An aqueous rechargeable formate-based hydrogen battery driven by heterogeneous Pd catalysis

Article abstract of DOI:10.1002/anie.201409500

The formate-based rechargeable hydrogen battery (RHB) promises high reversible capacity to meet the need for safe, reliable, and sustainable H₂ storage used in fuel cell applications. Described herein is an additive-free RHB which is based on repetitive cycles operated between aqueous formate dehydrogenation (discharging) and bicarbonate hydrogenation (charging). Key to this truly efficient and durable H₂ handling system is the use of highly strained Pd nanoparticles anchored on graphite oxide nanosheets as a robust and efficient solid catalyst, which can facilitate both the discharging and charging processes in a reversible and highly facile manner. Up to six repeated discharging/charging cycles can be performed without noticeable degradation in the storage capacity.

Full text of DOI:10.1002/anie.201409500

Angewandte
Chemie
DOI: 10.1002/anie.201409500
Hydrogen Storage
An Aqueous Rechargeable Formate-Based Hydrogen Battery Driven
by Heterogeneous Pd Catalysis**
Qing-Yuan Bi, Jian-Dong Lin, Yong-Mei Liu, Xian-Long Du, Jian-Qiang Wang, He-Yong He,
and Yong Cao*
Abstract: The formate-based rechargeable hydrogen battery
RHB) promises high reversible capacity to meet the need for
direct access to regenerative energy carriers based on waste
material from the energetic use of fossil fuels, opens a possible
route to convert abundantly available, inexpensive, and
(
safe, reliable, and sustainable H2 storage used in fuel cell
applications. Described herein is an additive-free RHB which
is based on repetitive cycles operated between aqueous formate
dehydrogenation (discharging) and bicarbonate hydrogena-
tion (charging). Key to this truly efficient and durable H2
handling system is the use of highly strained Pd nanoparticles
anchored on graphite oxide nanosheets as a robust and efficient
solid catalyst, which can facilitate both the discharging and
charging processes in a reversible and highly facile manner. Up
to six repeated discharging/charging cycles can be performed
without noticeable degradation in the storage capacity.
[
5a–c,e]
renewable CO2 into a variety of liquid fuels,
and is
discussed as a potential option for the development of
[
5e]
efficient sustainable energy supply chains. A full optimiza-
tion of the relevant reaction pathway is believed to be an
essential step toward the realization of a carbon-neutral
energy future. The vital challenge for implementing such
advanced energy storage concepts lies in the development of
new, innovative, economically feasible, and reliable catalytic
methods that do not only store H at high energy density but
2
that can operate reversibly under realistic working condi-
[
6]
tions.
I
dentifying and building a sustainable energy system are two
As a result of an ever-increasing demand for mobile
devices and the continued search for new improved energy
storage technologies for both transportation and grid-scale
[
1]
of the most significant issues facing modern society. Current
energy systems that are mostly based on fossil fuels, which
have limited supply and considerable negative environmental
impact, should ideally be replaced with systems based on
renewable fuels. To make alternative energy solutions more
competitive than existing ones, the challenge of reversible
energy storage and delivery must be solved to address
needs, a compact and regenerative H storage system that can
2
allow hydrogen uptake and release under identical conditions
[
7]
is a highly desired goal. Such a battery-like H handling
2
[8]
system, namely rechargeable hydrogen battery (RHB),
would be especially attractive in terms of its potential to
decouple energy production and energy consumption with
respect to both space and time, a key issue in relation to the
integration of renewable energies into our future energy
scenarios. One promising storage medium identified in this
regard is the formate/bicarbonate redox couple (Scheme 1),
[
1,2]
fluctuations in demand and supply.
flexible energy carrier and considered to be a globally
accepted clean energy feedstock. However, lack of safe,
Hydrogen is a very
[
3]
efficient, and economical H storage technologies hinders the
2
[
4]
viability of a hydrogen-based economy. Storing H2 in
chemical bonds is attractive because of the high energy
density by weight of chemical fuels. In this context, one of the
most promising strategies to store hydrogen entails the
[
5]
interconversion of H2 and CO2. This approach offers
[
*] Dr. Q. Y. Bi, J. D. Lin, Dr. Y. M. Liu, Prof. Dr. H. Y. He, Prof. Dr. Y. Cao
Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Collaborative Innovation Center
of Chemistry for Energy Materials, Fudan University
Shanghai 200433 (P. R. China)
E-mail: yongcao@fudan.edu.cn
Dr. X. L. Du, Prof. Dr. J. Q. Wang
Shanghai Institute of Applied Physics, Chinese Academy of Sciences
Shanghai 201800 (P. R. China)
Scheme 1. Aqueous RHB based on the formate/bicarbonate system.
[
**] This work was supported by the National Natural Science
Foundation of China (21273044, 21473035), the State Key Basic
Research Program of PRC (2009CB623506), the Research Fund for
the Doctoral Program of Higher Education (2012007000011), the
China Postdoctoral Science Foundation (2014M551314), and the
Science & Technology Commission of Shanghai Municipality
which has several desirable properties that makes it an
[9]
attractive candidate for an enabling RHB system. First,
a thermodynamically reversible and essentially water-based
H storage system can be achieved at near ambient conditions
2
ꢀ
(
08DZ2270500).
through the chemical equilibrium of HCOO + H O$H +
2
2
ꢀ
HCO3 . Second, it can be directly adapted to downstream
applications such as fuel-cell-based technologies for clean
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409500.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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1
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.
Angewandte
Communications
power generation, given the ultraclean nature of the resulting
catalysts and relevant characterization data are provided in
the Supporting Information (SI). Importantly, in line with our
previous research on low temperature formate activation,
H2 stream (essentially free of CO and volatile organic
x
[10]
[11]
compounds). To realize the full potential of this particular
energy storage system, the overall process should be per-
formed in the same vessel using one single catalyst. Two very
recent reports describe the feasibility of using a homogeneous
preliminary tests have shown that some of the catalysts used,
such as Au/CeO , Au/ZrO , and Pd/C, exhibit prominent
2
2
catalytic activity for transfer reduction of aldehydes with
aqueous PF (Table S1). We therefore set out to test these Au-
and Pd-based catalysts for PF dehydrogenation. For these
initial assays the conditions studied were 808C, 4.8m aqueous
PF, and a catalyst loading of 0.0265 mol% (the amount of the
active metal species relative to the substrate). The extremely
low catalyst loading employed in these screening assays is
noteworthy. Surprisingly, the Au catalysts tested were not
active at all (Table 1, entries 1 and 2). Of the various catalysts
examined, only Pd deposited on carbon-based materials
showed notable activity with appreciable turnover frequency
(TOF, over the first 20 min). In particular, the use of reduced
graphite oxide as a support (Pd/r-GO) afforded the highest
Ru catalyst to enable a reversible HCOONa/NaHCO -based
3
[
9]
H storage. However, apart from the practical inconven-
2
ience arising from the use of sophisticated ligands, the
application of these systems is greatly constrained by limited
durability and deliverable capacity.
Herein, we demonstrate the possibility to set up a viable
and truly rechargeable hydrogen storage device, simply by
applying a novel heterogeneous Pd-based catalyst capable of
facilitating efficient and reversible interconversion of formate
and bicarbonate in aqueous solution under mild and practical
conditions. The key to the unprecendented hydrogen storage/
release efficiency is an integrated development and optimi-
zation of the catalytic metal and the underlying support that
exploit the principles of metal–support synergy involving
carbon-based “flat-mat” materials such as reduced graphite
oxide (r-GO) nanosheets as the support as well as the
microstructural properties of firmly anchored Pd nanoparti-
cles (NPs) for reactivity control. The results not only show
a benchmark example of the metal–graphite-based hybrid
material used as an incredibly efficient, robust, and durable
catalyst in practical hydrogen release and storage but also the
ꢀ1
activity, providing an impressive initial TOF of 5420 h for
exclusive H evolution (Table 1, entry 3). Already, this result
2
represents the best H production efficiency ever reported for
2
[9,12]
dehydrogenation of aqueous formates (Table S2).
Pd/r-GO is a material in which palladium from PdCl has
2
been deposited onto solution-processable GO by a one-step
[
13]
co-reducing method (Scheme S1). Owing to their superior
electron mobility, considerable surface area, and chemical
tunability, GO and related carbon nanostructures have
emerged as a new class of very promising functional materials,
especially in sensing, optoelectronics, fuel cells, and related
benchmark example of the H handling system for a future
2
sustainable energy supply.
[14]
We began our research by exploring the decomposition of
potassium formate (HCOOK, PF) in neat water. The
decomposition of aqueous PF may proceed through simple
electrochemical energy conversion applications. By serving
as a unique carbon-based matrix, the GO sheets could be
applied for heterogeneous catalysis by anchoring metal NPs
[
14c,15]
dehydrogenation, producing H2 and KHCO . At high PF
on its surface.
However, approaches for growing nar-
3
conversions, observation of CO as a secondary decomposi-
rowly distributed NPs effectively in the presence of GO
sheets without the use of capping agents are still rare. Upon
2
tion product derived from a partial transformation of
bicarbonate to carbonate is also expected. The series of
catalysts that were examined in the preliminary study of their
catalytic activity toward PF dehydrogenation is summarized
in Table 1. The preparation procedure for these noble metal
using NaBH as the reducing agent, we have been able to
4
directly introduce stabilizer-free Pd NPs on the surface of
reduced GO nanosheets at room temperature. A typical TEM
image (Figure S1) shows spherical and uniform Pd particles
that are ca. 2.4 ꢁ 0.1 nm in size dispersed on the r-GO sheets
with only a small degree of particle aggregation. The XPS
0
Table 1: Study of various solid catalysts for the H generation from
analysis confirmed the predominant presence of Pd on r-GO
2
[
a]
aqueous HCOOK.
(Figure S3). In contrast to the samples in which Pd with an
identical metal loading (5 wt% Pd based on ICP analysis) is
supported on conventional carbon-based materials or metal
oxides, the Pd/r-GO hybrid exhibits remarkable activity
toward formate decomposition to yield H2 (Table 1,
entries 4–8). Importantly, controls using pristine r-GO nano-
sheets as support for Au, Ir, Pt, Ru, and Rh metals exhibit no
PF dehydrogenation activity (Table S3), that is, the catalytic
properties of Pd/r-GO are derived from the combination of
Pd NPs and r-GO.
Entry
Catalyst
Metal loading
Particle
size [nm]
VH2
[mL]
TOF
[
b]
[c]
[d]
ꢀ1 [e]
[wt%]
[h
]
1
2
3
4
5
6
7
8
Au/CeO2
Au/ZrO2
Pd/r-GO
Pd/C
Pd/XC-72
Pd/CNTs
Pd/TiO2
1
0.8
5
5
5
5
5
5
1
1.9
1.8
2.4
2.6
2.4
2.3
2.6
3.2
1.8
1.9
0
0
–
–
408
92
26
32
12
16
426
345
5420
1241
379
438
204
285
11299
8972
Pd/Al O3
2
[
f]
It is well known that the enhanced catalytic activity of
metal particles induced by support materials can in general be
9
1
Pd/r-GO
Pd/r-GO
[
f]
0
2
[
16]
attributed to either geometric or electronic effects. Given
the quite similar average particle size and chemical compo-
sition of Pd surfaces seen in the three carbon-supported
samples (see the TEM and XPS data provided in the SI), one
possibility is a lattice mismatch between the Pd “domain” and
[
a] Reaction conditions: 5 mL scale of 4.8m aqueous HCOOK, 6.4 mmol
metal, 808C. [b] Determined by inductively coupled plasma (ICP) bulk
composition analysis. [c] Evaluated by TEM. [d] Volume of H gas
generated in 1 h. [e] Initial TOF measured within the first 20 min.
[
2
f] 3.2 mmol Pd. CNTs=carbon nanotubes.
2
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Chemie
the underlying support surface that may cause an isomorphic
structural) effect on the electronic structure of Pd. The
achieving catalytic PF dehydrogenation, with catalyst load-
ings that are orders of magnitude lower than those that have
(
[
12]
created expansion or contraction in lattice parameters of Pd
at the interface, also frequently known as “lattice micro-
previously been employed for this transformation.
To
confirm the identity of the evolved gas, it was introduced
into a separate reaction vessel containing styrene, 1 mol%
commercial Pd/C (5 wt%), and 10 mL of heptane (see SI).
Product analysis of this diagnostic reaction performed at 408C
indicated a clean formation of ethylbenzene (90% conversion
[17]
strain”, could result in higher activity. We thus checked the
lattice strains of these carbon-supported Pd catalysts by
[
18]
synchrotron-radiation-based XRD (Figure S12). Indeed, it
was found that the dehydrogenation activity of these catalysts
critically depends on the degree of lattice expansion in the
corresponding Pd crystals, stressing the significance of the r-
GO support in providing a suitable catalytic interface
between the Pd NPs and the reaction media. This scenario
was further corroborated by the examination of a separate set
of Pd/r-GO samples with systematically varied Pd content,
wherein a strong positive linear correlation of the lattice
microstrain in the Pd NPs with their catalytic dehydrogen-
ation activity was identified (Figure S12). Taken together,
these results highlight the critical importance of tuning the
lattice distortion of supported Pd NPs to facilitate crucial
formate activation under mild aqueous conditions. More
specifically, the fact that the r-GO support can substantially
facilitate the creation of highly strained Pd NPs appears to be
the key factor for achieving high activity in the PF dehydro-
genation.
by GC), thereby in turn clarifying that H was produced from
2
the dehydrogenation reaction. In addition, the headspace gas
analysis by GC/MS unambiguously confirmed the exclusive
formation of H without CO emission in the dehydrogen-
2
19]
2
[
ation reaction. A further indication that both PF and water
are involved in the dehydrogenation (Scheme S2) came from
the observation of an intense signal at m/z = 3 (HD) and
a diminished signal at m/z = 2 upon replacing H O with D O
2
2
[
12a]
under otherwise identical reaction conditions (Table S7).
Additional investigation of the various reaction parame-
ters showed that the formate concentration has a profound
influence on the reaction outcome (Figure S17). The rates of
H release increased as the PF concentration increased from
2
1.0m to 4.8m; however, higher concentrations (> 5.0m) were
found to have a negative effect on dehydrogenation kinetics,
affording a volcano-shaped curve with the highest value at
4.8m. Despite the reduced activities at higher concentrations
of > 5.0m, the catalyst still exhibited enhanced activity
compared to previous results even at a high concentration
Consistent with the inference noted above, it is possible to
further improve the dehydrogenation efficiency by progres-
sively decreasing the Pd loading in the Pd/r-GO system. As
a result, the 1 wt% Pd/r-GO material possessing the highest
level of lattice expansion can deliver an unprecedented TOF
[
9,12]
of 14.2m,
which likely provides advantages for practical
applications. The most noteworthy aspect, however, is that the
reaction did not proceed at temperatures lower than 308C,
ꢀ
1
[9,12]
up to 11299 h (Table 1, entry 9).
gas can be generated over the 1 wt% Pd/r-GO hybrid
Figure 1), corresponding to almost full conversion (96.6%)
of PF into H and KHCO . As an illustration that this catalyst
Within 2 h, 550 mL of
thus underscoring the possibility to control H evolution by
2
(
varying the reaction temperature in a relatively narrow range.
This is indeed the case. Thus, as illustrated in Figure 1, the
catalyst showed a very low activity when the reaction was
2
3
remains active for an extended period of reaction time,
ꢀ
1
a continuous stream of 2.2 L of H can be released from
carried out at 408C (TOF < 2500 h ). After a rapid increase
of the reaction temperature to 808C, strong effervescence was
immediately observed and the reaction proceeded steadily
over time until the PF in the reaction medium was totally
consumed. The temperature was then lowered to 408C (258C
in the last cycle). This cycle was repeated four times (Fig-
ure S18). In all cycles, a reproducible catalytic performance
was achieved. This distinct feature, together with the fact that
the catalytic system is stable under a wide range of reaction
conditions, offers practical advantages over existing methods
to establish a more viable hydrogen storage device of great
potential for future applications.
2
2
0 mL of a 4.8m PF solution within 8 h (Figure S14). In this
case, the total turnover number (TON) approached the
[9,12]
highest ever-reported value of 142700,
thus further
demonstrating the effectiveness of 1 wt% Pd/r-GO for
Subsequent studies focusing on the comparison with other
alkali formates (Table S8) showed that, in contrast to
[
9]
previously reported experiments using Ru-based complexes,
PF is the substrate of choice for efficient and exclusive H2
production under the presented mild aqueous conditions.
Interestingly, when the dehydrogenation of PF was performed
in the presence of external H (20–30 bar), a greatly reduced
2
or complete loss of catalytic activity was observed (Fig-
ure S21). These results prompted us to examine whether the
reverse reaction, i.e., hydrogenation of KHCO to PF, could
3
be accomplished using the same catalyst (1 wt% Pd/r-GO). It
should be emphasized that in the only two related precedents
to this work the conversion of bicarbonate to formate
Figure 1. Kinetic traces for H evolution by dehydrogenation of aque-
ous HCOOK catalyzed by 1 wt% Pd/r-GO. Reaction conditions: 5 mL
scale of 4.8m aqueous HCOOK, 3.2 mmol Pd.
2
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3
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.
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Communications
[
a]
Table 2: Hydrogenation of aqueous KHCO using 1 wt% Pd/r-GO.
3
[
b]
Entry
T [8C]
PH2 [bar]
t [h]
Formate yield [%]
TON
[
c]
1
2
3
4
5
6
7
8
100
100
80
130
100
100
100
100
40
40
40
40
40
40
40
30
32
10
10
6
10
10
10
10
94.5
96.8
62.2
95.6
84.7
66.3
31.5
78.9
7088
2420
1555
2390
2117
1658
788
[
[
[
d]
e]
f]
1973
[
a] Reaction conditions: 5 mL scale of 4.8m aqueous KHCO , 9.6 mmol
3
Pd, 100 mL Parr autoclave. [b] Pressure at 258C. [c] 3.2 mmol Pd.
d] 2 wt% Pd/r-GO. [e] 5 wt% Pd/r-GO. [f] 5 wt% Pd/C.
[
Figure 2. Discharging process after the battery has been stored for
four months under ambient conditions. The inset shows the H2
release/storage cycles with 1 wt% Pd/r-GO. Storage was performed at
proceeds at a reasonable rate and to an acceptable degree
only under pressures of 80–100 bar. It is apparent from
[9]
Table 2 that a much lower H pressure level of 40 bar was
1008C under 40 bar H . Release was performed after decompression of
2
2
enough to achieve optimal activity with 1 wt% Pd/r-GO at
the system at 258C followed by heating to 808C (conditions: 5 mL
scale of 4.8m aqueous HCOOK, 9.6 mmol Pd).
8
0–1308C. For instance, 4.8m aqueous KHCO was reduced to
3
almost complete conversion in 10 h using 1 wt% Pd/r-GO
with a Pd loading of 0.0398 mol% at 1008C (Table 2, entry 2).
Similar to the PF dehydrogenation, performing the same
reaction with traditional Pd/C or Pd/r-GO samples with
higher Pd contents resulted in conversions of less than 85%
for practical H storage, regarding effectiveness, reversibility,
durability, and scalability.
Given the significant scope for optimization of the reactor
design, operating parameters, and materials properties of the
Pd-based catalysts, we anticipate that the present system will
have the potential to become an ideal solution for safe,
2
(
Table 2, entries 5–7) . These results again show the remark-
able benefit of using highly strained Pd catalysts for the
reduction. Consistent with the trend in formate dehydrogen-
ation, the hydrogenation of other bicarbonates were less
reliable, and affordable H storage used in low-temperature
2
fuel cells—either alkaline fuel cells or proton exchange
effective (Table S9). A lower H pressure was also unfavor-
membrane fuel cells. It was estimated that a H supply of
2
2
ꢀ
1
able for the hydrogenation reaction (Table 2, entry 8).
Upon establishing that 1 wt% Pd/r-GO could efficiently
promote both, the dehydrogenation of PF and the hydro-
23.9 molh is necessary to activate a 1 kW fuel cell, whereby
the efficiency of the fuel cell is 50%, the hydrogen con-
sumption efficiency is 95%, and the average voltage is
[
20]
genation of KHCO , we sought to explore the possibility of
0.779 V.
Based on the current best H production rates
2
3
a cyclic operation by coupling the two transformations. In
particular, we were curious wether the repetitive intercon-
obtained in this study (see Figure 2), this power output would
require 5 L of the above-mentioned 4.8m PF solution, using
approximately 42.8 g of 1 wt% Pd/r-GO. On the other hand,
only 1.7 L of saturated PF solution (14.2m at 258C) would be
sufficient to provide 1 kW of electric power, provided that
continuous hydrogen production could be performed at
version between PF and KHCO in one reaction vessel can be
3
realized through dehydrogenation–hydrogenation reactions
using 1 wt% Pd/r-GO as the single catalyst. Therefore,
alternating pressurized charging and pressure-free discharg-
ing procedures were employed to evaluate the feasibility in
a commercially available standard autoclave. As shown in the
inset of Figure 2, up to six consecutive cycles of nearly
quantitative H2 release from 5 mL of 4.8m PF and the
ꢀ
1
ꢀ1
a similar rate of 666.2 LH h gPd
.
2
In conclusion, we have developed the first heterogeneous
catalytic system for the efficient reversible dehydrogenation
of aqueous formates to bicarbonates. The r-GO nanosheets
were shown to be a distinct and powerful support allowing for
the generation of highly strained Pd NPs that enable facile
and repetitive formate/bicarbonate interconversion under
mild aqueous conditions. By controlling the temperature and
the hydrogen pressure, the reversible catalytic transforma-
tions can be repeated nearly quantitatively six times with
almost no loss of efficiency. This could lead to the construc-
hydrogenation of the resulting KHCO were possible. Note
3
that the cyclic operation was performed without isolating
either the formate or the bicarbonate to start a new cycle, an
important metrics by which any practical battery energy
storage system is judged. To further study the stability of the
present PF-based energy storage system for long terms, we
measured the H release properties of a catalyst-containing
2
charged solution system stored for four months at ambient
conditions. Gratifyingly, when warming this system to 808C,
a gas volume of 550 mL corresponding to a near-total
decomposition of the original PF is readily attained
tion of a simple, truly rechargeable HCOOK/KHCO -based
hydrogen storage device. These results highlight the substan-
tial promise that designed heterogeneous metal-based cata-
3
lyst hold for reversible and controlled H delivery, especially
2
(
Figure 2). To our knowledge, this is the first durable and
in the field of energy storage for sustainable energy supply.
simplified system capable of delivering reversible H storage
2
using a single solid catalyst and this cyclic formate formation/
decomposition process appears to be the most efficient way
Received: September 27, 2014
Published online: && &&, &&&&
4
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Keywords: batteries · energy conversion · hydrogen ·
lattice strain · Pd catalysis
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[
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HCOONa/H O and 2.02 wt% for HCOOK/H O, but they are
2
2
nontoxic, noncorrosive, nonflammable, and easy to handle. The
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[19] Note that under conditions, in which the HCOOK concentration
(> 8m) or reaction temperature (> 1508C) is high, a very small
amount of CO (< 0.05 vol%) along with H would be generated
2
2
as a result of the slow transformation of bicarbonate ions to
ꢀ
2ꢀ
[
carbonate (2HCO3 !CO + CO + H O).
3
2
2
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Hydrogen Storage
The formate/bicarbonate pair: A
rechargeable hydrogen battery based on
repetitive formate/bicarbonate intercon-
version in aqueous solution was devel-
oped. A hybrid material of Pd nanopar-
ticles and reduced graphite oxide serves
as the robust and efficient catalyst for
both steps. Multiple charging and dis-
charging cycles were performed with
comparable storage/release efficiency
Q. Y. Bi, J. D. Lin, Y. M. Liu, X. L. Du,
J. Q. Wang, H. Y. He,
Y. Cao*
&&&& — &&&&
An Aqueous Rechargeable Formate-
Based Hydrogen Battery Driven by
Heterogeneous Pd Catalysis
and the resulting H gas is free of CO and
2
CO2.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!