Nickel-Cerium Alloys for Borohydride Oxidation

Abstract

Nickel and nickel-cerium (Ni-Ce) alloys with different amounts of Ce (5 and 10 at.% Ce) have been prepared by arc melting and characterized by scanning electron microscopy and electron probe microanalysis based on energy dispersive X-ray spectroscopy, to investigate microstructure and to determine phase composition. Subsequently, Ni and Ni alloy-based electrodes were fabricated and studied for borohydride (BH4-) oxidation in alkaline media by cyclic voltammetry and chronoamperometry in the temperature range 25 - 45 ºC. Preliminary results showed comparable BH4- oxidation peak current densities obtained at three electrodes, with Ni95Ce5 electrode giving the highest current densities. Electrochemical data are modelled to obtain relevant kinetic parameters of BH4- oxidation at these electrodes. Effect of electrolyte temperature on the evaluated parameters has been investigated as well.

Full text

D. M. F. Santosa,*, B. Šljukića, L. Amarala, D. Macciòb, A. Sacconeb,
and C. A. C. Sequeiraa
1
a Materials Electrochemistry Group, ICEMS, Instituto Superior Técnico, University of Lisbon,
Av. Rovisco Pais, 1049–001 Lisboa, Portugal
b Università di Genova, Dipartimento di Chimica e Chimica Industriale, via Dodecaneso 31, I-16146 Genova, Italy
* E-mail: diogosantos@ist.utl.pt
224th ECS Meeting, San Francisco, CA, Oct. 27 –Nov 1, 2013

2
Sodium borohydride (NaBH4) as a fuel
Na
H
H
H
H
B
stable in alkaline media
readily available
gives water-soluble discharge products
carries a large amount of hydrogen (10.6 wt. %) in a safe and compact form
no carbon dioxide is produced during its oxidation in FCs
environmentally accepted oxidation product, sodium metaborate, with possibility of
recycling to NaBH4

3
Direct Borohydride Fuel Cells (DBFCs)
high theoretical specific
capacity, i.e., 5.7 Ah g-1
(based on NaBH4),
Type of cell DBFC DBPFC
Hydrogen FC
DMFC
Cell voltage / V 1.64 2.11 –3.02 1.24 1.19
Specific energy /
Whg-1 9.25 17.00 3.40 6.09
Anode: BH4
-+ 8 OH-→ BO2
-+ 6 H2O + 8 e-E0 = -1.24 V
Cathode: 2 O2+ 4 H2O + 8 e-→ 8 OH-E0 = 0.40 V
Overall: BH4
-+ 2 O2→ BO2
-+ 2 H2O E0 = 1.64 V
Cathode: H2O2+ 2 e-→ 2 OH-E0 = 0.87 V
Overall: BH4
-+ 4 H2O2→ BO2
-+ 6 H2O E0 = 2.11V

Electrocatalysts for BOR
4
Pt –high kinetics of BH4-oxidation
gold (Au) - the highest coulombic efficiency for BH4-oxidation
palladium (Pd)
silver (Ag)
alloys such as Pt-Au, Pt-Ag, Pt-ruthenium (Ru), Pt-rare earth (RE), Au-cobalt
(Co), rhodium-iridium (Rh-Ir) and osmium (Os) alloys
High cost, low reserves!
Non-noble metals:
nickel (Ni)
cobalt (Co) and its oxide CoO
zinc (Zn)
hydrogen storage AB5and AB2-type
Solution?
Bimetallic Ni alloys (with Rare Earth metals)
•high BOR rates
•increased coulombic efficiency, i.e., higher number of electrons transferred
NaBH4+ x OH-→NaBO2+ (x - 2) H2O + (4 - x/2) H2+ x e-

5
Experimental
One compartment glass cell with three-electrode set up
•Pt mesh (100 cm2)as counter electrode
•Saturated Calomel Electrode (SCE) as reference electrode
•Ni or NiCe alloy as working electrode
0.03 M NaBH4 in 2 M NaOH supporting electrolyte
Temperature controlled in the range from 25 to 45⁰C
Cyclic voltammetry (CV) measurements with scan rates in the range 0.5-3500 mVs-1
Chronoamperometry (CA) studies with different potentials (-0.9, -0.8, -0.7 and -0.6 V)
Fuel cell testing Ni-based electrode | NaBH4, NaOH|| HCl, H2O2|Pt

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Electrocatalysts characterisation
A - Ni-Ce with 95 at.% Ni: two phase alloy constituted by Ni black crystals (dendritic
shape) and eutectic formed by (Ni+CeNi5)
B - Ni-Ce with 90 at.% Ni: two phases consisting of CeNi5white crystals and eutectic
formed by (Ni+CeNi5). The average eutectic composition is around 90-91 at.% Ni.
Backscattered electron images of the Ni-Ce alloys: A) Ni95Ce5; B) Ni90Ce10.

7
CVs of Ni and Ni-Ce alloy electrodes in 0.03 M NaBH4solution
recorded at a scan rate of 50 mVs-1 and temperature of 25 ºC.
CV study of BOR at Ni and Ni-Ce electrodes
Ni + 2 OH-→α-Ni(OH)2+ 2 e-
Ni + H2O →NiO + 2 H++ 2 e-
Ni(OH)2+ OH-→NiOOH + H2O + e-
BOR

8
CV study of BOR at Ni and Ni-Ce electrodes
- reaction mechanism -
Two mechanisms have been proposed for BH4-oxidation at Ni and Ni-based electrodes.
Stepwise pathway: BHy-dissociation via four steps (y = 4, 3, 2, 1) to H atom and BHy-1-ion.
The generated ions combine with OH-forming BHy-1(OH)-with release of 1 electron in
each step (Eqs. 1 and 2).
BHy(OH)4-y-→BHy-1(OH)4-y-+ H (y = 4, 3, 2, 1) [1]
BHy-1(OH)4-y-+OH-→BHy-1(OH)5-y-+ e-[2]
Second proposed mechanism: rates of the simultaneous BH4-oxidation and hydrolysis
are determined by the potential, as well as the chemical state of electrode surface
(Eqs. 3 and 4).
BHy(OH)4-y-+ 2 OH-→BHy-1(OH)5-y-+ H2O + 2 e-[3]
BHy(OH)4-y-+ H2O→BHy-1(OH)5-y-+ H2[4]

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CV study of BOR at Ni and Ni-Ce electrodes
- charge transfer coefficient -
1/2
1/2 a
0
p
as
RT D (1 )n F
E E 0.78 ln ln
(1 )n F k RT

  

   



 



Electrode α
Ni 0.69
Ni95Ce50.65
Ni90Ce10 0.53
Dependence of peak potential, Ep, on logarithm of scan rate
for Ni95Ce5alloy electrode in 0.03 M NaBH4solution at 45 ºC.

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Number of
electrons exchanged
(n) in the BOR at
three studied
electrodes close to 1
Why number of
electrons exchanged
lower than
expected?
CV study of BOR at Ni and Ni-Ce electrodes
- coulombic efficiency -
 
1/2
5 1/2 1/2
pa
j = 2.99×10 1-α n nCD 


Dependence of peak current density, jp, on square root of scan rate, ν1/2,
for Ni95Ce5alloy electrode in 0.03 M NaBH4solution at 45 ºC.
Randles-Sevcik equation
for irreversible processes

11
CV study of BOR at Ni and Ni-Ce electrodes
- influence of temperature -
Anodic scans of Ni95Ce5electrode in 0.03 M NaBH4at a scan rate of 250 mVs-1
for temperatures of 25, 35 and 45 ºC

12
CV study of BOR at Ni and Ni-Ce electrodes
- heterogeneous rate constant -
 
a0
p s p
(1-α)n F
j = 0.227nFCk exp E -E
RT






Electrode ks
Ni 10-3 cm s-1 order of magnitude
independent of temperature
Ni95Ce510-6 to 10-4 cm s-1 order of magnitude
depending on temperature
Ni90Ce10 10-4 to 10-3 cm s-1 order of magnitude
depending on temperature

13
Chronoamperometry curves of the three studied electrodes recorded in
0.03 M NaBH4solution at potential of -0.9 V and temperature of 45 ºC
CA study of BOR at Ni and Ni-Ce electrodes
Ce + 2 OH-→Ce(OH)22+ + 4 e-
Ce(OH)22+ + 2 OH-→Ce(OH)4
Ce(OH)4→CeO2+ 2 H2O

14
Effect of the operation temperature on the polarization and power density
curves of a DBPFC employing Ni95Ce5anode.
Fuel cell testing

15
25 ºC
35 ºC
45 ºC
Peak power density / mW cm-2 58.1 73.2 110
Cell voltage at peak power density / V 0.45 0.55 0.50
Current density at peak power density / mAcm-2 129 133 219
Effect of temperature on Ni95Ce5| NaBH4, NaOH|| HCl, H2O2| Pt fuel cell.
Fuel cell testing

16
Conclusions
Ni and Ni-Ce alloys of different compositions were prepared by arc melting procedure
and characterized by SEM/EPMA and XRPD.
A thorough CV and CA study of BOR on three Ni-based electrodes, pure Ni, Ni95Ce5and
Ni90Ce50 alloy, has been performed.
Oxidation peak current densities obtained were of similar values at all studied
electrodes, with Ni95Ce5alloy electrode giving the highest current densities.
Kinetic parameters (charge transfer coefficient and heterogeneous rate constant) of
BOR were evaluated several different temperatures.
n values were found to be lower than expected.
Peak power density of 110 mW cm-2 was achieved at a cell voltage of 0.50 V and a
current density of 219 mAcm-2 at 45 ºC in a DBPFC employing Ni95Ce5alloy anode.

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THANK YOU
FOR YOUR ATTENTION
Acknowledgements
Fundação para a Ciência e a Tecnologia (FCT, Portugal)
Postdoctoral research grants no. SFRH/BPD/91853/2012 (D.M.F. Santos) and
SFRH/BPD/77768/2011 (B.Šljukić)
Research grant within the project PTDC/SEN-ENR/121265/2010 (L. Amaral)