Heteroatom-doped Transition Metal Electrocatalysts for Hydrogen

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Heteroatom-doped Transition Metal Electrocatalysts for Hydrogen Evolution Reaction Huanyu Jin, Xin Liu, Shuangming Chen, Anthony Vasileff, LAIQUAN LI, Yan Jiao, Li Song, Yao Zheng, and Shi-Zhang Qiao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00348 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Heteroatom-doped Transition Metal Electrocatalysts for Hydrogen Evolution Reaction Huanyu Jin,a,† Xin Liu,a,† Shuangming Chen,b,† Anthony Vasileff,a Laiquan Li,a Yan Jiao,a Li Song,b Yao Zheng,*a and Shi-Zhang Qiao*a a

School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia

b

National Synchrotron Radiation Laboratory, CAS Center for School of Chemical Engineering,

Excellence in Nanoscience, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China. Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

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ABSTRACT: Developing efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) is important for clean energy systems. Non-noble transition metals are the most promising candidates for replacement of conventional Pt group catalysts for the HER. However, most non-noble metals show poor HER activity due to their intrinsic electronic structures. Herein, we use a multifaceted heteroatom-doping method (nitrogen, sulfur and phosphorus) to directly and continuously fine tune the electronic structure and HER activity of non-noble metals without changing their chemical composition. As a proof-of-concept, a nitrogen and phosphorus dualdoped Ni catalyst is explored by precisely manipulating doping modes, revealing the best HER performance among all doped Ni catalysts tested. The doping-induced charge redistribution in the Ni metal significantly influences its catalytic performance for the HER in alkaline media, which is confirmed by merging theoretical calculation with synchrotron-based spectroscopy. The principle that can bridge the doping modes and HER activity of the doped-catalysts is built with a volcano relationship.

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Hydrogen is considered a sustainable and clean energy carrier for future energy systems. The hydrogen evolution reaction (HER) is core to producing high purity hydrogen in water electrolysis systems. To date, transition metal electrocatalysts (TMEs) are the most commonly utilized catalysts for hydrogen-related clean energy conversion processes.1-5 However, the most efficient TMEs for HER are noble metals such as Pt and Ru. These metals are expensive and scarce, which hinders their large-scale application. Consequently, activating and optimizing non-noble TMEs for HER is an alternative way for low-cost hydrogen production. One strategy for optimizing the catalytic activity of known non-noble metal TMEs is to modify their electronic structures.6-8 However, strategies such as alloying, lattice strain and single atom synthesis, face difficulty in achieving continuous control over their electronic structures and usually introduce additional variables such as ligand effect or composition change,9-10 leading to obstacles in identifying and optimizing the electronic structure of the TMEs alone.11-13 Thus, new methods that can flexibly and effectively control the electronic structure of non-noble TMEs for HER are needed. Heteroatom doping is a promising method that can tune the electronic structure of host materials by introducing charge redistribution.14-18 By doping heteroatoms (e.g. boron, nitrogen, sulfur, phosphorus) with different electronegativity, the resultant charge redistribution can change the electronic, catalytic and optical properties of the materials for energy related applications.19-21 Furthermore, one of the advantages of heteroatom doping is that it does not change the composition of the host materials (which can retain their desired intrinsic features).22 To date, heteroatom doping has been widely used in modifying the electronic structure of carbon materials and metalcompounds, demonstrating great potential for tuning the electrocatalytic performance of the matrix.23-26 However, it is difficult to apply this method to TMEs due to the formation of corresponding compounds like metal nitrides, sulfides, etc. Only a few works have reported the


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synthesis of single doped TMEs which showed significant HER activity enhancement in comparison to their non-doped counterparts and relevant metal compounds.5, 27-29 The heteroatom site on metal surface can promote the water dissociation step of HER by decreasing the free-energy barrier. However, these works did not realize multi-faceted regulation in electronic state due to the simple doping strategies used. Consequently, it is necessary to introduce multiple doping modes that can continuously control the electronic state of TMEs in order to precisely optimize HER catalysts. Recently, design principles for heteroatom-doped graphene materials for the HER in acidic media were established by continuously tuning their electronic structures through multiple heteroatom doping.14 The electrons from different dopants can be transferred directionally, leading to the formation of desired active sites for the HER. For example, in boron and nitrogen co-doping, the boron atom acts as an electron donor while nitrogen is the electron acceptor, due to differences in electronegativity. Thus, the adsorption energy of intermediates on the active sites can be continuously tuned to either a strong or weak state by changing the doping mode. By extension, continuous control of the electronic structure of TMEs could possibly be realized using different doping modes which could also unveil their activity mechanism. However, multi-faceted heteroatom doping on pure transition metals has not been achieved. In this work, for the first time, multi-faceted heteroatom doping on TMEs was realized and, in turn, the optimization of multiple intermediates for the HER was achieved. Using Ni metal as the example, the d band center of the TME was tuned by switching between single and dual doping modes (N, P, S). The effects of the doping mechanism on HER activity (for both single and dual doping) was explained by mutually corroborating experiments and theoretical computation. Significantly, the N, P co-doped Ni exhibited the best HER performance with an overpotential of


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24 mV at a current density of 10mA cm-2, which is even better than many noble metal catalysts. Importantly, a volcano plot between different doping modes on Ni catalysts and their HER performance was built, which demonstrates the possibility of using heteroatom doping methods to optimize the HER activity of TMEs. At last, we also observed similar enhancement for doped Co metal catalysts, manifesting the general applicability of this method. A series of single and dual heteroatom-doped (N, P, S) Ni nanoparticles (N-Ni, P-Ni, S-Ni, N-P-Ni, S-N-Ni, and S-P-Ni) were synthesized (Figure S1). The X-ray diffraction (XRD) patterns (Figure 1a, Figure S2a and 2b) indicate that the doped-Ni catalysts are isostructural to metallic Ni (JCPDS card No. 04-0850). Therefore, the doping method did not significantly change the crystal structure of the metal host and no detectable Ni-based compounds were formed. As shown in the Ni 2p X-ray photoelectron spectroscopy (XPS) spectra (Figure 1b and Figure S2c), the valence state of Ni is dominated by Ni (0) after heteroatom doping, indicating that the majority of the catalytic surface is metallic Ni and no metal compound is formed (e.g. Ni3N, Ni2P). The shoulder peak at 855.8 eV increased slightly after doping, indicating that the heteroatoms were successfully doped on the Ni. As confirmed by synchrotron-based XPS, the doping level of each element was about 0.5 at. % (Table S1), which is likely to be the saturation level. It was also found that formation of the corresponding metal compound (such as Ni2P) was favored upon increasing the amount of heteroatom precursor (Figure S3). Taking N-P-Ni as an example, the highly crystalline nanoparticles were distributed on the carbon support uniformly and had an average particle diameter of about 7 nm (Figure 1c and Figure S4). The exposed (111) plane of N-P-Ni nanoparticles is shown in the aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1d). Energy dispersive spectroscopy (EDS) mapping of N-P-Ni shows the spatial distribution of N and P on the Ni


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particles (Figure 1e). The consistent O 1s XPS, C 1s XPS and C K edge X-ray absorption near edge structure (XANES) spectra between the different catalysts indicate that the heteroatoms were doped on Ni instead of the carbon or oxygen (Figure 1f and S5). A series of synchrotron-based X-ray adsorption (XAS) measurements were then performed to characterize the local coordination of doped-Ni samples at the atomic level (Figure 2a). The spectrum of N-P-Ni is similar to Ni foil and different from the Ni3N and Ni2P, further indicating that no bulk Ni-compound was formed. The absorption edge of N-P-Ni shifts to higher energy than that of Ni foil, indicating the formation of positively charged Ni sites after heteroatom doping. Figure 2b shows the Ni K edge extended X-ray adsorption fine structure (EXAFS) spectra of the different catalysts. N-P-Ni shows a main coordination peak at 2.18 Å which is assigned to Ni-Ni bonding. The small peak around 1.3 Å can be assigned to Ni-N and Ni-P coordination,30 which indicates the successful doping of N and P on the Ni surface. The fitting results show that the coordination number of Ni-N and Ni-P increases after heteroatom doping (Figure S6 and Table S2). Wavelet transform (WT) was adopted to further analyze the Ni K edge EXFAS oscillations (Figure 2c and Figure S7). The WT contour plots of N-P-Ni exhibit one intensity maximum at 7.6 Å, which is closer to that of Ni foil (8 Å). The negative shift indicates that N and P were successfully doped on the Ni particle. Whereas the patterns for N-P-Ni are totally different from that of Ni2P and Ni3N, indicating that no nitride or phosphide was formed. As confirmed by N and P K edge XANES (Figure 2d and 2e), the peaks at 400.8 eV and 2151.8 eV correspond to the newly formed Ni-N and Ni-P species in N-P-Ni. The XANES spectra of the other samples also show characteristic peaks (Ni-N, Ni-S and Ni-P) at relevant positions (Figure S8), indicating that heteroatom doping occurred on Ni instead of the carbon.


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The electronic interaction and HER activity between N, P, S dopants and Ni atoms was then investigated. As confirmed by the Ni L3 edge XANES of different samples, the valence state of Ni slightly increased after single doping (Figure 3a, Figure S9). More interestingly, the magnitude of the shift for the Ni L3 edge peak is positively correlated to the electronegativity of the heteroatoms. Consequently, as N has the largest electronegativity, N-Ni demonstrated the biggest shift of 0.34 eV. The valence state of Ni increased after single doping, which can be explained by the electronic interaction between Ni and the different heteroatom dopants. In single doping, an electron is transferred from Ni to the more electronegative heteroatom (N, P or S). Therefore, the interaction between the fully occupied π-symmetry (t2g) d-orbitals of Ni and the bridging heteroatom is strong in the single doping mode, leading to an increase in valence state of the metal atoms.31 We then measured the electrocatalytic HER performance of single doped Ni catalysts to evaluate the relationship between the charge redistribution of single doped Ni and their electrocatalytic activity. Figure 3b shows the linear sweep voltammetry (LSV) curves of different single-doped catalysts. N-Ni exhibited an overpotential of 96 mV at a current density of 10 mA cm-2 (η10), which is much better than that of pristine Ni (285 mV). Further, S and P doped Ni also exhibited enhanced HER activity compared to Ni (Figure 3b and S10a, b). The activity enhancement of single-doped Ni nanoparticles is likely attributed to the changes in electron structure of the metal and heteroatom sites. For finer control of electronic state, we produced dual-doped structures to counter some of the effects of the electronegative dopant. As shown in Figure 3a, the valence state of Ni after codoping (judged by the peak position of Ni L3 edge XANES) still increased in comparison to pristine Ni. However, the overall valence state decreased compared to N-Ni (material with highest valence state). The reduction of Ni valence state indicates there is an interesting interaction between Ni


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and the two heteroatoms. Taking N-P-Ni as an example, the dopant atoms are bonded with Ni in co-doping mode. Since the electronegativity of N (3.04) is larger than that of P (2.19), electrons from P transfer to N and weaken the interaction between N and Ni. As the P provides extra electrons to the whole system, this leads to the overall valence state of Ni being lower than that of N-Ni, which is in accordance with the XANES and XPS data. As expected, all the dual-doped catalysts exhibited enhanced HER performance compared to their single-doped counterparts and the pristine Ni catalyst. This demonstrates the superiority of dual-heteroatom doping. Significantly, the N-P-doped Ni exhibited excellent performance with a small η10 of 24 mV which is better than those of other catalysts including Ni3N and Ni2P (Figure 4c, Figure S11). The i0 of N-P-Ni (1.22 mA cm-2) is very close to that of commercial Pt/C (1.3 mA cm-2) and 8-fold higher than that of pristine Ni (0.154 mA cm-2; Figure S10b). N-P-Ni also shows a small Tafel slope of 34 mV dec–1, indicating the Volmer step is no longer rate determining as water dissociation kinetics on this material are significantly improved. Significantly, the overall performance of N-P-Ni is even better than those of many reported noble-metal and other catalysts, as is shown in Figure 3e and Table S3.32-36 Regarding stability, chronoamperometry (Figure 3f) was carried out for 50 h at a constant overpotential of 30 mV. The stable current response further indicates that the catalyst is stable under operating conditions. Figure 3f insert depicts the LSV curves collected from N-PNi at the first cycle and 1000th cycle. Negligible change in the current response is observed after 1000 cycles. By spectroscopic and TEM measurements (Figure S12), N-P-Ni was also found to be very stable during operation. Furthermore, several control experiments were conducted to highlight the importance of the heteroatoms in TMEs (Figure S13 and S14). In order to explore the versatility of this method, cobalt metal-based catalysts were prepared and subjected to the same


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ACS Energy Letters

doping treatment (Figure S15). Similar enhancement is observed in N-P-Co which demonstrates the effectiveness of this method for improving the HER activities of inexpensive TMEs. DFT calculations studying the different catalysts were conducted to probe the electronic structure of Ni after heteroatom doping. The electron density difference in Figure 4a demonstrates the charge transfer induced by N-P dual doping: electrons transfer from the surrounding Ni atoms to the Ni-N bonding area, suggesting a strong interaction between the N and Ni as indicated by the large iso-surface; whereas P donates electrons to surrounding Ni atoms displayed as a blue electron depletion iso-surface. From the two-dimensional electron density difference plot, we clearly see that electron density accumulates at N and the surrounding Ni near the P dopant. For the N-P doping mode, electrons transfer from Ni to N (Figure 4a, top view). After P doping, the electrons transfer from P to Ni due to its unsaturated electron orbital. Consequently, the average valence state of Ni can be incrementally increased or decreased. Further, we calculated electronic structures of all the samples to reveal the underlying activity origin trend. As shown in Figure S16, the d band center position of different catalysts referenced to the Fermi level and slightly shifted is plotted against and experimental descriptor (η10). Interestingly, the η10 as a function of d band position (Figure 4b) shows a volcano plot, which indicates that there is an optimal value of d band position. This is a quantitative illustration of the Sabatier principle: an active catalyst binds reaction intermediate(s) neither too strongly nor too weakly.37 It is thus of great importance to continuously tune the electronic structure to reach the summit of the volcano rather than over-optimizing the electronic structure and missing the peak. Furthermore, the Gibbs free energy and theoretical HER overpotentials for each reaction step of Ni and N-P-Ni are shown in Figure 4c and 4d. The hydroxide desorption and hydrogen evolution steps are endergonic, while the water adsorption and dissociation are exergonic. Therefore, the potential determining step should either be hydroxide


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desorption or hydrogen evolution, of which the free energy difference should be the theoretical overpotential to drive the reaction at an appropriate rate. After N, P doping, both hydroxide desorption and hydrogen evolution on Ni (111) are optimized which therefore boost HER performance significantly. In summary, a multifaceted heteroatom-doping method was developed to optimze the HER activity of non-noble metal catalysts. Out of all the doped Ni catalysts tested, N-P co-doped Ni exhibited the best HER performance with a very small overpotential of 24 mV at a current density of 10 mA cm-2. As confirmed by experiments and DFT calculations, doping-induced charge redistribution on the Ni surface can change the electronic structure of the Ni metal, resulting in catalytic properties similar to nobel metal catalysts. Similar enhancement was also observed for Co metal catalysts, elucidating the versatility of this method. This study demonstrates the possibility of activating TMEs via heteroatom doping and provides new insight for activating unconventional materials for applications in other energy storage and conversion systems.

Supporting Information. Experimental section, TEM figures, spectroscopic characterization, and electrocatalytic data are available in the Supporting Information.

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Author Contributions †

H. J., X. L. and S. C. contributed equally.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by the Australian Research Council (DP160104866, DE160101163 and FL170100154). XANES measurements were undertaken on the soft X-ray beamline at the Australian Synchrotron. REFERENCES (1)

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Page 15 of 19 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

ACS Energy Letters

(36)

Chen, G.; Wang, T.; Zhang, J.; Liu, P.; Sun, H.; Zhuang, X.; Chen, M.; Feng, X.

Accelerated Hydrogen Evolution Kinetics on NiFe-Layered Double Hydroxide Electrocatalysts by Tailoring Water Dissociation Active Sites. Adv. Mater. 2018, 30, 1706279. (37)

Greeley, J.; Mavrikakis, M. Alloy Catalysts Designed from First Principles. Nat. Mater.

2004, 3, 810-815.


15

ACS Energy Letters

b

(111) (200) N-P-Ni P-Ni N-Ni

(220)

c

Ni (0)

Intensity (a.u.)

a Intensity (a.u.)

N-Ni

Ni

P-Ni N-P-Ni

Carbon

Ni

Ni

5 nm

870 860 850 20 30 40 50 60 70 80 880 Binding Energy (eV) 2 Theta (degree)

d

e

f C-C π*

10 nm

Ni

2 nm N

P

Intensity (a.u.)

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 16 of 19

280

C-C σ* Ni N-Ni P-Ni N-P-Ni

285 290 295 Photon Energy (eV)

Figure 1. (a) XRD patterns of single and dual doped Ni nanoparticles. (b) Ni 2p XPS spectra of single and dual doped catalysts. (c) TEM image of N-P-Ni. (d) HAADF-STEM of a N-P-Ni nanoparticle. (e) Elemental mapping of N-P-Ni. (f) Synchrotron-based C K edge XANES spectra of different catalysts.


16

c

Intensity (a.u.)

Ni K edge

N-P-Ni Ni foil Ni3N Ni2P

N-P-Ni

Ni foil

Ni-Ni

Ni3N

N-P-Ni Ni foil Ni3N Ni2P

Ni2P

Ni-N

4

2

Ni-N bonding

e

410 400 390 Photon Energy (eV) P K edge

Ni-P

0

N K edge

N-P-Ni Ni3N

8320 8340 8360 8380 8400 8420 Energy(eV)

b

d

Intensity (a.u.)

a

FT(k3c(k))

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

ACS Energy Letters

Intensity (a.u.)

Page 17 of 19

Ni-P bonding

N-P-Ni Ni2P

2160 2150 Photon Energy (eV)

6

R(Å)

420

2170

Figure 2. Synchrotron-based (a) XAS spectra, (b) FT curves and (c) WT-EXFAS plots of the asprepared N-P-Ni and other relevant materials. (d) N K edge XANES of Ni3N and N-P-Ni. (e) P K edge XANES of Ni2P and N-P-Ni.


17

ACS Energy Letters

-20 -0.5

S- i PN i

-N

66 mV dec-1

-0.4

-0.3

-0.2

-0.1

Ru@CN

PtNi-O/C

RuCo@NC

Ru/GnP

N-P-Ni

Ru/NC

20

Pt/C

η10

60

RuO2/NiO/NF

80

40 32 mV dec-1

0.00

10 Current (mA cm-2)

-10

-15

f

-0.1 0.0 Potential (V versus RHE)

0 -15

0

-30 -45 -60

0

Catalysts

-2 -4

1 th 1000 th

-6 -8

-10 -0.10 -0.05 0.00 0.05 Potential (V versus RHE)

-75

0 1

-5

-20 -0.2

0.0

e120

53 mV dec-1 34 mV dec-1

Pt/C N-P-Ni S-N-Ni S-P-Ni

Potential (V versus RHE)

100

0.05

0

Current (mA cm-2)

N

-P

i

-N -S

N

i

Pt/C N-P-Ni S-N-Ni S-P-Ni

0.10

-N

N

0.15

N

S-

N i PN

i

i

852.6

d

-15

Current density (mA cm-2)

852.7

-10

RuO2/CNx

852.8

-5

Ru/C3N4

In

852.9

Ni P-Ni S-Ni N-Ni

RuP2@NPC

ard

c

0

Current Density (mA cm-2)

b

cr

ea se

853.0

Ba ckw

Current Density (mA cm-2)

Ni L3 Peak Position (eV)

a 853.1

Overpotential (V)

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 18 of 19

10

20 30 Time (h)

40

50

Figure 3. (a) Synchrotron-based Ni L3 edge peak position of single doped and dual doped Ni catalysts. (b) LSV curves of the single doped Ni catalysts. (c) and (d) LSV curves and Tafel plot of the dual doped N-P-Ni, S-P-Ni, S-N-Ni and Pt/C. (e) Comparison of N-P-Ni (η10) with other noble metal-based catalysts. (f) Chronoamperometric curves and LSV (inset) of N-P-Ni in long term stability test.


18

Page 19 of 19

a

b 0 Overpotential (mV)

N-P-Ni

e-/Bohr3

2xE-3

S-P-Ni

S-N-Ni

-100

N-Ni

S-Ni

-200

P-Ni Break scaling relation Ni

-300 -1.42

-7xE-3

-1.40

-1.38

-1.36

d band center (eV)

c

d H2O + *

Ni (111)

-

OH + H*

-0.5

N-P-Ni (111)

H2O + *

OH- + 1/2 H2

0.0 Free Energy (eV)

0.0 Free Energy (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

ACS Energy Letters

OH- + 1/2 H2

OH-+ H*

-0.5

OH*+ H*

-1.0

OH*+ H*

-1.0

Reaction Pathway

Reaction Pathway

Figure 4. (a) Top: Interfacial electron transfer in N-P-Ni. Color: gray, Ni; blue circle, N; pink circle, P. Yellow and cyan iso-surface represents electron accumulation and electron depletion at an iso-surface cutoff of 0.003 e-/Bohr3, bottom: Electron density change plots of N-P-Ni. (b) Volcano plot of η10 as a function of the d band position for various heteroatom doped Ni catalysts. (c) and (d) Reaction path ways of Ni and N-P-Ni for alkaline HER.


19

Heteroatom-doped Transition Metal Electrocatalysts for Hydrogen

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