AuCo/TiO₂-NTs Anode Catalysts for Direct Borohydride Fuel Cells

Abstract

Gold-cobalt catalysts deposited on titania nanotubes surface (denoted as AuCo/TiO 2 -NTs) are studied as possible anode materials for direct borohydride/hydrogen peroxide fuel cells. The morphology and composition of the catalysts is characterized by field emission scanning electron microscopy and by inductively coupled plasma optical emission spectroscopy. The fuel cell measurements are performed at four temperatures, in the 25-55 • C range, on a lab-scale direct alkaline NaBH 4 -H 2 O 2 single fuel cell. Polarization curves are recorded to evaluate the fuel cell performance using each prepared anode catalyst. The peak power density dependence on temperature is in the 87-102 mWcm -2 range for Co/TiO 2 -NTs catalyst and in the 163-283 mWcm -2 range for the AuCo/TiO 2 -NTs catalysts with Au loadings ranging from 10 to 60 μg Au cm -2 . The present results demonstrate that power density is raised by over 150% due to the use of AuCo/TiO 2 -NTs as anode catalysts.The direct borohydride fuel cell (DBFC), using sodium borohydride (NaBH 4 ) aqueous solution as the fuel, has been the focus of many investigations during the last decade. 1-7 This low-temperature fuel cell has specific features that make it a promising power source for portable applications. Those features include high energy density (9.3 Wh g −1 at 1.64 V), low toxicity of its reactants and products, and their easy storage and good stability in alkaline solution. 1,[8][9][10][11] Noble metals like platinum (Pt) or gold (Au) have been extensively studied as anodes for borohydride (BH 4 − ) oxidation in DBFCs. But in contrast to Pt, which is also catalyst for the detrimental BH 4 − hydrolysis reaction, Au has demonstrated high coulombic efficiency for the borohydride oxidation reaction (BOR). However, the use of Au as an electrode material is limited by its high price. One way to reduce the Au amount is to disperse Au nanoparticles (NPs) on a technologically relevant substrate. [12][13][14][15][16] Recently, alloying Au with transition metals such as Ni, 17-21 Co 21-23 and Cu 20,24-26 enables reduced cost and better catalytic features for BOR. It has been determined that bimetallic catalysts usually have higher activity and stability than the monometallic ones. 17-26 Moreover, bimetallic catalysts are of wide interest since they lead to many interesting size-dependent electrical, chemical and optical properties. Bimetallic NP catalysts are particularly important because they usually consist of a primary metal that has high electrocatalytic activity and a secondary metal that can either enhance the catalytic efficiency toward BOR or inhibit BH 4 − hydrolysis. 20,27 The 3d transition metals (Fe, Co, Ni, Cu, Zn) are able to enhance the electrochemical activity of bimetallic catalysts for BH 4 − oxidation and reduce the total cost of the electrocatalyst. It has been shown that prepared Au-Co, Au-Ni and Au-Zn bimetallic catalysts with different ratios attained excellent electrochemical performances as anodes for the DBFC. [18][19][20][21][22][23]27 In the present ...