Sustainable Synthesis of Bismuth-Based Core-Shell and De-Alloyed Nanoparticles As Electrocatalysts for Proton Exchange Membrane Fuel Cells

Abstract

The dissolution of base metal cores made of Ni, Fe, and Co in the hot, acidic environment of proton exchange membrane fuel cells (PEMFCs) has impeded the use of novel core-shell nanoparticles (NPs) where the precious metals Pt and Ru are confined to surface. Since electrocatalysis occurs on surfaces, a substantial reduction in precious metal use could be achieved with core-shell NPs relative to pure systems, as long as the core remains stable during use. Bi is an abundant earth metal with a low environmental and human toxicity and water stability across a wide pH range, particularly under the acidic conditions of a fuel cell environment. These characteristics make Bi an ideal candidate for use as a durable and inexpensive core material to minimize precious metal use in highly dispersed core-shell Bi-Pt nanoparticle (NP) electrocatalyst in PEMFCs. Bi has been found to substantially promote direct alcohol fuel cell (DAFC) efficiency relative to pure Pd by mitigating catalyst poisoning due to irreversibly bound reaction intermediates such as CO and acetaldehyde. Unfortunately, the formation of Bi NPs is difficult and currently based on unsustainable methods employing toxic organic solvents, expensive organic precursors, and dangerous reducing agents such as hydrazine and lithium that frequently yield highly non-uniform structures. In this work, we describe an inorganic ligand metal redox synthesis approach which is used to synthesize a variety of well-defined core-shell as well as Pt-Bi alloy nanoparticle structures under sustainable, all-aqueous conditions. The approach uses SnCl3- as both a reducing agent and inorganic stabilizing ligand to provide unprecedented control over the size and structure of these nanoparticles. Furthermore, the ligand itself exists in a variety of structures that can be used to control electrostatic assembly onto various substrates. The materials to be discussed in this presentation have never been previously reported and their synthesis is unexpected given both that pure Bi has well-known instability in high salt, low pH environment and that Bi-Sn ligand formation has not been previously recognized. The implications of such metal NP synthesis are transformative with respect to the practical realization and wide application of alternative energy technologies given its simplicity and sustainability. We will provide data showing that such structures can double the electrocatalytic oxygen reduction reaction mass activity and increase the specific activity by nearly an order of magnitude relative to pure systems (see graphic below). As important, we observe simultaneous mitigation of carbon monoxide poisoning due to the persistence of Bi and Sn on their surface. These observations are discussed relative to the surface distribution of active sites as characterized utilizing electrochemical adatom stripping. Figure 1