An Integrated Single-Electrode Method Reveals the Template Roles of

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An Integrated Single-Electrode Method Reveals the Template Roles of Atomic Steps: Disturb Interfacial Water Networks and thus Affect the Reactivity of Electrocatalysts Xiao Zhao, Takao Gunji, Takuma Kaneko, Yusuke Yoshida, Shinobu Takao, Kotaro Higashi, Tomoya Uruga, Wenxiang He, Jianguo Liu, and Zhigang Zou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02049 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Journal of the American Chemical Society

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An Integrated Single-Electrode Method Reveals the Template Roles of Atomic Steps: Disturb

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Interfacial Water Networks and thus Affect the Reactivity of Electrocatalysts

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Xiao Zhao,†* Takao Gunji, † Takuma Kaneko, † Yusuke Yoshida, † Shinobu Takao, † Kotaro Higashi, † Tomoya Uruga,⊥ Wenxiang He,‡ Jianguo Liu‡* and Zhigang Zou‡

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†Innovation

Research Center for Fuel Cells, The University of Electro-Communications, Chofugaoka, Chofu, Tokyo 182-8585, Japan

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‡Jiangsu

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⊥Japan

Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, China. Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198, Japan

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KEYWORDS: oxygen reduction reactions • atomic steps • interfacial water networks • high-index

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facets/concave surface • surface defects

1 Environment ACS Paragon Plus

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: A method enabling the accurate and precise correlation between structures and properties

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is critical to the development of efficient electrocatalysts. To this end, we developed an integrated

3

single-electrode method (ISM) that intimately couples electrochemical rotating disk electrodes, in

4

situ/operando X-ray absorption fine structures and aberration-corrected transmission electron

5

microscopy on identical electrodes. This all-in-one method allows for the one-to-one, in situ/operando

6

and atomic-scale correlation between structures of electrocatalysts with their electrochemical reactivities,

7

distinct from common methods that adopt multi-samples separately for electrochemical and physical

8

characterizations. Because the atomic step is one of the most fundamentally structural elements in

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electrocatalysts, we demonstrated the feasibility of ISM by exploring the roles of atomic steps in the

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reactivity of electrocatalysts. In situ and atomic-scale evidence shows that low-coordinated atomic steps

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not only generate reactive species at low potentials and strengthen surface contraction but also act as

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templates to disturb interfacial water networks and thus affect the reactivity of electrocatalysts. This

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template role interprets the long-standing puzzle regarding why high-index facets are active for the

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oxygen reduction reaction (ORR) in acidic media. The ISM as a fundamentally new method for

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workflows should aid the study of many other electrocatalysts regarding their nature of active sites and

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operative mechanisms.


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INTRODUCTION

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The development of efficient electrocatalysts is indispensable for future energy storage and conversion

3

technologies.1 In this context, the ability to realize the accurate and precise correlation between the

4

structures of electrocatalysts and their electrochemical reactivities remains challenging. The

5

conventional

6

characterizations, i.e., the separate multi-samples method (SMM). However, electrocatalysts are made

7

of nanoparticles (NPs) that have inherently structural heterogeneity due to varying sizes, structural

8

defects, crystallographical orientations and chemical compositions. As a result, structural and property

9

heterogeneities exist between samples. Moreover, electroactive surfaces are likely to restructure in

10

reactive environments, adopting geometrical and electronic structures different from their initial or ex

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situ states.2 As such, SMM has inherent difficulties addressing the key challenges regarding structural

12

heterogeneity and in situ restructuring. Therefore, new characterization methods or techniques are

13

imperative and have been developed, such as surface science-based methods,3-5 photon-based

14

techniques6-12 and scanning probe microscopy.13 Despite great successes already made, most techniques

15

are limited to special samples and reactions. For example, Xu and coworkers successfully determined

16

the activation energy of single nanocatalysts using single-molecule fluorescence spectroscopy; the

17

approach, however, is limited to materials able to produce fluorescence signals.11 A general method

18

remains absent for practical nanoelectrocatalysts and electrochemical reactions.

methods

adopt

multi-samples

separately

for

electrochemical

and

physical

19

Atomic steps (briefly denoted as steps), as a type of linear defect, are one of the most basic structural

20

elements in electrocatalysts and thus have received intensive attention.14-21 There is a consensus in the

21

literature regarding that undercoordinated steps generate reactive hydroxyl species at low potentials to

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promote small molecule oxidation reactions (SMOR) via a bifunctional mechanism.20,

23

situations are complex for the roles of steps in the oxygen reduction reaction (ORR), which is crucial to

24

technologies such as fuel cells and rechargeable metal-air batteries.1, 17, 25-28 According to the theoretical


22-24

However,


28-29

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model proposed by Nørskov,

low-coordinated steps that bind too strongly to ORR intermediates

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(e.g., *O, *OH and *OOH where * denotes an adsorbed state) are inactive or low active (if any) for the

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ORR. Using thermal treatment to control the densities of atomic steps on Pt NPs, Yang et al. suggested

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that a small increase in surface steps on Pt NPs can remarkably enhance methanol oxidation activity but

5

not ORR activity.23, 30 In contrast, the promotional role of steps in ORR was reported in the studies of

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Feliu31-32 and Hoshi33-34 et al. on stepped Pt single crystals and the works of Sun,21 Xia,35 Huang36 and

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so on17 regarding shaped Pt-based NPs. Calle-Vallejo et al. rationalized the enhanced ORR activity of

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concave defective sites that possess steps on the basis of the concept of weighted average coordination

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numbers (CN) and the calculated adsorption energy of ORR intermediates37-38 and showed that optimal

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active sites for the ORR have CN ≈ 8.3 and *OH adsorption energies ~0.15 eV weaker than Pt(111).38

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The rationalization of these seeming contradictions and, more importantly, the clarification of

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underlying operative mechanisms are necessary and would enable devising new nanocatalysts via step

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engineering. Recent theoretical works39-44 and experimental studies on model surfaces26,

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indicated that the hydrated status of ORR intermediates is a key factor in ORR activity. However, the

15

discrepancies between model surfaces (single crystals and DFT calculations assume an atomically

16

accurate atom arrangement with infinite dimensions) and practical nanocatalysts (atomic scale

17

heterogeneity and finite size) make it highly challenging for the direct transition from the knowledge

18

acquired on model surfaces to that on practical nanocatalysts. Yet, the studies on model surfaces still

19

provide the guidelines for understanding and designing practical nanocatalysts given that the catalysis

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events occur at an atomic or molecular level. Overall, it is a murky issue whether and how interfacial

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water affects the ORR kinetic for practical Pt nanocatalysts with rich steps.

45-52

have

22

Herein, an integrated single electrode method (ISM) was first developed with the capacity to build a

23

one-to-one, in situ/operando and atomic-scale correlation between the structures of electrocatalysts and

24

their electrochemical reactivities. We demonstrated the feasibility of ISM by exploring the roles of


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atomic steps in the reactivity of electrocatalysts. Through the control of structural variables pertaining

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solely to atomic steps and the utilization of the new characterization platform ISM, we obtained in situ

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and atomic scale evidence to rationalize the long-standing puzzle regarding why high-energy stepped

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surfaces or high-index facets are active for the ORR in acidic media and the debate on whether steps

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benefit nanoscale electrocatalysts for the ORR. We discovered that low-coordinated steps not only

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generate reactive hydroxyl species at low potentials and strengthen surface contraction but also act as

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templates to disturb interfacial water networks. Whether steps significantly benefit the ORR for

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nanoscale Pt-based electrocatalysts depends on the density of steps. High-density in-plane steps that

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form continually concave surfaces or high-index facets with low-index terraces promote both the SMOR

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and ORR. In contrast, sparse edge-plane steps may only benefit the SMOR remarkably. The underlying

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reason is that steps promote the SMOR through a bifunctional mechanism that requires only a small

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amount of steps for a significant activity enhancement. In contrast, steps promote the ORR mainly by

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acting the templates to disturb interfacial water networks and thus change the hydration and adsorption

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characteristics of ORR intermediates on neighboring terrace sites. This template role requires

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considerable steps for a substantial enhancement of ORR activity.

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RESULTS AND DISCUSSIONS



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Scheme 1. Challenges (a) and workflows of SMM (b) and ISM (c) methods to build a structuresproperties relation (SPR). CE, RE, WE and RDE correspond to the counter electrode, reference electrode, working electrode and rotating disk electrode, respectively. XAFS and AEM represent X-ray adsorption fine structure and aberration-corrected transmission electron microscopy, respectively.

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Scheme 1 illustrates the challenges and the workflows of SMM and ISM methods for building an

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accurate and precise structures-properties-relationship (SPR). The first challenge originates from the

9

inherently structural heterogeneities in nanoelectrocatalysts, causing structural and property differences

10

between samples. For example, even for commercial Pt/C, nonnegligible differences between samples

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have been reported in the literature17 and observed in our measurements. Therefore, adopting multi-

12

samples separately for electrochemical and physical characterizations more or less introduces a

13

deviation in the precision of the SPR, although the general evaluation of the activity trend has been not

14

affected. The second challenge is the surface adsorption that spontaneously occurs in an electrochemical

15

environment in which supporting electrolyte anions and cations and/or reactants adsorb on specific

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surface sites, making them distinct from ex situ or vacuum states.53-56 The third challenge is the in situ

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restructuring that changes the nature of active sites significantly8-9, 49, 57-60 or even completely61-62 and

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thus affects the building of an accurate SPR. Particularly, in situ restructuring occurs frequently either


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during electrochemical pretreatment49,

57, 63

or in an operative process58,

61

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electrochemical environment.61 For example, voltammetric activation, the most commonly used

3

electrochemical pretreatment, results in the dissolution of nonnoble constituents in bimetallic alloys and

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concurrent restructuring.49, 57, 63 Even for monometallic Pt/C, their surface states can change irreversibly

5

after voltammetric pretreatments. Compared to the as-prepared state, the voltammetrically activated

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commercial Pt/C displays an increased electrochemical surface area and an enhanced intrinsic ORR

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activity indicative of a tailored electronic state on the surfaces of the electrode (Figure S1).4, 64 In line

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with electrochemical data, in situ X-ray absorption near edge structure (XANES) spectra directly

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demonstrate a tailored electronic state for a voltammetrically activated electrode (Figure S2). Finally,

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the applied electrode potentials tune the electronic states of the electrodes directly (Figure S3). Together,

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using the SMM may cause a one-to-another structure-property correlation due to challenges from

12

structural heterogeneity, surface adsorption, in situ restructuring and applied electrode potentials. These

13

challenges substantially impact our thinking of how to build an accurate and precise SPR—specifically,

14

how to ensure the identical sample and surface state between electrochemical and physical

15

characterizations.

or simply at rest in an

16

To this end, we developed an ISM that intimately couples an electrochemical rotating disk electrode

17

(RDE), an in situ/operando X-ray absorption fine structure (XAFS) and aberration-corrected

18

transmission electron microscopy (AEM) based on identical electrodes. The electrocatalyst-coated RDE

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is used as the working electrode simultaneously for electrochemical and in situ/operando XAFS

20

characterizations, ensuring the one-to-one and in situ/operando correlation between electrochemical

21

fingerprints, electronic states and local coordination environments, which is unique and particularly

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useful compared to common methods. The structural deviation caused by different samples is thereby

23

avoided. The influences of surface adsorption, in situ restructuring and applied electrode potentials on

24

structures are captured by in situ/operando XAFS. Finally, using AEM, we determined the atomically

25

resolved real-space structures that provide visual images regarding what structural details result in the



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observed electrochemical reactivity. These atomic-scale structures were measured after the operando

2

characterizations and thus correlated closely with the electrochemical reactivity of electrocatalysts,

3

although AEM was operated in an ex situ state. Together, a one-to-one, in situ/operando and atomic

4

scale correlation between the structures of electrocatalysts and their electrochemical reactivities could

5

be realized. Importantly, the ISM has no special limitation on the types of electrocatalysts and

6

electrochemical reactions, thus representing a general method. Additionally, the ISM requires only

7

microgram-level electrocatalysts, thereby saving materials and cost greatly compared to the methods

8

commonly used. Overall, the ISM, as a fundamentally new method, shows the ability to aid the study of

9

nanoelectrocatalysts regarding the nature of their active sites and operative mechanisms.

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Figure 1. Pt NPs with stepped nanofacets. (a-b) High-angle annular dark-field scanning transmission electron microscopy images with different magnifications. (c) Atomic resolution aberration-corrected transmission electron microscopy images. (d) Particle size distribution histograms. Scale bars in (a, b, c) are 20, 5 and 1 nm, respectively.

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With the unique merits of the ISM, we explored the roles of steps in the reactivity of electrocatalysts.

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The remaining difficulty was to control structural variables pertaining solely to steps for model Pt NPs

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that should have similar size and shape with but much richer steps than common Pt NPs. NPs enclosed


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by stepped nanofacets have a high surface energy and are thermodynamically metastable, resulting in a

2

challenge in their controllable synthesis. Herein, we developed an ultrafast reduction method using

3

chloroplatinic acid as a platinum precursor; oleic acid and oleylamine (1/1, v/v) as cosurfactants; 200

4

kPa CO as a reducing agent; and benzyl ether as a solvent at a reaction temperature of 503 K. Figure 1

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shows that as-prepared Pt NPs are monodisperse with an average size of 3.6±0.5 nm. Atomically

6

resolved AEM images demonstrated that rich steps and some vacancies exist on surfaces (Figure 1c).

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Herein, the maintenance of rich steps on the surfaces of Pt NPs relies on a combined thermodynamic-

8

kinetic control of nanocrystal growth. Undercoordinated steps have an upshifted d-band center and bind

9

strongly with amine and/or carboxyl groups of surfactants.65 In other words, the surface energies of

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stepped nanofacets could be tuned effectively by the adsorption of surfactants. Moreover, the reduction

11

process was ultrafast, completed within 2 min, which was enabled by the high reaction temperature and

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the high pressure of CO gas. The normal growth of low-index facets was kinetically interrupted and/or

13

terminated, thereby reserving surfactant-capped stepped nanofacets. These as-prepared Pt NPs were

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deposited onto a carbon support material Ketjen Black as a nanoscale model Pt/C catalyst with stepped

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nanofacets (denoted as S-Pt/C, Figure S4).



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Figure 2. Electrochemical fingerprints of C-Pt/C and S-Pt/C electrodes. (a) Cyclic voltammograms in N2-saturated 0.1 M HClO4 aqueous electrolyte, scan rate 50 mV/s. (b) *CO stripping oxidation voltammograms, scan rate 50 mV/s. (c) ORR polarization curves in O2-saturated 0.1 M HClO4 aqueous electrolyte, scan rate 20 mV/s. (d) Derived specific activity-based Tafel plots from (c). (e-f) Histograms for mass activity (MA, e) and specific activity (SA, f), SA and MA were calculated by the KouteckyLevich equation. All potentials are relative to RHE. The Pt loadings on the S-Pt/C and C-Pt/C electrodes were approximately 5.1 µgPt/cm2Geo. and 17.9 µgPt/cm2Geo. respectively. The possible influence of Pt loadings on the electrochemical performances is shown in Figure S5

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An electrocatalyst S-Pt/C ink was spin-coated onto RDE to prepare the S-Pt/C electrode (see

12

Experimental Section). A commercial Pt/C (TKK, TEC10E50E-HT) with an average size of 4.0-5.0 nm

13

was used as the benchmark to prepare the C-Pt/C electrode. The base cyclic voltammograms in N2-

14

saturated 0.1 M HClO4 aqueous electrolytes show four typical regions: hydrogen ad-/desorption (0.05-

15

0.35 VRHE), electric double layer (0.4-0.6 VRHE), *OH/*O ad-/desorption on terrace sites (0.7-1.0 VRHE)


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and PtO layer formation/reduction (>1.1 VRHE).66-67

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complicated issue with the changed potentials.4, 68-70 The previous studies suggested that several species

3

are involved in the platinum oxidation such as chemisorbed hydroxide and oxygen, initial Pt oxide

4

structures and even sub-surface oxygen, which interconvert and interact.68-70 For example, Pt(111) oxide

5

starts by water dissociation to *OH that is furtherly oxidized to *O at higher potentials. Those *O

6

species form a stable adlayer as a phase transition through a nucleation and growth mechanism and in

7

parallel transform to an initial Pt oxide structure.68 Herein, we made a simplified discussion mainly

8

assuming two types of surface sites (terraces and undercoordinated steps) according to coordination

9

numbers and considering *OH/*O ad-/desorption. The S-Pt/C electrode displays a positive shift in

10

potentials for *OH/*O ad-/desorption on terraces compared to the C-Pt/C electrode. The

11

electrochemical fingerprints from the stripping oxidation of the *CO adlayer reveal that compared to a

12

single peak of *CO oxidation on the C-Pt/C electrode at 0.876 VRHE, the S-Pt/C electrode shows a

13

prepeak at 0.774 VRHE along with a main peak at 0.861 VRHE (Figure 2b). The kinetics of *CO oxidation

14

can be promoted by a bifunctional mechanism in which *CO reacts with adjacent *OH/*O species that

15

are generated on oxophilic sites at low overpotentials.71-72 The small prepeak is therefore ascribed to the

16

most reactive configuration made of *OH/*O-steps and the nearest neighboring *CO. As *CO diffuses

17

fast on stepped surfaces under electrochemical conditions, the rate of main oxidation of *CO is

18

determined by the rate of *OH/*O formation.72 Thus, although the onset potential for *OH/*O

19

formation on terrace sites of the S-Pt/C electrode is more positive than that of the C-Pt/C electrode

20

(Figure 2a), the S-Pt/C electrode still displayed a faster rate or a more negative peak potential for the

21

main oxidation of *CO than did the C-Pt/C electrode (Figure 2b). Similarly, the oxidative kinetics of

22

methanol on the S-Pt/C electrode was also enhanced (Figure S6). Together, the introduction of

23

unsaturated steps that generated reactive hydroxyl species at low potentials promotes SMOR via a

24

bifunctional mechanism.72-73 The ORR electroactivities of the two electrodes were evaluated by the

25

anodic polarization in O2-saturated 0.1 M HClO4 aqueous electrolytes (Figure 2c). Smaller Tafel slopes

26

were observed on the S-Pt/C electrode at approximately 59 mV dec−1 than on the C-Pt/C electrode (71

Note that the oxidation/reduction of Pt is a



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mV dec−1, Figure 2d). The ORR performances calculated by the Koutecky-Levich equation show that

2

the S-Pt/C electrode possesses enhancement factors of mass activity normalized by the mass of Pt (MA,

3

Figure 2e) and specific activity normalized by the CO stripping charge based electrochemical surface

4

areas

5

electrochemical results suggested that a nanoscale electrocatalyst with rich steps can realize an

6

enhanced ORR activity.

(SA, Figure 2f) of 5.4 and 3.3, respectively, compared to the C-Pt/C electrode. These

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Figure 3. One-to-one and in situ correlation between electrochemical fingerprints and XANES results highlighting technologically important potential regions of 0.7-0.9 VRHE (green shade regions). (a) Base anodic polarization parts taken from Figure 2a. (b) Potential-dependent normalized Pt L3-white line peak intensities (µNorm.). (c) *CO stripping oxidation curves taken from Figure 2b. (d) ORR polarization curves shown in Figure 2c. (e) Potential-dependent Δ µNorm.-XANES plots. (f) Operando XANES data collected at 0.9 V. All potentials are relative to RHE. The Pt loadings on the SPt/C and C-Pt/C electrodes were approximately 5.1 µgPt/cm2Geo. and 17.9 µgPt/cm2Geo. respectively.

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We used the same electrodes employed in electrochemical measurements and adopted the operando

17

mode for XAFS measurements. The electrodes were examined before and after operando XAFS by 12 Environment ACS Paragon Plus

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1

cyclic voltammetry to ensure no noticeable changes in electrochemical surface states. Figure 3a-c shows

2

the correlation between the base anodic polarization curves (Figure 3a), the potential-dependent

3

normalized Pt L3 edge white line peak intensities (µNorm, Figure 3b) and the *CO stripping oxidation

4

curves (Figure 3c). The µNorm values for both electrodes increase with an increase in the applied

5

potentials from 0.4 to 0.9 VRHE due to the chemisorbed oxygenated species.30,

6

double layers region, there is no observable interfacial charge transfer. Interestingly, at 0.4 VRHE, the S-

7

Pt/C electrode displays a higher µNorm. than does the C-Pt/C electrode, suggesting the chemisorption of

8

*OH/*O species on some steps may occur in hydrogen regions.22, 46 At 0.7 VRHE, *OH/*O adsorption on

9

terrace sites begins, and the S-Pt/C electrode still has a higher µNorm. than does the C-Pt/C electrode. At

10

0.4-0.7 VRHE, a higher µNorm. on the S-Pt/C electrode structurally corresponds to its higher fraction of

11

low-coordinated steps and accounts for its improved *CO and methanol oxidation kinetics compared

12

with those on the C-Pt/C electrode, given that a bifunctional mechanism operates through the

13

chemisorption of *OH/*O species on steps of the S-Pt/C electrode. On the other hand, the

14

chemisorption of *OH/*O species on steps at low potentials suggests that it is difficult for steps to

15

directly contribute to ORR activity at potentials higher than 0.7 VRHE. At 0.9 VRHE, the rate of coverages

16

of oxygenated species on both electrodes approach their platform values (Figure 3a); however, their

17

adsorption characteristics are distinct and affect ORR kinetics as explained below.

74-75

Over the electric

18

As ΔµNorm. (Δμ = μ(E)-μ(0.4 VRHE)) is surface-sensitive and reflects the changes of coverages and the

19

natures of adsorbates on electrodes,53, 76 a correlation analysis was conducted between ORR polarization

20

curves and Δ µNorm. to examine the potential-dependent ORR kinetics. In a diffusion-controlled process,

21

the geometric area is the relevant parameter for uniform flat electrodes due to the fusion of the different

22

local diffusional contributions from neighbor nanoparticles. Meanwhile, for the ORR characterization of

23

nanocatalysts by using the technique of catalyst thin film RDE, the quality of catalyst thin-film, for

24

example the film-uniformity, affect the measured ORR current remarkably.77-78 At 0.4-0.6 VRHE, the

25

ORR current on Pt-based electrodes is governed by a mass diffusion-controlled process and the similar

26

diffusion-limiting currents on both electrodes suggest their similar quality of catalyst thin films. With 13 Environment ACS Paragon Plus


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the potential raised to 0.7 VRHE, ORR enters a mixed kinetic-diffusion control region where *OH/*O

2

species form on terraces and block them, i.e., the site-blocking effect. At this stage, the ORR current is

3

controlled by mass transport performance and the quantities of and the intrinsic activities of free active

4

sites. In the potential region of 0.4-0.7 VRHE, a slightly higher ΔµNorm. (0.7_0.4 V) on the S-Pt/C

5

electrode was observed than on the C-Pt/C electrode. Overall, at 0.7 VRHE, the slightly larger ORR

6

current on the S-Pt/C electrode than on the C-Pt/C electrode can be ascribed to an enhanced intrinsic

7

activity on the S-Pt/C electrode (Figure 3d). Interestingly, over the potential regime of 0.7-0.9 VRHE,

8

which is the voltage region for working fuel cells, the S-Pt/C electrode conversely presents an almost

9

negligible ΔµNorm., while the C-Pt/C electrode exhibits a clearly increased ΔµNorm. (Figure 3e).

10

Correspondingly, the electrochemical data show an enlarged difference in the ORR current (Figure 3d)

11

and the retarded formation of *OH/*O species (Figure 3a) on the S-Pt/C electrode compared to the C-

12

Pt/C electrode. At 0.9 VRHE, Pt-L3 edge XANES spectra show comparable µNorm. values and shapes for

13

the two electrodes (Figure 3f and Figure S7 ). However, combining electrochemical fingerprints and

14

potential-dependent XANES data, the contribution of chemisorbed *OH/*O to µNorm. on the S-Pt/C

15

electrode occurred predominantly at step sites and at potentials smaller than 0.7 VRHE, while that on the

16

C-Pt/C electrode mainly occurred at terrace sites and in the potential region of 0.7-0.9 VRHE. Overall,

17

operando XANES demonstrated that (i) over the potential region of 0.4-0.7 VRHE, the S-Pt/C electrode

18

possesses a higher Pt-L3 edge µNorm. than does the C-Pt/C electrode; however, (ii) over the potential

19

region of 0.7-0.9 VRHE, the S-Pt/C electrode conversely exhibits a negligible change in Pt-L3 edge µNorm.

20

in contrast to a significant increase for the C-Pt/C electrode. These in situ XANES behaviors were not

21

surprising and were observed for Pt alloy NPs in the study of Mukerjee et al.74 Further analysis was

22

combined with in situ Fourier transformed extended X-ray absorption fine structure (FT-EXAFS).


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1 2 3 4 5 6

Figure 4. (a, b) Operando Fourier-transformed k2-weighted EXAFS spectra. The green shaded region highlights the Pt-O scattering shell. (c) First shell EXFAS fitting in R space for spectra data at 0.9 V; reasonable fitting models for C-Pt/C require Pt-Pt and Pt-O paths, while for S-Pt/C spectra, the models require only a Pt-Pt path. (d) Δµ-XANES spectra (0.9 V_0.7 V). (e, f) Favorable interfacial water configurations on flat (111) surface (e) and stepped (553) surface79. All potentials are relative to RHE.

7 8

The FT-EXAFS spectra at the Pt-L3-edge emphasized a gradually definite Pt-O scattering shell on

9

the C-Pt/C electrode with the applied potentials raised from 0.4 to 0.9 VRHE (Figure 4a), while such a

10

change was negligible on the S-Pt/C electrode (Figure 4b). The EXAFS fits in R space (Figure 4c,

11

Figure S8, S9 and Table S1) suggest that the inclusion of Pt-O path for C-Pt/C is a more reasonable than

12

without it (Figure 4c and Table S1-S2), while the reasonable fit for S-Pt/C electrode requires the

13

exclusion of a Pt-O path (Figure 4c and Table S1, S3). Consistent with FT-EXAFS, Pt-L3 edge

14

ΔµNorm(0.9_0.7 V)-XANES spectra for the S-Pt/C electrode show a negligible change, while for the C-

15

Pt/C electrode, the spectra exhibit a clearly increased ΔµNorm. (Figure 4d). Thus, an interesting question 15 Environment ACS Paragon Plus


Page 16 of 30

1

is why those *OH/*O species that adsorbed on terraces during 0.7-0.9 VRHE are not visible for the S-

2

Pt/C electrode but are clearly detectable for the C-Pt/C electrode in their EXAFS (Figure 4a-c) and

3

ΔµNorm-XANES spectra (Figure 4d), although electrochemical signals for both electrodes clearly

4

respond (Figure 3a). We assumed that steps as the templates disrupt the normal water structure and thus

5

change the hydrated and adsorbed characteristics of *OH/*O species on terraces. The framework for

6

interpreting these differences is as follows. At electrode/electrolyte interfaces, adsorbates are hydrated

7

and embedded in water networks.40, 80 On the flat (111) surface, interfacial water molecules bind to each

8

other through hydrogen bonds to create a nearly flat hexagonal water adlayer, i.e., an ice-like structure

9

(Figure 4e).81 However, on stepped surfaces, water molecules preferably interact with *OH/*O species

10

generated on steps at low potentials because their hydrogen bonds are much stronger than that between

11

two water molecules (0.4 vs. 0.2 eV, respectively).82 Experimentally, Koper and coworkers observed

12

water on a Pt(553) surface adopting the structure of double-stranded lines forming water tetragons with

13

dissimilar hydrogen bonds within and between the lines.79 As a result, steps and neighboring terraces

14

constitute deformed geometric templates relative to the flat template for the water adlayer (Figure 4f).

15

The deformed water adlayer changes the hydrated and adsorbed characteristics of oxygenated species on

16

terrace sites. Specifically, the adsorbed oxygenated species are relatively disordered, delocalized and

17

loose on the terraces of the S-Pt/C electrode, unlike the ordered, localized and strong adsorption on the

18

terraces of the C-Pt/C electrode, which consistently interprets the positive shift in potentials for the ad-

19

/desorption of *OH/*O species on terrace sites, the enhanced ORR kinetics, the absence of a definite Pt-

20

O scattering path in operando EXAFS at 0.9 VRHE and the negligible change in operando ΔµNorm.-

21

XANES (0.9_0.7 VRHE) for the S-Pt/C electrode compared with the C-Pt/C electrode. Literature

22

reported such experimental phenomena in which the adsorption of ORR intermediates and thus the ORR

23

reactivity on extended Pt surface were affected by other noncovalent adsorbates and vice versa.48, 52, 55, 76,

24

83

25

(molar ratio 9/1) enhances ORR activity of Pt terraces on n(111)–(111) surfaces with the terraces n

26

greater than 7, while deactivates the ORR on n(111)–(111) surfaces with n less than 6.48 Ramaker et al.

For example, Saikawa showed that the adsorbed octylamine/an alkyl amine containing a pyrene ring


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1

observed that only at potentials above 0.6 VRHE—when other ions, such as *OH, are on the surface—is

2

the specifically adsorbed bisulfate on the Pt electrode visible in the EXAFS spectra.76 On the basis of

3

those literature and current experimental observations, we thus suggested that the operative mechanism

4

regarding the disturbance of surface defects or oxophilic sites (here atomic steps) or other non-covalent

5

adsorbates to the normal water structures and thus the change in the hydrated and adsorbed status of

6

reaction intermediates may work for other electrochemical reactions, especially for the those operating

7

in high-potential regions, such as the oxygen evolution reaction84 and the reactions occurring in alkaline

8

media67 as well as for other electrocatalytic materials.85 For example, Jia et al. observed a similar

9

ΔµNorm-XANES behavior for MoOx-modified octahedral Pt3Ni nanoparticles in which MoOx induced

10

the formation of *OH/*O at 0.7 V and subsequently suppressed the further formation of *OH/*O at 0.9

11

V on Pt surfaces. This report indicated that the metal oxides on Pt surface may also disturb interfacial

12

water and furtherly affect the reactivity of surface Pt atoms although the authors proposed a changed

13

coordination environment of surface Pt atoms to account for this unusual potential-dependent ΔµNorm-

14

XANES phenomenon.85 In light of the complexity in atomic scale structures and associated surface

15

electrochemistry of practical nanocatalysts as well as the unknown factors regarding the detailed

16

structures of interfacial water in electric double layers and their interactions with other adsorbates,

17

current understanding does not necessarily exclude other possible interpretations.

18

Operando EXAFS spectra also revealed a slightly shortened Pt−Pt bond in the S-Pt/C electrode (2.75

19

Å at 0.4 VRHE) compared to that in the C-Pt/C electrode (2.76 Å, at 0.4 VRHE). The metal bond length is

20

known to decrease with the coordination number86-89 either to stabilize the shared-electron-pair bonds87

21

or to smoothen the surface electron density.88-89 Thus, metal bonds relax inwardly more significantly for

22

stepped/curved surfaces than for closely/densely packed surfaces. A higher fraction of step and kink

23

atoms in S-Pt/C accounts for its shortened Pt-Pt bond relative to C-Pt/C, as further explored by the

24

atomic-scale structural analysis of individual Pt NPs.



Page 18 of 30

1 2 3 4 5 6 7

Figure 5. Atomically resolved real space structural details for representative NPs. a) Pt NP from the S-Pt/C electrode; the atomic columns marked by red dotted cycles are used as the benchmarks to estimate atomic dislocation; the atoms marked by green dotted cycles suggest the outermost atoms. b) Pt NP from the C-Pt/C electrode, and the atoms marked by green dotted cycles suggest the plane-edge steps. Scale bars in (a-b) are 1 nm. c) The integrated intensity profiles along three outermost atomic layers; see the indicative arrows and rectangles in images a-b.

8 9

After operando XAFS determinations, electrocatalysts on electrodes were transferred onto grids

10

made of lacey carbon films on copper for atomic-scale structural analysis using AEM. As shown in

11

Figure 5, atomic steps in S-Pt/C were preserved after electrochemical and XAFS measurements.

12

Relative to the regular arrangement, the surfaces viewed at the atomic scale were considerably distorted

13

and disordered for the Pt NPs in S-Pt/C due to the existence of atomic steps and surface dislocations

14

(see white dashed lines, Figure 4a). The structurally disordered surfaces support the formation of

15

deformed interfacial water networks that destabilize ORR intermediates. Notably, scatted steps located

16

on the edge plane were also observed for C-Pt/C, in line with a previous report;30 however, the C-Pt/C

17

electrode showed a definite Pt-O scattering shell in operando EXAFS at 0.9 VRHE. The combination of

18

operando XAFS and AEM data suggests that the substantial enhancement of ORR activity on nanoscale

19

catalysts may require the involvement of a group of interval atomic steps. This finding agrees with the

20

trend observed on extended Pt surfaces that display an increased ORR activity with an increase in step 18 Environment ACS Paragon Plus

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1

density and reach a maximum at short terrace lengths of 3-5 atoms.16, 34 Additionally, the ORR activity

2

determined by the RDE technique is an average value contributed by all surface sites. The realization of

3

a noticeable enhancement in ORR activity on nanocatalysts also require a high density of in-plane steps,

4

such as continually high-index facets or concave surfaces. Overall, steps are inactive or low active (if

5

any) for ORR but could act as templates to disturb interfacial water structures and thus destabilize ORR

6

intermediates on neighboring terraces, through which the ORR activity on neighboring terraces is

7

substantially enhanced. In addition, the improved the step density on surfaces of S-Pt/C results in more

8

inward surface contraction compared to C-Pt/C (Figure 4c), in line with theoretical predictions87-89 and

9

experimental reports using other techniques.86 The atomic dislocation and surface contraction can

10

modify the surface charge distribution and may also result in an enhanced intrinsic ORR activity.90-91

11

However, these differences are relatively small between the two electrodes based on the operando

12

EXAFS results.

13

CONCLUSIONS

14

In summary, we developed an integrated single electrode method (ISM) with the capacity for the

15

one-to-one, in situ/operando and atomic-scale correlation between the structures of electrocatalysts and

16

their electrochemical reactivities. No special limitation exists for the ISM, which represents a general

17

method for practical electrocatalysts and electrochemical reactions. The ISM requires only microgram-

18

level electrocatalysts, thereby greatly saving materials and cost. Overall, the ISM is a fundamentally

19

new workflow for the study of electrocatalysts. Utilizing the ISM, we discovered that low-coordinated

20

atomic steps not only generate reactive *OH/*O species at low potentials and strengthen surface

21

contractions but also act as templates to affect interfacial water structures. Whether steps significantly

22

promote the ORR for nanoscale electrocatalysts depends on the step density. With low-index terraces, a

23

group of in-plane steps that form continually concave surfaces or high-index facets promotes both the

24

SMOR and ORR. In contrast, sparse edge-plane steps may only benefit the SMOR remarkably. The

25

underlying reasons are that steps promote the SMOR through a bifunctional mechanism while 19 Environment ACS Paragon Plus


Page 20 of 30

1

enhancing the ORR mainly by disturbing interfacial water networks and thus changing the hydrated and

2

adsorbed characteristics of ORR intermediates on neighboring terraces. This template role requires

3

considerable steps for a substantial enhancement in ORR activity. The operative mechanism regarding

4

surface defects or oxophilic sites (herein steps) as templates to disturb interfacial water networks and

5

thus affect the electrochemical reactivity of electrocatalysts is suggested to function for other

6

electrocatalytic reactions, especially for those operating over high potential regions, for example, the

7

oxygen evolution reaction, and/or reactions occurring in alkaline media. The possibility suggests that

8

distinct adsorbates affect each other through a noncontact interplay using interfacial water as the

9

medium. Our works highlight the in situ, dynamical and site-specific consideration of electrocatalytic

10

behavior.

11

EXPERIMENTAL SECTION

12

Chemicals. Chloroplatinic acid hexahydrate, oleic acid, oleyl amine and benzyl ether were all

13

purchased from Sigma-Aldrich (research-grade). Perchloric acid (TraceSELECT®) and high-purity CO

14

gas (> 99.95 vol.%, Taiyo Nippon Sanso) were used without further purification. Millipore water (18

15

MΩ·cm) purified in a Millipore system was used in all experiments.

16

Preparation of Pt nanoparticles with Stepped Nanofacets. The synthesis of Pt NPs with stepped

17

nanofacets was based on a newly developed ultrafast reduction method. In a typical synthesis, 27 mg

18

chloroplatinic acid hexahydrate was dissolved in 0.5 mL H2O and mixed with a solution consisting of 1

19

mL oleyl amine, 1 mL oleic acid and 6 mL benzyl ether in a 100 mL pressure vessel. This precursor

20

mixture was dispersed by a combination of ultrasonication and stirring mixing. This reaction vessel was

21

then heated to 333 K and under a stirring rate of 1,000 revolutions per minute (rpm) and vacuum

22

conditions to remove internal water and air for 40 min. After cooling from 60 °C to room temperature,

23

the reaction vessel was charged with 200 kPa CO gas. Chloroplatinic acid was reduced by transferring

24

the abovementioned reaction vessel into an oil bath that was preheated at 503 K. Once the vessel was

25

dipped into the oil bath, the reaction solution immediately turned black. Within 2 min, the vessel was 20 Environment ACS Paragon Plus

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1

removed from the oil bath to end the reaction. To collect Pt NPs, 10 mL ethanol was added to the

2

product mixtures and then centrifuged at 10,000 rpm for 15 min. After decanting the supernatant, the

3

precipitated Pt NPs were redispersed in a mixed solution composed of 15 mL hexane and 5 mL ethanol

4

via a brief ultrasonication and shaking. Centrifugation and dispersion were repeated three times, and

5

finally, Pt NPs were dispersed in 10 mL hexane.

6

Preparation of S-Pt/C Catalyst

7

The abovementioned solution of Pt NPs was poured into 20 ml n-butylamine containing 90 mg

8

predispersed Ketjen black carbon and then dispersed thoroughly via ultrasonication. The black mixture

9

was stirred at room temperature for 24 h to allow for ligand exchange between oleyl amine/oleic acid

10

and n-butylamine. The product was then collected by centrifugation and washed with ethanol three

11

times. The S-Pt/C sample was dried in a vacuum oven at 353 K for 24 h. To ensure complete removal of

12

oleyl amine, oleic acid or n-butylamine ligand residuals on surfaces of Pt NPs, the S-Pt/C catalyst was

13

heated to 200 °C in air for 1 h in a muffle furnace. The bulk Pt mass percentage was estimated by X-ray

14

fluorescence (XRF) to be approximately 10.0 wt.%.

15

TEC10E50E-HT, which has an average particle size of 4-5 nm, was obtained from Tanaka Kikinzoku

16

Kogyo (TKK). TEC10E50E-HT was used as the reference catalyst and to prepare the C-Pt/C electrode.

17

Characterization Procedure for Integrated Single-Electrode Method (ISM)

18

Electrochemical Measurements Based on Rotating Disk Electrode (RDE).

19

A 0.1 M HClO4 solution was prepared using perchloric acid and 18.2 MΩ·cm Millipore water. The

20

electrocatalyst ink formulation was composed of 1 mg electrocatalyst/0.5 mL Millipore water/0.4 mL

21

isopropanol/0.005 ml 5 wt% Nafion®. The testing temperature was room temperature. Pt foil and an

22

RHE were used as counter and reference electrodes, respectively. Electrocatalyst-coated RDEs were

23

prepared by a spin-coating method on a 5 mm RDE. The Pt loadings on the S-Pt/C and C-Pt/C

24

electrodes were approximately 5.1 µgPt/cm2Geo.(1.0 µg Pt ) and 17.9 µgPt/cm2Geo. (3.5 µg Pt), 21 Environment ACS Paragon Plus


Page 22 of 30

1

respectively.

2

The sequence of electrochemical measurements was as follows: (1) The working electrodes were

3

voltammetrically pretreated by potential cycles (typically 50 cycles) between 0.02 and 1.2 VRHE at a

4

scan rate of 100 mV s-1 in N2-saturated 0.1 M HClO4. (2) Base cyclic voltammetry was performed at a

5

scan rate of 50 mV s-1 in N2-saturated 0.1 M HClO4 for three cycles. (3) The electrocatalytic

6

performances for ORR were estimated by linear sweep voltammetry (LSV) from 0.0 to 1.05 V at a scan

7

rate of 20 mV s-1 in O2-saturated 0.1 M HClO4 at 1600 rpm. (4) The solution resistance Rsol was

8

measured by an i-interrupter method and used for IR compensation. (5) The CO stripping test: First, the

9

CO preadsorption on surfaces of Pt/C electrodes was conducted through a chronoamperometry method

10

with an applied potential at 0.4 VRHE in CO-saturated 0.1 M HClO4 for 5 min, followed by removal of

11

residual CO in 0.1 M HClO4 by bubbling N2 for 15 min. Second, the working electrode with

12

preadsorbed CO was potentially cycled two times from 0.025 to 1.2 VRHE at a scan rate of 50 mV s−1,

13

where the second cycle was used to verify whether preadsorbed CO was removed completely. The CO

14

stripping Coulombic charges were used for the calculation of electrochemical surface areas (ECSACO)

15

(6). The electroactivity of electrodes for the methanol oxidation reaction was evaluated by a cyclic

16

voltammetry test at 50 mVs-1 in the electrolyte solution composed of 1 M CH3OH and 0.1 M HClO4.

17

The residual methanol on the electrodes was washed with ultrapure water. Subsequently, the electrodes

18

were tested by cyclic voltammetry in N2-saturated 0.1 M HClO4 electrolyte to ensure the complete

19

removal of methanol.

20

In situ/Operando X-ray Absorption Fine Structure (XAFS) Based on the Identical RDE.

21

The same electrodes used for electrochemical measurements were continually used to collect in

22

situ/operando XAFS spectra. Before the in situ XAFS measurement, we conducted a cyclic voltammetry

23

test and compared the cyclic voltammograms with the previous voltammograms in the electrochemical

24

part to ensure that there was no noticeable change in the surface states of the electrodes. At each set

25

potential (e.g., 0.4, 0.7, and 0.9 VRHE), the electrodes were first polarized for 5 min to make the

26

electrodes enter a steady state, and the in situ X-ray adsorption signals were then collected. Potential22 Environment ACS Paragon Plus

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1

dependent in situ XAFS spectra at the Pt L3-edge were acquired in a homemade electrochemical cell

2

(Figure S10) in fluorescence mode using a Si(111) double-crystal monochromator and an ion chamber

3

(I0: Ar 5% / N2 95%) for incident X-rays and a 21 Ge-element detector for fluorescent X-rays at the

4

BL36XU station in SPring-8.49,

5

normalized by Athena software.93 The normalization range for the analysis of XANES and ΔµNorm-

6

XANES is 20-100 eV relative to E0. The XAFS spectra were treated with the data analysis program

7

IFEFFIT (version 1.2.11c).94 Theoretical phase shifts and amplitude functions were calculated from

8

FEFF 8.4.95 The extracted EXAFS oscillations were k2-weighted and Fourier-transformed to R space,

9

and the curve fittings of k2-weighted EXAFS data in R space were carried out with Artemis. After in

10

situ XAFS measurement, we conducted a cyclic voltommetry test again to check whether there were

11

noticeable changes in the surface states of the electrocatalysts or the detachment of the electrocatalysts

12

from the electrodes.

13

Aberration-Corrected Transmission Electron Microscopy (ATEM) based on the Electrocatalysts

14

on the Identical RDE

15

The electrocatalysts on the same RDE were transferred to grids made of lacey carbon films on copper

16

for atomic-scale structural analysis using AEM. ATEM images were obtained using JEM-ARM002F at

17

200 kV. Partial TEM images were obtained using a JEM-2100F.

18

XRF. X-ray fluorescence (XRF) analysis for bulk composition was conducted with a Rigaku ZSX

19

Primus2.

20

Supporting Information. TEM, in situ XAFS and electrochemical results are available online.

21

Corresponding Author. [email protected]; [email protected]

22

ACKNOWLEDGMENT

23

This work was supported by the New Energy and Industrial Technology Development Organization

24

(NEDO), Ministry of Economy, Trade, and Industry (METI), Japan; XAFS measurements were

92

X-ray absorption near-edge structure (XANES) spectra were



Page 24 of 30

1

performed with SPring-8 subject numbers 2016A7800, 2016B7800, 2016B7806, 2017A7800,

2

2017A7803, 2017A7806, 2017B7800, 2017B7806, 2018A7800 and 2018B7800. ATEM measurements

3

were conducted under the support of the NIMS microstructural characterization platform through the

4

program "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and

5

Technology (MEXT), Japan. The authors from Nanjing University gratefully acknowledge financial

6

support from the National Key R&D Plan of China (2016YFB0101308), the National Natural Science

7

Foundation of China (21676135), the 333 High-Level Talent Project of Jiangsu (BRA2018007), and the

8

Graduate Innovation Foundation of Nanjing University (2017ZDL05). We thank the support of Prof.

9

Yasuhiro Iwasawa for this work.

10

REFERENCES

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(1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355, eaad4998. (2) Tao, F.; Salmeron, M. In Situ Studies of Chemistry and Structure of Materials in Reactive Environments. Science 2011, 331, 171-174. (3) Pfisterer, J. H. K.; Liang, Y.; Schneider, O.; Bandarenka, A. S. Direct instrumental identification of catalytically active surface sites. Nature 2017, 549, 74-77. (4) Jacobse, L.; Huang, Y.-F.; Koper, M. T. M.; Rost, M. J. Correlation of surface site formation to nanoisland growth in the electrochemical roughening of Pt(111). Nature Mater. 2018, 17, 277-282. (5) Faisal, F.; Stumm, C.; Bertram, M.; Waidhas, F.; Lykhach, Y.; Cherevko, S.; Xiang, F.; Ammon, M.; Vorokhta, M.; Šmíd, B.; Skála, T.; Tsud, N.; Neitzel, A.; Beranová, K.; Prince, K. C.; Geiger, S.; Kasian, O.; Wähler, T.; Schuster, R.; Schneider, M. A.; Matolín, V.; Mayrhofer, K. J. J.; Brummel, O.; Libuda, J. Electrifying model catalysts for understanding electrocatalytic reactions in liquid electrolytes. Nature Mater. 2018, 17, 592–598. (6) Chen, T.; Dong, B.; Chen, K. C.; Zhao, F.; Cheng, X. D.; Ma, C. B.; Lee, S.; Zhang, P.; Kang, S. H.; Ha, J. W.; Xu, W. L.; Fang, N. Optical Super-Resolution Imaging of Surface Reactions. Chem. Rev. 2017, 117, 7510-7537. (7) Su, H.-S.; Zhang, X.-G.; Sun, J.-J.; Jin, X.; Wu, D.-Y.; Lian, X.-B.; Zhong, J.-H.; Ren, B. RealSpace Observation of Atomic Site-Specific Electronic Properties of a Pt Nanoisland/Au(111) Bimetallic Surface by Tip-Enhanced Raman Spectroscopy. Angew. Chem. Int. Ed. 2018, 57, 13177-13181. (8) Yang, Y.; Wang, Y.; Xiong, Y.; Huang, X.; Shen, L.; Huang, R.; Wang, H.; Pastore, J. P.; Yu, S.-H.; Xiao, L.; Brock, J. D.; Zhuang, L.; Abruña, H. D. In Situ X-ray Absorption Spectroscopy of a Synergistic Co-Mn Oxide Catalyst for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2019, 141 1463–1466. (9) Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525-8534. 24 Environment ACS Paragon Plus

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