Elsevier

Journal of Power Sources

Volume 176, Issue 1, 21 January 2008, Pages 287-292
Journal of Power Sources

Short communication
Study of fuel efficiency in a direct borohydride fuel cell

https://doi.org/10.1016/j.jpowsour.2007.10.036Get rights and content

Abstract

In this study, direct borohydride fuel cells (DBFCs) potentialities are evaluated. These emerging systems make it possible to reach maximum powers of about 200 mW cm−2 at room temperature and ambient air (natural convection) with high concentrated borohydride solutions. On the other hand, a part of borohydride hydrolyses during cell operating which leads to hydrogen formation and fuel loss: the practical capacity represents about only 18% of the theoretical one. In order to improve fuel efficiency, thiourea is tested as an inhibitor of the catalytic hydrolysis associated with BH4 electro-oxidation on Pt. The practical capacity is drastically improved: it represents about 64% of the theoretical one. Against, electrochemical performances (IE curves) are affected by the presence of thiourea.

Last, both playing with platinum anodic mass loading and the nature of catalyst, a good compromise has been found in terms of both specific capacity and power.

Introduction

Currently, ‘low-temperature’ fuel cells developments are mainly focused on two technologies: DMFC (direct methanol fuel cell) and PEMFC (proton exchange membrane fuel cell supplied with hydrogen). The very fast oxidation kinetic of hydrogen makes it unassailable: the DMFC cannot compete with the PEMFC technology in terms of power density [1]. However, the advantage of the DMFC is the undoubted easiness of storage and management of its liquid fuel. Even if improvements have been realized to reduce the permeability of the membrane to methanol and to make cathodic catalysts insensitive with methanol, the cross over of methanol from the anodic compartment to the cathodic one leads to an irremediable downfall of the DMFC performances.

In this environment, other liquid fuel cells are emerging such as direct borohydride fuel cells (DBFCs). Indeed, from a thermodynamical point of view, the electromotive force of a DBFC working with borohydride as the fuel is higher (1.64 V) than the one of the PEMFC (1.23 V). It is the same way for the theoretical yield (0.91 for the DBFC working with borohydride against 0.83 for the PEMFC). Lastly, if the oxidation of borohydride ions is straight, which means that there is no hydrogen intermediate formation (Eq. (1)), 1 mol of borohydride releases 8 mol of electrons. Then it should make it possible to reach 9300 Wh kg−1 of NaBH4.

In practice, Amendola et al. [2], [3] were one of the first team to announce in 1999 the fabrication of a DBFC. Its fuel core was composed of an anionic hydroxide ion-exchange membrane and a Au–Pt bimetallic anodic catalyst. The authors succeeded in reaching 20 mW cm−2 at room temperature and about 60 mW cm−2 at 70 °C. More recently, Liu et al. [4], [5] carried out several efficient DBFC tests: one with non-noble metallic catalysts (nickel on the anodic side, silver on the cathodic one) which get at 35 mW cm−2 at room temperature and an other one which reached 290 mW cm−2 at 60 °C with humidified air (palladium and Zr–Ni alloy on the anodic side, platinum on the cathodic side).

Obviously, despite its good reported results, DBFC still needs some specific efforts of development and especially ones concerning fuel management: a part of borohydride hydrolyses during cell operating which leads to hydrogen formation and fuel loss (Eq. 4c).

This reaction involves the decrease of faradic capacity of borohydride solutions. In addition, the accumulation of H2 in the anodic side generates some safety and packaging problems for industrial applications. For these reasons, most of the published DBFC studies focused on the anodic side of the fuel cell: some mention the use of H2 production inhibitors (use of thiourea [6], [7] which is known to block hydrogen adsorption on several metals such as Pd and Pt, use of Si [8] which allowed the authors to decrease hydrogen losses and to enhance the use of NaBH4 from 20 to 95%) while others quote the use of selective anodic catalysts (Au [3], [6], Zr0.9Ti0.1Mn0.6V0.2Co0.1Ni1.1 [9] and Ni [4]).

In this work, we present our contribution to the development of DBFC. In particular, we tried to evaluate the ability of thiourea to be a good inhibitor of hydrogen production as it seems to be very discussed in different works [6], [10], [11], [12].

Section snippets

Experimental details

The core part of the DBFC consists basically of two electrodes (anode and cathode) separated by a anion-exchange membrane supplied by Solvay (Morgane® ADP).

The anode Pt catalyst is prepared by mixing 80 wt.% Pt/C (E-TEK 80% Pt on Vulcan XC72) and Pt black (E-TEK) with ethanol–water (1:1) and a 60 wt.% PTFE dispersion in H2O (Aldrich). The resultant ink is then pulverized on a 3 cm × 3 cm carbon cloth. The anodic mass load of Pt is about 1.3 mg cm−2 or 0.3 mg cm−2 depending on the experiments.

The anode

Voltametric study

Fig. 2 exhibits the voltametric second cycles at 25 mV s−1 from −1500 to −250 mV (vs. Hg/Hg2SO4) and at 100 mV s−1 from −1500 to 0 mV on a platinum disk electrode (DE) for 10−1 M NaBH4 and 1 M NaOH medium. Fig. 3 exhibits the voltametric second cycle from −1450 to −250 mV at 25 mV s−1 on a platinum disk electrode (DE) for different concentrations of thiourea in 10−1 M NaBH4 and 1 M NaOH medium. Open circuit potential of Pt DE have also been measured in the different solutions: −1500 mV is OCP for 10−1 M NaBH4

Conclusion

The DBFC presents great power performances: indeed, the electrochemical study of a DBFC supplied with an alkaline solution of 2 M NaBH4 and 1 M NaOH shows that it is possible to reach 200 mW cm−2 at room temperature and ambient air. On the other hand, the autonomy of such a system is quite poor: NaBH4 hydrolysis is catalysed by platinum, that leads to huge hydrogen loss. At 1 A only 18% of the fuel is effectively used in the DBFC while 58% is lost by hydrogen production. 24% of the fuel is not used

Acknowledgements

The authors are grateful to Solvay S.A. for providing anionic membranes and acknowledge the French Ministry of Research for financial support.

References (14)

  • S. Amendola et al.

    J. Power Sources

    (1999)
  • B.H. Liu et al.

    Electrochim. Acta

    (2005)
  • Z.P. Li et al.

    J. Alloy Compd.

    (2005)
  • E.L. Gyenge

    Electrochim. Acta

    (2004)
  • M. Atwan et al.

    Int. J. Hydrogen Energy

    (2005)
  • L. Wang et al.

    J. Alloy Compd.

    (2005)
  • Ü.B. Demirci

    Electrochim. Acta

    (2007)
There are more references available in the full text version of this article.

Cited by (0)

View full text