High Mass Activity and Stability of Core-Shell Pd-Ni Nanoparticles Electrodeposited on Diamond Electrodes for Direct Alcohol Fuel Cell Applications

Abstract

Sustainable ‘green’ energy applications have received increasing research efforts in the last few years to deal with issues such as climate change, energy shortage and environmental pollution. A subcategory of fuel cells, the direct alcohol fuel cells (DAFCs) have been established in that field due to their ability to convert the chemical energy stored in the fuel into electricity and can be utilised in portable devices and transportation applications. Electrocatalysis is an important aspect in the development of such sustainable technologies. Hybrid materials consisting of graphitic nanostructures and various metal or metal oxide complexes have been widely studied as functional components in catalysis for fuel cell applications. The development core-shell nanostructures and replacing or reducing noble metal content by the incorporation of non-noble metals promises to increase the overall mass activity and improve stability due to electronic interactions with the interfacial layer in the inner shell. Herein, the electrodeposition of Ni and Pd nanoparticles supported on oxygen terminated boron-doped diamond (OBDD) electrodes is described. The electrochemical modification involves two-steps; first electrodeposition of Ni as nanoparticles on OBDD followed by Pd onto the freshly deposited Ni nanoparticles. SEM imgage of the sequential deposition of Pd-Ni produced nanoparticles in the size range of 10-13 nm is shown in Fig. 1 (a). XPS and electrochemical characterization showed that Ni nanoparticles act as reactive centres on OBDD for the subsequent deposition of Pd to form core-shell nanostructures. The Pd-Ni/OBDD electrocatalysts were evaluated in an alkaline aqueous solution containing ethanol and compared to Pd/OBDD itself. The formation of core-shell nanostructures led to high current responses for ethanol oxidation. The phenomenon is attributed to the improved dispersion of Ni onto the BDD and the electronic interaction between the interfaces of the two metals. Of note is the fact the core-shell catalysts exhibited enhanced mass activity and operational stability. After chronopotentiometric measurements, there was only 12% current drop of the Pd-Ni/OBDD in comparison to 35% current drop of the Pd/OBDD catalyst itself. The Pd-Ni electrocatalysts even with 4 times less palladium loading showed higher amperometric response than the pure Pd/OBDD (Fig. 1(b)). The higher mass activity is likely due to an increased tolerance to carbon based reaction by-products poisoning effects at the bimetallic interface with the transition metal in the inner shell. Figure 1 (a) SEM image of Pd-Ni modified OBDD electrode (b) Cyclic voltammograms of Pd and Pd-Ni modified OBDD in 0.5 M KOH and 1 M EtOH at a potential scan rate of 100 mV s-1. Figure 1