Synthesis and characterisation of Pd–Ni–Sn electrocatalyst for use in direct ethanol fuel cells

Abstract

One critical challenge to commercialise direct ethanol fuel cells (DEFCs) is catalyst poisoning associated with the strongly adsorbed CO intermediate on catalyst's surface at low temperature. The present work therefore aimed to develop a highly electrochemically active catalyst which was durable to the presence of CO contaminant. Different Pd–Ni–Sn compositions impregnated on carbon black (CB) were synthesised by sodium borohydride reduction method which can be classified as mono-, binary- and ternary-catalyst systems at which 20%w Pd/CB was utilised as a base catalyst. The addition of 5–20%w Ni and/or 5–20%w Sn metals was found to increase catalytic activity and catalyst stability for ethanol oxidation reaction (EOR). Several diagnostic techniques were employed to assess their suitability for use in DEFCs including scanning electron microscopy–energy dispersive spectrometry (SEM–EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), CO-pulse chemisorption analysis and several modes of voltammetry. By using SEM–EDX, it was found that the actual ratios of metal loadings were relatively similar to the desired compositions and their surface morphology was uniformly distributed. The average particle size of the as-prepared electrocatalysts was in the range of 5.48–11.01nm, depending on the quantity of Sn loading. Accordingly, the introduction of Sn content over 15%w to the Pd-based catalysts (i.e. 20%Pd20%Sn/CB and 20%Pd5%Ni15%Sn/CB) resulted in a larger particle and agglomerate size due to the fast rate of SnCl2 reduction. This was also supported by the chemisorption measurement; the variation of both metal dispersion and number of active sites depended strongly on the extent of Sn loading and the values were lowest with 20%Pd20%Sn/CB. The XRD spectra confirmed that the existence of Pd, Ni and Sn in the as-prepared catalysts was in the oxidation state of Pd0, Ni2+ (in form of Ni(OH)2) and Sn4+ (in form of SnO2), respectively. The electrochemical characterisations suggested that 20%Pd10%Ni10%Sn/CB electrocatalyst exhibited superior ethanol electro-oxidation properties among various catalytic compositions with regard to current density for EOR (i.e. 146mAcm−2 or 2151mA mgPd−1), catalyst stability with an excellent CO tolerance and electrochemical surface area. This agreed well with its physical and chemical properties which could be implied that such suitable Pd–Ni–Sn composition enabled a uniform dispersion and distribution of metal nanoparticles and thus enhanced the number of active sites for EOR, CO oxidation and OH− adsorption/desorption.