Full Papers
cesium would minimize its bioavailability. Cesium formate is
not a reported carcinogen but is known to be a mild eye and
skin irritant. Chronic health effects are mainly associated with
high repeated oral doses of the compound which are highly
unlikely to be a significant factor with small- to medium-scale
hydrogen storage systems. Cesium bicarbonate has not been
as extensively studied and therefore far less data is available. It
is classified as a non-hazardous material according to European
standards. The bicarbonate ion itself has a wide natural occur-
rence as part of the photosynthetic mechanism of plants as
well as the CO2–water equilibria. Its toxicity is mainly associat-
ed to an increase in the pH of water bodies through the aque-
ous bicarbonate–carbonate equilibrium.
Results and Discussion
Evaluation of cesium formate and bicarbonate salts for H2
storage
The applicability of such a hydrogen storage system strongly
depends on both the maximum solubility and the conversion
of the substrates under the specific reaction conditions. In this
sense, cesium salts clearly offer an advantage over their
sodium and potassium analogues owing to their increased
water solubility, resulting in a higher volumetric and gravimet-
ric hydrogen content. Since alkali metal formate salts have
higher water solubility than the corresponding bicarbonate
salts, the solubility of the latter is the limiting factor. Consider-
ing that the solubility of cesium bicarbonate (209.0 g/100 gH O
2
Catalyst and system optimization
at 158C) is more than twenty and five times higher than the
solubilities of the respective sodium (10.3 g/100 gH O at 258C)
We are interested in a catalytic system active for both formate
dehydrogenation and bicarbonate hydrogenation. In this way
catalyst removal and replacement (depending on the reaction
of interest) is avoided, which would otherwise pose a signifi-
cant burden on the development of rechargeable “H2 batter-
ies”. However, any catalytic system active for both reactions
would tend towards an equilibrium position (Scheme 1). Data
on this phenomenon would be crucial for controlling the
charging (H2 storage) and discharging (H2 release) steps in con-
tinuous processes. Herein we report for the first time an at-
tempt to produce hydrogen for practical applications, by sub-
stantially increasing the substrate concentration and conse-
quently volumetric H2 density, owing to the very high water
solubility of cesium formate.
2
and potassium salts (36.2 g/100 gH O at 258C),[27] cesium salts
2
stand out for possible practical applications. At 158C a saturat-
ed aqueous solution of CsHCO3 (and consequently CsOOCH)
has
a concentration (storage capacity) of approximately
11 molkgÀ1 H2O and a volumetric H2 density of 14 gLÀ1 (the
solution volume increases approximately 57% upon addition
of the solute). It is however important to point out that this
value can be significantly increased at elevated temperatures,
which is relevant for stationary applications for which the in-
stallation of a heating system is straightforward. For reference,
the solubility of CsOOCH in 100 g H2O increases to 2012.0 g at
958C.[28] However it must be noted that such a saturated solu-
tion cannot be used for hydrogen storage systems because
water becomes the limiting reactant during formate dehydro-
genation. Using the maximal CsOOCH concentration of
18 molkgwaterÀ1, our system has a gravimetric hydrogen storage
The reverse hydrogenation reaction of dilute NaHCO3 solu-
tions by an in situ formed {RuCl2(mTPPMS)2}2 +mTPPMS cata-
lyst (TPPMS=triphenylphosphine monosulfonate) was previ-
ously studied as part of a charge–discharge device.[22] Here we
report a similar system making use of the trisulfonated analog
{RuCl2(mTPPTS)2}2 +2mTPPTS (1) (TPPTS=triphenylphosphine
trisulfonate) with increased water solubility (1100 gLÀ1 mTPPTS
vs. 28 gLÀ1 mTPPMS) and hence catalytic concentration.
NaOO13CH and CsOOCH were chosen as substrates for dehy-
drogenation, the former being isotopically enriched to en-
hance NMR detection at low concentrations. Initially, the stabil-
ity and activity of the catalytic system over extended time peri-
ods at elevated temperatures were optimized by varying the
metal-to-excess-phosphine ratio. The highest rate for formate
dehydrogenation as well as prolonged catalytic lifetime
(Figure 1, blue squares) was obtained with a twofold excess of
mTPPTS per ruthenium dimer (that is, total Ru/P=1:3). With
lower excesses, the initial rate for formate dehydrogenation
was equally high (Figure 1, red squares), but gradually de-
creased with the concurrent appearance of black ruthenium
nanoparticles within the reaction solution. It is known that in-
sufficient phosphine stoichiometry leads to poor stabilization
at ruthenium, which is also likely in this case.[34] On the contra-
ry, with larger excesses of mTPPTS the first coordination sphere
of the metal becomes saturated. This would hinder substrate
coordination and thus inhibit formate dehydrogenation at
higher Ru/P ratios.
capacity of 0.85 wt% at 808C and a volumetric H2 density of
À1
23 gLsolution
.
Toxicity
A considerable parameter for practical hydrogen storage appli-
cations is the impact of a system failure on both human health
and the environment. As cesium salts have never been report-
ed as hydrogen carriers, a short insight into their properties is
pertinent. The cesium ion itself is more toxic than sodium, but
less toxic than the potassium or lithium ion.[29] However, the
toxicity of alkali metal salts is strongly influenced by the partic-
ular anion. As cesium formate has widely replaced the toxic
and corrosive zinc bromide as a drilling and completion fluid
in the oil industry,[30] it has undergone extensive studies into
its biochemical compatibility, toxicology and environmental
impacts. Cesium formate is readily biodegradable;[31] it is char-
acterized as an environmental-friendly compound and an acci-
dental release is not expected to adversely affect the environ-
ment owing to the well-known formate degradation pathways.
Furthermore, elevated cesium levels in the environment have
not been reported to cause any adverse effects.[32,33] The dis-
charge of cesium in aquatic systems is likely to be extenuated
by dilution, whereas in terrestrial environments sorption of
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