activity origin and catalyst design principles for ...€¦ · activity origin and catalyst design...

SUPPLEMENTARY INFORMATIONARTICLE NUMBER: 16130 | DOI: 10.1038/NENERGY.2016.130

NATURE ENERGY | www.nature.com/natureenergy 1

Activity Origin and Catalyst Design Principles for Electrocatalytic Hydrogen-

Evolution on Heteroatom-Doped Graphene

Yan Jiao,# Yao Zheng,# Kenneth Davey, Shi-Zhang Qiao*

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

* Correspondence to: [email protected]

# These authors contributed equally to this work.

This PDF file includes:

Supplementary Figures 1-15

Supplementary Table 1

Supplementary Notes 1-5

Supplementary References 1-7

Activity origin and catalyst design principles for electrocatalytic hydrogen evolution on heteroatom-doped graphene

http://dx.doi.org/10.1038/nenergy.2016.130

2 NATURE ENERGY | www.nature.com/natureenergy

SUPPLEMENTARY INFORMATION DOI: 10.1038/NENERGY.2016.130

Supplementary Figures

280 285 290 295 300 305

B-G

G

P-G

S-G

O-G

Nor

mal

ized

inte

nsity

(a.u

.)

Photon energy (eV)

N-G

Carbon K edge

Supplementary Fig. 1 Carbon K-edge NEXAFS on various graphene-based samples. The

carbons in five (5) doped-samples maintained the graphitic framework.



SUPPLEMENTARY INFORMATIONDOI: 10.1038/NENERGY.2016.130

Supplementary Fig. 2 (a) Graphene sheet model; (b, c) graphene ribbon model with armchair

and zigzag edge.




Supplementary Fig. 3 Investigated sites and corresponding adsorption energy on (a) B-3C, (c)

B-2C-O and (e) B-C-2O in boron-doped graphene model. The possible active sites are circled

whilst the most active are indicated by a red circle (This labelling method is repeated in

Supplementary Fig. 4-7). The configuration and distance between hydrogen and carbon are

shown as insert. For models (c) and (e), the carbons adjacent the functional group were evaluated

also. The final structures however were found to be unreasonable, because the graphene framework

did not keep its original configuration, therefore they are not shown here. The corresponding DOS

for the configuration with strongest H* adsorption on each of the models was plotted in panels (b),

(d), and (f). To facilitate the next step DOS analysis, the total DOS and projected DOS on the active

carbon site and that for hydrogen were included.




Supplementary Fig. 4 Investigated sites and corresponding adsorption energy on (a) g-N, (c)

py-N, (e) N-O and (g) pr-N in nitrogen doped graphene model. (b) (d) (f) (h) are DOS and local

atomic configuration with the strongest hydrogen adsorption. For model c, the carbon adjacent

the functional group was evaluated. However the hydrogen atom moved spontaneously to the edge

site during geometry optimization.




Supplementary Fig. 5 Investigated sites and corresponding adsorption energy on (a) C-O-C, (c)

py-O, (e) C=O, and (g) C-OH in oxygen doped graphene model. (b) (d) (f) (h) are DOS and local

atomic configuration with the strongest hydrogen adsorption on these models.




Supplementary Fig. 6 Investigated sites and corresponding adsorption energy on (a) P-3C-(O)

and (c) P-2C-(2O) in phosphorus doped graphene model. (b) (d) are DOS and local atomic

configuration with the strongest hydrogen adsorption on these models.




Supplementary Fig. 7 Investigated sites and corresponding adsorption energy on (a) S-2O and

(c) th-S in sulfur doped graphene model (c). (b) (d) are DOS and local atomic configuration

with the strongest hydrogen adsorption for these models.




Supplementary Fig. 8 Atomic configuration of reaction intermediate states on the edge carbon

(a) state (M′-2H) + H+ and (b) state (M′-H2) + H+. The corresponding configurations for state

(M′-2H) + H+ on non-edge carbon is shown as (c). Green is carbon, blue is nitrogen, pink is

hydrogen, and the hydrogen atoms involved in the HER is highlighted by yellow. The inclusion

of state (M′-H2) + H+ for the edge carbon site is due to this carbon being connected by a hydrogen for

the purpose of saturating the dangling bond. Because the distance between two hydrogen atoms

(highlighted by yellow) connecting to the edge carbon is just 1.75 Å (panel a), the next step of the

reaction is the combination of these two hydrogen atoms to form an H2 (panel b). Whilst, for the non-

edge site (panel c), due to the longer distance between the two highlighted hydrogen atoms, a diffusion

course exists that leads to a high energy barrier (~1.3 eV for H hopping to the adjacent carbon, and

therefore the diffusion is limited).1,2 Therefore for this model, similar configurations as in panel b are

not energetically probable and are not shown on Fig. 2b; otherwise, the reaction on the non-edge site

should be followed by Heyrovsky route.




-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

(M'-H)+H2

(M'-H2)+H+

(M'-2H)+H+

(M'-H)+2H+

non-edge site edge site

Free

Ene

rgy

(eV

)

Reaction Pathway

bN-O

0.0

0.5

1.0

1.5

2.0

(M'-H)+H2

(M'-H2)+H+

(M'-2H)+H+

(M'-H)+2H+

non-edge site edge site

Free

Ene

rgy

(eV

)

Reaction Pathway

a Pr-N

Supplementary Fig. 9 Reaction pathways on edge (marked by blue circles) and non-edge

carbons (marked by red circles) in (a) pr-N and (b) N-O models. The overall free energy change

on the edge carbon is higher than the non-edge carbon. This indicates that the former site is less active

than the latter. The substrate is labelled M' to denote a standard substrate mode M less one hydrogen

atom adsorbed on the investigated adsorption centre i.e. M = M' - H.




-0.8 -0.6 -0.4 -0.2 0.0

Den

sity

of S

tate

s (a

.u.)

Energy (eV)

I II III IV V

b

-2 -1 0 1 2

Den

sity

of S

tate

s (a

.u.)

Energy (eV)

I II III IV V

a

1.5

1.0

0.5

0.0

1.0

1.2

1.4

1.6

1.8

PD

OS

Pea

k (e

V)

Activite carbon's distance to dopants

GH

*(eV

)

II

III

I

IV

V

Gc

IIIIII IV

V

Supplementary Fig. 10 (a) PDOS for sites I to V on py-N model. (b) Enlarged dotted zone in

panel a near the Fermi level. The model and proposed adsorption sites are shown as insert. (c)

Comparison of PDOS peak value with that of the adsorption strength. The adsorption energy

value on pristine graphene (G) is considered as a reference for the carbon atom that is located

at an infinitely large distance to the edge. The value of ΔGH* on various active carbons was

oscillatory, increasing with the distance to the dopant. When the carbon site is sufficiently far from

the dopant, ΔGH* should be the same value as that for pristine graphene. The oscillating trend in the

ΔGH value could be related to different electronic structures of each investigated site. Because of the

postulated bond formation scheme, with a higher PDOS peak value (for example site I), the bonding

between the adsorption site and the hydrogen is stronger, leading to a lower ΔGH value.




-0.6 -0.5 -0.4 -0.3 -0.2-10

-8

-6

-4

-2

0

Initial After 1,000 cycles

Cur

rent

Den

sity

(mA/

cm2 )

Potential vs. RHE (V)

B-G

-0.6 -0.5 -0.4 -0.3 -0.2-10

-8

-6

-4

-2

0

Cur

rent

Den

sity

(mA/

cm2 )


O-G Initial After 1,000 cycles

-0.6 -0.5 -0.4 -0.3 -0.2-10

-8

-6

-4

-2

0

Cur

rent

Den

sity

(mA/

cm2 )


N-G Initial After 1,000 cycles

-0.6 -0.5 -0.4 -0.3 -0.2-10

-8

-6

-4

-2

0

Cur

rent

Den

sity

(mA/

cm2 )


S-G Initial After 1,000 cycles

-0.6 -0.5 -0.4 -0.3 -0.2-10

-8

-6

-4

-2

0

Cur

rent

Den

sity

(mA/

cm2 )


P-G Initial After 1,000 cycles

Supplementary Fig. 11 Polarization curves recorded for various doped graphene samples

before and after 1,000 potential sweeps (+0.2 to -0.6 V versus reversible hydrogen electrode)

under 0.5 M H2SO4.




Supplementary Fig. 12 HER activity (expressed by i0/site) trend calculated from the kinetic

model plotted as a function of free energy for hydrogen adsorption at various α and kg0 values.

Based on the experimentally measured i0 per active site (square symbols), several computationally

derived i0/site lines are plotted with different α values. Among which, the line with α = 0.125 and kg0

= 0.01 s-1site-1 shows the best fit to the experimental data. Therefore this volcano plot was selected in

this study.

-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2

10-22

10-20

10-18

10-16

10-14

Nor

mal

ized

i 0 (A/s

ite)

GH* (eV)




-0.4 0.0 0.4 0.8 1.2-10

-8

-6

-4

-2

0

2

Plane catalyst

DO

S activ

e ca

rbon

(eV)

GH* (eV)

b

-15 -10 -5 0 5

DOS6

DOS5

DOS3

Plane catalyst Adsorption with H

DO

S (a

.u.)

EFa

Energy (eV)

DOS1

DOS4

DOS2

-0.4 0.0 0.4 0.8 1.2-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

Plane catalyst

Wei

ghte

d ce

ntre

of t

otal

DO

S (e

V)

GH* (eV)

c

-0.4 0.0 0.4 0.8 1.2-10

-9

-8

-7

GH* (eV)

d

Plane catalystWei

ghte

d ce

ntre

of t

otal

DO

S (e

V)

-0.4 0.0 0.4 0.8 1.2-5

-4

-3

-2W

eigh

ted

cent

re o

f tot

al D

OS

(eV)

GH* (eV)

e

Adsorption with H

-0.4 0.0 0.4 0.8 1.2-10.0

-9.5

-9.0

-8.5

-8.0

GH* (eV)

f

Adsorption with H

Wei

ghte

d ce

ntre

of t

otal

DO

S (e

V)

Supplementary Fig. 13 Analyses of key electronic structure indicator with that of ∆GH* on the

various graphene models. (a) Definition diagram of each indicator using g-N as an example. (b)

Active centre highest peak position vs ∆GH*; (c) total DOS without H* vs ∆GH*; (d) DOS below

Fermi level without H* vs ∆GH*; (e) total DOS with H* vs ∆GH*; (f) DOS below Fermi level with

H* vs ∆GH*.




Supplementary Fig. 14 Investigated sites and corresponding adsorption energies on dual-doped

models.




-20 -15 -10 -5 0 5

DO

S activ

e ca

rbon

(a.u

.)

Energy (eV)

N,S-grahpene th-S-grahpene

EFa

-20 -15 -10 -5 0 5

DO

S activ

e ca

rbon

(eV)

Energy (eV)

N,B-graphene B-3C-graphene

EF

c

-20 -15 -10 -5 0 5

DO

Sac

tive

carb

on (e

V)

Energy (eV)

N,P-graphene P-3C(-O) -graphene

EF

b

Supplementary Fig. 15 (a) DOS analysis of the most active carbon on N-S-graphene of

Supplementary Fig. 14b (orange line), that of th-S-graphene is included as comparison (black

line). (b) DOS analysis of the most active carbon on N-P-graphene of Supplementary Fig. 14e

(green line), that of P-3C(-O)-graphene is included as comparison (black line). (c) DOS analysis

of the most active carbon on N-B-graphene of Supplementary Fig. 14i (pink line), that of B-3C-

graphene is included as comparison (black line). The DOS analysis for the dual-doped graphene

follows the trend identified in Fig. 5. On N-S-graphene, the DOS of the active carbon moves toward

the Fermi level. This indicates a stronger adsorption of the hydrogen atom on this substrate and

therefore enhanced HER activity. Similarly, on N,P-graphene, a DOS peak appears near the Fermi

level. To the contrary, for N,B-graphene, a peak appears around -8 eV, and the overall DOS

distribution moves further away from the Fermi level. It is concluded H* adsorption is weaker than

that for boron doped graphene.




Supplementary Table 1 Summary of physicochemical characterization, electrochemical

measurement, and calculation results on various samples/models.

* data obtained from ref. 3

Sample Tafel (mV/dec)

Measured i0 (A)

Surface area*

(m2/ g) Dopant*

(%) Active species in

Dopants* (%) i0/site

exp (A /site)

∆GH* (eV)

B-G 112.3 7.5×10-8 113.7 5.4 9.0 9.0×10-

23 0.61

N-G 115.5 1.1×10-7 108.8 6.4 24.3 4.3×10-

23 0.81

O-G 113.0 4.8×10-8 124.6 6.3 18.0 2.2×10-

23 0.97

S-G 112.8 5.6×10-8 154.6 2.4 76.6 1.3×10-

23 1.01

P-G 120.6 4.2×10-8 103.1 1.9 50.2 2.9×10-

23 0.71




Supplementary Note 1

Contribution to activity as determined by different activation energy

According to the Arrhenius Equation, the reaction rate, k, is related to the activation energy, Ea, via

k = A' exp(-Ea/(kBT)) (1)

where kB is the Boltzmann constant, T is the reaction temperature (300 K to represent room-

temperature), and A' is a uniform pre-factor for the particular reaction. Consequently, for two

reactions to give k1/k2 > 10, the relationship between their activation energies Ea1 and Ea2 should

follow Ea2 – Ea1 > 0.06 eV. In other words, only when the difference between ΔGH* (see the definition

in the following text) on two atoms is smaller than 0.06 eV, will the reaction rate (expressed as i0 in

electrocatalysis) for these two sites be of the same order. In the present study, the difference between

ΔGH* on the most active sites and all other sites exceeds this threshold. Other sites therefore are

viewed as not contributing to the overall reaction.





Reaction Mechanism.

Generally, in the acidic solution, the overall HER pathway is:4

H+ + e– → H* (Volmer step), followed by

H* + H+ + e– → H2 (Heyrovsky step)

or

H* + H* → H2 (Tafel step)

where * refers to a potential active site. The free energy diagram for the overall HER process is

normally a three-state one, comprising by an initial-state (H+ + e–), an intermediate-state of H

adsorbed on the catalyst surface (H*), and a final-state product represented by ½H2.5 In the present

study, extra states of H2* or 2H* adsorption were also considered to take Heyrovsky or Tafel steps

into consideration. Beside these, additional energy barriers might exist but the values are negligible

and therefore were not considered on the free diagram.6





Normalization of i0

The normalization process from measured i0 to the activity per active site (i0/siteexp) for all samples

follows the same method that considers samples physical surface area and the most active doping

type percentage. Taking N-G as an example: i0 measured from Tafel plot is 1.1×10-7 A. The mass

loading of powder sample on working electrode is 40 μg, which yields the surface area of 40 × 10-6 ×

108.8 = 4.35 ×10-3 m2 (Supplementary Table 1). According to the geometric area of single carbon

atom, the number of total carbons contained in this area is 4.35×10-3 / 2.64×10-20 =1.65×1017.

Therefore, exact number of “active” carbon sites can be calculated based on the dopant’s

concentration and active species in dopant as 1.65 ×1017 × 6.4 % × 24.3 % = 2.57×1015

(Supplementary Table 1). Afterwards, the i0 on per active site is calculated as 1.1×10-7 / 2.57×1015 =

4.27 × 10-23 A.





DOS analysis

To evaluate the relationship between electronic structure and H* adsorption energy, a schema of six

(6) descriptors was introduced. This schema is shown in Supplementary Fig. 13a. The six are: the

position of highest peak of active site DOS without hydrogen adsorption (DOS1); weighted DOS

centre for graphene models without hydrogen adsorption – below Fermi level (DOS2); weighted DOS

centre for graphene models without hydrogen adsorption – overall range (DOS3); the position of

highest peak of active site’s DOS with hydrogen adsorption (DOS4); weighted DOS centre for

graphene models with hydrogen adsorption – below Fermi level (DOS5); and, weighted DOS centre

for graphene models with hydrogen adsorption – overall range (DOS6). The definition of weighted

DOS centre is

𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 1∑ 𝜀𝜀𝑖𝑖𝑖𝑖

∑ 𝜀𝜀𝑖𝑖𝑟𝑟𝑖𝑖𝑖𝑖 (2)

where ri is the DOS for each model at energy εi.





Electrode Current Calculation

According to the Butler-Volmer Equation, assuming the exchange current for an active site is i0/site,

the electrode current per site i/site under electrode potential U is7

i/site(U) = – i0/site × 10(-(U-U0)/b) (3)

where U0 = 0 vs RHE and b is the Tafel slope selected as 120 mV/dec. The current density, j, therefore

on an electrode under U is computed from

j(U) = i/site(U) × (Ssurface_area × mloading ÷ Scarbon) × ζdoping ÷ Selectrode (4)

where Ssurface_area is the surface area per mass loading (in m2 g-1), mloading is the loading amount of

catalyst material (g), Scarbon is the area per carbon atom (in 2.64×10-20 m2), ζdoping is the doping

percentage of active heteroatoms, and; Selectrode is the geometric area of the electrode (0.196 cm2).




Supplementary References

1 Ferro, Y., Marinelli, F. & Allouche, A., Density functional theory investigation of the diffusion

and recombination of H on a graphite surface. Chem. Phys. Lett. 368, 609-615 (2003).

2 Borodin, V. A., Vehviläinen, T. T., Ganchenkova, M. G. & Nieminen, R. M., Phys. Rev. B 84,

075486 (2011).

3 Jiao, Y., Zheng, Y., Jaroniec, M. & Qiao, S. Z. Origin of the electrocatalytic oxygen reduction

activity of graphene-based catalysts: a roadmap to achieve the best performance. J. Am. Chem. Soc.

136, 4394-4403 (2014).

4 Zheng, Y., Jiao, Y., Jaroniec, M. & Qiao, S. Z. Advancing the electrochemistry of the hydrogen-

evolution reaction through combining experiment and theory. Angew. Chem. Int. Ed. 54, 52-65

(2014).

5 Nørskov, J. K. et al. Trends in the exchange current for hydrogen evolution. J. Electrochem. Soc.

152, J23-J26 (2005).

6 Skulason, E. et al. Density functional theory calculations for the hydrogen evolution reaction in

an electrochemical double layer on the Pt(111) electrode. Phys. Chem. Chem. Phys. 9, 3241-3250

(2007).

7 Nørskov, J. K. et al. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode.

J. Phys. Chem. B 108, 17886-17892 (2004).


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