Borohydride-bromate fuel cell. (SHORT COMMUNICATION)
Yuriy V. Tolmachev, Ftorion, Inc., Boston, MA, USA.
Abstract:
A polymer electrolyte fuel cell with Pt grid electrodes employing acidic lithium bromate as the oxidant
and alkaline sodium borohydride as the reducer is demonstrated. Such system provides simultaneously
high energy (over 1,000 Ah/L and 660 Ah/kg in the tanks) and high power (0.5 W/cm2 at 60 oC) densities
on discharge. A regenerative fuel cell cycle employing hydrogen as the reducer is also discussed.
Highlights:
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



Sodium Borohydride solutions have a very high energy density and can be safely stored and used
at alkaline pH.
Lithium halate solutions also have very high energy density and can be safely stored at neutral
pH.
The electroreduction of bromate on platinum becomes facile upon acidification.
A discharge fuel cell system utilizing lithium bromate and sodium borohydride shows
simultaneously high energy and power densities.
When H2 is used as the reducer, a cost- and energy- efficient electric or solar regeneration of
LiBrO3 from LiBr is possible.
Keywords: flow battery, fuel cell, borohydride, bromate, comproportionation, air-independent
propulsion, high test peroxide.
Introduction:
Hydrogen peroxide is commonly employed nowadays for air-independent propulsion[1, 2] and for
portable electricity generation.[3-8] However, the use of H2O2 at various concentrations compromises
achieving simultaneously good safety, energy and power densities. Ftorion, Inc. recently disclosed fuel
cells utilizing lithium halates as oxidants.[9-11] These oxidants afford record-high specific charge
(Ah/kg) and charge density (Ah/L) due to their enormous solubilities in water[12-15] and their
multielectron redox character. In particular, aqueous solutions of LiBrO3 (13.27 m) and LiClO3 (41.15 m)
saturated at 20 °C carry, respectively, 765 and 1,402 Ah per kg of solution, which is competitive with
1,576 Ah/kg for High Test Peroxide (85 w/w % H2O2), especially when a higher energy conversion
efficiency and power density (see below) are taken into account. The final products of lithium halate
reduction, LiBr and LiCl, have even higher molal solubilities.[13] Whereas Ftorion, Inc. is currently
pursuing halate-based regenerative fuel cells with H2 reducer for stationary energy storage and for
automotive propulsion,[9-11] we report herein the performance of a non-rechargeable fuel cell with
sodium borohydride[16, 17] reducer and lithium bromate oxidant. The high areal power achieved with
this system, combined with outstanding specific energy and safety of the reagents, makes it interesting for
submarine, aerospace and portable applications.
Experimental:
A single fuel cell with a 50×50 mm2 active area, comprising machined carbon flow plates and gold-coated
copper endplates, was custom-made by Fuel Cells Etc. The inlet and outlet ports were made on the sides
of the carbon flowplates to avoid wetting the metal endplates. The positive flow field was five-serpentine
(1.0 mm deep, 1.0 mm wide, 1.0 mm wall) and the negative flow field was pin-type (1.3 mm deep, 1.6
mm square pins and 2.4 mm interpin space). Smooth platinum gauzes (Sigma-Aldrich # 298093-1.7G,
100 mesh, 0.16 mm thick, 50×50 mm2) surrounded by Parafilm™ gaskets (0.12 mm pre-compressed
thickness) and separated by Nafion™ XL membrane (27.5 µm thick, Fuel Cells Etc.) were used on both
sides as the electrodes.
200 mL of 10.0 M NaBH4 + 5.0 M NaOH ( d= 1.11 g/mL ) was used as the anolyte and 500 mL of 9.0 M
LiBrO3 + 5.0 M H2SO4 ( d= 1.19 g/mL ) was used as the catholyte. The data shown herein are for twoweek old solutions stored at 20°C. Despite the appearance of a gray precipitate in the alkaline borohydride
solution and of a brown discoloration in the in the acidic bromate solution, the discharge curves are not
significantly affected by these effects. Two separate peristaltic pumps with Viton™ tubings connected to
PFA tubings were used to recirculate the anolyte (0.13 mL/s ) and the catholyte (4.05 mL/s). Impedance
measurements on a compete cell at the open circuit voltage yielded 0.0058 Ohm as the high frequency
limit of the real part (not shown), which agrees well with the expected value for the membrane resistance.
The data shown in this work were not corrected for this series resistance. The discharge power was
controlled using a home-made electronic load. In order to maintain the temperature of the experiment, the
whole system, including the reagent bottles and the pumps (but not the electronic load), was placed into a
convection oven. The open circuit potential, the discharge curve and high-frequency series resistance
were reproducible over 0.5-4 h period, thus suggesting that the membrane behavior in this time scale can
be adequately described by steady-state models.
Results, discussion, conclusions and future outlook:
The borohydride solution used in this work nominally contains 2,114 Ah/L and 1,905 Ah/kg. The
bromate solution nominally contains 1,447 Ah/L and 1,216 Ah/kg. Together, these solutions (both are at
about 60-70% of the saturation limit of the redox species at 20 °C) give nominally 1,002 Ah/L and 661
Ah/kg at the tank level, which are ca. 60 and 30 times higher than corresponding numbers for Vanadium
Redox Flow batteries.[18, 19] When compared with a C6-LiFePO4 battery pack in Tesla Roadster™ ,
these fluids provide 14.6 and 5.5 higher energy (in Wh) content per weight and per volume respectively,
even when operating at the half-peak power (i.e. 1.40 V, see Fig. 1).
Fig. 1 shows discharge curves obtained in the same single fuel cell supplied with an alkaline borohydride
solution to the negative electrode and with an acidic bromate solution to the positive electrode at 20 and
60 °C. In both cases we observed a very high open circuit potential, between 2.3 and 2.5 V which is about
twice as large as the electrochemical stability window of water at a single pH but is expected nevertheless
from the theoretical values of the equilibrium potentials of the reducer and oxidant and their respective
pH’s. The discharge current remains below 100 mA/cm2 at the cell voltages above 1.5 V. We attribute
the high voltage portion of the discharge curve to a direct electroreduction of bromate on Pt:[20]
BrO3 ̅ + 6H+ + 6 e ̅ = 6Br ̅ + 3H2O
(1)
At the cell voltages below 1.5V we observed a steep increase in the current which we tentatively attribute
to the facilitation of the bromate reduction via a homogeneous com-proportionation (2) [21-23] with
bromide product yielding an electroactive (3) [24-26] bromine:
5Br ̅ + BrO3 ̅ + 6H+ = 3H2O + 3Br2
(2)
3Br2 + 6 e ̅ = 6Br ̅
(3)
The peak powers are 0.25 and 0.50 W/cm2 at 20 and 60 °C, respectively, which are significantly higher
than the peak powers (0.1 W/cm2) commonly reported for direct borohydride - peroxide fuel cells.[5] The
doubling of the peak power upon a 40 °C increase in the operating temperature also agrees with a
behavior commonly found with other liquid reagent fuel cells.[5, 7, 27-31] It is worth noting the peak
power that we reported earlier[9] for a similar cell supplied with 1 atm H2 / ca. 9 M HBrO3 is ca. 0.6
W/cm2 at 20 °C, thus tentatively suggesting that in the present case the power is limited by the oxidation
of borohydride rather than by the reduction of bromate.
Although not shown in this work, we disclosed earlier[10] that much higher acid concentrations in the
oxidant solution can be used in practice since an acid can be added to the halate solution right before the
discharge either by mixing or by using reagent-free Orthogonal Ion Migration Across Laminar Flow
(OIMALF) process akin to ion suppression of Ion Chromatography. Also disclosed therein, is the process
of comproportionation (2) - mediated electroreduction (3) of halates in acid which does not require the
use of noble metal electrocatalysts. Furthermore, in the case of hydrogen used as the reducer, the halate
flow battery allows for an efficient regeneration of the reducer and oxidant in splitting-disproportionation
cycle (4)-(5) using electricity or sunlight[32] as the only energy input(s), without consuming any
chemicals and without generating any waste:
6LiX + 6HA = 3H2 + 6LiA + 3X2
6LiA + 3X2 + 3H2O = LiXO3 + 5LiX+ 6HA
electrolysis or photoelectrolysis (4)
disproportionation (5)
According to literature,[21-23, 33-38] the disproportionation of Br2 to BrO3- and Br- is thermodynamically
favorable at pH above 5.2, although this reaction has the maximal rate near pH 8.0 due to the formation of
an intermediate hypobromite which is kinetically stable toward further disproportionation at pH >
pKa(HBrO) = 8.8. We found earlier that Li-MOPS buffer (pKa= 7.20) works well for such
regeneration.[9, 10]
The process of photoelectrochemical regeneration of LiBrO3 from aqueous LiBr can finally enable the
human civilization to use sunlight (with its average power striking the planet exceeding the 13 TW needs
of our civilization by a factor of over 10,000)[39, 40] as the primary energy source for transportation. In
fact, it has been already demonstrated[41, 42] that open-circuit potential photoelectrolysis of aqueous HBr
on semiconductor particles can be done at a lower cost and with a higher efficiency than the
photoelectrolysis of water advocated by proponents of hydrogen economy.
Acknowledgement:
This work was funded by Ftorion, Inc.
(+) 9M LiBrO3+ 5M H2SO4, 5-serpentine FF, Pt gauze 100
1200
1000
i, mA/cm
2
membrane Nafion XL (27.5 um)
(-) 10M NaBH4+5M NaOH, pin FF, Pt gauze 100
800
600
o
400
200
P, mW/cm
2
60 C
o
20 C
U, V
0
0.0
0.5
1.0
1.5
2.0
2.5
Fig.1. Discharge curves (current- solid symbols, power- open symbols) obtained with a borohydridebromate flow cell at 20°C (circles) and at 60°C (squares). Other conditions are specified in the figure and
in the text.
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