A rechargeable hydrogen battery based on Ru catalysis

Article abstract of DOI:10.1002/anie.201310972

Apart from energy generation, the storage and liberation of energy are among the major problems in establishing a sustainable energy supply chain. Herein we report the development of a rechargeable H₂ battery which is based on the principle of the Ru-catalyzed hydrogenation of CO₂ to formic acid (charging process) and the Ru-catalyzed decomposition of formic acid to CO₂ and H₂ (discharging process). Both processes are driven by the same catalyst at elevated temperature either under pressure (charging process) or pressure-free conditions (discharging process). Up to five charging-discharging cycles were performed without decrease of storage capacity. The resulting CO₂/H₂ mixture is free of CO and can be employed directly in fuel-cell technology. Simple but efficient: A readily accessible Ru catalyst is the basis for a reversible H ₂/CO₂-driven battery. At elevated temperatures both the reduction of CO₂ to formic acid and the decomposition of formic acid were realized with 0.075 mol % of the Ru complex. Several charging and discharging cycles were performed with comparable storage-release efficiency. Furthermore, the partial removal of defined gas amounts is possible under pressure.

Full text of DOI:10.1002/anie.201310972

Angewandte
Chemie
DOI: 10.1002/anie.201310972
Ru Catalysis
A Rechargeable Hydrogen Battery Based on Ru Catalysis
Shih-Fan Hsu, Susanne Rommel, Philipp Eversfield, Keven Muller, Elias Klemm,
Werner R. Thiel, and Bernd Plietker*
Abstract: Apart from energy generation, the storage and
liberation of energy are among the major problems in
establishing a sustainable energy supply chain. Herein we
report the development of a rechargeable H2 battery which is
based on the principle of the Ru-catalyzed hydrogenation of
CO2 to formic acid (charging process) and the Ru-catalyzed
decomposition of formic acid to CO2 and H2 (discharging
process). Both processes are driven by the same catalyst at
elevated temperature either under pressure (charging process)
or pressure-free conditions (discharging process). Up to five
charging–discharging cycles were performed without decrease
of storage capacity. The resulting CO2/H2 mixture is free of CO
and can be employed directly in fuel-cell technology.
Important advances were made in the field of adsorptive H2
storage.[5–7] More recently, H2 storage in chemical bonds is
considered as a complementary approach.[8] In particular H2
storage through the reduction of carbon dioxide to formic
acid, methanol, or methane opens up new perspectives.[9–20]
Furthermore, important studies in the field of H2 liberation by
the dehydrogenation of ammonia–borane adducts,[21] metal
hydrides,[22] and alcohols[23] have been reported.
Apart from the development of pilot plants for the
separate storage and liberation of hydrogen, systems that
allow the reversible storage and liberation of hydrogen gas
using an identical catalyst system (i.e. a hydrogen battery) are
also of interest. These systems are particularly interesting in
the context of autarkic energy supplies (e.g. island solutions,
automotive industry, etc.). In order to reach the goal of
a hydrogen-driven battery, the charged storage medium must
also promote the reverse reaction, that is, the liberation of
hydrogen, with full regeneration of the storage medium. In
this regard, N-ethylhexahydrocarbazol[8] and formic acid
present significant advantages since the storage medium (i.e.
N-ethylcarbazol or CO2) is completely regenerated, and
hence both hydrogen storage and liberation are possi-
ble.[8,24–28] Due to its abundance CO2 might have advantages
as an environmentally benign storage medium.
Since the ground-breaking work of Laurenczy et al. in the
field of CO2 reduction/formic acid decomposition, in which an
aqueous bicarbonate solution was reduced to the correspond-
ing formate solution, important contributions from the Beller
and Fujita groups have been published in which either the
temperature or the pH value was used as a switch for the
discharging process.[29–32] These systems are based on the
reduction of an aqueous bicarbonate solution, but recently
Beller et al. reported a reversible hydrogen-storage and
-liberation system employing amines as bases.[33] Whereas
the increased storage capacity is advantageous, the liberated
CO2/H2 gas mixture is disadvantageous since CO2 must be
recovered from the gas phase or the gas mixture can directly
be employed in fuel-cell technology.[34] However, this type of
battery must be stored at temperatures below room temper-
ature after the charging process.
T
he installation of a sustainable energy supply chain is one of
the greatest challenges to be addressed in this century.[1,2] The
focus of discussions in this field has shifted within the past
years from the generation of energy from regenerative
sources toward energy storage and liberation. Among the
manifold reasons for this change, the most prominent is the
fact that energy generation from regenerative sources is not
constant and depends on unpredictable climatic factors (like
wind, temperature, duration of sunshine, etc). Today the
development of efficient energy-storage media that allow
efficient energy regeneration is generally considered to be
decisive. Within this context, hydrogen as an energy carrier is
recognized to be of particular importance.[3] It can be
employed in fuel cells for energy generation with formation
of water. In the past years the field of fuel-cell technology has
reached a sophisticated state of development;[4] however, it is
still hampered by the lack of efficient hydrogen-storage
systems. Due to the inherent chemical properties of hydrogen,
some of these problems (diffusion through metal layers,
storage capacity, etc.) can be addressed only with significant
technical investment.[5,6] For these reasons the development
of practical hydrogen-storage devices plays a key role in
sustainable-energy politics. Two general main strategies for
tackling this problem have evolved during the past years.
[*] M. Sc. S.-F. Hsu, Dipl.-Chem. S. Rommel, Prof. Dr. B. Plietker
Institut fꢀr Organische Chemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Herein we report a prototype system for the reversible
storage and liberation of H2 based on the amine/CO2
technology (Figure 1). At elevated temperature both the
fast storage and the release of H2 gas is possible. Several
charging and discharging cycles can be performed without the
need to change the storage container (an autoclave) or the
catalyst. Moreover, the charged system can be stored for days
without loss of efficiency.
E-mail: bernd.plietker@oc.uni-stuttgart.de
M. Sc. P. Eversfield, Prof. Dr. E. Klemm
Institut fꢀr Technische Chemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Dr. K. Muller, Prof. Dr. W. R. Thiel
Fachbereich Chemie, TU Kaiserslautern
Recently our group reported the preparation and appli-
Erwin-Schrçdinger-Strasse 54, 67663 Kaiserslautern (Germany)
cation of the (PNNP)(acetonitrile)RuII complex 1 for selec-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310972.
[35]
À
tive C H oxidation. With regard to the catalyst structure
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Table 1: Reduction of CO2.[a]
Entry
1
C6F5OH
[mol%]
toluene
[m]
t
[h]
Yield
[%]
TON[b]
[mol%]
1
2
3
4
5
6
7
8
0.075
0.075
0.075
0.075
0.015
0.015
0.015
0.015
–
–
1.25
–
0.25
–
6
–
6
6
6
6
–
–
1
4
4
4
4
4
4
4
119
135
137
141
62
60
83
84
1587
1800
1827
1880
4133
4000
5533
5600
Figure 1. Prototype of a Ru-driven hydrogen battery.
0.25
–
we speculated that the same complex could also be active in
hydrogenations. Based on the fundamental reports from the
Jessop group[36] in the field of CO2 reduction, we performed
initial experiments using amine bases and catalytic amounts
of complex 1 in the hydrogenation of CO2. Gratifyingly, the
reduction of CO2 to the corresponding DBU formate salt
(DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) was possible
with only 0.015 mol% of 1 within 4 h at 1008C (entry 8,
Table 1).
[a] Conditions: 65.7 mmol DBU, 1008C, 70 bar H2, 20 g dry ice (freshly
prepared). [b] Calculated as mole of DBU formate salt per mole of
catalyst).
Moreover, CO impurities were not detectable in the gas
mixture down to 10 ppm.[37] A further decrease of the catalyst
concentration slowed down the reaction significantly.
At higher catalyst loadings under otherwise identical
conditions the bis-formic acid DBU product, and hence
In subsequent experiments the two catalytic transforma-
tions were coupled to form a cyclic process. The autoclave was
filled with DBU and 0.075 mol% complex 1 since at this
catalyst concentration the 2:1 adduct of formate and DBU
adduct was formed and yields higher than 100% were
observed. The autoclave was charged with dry ice and
pressurized with hydrogen gas (70 bar). The system was
closed and heated to 1008C with the pressure approaching
about 140 bar. After 2.5 h a pressure drop to about 100 bar
was observed. The autoclave was cooled to room temperature
and the gas atmosphere was flushed with N2 gas. The
autoclave was subsequently heated up under pressure-free
conditions and the released gas was collected in an inverted
gas burette. After 2 h gas evolution ceased and the crude
product gas was analyzed. The autoclave was cooled to room
temperature under an N2 atmosphere, charged again with dry
ice and hydrogen gas (70 bar), closed, and heated up again.
This algorithm of charging and discharging was repeated five
times (Figure 3). As shown in Figure 3 the H2 yields equili-
brated to about 100% (or a corresponding gas volume of
about 3.4 L) after the first cycle. The analysis of the gas
mixture showed that apart from traces of N2 (from flushing
the autoclave) equimolar amounts of CO2 and H2 were
formed. CO impurities were not detectable.[37]
a
significantly increased storage capacity (entries 1–4,
Table 1), was observed. Although no solvent is required for
the reduction process, the addition of a small amount of
toluene reduces the undesired crystallization of the salt and
suppresses some problems that were encountered with the
neat system (bad miscibility, catalyst deactivation through
formation of inclusion products, problematic reactivation of
the charged battery after a couple of days; entries 1, 3–6,
Table 1). Further optimization of the system showed that the
addition of pentafluorophenol had no beneficial influence on
the reactivity (entries 3 and 4, Table 1). This observation is in
strong contrast to previously reported systems[33,36] and might
indicate a different activation–reduction mechanism.
Knowing that complex 1 shows good activity in the CO2
reduction we subsequently investigated its activity in the
decomposition of formic acid (Figure 2). Under pressure-free
conditions the DBU formate salt decomposed at 1008C in the
presence of 0.075 mol% 1 within 70 min (Figure 2C). Low-
ering the temperature resulted in a significant decrease of the
decomposition rate. Furthermore, we investigated the influ-
ence of pentafluorophenol and toluene on the decomposition
rate (Figure 2B). We were pleased to find that the addition of
toluene led to an increase of the decomposition rate, whereas
the addition of pentafluorophenol had almost no effect.
In order to get insight into an undesired decomposition of
the formic acid DBU adduct in the presence of the catalyst at
2
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Figure 2. A) Activity of catalyst 1 in the decomposition of formic acid. B) Effects of additives and solvent; reagents and conditions: 1:1 adduct of
formic acid and DBU (5 mmol), 1 (0.075 mol%), C6F5OH (1.25 mol%, if necessary), toluene (1.66 mL). B) Temperature effects; reagents and
conditions: 1:1 adduct of formic acid and DBU (5 mmol), 1 (0.075 mol%), toluene (1.66 mL).
cooled to room temperature and
flushed with N2. The composition
of the condensed phase was ana-
lyzed by NMR spectroscopy on
a daily basis over a period of five
days. The composition of the battery
content did not change (Figure 4A)
and hence heating the system to
1008C after five days resulted in
complete H2 generation (chart (B),
Figure 4B). After the discharging
process was complete, the system
was charged again without loss of
efficiency.
Having tested the recyclability
and long-term stability of the stor-
age medium and the catalyst, we
evaluated the possibilities for
removing a defined amount of gas
in two final experiments. The
charged system was heated to
1008C and the internal pressure
Figure 3. Charging and discharging cycles of the H2 battery.
was monitored over the time.
room temperature and into the long-term stability of the
catalyst in the presence of the formic acid DBU adduct, we
investigated the long-term stability of the charged H2 battery
(Figure 4). After the battery had been charged the system was
Within 2.5 h an internal pressure of 22 bar was recorded
(Figure 5A). Hence, this battery can also be used in
combination with a suitable valve technology for high-
performance fuel cells in which defined gas volumes are
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injected under pressure. Apart from the use of dosing valves
the amount of the produced gas can also be directed by means
of the temperature. In order to prove this hypothesis the
system was subjected to an alternating heating–cooling
algorithm. The formation of the CO2/H2 gas mixture was
followed volumetrically over the time (Figure 5B). As can be
seen from this diagram the system is stable and allows the
controlled release of a defined amount of H2 gas either under
pressure or pressure-free by adjustment of the respective
heating period.
Herein we have reported the use of a readily accessible
(PNNP)(acetonitrile)RuII complex for the efficient and
reversible reduction of CO2 in the presence of DBU as
a base. At a catalyst loading of only 0.075 mol% both the
reduction using H2 pressure as well as the decomposition of
the formic acid adduct under pressure-free conditions were
demonstrated. As compared to the hydrogen storage via
reduction of CO2 to methanol The present system has a lower
hydrogen-storage capacity (4.4 wt%) than that achieved for
the reduction of CO2 to methanol (12.5 wt%). However, it
possesses the advantage of being a fully reversible hydrogen-
storage system that can be used in conventional fuel cells.[38]
Technical problems like the addition of solid CO2 and the
recycling of the released CO2 will be addressed in future work
by employing microreactors.
Figure 4. A) Long-term stability of the storage medium (determined by
1H NMR integration). B) Discharging process after the battery had
been stored for 5 days at room temperature.
Received: December 18, 2013
Published online: && &&, &&&&
Keywords: batteries · carbon dioxide · energy · hydrogenation ·
.
Ru catalysis
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Communications
Ru Catalysis
S.-F. Hsu, S. Rommel, P. Eversfield,
K. Muller, E. Klemm, W. R. Thiel,
B. Plietker*
&&&& — &&&&
A Rechargeable Hydrogen Battery Based
on Ru Catalysis
Simple but efficient: A readily accessible
Ru catalyst is the basis for a reversible H2/
CO2-driven battery. At elevated temper-
atures both the reduction of CO2 to
formic acid and the decomposition of
formic acid were realized with
0.075 mol% of the Ru complex. Several
charging and discharging cycles were
performed with comparable storage–
release efficiency. Furthermore, the partial
removal of defined gas amounts is pos-
sible under pressure.
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!