Co(111) single crystal with unprecedented activity for the oxygen reduction reaction. ACS Omega 2018, 3:154–158. https://doi.org/10.1021/acsomega.7b01793. 141. Jacobse L, Huang Y-F, Koper MTM, Rost MJ: Correlation of •• surface site formation to nanoisland growth in the electrochemical roughening of Pt(111). Nat Mater 2018, 17:277–282. https://doi.org/10.1038/s41563- 017- 0015- z. Using EC-STM, this paper presents direct evidence of the formation and growth of nanoislands during cyling up to potential regions relevant for ORR. 142. Pfisterer JHK, Liang Y, Schneider O, Bandarenka AS: Direct • instrumental identification of catalytically active surface sites. Nature 2017, 549:74–77. This work presents the use of tunneling noise to identify catalytic active regions in EC-STM with a resolution of only 1–2nm.

132. Ruge M, Drnec J, Rahn B, Reikowski F, Harrington DA, Carlà F, Felici R, Stettner J, Magnussen OM: Structural reorganization of Pt(111) electrodes by electrochemical oxidation and reduction. J Am Chem Soc 2017, 139:4532–4539. https://doi.org/10.1021/jacs.7b01039.

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