High-quality hydrogen generated from formic acid triggered by in situ prepared Pd/C catalyst for fuel cells

Article abstract of DOI:10.1039/c5cy00245a

High-quality hydrogen can be generated from formic acid triggered by facilely in situ prepared Pd/C catalyst in ambient conditions. The obtained gas can be directly fed into proton exchange membrane fuel cells indicating a very promising application.

Full text of DOI:10.1039/c5cy00245a

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High-quality hydrogen generated from formic
acid triggered by in situ prepared Pd/C catalyst
for fuel cells†
Cite this: DOI: 10.1039/c5cy00245a
Received 14th February 2015,
Accepted 5th March 2015
a
b
b
a
a
Qing Lv, Ligang Feng,* Chaoquan Hu, Changpeng Liu and Wei Xing*
DOI: 10.1039/c5cy00245a
www.rsc.org/catalysis
High-quality hydrogen can be generated from formic acid triggered
by facilely in situ prepared Pd/C catalyst in ambient conditions. The
obtained gas can be directly fed into proton exchange membrane
fuel cells indicating a very promising application.
It is desired but still challenging to develop facile and
selective evolution of CO-free H especially at lower tempera-
2
tures. Pd/C as a simple catalyst intrigued the researchers'
8
attention. However, the complicated preparation method or
the low activity of Pd/C catalyst hindered the practical appli-
cation. Herein, we demonstrated an approach to obtain high-
quality hydrogen triggered by the in situ generated Pd/C cata-
lyst in ambient conditions without any complicated post-
treatment. No CO was detected in the gas products that can
be directly fed into fuel cells and the power density as high
Hydrogen economy has received considerable attention due
to development by human society, but storage and transpor-
tation problems severely hinder the wide-spread application.
1
Recently, formic acid has been considered as a convenient
2
hydrogen carrier for portable hydrogen storage application.
Formic acid is a non-toxic liquid at room temperature which
is adaptable to the existing liquid-based distribution infra-
structure, thus makes it more competitive compared with
−2
as 80 mW cm was achieved, which competes well over sev-
eral alternative power supply approaches. Moreover, as a bi-
functional application, the resulted Pd/C catalyst also showed
comparable catalytic performance for formic acid electro-
3
other hydrogen storage approaches. Further, the hydrogen
generated from formic acid can be directly employed in com-
oxidation to a commercial Pd/C catalyst.
1
d,4
bination with the proton exchange membrane fuel cells.
2−
PdCl
4
can be reduced by formic acid, a kind of weak
However, it should be pointed out that the trace amount of
CO in the gas adsorbed on the catalyst active sites could result
reducing agent, but it will take a relatively long time. How-
ever, when the pH was adjusted by adding NaOH, Pd parti-
cles were formed in a very short time. Digital photos showed
that there was no colour change when the H PdCl solution
2 4
was mixed with formic acid after 3 hours (Fig. S1†); however,
5
in the fuel cell performance decay due to catalyst poisoning.
Thus, both activity and selectivity should be considered in the
6
catalyst preparation.
Recent studies of noble metal nanoparticles supported
on various materials showed promising catalyst systems
the colour change occurred shortly when the mixture of
sodium hydroxide and formic acid was added to the H PdCl
2 4
2
d,5b,7
for formic acid decomposition.
Unfortunately, the trace
solution. The colour changed from yellow to light grey soon
within 30 seconds, and finally changed to dark grey after 2
minutes. Fig. 1 shows the UV-vis spectral changes for
H PdCl solution in different mixed solutions. An obvious
amount of CO in the mixed gas is not suitable for fuel cell
application, and high temperature is needed to retain accept-
2
a,10a
able catalytic activity.
Moreover, such kinds of catalysts
2
4
need to be prepared in advance, which increases the addi-
2 4
peak at ca. 415 nm was observed for the pure H PdCl solu-
7
b,8
tional work for practical fuel cell application.
As a result,
tion, and almost no changes occurred when HCOOH was
added after 3 hours. However, there was no peak after the
mixture of formic acid and sodium hydroxide was added to
there were very limited reports about hydrogen application in
fuel cells by formic acid decomposition.
2 4
the H PdCl solution after 2 minutes. Thus, the presence of
a
State Key Laboratory of Electroanalytical Chemistry, Laboratory of Advanced
NaOH accelerated the reduction rate of Pd precursors.
Owing to the important effect of the formate in the solu-
Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, PR China. E-mail: xingwei@ciac.ac.cn
−
tion, the influence of HCOO on the rate and quantity of gas
b
Department of Applied Physics, Chalmers University of Technology, SE-41296,
9
evolution was compared with 1.5 ml of formic acid added,
Göteborg, Sweden. E-mail: ligang.feng@chalmers.se, fenglg11@gmail.com
in which the formate ratio was adjusted by the amount of
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00245a
NaOH added (Fig. S2 and see ESI† for experimental details).
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reduced Pd nanoparticles provide active sites for formic acid
decomposition. Besides, the initial turnover frequency (TOF)
−
1
for this system with 60% formate was calculated to be 79 h
after the initial 1 hour, which is also higher than some
1
0a,b
reports in similar conditions.
The effect of the tempera-
ture on the catalytic performance of formic acid decomposi-
tion was also evaluated (Fig. S3†). With the increase in the
temperature, the performance was improved, and the activa-
−
1
tion energy from Fig. S4† was calculated to be 38.9 kJ mol ,
1
a,2a,10
which is lower than some similar reports.
For example,
−
1
11
−1
it is 53 kJ mol for Au/Al O , 72 kJ mol for Pt/Al O
2 3
2
3
−
1
2b
(
ref. 11) and 49.3 kJ mol for Au/ZrO
2
catalyst.
The formic acid decomposition ability of Pd/C catalyst was
further evaluated by changing the amount of formic acid and
Fig. 1 UV-vis spectra of an aqueous solution of H
solution mixed with HCOOH after 3 hours (b), and H
mixed with NaOH and HCOOH after 2 minutes (c) in ambient conditions.
2
PdCl
4
(a), H
solution
2
PdCl
4
the formate ratio was maintained at 60% (Table S1†). As
2
PdCl
4
shown in Fig. 3, it takes about 100 min for Pd/C catalyst
12
(
10 mg Pd) to fully decompose 0.5 or 1 mL of formic acid.
With the further increase in formic acid volume to 1.5 ml,
the produced gas volume was increased to a maximum value
of 640 mL, and the gas evolution became very slow (0.35 mL
For a clear understanding, the composition of the different
−
1
mixtures is shown in Table S1.† With the increase in the
min ) after 2 hours. We should also notice that as a quick
response technology, it is still attractive that about 200 mL of
hydrogen (400 mL mixed gas) was released in one hour with
only 10 mg of Pd and 1 mL of initial formic acid. Here it
should be pointed out that the real amount of formic acid is
−
−
HCOO , the amount of gas was increased till 60% of HCOO
in the solution. Fig. 2 displays the plot of formate molar
percent dependence of the gas evolution rate in the linear
−
range from Fig. S2.† It is evident that with more HCOO in
1
3
the solution, the gas evolution rate was increased largely.
0.4 mL, thus theoretically the produced hydrogen is 250 ml;
a conversion as high as 80% is obtained. As the poor re-
cycling ability is a common problem for this kind of catalyst,
unfortunately in our case, the recycling performance was also
bad; the deactivation might be caused by the absorption of
hydrogen into the Pd bulk during the formation of Pd. Thus,
increasing the catalysts' recycling performance is also needed
in future work.
−
Further increasing the HCOO concentration, the formic acid
decomposed completely around 50 minutes. It was in agree-
ment with the above-mentioned result that the Pd reduction
−
rate increased with more HCOO in the solution; thus more
Pd was formed in less time, and would accelerate the formic
acid decomposition rate (Table S2†). Furthermore, a video of
one of the experiments (using 0.955 g NaOH and 1.5 mL
formic acid) starting after formic acid was added to the sus-
pension for 15 minutes is attached in the ESI.† It can be seen
that the gas-evolution rate was accelerated at first when the
The obtained gas was detected by gas chromatography
(GC), and no CO was detected (the CO concentration is <50 ppm,
Fig. S5 in ESI†). The quality of the reforming gas reaches the
fuel cell standard with the CO concentration lower than
2
−
PdCl4 was being reduced and then the rate became fast
and relatively steady, which demonstrated further that the
Fig. 3 The change in gas-evolution amount vs. time for different
formic acid and formate mixtures catalyzed by in situ prepared Pd/C at
313 K. Initially added formic acid of 0.5 mL (—), 1.0 mL ( ), 1.5 mL ( ),
2.0 mL ( ) and 2.5 mL ( ). The molar percent of formate was 60%.
Fig. 2 Formate molar percent dependence of the gas evolution rate in
the initial linear range.
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1
00 ppm. The molar ratio of H to CO in the reforming gas
82 and 87° correspond to the face-centered-cubic phase of
Pd. TEM images of the reduced Pd/C catalyst with different
formate ratios (0% to 100%; from Fig. 2) are shown in
Fig. S8.† We didn't observe obvious differences about the
2
2
was about 1 : 1. Because hydrogen was the only gaseous prod-
uct for formate hydrolysis
6,8c
and formic acid would give rise
to H2 and CO2 with a molar ratio of 1: 1, in this system,
formic acid should be the overwhelming hydrogen source.
Now, it is more clear that the presence of formate accelerates
the reduction of Pd ions, and the in situ generated Pd/C is
active for the formic acid decomposition.
2
−
morphology of Pd particles except that PdCl4 was not
reduced completely by pure formic acid. For all cases, the
particle size distribution is within the optimal particle size of
1
0b
Pd for formic acid decomposition in aqueous solution,
Here, taking an initial 1.5 ml of formic acid (60% formate)
as an example, the obtained gas was directly fed into home-
made fuel cells. The schematic diagram for this system is
and also it shows that such a small particle size could be
obtained facilely without adding any protective agent. Thus,
the gas evolution rate differences observed in Fig. 2 might be
influenced by the ratios of formic acid to formate. As pointed
out, the presence of formate in the solution considerably
improved the Pd reduction rate, thus largely increased the
2
shown in Scheme S1.† Without removal of CO , a power
−
2
density of ca. 60 mW cm was achieved (Fig. 4). It is evident
that the performance was affected by the mass transportation
due to the presence of CO
2
, and, moreover, CO
2
can be
2
production rate of H over the in situ generated Pd/C catalyst.
reduced to CO on Pt–H surfaces leading to a lower fuel cell
This appears to be consistent with the previously proposed
reaction mechanism based on HCOO as the rapidest H
donator at ambient temperatures (for details see ESI†). If this
is the only case, a pure formate solution with the use of the
1
4
−
performance. After CO removal, the power density as high
2
−
2
as 80 mW cm was obtained (Fig. 4), which competes well
over several alternative power supply approaches, such as fuel
−
2 15
−2 16
cells fed with methanol (30 mW cm , 45 mW cm ,
2
Pd/C catalyst should provide a faster H production rate at
−
2
−2 18
−2 19
6
3
1
5 mW cm (ref. 17)), ethanol (40 mW cm , 52 mW cm ,
0 mW cm (ref. 20)), sodium borohydride (30 mW cm ,
8 mW cm (ref. 22)), etc. It should be noticed that this power
least than a mixture solution containing formic acid and for-
mate. However, this is against the experimental results
observed with a pure formate solution over the catalyst. It is
likely that some aspects, such as Pd particle size, space repul-
sion, and bonding competition, may also have important
effects on the hydrogen evolution from the mixture solution
over the Pd surface. Further studies are required to fully clar-
−
2
−2 21
−
2
density is inspirational and acceptable because it was only
limited by the natural formic acid decomposition rate. The
power density curves at different times during the formic acid
decomposition were also recorded, which showed a stable per-
formance (Fig. S6†). The result demonstrated the promising
applications as a power supply approach.
Finally, the Pd/C was collected to analyse the properties by
physical measurements. Inductively coupled plasma optical
emission spectrometry (ICP-OES) indicated that the mass
fraction of Pd in Pd/C catalyst is ca. 20 wt.% which agreed
very well with the Pd amount added (Table S3†) initially. The
crystal structure of Pd/C was characterized by X-ray diffrac-
tion (XRD) technology, and is shown in Fig. S7.† The peak at
ca. 25° is evidently attributed to the (002) plane reflection of
Vulcan XC-72 carbon. The diffraction peaks at ca. 40, 47, 68,
ify the reaction nature of Pd-catalyzed H generation from an
2
aqueous solution containing formic acid and formate.
Finally, as a bi-functional application, the electrocatalytic
activity of several Pd/C catalysts toward formic acid oxidation
was compared with a commercial Pd/C catalyst (Fig. S9†). All
the catalysts showed comparable catalytic activity to the com-
mercial Pd/C catalyst that increased the economic value for
2
3
promising application.
In summary, high-quality hydrogen can be generated from
formic acid triggered by in situ prepared Pd/C catalyst in
−
ambient conditions. The HCOO concentration has a large
impact on Pd formation, thus a great effect on the gas evolu-
tion rate. Due to the complicated system, further studies
using calculation or spectroscopy are required to fully clarify
the reaction mechanism. Moreover, as bi-functional catalyst,
the used catalyst has comparable performance to the com-
mercial Pd/C catalyst for formic acid electrooxidation. Such a
facile approach makes it successful as a quick response tech-
nology to get high purity H2 from formic acid. The power
−
2
density as high as 80 mW cm was achieved in combination
with fuel cells indicating a very promising application as a
power supply approach.
Acknowledgements
Fig. 4 Steady-state polarization curves for proton exchange membrane
This work was financially supported by the National Natural
Science Foundation of China (no. 21373199), the National
High Technology Research and Development Program of
China (863 program, no. 2012AA053401, 2013AA051002), the
fuel cells with the produced gas and CO
2
removed gas. The experiment
was performed at room temperature and atmospheric pressure. 1.5 ml
−
2
of formic acid (60% formate); anode: Pt loading (1.0 mg cm ); cathode:
−
2
Pt loading (1.0 mg cm ).
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