Platinum-palladium electrodes for poly electrolyte membrane fuel cell
Polymer electrolyte (or proton exchange) membrane fuel cells (PEMFCs) are an energy efficient alternative to combustion engines and are on the cusp of mass production. Before its commercialization, advancements have to be made in terms of cost reduction and improved performance. A significant amount of the cost of PEMFC is incurred by the platinum (Pt) electro catalysts, which conventionally has Pt nanoparticles dispersed on carbon black particles to enhance the specific surface area significantly, but concomitant durability problems due to carbon support corrosion and loss of surface area under PEMFC working conditions have led to renewed interest in carbon-free nanostructured electrodes. In this context, low-cost and simple, salt solution-based print-expose-develop inkjet printing technique derived from well-developed silver halide chemistry in the field of photography, was used to obtain silver nanowires on flexible substrates like chromatographic paper and Nafion poly-electrolyte membrane. Owing to their excellent conductivity, nanoporosity and enhanced surface area, these Ag nano-structures were used as substrate to deposit Pt overlayers in the form of atomic shells through the self-terminating Pt monolayer deposition – a process for controlled growth of Pt monolayers tantamount to wet atomic layer deposition, but with a simple electrochemical setup. This type of layer by layer deposition of Pt has been well observed on Pd nanowires to make ultra-low PGM (platinum group metal) loaded electro-catalysts. As such, the activity for the oxygen reduction reaction (ORR) of Pd is a bit lower than that of Pt, but by addition of a suitable metal, such as Fe, the ORR activity of Pd may overcome that of Pt. Conversely, the activity for the hydrogen oxidation reaction (HOR) of Pd is very-much lower than that of Pt, but by adding small amount of Pt, the HOR activity of Pd attains that of pure Pt. In this context, Palladium was coated on silver nano-structures through galvanic displacement method – an easy to implement method. The palladium deposition was optimized to retain the high conductivity (< 10 Ω / sq) of Ag@Pd nano-structures. Further, these Ag@Pd nanostructures will be coated with Pt atomic layers via the self-terminating Pt electro-deposition process to form conductive, nano-porous, electro-catalytic active catalyst layers for PEMFC applications.
Keywords: fuel cells, nanoparticles, inkjet printing, silver nanowires, self-terminating, galvanic displacement