Platinum alloys core-shell

This section starts with a short overview of state-of-the-art catalysts, all of which are based on platinum, followed by an explanation of the carbon monoxide issue when designing a catalyst for fuel cell applications. As the oxygen reduction reaction is the main source of kinetic performance losses of the cell, the reaction is discussed in more detail to set guidehnes for new catalyst concepts. New approaches cover platinum-containing core shell catalysis as well as de-alloyed approaches, where both classes aim for a severely reduced overall loading, and platinum-free alternatives are discussed in more detail. Finally, the influence of carbon supports on performance is discussed and alternative catalyst supports are presented. 

In summary, the organometallic approach is also efficient to prepare bimetallic nanoparticles. By precisely selecting the reaction conditions (precursor, stabilizer, reactant), we could access to ruthenium-based bimetallic nanoparticles displaying a controlled chemical order, i.e. alloy, core-shell, or even nanoparticles decorated with a second metal such as platinum, iron, or tin. These nanoparticles, which display different surface properties, can pave the way towards synergetic and selective catalytic performances. 

Park and Cheon [213] discussed an interesting synthetization route to process solid solution and core-shell type cobalt-platinum nanoparticles via redox transniet-allation reaction, reporting they had obtained nanoparticles of solid solution and core-shell structures smaller than 10 nm. These alloys were formed by redox trans-metallation reactions between the reagents without the addition of reducing agents. The reaction between Co2(CO)s and Pt(hfac)2 (hfac = hexafluoroacetylacetonate) resulted in the formation of solid solution, while the reaction between Co nanoparticles and Pt(hfac)2 in solution resulted in Co-core - Pt-shell type nanoparticles. Narrow particle size distributions were achieved in both processes. 

Kaplan, D., Burstein, L., Rosenberg, Y., and Peled, E. (2011) Comparison of methanol and ethylene glycol oxidation by alloy and core-shell platinum based catalysts. J. Power Sources, 196, 8286-8292. 

Abstract One of the most critical fuel cell components is the catalyst layer, where electrochemical reduction and oxidation of the reactants and fuels take place kinetics and transport properties influence cell jjerformance. Fundamentals of fuel cell catalysis are explain, concurrent reaction pathways of the methanol oxidation reaction are discussed and a variety of catalysts for applications in low temperature fuel cells is described. The chapter highlights the most common polymer electrolyte membrane fuel cell (PEMFC) anode and cathode catalysts, core shell particles, de-alloyed structures and platinum-free materials, reducing platinum content while ensuring electrochemical activity, concluding with a description of different catalyst supports. The role of direct methanol fuel cell (DMFC) bi-fimctional catalysts is explained and optimization strategies towards a reduction of the overall platinum content are presented. 

Varying the kinetics of decomposition of the precursors was profitable for the controlled synthesis of RuPt NPs. Whilst the co-decomposition of [Ru(COD(COT)] and [Pt(dba)2] in the presence of polyvinylpyrrolidone as stabilizer led to a RuPt alloy with a fee structure, core-shell RuPt NPs were obtained in PVP, using [Pt(CH3)2(COD)] (i.e., [dimethyl(l,5-cyclooctadiene) platinum (II)]) instead of [Pt(dba)2] [91]. This is related to the slower rate of decomposition of [Pt(CH3)2(COD)]. In summary, the chemical order can be controlled via the choice of the precursor. The chemical segregation leading to core-shell RuPt results from kinetic (decomposition rate of the metal precursors) and thermodynamic (preferred location of each metal in the particle) parameters as well as from the steric... 

---