Electrocatalysts for the anodic oxidation of borohydrides

Article abstract of DOI:10.1016/j.electacta.2004.02.023

Anodic performances of alkali borohydrides on several electrocatalysts such as Ni, Raney Ni, Pd, Pt, Cu, Au have been studied in an effort to develop suitable electrode materials for a borohydride-fueled fuel cell. The open-circuit potentials (OCPs), polarization performances, and hydrogen evolution behaviors were examined in a three-electrode system. The open-circuit potential was found to be dependent on borohydride concentration and also influenced by the electrocatalyst. In concentrated borohydride solutions used in this work, electrode polarizations were less influenced by the fuel concentration. Borohydrides on different electrocatalysts showed different hydrogen evolution behaviors. The relation of hydrogen evolution rate with the anode current was found to change not only with the concentration of borohydride and but also with the electrode material. Comparison of anodic behaviors of borohydride on different electrodes implies that the anodic oxidation of borohydride, as a multi-step process, may take different reaction paths, depending on the electrocatalyst and reaction conditions such as the borohydride concentration.

Full text of DOI:10.1016/j.electacta.2004.02.023

Electrochimica Acta 49 (2004) 3097–3105
Electrocatalysts for the anodic oxidation of borohydrides
Bin Hong Liu∗, Zhou Peng Li, Seijirau Suda
Department of Environmental and Chemical Engineering, Kogakuin University, Hachioji, Tokyo 192-0015, Japan
Received 3 October 2003; received in revised form 19 January 2004; accepted 10 February 2004
Available online 27 March 2004
Abstract
Anodic performances of alkali borohydrides on several electrocatalysts such as Ni, Raney Ni, Pd, Pt, Cu, Au have been studied in an effort
to develop suitable electrode materials for a borohydride-fueled fuel cell. The open-circuit potentials (OCPs), polarization performances, and
hydrogen evolution behaviors were examined in a three-electrode system. The open-circuit potential was found to be dependent on borohy-
dride concentration and also influenced by the electrocatalyst. In concentrated borohydride solutions used in this work, electrode polarizations
were less influenced by the fuel concentration. Borohydrides on different electrocatalysts showed different hydrogen evolution behaviors.
The relation of hydrogen evolution rate with the anode current was found to change not only with the concentration of borohydride and but
also with the electrode material. Comparison of anodic behaviors of borohydride on different electrodes implies that the anodic oxidation of
borohydride, as a multi-step process, may take different reaction paths, depending on the electrocatalyst and reaction conditions such as the
borohydride concentration.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: Borohydride; Anodic oxidation; Open-circuit potential; Polarization; Hydrogen evolution
1. Introduction
methanol, borohydrides are thus expected to provide higher
cell voltage and power density.
The increasing demand for efficient and clean power
sources has greatly stimulated the research and develop-
ment of fuel cells in recent years. Fuel cells are not only
of interest in some large scale applications, but also being
considered as power sources for portable machines such
as lap-top computers and cellular phones. In these applica-
tions portable fuel sources are required. Currently the direct
methanol fuel cell (DMFC) is attracting considerable atten-
tion as the fuel cell for portable applications [1]. However,
it usually shows low cell voltage and power density. The
poor anode performance of methanol requires heavy load-
ing of noble metals like Pt or Pt–Ru alloy as the catalyst.
On the contrary, aqueous solutions of alkali borohydrides
(MBH4, M: Li, Na, K) as another liquid-type fuel have
high reactivity and non-precious materials like Ni and Cu
are good enough to catalyze their anodic reactions [2,7].
In theoretical calculations, the standard potential of boro-
hydrides is 0.43 V more negative than that of methanol in
an alkaline environment. With similar energy densities as
Although the anodic oxidation of borohydrides is postu-
lated as follows:
BH4− + 8OH− = BO2− + 6H2O + 8e, E◦ = −1.24 V
(1)
This reaction is barely found to happen in practice because
the anodic reactions on electrodes have to compete with the
hydrolysis reaction as follows:
BH4− + 2H2O = BO2− + 4H2
(2)
The actual anodic reaction will be as follows:
BH4− + xOH− = BO2− + (x − 2)H2O + (4 − x)H2 + xe
1
2
(3)
x here represents the actual number of the electrons released
−
by each BH4 ion and it is called columbic number in fol-
lowing paragraphs.
It was reported by Indig and Snyder [2] that on a porous
nickel electrode the anodic reaction was a 4e reaction, cor-
responding to x = 4 in the above formula. Kubokawa et al.
[3] reported a 6e reaction on a Pd-coated carbon electrode.
∗
Corresponding author.
E-mail address: bp97002@ns.kogakuin.ac.jp (B.H. Liu).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.02.023

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B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
Table 1
Details of the tested electrodes
Electrode
Powder
Ni
Raney Ni
Cu
Au
Pd
Pt
Inco type210
1.0 g
Raney Ni
0.01 g
Cu (10 ␮m)
1.0 g
Au (5 ␮m)
1.0 g
Pd/C (Pd 10 wt.%)
0.4 g
Pt/C (Pt 5 wt.%)
0.4 g
(Pd 40 mg)
(Pt 20 mg)
PTFE (g)
Area (cm2)
0.05
5
0.05
5
0.05
5
0.1
5
0.05
5
1.5
Elder and Hickling [4] found that on a Pt electrode the
columbic number is between 2 and 4. And Amendola et al.
[5] reported a 7e reaction on an Au electrode.
In a previous paper, we made a detailed study on the an-
odic oxidation of borohydride catalyzed by a nickel electrode
and also found a 4e anodic reaction [6]. In this work, we
studied the electrochemical performances of borohydrides
on several electrocatalysts including Ni, Raney Ni, Pd, Pt,
Au, Cu in an effort to select proper anode material for a
borohydride-fueled fuel cell. We also hope to clarify the re-
action mechanism of borohydride oxidation by comparison
of anodic behaviors on these electrodes.
2. Experimental
The electrodes were made by first thoroughly mixing and
grinding metal or carbon-supported noble metal powders
with polytetrafluoroethylene (PTFE) powder, then pasting
the mixture on a nickel foam with a size of 2.5 cm×2 cm and
then enveloping it using a 100 mesh nickel net, and finally
roller-pressing. A blank test showed that the effect of nickel
foam and net supporter on electrochemical performance of
Fig. 2. Dependence of the open-circuit potential on the borohydride
concentration for three electrodes at 293 K.
electrodes was negligible. Metal powders used in the exper-
iment had purities at least more than 99%. The Raney Ni
electrode was made of a Ni foam on which a layer of Raney
Ni was formed. The details of electrodes are described in
Table 1.
Anodic performances of borohydrides on the electrocata-
lysts were tested in a three-electrode system shown in Fig. 1.
The anode was put in the middle compartment and the
counter electrode and reference electrode were put in the
outside compartments, respectively. The anode compartment
was separated from other compartments by Nafion 117 mem-
branes. The anode had a distance of 3.3 cm with the cathode
and with the center of the reference electrode. The anolyte
was prepared by dissolving NaBH4 in 100 ml 6 M NaOH or
KBH4 in 100 ml 6 M KOH. The electrolyte in outside com-
partments was correspondingly 6 M NaOH or 6 M KOH.
Table 2
Open–circuit potentials exhibited on different electrodes at 293 K in a
2.4 M NaBH4 solution
Electrode
OCP vs. NHE (V)
Ni
Raney Ni
Cu
Au
Pd/C
Pt/C
−1.03
−1.03
−1.02
−0.99
−0.91
−0.91
Fig. 1. Schematic diagram of the experiment set-up: (1) anode, (2) anolyte,
(3) anolyte inlet, (4) H2 gas outlet, (5) cathode, (6) 6 M KOH or 6 M
NaOH electrolyte, (7) Hg/HgO reference electrode, and (8) Nafion 117
membrane.

B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
3099
−
The cathode was a metal hydride electrode and a Hg/HgO
electrode was used as the reference electrode. Open-circuit
potentials (OCPs) of electrodes at different borohydride con-
centrations were measured by immersing the electrodes in
the solution until the potential became stable within 10 min.
Polarizations were recorded when anodic currents passed the
electrode and at the same time hydrogen evolved from the
anode was collected and the volume was converted into the
value at the standard temperature and pressure (STP), then
the hydrogen evolution rate was calculated.
Fig. 2 shows the relation of OCP with the BH4 concen-
tration in a KBH4/6 M KOH solution for three electrodes.
It can be seen that a step appeared in the OCP curve both
for the Ni powder electrode and the carbon-supported Pd
electrode while the Raney Ni electrode did not show such
behavior and its OCP linearly changed with the logarithm
of the borohydride concentration. We suppose the step in
the OCP curves was related to−a change in surface states.
Below one certain level of BH4 concentration, the surface
of electrodes remained in a passive state. When the concen-
tration of borohydride was increased, the surface oxide or
hydroxide was reduced by the borohydride and then acti-
vated. The potential then reflected the real anode potential
of borohydride.
3. Results
3.1. Open-circuit potentials
The OCP was also a little influenced by the electrocata-
lysts. Table 2 shows the OCPs in a 2.4 M NaBH4 solution
for different electrodes. It seems that the Ni and Raney Ni
electrode exhibited the most negative potential, followed by
The open-circuit potentials of the anodes were found to
depend on the concentration of borohydride in the solution.
Fig. 3. Polarization curves for the (a) Ni electrode at different borohydride concentrations at 303 K; (b) Pd/C electrode at different borohydride
concentrations at 303 K; and (c) Pt/C electrode at different borohydride concentrations at 303 K.

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B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
Cu, Au, Pt, and Pd. The observed OCPs were all less neg-
ative than the calculated standard potential of the reaction
(1) but more negative than the hydrogen potential. The less
negative potential is supposed to be caused by the presence
of some intermediate oxidation products of BH4−, espe-
cially atomic hydrogen generated by the reaction (2). The
carbon-supported Pd and Pt electrodes showed less negative
potentials than other electrodes like Ni and Cu. The slight
differences among the observed potentials on different elec-
trocatalysts may be resulted from different anodic oxida-
tion mechanisms, which will be discussed in the following
section.
concentrations for the Ni, Cu, and also Pt electrode. The oc-
currence of maximum current behavior is usually attributed
to the limitation of mass transfer. However, in this case it is
also possible that the phenomenon was caused by a forma-
tion of oxide film on the electrode surface. When the poten-
tial goes into the range in which a surface oxidation reaction
occurs, the surface will be oxidized and the anodic oxida-
tion of borohydride could not continue without the active
electrocatalyst. Therefore, there would not be stable anodic
current beyond certain potential levels.
At a polarization current density of 100 mA/cm2, as can
been seen in Fig. 3(a), the potential of the Ni electrode was
still be more negative than the hydrogen potential, while
this was not the case for the carbon-supported Pd and Pt
electrode.
3.2. Polarization
Fig. 3 shows the polarization behaviors of borohydride on
different electrodes. From the figure, it can be seen that the
polarizations on these electrodes changed little with the in-
crease of the borohydride concentration in the concentrated
solutions. This result differs from our previous observation
that in dilute solutions the polarization on the Ni electrode
was largely dependent on borohydride concentration [6]. It
seems that the polarization became stable when the boro-
hydride concentration was increased to a certain level. This
has a practical meaning because stable cell voltage can be
achieved if the anode polarization is not so sensitive to the
fuel concentration.
3.3. Hydrogen evolution
One of the most important features for selecting the
electrode material for anodic oxidation of borohydrides is
to decrease the evolution of gaseous hydrogen because hy-
drogen evolution not only reduces fuel efficiency but also
cause other problems such as safety problem of the fuel.
In this work, we made observations on how the hydrogen
evolution rate changed with the anode current at different
borohydride concentrations on these electrodes. Fig. 4(a)
shows the relations of hydrogen evolution rate with anodic
current at different NaBH4 concentration levels on the Ni
electrode. The hydrogen evolution rate first kept or a little
decreased and then increased with increasing the anodic
Although the polarization did not change much when the
borohydride concentration varied from 0.5 to 2.4 M, maxi-
mum currents were observed at relatively low borohydride
Fig. 4. (a) Relation of the hydrogen evolution rate with the anode current at different borohydride concentration levels for the Ni electrode at 303 K. (b)
Changing of the potential, hydrogen evolution rate and borohydride concentration with the time when an anode current of 4 A was exerted on the Ni
electrode in a 2.4 M borohydride solution at 303 K.

B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
3101
Fig. 4. (Continued ).
current. Finally they fell into the stoichiometric relation of
the 4e reaction (corresponding to x = 4 in Eq. (3)). In-
creasing the initial concentration of borohydride led to an
increase of limiting current but did not change the 4e re-
lation. Fig. 4(b) shows the changing of hydrogen evolution
rate with the time at a constant anode current of four am-
pere. Because of the limited volume of 110 ml for the fuel,
the borohydride concentration decreased with the time. The
actual borohydride concentration in the solution was calcu-
lated from the obtained electricity and collected hydrogen
gas according to the reactions (1) and (2). Although the
hydrogen evolution behavior on the carbon-supported Pd
electrode. The hydrogen evolution rate first showed an ob-
vious decrease but finally turned to inc−rease again with the
increase of the current. When the BH4 concentration was
lower than 1.5 M, the hydrogen evolution rate and the cur-
rent −had a relation near to a 6e reaction. However, when the
BH4 concentration became higher, the relation changed to
a 4e reaction. Fig. 5(b) shows the changing of hydrogen evo-
lution rate with the time at a constant current of 4 A on the
Pd electrode. It can be seen that the hydrogen evolution rate
decreased continuously with the time and did not show a def-
inite stoichiometric relation with the current. It may rather
be dependent on the actual concentration of borohydride in
the solution as they showed a parallel relation in Fig. 5(b).
Fig. 6(a) shows the relation of hydrogen evolution rate
with the current on a carbon-supported Pt electrode. Similar
to the Pd electrode, the hydrogen evolution rate showed a
−
BH4 concentration continuously decreased from 2.4 M at
the start to about 0.6 M at the end, the hydrogen evolution
rate kept near the stoichiometric value of the 4e reaction.
It can be concluded that the anodic oxidation of borohy-
dride on the nickel electrode was essentially the 4e reaction
within the concentrations of borohydride used in this work.
On the other side hydrogen evolution was more com-
plicated on the Pd and Pt electrode. Fig. 5(a) shows the
−
changing relation with the current at different BH4 con-
centration levels. It is interesting to note that hydrogen

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B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
evolution decreased and almost ceased at large currents
when the BH4 concentration was less than 1.5 M. The
intermediates and different reaction routes for the oxidation
of borohydride. For example, Elder and Hickling [4] pro-
posed the following reaction scheme for the anodic behavior
of borohydride on a smooth platinum electrode:
−
anodic reaction appeared to be near the ideal 8e reaction.
However, the electrode potentials at these large currents
were not so negative. When the borohydride concentration
was further increased to more than 1.5 M, a complete stop
of hydrogen evolution did not happen before it turned to
increase with increasing the current. Fig. 6(b) shows a pro-
longed anodic oxidation of 4 A on the Pt electrod−e. It can
be seen that higher initial concentrations of BH4 led to
larger hydrogen evolution rate, corresponding to Fig. 6(a).
However, hydrogen was generated at the same rate during
• initial step
BH4− + Pt − 2e ꢀ Pt · · · BH3 + H+
Pt · · · BH3 + OH− → Pt · · · BH3OH−
(4.1)
(4.2)
• rate-determining step
Pt · · · BH3OH− − 2e → Pt · · · BH2OH + H+
(4.3)
−
the period though the BH4 concentration kept decreasing
Pt · · · BH2OH + H2O → Pt + BH2(OH)2− + H+ (4.4)
with the time. It seems that in thi−s case hydrogen generation
rate depended on the initial BH4 concentration rather than
the actual concentration at the time. Compared with that on
the Pd electrode, the Pt electrode showed different behavior
in this aspect.
Similar to the Ni electrode, the 4e reaction was found to
happen on the Raney Ni and Cu electrode. In a 2.4 M boro-
hydride solution, the Au electrode showed a 4.5e stoichio-
metric relation with the current.
• slow hydrolysis
Pt · · · BH3OH− + H2O → Pt + BH2(OH)2− + H2
(4.5)
• completion step
BH2(OH)2− + H2O → BH(OH)3− + H2
(4.6)
(4.7)
−
BH(OH)3 → H2BO3− + H2
The overall reaction −gives a columbic number between
2 and 4 for each BH4 ion. Mirkin et al. [9] suggested
the first step of borohydride oxidation on a Au electrode
was a 2e process with the BH3-like intermediate rather than
BH3OH−.
4. Discussion
The electrochemical oxidation of borohydride is supposed
to be a stepwise process [4], complicated by a stepwise
hydrolysis reaction [8]. Therefore, there may exist several
Fig. 5. (a) Relation of the hydrogen evolution rate with the anode current at different borohydride concentrations for the Pd/C electrode at 303 K. (b)
Changing of the hydrogen evolution rate and borohydride concentration with the time when the Pd/C electrode was under a current of 4 A at 303 K ((I)
initial borohydride concentration 1.5 M; (II) initial borohydride concentration 2.4 M).

B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
3103
Fig. 5. (Continuned ).
Our results shown above demonstrate changing columbic
numbers, depending not only on borohydride concentra-
tion but also on electrode materials. Although it is too
early for us to deduce reaction mechanisms without more
detailed analyses, we may get some clues to understand
the reaction mechanism by comparing the anodic be-
haviors of borohydride on these electrodes. Three major
differences can be concluded based on the experimental
results. First, the OCP of the Ni electrode was observed
to be more negative than those of Pd and Pt, which was
also reported by others [3,7,10]. Second, the potential at
100 mA/cm2 on the Ni electrode was still more negative
than that of atomic hydrogen, while this was not the case
for the carbon-supported Pd and Pt electrode. Third, the
anodic reaction−on the Ni was primarily a 4e process within
the tested BH4 concentrations and current densities. On
the contrary, the columbic number changed on the Pd an−d
Pt electrodes with changing current density and BH4
concentration.
Based on the reaction scheme proposed by Elder and
Hickling [4] and our observations on different electr−ocat-
alysts, we suppose that the anodic oxidation of BH4 on
the electrodes may take different reaction paths, depend-
ing on electrocatalysts and reaction conditions. On the Ni
electrode, we suppose that the reaction may be initiated
−
by the electrochemical reactions of BH4 and BH3OH−
shown as (4.1) and (4.3), and then followed by the hydrol-
−
ysis reaction of the derived intermediates BH2(OH)2 and
−
BH(OH)3 shown as (4.6) and (4.7). The overall reaction
gives a columbic number of four. On the other side, the
anodic reaction on the Pt and Pd electrode may be more
complicated as shown in Figs. 5 and 6. In less concentrated

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B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
solutions, the electrochemical reaction predominating on the
Pd or Pt electrode may be the ionization of atomic hy-
drogen, the product of the hydrolysis reaction. Then we
can explain why the OCPs on Pt and Pd were less nega-
tive than that on the Ni electrode and were near to that of
atomic hydrogen. The gradual decrease and final cease of
hydrogen evolution on Pt electrode with the increase of an-
ode current may be described as a gradual consumption of
the atomic hydrogen produced by the hydrolysis reaction.
towards the electrochemical and hydrolysis reaction of boro-
hydride ions, boron-containing intermediates and atomic
hydrogen.
Although hydrogen evolution was decreased or even
ceased on carbon-supported Pd and Pt electrodes, it
only took place at limited conditions when the con-
−
centration of BH4 and the temperature were not very
high and the current was large enough. At these con-
ditions, the polarizations were large. It may have little
practical meaning because the very negative potential of
borohydride was not obtained and therefore there was
a large energy loss. It may not be more advantageous
than an indirect oxidation process which means that
borohydride is first catalytically hydrolyzed to hydrogen
gas and then hydrogen is used as the fuel to feed fuel
cells.
Borohydride is a promising fuel considering its high
reactivity that results in a high open-circuit potential and
good anode performance. However, hydrogen evolution is
still a problem. Our results above show that the Ni elec-
trode is cheap and has good performance but it ends up
with the 4e reaction. Although it is possible to achieve
higher fuel efficiency on a Pd or Pt electrode under cer-
tain conditions, the process may not be so advantageous
compared with an indirect process via hydrogen. One more
problem is how to deal with the metaborate, the oxidation
product of borohydride. If these two problems are solved,
borohydride may serve as a suitable fuel for a portable
purpose.
−
When the concentration of BH4 increases, the direct an-
−
odic oxidation of BH4 or BH3(OH)− becomes more fa-
vorable. The anodic reaction may be predominated by the
direct anodic oxidation of borohydride ions and hydrogen
tends to evolve out. The columbic number then decreases
down at concentrated solutions. Although hydrogen evo-
lution behaviors on Pd and Pt showed some similarities,
they also showed differences. In the prolonged test shown
in Fig. 5(b), the hydrogen evolution rate on the Pd elec-
trode changed continuously with the time, while that on the
Pt electrode kept the same value during the period. It sug-
gests that the reaction path might change during the test
on the Pd electrode but the reaction path kept the same on
the Pt electrode and it only depended on the initial reaction
condition.
More experiments and analyses are needed to clarify the
reason why borohydride shows different anodic behaviors
on different electrocatalysts. We suppose the difference
in reaction mechanism on the Ni and Pt or Pd electrode
may be mainly due to their different catalytic abilities
Fig. 6. (a) Relation of the hydrogen evolution rate with the anode current at different borohydride concentrations for the Pt/C electrode at 303 K. (b)
Changing of the hydrogen evolution rate and borohydride concentration with the time when the Pt/C electrode was under a current of 4 A at 303 K ((I)
initial borohydride concentration 1.5 M; (II) initial borohydride concentration 2.4 M).

B.H. Liu et al. / Electrochimica Acta 49 (2004) 3097–3105
3105
Fig. 6. (Continued ).
5. Conclusions
References
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electrocatalysts was studied in this work. The open-circuit
potentials shown on electrodes were less negative than
the theoretical value but more negative than the hydrogen
potential at high borohydride concentrations. The Ni elec-
trode showed good anode performance with more negative
open-circuit potential and small polarizations but the re-
action was a 4e reaction where half of borohydride turned
into hydrogen gas. Higher efficiency can be achieved on
Pd and Pt electrode at some special conditions such as
relatively low borohydride concentration and large anode
current. An almost cease of hydrogen evolution was found
on the carbon-supported Pt electrode at certain conditions.
We suppose that the anodic reaction of borohydride may
change its path depending on reaction conditions and the
nature of electrocatalysts.
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