Supporting Information
Wiley-VCH 2012
69451 Weinheim, Germany
Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in
both Acidic and Basic Solutions**
Heron Vrubel and Xile Hu*
anie_201207111_sm_miscellaneous_information.pdf
S1
Chemicals and Reagents
All manipulations were carried out under an inert N
2
atmosphere using standard Schlenk and/or
glovebox techniques unless otherwise mentioned. Mo
2
C and MoB (-325 mesh, 99.5%) were
purchased from Aldrich and stored under nitrogen. Mo
2
C should be stored under N
2
. Nickel
(99.98%) and platinum (99.95%) wires were purchased from Advent Research Materials. A
commercial Pt electrode with a diameter of 2 mm was bought from CH Instruments and polished
with alumina powder prior to use. Unless noted, all other reagents were purchased from
commercial sources and used without further purification.
Physical methods
GC measurement was conducted on a Perkin-Elmer Clarus 400 GC with a TCD detector and a 5
Å molecular sieves packed column with Ar as a carrier gas. SEM secondary electron (SE)
images were taken in a Phillips (FEI) XLF-30 FEG scanning electron microscope.
Electrochemical measurements were recorded by an EG&G Princeton Applied Research
Potentiostat/Galvanostat model 273 or an IviumStat electrochemical analyzer. A three-electrode
configuration was used. For polarization and electrolysis measurements, a platinum wire was
used as the auxiliary electrode and an Ag/AgCl (KCl saturated) electrode was used as the
reference electrode. Potentials were referenced to a reversible hydrogen electrode (RHE) by
adding a value of (0.197 + 0.059pH)V. Ohmic drop correction was performed using the current
interrupt method. Pressure measurements during electrolysis were performed using a
SensorTechnics DSDX0500D4R differential pressure transducer. Pressure data was recorded
using an A/D Labjack U12 interface with a sampling interval of 1 point per second. ICP-OES
analysis was performed with an Optima 2000 spectrometer (Perkin-Elmer). Molybdenum and
Boron contents were determined using the intensity of the following emission lines: Mo -
203.845; B - 208.957. Standards TraceCERT were purchased from Aldrich and were used for
calibration. For the determination of the catalyst loading, the surface of the carbon paste
electrode was dissolved in hot aqua regia prior to measurement. XRD measurements were
carried out on an X'Pert Philips diffractometer in Bragg-Brentano geometry with CuK
α
1
radiation
and a fast Si-PIN multi-strip detector
(0.1540 nm).
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XPS measurements were performed at SuSoS AG (Surface Solutions – Switzerland) using a
PhI5000 VersaProbe spectrometer (ULVAC-PHI, INC.) equipped with a 180° spherical
capacitor energy analyzer and a multi-channel detection system with 16 channels. Spectra were
acquired at a base pressure of 5 x 10-8 Pa using a focused scanning monochromatic Al-Ka
source (1486.6 eV) with a spot size of 200 μm. The instrument was run in the FAT analyzer
mode with electrons emitted at 45° to the surface normal. Pass energy used for survey scans was
187.85 eV and 46.95 for detail spectra. Charge neutralisation utilizing both a cool cathode
electron flood source (1.2 eV) and very low energy Ar+–ions (10 eV) was applied throughout the
analysis.
The carbon paste pellets analyzed by XPS were prepared inside of a Glove Box filled with pure
nitrogen and activated in galvanostatic mode with a current density of 10 mA/cm
2
for 15
minutes. After activation, the pellets were washed with pure bidistillated water and let dry inside
of the Glove Box for 30 minutes. The pellets were transferred to a small desiccator inside the box
and transported under nitrogen for analysis. The samples were exposed briefly to air (≈1 minute)
during the transfer to the XPS antechamber.
Fabrication of electrodes
(a) Preparation of carbon paste electrode
8 g of powdered synthetic graphite (<20 μm) and 2 g of white paraffin wax were placed in a
round-bottom flask. 40 mL of hot toluene was added to the flask. The mixture was sonicated in
an ultrasonic bath for 5 minutes. The solvent of the resulting solution was removed under
vacuum to yield a conductive graphite powder. The powder was pressed to fill the empty body of
a home-made electrode, to give the carbon paste electrode. The homemade electrode consists of
a PEEK tube with a back contact made of brass. The back contact can be screwed to allow the
removal of the carbon paste pellet for surface analysis. The active area of the electrode is 0.1964
cm
2
(5 mm diameter).
(b) Preparation of MoB and Mo
2
C-modified electrodes
The surface of the carbon paste electrode was cleaned using a weighing paper. Powdered Mo
2
C
and MoB were pressed against the soft surface of the carbon paste electrode and were spread
evenly on the surface using a weighing paper.
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(c) Preparation of disk electrodes
0.450 g of powdered Mo
2
C and MoB were mixed with 0.050 g of Teflon powder (1μm). The
mixture was pressed in a conventional KBr pelletizer under 10 Tons to produce a 13mm
diameter pellet. A copper wire contact was glued to one side of the pellet using silver conductive
epoxy glue (CircuitWorks CW2400 - Chemtronics). A 5mm hole mask was glued to the other
side of the electrode to limit the surface area. With the exception of the active area, the whole
body of the electrode was insulated with molten polypropylene.
Pure Molybdenum rod (99.95% / 4mm diameter) was acquired from Advent Research Materials
and used to fabricate a 0.1257 cm
2
electrode.
Preparation of the electrolytes:
1M H
2
SO
4
was used as electrolyte for measurements at pH = 0. A phosphate buffer (metrohm
standard) was used for the experiments at pH = 7. 1M KOH was used as electrolyte for the
measurements conducted at pH = 14.
Polarization measurements
Polarization curves were measured in a T-shape cell under nitrogen. The Pt counter electrode
was separated from the main compartment by a porous glass frit (porosity 3). The Ag/AgCl
reference electrode was kept as close as possible from the working electrode. The scan rate was 1
mV s
-1
. Ohmic drop was corrected by the potentiostate or manually using the current interrupt
method or corrected mathematically as follows:
The overpotential
η
(V) observed during an experiment is given by equation (1):
η
= a + bln j + jR (1)
where a (V) is the Tafel constant, b (V dec
-1
) is the Tafel slope, j (A cm
-2
) is the current density and R
(Ω cm
2
) is the total area-specific uncompensated resistance of the system, which is assumed to be
constant. The derivative of Eq. (1) with respect to current density gives Eq. (2) from which b and R
can be easily obtained by plotting d
η
/dj as a function of 1/j.
d
η
/dj=b/j + R (2)
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The estimation of R allows correcting the experimental overpotential by subtracting the ohmic drop
jR according to equation (3):
η
corr =
η
− jR (3)
During the calculations, the derivative d
η
/dj was replaced by their finite elements Δ
η
/Δj estimated
from each pair of consecutive experimental points.
The overpotential was calculated as the difference of the operating potential and the
thermodynamic potential (RHE = -0.059 x pH V vs. NHE). Ag/AgCl = 0.197 V vs. NHE.
The current-potential curves for Pt were corrected against the real surface area of Pt electrode,
determined by the hydrogen adsorption peaks between -100 and -150 mV vs Ag/AgCl (J. M. D.
Rodríguez, J. A. H. Melían, J. P. Peña, J. Chem. Ed. 2000, 77, 1195-1197.)
Galvanostatic Electrolysis
Electrolysis experiments were performed in an H shape cell. A total liquid volume of 30 mL was
used to fill the cell. The headspace is 8.2 mL. The platinum counter electrode was separated from
the solution through a porous glass frit (porosity 3) and this whole assembly inserted into one
side of the H cell. The modified working electrode was inserted in the other side of the cell,
together with a magnetic stirring bar and a Luggin capillary. Solution resistance was not
corrected.
Two small inlets were present in the cell allowing the connection to the pressure monitoring
device and the other kept closed by a septum for sampling of the gas phase. The whole cell
apparatus is gas-tight and the pressure increase is proportional to the gases generated (H
2
+ O
2
).
Prior to each experiment, the assembled cell was calibrated by injecting known amounts of air
into the closed system and recording the pressure change. After the calibration, the cell was
purged with nitrogen for 20 minutes and the measurements were performed. Control experiments
were performed using platinum as a working electrode and a quantitative Faradaic yield was
obtained by measuring the pressure (97-102 %) and confirmed by GC analysis of the gas in the
headspace (92-96 %) at the end of the electrolysis.
The Faradaic yield was calculated as follow: