Electronic Supporting Information

Synthesis of Amorphous Boride Nanosheets by Chemical Reduction of Prussian Blue

Analogs for Efficient Water Electrolysis

Ting He, a Jean Marie Vianney Nsanzimana, b Ruijuan Qi, c Jun-Ye Zhang, a Mao Miao, a Ya Yan, d Kai Qi, a

Hongfang Liu, a and Bao Yu Xia a,*

a Key laboratory of Material chemistry for Energy Conversion and Storage (Ministry of Education), School

of Chemistry and Chemical Engineering, Wuhan National Laboratory for Optoelectronics, Huazhong

University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan 430074, PR China

b School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Avenue,

Singapore 639798, Singapore.

c School of Information Science and Technology, East China Normal University, 500 Dongchuan Road,

Shanghai 200241, PR China

d School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516

Jungong Road, Shanghai 200093, PR China

Corresponding author: byxia@hust.edu.cn (B. Y. Xia)

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2018

Experimental Section

1、Material

Cobalt chloride hexahydrate, sodium citrate, sodium borohydride, and commercial Platinum (20%) on

graphitized carbon (20% Pt/C) were purchased from Sigma-Aldrich. Potassium hydroxide from J. T. Baker

and Nafion 5% in lower aliphatic alcohols and water was purchased from Sigma-Aldrich. All chemical

reagents were used in the experiments directly without further treatment.

2、Method

Synthesis of Co-Ni PBA stacked nanoplates: In a typical procedure, 1.6 mmol of cobalt chloride hexahydrate

and 0.6 mmol of sodium citrate were dissolved in 40 mL of DI water to form solution A. 0.8 mmol of

potassium tetracyanonickelate (II) was dissolved in 40 mL of DI water to form solution B. Next, blend

solutions A and B under magnetic stirring for 5 min. The obtained homogeneous solution was aged for 24 h

at room temperature. After collected by centrifugation and washed several times with water and ethanol, the

precipitates were dried at 60 oC overnight.

Synthesis of metal boride(oxide): The as-obtained CNBO-NS were prepared by chemical reduction of the

as-prepared PBA stacked nanoplates at room temperature. In a typical synthesis, 60 mg Co-Ni PBA was

dissolved in 20 mL DI water under a continuous and vigorous stirring to form a homogeneous mixture.

Afterward, 20 mL of sodium borohydride (NaBH4) solution (0.1 mol L-1) was added and the mixture was

maintained at room-temperature for 1.5 hours under stirring. The final products were collected by centrifuging

and washing with DI water for several times, and then dried overnight for further characterizations. Different

metal borides were synthesized by at a range of reduction times.

Characterizations. The crystal phase of metal borides was characterized by X-ray diffraction (XRD) patterns

on Empyrean (PANalytical B.V. with Cu-Kα radiation). The scanning electron microscopy (SEM) was carried

out on JEOL JSM-7100F at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) was

performed on TecnaiG2 20 (Philips) at an accelerating voltage of 200 kV assembled with a linear scanning

EDX spectrum. X-ray photoelectron spectroscopy (XPS) experiment was implemented on a Kratos AXIS

Ultra DLD-600W XPS system with a monochromatic Al Kα (1486.6 eV) X-ray source.

Electrochemical Measurements: All the electrochemical tests were carried out in a three-electrode system

on an electrochemical workstation (Autolab 302N). In the three-electrode configurations, a glassy carbon

electrode was served as the working electrode, while the calibrated Hg/HgO electrode (1.0 M KOH) and a

graphite rod were used as the reference electrode and the counter electrode, respectively. All measured

potentials were converted to the reversible hydrogen electrode (RHE), according to the equation; ERHE =

EHg/HgO + 0.098 + 0.059*pH. A homogeneous catalyst ink dispersion was obtained by sonicating the samples

(5 mg) and Nafion/alcohol mixed solution (500 μL). The prepared ink (10 μL) was dropped onto the working

electrode and dried at room temperature naturally, the mass loadings of the catalyst were about 0.5 mg cm-2.

The electrochemical measurements were investigated in 1.0 M KOH medium. Linear sweep voltammetry

(LSV) measurements were performed at the scan rate of 5 mV s-1 at a potential range 0 V to -0.4 V for

cathodic hydrogen evolution and from 1.3 V to 1.7 V for anodic oxygen evolution. The overall electrochemical

water splitting was detected by LSV measurements at the potential from 1.2 to 2.0 V. The stability test was

performed at a current density of 10 mA cm-2 for 24 h. All the data presented were corrected for iR losses and

background current. Electrochemical impedance spectroscopy (EIS) detection was conducted in the frequency

range from 100 kHz to 0.01 Hz with an ac perturbation of 5 mV when the potential was 1.55 V vs RHE. The

electrochemical double-layer capacitance was determined from the CV curves measured in a potential range

without redox processes according to the following equation: Cdl = Ic/ν, where Cdl, Ic, and ν are the double-

layer capacitance (mF cm-2) of the electroactive materials, charging current (mA cm-2), and scan rate (mV s-

1). Hence, cyclic voltammogram (CV) cycles used for Cdl determination was performed in the potential range

from 0.22 V to 0.12 V at the scan rate of 20, 40, 60, 80, 100, 200 mV s-1, respectively. And the drainage

method is applied in faradaic efficiency test. The faradaic efficiency calculation efficiency is: F=(m*n*F)/Q,

m denotes the moles of product, n denotes transferred electrons, F is faradaic constant and Q is the consumed

charge.

Figure and Captions

Figure S1. (a) SEM and (b) XRD of Co-Ni PBA precursor.

Figure S2. XRD patterns of amorphous metal borides annealed at different temperatures.

.

Figure S3. SEM images of CNBO-NS abtained at 0.5h (a,b) and 3h (c,d).

Figure S4. (a) TEM image and corresponding, inset of (a) is HRTEM, (b) EDX spectrum accompanying the

atomic ratio of corresponding elements.

Figure S5. XPS spectra of Ni 2p (a) and Co 2p (b) of Co-Ni PBA precursor.

Figure S6. XPS spectra of Ni 2p (a), Co 2p (b), B 1s (c) and O 1s (d) of CNBO-NS product obtained at 3h

reduction time.

Figure S7. (a) Polarization curves, (b) corresponding Tafel plots and (c) Nyquist plot EIS of CNBO-NS

obtained at different reaction time.

Figure S8. Polarization curves of CNBO-NS and their annealed samples at different temperatures.

Figure S9. Electrochemical capacitance measurements to determine the surface area of the obtained electrodes

in 1.0 M KOH electrolyte. The capacitive current density on CNBO-NS obtained at 1.5h (a) and 3h (b) from

double layer charging can be measured from cyclic voltammograms in a potential range where no Faradic

reaction occur, and (c) the measured capacitive current plotted as a function of scan rate.

Figure S10. Electrochemical capacitance measurements to determine the surface area of the obtained

electrodes in 1.0 M KOH electrolyte. The capacitive current density on CNBO-NS obtained at 1.5h (a) and

PBA precursor (b) from double layer charging can be measured from cyclic voltammograms in a potential

range where no Faradic reaction occur, and (c) the measured capacitive current plotted as a function of scan

rate.

Figure S11. (a) Polarization curves and (b) corresponding Tafel plots of the amorphous boride of different

reaction time in 1.0 M KOH solution.

Figure S12. Tafel plots of the amorphous boride and Co-Ni PBA in 1.0 M KOH solution.

Figure S13. Faradic efficiency of hydrogen evolution.

Figure S14. (a) LSV of CNBO-NS couple (without iR corrected), PBA couple and Pt/C//IrO2. (b)

Chronopotentiometry test during 24h electrolysis by the CNBO-NS pair. Inset is the photograph of CNBO-

NS couple assembled electrolysis cell.

Movie S1

Movie S1. CNBO-NS pair electrolysis at current density=10 mA cm-2 in 1.0 M KOH solution.

Table S1. OER activity of recent reported catalysts.

Catalysts Electrolyte Overpotential

(η10)（mV）

Tafel slope (mV dec-1)

Reference

CNBO-NS 1.0 M KOH 300 60 This work

Co2B-500 0.1 M KOH 380 45 Adv. Energy Mater. 2016, 6, 1502313

Co–Ni–B@NF 1.0 M KOH 339 131 J. Mater. Chem. A 2017, 5, 12379

Ni-Co mixed oxide cages

1.0 M KOH 381 50 Adv. Mater. 2016, 28, 4601

FeNi@single layer graphene

1.0 M KOH 280 70 Energy Environ. Sci. 2016, 9, 123

Co3O4/NiCo2O4 nanocages

1.0 M KOH 340 88 J. Am. Chem. Soc. 2015, 137, 5590-5595

Hollow Co3S4 nanosheets

0.1 M KOH 363 90 ACS Nano 2014, 8, 10909-10919

NixB-300-GC 1.0 M KOH 380 / Adv. Energy Mater. 2017, 7, 1700381

Co-Bi NS/G 1.0 M KOH 290 53 Angew. Chem. Int. Ed. 2016, 55, 2488-2492

Ni-Bi@NB 1.0 M KOH 302 52 Angew. Chem. Int. Ed. 2017, 56, 6572-6577

CoP@N-doped Graphene

1.0 M KOH 338 54 Nanoscale 2016, 8, 10902

FeB2 1.0 M KOH 296 52.4 Adv. Energy Mater. 2017, 7, 1700513

Ni3Se2-GC 1.0 M KOH 310 79.5 Energy Environ. Sci. 2016, 9, 1771

NiCoP/NF 1.0 M KOH 320 37 Nano Lett. 2016, 16, 7718

NiCo LDH 1.0 M KOH 367 / Nano Lett. 2015, 15, 1421

Table S2. HER activity of recent reported catalysts.

Catalysts Electrolyte Overpotential

(η10)（mV）

Tafel slope (mV dec-1)

Reference

CNBO-NS 1.0 M KOH 140 116 This work

(Ni,Co)Se2-GA 1.0 M KOH 127 79 ACS Catal. 2017, 7, 6394-6399

NiCoFe LDHs 1.0 M KOH 200 70 ACS Energy Lett. 2016, 1, 445

Co–Ni –B@NF 1.0 M KOH 205 / J. Mater. Chem. A. 2017, 5, 12379

NiBx film 1.0 M KOH 135 88 Nano Energy 2016, 19, 98

Ni-Co-P-300 1.0 M KOH 150 60.6 Chem. Commun. 2016, 52, 1633-1636

CoP nanowire arrays 1.0 M KOH 209 129 J. Am. Chem. Soc. 2014, 136, 7587-7590

Co2B-500 1.0 M KOH 127 92.4 Adv. Energy Mater. 2016, 6, 1502313

NiB0.54@Ni Foil 1.0 M KOH 306 / Nano Energy 2016, 19, 98

FeCoNi 1.0 M KOH 149 77 ACS Catal. 2017, 7, 469

NiCo2S4 NWs/NF 1.0 M KOH 210 58.9 Adv. Funct. Mater. 2016, 26, 4661

CP@Ni-P 1.0 M KOH 117 85.4 Adv. Funct. Mater. 2016, 26, 4067

Ni-P 1.0 M KOH 350 132.3 Chem. Commun. 2016, 52, 1633-1636

Ni3Se2 nanoforest 1.0 M KOH 203 79 Nano Energy 2016, 24, 103

Table S3. Cell voltage of different catalysts for overall water electrolysis.

Catalysts Electrolyte Cell voltage (V)

Current density (mA cm-2)

Reference

CNBO-NS 1.0 M KOH 1.69 10 This work

(Ni,Co)Se2 1.0 M KOH 1.69 10 ACS Catal. 2017, 7, 6394-6399

NiSe 1.0 M KOH 1.63 10 Angew. Chem. Int. Ed. 2015, 54, 9351

NiFe LDH/NF 1.0 M KOH 1.7 10 Science 2014, 345, 1593

NiCo2O4 hollow microcuboids

1.0 M KOH 1.65 10 Angew. Chem. Int. Ed. 2016, 55, 1

Co–Ni –B@NF 1.0 M KOH 1.72 10 J. Mater. Chem. A 2017, 5, 12379

Co/NBC-900@NF 1.0 M KOH 1.68 10 Adv. Funct. Mater. 2018, 28, 1801136

Co2B-500 3.0 M KOH 1.81 10 Adv. Energy Mater. 2016, 6, 1502313

CoMnCH 1.0 M KOH 1.68 10 J. Am. Chem. Soc. 2017, 139, 8320

FeCoNi 1.0 M KOH 1.66 10 ACS Catal. 2017, 7, 469

Ni5P4 1.0 M KOH 1.7 10 Angew. Chem. Int. Ed. 2015, 54, 12361

Ni3S2/NF 1.0 M KOH 1.76 10 J. Am. Chem. Soc. 2015, 137, 14023