Rapid Adsorption Enables Interface Engineering of PdMnCo Alloy

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Rapid Adsorption Enables Interface Engineering of PdMnCo Alloy/Nitrogen-Doped Carbon as Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction Ruirui Zhang, Zhongti Sun, Ruilu Feng, Zhiyu Lin, Haizhen Liu, Mengsi Li, Yang Yang, Ruohong Shi, Wenhua Zhang, and Qianwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10016 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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ACS Applied Materials & Interfaces

Rapid Adsorption Enables Interface Engineering of PdMnCo Alloy/ NitrogenDoped Carbon as Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction Ruirui Zhang, [†]a Zhongti Sun, [†]a Ruilu Feng,a Zhiyu Lin, a Haizhen Liu, a Mengsi Li,a Yang Yang, a Ruohong Shi, a Wenhua Zhang a and Qianwang Chen * ab

a

Hefei National Laboratory for Physical Sciences at Microscale, Department of Materials Science & Engineering, Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China b

The Anhui Key Laboratory of Condensed Mater Physics at Extreme Conditions, High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China E-mail: [email protected] [†]

These authors contributed equally to this work.

Keywords: hydrogen evolution reaction, PdMnCo alloy, interfacial effect, Pyridinic nitrogen-doped carbon, hybrid structures

Abstract: The catalytic performance of Pd-based catalysts has long been hindered by surface

28

contamination, particle agglomeration and lack of rational structural design. Here we report a simple

29

adsorption method for rapid synthesis (about 90 seconds) of structure optimized Pd alloy supported

30

on nitrogen doped carbon without the use of surfactants or extra reducing agents. The material shows

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much lower overpotential than 30 wt% Pd/C and 40 wt% Pt/C catalysts while exhibiting excellent

32

durability (80 hours). Moreover, unveiled by the density-functional theory calculation results, the

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underlying reason for the outstanding performance is that the PdMnCo alloy/pyridinic nitrogen

34

doped carbon interfaces weaken the hydrogen adsorption energy on the catalyst and thus optimize

35

the Gibbs free-energy of the intermediate state (∆GH*), leading to a remarkable electrocatalytic

36

activity. This work also opens up an avenue for quick synthesis of high efficient structural optimized

37

Pd-based catalyst.

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Introduction

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Pd-based nanomaterials are well-known for their high affinity to hydrogen, which facilitates their

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broad use in organic coupling synthesis,1 hydrogen storage,2 and electrochemical reactions.3-4 The

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conventional wet-chemical methods of synthesizing Pd-based nanoparticles (NPs) generally include

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coating the NPs with stabilizers for preventing their agglomeration, which result in contaminated

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surface and thus lower their catalytic activity.5-7 Moreover, the synthesis process also involves time-

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consuming steps and strictly controlled reaction conditions, such as seed preparation, inert gas

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protection, and heating.8-9 Hence, for catalytic applications, the development of general and rapid

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routes to synthesize uncapped Pd-based NPs is of paramount significance.

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Hydrogen evolution reaction (HER) is a fundamental step for hydrogen generation to constitute

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reversible hydrogen fuel cell technology.10 Among numerous catalysts developed for HER, Pt is

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generally considered as the most effective one due to its low overpotentials, high current density and

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good corrosion resistance.11-12 But the high cost and scarcity also greatly hinder its

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commercialization.

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urgency in developing sustainable catalysts. Among many alternatives, Pd has been investigated as a

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promising one because of its notable catalytic activity, 3-fold higher abundance and lower price.

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At present, although continuous efforts in synthesizing surface-clean Pd-based materials have been

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made to enhance their performance,16-18 their catalytic efficiencies are still inferior to Pt, which

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prompts us to pay more attention to structural design rather than merely synthesis route modification.

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Addressing above issues, we turn to surface electronic structure modification and interface

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engineering, which act crucial roles in catalysis.19-20 With the development of computing power,

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theoretical simulations enable precise optimization of catalyst structures, such as the incorporation of

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transition metals can effectively tune surface electronic structure,21 and carbon/metal coupling will

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bring a moderate lower adsorption energy.22 In particular, by doping nitrogen into carbon, the

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resulting N-doped carbon (NC) gets obviously modified electronic structure for boosting

13-14

Replacing the scarce Pt with more abundant materials is therefore of great

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electrochemical reactions and different types of nitrogen dopant may also have various effect on

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catalytic reactions.23-27 In view of these facts, Pd alloy with a NC support might be a rational

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designed hybrid structure for efficient catalyst, because the alloying of Pd with other transition

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metals will optimize the surface electronic structure of NPs, improving the electrochemical activity

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of bare metal surface. Meanwhile, by coupling with NC, NPs can not only avoid aggregation owing

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to the anchor effect of carbon, which is free from stabilizers,

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electronic structure due to Pd alloy/NC engineered interface, leading to an enhanced overall catalytic

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activity. To verify our proposition, finding suitable synthesis method to realize this optimized

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structure is highly desirable.

6, 28-29

but might also obtain tailored

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Herein, we present a facile and rapid way to synthesize PdMnCo alloy (PdMnCo) supported on

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NC through a heterogeneous nucleation and methanol-mediated growth approach. First,

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Mn3[Co(CN)6]2 is selected as the precursor to fabricate NC with surface supported ultrasmall NPs

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containing metallic Mn and Co, which serve as seeds for Pd loading. Then, sonication facilitates the

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adsorption of Pd2+ for growing PdMnCo NPs in PdCl2 methanol solution and the adsorption process

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only lasts for 90s. This general and rapid route not only realizes the formation of designed hybrid

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structure, but also constructs a stable metal/NC interface due to the in-site transformation of

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supported transition metal NPs into Pd alloy NPs.

82 83

2. Results and Discussion

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2.1. PdMnCo/NC hybrid catalyst.

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The protocol for preparing PdMnCo/NC is schematically demonstrated in Figure 1. As the field-

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emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) images

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(Figure 2a-c) and the X-ray diffraction (XRD) data shown in Figure S1, the high purity as-prepared

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Mn3[Co(CN)6]2 nanocubes are monodispersed, which ensure the fabrication of high quality substrate.

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During the subsequent pyrolysis process in Argon (TGA curves see Supporting Information, Figure 3 ACS Paragon Plus Environment


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S2), these nanocubes are in-site converted into boxes of substrate with (Mn,Co)4N particles (verified

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by XRD and XRF discussed in Supporting Information, Figure S3-4) supported on NC. SEM and

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TEM images (Figure 2d-f) clearly present that the products have retained the size and cubic shape of

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the precursor particles, and the rough, porous surface provides a broad platform for Pd loading due to

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high Brunauer−Emmett−Teller specific surface area (Figure S5). We also conduct the room-

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temperature (300 K) M−H curves of the substrate (Figure S6), and the results indicate that it can be

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used as a substrate for constituting magnetically separable composite. Moreover, Raman

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spectroscopy for the substrate confirms the presence of carbon (Figure S7). The weak second-order

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band at ca. 2700 cm−1 is representative character for few-layered graphene,30-31 and the relatively

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high ID/IG band intensity ratio suggests the presence of structural imperfections, which may provide

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fast diffusion channels for transferring H into metal/NC interfaces and thus promotes HER activity.

b

a N C Mn Co

Ar

H

H

H+

PdCl2 /CH 3OH

e

PdMn Co

Sonication o) 4N (M n,C

102

c

N Co)4 (Mn,

N-doped carbon (NC)

d

103 104 105 106 107 108 109

Figure 1. Schematic illustration of the formation of PdMnCo/NC: (a) Mn3[Co(CN)6]2 nanocube with its crystalline structure inside, containing the framework of Mn-C≡N-Co; (b) an aggregate of (Mn,Co)4N supported on NC; (c) enlarged model of NC wrapped (Mn,Co)4N core and outer carbon surface supported ultrasmall (Mn,Co)4N NPs; (d) PdMnCo supported on NC; (e) the HER process at the interface of PdMnCo/NC.

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raised here: what happened in the pyrolysis procedure? Our inference is as follows. During the

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annealing process under inert atmosphere, Mn, Co and N atoms from the precursor form (Mn,Co)4N

Admittedly, apart from the characterization experiments mentioned above, a question might be


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NPs under high temperature while CN− group linkers directly lead to the formation of NC. As a

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result, (Mn,Co)4N/NC composite can be obtained. In fact, the TEM with energy dispersive X−ray

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analysis (TEM-EDX) elemental mappings observations confirm our inference. For instance, Figure

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S8 demonstrates that the big (Mn,Co)4N NPs are wrapped by NC, while the ultrasmall NPs are

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supported on the NC surface and the average particle size of NPs in the surface is about 2 nm. Since

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the substrate annealed at different temperature may result in distinct catalytic performance,32-33 we

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also take this factor into consideration.

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Figure 2. (a, b) FESEM and (c) TEM images of Mn3[Co(CN)6]2 nanocubes; (d, e) FESEM and (f) TEM images of annealed Mn3[Co(CN)6]2; (g) TEM images of PdMnCo/NC-2 and (h-l) elemental mapping of Mn, Co, C, N and Pd; (m) Enlarged TEM images of PdMnCo/NC-2; (n) HRTEM image of PdMnCo/NC-2; (o) TEM image of the PdMnCo/NC-2 and line-scan EDS analysis across a PdMnCo nanoparticle.



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In general, the PdMnCo/NC can be formed by following steps. Firstly, to find the optimal

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substrate, we test the HER performance of precursors that annealed at different temperature (600℃,

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700℃ and 800℃) and followed by loading the same amount of Pd. The one exhibiting the best

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performance is our choice. Then, we designated the samples with different Pd loading amount on the

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optimized substrate as PdMnCo/NC-1, PdMnCo/NC-2, and PdMnCo/NC-3, respectively. TEM-EDX

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images of as-prepared single nanobox shown in Figure 2(g-l) suggest the homogeneous distribution

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of Mn, Co, C, N and Pd, whereas the HRTEM images of the substrate surface demonstrate that the

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embedded particles have grown up to about 10 nm (Figure 2m) in comparison with NPs supported on

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the surface of pristine substrate (Figure S8b). Moreover, XAFS spectroscopy results (Figure S9-10)

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show that the Co K-edge and Mn K-edge spectra of PdMnCo/NC-2 demonstrates the reduced

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amplitude and slight phase shifting in comparasion with pure substrate, providing further evidence

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for the formation of alloy and existed at large-scale.34-35 HRTEM image (Figure 2n) shows that

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PdMnCo NP is anchored on the carbon outer layer and the interplanar spacing of the particle is 0.216

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nm, which might be assigned to the (111) plane of the PdMnCo. The results obtained above confirm

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the formation of surface anchored PdMnCo NPs, and these NPs are not only free from agglomeration,

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but also creating the exposed interfaces between PdMnCo NP and NC substrate, which provide a

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confined geometry for allowing the diffusion of small molecules. The overall process is shown in

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Figure 3a.

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2.2. XPS studies and PdMnCo/NC formation mechanism.

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X−ray photoelectron spectroscopy (XPS) spectra give us information about surface composition

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and chemical state changes between the substrate and the resultant PdMnCo/NC-2. And the peak

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deconvolutions are shown in Figure 3b (details are discussed in Supporting Information, Figure S11).

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As expected, we find the appearance of XPS peaks at 335.8 eV and 341.1eV, which corresponds to

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3d5/2 and 3d3/2 components of Pd0, suggesting the formation of metallic Pd. Two additional small


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peaks at 338.1 eV and 342.9 eV are also observed in the spectrum, which might be attributed to the

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small amount of adsorbed Pd2+ cations.9, 36 Interestingly, the content of metallic Mn and Co shown in

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PdMnCo/NC-2 are decreased comparing with the substrate, while the proportion of their oxidation

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state increased. These observations indicate that, during the sonication process, Pd2+ is reduced into

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metallic Pd, whereas (Mn, Co)4N is oxidized. (a)

(b) Intensity /(a.u.)

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Co 2p

Mn 2p Mn 2p2/3

Co 2p2/ 3

Ⅵ

Ⅴ

Co 2p1/ 2

Ⅵ

Mn 0

Mn 2p1/2

Pd 0 Pd 3d3/2

Pd 3d5/2

Ⅵ

Ⅴ

808 800 792 784 776 660

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Pd 3d

Co 0

Ⅴ

654

648

642

636

348 344 340 336 332

Binding Energy /eV

157 158 159 160 161 162

Figure 3. (a) Time-progression color changes of the (Ⅰ) PdCl2 methanol solution (Ⅱ) PdCl2 methanol solution with substrate (Ⅲ) PdCl2 methanol solution and substrate with sonication for 90s (Ⅳ) Magnetic separation of the obtained PdMnCo/NC-2; (b) XPS spectra for Co 2p, Mn 2p and Pd 3d of the (Ⅴ) substrate (Ⅵ) PdMnCo/NC-2.

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potentials, the ultrasmall metallic (Mn, Co)4N NPs that supported on outer carbon layer of the

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substrate partly react with adsorbed cations via a galvanic replacement reaction ， then these

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ultrasmall NPs serve as seeds for inducing PdMnCo growing to the obtained size with sonication.

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The anhydrous methanol might act as mild reductant and weak capping agent.9 In addition to

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boosting the rapid adsorption of Pd2+ in solution, sonication might also facilitate the diffusion

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between atoms, leading to the formation of the homogeneous alloy. Owing to the synergistic effects

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mentioned above, PdMnCo can be formed through a heterogeneous nucleation and methanol-

Therefore, the formation of PdMnCo can be explained as follows: due to different reduction



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mediated growth approach. Thus, PdMnCo/NC is the hybrid of NC inner surface wrapped

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(Mn,Co)4N and NC ourter surface supported PdMnCo NPs. Based on inductively-coupled plasma

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(ICP) analysis, the Pd loading in PdMnCo/NC-1, PdMnCo/NC-2 and PdMnCo/NC-3 are 13.6 wt%,

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26.1 wt% and 34 wt%, respectively. The prepared catalysts hold clean and high active surface in

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contrast to the reported capping agents and heating involved synthesis method.40-41

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2.3. Electrochemically-measured HER activity and material stability.

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M H2SO4 saturated with N2 in our experiment. The overpotentials required to get a 10 mA cm–2 HER

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current density is an important parameter for practical purpose. Figure S12 shows that the 700℃ is

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the optimal annealing temperature and our following samples are based on this substrate. Figure 4a

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demonstrates the polarization curves of PdMnCo/NC-1, PdMnCo/NC-2 and PdMnCo/NC-3. The

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PdMnCo/NC-2 exhibits the best HER performance, showing an potential of 34 mV. We also conduct

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electrochemically active surface area (ECSA) test, which is regarded as an indicative reference point

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to estimate active sites of an HER catalyst and the value of ECSA is relative to the electrochemical

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double-layer capacitance (Cdl). The Cdl of PdMnCo/NC-1, PdMnCo/NC-2, and PdMnCo/NC-3 are

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measured by voltammetry, and the consequence is exhibited in Figure S13. The rank of Cdl is

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following as PdMnCo/NC-1 < PdMnCo/NC-3 < PdMnCo/NC-2 (Figure S13d), confirming

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PdMnCo/NC-2 with the largest ECSA and the maximum number of functional sites among the three

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catalysts. Moreover, as references, the electrocatalytic performances of commercial Pt/C (40 wt%),

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Pd/C (30 wt%), graphene, along with Pd NPs and substrate are also investigated for comparison and

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the details (measurement and synthesis methods) are listed in the supporting information. Figure 4b

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demonstrates the polarization curves of these samples. The PdMnCo/NC-2 exhibits better

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performance than that of commercial Pt/C (58 mV), Pd/C (96 mV) and most reported Pd-based

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catalyst shown in Table S1. Meanwhile, the graphene is found to be inactive toward HER, as it

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shows a very high potential of 606 mV(vs.RHE). The substrate exhibits a passable HER catalytic

To evaluate the electrocatalytic HER activity of the catalyst, we use a three-electrode setup in 0.5


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activity in comparison with graphene, but is not comparable to PdMnCo/NC-2, implying the high

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catalytic activity is not from the substrate. In addition, Figure 4b also demonstrate that the pure Pd

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NPs (181 mV) show less active than Pd/C or PdMnCo/NC-2 in HER. 0

J / mA cm-2

(a)

PdMnCo/NC-1 PdMnCo/NC-2 PdMnCo/NC-3

-5

-10

(b)

0.0

J / mA cm-2

196

-2.5

Graphene Substrate PdMnCo/NC-2 Pd 30% Pd/C 40% Pt/C

-5.0

-7.5

-10.0

-15 -0.3

-0.2

-0.1

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

0.0

30% Pd/C 40% Pt/C PdMnCo/NC-2

E / V (vs. RHE )

-60

0 -20 -40 -60 -80

0

20

40

60

30% Pd/C 40% Pt/C PdMnCo/NC-2 3 mV/dec 4

0.10

20

J / mA cm -2

J / mA cm -2

(d)

0

-30

0.0

E / V ( vs. RHE )

E / V ( vs. RHE )

(c)

80

0.08 /de c 33 m V

0.06 /de c 31 m V

0.04

Time ( h )

-90 -0.3

199 200 201 202 203 204 205 206

-0.2

-0.1

0.02

0.0

1.0

PdMnCo/NC-2

(f)

1.1

1.2

1.3

1.4

200

Substrate PdMnCo/NC-2 30% Pd/C 40% Pt/C

2

15

100

-Z ′′ / ohm

-Z ′′ / ohm

150

1

1.5

log ( current / mA cm-2 )

Potential ( V vs. RHE )

(e) Mass Density (A⋅⋅ mgPd/Pt -1)

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10

5

50 40% Pt/C

0

Pd

0 0

30% Pd/C

5

10

15

20

Z ′ / ohm

0 0

50

Mass Activity

100

150

200

Z ′ / ohm

Figure 4. (a) Polarization curves of PdMnCo/NC-1, PdMnCo/NC-2, PdMnCo/NC-3. (b) Polarization curves of PdMnCo/NC-2, graphene, substrate, Pd, 30% Pd/C and 40% Pt/C for comparison. (c) Polarization curves of PdMnCo/NC-2, 30% Pd/C and 40% Pt/C for comparison; Inset in (c) is chronoamperometric test of PdMnCo/NC-2 at a constant applied potential of 55 mV (vs. RHE). (d) Tafel plots of PdMnCo/NC-2, 30% Pd/C and 40% Pt/C. (e) Comparison of the mass specific activity of Pd, 30% Pd/C, PdMnCo/NC-2 and 40% Pt/C at applied potential of 35 mV (vs. RHE). (f) Nyquist plots of substrate, PdMnCo/NC-2, 30% Pd/C and 40% Pt/C. 9 ACS Paragon Plus Environment


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Previous studies showed that using platinum as the counter electrode (CE) may significantly 42

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improve the activity of nonprecious metal catalysts for the hydrogen evolution reaction.

209

evaluate the influence of possible dissolved Pt towards our test, we re-test the HER catalytic activity

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of PdMnCo/NC-2 using Pd wire as CE. The results shown in Figure S14 demonstrate that Pt CE also

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has influence on HER performance of Pd-based catalyst, but the enhancement is very limited (~ 4

212

mV at 10 mA cm–2). Thus, for convenience of HER performance comparison with previously

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reported Pd-based catalyst, our following tests are using Pt as CE .

To

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The durability of PdMnCo/NC-2 is illustrated in Figure 4c. We carry out a chronoamperometric

215

measurement at an applied potential of 55 mV (vs.RHE). During the testing process, the current

216

density fluctuates little around 20 mA cm-2 for 80h, indicating the stability of this hybrid strucure.

217

Moreover, at a higher current density of 90 mA cm-2 (Figure 4c), PdMnCo/NC-2 needs only 96 mV

218

(vs.RHE), which is lower than that of both Pd/C (163 mV) and Pt/C (119 mV), further confirming

219

the excellent HER catalytic activity of this rational designed hybrid catalyst. Tafel plots analyses of

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as-prepared PdMnCo/NC-2, commercial Pt/C as well as Pd/C are examined, and the results are

221

shown in Figure 4d. PdMnCo/NC-2 demonstrates a Tafel slope of 31 mV dec-1, lower than that of

222

Pt/C (32 mV dec-1) and Pd/C (41 mV dec-1). Tafel curve of PdMnCo/NC-2 clearly illustrates that the

223

HER occurs through a Volmer-Tafel mechanism, that is, the recombination of two Hads is the rate-

224

determining step. Mass specific activities is way to evaluate the efficiency of hybrid catalysts.

225

Figure 4e shows the calculated mass specific activities at applied potential of 35 mV (vs.RHE).

226

PdMnCo/NC-2 exhibits a mass specific activity of 2.1 A•mg−1Pd, which is 37.1 and 112.4 times

227

greater than those obtained from commercial 30% Pd/C and pure Pd NPs, respectively, realizing the

228

efficient utilization of Pd. Notably, the mass specific activity of PdMnCo/NC-2 is 10.1 times greater

229

than that of commercial 40% Pt/C, demonstrating the high electrocatalytic activity of the sample.

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The kinetics of the catalytic processes on the samples were examined by electrical impedance

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spectroscopy (EIS; Figure 4f) to further illustrate the superior HER performance of the PdMnCo/NC-


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2. Compared to the 30 wt% Pd/C, 40 wt% Pt/C and pure substrate, the PdMnCo/NC-2 have the

233

lowest charge-transfer resistance, indicating its superior charge transport kinetics.

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2.4. HER free-energy diagram.

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To obtain a fundamental understanding of the origin of the unexpected high HER activity of

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PdMnCo/NC-2, DFT-based first principle simulations are carried out. According to the ICP results

237

(Pd:Mn:Co=25:2:1) and HRTEM image, the possible configurations of PdMnCo, PdMnCo/NC-2

238

and hydrogen adsorption sites shown in Figure S15-17 are tested, and Table S3-4 are the relevant

239

energies. Based on stable slab with the lowest energy, the steady adsorption configurations of

240

hydrogen are shown in Figure 5. (a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Pt Pd Mn Co N C H

241 242 243 244 245 246

Figure 5. The top (up) and side (down) view of stable adsorption configurations of hydrogen atom on the (a) Pt (111), (b) Pd (111), (c) PdMnCo (111), (d) graphene, (e) PdMnCo/C, (f) PdMnCo/NGC, (g) PdMnCo/2NGC, (h) PdMnCo/NPC.

247

models and the results are illustrated in Figure 6a-c and Table S5-6. The pristine graphene

248

demonstrates a large ∆GH* of 1.73 eV, suggesting a negligible adsorption ability of H*. On the

We calculate the ∆GH*, a good descriptor for theoretical prediction of HER activity, of these



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contrary, a negative ∆GH* of −0.26 eV suggests that Pd (111) exhibits a much better adsorption

250

ability than graphene. Furthermore, by alloying with Mn and Co, bare PdMnCo gets lower |∆GH*|

251

than Pd (111), but still higher than Pt (111). Figure 6d demonstrate the N1s spectrum of

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PdMnCo/NC-2, which can be deconvoluted into three individual peaks that are assigned to pyridinic

253

N (398.6 eV), pyrrolic N (400.9eV), and quaternary N (401.9 eV), respectively 37-39 corroborating the

254

formation of NC substrate with different types of nitrogen dopant. 1.8

(a)

1.8

(b)

Graphene

Graphene

H*

1.6

1.6

0.0

H + + e-

∆ GH* (eV)

∆ GH* (eV)

H*

1/2 H 2

0.0

Pt (111)

-0.2

H+ + e-

PdMnCo/NPC

PdMnCo/C

-0.2

PdMnCo

1/2 H2

Pt /C

Pd /C

Pd (111)

-0.4

-0.4

Reaction Coordinate

(c)

Reaction Coordinate

1.8

(d)

Graphene

N 1s

Intensity /(a.u.)

H*

∆ GH* (eV)

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

Page 12 of 18

1.6

0.0

H + + e-

1/2 H2 PdMnCo/2NG

-0.2

Quaternary N

PdMnCo/NG

-0.4

406

Reaction Coordinate

(e)

Pyridinic N Pyrrolic N

404

402

400

398

396

Binding Energy /eV

(g)

(f)

(h)

Pd Mn Co N C H

255 256 257 258 259 260

Figure 6. (a-c) The calculated free-energy diagram of HER at the equilibrium potential for the models; (d) The XPS result of the N1s spectrum for PdMnCo/NC-2; The charge density difference graph between carbon and hydrogen adsorbed PdMnCo system for (e) PdMnCo/C, (f) PdMnCo/NGC, (g) PdMnCo/2NGC and (h) PdMnCo/NPC; yellow and cyan contour represent electron accumulations and electron depletions, respectively; the iso-surface level is 0.0009e/bohr3.


Page 13 of 18

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261

Meanwhile, to study the effect of the engineered interfaces, we also conduct the calculations of

262

∆GH* for interfaces of Pt/C and PdMnCo/NC. PdMnCo/NC is further classified as PdMnCo/NPC

263

(NPC represents one pyridinic nitrogen atom doped carbon) and PdMnCo/NGC (NGC represents one

264

graphitic nitrogen atom doped carbon) according to XPS results. Interestingly, as shown in Figure 6b,

265

all of the interface models get improved ∆GH*, especially PdMnCo/NPC. It shows a smaller |∆GH*|

266

value of 0 eV than Pt/C interface (∆GH* = − 0.09 eV). In a word, whereas bare Pt surface shows

267

smaller free energy of hydrogen than bare PdMnCo, the |∆GH*| value of PdMnCo/NPC interface is

268

even smaller than that of Pt/C. This indicates that the reason why PdMnCo/NC exhibits superior

269

experimental HER performance to Pt/C is due to the contribution of high catalytic PdMnCo/NPC

270

interfaces. Apart from the comparison mentioned above, we also investigate the role of substrate

271

nitrogen content towards HER performance. To simplify the calculation, only graphitic N doping is

272

considered. The results (Figure 6c) demonstrate that the ∆GH* of PdMnCo/2NGC (2NGC represents

273

two graphitic nitrogen atoms doped carbon) is closer to zero than PdMnCo/NGC interfaces.

274

Accordingly, it suggests that the augment of nitrogen concentration in the carbon substrate improves

275

the catalytic activity of the interfaces, which is beneficial to overall HER performance.

276 277

2.5. HER-enhanced mechanism.

278

To investigate the difference of underlying mechanisms in HER activity between bare metal

279

surface and metal-carbon interface, we also conduct calculations about structural changes after

280

adsorption of hydrogen. As shown in Table S5-6, the distance between H* and Pd atom (d

281

PdMnCo/NC interfaces is shorter than that of bare PdMnCo, and the d

282

smallest in all PdMnCo/NC models. In fact, the shorter the distance, the closer the atoms will be,

283

which will thus increase the mutual superposition of electron clouds, leading to an enhanced

284

repulsion part of the interactions. Moreover, after hydrogen adsorption, the interface spacing will

285

also increase by ~ 0.18Å in Table S7, and this structural change may cause an extra energy penalty.

286

As a result, comparing with bare metal surface adsorbed H*, the hydrogen binding energy at the 13 ACS Paragon Plus Environment

H-Pd

H-Pd)

in

of PdMnCo/NPC is the


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

287

metal/carbon interface is decreased, and it is might be the cause of the improved HER performance

288

for having a carbon substrate.

289

In adsorption-dominated system (∆GH*

Rapid Adsorption Enables Interface Engineering of PdMnCo Alloy

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