Electrocatalytic Activity of Anode for Lithium Borohydride in Tetrahydrofuran

Introduction: Borohydrides (MBH₄, M = Li, Na, K, etc.) run the direct borohydride fuel cell (DBFC), a potential device that can supply a high working voltage and power at room temperature. The oxidation of borohydrides proceeds on the anode of the DBFC in an alkaline aqueous solutions in general. Finding a proper anode catalyst is thus necessary to efficiently generate the electricity from the DBFC. However, the non-electrochemical hydrolysis of borohydrides interferes with the evaluation of the electrocatalytic activity because the equilibrium potential of the ideal oxidation of borohydrides is lower than that of the H2-evolution from aqueous solution; i.e., BH₄^– + 8OH^– ↔ BO₂^– + 6H₂O + 8e^–: E^° = –0.41 V vs. RHE.¹

The present study investigates the anodic reaction of lithium borohydride (LiBH₄) in tetrahydrofuran (THF) on a varieties of metal electrodes in order to obtain a fundamental information on the catalytic activity of metals to oxidize hydride ion H^– in the borohydride to H₂. The result will be compared with the reduction of  H⁺ to H₂.

Experimental: 19 pure metals were investigated; namely, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au.  The metal wire (φ = 0.5 mm) fixed with glass was used as the working electrode. Prior to the measurement, the surface of metal wire was polished with diamond powder (1 µm) and buffered with alumina powder (0.05 µm).  The borohydride solution was prepared by dissolving 1 mol dm^–3 of LiBH₄in THF. Electrochemical properties, cyclic voltammetry, Tafel polarization and impedance spectroscopy, of the metal electrodes for the borohydride solution were examined in a three-electrode cell with stirring at 298 K and 1 atm using a Solartron Instruments SI 1280B. The counter and reference electrodes were Ni mesh and Li wire, respectively. Preparation of the cell and electrochemical measurements were carried out under the dried Ar atmosphere.

Result&discussion: Figure 1 shows the anodic polarization curves of borohydride solution on the 19 metal electrodes. In each case the relationship between the potential and the logarithmic current density follows the Tafel equation (η = a_a + b_alog i), suggesting that the charge transfer process is the rate-determining step in the anodic reaction of BH₄^–.

        The 19 metals can be classified into thre categories from the viewpoint of the slope b_a in Figure 1, the periodic group (PG) to which the metal belongs and the metal hydride formation: A. PG 4 and 5 (Ti, Zr, Hf, V, Nb, and Ta) & Pd; B. PG 6-10 (Mo, W, Re, Fe, Co, Rh, Ir, Ni, & Pt); and C. PG 11 (Cu, Ag, Au). The metals in Category A forms a strong bond with hydrogen to form metal hydrides. These metals have a low electrocatalytic activity for borohydride oxidation. Those in Category B are the transition metal which makes a weak bond with hydrogen. The activity increases with the increase in the atomic number. The traditional coinage metals, Category C, show the best electrocatalytic activity.

        The relationship between the exchange current density and the strength of the metal-hydrogen bond² will be discussed on meeting site.

1. H. Senoh, Z. Siroma, N. Fujiwara, K. Yasuda, J. Power Sources, 185, 1 (2008).

2. S. Trasatti, J. Electroanal. Chem. Inter. Electrochem., 39, 163 (1972).