Supporting Information
Surface Oxidized Dicobalt Phosphide Nanoneedles as a NonPrecious, Durable and Efficient OER Catalyst
Anirban Dutta,† Aneeya K. Samantara,‡ Sumit K. Dutta,† Bikash Kumar Jena*‡ and Narayan
Pradhan*†
†
Department of Materials Science and Centre for Advanced Materials, Indian Association for
the Cultivation of Science, Kolkata, India 700032
‡
Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials
Technology, Bhubaneswar, Odisha, India 751013
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EXPERIMENTAL
Materials
Cobalt (II) chloride hexahydrate (CoCl2.6H2O, acs reagent, 98%), Oleylamine (OLM,
tech.70%),
Calcium Phosphide (Ca3P2), Nafion 117 solution (5% in a mixture of lower
aliphatic alcohols and water), Potassium hexachloroiridate (IV), Sodium citrate dibasic
sesquihydrate (assay:≥99.0%) and carbon powder (Graphitised carbon black)
were
purchased from Aldrich. Titrisol® (standard KOH solution) and Hydrochloric Acid (HCl,
35%) were purchased from Merck.
Methods
Phosphine gas generation:
Phosphine gas was prepared by modified literature procedure.1 In a typical procedure, 8 gm
Ca3P2 was loaded in a 100 mL three necked flask and degassed with purging of Argon gas for
10 min. Then 10 mL of 4 M HCl was added to the flask. Released PH3 gas was passed
through CaCl2 before purging to the reaction medium to remove the moisture and the
unreacted gas from the reaction system is trapped by aqueous CuSO4 trap.
Gram-Scale synthesis of Co2P:
In a typical process, 2 gm of CoCl2.6H2O and 60 mL oleylamine were loaded in a 200 mL
three necked round bottom flask and the reaction mixture was degassed by purging Argon gas
at 130 °C for 30 min. Then the temperature was increased to 150 °C and kept at this
temperature for another 30 min and further heated to 230 °C and ex-situ produced PH3 gas
was passed through the reaction mixture. Immediate change of the colour of the solution from
bluish to blackish indicated the formation of Co2P and annealed for 30 min at 200 °C. Finally
the reaction was cooled down and the materials were purified using ethanol followed by
washing with chloroform and acetone as solvent-non solvent pair and finally dispersed in
chloroform.
Synthesis of IrO2/C:
The Synthesis of colloidal IrO2/C (20 wt %) was carried out by following a reported
procedure.2 In detail, 0.1 g of K2IrCl6 was added to 50 ml aqueous solution of 6.3×10-4 M
sodium hydrogen citrate sesquihydrate (0.167 g). The red-brown coloured solution was taken
S2
and the pH was adjusted to 7.5 by adding 0.25 M NaOH solution and heated at 95 °C in an
oil bath under continuous stirring. After 30 minute, the solution was cooled to room
temperature and again the pH adjusted to 7.5 by adding 0.25 M NaOH. The adjustment of pH
and heating at 95 °C was repeated until the pH value had stabilized to 7.5. Then 0.184 g of
carbon powder (Graphitised carbon black, Aldrich) added to the solution and well dispersed
properly by ultrasonication. After that the mixed solution was refluxed at 95 °C connected to
a reflux condenser for 2 hour with bubbling of O2 gas. The solution was cooled down to room
temperature and vacuum dried at 70 °C. In order to remove the organic contaminant, the as
dried IrO2/C sample was annealed at 300 °C for 30 minute. Then the sample was washed
repeatedly with double distilled water, dried at 60 °C and stored in vacuum desiccator for
future use.
Synthesis of Co3O4:
The Co3O4 was synthesized by following a Solvothermal synthetic route6 taking Co
(NO3)2.9H2O (0.6 g) and 3.3 g of sodium dodecyl benzene sulfonate (SDBS) in 40 ml of
absolute ethanol. The as prepared suspension was poured into a 50ml Teflon lined stainless
steel autoclave and placed in a hot air oven at 180°C for 6 hour. After cooled down to room
temperature, the obtained precipitates were washed repeatedly with deionised water and
ethanol followed by centrifugation at 4000 rpm to remove the unreacted precursors and dried
in vacuum oven at 60°C. Then the as collected sample was calcined at 250°C for 4 hour and
stored in a vacuum desiccator for future use. After synthesis the powder XRD data was
recorded to conform the formation of the Co3O4 and is presented in Figure S10.
Characterizations of Materials
The X-ray diffraction (XRD) study of the samples were carried out using a Bruker D8
Advance powder diffractometer, using Cu Kα (λ= 1.54 Å) as the incident radiation.
Transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were
captured on a UHR FEG-TEM, JEOL JEM-2100F electron microscope using a 200 kV
electron source and also low resolution TEM images were taken in JEOL JEM-1400 plus
using 120 kV electron source. The specimens were prepared by dropping a drop of
nanocrystal solution, dispersed in chloroform onto a carbon coated copper grid, and the grid
S3
was subsequently dried in air and stored in desiccator. ICP-OES experiment performed by
Perkin Elmer Optima 2100DV instrument.
Procedure for the removal of iron from 1M KOH:
Here the iron content in the as prepared 1 M KOH solution was removed by using the
procedure followed by literature method.3 In a typical procedure first the Co (OH)2 was
prepared by adding 2 gm of Co(NO3)2. 6H2O to 4 ml of deionised water followed by addition
of 20 mL of 0.1 M KOH solution. After shaking, the mixture was centrifuged at 4000 rpm
for 5 minute and the as obtained supernatant was decanted. The precipitate was repeatedly
washed with the deionised water by mechanical agitation and centrifugation for three times.
After the third washing the as prepared Co(OH)2 was used to purify 1 M KOH solution. 45
mL of 1 M KOH taken in a 50 mL centrifuge tube and to it the prepared Co(OH)2 added,
mechanically agitated for 10 minute to absorb the iron impurities. Then the brown coloured
precipitate formed was separated by centrifugation and the purified KOH was collected in a
separate centrifuge tube. Again by following the purification process the as decanted 1 M
KOH was washed for three times. After third purification, the KOH solution was used as the
iron free electrolyte for the electrochemical oxygen evolution reaction.
Electrochemical measurements:
The electrochemical behaviour of the samples towards the oxygen and hydrogen evolution
reaction was investigated by using a Bio-logic electrochemical work station equipped with a
rotor (Pine instruments, USA). All the electrochemical measurements were recorded in a two
compartment three electrode electrochemical cell with a bare platinum wire, saturated
calomel (SCE, containing saturated KCl solution) and sample modified Glassy carbon
rotating disk electrode (GC-RDE, geometric surface area: ~0.196 cm2) as the counter,
reference and working electrode.
Here the obtained current is normalised over the
geometrical surface area of the GC-RDE. At first the GC-RDE was polished with slurry of
alumina powder (1 µ, 0.3 µ and 0.05 µ sequentially), washed with deionised water followed
by ultrasonication in a bath sonicator for 10 minutes. After proper drying in the vacuum
desiccator, the GC-RDE was pre-treated electrochemically with 20 CV cycles at a potential
range of 0.5 to 1.9 V vs. RHE (at a scan rate of 100 mV s-1) in 1 M KOH to enhance the
hydrophilicity and wettability of the surface. Finally, the electrode was modified with the
catalyst ink of loading 0.2 mgcm-2 and dried properly in a vacuum desiccator prior to the
S4
measurement. Here, the catalyst ink was prepared by ultra-sonicating the mixture of as
synthesized Co2P and carbon black (in 2:1 proportion) and nafion in absolute ethanol. All the
polarization data were recorded at a sweep rate of 5 mV s-1 with rotation speed of 1600 rpm.
Initial 5 cycles of cyclic voltammetric scans (1.20–1.65 V vs. RHE) were used to activate the
catalysts. The potentials in this study were recorded by SCE and presented in reversible
hydrogen electrode (RHE) potential scale by using the following Nerst equation, 2
ERHE= ESCE + 0.0591(pH) + 0.244V …………………………… (1)
1 M KOH (pH=14) and 0.5 M H2SO4 (pH=0.3) are taken as the supporting electrolyte for
OER and HER respectively and 0.244 is the standard potential for the SCE.
The electrochemical Nyquist impedance spectroscopy was performed in a frequency range of
100 kHz to 0.1 Hz (in 1 M KOH for OER and 0.5 M H2SO4 for HER). All the potentials
presented here were iR compensated potentials denoted as E−iR, where “i” is the current and
“R” is the uncompensated ohmic electrolyte resistance determined from the Nyquist
impedance measurements. In this work the value of “R” was found to be 6.45 Ω and 5.72 Ω
in 1 M KOH and 0.5 M H2SO4 respectively. The correction was carried out to get the
intrinsic electrocatalytic activity of the electrode by eliminating the negative influence of the
electrolyte and catalyst resistance on the electrochemical behaviour. The Tafel slopes were
derived from the polarisation curves and were calculated according to the following Tafel
equation,
ղ = a + b log j ………………………………………………..….. (2)
Here ղ, a, b and j were denoted as the over potential, Tafel constant, the Tafel slope and the
current density. For OER, the over potential (ղ) was calculated by subtracting 1.23 V
(standard potential for water oxidation) using the following equation,
η = E (vs. RHE)−1.23 ...........…………………………………………………….……… (3)
Whereas in case of HER, the observed potential was subtracted from 0V. For both the cases,
the long term stability of the material was carried out by employing the chronopotentiometry
technique at 10 mA/cm2 current density.
S5
Electrical Double Layered Capacitance measurement:
Further the electrochemical accessible surface area (ECSA) was determined in terms of the
double layer capacitance (Cdl). For this, the GC-RDE was modified with the Co2P/C and
IrO2/C as discussed above with the catalyst loading of 0.2mg/cm2. By taking the bare Pt wire
and Ag/AgCl as the counter and reference electrode the cyclic voltamograms were recorded
in 1 M KOH electrolyte. Typically the CVs are recorded at different scan rate from 10 to 200
mV/s in a non-Faradic potential region (0.2 to 0.26 V vs. Ag/AgCl). In this small potential
region, the current only originates from the double layer charging and discharging instead of
the charge transfer reaction. Since the double layer current is directly proportional to the scan
rate as,
i= ν Cdl
So, the Cdl was derived as the average value of linear fitted slope by plotting both the anodic
and cathodic current at 0.23 V against the scan rate. Then the ECSA was calculated as
Here Cs is the specific capacitance of an atomically smooth surface of the material in the
same electrolyte conditions. For this estimation, the Cs is 0.04 mF cm-2.
4,5
After getting the
value of ECSA, the Rf been calculated simply dividing the geometrical surface area of the
working electrode.5
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Figure S1. Schematic presentation of the reaction set up for gram scale synthesis.
Figure S2. Wide view TEM image of needle type shape of Co2P nanostructures.
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Figure S3. (a) LSV of first and second scans using Co2P/C electrode in 1 M KOH and (b)
corresponding Tafel plots.
Figure S4. (a) Linear sweep voltamograms of Co2P/C before and after iR correction. (b)
Nyquist plot for Co2P/C for OER and the inset showing the circuit diagram.
S8
Figure S5. OER polarisation curves of the Co2P/C catalyst measured in 1 M KOH with
sweep rate 5 mVs-1 (catalyst loadings: ~0.2 mgcm-2). On average 310 mV overpotential
require to reach 10 mAcm-2 current density. (Standard deviation: 7 mV)
Figure S6. TEM image of Co2P obtained from the sample collected after OER.
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Figure S7. (a, c and e) CVs for Co2P/C, IrO2/C and Co3O4/C in 1 M KOH at different scan
rate (10 to 200 mVs-1) and (b, d and f) plot of current at 0.23 V vs. scan rate to determine the
double layer capacitance(Cdl) and the roughness factor (Rf).
S10
Figure S8. LSVs for the HER by Pt/C and Co2P/C at 5 mV/s in 0.5 M H2SO4 before and after
iR correction electrolyte.
Figure S9. (a) Long-term stability test for the HER by Co2P/C on glassy carbon plate in 0.5
M H2SO4 at 10 mAcm-2. (b) The LSV before and after stability test. Inset in (a) is the
photograph of the modified electrode before and at the time of stability test.
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Figure S10. Powder XRD data of the synthesised Co3O4.
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Table S1: Comparison of OER activities of Co2P with literature
Materials
Tafel
Slope
mV/dec
Electrolyte
(M, KOH)
Overpotential
(mV) at10
mA/cm2
Catalyst
Loading
mg/cm2
References
310
Onset
Potential
(V vs.
RHE)
-
Co2P
50
1
0.2
This Work
CoP NR/C
71
1
340
---
0.71
7
CoP/C
66
0.1
360
1.48
0.05(Co)
8
CoP-MNA
65
1
290
---
6.2
9
Ni2P Nanowires
60
1
400
1.54
0.1
10
Ni2P Nanoparticles
70
1
500
1.61
0.1
11
Ni2P
47
1
290
---
0.14
11
40
1
367
---
NiO/NiFe2O4
51
1
>375
---
0.5
13
N-CG–CoO
71
1
340
1.55
-----
14
IPNT
43
1
288
1.48
1.6
15
39
1 (NaOH)
325
1.48
LiCoO2
52
0.1
---
1.55
CoMn LDH
43
1
324
1.5
0.142
18
Au@Co3O4
60
0.1
310
1.53
0.064
19
NixCo3-xO4
64
1 (NaOH)
---
>1.53
2.3
20
Mn0.1Ni1
---
0.1
420
1.59
----
21
NiFe-LDH/CNT
31
1
---
1.45
Ni2/3Fe1/3-rGO
40
1
210
---
NiCo LDH
Nanoplates
Amorphous Ni-Co
Oxide
0.17
----0.25
(oxide)
1
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