Hydrogen evolution activity and electrochemical stability of selected transition metal carbides in concentrated phosphoric acid

Article abstract of DOI:10.1016/j.electacta.2014.06.087

Alternative catalysts based on carbides of Group 5 (niobium and tantalum) and 6 (chromium, molybdenum and tungsten) metals were prepared as films on the metallic substrates. The electrochemical activities of these carbide electrodes towards the hydrogen evolution reaction (HER) in concentrated phosphoric acid were investigated in a temperature range from 80 to 170 °C. A significant dependence of the activities on temperature was observed for all five carbide samples. Through the entire temperature range Group 6 metal carbides showed higher activity than that of the Group 5 metal carbides, attributable to the different electronic structures. Tungsten carbide among the studied electrode samples exhibited the highest HER activity. Upon anodic potential scans in the presence of oxygen, chromium, tantalum and tungsten carbides displayed passivation due to the formation of stable surface layers whereas niobium and molybdenum carbides seemed to undergo corrosion.

Full text of DOI:10.1016/j.electacta.2014.06.087

Accepted Manuscript
Title: Hydrogen evolution activity and electrochemical
stability of selected transition metal carbides in concentrated
phosphoric acid
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Author: Antonio Luis Tomas-Garcıa Jens Oluf Jensen Niels J.
Bjerrum Qingfeng Li
PII:
S0013-4686(14)01269-9
DOI:
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EA 22949
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6-4-2014
12-6-2014
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Please cite this article as: Antonio Luis Tomas-Garcia, Jens Oluf Jensen, Niels J.
Bjerrum, Qingfeng Li, Hydrogen evolution activity and electrochemical stability of
selected transition metal carbides in concentrated phosphoric acid, Electrochimica Acta
(2014), http://dx.doi.org/10.1016/j.electacta.2014.06.087
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Hydrogen evolution activity and electrochemical
stability of selected transition metal carbides in
concentrated phosphoric acid
Antonio Luis Tom´as-Garc´ıa, Jens Oluf Jensen, Niels J. Bjerrum, Qingfeng
Li1,∗
Department of Energy Conversion and Storate, Technical University of Denmark,
Kemitorvet 207, DK-2800 Kgs. Lyngby, Denmark
Abstract
Alternative catalysts based on carbides of Group 5 (niobium and tantalum)
and 6 (chromium, molybdenum and tungsten) metals were prepared as films
on the metallic substrates. The electrochemical activities of these carbide
electrodes towards the hydrogen evolution reaction (HER) in concentrated
phosphoric acid were investigated in a temperature range from 80 to 170°C.
A significant dependence of the activities on temperature was observed for all
five carbide samples. Through the entire temperature range Group 6 metal
carbides showed higher activity that that of the Group 5 metal carbides,
attributable to the different electronic structures. Tungsten carbide among
the studied electrodes samples exhibited highest HER activity. Upon anodic
potential scans in the presence of oxygen, chromium, tantalum and tungsten
carbides displayed passivation due to the formation of stable surface layers
whereas niobium and molybdenum carbides seemed to undergo corrosion.
∗Corresponding author. Telephone number: +45 45 25 23 18
Email address: qfli@dtu.dk (Qingfeng Li)
1IES member
Preprint submitted to Electrochimica Acta
June 20, 2014
Page 1 of 36

Keywords:
metal carbides, activity and stability, hydrogen evolution reaction,
phosphoric acid, high temperature
1. Introduction
An important issue in the field of renewable energy technologies is ef-
fective energy storage over time [1], with different competing technological
scenarios under consideration. Hydrogen, in its role as energy carrier for
electrochemical energy conversion technologies such as fuel cells and electrol-
ysers, is proposed as a reliable, secure and clean option in association with
renewable energy sources. Particularly, water electrolysis in proton exchange
membrane (PEM) cells provides favorable and practical means for hydro-
gen production using surplus electric energy. A significant challenge for the
widespread use of water electrolysis for hydrogen production is the economic
costs of this technology, related in part to the high price and scarce avail-
ability of Pt and related noble metals (such as Ir), which are the state of the
art catalyst materials for PEM electrolysers.
There is intensive research towards the investigation and development of
highly active and cost-effective catalysts for electrolysis[2, 3, 4]. Group 6
transition metal carbides, among others, have been known for years for their
catalytic properties, similar to those of noble metals [5, 6, 7, 8] and explored
for chemical[9] and electrochemical[10] applications.
The electrochemistry of WC in acidic aqueous media has been studied
by many authors. Early work [11, 12] was made to study the activity of the
material for hydrogen oxidation in sulfuric acid as well as the adsorption of
2
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hydrogen. Some work was also done regarding the behavior in hot phosphoric
acid [13]. The research in sulfuric acid media has shown that the activity
of WC itself as electrocatalyst for the hydrogen oxidation reaction (HOR)
showed significant activity, although lower than that of Pt[14].
Alternatively, carbides have been proposed to be used as a support for
noble metal catalysts. For example, WC-supported Pt catalysts exhibit a
synergistic effect with Pt for HOR[15, 16]. In this regard, an interesting
finding was the good resistance of WC supports and the corresponding sup-
ported catalysts towards CO poisoning[17], which this is a key issue for the
use of reformate hydrogen as fuel for both low and high temperature PEM
cells.
In connection with this, recent efforts were made to use metal carbides as
supports in catalysts for methanol oxidation[18, 19, 20, 21, 22, 23, 24]. CO
poisoning resistance is also relevant, with the carbide playing an analogous
role to that of Ru in methanol oxidation reaction. In addition to Pt, WC-
supported PdCo alloy nanoparticles were recently studied for formic acid
electrooxidation[25].
The oxidative stability of metal carbides is of critical concern, as the
oxidation occurs at potentials below the onset for the oxygen reduction re-
action (ORR). Nevertheless, it has been tested as support for Pt in ORR
electrodes[26, 27, 28, 29], where the presence of Pt seems to improve the cor-
rosion resistance of the carbide[30]. Moreover, other metals like Ag[31] and
Au-Pd[32] catalysts have also been investigated with carbides as the support.
Relatively limited work has been done to evaluate the catalytic activity of
metal carbides towards the hydrogen evolution reaction (HER). Using tung-
3
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sten carbides as supports for platinum catalysts Ham et al.[33] and Hsu et
al.[34] reported enhanced HER activity. In addition to tungsten carbides,
a few other Group 6 metal carbides have been investigated. Molybdenum
carbides seem to behave in a similar way as tungsten carbides as catalyst
supports[35, 36, 37], while chromium carbides show significant hydrogen evo-
lution activity in sulfuric acid[38]. In the case of Group 5 carbides, only lim-
ited research has done with e.g. TaC as support for IrO2 in oxygen evolution
electrodes[39].
Most of these studies were carried out in the lower temperature range.
Recent development of PEM technologies is aimed at operation at elevated
temperatures, above 100°C, for example, based on phosphoric acid doped
polybenzimidazole membranes[40] and phosphoric acid doped perfluorinated
sulfonic acid membranes for steam electrolysers[41, 42, 43]. Significant efforts
in the field are being made to explore solid acids e.g. CsH2PO4 as electrolytes
for intermediate temperature fuel cells[44]. A better chance might exist for
using metal carbides as catalysts in this temperature range[45]. In both
doped membranes and inorganic solid acids, phosphoric acids and phosphates
function as the proton conducting electrolyte. The electrochemical behavior
of metal carbides in these media, as demonstrated recently[46], shows an
important temperature dependency.
In the present work, the basic electrochemical behaviour of selected tran-
sition metal carbide surfaces in hot phosphoric acid were studied, focusing on
their electrocatalytical properties towards the hydrogen evolution reaction,
as well as their electrochemical stability in oxydizing conditions.
4
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2. Experimental
2.1. Electrode preparation
In order to obtain samples with controlled electrode area, films of five
metal carbides were prepared by using the parent metals as substrates. Bulk
carbide materials are not easy to prepare. In general, the insertion of carbon
in the metal structure requires the diffusion of carbon atoms from a carbon-
containing precursor, such as graphite or a hydrocarbon:
M(s) + C(s) → MCx(s)
(1)
(2)
M(s) + CmHn(g) → MCx(s) + H2(g)
This diffusion seems to have a high activation energy, and so all known
synthesis processes for metal carbide preparation require high temperatures,
as high as 1230℃. For instance, in the case of tungsten carbides, the solid-
solid W-C reaction[47] synthesis needs above 1200℃ to produce α-WC. In
this case, there is the added difficulty of controlling the amount of reactants to
have the right stoichiometric amount, lest significant amounts of non-desired
products are obtained.
In the case of carburization with carbon-containing gases, the tempera-
tures required are in general lower than for the solid-solid cases, in the order
of 750℃ and above. Another advantage over the solid-solid synthesis is that
generally better defined compositions are obtained, which is convenient for
good reproducibility in the synthesis and characterisation of these materi-
als. This class of synthesis is usually named the temperature programmed
reduction (TPRe) method in the literature[48, 49, 50, 51].
5
Page 5 of 36

In the present work, plates of Cr, Nb, Mo, Ta and W (Goodfellow Cam-
bridge Ltd.) were mechanically polished with SiC sandpaper (down to 4µm
particle size) and cleaned with NaOH solution in order to remove grease
and oxide impurities, and dried at 80°C in air. The plates were carbur-
ized with 20% vol. CH4 in H2, through the TPRe process as described
elsewhere[52, 53, 54, 55]. In a typical carburization, a W plate was placed in
a quartz oven under 300 mL/min CH4/H2 flow for 30 min at room temper-
ature. A temperature ramp of 8°C/min was then applied up to 880°C; the
final temperature was held for 1h. Other carbides were synthesized similarly,
albeit at different final temperatures, as listed in Table 1. After this, the
carburized plate was allowed to cool down to room temperature and stored.
2.2. Characterization
Reflexion X-ray diffraction (XRD) was used on the carburized plates. A
˚
Bruker D8 Advance diffractometer (Cu K, 1.54056 A) was used to detect
the obtained carbide phases. The surface morphology of the samples was
investigated with a scanning electron microscope (Carl Zeiss EVO MA10).
The electrochemical measurements were performed in a custom-made
borosilicate glass cell (Fig. 2). The electrolyte used was 100% H3PO4, pre-
pared by evaporation of 85% H3PO4 (Sigma-Aldrich) at 180°C to d(25°C)=1.88
g/mL. It is well known that, in the temperature range of the present study,
the water vapor pressure over concentrated phosphoric acid is quite signifi-
cant. In this setup, a water-cooled condenser was attached to the cell in order
to minimize water losses, allowing measurements up to 170°C. A reversible
hydrogen electrode (RHE) was used as the reference electrode and a Pt disk
as the counter electrode. The temperature in the cell was controlled with a
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hot plate within ±1°C. The electrolyte was purged with either pure H2 or O2
throughout the measurements.
HER activity measurements were performed in H2-saturated H3PO4. Lin-
ear scan voltammetry at 1 mV/s scan rate was used to extract the Tafel
equation parameters of the electrodes (j0 and Tafel slope):
RT
RT
η = 2.303
log(j0) − 2.303
log(j)
(3)
αCF
αCF
The electrode potential was corrected for the ohmic loss; this was esti-
mated from electrochemical impedance measurements, taking the real impedance
at 103 Hz as the cell ohmic loss. The activity measurements were performed
at 80, 120, 150 and 170°C, respectively.
The electrochemical stability of the carbide films in O2-saturated H3PO4
was studied as well. Cyclic voltammetry was used to study the effect of anodic
polarisation on the carbide films. 10 voltammetric cycles at 100 mV/s were
performed, with increasing anodic vertex potential after each experiment.
After the measurements, the electrodes were rinsed and dried in air at 80°C
for SEM imaging.
3. Results and Discussion
3.1. Physical characterization
The obtained XRD spectra for the carburized plates are shown in Figure
1. There are three main known chromium carbide phases (Cr3C2, Cr7C3 and
Cr23C6). Of these carbides, Cr3C2 is known to have high strength, good me-
chanical resistance and chemical stability. By reduction of chromium oxide
7
Page 7 of 36

with 30% CH4 in Ar at 850 to 1000°C, Cr3C2 was found to be the only car-
bide phase[56]. On the metal substrate, Figure 1 shows two major chromium
carbide phases, Cr7C3 and Cr3C2. No metallic chromium was detected, in-
dicating formation of a relatively thick carburized layer. Similarly, in the
case of tantalum only the TaC carbide phase was detected, with peaks at
2θ=34.908, 40.529, 58.728, 70.283 and 73.799◦.
For niobium, molybdenum and tungsten, also single carbide phases were
detected that were NbC, Mo2C and WC, respectively. A recent study using
the same mixture of methane and hydrogen at 880°C, both tungsten oxide
and nitride were found to be converted into WC[46]. With these three metal
substrates, however, the presence of the metal phase was also identified.
This is an indication that only a thin surface layer of the metal has been
transformed. The electron densities of metal and carbide phases dictates
in part the relative intensities of the corresponding XRD peaks, but the
thickness of the formed carbide layer is also important. A relatively thick
layer will effectively absorb or scatter the incoming X-Rays strongly enough
that no significant amount will reach the underlying metal.
In principle, the detection of metal in the XRD spectra could indicate
a partial coverage of the electrode with carbide phase; however, since the
carburizating agent, methane, has to react with the sample surface to form
the carbide, and excess methane is used in the synthesis, it is more likely
that the detect metal lies underneath the formed carbide phase, which is
completely covering the surface of the sample.
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Page 8 of 36

3.2. HER activity measurements
All five samples were studied as electrodes for the hydyogen evolution
reaction in hydrogen saturated 100% phosphoric acid at different temper-
atures. Figure 3 shows CV measurements on the carburized W film. At
80°C, the electrode showed little activity at potentials below 0.1 V versus
the reversible hydrogen reference electrode (RHE, the same below) while at
the same potential a noticeable cathodic current was observed at 120°C. As
the temperature was elevated to 150 and 170°C, a significant increase of the
electrode activity with temperature was observed at cathodic potentials. In
Figure 4, the voltammograms at 170°C for the different electrodes are shown.
By looking at the potentials of the anodic scan at a given current density (e.g.
-0.2 mA/cm2), it can be seen qualitatively that the order of HER activity is:
W>Cr>Mo>Ta>Nb. At anodic potentials, up to +0.2V no electrochemical
features other than capacitive current were observed.
Quantitative HER activity measurements were obtained in the same con-
ditions as for the cyclic voltammetry measurements. Linear scan voltamme-
try was performed under a quasi steady state of 1 mV/s scan rate. The Tafel
curves obtained for the tungsten carbide film electrode at different temper-
atures are shown in Figure 5, which confirming the strong dependence of
temperature as observed in Figure 3. The same trend was observed for
the other studied carbides, as shown in the Supplementary Materials. Tafel
curves for the different films are shown in Figure 6, indicating a differenti-
ation of the samples. It should be noted that, for these measurements, the
ohmic loss was corrected using AC impedance at 1kHz. Also, the measured
open circuit potential (OCP) was very close to 0V versus a RHE reference,
9
Page 9 of 36

as would be expected if the H+/H2 redox couple defined the electrode poten-
tial, which was, in short, considered equal to the HER reaction overpotential
for all activity measurements. Therefore, for each curve the corresponding
Tafel equation was fitted, from which the exchange current density and the
Tafel slope were calculated. In addition, the activation energy for the elec-
trode activities in the 80 to 170°C range was calculated by using the original
Arrhenius equation (Eq. 4). These results are listed in Table 1.
ꢀ
ꢁ
−Ea
j = A · exp
(4)
RT
A close look at Figures 4 and 6 shows interestingly that Group 6 element
(Cr, Mo, W) carbides have similar behaviors, with significantly higher HER
activity than that of Group 5 element (Nb, Ta) carbides. This difference
was observed across the whole range of temperatures in this study. Further
examination of the electrochemical activities of the five types of carbides
towards the HER was made by comparing the current densities at the po-
tential of -0.1V. The results through the entire temperature range are shown
in Figure 7 with similar activity order of the studied carbides as observed in
Figures 4 and 6. Also in Figure 7 is included the smooth platinum electrode
as a reference, which will be further discussed below. The calculated Tafel
equation parameters at 170°C, as well as the apparent activation energy for
the exchange current density, are compiled in Table 1.
HER mechanism on Group 6 carbide films. Although the HER activity of
tungsten carbide materials has been measured in sulfuric acid solutions[57]
as well as other, less acidic solutions, these studies have been mostly at or
close to room temperature. Earlier research on phosphoric acid fuel cells
10
Page 10 of 36

produced also some results, but these were limited to activity measurements,
e.g. the less stable W2C phase[13]. Here, a mechanism for hydrogen evo-
lution on the Group 6 metal carbides is proposed, taking into account the
information obtained in our experiments, as well as other prior research on
these materials.
In acidic conditions, the hydrogen evolution reaction is usually written
as:
2H+ + 2e− → H2(g)
(5)
Given the reactants and products involved, three basic steps can be pro-
posed in the HER or its reverse, the hydrogen oxidation reaction (HOR):
S + H+ + e− ꢀ Hads V olmer step
Hads + H+ + e− ꢀ H2(g) + S Heyrovsky step
Hads + Hads ꢀ H2(g) + 2S Tafel step
(6)
(7)
(8)
The HER will then occur through two basic routes: either Volmer step fol-
lowed by Tafel step (V-T mechanism), or Volmer step followed by Heyrovsky
step (V-H mechanism). In any case, the general reaction demands that there
has to be an electrochemical formation of adsorbed hydrogen atoms, followed
by desorption of H2 molecules. Hence the proposition of the two mentioned
mechanisms.
In the cases that the Heyrovsky step is rate determining, a 40 mV Tafel
slope (at room temperature) is expected[58], which can increase at higher
current densities to 120 mV. This corresponds to ca. 60 mV at 170°C. The
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WC film electrodes show Tafel slopes close to 60 mV at this temperature.
This would indicate electrochemical desorption as the rate determining step
(rds) of the reaction for this material.
Similar slopes were obtained for carburized chromium and molybdenum,
although slightly higher. While the experimental error for the measurement
of the slopes allows for some margin of error, as well as small temperature
variations within the cell, it is also possible that another mechanism may
be in competition, with higher Tafel slopes. As has been discussed for Pt
surfaces [59], the complexities of the H adatom behavior can vary the ob-
served Tafel slope beyond the values predicted by classic mechanism models
(usually relying on simple Langmuir adsorption models). Another possibil-
ity is the presence of relatively resistive oxide layers on the electrodes, which
would increase the apparent slope due to local resistance on the electrode-
electrolyte interface[58]. However, in general terms, the HER activity for the
three studied carbides from Group 6 metals show analogous behavior.
According to the Sabatier principle (which states that catalyst-substrate
binding should be neither too strong nor too weak), intermediate binding
energy for adsorbed hydrogen atoms gives the highest catalytical activity for
the HER/HOR process. Platinum is very close to the optimum value (ca.
250 kJ/mol)[60], which explains its good electrocatalytical behavior.
Research on the chemisorption of hydrogen on WC surfaces shows rela-
tively high values the H binding energy [61] compared to Platinum. For the
case of tungsten carbide, this binding energy seems to be higher than this
optimum. As was mentioned in the Introduction, the electronic structure of
WC is similar to that of metallic Pt, which is what started the interest in
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Page 12 of 36

this material as a catalyst in the first place. However, the relatively strong H
adsorption of WC surfaces seems to limit its catalytical activity, as evidenced
by the different mechanism for HER observed in this work.
Chromium and molybdenum are in the same group as tungsten, sharing
its (n−1)2nd4 electronic configuration. It is possible then that their carbides
share similar electronic properties to the tungsten counterpart.
For niobium and tantalum carbides, the results show a significantly differ-
ent behavior. Both materials show higher Tafel slopes and lower specific HER
activities. These two elements have (n−1)s2nd3 electronic configuration, dif-
ferent from that of Cr, Mo and W, which have (n − 1)s2nd4 configuration.
This can be one reason for their different behavior. Aizawa et al.[62] mea-
sured the strength of metal-hydrogen bonds for these two carbides (among
others) and found relatively short M-H bond lengths, corresponding to strong
binding energies.
3.3. Electrochemical stability in O2
Cyclic voltammetry experiments were performed in O2-saturated H3PO4
at 170°C. Starting at +0.2 VRHE, the anodic reversing potential was increased
every 10th cycle. Selected voltammograms of the different carbide electrodes
can be seen in Figure 8.
The voltammograms registered during the progressive oxidation of chromium,
tantalum and tungsten carbide films were quite different to those registered
for NbC and Mo2C. In general, for a given potential, the observed current
decreases with increasing vertex potential. That is, the increasingly oxida-
tive conditions of the cycling experiment decrease the HER activity of the
electrode. This behavior is indicative of passivation. In metal-containing
13
Page 13 of 36

materials, passivation is commonly due to formation of stable surface lay-
ers that protect the bulk of the material from further degradation. Thus,
it is possible that a protective oxide layer is formed during electrochemical
oxidation of the films.
In the case of NbC and Mo2C films, this behavior was not observed.
As the anodic vertex potential was steadily increased, no current decrease
was observed; on the contrary, a current increase consistent with the Butler-
Volmer kinetics was seen, in both cases well below the Nernst potential for
oxygen evolution (ca. +1.15 VRHE). It is apparent that the NbC and Mo2C
films undergo corrosion in these conditions.
SEM images of different carbide films were taken before and after the ox-
idation experiments (Figure 9). The comparison of the surface morphology
before and after oxidation in O2-saturated H3PO4 provides qualitative infor-
mation on the electrode degradation. In the case of Cr, Ta and W carbide
films, the electrodes did not seem to suffer significant changes. On the other
hand, the Nb and Mo carbide films showed noticeable degradation, with for-
mation of visible holes on the surface. Taking into account the analysis of
the voltammograms, this degradation can be indicative of material loss, most
likely due to corrosion of the carbide film.
4. Conclusions
Carbide films were prepared on top of Cr, Nb, Mo, Ta and W metal sub-
strates through the TPRe carburization. The hydrogen evolution activity of
the prepared films was studied in 100% phosphoric acid in the temperature
range between 80 and 170°C. The activities of Group 6 metal (Cr, Mo, W)
14
Page 14 of 36

were observed to be significantly higher than those of Group 5 metal(Nb, Ta)
carbides. The different behaviors were attributed to the electronic structure
of the metals. Among the studied carbides, WC showed the highest HER
activity through the studied temperature range, followed by chromium and
molybdenum carbides. The possible mechanism for the hydrogen evolution
reaction was discussed for the Group 6 metal carbides, suggesting the Volmer-
Heyrovsky route, with electrochemical desorption as rate-determining step.
The electrochemical stability of the carbide films in O2-saturated phosphoric
acid was qualitatively evaluated through CV scanning at anodic potentials,
as well as SEM imaging, indicating passive behavior for chromium, tanta-
lum and tungsten carbide films, whereas niobium and molybdenum carbides
seemed to undergo significant corrosion.
Acknowledgements
This work has received financial support from the Danish National Re-
search Foundation (the PROCON Center) and the Danish ForskEL pro-
gramme (Catbooster).
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5. Tables and figures
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1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
Table 1: Carburization temperature, obtained experimental kinetic parameters at 170 C
and activation energy for HER on carburized metals.
Metal Tsynthesis log(j0) mTafel
j
Ea
−100mV
C
A/cm2
mV
µA/cm2 kJ/mol
Cr
Mo
W
800
750
880
900
950
-4.81
-4.61
-4.41
-5.52
-6.44
76
67
318
763
73
71
56
2380
16.6
4.27
38.7
119
28.6
Nb
Ta
135
93
Figure 1: XRD spectra of carburized metal plates.
29
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