1

Electronic Supplementary Information (ESI)

Nitrogen and phosphorous co-doped graphitic carbon encapsulated ultrafine OsP2 nanoparticles: A pH universal highly

durable catalyst for hydrogen evolution reaction

Sukanta Chakrabartty, Barun Kumar Barman, and C. Retna Raj*

these authors have equal contribution

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2019

2

1. Materials and Methods

Materials

Ammonium hexachlorosmate ((NH4)2OsCl6), melamine, 20% Pt/C and Nafion (5 wt

%) were purchased from from Sigma-Aldrich. Triphenylphosphine (TPP) was obtained from

Merck. All other chemicals used in this work were of analytical grade and used without

further purification. All the solutions were prepared with Millipore water (Milli-Q system).

1.2. Synthesis of OsP2@NPC composite

In a typical synthesis, (NH4)2OsCl6 (100 mg), TPP (125 mg), and melamine (125 mg)

was added into a mortar-pestle and grounded well to form homogeneous mixture. Now, the

mixture was kept in a sealed quartz-tube filled with Ar gas and annealed at different

temperature (750, 850 and 950 ˚C) in a furnace with a ramping rate of 5 ˚C/min. OsP2

obtained at 950 ˚C is referred as OsP2@NPC in the text.

1.3. Synthesis of Os@NC composite

Os@NC was obtained by same procedure use for the synthesis of OsP2@NPC in the

absence of TPP.

2.4. Instrumentation

Powder X-ray diffraction profiles (PXRD) were acquired with BRUKER D8 advance

unit using Cu-Kα (λ = 1.54 Å) radiation. Raman measurements were performed with a

HORIBA JOBIN YVON (France, model no. T64000, 514.5 nm laser excitation). The field

emission scanning electron microscopy (FESEM) analysis performed in Merlin FESEM.

Scanning Transmission electron microscopy (STEM), TEM and high resolution-TEM

analysis was performed using JEM 2100F (JEOL, Japan). High angle annular dark field

3

(HAADF) image was acquired in JEOL energy dispersive spectrometer (EDS) attached with

JEM 2100F. The X-ray photoelectron spectroscopy (XPS) measurements were carried out

with PHI 5000 Versaprobe II. The specific surface area and pore size distribution were

characterized with Micromeritics 3FLeX adsorption analyzer. The surface area from the

adsorption data was obtained by Brunauer-Emmett-Teller (BET) equation. Inductively

coupled plasma optical emission spectrometry (ICP-OES) analysis was performed in Perkin-

Elmer Optima 8300.

2.5. Electrode modification

The catalyst ink was prepared by dispersing 1 mg of the catalyst in 100 L Nafion

solution of Nafion (5 wt%) and dimethylformamide (1:4 vol/vol)) in a bath sonicator for 30

min. An aliquot of 2 L of the as-prepared ink was drop casted on a GC electrode (3 mm

diameter) and dried at room temperature. The catalyst loading on the electrode surface was

optimised to be 0.285 mg/cm2. Faradic efficiency of OsP2@NPC was checked using nickel

foam (0.5 * 0.5 cm2) and loading was 2 mg/cm2.

2.6. Electrochemical measurements

All electrochemical experiments were performed with CHI643B electrochemical

analyzer (CH Instruments, USA) in a two-compartment three-electrode electrochemical cell.

Two compartments were joined with porous frit. Glassy carbon (GC) and graphite rod were

used as working and counter electrode, respectively. 0.5 M H2SO4, 1M PBS and 1M KOH

was used as acidic, neutral and alkaline solution, respectively. In acidic and neutral solution

Ag/AgCl (3M KCl) and in alkaline solution Hg/HgO was used as reference electrode,

respectively. Reference electrodes are calibrated to RHE as follow; ERHE = EAgCl + 0.22 (in

acid), EAgCl + 0.52 (in PBS) and ERHE = EHg/HgO + 0.95. Polarization curves were obtained

using linear sweep voltammetry (LSV) at scan rate of 5 mV/s. Current was normalized using

4

geometrical surface area of the GC (0.07 cm2). All polarization curves were not iR corrected

(until and unless specified). Electrochemically active surface area (ECSA) was calculated by

performing cyclic voltammetry (CV) at different scan rate (20-200 mV/s) at the non-faradic

region. Stability test was performed by CV and i-t amperometry. Electrochemical impedance

spectroscopy (EIS) measurements were performed on Autolab potentiostat galvanostat

(302N), using computer controlled NOVA 2.1 software within the range of 0.1-105 Hz with

AC amplitude of 10 mV.

Faradic efficiency calculation: Potentiostatic electrolysis of 0.5 M H2SO4 solution was

performed for the calculation of Faradic efficiency. Evolved H2 gas was collected by inverted

burette method. Volume of H2 produced was measured at different time interval up to 1 h.

Then Faradic efficiency was calculated by taking the ratio of experimentally measured

volume of H2 to that of theoretically expected amount.1 Theoretical amount of H2 was

calculated by applying Faraday Law: It/2F (where, I is current in ampere, t is time in second,

2 is number of electron and F is faraday constant (96,485 C/mol)).

5

Calculation of Turnover frequency (TOF):

Following steps were performed to calculate TOF using literature procedure.2

Calculation of ECSA (without mass normalization)

𝐴𝐸𝐶𝑆𝐴 = 𝑇ℎ𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑑𝑜𝑢𝑏𝑙𝑒 𝑙𝑎𝑦𝑒𝑟 𝐶𝑑𝑙

60 µ𝐹/𝑐𝑚2𝑝𝑒𝑟 𝑐𝑚 2𝐸𝐶𝑆𝐴

TOF is calculated using this formula:

𝑇𝑂𝐹 = 𝑛𝑜. 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝑡𝑢𝑟𝑛𝑜𝑣𝑟𝑒𝑠/𝑐𝑚2𝑜𝑓 𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑎𝑟𝑒𝑎

𝑛𝑜. 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠/𝑐𝑚2𝑜𝑓𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑎𝑟𝑒𝑎

Now, the total number of H2 turnover was calculated from current density as follow:

𝑛𝑜 𝑜𝑓 𝐻2 = ( 𝑗𝑚𝐴

𝑐𝑚2)(

1 𝐶𝑠 ‒ 1

1000 𝑚𝐴)(

1 𝑚𝑜𝑙 𝑒 ‒

96485.3 𝐶)(

1 𝑚𝑜𝑙 𝐻2

2 𝑚𝑜𝑙 𝑒 ‒)(

6.022 × 1023 𝐻2

1 𝑚𝑜𝑙 𝐻2)

𝑛𝑜 𝑜𝑓 𝐻2 = 3.12 × 1015 𝐻2/𝑠

𝑐𝑚2 𝑝𝑒𝑟

𝑚𝐴

𝑐𝑚2

All the atoms are considered to be active in this calculation.

And no. of active site for Os@NCwas calculated as follow:

𝑛𝑜. 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠 = [ 2 𝑎𝑡𝑜𝑚𝑠

𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙

27.96 𝐴3

𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙]2/3

𝑛𝑜. 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠 = 1.72 × 1015 𝑎𝑡𝑜𝑚𝑠 𝑐𝑚 ‒ 2𝑟𝑒𝑎𝑙

𝑇𝑂𝐹 = ( 3.12 × 1015

𝐻2

𝑠

𝑐𝑚2 𝑝𝑒𝑟

𝑚𝐴

𝑐𝑚2× |𝑗|

(1.72 × 1015 𝑎𝑡𝑜𝑚𝑠 𝑐𝑚 ‒ 2𝑟𝑒𝑎𝑙 )𝐴𝐸𝐶𝑆𝐴)

Total no of Os atom per unit cell = 2

6

And no. of active site for OsP2@NC was calculated as follow:

𝑛𝑜. 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠 = [ 6 𝑎𝑡𝑜𝑚𝑠

𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙

87.87 𝐴3

𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙]2/3

𝑛𝑜. 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠 = 1.67 × 1015 𝑎𝑡𝑜𝑚𝑠 𝑐𝑚 ‒ 2𝑟𝑒𝑎𝑙

As OsP2@NPC contains both OsP2 and Os as active site we have taken the average value of

and .1.72 × 1015 1.67 × 1015 𝑎𝑡𝑜𝑚𝑠 𝑐𝑚 ‒ 2𝑟𝑒𝑎𝑙

So, for OsP2@NPC the 𝑛𝑜. 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠 = 1.69 × 1015 𝑎𝑡𝑜𝑚𝑠 𝑐𝑚 ‒ 2𝑟𝑒𝑎𝑙

𝑇𝑂𝐹 𝑓𝑜𝑟 𝑂𝑠𝑃2@𝑁𝑃𝐶 = ( 3.12 × 1015

𝐻2

𝑠

𝑐𝑚2 𝑝𝑒𝑟

𝑚𝐴

𝑐𝑚2× |𝑗|

(1.69 × 1015 𝑎𝑡𝑜𝑚𝑠 𝑐𝑚 ‒ 2𝑟𝑒𝑎𝑙 )𝐴𝐸𝐶𝑆𝐴)

Total no. of atom per unit cell = 6 (2 Os + 4P)

7

Fig. S1 EDX-elemental mapping of OsP2@NPC corresponding to the FESEM image shown

in Fig.1.

8

Fig. S2 Nitrogen adsorption-desorption isotherm (a) and BJH pore size distribution plot (b) of OsP2@NPC.

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

250

300a

Qua

ntity

Ads

orbe

d (c

m³/g

STP

)

Relative Pressure (P/Po)5 10 15 20 25 30 35 40 45 50

Pore

vol

ume d

V/dD

(cm

3 g-1nm

-1)

Pore diameter (nm)

b

9

Fig. S3 Raman spectrum showing D and G band of N, P co-doped carbon in OsP2@NPC.

900 1200 1500 1800

G Band

Inte

nsity

(a.u

.)

Raman shift (cm-1)

D Band

10

Fig. S4 XPS surface survey spectrum of OsP2@NPC.

800 700 600 500 400 300 200 100 0

Binding Energy (eV)

Inte

nsity

(a.u

.)

Os-4fP-2p

N-1sO-2p

C-1s

11

Fig. S5 Deconvoluted C 1s XPS profile of OsP2@NPC.

290 288 286 284 282

C-N

In

tensit

y (a.u

.)

Binding Energy (eV)

C-C/C=CC 1s

12

Fig. S6 XRD profile of Os@NC.

20 40 60 80

C (201

)

(103

)

(102

)

(110

)(100

)

# 06-0662 Os(101

)(0

02)

Inte

nsity

(a.u

.)

2 (degree)

13

Fig. S7 TEM (a,b) and HRTEM (c,d) images of Os@NC.

14

Fig. S8 XPS surface survey (a,b) and deconvoluted N1s profile of OsP2 obtained at 750 (c)

and 850 oC (d).

404 402 400 398 396

d

Binding Energy (eV)

Inte

nsity

(a.u

.)

N 1s

PyridinicPyrrolic

Graphitic

800 700 600 500 400 300 200 100 0

a

P-2p

N-1s

O-2p

C-1s

Os-4f

In

tens

ity (a

.u.)

Binding Energy (eV)800 700 600 500 400 300 200 100 0

C-1s

Os-4f

P-2p

b

N-1sO-2p

Inte

nsity

(a.u

.)

Binding Energy (eV)

404 402 400 398 396

c N 1s

Inte

nsity

(a.u

.)

Binding Energy (eV)

15

Fig. S9 XRD profile of catalysts obtained at 750, 850 and 950 ̊C.

The intense peak at 43.5̊ for the catalyst obtained at 750 oC corresponds to unphosphidated

metallic Os (101). The intensity of (101) diffraction decreases with increasing the

temperature and only broad hump is observed in OsP2@NPC (950 ̊C). Moreover, the (100)

and (103) diffractions corresponding to metallic Os are not seen with OsP2@NPC obtained at

950 ̊C, suggesting that degree of phosphidation increases at high temperature.

20 40 60 80

(100

)

OsP2850

OsP2750

OsP2@NPCOsP2 (# 71-0168)

(020

)

Os (# 06-0662)

(110

)(1

10)

(100

)

(040

)

(103

)(1

03)

(321

)(3

21)

(321

)(2

40)

(240

)(2

40)

(112

)

(102

)(1

02)

(040

)(0

40)

(031

)(0

31)

(031

)

(130

)(1

30)

(130

)

(220

)(2

20)

(220

)(1

01)

(101

)(1

01)

(002

)(0

02)

(002

)

(101

)(1

01)

(101

)(0

11)

(011

)(0

11)

(020

)(0

20)

(110

)(1

10)

(110

)

2

Inte

nsity

(a.u

.)

16

Fig. S10 Cyclic voltammetric response of OsP2@NPC in 0.5 M H2SO4 and the corresponding

plot of peak current against sweep rate. The double layer capacitance was calculated using

the plot.

0.3 0.4 0.5 0.6-0.6-0.30.00.30.60.91.21.51.8

j (m

A/cm

2 )

E/V vs RHE0 30 60 90 120 150 180 210

0.00.20.40.60.81.01.2

j ( j

a-j c

)/mAc

m-2

Scan rate (mV/s)

17

Fig. S11 Cyclic voltammetric response of Os@NC in 0.5 M H2SO4 and the corresponding

plot of peak current against sweep rate. The double layer capacitance was calculated using

the plot.

0 40 80 120 160 200

0.3

0.6

0.9

j ( j

a-jc)/m

Acm-2

Scan rate (mV/s)0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

j (m

A/cm

2 )

E/V vs RHE

18

Fig. S12 Tafel plot used for the calculation of exchange current density (j0) in O.5 M H2SO4.

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.500.00

0.03

0.06

0.09

OsP2@NPC

Os@NC

log(j)

Ove

rpot

entia

l (V)

19

Fig. S13 iR corrected polarization curves of OsP2@NPC and Os@NC in acid. iR correction

was done according to the following equation: Ecorr = Eexp − iR (where Ecorr is the iR-

corrected potential, Eexp is the experimentally measured potential and R is the solution

resistance.)

Catalyst 10 before iR correction

in acid (mv)

10 after iR correction

in acid (mV)

OsP2@NPC 46 37

Os@NC 80 72

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-150

-120

-90

-60

-30

0

j (m

A/cm

2 )

Not corrected iR corrected

Os@NCOsP2@NPC

E/V vs RHE

20

Fig. S14 Linear sweep voltammogram compares HER performance of catalysts synthesized

at 750, 850 and 950 ̊C in 0.5 M H2SO4.

-0.4 -0.2 0.0 0.2-80

-60

-40

-20

0

j (m

A/cm

2 )

OsP2-750OsP2-850OsP2@NPC

E/V vs RHE

21

Fig. S15 Plot illustrating the turn over frequency (TOF) of OsP2@NPC and Os@NC in acid.

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4-1

0

1

2

3

4

5

6OsP2@NPCOs@NC

TOF

( H2 p

er a

ctiv

e site

)

E/V vs RHE

22

Fig. S16 Digital image showing the quantification of H2 (inverted burette approach)

generated during the electrocatalytic HER with OsP2@NPC in 0.5 M H2SO4.

23

Fig. S17 Amperometry i-t profile of commercial Pt/C (20%) obtained during the durability

test in 0.5 M H2SO4. The analysis is performed by holding the potential at 30 mV (vs RHE)

for 10 h.

0 2 4 6 8 10-20

-15

-10

-5

0

j (

mA/

cm2 )

Time/h

24

Fig. S18 XRD profile of OsP2@NPC after 10 h of durability test in acid (green) and base

(violet).

15 20 25 30 35 40 45 50 55 60 65 70 75 80

(240

)(3

21)

(112

)

(040

)

(310

)

(130

)(2

20)

(101

)

(101

)(0

11)

(020

)

2

OsP2 after durability in acid(110

)

(240

)(3

21)

(112

)

(040

)

(310

)

(130

)(2

20)

(101

)

(101

)(0

11)

(020

)

(110

)OsP2 after durability in base

Inte

nsity

(a.u

)

25

Fig. S19 FESEM image (a) and the corresponding EDX elemental mapping (b-f) of

OsP2@NPC after 10 h of durability test in acid.

26

Fig. S20 FESEM image (a) and the corresponding EDX elemental mapping (b-f) of

OsP2@NPC after 10 h of durability test in base.

27

Fig. S21 Impedance analysis of OsP2@NPC and Os@NC in acid at an overpotential of 25

mV. Equivalent circuit3,4 shown in the inset is used to fit the experimental data. Rs is the

solution resistance, CPE is the constant phase element and RCT is the charge transfer

resistance, respectively.

0 30 60 90 120 1500

25

50

75

100

125

150

OsP2@NPC

Os@NC

Zreal/

Z im

/

28

Table S1. Total N content and relative percentage of different N species present in OsP2

catalyst obtained different temperature 750, 850 and 950 ̊C.

Temperature(oC)

Pyrrolic-N Pyridinic-N Graphitic-N Total N (Atomic %)

750 27.1% 24.2% 48.7% 7.01

850 22% 28.7% 49.3% 4.43

950 31.4% 17.6% 51% 2.59

It is known that the annealing temperature has large control over the total amount of N

content and percentage of different N species in the carbon framework. The nitrogen content

can influence the electrocatalytic performance of the catalyst. In order to understand the

phosphidation process and the role of nitrogen content, OsP2 was synthesized at three

different temperatures. The N content and chemical nature of nitrogen was analysed by XPS

measurements. As shown in the table, the catalyst obtained at low temperature (750 oC ) has

the highest N content of 7.01 atomic %. OsP2@NPC obtained at 950 oC has only 2.59 atomic

%. However, the percentage of graphitic-N increases with increasing annealing temperature

and found to be highest (51%) in OsP2@NPC. It is known that pyridinic-N can facilitate

hydrogen evolution reaction.5 In OsP2@NPC, the % of pyridinic-N is found to be 17.6. It is

proposed that the synergistic effect between pyridinic-N and active OsP2 improves the overall

HER performance.

29

Table S2. Cdl and ECSA of OsP2@NPC and Os@NC in 0.5 M H2SO4.

Cdl (mF/cm2) ECSA (m2/g)Catalyst

OsP2@NPC 14.6

Os@NC

2.5

2.1 12.2

Calculation of ECSA (mass normalized):

ECSA (m2/g) is calculated according to literature procedure.6

Cs = k/(2*m)

Where, Cs, k and m is specific capacitance, linear fitting slope and loading of the catalyst over GC, respectively.

ECSA= Cs/ 60 µF/cm2

(60 µF/cm2 is the standard value)

30

Table S3. HER performance of OsP2@NPC, Os@NC and traditional Pt/C.

Acid Alkaline NeutralCatalyst

10(mV)

TafelSlope

(mV/dec)

j0(mA/cm2)

10(mV)

TafelSlope

(mV/dec)

j0(mA/cm2)

10(mV)

TafelSlope

(mV/dec)

j0(mA/cm2)

OsP2@NPC 46 43 1.23 90 54 0.820 144 64 0.565

Os@NC 80 63 0.77 145 85 0.391 217 71 0.235

Pt/C 21 31 1.57 51 38 1.32 74 42 0.797

31

Table S4. Table comparing the HER activity and cost of the metal (Os, Ru, Rh and Pd).

* Cost of the metal was obtained from https://www.metalary.com/

MediumCatalyst

Acid Alkaline Neutral

Loading Cost of Metal*

Ref.

OsP2@NPC 46 mV

38 mV

(iR-corrected)

90 mV 144 mV 0.285 mg/cm2

(13.9 gOs/cm2)

(ICP-OES 4.9 wt% Os)

400 $/ozt This work

RuP2@NPC 38 mV

(iR-corrected)

52 mV

(iR corrected)

57 mV

(iR corrected)

1.0 mg/ cm2

(ICP-OES 23.3 wt% Ru)

270 $/ozt Angew. Chem. Int. Ed., 2017, 56, 11559.

w-Rh2P NS/C 15.8 mV 18.3 mV 21.9 mV 10.7 μgRh/ cm2. 2465$/ozt Adv. Energy Mater., 2018, 8, 1801891.

PdP2@CB 27.4 mV 35.4 mV 84.6 mV 0.283 mg/cm-2

(ICP-OES 6.29 wt%)

1346 $/ozt Angew. Chem. Int. Ed., 2018, 57, 14862.

32

Table S5. Comparison of HER activity of OsP2@NPC with existing metal phoshide based

electrocatalyst in acidic, neutral and alkaline pH.

Catalyst Electrolyte Overpotential( mV)

@ 10 mA/cm2

Tafel

slope

(mv/dec)

Reference

OsP2@NPC

0.5M H2SO4

1M KOH

1M PBS

46

90

144

43

54

64

This work

WP2 NR

0.5M H2SO4

1M KOH

1M PBS

148

225

298

52

84

79

7

WP NPs@NC 0.5M H2SO4

1M KOH

1M PBS

102

150

96

58

--

--

8

WP NAs/CC 0.5M H2SO4

1M KOH

1M PBS

130

150

200

69

102

125

9

Ni2P/Ni 0.5M H2SO4

1M KOH

1M PBS

120

130

170

68

50

142

10

MoP NA/CC

0.5M H2SO4

1M KOH

1M PBS

12480187

588394

11

MoP Ns@NC 0.5M H2SO4

1M KOH

1M PBS

11580136

655971

12

MoP2 NS/CC 0.5M H2SO4

1M KOH

1M PBS

586785

63.670

98.3

13

MoP2 NPs/Mo 0.5M H2SO4

1M KOH

1M PBS

143

194

211

57

80

81

14

CoP@BCN 0.5M H2SO4

1M KOH

1M PBS

87215122

465259 15

33

WP2 NPs/W 0.5M H2SO4

1M KOH

1M PBS

143214201

669295

16

CoP/CC 0.5M H2SO4

1M KOH

1M PBS

6720965

5112993

17

Reference:

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